Keywords: Assembly, ceramic heat sink, adhesion layer, power modules, silver sintering

A new packaging technology allowing the development of high performance power modules for harsh environment as well as the optimization of the manufacturing process is presented. The assembly is achieved using an aluminum nitride ceramic heat sink and multilayer silver sintering to make both current track and die attach layers. In order to control the properties of silver sintered layers, a focus on the pressure-less sintering process parameters at conventional and unusual high temperatures was performed. Depending on the sintering temperature and dwell time, the relative density of the sintered layers was evaluated in the range of 56% to 88%. The electrical resistivity was measured in a large temperature range for various densities and the latter can only be 30% higher than the one of bulk silver once the paste sintered at 500°C for 1 h. Finally, the mechanical and microstructure properties of assemblies using several metal adhesion layers on the ceramic heat sink and silver sintered multilayers were investigated. Assemblies using Ti/Ni/Ag sputtered layers and silver ink sintered layer as adhesion layers on the AlN ceramic show acceptable shear stress values of about 20 MPa while values less than 10 MPa are obtained for W/Ni/Au and W/Ni/Ag adhesion layers. For W/Ni/Au adhesion layers, Ag-Au solid solution was formed at the interface between the sintered silver and the Au finish, leading to a depletion zone at the sintered silver interface and hence a low shear stress. For W/Ni/Ag adhesion layers, the low shear stress value is mainly due to the oxidation of the Ni layer.

I. INTRODUCTION

he past few years have seen the increase of the electrical energy in aircrafts. Manufacturers predict that onboard electrical power of commercial airplanes will exceed 10 MW in 30 years [START_REF] Revol | Overview and new trends in technology bricks for reliability enhancement in future wide band gap power converters for More Electrical Aircraft (MEA) and More Electrical Propulsion (MEP) systems[END_REF]. However, various technological locks must be removed to reach such onboard power. Among others, the use of electronic converters in harsh environment [-60°C; 200°C] can be of first importance since it will allow integrating converters in hot area environments and/or increasing the power density of electrical systems. Power modules are critical components of the converter that ensure electrical connections between the power semiconductor devices and the external electrical connectors, the heat management and the mechanical robustness of the package. The emergence of wide band gap semiconductor components like Silicon Carbide (SiC) and Gallium Nitride (GaN) power devices able to operate at high temperatures (above 200°C) paves the road toward the use of power modules in harsh thermal environment [START_REF] Kyung-Ah | GaN-Based High Temperature and Radiation-Hard Electronics for Harsh Environments[END_REF] [START_REF] Funaki | SiC JFET dc characteristics under extremely high ambient temperatures[END_REF]. However, to complete this breakthrough, the whole packaging system must be redesigned.

A cross section view of a conventional power module assembly is schematically presented in Fig. 1a. The semiconductor device of the module is attached to the substrate by soldering. Solders with an operating temperature above 200 °C exist but most of them contain a high amount of lead (Pb), which is not compliant with the regulations of Restriction of Hazardous Substances (RoHS). Some solders can meet those regulations but they suffer from high cost (Au80Sn20, Au97Si3, Au88Ge12) or are difficult to process (Zn95Al5) [START_REF] Khazaka | Survey of High-Temperature Reliability of Power Electronics Packaging Components[END_REF]. New die attach techniques like transient liquid phase bonding or metal sintering technologies have emerged during the last decade [START_REF] Welch | Gold-Indium Transient Liquid Phase (TLP) wafer bonding for MEMS vacuum packaging[END_REF] [START_REF] Knoerr | Power semiconductor joining through sintering of silver nanoparticles: Evaluation of influence of parameters time, temperature and pressure on density, strength and reliability[END_REF]. Pressureless silver sintering is particularly attractive as it is a simple process providing joints with good physical properties (high thermal and electrical conductivities and low elastic modulus) [START_REF] Khazaka | Review on Joint Shear Strength of Nano-Silver Paste and Its Long-Term High Temperature Reliability[END_REF] [START_REF] Siow | Die-Attach Materials for High Temperature Applications in Microelectronics Packaging[END_REF].

The substrate provides the electrical insulation and insures the circuitry in the module through the upper metallic layout. Ceramic based substrates can be used at temperature higher than 300 °C but most of them suffer from poor thermomechanical reliability. During large amplitude thermal cycling (300°C), it has been found that 20 to 30 cycles are typically needed to crack or spall Direct Bonded Copper (DBC) substrates made of Al2O3 or AlN ceramics [START_REF] Ning | SiC Wirebond Mutlichip Phase-Leg Module Packaging Design and Testing for Harsh Environment[END_REF]. The CTE mismatch between Cu and the ceramic as well as the copper hardening due to the plastic deformation result in the fracture of the ceramic and the peel off of the Cu foil [START_REF] Tilford | Thermo-mechanical modelling of power electronics module structures[END_REF] [START_REF] Gunther | Failure Mechanisms of Direct Copper Bonding Substrates (DCB)[END_REF]. The use of Si3N4 ceramics with copper bonded using Active Metal Brazing (AMB) technology improves the substrate reliability [START_REF] Wang | Power Device Packaging Technologies for Extreme Enironments[END_REF] [13], but the Si3N4 thermal conductivity is half of the AlN one and hence thin ceramic plates should be used. In such case, the voltage withstand and the warpage of the substrate become serious issues [START_REF] Zhang | Residual stress and warpage of AMB ceramic substrate studied by finite element simulations[END_REF]. Direct Bonded Aluminum (DBA) on AlN ceramic has shown a high level of reliability when large amplitude cycles were used (1500 cycles from -55 °C to +250 °C) [START_REF] Guangyin Lei | Effects of Large-Temperature Cycling Range on Direct Bond Aluminum Substrate[END_REF]. This augmented reliability is due to the difference in the plastic hardening behavior between Al and Cu and the lower modulus of Al. However, since Al is not compatible with most solders and should be protected from oxidation, a nickel (Ni) layer is always applied. Al has a high CTE (24 ppm/°C) compared to the one of Cu (16 ppm/°C), leading to strong plastic deformation. As a consequence, surface roughening of Al has been observed after thermal cycling as well as cracks in the Ni layer [START_REF] Fukumoto | Effects of Extreme Temperature Swings (-55°C to 250°C) on Silicon Nitride Active Metal Brazing Substrates[END_REF]. This may alter the die attach reliability. In order to allow a high heat spreading, the substrate can be thermally coupled to the metal base plate using solder. The power modules are assembled to their heatsinks using Thermal Interface Materials (TIMs), such as thermal greases, elastomers or phase change materials. These materials are used to reduce the contact thermal resistance between the underside of the power module and its heatsink. However, organic based TIMs have a low thermal conductivity and a very low stability at high temperature [START_REF] Due | Reliability of thermal interface materials: A review[END_REF], which has a negative impact on thermal management. Graphite or graphene foils could also be used as TIM, but they suffer from very low thermal conductivity in at least one direction (anisotropic materials) [START_REF] Forsbach | Reliability of Thermal Interface Materials for Power Semiconductor Devices[END_REF]. In addition, in order to minimize the thermal contact resistance, the use of such foils requires a perfect surface finish as well as the application of high pressure during operation. Metal springs are also seen as a solution to reduce the thermal resistance of contact, but the main obstacle to their use is their high cost [START_REF] Graham | Sn+ Heat-Spring Solder RIMs for Superior Thermal Management in IGBT Power Modules[END_REF]. Cooling technologies such as jet impingement or spray cooling allow the evacuation of a large amount of heat and the elimination of TIMs [START_REF] Buttay | Compact Inverter Designed for High-Temperature Operation[END_REF] [20] [START_REF] Kim | Spray cooling heat transfer: The state of the art[END_REF]. Their main drawbacks are the limited exchange surface, the complex cooling system as well as the risk of clogging the cooling nozzles.

In order to avoid the aforementioned limitations of metallized substrates, TIM and die attaches, a new assembly based on multilayers silver sintering on highly thermally conductive AlN ceramic heat sink is presented in this paper (Fig. 1b,1c,1d). The main objectives of this paper is to optimize the sintering processes used to make the power assembly presented in Fig. 1d and to evaluate the properties of such assembly. The tracks of the electrical circuit are done by a thick sintered silver layer called "Track layer" as shown in Fig. 1c. The main goal of this layer is to carry the current in the horizontal direction. Its electrical resistance should therefore be as low as possible. Note, that it can be sintered at unusual temperature, above 400°C. An "adhesion layer" and a "die attach layer" are necessary to ensure respectively a good contact of the "track layer" with the ceramic substrate and the semiconductor chips. Various adhesion layers will be studied in this paper while it is proposed to make the die attach layer by silver sintering in order to limit the number of materials in the power module. The die attach layer must be sintered at conventional temperature according to datasheet to avoid die degradation, around 300°C. The main contributions presented in this paper are detailed hereafter:

1. Investigation of pressure-less silver sintering kinetics at nonconventional high temperatures using in situ shrinkage measurement. 2. Identification of the relationship between process parameters, relative density and electrical properties for sintered silver layers used as current track and die attach layers.

