Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Utility Patent
1998-12-01
2001-01-02
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S296000
Utility Patent
active
06168890
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to microlithography reticles and masks, substrates for manufacture of the reticles and masks, and methods for manufacture of the same.
BACKGROUND OF THE INVENTION
Recent developments in semiconductor integrated-circuit technology have been remarkable with the miniaturization of the constituent semiconductor elements and trends toward increased integrated-circuit density. Up to the present, so-called optical lithography steppers conventionally have been used for performing lithographic exposure (projection-transfer) of integrated-circuit patterns onto semiconductor wafers. Unfortunately, current optical lithography techniques are (or soon will be) unable to provide the image resolution necessary to satisfy anticipated demands for ever decreasing miniaturization of semiconductor elements and increases in integrated-circuit density. Consequently, effort has been expended to develop microlithographic equipment employing a charged particle beam (e.g., electron beam or ion beam) or an X-ray beam rather than a light (ultraviolet) beam. That is, a charged particle beam or X-ray beam is used to project a pattern, defined by a mask or reticle (both terms are used interchangeably herein), with demagnification, onto a sensitive substrate, such as a semiconductor wafer coated with a suitable resist.
Known types of masks used in conventional charged-particle-beam (CPB) projection-transfer systems are shown in FIGS.
3
and
4
(
a
)-
4
(
c
).
The mask
21
of
FIG. 3
is a typical “absorbing stencil mask.” The absorbing stencil mask
21
comprises a silicon mask substrate
22
that defines through-holes
23
. The through-holes
23
, together with the remaining mask substrate
22
, define a pattern. The mask substrate
22
has a thickness (e.g., 50 &mgr;m) sufficient to absorb charged particles from a charged particle beam EB used to irradiate the mask
21
.
Charged particles in the charged particle beam EB irradiated onto the mask
21
pass only through the through-holes
23
. Charged particles that pass through the through-holes are converged on a resist-coated surface of a suitable substrate
25
(e.g., a silicon wafer coated with a resist that is reactive to the charged particles). Thus, a pattern corresponding to the mask pattern is transferred onto the substrate
25
.
The absorbing stencil mask of
FIG. 3
has, at least, the following shortcomings: (1) a single absorbing stencil mask cannot form “island” or “peninsula” features; and (2) an absorbing stencil mask generates large amounts of heat causing heat deformation of the mask with consequent pattern distortion.
More specifically, because the mask substrate of an absorbing stencil mask is typically relatively thick, charged particles that are absorbed by the substrate (i.e., by portions of the substrate
22
where through-holes are absent) generate large amounts of heat. The heat causes significant deformation of the mask with consequent pattern distortion. In an absorbing stencil mask, if the absorbing portion of the substrate is made of a relatively thin membrane, the problems of heat deformation can be reduced significantly. However, a relatively thin membrane effectively cannot absorb the charged particles, resulting in transferred pattern images that have inadequate resolution.
The mask
100
of FIG.
4
(
b
) is a “scattering transmission mask” in which the mask pattern is defined by a corresponding pattern of a scattering body
30
a
formed on the surface of a silicon mask substrate
20
. The mask substrate
20
is configured as a membrane that is sufficiently thin to allow transmission of charged particles. When a charged particle beam irradiates the scattering transmission mask
100
, charged particles passing through the scattering body
30
a
and the mask substrate
20
are forward-scattered and thus exhibit divergence from the mask (forming a “beam” EB
2
indicated by a dashed line in FIG.
4
(
b
)). Charged particles passing through the membrane
20
, but not passing through the scattering body
30
a
, are not scattered significantly (forming a “beam” EB
1
indicated by a solid line in FIG.
4
(
b
)).
Referring to FIG.
4
(
b
), the beam EB
1
passes through a projection lens
35
that converges the beam EB
1
at a crossover CO. An aperture plate
37
(defining an aperture
37
a
) is positioned typically at the crossover CO. The aperture plate
37
serves to block divergently scattered charged particles (such as many of the charged particles in the beam EB
2
). Blocking the divergently scattered charged particles allows formation of a high-resolution pattern image on a resist-coated surface of a suitable substrate
110
(e.g., a silicon wafer coated with a resist that is reactive to the charged particles).
The scattering transmission mask
100
of FIG.
4
(
b
) generally is not subject to the problems associated with absorbing stencil masks. Specifically, the scattering body
30
a
is supported by the mask substrate (membrane)
20
of the scattering transmission mask
100
. Therefore, such a mask can define peninsula or island features, such as the feature A shown in FIG.
4
(
a
). Also, in the scattering transmission mask
100
, it is unnecessary that the scattering body
30
a
completely shield the charged particle beam; this reduces the number of charged particles of the irradiating beam that are blocked by the mask. As a result, less heat is generated by charged particle irradiation of a scattering transmission mask than an absorbing stencil mask, and the scattering transmission mask does not experience the same extent of heat deformation as an absorbing stencil mask.
With a scattering transmission mask, the resolution of a projected pattern image may be improved by increasing the absorption and scattering of charged particles by the scattering body
30
a
. Alternatively, the resolution of the projected pattern image may be improved by reducing the extent of absorption and scattering of the charged particles by the mask substrate (membrane)
20
. Accordingly, the thickness of the membrane of a conventional scattering transmission mask is typically about 10 &mgr;m. However, the temperature of the membrane still increases due to absorption of charged particles by the scattering body
30
a
. The temperature increase is sufficient to result in mask deformation and, consequently, pattern distortion.
For example, referring to FIG.
4
(
a
), it is difficult to avoid heat being generated in feature A of the membrane
20
of the mask
100
due to the transfer of heat from the scattering body
30
a
. The portion of the scattering body
30
a
forming the feature A generates significant amounts of heat. Additionally, if the membrane
20
is too thin, then the membrane
20
will lack the strength to adequately support the scattering body
30
a.
Accordingly, the membrane
20
typically is made relatively thin, but in a manner such that the membrane has adequate strength to support the scattering body
30
a
. To such end, a scattering transmission mask (or a scattering stencil mask) has been developed to include a membrane defining a pattern, wherein the pattern is divided into a plurality of small regions termed “mask subfields.” The divided pattern includes non-pattern regions or “boundary regions” between the small regions. The mask further includes a support structure to support the membrane. The support structure may comprise a support grid of a comparatively rigid network of intersecting struts. The struts of the support grid are in registration with the boundary regions of the membrane.
For example, the scattering transmission mask
100
shown in FIGS.
4
(
a
)-
4
(
c
) includes CPB-scattering bodies
30
a
formed on each of a plurality of mask subfields
100
a
on an upstream-facing surface of the membrane
20
. Lattice-shaped struts X are in registration with boundary regions
100
b
on a downstream-facing surface of the membrane
20
. Each mask subfield
100
a
defines a respective portion of a pattern to be transferred to the sensitive substrate
110
. Typically, the
Klarquist Sparkman Cambell Leigh & Whinston, LLP
Nikon Corporation
Rosasco S.
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