Catarina Viegas

Margarida Gonçalves

Liliana Soares

Benilde Mendes

Bioremediation of Agro-Industrial Effluents Using Chlorella Microalgae

Keywords: microalgae, bioremediation, poultry industry, pig industry, Chlorella vulgaris, Chlorella protothecoides

Two microalgae species (Chlorella vulgaris and Chlorella protothecoides) were tested at lab scale in order to select the optimal conditions for biomass productivity and the efficient remediation of effluents from poultry and pig industries. Both microalgae showed biomass productivities in the agroindustrial effluents that were comparable to the Chlorella synthetic medium used as control. C. protothecoides presented the higher productivities both for poultry effluents (46.13 and 41.75 mg.L -1 .day -1 for raw and flocculated effluents) and for pig manure (95.86 mg.L -1 .day -1 ). The supplementation of pig effluents with biomass ash increased by 50% the microalgae productivity with the highest results obtained for C. protothecoides and C. vulgaris at ash concentrations of 1.5 g/L and 3.0 g/L, respectively. The optical density of both effluents was efficiently reduced by both microalgae but particularly by C. protothecoides and in the presence of added ash, indicating that significant reductions of suspended solids and organic matter occurred. The results showed that poultry and pig effluents may be efficiently remediated with microalgae and the fortification with biomass ash benefits the process.

Introduction

Current technologies for the treatment of urban wastewaters and agro-industrial effluents represent complex and costly procedures but are necessary because those effluents contain a high organic load and can cause eutrophication in freshwater and marine ecosystems if discharged without treatment [START_REF] Mo | Can municipal wastewater treatment systems be carbon neutral?[END_REF][START_REF] González-García | Life Cycle Assessment of broiler chicken production: A Portuguese case study[END_REF].

The animal production industry (poultry, pig and cattle industries) is a source of various wastes with significant environmental impact, including manure, effluents from cleaning activities and wastewaters from dead animal processing. All organic residues from the slaughterhouses are homogenized, treated thermally and sent to a solid-liquid separator. The decanted solid wastes are used for pet food production while the aqueous phase with a high fat content is subject to a series of unitary operations to reduce its organic load and sent to the wastewater treatment plants. In traditional productions the mix of urine, faeces and wastewater were disposed on the ground as fertilizer, but with the increase in intensive farms this option is no longer usable [3].

Moreover the direct use of agro-industrial effluents as fertilizers has negative environmental impacts such as unpleasant odour emissions, contamination with pathogenic microorganisms and contamination of ground waters with components of those effluents.

Microalgae present very high biomass production rates (much higher than the vascular plants) and have their low cultivation demands enable their production on degraded lands, wastelands, deserts and even on off-shore structures, thereby not competing with the food sector [START_REF] Demirbas | Biofuels from algae for sustainable development[END_REF].

In the case of agro-industrial effluents with high levels of nutrients but not contaminated with hazardous elements the produced algal biomass can be incorporated at different stages of the industrial process, in a circular economy strategy. Microalgae applications include feed supplement [START_REF] Fredriksson | Fatty acid and carotenoid composition of egg yolk as an effect of microalgae addition to feed formula for laying hens[END_REF], bio-fertilizers for feed crops [START_REF] Wang | The research progress of CO2 sequestration by algal biofertilizer in China[END_REF] and/or feedstock for biofuel production [START_REF] Mata | Sustainable production of biodiesel from tallow, lard and poultry fat and its quality evaluation[END_REF].

Microalgae can be used in food and feed due to their chemical composition, with a high protein content, but also because they are a source of almost all the essential vitamins (eg A, B1, B2, B6, B12 , C, E, biotin, folic acid, pantothenic acid, nicotinate) are rich in pigments such as chlorophylls, carotenoids and phycobilins [START_REF] Milledge | Commercial application of microalgae other than as biofuels: a brief review[END_REF][START_REF] Spolaore | Commercial Applications of Microalgae[END_REF][START_REF] Dufossé | Microorganisms and microalgae as sources of pigments for food use: a scientific oddity or an industrial reality?[END_REF].

This PhD program is focused in the optimisation of microalgae-based bioremediation processes for agro-industrial effluents and studying various applications of the produced microalgae biomass.

The different tasks to be implemented in order to achieve this main objective are: a) Optimization of the growth of different microalgae (Chlorella vulgaris, Chlorella protothecoides and Scenedesmus obliquus) in effluents from poultry and piggery industries. b) Mixing of animal production effluents with other agro-industrial effluents with nutrient limitations in order to achieve the remediation of both effluents. c) Testing the use of biomass ashes as inorganic additives for microalgae growth as an alternative to the use of pure inorganic components of the culture medium. d) Characterisation of the microalgae biomass as a supplement for animal feed. e) Use of the produced microalgae biomass to increase the organic load of soils and as a fertilizer for fast growing vegetable species. f) Application of the optimized methods in the construction and test of a pilot plant for the integrated bioremediation of mixed agro-industrial effluents.

In the present work aims it was studied the bioremediation of effluents from the poultry and pig industries using the microalgae Chlorella vulgaris and Chlorella protothecoides. Because these effluents have a strong organic load, the mixotrophic alga (Chlorella protothecoides) was used to evaluate the contribution of the heterotrophic metabolism to the microalgae productivity. Biomass ashes (raw or predigested) were evaluated as mineral supplements for the pig effluents, in order to test growth limitations due to inorganic factors.

Relationship to Cyber-Physical Systems

The production of microalgae in an industrial scale and their use in the remediation of industrial effluents requires as other industrial processes, the implementation of automated systems for process monitoring and control. In microalgae production the automated control of liquid and gaseous flows and the continuous measurement of parameters such as pH, conductivity, temperature and optical density is critical to achieve stable operation and apply the necessary correction actions whenever these parameters deviate from optimum values.

Cyber-physical systems (CPS) are physical and engineered systems whose operations are monitored and controlled by a computing and communication core [START_REF] Rajkumar | Cyber-Physical Systems: The Next Computing Revolution[END_REF]. In the case of microalgae-base bioremediation systems there are two levels of parameter monitoring that should be implemented: process parameters such as temperature or pH that must be constant for ideal operation and culture parameters such as optical density or concentrations of specific elements that are being consumed during algae growth.

During microalgae growth automated systems should contemplate stirring of the culture medium and may include mechanical systems to adjust the position of the bioreactors in order to achieve a maximum exposure to solar light.

Mechanical systems for continuous cleaning of the internal surface of the bioreactor may also be implemented to ensure maximum light penetration in the culture medium. When the culture achieves a given value of optical density a fraction of the culture medium should be discharged to a decantation pond and a given volume of fresh medium should be added to the bioreactor in order to replenish the level of nutrients to sustain microalgae growth.

The association of continuous sensors, automated valves and computational control center are essential elements for the implementation of large scale microalgae bioremediation systems in which the culture achieves a steady state of biomass production.

Methods

Two microalgae (Chlorella vulgaris and Chlorella protothecoides) were grown in two different effluents from the poultry industry (raw effluent and flocculated effluent) and a pig manure effluent mainly composed by urine. For the control it was used a Chlorella culture synthetic medium [START_REF] Vonshak | Laboratory techniques for the cultivation of microalgae[END_REF].

Biomass ashes were added to the pig manure effluent at concentrations of 1.5 g/L and 3g/L and in two different formulations: as received or pre-digested with concentrated sulfuric acid. The pig manure effluent was boiled and diluted (1:2) with tap water, before use.

The experiments were conducted at room temperature (22 ºC ± 2 ºC), under artificial lighting (10000 lux) with cycles of 12 h light/12 h dark, using 500 mL of effluent agitated by air bubbling. Algal growth was followed by measuring the culture optical density at 540 nm and dry weight. The pH was also measured and controlled in order to remain in the range of 7 to 8. The parameters evaluated in the effluents before and after microalgae growth were: total and suspended solids, total nitrogen, ammonia nitrogen, nitrate, nitrite, total phosphorus, chemical oxygen demand and biochemical oxygen demand. The growth experiments were performed in duplicate and the trials ended when the optical density started to decrease.

Results and Discussion

The characteristics of the tested effluents and the culture control medium were analyzed and presented in Table 1.

It can be observed that nitrogen concentrations in the effluents are comparable or higher than in the culture medium therefore this is not a limiting nutrient in these agro-industrial waste streams. On the other hand, the presence of various organic components namely proteins and lipids, some with a limited solubility in water, explains why the agro-industrial effluents have an optical density considerably higher than the synthetic culture medium.

The flocculated poultry effluent had an optical density lower than the raw effluent because the flocculation/decantation process is mainly effective in the removal of fat and suspended solids but does not affect dissolved nitrogen and phosphorus species, whose concentration in both effluents depends on the mix of wastes being treated.

The raw effluent and the flocculated effluent used in this trial were supplied by the same poultry industry but correspond to different moments of collection so their mineral composition is not necessarily related. The high content of nitrogen of the pig effluent is expected since it is mainly composed of pig urine. On the other hand the poultry and pig effluents may have limitations of some inorganic components that are essential for the microalgae growth since total ash content is significantly lower than the synthetic culture media. In a first set of experiments microalgae C. vulgaris and C. protothecoides were grown in the raw and flocculated poultry effluents and the productivities were comparable but inferior to those of the control medium (Fig 1 ),although C. protothecoides had a better performance both in the control medium and in the effluents.

Fig. 1. Microalgae productivity in poultry effluents (mean values and standard deviation).

The lower microalgae productivity in both poultry effluents when compared with the control medium indicates that their lower mineral content and phosphorus concentrations are probably limiting microalgae growth.

The effect of algal growth in the optical density of the poultry effluents is presented in Fig 2. The microalgae growth caused an important reduction on the optical density of the effluents indicating that total solids and organic matter were used by the microalgae or aggregated to their cell wall. This effect was more pronounced for C. protothecoides that achieved final optical densities comparable to the control medium, probably due to its ability to metabolize organic compounds using heterotrophic pathways.

