Study On Fabrication Of LED Micro Display Devices And Laser Bonding Technology

LED micro-display is a micro-display technology with Pixel density greater than1000PPI(Pixel Per Inch),active area diagonal is less than 1inch long and Pixel size less than 50?m,which is prepared by Micro-LED.It is a new micro-display technology with high contrast,low power consumption,long life and fast response compared with liquid crystal micro-display,Digital micromirror device(DMD)and organic light-emitting diode(OLED)micro-display technologies.The Blue,green LED display is mainly composed of Ga N based LED microarray and Si based CMOS circuit,through the way of flip chip bonding(FCB)to integrate the two together,because the thermal mismatch Between Ga N and Si is as high as 54%,temperature thermal stress applied on device will influence the overall device bonding quality in the process of Flip Chip Bonding,even causing bond convex point of fracture.In this paper,laser bonding technology by bonding pixels with laser irradiation is introduced to reduce the thermal stress caused by thermal mismatch between materials.The influence of laser irradiation on device performance and the feasibility of laser bonding technology in LED micro-display device integration are investigated.The main research contents are as follows:1.Through the micro-LED preparation process and equipment,different size and mesa shape of Micro-LED are prepared,the mesa of different size and shape on the Micro-LED the effect of electrical performance is explored,found that due to the reason of the series resistance of Micro-LED current increase with the size increasing,current density decreases with the size increasing for the junction temperature rise makes resistivity lower,and found that different mesa shape Micro-LED had no effect on the electrical properties;The experiment of irradiating green micro-LED by external light sources with different wavelengths of 628.6nm,531.4nm and 452.5nm was conducted to explore whether the irradiation between each pixel in the micro-display had an impact on the performance of the micro-display device.It was found that the illumination had a significant impact on the reverse leakage current,and the Micro-LED was basically unaffected when working at the forward voltage.2.Two LED micro-display preparation schemes A and B are designed:scheme A is wafer to wafer bonding preparation,scheme B is die to die bonding preparation.Coating photoresist on the Dummy IC wafer with In bump to protect the CMOS circuit,at the same time to ensure that In out of photoresist,the need to use photoresist thinning process,glue thinning process test for A scheme found in dry photoresist thinning than wet process is more simple,high controllable and no other negative effects,and test the plan A photoresist under the premise of can remove can withstand temperatures up to170?;The results show that the thermal expansion coefficient of Ga N is 5.59×10^-6/K,the Si is 2.59×10^-6/K,the mismatch is up to 54%,the maximum relative migration of the bond convex is up to 19?m,and the bond failure is caused by the fracture of the In bumps.3.To preparation of micro LED display device by plan B,four mask designed for the implementation of the plan B.The green and blue LED display device with 0.42inch size,resolution of 480 x 270 and pitch size of 20 microns is successfully prepared.the green light device turn on voltage of 2.36 V,10 A/cm² under the emitting wavelength is 517.8 nm,color coordinates(0.1087,0.7376),half wave width 32 nm,and external quantum efficiency is 3.52%,The brightness can reach 270000 cd/m²under current density of 47.9 A/cm²4.The green and blue LED micro-display device prepared exist some pixels have not been lighted due to thermal mismatch between Ga N and Si,through the wavelength of 532 nm,the energy density of 662 m J/cm²,pulse width is 13 ps,the spot size is 1?m laser irradiation,the pixels that cannot be lit up are successfully bonded,found the forward current increases about 50%,external quantum efficiency increases by about15%,the brightness increased by about 50%;However,with the increase of the number of laser bonding pixels,the electrical and optical properties are attenuated,which is due to the thermal damage caused by the high temperature generated by laser irradiation on Ga N materials and the introduction of a large number of defects.