Chemistry: analytical and immunological testing – Determination of water – In petroleum oil – hydrocarbon oil or organic fluid
Reexamination Certificate
2001-09-25
2004-12-21
Warden, Jill (Department: 1743)
Chemistry: analytical and immunological testing
Determination of water
In petroleum oil, hydrocarbon oil or organic fluid
C436S005000, C436S073000, C436S080000, C436S178000
Reexamination Certificate
active
06833273
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for quantitatively analyzing metals solid-solubilized in a silicon wafer.
BACKGROUND ART
It is known that, with recent use of finer semiconductor devices and higher integration degree thereof, metals contained in silicon wafers degrade device characteristics and markedly influence on yield of device production. In particular, it is known from many examples that Cu solid-solubilized in silicon wafers (also referred to as “bulk Cu” hereinafter) may be a cause of bad influences on the device characteristics. Therefore, a large number of gettering methods, cleaning methods and so forth have been researched for eliminating such metal impurities.
On the other hand, there is desired a method for analyzing such metal impurities, especially bulk Cu concentration, with high precision and high sensitivity for controlling the metal contamination during the wafer production processes such as polishing step and cleaning step.
As such a method for analyzing metals contained in silicon wafers, there are evaluation methods called one-drop method, step etching method and so forth. These are dissolution methods in which all or a part of a silicon wafer is dissolved with a mixture of hydrofluoric acid and nitric acid (also referred to as “mixed acid” hereinafter) or the like in a gaseous phase or liquid phase and metals in the solution are quantified by an analysis apparatus.
There is also a method called an annealing combined method or the like. In this method, metals present in a silicon wafer is transferred to a wafer surface (or captured thereby) by subjecting the wafer to a heat treatment, then an oxide film on the wafer surface is decomposed in a vapor phase, recovery solution is run over the entire surface of the wafer, and the recovery solution is subjected to quantitative analysis using an analysis apparatus.
Analysis apparatuses generally used for the analysis of dissolution solution or recovery solution obtained by these methods include frameless atomic absorption spectrophotometer (abbreviated as “AAS” hereinafter), inductively coupled plasma mass spectrometer (abbreviated as “ICP-MS” hereafter) and so forth.
Further, although it is not a method of directly evaluating metals contained in silicon wafers, there are also contemplated a method of directly analyzing metals contained in chemical solutions used in cleaning step and so forth, and other methods.
The conventional dissolution methods require enormous labor for maintenance and management of analysis apparatus and prevention of contamination from environment, and are likely to suffer from significant fluctuation of human factors. For example, when Cu is contained in a silicon wafer in an amount of about 1×10
13
atoms/cm
3
and the analysis is performed by the dissolution methods, detection cannot be achieved unless the analysis apparatus used for analyzing a final dissolution solution has an ability to detect about 0.1 ppt of the metal.
Further, for example, analysis may become possible at a level exceeding the ability of the analysis apparatus by concentrating the dissolution solution and so forth. In such a case, however, contamination which is newly introduced from a platinum crucible used for the concentration or external environment, i.e., interfusion of metals, is expected, and therefore good measurement precision could not necessarily be obtained.
In recent years, the sensitivity of analysis apparatuses, such as frameless atomic absorption spectrophotometer (AAS) and inductively coupled plasma mass spectrometer (ICP-MS), has shifted to sensitivity of ppt level. As for the ability of generally used apparatuses such as frameless atomic absorption spectrophotometer and inductively coupled plasma mass spectrometer, for example, the frameless atomic absorption spectrophotometer is at about 100 ppt level, and even the inductively coupled plasma mass spectrometer is at about 1 ppt level for Cu.
However, for evaluation of metals in silicon bulk, it is a problem how to extract metals contained in inside of silicon to the surface and collect them, in addition to the increase of sensitivity of the analysis apparatus itself.
When Cu is analyzed by the dissolution methods such as the one-drop method and the step etching method by using currently used apparatuses, the analysis could not actually be performed unless 1×10
14
to 1×10
16
atoms/cm
3
or more of Cu is contained in a silicon wafer, because the apparatuses show bad recovery yield from inside of the silicon.
Furthermore, in the annealing combined method, although metals comes to be likely to gather at the surface by the heat treatment, the recovery yield, i.e., the ratio (gettering efficiency) of metals transferred to the wafer surface (captured at the wafer surface) is as low as 0.1% or less for Cu in low resistivity silicon wafers which are doped with boron at a high concentration and so forth. Therefore, metals contained in the inside of silicon scarcely gather at the surface, and also there is resistivity dependency, so that measurement has a large error. In addition, it is expected that contamination and so forth are newly invited by the heat treatment at a high temperature (about 650° C.). Even by this method, the analysis could not actually be performed unless 1×10
13
to 1×10
14
atoms/cm
3
or more is contained in a silicon wafer.
Further, the method of directly analyzing metals contained in a chemical solution used in cleaning step etc., for example, suffers from problems that detection requires concentration of the solution by heating or the like because only a small amount of metal impurities are contained in a large volume of the chemical solution, the chemical solution may be an inhibition factor for the analysis depending on its nature, and strikingly decrease sensitivity of the analysis for metals when a large volume of the chemical solution is present. For example, these are caused in a case where Cu should be analyzed in a cleaning solution containing a large volume of sulfuric acid. For the analysis of Cu contained in such a solution, a special method such as an evaluation method utilizing a radioisotope must be used, and thus the method must be in a large scale in view of evaluation time and apparatus. Further, this is just an evaluation of metals in the chemical solution, and it is not for accurately determining concentration of metal impurities in a silicon wafer, which is the original matter of interest.
DISCLOSURE OF THE INVENTION
Accordingly, an object of the present invention is to provide a pretreatment method for analyzing a concentration of a metal, in particular, Cu, contained in a silicon wafer with high sensitivity in a simple manner.
In order to achieve the aforementioned object, the present invention provides a method for evaluating concentration of metal impurities contained in a silicon wafer, which comprises dropping concentrated sulfuric acid onto a surface of the silicon wafer to extract metal impurities solid-solubilized in the inside of silicon wafer into the concentrated sulfuric acid, and chemically analyzing metal impurities contained in the concentrated sulfuric acid.
If the metals in the bulk are recovered by using concentrated sulfuric acid as described above, the metals once recovered in the concentrated sulfuric acid scarcely diffuse again into the inside of the bulk, and thus the metals can be efficiently extracted to the wafer surface. Further, the wafer surface is unlikely to be roughened, and therefore favorable evaluation of wafer can be performed. Moreover, since the evaluation can be performed by using a few drops of concentrated sulfuric acid, the influences of sulfuric acid such as decrease of analytical sensitivity can be minimized.
Specifically, the method for extracting metal impurities solid-solubilized in the inside of a silicon wafer into concentrated sulfuric acid is performed by dropping an arbitrary amount of concentrated sulfuric acid onto the aforementioned silicon wafer surface, putting another uncontaminated wafer on th
Hogan & Hartson LLP
Shin-Etsu Handotai & Co., Ltd.
Siefke Sam P.
Warden Jill
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