U.S. patent application number 09/891643 was filed with the patent office on 2001-10-25 for combustible gas diode sensor.
This patent application is currently assigned to Ford Global Technologies, Inc.. Invention is credited to Gebremariam, Samuel Admassu, Rimai, Lajos, Samman, Amer Mohammad Khaled.
Application Number | 20010032493 09/891643 |
Document ID | / |
Family ID | 21833728 |
Filed Date | 2001-10-25 |
United States Patent
Application |
20010032493 |
Kind Code |
A1 |
Samman, Amer Mohammad Khaled ;
et al. |
October 25, 2001 |
Combustible gas diode sensor
Abstract
A combustible gas sensor diode including a SiC semiconductor
substrate, on top of which an AlN layer and a catalytic metal
"gate" electrode are deposited is disclosed. The combustible gas
sensor diode can be operated in either a D.C. forward conduction
mode or an A.C. reverse bias mode. Methods of detecting
combustibles in both D.C. and A.C. modes are further disclosed.
Inventors: |
Samman, Amer Mohammad Khaled;
(Dearborn, MI) ; Gebremariam, Samuel Admassu;
(Detroit, MI) ; Rimai, Lajos; (Ann Arbor,
MI) |
Correspondence
Address: |
Sangeeta G. Shah
Brooks & Kushman P.C.
1000 Town Center, 22nd Floor
Southfield
MI
48075-1351
US
|
Assignee: |
Ford Global Technologies,
Inc.
Dearborn
MI
|
Family ID: |
21833728 |
Appl. No.: |
09/891643 |
Filed: |
June 25, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09891643 |
Jun 25, 2001 |
|
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|
09026776 |
Feb 20, 1998 |
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Current U.S.
Class: |
73/31.06 |
Current CPC
Class: |
G01N 27/129
20130101 |
Class at
Publication: |
73/31.06 |
International
Class: |
G01N 007/00 |
Claims
What is claimed is:
1. A combustible gas sensor diode structure comprising: a SiC
semiconductor substrate; a metal back contact juxtaposed to the SiC
semiconductor substrate; an AlN layer deposited on the SiC
semiconductor substrate; and a catalytically active metal "gate"
electrode deposited on the AlN layer.
2. The diode structure of claim 1, wherein the SiC semiconductor
comprises a SiC single crystal substrate having an epitaxial layer
of SiC thereon.
3. The diode structure of claim 1, wherein the ohmic metal back
contact comprises a metal selected from the group consisting of
tantalum, titanium, nickel, and silicides thereof, with a platinum
layer deposited thereon, wherein the selected metal is placed in
contact with the SiC semiconductor substrate.
4. The diode structure of claim 1, wherein the metal back contact
comprises a laser ablated platinum film.
5. The diode structure of claim 1, wherein the catalytically active
metal gate comprises a catalytically active metal selected from the
group consisting of platinum, palladium, rhodium and alloys
thereof.
6. The diode structure of claim 1, wherein the AlN layer has a
thickness between 1000 and 2000 angstrom.
7. The diode structure of claim 1, wherein the AlN layer is laser
ablated onto the SiC semiconductor substrate.
8. The diode structure of claim 1, wherein the semiconductor
substrate comprises n-type SiC.
9. The diode structure of claim 1, wherein the catalytically active
metal gate has a thickness in the range of 1000-2000 angstroms.
10. The diode structure of claim 1, wherein the metal back contact
has a thickness in the range of 1500-2500 angstroms.
11. A method of detecting combustibles in a gas stream, comprising:
placing a reverse biased diode sensor in contact with the gas
stream, wherein the diode sensor comprises: a SiC semiconductor
substrate; a metal back contact juxtaposed to the SiC semiconductor
substrate; an AlN layer deposited on the SiC semiconductor
substrate; and a catalytically active metal gate deposited on the
AlN layer; measuring the capacitance of the diode sensor at
frequencies between 100 and 1000 KHz; and detecting a shift in the
capacitance versus voltage characteristics of the diode sensor.
12. A method of detecting combustibles in a gas stream comprising:
placing a forward biased diode sensor in contact with the gas
stream, wherein the diode sensor comprises: a SiC semiconductor
substrate; a metal back contact juxtaposed to the SiC semiconductor
substrate; an AlN layer deposited on the SiC semiconductor
substrate; and a catalytically active metal gate deposited on the
AlN layer; applying a constant forward current to the diode sensor;
and measuring a change in the voltage drop across the diode sensor
in response to changes in gas stream composition.
