WO2012020274A1

WO2012020274A1 - Process and apparatus for texturizing a flat semiconductor substrate

Info

Publication number: WO2012020274A1
Application number: PCT/IB2010/001980
Authority: WO
Inventors: Eckhardt Wefringhaus; Steffen Queisser; Denise Nagel
Original assignee: Rena Gmbh
Priority date: 2010-08-10
Filing date: 2010-08-10
Publication date: 2012-02-16
Also published as: TW201207928A

Abstract

Process for texturizing a flat semiconductor substrate (1), comprising arranging the semiconductor substrate (1) in such a way that an underside (2) to be texturized is being orientated facing down and contacted from below by a texturization solution (5) during a texturization time, providing an etching solution (7; 47) on a top side (3) of the semiconductor substrate (1) in order to etch off semiconductor material from the top side (3) of the semiconductor substrate (1) during an etching time, and choosing the etching solution (7) and the etching time such that less semiconductor material is etched off from the top side (3) than from the underside (2) of the semiconductor substrate (1), as well as apparatus for carrying out this method.

Description

Process and apparatus for texturizing a flat semiconductor substrate

The invention relates to a process for texturizing a flat semiconductor substrate according to the preamble of claim 1 and to an apparatus for carrying out this process. So far, flat semiconductor substrates like silicon wafers have been texturized on an industrial scale by immersing the substrate completely into a texturization solution, as it is described for example in WO 2004/100244 Al . In doing so, both sides of the flat substrate are etched and in this way textur^¬ ized roughly to the same extent. For that reason, it is some^¬ times, referred to this kind of texturization as symmetrical texturization .

Recently, interest in texturization methods for texturizing the two sides of a flat substrate to a significantly different extent has increased, particularly with respect to the production of silicon solar cells. Such texturizations are sometimes called single-side or asymmetrical texturizations. There exist several reasons for the increased interest in such asymmetrical texturizations. For example, latest types of solar cells require single side polished wafers^', which can be manufactured more easily and in a more efficient way, if the side of the wafer to be. polished is not or only slightly texturized. With respect to the use of more and more thinner wafers, it is moreover advantageous to reduce the amount of wafer material etched off during texturization. Furthermore, the consumption of chemicals can be reduced and lifetime of the texturization solution can be enhanced. Known methods for producing asymmetrical or single side tex- turizations are the dicing of V-shaped grooves into the substrates, as described for example by G. Willeke, H. Nussbaumer et al. in Solar Energy Materials and Solar Cells 26 (1992), pp. 345 - 356, or by plasma texturization described for example by Hendrick F. . Dekkers in Study and Optimization of dry process technologies for thin crystalline silicon solar cell manufacturing, dissertation Catholic University Leuven 2008. These methods are, however, elaborate and expensive . Moreover, ^' plasma texturization is a batch process and therefore requires additional handling steps and expensive handling devices.

Moreover, said additional handling steps might result in an increased rate of breakage of the semiconductor substrates. For that reasons, said methods are not used on an industrial scale .

WO 2005/093788 Al discloses a method for the treatment of substrate surfaces, according to which an underside of a silicon wafer is .lowered into a solution and horizontally transported through the solution, whereas a top side of the silicon wafer -is not covered by the solution. One might consider replacing said solution by a texturization solution in order to provide an asymmetrically texturized substrate. However, it has turned out that in doing so the substrate, or wafer respectively and_ particularly an as-cut-wafer, is deformed, so that it shows a bow. Such a bow increases the rate of substrate breakage during subsequent processing steps like, for example, cleaning or handling steps. Furthermore, the described bow of the substrate or wafer can cause undesirable inhomogeneities in several processing steps like, for example, cleaning steps.

Against the background described above, the present invention is based on the problem to provide a low-complexity process for asymmetrically texturizing a flat semiconductor substrate that at least partly avoids a bow of the semiconductor substrate.

This problem is solved by a process having the features of claim 1 or of claim 14.

Moreover, the present invention is based on the problem to provide an apparatus for carrying out the process according to the invention.

This problem is solved by an apparatus having the features of claim 27 or of claim 32.

In each case, dependent claims relate to advantageous developments .

The process according to the invention is based on the concept of arranging the semiconductor substrate in such a way that an underside to be texturized is being orientated facing down and contact from below by a texturization solution during a tex- ' turization time and providing an etching solution on a top side of the semiconductor substrate in order to etch off semiconductor material from the top side of the semiconductor substrate during an etching time. The etching solution and the etching time are chosen such that less semiconductor material is etched of from the top side than from the underside of the semiconductor substrate.

