Theory Research Of Key Controlling Techniques During Polar Laser Direct Writing

Diffraction optical elements are keys to aerospace, aviation, national defense and communications. The diffraction optical elements with high performance and high fabrication precision, especially with minuteness structure, are urgently demanded with the development of science and technology. Laser direct writing is one of the key techniques in diffraction optical elements fabrication due to its maskless and high precision, and polar laser direct writing is the mostly method to manufacture centrosymmetrical diffraction optical elements. However, the shortages of polar laser direct writing such as low joint quality, low linewidth stability and line center radial drift restrict the improvement of centrosymmetrical diffraction optical elements quality.The study of this dissertation is to get a control method to improve the line fabrication quality on the basis of polar laser direct writing technique at present. Concretely, radial drift is eliminated by radial position compensation, the joints quality is improved by increasing valid exposure dose transition length and the linewidth stability is improved by reducing the linewidth sensitivity to exposure dose error and exposure threshold error. The study provides an effective approach to resolve the problem of joints quality and linewidth stability, then to provide technical storage for improvement of diffractive optical elements performance and widen the application fields of diffractive optical elements.In order to reduce the fabrication error during polar laser direct writing process, the exposure dose distribution is profoundly analyzed correspond to exposure principle of polar laser direct writing. The linewidth broadening model and the line center radial drift model are established on the basis of studying the causes that linewidth broadening and radial drift appear. The coupling of linewidth broadening and radial drift is analyzed, and then the linewidth broadening power compensation model and radial drift compensation model are established.In order to improve the joint quality of closed line, an S-curve intensity control smooth close-up method is presented. The intensity of joint area is controlled by S-curve, and the valid transition length of exposure dose variation increases due to the characteristic of S-curve's slope rate gradual change. Further, the parameters of S-curve is optimized considering it to be optimal when the exposure dose distribution got an extremum at one point, then smooth close-up is realized.Linewidth stability is an important index of line quality. A laser power control method is presented to improve linewidth stability for defocusing laser direct writing on the basis of analyzing the factors which affect the linewidth stability. The writing power and defocusing amount are synchronously adjusted to set photoresist threshold at position where the linewidth variation rate is small, thereby the linewidth sensitivity to variation of actual exposure dose and photoresist threshold is weakened, and then the linewidth stability during defocusing laser direct writing is guaranteed.Finally, experiment are done to validate the methods presented in this dissertation, including validation of close-up effect when use S-curve to control the intensity of joint area, validation of linewidth model and linewidth stability when linewidth stabilizing method is adopted.