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| Photo 1. Chemical sensors represent one of the fastest growing sensor market segments because they are intimately involved in all aspects of our daily life. Chemical sensors and gas detection products are used to ensure the quality of the air we breathe, our food and water, and the heating and cooling systems in our homes. They can also be found in the automobiles, buses, and trains we routinely ride to and from work and school. |
New chemical sensors will need to be micromachined to provide customers with cost-effective turnkey measurements. Silicon-based metal-oxide chemical sensors are a first step. In fact, when fabricated using standard semiconductor manufacturing and micromachining techniques, these sensors deliver the key sensor attributes currently demanded by end users:
Sensor Technology and Operation
Photo 2 shows a MicroChemical Systems silicon-based metal-oxide chemical sensor. The sensor chip contains a thin film sensing element fabricated from tin oxide as well as a buried polysilicon microheater (see Figure 1). The gas-sensitive layer converts gas concentration to electrical conductivity when exposed to air. The thin film heater controls the operating temperature of the sensor to optimize selectivity and sensitivity.
The heater and sensing element are built on a micromachined diaphragm precision formed from a silicon oxy-nitride ceramic with thermal properties that minimize sensor power consumption and response time. Heater power consumption levels are <40 mW with drive currents <40 mA, meaning that the chemical sensor is fully compatible with standard digital electronics components. Better compatibility allows product engineers to design simpler, lower cost gas-detection products for consumer, industrial, and automotive applications. The underlying silicon micromachined substrate is a standard sensor platform containing electrical interconnects to the sensor and device heater, as well as electrostatic discharge (ESD) protection diodes.
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| Photo 2. The development of chemical microsensors now permits the design and fabrication of low-cost smart sensing modules with a high degree of onboard intelligence and ultra-low power consumption. Chemical sensing microsystems may be considered as a specialized class of smart sensor, which involves the merging of a solid-state chemical sensor with the electronics and firmware for signal conditioning, data processing, and control. |
The sensing film is constructed from an n-type tin oxide and ionic semiconducting material. During sensor operation, ambient O2 molecules chemisorb onto the sensing film surface and dissociate into oxygen ions. Heating the tin-oxide film to ~450°C using the buried heating element facilitates this process. Formation of the oxygen ions removes free electrons from the tin-oxide grain surfaces, producing an increase in the sensing film's resistance. The presence of a reducing target gas such as CO reacts with the O2 to produce CO2 and releases electrons back into the sensing film with a corresponding increase in conductance or a decrease in film resistance. As CO concentration increases, the sensing film resistance decreases further. If the target gas were an oxidizing agent, such as NO2, the opposite effect on sensing film resistance would occur.
Chemical Sensing Microsystems
A major concern for design engineers considering the use of any new sensor is the cost of switching from their existing sensor. Switching usually requires a complete redesign of the gas detector circuit, packaging, and, in some instances, control firmware. The solution MicroChemical Systems found to this problem was to bundle the new sensor with other detector elements into a chemical sensing microsystem that can be replaced as a simple building block in the larger gas detection product.
The key design challenges in developing a successful microsystem for chemical measurements are:
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| Figure 1. Silicon-based chemical sensor chips are first-level microsystems containing several on-chip devices. The heater and sensing element are built on a micromachined diaphragm that is precision formed from a silicon oxy-nitride ceramic with superior thermal properties to minimize sensor power consumption and response time. Heater power consumption levels are <40 mW with drive currents <40 mA. Electrostatic discharge protection diodes are also included on the same silicon. |
You can think of a chemical sensing microsystem as a "system platform" from which a multitude of new application-specific sensing products can be quickly developed. In fact, many different sensing applications could be satisfied by simply making building-block changes. For example:
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| Figure 2. Packaged chemical microsensors can include integral dust and chemical filters to keep out interfering agents. Filters are selectable, based on the application. A simple carbon filter is typically used in home CO detectors to keep out dust and background cooking gases that could interfere with the sensor's response to the target gas. A similar design is available for detecting leaks of methane, natural gas, or town gas from gas-fired home appliances such as water heaters, space-heating furnaces, or cook stoves. |
Manufacturing Challenges
Almost as difficult as producing a new microsensor design is developing a repeatable, cost-effective process to manufacture it. Micromachined chemical-sensor manufacturing combines standard and customized semiconductor processing steps with specialized sensor chip test systems to mass produce reliable products that perform to the manufacturer's specifications.
At MicroChemical Systems, silicon-based chemical sensor manufacturing begins with standard CMOS processing steps--the same process technology used to produce the ICs found in personal computers and digitally controlled consumer products. The microsensor starts out as a blank silicon wafer. Typical CMOS steps performed on the wafer include:
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| Figure 3. Sensor control algorithms optimize detection for a specific target gas. The silicon-based chemical sensor uses the on-chip polysilicon heater to control the temperature of the sensing element. Simple stepwise changes in heater voltage can be used to thermally tune the sensing element to produce a gas-dependent transient response. The resulting sensor response contains chemical kinetic information about the gas reactions taking place on the sensing film and can be used to distinguish one type of gas from another using the same sensor. |
Photolithography and chemical wet etching are also used to pattern and form the key device structures of the sensor. Because manufacturing is a standard CMOS process, other on-chip devices, such as thermistors and ESD protection diodes, can be easily formed on the same substrate adjacent to the sensing structures. This is a key advantage and design challenge of silicon-based sensors: tightly integrating the sensor with associated control and signal conditioning elements onto the same piece of silicon.
