Electric heating – Metal heating – By arc
Reexamination Certificate
2001-01-18
2003-09-16
Berman, Jack (Department: 2881)
Electric heating
Metal heating
By arc
C250S492100
Reexamination Certificate
active
06621044
ABSTRACT:
(B) CROSS-REFERENCE TO RELATED APPLICATIONS
(NONE)
(C) STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
(NONE)
(D) REFERENCE TO A MICROFICHE APPENDIX
(NONE)
(E) BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to a dual-beam materials-processing system for applying two collinear and coincident controlled pulses of radiation to materials for making physical changes; and more particularly, relates to a means for obtaining high-fluence patterned irradiation that is highly uniform, and highly controllable in shape, intensity, and pulse duration to achieve high resolution imaging for materials processing—without feeding a high fluence through the imaging system.
(2) Description of Related Art
Several techniques have been developed for converting thin amorphous silicon films into polycrystalline films. See Im and Sposili,
Materials Research Society Bulletin,
21, 39 (1996). This is desirable because better-performing thin-film transistor (TFT) devices can be fabricated from crystalline silicon than from amorphous silicon, and it is not possible to deposit silicon films directly in a high-quality crystalline state. Among the various crystallization techniques, those based on excimer laser irradiation (or pulsed-laser irradiation more generally) are prevalent because they are compatible with substrates that cannot withstand high-temperature processing; the techniques can be applied when the silicon film to be crystallized is deposited on such a substrate. Glass and other substrates that cannot withstand high-temperature processing (e.g., plastics) are required for use in many TFT applications, for example, in displays.
Sequential lateral solidification (SLS) is a particular type of excimer laser crystallization process that can produce previously unavailable large-grained and grain-boundary-location-controlled microstructures in thin silicon films. The technique is well documented; see Sposili and Im,
Applied Physics Letters,
69, 2864 (1996); Sposili and Im,
Applied Physics A,
A67, 273 (1998); Im, Sposili et al.,
Applied Physics Letters,
70, 3434 (1997); Sposili, Crowder et al.,
Mater. Res. Soc. Symp. Proc.,
452, 953 (1997); and Im, Crowder et al.,
Physica Status Solidi A,
166, 603 (1998). Films having these microstructures are superior to the types of random-microstructure polycrystalline silicon produced by other excimer laser crystallization processes (and by non-excimer-laser-based crystallization processes as well) in that the TFTs exhibit a combination of superior electronic performance (e.g., higher carrier mobility) and a high level of device-to-device uniformity. The availability of high-performance TFT devices enables numerous applications, such as integrated active-matrix liquid-crystal displays (IAMLCDs), where the driver circuits and other electronics are integrated directly onto the substrate along with the pixel-controlling transistors, and active-matrix organic light-emitting displays (AMOLEDs), among others.
SLS requires that the silicon film be completely melted in and only in a micron-sized spatially controlled region or regions with each irradiation, and that the film be translated with sub-micron precision in between irradiations such that the lateral crystallization induced by each irradiation overlaps with that produced previously. Various schemes have been proposed for conducting the SLS process, and different approaches exist for effecting the requisite spatial tailoring of the beam. Projection irradiation is a very flexible method, wherein a patterned mask is imaged onto the film in order to define the location and extent of the molten zones. Generally, any projection SLS system would contain the following elements: a pulsed-laser source, typically an excimer laser; an illumination system, including a homogenizer, to provide uniform illumination of the mask; an imaging system to image the mask pattern onto the film; and a high-precision sub-micron sample-translation system.
Such SLS projection systems are similar in many respects to photolithography and ablation systems based on excimer lasers; the basic components listed above are common to all. While the various components and subsystems have the same general purpose for each type of projection system, the requirements of the processes differ and therefore so does the configuration of the subsystems.
An excimer laser projection system designed for SLS requires a high fluence—sufficient to completely melt a silicon film in the exposed region or regions. Unlike in lithography and ablation, in which the desired material changes can be effected equally well whether the required dose is delivered in a single irradiation or distributed over several irradiations, the physics of melting and solidification involved in the SLS process requires that the dose be delivered in a single irradiation. As a consequence of this fact, combined with the high energy density needed to heat silicon above its melting point of approximately 1,400° C., successful execution of the SLS process requires that the films be irradiated at very high fluence.
The dual requirements of high fluence and projection imaging with micron-scale patterning present an engineering challenge because the mask materials commonly used, such as chrome-on-quartz, cannot withstand the level of fluence required to melt the silicon films. Conventional SLS system designs use demagnification to address this difficulty; the projection lens is designed to demagnify the image of the mask features. In this way, the fluence at the mask plane can be kept below the mask-damage threshold while achieving high fluence at the silicon film. Neglecting optical losses in the imaging system, the fluence at the silicon film (image plane) will be greater than the fluence at the mask plane (object plane) by a factor of the demagnification squared.
In a previously submitted patent, we described a number of innovations for SLS equipment including a novel illumination subsystem, which produces a self-luminous beam of selected cross-section, spatially uniform intensity, and selected numerical aperture; and a high-efficiency energy-recycling exposure system, which extends the duration of radiation pulses and conserves energy. The importance of these innovations can be understood in connection with the high-fluence requirements for SLS, and additionally in connection with the requirement that such high-fluence irradiation be spatially homogenous within the irradiated regions. If the intensity is too low in particular regions, the film will not melt completely, leading to failure of the process; if the intensity is too high, film damage can occur. In the previous patent, these inventions were described in the context of a conventional SLS system, of the type that utilizes demagnification imaging to keep the fluence level at the mask plane below the damage threshold of the mask materials. However, the utility of the inventions described in the previous patent is not limited specifically to the conventional SLS system design; rather the benefits of the previous invention are equally applicable to the type of system described herein.
Although demagnification is commonly used to address the fluence-limitation issues of SLS equipment, there are advantages to conducting the SLS process using equipment modeled after Anvik's large-area, 1:1-magnification patterning systems rather than using the conventional reduction design.
Anvik's large-area patterning technology is based on the Jain design [1-4] and is schematically illustrated in FIG.
1
. The substrate
11
and the mask
12
are rigidly mounted on a single-planar scanning stage
13
that is capable of moving them in synchronism in both x- and y-directions. The illumination system
14
comprises an excimer laser light source
15
and a beam-processing optical system
16
. The excimer laser
15
emits several tens of watts of UV radiation at 351 nm, a wavelength well-suited for imaging conventional photoresists designed for mercury arc lamp exposure. The systems can al
Jain Kanti
Klosner Marc A.
Sposili Robert S.
Zemel Marc I.
Anvik Corporation
Berman Jack
Kling Carl C.
Smith II Johnnie L
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