U.S. patent application number 14/212006 was filed with the patent office on 2014-09-18 for miniaturized gas sensor device and method.
This patent application is currently assigned to THE CLEVELAND CLINIC FOUNDATION. The applicant listed for this patent is THE CLEVELAND CLINIC FOUNDATION. Invention is credited to Azlin M. Biaggi-Labiosa, Carl W. Chang, Prabir K. Dutta, Gary W. Hunter, Suvra P. Mondal.
Application Number | 20140262835 14/212006 |
Document ID | / |
Family ID | 50625148 |
Filed Date | 2014-09-18 |
United States Patent
Application |
20140262835 |
Kind Code |
A1 |
Hunter; Gary W. ; et
al. |
September 18, 2014 |
MINIATURIZED GAS SENSOR DEVICE AND METHOD
Abstract
Various embodiments of a gas sensor device and method of
fabricating a gas sensor device are provided. In one embodiment a
gas sensor device includes a base substrate, an electrolyte layer
disposed on the base substrate and a plurality of potentiometric
sensor units electrically coupled to the base substrate. Each
potentiometric sensor unit includes an electrolyte layer disposed
on the base substrate, a sensing electrode comprising tungsten
oxide (WO.sub.3) and platinum (Pt), a reference electrode
comprising Pt, and a plurality of connectors coupled to the
plurality of potentiometric sensors to connect the plurality of
potentiometric sensors in series.
Inventors: |
Hunter; Gary W.; (Oberlin,
OH) ; Chang; Carl W.; (Westlake, OH) ; Dutta;
Prabir K.; (Worthington, OH) ; Mondal; Suvra P.;
(Tripura, IN) ; Biaggi-Labiosa; Azlin M.; (North
Ridgeville, OH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
THE CLEVELAND CLINIC FOUNDATION |
Cleveland |
OH |
US |
|
|
Assignee: |
THE CLEVELAND CLINIC
FOUNDATION
Cleveland
OH
|
Family ID: |
50625148 |
Appl. No.: |
14/212006 |
Filed: |
March 14, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61801106 |
Mar 15, 2013 |
|
|
|
Current U.S.
Class: |
205/785.5 ;
204/427; 204/431; 205/775; 427/526 |
Current CPC
Class: |
G01N 33/0037 20130101;
G01N 33/0031 20130101; Y02A 50/20 20180101; G01N 27/417 20130101;
Y02A 50/245 20180101 |
Class at
Publication: |
205/785.5 ;
204/431; 205/775; 204/427; 427/526 |
International
Class: |
G01N 27/407 20060101
G01N027/407 |
Claims
1. A microfabricated potentiometric gas sensor device comprising: a
base substrate; an electrolyte layer disposed on the base
substrate; a plurality of potentiometric sensor units connected in
series and coupled to the base substrate, each potentiometric
sensor unit comprising: an electrolyte layer disposed on the base
substrate; a two-part sensing electrode comprising a layer of
tungsten oxide (WO.sub.3) disposed on a platinum (Pt) contact; and
a reference electrode comprising platinum (Pt).
2. The microfabricated potentiometric gas sensor device of claim 1,
wherein the sensor device is a MEMS sensor device.
3. The microfabricated potentiometric gas sensor device of claim 1,
wherein the sensor device determines the gas level at a sensitivity
of at least 1 ppm.
4. The microfabricated potentiometric gas sensor device of claim 1,
wherein the sensor device determines the gas level at a sensitivity
of at least 500 ppb.
5. The microfabricated potentiometric gas sensor device of claim 1,
wherein the sensor device determines the gas level at a sensitivity
of at least 300 ppb.
6. The microfabricated potentiometric gas sensor device of claim 1,
wherein the sensor device determines the gas level of NO.sub.x at a
sensitivity of at least 300 ppb.
7. The microfabricated potentiometric gas sensor device of claim 1,
wherein the electrolyte comprises yttria-stabilized zirconia
(YSZ).
8. The microfabricated potentiometric gas sensor device of claim 1,
wherein the substrate comprises a material that is an
insulator.
9. The microfabriated potentiometric gas sensor device of claim 1,
wherein a thickness of the electrolyte layer is maximized a
sufficient amount to minimize the internal resistance of the
corresponding potentiometric sensor unit, and such that the
internal resistance of each of the plurality of potentiometric
sensor units is minimized so as to minimize the overall resistance
of the microfabricated potentiometric gas sensor device and
increase sensitivity of the microfabricated potentiometric gas
sensor device.
10. The microfabricated poteniometric sensor device of claim 1,
wherein the ratio of the surface area of the WO.sub.3 sensing
electrode to the surface area of the Pt reference electrode
disposed on the electrolyte is sufficiently high to increase the
sensitivity of the microfabricated potentiometric gas sensor
device.
11. The microfabricated gas sensor device of claim 10, wherein the
exposed surface of the electrolyte layer is minimized a sufficient
amount to increase the sensitivity of the microfabricated
potentiometric gas sensor device.
12. The microfabricated gas sensor device of claim 1, wherein the
electrolyte is YSZ and the surface area of the WO.sub.3 sensing
electrode on the electrolyte is fabricated using microfabrication
techniques so as to maximize the ratio to that of the WO.sub.3 to
the Pt reference electrode on the electrolyte while minimizing the
exposed layer of electrolyte layer.
13. The microfabricated gas sensor device of claim 1, wherein the
ratio of the surface area of the WO.sub.3 sensing electrode to the
surface area of the Pt contact is sufficiently high to increase the
sensitivity of the microfabricated potentiometric gas sensor
device.
14. The microfabricated gas sensor device of claim 10, wherein the
ratio of the surface area of the WO.sub.3 sensing electrode to the
surface area of the Pt contact of the sensing electrode is
sufficiently high to increase the sensitivity of the
microfabricated potentiometric gas sensor device.
