VCSEL Contact Hole Metallization With Photoresist Sidewall Masking
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Solution Overview
Problem
Conventional methods for forming electrical metal contacts in vertical cavity surface emitting lasers face challenges such as mechanical instability and sensitivity in high topological structures, particularly in contact holes deeper than 10 μm, due to uneven etching and native oxide removal issues, leading to potential cracking and voids.
Innovation Solution
A method involving the use of a photoresist mask within the contact hole for isotropic wet-chemical etching and self-aligned metal deposition, which protects the side walls and enables a smoother transition, allowing for the formation of a self-aligned electrical metal contact with reduced thermal resistance and improved chip layout efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If contact holes deeper than 10 μm are formed to provide contact areas at the bottoms, then electrical contact can be achieved on the epitaxial side, but mechanical instability and sensitivity occur due to high surface topology and exposed semiconductor layers at sidewalls
Solution Approach 1:
A photoresist mask is applied to the sidewalls of the contact hole before metal deposition. This preliminary protective action prevents damage to the exposed semiconductor layers at the sidewalls during subsequent processing steps, eliminating the need for extensive undercut etching while ensuring mechanical stability of the high-aspect-ratio contact structure
Solution Approach 2:
The photoresist mask acts as an intermediary protective layer between the metal deposition process and the sensitive semiconductor sidewalls. This intermediary prevents direct interaction that would cause mechanical instability, allowing formation of reliable electrical contacts in deep contact holes without compromising structural integrity
2Ease of manufacture
If metal contacts are formed by depositing metal over the whole VCSEL layer structure and patterning by photoresist and plasma etching, then electrical connection is achieved, but extreme criticality occurs in terms of homogeneity in topological high structure and stopping etching on sensitive surface
Solution Approach 1:
Instead of depositing metal over the entire structure and then etching away excess metal (conventional approach), the invention inverts the sequence by first protecting the sidewalls with photoresist mask and then depositing metal only where needed. This eliminates the critical plasma etching step entirely, replacing it with a simpler, more controllable process that does not require precise etching control on sensitive surfaces
Solution Approach 2:
The problematic plasma etching step is extracted and removed from the process sequence. By using photoresist mask to define the metal contact area before deposition, the method eliminates the need for subsequent metal etching, thereby removing the source of manufacturing precision issues related to etching homogeneity and control on high-topography surfaces
3Ease of manufacture
If lift-off process sequence is used to form metal contacts at the bottom of contact hole, then metal contact can be formed, but large free space is required on the bottom reaching up to 15 μm on each side, stretching the chip layout
Solution Approach 1:
The photoresist mask is applied to the sidewalls before metal deposition, preliminarily defining the exact area where metal will be deposited. This eliminates the need for large undercut spaces required by conventional lift-off methods, as the mask precisely controls metal placement without requiring additional free space for undercut formation
Solution Approach 2:
The photoresist mask creates a precise negative copy of the desired metal contact footprint on the sidewalls. This copied pattern guides the subsequent metal deposition to form contacts with exact dimensions and positioning, eliminating the need for the 15 μm free space requirement of conventional lift-off processes and enabling compact chip layouts
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach results in a stable and efficient formation of electrical metal contacts within high-aspect-ratio contact holes, reducing mechanical instability and enabling precise positioning without damaging the epitaxial structure, while allowing for thick contact metal layers and reduced thermal resistance.
Implementation Method 1
providing a photoresist mask inside the contact hole, the photoresist mask covering the side wall of the contact hole
Implementation Method 2
wet-chemical isotropic etching the bottom of the contact hole
Implementation Method 3
depositing a metal on the bottom of the contact hole
Data Source
AI summary
A method of forming an electrical metal contact within a semiconductor layer stack of a vertical cavity surface emitting laser includes forming a contact hole into the semiconductor layer stack. The contact hole has a bottom and a side wall extending from the bottom. The method further includes providing a photoresist mask inside the contact hole. The photoresist mask covers the side wall of the contact hole and has an opening extending to the bottom of the contact hole. The method additionally includes wet-chemical isotropic etching the bottom of the contact hole, depositing a metal on the bottom of the contact hole, and removing the photoresist mask so that the metal on the bottom of the contact hole is left as the electrical metal contact.