DE102011014828A1 - Fluid, useful to produce wafers, electronic components and photovoltaic cells, comprises a base glycol, additives, a glycol etherified with an alkyl group and water - Google Patents

Abstract

Fluid comprises at least a base glycol (I), optionally further additives, a glycol etherified with at least one alkyl group (II) and less than 2 wt.% of water. Fluid comprises at least a base glycol compound of formula (HO-[(CH 2) x-O] n-H) (I), optionally further additives, a glycol etherified with at least one alkyl group of formula (HO-[(CH 2) a-O] b-R) (II) and less than 2 wt.% of water. R : linear or branched 1-8C-alkyl; n : 1-14; x, a : 1-4; and b : 1-10. An independent claim is included for producing wafers, electronic components and photovoltaic cells, comprising sawing ingots from wafers, preferably with multiwire-saw using the separating fluid.

Description

The invention relates to improved fluid separation media, such as sawing fluids or slurries, in particular for sawing brittle-hard material blocks, as well as their use in particular for the production of wafers, and photovoltaic elements.

Wafer, z. As those made of silicon, serve as a support plate for electronic components, in particular for semiconductor devices and photovoltaic elements. They form the basis for the production of solar cells, which are further processed into ready-to-use solar modules. The wafers are obtained even by sawing multicrystalline cuboid blocks, columns or bricks or monocrystalline, round or squared ingots, wherein the sawing usually takes place by means of wire sawing, in which a plurality of parallel, closely adjacent wires along with a sufficiently high speed their longitudinal direction to be moved over the good to be sawn. Such a wire saw is for example in the US-A 2006/0249134 described.

When sawing cutting or cutting tools, so the saw wires, at least at the respective cutting point (cutting gap) brought into contact with a fluid separation medium, which among other things is used for cooling the saw surface and the wire and is therefore also referred to as Coolant. The fluid may optionally contain Abrasions- or abrasive particles and is therefore referred to as slurry. When using bonded grain saw wires, the fluid usually does not contain any of these abrasive particles.

In the former case, the abrasion particles contained in the slurry are entrained by the moving wire at high speed and cause their movement in the cutting gap chipping of brittle material in the cutting gap, which thereby constantly deepens and so to sever the brittle starting body and the separation of the individual wafer slices leads.

In the second case are the hard abrasion particles, which z. As may be made of diamond, tied on the surface of the saw wire and cut by the rapid movement of the wire in the kerf brittle material, whereby the cutting gap is constantly deepening and thus leads to severing the brittle starting body and its severing into individual wafer slices.

In both cases of the sawing process known as wafer sawing, different fluid separation media are used, the liquid phase of which is also referred to only as fluid, coolant, sawing fluid or carrier fluid, and z. Example of 1 or different glycols, which may be added rheologically active agents in addition.

The proportion of the production costs of the silicon wafers amounts to approximately 55% of the total costs of solar cell production, whereby again the largest share of the costs in the wafer production is assigned to sawing alone. For example, the use of slurries accounts for approximately 10% to 15% of the total sawing process. By economically and ecologically recycling such spent slurries or coolants, therefore, the costs associated with the production of such wafers, in particular when using slurries in the sawing process, can be significantly reduced.

In addition, the use of such slurries or coolants in the production of silicon wafers will gain in importance in the future, because these determine whether the production of silicon wafers with low cutting loss or low energy consumption or high yield with high surface quality of Wafer succeeds. The production of thin silicon wafers or sawing with low cutting loss (kerf) is also desired in order to produce the lowest possible use of material as high as possible number of wafers per Brick mass used.

The carrier fluids or carrier fluids used in this case usually differ, inter alia, by their viscosity. Frequently used carrier fluids are oil, and in particular glycols, such as PEG300, PEG400 or PEG200.

These carrier liquids also serve as coolants and have, above all, the task of picking up and removing the heat generated during the abrasion or cutting off of material particles on the cutting front in the cutting gap by friction. In addition, they also serve to remove the chopped-off in the kerf material particles (Kerf) from this. In addition, they also serve as suspending agents and transport vehicles in the use of loose unbound abrasive particles or cutting grains.

