Humidity and temperature compensation in work function gas sensor FETs

Humidity and temperature compensation in work function gas sensor FETs

Sensors and Actuators B 93 (2003) 271–275 Humidity and temperature compensation in work function gas sensor FETs$ M. Burgmair*, M. Zimmer, I. Eisele ...

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Sensors and Actuators B 93 (2003) 271–275

Humidity and temperature compensation in work function gas sensor FETs$ M. Burgmair*, M. Zimmer, I. Eisele Institute of Physics EIT 9.2, Universita¨t der Bundeswehr Mu¨nchen, Werner-Heisenberg-Weg 39, Neubiberg 85577, Germany

Abstract Field-effect transistors with suspended gate are able to detect gas species if a sensitive film is used as gate. The FET acts as a transducer which converts adsorption induced changes of work function DF at the surface to a corresponding change of the drain-source current DIDS in the FET channel. At operational temperatures below 80 8C, however, humidity induces contributions to the sensor signal and a base line drift which is not related to the sensitive film but to the transducer FET itself. An explanation will be given for this phenomenon and it will be shown that a guard ring and a second FET can largely suppress the influence of humidity at ambient temperatures. # 2003 Elsevier Science B.V. All rights reserved. Keywords: GasFET; Work function; Isothermic point; Humidity suppression; Guard ring; Reference FET

1. Introduction In recent years, a gas sensitive FET with an operational temperature of <100 8C has been developed which is capable of detecting gas species by measuring the induced shift of work function potential at the surface of a sensitive film [1]. The device consists of two parts which are processed in parallel. Part one is the sensitive film deposited on a polished, mechanically rigid and electrically conductive substrate, e.g. highly doped silicon. The second part consists of a field-effect transistor whose gate was not fabricated (gateless MOSFET). The channel between source and drain is only protected by a stack of 40 nm Si3N4 on 60 nm SiO2. In terms of work function changes LPCVD-Si3N4 turned out to be the most chemically inert material compared to thermal SiO2, laser ablated Al2O3 and LPCVD Ta2O5 [2]. A chip (3:5 mm  4 mm) with two gateless FETs can be seen in Fig. 1. The gateless FET acts as a transducer which converts changes of the electrical potential in the area above the source-drain channel to an electrical signal, i.e. a change of the drain-source current IDS. The surface above the channel is surrounded by a guard ring (100 nm Pt with a few nanometres of Ti for adhesion). By means of a flip-chip $

Presentation given at the IMCS in Boston, USA, 2002. Corresponding author. Tel.: þ49-89-6004-3877; fax: þ49-89-6004-4039. E-mail address: [email protected] (M. Burgmair). URL: http://www.unibwm-physik.de *

bonder, the substrate with the sensitive film is glued upside down onto the transducer FET [3]. Both are separated by an air gap of 1 mm. The gap is determined by the surface topology of the transducer. This hybrid device is called ‘‘hybrid suspended gate FET’’ (HSGFET). The glued substrate with the sensitive film will act as gate. A schematic cross-section of this hybrid assembled device is sketched in Fig. 2. Fig. 3 shows the equivalent circuit diagram. The gate capacitance is mainly determined by the width of the air gap. RS is the surface resistance between the area above the FET channel and the surrounding guard ring. Gas species can access the air gap and adsorb on the inner surfaces: the sensitive film and the Si3N4 film of the transducer FET. In the experiments, humidity was produced by moisturizing dry synthetic air at room temperature. Relative humidities between 10 and 90% could be adjusted (given at 23 8C). Since only the chip with the FETs was heated to operate it at constant temperatures between 30 and 150 8C the test gas was at room temperature, too, when reaching the sensor.

