Catalytic calorimetric gas sensor

Disclosed is a method to make and an apparatus for monitoring the exhaust gas conversion efficiency of a catalytic converter. The catalytic calorimetric sensor disclosed includes a sol-gel processed washcoat and sol-processed catalytically active metal particles. Sol-gel processing creates a washcoat with high surface area and controlled porosity which increases the sensitivity, durability, and reproducibility of the resultant sensor.

TECHNICAL FIELD 
The present invention is concerned with diagnostic methods and devices for 
monitoring exhaust gases generated from automotive engines. 
BACKGROUND OF THE INVENTION 
The Environmental Protection Agency (EPA) and the California Air Resources 
Board (CARB) have implemented stringent diagnostic requirements for 
automotive emissions. As part of their requirements, CARB has mandated 
on-board monitoring of the exhaust gas conversion efficiency of catalytic 
converters, under its On-Board Diagnostics phase 2 (OBD-II) plan. 
Exhaust gas constituents (EGC) sensors have been proposed as an answer to 
the new regulation. One such potential EGC sensor is the catalytic 
calorimetric sensor. 
In a catalytic calorimetric sensor combustible gases (such as hydrocarbons 
HC, carbon monoxide CO, hydrogen H.sub.2, etc.) are oxidized with the help 
of a catalytic layer. The generated heat, measured as the increase in 
substrate temperature, results in an electrical output signal proportional 
to the amount of combustible gases present in the gas mixture. 
Catalytic calorimetric gas sensors typically operate in the 250.degree. to 
500.degree. C. temperature range, making them in principle applicable for 
automotive applications. Although generally of lower sensitivity than 
semiconducting-type gas sensors, catalytic calorimetric sensors appear to 
be considerably more stable and faster responding. However, existing 
catalytic calorimetric sensors have been investigated and found not 
suitable for automotive use because of application-oriented limitations. 
Such limitations have included a lack of sensitivity, restrictive 
detection limits and response time, susceptibility to flow and temperature 
variation. 
These disadvantages of the prior art devices combine to limit the 
usefulness and applicability of catalytic calorimetric gas sensors. 
U.S. Patent No. 4,355,056 discloses a method of manufacturing a 
differential thermocouple combustible sensor which makes the sensor 
relatively insensitive to sulfur poisoning. The catalytic thermocouple 
junction of a catalytic/non-catalytic junction pair is formed by coating 
it with a gel to increase the surface area and then with a chloroplatinic 
acid solution to make it catalytic. The catalytic junction is then treated 
with H.sub.2 S to achieve a high catalyst surface area. In this patent, 
the noble metal catalyst is applied from a solution, which results in 
large particle sizes and an accordingly small number of catalytic sites, 
the resulting sensor lacks sensitivity. 
The prior art suffers from a lack of sensitivity. There thus exists a need 
for a more sensitive gas sensor which also exhibits durability. 
SUMMARY OF THE INVENTION 
The present invention relates to a sensitivity-enhanced catalytic 
calorimetric sensor. 
The present invention discloses a catalytic calorimetric sensor comprising: 
a substrate, a temperature measuring layer and a sol-gel processed 
catalytic layer. 
The invention also discloses a catalytic calorimetric sensor comprising: a 
substrate, a temperature measuring layer and a catalytic layer which 
comprises a sol-gel processed washcoat and a plurality of catalytically 
active metal particles loaded thereon. 
An alternative embodiment of the present invention teaches a catalytic 
calorimetric gas sensor, comprising: a substrate, a temperature measuring 
layer and a catalytic layer which comprises a sol-gel processed washcoat 
and a plurality of sol-gel processed catalytically active metal particles 
deposited on the washcoat. 
The present invention also discloses a silicon micromachining method for 
producing a catalytic calorimetric gas sensor to yield a highly 
reproducible and sensitive combustible gas sensor. 
Lastly, the present invention discloses a method to maximize deposition of 
the catalytically active metal particles in the pores of the washcoat. 
This method reduces agglomeration of the metal particles while making high 
surface area metal particles available for catalytic reactions. 
Sol-gel processed alumina/silica washcoats are beneficial for use with 
sensors due to the high surface area and controlled porosity that can be 
achieved. 
