Survivability of a silicon-based microelectronic gas-detector structure for high-temperature flow applications

r.

ELSEVIER

Sensors and Actuators B 37 (1996) 27-35

CHEMICAL

Survivability of a silicon-based microelectronic gas-detector structure for high-temperature flow applications Sanjay V. Patel ", Michael DiBattista a, John L. Gland a,b Johannes W. Schwank

"'*

a Department of Chemical Engineering, University of Michigan, Ann Arbor, M! 48109-2136, USA b Department of Chemisty, University of Michigan, Ann Arbor, M! 48109.2136, USA Received 15 May 1995, revised 17 May 1996; accepted 24 May 1996

Abstract

This investigation addresses the important question of whether or not silicon-based micromachined chemical sensors are a viable option for gas sensing in harsh, high-temperature flow applications such as automotive exhaust. Data are presented on the thermal and mechanical stability and long-term functionality of micromachined silicon devices containing ultra-thin Pt/TiOx films supported on a heated muitilayer silicon oxide/silicon nitride membrane. These gas detectors were originally designed for use in vacuum applications such as reactive ion etching systems. Significant modifications in device structure and materials are required to adapt these sensors for use in harsh thermal and chemical environments at elevated pressures. To test the long-term structural integrity of the sensors, they are subjected to a test protocol including pressure fluctuations, thermal shock, and mechanical vibrations. For characterization purposes, electrical resistance measurements, optical microscopy, atomic force microscopy (AFM), and Auger spectroscopy have been used. Our results indicate that properly designed micromachined silicon structures can survive long-term operation at high temperatures in ambient air, and can withstand rapid fluctuations of temperature, pressure, and flow rate. Keywords: Structural stability; Survivability; Microelectronic sensors; High temperature

1. Introduction

The sensor literature contains many accounts of sensor responses collected under mild and well-controlled conditions, ranging from vacuum to static air. However, there is a rapidly increasing need for chemical sensing in harsh environments, such as automotive exhaust systems. Cost considerations and ease of mass production make micromachined silicon-based sensors an attractive alternative to conventional chemical-sensing technologies. However, the viability of silicon-based sensors is still being questioned by the automotive industry. A major concern is the lack of data on the thermal and mechanical stability and long-term functionality of such silicon devices. Chemical sensors for industrial or automotive exhaust environments have to function reliably under harsh conditions of high gas-flow rate, accompanied by large temperature and pressure fluctuations and mechanical vibrations. In addition, these types of sensors will be exposed to contaminants and particulates from which they need to be protected by adequate packaging. * Corresponding author. Phone: + 1 313 764 3374. Fax: + ! 313 763 0459. 0925-4005/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved PI! S 0 9 2 5 - 4 0 0 5 ( 96 ) 0 1 9 7 7 - 6

As far as automotive exhaust emissions are concerned, only oxygen sensors have been implemented on a large scale. The Clean Air Act [ 1] provides a major motivating force in the development of reliable sensors for monitoring the emission of gases such as CO, NO~. ~nd uncombusted hydrocarbons. The Environmental Protection Agency (EPA) and the California EPA Air Resources Board (CARB) have defined various periods in the future where the emission rates of hydrocarbons, CO, and NOx have to be significantly reduced [ 2]. A greater number of cars produced in the USA will have to be 'zero emission' vehicles (ZEV). The EPA's On Board Diagnostic Act-II (OBDII) demands a diagnostic of all components of the engine-management system with a depoilution function. Also required is the detection of engine misfire, which allows a large amount of uncombusted hydrocarbons through the exhaust system [ 3 ]. Recent advances in silicon microfabrication technology of gas sensor~ represent great opportunities to address the challenging 'problem of in situ monitoring of industrial gas emissions ar,d automotive exhaust gases. This technology could be of great importance in the near future, not only for traditional oxygen sensing in automotive engines, but also for carbon monoxide, hydrocarbons, and toxic-gas detection in

28

S. V. Patel et al. / Sensors and Actuators B 37 (1996) 27-35

Table l Present and potential applications for chemical microsensors (adapted from [4l) Application

