METHOD FOR MANUFACTURING SILICONE WAFERS

Abstract

The method of manufacturing silicon wafers from one or more silicon blocks or bricks includes etching at least one lateral surface of a silicon block or brick using a mixture of highly oxidative acids and then forming a plurality of wafers by sawing the silicon block or brick. During the etching of the lateral surface a mean amount of material removed is 3 to 160 μm thick and the material is isotropically removed with a constant mean material removal speed of from 1 to 20 μm/min across the entire lateral surface. Prior to the etching treatment the silicon block or brick is advantageously subjected to an abrasive grinding or polishing. The mixture of acids is preferably a mixture of 50 to 70% nitric and 40 to 60% hydrofluoric acids in a ratio range of 8:1 to 4:1.

Description

CROSS REFERENCE TO A RELATED APPLICATION

The disclosure in German Patent Application DE 10 2007 040 390.0 of Aug. 27, 2007, is expressly incorporated herein by reference hereto. This German Patent Application also describes the same invention that is described herein below and provides the basis for a claim of priority of invention under 35 U.S.C. 119(a) to (d).

BACKGROUND OF THE INVENTION

The present invention relates to a method for manufacturing silicon wafers by separating a silicon block or brick, in the case of which at least one lateral surface of the silicon block is etched before separation is carried out. The present invention also relates to the silicon blocks and wafers manufactured using the method.

Silicon wafers are thin sheets of crystalline silicon and are used to manufacture solar cells and, therefore, solar modules. In accordance with the increasing usage of solar cells and the like, the demand for silicon wafers is also increasing from year to year.

The production of silicon wafers begins with the manufacture of blocks of crystalline silicon ingots that are made of monocrystalline and polycrystalline material and are cultivated in a round shape or as cuboids. In a further step, these crystalline ingots are separated or sawed to form further cuboid blocks. These blocks are also referred to as columns or bricks; in the case of monocrystalline silicon, the cross section is nearly square in shape, and in the case of multicrystalline silicon, the cross section is exactly square in shape.

In a further step, these blocks are sawed into the individual wafers, and, in fact, usually transversely to their longitudinal direction, so that the lateral surfaces of the column-shaped blocks or bricks created via sawing become the edge surfaces of the wafers produced in this manner. It has been shown that, when the bricks are manufactured, superfine, often microscopically small fissures are produced on the lateral surfaces created via sawing. These fissures penetrate the block, and therefore, the edges of the subsequent wafer, to a greater or lesser extent.

Silicon is an extremely brittle material. Unlike the situation with metals, a fissure in this material may therefore spread rapidly when stress is applied. Normal stressing, which occurs, e.g., during handling in cell production, may quickly result in fracture of the wafer if fissures are present, in particular in the edge of the wafer.

Since the costs required to manufacture the silicon wafer comprise approximately 55% of the total manufacturing costs for solar cells, a high percentage of broken silicon wafers, i.e., a high fracture rate, results in a marked increase in costs to manufacture solar cells.

Given that there is a worldwide shortage of silicon, the silicon wafers manufactured in the future will have to be extremely thin. If fissures form in these thin silicon wafers, they are extremely susceptible to fracture. Fissures in the edges of the wafers are particularly critical.

To avoid the problems described above, it is provided in US 2002 036182 AA to diminish the roughness of the surfaces of such silicon blocks before the silicon wafers are manufactured. The roughness of the surfaces is diminished via mechanical polishing, as described in JP 3,648,239. “Polishing” refers expressly to treatment with loose grains. Finally, a separating method for a semiconductor is described in U.S. Pat. No. 5,484,326, with which the surfaces of a silicon block are ground in advance.

It has been shown that this method does reduce the frequency of fracture of the cut silicon wafers, but that this is insufficient for very thin wafers in particular.

It is described in publication U.S. Pat. No. 6,099,748 that the surfaces of silicon wafers may be etched using alkaline solutions. It is assumed that the etching speed used on the <100> surface is basically 60 to 100-fold of that used on the <111> crystal surface. Based on this, it is provided that the wafer surfaces be etched using an aqueous alkali solution with an alkali content of 50.6% to 55.0% by weight. Etching should be carried out at a temperature of 65° C. to 85° C., with 15 μm to 40 μm of the total thickness of the wafer surface being removed via etching on both sides of the wafer.