15. The microfabricated gas sensor device of claim 11, wherein the
ratio of the surface area of the WO.sub.3 sensing electrode to the
surface area of the Pt contact is sufficiently high to increase the
sensitivity of the microfabricated potentiometric gas sensor
device.
16. The microfabricated sensor device of claim 15, wherein a
thickness of the electrolyte layer of each sensor unit is maximized
a sufficient amount to minimize the internal resistance of the
corresponding potentiometric sensor unit, and such that the
internal resistance of each of the plurality of potentiometric
sensor units is minimized so as to minimize the overall resistance
of the microfabricated potentiometric gas sensor device and
increase sensitivity of the microfabricated potentiometric gas
sensor device.
17. The gas sensor device of claim 1, wherein the surface area of
the WO.sub.3 electrode on the electrolyte is at least two times
greater than the surface area of the Pt electrode on the
electrolyte.
18. The microfabricated potentiometric gas sensor device of claim
1, wherein the surface area of the WO.sub.3 electrode on the
electrolyte is at least five times greater than the surface area of
the Pt electrode on the electrolyte.
19. The microfabricated potentiometric gas sensor device of claim
1, wherein the layer of the WO.sub.3 has a lateral projection on
the surface of the electrolyte forming a torturous path.
20. The microfabricated potentiometric gas sensor device of claim
1, wherein the layer of the WO.sub.3 has a plurality of lateral
projections on the surface of the electrolyte.
21. The microfabricated potentiometric gas sensor device of claim
20, wherein the at least one of the plurality of lateral
projections of WO.sub.3 has at least two edge interfaces along the
surface of the electrolyte layer that increase the triple point
boundary.
22. The microfabricated potentiometric gas sensor device of claim
1, wherein the surface area of the platinum (Pt) contact is at
least 5 times smaller than the surface area of the layer of
tungsten oxide (WO.sub.3).
23. The microfabricated potentiometric gas sensor device of claim
1, wherein the surface area of the platinum (Pt) contact is at
least 9 times smaller than the surface area of the sensing
electrode.
24. The microfabricated potentiometric gas sensor device of claim 1
further comprising: a first electrical interconnect coupled to the
sensing electrode of a first potentiometric sensor within the
series of potentiometric sensor units; a second electrical
interconnect electrically coupled to the reference electrode of a
last potentiometric sensor within the series of potentiometric
sensors; and wherein the combined potential difference is
measurable at the first and second electrical leads.
25. The microfabricated potentiometric gas sensor device of claim
23, further comprising a third potentiometric sensor unit
electrically coupled between the first and last potentiometric
sensor units within the series of potentiometric sensors, wherein
the sensing electrode of the first potentiometric sensor is
connected to the reference electrode of the third potentiometric
sensor.
26. The microfabricated potentiometric gas sensor device of claim
2, wherein the combined potential difference comprises a sum of the
potential differences of each of the potentiometric sensor
units.
27. The microfabricated potentiometric gas sensor device of claim
1, wherein the sensor device is made using the photolithography
fabrication method.
28. The microfabricated potentiometric gas sensor gas sensor device
of claim 1, wherein the sensor device is made using the shadow mask
fabrication method.
29. The microfabricated potentiometric gas sensor device of claim
1, wherein the sensor device was fabricated using photolithography
method, wherein the surface of the sensor unit has been annealed
with oxygen to remove a photoresist mask.
30. A method for making a microfabricated potentiometric gas sensor
comprising: applying a photoresist mask to a surface of substrate;
depositing an electrolyte layer on the substrate; removing the
photoresist mask; and annealing the substrate with oxygen to remove
residual photo resist caused by a photoresist mask.
31. The method of claim 30, further comprising: applying a second
photoresist mask over the electrolyte layer; depositing a reference
electrode material over the second photoresist mask to form a
reference electrode, an electrical interconnect, and an electrical
contact portion of a sensing electrode; removing the second
photoresist mask; and annealing the substrate to remove residual
photo resist caused by the second photoresist mask.
32. The method of claim 31, further comprising: apply a third
photoresist mask over the electrolyte layer, the reference
electrode layer, electrical interconnect layer, and the electrical
contact for the portion of the electrode layer; depositing material
for a second portion of a sensing electrode over the third
photoresist mask to form a sensing electrode; removing the third
photoresist mask; and annealing the substrate to remove residual
photo resist caused by the another photoresist mask.
33. The method of claim 32, wherein the sensing electrode comprises
a layer of tungsten oxide (WO.sub.3) disposed on a platinum (Pt)
contact, and the reference electrode comprising platinum (Pt).
34. A method of sensing gas, the method comprising: receiving an
original sample; and generating a potential difference in a
microfabricated potentiometric sensor in response to presence of
gas in the sample, wherein the microfabricated potentiometric
sensor comprises: a base substrate; an electrolyte layer disposed
on the base substrate; a plurality of potentiometric sensor units
connected in series and coupled to the base substrate, each
potentiometric sensor unit comprising: an electrolyte layer
disposed on the base substrate; a two-part sensing electrode
comprising a layer of tungsten oxide (WO.sub.3) disposed on a
platinum (Pt) contact; and a reference electrode comprising
platinum (Pt).
35. The method of claim 34, comprising: determining a level of gas
within the original sample based on the potential difference
generated by the microfabricated potentiometric sensor; and wherein
the sensor device determines the gas level at a sensitivity of at
least 1 ppm.
36. The method of claim 35, wherein the gas is NOx.
Description
RELATED APPLICATION
[0001] This patent application claims priority to Application Ser.
No. 61/801,106 entitled "Miniaturized Gas Sensor and Method" filed
on Mar. 15, 2013, the entirety of which is incorporated by
reference herein.
TECHNICAL FIELD
[0002] The embodiments of the present invention relate generally to
gas sensors. More specifically, the disclosure relates to
miniaturized gas sensors that detect NO.sub.x gas.