Examples

Environmental monitoring of toxic chemicals

Trichloroethylene in air, soil, and ground water

Determining the performance of expensive equipment

Transformers, turbines, airplanes

Internal combustion engine exhaust analysis

0:, NO:, CO. hydrocarbons

Inexpensivesensorsfor

CO, natural g~

home hazmds

Critical monitoring of

chemicalprocesses Researchin chemical reactions

Material separation, manufacturing and waste stream control Reactive flows

the home and in industrial facilities. Table 1 displays some of the potential applications for silicon-based microelectronic gas detectors. Current oxygen sensors require adsorption and diffusion of oxygen into the bulk of solid electrolytes at elevated temperatures. Such diffusion-based gas-sensing methods tend to have slow response times and suffer from hysteresis. Miniaturized gas sensors relying on surface adsorption of gases on detecting films tend to be much faster in response, but become vulnerable to oxidation, physical degradation, sintering, and poisoning. Miniaturization made possible by state-of-the-art silicon micromachining brings the additional advantage of accurate temperature control due to the low thermal mass of the sensing film. Furthermore, the case of manufacturing of silicon-based devices brings the unit cost itlto a range where the deployment of distributed sensors or sensor arrays becomes economical. Sensing arrays offer the possibility to differentiate between the components of complex gas mixtures, giving sufficient numbers of measured variables to solve for the unknowns. While enormous progress has been made in integrated silicon-based sensors for pressure, flow, acceleration, and temperature, the implementation of chemical microsensors has lagged behind. Since chemical sensors operate on the basis of chemical interactions between the environment and the sensor surface, a host of surface and interface phenomena, many of them not yet fully understood, need to be taken into consideration. The inherent complexity of the interactions possible between a chemical agent and a sensing device is compouvded by the multivariant analyses required to deconvolute the resulting electrical signals. Furthermore, it is yet to be established whether or not silicon-based chemical senso~s can meet the long-term stability and survivability requirements for industrial and automotive exhaust conditions. The challenge is not only to develop an active sensing element that can function over many years of exposure to harsh conditions, but also to design and implement a durable and reliable device structure and protective packaging. For all

practical purposes, placing a silicon-based chemical microsensing device into an industrial effluent or automotive exhaust stream corresponds to insertion of an active integrated circuit into this hostile environment. In order to use the detector for such demanding applications, it must be reliable for several years and able to withstand an ever-changing atmosphere including moisture and potential surface contaminants. The device must be physically rugged to withstand vibrations and temperatures extremes from below zero to well over 600°C. The packaging must protect the sensing films from bombardment by particulates, such as dust or soot. This paper addresses the issue of device stability, singling out some of the effects of individual contributing factors that are of concern in an industrial or automotive exhaust environment. Specifically, the effects of temperature fluctuations, pressure fluctuations, and mechanical vibrations are explored. This is a first step towards probing the devicc stability and functionality in more complex environments where all these and additional factors are simultaneously present, possibly compounding each other.

2. Sensing-device structure The sensing-device structure is based on technology originally developed by Johnson et al. for monitoring of ppm amounts of oxygen in a CF4 background under high-vacuum conditions [5 ]. To adapt this type of device for atmospheric pressure-flow e~plications at high temperatures required substantial redesign and structural modifications to improve the thermal and mechanical stability. The integrated microelectronic gas sensors are produced with an eight-mask process, involving several lithographic and micromachining steps [ 59]. On a 4 inch silicon wafer, 500 sensing devices can be produced. Fig. l shows magnified top views of a device mounted and wired for operation. The close-up view of the central region of the device shows the major features that are required for operation. A schematic cross section of the J~,i~:e is shown in Fig. 2. The device fabrication process begins with a silicon wafer which is patterned for deep boron diffusion on the front- and backsides. The frontside pattern defines the meandering heater, two temperature sensing resistors (TSRs), and an outline for a dielectric membrane. The backside boron diffusion defines a 300 to 500 lam tall support rim, which remains during the final etch step when most of the rest of the wafer is removed. After the boron diffusion, the dielectric membrane is deposited on the frontside by low-pressure chemical-vapor deposition of three layers of silicon dioxide (SiO2), silicon nitride (Si3N4), and SiO2. The dielectric membrane is 1 mm in diameter, 1.3 p,m thick, and becomes transparent after the final etch. This three-layer design offers stress compensation by layering SiO2 which is compressive and Si3N4 which is tensile. The SiO2/Si3N4 ratio has been carefully optimized to provide a stress-relieved membrane at 2(X)-A00°C. After the dielectric membrane is deposited, con-

