Method for producing a semiconductor wafer by means of epitaxial deposition, seed wafer and method for seed wafer production

Abstract

Method (1) for producing a semiconductor wafer (72) by means of epitaxial deposition (28) of semiconductor material (70) on a seed wafer (50), wherein a cast-mono semiconductor wafer (50) is used as said seed wafer, cast-mono semiconductor wafer and method for its production.

Description

[0001] NXWO O IWO 26 . 11 . 2025

[0002] 1

[0003] Method for producing a semiconductor wafer by means of epitaxial deposition , seed wafer and method for seed wafer production

[0004] Then invention relates to a method for producing a semiconductor wafer by means of epitaxial deposition and to an epitaxial semiconductor wafer produced by this method . Moreover, the invention relates to a seed wafer for use in the production of semiconductor wafers by means of epitaxial deposition of semiconductor material on the seed wafer and to a method for its production .

[0005] Epitaxially grown semiconductor wafers , in particular silicon wafers , show several advantages as compared to semiconductor wafers grown by conventional casting or pulling methods . Among others , epitaxially grown semiconductor wafers can be directly produced in the required thickness . There are no kerf losses and ultra-thin wafers can be produced reliably . As compared to wafers grown using the Czochralski ( CZ ) technique , semiconductor wafers with lower oxygen content can be produced . This is of particular relevance for wafers intended for photovoltaic applications , as the oxygen-related drop of solar cell ef ficiency in silicon solar cells can be reduced . For these reasons there is a great interest in semiconductor wafers produced by means of epitaxial deposition techniques l ike liquid phase epitaxy or epitaxial chemical vapour deposition .

[0006] Against this background, it is an obj ect of the present invention to reduce the ef forts required for producing an at least solar grade semiconductor wafer by means of epitaxial deposition . In this context , solar grade wafer means that the semiconductor wafer is suitable for producing solar cel ls of good quality . NXWO O IWO 26 . 11 . 2025

[0007] 2

[0008] This problem is solved by a method with the features of claim 1 .

[0009] It is a further obj ect of the invention to provide a seed wafer that is suitable for being used in the method for producing a semiconductor wafer .

[0010] This problem is solved by a semiconductor wafer with the features according to claim 20 .

[0011] A further obj ect of the invention is to provide a method for providing such a semiconductor wafer .

[0012] This problem is solved by a method having the features of claim 23 .

[0013] Moreover, it is an obj ect of the present invention to provide an epitaxial semiconductor wafer of desired quality that can be ef ficiently produced .

[0014] This problem is solved by the epitaxial semiconductor wafer having the features of claim 24 .

[0015] Advantageous improvements are in each case the obj ects of dependent subordinate claims .

[0016] The method according to the invention provides that , for the purpose of producing a semiconductor wafer by means of epitaxial deposition of semiconductor material on the seed wafer, a cast-mono semiconductor wafer is used as the seed wafer . A cast-mono wafer within the meaning of the present application means a wafer made out of cast-mono semiconductor material . NXWO O IWO 26 . 11 . 2025

[0017] 3

[0018] Such cast-mono semiconductor material is grown in a large crucible similar to multi-crystalline material like well-known multicrystalline silicon material . In contrast to the production of multicrystalline silicon material , however, monocrystalline seeds are placed at the bottom of the crucible . In doing so , semiconductor blocks can be produced which are essentially monocrystalline . As such cast-mono semiconductor material is not perfectly monocrystalline , it is sometimes also referred to as quasi-mono semiconductor material . The most commonly used cast-mono semiconductor material is cast-mono silicon which is sometimes simply called CM silicon or CM-Si .

[0019] So far, semiconductor wafers that have been grown using the CZ technique , so called CZ wafers , have been used as seed wafers . CZ wafers are perfectly monocrystalline and ensure a good quality of the semiconductor wafer that is epitaxially deposited on the CZ seed wafer and which will be called epitaxial semiconductor wafer, or in short words epitaxial wafer, in the following . The perfect crystallinity of the CZ seed wafer was considered an indispensable requirement for the production of an epitaxial wafer of good quality .

[0020] Up to now, it has been common opinion that CZ seed wafers cannot be replaced by seed wafers of lower crystal quality without deteriorating the epitaxial wafers ' quality . However, seed wafers are typically reused several times for the production of several epitaxial wafers . And it has turned out that oxygen contained in the CZ wafers , particularly in CZ silicon wafers , puts severe limits to the reuse of CZ wafers .

[0021] Within each cycle of reuse the seed wafer undergoes at least one high temperature step . During high temperature steps the oxygen contained in the seed wafer, and particularly in a silicon seed wafer, becomes mobile and forms so called oxygen NXWO O IWO 26 . 11 . 2025

[0022] 4 precipitates . And the si ze of oxygen precipitates increases with each cycle . This is the reason why the oxygen content of CZ wafers and resulting oxygen precipitates do not carry the same weight in cases where solar cells are produced directly from CZ wafers , in particular from CZ silicon wafers . In such production processes the CZ wafers undergo much les s high temperature steps than a seed wafer that is reused in many epitaxial deposition cycles . In case of the CZ seed wafers , instead, the growing oxygen precipitates can reach surface of the CZ seed wafer . When this happens , the oxygen precipitates deteriorate the crystallinity of epitaxial wafers grown on the CZ seed wafer surface . This finally leads to reduced minority charge carrier li fetimes in the grown epitaxial wafers .

