Method and system for controlling beam scanning in an ion implantation device

Abstract

A method and apparatus for controlling ion beam scanning in an ion implanter is disclosed. Before an implant process is commenced, a scan waveform to create a uniform distribution along a magnetic scan axis is determined, using a travelling Faraday detector ( 24 ). Charge data from the travelling Faraday ( 24 ) is collected into a small, finite number of channels and this is used to create a histogram of collected charge vs. beam crossing time. This is in turn used to correct a target scan velocity to compensate for any dose non-uniformity. The target scan velocity is used as a first input to a fast feedback loop. A second input is obtained by digitizing the output of an inductive pickup in the magnet of the magnetic scanner in the ion implanter. Each input is separately integrated and Fast Fourier Transformed Error coefficients F error are obtained by dividing Fourier coefficients from the target scan velocity by Fourier coefficients from the inductive pickup signal. These error coefficients are used to control the command waveform to the magnetic scanner in the ion implanter.

1 . A method of controlling a periodic product waveform produced by a system having a linear transfer function in response to a drive signal so as to match a target waveform, comprising the steps of: 
 a) sensing at least one period of the product waveform and generating control input digital data representative of said waveform;    b) performing a Fourier transform on said control input digital data to generate control input Fourier coefficients corresponding to said product waveform;    c) performing a Fourier transform on target digital data representative of a period of said target waveform to generate target Fourier coefficients corresponding to said target waveform;    d) comparing said control input Fourier coefficients and said target Fourier coefficients to generate a set of error coefficients;    e) maintaining control output Fourier coefficients for use in generating said drive signal for said system;    f) modifying said control output Fourier coefficients in accordance with said error coefficients to generate improved control output Fourier coefficients;    g) performing an inverse Fourier transform on said improved control output Fourier coefficients to generate control output digital data representative of said drive signal for said system; and    h) using said control output digital data to produce said drive signal for a subsequent period of the periodic product waveform.    
   
   
       2 . An analogue system producing a desired periodic waveform from a periodic drive signal to the system, the system having a linear transfer function, wherein the system comprises: 
 a) a sensor producing a control input signal having a predetermined relationship to said product waveform;    b) an A/D converter generating control input digital data representative of a period of said product signal;    c) a source of target digital data representative of a period of a target waveform having said predetermined relationship to said desired waveform;    d) a digital signal processor receiving said control input digital data and said target digital data and generating control output digital data representative of a period of said drive signal to the system; and    e) a D/A converter generating said drive signal from said control output digital data;    f) said digital signal processor being arrange to 
 i) perform a Fourier transform on said control input digital data to generate control input Fourier coefficients corresponding to said control input waveform;  
 ii) perform a Fourier transform on said target digital data to generate target Fourier coefficients corresponding to said target waveform;  
 iii) compare said control input Fourier coefficients and said target Fourier coefficients;  
 iv) maintain control output Fourier coefficients for use in generating said control output digital data,  
 v) modify said control output Fourier coefficients in accordance with said error coefficients to generate improved control output Fourier coefficients, and  
 vi) perform an inverse Fourier transform on said improved control output Fourier coefficients to generate said control output digital data.