BACKGROUND OF THE INVENTION
[0003] Nitric oxide (NO) sensing is a critical capability for a
variety of applications ranging from high temperature combustion to
clinical analysis. In high temperature combustion applications,
detection of nitrogen oxides (NO.sub.x) is critical in controlling
the processes used to reduce the NO.sub.x emissions produced by the
leaner combustion processes being developed to improve fuel
efficiency. NO.sub.x sensors that are high temperature capable may
also find use in other high-temperature applications. Another area
where NO.sub.x sensing is required is in the medical industry,
specifically in breath analysis. These do not typically involve
applications where the sensor operates in a high temperature
ambient environment, but it is one where the detection of nitric
oxide (NO) itself has high importance.
[0004] There are a variety of ways to detect NO, with solid-state
electrochemical sensors being one such technique. Such sensors also
have the added benefit of being easier to miniaturize compared to
other techniques. A variety of solid-state electrochemical sensors
for NO have been demonstrated previously. These techniques vary and
a continuing challenge is to design sensitive systems with limited
size, weight and power consumption so as to allow for portable
sensor systems. Such advancements would have notable impact on the
healthcare industry in enabling homecare monitoring units.
[0005] NO sensors capable of detecting NO at concentrations as low
as 7 ppb have been demonstrated using an array of sensor units in
series to increase the resulting sensor signal for a given NO
concentration. However, these sensors were made using hand assembly
techniques and also were assembled into arrays by hand. This manual
fabrication limits the minimum size to which the sensors can be
reduced.
[0006] Miniaturized sensors based on microelectromechanical systems
(MEMS) fabrication technology have been demonstrated for aerospace
applications. Sensors made by MEMS fabrication are very small
devices that can be made up of components and features between 1 to
100 micrometers in size (0.001 to 0.1 mm). Fabrication is a
challenge at these size scales for several reasons. Large surface
area to volume ratio of MEMS, and the resulting surface effects
which dominate over volume effects can improve sensor performance.
However, the overall surface area of a MEMS sensor unit may be
notably smaller than corresponding macro sensor devices. This may
decrease the overall number of chemical reactions involved,
resulting in a decreased signal. Thus, improved sensor design is
mandatory to enable miniaturization of sensor systems. Such
optimization may be different on the macro level then for micro
sensors, and simple application of design principle that are
successful for macro sensor can lead to significantly degraded
performance for micro sensors.
[0007] A reduction in size of the sensors using MEMS techniques
would not only decrease the size for better implementation in a
handheld home monitoring unit, but the reduced size would also
decrease the power required to bring the sensors up to operating
temperature. In addition, the utilization of MEMS fabrication
techniques introduces batch fabrication that allows for multiple
sensors to be made at one time, thus reducing costs.
SUMMARY
[0008] Various embodiments of a microfabricated gas sensor device
and method of fabricating a miniaturized gas sensor device are
provided. In one embodiment a microfabricated gas sensor device
includes a base substrate, an electrolyte layer disposed on the
base substrate and a plurality of potentiometric sensor units
electrically coupled together on the base substrate. Each
potentiometric sensor unit includes an electrolyte layer disposed
on the base substrate, a sensing electrode comprising tungsten
oxide (WO.sub.3), a reference electrode comprising platinum (Pt),
and a plurality of connectors coupled to the plurality of
potentiometric sensors to connect the plurality of potentiometric
sensors in series. The structure of each of these potentiometric
sensor units is designed to greatly improve sensor response.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The example embodiments of the present invention can be
understood with reference to the following drawings. The components
in the drawings are not necessarily to scale. Also, in the
drawings, like reference numerals designate corresponding parts
throughout the several views.
[0010] FIG. 1 illustrates a miniature sensor device, which is a
first generation design using shadow masks, according to an
embodiment of the present invention;
[0011] FIG. 2 illustrates a miniature sensor device, which is a
second generation design using shadow mask, according to an
embodiment of the present invention;
[0012] FIGS. 3 through 6 illustrate a sensor device at various
steps of a fabrication process, according to an embodiment of the
present invention;
[0013] FIG. 7 is a flow chart of the method for making the sensor
device, according to an embodiment of the present invention;
[0014] FIG. 8 illustrates a photographic image taken under a
microscope of one of the sensor units in the first generation,
shadow mask design of FIG. 1, according to an embodiment of the
present invention;
[0015] FIG. 9 illustrates aphotographic image taken under a
microscope of a sensor unit of the second generation, shadow mask
design of FIG. 2, according to an embodiment of the present
invention;
[0016] FIG. 10 illustrates a photographic image taken under a
microscope of an alternative miniature sensor unit of a second
generation, shadow mask design, according to an embodiment of the
present invention;
[0017] FIG. 11 illustrates the spectrum from XPS analysis of
sputter deposited WO.sub.3 film, in accordance with an embodiment
of the present invention;
[0018] FIG. 12 illustrates a miniature sensors of the second
generation shadow mask design, and a photoresist version of the
miniature sensor compared to a hand fabricated sensor, according to
an embodiment of the present invention;
[0019] FIG. 13 illustrates the test results of a miniature five
sensor unit device of the first generation, shadow mask sensor
design at 50 ppm exposure to NO, according to an embodiment of the
present invention;
[0020] FIG. 14 illustrates the test results of a miniature five
sensor unit device of a second generation, shadow mask sensor
design at 33 ppm exposure to NO, according to an embodiment of the
present invention;
[0021] FIG. 15 illustrates test results of a miniature ten sensor
unit device of a second generation, shadow mask design in a plot of
sensor voltage response to 20 ppm, 50 ppm, and 100 ppm NO exposure,
according to an embodiment of the present invention;
[0022] FIG. 16 illustrates the test results of a miniature fifteen
sensor unit device of a second generation, photolithographic design
in a plot of sensor response to 0.5 ppm, 2 ppm, 5 ppm, and 10 ppm
NO exposure, according to an embodiment of the present
invention;
[0023] FIG. 17 illustrates a photographic image showing a six
sensor unit device of a second generation, photolithographic
design, according to an embodiment of the present invention;
[0024] FIG. 18 illustrates a photographic image showing a six
sensor unit device of an alternative second generation,
photolithographic design, according to an embodiment of the present
invention;
[0025] FIGS. 19a and 19b are SEM images of sensor showing cracks of
the YSZ following the photolithographic process, according to an
embodiment of the present invention;
[0026] FIG. 20 illustrates a plot of test results of a sensor
device before extended heat exposure, according to an embodiment of
the present invention;
[0027] FIG. 21 illustrates a plot of test results after 48 hours of
continuous heat exposure, according to an embodiment of the present
invention;
[0028] FIGS. 22 and 23 before and after, respectively, extended
heat exposure, according to an embodiment of the present invention;
and
[0029] FIGS. 24 a-d illustrate SEM images of sensor region
containing WO3 after heat exposure, according to an embodiment of
the present invention.