In a second set of experiments pig effluents were used as media to grow the same microalgae (Fig. 3). This effluent already has nitrogen and phosphorus contents higher than the poultry effluents so is expected to better fulfil the requirements of microalgae growth and the incorporation of biomass ashes as mineral supplements aims at testing the role of inorganic components as limiting nutrients. Effectively both C. vulgaris and C. protothecoides had better productivities in the pig effluent than in the poultry one achieving values comparable to the control medium (Fig. 3). The incorporation of biomass ashes had a positive effect in the growth of both microalgae but C. protothecoides was again the microalga with higher specific growth rate. The maximum productivity for C. protothecoides (95.9 mg.L -1 .dia -1 ) was attained in the experiment with 1.5 g/L of digested ashes while for C. vulgaris a maximum productivity of 80.9 mg.L -1 .dia -1 was obtained with incorporation of 3 g/L of undigested ashes.

The yield obtained for C. vulgaris grown in pig manure (39,4 mg.L -1 .day -1 ) was higher than that obtained by Reda et al. (23 mg.L -1 .day -1 ) which used biologicallytreated piggery wastewater effluent [START_REF] Reda | Microalgal species growing on piggery wastewater as a valuable candidate for nutrient removal and biodiesel production[END_REF]. Other authors used piggery wastewater (COD of 11g/L) autoclaved at 120ºC and diluted about 10 times with distilled water to grow the microalgae Chlorella pyrenoidosa and obtained a productivity of 6.3 mg.L -1 .day -1 [START_REF] Wang | Mixotrophic cultivation of Chlorella pyrenoidosa with diluted primary piggery wastewater to produce lipids[END_REF]. The incorporation of biomass ash resulted in a 50% increase in growth indicating that this mineral waste could be thus valorised as an inorganic supplement and no significant toxic effects were detected. The effect of the microalgae growth on the optical density of the pig effluent is presented in Fig. 4.

The optical density of the pig effluent used for microalgae growth with or without ash supplementation suffered a significant reduction achieving in some cases values comparable to the control medium (Fig. 4). The addition of biomass ashes contributed to an increase of the initial optical density of the pig effluent, especially for the 1.5 g/L dose in the undigested formulation. Nevertheless the final optical densities of the pig effluents used with added ash were generally lower than the pig effluents used without ash supplementation either because a higher microalgae productivity resulted in a better remediation of the effluent or because the biomass ashes induce the aggregation of the organic matter present in the effluent by altering their electrostatic status.

Conclusions

Microalgae can have an important role in the low cost remediation of complex effluents whose chemical and physical treatment methods are complex and expensive.

The results obtained in this series of experiments demonstrate the ability of C. vulgaris and C. protothecoides to bioremediate poultry and pig effluents, achieving a significant reduction in total solids and organic matter contents.

The incorporation of an inorganic waste (biomass ash) had a positive effect both in microalgae growth and effluent remediation. This approach constitutes an alternative use for these mineral wastes and reduces microalgae culture requirements of inorganic supplements, and therefore the production costs. Future work comprehends the conception of a pilot scale microalgae remediation system applied to a small scale agroindustry to achieve an efficient and fast remediation of wastewaters and production of a biofertilizer. This system will comprise a cyber-physical monitoring system with automated data acquisition linked with alert systems to ensure maintenance of equilibrium conditions. Automatic replacement of culture medium with fresh effluent to achieve a steady state culture is another goal that could be attained by incorporating electronically actuated mechanical devices.

Fig. 2 .

 2 Fig. 2. Optical density of the poultry effluents measured before and after the microalgae growth experiments (mean values and standard deviation).

Fig. 3 .

 3 Fig. 3. Microalgae productivity in pig manure effluent, with and without ash addition (mean values and standard deviation).

Fig. 4 .

 4 Fig. 4. Optical density of the pig effluents measured before and after the microalgae growth experiments (mean values and standard deviation).

Table 1 .

 1 Characterization of the effluents used in the trials.

	Effluent	Total Nitrogen (mg N/L)	Total Phosphorus (mg P/L)	O.D. (540nm)	Ash Content (g/L)
	Raw Effluent (poultry)	160.78 ± 6.5	37.3 ± 0.5	0.778	0.46 ± 0.03
	Floccul. Effluent (poultry)	202.03 ± 2.6	105.4 ± 8.4	0.372	0.54 ± 0.04
	Manure Effluent 1:2	313.60 ± 15.8	83.7 ± 0.5	1.561	1.66 ± 0.01
	Synthetic Culture Medium	173.1	283.9	0.087	4.64


Ana Correia
email: a.correia@campus.fct.unl.pt

João Goes

Pedro Barquinha

Oxide TFTs on Flexible Substrates for Designing and Fabricating Analog-to-Digital Converters

Keywords: Amorphous oxide semiconductors, IGZO TFTs, circuit integration, ADCs, ∑∆ modulator

Thin-film transistors (TFTs) employing oxide semiconductors have recently emerged in electronics, offering excellent performance and stability, low processing temperature and large area processing, being indium-galliumzinc oxide (IGZO) the most popular amorphous oxide semiconductor. In this work it is shown how IGZO TFTs can be integrated with multilayer high-κ dielectrics to obtain low operating voltages, both on glass and flexible PEN substrates. Then, the electrical properties extracted from these devices are used to design and simulate a 2 nd -order Sigma-Delta (∑∆) analog-to-digital converter (ADC), showing superior performance (e.g. SNDR≈57 dB, and DR≈65 dB) over ADCs using competing thin-film technologies.

TFTs for Large-Area Electronics: A Materials Perspective

Displays are deeply incorporated in our daily lives in mobile phones, TVs, monitors, among others. The key electronic elements of these displays are thin-film transistors (TFTs) responsible for switching each pixel on and off. A TFT is a particular case of a field-effect transistor (FET) based on thin-film technologies, presenting a semiconductor located between drain and source electrodes and the dielectric layer placed between gate electrode and the semiconductor. Using these three terminals (drain, source and gate), the principle behind this device is to control the current between drain and source electrodes (IDS) through the gate-source potential (VGS) [START_REF] Lee | Introduction To Flat Panel Displays[END_REF].

The first conceptual patents on TFTs were reported in 1930, however these devices only acquired a high relevance in the 70s, with the advances on flat-panel displays, specially with the appearance of active matrix liquid-crystal displays (AMLCDs). Hydrogenated amorphous silicon (a-Si:H) soon started to be the preferred choice for semiconductor owing to its amorphous structure perfectly suitable for large area fabrication. Still, its low field-effect mobility (µFE<1 cm 2 /Vs) was not compatible either with the fabrication of the peripheral drivers of the displays or for other circuits operating in the MHz range. Poly-Si was introduced to deal with this limitation, enabling µFE>100 cm 2 /Vs, but at the cost of larger processing temperatures and a polycrystalline structure, hindering low cost and uniform fabrication in large areas. On the other hand, organic semiconductor based TFTs were also introduced in the late 80s, as the ultimate technology, when low-cost and mechanical flexibility are major requirements. However, their electrical performance was (and is) comparable to a-Si:H TFTs and degradation under normal environmental conditions is a major concern.

The new millennium set the stage for a new transistor technology, able to fulfil the needs of flexible, low cost but nonetheless fast electronics. Oxide semiconductor TFTs are such technology, offering excellent uniformity in large areas using temperatures below 150 °C and µFE exceeding 10 cm 2 /Vs. In addition, most oxides have bandgaps (EG) above 3 eV, turning them transparent materials in the visible spectrum, enabling exciting concepts such as fully transparent and flexible devices [START_REF] Grundmann | Transparent semiconducting oxides: materials and devices[END_REF]. Even if some initial oxide TFTs were reported in the 60s, the most relevant works showing the potential of such technology started in 2002 with polycrystalline ZnO TFTs [START_REF] Masuda | Transparent thin film transistors using ZnO as an active channel layer and their electrical properties[END_REF][START_REF] Carcia | Transparent ZnO thin-film transistor fabricated by rf magnetron sputtering[END_REF][START_REF] Hoffman | ZnO-channel thin-film transistors: Channel mobility[END_REF][START_REF] Fortunato | Wide-bandgap high-mobility ZnO thinfilm transistors produced at room temperature[END_REF]. Despite the encouraging results obtained using ZnO, rapidly amorphous multicomponent materials started to be used, specially indium gallium zinc oxide (IGZO). In fact, in these amorphous oxides a "continuous path" created by spherical isotropic ns orbitals of the metallic cations, when the radii of these orbitals is larger than the distance between cations, allows obtaining excellent µFE even in amorphous structures [START_REF] Nomura | Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors[END_REF]. Although IGZO is the most widely used amorphous oxide semiconductor (AOS), some additional combinations have been considered, as indium-free alternatives such as zinc-tin oxide (ZTO).

Despite the semiconductor used in a TFT, the dielectric choice critically affects the device performance and stability. High-κ dielectrics have been used instead of traditional SiO2 due to different reasons. When low temperature deposition processes are used, semiconductors and their interfaces are more prone to have large defect density and poor compactness. Hence, the large capacitance per unit area of high-κ dielectrics compensates this, by inducing a larger charge density per unit voltage. This also allows for low-voltage operation: as will be seen in this work, high-κ results in less than half of the operation voltage (seen by transconductance saturation) compared to thermal SiO2 with similar thickness. Furthermore, the larger film thickness (without compromising a good overall capacitance) enabled by high-κ dielectrics compensates the more degraded dielectric film properties fabricated at low temperatures, avoiding for instance quantum tunnelling effects. Additionally, selecting high-κ materials with an amorphous structure typically results in smoother films and, consequently, in improved interfaces with amorphous semiconductors, while the lack of grain boundaries of amorphous structures decreases the gate leakage current (IG). However, most of the high-κ dielectrics have lower EG than lower-κ materials as SiO2, which based on band alignment considerations can be problematic to obtain low IG. Hence, multicomponent dielectrics based on combinations of materials with high-κ and high-EG, such as Ta2O5 and SiO2, respectively, has been studied in different configurations, taking advantage of structural and electrical properties of both materials [START_REF] Zhang | High-performance ZnO thin film transistors with sputtering SiO2/Ta2O5/SiO2 multilayer gate dielectric[END_REF].