SUMMARY OF INVENTION

An object of the present invention, therefore, was to improve the fracture rate in the manufacture of silicon wafers, and to reduce it considerably. In particular, the object is to reduce the fracture rate in the production of extremely thin wafers with a thickness of <230 μm.

This object is attained via a method for manufacturing silicon wafers that include a top surface and side edges, and that are obtained by separating a silicon block or brick that includes lateral surfaces that are cuboid in particular, the silicon blocks or bricks having been previously subjected to etching as described in Claim 1.

It was found, according to the present invention, that micro-fissures and defects that formed on the surface of the silicon blocks or columns—in particular due to the material having been sawed to form cuboids of the ingots—may expand as a result of isotropic etching to the point that these defects or structures located on the eventual outer edge or edge surface of the wafer no longer elongate spontaneously when stress is applied, or they propagate into the interior of the material, and therefore result in fracture of the wafer.

It was also found that the roughness of the silicon block is not the main cause of wafer fracture. Instead, subsurface defects, e.g., micro-fissures in the layers close to the surface, are also responsible for fractures that start on the edge of the wafer. In particular, the tips and course of the micro-fissures are decisive factors that determine the fracture behavior of the silicon wafer. The tip of the fracture is widened or deflected via the inventive method.

The lateral surfaces of the silicon brick or the silicon column or block are essentially etched isotropically such that more than 90% of all cracks per unit surface area on the sawed surface have disappeared after etching, as determined via light microscope. It is permissible—to improve countability—to polish away the cracks that characterize the starting state, i.e., the starting surface, by the amount of the profile depth P_x as stated in DIN ISO 4287. Experiments have shown that the fewer than 10% of the cracks still noticeable after etching have widened—and, mainly—have a crack tip that is rounded off to the point that the remaining cracks no longer pose a risk of fracturing the sawed wafer. According to the present invention it was found that the cracks and micro-fissures result in fracture of the wafers manufactured in this manner when the end of the crack trough or the deep peak are particularly pointy. That is, the wider the crack or cavity is, the lesser is the risk that it will spread spontaneously and result in a fracture. The decisive factor is the depth of the particular crack or indentation in the material that is the trough formed by the crack, and, in particular, only on its more or less pointy course. According to the present invention, etching is therefore carried out isotropically such that the particular crack expands or widens, and the tip of the crack rounds off, thereby drastically reducing the spread of a crack of this type.

After the wafer has been sawed, the lateral surfaces of the silicon block become the edges and/or the circumferential surface of the silicon wafer that has been cut or sawed out of it.

Within the sense of the present invention, “etching” refers to the chemical removal and/or dissolution of material, i.e., silicon material, and, in fact, also in the recesses and/or fissures present on or under the surface. According to the present invention, the term “isotropic etching” therefore refers to the removal of material using a chemical process that takes place at a more or less even speed in every crystal direction, i.e., the etching speed is essentially uniform along the <100>-plane, the <111>-plane, and the <110>-plane. According to the present invention, etching is considered to be isotropic when the etching speed is essentially uniform in various crystal directions. Only the differences in etching speed between grains having different orientations are considered. The etching speeds preferably do not differ from each other by more than 1.3-fold, preferably by not more than 1.1-fold, and particularly preferably by not more than 1.05-fold. Differences in the etching behavior at grain boundaries or dislocations are not included in this definition of the isotropic etching behavior.

In addition, according to the present invention, etching is carried out such that the material is removed from the particular surface of the silicon brick in a more or less uniform manner. Within the framework of the present invention, an essentially uniform removal of material means that the extent of material removal over the entire particular lateral surface does not fluctuate by more than 10 μm, with maximum fluctuations of 8 μm, and, in particular, 5 μm or 4 μm being particularly preferred.

With the inventive method, etching is typically carried out with a mean material-removal speed of 1 μm to 20 μm per minute, in particular of 3 μm to 15 μm per minute.