DETAILED DESCRIPTION
[0030] The various embodiments of miniature NO sensors disclosed
herein is an electrochemical sensor whose structure includes sensor
units of solid electrolyte, a reference electrode and a working
electrode. An electromotive force (EMF) is induced between the
working and reference electrodes when NO impinges on the sensor due
to the dissimilar chemical activity at each electrode. In one
embodiment, the reference electrode is platinum (Pt), while the
sensing or working electrode is tungsten oxide (WO.sub.3). The
solid electrolyte is yttria-stabilized zirconia (YSZ). These sensor
material choices are based on larger hand-made sensors that are
described further in the examples below.
[0031] FIG. 1 shows a miniature sensor device 10 having individual
sensor units 50 and fabricated on aluminum oxide substrates 12.
Each sensor on the substrate comprises a YSZ island 50 upon which
the rest of the sensor is built. Pt reference electrode 52 and
WO.sub.3 sensing or working electrode 54 lie on top of the YSZ
island 51. A Pt contact 55 of the sensing electrode is located
below the WO.sub.3 to make contact to the WO.sub.3 of this two-part
sensing or working electrode. As each sensor 50 is an
electrochemical cell, the sensors can be connected in series to
generate a larger signal response. Thus, an array of sensors can be
used to improve the signal response to NOx, including NO. The
sensors are electrically connected together as in the cells of a
battery such that the induced EMF's are additive, thereby
increasing the response for a given NOx concentration over a single
sensor. Sensor devices of 5, 10, 15 and 20 sensors were connected
electrically and tested. Each sensor array is interconnected
electrically via Pt leads. This fabrication approach represents a
simple reduction in size of the larger in the sensors and it has
been found herein, as demonstrated in the examples below, that
simple reduction in size does not result in the desired sensitivity
needed to improve sensor performance.
[0032] With reference to FIG. 1, the gas sensor device 10 includes
a plurality of, i.e. or at least two, sensor units 50 in a dice
unit row 12. Each of the sensor units 50 includes a reference
electrode 52 and a sensing electrode 54. In one embodiment, the
sensing electrodes 54 comprise WO.sub.3, and the reference
electrodes 52 comprise platinum (Pt).
[0033] In one embodiment, the gas sensor device 10 includes 3 rows
14 of sensor units 14, but a variety of sensor units 50 is
possible. Each of the sensor units 50 is electrically coupled to at
least one adjacent sensor unit 50. For example, the sensor units 50
are electrically connected together in series. The combined
potential difference of the plurality of sensor units 50 is
approximately a sum of the potential differences of each of the
individual sensor units 50 electrically connected to one
another.
[0034] Experimental tests that have been conducted herein show that
the sensitivity of the system 10 is based on the number of sensor
units 50. Each of the sensor units generates a potential difference
in the response to a gas, for example, NO.sub.x gas. Generally, a
system including more sensor units 50 has been found to be
relatively more sensitive to NO, and a system including less sensor
units has been found to be relatively less sensitive to NO.
However, there is a point at which additional sensor units 50 will
not improve the sensitivity, and it has been found that sensor
devices that have 15-20 sensor units have increased sensitivity.
This is due to lack of previous recognition of the various elements
of sensor design including, for example, the internal resistance of
each sensor element. As noted above, a corresponding reduction of
the size of the sensor having the same materials of construction do
not result in improved sensitivity. The impact of the internal
resistance of the individual sensor units, and the design features
that contributed to higher sensitivity is described herein and was
discovered during the course of the fabrication of sensors as
discussed in the examples below. Overcoming internal resistance is
core to even higher levels of sensitivity.
[0035] It has been found herein, in accordance with various
embodiments of the present invention, that reducing the exposed
surface area of Pt reference electrode on the YSZ electrolyte and
increasing the surface area of WO.sub.3 electrode covering the YSZ
electrolyte improves the sensitivity of the sensor as can be seen
in comparing the results of the first generation, shadow mask
design of FIG. 1 and second generation, shadow mask design of FIG.
2 and shown in the results of the examples. FIG. 2 shows a
miniature sensor device having individual sensor units 60 and
fabricated on a substrate, for example, aluminum oxide substrates.
Each sensor on the substrate comprises a YSZ island 50 upon which
the rest of the sensor is built. Pt reference electrode 62 and
WO.sub.3 sensing or working electrode 64 lie on top of the YSZ
island 61. A Pt contact 65 of the sensing electrode is located
below the WO.sub.3 to make contact to the WO.sub.3 of this two-part
sensing or working electrode. As each sensor unit 60 is an
electrochemical cell, the sensors can be connected in series as a
sensor device to generate a larger signal response. Thus, an array
of sensors can be used to improve the signal response to NO.