For the purposes of the present invention, underside and top side of the semiconductor substrate, in short referred to as substrate, mean those sides of the flat semiconductor substrate, which have the largest areas. The underside of the semiconductor substrate, which is orientated facing down, is the one that shall be texturized. In case of a solar cell sub- strate, for example a silicon wafer, that underside usually corresponds to a front side of the solar cell.

Texturization time starts when the underside^' of the substrate is brought into contact with the texturization solution and ends when this contact is disrupted, for example due to removal of the texturization solution from the underside of the substrate by rinsing the substrate in^" water. Etching- time starts when the top side of the substrate gets into contact with the etching solution and ends when this contact is disrupted, for example by removal of the etching solution from the top side of the substrate.

The process according to the invention is a wet chemical process and therefore less elaborate than plasma processes or dicing processes. Furthermore, it can be easily integrated ^"into widely used industrial inline production processes. Well known texturization solutions that have been used so far for symmetrical texturization of substrates can also be used for the process according to the invention. It has turned out that a bending of the semiconductor substrate can be avoided by etching off semiconductor material from the top side although less material is etched of from the topside than from the underside of the semiconductor substrate. Therefore, the process according to the invention makes it possible, to provide asymmetrically texturized semiconductor substrates without bow.

Surprisingly, it has been found that, as compared to the amount of material etched off from the underside by the texturization solution, only a small amount of semiconductor material needs to be etched off from the top side of the substrate in order to prevent, or at least significantly^; reduce, a bow of the texturized substrate. In an advantageous embodiment of the process according to the invention therefore less than 50%, preferably less than 20%, of the amount of semiconductor material etched off from the underside of the semiconductor substrate is etched off from the top side of the semiconductor substrate.

In practice, it has proven advantageous to etch off a layer having a thickness of less than 10 μπι, preferably of less than 3 μιη and especially preferred of less than 1 μπι, from the top side of the semiconductor substrate.

In a preferred embodiment of the process according to the invention the semiconductor substrate is arranged at the surface of the texturization solution such that the texturization solution is prevented from contacting the top side of the semi- conductor substrate. WO 2005/093788 Al illustrates how this might be realized. The substrate not necessarily needs to be arranged at the surface of the texturization solution in the described way during all of the texturization time. In one embodiment of the process according to the invention the etching solution is provided on the top side of the semiconductor substrate by temporarily arranging said top side below the surface of the texturization solution during a part of the etching time. For the purposes of the present invention the top side is below the surface, if the substrate is arranged in the texturization solution such that the top side is completely wetted by the texturization solution. This can for example be realized by dipping the substrate completely into the texturization solution. In this embodiment the texturiza- ^' tion solution is simultaneously used as etching solution.

In an alternative embodiment of the process according to the invention the etching solution is sprayed onto the top side the semiconductor substrate. In this way etching solutions different from the texturization solution can comfortably be arranged on the top side of the semiconductor substrate. Such etching solutions can therefore be easily adapted to the required etching time and the amount of semiconductor material that shall be etched off during the etching time. Nevertheless, in this and in all other embodiments of the invention the etching solution can be of the same kind as the texturiza tion solution. Compositions of these solutions might be the same or differ. In particular, a diluted texturization solu- tion might be used as etching solution. Solutions containing HF, HN0₃ and H₂0, for example, can be used as texturization so lutions .

In principle, etching time can start after the end of the tex turization time or texturization time can begin after the end of the etching time.

In an advantageous embodiment of the process according to the invention the etching solution is removed from the top side o the semiconductor substrate by the end of the etching time, preferably by blowing it off said top side of the semiconductor substrate. In this way etching of the top side can be com^¬ fortably stopped at a suitable point of time, particularly be^¬ fore the end of the texturization time, i.e. when the underside of the semiconductor substrate is still contacted by the texturization solution. Blowing off the etching solution can in principle be realized by means of any known blow off device, particularly by means of one or more so called air knives, whereas the used gas not necessarily needs to be air. Also other gases or gas mixtures can be used.

In a preferred embodiment of the process according to the in- vention the etching solution is provided on the top side of the semiconductor substrate within a first half of the tex- turization time, preferably within a first fifth of the texturization time, and removed from said top side before the end of the texturization time. In this way semiconductor material is etched off from the top side of the semiconductor substrate quite shortly after beginning of the texturization time, so that bending of the semiconductor substrate, in short referred to as substrate, during the texturization time can be avoided. Consequently, the substrates have no bow during the- texturization time and can be homogeneously texturized. In an especially preferred embodiment the etching solution is provided on the top side of the semiconductor substrate at the beginning of the texturization time, so that etching and texturization time begin simultaneously.

Removal of the etching solution can, for example, be realized by blowing it off the top side as described above. Advantageously, the etching solution is removed within the first half of the texturization time, preferably within' the first fifth of the texturization time.

