JP2010165960A - Method for cleaning of silicon wafer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for cleaning of a silicon wafer which is carried out after finish polishing. <P>SOLUTION: In the method for cleaning of the silicon wafer 18, an oxide film 22 is formed on a surface of the silicon wafer and is removed to remove impurities on the surface after the finish polishing carried out after rough polishing of the silicon wafer 18, the removal of the oxide film 22 is carried out until a processing altered layer 22 formed on a polished surface of the silicon wafer 18 during the finish polishing is removed in an oxide film removing process. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a technique for cleaning a semiconductor silicon wafer.

  In general, a semiconductor silicon wafer is cut out from an ingot using a diamond saw, a wire saw or the like, and then shipped as a product through steps of lapping, etching, polishing, cleaning, and inspection. Of these, the polishing step usually comprises two or three stages. Specifically, rough polishing (primary polishing) for the purpose of removing the lapping surface and finish polishing for the purpose of preventing light scattering from the surface called haze (cloudiness), or the above rough polishing It consists of three stages in which intermediate polishing (secondary polishing) is introduced during final polishing. The role varies depending on each polishing stage, and an appropriate polishing liquid, pad, or the like is selected according to the role.

For example, in the rough polishing, as described above, the purpose is to efficiently remove the lapping surface, and therefore, polishing processing is performed with an emphasis on the polishing rate. Specifically, in rough polishing, a polishing liquid mainly composed of alkaline colloidal silica and artificial leather with relatively high hardness are used as a pad. All of these are adjusted appropriately from the viewpoint of increasing the removal rate and improving productivity, thereby aiming to raise the level of TTV (total thickness variation). Furthermore, the productivity is further improved by setting the polishing pressure as high as 300 gf / cm ³ or more (see Patent Document 1).

  By the way, in the machining process related to the above-described rough polishing, a damaged layer (worked layer) may be formed on the entire surface of the wafer at a depth of several μm. Such a work-affected layer induces crystal defects such as slip dislocation in the device manufacturing process, lowers the mechanical strength of the wafer, and adversely affects electrical characteristics. Therefore, Patent Document 2 discloses a single wafer cleaning apparatus for removing a work-affected layer generated in this rough polishing.

  On the other hand, finish polishing aims to improve micro-roughness that is closely related to the above-described haze. This micro roughness can be measured by an AFM (Atomic Force Microscope), but the period of the unevenness is as short as several tens of nm. The above micro-roughness governs the roughness of the interface between the oxide film and silicon layer formed on the surface, but in particular, the roughness of the interface greatly affects the electrical characteristics such as the oxide film breakdown voltage. Therefore, in order to obtain a uniform boundary surface, it is necessary to improve the micro roughness. As for the polishing liquid (slurry) used in the final polishing, a polishing liquid made of alkaline colloidal silica is added with both monohydric alcohol having 3 to 5 C atoms and polyvinyl alcohol (PVA), and HLB is added to alkaline colloidal silica. 13 or more and less than 20 nonionic surfactant added, alkaline colloidal silica added sulfonate type, sulfate type, carboxylate type or phosphate type anionic surfactant Is used (see Patent Document 1).

  The fine particles adhering to the surface of the silicon wafer may cause disconnection or short-circuiting of the device wiring, or cause abnormal diffusion of dopants or abnormal oxide film thickness in the diffusion and oxidation processes. On the other hand, metal oxides such as Fe and Cu diffuse into the silicon oxide film and silicon wafer in the heat treatment process, and lower the dielectric breakdown voltage and the lifetime of carriers.

  Therefore, after the finish polishing, a cleaning operation for removing the polishing agent, metal, and the like on the wafer surface is performed. The cleaning operation is performed using the single wafer cleaning apparatus as described above. (A) Ozone treatment is performed on the wafer surface for 10 seconds to form an oxide film having a thickness of about 1 nm for the purpose of removing metal adhering to the wafer surface. Then, (b) hydrofluoric acid treatment for reduction (removal) of the oxide film is performed for 10 seconds, (c) (a) and (b) are repeated several times, and (d) spin drying is performed. In (b), the oxide film is not completely removed in order to prevent the active Si surface from adsorbing dust.

