THE LONG TERM RELIABILITY OF A SWITCHED-CAPACITOR RELATIVE HUMIDITY SENSOR SYSTEM

Denice D. Denton, Maha A. S . Jaafar, and Andrew R.K. Ralston

Department of Electrical and Computer Engineering University of Wisconsin-Madison

Madison, WI 53706 608-263-2354

Abstract - This paper investigates the reliability of a solid state relative humidity (RH) sensor system. The sensor is a 15 pF parallel plate capacitor that employs polyimide (PI) as the dielectric. The integrated driving circuit is based on a charge sharing technique. The frequency response, hysteresis, dielectric stability and long term reliability for the sensor system are determined experimentally. The experimental results indicate that the sensor device capacitance-RH characteristic changes when aged at 85°C and 85% RH. The capacitance values at the lowest frequencies and highest ambient RH are effected the most by aging. The performance, however, of the CMOS measuring circuit is not effected when aged at 85°C and 85% RH for a period of 390 hours. The effects of change in the aged sensor device on the system output signal are determined. Suggestions are made for maximizing the system stability.

I. INTRODUCTION

The fabrication of solid state relative humidity sensors is a topic of much investigation in the literature [1]-[8]. Several commercial products are currently available [9]-[ 101. However, many questions remain conceming the hysteresis, long-term reliability, stability, and temperature coefficient of these devices. We have reported previously that exposure to high temperature and high humidity environments leads to drift and loss of calibration in polymeric capacitive humidity sensors ranging from 500 pF to 2000 pF [ 111-[ 121. In this study, we examine sensor devices with smaller geometries that would be used in an integrated "smart" sensor. The smaller geometry, 1425x1400 p, gives a dry capacitance value of approximately 15 pF suitable to be interfaced with the integrated driving circuit reported in [12]. The PI capacitor fabrication and the integrated circuit are briefly described and are reported in more detail elsewhere [5],[ 111-[12]. The experimental results of the performance of the integrated circuit as a function of exposure at 85"C/85% RH are also reported. The sensor device exhibits a dispersion in capacitance at high humidities when aged for 215 hours in a high temperature and high humidity (85"C/85% RH) environment. The effect of aging on the output of the driving circuit is measured at different operating frequencies. Recommendations for improving the stability of a polyimide solid state RH sensor system are made based on experimental observations.

11. POLYIMIDE CAPACITOR FABRICATION

The humidity sensing device is a parallel plate capacitor in which a polyimide @'I) film acts as the dielectric. The lower electrode is a 0.3 pm layer of aluminum sputtered onto a three inch silicon wafer. Two layers of PMDA-ODA polyimide precursor dissolved in N-methyl-2-pyrrolidinone (NMP) are spun-cast onto the wafer and baked at 135°C for 30 minutes each

to drive off excess solvent and partially imidize the PI. The wafer is then cured at 350°C for one hour to fully imidize the PI. The final polyimide thickness is approximately 2.1 pm. A second layer of aluminum is then sputtered onto the cured polyimide and pattemed photolithographically to produce capacitors consisting of approximately 25 pm lines separated by 25 pm spaces and shorted at each end by a 25 pm shorting bar. This geometry provides a diffusion path for water into the polyimide film. For this study, capacitor stmctures of 1400x1425 p2 geometry were constructed. The pads of the two electrodes are 125x125 p2. A probe station using fifteen mil tungsten probes with a 1 mil tip radius was used to contact the two pads of the sensor electrodes.

111. MEASURING CIRCUIT

PRINCIPLE OF OPERATION:

The measuring circuit for the relative humidity sensor system was reported previously [6],[9],[11]-[12]. The circuit configuration is shown in Fig. 1. It consists of a capacitance measuring circuit, based on a charge sharing technique, and a sample-and-hold circuit. Switches S1 and S2 are driven by a non- overlapping two phase clock. The circuit compares the charge stored on the polyimide capacitor C i with the charge stored on a reference capacitor Cref and stores &e difference on the feedback capacitor CO. This is done by first charging Cpi while discharging Cref and CO in the first phase, where the charge on Cpi is:

Then, in the second phase the reference capacitor is charged to:

The difference in charge between Q1 and 4 2 will be stored on the capacitor (CO) as: Q1- Q = C, X Vout

Q1 =Cpi Xvref. (1)

Q2 = Cref x V,f. (2)

(3)

Fig. 1 Circuit configuration of the capacitance measuring system.