Evaluation of microstructural and mechanical

properties of the assemblies using various metal adhesion layers on AlN ceramic. The paper is organized as follows: section II describes the selected materials, the samples preparation and the experimental techniques that have been used. In section III, experimental results on silver paste sintering for track and die attach layers are displayed and discussed. Section IV presents the relationship between the sintering process and the physical properties of the sintered material. Finally, different assemblies (various adhesion and finish layers and various multilayer sintering processes) are compared in Section V.

II. MATERIALS AND EXPERIMENTS

A. Materials

AlN ceramic plates with a thickness of 1 mm supplied by Ceramtec and 350 µm thick Si diodes with Ni/Ag bottom metallization supplied by ABB (5SLY12E) were used in this study. AlN plates were used for this study instead of heat sink to ease the survey.

Various metal adhesion layers on the AlN ceramic have been tested using screen printing and/or sputtering technologies. The objective is to compare these different adhesion layers in order to select the best one for the proposed assembly. The first evaluated adhesion layer is a screen printable silver ink sintered at 850°C in air for 10 minutes. For this metallization technique, the main interest is to reduce the number of materials in the final assembly: only silver is present between the ceramic and the die metallization. The second studied technique involves layers of Ti/Ni/Ag sputtered onto the AlN plate (thickness 100nm/500nm/100nm). In this stacking, the Ti is used for adhesion on the AlN ceramic, Ni acts as diffusion barrier and Ag is used to facilitate the adhesion with the sintered silver layer and to prevent Ni oxidation. Finally, the third studied technique consists of a W layer sintered at high temperature to insure the adhesion with the AlN. To avoid element diffusion, NiP layer was then electroless plated on W, followed by a gold or silver finishing sputtered layer to protect nickel from oxidation (thickness 7µm/2µm/100nm). Au finishing has been selected for its good property to protect the lower layers from oxidation. Ag finishing is less effective in protecting against oxidation, nevertheless silver will be sintered on this layer. The Ag finishing will therefore avoid harmful diffusion phenomena that could occur between Ag and Au in the case of Au finishing as it will be observed later in the paper. Since the coefficient of thermal expansion of W (4.6 ppm/°C) is close to the one of AlN (4.5 ppm/°C), this solution allows reducing the mechanical stresses at the interface between the AlN and the metal adhesion layer. Hence a good thermomechanical reliability of the interface under thermal cycles can be expected.

For track and die attach layers, a screen printable pressureless silver paste has been used [START_REF] Botter | Thermal aging of power module assemblies based on ceramic heat sink and multilayers pressureless silver sintering[END_REF]. The silver particles have been observed using scanning electron microscopy (SEM -Zeiss Ultra 55) and the results indicate that their size are in the range of 1 µm. The density of the paste is 4.3 g.cm -3 , which suggests a large amount of organic solvent and additives in its composition (silver density is 10.5 g.cm -3 ). Organic components allow preventing the premature agglomeration of silver particles during storage and give to the paste the viscosity required for screen-printing.

B. Samples preparation and experiments

The control of electrical and thermal conductivities of the sintered layers is mandatory for power module design. It is argued that the sintering process has a direct impact on the density of the sintered layers and the density directly governs the electrical and thermal properties of the layers [START_REF] Bai | Low-Temperature Sintered Nanoscale Silver as a Novel Semiconductor Device-Metallized Substrate Interconnect Material[END_REF] [START_REF] Rmili | Quantitative Analyses of Porosity and Transport Properties by FIB-SEM 3D Imaging of a Solder Based Sintered Silver for a New Microelectronic Component[END_REF]. Hence, a deep investigation of the effect of various sintering process parameters (atmosphere, heating rate, paste drying, sintering temperature, dwell time) on the sintered layer density was investigated by using in situ monitoring of the mass loss and the layer shrinkage and will be presented in the next sections.

Mass loss during de-binding was evaluated with 800 mg of paste by thermogravimetric analysis (SETARAM TGA 96) in order to optimize the thermal profile. The impact of oxygen on the organic evaporation was investigated by analyzing the losses in air and inert argon atmosphere.

Dilatometry (SETARAM TMA 96) was used to measure the variation of the thickness of the samples during sintering. The silver paste was sandwiched between two AlN plates with 8×8 mm 2 area and a thickness of 1 mm each. Silver ink with a thickness of 10 µm was used as adhesion layer on the AlN. The silver paste was screen printed on the sintered silver adhesion layer by using a 100 µm thick stainless stencil. The top AlN plate was deposited using a die bonder with a low force to avoid the smearing of the paste (0.1 N for 100 ms). The samples prepared for dilatometry tests are presented in Fig. 2c and2c'. In order to eliminate the effect of the thermal deformation of the sandwiched structure, a blank test was systematically implemented with two metallized AlN plates without silver paste before each test. The displacement measured during the blank test was then subtracted to the displacement measured during the paste sintering test. The electrical conductivity was measured from room temperature until 200°C for various sintering profiles using conventional four-probe method. The thermal conductivity was deduced from the Weidemann-Franz law. The samples prepared for electrical conductivity tests are presented in Fig. 2b'. The sintered layer density was deduced from weight and volume measurements on samples presented in Fig. 2c'. The impact of various adhesion metal layers and sintering parameters on the microstructure of the assembly was investigated using SEM. Samples are constituted of a Si device assembled on an AlN plate using three metallic layers: an adhesion layer, a track layer and a die attach layer Fig. 3b, c andd. One example of metallography with W/Ni/Ag as adhesion layer is presented in Fig. 4.

The shear tests were carried out using a Royce 600 series bond test equipment with a head tool length of 8 mm (larger than the device length) (Fig. 3e). The head tool was positioned at 20 µm above the ceramic surface and the displacement speed was fixed at 1mm.min -1 . The applied force is measured during the test until the failure detection. The shear stress is calculated by dividing the maximum measured force by the device area (6.3×6.3 mm 2 ). At least 4 samples have been tested for each condition to have an acceptable statistic distribution. In order to identify the weak points of each assembly, fracture analyses have been achieved using optical microscopy and SEM coupled with EDX elements analyses.

III. SILVER PASTE SINTERING FOR TRACK AND DIE ATTACH LAYERS

In the following section, the impact of various parameters of the thermal pressure-less sintering process (such as atmosphere, heating rate, maximal temperature and dwell time) of the silver layer densification will be presented thanks to thermogravimetry and dilatometry analyses.

A. Impact of the atmosphere and the heating rate on organic evaporation

As aforementioned, silver pastes contain organic components that must be removed before sintering. Hence, the optimization of the organic removal is necessary to reach the best conditions for subsequent sintering. Results of two TGA experiments with a heating rate of 1°C.min -1 from ambient temperature in argon flow and in air flow are presented in Fig. 5. On each curve, two separate loss steps can be identified. The first one with a large mass loss of 10.1 wt.% corresponds to the solvent evaporation and it is independent of the atmosphere. The second loss step corresponds to dispersant evaporation and it occurs between 250°C and 280°C with only 0.6 wt.% loss. It takes place at lower temperature in air due to the oxygen presence that promotes the dispersant elimination. Accordingly, the next experiments have been achieved in air, as it facilitates the dispersant elimination. Above 300°C, the mass remains unchanged in both tests, which suggests that the whole organic materials have been removed from the paste. The mass and volume fractions of silver inside the paste were calculated to be 89.3 % and 36 % respectively. Dilatometry tests were carried out to analyze the effect of the heating rate on the shrinkage during solvent removal. Fig. 6 shows the shrinkage measured for two heating rates: 1 and 5°C.min -1 from ambient until 300°C. At 1°C.min -1 , shrinkage is observed between 50 and 150°C, with a total value of 10%. Then, the measured displacement did not significantly evolve until 300°C. This shrinkage is due to particle rearrangement during evaporation of the solvent, until the achievement of a more compact packing with particles in contact. On the other hand, during the test with a heating rate of 5°C.min -1 , a strong swelling is observed around 150°C, followed by an equivalent shrinkage and a flat period until the start of sintering at 300°C. Since a high quantity of liquid is suddenly evaporated during the 5°C.min -1 heating, the internal pressure increases and can create macropores inside the silver layer. The lower shrinkage measured at 300°C for the 5°C.min -1 test confirms the higher porosity of the silver particle packing after drying. Since macropores are difficult to eliminate during sintering, it is recommended to evaporate the organic of the paste by using a heating rate of 1°C.min -1 .

B. Impact of the heating rate on the joint sintering

In order to evaluate the impact of the heating rate on sintering and to avoid the swelling during the organic evaporation, a temperature profile with a slow heating rate until a drying step of 1h at 150°C has been selected. In this way, most of organic components are removed without swelling before reaching the sintering temperature. After the drying step, heating rates of 5°C.min -1 and 20°C.min -1 have been selected to reach the sintering temperature. Results showing the shrinkage and the temperature as functions of time are presented in Fig. 7. A small swelling related to the dispersant evaporation is still recorded at about 250°C. However, since the swelling leads to only 2% deformation, it has been decided to ignore it. At the end of the sintering process, the shrinkage difference is less than 1% between the two curves, meaning that the heating rate has little effect in this range. Hence, in order to reduce the process time, the heating rate of 20°C.min -1 will be used in the next presented experiments.