[0023] Cast-mono semiconductor wafers , to which will be re ferred to in the following also as CM semiconductor wafers , typically contain less oxygen than CZ semiconductor wafers . The problem of growing oxygen precipitates described above is therefore less severe than in case of CZ seed wafers . This makes it possible to grow epitaxial wafers more ef ficiently . Moreover, the oxygen content varies signi ficantly less within CM semiconductor material than in CZ semiconductor material . Thi s is particularly true for silicon materials and is valid for the variation of oxygen content within an ingot and from ingot to ingot . Consequently, material losses which result from cutting of f ingot sections comprising too high oxygen contents or too strongly varying oxygen contents from an ingot can be reduced or even completely avoided by using CM semiconductor wafers as seed wafers . Moreover, homogeneity of minority charge carrier li fetimes within produced epitaxial wafers can be improved . Both ef fects can improve ef ficiency of the production of the epitaxial wafers . NXWO O IWO 26 . 11 . 2025

[0024] 5

[0025] Dopant is typically more homogeneously distributed over ingot length and width within CM semiconductor ingots than within CZ semiconductor ingots . In epitaxial deposition processes requirements for speci fic resistance of the used seed wafer need to be met . Therefore , the use of CM semiconductor seed wafers contributes to improve the process ef ficiency also in this respect .

[0026] The production of CM semiconductor wafers is less complex and consequently less costly than processes for producing CZ semiconductor wafers . This further improves the ef ficiency of the method according to the invention .

[0027] As less energy is required for the production of CM semiconductor wafers than for CZ semiconductor wafers , the use of CM semiconductor wafers as seed wafers may reduce the carbon footprint of the epitaxial wafer production process . With regard to wafer si zes , the production process of CM semiconductor wafers is much more flexible than the production process of CZ semiconductor wafers . Using CM seed wafers therefore offers more flexibility concerning the si ze of produced epitaxial semiconductor wafers .

[0028] In a preferred embodiment the CM silicon wafer is used as seed wafer and silicon is epitaxially deposited on the seed wafer . In doing so , an epitaxial silicon wafer is produced . In this embodiment the reduced oxygen content in the used seed wafer has proven particularly advantageous .

[0029] Preferably, the semiconductor material is deposited epitaxially on the seed wafer by means of chemical vapour deposition . In this case the epitaxial wafer is doped directly from a gas phase . Consequently, segregation ef fects in a melt cannot im- NXWOOIWO 26.11.2025

[0030] 6 pact the specific resistivity of the epitaxial wafers and epitaxial wafers with enhanced homogeneity in resistivity distribution can be realised. In principle, however, also other epitaxial deposition technologies may be applied.

[0031] In an advantageous embodiment the CM semiconductor wafer used as the seed wafer comprises a dopant concentration of at least 1.0 -IO18atoms per cm3. Preferably, the dopant concentration amounts to at least 3.0 -IO18atoms per cm3, and especially preferably to at least 1.0 -IO19atoms per cm3. The mentioned high dopant concentrations enable electrochemical porosifica- tion of the seed wafer surface. By means of electrochemical porosif ication porous semiconductor layers can be formed in a reliable and mass production capable way. The mentioned dopant concentrations can be efficiently realised by adding dopant, for example boron, during a casting process of the CM semiconductor wafer.

[0032] A p-type doped CM semiconductor wafer may be used as the seed wafer (50) , preferably a boron doped CM semiconductor wafer and especially preferably a boron doped CM silicon wafer. In this way the method is compatible with widely used solar cell materials .

[0033] In a preferred embodiment the seed wafer is prepared by performing a gettering step before depositing the semiconductor material on the seed wafer. Said gettering step forms a seed wafer preparation step. In this way metal impurities, which may be present in a larger amount in CM semiconductor material than in CZ semiconductor material, can be removed from the seed wafer. In particular, a phosphorus gettering step, in short referred to as P-gettering, or a boron gettering step, in short referred to as B-gettering, may be performed. Preferably, a phosphorous diffusion gettering step is chosen as the NXWOOIWO 26.11.2025

[0034] 7 gettering step. This kind of gettering has proven itself in connection with CM semiconductor material and is particularly advantageous in connection with CM silicon seed wafers. For the purpose of gettering CM silicon seed wafers, the phosphorous diffusion gettering step can be realised for example by means of an open-tube POCla-dif fusion for 30 to 90 minutes at a temperature of 780°C to 900°C followed by a 10 minutes drive- in phase.

[0035] Advantageously, a diffusion layer formed during the gettering step is at least partly removed. Preferably, it is completely removed. In this way impurities, like the mentioned metal impurities that diffused into the diffusion layer during the gettering step, can be removed. As a result, the seed wafer is purified.

[0036] The diffusion layer can be removed by means of etching. It is especially preferred to remove it by wet chemical etching, for example by means of so a called CP4 etching process using an aqueous solution comprising HNOa and HF.

[0037] In a preferred embodiment the following two seed wafer preparation steps are performed before depositing the semiconductor material on the seed wafer in order to prepare the seed wafer: As a first step, the seed wafer is locally molten by means of irradiating a first type laser beam. As a second and subsequent step, seed wafer material that has been previously locally molten by irradiating the first type laser beam is recrystallized. There is no need to separate the first step and the second step completely in time. This means that molten seed wafer material may already be recrystallized at one position, whereas at a different position the step of locally melting the seed wafer is still ongoing. NXWO O IWO 26 . 11 . 2025

[0038] 8

[0039] As explained above , CM seed wafers may not be perfectly monocrystalline . By means of the described seed wafer preparation steps of locally melting and recrystalli zing wafer parts crystal defects can be cured . For example , slightly misoriented grains can be cured which otherwise might cause recombination of minority charge carriers or accumulate metal impurities that cause recombination of minority charge carriers .

