Measuring and testing – Vibration – Resonance – frequency – or amplitude study
Reexamination Certificate
2000-09-07
2002-10-01
Williams, Hezron (Department: 2856)
Measuring and testing
Vibration
Resonance, frequency, or amplitude study
C073S602000, C073S800000, C356S035500
Reexamination Certificate
active
06457359
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to apparatus and methods for measuring stress in thin films (membranes) in specimens, particularly specimens such as reticles and reticle blanks as used in charged-particle-beam microlithography apparatus and methods.
BACKGROUND OF THE INVENTION
In recent years, as individual circuit elements in semiconductor integrated circuits have become increasingly miniaturized, microlithographic exposure systems and methods offering prospects of finer resolution than obtainable by optical microlithography (which is encumbered by the diffraction limit of light) have received great R&D attention. For example, microlithographic technologies utilizing a charged particle beam (e.g., ion beam or electron beam) or X-ray beam are currently under intensive development.
One conventional microlithographic system utilizes a single electron beam to “draw” the pattern on a substrate (e.g., semiconductor wafer) line-by-line. With such technology, a fine pattern having a linewidth of 1&mgr;m or less can be produced since the electron beam can be constricted to a diameter of a few Ångstroms. Unfortunately, progressive miniaturization of a circuit typically results in a more condensed circuit with a correspondingly greater number of lines. Because these systems “draw” the pattern line-by-line, a more condensed circuit has more lines and requires a more constricted beam. Consequently, drawing time is increased, and “throughput” (number of circuits or wafers that can be processed per unit time) is correspondingly decreased. Low throughput is a key factor in making this technique unsuitable for mass-production of integrated circuits.
Another approach receiving substantial current attention is charged-particle-beam (CPB) projection microlithography in which a pattern defined on a reticle or mask is illuminated and projected (“transferred”), using the charged particle beam, to the substrate. Referring to FIG.
3
(
a
), one type of reticle used with CPB projection microlithography is a scattering-membrane reticle
21
comprising a membrane
22
on which a pattern of scattering bodies
24
is formed. The scattering bodies
24
scatter particles of an incident charged particle beam, while the membrane
22
is relatively transmissive to such particles without scattering the beam. Referring to FIG.
3
(
b
), another type of reticle used with CPB projection microlithography is a scattering-stencil reticle
31
comprising a membrane
32
defining a pattern of voids (through-holes). The membrane
32
has a thickness sufficient to scatter particles of an incident charged particle beam, whereas the particles pass freely through the voids.
In each of the reticles shown in FIGS.
3
(
a
)-
3
(
b
), the pattern is divided into multiple small regions
22
a,
32
a
each defining a respective portion of the overall pattern defined by the respective reticle. The small regions
22
a,
32
a
are separated from each other by respective boundary regions
25
,
35
that define no portion of the pattern. The boundary regions typically include struts
23
,
33
extending therefrom. The struts add substantial rigidity and mechanical strength to the reticle.
Conventionally, reticles such as those shown in FIGS.
3
(
a
)-
3
(
b
) are fabricated from “reticle blanks,” which are essentially reticles lacking any pattern. Reticle blanks used for making scattering-stencil masks can be manufactured using, for example, a manufacturing process described in Japanese Kôkai Patent Publication No. Hei 10-106943. In this process, fabrication of a reticle blank begins with formation of an SOI (silicon-on-insulator) substrate. The SOI substrate is prepared from a supporting silicon substrate, a silicon oxide layer on the silicon substrate, and a silicon “active” (doped) layer on the silicon oxide layer. The silicon substrate is etched to form the struts, with regions of membrane extending between the struts. In forming the SOI substrate, the silicon oxide layer is formed by thermal oxidation of the respective surface of the silicon substrate. This oxidation step, and a subsequent step in which the silicon active layer is fused thermally with the silicon oxide layer, involve heating to respective temperatures greater than 1000 degrees. Unfortunately, due to differences in the thermal-expansion coefficients of silicon oxide versus the silicon substrate, residual thermal compressive and tensile stresses are generated in the silicon active layer as the reticle blank returns to a normal temperature. These residual stresses cause deformation of the membrane. If the residual stresses are excessively large, then substantial deformation of the pattern can arise when the reticle blank is made into a reticle.
Accordingly, to form a reticle in which the pattern formed on the reticle membrane has an optimal level of internal stress, it is highly desirable to measure and evaluate the residual internal stress of the membrane. An important known technique for obtaining such measurements and evaluations of residual membrane stress is discussed in Anderer and Behringer, “Determination of the Average Stress and Its Adjustment in Thin Silicon Membranes Used in Various Lithographies,”
Microelectronic Engineering
5:67-71,1986.
A conventional stress-measuring apparatus is shown in FIG.
4
. The
FIG. 4
apparatus includes a stage
42
on which the specimen (i.e., reticle or reticle blank) is mounted, a vibration-applying electrode
43
used to apply a vibration to a membrane
41
a
of the specimen
41
, and a detection electrode
44
situated on the opposite side of the specimen
41
from the electrode
43
. An AC voltage (at a desired selectable frequency) is supplied to the electrode
43
to cause the electrode
43
to generate an AC electrical field. The vibration-applying electrode
43
and detection electrode
44
normally are situated in extremely close proximity to respective surfaces of the membrane
41
a.
The AC electrical field electrostatically extends from the electrode
43
to the membrane
41
a.
Hence, the vibration-applying electrode
43
applies a vibration (corresponding to the frequency of the AC electrical field) electrostatically to the membrane
41
a.
As the membrane
41
a
is being energized in this manner, a resonance frequency of vibration of the membrane
41
a
is detected as corresponding changes in capacitance between the membrane
41
a
and the detection electrode
44
.
A stress-frequency table is prepared for respective calibration membranes as determined by finite-element analysis from data concerning the length, thickness, density, Poisson ratio, and Young's modulus values of calibration membranes. The table is stored in a memory. A stress value corresponding to a particular measured resonance frequency is determined from the data in the table.
As noted above, the vibration-applying electrode
43
must be situated in extremely close proximity to the membrane
41
a
of the specimen
41
, i.e., at a distance of 300 &mgr;m or less. The detection electrode
44
also must be placed in similar close proximity to the membrane
41
a
of the specimen
41
. Unfortunately, because accurate placements of the electrodes
43
,
44
relative to the membrane
41
a
require significant time to perform, measurement throughput is low.
Furthermore, because the conventional stress-measuring device summarized above utilizes a vibration-applying electrode
43
and detection electrode
44
, the device only can be used to measure stress in a conductive membrane.
SUMMARY OF THE INVENTION
In view of the shortcomings of the prior art as summarized above, an object of the invention is to provide stress-measurement apparatus and methods exhibiting improved measurement throughput. Another object is to provide such apparatus and methods that can be used to measure stress in any desired membrane, including non-conductive membranes.
According to a first aspect of the invention, apparatus are provided for measuring stress in a membrane of a specimen. A representative embodiment of such an apparatus comprises a
Klarquist & Sparkman, LLP
Nikon Corporation
Saint-Surin Jacques
Williams Hezron
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