Method for separating a semiconductor wafer from a layer stack and method for producing a semiconductor wafer

Abstract

A method for separating a semiconductor wafer (4') from a layer stack (1) comprising the following steps: A) Providing the layer stack (1) that comprises a seed substrate (2), a separation layer (3) on the seed substrate (2), and a semiconductor layer (4) formed on the separation layer (3), which preferably is produced epitaxially on the separation layer (3); B) Separating the semiconductor layer (4') from the layer stack (1), further comprising a first step B1), wherein the semiconductor layer (4) is pre-damaged in an area such that a fracture stress of the semiconductor layer (4) in this area is reduced, wherein step B1) is performed before and / or during step B); and a second step B2), wherein thermal energy means from a source is applied to the semiconductor layer (4) along a separation line (11), wherein the separation line (11) starts from the pre-damaged area of the semiconductor layer (4) and wherein the thermal energy means from a source heats and / or cools the semiconductor layer (4) locally such that a thermally induced stress is generated at least in the heated and / or cooled region of the semiconductor layer (4) in order to cause a crack (13) that runs along the separation line (11) and which separates the semiconductor wafer (4') from the layer stack (1). In addition, the invention relates to a method for producing a semiconductor wafer (4').

Description

[0001] Method for separating a semiconductor wafer from a layer stack and method for producing a semiconductor wafer

[0002] Description

[0003] The invention relates to a method for separating a semiconductor wafer from a layer stack according to claim 1 and a method for producing a semiconductor wafer according to claim 19.

[0004] For the production of electronic components, such as large-area light-emitting elements or solar cells, there is a need for low-cost semiconductor wafers. Semiconductor wafers are typically flat substrates that are made of semiconductor materials such as silicon (Si), gallium arsenide (GaAs), germanium (Ge) or silicon carbide (SiC).

[0005] The process of manufacturing or producing semiconductor wafers typically begins with a single large crystal of the semiconductor material, which may be cut in order to separate single semiconductor wafers. For this purpose, such a semiconductor may be sawn by means of a machine tool, like a diamond wire sawing tool. Even though a high quality can be achieved, manufacturing costs are high, especially due to the loss of material.

[0006] Epitaxial chemical vapor deposition (CVD) processes make it possible to avoid these problems by creating a layer stack in which the desired semiconductor wafer is created as a layer on a seed substrate, hereinafter referred to as semiconductor layer. A porous separation layer is typically arranged between said seed substrate and the semiconductor layer and makes it possible to separate the semiconductor layer from the seed substrate almost without losing any seed wafer material whereby both a high quality of the semiconductor wafer and a good reusability of the seed substrate can be achieved.

[0007] To detach the semiconductor wafer from the seed substrate, separation cuts are typically made by means of a laser beam using laser ablation. In this way, the laser beam typically penetrates the semiconductor layer completely and reaches also into the separation layer and / or the seed substrate, whereby the seed substrate is also damaged or reduced in size.

[0008] However, the seed substrate and the separation layer are also affected by these cuts, so that the seed substrates often cannot be reused for the production of a further semiconducting layer or have to be processed at great expense, which limits the reusability (number of reuse cycles) of the seed substrate.

[0009] It is also possible to trim the layer stack from the side by means of laser ablation, but this also reduces the size of the seed substrate and therefore limits the reusability of said seed substrate. In addition, the edge quality of the cut-out semiconductor layer is strongly affected by laser ablation processes in the form of laser-induced damage, which can decrease the breaking strength of the resulting wafer and the material quality in terms of minority carrier lifetime (MCLT).

[0010] A further problem is that the semiconductor layer is formed with a non-planar geometry, in particular at the edges, which is undesirable for the production of electronic components based on this layer. For this reason, it is also desirable to separate individual areas of the semiconductor layer from other areas of the semiconductor layer.

[0011] To detach the semiconductor wafer, separation cuts can be provided by means of laser radiation, which is for example described in DE 10 2015 118 042 A1. However, by using laser radiation, there is a risk that said laser radiation penetrates the semiconductor layer and reaches into the separation layer and the seed substrate. Thus, the seed substrate can be damaged such that its further use for growing further semiconductor layers is impaired. On the other hand, if the laser power is set too low, it may not be sufficient to separate individual areas from the semiconductor wafer and to detach them from the layer stack or it might result in a low cutting speed limiting throughput in wafer production. Another disadvantage of conventional laser ablation is the generation of particles, in particular on the wafer, which needs to be handled properly in a manufacturing process. US2014 / 0038392 A1 discloses a system and method for laser splitting of a semiconductor wafer, wherein a focused laser beam is scanned over the entire surface of a semiconductor layer to produce a laser beam array within the layer, wherein thermal and / or external mechanical stress after laser irradiation may enhance the separation of the wafer from the semiconductor layer. Scanning the entire surface of the wafer is time consuming and cost intensive and has to be done carefully, so that the whole area is processed. Otherwise, the wafer may be destroyed during detachment.

[0012] It is therefore an objective of the invention to provide a reliable method for separating a semiconductor wafer from a layer stack, enabling a good reusability of the seed substrate and a high quality of the semiconductor wafer at a high process speed.

[0013] The objective is solved by a method for separating a semiconductor wafer from a layer stack according to claim 1 and a method for producing a semiconductor wafer according to claim 19. Preferred embodiments are subject to the dependent claims.

