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
2001-09-13
2004-11-30
Thornton, Yvette C. (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
C430S271100, C430S320000, C430S156000
Reexamination Certificate
active
06824952
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to selected resist compositions useful as the lower release layer or lift-off resist in a bilayer metal lift-off photoresist process or as base-soluble anti-reflective coating for conventional lithographic applications. In particular, this invention relates to specific coating compositions useful for the bottom layer for these uses, that include at least one solvent, at least one polydimethylglutarimide (PMGI) resin and at least one selected deep-ultraviolet (DUV) absorbing additive. More specifically, this resist composition comprising a mixture of one or more selected light-absorbing additives that when mixed with PMGI reduces the amplitude of standing waves caused by reflected light and prevents intermixing in a bilayer photoresist system, while maintaining a controllable dissolution rate in aqueous alkali developer in order to obtain a predetermined degree of undercut.
2. Brief Description of Art
The Lift-off process is the name used to describe the process in which photolithography is used to form a relief pattern on a substrate in a light sensitive thin film such as a photoresist, followed by blanket deposition of a metal layer such that the metal is deposited onto the substrate in regions that are uncovered by photoresist, followed by removal of the photoresist and excess metal by wet methods. The most widely used lift-off processes involve a bilayer lithographic process (sometimes referred to as a bilevel process). Such bilayer lift-off processes have been used to fabricate the metallic “read stripe” in the manufacture of thin film heads for magnetic hard drives and in the fabrication of the gate oxide for gallium arsenide 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 Inc.) and U.S. Pat. No. 4,814,258 (assigned to Motorola, Inc.).
In bilayer lift-off processes, a solution of a non-photoimagable lift-off resist (LOR) is first deposited by spin-coating to form a uniform thin film on top of the substrate to be metallized. Lift-off processes have been described in detail by W. Moreau, in
Semiconductor Lithography, Principles, Practices, and Materials
(Plenum Press, New York, N.Y., 1988), Chapter 12. After application by spin-coating, 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 photo imaging photoresist layer is then deposited by spin-coating on top of the LOR followed by a soft-bake to evaporate most of the solvent, leaving a solid uniform coating. Especially for lift-off applications in which high resolution features must be delineated, it is essential that no intermixing occurs between the LOR and the top photoresist. Therefore, the LOR layer should have low solubility in the solvent system used to formulate the top photoresist. A pattern is then transferred from a mask to the top resist film by patterned radiation using a conventional photolithographic imaging process such as contact, proximity, or projection printing. For a positive resist, which is made soluble by exposure to visible or ultraviolet radiation, the exposed regions are dissolved away by the developer, yielding a positive image of the photomask. This in turn exposes the LOR layer to developer in these regions, 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 remove too much unexposed photoresist, or too short to make the process irreproducible. In any practical bilayer 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 an aqueous alkaline developer which is compatible with, and provides a wide process latitude for the positive photoresist imaging layer.
There is a need for high resolution lithographic patterning as the critical dimension in device manufacture becomes smaller. Although traditionally the g-line (436 nm), h-line (404 nm), and i-line (365 nm) of the mercury lamp output have been used in bilayer lift-off lithography, there is a motivation to utilize D-UV radiation, typically below 300 nm, which allows higher resolution patterning since the diffraction-limited resolution increases approximately linearly with decrease in wavelength. It is especially useful to provide absorption at the wavelength 248 nm of a krypton fluoride (KrF) laser, which is widely used in DUV lithography.
It is widely known that in thin films propagating waves and reflected waves can produce standing waves through interference effects. The amplitude of these standing waves depends on the reflectivity, the wavelength of the radiation and the thickness of the films. Standing waves result in varying light intensity in the resist film, which depends critically on the thickness of the resist layer, and the underlying substrate topography, all of which vary in practice, thus reducing the process latitude. The amplitude of standing waves is high when the reflectivity of the bottom substrate is high, as is the case for most metals, silicon, and GaAs. Furthermore the reflectivity tends to increase with decrease in wavelength. It is therefore desirable to increase the absorption of the LOR, especially if the LOR is used in DUV lithography, such that a thin film will reduce the intensity of the reflected light and hence suppress the standing waves in a manner similar to that used in conventional lithography by utilizing a highly absorbing anti-reflective coating.
Partially or fully imidized acrylic polymers referred to as polydimethylglutarimides or PMGI resins which are suitable for use as the bottom resist layer in a lift-off process have been described in U.S. Pat. No. 4,524,121 (assigned to Rohm and Haas Co.). PMGI resins are manufactured commercially by the process described in U.S. Pat. No. 4,246,374 (assigned to Rohm and Haas). In this process, poly(methyl methacrylate) (PMMA) having a weight-average molecular weight (M
w
) of about 70,000 to 110,000 is partially imidized with ammonia gas in an extruder at high pressure and relatively high temperature such that the resulting polymer contains about 65-80% of imidized methacrylate moieties, the nitrogen atoms of which bear either a methyl group (N—Me) or hydrogen (N—H). The percentage of (N—H) groups determines the alkaline solubility. And is typically about 65-80%. PMGI resins produced by this process have negligible optical absorbance at actinic wavelengths and a fairly narrow range of alkaline solubility. This limitation creates the need for other methods of modifying the absorbance and concurrently controlling the dissolution rate of PMGI for critical high resolution applications in lift off processes such as those using DUV lithography.
Radiation absorbers which are suitable for use in anti-reflective coatings or as additives to photoresists may not be suitable for use in bilayer lift-off applications because of the different requirements. In particular, it is especially difficult to select an absorber which can be added to an LOR at a concentration sufficiently high to provide high absorbance, yet capable of meeting the processing requirements described above, namely (1) insoluble in resist solvent, (2) soluble in a resist developer, and (3) non-chemically reactive towards the photoresist. The absorber must also be non-subliming or non-volatizing at high temperatures, miscible with PMGI polymer, and must not dissolve in or leach into positive photoresist, which is typically used as the imaging layer.
Several approaches have been taken to solve these problems, including the following:
The addition of an non-actinic wavelength absorbing dye to
Minsek David W.
Nawrocki Daniel J.
Geschke Elizabeth
MicroChem Corp.
Simons William A.
Thornton Yvette C.
Wiggin and Dana LLP
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