Erwan Pannier
email: erwan.pannier@centralesupelec.fr

Christophe O Laux
email: christophe.laux@centralesupelec.fr

Analysis of the JAXA Nonequilibrium Infrared Emission Spectra for Mars Entry Conditions

Nomenclature p = pressure [bar]

T r ot = rotational temperature of the gas [K]

T tr ans = translational temperature of the gas [K]

T vib = vibrational temperature of the gas [K]

T vib 1 , T vib 2 , T vib 3 = for CO 2 , temperature of each vibration mode: symmetric, bending and asymmetric vibration, respectively [K] x CO , x CO 2 = mole fractions

I. Introduction

CO 2 radiation plays an important role in forebody and afterbody heating of vehicles entering the Martian atmosphere.

In the forebody, the radiation is dominated by visible and UV emission of CO, CN and O 2 [2? ]. In the afterbody, the recombination of CO 2 is responsible for a radiative heat flux larger than convective heating [START_REF] Da Silva | Contribution of CO2 IR Radiation to Martian Entries Radiative Wall Fluxes[END_REF], but thee are still large uncertainties on the radiative fluxes [START_REF] Cruden | Radiative Heating During Mars Science Laboratory 12 Entry: Simulation, Ground Test, and Flight[END_REF]: on the eve of new Mars missions, a renewed computational [START_REF] Rouzaud | Influence of Radiative Heating on a Martian Orbiter[END_REF] and experimental effort was engaged at NASA [START_REF] Cruden | Radiative Heating During Mars Science Laboratory 12 Entry: Simulation, Ground Test, and Flight[END_REF], JAXA [START_REF] Takayanagi | Measurements of Carbon Dioxide Nonequilibrium Infrared Radiation in Shocked and Expanded Flows[END_REF] and UQ [START_REF] Gu | Study of the Afterbody Radiation during Mars Entry in an Expansion Tube[END_REF] to model nonequilibrium CO 2 radiation.

The JAXA expansion tube experiment of Takayanagi et al. [START_REF] Takayanagi | Measurements of Carbon Dioxide Nonequilibrium Infrared Radiation in Shocked and Expanded Flows[END_REF] was one of the first experiment to measure absolute intensity CO 2 infrared spectra in an expanding flow. The experiment was performed in the JAXA HVST expansion-tube facility, with a model placed at the exit of the tube, and a flow velocity of 5.68 km/s. Using CFD simulations to reproduce the measured shock standoff distance, the authors determined the following nonequilibrium conditions in the free flow upstream of the model: T trans =1690 K, T vib =4000 K, p=17 mbar, x CO 2 =0.61, x CO =0.26, x O 2 =0.12 and x O =0.01.

The authors also measured absolute emission spectra of the 4.2 µm band of CO 2 at three locations: in the free flow, in front of the model, and behind the model. Takayanagi et al. [START_REF] Takayanagi | Measurements of Carbon Dioxide Nonequilibrium Infrared Radiation in Shocked and Expanded Flows[END_REF] attempted to fit the free flow experimental spectrum with the SPRADIAN code [START_REF] Fujita | SPRADIAN, Structured Package for Radiation Analysis:Theory and Application[END_REF] assuming equilibrium at temperatures ranging from 1500 to 4000 K. However, they obtained a surprisingly large disagreement for all conditions considered (see Fig. 22 in Ref. [START_REF] Takayanagi | Measurements of Carbon Dioxide Nonequilibrium Infrared Radiation in Shocked and Expanded Flows[END_REF]). They attributed the discrepancies to flow heterogeneities and to a nonequilibrium vibrational distribution. The forebody radiation was not analyzed.

In this work, we reanalyze the nonequilibrium spectra measured by Takayanagi et al. [START_REF] Takayanagi | Measurements of Carbon Dioxide Nonequilibrium Infrared Radiation in Shocked and Expanded Flows[END_REF]. We first consider the free-flow to confirm its nonequilibrium state. Then, the forebody radiation will be examined. The path length associated with the afterbody radiation is not clear, hence it is not studied in the present work.

Figure 1 shows the experimental configuration and measurement locations. The shock wave position (dashed) is inferred from the Schlieren images of Ref. [START_REF] Takayanagi | Measurements of Carbon Dioxide Nonequilibrium Infrared Radiation in Shocked and Expanded Flows[END_REF]. The measurement locations are inferred from Fig. 21 of Ref. [START_REF] Takayanagi | Measurements of Carbon Dioxide Nonequilibrium Infrared Radiation in Shocked and Expanded Flows[END_REF]. The spectra were measured tangentially to the model so the total line-of-sight length is always 7 cm. In front of the model (blue arrow in Fig. 1), part of the line-of-sight is in the free-flow (red portion) and part is in the shock layer (blue portion).

Because of the model curvature, the distance to the model wall is not constant (see the upper part of Fig. 1, estimated from Fig. 17 of Ref. [START_REF] Takayanagi | Measurements of Carbon Dioxide Nonequilibrium Infrared Radiation in Shocked and Expanded Flows[END_REF]) and the integration path is not homogeneous: in the following, we consider that the shock region is equivalent to a segment of uniform gas of length 1 cm.

The spectra measured by Takayanagi et al. [START_REF] Takayanagi | Measurements of Carbon Dioxide Nonequilibrium Infrared Radiation in Shocked and Expanded Flows[END_REF] in the free-flow and in front of the model are reproduced in Figure 2. They are red-shifted by 30 nm so as to match the CO 2 bandhead in the later calculations. This shift could be attributed to a small offset in the experimental wavelength calibration procedure. For illustration purpose, the forebody emission (dashed, blue) is estimated in the optically thin approximation by subtracting the free-flow emission scaled down to 6 cm to the line-of-sight measurement in front of the model. In the later calculations, the full line-of-sight is calculated solving the radiative transfer equation.

II. Spectral code

The radiation of CO 2 and CO radiation is modeled using RADIS [START_REF] Pannier | RADIS: a Nonequilibrium Infrared Radiative Solver: Source Code and User Guide[END_REF][START_REF] Pannier | RADIS : A Nonequilibrium Line-by-Line Radiative Code for CO2 and HITRAN-like database species[END_REF], an open-source nonequilibrium infrared line-by-line code. In Ref. [START_REF] Pannier | RADIS : A Nonequilibrium Line-by-Line Radiative Code for CO2 and HITRAN-like database species[END_REF], the RADIS solver was validated against infrared measurements from an equilibrium microwave torch [START_REF] Depraz | Infrared emission spectroscopy of CO2 at high temperature. Part II: Experimental results and comparisons with spectroscopic databases[END_REF], a low-pressure nonequilibrium laser discharge [START_REF] Dang | Detailed vibrational population distributions in a CO2 laser discharge as measured with a tunable diode laser[END_REF], an equilibrium RF plasma torch [START_REF] Packan | Measurement and Modeling of OH, NO, and CO Infrared Radiation at 3400 K[END_REF] and a NASA shock tube experiment [START_REF] Cruden | Measurement and Characterization of Mid-wave Infrared Radiation in CO2 Shocks[END_REF]. RADIS takes HITRAN-like line databases as input, and either reads rovibrational energies from precomputed databases or calculates them from spectroscopic constants. RADIS can model a variety of vibrational population distributions (Boltzmann, Treanor, overpopulation factors...), as well as line-of-sight emission spectra of heterogeneous media: for these simulations, the radiative transfer equation is solved along multiple slabs, each slab being assumed to have uniform thermochemical properties.

In this work, the CDSD-4000 line database [START_REF] Tashkun | CDSD-4000: High-resolution, high-temperature carbon dioxide spectroscopic databank[END_REF] is used to model CO 2 radiation at high temperature (T > 2200 K), and the HITEMP-2010 database [START_REF] Rothman | HITEMP, the high-temperature molecular spectroscopic database[END_REF] for CO radiation or CO 2 radiation at lower temperature (T ≤ 2200 K). A sensitivity analysis [START_REF] Pannier | RADIS : A Nonequilibrium Line-by-Line Radiative Code for CO2 and HITRAN-like database species[END_REF] showed that HITEMP-2010 and CDSD-4000 give similar results at T ≤ 2200 K for the 4.2µm CO 2 band. The rovibrational energies of CO are calculated with a coupled rotating vibrator model. Dunham expansions are calculated with the spectroscopic constants of Ref. [START_REF] Guelachvili | Dunham coefficients for seven isotopic species of CO[END_REF], which include rotational-vibrational interaction terms for seven isotopes of CO. The rovibrational energies of CO 2 are calculated with an uncoupled rotating vibrator model. The spectroscopic constants reviewed by Klarenaar et al. [START_REF] Klarenaar | Time evolution of vibrational temperatures in a CO 2 glow discharge measured with infrared absorption spectroscopy[END_REF] are used for the three main isotopes of CO 2 , and do not include rotational-vibrational interaction terms. Boltzmann distributions are used for both the rotational and the vibrational levels.

Nonequilibrium CO 2 spectra are calculated with a two-temperature model (T vib , T rot ). In a first model, the rotational temperature is assumed to be equal to the translational temperature, and the three vibrational modes of CO 2 are assumed to be in equilibrium, i.e.:

T rot = T tr ans

(1)

T vib = T vib 1 = T vib 2 = T vib 3 (2)
However, as the asymmetric mode of CO 2 (ν 3 ) relaxes slower than the symmetric (ν 1 ) and bending (ν 2 ) vibrational modes [START_REF] Witteman | The CO2 laser[END_REF], another two-temperature model can be considered where the asymmetric vibration is not in equilibrium with the other vibration modes. We then define it as the vibrational temperature, and consider all other temperatures equal to the rotational temperature:

T rot = T tr ans = T vib 1 = T vib 2 (3)
T vib = T vib 3 (4)
No previous measurements have been reported in the literature that would suggest either of the two models to be more appropriate for expansion tube conditions. Measurements under similar pressure conditions, such as those of Dang et al. [START_REF] Dang | Detailed vibrational population distributions in a CO2 laser discharge as measured with a tunable diode laser[END_REF] in a 20 mbar CO 2 laser, and those of Klarenaar et al. [START_REF] Klarenaar | Time evolution of vibrational temperatures in a CO 2 glow discharge measured with infrared absorption spectroscopy[END_REF] in a 6.7 mbar DC glow discharge, showed symmetric and bending vibrational mode temperatures close to the rotational temperatures, whereas the asymmetric mode temperature was significantly different. These measurements would support the second model (Eqn. 3, 4), however, the thermodynamic conditions in the expansion tube are significantly different. Furthermore, the latter model requires that the contribution of all vibration modes to the total rovibrational energies be identified, which is not possible for some of the high energy levels featured in the CDSD-4000 [START_REF] Tashkun | CDSD-4000: High-resolution, high-temperature carbon dioxide spectroscopic databank[END_REF] database. Therefore, such a model is limited to low temperature conditions that can be modeled with the HITEMP-2010 [START_REF] Rothman | HITEMP, the high-temperature molecular spectroscopic database[END_REF] database (T ≤ 2200 K). In this work, both models will be compared in the lower temperature free-flow region.

