JP2005112917A - Slurry for cutting silicon ingot and method for cutting silicon ingot therewith - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a slurry which is used for cutting silicon ingots and can reduce cutting resistance on the cutting of the silicon ingots to efficiently obtain wafers having high qualities, and to provide a method for cutting a silicon ingot therewith. <P>SOLUTION: This slurry which is used for cutting the silicon ingots and contains abrasive grains is characterized by furthermore containing nitric acid and hydrofluoric acid in a mass ratio of the nitric acid to the hydrofluoric acid of 0.5 to 25 : 1. The slurry may further contain acetic acid. The slurry is preferably used at 40 to 110°C. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  TECHNICAL FIELD The present invention relates to a slurry for cutting a silicon ingot used when cutting a single crystal, polycrystalline or amorphous silicon ingot for manufacturing semiconductor and solar cell wafers and a method for cutting a silicon ingot using the same. .

  Conventionally, a wire saw that can cut a silicon ingot with a small cutting allowance and a uniform thickness or can cut a large number of wafers at a time has been used. Cutting a silicon ingot using this wire saw is performed by introducing a slurry for cutting containing abrasive grains into the cutting interface while pressing the silicon ingot against a traveling wire. In the cutting of silicon ingots using such wires, it is required to maintain high wafer quality, increase the cutting speed, reduce the cutting allowance and the cutting pitch, and reduce the wafer processing cost. .

In order to reduce the cutting allowance, the wire diameter may be reduced. However, since the breaking strength of the wire is reduced accordingly, it is necessary to reduce the tension applied to the wire. When the tension is reduced, cutting resistance is generated due to poor dispersion of abrasive grains at the cutting interface, and the displacement (deflection) of the wire increases, so the cutting speed decreases, wafer warpage, thickness unevenness, and minute unevenness. (Saw mark) occurs, and the quality of the wafer decreases. Also, if the ingot feed rate is increased forcibly, a large tension is generated in the wire, resulting in breakage of the wire. On the other hand, when the wafer is thinned by reducing the cutting pitch, a minute crack is generated in the wafer due to the cutting resistance, or the wafer is dropped from the holding member during the cutting process.
Accordingly, it is necessary to reduce the cutting resistance in order to maintain high wafer quality, increase the cutting speed, and reduce the cutting allowance and cutting pitch of the silicon ingot.

  Then, the method of cut | disconnecting a silicon ingot using the fixed abrasive wire and the slurry containing a free abrasive grain or the KOH alkaline solution whose density | concentration is 2% or less is proposed (for example, refer patent document 1).

JP 2000-343525 A

However, in the conventional cutting method using a fixed abrasive wire and a slurry containing free abrasive grains, the fixed abrasive wire is simply used as a medium for transporting the free abrasive grains, and the silicon ingot is cut by the lapping action of the free abrasive grains. In appearance, the cutting resistance due to the fixed abrasive wire is merely lowered. Furthermore, there is a problem that it becomes difficult to discharge cutting waste and free abrasive grains as compared with the case of using a bare wire. In addition, fixed abrasive wires are very expensive and their use is significantly less economical.
In addition, in the conventional cutting method using a fixed abrasive wire and an alkaline solution, the cutting resistance cannot be lowered sufficiently, and there is a problem that wafer warpage, thickness unevenness, minute unevenness, and micro cracks are likely to occur. .

  Therefore, the present invention is intended to solve the above-described problems, and a silicon ingot cutting slurry capable of efficiently obtaining a high-quality wafer by reducing cutting resistance during silicon ingot cutting processing and It aims at providing the cutting method of a silicon ingot using it.

  The present invention is the slurry for cutting a silicon ingot containing abrasive grains, wherein the slurry further contains nitric acid and hydrofluoric acid, and the mass ratio of the nitric acid to the hydrofluoric acid is 0.5 to 25. A slurry for cutting a silicon ingot characterized by

  Further, the present invention provides a method for cutting a silicon ingot using a silicon ingot cutting slurry containing abrasive grains, wherein the slurry further contains nitric acid and hydrofluoric acid, and the mass of the nitric acid relative to the hydrofluoric acid The silicon ingot cutting method is characterized in that the ratio is 0.5 to 25, and the method is used at 40 to 110 ° C.

