Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From carboxylic acid or derivative thereof
Reexamination Certificate
2001-09-18
2003-07-01
Hampton-Hightower, P. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From carboxylic acid or derivative thereof
C528S322000
Reexamination Certificate
active
06586560
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to selected alkaline soluble maleimide tetrapolyrners prepared by the free radical polymerization of maleimide, N-alkyl maleimide, methyl(meth)acrylate and either (meth)acrylic acid or (meth)acrylamide. Furthermore, the present invention relates to their use as non-imageable resists or in anti-reflective coatings in multilevel photolithographic processing or a polymer component in an imageable photoresist.
2. Brief Description of Art
The additive process of depositing patterned metal films is known as the lift-off process or metal lift-off process. There are several variations of the metal lift-off process. Those which are most widely used in practice, for example, to deposit the metallic “read-stripe” in the manufacture of thin film heads for magnetic hard drives, and in the fabrication of the gate metal for GaAs FET devices, involve a bilevel lithographic process which is the subject of the present invention. Variants of the bilevel lift-off process are described in detail in European Patent Application No. 0341843 A2 assigned to International Business Machines Corp., and U.S. Pat. No. 4,814,258 assigned to Motorola Inc.
A solution of a non-imaging lift-off resist is deposited by spin-coating to form a uniform thin film on top of a substrate to be metallized. The lift-off layer is soft-baked by heating at a sufficiently high temperature to remove most of the solvent. A conventional imaging positive resist layer is deposited on top of the lift-off resist. There must be no intermixing between the top resist and the bottom layer. Therefore, the lift-off resist should have a low solubility in conventional positive resist solvents. After a second soft-bake to remove most of the residual solvent in the imaging resist layer, the pattern is transferred from a mask to the resist film using a conventional microlithographic imaging tool such as a contact and proximity printer or stepper. The exposed areas in the resist layer represent the areas to be metallized. The exposed resist is developed through to the lift-off resist layer which then dissolves both vertically through to the substrate and laterally, to penetrate the underneath the unexposed areas of the top resist a small predefined distance to produce a controlled degree of undercut in a development time which is neither too long to make the process impractical or to remove unexposed photoresist, or too short to make the process irreproducible.
In one variation of the multilevel photolithographic process, often referred to as the PCM (portable conformable mask) variation, the underlying lower level resist is photosensitive in the deep ultra-violet (DUV) spectral range and the positive imaging top resist is of the novolak-diazo-naphthoquinone type. The latter absorbs in the DUV and acts as a mask to an intermediate DUV flood exposure. This renders the lower level resist more soluble in a selected developer in the exposed areas that are to be removed during the development process.
After the desired degree of undercut is developed in the lower resist layer, the metal layer is blanket-deposited by sputtering. The undercut ensures a discontinuity between the metal on top of the resist and the metal in the trench formed by the lithographic process. By this means, upon subsequent stripping of the remaining top photoresist and the lower resist, the metal deposited on top of the resist is cleanly separated from the metal deposited on the substrate, ensuring consistent profiles and critical dimensions of the metal pattern. The degree of undercut, and hence the lateral dissolution rate, must be carefully controlled.
In any practical lift-off process it is desirable to adjust and maintain precise control of the dissolution rate of the lift-off resist layer, so that the required degree of undercut is always obtained in a relatively short time using a developer which it is compatible with, and provides a wide process latitude for the imaging positive photoresist layer.
In the fabrication of semiconductor devices on highly reflective substrates such as silicon, or metals, a bottom anti-reflective coating which absorbs actinic radiation, thus suppressing standing waves, which is soluble in an alkali developer and is, therefore, easily removed without the need for plasma etching is especially useful. The properties required for a film-forming polymer which is to be used in combination with a dye or other absorber in an alkali soluble anti-reflective coating are very similar to those required for a lift-off resist.
Only one known class of commercially available thermoplastic polymers known as polyglutarimides, especially polydimethylglutarimide (PMGI), have the desirable combination of properties including poor solubility in positive resist solvents such as ethyl lactate, 2-heptanone and propylene glycol methyl ether acetate, good solubility in polar solvents such as cyclopentanone and N-methyl-2-pyrrolidinone, a desirable range of alkaline solubility to give controlled undercut rates with aqueous developers typically used for the development of conventional positive resists, and have a sufficiently high glass transition temperature ideally required for the lower level resist in the above described bilevel process.
Polyglutarimide or polydimethylglutarimide refers to a class of polymers containing partially cyclized imide and N-alkyl imide moieties and uncyclized polymethacrylate ester, in which the degree of cyclization as well as the ratio of N-alkyl to N—H can vary widely depending on the starting materials and the process used in the preparation. PMGI polymers for lift-off applications are generally found to comprise about 65-85% or more of cyclized imide moieties of which about 50-75% are N—H and the remainder N-methyl substituted, and are made commercially by the reaction of ammonia at high pressure with polymethyl methacrylate (PMMA) in a reactive extruder as disclosed in U.S. Pat. No. 4,246,374 (assigned to Rohm and Haas Company). The process conditions required to cause sufficient imidization of PMMA, are severe, requiring anhydrous ammonia at a pressure above 60 atmospheres and a reaction temperature of greater than 300° C. The high reaction temperature can result in partial decomposition of the PMMA resin. Unreacted ammonia and volatile bi-products must be treated and safely disposed of.
The alkaline solubility of PMGI is critically dependent on the degree of imidization, the value of which is sensitive to small variations in the extruder reaction parameters. Thus, it is difficult to obtain a resin composition having a specified uniform alkaline solubility required for a lift-off resist.
PMGI resins produced by this process have a fairly narrow range of alkaline solubility, and exhibit poor adhesion on widely used substrates such as bare silicon, tantalum nitride, gallium arsenide and metals, especially if the bake temperature used is below the glass transition temperature of the PMGI resin film. Additionally, in the case of a lift-off process which requires a small degree of undercut, and hence a low dissolution rate, the use of a high molecular weight PMGI as the lift off-resist it may lead to the formation of residues (scum) due to incomplete solubilization during development. Furthermore, in the case of a lift-off process which requires a large degree of undercut, and hence a high dissolution rate, the use of a high molecular weight PMGI as the lift-off resist and along with a developer compatible with conventional positive resists may not produce the desired degree of undercut.
This limitation creates the need for other methods of modifying the dissolution rate of these PMGI resins. One such method is to reduce the molecular weight of PMGI by exposing the polymer to DUV or electron beam radiation. This method has been described in U.S. Pat. No. 4,636,532 (assigned to Shipley Co.). By this means, the dissolution rate of PMGI, can be increased to some extent. However, the amount of increase in the dissolution rate may be insufficient for certain li
Chen Cindy X.
Hurditch Rodney
Hampton-Hightower P.
MicroChem Corp.
Simons William A.
Wiggin & Dana
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