In this manner, fissures in the wafer are prevented from propagating from the wafer edge toward the interior of the wafer. The fracture rate of the silicon wafers may therefore also be reduced markedly in this manner.

The silicon block is preferably etched for at least 30 seconds, preferably for at least 45 seconds, and particularly preferably for at least 60 seconds. A typical maximum duration of treatment is 300 seconds, with a maximum of 250 seconds being preferred, and a maximum of 200 or 150 seconds being particularly preferred. The duration also depends on the size of the surface, the temperature, and the etching formulation, and it is selected such that it is possible to perform isotropic etching or etching in the isotropic/anisotropic boundary region. The particular conditions for etching in the isotropic region and/or isotropic/anisotropic boundary region are to be determined by one skilled in the technical art based on a few trials.

The mean layer thickness to be removed depends on the starting state of the jacket surfaces of the brick before etching is begun. If the jacket surface of the brick is sawed raw, i.e., it is the result of a wire-sawing process (squares), then a mean material removal of 25 μm to 100 μm is required. If the jacket surface of the brick—after it has been sawed into a square shape—will be coarse-ground further (dimensional grinding), then a mean amount of 8 μm to 50 μm should be removed via etching. If the jacket surface of the brick will also be fine-ground, then a mean amount of only 5 μm to 25 μm should be removed via etching. If the jacket surface of the brick has been completely fine-ground in the ductile region, with the result that the surface meets the criterium for polished surfaces, i.e., the maximum roughness height R_x—at λ/2 to λ/60—is below the wavelength of light, then etching may result in success, even though it may be less pronounced. Since a surface of this type is not 100% free of subsurface defects, due to abrasive grains that break free as a result of fine grinding, the risk of fracture of the wafer may be further reduced slightly by performing etching very briefly for 20 to 30 seconds.

The silicon block is preferably etched at the lowest temperature possible. For economic reasons, temperatures of at least 18° C. have proven advantageous, and temperatures of at least 20° C. have proven particularly advantageous. Further advantageous maximum temperatures are 25° C., in particular up to 23° C., with maximum temperatures of 22° C. being particularly preferred.

It was also determined according to the present invention that an isotropic behavior may be increased by fluctuating the particular selected temperature during etching and at the various etching points, i.e., at different points on the silicon brick, by a maximum of ±2° C., with maximum differences of ±1° C., and in particular, ±0.5° C., being preferred. Fluctuations that do not exceed ±0.2° C. or ±0.1° C. are very particularly preferred. These conditions are maintained more easily by using a large quantity of fluid and by providing effective cooling. The behavior must be isotropic during etching in order for the most constant temperature possible to be maintained.

The etching procedure used is a wet etching procedure, which is preferably carried out in an acidic medium. Preferred acids are highly oxidative acids such as sulphuric acid, nitric acid, and/or hydrofluoric acid, and the related peroxo acids thereof.

Preferably, the silicon block is etched with a solution of 50-70% nitric acid and 40-60% hydrofluoric acid in a ratio in the range of 10:1 to 1:1, and preferably in the range of 8:1 to 4:1, these ratios being volume ratios.

Further additives are typically added to this mixture, with the aim of moderating the reaction. A fluid such as water, acetic acid, hydrogen peroxide, or a surfactant (wetting agent) that lowers the surface energy is preferably added to the solution.

It has been shown that it is advantageous to add additional quantities of the acids that are consumed in the etching process. The addition may take place continually or in lots. The addition may take place after a certain period of usage based on previously determined values from experience, or by performing an accurate, on-line measurement of the acid concentrations. If the particular acid concentration changes, the required amount is added. This addition or refreshment of acid may greatly extend the period of time until the etching solution must inevitably be replaced entirely.

The etching itself is preferably carried out by immersing the particular silicon brick or silicon column or block to be treated in an etching bath. Etching baths are preferably used in which the fluid is moved continually. It is very particularly preferred to allow the etching fluid to flow evenly over the particular surfaces. It is also basically possible, however, spray the silicon column or brick with the etching solution. In this case, a more or less thick film of etching fluid flows over the surface of the silicon block. The etching solution removes material from the surface of the brick.

