Abstract
In precision micro-nano fabrication, lithography is one of core technology. Recently, lithography technology is applied to diverse field of technologies. These technologies include Micro Electro Mechanical System (MEMS) devices, Flat Panel Display (FPD), and semiconductor industry. In the lithography process, the optical exposure is a critical and essential step and it includes a set of masks. In optical exposure process, a pattern is first created on mask at four times the desired final size and the image of the mask is projected onto a substrate by a large and very expensive reduction lens. As increasing exposure area and requirement of high integration, cost for mask fabrication becomes significant in the total production budget. So, how to reduce the cost of the mask and employing maskless lithography technology has become an important issue to engineers. There are many alternative methods were developed for reducing the mask manufacturing cost or eliminating the entire mask dependent procedures. One of the notable optical exposure processes is Digital Mirror Device (DMD) based maskless lithography. Although there being both advantages and disadvantages, maskless lithography is a receiving substantial attention from engineers in the fields of micro-nano fabrication. The maskless lithography technique provides benefits not only on cost reduction but also on great flexibility during product design and development stages. The basic principle for creating maskless images is that the DMD mirrors are free to rotate from the scan direction, and then the image processing board creates a series of pattern data and transfers it to the DMD. The most important thing in maskless lithography exposure process is how to generate line/space patterns. If individual incident light is not perfectly aligned, the image will be distorted. In order to achieve the accurate alignment of optical head within a reasonable amount of tack time, an autonomous position alignment of micro parallel manipulator system should be installed to each optical head. Moreover, since the optical heads have uncommon aspect ratio and are installed in very limited spatial dimension, the manipulator should also be packaged in a very compact form factor. For the accurate operation of the exposure process, motions of the optical heads should be sufficiently precise than the motion of the chuck which moves in a plane. Optical head alignment process requires high accuracy, but in the practical production process usually manually handle alignment task, consequently increase the cost and reduce the speed of production. In this work, Particle Swarm Optimization (PSO) algorithm is employed to align spot array and judging image sharpness with Auto-Focusing (AF) image evaluation function. To satisfy the requirement of optical alignment system, a high precision manipulator has to be developed. Applied parallel manipulator that consists of four 2-degree of freedom (DOF) decoupled actuator gives chances to provide 6-DOF independence motion, and has strength in high accuracy. This paper covers the follows: Firstly, we reported the DMD-based maskless lithography system to introduce spot array method in maskless digital exposure process. Secondly, in order to precisely control moving platform, applying a redundant parallel micro-manipulator and analyzing kinematic characteristic of the 4-[PP]PS parallel manipulator is described. Thirdly, proposing a suitable error model of the system and applying the PSO and Z axis auto-focusing algorithm to align spot array concerning position correction. Finally, we designed experiment which is applied a vision sensor and alignment unit to verify proposed positioning algorithm. Experimental result will be shown that proposed can solve the alignment problem by reducing the position error of the system.