The use of a sol-gel processed catalytically active metal particles results 
in a catalyst comprising smaller metal particles of better uniformity than 
those provided from conventional coating systems, such as sputtering and 
the like. 
It is an object of the present invention to provide a catalytic 
calorimetric gas sensor, where some or all of the catalytic layer is 
processed using a sol-gel technique to create a sensor having an increased 
number of active catalytic sites for catalytic oxidation of the 
combustible gas molecules. 
It is also an object of the present invention to provide a 
sensitivity-enhanced calorimetric gas sensor using a sol-gel technique to 
process some or all of the catalytic layer. 
It is another object of the present invention to provide a catalytic 
calorimetric gas sensor that is more durable and more easy to manufacture. 
It is a further object of the present invention to provide a method for 
fabricating catalytic calorimetric sensors with lower power consumption at 
potentially lower manufacturing costs using silicon micromachining.

BEST MODE FOR CARRYING OUT THE INVENTION 
The present invention generally relates to the application of a sol-gel 
processed, high-surface area alumina and/or silica washcoat and 
catalytically active metals impregnated thereon to fabricate the catalytic 
layer of a combustible gas sensor. Sol-gel processed alumina-silica 
materials are beneficial in sensor applications because such materials can 
be processed easily and have desired properties such as high-surface area 
and controlled porosity resulting in a sensor with increased sensitivity 
and durability. 
Sensitivity is determined by two factors, the rate of gas diffusion and the 
rate of oxidation. In this two step process, the gas molecules are 
transported by diffusion to the catalytic layer, and then oxidized. 
Sol-gel processing provides a way to increase the number of catalytic 
active sites and thus increase the rate of oxidation for certain specified 
exhaust gases. However, in the beginning stages of sensor usage, the 
increased number of catalytic sites may not drastically increase the 
sensitivity of the sensor. It is believed that initially, once a given 
amount of active sites are created, the marginal utility of each 
subsequent active site decreases. One theory is that during this initial 
period, because of the large number of active sites created, the rate 
limiting step becomes the rate of gas diffusion and not the rate of 
oxidation. Nonetheless, over a period of years the number of catalytic 
sites decreases as a result of poisoning of the catalyst and thermal 
sintering. Thus by increasing the number of catalytic sites produced, the 
enhanced level of sensitivity will be sustained over the course of 
operation. Accordingly, the durability and life of the sensor 
significantly increases through the use of sol-gel processed catalytic 
sensors. 
It is further believed that the instant sol-gel technique provides a 
catalytic layer with an increased number of active catalytic sites 
compared to those provided from conventional coating systems. This then 
may allow the formed coating to be thinner, resulting in a catalytic 
calorimetric sensor having an enhanced sensitivity. 
A schematic diagram of a catalytic calorimetric sensor is shown in FIG. 1. 
It consists of a substrate 30. On top of this substrate is a layer to 
measure the temperature 32. The temperature measuring device which 
comprises the temperature measuring layer can be selected from the group 
consisting of a thermocouple, a temperature dependent metal resistor, a 
temperature dependent semiconductor resistor, a p-n junction semiconductor 
and a thermopile. This layer can be made by sputtering, screen printing, 
sol-gel process, etc. Additionally, a catalytic layer 34 is placed on top 
of the temperature measuring layer to enable the oxidation of combustible 
gases in the 300.degree.-500.degree. C. temperature range. The substrate 
should be as thin as possible. The substrate should preferably have a 
thickness in the range between 500 to 1000 nm. The substrate should also 
be comprised of materials with a low thermal conductivity, such as a 
ceramic or a silicon micromachined structure. Examples of suitable ceramic 
materials include aluminum oxide, silicon oxide, polysilicon, silicon 
nitride or combinations thereof. 
