JP2024078489A - Method for removing device patterns and method for evaluating crystal defects in silicon wafers using this method - Google Patents

Abstract

[Problem] To peel off (remove) a device pattern (device structure film) from a silicon wafer in a short time without damaging the surface of the silicon wafer. [Solution] In a method for removing a device pattern from a silicon wafer on which a device pattern has been formed after a semiconductor device process, the device pattern is removed by immersing the silicon wafer on which the device pattern has been formed in a hydrofluoric-nitric acid solution using a solution of hydrofluoric acid with a weight concentration of 0.05% or more and 0.1% or less and nitric acid with a weight concentration of 69% or more and 71% or less for 5 seconds or more and 1 minute or less. [Selected Figure] None

Description

The present invention relates to a method for removing a device pattern (device structure film) from a silicon wafer on which the device pattern is formed, and a method for evaluating crystal defects in a silicon wafer using this method.

Silicon wafers (hereinafter also referred to as semiconductor substrates) are generally manufactured by producing a cylindrical single crystal ingot by the Czochralski (CZ) method or the floating zone (FZ) method, cutting (slicing) the single crystal ingot into thin plates to produce wafers, and then grinding, etching, and polishing the obtained wafers.
The silicon wafer thus manufactured undergoes a semiconductor device process, and a device pattern (device structure film) is formed on the silicon wafer, thereby manufacturing memories, LSIs, and the like.

Incidentally, in recent years, the integration density of semiconductor circuits such as NAND type flash memories has improved significantly, leading to miniaturization of device patterns (elements).
Therefore, in order to obtain circuits with high performance, reliability, and yield, it is required that the wafer surface and surface layer be high-purity and have few defects such as dislocations, and that the wafer interior be a high-quality semiconductor substrate having a gettering layer of high-density BMDs (bulk micro defects) formed therein.

One way to evaluate whether a semiconductor substrate is of high quality is to evaluate crystal defects. In order to evaluate crystal defects of a silicon wafer on which a device pattern (device structure film) is formed, it is first necessary to remove the device pattern (device structure film) formed on the surface of the silicon wafer.
This device structure film is a silicon nitride film, a silicon oxide film, a polysilicon film, a metal film, or the like, and conventionally, the device pattern (device structure film) has been removed by etching or grinding.

Then, after removing the device pattern (device structure film), the silicon wafer is selectively etched with a selective etching solution, and then pits on the wafer surface and cross section are observed to evaluate crystal defects such as OSFs (oxidation-induced stacking faults) and dislocations.
As an etching solution for this selective etching, a solution that does not contain chromium is standardized as JIS H0609 of the JIS standard.

Furthermore, methods for removing (peeling off) device patterns (device structure films) or thin films formed on the silicon wafer will be described with reference to Patent Documents 1 to 5 as examples.
Patent Document 1 proposes a method for removing (peeling off) a thin film formed on a silicon wafer, in which a polysilicon film is etched with heated potassium hydroxide, and a silicon dioxide film is etched with a mixture of hydrofluoric acid and hydrochloric acid.

Furthermore, as a method for removing (peeling off) a device pattern (device structure film) by wet etching similar to that in Patent Document 1, Patent Document 2 proposes a technique in which a substrate is immersed in NH ₄ F or 33% to 49% HF for 15 to 30 minutes, and then immersed in dilute HF for 30 minutes to peel off the device pattern, thereby making it possible to observe surface defects by selective etching.

Patent Document 3 also proposes a method of combining etching and mechanical cleaning, in which a silicon substrate on which a device film has been formed is immersed in hydrofluoric acid with a weight concentration of 20% to 50%, and then the surface of the silicon substrate is mechanically cleaned with a water-absorbent resin brush or the like that has absorbed pure water, thereby removing the device film and exposing the silicon substrate surface, and then performing defect observation by selective etching.

In addition to the above-mentioned wet etching peeling method, methods using grinding and sandblasting have been proposed as methods for removing (peeling) device patterns (device structure films) from silicon wafers, as shown in Patent Documents 4 and 5.

