Download New Silicon-Based Metal-Oxide Chemical Sensors

New Silicon-Based Metal-Oxide Chemical Sensors
Microfabricated chemical sensors offer greatly reduced power
consumption, low operating voltage that is compatible with low-power CMOS
devices, and rapid response time--all in a very small package size.
Stephan Trautweiler
and Nicholas Mosier,
MicroChemical
Systems SA;
Edward Zdankiewicz,
MicroChemical Systems Inc.
Tomorrow's chemical sensing needs will require more than the stand-alone sensors and
signal conditioning circuits currently offered by many of today's sensor manufacturers.
Customers are asking for turnkey chemical sensing systems and sensors that can be easily
designed into their consumer gas detectors or OEM gas sensing products. One way to
accomplish this is to create chemical sensing microsystems by bundling chemical sensors
with control algorithms, electronic circuit elements, and gas sample conditioning elements
(see Photo 1).
Photo 1. Chemical sensors
represent one of the fastest
growing sensor market segments
because they are intimately
involved in all aspects of our
New chemical sensors will need to be micromachined
daily life. Chemical sensors and
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:
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.
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Low cost
Lower power
Robustness
Small size
Ease of use
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.
The sensing film is constructed from an n-type tin oxide and
Photo 2. The development
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
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
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
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.
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:

System partitioning. Where should the system elements be packaged with respect
to the microsensor and gas sample conditioning elements within the system? Photo
2 shows an example of a simple chemical sensing microsystem. A charcoal filter
and mesh screen are packaged in the same housing as the sensor chip to keep out
dust and background gases that could interfere with the sensor response to the
target gas [1,2].
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Package design. The sensor package
must be designed along with the
microsensor. As shown in Photos 1
and 2, MicroChemical's microsystem
uses a standard TO-series package
carrier for mechanical attachment of
the sensor chip and provides die bond
wires to interface the on-chip heater
and sensing element to connector
pins. The pins then can be plugged
into simple electrical sockets. The
package housing consists of a
combination inner TO-series metal
can and outer nylon cap to capture
the charcoal filter (see Figure 2).

Process optimization. The
microsystem needs the ability to
exploit basic material and thermal
characteristics of the metal-oxide
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
sensing film to optimize its response
diodes are also included on the
to a specific target gas. Simple
same silicon.
stepwise changes in heater voltage
can be used to thermally tune the
metal oxide to produce a gas-dependent transient response, as shown in Figure 3
[2,3]. This phenomenon can be exploited to optimize the sensor response to a
specific gas against a background of interfering gases, or can even allow a single
gas sensor to speciate a mixture of gases, such as hydrogen, carbon monoxide, and
methane.
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:

Different control algorithms are possible with
a simple change in device firmware.

Combined control algorithms can
automatically reconfigure the sensor on the fly
to detect several different gases sequentially or
can put the sensor into a safe standby mode to
protect it from adverse operating conditions.

The customer can select different
factory-installed chemical filters for removal
of specific chemical compounds.
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Figure 2. Packaged chemical
Combined chemical filters, the physical analog
microsensors can include integral
to combined control algorithms, can be
dust and chemical filters to keep
customer specified and factory installed.
out interfering agents. Filters are
Different metal-oxide films deposited onto the selectable, based on the
application. A simple carbon filter
same silicon chip can create different silicon
chip platforms, just as the "core circuitry" of a is typically used in home CO
detectors to keep out dust and
microprocessor IC is changed for different
background cooking gases that
control applications.
could interfere with the sensor's
Manufacturing Challenges
response to the target gas. A
similar design is available for
Almost as difficult as producing a new microsensor
detecting leaks of methane,
design is developing a repeatable, cost-effective
natural gas, or town gas from
process to manufacture it. Micromachined
gas-fired home appliances such as
chemical-sensor manufacturing combines standard and
water heaters, space-heating
customized semiconductor processing steps with
furnaces, or cook stoves.
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:

Chemical vapor deposition (CVD)
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Oxidation
Doping
Diffusion
Metallization
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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.
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
A hybrid CMOS process is required to deposit, pattern, element to produce a
and activate the metal-oxide sensing element.
gas-dependent transient response.
Materials purity and process cleanliness during
The resulting sensor response
deposition are critical. Semiconductor-class tooling is contains chemical kinetic
required because contaminants will adversely affect
information about the gas
the grain structure and electrical properties of the
reactions taking place on the
sensing element, resulting in parts that fail to meet
sensing film and can be used to
product specifications.
distinguish one type of gas from
another using the same sensor.
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
chemical sensitivity of the sensing film. Thinner
diaphragms will reduce sensor operating life.
Traditional microdiaphragm structures made from
doping the silicon wafer to act as an etch stop are very
difficult to control, resulting in wide variations in
diaphragm thickness.
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.
Photo 3. Silicon-based chemical
sensor manufacturing begins with
standard CMOS processing steps.
For example, chemical vapor
deposition tooling is used to
Production testing of chemical microsensors is
precisely form the
necessary to ensure a consistent, high-quality product. microdiaphragm. This is the same
Traditional IC testing techniques such as parametric
process technology that has been
test and automated probing subject each die on the
perfected for over 40 years to
wafer to a 100% electrical test to guarantee their
produce today's ICs built into PCs
conformance to manufacturing specifications. After the and digitally controlled consumer
sensor dies are cut and packaged, they must undergo
products.
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.
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
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.
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, [email protected]