Below we will also discuss the most preferred embodiment, a differential 
microcalorimeter structure as shown in FIGS. 2 and 3. It is a differential 
microcalorimeter in which one membrane is covered with a catalytic layer 
and the other membrane acts as a reference to compensate for temperature 
fluctuations in the gas. FIG. 2 is a perspective view of one embodiment of 
a catalytic differential calorimetric sensor having a silicon frame 40 
with two membranes 44 and 45 with temperature measuring layers 48 and 49 
placed on top of the membrane. The temperature measuring layers are 
covered by a passivation layer 41. A catalytic layer 46 is placed on top 
of membrane 45. The temperature measuring layer 49 on membrane 44 is used 
to measure the temperature of the surrounding gas, while the temperature 
measuring layer on membrane 45 measures the additional heat generated by 
the catalytic layer. Both membranes are thermally insulated from each 
other, because both membranes have a low thermal conductivity. A 
cross-sectional view of the sensor of FIG. 2 taken along lines 4--4 is 
shown in FIG. 3. The various methods of operating differential 
calorimetric sensors are known and have been described in, for example, 
CALORIMETRY FUNDAMENTALS AND PRACTICE by W. Hemminger (1984), herein 
incorporated by reference. 
The membranes 44 and 45 can be made of silicon nitride. The preferred 
embodiment includes membranes made of a composite of silicon nitride and 
silicon oxide layers, most preferably silicon nitride, silicon oxide and 
silicon nitride layers. 
Aluminum oxide can be deposited on top of the membrane structure to improve 
the adhesive properties of the catalytic layer on the membrane. The 
resultant membranes have a low thermal conductivity and are tensile so 
they stay flat over a wide temperature range without buckling. 
Although microfabricated catalytic microcalorimeters (FIGS. 2 and 3) are 
known, the present invention provides a chemical and more effective means 
of fabrication using the sol-gel process. Metals are traditionally loaded 
on the membrane by sputtering; however, this technique can produce only a 
limited number of active sites for catalytic oxidization of the 
combustible gas molecules. This limited number of active catalytic sites 
might reduce the sensitivity of a catalytic calorimetric gas sensor. To 
increase the sensitivity and durability of the sensor for automotive 
applications, this invention describes sol-gel methods which increase the 
number of catalytic sites without increasing the size of the device. The 
number of catalytic sites are increased by the use of sol-gel processed 
alumina/silica based materials which provide a high-surface area washcoat 
having controlled porosity with narrow pore-size distribution. 
In addition to the sol-gel processed washcoat, the present invention 
discloses a method to maximize deposition of the catalytically active 
metal particles in the pores of the washcoat. This method reduces 
agglomeration of the metal particles available for catalytic reactions. 
Accordingly, the present invention directed efforts to preparing 
controlled, small metal particles, using a technique disclosed by Schmid 
in Chem. Rev. 92 (1992) 1709-1727. With Schmid's technique, small noble 
particles were stabilized by small organic molecules such as sulfanilic 
acid salts. Another method for preparing controlled size catalytically 
active metals includes impregnation, whereby the particles are prepared on 
the washcoat itself. These small noble metal particles in conjunction with 
high-surface area alumina-based membranes provide a substantially larger 
number of catalytic sites for calorimetric sensors as compared to 
conventional fabrication routes. 
As used herein, the term washcoat refers to the supporting material on 
which the catalytically active metal particles are loaded. The term 
substrate refers to the material supporting the temperature measuring 
layer and the catalytic layer. The substrate is generally a ceramic 30 or 
a silicon micromachined structure consisting of a silicon frame 40 and 
membranes 44 and 45. 
There are several metals which act as catalytically active metals, 
including but not limited to Fe, Cu, Co, Cr, Ni, Mn, Zn, Cd and Ag and 
mixtures thereof; however, the preferred active metals are generally noble 
metals. Noble metals are preferred in large part due to their stability 
towards catalyst poisons. Noble metals include but are not limited to 
platinum, palladium, silver, gold, ruthenium, rhodium, osmium, iridium and 
mixtures thereof. 
In one washcoat embodiment, alumina sols were prepared by hydrolyzing 
aluminum alkoxides in water followed by peptization in the presence of 
dilute mineral acid. The sol was concentrated to a gel which was then 
heated at 250.degree. C. and 600.degree. C. in a nitrogen atmosphere. An 
alumina washcoat was then prepared from the alumina sols and showed a 
surface area of 230 m.sup.2 /g and a pore size of 45 .ANG.. 
Another washcoat embodiment includes preparation of alumina/silica 
washcoats. Alumina/silica washcoats were prepared from a mixture of 
alumina and silica sol which were gelled, dried and aged at 600.degree. C. 