Isotropic etching or nearly isotropic etching results in a reduction in surface roughness.

Within the framework of the present invention, the term “surface roughness” refers to the uneveness of the surface height.

The ratio of the surface roughness R_max of the particular lateral surface being treated according to the present invention after etching to the surface roughness R_max of the lateral surface before the etching process is preferably 10:1 to 0.5:1, typically from 8:1 to 0.5:1, and very particularly preferred in a range of 7:1 to 1:1.

The depth of the cracks that are detectable after the etching process has preferably been reduced by at least 30%, preferably by at least 50% or 60%, although at least 80%, and, in particular, at least 85% has proven to be common. Particularly preferably, the depth of the detectable cracks has been reduced by 90%, and, in particular, by 95%. According to the present invention, it is possible to eliminate the micro-fissures more or less entirely, i.e., to reduce the depth by nearly 100%. This does not mean that etching necessarily reduces the surface roughness, however. A distinction must be made here in particular between unevenness in the surface and the micro-fissures. According to the present invention, the surface roughness defined by the unevenness in the surface may be very great, provided that the width of the particular wave troughs and/or roughness troughs is great, and that their deepest points are rounded off. This is precisely what is attained using the inventive etching procedure. In a particularly preferred inventive embodiment, the lateral surfaces of the silicon block are ground before etching is carried out. In a very particularly preferred embodiment, the lateral surface is ground and/or polished essentially parallel to the eventual cutting plane, with the grinding or polishing motion extending along the edge surface of the eventual wafer. Once this grinding/polishing procedure is carried out, the silicon block is subjected to the inventive etching procedure, and it is then sawed or cut to form the actual wafer, in a further step.

In the present invention, the lateral surfaces of the silicon block or brick represent—after the block has been cut—the circumferential surfaces, i.e., the edges of the wafer.

It was found that the roughness of the lateral surfaces of the silicon block is not the main cause of wafer fracture. Instead, superfine, microscopically small fissures and defects located below the surface and in layers close to the surface, i.e., subsurface defects, e.g., micro-fissures, are responsible for fractures that start at the edge of the wafer. It is precisely these defects close to the surface that are formed—and existing subsurface defects are covered—via the plastic deformation. It has been shown that the depth and course of the micro-fissures, in particular, are decisive factors in determining the fracture behavior of the silicon wafer. Fissures that extend perpendicularly to the cutting plane, i.e., perpendicularly to the lateral surface of the silicon brick or block, i.e., in the direction of the subsequent wafer surface, are particularly critical in terms of the fracture of the silicon wafer. In contrast, fissures that extend parallel to the cutting plane, i.e., parallel to—that is, along—the subsequent wafer edge have minimal influence on the fracture of the silicon wafer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a preferred embodiment, the silicon block or brick is ground and/or polished using a tool with a cylindrical shape. In a preferred embodiment, the tool has the shape of a hollow cylinder. The end surfaces of the jacket of the hollow cylinder therefore serve as the grinding and/or polishing surfaces. Tools of this type are typically also referred to as grinding cups. A grinding/polishing treatment of this type is described in application DE 10 2007 040 385.4, which has the same priority.

Preferably, the cylindrical tool has a diameter that is at least 1.5 times that of the width of the surface of the silicon brick to be treated, is particularly preferably 1.55 times, and is very particularly preferably 1.75 times that of the width of the surface of the silicon brick to be treated.

Grinding tools of this type that include a grinding cup are available, e.g., from Saint-Gobain Diamantwerkzeuge GmbH & Co KG in Norderstedt, Germany, from Wendt GmbH in Meerbusch, Germany, from Günter Effgen GmbH in Herstein, Germany, and from Herbert Arnold GmbH & Co KG in Weilburg, Germany.

The thickness of the hollow cylindrical jacket of the grinding tool and/or the grinding cup is typically at least 3 mm. Maximum reasonable thicknesses are typically 2 to 3 cm, and thicknesses of 1 cm to 2 cm are preferred. Thicknesses of 12 mm to 17 mm—and preferably to 13 mm—are very particularly preferred.