[0040] First type laser beams having a laser wavelength of 1000 nm or 1100 nm have proven themselves in connection with CM silicon seed wafers . Advantageously, the irradiated laser energy of the first type laser beam is chosen such that the seed wafer is molten up to a depth of at least 50 % of its thickness , preferably up to a depth of at least 70 % of its thickness , and especially preferably up to a depth of at least 90 % of its thickness . In this way it is possible to make use of the curing ef fects of the seed wafer preparation steps of locally melting and recrystalli zing seed wafer material for many cycles of reusing the seed wafer and consequently for many produced epitaxial wafers .

[0041] It is preferred to move the first type laser beam in such a way across a selected area of the seed wafer that a melting front and a recrystalli zation front are moved through a seed wafer volume located below the selected area . In doing so , the melting front and the recrystalli zation front are formed at a distance from each other . This makes it possible to ef ficiently cure crystal defects , like the slightly misoriented grains mentioned above , for example . In order to realise said melting and recrystalli zation fronts , the first type laser beam is moved at a suitable velocity and in a suitable path across the selected area . And the energy of the first type laser beam is suitably chosen . NXWO O IWO 26 . 11 . 2025

[0042] 9

[0043] In an advantageous embodiment , a closed recrystalli zation front is formed . Preferably, a recrystalli zation front having an elliptic shape is formed . Said recrystalli zation front is moved through the seed wafer volume in such a way that it becomes smaller . In this way it can be realised that the recrystalli zation front collapses in a single point . It has turned out that this approach enables an ef ficient curing of crystal defects . In praxi , it has proven advantageouss to move the closed recrystalli zation front in such a way through the seed wafer volume that it becomes continuously smaller . A closed melting front may be analogously formed, preferably a closed melting front having an elliptic shape .

[0044] The first type laser beam is preferably moved acros s the selected area in a spiral path . In this way a closed recrystalli zation front as well as a closed melting front can be realised with rather low ef forts .

[0045] An advantageous embodiment provides for locating a defect area comprising disturbing crystal defects within the seed wafer in a step ( a ) . In a step (b ) , the first type laser beam is moved across the selected area in such a way that the melting front and the recrystalli zation front are generated in an area being free from disturbing crystal defects and are subsequently moved across the defect area . As not all CM semiconductor wafers comprise disturbing crystal defects , this makes it possible to avoid unnecessary seed wafer preparation steps and to increase the ef ficiency of the method for producing epitaxial wafers . Also in cases where the disturbing crystal defects are not evenly distributed within the seed wafer, this embodiment can improve ef ficiency, as areas that are free from disturbing crystal defects are not locally molten and recrystalli zed . Furthermore , detected disturbing defects can be categorised NXWO O IWO 26 . 11 . 2025

[0046] 10 according to their disturbing impact . Defects with low disturbing impact may be left uncured i f the performance of the concerned seed wafer is determined by other influences . This may also help to improve the total ef ficiency of the process . In order to produce epitaxial wafers with the best quality possible , however, it is advantageous to locate all defect areas comprising disturbing crystal defects within the seed wafer and to perform the step (b ) for each of the located defect areas . In principle , all suitable measurement technologies can be used for locating the defect areas . For example photo luminescence , electro luminescence , or photoconductance decay measurement technologies .

[0047] It has proven useful to perform the steps of locally melting the seed wafer and recrystalli zing the in this way molten seed wafer material in an oxygen- free atmosphere . Preferably, said steps are performed in an atmosphere that comprises hydrogen or hydrogen and argon . In this way contamination of the seed wafer, in particular oxygen contamination, can be avoided or at least signi ficantly reduced . Moreover, hydrogen may passivate disturbing defects contained in the seed wafer . In praxi , it has proven that said atmosphere comprises 0 vol% to 30 vol% argon and 100 vol% to 70 vol% hydrogen . In order to ensure a defined composition of the oxygen- free atmosphere , the atmosphere may be controlled .

[0048] In a preferred embodiment the seed wafer is preheated to a temperature lower than a melting temperature of the seed wafer material prior to irradiating the first type laser beam . In doing so , a temperature di f ference between an irradiation area in which the first type laser beam is irradiated and other parts of the seed wafer can be reduced . This helps to reduce thermal stress induced by the described laser treatment and, consequently, reduces the risk of damages to the seed wafer . NXWO O IWO 26 . 11 . 2025

[0049] Said preheating of the seed wafer may be realised by placing the seed wafer on a heated wafer chuck full . It has turned out that this enables a homogeneous preheating of the seed wafer which is beneficial for reducing the risk of damages . Preferably, the wafer chuck is heated by means of resistance heating .

[0050] In a further embodiment the seed wafer is annealed in an annealing step . In this way further disturbing defects can be cured . Said annealing is preferably carried out in an annealing atmosphere comprising hydrogen or hydrogen and argon . It is especially preferred to anneal locally molten and recrystalli zed parts of the seed wafer in said annealing step . Good results have been achieved for CM silicon wafers us ing an annealing atmosphere that comprises hydrogen or hydrogen and argon . In order to ensure a defined composition of the annealing atmosphere , the atmosphere may be controlled .

[0051] For the purpose of heating the seed wafer during the annealing step, the seed wafer may be arranged on a heated wafer chuck . Again the wafer chuck can be heated by means of res istance heating .