S. V. Patel et al. / Sensors and Actuators B 37 (i 996) 27-35

29

Fig. I. Top views of the Universityof Michigangas sensor. =a,-,,-,ai,~,,

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the contact metallization, the sensing film region, conductance electrodes, and bonding pads are deposited sequentially on top of the membrane to complete the structure. Typical detectors like the one shown in Figs. 1 and 2 contain a thin film that is discontinuous and deposited by evaporation as 65/~ Ti and 35/~, Pt. Typical film resistance values vary between 20 and 75 IL Layers of 250 ~ Ti under 1200/t Pt are deposited to define the four-point probe electrode regions. The final step is an ethylenediamine pyrocatechol (EDP) etch. This removes the silicon from the backside and defines the heater and TSRs such that they are suspended from the dielectric membrane, which itself is supported by the thick silicon rim. The as-deposited films contain metallic titanium covered by a thin over-layer of platinum. These thin films also tend to be contaminated by carbon residue from the clean-room manufacturing process. Fig. 4 shows the Auger electron spectra of a sensing film as deposited and after 150 s of argon sputtering. Sputtering a small area of the sensing-film surface removes the carbon-containing residues in that area and reveals the platinum, titanium, and oxygen in the sensing film. The freshly fabricated devices show poor gas-sensor

&Reslslance

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100

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150

Temperature (*C) Fig. 3. Temperaturecoefficientof resistance. The sensors were heated and cooled repeatedly in an oven while the heater and temperature sensing resistor (TSR) resistances were measured. The slope of the lines is the temperature coefficient of resistance (TCR). The average TCR is 1858 ppm °C-=

tact channels are cut from the top through the membrane to the boron-doped regions of the heater and TSRs. Typical resistances of the boron-doped heaters are in the range 450600 l~ (and 180-300 l~ for the TSRs) with a temperature coefficient of resistance (TCR) of ! 858 ppm °C- ~, as shown in Fig. 3. The boron-diffused silicon heater is capable of rapidly heating the detecting film to well over 1000°C with minimal power. The heater and TSR contact channels are filled with metal to ensure good electrical contact. Following

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Electron Energy (eV) Fig. 4. Auger electron spectra of the sensing fihn as-deposited, and after 150 s of argon sputtering. Sputtering removes carbon residues, revealing well-defined platinum,titanitimand oxygenpeaks.

30

S. V. Patei et al. / Sensors and Actuators B 37 (1996) 27-35

response until the carbon residue is removed. The most likely source of carbon contamination is the EDP etch solution used to etch the window structure. Heating in oxygen at 500°C is required to burn off this carbon contaminetion completely. This treatment also partially oxidizes the titanium, disperses the platinum overlayer, and creates a roughened surface morphology with increased specific surface area, which is beneficial for gas sensing. After completion of the microfabrication process the sensots, called dies at this stage, are separated from the bulk wafer by severing thin silicon connections between each of the 500 dies with a sharp die-separation tool. Each individual sensor die is then mounted, with vacuum epoxy applied to the bottom of the die, on a commercially available 10-pin "1"O5header. After curing the epoxy, the bonding pads on the devices are connected to the TO5 header using a standard gold-wire bonding tool. The device is then ready to be used as a sensor, It should be noted that these devices are moun*ed on TO5 headers just for our testing purposes. The TOS header, epoxy mounting technique, and gold wire bonds are not intended to be used in commercial applications.

3. Sensor operation The sensing-film temperature is controlled by a proportional-integral controller using the TSR resistance as an input measure of the device temperature, and the current sent to the heater as the output control parameter. The sensing film can be operated either isothermally or in a temperature-programmed mode. In the isothermal mode of operation, inert gases, regardless of their temperature, will not cause changes in film resistance, as the temperature-control system can easily compensate for any heat loss by convection. The sensor will only respond to gases chemisorbing or reacting on the surface, causing a change in film resistance. In the temperature-programmed mode of operation, the device may be operated using preprogrammed temperature ramp and hold steps suitable for temperature-programmed desorption (TPD) studies. For example, tint a device may be ramped in an inert atmosphere between two known temperatures, generating a baseline of data relating the sensing-film resistance to temperature. Gases may then be dosed, and a second temperature ramp can be performed between the same two temperatures. The difference between the two data sets may be used to identify a particular gas s~cies or concentration by the direction, magnitude, and temperature region of the resistance deviation from baseline. The flexibility to operate the sensor over a wide temperature range allows optimization for specific gases by selecting an appropriate film composition and temperature regime where high surface coverage of the gas of interest is observed. In the case of Pt/TiO, thin films, thermal treatment of the film in either oxidizing or reducing atmospheres can be used to modify the microstructure and surface reactivity [5-9]. For example, a continuous Pt film can be broken up into