[0014] According to the invention, the method for separating a semiconductor wafer from a layer stack comprises the following steps:

[0015] A) Providing the layer stack that comprises a seed substrate, a separation layer on the seed substrate, and a semiconductor layer on the separation layer, preferably epitaxially grown on the separation layer;

[0016] B) Separating the semiconductor wafer, which is at least a part of the semiconductor layer (4), from the layer stack.

[0017] It is essential that in a first step B1) the semiconductor layer is pre-damaged in an area with a pre-damage such that a fracture stress of the semiconductor layer in this area is reduced. Step B1) is performed before and / or during step B). Furthermore, in a second step B2), thermal energy means from a source is applied to the semiconductor layer along a separation line, wherein the separation line comprises the pre-damage or in particular starts from the predamage of the semiconductor layer. The thermal energy means heats and / or cools the semiconductor layer locally, in particular below the surface of the semiconductor layer, such that a thermally induced stress is generated in order to cause a crack, which runs essentially along the separation line.

[0018] The invention is based on the fact that heating and / or cooling by thermal energy means alone can cause thermally induced stresses within the semiconductor layer, which are relieved by crack formation from the pre-damage - especially without a change in the microstructure of the semiconductor layer. The invention is based on the realization that thermal energy means can be used to create a clean separation line along which the semiconductor wafer can be separated from the rest of the layer stack without any effect to the quality of the separation layer and / or the seed substrate and without the creation of particulate matter, e.g. by laser ablation. It has been found that a separation line is sufficient to separate the semiconductor wafer from the layer stack. A processing of the whole surface area is not necessary, which enables a faster processing by less costs.

[0019] Preferably, the thermal energy means are designed in such a way that the semiconductor layer is heated and / or cooled directly, in particular without heating and / or cooling any intermediate layer, in particular any layer on top of the semiconductor layer in the layer stack. More preferably, the thermal energy means are designed in such a way that the semiconductor layer is heated and / or cooled within the layer thickness without applying thermal energy on the surface of the semiconductor layer.

[0020] By applying a thermally induced stress to the semiconductor layer, one or multiple cracks are created, along which the semiconductor wafer is separated from the rest of the layer stack. By applying a detachment force on the semiconductor wafer, the wafer is detached from the separation layer.

[0021] Preferably, the semiconductor wafer still adheres at least partially to the separation layer after applying the thermally induced stress. Such a separation is efficient compared to known methods, since the reusability of the seed structure is not impaired and an undesirably high loss of material can be avoided. Another advantage is that the generation of the separation line is not limited to a specific geometry. It is within the scope of the invention that the separation line can in principle be set with any course. This is particularly advantageous because the course of the separation line can be adapted to any contour of the layer stack or to the semiconductor wafer to be cut out, which can have both straight and curved edges. In a preferred embodiment, the separation line is completely closed, in particular such that the separation line is formed substantially rectangular.

[0022] The present invention relates to the separation of a layer from a layer stack to produce a wafer, which can be further processed, e.g. for the production of electronic components or solar panels. In particular, the semiconductor wafer is a single element with a flat surface. The present invention in particular does not relate to process a wafer to produce partial areas.

[0023] In particular, detaching the semiconductor layer from the layer stack means that the semiconductor layer does not have any longer a connection to the layer stack and, in particular, is removed from the layer stack. Separating the semiconductor layer from the layer stack means that the semiconductor layer is separated from the layer stack but can still adhere to the layer stack and a detachment force is necessary for detaching the semiconductor layer from the layer stack.

[0024] In a preferred embodiment, the thermal energy means is a laser radiation from a laser source as source. Preferably the laser radiation is emitted continuously and, in particular the laser source is a continuous wave laser.

[0025] The invention is based on the realization that laser radiation with a comparatively low intensity, in particular a low light intensity, can be used to create a clean separation line along which the semiconductor wafer is separated and can be detached from the rest of the layer stack. In particular, it is not necessary to set the laser power such that a laser ablation takes place, which in particular can also affect the quality of the separation layer and / or the seed substrate. Instead, the heating through the laser radiation alone can cause thermally induced stresses within the semiconductor layer, which are relieved by crack formation - especially without a change in the microstructure of the semiconductor layer. In comparison to a pulsed laser, a continuous wave laser emits laser radiation without interruption. It is thus possible to generate the thermally induced stresses homogeneously and, in particular, without spatial interruptions along the separation line. This facilitates crack initiation and propagation. Moreover, pulsed lasers may be disadvantageous since the energy density of each pulse may be higher than a limit above which the crystalline structure of the semiconductor layer could be changed, which is undesirable.

[0026] In a preferred embodiment, therefore, a power of the laser radiation as thermal energy means in step B2) is selected such that it is essentially absorbed in the semiconductor layer and that the power of the laser radiation is selected such that the semiconductor layer retains its original crystalline structure. The predamage generated in step B1) serves to reduce the fracture stress of the semiconductor layer, so that even a low thermally induced stress can be sufficient to cause the desired cracking within the semiconductor layer along the separation line. Additionally, the pre-damage can serve as a guiding line and / or starting point and / or end point for the propagation of the thermally induced crack.

