JP2008181941A - Method for removing metallic impurities - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily and efficiently remove metallic impurities inside a silicon wafer.
In a step S1, a first cleaning is performed on a silicon wafer whose surface is exposed. Here, the silicon wafer surface is cleaned. Next, in step S2, the silicon wafer is stored at room temperature and left in an air atmosphere at a temperature close to room temperature, for example, for 200 hours to 800 hours. By this step, metal atoms inside the silicon wafer are diffused outward, come out on the surface of the silicon wafer, and aggregate on the surface portion. Next, in step S3, the silicon wafer is subjected to a second cleaning to remove the metal impurities aggregated on the surface of the silicon wafer. In this way, metal impurities inside the silicon wafer can be easily and efficiently removed.

Description

  The present invention relates to a method for removing metal impurities, and more particularly, to a method for removing metal impurities in a silicon wafer.

  Silicon wafers used for semiconductor device manufacturing are grown after single crystal ingot, for example, slicing process, chamfering process, lapping process, etching process, mirror polishing process, cleaning process for slicing the above ingot to obtain a thin disk-shaped wafer The product is manufactured through many processes such as a heat treatment process. The silicon wafer includes a silicon epitaxial wafer in addition to a silicon bulk wafer made of single crystal silicon. A silicon epitaxial wafer is obtained by forming a strained silicon layer including a single crystal silicon layer or a SiGe layer as an epitaxial layer on a silicon bulk wafer by, for example, a CVD (chemical vapor deposition) method at a temperature of 1000 to 1200 ° C. .

  Such a silicon wafer is subjected to a slight amount of contamination with metal impurities such as Fe, Cu, Ni, Cr in the above-described wafer manufacturing process. Similar contamination occurs in the process of manufacturing semiconductor devices. The metal impurities are precipitated in the silicon wafer to cause crystal defects, or become particle nuclei on the silicon wafer surface. In addition, an electrically deep level is formed in the silicon wafer to deteriorate the performance of the semiconductor device, or it is present in the silicon oxide film formed on the surface of the silicon wafer to reduce its insulation. As described above, the metal impurities have various adverse effects on the silicon wafer depending on the type of the metal.

Therefore, various methods for removing contaminated metal impurities have been proposed, and some of the techniques have been put into practical use. In addition, there is a method for reducing or preventing the above-mentioned adverse effects of the metal impurities by gettering contaminated metal to a predetermined portion of the silicon wafer. As a method for removing metal impurities, cleaning of the surface of a silicon wafer with a chemical solution (referred to as HPM) mixed with HCl (hydrochloric acid) -H ₂ O ₂ (hydrogen peroxide) -H ₂ O (pure water) is well known. It has been. In addition, for example, there is a method of removing by washing with a chemical chemical solution of ozone-dissolved water-H ₂ O ₂ . A method of removing metal impurities with a chemical solution of H ₂ O ₂ —H ₂ O—citric acid and forming an extremely thin silicon oxide film on the silicon wafer surface has also been proposed.

  As a method for gettering a metal impurity, so-called extrinsic gettering (EG) and intrinsic gettering (IG) are well known. Here, a portion where metal impurities are gettered (hereinafter referred to as a gettering site) is a region away from the active layer of the semiconductor device in the silicon wafer. In the EG method, a method called backside damage (BSD), poly-Si back seal (PBS), or ring gettering has been used for a long time, with the back side of the silicon wafer as a gettering site. In the IG method, a SiOx oxygen precipitate is formed in a substantially central region inside a silicon wafer, and a crystal defect-induced gettering site is used.

In addition, in a silicon epitaxial wafer having a so-called p / p ⁺ structure, a method of using the epitaxial substrate p ⁺ substrate as a metal gettering site has been proposed, and such a silicon epitaxial wafer is also put into practical use (for example, , See Patent Document 1).
Japanese Patent Laid-Open No. 2004-165589

  However, the above-described method for removing metal impurities can remove the metal on the surface of the silicon wafer, but there is a problem that it is difficult to remove the metal inside the silicon wafer. Here, a method is also conceivable in which a silicon wafer is subjected to a heat treatment, and the metal inside the silicon wafer is exposed to the surface by outward diffusion, and then the chemical chemical solution is subjected to surface cleaning to remove the metal. However, in such a method, the back contamination of the silicon wafer due to the heat treatment is likely to occur, and its practical use is difficult.