discrete Pt particles, and the TiO,, surface can be exposed by thermal treatment in oxygen e.t 750°C. There is a vast body of literature regarding surface coverages and temperatureprogrammed desorption of species such as hydrogen and oxygen on platinum and other surfaces. Chemisorbed gases tend to desorb within a fairly narrow temperature regime, where there is sufficient activation energy for breaking the bonds between the surface atoms and the adsorbed species. These gas sensors are typically operated in temperature regimes where high surface coverage with hydrogen or oxygen has been reported in the surface-science literature. The surfacecoverage measure is an indirect one, utilizing sensor resistance changes (i.e., deviations from the inert-atmosphere baseline) as the indicator for relative surface coverage. The challenge here is that temperature-programmed desorption from the sensors cannot be measured directly due to the extremely small size of the surface and the resulting small amount of chemisorbed gas.

4. Chemical-sensing, experiments Sensors operating under atmospheric conditions have to be able to detect pollutants in the presence of a large nitrogen background. Ideally, no sensor response should occur when the nitrogen flow rate fluctuates. To test the devices for inert behavior in nitrogen, several sensors were set to various fixed temperatures and the flow of room-temperature nitrogen past the sensors was changed in a stepwise fashion from no flow to a variety of flow rates even higher than those encountered in an automotive exhaust stream. The results shown in Fig. 5 prove that the sensing-film resistance did not respond to nitrogen flow-rate changes. Furthermore, the temperature-control system was able to maintain the sensor-film temperature precisely at the setpoint ( 5: 0.7°C), even when the flow rate 17(

5000 Roslslanceat 2000C

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s. v. Patei et el./Sensors and Actuators B 37 (1996) 27-35

Of N 2 w a s suddenly changed from 0 to 2000 sccm. The nitrogen flow e x p e r i m e n t s arc important to e~tablish a baseline for future experiments where the flow rate of gases may change around the sensor. Chemisorption and surface reaction of gases such as hydrogen and oxygen on discontinuous Pt/TiO~ thin films can alter the film conductance due to changes in either charge-carrier concentration or mobility. In addition, conductance changes in systems containing highly dispersed metal particles such as Pt in contact with reducible oxides such as TiOx can be caused by partial reduction or oxidation of the oxide. Heating the sensors in the presence of oxygen leads to an increase in film resistance [6]. When 1% hydrogen is added to an oxygen/nitrogen stream, the sensor responds quickly, giving a decrease in film resistance (Fig. 6). The magnitude of the hydrogen-induced decrease in sensing-film resistance is a function of temperature. Fig. 7 shows a correlation of the resistance response to hydrogen and sensing-film temperature. The sensor response to hydrogen can be attributed to the partial reduction of the Pt/TiOx film to a more metallic state. In atmospheric pressure conditions, the Pt/TiO~ surface is an active catalyst for reactions between hydrogen and oxygen. Reactions between these two gases play an important role in the response mechanism of the sensor. Atomic hydrogen generated during the dissociative adsorption of hydrogen on the 195' 185'

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Time (s) Fig. 6. Sensorresponse to hydrogen. The sensing-filmresistance decreases with the introduction of hydrogen flow pulses (5 seem) into a stream of 25 sccm oxygenand 475 sccm nitrogen.

31

sensing-film surface may react with pre-adsorbed oxygen species, donating electrons and forming hydroxyl groups and/or water molecules. The increase in the free electron density resulting from electron donation by hydrogen decreases the film resistance. In the presence of oxygen, the baseline resistance of the sensor is shifted to higher values due to the filling of oxygen vacancies in the TiO~ lattice. Adding small amounts of hydrogen to an oxygen stream removes some of this lattice oxygen, causing the resistance to shift to lower values.

