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CMOS Hydrogen Sensor
1 of 10 Fluence 541-390-9572
CMOS Hydrogen Sensor
Prepared by:
Steve Pyke
Peterson Ridge LLC (dba. Fluence) PO Box 1257
Sisters, OR 97759
541-390-9572
Abstract Peterson Ridge LLC (dba Fluence) has developed a silicon based CMOS FET (field effect transistor) sensor for hydrogen. The CMOS sensor technology described below has the potential for being part of a low-cost product. Sensitivity to hydrogen, resolution from interference sources and reliability is enhanced when the sensor is used in combination with two other CMOS FET sensors in an array. Additional cost reduction can be cost-effective for high-volume sales by the custom integration of the sensor with ASIC electronics for signal processing and communication. An ASIC approach to integrating sensor control, signal processing, analysis and communications can be produced in CMOS on silicon. Depending on the volume, this approach to system development can also be one of the lowest cost approaches to WAN and LAN available.
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The Fluence technology is an array of silicon-based field effect transistors (FETs) for the low-cost, selective, sensitive and reliable detection of hydrogen. Two FET sensors will have a catalytic gate and one will have an inert gate for temperature and humidity compensation. The two catalytic gate sensors are sensitive to and selective for hydrogen and insensitive to saturated and unsaturated hydrocarbon gases and vapors (e.g. methane, ethylene). The two sensors with catalytic gates will be produced side by side on the silicon wafer and physically identical in-so-far as can be achieved by the CMOS process. These two sensors will have identical sensitivities to hydrogen. This redundancy increases the product reliability. Any difference in the response of these two sensors would be a fault of either one or both sensors and will alert the user to a problem requiring his or her attention. The FET sensor with the inert gate will respond to temperature and humidity in the same way as the pair with catalytic gates. All three sensors will have very similar dependence on ambient temperature and humidity. The inert sensor will not respond to hydrogen, and a true hydrogen effect can be resolved from changes due to temperature and humidity interference and can be measured with greater accuracy, precision and reliability. Sensor Background The need for low-cost hydrogen sensors (Hoffheins et al, 1999) and gas sensors for combustible and toxic gases has supported a variety of technologies (Ho, 2001). The catalytic bead sensor is the oldest of the commercial combustible gas sensors. Any combustible gas including hydrogen is oxidized by atmospheric oxygen on a heated wire coated with a catalyst. The reaction increases the temperature of the wire and decreases its electrical resistance. The change in resistance is proportional to the concentration of combustible gas. A related sensor is the metal-oxide-semiconductor sensor. Metal oxide semiconductors (e.g. tin oxide, iron oxide) are electrically conductive as the result of oxide ion conductivity at high temperatures typically above 400C. These materials when pressed into pellet form and sintered at high temperature form the basis for a sensor which can detect combustible gases by the change to the electrical conductivity of the material. The technology was commercialized by Figaro Engineering in the early 1970s (Taguchi, 1972) and was reviewed (Morrison, 1982 and Kohl, 1989). The selectivity of the tin oxide Figaro sensors was very poor to start but has been improved by diluting the powdered starting materials with small amounts of catalyst (e.g. palladium, platinum and other metals) before sintering. Arrays of these sensors have been proposed for resolving the composition of gas mixtures using pattern recognition software for analysis, but temperatures above 300 C are required for the measurement. The large heater current requires large batteries precluding products such as small monitors powered by watch batteries. Another disadvantage is drift due to temperature due to field driven ionic migration and recrystallization of the sintered material. Electrochemical sensors were recently reviewed (Bakker and Telting-Diaz, 2002). These sensors produce a current from hydrogen and using atmospheric oxygen as the oxidant. The electrochemical sensor typically uses a liquid based electrolyte, typically water, and the operational temperature range is limited, and they cannot operate without a source of oxygen. Electrochemical sensors with solid oxide electrolytes have been reported recently (Martin, 2003), but like the Figaro technology, solid oxide electrolytes typically require high temperature for electrical conductivity. These technologies are limited by the need for high temperature or oxygen and sometimes both and a custom manufacturing process developed specifically for the sensor product. The Fluence CMOS FET sensor does not require high temperature or oxygen to detect hydrogen.
