Grain Boundary Confinement in Silicon-on-Insulator Films

This invention relates to a method of growing regular arrays of grain boundary free single crystal silicon islands in a silicon-on-insulator film.

In a standard method for making single crystal silicon islands, a patterned antireflection coating is used to define single crystal island areas. The antireflective coating pattern is made up of a series of stripes which are deposited over a layer of polysilicon. A laser beam is then scanned parallel to the antireflective stripes thereby producing a single crystal silicon island between the antireflective stripes [see e.g., J. P. Colinge, E. Demoulin, D. Bensahel, and G. Auvert, "Use of Selective Annealing for Growing Very Large Grain Silicon-On-Insulator," Applied Physics Letter, Vol. 41, No. 14 pp.346-347, August 1982; J. P. Colinge, E. Demoulin, D. Bensahel, G. Auvert, and H. Morel, "Transistors Made in Single-Crystal SOI Films," IEEE Electron Device Letters, Vol. EDL-4, No. 4 April 1983]. Other methods for growing single crystal silicon islands in a silicon-on-insulator film are described in an article by G. K. Celler, L. E. Trimble, K. K. Ng, H. J. Leamy and H. Baumgart entitled "Seeded Oscillatory Growth of Si Over SiO.sub.2 by CW Laser Irradiation," in Applied Physics Letter, Vol. 40, No. 12, June 1983.

One of the drawbacks of the above method is the difficulty of obtaining high quality single crystal silicon near the edges of the beam. In addition, overlapped successive scans can destroy the single crystal produced by the earlier scans causing random nucleation in the overlap region. This random nucleation prevents the making of single crystal silicon regions larger than 30 microns in width.

In accordance with the illustrated preferred embodiment of the present invention, a method for making a regular array of grain boundary free silicon islands is disclosed. These islands have been grown in a silicon-on-insulator layer by using a patterned antireflective coating in combination with a laser scanning procedure. The antireflective coating pattern is made up of a series of parallel stripes terminating in seeding windows for seeding the crystalline growth of the silicon islands. A laser beam of elliptical cross-section is scanned perpendicular to the stripes, with the long axis of the beam parallel to the scan direction. The beam is stepped between successive scans to advance the single crystal growth along the direction of the antireflection stripes. Grain boundaries are confined to the region under the antireflective stripes.

FIG. 1A shows a cross-sectional structure of the preferred embodiment of the present invention.

FIG. 1B shows a top view of the preferred embodiment as shown in FIG. 1A of the present invention.

FIG. 2 illustrates a completed array of single crystal stripes according to the preferred embodiment.

As illustrated in FIG. 1A, a silicon dioxide layer 110 is formed on the surface of a silicon wafer 100 by conventional oxidation methods. A polysilicon layer 120 is deposited over the silicon dioxide layer 110 by low pressure chemical vapor deposition (LPCVD) techniques. Seed windows 111 connecting the polysilicon to the silicon wafer are provided periodically. For best results the crystal orientation of the edges of the seeding window are in the [110] direction. A capping layer of an antireflective coating 130 is then deposited over the polysilicon layer 120 and seed windows 111, also by LPCVD techniques. The antireflective coating 130 is typically a combination of silicon dioxide and silicon nitride having a total thickness in the range of 750 to 850 angstroms. However, other antireflective coatings can also be used e.g., a single layer of silicon nitride or silicon dioxide.

The antireflective coating is then patterned, as shown in FIG. 1B, using a masking step to delineate stripes 112, which in the best mode are generally 10 microns wide and 10 microns apart (center to center). Antireflective stripes having widths in the range of 5 to 25 microns and spacings in the range of 10 to 50 microns can also be used. This masking step thereby defines polysilicon islands between the antireflective coating stripes. An insulating layer, e.g., a layer of silicon dioxide, may be formed between the antireflective stripes. Additionally an inorganic layer, e.g., a layer of silicon nitride, may be formed on top of the insulating layer. The wafer is then processed with a cw argon laser, typically at a scan speed in the range of 10 to 50 centimeters per second, a beam size of typically 50 by 250 microns, an overlap of 90 to 98 percent, and a power ranging from 5 to 15 watts. During laser processing the wafer is kept at a temperature in the range of 350 to 500 degrees celcius. The laser beam is scanned substantially perpendicular to the long axis of the antireflective stripes with the beam spot covering at least two antireflective stripes simultaneously. Each time the beam scans across an island between adjacent antireflective stripes, the silicon which was recrystallized in the previous scan is remelted. New crystallization starts at the center of the island because it is cooler there than at the edges. A melt front 113 is shaped by the antireflective coating to produce concave regions in the space between the stripes and convex regions under the stripes. Defects left from previous scans are eliminated in the remelted silicon by scanning overlaps.

After laser processing, long single crystals of silicon are formed in the region between adjacent antireflective coating stripes. The antireflective coating is removed, and the polysilicon surface is decorated with an etch to reveal the grain boundaries. FIG. 2 shows an optical photograph of an array of single silicon crystals grown using the method of the present invention. The grain boundaries 114 are typically straight lines, and are located beneath the center of the antireflective stripes 112.