Electron beam lithography systems require fast hardware to generate the deflection signals for the exposure of the required shapes. Today's standard shape primitives are rectangles, triangles and trapezoids, which is a sufficient set for typical semiconductor applications. New methods of proximity correction or the production of microoptical devices require shapes with curved, smooth borders. Today, these shapes will usually be approximated by a set of these standard shape primitives, but a high quality of the approximations increases the number of needed shape primitives dramatically.
One possible way out of this problem is described by Anderson et. al. [1]. Generalized curvilinear structures are splitted into 2nd order line segments, which are computed by an array of fast digital signal processors (DSPs) while writing the previous line segment.
A different implementation was realized by Vasey et. al. [2]. Here, the endpoints of line segments on curved conic shapes are calculated by a DSP while linescanning hardware is filling in the shape with these line segments until reaching the calculated endpoint.
We develop a new pattern generator, which is able to produce the deflections signals for shapes with two arbitrairily curved borders. The borders can be described as 3rd order polynomials providing high flexibility in the possible shapes while strongly reducing the amount of data. The shapes are enitirely filled by hardware and the computational power of the system's processor does therefore not impose any limits to the pixel rate. Hence pixel rates greater than 20 MHz can be achieved without expensive computing hardware. By using faster components (faster FPGAs, ASICs), pixel rates can be increased above 50 MHz. The exposure data can be easily prepared offline. This new pattern generator includes all necessairy components like a direct digital synthesis clock, field correction unit and standard PC-interfaces for data handling and control.
Prepared raw data are sent to the pattern genrarator via the centronics interface and bufferd in a 4 kword FIFO. These data set up two accumulators that calculate the endpoints of the shapes on the polynomial border. Between the two borders, the shapes are filled with line segments by a linescanning logic that compares the actual scanposition with the calculated endpoints, as shown in figure 1. The next line starts with no clock delay as soon as an endpoint is reached. Linescan and polynomial logic as well as the interfaces are controlled by a DSP. The calculated digital data are converted to analog signals by 2 digital to analog converters (DACs) per axis, including precise self calibrating 16bit-DACs.
The complete system is designed as modular 19" system, allowing to configure it with only the components required by the desired applicaton (DDS-clock, field correction, interfaces). Although we do not make any compromise concerning the quality of the components in the analog and digital sections, the complete system can be considered as a cost sensitive solution for digital generation of curved shapes due to the absence of expensive powerful computational hardware, the use of programmable logic (FPGAs) and the standard interfaces (Centronics for the exposure data, V24 for controlling).
References
[1] Erik H. Anderson, Volker Boegli and Lawrence Mury, J. Vac. Sci. Technol. B, 13, 2529 (1995)
[2] F. Vasey, H. Rothuizen, P. Vettiger, J. Vac. Sci. Technol. B, 12, 3460 (1994)