Since such carrier liquids can only absorb the material particles (kerf) abraded or cut from the blocks, columns, bricks or ingots in the kerfs in a limited amount, the slurries or coolants used must be replaced from time to time. In addition, it has been shown that, in particular when sawing silicon blocks, the viscosity of the sawing liquid increases sharply with increasing loading of abraded or chipped material particles (kerf), which leads to an increase in the Energy consumption during sawing leads. At the same time, the generated heat of friction increases sharply, which in turn leads to an increase in energy consumption in the cooling of the Coolants or the slurry.

In addition, it has been found that when saw wires are used, especially with increased loading of the slurry or coolant with abraded particles, the saw wires are deflected laterally, resulting in non-constant thickness wafers which are not perfectly flat Have side surfaces but the sawing depths, also called Saw Marks have.

The EP 1752521 A1 describes an aqueous carrier fluid for abrasive particles. This aqueous carrier fluid contains glycols and glycol ethers in a ratio of 1: 1 to 99: 1 and 2-50% by weight of water. However, it has been found that a slurry prepared from such a carrier fluid undergoes an abrupt increase in viscosity during prolonged use, ie "it tilts". Also, this slurry is then difficult to recycle and is therefore usually discarded.

In the JP 2002-080883 A For example, there is described a water-soluble carrier fluid for a wire saw which contains 90-99,499% by weight of a water-soluble glycol, water-soluble glycol ether and water-soluble glycol ester and up to 5% by weight of an organic acid and up to 5% by weight of an organic smectite. Such a fluid, which is mixed with water to the finished Sägeslurry should show a reduced hydrogen formation.

The invention therefore has for its object to provide a carrier fluid or sawing fluid, which avoids the aforementioned disadvantages and which, when used as a cooling component or wafer component in wafer sawing in addition to an increased wafer yield, has a prolonged service life and which energy in the production of wafers is reduced.

The invention also has for its object to provide a sawing fluid or a sawing slurry, which can be recycled many times and can be used to saw good wafers even after repeated use.

This also means that the Sägeslurry does not tip over prolonged use.

These objects are achieved by the features defined in the claims.

In fact, it has been found that by adding glycols etherified with an alkyl radical to a carrier fluid containing one or more glycols and using this carrier fluid directly as a sawing fluid or coolant or to produce a slurry, not only the energy used for sawing is reduced can, but that also reduces the necessary for cooling the slurry or the Coolants energy. The fluid is substantially free of water or has the lowest possible water content.

The sawing fluid therefore has a maximum of 5% by volume of water, which corresponds to about 4.24% by weight (about 1.13 g / cm ³ ), but typically contains less than 2% by weight of water. Usually, no water is added, ie the water content is formed only by unavoidable entrainment with the starting materials and by entry via the humidity.

In addition, it has been found that with the procedure according to the invention, the loading of the sawing fluid with kerf can also be significantly higher, which drastically increases the service life of the coolant or the slurry.

The fluid is substantially free of water or has the lowest possible water content.

In particular, the sawing fluid contains less than 1.8% by weight or by weight of water, with water contents of less than 1.3 or less than 1% by weight being particularly preferred. Typical levels of water are less than 0.8 and 0.5 wt .-%, with very particular maximum water contents 0.3 or 0.2 wt .-% are preferred. In the unused condition, the mixed sawing fluid mixed with the components is down to a specification of water content of the components down to a minimum proportion of typically max. 0.2 wt .-% free of water. This content increases during use due to entry of atmospheric moisture or entrainment in other ways. Especially on days or at locations with increased humidity increases the water content of the sawing fluid during sawing unintentionally, thereby reducing the life of the inventive anhydrous or low-water separation fluid or Coolants or the slurry. Even in such unfavorable cases, water contents of the sawing fluid or the liquid phase of the slurry of greater than or equal to 2 wt .-% are deliberately excluded by the addition of new unused fluids or recycled used sawing fluid or slurries or by influencing the environmental conditions during sawing ,

In addition, spot-free wafers are obtained with the sawing fluid or separating fluids according to the invention when using this sawing fluid in a slurry, or staining is greatly reduced. Finally, even sawn wafers can be easily rinsed off with water, even with heavy sludge.