In a preferred embodiment, the grinding tool includes a matrix in which the particles of the abrasive device are embedded and/or are more or less fixedly bound.

Advantageously, the abrasive grains have a hardness that is much greater than that of crystalline silicon.

The abrasive grains are preferably composed of diamond, and grinding or cutting grains made of silicon carbide and/or silicon nitride have also proven to be suitable.

Preferably, a matrix made of a material is used that is selected from the group composed of a soft metal, e.g., bronze, at least one polymer, and at least one resin.

If abrasive grains are torn out of their matrix bond during the treatment process, the soft matrix material is also removed quickly and in an accelerated manner, thereby exposing new abrasive grains. This process is also referred to as “self-sharpening”. In this case, the tool does not need to be replaced until the abrasive grains are removed, i.e., used up.

In an embodiment preferred according to the present invention, abrasive grains with a larger particle diameter, i.e., particles used for coarse pre-grinding, are embedded and/or bonded in a bronze matrix, but particles used for fine-grinding or post-grinding are embedded and/or bound in a plastic matrix.

The abrasive grains preferably have a mean diameter in the range of 3 μm to 160 μm. Various tools with grinding or polishing particles of various sizes and/or hardnesses are used depending on whether the block or brick is now ground to an exact size, or if it is ground particularly smooth and with little damage, and/or if the aim is to attain a surface with a polished quality. For example, if the dimension of a block is to be fixed, a grinding cup is used that includes abrasive or cutting grains or particles with a mean diameter of at least 80 μm and up to 160 μm. Particularly preferably, particles with a mean diameter of at least 85 μm and up to 130 μm are used. If a surface that is particularly damage-free is to be attained, however, as is carried out, e.g., in a further process that results in a second level of grinding and/or polishing quality according to a preferred inventive embodiment, then mean abrasive particle diameters of at least 3 μm and, in particular, at least 10 μm are typically used, while a maximum size of 40 μm, and preferably 25 μm has proven suitable. In a preferred inventive embodiment, the silicon block or brick is initially treated with a coarse-grain abrasive tool, and then with a fine-grain abrasive tool.

If the grinding tool includes abrasive grains with a mean diameter in the range of 80 μm to 160 μm, it is referred to as a coarse-grain grinding tool. A grinding or polishing procedure in which case gritty abrasive grains or particles are used, is therefore also referred to as rough grinding.

When the grinding tool includes abrasive grains with a mean diameter in the range of 3 μm to 40 μm, it is referred to as a fine-grained grinding tool. A related process in which fine abrasive grains or particles are used is therefore also referred to as fine grinding or fine polishing.

A related process in which a grinding tool with fine abrasive grains is used can result in surface qualities that correspond to those of polished surfaces. When it is ensured via the process parameters that the penetration depth of the individual grinding particles is fixed at less than 40 nm or 30 nm—so that material removal takes place in the ductile region—it is ensured that the maximum roughness height R_x is below the wavelength of light, i.e., in the range less than λ/2. “Removing material in the ductile region” means that the material is plastically deformed when it is removed, without the treated surface becoming damaged due to stress or strain. Ductile material removal is carried out under conditions that permit a plastically deformable removal of material, without the material underneath becoming damaged, as is the case with brittle material removal. This is typically attained using rapid grinding at a high rate of speed, which results in local warming of the material to be removed, so that the abrasive grain removes and/or abrades the plastic material in a material-removing manner. The surface that is attained is then considered to be polished, within the sense of the present invention.

For coarse-grained grinding tools, a matrix made of a soft metal, e.g., bronze, is preferably used. For fine-grained grinding tools, however, a polymer or a synthetic resin is typically used as the matrix. According to the present invention, as a measure of the concentration of cutting grains in a plastic matrix or a synthetic resin matrix, a concentration is used that is available as a synthetic bond in the range C60 to C85. A concentration of C75 is preferably used.