[0036] The ratio of the exposed WO.sub.3 to the exposed platinum Pt
is maximized to increase the sensitivity and obtain a low end
sensor reading. Furthermore, in another embodiment, the ratio of
the exposed WO.sub.3 to the exposed platinum Pt is maximized while
also decreasing the size, for example the surface area, of the Pt
contact of the sensing electrode that is contact with WO.sub.3
Decreasing the size of the contact 65 underneath the WO.sub.3 so
that it is minimized to the extent of fabrication (i.e. within the
resolution of the fabrication approach, such as 2 microns) is found
to increases the sensitivity of the microfabricated sensors units
60. This electrode structure is not a simple one component
electrode, but rather composed of both an oxide and metallic
electrode combination that together are designed for improved
response. In accordance with an embodiment of the present
invention, the electrolyte layer of the microfabriated
potentiometric gas sensor device has a thickness of the electrolyte
layer that is maximized a sufficient amount to minimize the
internal resistance of the potentiometric sensor unit, and such
that the internal resistance of each of the plurality of sensor
units is minimized so as to minimize the overall resistance of the
sensor device to increase the sensitivity of the sensor device.
[0037] For example, in one embodiment the surface area of the
WO.sub.3 electrode on the electrolyte is greater than the surface
area of the Pt electrode. In another embodiment the WO.sub.3 covers
all of the available surface on the YSZ unused by the Pt electrode
within the resolution of the fabrication approach (approximately 2
microns depending on the equipment used). In another embodiment the
surface area of the WO.sub.3 electrode is at least two times
greater than the surface area of the Pt electrode, and in another
embodiment, the WO.sub.3 electrode is at least 5 times greater than
the surface area of the Pt electrode, and in yet another embodiment
the WO.sub.3 electrode is at least 10 times greater than the
surface area of the Pt electrode.
[0038] The increased surface area of the WO.sub.3 boundaries does
increase the triple point boundary of the WO.sub.3 electrode, the
YSZ electrolyte and the gas, for example, NO gas compared to those
of the Pt electrode. The decreased surface area of the Pt electrode
decreases the triple point boundary of the Pt, the YSZ electrolyte
and the gas. The limitation on the amount of YSZ surface area that
is not also a triple point boundary is believed to decrease the
sensitivity of the sensor. As a result it has been found that the
sensitivity of the sensor device can be increased.
[0039] In addition FIG. 9 shows that the surface of the WO.sub.3
electrode 310 of sensor unit 300 has at least one lateral
projection 320. The at least one lateral projection has at least
two edge interfaces 330, 340 along the surface of the electrolyte,
and in another embodiment at least three edge interfaces, thereby
creating additional triple point boundary points between the
WO.sub.3 electrode and YSZ electrolyte. In another embodiment, FIG.
10 shows that the surface of the WO.sub.3 electrode has three
lateral projections 420, 430 and 440. Each lateral projection has
at least three edge interfaces 510, 520 and 530 along the surface
of the electrolyte which forms a triple point boundary between the
WO.sub.3, the YSZ electrolyte and the gas. As shown the lateral
projections form corners formed by the edge interfaces 520 and 530.
These lateral projections increased the torturous nature of the
pattern and increase the number of triple points. For example,
these lateral surface contours increase the lineal length of edge
interface between the WO.sub.3, the electrolyte, and the
surrounding gas as illustrated in FIGS. 9 and 10.
[0040] Further, the size of the YSZ patterns in FIG. 2 compared to
FIG. 1 decreases the distance, and thus corresponding resistance,
between the electrodes. Other factors decreasing this resistance
include the thickness of the electroylyte,
yttria-stabilized-zirconia, and thickness of the various metal
layers that are micro-deposited on the surface. Such features are
not apparent in macro sensor since, for example, a 5 micron change
in the thickness of the zirconia in a macro sensor has much less
effect on the internal resistance of the sensor unit where it would
nearly eliminate the zirconia layer for a microfabricated
sensor.
[0041] It should be noted that this is a potentiometric sensor
(voltage difference), rather than an amperiometric sensor (current
flow). In a sensor that measures current flow, the effect of
resistance is known and too large a resistance can readily be seen
to limit the measurement. Such amperometric sensors are not linked
in series like batteries (as are the potentiometric sensors in our
work) and the effect of increase resistance is directly noticeable
in the measurement. It is discovered that an aspect of the
potentiometric sensor device that includes sensor units linked in
series, is that high resistance of each sensor unit was found to
limit the lower detection limit of the sensor overall. Thus, while
each sensor unit might have a resistance that did not notably
affect its operation; the combined resistance of each of the
potentiometric sensor unit in series can change the lower limit
detection capabilities of the overall sensor. This may not be
obvious at higher concentrations, but was found to have significant
effect on sensitivity for lower concentration measurements. It was
found that decreasing this overall resistance is a feature of
increasing the sensor's lower detection limit.
[0042] The microfabricated potentiometric gas sensor device senses
gas at a broad range of temperatures, including but not limited to,
high temperatures that range from about 500.degree. C. to about
700.degree. C., in another embodiment, from about 550.degree. C. to
about 650.degree. C.
Method of Fabrication
[0043] MEMS fabrication has been successfully implemented in the
examples herein, where sensors are batch fabricated on a single
wafer, with each wafer containing multiple sensors units. These
examples show that applying the concepts above are not only
achievable but improve the capability of the sensor. These examples
are meant to show different aspects of the design optimization from
large to smaller sensors and so while one example may show improved
response time but decreased response, it is the combination of the
various design features that is understood to provide an improved
sensor system, or may be used as needed to emphasize certain
aspects of the sensor response. The sensors are fabricated using
masks and thin film deposition techniques. Each layer of the
structure is deposited via sputtering from a target containing the
desired material or a component of the desired material, with the
masks serving to define the shape of the resulting deposited film.
The fabrication of these sensors was carried out using thin metal,
shadow masks or photoresist masks. The sensors were fabricated on
two-inch alumina wafers.
[0044] The general process flow for the sensors is shown in FIG. 7.