[0052] Alternatively or additionally, a second type laser beam can be swept over parts of the seed wafer for the purpose of heating the seed wafer during the annealing step . Preferably, the second type laser beam is swept over the said locally molten and recrystalli zed parts of the seed wafer . The described usage of the second type laser beam makes it possible to locally heat selected parts of the seed wafer, in particular the said locally molten and recrystalli zed parts of the seed wafer . However, the second type laser beam may also be swept over an essentially whole side surface on which the first type laser beam was irradiated for the purpose of locally melting the NXWO O IWO 26 . 11 . 2025

[0053] 12 seed wafer . In this way the essential whole seed wafer can be heated by means of the second type laser beam . A laser device used for generating the first type laser beam may be used to also generate the second type laser beam . In this way the equipment required, and consequently plant expenditure , can be reduced .

[0054] In a preferred embodiment a porous layer is formed at least on a first surface side of the seed wafer . This is done after the seed wafer preparation steps have been performed, i f any . Subsequent to the forming of the porous layer, the semiconductor material is epitaxially deposited on the porous layer by means of chemical vapour deposition . In this way the semiconductor wafer is grown . In the following the semiconductor wafer, which may also be called epitaxial wafer, is detached by means of breaking the porous layer . The porous layer can advantageously be formed by means of wet chemical etching, preferably by single side wet chemical etching . Moreover, the porous layer may be formed out of several porous layers . Further details and references concerning the formation of the porous layer can be found DE 10 2015 118 042 Al . More information concerning the step of detaching the semiconductor wafer are disclosed in DE 10 2015 118 042 Al . Preferably, the semiconductor wafer is detached in a way disclosed in 10 2023 101 592 Al .

[0055] In an especially preferred variant , a CM silicon seed wafer is used, a porous silicon layer is formed, and silicon is deposited as semiconductor material .

[0056] After the grown semiconductor wafer has been detached from the seed wafer, the seed wafer can be reused in a next deposition cycle . The seed wafer preparation steps do not need to be performed before every next deposition cycle . But they may be NXWO O IWO 26 . 11 . 2025

[0057] 13 performed again after a certain number of deposition cycles , if required . This depends on a starting quality of the seed wafer and the way in which initial seed wafer preparations steps have been performed .

[0058] Advantageously, a closed semiconductor material surface , which is usually called template layer, is formed on a side of the porous layer that faces away from the volume of the seed wafer . This is preferably realised by annealing the porous layer . Subsequently, the semiconductor material is epitaxially deposited on the closed semiconductor material surface . In this way the quality of the grown semiconductor wafers , or in other words of the epitaxial wafers , can be improved . Said annealing of the porous layer can be realised by annealing the seed wafer as a whole , for example in a continuous furnace .

[0059] Annealing temperatures in the range of 1050 ° C to 1200 ° C have proven useful . The annealing is preferably performed in an annealing atmosphere that is free of oxygen . Instead, it may contain hydrogen .

[0060] The epitaxial semiconductor wafer according to the invention is produced by means of the method according to the invention described above . In a preferred embodiment it is an epitaxial silicon wafer .

[0061] The cast-mono semiconductor wafer according to the invention comprises a radial resistivity variation of less than 7 % , preferably less than 5 % , and especially preferably less than 3 % . The said radial resistivity variation is determined pursuant SEMI standard MF81- 1105 ( reapproved 1221 ) .

[0062] Said cast-mono semiconductor wafer, or in short words CM semiconductor wafer, can advantageously be used as the seed wafer in the above-described method for producing a semiconductor NXWO O IWO 26 . 11 . 2025

[0063] 14 wafer by means of epitaxial deposition of semiconductor material on the seed wafer . Providing the CM semiconductor wafer according to the invention in the form of a CM silicon wafer has especially proven itsel f in industrial applications .

[0064] In an advantageous embodiment the CM semiconductor wafer is doped and comprises a dopant concentration of at least 1 . 0 - I O18atoms per cm3, preferably of at least 3 . 0 - I O18atoms per cm3, and particularly preferably of at least 1 . 0 - I O19atoms per cm3. This enables a convenient formation of a porous layer on the surface of the CM semiconductor wafer as explained above .

[0065] In one embodiment the CM semiconductor wafer is p-type doped, preferably boron doped . Moreover it is a silicon wafer that is provided as CM semiconductor wafer . Such a CM silicon wafer has especially proven itsel f as seed wafer in the above-described method for producing a semiconductor wafer by means of epitaxial deposition of semiconductor material on the seed wafer .

[0066] The method according to the invention for producing the CM semiconductor wafer provides that the CM semiconductor wafer is prepared for being used as a seed wafer by performing any combination of seed wafer preparation steps that is disclosed in the present application in connection with or relating to the described method for producing a semiconductor wafer by means of epitaxial deposition of semiconductor material on a seed wafer .

[0067] In the following, the invention is described in more detail with the aid of figures . Where appropriate , identical reference numbers are assigned to elements having the same ef fects . The invention is neither with respect to functional features nor with respect to other features limited to the embodiments NXWO O IWO 26 . 11 . 2025

[0068] 15 shown in the figures . The description text above as well as the text below describe many details of the invention, which can be combined with each other in many ways . All these features are to be considered isolated and can be combined in every possible and suitable combination . In particular, all of these features can be combined, alone or together with other features , with the method for producing a semiconductor wafer of claim 1 , the CM semiconductor wafer of claim 20 , the method for producing a CM semiconductor wafer of claim 23 , or the epitaxial semiconductor wafer of claim 24 . The figures show :

[0069] Figure 1 : A schematic illustration of an embodiment of the method for producing a semiconductor . This illustration simultaneously illustrates an embodiment of the method for producing a CM semiconductor wafer according to the invention .

