Novel design of att-PSM structure for extreme-ultraviolet lithography and enhancement of image contrast during inspection

Abstract

Attenuated Phase Shift Masks (Att-PSM) have been actively investigated and developed for conventional optical lithography to enhance the lithographic performance. In this paper, Att-PSM for Extreme Ultraviolet Lithography (EUVL) is compared to binary EUVL masks through simulation. Additionally, a new structural design for EUVL Att-PSM that is intended to enhance the image contrast during the inspection is also presented. Aerial image simulation for 50 and 35-nm wide contact holes was performed using an internally developed optical projection lithography simulator. Analysis of phase shift and attenuation for various combinations of Att-PSM layers was also done using an internally developed simulator based on optical multilayer thin film theory. The results of aerial image simulation agree with previously published results in that Att-PSM for EUVL provide steeper edge profile and higher peak intensity compared to the binary EUVL mask. These enhanced aerial images provide greater exposure latitude and 28% to 80% greater depth of focus for Att-PSM compared to binary masks for printing contacts. The simulations were also used to set initial targets for phase and reflectance control of the PSM stack for 35-nm contacts. Mean reflectance between 3 and 6% and phase of 180±10° result in significantly larger DOF than for binary masks. The prototype structure simulated for an EUVL Att-PSM consists of an upper dielectric layer (SiON) and a lower metal layer (TaN or Cr) on top of Mo/Si multilayer mirror. With this dual layer scheme, satisfying the optical requirements (180° phase shift and 3~15% attenuation) is easier than with a single layer structure because accurate control of phase shift and attenuation is possible by controlling the thickness of both the upper and lower layers. Obtaining the desired phase shift and transmission using a single layer is difficult. The advantage of having a dielectric (SiON) top layer is that the thickness of dielectric layer can be optimized to enhance the image contrast at inspection wavelength (normally DUV) as well as to provide the desired phase shift at exposure wavelength (13.4-13.5nm). Another advantage of Att-PSM for EUVL is the reduced height of patterned structure on the multilayer mirror which provides a relative advantage in resolution by reducing the image blur caused by the shadowing effects of the taller absorber stack.

Design and method of fabricating phase-shift masks for extreme-ultraviolet lithography by partial etching into the EUV multilayer mirror

“…Scalar aerial image simulations were used to determine the possible exposure latitude and depth of focus or process window enhancements that might be obtained using PSMs. Han et al 2 showed that a significant increase in depth of focus is attainable using AttPSM for patterning contacts, especially for contact width less than 35 nm. These simulations were also used to determine initial phase and relative reflectance or reflectance ratio tolerances, which are analogous to phase and transmission tolerances, respectively, for AttPSM in optical lithography.…”

“…1 The concept of fabricating PSMs for EUVL has been studied before. [2][3][4] Initial concepts, which were described by Deng et al 3 and Yan, 4 used a height difference between adjacent lithographic patterns on the multilayer to induce 180° of phase shift. One method is to form a phase step by patterning surface relief on the substrate prior to the Mo/Si multilayer deposition.…”

EUV actinic imaging tool aerial image evaluation of EUVL embedded phase shift mask performance

“…EUVL EPSM application and performance have been previously studied by several authors. [1][2][3][4][5] Last year, we have conducted detailed wafer level study of a 6% EUVL EPSM and EUVL binary mask via ADT wafer exposure. [6][7] In the experiment, we used SEVR59 resist and the exposure conditions of numeric aperture (NA) NA=0.25, partial coherence =0.5.…”

Pattern printability for reflectance degradation of Mo/Si mask blanks in EUV lithography

“…However, for an attenuated phase-shifting mask (PSM) [2][3][4] variations in thickness not only degrade the uniformity of the exposure energy but also change the phase and attenuated reflectance because of large changes in the complex index of refraction, even for narrow-spectrum light. This paper discusses how two types of thickness variation in a binary mask and an attenuated PSM affect printability.…”