2 Relationship to Cyber-Physical Systems: From Oxide TFTs to a Complete System-on-Foil

The current advances in oxide TFTs are changing and expanding the display market, enabling higher refresh rate combined with ultra-definition and stimulating new approaches such as curved or transparent displays. The big players in the multi-billion dollar display industry are naturally all aware of this TFT technology, with an increasing number of prototypes and even commercial products being available from companies as Samsung and LG. One of the most relevant examples is the line of curved OLED TVs by LG, making use of oxide TFT backplanes. Despite all this activity in taking oxide TFTs to displays, efforts have also been done to take full advantage of oxide TFTs and integrate them in other electronic circuits and systems. In fact, good uniformity over large area, excellent electrical performance and stability, low production costs and the flexible or even recyclable concepts encourage their integration in full system-on-foil concepts, expanding their functionality and their applicability. To the author's best knowledge, the first circuits using oxide TFTs appeared in 2006, in which inverters and a five-stage ring oscillator (RO) have been reported using indium-gallium oxide (IGO) TFTs. Regarding inverters, a peak gain magnitude close to 1.5 was measured and a maximum oscillation frequency of 9.5 kHz has been experimentally demonstrated [START_REF] Presley | Transparent ring oscillator based on indium gallium oxide thin-film transistors[END_REF]. Several RO performance improvements have been achieved by decreasing the propagation delay and, therefore, allowing to increase the maximum oscillation frequency and the number of cascaded stages [START_REF] Ofuji | Fast Thin-Film Transistor Circuits Based on Amorphous Oxide Semiconductor[END_REF][START_REF] Mativenga | Transparent Flexible Circuits Based on Amorphous-Indium-Gallium-Zinc-Oxide Thin-Film Transistors[END_REF]. Shifter registers, current mirrors and a digital-toanalog converter (DAC) were also produced using oxide TFTs. Regarding DAC, which is already a circuit realization with a considerable level of complexity, encouraging results were achieved using IGZO TFTs, showing a spurious-free dynamic range (SFDR) above 30 dB up to 300 kHz and a sampling rate of 1 MS/s [START_REF] Raiteri | A 6b 10MS/s current-steering DAC manufactured with amorphous Gallium-Indium-Zinc-Oxide TFTs achieving SFDR > 30dB up to 300kHz[END_REF]. It is also relevant to notice the relatively recent developments of using TFTs in applications spanning from near-field communications (NFC) to radio-frequency identification (RFID) tags. In 2011, the first RFID chip was produced using IGZO TFTs, showing a small power dissipation (20 µW) making it suitable for short-range wireless operation [START_REF] Cui | Simple Noise Margin Model for Optimal Design of Unipolar Thin-Film Transistor Logic Circuits[END_REF]. In 2015, a fully flexible NFC tag also using IGZO TFTs has been fabricated, exhibiting excellent performance, as a maximum data rate close to 72 kbits/s [START_REF] Myny | 3 Flexible thin-film NFC tags powered by commercial USB reader device at 13.56MHz[END_REF]. These last examples intensify the potential of using oxide TFTs for an entire system-on-foil concept where, besides performance and processability of electronic devices in low cost and flexible substrates, multifunctionality of the platform using only one semiconductor technology is a paramount advantage in terms of reliability and cost of the complete system.

The referred system-on-foil concept plays a relevant role nowadays in the global cyber-physical systems (CPS) approach, which combines computation and physical processes networking and it is transversal to multiple fields of knowledge, involving areas such as healthcare, energy or consumer appliances. In fact, the excellent performance and stability of oxide technologies are key enablers to develop innovative, more efficient and high frequency systems, improving strongly the communication in complex systems.

Oxide TFTs Produced at FCT-NOVA

Oxide TFTs have been studied using different approaches in order to optimize and enhance TFT performance, and reducing at the same time the involved fabrication costs. For that end different materials, structures and processes, such as the usage of organic insulators, nanowire structures and solution processes, respectively, have been studied in different research groups worldwide, including CENIMAT and CEMOP at FCT-NOVA. This section summarizes a small part of these developments, which were then used as the basis to design the ADC case study presented in section 4. In this case, since complex circuit design demands for good performance and stable devices, sputtered IGZO was selected as the semiconductor, given that it is the most mature process/material available in-house. IGZO was then coupled with different sputtered high-κ dielectrics based on Ta2O5 and SiO2, evaluating the effect of the dielectric on important device parameters such as µFE, turn-on voltage (Von) and IG.

Devices Fabrication and Characterization

IGZO TFTs were produced on Corning Eagle glass and on flexible Polyethylene Naphthalate (PEN), depositing all layers using RF magnetron sputtering. These devices present a staggered bottom gate, top contact structure and were fabricated as follows: Molybdenum (Mo) gate electrodes (≈60 nm thick) were deposited in an Ar atmosphere; for the dielectric a multilayer structure with alternating SiO2 and cosputtered Ta2O5+SiO2 (denoted TSiO) layers was fabricated, with a thickness of 100-150 nm. The semiconductor, IGZO (≈40 nm thick and 2:1:1 In:Ga:Zn atomic ratio), was deposited in an Ar+O2 atmosphere. At the end, Mo source and drain electrodes were fabricated using the same procedure as for gate electrode. Both the electrodes and the semiconductor were patterned by lift-off and, on the other hand, the dielectric layer by dry-etching, in a SF6 atmosphere. To conclude the production, the devices fabricated on glass and on PEN were annealed at 180-200 and 150 °C, respectively, on a hotplate, in air.

Electrical characterization of devices was performed using a semiconductor parameter analyzer (Keithley 4200-SCS) and a probe station (Cascade Microtech M150) under dark room conditions.

IGZO TFTs on Glass Substrate

As discussed in [START_REF] Correia | A Second-Order ΣΔ ADC Using Sputtered IGZO TFTs[END_REF] a multilayer dielectric comprising TSiO and thin SiO2 (with the latter being in contact with IGZO) results in considerably improved device yield over single layer TSiO or Ta2O5. In fact, recent improvements in tuning the thickness of these multilayer dielectrics enabled breakdown field (EB) >7 MV/cm and κ≈10, resulting in IGZO TFTs with IG<1 pA and low operation voltage (yielding a transconductance saturation for VGS<6 V). Fig. 1 shows transfer and output curves of one IGZO TFT with a 150 nm thick multilayer dielectric comprising 7 alternating SiO2/TSiO layers, annealed at 180 °C, with a width-to-length ratio (W/L) of 40/20 (µm). Excellent performance is obtained: µFE≈14 cm 2 /Vs, On-Off ratio≈10 9 , Von=-1.5 V, subthreshold slope (S) of 0.16 V/dec, negligible hysteresis, hard saturation and no current crowding at low VDS. Note that these results are quite similar to the ones obtained when the same IGZO layers are deposited on state-of-the-art thermal dry SiO2. In fact, S and operating voltage are even improved with the present multilayer dielectric, owing to its larger κ (in thermal SiO2 S=0.20 V/dec and transconductance saturation occurs for VGS=15 V).

IGZO TFTs on Flexible PEN Substrate

The possibility of using flexible substrates brings significant advantages to a TFT technology. First, it enables the usage of roll-to-roll fabrication tools, highly desirable for large area electronics. Then, at a system level they enable weight and thickness reduction over conventional glass. Finally, bendable or even rollable products can be conceived. Given the low temperature fabrication required for IGZO TFTs with sputtered multilayer dielectrics, their performance was also analyzed on PEN substrates. Fig. 2 shows the transfer curves and a photo of such devices with a 7 layer dielectric, having a total thickness ≈100 nm, annealed at only 150 °C. It is noteworthy the excellent electrical performance obtained: in fact, besides the larger hysteresis, all the other parameters reveal to be even slightly improved over the ones achieved on glass substrates (Fig. 1), which is also a consequence of the thinner dielectric layer used on PEN. Electrical characterization under different bending radius is currently underway.

ADCs using Oxide TFTs Produced at FCT-NOVA

As mentioned before, circuit integration is a crucial demonstration of the potential of a new transistor technology. ADCs are one of the most difficult circuits to implement and, given their importance in a variety of fields such as electronic, medical, telecommunications or other systems that need signal processing, their implementation using a new technology is a significant achievement. Despite the lack of maturity compared to the dominant TFT technology (i.e., a-Si:H), oxide TFTs have been implemented in systems with a reasonable level of complexity and integration but, to the best of the authors' knowledge and up to date, no kind of reference was found regarding the use of oxide TFTs in the design and practical implementation of ADCs. Some works related with practical realizations of ADCs using other thin-film technologies are summarized in [START_REF] Dey | Ieee: Amorphous Silicon 5 Bit Flash Analog to Digital Converter[END_REF][START_REF] Lin | A CBSC second-order sigma-delta modulator in 3 µm LTPS-TFT technology[END_REF][START_REF] Marien | A Fully Integrated Delta Sigma ADC in Organic Thin-Film Transistor Technology on Flexible Plastic Foil[END_REF]. Although being a small part of all developed work, it contains examples of ADCs employing a-Si:H, low-temperature poly-silicon (LTPS) and organic TFTs. Excellent performance, good stability, large EB and low IG are some of the key characteristics of IGZO TFTS with multilayer dielectric previously optimized, encouraging to proceed to the design of a 2 nd -order Sigma-Delta (∑∆) ADC using the devices produced at FCT-NOVA. An ADC architecture based on oversampling and noise-shaping techniques has been adopted, since due its intrinsic feedback nature, it can easily deal with high process variations and mismatch errors in the physical/electrical parameters of the TFT devices during fabrication. It is important to notice that, due to the lack of a reproducible and stable p-type oxide TFTs, only ntype devices were used in the circuit design. Additionally, due to the expected high device mismatch and poor absolute accuracy when these passive elements are fabricated, SMD passive elements have been employed in order to minimize additional risks in the early phase of proof-of-concept.