C. Impact of the temperature and dwell time on the silver densification

The aforementioned results show the interest of using an oxygen atmosphere, a low heating rate during solvent evaporation and a high heating rate to reach the sintering temperature. Hence, for the next experiments aiming at evaluating the effect of the sintering temperature and dwell time on the densification, air atmosphere has been chosen and the following thermal cycle has been followed: slow heating rate of 1°C.mm -1 until drying for 1h at 150°C, followed by 20°C.min -1 heating until the maximum sintering temperature. Four temperatures have been compared: 325°C, 400°C, 500°C and 850°C. The value of 325°C has been selected for studying the sintering of the die attach layer (maximum recommended temperature in the SiC MOSFET datasheet CPM3-0900-0010A), while the other values have been selected for the sintering of the track layer. In Fig. 8a, the swelling induced by the dispersant evaporation is systematically found after approximately 185 min. The densification begins just after this swelling and the shrinkage of the sintered joint increases with increasing sintering temperature and dwell time. For the temperatures of 325°C, 400°C and 500°C, the shrinkage value after 60 min is 22, 32.5 and 40%, respectively. The higher shrinkage of about 47% is observed for sintering at 850°C and more than 90% of the shrinkage takes place during heating. In the latter case, the dwell time does not impact the densification. The link between densification, time and temperature can be explained by the fact that sintering is controlled by diffusion mechanisms which are thermally accelerated.

Plotting the shrinkage rate shows that the densification phenomenon takes place in 3 steps pointed by red arrows in Fig. 8b. During the first step, just after swelling, the shrinkage rate increases drastically. A second step can be detected after about 190 min when the rate stabilizes and reaches a plateau with a value of about 1%.min -1 . Finally, when the temperature reaches 350°C, the shrinkage rate increases again to reach its maximum value. The first acceleration of densification occurs after complete dispersant evaporation, when particle get in contact with each other. The second acceleration observed at 350°C can be due to the activation of new sintering mechanisms that take place at high temperature.

IV. RELATIONSHIP BETWEEN THE PROCESS AND THE SILVER LAYER PHYSICAL PROPERTIES

The microstructure after sintering at various temperatures has been investigated on the top surface of bare sintered silver layers (Fig. 9). For the sample sintered at 325°C for 10 min, it can be seen that diffusion mechanisms have already created necks between particles. These necks have greatly increased in size after 10 min sintering at 400°C. Finally, in the sample sintered at 500°C during 1h, the number of pores has greatly decreased, revealing a porosity that is probably closed. Usually, cross section observation combined with image analysis is used to evaluate the porosity of sintered materials. However, the polishing of sintered silver can severely compromise the relevance of porosity quantification because of pore smearing and smudging. In order to avoid complex and expensive preparation techniques, the porosity of the samples was deduced from volume and mass measurements. Results for various sintering conditions are presented in Table 1. The higher the temperature and the longer the sintering time, the higher the densification. Once the density of sintered materials is calculated for various temperature profiles, the objective is to link it to the physical properties of the materials. The electrical resistivity of different sintered silver samples at for 10min, 400°C for 10min, 400°C for 1h and at 500°C for 1h was measured. The measurements were carried out at various temperatures between 20 and 200°C, as the aim is to improve the performance of the power modules in high-temperature environment. The results are presented in Fig. 10. As expected, the higher the sintering temperature, the lower the resistivity. A resistivity of 21 nΩ.m (30% higher than the one of bulk silver) is found for the paste sintered at 500°C for 1 h. All samples show a thermal electrical coefficient similar to that of bulk silver which is 0.0038°C -1 . The thermal conductivity as function of sintered layer density was determined from electrical resistivity thanks to the Wiedemann-Franz equation [START_REF] Mahan | Wiedmann-Franz law at boundaries[END_REF] and both values found at 20°C as function of joint porosity rate are shown in Fig. 11. In the whole density range, both electrical and thermal conductivity values are comparable with the ones obtained after pressure assisted silver sintering [START_REF] Shang | Pressure-Assisted Low-Temperature Sintering of Silver Paster as an Alternative Die-Attach Solution to Solder Reflow[END_REF]. In the worst case where the sample presents a porosity of 44%, the thermal conductivity is about 150 W.m - 1 .K -1 which remains three times higher than the one of conventional solders [START_REF] Duch | Comparison of Thermal Measurement Methods and Determination of Thermal Resistance of Die Attach Materials used in Power Electronic Modules[END_REF]. V. MECHANICAL EVALUATION OF THE ASSEMBLIES In this section, the mechanical strength of the whole assembly (die attach layer, track layer, adhesion layer and ceramic) will be evaluated. Regarding previous dilatometry results, sintering the track layer at a temperature of 850°C seems to be the best option. However, due to the small differences in final porosities between the 850°C and 500°C tests, and in order to reduce residual thermomechanical stresses in the assembly after the process, the sintering temperature at 500°C for 1h was selected. In addition, to evaluate the importance of the sintering profile of the track layer, more porous samples sintered at 400°C for 10 min have also been fabricated. For all assemblies, the die attach layer has been sintered at 325°C during 10 min to avoid die degradation. In the following, different adhesion metal layers will be compared. Results of shear stress for different adhesion metal layers with track layers sintered at two different temperatures are presented in Fig. 12. For Ti/Ni/Ag and silver ink adhesion layers, samples with a track layer sintered at 500°C for 1 h show average shear stress values of 25 MPa and 21 MPa respectively. The fracture surface as well as the thickness of the metal layer remained on the substrate after the shear test in both cases are presented in Fig. 13a andb. It comes that the fracture took place at the die attach level. The difference in shear strength between silver ink and Ti/Ni/Ag samples with a track layer sintered at 500°C for 1h comes probably from the difference of thickness of the assembly, visible in Fig. 13e. Average shear stress for samples with a track layer sintered at 400°C during 10 min show values of about 19 MPa irrespectively of the adhesion layer. The fracture location is similar to the one obtained for track layers sintered at 500°C for 1h. For both adhesion layers, a good contact is achieved with the track layer shown by SEM cross section images in Fig. 14a andb. This analysis highlights that the adhesion layer is not the limiting factor of the assembly in the cases of a silver ink and Ti/Ni/Ag adhesion layers.

For the W/Ni/Au adhesion layer sample, the mean shear strength is between 7 and 8 MPa. This result is independent of the track layer sintering temperature, which suggests that the weak point of the assembly is the adhesion layer. This hypothesis is confirmed by the measurement of the silver thickness remaining on the substrate, as shown in Fig. 13e. This measurement indicates that the samples delaminate at the adhesion layer level. The samples with a track layer sintered at 500°C for 1h showed the same fracture surface with a delamination at the adhesion layer also. The low shear strength can be explained by the presence of a silver depletion zone at the interface. This depletion zone can be seen in Fig. 14c. The Energy Dispersive X-ray spectroscopy (EDX) in Fig. 15 performed on the fracture surface clearly shows the presence of Ag and Au, further confirming the location of the fracture. Low shear strength of silver sintered joints on gold finishes have been already observed [START_REF] Yu | Reliability of Ag Sintering for Power Semiconductor Die Attach in High-Temperature Applications[END_REF].

For the W/Ni/Ag adhesion layer sample, the mean shear strength value is around 9MPa. Analyses of fracture surface in Fig. 13d and Fig. 13e show that the failure occurs systematically at the adhesion layer. Unlike the W/Ni/Au adhesion layer, no depletion zone is visible in Fig. 14d. However, EDX analysis of the interface with the silver in Fig. 15 reveals that the NiP layer was partially oxidized during processing and could be the cause of the low shear stress values. This analysis confirms the color of the fracture surface, visible in Fig. 13d. The previous results suggests that metallization with Au finish is not suitable for the assembly proposed in this work. The W/Ni/Ag based metallization also shows poor shear strength due to the partial oxidation of the NiP layer. However, the shear strength of the assemblies with silver ink and Ti/Ni/Ag sputtered layer based metallization present satisfactory shear strength, higher than 20 MPa. Finally, the sintering temperature of the track layer is also important for the mechanical strength of the assembly. Samples with track layers sintered at 500°C for 1h have a better mechanical strength than those with a track layer sintered at 400°C for 10min. This is probably due to a stronger adhesion at the interface with the 500°C sintered layer, as this layer is much denser with larger contact area. Therefore, it is recommended to select a metallization layer based on silver ink or Ti/Ni/Ag sputtered layer, with a 500°C sintered track layer.

