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A capacitive humidity sensor integrated with micro heater and ring oscillator circuit fabricated by CMOS–MEMS technique
Sensors and Actuators B 122 (2007) 375–380
A capacitive humidity sensor integrated with micro heater and ring oscillator circuit fabricated by CMOS–MEMS technique Ching-Liang Dai ∗ Department of Mechanical Engineering, National Chung Hsing University, Taichung, 402 Taiwan, ROC Received 5 April 2006; received in revised form 30 May 2006; accepted 31 May 2006 Available online 10 July 2006
Abstract This study presents the fabrication of a humidity sensor with a micro heater and a ring oscillator circuit using the commercial 0.35 m complementary metal oxide semiconductor (CMOS) process and a post-process. The micro heater is utilized to provide a super-ambient working temperature to the humidity sensor, which can avoid the humidity sensor to generate the signal drift. The humidity sensor is a capacitive type sensor. The structure of the humidity sensor consists of interdigital electrodes and a sensing film. The sensing film, which is polyimide, is coated on interdigital electrodes. The humidity sensor changes in capacitance when the sensing film absorbs or desorbs water vapor. The ring oscillator circuit is employed to convert the capacitance of the humidity sensor into the oscillation frequency output. Experimental results show that the sensitivity of the humidity sensor is about 25.5 kHz/% RH at 80 ◦ C. © 2006 Elsevier B.V. All rights reserved. Keywords: Humidity sensor; CMOS; Micro heater; Ring oscillator circuit
1. Introduction Humidity sensors, which are used to sense humidity and moisture, are important devices in many industrial and biomedical applications [1,2]. Micro electro mechanical systems (MEMS) technology has recently become popular for the miniaturization of sensors. Humidity sensors fabricated by MEMS technology have the benefit of small size, low weight, high performance, easy mass-production and low cost [3]. Several researchers have recently used MEMS technology to fabricate micro humidity sensors. For instance, Rittersma et al. [4] employed surface micromachining technique to fabricate a capacitive porous silicon humidity sensor. O’Halloran et al. [5] fabricated a capacitive porous silicon humidity sensor using a bulk micromachining technique. A thin-film surface micromachining technique, proposed by Park et al. [6], was used to manufacture humidity and temperature sensors. Story et al. [7] manufactured a polymetric humidity sensor, which had a sensitivity of 10 k/% RH. Nahar [8] developed a capacitive humidity sensor with anodized aluminum oxide (Al2 O3 ) thin-film, which
∗
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had a sensitivity of 100 pF/% RH. Lee et al. [9] investigated a micromachined robust humidity sensor using a pair of thermally isolated membranes including meander shape metal resistive elements, and its sensitivity was 0.054 mV/% RH. Yang et al. [10] presented a polyimide capacitive humidity sensor achieved by reactive ion etching microstructures in the polyimide film. The humidity sensor had a sensitivity of 1.24 pF/% RH. The standard CMOS process is an integrated circuit (IC) process that can fabricate microprocessors and memory circuits. Several studies have recently developed to use the standard CMOS process to fabricate MEMS devices, such as micromirrors [11], microwave switches [12], gyroscopes [13] and suspended inductors [14]. This technique that uses the CMOS process to manufacture MEMS devices is called as CMOS–MEMS [15,16]. Many micro devices fabricated by the CMOS–MEMS technique require a post-process to release the suspended structures or coat the functional films. The advantage of micromachined devices fabricated by the CMOS–MEMS technique is the capability of integration with readout circuits as a system on chip (SOC) due to their compatibility with the CMOS process. In this study, we utilize the CMOS–MEMS technique to fabricate a capacitive humidity sensor. The humidity sensor is integrated with a micro heater and a ring oscillator circuit on chip. The micro heater is employed to provide a super-ambient
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Fig. 1. Schematic structure of the humidity sensor with heater and circuit.