A hybrid CMOS process is required to deposit, pattern, and activate the metal-oxide sensing element. Materials purity and process cleanliness during deposition are critical. Semiconductor-class tooling is required because contaminants will adversely affect the grain structure and electrical properties of the sensing element, resulting in parts that fail to meet product specifications.
The last hybrid process step is a precisely controlled wet chemical etch to release the microdiaphragms from the surrounding silicon. Tight dimensional control of diaphragm thickness is crucial to sensor performance. Thicker diaphragms will shift the operating temperature and
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| Photo 3. Silicon-based chemical sensor manufacturing begins with standard CMOS processing steps. For example, chemical vapor deposition tooling is used to precisely form the microdiaphragm. This is the same process technology that has been perfected for over 40 years to produce today's ICs built into PCs and digitally controlled consumer products. |
Using a cross-functional team of materials, MicroChemical's device and fab process engineers solved this problem. They redesigned the diaphragm using an oxy-nitride ceramic. The patented diaphragm layer can be precisely deposited onto the silicon substrate using a modified CVD process tool (see Photo 3). The oxy-nitride material acts as a natural etch stop, eliminating the production tolerance problem. An added benefit is a 50% reduction in heater power consumption for normal sensor operation.
Production testing of chemical microsensors is necessary to ensure a consistent, high-quality product. Traditional IC testing techniques such as parametric test and automated probing subject each die on the wafer to a 100% electrical test to guarantee their conformance to manufacturing specifications. After the sensor dies are cut and packaged, they must undergo further electrical testing for functionality and be characterized chemically with a calibration gas at a controlled temperature and relative humidity to confirm product performance. Because off-the-shelf test systems don't exist for "chemical parametric" testing, MicroChemical had to build one (see Photo 4).
Future Improvements
As the first silicon-based chemical sensors enter into worldwide commercial use, improvements in the gas sensitive layer are already under way. A prime area of focus is sensitivity. The chemical sensitivity of a metal-oxide film depends on the film's grain size and structure [4,5]. Smaller grain size is preferable because it provides more surface area and film porosity, which improves sensitivity and long-term stability of the film. Nano-structured metal-oxide films have been recently demonstrated with a 20 nm (0.02 mm) grain size [6]. Initial test results for nano-structured metal-oxide films indicate that fourfold improvements in sensitivity are possible. Future metal-oxide chemical sensors using nano-structured films will dramatically improve sensor stability, response, selectivity, and gain.
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| Photo 4. High-volume production testing of chemical microsensors is as complex as the chip manufacturing steps that precede it. Off-the-shelf test systems don't exist for "chemical parametric" testing and must be custom built. This is a large investment that does not add value, but does ensure customer satisfaction. |
MicroChemical is also working on product improvements in sensor control and signal processing. Pulsed-heater algorithms have shown that the resulting sensor response contains kinetic information about the gas reactions taking place on the sensing film. The shape of the response curve holds further information about the difference in adsorption and desorption kinetics as a function of sensor temperature [7]. Such advanced signal processing techniques as chemometrics, fuzzy logic, or neural networks [8] could be used to "chemically image" the gas mixture. This has the potential of extending the operation of a single or a few gas detectors to do the job of many sensors or a sensor array. A chemical microsensor packaged with an off-the-shelf microcontroller IC could contain the signal processing firmware. The benefits to customers would be improved compensation for humidity and the ability to resolve the target gas signal from a high background noise of interfering gases. It would be another successful microsystem.
Acknowledgments
The authors wish to thank John Skardon of Salmon Creek Consulting Inc., Vancouver, Washington, for his support, reviews, and constructive input.
References
1. MicroChemical Systems SA. 1998. "Carbon Monoxide Gas Sensor, MiCS 1110," Product Data Sheet.
2. MicroChemical Systems SA. 1999. "Application of the MiCS 1110P for Carbon Monoxide Detection," Application Note 1110.0.
3. Tohuru Nomura et al. 1998. "Battery Operated Semiconductor CO Sensor Using Pulse Heating Method," Sensors and Actuators B, 52:90-95.
4. Noburu Yamazoe et al. 1991. "Grain Size Effects on Gas Sensitivity of Porous SnO2-based Elements," Sensors and Actuators B, 3:147-155.
5. Chung-Chiun Liu et al. 1998. "Application of Nano-Crystalline Porous Tin Oxide Thin Film for CO Sensing," Sensors and Actuators B, 52:188-194.
6. "Nanoparticles Improve Gas Sensors' Performance." Dec. 1998. Chemical Engineering:25.
7. Alain Seube et al. 1996. "Pulsed Mode of Operation and Artificial Neural Network Evaluation for Improving the CO Selectivity of SnO2 Gas Sensors," Motorola 31023, Toulouse Cedex, France, IPC.
8. Wolfgang Gopel. 1998. "Chemical Imaging I: Concepts and Visions for Electronic and Bioelectronic Noses," Sensors and Actuators B:125-142.
Stephan Trautweiler is a Product Manager and Nicholas Mosier is an Applications Engineer, MicroChemical Systems SA, Corcelles, Switzerland.
Edward Zdankiewicz, P.E., is General Manager, MicroChemical Systems Inc., 1128 W. Pleasant Valley Rd., PMB 307, Cleveland, OH 44134-6711, 216-374-7320, fax 216-274-9116, edz@microchemical.com
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