Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
1999-02-24
2001-05-08
Huff, Mark F. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S330000
Reexamination Certificate
active
06228540
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a photomask forming method and heat treatment equipment for forming a photomask where a resist pattern is formed on a photomask substrate.
When forming a resist pattern on a photomask substrate, an electron beam (referred to as EB hereinafter) resist has conventionally been used widely. This EB resist is not the so-called chemical amplification type resist but a polymeric material for effecting development by molecular weight difference through EB application. The thermophysical property of a positive type resist for removing the EB-applied portion by the development selectivity (molecular weight difference) will be described below.
When PMMA (polymethyl methacrylate) which is a polymeric material of a hydrocarbon system is used as the EB resist, in a process of a baking (referred to as pre-baking hereinafter) of heat treatment after the application of the resist, there is observed a close relation between a prebaking temperature and a resist sensitivity as shown in FIG.
4
. In
FIG. 4
, the horizontal axis represents the pre-baking temperature in an arbitrary scale, while the vertical axis represents the resist sensitivity in an arbitrary scale. In
FIG. 4
, the resist sensitivity reduces as a position in relation to the vertical axis ascends. In a temperature range “a” exceeding a glass transition point Tg of the EB resist, a portion (e.g., a double-bond portion on the side chain) having a weak bonding strength of the constituent elements in the resist is once broken in the pre-baking stage. The broken portion is recombined in its easily stabilized state (a slackly re-cross-linked state) into a polymer in a cooling stage after the pre-baking. This increases a cross-linking ratio in accordance with the increase in the pre-baking temperature, so that the EB resist is gradually reduced in sensitivity. In a temperature range “b” higher than the temperature range “a” shown in
FIG. 4
, the cross-linking becomes saturated, so that the sensitivity of the EB resist enters a stable region. Furthermore, in a temperature range “c” higher than the temperature range “b” shown in
FIG. 4
, the EB resist material itself is decomposed by the heat treatment in the pre-baking stage to have a reduced molecular weight, and this makes the resist have a high sensitivity.
As shown in
FIG. 5
, there is observed a close relation between a cooling temperature and the resist sensitivity in a cooling process after the pre-baking process. In
FIG. 5
, the horizontal axis represents the cooling temperature, while the vertical axis represents the resist sensitivity in an arbitrary scale. As explained in connection with the temperature range “a” of
FIG. 4
, the cooling temperature after the pre-baking process dominates the polymeric bond state and the rate of progress of the cross-linking and operates as a very great factor for finally determining the resist sensitivity. This is because the rate of molecules that can be re-cross-linked in the cooling stage after the pre-baking process depends on a cooling rate, in particular, a rate of transition over the glass transition point. That is, the rate of progress of the cross-linking is determined by the cooling rate (cooling temperature). The cross-linking sufficiently progresses at the time of slow cooling, that is, at the time of the high cooling temperature, and therefore the EB resist comes to have a low sensitivity.
As described above, the sensitivity of the EB resist that is currently applied to the mass-production of photomasks depends on the pre-baking temperature in the pre-baking process and the cooling temperature in the subsequent cooling process. To accurately control the cooling temperature, which has no specific condition of stabilization in terms of thermophysics, is indispensable for obtaining a photomask of high dimensional accuracy.
For the method of forming a photomask using the above-mentioned EB resist, a convection type oven
20
as shown in
FIG. 6 and a
hot-plate oven
40
as shown in
FIG. 7
, each of which serves as heat treatment equipment, are widely used.
As shown in
FIG. 6
, the convection type oven
20
includes an inner vessel
24
arranged inside an outer vessel
25
having a door
26
and a heater
28
arranged on the lower side of the inner vessel
24
and operates to heat air by the heater
28
under the inner vessel
24
and circulate the heated air through a space between the inner vessel
24
and the outer vessel
25
by means of a fan
32
and a fan
22
, thereby heating a photomask blank
21
arranged roughly at the center of the inner vessel
24
, thereby a photomask blank
21
arranged generally at the center of the inner vessel
24
is heated. The convection type oven
20
is provided with filters
33
and
23
on the downwind side of the fans
32
and
22
, respectively. Although the convection type oven
20
has the advantage that it can perform batch processing of a plurality of photomask blanks
21
, it is structurally difficult to make uniform the intra-planar temperature distribution of the photomask blanks
21
due to the heat treatment by the convection inside the oven. The cooling process after a preheating process is generally performed by natural cooling inside a clean bench or the like. Therefore, it is actually impossible to achieve adjustment to the desired resist sensitivity, and variations in temperature of the photomask blanks
21
between photomasks and inside the photomask plane are also remarkable in the cooling stage.
FIG. 7
shows a perspective view of the hot-plate oven
40
of a horizontal plate placing system that solves the aforementioned problems. The hot-plate oven
40
has a plurality of base plates
41
arranged linearly and a conveyance arm
42
provided in the vicinity of each of the base plates
41
as shown in FIG.
7
. By moving the photomask blank between the base plates
41
by means of the conveyance arm
42
, the pre-baking process and the cooling process are sequentially performed.
FIG. 8
shows an enlarged sectional view of the essential part of the hot-plate oven
40
for performing the preheating process and the cooling process. Teflon (a trade name, PTFE) spacers
52
and
52
are arranged at a specified interval on a base plate
51
. A photomask blank
53
is arranged horizontally so that both the end portions thereof are placed on the Teflon spacers
52
and
52
. Thereby, a specified interval between the base plate
51
and the photomask blank
53
is placed by means of the Teflon spacers
52
and
52
. Then, the base plate
51
under the photomask blank
53
is heated by a heater (not shown), thereby proximity-heating the photomask blank
53
. According to the photomask forming method by means of this hot-plate oven
40
, heat can be uniformly applied to the inside of the plane of the photomask blank
53
. By setting the temperature of the base plate
51
to or around the normal temperature by water cooling or similar means, the method can also be applied to the cooling of the photomask blank
53
. Therefore, taking advantage of the physical property of the EB resist shown in
FIG. 5
, the resist sensitivity can also be adjusted to the desired value by setting the temperature of the base plate
51
that serves as a cooling plate.
FIG. 10
shows an intra-planar dimensional distribution of a resist pattern formed by EB lithography using a photomask blank
60
of a plate thickness of 0.09 inch, which has undergone the pre-baking process and the cooling process in this hot-plate oven
40
. In
FIG. 10
, the size of the hatched square mark
61
represents the positive shift amount relative to an intra-planar dimensional mean value, while the size of the white square mark
62
represents the negative shift amount relative to the intra-planar dimensional mean value. By uniform heat treatment for the photomask blank
60
in the pre-baking stage and the cooling stage, a satisfactory resist pattern intra-planar dimensional distribution can be obtained.
However, as shown in
FIG. 11
, if the resist pattern intra-p
Huff Mark F.
Mohamedulla Saleha R.
Nixon & Vanderhye P.C
Sharpe Kabushiki Kaisha
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