III-Nitride VCSEL Monolithic Grating for Simpler DBR Fabrication
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Solution Overview
Problem
The fabrication of distributed Bragg reflectors in III-nitride vertical cavity surface emitting lasers (VCSELs) is challenging due to complex processes like chemical mechanical polishing, substrate removal, and etching, which can degrade crystalline quality and increase production costs.
Innovation Solution
A method involving epitaxial lateral overgrowth (ELO) is used to form a monolithic grating and dielectric layers, allowing the growth of high-quality III-nitride regions without direct exposure to etching, thereby simplifying the fabrication process and reducing substrate damage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If chemical mechanical polishing (CMP) is used to remove substrate, then substrate can be removed to form bottom mirror, but the process becomes complex, tedious, and difficult to control, and expensive substrates are wasted
Solution Approach 1:
The patent extracts the substrate removal function from the complex CMP process by using selective layer deposition and patterned etching. The bottom mirror is formed by removing only the necessary substrate portion through controlled etching of sacrificial layers, rather than using comprehensive CMP that removes excessive material and requires complex process control.
Solution Approach 2:
The patent applies preliminary action by depositing sacrificial layers and forming patterned structures on the substrate before the actual bottom mirror formation. These preliminary structures guide the subsequent etching process, enabling precise substrate removal without requiring complex CMP process control.
2Reliability
If epitaxial DBR is used to form bottom mirror, then substrate can be retained, but the formation process becomes complex and time-consuming with degraded crystalline quality
Solution Approach 1:
The patent segments the bottom mirror formation into distinct functional layers: a sacrificial layer for substrate release, a reflective layer for mirror function, and a protective layer for structural integrity. This segmentation allows each layer to be optimized independently, maintaining crystalline quality while achieving substrate retention.
Solution Approach 2:
The patent introduces intermediary sacrificial layers that mediate between the substrate and the bottom mirror structure. These sacrificial layers enable controlled substrate release and mirror formation without requiring complex epitaxial DBR processes that degrade crystalline quality.
3Loss of energy
If substrate is thinned from 300-400 micrometers to 10-30 micrometers to reduce absorption loss, then absorption loss is reduced, but the thinning process is difficult to control and may damage the wafer
Solution Approach 1:
The patent applies preliminary action by forming the bottom mirror structure and sacrificial layers on the thick substrate before thinning operations. This allows the substrate to be thinned to the optimal 10-30 micrometer range using controlled etching of sacrificial layers, rather than attempting to thin the substrate first which is difficult to control and may damage the wafer.
Solution Approach 2:
The patent uses sacrificial layers as intermediaries to enable controlled substrate thinning. These sacrificial layers are etched selectively to remove substrate material to the precise desired thickness, avoiding the uncontrolled thinning process that could damage the wafer while achieving the thin substrate needed to reduce absorption loss.
4Manufacturing precision
If e-beam lithography and etching are used to fabricate high-index contrast grating, then grating can be formed, but device layers are damaged requiring additional protective layers
Solution Approach 1:
The patent applies preliminary action by forming the grating structure on the substrate surface before depositing the device layers. The grating is created using controlled etching of sacrificial patterns, and once formed, the device layers are deposited over it. This eliminates the need for subsequent etching that would damage the device layers, removing the requirement for additional protective layers.
Solution Approach 2:
The patent inverts the conventional fabrication sequence by forming the grating structure first on the substrate, then depositing the device layers over it. This reversal of the normal process order (where device layers are formed first and then etched to create gratings) prevents damage to the device layers during grating fabrication, eliminating the need for protective layers.
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 high-quality VCSELs with improved crystalline integrity and reduced production costs by eliminating complex etching processes, enabling efficient laser operation at visible and ultraviolet wavelengths.
Implementation Method 1
a monolithic grating having a periodic one-dimensional pattern... the periodic one-dimensional pattern extends in a second axial direction that intersects the first axial direction
Implementation Method 2
two distributed Bragg reflectors, DBRs, which act as high reflective mirrors
Data Source
AI summary
A vertical cavity surface emitting laser (VCSEL) includes a distributed Bragg reflector (DBR) including a first dielectric layer and a second dielectric layer alternately arranged in a first axial direction; and a semiconductor section including a p-type III nitride region, a III nitride region, and a Ill nitride active region between the p-type Ill nitride region and the III nitride region, the p-type III nitride region, the Ill nitride active region, and the Ill nitride region being arranged in the first axial direction, the Ill nitride region including an n-type III nitride region. The semiconductor section includes a monolithic grating having a periodic one-dimensional pattern. The monolithic grating, the Ill nitride active region, and the distributed Bragg reflector are arranged in the first axial direction to form an optical cavity. The periodic one-dimensional pattern extends in a second axial direction that intersects the first axial direction.