Defocusing image to pattern contact holes using attenuated phase shift masks

Abstract

The patterning of contact holes by selecting out-of-focus image plane (defocus) using attenuated phase shift masks (APSM) has been studied. Defocus is found to enhance the image modulation at low partial coherence for contact holes with negative local average of mask function. Semi-dense holes up to 130 nm in 8% APSM have been printed by 0.5 μm defocus at a partial coherence of 0.31 using KrF scanner with highest numerical aperture of 0.68. However, these holes were closed with in-focus imaging. Defocus is also found to be beneficial for patterning the pitches that have extensive side lobes with in-focus imaging.

Introduction

The resolution, whether astronomic or lithographic, has been defined by Rayleigh's formula [1], [2]

$$\text{R=K}{}_1\text{λ/}\text{NA}\text{,}$$

where λ is the wavelength of the exposing radiation; NA, the numerical aperture of the projection system; and K₁ is a process dependent parameter known as Rayleigh's constant. In practical terms, the resolution in microlithography is equal to the half of the smallest pitch resolved with specified depth of focus (DOF). Theoretically, the isolated features do not have any resolution limit and can always be patterned by changing the exposure energy. However, the aerial image size, which equals to the convolution of object function (amplitude or intensity) with respective impulse response [3], is defined by the size of impulse response until it is wider than the feature. The impulse response of the optical imaging system for infinitesimally small hole (point source) and infinitesimally fine space (line source) are point spread function (PSF) and line spread function (LSF), respectively, and are governed by the equations [4], [5]

$$\text{PSF}\text{(ρ)=}\text{2J}{}_1\text{(2}\text{π}\text{ρ)}\text{2}\text{π}\text{ρ}\text{,}$$

and

$$\text{LSF}\text{(x)=}\text{∫}\text{−∞}\text{∞}\text{PSF}\text{(x,y)}\text{d}\text{y.}$$

The symbol J₁ is the Bessel function of first kind, order 1 and ρ is the radial distance from the center of the image. The aerial image size has been reduced below PSF or LSF by using the phase shifting principle [5]. The use of attenuated phase shift masks (APSM) has been a popular method for enhancing resolution in optical lithography. Moreover, the image size is wider than PSF or LSF if the image plane is out of focus (defocus). Hence it would be difficult to believe that the defocus could be helpful in patterning the sub resolution features. However, the phenomenon of optical image reversal that occurs with APSM leads towards the use of defocusing image to pattern sub resolution features. The increase of non-lithographic lateral resolution in microscope and telescope by selecting out-of-focus image planes (defocus) has already been recognized [6].

This paper discusses the defocus approach to pattern the sub-resolution holes down to 130 nm in 8% APSM using KrF (248 nm exposing wavelength) 0.68 NA lithography scanner. Scanning electron microscope (SEM) top view pictures and PROLITH (lithography simulation tool from KLA Tencor) simulation graphs have been used to evaluate the defocus effect.