An optimization method for S-curve intensity control model is proposed for further improvement of joint quality during direct writing closed line. The relationship between joint quality and S-curve parameters is established first

then the radian θ and radius R used in the control function is optimized

and finally the model of joint length after optimization is established. The joint length after optimization can be determined by the radian θ. Experimental results indicate that the proposed optimization method can further increase the valid transition area length and reduce the exposure dose distribution variation rate at the joint area. And the joint length using the proposed optimization method is just 2 times of joint length using slope-line control method. Therefore we can conclude that the proposed method improves the line quality with guarantee of writing efficiency.