JP2003100701A - Method for etching silicon wafer and method for differentiating front and rear surface of silicon wafer using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an etching method of a silicon wafer, by which excellent mirror finish flatness can be obtained on the front surface and the rear surface is a little rough. SOLUTION: This is to provide an improved version of a silicon wafer etching method in which an acid etchant and an alkaline etchant are individually stored in a plurality of tanks, and a silicon wafer having a process modified layer formed through a lapping process and a subsequent cleaning process is sequentially immersed in the acid etchant and the alkaline etchant. In this method, the alkaline etching is treated after the acid etching, the concentration of the alkaline etchant is set to be 8 mol/l or more, and the etching rate of the acid etching is set to be 0.2 μm/sec or more in total in the front and rear surfaces of the silicon wafer.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in a method for etching and removing a work-affected layer on a surface of a wafer which is generated in a silicon wafer manufacturing process. More specifically, it relates to a method of differentiating the front and back surfaces of a wafer by mirror-polishing only the surface of the etched wafer.

[0002]

2. Description of the Related Art Generally, a semiconductor silicon wafer is manufactured by cutting a silicon single crystal ingot,
The wafer obtained by slicing is composed of the steps of chamfering, mechanical polishing (lapping), etching, mirror polishing (polishing) and cleaning, and is produced as a wafer having high precision flatness. Depending on the purpose, some of these steps are replaced with each other, repeated a plurality of times, or other steps such as heat treatment and grinding are added or replaced to perform various steps. A silicon wafer that has undergone a machining process such as block cutting, outer diameter grinding, slicing, and lapping has a damaged layer, that is, a work-affected layer on the surface. The work-affected layer induces crystal defects such as slip dislocations in the device manufacturing process, lowers the mechanical strength of the wafer, and adversely affects the electrical characteristics, and therefore must be completely removed.

An etching process is performed to remove the work-affected layer. The etching treatment includes acid etching using an acid etching solution such as mixed acid and alkali etching using an alkali etching solution such as NaOH.
However, by performing acid etching, the flatness obtained by lapping is impaired, and waviness on the order of mm and unevenness called peeling occur. In addition, there is a problem that the pits (hereinafter referred to as facets) having a local depth of several μm and a size of several to several tens μm are generated by performing alkali etching.

As a method for solving the above-mentioned problems, acid etching is performed after alkali etching, and a method of processing a wafer in which an acid etching removal margin is larger than an acid etching removal allowance and a wafer processed by this method Has been proposed (JP-A-11-233).
485). By the above method, the work-affected layer is removed while maintaining the flatness after lapping, the surface roughness is improved, and especially the local facets have a shallower and smooth uneven shape, and particles and contamination are unlikely to occur. It becomes possible to produce a wafer having an etching surface. on the other hand,
Since the presence / absence of the wafer is detected by the back surface of the wafer in the transfer system of the device process, if the back surface of the wafer whose surface is mirror-polished is mirror-like, there are problems such as detection difficulty and erroneous detection.

[0005]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
A wafer whose surface is mirror-polished (hereinafter referred to as PW; Polished Wafer) disclosed in Japanese Patent No. 3485 has a good flatness desired by a device maker and a small back surface roughness of the PW. There was a problem that a wafer could not be obtained.

An object of the present invention is to provide a method for etching a silicon wafer whose surface is mirror-polished, which has a good flatness and a small back surface roughness. Another object of the present invention is to provide a method for differentiating the front and back surfaces of a silicon wafer, which has a highly accurate flatness and a small surface roughness on both surfaces of the wafer and enables the front and back surfaces of the wafer to be visually identified. .

[0007]

The invention according to claim 1 is
An etching method for a silicon wafer in which an acid etching solution and an alkali etching solution are respectively stored in a plurality of etching tanks, and a silicon wafer having a work-affected layer that has undergone a cleaning step following a lapping step is sequentially immersed in an acid etching solution and an alkali etching solution. Is an improvement. In this characteristic structure, alkali etching is performed after acid etching, the concentration of the alkali etching solution is 8 mol / l or more, and the etching rate of acid etching is 0.2 μm / total when the front surface and the back surface of the silicon wafer are combined.
It is in the place of more than a second. In the invention according to claim 1,
After the acid etching, alkali etching is performed, and the wafer subjected to the etching treatment by sequentially immersing the wafer in the alkali and the acid etching solution by defining the concentration of the alkali etching solution and the etching rate of the acid etching in the above conditions is subjected to the lapping step. It is possible to maintain the obtained flatness and reduce the back surface roughness.

