Method of making masks and electronic parts

Details Method of making masks and electronic parts Method of making masks and electronic parts

2000-02-11

2003-07-22

Huff, Mark F. (Department: 1756)

Radiation imagery chemistry: process, composition, or product th

Imaging affecting physical property of radiation sensitive...

Including heating

C430S311000, C430S270100

Reexamination Certificate

active

06596469

ABSTRACT:

This invention relates to masks and electronic parts and to their production using positive working radiation sensitive compositions.
We have developed novel radiation sensitive compositions and in our earlier-filed patent, application PCT/GB97/01117, published on Oct. 30, 1997, we disclose these novel compositions and their use as imagable coatings for lithographic printing forms. We have now determined that such compositions are suitable as radiation sensitive coatings for masks and for printed circuit and other electronic parts.
The coatings used in pattern forming methods for electronic parts such as printed circuits are classified into two types: negative working and positive working. After exposure to radiation and development, the coating pattern is used as a screen for forming the patterns onto the underlying electronic elements—for example by etching an underlying copper foil. Due to the high resolution demands and the requirements of high resistance to etching techniques, positive working systems are widely used. In particular, in the main there have been used alkali developable positive working coatings mainly composed of alkali-soluble novolac resins as disclosed in J. C. Streiter, Kodak Microelectronics Seminar Proceedings, 1979, p. 116. The primary active component of such positive working compositions, both in the context of lithographic printing forms and electronic parts, is a naphthoquinonediazide (NQD) derivative.
The types of electronic parts whose manufacture may use a radiation sensitive coating include printed wiring boards (PWBs), thick- and thin-film circuits, comprising passive elements such as resistors, capacitors and inductors; multichip devices (MDCs); integrated circuits (ICs); and active semiconductor devices. The electronic parts may suitably comprise conductors, for example copper board; semiconductors, for example silicon or germanium; and insulators, for example silica as a surface layer with silicon beneath, with the silica being selectively etched away to expose portions of the silicon beneath (a step in the manufacture of e.g. field effect transistors). In relation to masks, a required pattern may be formed in the coating on the mask precursor, which is then used as a mask in a later processing step, in forming a pattern on, for example, a printing or electronic part substrate.
An example of a positive working, direct laser addressable coating is described in U.S. Pat. No. 4,708,925. This patent describes a composition which comprises a phenolic resin and a radiation-sensitive onium salt. As described in the patent, the interaction of the phenolic resin and the onium salt produces an alkali-insoluble composition which is restored to alkali solubility upon photolytic decomposition of the onium salt. The compositions can be utilised as positive working compositions or as negative working compositions using additional process steps between exposure and development as detailed in British Patent No. 2,082,339. The compositions are intrinsically sensitive to UV radiation and can be additionally sensitised to visible and infra-red radiation. The compositions are primarily for use as printing plate coatings but there is mention that they may be used as positive coatings for printed circuit boards.
The compositions described in U.S. Pat. No. 4,708,925 cannot be handled extensively without due consideration for the lighting conditions in the working area. Special safelighting conditions are required which prevent unwanted exposure to UV radiation. The precursors coated with such compositions may be utilised for limited periods only in white light working conditions dependent on the output spectrum of the white light source. It would be desirable to utilise digital imaging hardware and to handle the precursors in an unrestricted, white light environment in order to streamline workflows and UV sensitivity would be a disadvantage in these areas. In addition, white light handling would provide an improved working environment in areas which currently have to be under restrictive safelight conditions.
Moreover, the compositions of U.S. Pat. No. 4,708,925 have constraints on their components which create difficulties in optimising properties to provide optimum performance across a range of performance parameters. The presence of functional groups which would crosslink the phenolic resin in the presence of onium salts upon irradiation cannot be allowed, either as a modification to the alkali soluble resin or as additional components in the composition, as this would lead to reduced solubilisation on exposure.
The heat-sensitive composition we have discovered, initially proposed as a coating for a printing form precursor and now proposed also as a coating for a mask or for an electronic part precursor (without the necessity for the composition to be ink-accepting) is suitable for application as a heat-sensitive positive working composition for heat mode imaging which does not exhibit the disadvantages of the prior art as described hereinabove.
The composition of the present invention is heat-sensitive in that localised heating of the composition, preferably by suitable radiation, causes an increase in the aqueous developer solubility of the exposed areas.
Therefore according to one aspect of the present invention there is provided a method of making a mask or an electronic part, the method comprising the steps of:
a) delivering heat selectively to regions of a precursor of the mask or electronic part, the precursor comprising a surface coated with a coating, the coating comprising a heat-sensitive composition itself comprising an aqueous developer soluble polymeric substance (hereinafter called the “active polymer”) and a compound which reduces the aqueous developer solubility of the polymeric substance (hereinafter called the “reversible insolubiliser compound”) wherein the aqueous developer solubility of the composition is not increased by incident UV radiation but is increased by the said delivery of heat; and
b) developing the precursor in a said aqueous developer in order to remove heat-sensitive composition in regions to which said heat was delivered.
Preferably the removal of heat-sensitive composition is complete at those regions, so as to reveal the said surface at those regions, but certain methods, in particular to make certain types of mask, may require the removal of only a proportion of the full depth of the composition where heated, rather than the full depth thereof.
The electronic part may, for example, be any of the electronic parts set out earlier in this specification. The said surface may thus be a semiconductor in which case the method may involve one or more subsequent doping steps selectively in regions in which the composition has been removed, such doping steps being for example by vapour phase, liquid immersion or by ion implantation methods. It may be an insulator which is etched to reveal a conductive or semiconductor layer below. For example it may be silica etched to reveal silicon. Alternatively it may be a conductive surface on which a wiring pattern is to be formed, for example by etching the printed circuit precursor in an etchant to remove said conductive surface selectively in the said regions.
A said surface of an electronic part precursor referred to herein is different from the base used for lithographic printing plates, the context in which the compositions were first proposed. In particular, a said surface is not an aluminium plate which has undergone the usual anodic graining and post-anodic treatments well known in the lithographic art for enabling a radiation sensitive composition to be coated thereon and for the surface of the plate to function as a printing surface. More preferably it is not a metallic plate treated for use as a printing plate. More preferably when the surface is conductive it is a copper or a copper-rich alloy. Preferably such a surface is supported by an insulating substrate. The insulating substrate is suitably a dimensionally stable plastics board, for example of epoxy resin reinforced by

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