Yi Ta Hsieh

AlGaInP LEDs Prepared by Contact-Transferred and Mask-Embedded Lithography

The authors applied a simple, low-cost, mass-producible contact-transferred and mask-embedded lithography (CMEL) to texture p-GaP window layer for the fabrication of AlGaInP light-emitting diodes (LEDs) emitting at 612 nm. Under 20 mA current injection, it was found that forward voltages were 2.25, 2.39, 2.29, 2.39, 2.24, 2.21, and 2.25-V while the 20 mA output powers were 1.43, 1.42, 1.38, 1.35, 1.28, 1.22, and 1.16 mW for CMEL-400-nm LED, CMEL-600-nm LED, CMEL-800-nm LED, CMEL-1-μm LED, CMEL-2-μm LED, CMEL-3-μm, and the conventional LED without CMEL, respectively.

Metal contact printing photolithography for fabrication of submicrometer patterned sapphire substrates for light-emitting diodes

This paper reports an improved method that combines a metal film contact printing method with traditional photolithography for fabrication of submicrometer-scale patterned sapphire substrates (PSSs) used for high-brightness light-emitting diodes (LEDs). First, a patterned metal thin film is transferred from the surface of a mold onto a photoresist (PR) layer deposited on top of the sapphire substrate. The transferred metal pattern acts as a photomask for subsequent photolithographic processes. PR structures with a high aspect ratio of 5 and a small line width of 500 nm are fabricated on 2 and 4 in. sapphire wafers. Finally, inductively coupled plasma etching is performed on the sapphire substrates to obtain PSSs by using the patterned PR microstructures as an etching mask. Experiments have been performed and both 2 and 4 in. PSSs with submicrometer-scaled and cone-shaped surface features were successfully obtained. These PSSs can be used in the LED industry to obtain high-brightness LEDs.

A soft photo-mask with embedded carbon black and its application in contact photolithography

This paper presents a new type of soft photo-mask which can be used in contact photolithography for achieving small line-width, large area, and high throughput ultraviolet (UV) patterning. It starts from a polydimethylsiloxane (PDMS) mold replicated from a silicon master mold. A carbon black photo-resist (PR) is spin-coated on top of the PDMS mold and then thermally cured. After a contact transfer process, the solidified carbon black PR exists only in the concave region of the PDMS mold, which converts the PDMS mold into a carbon-black/PDMS soft photo-mask. Due to its flexibility, this soft photo-mask can be used in contact photolithography on a slightly curved substrate. Experiments on preparing this new soft photo-mask and its application for fabricating patterned sapphire substrates (PSSs) used in the light-emitting-diode (LED) industry are carried out. Successful results are observed.

A Soft PDMS/Metal-Film Photo-Mask for Large-Area Contact Photolithography at Sub-Micrometer Scale With Application on Patterned Sapphire Substrates

This paper reports a new type of soft PDMS/metal-film photo-mask that can be applied in contact photolithography with a resolution at sub-micrometer scale and a patterning area over a 4-in wafer. This new type of photo-mask is made from a soft PDMS mold that contains a patterned metal film on the concave surface of its microstructures. The metal film can selectively block incident UV light, while the convex PDMS microstructures can guide the incident UV light to expose a photo-resist (PR) layer. Due to its soft and compliant property, this new soft photo-mask can form intimate contact with a substrate and carry out UV exposure to form PR microstructures. It is particularly useful in patterning slightly curved substrates such as sapphire wafers, and therefore has a great potential on manufacturing patterned sapphire substrates (PSSs) in light-emitting diodes. In this paper, both 2 and 4 in PSSs with sub-micrometer feature sizes are successfully achieved. This new type of soft photo-mask and its contact photolithography can be easily implemented at a low cost for large-area, nonflat, and sub-micrometer scaled patterning, and therefore has great potential in many applications.

