Method of evaluating adsorption of contaminant on solid surface

Details Method of evaluating adsorption of contaminant on solid surface Method of evaluating adsorption of contaminant on solid surface

2001-03-09

2003-08-26

Larkin, Daniel S. (Department: 2856)

Measuring and testing

Surface and cutting edge testing

C073S031030, C073S865800, C073S866000

Reexamination Certificate

active

06609415

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of evaluating the adsorption of contaminants on a solid surface, particularly, to a method adapted for measuring the amount of adsorption of contaminants floating in a cleansing room, such as a clean room, on a workpiece, such as a silicon wafer, and for evaluating the cleanliness within the cleansing room and the contamination of the workpiece itself or on the cut solid surface.
2. Description of the Related Art
In accordance with progress in the degree of integration of the semiconductor device, the semiconductor device has been miniaturized and a multi-layered structure has come to be widely employed. In this connection, the manufacturing process of the semiconductor device has come to be affected by various impurities. It is known to the art that the manufacturing process of the semiconductor device is more or less affected by traces of, i.e., about picograms (10
−12
g) of, all the atoms and molecules except silicon. Particularly, many kinds of organic compounds represented by carbon compounds are used in the manufacturing process of the semiconductor device. For example, a photoresist material, which is a typical organic compound, is used twenty times or more in the manufacturing process of the semiconductor device. Also, many materials generating organic compounds are used in a cleansing room, such as a clean room. For example, a plastic material is used in the duct, piping, wall material, and floor material. What should be noted is that a large amount of an organic compound, such as dibutyl phthalic acid (DBP) or dioctyl phthalic acid (DOP), is contained as a plasticizer in the plastic material. Such an organic compound contained in the plasticizer, which provides a contaminant, is evaporated from the duct, piping, wall material, floor material, etc. even under a low pressure and room temperature so as to float within the clean room. Such a floating contaminant is adsorbed on the workpiece, such as a silicon wafer, so as to contaminate the workpiece. Other contaminants, such as silicon, quartz, glass, and a metal material, also float within the clean room.
The methods for measuring the contamination includes, for example, a method for evaluating the contaminant, such as an organic compound, which is adsorbed on the wafer surface. In this method, the wafer itself is left to stand within an atmosphere so as to be measured by a wafer heating desorption type gas chromatograph for the mass analysis.
The conventional evaluation method referred to above will now be described with reference to FIG.
11
. As shown in the drawing, a silicon wafer transferred and stored in a plastic container is taken out of the plastic container (steps
1
and
2
), and is heated in a heating device for cleansing the silicon wafer so as to remove the contaminant from the silicon wafer (step
3
), thereby cleansing the silicon wafer. Alternatively, it is possible to cleanse the silicon wafer by using a heating-desorbing device of a silicon wafer. In this case, however, the number of analyses and measurements performed in a unit time is reduced by half. The cleansed silicon wafer is housed again in the plastic container and is transferred to a measuring site within a chamber, such as a clean room (step
4
). The silicon wafer is taken out of the plastic container at the measuring site and is left to stand for a predetermined time (one to two hundred hours) in the measuring site (step
5
). The silicon wafer having an organic compound adsorbed thereon is housed again in the plastic container so as to be transferred to the point of a measuring device. Then, the silicon wafer is taken out of the plastic container and, is set in a heating-desorbing device for a silicon wafer so as to be heated at 300° C. for thirty minutes (steps
6
and
7
).
The organic compound generated from the heated silicon wafer is caught by an absorption tube (trade name of Tenax Absorption Tube) containing an activated carbon (step
8
). Then, the adsorption tube is subjected to a heat treatment at 280° C. for three hours by using an adsorption tube purging apparatus (trade name of Tenax adsorption tube purging apparatus), followed by washing the adsorption tube (step
9
). Since the adsorption tube set in the heating-desorbing apparatus for the silicon wafer is heated, the organic compound is liberated (step
10
). Further, the adsorption tube is mounted to a gas chromatograph for the mass analysis for the measurement of the material and mass of the liberated organic compound (step
11
). It is evaluated whether or not the measured point is contaminated with the organic compound based on the result of the measurement (step
12
).
Also known is a method in which a silicon powder is put in an adsorption tube and the air is sucked from, for example, a clean room into the adsorption tube for performing a desired measurement.
However, in the conventional method described above, in which a silicon wafer is used and the organic compound adsorbed on the wafer surface is evaluated, it is necessary to use first a heating apparatus for cleansing the silicon wafer and a heating-desorbing apparatus for the silicon wafer for liberating the organic compound adsorbed on the silicon wafer. The heating apparatus for cleansing the silicon wafer and the heating-desorbing apparatus for the silicon wafer give rise to problems. Specifically, these apparatuses are bulky, require a large mounting area, and are costly. Also, required is a Tenax adsorption tube.
It should also be noted that the cleansed silicon wafer and the silicon wafer having a contaminant, such as an organic compound adsorbed thereon, are housed in the same plastic container for the transfer and storage. As a result, the contaminant, such as an organic compound, is evaporated from the plastic container and is adsorbed on the silicon wafer so as to lower the measuring accuracy. Further, since the silicon wafer whose contaminant has been measured is stored in the plastic container for the transfer and storage, making it impossible to store the silicon wafer for a long time. It follows that the measuring site is limited to locations in the vicinity of the heating apparatus for cleansing the silicon wafer and the heating-desorbing apparatus for the silicon wafer. As a result, it is necessary to newly install the bulky and costly heating apparatus for cleansing the silicon wafer and heating-desorbing apparatus for the silicon wafer in the case where the measuring site is located far away, e.g., where the measuring site is located overseas. An additional problem to be noted is that the use of the heating apparatus for cleansing the silicon wafer and the heating-desorbing apparatus for the silicon wafer leads to an increase in the number of measuring steps, making it difficult to carry out the required evaluation promptly.
Also, the evaluating concentration of the contaminant, e.g., dioctyl phthalic acid (DOP), adsorbed on the silicon wafer is 0.2 nanogram (ng)/cm
2
. On the other hand, the gas chromatograph mass analyzing apparatus for detecting the contaminant has a high sensitivity and is capable of detecting DOP to an absolute concentration of 0.1 ng. However, in the case of using a silicon wafer, the silicon wafer has a surface area of 314 cm
2
on one surface because the silicon wafer has a diameter of 200 mm. Therefore, the concentration corresponding to the detectable sensitivity noted above is 0.1 ng/314 cm
2
=0.00032 ng/cm
2
. Naturally, the sensitivity is excessively high, compared with the required sensitivity of 0.2 ng/cm
2
.
Also known is a method in which a silicon powder or pellets are put in an adsorption tube, and air within, for example, a clean room is sucked into the adsorption tube for carrying out the required measurement. In this method, however, the silicon powder or pellets put in the adsorption tube are caused to overlap with each other, with the result that the adsorption of the organic material on the silicon wafer is deviated. Clearly, this method i

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