JP3257518B2 - How to store silicon wafers in liquid - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for storing a silicon wafer in a liquid, and more particularly to a method for storing a silicon wafer having a hydrophobic surface in a liquid.

[0002]

2. Description of the Related Art Generally, a method for producing a semiconductor wafer includes a slicing step of slicing a single crystal ingot to obtain a thin disk-shaped wafer, and a slicing step for preventing the wafer obtained by the slicing step from cracking or chipping. A chamfering step for chamfering the outer peripheral portion, a lapping step for flattening the wafer, an etching step for removing processing distortion remaining on the chamfered and wrapped wafer, and a polishing (polishing) step for mirror-polishing the wafer surface. Then, a cleaning step of cleaning the polished wafer and removing abrasives and foreign matters adhering thereto is performed.

[0003] The above steps show the main steps, and other steps such as a heat treatment step are added or the order of the steps is changed. Storage may be required and must be performed in a manner appropriate for the state of the silicon.

[0004] For example, a wafer after the polishing step has a washing step in the next step, and may be stored in a liquid during a waiting time before being sent to this step. This is because if the wafer is left in the air, the slurry will adhere to the slurry due to drying of the polishing slurry, and it will be difficult to remove it by washing in the next step.

[0005]

It is well known that when heat treatment is performed with impurities, particularly heavy metals, adsorbed on the wafer surface, the impurities adversely affect the electrical characteristics of the device. Therefore, a cleaning step for removing such impurities is usually performed before the heat treatment.

[0006] Therefore, it is extremely important to grasp the contamination state in the cleaning solution and the contamination state on the silicon wafer surface and the like after the cleaning step, and these have been studied.

[0007] However, this is a case in which the substances adhered in the step before cleaning are removed, and those contaminated in the step before cleaning are removed by this cleaning, or the contamination is not performed during the cleaning. This is to obtain a silicon wafer having a clean surface.

However, even if a silicon wafer having a clean surface is obtained by controlling the contaminant concentration in the above-mentioned cleaning step and performing cleaning, thermal oxidation is thereafter performed to form an oxide film. May occur.

It is known that this oxide film breakdown voltage defect is generally a wafer contaminated with metal, and if the metal remains on the wafer, the breakdown voltage deteriorates. Therefore,
Metal impurity analysis on the surface of the silicon wafer after cleaning was performed, and a correlation between the analysis value and the wafer having an oxide film breakdown voltage defect was performed. No remarkable difference was found between the defective wafer and the non-defective wafer. Was. That is, it was found that the defective wafer in which the oxide film breakdown voltage defect occurred was also properly cleaned, and the impurities were removed.

The present inventor has analyzed the cause of such a breakdown voltage failure, and as a result, it has been found that the cause of the failure lies in the method of storing the silicon wafer in the process before cleaning. In particular, metals with a lower ionization tendency than silicon,
For example, when ions such as copper and silver are present in a storage solution, when a silicon wafer having a hydrophobic surface immediately after polishing is stored in this solution, the oxide film quality when the wafer is subjected to thermal oxidation is thereafter It was found that the deterioration of the steel was remarkable.

This phenomenon is not limited to wafers immediately after polishing, but applies to all silicon wafers having a hydrophobic surface. For example, it has been confirmed that an epitaxial wafer epitaxially grown on a silicon wafer substrate has a hydrophobic surface after epitaxial growth and a similar phenomenon is observed.

Further, as a result of the investigation, it has been found that, particularly in the case of copper, even if the concentration of copper ions in the storage solution is 1 ppb or less, the oxide film withstand voltage failure occurs. Contamination of the storage solution by such low levels of copper ions can occur due to various factors such as carry-in from the previous process. Therefore, development of a technique for preventing deterioration of the oxide film breakdown voltage characteristics due to the above-described causes has been desired.

The present invention has been made in view of the above circumstances, and a method of storing a silicon wafer in a liquid, particularly, a method of preventing a silicon wafer having a hydrophobic surface from causing an oxide film withstand voltage failure after storage. It is an object of the present invention to provide a method for storing a silicon wafer in a liquid.

[0014]

Means for Solving the Problems The present invention has been made to solve the above-mentioned problems, and the invention according to claim 1 of the present invention provides a method for storing a silicon wafer in a liquid, comprising the steps of: A method for storing a silicon wafer in a liquid, characterized by storing the wafer in water or a chemical solution to which silicon has been added.

