Donald W. Pettibone

Wafer printability simulation accuracy based on UV optical inspection images of reticle defects

As semiconductor processes have moved towards lower k1 and mask inspection equipment has moved into the UV range, more subtle reticle defects have been found to cause manufacturing problems. Lower k1 and new lithography processes and reticle technologies, such as OPC and PSM, have made it difficult to determine the significant and these defects. This paper reports on the development of a simulation tool that will improve the yield and productivity of photomask manufacturers and wafer manufacturers by improving reticle defect assessment. This study demonstrates the accuracy of simulation software that predicts resist patterns based on sophisticated modeling software that uses optical images obtained from a state-of-the-art UV optical inspection system. A DUV 4X reduction stepper was used to print a reticle with programmed defects across an exposure/focus matrix, with the minimum feature size being 200 nm. Quantitative comparisons between predicted and measured wafer CDs were made. In summary, it was found that the simulation software based solely on aerial images predicted absolute CDs with limited accuracy, but differential CDs with limited accuracy, but differential CDs, obtained by utilizing both the reference and defect images, were predicted accurately. Comparison of simulations using both reticle SEM images and optical reticle inspection images showed good agreement, demonstrating the accuracy and high resolution of the optical reticle inspection images. Application of differential aerial images to a simple test case showed that it was possible to identify and therefore eliminate a significant number of defects that did not print, thereby improving defect assessment.

Inspection of EUV reticles

This paper presents the results of patterned and unpatterned EUV mask inspections. We will show inspection results related to EUV patterned mask design factors that affect inspection tool sensitivity, in particular, EUV absorber material reflectivity, and EUV buffer layer thickness. We have used a DUV (257nm) inspection system to inspect patterned reticles, and have achieved defect size sensitivities on patterned reticles of approximately 80 nm. We have inspected EUV substrates and blanks with a UV (364nm) tool with a 90nm to a 120 nm PSL sensitivity, respectively, and found that defect density varies markedly, by factors of 10 and more, from sample to sample. We are using this information in an ongoing effort to reduce defect densities in substrates and blanks to the low levels that will be needed for EUV lithography. While DUV tools will likely meet the patterned inspection requirements of the 70 nm node in terms of reticle defect sensitivity, wavelengths shorter than 200 nm will be required to meet the 50 nm node requirements. This research was sponsored in part by NIST-ATP under KLA-Tencor Cooperative Agreement #70NANB8H44024.

UV inspection of EUV and SCALPEL reticles

A UV inspection tool has been used to image and inspect Next Generation Lithography (NGL) reticles. Inspection images and simulations have been used to provide feedback to mask makers so that inspectability of NGL masks can be optimized. SCALPEL masks have high optical contrast and look much the same in reflection as conventional chrome on glass masks do in transmission. EPL stencil masks can be imaged well in reflection, but defects below the top surface, in the cutouts, may not be detectable optically. EUV masks that have been made to date tend to have relatively low contrast, with line edge profiles that are complex due to interference effects. Simulation results show that improved EUV inspection images can be obtained with a low reflectivity absorbing layer and proper choice of buffer layer thickness.

Next-generation lithography mask inspection

KLA-Tencor and industry partners are collaborating on a project for developing early capabilities of inspecting NGL masks. The project, partially funded by NIST as part of the ATP program, is focusing on building a research tool that will provide experimental data for development of a production capable tool. Some of the key technical issues include contrast in transmission and reflection, defect sources and types, and maintaining mask cleanliness in the absence of pellicles. The masks need to be inspected at multiple process stages, starting with unpatterned substrates, and ending with the pattern inspection. System issues include defect sensitivity and inspection time, which need to be balanced.

Inspection and repair issues for Step and Flash Imprint Lithography templates

Step and Flash Imprint Lithography (S-FIL) 1X templates must eventually achieve and maintain the very low defect counts commensurate to current production masks. This requires typically fewer than ten or even no defects over the entire field and to minimize template fabrication costs and techniques must be identified to repair defects on templates when they do occur. We describe inspection and repair methodologies and how it can be applied to the imprint template. For inspection, test patterns etched onto the template enable both a die-to-die comparison, to find nuisance defects, and also calibration of sensitivity to different types of preprogrammed defects. A state of the art deep UV photomask inspection system (KLA-Tencor model 526) can detect these events with about 70 nm threshold for imprint masks using reflection mode contrast. Initial scans are made at various stages of the imprint process: the processed mask, after dicing, and after several imprints. The scans show mostly isolated point defects at a density of ~ 10 to 100 per mm2. To repair defects, studies were undertaken using RAVE’s nm650 tool which is essentially an AFM platform that relies upon a nano-machining technique for opaque defect removal. On S-FIL templates, the standard deviation for depth repairs in quartz from the target depth was found to be 3.1 nm (1σ). The spread in edge placement data for opaque line protrusions was 21.5 nm (1σ). Trench cuts through lines were successfully created with a minimum size of about 55nm. The repairs on the template were verified by imprinting the features on wafers. The range of depth offsets studied (-15 to +15) had no bearing on the imprinting process and the edge placement on wafers replicated the edge placement of the repaired templates. Trench cuts on the template were successfully filled with the imprint monomer and measured slightly larger than the minimum gap size. Finally, the imprinted wafers were used to pattern transfer features into 100nm of oxide.