The sawing fluid or separating fluid according to the invention usually contains at least one base glycol. Typical basic glycols have the formula HO - [(CH ₂ ) _x -O] _n -H on.

Where n is an integer. Usual maximum values for n are 14 or in particular 10, with maximum values of 7 or 5 being particularly preferred. In particular, a preferred lower value for n is 2, although a lower value of 3 has proven to be suitable. x is 1, 2, 3 or 4.

Also suitable glycols have as repetition units on such glycols, which are composed of ethylene, propylene and butylene units. The units may be constructed both linearly and from branched or isomeric units. They can also be composed of mixed units.

Typical basic glycols are PEG200, as well as PEG300 and PEG400. However, preferred are ethylene glycols and propylene glycols and mixtures thereof. Particularly preferred are ethylene glycol, diethylene glycol and triethylene glycol or dipropylene glycol.

The etherified glycol added according to the invention is in particular a polyethylene glycol which has been etherified with an alkyl radical. It preferably has the formula HO - [(CH ₂ -) _a -O] _b -R on.

In this case, b is an integer of at least 1 or 2, in particular 1-8 and a is 1, 2, 3 or 4. R is a linear or branched alkyl radical having 1-10 or 1-8 C atoms.

If the base glycol PEG300 or PEG400 can according to the invention, the addition of an ethylene glycol of the general formula HO [CH ₂ -O) _m -H with m = 1 or 2 take place. In this particular case, it is possible to dispense with the addition of a glycol ether etherified with an alkyl radical.

In a preferred embodiment, a = 2. R is an alkyl radical having 1-10 C atoms, preferably 2-8 C atoms.

Particularly preferred are etherified alkyl radicals having a maximum proportion of C atoms of 6, in particular of 4.

Typically, the proportion of glycol etherified with an alkyl radical to the total amount of sawing fluid (i.e., without optional cutting grains, as well as other additives) is at least 1 weight percent, with a minimum level of 3 being especially preferred, 5 weight percent. Particular preference is given to minimum amounts of 10% by weight. A usual maximum proportion of the etherified with an alkyl glycol content of the total fluid is at most 50 wt .-%, with a maximum of 40 wt .-% is preferred. Particularly preferred are maximum values of 35 and 30 wt .-%.

It has been found that the fluid according to the invention can still be treated with high sludge, ie. H. a high content of abgespanten material particles, is usable. In a particularly preferred embodiment, used from time to time consumed sawing slurry or Coolant and partially replaced by new.

It has proven to be particularly useful to set a content of abgespanten material particles (Kerf) of 2-8 wt .-%, preferably from 2-6 wt .-%.

In a preferred embodiment, z. B. the slurry up to 5 vol .-% water, but preferably the liquid phase of the slurry, the sawing fluid only less than 2 wt .-% water.

In a further embodiment of the invention, the water has a slightly acidic pH of 3-6, with a pH of 3 to 5, in particular a pH of 4 +/- 0.5 being particularly preferred. The content of acid in this water is between 0.1 and 3 mol / l, with a value between 0.5 and 2 mol / l being particularly preferred. A typical concentration is 1 +/- 0.5 mol / l. According to the invention all acids can be used, as long as they attack neither sawing wire nor the material to be cut in use. However, it has been shown here that citric acid is particularly suitable.

Typical materials to be diced are all brittle-hard, especially crystalline materials, with mono- or polycrystalline silicon being particularly preferred.