working temperature to the humidity sensor, and to avoid the signal drift. The post-process uses reactive ion etching (RIE) to etch the sacrificial layers of the humidity sensor, and then a polyimide film is coated on the humidity sensor. The capacitance of the humidity sensor changes when the sensing film absorbs or desorbs water vapor. Then the ring oscillator circuit is used to convert the capacitance of the humidity sensor into the oscillation frequency output. 2. Structure of the sensors Fig. 1 illustrates structure of the integrated chip, which contains a humidity sensor, a heater and a ring oscillator circuit. The humidity sensor, which is composed of interdigital electrodes and a sensing film, is a capacitive type sensor. Material of interdigital electrodes is the metal and via layers of the CMOS process. The sensing film is polyimide that is coated on the interdigital electrodes. By neglecting the fringing effects, the capacitance Csensor of the humidity sensor can be expressed as: Csensor = nε
lt d
(1)
where ε represents the dielectric constant of sensing film, n the fingers number of interdigital electrodes, l and t the length and thickness of interdigital electrodes, respectively, and d is the gap of interdigital electrodes. The dielectric constant of polyimide depends on humidity. The dielectric constant ε of sensing film generates a change when the sensing film absorbs or desorbs water vapor. According to Eq. (1), we know that the humidity sensor capacitance Csensor produces a variation as the dielectric constant of the sensing film changes. In this design, the length,
Fig. 2. Structure of the micro heater.
width, thickness and gap of interdigital electrodes in the humidity sensor are about 1000, 4, 6 and 4 m, respectively. There are 125 fingers of interdigital electrodes. The capacitance Csensor can be evaluated in accordance with Eq. (1) if the dielectric constant of the sensing film is given. Fig. 2 illustrates schematically structure of the micro heater, which is embedded under the interdigital electrodes of the humidity sensor. The micro heater is designed as the shape of gratings, which can generate a uniform heat to the humidity sensor. The gratings number in the micro heater is 125. Each grating in the micro heater has about 1000 m long, 4 m wide and 1 m thick. Material of the micro heater is the polysilicon layer of the CMOS process. The polysilicon layer is a good thermal resistive material. The micro heater is used to provide a super-ambient operating temperature to the humidity sensor. The humidity sensor integrates with a ring oscillator circuit on chip. Fig. 3 shows the ring oscillator circuit, which is the cascade connection of three identical inverters. The output node of the third inverter, as shown in Fig. 3, is connected to the input node of the first inverter. The three inverters form a voltage feedback loop. It can be found that this circuit does not have a stable operating point, and the only possible operating point, at which the input and output voltage of all inverters are equal to
Fig. 3. Three-stage ring oscillator circuit consisting of identical inverters.
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the logic threshold voltage, is inherently unstable. Hence, the circuit will oscillate once when any of the inverters input or output voltage diverges from the unstable operating point. The oscillation frequency f of the ring oscillator circuit is given by [17] f =
1 1 = 6τp + 2τs (6Cload V/Iavg ) + (2Csensor V/Iavg )
(2)
where Cload is the load capacitances, Csensor the humidity sensor capacitance, τ p the average propagation delay time associated with the load capacitances, τ s the delay time associated with the humidity sensor capacitance, and V and Iavg are the threshold voltage and average current, respectively. The load capacitances are formed from the parasitic capacitances of interconnection metals in the circuit. According to Eq. (2), it can be known that the oscillation frequency of the ring oscillator circuit is changed as the humidity sensor capacitance varies. The ring oscillator circuit can be employed to convert the humidity sensor capacitance into the oscillation frequency output. The HSPIE, professional circuit simulation software is used to analyze the ring oscillation circuit. Fig. 4 reveals the simulated results of the ring oscillation circuit at different temperatures. In this simulation, the load capacitance is given with 0.1 pF and the humidity sensor capacitance varies from 10 to 40 pF. The oscillation frequency of the ring oscillator circuit changes from 56.6 to 49.3 MHz and from 51.9 to 42.5 MHz at a constant temperature of 25 and 80 ◦ C, respectively, as the humidity sensor capacitance varies from 10 to 40 pF. The result shows that the humidity sensor has a signal drift when ambient temperature changes. In order to avoid the signal drift, the micro heater is employed to provide a super-ambient working temperature for the humidity sensor. For instance, the micro heater supplies a constant working temperature of 80 ◦ C to the humidity sensor. Then, the humidity sensor does not produce the signal drift if the ambient temperature is lower than the working temperature of 80 ◦ C.
Fig. 4. Simulated oscillation frequency of the ring oscillator circuit at different temperatures.