The first step in the fabrication process is to clean the alumina
wafer using a combination of solvents, generally acetone followed
by isopropanol. The YSZ islands are sputter deposited using the
first mask, forming individual rectangular islands of YSZ. A
thermal anneal is then carried out at 1000.degree. C. for two hours
in air ambient to densify the YSZ. The Pt electrodes are then
deposited using the next mask. A second platinum layer is sputter
deposited to form the interconnects, electrically connecting the
sensors in the array. It should be noted that in some designs of
the sensor device the Pt interconnects are deposited at the same
time as the first Pt electrode deposition. Finally, the WO.sub.3 is
reactively sputter deposited from a tungsten target in an
argon/oxygen atmosphere using the third mask. The sensors are then
diced into individual arrays following the final film
deposition.
[0045] The three films that are deposited are the YSZ, Pt, and
WO.sub.3 films. Both the YSZ and Pt films are deposited by a
sputter deposition process. The WO3 also is deposited by a sputter
process. However, the sputter process is a reactive sputter process
using a tungsten (W) target whereby a W target is sputtered to
produce W atoms that are then reacted with an oxygen gas flow in
the chamber prior to impingement on the substrate where they are
deposited as WO.sub.3. The deposition is done at room temperature
using a cooled substrate to keep the substrate cool. XPS analysis
on the films confirmed the proper stoichiometry of the films after
the sputter deposition processes, as shown in FIG. 10. No sensors
fabricated using thin film microfabrication techniques that are
significantly smaller than such sensors are herein demonstrated.
Changes made from the initial design to the size of both the
reference and working electrode as well as the contacting Pt
electrode under the working electrode resulted in improved
sensitivity of the sensor. These sensors have demonstrated
sensitivity below the ppm level.
[0046] FIGS. 3 through 6 illustrate a sensor array at various steps
of a fabrication process, according to an embodiment of the present
invention. Cross-sectional and top views are shown left and right,
respectively: (a) Alumina substrate, (b) deposition of YSZ islands,
(c) following deposition of Pt electrodes, (d) sensor after
deposition of WO.sub.3, according to an embodiment of the present
invention;
[0047] FIGS. 3A and 3B show the base layer of alumina used in the
substrate 100 in an embodiment of the present invention. FIGS. 4A
and 4B show a side view and top view, respectively, of the alumina
substrate 100 with an electrolyte layer 102 deposited on the
substrate. FIGS. 5A and 5B show a side view and top view,
respectively, of the substrate 100 with the electrodes 104
deposited on top of the electrolyte layer 102. FIGS. 6A and 6B show
the side view and top view of the tungsten oxide WO.sub.3 electrode
106 which is the working electrode deposited on top of the Pt
contact 104.
[0048] FIG. 7 illustrates the process step for making the gas
sensor. In one embodiment the box 205 shows the first step of
cleaning the alumina wafer substrate. Solvents or combinations of
solvents, for example alcohols acetone and isopropanol, or an
application of solvent in a series of acetone followed by
isopropanol can be used to clean the substrate. In box 210 the mask
is placed on the substrate, a thin metal mask for the shadow mask
design and a photoresist mask in the photoresist design n box 210,
for the photoresist design the photo resist mask is applied to the
surface of the substrate. Once the photoresist mask is deposited,
the substrate is soft baked to remove some of the solvents in the
liquid photoresist. Then the photoresist is placed under a UV light
source and the UV light is selectively passed through a glass mask
with defined openings through which light may pass to the
photoresist. Depending on whether the photo resist is positive or
negative, the resulting regions exposed to light will either become
more soluble or less soluble respectively after exposure to the UV
light. The substrate is then placed into a developing solution that
removes the more soluble regions of the photo resist. The substrate
is then placed under heat for a hard bake of the photo resist mask.
In the next step of box 215 the process includes depositing a layer
of electrolyte using a sputter deposition of the desired film. The
photo resist is then removed by a solvent, usually acetone, and the
sputtered film on top of the photo resist is also removed leaving
behind the thin film that was defined by the openings in the photo
resist mask. The photo resist mask has precisely defined features
down to below 100 micrometers which is a much smaller resolution
than the shadow mask used in the embodiments described above. Next
in box 220 the substrates were annealed in the presence of oxygen
in order to remove or clean the residual photo resist from the
surface. It was found that standard techniques to remove the photo
resist perform poorly and the miniaturization process did not
provide good results compared to the shadow mask method of making
the sensor when using the standard software-based photoresist
removal techniques. The oxygen annealing temperature to remove the
residual photo resist on the surface or in the pores of the films
of the sensor can be carried out at a temperature that is at least
350.degree. C., and another embodiment from about 350.degree. C. to
about 450.degree. C., and in another embodiment from about
390.degree. C. to 425.degree. C. It has been found that residual
photoresist can notably affect sensor response and the oxygen
annealing step, in accordance to an embodiment of the present
invention was found to improve sensitivity.
[0049] Still referring to FIG. 7, the process for making the gas
sensor further includes applying another mask over the electrolyte
layers and next in box 230 the process further includes depositing
the reference electrode material and contact material for the sense
electrode over the mask but through sputtering. The mask is removed
and for the photoresist-based sensor the substrate is again
annealed to remove residual mask material in box 235. Next in box
240 another mask is again applied over the substrate and over the
electrodes to deposit the connectors in box 250. In another
embodiment the reference electrode, contact for the sense
electrode, and the connectors can be applied in a single step after
application of the mask in box 210. Next in box 260 the mask is
removed and for the photoresist-based sensor the substrate then
undergoes the annealing process to remove the residual photoresist.
Next in box 265 the mask is applied again and then in box 270 the
sensing electrode is deposited by reactive sputtering. Next the
mask is removed and for the photoresist-based sensor the substrate
undergoes another annealing process as shown in box 270 to remove
all residual photoresist. Finally, in box 280 the substrate can
undergo dicing to separate the individual sensor arrays 10 on the
substrate. The oxygen annealing temperatures of boxes 235, 260, and
275 are carried out and are the same as the temperature ranges
described above with respect to box 220.
