JP5385566B2 - Method for evaluating metal contamination of polishing cloth - Google Patents

Description

  The present invention relates to a method for evaluating metal contamination of a polishing cloth used in a polishing process of a silicon wafer.

  A silicon wafer (hereinafter also simply referred to as “wafer”) is obtained by thinly slicing a silicon single crystal ingot. The surface of the wafer is smoothed by a polishing process. There are multiple types of polishing processes, but the final polishing process that is performed before shipping silicon wafers is to prepare irregularities ranging from small irregularities such as microroughness to large levels of undulations, and contamination sources attached to the wafer surface. The main purpose is to remove certain heavy metals.

  The final polishing step is usually a step of performing polishing while pressing a wafer against a polishing cloth called a polishing cloth while supplying a liquid containing abrasive grains (slurry). Although polishing of a wafer using such a polishing cloth is widely performed, since the wafer is required to have high smoothness, the surface state of the polishing cloth used there has a great influence on the quality of the wafer. Therefore, in order to stabilize the quality of the wafer, it is desired to grasp the surface state of the polishing cloth used in the polishing process by a simple method.

From such a viewpoint, for example, a method has been proposed in which the surface state of the polishing cloth is grasped by transferring the unevenness on the surface of the polishing cloth to a replica film and measuring the unevenness of the transfer replica film (for example, Patent Documents). 1).
JP-A-5-306924

  However, even if a polishing cloth whose surface condition is strictly controlled is used, a minute defect called silicon pit is generated on the surface of the wafer when the etching process is repeated to produce a semiconductor device from the wafer. Many troubles occurred. This is because the wafer is contaminated by heavy metal components such as copper, silver, gold, etc. slightly contained in the polishing cloth, but every time the polishing cloth is used or the lot of the polishing cloth changes, the polishing is performed. It was complicated to measure the content of heavy metal components in the cloth.

  Therefore, an object of the present invention is to provide a method capable of evaluating the amount of a heavy metal component (that is, the degree of metal contamination) contained in a polishing cloth by a simple means.

  As a result of intensive studies, the present inventors have made a metal component transfer from the polishing cloth to the evaluation wafer if the polishing cloth to be evaluated is attached to the evaluation wafer, and then the metal contamination of the evaluation wafer. By investigating the degree of the above, it was found that the amount of the metal component contained in the polishing cloth to be evaluated can be indirectly grasped. The present invention has been completed based on such findings.

  (1) The method for evaluating metal contamination of a polishing cloth according to the present invention is a method for evaluating metal contamination of a polishing cloth used in a silicon wafer polishing step, and a cloth attaching step for attaching the polishing cloth to the silicon wafer; And a determination step of determining the degree of metal contamination of the polishing cloth by determining the silicon wafer that has undergone the cloth adhesion step.

  (2) The determination step includes an etching step for etching the silicon wafer that has undergone the cross adhesion step, and counts silicon pits generated on the surface of the silicon wafer by the etching step, and a counting result obtained by counting. To a first measurement step of measuring the degree of metal adhesion on the surface of the silicon wafer.

  (3) The time of the etching process is preferably longer than the time of the final cleaning process by etching in the manufacturing process of the silicon wafer.

  (4) It is preferable that the etching liquid used for the said etching process contains SC (Standard Cleaning) -1 liquid and / or hydrofluoric acid.

  (5) In the determination step, an oxide film forming step of forming a gate oxide film on the silicon wafer that has undergone the cross-adhering step, and an insulation property of the gate oxide film are measured, and the silicon measurement result is used to determine the silicon And a second measuring step for measuring the degree of metal adhesion on the surface of the wafer.

  According to the present invention, the degree of the metal component contained in the polishing cloth can be investigated by a simple means.