Silica sol was prepared from Si(EtO).sub.4 in ethanol, water and dilute 
mineral acid. The resultant alumina/silica washcoat showed a surface area 
of 390 m.sup.2 /g and a pore size of 45 .ANG.. In a second experiment the 
alumina/silica washcoat was prepared by treating an alumina washcoat with 
a silica sol, resulting in a pore size of between 30 and 40 .ANG.. In 
another embodiment, alumina/silica materials were prepared from 
(tBuO).sub.3 Si-O-Al(OR).sub.2, and Al(OR).sub.3 in parent alcohol. These 
materials also retained a high surface area after aging at 1100.degree. C. 
in air. 
The alumina sol is preferably prepared by Burgraff's method of preparation 
of supported and unsupported alumina washcoats. Aluminum sec-butoxide is 
hydrolyzed in water and heated to remove iso-propanol. The residue is 
treated with dilute nitric acid and peptized for sixteen hours to obtain 
boehemite sol. The surface area of bulk gel derived from boehemite sol and 
fired to 600.degree. C. is 200 m.sup.2 /g and pore size is 45 .ANG.. 
Notably, Burgraff reports a pore size of 60 .ANG. while our experiments 
yielded a pore size of 45 .ANG.. 
The concentration and viscosity of alumina sol is then adjusted to make it 
suitable for deposition on the substrate or alternatively on one of the 
two membranes (in FIGS. 2 and 3) of prefabricated wafers by microcapillary 
device. The amount of liquid dispensed and the wetting characteristics of 
the fluid to the substrate determine the thickness of the alumina 
washcoat. In practice, it is difficult to generate very small drops, 
however, part of the liquid can be removed by suction leaving only a 
controlled amount of alumina on the substrate. After drying, the sol is 
slowly heated to 400.degree. C. and cooled to room temperature, leaving a 
thin supported alumina washcoat on the substrate. 
The alumina washcoat, prepared by this method, has low residual stress, is 
rather uniform with the exception of the border region and retains high 
surface area and porosity. The thickness of the alumina washcoat can be 
varied in the 20-100 nm range and a thicker washcoat can be deposited by 
repeated applications of alumina sol. An advantage of this method is that 
it results in an alumina washcoat having relative uniformity and thinness 
which improves thermal contact between the alumina washcoat and the 
substrate. 
Additionally, the ability to make a thin layer using the sol-gel process 
provides good thermal contact between the catalytic layer and the 
temperature measuring layer. A thin layer allows the temperature measuring 
layer to accurately measure the heat produced by the catalytic layer. 
The preferred method of metal sol preparation includes the use of either 
platinum or palladium as the metal of choice to produce particles having a 
small and controlled size as described by Schmid. 
Platinum or palladium sols are prepared by reduction of chloroplatinic or 
chloropalladic acids in water. The sols are then stabilized by treatment 
with sodium salt of sulphanilic acid. After concentrating the sols, metal 
particles are isolated in solid state (particle size 50-200 .ANG.) and 
redissolved in water. Practically any concentration of metal particles in 
water yields acceptable results. Metals are then deposited by 
microcapillary device on the alumina washcoat of the prefabricated device 
described above. No further processing is necessary, because the organic 
component is lost when heated to the operating temperature. The uniform 
small particle size of the metals and the controlled amounts contribute to 
a large number of catalytic sites on the sensor device. 
The catalytic layer that would be utilized would preferably comprise 
sol-gel processed catalytically active metal particles in combination with 
a sol-gel processed washcoat. The metal particles can be deposited on the 
washcoat or mixed with the washcoat. The term "sol-gel processed catalytic 
layer" includes: 1) a sol-gel processed washcoat together with sol-gel 
processed catalytically active metal particles; and 2) a sol-gel processed 
washcoat together with catalytically active metal particles. 
The preferred method of providing a washcoat on a substrate, includes a 
channelled substrate, such as a honeycomb ceramic structure or the like. 