5. Thermal cycles If the sensor is to operate for long periods of time in a temperature-programmed mode, it must be able to withstand many temperature ramps. To determine the longevity of the heater and the membrane structure, a standard test cycle was used consisting of a 2 s linear ramp from room temperature to a predetermined peak temperature, followed by a 2 s span in which the heater power was cut and the sensor was cooled back to room temperature. To facilitate the simultaneous testing of up to 12 sensors, a thermal cycler apparatus was designed and built allowing for a wide range of sensor operating temperatures, and for various pressures and flow rates. The gas composition can also be varied as needed. Each sensor is connected by feedthroughs to a data-acquisition and control system. This allows individual peak temperatures to be set for each device, and to measure the resistance of the heater, sensing film, and temperature sensor simultaneously. The heater resistances are checked for drift from their original room-temperature values as an indicator for structural stability of the membrane. Two different groups of sensors were tested. The first group of eight sensors, as shown in Fig. 8, was cycled 408 000 times in static room air from 25°C to peak temperatures ranging from 185 to 671°(:. The second group of 15 sensors was heated isothermally in different flow rates of room-temperature air at temperatures ranging from 125 to 850°(:. ... 8(H~]

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Fig. 7. Sensor response vs. temperature. The changes in sensing-filmresistance vs. temperatureforthe 1%pulsesof hydrogenand nitrogen flowshocks are displayed.

31;9

404

434

474

51)4

573

67 I

Average Peak "l'¢nq~eralure (':'C)

Fig. 8. Drift in healer resistance during thermal cycling to indicated peak temperature in i arm air. Each device was cycledfromroomtemperature to the indicated average peak temperature in 2 s and cooled back to room temperature in 2 s.

32

S. V. Patel et al. / Sensors and Actuators B 37 (1996) 27-35 A

.~

2o II A S,mu,.o. I 1 I . Mo~u~od I

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O'~g . . . . . . - • 0 200 400 600 800 1000 Temperature (°C) Fig. 10. Deflection study. Measured and simulated deflection of the center of the membrane with temperature. The deflection angle for a 1000 p,m diameter membrane, with the center deflected by 15 p,m, is less than 2*.

Fig. 9. Top view of a thermally cycled sensor. The discoloration, as seen through the thin, transparent membrane, is a result of the redeposition of tht~

epoxy usedin the mountingprocess. This sensorwas cycled408 000 times to an averagepeaktemperatureof 67IOC. Structural examination of the devices in group 1 using optical microscopy and atomic force microscopy (AFM) focused on macroscopic device-failure modes such as cracks in the device or breakage of the wire bonds, and also on microstructural changes such as delamination of layers of the dielectric membrane and reconstruction of the thin films. The wire bonds showed no changes at all, due to their distance from the heaters. By visual inspection, no changes were noticed on the membrane. None of the membranes had cracked or broken. Microscopic examination, however, revealed discoloration of the membrane heated to 67 I°C, as shown in Fig. 9, which presents a top view of the device, with the discolored regions seen through the thin, transparent membrane. Possible explanations for the discoloration are the incomplete combustion of process residues or the vaporization and redeposition of epoxy used for mounting the device to the TO5 header. The sensors heated to lower peak temperatures showed no such discoloration, Long-term thermal cycling caused only some initial drift in the heater resistance, most likely due to contact oxidation or surface oxidation of the boron-doped silicon heater. The contacts to the heater are titanium and iridium, and may become oxidized after long periods of time and exposure to heat. Since the heater resistance measurements are two-point measurements, any changes in the metals attached to the boron-doped heater may influence the measured value. On all devices there was zero drift during the last 100 000 heating and cooling cycles, proving that the devices had stabilized. As the membranes were ramped up to the peak temperature and rapidly cooled back to room temperature, slight flexing was observed by magnified visual inspection. The flexing was not severe enough to break any of the membranes. Examination under a microscope revealed that at 773°C a maximum deflection of 15 p,m occurred at the center of the membrane (Fig. 10). Since the membrane has a diameter of 1000 p,m, a 15 Ixm center deflection results in less than 2° deflection

