Performance Degradations of MISFET-Based Hydrogen Sensors with Pd-Ta₂O₅-SiO₂-Si Structure at Long-Time Operation ^†

Abstract

There are presented the generalized results of studies of performance degradation of hydrogen sensors based on MISFET with structure Pd-Ta₂O₅-SiO₂-Si. It was shown how responses’ parameters change during long-term tests of sensors under repeated hydrogen impacts. There were found two stages of time-dependence response’ instability, the degradation degree of which depends on operating conditions, hydrogen concentrations and time. To interpret results there were proposed the models, parameters of which were calculated using experimental data. These models can be used to predict performances of MISFET-based devices for long-time operation.

1. Introduction

The gas sensors based on the metal-insulator-semiconductor devices (MIS-capacitors and field-effect transistors called as MISFETs) have been studied by many investigators (e.g., [1,2,3,4,5,6,7,8,9,10]). A great contribution to the developments of gas-sensitive MIS devices has been made by the researchers at Linköping University [5]. MIS sensors with different gates’ material (palladium, platinum and iridium), with dielectric films SiO₂, Si₃N4-SiO₂, TiO₂-SiO₂, Ta₂O₅-SiO₂ have been investigated. Semiconductors Si, GaAs [3] and SiC [7] were used in MIS gas sensors to detect the low concentrations of gases H₂ [2,3,4], NH₃ [5], H₂S [6] and CO [7]. The studies have shown that performances characteristics of MISFET-based hydrogen sensors depend on technological parameters [2], electrical modes [8], chip temperature [10] and external factors (other gases, irradiation [9]). The researchers at National Research Nuclear University MEPhI have developed and investigated the number of hydrogen sensors based on MIS-capacitors and MISFETs with structures Pd (or Pt)-SiO₂-Si, Pd/Ti-SiO₂-Si, Pd (or Pt)-Ta₂O₅-SiO₂-Si. The experiments have demonstrated, that among developed MIS-sensors the integrated cells, containing on single chip MISFET with Pd-Ta₂O₅-SiO₂-Si-structure, heater-resistor and temperature sensor, possess the best stability and reproducibility of characteristics [4].

Performances characteristics (sensitivity, stability and speed) are determined by sensor responses’ parameters, the repeatability of which becomes an important characteristic at long-term operation of gas-analytical devices. The motivations of this paper are to generalize data on researches of responses’ parameters changes of MISFET-based sensors on structure Pd-Ta₂O₅-SiO₂-Si at long-term operation and to propose models, taking in to account the performance degradations.

2. Materials and Methods

The testing n-channel MISFET element based on Pd-Ta₂O₅-SiO₂-Si structure was fabricated on single chip (2 × 2 mm²) together with (p–n)-junction temperature sensor and heater-resistor by means of conventional n-MOS-technology using laser evaporation Pd-films. To measure the sensor’s hydrogen responses there was used the special circuitry shown in Figure 1. The circuit provides the constant drain current I_D = 0.1 mA and source-drain voltage V_D = 1.0 V. In this circuitry the voltage V is equal to the gate voltage V_G. The constant chip temperature (T = 130 ± 2 °C) was supported by the temperature-stabilization circuitry with feedback loop using on-chip thermo-sensor and heater.

3. Results

We have tested the sensors for 5 or 8 weeks (5 days a week in a row with 2 day breaks) by 5 of repeated hydrogen j-impacts at values δV_0j and maximum response amplitudes δV_CM of vs. C for each j-cycle (Figure 4). In each j-impact sensors were consecutively exposed to hydrogen pulses of different concentrations C_i (Figure 2). The indexes i, j, k and l are serial numbers of responses, ordinary, day and weak cycles respectively. There were measured the parameters of each i-response (Figure 3). There were calculated the sums of residual characteristics, instability responses’ parameters and its designations are presented in

Table 1. Designations and n_lkji-responses’ parameters (Y_lkji) for the characterization of instabilities and performance degradations’ parameters of output voltage V(C,t,t_C).

i-Response	j-Cycle	k-and l-Cycles	General Values
V00—primary voltage	Maximum amplitude ΔVCM	Amplitude’ changes: δVCM lk = ΔVCM − ΔVCMlk	N—number of sensors
τi—H2 pulse duration	Amplitude’ changes: δVCM j= ΔVCM − ΔVCMj		Y0—primary value of Y
Ci—H2 concentration		break times tbk and tbl	average of Y, variation indices, degradation degree:
V0i—initial voltage	differential sensitivity: Sdj = d(ΔVC)/dC	cycle time tk= tj × jmax+ tbk
ΔVCi—response amplitude		cycle time tl = tk × kmax + tbl
δV0i—residual value	maximum sensitivity SdM lk	summary ZLD (j = 1; k = 1)
τ1i—response time	break time tbj	ΔV0Sk= V0k − V0(k+1)
τ2i—relaxation time	cycle time tj = tji × imax+ tbj	ΔV0Sl= V0l − V0(l+1);
ti—response period	ZLD ΔV0j= V0j1 − V0j i max	ΔV0S= V0111 − V0( lkji) max
Si = ΔVCi /Ci − sensitivity	summary SZLD (I = 1):	C-time factor D = nk∙nl∙Djmax
Di= (Ci·τi) is Ci-time factor	ΔV0Slkj = V0lk1 − V0lk(j+1)	Degradation rate vY = dY/dt

. Responses’ and models’ parameters changes are demonstrated in Figure 4, Figure 5, Figure 6 and Figure 7 and in

Table 2. Average values of responses’ and model parameters of conversion function ΔV_C (C) for lkj-cycles.

