JP3188902B2 - Silicon wafer etching method and etching solution using the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for chemically etching a silicon semiconductor wafer, in particular, a wafer for manufacturing an integrated circuit device, and an etching solution used for the method.

[0002]

2. Description of the Related Art A silicon semiconductor wafer is processed by pulling up a silicon single crystal ingot, lapping a thin disk obtained by cutting the ingot, etching and polishing. In recent years, as the degree of integration of devices has increased, extremely high shape accuracy has been required for VLSI wafers.

[0003] In the etching step, for the purpose of removing residual abrasive grains and processing-affected layers, 40 to 5 wafers processed simultaneously with simultaneous lapping on both surfaces and processed with high precision in both flatness and plate thickness.
This is performed to remove chemically about 0 μm, but it is very difficult to control the shape. Usually, a mixed solution of hydrofluoric acid, nitric acid, acetic acid, and water, which has no selective etching property, has a small surface roughness, and has a high etching efficiency, is used for etching a wafer.

As disclosed in JP-A-59-219476, hydrofluoric acid (about 50% by weight): nitric acid (about 70% by weight): acetic acid (about 100% by weight) has conventionally been used in terms of a weight percentage.
A mixed solution having a high nitric acid concentration represented by a mixing ratio of 3: 5: 3 (volume ratio) has been used. As described in detail in the publication, in the above-mentioned etching solution, etching proceeds under the condition of diffusion control. Under diffusion-controlled conditions, the reaction rate does not depend on the plane orientation, crystal defects, etc. of the crystal surface. Diffusion on the crystal surface has the main effect, so the surface roughness of the silicon wafer surface is flattened and the micro shape is improved. There is an advantage of doing so. However, there is a disadvantage in that the outer periphery of the wafer sags with the progress of etching, and flatness is impaired as a macro shape.
An etching solution having a high concentration of hydrofluoric acid has not been put into practical use due to problems such as an increase in the etching rate, a deterioration in the microscopic shape of the wafer surface, and a decrease in glossiness.

For this reason, in the conventional etching process, as represented by the above-mentioned Japanese Patent Application Laid-Open No. Sho 59-219476, the flow of an etching solution is controlled under a diffusion-controlled condition with a high nitric acid concentration, thereby obtaining a wafer. Efforts have been made to suppress the deterioration of the macro shape.

When etching a silicon semiconductor wafer, another consideration is etching unevenness. After etching the silicon wafer,
When the etched wafer is immersed in pure water, the wafer is pulled up, and the surface of the wafer is observed, it may be observed as unevenness under a fluorescent lamp (hereinafter, this unevenness is referred to as etch unevenness I). Next, some thin film is formed on the wafer outermost surface, it may be observed as color unevenness in the lower (device for irradiating light of about 10 to 300,000 lux) for Atsumarihikarito (hereinafter, the uneven etch nonuniformity II ).

[0007] Under the condition of high nitric acid concentration and diffusion control, the etch unevenness I does not occur, but the etch unevenness II tends to occur. Therefore, in a conventional post-etching step, etching has been performed by using an alkaline solution to remove the etch unevenness II, thereby removing the etch unevenness II. Minor etch unevenness II can be removed by this alkaline etching.
In the case where the degree is extremely severe, there is a problem that it cannot be completely removed.

[0008]

The lapping wafer etchant is, for example, hydrofluoric acid (about 50%): nitric acid (about 70%): acetic acid (about 100%) = 3: 5: 3 (volume ratio). As typified, chemical etching was performed under diffusion-controlled conditions. Alternatively, as disclosed in Japanese Unexamined Patent Publication No. 3-1537, a diffusion-controlled etching solution in which the amount of water in the solution is reduced by using fuming nitric acid is focused on by focusing on the moisture in the etching solution. Had been etched. In any case, the etching solution is a diffusion-controlled etching solution, and the wafer subjected to etching in terms of shape accuracy is used.
Since the diffusion layer becomes thinner than the inside of the wafer surface, the corner of the wafer becomes rounded, and the outermost edge of the wafer is cut off.