The cutting speed of the grinding cup in the process described above is preferably set at approximately 20 m/s to 50 m/s. Per the relationship

V_s=π*D*n

• V_s=cutting speed
• π=3.1416
• D=diameter of cutting tool (grinding cup)
• N=rotational speed
  suitable values may be selected for D and n in order to set the desired cutting speed. A comparable speed may also be used for polishing as defined per the present invention. The grinding cup may have a diameter that is substantially greater than the width of the silicon blocks to be ground. This makes it possible to use grinding cups with a diameter of 200 mm or greater to process silicon blocks of various widths, e.g., to process 5-inch, 6-inch or 8-inch silicon blocks, in order to cover the entire width of the silicon block in every processing step. Material is removed from the silicon block in a material-removing and as gentle a manner as possible, in particular using a grinding tool, with a large diameter per the present invention, e.g., 200 mm to 350 mm, and with a high rotational speed, typically in the range of at least 1500, with at least 1800 or 2000 revolutions per minute being preferred. Typical advantageous maximum rotational speeds are, in particular, a maximum of 6000 revolutions per minute, and typically a maximum of 5000 revolutions per minute, with a maximum of 4000 revolutions per minute being preferred. Particularly preferred are maximum revolutions of 3500, in particular 3100 revolutions per minute, and rotational speeds of up to 3000 revolutions per minute are very particularly preferred. The inventive application of a large grinding cup therefore ensures that material will be removed quickly and that surface damage will be minimized.

Advantageously, the silicon block in the inventive method is essentially rectangular in shape. There are no limitations on the shape of the silicon block, however. As a result, the inventive method may also be used with silicon blocks that have a different shape. Advantageously, the silicon block has a square cross-sectional or main surface in the range between 220×220 mm² and 100×100 mm², preferably in the range between 125×125 mm² (5 inches), 156×156 mm² (6 inches) and 210×210 mm² (8 inches). Typical heights and lengths are at least 150 mm, in particular up to 600 mm, in particular up to 500 mm, and preferably a minimum of 200 mm and a maximum of 450 mm.

A thickness of at least 20 μm is ground off of the lateral surfaces of the silicon block or brick, and a minimum of 25 μm or 30 μm is particularly preferred. Minimal abrading thickness of at least 50 μm have proven to be extremely suitable, in particular at least 150 μm, with at least 200 μm being particularly preferable. For economic reasons, maximum abrading thicknesses of up to 500 μm, and in particular up to 400 μm have proven to be suitable, with a maximum of 300 μm being preferred. Abrading thicknesses of up to 280 μm and 250 μm have proven to be very particularly preferred.

A further subject of the present invention is a method for manufacturing particularly thin silicon wafers by separating a silicon block or brick, in which case the lateral surfaces of the silicon block or brick are ground and/or polished—in the sense of the present invention—essentially parallel to the outer surface of the eventual wafer edge (cross-sectional edge of the brick and/or cutting plane) of the silicon block, and the silicon block is then cut into wafers parallel to the cross-sectional area of the brick (cutting plane) and/or perpendicularly to its longitudinal axis.

The lateral surfaces of the silicon brick are preferably ground or polished essentially parallel to the eventual cutting plane, in two steps. In a first step, a grinding tool is used that includes abrasive grains with a mean diameter that is greater than 70 μm, preferably greater than 80 μm, and in particular greater than 90 μm. In a further abrasive treatment, a grinding tool is used that includes abrasive grains with a mean diameter that is preferably less than 30 μm, in particular less than 20 μm, and very particularly preferably less than 15 μm. This second processing step may be carried out by selecting a slower infeed rate, so that the penetration depth of the individual grinding grain does not exceed 40 nm or 30 nm. As a result, the material is removed in the ductile region, and a surface quality is attained that corresponds to the quality criterium for polished surfaces. The grinding of the silicon block in two steps may take place by using grinding cups with abrasive grains having different diameters, the grinding cups being moved across the lateral surfaces of the silicon block in succession.

