US-12620779-B2 - VCSEL with self-aligned microlens to improve beam divergence

US12620779B2US 12620779 B2US12620779 B2US 12620779B2US-12620779-B2

Abstract

A method of making a microlens for a VCSEL device includes forming a first lens layer over a second reflector layer. The first lens layer has a first average concentration of a first element. A first additional reflector layer is formed over the first lens layer. A second lens layer is formed over the first additional reflector layer. The second lens layer has a second average concentration of the first element greater than the first average concentration. A second additional reflector layer is formed over the second lens layer. An oxidation process is performed to oxidize peripheral portions of the first and second lens layers to form oxidized peripheral portions of the first and second lens layer. The oxidized peripheral portions of the second lens layer are wider than the oxidized peripheral portions of the first lens layer.

Inventors

Jhih-Bin CHEN
Ming Chyi Liu

Assignees

TAIWAN SEMICONDUCTOR MANUFACTURING COMPANY, LTD.

Dates

Publication Date: 20260505
Application Date: 20220616

Claims (20)

1 . A method of making a microlens for a vertical cavity surface emitting laser (VCSEL) device, the method comprising: forming a first lens layer over a reflector layer and comprising a first average concentration of a first element and a first variable concentration of the first element, wherein the first lens layer continuously extends between a top surface and a bottom surface of the first lens layer, the first variable concentration changing between the top surface and the bottom surface; forming a first additional reflector layer over the first lens layer; forming a second lens layer over the first additional reflector layer and comprising a second average concentration of the first element greater than the first average concentration and a second variable concentration of the first element that is different than the first variable concentration; forming a second additional reflector layer over the second lens layer; performing an oxidation process to oxidize peripheral portions of the first and second lens layers to form oxidized peripheral portions of the first lens layer and oxidized peripheral portions of the second lens layer, wherein the oxidized peripheral portions of the second lens layer are wider than the oxidized peripheral portions of the first lens layer; and wherein a concentration of the first element at the top surface of the first lens layer is approximately equal to a concentration of the first element at a bottom surface of the second lens layer and different than a concentration of the first element at a top surface of the second lens layer.
2 . The method of claim 1 , further comprising: performing an etch process according to a masking layer to remove outermost edges of the first lens layer, the first additional reflector layer, the second lens layer, and the second additional reflector layer, such that the reflector layer is wider than the first additional reflector layer, wherein the etch process is performed after forming the second additional reflector layer and before performing the oxidation process.
3 . The method of claim 2 , further comprising: forming sidewall spacers along sidewalls of the masking layer, the first lens layer, the first additional reflector layer, the second lens layer, and the second additional reflector layer after performing the oxidation process.
4 . The method of claim 3 , further comprising: forming the first lens layer over a reflector stack comprising an optically active region configured to generate electromagnetic radiation; and performing a second etch process to etch the reflector stack according to the sidewall spacers and the masking layer.
5 . The method of claim 1 , wherein the first lens layer has a higher concentration of the first element at an upper portion than at a lower portion, and wherein the upper portion undergoes the oxidation process at a higher rate than the lower portion.
6 . The method of claim 1 , wherein the oxidized peripheral portions of the first lens layer and the second lens layer laterally protrude outward past an outermost sidewall of the first additional reflector layer and the second additional reflector layer to different non-zero distances.
7 . The method of claim 1 , wherein the first variable concentration increases from the bottom surface of the first lens layer to the top surface of the first lens layer.
8 . A method of making a microlens for a vertical cavity surface emitting laser (VCSEL) device, the method comprising: forming a reflector stack comprising a plurality of first reflector layers alternatingly stacked with a plurality of second reflector layers, wherein the plurality of first reflector layers comprise a first composition and the plurality of second reflector layers comprise a second composition different from the first composition; forming a microlens stack comprising a plurality of lens layers alternatively stacked with a plurality of additional reflector layers; performing a first etch process according to a masking layer to remove outermost edges of the microlens stack; performing an oxidation process to oxidize peripheral portions of the plurality of lens layers after the first etch process, wherein the oxidized peripheral portions increase in width as a distance over the reflector stack increases; forming one or more upper sidewall spacers along sidewalls of the masking layer and the microlens stack; performing a second etch process to etch the reflector stack after performing the oxidation process, the second etch process etching the reflector stack with the masking layer and the one or more upper sidewall spacers in place; forming one or more lower sidewall spacers along sidewalls of the reflector stack after the second etch process; forming a molding layer on outermost sidewalls of the one or more lower sidewall spacers, wherein the one or more lower sidewall spacers have an exposed part that protrudes outward from a top surface of the molding layer; removing the exposed part, but not all, of the one or more lower sidewall spacers after the second etch process is completed and removing a part, but not all, of the one or more upper sidewall spacers after the second etch process is completed; removing the molding layer after removing the exposed part of the one or more lower sidewall spacers; and forming top electrode contacts onto a top of the reflector stack and laterally between an outermost sidewall of the reflector stack and the one or more upper sidewall spacers after removing the part of the one or more upper sidewall spacers.