If an etching solution with a smaller etching rate than the texturization solution is used, the etching solution can in principle also remain on the top side of the substrate until the end of the texturization time. However, the usage of an etching solution that differs from the texturization solution increases complexity of the process and might cause, contamination of the texturization solution by etching solution sprayed in or blown into the texturization solution. If the etching rate of the etching solution decreases during the etching time, e. g. because the etching solution is depleted during the etching time, the same kind of solution might be used as _. texturization solution and as etching solution and the etching solution might, nevertheless, remain on the top side of- the substrate until the end of the texurization time. In another embodiment of the process according to the invention the etching solution is provided on the top side of the semiconductor substrate before the end of the texturization time and removed together with the texturization solution at the end of the texturization time. Preferably, the etching solution is provided on the top side of the substrate within the last half of the texturization time, and especially preferred it is provided within 'the last fifth of the texturization time. Simultaneous removal of etching solution and texturization solution is preferably realized by rinsing the semiconductor substrate in water. In this way complexity of the process can be further reduced, as no separate step for removal of the etching solution is required. However, the substrates might become deformed during the texturization time before the etching solution is provided on the top side. After the etching solution is provided on the top side and a sufficient amount of semiconductor material is etched off from the top side of the substrate, the possibly previously formed bow diminishes again. But the bow existing temporarily during the texturization time might in certain cases already have caused an inhomogeneous texturization on the underside of the substrate.

In practice, it has proven useful, if the underside of the substrate is^' contacted by the texturization solution during all of the etching time.

According to a preferred embodiment of the process according to the invention the semiconductor substrate is transported horizontally through a basin filled with texturization solution during the texturization time. For the purposes of the present invention a filled basin shall not be understood in such that the basin necessarily needs to be completely filled, although this might be the case.

According to an improvement of the process according to the invention the top side of the semiconductor substrate is single side etched in a chemical polishing solution after expiration of the etching time and the texturization time. As explained above, an only slightly texturized surface of the semiconductor substrate can be polished less elaborately.

Therefore, this improvement results in a low-complexity process for providing a flat ^'semiconductor substrate with one side polished and the other side texturized as it is required for the production of latest types of solar cells.

It has turned out that it is not mandatory to texturize the underside of the semiconductor substrate in order to solve the problem mentioned above. Instead, also the top side can be texturized. In an alternative form of the process according to the invention therefore the semiconductor substrate is arranged in such a way that an underside is being orientated facing down and contacted from below, by an etching solution during an etching time. A texturization solution is provided on a top side of the semiconductor substrate . in order to texturize the top side of the semiconductor substrate during a texturization time. The etching solution and the etching time are chosen such that less semiconductor material is etched off from the underside than from the top side of the semiconductor substrate.

In case of a solar cell, in this form of the process according to the invention the top side of the semiconductor substrate corresponds to a front ^"side of the solar cell. The texturization time starts when the top side of the substrate is brought into contact^' with the texturization solution and ends when this contact is disrupted. Etching time starts when the under^¬ side of the substrate gets into contact with the etching solu^¬ tion and ends when this contact is disrupted.

The additional form of the process according to the invention offers the same advantages as those explained above. In particular, asymmetrically texturized semiconductor substrates without bow can be provided.

In an advantageous embodiment of the process according to the invention less than 50%, preferably less than 20%, of the amount of semiconductor material etched off from the top side of the semiconductor substrate is etched off from the underside of the semiconductor substrate.

In practice, it has in this case proven advantageous to etch off a layer having a thickness of less than 10_. μπι, preferably of less than 3 μπι and especially preferred of less than 1 μηα, from the underside of the semiconductor substrate.

In a preferred embodiment the semiconductor substrate is arranged at a surface of the etching solution such that the etching solution is prevented from contacting the top side of the semiconductor substrate. Again WO 2005/093788 Al illustrates how this might be realized. The substrate not necessarily needs to be arranged at the surface of the etching, solution in the described way during all of the etching time.

In one embodiment of the additional form of the process according to the invention a texturization solution is used as etching solution. The texturization solution is then provided on the top side of the semiconductor substrate by temporarily arranging said top side below the surface of the etching solu^¬ tion during a part of the texturization time. As analogously explained above, the top side is below the surface, if the substrate is arranged in the etching solution such that the top side is completely wetted by the etching solution.

In an alternative embodiment the texturization solution is sprayed onto the top side of the semiconductor substrate. In this way, texturization solutions different from the etching solution can comfortably be arranged on the top side of the semiconductor substrate. Nevertheless, in this and in all other embodiments the etching solution can be of the same kind as the texturization solution. Compositions of these solutions might be the same or differ.