[0070] Figure 2a : Movement of a first type laser beam across a selected area of a seed wafer in a spiral path .

[0071] Figure 2b : Formation of closed melting and recrystalli zation fronts and their movement through the seed wafer volume during the movement of the first type laser beam depicted in Fig . 2a .

[0072] Figure 2c : Schematic sectional view through the seed wafer depicted in Fig . 2a along line A-A.

[0073] Figure 3 : A further embodiment of moving the first type laser beam across selected areas of a seed wafer .

[0074] Figure 4 : A further embodiment of moving the first type laser beam across selected areas of a seed wafer . NXWO O IWO 26 . 11 . 2025

[0075] 16

[0076] Figure 5 : A further embodiment of moving the first type laser beam across selected areas of a seed wafer .

[0077] Figure 6 : Schematic illustration of preheating or annealing a seed wafer on a wafer chuck .

[0078] Figure 7 : Sweeping a second type laser beam over a seed wafer for the purpose of heating the seed wafer ( schematic illustration) .

[0079] Figure 8 : Schematic illustration of a epitaxial wafer deposited on a seed wafer .

[0080] Figure 9 : Epitaxial wafer of Fig . 8 after its detachment from the seed wafer .

[0081] Fig . 1 shows a method 1 for producing a silicon wafer . This method one is an embodiment of the method for producing a semiconductor according to the invention . In this embodiment a CM silicon wafer is used as a seed wafer . At the beginning, said CM silicon seed wafer is cleaned 10 . This is realised in the present embodiment by means of a so called CP4 etching procedure using a HNOa-HF-mixture in combination with a subsequently performed cleaning step in an etching solution comprising HNa and H2O2 , whereas the latter cleaning step is well known in literature as a so-called RCA1 cleaning step . Said RCA1 cleaning step is followed by an HF-etching step in a 2 vol% aqueous HF solution which serves to remove a native oxide from the surface of the CM silicon seed wafer . Said HF-etching step still forms a part of the cleaning 10 of the CM sil icon seed wafer . It is rather immediately followed by a step of P-get- tering 12 the seed wafer . In the depicted embodiment this phosphorus gettering step is realised by means of an open-tube NXWOOIWO 26.11.2025

[0082] 17

[0083] POCla-dif fusion for 30 to 90 minutes at a temperature of 780°C to 900°C followed by a 10 minutes drive-in phase. A diffusion layer formed during P-gettering 12 is removed 14 afterwards.

[0084] In a next step, defect areas comprising disturbing crystal defects are located 16 within the seed wafer. As described above, all suitable measurement technologies can be applied. For example photo luminescence, electro luminescence, or photoconductance decay measurements. If no disturbing defects, and consequently no defect areas, are located, three following steps are omitted as indicated in Fig. 1 and a porous silicon layer is formed 24 on the seed wafer. If, on the other hand, defect areas comprising disturbing defects can be located, further seed wafer preparation steps are performed. The embodiment of Fig. 1 provides for this case a preheating 18 of the seed wafer as a next seed wafer preparation step. Said preheating 18 is realised by placing the seed wafer 50 on a heated wafer chuck as schematically illustrated in Fig. 6. As explained above, a risk of damages to the seed wafer 50 can be reduced in this way.

[0085] Preheating 18 is followed by locally laser melting 20 and recrystallizing the defect areas. Further details are schematically shown in Figs. 2a to 2c. As depicted in Fig. 2a, a first type laser beam 53 is irradiated on the seed wafer 50 and moved across a selected area 55. The selected area 55 is chosen such that a detected defect area 52 lies within the selected area 55. The first type laser beam 53 is moved across the selected area 55 in a path 54 which has, in the embodiment of Fig. 2a, a spiral form. In doing so, the first type laser beam is moved in such a way that a melting front 56 and a recrystallization front 58 are moved through a seed wafer volume 16 located below the selected area 55. As schematically shown in Fig. 2b, the recrystallization front 58 is realised as NXWO O IWO 26 . 11 . 2025

[0086] 18 closed recrystalli zation front 58 and the melting front 56 is realised as closed melting front 56 . The melting front 56 and the recrystalli zation front both have a roughly circular and therefore elliptic shape as schematically indicated in Fig . 2b . The melting front 56 as well as the recrystalli zation front 58 are moved through the seed wafer volume 60 in such a way that they become smaller . This is indicated in Fig . 2b by means of arrows . Preferably, the closed recrystalli zation front 58 is moved through the seed wafer volume 60 in such a way that it becomes continuously smaller . In order to realise said closed melting front 56 and said closed recrystalli zation front 58 and their described movement through the seed wafer volume 60 , a laser energy of the first type laser beam 53 is suitably chosen and the first type laser beam 53 is moved at a suitable velocity across the selected area 55 . The melting front 56 and the recrystalli zation front 58 are generated in an area being free from disturbing crystal defects and are subsequently moved across the defect area 52 .

[0087] Figs . 2c shows a sectional view through the CM silicon seed wafer 50 along line A-A depicted in Fig . 2a . As can be seen in the schematic illustration of Fig . 2c, the irradiated laser energy of the first type laser beam 53 is chosen such that the seed wafer is molten up to a depth of at least 90 % of a thickness of the seed wafer 50 . As explained above , this makes it possible to reuse the seed wafer 50 in a large number of deposition cycles and to produce thereby epitaxial wafers of constantly good quality . However, the embodiment shown in Fig . 1 , 2a, and 2b can be very easily adapted to melt the seed wafer j ust up to a depth of 70 % or less , i f this should be appropriate in the respective use case .