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3. Fabrication of sensor chip The integrated sensor chip is fabricated using the commercial 0.35 m CMOS process of Taiwan Semiconductor Manufacturing Company (TSMC). The humidity sensor needs a postprocess to etch the sacrificial layers and to coat a sensing film after completion of the CMOS process. Fig. 5 illustrates the process flow of the chip with the humidity sensor, the heater and the circuits. Fig. 5(a) depicts a cross-section of the chip with the humidity sensor, the heater and the circuits after the CMOS process. The gaps of interdigital electrodes in the humidity sensor are filled with the silicon dioxide layers, which are the sacrificial layers. The interdigital electrodes of the humidity sensor are stacked with the metal and via layers. The metal layers are aluminum and the via layers are tungsten. The thickness of the interdigital electrodes is approximately 6 m. Fig. 5(b) illustrates a photoresist (PR) layer that is coated onto the chip and patterned by a photo mask to generate a PR mask on the chip. The PR mask is applied to protect the unneeded etched regions of the humidity sensor, the heater and the circuits during RIE etching. Fig. 5(c) displays that CF4 /O2 RIE is used to etch the silicon dioxide in the gaps of interdigital electrodes of the humidity sensor, and then the PR mask on the chip is removed. Fig. 6 shows a photograph of the humidity sensor with the circuits after RIE etching process. Finally, a polyimide (SE7492, Nissan Chemical Instruments) film is coated on the chip and patterned by the other photo mask to generate a sensing film on the interdigital electrodes, as shown in Fig. 5(d). 4. Results A test chamber (GTH-099-40-1P, Giant Force Instruments Enterprise Co.) and a spectrum analyzer were employed to characterize the performance of the humidity sensor. The sensor was set in the test chamber and connected to the spectrum analyzer, which was used to record the signal response of the sensor to humidity changes. The test chamber was capable of providing a humidity range of 25–95% RH and a temperature range of 0–100 ◦ C. The humidity and temperature in the test chamber could be tuned separately and maintained at constant levels. The test chamber provides different humidity for the humidity sensor, and the humidity sensor changes in capacitance as the humidity rises or drops. The ring oscillation circuit is employed to convert the humidity sensor capacitance into the oscillation frequency. The output frequency of the ring oscillator circuit is measured using the spectrum analyzer. Fig. 7 presents the oscillation frequency changes of the ring oscillator circuit for the humidity sensor. In this investigation, the temperature was maintained constant at 25 ◦ C, while the humidity was increased from 25 to 95% RH in 40 min and then dehumidified to 25% RH at the same rate. The experiment revealed that the humidity sensor exhibited a small degree of humidity hysteresis. In order to determine the influence of temperature on the humidity sensor, the oscillation frequency of the ring oscillator circuit was measured in the humidity range from 25 to 95% RH at constant temperature of 25, 35, 50, 65 and 80 ◦ C. Fig. 8 displays the measured results of the humidity sensor at different
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Fig. 6. Photograph of the integrated chip after the CMOS process.
Fig. 7. Humidity hysteresis curve for the humidity sensor at 25 ◦ C.
Fig. 5. Process flow of the chip; (a) after completion of the CMOS process; (b) patterning a PR mask; (c) using RIE to etch the sacrificial layers; (d) coating polyimide on the interdigital electrodes.
Fig. 8. Tested results of the humidity sensor at different temperatures.
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supply provides different voltages, which are 2, 4, 6, 8 and 10 V, to the micro heater. In this experiment, the humidity sensor is heated by the micro heater, and is not heated by the test chamber. Fig. 10 shows the tested results of the humidity sensor with the micro heater under different voltages. Comparing Figs. 8 and 10, the curve at the voltage of 8 V (Fig. 10) is similar to the curve at a constant temperature of 80 ◦ C (Fig. 8). The result reveals that the micro heater has a good heated effect; the voltage of 8 V can produce approximately the working temperature of 80 ◦ C. The humidity sensor should not generate the signal drift if the ambient temperature is lower than the working temperature from the micro heater. 5. Conclusion Fig. 9. Relation between humidity sensitivity and temperature.