13. A method of detecting combustibles in a gas stream comprising:
placing a forward biased diode sensor in contact with the gas
stream, wherein the diode sensor comprises: a SiC semiconductor
substrate; a metal back contact juxtaposed to the SiC semiconductor
substrate; an AlN layer deposited on the SiC semiconductor
substrate; and a catalytically active metal gate deposited on the
AlN layer; applying a constant forward voltage to the diode sensor;
and measuring the change in the current through the diode sensor in
response to changes in gas stream composition.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a combustible gas diode sensor
including a SiC semiconductor substrate on top of which an AlN
layer and a catalytic metal "gate" electrode are deposited. The
resulting devices can be operated in either a D.C. forward
conduction or an A.C. reverse bias mode.
BACKGROUND
[0002] MOS combustible gas sensors operate by catalytic oxidation
of combustible gases at the "gate". Substantial efforts have been
expended in recent years towards the development of combustible gas
sensors using semiconductor MOS technology.
[0003] Generally the MOS gas sensor consists of a semiconductor
substrate with an ohmic contact on one side and with the other side
covered by a SiO.sub.2 insulating layer with a metal gate on top.
The metal gate is composed of a metal capable of catalyzing the
oxidation of combustible gases. As a result of catalytic redox
reactions on the gate surface, certain atomic and molecular species
are generated which can diffuse through the porous gate to the
metal gate/insulator interface where they can ionize. These ions
can penetrate through the insulator thereby changing the potential
distribution across the device. This changes the potential of the
insulator/semiconductor interface and thus the depletion layer
inside the semiconductor which in turn shifts the voltage dependent
A.C. admittance characteristic of the device along the voltage
axis.
[0004] In order to be sensitive to combustibles other then H.sub.2,
the catalytic gate and, therefore, the device have to be operated
at temperatures above 400.degree. C., requiring the use of a wide
band gap semiconductor such as SiC instead of Si. However, at such
high temperatures the SiO.sub.2 layer becomes less insulating as
the ionic charges within the layer become mobile. Under these
conditions the device acts as a true capacitor only when biased in
depletion (e.g., for n-type SiC, the gate voltage is negative with
regard to the back contact). In accumulation, it begins showing
some D.C. conduction. However, as the conduction process in the
SiO.sub.2 is different from that in the SiC, there will be a finite
voltage (the barrier potential) at which there is onset of this
forward conduction. This barrier potential will depend on the
charges injected into the SiO.sub.2 insulator by the chemical
processes at the gate. As changes in this barrier voltage are
directly reflected as changes in the current/voltage D.C.
characteristic in the forward direction, a sensor response can be
obtained by measuring changes in this characteristic as a function
of changes in the combustible concentration near the catalytic
gate. However, the mobility of charges in SiO.sub.2 is still
relatively low. Therefore, to obtain a reasonable forward current
with reasonable applied voltage the thickness of this SiO.sub.2
layer has to be very small, leading to breakdown instabilities.
SUMMARY OF THE INVENTION
[0005] The present invention provides a combustible gas sensor
consisting of a diode structure which includes a silicon carbide
semiconductor substrate having a metal back contact juxtaposed
thereto. An AlN layer is deposited onto the SiC semiconductor
substrate and a thin porous catalytically active metal electrode
"gate" is deposited on the AlN layer.
[0006] This diode structure allows the detection of combustibles,
such as hydrocarbons and carbon monoxide in a gas flow in a wide
concentration range with the adjustment of certain parameters, such
as the oxygen concentration. This device is compatible with
semiconductor electronic technology and is substantially more
robust than devices requiring membrane support for low thermal mass
as the micro-calorimeter. The device further can be operated in
either a forward conduction D.C. or a reverse bias A.C. response
mode, the former not requiring radio frequency techniques.
[0007] In the D.C. mode embodiment, there is provided a method of
detecting combustibles in a gas stream by placing a forward biased
diode sensor in contact with the gas stream, wherein the diode
sensor comprises a silicon carbide semiconductor substrate; a metal
back contact juxtaposed to the silicon carbide semiconductor
substrate; an AlN layer deposited onto the silicon carbide
semiconductor substrate; and a catalytically active metal "gate"
electrode deposited on the AlN layer. The method further includes
the step of applying a constant forward current to the diode sensor
and measuring changes in the forward voltage drop across the diode
sensor as the response to changes of the combustible concentration
in the gas stream. Alternatively, a constant forward bias may be
applied across the sensor measuring changes in the forward current
through the device as the response to changes of the combustible
concentration in the gas stream.
