David J. Gerold

Multiple pitch transmission and phase analysis of six types of strong phase-shifting masks

Our previous work showed that for 100nm lines, the Sidewall Chrome Alternating Aperture (SCAA) mask structure could overcome the problem of transmission and phase imbalance among multiple pitch structures. In that work, we explained the SCAA mask concept, showed a brief electromagnetic field (EMF) simulated comparison to two subtractive etch techniques and proposed a fabrication paradigm that could make SCAA a reality. What we did not show, however, was the detail of our EMF simulation work for any of these masks. Our current work provides this missing item and explores across pitch performance at 248nm wavelength for several masks meant to optimize alternating phase-shift (altPSM) mask phase and transmission: SCAA, asymmetric lateral biased, additive, undercut, dual trench (with and without undercut), mask-phase-only, and uncompensated. First, we discuss why vector electromagnetic field (EMF) simulation is necessary. Then we describe a typical optimization approach. There we describe how two simulators, ProMAX (FINLE Technologies, Inc.) and TEMPESTpr (Panoramic Technologies), were set up to reduce grid snapping and other simulation pitfalls, as well as EMF output analysis and topography optimization techniques using one mask type as an example. The optimization approach was to find the best topography for the 100:200nm line:space mask of each type according to the phase and transmission errors extracted from the EMF simulated diffraction orders. Because phase and transmission errors in an alternating PSM are both coupled to the existence of a non-zero central diffraction order, we screened mask topographies according to the zero diffraction order power, relative to power in the first orders. Monitoring the central diffraction order did prove be a useful technique for optimizing topographies because it is a single attribute that correlates to both phase and transmission errors, which are coupled and thus difficult to optimize concurrently. The same topography adjustments from the 300nm pitch optimization were then applied through pitch with fixed 100nm line. Next we summarize the EMF results for each mask compensation technique. Mask types were ranked according to best sum of central diffraction order power through pitch, effectively ranking phase and transmission performance across pitch by mask type. The highest ranking masks were SCAA (with 15nm ARC on chrome and no topography adjustments from ideal) and the asymmetric biased mask (with no ARC but with 40nm increase in each side of shifter space width at mask scale). The lowest performing masks were dual-trench (mainly because of phase errors) and the unadjusted mask (mainly due to transmission errors). Finally we move from EMF to lithographic simulation of the best two masks according to EMF simulation. For SCAA and asymmetric bias we examine the NILS and MEEF (with line size 90nm, 100nm, and 110nm) for 300nm pitch. Responses for the process window analysis include resist linewidth, resist retention, sidewall angle and feature placement. The analysis showed that SCAA and optimized asymmetric bias had identical NILS through focus, but that image CD was less sensitive to focus on a SCAA mask than on an asymmetric biased mask. The MEEF results were 0.9 for both masks, while SCAA had better depth of focus than the asymmetric biased mask for single line sizes. While the asymmetric biased mask is simpler to build with existing mask production processes, it requires EMF simulation to determine optimum topography (as do all the other compensation techniques in this study). SCAA requires a non-standard chrome deposition, but performed well according to lithographic simulations without any EMF simulation and topography adjustment. Both SCAA and asymmetric biased masks, it should be noted, did not require any undercut. Future work aimed at the most promising altPSM mask types is needed to further quantify sensitivity to expected fabrication variations and to gain experience with physical wafer prints.

Robust methodology for state-of-the-art embedded SRAM bitcell design

A design and verification methodology of advanced SRAM bitcell design is described. Dense bitcells, drawn for embedded SRAM memory applications, are drawn and simulated for cell functionality and stability. After first-pass design, lithography correction is determined using analytical and iterative simulation routines. Analytical corrections are tailored to comprehend not only specific tool and material platforms associated with the process technology, but are also optimized to account for process integration issues arising from mask layer to mask layer interactions. Lithography process windows are modeled though simulations based on specific stepper illumination scheme, and material systems. Process integration windows are modeled through overlay of simulated patterns while taking into account process control limits of misalignment and critical dimension. Comparison of simulated to electrical bitcell results are discussed and manufacturability considerations are addressed through electrical responses of bitcell-specific diagnostic test structures.