[0027] Studies have also shown that it is typically necessary for the separation line, along which the semiconductor layer is separated from the layer stack, to run at a distance of 1-3 mm from the edge of the layer stack. However, by predamaging, it is possible to better control the crack path so that shorter distances can be realized, which leads to a better yield in the production of the semiconductor wafer.

[0028] The crack path runs in particular along the preset separation line. However, it is also possible that a crack is realized with an offset to the intentional separation line. This offset can be realized “naturally” due the presence of stresses in particular near the edge of the semiconductor wafer or by individual setting of the pre-damage and / or the laser radiation.

[0029] The separation line preferably represents a line along which the cracking essentially propagates. However, the separation line and the crack path do not always have to lie on top of each other and can also deviate from each other up to a certain extent.

[0030] The invention has the advantage that very high cutting speeds of typically 150- 500 mm / s or even multiple meters per second are possible for the creation of the crack. Furthermore, there are fewer wear parts for a laser-based separation of the semiconductor layer compared to mechanical machining. In addition, the contamination or soiling of the wafers by abrasion and / or particle generation by saw blade or laser ablation is reduced respectively. Additionally, with the presented invention laser-induced material damage in the heat affected zone is reduced or can be completely avoided. Moreover, there is no formation of so- called fiducials or alignment markers required, such that the semiconductor layer remains usable on both surfaces. Also chipping of the edges may be avoided easily, which leads to a good edge quality and a high mechanical stability of the detached semiconductor layer. In addition, the entire process can be carried out entirely without the use of coolants, so that any drying steps of the semiconductor layer can be avoided.

[0031] The semiconductor layer and semiconductor wafer are preferably made of silicon, in particular monocrystalline silicon. However, it is also within the scope of the invention that the semiconductor layer and semiconductor wafer are made of germanium or silicon carbide or lll-V compound semiconductor materials like gallium arsenide, indium phosphate or indium arsenide or based on group-ill nitrides like gallium nitride, aluminum nitride or indium nitride or compounds therefrom.

[0032] It is within the scope of the invention that the semiconductor wafer to be separated is an epitaxially grown layer that has a higher density than the underlying separation layer. In particular, the porosity of the separation layer is higher than that of the semiconductor layer.

[0033] Preferably, the semiconductor layer has a thickness in the range from 5 pm to 2000 pm, preferably from 10 pm to 500 pm, particularly preferred from 50 pm to 200 pm, in particular from 60 pm to 160 pm. The seed substrate can have a small thickness of 1 pm to 800 pm, preferably a thickness of 200 pm to 600 pm, but can also be several millimeters thick.

[0034] According to the invention, the thermal stress results from a temperature gradient in the semiconductor layer due to local heating by a thermal energy means, in particular by a laser, in step B2. In particular, it is sufficient if the temperature gradient occurs as a result of a temperature difference between the heated or cooled region and a non-heated or non-cooled region of the semiconductor layer. Thus, it may be sufficient that at least the semiconductor layer in the non-heated or non-cooled region is at ambient temperature.

[0035] Preferably, method step B2) is performed using laser direct cleaving (LDC), wherein only the semiconductor layer is machined without influencing the separation layer and / or the seed substrate.

[0036] In a preferred embodiment, the thermal energy means is a heating means and a cooling medium is applied along the separation line before, during and / or after applying the heating means. Preferably, the heating means and the cooling medium are applied laterally displaced. As a result, it is possible to increase the temperature difference and the resulting thermally induced stress in the semiconductor layer and to better guide the propagating crack along the separation line. This favors the formation of cracks within the semiconductor layer along its thickness and makes it easier to separate the semiconductor wafer from the rest of the layer stack. The invention is not limited as to which cooling medium is used, however it has been found advantageous to use an aerosol based on water. Preferably, the cooling medium itself and / or the application of the cooling medium can be adjusted by setting the temperature of the cooling medium before and / or during its application on the semiconductor layer or by setting the pressure and / or the flow of the cooling medium during its application on the semiconductor layer. When using an additional cooling medium, method step B2) is preferably performed using Thermal Laser Separation (TLS).

[0037] In another preferred embodiment, the thermal energy means is a cooling means, and a heating medium is applied along the separation line before, during and / or after applying the cooling means. Preferably, the heating medium and the cooling means are applied laterally displaced.

[0038] In a preferred embodiment, the wavelength of the laser radiation is in the infrared range, preferably between 780-2000 nm, highly preferably between 800- 1500 nm, particularly preferably between 1000 and 1100 nm, especially at 1060 nm or 1064 nm. Studies have shown that the named wavelength ranges are particularly advantageous for achieving the benefits of the invention. Apart from this, however, the wavelength range is in principle not limited within the scope of the invention and may depend on the material of the semiconductor layer.

[0039] In another preferred embodiment, the laser radiation is deflected by means of mirrors and / or a galvo scanner and applied to the semiconductor layer. By means of this embodiment, it is basically not necessary to affect a relative movement between the semiconductor layer and the laser source in order to be able to generate the separation line. However, it is also within the scope of the invention that a spatial displacement is first carried out between the semiconductor layer and the laser source in order to bring the laser source in a suitable position relative to the layer stack and afterwards to heat the semiconductor layer along the separation line by deflecting the laser radiation.

[0040] Preferably the thermal energy means is applied continuously or in a pulsed manner on the semiconductor layer.

