Extended range chemical sensing apparatus

An apparatus for sensing chemicals over extended range of concentrations. In particular, first and second sensors each having separate, but overlapping ranges for sensing concentrations of hydrogen are provided. Preferably, the first sensor is a MOS solid state device wherein the metal electrode or gate is a nickel alloy. The second sensor is a chemiresistor comprising a nickel alloy.

BACKGROUND OF THE INVENTION 
The present invention relates generally to chemical sensing apparatus and, 
more particularly, to apparatus for sensing hydrogen over an extended 
range of concentrations. 
Providing chemical sensing apparatus, and especially apparatus for sensing 
hydrogen, having the capability to detect chemicals over an extended range 
of concentrations continues to present a technological challenge in spite 
of recent advance in the field of chemical sensing apparatus. While a wide 
variety of solid state chemical sensors have been developed, such sensors 
have generally been limited to detecting low concentrations of hydrogen. 
Exemplary of such solid state sensors include: 
metal-insulator-semiconductor (MIS) or metal-oxide-semiconductor (MOS) 
capacitors and field effect transistors (FET) as well as palladium gated 
diodes. A first MISFET sensor is described by Svensson et.al. in U.S. Pat. 
No. 4,058,368 wherein the metal electrode or gate is preferably palladium; 
although, Svennson et al. suggest that less sensitive devices could also 
be made with a nickel or platinum metal electrode. Improvements on such 
MISFET sensor are described by Sibbald et. al. in U.S. Pat. No. 4,931,851, 
wherein the metal electrode or gate comprises, either platinum or 
palladium and SiO.sub.2 mixed with or deposited on an exposed surface of 
the metal electrode. Whereas, Raul in U.S. Pat. No. 4,892,834, describes 
the use of sandwiched layers of metal oxides, pure catalytic metals, 
insulated oxides and semiconducting material in forming his solid state 
chemical sensor. More recently, Hughes et al. describe a palladium silver 
alloy diode for sensing hydrogen in "Thin-film palladium and silver alloys 
and layers for metal-insulator-semiconductor sensor" J. App. Phys. 63(1), 
1987 pgs. 1074-1083. Alternatively, McNally in U.S. Pat. No. 4,313,907 
describes a chemical sensor employing a wheatstone bridge wherein one of 
the resistive legs comprises a platinum wire coated with a mixture of 
palladium, palladium oxide and nickel oxide. While Johnson et. al. in U.S. 
Pat. No. 4,953,387 describes an alternative resistive semiconductor 
device. 
In spite of such advances, there remains a need for chemical sensing 
apparatus capable of detecting a dynamic range of hydrogen concentrations 
over at least six orders of magnitude (i.e. 10.sup.6). Moreover, there 
remains a need for a chemical sensing apparatus which responds rapidly and 
reversibly to changes in concentrations at room temperatures while 
resisting the poisoning effects of materials such as H.sub.2 S. 
SUMMARY OF THE INVENTION 
The present invention relates generally to apparatus for sensing chemicals. 
In particular, a chemical sensing apparatus is described for sensing 
hydrogen over an extended range of concentrations of at least six orders 
of magnitude. The chemical sensing apparatus includes a first sensor for 
sensing a selected chemical over a first range of concentrations as well 
as a second sensor for sensing the selected chemical over a second range 
of concentrations; wherein, the first and second ranges overlap. In 
particular, the first sensor can include either a MOS transistor or MOS 
capacitor wherein the electrode or metal gate comprises an alloy adapted 
to resist formation of a hydride phase over an extended range of hydrogen 
partial pressures or concentrations. The second sensor can include a 
resistor formed from an alloy adapted to resist hydride phase formation 
over an extended range of hydrogen partial pressures or concentration. 
Most advantageously, such alloys comprise about 8-20% (atom %) nickel and 
can include alloys of palladium and at least one metal selected from the 
group including platinum, chromium, rhodium, copper, and nickel. Chemical 
sensing apparatus according to the present invention provide a dramatic 
improvement over existing chemical sensing apparatus. In particular, they 
develop reproducible, large signals, as well as respond rapidly and 
reversibly to changes in hydrogen concentration, and exhibit resistance to 
poisoning. Unexpectedly, such sensing apparatus can also distinguish 
between hydrogen and hydrogen containing materials, such as formic acid. 
These and other advantages of the present invention will be discussed more 
completely below. However, it will be understood that the detailed 
description and specific examples are illustrative of the present 
invention and that those skilled in the art will recognize that various 
changes and modifications in materials and apparatus will be apparent 
without departing from the spirit and scope of the present invention.

