Method of evaluating silicon wafers

The method of evaluating silicon wafers according to this invention is capable of predicating degradation of the quality of oxide film insulation, which is incurred, on the silicon wafers, by process faults or local residual strains undetectable by the naked eye. The method includes the following steps of: removing selectively a surface of a silicon wafer treated by mirror polishing by using an etching selectivity caused by an unordinary surface state; counting the number of etch pits on the surface of the silicon wafer with the aid of an optical microscope; and judging the quality of the silicon wafer based on the etch pit density, which is calculated from the above number of etch pits, and the threshold value of etch pit density. The threshold value of etch pit density of the silicon wafer treated by selective etching is set to be below 5.times.10.sup.5 pits/cm.sup.2, and improvements to the processing of production lines relating to low-quality silicon wafers can be made.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method of evaluating silicon wafers, 
particularly to a method adapted to evaluate the quality of mirror 
polishing of silicon wafers and the quality of the grinding process for 
silicon wafers, which is an important factor affecting the quality of 
silicon wafers. 
2. Description of Prior Art 
Silicon wafers employed in semiconductor devices are sliced from silicon 
single crystals, which are usually manufactured by the CZ or FZ method. 
The outer peripheries of the sliced silicon wafers are chamfered, then 
processes such as grinding, lapping, and polishing are performed on both 
of the front and rear surfaces so as to mirror-finish at least one 
surface. 
Grinding and polishing semiconductor wafers to be mirror-finished produces 
process faults (hereinafter referred as defects) such as scratching 
capable of being detected by eyesight inspection and local residual 
strains undetectable by the naked eye. According to JP-A 8-70009 (JP-A: 
Japanese unexamined Patent Publication) entitled "METHOD OF MANUFACTURING 
SEMICONDUCTOR SILICON WAFERS", the existence of residual strains 
undetectable by the naked eye can be recognized by using a microscope if 
selective etching is performed on a silicon wafer. Furthermore, the above 
residual strains can be removed by applying heat treatment to silicon 
wafers within a short time period at low temperature. 
The above method of manufacturing semiconductor silicon wafers is adaptable 
for removing residual strains induced during mirror polishing on surfaces 
of silicon wafers, the result can be recognized by performing selective 
etching. However, even if selective etching has been performed, it is 
impossible to predict the extent to which the recognized defects will 
adversely affect the important qualities of semiconductor devices, for 
example the electrical characteristic such as oxide film insulation. 
Therefore, it is difficult to isolate the quality-control problems in the 
processing of silicon wafers and improve them. 
SUMMARY OF THE INVENTION 
In view of the above drawbacks, the object of the present invention is to 
provide a method of evaluating silicon wafers, which can contribute to the 
management and improvement of the processing of silicon wafers. The method 
of evaluating silicon wafers is capable of easily evaluating the 
relationship between the quality of oxide film insulation and the 
proceeding steps such as mirror polishing and washing. 
A first aspect of the method of evaluating silicon wafers is a method 
according to this invention, which comprises the following steps of: 
removing selectively a surface of a silicon wafer treated by mirror 
polishing by using a etching selectivity caused by unordinary surface 
state; 
counting the number of etch pits on the surface of the silicon wafer by the 
aid of an optical microscope; and 
judging the quality of the silicon wafer based on the etch pit density, 
which is calculated from the above number of etch pits, and the threshold 
value of etch pit density. 
A third aspect of the method of evaluating silicon wafers is a method 
according to the first aspect, wherein the step of removing is conducted 
after mirror polishing and washing the surface of a silicon wafer. 
A fourth aspect of the method of evaluating silicon wafers is a method 
according to the first aspect, wherein the step of judging the quality of 
the silicon wafer comprises a step of judging the quality to be good in 
the case that the threshold value of etch pit density of the silicon wafer 
treated by selective etching is less than 5.times.10.sup.5 pits/cm.sup.2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
After removing a layer of depth 0.5-5 .mu.m by performing selective etching 
on the surface of a silicon wafer, the defects on the surface of the 
silicon wafer are capable of being enlarged to a size visible by the aid 
of an optical microscope. "Selective etching" used in the present 
invention is based on the steps of oxidizing a silicon wafer and being 
solved by hydrogen oxide. In the selective etching process, an oxidation 
rate caused by the oxidation agent in the defective region which has a 
crystal defect or strain is distinct from that in the complete crystal 
region (non-defective region). Namely, oxide formed in the defective 
region is usually etched preferably to be a concave shaped pit. On the 
other hand the defective region where etching rate gets slower, is etched 
to be convex shaped mount. 
In the case that the step of etching is accompanied with plurality of 
reactions, etching rate are determined by the ratio of the etching agent 
(HF etc.) reaching to the surface of the crystal rather than the surface 
state of the crystal. Therefore by stirring an etching solution, the 
etching rate can be increased and the etching reaction advances uniformly. 
Alternatively, by adding the buffer agent (for example H.sub.2 O, CH.sub.3 
COOH), the etching rate can be restrained. As the oxidizing agent is 
increased, the etched surface is apt to be made dim. 
