Radiation imagery chemistry: process – composition – or product th – Imaged product
Patent
1995-08-18
1998-10-13
Rosasco, S.
Radiation imagery chemistry: process, composition, or product th
Imaged product
427555, 427584, G03C 300
Patent
active
058210172
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a method of depositing substances, for example metals. The method may be used in the manufacture of integrated circuits and photomasks.
Modern technological demands in integrated circuitry, for example opto-electronics and electronic surgical implants, now require methods by which ultra fine metal lines of submicron dimensions can be deposited onto inert substrate materials. There is a considerable demand for submicron technology in a wide variety of disciplines, but there are many difficulties in developing nanoscale metal deposition processes by conventional lithographic methods. Lithography is the process by which a pattern is transferred to the surface of a substrate material. Before this process can occur a photomask has to be prepared which defines the pattern ultimately. to be achieved on the substrate layer.
Conventionally the photomasks required in the manufacture of integrated circuits are made by applying a film layer of metal, usually chromium since this resists scratching or etching and has good adhesion properties, by metal vapour deposition to a highly pure quartz substrate which has been polished to give an extremely flat surface. The exposed metal is then covered with electron beam resist (which is a material that is sensitive to a beam of electrons and is disrupted on exposure thereto) by placing a drop of resist in the middle of a spinning disk of mask material. The resist is then baked and dried in an oven. The usual electron beam resist is polybutylsulphone (PBS).
An electron beam is directed at the resist face of the resist/metal/substrate composite to degrade portions of the resist, leaving unaffected a pattern required for the integrated circuit. The degraded resist is then dissolved away using a proprietary etchant to expose the layer of metal in the areas where the resist has been degraded, and this is subjected to wet etching by ceric ammonium nitrate to remove the metal and expose the quartz substrate. The remaining resist is then burnt off from the residual metal, resulting in a product through which light can pass apart from the areas masked by metal.
The photomask thus produced is used to define a pattern on a silicon chip coated with photoresist, by passing ultraviolet light through the photomask to degrade the photoresist in the defined areas.
This method of manufacturing photomasks has a number of disadvantages. For example, the dissolution of the degraded photoresist may not be entirely uniform, and while the result is adequate for many products current computer technology demands increasingly dense patterns on integrated circuits and therefore higher-definition photomasks, down to the nanoscale level.
A further area of loss of definition is the wet etch process for removing the unwanted chromium. The etch material attacks the upper, exposed face of the chromium but as it penetrates into the chromium layer its effect is not unidirectional, so instead of producing vertical walls for the residual chromium it erodes and undercuts these walls. This can produce a scattering effect on the ultraviolet light during manufacture of the integrated circuits, reducing definition of the applied pattern. This is one of the principal problems in mask-making.
Where several photomasks are used successively in the manufacture of a chip, accurate registration of each mask is absolutely essential so that each feature appears in the correct place on the finished chip. Problems in epitaxy can cause pattern shift and thus registration. Usually registration is accurate to +/- 0.2 .mu.m. Registration problems, epitaxy growth problems, undercutting, constructive and destructive interference during lithographic process at micron and submicron dimensions, all contrive to indicate that mask-making at nanometer dimensions may require an entirely new manufacturing process.
For nanoscale circuits, conventional methods using metallic inks do not as yet possess the specifications which can ensure the deposition of accurate straight lines of high specification, and the formation of
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Cairns James
Thomson James
Rosasco S.
The University Court of The University of Dundee
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