METHOD FOR PRODUCING A VIA 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, whereby the disclosed method is intended to avoid, among other things, 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 YAG laser. To prepare the very rough surfaces on the side walls of the through-hole, which are created during the laser bombardment, for subsequent metallization, in a second process step, the side walls are coated with a polyimide or, in the case of smaller through-holes, the opening is completely filled with polyimide. In a subsequent process step, the polyimide is heated to cure.

[0003] The polyimide is referred to as a dielectric coating. In a fourth process step, a small-diameter through-hole is created within the polyimide using a laser. After the laser irradiation, the polyimide has a smooth surface on the newly created side surfaces. The polyimide side surfaces are then covered with a metal coating in a subsequent process step.

[0004] The application of polyimide also avoids, among other things, the need for smoothing the side surfaces using aggressive chemical etching, and protects the active area of ​​the solar cell and the GaAs or Ge substrate from the etching attack. Furthermore, the polyimide exhibits good adhesion both to the substrate and to any feedthrough metallization that may be applied.

[0005] DE 10 2019 006094 A1 also discloses a laser cutting method for producing through holes for vias in GaAs or Ge solar cells, wherein overlapping openings are created by photolithography and etching, which requires several process steps.

[0006] Against this background, the object of the invention is to provide a method with which the state of the art is further developed.

[0007] The object is achieved by a method for producing a via in a III-V multi-junction solar cell having the features of patent claim 1. Advantageous embodiments of the invention are the subject of subclaims.

[0008] According to the subject matter, a method for manufacturing a via in a III-V multi-junction solar cell is provided.

[0009] The III-V multi-junction solar cell has a top side and a bottom side, with the via being continuous from the top side to the bottom side.

[0010] The III-V multi-junction solar cell comprises a substrate arranged on the underside.

[0011] The substrate has a top surface and an epi-layer system with several III-V layers formed on the top surface of the substrate.

[0012] The epi-layer system comprises at least one first III-V solar cell. An organic layer is arranged on top of the first III-V solar cell.

[0013] In a first method step, an opening with a width X and with a first bottom surface formed in the epi layer system is created by means of a laser.

[0014] In a second method step, an opening with a width Y and with a second bottom surface formed in the substrate is created by means of the laser, wherein the width Y is smaller than the width X.

[0015] In a third method step, an opening with a width Z is created by means of the laser to form the continuous via, wherein the opening has no bottom surface and the width Z is smaller than the width Y.

[0016] The above-mentioned procedural steps are carried out in the specified order or in any order.

[0017] If the aforementioned method steps are performed in the specified order, the second method step is performed after the first method step, followed by the third method step. It is understood that in the second method step, the opening with the second base surface is created in the first base surface, and in the third method step, the through opening, referred to as a via, is created in the second base surface.

[0018] If the process steps are not carried out in the specified order, the third process step is carried out first and then either the second process step or the first process step is carried out second.

[0019] Subsequently, the process step not yet performed, i.e., the first process step or the second process step, is carried out. It is understood that, depending on the sequence, the second floor surface is formed after the through opening, and the first floor surface is formed in the final process step.

[0020] In another refinement, the second process step is performed first, thereby forming the second base surface. Subsequently, either the first process step is performed, forming the first base surface, or the third process step is performed, forming the through-opening. Subsequently, the process step not yet performed, i.e., the third process step or the first process step, is performed.

[0021] It goes without saying that after all three process steps have been performed, regardless of the order in which they are performed, a through-hole with two stages is always formed. In other words, in a cross-sectional view, the through-holes produced using different methods correspond to one another or have exactly the same structure.

[0022] It should be noted that the organic layer comprises a resist layer and / or a polyimide layer and / or a plastic layer. It should be understood that the layers are preferably unstructured to avoid a masking step.

[0023] Furthermore, it should be noted that the via is always an opening extending through the entire layer, perpendicular to the front side. Such openings can be used for electrical through-hole plating by forming a metal layer in the via.

[0024] Furthermore, it is understood that the substrate is a semiconductor substrate or a semiconductor wafer with a diameter of at least or exactly 100 mm or at least or exactly 150 mm.

[0025] It should also be noted that all process steps are carried out using a laser starting from the top side.