Japanese Patent Application Laid-Open No. 07-122532 JP 2004-179638 A JP 2003-318242 A JP 2011-211012 A JP 2020-061515 A

Incidentally, when observing defects on the surface of a silicon wafer (semiconductor substrate) that has been subjected to a semiconductor device process, if the method described in Patent Document 1 is used as a method for removing (peeling) a device pattern (device structure film), a removal (peeling) operation is performed using a hydrofluoric acid solution after immersion in an alkaline solution.
This requires two etching solution tanks, one for alkali and one for acid, and cleaning devices for cleaning the respective etching solutions, which results in an increase in the size of the equipment.

The same is true for the method described in Patent Document 2, which requires two etching solution baths to remove (peel off) the device pattern (device structure film), and the processing time is more than one hour, resulting in poor work efficiency.

In addition, when using the method described in Patent Document 3, the device pattern is peeled off with a brush soaked in pure water after immersion in a hydrofluoric acid solution, which can scratch the surface of the silicon wafer (semiconductor substrate), making it difficult to distinguish from minute crystal defects and making it difficult to observe the crystal defects.

There are also methods for removing the film by sandblasting or surface grinding, as described in Patent Documents 4 and 5, but these methods also leave scratches on the surface of the silicon wafer (semiconductor substrate), which can be difficult to distinguish from minute crystal defects, and there is a risk that the crystal defects cannot be observed.

In order to solve the above problems, the present inventors have earnestly studied a method for removing (peeling off) a device pattern (device structure film) formed on a silicon wafer.
In this study, we conducted extensive research under the premise that the size of the equipment should not increase, the processing time should not be long, and the surface should not be damaged by mechanical means.
As a result, the inventors discovered that when removing (peeling) a device pattern (device structure film) from a silicon wafer, an oxide film is formed between the device pattern (device structure film) and the silicon substrate by immersing the silicon wafer in a nitric acid solution and dilute hydrofluoric acid, both of which have strong oxidizing properties, and the device pattern (device structure film) can be removed (peeled) by dissolving this chemical oxide film, which led to the invention.

The present invention was made in light of the above situation, and aims to provide a method for removing a device pattern that can peel off (remove) a device pattern (device structure film) from a silicon wafer in a short time without damaging the surface of the silicon wafer, and a method for evaluating crystal defects in a silicon wafer using this method.

The device pattern removal method of the present invention, which has been made to solve the above problems, is a method for removing a device pattern from a silicon wafer on which a device pattern has been formed after a semiconductor device process, characterized in that the device pattern is removed by immersing the silicon wafer on which the device pattern has been formed in a hydrofluoric-nitric acid solution using a solution of hydrofluoric acid with a weight concentration of 0.05% or more and 0.1% or less and nitric acid with a weight concentration of 69% or more and 71% or less for 5 seconds or more and 1 minute or less.

In the present invention, a silicon wafer on which a device pattern (device structure film) has been formed is immersed in a fluoronitric acid solution of a predetermined concentration for a short period of time (5 seconds to 1 minute) to remove (peel off) the device pattern (device structure film).
That is, when removing (peeling) the device pattern (device structure film) of a silicon wafer, the silicon wafer is immersed in a mixture of a nitric acid solution and dilute hydrofluoric acid, both of which have strong oxidizing power. This forms a chemical oxide film between the device pattern (device structure film) and the silicon wafer. Then, the device pattern (device structure film) is removed (peeled) by dissolving this chemical oxide film.

The hydrofluoric and nitric acid solution used herein is a solution containing hydrofluoric acid with a weight concentration of 0.05% or more and 0.1% or less, and nitric acid with a weight concentration of 69% or more and 71% or less.
In this manner, since the treatment is performed with a solution having a dilute concentration of hydrofluoric acid, the surface (surface layer) of the silicon wafer is unlikely to be etched more than necessary, and is unlikely to become rough.

Furthermore, in the present invention, the device pattern (device structure film) of the silicon wafer can be formed in one etching solution tank, so that the size of the apparatus does not need to be increased.
In addition, since the immersion time in the fluoronitric acid solution is short, the surface (surface layer) of the silicon wafer is not etched and roughened, and the device pattern (device structure film) can be removed efficiently in a short time.