angle. Our experimental observations are in excellent agreement with a finite-element simulation [ 10 ] using a computeraided engineering tool for mieroelectromcchanical systems (CAEMEMS-D) developed by Crary et al. [11-13]. This simulation package integrates design-analysis tools and contains four principal components: a database for storing materials properties and process model parameters, a process modeler which reads mask and process information and produces a model for structures to be analyzed, a visualization and design verification feature, and a device modeler that performs finite-element simulations. The temperature dependence of membrane stress is derived from the equation T = ( ! - v ) o ' / E a , where Tis the temperature, v is Poisson's ratio, o" is the internal stress in the membrane, and a is a thermal expansion coefficient. The sensors in group 2 were held isothermally at various temperatures and different flow rates of dry air passed into the sensor test chamber at room temperature. The sensors showed excellent durability of the heater and temperaturesensing resistor (TSR). Up to temperatures of 650°C, there was only a slight drift of the resistances of the heater and TSR over approximately 100 h. For example, in the low-temperature range between 120 and 150°C, five sensors tested showed negligible TSR and heater resistance drift even when the air flow was as high as 1175 seem. Five sensors tested for 97 h in 50 sccm air flow at 213-465°C showed less than 1% drift in TSR and heater resistances. Heating at 576°(2 for 71 h at 100 sccm air led to a 2% drift of both TSR and heater resistances. After 141 h at 644°C in 250 sccm air flow, there was no drift in TSR, but approximately 4% drift in heater resistance. Heating for 141 h at 850°C under the same flow conditions resulted in a decrease of TSR by 10%, while the heater resistance increased by almost 50%. The dielectric membrane on this sensor showed the same type of discoloration as on the sensor that was thermal cycled to 671 °C. This clearly shows that long-term exposure to such a high temperature causes changes in the heater, most likely due to the growth of an oxide layer on the silicon heater and temperature-sensing resistors and oxidation of the metal contacts to these structures. The scanning force microscopy analysis in Fig. 11 shows two regions of the structure of the dielectric window. The two regions are (a) the cool area away from

S. V. Patel et aL /Sensors and Actuators B 37 (1996) 27-35

o

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2.00

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(a)

(b)

Fig. ! I, Scanningforce microscopyimageof the sensor window. (a) Coolregionaway from the heater,with polymeri¢siduedecoratingthe S i O 2 surface. (b) Hot regionabovethe heater, after removalof the I)olymerresidue.The sensorwas heated isothermallyat 850°Cfor over 100 h. The SiO2particlesare the same size in both legions, the heaters and (b) the hot region of the window near the heaters and in the discolored region. The window in both images shows no signs of reconstruction or damage. This analysis also leads us to believe that the discoloration seen on the hottest sensors was due to a redeposition of the adhesive material used to bond the sensors to the TO5 electrical mounts.

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6. Pressure shocks Sensors in industrial or automotive exhaust streams must be able to survive fluctuations in pressure and flow conditions. A conventional flow system with stainless-steel sample chamber, mass-flow controllers, and Baratron pressure gauges was used to test the survivability of the sensor devices. Ventilation of the underside of the membrane is important to eliminate large pressure gradients across the membrane. Two sets of experiments were performed. The first set involved sensors at room temperature exposed to air also at room temperature. A typical experiment involved pumping down the sensor chamber to 133 Pa, followed by a sudden increase of the chamber pressure to 373 000 Pa. The time required to equilibrate fully the pressure between chamber and inlet lines depended on the peak pressure. It took 0.6 s to reach 0.1 MPa, and 4 s to reach 0.373 MPa. The heater resistance was used to monitor the integrity of the membrane during pressure cycles. For a sensor having a room-temperature heater resistance of 442 f'Z at a pressure of 133 Pa, the resistance increased by 21~ at the peak pressure of 0.373 MPa. The resistance change was completely reversed when the pressure was lowered. A second set of pressure-shock experiments was performed on sensors that were controlled isothermally at various temperatures to study the survivability of hot membranes.

