Radiation imagery chemistry: process – composition – or product th – Diazo reproduction – process – composition – or product – Composition or product which contains radiation sensitive...
Reexamination Certificate
2000-03-31
2001-10-16
Chu, John S. (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Diazo reproduction, process, composition, or product
Composition or product which contains radiation sensitive...
C430S329000, C430S519000
Reexamination Certificate
active
06303260
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to selected compositions useful as a lift-off photoresist in a bilayer metal lift-off process. In particular, this invention relates to specific compositions useful for that purpose that include at least one solvent, at least one polyglutarimide resin and at least one selected actinic-absorbing dye useful as dissolution rate modifier.
2. Brief Description of Art
The additive process of depositing patterned metal films onto microelectronic substrates is known as the lift-off process or metal lift-off process. There are several variations of this lift-off process. The most widely used lift-off processes involve a bilayer lithographic process (sometimes also referred to as a “bilevel” process). Such bilayer lift-off processes have been used to deposit the metallic “read-stripe” in the manufacture of thin film heads for magnetic hard drives and in the fabrication of the gate oxide for GaAs Field Effect Transistor (FET) devices. Variants of these bilayer lift-off processes are described in detail in European Patent Application No. 0341843 (assigned to International Business Machines Corp.) and U.S. Pat. No. 4,814,258 (assigned to Motorola Inc.).
In bilayer lift-off processes, a solution of a non-imaging lift-off resist (LOR) is first deposited by spin-coating to form a uniform thin film on top of a substrate to be metallized. The LOR layer is then soft-baked by heating at a sufficiently high temperature to remove most of the solvent contained in it. A conventional positive-imaging resist layer is then deposited on top of the LOR. The top resist and the lower LOR layer should not be intermixed. Therefore, the LOR should have a low solubility in conventional positive resist solvents.
After a second soft-bake to remove most of the residual solvent in the top resist layer, a pattern is transferred from a mask to the top resist film using a conventional microlithographic imaging tool such as a contact-proximity printer or stepper. The exposed areas in the top resist layer represent the areas to be metallized. The exposed resist is developed with an aqueous developer through to the LOR layer, which then dissolves both vertically through to the substrate and laterally to penetrate a small predefined distance into the adjacent unexposed areas of the photoresist layer. This lateral dissolution produces a controlled degree of undercut in a development time which is neither too long to make the process impractical or to remove too much unexposed photoresist, or too short to make the process irreproducible. In one variation of the process, often referred to as the PCM (portable conformable mask) variation, the underlying LOR 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 LOR layer more soluble in a selected developer in the exposed areas that are to be removed during the development process. It is preferred to avoid a DUV intermediate exposure step and rely instead on the LOR having the desired rate of dissolution in the positive imaging resist developer. Moreover, this PCM process cannot be used with a positive top resist of the chemically amplified type designed to be photosensitive to DUV wavelengths.
After the desired degree of undercut is developed in the LOR 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 LOR, 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.
Partially or fully imidized acrylic polymers referred to as polyglutarimides, especially polydimethylglutarimide (PMGI), have been described in U.S. Pat. No. 4,524,121 (assigned to Rohm and Haas Co.). Polyglutarimide refers to a class of polymers containing partially cyclized imide and N-alkyl imide moeties and uncyclized polymethacrylate, 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. In the case where the alkyl group is methyl, the polymer is more correctly referred to as polydimethylglutarimide, or PMGI. If PMGI is made from polymethacrylic acid or a PMMA/methacrylic acid copolymer, (uncyclized) poly(methacrylicacid) units may also be present. PMGI polymers for lift-off applications are generally found to comprise about 65%-80% or more of cyclized imide moieties of which about 50-60% are N-H and the remainder N-methyl substituted. These compounds have several desirable properties, especially good solubility in aqueous bases typically used for the development of conventional positive resists, and poor solubility in positive resist solvents such as ethyl lactate, 2-heptanone and propylene glycol methyl ether acetate, which make them suitable for use in lift-off resists for bilayer lift-off process applications. Additionally, their solubility may be increased by exposure to high energy radiation such as deep ultra-violet (DUV) or electron beam.
The basic reaction to form poly(N-alkylimides) from the reaction of poly(methylmethacrylate)(PMMA) or poly(methacrylic acid) with an amine is disclosed in Graves U.S. Pat. No. 2,146,209, (assigned to E. I. du Pont de Nemours & Co.), see German Patent No. 1,077,872 and
Makromol. Chem
. 96, 227 (1966).
European Patent Application No. A0275918 (assigned to Verdril S.p.A.) discloses a solution process for making imidized acrylic polymers by reaction of acrylic resin with an amide
U.S. Pat. No. 4,689,243 (assigned to Mitsubishi Rayon Co.) discloses a process for forming polyglutarimide polymers by reaction of a solution of PMMA with ammonia or an amine, followed by separation of the polymer from non-polymeric reaction products and solvents under vacuum in a vent extruder. As described in U.S. Pat. No. 3,284,425, the same reaction is carried out in a suspending solvent in an autoclave.
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 is compatible with, and provides a wide process latitude for the imaging positive photoresist layer.
Commercially available PMGI has been manufactured by the process described in U.S. Pat. No. 4,246,374 (assigned to Rohm and Haas). In this process, poly(methyl methacrylate) (PMMA) is imidized with ammonia gas in an extruder at high pressure and relatively high temperature. This reaction is practical only if the weight-average molecular weight (M
w
) of the starting PMMA is sufficiently high (i.e. greater than 60,000 and typically 60,000 to 120,000). The resulting polymer should also contain about 20-35% of unreacted methacrylate moieties and about 30-60% of the nitrogen atoms on the imide groups should be methylated. The percentage of the remaining imide groups (N-H) determines the alkaline solubility. PMGI resins produced by this process have a fairly narrow range of alkaline solubility. 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 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, and hence the rate of undercut, can be increased to some extent. However, the amount of increase in the dissolution rate may be insufficient for certain lift-off processes requiring a relatively lar
Hurditch Rodney J.
Johnson Donald W.
Joshi Neela
Chu John S.
MicroChem Corp.
Simons William A.
Wiggin & Dana
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