VI. CONCLUSIONS

An innovative power module packaging including an adhesion layer on an AlN heat sink and pressure-less sintered silver layers has been presented. The impact of the process parameters such as the sintering atmosphere, the de-binding conditions, the heating rate, the sintering temperature and the dwell time of silver paste has been evaluated using in situ shrinkage measurement and thermogravimetry analysis. It has been shown that the de-binding stage should be achieved with a low heating rate to avoid swelling during silver sintering. The relationship between the process, the microstructure and the physical properties of sintered silver such as porosity and thermal and electrical conductivities has been established. It has been shown that thermal conductivities as high as 150 W.m -1 .K - 1 and 300 W.m -1 .K -1 for the die attach and the track layer respectively can be obtained. These results as well as the dilatometric results will be very useful to allow optimizing future power assemblies. Finally, the mechanical properties of the complete assembly with various adhesion layers on the AlN substrate have been investigated. The study revealed that classical gold finish layer leads to low shear strength (about 8MPa) due to the creation of a depletion zone at the interface with the sintered silver. Results are equivalent for W/Ni/Ag metallization. For this adhesion layer, it seems that the sputtered layer does not adhere well to the NiP layer and fails to protect it from oxidation. Nevertheless, a high strength (of the order of 20MPa) has been found for assemblies with silver ink and Ti/Ni/Ag adhesion layer. For these assemblies, the fracture path takes place essentially at the level of the sintered die attach layer. The study of the evolution of the assemblies as a function of aging is ongoing in order to consolidate the results and will be presented in a future paper.

Fig. 1 :

 1 Fig. 1: (a) Cross section of standard power module assembly, (b) proposed power module assembly using ceramic heat sink and multilayer silver sintering, (c) Schematic cross section view of proposed assembly, (d) Top view of power assembly on ceramic plate.

Fig. 2

 2 Fig. 2 Cross section of the ceramic and the silver ink adhesion layer (a), cross section and top view of samples used for electrical resistivity measurement (b) and (b') respectively, cross section and top view of samples usé for dilatometry measurement (c) and (c') respectively.

Fig. 3 :

 3 Fig. 3: Schematic view of the assembly preparation: (a) cross section of the ceramic with the various tested adhesion layers, (b) track layers sintered using two thermal profiles on the various metal adhesion layers, (c) die attach of the device. (d) Top view picture of the assembled samples and (e) picture of the sample under shear test.

Fig. 4 :

 4 Fig. 4: SEM cross section view of assembly.

Fig. 5 :

 5 Fig. 5: TGA results with a heating rate of 1°C.min -1 in argon and air flows.

Fig. 6 :

 6 Fig. 6: Shrinkage versus temperature measured during drying with two heating rates.

Fig. 7 :

 7 Fig. 7: Dilatometric results showing the impact of the heating rate used to reach the sintering temperature.

Fig. 8 :

 8 Fig. 8: Dilatometric results showing the impact of the sintering temperature on the measured shrinkage (a) and the shrinkage rate for different sintering temperatures (b).

Fig. 9 :

 9 Fig. 9: Top view of various sintered silver layers observed by SEM: (a) 325°C-10min, (b) 400°C-10min and (c) 500°C 1h.

Fig. 10 :

 10 Fig. 10: Electrical resistivity as a function of temperature for silver sintered layers with different thermal profiles.

Fig. 11 :

 11 Fig. 11: Electrical resistivity and thermal conductivity at 20°C as function of porosity.

Fig. 12 :

 12 Fig. 12: Shear stress of different assemblies using various metal adhesion layer and two sintering profiles for track layers.

13 :

 13 Fracture surface after shear tests of assemblies with: (a) Ti/Ni/Ag sample and track layer sintered at 500°C during 1h, (b) silver ink sample and track layer sintered at 500°C during 1h, (c) W/Ni/Au sample and track layer sintered at 400°C during 10min, (d) W/Ni/Ag sample and track layer sintered at 400°C during 10min, (e) Thickness measurement of silver thickness remaining on the substrate after shear test.

Fig. 14 :Fig. 15 :

 1415 Fig. 14: SEM images of the interface between the track layer (sintered at 400°C for 10min) and various metal adhesion layers: (a) Ti/Ni/Ag, (b) silver ink, (c) W/Ni/Au, (d) W/Ni/Ag.

Currently, he is a researcher with the Electronic and Electrical Systems Research Group at Safran Tech. His current research fields focus on the study of materials, packaging technologies and reliability modeling of power electronic modules for harsh environments. Mr Khazaka is the author and coauthor of 18 papers in international peer-reviewed scientific journals, 18 presentations at international conferences, and one book chapter. He holds more than 15 patents in the field of materials and power electronic packaging.

Yvan Avenas received his Ph.D. degree from the National Polytechnic Institute of Grenoble, France, in 2002. He is currently an associate professor at the Grenoble Institute of Technology / University Grenoble Alpes. His main research topic is three-dimensional packaging of power semiconductor devices. In this context, he is particularly interested by thermal management and junction temperature measurements and also by multiphysics problems. He was implied in many French collaborative and industrial projects and some as coordinator. He is currently the coordinator of the DESTINI project funded by the French National Agency and of the TAPIR project funded by Region Auvergne-Rhone-Alpes.

Didier Bouvard is a professor at Université Grenoble Alpes. He has been working for 35 years on powder processing of metal and ceramic parts by coupling experiments and modelling, in close connection with industrial companies.

Jean-Michel Missiaen received his

Master degree in Materials Science and Engineering at Université de Caen (France) in 1985 and a PhD in Process Engineering at Ecole des Mines de Saint-Etienne (France) in 1989. His PhD research was on the characterization of granular media in relation with the reactivity of powders. After a postdoctoral research at Ecole des Mines de Saint-Etienne, he obtained an academic permanent position at Université Grenoble Alpes in 1991 where he is currently Professor in Materials Science and Engineering. He developed a research activity in the field of Materials Processing through the Powder Metallurgy route applied to various systems (WC-Co, W-Cu, Cu, Al alloys, steels, Nd-Fe-B, Si,…). The focus of his research are the relations between microstructural evolution, densification, interfacial processes and their kinetics, with the aim to understand sintering mechanisms in metallic singlephase or multi-phase materials. He is the author of 66 articles in international peer-reviewed scientific journals and 52 articles in international conference proceedings and 4 contributions in scientific books.


Tadeu Mota Frutuoso

Joao Lé

Yoann Berthoud

Julio Mota Pinheiro

Marc Margalef-Rovira

Senior Member, IEEE Philippe Ferrari

Gustavo Plamplona Rehder

Ariana Maria

Conceiçao Lacorte

Caniato Serrano

João Lé

Member, IEEE Ariana L C Serrano

Nanowire-Based 3-D Transmission-Line Transformer for Millimeter-Wave Applications

Keywords: 3-D circuits, millimeter-wave circuits, system integration, transformer

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

I. INTRODUCTION

T RANSFORMERS play an important role in microwave and mm-wave circuits' and systems' design. Their main applications concern matching and biasing networks [START_REF] Pepe | Transformer-based input integrated matching in cascode amplifiers: Analytical proofs[END_REF], polyphase networks [START_REF] Liu | A Ka-band vector-sum phase shifter in 65 nm CMOS for phased array receivers[END_REF], signal generation [START_REF] Piri | A >40dB IRR, 44% fractional-bandwidth ultra-wideband mm-wave quadrature LO generator for 5G networks in 55 nm CMOS[END_REF], baluns [START_REF] Iotti | Design of lowpower wideband frequency quadruplers based on transformer-coupled resonators for E-band backhaul applications[END_REF], and power divider/combiner [START_REF] Pan | A 76-81 GHz CMOS PA with 16-dBm PSAT and 30-dB amplitude control for MIMO automotive radars[END_REF]. Their implementation in monolithic low-cost integrated technologies such as (Bi)CMOS at mm-wave frequencies is very common today. Most of the works refer to traditional transformers that use two windings (primary and secondary) aiming at low loss and high magnetic coupling coefficient. The primary and secondary spirals can be placed in the same layer [START_REF] Long | Monolithic transformers for silicon RF IC design[END_REF], [START_REF] El-Gharniti | Modeling and characterization of on-chip transformers for silicon RFIC[END_REF] or in different layers [START_REF] Vanukuru | High efficiency millimeterwave stacked two turn transformer using only top two thick metals[END_REF]- [START_REF] Leite | Design of high transformation ratio millimeter-wave integrated transformers[END_REF]. Using different layers with the spirals vertically aligned has shown better performance and size/cost reduction [START_REF] Dickson | 30-100-GHz inductors and transformers for millimeter-wave (Bi)CMOS integrated circuits[END_REF]. Tadeu M. Frutuoso is with CEA, 38000 Grenoble, France (e-mail: tadeu.motafrutuoso@cea.fr).

João Lé, Julio M. Pinheiro, Gustavo P. Rehder, and Ariana L. C. Serrano are with LME, Escola Politécnica da Universidade de São Paulo, Sao Paulo 05508-010, Brazil (e-mail: joao.le@usp.br; juliomota@alumni.usp.br; gprehder@usp.br; aserrano@usp.br).

Yoann Berthoud is with G2ELab, 38000 Grenoble, France, and also with Schneider Electric, 38320 Eybens, France (e-mail: yoann.berthoud@ icloud.com).

Marc Margalef-Rovira is with CNRS, Centrale Lille, UMR 8520-IEMN, Univ. Polytechnique Hauts-de-France, Univ. Lille, F-59000 Lille, France (e-mail: marc.margalef-rovira@iemn.fr).

