More On TCZ's Low-Temperature Polysilicon Process

According to market researcher DisplaySearch, the flat-panel display (FPD) market is about to explode. Growth rates for active-matrix LCDs (AMLCDs) should reach 35% over the next five years, while organic LED (OLED) growth will more than double each year beyond 2007 (Fig. 1). Team Cymer Ziess’ latest system should propel that growth.

TCZ’s TCX900X should help thin-film-transistor (TFT) FPD manufacturers produce low-cost, high-quality AMLCD and OLED system-on-panel (SoP) and system-on-glass (SoG) FPDs. Key to TCZ’s development is a novel and long 351-nm excimer laser thin-directional crystallization or x’tallization (TDX) beam.

TDX uses an optical system that processes a location on a silicon wafer using only about two or three pulses, versus 20 to 30 pulses for conventional excimer laser annealing (ELA), providing much higher panel throughput. In addition, the process window is much larger than that for an ELA technique because it does not rely on partial melting. That helps to improve yield and prevents possible damage to the material at high energy density due to silicon agglomeration. Because the entire panel is exposed in a single pass, the TDX process also prevents the non-uniformity caused by the overlapping regions seen in multipass exposure techniques, such as ELA and sequential lateral solidification (SLS) methods.

Each laser beam pulse exposes an 5-µm wide by 730-mm long area at an energy level of 150 mJ/pulse. The laser beam sweeps the silicon wafer laterally. It scans the entire width of a flat panel without the stitching effects of conventional laser annealing, while allowing for lateral growth on a planar surface topology.

By overlapping each new beam stripe over the previous one, the new stripe is “seeded” from the good polysilicon of the previous stripe. The system then achieves continuous growth of long, uniform crystal grains across the entire substrate (Fig. 2).

Because the melt region is only 5 µm wide, the silicon solidifies by lateral growth crystallization, resulting in high-mobility polysilicon. To process the entire substrate, the glass is scanned beneath the beam so that the crystallization occurs in a single pass. The glass moves at a constant velocity, and the laser is triggered to fire after a translation of approximately 2 µm.

Developed by Cymer and in production for the last three years, the laser beam has an output power of 900 W, which is about three times the present ELA power levels available. The Laser Optics Division of Carl Zeiss produced the optics part of the system, which was designed for an optimal beam shape (i.e., a beam long enough to cover the entire width of the substrate being scanned and narrow enough to optimize the crystallization process). The stage for the entire system, developed by Carl Zeiss’ Industrial Metrology Division, features a scan rate of 12 mm/s for a 2- µm pitch at 6 kHz.

An entire fourth-generation (Gen4) panel can be processed with TDX in as little as 75 seconds. More than 25 Gen4 substrates can be processed per hour (see the table). A demonstration system is now available. Shipments will begin in the second and third quarters of next year.