Digital transducer

Excimer lasers for lithography Experiments reported by scientists of the IBM San Jose Research Laboratory indicate that the intense ultraviolet light from 'excimer' lasers may be a key element in extending optical lithography into the sub-micrometer region. The San Jose researchers have produced excellent 0.5/Jm lines in resists using contact masking with exposure times as short as a few tens of nanoseconds. Excimer lasers have two properties that are potentially of great significance for microcircuit manufacture. First, they provide very intense short wavelength light with wavelengths ranging from the near uv (350 nm) into the vacuum uv (less than 200 nm). The intensity is more than a hundred times greater than that of conventional mercury or deuterium lamps. Second, excimer lasers have less spatial coherence than most other lasers. The IBM experiments show that this is an advantage for lithography because it eliminates the 'speckle' or interference effects that have previously limited the usefulness of lasers in lithography. The intense light flux from excimer lasers provides a number of desirable options in semiconductor processing: an increase in the speed of wafer exposure; steeper wall profiles made possible by the greater coherence available with excimer lasers as compared to completely incoherent sources; and a greater flexibility in choosing resist properties such as resolution and etching characteristics. Most research on resists has necessarily emphasized photosensitivity because of the low intensity of available uv light sources. The operation of excimer lasers involves somewhat complicated reactions among noble gases (xenon, krypton, argon) and halogen molecules, usually chlorine or fluorine. All excimer lasers are pulsed; typically, an electrical discharge initially produces a noble gas-halogen molecule in an excited state. This molecule decays to a weakly bound ground state, giving off ultraviolet light in the process. The intensity of excimer light is known to be essential to a process known as stimulated Raman scatter-

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ing, in which the frequency of the laser light is shifted over a considerable range. This permits laser light to be tuned to the most sensitive spectral region of a resist. (Raman scattering involves the absorption and re-emission of light by matter. In the process, the frequency of the light is increased or decreased because the vibrational energy of the active molecules is added to or subtracted from the light. If the laser light is intense enough, the Raman process becomes stimulated, much as in the laser itself, and the conversion efficiency is quite high, up to as much as 70%.) The San Jose researchers also measured the effects of 'reciprocity failure' in resists at the high intensity exposures produced by excimer lasers. Reciprocity failure here means a saturation of the absorption of the resist, a condition in which higher intensity of light has little or no further effect on resist behaviour. They found that reciprociW failure had no more than a factor of three effect on resist response, although the peak light intensities in the laser experiments were more than 100 million times greater than those from conventional uv lamps. Thus, most of the higher uv intensity of excimer lasers can be exploited in faster exposure of the resist. The IBM work on excimer lasers for lithography was reported by Kanti Jain and Grant Willson of the IBM San Jose Research Laboratory in collaboration with B.J. Lin of the IBM T.J. Watson Research Center in Yorktown, New York, at the Optical Microtithography conference of the Society of Photo-Optical Instrumentation Engineers in Santa Clara, California. Additional details are published in the March issue of the IBM Journal of Research and Development, and the March issue of the I EEE Electron Devices Letters.

Digital transducer Miniaturisation of many engineering components has brought with it a need to resolve linear displacements to 1 or even 0.5/~m with a high degree of repeatability and accuracy. A new form of optical transducer can now be sup-

plied which is particularly designed for use in gauging fixtures requiring these orders of precision. The device consists of a high precision cross roller linear bearing slide to which is attached directly a diffraction grating scale and stabilised reflection reading head. Adequate fixing points for contact probes and secure mounting of the assembly are provided. The system is supplied in a prealigned and tested form, thus eliminating any need for expertise in electronics on the part of the user. This new product bridges the gap between systems available in component form for oem use and fully enclosed systems for end users. Full compatibility with digital display systems and various forms of computer interfacing have been retained. Standard transducers provide for a range up to 100 mm and specials for larger ranges can also be supplied. Metronic Technology Ltd, PO Box 404, Kings Langley, Herts, UK, WD4 8ND

Grinding wheels Semi-friable grinding wheels suitable for the majority of precision grinding operations have been introduced by the Universal Grinding Wheel Company. This 47A range of vitrified wheels is claimed to combine the toughness and wear resistance of conventional aluminium oxide with the free cutting properties of high purity, white, aluminium oxide. This produces a quality product able to outperform conventional abrasives on cylindrical, centreless, crankshaft and some surface grinding operations. The main features of the 47A are its free cutting properties (up to 35% higher stock removal rates), more components per dress (up to 35% more before re-dressing the wheel), improved surface finish (grit for grit size, components ground with 47A will have a better surface finish) and longer wheel life (up to 20% longer). These in turn lead to reduced diamond costs, reduced dressing cycle time and less downtime. Universal Grinding Wheel Company Ltd, Doxey Road, Stafford, UK, ST16 1 EA

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