The efficiency of semiconductor and integrated circuit fabrication process is sensitive to not only the quality of the materials utilized in the fabrication procedure but also the environment of the fabrication location. In the prior U.S. Pat. No. 5,085,760, the invention was directed to sensing or monitoring the very low levels of oxygen, in the parts per billion range, present in the inert blanketing gases required in the fabrication procedures. A similar problem in the fabrication process is that the presence of very low levels of moisture in the gas feed stream can severely effect both the yield and the quality of the integrated circuits or the like. In the past, a variety of sophisticated analytical instruments to monitor the moisture in a gas feed stream from high parts per million (ppm) to sub-parts per billion (ppb) levels were developed. Many of these prior art instruments utilized for sub-parts per billion (ppb) level measurements are relatively complex and expensive. The prior art instruments based on the electrolytic mechanism for decomposing or splitting the water in gases are relatively simple and inexpensive but lack the sensitivity to measure moisture in sub parts per billion levels and exhibit the sluggish response and recovery time. These electrolytic hygrometers utilized the principles disclosed by F. A. Keidel in Analytical Chemistry, Vol. 12, page 2043(1959). This well known, prior art hygrometer consists of an electrolytic cell having a pair of electrodes covered with a hygroscopic electrolyte such as Phosphorous Pentoxide P.sub.2 O.sub.5. In this sensor, the water vapors in the gas are absorbed at the surface of the hygroscopic layer and electrolyzed to gaseous oxygen and hydrogen. The current drawn by the electrolytic cell is a direct measure of the water being electrolyzed. At equilibrium, the cell current and the gas flow rate gives the absolute and continuous measurement of the amount of water in the gas stream.
Electrolytic cells with the different physical configurations based on the Keidel principles have been disclosed in the literature. The most noticeable and frequently used physical configuration is in the form of a tubular conduit having a pair of electrode wires helically positioned in parallel on the inner wall of the conduit from end to end or alternatively an interdigitated grid of electrodes deposited on an insulating surface that is coated with a thin layer of a hygroscopic film, P.sub.2 O.sub.5, derived from the dehydration of a thin layer of phosphoric acid by electrolysis at elevated temperatures. The major problems experienced with such a configuration is that the electrolytic layer(1) upon prolonged exposure to high moisture levels, is converted to phosphoric acid that tends to run off the electrodes and (2) upon the prolonged exposure to gases with moisture levels in the sub parts per billion level causes the electrolytic layer to crack. Both of these undesirable conditions cause discontinuity in the electrolyte layer and as a result the sensor loses its sensitivity. In addition, when the electrolyte layer is fully dehydrated, the sensor's response to small changes in the moisture content becomes very sluggish.
A solid, perfluorinated, ion-exchange polymer of the prior art, commercially available from E.I. du Pont de Nemours & Company, Inc. of Wilmington, Del. and commercially identified as a "Nafion" element, has been used as the electrolyte element in an electrolytic sensor. This ion-exchange polymer in the form of a thin film sandwiched between a pair of electrodes for trace water sensing and is disclosed in the U.S. Pat. No. 4,514,278 granted on Apr. 30, 1985. U.S. Pat. No. 4,954,238 granted on Sep. 4, 1990, discloses a moisture sensing hygrometer element in the form of an ion-exchange film of the "Nafion" type cast on the pair of electrodes on a tubular substrate. These "Nafion" devices are considered to have less disadvantages than those inherent in the Keidel-phosphorous pentoxide films. However, these "Nafion" based humidity sensors are less sensitive by more than two orders of magnitude compared to those using P.sub.2 O.sub.5 with identical physical configurations and with a proportionate effect on the lower detection limit. The lower sensitivity with perfluorinated ion-exchange polymer electrolyte ("Nafion") is primarily due to the decreased ionic conductivity of the "Nafion" polymer in the presence of lower levels of moisture. It is well known that water is essential for solvating the hydrogen ions of the sulfonic acid groups of the "Nafion" polymer for the ionic conduction to occur in the "Nafion" membrane. This is considered to preclude the use of the ion-exchange polymer as the electrolyte element in either very dry environments or at temperatures above 100 degrees Centigrade with unhumidified gases and without the means to supply water to the polymer.