[0041] Preferably, the layer stack is moved relative to the source according to a shape of the separation line. It is within the scope of the invention that either the layer stack is fixed and the source or a part of the source is moved relative to the layer stack or vice versa. Alternatively, both the source or a part of the source and the substrate are moved. In particular, guiding means and kinematics may be provided to enable a relative movement between the source, in particular a part of the source, and the layer stack. In preferred embodiment, for a laser source as source, a part of the laser source could be a laser head or an optic element of the laser source.

[0042] In another preferred embodiment, the layer stack is at rest relative to the source, and the thermal energy means is guided by guiding means to the semiconductor layer. By means of this embodiment, it is basically not necessary to affect a relative movement between the semiconductor layer and the source in order to be able to generate the separation line. However, it is also within the scope of the invention that a spatial displacement is first carried out between the semiconductor layer and the source in order to bring the source of the thermal energy means in a suitable position relative to the layer stack and afterwards to heat and / or cool the semiconductor layer along the separation line by guiding the thermal energy means through the guiding means. In a preferred embodiment, the pre-damage is affected by means of the laser source or a further laser source, in particular by means of a pulsed laser source, preferably a laser source with a pulse length in the nanosecond range. Alternatively, or preferably in addition, the pre-damage is affected by means of a scribe.

[0043] Studies have shown that pre-damage serves to establish a starting point for cracking and separation of the semiconductor layer. In this respect, it may be sufficient to achieve the advantages according to the invention if the predamaged area is generated essentially punctually, i.e. , without a preferred direction of extension on the semiconductor layer. Preferably, the pre-damage is essentially linear in shape. In particular, it is within the scope of the invention that the separation line begins in the pre-damaged area and spatially extends beyond it.

[0044] In a preferred embodiment, the pre-damage is an initial pre-damage, wherein the initial pre-damage covers only a small area of the semiconductor layer and preferably may be formed as an initial scribe with a length of about at least 1 pm to 2000 pm, more preferable with a length of about at least 100 pm to 1000 pm. The initial pre-damage can be also visible on the surface of the semiconductor layer.

[0045] In another preferred embodiment of the invention, the pre-damage is a deep pre-damage, which extends in particular along the separation line and has a length up to the dimensions of the semiconductor layer. Preferably, the deep pre-damage extends within the semiconductor layer and not on its surface. The deep pre-damage may be also referred as deep scribe. The deep scribe may provide a separation line, in particular a closed separation line, within the semiconductor layer. Therefore, the deep long pre-damage covers a larger area than the initial short pre-damage. However, the deep pre-damage does not cause a complete cracking of the semiconductor layer surrounded by the deep pre-damage. A cracking is realized in step B2 through thermal energy means, in particular by laser radiation. Preferably, the deep pre-damage can be a modification area within the semiconductor layer, wherein the semiconductor material in the modification area is modified, in particular through laser absorption, in respect to the surrounding semiconductor material. A modification can be a change in the crystalline structure of the semiconductor material. This is achieved for example by briefly melting or vaporizing the material in the semiconductor layer, in particular through laser absorption, and recrystallizing or solidifying the molten material. For a deep scribe, it is preferred to analyze the surface structure and in particular the thickness variation of the semiconductor layer for setting the deep pre-damage at the required depth. However, it is not mandatory to analyze the surface structure of the semiconductor layer in advance of producing a deep pre-damage.

[0046] In a further preferred embodiment, a plurality of pre-damages is created on the semiconductor layer, wherein the plurality of pre-damages is distributed on the semiconductor layer according to the course of the separation line.

[0047] Preferably, a further pre-damage is created at an end and / or at an edge and / or at a curve of the separation line and / or at a crossing of at least two separation lines. The further pre-damage prevents a break-out of the separation line, in particular at the end and / or at the edge and / or at a curve of the separation line. In addition, the further pre-damage can also provide a direction for the further propagation of the separation line and / or the cracking path.

[0048] It is also within the scope of the invention that a plurality of pre-damaged areas is created, which are distributed on the semiconductor layer according to the course of the separation line. However, it is also within the scope of the invention, that the pre-damaged area is created continuously according to the course of the separation line.

[0049] In a further preferred embodiment, in which the semiconductor layer is heated by a continuous wave laser as a first mode, the laser can have a further mode, in which the laser radiation deviates from the first mode to generate the predamage. In particular, the laser radiation in the further modes can be pulsed and / or can have a different focal point and / or the intensity deviates from the first mode. Alternatively or preferably in addition, the wavelength of the laser radiation may differ in the first mode and the further mode. This makes it possible to effect both the pre-damage and the heating with one and the same device.

[0050] The method according to the invention is advantageous regardless of how the semiconductor layer is arranged on the layer stack. In particular, it is possible to divide the semiconductor layer by the separation line in such a way that one part of the semiconductor layer, forming the semiconductor wafer, is separated from another part of the semiconductor layer, which remains on the layer stack. In other words, the semiconductor layer can overlap a side surface of the seed structure and / or the separation layer, so that it may be desirable to separate a planar area or high-quality area of the semiconductor layer from a non-planar or erroneous area.

[0051] Preferably, the semiconductor layer comprises an overlap portion which projects beyond one side surface of the seed substrate and / or of the separation layer and in particular at least partially covers one or more side surfaces of the seed substrate and / or of the separation layer. In a preferred embodiment, the overlap portion is formed of the same material as the semiconductor layer.