JP-A-11-140427 JP 2008-218545 A

  By the way, even in finish polishing, a work-affected layer may occur due to foreign matters mixed in the polishing liquid. In addition, the work-affected layers to be formed are scattered on the wafer surface, the depth thereof is about 1 nm, and a layer having an order completely different from that in the case of rough polishing is formed.

  When the above-described finishing cleaning operation is performed with the work-affected layer formed at the time of finish polishing, the work-affected layer is oxidized by the oxidation process, but the property as a single crystal is lost. And an etching rate difference occurs in a region other than the work-affected layer, and a step is formed in a state where the work-affected layer is one step higher than the oxide film. Then, as described in (c), since the oxide film is formed and the process of etching the oxide film is repeated, this step is enlarged, the particle level is lowered, and the above-mentioned electrical characteristics such as the oxide film breakdown voltage are obtained. It will have a big impact.

  Therefore, the present invention pays attention to the above problems, and an object of the present invention is to provide a silicon wafer cleaning method capable of removing a work-affected layer formed during finish polishing while removing surface deposits.

  In order to achieve the above object, a silicon wafer cleaning method according to the present invention firstly forms an oxide film on the surface of the silicon wafer after finish polishing performed through rough polishing of the silicon wafer, A method of cleaning a silicon wafer in which impurities on the surface are removed by removing an oxide film, wherein the oxide film is removed when a work-affected layer formed on the polished surface of the silicon wafer during the final polishing is removed by an oxide film It is characterized by being carried out until it is removed by the process.

Second, the oxide film is formed by supplying ozone to the surface.
Thirdly, in the oxide film removing step, the oxide film is removed using dilute hydrofluoric acid.

  According to the silicon wafer cleaning method of the present invention, since the oxide film removing step is performed once, it is possible to remove deposits attached to the silicon wafer surface. Although the oxide film in the region other than the work-affected layer has a higher etching rate than the oxide film in the work-affected layer, the oxide film in the region other than the work-affected layer is removed and Si is exposed at the stage where Si is exposed. While the etching is finished, the oxide film removing process itself is performed until the work-affected layer is removed. Therefore, since the difference between the thickness of the oxide film in the region other than the work-affected layer and the oxide layer in the region other than the work-affected layer is only formed as a step, the region in which the oxide film in the region other than the work-affected layer is formed, The level difference with the region to be formed can be reduced, and the increase in level difference due to the repeated oxide film removal process can be prevented. Therefore, the particle level is improved, and the processing alteration is applied to the part where the processing alteration layer is removed. Since an oxide film having the same quality as that of the oxide film formed in the region having no layer can be formed, the electrical characteristics of the silicon wafer can be improved. Further, by using ozone as an oxidizing agent for forming an oxide film and dilute hydrofluoric acid as a reducing agent for removing the oxide film, the silicon wafer can be cleaned without cost.

It is a schematic diagram of the washing | cleaning apparatus which concerns on this embodiment. It is a schematic diagram of the washing | cleaning apparatus which concerns on this embodiment. It is a figure which shows the etching rate of the oxide film of a single crystal silicon, and the oxidized work-affected layer. It is a schematic diagram which shows the cleaning mechanism of the silicon wafer which concerns on a prior art. It is a schematic diagram which shows the cleaning mechanism of the silicon wafer which concerns on this embodiment. It is a graph which shows the size change of LPD of the silicon wafer surface. It is a graph which shows the LPD number after the washing | cleaning process of the silicon wafer surface.

  Hereinafter, the present invention will be described in detail with reference to embodiments shown in the drawings. However, the components, types, combinations, shapes, relative arrangements, and the like described in this embodiment are merely illustrative examples and not intended to limit the scope of the present invention only unless otherwise specified. .

  In the silicon wafer cleaning method according to the present embodiment, after finish polishing performed through rough polishing of the silicon wafer, an oxide film is formed on the surface of the silicon wafer, and the oxide film is removed to remove impurities on the surface. In this method, the oxide film is removed until the damaged layer formed on the polished surface of the silicon wafer is removed by the oxide film removing step during the final polishing. FIG. 1 and FIG. 2 show a single wafer cleaning apparatus that embodies this method. FIG. 1A is a schematic view of the entire cleaning apparatus, and FIG. 1B is a plan view of a wafer chuck constituting the cleaning apparatus. FIG. 2A shows the arrangement of the cleaning apparatus when supplying the etching solution, and FIG. 2B shows the arrangement of the cleaning apparatus when supplying the oxidizing agent.