  According to the present invention, by setting the mass ratio of nitric acid to hydrofluoric acid to be 0.5 to 25, it is possible to reduce the cutting resistance at the time of silicon ingot cutting processing, so that a high-quality wafer is efficiently obtained. be able to.

The slurry for cutting a silicon ingot according to the present invention contains hydrofluoric acid and nitric acid. And the mass ratio of nitric acid to hydrofluoric acid is 0.5 to 25, preferably 0.5 to 10.
When the slurry for cutting a silicon ingot according to the present invention is introduced into the cutting interface, it is lapped by the abrasive grains and, as a first step, silicon is oxidized by nitric acid as shown in the following formula (1).
Si + 2HNO ₃ → SiO ₂ + 2HNO ₂ (1)
As the second stage, as shown in the following formula (2), silicon oxide reacts with hydrofluoric acid to form hexafluorosilicic acid and dissolve in the slurry.
SiO ₂ + 6HF → H ₂ SiF ₆ + 2H ₂ O (2)

  For this reason, when the mass ratio of nitric acid to hydrofluoric acid is less than 0.5, the first stage reaction is not performed rapidly, and the amount of silicon oxide that hydrofluoric acid can act on is not sufficient. The reaction in the first stage is rate limiting, and the chemical action of the slurry cannot be fully utilized. On the other hand, when the mass ratio of nitric acid to hydrofluoric acid exceeds 25, the second stage reaction is not performed rapidly, and silicon oxide remains, making it impossible to make full use of the chemical action of the slurry. The reason is considered that silicon oxide has almost the same hardness as the silicon base material of the ingot, so that hexafluorosilicic acid is not generated by the second stage reaction.

In addition, when a polycrystalline silicon ingot is cut using a conventional alkaline cutting slurry, the etching rate has anisotropy due to crystal orientation (for example, the etching rate in the [111] direction is higher than that in the [100] direction). Therefore, non-uniform etching proceeds at the cutting interface, resulting in minute irregularities on the wafer surface after cutting. Such minute unevenness on the surface is not a big problem in the current solar cell wafer, but when the wafer thickness is reduced in the future and the wafer thickness becomes 200 μm or less, the unevenness on the cut surface becomes the wafer thickness. May not be negligible and may cause problems such as cracks and increased cracks.
On the other hand, when the polycrystalline silicon ingot is cut using the silicon ingot cutting slurry according to the present invention, uniform etching proceeds by the reaction of the silicon ingot and the first stage and the second stage. The wafer surface can be made extremely smooth. Furthermore, the removal process of the surface damage layer in a later process can be facilitated by making the wafer surface smoother.
As a method for evaluating the surface of the etched wafer as described above, for example, a polycrystalline silicon ingot sample is immersed in a slurry for cutting silicon ingot for a predetermined time, washed with water, dried, and then ingot with a microscope. A method of observing the surface of the film.

  In this invention, although content of nitric acid is not specifically limited, It is preferable that it is 15 mass%-50 mass% with respect to the mass of the whole liquid component in the slurry for silicon ingot cutting. When the content of nitric acid is too small, the production of silicon oxide is not promoted. When the content is too large, the production of silicon oxide is not promoted as much as it is added, and the cost is wasted. The content of hydrofluoric acid is not particularly limited, but is preferably 2% by mass to 30% by mass with respect to the mass of the entire liquid component in the slurry for silicon ingot cutting. When the content of hydrofluoric acid is too small, the production of hexafluorosilicic acid is not promoted, and when it is too much, the production of hexafluorosilicic acid is not promoted as much as it is added, which is wasteful in cost. It is not preferable.

Moreover, the slurry for cutting a silicon ingot according to the present invention may further contain acetic acid. Since acetic acid acts as a pH buffer, fluctuations in the pH of the slurry during the cutting process can be suppressed, and the wafer processing quality can be further improved. When acetic acid is contained, the mass ratio of acetic acid to hydrofluoric acid is 0.05 to 10, preferably 0.05 to 5. When the mass ratio of acetic acid to hydrofluoric acid is less than 0.05, no significant buffering effect is observed, and when the mass ratio of acetic acid to hydrofluoric acid exceeds 10, the chemical action of the slurry Becomes dull and the cutting speed is lowered, which is not preferable.
In this invention, although content of acetic acid is not specifically limited, It is preferable that it is 1.5 mass%-40 mass% with respect to the mass of the whole liquid component in the slurry for silicon ingot cutting.

