JPH0680654B2 - Silicon semiconductor wafer etching method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for chemically etching a silicon semiconductor wafer, particularly a wafer for manufacturing integrated circuit devices.

[Prior Art] The silicon semiconductor wafer is processed by lapping, etching, and polishing a thin disk obtained by cutting a silicon single crystal in this order. As the degree of integration has improved in recent years, extremely high shape precision has been required for wafers for VLSI, for example.

When the polishing process is performed by so-called mechanical chemical polishing, in order to reduce processing scratches on the finished mirror surface as much as possible, submicron silica sol dispersed in an alkaline aqueous solution is used as an abrasive, and the wafer is a soft porous urethane foam in a wet state. Light load on top, for example 50-100g / c
Since the polishing is performed with m ₂ (wafer), if the shape accuracy of the wafer surface before the polishing step is poor, it is impossible to expect a higher mirror surface shape accuracy. Therefore, for highly integrated wafers, it is strongly desired to improve the surface shape accuracy in the etching process.

In order to improve the shape accuracy of the etched surface of the silicon wafer, various chemical etching aqueous solutions for chemical etching have been used, and a catalyst has been added thereto.

Also, B. Schwartz and H. Robbins “Chemical Etching
of Silicon IV .Etching Technology "J. Electrochem.
Soc.:SOLID-STATESCIENCE AND TECHNOLOGY, 123, Vol.12
3, No. 12, pp. 1903-1909 also describes the etching of silicon wafers. That is, the observation results of the shape accuracy of an etching target in relation to the dissolution rate of silicon in a dilute mixed acid aqueous solution of hydrofluoric acid, nitric acid and water or acetic acid, its reaction mechanism, and the reaction mechanism are described in detail.
According to this, when the nitric acid concentration is high, the diffusion rate is controlled, and when the hydrofluoric acid concentration is high, the surface reaction rate is controlled, and both water and acetic acid act as diluents. Under the surface reaction rate-determining condition, the macroscopic shape accuracy is maintained, but the microscopic shape accuracy, that is, the surface roughness is inferior. On the contrary, under the diffusion-controlled condition, the micro shape accuracy, that is, the surface roughness is improved, but the macro shape accuracy is deteriorated.

Conventional semiconductor wafers have been chemically etched under diffusion-controlled conditions. For example, hydrofluoric acid (about 50%): nitric acid (about 70%)
%): Acetic acid (about 100%) = 3: 5: 3 (volume ratio) is a typical etching mixed acid. Under the diffusion-controlled condition, the reaction rate does not depend on the orientation of the crystal surface, crystal defects, etc., and the stirring effect on the crystal surface has the main effect. Therefore, since selective etching is not performed, the surface roughness of the silicon wafer surface is smoothed, and at least the microscopic shape accuracy is improved. However, as the etching progresses, on the contrary, the macroscopic shape accuracy of the silicon wafer surface deteriorates. In the conventional semiconductor wafer processing step, it is the actual situation that the etching conditions are selected by balancing the macro and micro shape accuracy of the surface of the silicon wafer and sacrificing either shape accuracy.

[Problems to be Solved by the Invention] The present inventors have proposed a conventional smoothing etching mixed acid such as the above-mentioned hydrofluoric acid (about 50%): nitric acid (about 70%): acetic acid (about 100%).
＝ 3: 5: 3 (capacity ratio) while varying the composition of the mixed acid, the macro and micro shape accuracy after etching of the silicon wafer surface is measured, and the micro shape accuracy is not deteriorated. We examined the conditions that could effectively improve it, but were unsuccessful. Therefore, when the micro shape accuracy was examined in detail, the surface roughness was composed of two types of pitches, large and small, and the height of these peaks and valleys was also affected by the composition of the etching solution. Then, it was difficult to improve the surface roughness of the large pitch even if the surface roughness of the small pitch was improved.