III. Modeling of the free-flow radiation

The 7-cm line-of-sight in the free-flow region is assumed to be homogeneous. CO 2 spectra are calculated with RADIS using the HITEMP-2010 [START_REF] Rothman | HITEMP, the high-temperature molecular spectroscopic database[END_REF] database, a pressure of 17 mbar and a frozen CO 2 concentration of x CO 2 =0.61 as given in Takayanagi et al. [START_REF] Takayanagi | Measurements of Carbon Dioxide Nonequilibrium Infrared Radiation in Shocked and Expanded Flows[END_REF]. Nonequilibrium spectra are first calculated using the model of Eqn. 1, 2, which assumes equilibrium between all CO 2 vibration modes. Figure 3 shows the spectra for various vibrational and rotational temperatures.

: In general, the calculated spectra are much more similar in shape and amplitude to the experimental spectrum than those computed with SPRADIAN and presented in Ref. [START_REF] Takayanagi | Measurements of Carbon Dioxide Nonequilibrium Infrared Radiation in Shocked and Expanded Flows[END_REF] (Fig. 22). The best agreement is obtained for T rot =1700 K and T vib =2200 K. The bandhead at 4.2 µm is correctly reproduced. However, the agreement is not perfect, in particular for the region of the CO 2 hot bands (> 4.4 µm). As suggested by [START_REF] Takayanagi | Measurements of Carbon Dioxide Nonequilibrium Infrared Radiation in Shocked and Expanded Flows[END_REF], the presence of a heterogeneous flow with hotter CO 2 slabs may be an explanation for the discrepancy. However, adding a hotter CO 2 slab would also overpredict the radiation around 4.6 µm, as shown in Fig. 3 for the calculation at 2600 K, which is already underestimated because it is slightly above the vaid temperature range of HITEMP-2010. Spatially-resolved spectra would be needed to close this issue.

Another possible reason for the discrepancy could be the presence of CO infrared emission. We added nonequilibrium CO emission to our simulations, assuming that the CO concentration remains frozen with x CO =0.26. The CO rotational temperature is set equal to that of CO 2 , and the CO vibrational temperature T CO is set equal to that of CO 2 . However, the inclusion of CO made no significant changes to the spectra of Fig. 3. Only CO with vibrational temperature much higher than the CO 2 vibrational temperature can yield quantitative changes. In Figure 4, we include the emission of nonequilibrium CO with a vibrational temperature of 4000 K to the T rot =1700 K, T vib =2200 K spectrum of Fig ??. The emission of CO with a vibrational temperature of 2200 K is shown for reference. The addition is enough to reproduce the experimental spectrum in the 4.8 -4.9 µm region, however, there is about a factor of 2 between the vibrational temperature of CO 2 and CO. In the measurements of Klarenaar et al. [START_REF] Klarenaar | Time evolution of vibrational temperatures in a CO 2 glow discharge measured with infrared absorption spectroscopy[END_REF] in a DC glow at similar pressure, the CO temperature was found to be generally much higher than the vibrational temperature of the bending and symmetric modes of CO 2 , but it remained comparable with the asymmetric temperature T vib 3 . In our model, T vib 3 remained equal to 2200 K. Without further confirmation of the possibly large nonequilibrium between CO and CO 2 vibrational temperatures, we did not retain the adjustment and the CO vibrational temperature is set equal to that of CO 2 .

:DYHOHQJWKQP A broad range of nonequilibrium conditions was calculated in RADIS using the CO 2 nonequilibrium model of Eqn. 1, 2, and the CO nonequilibrium model described previously. Figure 5 shows the residual difference between the computed and measured spectra for various combinations of vibrational and rotational temperatures between 1600 and 2600 K (white dots). The best agreement with the experimental spectrum is obtained for T rot =1700 K and T vib 1 =T vib 2 =T vib 3 =T CO =2200 K. The fitted rotational temperature is close to the value measured by Takayanagi et al. [START_REF] Takayanagi | Measurements of Carbon Dioxide Nonequilibrium Infrared Radiation in Shocked and Expanded Flows[END_REF],

5DGLDQFH:FP 2 VUP ([SHULPHQW &2. &2. &2 2 &2. &2 2 &2.
T rot =1690 K. The vibrational temperature is also higher than the rotational temperature, as suggested by the adjusted parameters of [START_REF] Takayanagi | Measurements of Carbon Dioxide Nonequilibrium Infrared Radiation in Shocked and Expanded Flows[END_REF], and characteristic of a fast expanding flow. However, the measured departure from nonequilibrium in the free flow is much smaller than predicted by the CFD simulations of Ref. [START_REF] Takayanagi | Measurements of Carbon Dioxide Nonequilibrium Infrared Radiation in Shocked and Expanded Flows[END_REF] (T rot =1690 K and T vib =4000 K). We then consider the second CO 2 nonequilibrium model (Eqn. [START_REF] Da Silva | Contribution of CO2 IR Radiation to Martian Entries Radiative Wall Fluxes[END_REF][START_REF] Cruden | Radiative Heating During Mars Science Laboratory 12 Entry: Simulation, Ground Test, and Flight[END_REF], where the vibrational temperature of the CO 2 asymmetric mode T vib 3 is in nonequilibrium. CO emission is also included. In this model, the CO vibrational temperature is set equal to T vib 3 , as suggested by the measurements of Klarenaar et al. [START_REF] Klarenaar | Time evolution of vibrational temperatures in a CO 2 glow discharge measured with infrared absorption spectroscopy[END_REF] at similar pressure. However,the contribution of CO is expected to be negligible as was shown in Fig. ??.

Figure 6 shows the spectra calculated with this model. The agreement with the experimental spectrum is not as good as when using the first model (Fig. 3). In particular, the bandhead at 4.2 µm is not as well reproduced as when all of the vibrational modes of CO 2 share a same vibrational temperature. All conditions of Fig. 5 were also calculated and are shown on Figure 7. The best agreement is achieved for temperatures of T rot =T vib 1 =T vib 2 =2200 K, T vib 3 =2500 K.

The measured rotational temperature is slightly higher than the 1690 K temperature of Takayanagi et al. [START_REF] Takayanagi | Measurements of Carbon Dioxide Nonequilibrium Infrared Radiation in Shocked and Expanded Flows[END_REF], and the residuals in Fig. 7 are about twice higher as in Fig. 5.

In conclusion, our calculations suggest that in this particular experiment, the free-flow spectrum is better reproduced by a nonequilibrium model assuming that all CO 2 vibration modes share a same vibrational temperature different from the rotational temperature, as in Eqn. 1, 2. Specifically, we found T rot =1700 K and T vib departure from equilibrium is limited, and all vibration modes seem to be in equilibrium. The spectrum is dominated by CO 2 emission and the contribution of CO is negligible.

Table 1 Simplified For these calculations, RADIS uses the CDSD-4000 [START_REF] Tashkun | CDSD-4000: High-resolution, high-temperature carbon dioxide spectroscopic databank[END_REF] database and is interfaced with the CANTERA code [START_REF] Goodwin | Cantera: An Object-oriented Software Toolkit for Chemical Kinetics, Thermodynamics, and Transport Processes[END_REF] to calculate the equilibrium CO 2 and CO concentrations. At very high temperature (11,000 K), CO and CO 2 are both fully dissociated in the shock-layer and the line-of-sight emission spectrum is entirely due to the free flow component.

At 3500 K, the emission spectrum is dominated by the contribution of CO 2 in the shock-layer. As the temperature increases, the CO 2 equilibrium concentrations falls and the overall radiation intensity decreases. CO emission becomes stronger than CO 2 emission above 4000 K.

The high temperature CO emission spectrum starts above 4294 nm, which corresponds to the reversal of the v'=1→v"=0 band at about J=92. Therefore, all emission below 4294 nm in the experimental spectrum can only be attributed to CO 2 emission. Above 4294 nm, the calculated CO emission spectrum features characteristic high temperature vibrational bandheads that are regularly spaced every about 55 nm. The shifted experimental spectrum also features similar regularly spaced peaks between 4410 and 4790 nm. These peaks are more visible when subtracting the free-flow component, as shown in Fig. 2. Another characteristic of CO emission is the emission plateau in the 4.8 -5 µm region, which corresponds to optically thick CO emission lines in the calculated spectra.

The 3500 K spectrum (Fig. 8) does not reproduce the 4.8 -5 µm region well, and significantly overestimates the total intensity. The 4500 K spectrum (Fig. 10) also fails to reproduce the experimental spectrum, particularly in the 4.2 -4.3 µm region and above 4.6 µm. the measured intensity in the 4.2 -4.3 µm region is slightly higher than in the case of the free-flow (Fig. 2), which proves the presence of additional CO 2 emission in the shock layer slab. However, under the chemical equilibrium assumption, the CO 2 emission is negligible at 4500 K. Finally, the line-of-sight spectrum corresponding to a shock-layer temperature of T=4000 K gives the best agreement with the experimental spectrum, both in terms of shape and absolute intensity. We conclude that the emission of the shock-layer can reasonably be approximated by that of a 1 cm slab in thermochemical equilibrium at T=4000 ± 500 K.

Conclusion

The RADIS [START_REF] Pannier | RADIS: a Nonequilibrium Infrared Radiative Solver: Source Code and User Guide[END_REF][START_REF] Pannier | RADIS : A Nonequilibrium Line-by-Line Radiative Code for CO2 and HITRAN-like database species[END_REF] nonequilibrium spectral code was used to reanalyze the free-flow radiation measured in the expansion-tube experiment of Ref. [START_REF] Takayanagi | Measurements of Carbon Dioxide Nonequilibrium Infrared Radiation in Shocked and Expanded Flows[END_REF]. Excellent agreement was achieved using a homogeneous spatial model and a slightly nonequilibrium distribution, which confirms the nonequilibrium hypothesis of Takayanagi et al. [START_REF] Takayanagi | Measurements of Carbon Dioxide Nonequilibrium Infrared Radiation in Shocked and Expanded Flows[END_REF]. A comparison between different nonequilibrium models suggested that all CO 2 vibration modes share a common vibrational temperature (2200 K) which is equal to the CO vibrational temperature and slightly above the rotational temperature (1690 K). The study was extended to do the first modeling of the spectral emission within the shock-layer in front of the model. It was shown that the forebody radiation corresponds to that of a plasma in thermochemical equilibrium at a temperature of 4000 ± 500 K, and that the measured experimental spectrum comprises an about equal contribution of CO 2 and CO emission.

