Radiation imagery chemistry: process – composition – or product th – Registration or layout process other than color proofing
Reexamination Certificate
2000-06-15
2002-04-23
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Registration or layout process other than color proofing
C430S030000, C430S296000, C430S942000, C250S492200, C250S492220, C250S492300
Reexamination Certificate
active
06376137
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to microlithography (projection-transfer) of a pattern, defined by a reticle, to a sensitive substrate using a charged particle beam as an energy beam. Microlithography is used generally in the fabrication of semiconductor integrated circuits and displays. More specifically, the invention pertains to correcting positioning errors of the reticle stage and/or wafer stage in such microlithography apparatus.
BACKGROUND OF THE INVENTION
Charged-particle-beam (CPB) microlithography (e.g., microlithography performed using an electron beam) is a promising method for used in fabrication of semiconductor integrated circuits, displays, and other devices demanding the accurate transfer of extremely fine patterns (having linewidths of 0.1 &mgr;m or less).
Although CPB microlithography offers prospects of very high resolution, a disadvantage with current CPB microlithography techniques is low “throughput,” by which is meant the number of wafers that can be processed per unit time. Various technical approaches have been investigated to improve throughput.
One group of current approaches is the so-called “partial pattern block exposure” methods such as cell projection, character projection, and block exposure. In partial pattern block exposure, small repeatable portions of the circuit pattern (on the order of 5 &mgr;m on the wafer) are exposed repeatedly onto the wafer from a reticle (mask) defining a number of different such small portions. Unfortunately, before or after exposing the repeated portions, the non-repeated portions of the pattern must be exposed, which is achieved typically by “direct writing” the pattern elements in the non-repeated portions element-by-element using variable-shaped beam. Consequently, throughput is too low for mass-production of wafers.
CPB “projection” microlithography was proposed as a possible solution to the throughput problem. For example, it has been proposed to projection-expose an entire pattern from the reticle to the wafer in one “shot”(usually with the image on the wafer being “demagnified” relative to the pattern on the reticle). Whereas this approach offers tantalizing prospects of high throughput, it has daunting technical problems. First, it is extremely difficult to fabricate a reticle for CPB projection microlithography that can be exposed in one shot. Attempts to fabricate such a reticle have resulted in reticles that have inadequate mechanical strength and rigidity for use. Second, it is extremely difficult to fabricate CPB optical systems (comprising electromagnetic lenses, deflectors, and the like) having a sufficiently wide optical field to expose an entire reticle in one shot. It is especially difficult to fabricate such lenses that do not exhibit substantial off-axis aberrations that prevent achieving the desired fidelity and dimensional accuracy of the transferred pattern.
In view of the above, so-called “divided-reticle” CPB microlithography methods have received considerable research attention. In divided-reticle approaches, the CPB optical system has a relatively large field. However, rather than exposing an entire die (typically corresponding to a single “chip” on the wafer), or even multiple dies, in one shot, the pattern as defined on the reticle is divided into multiple small regions (termed “subfields”), that are exposed individually onto the wafer in a sequential manner. As exposure progresses, the charged particle beam is deflected to the subfields in succession. Meanwhile, adjustments are made as required to correct the focus of the corresponding subfield image as formed on the wafer surface, and to correct aberrations such as distortion. The images of the subfields are formed contiguously (i.e., “stitched” together) on the wafer in the proper order to form the entire pattern.
Unlike CPB microlithographic approaches in which an entire die is transferred in one shot, divided-reticle microlithography can provide accurate pattern exposures with high resolution over an optically large field. By continuously moving the reticle and wafer during exposure, a large exposure area can be patterned at an acceptable throughput.
Certain aspects of a conventional divided-reticle electron-beam microlithography system are depicted in
FIG. 3
, in which the most upstream component shown is an electron gun
1
. The electron gun
1
emits an electron beam EB that propagates in a downstream direction along an optical axis AX. Downstream of the electron gun
1
are first and second condenser lenses
2
,
3
, respectively. The electron beam EB passes through the condenser lenses
2
,
3
to form a crossover image C.O.
1
. The crossover image C.O.
1
is located on the optical axis AX at a blanking aperture
7
.
A beam-shaping aperture
4
is situated between the second condenser lens
3
and the blanking aperture
7
. The beam-shaping aperture
4
defines an axial opening that is sized and shaped to pass therethrough only a portion of the electron beam EB sufficient to illuminate a single exposure unit (“subfield”) of a downstream reticle
10
. For example, if the subfields on the reticle
10
are rectangular in shape (each subfield on the reticle usually is sized and shaped identically), then the beam-shaping aperture
4
defines a corresponding rectangular axial opening. If the subfields on the reticle
10
are square in shape and have an area of, e.g., (1 mm)
2
, then the beam-shaping aperture
4
defines an axial opening sufficient to provide the electron beam, as incident on the reticle, with a square transverse profile with each side of the square being slightly greater than 1 mm. An image of the axial opening defined by the beam-shaping aperture
4
is formed on the reticle
10
by a collimating lens
9
situated between the blanking aperture
7
and the reticle
10
.
The electron beam EB propagating between the electron gun
1
and the reticle
10
is termed herein the “illumination beam” IB. The portion of the electron-optical system (including the lenses
2
,
3
,
9
and the apertures
4
,
7
) is termed herein the “illumination-optical system.”
The illumination-optical system also includes a blanking deflector
5
disposed downstream of the beam-shaping aperture
4
. The blanking deflector
5
, when energized, deflects the illumination beam IB laterally so as to cause the entire illumination beam IB to be blocked as required by the blanking aperture
7
during moments when no exposure is occurring or desired.
The illumination-optical system also includes a selection deflector
8
situated downstream of the blanking aperture
7
. The selection deflector
8
deflects the illumination beam IB mainly in the X-, or left-right, direction (note axes shown in the figure) in a scanning manner. By scanning the illumination beam IB in this manner, successive subfields on the reticle
10
located within the field of the illumination-optical system are illuminated. Thus, the FIG.-
3
apparatus exposes the subfields of the reticle
10
in a scanning manner. The collimating lens
9
, situated downstream of the selection deflector
8
, collimates the illumination beam IB for impingement on a desired subfield of the reticle
10
. Thus, an image of the axial opening defined by the beam-shaping aperture
4
is focused on the reticle
10
.
In
FIG. 3
, only a single subfield (centered on the optical axis AX) is shown. An actual reticle
10
extends outward in the X-Y plane and defines many subfields. In any event, the reticle
10
defines a pattern (chip pattern) for a single semiconductor device (“die”) to be formed on a downstream substrate
23
, and each subfield defines a respective portion of the pattern.
As noted above, the illumination beam IB is deflected laterally to illuminate successive subfields situated within the field of the illumination-optical system. (Hence, multiple subfields desirably fall within the field of the illumination-optical system.) To illuminate a subfield situated outside the field of the illumination-optical system, the reticle
10
is moved relative to the illumination-optical
Klarquist & Sparkman, LLP
Nikon Corporation
Young Christopher G.
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