Again, etching time can in principle start after the end of the texturization time or texturization time can begin after the end of the etching time.

Advantageously, the texturization solution is removed from the top side of the* semiconductor substrate by .the end of the etching time, preferably again by blowing it off said top side of the semiconductor substrate. As discussed above, blowing off the texturization solution can in principle be realized by means of any known blow off device, particularly by means of one or more air knives, whereas the used gas not necessarily needs to be air. Also other gases or gas mixtures can be used.

In a preferred embodiment the texturization solution is provided on the top side of the semiconductor substrate within a first half of the etching time, preferably within a first fifth of the etching time. In an especially preferred embodiment the texturization solution is provided on the top side of the semiconductor substrate at the beginning of the etching time, so that etching and , texturization time begin . simultaneously. In doing so, the substrates have no bow during the tex- turization time and can be homogeneously texturized. The texturization solution is preferably removed from the top side before the end of the etching time.

In another embodiment the texturization solution is provided on the top side of the semiconductor substrate before the beginning of the etching time and removed together with the etching solution at . the end of the etching time. Simultaneous removal of etching solution and texturization solution is preferably realized by rinsing the . semiconductor substrate in water. In this way complexity of the process can be further reduced, as no separate step for removal of the texturization solution is required. However, the substrates might become deformed during the texturization time before the etching solution is brought into contact with the underside of the substrate. After the etching solution is brought into contact with said underside and a sufficient amount of semiconductor material is etched off from said underside, the possibly previously formed bow diminishes again. But the bow existing temporarily during the texturization time might in certain cases already have caused an inhomogeneous texturization on the top side of the substrate.

In practice, it might be useful, if the underside of the substrate is contacted by the etching solution during all of the^' texturization time.

Preferably, the semiconductor substrate is transported horizontally through a^' basin filled with etching solution during the etching time. in the alternative form of the process according to the invention the underside of the semiconductor substrate is single side etched in a chemical polishing solution after expiration of the etching time and the texturization time. As explained above, this results in a low-complexity process for providing a flat semiconductor substrate with one side polished and the other side texturized as it is required for the production of latest types of solar cells.

An apparatus according to the invention comprises a basin filled with a first etching solution, a bearing device for arranging a semiconductor substrate such that ' an underside of the semiconductor substrate is orientated facing down and contacted from below by the first etching solution. Furthermore, it comprises a spraying device for spraying a second etching solution onto a top side of the^' semiconductor substrate. For the .purposes of the present invention a^'" filled basin does not necessarily mean that the basin is completely filled with the first etching solution. A partly filled basin is, in principle, sufficient.

In a first embodiment of said apparatus a texturization solu- tion is provided as_. first etching solution. In this way the apparatus can be used for performing the form of the process according to the invention, wherein the underside of the substrate is to be texturized. In a second embodiment of said apparatus a texturization solution is provided as second etching solution.. Such an apparatus can be used for performing the alternative form of the process according to the invention, wherein the top side of the substrate is texturized.

Preferably, the same kind of texturization solution is provided as first etching solution and as second etching solu- ^■ tion. Especially preferred a with regard to its composition identical texturization solution is provided as first etching solution and as second etching solution.

The spraying device makes it possible to provide the second etching solution on the top side of the substrate at a suitable point .of time. Preferably, the spraying device is arranged above a surface of the first etching solution.

Advantageously, a horizontal transport device for transporting the semiconductor substrate horizontally through the basin is provided as bearing device. Such · a horizontal transport device might for example be formed by an appropriately arranged set of transport rollers .

In one embodiment of the apparatus according to the invention the spraying device is arranged in an exit area of the basin. For the purposes of the present invention an exit area of the basin means an area where the transport of the semiconductor substrate through the basin ends and includes also a region above the basin.

In a preferred alternative embodiment the spraying device is arranged in an entrance area of the basin, so that the second etching solution can be sprayed onto the top side of the substrate quite shortly after the underside of the substrate is contacted from below by the first etching solution; i.e. at the beginning of the texturization or etching time, or within the first half of the texturization or etching time or preferably within the first fifth of the texturization or etching time. For the purposes of the present invention entrance area of the basin means an area where the transport of the semiconductor substrate through the basin starts and includes also a region above the basin. An advantageous embodiment of the apparatus further comprises a blow-off device for blowing off the sprayed-on second etching solution from the top side of the semiconductor substrate. This makes it possible to remove the second etching solution from the substrate easily and to set comfortably the etching time, or texturization time respectively. In principle, any blow-off device that is appropriate for the specific application can be used. Particularly, one or more so called air knives can be provided as blow-off device. Air is preferably used with the blow-off device as medium for blowing off the second etching solution. However, also other gases or gas mixtures can be used in principle.