[0026] It should be noted that in the present case the term dielectric layer includes exclusively inorganic layers, in particular oxide and nitride layers.

[0027] It is understood that the term epi-layer system refers exclusively to III-V layers produced on the substrate by means of an epitaxial process.

[0028] Furthermore, it should be noted that the first bottom surface formed in the epi layer system or in the substrate has an opening or a hole, so that the first bottom surface is formed as a circumferential edge surface starting from the opening or the hole.

[0029] It is understood that the surface formed in the base region, ie the first base surface or the peripheral edge surface, is flat or is flat in a first approximation and that the surface at the base in the epi-layer system consists exclusively of a III-V material or, in the case of the base surface in the substrate, exclusively of the substrate material.

[0030] Furthermore, it should be noted that the resulting side surfaces in the laser-based process steps are either exactly vertical or nearly vertical. The term "nearly vertical" refers to a deviation from the vertical of less than 5°.

[0031] In a further development, the opening angle is less than 20° or less than 10°. The opening angle is understood to be the angle inclined relative to the vertical, with the inclination being "outward," so that the total angle is more than 90°. In other words, the hole has a larger diameter at the top, in a non-vertical design, than at the bottom; the hole is conical.

[0032] One advantage of this process is that the organic layers, especially resist layers, can be flexibly arranged anywhere on the top side of the III-V multi-junction solar cell without the need for further structuring using complex and expensive lithography processes. These vias allow the top side—i.e., the front side—of the III-V multi-junction solar cell to be electrically contacted from the back side.

[0033] A further advantage is that the at least three-stage laser process is surprisingly insensitive to minor changes in the layer structure, particularly in the thickness and number of III-V layers. The process is also insensitive to changes in the substrate material from Ge to GaAs and vice versa. Insignificant changes include, in particular, a change in the III-V material composition and stoichiometry, and a change in the total layer thickness of less than 20 µm.

[0034] Another advantage is that the laser settings can be easily adjusted even if the substrate thickness changes significantly. A significant change is defined as a change in substrate thickness of at least 50 µm or at least 100 µm.

[0035] Another advantage of laser-based processing steps compared to wet-chemical processing steps is that no undercutting occurs. Furthermore, wet-chemical processing steps generally require a structured application of protective coatings to the surface and / or side surfaces of the parts to be manufactured, especially in multi-level structures.

[0036] In one embodiment, by using the laser, the laser process steps are carried out one after the other without intermediate steps such as wet etching steps or dry etching steps.

[0037] However, it is understood that in another embodiment, instead of the three consecutive laser process steps, more than three, for example four or five, process steps are carried out using the laser.

[0038] In another embodiment, further process steps without laser are carried out between or before or after the at least three-stage opening process.

[0039] In a further development, at least one further III-V solar cell is formed between the substrate and the first III-V solar cell.

[0040] In one embodiment, the first III-V solar cell as the top III-V solar cell has a larger band gap or a band gap of the same size as the underlying III-V solar cells.

[0041] In order to electrically connect the stacked III-V solar cells in series, several tunnel diode layers are arranged between the two III-V solar cells.

[0042] In another embodiment, the two III-V solar cells comprise the same or different materials. In one embodiment, the uppermost III-V solar cell comprises an InGaP compound. In another embodiment, the further III-V solar cell is designed as a second III-V solar cell and comprises a GaAs compound or an InGaAs compound.

[0043] 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. The layers are preferably grown epitaxially using a MOVPE system.

[0044] In another embodiment, a substrate solar cell is formed in or on the top surface of the substrate. In the case of a GaAs substrate, the substrate solar cell is a III-V solar cell, while in the case of Ge, it is a VI solar cell.

[0045] 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 formed by diffusion of dopants and not by epitaxy.

[0046] In one embodiment, the substrate has a thickness in a range between 80 µm and 850 µm. In another embodiment, the thickness of the substrate is in a range between 150 µm and 750 µm.

[0047] In one embodiment, further layers are formed above the first III-V solar cell and below the organic layer.

[0048] In a further development, the width X is in a range between 5 µm and 5 mm or in a range between 50 µm and 1 mm.

[0049] In a further development, the width y is in a range between 5 µm and 5 mm or in a range between 50 µm and 1 mm.

