Electron beam induced conductivity effect of polymer resists and charging induced electron beam deflection simulation in electron beam lithography

The electron beam induced conductivity (EBIC) effect of polymer materials was investigated and three electron beam resists, PBS, EBR900, and ZEP7000, were used as target materials in this research. An external bias method was applied for the direct measurements of EBIC current and the EBIC conductivity was then derived. The change of carrier drift velocity and the carrier concentration under field is believed to be the reason for the conductivity increase and the approach of the asymptotic values in the initial state EBIC characteristics. The temperature effect on EBIC was illustrated by EBR900 and a simple single trap level analysis gave good agreement in explanation of initial state conductivity dependence on temperature changeThe dependence of initial state EBIC values on dose rates and bias was also determined and an empirical from of EBIC-bias relationship was established for the electron beam deflection modeling.

The long term EBIC characteristics of resists were investigated under different dose rates. The variation of EBIC versus time was explained by a transient theory that considers rates of carrier generation, carrier trapping, carrier detrapping, and carrier recombination contribution. The trapping model analysis on steady state EBIC results revealed that PBS has a uniform distribution of traps, while EBR900 and ZEP7000 fall in the same category that has exponential distribution of traps. The steady state EBIC gain was also determined for the three resists and the onset of the plasma region was observed for EBR900 and ZEP7000.

Monte Carlo simulation on electron beam range advised that only the positive charging layer needed to be considered for charging simulation and the image method was applied for the construction of the surface potential calculation.

A simulation model for the calculation of electron beam deflection owing to resist charging was developed in the second part of this research. Charging patterns of circles and squares of different dimensions were considered. Both SIMION and MATHEMATICA were used as simulation tools and the limitation of SIMION was illustrated and discussed. The model programmed in MATHEMATICA simulated electron beam deflection during e-beam writing. With the combination of the empirical EBIC-bias relationship of EBR900, the model simulated surface potential decay after irradiation and the results showed good agreement with literature values. The extent of deflection per surface potential was shown to increase with pattern dimension size under the same secondary electron emission yield, while the maximum deflection per surface potential per unit area decreased as the pattern dimension decreased.