The blow-off device is advantageously arranged downstream of the spraying device in a transport direction of the transport device .

A further- apparatus for carrying out the process according to the invention comprises a basin- filled with a texturization solution, and a bearing device for arranging a semiconductor substrate such that an underside of the semiconductor substrate is orientated facing down and contacted from below by the texturization solution. The bearing device is set up to lower the semiconductor substrate into the texturization solution in such a way that a top side of the semiconductor substrate is temporarily located below the surface of the texturization solution. Furthermore, a blow-off device for subsequently blowing off the texturization solution from the top side of the semiconductor substrate is provided. For the pur-

I

poses of the present invention the top side is regarded as being located below the surface of the texturization solution, if the substrate is lowered into the texturization solution in such a way that the top side is completely wetted by the texturization solution. In analogy to the above-described embodiments having a spraying device, the bearing device can be either set up to lower the substrate into the texturization solution in an exit area of the basin or to lower them into the texturization solution in an entrance area of the basin.

Preferably, a blow-off device is provided for blowing off the texturization solution from the top side of the substrate. Again, one or more air knives might be provided as blow-off device. This is particularly advantageous, if the bearing device is set up to lower the substrate into the texturization solution in the entrance area of the basin. As medium of the blow-off device gases or gas mixtures described above can be used.

In an advantageous embodiment the bearing device is realiz by means of transport rollers. Preferably, some of those transport rollers are set up to transport the semiconducto substrate horizontally through a part of the basin.

The invent 1on will be explained in more detail below with ref- erence to figures . Elements exercising essentially similar ef- fects are, as far as this appears appropriate, marked with equal reference signs . The figures show:.

Fig. 1 Schematic illustration of a first embodiment of the process according to the invention and of a first embodiment of an apparatus according to the invention

Fig. 2 Schematic illustration of a second embodiment of the process according to the invention and of a second embodiment of an apparatus according to the invention

Fig. 3 Schematic illustration of a third embodiment of the process according to the invention and of a third embodiment of an apparatus according to the invention

Figure 1 schematically illustrates a first embodiment of a process as well as of an apparatus according to the invention. The apparatus comprises a basin 4 filled with a texturization solution 5. In this embodiment said texturization solution is provided as first etching solution. Transport rollers 9 are -provided as a bearing device for semiconductor substrates 1. Said transport rollers 9 are arranged in a horizontal plane and in this way form a horizontal transport device for trans- porting the semiconductor substrates 1, in short referred to as substrates 1, horizontally through the basin 4. The transport rollers 9 are arranged such that undersides 2 of the substrates 1 are orientated facing down and contacted from below by the texturization solution 5. As Figure 1 schematically il- lustrates, the transport rollers 9 are furthermore positioned such that the substrates 1 are arranged at a surface 6 of the texturization solution 5 such that the texturization solution ^' 5 is prevented from contacting a top side 3 of the substrates 1.

In an entrance area 13 of the basin 4 a spraying device 11 for spraying an etching solution 7 onto the top sides 3 of the substrates 1 is provided. In the present embodiment, said etching solution 7 forms the second etching solution. As de- picted in Figure 1, the spraying device is arranged above the surface 6 of the texturization solution. Furthermore, an air knife 15 is provided for blowing off the sprayed-on etching solution 7 from the substrates 1. Said air knife 15 uses a gas 16 as medium for blowing off the etching solution 7. In principle, any gas 16 or gas mixture 16 suit- able for the specific application might be used as medium. Preferably, air is used.

As schematically depicted in Figure 1, the substrates 1 are arranged in such a way that their undersides 2 are orientated facing down and contacted from below by a texturization solution 5. This is realized by means of the transport rollers 9. The undersides 2 of the substrates 1 are texturized by the texturization solution 5. Therefore, texturization starts as soon as the substrates enter the basin 4 and are brought into contact with the texturization solution 5. This is also the point of time when · the texturization time begins. Said texturization time lasts until the substrates l.exit from the basin 4 and the texturization solution is removed from their undersides 2, for example by rinsing them in water.

By means of the spraying device 11, the etching solution 7 is sprayed onto the top .sides 3 of the substrates in the entrance area 13 of the basin 4. At this point of time the etching time starts and the top sides 3 are etched. Later on, i.e. at the end of the etching time, the etching solution 7 is removed from the top sides 3 of the substrates 1 by blowing it off from the top sides 3 using the air knife 15. As the air knife 15 is arranged away from an exit of the basin 4, the etching solution 7 is removed from the top sides 3 before the end of the texturization time.