  The cleaning apparatus 10 is accommodated in a chamber (not shown), and a wafer chuck 12 that holds a single thin disc-shaped silicon wafer 18 and holds it horizontally, and the silicon wafer 18 rotates in a horizontal plane around its vertical center line. A first nozzle 14 for supplying an etching solution 14a such as hydrofluoric acid to the upper surface of the silicon wafer 18 held by the wafer chuck 12, and a second nozzle for supplying an oxidizing agent 16a such as ozone. 16. The silicon wafer 18 is obtained by slicing a silicon single crystal ingot, and then subjected to rough polishing for the purpose of removing the lapping surface and finish polishing for preventing light scattering from the surface called haze. It is.

  The wafer chuck 12 includes a shaft portion 12a provided extending in the vertical direction, a large-diameter wafer receiving portion 12b integrally formed with the shaft portion 12a on the upper surface of the shaft portion 12a, a shaft portion 12a, and a wafer receiver. A through hole 12c formed in the center of the portion 12b extending vertically from the lower surface of the shaft portion to the center of the wafer receiving portion, and one end communicating with the upper end of the through hole 12c, and the wafer receiving portion 12b around the through hole 12c. A plurality of communication holes 12f radially extending outward in the radial direction and closed at the other end, a plurality of ring grooves 12d formed concentrically on the upper surface of the wafer receiving portion 12b, a communication hole 12f and a ring groove 12d. A plurality of small holes 12e connected in communication and a vacuum pump (not shown) connected to the lower end of the through hole 12c are provided.

  A silicon wafer 18 is placed concentrically with the wafer receiving portion 12b on the upper surface of the wafer receiving portion 12b, and a vacuum pump (not shown) is driven to pass through the through hole 12c, the communication hole 12f, the small hole 12e, and the ring groove 12d. When the pressure becomes negative, the lower surface of the silicon wafer 18 is attracted to the wafer receiving portion 12b and the silicon wafer 18 is held horizontally. The rotating means (not shown) has a drive motor (not shown) that rotates the shaft portion 12a. In this embodiment, it shall rotate in the direction (clockwise) of the arrow shown to Fig.1 (a) and FIG.1 (b). By rotating the shaft portion 12a by a drive motor (not shown), the silicon wafer 18 held by the wafer receiving portion 12b is configured to rotate together with the shaft portion 12a and the wafer receiving portion 12b.

  The first nozzle 14 and the second nozzle 16 can supply and stop supply of a predetermined amount of etching solution and oxidizing agent (ozone aqueous solution) per unit time by ON / OFF control of a pump (not shown), respectively. (A) As shown in FIG. 2B, the first nozzle 14 and the second nozzle 16 alternately move the supply ports 14a and 16a on the rotation axis of the silicon wafer 18 on the silicon wafer 18, An etching solution and an oxidizing agent can be supplied while rotating the silicon wafer 18 in the direction of the arrow (clockwise) in the figure. As a result, the etching solution and the oxidizing agent can be uniformly supplied onto the silicon wafer 18.

  The cleaning process includes (a) forming an oxide film by performing ozone treatment on the silicon wafer surface for the purpose of removing metal adhering to the silicon wafer surface, and (b) reducing the oxide film and removing the work-affected layer. Hydrofluoric acid treatment (oxide film removal step) is performed for 30 to 60 seconds, (c) ozone treatment is performed on the wafer surface for 10 seconds to form an oxide film for surface protection, and (d) spin drying is performed. .

Here, in (a), the ozone treatment time is about 10 seconds. This is because the thickness of the oxide film formed by one ozone treatment is about 1 nm to 1.5 nm, and when the oxide film is formed to this extent, the growth of the oxide film is saturated. In (b), the hydrofluoric acid treatment is preferably performed using dilute hydrofluoric acid having a concentration of 1 to 3%. If the concentration is 1% or less, the etching cannot be sufficiently performed. Conversely, if the concentration exceeds 3%, haze deterioration is caused. Note that it is also preferable to use an SC1 (NH ₃ / H ₂ O ₂ ) cleaning solution in addition to the dilute hydrofluoric acid described above. (C) In (d), it is the same as that of the above-mentioned prior art.

