Method for etching multi-layer epitaxial material

Abstract

A single-step wet etch process is provided to isolate multijunction solar cells on semiconductor substrates, wherein the wet etch chemistry removes semiconductor materials nonselectively without a major difference in etch rate between different heteroepitaxial layers. The solar cells thus formed comprise multiple heterogeneous semiconductor layers epitaxially grown on the semiconductor substrate.

Claims

1. A method comprising the steps of: providing a wafer comprising a substrate and three or more subcells of a multijunction solar cell overlying the substrate, wherein, the substrate comprises a gallium arsenide-containing material or a germanium-containing material; at least one of the three or more subcells comprises GaInNAsSb, GaInNAsBi, GaInNAsSbBi, GaNAsSb, GaNAsBi, or GaNAsSbBi; each of the three or more subcell comprises a back surface field, a base, a depletion region, an emitter and a front surface field; and an interconnection region overlying each of the three or more subcells; patterning the wafer with a mesa etch pattern using photolithography; and etching in exposed areas the three or more subcells and part or all of the substrate according to the mesa etch pattern using a single nonselective etchant mixture comprising hydrochloric acid, iodic acid, and water, for yielding mesa isolation of individual multijunction solar cells characterized by a macroscopically smooth substantially inwardly curved sidewall profile, wherein the etchant mixture comprises a molar ratio of 0.95 moles to 1.05 moles iodic acid:59 moles to 65 moles hydrochloric acid:760 moles water.

2. The method of claim 1, wherein the etchant mixture has a temperature 10 C. to 140 C.

3. The method of claim 1, wherein the etchant mixture has a temperature of 30 C. to 45 C.

4. The method of claim 1, wherein patterning comprises using a photoresist, using a dielectric hard mask, or using both a photoresist and a dielectric hard mask.

5. The method of claim 1, wherein patterning comprises using a photoresist to pattern a dielectric hard mask, and the photoresist is retained during removal of the heteroepitaxial layers during mesa etching.

6. The method of claim 1, wherein etching comprises agitating the wafer.

7. The method of claim 1, wherein the three or more subcells comprises: a multijunction photovoltaic cell comprising a first subcell overlying the substrate; and a second subcell overlying the first subcell; wherein the first subcell comprises GaInNAsSb, GaInNAsBi, GaInNAsSbBi, GaNAsSb, GaNAsBi, or GaNAsSbBi.

8. The method of claim 1, wherein each of the three or more subcells is lattice matched to the substrate.

9. The method of claim 1, wherein, the substrate comprises Si, Ge, SiGe, or GaAs; and each of the three or more subcells is lattice matched to the substrate.

10. A multijunction solar cell fabricated using the method of claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1A is a cross-sectional diagram of a multijunction solar cell in which the invention could be used.

(2) FIG. 1B is a top plan view of cells as found in the prior art.

(3) FIG. 2 is a close-up view of a portion of the mesa isolation processed structure of the prior art. Individual cells on a semiconductor substrate are isolated by removing grown epitaxial layers and a portion of the substrate in areas between adjacent mesa structures.

(4) FIG. 3 is a graph showing the lattice constant-bandgap relationship of common compound and elemental semiconductors and the alloys typically used in multijunction solar cells.

(5) FIG. 4 is a cross-sectional view of an exemplary multijunction solar cell of the prior art.

(6) FIGS. 5A, 5B, and 5C show results of processing steps in accordance with the present invention.

(7) FIG. 6 is a cross-sectional view of a multijunction solar cell manufactured using the wet etch process of the present invention.

(8) FIGS. 7A, 7B, and 7C show results of processing steps of one embodiment of the present invention wherein a patterned photoresist mask 21 is used to pattern the mesa structures.

(9) FIGS. 8A, 8B, and 8C show results of processing steps of another embodiment of the present invention wherein patterned photoresist 10 is first used to pattern dielectric 21, and thereafter, retained during mesa etch.

