Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making named article
Reexamination Certificate
2000-06-26
2002-07-23
McPherson, John A. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making named article
C430S321000, C430S328000, C430S330000
Reexamination Certificate
active
06423477
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of manufacturing a stamper for producing optical discs, comprising the application of a photoresist to a stamper plate and the structuring of the applied photoresist film, wherein said structuring comprises the successive exposure, development and heating of said photoresist film, wherein the developed photoresist film is subjected to an additional exposure step in the deep UV range prior to the final heat treatment. The present invention furthermore relates to a stamper for producing optical discs and to optical discs obtained by means of such a stamper.
BACKGROUND OF THE INVENTION
A method of this kind is known from Dutch patent application no. 9400225 filed in the name of the present applicant. According to the method of manufacturing a stamper described therein, a photoresist film is applied to a stamper plate, after which the applied photoresist film is structured. The structuring of the applied photoresist film comprises the successive selective exposure and development of the applied photoresist film, wherein the structuring of the applied photoresist film furthermore comprises, in particular as an additional step, the heating of the selectively exposed photoresist film. In addition to that, the structuring of the applied photoresist film furthermore comprises the integral exposure of the photoresist film prior to developing the photoresist film. The successive steps of selective exposure, heating and integral exposure prior to the developing step are also known as “image reversal process” to those skilled in this field of the art. According to the example carried out in said Dutch patent application, a negative photoresist was coated, by means of spin-on deposition, onto a blank 300 &mgr;m thick nickel shell, which was glued on a standard 8-inch CD glass substrate for this experiment. After dry-spinning of the nickel shell/glass substrate assembly at a constant temperature of 80° C., the applied negative photoresist film was dried. After cooling, the assembly was exposed by means of a master registration system (MRS) at a wavelength of 459 nm. Directly after said exposure, the assembly was heated and subsequently cooled, after which integral exposure took place for 4 minutes. Thus, the non-exposed parts of the negative photoresist film applied to the nickel shell were exposed as yet. The assembly was subsequently developed and heated again, in the same manner as after the exposure, for the purpose of fixating the structure of the photoresist posts, after which cooling took place, with this difference that the oven temperature set for heating was now 140° C.
In practice, however, this latter heat treatment, also called hardbake, has been carried out at a temperature of 200° C. the last few years. A drawback of this relatively high temperature of the final heat treatment is the fact that the so-called photoresist posts formed after the developing step lose their structure or geometry in an uncontrolled manner, as a result of which they will flow out more or less. Since the geometry of the photoresist posts will change as a result of the necessary heat treatment at a high temperature, the obtained stamper often does not meet the precise specifications that are required for producing optical discs, which is undesirable.
SUMMARY OF THE INVENTION
Consequently, it is an object of the present invention to provide a method of manufacturing a stamper as referred to in the introduction, which method eliminates the aforesaid problems.
Another object of the present invention is to provide a method of manufacturing a stamper as referred to in the introduction, which method allows or effects a certain amount of flowing of the photoresist posts, making it possible to control the desired post geometry which is required for mass replication.
DETAILED DESCRIPTION
Surprisingly, the above objectives have been accomplished by the present inventors by providing a method as referred to in the introduction, which is characterized in that the effective dose of said additional exposure step ranges between 4·10
−4
and 5·10
−2
J/cm
2
with a wavelength of 200-320 nm.
According to the present invention, the effective dose or energy density of the additional exposure step thus ranges between 4·10
−4
and 5·10
−2
J/cm
2
, preferably between 8·10
−4
and 1.2·10
−2
J/cm
2
. If the effective dose of the additional exposure step according to the present invention is less than 4·10
−4
J/cm
2
, no suffficient crosslinking effect will occur in the photoresist film. If the effective dose of the additional exposure step according to the present invention is more than 5·10
−2
J/cm
2
, no significant crosslinking effect will occur in the photoresist film. According to the present invention, the term effective dose is understood to mean the product of the dose and the sensitivity of the exposed material at the wavelength used. For novolac resin, for example, the maximum sensitivity is at a value of &lgr;=250 nm, as a result of which the energy dose being used at a wavelength of 250 nm substantially corresponds with the effective dose. It is apparent that a wavelength other than 250 nm will require a higher energy dose in order to obtain the same effective dose and thus the same effect as when a wavelength of 250 nm is used.
Although it is known from the article “Deep UV hardening of positive photoresist patterns”, Journal of the Electrochemical Society, part 29, No. 6, June 1982, pages 1379-1381 in the name of Allen R. et al. to reduce the thermal deformation, in particular the flowing of photoresist patterns, during treatments at a high temperature of 180° C. for 30 minutes by exposing the photoresist in the deep UV range by means of a low-pressure mercury vapour lamp, said article discloses an exposure of for example 8 mW/cm
2
for 20 minutes, which corresponds with an energy dose of 9,6 J/cm
2
. Such a dose in accordance with the article of Allen R. et al. is substantially higher than the effective dose in the range of 4·10
−4
-5·10
−2
J/cm
2
and a wavelength of 200-320 nm which the present invention employs, which dose makes it possible to control said flowing, thus making it possible to control the geometry of the photoresist posts being formed.
The use of an additional exposure step before the hardbake step is also known from the articles “UV-hardening of Resist Patterns”, IBM Technical Disclosure Bulletin, part 24, No. 3, August 1981, and from the article “Photoresist Stabilization System”, Solid State Technology, part 27, No. 7, June 1984, Washington, USA, but neither article discloses any exposure energy values. Said use corresponds with the aforesaid article by Allen, R et al., however, in particular IC lithography, so that it is likely that the energy values being employed will also be the same as in said article, that is, considerably higher than the values employed according to the present invention.
Such an additional exposure step according to the present invention with a very effective dose will probably result in oxidation reactions in the photoresist, in particular in the outer layers of the photoresist, which oxidation reactions lead to curing and a higher glass transition temperature. As a result of that, the structure of the photoresist posts formed during the developing step will not be adversely affected by the subsequent heat treatment at a high temperature, and it will probably be controllable. However, the invention is not limited by this assumption regarding the oxidation reaction that will probably occur.
The additional exposure step according to the invention, which is to be carried out prior to the final heat treatment, is preferably carried out in the range in which the photochemical curing of the photoresist takes place, that is, in the range in which absorption of the electromagnetic radiation takes place, in particular in the range of 200-320 nm, more in particular in the range of 240-260 nm.
The selection of the wavelength range to be used
Engelen Thomas Wilhelmus
Stals Walter T. M.
Vermeijlen Jacobus J. T. T.
McPherson John A.
Odme International B.V.
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