[0050] In one embodiment, the ratio of width X to width Y is less than a factor of 10 or less than a factor of 5. In other words, although width X is always greater than width Y, width X is a maximum of 10 times greater than width Y or a maximum of 5 times greater than width Y.

[0051] In another embodiment, the width Z is in a range of 1 µm to 1 mm or the width Z of the vias is in a range of 20 µm to 0.5 mm.

[0052] In another embodiment, the organic layer is formed as a continuous layer covering the entire upper 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, at least one III-V layer and / or at least one dielectric layer is formed between the first III-V solar cell and the organic layer. In another embodiment, the dielectric layer comprises or consists of SiO 2 .

[0054] In another embodiment, an anti-reflective layer is formed between the first III-V solar cell and the organic layer.

[0055] In one embodiment, a metal layer is arranged on the top side of the III-V multi-junction solar cell before the first process step in the region of the via between the first III-V solar cell and the organic layer, or precisely no metal layer is formed. In other words, the process steps can be performed regardless of whether or not a metal layer is formed at the location of the via to be produced.

[0056] In another embodiment, the via has an oval shape. In a further development, the via has a circular opening.

[0057] In a further development, at least two steps are formed in the via in a direction from the top side to the bottom side of the III-V multi-junction solar cell during the laser process.

[0058] In another further development, after the three process steps have been carried out with the laser, a wet chemical etching is carried out, in particular to clean the surfaces and the side surfaces.

[0059] In a further development, depending on the etching solution used and its duration, only one of the two steps is visible after etching. The wet-chemical etching step is preferably performed immediately after the three laser processing steps.

[0060] In a further development, the organic layer is removed only after a wet-chemical etching step. One advantage is that the organic layer protects the surface from etching.

[0061] In one embodiment, the via has a constant diameter or a diameter that is constant in a first approximation, viewed in a plane that is parallel to the underside.

[0062] The invention will be explained in more detail below with reference to the drawings. Similar parts are labeled with identical designations. The illustrated embodiments are highly schematic, ie the distances and the lateral and vertical extensions are not to scale and, unless otherwise stated, do not have any deducible geometric relationships to one another. Figure 1 shows a cross-sectional view of an unprocessed multi-solar cell structure, Figure 2a-c shows cross-sectional views of a resulting VLA structure after different laser processing steps, Figure 3 shows a cross-sectional view of a via after a wet etching step.

[0063] In the cross-sectional views shown below, a section of a multi-junction solar cell structure MS is shown as part of an entire semiconductor wafer, wherein the multi-junction cell structure MS, in an embodiment also not shown, is also formed as a plurality of multi-junction cell structures MS on a semiconductor wafer with a diameter of at least 100 mm.

[0064] The following figures also show only one embodiment in which the process steps one to three are carried out in the specified order, ie the first process step is carried out first, the second process step second and the third process step third.

[0065] The illustration of the Figure 1 shows a cross-sectional view of the unprocessed 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 multi-junction solar cell structure MS.

[0066] An epi-layer system ES is formed on a top surface of the substrate layer SUB. The epi-layer system ES is arranged in a material-to-material bond on the substrate layer SUB. An organic layer SL, designed as a protective coating, is arranged on the epi-layer system ES.

[0067] In the embodiment shown, the organic layer SL is integrally connected to the upper side of the III-V epi layer system ES.

[0068] In an embodiment not shown, further layers, in particular passivation layers, are formed cohesively on the epi-layer system ES between the epi-layer system and the organic layer SL.

[0069] In an embodiment not shown, further organic and / or inorganic layers are formed between the organic layer SL and the epilayer system ES. The further layers preferably comprise or consist of passivation layers made of, for example, a silicon oxide and / or a silicon nitride.

[0070] It goes without saying that the aforementioned layers are arranged in the aforementioned order. It should also be noted that all of the aforementioned layers are formed over the entire surface.

[0071] In the illustration of the Figure 2a to 2c Individual cross-sectional views of a developing via structure are shown, with each image showing the resulting via structure after one of the three laser processing steps has been carried out. In the following, only the differences to the image of the Figure 1 explained.

[0072] In a first step, shown in the Fig. 2a Using a laser, an opening with a width X is created through the organic layer SL and with a first bottom surface BO1 formed in the epi-layer system. The resulting side surfaces are formed almost vertically.

