Mask Repair Systems Show Promise

Ruth DeJule, Associate Editor -- Semiconductor International, 11/1/1998

From binary to phase shift masks (PSM), the driver of mask costs is the ability to repair reticles. As the resolution of optical lithography is pushed to the limits, the introduction of enhancement techniques such as optical proximity correction (OPC) and phase shifting become more prevalent. This has meant the need for more stringent requirements from mask repair systems.

Traditionally, focused ion beams and nanosecond pulsed lasers have been used to repair chrome-on-glass (COG) masks. Each has limitations. Gallium (Ga) focused ion beams with a spot diameter well below 25 nm, repair opaque defects by sputtering excess chrome. Despite excellent spatial resolution, during repair, Ga is implanted into the quartz substrate causing significant reduction in optical transmission in the repair area. Toward the end of the process, the ion beam can erode and damage the quartz substrate, impacting optical quality.

Nanosecond laser repair ablates excess chrome using pulses of laser light. The primary limitation of this technique is the heat generated during the 'long'' pulse. Curling or balling of the edges of chrome features and chrome splattering create small defects that can reduce optical transmission. Femtosecond laser mask repair directly addresses limitations caused by heating. The light pulse excites the material in less time than it takes to heat the material. With no electrons to bind the atoms together, the metal vaporizes. The advantages of this non-thermal ablation process include no thermal diffusion to degrade the resolution, no metal splattering or balling and no damage to the quartz substrate. Two approaches to this technology come from the IBM T.J. Watson Research Center and Nanonics Lithography (Jerusalem, Israel).

Fig. 1. Arrows indicate the repair of a bridge defect (top). The corresponding AIMS plot (bottom) shows that optical transmission of the repair is reduced ~1% relative to adjacent spaces. (Source: IBM)

IBM's system integrates two commercially available tools, a femtosecond laser and an imaged aperture for delivering the pulses to the mask. The rectangular aperture is illuminated with laser light, reimaged onto the mask and demagnified by a factor of 100. The automated system accurately positions the mask with an interferometrically controlled stage and a CCD camera. Laser cuts to less than 0.5 µm and ~1% reduction in optical transmission of the repair region to that of adjacent clear spaces (Fig. 1, top) have been demonstrated. Over 300 COG masks have been repaired by this manufacturing tool. This technique is expected to be used for repairs to 100 nm design rules.

Near-field imaging, used in the Nanonics Lithography repair system, is the only optical technique for high resolution repairs beyond the far field diffraction limit, said Dr. Yosi Shani, physics group leader. When an aperture is smaller than the wavelength of a light source, there is a near-field region that extends to a distance on the order of the diameter of the aperture. If a surface is placed within the near-field region and illuminated by a sub-wavelength spot of light, the dimension of the spot is dependent solely on the size of the aperture, not its wavelength. To ensure that the aperture is tracking the near-field of an arbitrary surface, it is necessary to locate the aperture at the tip of a sharp probe. Nanonics uses micropipettes that act as an atomic force sensors for tracking surface topography. Femtosecond pulses are transmitted through the well defined apertures of the micropipettes. Edge placement of 50 nm can be achieved, Shani said. With further system improvements, 30 nm edge placement is expected.

Currently, femtosecond mask repair has been used for COG and CrO attenuated phase shift masks. Though femptosecond repairs have yet to be performed on MoSi alternating PSMs, Shani anticipates the same quality of repair, since ablation quality is independent of the material.