Photoresist Thickness Measurement Method Based On Monochromatic Light Interference

The measurement of photoresist thickness is an essential process in the photolithography process.To obtain components that meet the requirements,it is necessary to get an accurate photoresist thickness.Adjusting the exposure time based on inaccurate thickness information often leads to a short circuit of the metal electrode or blurred edges.The traditional photoresist thickness measurement is to roughly judge the range of the thickness by the experimenter according to the rotation speed-thickness curve provided by the photoresist manufacturer.The photoresist thickness information obtained in this way is inaccurate and the method to judge the thickness is inefficient and high-cost.To solve this problem,a photoresist thickness measurement method based on monochromatic light interference was studied,including the study of the automatic counting algorithm of interference fringes,the review of the thickness calculation formula at the apex of the photoresist edge,and the preparation of the photoresist experimental samples,the construction of hardware system and software system,the experimental verification and analysis.Based on the light transmittance,viscosity,adhesion,and sensitivity of shortwave light of the photoresist,design the optical path of the system,use the principle of film interference to obtain the interference fringe pattern of the photoresist sample,and design the photoresist thickness Measure the platform and calibrate the microscope.Using monochromatic light interference to measure the thickness,it is necessary to determine the number of interference fringes manually.For this purpose,an automatic counting algorithm for interference fringes is designed.To avoid counting the redundant parts of the interference fringe pattern,the region of interest(ROI)on the interference fringe pattern is obtained based on the method of human-computer interaction.For the noise and impurities in the image,the image of the region of interest is filtered and denoised.To better process the image information,the interference fringe image is binarized based on the Otsu algorithm,and the pixel value of the image is changed to 0 or 1,and the image data is inverted.Refine the interference fringes to a single-pixel width skeleton based on the Hilditch refinement algorithm.Design an automatic deburring algorithm for the burrs that appear on the fringes.Design an automatic counting algorithm for interference fringes by scanning all columns in the middle row of the image.The principle of light interference,the principle of relative light intensity based on the monochromatic light interference,the thickness calculation formula for any interference half-level is introduced.Finally the thickness calculation formula at the apex of the photoresist edge is proposed.Experimental samples of photoresist with different thicknesses were prepared.The experimental hardware system and software system are built.The hardware system can obtain the interference fringe pattern at the photoresist's edge based on the principle of interference and transfer the image to a PC for processing.The software system can realize the fitting of the thickness curve at the edge and get the thickness value at the vertex.Carry out the experiment of interference fringe automatic counting algorithm and photoresist film thickness measurement.The experimental verification shows that the interference fringe automatic counting algorithm can correctly get the total fringes.When the interference fringe spacing is too dense,some impurities on the photoresist film,and the light field is too dark,the algorithm can basically correctly determine the number of fringes.The repeatability test of the film thickness at the photoresist's apex was carried out and compared with the film thickness data obtained by the step meter and the F20 film thickness meter.The experiment verified the accuracy of the photoresist film thickness measurement method.