Immersion lithography offers the semiconductor industry the chance to extend the current ArF processes to smaller
nodes before switching to a shorter wavelength. The move to immersion will require increased attention to the
photomask along with new effects influencing the aerial image formation as the numerical apertures (NA) of scanners
move up to at least 0.93 and even higher. Feature sizes on the photomask become comparable or even smaller than the
wavelength and hence act more like wire grid polaris ers which lead to polarisation effects. As of today AIMSTM fab
tools are in operation worldwide. The novel AIMSTM fab 193i tool with a maximum NA of 0.93 is the latest aerial image
measurement system for ArF-lithography emulation down to the 65nm node. Common adjustments are numerical
aperture, illumination type and partial illumination coherence to match the conditions in 193nm scanners. In addition to
non-polarized illumination, the AIMSTM fab 193i allows the user to select linear x and y polarised light for different
settings and types, e.g. off-axis annular, quadrupole or dipole illumination. In this paper the polarisation effects of
different photomask features are explored by comparing measurement results using linear polarised illumination parallel
and perpendicular to line and space patterns and non-polarised illumination. Different MoSiON embedded phase shift
masks have been investigated at the highest possible NA=0.93 and for different half-pitches from 500nm to 260nm, the
latter corresponding to the 65nm node at the wafer level.
The Aerial Image Measurement System, AIMS, for 193nm lithography emulation is established as a standard for the rapid prediction of wafer printability for critical structures including dense patterns and defects or repairs on masks. The main benefit of AIMS is to save expensive image qualification consisting of test wafer exposures followed by wafer CD-SEM resist or wafer analysis. By adjustment of numerical aperture (NA), illumination type and partial coherence (σ) to match any given stepper/ scanner, AIMS predicts the printability of 193nm reticles such as binary with, or without OPC and phase shifting. A new AIMS fab 193 second generation system with a maximum NA of 0.93 is now available. Improvements in field uniformity, stability over time, measurement automation and higher throughput meet the challenging requirements of the 65nm node. A new function, “Global CD Map” can be applied to automatically measure and analyse the global CD uniformity of repeating structures across a reticle. With the options of extended depth-of-focus (EDOF) software and the upcoming linear polarisation capability in the illumination the new AIMS fab 193 second generation system is able to cover both dry and immersion requirements for NA < 1. Rigorous simulations have been performed to study the effects of polarisation for imaging by comparing the aerial image of the AIMS to the resist image of the scanner.
The "AIMS fab 193" tool is an aerial image measurement system for ArF-lithography emulation and is in operation worldwide. By adjustment of numerical aperture, illumination type and partial coherence parameter to match the conditions in 193nm steppers or scanners, it can emulate lithographic exposure tools for any type of reticles such as binary masks, OPC and phase shift structures, down to the 65nm node. The AIMSTM fab 193 allows the rapid prediction of wafer printability of critical features, such as dense patterns or contacts, defects or repairs on masks without the need to prepare real wafer prints using the stepper or scanner. Recently, a high resolution mode has been introduced based on a sophisticated microscope objective, characterized by a high numerical aperture (NA) and large working distance that allows working with pellicle mounted mask. With this lens system a high contrast image with resolution down to 150 nm lines and spaces (L/S) on mask has been demonstrated. In addition to the AIMSTM through-focus mode for printability which is optically equivalent to the latent image in the photo resist of a wafer, the high resolution mode allows the imaging of mask structures in focus and at printing wavelength to review defects or repairs. Such viewing capability is also helpful at the binary stage of a first writing step in the mask manufacturing process. In this work we will present application results for defects and critical features using both, aerial imaging and high resolution mode.
A new, second generation AIMS fab 193 system has been developed which is capable of emulating lithographic imaging of any type of reticles such as binary and phase shift masks (PSM) including resolution enhancement technologies (RET) such as optical proximity correction (OPC) or scatter bars. The system emulates the imaging process by adjustment of the lithography equivalent illumination and imaging conditions of 193nm wafer steppers including circular, annular, dipole and quadrupole type illumination modes. The AIMS fab 193 allows a rapid prediction of wafer printability of critical mask features, including dense patterns and contacts, defects or repairs by acquiring through-focus image stacks by means of a CCD camera followed by quantitative image analysis. Moreover the technology can be readily applied to directly determine the process window of a given mask under stepper imaging conditions. Since data acquisition is performed electronically, AIMS in many applications replaces the need for costly and time consuming wafer prints using a wafer stepper/ scanner followed by CD SEM resist or wafer analysis. The AIMS fab 193 second generation system is designed for 193nm lithography mask printing predictability down to the 65nm node. In addition to hardware improvements a new modular AIMS software is introduced allowing for a fully automated operation mode. Multiple pre-defined points can be visited and through-focus AIMS measurements can be executed automatically in a recipe based mode. To increase the effectiveness of the automated operation mode, the throughput of the system to locate the area of interest, and to acquire the through-focus images is increased by almost a factor of two in comparison with the first generation AIMS systems. In addition a new software plug-in concept is realised for the tools. One new feature has been successfully introduced as "Global CD Map", enabling automated investigation of global mask quality based on the local determination of wafer level CD at multiple pre-defined measurement points on the mask. This feature supports both reticle manufacturing in mask shops and lithographic process analysis in the Wafer Fab environment. Based on the newly developed software the AIMS can speed up mask development in both the design process and OPC verification.