  Moreover, the silicon ingot cutting slurry of the present invention is used at 40 ° C to 110 ° C. If the temperature at which the slurry is used is too low, the reaction is not activated and the cutting resistance is not sufficiently reduced. If it is too high, the hydrofluoric acid evaporates and the working environment is deteriorated, This is not preferable because the amount of hydrofluoric acid is insufficient and silicon oxide tends to remain.

In the present invention, the abrasive grains may be anything that is generally used as an abrasive, and examples thereof include silicon carbide, cerium oxide, diamond, boron nitride, aluminum oxide, zirconium oxide, and silicon dioxide. These can be used alone or in combination of two or more. Compounds that can be used for such abrasive grains are commercially available. Specifically, as silicon carbide, trade names GC (Green Silicon Carbide) and C (Black Silicon Carbide) (manufactured by Fujimi Incorporated) ) And aluminum oxide include trade names FO (Fujimi Optical Emery), A (Regular Fused Aluminum), WA (White Fused Aluminum), and PWA (Platelet Calcined Aluminum) (manufactured by Fujimi Incorporated). .
Although the average particle diameter of an abrasive grain is not specifically limited, Preferably it is 1 micrometer-60 micrometers, More preferably, it is 5 micrometers-20 micrometers. If the average particle size of the abrasive grains is less than 1 μm, the cutting speed is remarkably slow, which is not practical. If the average particle size of the abrasive grains exceeds 60 μm, the surface roughness of the wafer surface after cutting increases. This is not preferable because the wafer quality may deteriorate.
Further, the content of the abrasive grains is not particularly limited, but is preferably 30% by mass to 70% by mass with respect to the mass of the entire silicon ingot cutting slurry. When the content of abrasive grains is less than 30% by mass, the cutting speed becomes slow and the practicality may become poor. When the content of abrasive grains exceeds 70% by mass, the viscosity of the slurry becomes excessive. Thus, it may be difficult to introduce the slurry to the cutting interface.

In the present invention, water, a known coolant, and a mixture thereof can be used as the liquid component of the slurry. The water used here is preferably one having a small impurity content, but is not limited thereto. Specific examples include pure water, ultrapure water, city water, and industrial water. Although content of water is not specifically limited, Preferably it is 10 mass%-40 mass% with respect to the mass of the whole slurry for silicon ingot cutting.
Moreover, as a coolant, what is generally used as a cutting auxiliary liquid mixture containing a moisturizer, a lubricant, a rust inhibitor, a viscosity modifier, for example, polyethylene glycol, benzotriazole, oleic acid and the like may be used. Such a coolant is commercially available, and specific examples include trade names such as Multi-Licanol (manufactured by Rika Trading Co., Ltd.), Luna coolant (manufactured by Ochi Chemical Industry Co., Ltd.) and the like. Although content of a coolant is not specifically limited, Preferably it is 10 mass%-40 mass% with respect to the mass of the whole slurry for silicon ingot cutting.

  Various known additives may be added to the slurry for cutting a silicon ingot according to the present invention in accordance with the purpose of maintaining product quality and stabilizing the performance, the type of silicon ingot, processing conditions, and the like. Examples of such additives include humectants, lubricants, rust inhibitors, chelating agents such as ethylenediaminetetraacetic acid sodium salt, and abrasive dispersion aids such as bentonite.

  The slurry for cutting a silicon ingot of the present invention can be prepared by mixing each of the above components at a desired ratio. The method of mixing each component is arbitrary, For example, it can carry out by stirring with a wing-type stirrer. The order of mixing the components is also arbitrary. Further, for the purpose of purification, the prepared silicon ingot cutting slurry may be subjected to further treatment, for example, filtration treatment, ion exchange treatment and the like.