In the embodiment of Figure 1 the etching solution 7 is provided on the top sides 3 of the substrates 1 within a first fifth of the texturization time. The position of the air knife 15 can vary according to the etching time to be realized. In case of shorter etching times it would be arranged in Figure 1 more to the left, otherwise more to the right. Preferably, the same kind of solution used as texturization solution 5 is also used as etching solution 7.

In each case, however, spraying device 11 and air knife 15 are positioned such that the resulting etching time in combination with the used kind of etching solution ensures that less semi- conductor material is etched off from the top sides 3 than from the undersides 2 of the substrates 1.

If one replaces the texturization solution 5 in Fig. 1 by an etching solution 55 and the etching solution 7 a texturization solution 57, he arrives at the additional form of the process according to the invention. Therein, the underside 2 is contacted from below by the etching solution 55 during the etching time. Etching time starts as soon as the substrates enter the basin 4 and are brought into contact with the etching so- lution 55. Said etching time lasts until the substrates 1 exit from the basin 4 and the. etching solution 55 is removed from their undersides 2, for example by rinsing them in water. The texturization solution 57 is sprayed onto the top side 3 of the substrate 1 by means of the spraying device 11. At this point of time the texturization time starts and the top sides , 3 are texturi^'zed. Later on, i.e. at the end of the texturization time, the texturization solution 57 is removed from, the top sides 3 of the substrates 1 by blowing it off from the top sides 3 using the air knife 15. As the air knife 15 is ar- ranged away from an exit of the basin 4, the texturization solution 57 is removed from the top sides 3 before the end of the etching time. In this embodiment of the- additional form of the process according to the invention, spraying device 11 and air knife 15 are positioned such that the resulting texturiza- tion time in combination with the used etching solution 55 and the used texturization solution 55 ensures that less semiconductor material is etched off from the undersides 2 than from the top sides 3 of the substrates 1.

The embodiment of Figure 2 differs from the one of Figure 1 in that the spraying device 11 is arranged in an exit area 25 of the basin 4. No air knife is provided. Instead, a water basin 27 filled with water, further on referred to as rinsing water 29, is provided. When substrates 1 exit from the basin 4, a film 23 of texturization solution remains on their undersides 2. This film 23 is subsequently removed together with the etching solution 7 on the top sides 3 of the substrates 1 by rinsing them.- in the rinsing water 29. -Consequently, in the em-

(

bodiment of Figure 2 the etching solution 7 is provided on the top sides 3 of the wafers before the end of the texturization · time and etching time ends together with the texturization time when substrates 1 are rinsed in the rinsing water 29. The begin of the etching time is set by the position of the ^"spray- ing device 11. In the schematic illustration of Figure 2, which is not true to scale, the spraying device is arranged such that the etching solution 7 is provided within the last half or last fifth of the texturization time. For certain applications it can nevertheless be advantageous to provide at least one air knife in the exit area 25 in order to avoid tracking out of the etching solution 7 into the water basin 27.

In Figure 2 the etching solution 7 can, in principle, again be replaced by the texturization solution 57 and texturization solution 5 can again be replaced by etching solution 55 in order to provide a further embodiment of the additional form of the process according to the invention. In this connection it can be appropriate to move the spraying device 11 more to the ^■ left in order to lengthen the texturization time during which the texturization solution 57 texturizes the top sides 3 of the substrates 1. In Figure 2 also transport rollers 9 are provided as bearing device. The transport rollers 9 again form a horizontal plane and transport the substrates 1 horizontally through the basin 4. As compared to these transport rollers 9, there exist displaced transport rollers 30 for transporting the substrates 1 out of the basin. Further, transport rollers 33 are provided for transporting the substrates into the water basin 27, which comprises transport rollers 31 for transporting the substrates 1 through the water basin 27. Figure 3, which is like all other figures not true to scale, differs from Figure 1 in such that no spraying device is provided. Instead, a bearing device 9, 49 formed by means of transport rollers 9, 49 is set up to lower the substrates 1 into the texturization solution such that the top sides 3 of the substrates 1 are temporarily located below the surface 6 of the texturization solution.5.' This is realized by means of displaced transport rollers 49 which are displaced as compared to transport rollers 9 being arranged in a horizontal plane as explained above. Due to the displaced transport rollers 49, texturization solution that serves as etching solution 47 is provided on the top sides 3 of the substrates 1. Etching time starts when texturization solution 5 gets in contact with the top sides 3 of the substrates 1. In analogy with Figure 1, etching time ends when the texturization solution 47 used as etching solution is blown off from the top sides^■ 3 of the substrates by the air knife 15.