  In the method of gettering metal impurities at the gettering site of the silicon wafer, the contaminated metal remains at the silicon wafer site and is not removed from the silicon wafer. As a result, there is a problem that the above-mentioned adverse effects occur when the semiconductor device enters the active region of the semiconductor device.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for removing a metal impurity that can easily and efficiently remove a metal impurity present in a silicon wafer. .

  In order to achieve the above object, a method for removing a metal-based impurity according to the present invention is a method for removing metal inside a silicon wafer, wherein the silicon wafer is placed in a state where the surface of the silicon wafer is exposed. The process has a process of storing at a temperature close to room temperature, and a process of cleaning and removing the metal aggregated on the surface of the silicon wafer with the chemical solution after the storage.

  In the above invention, in a preferred embodiment, the storage time is in the range of 300 to 600 hours. And the temperature near the said room temperature exists in the range of 10-40 degreeC.

  According to the above invention, the metal-based impurities present inside the silicon wafer come out on the surface of the silicon wafer by outward diffusion and aggregate at the surface portion. The agglomerated metal impurities are easily and efficiently removed by washing.

  In addition, this method for removing metal impurities does not require a new and special apparatus or facility for removal, and is a very simple method that is easy to reduce costs and is highly practical.

  In the above invention, in a preferred aspect, the storage of the silicon wafer at a temperature near the room temperature and the cleaning removal with the chemical solution are repeated a plurality of times. The metal impurities are Cu metal or Ni metal.

  According to the configuration of the present invention, metal impurities existing in the silicon wafer, particularly Cu and Ni metals, can be easily and efficiently removed.

  Preferred embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a process flow diagram showing an example of a method for removing metal impurities according to the present embodiment, and FIG. 2 is a cross-sectional view by process schematically showing a Cu metal removal process as an example. FIG. 3 is a schematic diagram for explaining a mechanism for removing metal impurities in the present embodiment.

As shown in FIG. 1, the first cleaning in step S1 is performed on the silicon wafer W whose surface is exposed. Then, the silicon wafer W subjected to the first cleaning is dried. Here, the cleaning liquid used for the first cleaning is, for example, a chemical chemical liquid (referred to as APM) in which NH ₄ OH (ammonia water) —H ₂ O ₂ —H ₂ O is mixed, and the temperature thereof is, for example, about 80 ° C. Is set as follows. In addition, it is preferable that the impurity component in the chemical solution has a high purity of, for example, a ppb level or less.

  By the above step S1, as shown in FIG. 2A, the surface of the silicon wafer W in which Cu metal is present is cleaned by removing particles and the like. In the first cleaning using the APM as a cleaning liquid, although not shown, a natural oxide film having a thickness of about several nm is formed on the surface of the silicon wafer W. Such a natural oxide film is porous, and many dangling bonds (unbonded portions) of Si are present on the surface of the silicon wafer W.

  Next, in step S <b> 2, the silicon wafer W subjected to the first cleaning and having the dried surface is subjected to, for example, 200 hours to 800 hours in an air atmosphere at a temperature near room temperature (also referred to as room temperature in the description of the present invention). Store at room temperature. Here, although details will be described later, the temperature at room temperature storage is preferably in the range of 10 ° C. to 40 ° C., more preferably room temperature (20 to 25 ° C.). In practice, the storage time is preferably in the range of 300 hours to 600 hours. Such silicon wafer storage is preferably performed in an air atmosphere that has been cleaned to a class 100 or less with few particles.

  As shown in FIG. 2 (b), Cu atoms existing inside the silicon wafer W are diffused outward and come out on the surface of the silicon wafer W and aggregate at the exposed surface portion by the step S2. . Since the metal agglomerated on the surface portion is liable to become an adhesion nucleus of particles, it is preferable to store the silicon wafer in the cleaned portion as described above. The mechanism of the aggregation phenomenon will be described later with reference to FIG.

  Next, in step S3, the silicon wafer W stored for a long time, for example, at a temperature close to room temperature, is subjected to the second cleaning, and the metal impurities aggregated on the surface portion of the silicon wafer W are cleaned and removed. To do. Here, the cleaning liquid used for the second cleaning is preferably HPM, for example. Alternatively, the APM may be used. In the second cleaning, the temperature of the chemical solution is preferably set so as not to be too high, for example, 60 ° C. or less. Moreover, it is preferable that this chemical liquid also has high purity. Then, the silicon wafer W subjected to the second cleaning is dried.