Based on this finding, the present inventors pay particular attention to the water content in the mixed acid, and instead of the usual nitric acid (about 70%), a solution of silicon in fuming nitric acid having a low water content is used. When the silicon wafer was etched by the method,
Surprisingly, there is little deterioration of macro shape accuracy, the surface roughness of large pitch is improved even in micro shape accuracy, and of course the desired surface roughness can be obtained even for surface roughness of small pitch. And has reached the present invention.

[Means for Solving the Problems] An object of the present invention is to obtain a silicon wafer having excellent smoothness, paying attention to the composition of an etching solution in an etching process of a silicon semiconductor wafer without sacrificing macroscopic shape accuracy. An object of the present invention is to provide an etching method capable of easily obtaining a mirror surface having excellent macroscopic shape accuracy in the next polishing step.

The method of the present invention uses an intermediate product of a silicon semiconductor wafer, for example, a silicon lap wafer, to obtain HF / HNO ₃ = 0.25.
_{~1.00, HF + HNO 3 / CH} 3 COOH = 1.5~5.0, HF + HNO 3 / H 2 O =
Etching for a required time with an etching solution in which 11 to 17 g / l of silicon is dissolved in a mixed acid consisting of four components of hydrofluoric acid, nitric acid, acetic acid and water having a composition of 3 to 9 (all in weight ratio) With this, it is possible to obtain a silicon etching wafer with effectively improved micro surface accuracy without deteriorating the macro surface accuracy of the wafer.

[Operation] In the practice of the method of the present invention, it has been confirmed that the composition of the etching solution can be considered to be in the diffusion-controlled region, and the etching solution for an intermediate of a silicon semiconductor wafer, for example, a lap wafer is the solution. It is important that the whole composition and the vicinity of the surface of the wafer are sufficiently stirred.

The amount of silicon dissolved in the mixed acid in the present invention is adjusted by first preparing a mixed acid of hydrofluoric acid, nitric acid, acetic acid and water, and dissolving the lower limit amount of silicon of the present invention, which is silicon, in the mixed acid. After that, the etching of the silicon wafer progressed,
If the same liquid is used repeatedly, it is necessary to remove a part and replace it with a fresh mixed acid, which can also be carried out continuously using a circulation system. In the examples, the increase in silicon dissolution during etching of the silicon wafer was negligible.
In the case of industrial use, hydrofluoric acid and nitric acid are also consumed by the progress of etching, so the amount of dissolved silicon must be adjusted in the same manner.

The etching liquid in the method of the present invention is a mixed acid consisting of four components of hydrofluoric acid, nitric acid, acetic acid and water, and the silicon dissolved therein has a value within a range of 11 to 17 g relative to 1 of the mixed acid. And the mixed acid composition is: HF / HNO ₃ = 0.25 to 1.00, HF + HNO ₃ / CH ₃ COOH = 1.5 to 5.0, HF + HNO ₃ / H ₂ O = 3 to 9 (all by weight) As far as possible, the silicon wafer excels in macro and micro shape accuracy after etching.
Since acetic acid and water are diluents, the etching rate decreases when the ratios of the above formulas are 1.5 and 3 or less, respectively.

As the amount of dissolved silicon increases, the surface roughness of small pitches in the micro shape accuracy becomes worse, but the surface roughness of large pitches in the macro and micro shape accuracy tends to improve conversely. . Therefore, there is a certain preferred range for the amount of silicon dissolved, which is about 10 g / l.
Between 18 and 18 g / l. Of the micro shape accuracy, the small-pitch surface roughness is almost constant below 13g / l, and of the macro shape accuracy and the micro shape accuracy, the large-pitch surface roughness is almost constant above 10g / l. Therefore, it can be considered that the optimum amount of silicon dissolved is around 12.5 g / l. However, as long as the amount of silicon dissolved is in the range of 11 to 17 g / l, it is possible to obtain a wafer having an overall better shape accuracy than the conventional etching method.