DETAILED DESCRIPTION

(10) FIG. 1B and FIG. 2 illustrate of a portion of a mesa isolation processed solar cell structure 100 on a wafer 12. In top view, the structures of the prior art and of the present invention are indistinguishable. Individual solar cells 100 on the semiconductor substrate 12 are isolated by removing grown epitaxial layers and a portion of the substrate 12 in areas 8 (etched regions) between adjacent mesa structures. Bus bars 22 connect grid lines 2. The mesa structure 6 is bounded by edges along the boundaries of the etched regions 8 as hereinafter explained.

(11) FIG. 3 is a graph showing the lattice constant-bandgap relationship of common compound and elemental semiconductors and the alloys typically used in multijunction solar cells. In mesa isolation it is a requirement to etch through all of the semiconductor materials used in multijunction solar cells. Whereas the prior art as represented by FIG. 4 yields an inconsistent cross section during etch, the present invention, as shown in FIG. 6, does not.

(12) The mixture according to certain embodiments of the wet etch process of the invention, comprises iodic acid, hydrochloric acid, and water prepared in the molar ratios of 1:62:760, respectively. The said molar ratios of iodic acid and hydrochloric acid can be within a variance of 5.0%, such that the molar ratios in the mixture are within the ranges (0.95-1.05):(59-65):760, for iodic acid, hydrochloric acid, and water, respectively. Preparation in the laboratory or manufacturing process is in a 1:2:3 ratio by volume, wherein the aqueous solution of hydrochloric acid is 38.0%3.0% by weight and the aqueous solution of iodic acid is 6.6%1.0% by weight. It is within the contemplation of the invention to use another solute or liquid mixtures besides water in the wet etch process, although water is the most readily available. Similarly, other acids of different molar concentration could be substituted for hydrochloric acid to yield the same result. However, it is the iodic acid HIO.sub.3 in the above concentration range that is considered efficacious for the purposes of this invention to produce substantially inwardly curved sidewalls through all heterogeneous layers of epitaxy.

(13) FIGS. 5A, 5B, and 5C show processing steps in accordance with the present invention as previously summarized. The layers of a multijunction solar cell are represented by heteroepitaxial material stack 11 on an unmodified substrate 12. Photoresist 10 overlays the epitaxial stack 11 (FIG. 5A). When etched according to the invention (FIG. 5B) in a reservoir of the aqueous etch solution, heteroepitaxial stack 11 is transformed into individual elements 13 that form cells 100 (FIG. 1) penetrating the substrate 4, now designated 121, and producing mesa structures 6, as hereinafter depicted in FIG. 6 according to the invention. (Not shown is the optional process of protecting the underside of the substrate 12 against the etch solution.)

(14) The resulting cross-sectional shape (FIG. 6) after the mesa isolation step is a side wall profile characterized by a substantially inwardly curved profile, that is, having a substantially macroscopically smooth surface without significant undercutting of a junction region compared to other junction regions. As compared with the prior art, a substantially inwardly curved profile is indicative of nonselective etch produced by the wet etch process of the invention. Interconnection regions 17 and 18 between junctions 14, 15, and 16 often exhibit different etch rates compared to the semiconductors used in the junctions. As shown in FIG. 6, methods provided by the present disclosure produce a mesa side wall profile that is characterized by a macroscopically smooth profile from junction 14 and gradually widening toward and along substrate 121. The mesa sidewall profile is further characterized by minimal undercutting beneath the uppermost interconnection layer and interconnection layer 17, and a modest etch-back at interconnection layer 18. In particular, aluminum containing layers may show a larger undercut compared to other layers. It is to be understood that traces of iodine may be found on etched surfaces 19 (FIG. 6) as a result of the present invention, including sidewalls of mesa structures 9 and exposed portions of the substrate 121 (FIG. 5C).