Fig. !2. Temperaturecontrol summary.As a result of the pressure testing, the temperatureresponseof the sensorsto air was studied. A I°C deviation at room temperaturewas the maximumdeviationobserved. Another objective of these tests was to examine the ability of the membranes to maintain constant temperature in the presence of pressure fluctuations. The device was found to survive a wide range of pressure changes. Similar to the first set of pressure tests, the sensors were exposed to cycles from 133 Pa to a maximum of 0.308 MPa. After completing each pressureshock experiment at a given temperature, the film temperature was raised, and the pressure shocks were repeated. Fig. 12 displays the disturbance in control caused by the pressure shocks. The widest fluctuations in temperature never exceeded l°C and occurred with a sensor near room temperature and at the highest temperatures. No heater or TSR resistance drift was observed.

7.

Vibrations

In order for the microelectronic gas sensors to operate in an industrial or automotive environment, they will have to

34

$. V. Patel et ai. / Sensors and Actuators B 37 (1996) 27-35

Fig. 13. Vibration tester with five sensors.

Fig. 14. Five.sensor vibration tester on an engine.

endure vibrations. As an extreme case in automotive applications, the sensors would be exposed to vib:ations caused by the engine, exhaust, and suspension systems. The device structure components of interest are the wire bonds, the dielectric mer,'~rane, and sensing film. Vibration tests were performed in the engine compartment era four-wheel drive sport utility vehicle. A special, bracket-mounted test chamber that attached directly to the engine block of the test vehicle pro(a)

vided direct exposure of the devices to engine vibrations, as well as vibrations due to day-to-day driving. The test chamber is shown in Figs. 13 and 14. The stiff mounting of the sensor housing directly on the engine exposed the sensor not only to the higher-frequency vibrations from engine activity but also to the vibrations transmitted from the road during driving. The criteria for a successful survival of the vibration tests were the absence of any cracks in the dielectric membrane and the integrity of all wire bonds on each sensor. Sensors were visually inspected and photographed prior to, and at certain intervals during, the vibration test. A compound microscope with a 35 mm camera attachment was used for this purpose. Two sets of five sensors were used for vibration testing in a specially designed chamber capable of housing five individual sensors. The chamber was constructed from aluminum and contained five Teflon sockets serving as electrical feedthroughs. In an initial test, all five sensors from group 1 survived a road test of 203 miles of mostly city travel. The second group of five sensors was inspected five times during 2363 miles worth of highway and city travel. None of the sensors was damaged during the vibration testing. This was established by visual comparison of optical micrographs; no cracks were observed, and all wire bonds stayed intact. As a second measure of survivability, the probe and heater resistances were measured. Maximum film and heater resistance drifts of 4.8 and 0.5%, respectively, were observed. Such slight changes in film resistance are not surprising in view of the exposure of the film surface to moisture and air at high engine temperatures. Fig. 15 shows that no damage has occurred to one of the dielectric membranes after 2363 miles of vibration testing.

8. Conclusions The results presented here show great promise for the eventual use of silicon-based integrated seesors in conditions (b)

after 0 mi. after 2363 mi. Ftg. 15, Vibration-tested sensor. The sensors survived 2363 miles of vibration testing with no damage to their structures. They were mounted on the engine of a sport utility vehicle.

s. v. Patei et al. / Sensors and Actuators B 37 (1996) 27-35

ranging from vacuum to elevated pressure, high-temperature flow conditions. This work represents an important step to determine whether or not silicon-based chemical sensors can meet the long-term stability and survivability requirements for the extreme case of automotive exhaust conditions. Although the sensors proved to be remarkably stable when exposed to individual conditions, it is not clear to what extent the environmental variables will compound each other when present simultaneously. It is also clear that adequate packaging must be explored.

Acknowledgements

The authors would like to acknowledge Dr Kensall Wise for his help with the manuscript and the work of Dr Yafan Zhang and Dr Paul Bergstrom with regards to the deflection study. We would also like to thank Dieter Schweiss, Diane Moon, Kamal Nainani, Herman Surjono, and Eric Bohnert for their help with the experiments.

References

[ ! ] 1990CleanAir Act EPA400-F-92-013 August 1994,Fact Sheet OMS-

35

Biographies

Sanjay V. Patel received his B.S. degree in chemical engineering from the University of California at Berkeley, Berkeley, CA, in 1992, and his M.S. degree in chemical engineering from the University of Michigan, Ann Arbor, MI, in 1993. He is a member of the American Institute of Chemical Engineers, the Materials Research Society, and the American Chemical Society. His research interests include characterization of chemical interactions with thin films for gas-sensing applications, He expects to receive his Ph.D. degree in 1998, also from the University of Michigan.