Virtuoso TM Platform and Spectre TM Simulator from CADENCE have been used to design and electrically simulate all building blocks. Furthermore, a model for IGZO TFTs was adapted from an a-Si:H TFT model developed by Semiconductor Devices Research Group at RPI, with good fitting to experimental data.

Given the relevance of the comparator in the ∑∆ modulator (∑∆M), the active block in circuit, it was designed and simulated in order to obtain the best performance. It employs a cascade of three pre-amplification stages, a positivefeedback analog latch stage and four logic inverters implementing a fully-dynamic digital latch. Results show a low offset, working at several tens of kHz, with an accuracy ≈10 mV. Moreover, it supports the Von variations of fabrication [START_REF] Correia | Design of a robust generalpurpose low-offset comparator based on IGZO thin-film transistors[END_REF].

Regarding the final design of continuous-time ∑∆M, it employs an analog loop filter (a cascade connection of two fully passive RC-type integrators), implementing the 2 nd -order noise shaping transfer function. ADC simulations reveal: SNDR≈57 dB, DR≈65 dB, and power dissipation, approximately, of 22 mW (VDD=10 V). Comparing these simulation results, shown in Table 1, with the current state-of-the-art of ∑∆Ms, it can be observed that this work is clearly beyond prior art, for competing thin-film technologies such as organics or even LTPS, supporting the continuity of this work. However, the operation frequency and intrinsic gain of these devices can limit the performance of the ∑∆M. Nevertheless, results give an excellent stimulus to produce one of the first 2 nd -order ΣΔ ADC using oxide TFTs.

Another paramount step in this kind of work is the layout, specially using new technologies for which a process-design-kit (PDK) is not available. Consequently, a parameterized cell (PCELL) needs to be constructed taking into account the structure of devices and all process/material constraints. Given the absence of PDK, automatic design rule check (DRC) and layout versus schematic (LVS) could not be performed using EDA (electronic design automation) tools. Hence, layout of the entire circuit with a die area close to 10 mm 2 still needs to be checked manually. More details regarding design, simulation results and layout can be found in [START_REF] Correia | A Second-Order ΣΔ ADC Using Sputtered IGZO TFTs[END_REF].

Conclusions

Fully sputtered IGZO TFTs have been produced both on glass and PEN exhibiting comparable electrical performance, with µFE≈14 cm 2 /Vs, On-Off ratio≈10 8 -10 9 , Von=-1.0 to -1.5 V and subthreshold slope (S) of 0.16 V/dec. These properties are achieved with only 150-180 °C annealing temperature and are even superior to the ones obtained with high-temperature 100 nm thick dry-SiO2 dielectric from conventional CMOS technology. A 2 nd -order ∑∆M was designed and simulated based on the measured IGZO TFTs, showing a SNDR≈57 dB, and a power dissipation of 22 mW. Circuit layout with a die area of 10 mm 2 is currently under final LVS and DRC verifications. Fabrication and experimental evaluation will be the next steps.

Fig. 1 .

 1 Fig. 1. Transfer (left) and output (right) curves of an IGZO TFT on glass with a sputtered 7 layer SiO2/TSiO dielectric.

Fig. 2 .

 2 Fig. 2. Measured transfer curves (left) and a photo of IGZO TFTs on PEN substrate with W/L=40/80 µm/µm and annealed at 150 ⁰C (right) using a multilayer dielectric (7 layers).

Table 1 .

 1 Simulated main performance parameters of the ΣΔM using IGZO TFTs.

	Characteristic	Value
	Sampling Frequency (Fs)	128 kHz
	Input Signal Amplitude (Ain)	5 Vdiff
	Input Signal Frequency (Fin)	101.5625 Hz
	Oversampling ratio (OSR)	128
	Bandwidth (BW)	500 Hz
	Signal-to-noise distortion ratio (SNDR)	57 dB
	Dynamic Range (DR)	65 dB

Acknowledgments. This work has been funded by FEDER funds through the COMPETE 2020 Programme and National Funds through FCT -Portuguese Foundation for Science and Technology, under the projects "Multifunctional nanoscale oxide materials" (EXCL/CTM-NAN/0201/2012), DISRUPTIVE (EXCL/EEI-ELC/0261/2012), INCENTIVO (EEI/UI0066/2014) and Strategic projects (UID/CTM/50025/2013) and (UID/EEA/00066/2013). The work has also received funding from the European Communities 7 th Framework Programme under grant agreement i-FLEXIS project (ICT-2013-10-611070) and H2020 Programme under grant agreement ROLLOUT project (ICT-03-2014-644631).


Paul Grey

Luís Pereira

Sónia Pereira

Pedro Barquinha

Inês Cunha

Rodrigo Martins

Elvira Fortunato

Electrochemical Transistor Based on Tungsten Oxide with Optoelectronic Properties

Keywords: tungsten oxides (VI), electrochromism, thin films, electrolyte-gated transistors, polymer electrolytes

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Electrolyte-gated transistors (EGTs) and electrochemical transistors (ECTs) have received special attention over the last decade due to their advantages, mainly deriving from the use of solid electrolytes as gate dielectrics. These include low operating voltages, printability and solution processability, low contact resistances, the possibility to fabricate new device architectures, and high driving currents. [START_REF] Kim | Electrolyte-Gated Transistors for Organic and Printed Electronics[END_REF] The understanding of the involved phenomena in electrolyte-gated transistor (EGT) operation is an important aspect to enable their integration in applications like biosensors, [START_REF] Berggren | Organic bioelectronics[END_REF] microelectronics [START_REF] Herlogsson | Polyelectrolyte-Gated Organic Complementary Circuits Operating at Low Power and Voltage[END_REF] or even the emergent area of textronics. [START_REF] Hamedi | Fiber-Embedded Electrolyte-Gated Field-Effect Transistors for e-Textiles[END_REF] EGTs using electrochromic (EC) semiconductors are particularly interesting as they combine electrical/ electrochemical effects with a simultaneous optical modulation. The optical absorption of an EC material can be modified through double insertion of electrons and charge compensating ions, [START_REF] Granqvist | Electrochromic tungsten oxide films: Review of progress 1993-1998[END_REF] which can be easily achieved by the gate field in an EGT structure. Being a reversible process, this gives an EC material the capability of transiting between two distinct oxidation states: colored and bleached.

During the electrochemical reaction WO3 also undergoes a change in its electrical properties, namely in its conductivity, making it a candidate for implementation as the semiconductor layer in transistors, as initially demonstrated by Natan et al. [START_REF] Natan | PH-SENSITIVE WO3-BASED MICROELECTROCHEMICAL TRANSISTORS[END_REF] Furthermore, inorganic oxide semiconductors as WO3 possess higher chemical stability and higher conductance modulation than the organic semiconductors that compose most of the reported EGTs to date, which are great advantages for digital and analog circuit design with improved longevity.

The combination of both optoelectronic properties of EC-T opens possibilities in a wide range of applications and shows the value of the integration of smart materials into electronic circuitry.

Relationship to Cyber-Physical Systems

The use of electrolytes in active electronic components will be one of the fundamental building blocks of the interaction between common solid-state devices and ionic charge carriers. Figure 1 exemplifies the proposed interaction between an electrolyte and an ion permeable semiconductor. Electrolytes are a big part of modern society and naturally occur throughout the whole spectrum of science. For instance solely in physiological systems there exist a plethora of electrolytes, responsible for intra-and extracellular interactions as well as nerve and muscle function control, blood pressure and pH. Consequently, reliable readout and double redundant values (electrical and optical) for electrolyte interpretation is crucial. Thus, creating a consistent interface between semiconductors and such electrolytes potentiates a main challenge for the development and study of EGTs. The technology of EGTs is therefore capable of adding gross value to a diverse range of cyber-physical systems (CPS), such as in chemical processes, healthcare and bio sensing schemes.

Results and Discussion

The complete device structure is depicted in Figure 2. An interdigital source and drain titanium contact architecture was chosen in order to increase channel width (3000 µm) and consequently the reached drain current of the devices. Furthermore an encapsulated device was fabricated, where the gate electrode (in this case an ITO coated PET foil) was extended over the electrolyte, resulting, not only, in a vertical architecture, but also in a self-passivation effect (Fig. 2 b andd). Subsequent characterization of the most crucial layers of the device was carried out. This includes electro-optical characterization of the sputtered WO3 thin-films (100 nm), Electrochemical impedance spectroscopy (EIS) of the drop-casted electrolyte (a thermosetting composite solid electrolyte -TCSE) and lastly electrical characterization of the final devices.