As described above, in the method of storing a silicon wafer in a liquid, by adding a chelating agent to the storage liquid, even if harmful metal ions such as Cu are mixed in the storage liquid, the metal of the wafer can be stored. It is possible to prevent the contamination and the accompanying deterioration of the oxide film breakdown voltage characteristics.

The method for storing a silicon wafer in a liquid according to the present invention is effective regardless of whether the surface of the silicon wafer is a hydrophilic surface or a hydrophobic surface.
In particular, the effect is remarkable when a silicon wafer, which is a hydrophobic surface having no oxide film on the wafer surface, is stored in a liquid. In other words, after the polishing process is completed in the silicon wafer manufacturing process, in the waiting time until the silicon wafer immediately after polishing is sent to the next cleaning process,
The method for storing a silicon wafer in a liquid according to the present invention can be used (claim 2).

When the storage liquid is water, the solubility of the chelating agent in water is low depending on the chelating agent.
When the metal ion concentration in the storage water becomes high,
If the concentration of the chelating agent is such that it dissolves in water, the effect of suppressing metal contamination may be insufficient. In this case, an alkaline aqueous solution having a high solubility of the chelating agent may be used as a storage chemical, and the chelating agent may be added to the chemical (claim 3).

When water or a chemical solution containing a chelating agent is used as a storage solution, particularly when an alkaline aqueous solution is used as a storage solution, the surface of the wafer may be roughened due to the etching action with alkali. . in this case,
It is desirable to add an appropriate amount of a surfactant to the storage solution (claim 4).

The chelating agent is more effective as long as it has a chelating compound-producing ability higher than that of NTA (nitrotriacetic acid).

The chelating agent is NTA
Besides, EDTA (ethylenediaminetetraacetic acid), DTP
A (diethylenetriamine-N, N, N ', N ", N"
-Pentaacetic acid), CyDTA (diaminocyclohexane-
N, N, N ', N'-tetraacetic acid) and the like, and their sodium salts. These are effective when used alone or in a mixed state (claim 6).

Hereinafter, the present invention will be described in more detail.
The present invention is not limited to these. The present inventor has analyzed the cause of the oxide film breakdown voltage failure as described above.As a result, when a silicon wafer having a hydrophobic surface and metal impurities, particularly a metal having a smaller ionization tendency such as copper and silver coexist with silicon coexist. It is found that a problem occurs, and as a case where such a state exists, there is a problem particularly when a wafer immediately after polishing is temporarily stored in a liquid before cleaning between a polishing step and a cleaning step. I found a thing.

This is particularly problematic when the metal impurity is copper,
It was found that even if the concentration of copper ions in the storage solution was as low as 1 ppb or less, the oxide film withstand voltage failure occurred. In addition, this phenomenon is not limited to the wafer immediately after polishing, but it is a problem when the silicon wafer is stored in a liquid, and it is found that the phenomenon occurs particularly when the silicon wafer that is a hydrophobic surface is stored. . For example, it was confirmed that a similar phenomenon was observed on an epi-wafer having a hydrophobic surface.

Although the cause of the poor quality of the oxide film is not clear, when a metal having a lower ionization tendency than silicon, such as copper ions, is present in the storage solution, it is electrochemically reacted on the silicon surface to deposit. This is considered to form a defect on the silicon wafer.

In this case, it should be possible to remove copper and the like deposited on the surface in the subsequent washing, but defects such as distortion of the surface of the silicon wafer caused by the reaction remain, resulting in poor oxide film quality. It seems to be connected.

Therefore, the present inventor decided to add a chelating agent to the storage solution in order to render the above metal ions such as copper harmless, and to use water or a chemical solution to which the chelating agent was added, to prepare a silicon wafer. By storing in liquid
The present invention has been completed based on the idea of preventing deterioration of the quality of an oxide film of a silicon wafer.

That is, by using water or a chemical solution to which a chelating agent has been added as a storage solution, even when metal ions such as copper are mixed in the storage solution, the metal ions can be converted into a chelate compound to form a metal. It is possible to prevent ions from being electrochemically adsorbed on the silicon wafer. As a result, it is possible to prevent the silicon wafer from having an oxide film withstand voltage defect due to metal contamination of the storage solution.