2. Guard ring Ideally, only the sensitive film reacts with gas species, whereas the Si3N4 film should be chemically inert. This ideal behaviour can be observed with dry synthetic air or at operational temperatures above 80 8C. However at lower operation temperatures, two phenomena occur which can become dominant in the case of high ambient relative

0925-4005/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0925-4005(03)00232-6

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Fig. 4. Influence of humidity pulses (given at 23 8C) to an HSGFET without guard ring (Top ¼ 30 8C). Fig. 1. Surface of gateless GasFET chip (3:5 mm  4 mm) with guard rings surrounding each transistor channel.

Fig. 2. Schematic cross-section of the hybrid suspended gate GasFET (HSGFET).

humidities. These are the humidity induced baseline drift and the contribution of the gate insulator surface Si3N4 to the signal DF at exposure to humidity in combination with various gases. Fig. 4 depicts the sensor signal IDS of an HSGFET without guard ring at an operational temperature of 30 8C. The same would be observed if the guard ring is electrically floating. Apparently, the sensor responds by a baseline drift when the humidity is turned on. This drift increases with humidity level. It is already visible at 10% RH and becomes dramatic at levels over 40% RH. Conversion of the drain-source current IDS to a corresponding work function shift yields a value of as big as 22 V, which is nearly 100 times the value that is observed for hydrogen detection [4]. A guard ring kept at a constant electric potential (here 0 V) suppresses the base line drift enormously, as seen in Fig. 5 where a gateless FET (details in Section 3)

Fig. 5. Gateless FET: suppression of humidity influence (given at 23 8C) by means of guard ring at ground potential (Top ¼ 30 8C).

with guard ring is operated at 30 8C. Even high humidity levels of 90% RH do not cause baseline drift. The reason for the baseline drift is assumed to be the condensation of a thin film of water at the surface of the passivation layer. Since LPCVD-Si3N4 is hydrophilic (own observation and [5]) this film closely covers the surface and reduces the surface resistance dramatically by enabling transportation of ions across the surface. At dry conditions these charges are immobilized. By means of the guard ring, which is kept at constant electric potential and which surrounds the area of the channel, these ions cannot enter or leave this area and leakage currents due to potential gradients are suppressed. Initial inhomogeneous charge distributions within that area can be equalized at first exposure of the device with high humidity levels. The resistance RS in the equivalent circuit diagram (Fig. 3) reflects this surface conductance. The value of RS depends on the material used for passivation (here LPCVD-Si3N4) and the degree of condensation of water vapour on its surface.

3. Gate insulator surface

Fig. 3. Equivalent circuit diagram of the HSGFET.

The second phenomenon concerns the fact that the passivation film (i.e. gate insulator) is not ideally chemically inert. Previous and recent work performed by Kelvin probe

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and FET, respectively, revealed that the Si3N4 surface reacts to different gases like H2, NO2, NH3 [2,6] as well as humidity. Surface reactions lead to contributions to the measured work function shift DF which can account for some tens of mV for gases and up to 100 mV for 90% relative humidity as seen in Fig. 5. Using the transducer FET with guard ring but without a gas sensitive gate allows to detect the contribution of the transducer surface itself. Removing the gate leads to a more unstable baseline of the transducer signal IDS on a long-term scale because the source-drain channel is more exposed to any external potential fluctuations. But since the gas exposures happen within a few seconds the immediate response of the gate insulator film above the channel can be well discriminated from the long-term drift. The gateless FETwith its Si3N4 surface was exposed to relative humidity pulses between 10 and 90% (at 25 8C). The temperature of operation was 30 8C. Clearly, the baseline is stable even at high humidity levels which is attributed to the guard ring. However, humidity induces reversible signal pulses of up to DIDS ¼ 46 nA. Conversion with the transducer’s sensitivity factor of ð1=0:12 VÞð@IDrain [email protected]Þ ¼ 4 mS=V (drain-source voltage VDS ¼ 0:12 V) yields ‘‘DF’’ of about 100 mV. This is fairly high as compared to gas measurements with a sensitive gate. A second FET is necessary which exhibits identical electrical characteristics except for gas sensitivity. This FET is called reference FET (RefFET). Due to the same humidity effect on its passiviation surface the RefFET should show the same signal response to humidity. A magnified view of the drainsource currents of Gas and RefFET is depicted in the upper half of Fig. 6. Below, the difference of both signals is shown: The influence of even high humidity could be reduced from DF ¼ 96 to 17 mV and, in addition, the baseline could be stabilized. These results were applied for an ammonia sensor based on Cr1.8Ti0.2O3 (CTO) as sensitive material for the GasFET [7]. Subtraction was done by means of operational amplifiers. The RefFET IDS was used as a reference in an electrical feedback circuit which kept the GasFET IDS constant by controlling the

