Laser crystallization of an amorphous silicon film that has been deposited on a substrate, e.g., glass, represents a promising technology for the production of material films having relatively high electron mobilities. Once crystallized, this material can then be used to manufacture thin film transistors (TFT's) and in one particular application, TFT's suitable for use in relatively large liquid crystal displays (LCD's). Other applications for crystallized silicon films may include Organic LED (OLED), System on a Panel (SOP), flexible electronics and photovoltaics. In more quantitative terms, high volume production systems may be commercially available in the near future capable of quickly crystallizing a film having a thickness of about 90 nm and a width of about 700 mm or longer.
Laser crystallization may be performed using pulsed laser light that is optically shaped to a line beam, e.g., laser light that is focused in a first axis, e.g., the short-axis, and expanded in a second axis, e.g., the long-axis. Typically, the first and second axes are mutually orthogonal and both axes are approximately orthogonal to a central ray traveling toward the film. An exemplary line beam for laser crystallization may have a beam width at the film of less than about 20 microns, e.g. 3-4 microns, and a beam length of about 700 mm. With this arrangement, the film can be scanned or stepped in a direction parallel to the beam width to sequentially melt and subsequently crystallize a film having a substantial length, e.g., 900 mm or more.
In some cases, e.g. sequential lateral solidification processes, it may be desirable to ensure that the silicon film is exposed using a beam having an intensity that is relatively uniform across the short-axis and that drops off sharply at the short-axis edges (i.e. a beam having relatively steep, short-axis sidewalls). More specifically, failure to obtain a steep sidewall on the trailing short-axis edge may result in the undesirable crystal quality of new grains near the short-axis edge due to insufficient overlap between adjacent pulses. Also, in some implementations, it may be desirable to have a steep sidewall on the leading short-axis edge to reduce surface variations and provide more consistent lateral growth. One way to achieve this shape is to focus a laser at a short-axis stop, e.g. field stop, which is shaped as an elongated slit that is aligned in the direction of the long-axis. An optic may then be used to produce an image of the short-axis stop at the film. With this arrangement, a beam having relatively steep, short-axis sidewalls may be obtained. For the dimensions contemplated above, e.g. a beam width at the film of less than 20 microns, it may be important to control the dimensions of the short-axis stop to relatively close tolerances.
In some cases, it may be desirable to ensure that each portion of the silicon film is exposed to an average laser energy density that is controlled within a preselected energy density range during melting. In particular, energy density control within a preselected range is typically desired for locations along the shaped line beam, and a somewhat constant energy density is desirable as the line beam is scanned relative to the silicon film. High energy density levels may cause the film to flow resulting in undesirable “thin spots”, a non-flat surface profile and poor grain quality. This uneven distribution of film material is often termed “agglomeration” and can render the crystallized film unsuitable for certain applications. On the other hand, low energy density levels may lead to incomplete melting and result in poor grain quality. By controlling energy density, a film having substantially homogeneous properties may be achieved.
One factor that can affect the energy density within an exposed film is the spatial relationship of the thin film relative to the pulsed laser's depth of focus (DOF). This DOF depends on the focusing lens, but for a typical lens system configured to produce a line beam having a 20 micron beam width, a good approximation of DOF may be about 20 microns.
With the above in mind, it is to be appreciated that a portion of the silicon film that is completely within the laser's DOF will experience a different average energy density through the film's thickness than a portion of the silicon film that is only partially within the laser's DOF. Thus, surface variations of the silicon film, the glass substrate and the vacuum chuck surface which holds the glass substrate, even variations as small as a few microns, if unaccounted for, can lead to unwanted variations in average energy density from one film location to another. Moreover, even under controlled manufacturing conditions, total surface variations (i.e., vacuum chuck+glass substrate+film) can be about 35 microns. It is to be appreciated that these surface variations can be especially problematic for focused thin beam having a DOF of only about 20 microns.
As indicated above, for some implementations, a desirable beam may have a relatively flat so-called “tophat” type intensity profile at the film with relatively sharp sidewall slopes. To achieve this shape, it may be desirable to use a relatively large NA optical system between the short axis stop (see description above) and the film to obtain a good reproduction of the slit and sharp sidewalls at the film. However, an increase in the numerical aperture of the optical system typically results in a commensurate decrease in the depth of field. Thus, it would be desirable to reduce a system's dependence on a relatively large DOF to allow the use of a higher NA optical system, which in turn, may produce a better intensity profile shape at the film.
With the above in mind, Applicants disclose systems and methods for implementing an interaction between a shaped line beam and a film deposited on a substrate.