Research On Thin Layer Resistivity Test System For Silicon Wafer Based On Pseudo Measured Value Method

As silicon wafer is the substrate material for making IC chips,its electrical properties have a great influence on the performance of electronic products.And the thin layer resistivity of silicon wafer is directly related to the product yield.Therefore,the accuracy of resistivity measurement is very important.In fact,the conventional test method has many limitations,such as long test time,complex operation steps,high accuracy,high pollution and low resolution.In order to avoid these problems,a pseudo measurement value method is put forward to reconstruct the resistivity distribution image of silicon wafer based on the node backprojection algorithm,and applies to the thin layer resistivity test of silicon wafer.This method is used to get the boundary voltage distribution function by fitting the polynomial function with the voltage containing the impedance information between the 16 electrodes on the boundary of the silicon wafer.With this function,we can get more the voltage informations on locations of the boundary.Through these voltage informations,we use the back projection algorithm to reconstruct the resistivity distribution image.The constant current source is an important component of testing system,and its performance determines the accuracy and the quality of reconstructed image.The resistivity of the silicon wafer is greatly influenced by the current,and different currents are needed to be injected during the testing of silicon wafers with different resistance.In order to satisfy this demand,a constant current source based on fuzzy control algorithm is proposed in this paper.A fuzzy controller is designed through practical experience so that the constant current source can output the optimal currents.The relevant hardware circuits of the thin layer resistivity test system with DSP28335 as the core control device are designed.It includs constant current source module,multi-channel analog channel module,signal amplification module and serial communication module.And the parameters of the main components of each module are given.During the design process,the basic physical properties of silicon wafer were studied,and the contact resistance between silicon wafer and metal electrode was analyzed.Appropriate electrode materials were selected through analysis and comparison.Finally,we can use the system to realize the acquisition of voltage.After analyzing and fitting the collected voltage data based on the pseudo measuredvalue method,the boundary voltage distribution function is obtained.And the resistivity distribution map is reconstructed by using image reconstruction algorithm throught this function.The results by comparing the conventional measurement methods show that the proposed scheme is correct.It also shows the superiority of the scheme in the resistivity resolution,detection speed and nondestructive measurement.