<First embodiment>
A first embodiment of the method for evaluating metal contamination of a polishing cloth of the present invention (hereinafter also simply referred to as “metal contamination evaluation method”) will be described. FIG. 1 is a process diagram of a metal contamination evaluation method according to the first embodiment of the present invention. FIG. 2 is a perspective view showing an example of a cloth attaching step S1 for attaching the polishing cloth 3 to the silicon wafer 4 for evaluation.
As shown in FIG. 1, the metal contamination evaluation method according to the first embodiment is a polishing method by determining a cloth adhesion step S1 for attaching a polishing cloth to be evaluated to a silicon wafer and the silicon wafer that has undergone the cloth adhesion step S1. Determination step S2 for determining the degree of metal contamination of the cloth. Further, the determination step S2 includes an etching step S3 for etching the silicon wafer that has undergone the cross attachment step S1, and silicon pits generated on the surface of the silicon wafer by the etching step. First measurement step S4 for measuring the degree of metal adhesion on the surface of the wafer.
Hereinafter, the metal contamination evaluation method of the first embodiment will be described with reference to the drawings.

[Cross adhesion process]
The cloth adhesion step S <b> 1 is a step of transferring the metal component contained in the polishing cloth 3 to the evaluation wafer 4. As shown in FIG. 2, in the cloth attaching step S <b> 1, the polishing cloth 3 is placed on the surface of the evaluation wafer 4 in order to attach the evaluation wafer 4 and the polishing cloth 3. For this reason, the evaluation wafer 4 and the polishing cloth 3 come into contact with each other due to the gravity applied to the polishing cloth 3. Although it does not specifically limit as a magnitude | size of the grinding | polishing cloth 3, A square with a side of about 5-10 cm can be illustrated. The size of the evaluation wafer 4 is not particularly limited. For example, if the wafer has a diameter of 150 mm or 200 mm or more, it can be used as it is.

  As a pretreatment for evaluation, the surface layer portion of the polishing cloth 3 is washed with secondary pure water using a brush having a relatively high cleanliness. At this time, it is preferable not to break the surface layer structure by rubbing the surface layer portion too much. A nylon brush is exemplified as the brush used for cleaning, but it is not particularly limited as long as it has a high degree of cleanliness and is flexible enough not to break the structure of the surface layer portion of the polishing cloth 3. Further, since the polishing cloth 3 used for evaluation may be a fragment, for example, when a new lot of the polishing cloth 3 is used, a part of the lot is collected as an evaluation sample and the metal contamination evaluation method of the present invention is used. It can be performed. Thereby, metal contamination of the silicon wafer caused by the polishing cloth can be effectively prevented.

It is preferable that the wafer 4 used for evaluation has already finished the mirror surface processing and the final cleaning process. If the evaluation wafer 4 is contaminated with metal or the surface of the wafer is uneven, silicon pits resulting from the original contamination of the evaluation wafer 4 are generated in the etching step S3 described later, and correct evaluation is performed. It is because it cannot be done. Note that the final cleaning step referred to here is cleaning using a cleaning liquid having an etching action such as SC (Standard Clean) -1 solution or hydrofluoric acid. The SC-1 solution is a mixed solution in which NH ₄ OH, H ₂ O ₂ , and H ₂ O are mixed at a predetermined volume ratio.

  The polishing cloth 3 cleaned by the above method is placed on the surface of the evaluation wafer 4. At this time, since the polishing cloth 3 does not need to be dried, the evaluation may be performed with the secondary pure water used for cleaning in a wet state.

