Method of forming an LCD with predominantly <100> polycrystalline silicon regions

#This invention relates generally to semiconductor technology and more particularly to the method of forming liquid crystal displays (LCDs) having polycrystalline silicon regions within an amorphous silicon film, which overlies a display substrate.

#Polycrystalline silicon is formed by crystallizing amorphous silicon films. One method of crystallizing amorphous silicon films is excimer laser annealing (ELA). Conventional ELA processes form polycrystalline films having a random polycrystalline structure. Random, as used here, means that no single crystal orientation is dominant and that polycrystalline structures consist of a mixture of crystallographic orientations in silicon. These crystallographic orientations in silicon are commonly denoted as <111>, <110>, and <100>, along with their respective corollaries, as is well known in the art. Control of crystallographic orientation is generally desirable because the electrical characteristics of a polycrystalline silicon film depend upon the crystallographic orientation of the film. In addition, the uniformity of the electrical characteristics will improve if the majority of the film has a controllable texture.

#ELA, as well as many other annealing methods, has not provided a means to control these microstructural characteristics and achieve a predictable and repeatable preferential crystal orientation and film texture within an annealed film.

#FIG. 1 is a schematic cross-sectional view showing an excimer laser anneal (ELA) apparatus used in connection with the present method.

#FIG. 2 (prior art) is a cross-sectional view showing polycrystalline film crystallized using an interface-seeded ELA (IS-ELA) process

#FIG. 3 illustrates a step in the process of lateral-seeded ELA (LS-ELA).

#Referring to FIG. 1 a lateral-seeded excimer laser annealing (LS-ELA) apparatus 10 is shown. LS-ELA apparatus 10 has a laser source 12. Laser source 12 includes a laser (not shown) along with optics, including mirrors and lens, which shape a laser beam 14 (shown by dotted lines) and direct it toward a substrate 16, which is supported by a stage 17. The laser beam 14 passes through a mask 18 supported by a mask holder 20. The laser beam 14 preferably has an output energy in the range of 0.8 to 1 Joule when the mask 18 is 50 mm×50 mm. Currently available commercial lasers such as Lambda Steel 1000 can achieve this output. As the power of available lasers increases, the energy of the laser beam 14 will be able to be higher, and the mask size will be able to increase as well. After passing through the mask 18, the laser beam 14 passes through demagnification optics 22 (shown schematically). The demagnification optics 22 reduce the size of the laser beam reducing the size of any image produced after passing through the mask 18, and simultaneously increasing the intensity of the optical energy striking the substrate 16 at a desired location 24. The demagnification is typically on the order of between 3× and 7× reduction, preferably a 5× reduction, in image size. For a 5× reduction the image of the mask 18 striking the surface at the location 24 has 25 times less total area than the mask, correspondingly increasing the energy density of the laser beam 14 at the location 24.

#The stage 17 is preferably a precision x-y stage that can accurately position the substrate 16 under the beam 14. The stage 17 is preferably capable of motion along the z-axis, enabling it to move up and down to assist in focusing or defocusing the image of the mask 18 produced by the laser beam 14 at the location 24. The mask holder 20 is also capable of x-y movement.

#FIG. 2 illustrates aspects of a prior art ELA process. This process is sometimes referred to as Interface-Seeded ELA (IS-ELA). An amorphous silicon film 30 has been deposited over the substrate 16. A laser pulse is directed at the amorphous silicon film 30, which melts and crystallizes a region 32. The laser pulse melts a region on the order of 0.5 mm. Small microcrystalline seeds 34 remain, or form, at the interface. As the surrounding amorphous silicon crystallizes these seeds affect the crystal orientation. Since the seeds 34 have a variety of crystal orientations, the resulting films will accordingly have a wide mix of crystal orientations. This is illustrated by previously crystallized region 36. In actuality, since a large number of seeds would be present at the interface, a large number of crystal orientations would form.