The invention according to claim 7 relates to claims 1 to 6.
This is a method in which only the front surface of a silicon wafer etched by any of the methods is mirror-polished to differentiate the front and back surfaces of the wafer. In the invention according to claim 7, good flatness is obtained by etching, and only the wafer surface whose back surface roughness is reduced is mirror-polished, so that both surfaces of the wafer have high-precision flatness and small surface roughness. In addition, the front surface and back surface of the wafer can be visually identified because the front surface of the wafer has a glossiness desired by the device manufacturer.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described with reference to the drawings. The method for etching a silicon wafer according to the present invention stores an acid etching solution and an alkali etching solution in a plurality of etching tanks, respectively, and a silicon wafer having a work-affected layer that has undergone a cleaning step following a lapping step is subjected to an acid etching solution and an alkali etching solution. It is an improvement of the method of sequential immersion in liquid, and this characteristic configuration is
Alkaline etching is performed after acid etching, and the concentration of the alkali etching solution is set to 8 mol / l or more, and the etching rate of acid etching is set to 0.2 μm / sec or more in total for the front surface and the back surface of the silicon wafer. is there.

If the concentration of the alkaline etching solution is less than the lower limit value, the shape of facets formed on the wafer becomes large, and deep pits having a size of several microns or less and a depth of about 10 to several tens of microns are generated. However, the surface roughness becomes large, and it is necessary to increase the polishing allowance of the chemical mechanical polishing performed in the subsequent step. The concentration of the alkaline etching solution is preferably 10 mol / l or more. The etching rate of acid etching is preferably 0.2 to 0.8 μm / sec.

Here, the glossiness is the JIS standard (JIS Z 8
741). According to this standard, the glossiness is relative to the specular reflection luminous flux Ψs ₀ of the glass surface having a refractive index of 1.567 in the same measurement system of the specular reflection luminous flux Ψs of the light incident on the sample surface at the incident angle θ. It is expressed as a number, which is a percentage. Gloss Gr
(θ) can be expressed by the following formula (1), and the incident angle θ when measuring the glossiness of the silicon wafer surface
Is 60 °.

[0012]

[Equation 1]

The etching mechanism of acid etching consists of oxidation of silicon by an oxidizing species contained in nitric acid or the like or contained in another compound, and removal of oxide by hydrofluoric acid or another reducing compound. . In order to control the etching rate of this acid etching, acetic acid, sulfuric acid, phosphoric acid, water or the like is added as a diluent. The additive solution used for these diluents has another effect of changing the surface tension and viscosity of the acid etching solution, and can be used properly depending on its purpose. Generally, the addition of a diluent reduces the etching rate of the acid etching solution. The surface roughness tends to increase as the etching rate decreases. Therefore, Ra, which is an index of the surface roughness, becomes large, and the glossiness becomes small. Further, regarding the heat generated by the etching reaction, the flatness tends to be improved as the etching rate is reduced, for the reason that the in-plane uniformity of the wafer is improved.

The total removal amount of the acid etching tank is 13 to 25 μm in total including the front surface and the back surface of the silicon wafer,
The total etching allowance of the alkali etching tank is such that the total of the front surface and the back surface of the silicon wafer is 5 to 13 μm. If the total removal allowance of acid etching is less than the lower limit, problems such as increased surface roughness and inability to accurately control the allowance occur, and above the upper limit, a few mm called nanotopography.
This causes a problem that the swell of the order becomes large. If the total removal amount of alkali etching is less than the lower limit value, the glossiness does not reach a desired value, and if it exceeds the upper limit value, nanotopography occurs. The total removal allowance of the acid etching bath is preferably 13 to 20 μm, and the total removal allowance of the alkali etching bath is preferably 5 to 10 μm.

The number of etching baths of the present invention is 2 to 6. Table 1 shows combinations of the acid etching bath and the alkali etching bath.

[0016]

[Table 1]

If the number of etching baths exceeds the upper limit, the surface roughness of the wafer deteriorates. The preferred number of etching baths is two to three, and the optimum mode in this case is one acid etching bath and two or less alkaline etching baths.
For example, when the number of etching baths is two, the wafer is immersed in the order of the acid etching bath and the alkali etching bath.
When the number of etching baths is 3, the wafer is immersed in the order of the acid etching bath, the alkali etching bath, and the alkali etching bath.

A rinsing step of immersing in a rinsing bath may or may not be performed between the acid etching step and the acid etching step and between the alkali etching step and the alkali etching step. A rinse process is always performed between the etching process and the alkali etching process. By inserting this rinsing step in between, the acid adhering to the wafer is washed off, and there is no risk of reacting with the alkali in the next step. The acid etching solution contains hydrofluoric acid and nitric acid, respectively, and preferably further contains at least one kind of acetic acid, sulfuric acid or phosphoric acid. A solution containing sodium hydroxide or potassium hydroxide is used as the alkaline etching solution.

The wafer obtained by mirror-polishing only the surface of the silicon wafer etched by the etching method of the present invention has a higher gloss than the back surface of the wafer. Can be converted.