Fig. 1

 1 Fig. 1 Schematic of the measurement locations of Takayanagi et al. [6], without the model (red arrow) and in front of the model (blue arrow). Refer to text for more details.

Fig. 2 IR

 2 Fig. 2 IR radiation measured in the expansion-tube experiment of Ref. Takayanagi et al. [6] in the free flow (red) and in front of the entry model (solid blue). Forebody emission (dashed, blue) is estimated in the optically thin approximation.

Fig. 4

 4 Fig. 4 Contribution of nonequilibrium CO at T vib = 2200 K (green) and 4400 K (blue) to the total line-of-sight emission (dashed and solid red, respectively). x CO 2 =0.61, x CO =0.26, p=17 mbar, T rot =1700 K, T vib 1,2,3 =2200 K.

Fig. 5

 5 Fig. 5 Residual of nonequilibrium calculation relative to the free flow spectrum [6]. The minimum is obtained for T rot =1700 K, T vib 1,2,3 =2200 K. Red circle spectra are shown in Fig. 3.

Fig. 7

 7 Fig. 7 Residual of nonequilibrium calculation relative to the free flow spectrum [6]. The minimum is obtained for T rot =T vib 1,2 =2200 K, T vib 3 =2500 K. Red circle spectra are shown in Fig. 6.

3 - 8 Fig. 9 Fig. 10 Free

 38910 slab model for RADIS simulations of the forebody line-of-sight emission. Line-of-sight Fig. -stream: Two 3-cm nonequilibrium slabs T rot = T trans = 1690 K T vib (CO 2 ) = T vib1 = T vib2 = T vib3 = T vib (CO) = 2200 K x CO = 0.257, x CO2 = 0.606, p = 17 mbar Shock-layer: 1-cm slab under thermochemical equilibrium T = 3500 K, p = 1 bar x CO 2 =0.130, x CO =0.514 T = 4000 K, p = 1 bar x CO 2 =0.027, x CO =0.520 T = 4500 K, p = 1 bar x CO 2 =0.005, x CO =0.506

Fig. 8 Fig. 9

 89 Fig. 8 Measured spectrum across the shock layer [6] (black), compared to the line-of-sight calculation (red) including two nonequilibrium free-flow slabs (dashed) and one equilibrium slab at 3500 K (CO 2 : blue, CO: green).

Fig. 10

 10 Fig. 10 Measured spectrum across the shock layer [6] (black), compared to the line-of-sight calculation (red) including two nonequilibrium free-flow slabs (dashed) and one equilibrium slab at 4500 K (CO 2 : blue, CO: green).

3 Emission spectra in the free-flow (black [6]), compared with CO 2 calculations in a frozen state (x CO 2 =0.61, p=17 mbar) with the following nonequilibrium model: T rot , T vib =T vib 1 =T vib 2 =T vib 3

  vib .7 rot . 7 vib .7 rot . 7 vib .7 rot . 7 vib .7 rot .

	VUP	7 vib .7 rot . ([SHULPHQW 7
	5DGLDQFH:FP 2
		DYHOHQJWKQP

Fig.

6 Emission spectra in the free-flow (black [6]), compared with calculations with CO 2 in a frozen state (x CO 2 =0.61, , p=17 mbar) with the following nonequilibrium model: T rot =T vib 1,2 , T vib =T vib 3 .

  1 =T vib 2 =T vib 3 =2200 K. The vib 3 .7 rot . 7 vib 3 .7 rot . 7 vib 3 .7 rot .

	5DGLDQFH:FP 2 VUP 7 rot .	:DYHOHQJWKQP ([SHULPHQW 7 7 vib3 .	UHVLGXDO

Fig.

IV. Modeling of the forebody radiation

We now consider the line-of-sight spectrum measured in the front part of the model. This spectrum has not been analyzed previously. This spectrum contains radiation from both the free flow and from the forebody shock layer, as shown in Fig. 1. Using Fig. 21 in Ref. [START_REF] Takayanagi | Measurements of Carbon Dioxide Nonequilibrium Infrared Radiation in Shocked and Expanded Flows[END_REF], we estimated the forebody path length to be about 1 cm. This is an approximation, but it is sufficient to analyze the main features of the forebody emission. Thus, our line-of-sight model is a 1 cm slab corresponding to the shock layer, surrounded by two 3 cm slabs corresponding to the free-flow, for a total of 7 cm. The free-flow conditions are taken as those determined in Section III, i.e., T rot =1700 K and T vib 1 =T vib 2 =T vib 3 =T vib (CO)=2200 K. Because the free-flow slabs are not optically thin, the RADIS line-of-sight module is used to solve the radiative transfer equation. «««< Updated upstream The shock-layer conditions are calculated with the CEA code [START_REF]CEARUN[END_REF] for pre-shock conditions corresponding to those of the free-flow, i.e., v=5.8 km/s and mole fractions x CO 2 =0.61, x CO =0.26. Takayanagi et al. [START_REF] Takayanagi | Measurements of Carbon Dioxide Nonequilibrium Infrared Radiation in Shocked and Expanded Flows[END_REF] (Table 7) calculated parameters of 17 mbar and T vib =4000 K based on an initial parameters of 12 mbar, T vib =1690 K.

Shock-layer conditions are very sensitive to these parameters. To estimate the pressure in the shock-layer, we approximate the free flow as being in thermal equilibrium at a temperature of 1690 K. For free-flow pressures of 12 and 17 mbar, the post-shock pressures are 0.97 and 1.38 bar, respectively. In the following discussion, we assume that the pressure is p=1 bar. Radiation in the shock layer is calculated assuming that the gas is under thermochemical equilibrium. Figures 8,[START_REF] Pannier | RADIS: a Nonequilibrium Infrared Radiative Solver: Source Code and User Guide[END_REF] and 10 show the line-of-sight emission spectra calculated for shock-layer temperatures of 3500, 4000 and 45000 K. ======= The pressure in the shock-layer is calculated with the CEA code [START_REF]CEARUN[END_REF] for pre-shock conditions corresponding to those of the free-flow, i.e., v=5.8 km/s and mole fractions x CO 2 =0.61, x CO =0.26. Takayanagi et al. [START_REF] Takayanagi | Measurements of Carbon Dioxide Nonequilibrium Infrared Radiation in Shocked and Expanded Flows[END_REF] (Table 7) calculated free-flow parameters of 17 mbar, T vib =4000 K, T rot =1690 K to match the shock standoff distance.

For a free-flow pressure of 17 mbar, assuming thermal equilibrium at a temperature of 1690 and 4000 K, the post-shock pressure is 1.49 and 0.64 bar, respectively. In the following discussion, we assume that the pressure in the shock-layer is on the order of p=1 bar. Furthermore, the gas in the shock layer is assumed to be at thermochemical equilibrium. This assumption is supported by the CFD calculations of [START_REF] Takayanagi | Measurements of Carbon Dioxide Nonequilibrium Infrared Radiation in Shocked and Expanded Flows[END_REF] (Fig. 18), who observed that the shock-layer flow was close to thermodynamic equilibrium. Line-of-sight emission spectra are calculated for shock-layer temperatures of 3500, 4000 and 4500 K. »»»> Stashed changes The conditions of the simulations are summarized in Table 1. Other calculations were also performed from 3000 K to 11,000 K.


Mustaque A Khan

Karuna K Nanda

Saluru B Krupanidhi

Reduced Graphene Oxide Film Based Highly Responsive Infrared Detector

Keywords: reduced graphene oxide, infrared detector, mid-gap state, temperature sensor

Due to the unique optical properties, graphene can effectively be used for the detection of infrared light. In this regard, reduced graphene oxide (RGO) has drawn considerable attention in scientific society because of simplicity of preparation and tunable properties. Here, we report the synthesis of RGO by solvothermal reduction of graphene oxide (GO) in ethanol and the detection of infrared light (1064 and 1550 nm) with metal -RGO -metal configuration.

We have observed that photocurrent, responsivity as well as the external quantum efficiency increase with C/O ratio. The responsivity value in near-infrared region can be as high as 1.34 A.W -1 and the external quantum efficiency is more than 100%. Response times of these devices are in the order of few seconds. Overall, the responsivity of our device is found to be better than many of the already reported values where graphene or reduced graphene oxide is the only active material. The high value of quantum efficiency is due to strong light absorption and the presence of mid-gap state in RGOs.

INTRODUCTION

Now-a-days, infrared detectors are being used in many applications such as rail safety, gas leak detection, flame detection, night flying, night surveillance, night vision goggles and also space operations. [START_REF] Rogalski | Infrared detectors for the future[END_REF] Graphene is a zero band gap material so it absorbs a broad spectrum of light. This property has made graphene a highly potential material for infrared detector. Unfortunately, a single layer graphene absorbs only ~ 2.3% of incident white light, [START_REF] Nair | Fine structure constant defines visual transparency of graphene[END_REF] and photo-generated electrons and holes in graphene recombine within few picoseconds. [START_REF] Breusing | Ultrafast carrier dynamics in graphite[END_REF][START_REF] George | Ultrafast Optical-Pump Terahertz-Probe Spectroscopy of the Carrier Relaxation and Recombination Dynamics in Epitaxial Graphene[END_REF][START_REF] Rana | Carrier recombination and generation rates for intravalley and intervalley phonon scattering in graphene[END_REF] Lower absorption and faster carrier recombination lead to low photo-response. [START_REF] Zhang | Broadband high photoresponse from pure monolayer graphene photodetector[END_REF][START_REF] Chang | Regulating Infrared Photoresponses in Reduced Graphene Oxide Phototransistors by Defect and Atomic Structure Control[END_REF][START_REF] Gowda | Photoresponse of double-stacked graphene to Infrared radiation[END_REF] In order to use graphene as an effective photodetector, absorption as well as life time of photo-excited electrons and holes should be increased. Built-in electric field near metalcarbon nanotube junction can separate photogenerated electron and holes, preventing quick recombination. [START_REF] Lu | Photoconductivity in single wall carbon nanotube sheets[END_REF] But this kind of effects are limited at the junctions. Introduction of defects in graphene sheets leads to creation of band gap and also limits the recombination of charge carriers. [START_REF] Zhang | Broadband high photoresponse from pure monolayer graphene photodetector[END_REF][START_REF] Chang | Regulating Infrared Photoresponses in Reduced Graphene Oxide Phototransistors by Defect and Atomic Structure Control[END_REF][START_REF] Gowda | Defect-Induced Enhancement and Quenching Control of Photocurrent in Few-Layer Graphene Photodetectors[END_REF] Though plasmonic nanostructures, micro-cavities have been used to enhance the absorption efficiency of graphene, [START_REF] Liu | Plasmon resonance enhanced multicolour photodetection by graphene[END_REF][START_REF] Echtermeyer | Strong plasmonic enhancement of photovoltage in graphene[END_REF][START_REF] Zhu | Manipulating light absorption of graphene using plasmonic nanoparticles[END_REF][START_REF] Furchi | Microcavity-Integrated Graphene Photodetector[END_REF] responsivity still remains in few tens of mA.W -1 . The light absorption efficiency can also be increased by simply increasing the number of layers in graphene. [START_REF] Bae | Roll-to-roll production of 30-inch graphene films for transparent electrodes[END_REF] Therefore, few layers graphene with considerable amount of defects on the sheets is a potential material for highly responsive infrared photo-detector. [START_REF] Loomis | Large photocurrents in single layer graphene thin films: effects of diffusion and drift[END_REF][START_REF] Yang | Highly conductive free-standing reduced graphene oxide thin films for fast photoelectric devices[END_REF] Few layers structure and ease preparation makes reduced graphene oxide (RGO) a very important candidate of the graphene family. Properties of RGO can be tuned by tuning the amount of reduction [START_REF] Gómez-Navarro | Electronic Transport Properties of Individual Chemically Reduced Graphene Oxide Sheets[END_REF][START_REF] Wei | Nanoscale Tunable Reduction of Graphene Oxide for Graphene Electronics[END_REF][START_REF] Eda | Large-area ultrathin films of reduced graphene oxide as a transparent and flexible electronic material[END_REF][START_REF] Mathkar | Controlled, stepwise reduction and band gap manipulation of graphene oxide[END_REF][START_REF] Tang | Graphene-related nanomaterials: tuning properties by functionalization[END_REF] and also it can be reduced by many different reducing agents. [START_REF] Stankovich | Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide[END_REF][START_REF] Zhang | Reduction of graphene oxide via L-ascorbic acid[END_REF][START_REF] Pham | Chemical reduction of an aqueous suspension of graphene oxide by nascent hydrogen[END_REF][START_REF] Barman | Si-mediated fabrication of reduced graphene oxide and its hybrids for electrode materials[END_REF] We have prepared RGO without using any extremely harmful chemicals like hydrazine and also the method of preparation is simple.