  The inventor of the present application examined the etching rate of etching with hydrofluoric acid, such as an oxide film of single crystal silicon and an oxidized work-affected layer as shown in FIG. As described above, the work-affected layer is formed by foreign matter in the slurry in the final polishing step of the single crystal silicon surface.

  As shown in FIG. 3 (a), the etching rate of single crystal silicon is almost zero (that is, it is hardly etched), and the etching rate of the work-affected layer is considerably low. It was found that the etching rate of the layer was considerably slower than that of the single crystal silicon oxide film. Therefore, as shown in FIG. 3B, when the oxide film of single crystal silicon and the oxidized work-affected layer have the same thickness, the oxidized work-affected layer has the same structure as the oxide film of single crystal silicon. In comparison, it takes a considerable amount of time to disappear by etching.

  Based on the above knowledge, the cleaning mechanism of the silicon wafer surface is shown in FIGS. FIG. 4 shows a cleaning mechanism according to the prior art, and FIG. 5 shows a cleaning mechanism according to this embodiment.

  First, as described in the prior art, the silicon wafers 18 and 19 used in any of the cleaning processes have the work-affected layers 20 and 21 formed by foreign matters in the slurry in the final polishing step. Ozone is supplied to the surfaces of the silicon wafers 18 and 19 on which the work-affected layers 20 and 21 are formed for 10 seconds, respectively, on the surface of the single crystal silicon, and in regions other than the work-affected layers 20 and 21, oxide films 22 and 23 is formed. Up to this stage, the prior art and this embodiment are the same (see FIG. 4A, FIG. 4B, FIG. 5A, and FIG. 5B). At this time, the work-affected layers 20 and 21 have structures different from those of the silicon wafers 18 and 19, so that even if oxidized by ozone like the surfaces of the silicon wafers 18 and 19, the structures are different from those of the oxide films 22 and 23. It is thought that it becomes.

  Then, the oxide films 22 and 23 are removed by etching. At this time, as shown in FIG. 4C, in the conventional technique, most of the work-affected layer 21 remains in one oxide film removal step, and a part of the oxide film 23 in a region other than the work-affected layer is partially formed. Since the oxide film 23 is removed in the remaining state, a step is formed between the work-affected layer 21 and the oxide film 23 in a region other than the work-affected layer. And as shown in FIG.4 (d), whenever it repeats the process of supplying ozone again and growing the oxide film 23 of area | regions other than a work-affected layer to a predetermined thickness, the process-affected layer 21 and the parts other than the work-affected layer are repeated. The step between the region and the oxide film 23 is enlarged, and as shown in FIG. 5E, the step increases until the work-affected layer 21 disappears due to repeated etching.

  On the other hand, as shown in FIG. 5C, in the present embodiment, the oxide film 22 in the region other than the formed work-affected layer is completely removed by etching, and the surface of the silicon wafer 18 is exposed. Since the silicon wafer 18 is not etched by dilute hydrofluoric acid, the region where the oxide film 22 is formed other than the work-affected layer is not further etched. And as shown in FIG.5 (d), since etching is performed for about 30 second-60 second, it will be continued until the process-affected layer 20 is removed. Therefore, the difference between the thickness of the oxide film 22 in the region other than the work-affected layer and the thickness of the work-affected layer 20 indicates that the position of the silicon wafer 18 where the work-affected layer 20 was present and the position where the work-affected layer 20 was originally absent. A step is formed by this oxide film removal step. And since this process is performed only once, the step is not further expanded. Therefore, as shown in FIGS. 4 (f) and 5 (e), even after the oxide films 24 and 25 for surface protection are formed, the step on the surface becomes smaller than that in the prior art, and silicon The silicon wafer 18 maintains a flatness as compared with the wafer 19.