In the above referenced prior U.S. Pat. No. 5,164,053, an improved electrochemical sensing cell is disclosed that utilizes a solid, perfluorinated ion-exchange polymer (PFIEP) element as an electrolyte element in the sensor. The electrolyte element taught to the art by the aforementioned patent utilizes a solid polymer electrolyte that has been equilibrated with phosphoric acid for maintaining the ionic conductivity of the polymer up to temperatures on the order of 180 degrees Centigrade. This prior patent teaches the solvation of the hydrogen ions of the sulfonic acid groups of the ion-exchange by the phosphoric acid that provides with a hydrogen bonding network for maintaining the ionic conductivity of the polymer without the need to add water to the polymer. This general structure is represented as follows: ##STR1##
It has only recently become known that it has been reported in the literature that incorporating phosphoric acid in a solution of the "Nafion" material for an electrolytic moisture sensor has been found to function in relatively dry environments with better sensitivity than the known prior art. This is disclosed by Huang et al. in Analytical Chemistry, Vol. 63, No. 15 for Aug. 1, 1991 on pages 1570-1573. The known humidity sensors for detecting sub parts per million moisture levels are typically powered at 30-100 volts. Under such electrical operating conditions phosphoric acid is readily converted into the dehydrated form of phosphorous pentoxide, P.sub.2 O.sub.5. This causes the electrolyte layer to lose its continuity and thereby the sensor loses its sensitivity. I have found that in a phosphoric acid, equilibrated perfluorinated ion-exchange polymer electrolyte used for electrochemical sensors, where the voltage between two electrodes is typically less than 1.23 volts (1.23 volts is the thermodynic voltage between the hydrogen and oxygen electrodes), as disclosed in U.S. Pat. No. 5,164,053, that the phosphoric acid is not converted to the dehydrated form and thus the conductivity of the electrolyte element is maintained. I have also determined that a moisture sensor utilizing a phosphoric acid treated, solid, perfluorinated ion-exchange polymer electrolyte element where the voltage is typically between 30-100 volts, over a prolonged period of time and in a very dry environment, (moisture levels in the lower end of the parts per billion levels), phosphoric acid is partially converted to the P.sub.2 O.sub.5 form and thereby results in decreasing the sensitivity of the sensor.
A further aspect of the prior art is found in the U.S. Pat. No. 4,900,405 granted on Feb. 13, 1990 for a "Surface Type Microelectronic Gas and Vapor Sensor". This prior art principally discloses various micro-sensing structures having two, three or more electrode structures deposited in close proximity to one another on the same side of an active area on a surface of the substrate so that the ion migration is fast. The substrates may be constructed of an electrically insulative material or of a semiconductor material. These structures utilize solid, electrolytes including ion-exchange polymers of the "Nafion" type and takes advantage of the hydrophobic nature of such a polymer. All of the disclosed structures, however, utilizing the "Nafion" type ion-exchange member require the use of an aqueous reservoir in contact with the solid electrolyte medium to keep it from drying out and inactivating the disclosed microsensor. This problem is solved by the disclosure in the aforementioned U.S. Pat. No. 5,164,053 and further improved upon herein.
Another piece of prior art, utilizing an aqueous electrolyte with 3 electrodes, discloses a means to enable the various impurities in the environmental air to be distinctly detected and measured is found in U.S. Pat. No. 3,776,832. The gases, noxious atmospheric pollutants were selected from the group consisting of carbon monoxide, nitric oxide, hydrocarbons, ethanol and methanol in air. This is accomplished by conveying the same air sample through a series of individual cells, with each cell being constructed and defined to detect only a single impurity. This prior art patent discloses the use of potentiostat circuit means for maintaining a constant or fixed relative potential difference between the anode or sensing electrode and the reference electrode of the 3 electrode sensors with the voltage selected in accordance with the gas species to be sensed.
There is, then, a present need for an improved, relatively inexpensive electrochemical/electrolytic sensor with a greater stability and faster responses and recovery times than those presently known and in commercial use that are not subject to drying out during continuous use.