[0052] Preferably, the separation line may be formed such that the semiconductor wafer is separated from the overlap region and / or from the layer stack in process step B).

[0053] Preferably, the semiconductor layer comprises a core area, wherein the core area is an area of the semiconductor layer within the separation line and circumscribed by the separation line. In particular, the core area does not include the overlap portion. Preferably, the core area builds the semiconductor wafer.

[0054] As mentioned above, the objective of the invention is also solved by a method for producing a semiconductor wafer, comprising the following steps

[0055] V1 ) Providing a seed substrate;

[0056] V2) Creating a separation layer on the seed substrate;

[0057] V3) Forming a semiconductor layer on the separation layer, preferably by chemical vapor deposition epitaxy, in order to provide a layer stack comprising the seed substrate, the separation layer, and the semiconductor layer; and

[0058] V4) Detaching the semiconductor wafer, which is at least a part of the semiconductor layer, from the layer stack, in particular by a detachment force.

[0059] It is essential that prior to or during method step V4) a method for separating a semiconductor wafer according to the invention or a preferred embodiment thereof is performed.

[0060] Preferably, the thermal energy means heats and / or cools the semiconductor layer directly, in particular without heating and / or cooling any intermediate layer, in particular any layer on top of the semiconductor layer in the layer stack. More preferably, the thermal energy means heats and / or cools the semiconductor layer within the layer thickness without applying thermal energy on the surface of the semiconductor layer.

[0061] Regarding the producing or manufacturing process according to the invention, reference is made to the achievable advantages with the method for separating a semiconductor layer according to the invention or a preferred embodiment thereof.

[0062] The advantages of the invention are explained by way of example with reference to embodiments and the figures.

[0063] Figure 1 shows a layer stack in a side view a) and top view b);

[0064] Figure 2 shows a first embodiment for separating a semiconductor wafer from a layer stack using a deep pre-damage and subsequent thermal laser separation in perspective views a)-d);

[0065] Figure 3 shows the first embodiment for separating a semiconductor wafer from a layer stack using deep pre-damage and subsequent thermal laser separation in side views a)-d); Figure 4 shows a second embodiment for separating a semiconductor wafer from a layer stack using initial pre-damage by laser ablation and subsequent thermal laser separation in perspective views a)-d);

[0066] Figure 5 shows the second embodiment for separating a semiconductor wafer from a layer stack using initial pre-damage by laser ablation and subsequent thermal laser separation in side views a)-d);

[0067] Figure 6 shows a third embodiment for separating a semiconductor wafer from a layer stack using laser direct cleaving in perspective views a)-d);

[0068] Figure 7 shows the third embodiment for separating a semiconductor wafer from a layer stack using laser direct cleaving in side views a)-d);

[0069] View a) of Figure 1 is a side view of a layer stack 1 , which comprises several layers deposited on top of each other. According to the embodiment shown in view a) of Figure 1 , a crystalline seed substrate 2 is covered by a separation layer 3, which is a porous silicon. A semiconductor layer 4 is arranged on top of the separation layer 3 by epitaxial growth, wherein the semiconductor layer 4 in this embodiment consists of monocrystalline silicon.

[0070] The porous separation layer 3 facilitates a clean separation of the semiconductor layer 4 from the seed substrate 2, such that the semiconductor wafer 4’, which forms at least a part of the semiconductor layer 4 can be used for the manufacture of electronic components and the seed substrate 2 can be reused for growth of further semiconductor layers 4.

[0071] As can be seen from view a) of Figure 1 , the semiconductor layer 4 comprises a core area 5, which is arranged above the flat surfaces of the seed substrate 2 and the separation layer 3. Furthermore, the semiconductor layer 4 has an overlap portion 6 with which the semiconductor layer 4 extends beyond the side surfaces of the seed substrate 2 and the separation layer 3 and encloses them laterally. This is typically due to the fact that the epitaxially grown layers are formed on the entire surface of the seed substrate 2 or the separation layer 3, for example as a result of chemical vapor deposition, and are therefore not limited to certain spatial areas. View b) of Figure 1 is a top view of the layer stack 1 , whereby it can be seen that the layer stack 1 has a substantially rectangular lateral outline / shape and the overlap portion 6 surrounds the core area 5 on the circumference.

[0072] In order to use the semiconductor wafer 4’ for the production of electronic components, it is often necessary that it has a surface with a low total thickness variation. In particular, the thickness of the semiconductor layer 4 should be in the range that a total thickness variation is below 25 pm. Therefore, it is necessary to separate the core area 5 of the semiconductor layer 4 from the layer stack 1 and especially from the typically uneven overlap portion 6. The methods used for this are explained below with reference to Figures 2-5.

[0073] According to view a) of Figure 2, a layer stack 1 is provided, which is the same as described with respect to Figure 1. As already explained in Figure 1 , the layer stack 1 has an external semiconductor layer 4, which comprises a core area 5 that shall be separated from the layer stack 1 and in particular the overlap portion 6.

