Study Of Key Technologies Of Excimer Laser Annealing Equipments

Excimer laser annealing equipment is one of the key equipments in the industrial production of high performance active matrix liquid crystal display and active matrix organic light emitting diode display.The amorphous silicon thin films on large size substrates can be crystallized efficiently with high quality by employing this equipment so that the performance of the displays can be improved significantly.However,frontier technologies such as excimer laser,beam shaping are involved in the equipment,which brings great development difficulty.A few companies like the American Coherent company could supply high performance commercialized excimer laser annealing equipment so far,and importation is the only way to obtain the mentioned equipment in our country.Excimer laser annealing equipment is built based on large energy high repetition rate laser technique,uniform illumination technique and so on.It is of great significance to the development of the display industry to carry out researches on the referred key techniques.This paper begins at the point of liquid crystal display technology and organic light emitting diode display technology based on thin film transistor pixel switches.First,the laser annealing process is summarized.Second,the key techniques such as large volume uniform discharge,high voltage fast pulse excitation,spot homogenization,projection,line beam detection of the excimer laser annealing equipment are introduced and studied.Based on the key techniques,a line beam annealing prototype is integrated.Finally,the output line beam of the prototype is used to crystallize the large area amorphous silicon thin film to prove the correctness of the technical route and the design method.Furthermore,the effects of the process parameters on the quality of the poly-silicon thin films are discussed.A high voltage fast pulse excitation system is designed and assembled which makes use of reverse doubling voltage structure based on hydrogen thyratron and energy transfer structure based on magnetic pulse compression switch.The pulse with peak voltage of 42 kV and rising edge of 100 ns is achieved.The electrodes fitting for large volume uniform discharge are designed and fabricated with Chang theory.With the help of spark pre-ionization structure,the XeCl work gas is pumped.The length of the activation area is 510 mm,the distance between the electrodes is 30 mm and the electric field inhomogeneity is 27.8%.The effects of components of work gas on pulse energies are experimentally studied.Results show that when Ne is used as buffer gas,the content of HCl and Xe are 0.11%and 2.86%,the total pressure is 2800 mbar,308 nm single pulse energy of 440 mJ can be achieved.The output 308 nm near-field spot appears rectangular with long axis of 30 mm and short axis of 13 mm.The laser spot uniform function of the lens array is discussed by physical and geometrical optics respectively.A design method of line beam shaping systems is proposed.First,projector design is carried out according to working condition.Second,long and short beam homogenizing systems are constrained by spot size,telecentric irradiation,aperture angle and one to one principle.Finally,up or down-collimator parameters are solved according to homogenization requirement.Based on the method,telecentric and non-telecentric illumination systems are designed and comparatively analyzed.Results show that in the telecentric system,most of the optical elements have similar lengths with the line beam,what's more,the total length of the system is large,which makes processing and installation of great difficulty.Effects of key parameters like ccentric errors of the arrays,deviation of work plane on line beam quality are discussed with the help of optical design and simulation software.Results show that in the range of ±0.3 mm of work plane deviation,no obvious influence is found on line beam energy distribution in both systems.Long axis homogenized spot of telecentric system is insensitive to large variation of work plane.Based on the lens arrays,a non-telecentric uniform illumination system is fabricated and assembled.A refraction triplet projector with large aperture and small field of view is designed and assembled by using scaling method,projecting the short axis homogenized spot at the field stop to the surface of the work piece with a reduction ratio of 20:3.Besides,a beam conversion module is installed to make horizontal axis and vertical axis of the beam cross section change their position with each other.An expander is severed to collimate short axis beam.In addition,its magnification determines beam size to match the aperture of the short axis homogenizer.One of the testing result show that the energy transmitting efficiency of the beam shaping system is 33%.Beam size on the work piece is 100 mm×0.3 mm with the average energy density of 520 mJ·cm-2.Homogeneity of part of the long axis beam is 93.95%.Large size amorphous silicon films is annealed by the line beam and the effects of energy density and irradiation pulse number of the line shape laser beam on the crystallization of amorphous silicon films are researched.The crystallization volume fraction and the uniformity of the acquired large size poly-silicon film are discussed.The experimental results show that the crystallization threshold of the amorphous silicon film is 194 mJ·cm-2.The crystallization volume fraction increases with a linear factor of 0.3 first,and then decreases slowly.When the energy density is 432 mJ·cm-2,the crystallization degree reaches its peak.The crystallization volume fraction keeps stable when the number of irradiation is above 20.The crystallization volume fraction of the acquired large size poly-silicon film is 92.26%with the relative standard deviation of 1.56%in the right region under the overlap ratio of 93.7%.