Here, the water to which the chelating agent is added is preferably pure water, preferably ultrapure water.
Examples thereof include an acid, an alkali solution, and an organic solvent, and more specifically, trichloroethylene and ethanol, but are not particularly limited thereto.

The amount of the chelating agent depends on how much the metal ion concentration in the storage solution increases. If the concentration of metal ions in the storage solution is high,
Increasing the amount of the chelating agent can prevent the oxide film from having a withstand voltage. For example, if the addition concentration of the chelating agent in the storage solution is at least 0.05 mol / L, the effect of preventing the oxide film from withstanding pressure becomes more remarkable.

Further, the storage method of the present invention is effective regardless of whether the surface of the silicon wafer to be stored is a hydrophilic surface or a hydrophobic surface. However, a silicon wafer which is a hydrophobic surface having no natural oxide film has an active surface and is easily contaminated. Therefore, it is particularly advantageous to use the storage method of the present invention.

Therefore, the present invention is most suitable when a silicon wafer, which is a hydrophobic surface having no natural oxide film immediately after polishing, is stored in a liquid. The storage method of the present invention can be used when the wafer is stored in liquid during the waiting time between steps. As another application scene of the present invention, application to silicon wafers after etching or cleaning using hydrofluoric acid (HF) may be mentioned. This is because an oxide film on the surface of the silicon wafer is removed by hydrofluoric acid (HF), and the silicon wafer becomes active and is easily contaminated with Cu.

Thus, the use of the present invention not only causes the polishing slurry to stick when left in the air during the waiting time between processes, but also causes contamination from storage water, which has been a problem in conventional underwater storage. It is possible to prevent the oxide film breakdown voltage characteristic from deteriorating.

When the storage solution is water, the solubility of the chelating agent in water is low depending on the chelating agent. At a chelating agent concentration, the effect of suppressing metal contamination may be insufficient. In this case, an alkaline aqueous solution having a high solubility of the chelating agent is used as the storage chemical,
A chelating agent may be added to this chemical.

The alkaline aqueous solution includes, for example, aqueous ammonia, sodium hydroxide, potassium hydroxide and a mixture thereof, but is not particularly limited as long as it can sufficiently dissolve the chelating agent. Not something.

Furthermore, when water or a chemical solution to which a chelating agent is added is used as a storage solution, particularly when an alkaline aqueous solution is used as a storage solution, the wafer surface may be roughened by the etching action with alkali. . In this case, it is desirable to add an appropriate amount of a surfactant to the storage solution.

As described above, even when the surface of the wafer is roughened by the etching action of the alkali, the surface roughening can be prevented by adding the surfactant to the storage solution. This is considered to be because, when the surfactant is added to the storage solution, the surfactant molecules are physically adsorbed to the silicon wafer, and the alkali in the storage solution cannot react with the silicon wafer.

The above-mentioned surfactants include, for example, neutral surfactants such as polyoxyethylene nonylphenyl ether. However, other neutral surfactants, such as polyoxyethylene lauryl ether, or anionic surfactants, such as sodium polyoxyethylene alkylphenyl sulfate, have similar efficacy.

The addition amount of the surfactant is determined by the addition amount of the chelating agent and the amount of the alkaline aqueous solution necessary for dissolving the chelating agent, and there is no particular limitation as long as the effects of the present invention are achieved. Absent.

Any of the above chelating agents can exert the effects of the present invention as long as the metal ions mixed in the storage solution can be used as a chelating compound. If a chelating agent having a large compound-forming ability is used, a more remarkable effect can be obtained. This is because a metal having a lower ionization tendency than silicon such as copper is sufficiently chelated and trapped, so that a chelate compound generation ability equal to or higher than NTA is required.

Other chelating agents having a higher chelating compound generating ability than NTA include EDTA, DTPA and Cy.
DTA and the like, and sodium salts thereof and the like. By using a sodium salt or the like, a larger amount of a chelating agent can be made water-soluble without using an alkaline aqueous solution. These are effective when used alone or in a mixed state.

As described above, the amount of the chelating agent depends on how much the metal ion concentration in the storage solution increases. 1 × 10 ⁻⁵ mol if it is known that external contamination such as the pre-process is small (about several ppb)
/ L is sufficient. However, when the concentration of the chelating agent is 0.05 mol / L or more, the present invention exhibits more remarkable effects even when a high concentration of metal ions is present. In this case, when there is a possibility that the surface of the wafer may be roughened, the amount of the surfactant added for preventing the surface roughening may be increased.