[0074] To separate the semiconductor wafer 4’ from the layer stack 1 , a pre-damage 8 is created by means of a pre-damaging device 7, e.g. a laser, which is shown in view b) of Figure 2. This pre-damaging device 7 in the form of a laser creates a pre-damage 8 in the semiconductor layer 4 in such a way that its fracture stress is locally reduced. Rather, the laser power is adjusted in such a way that the pre-damage 8 is formed within the semiconductor layer 4 and does not necessarily have to extend to the surface of the layer stack 1 or the semiconductor layer 4 respectively. Alternatively, the laser power of the predamaging device 7 is adjusted in such a way that a superficial depression in the form of the said trench is created by laser ablation. A trench within the semiconductor layer 4 could be also achieved by means of scratching.

[0075] In principle, it is sufficient for a pre-damage 8 to be limited to a selective area, in particular a small point or line, which only covers a part of the separation line 11 along which the semiconductor wafer 4’ with the area 5 shall be separated from the layer stack 1 as shown in the further embodiments in figures 4 to 7. In this case, the pre-damage 8 has a length of about 100 pm to 1000 pm. The pre-damaging device 7 in figures 2 and 3 comprises a pulse laser with which the desired pre-damage 8 is introduced into the semiconductor layer 4 as part of the layer stack 1. By moving the pre-damaging device 7 relative to the layer stack 1 , the pre-damage 8 can also be positioned at any point from which the separation line 11 is to be generated. In this embodiment, the pre-damaging device 7 is moved relative to the layer stack 1 in such a way that the predamage 8 is formed with the length of the layer stack 1. This allows also to cause a deep pre-damage 8 within the semiconductor layer 4. In another embodiment, the layer stack 1 may also rest relative to the pre-damaging device 7 and the laser radiation of the pre-damaging device 7 may be deflected by means of mirrors. Through such mirrors, the laser radiation 10 is directed and applied on the semiconductor layer 4 according to the desired separation line 11.

[0076] Subsequently, according to view c) of Figure 2, a device 9 for thermal laser separation (TLS) is used to apply laser radiation 10 to the semiconductor layer 4 along a separation line 11 , along which the semiconductor layer 4', built by the area 5, should be separated from the layer stack 1 through a crack 13.

[0077] The starting point for device 9 is built by the pre-damage 8 created in the previous step. The laser radiation is applied directly within the semiconductor layer 4. As a result, the semiconductor layer 4 is heated locally leading to the formation of thermally induced stress within the semiconductor layer 4. This causes a formation of a crack 13 in the pre-damaged area, which propagates along the separation line 11. This makes it possible to separate the area 5 of the semiconductor layer 4 from its overlap region 6 and from the rest of the layer stack 1 , wherein the semiconductor wafer 4’, built by the area 5, is processed further to electronic devices. The crack 13 is formed along the separation line 11. In this embodiment, the crack 13 is formed substantially identical to the separation line 11. However, the crack 13 may have an offset to this separation line 11 , e.g. by stress through defects or crystal faults in the crystalline structure at the edge area within the semiconductor layer 4.

[0078] In order to enhance the formation of thermally induced stress, the device 9 is designed to apply a cooling medium 12, in particular an aerosol, during heating of the semiconductor layer 4 by laser radiation 10, wherein the cooling medium

[0079] 12 is applied laterally displaced to the laser radiation 10. The aerosol serves as a cooling medium 12 and, as already mentioned, reinforces and directs the formation of thermally induced stresses, which lead to the formation of a crack

[0080] 13 and a controlled and directed propagation of the crack 13 within the semiconductor layer 4.

[0081] The laser radiation 10 is generated by means of a continuous wave laser in device 9. This enables the thermally induced stresses to be applied continuously into the semiconductor layer 4. The wavelength of the laser is 1060 nm or 1064 nm. In an embodiment not shown here in detail, the layer stack 1 may also rest relative to the device 9 and the laser radiation 10 may be deflected by means of mirrors or other guiding means. Through such mirrors, the laser radiation 10 is directed and applied on the semiconductor layer 4 according to the desired separation line 11.

[0082] It is essential that the laser power of laser radiation 10 is selected in a way that besides heating the laser itself does not cause any other degradation of the semiconductor layer 4. In particular, there is no change in the microstructure or crystalline structure through application of a laser radiation 10 on the semiconductor layer 4 nor on the separation layer 3 or the seed substrate 2.

[0083] Once the cracks 13 have formed, the semiconductor wafer 4’, which is built by the core area 5, is separated from the layer stack 1 and can be detached from the layer stack 1 and the overlap portion 6, as shown in view d) of Figure 2.

[0084] Figure 3 shows the process steps shown in Figure 2, wherein the side views a)- d) of Figure 3 correspond to the perspective views a)-d) of Figure 2.

[0085] View a) of Figure 3 is identical to view a) of Figure 1 and shows a layer stack 1 with a seed substrate 2, a separation layer 3 and a semiconductor layer 4 comprising an area 5 and an overlapping portion 6.

[0086] In view b) of Figure 3, it is shown as one example how the pre-damage 8 of the semiconductor layer 4 is created, with the pre-damaging device 7 creating a deep scribe as deep pre-damage 8. As can be clearly seen from view b), the pre-damage 8 in form of a deep scribe is created with a depth that does not reach the separation layer 3, so that this separation layer 3 and the underlying seed substrate 2 remain unaffected. In addition, the deep pre-damage 8 does also not reach the surface of the semiconductor layer 4. Rather, the generation of the pre-damage 8 only serves to reduce the fracture stress of the semiconductor layer 4 in order to separate the area 5 from the remaining part of the layer stack 1 and the overlap portion 6. The pre-damage 8 is created along the length of the layer stack 1 .

