Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2000-07-21
2003-06-10
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S053000
Reexamination Certificate
active
06577380
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
(NONE)
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
(NONE)
REFERENCE TO A MICROFICHE APPENDIX
(NONE)
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to a materials-processing system for applying controlled pulses of radiation to materials for making physical changes, and more particularly relates to a highly uniform, high-efficiency optical illumination system which is highly controllable in shape, intensity, and pulse duration to achieve high resolution imaging for materials processing.
(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(3) (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 with each irradiation the silicon film be completely melted in and only in a micron-sized spatially controlled region or regions, 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, typically at 5:1 or greater reduction, although 1:1 imaging is a possibility; 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. For example, a system intended for photolithography might require imaging resolution on the order of 1 &mgr;m, whereas a resolution of 3-5 &mgr;m is usually adequate for SLS. Conversely, SLS has more-stringent requirements than photolithography and ablation in other respects.
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. Spatially homogenous illumination is also essential so that all of the irradiated areas are irradiated at a fluence greater than the complete-melting threshold of the film. 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.
For certain high-throughput configurations of SLS, it would be beneficial to be able to configure the illumination into a high-aspect-ratio form (e.g., a long, narrow rectangle) while maintaining a high degree of spatial uniformity. Techniques describing an internally reflective homogenizer, using a polygonal cross-section, fully internally mirrored chamber to convert collimated light into self-luminous light at an output aperture have been reported in Jain, U.S. Pat. No. 5,059,013 and Farmiga, U.S. Pat. No. 5,828,505. These homogenizer designs offer the additional benefit of preserving numerical aperture (NA) of the illumination, which allows for high optical efficiency. Further, the ability to vary the configuration of the illumination between the high-aspect-ratio shape and some other polygonal shape would also be extremely useful, as it would enable the equipment to be reconfigured between the high-throughput and other variants of the SLS process. For example, a homogenizer with a rectangular cross-section could be constructed wherein one of the sides would be adjustable, enabling one dimension of the rectangle (and thus the aspect ratio) to be varied. An system with such a homogenizer could conveniently be reconfigured to perform different variants of the process as desired. Existing illumination and homogenization schemes that have been applied to SLS cannot provide the requisite level of spatial uniformity in a high-aspect-ratio shape, and are generally limited to providing square or near-square illumination.
The beam area over which a sufficiently high fluence can be maintained is defined primarily by the energy output of the excimer laser source, but for a given laser, fluence can be optimized by a highly efficient optical system, especially one incorporating an energy-recycling scheme. Large-field homogeneous illumination is a prerequisite for a large-working-area beam, which is necessary in order that the process have a high throughput.
A high-efficiency energy-recycling exposure system has been reported. See Hoffman and Jain, U.S. Pat. No. 5,473,408. This scheme offers the additional feature, by the very nature of the energy-recycling scheme with its multiple reflections of portions of the pulse through the optical system, of extending the effective duration of the excimer laser pulse reaching the substrate. Pulse extension beyond the approximately 30-ns FWHM (full-width half-maximum) typically provided by most commercially available excimer lasers offers two benefits to the SLS process: (1) the longer pulse provides some amount of substrate heating, which delays the onset of nucleation during solidification, allowing for a longer lateral growth distance, and therefore increases the throughput of the process; and (2) the substrate heating reduces the thermal gradients during solidification, which in turn reduces the number of intragrain defects, further improving the quality of the crystallized films.
The energy-recycling sche
Farmiga Nestor O.
Jain Kanti
Sposili Robert S.
Adams Russell
Anvik Corporation
Esplin D. Ben
Kling Carl C.
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