RU2429964C1 - Procedure for wire cut of silicon ingot into plates - Google Patents

Abstract

FIELD: machine building.

SUBSTANCE: procedure consists in fixation of ingot on mandrel by means of gluing resin, in feeding ingot in horizontal direction with its side surface through vertical rows of wire up to complete ingot cutting. Vertical rows wetted with abrasive suspension perform cyclic reciprocating motion. With its end section the ingot is fixed on the mandrel. A layer of viscous resin is placed between side surface of the ingot and the mandrel. The mandrel with a glued ingot is arranged vertically with a free end down at angle not less, than 7° to virtual plane wherein there are set rows of wire. The ingot is fed through rows of wire performing cyclic reciprocating motion in horizontal plane.

EFFECT: increased efficiency of cutting process.

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Description

The invention relates to the field of processing poly- and single-crystal silicon ingots in order to separate them into wafers and can be used in the manufacture of wafers used in the manufacture of semiconductor photoelectric converters, semiconductor devices and integrated circuits.

A known method of wire cutting a silicon ingot, which involves fixing the cut ingot on the holder and cutting the ingot moving at a speed of 700 ÷ 800 m / s in a horizontal plane perpendicular to the axis of the ingot with wire reinforced with diamond chips [1].

The specified method has the following disadvantages:

- the high cost of the tool, due to the use of high-strength steel wire reinforced with abrasive material from artificial or natural diamonds as a cutting tool;

- large losses of silicon during cutting, due to the fact that the cutting is carried out at a very large (V ~ 5 ÷ 10 mm / min) speed, while the width of the cut is very significant, and significant mechanical damage occurs in the region of the cut;

- low productivity of the method, due to the fact that the cutting is performed by a single wire saw.

Currently, this method of cutting is used only for cutting an ingot, cutting an ingot into measured billets, as well as for cutting thick test washers from an ingot for control operations.

There is a method of wire cutting a silicon ingot into wafers, including gluing the ingot with adhesive on the side surface to the mandrel mounted on the holder, feeding the ingot from top to bottom with the side surface through rows of wire reciprocating in the horizontal plane, and cutting the ingot in rows wire with feeding the abrasive slurry into the cutting area up to cutting the ingot to the mandrel [2].

The main advantage of cutting in parallel rows of wire is that this method produces cut plates with minimal surface structural defects due to the small thermodynamic stresses arising in the contact zone of the tool with the cut ingot.

This method allows you to simultaneously cut the ingot into a large number of plates, while ensuring the least possible of all existing methods of cutting the thickness of the cut plates and the width of the cut, large savings are achieved in the processed materials [3].

The essence of the method is illustrated in figure 1, which shows a diagram of a device that provides the implementation of the cutting process, where:

1 - silicon ingot;

2 - a mandrel of an epoxy composition;

3 - a layer of adhesive mastic;

4 - holder;

5 - wire;

6 - cylindrical drums;

V _c - ingot feed rate through the rows of wire, mm / min;

V _p - wire speed, m / s;

- направления перемещения проволоки.

- the direction of movement of the wire.

The wire saw is a wire 5 of high-strength steel with a thickness of 0.08 ÷ 0.15 mm with an outer coating with a thickness of up to 20 μm made of soft metal (usually copper). Using an automated stacker of the cutting machine, rows of wire 5 are formed between the drums 6.

Drums 6 are cylinders ~ 300 mm long, on the surface of which up to 800 pieces of annular grooves are formed, located at a distance of 100 ÷ 200 μm from each other, into which the wire is laid by an automated stacker [3].

The ingot 1 is fixed with a lateral surface on the mandrel 2 using adhesive mastic 3, after which the mandrel with the ingot fixed on it is fixed in the holder 4. As a rule, the mandrel is fixed on the holder with a dovetail mount.

The wire is driven at a speed of V _p = 4 ÷ 12 m / s, cyclically changing the direction of the wire in the opposite direction, resulting in a synchronous reciprocating movement of the rows of wire 5 between the reels 6.

The ingot 1 is lowered by its lateral surface until it touches the virtual plane formed by the moving rows of wire 5, and the specified ingot feed speed V _c = 0.3–1.0 mm / min is set.

At the same time, an abrasive slurry is continuously fed to the wire through the slot nozzles of the nozzles (not shown in Fig. 1), the abrasive particles from which (usually these are silicon carbide particles SiC 4-15 microns in size), captured by the surface of the moving wire, actually carry out the cutting process.

The interaction of abrasive grains with a semiconductor material leads to the appearance of microcracks and punctures in the last and layer-by-layer removal of the processed material.

At low feed rates of the silicon ingot (V _c = 0.3 ÷ 0.4 mm / min, i.e., when the applied force is small), its slight destruction occurs. In this case, the cutting width is provided at the level of 0.08 ÷ 0.2 mm.