Development of a sub-100-nm integrated imaging system using chromeless phase-shifting imaging with very high NA KrF exposure and off-axis illumination

Examining features of varying pitch imaged using phase-shifting masks shows a pitch dependence on the transmission best suited for optimum imaging. The reason for this deals with the relative magnitude of the zero and higher diffraction orders that are formed as the exposing wavelength passes through the plurality of zero and 180-degree phase-shifted regions. Subsequently, some of the diffraction orders are collected and projected to form the image of the object. Chromeless Phase-Shift Lithography (CPL) deals with using halftoning structures to manipulate these relative magnitudes of these diffraction orders to ultimately construct the desired projected image. A key feature of CPL is that with the ability to manipulate the diffraction orders, a single weak phase-shifting mask can be made to emulate any weak phase-shifting mask and therefore the optimal imaging condition of any pattern can be placed on a single mask regardless of the type of weak phase-shifter that produces that result. In addition, these structures are used to render the plurality of size, shape and pitch such that the formed images produce their respective desired size and shape with sufficient image process tolerance. These images are typically made under identical exposure conditions, but not limited to single exposure condition. These halftoning structures can be used exterior, as assist features, or interior to the primary feature. These structures can range in transmission from 0% to 100% and they can be phase-shifted relative to the primary features or not. Thus CPL deals with the design, layout, and utilization of transparent and semi-transparent phase-shift masks and their use in an integrated imaging solution of exposure tool, mask and the photoresist recording media. This paper describes the method of diffraction matching, provides an example and reviews some experimental data using high numerical aperture KrF exposure.

Phase phirst! An improved strong-PSM paradigm

The remaining difficulties in applying dual exposure dark-field strong-PSM technology can be overcome using the Sidewall Chrome Alternating Aperture (SCAA) mask structure, first proposed in 1992 and now fabricated. With all silica sidewalls covered and all chrome supported, the SCAA mask is largely immune to the phase and amplitude anomalies that cause spacewidth alternation as well as the design, fabrication and cleaning difficulties that plague other structures. Maxwells equation solvers predict that the optical phase will be essentially independent of aperture size. Chips designed with their fmest features on a pre-defined regular grid can employ generic SCAA mask substrates in which the topography has been pre-patterned using wafer fab techniques. Guaranteed defect-free SCAA mask substrates will be manufactured in large quantity and low cost if the design grids become standardized. Fabricating strong-PSMs using these Phase Phirst mask substrates will prove no more difficult for mask-makers than COG masks, and the reduced MEEF will permit loosened CD specifications, among other advantages. The Phase Phirst Paradigm promises to reduce optical lithography costs —even for ASIC manufactures — and to delay the need for NGL technologies.

Extending KrF to 100-nm imaging with high-NA- and chromeless phase lithography technology

In this paper the concept of chromeless phase lithography (CPL) is introduced and experimental results on an ASML PAS 5500/800 are presented. CPL is a single exposure technique and is capable of resolution enhancement on all device layers (bright and dark field masks). Line space structures through pitch are measured with cross section and have O.35jim depth of focus (DOF) at 10% exposure latitude without forbidden pitches. CPL experimental results for a k1 of 0.38 (½ pitch) are presented for three DRAM device layers, isolation brick wall, storage capacitor, and honeycomb contact. Each of these layers have a DOF of O.35jim at 10% exposure latitude. CPL experimental results are presented for a SRAM gate and contact with lOOnm feature size (k1=O.32) and have a DOF of O.35jim at 10% exposure latitude.

Programmable lithography engine (ProLE) grid-type supercomputer and its applications

There are many variables that can affect lithographic dependent device yield. Because of this, it is not enough to make optical proximity corrections (OPC) based on the mask type, wavelength, lens, illumination-type and coherence. Resist chemistry and physics along with substrate, exposure, and all post-exposure processing must be considered too. Only a holistic approach to finding imaging solutions will accelerate yield and maximize performance. Since experiments are too costly in both time and money, accomplishing this takes massive amounts of accurate simulation capability. Our solution is to create a workbench that has a set of advanced user applications that utilize best-in-class simulator engines for solving litho-related DFM problems using distributive computing. Our product, ProLE (Programmable Lithography Engine), is an integrated system that combines Petersen Advanced Lithography Inc.’s (PAL’s) proprietary applications and cluster management software wrapped around commercial software engines, along with optional commercial hardware and software. It uses the most rigorous lithography simulation engines to solve deep sub-wavelength imaging problems accurately and at speeds that are several orders of magnitude faster than current methods. Specifically, ProLE uses full vector thin-mask aerial image models or when needed, full across source 3D electromagnetic field simulation to make accurate aerial image predictions along with calibrated resist models;. The ProLE workstation from Petersen Advanced Lithography, Inc., is the first commercial product that makes it possible to do these intensive calculations at a fraction of a time previously available thus significantly reducing time to market for advance technology devices. In this work, ProLE is introduced, through model comparison to show why vector imaging and rigorous resist models work better than other less rigorous models, then some applications of that use our distributive computing solution are shown. Topics covered describe why ProLE solutions are needed from an economic and technical aspect, a high level discussion of how the distributive system works, speed benchmarking, and finally, a brief survey of applications including advanced aberrations for lens sensitivity and flare studies, optical-proximity-correction for a bitcell and an application that will allow evaluation of the potential of a design to have systematic failures during fabrication.