[0041]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described with reference to examples, but the present invention is not limited to these.

[0042]

EXAMPLES Examples of the present invention and comparative examples will be described below. (Example 1, Comparative Example 1) Into water intentionally contaminated to contain 1 ppb of copper ions, NTA, EDTA, DTPA,
Each storage solution to which CyDTA was added at a concentration of 0.05 mol / L was prepared. For comparison, pure water without intentional contamination and a storage solution containing 1 ppb copper ions but containing no chelating agent were prepared. Next, an FZ silicon wafer manufactured by a floating zone method (FZ method) was prepared, and a natural oxide film on the surface thereof was removed with hydrofluoric acid. This process is performed to remove the natural oxide film on the surface of the silicon wafer to make the silicon surface hydrophobic, and to bring the silicon wafer into the same state as the silicon wafer immediately after polishing. The FZ silicon wafer from which the natural oxide film was removed was immersed in each storage solution for 30 minutes. afterwards,
A known RCA cleaning was performed to evaluate the oxide film breakdown voltage. The evaluation of the oxide withstand voltage quality is based on the oxide withstand voltage (Time Zero Dielec
C-mode yield of tric breakdown (TZDB), more specifically, a dielectric breakdown electric field of 8 MV / cm or more evaluated by making a phosphorus-doped polysilicon electrode (oxide film thickness 25 nm, electrode area 8 mm ² ) and evaluating at a judgment current value of 1 mA / cm ² Of non-defective products.

Here, the RCA cleaning is a standard cleaning method for silicon wafers proposed by Kern, and includes three steps. The first step is to remove an oxide film and a silicon surface with NH ₄ OH / H ₂ O _2. The second step is a method for cleaning a silicon wafer in which an oxide film formed in the previous step is removed by HF and a heavy oxide is removed in a third step by HCl / H ₂ O ₂ to form a natural oxide film. The higher the numerical value of the C-mode yield, the better the quality.

Table 1 shows the results.

[Table 1]

As is clear from Table 1, when the silicon wafer is stored without adding a chelating agent to the water contaminated with copper, the withstand voltage quality of the oxide film is remarkably deteriorated. On the other hand, when a wafer is stored in a storage solution containing a chelating agent, the wafer is stored in pure water regardless of which chelating agent is added, despite intentional metal contamination. It can be seen that, similarly to the case of the above, deterioration of the oxide film breakdown voltage quality did not occur.

(Example 2, Comparative Example 2) Storage solutions were prepared under the conditions shown in Table 2 and prepared. In this embodiment, an aqueous ammonia solution, which is an alkaline aqueous solution, was used as a storage chemical solution in order to dissolve a large amount of a chelating agent. Then, as in Example 1, the FZ silicon wafer from which the natural oxide film had been removed with hydrofluoric acid was immersed in each storage water tank for 30 minutes. Thereafter, RCA cleaning was performed, and the oxide film breakdown voltage evaluation and the micro roughness were evaluated. The micro roughness is LS-6000 manufactured by Hitachi Electronics Engineering Co., Ltd.
And the detection voltage was 900 V. The larger the evaluation value of the micro roughness, the higher the surface roughness.

Table 2 shows the results.

[Table 2]

As shown in Table 2, when the concentration of copper ions in the storage solution intentionally contaminated is as high as 1000 ppb, and when the concentration of EDTA is 0.05 mol / L, the deterioration of the withstand voltage quality of the oxide film is deteriorated. Has the effect of preventing
Deterioration has not been completely prevented. However, when the concentration of EDTA is 0.5 mol / L, it is possible to completely prevent the deterioration of the withstand voltage quality of the oxide film, and to obtain the same oxide film quality as a wafer immersed in pure water that has not been intentionally contaminated. I understand.

That is, by adding a chelating agent according to the metal ion concentration in the storage solution, it is possible to prevent the oxide film breakdown voltage from deteriorating even if the metal ion concentration in the storage solution is at a high level. It is.

In this example, ammonia water was used as a storage chemical to dissolve a large amount of EDTA.
When the surfactant is not added, although the deterioration of the withstand voltage quality of the oxide film can be prevented, the surface of the silicon wafer is roughened. However, it is understood that the surface roughness can be prevented by adding the surfactant, and the wafer surface can be maintained equivalent to that when the wafer is stored in pure water, even though the wafer is stored in the alkaline aqueous solution.