  In the method for cutting a silicon ingot according to the present invention, a cutting device is used. As the cutting device used here, any device can be used, and examples thereof include a band saw, a wire saw, a multi-band saw, a multi-wire saw, an outer peripheral blade cutting device, and an inner peripheral blade cutting device. Among these, a wire saw and a multi-wire saw are preferable when cutting an ingot having a large diameter, for example, 6 inches or more. This is because the ingot can be cut with a small cutting allowance and a uniform thickness as compared with other cutting apparatuses, and a large number of wafers can be cut at a time.

  Here, the silicon ingot cutting method according to the present invention will be described taking as an example a case where a multi-wire saw is used as a cutting device. As shown in FIG. 1, a multi-wire saw 10 supplies an ingot feeding mechanism 1 for fixing and pushing down a silicon ingot 2, a wire feeding mechanism for feeding a bare wire 3, and a silicon ingot cutting slurry. A slurry agitation / supply tank 8, a slurry application head 9 for applying a slurry for silicon ingot cutting to the bare wire 3, a wire delivery mechanism 5 for sending out the bare wire 3, and a bare wire 3. A wire winding mechanism 6 for winding and a tension control roller 7 for keeping the tension of the bare wire 3 constant are provided. The wire feeding mechanism includes two rotating rollers 4 that rotate synchronously, and a groove that guides the wire 3 is formed on the outer periphery of the rotating roller 4. Moreover, as a bare wire used here, the thing made from a metal and resin are mentioned, From a viewpoint of cutting efficiency, a metallic thing is more preferable.

  When the silicon ingot is cut with such a multi-wire saw, the silicon ingot 2 fixed to the ingot feeding mechanism 1 is brought into contact with the bare wire 3. The bare wire 3 is sent out from the wire sending mechanism 5 synchronized with the wire feeding mechanism, and is taken up by the wire winding mechanism 6. Also, the silicon ingot cutting slurry supplied from the slurry agitation / supply tank 8 is applied onto the wire 3 via the slurry application head 9. As shown in FIG. 2, when the silicon ingot cutting slurry is conveyed to the silicon ingot cutting portion by the traveling bare wire 3, the silicon ingot 2 is scraped and cut by a lapping action.

Next, an evaluation method in cutting the silicon ingot 2 using the multi-wire saw 10 will be described with reference to FIG. FIG. 3 is a diagram showing the relationship of each parameter in the cutting of the silicon ingot 2 using the multi-wire saw 10. FIG. 3 (a) is a schematic diagram showing a method for cutting the silicon ingot 2, and FIG. It is AA arrow sectional drawing of (a) of FIG. In FIG. 3, the feeding speed of the silicon ingot 2 is V, the feeding speed of the wire 3 is U, the cutting resistance is P, the displacement of the wire 3 in the direction perpendicular to the cutting direction is δ _x , and the displacement of the wire 3 in the cutting direction is δ _{When y} and the tension of the wire 3 are T, the following empirical formula is generally known.
P∝V / U (3)
δ _x ∝P / T (4)
δ _y ∝P / T (5)
Based on these equations, in cutting the silicon ingot 2 using the multi-wire saw 10, by measuring the displacement δ _x of the wire 3 in the direction perpendicular to the cutting direction and the displacement δ _y (deflection) of the wire 3 in the cutting direction The cutting speed and cutting resistance can be evaluated.

This will be described in detail. First, slurry containing abrasive grains 22 is introduced to the cutting interface of the silicon ingot 2 by the wire 3. Then, due to the uneven distribution of the abrasive grains 22 in the slurry, a displacement δ _x of the wire 3 in a direction perpendicular to the cutting direction and a displacement δ _y of the wire 3 in the cutting direction are generated. Since [delta] _x is the displacement in the direction perpendicular of the wire 3 in the cutting direction, if this value increases, warp of the wafer obtained by cutting a silicon ingot 2, the thickness unevenness, minute unevenness (saw marks) is generated The quality of the wafer is degraded. Therefore, the smaller δ _x is, the better. Further, if δ _y is increased, the wire 3 at the cutting interface is delayed in the cutting direction, and a desired cutting speed cannot be obtained. Therefore, δ _y is preferably as small as possible. Assuming that the tension T of the wire 3 is constant, the cutting resistance P may be reduced in order to reduce δ _x and δ _y from the equations (4) and (5). As can be seen from Equation (3), in order to reduce the cutting resistance P, the feed speed V of the silicon ingot 2 can be reduced or the feed speed U of the wire 3 can be increased. However, since the feed speed V of the silicon ingot 2 is proportional to the cutting speed of the silicon ingot 2, it cannot be made too small. When the feeding speed U of the wire 3 is increased, it is necessary to increase the wire length, and the wire cost increases. Therefore, U cannot be made too large. Since the parameters are closely related to each other as described above, the parameters are set so as to be balanced in consideration of the cutting efficiency and quality of the wafer, and the cutting speed and cutting resistance are evaluated.
In addition, although each parameter was demonstrated taking the case of using a multi wire saw as an example, it is the same also when using a wire saw.

