Impurity measuring device

Details Impurity measuring device Impurity measuring device

1999-01-04

2001-02-20

Tung, T. (Department: 1743)

Chemistry: electrical and wave energy

Apparatus

Electrolytic

C204S400000, C205S775000, C205S789500, C205S790000

Reexamination Certificate

active

06190520

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of measuring a small amount of impurities and an impurity measuring device.
2. Description of the Related Art
In a process of manufacturing a semiconductor apparatus, metallic impurities, such as Na, K and Fe, are undesirably mixed to a thin film such as a polycrystal silicon film which is formed in or on a semiconductor substrate. It is generally known that if a very small amount of such metallic impurities are mixed, a bad influence is exerted on an electric characteristic of the semiconductor apparatus. For this reason, in order to improve the electric characteristic of the semiconductor apparatus, it is necessary to accurately grasp an amount of the mixed metallic impurities in each manufacturing steps and take measures to suppress the mixing of impurities.
The following explains a conventional device and method of measuring an amount of metallic impurities existing in and on a thin film which is formed in or on a semiconductor substrate or on a surface of a semiconductor substrate on reference to
FIGS. 4
,
5
A and
5
B.
FIG. 4
is a perspective view which shows an arrangement of a conventional impurity measuring device,
FIG. 5A
is a top view of the conventional impurity measuring device of
FIG. 4
, and
FIG. 5B
is a cross sectional view taken along line VB—VB of the impurity measuring device. The conventional impurity measuring device is composed of a substrate holding stand
1
, a frame
2
and four screws
3
. The substrate holding stand
1
has a concave section for holding a semiconductor substrate
10
, and a depth of the concave section is smaller than a thickness of the semi-conductor substrate
10
. The frame
2
has an opening which penetrate the frame
2
at its center portion, and a diameter of the opening is smaller than a diameter of the semiconductor substrate
10
. Moreover, the substrate holding stand
1
and the frame
2
respectively have holes for the screws
3
at each corner. The holding stand
1
, the frame
2
and the four screws
3
are made of, for example, Teflon.
In the conventional impurity measuring method, first, the semiconductor substrate
10
on which a film is not formed or the semiconductor substrate
10
on which a thin film is formed, is placed on the concave section of the holding stand
1
, then the frame
2
is placed on the semiconductor substrate
10
, and the corners of the holding stand
1
are fitted on the corresponding corners of the frame
2
so that the corners are fastened with the screws
3
. As a result, the frame
2
is fixed tight on the semiconductor substrate
10
. Next, a solution
4
, which dissolves the semiconductor substrate
10
or the thin film on the semiconductor substrate, is poured into a cavity formed by the opening of the frame
2
and the substrate
10
, and the semiconductor substrate
10
or the thin film of the semiconductor substrate are dissolved by the solution
4
. Then, an impurity, which exists in the thin film and on the surface of the thin film formed in or on the semiconductor substrate
10
or on the surface of the semiconductor substrate
10
, exists in the solution
4
as ion. Thereafter, the impurity contained in the solution
4
is measured by, for example, an atomic absorption measuring device.
At this time, the higher the concentration of the impurity in the solution is, the more the sensitivity of the measurement is improved. Therefore, in the case where an extremely small amount of impurities are measured, it is necessary to decrease an amount of the solution to minimum amount. However, in the above method, since a comparatively large amount of the solution
4
, namely, at least about 10 ml is required for dissolving the substrate
10
, the impurity concentration becomes low and thus the sensitivity was not so high.
For this reason, in order to improve the sensitivity of the measurement, after the semiconductor substrate or the thin film on the semiconductor substrate is dissolved, a method of concentrating the solution is used for measuring impurities contained in the solution. However, this method requires a long time for the concentration, and the solution might be possibly contaminated by more impurities during the concentrating process.
Furthermore, in the case where impurities in a silicon substrate are measured, the silicon substrate is dissolved by a volatile solution such as fluoric acid, and the dissolved silicon substrate is changed into fluoride silicon having volatility. As a result, the silicon can be easily removed, and thus the impurities can be measured easily. However, since it is difficult to remove materials such as Al and Ti other than the silicon, a very small impurities contained in an Al thin film and a Ti thin film could not be measured.
In addition, as the diameter of the semiconductor substrate is increased, a larger amount of solution and longer time are required for dissolving the semi-conductor substrate or the thin film on the semi-conductor substrate. Moreover, there arises a problem that time required for concentrating a lot of solutions becomes longer.
As mentioned above in the conventional impurity measuring method and impurity measuring device, since it is necessary to concentrate the solution in order to measure a very small of impurities, a longer time is required for the concentration, and the solution might be contaminated by impurities during the concentration. As another problem, since materials such as Al and Ti other than silicon are not removed, impurities contained in the Al thin film and the Ti thin film cannot be measured.
SUMMARY OF THE INVENTION
The present invention has been achieved under the above mentioned circumstances, and it is an object of the present invention to provide an impurity measuring method and an impurity measuring device which are capable of measuring a very small of impurities existing in a sample or on a surface of the sample at high sensitivity.
In order to solve the above mentioned problems and to achieve the above mentioned object, an impurity measuring method of the present invention is characterized by comprising the step of dissolving impurities which exist in a sample or on a surface of the sample and the sample in a solution by bringing the solution into contact with the surface of the sample; the step of putting a plurality of electrodes in the solution so as to apply a voltage across the electrodes; the step of depositing the impurities on a surface of the electrodes; and the step of analyzing the deposited impurities so as to measure the impurities.
In addition, an impurity measuring method of the present invention is characterized by comprising the step of dissolving impurities which exist in a sample or on a surface of the sample and the sample in a first solution by bringing the surface of the sample contact with the first solution; the step of putting a plurality of electrodes into the first solution so as to apply a voltage across the electrodes; the step of depositing the impurities on the surfaces of the electrodes; the step of dissolving the deposited impurities in a second solution; and the step of measuring the impurities dissolved in the second solution.
In addition, an impurity measuring device of the present invention is characterized by having means for soaking a sample in a solution for dissolving impurities which exist in the sample or on a surface of the sample, and the sample therein; a plurality of electrodes which are put into the solution; means for applying a voltage, which deposits the impurities dissolved in the solution on surfaces of the electrodes, across the plurality of electrodes; and means for detecting the deposited impurities.
The applying voltage may be such that the applying voltage deposits impurities to be measured but does not deposit the other impurities which are expected to be dissolved in the solution.
The applying voltage may be such that the applying voltage deposits only impurities to be measured and does not deposit the sample dissolved in the solution.
T

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