[0008] In the A.C. mode, the present invention also provides a
method of detecting combustibles in a gas stream, by placing the
reverse biased diode sensor in contact with the gas stream, wherein
the diode sensor comprises a silicon carbide semiconductor
substrate; a metal back contact in mating engagement with the
silicon carbide semiconductor substrate; an AlN layer deposited
onto the SiC semiconductor substrate; and a catalytically active
metal gate deposited on the AlN layer. The method next involves the
step of measuring the capacitance of the diode sensor at
frequencies between 10 and 3000 KHz. A constant reverse bias
voltage is applied across the diode sensor. Lastly, the method
requires the step of detecting a change in the capacitance in
response to a change of the combustible concentration in the gas
stream.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 illustrates a simple diagram of a gas sensor diode
structure;
[0010] FIG. 2 is a plane view optical microphotograph of a set of
nine Pt/AlN/SiC sensor diodes fabricated on a common substrate. The
circular patches are the 1 mm diameter Pt gates;
[0011] FIG. 3 is a graph illustrating the D.C. current-voltage
characteristic I(V) of a Pt/AlN/SiC sensor diode at 520.degree. C.
with and without propane in the ambient gas (nitrogen). Positive
voltage corresponds to the gate positively biased with respect to
the back contact;
[0012] FIG. 4 is a graph illustrating the D.C. real time response
of a Pt/AlN/SiC sensor diode to propane in a background
concentration of O.sub.2 in 1000 sccm N.sub.2. The propane and
oxygen concentrations are indicated by the right vertical axis;
[0013] FIG. 5 is a graph illustrating the D.C. real time response
of a Pt/AlN/SiC sensor diode to CO in the absence of O.sub.2. The
ordinate for the CO concentration is indicated by the right
vertical axis. The total flow is in 1000 sccm N.sub.2;
[0014] FIG. 6 is a graph illustrating the D.C. responses of a
Pt/AlN/SiC sensor diode to propane and CO plotted as a function of
the combustible to oxygen ratio normalized to exact stoichiometry
for a wide range of oxygen concentrations. The response is measured
by the shift in the forward voltage (at a forward current of 5 A)
across the device; and
[0015] FIG. 7 is a graph illustrating the real time reverse bias
capacitance response at 1 MHz of a Pt/AlN/SiC sensor diode to a
sequence of rectangular 5 min duty cycle pulses of CO 0 to 100 ppm
in amplitude and increasing in steps of 20 ppm.
DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
[0016] A simple model of the gas sensor structure 10 is shown
schematically in FIG. 1. The sensor structure 10 includes a
semiconductor 12, a metal back contact 14, an AlN layer 16, and a
catalytic metal "gate" electrode 18. For high temperature
applications, the preferred semiconductor is SiC, due to its
ability to operate at temperatures well in excess of 400.degree. C.
necessary for achieving catalytic oxidation-reduction of
hydrocarbons as well as CO. The metal back contact 14 is preferably
made of tantalum, titanium, nickel or their silicides with an
outside layer of Pt to prevent high temperature oxidation. Laser
ablated Pt film on SiC by itself will also make an acceptable ohmic
metal back contact at such high temperatures.
[0017] The structure of the present invention can be used as a
sensor in two different modes depending on the range of the gate
bias: (1) in a D.C. mode by measuring the voltage across the device
needed to maintain a constant forward current, or by measuring the
current through the device when a constant forward voltage is
applied across it; and (2) using the small signal A.C. measurement
of the device capacitance when a reverse voltage is applies across
it.
[0018] Under the present invention, and as depicted in FIG. 1, a
diode structure is disclosed which includes a metal gate 18 over an
AlN layer 16 grown over a SiC semiconductor substrate 12 with a
metal back contact 14. The 2H--AlN layer is deposited by laser
ablation of a stoichiometric AlN target, onto a SiC carbide 6H
wafer with a 10 .mu.m n-type homoepilayer, nitrogen doped with a
10.sup.16 cm.sup.-3 nominal donor concentration. This substrate was
cut at vicinal (0001) orientation (3.5.degree. inclination) and is
a standard commercially available item. The AlN deposition was
carried out in a vacuum chamber at a base pressure of
5.times.10.sup.-7 torr, onto the heated substrate held at
900.degree. C. The 351 nm radiation from the XeF excimer was used
at fluences in the range of 1.5 to 3 J/cm.sup.2. At room
temperature the AlN films were insulating. For the large majority
of films on SiC, the .theta.-2.theta. x-ray diffraction scans were
strongly dominated by the 0006 SiC and 0002 AlN reflections. The
ohmic back contact consisted of a tantalum layer 24 covered by a
platinum film 26 deposited sequentially by laser ablation of the
respective targets. Laser ablation was also used to deposit the
metal gate electrodes 18, preferably platinum, on top of the AlN
layer 16. In the preferred embodiment, the metal gate electrode
consists of platinum having a thickness in the range of 1000-2000
angstroms deposited onto the AlN layer having a thickness between
1700-2000 angstroms. The metal back contact consists of a 500
angstrom tantalum layer with a 1000-2000 angstroms platinum layer
deposited thereon.
