Chemistry: analytical and immunological testing – Nitrogen containing
Reexamination Certificate
1999-01-14
2001-03-27
Snay, Jeffrey (Department: 1743)
Chemistry: analytical and immunological testing
Nitrogen containing
C422S091000, C422S093000, C422S062000, C422S052000, C422S116000, C436S111000, C436S113000, C436S172000, C436S106000
Reexamination Certificate
active
06207460
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to the detection of base contaminants in a gas sample, especially amine contaminants, and to systems employing such detection, including semiconductor fabrication systems and systems for filtering gases for semiconductor fabrication and other processes that require uncontaminated atmospheres 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 airborne basic compounds such as normal methyl pyrrolidinone (NMP) and ammonia. Such contaminants 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 development 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 that 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 atmospheric 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, some have developed 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
, machines have been made that employ an ammonia scrubber or absorber coupled with a thermal/catalytic converter with or without a molybdenum catalyst. For instance, in one 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.
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.
DISCLOSURE OF THE INVENTION
The measurement of ammonia, exclusive of other basic contaminants, is unsatisfactory in photolithographic processes that are affected by low concentrations of any basic contaminant gas. One process that is sensitive to low levels of any basic contaminant gas is chemically-amplified deep-ultraviolet (DUV) photoresist processing. Typically, most or all of such basic contaminants that can affect the process include nitrogen. Measurement of total fixed-nitrogen species is not applicable, however, because many of the fixed-nitrogen species (e.g., HCN, NO, NO
2
) are not basic in nature and do not affect the process.
None of the techniques mentioned above have suggested the concept of the present invention of measuring—in a single, non-specific reading—a low-level concentration of multiple basic nitrogen compounds in a gas sample exposed to photolithographic processes and the like.
The invention is based in part on the realization that semiconductor manufacturing and certain other processes, which are recognized to be sensitive to NMP, ammonia, or other basic nitrogen compounds, are in fact sensitive to the total proton-bonding capability of all nitrogenous base contaminants present, regardless of the specific identity of the nitrogenous base contaminants. 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 nitrogenous base contaminants that may be present in the monitored air. In this way, a “total basic-nitrogen-compound detector” is provided.
As explained further below, what is recognized to be of use is a measurement of the totality of those multiple basic-nitrogen-compound contaminants in the gas sample that can adversely affect the process being monitored. For instance, currently-employed DUV photolithography processes are sensitive to both strong and weak bases; hence, according to the present invention, all airborne basic nitrogen compounds are measured down to low concentration levels. In other cases, where the process is sensitive only to bases greater than a certain pK
b
, then the system is implemented, according to this idea of the invention, to measure the totality of the multiple basic nitrogen compounds within the pK
b
range to which the process is sensitive, even at low-concentration levels.
The present invention focuses specifically on a basic-nitrogen-compound scrubber system as an important component of the entire basic-nitrogen-compound detection system. While a single ion exchange bed can be provided in a channel to remove ammonia from a gas sample flowing through the channel, this solution is not without drawbacks. Of particular concern is that ion exchange beds over time do not effectively filter basic nitrogen compounds other than ammonia. For example, NMP, a higher-molecular-weight imide, can pass through a 2-inch-deep, ½-inch-diameter scrubber in about 15 hours triggering false-negative signals in a measurement of total basic nitrogen compounds. This is a major problem which affects the functionality and reliability of the instrument's output.
In a detection system of this invention, a primary channel and a plurality of scrubbing channels are connected to a detector. The term, “detector,” as used herein includes a single detection device/unit as well as a plurality of detection devices/units. Each of the scrubbing channels include a basic-nitrogen-compound scrubber. Downstream from each of the scrubbers is a converter which converts gaseous nitrogen compounds into an indicator gas and a detector for detecting the indicator gas. A primary channel also leads to the converter and detector. As used, herein, the term, “converter,” refers either to a single converter unit to which all channels are connected, in parallel, or to multiple converter units individually associated with a single or multiple channels.
In a method of this invention, reference gas samples are passed through first and second scrubbers to remove basic nitrogen compounds from the gas samples. The second scrubber is purged to remove reversibly-bound nitrogen compounds while a gas sample passes through the second scrubber.
Goodwin William M.
Kishkovich Oleg P.
Extraction Systems, Inc.
Gordon Brian R.
Hamilton Brook Smith & Reynolds P.C.
Snay Jeffrey
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