If the silicon wafer etched by the method of the present invention is used in the polishing step, a mirror surface with good shape surface accuracy can be efficiently achieved. This is because the large-pitch surface roughness of the microscopic shape is well controlled, so that the polishing amount in the polishing step is small and desired mirror surface processing can be performed.

On the other hand, since the silicon wafer etched by the conventional method does not have a good surface roughness with a large pitch among the microscopic shape accuracy, the surface roughness observed on the polished surface after the polishing step remains almost unchanged. .

[Example] A wafer was prepared by slicing a silicon single crystal having a diameter of 4 "n type and about 10 Ωcm, and the wafer was lapped on both sides with # 1200 alundum abrasive grains.
A 0 μm lap wafer was washed and dried. Next, as shown in FIG. 1, the etching solution 2 was placed in a Teflon beaker (500 cc) 1 and the lap wafer 3 was placed almost in the center of the solution.
Is held upright, the whole liquid is agitated by the magnet rotor 4 at the bottom of the beaker to generate a liquid flow 5 and etching is performed.
38-42 μm was etched off and the shape accuracy of the wafer was measured. When a silicon wafer is etched, the corners of the cross-sectional shape of the silicon wafer are etched off at the periphery, and as shown in FIG. 2, the silicon wafer is rounded and so-called sagging occurs.

Reference numeral 6 in FIG. 2 (a) indicates a lap wafer, and reference numeral 7 in FIG. 2 (b) indicates an etch wafer. The distance L in the radial direction between the point A that starts to be rounded on the edge of the etch wafer and the outermost side B of the edge is defined as sag.

Next, for microscopic shape accuracy, a stylus type surface roughness measuring instrument (Kosaka surface roughness meter SE-3F) is used to express the surface roughness of a small pitch with the roughness Ra obtained from this, and the Maximum waviness W _CM is used to represent large surface roughness (usually called waviness). The relationship between this Ra and W _CM is
This is as shown in FIG.

The part (1) of FIG. 3 shows the situation of a small pitch, and the part (2) of FIG. 3 shows the situation of a large pitch. R _max and W in the figure
_CM is the maximum distance between the small pitch and the large bitch's peaks and valleys, respectively. The ratio of the maximum value of the distance between the peak and the valley to the average height of the profile of the small pitch peak and valley is expressed separately as Ra.

Table 1 is a list of shape accuracies when etching with etching solutions of various compositions according to the present invention and the conventional method.

According to the method of the present invention, L, W _CM and Ra are all smaller than those of the conventional method. Although Ra may be close to Ra of the present invention in some conventional methods, L and W _CM are large. Further, when the dissolved amount of silicon is large beyond the range of the method of the present invention,
Although the conventional method shows shape accuracy close to that of the present invention, W _CM and Ra are not particularly satisfactory. If Ra is large, the glossiness of the surface is lost, so it can be visually discerned.

[Effects of the Invention] The method of the present invention is characterized in that the amount of dissolved silicon is limited to a certain range in a region where the water content is small in the composition ratio of the conventional etching solution. It excels in shape accuracy and facilitates the subsequent polishing process.
It is possible to provide an etch wafer suitable for producing a mirror-finished silicon semiconductor wafer that is economical in general and excels in shape accuracy.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal sectional view of an etching apparatus in an embodiment, FIG. 2 (a) is a lap wafer, FIG. 2 (b) is a side view of an etch wafer, and FIG. It is explanatory drawing of the surface roughness of a small pitch. 1 ... Beaker, 2 ... Etching liquid, 3 ... Lap wafer, 4 ... Magnet rotor, 5 ... Liquid flow, 6 ... Lap wafer, 7 ... Etch wafer.

Claims (1)

[Claims] 1. HF / HNO ₃ = 0.25 to 1.00, HF + HNO ₃ / CH ₃ COOH
_{= 1.5~5.0, HF + HNO 3 /} H 2 O = 3~9 ( both by weight)
A method for etching a silicon semiconductor wafer, which comprises using an etching solution in which 11 to 17 g / l of silicon is dissolved in a mixed acid having the composition of.