temperatures. The humidity sensitivity, which can be obtained by the linear fitting to the data in Fig. 8, was 14.5 kHz/% RH at 25 ◦ C, 16.4 kHz/% RH at 35 ◦ C, 21.2 kHz/% RH at 50 ◦ C, 24.2 kHz/% RH at 65 ◦ C and 25.5 kHz/% RH at 80 ◦ C. The results show that the humidity sensitivity increases as the temperature rises. According to the above results, the relation between the humidity sensitivity and temperature is plotted in Fig. 9. The fitted curve equation of the data (Fig. 9) is expressed by S = 0.212T + 9.55
(3)
where S represents the humidity sensitivity (kHz/% RH) and T is the ambient temperature (◦ C). According to Eq. (3), the humidity sensor has the disadvantage of the variation in humidity sensitivity with ambient temperature. In the above investigation, the micro heater in the chip is not used to provide heat, and the ambient temperature is from the test chamber. In order to solve the influence of ambient temperature, the micro heater in the chip is adopted to provide a super-ambient working temperature to the humidity sensor. The test chamber is maintained at constant temperature of 25 ◦ C, and a dc power
This study has successfully fabricated a capacitive humidity sensor integrated with a micro heater and a ring oscillator circuit using the commercial 0.35 m CMOS process and the postprocess. The post-process used RIE to etch the sacrificial layers of humidity sensor, and then coated a polyimide film on the interdigital electrodes of humidity sensor. The ring oscillator circuit is employed to convert the capacitance of the humidity sensor into the oscillation frequency output. The micro heater provides a super-ambient working temperature to the humidity sensor, which can avoid the humidity sensor to produce the signal drift. Experiments showed that the micro heater with the voltage of 8 V generated the working temperature of about 80 ◦ C and the humidity sensitivity of the humidity sensor was 25.5 kHz/% RH at 80 ◦ C. Acknowledgements The authors would like to thank National Center for Highperformance Computing (NCHC) for chip simulation, National Chip Implementation Center (CIC) for chip fabrication and the National Science Council of the Republic of China for financially supporting this research under Contract No. NSC 94-2212E-005-001. References
Fig. 10. Tested results of the humidity sensor with the micro heater under different voltages.
[1] A. Tetelin, C. Pellet, C. Laville, G. N’Kaoua, Fast response humidity sensors for a medical microsystem, Sens. Actuators B 91 (2003) 211–218. [2] G. Bokmann, B. March, D. Romhild, A. Steinke, Integrated Multisensors for Industrial Humidity Measurement, Int. Conf. Multi-Sensor Fusion Integ. Intelligent Systems, Baden-Banden, Germany, August 2001, pp. 201–203. [3] Z. Fang, Z. Zhao, Y. Wu, B. Zhang, Y.Z. Wang, Integrated Temperature and Humidity Sensor Based MEMS, Int. Conf. Information Acquisition, Hefei, China, June 2004, pp. 84–87. [4] Z.M. Rittersma, A. Splinter, A. Bodecker, W. Benecke, Novel surfacemicromachined capacitive porous silicon humidity sensor, Sens. Actuators B 68 (2000) 210–217. [5] G.M. O’Halloran, P.M. Sarro, J. Groeneweg, P.J. Trimp, P.J. French, Bulk Micromachined Humidity Sensor Based on Porous Silicon, Int. Conf. Solid-State Sens. Actuators, Chicago, IL, USA, June 1997, pp. 563–566. [6] S. Park, J.H. Kang, J. Park, S. Mun, One-bodied humidity and temperature sensor having advanced linearity at low and high related humidity range, Sens. Actuators B 76 (2001) 322–326.
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[7] P.R. Story, D.W. Galipeau, R.D. Mileham, A study of low-cost sensors for measuring low relative humidity, Sens. Actuators B 24–25 (1995) 681–685. [8] R.K. Nahar, Study of the performance degradation of thin film aluminum oxide sensors at high humidity, Sens. Actuators B 63 (2000) 49–54. [9] D.H. Lee, H.K. Hong, C.K. Park, G.H. Kim, Y.S. Jeon, J.U. Bu, A Micromachined Robust Humidity Sensor for Harsh Environment Applications, IEEE Int. Conf. Micro Electro Mechan. Syst., Interlaken, January 2001, pp. 558–561. [10] Y.L. Yang, L.H. Lo, I.Y. Huang, H.J.H. Chen, W.S. Huang, S.R.S. Huang, Improvement of Polyimide Capacitive Humidity Sensor by Reactive Ion Etching and Novel Electrode Design, IEEE Int. Conf. Sensors, Orlando, FL, USA, June 2002, pp. 511–514. [11] Y.C. Cheng, C.L. Dai, C.Y. Lee, P.H. Chen, P.Z. Chang, A circular micromirror array fabricated by a maskless post-CMOS process, Microsystem Technol. 11 (2005) 444–451. [12] C.L. Dai, H.J. Peng, M.C. Liu, C.C. Wu, H.M. Hsu, L.J. Yang, A micromachined microware switch fabricated by the complementary metal-oxide semiconductor post-process of etching silicon dioxide, Jpn. J. Appl. Phys. 44 (2005) 6804–6809. [13] H. Lou, X. Zhu, H. Lakdawala, L.R. Carley, G.K. Fedder, A Copper CMOSMEMS Z-axis Gyroscope, IEEE Int. Conf. Micro Electro Mechan. Syst., Las Vegas, NV, USA, January 2002, pp. 631–634.