[0050] The results of the testing on these various generations of
designs indicate that the changes that were made between each
generation were indeed beneficial to the overall performance of the
sensor. Reducing the exposed Pt on the YSZ and increasing the WO3
covering the YSZ improved the sensitivity of the sensor as can be
seen in comparing the results of the first and second generation
shadow mask designs. The resulting design changes were applied to
the photomask-based design, which is basically the second
generation shadow mask design reduced in size by a factor of 0.35.
The test results of the photoresist design indicate that the sensor
array should be capable of sensing down to at least 500 ppb level,
and in another embodiment down to about 300 ppb level.
[0051] In another embodiment, the several embodiments of the NO
sensor device described above can be used in an apparatus for
measuring the level of NO. The apparatus includes the sensor device
and an inlet for receiving a gas sample. The gas sample, for
example, NO gas, is in fluid communication with the sensor. The
potential difference is indicative of a level of NO within the
original sample. In one embodiment, the gas sample is a breath
sample from the subject. In another embodiment, the gas sample that
enters the apparatus may be treated by humidification or
dehumidification to improve the sensitivity. The potential
difference of the sensor array 10 is a summation of the individual
potential differences across the individual sensor units in
response to presence of the NO in the gas sample.
EXAMPLES
Examples
Shadow Mask Fabrication
[0052] The shadow mask version of the sensor arrays utilized a
metal shadow mask during each of the deposition processes to define
the deposited films into the desired features. The metal masks were
placed onto the substrate and clamped at the edges of the
substrate. The shadow masks are easy to use and are aligned from
one mask layer to the next. However, the defined features are
rather large in size, there can be distortion in the shadow mask
resulting in edges of the defined shapes that are not sharp, and
after multiple uses the resulting film build up on the shadow masks
may cause warpage of the shadow mask and/or micromasking during the
deposition process as particles fall off of the shadow mask and
land on the openings defined by the mask.
[0053] For comparison, our initial sensor design is shown in FIG. 1
with corresponding dimensions, along with the second generation,
shadow mass that is illustrated in FIG. 2. FIG. 1 shows that the
electrolyte island 51 dimension A is 1.55 mm by 2.88 mm; the Pt
reference electrode 52 of dimension B is 1 mm.times.0.21 mm; the Pt
interconnect 53 of dimension C has an outer length of 1.56 mm and
an inner length of 1.15 mm; the sensing electrode 54 of WO.sub.3
has a dimension D of 1.1 mm.times.0.31 mm; and the Pt contact 55 of
the sensing electrode E is approximately the same surface area as
the reference electrode 52. FIG. 2 shows that the electrolyte
island 61 dimension N is 1.328 mm by 1.55 mm; the Pt reference
electrode 62 of dimension I is 0.4 mm.times.0.21 mm; the Pt
interconnect 63 of dimension G has dimensions of 0.33 mm.times.1.4
mm; the WO.sub.3 of sensing electrode 54 has an "L" shape that can
be calculated by dimensions L (0.54 mm), M (1.4 mm), O (1.22), and
K (0.88); and the Pt contact 65 of the sensing electrode has
dimensions H, 1.5 mm.times.0.1 mm.
[0054] FIG. 12 illustrates a comparison of sensors to hand
fabricated sensor on the far right, with a quarter shown for size
reference. The shadow mask version of the sensor is to the left of
the quarter and is 10 mm by 12 mm. The photoresist version of the
sensor is to the far left and is 3 mm by 4 mm in size.
[0055] In comparing the first and second generation shadow mask
designs, the size of the WO.sub.3 covering the YSZ is increased by
a factor of 4.0 from 0.341 mm.sup.2 to 1.35 mm.sup.2 for the
WO.sub.3 on the YSZ in moving from design generation one to two.
Similarly the Pt reference electrode was decreased in size by a
factor of 2.5 from 0.21 mm.sup.2 to 0.084 mm.sup.2 in moving from
design generation one to two. The size of the YSZ island for each
individual sensor is 1.55 by 2.28 mm.sup.2 for generation one and
1.328 by 1.55 mm.sup.2 for generation two designs. These designs
are shown in FIGS. 1 and 2.
Examples
Photoresist Process
[0056] The second variation of the sensors that was fabricated was
a photoresist-based version of the sensors. In this version of the
sensor arrays, the deposited films are defined by photoresist
layers deposited on the substrate. In the photoresist process a
liquid photoresist film is spun onto the surface of the substrate.
Once the photoresist is soft baked to remove some of the solvents
in the liquid photoresist, the substrate is placed under a UV light
source that is defined by a glass mask. A thin metal film on the
glass mask defines openings through which light may pass to the
substrate. Depending on whether the photoresist is positive or
negative, the resulting regions exposed to light will either become
more soluble or less soluble, respectively, after exposure to the
UV light. The substrate is then placed into a developer solution
that removes the more soluble regions of the photoresist. After a
hard bake the photoresist mask is ready to be used as the mask for
sputter deposition of the desired film. Once the sputter deposition
process is finished the sputtered film is defined by a lift-off
process whereby the photoresist is removed by a solvent, usually
acetone, and the sputtered film on top of the photoresist is also
removed leaving behind the thin film that was defined by the
openings in the photoresist mask.
[0057] The advantage of the photoresist mask versus the shadow mask
is that the photoresist mask can define features to a much smaller
resolution (e.g., down to about 2 micrometers). The photoresist
version of the sensor was decreased to 0.35 times the size of the
second generation shadow mask version, the sensors are otherwise
identical in layout design. The downside to the photoresist mask is
the possible contamination of the underlying materials with
photoresist if they are porous. If the photoresist is not
completely removed following the thin film deposition the resulting
remnants may react at higher temperatures and form a barrier to gas
reaction at the surfaces of the sensor. In fact, sensors that were
initially fabricated using standard photoresist development and
removal techniques performed poorly compared to the shadow mask
versions. This link between photoresist contamination and degraded
sensor performance was confirmed when shadow mask versions of the
sensors that were covered with photoresist prior to dicing into
individual arrays by the dicing saw were found to perform poorly
compared to similar shadow mask sensors not subjected photoresist
coating that were partially diced (pre-scribed) prior to
fabrication. Several methodologies were attempted in removing any
residual photoresist from the sensor surfaces. Continued solution
in acetone and application of ultrasonic in acetone solution were
both tried with little change in results. A typical solution to
such a problem is to use an oxygen plasma clean to remove such
residual photoresist. However, due to the nature of the films that
were depositing, there was a small percentage of Na in the
resulting films that precluded the employment of this oxygen plasma
system as this was designated a MOS piece of equipment that should
be free of exposure to salt containing films. Surprisingly, it was
discovered that when the sample substrates were exposed to an
oxygen annealing in a tube furnace after each photoresist step,
residual photoresist was removed. This higher temperature process
of about 400.degree. C. removed the residual resist on the surface
or in the pores of the films on the sensors. Sensors fabricated
using this method produced the same or better results compared to
equivalent shadow mask sensor arrays validating the employment of
oxygen anneal to remove residual photoresist.