The silicon wafer crystal defect evaluation method of the present invention, which has been made to solve the above problems, is characterized in that after removing a device pattern from a silicon wafer on which the device pattern is formed using the above device pattern removal method, the crystal defects in the silicon wafer are made visible using a selective etching solution, and the crystal defects are observed and evaluated.

In this way, since the device pattern is removed in a short time using a solution with a low concentration of hydrofluoric acid, the silicon wafer is hardly etched and surface roughness is unlikely to occur.
Therefore, by performing selective etching after removing the device pattern, crystal defects on the silicon wafer surface can be easily observed and evaluated.

Here, it is preferable that the selective etching solution contains at least hydrofluoric acid and nitric acid, and further contains acetic acid.
This method for evaluating crystal defects is suitable for observing and evaluating OSFs or dislocation pits present on the surface of a silicon wafer.

The present invention provides a method for removing a device pattern that can peel off (remove) a device pattern (device structure film) from a silicon wafer in a short time without damaging the surface of the silicon wafer, and a method for evaluating crystal defects in a silicon wafer using this method.

FIG. 2 is a diagram showing a surface state of a silicon wafer after selective etching in Example 1. FIG. 1 is a diagram showing a surface state of a silicon wafer after selective etching in Comparative Example 1.

Below, an embodiment of the present invention will be described with reference to the drawings. However, the present invention is not limited to the embodiment described below.

(Method of removing device patterns)
The method for removing a device pattern according to the present invention is a method for removing a device pattern (device structure film) from a silicon wafer on which the device pattern (device structure film) has been formed through a semiconductor device process.
One example of a method for removing this device pattern is to immerse a silicon wafer on which a device pattern (device structure film) has been formed in a fluoronitric acid solution of a predetermined concentration for 5 seconds to 1 minute to remove the device pattern.

As described above, this method for removing a device pattern is carried out on a silicon wafer on which a device pattern (device structure film) has been formed through a semiconductor device process.
Then, the device pattern (device structure film) is removed from the silicon wafer to expose the surface of the silicon wafer. After the surface of the silicon wafer is exposed, the silicon wafer is selectively etched, and surface defects of the silicon wafer are observed and evaluated.
That is, by removing the device pattern (device structure film) and observing and evaluating the surface defects of the silicon wafer, it is possible to clarify whether the defects on the silicon wafer are defects caused by crystals or defects caused by stress or contamination due to the semiconductor device process.

As described above, the device pattern (device structure film) is removed (peeled off) by immersing the silicon wafer in a mixture of a nitric acid solution and dilute hydrofluoric acid, both of which have strong oxidizing properties.
By immersing the wafer in a mixture of nitric acid and dilute hydrofluoric acid, a chemical oxide film is formed between the device pattern (device structure film) and the silicon wafer.
It is believed that the formed chemical oxide film is dissolved by the action of hydrofluoric acid, thereby removing the device pattern (device structure film).

The concentration of the fluoronitric acid solution in which the silicon wafer is immersed affects the processing time (immersion time). From the perspective of efficient removal of device patterns (device structure films), an immersion time of 5 seconds or more and 1 minute or less is desirable.

In order to set the immersion time to 5 seconds or more and 1 minute or less, for example, a mixed solution of hydrofluoric acid having a weight concentration of 0.05% or more and 0.1% or less and nitric acid having a weight concentration of 69% or more and 71% or less is used as the hydrofluoric-nitric acid solution.
In this way, by using the hydrofluoric nitric acid solution, the device pattern (device structure film) can be removed in a short time. In addition, since the treatment is performed with a solution with a low concentration of hydrofluoric acid, the surface (surface layer) of the silicon wafer is less likely to be etched and less likely to become rough.
As a result, by performing selective etching after removing the device pattern, crystal defects on the silicon wafer surface can be easily observed and evaluated.