Under these conditions, an ingot ⌀ 150 mm during the cutting process will be divided into 600 ÷ 700 plates with a thickness of up to 200 μm in 6-7 hours.

The disadvantages inherent in this method include the following:

- increased wear of the lower drum, due to the fact that the abrasive slurry supplied to the upper rows of the wire partially flows down and falls on the lower drum, which leads to its premature wear in comparison with horizontal drums;

- the time required to separate the cut ingot into separate plates is up to 10% of the cutting time, which is determined by the separation technology: the mandrel with the plates attached to it is removed from the holder and placed obliquely in a bath with cold (T = 15 ÷ 20 ° C) water , then for 30–40 min the water temperature is gradually raised to T ~ 70 ° C, as a result of which the mastic softens, and the plates are separated from the mandrel.

There is a method of wire cutting a silicon ingot into wafers, including gluing the ingot with adhesive with the side surface to the mandrel mounted on the holder, feeding the ingot in the horizontal direction with the side surface through rows of wire reciprocating in the vertical plane, and cutting the ingot in rows of wire with the supply of abrasive slurry to the cutting area up to cutting the ingot to the mandrel [4].

The essence of the method is illustrated in figure 2, which shows a diagram of a device that provides the implementation of the cutting process, where:

1 - silicon ingot;

2 - a mandrel of an epoxy composition;

3 - a layer of adhesive mastic;

4 - holder;

5 - wire;

6 - cylindrical drums;

7 - nozzles feed abrasive suspension;

8 - drive horizontal movement of the ingot;

V _c - ingot feed rate through the rows of wire, mm / min;

V _p - wire speed, m / s;

- направления перемещения проволоки.

- the direction of movement of the wire.

The disadvantage of this method of wire cutting is the decrease in process productivity due to the need to dismantle the mandrel with a cut ingot, placing the mandrel in a cold water bath with smooth heating of the water until the mastic softens and the plates are separated from the mandrel.

The objective of the invention is to increase the productivity of the cutting process.

This is achieved by the fact that when cutting a silicon ingot onto wafers, which involves fixing the ingot on the mandrel using adhesive mastic, feeding the ingot horizontally with its lateral surface through vertical rows of wettable abrasive suspension of the wire until the ingot is completely cut, the ingot is fixed on the mandrel the end part, and between the side surface of the ingot and the mandrel, a layer of viscous mastic is mixed t vertically with a free end face down at an angle of not less than 7 ° to the virtual plane in which the rows of wire are located, and the ingot is fed through rows of wires that perform cyclic reciprocating motion in the horizontal plane.

In the solutions of a similar problem known to science and technology, the use of the vertical orientation of the ingot at a slight angle to the plane of the rows of wire was not found, with a layer of viscous mastic placed between the side surface of the ingot and the mandrel, in order to exclude the operation of subsequent separation of the cut plates from the mandrel.

The invention is illustrated in figure 3, where:

1 - silicon ingot;

2 - a mandrel of an epoxy composition;

3 - a layer of adhesive mastic;

4 - holder;

5 - wire;

8 - drive horizontal movement of the ingot;

9 - a layer of viscous mastic;

10 - virtual plane of the arrangement of rows of wire;

11 - axis of the ingot;

φ is the angle of inclination of the ingot to the virtual plane of the arrangement of rows of wire, °;

V _c - ingot feed rate through the rows of wire, mm / min;

V _p - wire speed, m / s;

- направления перемещения проволоки.

- the direction of movement of the wire.

To implement the proposed method using a L-shaped mandrel 2 made of an epoxy composition.

Ingot 1 is glued with an end face to the short shoulder of mandrel 2 using adhesive mastic 3 (a mixture of 60 g of APALDITE AV 144-2 glue and 40 g of HARDENER HV 997 glue), and placed between the side surface of the ingot 1 and the long shoulder of mandrel 2 a layer of viscous mastic 9. The mandrel 2 with the fixed ingot 1 is fixed in the holder 4, after which the axis of the ingot 11 is oriented relative to the virtual plane 10 formed by the moving rows of wire 5 at an angle φ ~ 7 °.

An ingot 1 with a lateral surface using a horizontal displacement drive 8 is fed towards the rows of wire 5, while an abrasive slurry containing particles of silicon carbide SiC is fed to the wire through slotted nozzles of nozzles (not shown in FIG. 3).

The ingot 1 oriented in this way with a speed of V _p = 0.3 ÷ 0.4 mm / min is fed through the rows of wire 5, while the cutting of the ingot starts from the side of the lower free end. The ingot is heated to a temperature of T ~ 45 ° C during cutting.

When the lower rows of wire cut through the ingot to the layer of viscous mastic 9, the cut off plates begin to deviate under their own weight in the opposite direction from the axis of the ingot, since they are glued to the mandrel with a layer of viscous mastic. The viscosity of the mastic is selected so that the cut plate, deviating from the mandrel 2 to a vertical position, under its own weight, breaks off from the mandrel.

