Thinned GaN LED Pillars for Ultra-Dense Retinal Projection
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Solution Overview
Problem
Conventional LED displays are limited in achieving ultra-dense and high-resolution due to large pixel spacing and size, which is not suitable for applications like contact lens projectors that require smaller pixel sizes to match the density of light receptors in the retina.
Innovation Solution
The approach involves thinning the gallium nitride (GaN) region in LED fabrication by chemical mechanical polishing and etching, reducing the height of LED pillars and the aspect ratio of trenches, allowing for smaller pixel sizes and improved optical performance, and using a backplane with integrated driver circuits and a frontplane with a hexagonal LED array to create an ultra-dense LED display.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional LED fabrication is used with standard GaN thickness, then manufacturing is simpler, but pixel size cannot be reduced below 5-10 um pitch
Solution Approach 1:
The patent applies preliminary action by performing chemical mechanical polishing and etching of the GaN region before LED pillar formation. This pre-thinning of the GaN layer reduces the aspect ratio of subsequent trenches and allows for smaller pixel pitches (2 um or less) that would be impossible with conventional thickness GaN layers
Solution Approach 2:
The patent changes the physical parameter of GaN thickness from conventional standards to ultra-thin dimensions through chemical mechanical polishing and etching. This parameter change enables the formation of LED pillars with 2 um pitch while maintaining structural integrity and reducing optical loss
2Measurement precision
If pixel size is reduced to increase resolution, then image resolution improves, but optical loss increases due to higher aspect ratio trenches
Solution Approach 1:
The patent performs preliminary thinning of the GaN region through chemical mechanical polishing and etching before forming the LED pillars. This reduces the trench depth and aspect ratio, enabling sub-1 um2 pixel areas with 2 um pitch while minimizing optical loss from sidewall reflections
Solution Approach 2:
The patent changes the GaN layer thickness parameter to ultra-thin dimensions, which directly reduces the optical path length and minimizes optical loss. This parameter change enables high-resolution pixels with reduced optical attenuation
3Measurement precision
If discrete LED dies are used for each color, then color accuracy is improved, but pixel spacing increases to 25 um pitch
Solution Approach 1:
The patent merges red, green, and blue emitting LEDs onto a single GaN-based die, creating a monolithic micro-display. This integration reduces pixel pitch from 25 um (discrete dies) to 2 um or less while maintaining color accuracy through precise spatial arrangement of sub-pixel LEDs
Solution Approach 2:
The patent transitions from discrete three-dimensional die stacking to a two-dimensional monolithic integration on a single GaN die. This dimensional change enables ultra-dense packing with 2 um pitch while maintaining all three color emissions on the same plane
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 method enables pixel sizes of less than 1 um2 with a pixel pitch of 2 um or less, significantly increasing image resolution and reducing optical loss, making it feasible to project high-resolution images onto the retina within a contact lens.
Implementation Method 1
The approach involves thinning the gallium nitride (GaN) region in LED fabrication by chemical mechanical polishing and etching
Data Source
AI summary
A small projector uses an ultra-dense array of gallium nitride (GaN) LEDs. However, epitaxial growth of GaN typically produces a GaN region that is 5 um or thicker. To achieve high pixel density, the LEDs have small area, so the resulting LED structures are tall and skinny. This is undesirable because it makes further processing more difficult and has higher optical losses. As a result, it is beneficial to reduce the thickness of the GaN region. In one approach, a wafer with the GaN region on substrate is bonded to a backplane wafer containing LED driver circuits. The substrate is then separated from the GaN region, exposing a buffer layer of the GaN region. The GaN region is thinned and then patterned into individual LEDs. Typically, the buffer layer is removed entirely.