Method for fabricating trenches in group III-V multi-junction solar cells

Abstract

The invention relates to a method for producing a trench-like structure in a group III-V multi-junction solar cell, which has an upper side and a lower side, and which comprises a substrate arranged on the lower side, and which has an epitaxial layer system on the front side, said epitaxial layer system comprising at least one trench-like structure, said trench-like structure comprising at least one trench-like structure, said trench-like structure comprising at least one trench-like structure and at least one trench-like structure, said trench-like structure comprising at least one trench-like structure, said trench-like structure comprising at least one trench-like structure and at least one trench-like structure. According to the invention, the epitaxial layer system comprises at least one first III-V solar cell formed on the upper side, on which an organic layer is arranged, and in a first method step, a first trench having a width X is produced by means of a laser, and in a second method step, a second trench having a width X is produced by means of a laser. A second trench having a width Y and a bottom surface formed in the substrate is produced by means of the laser, the width Y being smaller than the width X, such that the first step is formed by the implementation of the second method step.

Description

Background Art

[0001] DE 69 216 502 T2 discloses a laser cutting method for producing through-holes for through-hole contacts in GaAs or Ge solar cells, wherein the disclosed method is intended in particular to avoid CVD SiO 2 Disadvantages of coating.

[0002] In a first method step, a continuous opening with a large diameter is produced by means of laser pulses from a solid-state laser. In order to prepare the very rough surface produced during the laser bombardment on the side walls of the through-opening for subsequent metallization, in a second method step, in particular the side walls are impregnated with polyimide or, in the case of smaller continuous openings, the openings are completely filled with polyimide.

[0003] In a subsequent method step, the polyimide is heated to harden. The polyimide is referred to as a dielectric coating.

[0004] In a fourth method step, through-holes with a small diameter are produced in the polyimide by means of a laser, wherein the polyimide has a smooth surface on the newly produced side surfaces after the laser bombardment. In a subsequent method step, the polyimide side surfaces are coated with a metal.

[0005] Coating with polyimide also avoids the need to smooth the side surfaces by aggressive chemical etching and protects the active region of the solar cell and the GaAs substrate or Ge substrate from etching attacks. In addition, polyimide has good adhesion to the substrate and to the feedthrough metallization to be applied. Summary of the invention

[0006] Against this background, the object of the present invention is to specify a method for further developing the prior art.

[0007] This object is achieved by a method for producing trenches in a III-V multijunction solar cell having the features of claim 1. Advantageous embodiments of the invention are the subject matter of the dependent claims.

[0008] According to this subject matter, a method for fabricating trenches in a III-V multi-junction solar cell is provided.

[0009] The III-V multi-junction solar cell has an upper side and a lower side and includes a substrate arranged on the lower side.

[0010] The substrate is in the form of a semiconductor wafer and has an epitaxial layer system with a plurality of III-V layers on the front side.

[0011] The epitaxial layer system comprises at least one first III-V solar cell constructed on the upper side of the III-V multi-junction solar cell, wherein an organic layer is arranged on the first III-V solar cell.

[0012] In a first method step, a first trench having a width X and a bottom surface formed in the epitaxial layer system is produced by means of a laser.

[0013] In a second method step, a second trench having a width Y and a bottom formed in the substrate is produced in the bottom of the first trench by means of a laser, wherein the width Y is smaller than the width X, so that the first step is formed by carrying out the second method step.

[0014] The aforementioned method steps are carried out in the order given, or, in an alternative, the second method step is carried out before the first method step, wherein, in an alternative implementation, the second trench (GA2) is produced before the first trench (GA1).

[0015] It should be noted that the organic layer includes a lacquer layer and / or a polyimide layer and / or a plastic layer. It should be understood that these layers are constructed to be unstructured in order to avoid a masking step, ie, a photolithography step.

[0016] Furthermore, it is to be understood that the substrate is a semiconductor substrate having a diameter of at least or exactly 100 mm or at least or exactly 150 mm.

[0017] It should be noted that, currently, the term "dielectric layer" exclusively includes inorganic layers, especially oxide layers and nitride layers.