Michael DiBattista received his B.S. and M.S. degrees in chemical engineering from the University of Michigan, Ann Arbor, MI, in 1992 and 1994, respectively. He expects to receive his Ph.D. degree in 1998 from the University of Michigan. He is a member of the American Institute of Chemical Engineers, the Materials Research Society, and the Microscopy Society of America. His research interests include microscopic investigations of microfabricated device structures and structural characterization of thin films for g,~, sensors.

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[2] B. Cooper, The future of catalytic systems, Automotive Eng., !00 (1992) 9-12. [3] J. Binder, New generation of automotive sensors to fulfill the requirements of fuel economy and emission control, Sensors and Actuators A, 31 (1992) 60--67. [4] R.C. Hughes, A.J. Ricco, M.A. Butler and S.J. Martin, Chemical microsensors, Science, 254 ( 1991) 74-80. [5 ] K.D.Wise, J.L. Glandand J.W. Schwank,in S. Semancik(ed.), Prec. NIST Workshop on Gas Sensors: Strategies For Future Technology, Gaithersburg, MD,, USA. 8-9 Sept 1993, NIST Special Publication,

865, 1994. [6] C.L.Johnson, J.W. Schwankand K.D.Wise, Integratedultra-thin-film gas sensors, Sensors and Acmators B, 20 (1994) 55-62. [7] C.L. Johnson, Ultra-thin film monolithically-integratedsilicon-based gas detectors, Ph.D. Dissertation, The Universityof Michigan (1990), pp. 22-33. 18] C.L. Johnson, J.W. Schwank and K.D. Wise, Ultrathin-fihn gas detector, US Patent No. 4 935 387 (4 Sept., 1990). [9] N. Najafi, K.D. Wise and J.W. Schwank, A micromachinedultra-thinfilm gas detector, IEEE Trans. Electron Devices, 41 (1994) 17701777. [10] Y. Zhang, Non-planar diaphragm structures for high-performance silicon pressure sensors, Semiconducwr Research Corporation Tech. Report No. 234, July 1994. [11 ] Y. Zhang, S.B. Crary and K.D. Wise, Pressure sensor design and simulationusingthe CAEMEMS-Dmodule,Tech. Digest, IEEESolidState Sensors and Actuators Workshop, Hilton Head Island, SC. USA. 4-7 June 1990, pp. 36-41.

[ 12] S. Crary and Y. Zhang, CAEMEMS:an integrated computer-aided engineering workbench for micro.electro-mechanical systems,Prec. IEEE Workshop on Micro Electro Mechanical Systems, Napa, CA. USA, 11-14 Feb., 1990, pp. 113-114.

[ ! 3] S. Crary,O. Jumaand Y. Zhang,Softwaretoolsfor designersofsensors and actuator CAE systems, Prec. 6th Int. Cm~ Solid-State Sensors and Actuators (Transducers '91), San Francisco. CA, USA. 24-28 June, 1991, pp. 498-501.

John L. Gland received his Ph.D. in physical chemistry from the University of California, Berkeley, in 1973. He joined the Department of Chemistry at the University of Michigan, Ann Arbor, and holds a joint appointment in the Department of Chemical Engineering. Dr Gland is the author of over 100 publications including three patents. He is a member of numerous professional societies, including the American Chemical Society, American Physical Society, and the American Vacuum Society. His current research interests include structure-reactivity relationships on model surfaces, advanced methods for characterizing surfaces, and surface chemistry on compound surfaces.

J o h a n n e s W. S c h w a n k received his Ph.D. in physical chemistry from the University on Innsbruck, Austria, in 1978. He joined the Department of Chemical Engineering at the University of Michigan, Ann Arbor, where he is now a professor. He also serves as director of the Center for Catalysis a~d Surface Science. Dr Schwank is the author of over 70 technical publications, including six US patents, and he has presented more than 90 papers at national and international conferences. He is a member of numerous professional societies, including the American Institute of Chemical Engineers, theAmerican Chemical Society, the American Society for the Advancement of Science, and the North American Catalysis Society. His present research interests include microelectronic gas sensors, heterogeneous catalysis, and surface science.