Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making printing plates
Reexamination Certificate
1999-11-03
2002-09-24
Baxter, Janet (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making printing plates
C430S300000, C430S322000, C430S309000, C430S325000, C430S270100, C430S284100, C430S277100, C430S927000
Reexamination Certificate
active
06455231
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to new photoresist dry film constructions and compositions. Preferred dry film of the invention are characterized as including a base carrier sheet, but without any type of cover sheet. These preferred dry film resists can be photoimaged in the absence of the carrier sheet, enabling significant cost and waste reductions as well as significant improvements in resolution of the patterned resist.
2. Background
Photoresists are photosensitive films used for the transfer of images to a substrate. A coating layer of a photoresist is formed on a substrate and the photoresist layer is then exposed through a photomask to a source of activating radiation. The photomask has areas that are opaque to activating radiation and other areas that are transparent to activating radiation. Exposure to activating radiation provides a photoinduced chemical transformation of the photoresist coating to thereby transfer the pattern of the photomask to the photoresist-coated substrate. Following exposure, the photoresist may be developed in a suitable developer to provide a relief image that permits selective processing of a substrate.
Photoresists are often formulated and applied as liquid compositions and the liquid carrier is removed after coating onto a substrate surface.
For many applications however including printed circuit board manufacture, dry film resists are frequently employed. See, for instance, U.S. Pat. Nos. 5,300,913, 4,943,513 and 4,610,951 for background on dry film resists. Such resists are applied as a solid (dry film) coating layer to a copper laminate or other substrate. A typical dry film contains a base or carrier sheet over which is coated a photoimageable composition with subsequent removal of the composition liquid carrier. Polyethylene terephthalate (PET) is a common carrier sheet.
To protect the unprotected surface of the photoimageable coating and to facilitate rolling of the carrier sheet/photoimageable construction, the unprotected composition surface is covered with a removable protective sheet such as a polyethylene (PE) layer, polypropylene layer or a polyethylene terephthalate (PET) layer.
Despite the nomenclature, a typical dry film photoresist is a highly viscous liquid that contains a certain amount of tack after drying. As a result, when a carrier sheet (e.g. PET) is coated and the photoresist is applied and dried, a protective cover sheet such as polyethylene is required. Without that cover sheet, the photoresist would adhere to the backside of the PET when it is wound around a plastic core into a roll, which is a typical dry film construction form.
In use, the cover sheet (typically PE) is removed from the dry film construction and the exposed side is applied to a copper laminate or other substrate surface. The PET carrier sheet remains, overlaying the photoresist through exposure. A sheet that contains the desired pattern (photomask) is placed on top of the PET carrier sheet and through which activating radiation is applied and the pattern is transferred to the resist. The carrier sheet is required through exposure. If the carrier sheet were removed prior to exposure, the photomask sheet would adhere to the surface of the tacky photoresist.
Hence, the PET or other carrier sheet must have high optical quality to avoid undesired interference with the radiation passed therethrough to the resist layer. Interference with the exposure radiation can compromise resolution of the image patterned into the resist layer and transferred to the underlying copper or other substrate surface. After the exposure step, the carrier sheet is removed and discarded in order to permit development of the latent image patterned in the resist layer.
Moreover, as a viscous liquid sandwiched between the carrier and protective sheets, the dry film is subject to flow over time. Such “cold” flow can result in thin spots, thin lines, or other areas where film thickness is less than desired, all of which can significantly compromise lithographic performance and reduce production yields. In particular, any surface protrusions in the carrier and protective sheets will cause thin spots and voids. Such protrusions are generally present in the PE cover sheet. Additionally, flow can occur at the end of a dry film roll where resist flows out and fuses with other areas of flow. Such end flow requires slitting of the end prior to use of the dry film, resulting in significant waste and again potentially compromising lithographic results. Additionally, when the dry film is unwound, the “fused” resist will flake and can be caught between the photoresist and substrate resulting in processing variations and subsequent yield losses.
It thus would be desirable to have new dry film constructions.
SUMMARY OF THE INVENTION
I have now found new photoresist compositions and dry film constructions that are highly useful in negative-tone dry film resist constructions.
Preferred dry film resist constructions of the invention do not require the use of a protective cover sheet due to the resist's very dry nature. In particular, because of such high dryness of the composition, a carrier sheet (e.g. PET) is not required during the exposure step since the pattern containing sheet (photomask) will not adhere to the resist composition during exposure. Moreover, enhanced resolution of the image patterned in the resist can result since the carrier sheet (e.g. PET) will not be present to potentially interfere (e.g. diffraction or other optical interference) with the exposure radiation.
Dry film resists of the invention preferably comprise at least a photoactive component, and a highly viscous or solid (at room temperature, ca. 25° C.) crosslinker component. A flexibilizing agent also may be added, e.g. depending on other resist composition choices. Additionally, a film forming polymer can be added to aid in coating and processing of the resist.
The crosslinker component of prior dry film resists conventionally has been comprised in at least significant part of one or more monomers (e.g. unsaturated compounds) or other relatively low molecular weight compounds. It has been found that such low molecular crosslinkers are a predominant cause of undesired cold flow during storage of a dry film resist roll.
I have found that use of oligomeric or other higher molecular weight crosslinkers can provide a dry film resist that is essentially dry and tack free. In fact, unlike prior dry film compositions, resist compositions of the invention have melt point or sharp T
g
, e.g. about 35° C. or 40° C. or greater, more preferably about 45° C. or 50° C. or greater. Exemplary preferred crosslinkers include acrylated urethanes, preferably having a weight average molecular weight of at least about 500 daltons, more typically an Mw of from about 500 to 100,000, still more typically from about 1,000 to 50,000. Another preferred crosslinker is an acrylate oligomer, typically having a weight average molecular weight of from about 100 to 5,000 daltons. In general, preferred crosslinkers of the compositions of the invention will have a molecular weight of at least about 400 daltons, more preferably at least about 500, 600, 700, 800, 900 or 1000 daltons. Generally preferred crosslinkers are polymeric compounds. Generally preferred compositions of the invention contain a crosslinker component that is predominately (greater than 50 weight %) comprised of compounds having a molecular weight (Mw) of about 15,000 daltons or greater, more preferably a molecular weight (Mw) of about 75,000 or greater. Even more preferably, the crosslinker is comprised of at least about 30, 50, 60, 70, 80, 90, or even 95 weight percent of such high molecular weight crosslinker compounds. The crosslinker component may contain a relatively minor portion of low molecular weight monomer-type compounds, although such use of such compounds is less preferred.
In preferred compositions, a film forming polymeric material may be added that has a Tg sufficiently high to allow use of low molecular we
Baxter Janet
Clarke Yvette M.
Corless Peter F.
Piskorski John J.
Shipley Company L.L.C.
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