[0088] Figs . 3 and 4 show alternative embodiments of implementing the local laser melting 20 and recrystalli zation of defect areas . NXWOOIWO 26.11.2025

[0089] They are fully compatible with the embodiment shown in Fig. 1. In both cases again the first type laser beam 53 is irradiated and moved across the selected area 55. However, different paths of the first type laser beam are chosen. In case of the embodiment of Fig. 3, the first type laser beam 53 is moved across the selected area 55 in a path that it is composed of several linear parts 54a, 54b, 54c 54d, 54e, and 54f each of which start at an edge of the seed wafer 50 and extend to an opposite edge of the seed wafer 50 thereby spanning two defect areas 52. The melting front and the recrystallization front are in case of each linear path 54a to 54f generated in an area being free from disturbing crystal defects and are subsequently moved across the defect areas 52.

[0090] The embodiment of Fig. 4 differs from the embodiment of Fig. 3 in that the parts 54a to 54e of the first type laser beam 53 start at an edge of the seed wafer 50 and extend to a neighbouring edge of the seed wafer 50 and not to an opposite edge. Again the paths 54a to 54e of the first type laser beam 53 start outside the defect area 52 so that the melting front and the recrystallization front are in each case generated in an area being free from disturbing crystal defects. They are subsequently moved across the defect area 52.

[0091] Fig. 5 shows a further embodiment of moving the first type laser beam across selected areas of a seed wafer. This embodiment is fully compatible with the embodiment shown in Fig. 1. Analogously to the embodiment of Fig. 4, the first type laser beam 53 is moved across the selected area 55. In the case of the embodiment of Fig. 5, however, the first type laser beam 53 is moved across the selected area 55 in a path that is composed of several parts 54a, 54b, 54c, and 54d each of which have a curved shape, preferably the shape of a circular arc. Said curved paths 54a to 54d span the defect area 52. The NXWO O IWO 26 . 11 . 2025

[0092] 20 paths 54a to 54d of the first type laser beam 53 can be chosen such that they start outside the defect area 52 so that the melting front and the recrystalli zation front are in each case generated in an area being free from disturbing crystal defects and can subsequently be moved across the defect area 52 .

[0093] In the embodiment of Fig . 1 , the local laser melting 20 and recrystalli zation of defect areas is followed by an annealing 22 of the seed wafer 50 . Said annealing 22 is performed in an annealing atmosphere comprising hydrogen or hydrogen and argon . During the annealing step 22 the seed wafer is heated 36a by means of placing it on a heated wafer chuck 62 . Fig . 6 serves to also illustrate this heating 36a of the seed wafer 50 during the annealing step 22 . Fig . 7 schematical ly illustrates a further possibility for heating 36b the seed wafer during the annealing step 22 . In this embodiment a second type laser beam 64 is swept over the surface of the seed wafer 50 . This is schematically indicated in Fig . 7 with the help of straight arrows . The second type laser beam 64 can be swept over an essentially whole side surface in order to heat the essential whole seed wafer 50 by means of the second type laser beam 64 . Alternatively, the 2nd type laser beam 64 may be swept over locally molten and recrystalli zed parts of the seed wafer 50 only . In this way the locally molten and recrystalli zed parts of the seed wafer 50 can be locally heated . The variants of heating the seed wafer 36a, 36b during the annealing step 22 depicted in Figs . 6 and 7 can be combined or used independently from each other .

[0094] The schematic illustration of Fig . 1 provides as following step the forming 24 of a porous silicon layer on the seed wafer 50 . This step as well as all further method steps depicted in Fig . 1 will be explained in the following explained with the help of Figs . 8 and 9 : NXWO O IWO 26 . 11 . 2025

[0095] Said forming 24 of the porous silicon layer 66 is realised by means of wet chemical single side etching of the seed wafer 50 . The resulting porous silicon layer 66 is schematically indicated in Figs . 8 and 9 . In the following, the porous silicon layer 66 is annealed such that a closed silicon surface 68 is formed 26 on a side of the porous layer 66 that faces away from a volume of the seed wafer 50 . Said annealing is performed in an annealing atmosphere and can be realised in a continuous furnace as explained above in further detail .

[0096] Subsequently, silicon material 70 is epitaxially deposited 28 on the closed silicon surface 68 . In the present embodiment this is realised by means of chemical vapour deposition . In the following, the porous silicon layer 66 is broken and the epitaxial silicon wafer 72 is detached 30 from the seed wafer . This results in the epitaxial silicon wafer 72 schematically depicted in Fig . 9 . Fig . 9 shows the detached epitaxial silicon wafer 72 in principle . Depending on the detaching technology applied, the epitaxial silicon wafer obtained may look somewhat di f ferent . For example , overgrown sections at the edges of the epitaxial silicon wafer may have been cut of f before , during, or after detaching the epitaxial silicon wafer . In particular, detaching technologies disclosed in DE 10 2023 101 592 Al or DE 10 2015 118 042 Al may be applied . Finally, the seed wafer 50 is prepared 32 for reuse . This is schematically indicated in Fig . 1 by means of arrow 32 . The step of preparing 32 the seed wafer 50 for reuse comprises removing porous silicon residuals from the seed wafer 50 . This can be realised by means of wet chemical etching .

