Method to form a self-aligned evaporated metal contact in a deep hole and VCSEL with such contact
A method for forming a metal contact in a deep hole in a workpiece. A first hole is formed that extends from the upper surface of the workpiece to a substrate at the bottom of the hole. The hole is then filled with photoresist. Next, a photolithographic process is performed to create a second hole within the photoresist and to expose the substrate; and a wet etch is performed to remove a portion of the substrate. A layer of contact metal is then deposited on the surface of the photoresist. 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 deposited on the photoresist while leaving the metal at the bottom of the second hole in contact with the substrate.
1. A method for forming an evaporated metal contact in a hole in a workpiece comprising:
forming a first hole through a plurality of layers in the workpiece to expose an underlying substrate;
depositing a photoresist material over a top major surface of the workpiece, the photoresist material also filling the first hole;
removing some of the photoresist material to form a second, smaller diameter hole within the first hole in which a portion of a substrate is exposed at the bottom of the second hole while leaving photoresist material as a covering on an outer perimeter region of the first hole;
etching the exposed substrate at the bottom of the second smaller hole formed within the first hole, the etching to remove a portion of the substrate and to undercut some of the photoresist material covering the outer perimeter region of the first hole;
depositing a layer of contact metal on the photoresist material and the etched substrate at the bottom of the second hole, thereby forming a continuous metal layer on the exposed portion of the substrate at the bottom of the second hole and discontinuous metal layers on the photoresist material covering the outer perimeter region of the first hole; and
performing a liftoff process to remove both the photoresist material on the sidewalls of the second hole and the metal layers on the photoresist material to prevent the formation of metal stringers, while leaving the metal on the exposed portion of the substrate.
2. The method of claim 1 wherein the workpiece is a semiconductor.
3. The method of claim 1 wherein the workpiece is a plurality of epitaxially grown layers.
4. The method of claim 1 wherein the workpiece is a distributed Bragg reflector in a vertical cavity surface emitting laser.
5. The method of claim 1 further comprising the step of forming an electrical contact to the metal on the exposed portion of the substrate.
6. The method of claim 1 wherein the step of removing some photoresist material to form a second, smaller diameter hole includes the steps of
placing a mask on the photoresist, the mask defining a selected portion associated with a location for the second, smaller diameter hole;
exposing the masked photoresist material; and
removing exposed photoresist material to form the second hole.
7. The method of claim 6 wherein actinic radiation is used in the exposing step.
8. The method of claim 1 wherein the step of etching the substrate at the bottom of the second hole uses a wet chemical etching process to remove the portion of the substrate and to undercut the photoresist material covering the outer perimeter region of the first hole.
9. A method for forming an evaporated metal contact in a hole in a first distributed Bragg reflection (DBR) of a vertical cavity surface emitting laser (VCSEL) including the first DBR formed on a substrate, as well as an active region and a second DBR formed over the first DBR, the method including:
forming a first hole through the second DBR, the active region and the first DBR to expose a portion of the substrate;
depositing a photoresist material to cover a top major surface of the second DBR and also fill the first hole formed through the second DBR, the active region and the first DBR;
removing some of the photoresist material within the first hole to form a second hole of a smaller diameter within the first hole, using the steps of:
placing a mask on the photoresist, the mask defining a selected portion associated with a location for the second hole;
exposing the masked photoresist material; and
removing exposed photoresist material to form the second hole in which a portion of the substrate is exposed at the bottom of the second hole while leaving photoresist material covering a perimeter portion of the first hole;
wet chemical etching the substrate at the bottom of the second hole to remove a portion of the substrate and to undercut some of the photoresist material covering the perimeter portion of the first hole;
depositing a layer of contact metal on the photoresist material and the etched substrate at the bottom of the second hole, thereby forming a continuous metal layer on the exposed portion of the substrate at the bottom of the second hole and discontinuous metal layers on portions of the photoresist material covering the perimeter portion of the first hole; and
performing a liftoff process to remove both the vertical photoresist material on the sidewalls of the second hole and the metal layers on the photoresist material to prevent the formation of metal stringers, while leaving the metal on the exposed portion of the substrate.
10. The of method of claim 9 further comprising the step of forming an electrical contact to the metal on the exposed portion of the substrate.
11. The method of claim 9 wherein actinic radiation is used in the exposing step.