We have used ethanol as reducing agent. [START_REF] Dreyer | Reduction of graphite oxide using alcohols[END_REF] It effectively reduces graphene oxide. We also A c c e p t e d M a n u s c r i p t report the detection of infrared light. The devices consisting of "metal -RGO -metal" configuration show high responsivity even at very low bias. The responsivity in NIR region can be as high as 1.34 A.W -1 and the external quantum efficiency >100% has been achieved.

Response times of these devices are in the order of few seconds. To our knowledge, the responsivity is much better than most of devices based on graphene or RGO as the only active material.

EXPERIMENTAL SECTION

2.1 Materials and Method. Graphite flakes were purchased from Sigma Aldrich. Potassium permanganate (KMnO 4 ) and sulfuric acid (H 2 SO 4 ) were purchased from Fisher Scientific. Sodium nitrate and hydrochloric acid were purchased from SDFCL. Ethanol was purchased from local supplier. All the chemicals were at least more than 99% pure and used without further purification.

Graphitic oxide (GO) is prepared according to Hummers method. Details of the method are reported elsewhere. [START_REF] Hummers | Preparation of graphitic oxide[END_REF] Reduction of GO is achieved by solvothermal method with ethanol as solvent. First, 80 mg of GO is added to 40 ml of ethanol. Then these mixtures are sonicated for 200 min. in a 30 kHz TELSONIC ULTRASONICS bath sonicator. After sonication, GO dispersion is transferred into a 50 ml teflon lined autoclave and kept in a muffle furnace at 180°C for 3, 6, 12, 24 and 36 h. After solvothermal reduction, the furnace is allowed to cool naturally. After that the product mixtures are centrifuged at 5000 rpm and washed with ethanol. Then the products are dried in vacuum overnight. The RGOs are named as RGO1, RGO2, RGO3, RGO4, and RGO5 for 3, 6, 12, 24, and 36h of reduction respectively.

A c c e p t e d M a n u s c r i p t 2.2 Characterizations. X-ray diffraction (XRD) studies have been carried out using RigakuSmartLab X-ray diffractometer with Cu Kα as X-ray source. Raman spectra have been acquired using LabRAM HR system with 532 nm excitation source. Absorption and infrared (IR) spectra have been recorded using PerkinElmer UV/VIS/NIR Lambda 750 spectrometer and Bruker FTIR instrument, respectively. X-ray photoelectron spectroscopy (XPS) measurements have been done using KRATOS ANALYTICAL's AXIS Ultra photoelectron spectrometer with Al Kα as X-ray source. Scanning electron microscopy (SEM) images are taken with ZEISS's GEMINI ULTRA 55 FE-SEM, while transmission electron microscopy (TEM) images are taken with FEI Tecnai F30 S-TWIN TEM.

Device Fabrication and Opto-electrical Measurement. Silver electrodes with 200 nm

thickness and a gap of 95 μm are deposited on cleaned quartz substrates by thermal evaporation (Figure S1). To fabricate the devices, 4.8 mg of RGO is dispersed in 150 μL toluene by sonication and drop-casted in between the electrodes with a micro pipette. The devices are dried at ~ 40°C on a hotplate for ~45 min. and cooled to the room temperature. A CNI's MDL-III 1550 nm laser (with an optical coupler of ~2 mm radius) and MIL-III 1064 nm (with a beam radius 2.5 mm) have been used as the IR source. Optical chopper SR540 has been used for chopping measurements. Tektronix Keithley 2400 source measure instrument is used for electrical studies.

RESULTS AND DISCUSSION

SEM images [Figure S2 S3) of GO shows a peak at around 11.86° 2θ. Oxygen functionalities incorporation in between the (002) planes lead to larger interplanar distances. [START_REF] Park | Hydrazine-reduction of graphite-and graphene oxide[END_REF] After reduction, diffraction peak restores around graphitic peak (26.51° 2θ) at 25.46° 2θ. Very broad nature of the peaks is due to few layers structure and presence of defects in RGO.

Absorption spectrum of GO and RGOs have been taken in N-Methyl-2-pyrrolidone (NMP). As the reduction increases, the absorption increases in visible and infrared spectrum (Figure 1 a).

Infrared spectra of GO and RGO (Figure S4) also indicate restoration of aromaticity after reduction. Alkane C-H stretching and C-H rocking absorption intensity also decreases. On the other hand C-C aromatic stretching absorption and C-H in plane bending absorption increases.

EDS and XPS survey spectra of GO and RGO ( Figure S5) show vivid difference of carbon and oxygen contain before and after reduction of graphitic oxide. Figure 1b shows the XPS C1s spectrum of GO which can be deconvoluted to four peaks. These peaks are arising from C-C/C=C (~284.5eV), C-O (~286.5eV), C=O (~288.3eV) and CO(OH) (~290.2eV). [START_REF] Some | Binol salt as a completely removable graphene surfactant[END_REF] The very nature of C1s peak of GO indicates oxygen incorporation on the layers of graphite flakes and the C/O ratio is ~ 1.96. XPS C1s spectrum (Figure 1c) show that oxygen functionalities are mostly removed during reduction process. The C/O ratio for different RGOs has been evaluated from the XPS spectra and produced in table 1. Raman spectra of GO, RGO3 (Figure 1d) show four major peaks. The strong peak at ~1590 cm -1 corresponds to G peak. [START_REF] Ferrari | Raman spectroscopy of graphene and graphite: disorder, electron-phonon coupling, doping and nonadiabatic effects[END_REF] Raman peaks at 1345 and 2695 cm -1 are called as D peak and 2D (or G') peak and the peak around 2935 cm -1 is known as D+G peak. [START_REF] Ferrari | Raman spectroscopy of graphene and graphite: disorder, electron-phonon coupling, doping and nonadiabatic effects[END_REF][START_REF] Wang | Raman spectroscopy of carbon materials: structural basis of observed spectra[END_REF] For perfectly crystalline graphene Dband remains absent because the corresponding A 1g mode does change the overall polarizability. [START_REF] Tuinstra | Raman spectrum of graphite[END_REF] It requires defect or disorder in the crystal to get activated. [START_REF] Tuinstra | Raman spectrum of graphite[END_REF][START_REF] Ferrari | Interpretation of Raman spectra of disordered and amorphous carbon[END_REF] D peak only appears when size of the graphene sheets decreases or defects gets incorporated on graphene sheets and can be used to quantify amount of defects in graphene. Though, the ratio of I D /I G is the measure of defects in graphene, [START_REF] Ferrari | Raman spectroscopy of graphene and graphite: disorder, electron-phonon coupling, doping and nonadiabatic effects[END_REF][START_REF] Tuinstra | Raman spectrum of graphite[END_REF][START_REF] Ferrari | Interpretation of Raman spectra of disordered and amorphous carbon[END_REF][START_REF] Pimenta | Studying disorder in graphite-based systems by Raman spectroscopy[END_REF] the ratio obtained for GO and RGO is inconclusive as the value is very much position-dependent.

Non-uniform size, presence of heteroatom in GO and RGO leads to position dependent I D /I G value. A c c e p t e d M a n u s c r i p t of electron-hole generation, trapping of electrons and recombination. [START_REF] Zhang | Broadband high photoresponse from pure monolayer graphene photodetector[END_REF][START_REF] Kind | Nanowire ultraviolet photodetectors and optical switches[END_REF][START_REF] Rose | Concepts in photoconductivity and allied problems[END_REF] In this context, it is worthy to note that photocurrent is linear with bias voltage as shown in figure S6. Bolometric temperature rise of RGO can also lead to generation of photocurrent. [START_REF] Bae | Sensitive photo-thermal response of graphene oxide for mid-infrared detection[END_REF][START_REF] Liang | Mid-infrared response of reduced graphene oxide and its high-temperature coefficient of resistance[END_REF] In order to shed light on this, we studied the effect of temperature on electrical properties of the devices

        (a) (b) (c) (d)
and measured the temperature rise by irradiation (Figure S7 and Table S1). No appreciable rise in temperature (< 0.1 degree) could be detected (with a resistance temperature detector, resolution of 0.1 K) for ~50 s exposure of infrared radiation (~ 80 mW.cm -2 ). We have also carried out photocurrent studies using a chopper [Figure S8]. It is well known that bolometric temperature rise (ΔT B ) depends on chopper frequency and the bolometric effect is dominant at lower frequency. This indicates that bolometric effect can be avoided at higher frequency. As discussed earlier, the band gap decreases with increase of C/O ratio that leads to increase of photocurrent as well as the EQE. Higher absorption at 1064 nm is believed to be the underlying reason for higher γ value for 1064 nm radiation as compared to 1550 nm. In this context, the role Temperature dependent measurement shows that photocurrent decrease with increase in temperature (Figure 4). Trapping in mid-gap state band is efficient at low temperature. As the In most cases, the responsivity of graphene or RGO is in range of few mA.W -1 or less. [START_REF] Zhu | Manipulating light absorption of graphene using plasmonic nanoparticles[END_REF][START_REF] Furchi | Microcavity-Integrated Graphene Photodetector[END_REF][START_REF] Xia | Ultrafast graphene photodetector[END_REF][START_REF] Gabor | Hot carrier-assisted intrinsic photoresponse in graphene[END_REF][START_REF] Fernandes | Reduced graphene oxide mid-infrared photodetector at 300 K[END_REF][START_REF] Freitag | Increased responsivity of suspended graphene photodetectors[END_REF][START_REF] Pospischil | CMOS-compatible graphene photodetector covering all optical communication bands[END_REF][START_REF] Gan | Chip-integrated ultrafast graphene photodetector with high responsivity[END_REF][START_REF] Xia | Photocurrent imaging and efficient photon detection in a graphene transistor[END_REF][START_REF] Mueller | Graphene photodetectors for high-speed optical communications[END_REF][START_REF] Radoi | Ultrabroadband photodetection based on graphene ink[END_REF][START_REF] Chowdhury | Thermally reduced solutionprocessed graphene oxide thin film: An efficient infrared photodetector[END_REF] Due to poor absorption and fast carrier recombination, EQE is very low for monolayer or very few layer graphene based photodetector. The responsivity and EQE values of our RGOs are comparable with the graphene nano-ribbon as an active material. [START_REF] Chitara | Infrared photodetectors based on reduced graphene oxide and graphene nanoribbons[END_REF] Using RGO or graphene as the only active material, the responsivity of prepared RGO is found to be better than many of the reported literature.