  By the above step S3, as shown in FIG. 2C, almost all of the Cu metal aggregated on the surface portion of the silicon wafer W in step S2 is removed. In this way, the Cu metal inside the silicon wafer W is removed very easily.

  Here, as shown by the broken lines in FIG. 1, the above-described processing in steps S2 and S3 or the processing in steps S1 to S3 is repeated twice and three times to remove the metallic impurities present inside the silicon wafer W. You may make it remove. In this case, it is important to pay attention to reduce the recontamination of the metal impurities from the chemical solution or the storage atmosphere.

  As described above, the characteristic technical matter in the present embodiment is that the silicon wafer W is stored for a relatively long time in a state close to room temperature, and the metal inside the silicon wafer W is surfaced by outward diffusion. It is in a place where it is agglomerated on the surface and washed away.

  The mechanism for removing the metal inside the silicon wafer W will be described with reference to FIG. 3 by taking the case of Cu metal as an example. Here, FIG. 3A is a partially enlarged sectional view of the silicon wafer W, and FIG. 3B schematically shows the distribution of the thermodynamic chemical potential (μ) of Cu in the silicon wafer W. It is a graph. Here, the temperature of the silicon wafer W is assumed to be close to room temperature.

  As shown in FIG. 3A, when a silicon wafer is stored for a long time of several hundred hours near room temperature, Cu metal diffuses outwardly on the surface of the silicon wafer W as described in FIG. Then, it aggregates on the exposed silicon wafer surface. From the viewpoint of thermodynamic phenomenology, the agglomeration phenomenon of Cu metal on the surface portion of the silicon wafer is because the chemical potential (μ) of Cu metal in the silicon wafer rapidly decreases in the surface portion of the silicon wafer. It is thought to occur. In other words, Cu atoms inside the silicon wafer are in thermal motion inside the silicon wafer, but when they reach the silicon wafer surface due to thermal motion, they fall into a chemical potential that suddenly decreases at the surface portion and accumulate and aggregate as a result of thermal stabilization. it is conceivable that.

  The agglomeration phenomenon described above may be caused by the fact that the gettering site described in the prior art exists on the surface of the silicon wafer, but it is unclear what specific structure the gettering site is. It is. Whether it is related to the Si dangling bond on the surface or the interfacial energy with the natural oxide film on the surface of the silicon wafer is not certain at present.

  Here, considering that the Cu metal aggregated on the surface of the silicon wafer can be easily removed by the second cleaning process in step S3, it is considered that the Cu—Si bond between Cu and Si does not occur in the surface portion. . Further, it is considered that no Cu—O bond between O (oxygen) and Cu in the natural oxide film occurs. This is because in the second cleaning in step S3 of FIG. 1, Cu metal aggregated on the silicon wafer surface can be cleaned and removed very easily by a chemical solution such as APM.

  In any case, in the trial experiment of the present inventor, it has been confirmed that the metal impurity removal method described in the above embodiment is effective regardless of the formation state of the natural oxide film on the surface of the silicon wafer. For example, even when the silicon wafer surface has a high proportion of termination (termination) with hydrogen atoms, the same Cu metal removal effect has been confirmed.

  Considering the above, in the above embodiment, a natural oxide film may be formed on the exposed silicon wafer surface and stored for a long time at a temperature close to room temperature, or stored without a natural oxide film formed. May be made. Alternatively, a natural oxide film may be formed during storage as in the case of storage in air. In any case, it is preferable to store the silicon wafer surface at a temperature close to the room temperature in an exposed state where no other material film is deposited. Here, one side or a part of the surface of the silicon wafer may be exposed.

  The storage temperature is preferably in the range of 10 to 40 ° C, more preferably in the range of 20 to 25 ° C. This is because the metal surface aggregation mechanism described in FIG. 3 works effectively in such a temperature range. Here, when the temperature at the time of storage of the silicon wafer is higher than 40 ° C., for example, the surface-aggregated metal easily enters the silicon wafer by re-diffusion, and the removal efficiency by the second cleaning is lowered. On the other hand, when the temperature during storage of the silicon wafer is lower than 10 ° C., for example, the outward diffusion due to the thermal motion of the metal atoms in the silicon wafer is reduced, and the time until the metal inside the silicon wafer reaches the surface portion Becomes very long and impractical. In addition, when the temperature of storage becomes 30-40 degreeC and higher than room temperature, a simple hotplate etc. can be used.