  Next, the evaluation wafer 4 is placed on the polishing cloth 3 so that the mirror surface side of the evaluation wafer 4 is in contact with the polishing cloth 3. As a result, the mirror surface side of the evaluation wafer 4 is pressed against the polishing cloth 3 by gravity applied to the evaluation wafer 4. The contact state between the polishing cloth 3 and the evaluation wafer 4 is maintained for a predetermined time, and the metal component contained in the polishing cloth 3 is transferred to the evaluation wafer 4. The time during which the polishing cloth 3 and the evaluation wafer 4 are kept in contact with each other is preferably 0.1 to 10 minutes, more preferably 0.5 to 5 minutes, and most preferably 1 to 2 minutes. By bringing the polishing cloth 3 and the evaluation wafer 4 into contact with each other for 0.1 minute or longer, the metal component can be sufficiently transferred from the polishing cloth 3 to the evaluation wafer 4. Moreover, the influence of the surrounding environment can be reduced by setting the contact time between the polishing cloth 3 and the evaluation wafer 4 to 10 minutes or less.

  As a method of contacting the polishing cloth 3 and the evaluation wafer 4, a method other than the method using gravity as described above can be adopted. However, from the viewpoint that it is better not to attach the fibers to the surface of the evaluation wafer 4 for accurate evaluation, a contact method in which the two are not rubbed or attached with a strong force is preferable.

[Judgment process]
The evaluation wafer 4 that has undergone the cloth adhesion step S1 is subjected to the determination step S2 in order to determine the degree of metal contamination of the polishing cloth 3. In the determination step S2, an etching step S3 for etching the evaluation wafer 4 that has undergone the cross attachment step S1, and silicon pits generated on the surface of the evaluation wafer 4 by the etching step S3 are counted, and the count obtained by counting is counted. A first measurement step S4 for measuring the degree of metal adhesion on the surface of the evaluation wafer 4 from the result is included. Hereinafter, these steps will be described.

[Etching process]

  The evaluation wafer 4 is immersed in the etching solution with the surface in contact with the polishing cloth 3 facing up in the cloth attaching step S1.

The etching solution used in this step is preferably the same as the etching solution used in the final cleaning step of the silicon wafer manufacturing process. Examples of such an etchant include those containing SC-1 solution and hydrofluoric acid. When using the SC-1 solution in an etching solution, concentration of SC-1 solution in the etching _solution, NH 4 OH: 0.4 to 3 _{wt%, H} 2 O 2: a 1.5 to 7 wt% It is preferable that NH ₄ OH: 0.8 to 1.5% by mass and H ₂ O ₂ : 3.0 to 3.5% by mass are more preferable. When hydrofluoric acid is used as the etching solution, the concentration of hydrofluoric acid in the etching solution is preferably 0.5 to 3% by mass, and more preferably 1 to 1.5% by mass.

  The time for immersing the evaluation wafer 4 in the etching process S3 is longer than the time for immersing the silicon wafer in the etching solution in the final cleaning process of the silicon wafer manufacturing process, and is about 2 to 12 times the final cleaning process in the silicon wafer manufacturing process. Is preferred. Therefore, if the final cleaning process requires 3 minutes, it is preferable to immerse in the etching process S3 for about 10 minutes, for example. By setting the immersion time to more than twice the time of the final cleaning step, contamination of the silicon wafer by the metal derived from the polishing cloth 3 that does not become apparent in the normal final cleaning step can be manifested in the form of silicon pits. it can. Moreover, it can prevent that it becomes difficult to evaluate by being etched excessively by making immersion time into the time 12 times or less of the last washing | cleaning process. A more preferable range of immersion time is 4 to 6 times the final cleaning step.

  In this etching step S3, when a silicon wafer free from metal contamination is etched, the entire silicon wafer is etched uniformly, whereas when etching a silicon wafer with metal contamination, only the portion with metal contamination is etched. It uses the fact that the speed becomes slow or fast, and the portion becomes a silicon pit that is a fine unevenness of a fluttering shape. The silicon pits formed in the etching step S3 are counted in the next first measurement step S4.

[First measurement process]
Next, silicon pits generated on the surface of the evaluation wafer 8 by the etching step S3 are counted, and a first measurement for measuring the degree of metal adhesion on the surface of the evaluation wafer 8 from the counting result obtained by the counting. Step S4 will be described.