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0-7803-0593-0/92 $3.00 1992 IEEE

Hence, the output voltage of the circuit is:

and it is proportional to the change in Cpi with the relative humidity. The sensitivity (S) of the circuit to this change, as can be seen from equation (4), is:

Vout = Vref x (Cpi-Cref)G

S = A Vout /A (Cpi-Cref) = Vref/C,

(4)

(9 DESCRIPTION OF THE INTEGRATED SYSTEM:

The switched-capacitor driving circuitry was fabricated with the MOS Implementation System (MOSIS) using a 2.0 pn, p-well, double poly, double metal CMOS process. Its performance with a standard variable capacitor was reported previously [11]-[12]. In this paper, the performance of the integrated CMOS circuit with the 15 pF PI humidity sensor will be reported as a function of aging. The operational amplifier design used in the circuit is a simple two stage amplifier with a cascoded output. Table I shows the characteristics of the operational amplifier. Its actual measured values were very close to the simulated values given by SPICE. On-chip double poly capacitors of approximately 25 pF value were used for both Clef and Co. Hence, from equation (9, the designed sensitivity (S) is 0.2 V/pF for a +5.0 volts reference voltage (VEf ). MOSIS supplies the chips in 40 pin DIP high quality ceramic, cavity-up, packages. The lid over the cavity was sealed prior to aging with an epoxy sealing to protect the circuitry from the environment.

I PARAMETER VALUE I Input Offset Voltage <5mV

Open Loop DC Gain > 70 dB

Biasing Current 3 @ Supply Current @ f 5V -*140@

Power Dissipation -1.5 mWatts Slew Rate (CL-30 pF) 5 V/ mSec

Table I Measured characteristics of the operational amplifier used in the CMOS circuit.

IV. EXPERIMENTAL PROCEDURES

The sensor samples were placed in an enclosed microprobe chamber. The CMOS chip used to measure the change in capacitance was placed in a second chamber. Air f” a moisture generator system was introduced into both chambers. The moisture generator system is from Watlow/Winona Inc. and is a microprogrammable unit that can generate controlled RH values from 5 to 90 % RH. A General Radio 1689 Digibridge was used to measure the capacitance of the sensor device under test @UT). Electrical contact to the DUT was made with tungsten probes. The CMOS chip under test is connected to the DUT with two micro-clips 1.63 inches in length. A f5.0 volts power supply was used. A WaveTek function generator (model 130) was used for the switching clock of the circuit. The output signal of the driving circuit was measured with a Metex (m-4650) digital multimeter.

Measurements were taken at 5%, 25%, 45%, 65%, and 85% with a fifteen to twenty minute period between each change in humidity. Diffusion studies [5] verified that this is an adequate equilibration time. At each RH, the capacitance of the sensor was first measured at 12 Hz, 100 Hz, 1 kHz, 10 kHz and 100 kHz. Ten points were averaged for each measurement at each frequency. The sensor sample was then connected to the CMOS circuit and the output voltage was measured at each different RH. Measurements of the voltage were taken at switching clock frequencies of 100 Hz, 1 kHz and 10 kHz. All measurements were done at m m temperature (approximately 24OC).

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A baseline measurement of the capacitance and output voltage of the sensor system was made before aging the samples at high temperature and high humidity. Other samples were stored in ambient lab air as controls. The aging environment was provided by a Parameter Generation and Control, Inc. (PGC) environmental chamber set to provide 85% RH at 85OC. The water used for the humidity was semiconductor processing grade, triply distilled, deionized water. The four sensor samples and the CMOS chip were aged in the PGC and removed for about two days when measurements were done.

