VCSEL Including A Self-Aligned, Deep Hole Evaporated Metal Contact
A vertical cavity surface emitting laser (VCSEL) including a first ohmic contact to the substrate formed on an upper surface of the device, instead of the conventional substrate bottom-side contact. The VCSEL is formed to include a hole made through the first distributed Bragg reflector (DBR) and into the material of the substrate itself. A metal layer is deposited at the bottom of the hole to contact the substrate, where the deposited metal layer creates a high quality ohmic contact by not also contacting the inner sidewalls of the hole (i.e., no “stringers” are formed within the hole).
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This application is a divisional of U.S. patent application Ser. No. 14/958,389, which claims benefit of provisional application serial no. 62/088,259, filed Dec. 5, 2014, which applications are incorporated by reference herein in their entirety.
BACKGROUNDIn semiconductor manufacturing, in microelectronic mechanical systems (MEMS), and in other nanotechnology applications, there frequently is a need to form a metal contact at the bottom of a relatively deep hole. Unfortunately, the dimensions of the hole and, in particular, its very small size and its high aspect ratio (hole depth to hole diameter) often present challenges to forming good contacts at acceptable yields. For example, the sidewalls of the hole often create shadowing effects on the surface at the bottom of the hole where the contact is to be formed. These challenges are often exacerbated if the hole is formed in a multi-layered structure such as a series of epitaxially grown layers. Such layers are found, for example, in a distributed Bragg reflector (DBR) in a vertical cavity surface emitting laser (VCSEL).
SUMMARYThis invention is a method for forming a metal contact in a relatively deep hole in a workpiece. In an illustrative embodiment, a first hole is formed in the workpiece at a desired location. The first hole extends from the upper surface of the workpiece to a substrate at the bottom of the hole. The first hole is then filled with photoresist by coating the upper surface of the workpiece with a layer of photoresist. Next, a photolithographic process is performed to create a second hole within the photoresist on the sidewalls of the first hole and to expose the substrate at the bottom of the second hole. A wet etch is then performed to remove a portion of the substrate at the bottom of the second hole and to undercut some of the photoresist remaining at the bottom of the second hole. Next, a layer of contact metal is deposited on the surface of the photoresist. This layer is a continuous layer except where the second hole is formed. In the second hole, the metal layer is formed on the exposed surface of the substrate and on discontinuous portions of the photoresist on the sidewalls. A liftoff process is then used to remove the photoresist and the metal that was deposited on the photoresist while leaving the metal at the bottom of the second hole in contact with the substrate.
Advantageously, the foregoing method may be practiced to make a VCSEL in which an ohmic contact to the substrate is made through a hole in the epitaxial layers of a DBR.
These and other objects, features and advantages of the invention will be more readily apparent from the following detailed description in which:
As shown in
Method 200 begins at step 210 as a first hole is formed at a desired location in a workpiece. The first hole extends from an upper surface of the workpiece to a substrate at the bottom of the hole. At step 220, the first hole is filled with photoresist by coating the upper surface of the workpiece with a layer of photoresist. At step 230, a photolithographic process is then performed to create a second hole within the photoresist on the sidewalls of the first hole and expose the substrate at the bottom of the second hole. Illustratively, the photolithographic process includes the placement on the photoresist of a mask that defines selected portions of the photoresist that are to be removed, selective exposure of the photoresist by actinic radiation directed through the mask, and removal of the selected portions of the photoresist.
A wet etch is then performed at step 240 to remove a portion of the substrate at the bottom of the second hole and to undercut some of the photoresist covering an outer perimeter of the first hole, this photoresist perimeter labeled as “P” in
Typically, the current is confined to a narrow region in the VCSEL by limiting the lateral dimensions of the active region and the second DBR and/or by inserting an aperture between the active region and the second DBR. In the embodiment shown in
Ohmic electrical contacts to the VCSEL are typically made to the substrate and the second DBR. This, however, is often a problem since it frequently is desirable to have the electrical leads for both contacts located on the same side of the VCSEL. To contact the substrate from the same side of the VCSEL as the second DBR, it is necessary to form a hole 380 in the epitaxial layers that extends from an upper surface of the VCSEL down to the substrate and to form a metal contact on the exposed surface of the substrate at the bottom of the hole.
As shown in
To contact substrate 910 from the same side of the VCSEL as the connection to second DBR 940, a hole 980 is formed in the epitaxial layers of the first DBR on the portion of the first DBR that is not covered by the active region and second DBR. Hole 980 extends from an upper surface of the first DBR down to substrate 910 and a metal layer 985 makes ohmic contact to the exposed surface of the substrate at the bottom of the hole. An electrical lead 987 extends through the hole to connect to metal layer 985. Lead 988 is electrically insulated from the epitaxial layers of first DBR 920 by a suitable insulating layer 989. A second metal layer 990 makes an ohmic contact with second DBR 940.
Hole 980 and metal layer 985 may be formed using the same steps as described in conjunction with
As will be apparent to those skilled in the art, numerous variations may be practiced within the spirit and scope of the present invention.
Claims
1. A vertical cavity surface emitting layer (VCSEL) comprising:
- a substrate having first and second major surfaces;
- a first distributed Bragg reflector (DBR) disposed on the first major surface of the substrate;
- an active region disposed on a top surface of the first DBR;
- a second DBR disposed over the active region;
- a hole that extends through at least the first DBR and into the substrate, where a bottom surface of the hole exposes a portion of the substrate;
- a first metal layer formed to cover a major extent of the bottom surface of the hole, the coverage of the first metal layer sufficient to form a first ohmic contact with the substrate;
- a first electrically conductive material extending through the hole to make electrical contact between the first metal layer and the first DBR; and
- a second ohmic contact coupled to the second DBR.
2. The VCSEL of claim 1 wherein the active region and the second DBR have a lateral extension that is less than that of the first DBR, exposing a portion of the top surface of the first DBR, with the hole formed in the exposed portion of the first DBR.
3. The VCSEL of claim 2 wherein the active region and the second DBR comprise a mesa geometry.
4. The VCSEL of claim 1 further comprising an aperture disposed between the active region and the second DBR.
5. The VCSEL of claim 4 wherein the aperture confines current flow in the VCSEL.
6. The VCSEL of claim 1 wherein the first DBR comprises a plurality of epitaxially grown layers.
7. The VCSEL of claim 1 wherein the first metal layer does not overlap with a sidewall of the hole.
8. The VCSEL of claim 1 wherein the VCSEL further comprises an insulating layer disposed between the first electrically conductive material and the first DBR.
9. The VCSEL of claim 8 where the insulating layer is disposed vertically along an inner wall of the hole from the first metal layer to a contact area on the first DBR.
Type: Application
Filed: Jan 19, 2022
Publication Date: Jun 30, 2022
Applicant: II-VI Delaware, Inc. (Wilmington, DE)
Inventors: Omar Husam Amer El-Tawil (North Brunswick, NJ), Kevin Chi-Wen Chang (West Windsor, NJ)
Application Number: 17/579,106