[0087] View c) of Figure 3 shows that device 9 applies laser radiation 10 starting from the pre-damage 8 and then immediately cools the semiconductor layer 4 with cooling medium 12, wherein the cooling medium 12 is applied laterally displaced to the laser radiation 10. This causes thermally induced stresses within the semiconductor layer 4, which lead to the formation of a crack 13 starting from the pre-damage 8 and extending along the separation line 11. This crack 13 along the separation line 11 causes the core area 5, which forms the semiconductor wafer 4’ to be further processed, to separate from the layer stack 1 and in particular from the overlap portion 6. For the detachment of the semiconductor wafer 4’ from the layer stack 1 , as shown in view d) of Figure 3, a detachment force is applied on the semiconductor wafer 4’.

[0088] Figures 4 and 5 show an alternative method to separate the semiconductor wafer 4’ from the layer stack 1. In contrast to figures 2 and 3, the pre-damage 8 is an initial pre-damage 8 and a terminal pre-damage 8’ at the end of the cutting line, which are generated in a small area of the semiconductor layer 4 and do not run along the length of the layer stack 1 compared to the deep scribe as deep pre-damage 8 in figures 2 and 3.

[0089] The initial scribe as initial pre-damage 8 and terminal pre-damage 8’ are formed in this embodiment as a trench with a length of about 200 pm as shown in figures 4b) and 5b). This trench is also formed by a laser. However, the trench could be also formed by other methods like scribing. In addition to the first predamage 8 in figure 4b) at the right side, a further pre-damage 8’ is created on the left side in figure 4b), which should avoid that the crack 13 breaks out at the end of the separation 11 and in particular at the edge of the semiconductor layer 4. In contrast to the separation line 11 in figures 2 and 3, along which the deep pre-damage 8 extends, the separation line 11 in figures 4 and 5 is only a path, along which the device 9 is moved.

[0090] Beyond this difference in the pre-damages 8 and 8’, the process shown in Figures 4 and 5 does not differ in principle from the process as shown in Figures 2 and 3. According to view a) of Figures 4 and 5, a layer stack 1 is provided and, according to view b) of Figures 4 and 5, provided with initial pre-damages 8, 8’ within the semiconductor layer 4, in particular to reduce fracture stress, as mentioned above. Furthermore, according to view c) of Figures 4 and 5, the semiconductor layer 4 is exposed to laser radiation 10, which results in thermal stresses within the semiconductor layer 4 as described above and leads to the formation of a crack 13 along a separation line 11 , along which the semiconductor wafer 4’ is separated from the layer stack 1 and can be detached from the layer stack 1 by a detachment force.

[0091] Figures 6 and 7 show an alternative method to separate the semiconductor wafer 4’ from the layer stack 1. In contrast to Figures 2 to 5, no additional cooling medium 12 is used in this process to generate thermally induced stresses. Rather, the temperature difference between the heated region of the semiconductor layer 4 and the ambient temperature, which can be approximately 300K, is sufficient to generate the thermally induced stresses required for crack propagation of the crack 13 in the semiconductor layer 4.

[0092] Beyond this, the process shown in Figures 6 and 7 does not differ in principle from the process as shown in Figures 2 and 3. According to view a) of Figures 6 and 7, a layer stack 1 is provided and, according to view b) of Figures 6 and 7, provided with a deep pre-damage 8 in form of a deep scribe within the semiconductor layer 4, in particular to reduce fracture stress. The deep predamage 8 forms a separation line 11 within the semiconductor layer 4. In this embodiment, the layer stack 1 rests relative to the pre-damaging device 7 and the laser radiation 10 of the pre-damaging device 7 is deflected by means of mirrors. For the deflection of the laser radiation 10, the pre-damaging device 7 comprises one or more galvo scanners. Through such mirrors, in particular the galvo scanners , the laser radiation 10 is directed and applied on the semiconductor layer 4 according to the desired separation line 11. As mentioned above, and according to view c) of Figures 6 and 7, the semiconductor layer 4 is exposed to laser radiation 10, which results in the thermal stresses also described above and leads to the formation of a crack 13 along the separation line 11 , along which the semiconductor wafer 4’ is separated from the layer stack 1 and is further detached from the layer stack 1 by a detachment force.

[0093] List of reference numbers

[0094] 1 layer stack

[0095] 2 seed substrate

[0096] 3 separation layer

[0097] 4 semiconductor layer

[0098] 4’ semiconductor wafer

[0099] 5 area

[0100] 6 overlap portion

[0101] 7 pre-damaging device

[0102] 8, 8’ pre-damage

[0103] 9 device

[0104] 10 laser radiation

[0105] 11 separation line

[0106] 12 cooling medium

[0107] 13 crack

Claims

Claims1. A method for separating a semiconductor wafer (4’) from a layer stack (1) comprising the following steps:A) Providing the layer stack (1) that comprises a seed substrate (2), a separation layer (3) on the seed substrate (2), and a semiconductor layer (4) formed on the separation layer (3), which preferably is produced epitaxially on the separation layer (3);B) Separating the semiconductor wafer (4’), which is at least a part of the semiconductor layer (4), at least partially from the layer stack (1), characterized by a first step B1), wherein the semiconductor layer (4) is pre-damaged in an area with a pre-damage (8) such that the fracture stress of the semiconductor layer (4) in this area is reduced, wherein step B1) is performed before and / or during step B); and a second step B2), wherein thermal energy means from a source is applied to the semiconductor layer (4) along a separation line (11), wherein the separation line (11) comprises the pre-damage (8) or in particular starts from the pre-damage (8) of the semiconductor layer (4) and wherein the thermal energy means heats and / or cools the semiconductor layer (4) locally such that a thermally induced stress is generated in order to cause a crack (13) that runs along the separation line (11).