Fabrication of sub-micrometer surface structures on sapphire substrate for GaN-based light-emitting diodes by metal contact printing method

Nano-scale pattern sapphire substrate (NPSS) used in light-emitting diodes (LEDs) was reported that could enhance their light extraction efficiency. This paper describes a novel method to fabricate sub-micrometer surface structures on sapphire substrate. This metal contact printing method can directly transferred a metal film pattern from a silicon mold to a sapphire substrate, and subsequently use the transferred metal film pattern as the etching mask for inductively coupled plasma (ICP) etching on the sapphire substrate. Because of excellent etching selectivity of the metal films, it is easy to obtain deeper etching depth on sapphire. Experimental tests have been successfully fabricated six different feature size hexagon surface structures on a 2 inch sapphire substrate and the etching depth is about 400 nm. The LED structures were grown on the patterned sapphire substrate by metal-organic chemical vapor deposition (MOCVD). The measurement forward voltages of the LEDs grown on different pattern size of the PSS were similar, and it is found that the luminous intensity was increasing with decreasing pattern size. It indicates that the pattern size of the PSS is related to the capability of light extraction, and the maximum increase intensity is 84.7% higher than conventional LEDs.

Investigation of GaN-Based Light-Emitting Diodes Grown on Patterned Sapphire Substrates by Contact-Transferred and Mask-Embedded Lithography

The influences of pattern size and etching depth of patterned sapphire substrates (PSSs) on crystal quality and light output power of light-emitting diodes (LEDs) were investigated by contact-transferred and mask-embedded lithography. The present results indicate that a smaller pattern size facilitates superior light extraction efficiency. However, a suitable pattern size and etching depth should be chosen to obtain the highest quality of GaN film. In comparison with the conventional sapphire substrate, the largest light output enhancement (~28.9%) was observed when the pattern diameter and the etching depth of PSS were 400 and 400 nm, respectively.

Metal contact printing photolithography for fabricating sub-micrometer patterned sapphire substrates in light-emitting diodes

This paper reports a novel process which is combine the contact metal transfer method and traditional photolithography process for fabricate nano-scale pattern sapphire substrate (NPSS) used in high brightness light emitting diodes (LEDs). The novel process can directly transfer a metal pattern onto the PR layer which above the sapphire substrate, the transferred metal pattern can as a perfect photo-mask for subsequent photolithography process. In this work, the high aspect ratio PR structures with the aspect ratio of 5 and line width of 500 nm are created by this novel process. Furthermore, the PR structure can as a etching mask for inductively coupled plasma (ICP) etching on the sapphire substrate. During the ICP etching, we successfully to obtain the NPSS with a perfect cone shape. Experiments have been demonstrate the feasibility of using this new approach for obtaining sub-micrometer surface structures on the complete surface area of a 2 inch and 4 inch sapphire substrates.

Nano-patterned AlGaInP light-emitting diode based on UV-Kiss metal transfer technology

This paper describes a new nano-patterning technique, the UV-Kiss Metal Transfer (UV-KMT) method, and applies it for patterning micro/nano-structures on AlGaInP light-emitting diodes (LEDs) for enhancing their light extraction efficiency. First of all, an ETFE mold with micro/nano-features is replicated from a silicon master mold. A thin metal film is then deposited on the ETFE mold which has very low surface energy. A layer of UV curable polymer solution is spin-coated on an AlGaInP LED surface. The metal-film coated EFTE mold and the UV-polymer coated LED are brought into contact with a uniformly distributed pressure of 0.1 MPa, and UV light is radiated through the ETFE mold and solidifies the UV polymer. The solidified UV polymer has stronger adhesion to the metal film in contact with, and therefore can transfer the metal pattern defined by the convex surface feature of the ETFE mold onto the AlGaInP LED surface. The transferred metal pattern is then serving as an etching mask for RIE etching on the underlying UV polymer layer. Finally, a patterned structure consisting of a metal film on top and an underlying UV polymer layer is formed on the LED surface. This metal/polymer surface structure can well serve as an etching mask again for ICP etching on the LED, and hence complete the fabrication of micro/nano-structures on the top surfaces of AlGaInP LEDs for enhancing their light extraction efficiency. The optical power measurement using an integrating sphere shows that the extraction efficiency of the patterned LED is 25% higher than that of the conventional LED. In short, we demonstrate an easily implemented, cost effective, and powerful method to pattern LED substrate.