  = =  (a) (b) (c) (d)
Both rise (figure 5a) and decay (figure 5b Reproducibility of photocurrent is checked over 20 cycles (Figure S11). A brief summary of results from previous works on graphene and RGO based photodetector have been listed in table

  (a) (b)
S3.It may be noted that in most of the graphene (or RGO) based photodetector, the responsivity value is much lower and time constant values are much higher compared to our photodetectors.

Table 3. Time constants, response and recovery time of photodetectors for 1550 nm infrared irradiation (V S-D = 0.3V, I L = 80 (± 0.6) mW.cm -2 ).

CONCLUSIONS

We have prepared highly reduced RGO by solvothermal reduction in ethanol. Devices have been fabricated by drop casting RGO on silver electrodes. Infrared photoresponse studies of the devices have been carried out with 1550 and 1064 nm laser source and the photocurrent is found to be photovoltaic in nature. Interestingly, EQE increases with C/O ratio and higher than 100% is obtained for RGO with higher C/O ratio which is believed to be due to the combined effects of strong absorption and mid-gap states assisted gain. Highest responsivity obtained in our experiment is 1. [START_REF] Ferrari | Interpretation of Raman spectra of disordered and amorphous carbon[END_REF]

  (a) and (b)] reveal some amount of agglomeration in RGO. TEM images [Figure S2(c) and (d)] reveal the layered nature of RGO. Inter-layer spacing is ~3.5 Å. SAED pattern [Figure S2(d) inset] reveals hexagonal symmetry with inner most diffraction spots corresponding to (1100) planes and diffraction spots next to that are from (2110) planes. (1100) A c c e p t e d M a n u s c r i p t planes correspond to a d-spacing of ~ 0.19 nm and (2110) planes correspond to a d-spacing of ~ 0.11 nm. XRD pattern (Figure

Figure 1 .

 1 Figure 1. (a) Absorption spectra of RGO samples, (b and c) XPS C1s spectra of GO and RGO3 respectively, (d) Raman spectra of GO and RGO.

Figure 2 .

 2 Figure 2. Photoresponse of RGO3 to (a) 1550 nm and (c) 1064 radiation with different radiation intensity at constant bias of 0.3V. (b and d) variation of photocurrent with intensity.

  However, we observed no change in photocurrent with the variation of chopper frequency [FigureS8 (b) and (c)]. The initial decrease in photocurrent during chopping is due to reduction of laser exposure time to half of its initial value. These two measurements suggest photocurrent is due photovoltaic effect and not bolometric effect.

Figure 3 (Figure 3 .

 33 Figure 3(a) and (b) display the temporal photoresponse of RGOs with 1550 and 1064 nm radiation of 80 mW.cm -2 at 0.5 V. Irrespective of the excitation wavelength, it is interesting to

have been shown in figure 3

 3 (d). Data points are well fitted with the equation, 𝑬𝑸𝑬 = 𝜿 * 𝒙 𝜸 , where x = C/O ratio and κ and γ are constants. γ for 1550 and 1064 nm radiation is ~6.2 and ~7.2, respectively indicates faster increase of EQE for 1064 nm excitation as compared to 1550 nm. I-V characteristic shows Coulomb blockade effect specially at low temperature (Figure S9 a). Transport of charge carriers through RGO sheets occurs via hopping mechanism (Figure S9 b).

A c c e p

  photocurrent vs. temperature indicates almost linear variation of photocurrent with temperature

Figure 4 .

 4 Figure 4. (a) Temporal photoresponse of RGO 4 at different temperature (λ=632 nm), (b) Variation of photocurrent with temperature.

Figure 5 .

 5 Figure 5. (a) Rise and (b) decay of photocurrent (1550 nm excitation) of different RGOs (V S-D = 0.3V, I L = 80 (± 0.6) mW.cm -2 ).

A c c e p t e d M a n u s c r i p t

Table 1 .

 1 Atomic percentage of carbon and oxygen in RGO as obtained from XPS analysis.

	𝑰 𝑷𝑪 = 𝜶 * 𝑰 𝑳 𝜷 ,
	where α and β are constants. β is 1.14, 1.27, 1.45, 1.28 and 1.28 for RGO1, RGO2, RGO3,
	RGO4, and RGO5, respectively with 1550 nm irradiation (Figure 2b). Temporal responses of
	RGO3 at different intensities of 1064 nm excitation (at a constant voltage of 0.3 V) are shown in

Photocurrent measurements of the devices are carried out with 1550 and 1064 nm laser irradiation. Figure

2a

shows the temporal response curves for RGO3 with 1550 nm excitation with a bias of 0.3 V. As soon as IR radiation falls on the device, photocurrent quickly increases and reaches its saturation level and the saturated photocurrent increases with increasing IR radiation intensity. Increase in intensity of IR radiation generates higher number of excited electrons and holes that leads to higher photocurrent. To determine the relation between photocurrent (I PC = current after illuminationdark current) and intensity of radiation (I L ) we have plotted I PC vs. I L at a constant bias of 0.3 V and fitted with the equation, figure 2(c). β is found to be 1.64, 1.58, 1.57, 1.64 and 1.55 for RGO1, RGO2, RGO3, RGO4 and RGO5, respectively [Figure

2(d)

]. The non-unity of β values is indicative of complex processes

Table 2 .

 2 Responsivity and EQE of RGOs at I L = 80 (± 0.6) mW.cm -2 and V S-D = 0.5 V.

Responsivity and EQE values for 1550 nm and 1064 nm excitation are listed in table 2. Variation of EQE values with the C/O ratio in RGO for both 1064 nm and 1550 nm infrared irradiation

  of mid-gap state band (MGB) needs to be discussed. Although RGOs are highly reduced, still they contain ~14.3% to 12.6% oxygen (atomic percentage). The reduction of sheet size, incorporation of defects during sonication, high temperature reduction and presence of oxygen functionalities are believed to create MGB in RGOs.[START_REF] Zhang | Broadband high photoresponse from pure monolayer graphene photodetector[END_REF][START_REF] Wehling | Impurities on graphene: Midgap states and migration barriers[END_REF][START_REF] Stauber | Electronic transport in graphene: A semiclassical approach including midgap states[END_REF][START_REF] Chen | Defect scattering in graphene[END_REF] MGB can trap photoexcited electrons.[START_REF] Zhang | Broadband high photoresponse from pure monolayer graphene photodetector[END_REF][START_REF] Ito | 3D Bicontinuous Nanoporous Reduced Graphene Oxide for Highly Sensitive Photodetectors[END_REF] During the lifetime of trapped electrons corresponding holes recirculate trough the circuit generating a gain in photocurrent (Schematic 1).[START_REF] Zhang | Broadband high photoresponse from pure monolayer graphene photodetector[END_REF][START_REF] Ito | 3D Bicontinuous Nanoporous Reduced Graphene Oxide for Highly Sensitive Photodetectors[END_REF][START_REF] Chen | Ultrahigh photocurrent gain in m-axial GaN nanowires[END_REF][START_REF] Li | Achieving EQE of 16,700% in P3HT: PC71BM based photodetectors by trap-assisted photomultiplication[END_REF][START_REF] Li | Trap-Assisted Photomultiplication Polymer Photodetectors Obtaining an External Quantum Efficiency of 37500%[END_REF][START_REF] Li | A trap-assisted ultrasensitive near-infrared organic photomultiple photodetector based on Y-type titanylphthalocyanine nanoparticles[END_REF][START_REF] Bhattacharya | Semiconductor Optoelectronic Devices[END_REF] Thus, combined effects of higher absorption and presence of MGB in RGO lead to high responsivity and EQE values of our RGO based photodetectors.EQE exceeds 100% for both 1550 and 1064 nm irradiation for highly reduced graphene oxide based devices. Electron trapping in mid-gap state and hole hopping.

	Sample RGO 1 RGO 2 RGO 3 RGO 4 RGO 5 A c c e p t e d M R (A/W) λ(1550 nm) λ(1064 nm) 0.55 0.42 0.72 0.61 1.09 0.91 1.23 1.11 1.34 1.21 a n u s c r i p t EQE(%) λ(1550 nm) λ(1064 nm) 44 49 58 72 87 106 98 129 107 141 Hole Electron Carbon Oxygen Schematic 1:

  I t is photocurrent at any time t and I f is the final photocurrent, A 1 and A 2 are constants (depends on incident light intensity), and  1 and  2 are time constants.[START_REF] Chang | Regulating Infrared Photoresponses in Reduced Graphene Oxide Phototransistors by Defect and Atomic Structure Control[END_REF][START_REF] Chitara | Infrared photodetectors based on reduced graphene oxide and graphene nanoribbons[END_REF][START_REF] Ito | 3D Bicontinuous Nanoporous Reduced Graphene Oxide for Highly Sensitive Photodetectors[END_REF] Rise and decay of photocurrent for 1064 nm excitation have been shown in figureS10(a) and (b). Values of time constants are evaluated for RGOs and provided in table 3 (1550 nm excitation) and tableS2(1064 nm excitation). All the cases,  values indicate a faster growth (or decay) followed by a slower saturation. These results are in accordance with the previous reports.[START_REF] Chang | Regulating Infrared Photoresponses in Reduced Graphene Oxide Phototransistors by Defect and Atomic Structure Control[END_REF][START_REF] Chitara | Infrared photodetectors based on reduced graphene oxide and graphene nanoribbons[END_REF] The values of time constants indicate that the responses of our RGO based photodetectors are faster than many other reported values.