  In the metal impurity removal method described in the above embodiment, Ni metal is removed even in a relatively short storage time in addition to Cu metal inside the silicon wafer. This is because the diffusion coefficient of these metals in the silicon wafer at room temperature is larger than that of other metals. Moreover, although the detailed reason is not clear at present, removal of metal impurities is also influenced by the kind of dopant in the silicon wafer. For example, in the case of an n-type silicon wafer having P (phosphorus) as a dopant, it has been confirmed that effective removal of Cu metal in a practical time becomes difficult. This is presumably because Cu-P bonding is likely to occur inside the silicon wafer, and Cu thermal diffusion at temperatures near room temperature is hindered as in this embodiment.

The chemical solution used for the first cleaning in step S1 of FIG. 1 of this embodiment is a chemical solution (referred to as FPM) in which HF (hydrofluoric acid) -H ₂ O ₂ —H ₂ O is mixed, or HF—. A natural oxide film may be removed as a chemical solution (referred to as DHF) mixed with H ₂ O. If the surface of the silicon wafer W is exposed, the step S1 may be omitted. Further, the chemical chemical solution used for the second cleaning in step S3 of FIG. 1 may be a chemical chemical solution (referred to as SPM) in which H ₂ SO ₄ (sulfuric acid) —H ₂ O ₂ —H ₂ O is mixed. . A chemical solution containing F (fluorine) such as FPM or DHF can also be used for the second cleaning.

  As described above, in the present embodiment, the metal impurities inside the silicon wafer can be easily and efficiently removed. Then, for example, the metal impurity concentration inside the silicon wafer can be lowered to the ppb to ppt level. In particular, in a silicon wafer having a p-type conductivity, Cu metal and Ni metal can be removed by cleaning even when the storage time is relatively short. In addition, according to the method for removing a metal-based impurity of the present embodiment, the amount of metal-based impurity in a silicon wafer can be made lower than its metal solid solubility at room temperature.

  Further, the method for removing metallic impurities according to the present embodiment does not require a new and special apparatus or facility for that purpose, and is an extremely simple and low-cost method. And the increase in the manufacturing cost of a silicon wafer hardly occurs, and it becomes highly practical.

Next, the effects of the present invention will be specifically described with reference to examples. Here, a 6-inch φ, p-conductivity type, double-sided mirror-finished (100) silicon wafer was used in a trial experiment for removing metallic impurities. In order to investigate the resistivity dependency of the p conductivity type, silicon wafers having a low resistance (0.01 Ω · cm) and a high resistance (10 Ω · cm) were prepared. These silicon wafers were subjected to forced contamination with Cu or Ni. In this forced contamination, a heat treatment was performed at 800 ° C. for about 1 hour in an N ₂ gas atmosphere after applying the metal-containing solution on the silicon wafer surface. After this heat treatment, the silicon oxide film formed on the surface of the silicon wafer was removed with an FPM chemical solution.

Then, the metal in the silicon wafer was removed by the removal method as shown in FIG. 1, and the removal recovery rate of the forcibly contaminated metal was examined. In this removal recovery rate, a forcedly contaminated silicon wafer was melted with a chemical chemical solution of HF-HNO ₃ , and a standard contamination amount was measured from an analysis by ICP-MS (Inductive Coupled Plasma-Mass Spectroscopy). Then, after the forced contamination, the silicon wafer subjected to the metal removal of this embodiment is melted, the amount of metal remaining on the silicon wafer is measured by ICP-MS, and the residual metal amount is subtracted from the standard contamination amount. The amount of metal removal was calculated. The metal removal amount thus determined was divided by the standard contamination amount and evaluated as a removal recovery rate. It should be noted that the present invention is not limited to such examples.

(Example 1)
As described above, Cu metal was forcibly contaminated with the silicon wafer. Here, the amount of contamination was a stable Cu metal contamination amount showing a value of slightly less than 1 × 10 ¹⁴ atoms / cm ³ in a plurality of silicon wafers. Moreover, the metal removal which changed the room temperature storage time demonstrated in FIG. 1 into 1 hour, 150 hours, 350 hours, and 800 hours with respect to the silicon wafer which contaminated Cu metal by this same lot was performed. In this metal removal, the cleaning liquid used in step S1 was APM, the silicon wafer was stored in a clean box placed on a clean bench of class 100 or lower, and the storage temperature was approximately room temperature 20 ° C. The cleaning liquid used in step S3 was also APM.