Philippe Ferrari is with the RFIC-Laboratory, Grenoble INP, Université Grenoble Alpes, 38000 Grenoble, France (e-mail: philippe.ferrari@ univ-grenoble-alpes.fr).

Color versions of one or more figures in this letter are available at https://doi.org/ Some other CMOS works refer to transmission-line transformers (TLTs) [START_REF] Cheung | Design and modeling of mm-wave monolithic transformers[END_REF], [START_REF] Hou | Distributed modeling of six-port transformer for millimeter-wave SiGe BiCMOS circuits design[END_REF]. TLTs consist of one or more pairs of transmission lines (TLs) interconnected in a way to provide a specific impedance ratio and have been widely studied in the whole frequency spectrum since their publication [START_REF] Gunella | Novel matching systems for high frequencies[END_REF], [START_REF] Ruthroff | Some broad-band transformers[END_REF]. In this case, evaluating the transformer performance based on the spirals' performance and their magnetic coupling factor is no longer valid. A more suitable manner to compare the performance of all types of transformers is the power transfer from the input to the output, in terms of the insertion loss (IL).

No matter which topology, (Bi)CMOS transformers are built as far as possible from the lossy silicon substrate [START_REF] Ng | Substrate effects in monolithic RF transformers on silicon[END_REF], reducing eddy current and parasitic capacitances, and preferable at the top thick layers, reducing losses due to series resistance. Despite the good performance, the maximum impedance ratio is limited by the technology considering the limits of the strip width allowed [START_REF] Leite | Design of high transformation ratio millimeter-wave integrated transformers[END_REF]. Their size is still large compared with the rest of the system [START_REF] Chowdhury | Design considerations for 60 GHz transformer-coupled CMOS power amplifiers[END_REF], [START_REF] Xu | A 100-117 GHz W-band CMOS power amplifier with on-chip adaptive biasing[END_REF] easily reaching 100 × 100 μm 2 , when considering not only the spirals but also the required outer spacing from other metals. For instance, removing the power amplifier input and output transformers in [START_REF] Chowdhury | Design considerations for 60 GHz transformer-coupled CMOS power amplifiers[END_REF], it would be possible to reduce the size (and therefore the cost) by half.

Heterogeneous 3-D integration (off chip solution) is an interesting alternative solution that brings flexibility and performance to the transformer design, addressing the issues related to monolithic integration. Transformers have been presented in high-resistivity silicon substrate in lower microwave frequencies [START_REF] Feng | Highperformance solenoidal RF transformers on high-resistivity silicon substrates for 3D integrated circuits[END_REF]- [START_REF] Li | Fully 3-D symmetrical TSV monolithic transformer for RFIC[END_REF], but the numerous vias necessary for their fabrication increases significantly the loss, becoming not suitable for mm-wave applications.

In this letter, we propose TLT transformers operating at mm-waves with broadband characteristic. The TLT is formed by two TLs 3-D meandered around each other forming a solenoid-like topology, as shown in Fig. 1. Since they are TLs, they require return current paths, formed by the metallic rings (one for each layer) drawn around the "solenoids." The transformers were built using a cost-effective technology based on an alumina metal-filled nanoporous membrane (MnM), where vias can be very easily realized, since the fabrication of the vias enters a standard fabrication process flow, with outstanding form factors and performance [START_REF] Pinheiro | 110-GHz through-substrate-via transition based on copper nanowires in alumina membrane[END_REF]. The advantage of the MnM technology is its low cost and good performance. The technology has been first introduced in [START_REF] Serrano | Modeling and characterization of slowwave microstrip lines on metallic-nanowire-filled-membrane substrate[END_REF] for the realization of slow wave TLs, with its electrical model presented in [START_REF] Pinheiro | Slow-wave microstrip line model for PCB and metallic-nanowire-filledmembrane technologies[END_REF]. The availability of small vias allowed the development of compact 3-D inductors with high-quality factor at mmwaves, as shown in [START_REF] Serrano | 3D inductors with nanowire through substrate vias[END_REF]. Therefore, the MnM technology is a high-performance interposer very suitable for heterogeneous 3-D integration [START_REF] Pelegrini | Interposer based on metallic-nanowire-membrane (MnM) for mm-wave applications[END_REF], embedding passive components and circuits into the membrane.

II. MNM TRANSFORMERS' PRINCIPLE, MODEL, AND DESIGN

The proposed 3-D transformer is formed by two 3-D meandered TLs, referred as primary and secondary, both fabricated in two metal layers, at the top (blue) and at the bottom (yellow) of a 50-μm-thick nanoporous alumina membrane. These layers are interconnected by vias realized by a forest of metallic nanowires, which are grown inside the nanopores electrochemically as described in [START_REF] Serrano | 3D inductors with nanowire through substrate vias[END_REF]. The vias used here are 25 × 25 μm 2 to follow the width of the TLs, but smaller vias and lines could be fabricated if needed.

The relative dielectric constant ε r and loss tangent tanδ of the porous alumina substrate are 6.7 and 0.01, respectively, as determined in [START_REF] Serrano | Modeling and characterization of slowwave microstrip lines on metallic-nanowire-filled-membrane substrate[END_REF]. Since the membrane used as substrate is translucid, the bottom layer of the 3-D transformers can be as seen in Fig. 2. The end of each TL was grounded to simplify the measurement to a two-port setup, as indicated in Fig. 2(a). The return current paths required for the TLs have a strong impact on the transformer response and must be considered from the earliest stages of design. An asymmetric TL topology was used to show impedance matching flexibility in the design when compared with the symmetric layout in [START_REF] Feng | Highperformance solenoidal RF transformers on high-resistivity silicon substrates for 3D integrated circuits[END_REF].

The model of an N:N transformer is shown in Fig. 3, showing the access lines and the transformer core. Despite the similarity to the traditional transformer model, the proposed model deals with distributed capacitances and inductances related to the coupled TLs. All the values of the model parameters can be extracted from simple capacitance and inductance simulations. The relevance of such an approach is that it is physically related, and therefore, the designer can change any dimension of the TLs to obtain the desired response. The model is also scalable, so it can be used for TLs with any number of turns.

The access lines are modeled by the series inductance L a and resistance R a , the shunt capacitance C ag , and the capacitance between access C a . As the lengths of the two access lines of a TL are not symmetrical, in one end we have L a1 and R a1 and in the other end L a2 and R a2 . The magnetic coupling between accesses is modeled by the mutual inductance M a . Regarding the transformer core, the variables for the primary TL (L p , R p , C pg ) and the secondary TL (L s , R s , C sg ) were defined for each turn of the coupled TL (1:1). The electric coupling between the two TLs is modeled by C ps . Due to its asymmetry, we have included in the model the coupling between adjacent turns, which is not used in the symmetrical case. Each coupled turn is magnetically coupled through two mutual inductances, M 1 and M 2 , representing the mutual inductance of the coupled turn and between adjacent turns, respectively.

The model uses the following equations:

L ( p or s)tot = N • L ( p or s) + L a1 + L a2 (1)
C ( p or s)gtot = N • C ( p or s)g + C ag ( 2
)
C ps tot = N • C ps + 2C a ( 3
)
M tot = N • M 1 + 2(N -1) • M 2 + 2M a ( 4
)
where N is the number of turns. The transformers' accesses and TLs were simulated in ANSYS Maxwell for different numbers of turns without connections to the return path. The values of L p , L s , C pg , C sg , and C ps were extracted from the slope of their curves and the values of C a , L a1 , and L a2 by the value where their curves intercept the y-axis following (1)-(4). C ag and M a were extracted from the simulation of the accesses alone. Then M 1 and M 2 were calculated from (4). The TLs and access lines' resistances were calculated using the dimensions of each strip, the copper conductance, and a simple skin effect model. As an example, the extracted parameters for each element of the model of a 2:2 transformer are presented in Table I and its comparison with a full-wave 3-D electromagnetic (EM) simulation and its measurement is shown in Fig. 4.

FABRICATION AND MEASUREMENT

The of the transformers follows the steps as the inductors presented in [START_REF] Serrano | 3D inductors with nanowire through substrate vias[END_REF]. The 3-D transformers were characterized up to 110 GHz using groundsignal-ground (GSG) probes (MPI Titan probes with 100 μmpitch) in a cascade manual probe station (EPS-150MMW) and a Keysight PNA 5227B vector network analyzer. A line-reflect-reflect-match (LRRM) calibration was performed. To prevent short-circuiting the bottom of the transformers with the metallic chuck of the probe station, a 5-mm-thick ROHACELL 51 IG foam was used as spacing.

Six different transformers were designed, simulated, fabricated, and tested with different numbers of turns: 1-6 turns, i.e., 1:1-6:6 ratios. The circuit model and the simulations performed in Keysight Advanced Design System (ADS) were compared with the measurements in Fig. 4 and are in very good agreement. The frequency where the circuit presents the minimum IL is inversely proportional to the number of turns, and hence, to the TLs' length, as expected. The summary of the measured performance is given in Table II showing that the proposed transformers have a performance similar or even better than those manufactured using CMOS technologies. The transformers show working frequencies up to 110 GHz, with BWs wider than 12 GHz and IL better than 1.6 dB.

