Wafer-Level Phosphor Coating Using Conductive Pedestals
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Solution Overview
Problem
Conventional methods for coating light emitting diodes (LEDs) with phosphor layers face challenges in controlling geometry and thickness, leading to non-uniform color temperature and inconsistent emission characteristics, and often require complex processes that hinder access to wire bond pads and are not scalable for large wafer sizes.
Innovation Solution
The method involves forming electrically conducting pedestals on LED contacts, allowing for wafer-level coating with a down-converter layer that can be planarized to expose the pedestals for wire bonding, enabling uniform thickness control and simplifying the fabrication process without the need for alignment, using techniques like spin-coating or electrophoretic deposition.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If conventional syringe or nozzle method is used for phosphor coating, then the coating process is simple, but the geometry and thickness control is difficult resulting in non-uniform color temperature
Solution Approach 1:
Electrically conducting pedestals are formed on the LED contacts before the phosphor coating process. This preliminary action creates raised structures that will protrude through the eventual phosphor layer, ensuring that the wire bond pads remain accessible after coating without requiring complex alignment or registration steps during the coating process itself.
Solution Approach 2:
The coating process is divided into multiple sequential steps: first applying a clear encapsulant layer, then applying the phosphor-containing encapsulant layer. This segmentation allows each layer to be optimized independently - the clear layer provides a uniform base that ensures consistent pedestals, while the phosphor layer can be controlled to the precise thickness needed for uniform color temperature.
2Productivity
If stencil printing method is used for phosphor coating, then multiple LEDs can be coated simultaneously, but the geometry and layer thickness control remains difficult and alignment is complex
Solution Approach 1:
The pedestals are formed on all LED contacts before the batch coating process. This preliminary action eliminates the need for alignment during coating, as the pedestals will naturally protrude through the phosphor layer regardless of slight variations in coating thickness or LED positioning, enabling simple blanket coating of entire wafers.
Solution Approach 2:
The clear encapsulant layer serves multiple functions: it provides a uniform surface for subsequent phosphor layer application, ensures consistent pedestal formation across all LEDs, and acts as an adhesive base layer. This multi-functionality simplifies the overall process while maintaining precision.
3Manufacturing precision
If phosphor coating is applied before wire bonding, then uniform coating can be achieved, but access to wire bond pads becomes difficult
Solution Approach 1:
Electrically conducting pedestals are formed on the LED contacts before the phosphor coating process. These pedestals are designed to protrude through the phosphor-containing encapsulant layer, ensuring that wire bond pads remain accessible after coating. This preliminary action resolves the contradiction by maintaining both coating uniformity and pad accessibility.
Solution Approach 2:
The solution moves the wire bond pad accessibility problem from the lateral plane to the vertical dimension. By creating raised pedestals that extend through the coating thickness, the pads remain accessible from the top surface even though the LEDs are uniformly coated, effectively solving the accessibility issue through dimensional change.
4Adaptability or versatility
If conventional coating methods are used, then the process can be applied to existing LEDs, but the process is not scalable for large wafer sizes
Solution Approach 1:
The pedestals are formed on LEDs while they are still mounted on the wafer in their existing configuration. This preliminary action allows the subsequent coating process to be performed as a blanket operation across the entire wafer surface, enabling scalability to large wafer sizes without requiring individual LED handling or complex alignment systems.
Solution Approach 2:
The coating process merges multiple functions into a single operation: the clear encapsulant layer and phosphor-containing encapsulant layer are applied in sequence as part of one integrated process, and the pedestals serve both as electrical contacts and as alignment/reference structures for the coating. This merging enables efficient scaling to large wafers.
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 allows for consistent production of white LEDs with controlled color points, facilitates access to wire bond pads, and is scalable to large wafer sizes, ensuring reliable and uniform emission characteristics across different viewing angles.
Implementation Method 1
The surrounding phosphor material 'downconverts' the wavelength of some of the LED' s blue light, changing its color to yellow
Implementation Method 2
using techniques like spin-coating or electrophoretic deposition
Implementation Method 3
using techniques like spin-coating or electrophoretic deposition
Data Source
Figure 1a~1c
Figure 1d~2a
Figure 2b~2d
AI summary
Methods for fabricating light emitting diode (LED) chips comprising providing a plurality of LEDs typically on a substrate. Pedestals are deposited on the LEDs with each of the pedestals in electrical contact with one of the LEDs. A coating is formed over the LEDs with the coating burying at least some of the pedestals. The coating is then planarized to expose at least some of the buried pedestals while leaving at least some of said coating on said LEDs. The exposed pedestals can then be contacted such as by wire bonds. The present invention discloses similar methods used for fabricating LED chips having LEDs that are flip-chip bonded on a carrier substrate and for fabricating other semiconductor devices. LED chip wafers and LED chips are also disclosed that are fabricated using the disclosed methods.