Chemical apparatus and process disinfecting – deodorizing – preser – Analyzer – structured indicator – or manipulative laboratory... – Chemiluminescent
Reexamination Certificate
1997-12-23
2001-10-02
Snay, Jeffrey (Department: 1743)
Chemical apparatus and process disinfecting, deodorizing, preser
Analyzer, structured indicator, or manipulative laboratory...
Chemiluminescent
C422S062000, C422S093000, C436S111000
Reexamination Certificate
active
06296806
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to the detection of base contaminants in air, especially amine contaminants, and to systems employing such detection, including semiconductor fabrication systems and systems for filtering air for semiconductor fabrication and other processes that require uncontaminated air of high quality.
A particular purpose of the invention is to reliably measure low concentrations of airborne base contaminants in a semiconductor manufacturing environment that may adversely affect base-sensitive photolithographic processes being employed.
In semiconductor manufacturing it has been found desirable to detect an organic amine such as NMP (normal methyl pyrrolidinone) or ammonia. Such a contaminant may interfere for instance with a photolithography process used in semiconductor fabrication. The base contaminant may react with protons produced as a result of exposure of a photoresist layer to light. This can interfere with proper exposure and can harm the yield of the process and the rate of production of the semiconductor wafers.
For this reason, semiconductor manufacturers have sought to measure and control the concentration of airborne molecular contamination during the critical steps of the photolithography process which are sensitive to it. A detecting instrument specific to the detection of NMP and a detecting instrument specific to the detection of ammonia have been employed in semiconductor manufacturing facilities to monitor the air quality in the vicinity of production tools.
To understand the novel aspects of the invention it is useful to mention some detection techniques that have been used in other contexts.
For study of combustion processes or atmospheric pollution, U.S. Pat. Nos. 4,333,735 and 3,647,387 disclose processes for measuring the total fixed gaseous nitrogen species, including NH
3
, NO, NO
2
, HCN and organic amines in gaseous mixtures. The process involves catalytic conversion at elevated temperature of all fixed nitrogen species to NO, followed by chemiluminescent measurement of the resulting NO concentration.
For detection of ammonia, NO and NO
x
, U.S. Pat. No. 3,904,371 and products from various instrument manufacturers, including Thermo-Environmental, Advance Pollution Instruments, and Instrumatics International, employ an ammonia scrubber or absorber coupled with a thermal/catalytic converter with or without a molybdenum catalyst. For instance, in the Instrumatics instrument for stack gas analysis, a diluted sample is directed by a valve to alternatively flow through or past an absorber that specifically removes ammonia. The alternating samples proceed along a common line through a thermal converter to a chemiluminescent detector that operates in the 650-750 millibar range. By subtracting signals, the ammonia concentration can be calculated.
U.S. Pat. No. 5,057,436 discloses the use of an ammonia scrubber, positioned between two ammonia detectors, to measure the ammonia level in air.
Another aspect of the invention relates to the use of air filters for the ambient air in semiconductor manufacturing. To avoid harm to the process from NMP or ammonia, semiconductor manufacturers have used chemical filters to remove the contaminants. These filtering systems employ filter stages within an enclosure, the filter media of each stage being penetrable by air with acceptable pressure drop. As air flows through the filtering system, unwanted contaminants are retained on the chemically active surface of the various stages of the filter system. A problem associated with such filtering systems has been to accurately predict the remaining life of the filter so that the filter media can be changed at appropriate times with minimal disruption to the use of the expensive production facility. In the case of semiconductor fabrication facilities, typically, filter life has been estimated by measuring the concentration of ammonia in the air flow associated with the filter system.
SUMMARY OF THE INVENTION
The measurement of ammonia only is not satisfactory to photolithographic processes that are affected by low concentration of any basic material or amines, such as chemically amplified DUV photoresist processing. The measurement of total fixed nitrogen species is not applicable because many of the species (e.g., HCN, NO, NO
2
) are not basic in nature and do not affect the process. Detection of ammonia at high concentrations is not useful for the monitoring of amines at low levels.
None of the techniques mentioned above have suggested the concept of the present invention of measuring in a single, non-specific reading, the concentration at low levels of the multiple amines in air exposed to photolithographic processes and the like, that will be described more fully below.
The invention is based at least in part on the realization that semiconductor manufacturing and in certain other processes, which are recognized to be sensitive to NMP, ammonia, or other amines, are in fact sensitive to the total proton-bonding capability of all base contaminants present, regardless of the specific identity of the amine contamination. According to the invention, rather than determine the presence and concentration of each individual contaminant by a separate detector, it is realized that important advantages can be obtained by providing a detector that provides a single reading that is stoichiometrically related to the aggregate proton-bonding characteristic of various base contaminants that may be present in the monitored air. In this way a “total amine detector” is provided.
As explained further below, what is recognized to be of use is a measurement of the totality of those multiple amine contaminants in the air that can adversely affect the process being monitored. For instance, currently-employed deep UV photolithography processes are sensitive to both strong and weak bases, hence, according to the present invention, all airborne amines are measured down to low concentration levels. In other cases, where the process is sensitive only to bases greater than a certain pH, then the system is implemented, according to this idea of the invention, to measure the totality of the multiple amines within the pH range to which the process is sensitive down to low concentration levels.
In important implementations of the invention, a system and method are provided that employ a converter to convert ammonia and other low or medium molecular weight amines, to a single detectable gas, which is then detected. Preferably, the contaminants are converted to NO (nitric oxide) molecules and the NO is detected with an NO
x
detector by subtracting from the total NO
x
reading, the NO
x
originally present in the sample as it was introduced into the converter. In a preferred implementation, the conversion is produced by thermal oxygenation. In various specific implementations, a heated stainless steel surface, a heated quartz surface or a catalytic conversion surface is employed to accomplish the oxygenation.
According to other aspects of the invention, advantages are obtained by placing the converter near the sampling site. In various implementations, the sampling site is a stage of a process affected by the contaminant, such as the stepper or track stage of a photolithographic tool cluster, or a part of the air filtration system, or the incoming air, or a region where contamination could arise such as a chemical storage locker. In these cases, sampling lines for unconverted and converted samples extend from the local converter to a remote NO detector. In certain cases, it is also advantageous that a number of such converters are employed to provide sampling capabilities in different locations, with each converter connected to the centralized detector.
One of the advantages of having the converter near the tool or other sampling site concerns the ability to obtain rapid stabilization of the detection cycle and, correspondingly rapid accurate readings, to give early warning of any contamination problem. Amine contaminants have a high adsorption coefficient relative to the
Kinkead Devon A.
Kishkovich Oleg P.
Bowditch & Dewey LLP
Extraction Systems, Inc.
Snay Jeffrey
LandOfFree
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