The model is proven to be scalable, which provides a powerful tool for the design of new and improved TLTs. All the dimensions provided in Fig. 1 can be modified according to the model to reach the designer's needs, being the minimum distance between lines and minimum linewidth of 5 μm. Moreover, any topology with TLs with any number of turns can be easily designed using this model. For high impedance ratios, different connections between the TLs can be designed as indicated in [START_REF] Rotholz | Transmission-line transformers[END_REF], giving a high degree of freedom for the designer. By considering an effective dielectric constant equal to 3.5, which is almost the average between the air and the dielectric constant of the nanoporous membrane, it can be noted that each TL has half-wavelength in average, which corroborates the resonant frequencies observed in the return loss presented in Fig. 4. This can be used to understand how to optimize transformers. To illustrate this, two different 1:1 transformers were fabricated with different dimensions: smaller turns (shorter TL) and closer return current path, to increase the frequency and enlarge the 1-dB BW, as shown in Fig. 5.

IV. CONCLUSION

In this letter, a cost-effective technology has been proposed for the realization of 3-D mm-wave transformers. Measurements showed working frequencies from 14.5 to 110 GHz and IL between 1.1 and 1.6 dB. To our knowledge, the presented result at 110 GHz is the highest in a low-cost, interposer technology. The circuit elements of the proposed model are physical related to layout features, allowing an accurate insight of its frequency response and performance. It has also been shown that each TL's length leads to halfwavelength resonances, and their characteristic impedance limits the transformer 1-dB BW to around 20 GHz. Future work will be focused toward the development of a baluns and high ratio impedance transformers.

Manuscript received 17

 17 December 2021; accepted 2 February 2022. Date of publication 5 May 2022; date of current version 7 October 2022. This work was supported in part by FAPESP under Grant 2016/25779-9 and Grant 2018/08782-1, and in part by CNPq under Grant 309577/2020-9, Grant 311031/2018-8, and Grant 431200/2018-1. (Corresponding author: Ariana L. C. Serrano.)

Fig. 1 .

 1 Fig. 1. (a) Illustration of the 2:2 3-D transformer. (b) Three-dimensional view of the 2:2 transformer. (c) Side view of the transformers.

Fig. 2 .

 2 Fig. 2. (a) Transformer schematic. (b) Photographs of the fabricated 3-D transformers (4:4, right; 2:2, left).

Fig. 3 .

 3 Fig. 3. of N :N 3-D transformers including access lines and transformer core.

Fig. 4 .

 4 Fig. 4. Comparison between measurement, model, and simulation results for the 2:2, 4:4, and 6:6 3-D transformers.

Fig. 5 .

 5 Fig. 5. 1:1 3-D transformers. The 1:1 WB has shorter TLs and the return current path is closer, resulting in higher frequency operation and larger 1-dB BW.

TABLE II COMPARISON

 II OF MM-WAVE TRANSFORMERS

Authorized licensed use limited to: University of Grenoble Alpes. Downloaded on October 23,2023 at 16:03:05 UTC from IEEE Xplore. Restrictions apply.


Quentin Lenne

Zaynab Atyf

Jalal Ghilane
email: jalal.ghilane@u-paris.fr

When ionic liquids meet diazonium salts to generate thin layers modified surfaces

Immobilization of thin organic layers bearing specific functional groups onto materials is one of the main strategies to modulate their properties and develop task specific surfaces. Besides that, ionic liquids (IL's) and their peculiar physicochemical properties have been a major topic of academic research and are now attracting industrial interest. This short review highlights the growing attention given to the encounter between the ionic liquids and diazonium compounds for building organized interfaces. The review explore the different grafting procedures of diazonium in ionic liquid including the electrochemical assisted grafting. Particular emphasis will be devoted to the properties of the generated layers in ionic liquid and the added value of the IL as a grafting medium.

Introduction

Ionic liquid and their derivatives have attracted a lot of interest from the scientific world and have become a real multidisciplinary field. According to the common definition, an ionic liquid is a molten salt with a melting temperature below 100 °C. [START_REF] Fabre | A review of the thermophysical properties and potential of ionic liquids for thermal applications[END_REF] The characteristics that can be attributed to ionic liquids are a wide liquid range, very low vapor pressure, and thermal stability with unusual solvent properties. [START_REF] Minea | State of the art on the development of 1-Butyl-3methylimidazolium tetrafluoroborate ionic liquid and its nanocolloids[END_REF] Overcome with knowledge about ionic liquids at this time, researchers' and scientists' imaginations make ILs more useful than only being used in organic synthesis. Thus, the limits are being further pushed to tailor the physical-chemical properties of ionic liquids by generating new structures for use in the field of electrochemistry, in which the development of this type of compound becomes unlimited. [START_REF] Hapiot | Electrochemical reactivity in room-temperature ionic liquids[END_REF] The interest of ionic liquids in electrochemistry is much more considerable than just being limited to their use as a solventelectrolyte. Recently, new families of ionic liquids known as "task specific ionic liquids" have been introduced that are based on ionic liquids bearing functional groups. [START_REF] Pillai | Mini-review on Recent Advances in the Application of Surface-Active Ionic Liquids: Petroleum Industry Perspective[END_REF][START_REF] Watanabe | Application of Ionic Liquids to Energy Storage and Conversion Materials and Devices[END_REF] From laboratory research to real-life applications, the role of surface modification with organic layers is always crucial. The properties of surfaces are dictated by the intrinsic characteristic of the material. Tuning the properties of the surface enable to extend their potential applications. To this end, surface modifications by the attachment of thin organic layers have been proposed to add new functionalities to the surface. Numerous strategies are reported in the literature and several reviews were devoted to this topic as well as the potential applications of the modified surfaces in various fields including, molecular electronics, energy storage and conversion and sensor devices. [START_REF] Li | Surface functionalization of nanomaterials by aryl diazonium salts for biomedical sciences[END_REF][START_REF] Hetemi | Grafting of diazonium salts on surfaces: Application to biosensors[END_REF][START_REF] Lenne | Surface Modification for Promoting Durable, Efficient, and Selective Electrocatalysts[END_REF][START_REF] Mishyn | Controlled covalent functionalization of a graphene-channel of a field effect transistor as an ideal platform for (bio)sensing applications[END_REF][START_REF] Cao | Covalent Grafting of Dielectric Films on Cu(111) Surface via Electrochemical Reduction of Aryl Diazonium Salts[END_REF][START_REF] Olloqui-Sariego | Immobilizing redox enzymes at mesoporous and nanostructured electrodes[END_REF] All the described strategies for the immobilization of organic layers share a common principle, thus, the molecule should contain anchoring groups (SH, NH2, Si(OR)3, COOH, N2 + ..) and the desired functional groups that will be attached to the extreme electrode interface.