A further subject of the present invention, therefore, is a method for manufacturing silicon wafers, with which

• in a step a1)
  the lateral surfaces of the silicon block are ground with a coarse-grained, hollow-cylindrical tool (a grinding cup). The tool preferably contains cutting and/or abrasive grains with a mean diameter that is greater than 80 μm, in particular greater than 90 μm, and up to 160 μm, and
• in a step a2)
  grinding or polishing is carried out with a fine-grained, hollow-cylindrical tool that includes grinding and/or cutting grains with a mean diameter than is less than 30 μm, preferably less than 20 μm, and particularly preferably less than 15 μm, and typically has a minimum diameter of 3 μm.

In a further step b), the inventive etching is then carried out after step a1) or after steps a1)+a2). When the amount of material to be removed via etching is selected to be great enough, as described above, etching of the raw-sawed block is also suitable for preventing an eventual fracture of the wafer.

• In a step c)
  the silicon block is cut or sawed as described above.

If rough grinding is followed by fine grinding, it has proven particularly advantageous for the ratio of material removal of rough grinding to fine grinding to be at least 5:1, and preferably at least 8:1 or 9:1. The maximum ratio of rough grinding to fine grinding is 12:1, with 10:1 being preferred.

Within the framework of the present invention, a micro-fissure is understood to mean a separating gap created via mechanical damage to the material, in which parts of a solid that was previously one piece are in contact with each other. The cross section of a fissure usually has a sharp tip.

When a hollow-cylindrical cutting or polishing tool that rotates around its hollow-cylindrical axis is used, then—in a design that is very particularly preferred according to the present invention—the rotation axis is tilted slightly relative to the surface normal to be ground, i.e., relative to the normal that is oriented at a right angle to the surface to be treated, and, in fact, preferably in the direction of the longitudinal axis of the brick. The rotation axis may be tilted toward the front or the rear. Typical maximum inclination angles are 0.1 or 0.07 angular degrees, with a maximum of 0.05 angular degrees being preferred, and a maximum of 0.04 angular degrees being particulary preferred. Minimal inclination angles are typically 0.001 or, in particular, 0.003 angular degrees, with a minimum of 0.005 angular degrees being preferred, and a minimum of 0.008 angular degrees being particulary preferred. According to the present invention, it was also discovered that, by tilting the angle of rotation, the particularly sensitive longitudinal edges of the bricks are subjected to less stress, since the grinding and polishing pressure applied by the tool is reduced there. In addition, the inclination (tilt) prevents the side of the tool that is opposite—by 180°—to the side that is engaged with the material from coming in contact with the surface to be processed. Material-engagement states that fluctuate and are therefore undefined are thereby prevented.

When silicon wafers manufactured according to one of the aforementioned methods are used to make solar cells, the yield of the solar cells is increased, since the fracture rate of the silicon wafers is markedly lower. The wafers obtained according to the present invention preferably have a thickness of less than 230 μm, in particular less than or equal to 210 μm, preferably less than 200 μm, in particular less than or equal to 180 μm, with thickness of less than 170 μm, in particular less than or equal to 150 μm or less than or equal to 120 μm being particularly preferred.

Advantageously, the silicon block or brick is cut into silicon wafers using a wire saw, as described, e.g., in EP 1 674 558 A1.

Silicon wafers that were manufactured using one of the aforementioned methods may be used in conventional methods known from the related art to produce solar cells and/or solar modules.

Advantageously, the silicon block in the inventive method is essentially rectangular in shape. There are no limitations on the shape of the silicon block, however. As a result, the inventive method may also be used with silicon blocks that have a different shape. Advantageously, the silicon block has a square cross-sectional or main surface in the range between 220×220 mm² and 100×100 mm², preferably in the range between 125×125 mm² (5 inches), 156×156 mm² (6 inches) and 210×210 mm² (8 inches). Typical heights and lengths are at least 150 mm, in particular up to 600 mm, in particular up to 500 mm, and preferably a minimum of 200 mm and a maximum of 450 mm.

If only one raw-sawed quartz block is etched using the inventive method, it has proven advantageous for the mean amount of material removed via etching to be at least 25 um, with at least 30 um, and, in particular, at least 35 pm being particularly preferred. For all cracks to be removed completely, a mean material removal must be realized that is at least the sum of R_max of the roughness profile plus the length of the longest crack that extends from the surface into the material. In this case, it is recommended to select a mean amount of material removal that is markedly greater than the sum, since etching also deepens existing cracks.