20

19

18

0, 17

16 5

15

14 0 20 40 60 80

(a) KRH - C(pF)@lZHz - C(pF)@lOOHz - C(pF) @lkHz

. - C(pF)@lOkHz - C(pF)@lOOkHz

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10- . , * , . , . , . 0 60 80 1 20 40 (b) KRH

_- - C(pF)@12Hr - C$FJ@lkHz - C(pF) @ 100kHz

5 . 16 - 14 - 1 2 - 1 . I . I . I . I . I

0 20 40 60 80 100 (4 KRH

Fig. 2 PI capacitance of 1400x1425 p2 geometry vs. ambient RH for five test frequencies, (a) virgin sample, (b) aged at 8S0C/85% RH for 215 hours, and (c) aged for an additional 175 hours and “dry baked”.

V. EXPERIMENTAL RESULTS

Figure 2(a) shows a plot of the unaged PI capacitance (1400x1425 p n 2 geometry) as a function of ambient RH at five test frequencies. Note that the response is more linear as the frequency increases. A least squares linear fit of these data yields a correlation coefficient of greater than 0.99 for the 100 Hz, 1 kHz, 10 kHz, and 100 kHz frequencies. The correlation coefficient is 0.926 for the 12 Hz frequency. Note that there is an

increased sensitivity for RH values above 65%. A slight drift upwards for humidities higher than 65% is also noticeable for the 100 Hz measurement. The capacitance increases by approximately 18% over the range of 5% to 85% RH at a test frequency of 100 kHz.

The long term stability of the dielectric permittivity was monitored as a function of exposure time to 85°C. 85% RH. Figure 2(b) shows the capacitance-RH characteristics for the same sample after aging for 215 hours. This figure shows a significant dispersion of capacitance as the frequency decreases and the relative humidity increases. A least squares linear fit of these data yields a correlation coefficient of 0.948 for the 100 kHz, and lower than 0.8 for the 10 kHz, 1 kHz, 100 Hz and 12 Hz.

The capacitance-RH characteristics for the same samples were also measured after aging in 85"C/85% RH for an additional 175 hours. However, during the last few hours of aging, tk relay that controls the water supply to the PGC stopped operating. This led to an increase in temp to about 140°C and a dry environment condition inside the PGC. As seen in Fig. 2(c), this "dry bake" partially cured the samples improving the linearity of the sensors. A least squares linear fit of these data yields a correlation coefficient of greater than 0.96 for the 1 kHz, 10 kHz, and 100 kHz frequencies. The correlation coefficient is 0.91 for the 100 Hz frequency, where an increased sensitivity is noted for RH values above 65%. The correlation coefficient is 0.861 for the 12 Hz frequency, where an increased sensitivity is also noted for RH values above 65%.

Figure 3 shows the capacitance-RH characteristics for the virgin and aged data at 12 Hz and 100 kHz. The RH is raised from 5% to 85% and lowered back to 5%. Figure 3(a) shows that there is no hysteresis effect for the sensor device before aging (virgin sample) at all test frequencies. Figure 3(b) indicates that there is no significant change in the hysteresis of the device after 215 hours of aging at 85"C/85% RH. Hysteresis, however, is present after aging for an additional 175 hours, as shown in Figure 3(c). This effect is more pronounced at 12 Hz than at 100 kHz.

Measurements on the fabricated driving circuit circuit were first done using a variable ceramic capacitor to test the performance of the CMOS chip itself. Figure 4 shows the performance of the CMOS circuit before and after it was left in the aging environment. The response is linear and does not change even after being left in the aging environment. The least squares liiear fit of these data yields a correlation coefficient greater than 0.99. The measured value of sensitivity is very close to the designed value of 0.2 V/pF. From equation (4) and a reference capacitor of approximately 25 pF, the circuit was designed to operate for a capacitance measurement range of about 25 pF to 50 pF. The actual range however is approximately between 15 pF and 35 pF, due to the additional extemal capacitances of the wires and the micro-probes. Figure 4 indicates that the system performance is limited primarily by the linearity of the PI sensor response and not by the circuit after aging under these conditions.

Measurements were then performed on the circuit with the PI sensor capacitor to test the performance of the system as a whole. Figure 5(a) shows the measured output voltage versus the change in RH for an unaged sample at three test frequencies. The response is linear for the 1 kHz and 10 kHz frequencies, with a least squares linear fit of these data yielding a correlation coefficient greater than 0.98. The correlation coefficient is 0.844 for the 100 Hz frequency. The measured sensitivity of the system at 10 kHz and 1 kHz is approximately 0.006 V/% RH.