[0097] Apart from the method 1 for producing the epitaxial silicon wafer 72 , Fig . 1 simultaneously illustrates an embodiment 5 of the method for producing a CM semiconductor wafer according to NXWOOIWO 26.11.2025

[0098] 22 the invention. This embodiment 5 is a method for producing a CM silicon wafer, namely the CM silicon seed wafer 50. In view of Figs. 1, 2a to 2c, 8, and 9 and related description passages above, this CM silicon seed wafer is obviously suitable for use as seed wafer. Every step of said method 5 for producing a CM silicon wafer has already been explained above in connection with the method 1 for producing the epitaxial silicon wafer 72. The CM silicon wafer resulting from method 5 is the seed wafer 50 depicted in Fig. 8. It is boron doped and comprises a dopant concentration of at least 1.0 -IO19atoms per cm3. Furthermore, it shows a radial resistivity variation of less than 7 %, preferably less than 5 %, and especially preferably less than 3 %.

[0099] NXWO O IWO 26 . 11 . 2025

[0100] List of references

[0101] 1 Method for producing silicon wafer

[0102] 5 Method for producing cast-mono silicon wafer

[0103] 10 Cleaning CM silicon seed wafer

[0104] 12 P-gettering seed wafer

[0105] 14 Removing di f fusion layer

[0106] 16 Locating defect areas in seed wafer

[0107] 18 Preheating seed wafer

[0108] 20 Local laser melting and recrystalli zation of defect areas

[0109] 22 Annealing seed wafer

[0110] 24 Forming porous silicon layer on seed wafer

[0111] 26 Annealing porous silicon layer and forming closed silicon surface on a side of porous silicon layer

[0112] 28 Depositing silicon epitaxially on closed silicon surface

[0113] 30 Breaking porous silicon layer and detaching silicon wafer

[0114] 32 Preparing seed wafer for reuse

[0115] 36a Heating seed wafer during annealing step

[0116] 36b Heating seed wafer during annealing step

[0117] 50 Seed wafer

[0118] 52 Defect area

[0119] 53 First type laser beam

[0120] 54 Path of first type laser beam

[0121] 54a Path of first type laser beam

[0122] 54b Path of first type laser beam

[0123] 54c Path of first type laser beam

[0124] 54d Path of first type laser beam

[0125] 54e Path of first type laser beam

[0126] 54 f Path of first type laser beam

[0127] 55 Selected area

[0128] 56 Melting front

[0129] 58 Recrystalli zation front

[0130] 60 Seed wafer volume NXWOOIWO 26.11.2025

[0131] 24

[0132] 62 Heated wafer chuck

[0133] 64 Second type laser beam

[0134] 66 Porous silicon layer

[0135] 68 Closed silicon surface 70 Silicon material

[0136] 72 Epitaxial silicon wafer

Claims

NXWOOIWO 26.11.202525Claims1. Method (1) for producing a semiconductor wafer (72) by means of epitaxial deposition (28) of semiconductor material (70) on a seed wafer (50) , c h a r a c t e r i z e d i n t h a t a cast-mono semiconductor wafer (50) is used as said seed wafer .

2. Method (1) according to claim 1, c h a r a c t e r i z e d i n t h a t the cast-mono semiconductor wafer used as the seed wafer (50) comprises a dopant concentration of at least 1.0 -IO18atoms per cm3, preferably of at least 3.0 -IO18atoms per cm3, and particularly preferably of at least 1.0 -IO19atoms per cm3.

3. Method (1) according to any of the preceding claims, c h a r a c t e r i z e d i n t h a t a p-type doped cast-mono semiconductor wafer is used as the seed wafer (50) , preferably a boron doped cast-mono semiconductor wafer and especially preferably a boron doped cast-mono silicon wafer.

4. Method (1) according to any of the preceding claims, c h a r a c t e r i z e d i n t h a t- before depositing (28) the semiconductor material (70) on the seed wafer (50) , the seed wafer (50) is prepared by performing, as a seed wafer preparation step, a get- tering step ( 12 ) ;- wherein a phosphorous diffusion gettering step (12) is preferably chosen as the gettering step (12) .NXWOOIWO 26.11.2025265. Method (1) according to claim 4, c h a r a c t e r i z e d i n t h a t a diffusion layer formed during the gettering step (12) is at least partly removed (14) .

6. Method (1) according to any of the preceding claims, c h a r a c t e r i z e d i n t h a t before depositing (28) the semiconductor material (70) on the seed wafer (50) , the seed wafer (50) is prepared by performing the following seed wafer preparation steps:- locally melting (20) the seed wafer (50) by means of irradiating a first type laser beam (53) ;- subsequently recrystallizing (20) in this way locally molten seed wafer material.

7. Method (1) according to claim 6, c h a r a c t e r i z e d i n t h a t by means of the said irradiation of the first type laser beam (53) , the seed wafer (50) is locally molten (20) up to a depth of at least 50 % of its thickness, preferably up to a depth of at least 70 % of its thickness, and especially preferably up to a depth of at least 90 % of its thickness.

8. Method (1) according to any of claims 6 to 7, c h a r a c t e r i z e d i n t h a t the first type laser beam (53) is moved across a selected area (55) of the seed wafer (50) in such a way that a melting front (56) and a recrystallization front (58) are moved through a seed wafer volume (60) located below the selected area (55) , whereas the melting front (56) and the recrystallization front (58) are formed at a distance from each other .NXWOOIWO 26.11.2025279. Method (1) according to claim 8, c h a r a c t e r i z e d i n t h a t- a closed recrystallization front (58) , preferably a recrystallization front (58) having an elliptic shape, is formed;- said closed recrystallization front (58) is moved through the seed wafer volume (60) in such a way that it becomes smaller;- wherein the closed recrystallization front (58) is preferably moved through the seed wafer volume (60) in such a way that it becomes continuously smaller.