2. Method according to claim 1 , characterized in that the thermal energy means is a heating means, and a cooling medium (12), preferably an aerosol, is applied along the separation line (11) before, during and / or after applying the heating means, in particular that the cooling medium (12) and the heating means are applied laterally displaced or that the thermal energy means is a cooling means, and a heating medium, is applied along the separation line (11) before, during and / or after applying the cooling means, in particular that the heating medium and the cooling means are applied laterally displaced.

3. Method according to one of the preceding claims, characterized in that the thermal energy means is a laser radiation (10) from a laser source as source, preferably that the laser radiation (10) is emitted continuously and, in particular, that the laser source is a continuous wave laser.

4. Method according to claim 3, characterized in that a power of the laser radiation (10) is selected such that it is essentially absorbed in the semiconductor layer (4) and that the power of the laser radiation (10) is selected such that the semiconductor layer (4) retains its original crystal structure.

5. Method according to any one of the preceding claims 3 to 4, characterized in that the wavelength of the laser radiation (10) is in the infrared range, preferably between 780 nm and 2000 nm, highly preferably between 800 nm and 1500 nm, particularly preferably between 1000 and 1100 nm, especially between 1060 nm and 1064 nm, and / or that the laser radiation (10) is deflected by means of mirrors and / or a galvo scanner and applied to the semiconductor layer (4).

6. Method according to any one of the preceding claims, characterized in that the thermal energy means is applied continuously or in a pulsed manner and / or that the layer stack (1) is moved relative to the source according to a shape of the separation line (11).

7. Method according to any one of the preceding claims, characterized in that the layer stack (1) is at rest relative to the source, and the thermal energy means is guided by guiding means to the semiconductor layer (4).

8. Method according to any one of the preceding claims, characterized in that the pre-damage (8) is affected by means of the laser source or a further laser source, in particular by means of a pulsed laser source, preferably a laser source with a pulse length in the nanosecond range, and / or is affected out by means of a scribe.

9. Method according to any one of the preceding claims, characterized in that the pre-damage (8) is an initial pre-damage, wherein the initial pre-damage covers only a small area of the semiconductor layer (4) and has preferably a length of about at least 1 pm to 2000 pm, more preferable with a length of about at least 100 pm to 1000 pm.

10. Method according to any one of the preceding claims, characterized in that the pre-damage (8) is a deep pre-damage, which extends in particular along the separation line (11) and has a length up to the dimensions of the semiconductor layer (4), wherein the deep long pre-damage preferably extends within the semiconductor layer (4) and not on its surface.

11. Method according to any one of the preceding claims, characterized in that a plurality of pre-damages (8) is created on the semiconductor layer (4), wherein the plurality of pre-damages (8) is distributed on the semiconductor layer (4) according to the course of the separation line (11).

12. Method according to claim 11 , characterized in that a further pre-damage (8) is created at an end and / or at an edge and / or at a curve of the separation line (11) and / or at a crossing of at least two separation lines (11).

13. Method according to any one of the preceding claims, characterized in that the semiconductor layer (4) comprises an overlap portion (6) which projects beyond one side surface of the seed substrate (2) and / or of the separation layer (3) and in particular at least partially covers one or more side surfaces of the seed substrate (2) and / or of the separation layer (3).

14. Method according to claim 13, characterized in that the overlap portion (6) is formed of the same material as the semiconductor layer (4).

15. Method according to claim 13 or 14, characterized in that the separation line (11) is formed such that the overlap portion (6) is separated from the semiconductor wafer (4’) and / or from the layer stack (1) in process step B).

16. Method according to any one of the preceding claims, characterized in that the separation line (11) is completely closed, in particular that the separation line (11) is formed such that the detached semiconductor wafer (4’) is substantially rectangular.

17. Method according to any one of the preceding claims, characterized in that the semiconductor layer (4) is made of silicon, in particular monocrystalline silicon, and / or that the separation layer (3) is porous silicon.

18. Method according to any one of the preceding claims, characterized in that the semiconductor layer (4) has a thickness in the range from 5 pm to2000 pm, preferably from 10 pm to 500 pm, particularly preferably from 50 pm to 200 pm.

19. Method for producing a semiconductor wafer (4’), comprising the following stepsV1) Providing a seed substrate (2);V2) Creating a separation layer (3) on the seed substrate (2);V3) Forming a semiconductor layer (4) on the separation layer (3), preferably by chemical vapor deposition epitaxy, in order to provide a layer stack (1) comprising the seed substrate (2), the separation layer (3), and the semiconductor layer (4); andV4) Detaching the semiconductor wafer (4’), which is at least a part of the semiconductor layer (4), from the layer stack (1), in particular by a detachment force, characterized in that prior to or during method step V4) a method according to any one of claims 1 to 18 is performed.

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