DISPLAY MANUFACTURING

Excimer laser annealing is a process by which amorphous silicon (a-silicon) is transformed to polysilicon (p-silicon). This process is of growing importance to the flat-panel display industry because it allows low-temperature processing of silicon films on glass substrates instead of on more-expensive quartz, which is needed for thermal annealing. Single-shot laser annealing of a-silicon can be used to produce small displays--those smaller than the beam size--but larger substrates require the ex

DISPLAY MANUFACTURING

Camera monitors laser-annealed silicon

Excimer laser annealing is a process by which amorphous silicon (a-silicon) is transformed to polysilicon (p-silicon). This process is of growing importance to the flat-panel display industry because it allows low-temperature processing of silicon films on glass substrates instead of on more-expensive quartz, which is needed for thermal annealing. Single-shot laser annealing of a-silicon can be used to produce small displays--those smaller than the beam size--but larger substrates require the excimer laser beam to be scanned across the surface in a step-and-repeat pattern. SensorPhysics (Oldsmar, FL) demonstrated that a modified optical microscope and analysis software can be used to ma¥the p-silicon surface and assess its uniformity, providing insight into the annealing process.

Annealing requires a shaped beam with an aspect ratio that is different from a raw excimer laser beam; the shaping process can introduce beam artifacts. Two issues are important in the excimer laser process: the beam must have even energy distribution over an area of about 1 ¥ 200 mm, and, as the beam is scanned over the a-silicon, the overla¥must be controlled for uniformity. Previously, SensorPhysics applied its UV-sensitive SensorCards to the characterization of the annealing laser beam (see Laser Focus World, May 1996, p. 40). According to developer Gary Forrest, the new camera-based technique permits direct resolution of diffraction effects in the annealed silicon film (see figure on p. 24). Forrest worked with microscope manufacturer Olympus (Melville, NY) and local Olympus distributor C. Squared Corporation (Gainesville, FL) to develo¥the dark-field illumination technique for a BX-60 optical microscope. A charge-coupled-device (CCD) camera recorded the image, and beam-analysis software was used to plot the x and y profiles of the intensity.

In these annealing experiments, the 308-nm excimer laser was deliberately misaligned to generate the diffraction pattern shown. The contrast in the dark-field image is evident by the peaks and valleys in the two-dimensional intensity profiles. Pixel sizes in a final display are on the order of about 10-20 µm; the distance (about 5 µm) over which uniformities can be measured gives an example of the resolution of the technique.

The goal of the excimer-laser annealing process is to make the material so it has uniform electrical properties. Then the transistors can be placed directly on the polysilicon and pixels can be defined. According to Forrest, in some instances, the initial beam characterization ste¥indicates that the beam is suitable for annealing but the final material does not have the proper electrical properties. He says, "What you are seeing in the image is an optical difference between amorphous and polysilicon." Observing the difference could explain the variance in electrical properties.

Recently, Toshiba (Tokyo, Japan) revealed it will begin production of 12-in. polysilicon active-matrix LCD screens in Fukaya, Japan, using the laser-annealing process (see Laser Focus World, May 1997, p. 77). Its pilot line reportedly will have a capacity of six thousand 400 ¥ 500-mm panels per month.1 Camera-based assessment of the annealing process may prove useful in a manufacturing environment.

Heather W. Messenger

REFERENCE

1. J. Robertson, Electron. Buyers News, March 31, 1997.

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