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CMOS FET Sensor for Hydrogen The manufacture of the CMOS sensor exploits the tightly controlled contract performance and quality specifications and the maturity and sophistication of the CMOS process and is the most important advantage for pursuing this technology for hydrogen sensors. The FET sensors can be combined with a CMOS application specific integrated circuit (ASIC) for sensor control and communication. The FET sensor has been the subject of research and development for thirty years beginning in Sweden and including recently the development of GaN FET sensors for the resolution and measurement of carbon monoxide (CO) in process hydrogen (Pyke and Sadwick, 2003). The CMOS approach brings state-of-the-art silicon processing to the concept of the FET sensor with a catalytic gate metal. This technology offers three major advantages in comparison with all of the commercial sensors listed above:
• CMOS foundry capabilities and resources have resulted in very low unit manufacturing cost • The sophistication and maturity of the semiconductor fabrication process used for both CMOS
electronics and the FET sensors results in much greater product consistency than is possible with current commercial sensor technology
• The FET does the initial signal conditioning and amplification simplifying the electronics for control and analysis
The cost of manufacturing commercial sensor products is dominated by the time and resources required to select, test and calibrate each sensor that is shipped in a product. Consistent sensor performance will be significantly greater for CMOS FET sensors, and the reduction in the cost for selecting, testing and calibration will be significantly reduced. Consistent sensor performance is directly related to the repeatability of key process parameters that quality improvement teams work to improve daily. The uniformity of the electrical properties of the typical silicon wafer continues to improve. Process controls necessary for consistent CMOS memories and large area microprocessors have become more stringent as device geometry shrinks. The CMOS FET sensor for hydrogen will completely capture the advantage of the industry standard CMOS process. The use of the FET to transduce the effect of hydrogen on the catalytic gate is an effective way to convert the chemical effect to an electrical effect which can be easily measured. FET Sensor Background Lundstrom first described the silicon FET with a palladium gate for hydrogen detection (Lundstrom et al, 1974). Figure 1 illustrates the structure of the FET. The palladium gate FET was part of a hydrogen leak detector manufactured by Sensistor AB starting in 1981. The selectivity to hydrogen is due to the solubility of hydrogen in palladium and the formation of palladium hydride. The change in composition results in a change in the chemical potential or work function of the metal and is measured by a change in the threshold voltage VT of the FET shown in Figure 2. The effect on VT was clearly demonstrated by current/voltage measurements with FETs. The hypothesis that work function was changing with hydrogen was the natural extension of these measurements since the correlation of metal work function with VT had been established early in the development of transistors. While there are other hypotheses that could also account for the change in VT, the work function explanation is now commonly used to explain the hydrogen sensitivity. The palladium gate had disadvantages overcome by different catalytic metals. While the palladium gate provides sensitivity to very low concentrations of hydrogen, the sensor saturates well below 1%(vol)