Electro-Optical Measurements

Electrochromism for WO3 is commonly introduced by reference to the following simplified redox reaction, also known as the double-injection model: [START_REF] Granqvist | Progress in electrochromics: tungsten oxide revisited[END_REF] 𝑊𝑂 3 (𝑡𝑟𝑎𝑛𝑠𝑝𝑎𝑟𝑒𝑛𝑡) + 𝑥𝑒 -+ 𝑥𝑀 + ↔ 𝑀 𝑥 𝑊𝑂 3 (𝑑𝑎𝑟𝑘 𝑏𝑙𝑢𝑒) [START_REF] Kim | Electrolyte-Gated Transistors for Organic and Printed Electronics[END_REF] where M + is an ion of the alkali metal group (e.g. H + , Li + , Na + or K + ), provided by the electrolyte, e -represents electrons and parameter 𝑥, designated as the insertion coefficient, represents the proportion of electro-reduced tungsten sites. Following Equation 1, the pristine WO3 is transparent and the resulting tungsten bronze (MxWO3), for low x, exhibits its characteristic blue coloration due to photointervalence charge transfer (CT) between adjacent W V and W VI sites. [START_REF] Somani | Electrochromic materials and devices: present and future[END_REF] It is widely accepted that the electro-optical centers are activated by the insertion of electrons and charge compensating ions, which distort and expand the lattice of the guest oxide, resulting in an overall modification of the optical properties and electronic structure of WO3. Charge carriers are trapped by W 6+ octahedra, reducing them to W 5+ , while the alkali metals remain ionized in the interstitial space. [START_REF] Azens | Electrochromism of W-oxide-based thin films: Recent advances[END_REF] This results in a delocalization of electrons in the lattice and a charge carrier concentration increase of several orders of magnitude from the pristine oxide. This consequently increases the conductivity, which in terms of transistor operation is reflected in the transition between the Off-and On-states (oxidized and reduced states, respectively).

The WO3 film was deposited on an ITO coated glass substrate, which was submersed in a Li + rich liquid electrolyte in a conventional three-electrode setup. By applying a potential of 1 and -1 V to the ITO film it is possible to influence the oxidation states (oxidized and reduced, respectively) and consequently monitor the change in transmittance at a specific wavelength (λ = 633 nm), as well as the inserted charges into the WO3 film (Fig. 3). The wavelength corresponds to the transmittance of the characteristic blue coloration of reduced WO3 and it can be observed that a transmittance variation of over 90% is achieved. The inserted charges during reduction (-1 V) remain as free available charge carriers in the film and are extracted upon oxidation (1 V). A maximum of 26 mC of charge induction was achieved, yielding (with a contact area of 0.833 cm 2 ) a charge density of about 2×10 17 cm -2 . However the process showed to be not completely reversible as charges might remain in the film after the reduction reaction (charge does not reach 0 mC at 90 s, see Fig. 3). Nonetheless sputtered WO3 films from previous works are capable of reaching very high lifetimes with more than 1000 cycles. [START_REF] Barquinha | Flexible and Transparent WO3 Transistor with Electrical and Optical Modulation[END_REF] This characterization technique proves the inherent optoelectronic behavior of WO3 and reveals its potential application in semiconducting devices, owing to reversible insulator-to-metal transitions, resulting from the electrochemical reaction.

Electrochemical Impedance Spectroscopy (EIS)

EIS is a well-known technique to determine the electrical properties of ionic materials, where an ac potential at different frequencies (f) is applied to an electrochemical cell, so the total capacitor impedance (Z) can be measured as a function of the frequency. This gives valuable information about ion migration, electric double layer (EDL) formation and its capacitance (CDL). The electrochemical cell prepared for EIS is, as shown in Figure 4 a), composed of 3 distinct layers. Explicitly two ITO coated glass substrates and, sandwiched in between, the TCSE. The behavior and the obtained data of this cell can be described by an equivalent circuit model (ECM) for parameter determination (see Fig. 4 b).

This ECM (describing an ideally polarizable electrochemical cell), is composed of a resistance (Rext), associated to contact resistances (ITO), in series with a parallel RbCb circuit and a constant phase element (CPE). The RbCb circuit is associated to bulk properties of the electrolyte, namely the bulk resistance and capacitance, respectively. Whereas Rb gives information about σi, Cb accounts for the dipolar relaxation of electrolyte solvent molecules at higher frequencies. The CPE mimics a non-ideal capacitive behavior of the electric double layer, associated to interface inhomogeneities. [START_REF] Barsoukov | Impedance Spectroscopy Theory, Experiment, and Applications[END_REF] Upon polarization of the electrochemical cell, ions migrate from one electrode to the other, causing charge built-ups and EDL formation. As can be observed in Figure 4 c) the ECM describes well the behavior of the cell throughout the whole frequency spectrum and serves for the determination of CDL and the ionic conductivity (σi).

Here, as we are dealing with an electrochemical transistor (ECT), σi will be the parameter of interest. This parameter will determine how pronounced the effective net ion diffusion into the semiconductor will be and how fast the insulator-to-metal transition occurs. However, from an electrochemical point-of-view it is also important to determine the amount of accumulated charges at the interfaces (CDL), which, in terms of transistor characterization, accounts for charge induction (field-effect) into the semiconductor. It was shown that both effects (electrochemical reaction and fieldeffect) can contribute to the drain current. The ionic conductivity (for cell dimensions: A = 5 cm 2 and d = 0.1 mm) and Rb = 87.34 Ω from the fitting, σi of the electrolyte was determined as 2.29x10 -5 Scm -1 , using the standard resistivity equation. For CDL we considered the approach taken by Jović et al. [START_REF] Jovic | EIS and differential capacitance measurements onto single crystal faces in different solutions -Part I: Ag(111) in 0.01 M NaCl[END_REF] to empirically determine this value by using the fit for the CPE, yielding CDL = 5.10 µFcm -2 .

A clear advantage for this type of dielectric becomes evident as the achieved dielectric capacitance reaches very high values. However, as both characteristic parameters (σi and CDL) are highly frequency-dependent, transistors working with these materials are often restricted to lower frequencies, where complete ionic relaxation can take place (capacitive regime in Fig. 4 c). Nonetheless, efforts are made towards an improvement of overall electrolyte behavior in order to augment the EGTs cut off frequency (fco -frequency where IOn/IOff = 10). [START_REF] Liu | Vertical polyelectrolyte-gated organic field-effect transistors[END_REF]

Electrical Characterization

Electrical characterization of the final devices (as depicted in Fig. 5 a) was conducted with a drain voltage (VD) as low as 1 V and a voltage sweep at the gate (VG) from -4 to 4 V and back at a scan-rate of 20 mVs -1 . Low-voltage operation of EGTs were found to be another strong factor of this type of devices, making their application in low power consuming applications feasible. The applied VG drives ion migration in the electrolyte (as schematized in Fig. 1) and promotes reversible electrochemical doping (reduction and oxidation) of the channel region. As can be observed in Figure 5 a, the fabricated EC-Ts start to switch close to VG = 0 V to their respective On-states, remaining however with a normally On characteristic. As the channel reduces (and colors from transparent to blue, as seen in Fig. 5 b) the drain current increases until reaching mA values around VG = 2 V, with leakage currents (IG) remaining in the order of µA (at most). The obtained On-Off ratios are considerable, reaching almost 7 orders of magnitude (vertical architecture). A hysteresis behavior is observed for both architectures planar and vertical, which is connected to the time-dependency of the electrochemical reaction, as well as ion migration processes. Nevertheless, it is possible to open and close both EC-Ts with symmetric VG (-4 and 4 V and -2 and 2 V for the planar and vertical architecture, respectively).

Table 1 resumes crucial device parameters, evidencing an advantage for the vertical and encapsulated EC-T, reaching higher values in each aspect. Furthermore, the vertical EC-T showed higher lifetimes than the planar one due to the selfpassivation effect of the organic electrolyte, which is always subjected to environmental degradation factors. Analyzing Figure 5 a and b together, the double functionality of the EC-Ts becomes visually clear, as it is possible to obtain a simultaneous electrical and optical response. The obtained information from these devices, regarding, for instance, biosensing applications, is on the one hand quantitative (ID output) and on the other hand qualitative (color output). This leads to the above discussed double redundancy outputs, turning these devices highly reliable for sensing applications in healthcare or chemical and biological schemes. The results show how important but also how effective a controlled interaction between semiconductor/electrical components and ionic conductors is. A consistent integration of such devices into cyber-physical systems or as such will be feasible in the near future.

Conclusion

With this work we show an approach for a possible integration of ionic conductors (electrolytes) into electronic circuitry. The developed electrolyte-gated transistors (EGTs) based on the semiconductor WO3 exhibit double functionality (electrical and optical), as the electrochemical reaction of WO3 not only promotes a change in its coloration, but also a change of conductivity of several orders of magnitude. Characterization goes through electro-optical analysis of the WO3 film, electrochemical impedance spectroscopy of the electrolyte (a thermosetting composite solid-electrolyte -TCSE) and electrical characterization of the final devices. Two transistor architectures with identical W/L = 12 were implemented (planar and vertical) in order to investigate structure influences and self-passivation effects. The best device (vertical) showed an On-Off ratio of almost 7 orders of magnitude (one of the highest ever reported for EGTs) for very low operation voltages (VD = 1 V and VG between -4 and 4 V) and additional hysteresis reduction. In future works the dynamical switching of such devices will be in focus, as well as testing and optimizing different types of electrolytes.

The presented research paves the way for sophisticated systems, where the analysis of electrolytes and the interaction of these with semiconductors is crucial. These devices could be integrated into applications as diverse as lab-on-chip, wearable electronics, healthcare monitoring and biosensors.

Fig. 1 .

 1 Fig. 1. Schematic representation of the interaction between an ion permeable semiconductor and an electrolyte in a transistor setup with source, drain and gate (S, D and G, respectively). In the present paper WO3 not only generates an electric response (conductivity change), but also an optical one (transmittance change), turning the system double redundant.

Fig. 2 .

 2 Fig. 2. Schematic device representation, where a) represents the planar and b) the vertical and encapsulated architecture and c) and d) show a longitudinal cross-section representation of both of the device architectures. The PET/ITO foil is kept in place by capillary forces of the electrolyte and a silver glue (Ag) on the gate electrode.

Fig. 3 .

 3 Fig.3. Electro-optical measurements of a 100 nm thick WO 3 film on ITO/Glass Substrate in a Li + rich electrolyte. Transmittance was measured during a full bleaching/coloration/bleaching cycle (30 s each) for the characteristic wavelength of reduced WO3 (633 nm). The inserted and extracted available charge-carriers were also measured as a function of the duration of the applied potential.

[ 12 ]Fig. 4 .