  In FIG. 4, the horizontal axis represents the storage time of step S2, and the vertical axis represents the Cu removal recovery rate. In the case of a silicon wafer having a specific resistance of ○ Ω in FIG. 4 of 0.01 Ω · cm, the Cu removal recovery rate increases rapidly until the storage time of 200 hours, and 99.5% of the forcedly contaminated Cu metal is removed. It becomes like this. And almost 100% is removed by storage for about 800 hours. Here, when the storage time is 200 hours to 800 hours, the removal recovery rate increases monotonously.

  In contrast, in the case of a silicon wafer having a specific resistance of 10 Ω · cm, the removal recovery rate is generally higher than that of a silicon wafer having a specific resistance of 0.01 Ω · cm. It can be seen that almost 100% is removed when the storage time is 200 hours or longer.

(Example 2)
Similarly, Ni metal was forcibly contaminated with a silicon wafer. In this case, the amount of contamination was stable on average at a value of just over 1 × 10 ¹⁴ atoms / cm ³ . The same metal removal as in Example 1 was performed on the silicon wafer contaminated with Ni metal by the same lot. In FIG. 5, the horizontal axis represents the storage time, and the vertical axis represents the Ni removal recovery rate. In the case of a silicon wafer having a specific resistance of 0.01 Ω · cm indicated by ○ in FIG. 5, contrary to the case of Example 1, it is removed and recovered as compared with the case of a silicon wafer having a specific resistance of 10 Ω · cm. It is easy. Then, 80% is recovered by storage for 1 hour, increases monotonically in 200 hours to 800 hours, and is almost 100% recovered in 800 hours.

  In the case of a silicon wafer having a specific resistance of 10 Ω · cm, the Ni removal recovery rate rapidly increases up to the storage time of 200 hours, and about 85% of the forcedly contaminated Cu metal is removed. And almost 100% is removed by storage for about 800 hours. Also in this case, the removal recovery rate increases monotonically when the storage time is 200 hours to 800 hours.

  From the above results, it was confirmed that the Cu metal and Ni metal inside the silicon wafer can be easily and highly efficiently removed by the metal impurity removal method shown in FIG. Further, it was found that the storage time of the silicon wafer is preferably at least 200 hours or more, and can be almost removed after about 800 hours. In view of stable metal removal and productivity in the silicon wafer manufacturing process, it has been found that the storage time is preferably in the range of 300 hours to 600 hours. Such a suitable storage time does not change if the storage is at a temperature close to room temperature.

  Although the preferred embodiments of the present invention have been described above, the above-described embodiments do not limit the present invention. Those skilled in the art can make various modifications and changes in specific embodiments without departing from the technical idea and technical scope of the present invention.

  For example, the metal impurity removal method described in the present embodiment may be applied to the case of removing metal inside a so-called SOI (Silicon on Insulator) wafer or silicon single crystal ingot in addition to a silicon wafer. Further, this embodiment may be applied to the case of a compound semiconductor in addition to the silicon semiconductor.

It is a process flow figure showing an example of a removal method of metal impurities concerning an embodiment of the present invention. It is sectional drawing according to process which showed typically the removal process of Cu metal which is an example of the embodiment of the present invention. It is a schematic diagram with which it uses for description of the mechanism of metal removal in embodiment of this invention. It is a graph which shows the result of the metal type impurity (Cu) removal in Example 1 of this invention. It is a graph which shows the result of metal type impurity (Ni) removal in Example 2 of the present invention.

Explanation of symbols

  W Silicon wafer

Claims (5)

A method for removing metal inside a silicon wafer,
Storing the silicon wafer at a temperature close to room temperature with the surface of the silicon wafer exposed; and
Cleaning and removing the metal aggregated on the surface of the silicon wafer after the storage with a chemical solution;
A method for removing metal impurities, comprising:   The method for removing metallic impurities according to claim 1, wherein the storage time is in the range of 300 to 600 hours.   The method for removing metallic impurities according to claim 1 or 2, wherein the temperature near room temperature is in the range of 10 to 40 ° C.   4. The method for removing metallic impurities according to claim 1, wherein the storage of the silicon wafer at a temperature near the room temperature and the cleaning removal with the chemical solution are repeated a plurality of times. The method for removing a metal-based impurity according to any one of claims 1 to 4, wherein the metal-based impurity is Cu metal or Ni metal.

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