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Fig. 7. Ammonia detection with Cr1.8Ti0.2O3, electrical feedback control using a RefFET for humidity compensation: (a) without guard ring; (b) with guard ring.

gate voltage VG of the GasFET. This voltage was taken as sensor signal. The resulting sensing behaviour can be seen in Fig. 7b where RefFET and guard rings on both FETs were deployed. The operational temperature was 40 8C. Rising the background humidity level from 0 to 30% did not shift the sensor baseline. In contrast, using GasFET and RefFET without guard rings resulted in a huge baseline drift upon rising the humidity level from 0 to 30% (Fig. 7a). This can be explained by different charge distributions at both transducer surfaces which were not suppressed by a guard ring. This shows that a reference FET is not enough to suppress the humidity effect if no guard rings are used. The deteriorated signal-to-noise ratio in Fig. 7b can be attributed to a bad capacitive coupling between the gate potential and the transducer due to a bad hybrid assembly, i.e. increased air gap. However, with a feedback circuit control the signal heights remain constant.

4. Temperature stabilization Fig. 6. Gateless gas and RefFET: individual IDS of each FET (top), difference of both IDS (below). Relative humidities range between 10 and 90% (given at 23 8C).

A reasonable sensor operation with a stable signal baseline under ambient conditions requires a small influence of

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temperature. This can be achieved either by keeping the operational temperature of the device constant at an elevated level above RT or by reducing the influence of temperature to the device as much as possible. In practice, both methods are applied. The first one can be realized by an external temperature controlled heater. For the second one at first a good comprehension of the temperature influence to the GasFET is required. Temperature influences the adsorption processes at the sensitive surface. Both the quantity of adsorbed species and the adsorption dynamics will be affected. In terms of work function sensing, this means that the value of DF and the time for saturation depends on temperature. Without gas exposures it can be assumed that temperature variations between 0 and 100 8C do not affect the sensitive film. The hybrid assembly of the sensitive gate by flip-chip gluing should not cause any trouble if the gate rests entirely on the transducer. Concerning the transducer FET temperature affects both the mobility m of charge carriers and the level of the Fermi energy eCF. For the linear regime of the IDS–VG characteristics the following temperature dependence can be derived from [8]: dIDS W dm ¼ CTot ðVG  VT ÞVDS L dT dT   W 1 QD dCF  m CTot VDS 2  L CTot 2CF dT where L is the effective channel length, W the effective channel width, Ctot the total capacitance across the air gap of the GasFET, VT the threshold voltage, VDS the drain-source voltage, m the inversion charge carrier mobility, QD the charge of depletion layer. Due to the counteracting behaviour of both contributions (subtraction) a point can be found with dIDS/dT being minimal. This point can be chosen by variation of the gate voltage VG. Fig. 8 shows the IDS–VG characteristics of a GasFET taken at seven different temperatures between 20 and 80 8C. Drain-source voltage was VDS ¼ 0:1 V. At VG  12 V all curves intersect in one point. This is called the isothermic point because the

Fig. 9. Temperature dependence of a feedback circuit driven HSGFET operated outside the isothermic point. Without (&) and with (*) RefFET.

