Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Radiation sensitive composition or product or process of making
Reexamination Certificate
2000-12-26
2002-05-21
Hamilton, Cynthia (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Radiation sensitive composition or product or process of making
C522S066000, C522S031000
Reexamination Certificate
active
06391523
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to photoimageable epoxy compositions useful as photoresists for applications requiring thick films. In particular, this invention relates to specific composition useful for that purpose that contains cyclopentanone as a solvent, at least one epoxidized polyfunctional bisphenol A formaldehyde novolak resin and at least one photoacid generator.
2. Brief Description of Art
Advanced packaging applications requiring solder bumps having a high aspect ratio (i.e., ratio of the film thickness or height to the width of the developed imaged feature) and applications involving the fabrication of micro electromechanical machines (MEMS) require photoresists which are capable of producing uniform layers by spin-coating, providing high aspect ratio images up to thicknesses of several hundred microns. Conventional positive resists based on diazonaphthoquinone-novolak chemistry are not suitable for applications for which the thickness is required to be above about 50-60 microns. This thickness limitation is primarily caused by the relatively high optical absorption of the diazonapthoquinone-type photoactive compound at the near-ultraviolet wavelengths (350-450 nm) which are typically used to expose the resist. Optical absorption necessarily reduces the radiation intensity as it traverses from the top to the bottom of the film, such that if the optical absorption is too high, the bottom of the film will be underexposed relative to the top, causing a tapered or otherwise distorted profile of the developed image.
A negative spin-coating thick-film resist of the chemically amplified type, which has very low optical absorbance at wavelengths in the 350-450 nm range has been described in the literature (N. LaBianca and J. D. Gelorme, Proc. SPIE Vol. 2438, page 846 (1995). This resist comprises a solution in a coating (casting) solvent of a polyfunctional epoxy novolak resin EPON® SU-8 from Shell Chemical and a triphenyl sulfonium hexafluoroantimonate salt (PAG), such as CYRACURE UVI 6974 from Union Carbide. The EPON SU-8 has a high functionality which results in efficient cross-linking, so the resist is capable of forming images having a high aspect ratio in films which may be 300 microns or more thick. The PAG is added at a concentration of less than 10% of the total solids. The disclosed coating solvent uses a mixture of gamma-butyrolactone (GBL) and methyl ethyl ketone (2-butanone). The resulting photoresist solution may be spin-coated onto a wide variety of substrates, pre-baked to evaporate the solvent, leaving a solid photoresist film of up to several hundred microns thickness depending on the solids content of the solution. A pattern is transferred to the photoresist by exposing the film through a mask to near-ultraviolet radiation by contact, proximity, or projection exposure. Subsequent immersion in developer solution dissolves away the unexposed regions, leaving behind a high resolution three dimensional negative image of the mask.
A further reference (K. Y. Lee, N. LaBianca, S. A. Rishton, J. D. Gelorme, J. Shaw and T. H. P. Chang, J. Vac. Sci. Technology B 13(6), 1995) discloses similar spin-coating photoresist compositions, in which the solvent is the single solvent gamma-butyrolactone.
Previous disclosures of thick-film negative photoresist compositions containing SU-8 resin, and a solvent are as follows: U.S. Pat. No. 4,882,245, assigned to International Business Machines Corp. discloses preferred photocurable compositions suitable for use in a printed circuit board which contain up 88% by weight of SU-8 resin, CYRACURE PAG, a reactive diluent epoxy resin and methyl ethyl ketone or methyl isobutyl ketone (4-methyl-2-pentanone) or mixtures thereof as the solvent. The preferred solvent was methyl ethyl ketone alone. These compositions are applied to a cured epoxy resin substrate by means of a doctor-blade and not by the spin-coating method of the present invention.
U.S. Pat. No. 5,026,624, assigned to International Business Machines Corp. discloses a preferred photocurable compositions suitable for use as a solder mask which contain up 90% by weight of SU-8 resin, CYRACURE PAG, a reactive diluent epoxy resin and a solvent selected from gamma-butyrolactone, N-methyl pyrrolidinone, 2-ethoxyethyl acetate, propylene carbonate, 2-methoxyethanol, propylene glycol monomethyl ether or propylene glycol monomethyl ether acetate. The preferred solvent was propylene glycol monomethyl ether acetate. These compositions are applied to a cured epoxy resin substrate by means of curtain coating and not by the spin-coating method of the present invention.
Negative photoresists based on the above disclosed compositions which are suitable for spin-coating, in which the solvent is gamma-butyrolactone alone, are sold by MicroChem Corp., Newton, Mass., USA and used commercially, especially in the fabrication of MEMS devices. For example, a typical product offered by MicroChem Corp., “SU8-50” can be spin-coated at 1000-3000 rpm to produce films of thickness in the range 30-100 microns, which after exposure and development according to a standardized process can produce images having an aspect ratio greater than 10:1 at film-thicknesses greater than 100 microns. Higher or lower solids versions extend the film-thickness range obtainable by a single coat process to below 5 microns and above 200 microns. Gamma-butyrolactone is selected as the solvent for these photoresist compositions primarily because of its high solvency of the SU-8 epoxy bisphenol A resin, and because it is non-flammable. Additionally, the viscosity of a solution containing a high concentration of solids required for a very thick film resist can reduced substantially by heating to a moderate temperature, thus making it practical to be filtered under reasonable pressure to remove particles or gels.
The choice of the coating (casting) solvent is critical. The use of a low boiling point high volatility coating solvent, such as methyl ethyl ketone, which has a vapor pressure of 100 mm at 25° C., may cause the photoresist film to dry too quickly resulting in an uneven surface. The mask pattern to be imaged cannot then be transferred uniformly across the area of the film. If the film is not dried sufficiently, the surface will remain tacky, in which case the film will stick to the mask if contact printing is used to transfer the pattern. Additionally, a stable image may not be formed. It is thus generally preferable to use a relatively high boiling point low volatility coating solvent with a vapor pressure of less than about 10 mm for photo resists which are to be deposited as uniform films by spin-coating.
However, the drying time, which increases rapidly with film-thickness, may become rather long if a low volatility solvent is used for a thick-film forming resist composition. This makes the resist unsuitable for use in production processes in which high throughput is important. In the case of the above disclosed compositions using as the solvent gamma-butyrolactone which has an extremely low vapor pressure of 0.3 mm, the drying time at practical bake temperatures (typically below about 100° C.) is in the range of 10-90 minutes for film-thicknesses in the 20100 micron range. Furthermore, the coating uniformity tends to be poor towards the outer edge of the substrate due to the formation of a rather large edge-bead, and the resist solution may not wet the surface of the substrate sufficiently resulting in uneven spreading, de-wetting or pulling-back of the film from the edge of the substrate after spin-coating or after soft bake. This occurs particularly on substrates such as bare silicon, silicon nitride, glass, or certain metals such as copper. Although the wettability of the substrate may be improved, for example, by treating bare silicon by means of sulfuric acid/hydrogen peroxide followed by hydrofluoric acid etching (piranha etch), or by using a selected primer, this adds to the complexity of the coating process, and in the case of certain primers can lead to a
Hurditch Rodney J.
Johnson Donald W.
Nawrocki Daniel J.
Hamilton Cynthia
MicroChem Corp.
Simons William A.
Wiggin & Dana
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