[14] C.L. Dai, C.H. Tsai, Fabrication of integrated chip with microinductors and micro-tunable capacitors by complementary metal-oxidesemiconductor postprocess, Jpn. J. Appl. Phys. 44 (2005) 2030– 2036. [15] H. Batles, O. Brand, A. Hierlemann, D. Lange, C. Hagleitner, CMOS MEMS—Present and Future, IEEE Int. Conf. Micro Electro Mechan. Syst., Las Vegas, NV, USA, January 2002, pp. 459–466. [16] C.L. Dai, F.Y. Xiao, Y.Z. Juang, C.F. Chiu, An approach to fabricating microstructures that incorporate circuits using a post-CMOS process, J. Micromech. Microeng. 15 (2005) 98–103. [17] S.M. Kang, Y. Leblebici, Digital Integrated Circuits, McGraw-Hill, New York, 1996, 214.
Biography Ching-Liang Dai received the MS degree in the applied mechanics from National Taiwan University, Taiwan, in 1993, and the PhD degree in the mechanical engineering from National Taiwan University, in 1997. He is currently an associate professor at the Department of Mechanical Engineering, National Chung Hsing University, Taiwan. His research interests are CMOS–MEMS, integrated microsensors and RF MEMS.
A capacitive humidity sensor integrated with micro heater and ring oscillator circuit fabricated by CMOS–MEMS technique Ching-Liang Dai ∗ Department of Mechanical Engineering, National Chung Hsing University, Taichung, 402 Taiwan, ROC Received 5 April 2006; received in revised form 30 May 2006; accepted 31 May 2006 Available online 10 July 2006
Abstract This study presents the fabrication of a humidity sensor with a micro heater and a ring oscillator circuit using the commercial 0.35 m complementary metal oxide semiconductor (CMOS) process and a post-process. The micro heater is utilized to provide a super-ambient working temperature to the humidity sensor, which can avoid the humidity sensor to generate the signal drift. The humidity sensor is a capacitive type sensor. The structure of the humidity sensor consists of interdigital electrodes and a sensing film. The sensing film, which is polyimide, is coated on interdigital electrodes. The humidity sensor changes in capacitance when the sensing film absorbs or desorbs water vapor. The ring oscillator circuit is employed to convert the capacitance of the humidity sensor into the oscillation frequency output. Experimental results show that the sensitivity of the humidity sensor is about 25.5 kHz/% RH at 80 ◦ C. © 2006 Elsevier B.V. All rights reserved. Keywords: Humidity sensor; CMOS; Micro heater; Ring oscillator circuit
1. Introduction Humidity sensors, which are used to sense humidity and moisture, are important devices in many industrial and biomedical applications [1,2]. Micro electro mechanical systems (MEMS) technology has recently become popular for the miniaturization of sensors. Humidity sensors fabricated by MEMS technology have the benefit of small size, low weight, high performance, easy mass-production and low cost [3]. Several researchers have recently used MEMS technology to fabricate micro humidity sensors. For instance, Rittersma et al. [4] employed surface micromachining technique to fabricate a capacitive porous silicon humidity sensor. O’Halloran et al. [5] fabricated a capacitive porous silicon humidity sensor using a bulk micromachining technique. A thin-film surface micromachining technique, proposed by Park et al. [6], was used to manufacture humidity and temperature sensors. Story et al. [7] manufactured a polymetric humidity sensor, which had a sensitivity of 10 k/% RH. Nahar [8] developed a capacitive humidity sensor with anodized aluminum oxide (Al2 O3 ) thin-film, which
∗
Tel.: +886 4 22840433; fax: +886 4 22877170. E-mail address: [email protected].