 124 Fig. 4. Electrochemical Impedance Spectroscopy of the used Thermosetting Composite Solid-Electrolyte (TCSE). Where a) shows the electrochemical cell (TCSE sandwiched in between two ITO (contact) coated glass substrates), b) represents the equivalent circuit model (ECM) used for the fittings and c) depicts the effective capacitance and the phase of the electrochemical cell in a frequency range between 10 -2 and 10 6 Hz.

Fig. 5 .

 5 Fig. 5. a) Transfer characteristics of the fabricated planar and vertical EC-Ts and b) micrographs of channel region with distinct colorations between the On-and the Off-state. Curves obtained with VD = 1 V and VG = (-4, 4, -4) V at a scan-rate of 20 mVs -1 . The arrow next to the main graphs indicates hysteresis direction.

Table 1 .

 1 Crucial EC-T parameters, comparing the planar with the vertical structure. Values calculated from the curves of Figure5a.

		IOn/IOff	VOn (V)	Ss (Vdec -1 )	gm (mS)
	Planar	6.55 × 10 5	0	0.28	2.24
	Vertical	5.10 × 10 6	-1	0.22	3.59

Acknowledgments. This work was supported by the FCT -Portuguese Foundation for Science and Technology, by the project EXCL/CTM-NAN/0201/2012 and by FEDER funds through the COMPETE 2020 Programme and National Funds through FCT under the project UID/CTM/50025/2013. The work was also supported by FP7 APPLE (grant number 262782-2) and SMART EC (grant number 258203) projects.


Jorge Martins
email: jorge.souto.martins@gmail.com

Pedro Barquinha

João Goes

TCAD Simulation of Amorphous Indium-Gallium-Zinc Oxide Thin-Film Transistors

Keywords: IGZO, TCAD simulation, DOS, TFT, CPS

de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Oxide TFTs are nowadays starting to play an important role in display industry, with leading companies as LG using them in the backplane of recent products, such as curved organic light emitting device (OLED) TV sets. Still, significant advances in the current state-of-the-art of oxide TFT technology are required before high performance fully transparent, flexible, low-cost and low-power dissipation electronics reach the maturity level required for commercialization. The implementation of Technology Computer-Aided Design (TCAD) [START_REF] Software | ATLAS User ' s Manual[END_REF][2] tools for simulation of devices at a physical level can lead to understanding on how to further improve fabrication processes, material properties and device architectures. The ultimate goal is to enhance the performance of amorphous oxide TFTs, mimicking the routes followed for single-crystalline CMOS, a-Si and poly-Si technologies. Embedding physically-extracted parameters from the fabricated devices into the models for simulation, accurate models can be built and prediction on how changes in materials and processes affect physical response of the oxide TFTs can be achieved. This can allow viable process and device development and, simultaneously, understanding, the otherwise difficult or even impossible to extract physical characteristics behind device operation. Some specific goals to be addressed are referred next.

As performance of amorphous oxide semiconductors (AOS) is controlled by the density of states (DOS) in a given material, and TCAD simulation is based in the DOS [START_REF] Hsieh | Modeling of amorphous InGaZnO4 thin film transistors and their subgap density of states[END_REF], it can provide understanding of the specific nature of states causing performance differences among different AOS. This can be used for instance to achieve a suitable and sustainable indium-free replacement for IGZO. Modeled IGZO TFTs based on DOS extracted from different techniques have shown to agree very well with measured characteristics, without employing any fitting parameter [START_REF] Jeon | Modeling of amorphous InGaZnO thin-film transistors based on the density of states extracted from the optical response of capacitance-voltage characteristics[END_REF] [START_REF] Shi | Influence of Deep States in Active Layers on the Performance of Amorphous In-Ga-Zn-O Thin Film Transistors Junfei Shi , Chengyuan Dong and Yikai Su[END_REF]. Instabilities in oxide TFTs can be further investigated with TCAD, by implementing charge-trapping and defect creation in the simulations, allowing to understand the specific physical causes of degradation in TFTs, typically associated with Gate/Drain Bias Stress, illumination and temperature. Correlations between the defect-creation and the device performance degradation have been reported [START_REF] Im | Effects of defect creation on bidirectional behavior with hump characteristics of InGaZnO TFTs under bias and thermal stress[END_REF], with localization of defect creation being also investigated by means of simulations [START_REF] Lee | Modeling and Characterization of the Abnormal Transistors After High Positive Bias Stress[END_REF]. Some effects that limit device performance are related to device architecture, as parasitic capacitances that limit the maximum operation frequency, and short channel effects (SCE), which limit the scalability of the technology. As architectures can be arbitrarily defined in TCAD simulation optimization and understanding of physical phenomena in different geometries appears as a great advantage. Moreover, as recently more and more TFT architectures are proposed (e.g. dual-gate, floating-capping-metal, bulk-accumulation [START_REF] Mativenga | Bulk accumulation a-IGZO TFT for high current and turnon voltage uniformity[END_REF] and dual-active layer TFTs), TCAD can accelerate the optimization of those configurations, reducing fabrication runs and elucidating the physics behind the electrical behavior of each architecture [START_REF] Lu | Investigating electron depletion effect in amorphous indium -gallium -zinc-oxide thin-film transistor with a floating capping metal by technology computer-aided design simulation and leakage reduction[END_REF].

In this work the short channel effect Drain Induced Barrier Lowering (DIBL) is shown with TCAD simulation in μm-scale IGZO TFTs. To the best of the authors' knowledge it's the first time that the barrier level for different drain bias is displayed for μm-scale oxide TFTs.

Integration in Cyber Physical Systems

Amorphous oxide electronics allow for production in a wide range of substrates, notably, inexpensive flexible and/or transparent substrates, as they can be processed at low temperatures (while still maintaining good performance) and have large area uniformity due to their amorphous nature. This way, they can be integrated into a wider range of systems, rather than their silicon technology counterparts. This technology thus shows to have great applicability in Cyber Physical Systems (CPS). In the CPS 5Cs architecture, amorphous oxides can be integrated in the Connection and the Conversion layers. In the Connection level they present the already discussed advantages of being applicable in wide range of substrates, as flexible and transparent ones. Several applications have already been reported as gas sensors [START_REF] Chen | Gas sensing properties of indium-gallium-zinc-oxide gas sensors in different light intensity[END_REF], touchpanels [START_REF] Fu | An IGZO TFT based in-cell capacitance touch sensor[END_REF] and even biological sensors [START_REF] Shen | IGZO thin film transistor biosensors functionalized with ZnO nanorods and antibodies[END_REF]. Integrated with the connection level, the technology can also be in the conversion level as they have performance that allow for the processing of the information received at the Connection level. In fact, one of the current trends of oxide TFT technology is precisely focused on full integration and in increasingly more complex circuits, including signal processing, amplification and, ultimately, analog-to-digital conversion [START_REF] Correia | A Second-Order ΣΔ ADC Using Sputtered IGZO TFTs[END_REF].

TCAD Simulation

In this section some preliminary simulations are presented, addressing specific scenarios in which IGZO TFT devices, as produced in our group, show the need for improvement. Sputtered staggered bottom-gate TFTs on glass have been produced. A 60 nm thick Molybdenum (Mo) gate electrode was sputtered. The dielectric layer is a sputtered 100 nm thick 7 layer multicomponent oxide (alternating layers of SiO2 and Ta2O5). The active layer is a 40 nm thick amorphous IGZO (with 2:1:1 composition in In:Ga:Zn atomic ratio), also sputtered. Finally, 60 nm thick Mo source and drain electrodes have been sputtered. All layers were patterned by photolithography and lift-off with exception of the dielectric layer, in which, photolithography and dryetching have been used. A schematic of the fabricated devices can be seen in Fig. 1. After all layer fabrication devices were annealed at 180 o C for 10 minutes in air in a rapid thermal annealing system. This fabrication process is compatible with commercial polymeric films. For simulation purposes the TCAD Silvaco's 2D ATLAS TM was used. A similar configuration than that of fabricated devices was employed. An electron mobility of 16 cm 2 (V s) -1 and electron concentration of 10 16 cm -3 in the a-IGZO was assumed. Both fabricated and simulated TFTs have a channel length of 20 μm unless otherwise is specified. Channel width of the characterized TFTs is 20 μm and for simulated devices the obtained drain current in 2D simulation is factored in order to correspond to a 20 μm width channel TFT.

Short Channel Effects

Sputtered IGZO TFTs with channel length (L) below 5 μm have exhibited SCEs, which are well known in silicon technology, where devices are of nm-scale. As low L (5 μm and below) IGZO TFTs have been reported with [START_REF] Chen | Self-aligned top-gate InGaZnO thin film transistors using SiO2/Al2O3 stack gate dielectric[END_REF] and without [START_REF] Park | Characteristics of short-channel amorphous In-Ga-Zn-O thin film transistors and their circuit performance as a load inverter[END_REF] SCEs, TCAD simulation was used to investigate this physical limitation further, as the the mechanisms for enabling or not the SCE are not clear yet. Preliminary simulations for similar TFTs showed that the short channel effects occur for L = 0.5 μm. As shown in Fig. 2, turn-on voltage (Von) shifts negatively with increasing drain bias in the simulated transfer characteristics. This effect is explained as Drain Induced Barrier Lowering (DIBL), as for low L drain bias can reduce the Source-Semiconductor schottky junction barrier. By extracting the conduction band energy level across the channel the decreasing of the barrier between source and semiconductor with increasing drain bias is clear, as shown in Fig. 3. x (m)

V DS = 0 V V DS = 0.2 V V DS = 15 V E F Fig. 3.
Conduction band energy of IGZO in respect to the fermi level (EF) for a L=0.5 μm IGZO TFT, showing source-semiconductor barrier lowering (at x=0μm) for increasing drain bias.

Hump in Transfer Characteristics.