	(a)	(b)
		
		) of photocurrent are fitted with the following
	exponential relation,

where

  and 1.21 for 1550 nm and 1064 nm infrared irradiation, respectively. The responsivity and response time of our photodetectors is much better than many of those values which are already reported for RGO based photodetector and comparable with commercial ones.

	Active	Rise(s)		Decay (s)
	Material	 1	 2	 1	 2
	RGO 1	0.25	8.78	0.32	4.27
	RGO 2	0.07	4.30	0.34	4.22
	RGO 3	0.17	5.12	0.35	5.19
	RGO 4	0.25	6.10	0.12	3.66
	RGO 5	0.20	7.20	2.03	10.49


M Laghrouchel

D Meunier

J Boussey

S Tardu

A Adane

A Microcontroller Based Wall Shear Stress Measurement

Keywords: MEMS, sensot, data acquisition, wall shear stress sensm•

A nl.'w typE' of hot wit"l.' anl.'moml.'tl.'r was dl.'wl- opl.'d by using surfacl' micro machining tl.'chniqul.'s. Thl' rl.'duction of thl' microprobl's Sl.'ction and thl' dl.'wlopml.'nt of a thHm al isolation cavity bl.'low thl' hot polycrystallinl' silicon wil"l' rl.'dUcl.'d considl.'rably indirl.'ct thHmall.'xchangl.'s bl.'tlnl.'n thl' hot wirl' and its substmte. As a consl'qul.'nCI.', thl' rl.'sponsl.' tim I.' of the sensm• is considerably improved allowing thl' dl.'tl.'ction of fast fluctuations and turbulent phl.'noml.'na in fluid ml.'chanics. In this paper WI.' bl"il'fly dl.'scribl' thl' realisation technology of thl' dl.'velopl.'d sensot•s, its opl.'l"ation principll's and the data acquisition circuit especially concl.'ived and rl'alisl.'d to JX"rform turbulent flow detection in a low SJX"ed wind tunnl'l.

I. INTRODUCTION

The control of turbulence in fluid mechanics is a very important issue which may has strong impact in several industrial fields. The more important one is the civil and military aviation where the reduction of air friction on the delta wings still a big economics stake. Since the early 90's. several research teams have been working on this application developing either integrated sensors or microactuators that have to be implemented in a specific critical points on the aeroplane. The work presented in this paper is related to a specific type of those sensors, i.e., hot wire anemometers which are usually dedicated for flow speed measurement. In aerodynamics applications, it has been established that hot wire anemometers can be used to detect tutbulent mechanisms inside boundary layers, under the only condition to be mounted tangential to the aeroplane surface. The tutbulent phenomena induce swirling structures which are a few hundred of microns width and a few rnillimetres length [ L 2] and have a very short life span (on the order of the millisecond). Classical anemometer (with Platinum wire of lOJ.lm diameter and 400 Jlm length. inserted inside macroscopic tubes) [START_REF] Stainback | A Nagabushana, Review of hot wire Anemometry techniques and the range of their applicability for various flows[END_REF] were reported to have very good dynamic performance (time response of few milliseconds) but still are incompatible with the need to obtain an array of identical, and ultra-miniaturised, sensors The spatial and temporal sensing of the swirling structures (responsible for parietal friction) is a big issue and is made progressively possible thanks to the use of integrated sensors. We will show in the presented work how the use of adapted microtechnologies allowed us to obtain highly miniaturised hot wire ane-mometers. Several microsensors of various geometries were produced b:y surface micro machining. using thennally sensitive materials like poly crystalline silicon [START_REF] Meunier | Therm<r(luidic FEM simulation of ultra-miniaturised wall shear sensors[END_REF][START_REF] Uifdahl | The m(luence of temperalllre on the measurements of Reynolds Stresses in shear free turbulence near a wall[END_REF]. The response time was also optimised via a suitable thennal isolation architecture. Finally. electrical and functional (in wind tunnel) characterisations were perforn1ed and the firstly obtained results are presented ..

II. REALIZATION OF A SHEAR STRESS SENSOR

Fig. 1 shows top and cross section scheme of the adopted technology. PECVD (Plasma Enllanced Chemical Vapor Deposition) silicon nitride was first deposited on p type 4" wafers to a thickness of 0.3 Jlm. Then the silicon nitride film was patterned and wet tl1ennal oxidation of silicon was made through the nitride window. Oxidation time was adjusted with technological step simulation for achieving a flat surface. A 0.3 )lm depth silica cavity was obtained. A 0.5 )1m-thick polycrystalline silicon film was tl1en deposited inside a LPCVD (Low Pressure Che1nical Vapor deposition) furnace and doped by boron ions implantation at appropriate parameters (energy and dose of ion implantation were chosen to opti1nise t11e thennal sensibility of the soobtained semiconductor. After the thermal mmealing. poly-CI)'Stalline silicon was patterned into to obtain variable section and Iengtl1 of •wires. Electrical contacts were finally taken with chromium pads. The whole wafer was then diced into individual chips and releasing of the poly silicon wire was performed by wetetching of the silica cavity performed on each san1ple. The presence of the cavity underneath the wire ensures the more efficient thennal isolation between the bot wire and its substrate. TI1e depth of cavity is only of 0.5 11m which prevents the fonnation of parasitic tmbulence inside the cavity [START_REF] Meunier | Simulation of wall shear stress integrated sensors[END_REF].

Ill. ELECTRIC SENSITMTY OF THE SENSORS TI1e sensors were firstly characterized under 2 points prober, in order to detennine their static electrical properties (current-bias characteristics) and their behavior with temperature. Two types of parameters were deduced for each type of sensors:

the resistance variation as a function of the temperature for determining the TCR (Temperature Coefficient Resis-1 ()R tance = --where R.,, is the extrapolated resistance R 0 dT .

taken at the temperature of 0°C). It was found out that the TCR values are independent of the hot wire geometry and relatively uniform on all the surface of the wafer. An aver- age value of ..0.5.10-4/oc was measured for samples doped at 10 16 cm• 2 (80 keY) of boron.

The current value to be injected inside the polysilicon wire in order to elevate its temperature. Tiris overheating polarisation current was deduced from TCR value and current-voltage characteristics of each sensor. As an example, for the 0.5 x 2 x 50 11m 3 having the above mentioned TCR value, a current of 2351J.A was used to increase the wire temperature of70°C.

A. Dynamic and Electric Time Response

Although static measurement is founded on a simple principle, it is now accepted that the frequency response of such sensors is in general complex because of convective and conductive heat exchanges. Tiris can be illustrated by the significant difference between the electrical time response and the thermal time response. The electrical time response defined as being the response time of the conductor when it is biased with a perfect square signal (pulse). The electrical time response of the sensor is 31J.S (Fig 2 ).

. •:• -• • •• •:••• -•• -• • ':"------~--••; - ! . SI~: ~---••-;, __ ---.:... _ __ _;_ __ ~ . . -----L-----4~---• -•.. . .. . -. . . . . ..... _., ___ _

Fig. 2 Electrical time response

An electric circuit based on the internal heating method was designed and carried out for the dynamic response time determination of the sensors (Fig. 3) The circuit makes possible to simulate. at the sensor leveL the fluctuations of temperature by carrying out an internal modulation of the wire heating created by a D.C. current (rated current of the probe). An alternative current, AC. with a frequency of I :MHz modulated by a square signal with a frequency of I kHz is superimposed. Fig. 4 shows the time response of a 60x2xi>.51J.m 3 hot wire. which is about 1501J.s. The bandwidth sensor lies between 0 and 1KHz. This bandwidth is not large enough for detecting all twbulence phenomena associated with airflow over aircraft's.

JhtTE: ~ij.lZ.~¥32 Tift[ I l!i )5
S!flRP>mUE!l: Fig. 4 Time response measurement of a 50x2x0.5f.U11 3 wall shear stress sensor.

B. Data Acquisition System

The Data Acquisition System is composed of two main subsystems: the programmable digital data acquisition system and the sensors with their associated circuits [START_REF] Ameur | Monitoring a greenhouse using a microcontroller-ba~ed meteorological data acquisition[END_REF] . The requirement of data acquisition system. in terms of speed and amount, dictated the use of a microcontroller. In our case the Motorola 68HC11 CMOS microcontroller integrated circuit was chosen for this purpose thanks to its low power consumption, small size and built-in analog to digital (AID) converter with 8 input multiplexer.

The AID converter of the microcontroller converts simple signals into digital values and transfers them to a personal computer with RF circuit via an RS 232 serial port.

The RF module transmits the data through the Amplitude Modulation of 464 :MHz frequency, so the consumed poweroftransmission is 13m W.

C. Conditioning circuit

As shown in Fig. 7, the associated analogue circuit is divided into 3 stages: a Wheatstone bridge circuit a Constant Temperature Anemometer (CTA) feedback circuit with a power stage and a differential amplifier circuit with an offset compensation network.

The Wheatstone circuit is formed with two precision resistors of 0.01% accuracy and a tuneable resistor to control the initial voltage reference. In the fourth branch we place the sensor near the resistors and the conditioned circuit in order to minimise the noise and any eAiernal perturbations.

The AD620 circuit component, is a low cost, high accuracy instrumentation amplifier which requires only one external resistor to set tnain gains from 1 to 1000. This latter is working as preamplifier due to its low voltage noise.

In order to reduce the circuit size, we have pasted the sensor and the Wheatstone bridge on the ceramic support. The conditioning circuit is realized with a very small circuit under the ceramic holder in which the Surface Mounted Circuits (CMS) technology is used. The hot wire anemometer is mounted in a Wheatstone bridge: Tllis is a self-balancing bridge which maintains the resistance R 1 of the sensor at a constant value R. An increase in fluid velocity causes a decrease of the temperature T as well as of the Rr values inducing an unbalancing of the bridge. This causes the amplifier output current and current through the sensor to increase thereby restoring T and R 1 to their required values.

The advantage of such CTM circuit [START_REF] Gale | An Elementcuy IntroductiOn to Hot ff'ire and Hot Film A nemometry[END_REF] is to mininlise thermal inertia of the sensors. which was leading to rather bad time response.

IV. STATIC SPEED CALffiRATION

The sensor was also characterised in a wind tunnel in order to calibrate it for static speed variation.

EA-perimentally. the hot wire anemometer is mounted at the wall surface of the wind tunnel. A Pitot tube measures the pressure in the centre of the tunnel. This pressure is then transformed in order to know the macroscopic speed. Tllis calibration was performed for velocity ranging from 7mls up to 15mls.

