Radiant energy – Means to align or position an object relative to a source or...
Reexamination Certificate
2002-06-04
2004-01-20
Lee, John R. (Department: 2881)
Radiant energy
Means to align or position an object relative to a source or...
C250S492200
Reexamination Certificate
active
06680481
ABSTRACT:
FIELD
This disclosure pertains to microlithography (transfer of a pattern to a sensitive substrate), especially as performed using a charged particle beam. Microlithography is a key technology used in the fabrication of microelectronic devices such as integrated circuits, displays, and micromachines. More specifically, the disclosure pertains to charged-particle-beam (CPB) microlithography methods and apparatus in which certain alignment marks are detected so as to provide improved accuracy of lithographic exposure.
BACKGROUND
Microlithographic pattern transfer using a charged particle beam is regarded as highly accurate and capable of achieving very fine pattern-transfer resolution. However, charged-particle-beam (CPB) microlithography disadvantageously has low “throughput” compared to optical microlithography (performed using deep UV light). (“Throughput” as used herein refers to the number of lithographic substrates, such as semiconductor wafers, that can be processed lithographically per unit time.) Various approaches have been investigated with the object of substantially improving throughput.
For example, several types of partial-pattern single-shot exposure systems (termed “cell projection,” “character projection,” and “block exposure” systems) have been devised. In each of the partial-pattern single-shot systems, certain circuit sub-patterns that are highly repeated in the layer being formed are repetitively transferred and exposed using an aperture mask on which one or more of the basic sub-patterns have been defined. An example of such a highly repeated sub-pattern is a memory cell dimensions of approximately 5-&mgr;m square on the lithographic substrate (“sensitive substrate”). Unfortunately, with any of these techniques, variable-shaped-beam tracing is required to form on the substrate those portions of the pattern that are relatively non-repetitive. Consequently, overall throughput is too low for practical application for mass-production of wafers.
An attractive solution to the problem of substantially improving the throughput of CPB microlithography is the so-called “one-shot” pattern-transfer approach, in which the entire pattern for a layer in a single die (“chip”) or even for multiple dies is exposed in one “shot.” Unfortunately, this approach has not been realized from a practical standpoint for two main reasons. The first reason is that reticles suitable for exposing an entire die pattern in one shot are currently impossible to fabricate. The second reason is that CPB optical systems having optical fields sufficiently large for exposing an entire die pattern without significant off-axis aberrations are currently impossible to fabricate. Consequently, whereas the excitement over the potential of this approach remains high, engineering development work has been directed to other, more feasible, approaches.
One approach receiving much current attention involves dividing a reticle, defining a die pattern, into multiple portions usually termed “subfields.” Each subfield defines a respective portion of the overall pattern. The subfields are arrayed on the reticle in an ordered manner and are exposed in a sequential manner from the reticle to the substrate. This approach is termed the “divided reticle” method, and apparatus configured for performing this method are termed “divided reticle” projection-microlithography apparatus. By performing exposure subfield-by-subfield, the optical field can be sufficiently small to keep aberrations within specifications. Furthermore, any specific aberrations or other errors (e.g., distortion or errors in focus) that arise while exposing a particular subfield can be corrected, on the fly, in a manner that is most suitable for the particular subfield being exposed. The subfield images are placed contiguously on the substrate so as to form, after all the subfields are exposed, the complete die pattern on the substrate surface. Thus, overall exposure is performed with excellent resolution, accuracy, and precision across an optically much wider range than possible with the one-shot transfer method.
So as to expose various subfields on the reticle and form the respective images at proper locations on the substrate, a deflector is provided in the CPB optical system of the microlithography apparatus. This deflector imparts appropriate lateral deflections of the beam to reach the selected subfields. Conventionally, the deflector is an electromagnetic deflector. Since the subfields (which normally are arranged in a highly ordered array) are exposed sequentially by laterally deflecting the beam as required, the deflector experiences a predetermined, repetitive, energization sequence during exposure of the subfields. But, this ordered scheme of energizing the deflector must be stopped temporarily to allow use of the deflector in detecting alignment marks on the substrate by beam irradiation.
An electromagnetic deflector exhibits certain magnetic hysteresis characteristics that are best controlled when the deflector is being energized in a highly ordered manner. The reproducibility of its deflection characteristics is significantly lowered whenever the deflector is being energized in a non-ordered manner (e.g., for irradiating alignment marks). In addition, whenever the electromagnetic deflector is not being energized, the deflector temperature is reduced. Even a slight temperature reduction normally experienced after a shift from a highly ordered energization sequence (for exposing subfields) to a non-ordered energization (for exposing alignment marks) causes significant changes in the deflection characteristics exhibited by the deflector. These changes in deflection characteristics, in turn, cause corresponding errors in positional measurements performed using the beam, such as errors in detecting the positions of alignment marks relative to the axis of the optical system and/or the axes of the reticle stage and substrate stage. These errors generate subtle shifts in the subfield images as formed on the substrate, which decreases the accuracy of pattern transfer.
SUMMARY
In view of the shortcomings of conventional methods and apparatus as summarized above, the invention provides, inter alia, charged-particle-beam (CPB) microlithography apparatus and methods that achieve more accurate detection of position-measurement marks than conventionally. I.e., detections of such marks are less influenced by temperature changes and hysteresis effects of deflection(s) used for performing such detection, which allows more accurate positional detections than currently achievable by current methods and apparatus.
To such ends and according to a first aspect of the invention, methods are provided, in the context of a CPB microlithography method, for detecting a position of a position-measurement mark situated within a deflection field of a CPB optical system. The CPB microlithography method is performed using a CPB optical system that is configured to projection-transfer respective images of exposure units of a pattern, defined on a divided reticle, to a sensitive substrate. In an embodiment of the subject method a charged particle beam (e.g., an electron beam) is deflected within the deflection field, according to a predetermined exposure sequence. While the charged particle beam is being deflected within the deflection field in a sequential manner, the position-measurement mark is irradiated with the charged particle beam. The position of the mark is detected at a moment in which the charged particle beam deflected according to the exposure sequence encounters the position-measurement mark in the deflection field.
The charged particle beam is deflected within the deflection field using a primary deflector in the CPB optical system. In this instance, the method can further comprise the step of scanning the charged particle beam over the position-measurement mark at the moment in which the charged particle beam encounters the position-measurement mark. Scanning of the charged particle beam over the position-measurement mark can be performed using a mark-s
Kalivoda Christopher M.
Klarquist & Sparkman, LLP
Lee John R.
Nikon Corporation
LandOfFree
Mark-detection methods and charged-particle-beam... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Mark-detection methods and charged-particle-beam..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Mark-detection methods and charged-particle-beam... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3205737