0925-4005/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2006.05.042
had a sensitivity of 100 pF/% RH. Lee et al. [9] investigated a micromachined robust humidity sensor using a pair of thermally isolated membranes including meander shape metal resistive elements, and its sensitivity was 0.054 mV/% RH. Yang et al. [10] presented a polyimide capacitive humidity sensor achieved by reactive ion etching microstructures in the polyimide film. The humidity sensor had a sensitivity of 1.24 pF/% RH. The standard CMOS process is an integrated circuit (IC) process that can fabricate microprocessors and memory circuits. Several studies have recently developed to use the standard CMOS process to fabricate MEMS devices, such as micromirrors [11], microwave switches [12], gyroscopes [13] and suspended inductors [14]. This technique that uses the CMOS process to manufacture MEMS devices is called as CMOS–MEMS [15,16]. Many micro devices fabricated by the CMOS–MEMS technique require a post-process to release the suspended structures or coat the functional films. The advantage of micromachined devices fabricated by the CMOS–MEMS technique is the capability of integration with readout circuits as a system on chip (SOC) due to their compatibility with the CMOS process. In this study, we utilize the CMOS–MEMS technique to fabricate a capacitive humidity sensor. The humidity sensor is integrated with a micro heater and a ring oscillator circuit on chip. The micro heater is employed to provide a super-ambient
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Fig. 1. Schematic structure of the humidity sensor with heater and circuit.
working temperature to the humidity sensor, and to avoid the signal drift. The post-process uses reactive ion etching (RIE) to etch the sacrificial layers of the humidity sensor, and then a polyimide film is coated on the humidity sensor. The capacitance of the humidity sensor changes when the sensing film absorbs or desorbs water vapor. Then the ring oscillator circuit is used to convert the capacitance of the humidity sensor into the oscillation frequency output. 2. Structure of the sensors Fig. 1 illustrates structure of the integrated chip, which contains a humidity sensor, a heater and a ring oscillator circuit. The humidity sensor, which is composed of interdigital electrodes and a sensing film, is a capacitive type sensor. Material of interdigital electrodes is the metal and via layers of the CMOS process. The sensing film is polyimide that is coated on the interdigital electrodes. By neglecting the fringing effects, the capacitance Csensor of the humidity sensor can be expressed as: Csensor = nε
lt d
(1)
where ε represents the dielectric constant of sensing film, n the fingers number of interdigital electrodes, l and t the length and thickness of interdigital electrodes, respectively, and d is the gap of interdigital electrodes. The dielectric constant of polyimide depends on humidity. The dielectric constant ε of sensing film generates a change when the sensing film absorbs or desorbs water vapor. According to Eq. (1), we know that the humidity sensor capacitance Csensor produces a variation as the dielectric constant of the sensing film changes. In this design, the length,
Fig. 2. Structure of the micro heater.
width, thickness and gap of interdigital electrodes in the humidity sensor are about 1000, 4, 6 and 4 m, respectively. There are 125 fingers of interdigital electrodes. The capacitance Csensor can be evaluated in accordance with Eq. (1) if the dielectric constant of the sensing film is given. Fig. 2 illustrates schematically structure of the micro heater, which is embedded under the interdigital electrodes of the humidity sensor. The micro heater is designed as the shape of gratings, which can generate a uniform heat to the humidity sensor. The gratings number in the micro heater is 125. Each grating in the micro heater has about 1000 m long, 4 m wide and 1 m thick. Material of the micro heater is the polysilicon layer of the CMOS process. The polysilicon layer is a good thermal resistive material. The micro heater is used to provide a super-ambient operating temperature to the humidity sensor. The humidity sensor integrates with a ring oscillator circuit on chip. Fig. 3 shows the ring oscillator circuit, which is the cascade connection of three identical inverters. The output node of the third inverter, as shown in Fig. 3, is connected to the input node of the first inverter. The three inverters form a voltage feedback loop. It can be found that this circuit does not have a stable operating point, and the only possible operating point, at which the input and output voltage of all inverters are equal to
Fig. 3. Three-stage ring oscillator circuit consisting of identical inverters.