Since the complete cutting of the ingot 1 to the layer of viscous mastic 9 due to the inclination of the ingot occurs simultaneously along the length of the ingot, the separation of the cut plates from the mandrel 2 also occurs with a slight time shift (for example, when cutting an ingot ⌀ 150 mm and 300 mm long at a cutting speed V _c = 0.4 mm / min, the separation of the plates from the mandrel begins about 10 ÷ 12 minutes before the end of the cutting process, i.e. the speed of separation of the plates is 50 ÷ 60 plates / min). The plates torn off from the mandrel are removed from the cutting zone either along an inclined tray additionally placed under the ingot, or due to a natural fall down into a container with warm (T ~ 45 ° С) water located under the ingot.

The lower limit of the angle of inclination of the ingot φ is limited to 7 °, because with a smaller angle of inclination, the speed of separation of the plates from the mandrel increases significantly. This leads to an avalanche sliding of the plates from the mandrel and capture of an part of the viscous mastic by an array of separated plates, which makes it necessary to include in the process an unintended operation of washing the plates from traces of viscous mastic.

The upper limit of the angle of inclination φ is theoretically unlimited, however, in practice, at angles φ> 7 °, the distortion of the shape of the cut off plates (ellipse when cutting cylindrical ingots, rectangularity when cutting square ingots) sharply limits their use in the further production of photoelectric converters, semiconductor devices and integrated circuits (for example, the standard maximum permissible deviation for plates ⌀ 150 mm should not exceed ± 1 mm, which exactly corresponds to the deviation of the axis of the ingot by 7 from the virtual plane in which the rows are arranged wire [5]).

Execution example.

According to the proposed method, 3 processes were performed for cutting ingots of silicon monocrystals of ⌀150 mm of the KDB-10 (111) -4 ° grade on an upgraded wire cutting machine from Mayer & Burger (model DS-265).

The length of the ingots ranged from 290 to 310 mm. In this case, TA-63 wire (manufactured by Trefil Arbed) with a carbon content of more than 0.6 wt.% Was used. The diameter of the wire is 140 microns, the thickness of the copper layer deposited on the wire is 20 microns. The length of the wire on the reel is 65,000 m. To wet the rows of wire during cutting, an abrasive slurry of the PEG-300 grade based on polyethylene glycol (abrasive SiC, grain size 10–11 μm) was used.

The ingot feed rate was V _c = 0.3 mm / min, the wire speed was V _p = 11 m / s (the reciprocating mode of wire transfer is a cyclic feed of 3000 m in the forward direction and 2700 m in the opposite).

The average duration of the cutting processes (taking into account the preparatory operations for the formation of rows of wire on the drums) was about 8 hours.

The time from the beginning of the separation of the first cut-off plate to the last (a total of 600 plates was cut off) averaged 11 minutes.

Cut off plates were removed from the cutting zone using an inclined tray located under the lower end of the ingot.

Thus, the productivity of the cutting process (average time spent on cutting 1 plate, calculated according to the results of 3 processes), amounted to 1.25 plates / min.

The performance of the process when cutting a similar silicon ingot using the technology of the prototype (including the separation of the cut ingot into plates in a bath with warm water for 1.17 hours) was 1.08 plates / min.

Thus, the proposed method in comparison with the prototype provides an increase in productivity of the process of cutting silicon ingots into wafers by at least 13%.

Information sources

1. US patent, IPC B28D 1/02, No. 5878737 dated March 09, 1999

2. US Patent, IPC B28D 1/06, No. 6237585 of May 29, 2001

3. At the cutting edge of precision silicon technology. Manual (Wire Saw DS 265). - “Mayer Burger Swiss Slicing Systems” (CH-3613, Steffisburg / Switzerland), 2006, pp. 87 ÷ 122.

4. US patent, IPC B28D 1/08, No. 6067976 of May 30, 2000 - a prototype.

5. SEMI M6.6-85. Standard for 150 mm monocrystalline solar cell grade silicon clices. - “Book of SEMI Standards” (805 East Middlefield Road, Mountain View, CA 94043-4080, USA), 1997, p. 1.

Claims (1)

A method of wire cutting a silicon ingot into wafers, which involves fixing the ingot on a mandrel using adhesive mastic, feeding the ingot horizontally with its lateral surface through vertical rows of wettable abrasive wire suspension until the ingot is completely cut, characterized in that the ingot is fixed on the mandrel end part, and between the side surface of the ingot and the mandrel place a layer of viscous mastic, mandrel with glued ingot m has a free end vertically downward at an angle of not less than 7 ° to a virtual plane in which are arranged rows of wire, and feeding the ingot carried out through the rows of wire to perform cyclic reciprocating movement in a horizontal plane.

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