Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2002-04-24
2003-09-30
Lee, John R. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S491100, C250S492200, C250S492220, C430S004000
Reexamination Certificate
active
06627906
ABSTRACT:
FIELD
This disclosure pertains to microlithography of a pattern, defined by a reticle or mask (generally termed “reticle” herein) from the reticle to a sensitive substrate (e.g., a semiconductor wafer coated with a substance, termed a “resist,” that is imprintable with an image of the pattern) using an energy beam. Microlithography is a key technique used in the manufacture of microelectronic devices such as integrated circuits, displays, magnetic pickup heads, and micromachines. More specifically, the disclosure pertains to microlithography performed using a charged particle beam (e.g., electron beam or ion beam) as the energy beam. Even more specifically, the disclosure pertains, in the context of charged-particle-beam microlithography, to obtaining a desired exposure dose on the substrate regardless of the thickness of the membrane portion of the reticle, thereby allowing throughput to be increased.
BACKGROUND
Charged-particle-beam (CPB) microlithography offers prospects of substantially higher pattern-transfer resolution than currently achievable using optical microlithography. Unfortunately, throughput achievable with CPB microlithography still is substantially less than the throughput achievable with optical microlithography. (“Throughput” denotes the number of production units such as wafers that can be processed per unit time using the subject technology.) Substantial development efforts in CPB microlithography have been directed to improving throughput.
The main reason for low throughput with CPB microlithography is the current impossibility of fabricating a reticle defining the entire pattern in a manner allowing the entire pattern to be exposed in a single exposure “shot.” Another reason is the current impossibility of providing illumination and projection “optics” capable of projecting an entire pattern in a single shot while also providing satisfactorily low aberrations in the projected image.
One current example of a CPB microlithography technology that has been put to practical use is “cell projection,” which involves the repeated transfer and exposure of a limited number of types of graphic portions of a device pattern (hence, this technique also is termed “graphic-portion batch exposure”). The graphic portions are defined as respective apertures in an aperture plate that are exposed in a mix-and-match manner so as to reconstruct the pattern portion-by-portion on the substrate. Unfortunately, the maximal throughput obtainable using graphic-portion batch exposure is about one order of magnitude too low for practical application in actual mass-production of semiconductor chips.
To improve throughput over that obtainable using cell projection, so-called “divided reticle” reduction projection-exposure apparatus have been developed for CPB microlithography. In the divided-reticle technique, the entire chip pattern as defined on the reticle is divided into multiple “subfields” each defining a respective portion of the overall pattern. The apparatus directs a charged-particle illumination beam to illuminate a selected subfield on the reticle. Portions of the illumination beam passing through the illuminated portion of the reticle are projected by a projection-optical system onto a corresponding selected region on the substrate. The subfields are projected according to a predetermined order, and are imaged such that the images of the subfields are placed contiguously with each other (i.e., “stitched” together) on the substrate. Projection of the images normally is performed with “reduction” (demagnification); hence, the image of a subfield on the substrate is smaller (usually by an integer factor such as 4 or 5) than the corresponding subfield on the reticle. Further details of this technique can be found in U.S. Pat. No. 5,260,151, incorporated herein by reference, and in Japan Kôkai (Laid-Open) Patent Document No. Hei 8-186070.
In divided-reticle CPB microlithography, generally two types of reticles are used: non-stenciled-membrane reticles and stenciled-membrane reticles, as shown in FIGS.
7
(A) and
7
(B), respectively. Both types of reticles are fabricated from a semiconductor wafer (typically silicon). The reticles are shown in the figures as respective elevational sections of a portion of each. The non-stenciled-membrane reticle also is termed a “scattering-membrane” reticle, and the stenciled-membrane reticle also is termed a “scattering-stencil” reticle.
The non-stenciled-membrane reticle
10
-
1
shown in FIG.
7
(A) comprises, for example, a SiN
x
(silicon nitride) layer
62
formed on the “lower” (downstream-facing) surface of a Si (silicon) substrate
61
. The SiN
x
layer
62
is relatively non-scattering to charged particles of an illumination beam EB incident from an upstream direction. I.e., incident charged particles pass through the SiN
x
layer
62
with little to no forward scattering. On the “lower” surface of the SiN
x
layer
62
is a W (tungsten) layer
63
. The W layer
63
is highly scattering to charged particles of the incident illumination beam EB. I.e., incident charged particles pass through the W layer
63
with substantial forward scattering. After forming the layers
62
,
63
the Si substrate
61
is removed by anisotropic dry-etching or the like from regions corresponding to subfields
64
, thereby forming in the subfields
64
respective self-supporting membrane portions
65
composed of the SiN
x
layer
62
and the W layer
63
. In the subfields
64
, portions of the W layer
63
are selectively removed to form non-scattering regions
66
and scattering regions
67
. In each subfield
64
, the non-scattering regions
66
and scattering regions
67
collectively define, on the membrane
65
, a respective portion of the chip pattern to be projected onto a sensitive substrate (not shown).
Whenever the non-stenciled-membrane reticle
10
-
1
is irradiated with a CPB illumination beam (e.g., electron beam EB), the non-scattering regions
66
transmit respective portions of the beam, with substantially no scattering, to the sensitive substrate. The scattering portions
67
, on the other hand, greatly scatter respective portions of the incident beam. The scattered particles propagating downstream of the reticle are absorbed and thus blocked by a contrast aperture or the like (not shown, but see
FIG. 1
) to prevent them from reaching the sensitive substrate. The non-scattered particles pass through the contrast aperture to the sensitive substrate. Thus, an image of the pattern can be formed with satisfactory contrast on the sensitive substrate.
Referring now to the stenciled-membrane reticle
10
-
2
shown in FIG.
7
(B), the silicon substrate
71
is removed by anisotropic dry-etching or the like from regions corresponding to subfields
74
, thereby forming in the subfields
74
respective self-supporting membrane portions
75
composed of residual silicon substrate
71
. Portions of the membranes
75
are selectively removed to form beam-transmissive apertures
76
. The apertures
76
and remaining portions
77
of the membranes
75
collectively define a pattern to be projected onto a sensitive substrate (not shown).
Whenever the stenciled-membrane reticle
10
-
2
is irradiated with a CPB illumination beam (e.g., electron beam EB), the apertures
76
transmit respective portions of the beam, with substantially no scattering, to the sensitive substrate. The membrane portions
77
, on the other hand, greatly scatter respective portions of the incident beam. The scattered particles propagating downstream of the reticle are absorbed by a contrast aperture or the like to prevent them from reaching the sensitive substrate. The non-scattered particles pass through the contrast aperture. Thus, an image of the pattern can be formed with satisfactory contrast on the sensitive substrate.
The transmissivity of a CPB-transmitting portion of a non-stenciled-membrane reticle used in an electron-beam microlithography apparatus is a function of the thickness of the CPB-transmissive layer, as discussed in
J. Vac. Sci. Technol.
B16:3385, 1998.
F
Klarquist & Sparkman, LLP
Lee John R.
Nikon Corporation
Souw Bernard E.
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