DETAILED DESCRIPTION OF THE INVENTION 
In order to better understand the present invention, the following 
introductory discussion is provided. The use of palladium gated MOS solid 
state devices as chemical sensors has been quite extensive since the 
discovery of their sensitivity to low concentrations of hydrogen gas. It 
is generally believed that such solid state sensors work because of the 
catalytic action of palladium and the selective diffusion of hydrogen in 
palladium. Unfortunately, the catalytic action of such solid state sensors 
can often times be inhibited or poisoned by materials such as H.sub.2 S. 
In bulk palladium, attainment of equilibrium concentrations of absorbed 
hydrogen on the surface of the catalytic metal and dissolved hydrogen at 
the catalytic metal-oxide layer interface can take minutes to hours for 
low concentrations of hydrogen (at room temperatures) depending on the 
particular composition of the catalytic metal. Once equilibrium has been 
achieved, such sensors do not generally respond accurately to decreases in 
hydrogen concentration (i.e. not reversible). While thin films of 
palladium can respond more rapidly, even in thin film alloys presently 
employed in many solid state chemical sensors have their limitations. In 
particular, thin films of palladium can undergo a transformation to a 
hydride phase when exposed to hydrogen partial pressures as low as about 5 
Torr at ambient temperature. The resulting hydride phase causes the 
palladium films to blister and delaminate. The present invention provides 
a novel chemical sensing apparatus for detecting chemicals over an 
extended range of concentrations and one which can respond rapidly and 
reversibly to changes in the concentrations of the chemical being sensed 
as well as have enhanced resistance to poisonings and hydride phase 
formation as will be described below. 
Looking now to FIG. 1, the present invention will initially be described. A 
chemical sensing apparatus 10 according to the present invention comprises 
a first sensor 12 for detecting a first range of concentrations of a 
chemical and a second sensor 14 for detecting a second range of 
concentrations of the chemical. The first and second concentration ranges 
overlap. The chemical sensing apparatus 10 can also include means 16 for 
compensating for variations in the operating temperature of the chemical 
sensing apparatus 10 as well as display means 18 for providing a measure 
of the concentration of the chemical sensed. The present invention is 
particularly useful in measuring changes in concentration of hydrogen and 
has also provided an unexpected ability to distinguish hydrogen (H.sub.2) 
from hydrogen containing materials such as formic acid. 
The first sensor 12 comprises a gated metal-oxide-semiconductor (MOS) or 
metal-insulator-semiconductor (MIS) solid state device which respond 
rapidly and reversibly to changes in hydrogen concentration. In one 
embodiment, the MOS device preferably comprises a MOS transistor and can 
alternatively comprise a MOS capacitor. As depicted in FIGS. 2 and 3 
respectively, MOS transistors and MOS capacitors, according to present 
invention, can respond rapidly and reversibly to changes in the 
concentration of hydrogen. In particular, FIG. 2 depicts the response time 
of a MOS transistor according to present invention to a cyclic exposure to 
gas containing 1% hydrogen followed by a purge of the hydrogen as well as 
the simultaneous response of a chemiresistor, which will be described 
below. FIG. 3 depicts the response time of a MOS capacitor according to 
the present invention to a variety of concentrations of hydrogen ranging 
from about 30 ppm to 800 ppm. 
While MOS capacitors are simpler to fabricate than MOS transistors, MOS 
capacitors require measuring changes in capacitance to measure changes in 
the concentration of hydrogen. The measurement of capacitance requires 
measuring changes in AC signals which can be susceptible to electric noise 
and interference; whereas, MOS transistors require only the measurement of 
changes in DC voltage signals. Additionally, because of the smaller 
catalytic metal gate area of MOS transistors than MOS capacitors, the long 
term reliability of MOS transistors can be greatly enhanced. Typically, 
both such MOS devices can detect concentrations of hydrogen over a range 
of about 1 ppm to 1000 ppm. The lower detection limit of the MOS capacitor 
or MOS transistor is determined by the response time at lower 
concentrations. The limit of 1 ppm is a practical one since the time 
required for 90% of full scale signal is roughly proportional to the 
hydrogen pressure and can also depend on the operating temperature. The 
upper limit of 1000 ppm is the partial pressure at which the signal is 
almost saturated. 
The second sensor 14 comprises a catalytic metal resistor which is 
typically referred to as chemiresistor. As depicted in FIGS. 2 and 4, 
chemiresistors, according to the present invention, respond rapidly and 
reversibly to changes in the concentration of hydrogen. Typically, such 
chemiresistors can detect concentrations of hydrogen ranging from about 
100 ppm to 100% (i.e., 1,000,000 ppm). As with the MOS solid state 
devices, the speed of response of the chemiresistor is proportional to the 
hydrogen partial pressure. The fastest response is at the highest partial 
pressure. At the lower partial pressures, the chemiresistor signal can be 
lost in noise. 
An important aspect of the present invention is the composition of the 
metal alloy used as the metal electrode or gate in MOS solid state devices 
and in chemiresistors. Unlike typical sensing apparatus employing 
palladium, the alloys of the present invention effectively resist the 
formation of a hydride phase of the catalytic metal contained therein. 