The sawing fluid according to the invention is particularly suitable for use with wire saws, in particular so-called multiwire saws. The sawing fluid is suitable both for sawing with loose and with bound grain. When sawing with loose grain is a slurry that consists of a suspension Abrasionskörnern, usually SiC, diamond or corundum, and the sawing fluid is applied to a formed on a wire of many wires. Each of the individual wires routed parallel to one another undergoes wetting by the slurry before it penetrates into the brittle-hard material. The slurry is entrained by each of the rapidly moving wires and penetrates into the cutting gap along with the wire. In the narrow cutting gap, the abrasive grains unfold their abrading effect by temporarily touching both the wire and the brittle-hard material or jamming in this gap, causing material to flake off the contact pad or contact surface. The wires intersect more and more deeply into the brittle-hard material, forming cutting gaps and ultimately lead to a complete severing of the material to be sawn z. B. a block, a column, a brick.

When sawing with bonded grain, a sawing wire is used, at the surface of which the correspondingly hard abrasion particles are firmly connected to this sawing wire. In contrast to the above-described sawing with loose grain, the abrasion particles adhering to the wire grind over the surface of the cutting gap and thereby chip somewhat brittle-hard material. The sawing fluid according to the invention is used in this method for cooling and recording on abgespanntem brittle material. Again, the wires intersect deeper and deeper into the brittle-hard material, forming cutting gaps and ultimately lead to a complete severing of zersägenden material z. B. a block, a column, a brick or a squared or round ingot leads.

The invention also relates to the use of such a fluid in the manufacture of electronic components, such as computer chips and in particular photovoltaic cells, as well as such systems.

The invention will be explained in more detail by the following examples:
A SägefFuid of diethylene glycol and diethylene glycol monobutyl ether in the ratio 4: 1 was prepared. This sawing fluid was mixed into a slurry at a ratio of 52% by weight sawing fluid to 48% silicon carbide. With this slurry, a 6-inch-long silicon ingot was sawn into wafers having a thickness of 200 μm. The wire speed was 14 m / s and the feed rate 400 μm / min. A wire of 130 μm in diameter was used, the pitch (the distance between 2 wires) being 360 μm.

The same experiment was carried out with a commercially available slurry using PEG200 as the sawing fluid. When comparing the slurry according to the invention to a commercially available slurry, it was found that in the case of the PEG200-based slurry according to the prior art, the starting viscosity was already substantially higher and continuously increased during loading with chopped silicon material particles (Kerf). In contrast, the starting viscosity of the slurry of the invention was already significantly lower and even decreased slightly during the loading with silicon particles (Kerf).

The course is in 1 shown.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 2006/0249134 A [0002]
• EP 1752521 A1 [0013]
• JP 2002-080883A [0014]

Claims (10)

Fluid for sawing brittle-hard material blocks especially with Multiwire-sag, the at least one base glycol of the general formula HO - [(CH ₂ -) _x -O] _n -H in which n is an integer from 1-14 and x is 1, 2, 3 or 4, and optionally contains further additives, characterized in that it additionally contains at least one glycol etherified with an alkyl radical of the general formula HO - [(CH ₂ -) _a -O] _b -R wherein a is 1, 2, 3 or 4 and b is an integer from 1-10 and R is a linear or branched alkyl radical having 1-8 C atoms, characterized in that the proportion of water less than 2 wt .-% is. Fluid according to claim 1, characterized in that the glycol is an ethylene glycol. Fluid according to one of the preceding claims, characterized in that the base glycol comprises PEG200, PEG300, PEG400 or a mixture thereof. Fluid according to one of the preceding claims, characterized in that the proportion of etherified glycol in the total fluid is 1-50 wt .-%. Fluid according to one of the preceding claims, characterized in that the etherified glycol is diethylene glycol monobutyl ether, Fluid according to one of the preceding claims, characterized in that the base glycol is a diethylene glycol. Fluid according to one of the preceding claims, characterized in that it contains a maximum of 1 wt .-% water. Fluid according to one of the preceding claims, characterized in that the water contains 0.1-3 mol / l of citric acid. Process for the production of wafers, electronic components and photovoltaic cells by dicing ingots into wafers, in particular with multiwire saws by means of a separating fluid according to one of claims 1-8. Use of a fluid according to any one of claims 1-8 for the production of wafers, electronic components and photovoltaic cells.

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