(Example 3 and Comparative Example 3) Storage solutions were prepared under the conditions shown in Table 3 and prepared. Then, as in Example 1, the FZ silicon wafer from which the natural oxide film was removed with hydrofluoric acid was immersed in each storage water tank for 12 hours. Thereafter, RCA cleaning was performed to evaluate the withstand voltage quality of the oxide film. In this embodiment, the evaluation of the oxide film breakdown voltage quality cannot be detected by the TZDB C-mode yield described above.
In order to evaluate a more subtle difference, TDDB (Time Dependent Dielectric) was used instead of TZDB C-mode yield.
Breakdown) was evaluated by the γ-mode yield. In this method, a phosphor-doped polysilicon electrode similar to that of Example 1 was formed on a wafer, a stress current of 0.01 mA / cm ² was continuously applied, and a dielectric breakdown occurred when the charge amount was 25 C / cm ² or more. The evaluation was based on the non-defective rate.

Table 3 shows the results.

[Table 3]

As shown in Table 3, when the concentration of copper ions in the storage solution intentionally contaminated is as low as 1 ppb, the concentration of EDTA to be added is as small as 1 × 10 ⁻⁵ mol / L. It can be seen that the effect of preventing the deterioration of the breakdown voltage quality of the oxide film can be improved. In this case, the concentration of EDTA is 1
When it is increased to × 10 ⁻⁴ mol / L, the withstand voltage quality of the oxide film is slightly deteriorated. This deterioration in TDDB is due to EDTA
It is considered that the surface roughness of the wafer was affected by the increase in the amount. However, even in such a case, as the amount of EDTA increases,
It can be seen that by increasing the amount of the surfactant added, it is possible to prevent surface roughness and deterioration of the withstand voltage quality of the oxide film.

In the above embodiment, a silicon wafer from which a natural oxide film was removed with hydrofluoric acid was used, but similar results are obtained with a silicon wafer having no oxide film, such as a silicon wafer immediately after polishing. . In this embodiment, intentional contamination was caused by copper ions. However, similar results were obtained when the storage solution was contaminated by other metal ions such as silver.

The present invention is not limited to the above embodiment. The above embodiment is an exemplification, and the present invention has substantially the same configuration as the technical idea described in the claims of the present invention, and has the same function and effect as that of the present invention. Within the technical scope of

For example, the scene to which the present invention is applied is not limited to the case where the wafer is temporarily stored in the process of manufacturing the silicon wafer, but may be any case where the silicon wafer is stored in the liquid. However, the present invention is also applicable and has the effects described above.

[0057]

As described above, according to the present invention, it is possible to prevent an oxide film withstand voltage defect caused by metal contamination from a storage solution, which has been a problem when the silicon wafer is stored in the solution. it can. In particular, when the silicon wafer, which is a hydrophobic surface after the polishing step, is stored in a liquid during a waiting time before being sent to the cleaning step, if the present invention is used, polishing when the wafer is left in the air While preventing the slurry from sticking, it is possible to prevent the silicon wafer from having an oxide film breakdown voltage defect.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int. Cl. ⁷ , DB name) H01L 21/304 H01L 21/68

Claims (6)

(57) [Claims] 1. A method for storing a silicon wafer in a liquid, comprising storing the silicon wafer in a liquid or a chemical solution to which a chelating agent has been added. 2. The method for storing a silicon wafer in a liquid according to claim 1, wherein the silicon wafer to be stored is a hydrophobic surface. 3. The method for storing a silicon wafer in a liquid according to claim 1, wherein the chemical solution to which the chelating agent is added is an alkaline aqueous solution. 4. The method according to claim 1, wherein a surfactant is added to water or a chemical solution to which the chelating agent is added.
A method for storing the silicon wafer according to any one of claims 3 to 5 in a liquid. 5. The silicon wafer according to claim 1, wherein the chelating agent is a chelating agent having a higher chelating compound generating ability than NTA. how to. 6. The chelating agent is NTA, EDTA, D
The method for storing a silicon wafer in a liquid according to any one of claims 1 to 5, wherein the silicon wafer is TPA, CyDTA, a salt thereof, or a mixture thereof.