[0018] Furthermore, it should be noted that the term “groove” refers to an elongated recess having a length of at least 100 μm or at least 200 μm.

[0019] It should be understood that the term “epitaxial layer system” exclusively denotes III-V layers produced on a substrate by means of an epitaxial process.

[0020] Furthermore, it should be noted that the side surfaces generated in each case in all laser-based method steps are designed to be exactly vertical or approximately vertical, wherein the term “approximately vertical” is to be understood as meaning a deviation of less than 5°.

[0021] In one embodiment, the opening angle is less than 20° or less than 10°. In this context, the “opening angle” is understood to be an angle of inclination relative to the vertical, wherein the inclination is “outward” so that the total angle is greater than 90°. In other words, in a non-vertical embodiment, the groove has a larger diameter on the top side than in the region of the bottom, i.e. is conically configured, and the groove has the shape of a conical configuration in cross section.

[0022] One advantage is that with this method, an organic layer, in particular a lacquer layer, can be flexibly arranged with continuous trenches at any location on the top side of a III-V multijunction solar cell without further structuring by means of complex and expensive photolithographic processes.

[0023] Another advantage is that the at least two-stage laser method is surprisingly insensitive to insignificant changes in the layer structure, especially the thickness and number of III-V layers. The method is also insensitive to changes in substrate material from Ge to GaAs and vice versa.

[0024] Insignificant changes are especially changes in the III-V material composition and stoichiometry and changes in the total layer thickness less than 20 μm.

[0025] A further advantage is that the settings of the laser can be easily modified in the laser-based method steps even in the case of significant changes in the substrate thickness.

[0026] Currently, a variation of at least 50 μm or at least 100 μm in the thickness of the substrate is referred to as a significant variation.

[0027] Another advantage of the laser-based method step compared to the wet-chemical method step is that under-etching does not occur. In addition, the wet-chemical method step is basically based on the structured application of the protective lacquer on the surface and / or side surfaces of the groove-shaped structure, especially when the groove-shaped structure includes at least one step.

[0028] In one embodiment, in a third method step, a slit is produced along the trench-shaped structure starting from the upper side or the lower side into the bottom of the second trench by means of a separation process in order to separate the substrate. In one embodiment, the slit has a length of at least 50 μm. It should be understood here that two method steps, namely the first method step and the second method step, have already been carried out.

[0029] In another embodiment, the slit is designed continuously along a straight line and is produced with a width Z, wherein the width Z is in the range between 5 μm and 120 μm or between 10 μm and 80 μm.

[0030] In one configuration, in the third method step, the substrate is singulated starting from the underside of the III-V multi-junction solar cell or the underside of the substrate by means of a saw.

[0031] In another embodiment, a third method step is performed starting from the top side of the III-V multijunction solar cell using a laser to produce the slit. It should be understood that the laser-based method step is preferably performed directly after the other two laser-based method steps.

[0032] In other words, the third method step is performed either by means of a laser process starting from the upper side or alternatively by means of a saw starting from the lower side of the substrate. The further side surfaces in the slit produced during the implementation of the third method step are also designed to be approximately vertical or exactly vertical.

[0033] In addition, it should be noted that in the epitaxial layer system, the bottom surface constructed in the first trench includes a second trench, wherein the bottom surface of the second trench is constructed in the substrate. Correspondingly, the bottom surface of the first trench is divided into two parts by the second trench, wherein the two parts of the bottom surface of the first trench respectively run along the side surface made of the III-V group material.

[0034] If the second trench includes a slit extending to the underside of the substrate, the bottom surface of the second trench is divided into two parts, namely by the slit, wherein the two parts of the bottom surface of the second trench respectively run along the side surface formed by the material of the substrate.

[0035] It is understood that the surface formed in the bottom region is flat or in a first approximation flat or has unevenness and that it consists exclusively of III-V materials on the bottom in the epitaxial layer system or exclusively of substrate material in the case of the bottom surface in the substrate.

[0036] If the method steps are not carried out in the specified order, in one embodiment the second method step is carried out first and then either the first method step or the third method step is carried out. Then the method step that has not yet been carried out, ie the third method step or the first method step, is carried out.

[0037] It will be understood that after carrying out all three method steps, regardless of the order in which they are carried out, at least one first step or, in the case of the formation of the slits, two steps are always formed.

[0038] In other words, even if the method steps are carried out in a different sequence or the third method step is carried out in a different manner, the resulting step shapes in the cross-sectional view correspond to one another or are in particular designed to be exactly identical.

[0039] In one embodiment, the first two method steps (ie the first method step and the second method step) are performed sequentially without intermediate steps (eg a wet etching step or a dry etching step).

[0040] In a further embodiment, the third laser method step or all laser-based method steps are carried out successively without intermediate steps, such as, in particular, wet etching steps or dry etching steps.

[0041] In one development, instead of three successive laser method steps, more than three (for example four or five) method steps can also be carried out using a laser.

[0042] In one configuration, at least one further III-V solar cell is formed between the substrate and the first III-V solar cell.

[0043] In one embodiment, the first III-V solar cell as the uppermost III-V solar cell has a larger band gap or the same size band gap as the underlying II-V solar cell.

[0044] In order to electrically connect III-V solar cells arranged in a stack to one another in series, a plurality of tunnel diode layers are arranged between two III-V solar cells.

[0045] In a further embodiment, the two III-V solar cells have the same or different materials. In one embodiment, the uppermost III-V solar cell comprises an InGaP compound. In a further embodiment, the further II-V solar cell is configured as a second III-V solar cell and comprises a GaAs compound or an InGaAs compound.

[0046] In one embodiment, the substrate comprises or is made of Ge or GaAs. It will be appreciated that a large number of III-V layers are arranged on the substrate. Preferably, these layers are epitaxially grown by means of a MOVPE apparatus.

[0047] In a further embodiment, a substrate solar cell is formed in or on the upper side of the substrate. The substrate solar cell is a III-V solar cell in the case of a GaAs substrate and a VI solar cell in the case of a Ge substrate.

[0048] It should be noted that when the substrate is constructed as a Ge substrate, the Ge substrate is generally constructed as a p-Ge substrate, and when an n-Ge layer is constructed to construct a Group VI solar cell on or in the surface of a p-Ge substrate, the n layer is produced by dopant internal diffusion rather than by epitaxy.

[0049] In one embodiment, the substrate has a thickness in the range between 40 μm and 850 μm. In another embodiment, the thickness of the substrate is in the range between 150 μm and 750 μm.

[0050] In one embodiment, a further layer is constructed above the first III-V solar cell and below the organic layer, wherein the further layer is constructed as an organic layer and / or an inorganic layer. Preferably, the further layer comprises a passivation layer or consists of a passivation layer, for example at least one III-V layer and / or at least one silicon oxide layer and / or at least one silicon nitride layer.

[0051] In a further configuration, an antireflection layer is formed between the first III-V solar cell and the organic layer.

[0052] In another embodiment, the organic layer is formed as a continuous layer which covers the entire top side of the first III-V solar cell. In this case, the organic layer can be applied by spin coating or by other coating methods.

[0053] In one embodiment, a metal layer is arranged on the upper side of the III-V multi-junction solar cell between the first III-V solar cell and the organic layer in the region of the trenches or no metal layer is formed. In other words, the first method step and / or the second method step can be performed with or without forming a metal layer at the location of the trenches to be produced.

[0054] In one configuration, during the laser process at least one second step is formed in the trench in a direction from the upper side to the lower side of the III-V multi-junction solar cell.

[0055] In a further embodiment, wet-chemical etching is carried out after the first two method steps have been carried out using a laser, in particular for cleaning the surface and the side surfaces.

[0056] In one embodiment, depending on the etching solution used and its usage time, only one of the two steps is visible after etching. Preferably, the wet chemical etching step is carried out immediately after the second laser method step or after the first laser-based method step and the second laser-based method step.

[0057] In the wet etching step, the width of the first trench and the width of the second trench are respectively increased and / or the depth of the first trench and / or the depth of the second trench are increased. In one embodiment, the enlargement of the first trench and / or the second trench is in the range between 0.5 μm and 30 μm.

[0058] In one embodiment, the removal of the organic layer is carried out only after the wet-chemical etching step has been carried out. One advantage is that the surface is protected from etching attacks by the organic layer.

[0059] In one embodiment, the width X of the first groove is in the range between 20 μm and 300 μm. In another embodiment, the width Y of the second groove is in the range between 5 μm and 200 μm. However, it should be understood that the width of the first groove is always configured to be wider than the width of the second groove. Alternatively, the ratio of the width of the first groove to the width of the second groove is in the range between 1 and 10.

[0060] In one embodiment, a wet etching step is performed after all laser-based method steps have been carried out. One advantage of the wet etching step is that the surface is cleaned of residues from the laser-based method steps.

[0061] In one embodiment, a wet etching step is performed before the sawing step.

[0062] In another embodiment, the wet etching step is performed immediately after the second method step or after the first two method steps have been performed (i.e. before the organic layer is removed). One advantage is that the organic layer not only protects the top side or the surface of the multijunction solar cell from contamination during the laser-based method steps, but also from etching attacks.

[0063] In another embodiment, the organic layer is removed only after all laser-based method steps and wet etching steps have been performed. It should be noted that the organic layer is preferably removed by means of a wet chemical or dry etching process.

[0064] In one configuration, the substrate is designed as a semiconductor wafer having a diameter of at least or exactly 100 mm or at least or exactly 150 mm.

[0065] In a further configuration, the substrate is separated only along the trenches.

[0066] In one embodiment, the third method step is not carried out in the predefined region, so that the second bottom remains and the substrate is not severed in the predefined region. BRIEF DESCRIPTION OF THE DRAWINGS

[0067] The invention is explained in more detail below with reference to the drawings. Here, like parts are marked with the same reference numerals. The embodiments shown are highly schematic, i.e., the distances and the lateral and vertical extensions are not to scale and, unless otherwise indicated, do not have any inferable geometrical relationships with each other. It is shown:

[0068] Figure 1a-1c Cross-sectional view of the resulting trench structure after different laser process steps,

[0069] Figure 2 After the wet etching step and the paint stripping process Figure 1cA cross-sectional view of the trench structure of the embodiment shown in,

[0070] Figure 3 Cross-sectional view of the trench structure with slits after the third laser-based method step,

[0071] Figure 4 Cross-sectional view of a trench structure with slits after the sawing step,

[0072] Figure 5 Top view of a semiconductor wafer with trench structures and slits. DETAILED DESCRIPTION

[0073] The cross sections shown below each show a part of a multijunction solar cell structure MS, wherein the multijunction cell structure MS or a plurality of multijunction cell structures are formed on a semiconductor wafer having a diameter of at least 100 mm.

[0074] The following figures also only show one embodiment, in which method steps one to three are performed in the given order, that is, the first method step is performed first, the second method step is performed second, and if the third method step is performed, the third method step is performed after the second method step.

[0075] exist Figures 1a to 1c , various cross-sectional views of the resulting trench structure are shown, wherein Figure 1b The illustration shows the resulting trench structure after carrying out the first laser method step, and Figure 1c The illustration of shows the resulting trench structure after carrying out the second laser method step.

[0076] Figure 1a The illustration shows a cross-sectional view of an unprocessed multi-junction solar cell structure MS having a bottom side US and an upper side OS. A substrate layer SUB made of germanium or GaAs is formed on the bottom side US of the multi-junction solar cell structure MS. The substrate layer SUB has a bottom side, wherein the bottom side of the substrate layer SUB is formed on the bottom side US of the multi-junction cell structure MS.

[0077] An epitaxial layer system ES is formed on the upper side of the substrate layer SUB. The epitaxial layer system ES is arranged on the substrate layer SUB in a material-locking manner. It will be appreciated that the epitaxial layer system ES comprises a plurality of III-V layers.

[0078] The epitaxial layer system ES comprises at least one first solar cell (not shown), wherein the first solar cell is always designed as the uppermost solar cell even if in further embodiments (not shown) one or more further solar cells are designed between the first solar cell and the substrate SUB.

[0079] An organic layer SL designed as a protective lacquer is arranged on the epitaxial layer system ES. In the embodiment shown, the organic layer SL is connected to the top of the III-V epitaxial layer system ES on the semiconductor wafer over the entire surface and in a material-bonded manner.

[0080] In an embodiment not shown, a further layer or a further layer between the epitaxial layer system and the organic layer SL, in particular a passivation layer, is formed on the epitaxial layer system ES in a material-bonded manner.

[0081] exist Figure 1b In a first method step shown in , a first trench GA1 is produced by means of a laser, which first trench has an opening with a width X through the organic layer SL and has a first bottom surface BO1 formed in the epitaxial layer system ES. The produced side surfaces S1 are formed approximately vertically.

[0082] exist Figure 1c In the second method step shown in , an opening with a width Y and with a second bottom surface BO2 formed in the substrate SUB is produced in the bottom surface BO1 of the first trench GA1 by means of a laser, wherein the width Y is smaller than the width X. By producing the second trench GA2 only in a part of the first bottom surface BO1 of the epitaxial layer system, a first step STU1 is produced. The side surface S2 produced in the second method step is also designed approximately vertically.

[0083] It should be understood that in embodiments not shown, the method steps performed by means of a laser can also be performed in a different order. In particular, the second method step can be performed before the first method step.

[0084] exist Figure 2 The diagram shows the wet etching step and the paint stripping process after the Figure 1c The cross-sectional view of the groove structure of the embodiment shown in FIG. Figure 1c The difference between the icons.

[0085] In a wet etching step, the wet etching step is performed to clean the side walls S1, S2 and the first step STU1 as well as the second bottom surface BO2. The side surfaces S1 and S2 are set back, i.e. the width X of the first trench GA1 and the width Y of the second trench are slightly increased. The depth of the second trench GA2 is also slightly increased, i.e. the second bottom surface GA2 BO2 is set deeper into the substrate SUB.

[0086] In order to protect the top side OS of the multijunction solar cell MS from etching attacks, the protective lacquer on the top side OS is removed only after the wet etching process by means of a lacquer stripping process.

[0087] exist Figure 3In the diagram of FIG. , the combination after the third method step performed by means of a laser is shown Figure 1c The cross-sectional view of the groove structure of the embodiment shown in the figure is only described below. Figure 1c The difference between the icons.

[0088] In a third method step, an opening with a width Z is produced by means of a laser to form the slit SLI, wherein the opening has no bottom surface since the slit SLI is continuous, i.e. the slit SLI extends to the underside US, and the width Z is smaller than the width Y. By producing the slit only in a part of the second bottom surface BO2 of the epitaxial layer system, a second step STU2 is produced.

[0089] exist Figure 4 In the illustration of FIG. 1 , the subsequent joining of the slits produced in the third method step by means of the sawing step is depicted. Figure 2 The following is a cross-sectional view of the groove structure shown in the figure. Figure 2 The difference from the previous embodiment shown in the figure.

[0090] In a method step not shown, the substrate SUB is thinned from the underside US before the third method step is carried out.

[0091] After the semiconductor wafer has been thinned, starting from the underside US of the substrate SUB, slits SLI are produced in a sawing step by means of a saw in order to separate the solar cell stacks MS formed on the front side.

[0092] Figure 5 The illustration shows a plan view of a semiconductor wafer, wherein both trench structures and slits are formed on the semiconductor wafer.

[0093] The following only shows Figure 2 In the present embodiment, a slit is produced along the trench structure in a third method step. The slit always starts and ends at the edge of the semiconductor wafer.

[0094] On the top side OS of the multijunction solar cell structure MS, both a slit structure SLI and a trench structure without slits are shown. One advantage is that both structures are formed on the same semiconductor wafer.

[0095] It has been shown that structural elements of different sizes can be manufactured on semiconductor wafers.

Claims

1. A method for producing a groove-shaped structure (GA) in a III-V multi-junction solar cell (MS), the III-V multi-junction solar cell having an upper side (OS) and an underside (US), and the III-V multi-junction solar cell (MS) comprising a substrate (SUB) arranged on the underside (US), and the substrate (SUB) is designed as a semiconductor wafer and has an epitaxial layer system (ES) with a plurality of III-V layers on the front side, wherein: The epitaxial layer system (ES) comprises at least one first III-V solar cell constructed on the upper side (OS) of the III-V multi-junction solar cell (MS), an organic layer (SL) being arranged on the first III-V solar cell, - in a first method step, a first trench (GA1) having a width X and a bottom surface (BO1) formed in the epitaxial layer system (ES) is produced by means of a laser, - in a second method step, a second trench (GA2) having a width Y and a bottom surface (BO1) formed in the substrate is produced in the bottom surface (OS) of the first trench (GA1) by means of the laser, wherein the width Y is smaller than the width X, so that a first step is formed by carrying out the second method step, The aforementioned method steps are carried out in the order given or, in an alternative, the second method step is carried out before the first method step and the second trench (GA2) with width Y is produced before the first trench (GA1) with width X.

2. The method for producing a trench-like structure (GA) in a III-V multi-junction solar cell (MS) according to claim 1, characterized in that In a third method step, slits are produced along the trench-like structure (GA) starting from the upper side (OS) or starting from the lower side (US) into the bottom (BO2) of the second trenches by means of a separation process in order to separate the substrate.

3. The method for producing a groove-shaped structure (GA) in a III-V multi-junction solar cell (MS) according to claim 2 or 3, characterized in that In the third method step, the substrate (SUB) is singulated starting from the underside (US) of the III-V multi-junction solar cell (MS) by means of a saw.

4. The method for producing a groove-shaped structure (GA) in a III-V multi-junction solar cell (MS) according to claim 2 or 3, characterized in that The third method step is performed starting from the upper side (OS) of the III-V multi-junction solar cell using the laser.

5. The method for producing a groove-like structure (GA) in a III-V multi-junction solar cell (MS) according to claim 1 or claim 2 or claim 4, characterized in that After all laser-based method steps have been carried out, a wet etching step is performed.

6. The method for producing a groove-like structure (GA) in a III-V multi-junction solar cell (MS) according to claim 1 or claim 2 or claim 4, characterized in that The organic layer (SL) is removed only after all laser-based method steps and wet etching steps have been carried out.

7. Method for producing a trench (GA) in a III-V multi-junction solar cell (MS) according to any of the preceding claims, characterized in that The substrate (SUB) includes or is made of Ge or GaAs.

8. Method for producing a trench (GA) in a III-V multi-junction solar cell (MS) according to any of the preceding claims, characterized in that Further III-V layers are constructed above the first III-V solar cell and below the organic layer (SL).

9. Method for producing a trench (GA) in a III-V multi-junction solar cell (MS) according to any of the preceding claims, characterized in that The organic layer (SL) is configured as a continuous layer covering the entire upper side (OS) of the III-V multi-junction solar cell (MS).

10. Method for producing a trench (GA) in a III-V multi-junction solar cell (MS) according to any of the preceding claims, characterized in that At least one III-V layer and / or at least one dielectric layer (SiO 2 ) is arranged between the first III-V solar cell and the organic layer (SL).

11. Method for producing a trench (GA) in a III-V multijunction solar cell (MS) according to any of the preceding claims, characterized in that An anti-reflection layer is configured between the first III-V solar cell and the organic layer (SL).

12. Method for producing a trench (GA) in a III-V multi-junction solar cell (MS) according to any of the preceding claims, characterized in that On the upper side (OS) of the III-V multijunction solar cell (MS), before the first method step, a metal layer is arranged in the region of the trench between the first III-V solar cell and the organic layer (SL) or no metal layer is formed.

13. Method for producing a trench (GA) in a III-V multi-junction solar cell (MS) according to any of the preceding claims, characterized in that The first groove (GA1) extends only along straight lines and / or curves and a first step (STU1) or at least two steps (STU1, STU2) or exactly two steps (STU1, STU2) are formed along the extension of the first groove (GA1) and the second groove (GA2) by means of the laser-based method steps.

14. Method for producing a trench-like structure (GA) in a III-V multijunction solar cell (MS) according to any of the preceding claims, characterized in that The third method step is not carried out in the predefined region, so that the second bottom (BO2) remains and the substrate is not singulated in the predefined region.

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