Study On GaN Film Transfer Process In Vertical Structure LED Fabrication And Substrate Reuse

The light-emitting diode(LED)is a new light source concept.LED has been widely used in many fields such as lighting,backlighting and display,due to its safety,reliability,high efficiency,energy saving and environmental protection.The GaN-based LED grown on the sapphire substrate occupies the main solid-state lighting market.However,with the continuous expansion of LED applications,the electrical insulation and poor thermal conductivity of sapphire substrate has become a major issue in limiting the development of the LED.The LED epitaxial film is transferred to a substrate with excellent electrical and thermal conductivity,and the preparation of the vertical structure LED can effectively solve the above problems.In this dissertation,the transfer process of GaN-based LED epitaxial film was studied.A 2-inch LED epitaxial wafer was bonded with a Si wafer substrate by using Au-In wafer bonding technique.Then,the laser lift-off technique was used to separate the GaN epitaxial film from the sapphire substrate.The sapphire substrate was processed with CMP for its reuse.The following research results have been obtained in the experiments:(1)When Au-In bonding technique was used to bond 2-inch LED epitaxial wafers and Si wafers,both heating and pressurization contribute to increasing the Au-In bonding yield.But the higher the bonding temperature,the greater thermal stress introduced,which easily leads to the wafers damaged during bonding or subsequent processing.The bonding yield between the 2-inch LED epitaxial wafers and the Si wafers can reach over 98%,and the bonding strength is greater than 2.5J/m~2 when the bonding temperature was 180°C,the bonding pressure was 3.6 bar,and the bonding time was 20 minutes.(2)When the KrF excimer laser with a pulse width of 38ns was used to irradiate the sapphire substrate,it is unsuitable when laser energy density was below 600mJ/cm~2 or above 680mJ/cm~2.The GaN epitaxial film can't be separated from the sapphire substrate when energy density of the laser was insufficient,and high energy density of the laser will cause damage to the epitaxial layer of the LED.Reducing the area of the laser spot helps reduce damage to the epitaxial layer during stripping.The epitaxial layer of the LED maintained the structural integrity,and there was a red shift of the emission peak wavelength of 2.314 nm after the epitaxial layer was separated from the sapphire substrate using the optimized process.(3)The reused substrate was used to grow the LED after CMP.Compared with the LED on original substrate,the light emission intensity of the LED on reused substrate was slightly attenuated about 4%,and there was a red shift of peak wavelength from 3.7 to 6.8nm.The epitaxial layer on reused substrate basically satisfies the requirements of the LED industry.