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1、原文 A Smart Single-Chip Micro-Hotplate-Based Gas Sensor System in CMOS-Technology Abstract This paper presents a monolithic chemical gas sensor system fabricated in industrial CMOS- technology combined with post-CMOS micromachining. The system comprises metal -oxide-covered (SnO2) micro-hotplates and
2、 the necessary driving and signal-conditioning circuitry. The SnO2 sensitive layer is operated at temperatures between 200 and 350. The on-chip temperature controller regulates the temperature of the membrane up to 350 with a resolution of 0.5. A special heater-design was developed in order to achie
3、ve membrane temperatures up to 350 with 5 V supply voltage. The heater design also ensures a homogeneous temperature distribution over the heated area of the hotplate (12% maximum temperature fluctuation). Temperature sensors, on- and off-membrane (near the circuitry), show an excellent thermal isol
4、ation between the heated membrane area and the circuitry-area on the bulk chip (chip temperature rises by max 6 at 350 membrane temperature). A logarithmic converter was included to measuring the SnO2 resistance variation upon gas exposure over a range of four orders of magnitude.An Analog Hardware
5、Description Language (AHDL) model of the membrane was developed to enable the simulations of the complete microsystem.Gas tests evidenced a detection limit below 1ppm for carbon monoxide and below 100 ppm for methane. Key Words: metal-oxide gas sensors； analog IC design；AHDL；CMOS compatible micromac
6、hining； microhotplates 1. Introduction There is a strong interest in CMOS-based microsensors and, in particular, in micro-hotplate-based gas sensors, since miniaturization and the possibility of monolithic integration of transducer and circuitry offer significant advantages such as lowpower consumpt
7、ion, potentiall low costs, and the possibility of applying new dynamic sensor operation modes. Tin dioxide (SnO2) is a widely used sensitive material for gas sensing in ambient air. Tin dioxide is heated to temperatures between 200 and 350 and changes its conductivity upon gas exposure.To achieve su
8、ch high operation temperatures with the lowest possible power consumption, micro-hotplate struc- tures have been developed for metal oxide gas sensors during the past few years. A micro-hotplate consists of a thermally isolated area with a heater structure, a temperature sensor and contact electrode
9、s for the sensitive layer. Many micro-hotplates were termed“CMOScompatible”,but only a few have been produced in standard CMOS processes. Micro-hotplate-based gas sensors systems were implemented in a multi-chip approach.the micro-hotplates were placed on one chip, and the necessary driving and sign
10、al conditioning circuitry was hosted by a separate Application Specific Integrated Circuit (ASIC). It was established that integrating micro-hotplates and the corresponding circuitry on the same substrate is feasible without over-heating the electronics.Additional features of the monolithic solution
11、 include low power consump- tion, low noise, reliable electrical signals, high yield, and low production costs as compared to a multi- chip approach. The integration of multiple gas sensors in sensor arrays, the use of multi-component analysis algorithms, such as principal component regression (PCR)
12、 or artificial neural networks(ANN), and the application of new dynamic sensor operation modes 14, 15 help to overcome the problems associated with poor selectivity and drift of individual gas sensors 16. 2. System Description 2.1. Membrane Figure 1 shows a micrograph of the microsensor system.The c
13、hip was fabricated using a standard doublepoly,double -metal, 0.8mCMOS-process as provided by 17. The chip hosts a micro-hotplate (close-up in Fig. 2), which is thermally isolated from the rest of the chip by placing it on a very thin membrane with low heat- conductivity. The membrane consists of th
14、e dielectric layers (silicon oxide/nitride) and features an additional N-well island (no connection to the bulk), which remains after applying a potassium-hydroxide (KOH) wet-etch with an electrochemical etch stop 18. After wet etching, the SnO2 sensitive layer is deposited on the membrane as a nano
15、crys- talline thick film (crosssection in Fig. 3). The membrane is 500 by 500 m in size with a heated area (micro-hotplate) of 300 by 300 m. The heated area exhibits a 6 m-thick N-well silicon island underneath for homogeneous heat distribution and membrane stiffening. The membrane features a polysi
16、licon ring-heater, which provides symmetric heat generation and dissipation. Four resistive polysilicon temperature sensors (Fig. 2) are integrated on the membrane to monitor the heat distribution. The polysilicon temperature sensor located in the center of the membrane provides input for the temper
