MARS2: An Advanced Femtosecond Laser Mask Repair Tool

Abstract

Femtosecond pulsed lasers offer fundamental advantages over other techniques for repairing lithographic masks. Since the femtosecond ablation process is non-thermal, the spatial resolution is not degraded by thermal diffusion and is therefore limited only by optical diffraction. In addition, metal splatter, gallium staining, reduced optical transmission, beam induced charging, quartz damage, and phase errors inherent in other repair methods are eliminated.A second generation femtosecond laser repair tool is described. The tool utilizes DUV optics which allow~100nm mask features to be imaged. The laser beam is focused to a round, gaussian spot. This gaussian spot is scanned over the defect, thus allowing arbitrarily shaped repairs to be performed with a spatial resolution of~100nm. Since the mask is not degraded in any way during the repair process, repairs can be performed iteratively by ablating small slices of the defect. Mask features can be trimmed to an RMS precision of~5nm. The system is also highly automated: masks are loaded into the tool from a SMIF pod via a robot and the tool is controlled from a single screen operator interface. This new tool has been operating successfully in the IBM Burlington mask house since late 2001, and is currently IBM's primary repair tool for 248 and 193nm chrome on glass and phase shift masks.

Structuring of thin films by ultrashort laser pulses

“…The second generation of the mask repair system (MARS II) has been installed in late 2001 as IBM's primary repair tool for 248 nm and 193 nm wavelength chrome on glass phase shift photolithographic masks [93]. In this system, a frequency-tripled Ti:sapphire femtosecond laser beam (260 nm, 100 fs, 1 kHz) is focused to a diffraction limited 150 nm Gaussian spot and scanned over the defect to remove it.…”

“…An ablation resolution of 80 nm together with an increased flexibility in ablating complex structures were demonstrated. Mask features were trimmed to a root-meansquare precision of ~ 5 nm [93].…”

Advanced femtosecond DUV laser mask repair tool for large area photomasks

“…Extended defects were removed by scanning the photomask stage under the focused beam. It was reported that the imaged aperture approach did not produce high-quality sub-wavelength laser zaps due to diffraction effects, thus the focused beam was chosen for semiconductor mask repairs where sub-wavelength nanometer-scale opaque defect ablation has priority 14 .…”

“…By carefully matching Gaussian distribution of the laser beam intensity profile to the ablation threshold of either Chrome or Mo-Si film on the glass substrate, sub-wavelength spatial resolution of ablation was achieved with an ultrafast DUV laser (266 nm, 100 fs) and 0.9NA DUV objective lens 8,14 . Extended defects were removed by scanning the photomask stage under the focused beam.…”