With this constant temperature configuration, we can find the various parameters a, ~ and n of the electrical calibration of the sensor described by a law as:

E 2 = a.(ur ) 2
n + fJ [START_REF] King | On the convection of heat fi'om small cylinders in a stream o.f.fluid[END_REF]. where E is the output voltage and ur ""u~ I 22 is the friction speed.

In measuring the output voltage variations, we can determine the other coefficients. In Fig. 8, one can see that the above mentioned law fits perfectly the experimental values when n is taken equal to 1/3. This is in very good agreement with the Leveque solution which states that n coefficient has to be close to 1/3.

V. CONCLUSION

Hot wire integrated anemometers obtained with polysilicon material were fabricated and characterised. Very tiny wires suspended upon cavity were on-wafer tested and demonstrated to have a good temperature sensitivity. The TCR value was found on the order of-O.Sxl0-4 ;oc and proven to be constant on the whole wafer. Time response with constant current model (CCM) and Constant temperature model (CTM) was measured. The latter gives a value of 150~s which is compatible with the required performance and in good agreement with our previous results obtained by simulation [START_REF] Meunier | Therm<r(luidic FEM simulation of ultra-miniaturised wall shear sensors[END_REF][START_REF] Uifdahl | The m(luence of temperalllre on the measurements of Reynolds Stresses in shear free turbulence near a wall[END_REF][START_REF] Jiang | Silicon Jvlicromachined Flow Sensors. 1l1esis[END_REF].

  Fig.! Technological realization ofthe hot-wire probe suspended on amicro-cavity.

Fig. 3

 3 Fig. 3 Time response measurement circuit.

Fig. 5 Fig. 6

 56 Fig. 5 Block diagrams of the microcontroller data acquisition

  Fig.? Schematic diagram of hot wire anemometer mounted in Constant Temperature Mode.

Fig. 8 3 tunnel 15 nun l 60mm 1 Fig. 9

 8319 Fig. 8 Static speed calibration in wind tunnel: E' in function of (t~) 2 ' 3


Dimitrios M Tsamados
email: tsamados@enserg.fr

Delphine Meunier

Mourad Laghrouche

Jumana Boussey

Sedat Tardu

Wall Shear Stress and Pressure Sensors Development for Active Control of Flow

Keywords: MEMS, SOI, pressure sensors, wall shear stress sensors

In this paper we present a technology for wall shear stress and pressure integrated sensor fabrication. Thanks to the use of SOI wafers and wafer bonding technique, we came up with an innovative technology that provides high on-chip density of sensors required for arrays utilized in numerous microfluidic applications like active control of flow. At the end some wall shear stress results are presented.

INTRODUCTION

Active control of flow is a difficult task to achieve because it necessitates sensors of high sensitivity, quick response and high density of integration. For instance, the dynamic response of a pressure sensor should be fast enough to immediately detect the formation of local turbulence mechanisms of which the lifetime is of the order of the millisecond. Moreover, the use of these sensors is only interesting if they can be disposed in an array configuration with a small enough mesh to allow spatial resolution compatible with the length of the smallest boundary layer eddies, i.e. <200µm.

In addition, three parameters are fundamental for accurate understanding of complex phenomena such as turbulence or separation occurring within the boundary layer of a fluid flowing through a microchannel or tangentially to a wing surface. These parameters are wall shear stress, pressure fluctuations and temperature. Therefore, we should add to the sensors' properties mentioned above the coexistence of three different types of sensors on the same chip in order to reach our goal. For avionics and automotive applications, a big challenge is to simultaneously monitor these three parameters in order to monitor their evolution in time and space.

Since the early nineties, some teams have been working on MEMS-based silicon technology providing highly sensitive sensor arrays. Their published reports either mention high integration density of wall shear sensors [START_REF] Jiang | Flexible shear stress sensor skin for aerodynamics applications[END_REF][START_REF] Löfdahl | Small Silicon Pressure Transducers for Space-Time correlation measurements in a flat plate turbulent boundary layer[END_REF], or good dynamic performance for pressure fluctuation sensing [START_REF] Löfdahl | MEMS applications in Turbulence and Flow Control[END_REF], but, to our knowledge, no team realized sensors complying with the necessity to co-integrate the three types of sensor arrays with good performance on the same chip.

With the help of Finite Element Analysis (FEA) tools (microfluidic, thermal and mechanical), we have determined the optimal dimensions of our sensors. This FEA approach has been developed elsewhere [START_REF] Meunier | Realization and simulation of wall shear stress integrated sensor[END_REF] and proved that the technology that best fits the desired specifications is the SOI one, which allows the realization of these prototypes briefly described in the following section.

Sensors properties

Pressure sensing is based on the use of piezoresistive gauges placed on very small diaphragms (<100x100µm 2 ). The design proposed in this paper permits the pressure sensors to operate in a differential or absolute pressure configuration. This last option necessitates the design of a specific ventilation channel, eventually pressure control during bonding (if absolute pressure sensor is used) as well as a high degree of thin layer thickness uniformity and reproducibility when elaborating such sensors. The wall shear stress sensor's operation is based upon the principle of hot wire anemometry. The sensing element is made of polysilicon or eventually platinum resistors. The latter has to be thermally isolated from the substrate by a sealed or open cavity to minimize the heat exchange with the substrate [START_REF] Meunier | Realization and simulation of wall shear stress integrated sensor[END_REF][START_REF] Meunier | Fil chaud dans une micro cavité et capteur de pression en technologie MEMS[END_REF]. Finally, simple miniaturized p-n diodes have been adopted for local temperature sensing. The principle of measurement is based on the temperature dependence of the forward voltage drop in a p-n junction biased at a constant current. In the following paragraphs, we present the flowchart of the adopted technology solution for the co-fabrication of the above-described sensors highlighting the specific contribution of selected technological parameters.

TECHNOLOGY

Fabrication

Two SOI wafers are the starting materials for the fabrication of the sensors. The first one, called "base" wafer, is a BESOI with a device layer thickness chosen equal to the pressure sensor's air-cavity depth. In case of KOH or TMAH bulk micromachining the "base" wafer should be <100>-oriented and p-type doped (typical resistivity range: 14-20Ωcm). In the following paragraphs we will see that the base wafer can be a regular double-sides-polished wafer.

The second or "top" wafer should be Unibond or SIMOX finished with epitaxial growth. The preferred SOI device layer thickness ranges from 0.5 to 2 µm, a 0.5µm-thick buried oxide layer (BOX) and can be n or p-type doped, allowing consequently boron or phosphor ion implantation to maximize the piezoresistance coefficients of the piezoresistive gauges which are orientation dependent (in each case the "top" wafer has to be correctly oriented prior to implantation). The first step (Fig. 1&2) consists in etching the pressure sensor cavities in the "base" wafer using a deep reactive ion silicon etching (DRIE) or KOH or TMAH etching. The buried oxide of the "base" wafer constitutes an excellent etch stop layer with respect to the KOH-TMAH selectivity between SiO 2 and Si. This provides a nearly perfect reproducibility of the cavity depth over the whole wafer surface. The substrate's BOX can also be used as an etch barrier for DRIE process although recent etching tests carried out in our clean room show that DRIE process can give very good homogeneity of cavity depths over the whole wafer surface without the use of a SOI wafer. This is very important for the final production because it drastically reduces the cost of the sensors. During this step we also patterned the vent channels for the pressure sensor. Subsequent alignment and lithography steps (No.3 step) require double-side lithography to pattern appropriate alignment features on the backside of this "base" wafer. During the second step (Fig. 3) the thermal isolation cavities on the "top" wafer necessary for the hot wire anemometer operation are patterned. This step is conducted under the same technological conditions than the first step and leads to an almost perfect reproducibility of the cavity depth. In this case DRIE is preferred because it does not require a protective oxide layer for the patterning (the photo-resist suffices), contrarily to KOH-TMAH etching. Indeed, this cavity depth becomes equal to the SOI thickness of the initial "active" wafer.

Thorough cleaning of the wafers' surface takes place before being assembled in a substrate bonder tool. The surface preparation prior to bonding is of great importance as it is already proven by many studies [START_REF] Tong | Semiconductor Wafer Bonding: Science and Technology[END_REF] and great care has been taken regarding this issue. Thermal oxidation followed by special chemical treatment has been performed before bonding (Fig. 5) for rendering the wafers' surfaces hydrophilic. Thermal anneal was the next step in order to increase the bonded surface energy. Subsequently, removal of all of the "active" wafer's silicon substrate reduces the whole pair thickness and creates the pressure sensors' membranes (Fig. 4). Relatively low residual stresses are observed in the membranes after the thinning-down of the "top" wafer (Fig. 7) by wet selective etching (TMAH solution). Another technique has recently being tested by utilizing chemical-mechanical polishing (CMP), a technique which is not used as stand-alone method of thinning-down but its role is complementary to TMAH etching. The first 300-400 microns of the "top" wafer's substrate are removed by CMP and the finishing is made by the TMAH solution. The reason for selecting this approach is that for removing around 400 microns of silicon several hours of etching in TMAH are necessary, risking the infiltration of TMAH into the bonding interface and the destruction of the pressure sensors' membranes by TMAH penetration through the sensors' vent channels. Fig. 4. A new substrate is obtained by bonding together the above-processed followed by thinning down the upper "active" layer. The next lithography step is dedicated to the patterning of the p-n junction and the piezoresistive gauge surfaces to be implanted. A boron or phosphorus ion-implantation (dose and energy are estimated by numerical simulation using Silvaco ® ) is then conducted and followed by an activation annealing. It is important to underline that one of the advantages of this technology is that it allows piezoresistive gauges to be monocrystaline. The piezoresistance coefficient of mono-Si is higher than the corresponding one of poly-Si, a fact that confers higher sensitivity for mono-Si gauges. After the ion implantation follows a deposition of a polysilicon film, p-type doped. A fifth lithography allows the hot wire resistor patterning and that of some of poly-Si gauges in order to compare their response and sensitivity to that of mono-Si ones.

DESIGN AND TEST

Design of the pressure sensor

As it is mentioned earlier in the introduction, the pressure sensor has to comply with quite strict technical specifications in order to be suitable for use in active control of flow. First of all, the detection of small-scale air eddies necessitates a high sensitivity pressure sensor of a relatively small size. Typically the size of these eddies is of the order of 100 microns or smaller and the pressure fluctuations as low as 10 3 Pa and some 10KHz of frequency. As a result the design of the pressure sensor had to be carried out carefully by adapting one of the already existing solutions for pressure sensing (piezoresistive gauge sensor) to the sensitivity, size and response time imposed by the application.

