Computer aids to the design of thin film circuits.
The work described in this thesis is concerned with the ways in which a digital computer can assist in the design of thin film circuits. One of the main advantages of thin film technology is that very accurate resistors can be fabricated on the slice. A set of programs have been developed such that the resistor geometries can be synthesised entirely automatically. The results are difficult to match by a conventional manual approach because of the iterative nature of the problem - manual techniques tend to make over-simplifications in the design. This increases the amount of adjustment required to each resistor after manufacture. The user can merge resistor geometric information with additional topological circuit data to drive the main interactive graphics programs. These combine automatic placement and routing algorithms with general purpose manipulative facilities, to provide a substitute for the traditional pencil and paper. The automatic placement facility incorporates the concept of "planarity", which attempts to minimise the number of conductor crossovers required by the automatic routing algorithm. This is important since each crossover has to be added by hand after fabrication. Initially developed for the design of single-sided printed circuit boards, the planarity concept is equally applicable to thin film circuits, which are made with a single layer of interconnections. The user has complete control over the design process at each step - the programs allow him to adjust the layout interactively after each placement or routing phase. This ensures that any inadequacy in the automatic routines is corrected before it can be compounded in further processing. When the layout has been finalised, it is possible to fabricate a set of photolithographic masters directly from the data structure. The programs which have been written for this purpose, produce paper tapes to drive a flat bed plotter fitted with a light attachment. The mask-shapes are systematically exposed to light on photographic film using a variety of different aperture shapes. The design suite is completely modular in structure and may easily be extended. Future improvements should serve to reduce the amount of manual interaction required at each stage.
Thin film circuits, Computer, Planarity, Photolithography
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