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hydrogen in air or nitrogen. Also, multiple cycles of hydrogen exposure followed by air or nitrogen result in the expansion and contraction of the crystal structure leading to stress and mechanical failure of the material. Work in the early 1980s at Bell Labs (Poteat et al, 1983) demonstrated sensitivity to hydrogen, ethylene and carbon monoxide on platinum and iridium metal-insulator-semiconductor (MIS) capacitors. This work received little attention, and an explanation for the mechanism was never given. By the mid 1980s, using FETs and MIS capacitors for detecting hydrogen with palladium gates had been reproduced in many laboratories. Continuing work on the MIS structure at Sandia showed the mechanical problems with pure palladium could be eliminated with palladium-nickel alloys increasing the hydrogen concentration where the structural problems occur. Hughes et al 1987 reported sensitivity to hydrogen sulfide, propylene oxide, ethylene, formic acid, CO and NO2 using catalytic metals in MIS capacitors in the presence of hydrogen. This work formed the basis for a hydrogen detector manufactured currently by H2Scan, Valencia, CA. This H2Scan product combines a palladium-nickel FET for low concentration measurements and a palladium-nickel resistor for higher concentrations of hydrogen. The catalytic gate was suspended over the channel region of the FET (SGFET) to allow gas penetration to the metal surface facing the channel and increase the sensitivity of catalytic metals like platinum and iridium where the solubility for hydrogen is orders of magnitude less than palladium (Cassidy et al 1986, Eisele et al, 2001, Feinstein et al, 1997, Fleitner et al, 1994, and Pyke, 1987). As with the work reported by Poteat, sensitivity to hydrogen and gases other than hydrogen was observed. The suspended catalytic gate has the disadvantage of involving several fabrication steps not part of the standard CMOS foundry process. Recent work in this laboratory on aluminum gallium nitride-gallium nitride (AlGaN-GaN) modulation doped MODFET sensors with solid platinum and rhodium gates produced using standard planar metallization has demonstrated hydrogen sensitivities equivalent to the palladium with greater dynamic range (Pyke and Sadwick, 2003 and references therein). The planar gate configuration eliminates the need for the complex suspended gate. Figure 3 shows the layout of the AlGaN-GaN FET sensor. Figure 4 is a plot of the sensitivity of both metals. The gate voltage depends on the logarithm of the hydrogen concentration and does not saturate.
AlGaN/GaN MODFET Platinum Gate Rhodium Gate Sensitivity (mV/decade) -93 -150 Range in Air*§ 0-2%vol 0-2%vol Range in Nitrogen* 0-100%vol 0-100%vol CO2 Sensitivity <10mV (0-15%vol) <10mV (0-15%vol) Methane Sensitivity <10mV (0-1%vol) <10mV (0-1%vol)
* Neither gate is saturated at with hydrogen (tested to 1atm). § There is a dependence on oxygen that would affect the hydrogen sensitivity, but the data indicate a logarithmic response up to 2% hydrogen.
This work also confirmed the reversible sensitivity to carbon monoxide in a hydrogen background reported earlier (Hughes et al, 1987). Detection of CO in hydrogen used as a fuel is important in order to avoid poisoning the electrodes in a fuel cell. Platinum and rhodium gate metals were used to resolve CO in the 0-80 ppm level with hydrogen concentration varied 30-70%. CMOS Processing The standard CMOS process cannot be customized without becoming a nonstandard process. Any process steps not part of the standard CMOS process will in a higher price and more inconsistency in the product. Fewer foundries are willing to engage in a nonstandard process and when they do the work invariably will take longer to complete and result in lower yields. The CMOS process flow is shown in Figure 5. While aluminum metal gates are a standard in CMOS processing, the catalytic metals are not.
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Platinum is exotic, and this part of the process is usually outsourced. There are few resources available for exotic metal deposition collaborating with CMOS foundries. Batch to batch variance will be greater than for the standard process, but sensor performance within a batch should be as consistent as the performance of any device produced in a standard CMOS process. Dielectric Layer Separating the Gate Metal from the Silicon The dielectric layer plays an important role in determining threshold voltage and sensitivity. The standard primary dielectric is an oxidized layer of silicon grown at high temperature. This layer of silicon oxide form a barrier to the flow of electric current, but it is a medium through which ions and hydrogen can diffuse. Low-pressure chemical vapor deposition (LPCVD) silicon nitride was