9 . The method of claim 8 , wherein the microlens stack comprises: a first lens layer; a first additional reflector layer over the first lens layer; and a second lens layer over the first additional reflector layer, wherein a concentration of a first element at a top surface of the first lens layer is approximately equal to the concentration of the first element at a bottom surface of the second lens layer and different than a concentration of the first element at a top surface of the second lens layer.
10 . The method of claim 8 , wherein the reflector stack comprises: a focusing layer comprising interior sidewalls that face and laterally surround one or more of the first reflector layers or the second reflector layers, the interior sidewalls being directly below the microlens stack after the first etch process; and an active layer vertically separated from the focusing layer by one or more of the first reflector layers and the second reflector layers, the active layer laterally extending past the interior sidewalls of the focusing layer.
11 . The method of claim 8 , wherein the top electrode contacts are symmetric about a center of the microlens stack in a cross-sectional view.
12 . The method of claim 8 , wherein the molding layer laterally contacts the outermost sidewalls of the one or more lower sidewall spacers and vertically extends below a bottommost surface of the one or more lower sidewall spacers.
13 . The method of claim 8 , further comprising: forming a dielectric structure laterally surrounding the one or more lower sidewall spacers, the reflector stack, and the microlens stack after performing the second etch process, wherein the dielectric structure vertically and physically contacts one of the additional reflector layers; and forming a vertical contact via extending from a top of the dielectric structure to a bottom of the dielectric structure, wherein the vertical contact via comprises a sidewall that is laterally separated from the microlens stack and that vertically extends from below a topmost surface of the microlens stack to above the topmost surface of the microlens stack.
14 . The method of claim 13 , further comprising: removing a central portion of the dielectric structure from over the microlens stack; and forming a connecting portion along a sidewall of the dielectric structure, wherein the connecting portion couples the vertical contact via to one of the top electrode contacts formed on the top of the reflector stack.
15 . The method of claim 8 , wherein the reflector stack comprises a first layer comprising aluminum gallium arsenic and a second layer comprising indium gallium arsenide.
16 . A method of making a microlens for a vertical cavity surface emitting laser (VCSEL) device, the method comprising: forming a reflector stack comprising a plurality of first reflector layers alternately stacked with a plurality of second reflector layers; forming a microlens stack comprising a plurality of lens layers alternately stacked with a plurality of additional reflector layers, wherein the plurality of lens layers have variable concentrations of a first element, and wherein concentrations of the first element at a top surface and a bottom surface of closest neighboring ones of the plurality of lens layers are approximately equal; performing a first etch process according to a masking layer to remove outermost edges of the microlens stack; performing an oxidation process to oxidize peripheral portions of the plurality of lens layers after the first etch process, wherein the oxidized peripheral portions increase in width as a distance over the reflector stack increases; forming one or more upper sidewall spacers along opposing sides of the microlens stack after performing the oxidation process; performing a second etch process to etch the reflector stack; forming one or more lower sidewall spacers continuously extending from a conductive layer below the reflector stack, to along a side of the reflector stack, and to along a sidewall of the one or more upper sidewall spacers; and removing a part of the one or more lower sidewall spacers and a part of the one or more upper sidewall spacers after forming the one or more lower sidewall spacers.
17 . The method of claim 16 , further comprising: forming a top electrode contact over a top of the reflector stack and along a side of the microlens stack; and forming a vertical contact via onto the top electrode contact, wherein the vertical contact via comprises a first sidewall that is laterally separated from the microlens stack and that vertically extends from below a topmost surface of the microlens stack to above the topmost surface of the microlens stack and an opposing second sidewall that vertically extends from over the topmost surface of the microlens stack to below a bottom of the reflector stack.
18 . The method of claim 16 , further comprising: forming a molding compound around the one or more lower sidewall spacers, wherein the one or more lower sidewall spacers protrude outward above a top of the molding compound; performing a third etch process with the molding compound in place to remove the part of the one or more lower sidewall spacers; and removing the molding compound.
19 . The method of claim 16 , wherein the oxidation process causes the lens layers to collectively form a microlens configured to focus radiation generated within the reflector stack, the microlens extending to a top of the one or more upper sidewall spacers.
20 . The method of claim 16 , wherein the one or more upper sidewall spacers comprise a first material and a second material covering an outer sidewall of the first material; and wherein removing the part of the one or more upper sidewall spacers removes the second material, the first material remaining along the opposing sides of the microlens stack after removing the part of the one or more upper sidewall spacers.