Specifically, if the hydrofluoric acid concentration exceeds 0.1% by weight, the reaction with the hydrofluoric acid-nitric acid mixture becomes intense, which is undesirable as it can cause surface roughening.
On the other hand, if the hydrofluoric acid concentration is less than 0.05% by weight, the reaction becomes diffuse and the treatment time becomes long, which is undesirable.
Also, if the nitric acid concentration is less than 69% by weight, the reaction becomes diffuse, which is not preferable, whereas if the nitric acid concentration is more than 71% by weight, it is not preferable because it is a special specification that is not used in the electronics industry.

(Method for evaluating crystal defects in silicon wafers)
After the device pattern is removed from the silicon wafer by carrying out the device pattern removing method, the silicon wafer is washed with water and selectively etched with a selective etching solution, and then the surface of the silicon wafer is observed and evaluated.

This selective etching is intended to expose crystal defects present on the surface of the silicon wafer, and any selective etchant can be used without particular limitation as long as it can selectively etch and expose the crystal defects in the silicon wafer.
In particular, it is desirable that the selective etching solution contains at least hydrofluoric acid and nitric acid, and further contains acetic acid.

Specifically, a selective etching solution described in JIS H0609 of the JIS standard is preferred. When the resistance of the silicon wafer is 0.05 Ωcm or less (but 0.02 Ω or more), Sato B solution or Sato G solution can be used, but when the resistance of the silicon wafer is 0.02 Ωcm or less, it is particularly preferred to use Sato G solution.
Incidentally, the volume ratio of Sato B solution is HF:HNO ₃ :CH ₃ COOH:H ₂ O = 1:12.7:3:5.7, and the treatment time when Sato B solution is used is 1.5 to 2.0 minutes, while the volume ratio of Sato G solution is HF:HNO ₃ :H ₂ O = 1:12.7:6.3, and the treatment time when Sato G solution is used is 1 to 3 minutes.

By performing selective etching on a silicon wafer using such a selective etching solution, crystal defects are made apparent and the crystal defects are observed and evaluated.
This makes it possible to clarify whether defects on silicon wafers are defects caused by crystals or by stress or contamination resulting from semiconductor device processes.

Crystal defects in silicon wafers that have been subjected to the above-mentioned treatment can be evaluated using standard methods, for example by observing OSFs or dislocation pits with an optical microscope. This allows the quality of silicon single crystal substrates to be evaluated quickly, at low cost, and with high accuracy.

〔Example〕
Example 1
In Example 1, a silicon wafer on which a device pattern was formed was immersed for 5 seconds in a mixed solution of hydrofluoric acid with a concentration of 0.05% by weight and nitric acid with a concentration of 70% by weight, and then thoroughly washed with water.
After immersion, the surface of the silicon wafer was clean and shiny, with the device pattern removed. The wafer was then etched for 1.5 minutes with a selective etching solution (Sato B solution). The surface of the silicon wafer was then observed using a differential interference microscope. As a result, pits due to selective etching were observed (Figure 1).

(Comparative Example 1)
As Comparative Example 1, a silicon wafer on which the same device pattern as in Example 1 was formed was immersed in 50% hydrofluoric acid for 48 hours, and then etched with the same selective etching solution as in Example 1 under the same conditions as in Example 1.
After that, the silicon wafer was thoroughly rinsed with water. After immersion, the surface condition of the silicon wafer was such that the device pattern had not been completely removed, with most of it remaining.
After that, the silicon wafer was etched for 1.5 minutes with a selective etching solution (Sato B solution). The surface of the silicon wafer was then observed using a differential interference microscope. As a result, the defects could not be observed sufficiently due to the remaining film (Figure 2).

(Examples 2 to 5, Comparative Examples 2 to 5)
Furthermore, the surface state of the silicon wafer after immersion in hydrofluoric acid was examined by changing the concentration of the hydrofluoric acid.
In Example 2, a silicon wafer on which a device pattern was formed was immersed in a mixed solution of 0.05% by weight hydrofluoric acid and 70% by weight nitric acid for 5 seconds. The wafer was then thoroughly rinsed with water. The surface condition of the silicon wafer after this immersion was examined. The results are shown in Table 1.

