Research On Laser Annealing Of Metal Oxide Semiconductor Thin Films And Transistors

The display market is developing in the direction of flexibility and large size.The metal oxide thin film transistor（MOTFT）has the advantages of high mobility（1-100cm²/Vs）and good film uniformity,making it a strong competitor in the display backplane industry represented by Active Matrix Liquid Crystal Display and Active Matrix Organic Light Emitting Diodes.Usually MOTFT needs to optimize the performance through the annealing process,and the traditional thermal annealing has the disadvantages of higher temperature,long time and easy damage to the flexible substrate.Laser annealing has the advantages of fast annealing speed and small damage to the substrate,and has great application potential in flexible and large-size panels.However,the domestic research on the laser annealing mechanism of metal oxide semiconductors is still immature.Therefore,it is very meaningful to carry out laser annealing research on metal oxide semiconductor thin film and transistors.First,in this paper,355nm and 266nm solid-state lasers are used for the laser annealing of silicon-doped tin dioxide（Sn O₂:Si,STO）film.The study found that:1.The STO film annealed by solid-state lasers with wavelengths of 355nm and 266nm,the phase structure remains amorphous,and the film density increases with the laser annealing energy density.And at a lower energy density,the 266nm laser annealing achieves an annealing effect similar to the355nm laser annealing.2.The 355nm laser annealing narrows the optical band gap of the STO film,reduces the relative content of oxygen vacancies,and increases the relative content of lattice oxygen.The reasons for the similarities and differences between 355nm laser annealing and 266nm laser annealing on STO film are as follow:1.The thermal effect generated by laser annealing can induce film densification,the higher the laser energy density,the stronger the thermal effect;and the 266nm laser photon energy（4.7e V）is greater than the STO optical band gap（3.62e V）,which can be recombined by electron-hole to produce additional thermal effects.This proves that the density of STO film can be increased by increasing the laser energy density and increasing the photon energy.2.The lower photon energy of the 355nm laser leads to a lower dissociation efficiency for the metal/oxygen bond,with the thermal effect as the dominant role.The thermal effect can eliminate the defect state of the STO film,reduce the relative content of oxygen vacancies,and reduce the relative content of oxygen vacancies to reduce the carrier concentration,resulting in a narrow optical band gap.The 266nm laser has higher photon energy and high dissociation efficiency for the metal/oxygen bond,which can induce the formation of more oxygen vacancies,increase the carrier concentration,enter the conduction band at the Fermi level,and widen the optical band gap.The 266nm laser photon energy is higher,so the dissociation efficiency of the metal/oxygen bond is high,which can induce the formation of more oxygen vacancies,increasing the film carrier concentration,and increasing the carrier concentration makes the Fermi level enter the conduction band,as a result,the optical band gap becomes wider.Second,in this paper,355nm and 266nm solid-state lasers were used for the study of STO-TFT laser annealing.The study has shown that 266nm laser annealing is more efficient than355nm laser annealing,and the corresponding annealed STO-TFT can obtain higher switching characteristics at lower energy density.The best process condition is to use a solid-state laser with a wavelength of 266nm.After the energy density of 70 m J/cm² laser annealing,the mobility of STO-TFT is 0.079 cm²/Vs,the switching ratio is 1.05×10⁶,the threshold voltage is3.68V,and the subthreshold swing the amplitude is 3.11V/dec.Finally,this paper independently built a 248nm excimer laser annealing platform,using this platform for indium gallium zinc oxide（In-Ga-Zn-O,IGZO）film and praseodymium-doped indium zinc oxide（In-Zn-O:Pr,Pr IZO）248nm excimer laser annealing of thin film.The study found that laser annealing with an energy density of 200m J/cm² caused the IGZO film to crystallize,and the laser-annealed IGZO and Pr IZO film under other conditions remained amorphous.After 248nm excimer laser annealing,the transmittance absorption edge of IGZO and Pr IZO film shifted red,the optical band gap narrowed,the oxygen vacancy content decreased,and the lattice oxygen content increased.The possible mechanism for the above results is that the excimer laser is a flat-top beam that is more uniform than the Gaussian beam of the solid laser,and the photon energy is higher,resulting in stronger thermal effects caused by laser annealing.The 248nm excimer laser annealing is supplemented by the thermal effect leading to dissociation.The thermal effect reduces the oxygen vacancies in the film and reduces the carrier concentration in the film,resulting in narrowing of the optical band gap of the film and red shift of the transmittance absorption edge.The optimized 248nm excimer laser annealed Pr IZO-TFT achieves good device performance.The energy density of 200m J/cm² laser annealed device has a mobility of up to 20.01cm²/Vs,a current switching ratio of up to 2.21×10⁸,and a threshold voltage as low as-2.11V,the subthreshold swing is as low as 1.97 V/dec.