[0020]

EXAMPLES Next, examples of the present invention will be described in detail together with comparative examples. <Example 1> First, a silicon wafer having a work-affected layer that has undergone a lapping process and a cleaning process was prepared. Next, hydrofluoric acid 50wt%, nitric acid 70wt%, acetic acid 90wt
% And water were mixed to prepare an acid etching solution having an etching rate of 0.5 μm / sec. Also, the concentration is 8.5
An alkaline etching solution containing mol / l potassium hydroxide as a main component was prepared. The prepared acid etching solution was stored in one etching tank and the liquid temperature was maintained at 30 ° C. Similarly, the alkali etching solution was stored in one etching tank and the liquid temperature was maintained at 80 ° C. Then, the above wafer was immersed while stirring the etching solution in the acid etching tank, and the removal margin of the wafer was immersed for 30 seconds with the total of the front surface and the back surface of the silicon wafer being 15 μm as a guide for etching. The wafer after acid etching was immersed in ultrapure water for rinsing. Next, the rinsed wafer is immersed while stirring the etching solution in the alkaline etching tank, and the wafer removal margin is immersed for 240 seconds with the total of the front surface and the back surface of the silicon wafer being 10 μm as a guide. Etching was performed. The wafer after the alkali etching was immersed in ultrapure water to rinse it, and then dried.

<Example 2> 50 wt% hydrofluoric acid, 70 nitric acid
The wafer was subjected to an etching treatment in the same manner as in Example 1 except that the acid etching rate was 0.3 μm / sec with a wt% and acetic acid of 90 wt%.

<Comparative Example 1> 50 wt% hydrofluoric acid, 70 nitric acid
The wafer was subjected to the etching treatment in the same manner as in Example 1 except that the acid etching rate was changed to 0.1 μm / sec by changing the wt% and acetic acid to 90 wt%.

<Comparative Test> The surface roughness and gloss of the back surface of the wafer after the etching treatment of Examples 1 and 2 and Comparative Example 1 were measured. The surface roughness is measured by an optical surface roughness measuring device (manufactured by chapman), and the glossiness is measured using a gloss meter (manufactured by Nippon Denshoku Co., Ltd.) according to JIS standard (JISZ 874).
1), and the obtained value was divided by 360%, which is the value of the surface gloss after mirror polishing, and the obtained value was taken as a percentage, and the surface gloss was defined as 100%. And Table 2 shows the measurement results.

[0024]

[Table 2]

As is clear from Table 2, in Comparative Examples 1 in which the acid etching rate is small, in Examples 1 and 2 in which the acid etching rate is large, the numerical value of the roughness Ra of the back surface of the wafer after the etching treatment becomes small. Moreover, it can be seen that the glossiness is in the range where the front and back can be distinguished.

[0026]

As described above, according to the present invention, a silicon wafer having a work-affected layer that has been subjected to a lapping step and a cleaning step after respectively storing an acid etching solution and an alkaline etching solution in a plurality of etching tanks is provided. This is an improvement of the method for etching a silicon wafer, which is sequentially immersed in an acid etching solution and an alkali etching solution. Alkaline etching is performed after acid etching, and the concentration of the alkali etching solution is set to 8 mol / l or more, and the etching rate of acid etching is set to 0.2 μm / sec or more in total for the front surface and the back surface of the silicon wafer. is there. By defining the acid and alkali etching under the above conditions, the back surface flatness, glossiness and surface roughness desired by the device manufacturer can be obtained.

Therefore, by subjecting only the surface of the wafer obtained by this etching to a mirror-polishing process, which is a post-process, the surface of the wafer has a higher glossiness than the back surface of the wafer, and both sides of the wafer have a highly accurate flatness and a small degree. It has a surface roughness and does not cause problems such as detection difficulty or erroneous detection when detecting the presence or absence of a wafer in the transfer system of the device process,
The front and back surfaces of the wafer can be differentiated to the extent that they can be visually identified.

Claims (7)

[Claims] 1. An acid etching solution and an alkali etching solution are respectively stored in a plurality of etching tanks, and a silicon wafer having a work-affected layer that has undergone a lapping step and a cleaning step is successively immersed in the acid etching solution and the alkali etching solution. In the method of etching a silicon wafer, alkali etching is performed after acid etching, the concentration of the alkali etching solution is 8 mol / l or more, and the etching rate of acid etching is 0 in total when the front surface and the back surface of the silicon wafer are combined. .2 μm /
A method for etching a silicon wafer, wherein the etching time is at least 2 seconds. 2. The total removal amount of the acid etching tank is 13 to 25 μm in total including the front surface and the back surface of the silicon wafer.
2. The etching method according to claim 1, wherein the total removal amount of the alkali etching tank is 5 to 13 μm in total including the front surface and the back surface of the silicon wafer. 3. The etching method according to claim 1, wherein the number of etching baths is 1 to 3 and the number of alkali etching baths is 1 to 3. 4. The etching method according to claim 1, wherein the acid etching solution contains hydrofluoric acid and nitric acid, respectively. 5. The etching method according to claim 4, wherein the acid etching solution further contains at least one of acetic acid, sulfuric acid and phosphoric acid. 6. The etching method according to claim 1, wherein the alkaline etching solution contains sodium hydroxide or potassium hydroxide. 7. A method of differentiating the front and back surfaces of the wafer by mirror-polishing only the surface of the silicon wafer etched by the method according to claim 1. Description: