METHOD FOR PRODUCING A TRENCH IN A III-V MULTI-JUNCTION SOLAR CELL

Description

[0001] From DE 692 16 502 T2 a laser cutting method for producing through holes for vias in GaAs or Ge solar cells is known, wherein the disclosed method is intended to avoid, among other things, the disadvantages of a CVD SiO 2 coating in through holes.

[0002] In a first process step, a large-diameter through-hole is created using laser pulses from a solid-state laser. To prepare the very rough surfaces on the sidewalls of the through-hole, which are created by the laser bombardment, for subsequent metallization, the sidewalls are coated with a polyimide in a second process step, or, in the case of smaller through-holes, the entire opening is filled with polyimide.

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

[0004] In a fourth process step, a small-diameter through-hole is created within the polyimide using a laser, resulting in smooth surfaces on the newly formed sides of the polyimide after laser irradiation. These polyimide sides are then coated with a metal in a subsequent process step.

[0005] Applying polyimide eliminates the need for aggressive chemical etching to smooth the surface areas and protects the active area of ​​the solar cell and the GaAs or Ge substrate from etching. Furthermore, the polyimide exhibits good adhesion to both the substrate and any subsequent feedthrough metallization.

[0006] WO 2010 / 042981 A1 also discloses a laser cutting process for the production of III-V multi-junction solar cells, in which continuous grooves are produced by combining a laser process with a photolithography process.

[0007] Against this background, the object of the invention is to provide a method by which the prior art is further developed.

[0008] The problem is solved by a method for producing a trench in a III-V multi-junction solar cell with the features of claim 1. Advantageous embodiments of the invention are the subject of dependent claims.

[0009] According to the subject matter, a method for producing a trench in a III-V multi-junction solar cell is provided.

[0010] The III-V multi-junction solar cell has a top and a bottom and includes a substrate arranged on the bottom.

[0011] The substrate is designed as a semiconductor disk and has an Epi-layer system with several III-V layers on one front side.

[0012] The Epi-layer system comprises at least one first III-V solar cell formed on the top side of the III-V multi-junction solar cell, wherein an organic layer is arranged on the first III-V solar cell.

[0013] In a first process step, a first trench with a width X and a soil surface formed in the Epi layer system is created using a laser.

[0014] In a second process step, a second trench with a width Y and a bottom surface formed in the substrate is created in the bottom surface of the first trench using the laser, wherein the width Y is smaller than the width X, so that a first stage is formed with the execution of the second process step, the aforementioned process steps are carried out in the specified order or alternatively the second process step is carried out before the first process step, wherein the second trench (GA2) is created before the first trench (GA1) in the alternative execution.

[0015] It should be noted that the organic layer comprises a lacquer layer and / or a polyimide layer and / or a plastic layer. It is understood that the layers are unstructured, specifically to avoid a masking step, i.e., a lithography step.

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

[0017] It should be noted that in the present context the term dielectric layer refers exclusively to inorganic layers, in particular oxide and nitride layers.

[0018] Furthermore, it should be noted that the term "trench" refers to an elongated depression with a length of at least 100 µm or at least 200 µm.

[0019] It is understood that the term "epi-layer system" refers exclusively to the III-V layers produced on the substrate by means of an epitaxy process.

[0020] Furthermore, it should be noted that the side surfaces created in all laser-based process steps are formed exactly perpendicular or almost perpendicular, whereby the term "almost perpendicular" is understood to mean a deviation of less than 5°.

[0021] In further training, the opening angle is less than 20° or less than 10°. The opening angle here refers to the angle inclined to the vertical, with the inclination being outwards, so that the overall angle is greater than 90°. In other words, the trench, in a non-vertical design, has a larger diameter at the top than at the bottom; it is therefore conical, meaning the trench has a conical cross-section.

[0022] One advantage is that, with this method, the organic layers, especially lacquer layers, can be flexibly arranged in continuous trenches at any point on the top of the III-V multi-junction solar cell without further structuring using elaborate and expensive lithography processes.

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

[0024] Non-essential changes include, in particular, changes to the III-V material composition and stoichiometry, and changes to the total layer thickness of less than 20 µm.

[0025] Another advantage is that even with a significant change in substrate thickness, the laser settings can be easily adjusted in laser-based process steps.

[0026] In the present case, a significant change is defined as a change in the thickness of the substrate by at least 50 µm or by at least 100 µm.

[0027] Another advantage of laser-based processes compared to wet-chemical processes is that no undercutting occurs. Furthermore, wet-chemical processes generally require the structured application of protective coatings to the surface and / or sides of trench-like structures, especially if the trench-like structure includes at least one step.

[0028] In one embodiment, a third process step involves creating a slit in the bottom of the second trough along the trench-shaped structure, either from the top or the bottom, by means of a separation process, in order to divide the substrate. In a further development, the slit has a length of at least 50 µm. It is understood that the first and second process steps have already been carried out.

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

[0030] In a further training, in the third process step the substrate is cut using a saw starting from the underside of the III-V multi-solar cell or from the underside of the substrate.

[0031] In another further development, the third process step for generating the slot is carried out using the laser, starting from the top side of the III-V multi-junction solar cell. It is understood that the laser-based process step is preferably carried out immediately after the other two laser-based process steps.

[0032] In other words, the third process step is carried out either using a laser process starting from the top side or, alternatively, starting from the underside of the substrate using a saw. The additional side surfaces created during the execution of the third process step at the slots are also formed almost vertically or exactly vertically.

[0033] Furthermore, it should be noted that the soil surface formed in the first trench within the epi-layer system comprises a second trench, the soil surface of which is formed within the substrate. Accordingly, the soil surface of the first trench is divided into two parts by the second trench, with each part of the soil surface of the first trench extending along the sides composed of III-V material.

[0034] If the second trench includes a slit extending to the underside of the substrate, the bottom surface of the second trench is also divided into two parts, namely by the slit, with the two parts of the bottom surface of the second trench each running along the side surfaces formed from the material of the substrate.

[0035] It is understood that the surfaces formed in the soil areas are flat or approximately flat or have unevenness, and that the surface on the ground in the Epi-layer system consists exclusively of a III-V material, or in the case of the soil surface in the substrate, consists exclusively of the substrate material.

[0036] If the process steps are not executed in the specified order, in a further training session the second process step is executed first, followed by either the first or the third process step. After this, the remaining process step, i.e., either the third or the first process step, is executed.

[0037] It is understood that after all three process steps have been carried out, regardless of the order in which they are performed, at least one first stage is always formed, or in the case of the formation of the slot, two stages are formed.

[0038] In other words, even if the process steps are carried out in different sequences or the third process step is performed in different ways, the resulting step shapes correspond to each other in a cross-sectional view or are, in particular, exactly the same.

[0039] In one embodiment, the first two process steps, i.e., the first process step and the second process step, are carried out one after the other without intermediate steps, such as wet etching steps or dry etching steps.

[0040] In another embodiment, the third laser process step or all laser-based process steps are carried out sequentially without intermediate steps, in particular wet etching steps or dry etching steps.

[0041] In further training, instead of the three consecutive laser process steps, more than three process steps, for example four or five, are carried out using the laser.

[0042] In a further development, at least one additional 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 a band gap of the same size as the III-V solar cells below it.

[0044] To electrically connect the stacked III-V solar cells in series, several tunnel diode layers are arranged between the two III-V solar cells.

[0045] In another embodiment, the two III-V solar cells comprise the same or different materials. In one embodiment, the uppermost III-V solar cell includes an InGaP compound. In another embodiment, the second III-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 consists of Ge or GaAs. It is understood that a plurality of III-V layers are arranged on the substrate. Preferably, the layers are grown epitaxially using a MOVPE system.

[0047] In another embodiment, a substrate solar cell is formed in or on the top surface of the substrate. In the case of the GaAs substrate, the substrate solar cell is a III-V solar cell; in the case of Ge, it is a VI-solar cell.

[0048] It should be noted that when the substrate is formed as a Ge substrate, the Ge substrate is usually formed as a p-Ge substrate, and in the case of the formation of an n-Ge layer for the formation of the group VI solar cell on the surface or in the surface of the p-Ge substrate, the n-layer is generated by diffusion of dopants and not by epitaxy.

[0049] In one advanced training, the substrate has a thickness ranging from 40 µm to 850 µm. In another advanced training, the substrate thickness ranges from 150 µm to 750 µm.

[0050] In one embodiment, further layers are formed above the first III-V solar cell and below the organic layer, wherein the further layers are formed as organic and / or inorganic layers. Preferably, the further layers comprise or consist of passivation layers, for example, at least one III-V layer and / or at least one silicon oxide and / or at least one silicon nitride layer.

[0051] In another advanced training, an antireflective 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 covering the entire top surface of the first III-V solar cell, i.e., it is unstructured. In this case, the organic layer can be applied either by spin coating or by another coating method.

[0053] In one embodiment, a metal layer is arranged on the top side of the III-V multi-junction solar cell in the area of ​​the trench between the first III-V solar cell and the organic layer, or, conversely, no metal layer is formed. In other words, the first process step and / or the second process step can be carried out depending on whether or not a metal layer is formed at the location of the trench to be created.

[0054] In a further training, at least a second stage is formed in the trench in one direction from the top to the bottom of the III-V multi-solar cell during the laser process.

[0055] In another training course, after the first two process steps with the laser, a wet chemical etching is carried out, especially for cleaning the surfaces and the side surfaces.

[0056] In a further development process, depending on the etching solution used and its application time, only one of the two stages is visible after etching. Preferably, the wet chemical etching step is carried out immediately after the second laser process step, or after the first laser-based process step and the second laser-based process step.

[0057] In the wet etching step, the width of the first groove and the width of the second groove are each increased, and / or the depth of the first groove and / or the second groove is increased. In a further development, the enlargement of the first groove and / or the second groove is in a range between 0.5 µm and 30 µm.

[0058] In a further training course, the removal of the organic layer is carried out only after a wet chemical etching step has been performed. One advantage is that the surface is protected from etching by the organic layer.

[0059] In one embodiment, the width X of the first trench lies in a range between 20 µm and 300 µm. In another embodiment, the width Y of the second trench lies in a range between 5 µm and 200 µm. It is understood, however, that the width of the first trench is always wider than the width of the second trench. Alternatively, the ratio of the width of the first trench to the width of the second trench lies in a range between 1 and 10.

[0060] In a training course, a wet etching step is performed after all laser-based process steps have been completed. One advantage of the wet etching step is that the surfaces are cleaned of residues from the laser-based process steps.

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

[0062] In another embodiment, a wet etching step is performed immediately after the second process step or after the first two process steps, i.e., before the organic layer is removed. An advantage is that the organic layer protects the top surface of the multi-junction solar cell not only from contamination during the laser-based process steps, but also from etching.

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

[0064] In a further training, the substrate is designed as a semiconductor disk with a diameter of at least or exactly 100 mm or at least or exactly 150 mm.

[0065] In another training method, the substrate is only cut along the trench.

[0066] In one embodiment, the third process step is not carried out in specified areas, so that the second layer is preserved and the substrate is not divided in the specified areas.

[0067] The invention is explained in more detail below with reference to the drawings. Similar parts are labelled with identical designations. The illustrated embodiments are highly schematic; that is, the distances and the lateral and vertical extents are not to scale and, unless otherwise indicated, do not exhibit any derivable geometric relationships to one another. The drawings show that Figure 1a-c shows cross-sectional views of an emerging trench structure after different laser processing steps. Figure 2 shows a cross-sectional view of the trench structure as depicted in the following illustration. Fig. 1cafter a wet etching step and paint stripping process, Figure 3 a cross-sectional view of the trench structure with a slot; after a third laser-based process step, Figure 4 a cross-sectional view of the trench structure with a slot; after a sawing step, Figure 5 a top view of a semiconductor disk with trench structure and slot.

[0068] The cross-sections shown below each depict a section of a multi-cell solar cell structure MS, wherein the multi-cell structure MS or a plurality of multi-cell structures is / are formed on a semiconductor wafer with a diameter of at least 100 mm.

[0069] The following illustrations also show only one embodiment in which the process steps one to three are carried out in the specified order, i.e. the first process step is carried out first, the second process step second, and if the third process step is carried out, the third process step is carried out after the second process step.

[0070] In the illustration of the Figure 1a to 1c Individual cross-sectional views of an emerging trench structure are shown, with the illustration of the Figure 1b the resulting trench structure after the first laser process step and the imaging of the Figure 1c The resulting trench structure after the second laser process step is shown.

[0071] The illustration of Figure 1aFigure 1 shows a cross-sectional view of an unmodified multi-junction solar cell structure (MS) with a bottom surface (US) and a top surface (OS). A substrate layer (SUB) made of germanium or GaAs is formed on the bottom surface (US) of the MS. The substrate layer (SUB) has a bottom surface, which is located on the bottom surface (US) of the MS.

[0072] An epilayer system ES is formed on the upper surface of the substrate layer SUB. The epilayer system ES is metallurgically bonded to the substrate layer SUB. It is understood that the epilayer system ES comprises a plurality of III-V layers.

[0073] The Epi-layer system ES comprises at least one first solar cell (not shown), wherein the first solar cell is always configured as the uppermost solar cell, even if, in further embodiments not shown, one or more additional solar cells are configured between the first solar cell and the substrate SUB.

[0074] An organic layer SL, designed as a protective coating, is arranged on the epi-layer system ES. In the illustrated embodiment, the organic layer SL is bonded across its entire surface to the top surface of the III-V epi-layer system ES, forming a metallurgical bond.

[0075] In an embodiment not shown, a further layer or layers, in particular passivation layers, are formed between the Epi-layer system and the organic layer SL in a materially bonded manner on the Epi-layer system ES.

[0076] In the first procedural step, as shown in the Fig. 1b A first trench GA1 with an opening of width X is created through the organic layer SL with a first bottom surface BO1 formed in the epi-layer system ES using a laser. The resulting side surfaces S1 are almost vertical.

[0077] In a second procedural step, as shown in the Fig. 1c Using a laser, an opening with a width Y is created in the base surface BO1 of the first trench GA1, and a second base surface BO2 is formed in the substrate SUB, where the width Y is smaller than the width X. By creating a second trench GA2 only in a portion of the first base surface BO1 of the Epi-layer system, a first stage STU1 is formed. The side surfaces S2 created in the second process step are also almost vertical.

[0078] It is understood that in an embodiment not shown, the process steps carried out using the laser can also be performed in a different order. In particular, the second process step can be performed before the first process step.

[0079] In the illustration of the Figure 2 Figure 1 shows a cross-sectional view of the trench structure of the embodiment. Fig. 1c The image is shown after a wet etching step and paint stripping process. The following only highlights the differences compared to the illustration of the Figure 1c explained.

[0080] In a wet etching step, the side walls S1, S2, the first stage STU1, and the second bottom surface BO2 were cleaned. The side surfaces S1 and S2 were recessed, meaning the width X of the first trench GA1 and the width Y of the second trench were slightly increased. The depth of the second trench GA2 was also slightly increased, meaning the second bottom surface GA2 BO2 was recessed deeper into the substrate SUB.

[0081] To protect the top surface OS of the multi-junction solar cell MS from etching, the protective coating on the top surface OS is only removed after the wet etching process by means of a coating removal process.

[0082] In the illustration of the Figure 3 is a cross-sectional view of the trench structure of the embodiment, shown in conjunction with the illustration of the Fig. 1c , after a third process step carried out using the laser. The following only highlights the differences compared to the illustration of the Figure 1c explained.

[0083] In a third process step, a laser is used to create an opening with a width Z for the slot SLI. The opening is continuous, meaning the slot SLI extends to the underside US, has no bottom surface, and its width Z is smaller than its width Y. By creating a slot only in a portion of the second bottom surface BO2 of the Epi slot system, a second stage STU2 is formed.

[0084] Figure 4 shows a cross-sectional view of the trench structure, in conjunction with the illustration of the Figure 2 , after a slot produced in the third process step by means of a sawing step. In the following, only the differences to the previous embodiment are shown, in conjunction with the illustration of the Fig. 2 , shown.

[0085] In a process step not shown, the substrate SUB is thinned from the underside US before the third process step is carried out.

[0086] After thinning the semiconductor wafer, a slot SLI is created from the underside US of the substrate SUB in a sawing step using a saw to divide the solar cell stacks MS formed on the front side.

[0087] Figure 5 shows a top view of a semiconductor disk, in which both the trench structure and the slot are formed on the semiconductor disk.

[0088] The following only highlights the differences from the previous illustration. Fig. 2 As shown, in the present embodiment, a slot is created along the trench structure in the third process step. The slot always begins and ends at the edge of the semiconductor wafer.

[0089] The top surface OS of the multi-junction solar cell structure MS shows both slotted SLI structures and trenchless structures. An advantage is that both structures are formed on the same semiconductor wafer.

[0090] It turns out that different sizes of components can be manufactured on a semiconductor wafer.

Claims

1. Method of producing a trench-shaped structure (GA) in a III-V multi-junction solar cell (MS) with an upper side (OS) and a lower side (US) and the III-V multi-junction solar cell (MS) comprises a substrate (SUB) arranged at the lower side (US), and the substrate (SUB) is constructed as a semiconductor wafer and has on a front side an epitaxial layer system (ES) with a plurality of III-V layers, wherein the epitaxial layer system (ES) comprises at least one first III-V solar cell formed at the upper side (OS) of the III-V multi-junction solar cell (MS), on which first III-V solar cell an organic layer (SL) is arranged, characterised in that - in a first method step a first trench (GA1) with a width X and with a base 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) with a width Y and with a base surface (BO2) formed in the substrate is produced in the base 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 the execution of the second method step, - the aforesaid method steps are executed in the indicated sequence or in an alternative the second method step is executed before the first method step and the second trench (GA2) with the width Y is produced before the first trench (GA1) with the width X.

2. Method of producing a trench-shaped structure (GA) in a III-V multi-junction solar cell (MS) according to claim 1, characterised in that in a third method step a slot is produced by means of a separating process in the base (BO2) of the second trench along the trench-shaped structure (GA) starting from the upper side (OS) or starting from the lower side (US) so as to divide the substrate.

3. Method of producing a trench-shaped structure (GA) in a III-V multi-junction solar cell (MS) according to claim 1 or claim 2, characterised in that in the third method step the substrate (SUB) is divided by means of a saw starting from the lower side (US) of the III-V multi-junction solar cell (MS).

4. Method of producing a trench-shaped structure (GA) in a III-V multi-junction solar cell (MS) according to claim 2 or 3, characterised in that the third method step is performed with use of the laser starting from the upper side (OS) of the III-V multi-junction solar cell.

5. Method of producing a trench-shaped structure (GA) in a III-V multi-junction solar cell (MS) according to claim 1 or claim 2 or claim 4, characterised in that a wet-etching step is performed after execution of all laser-based method steps.

6. Method of producing a trench-shaped structure (GA) in a III-V multi-junction solar cell (MS) according to claim 1 or claim 2 or claim 4, characterised in that the organic layer (SL) is removed only after execution of the laser-based method steps and a wet-etching step.

7. Method of producing a trench (VA) in a III-V multi-junction solar cell (MS) according to any one of the preceding claims, characterised in that the substrate (SUB) comprises Ge or GaAs or consists of Ge or GaAs.

8. Method of producing a trench (VA) in a III-V multi-junction solar cell (MS) according to any one of the preceding claims, characterised in that further III-V layers are formed above the first III-V solar cell and below the organic layer (SL).

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

10. Method of producing a trench (VA) in a III-V multi-junction solar cell (MS) according to any one of the preceding claims, characterised in that at least one III-V layer and / or at least one dielectric layer (SiO2) is or are formed between the first III-V solar cell and the organic layer.

11. Method of producing a trench (VA) in a III-V multi-junction solar cell (MS) according to any one of the preceding claims, characterised in that an anti-reflection layer is formed between the first III-V solar cell and the organic layer.

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

13. Method of producing a trench (VA) in a III-V multi-junction solar cell (MS) according to any one of the preceding claims, characterised in that the first trench (GA1) extends exclusively along a straight line and / or a curve and a first step (STU1) or at least two steps (STU1, STU2) or exactly two steps (STU1, STU2) is or are formed along the length of the first trench (GA1) and the second trench (GA2) by means of the laser-based method steps.

14. Method of producing a trench (VA) in a III-V multi-junction solar cell (MS) according to any one of the preceding claims, characterised in that the third method step is not executed in predetermined regions so that the second base (BO2) is obtained and the substrate is not divided in the predetermined regions.