[0019] FIG. 2 shows an optical microphotograph of a set of typical
sensor devices.
[0020] A study of the D.C. current versus voltage (I(V)) profiles
of a number of these AlN gas sensor diodes demonstrate rectifying
characteristics, as exemplified in FIG. 3, where the dependence of
the high temperature forward I(V) profile on the presence of
combustibles is clearly shown.
[0021] Hydrocarbon sensitivity of the I(V) profile has been
demonstrated for SiC MOS structures with extremely thin oxide
layers, thicknesses in the order of 20 to 50 angstroms, where the
forward current can be explained either by tunneling through such a
narrow potential barrier, or by direct thermal emission in the
Schottky diodes that might have formed directly between the SiC and
the gate in regions of discontinuity of such a thin SiO.sub.2 film.
In the present case, the AlN layer has a thickness between 1700 and
2000 angstroms making current conduction by such mechanisms
unlikely. AlN is actually a wide band gap semiconductor, with an
energy gap in the vicinity of 6 eV. This would make the device
structure that of a heterojunction with metal/semiconductor
contacts on both outside surfaces. Such structures generally will
have rectifying I(V) characteristics. AlN being a semi-conductor
with electrons as charge carriers exhibits a greater mobility and
is thus a suitable material to replace the much thinner SiO.sub.2
layer used in prior diode devices. It is conceivable that in the
present structures, charge carriers in the AlN are provided by
doping with donors or acceptors, either due to a small deviation
from stoichiometry, or due to inclusion of impurities such as Si or
C from the substrate. A voltage barrier will still exist at the
AlN-SiC interface as the band gaps of the two materials are widely
different, and this voltage barrier will still be affected by any
charges injected at the gate, as such charges will affect the
distribution of electrons in the AlN. Thus, such heterojunction
diodes will behave as combustible sensors when used in the forward
bias D.C. mode.
[0022] These devices can also be used in the reverse bias A.C. mode
as capacitive combustible sensors. The tantalum/platinum contact on
the back of the SiC substrate can be considered as ohmic.
[0023] FIGS. 4 and 5 illustrate the real time D.C. response of an
AlN gas sensor to time dependent combustible concentration changes
in the surrounding flow. The output on the ordinate axis
corresponds to the voltage across the device for a forward current
of 5 .mu.A maintained through the device (with the gate biased
positively with respect to the back contact). The combustibles are
propane in FIG. 4 and CO in FIG. 5. The data for propane shows that
there is saturation under very rich conditions, but that the device
sensitivity can be adjusted by varying the oxygen concentration.
Such effects were also recorded for CO, and propylene. By
maintaining the oxygen concentration in an appropriate range,
sensitivity to a few ppm per carbon atom can be realized.
[0024] FIG. 6 summarizes the responses obtained for CO and propane,
in terms of the absolute value of the shift of the voltage across
the device from its value for zero hydrocarbon concentration
(ordinate). The abscissa is the relative combustible to oxygen
ratio, normalized to its value at exact stoichiometry. Clearly the
sensitivity is not solely a function of this fuel/oxidizer ratio,
but also depends on the specific combustible. This is related to
the fact that the flow gas composition is being sensed via some
molecular or atomic species present in the reacting mixture in the
vicinity of the catalytic gate, and which is not in thermal
equilibrium with the main gas flow. This circumstance, albeit a
complicating factor, endows these devices with the potential for
use as selective combustible sensors.
[0025] FIG. 7 illustrates the real time capacitance change of a
reverse bias AlN device in response to CO concentration changes. CO
concentration was varied in rectangular pulses of increasing
amplitudes between 0 and 100 ppm.
[0026] While the present invention has been described in detail in
connection with the above examples, it is to be understood that
such detail is solely for that purpose and that variations can be
made by those skilled in the art without departing from the spirit
of the invention.
* * * * *