chosen as a second layer of dielectric to prevent hydrogen from diffusing into and altering the electrical properties of the silicon under the dielectric layer. Gate Architecture and Composition Fluence planar catalytic gate starts with a robust bond to the LPCVD silicon nitride. The gate pattern increases the surface area for hydrogen adsorption for a maximum sensitivity. The sensors have longer and wider channels where the effects of defects in the silicon are averaged over a larger area. The design for the gate is a ladder with the rungs extending from drain to source and connected at each end by two lines running along the inside edge of the source and drain regions respectively (see Figure 6). The gate-source voltage VGS and VT control the current in the channel region directly under the metallization. Current will flow in the channel region between the rungs depending on the applied VGS and any influence on VT by exposure to gas (see Figure 7). Both the catalytic and aluminum gates will share the same gate structure. Platinum was sputtered onto to the channel with an intermediate adhesion layer of thin film sputtered titanium. A lift-off process was used to remove the metals everywhere on the wafer except for the platinum fingers spanning the channel regions of the FETs. The CMOS foundry produced the aluminum gates using their standard process. Cross-Sensitivity: Temperature, Interfering Gases and Moisture Temperature testing and calibration will cover the range from �20C to 100C. In previous work (see Figure 8), we observed a value of drain-source current IDS and VGS where the dIDS/dVGS curves crossed. At the crossing point the device temperature sensitivity approached zero. Multiple devices will be tested to determine the wafer and batch variance of these values with the goal of a temperature independent sensor output. Interference or cross sensitivity by other gases and moisture are potential barriers to this and any sensor technology. Sensitivity to other gases and environmental conditions greater than 10% of the sensitivity to hydrogen is a potential interference. Moisture is a known interferent in silicon based devices. Water changes the surface composition of the native oxide which forms on the LPCVD silicon nitride layer between the gate fingers. The aluminum gate FET has the identical native oxynitride and will be used to directly compensate for the effect of moisture.
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Control Circuit and Reliability Electronics developed in this laboratory is used for an array of three sensors. The sensor control circuit is adjusted for specific value of drain current IDS, and a feedback loop adjusts the gate-source voltage VGS to compensate for any changes in IDS due to hydrogen. Two platinum gate sensors will provide redundant measurements for reliability. Equivalent changes in VGS on both sensors will be sufficient evidence for a hydrogen effect and will provide a real-time measurement of concentration and precision. The absence of a simultaneous and equivalent change is evidence of a fault condition and the need for maintenance or sensor replacement. References Bakker, E., and M. Telting-Diaz, 2002, Anal. Chem., 74, 2781. Cassidy, J., S. Pons and J. Janata, 1986, Anal. Chem., 58, 1757. Feinstein, D.I., C. Renn, M. Scharff and S.C. Pyke, 1997, �Metal-Insulator-Semiconductor (MIS) Gas Sensor Array for Gas Analysis and Diagnosing Faults in Oil-Filled Power Transformers�, 191st Meeting of the Electrochemical Society, Montreal, Quebec, Canada, May 4-9. Hedborg, E., F. Winquist, and I. Lundstrom, 1994, Appl. Phys. Lett., 64(4), 420. Ho, C.K., M.T. Itamura, M. Kelley and R.C. Hughes, 2001, �Review of Chemical Sensors for In-Situ Monitoring of Volatile Contaminants�, Sandia Report SAND2001-0643, http://www.sandia.gov/sensor/SAND2001-0643.pdf Hoffheins, B.S., L.C. Maxey, W. Holmes Jr., R.J. Lauf, C. Salter and D. Walker, 1999, �Development of Low Cost Sensors for Hydrogen Safety Applications�, DOE Report, http://www.eere.energy.gov/hydrogenandfuelcells/pdfs/26938rr.pdf Hughes, R.C., W.K. Schubert, T.E. Zipperian, J.L. Rodriguez and T.A. Plut, 1987, J. Appl. Phys., 62, 1074 Kohl, D., 1989, Sensors and Actuators, 18, 71. Lundstrom, I., M.S. Shivaraman, C. Svensson, and