The disclosure of this method, in the U.S. Pat. No. 5,210,062 disclosed by 
Narula et al., herein incorporated by reference, showed that the sol-gel 
process was suitable for deposition of a washcoat on a catalytic honeycomb 
substrate. The '062 patent also demonstrated that the sol-gel process 
reduced alumina buildup in the corners of the channelled substrate, 
thereby alleviating the back pressure problem which existed prior to the 
'062 patent. 
The method disclosed in the '062 patent comprises first applying a coating 
of a reactive mixture on the substrate. A reactive mixture is made by 
combining a certain type of aluminum alkoxide containing hydrolyzable 
alkoxy groups with water and acid, generally with stirring, wherein a 
suspension is formed. The aluminum alkoxide useful with this invention has 
the chemical formula: Al(OR).sub.3, wherein R comprises alkyl group, 
branched alkyl group, or aryl group having between 3 and 6 carbon atoms. 
Aluminum alkoxides which may be used in this invention include, but are 
not limited to, ethoxides, (n-, or iso)propoxides, (n, sec, or tert-) 
butoxides, or (n, sec, or tert-) amyloxides. The excess coating from the 
channels can be removed by blowing gas through the channels. The reactive 
coating is then hydrolyzed with the addition of water. The coating on the 
substrate is then dried at a temperature suitable to remove water present 
in the coating, preferably at or below about 100.degree. C. The method 
also includes calcining the coating, preferably at a temperature greater 
than about 300.degree. C., most preferably between about 300.degree. and 
900.degree. C., to densify the coating and convert it to .gamma.-alumina. 
The method may additionally comprise repeatedly applying and drying the 
coating followed by calcining or doing all three steps until a coating of 
desired surface area is obtained. 
The reactive mixture may further comprise other components such as 
compatible salts of materials like barium and cerum which would also form 
oxides thereof in the washcoat. The presence of barium oxide and cerium 
oxide in the washcoat improves the high temperature stability of the 
washcoat and the oxidation efficiency of the catalytic layer during use. 
For the same reasons, the above method provides a more efficient catalyst 
for combustible sensors. As with catalytic converters, the catalytic layer 
of the calorimetric sensor should be disposed on the substrate such that 
the catalytic active sites are maximized, and to provide an unrestricted 
flow of exhaust gases to pass through the catalytic layer. 
In the above preferred method, the substrate is made preferably of a 
substantially chemically inert, rigid solid material capable of 
maintaining its shape and strength at high temperatures. The substrate may 
be metallic or ceramic in nature or a combination thereof. Suitable 
materials are .alpha.-alumina, cordierite, alpha-alumina, and zirconium 
silicate. The preferred substrate is the honeycomb ceramic structure. The 
preferred aluminum alkoxide comprises aluminum tris (sec-butoxide) and the 
preferred solvent is sec-butanol and the preferred sol is alumina/silica. 
In sensor applications, it is also desirable to incorporate at least one 
other metal atom in the aluminum oxide washcoat. For example, the U.S. 
Pat. No. 5,134,107 issued to Narula, herein incorporated by reference, 
teaches a method for making single phase lanthanide-aluminum-oxide 
materials. Research has shown that when employing aluminum oxide materials 
as a catalyst washcoat it is desirable to include lanthanum or cerium 
atoms or both in the aluminum oxide matrix. Incorporating either or both 
of those metal atoms in the aluminum matrix tends to prevent structural 
changes that occur in unstabilized .gamma.-alumina at high temperatures, 
which would tamper with the efficiency of a catalytic sensor. When using 
sol-gel techniques to make the alumina material, these other metal atoms 
are added by co-hydrolyzing one or more metal-alkoxides with aluminum 
alkoxide. 
Prior to the '107 patent, such alkoxides when combined in water hydrolyzed, 
resulting in a mixture of hydroxides. The undesirable final product of 
such a mixture comprises a non-uniform 2-phase distribution of metal oxide 
in an aluminum oxide matrix. To overcome these disadvantages, the '107 
patent teaches a method which comprises reacting, according to sol-gel 
techniques, water and heterobimetallic alkoxides comprising 
tribis(2-propanolato)alumina)hexakis-(2-propanolato))!lanthanide 
represented by the general chemical formula LnAl(OPri).sub.4 !.sub.3, Ln 
being a lanthanide. Lanthanide is meant to include the members of the 
lanthanide series of the periodic table such as lanthanum and cerium. 