influence of temperature on IDS is minimal (0.01 mA K1). The remaining dependence could originate from variations of ohmic resistances, e.g. of the supply lines. Unfortunately, the isothermic point is far from VG ¼ 0 V, which makes mobile battery driven applications difficult to realize. Other process parameters during the fabrication of the transducer FET and other coupling capacitances CTot between the sensitive gate and the channel might yield GasFETs with isothermic points near VG ¼ 0 V. If this is not possible a reference FET (RefFET) can help to compensate the temperature influence if its IDS–VG characteristics and thermal dependence are identical with the GasFET. Fig. 9 demonstrates the effect of the RefFET if the device is not operated in the isothermic point. In this case the RefFET was not perfectly identical to the GasFET because the gate was micromachined to achieve a recessed suspended gate. Due to this increased air gap the capacitive coupling to the channel is poor, i.e. CTot is small and hence the gas sensitivity of the device is small. This modified FET was used as RefFET. But the IDS–VG characteristics and the temperature dependence has changed, too, because— according to the above equation—a variation of the total capacitance CTot influences IDS and its temperature behaviour. Nevertheless, an improvement can be achieved with a RefFET as can be seen in Fig. 9. The GasFET was driven by a feedback circuit with and without the RefFET. Without RefFET an average temperature dependence of about 25 mV K1 is achieved, with the RefFET the dependence can be reduced to about 5 mV K1. An even better performance should be expected for an identical but non-sensitive RefFET.

5. Summary

Fig. 8. IDS–VG characteristics of a GasFET at temperatures between 20 and 80 8C in steps of 10 8C. The intersection is the isothermic point.

It could be demonstrated that for low power application of the gas sensitive FET a guard ring surrounding the area above the FET channel helps to suppress the humidity induced baseline drifts by preventing the transport of ions

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at the surface. Although fairly stable, the gate insulator is not inert enough to be treated insensitive to gas exposures. Humidity still produces a contribution of the Si3N4 surface to the measured work function shift DF. Only by means of a reference FET (RefFET), which is identical to the GasFET in electrical terms but not gas sensitive, and which in particular has the same gate insulator material, helps to suppress its contribution. Temperature mainly influences the transducer part of the GasFET causing significant baseline drifts. Operation in the isothermic point and/or again using a reference FET, which should exhibit the same temperature dependence as the GasFET, reduces the temperature effect.

References [1] I. Eisele, T. Doll, M. Burgmair, Low power gas detection with FET sensors, Sens. Actuators B 78 (2001) 19–25.

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[2] M. Burgmair, I. Eisele, Contribution of the gate insulator surface to work function measurements with a gas sensitive FET, in: Proceedings of the 1st IEEE Sensors Conference, Orlando, FL, USA, 12–14 June 2002, pp. 439–442. [3] A. Fuchs, T. Doll, I. Eisele, Flip-chip mounting of hybrid FET gas sensors with air gap, in: Proceedings of the Micro Systems Technology, Potsdam, Germany, 1998. [4] K. Scharnagl, A. Karthigeyan, M. Burgmair, M. Zimmer, T. Doll, I. Eisele, Low temperature hydrogen detection at high concentrations: comparison of platinum and iridium, Sens. Actuators B 80 (2001) 163–168. [5] T. Hattori (Ed.), Ultraclean Surface Processing of Silicon Wafers, Secrets of VLSI Manufacturing, Springer, Berlin, 1998. [6] T. Doll, K. Scharnagl, R. Winter, M. Bo¨ gner, I. Eisele, B. Ostrik, M. Scho¨ ning, Work function gas sensors—reference layers and signal analysis, in: Proceedings of the 12th European Conference on SolidState Transducers, Southampton, UK, September 1998. [7] M. Burgmair, J. Wo¨ llenstein, H. Bo¨ ttner, A. Karthigeyan, K. Anothainart, I. Eisele, Ti-substituted chromium oxide in work function type sensors: ammonia detection at room temperature with low humidity cross sensitivity, IEEE Sens. J., submitted for publication. [8] S.M. Sze, Physics of Semiconductor Devices, Wiley, New York, 1981.