Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2002-08-29
2003-11-04
Lee, John R. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C430S296000
Reexamination Certificate
active
06642532
ABSTRACT:
FIELD
This disclosure pertains to microlithography, which is a key technique used in the manufacture of microelectronic devices such as semiconductor integrated circuits, displays, and the like. More specifically, the disclosure pertains to reticles for use in microlithography performed using a charged particle beam such as an electron beam or ion beam, wherein the reticle defines a pattern to be transferred lithographically to a suitable substrate. Even more specifically, the disclosure pertains to determining the pattern to be defined on the reticle.
BACKGROUND
As the degree of integration of active circuit elements in microelectronic devices has continued to increase, with corresponding decreases in the size of individual active circuit elements in such devices, the resolution limitations of conventional optical microlithography increasingly have become apparent. Consequently, substantial effort is being expended to develop a practical “next generation” microlithography (NGL) technology. One promising candidate NGL technology is microlithography performed using a charged particle beam, which offers prospects of better resolution than optical microlithography for reasons similar to reasons for which electron microscopy yields better image resolution than optical microscopy. Charged-particle-beam (CPB) microlithography can be performed using an electron beam or ion beam. Most effort is being expended to develop a practical electron-beam microlithography apparatus.
With current CPB microlithography apparatus, it is not possible to transfer-expose an entire pattern or even a large portion thereof in a single exposure “shot” due to various factors such as the aberration and distortion exhibited by conventional CPB optical systems. For this reason, transfer-exposure using a “divided” reticle has been developed. In a divided reticle, the pattern (corresponding in area to one “chip” or “die” on the lithographic substrate) as defined on the reticle is divided, or “segmented,” into a large number of exposure units, usually termed “subfields,” that define respective portions of the pattern. Exposure of the pattern from the reticle occurs subfield-by-subfield, wherein the respective images of the subfields are transferred to respective locations on the substrate such that the individual subfield images are “stitched together” in a contiguous manner to form the desired chip or die on the substrate. Typically, multiple chips are formed on a single substrate. So as to be imprintable with die patterns, the upstream-facing surface of the substrate is coated with a thin film of a substance termed a “resist.”
A typical manner of dividing the pattern into subfields is shown in FIG.
8
. First, as noted above, multiple chips are transfer-exposed onto a “transfer body” or lithographic substrate (usually a semiconductor “wafer,” which is the term used herein). The chip pattern, as transferred, is divided into one or more “stripes,” and each stripe is subdivided into multiple subfields. The respective subfields in each stripe are arranged rectilinearly in multiple rows, each containing multiple respective subfields. The pattern on the reticle, and thus the reticle itself, similarly is divided into stripes and subfields.
Transfer-exposure performed using a CPB microlithography apparatus and a divided reticle typically is performed in a manner as shown in FIG.
9
. First, the reticle and wafer are mounted on respective stages that provide support and controlled movements of the reticle and wafer, respectively, as required for exposure. During exposure, the respective stages position the reticle and wafer such that the optical axis of the CPB optical system intersects the respective centerlines of the selected stripe on the reticle and wafer. Exposure of a stripe is achieved by appropriate lateral deflections of the beam (performed by the CPB optical system), accompanied by respective continuous motions of the stages at respective constant velocities along the respective stripes, to expose the subfields in the selected stripe subfield-by-subfield and row-by-row.
The respective stage-movement velocities roughly correspond to the “demagnification” (reduction) ratio of the portion of the CPB optical system used to form the images on the wafer. For example, with a demagnification ratio of 1/4, each subfield image formed on the wafer is 1/4 the size of the respective subfield on the reticle; hence, during exposure the wafer stage moves at about 1/4 the velocity of the reticle stage.
For exposure, the CPB optical system includes an “illumination-optical system” for illuminating the subfields on the reticle and a “projection-optical system” for carrying respective aerial images of the illuminated subfields to the wafer and for resolving the images on the surface of the wafer. The charged particle beam propagating through the illumination-optical system is termed the “illumination beam,” and the charged particle beam propagating through the projection-optical system is termed the “patterned beam” or “imaging beam.”
Thus, during exposure of a stripe, the illumination beam is deflected laterally in a direction approximately perpendicular to the reticle-stage-movement direction to expose each row subfield-by-subfield. As exposure of a particular row ends, respective stage movements bring the next row into position for exposure, with a corresponding reverse in the deflection direction of the beam to expose the constituent subfields of the new row, and so on to the end of the stripe. Hence, exposure of the stripe progresses in a raster manner, which minimizes time lost between exposures of adjacent rows and thereby increases throughput. As exposure of a particular stripe ends, respective stage movements bring the next stripe into position for exposure.
The reticle used in the exposure method described above differs substantially in structure from a reticle used for optical microlithography. Whereas a reticle for optical lithography can be exposed in a single “shot” and is self-supporting, the reticle for CPB microlithography is structured to define individual subfields (each defining a respective portion of the pattern) and intervening structural members termed “struts.” The struts extend across the reticle in a lattice manner and separate the subfields one from another. Contiguous with the struts are frame members extending around the circumference of the reticle. The struts and frame provide structural strength and rigidity for the reticle. Each subfield on the reticle includes a respective membrane portion that includes a respective patterned portion and a respective skirt. The patterned portion defines the respective portion of the pattern defined by the reticle. The skirt surrounds the patterned portion. The patterned portion is transmissive to the illumination beam such that, as the illumination beam passes through the patterned portion, the beam acquires an aerial image of the respective pattern elements defined in the patterned portion. The outer edges of the illumination beam fall within the skirt as each subfield is illuminated. The skirt and the struts surrounding the skirt effectively isolate each subfield for individual exposure without crosstalk between adjacent subfields during exposure.
CPB microlithography is subject to a phenomenon known as the space-charge effect (also termed a “Coulomb effect”) caused by mutual electrostatic repulsion of charged particles in the beam. The mutual repulsion causes widening of the beam, with an accompanying drop in pattern-transfer resolution. To reduce the space-charge effect, the beam-acceleration voltage may be increased to increase the velocity of particles in the beam and correspondingly reduce the particle—particle interaction time during propagation from the reticle to the wafer. Hence, increasing the beam-acceleration voltage conventionally is a favored means for increasing pattern-transfer resolution.
However, increasing the beam-acceleration voltage causes certain problems, notably undesired changes in the profiles of pattern elements as transfer-expos
Gill Erin-Michael
Klarquist & Sparkman, LLP
Lee John R.
Nikon Corporation
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