Performance degradation by irradiation, temperature and/or bias stress is one the main challenges for the application of oxide electronics. Fabricated devices showed hump like behavior in transfer characteristics after gate bias stress. This hump has been reported on several scenarios, and many different explanations have been reported so far. Some authors report it as a result of an effect of shallow donor-like states creation in the semiconductor [START_REF] Im | Effects of defect creation on bidirectional behavior with hump characteristics of InGaZnO TFTs under bias and thermal stress[END_REF]. Fig. 4 shows a simulation with increasing peak value of this shallow donor state, assuming 0.1eV of peak variance and a state energy 0.2 eV below conduction band, since energy states at this level have been reported [START_REF] Oba | Energetics of native defects in ZnO[END_REF]. The concentration increase in these states leads to the creation of an increasingly more visible hump, suggesting that it might be the (or at least, one of the) creation mechanisms for the hump.

Dual-Gate TFT

Typically, fabricated IGZO TFT in our research group at CENIMAT/I3N at FCT NOVA show a depletion mode operation, which complicates its application in circuits. Some architectures for threshold voltage modulation have been already reported, as applying a second-gate on the back-interface of the semiconductor. Fig. 5 shows simulated transfer characteristics for dual-gated IGZO TFTs, with different fixed bias on the second gate. The added top gate and top gate oxide for dual-gate simulation are identical to the bottom gate and bottom gate oxide. As expected the Von is modulated, which can be used to achieve enhancement mode devices.

V GS (V) V GS(Top-Gate) = 0 V V GS(Top-Gate) = -1 V V GS(Top-Gate) = 1 V V DS = 0.1 V

Conclusions and Future Work

TCAD simulations have demonstrated to be a powerful tool to improve understanding on amorphous oxide TFTs and reduce fabricated batches and costs. Short channel effects have been shown in simulations of IGZO TFTs, and these are in agreement to what was verified in fabricated devices by sputtering. Transfer characteristics shifted with drain bias, explained as DIBL, and the barrier has been effectively seen to reduce with drain bias observing the simulated conduction band energy in the IGZO. Further investigation on the SCEs, such as understanding the role of the schottky barrier, might elucidate on how to suppress these effects. As for the hump behavior, simulation showed that it may be somehow related to shallow donor-like state creation. Further investigation on how specific DOS affect the device characteristics will lead to understanding the mechanisms of degradation in the oxide TFTs. Finally, simulation of a double-gate TFT was employed, and the Von modulation by the second gate bias was observed. This modulation can be used to achieve enhancement mode TFTs as preferred for application in circuit.

Fig. 1 .

 1 Fig. 1. Schematic of the sputtered IGZO TFTs.

Fig. 2 .

 2 Fig. 2. VOn shift with increasing drain bias for simulated L = 0.5 μm.

Fig. 4 .

 4 Fig. 4. Increasing of hump-like behavior in transfer characteristics of IGZO TFTs with increasing concentration of shallow donor-like states.

Fig. 5 .

 5 Fig. 5. Modulation of Von in dual-gate IGZO TFT by second gate bias.

Acknowledgements

This work is funded by FEDER funds through the COMPETE 2020 Programme and National Funds through FCT -Portuguese Foundation for Science and Technology under the Project Nos. UID/CTM/50025/2013 and EXCL/CTM-NAN/0201/2012. The

work has also received funding from the European Communities 7th Framework Programme under grant agreement ICT-2013-10-611070 (i-FLEXIS project) and from H2020 program under ICT-03-2014-644631 (ROLL-OUT project).


José Ribeiro
email: jribeiro@deea.isel.ipl.pt

Fernando Melício
email: fmelicio@deea.isel.ipl.pt

Luis Gomes

Extending IOPT Nets with a Module Construct

Keywords: Modularity, Composition, low-level Petri nets, IOPT nets

Input-output place-transition nets (IOPT nets) are a Petri net based formalism targeted for the development of embedded systems controllers. It is an extension to common place-transition Petri nets, introducing constructs to model the communication between the controller and the environment and using an execution semantics assuring a deterministic behavior. However, IOPT nets and the supporting tools framework -the IOPT-Tools -do not have a mechanism to support model structuring. Since models are flat, all the graphical components and annotations are visualized in the same page. Systems with several dozens of nodes become very difficult to manage. In this paper a modular construct for IOPT nets is presented, helping to manage large-scale systems, and the reuse of model components across projects. The algebraic specification of the model is provided and an example illustrating the concept is presented.

Introduction

The development of embedded systems is a challenging task due to their strict requirements of safety, correctness and real time constraints, among others restrictions [START_REF] Edwards | Design of embedded systems: formal models, validation, and synthesis[END_REF]. Embedded systems are dedicated computational devices, which controls a physical system through a communication interface. These systems are characterized by states, changing from one state to another by the occurrence of discrete events. Among several formal methods to model discrete event systems, Petri nets have the advantage of allowing to model the structure and the dynamics of the system and are provided with powerful analysis methods [START_REF] Murata | Petri nets: Properties, analysis and applications[END_REF]. Aspects as concurrent execution and synchronization of actions are naturally modeled with Petri nets. There are also a vast number of extensions that allow the modeling of distributed components or the reactive behavior of systems [START_REF] Moalla | Synchronized Petri nets: A model for the description of non-autonomous sytems[END_REF][START_REF] Moutinho | Asynchronous-Channels Within Petri Net-Based GALS Distributed Embedded Systems Modeling[END_REF]. The use of formal methods allows the analysis and validation of the system, which is crucial, in particular when the compliance with the requirements has to be assured previously to their implementation [START_REF] Girault | Petri Nets for Systems Engineering[END_REF]. Formal methods, particularly those with a graphical representation, as Petri nets, also enable the implementation of development approaches, based on models, that supports the entire development flow and the automation of some tasks.

The IOPT nets are a Petri net based modeling language, extending the place/transition Petri net class, addressed to the development of embedded systems controllers. Its main features are the constructs to explicitly model the communication between the net and the environment and a deterministic execution semantics. One aspect in which the Petri nets exhibit a major limitation is in the structuring of models. The representation of large-scale systems with flat models is highly inadequate. Modular mechanisms are essential for the expressivity and compactness of the models and to raise the efficiency of the modeling development process [START_REF] Girault | Petri Nets for Systems Engineering[END_REF].

Concerning the IOPT nets, the lack of a structuring mechanism makes difficult the construction and management of large-scale and complex models. All the models must be built from scratch, with basic primitives, becoming the design task inefficient and preventing the reuse of model components into other projects.

Considering this, we state the following research question: What structuring mechanisms should be defined to build generic subnets, reusable across different projects, allowing to manage complex and large models, while keeping all analysis capabilities?

Hypothesis: Modules with dynamic interfaces and configurable parameters, at instance level, allow the design of more compact models and its use in different situations. Converting a structured model into an equivalent flat model assures the use of existing analysis tools to validate the model.

The modular mechanism presented here is an extension for the IOPT nets [START_REF] Gomes | Extending Input-Output Place-Transition Petri nets for Distributed Controller Systems Development[END_REF][START_REF] Gomes | The Input-Output Place-Transition Petri Net Class and Associated Tools[END_REF] and is being implemented in the IOPT-Tools development framework [START_REF] Pereira | Web based iopt Petri net editor with an extensible plugin architecture to support generic net operations[END_REF] (available online at http://gres.uninova.pt/IOPT-Tools), the supporting tool for the IOPT nets. Fig. 1 shows a diagram with the IOPT nets extensions and the IOPT-Tools features. Concerning the proposing modular extension, the interface of the modules are nodes from the encapsulated net. Modules may be defined with more than one inter-face for being used in different situations. Several instances of the same module may be used in a containing net with different parameters.

The following Section of this paper cover the relationship of this work with the Cyber Physical Systems (CPS). In Section 3 it is reviewed some of the proposals of modular constructs that have been made in the context of Petri nets. The description of the modules for IOPT nets and its mathematical model is presented in Section 4. An example of a net with modules is presented in Section 5, followed by a discussion and conclusions.

Relationship to Cyber-Physical Systems

This work is about a modular extension for a modeling language (IOPT nets) addressed to the development of embedded systems controllers. An embedded system (ES) is a dedicated computational system embedded in a physical system which controls it. In [START_REF]Cyber-Physical System (CPS)[END_REF] Cyber-Physical Systems (CPS) are defined as "the tight conjoining of and coordination between computational and physical resources". The main difference between ES and CPS is in the focus that is given to the physical system, being the CPS a more complete approach. While in embedded systems the main emphasis is on the computational part, from the point of view of a CPS an additional attention is given to the physical system, resulting in a greater integration of computational and physical systems. Although IOPT nets are suited to the development of ES controllers, the modular structuring presented here allows the modeling of the controller and the controlled physical system. Thus one can use the whole model for simulation and analysis purposes and the controller part of the model for the synthesis of the controller using a CPS development approach.

Related Literature

There are multiple languages for the development of embedded systems controllers [START_REF] Gomes | Models of Computation for Embedded Systems[END_REF], namely those where the communication with the controlled system has a fundamental role, as Grafcet [START_REF] Thomas | Using Grafcet to design generic controllers[END_REF] or Mark Flow Graphs [START_REF] Hasegawa | Proposal of Mark Flow Graph for Discrete System Control[END_REF].

In this section we overview some of the structuring mechanisms proposed in the field of Petri nets, namely those related with the modular construct presented in this work.

Many structuring mechanisms have been proposed, in order to provide Petri nets (PNs) with abstraction and composition capabilities, either for low-level PNs [START_REF] Zuberek | Hierarchies of place/transition refinements in Petri nets[END_REF] or high-level PNs [START_REF] He | A formal definition of hierarchical predicate transition nets[END_REF][START_REF] Huber | Hierarchies in coloured Petri nets[END_REF]. Concerning composition, most of the proposals uses a module, or a similar entity, which models a small part of a system and provides an interface to communicate with the container net.