  FIG. 6 shows a comparison between scattering defects (Light Point Defect, hereinafter referred to as LPD) before and after cleaning on the surface of a silicon wafer and the above-mentioned conventional LPD. FIG. 6A shows a change in size of the LPD according to the conventional technique, and FIG. 6B shows a change in size of the LPD according to the present embodiment. In the LPD measurement, the surface of the silicon wafer is irradiated with laser light, and when particles or COP (Crystal Original Pit) is present, the reflected light is scattered. The light scatterer is detected, and the size of the light scatterer is measured from the intensity of the scattered light. In this LPD measurement, the particle size of the light scatterer (35 nm in FIG. 6) is set, and the number of scattered light having an intensity corresponding to a size equal to or larger than the set particle size is counted as the number of light scatterers. To do. Although it is difficult to remove the COP on the silicon wafer by post processing, the particles can be removed by post processing.

  In FIG. 6 (a) and FIG. 6 (b), a 45 degree line is drawn, but the plot below this line means that the LPD size has decreased after washing, and above this line. One plot means that the LPD size has increased after washing. As can be seen from FIG. 6A, when the cleaning is performed by the method according to the prior art, the plot is entirely located above the 45 degree line, and the LPD size is increased after the cleaning, and the flatness of the wafer surface is increased. It is falling. On the other hand, as shown in FIG. 6B, when the cleaning is performed by the method according to the present embodiment, the plot is positioned almost on the line of 45 degrees and the LPD size is maintained, so that the flatness of the wafer surface is maintained. You can see that. These causes are thought to be due to the mechanism described above.

  FIG. 7 counts the number of defects (LPD number) in a plurality of silicon wafers (particle diameter is 35 nm or more), and shows the maximum value and the minimum value for each of the present embodiment and the prior art. As shown in FIG. 7, in the case of conventional cleaning, the number of LPDs was 410, and the standard deviation (variation in particle diameter) was large. On the other hand, when the cleaning method according to the present embodiment is used, it can be seen that the number of LPDs is considerably improved to 169, and the variation is also reduced.

  As described above, according to the method for cleaning a silicon wafer according to the present embodiment, the removal process of the oxide film 22 is performed once, so that the adhering matter can be removed by adhering to the surface of the silicon wafer 18. Although the oxide film 22 has an etching rate higher than that of the work-affected layer 20, the etching is completed when the oxide film 22 is removed and Si is exposed. On the other hand, the oxide film removal process itself is performed until the work-affected layer 20 is removed. Will be done. Therefore, since the difference in thickness between the oxide film 22 and the work-affected layer 20 is only formed as a step, the step between the area where the oxide film 22 is formed and the area where the work-affected layer 20 is formed is reduced. Since the increase in the level difference due to the repetition of the oxide film removal process can be prevented, the particle level is improved, and the oxidation formed in the region where the work-affected layer 20 is not present in the portion where the work-affected layer 20 is removed. Since the oxide film 24 having the same quality as the film 22 can be formed, the electrical characteristics of the silicon wafer 18 can be improved. Further, by using ozone as an oxidizing agent for forming the oxide films 22 and 24 and using dilute hydrofluoric acid as a reducing agent for removing the oxide film, the silicon wafer can be cleaned without cost.

  It can be used as a method for cleaning a silicon wafer that can remove deposits on the surface of the silicon wafer while maintaining low cost.

DESCRIPTION OF SYMBOLS 10 ......... Washing apparatus, 12 ......... Wafer chuck, 14 ......... 1st nozzle, 16 ......... 2nd nozzle, 18 ......... Silicon wafer, 19 ......... Silicon wafer, 20 ......... Process alteration Layer 21... Processed layer 22... Oxide film 23... Oxide film 24... Oxide film 25.

Claims (3)

A method of cleaning a silicon wafer, wherein after finishing polishing performed through rough polishing of the silicon wafer, an oxide film is formed on the surface of the silicon wafer, and impurities on the surface are removed by removing the oxide film,
The method of cleaning a silicon wafer, wherein the removal of the oxide film is performed until the work-affected layer formed on the polished surface of the silicon wafer at the time of the final polishing is removed by the oxide film removal step.   2. The silicon wafer cleaning method according to claim 1, wherein the oxide film is formed by supplying ozone to the surface.   3. The silicon wafer cleaning method according to claim 1, wherein the oxide film is removed using dilute hydrofluoric acid in the oxide film removing step.

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