The lanthanide-aluminum-oxide materials according to the present invention 
are made from single phase sols. The sol may be made by forming a reaction 
mixture of the heterobimetallic alkoxides with water, and adding acid to 
the reaction mixture to form a sol. Acids employed embodiments of the 
present invention may be selected from any organic and inorganic acids 
which may include, but are not limited to, nitric, hydrochloric, sulfuric, 
acetic and propionic acid. Alcohol is generally employed as a solvent for 
the alkoxide prior to it being combined with water. Alcohols which may be 
broadly employed according to embodiments of the present invention include 
2-propanol, n-butanol and sec-butanol, with 2 propanol being preferred. 
The preferred heterobimetallic peroxide is 
tris(bis2-propanolato)alumina)hexakis(.alpha.-)2-propanolato))!lanthanum. 
The sol is preferably stabilized by maintaining the reaction mixture for a 
time and a temperature sufficient to form a stable sol. A stable sol is 
one that maintains its sol properties and does not experience any 
substantial gelling when exposed to air or moisture for a significant 
period of time, e.g., months. 
To form a washcoat, the sol is coated on the substrate and then the coating 
is dried and subsequently calcined at an elevated temperature. Generally 
calcination is carried out at a temperature above 300.degree. C., 
preferably between about 300.degree. C. and 900.degree. C. to form a 
lanthanide-aluminum-oxide material. 
Rather than forming a gel from the sol above, gels may be made more 
directly from lanthanum aluminum alkoxide. For example, the addition of a 
wet alcohol, generally meant to be one containing more than six 
equivalents of water, to a solution of the alkoxide in an alcohol at room 
temperature results in gel formation instantaneously at the contact layer. 
These sol-gel techniques may also be employed to make aluminum materials 
comprising more than one lanthanide as a single phase material. 
Further, the U.S. Pat. No. 5,234,881 issued Narula et al., herein 
incorporated by reference, discloses a method of making binary 
lanthanum-palladium oxides useful as an automotive exhaust catalyst 
washcoat at high temperatures. The teachings of the '881 patent can be 
readily applied to catalytic sensors. 
Prior to the '881 patent efforts to deposit the binary lanthanum-palladium 
oxide catalysts from their suspension in water followed by sintering 
resulted in the loss of lanthanum-palladium-oxides. The above-problem was 
solved in the '881 patent by depositing such oxides from their suspension 
in an alumina sol on a honeycomb substrate precoated with a commercial 
washcoat such as .gamma.-alumina. The alumina sol for this purpose can be 
readily prepared by hydrolyzing aluminum sec-butoxide in water at 
70.degree. to 90.degree. C., boiling off sec-butanol at 90.degree. C. and 
acidifying. Two different samples were made by suspending the individual 
binary oxides (La.sub.2 Pd.sub.2 O.sub.5 or La.sub.4 PdO.sub.7) in such 
sols in depositing such suspension onto catalyst substrate, such as 
monolithic cellular cordierite, which has been previously coated with a 
commercial washcoat such as .gamma.-alumina. On drying, alumina sol forms 
a gel and traps the particles of the binary lanthanum-palladium-oxide. The 
catalyst is then sintered, preferably at 600.degree. C. 
For the '881 patent other suitable sols such as SiO.sub.2, TiO.sub.2 and 
ZrO.sub.2 can be substituted for an alumina sol, Al.sub.2 O. 
Alumina sols were also prepared in the '881 patent by hydrolyzing aluminum 
alkoxide (eg. Al(OR.sub.3) in water followed by peptization in the 
presence of diluted mineral acid. The sol was then concentrated to a gel 
which was heated at between 250.degree. C. and 600.degree. C. in a 
nitrogen atmosphere. Alumina/silica washcoats were prepared for a mixture 
of alumina sol and silica sol which is obtained from Si(ETO).sub.4 in 
ethanol, water, and diluted mineral acid. A sample of alumina/silica 
washcoat was then made with uniform pore size around 45 .ANG. and 390 
m.sup.2 /g. 
