Laser assists micro led display in breaking through technological bottleneck.
The display structure of the micro led is a miniaturized led array. At present, the size of a single micro led chip can be fewer than 10 μ m, which is 1% of the size of the conventional led. Each micro led can be regarded as a pixel, each pixel can achieve address control, separate drive and self-luminous. Structurally, micro led is a p-n junction contact diode that is formed by p-type GaN material and n-type GaN material while both the two materials are in close contact. When a forward voltage is applied on both sides of the p-n junction, that is, the positive voltage is applied on the p-type GaN side, and the negative voltage is applied on the n-type GaN side, the carriers on both sides of the p-n junction can easily move between each other. Then the radiative recombination of electron-holes occurs to produce light emission. The micro led array connects the positive and negative electrodes of each micro led through a vertical staggered positive and negative grid electrode and lights up the micro led to display the image by scanning through the sequence of the electrode lines.
Due to the special structure and light-emitting principle of micro led chip, the display screen based on micro led chip has the advantages of high pixel, high color saturation, low power consumption, high brightness, fast response and long life. In terms of display quality, the single chip size of the micro led can be fewer than 10 μ m, so the micro led display can integrate extremely high pixel density. According to relevant data, the iPhone X pixel density of the OLED screen in the market is 458ppi, while the micro-led display can reach more than 2000ppi; In terms of performance, micro led is composed of inorganic materials, and the service life of the micro led screen is longer than that of the easily burned OLED screen which is composed of organic light emitting material; In technical level, micro led display adopts single-point driven self-luminescence technology, with nanosecond (ns)-grade ultra-fast reaction speed and efficiency. For example, the application in VR industry. Since the response speed of the micro led is extremely fast, it can effectively solve smear and delay issue, eliminate the vertigo of the VR user, can greatly improve the immersive feeling and enhance the user experience; In terms of energy consumption, the micro led which uses p-n junction as a typical structure has the characteristics of high brightness and low energy consumption, which can improve the life time of wearable devices.
Key Laser Stripping Technology in Micro Led Display
Micro led display has excellent performance, but it is remained to break through in technical level. One of the key technologies is the exfoliation of epitaxial substrate. For Micro Led chips that based on luminescent materials, because of the low lattice mismatch between GaN and sapphire and the low price, the sapphire substrate becomes the mainstream substrate for epitaxial growth GaN material. However, the non-conductivity and poor thermal conductivity of sapphire substrate affect the luminous efficiency of micro led devices; Meanwhile, brittle sapphire is disadvantageous to the application of micro led in the direction of flexible display. Based on above reasons and the advantages of micro led display itself, such as high resolution, high brightness, high contrast, etc.，laser stripping of sapphire is a necessary and key segment, and laser stripping technology can highlight the advantages of micro led.
Laser stripping is essentially a single pulse scanning process, so there is very high demand for uniformity and stability of laser beam. Because of the high demand for laser technology and process stability, there are not many enterprises in the world have this technology and can apply it in stable production. At present, Han’s Laser is the only representative of stable production in our country.
Laser stripping technology uses high energy pulsed laser beam to penetrate the sapphire substrate while the photon energy is between the sapphire band gap and the GaN band gap, the interface between the sapphire substrate and the epitaxial GaN material is uniformly scanned by laser stripping technique; GaN layer absorbs a large amount of photon energy and decomposes into liquid Ga and nitrogen, the separation of Al2O3 substrate from GaN thin film or GaN led chip is realized, so that the sapphire substrate can be stripped almost without external force.
Laser assists micro led display in breaking through technological bottleneck
Han’s DSI has been researching and developing laser stripping (laser lift off, llo) technology since 2013 which is aimed at the stripping of GaN-based micro led and vertically structured led wafer sapphire substrates and has successfully developed and launched automatic llo laser stripping equipment.
The equipment uses all-solid-state semiconductor (dpss) laser as light source. Self-developed 266nm wavelength can be used in stable produced laser frequency doubling module. It has the advantages of long stable cycle and low maintenance cost; Laser beam, power stability and laser efficiency are far better than the excimer equipment abroad. Superior processing performance, especially for stripping of some special epitaxial substrate (such as graphic sapphire substrate, etc.); The depth of focus reaches 1000 μ m. For warped wafers, a stable peeling effect can also be ensured within a certain range; Good stability of the equipment, automatic loading and unloading, 24-hour continuous production, significant savings in manpower cost; Large processing area, can selectively process 2~6 inches’ wafer, equipped with a high-precision CCD camera, can accurately achieve regional stripping; The processing efficiency is high, such as 4-inch wafer, it takes about 140 seconds to complete the single chip processing, while the traditional excimer device takes more than 300 seconds. Excellent performance has enabled the equipment to pass performance verification and run stably in many photoelectric companies, research institutes and universities in industry.
In the field of LLO laser stripping technology, Han’s DSI constantly accumulates industry experience, perfect technical reserve, continuously improve and optimize the process, and actively upgrade and replace the equipment, so that the technology and equipment have core competitiveness; At the same time, Han’s DSI have submitted related patent application of laser stripping technology and equipment.
Han’s DSI is also actively carrying out strategic layout in other technical areas of micro led. For more technical problems encountered in their development, it can also provide corresponding solutions:
- At present, how large is the unit micro led chip that can be stripped with stable and high yield laser stripping? Han’s DSI can strip off the micro led which has appeared in the market with the smallest unit particle that is 10-15 μ m.
- How to transfer these micro led to the substrate, how to bond, how to drive the circuit? The huge amount of transfer after laser stripping has become the research and development focus of various led plants and panel plants at present. Han’s DSI is committed to the research and development of micro led-related processes. At present, the related large transfer schemes in cooperation are laser d-bounding transfer scheme and contact transfer scheme.
- Because of the poor wavelength consistency of micro led, the color of led is uneven. To make a whole micro led panel which has the same size as that of smartphones, the rate of good product is very low. The detection of optical and electrical properties of micro led chips is also a major difficulty in follow-ups. At present, Han’s DSI is carrying out technical research in the field of detection.
There are still many bottlenecks in the development of micro led technology, but thanks to the advantages of micro led, this technology is still a bright spot for development in the future. As a leading supplier of domestic high-end equipment, Han’s DSI has always followed the market demand, walked in the forefront of the application technology of related industries, and provided professional laser solutions for enterprises.