Another method for evaluating the cutting resistance is a method using a polishing apparatus as shown in FIG.
The polishing device 21 measures a temperature of the beaker 12 for storing the silicon ingot cutting slurry 11, a heater / stirrer 14 for heating and stirring the slurry 11 by the magnet rotor 13, and the slurry 11. A thermometer 15, a rotary table 17 with a polishing pad 16 attached thereto, a liquid feed pump 19 for supplying the slurry 11 onto the polishing pad 16 via a liquid feed tube 18, and the silicon ingot 2. And a polishing head 20 for pressing against the polishing pad 16.

When the silicon ingot 2 is polished in such a polishing apparatus 21, the silicon ingot cutting slurry 11 is heated while being stirred by the heater / stirrer 14. The rotary table 17 is rotated at a predetermined number of revolutions, and the silicon ingot cutting slurry 11 is applied onto the polishing pad 16 by a liquid feed pump 19 and the silicon ingot 2 fixed to the tip of the polishing head 20 is pressed at a predetermined pressure. Press against the polishing pad 16. Then, the polishing rate is obtained from the mass change of the silicon ingot 2 after a certain time has elapsed, and the magnitude of the polishing resistance (this corresponds to the cutting resistance when a wire saw is used) can be known. As a result of the preliminary experiment, there is a correlation of Ew / Ep = 3/5 between the polishing speed Ep measured by the method shown in FIG. 4 and the cutting speed Ew in the multi-wire saw 10 (see FIG. 1). I understood.
Therefore, the cutting resistance when using a wire saw can be evaluated by evaluating the polishing rate in such a polishing apparatus.

EXAMPLES Hereinafter, although an Example demonstrates this invention in more detail, this invention is not limited to these.
45 parts by mass of a 70% by mass nitric acid aqueous solution, 5 parts by mass of a 50% by mass hydrofluoric acid aqueous solution, and 50 parts by mass of a coolant (Luna Coolant # 691 manufactured by Daichi Chemical Industry Co., Ltd.) were mixed. At this time, the mass ratio of nitric acid to hydrofluoric acid is (45 × 0.7) ÷ (5 × 0.5) = 12.6. To this mixed solution, 100 parts by mass of SiC abrasive grains (manufactured by Fujimi Incorporated, GC # 1200, average particle diameter of about 10 μm) was further added and stirred to prepare slurry A for silicon ingot cutting.

  A silicon ingot cutting slurry B was prepared in the same manner as in Example 1 except that 25 parts by mass of acetic acid was further added. At this time, the mass ratio of acetic acid to hydrofluoric acid is 25 ÷ (5 × 0.5) = 10.

  4 parts by mass of sodium hydroxide was dissolved in 46 parts by mass of water to form a basic aqueous solution, and this aqueous solution and 50 parts by mass of coolant (Luna Coolant # 691 manufactured by Daichi Chemical Industry Co., Ltd.) were mixed. To this mixed solution, 100 parts by mass of SiC abrasive grains (manufactured by Fujimi Incorporated, GC # 1200, average particle size of about 10 μm) was further added and stirred to prepare slurry C for silicon ingot cutting.

  50 parts by mass of water and 50 parts by mass of coolant (Luna Coolant # 691 manufactured by Ochi Chemical Industry Co., Ltd.) were mixed, and 100 parts by mass of SiC abrasive grains (manufactured by Fujimi Incorporated, GC # 1200, Further, an average particle diameter of about 10 μm) was added and stirred to prepare a slurry D for cutting silicon ingot.