Further, the sol-gel processed washcoat, can also include as the 
catalytically active metal, transition-metals. When tested experimentally, 
silver-containing sol, such as AgNO.sub.3, which is water soluble was 
dissolved in distilled water. The silver solution was then used to 
impregnate the sol-gel washcoat. The ratio of the silver amount to the 
sol-gel weight was dependent on the desired silver loading on the sol-gel 
washcoat. After the impregnation, the material was dried up to 120.degree. 
C. and then heated in air inside a furnace of 500.degree. to 600.degree. 
C. for four hours. 
Additionally, the present invention's preferred method for fabricating a 
sensitivity enhanced calorimetric device, includes using 
silicon-micromachining. Silicon micromachining offers the capability of 
fabricating devices with low power consumption at potentially lower 
manufacturing costs. A micromachined device can also have a faster 
response time because the membrane temperature measuring layer and the 
catalytic layer have a smaller thermal mass than conventional calorimetric 
devices. 
The preferred embodiment includes fabrication of a silicon micromachined 
differential microcalorimeter. To improve the detection limit of the 
sensor the temperature rise is preferably measured differentially by 
adding a second element with thermal characteristics identical to those of 
the temperature measuring device, but without a catalytic layer. The 
resistances of the two elements are represented by the following equation: 
EQU R.sub.catalytic =R.sub.o 1+.alpha.(T+.DELTA.T.sub.comb ! 
EQU R.sub.reference =R.sub.o 1+.alpha.T! 
with R.sub.o the resistance at 0.degree. C., the temperature coefficient of 
resistance, T the temperature of operation in .degree.C. and 
.DELTA.T.sub.comb the rise in temperature caused by the oxidation of 
combustible gases on the catalytic layer, .DELTA.T.sub.comb is given by: 
EQU .DELTA.T.sub.comb =(R.sub.catalytic -R.sub.reference)/.alpha.R.sub.o 
=.DELTA.R/.alpha.R.sub.o 
with .DELTA.T.sub.comb /1000 ppm of combustible gas defined as the 
sensitivity of the sensor. 
FIG. 2 shows a perspective view and FIG. 3 shows a schematic 
cross-sectional diagram of one of the device configurations fabricated and 
studied. Two thin-film resistors are fabricated on two micromachined 
membranes of low thermal conductivity, and one is covered by a catalytic 
layer. For simplicity, no heater was incorporated in the design and the 
devices were heated externally. 
The average temperature rise in a microcalorimeter is dictated by the 
balance of heat produced by the chemical reaction and the heat lost to the 
environment. In order to maximize the detection limit of the sensor, 
effects such as the reactant mass transfer, the reaction kinetics at the 
catalyst, the heat loss by conduction/convection to the ambient gas and by 
conduction to the substrate, thermal fluctuations in the environment, and 
the electrical characteristics of the thermometer must all be taken into 
account. 
The key elements of a Si-based microcalorimeter are the catalytic layer, 
the temperature measuring layer, the heater, and the supporting structure 
or substrate for all of the previous elements. The substrate consists of a 
bulk silicon frame with either a membrane layer or a more complex 
plate/teeter element which in both cases are obtained by etching the 
underlying bulk silicon frame. The membrane or plate/teeter acts as a 
support for the temperature measuring layer. Multiple silicon dies can be 
fabricated from a single silicon wafer. The membrane or plate/teeter 
should have a small thermal mass for fast response time, but must be 
mechanically robust to support the temperature measuring layer and the 
catalytic layer and survive temperature cycling, pressure shocks, water 
mist and small particle impingement. It should also be configured in such 
a way as to minimize the heat loss to the silicon frame and to the ambient 
gas for increased sensitivity. The catalytic layer should have a large 
specific surface area for the device to operate in mass-transport limited 
regime. This surface area is achieved by using sol-gel processed 
alumina-silica washcoat and/or sol-gel processed catalytically active 
metal particles. 
Additionally, good thermal contact between the catalytic layer and the 
underlying temperature measuring layer is also important for increased 
sensitivity. The catalytic layer should not substantially change the 
thermal characteristics of the membrane, otherwise the sensor temperature 
compensation may be compromised. For greater sensitivity, the temperature 
measuring layer should mainly measure the central region of the membrane 
where the temperature rise due to the reaction is the largest, without 
substantially contributing to conductive heat loss. A thin-film resistor 
with stable resistance and temperature coefficient of resistance (TCR) is 
desirable as the temperature measuring layer. The film resistor is 
patterned as a winding element to increase its resistance (i.e., the 
output signal) and distribute the stress induced by the thermal mismatch 
with the membrane. 