L. Lundqvist, 1975, J. Appl. Phys., 26, 55. Lundstrom, I. and L.G. Petersson, 1996, J. Vac. Sci. Technol., A, 14(3), 1539 Martin, L.P., R.S. Glass, 2003, �Electrochemical Sensors for Proton Exchange Fuel Cell Vehicles�, DOE Report, http://www.eere.energy.gov/hydrogenandfuelcells/pdfs/viib2_martin.pdf Morrison, S.R., Sensors and Actuators, 1982, 2, 329. Poteat, T.L., B. Lalevic, B. Kuliyev, M. Yousef and M. Chem, 1983, J. Electron.Mater., 12, 181 Pyke, S.C., 1987, US Patent No. 4,671,852, 1990, US Patent No. 4,947,104, 1995, US Patent 5,417,821, 1997, US Patent 5,591,321, and 2000, US Patent Application 09/820,037. Pyke S.C. and L. Sadwick, 2002, �Gallium Nitride Integrated Gas/Temperature Sensors for Fuel-Cell System Monitoring for Hydrogen and Carbon Monoxide�, DOE Report, http://www.eere.energy.gov/hydrogenandfuelcells/pdfs/32405b16.pdf Pyke S.C., and L. Sadwick, 2001, �Gallium Nitride Integrated Gas/Temperature Sensors for Fuel-Cell System Monitoring for Hydrogen and Carbon Monoxide�, DOE Report, http://www.eere.energy.gov/hydrogenandfuelcells/pdfs/30535ay.pdf Pyke, S.C., J-H. Chern, J. Hwu and L. Sadwick, 2000, �Gallium Nitride Integrated Gas/Temperature Sensors for Fuel-Cell System Monitoring for Hydrogen and Carbon Monoxide�, DOE Report, http://www.eere.energy.gov/hydrogenandfuelcells/pdfs/28890ii.pdf Pyke S.C. and L. Sadwick, 2003, �Resolution of Hydrogen and Carbon Monoxide on Metal Gate GaN MODFET Sensors�, IEEE Sensors 2003, Toronto, Canada October 22-24 Taguchi, N., 1972, US Patent No. 3,676,820.
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Figure 1. Crossection of the catalytic gate FET showing source, drain and gate electrodes and the dielectric between the gate and substrate. The region under the dielectric between the source and drain is the channel. The electrical conductivity of the channel is a sensitive measure of the electric field between the gate and the substrate.
Label1
Figure 2: The plot of IDS vs VDS on the left shows FET electrical behavior in forward bias where current can flow in the transistor. When the drain-source voltage reaches the �saturation value�, the gate-source voltage VGS controls the current. The intercept at zero current in the plot on the right is the threshold voltage VT .
7
GaN FET Layout
Slice 1Slice 2Slice 3Slice 4
Single Gate
Dual Gate
GateContact
GateContact
Source/DrainContact
Source/DrainContact
Source/DrainContact
Source/DrainContact
ThermocoupleContacts
Figure 3. Layout of the first generation AlGaN-GaN MODFET sensor with catalytic gate and titanium aluminum source and drain metallization. The area of the layout in this example is about 1 mm2. The CMOS sensors would be about the same size.
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28
Hydrogen Response in Air - 30 ºC
0 . 4
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V g s = - 0 . 0 9 3 L o g [ H 2 ]Pt
RhV g s = - 0 .1 5 L o g [ H 2 ]
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Figure 4: Sensitivity of the platinum and rhodium gate AlGaN-GaN MODFET sensors to hydrogen in air follows a nearly logarithmic isotherm in the full range from 100 ppm to 2% in air. There is no saturation unlike the palladium gate FETs. In nitrogen, there is no evidence of saturation up to 100% hydrogen.
Figure 5. Typical stepwise CMOS process steps.
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Figure 6. Fluence CMOS sensor layout showing NMOS and PMOS designs (gate length variation) and finger density. The pink gates are reference electrodes with complete channel coverage.
Gate Metal Finger
Gate Metal Finger
Environmental
Dielectric
Sillicon Depletion Layer
Figure 7. Crossection of gate fingers showing the capacitive coupling between the metal and the channel which controls the channel conductivity. Hydrogen adsorbed on the surface of the metal fingers will influence this coupling in proportion to its concentration in the medium being measured.
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Figure 8. Current voltage data for three platinum gate SGFET sensors. Where the curves cross, the drain source current is independent of temperature. Consistency in sensor performance will lead to consistent values where the curves cross and a temperature independent sensor.
