Laser scanning for semiconductor mask pattern generation

The use of laser scanning to generate semiconductor masks is reviewed. Following a brief historical introduction that describes early pattern generator implementations, current and future industry mask requirements are described with the consequences for pattern generator design: the need for small features, tight CD control, and high pixel delivery rates. The system architecture of a current deep UV scanning system is described in detail along with important print strategies, such as grayscale printing and multipass error averaging. Several subsystem technologies are then explored with emphasis on the application to short wavelengths and multiple beams. Today, frequency-doubled lasers generate the 257-nm radiation used by DUV pattern generators; tomorrow, sum frequency generation will be required to reach the wavelengths at or below 200 nm. Acousto-optic modulation (AOM) technology is shown to scale favorably with shorter wavelengths and to have the bandwidth capability for future system. Acousto-optic beam deflection, polygonal mirror beam deflection, and the reduction of scan bow error through the use of an f · sin(θ) lens are examined. A section on scan optics and image formation presents the differences between partially coherent imaging as used by a wafer stepper and the incoherent superposition of Gaussian beams as used by a laser scanner. Partially coherent imaging is shown to have a sharper image slope but worse feature size linearity. This section also discusses the effect of finite AOM turn-on time on the aerial image in the scan direction.