Among the various surface modification strategies, the reduction of diazonium derivatives was widely employed. [12,13•] Diazonium grafting can be initiated by several methods depending on the nature and the size of the substrates. For instance, chemically and radiatively induced reduction newly led to functional fabric and polymers under very smooth conditions. [START_REF] Li | One-step spontaneous grafting via diazonium chemistry for the fabrication of robust bionic multifunctional superhydrophobic fabric[END_REF][START_REF] Chernozem | Diazonium chemistry surface treatment of piezoelectric polyhydroxybutyrate scaffolds for enhanced osteoblastic cell growth[END_REF] More recently, the plasmon-assisted grafting of diazonium allowed to obtain multifunctional anisotropic objects, such as nanorods or nanocrosses by simply tuning the wavelength to the corresponding plasmon resonances. [START_REF] Therien | Three-color plasmon-mediated reduction of diazonium salts over metasurfaces[END_REF][START_REF] Olshtrem | Plasmon-assisted grafting of anisotropic nanoparticles-spatially selective surface modification and the creation of amphiphilic SERS nanoprobes[END_REF] Combining properly defined nanostructures and confinement effect even led to inhomogeneous growth of diazonium films on isotropic gold nanoparticles. [START_REF] Nguyen | Confinement effect of plasmon for the fabrication of interconnected aunps through the reduction of diazonium salts[END_REF] However, the most common reductive method remains the electrochemical assisted grafting. It is observed at soft reducing potential ranging from (0 to -0.6 V/SCE) which makes the grafting suitable over a wide range of conductive surfaces (metal, carbon-based materials, semiconductors). [13•] One should note that, while vastly explored, different aspects of this electrochemical reduction were only recently unveiled. For instance, this reduction was widely accepted to be a concerted mechanism. However, recent electrochemical spectroscopy coupled with ultrafast voltammetry measurements evidenced the presence of diazenyl radical species intermediate, suggesting that the diazonium electroreduction occurs through a stepwise mechanism. [START_REF] Pichereau | Real-time spectroelectrochemical monitoring of the diazonium electrografting[END_REF][START_REF] Pichereau | Highly Reactive Diazenyl Radical Species Evidenced during Aryldiazonium Electroreduction[END_REF] Thanks to these multiple modification strategies, we reached a point where almost any desired molecules moieties could be grafted with diazonium chemistry, either directly or through postfunctionalization. [START_REF] Wu | Building Tailored Interfaces through Covalent Coupling Reactions at Layers Grafted from Aryldiazonium Salts[END_REF][START_REF] Gautier | A post-functionalization toolbox for diazonium (electro)-grafted surfaces: Review of the coupling methods[END_REF] Subsequently, particular efforts were made in the recent years to tune the geometry of these immobilized layers to finely modulate the properties of the modified surfaces. While obtaining dense and thin layers is possible through the optimization of experimental parameters (time / scan rate, concentration, potential), [START_REF] Phal | Electrografting of 4-carboxybenzenediazonium on glassy carbon electrode: The effect of concentration on the formation of mono and multilayers[END_REF][START_REF] Kang | Tunable Electrochemical Grafting of Diazonium for Highly Sensitive Impedimetric DNA Sensor[END_REF][START_REF] Daukiya | Covalent functionalization of molybdenum disulfide by chemically activated diazonium salts †[END_REF] more efficient strategies were reported over the last years to obtain monolayers. Namely, it could either be performed using sterically hindered ligands (protecting groups, calix [START_REF] Pillai | Mini-review on Recent Advances in the Application of Surface-Active Ionic Liquids: Petroleum Industry Perspective[END_REF]arenes), [START_REF] Mishyn | Controlled covalent functionalization of a graphene-channel of a field effect transistor as an ideal platform for (bio)sensing applications[END_REF][START_REF] Mattiuzzi | Strategies for the Formation of Monolayers From Diazonium Salts: Unconventional Grafting Media, Unconventional Building Blocks[END_REF] redox inhibitors, [START_REF] Cesbron | Enhanced electrocatalytic activity on TEMPO mixed film grafted by diazonium reduction[END_REF] long alkyl chains, [START_REF] Shkirskiy | From Monolayer to Multilayer: Perylenediimide Diazonium Derivative Acting Either as a Growth Inhibitor or a Growth Enhancer[END_REF] enantioselective species, [START_REF] Zong | Chiral functionalization of solid surfaces with amino acid derivatives: diazonium grafting regulated by enantioselective processes[END_REF] or simply through ascorbic acid mediated dediazoniation. [START_REF] González | Selflimiting covalent modification of carbon surfaces: diazonium chemistry with a twist[END_REF] Although countless methods have been developed to modify surfaces through diazonium chemistry and finely tune the geometry of the grafted layer, one should notice that all the investigations are performed in classical solvents (water and acetonitrile). However, as discussed above, non-conventional solvents such as ionic liquids exhibit very favorable properties for the modification of surfaces using diazonium chemistry. This review focuses on the recent achievements that combine ionic liquids as solvent and the surface functionalization through diazonium molecules.

Ionic liquids as solvent for the electrochemical grafting

The possibility to perform the electrochemical grafting of diazonium compound in ionic liquid has been reported for the first time by Ghilane et. al. [START_REF] Ghilane | Modification of carbon electrode in ionic liquid through the reduction of phenyl diazonium salt. Electrochemical evidence in ionic liquid[END_REF] In this work, the nitrophenyl diazonium was electrochemically reduced in the presence of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [EMIM] [NTf2] (Scheme 1 in Figure 1). The electrochemical grafting, performed by cyclic voltammetry, displays the classical irreversible monoelectronic wave of diazonium reduction followed by it disappearance after successive cycles. This behavior is similar to that observed in classical solvent confirming the occurrence of the grafting in the ionic liquid. However, the peak current was found to be lower in ionic liquid due to its high viscosity. The presence of the attached nitrophenyl layer was evidenced by electrochemical measurement in ionic liquid, through the redox signal of the immobilized nitrophenyl (Scheme 2 in Figure 1). Interestingly, the authors report that the radical anion of the immobilized NO2-phenyl, in the ionic liquid [EMIM], undergoes rapid protonation leading to the formation of NHOH-phenyl (4e -reduction process) and NH2-phenyl groups (6e - reduction process) due to the presence of acidic proton at the position 2 in the ionic liquid cation [. It is therefore possible that the ionic liquid cation exchange protons with the reduced NO2phenyl. Thereafter, the same group performed a systematic study to investigate the electrochemical grafting of diazonium in various dry ionic liquids (100-200 ppm H2O) (Figure 1a andb). For all the investigated ionic liquids, the reduction of diazonium displays an irreversible reduction wave corresponding to the reduction of diazonium in the ionic liquid (Figure 1a). The comparison of the CV's display a current decreases of the diazonium reduction peak from 20 µA in ACN (0.34 cP) to 5.5 µA in [EMIm][NTF2] (32 cP) and finally reaches a value around 1.5 -2 µA in more viscous media (≈100-500 cP).Upon continuous cycling, this wave disappears suggesting the attachment of nitrophenyl layers onto the electrode surface. The performed electrochemical characterization of the modified electrode leads to estimate the surface concentration obtained in different ionic liquids. The surface concentration (Γ) was calculated using the formula Γ = Q/nFS, where Q corresponds to the charge of the immobilized redox

E (V vs SCE) i / µA [EMMIm] [NTf2] [Bu3MeN] [NTf2] [EMIm] [NTf2]
a moieties measured by integration of the redox peak, n is the number of electron involved, F is the Faraday constant and S represents the surface area. As an important result, the change in viscosity from 0.34 to 100 cP leads to a rapid decrease of the surface concentration, while, for a viscosity greater than 100 cP, the surface concentration reaches a constant value around 10 -10 mol.cm -2 . A linear correlation between ln(Γ) as a function of ln(1/η) was proposed. This work strongly supports the established hypothesis that the surface concentration of the immobilized molecules depends on the viscosity of the grafting medium. The use of ionic liquids as a grafting medium results in modulating the quantity of the attached layer and in forming a thin layer compared to the conventional solvent. Even though, the reason of the self-limitation of layer growth in ionic liquids remain somewhat unclear, the origin could be ascribed to several factors including the high viscosity of the IL which lower the diffusion of the diazonium, the reaction between the radical diazonium and the imidazolium cation, and the effect of the ionic liquid double layer.

In the literature, the electrochemical grafting of diazonium is performed using direct electrochemical reduction of diazonium compounds or using in-situ generated diazonium in the electrochemical cell. The latter is based on the use of amino-phenyl derivatives followed by the addition of the necessary chemical compounds (generally NaNO2 and HClO4) to perform the diazotation reaction. [START_REF] Salehzadeh | General approach for electrochemical functionalization of glassy carbon surface by in situ generation of diazonium ion under acidic and non-acidic condition with a cascade protocol[END_REF] One should note that this strategy avoids the synthesis and the isolation of diazonium salts (caution: risk of explosion). D. Bélanger et al. have investigated the possibility to generate in-situ diazonium in the presence of protic ionic liquid (Scheme in Fig 2)

. [START_REF] Shul | Electrochemical functionalization of glassy carbon electrode by reduction of diazonium cations in protic ionic liquid[END_REF] Figure 2. (a) Scheme illustrating the in-situ generation of diazonium in protic ionic liquid followed by the electrochemical grafting and the AFM images as well as the line profile of the grafted nitrophenyl. (Reprinted with permission from ref. [START_REF] Shul | Electrochemical functionalization of glassy carbon electrode by reduction of diazonium cations in protic ionic liquid[END_REF]. Copyright 2013 Elsevier). (b) Electrochemical grafting of diazonium onto glassy carbon and multi-walled carbon nanotubes using in-situ generated diazonium in ionic liquid. (Reprinted with permission from ref. [START_REF] Carvalho Padilha | Functionalization of Carbon Materials by Reduction of Diazonium Cations vvvvv Produced in Situ in a Brønstedt Acidic Ionic Liquid[END_REF].

Copyright 2016 Wiley).

The occurrence of the grafting was confirmed by electrochemical measurements and by surfaces analysis. As an important result, the attached layer presents characteristic close to that expected for a monolayer (AFM images in Fig 2). The measured thickness 1.5 nm corresponds a b to the formation of 1-2 layers of nitrophenyl groups which is found to be thinner than what was obtained in classical solvents (water or acetonitrile). To explain this difference, the authors proposed two hypotheses. First, this phenomenon might arise from the high viscosity of protic ionic liquid and thus lowering the diffusion coefficient of the diazonium. Secondly, it was postulated that the presence of larger ionic liquid ions within the electrochemical double layer may limit the growth of the grafted film to a few layers.

In another study, Lagrost et al. investigated the reduction of diazonium cations, produced insitu in the presence of the acidic ionic liquid, onto glassy carbon electrode and multi-walled carbon nanotubes (MWCNTs) (Figure 2b). [35•] In this work, the electrochemical characterization of the modified electrode displays higher surface concentrations than those obtained from the conventional HCl electrolyte. This funding is different than that reported earlier, however, this result was determined by following the electrochemical response of the attached redox group. It is well known that this electrochemical measurement provides only an estimation of the surface concentration due to the presence of silent redox groups. This difference was clarified when considering the AFM results where a thinner layer is obtained in the presence of the acidic ionic liquid compared to aqueous HCl. Overall, taking into account all the results, thinner and dense layers are probably produced in the presence of the ionic liquid when compared to classical solvent. Similar procedure was successfully performed to generate modified MWNTs for their potential use in energy storage applications with increased electrochemical performance.