Apparently, the bearing device can be set up to lower the substrates 1 into the texturization solution by other means than displaced transport rollers 49, e.g. by lifting systems or conveyor belts. However, systems that are compatible with inline production are preferred. In the embodiment depicted in Figure 3 the displaced transport rollers 49 are arranged in an entrance area 43 of a basin 45 filled with texturization solution 5. Alternatively, the displaced transport rollers 49 could be arranged in an exit area 44 of the basin 45 and the air knife 15 could be removed. Fur- ther adding of a water basin as the one depicted in Figure 2 would result in an embodiment analogous to the one of Figure 2, but in which the spraying device 11 has been replaced by the displaced transport rollers 49 for lowering substrates 1 into the texturization solution 5 such that the top sides 3 of the substrates 1 are temporarily located below the surface 6 of the texturization solution 5.

The apparatus shown in Figure 4 can be used to perform such a process according to the invention, wherein the underside of the substrate is to be texturized and contacted from below by a texturization solution as described above. Furthermore, said apparatus can apparently be used to perform the additional form of the process according to the invention, wherein the top side of the substrate is texturized.

At the end of all processes described above a per se known single-side polishing etching step can be added for chemically polishing the top sides 3 of the substrates 1. Corresponding chemical polishing devices can be added to the apparatuses de- scribed above. Preferably, the polishing etching step is realized using a chemical polishing solution. List of reference numbers

1 semiconductor substrate

2 underside of semiconductor substrate

3 top side of semiconductor substrate

4 basin

5 texturization solution

6 surface of texturization solution

7 etching solution

9 transport roller

11 spraying device

13 entrance area

15 air knife

16 gas - -

23 film of texturization solution

25 exit area

27 water basin

29 rinsing water

30 displaced transport roller

31 transport roller of water basin

33 further transport roller

43 entrance area

44 exit area

45 basin ,

47 texturization solution serving as etching solution 49 displaced transport roller

55 etching solution

57 ^' texturization solution

Claims

Patent claims

1. Process for texturizing a flat semiconductor substrate (1), comprising

- arranging the semiconductor substrate (1) in such a way that an underside (2) to be texturized is being orientated facing down and contacted from below by a texturi- zation solution (5) during a texturization time,

c h a r a c t e r i z e d i n t h a t ,

- an etching solution (7; 47) is provided on a top side ^■

(3) of the semiconductor substrate (1) in order to etch off semiconductor material from the top side (3) of the semiconductor substrate (1) during an etching time, and - in that the etching solution (7; 47) and the etching

time are chosen such that less semiconductor material is etched off from the top side (3) than from the underside (2) of the semiconductor substrate (1).

2. Process according to claim 1, wherein

less than 50%, preferably less than 20%, of the amount of semiconductor material etched off from the underside (2) of the semiconductor substrate (1) is etched off from the top side (3) of the semiconductor substrate (1) .

3. Process according to any of the preceding claims, wherein a layer with a thickness of less than 10 μπι, preferably of less than 3 μπι and especially preferred of less than 1 pm, is etched off from the top side (3) of the semiconductor substrate ( 1 ) .

4. Process according to any of the preceding claims, wherein the semiconductor substrate (1) is arranged at the surface (6) of the texturization solution (5) such that the .tex- turization solution (5) is prevented from contacting the top side _.(3) of the semiconductor substrate (1) .

Process according to any of the preceding claims, wherein the etching solution (47) is provided on the top side (3) of the semiconductor substrate (1) by temporarily arranging said top side (3) below the surface (6) of the texturization solution (5) during a part of the etching time.

Process according to any of claims 1 to ^'4, wherein

the etching solution (7) is sprayed onto the top side (3) of the semiconductor substrate (1).

Process according to any of the preceding claims, wherein the etching solution (7; 47) is removed from the top side (3) of the semiconductor substrate (1) by the end of the etching time by blowing it off said top side (3) .

Process according to any of the preceding claims, wherein the etching solution (7; 47) is provided on the top side (3) of the semiconductor substrate within a first half of the texturization time and removed from said top side (3) before the end of the texturization time.

Process according to any of claims 1 to 7, wherein

the etching solution (7) is provided on the top side (3) of the semiconductor, substrate (1) before the end of the texturization time and removed together with the texturization solution (5) at the end of the texturization time, preferably by rinsing the semiconductor substrate (1) in water (29) .

10. Process according to any of the preceding claims, wherein the same kind of solution is used as texturization solution (5) and as etching solution (7; 47).

11. Process according to any of the preceding claims, wherein the underside (2) is contacted by the texturization- solution (5) during all of the etching time.

12. Process according to any of the preceding claims, wherein the semiconductor substrate (1) is transported horizontally through a basin (4; 45) filled with texturization solution (5) during the texturization time.

13. Process according to any of the preceding claims, wherein the top side (3) of the semiconductor substrate (1) is single-side etched in a chemical polishing solution after expiration of the etching time and the texturization time.