  In order to count silicon pits generated on the surface of the evaluation wafer 8 by the etching step S3, for example, appropriate image processing may be performed by a confocal optical system defect inspection apparatus. A confocal optical system irradiates a sample with a small spot that converges a laser beam, refocuses the reflected light on a pinhole located on the entire surface of the receiver, and detects the amount of light that has passed through the pinhole. is there.

  FIG. 3 shows the result of measuring the surface of the evaluation wafer 4 with a confocal optical system defect inspection apparatus and performing image processing. FIG. 3A shows a state in which the generation of silicon pits is small. It is a figure and (b) is the figure which showed the state with many generation | occurrence | production of a silicon pit.

  In the case of FIG. 3A in which the generation of silicon pits 41 is small, it is determined that there is little migration of metal contamination from the polishing cloth 3 to be evaluated to the evaluation wafer 4. It is judged that there are few. On the other hand, in the case of FIG. 3B in which the generation of silicon pits 41 is large, it is determined that there is a lot of migration of metal contamination from the polishing cloth 3 to be evaluated to the evaluation wafer 4. It is determined that the degree is large.

  Note that the threshold for detecting silicon pits when performing image processing with a confocal optical system defect inspection apparatus, and how much silicon pits are generated are determined as the degree of metal contamination of the polishing cloth 3 being high. What is necessary is just to set suitably according to the use of a wafer. For example, when MAGICS M350 (trade name, manufactured by Lasertec Co., Ltd.) is used as a confocal optical system defect inspection apparatus, if the number of silicon pits is 10-20 or less when the threshold is set to 18 pixels or more, for evaluation It is determined that the metal adhesion to the wafer 4 is small, and if the number of silicon pits exceeds 30, it is determined that the metal adhesion to the evaluation wafer 4 is large.

  As a result of the first measurement step S4 described above, if it is determined that the amount of metal attached to the evaluation wafer 4 is small, it is evaluated that the amount of metal contained in the polishing cloth 3 to be evaluated is small. If it is determined that there is much adhesion of the metal, it is evaluated that the amount of metal contained in the polishing cloth 3 to be evaluated is large.

<Second embodiment>
Next, the metal contamination evaluation method of the second embodiment of the present invention will be described. FIG. 4 is a process diagram of the metal contamination evaluation method of the second embodiment of the present invention. FIG. 5 is a partial cross-sectional view showing a state in which the gate oxide film 5 is formed on the surface of the evaluation wafer 4.
As shown in FIG. 4, in the second embodiment, a cloth adhesion step S1 for attaching a polishing cloth to be evaluated to a silicon wafer, and a metal of the polishing cloth by determining the silicon wafer that has undergone the cloth attachment process S1. Determination step S2 ′ for determining the degree of contamination. In the determination step S2 ′, an oxide film forming step S5 for forming the gate oxide film 5 on the silicon wafer that has undergone the cross attachment step S1, and the insulation of the gate oxide film 5 are measured. And a second measurement step S6 for measuring the degree of adhesion.
Hereinafter, although the metal contamination evaluation method of the second embodiment of the present invention will be described, the cloth adhesion step S1 in the second embodiment is the same as that of the first embodiment, and the description thereof will be omitted.

[Judgment process]
The evaluation wafer 4 that has undergone the cloth adhesion step S1 is subjected to a determination step S2 ′ in order to determine the degree of metal contamination of the polishing cloth 3. In the determination step S2 ′, the oxide film forming step S5 for forming the gate oxide film 5 on the evaluation wafer 4 that has undergone the cross adhesion step S1, and the insulating property of the gate oxide film 5 are measured. And a second measurement step S6 for measuring the degree of metal adhesion on the surface of the evaluation wafer 4. Hereinafter, these steps will be described.

[Oxide film formation process]
As shown in FIG. 5, in order to form the gate oxide film 5 on the surface of the evaluation wafer 4, the wafer 4 that has undergone the cross-adhesion step S <b> 1 is, for example, based on the GOI evaluation method described in the SEMI standard M51. What is necessary is just to form.