↓parameters/lkj→	111	155	255	355	455	555	655	755	855	Total ΔYD,%
ΔVCM, V/ρVCM, %	0.46/4.3	0.41/3.7	0.39/3.8	0.38/3.9		0.37/4.0		0.36/4.2		21.7
kC, 1/%/ρkCM, %	12/8.2	10/8.8	8.5/10.6	8.2/10.6		8.1/11.5		8.0/11.5		33.3
SdM, V/%/ρSdM, %	5.52/12.5	4.1/12.5	3.32/14.4	3.12/14.5		2.96/15.5		2.88/15.7		47.8
vS, V/(% × day)	-	0.28	0.16	0.024		0.016		0.08		-
Sj5, V/%/δSj5, %	2.1/2.4	1.8/2.8	1.6/3.1	1.55/3.2		1.50/3.3		1.45/3.45		31.0
ΔV0Slkj, mV	33	37	16	7		3				-
vΔV0, mV/(day)	-	7.4	3.2	1.4		0.6				-
τ1i, s/τ2i, s (i = 2)	10/15	9/15	8/15	7/16		7/15				30/6.6
τ1i, s/τ2i, s (i = 5)	7/17	7/16	6/16	6/16		7/15				14.3/11.8
SdM, V/%/ρSdM, % (for Ci max = 1.0%)	5.5/8.7	3.9/9.2	3.65/9.8	3.5/10.2	3.4/9.5		-			38.2
D, % × min	0.26	6.5	13	19.5	26	32.5	39	45.5	52	-

.

4. Discussion

It was found that all MISFET-sensors at long-term operation have the following degradations’ features: (1) so-called “zero-line drift” (ZLD) – the changes of output voltage V at zero hydrogen concentration C; (2) reduction of hydrogen sensitivity S being equal |dV/dC|, if hydrogen exposition time t_C increases. The following 4-component model was used to interpret the results:

V (C, t, t_C) = V₀₀ + ΔV₀₀(t) − ΔV_0S(C, t_C) − ΔV_C(C, t_C)

(1)

ΔV₀₀(t) =ΔV_0M∙[1 − exp(−t/τ)]; ΔV_0S(C, t_C) = ΔV_0SM(C, t_C)·[1 − exp(−D/D₀)];
ΔV_C(C,t_C) = ΔV_CM(C, t_C)∙[1 − exp(−k_C×C)]; S_dM = k_C(C, t_C) × ΔV_CM(C, t_C),

(2)

where V₀₀ is a primary voltage being about 1.4 V (at T ≈ 130 °C). There are two sorts of ZLD: the initial time drift ΔV₀₀ (t) and the drift ΔV_0S(C,t_C) associated with the total hydrogen dose D = ∫C(t)dt (C− time factor). The first one appears immediately after the turning the sensor in the operating mode (e.g., maximum changes of value ΔV₀₀(t) can be equal ΔV_0M in ranges from ±10 to ±50 mV during 1–5 min, and τ ≈ 75 s). The second sort of ZLD occurs due to operation of the sensor in a hydrogen environment. Summary ZLD (ΔV_0Slkj) increases, if hydrogen exposition time t_C is rising. These changes depend on time t_C, the primary operating conditions (preliminary temperature-hydrogen treatments), chip temperature and electrical modes. Maximum changes of responses’ parameters are manifested in the first stages at values C-time factor D less than 25 (% vol.)×min, at D₀ being equal to about 8 (% vol.)×min.

5. Conclusions

This paper generalized data on researches of responses’ parameters changes of MISFET-based sensors on structure Pd-Ta₂O₅-SiO₂-Si at long-term operation and proposed models, taking into account the performance degradations. The all tested MISFET-sensors at long-term operation have degradations’ features: reduction of hydrogen sensitivity and “zero-line drift” (ZLD), which depend on operating conditions and accumulated hydrogen dose D (C-time factor). This can be explained by the effects of palladium aging (accumulation of Pd-H compounds and irreversible palladium swelling). Basic degradations’ parameters are manifested in the first stages at values D less than 25 (% vol.)×min. These effects for practical applications of hydrogen sensors were taken into account as the additive errors of “zero” (basic line drift).