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the logic threshold voltage, is inherently unstable. Hence, the circuit will oscillate once when any of the inverters input or output voltage diverges from the unstable operating point. The oscillation frequency f of the ring oscillator circuit is given by [17] f =
1 1 = 6τp + 2τs (6Cload V/Iavg ) + (2Csensor V/Iavg )
(2)
where Cload is the load capacitances, Csensor the humidity sensor capacitance, τ p the average propagation delay time associated with the load capacitances, τ s the delay time associated with the humidity sensor capacitance, and V and Iavg are the threshold voltage and average current, respectively. The load capacitances are formed from the parasitic capacitances of interconnection metals in the circuit. According to Eq. (2), it can be known that the oscillation frequency of the ring oscillator circuit is changed as the humidity sensor capacitance varies. The ring oscillator circuit can be employed to convert the humidity sensor capacitance into the oscillation frequency output. The HSPIE, professional circuit simulation software is used to analyze the ring oscillation circuit. Fig. 4 reveals the simulated results of the ring oscillation circuit at different temperatures. In this simulation, the load capacitance is given with 0.1 pF and the humidity sensor capacitance varies from 10 to 40 pF. The oscillation frequency of the ring oscillator circuit changes from 56.6 to 49.3 MHz and from 51.9 to 42.5 MHz at a constant temperature of 25 and 80 ◦ C, respectively, as the humidity sensor capacitance varies from 10 to 40 pF. The result shows that the humidity sensor has a signal drift when ambient temperature changes. In order to avoid the signal drift, the micro heater is employed to provide a super-ambient working temperature for the humidity sensor. For instance, the micro heater supplies a constant working temperature of 80 ◦ C to the humidity sensor. Then, the humidity sensor does not produce the signal drift if the ambient temperature is lower than the working temperature of 80 ◦ C.
Fig. 4. Simulated oscillation frequency of the ring oscillator circuit at different temperatures.
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3. Fabrication of sensor chip The integrated sensor chip is fabricated using the commercial 0.35 m CMOS process of Taiwan Semiconductor Manufacturing Company (TSMC). The humidity sensor needs a postprocess to etch the sacrificial layers and to coat a sensing film after completion of the CMOS process. Fig. 5 illustrates the process flow of the chip with the humidity sensor, the heater and the circuits. Fig. 5(a) depicts a cross-section of the chip with the humidity sensor, the heater and the circuits after the CMOS process. The gaps of interdigital electrodes in the humidity sensor are filled with the silicon dioxide layers, which are the sacrificial layers. The interdigital electrodes of the humidity sensor are stacked with the metal and via layers. The metal layers are aluminum and the via layers are tungsten. The thickness of the interdigital electrodes is approximately 6 m. Fig. 5(b) illustrates a photoresist (PR) layer that is coated onto the chip and patterned by a photo mask to generate a PR mask on the chip. The PR mask is applied to protect the unneeded etched regions of the humidity sensor, the heater and the circuits during RIE etching. Fig. 5(c) displays that CF4 /O2 RIE is used to etch the silicon dioxide in the gaps of interdigital electrodes of the humidity sensor, and then the PR mask on the chip is removed. Fig. 6 shows a photograph of the humidity sensor with the circuits after RIE etching process. Finally, a polyimide (SE7492, Nissan Chemical Instruments) film is coated on the chip and patterned by the other photo mask to generate a sensing film on the interdigital electrodes, as shown in Fig. 5(d). 4. Results A test chamber (GTH-099-40-1P, Giant Force Instruments Enterprise Co.) and a spectrum analyzer were employed to characterize the performance of the humidity sensor. The sensor was set in the test chamber and connected to the spectrum analyzer, which was used to record the signal response of the sensor to humidity changes. The test chamber was capable of providing a humidity range of 25–95% RH and a temperature range of 0–100 ◦ C. The humidity and temperature in the test chamber could be tuned separately and maintained at constant levels. The test chamber provides different humidity for the humidity sensor, and the humidity sensor changes in capacitance as the humidity rises or drops. The ring oscillation circuit is employed to convert the humidity sensor capacitance into the oscillation frequency. The output frequency of the ring oscillator circuit is measured using the spectrum analyzer. Fig. 7 presents the oscillation frequency changes of the ring oscillator circuit for the humidity sensor. In this investigation, the temperature was maintained constant at 25 ◦ C, while the humidity was increased from 25 to 95% RH in 40 min and then dehumidified to 25% RH at the same rate. The experiment revealed that the humidity sensor exhibited a small degree of humidity hysteresis. In order to determine the influence of temperature on the humidity sensor, the oscillation frequency of the ring oscillator circuit was measured in the humidity range from 25 to 95% RH at constant temperature of 25, 35, 50, 65 and 80 ◦ C. Fig. 8 displays the measured results of the humidity sensor at different
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Fig. 6. Photograph of the integrated chip after the CMOS process.