Therefore, etch pit density can be easily calculated, if the number of the 
etch pits appearing on the surface of the silicon wafer treated by 
selective etching is counted by the aid of the optical microscope. 
An etch pit density obtained by following the above sequence was compared 
with a threshold value of etch pit density to judge the quality of 
processing the silicon wafer. It was found that only silicon wafers with 
etch pit density below 5.times.10.sup.5 pits/cm.sup.2 have an oxide film 
insulation larger than 8 MV/cm. Therefore, it is possible to properly 
judge whether the oxide film insulation satisfies the requirement of 
semiconductor devices or not. Then, improvements can be made to the 
silicon wafers judged as bad ones. 
The following is a description of an embodiment of the method of evaluating 
silicon wafers, according to this invention with references made to the 
drawings. FIG. 1 is a block diagram showing the sequence of the steps of 
evaluating silicon wafers, wherein the numerals shown on the left side of 
each step denote the step numbers. 
After performing mirror polishing, the surface of the silicon wafer was 
washed. Then, in the first step, the silicon wafer to be evaluated was 
etched to a depth of 0.5-5 .mu.m by employing a selective etching solution 
consisting of HF: HNO.sub.3 : CH.sub.3 COOH: H.sub.2 O=1: 15: 3: X 
(wherein the volume ratio X is usually set to be 3). At the beginning of 
etching, the temperature of the selective etching solution was set to be 
20-25.degree. C. Then, in the second step, the surface of the silicon 
wafer etched by the above selective etching solution was inspected by the 
aid of an optical microscope, and the number of the etch pits existing on 
the surface of the silicon wafer was counted. In the third step, etch pit 
density was calculated, based on the number of the etch pits counted, and 
the etch pit density obtained was compared with a threshold value to judge 
the quality of the silicon wafer. On this occasion, the threshold value of 
etch pit density was set to be below 5.times.10.sup.5 pits/cm.sup.2. Next, 
silicon wafers judged as bad according to the above procedure were 
evaluated with respect to their fabrication conditions, and improvement 
was made, based on the above evaluation results. 
To examine whether the threshold value of etch pit density of this 
invention is proper or not, the following experiment was carried out. A 
silicon wafer was heat-treated within an oxidation atmosphere so as to 
form a thermal oxide film on the wafer surface. The above silicon wafer 
was sliced under the same condition and was sliced from the same crystal 
as the silicon wafer whose etch pit density had been calculated by the aid 
of the optical microscope. Next, a polycrystalline silicon layer was 
formed on the above thermal oxide film by the CVD method. Then, a preset 
number of polycrystalline electrodes having a predetermined size were 
formed by photolithography technology. Through this procedure, MOS 
capacitors were constructed by disposing an insulation layer of thermal 
oxide film between the silicon wafer and the polycrystalline electrodes. 
Subsequently, an electrical voltage was applied between the silicon wafer 
and the polycrystalline electrodes so as to measure the insulation 
strength of the oxide film. 
FIG. 2 shows the relationship between the etch pit density of the silicon 
wafer and the insulation strength of the oxide film in the above MOS 
capacitor structures. In FIG. 2, the symbols .box-solid., .circle-solid., 
.tangle-solidup., respectively denote silicon wafers mirror polished under 
polishing conditions A, B, and C. Furthermore, the symbol "O" denotes the 
wafers proceeded under B polishing condition and already being treated to 
change its surface state. Based on the results of measuring the insulation 
strength, the above wafers were classified into two groups. In other 
words, they were classified into a high-quality group having insulation 
strengths larger than 10 MV/cm and a poor-quality group having insulation 
strengths ranging from 2 to 6 MV/cm. The etch pit densities of the 
high-quality group were located to the left side of a quality judgment 
line, in other words, the values of their insulation strength were less 
than 5.times.10.sup.5 pits/cm.sup.2. The etch pit densities of the 
poor-quality group were located to the right side of the quality judgment 
line, namely, the values of their insulation strength were larger than 
5.times.10.sup.5 pits/cm.sup.2. The above measurement confirmed that 
wafers having etch pit densities less than 5.times.10.sup.5 pits/cm.sup.2 
have good insulation. 
As above described, conventionally, the influence of mirror polishing of 
silicon wafers on the insulation was difficult to predict. However, 
according to this invention, the quality of the mirror polishing of 
silicon wafers can be easily evaluated by only measuring etch pit 
densities of the wafers subjected to mirror polishing and selective 
etching. Therefore, it is possible to evaluate the quality of proceedings 
without performing the evaluation of oxide film insulation, and it is 
possible to manage and improve the processing of silicon wafers. 
As an etching solution used in a selective etching process, etching 
solutions as shown in FIG. 3 can be used. It is preferable to use a 
solution without containing Chromium in the view of a problem of treatment 
of waste liquid. 
Furthermore, the evaluation method of this invention is not limited to the 
evaluation of the mirror polishing of silicon wafers. It also can be used 
as a simple substitute procedure for evaluating the electric 
characteristics of a silicon wafer.