It is worthy to note that this functionalization strategy in ionic liquid is not only limited to carbonaceous surfaces as it was successfully applied to gold surfaces. [START_REF] Descroix | Regular poly(para-phenylene) films bound to gold surfaces through the electrochemical reduction of diazonium salts followed by electropolymerization in an ionic liquid[END_REF] First, benzenediazonium salt was electrochemically reduced to generate a thin layer of poly(paraphenylene) (PPP) covalently bound to gold substrate. Next, on top of this layer, a thicker layer of poly(para-phenylene) (PPP) is grown by oxidative electro-polymerization of biphenyl in 1butyl-3-methylimidazolium hexafluorophosphate [BMIm][PF6]. The generated material was characterized by various surface analyses methods including IR, ToF-SIMS and fluorescence spectroscopies demonstrating the formation of well regular PPP layers that are covalently linked to the gold substrate. The key role of the ionic liquid in the structuration of the polymer is emphasized. As a consequence, combining IL and diazonium chemistry led to extremely robust and organized films. [START_REF] Descroix | Regular poly(para-phenylene) films bound to gold surfaces through the electrochemical reduction of diazonium salts followed by electropolymerization in an ionic liquid[END_REF] In another study, electrochemical response of 4-nitrobenzene layers previously dissolved in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) imide ([BMP]NTf2]) was measured on gold surfaces, evidencing the grafting process. Similarly to what is observed on carbon surfaces, the use of ionic liquid as a grafting media resulted in thinner layers, supporting the idea that viscosity has an influence over compactness. [START_REF] Vacca | Functionalization of polycrystalline gold through the electroreduction of aryldiazonium salts in ionic liquids[END_REF] Thanks to the flexible synthesis of ionic liquid, it has also been possible to develop a unique example of diazonium-based IL consisting of p-butylbenzenediazonium ions and bis(trifluoromethanesulfonyl) imide (NTf2). [START_REF] Liang | A diazonium salt-based ionic liquid for solvent-free modification of carbon[END_REF] This flaccidity of the NTf2 anions renders the resulting diazonium to be liquid at ambient temperature. By soaking a graphitic rod in this hybrid liquid ionic, it was possible to observe a covalently grafted organic layer on the surface of the carbonaceous material, either by thermal (85°C overnight) or electrochemical activation (-0.8V vs Pt during 10 min). [START_REF] Liang | A diazonium salt-based ionic liquid for solvent-free modification of carbon[END_REF] However, the study underlines that electrochemical grafting resulted in a more homogeneous organic film, as a result of milder conditions. This class of task-specific ionic liquid therefore makes the use of solvent obsolete for the functionalization of material through diazonium chemistry.

Surface functionalization without electrical bias

While the coupling of electrochemistry and the presence of ionic liquid has been proven to be a powerful strategy, alternative techniques emerged for the covalent grafting of diazonium moieties onto surfaces without any external electrical bias.

In this regard, ILs were first employed to generate nano-carbon materials such as single wall carbon nanotubes (SWCNTs) and graphene using exfoliation process since they have been found to be particularly efficient to overcome the cohesive forces in these materials. [START_REF] Price | Green chemical functionalization of single-walled carbon nanotubes in ionic liquids[END_REF][START_REF] Jin | Mechanically assisted exfoliation and functionalization of thermally converted graphene sheets[END_REF] Exfoliation process coupled with the presence of diazonium salt led to a reasonable amount of functionalized carbon sites after a simple grinding step which allowed to obtain highly dispersible CNTs and single graphene sheets (Figure 3). The absence of ILs chemical components in the XPS analyses confirmed that the ILs only served as dispersive media and not grafted/adsorbed onto modified carbon nanomaterials. For such application, ILs with long alkyl chains (1-octyl-3-methylimidazolium tetrafluoroborate, OMIBF4) are more favorable as they have been shown to improve the debundling of the stacked 1D/2D objects. The versatility of this strategy has been evidenced by monitoring the solubility of the modified nano-objects as a function of the functional groups located at the interface, which were all better than their unmodified counterparts. Depending on the nature of the ionic liquid used, this method could then be an interesting environmentally friendly alternative for the functionalization of 1D/2D nano-carbon and could easily be implemented in an up-scalable setup thanks to its simplicity. The inherent conductivity of ionic liquid can also be a powerful tool to induce the necessary electron transfer for the diazonium reduction processes. For instance, boron doped diamond has been successfully modified with a thin layer of 4-nitroazobenzene, by simply stirring of diazonium salts at room temperature in the presence of hydrophobic and hydrophilic ionic liquid.[41•] The electrochemical characterizations of the attached layers confirm that the modified electrodes in hydrophilic and hydrophobic ionic liquids have similar electrochemical characteristics. However, the use of hydrophilic ionic liquid leads to the generation of more compact layers. In another study, imidazolium based ionic liquids were investigated using the thermal grafting of diazonium salts and led to the covalent grafting of organic moieties onto SWCNTs (Figure 3c).[42••] However, during classical heating procedures, no disruptive improvement could be observed when using ionic liquid rather than classical solvents. Interestingly, in the same study, the introduction of microwave radiations led to a tunable functionalization/retro-functionalization of SWCNTs depending on the reaction time. This phenomenon, taking place at high surface coverage, was explained by the concomitant action of the hot spots generated by the microwave irradiations in ionic liquids, causing the detachment of the ligands, and the presence of aryl radicals acting as "inhibitors", preventing the re-attack of the surface (Figure 3d).[42••] Those properties are quite unusual in diazonium chemistry where the radical grafting is not a reversible process. Such observations are promising to promote the restoration of the π-conjugated structure of CNTs, even if a subtle choice of IL / diazonium couple is needed for a fine tuning of the retro-functionalization process.

Concluding remarks

Using ionic liquid as an electrolyte for diazonium grafting offers various possibilities for the functionalization of different surfaces. While the inherent conductivity of ILs is an indisputable advantage for electrochemistry, the tunable properties of these non-conventional solvents (structure, viscosity) allow to finely modulate the resulting grafted layers. Overall, this review highlights the possibility to use ionic liquid media for the electrochemical grafting of thin layers based on the reduction of diazonium molecules. Compared to classical solvent, the use of IL leads to generate thinner layers which is attributed to several factors including the high viscosity of the ILs and the interaction between the highly reactive aryl radical and the ionic liquid cation. However, the parameters regulating this phenomenon need further rationalization and additional investigations are necessary to elucidate the role of the ionic liquids including the grafting mechanism, the role of the electrochemical double layer and the self-termination of the grafting. Moreover, several studies in this short review show that the use of ILs was not limited to the electrochemical grafting and report the successful grafting of diazonium using thermal, radiative, or even spontaneous activation on several surfaces. Given the number of available ILs and the functional diazonium salts and/or in-situ generated diazonium salts, one can easily imagine that the work reported here is just the mere beginning of this research area, especially if the upcoming progress in materials-genome prediction and design is dedicated to target the most promising ionic liquids candidates.

Figure 1 .

 1 Figure 1. Scheme illustrating the electrochemical grafting of diazonium in ionic liquid (1) and the redox reaction of the immobilized nitrophenyl groups in ionic liquid (2) (Reprinted with permission from ref. [31]. Copyright 2008 Elsevier). (a) CV of the electrochemical reduction of diazonium on a carbon electrode in ionic liquids, 1-ethyl-3-methylimidazolium [EMIm][NTf2], 1-ethyl-2,3-dimethylimidazolium [EMMIm] [NTf2] and N-tributyl-Nmethylammonium [Bu3MeN][NTf2]. (b) Variation of the surface concentration of the attached layer (electrochemical characterization in 0.1 M H2SO4) with the viscosity of the grafting media. (♦) grafting in [Bu3MeN][NTf2] with the addition of various amount of ACN co-solvent . (O) grafting in three different ionic liquids and ACN. (Reprinted with permission from ref. [32]. Copyright 2010 American Chemical Society).

Figure 3 .

 3 Figure3. a) Exfoliation and functionalization of graphene sheets by grinding thermally converted graphene (TCG) in ionic liquid in the presence of a diazonium salt. b) AFM image of TCG sheet functionalized with 2a in OMIBF4 on a mica surface and c) its corresponding sectional analysis from 3b. (Adapted from Ref.[START_REF] Jin | Mechanically assisted exfoliation and functionalization of thermally converted graphene sheets[END_REF]). d) Diazonium salt addition to CNTs yielding different modified CNTs under microwave radiations in different ILs. e) Schematic representation of the reversible functionalization/retro-functionalization process. (Adapted from Ref.[START_REF] Barrejón | Ionic liquids plus microwave irradiation: A •• general methodology for the retro-functionalization of single-walled carbon nanotubes[END_REF]).

Acknowledgements

This work was funded by ANR (pHSELECT, ANR-22-CE04-0006-02).

References and recommended reading

Papers of particular interest, published within the period of review, have been highlighted as:

• Paper of special interest.

•• Paper of outstanding interest.