The frequent occurrence of crystal growth defects on the patterned surface and back glass of critical layer reticles in 193nm lithography has been seen at most advanced fabs around the world. While frequent contamination inspections using regimented sample plans help monitor the growth of crystals and protect yield, no clear solutions have been found to eliminate this progressive defect growth. The recently proposed “Advanced Reticle Defect Disposition Process” (ARDD) was applied successfully for the first time. This process employs a high-throughput inspection system based on the STARlight architecture and - after defect reduction through algorithms - a high-resolution AIMS review system, utilizing the newest networked data connectivity to directly exchange inspection report data and review results. The printability of crystal growth defects is highly variable depending on which surface the defects occur, the size of the defects, and the proximity of the defect to a printing pattern. Crystal growth defects can have different transmittance and phase depending on the lithography wavelength and we found in our investigations a significant change in transmission loss depending on lithography settings like NA and sigma. Such effects may result in severe reduction of the process window, and affect yield. Progressive reticle defects have been characterized on a production reticle applying the ARDD process. It is shown that emulating any given stepper/ scanner settings is necessary to measure the effect of these types of defects on transmittance and that through-focus AIMS evaluation is required to accurately assess the printability of crystal growth defects in terms of process window on wafer. Both features are important components of an overall effective and economical reticle monitor strategy, e.g. in order to optimize the reticle cleaning cycles and thus the reticle lifetime.
Reticle inspection and qualification is getting very important due to the overall shrinking feature sizes on chips and CD values less than the exposure wavelength. Mask defects will matter increasingly and successful defect disposition and image qualification is becoming essential to improve yield. Currently ongoing studies demonstrate the beneficial use of AIMSTM* (Aerial Image Measuring System) -besides its application in mask shops like repair verification- for various wafer fab applications like Incoming Qualitiy Check (IQC), Automated Reticle Defect Disposition (ARDD)1, OPC verification or litho process evaluation in engineering without the use of stepper time and image qualification through wafer SEM evaluation.
Among the important questions for the use of an aerial image measuring system is the level on which different tools compare to each other in terms of critical system performance parameters in order to judge the results of the data analysis in a global way.
In this work we conducted a tool to tool comparison study of AIMSTM fab 193 systems investigating parameters like: Normalized illumination uniformity, CD (critical dimension) uniformity over field, and static CD repeatability over time in x- and y-directions. The study is based on the evaluation of a data base collected with typical feature sizes of 1μm on the mask, ensuring with such feature sizes that tool results are independent of mask features being close to the resolution limit or the printability capability. Typical settings are NA = 0.7 and circular sigma = 0.6 on a set of tools in the field as well as in-house. In addition the performance of the tools will be discussed in terms of a specific application, global CD mapping, for use in process control. It can be applied for different use in wafer fab and mask shop environment.
The Aerial Image Measurement System (AIMS)* for 193 nm lithography emulation has been brought into operation successfully worldwide. A second generation system comprising 193 nm AIMS capability, mini-environment and SMIF, the AIMS fab 193 plus is currently introduced into the market. By adjustment of numerical aperture (NA), illumination type and partial illumination coherence to match the conditions in 193 nm steppers or scanners, it can emulate the exposure tool for any type of reticles like binary, OPC and PSM down to the 65 nm node. The system allows a rapid prediction of wafer printability of defects or defect repairs, and critical features, like dense patterns or contacts on the masks without the need to perform expensive image qualification consisting of test wafer exposures followed by SEM measurements. Therefore, AIMS is a mask quality verification standard for high-end photo masks and established in mask shops worldwide. The progress on the AIMS technology described in this paper will highlight that besides mask shops there will be a very beneficial use of the AIMS in the wafer fab and we propose an Automated Reticle Defect Disposition (ARDD) process. With smaller nodes, where design rules are 65 nm or less, it is expected that smaller defects on reticles will occur in increasing numbers in the wafer fab. These smaller mask defects will matter more and more and become a serious yield limiting factor. With increasing mask prices and increasing number of defects and severability on reticles it will become cost beneficial to perform defect disposition on the reticles in wafer production. Currently ongoing studies demonstrate AIMS benefits for wafer fab applications. An outlook will be given for extension of 193 nm aerial imaging down to the 45 nm node based on emulation of immersion scanners.