Figure 5@) shows the performance of the system after 215 hours of aging at high humidity and high temperature. The response at 100 Hz is linear below 45% RH. Above 45% RH, a significant drift upwards is observed. The response at 1 kHz is also linear below 45% RH. Above 45% RH, a drift upwards is also observed but with a smaller slope than that at 100 Hz. The response at 10 kHz is linear below 65% RH. Above 65% RH, a drift upwards is also observed. These drifts in voltage reflect the drifts seen in Figure 2(b) for the capacitance after aging for 215 hours. Note that the circuit is measuring a slightly larger capacitance than what the Digibridge is reading due to wiring parasitics.

4 -

3 -

- 2 2 -

5 1 - > " .

0 . - '

0 -

1 -

2 - r

Figure 5(c) shows the performance of the system after being partially cured. The response is linear for the 1 kHz and 10 kHz frequencies, with a least squares linear fit of these data yielding a correlation coefficient greater than 0.96. The correlation Coefficient is 0.889 for the 100 Hz frequency.

19 - 18 - 17-

a . 3 16-

15 - 14 - 13 I 0 2 0 4 0 60 80 100

(a) XRH

P I

/ - C(pF) @ lWkHz(Down)

I O ! . , . I , I . I ' I 0 2 0 4 0 60 80 100

(W XRH

I

0 20 40 6 0 80 100 (C) %RH

Fig. 3 Hysteresis of PI capacitance vs % RH at two test frequencies, (a) for virgin sample, (b) after agng at 85"C/85% RH for 215 hours, and (c) after aging for an adhbonall75 hours.

y = - 3 2017 t 0 1910% R V = 0 998

10 1 5 2 0 2 5 30 3 5 4 0

. vovt (vlqln)

Vovt(Z15HlsAgedl

Vovt (390Hrr Agedl

ClpFI

Fig. 4 The measured output voltage of the CMOS circuit vs. the capacitance of a standard variable capacitor before and after aging.

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VI. FUTURE WORK

12- - @kHz - @JlOhnZ s 0.8

0.6

0.4 20 40 6 0 80 100

(.I KRH

0 20 40 60 B O

(b) KRH

Fig. 5 The measured output voltage of the CMOS circuit vs. ambient RH at three test frequencies, (a) with a virgin PI sample, (b) with the same PI sample after aging for 215 hours at 85"C/85% RH, and (c) after aging for an additional 175 hours and partially curing the PI sample with the "dry bake".

V. SUMMARY

The long term reliability of a solid state sensor system has been investigated. The capacitance-RH characteristic exhibits a dispersion at higher humidities and lower frequencies after 215 hours of aging at 8SoC/85% RH. The device drift is the greatest at lower frequencies (12 Hz). The stability is best at higher frequencies (100 kHz). In addition, a switched-capacitor driving circuit for the solid state relative humidity sensor has been demonstrated. This CMOS circuit was fabricated using a 2.0 prn p-well double poly double metal process. Measurements to test the performance of the integrated circuit with the sensor during exposure to various relative humidities and aging at 8S0C/85% RH were performed. The performance of the measuring circuit is maintained even after aging. The performance of the RH system depends only on the performance of the PI sensor device. The linearity of the PI sensor device can be recovered if baked for several hours at high temperatures (140°C) in a dry environment. An operating frequency of 100 kHz is recommended for improved

The measurement circuit has been redesigned and tested. The new circuit has an improved sensitivity of 0.5 V/pF. 'Ihese chips will be tested with a new batch of sensor samples that wilI be fabricated and packaged. Both the CMOS chip and the packaged sensor will be mounted on a circuit board. Aging at high humidity and high temperature will be performed on this RH measurement system prototype. This experiment will yield valuable information regarding the performance of a fully packaged RH sensor system in the field.

Acknowledgment - The authors would like to thank Mr. John Moe and Watlow/Winona Inc. for the donation of the moisaue generating system and Mr. Jack Hile of parameter Generation and Controls for the donation of the environment chamber. This work was supported in part by the National Science Foundation under Denton's Presidential Young Investigator Award (Ref. No. ECS- 8657655).

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MC-2, Wdtham, MA, USA.