Radiant energy – Means to align or position an object relative to a source or...
Reexamination Certificate
2002-10-02
2003-12-16
Lee, John R. (Department: 2881)
Radiant energy
Means to align or position an object relative to a source or...
C250S492200
Reexamination Certificate
active
06664551
ABSTRACT:
FIELD
This disclosure pertains to microlithography performed using a charged particle beam such as an electron beam or ion beam. Microlithography is a key technology used in the manufacture of microelectronic devices such as semiconductor integrated circuits, displays, and the like. More specifically, the disclosure pertains to methods, in the context of charged-particle-beam (CPB) microlithography, for detecting and maintaining orthogonality of the charged particle beam as incident on the lithographic substrate (e.g., semiconductor wafer).
BACKGROUND
Substantial research and development effort has been expended recently in the development of charged-particle-beam (CPB) microlithography systems (especially electron-beam microlithography systems) capable of producing extremely high pattern-transfer resolution as well as high throughput.
To such end, one system that has been researched extensively in the past is the “one-shot transfer” system, in which an entire “die” or “chip” (or even multiple dies or chips) is exposed in a single exposure “shot,” similar to the usual manner of exposure in optical microlithography. However, a practical one-shot transfer system for use in CPB microlithography has defied realization for several reasons. One reason is that a reticle configured for one-shot exposure using a charged particle beam is extremely difficult to fabricate. Another reason is that it is extremely difficult, with CPB microlithography, to keep aberrations sufficiently corrected over an optical field sufficiently large for exposing an entire die in a single shot. For these reasons, one-shot transfer systems utilizing a charged particle beam currently are deemed impractical, and research interest in such systems has waned.
As a result, substantial interest has been directed to so-called “divided-reticle” CPB microlithography systems that utilize a reticle in which the pattern to be transferred is divided, or “segmented,” into a large number of portions, termed “subfields,” each defining a respective portion of the pattern. The subfields are exposed sequentially while the respective images of the subfields are formed on the lithographic substrate. Each subfield image is located so that, after exposing all the subfields of the pattern, the individual images collectively form a contiguous image of the pattern for a particular die. Such subfield images are termed “stitched” together. An advantage of performing pattern transfer in this manner is that subfield exposure can be performed while correcting aberrations and other imaging faults in real time for each subfield. Thus, lithographic exposures can be performed with greater resolution, accuracy, and precision across a very wide optical field, including optical fields wider than the one-shot fields exposed in optical microlithography.
One parameter not addressed by conventional CPB microlithography systems is the orthogonality of the charged particle beam as incident at the surface of the lithographic substrate. If the incidence orthogonality (i.e.,normal incidence) of the beam at the surface of the substrate is not maintained, then stitching errors arise between adjacent subfield images on the substrate. These stitching errors, manifest as, e.g., reduced smoothness and integrity of connections between adjacent subfield images, yields correspondingly reduced pattern-transfer accuracy. Many microelectronic devices produced under these conditions exhibit substandard performance and unacceptably high failure rate.
SUMMARY
In view of the shortcomings of conventional methods as summarized above, the present invention provides, inter alia, charged-particle-beam (CPB) microlithography methods in which incident-beam orthogonality at the substrate surface is detected and maintained, thereby allowing pattern-transfer to be performed at higher accuracy than conventionally.
According to a first aspect of the invention, methods are provided, in the context of a CPB microlithography method, for determining an incidence orthogonality of the patterned beam on the substrate. In an embodiment of such a method, first and second reticle fiducial marks are placed laterally spaced apart from each other at a reticle plane. A charged particle beam is passed individually through each of the first and second reticle fiducial marks to form respective laterally spaced-apart images of the first and second reticle fiducial marks at each of first and second locations situated near a substrate plane. The first and second locations are separated from each other in an optical-axis direction by a distance &Dgr;H. A lateral distance L
1
, L
2
between the respective images of the first and second reticle fiducial marks is measured at each of the first and second locations, respectively. The incidence-orthogonality error &Dgr;&thgr; at the substrate surface is determined by calculating &Dgr;&thgr;=(L
1
−L
2
)/2&Dgr;H for the first and second reticle fiducial marks.
In this method embodiment, the first and second reticle fiducial marks can be located in a subfield situated at the reticle plane. The first and second reticle fiducial marks desirably are located along respective opposing edges of the subfield. The subfield can be situated on a reticle or on a reticle stage, for example.
The first and second reticle fiducial marks desirably are laterally spaced from each other in an X direction, in which instance the respective images formed at each of the first and second locations are separated from each other in the X direction. Furthermore, the step of placing reticle fiducial marks desirably further comprises placing third and fourth reticle fiducial marks laterally spaced from each other in a Y direction at the reticle plane. The step of passing the charged particle beam desirably further comprises passing the charged particle beam individually through each of the third and fourth reticle fiducial marks to form respective laterally spaced-apart images of the third and fourth reticle fiducial marks at each of the first and second locations situated near the substrate plane. The measuring step desirably further comprises measuring a lateral distance L
3
, L
4
between the respective images of the third and fourth reticle fiducial marks at each of the first and second locations, respectively. The determining step desirably further comprises determining an incidence-orthogonality error &Dgr;&thgr; at the substrate surface by calculating &Dgr;&thgr;=(L
3
−L
4
)/2&Dgr;H for the third and fourth reticle fiducial marks.
Also with respect to this method embodiment, the measuring step desirably comprises placing first and second substrate fiducial marks laterally spaced apart from each other near the substrate plane such that the substrate fiducial marks can be moved to each of the first and second locations. At each of the first and second locations, the respective images of the first and second fiducial marks are scanned over the first and second substrate fiducial marks, respectively. Backscattered electrons produced by scanning the respective images over the first and second substrate fiducial marks are detected.
Another embodiment of a method for determining incidence orthogonality of the patterned beam on the substrate comprises placing a first reticle fiducial mark on a movable body (e.g., reticle stage) located at a reticle plane. The movable body is laterally placeable at each of a first and second laterally displaced position on the reticle plane. A charged particle beam is passed through the first reticle fiducial mark at each of the two laterally displaced positions to form respective laterally spaced-apart images of the first reticle fiducial mark at each of first and second locations situated near a substrate plane. The first and second locations are separated from each other in an optical-axis direction by a distance &Dgr;H. A lateral distance L
1
, L
2
between the respective first and second images of the first reticle fiducial mark at each of the first and second locations, respectively, is measured. The incidence-orthogonality err
Gurzo Paul M.
Klarquist & Sparkman, LLP
Lee John R.
Nikon Corporation
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