17、ature control circuitry. The experimentally assessed maximum temperature variation across the membrane is 12% up to 350, and the membrane power efficiency is 4.8/mW. 2.2. Circuitry A schematic of the chip is shown in Fig. 4 as a block diagram.The circuitry can be grouped in three functional units: (
18、i) Membrane temperature control loop, (ii) bulk chip temperature measurement, and (iii) SnO2 resistance measurement. The chip is biased with an external current source (Hewelett Packard 3245A Universal Source), which is temperature-independent. This bias current is then copied and used for biasing t
19、he membrane temperature sensor. Fig. 1. Micrograph of the integrated chemical sensor chip. Fig. 2. Close-up of the microhotplate. F Fig. 3. Cross-section of the microhotplate. Fig. 4. Block diagram of the monolithic sensor system. Fig. 5. Proportional temperature controller. The proportional tempera
20、ture controller (Fig. 5) is implemented with an operational amplifier 20, 21 and an internal stabilization capacitor of 8 pF. The membrane temperature is controlled from room tem- perature up to 350. The operational amplifier drives a power transistor, which provides the current to the polysilicon h
21、eater (RHEATER). The inputs of the operational amplifier consist of the control voltage (VCONTROL) and the voltage drop on the polysilicon temperature sensor (RTEMP SENSOR), which provides the feedback signal for the temperature controller. The polysilicon temperature sensor is biased with a tempera
22、ture-independent current (IBIAS). The aging effects in the polysilicon temperature sensor cannot be compensated by the electronics on chip. The dominant pole of the temperature control system is determined by the thermal time constant of the membrane, which is approximately 20 ms. An on-chip stabili
23、zation capacitor was included to cancel the low frequency pole of the operational amplifier. The main advantage of the proportional temperature controller is that it occupies minimum silicon area. The most important trade-off in the design of the proportional controller is between its stability (pha
24、se margin) and the steady-state error. When the open-loop gain of the controller increases, the phase margin and the steady-state error decrease and vice versa. A good trade-off between stability (phase margin) and steadystate error was found to be an open-loop gain of 76 dB, with a phase margin of
25、approximately 88?. The main source of error for the temperature controller is the steady-state error introduced by the offset voltage of the operational amplifier, which is independent of the open-loop gain of the system (i.e., the steady-state error introduced by the offset voltage of the operation
26、al amplifier does not decrease when the open loop gain increases). During calibration of the sensor, the offset voltage is compensated for at room temperature (i.e., the steady state error is compensated for at room temperature), but the temperature dependence of the offset voltage remains. The stea
27、dy-state error measured over the sensor system operation temperature range (ambient temperature between ?40 and 120) is less than 1% of the membrane temperature. The bulk chip temperature is assessed via the baseemitter voltage difference between a pair of diodec- onnected vertical PNP transistors (
28、parasitic transistors available in the CMOS process, collectors tied to substrate) working at different current densities (Fig. 6). The resistance of the SnO2 sensitive layer can vary over a wide range (up to six decades) and is measured from 1 k_hm to 10 M_hm using a logarithmic converter(Fig. 7),
29、which was implemented with a voltageto- current converter and a pair of diode-connected vertical PNP transistors. The dc level of the logarithmic converter can be changed with the reference current (IREF). The bulk chip temperature is used to compensate for the temperature dependence of the logarith
30、mic converter. The circuitry is located at 1 mm distance from the micro-hotplate owing to packaging design consid- erations. A partial epoxy cover shields the circuitry and the bond wires from the environment, but en- ables free access of the analyte gas to the gas-sensitive material on the micro-ho
31、tplate. VBE = VBE1 - VBE2=VT .(I40A /I10A) VBE= VBE1 - VBE2=VT .(ISnO2/IREF) Fig. 6. Bulk chip temperature measurement circuitry. Fig. 7. Logarithmic converter Fig. 8. Membrane modeling using AHDL. 3. Simulation Results The ADHL model was used for simulating the stability of the temperature controll
32、er. Figure 9 shows the Bode plots (magnitude and phase) of the output of the control system (voltage drop over the polysilicon temperature sensor). The phase margin is around 88?. The dominant pole at 8Hz is resulting from the thermal time constant. Transient simulations were done using a sinusoidal