[0073] In a second process step, the laser creates an opening in the base surface with a width Y and a second base surface BO2 formed in the substrate, where the width Y is smaller than the width X. By creating a hole in only a portion of the first base surface BO1 of the epi-layer system, a first step STU1 is created. The side surfaces created in the second process step are also nearly vertical.

[0074] In a third process step, an opening with a width Z is created using the laser to form the continuous via VA. The opening has no bottom surface and the width Z is smaller than the width Y. By creating a hole only in a portion of the second bottom surface BO2 of the epi layer system, a second step STU2 is created. The side surfaces created in the third process step are also nearly vertical.

[0075] In the figure Figure 3is a cross-sectional view of the via, shown in conjunction with the figure of the Figure 2c , after a wet etching step. Only the differences from the previous figure are shown below.

[0076] On the epi-layer system ES, the organic layer SL was removed using a stripping process. Preferably, a wet etching step was performed before stripping the organic layer SL to clean the side surfaces in the via and the other surfaces, in particular the surfaces of the two steps STU1 and STU2 and the side surfaces, of contaminants from the three laser processing steps.

[0077] During the wet etching step, the first step STU1 is reshaped in such a way that the step surface from the epilayer system ES is relocated to the interface between the epilayer system ES and the top surface of the substrate SUB. The depth of the surface of the first step STU1 is also increased. Even after the etching step, the side surfaces within the via VA remain nearly vertical.

[0078] The step surface of the second step is also laid a little deeper in the substrate SUB.

[0079] It is understood that in an embodiment not shown, the process steps performed by means of the laser can also be carried out in a different order. In particular, the second process step can be carried out before the first process step.

Claims

1. Method of producing a via (VA) in a III-V multi-junction solar cell (MS) with an upper side and a lower side and the via (VA) is formed to be continuous from the upper side (OS) to the 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) has an upper side and an epitaxial layer system (ES), which is formed on the upper side, with a plurality of III-V layers, and the epitaxial layer system (ES) comprises at least one first III-V solar cell and an organic layer (SL) is formed at the upper side of the first III-V solar cell, - in a first method step an opening with a width X and with a first base surface (BO1) formed in the epitaxial layer system (ES) is produced by means of a laser, - in a second method step an opening with a width Y and with a second base surface (BO2) formed in the substrate is produced by means of the laser, wherein the width Y is smaller than the width X, - in a third method step an opening with a width Z is produced by means of the laser for formation of the continuous via (VA), wherein the opening has no base surface and the width Z is smaller than the width Y, - the aforesaid method steps are executed in the indicated sequence or in a desired sequence.

2. Method of producing a via (VA) in a III-V multi-junction solar cell (MS) according to claim 1, characterised in that at least one further III-V solar cell is formed between the substrate and the first III-V solar cell and a plurality of tunnel diode layers is arranged between the two III-V solar cells and the two III-V solar cells comprise the same or different materials.

3. Method of producing a via (VA) in a III-V multi-junction solar cell (MS) according to claim 1 or claim 2, characterised in that the substrate (SUB) comprises Ge or GaAs or consists of Ge or GaAs.

4. Method of producing a via in a III-V multi-junction solar cell (MS) according to any one of the preceding claims, characterised in that the substrate (SUB) has a thickness in a range between 80 µm and 850 µm or the thickness of the substrate (SUB) lies in a range between 150 µm and 750 µm.

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

6. Method of producing a via (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 of the first III-V solar cell.

7. Method of producing a via (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.

8. Method of producing a via (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.

9. Method of producing a via (VA) 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 via between the first III-V solar cell and the organic layer (SL) prior to the first method step.

10. Method of producing a via (VA) in a III-V multi-junction solar cell (MS) according to any one of the preceding claims, characterised in that the via has an oval shape and at least one first step (STU1) or at least two steps or exactly two steps is or are formed in the via in a direction from the upper side to the lower side of the III-V multi-junction solar cell (MS).

11. Method of producing a via (VA) in a III-V multi-junction solar cell (MS) according to any one of the preceding claims, characterised in that in the case of the via (VA) a constant diameter or a diameter constant to a first approximation is formed as seen in a plane formed parallelly to the lower side.