Optics: measuring and testing – Material strain analysis – By light interference detector
Reexamination Certificate
2000-05-11
2003-05-13
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
Material strain analysis
By light interference detector
Reexamination Certificate
active
06563570
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to microlithography (projection-exposure) of a pattern from a reticle to a sensitive substrate using a charged particle beam such as an electron beam. Microlithography is a key technology used in the manufacture of semiconductor integrated circuits, displays, and the like. More specifically, the invention pertains to reticles for charged-particle-beam microlithography, and even more specifically to apparatus and methods for evaluating a self-supporting reticle membrane of a reticle blank, from which blank a reticle is made.
BACKGROUND OF THE INVENTION
The dramatic progressive reduction in the sizes of circuit elements in integrated circuits that has occurred in recent years has created a need for image resolution better than that obtainable using optical microlithography systems that are limited by the diffraction of light. This has led to the ongoing development of microlithography (projection-exposure) systems that, instead of using ultraviolet light, employ an X-ray beam or a charged particle beam such as electron beam or an ion beam.
Current charged-particle-beam (CPB) systems include electron-beam pattern-drawing (“direct-write”) systems in which an electron beam is used to form a pattern directly (i.e., without having to project a pattern onto the wafer). Because of the current ability to stop an electron beam down to a spot diameter of a few nanometers, high-resolution sub-micron patterns can be formed in this way. A major drawback of direct-write systems is the fact that the pattern is drawn element-by-element and line-by-line (i.e., by “direct writing”). To draw a finer element, the electron beam simply is stopped down further to a smaller spot diameter. However, reducing the spot diameter increases the amount of time (“writing time”) that must be expended to draw the entire pattern. Increasing the writing time correspondingly reduces throughput and increases device-production costs. Consequently, direct-write systems are impractical for mass production of chip-containing wafers.
The shortcomings of direct-write systems have motivated a large amount of development effort currently being directed to the development of a practical CPB microlithography system that projects (with demagnification) a pattern image from a “reticle” or “mask” to the wafer. Such systems are termed “reduced-image projection-exposure” CPB microlithography systems, in which a reticle defining the prescribed pattern is illuminated by a charged particle beam. (e.g., electron beam), and a reduced (demagnified) image of the pattern located within the range of illumination is transferred onto the wafer by a projection lens.
By “demagnification” is meant that the image as formed on the wafer is smaller (usually by a factor such as ¼ or ⅕) than the corresponding illuminated region on the reticle.
As noted above, the pattern is defined on a “reticle” (sometimes termed a “mask,” but generally herein the term “reticle” is used). Two general types of reticles are known. A first type is termed a “scattering-membrane” reticle
21
, a portion of which is shown schematically in FIG.
11
(
a
). The scattering-membrane reticle
21
comprises a reticle membrane
22
on which regions
24
are formed. The regions
24
are of a substance that scatters particles of a charged particle beam incident from above. The reticle membrane
22
is sufficiently thin to be transmissive to particles of the incident beam and thus exhibit essentially no scattering. The regions
24
, in combination with the transmissive membrane
22
, define the elements of the pattern. A second type of reticle is termed a “scattering-stencil” reticle
31
, a portion of which is shown schematically in FIG.
11
(
b
). The scattering-stencil reticle
31
comprises a reticle membrane
32
(typically made of silicon) having a thickness (approximately 2 &mgr;m) sufficient to scatter particles of the incident beam. The membrane
32
defines through-holes
34
that are transmissive to particles of the incident beam. The through-holes
34
, in combination with the membrane
34
, define the elements of the pattern.
In CPB microlithography, it currently is impossible to project an entire pattern in one “shot.” As a result, the pattern as defined on the reticle is divided or “segmented” into multiple small portions termed “subfields”
22
a
,
32
a
each defining a respective portion of the overall pattern and each containing a respective portion of the reticle membrane
22
,
32
. The subfields
22
a
,
32
a
are separated from one another on the reticle by boundary zones
25
,
35
, respectively, that do not define any portion of the pattern. Extending outwardly from the boundary zones
25
,
35
are support struts
23
,
33
, respectively, that add substantial rigidity and strength to the reticle
21
,
31
, respectively.
Each subfield
22
a
,
32
a
represents an area of the reticle
21
,
31
, respectively, that can be exposed at any one instant, and each subfield is typically approximately 1-mm square in size. Hence, on the reticle, the entire pattern to be transferred to a chip-sized area (a “die” corresponding to a semiconductor chip) on the wafer) is divided into a large number of, typically, 1-mm square subfields. The subfields
22
a
,
32
a
are exposed individually. As the subfields are thus “transferred” to the wafer, the respective images of the subfields are “stitched” together contiguously to form the entire pattern in each die.
Reticles for CPB microlithography can be produced from a “reticle blank” which typically comprises a reticle membrane and supporting struts. One reference describing the manufacture of a reticle blank useful for making a scattering stencil reticle is Japanese Kôkai (laid-open) Patent Document No. Hei 10-106943.
More specifically, a reticle blank comprises a reticle membrane, an “outer frame” that fixes and supports the periphery of the reticle membrane, and a lattice of support struts that support the reticle membrane and divide the membrane into multiple subfields or other suitable “exposure units” (portions of the reticle that are exposed at any one instant in the CPB microlithography system).
In a reticle blank (and in a reticle made from the reticle blank), the reticle membrane is subject to certain tensile and compressive stresses. Unless stress on the reticle membrane of a reticle blank is maintained within an acceptable range, excessive strain may be manifest in the pattern subsequently defined on or in the membrane.
Hence, it is desirable to evaluate the internal stress and Young's modulus of the reticle membrane in a reticle blank. It is important, therefore, to have good techniques for performing these evaluations.
A “bulge” technique is one way in which to simultaneously measure the Young's modulus and internal stress of a membrane. Allen et al.,
Appl. Phys. Lett.
51:241-243, 1987. In a bulge technique, a pressure differential P is applied to the obverse and reverse surfaces of a membrane of a measurement sample to which a load has been applied, and any deflection (distention or bending) h of the membrane caused by the load is measured. The relationship between the applied pressure differential P and the deflection h is expressed by Equation (1) below:
P[r
2
/(
th
)]=
K
1
&sgr;+K
2
[E/
(1−
v
)](
h/r
)
2
(1)
wherein &sgr; is the membrane stress, E is the Young's modulus, v is the Poisson ratio, r is the radius (in the case of a circle) or half a length of a side (in the case of a square), t is the film thickness, and K
1
and K
2
are constants determined by the shape of the membrane.
An apparatus is shown in
FIG. 10
for evaluating a sample
12
according to the principle summarized above. Tabata et al.,
Sensors and Actuators
20:135-141, 1989. The sample
12
comprises a thin-film self-supporting membrane. The
FIG. 10
apparatus comprises an optical system including a He—Ne laser
14
and a beamsplitter
15
. The membrane of the sample
12
to be evaluated extends across an
Klarquist & Sparkman, LLP
Nikon Corporation
Turner Samuel A.
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