There are three ways to process the sol-gel catalytic layer. One method 
includes using a micro syringe to deposit the catalytic layer onto the 
membrane. With this method, the catalytic layer is deposited to create 
catalytic active sites specifically at the desired locations. Although 
this technique provides accurate deposition of the catalytic layer, this 
technique may not be suitable for mass manufacturing. 
A second method involves dipping the silicon wafer into a sol-gel solution 
to coat the silicon die with a sol-gel processed catalytic layer. This 
second method is preferred as it provides a controllable and efficient 
method to batch fabricate catalytic calorimetric gas sensors in a way that 
is compatible with silicon micromachined structures. The thickness of the 
catalytic layer can be readily controlled by varying the speed with which 
the silicon wafer is immersed and removed from the sol-gel solution. This 
dipping method further requires removal of the sol-gel catalytic layer 
from specific areas to create the catalytic active sites at the specific 
desired locations. Selective removal of the catalytic layer can be 
effectuated in two ways. One involves fabricating a mask on the silicon 
wafer and placing the mask over the substrate which is then followed by 
etching away the sol-gel catalytic layer from the undesired locations. The 
second involves heating the catalytic layer by resistive heating of the 
membrane on which a catalytic active layer is desired such that the 
solvents within the sol-gel catalytic layer burn off, resulting in the 
affixation of the sol-gel catalytic layer in the desired areas. This step 
is then followed by washing the sol-gel catalytic layer with a solvent to 
strip the remaining sol-gel solution from the catalytic layer. An acid 
solvent could strip the solution from the undesired locations. The result 
in either case is a silicon wafer having a sol-gel processed catalytic 
layer selectively placed thereon that is easily reproducible. At this 
time, the preferred method includes dipping the silicon wafer and then 
etching away the sol-gel solution from the undesired locations. 
With the preferred method of fabricating a silicon microcalorimeter, the 
membrane is deposited first on a silicon wafer, 100 mm in diameter, 400 
.mu.m thick. The membrane is in total preferably between 500-1000 nm, such 
that it is mechanically stable while being thin enough to prevent loss of 
heat by thermal conduction through the substrate. Either a 0.6 .mu.m thick 
layer of low-stress, low pressure chemical vapor deposition (LPCVD) 
silicon nitride, or a composite of plasma enhanced chemical vapor 
deposition (PECVD) silicon oxide/nitride layers (about 0.5 .mu.m and 0.1 
.mu.m, respectively) deposited over 0.1 .mu.m of LPCVD nitride can be 
used. After annealing at 600.degree. C., the latter composite layer has a 
small residual stress (tensile) of about 6.times.10.sup.8 dynes/cm.sup.2, 
the compressive state of the oxide being compensated by the tensile 
nitride. A Pt film resistance thermometer, 100 nm thick, is sputter 
deposited. The film resistors, acting as temperature measuring devices 
and/or heaters, are delineated by lithography and wet etching. After 
annealing the Pt resistors at 500.degree. C. in nitrogen to stabilize 
their resistance and temperature coefficient of resistance (TCR), the 
wafers are coated with 0.2-0.3 .mu.m of PECVD silicon nitride for 
passivation and annealed at 500.degree. C. The passivation is then removed 
on the contact pads with plasma etching. While defining the opening for 
the contact pads, an etch-mask pattern is also defined on the back side of 
the wafer using a double-sided aligner. A 30% aqueous solution of KOH at 
80.degree. C. is used to completely etch the silicon underneath the 
membrane. The membrane has sufficient mechanical strength to allow the 
wafer to be diced with a diamond saw. For ease of handling, a die size of 
7.times.7 mm.sup.2 is used, although only a 3.5.times.3.5 mm.sup.2 area is 
needed for the device with the smallest membrane. 
While the best mode for carrying out the invention has been described in 
detail, those familiar with the art to which this invention relates will 
recognize various alternative designs and embodiments for practicing the 
invention as defined by the following claims.