The present invention solves the above-mentioned problems by changing the concentration of hydrofluoric acid and nitric acid in an etching solution composed of hydrofluoric acid, nitric acid, acetic acid and water to control the etching rate, thereby minimizing the problem on the wafer surface. It is an object of the present invention to provide an etching method in which the shape of the wafer is improved, the edge at the edge of the wafer is not removed, and the etching is not uneven.

[0010]

Means for Solving the Problems The present invention has solved the above-mentioned problems, and the gist thereof is as follows.

[0011] To a solution having a composition of _{HF / HNO 3 = 0.4~1.0 (HF} + HNO 3) / CH 3 COOH = 0.8~3.0 (HF + HNO 3) / H 2 O) = 0.8~ 1.65 weight% ratio Using an etching solution obtained by dissolving 0.05 to 0.80 mol / l of silicon, and controlling the temperature of the etching solution.
The temperature is adjusted to 20 to 60 ° C., and the wafer is immersed and etched.

[0012]

The operation of the present invention will be described below in detail.

In the present invention, the lapping wafer is immersed in an etching solution comprising four components of hydrofluoric acid, nitric acid, acetic acid, and water to perform etching, and the initial composition is HF / HNO ₃ = 0.4 to 1.0 (% by weight). HF + HNO ₃ ) / CH ₃ COOH = 0.8 to 3.0 (HF + HNO ₃ ) / H ₂ O) = 0.8 to 1.65 , and a solution obtained by dissolving 0.05 to 0.80 mol / l of silicon in the solution is used as an etching solution. The temperature during the etching is set to 20 to 60 ° C. HF / HNO ₃ concentration ratio is high, (HF + HNO
₃ ) The low ratio of / H ₂ O is a big difference from the conventional etching solution. Another significant feature is that there is a temperature range that is optimal for etching.

When silicon is dissolved, a chemical reaction represented by the following equation occurs.

3Si + 4HNO ₃ + 18HF → 3H ₂ SiF ₆ + 4NO ↑ + 8H ₂ O When silicon is dissolved, hydrofluoric acid and nitric acid are consumed,
Hexafluorosilicic acid (H ₂ SiF
_6), nitric oxide (NO), water (H ₂ O) to generate.

Accordingly, the liquid composition at the time of etching can be calculated from the initial composition and the dissolved silicon concentration by the above equation.

The HF / HNO ₃ weight% ratio is defined as the first factor which determines the etching characteristics of the HF / HNO ₃ ratio. The HF / HNO ₃ ratio is mainly determined by the etching rate, the wafer surface condition, and the microstructure of the wafer surface. This is for determining the shape and the like. In particular, the etch nonuniformity II is mainly are substantially governed by the ratio of HF / HNO _3, the ratio of HF / HNO ₃ is likely to occur with lower. In this etch unevenness II, some thin film or porous thin film is formed on the silicon wafer surface,
It is observed as uneven color under a condensing lamp.

In the present invention, the lower limit of the ratio of HF / HNO ₃ is
The reason that 0.4 is specified is that if it becomes less than 0.4, the aforementioned etch unevenness
This is because II occurs. Moreover, the defined upper limit 1.0 and it comes to greater than 1.0 cause poor microscopic shape of the wafer surface glossiness of the surface is lowered.

Acetic acid is not directly involved in the dissolution of silicon, but acts as an inhibitor that reduces the etch rate. It also has the function of improving the microscopic shape of the wafer surface.

In the present invention, the lower limit of the ratio of (HF + HNO ₃ ) / CH ₃ COOH is specified to be 0.8, because when the ratio is less than 0.8, the function as an inhibitor becomes excessively strong. This is because the etching reaction easily proceeds unevenly, and as a result, the above-mentioned uneven etching I occurs.

