Determination Of Optical Constants And Thickness Of Wafer Film By Reflectance Spectrum

Wafers are widely used in ultra-precision processing and inspection fields such as electronic circuits and optical instruments.Film thickness and optical constants are important parameters of the wafer film and play a decisive role on the wafer.At present,there are still problems of low accuracy,long time of inversion of wafer film thickness and optical constants,and accurate determination of the end point of wafer processing and polishing.We propose two inversion algorithms based on white light reflection spectrum and a CMP endpoint determination method based on white light reflection method,which improves the accuracy and reduces the running time.The main contents of this article are as follows:1)We use a white light source,QE PRO spectrometer,optical fiber and other equipment to set up a white film reflectance measurement device for wafer film,and measure the corresponding reflectance value of the wafer film through experiments.2)Usually the reflectance extreme envelope method is used to calculate the estimated film thickness,but this method requires the film to have a certain thickness,otherwise the calculation accuracy is not high.Therefore,we use the sampling fitting method to improve it which can be used when the film thickness is thinner.The film thickness is calculated by adjusting the step size,which increases the applicable range and greatly reduces the model space for subsequent calculations.3)We combine the improved adaptive genetic algorithm(AGA)with the simplex method(SM)to construct a hybrid algorithm with two different combinations.Compared with SGA,the accuracy of the algorithm has been significantly improved.SM-AGA1 has a high accuracy and a significant increase in calculation speed.SM-AGA2 has slightly poor stability,and its accuracy is slightly lower but the calculation speed is the fastest.4)We proposed a method for determining the CMP on-line endpoint based on white light reflection spectrum to solve the dynamic polishing endpoint detection problem.The thickness and refractive index of the film are fitted by SM-AGA1 before CMP,and the average value of the reflectance between 390-780 nm is used as the characteristic value to obtain different films.The average reflectance under thick conditions is referred to the table.Finally,the difference between the real-time reflectivity and the preset film thickness reflectance,the eigenvalue size and the change trend are used to quickly determine the change of the polishing material during the CMP process and determine when the CMP end point is reached.