[0058] FIG. 12 is an optical image comparing arrays of sensors from
hand fabricated down to photoresist based sensors. Each successive
version of the sensor array is smaller in size, moving from the
original hand built sensor array to the shadow mask version to the
final photoresist-based version. As can be seen in FIG. 12, the
photoresist sensor is over a magnitude smaller in each dimension
compared to the hand fabricated version. The reduction in size
allows for smaller heater stages to be used to heat the sensor and
thus reduced power requirements.
Sensor Performance--Shadow Mask
[0059] Sensors were tested at temperatures ranging from 500 to 600
degrees Celsius to determine the efficacy of the sensors. Generally
sensors were found to work best when operated between 550 and 600
degrees Celsius. The sensors were tested by bringing the sensors to
temperature and awaiting the sensor's stabilization. Once the
temperature was stable, various NO gas concentrations were
introduced to the sensor. Air was generally used as the baseline
gas for these experiments. In general, the electrical connections
to the sensors were made via either probe tips or wires attached to
the contact pads at either end of the sensor array.
[0060] Test results of the initial first generation design are
shown in FIG. 13. The tested sensor was a five sensor array. As can
be seen from the results, the sensor response at 50 ppm was less
than 10 mV for this concentration of NO. A photographic image of
one of the sensors in this array is shown in FIG. 8. For comparison
in FIG. 14, the results of testing on a second generation shadow
mask design are shown for a five sensor array. This second
generation five sensor array shows a response of nearly 10 mV for a
33 ppm concentration of NO. It can be deduced from these
measurements that the sensor response increased quite significantly
(showing an equivalent response for a concentration reduction of
1.5 times) due to the changes made from the first to the second
generation shadow mask design. Also shown in FIGS. 9&10 are
photographic images of two sensors, showing the two different
designs of the sensor in the second generation design. FIG. 15
shows the results of testing with a 10 sensor array from this same
wafer. The results are shown for the sensor tested at 550 degrees
Celsius.
[0061] In addition it was found that sensor response of each
individual sensor in the array could be maximized by applying
and/or modifying several parameters. For the working electrode
minimizing the Pt exposed on the YSZ and maximizing WO.sub.3 film
exposure were found to increase the sensor response for a given NO
concentration. Pt exposure on YSZ was minimized at both the
reference and working electrodes. This change was done to minimize
the triple point boundaries between the gas, Pt, and YSZ and thus
the reactions at the exposed Pt surfaces on YSZ, thereby decreasing
competing reactions that would decrease the induced potential
across the sensor. Similarly, it was found that maximizing the
WO.sub.3 film on top of the YSZ was found to increase the induced
potential across the sensor. In this case, this was due to an
increase in the number of triple-point boundaries between the gas,
WO.sub.3, and YSZ.
Sensor Performance--Photoresist
[0062] The results of the photoresist-based version are shown in
FIG. 16. As can be seen in the test results, the photoresist-based
version of the sensor array was capable of reaching below 500 ppb.
These results are from a sensor array of 15 sensors connected in
series on a chip. These sensors were tested using a catalytic
filter to improve signal response and remove possible interfering
gases.
[0063] The results of the testing on these various generations of
designs indicate that the changes that were made between each
generation were indeed beneficial to the overall performance of the
sensor. Reducing the exposed Pt on the YSZ and increasing the WO3
covering the YSZ improved the sensitivity of the sensor as can be
seen in comparing the results of the first and second generation
shadow mask designs. The resulting design changes were applied to
the photomask-based design, which is basically the second
generation shadow mask design that was a factor of 0.35 in size
compared to the shadow mask design. The test results of the
photoresist design indicate that the sensor device is capable of at
least 500 ppb level sensitivity and lower.
[0064] It was found during testing of the sensor arrays is the
increased impedance of the connected connector array. In general,
the 15 sensor array was found to be in the 60 MOhm range at
operating temperature. The high impedance made the sensor array
very sensitive to electrical Noise in the surrounding environment.
A thicker YSZ film can decrease the impedance of the sensor due to
the thicker film increasing the area through which ions could move
from one end to the other of the sensor, however, it is found that
the residual stress in the YSZ film increases and at higher
thicknesses this stress can cause cracking in the film, especially
after a thermal exertion to the operating temperature of the
sensor. These cracks often run through the film, as seen in FIGS.
24a-24d, and could possibly result in lower conductivity of the
film.
[0065] Another possible issue is the longevity of the sensors.
Although the sensors were capable of repeated performance during
testing it was found that over a longer period of time (several
days of continuous testing) that the sensor performance would
gradually decrease. From optical and SEM examination it appears
that the films may be reacting at temperature and migrating from
their original deposited locations.
[0066] Although the invention has been described with reference to
several specific embodiments, this description is not meant to be
construed in a limited sense. Various modifications of the
disclosed embodiments, as well as alternative embodiments of the
invention will become apparent to persons skilled in the art upon
the reference to the description of the invention. It is,
therefore, contemplated that the appended claims will cover such
modifications that fall within the scope of the invention.
* * * * *