The reason for defining the upper limit to 3.0 is that the HF /
In the range of the ratio of HNO _3, it becomes difficult to control the the _{(HF + NO 3) / CH} 3 COOH is 3.0 ultra <br/>, and the etching rate becomes too large, the thickness of the wafer after etching That's why.

Water, which is a component of the etching solution, also functions as an inhibitor like acetic acid. In the present invention, the reason why the lower limit of the ratio of (HF + HNO ₃ ) / H ₂ O is 0.8 is that when the ratio is less than 0.8, etch unevenness I tends to occur, and the appearance of the wafer deteriorates. Also, the upper limit of 1.65 is specified because if it exceeds 1.65, it becomes difficult to prepare an etchant, and the etching rate becomes too high, so that it becomes difficult to control the thickness of the etched wafer. . When preparing an etchant using commercially available chemicals,
Since there are many 50 wt% hydrofluoric acid and 70 wt% nitric acid, it is inevitable that the water component in the etching solution is inevitably increased. _{Therefore, (HF / HNO 3) /} H 2 that O upper gel ratio of is very difficult, as is described in the specification of JP-A-3-1573,
It cannot be prepared unless chemicals such as fuming nitric acid are used. For such a reason, if the ratio exceeds 1.65, it becomes difficult to prepare an etching solution.

When performing etching industrially, the amount of consumed hydrofluoric acid, the amount of nitric acid, the amount of increased silicon, and the amount of water cannot be ignored. Therefore, after etching the silicon, a part of the etching solution is extracted to adjust the amount of silicon, the amount of hydrofluoric acid, the amount of nitric acid, the amount of acetic acid, and the amount of water, and a predetermined amount of hydrofluoric acid, nitric acid, acetic acid and water are added, and the etching is performed. The composition of the solution is prepared so as to return to the original state, and is used repeatedly.

The lower limit of the amount of dissolved silicon is 0.05 mol / l
It is necessary to remove a large amount of the etching solution in order to return the etching solution to the original etching solution composition, especially the amount of dissolved silicon, when the amount is less than 0.05 mol / l, and industrially, Cannot be performed.

The upper limit of the amount of dissolved silicon is 0.80 mol / l
The defining the it comes to 0.80 mol / l greater than, the number component of the water in the etching solution by the dissolution of silicon, as a result, easily etched uneven I described above is generated, also, the glossiness is lowered That's why.

The reason for defining the temperature range in the present invention is that the temperature of the etching solution is an important factor that greatly affects the surface condition of the wafer and the shape of the wafer after etching. The defining the 20 ° C. The lower limit is the temperature Not
This is because when it is full , the uneven etch I tends to occur. The reason why the upper limit is set to 60 ° C. is that if the temperature is exceeded, the composition of the etchant is partially evaporated to cause a composition change.

[0027] etching the lapped wafer in the method of the present invention to improve the microscopic shape of the wafer surface, anyone rather than the top edge portion of the wafer, and further does not occur etch nonuniformity. As shown in Table 1 of the example, the etching rate of the conventional etching liquid was different depending on the rotation speed of the wafer, and was dependent on the rotation speed of the wafer. This indicates that the reaction when the lap wafer is etched is diffusion controlled. Therefore, when etching the wafer, it is necessary to sufficiently diffuse the liquid on the wafer surface. On the other hand, in the etching solution of the present invention, the etching rate does not change depending on the rotation speed of the wafer. Accordingly, the present invention etchant is not diffusion limited, the etching solution of the present invention etching grayed by reaction rate
Is thought to progress.

[0028]

EXAMPLE A silicon single crystal having a diameter of 8 inches and 8 to 12 Ωcm was sliced, beveled, and then wrapped on both sides to prepare a wrapping wafer. The etching was performed by cleaning and drying the wrapping wafer.