Application of CPL reticle technology for the 65- and 50-nm node

Each generation of semiconductor device technology drive new and interesting resolution enhancement technology (RET’s). The race to smaller and smaller geometry’s has forced device manufacturers to k1’s approaching 0.40. The authors have been investigating the use of Chromeless phase-shifting masks (CLM) exposed with ArF, high numerical aperture (NA), and off-axis illumination (OAI) has been shown to produce production worthy sub-100nm resist patterns with acceptable overlapped process window across feature pitch. There have been a number of authors who have investigated CLM in the past but the technology has never received mainstream attention due to constraints such as wet quartz etch during mask fabrication, limited approach to optical proximity correction (OPC), and exposure tool limitations such as on-axis illumination and too low of NA. With novel binary halftone OPC and a capable modern mask making process, it has become possible to achieve global and local pattern optimization of the phase shifter for a given layout especially for patterning features with dimension at sub-half-exposure wavelength. The authors have built a number of test structures that require superior 2D control for SRAM gate structures. In this paper the authors will focus on image process integration for the 65nm node. Emphasis on pattern layout, mask fabrication and image processing will be discussed. Furthermore, the authors will discuss defect printing, inspection and repair, mask error enhancement factor (MEEF) of 2D structures coupled with phase error, layout, and mask fabrication specifications.

Developing an integrated imaging system for the 70-nm node using high numerical aperture ArF lithography

At its conception, 193 nm lithography was thought to be the best way to take optical lithography to the 180 nm node. It was expected that 193 nm could support the now-defunct 160 nm node before optical lithography would have to yield to an undetermined non-optical solution. Today, 193 nm must compete with 248 nm for the 130 nm node and is expected to support lithography until it is replaced by 157 nm at the 70 nm node. Given the challenges facing 157 nm, it is likely that lithographers will attempt to extend the utility of 193 nm to its theoretical limits.

Evaluation of SCAA mask technology as a pathway to the 65-nm node

This study takes an integrated approach utilizing a combination of high NA 193 nm lithography, a sidewall chrome alternating aperture (SCAA) phase shift mask, optical proximity correction (OPC) and customized illumination in an attempt to demonstrate the feasibility of using 193 nm lithography to support the 65 nm node. A SCAA mask was designed and built with line/space patterns ranging in pitch from 300 nm down to 140 nm. A range of mask biases were applied to the zero and pi spaces in order to examine to response of the lithography to a combination of the SCAA approach and asymmetric biasing. In combination to the asymmetric biasing, overlay bracketing was applied in order to measure the chrome overlay tolerances of the mask. Simulations suggested that an unconventionally small sigma of 0.15 would be the optimum coherence for a high 193 nm optical system. A custom 0.15 sigma partial coherence illuminator was, therefore, built and installed in the experimental ASML Micrascan V 0.75 NA 193 nm scanner. Wafers were exposed using 190 nm of 193 nm resist and an organic BARC. The 70 nm 1:1 line/space patterns resolved with a depth of focus of about 0.2 μm. The 75 nm 1:1 line/space patterns showed a 0.3-0.4 μm depth of focus. Both of these process windows were limited by pattern collapse. Addressing the pattern collapse may improve the depth of focus. Comparing mask measurements to wafer measurements show that little or no asymmetric biasing in necessary to balance the pitch. Moreover, the measured pitch was stable over a focus range of at least 0.4 microns demonstrating that any phase imbalance present was not significantly affecting the observed lithography.

Development of a sub-100nm integrated imaging system using chromeless phase-shifting imaging with very high NA KrF exposure and off-axis illumination

Examining features of varying pitch imaged using phase- shifting masks shows a pitch dependence eon the transmission best suited for optimum imaging. The reason for this deals with the relative magnitude of the zero and higher diffraction orders that are formed as the exposing wavelength passes through the plurality of zero and higher diffraction orders that are formed as the exposing wavelength passes through the plurality of zero and 180- degree phase-shifted regions. Subsequently, some of the diffraction orders are collected and projected to form the image of the object. chromeless Phase-Shift Lithography (CPL) deals with using half-toning structures to manipulate these relative magnitudes of these diffraction orders to ultimately construct the desired projected image. A key feature of CPL is that with the ability to manipulate the diffraction orders, a single weak phase-shifting mask can be made to emulate any weak phase-shifting mask and therefore the optimal imaging condition of any pattern can be placed on a single mask regardless of the type of weak phase- shifter that produces that result. In addition, these structures are used to render the plurality of size, shape and pitch such that the formed images produce their respective desired size and shape with sufficient image process tolerance. These images are typically made under identical exposure conditions, but not limited to single exposure condition. These half toning structures can be used exterior, as assist features, or interior to the primary feature. These structures can range in transmission from 0 percent to 100 percent and they can be phase-shifted relative to the primary features or not. Thus CPL deals with the design, layout, and utilization of transparent and semi- transparent phase-shift masks and their use in an integrated imaging solution of exposure tool, mask and the photoresist recording media. This paper describes the method of diffraction matching, provides an example and reviews some experimental data using high numerical aperture KrF exposure.