Finite element analysis (FEA) is the first candidate when it comes to the sensor's initial design and later to its optimization. A parametric 3D model of the pressure sensor has been created using ANSYS ® Multiphysics FEA (Fig. 10,11,12) tool in order to numerically simulate the sensor's behaviour for different designs and different conditions of operation (e.g. pressure change). At the same time, the influence of a fluid (air in our case) in the pressure sensor's cavity was investigated during the designing of the sensor using fluid-solid interaction (FSI) (Fig. 13,14) finite element analysis. This kind of numerical simulation was important in order to compare the results with purely structural FEA where no air is present which is easiest to perform and less time consuming. The use of a parametric model facilitated the design and the optimization of the sensor by greatly reducing the development time. Many parameters were studied and the most important of them are of course the positioning and size of the piezoresistive gauges (Fig. 15) in the pressure sensor's membrane, the size of the membrane, its overall thickness (Fig. 16) and the cavity depth. The need of differential pressure sensing has to be addressed also by minimizing its impact on the sensor's sensibility and on its overall cost. Finally, the solution of vent channels originating from the sensor's cavity corners has been adopted. The reason is that there the stresses are minimal so their presence has practically no effect on the sensor's sensitivity as it is shown by the FE analysis. In this case also an FSI numerical simulation has been carried out in order to optimize the design and avoid any undesired effects as much as possible prior to fabrication. As it is already mentioned in the first part, the piezoresistive gauges are incorporated into the sensor's membrane by ion-implanting the silicon layer in order to achieve greater piezoresistance coefficients. The dose and energy of the ion implantation have been determined by using Silvaco ® Athena technological simulation FE tool. The doping profile has been extracted in order to calculate more accurately the sensitivity of the gauges. problems. We hope that until the venue of the SPIE conference the first results will be ready for publication and for presentation.

Hotwire anemometer and thermal wall shear stress measurements

Various characterization methods of wall shear stress sensors were undertaken. First we have calibrated the sensors in temperature. After that, we have estimated the response time in constant current and constant temperature modes. Finally, the sensors were calibrated in a low-speed wind tunnel.

Temperature -Resistance setup and results

The resistance-temperature behaviour, specially the Temperature Coefficient Resistance (TCR), is necessary to calculate the sensor overheat ratio (a R ) for a given resistance.

The TCR is experimentally defined with an indirect heating achieved by Joule effect: TCR =

C T T R R °= ∂ ∂ 0 0 1 . The resistance overheat ratio is defined as a R = R -R a R a
, where R is the sensor resistance at the operating temperature and Ra is the wire resistance at the ambient temperature.

The obtained TCR values are very depending of material, deposition method and thickness. The nominal resistance value of a 50 x 2 x 0.5µm 3 polysilicon wire, doped at 10 20 cm -3 , taken at room temperature is about 3 kΩ. Thermal electrical measurements give a TCR of 0.145%/°C (Fig. 17).

Time Response and measurement methods

Although static measurement is founded on a simple principle, it is now accepted that the frequency response of such sensors is in general complex because of convective and conductive heat exchanges. An electric circuit based on the internal heating method was designed and carried out for the dynamic response time determination of the sensors (Fig. 20).

The circuit makes possible to simulate, on the sensor level, the fluctuations of temperature by carrying out an internal modulation of heating of the probe created by a D.C. current (rated current of the probe). To this current, a AC current of 1 MHz frequency modulated by a square signal of 1 kHz frequency is superimposed. Figure 21 shows the time response of a 58x2x0.5µm 3 hot wire, which is about 150µs. The bandwidth sensor lies between 0 and 1KHz, which is not enough to detect all turbulence phenomena associated with airflow over aircrafts in wind tunnels.

Time response study in function of the geometry

A study comparing the time response of various hot wire geometry was undertaken. The obtained results are in agreement with the analytical model [START_REF] Meunier | Fil chaud dans une micro cavité et capteur de pression en technologie MEMS[END_REF][START_REF] Tong | Semiconductor Wafer Bonding: Science and Technology[END_REF][START_REF] Bentley | Principles of measurements systems[END_REF] where the time response is calculated for constant current mode (CCM), with the next formula:

θ τ K i A v U MC v 2 0 0 ) ( - =
, where M is the sensor mass, C, the sensor specific heat, U the convection heat transfer coefficient between fluid and sensor, equal to 0.65 Wcm -2 °C-1 , A is the cross section, i 0 the current passing through the wire and T R K ∆ ∆ = θ θ . On figure 23, one can see that the time response is dependent on the cross section wire: the smaller is the width, the shorter is the response time. We can also note that the experimental values are fitting very well the theoretical model. The time response could be improved in mounting the hot wire in a Constant Temperature Mode (CTM) (Fig. 22). The hot wire anemometer is mounted in a Wheatstone bridge: This is a self-balancing bridge that maintains the resistance R T of the sensor at a constant value R. An increase in fluid velocity v causes a decrease of T and R T values inducing an unbalancing of the bridge. This causes the amplifier output current and current through the sensor to increase thereby restoring T and R T to their required values.

The advantage of such CTM circuit is to minimize thermal inertia of the sensors, which was leading to rather bad time response. It was demonstrated [START_REF] Meunier | Fil chaud dans une micro cavité et capteur de pression en technologie MEMS[END_REF][START_REF] Tong | Semiconductor Wafer Bonding: Science and Technology[END_REF][START_REF] Bentley | Principles of measurements systems[END_REF] that the time response in CTM becomes:

B A I v CTA K K K + = 1 τ τ , with ] [ 2 2 0 0 0 0 θ θ θ K i A U R i K K v I - = , 4 - ≈ A K and R V K s B 4 1 =
where Vs is the bridge supply voltage and R the variable resistance. In figure 23 the theoretical time response comparison between CCM and CTM is presented. For our sensors, it means that the sensor time response will decrease of about 22 times. The covered bandwidth will lie between 0 and 25 kHz, which is largely sufficient for most of turbulence measurement applications.

Calibration in Wind Tunnel

Wall shear stress sensors were also characterized in a wind tunnel in order to calibrate them for static speed variation. Experimentally, the hot wire anemometer is mounted at the wall surface of the wind tunnel. A Pitot tube measures the pressure in the centre of the tunnel. This pressure is then transformed in order to know the macroscopic speed. This calibration was done for speed ranging between 5m/s and 15m/s. With this constant temperature configuration, we can find the various parameters α , β and n of the electrical calibration of the sensor described by a law as:

β α τ + = n u E 2 2
) .(

, where E is the output voltage and u τ the friction speed.

In measuring the output voltage variations, we can determine the other coefficients. According to Levêque solution, n coefficient has to be close to 1/3. In figure 12, one can see that the above-mentioned law fits perfectly the experimental values when n is taken equal to 1/3.

CONCLUSIONS

This technology based on the use of SOI wafers and wafer-bonding technique requires only 8 photolithographies and masks. The principal advantage of such technology is to keep reproducible and constant pressure sensor cavity's depth and membrane's thickness, which is a mandatory criterion for the arrays configuration. Another advantage is that the

Fig. 1 .

 1 Fig.1. Pressure sensor cavity elaboration in the "base" BESOI wafer by deep RIE etching of <100> silicon.

Fig. 2 .

 2 Fig.2. Top view of the base wafer after deep reactive ion etching of pressure sensors' cavities and vent channels.

Fig. 3 .

 3 Fig.3. Patterning within the "active" SOI wafer of the cavity underneath the hot wire anemometer.

Fig. 5 .

 5 Fig.5. Infra-red photograph of the two bonded wafers prior to thinning-down.

Fig. 6 .

 6 Fig.6. Schematic section view of the final structure with the 3 types of sensors co-existing on the same die.

Fig. 7 .Fig. 8 .

 78 Fig. 7. Deformation of a100x100µm2 membrane due to residual stresses (image taken with Fogale ® Nanotech's ZoomSurf3D ™ optical profilometer).

Fig. 9 .

 9 Fig. 9. Hot-wire anemometers in a 90 deg. setup.

Fig. 10 .

 10 Fig. 10. Compressive (S3) stresses [Pa] in the pressure sensor's structure due to pressure of 10 5 Pa applied (memb.= 100x100x2µm 3 ).

Fig. 11 .

 11 Fig. 11. Tensile (S1) [Pa] stresses in the pressure sensor's structure submitted to 10 5 Pa pressure (membrane=100x100x2µm 3 ).

Fig. 12 .

 12 Fig. 12. Meshing details of the 1/4 (symmetry used) 3D model of the pressure sensor used for FSI simulations (membrane=100x100x2µm 3 ).

  Fig. 13. FSI results in the fluid domain. The velocity of the fluid (air) at a specific section of the fluid and the traces of the fluid flow are presented (velocities are in [m/s]).

Fig. 14 .

 14 Fig. 14. Vector plot of the velocity of the fluid (air) inside the pressure sensor's cavity and vent channels after a pressure of 10 5 Pa has been applied on the sensor's membrane.

Fig. 15 .

 15 Fig. 15. Simulation results of the evolution of the resistance of the piezoresistive gauges as a function of their length. The pressure on the membrane was 10 4 Pa and the gauges' width equal to 2µm. The gauges were positioned at 2µm off the membrane's edge.

Fig. 16 .

 16 Fig. 16. Evolution of the gauges resistance as a function of the silicon layer thickness. The gauges thickness has been considered equal to the silicon layer thickness. The results may differ depending on the doping profile of the gauges.

Fig. 17 .

 17 Fig. 17. Resistance variation as a function of temperature (TCR) of a 50x2x0.5µm 3 polysilicon hotwire.Fig. 18. I(V) characteristics as obtained for various wire geometries.

Fig. 21 .

 21 Fig. 21. Time response measurement of a 58x2x0.5µm 3 wall shear stress sensor.

Fig. 22 .

 22 Fig. 22. Schematic diagram of hot wire mounted in Constant Temperature Mode.

Fig. 23 .

 23 Fig. 23. Theoretical time response comparison between Constant Current Mode and Constant Temperature Mode. Fitting of experimental values for the CC Mode.

Fig. 24 .

 24 Fig. 24. Static speed calibration in wind tunnel: E 2 plot as a function of (u τ ) 2/3 , where u τ is the friction speed.

We have also measured I(V) curves of sensors with different widths and lengths . As we can see on Fig. 18, the thinnest and longest one exhibits higher voltage for small current. The over heat ratio a R gives the sensors sensitivity. 20% of variation can be achieved with a constant current of 1.4mA for a 50 x 2 x 0.3 µm 3 wire (Fig. 19), but for a 50 x 4 x 0.5 µm 3 , the needed current is about 4.55mA, so about three times more.

pressure gauges are in monocrystaline silicon and exhibit consequently very high sensitivity. Finally, this technology offers various thermal isolation architectures for the hot wire anemometer. In the future, signal processing integrated circuit will be integrated on the same chip. The first wall shear stress calibrations demonstrate the feasibility of most turbulence fluctuation measurements. Pressure sensors measurements will be added at the conference.