The etching solution is obtained by dissolving silicon in a solution containing a predetermined initial composition of hydrofluoric acid, nitric acid, acetic acid and water.
An etchant was prepared. Since a large amount of NOx is generated when dissolving silicon, etching was performed one hour after mixing an etching solution having a predetermined concentration. An etching solution 3 having a predetermined concentration was put in an etching bath 1 shown in FIG. 1 and etching was performed while rotating a lapping wafer 2. Etching cost is 3 for both sides
The thickness was 5 to 40 μm.

FIG. 2 shows the shape of a wrapping wafer, and FIG. 3 shows the shape of a wafer etched with a conventional etching solution.
FIG. 4 shows the shape of a wafer etched by the etching method of the present invention. In the conventional method, when the lapping wafer is etched, as shown in FIG. 3, the etching is promoted at the uppermost edge portion of the wafer, the corner is etched off, and the edge portion is rounded and drooped. Since the wafer is rotated and the wafer is etched, the thickness of the diffusion layer is different between the central part of the wafer and the outer peripheral part of the wafer due to the difference in the relative speed with respect to the liquid, and the diffusion layer is thinner at the outer periphery of the wafer. This is because the etching rate has increased. The distance L in the radial direction from the outermost peripheral portion A of the etched wafer to the point B that has begun to be rounded is defined as who.

Table 1 shows the composition of the etching solution and the etching characteristics in the present invention and the conventional method.

In the conventional etching solution, the etching rate varies depending on the number of rotations of the wafer, and has a dependency on the number of rotations of the wafer. On the other hand, in the etching solution of the present invention, the etching rate does not change depending on the rotation speed of the wafer. Therefore, it is considered that the etching solution of the present invention does not have a diffusion-determining rate, and the etching solution of the present invention advances by a reaction-limiting rate.

In the conventional etching method, etch unevenness II was observed, but was not observed at all in the method of the present invention.

[0034]

[Table 1]

[0035]

The method of the present invention is characterized in that the HF / HNO ₃ concentration ratio is increased in the composition ratio of the conventional etching solution, and the acetic acid concentration and the water concentration are increased in a region where the HF concentration is high. In addition, the etched wafer has an excellent macro shape and a good micro shape, and can provide a wafer free from etch unevenness. Further, it is possible to provide an etching wafer capable of obtaining a wafer having high surface shape accuracy in a polishing step which is a subsequent step.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of an etching mode in an embodiment of the present invention.

FIG. 2 is a diagram showing a cross-sectional shape of an outer peripheral portion of a lapping wafer.

FIG. 3 is a diagram illustrating a cross-sectional shape of an outer peripheral portion of a wafer that has been etched by being immersed in a conventional etchant.

FIG. 4 is a view showing a cross-sectional shape of an outer peripheral portion of a wafer immersed and etched in an etching solution of the present invention.

[Explanation of symbols]

1. Etching tank 2. Silicon semiconductor wafer 3. Etching solution

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-3-1537 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01L 21/306, 21/308

Claims (2)

(57) [Claims] 1. A solution having a composition of HF / HNO ₃ = 0.4 to 1.0 (HF + HNO ₃ ) / CH ₃ COOH = 0.8 to 3.0 (HF + HNO ₃ ) / H ₂ O = 0.8 to 1.65 in weight% ratio. Then, using an etching solution obtained by dissolving 0.05 to 0.80 mol / l of silicon, the temperature of the etching solution was
A method for etching a silicon wafer, comprising adjusting the temperature to 20 to 60 ° C., immersing the wafer, and etching. 2. An etching solution for a silicon wafer, wherein HF / HNO ₃ = 0.4 to 1.0 (HF + HNO ₃ ) / CH ₃ COOH = 0.8 to 3.0 (HF + HNO ₃ ) / H ₂ O in weight% ratio. An etching solution for silicon wafers, wherein silicon is dissolved in a solution having a composition of 0.8 to 1.65 in an amount of 0.05 to 0.80 mol / l.