In Example 3, a silicon wafer on which a device pattern was formed was immersed in a mixed solution of 0.05% hydrofluoric acid by weight and 70% nitric acid by weight for 60 seconds. It was then thoroughly rinsed with water. The surface condition of the silicon wafer after this immersion was examined. The results are shown in Table 1.

In Example 4, a silicon wafer on which a device pattern was formed was immersed for 5 seconds in a mixed solution of 0.1% hydrofluoric acid by weight and 70% nitric acid by weight. The wafer was then thoroughly rinsed with water. The surface condition of the silicon wafer after this immersion was examined. The results are shown in Table 1.

In Example 5, a silicon wafer on which a device pattern was formed was immersed in a mixed solution of 0.1% hydrofluoric acid by weight and 70% nitric acid by weight for 60 seconds. It was then thoroughly rinsed with water. The surface condition of the silicon wafer after this immersion was examined. The results are shown in Table 1.

In Comparative Example 2, a silicon wafer on which a device pattern was formed was immersed for 3 seconds in a mixed solution of 0.05% hydrofluoric acid by weight and 70% nitric acid by weight. It was then thoroughly rinsed with water. The surface condition of the silicon wafer after this immersion was examined. The results are shown in Table 1.

In Comparative Example 3, a silicon wafer on which a device pattern was formed was immersed in a mixed solution of 0.05% hydrofluoric acid by weight and 70% nitric acid by weight for 90 seconds. It was then thoroughly rinsed with water. The surface condition of the silicon wafer after this immersion was examined. The results are shown in Table 1.

In Comparative Example 4, a silicon wafer on which a device pattern was formed was immersed for 3 seconds in a mixed solution of 0.1% hydrofluoric acid by weight and 70% nitric acid by weight. It was then thoroughly rinsed with water. The surface condition of the silicon wafer after this immersion was examined. The results are shown in Table 1.

In Comparative Example 5, a silicon wafer on which a device pattern was formed was immersed in a mixed solution of 0.1% hydrofluoric acid by weight and 70% nitric acid by weight for 90 seconds. It was then thoroughly rinsed with water. The surface condition of the silicon wafer after this immersion was examined. The results are shown in Table 1.

In Comparative Examples 6 to 9, silicon wafers on which device patterns were formed were immersed for 30 seconds in a mixed solution of hydrofluoric acid and nitric acid with the weight concentrations shown in Table 1. After that, they were thoroughly rinsed with water. The surface condition of the silicon wafers after this immersion was examined. The results are shown in Table 1. Note that nitric acid with a concentration of more than 71% was not examined because it is not used in the electronics industry.

From the above examples and comparative examples, it was confirmed that the device pattern can be removed by immersing for 5 seconds to 1 minute in a hydrofluoric-nitric acid solution using a solution of hydrofluoric acid with a weight concentration of 0.05% or more and 0.1% or less and nitric acid with a weight concentration of 69% to 71% or less.

Claims (4)

1. A method for removing a device pattern from a silicon wafer on which a device pattern has been formed through a semiconductor device process, comprising the steps of:
The silicon wafer on which the device pattern is formed is
A hydrofluoric acid solution containing 0.05% or more and 0.1% or less by weight of hydrofluoric acid and 69% or more and 71% or less by weight of nitric acid is used.
By immersing for 5 seconds or more and up to 1 minute,
A method for removing a device pattern, comprising removing the device pattern. Using the device pattern removal method described in claim 1,
After removing the device pattern from the silicon wafer on which the device pattern is formed,
A method for evaluating crystal defects in a silicon wafer, comprising the steps of: making crystal defects in a silicon wafer visible by using a selective etching solution; and observing and evaluating the crystal defects. The method for evaluating crystal defects in silicon wafers according to claim 2, characterized in that the selective etching solution contains at least hydrofluoric acid and nitric acid, and further contains acetic acid. The method for evaluating crystal defects in silicon wafers according to claim 2, characterized in that the crystal defects evaluated are OSFs or dislocation pits present on the silicon wafer surface.