Radiant energy – Ionic separation or analysis – Methods
Reexamination Certificate
1999-02-03
2001-06-19
Berman, Jack (Department: 2881)
Radiant energy
Ionic separation or analysis
Methods
C250S288000
Reexamination Certificate
active
06248997
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an analytical method of substances and more particularly, to a method of analyzing substances existing in a gas (e.g., air) to find and identify contaminants existing therein, which is preferably used for evaluating the environment or atmosphere in a cleanroom that has been popularly used in fabrication of semiconductor devices.
2. Description of the Prior Art
Usually, the air existing in a cleanroom used for fabrication of semiconductor devices contains various organic substances that apply bad effects to the devices. For example, if some organic substances (i.e., organic contaminants) are adhered onto a single-crystal silicon (Si) wafer, the adhered substances or contaminants may lower the dielectric strength of silicon dioxide (SiO
2
) films provided for the electronic elements (e.g., transistors) formed on or over the Si wafer. Alternately, the adhered substances or contaminants may weaken the cleaning effects of cleaning chemicals used in photolithography and etching processes or the like, resulting in insufficient removal of native oxides and/or metallic impurities.
Conventionally, the organic substances existing in the air in the cleanroom, which serve as contaminants for the popular Si devices, have been measured or identified in the following way.
First, a resin-based absorbent is packed into an absorption tube and then, the air existing in the cleanroom is injected into the tube as a sample by using a proper pumping system. Thus, organic substances contained in the injected sample air are absorbed or trapped by the absorbent. Thereafter, the absorption tube is heated to thermally desorb the absorbed substances from the absorbent at a high temperature of approximately 250° C. to 300° C.
Subsequently, the thermally-desorbed organic substances are analyzed by the use of a gas chromatograph and a mass spectrometer. Specifically, the thermally-desorbed substances in the absorption tube are first sent to the gas chromatograph by a carrier gas and physically separated therein. Next, the substances thus physically separated are sent to the mass spectrometer and analyzed qualitatively and quantitatively therein. As a result, the organic substances existing in the sample air are identified.
With the above-described conventional method, a lot of undesired organic substances that apply no bad effect to the semiconductor devices are detected. This is because the resin-based absorbent used in the conventional method is made of an organic material having a property to absorb almost all the existing organic substances. Therefore, the desorbed organic substances from the absorbent are likely to contain the undesired organic substances.
The undesired organic substances serve as a noise source in the analysis for detecting the desired organic substances that apply some bad effect to the semiconductor devices. In other words, the undesired organic substances cause insufficient physical separation of the organic substances to be analyzed in the gas chromatograph, resulting in incorrect qualitative and quantitative analysis in the mass spectrometer.
Consequently, there arises a problem that the desired organic substances are unable to be analyzed with satisfactory correctness.
Moreover, to thermally desorb the absorbed organic substances from the absorbent efficiently, it is preferred that the thermally desorbing temperature is as high as possible. However, because the resin-based absorbent used in the conventional method is usually made of an organic material, the thermally desorbing temperature needs to be set at a comparatively low temperature of approximately 250 to 300° C.
Thus, there arises another problem that the thermally desorbing temperature is unable to be raised as desired.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention to provide a method of analyzing substances that improves the correctness in analysis of desired substances applying some bad effect to a semiconductor device.
Another object of the present invention to provide a method of analyzing substances that makes it possible to selectively analyze the desired substances.
Still another object of the present invention to provide a method of analyzing substances that decreases the noise occurring in analysis.
A further object of the present invention to provide a method of analyzing substances that raises the thermally desorbing temperature of the absorbed substances.
The above objects together with others not specifically mentioned will become clear to those skilled in the art from the following description.
A method of analyzing substances existing in a gas according to the present invention is comprised of the following first to third steps.
In the first step, a gas to be analyzed is contacted with an absorbent, thereby absorbing substances existing in the gas to the absorbent. The absorbent is made of a same material as that of a semiconductor material to be processed in the gas.
In the second step, the absorbent is heated to thermally desorb the absorbed substances from the absorbent at a specific thermally desorbing temperature.
In the third step, the desorbed substances are separated and identified by using an analytical system.
With the method of analyzing substances according to the present invention, in the first step, the gas to be analyzed is contacted with the absorbent, thereby absorbing the substances existing in the gas to the absorbent. The absorbent is made of a same material as that of the semiconductor material to be processed in the gas.
Therefore, each of the substances absorbed to the absorbent has a property of being absorbed to the semiconductor material to be processed in the gas. In other words, any substance having a property of not being absorbed to the semiconductor material is not absorbed to the absorbent.
As a result, the desorbed substances from the absorbent do not contain undesired substances applying no bad effect to a semiconductor device formed by the use of the semiconductor material. This means that the undesired substances are not analyzed by the analytical system in the third step, and that the noise due to the undesired substances is decreased or eliminated.
Thus, the desired substances applying some bad effect to the semiconductor device are selectively analyzed in the third step, which improves the correctness in analysis of the desired substances.
Moreover, the absorbent used in the first step is made of a same material as that of the semiconductor material to be processed in the gas. Therefore, the thermally desorbing temperature in the second step can be raised compared with the conventional method using the resin-based absorbent.
In a preferred embodiment of the method according to the present invention, the absorbent used in the first step is made of bits or particles of Si. Si may be polycrystalline, single-crystalline, or amorphous. In this embodiment, the advantages of the present invention are remarkably exhibited, because Si is a very popular material in the fabrication processes of the semiconductor device.
It is more preferred that the absorbent is made of bits or particles of polycrystalline Si (i.e., polysilicon) because this is readily accessible in the fabrication processes of the semiconductor device.
In another preferred embodiment of the method according to the present invention, in the first step, the absorbent is located in a hollow refractory tube and the gas is injected into the tube. In this embodiment, it is more preferred that the tube is made of quartz, because quartz has a high heat-resistant property and is readily accessible.
In still another preferred embodiment of the method according to the present invention, the third step is carried out by using a gas chromatograph. In this embodiment, it is more preferred that a mass analyzer is used as a detector for the gas chromatograph in the third step.
In a further preferred embodiment of the method according to the present invention, the gas to be analyzed is the air existing in a cleanroo
Berman Jack
NEC Corporation
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