[Second measurement process]
Next, the second measurement step S6 for measuring the insulation of the gate oxide film 5 and measuring the degree of metal adhesion on the surface of the evaluation wafer 4 from the measurement result of the insulation will be described.
Various methods for evaluating the insulating property of the gate oxide film 5 have been published, and may be appropriately selected and used. Examples of such an evaluation method include the method described in the GOI evaluation method described in the SEMI standard M51. As a result of this evaluation, if the insulating property of the gate oxide film 5 formed on the evaluation wafer 4 is good, it is determined that there is little metal adhesion to the evaluation wafer 4. On the other hand, if the insulating property of the gate oxide film 5 formed on the evaluation wafer 4 is poor, it is determined that the metal adheres to the evaluation wafer 4 frequently.

  As a result of the second measurement step S6, if it is determined that the amount of metal attached to the evaluation wafer 4 is small, it is evaluated that the amount of metal contained in the polishing cloth 3 to be evaluated is small, and the metal to the evaluation wafer 4 is If it is determined that there is much adhesion, it is evaluated that the amount of metal contained in the polishing cloth 3 to be evaluated is large.

  The embodiments of the present invention have been described in detail with reference to the drawings. However, the present invention is not limited to the above embodiments, and appropriate modifications are made within the scope of the object of the present invention. Can be implemented.

It is process drawing of the metal contamination evaluation method of the first embodiment of the present invention. It is the perspective view which showed an example of cloth adhesion process S1 which adheres the polishing cloth 3 to the silicon wafer 4 for evaluation. The surface of the evaluation wafer 4 is measured with a confocal optical system defect inspection apparatus, and the result of image processing is illustrated. (A) is a diagram showing a state in which the generation of silicon pits is small, (B) is the figure which showed the state with many generation | occurrence | production of a silicon pit. It is process drawing of the metal contamination evaluation method of the 2nd embodiment of this invention. 4 is a partial cross-sectional view showing a state in which a gate oxide film 5 is formed on the surface of an evaluation wafer 4. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 3 Polishing cloth 4 Wafer for evaluation 41 Silicon pit 5 Gate oxide film S1 Cross adhesion process S2, S2 'determination process S3 Etching process S4 First measurement process S5 Oxide film formation process S6 Second measurement process

Claims (5)

A method for evaluating metal contamination of a polishing cloth used in a polishing process of a silicon wafer,
A cloth attaching step for attaching the polishing cloth to the silicon wafer for evaluation ;
A determination step of determining the degree of metal contamination of the polishing cloth by determining the evaluation silicon wafer that has undergone the cloth adhesion step,
Method for evaluating metal contamination of polishing cloth. The determination step includes
Etching process for etching the silicon wafer for evaluation that has undergone the cloth adhesion process,
A first measurement step of counting silicon pits generated on the surface of the evaluation silicon wafer by the etching step, and measuring a degree of metal adhesion on the surface of the evaluation silicon wafer from a counting result obtained by counting; ,including,
The method for evaluating metal contamination of a polishing cloth according to claim 1. 3. The method for evaluating metal contamination of a polishing cloth according to claim 2, wherein the time of the etching step is longer than the time of the final cleaning step by etching in the manufacturing process of the evaluation silicon wafer.   The method for evaluating metal contamination of a polishing cloth according to claim 2 or 3, wherein the etching solution used in the etching step contains SC (Standard Cleaning) -1 solution and / or hydrofluoric acid. The determination step includes
An oxide film forming step of forming a gate oxide film on the silicon wafer for evaluation that has undergone the cross adhesion step;
Measuring the insulating property of the gate oxide film, and measuring the degree of metal adhesion on the surface of the silicon wafer for evaluation from the insulating measurement result,
The method for evaluating metal contamination of a polishing cloth according to claim 1.

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