10. Method (1) according to any of claims 8 to 9, c h a r a c t e r i z e d i n t h a t the first type laser beam (53) is moved across the selected area (55) in a spiral path (54) .

11. Method (1) according to any of claims 8 to 10, c h a r a c t e r i z e d i n t h a t- in a step (a) a defect area (52) comprising disturbing crystal defects is located (16) within the seed wafer (50) ;- in a step (b) the first type laser beam (53) is moved across the selected area (60) in such a way that the melting front (56) and the recrystallization front (58) are generated in an area being free from disturbing crystal defects and are subsequently moved across the defect area (52) ;- wherein preferably all defect areas (52) comprising disturbing crystal defects are located within the seed wafer (50) and step (b) is performed for each of the located defect areas (52) .NXWOOIWO 26.11.20252812. Method (1) according to any of claims 6 to 11, c h a r a c t e r i z e d i n t h a t said steps of locally melting (20) the seed wafer (50) and recrystallizing (20) the in this way molten seed wafer material are both performed in an oxygen-free atmosphere, preferably in an atmosphere that comprises hydrogen or hydrogen and argon, whereas said atmosphere particularly preferably comprises 0 vol% to 30 vol% argon and 100 vol% to 70 vol% hydrogen.

13. Method (1) according to any of claims 6 to 12, c h a r a c t e r i z e d i n t h a t the seed wafer (50) is preheated (18) to a temperature lower than a melting temperature of the seed wafer material prior to irradiating the first type laser beam (53) .

14. Method (1) according to claim 13, c h a r a c t e r i z e d i n t h a t the seed wafer (50) is preheated (18) by placing it on a heated wafer chuck (62) , whereas said wafer chuck (62) is preferably heated by means of resistance heating.

15. Method (1) according to any of the preceding claims, c h a r a c t e r i z e d i n t h a t the seed wafer (50) is annealed (22) in an annealing step, whereas said annealing (22) is preferably carried out in an annealing atmosphere comprising hydrogen or hydrogen and argon, and whereas it is particularly preferred to anneal locally molten and recrystallized parts of the seed wafer (50) in said annealing step.

16. Method (1) according to claim 15, c h a r a c t e r i z e d i n t h a t for the purpose of heating (36a) the seed wafer (50) duringNXWOOIWO 26.11.202529 the annealing step (22) , the seed wafer (50) is arranged on a heated wafer chuck (62) .

17. Method according to any of claims 15 to 16, c h a r a c t e r i z e d i n t h a t for the purpose of heating (36b) the seed wafer (50) during the annealing step (22) , a second type laser beam (64) is swept over at least parts of the seed wafer (50) , preferably over said locally molten and recrystallized parts of the seed wafer (50) .

18. Method (1) according to any of the preceding claims, c h a r a c t e r i z e d i n t h a t subsequent to the seed wafer preparation steps the following steps are performed:- forming (24) a porous layer (66) at least on a first surface side of the seed wafer (50) ;- subsequently depositing (28) the semiconductor material (70) epitaxially on the porous layer (66) by means of chemical vapour deposition and in this way growing the semiconductor wafer (72) ;- subsequently detaching (30) the semiconductor wafer (72) by means of breaking the porous layer (66) .

19. Method (1) according to claim 18, c h a r a c t e r i z e d b y- forming (26) a closed semiconductor material surface(68) on a side of the porous layer (66) that faces away from the volume of the seed wafer (50) , preferably by annealing (26) the porous layer (66) ;- depositing (28) the semiconductor material (70) epitaxially on the closed semiconductor material surface (68) .NXWOOIWO 26.11.202520. Cast-mono semiconductor wafer (50) for use as a seed wafer in a method for producing a semiconductor wafer by means of epitaxial deposition of semiconductor material on the seed wafer, c h a r a c t e r i z e d b y a radial resistivity variation of less than 7 %, preferably less than 5 %, and especially preferably less than 3 %, wherein the radial resistivity variation is determined pursuant to SEMI standard MF81-1105.

21. Cast-mono semiconductor wafer (50) according to claim 20, c h a r a c t e r i z e d i n t h a t the cast-mono semiconductor wafer (50) is doped and comprises a dopant concentration of at least 1.0 -IO18atoms per cm3, preferably of at least 3.0 -IO18atoms per cm3, and particularly preferably of at least 1.0 -IO19atoms per cm3.

22. Cast-mono semiconductor wafer (50) according to any of claims 20 to 21, c h a r a c t e r i z e d i n t h a t the cast-mono semiconductor wafer is p-type doped, preferably boron doped, and especially preferably a boron doped silicon wafer is provided as cast-mono semiconductor wafer.

23. Method (5) for producing a cast-mono semiconductor wafer(50) according to any of claims 20 to 22, c h a r a c t e r i z e d i n t h a t the cast-mono semiconductor wafer is prepared for being used as seed wafer by performing the seed wafer preparations steps according to any of claims 4 to 17.

24. Epitaxial semiconductor wafer (72) , c h a r a c t e r i z e d i n t h a tNXWOOIWO 26.11.202531 it is produced by means of the method (1) according to any of claims 1 to 19.

25. Epitaxial wafer according to claim 24, c h a r a c t e r i z e d i n t h a t it is an epitaxial silicon wafer.

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