As described above, it was also discovered according to the present invention that increasing the surface roughness does not result in any disadvantageous fracture properties. According to the present invention, the surface roughness defined by the unevenness in the surface created by etching may be very great—as stated above—provided that the width of the particular wave troughs and/or roughness troughs is great, and that their deepest points are rounded off.

According to the present invention, material may therefore be easily removed up to large etching depths of, e.g., 100 μm or 80 μm, although a maximum mean material removal of 70 μm is typically preferred. A particularly advantageous mean material removal of a surface that has been sawed but not treated further is a maximum of 50 μm or 40 μm.

If a silicon block pretreated with coarse grain is etched, minimum mean etching depths of at least 5 μm, in particular of at least 7 μm, and preferably of at least 8 μm have proven advantageous. Mean minimal etching depths of 10 μm are particularly preferred. The maximum mean etching depth in coarsely pre-ground materials is typically 50 μm, with a maximum of 40 μm being preferred, and a maximum of 30 μm being particularly preferred. In cases such as these, etching depths of up to 25 μm are very particularly preferred.

If a silicon block is etched that was ground and/or polished coarsely at first and then finely, minimum mean etching depths of at least 3 μm, and, in particular, of at least 4 μm have proven reasonable, with at least 5 μm and, in particular, at least 7 μm having proven to be particularly advantageous. Minimum etching depths of 8 μm are very particularly preferred. With silicon blocks that have been pretreated in this manner, a maximum thickness of material removal via etching of up to 30 μm has proven reasonable, with a maximum of 25 μm being preferred, and a maximum of 20 μm being particulary preferred. Maximum material-removal depths of up to 18 μm, and, in particular, up to 16 μm, are very particularly preferred.

It has been shown, however, that, for economic reasons, an optimal duration of etching at an etching temperature and with material removal via etching is advantageously at least 1 minute, and, in particular, at least 2 minutes. A lower limit of 3 minutes has proven to be particularly advantageous. Maximum etching durations are typically 25 minutes, with 20 minutes being preferred, and 15 or 12 minutes being very particularly preferred.

Claims

1. A method for manufacturing silicon wafers by separating silicon blocks, with which material is removed on at least one lateral surface of the silicon block via etching,

wherein

the mean amount of material removed is 3 μm to 160 μm, and material is removed isotropically with a constant mean material-removal speed of 1 μm to 20 μm per minute across the entire lateral surface.

2. The method as recited in claim 1,

wherein

the etching is carried out using a highly oxidative acid or a mixture of such acids.

3. The method as recited in claim 1,

wherein

the mean amount of material removed is 3 μm to 15 μm per minute.

4. The method as recited in claim 1,

wherein,

the silicon block is etched for 1 to 25 minutes.

5. The method as recited in claim 1,

wherein

a constant mean amount of material removed from the lateral surfaces is 3 μm to 150 μm.

6. The method as recited in claim 1,

wherein

the silicon block is etched at a temperature of 20 ° C. to 25° C.

7. The method as recited in claim 5,

wherein

the temperature of the etching medium fluctuates in the range of ±1° C. at the most.

8. The method as recited in claim 1,

wherein

the silicon block is etched with a solution of 50-70% nitric acid and 40-60% hydrofluoric acid in the range of 8:1 to 4:1.

9. The method as recited in claim 7,

wherein

the solution contains at least one further fluid that is selected from the group composed of water, acetic acid, and hydrogen peroxide.

10. The method as recited in claim 1,

wherein

the ratio of surface roughness Rmax of the lateral surface after the etching process to the surface roughness Rmax of the lateral surface before the etching process is 10:1 to 0.5:1.

11. The method as recited in claim 1,

wherein

the silicon block is subjected to an abrasive grinding or polishing treatment before etching is carried out.

12. A crack-free silicon block that is obtained using the method as recited in claim 1.

13. A wafer that is free of micro-fissures and has improved resistance to fracture that is obtained using the method as recited in claim 1.