Laser bandwidth and other sources of focus blur in lithography

Abstract

It is well known that the refractive optics used in today's exposure tools are highly chromatic, meaning that small wavelength shifts will cause large focus shifts. Even a line-narrowed excimer laser has a large enough range of wavelengths that we can no longer think of an infinitely thin image plane. The concept of "focus blur" can be generalized to encompass the effect of laser bandwidth chromatic aberrations, vertical stage vibrations (MSDz) and stage tilts which cause focus to change during the scan. This paper will introduce a new parameter called Mean Absolute Defocus (MAD) that can characterize the focus blur, and will be shown to correlate with the lithographic effects.Focus blur can be incorporated into simulation models, in a manner similar to the way that stage vibration is modeled. New simulation results will illustrate the impact of focus blur on modern lithographic processes. Process stability and machine-to-machine matching issues will be discussed.

Convolution-variation separation method for efficient modeling of optical lithography

“…(10), with the regularization weight η 0.02. We adopted two focus blur functions to model focus variations [16]. One is a Gaussian distribution ζ 1 h exp−h 2 ∕2σ 2 with σ 20 nm to characterize the stage vibration, and the other is a modified Lorentzian function ζ 2 h Γ 2 ∕Γ 2 j2hj 2 with Γ 30 nm to represent the laser bandwidth.…”

The Focus Margin Gain and Limitation Realized by Focus Drilling in 28 nm Hole-Like Design Rules

“…When semiconductor technology node achieves to 28 nm, some function of scanner comes out contribution on process window increase. Focus drilling is the scanner implementation to increase the depth of focus (DoF) for contact-hole or via fabrication processes [1][2][3][4][5][6][7]. The basic principle is to smear out the maximum image contrast at best focus (BF) over a specific defocus range.…”