14. Process for texturizing a flat semiconductor substrate (1), comprising

- arranging the semiconductor substrate (1) in such a way that an underside (2) is being orientated facing down and contacted from below by an etching solution (55) during an etching time, .

c h a r a c t e r i z e d i n t h a t ,

- a texturization solution (57) is provided on a top side (3) of the semiconductor substrate (1) in order to tex- turize the top side (3) of the semiconductor substrate (1) during a texturization time, and

- in that the etching solution (55) and the etching time are chosen such that less semiconductor material is etched off from the underside (2) than from the top side (3) of the semiconductor substrate (1).

15. Process according to claim 14, wherein

less than 50%, preferably less than 20%, of the amount of semiconductor material etched off from the top side (3) of the semiconductor substrate (1) is etched off from the underside (2) of the semiconductor substrate (1).

16. Process according to any of claims 14 to 15, wherein

a layer with a thickness of less than 10 um, preferably of less than 3 um and especially preferred of less than 1 μπι, is etched off from the underside (2) of the semiconductor substrate ( 1 ) .

17. Process according to any of claims 14 to 16, wherein

the semiconductor substrate (1) is arranged at the surface (6) of the etching solution (55) such that the etching solution (55) is prevented from contacting the top side (3) of the semiconductor substrate (1) .

18. Process according to any of the preceding claims, wherein a texturization solution is used as etching solution and the texturization solution is provided on the top side (3) of the semiconductor substrate (1) by temporarily arranging said top side (3) below the surface (6) of the etching solution during- a part of the texturization time.

19. Process according to any of claims 14 to 17, wherein

the texturization solution (57; 47) is sprayed onto the top side (3) of the semiconductor substrate (1).

20. Process according to any of claims 14 to 19, wherein

the texturization solution (57) is removed from the top side (3) of the semiconductor substrate (1) by the end of the texturization time by blowing it off said top side (3).

21. Process according to any of the preceding . claims , wherein the texturization solution (57) is provided on the top side (3) of the semiconductor substrate within a first half of the etching time.

22. Process according to any of claims 14 to 20, wherein

the texturization solution is provided on the top side (3.) of the semiconductor substrate (1) before the beginning of the etching time and removed together with the etching solution at the end of the etching time, preferably by rins- ing the semiconductor substrate (1) in water (29).

23. Process according to any of claims 14 to 22, wherein

the same kind of solution is used as texturization solution

(57) and as etching solution (55).

24. Process according to any of claims 14 23, wherein

the underside (2) is contacted by the etching solution (55) during all of the texturization time.

25. Process according to any of claims 14 to 24, wherein

the semiconductor substrate (1) is transported horizontally through a basin (4; 45) filled with etching solution (55) during the etching time.

26. Process according to any of claims 14 to 25, wherein

the underside (2) of the semiconductor substrate (1) is single-side etched in a chemical polishing solution after expiration of the etching time and the texturization time.

27. Apparatus for carrying out the process according any of

claims 1 to 26, comprising - a basin (4; 45) filled with a first etching solution (5; 55),

- a bearing device. (9; 49) for arranging a semiconductor substrate (1) such that an underside (2) of the semiconductor substrate (1) is orientated facing down and contacted from below by the first etching solution (5; 55) , c h a r a c t e r i z e d b y

a spraying device (11) for spraying a second etching solution (7; 57) onto a top side (3) of the semiconductor substrate ( 1 ) .

28. Apparatus according to claim 27, wherein

a horizontal transport device (9) for transporting the semiconductor substrate (1) horizontally through the basin is provided as bearing device (9) .

29. Apparatus according claims 28, wherein

the spraying device is (11) arranged in an exit area (25) of the basin ( 4 ) .

30. Apparatus according to claim 28, wherein

the spraying device (11) is arranged in an entrance area (13) of the basin (4) .

31. Apparatus according to any of claims 27 to 30, further comprising

a blow-off device (15) for blowing off the sprayed-on second etching solution (7; 57) from the top side (3) of the semiconductor substrate (1).

32. Apparatus for carrying out the method according any of

claims 1 to 26, comprising

- a basin (45) filled with a texturization solution (5), - a bearing device (49) for arranging a semiconductor substrate (1) such that an underside (2) of the semiconductor substrate (1) is orientated facing down and contacted from below by the texturization solution ·( 5 ) , c h a r a c t e r i z e d i n t h a t

- the bearing device (49) is set up to lower the semiconductor substrate (1) into the texturization solution (5) such that a top side (3) of the semiconductor substrate (1) is temporarily located below the surface (6) of the texturization solution (5), and

- in that a blow-off device (15) for subsequently blowing off the texturization solution (47) from the top side (3) of the semiconductor substrate (1) is provided.