Fig. 7. Humidity hysteresis curve for the humidity sensor at 25 ◦ C.
Fig. 5. Process flow of the chip; (a) after completion of the CMOS process; (b) patterning a PR mask; (c) using RIE to etch the sacrificial layers; (d) coating polyimide on the interdigital electrodes.
Fig. 8. Tested results of the humidity sensor at different temperatures.
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supply provides different voltages, which are 2, 4, 6, 8 and 10 V, to the micro heater. In this experiment, the humidity sensor is heated by the micro heater, and is not heated by the test chamber. Fig. 10 shows the tested results of the humidity sensor with the micro heater under different voltages. Comparing Figs. 8 and 10, the curve at the voltage of 8 V (Fig. 10) is similar to the curve at a constant temperature of 80 ◦ C (Fig. 8). The result reveals that the micro heater has a good heated effect; the voltage of 8 V can produce approximately the working temperature of 80 ◦ C. The humidity sensor should not generate the signal drift if the ambient temperature is lower than the working temperature from the micro heater. 5. Conclusion Fig. 9. Relation between humidity sensitivity and temperature.
temperatures. The humidity sensitivity, which can be obtained by the linear fitting to the data in Fig. 8, was 14.5 kHz/% RH at 25 ◦ C, 16.4 kHz/% RH at 35 ◦ C, 21.2 kHz/% RH at 50 ◦ C, 24.2 kHz/% RH at 65 ◦ C and 25.5 kHz/% RH at 80 ◦ C. The results show that the humidity sensitivity increases as the temperature rises. According to the above results, the relation between the humidity sensitivity and temperature is plotted in Fig. 9. The fitted curve equation of the data (Fig. 9) is expressed by S = 0.212T + 9.55
(3)
where S represents the humidity sensitivity (kHz/% RH) and T is the ambient temperature (◦ C). According to Eq. (3), the humidity sensor has the disadvantage of the variation in humidity sensitivity with ambient temperature. In the above investigation, the micro heater in the chip is not used to provide heat, and the ambient temperature is from the test chamber. In order to solve the influence of ambient temperature, the micro heater in the chip is adopted to provide a super-ambient working temperature to the humidity sensor. The test chamber is maintained at constant temperature of 25 ◦ C, and a dc power
This study has successfully fabricated a capacitive humidity sensor integrated with a micro heater and a ring oscillator circuit using the commercial 0.35 m CMOS process and the postprocess. The post-process used RIE to etch the sacrificial layers of humidity sensor, and then coated a polyimide film on the interdigital electrodes of humidity sensor. The ring oscillator circuit is employed to convert the capacitance of the humidity sensor into the oscillation frequency output. The micro heater provides a super-ambient working temperature to the humidity sensor, which can avoid the humidity sensor to produce the signal drift. Experiments showed that the micro heater with the voltage of 8 V generated the working temperature of about 80 ◦ C and the humidity sensitivity of the humidity sensor was 25.5 kHz/% RH at 80 ◦ C. Acknowledgements The authors would like to thank National Center for Highperformance Computing (NCHC) for chip simulation, National Chip Implementation Center (CIC) for chip fabrication and the National Science Council of the Republic of China for financially supporting this research under Contract No. NSC 94-2212E-005-001. References
Fig. 10. Tested results of the humidity sensor with the micro heater under different voltages.
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Biography Ching-Liang Dai received the MS degree in the applied mechanics from National Taiwan University, Taiwan, in 1993, and the PhD degree in the mechanical engineering from National Taiwan University, in 1997. He is currently an associate professor at the Department of Mechanical Engineering, National Chung Hsing University, Taiwan. His research interests are CMOS–MEMS, integrated microsensors and RF MEMS.