Using each of the obtained silicon ingot cutting slurries A to D, a polycrystalline silicon ingot sample (3 mm × 3 mm × thickness 1 mm) was polished under the following polishing conditions. The amount of polishing was determined from the change in mass of the sample before and after polishing, and the polishing rate was determined by dividing by the polishing time.
<Polishing conditions>
Polishing equipment: Ecomet polishing machine type 3 (Buhler)
Polishing pad: 200 mm in diameter (Buhler, polishing buff, ultrapad 8 inch polishing)
Sample position: 65 mm from the center of the pad
Polishing table rotation speed: 200 rpm
Polishing time: 5 minutes Slurry supply amount: 65 cc / minute Slurry supply position: 65 mm from the center of the pad, 30 ° rotating slurry temperature of the sample: 25 ° C., 40 ° C., 60 ° C., 80 ° C., 100 ° C., 110 ° C. and 120 ° ℃
Sample pressing: 10N

Next, after immersing a new polycrystalline silicon ingot sample in each of the silicon ingot cutting slurries A to D for 60 minutes, washing with water and drying, the etched surface of the ingot was observed with a microscope. Evaluation based on the criteria. The results are shown in Table 1.
<Evaluation criteria>
◎: Ingot etching surface has very little unevenness ○: Ingot etching surface has little unevenness Δ: Ingot etching surface has many unevenness ×: Ingot etching surface has many unevenness

As apparent from Table 1, the slurry A for silicon ingot cutting had a polishing rate as high as 21 μm / min, especially at a slurry temperature of 40 ° C., and there were few irregularities on the etching surface of the ingot. Therefore, in this method of cutting a silicon ingot with a wire saw using this slurry, the cutting resistance can be reduced, so that a high-quality wafer can be obtained efficiently.
In addition, the silicon ingot cutting slurry B containing acetic acid had a polishing rate as high as 21 to 23 μm / min and reduced unevenness on the etching surface, particularly at a slurry temperature of 40 to 110 ° C. Therefore, the method of cutting the silicon ingot using this slurry can reduce wafer warpage, surface step difference, and thickness unevenness, and can more efficiently obtain a high-quality wafer.
Furthermore, since the silicon ingot cutting slurry according to the present invention can increase the feed speed of the ingot as much as the cutting resistance is reduced, the cutting speed can be further increased.
On the other hand, in the silicon ingot cutting slurry C containing sodium hydroxide, although the polishing rate was as high as 23 μm / min, the etching surface had very large irregularities. Further, with the silicon ingot cutting slurry D, a sufficient polishing rate could not be achieved.

It is the schematic of the multi wire saw used in one Embodiment of this invention. It is a cutting part enlarged view of a silicon ingot in one embodiment of the present invention. It is a figure which shows the relationship of each parameter in the cutting | disconnection of the silicon ingot using a multi-wire saw. It is the schematic of the grinding | polishing apparatus used in one Embodiment of this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Ingot feeding mechanism, 2 Silicon ingot, 3 Wire, 4 Rotating roller, 5 Wire delivery mechanism, 6 Wire winding mechanism, 7 Tension control roller, 8 Slurry stirring and supply tank, 9 Slurry coating head, 10 Multi-wire saw, 11 Slurry for cutting silicon ingot, 12 beaker, 13 magnet rotor, 14 heater / stirrer, 15 thermometer, 16 polishing pad, 17 rotating table, 18 liquid feeding tube, 19 liquid feeding pump, 20 polishing head, 21 polishing apparatus, 22 Abrasive grain.

Claims (3)

In the silicon ingot cutting slurry containing abrasive grains,
The slurry for silicon ingot cutting, wherein the slurry further contains nitric acid and hydrofluoric acid, and a mass ratio of the nitric acid to the hydrofluoric acid is 0.5 to 25.   The slurry for silicon ingot cutting according to claim 1, wherein the slurry further contains acetic acid, and a mass ratio of the acetic acid to the hydrofluoric acid is 0.05 to 10. In a method of cutting a silicon ingot using a slurry for cutting a silicon ingot containing abrasive grains,
The slurry further includes nitric acid and hydrofluoric acid, the mass ratio of the nitric acid to the hydrofluoric acid is 0.5 to 25, and the slurry is used at 40 ° C to 110 ° C. Cutting method of silicon ingot.

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