Radiant energy – With charged particle beam deflection or focussing – Magnetic lens
Reexamination Certificate
1998-08-07
2001-01-30
Anderson, Bruce C. (Department: 2881)
Radiant energy
With charged particle beam deflection or focussing
Magnetic lens
C250S3960ML
Reexamination Certificate
active
06180947
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to optical correction of charged particle beam tools and, more particularly, to reduction and compensation of aberrations in electron beam (e-beam) projection systems.
2. Description of the Prior Art
Numerous techniques are known utilizing charged particle beams and are in widespread use for manufacture of integrated circuit devices, in particular. For example, charged particle beams are used for implantation of impurities, inspection (e.g. with scanning electron microscopes) of structures for process evaluation and development and for lithographic patterning of substrates and layers deposited thereon.
Essentially, lithography processes define potentially very minute areas and shapes on a surface through selective exposure and removal of portions of a layer of resist to expose areas of the surface for further processing by, for example, etching, implantation and/or deposition. In general, for semiconductor device manufacturing, such exposures of the resist have predominantly been made with electromagnetic radiation (EMR) rather than with charged particle beams.
However, there is a strong incentive in the manufacture of integrated circuits to increase integration density to the greatest possible degree consistent with acceptable manufacturing yields. Device arrays of increased density provide increased performance since signal propagation time is reduced and noise immunity is increased with reduced connection length and capacitance. Further, increased device density on a chip allows greater chip functionality as well as greater numbers of devices which can be manufactured on a chip of a given area. This, in turn, results in increased economy of manufacture if manufacturing yields can be maintained.
Device size and density is a function of the minimum feature size which can be reliably produced in the course of patterning the resist. Minimum feature size is limited by the resolution of the exposure which, in the case of EMR, is essentially determined by the wavelength of the radiation utilized to expose the resist. Wavelengths corresponding to deep ultra-violet (DUV) is used almost exclusively for current integrated circuit manufacturing processes and can produce minimum feature sizes as small as 0.25 microns. While use of DUV lithography (DUVL) may be extended to minimum feature sizes of about 0.13 microns (130 nanometers) it is generally considered that other lithographic exposure techniques such as extreme ultra-violet lithography (EUVL), x-ray lithography (XRL), and charged particle lithography (ion beam projection lithography (IPL) and electron beam projection lithography (EBPL)) will be required at smaller feature sizes.
Among these techniques, electron beam projection lithography has the advantage that electrons can readily be controlled and manipulated by electromagnetic fields acting as lenses, deflectors and correctors. Electron beam projection lithography is also able to produce a plurality of pattern elements or pixels in a single exposure. However, it can be appreciated that e-beam exposure is only viable as an exposure medium for high volume production of integrated circuits (ICs) if a throughput comparable to EMR exposure techniques such as DUVL can be realized. To date, EBPL tools have employed beams which are limited in transverse dimensions to about five microns and therefore contain only limited numbers of pattern elements or pixels, generally on the order of about one thousand pixels or less per exposure. These systems are often referred to (collectively with individual pixel exposure systems) as probe forming systems.
Current IC chip designs, however, include on the order of one billion pixels. This number may increase by a factor of ten to one hundred or more in the foreseeable future. Therefore, a very large number of sequential exposures is required to make an exposure of a complete chip area pattern and throughput of probe-forming systems is unacceptably low. This problem cannot realistically be approached by reduction of shot exposure time in view of the large number of exposures which must still be made.
Therefore, any practical solution must increase the number of pixels which can be exposed simultaneously, in parallel. Ultimately, it would be desirable to expose an entire chip area pattern with a single exposure. However, exposure of such an extensive area is not currently practical for various reasons including distortion and field curvature of the electron-optical system (which are correctable to tolerable residual error over only small areas) and available flatness of a wafer surface over a chip area. Lack of wafer flatness can derive from both wafer manufacture and/or mounting of the wafer in the e-beam exposure tool and is collectively referred to as target height variation. That is, among other practical considerations, focus must be adjusted within sub-fields of chip areas to compensate for field curvature and distortion as well as irregularities of topography of the wafer or other structures formed thereon so that resolution of features of a size that would require e-beam exposure, in the first instance, can be achieved.
A solution which is currently feasible, however, is to project a portion of a chip pattern or sub-field which, while small relative to chip area (e.g. 0.01% of the chip pattern), is large compared to a pixel in probe-forming systems. Generally systems capable of projecting increased numbers of pixels also employ optical reduction of projected image size from a sub-field pattern provided on a reticle, employing what is referred to hereinafter as large area reduction projection optics (LARPO). However, such systems have not heretofore been successfully applied to minimum feature sizes approaching or below one-tenth micron and in which the pattern may include one million pixels or more.
The consequences of extending this approach to smaller minimum feature size regimes include a need to project a sub-field having dimensions of about several tenths of a millimeter on a side as flawlessly as required for pattern fidelity commensurate with ground rules of 100 nm and smaller (e.g. a dimensional tolerance of the image of about 10% to 15% of minimum feature size). Further, in order to cover a chip area which may measure several centimeters on a side, the positioning and shape of the image must be of comparable accuracy and fidelity and achieved at very high speed. This latter requirement further implies that the beam must be deflected off the central axis of the beam generating particle-optic system.
In this regard, those skilled in the art will recognize that a projected electron beam will include imperfections or geometric aberrations of many types and that the number of types of aberrations and their size will increase when the beam is deflected off-axis. Fortunately, some of these aberrations can be corrected dynamically in accordance with beam position by appropriate driving of lenses and correctors in synchronism with deflectors.
In probe-forming systems, there are only two dynamically correctable aberrations: astigmatism and field curvature. These aberrations are respectively correctable with one dual axis quadrupole stigmator and one focusing device forming a correction lens. (The latter is often referred to as a focus coil since it is generally small and has much less inductance than the major lenses of the system. The major lenses of the system cannot, as a practical matter, be dynamically driven for that reason.) Development of appropriate corrections for probe-forming systems is well-known, as is the fact that the alteration of focus will cause both a change in image magnification and rotation of the image.
The effects of magnification change and rotation are, however, negligible for beam sizes (corresponding to a much larger sub-field in LARPO systems) characteristic of probe-forming systems. That is, the beam size and orientation error which results from magnification change or rotation over the small beam transverse dimension
Golladay Steven D.
Gordon Michael S.
Pfeiffer Hans C.
Stickel Werner
Anderson Bruce C.
McGuireWoods LLP
Nikon Corporation
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