Alloys of nickel and palladium have been most effective; however, alloys 
of nickel with other catalytic metals such as platinum, and rhodium, as 
well as alloys of palladium and copper, palladium and platinum and 
palladium and chromium are also believed to be effective. Alloys of about 
8 to 20% (by atom %) nickel and the balance palladium have been found to 
be most effective. However, alloys containing greater than approximately 
56% (by atom %) nickel and the balance palladium have been found 
insensitive to changes in hydrogen concentration. 
To ensure a uniform alloy of nickel in the MOS solid state devices and 
chemiresistors, thin films (500 to 2000 .ANG.) of such alloys can be 
deposited using a dual electron beam evaporator with dual thickness 
monitors to give accurate, complete mixing of the alloys. It is believed 
that sputtering techniques can also produce similar results. Composition 
of the deposited nickel alloys can be verified by Auger electron 
spectroscopy. 
As with almost all metal films, the chemiresistor has a temperature 
coefficient of resistance (TCR) that is linear. The TCR is large enough 
that a temperature correction to the signal must be made. As such, 
temperature control means 16 are provided for maintaining the temperature 
of operation of the sensing apparatus 10 and can include a feed back 
circuit having a resistor insensitive to changes in the concentration of 
hydrogen to act as a heat source to maintain a selected operating 
temperature as well as a resistor to act as a temperature sensor. As 
depicted in FIG. 1, the temperature control means 16 operates to maintain 
a fixed temperature for both sensors. For example, a chemiresistor of 
composition of at least 56% nickel and the balance palladium could be 
adapted for both uses. 
The sensing apparatus 10 can also include display means 18 by providing a 
measure of the hydrogen concentration sensed. By way of example, such 
display means 18 can include a computer for processing the signals 
developed by the first and second sensors as well as a display device. 
Fabrication of the sensing apparatus 10 can be achieved by shadow masking 
or photolithography deposition technologies. In particular, both MOS solid 
state devices and chemiresistors can be fabricated on a single chip. 
Additionally, placement of temperature control means 15 for maintaining 
the operating temperature of the apparatus 10 can similarly be fabricated. 
EXAMPLE 
The construction and testing of a chemical sensing apparatus according to 
present invention will now be described. A 500 .ANG. film, having a 
composition of about 8% Ni, and 92% Pd, was evaporated onto a flat 
substrate of SiO.sub.2 deposited on a silicon wafer. In this example, the 
Si with SiO.sub.2 wafer is a convenient flat, insulating, durable 
substrate to form a chemiresistor. A MOS capacitor was formed using a 
similar deposition technique. The responses were measured at 22.degree. C. 
over seven decades of partial pressures of hydrogen as depicted in FIG. 5, 
The solid boxes depict the response of the chemiresistor, with changes in 
resistance shown on the right hand scale. The solid dots show the effect 
of a mixture of 20% partial pressure of oxygen on the signals from the 
chemiresistor. The open boxes depict the response of the MOS capacitor, 
with changes in surface covered on the left scale, which is proportional 
to changes in flat band voltage shift. The open circles depict the effect 
of a mixture of 20% partial pressure of oxygen on the signals from the MOS 
capacitor. The open triangles refer to a MOS capacitor having a pure 
palladium gate. The vertical line represents a demarcation between 
explosive and non explosive mixtures of hydrogen when an oxidant is 
present. The critical difference between the signal at 1% and 4% 
concentration of hydrogen can easily be seen. (i.e. non-explosive vs. 
explosive mixtures). The response of the MOS device depicted in FIG. 5 is 
the same alloy as the chemiresistor, but it responds to H.sub.2 by a 
fundamentally different mechanism and responds to much smaller 
concentrations of H.sub.2 (down to 1 ppm H.sub.2) than the chemiresistor 
(generally no less than 100 ppm H.sub.2). 
Palladium and its alloys are notorious for losing their activity due to 
surface poisoning, and H.sub.2 S is one of the worst poisons. A 
chemiresistor, according to the present invention, was exposed to about 
100 ppm of H.sub.2 S at 22.degree. C. for 5 min. and showed little effect 
of poisoning as depicted in FIG. 6. Earlier tests on diodes with pure Pd 
gates showed a factor of 100 slowing of response after exposures to the 
same amount of H.sub.2 S. Additionally, chemical sensing apparatus 
according to the present invention have also shown a surprising and 
unexpected ability to distinguish hydrogen from hydrogen containing 
materials such as formic acid. 
While particular embodiments of the present invention have been described 
for chemical sensing apparatus, it is not intended that the invention be 
limited thereby. Moreover, it will be apparent to those skilled in the art 
that various changes and modifications may be made to the invention as 
described without departing from the scope of claims appended hereto.