(15) In certain embodiments the wafers are agitated in the etch solution to control etch rate and provide etch uniformity across wafers.

(16) In another embodiment, anti-reflection coating (ARC) is used as a dielectric hard mask for mesa isolation. The process steps in this embodiment are depicted in FIG. 7. Anti-reflection coating deposition 20 is done before the mesa isolation step and patterned ARC 21 is used to protect the solar cells during the removal of heteroepitaxial layers around the cells using the present invention. In a variation of this embodiment (FIG. 8), the ARC 20 is patterned using photoresist 10. After the ARC etch is completed the photoresist 10 is retained during mesa isolation as well.

(17) In certain embodiments of the invention, the volumetric ratio of hydrochloric acid in the mixture is 10%-50% and the volumetric ratio of iodic acid in the mixture is 10%-50%, wherein the aqueous solution of hydrochloric acid is 38.0%3.0% by weight and the aqueous solution of iodic acid is 6.6%1.0% by weight. It is to be understood the same molar ratios of the constituent chemicals can be provided using different volumetric ratios with different molarities in the aqueous solutions used. During processing, the temperature of the mixture is maintained between 10 C. and 140 C.

(18) In another specific embodiment of the invention, the volumetric ratio of hydrochloric acid is 30%-35% and the volumetric ratio of iodic acid is 14%-19%, using the said molarities in the aqueous solutions of the constituent chemicals, and the temperature of the mixture is maintained between 30 C. and 45 C.

(19) The etching methods provided by the present disclosure can be used to fabricate solar cells including multijunction solar cells. Accordingly, solar cells including multijunction solar cells fabricated using the methods disclosed herein are provided.

(20) In certain embodiments, a solar cell device comprises a multijunction photovoltaic cell comprising a first subcell and a second subcell overlying the first subcell, wherein the multijunction photovoltaic cell is characterized by mesa sidewalls formed by a nonselective wet etch process, wherein the mesa sidewalls are substantially inwardly curved.

(21) In certain embodiments of a solar cell, at least one of the first subcell and the second subcell comprises a base layer comprising an alloy comprising one or more elements from group III of the periodic table, nitrogen, arsenic, and an element selected from Sb, Bi, and a combination thereof; and the first subcell and the second subcell are substantially lattice matched.

(22) In certain embodiments of a solar cell, the first subcell and the second subcell are substantially lattice matched to a material selected from Si, Ge, SiGe, GaAs, and InP.

(23) In certain embodiments of a solar cell, the first subcell comprises a base layer selected from GaInNAsSb, GaInNAsBi, GaInNAsSbBi, GaNAsSb, GaNAsBi, and GaNAsSbBi.

(24) In certain embodiments of a solar cell, the solar cell comprises a gallium arsenide-containing material underlying the first subcell.

(25) In certain embodiments of a solar cell, the solar cell comprises a germanium-containing material underlying the first subcell.

(26) In certain embodiments of a solar cell, the first subcell comprises a germanium-containing material.

(27) In certain embodiments of a solar cell, the nonselective wet etch process used to form the mesa sidewalls comprises the use of a single etchant mixture comprising hydrochloric acid, iodic acid, and water.

(28) In certain embodiments of a solar cell, the etchant mixture used to form the mesa sidewalls comprises a volumetric ratio of hydrochloric acid of 10% to 50%; and a volumetric ratio of iodic acid of 10% to 50%; and the etchant mixture is characterized by a temperature from 10 C. to 140 C.

(29) In certain embodiments of a solar cell, the etchant mixture used to form the mesa sidewalls comprises a volumetric ratio of hydrochloric acid of 30% to 35%; and a volumetric ratio of iodic acid of 14% to 19%; and the etchant mixture is characterized by a temperature from 30 C. to 45 C.

(30) The invention has been explained with respect to specific embodiments. Other embodiments will be evident to those of ordinary skill in the art. Therefore, the invention is not intended to be limited, except as indicated by the appended claims.