Description

REFERENCE TO RELATED APPLICATION This Application is a Divisional of U.S. application Ser. No. 16/579,692, filed on Sep. 23, 2019, the contents of which are hereby incorporated by reference in their entirety. BACKGROUND Laser diodes are used in many kinds of modern day devices. A vertical cavity surface emitting laser (VCSEL) is one promising candidate for next generation laser diodes. Compared to current laser diodes, such as edge-emitting devices, the emission from a VCSEL is normal to the plane of the device, and therefore it can be processed using standard processing techniques. Furthermore, the advantageous emission from the VCSEL device allows for production of a large plurality of lasers on a single wafer. BRIEF DESCRIPTION OF THE DRAWINGS Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. FIGS. 1A and 1B illustrate cross-sectional views of some embodiments of a VCSEL device having a self-aligned microlens comprising lens layers surrounded by oxidized lens layers. FIG. 2 illustrates a cross-sectional view of some additional embodiments of a VCSEL device having a self-aligned microlens coupled to a transistor. FIGS. 3-19 illustrate cross-sectional views of some embodiments of a method of forming a self-aligned microlens for a VCSEL device using an oxidation process. FIG. 20 illustrates a flow diagram of some embodiments of a method corresponding to FIGS. 3-19. DETAILED DESCRIPTION The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly. A vertical cavity surface emitting laser (VCSEL) device includes distributed Bragg reflectors (DBR) stacks and an optically active region arranged between top and bottom electrodes. The DBR stacks comprise alternating first and second reflector layers. During operation of the VCSEL device, a bias may be applied across the top and bottom electrodes, which causes the optically active region to emit light. The light reflects multiple times through the first and second reflector layers, and due to the effects of interference, some patterns and/or frequencies of light are amplified by constructive interference while other patterns and/or frequencies are attenuated by destructive interference. In this way, after multiple reflections between the first and second reflector layers, light is directed through an optical aperture (e.g., an opening within an optically opaque material disposed within the DBR stack, an opening within a focusing material having a different refractive index than layers of the DBR stacks) and a focused laser is emitted through a microlens at a direction that is normal to the first and second reflector layers. Typically, during the manufacturing of a VCSEL device, the microlens is manufactured separately from the DBR stacks, and is subsequently aligned and bonded to a top surface of the DBR stacks. The alignment of the microlens over the DBR stacks assures that the laser is focused when emitted. However, oftentimes, the precision of the alignment is difficult, and the microlens is misaligned over the DBR stacks. For example, in some embodiments, the optical aperture is within the DBR stacks to aid in the focus of the light, and the micro