In some mechanisms the interface is composed by places or transitions, as Modular PNML [START_REF] Kindler | Towards a standard for modular Petri nets: A formalisation[END_REF][START_REF] Kindler | A universal module concept for Petri nets-an implementationoriented approach[END_REF] and Hierarchical Coloured Petri nets (HCP nets) [START_REF] Huber | Hierarchies in coloured Petri nets[END_REF]. In some others it is used new elements, as events or signals in Signal Nets [START_REF] Juhás | Modeling and control with modules of signal nets[END_REF] and Net Condition Event Systems (NCES) [START_REF] Rausch | Net condition/event systems with multiple condition outputs[END_REF].

The kind of communication can be synchronous, usually made through transition fusion, as in HCP [START_REF] Huber | Hierarchies in coloured Petri nets[END_REF] nets and modular PNML [START_REF] Kindler | Towards a standard for modular Petri nets: A formalisation[END_REF] or asynchronous, usually made by place fusion or by message sending, as in Object Petri nets (OP nets) [START_REF]Object oriented modelling with object Petri nets[END_REF].

Concerning instantiation it can be static or dynamic. Static instantiation has a substitution semantics, where an instance, or a macro, is substituted by the module. In this kind of instantiation the structure of the net remains unchanged during their execution and can be transformed into an equivalent unstructured net. Transition substitution in HCP nets and modular PNML are examples of static instantiation. Dynamic instantiation consists in the creation of a module instance, during the execution of the net, and its subsequent destruction. The creation and destruction of the instance is governed by an event of the net, as the firing of a transition or a marking in a place. This change in net structure makes it very difficult to analyze. Dynamic instantiation is used in classes inspired in object-oriented paradigm as OP nets and Reference Nets [START_REF] Kummer | Referenznetze[END_REF]. The invocation transition mechanism of HCP nets also uses dynamic instantiation although it was never implemented in the CPN Tools, the supporting tool of the HCP nets.

Research Contribution and Innovation

This section contains the definition of the modules for IOPT nets. This mechanism uses nodes as interface and static instantiation. At first it is presented the definition of unstructured IOPT nets, their main features and semantics. Then the modular extension is presented and formalized.

IOPT Nets

As stated in the introduction, the IOPT nets have explicit constructs to model the communication with external devices. This communication is done through input and output signals and events. An IOPT net models a controller for an embedded system, thus must have a deterministic behavior. To accomplish this behavior it was adopted an execution semantics with maximal step combined with a single server semantics. Maximal step semantics means that all the transitions ready to fire, will fire in a given step. With single server semantics a transition fires only once in one step, even if it remains ready. The firing of transitions is synchronized with an external clock. A step is the occurrence of all the transitions ready to fire. A transition t , is enabled if for all its input places, •t , M(•p) ≥ w( p, t ) with (p, t) ∈ A. An enabled transition is ready to fire if the transition guard is true and the event associated with the transition occurs.

Output events can be generated by the firing of a transition, and output signals can be updated by expressions evaluated by the firing of a transition, or by a specific marking of a place.

IOPT Net Mathematical Model

An IOPT net is defined as a tuple: IOPT = ( NG, IO, AN, Const, M0 ), where, An interface node may be of type input, output or, by default, input/output. Input nodes can only have incoming arcs from the container net. Output nodes can only be linked with outgoing arcs.

An IOPT net composed with modules has an equivalent at net, where the instances are substituted by their referenced modules. The interface nodes, from the instance, are fused with the respective nodes of the module. In the following definition of module it is considered a simple module, that is, a module without instances of other modules in its composition. A module is a structure Module = (IOPT, ITF, IT) where, -IOPT is a IOPT net as defined in 4.2.

-ITF = itf1, itf2, ... is a set of interfaces, with, itfi : P ∪ T ↛ {in, out, in/out}, each interface is a partial function that assigns nodes with an attribute in, out, in/out, for input, output or input/output nodes. -IT : Sin ∪ Ein ∪ Const → {module, instance} is a function that assigns an attribute to each input signal, input event and symbolic constant declared in the module. For signals and events, if the attribute is module the signal/event is unique and shared by all the instances. If the attribute is instance, each instance has its own input signal/event. Constants with the attribute instance may be overridden and have a different value in each instance. A constant with an attribute module cannot be overridden at instance level.

Composition of Modules

A module is used through instances. An instance is graphically represented by a rectangle, a round rectangle or an oval. The interface nodes are drawn over the instance object. The nodes in the instance are copies of the respective interface nodes of the module. The encapsulated net of a module may be composed with instance of other modules. However it is not allowed to define a module with instances of itself. When more than one instance of the same module exists in the same net, input signals, input events and symbolic constants may be shared by all the instances (attribute module), or exist as separate entities in each instance (attribute instance). Output signals and output events are unique for each instance of the same module.

The equivalent plain net is obtained by substituting the instances by the net of the respective module. The interface nodes of the instance are fused with the nodes of the module and the element names are prefixed with the name of the instance.

Example

The following example illustrates the definition of modules and a net built with instances of that modules. The equivalent flat net, obtained from the conversion of the net with module instances into an equivalent net without module instances, shows the semantics of the modules. The example is adapted from [START_REF] Gomes | Structuring and Composability Issues in Petri Nets Modeling[END_REF]. It is used a parking lot with an entrance and an exit as shown in the Fig. 2. When a car is detected near the entrance gate, a ticket is printed. When the driver gets the ticket, the gate opens and he can enter the park. To leave the park, the car must be detected near the exit gate and after paying the ticket, the gate opens. The sensors entrance and exit detect the arrival of a car at the entrance and exit, respectively. The signal of these sensors have a positive edge trigger when a car is detected and a negative edge trigger when the detection ceases. The models of the park entrance and park exit are implemented, respectively, in the module Entrance (Fig. 3), and in the module Exit (Fig. 4). Each of these modules have a declaration of two interfaces.

A module may have multiple interfaces declared, supporting its use in different situations. For each instance of a module the designer chooses the most suited interface to communicate with the surrounding elements of the net.

In this example the interface i1 of the module Entrance (composed by the node got_ticket) is used to decrease de number of available places and increase the occupied ones. The interface i2 (nodes got_ticket and arrive-) could be used to, additionally, control a traffic light, using the transition got_ticket to switch to green and arriveto switch to red. It was presented a modular extension for IOPT nets, a Petri net based language targeted for the development of embedded systems. The aim of the work was to create a structuring mechanism to represent models in a more compact way, while ensuring a greater distinction among different components of a system, whether they are physical or logical.

Comparing with similar proposals, as the substitution transition in HCP nets [START_REF] Huber | Hierarchies in coloured Petri nets[END_REF] and modular PNML [START_REF] Kindler | A universal module concept for Petri nets-an implementationoriented approach[END_REF], the mechanism presented here distinguishes by supporting multiple interfaces composed by concrete nodes (interface nodes in [START_REF] Kindler | A universal module concept for Petri nets-an implementationoriented approach[END_REF] are reference nodes). Defining multiple interfaces for the same module, supports different ways to connect the module within the container net and therefore a more flexible usage.

The parameterization of some module elements as instance elements (events, signals and constants) also increase the flexibility, enabling that different instances of the same module have its one parameters.

Although it is used the fusion of nodes as the composition mechanism between the surrounding net and an instance of the module, this is done in a transparent manner for the designer. The connections are made through arcs, or any kind of channels, as synchronous or asynchronous, which facilitates the composition.

Fig. 1 .

 1 Fig. 1. IOPT Net and IOPT-Tools main features.

Fig. 2 .

 2 Fig. 2. Parking lot scheme.

Fig. 3 .

 3 Fig. 3. Parking lot Entrance model.

Fig. 5

 5 Fig.5is the net composed by the instances of the modules. The places occupied and free models the number of places occupied/free in the parking lot. The equivalent plain net is shown in Fig.6.

Fig. 4 .

 4 Fig. 4. Parking lot Exit model.

Fig. 5 .

 5 Fig. 5. Parking lot model with modules.

Fig. 6 .

 6 Fig. 6. Parking lot equivalent net.

-NG is the net graph, NG = ( P,T, F ) where,

• P ∪ T are the set of nodes, satisfying the condition P ∩ T=∅,

• F is the set of arcs which defines the flow relation. Each arc has a type from the set AT = {normal, test}, such that Fnormal ⊆ ( P T ) ∪ ( T P ) and Fnormal ⊆ ( P T ). • A : F → ℕ, is the weight function, assigning each arc with a positive integer.

• TG : T ↛ Exp(bool) is a partial function that annotates transitions with boolean guard expressions.

• TP : T → ℕ, is a function that annotates each transition with a priority value.

• TIE : T ↛ Ein, is a partial function that annotates transitions with input events. • TOE : T ↛ Eout, is a partial function that annotates transitions with output events. • POS : P ↛ (Sout, Exp(bool)) is a partial function that annotates places with output signals. The signal is updated when the place is marked and the boolean expression is true. -Const is a set of symbolic constants whose values are naturals. A constant may be used as the initial marking of a place.

-M0 : P → ℕ0, is the initial marking of the net, assigning a nonnegative integer to each place.

The transition guard and place output expression are built with operands from signal values, constants and place marking values (the number of dots), and operators from the set O = {Oarithmetic, Ocomparison, Ologic}, with Oarithmetic = {+, -, *, /, %}, being the usual arithmetic operators, Ocomparison = {<, >, =, <=, >=, =}, the set of comparison operators and Ologic = {and, or, xor, not} the set of logic operators.

Modules

An IOPT net uses the element page as the container of the net elements: graph elements, annotations and input/output declarations through which is done the communication with the environment. A module is also a container, with an encapsulated net, with one or more interfaces. The interface of a module is a subset of the module set of nodes.

A module is used in a net by their instance. An instance of a module is a copy of the module, represented in the net by a rectangle with the interface nodes. Each instance uses one interface. The interface nodes of the instance are copies of its counterpart in the module net.