33、 variation of the control voltage with an amplitude of 800mVaround a common mode of 1Vand a frequency of 0.1 Hz (Fig. 10). Such low frequency of the control voltage was selected in the simulations because this frequency is commonly used in the temper- ature modulation mode of hotplate-based gas sens
34、ors (speed of the chemical reaction at the interface). The simulations show, that the circuitry can drive the membrane temperature to 350 without distorting the shape given by the control voltage. When the control voltage is lower than the voltage drop across the polysilicon temperature sensor at am
35、bient temperature (around 1 V at 27), the membrane temper- ature equals ambient temperature. The simulations also show, that the simulated membrane temperature is 350 for a control voltage of 1.8 V, which results in an error of 2% in comparison to the measured membrane temperature (Fig. 11). 4. Expe
36、rimental Results 4.1. Electrical Measurements The tracking mode performance of the temperature controller was measured and is shown in Fig. 11. The measurement was done at room temperature, and the control voltagewas increased in steps of 10mV.Acontrol voltage of 1.80 V produced, e.g., a membrane te
37、mperature of 343. The measured sensitivity was about 0.63/mV. The controller showed an excellent performance with a resolution of 0.5, which was assessed at the center temperature sensor of the membrane (temperature inhomogen- ities over the membrane were not taken into account). The dynamic behavio
38、r of the temperature controller was assessed by subjecting the membrane to a temperature step from 27 to 300. The temperature controller was found to be very stable showing no overshoot or ringing. The performance of the temperature controller in the stabilization modewas also assessed (Fig. 12). Th
39、e ambient temperature was ramped from ?40 to 120 in steps of 5. A control voltage of 1.77 V was applied, which produced a membrane temperature of 330.9.The steady-state error measured over the operating temperature range is less than 1% of the membrane temperature. The performance of the temperature
40、 sensor on the bulk chip was also measured. The ambient temper- ature was swept from ?40 to 120 in steps of 5, and the temperature controller was switched off. A twopoint calibration at ?20 and 85 was performed. The measured sensitivity was about 128 V/, and the resolution was 1.5. 4.2. Chemical Mea
41、surements Gas test measurements were carried out with various concentrations of carbon monoxide (CO) and methane(CH4) in air. The CO concentrations ranged between 10100 ppm (Fig. 14), and the CH4 concentrations between 75010000 ppm (Fig. 15). The measurements were performed at relative humidities (r
42、.h.) of 0 and 50%. The sensor was operated between 250 and 350 in steps of 50. The gas flow rate was 0.2 liters per minute for all measurements. The exposure time of 30 min to the analyte gas is sufficient to achieve stable steady-state signals (response time approximately 30 s). A recovery of the s
43、ensor is reached with- in 15 min. The exposure time to pure carrier gas was set to 30 min to be on the safe side.The sensor resp- onses to CO and CH4 show different temperature dependence, which facilitates the quantitative detec- tion of these compounds in a mixture using multi-component analysis m
44、ethods. Detectability of less than 1 ppm CO and less than 100 ppm CH4 enables the use of this chip in a wide range of applications. 5. Conclusion and Outlook A monolithic gas sensor system fabricated in standard 0.8mCMOS process combined with post-CMOS micromachining was presented. It comprises meta
45、l-oxidecovered (SnO2) micro-hotplates and the necessary driving and signal-conditioning circuitry on the same chip. The on-chip proportional temperature controller can accurately adjust the membrane temperature up to 350 with a resolution of 0.5. The steady-state error measured over the operating te
46、mperature range is less than 1% of the membrane temperature. The temperature controller is very stable and does not show any overshoot or ringing. The steady-state error in the operation temperature range can be further decreased if a proportional-integral-derivative (PID) temperature controller is
47、used. The integral part will decrease the steady-state error, but it will also slowdown the time response of the controller. This problem can be addressed with the derivative part of the controller. The modeling of the membrane in AHDL can be used for design and simulation of the temperature control
48、ler without the need of changing the simulation environment. The logarithmic converter enables resistance measurements of the SnO2 sensitive layer between 1 k_hm and 10 M_ with an error of 1%. Gas tests yielded limits of detection below 1 ppm CO and below 100 ppm CH4. The problems associated with poor selectivity and drift of individual gas sensors will be o