Optical inspection of NGL masks

For the last five years KLA-Tencor and our joint venture partners have pursued a research program studying the ability of optical inspection tools to meet the inspection needs of possible NGL lithographies. The NGL technologies that we have studied include SCALPEL, PREVAIL, EUV lithography, and Step and Flash Imprint Lithography. We will discuss the sensitivity of the inspection tools and mask design factors that affect tool sensitivity. Most of the work has been directed towards EUV mask inspection and how to optimize the mask to facilitate inspection. Our partners have succeeded in making high contrast EUV masks ranging in contrast from 70% to 98%. Die to die and die to database inspection of EUV masks have been achieved with a sensitivity that is comparable to what can be achieved with conventional photomasks, approximately 80nm defect sensitivity. We have inspected SCALPEL masks successfully. We have found a limitation of optical inspection when applied to PREVAIL stencil masks. We have run inspections on SFIL masks in die to die, reflected light, in an effort to provide feedback to improve the masks. We have used a UV inspection system to inspect both unpatterned EUV substrates (no coatings) and blanks (with EUV multilayer coatings). These inspection results have proven useful in driving down the substrate and blank defect levels.

Defect printability and inspection of EUVL mask

Defect printability and inspection studies were conducted on a programmed EUV defect mask. The mask was fabricated using Ta-based absorber stack on a Mo/Si multilayer coated 6025 plate. The defect pattern contains a variety of types of defects. The defect printing was performed on the Engineering Test Stand (ETS), which is the 0.1 NA EUV scanner at Sandia National Laboratories in Livermore, CA. The result showed that the printability of defects depended on the defect type and that either notches in or protrusions from absorber lines were the first to print. The minimum printable defect size was approximately 15 nm (1X). Defect inspection was performed on a 257-nm wavelength mask inspection system in die-to-die mode. Seventy-eight out of 120 programmed defects were detected when using 50% detection sensitivity. Maximum detection sensitivity was also tried. However, the number of defects is overwhelmed by the nuisance defects. The minimum defect detected was 52 nm in width. Simulations with a 2-D scalar model are used to verify the results.

Integration of Anti-reflection Coatings on EUV Absorber Stacks

Extreme ultraviolet lithography (EUVL) is the leading candidate for next generation lithography with the potential for extendibility beyond the 50-nm node. The inspection contrast of DUV and 193nm optical reticles is essentially 100%; however, EUVL reticles are reflective in nature and do not allow for transmissive inspection. The Mo/Si multilayer (ML) mirror has a reflectivity of 55-60% with 257nm illumination. The reflectivity of the multilayer at the inspection wavelength dictates that the patterned areas of the mask must be dark to achieve high inspection contrast (i.e., 0% reflectivity at the inspection wavelength). Furthermore, the reticle should retain the same tone during the pre-repair stage and the final reticle stage to allow reuse of inspection algorithms and easier defect repair verification. The use of an anti-reflection coating (ARC) on a TaN absorber has been shown . This article will describe additional options for a wide range of anti-reflection coatings and their impact on the design and fabrication of the EUV absorber stack. Both experimental and modeling results will be presented for different absorber stack configurations.

Optical inspection of EUV and SCALPEL reticles

Next Generation Lithography (NGL) reticle inspection poses some difficult problems. The masks dictate that reflection images, rather than the more usual transmission images, be used for inspection. The smaller linewidths and feature sizes of NGL will require the optical inspection images to have better resolution than has been needed for conventional masks. In this paper we present inspection images and inspection results for EUV and EPL programmed defect test reticles using both UV and DUV reticle inspection systems. Our emphasis has been on providing feedback to the mask manufacturing process to help optimize the inspectability of NGL masks, as well as determining whether the required sensitivity for the 100 nm and 70 nm nodes can be met with optical inspection. Simulated and actual images of NGL masks have proven useful in identifying the important factors in optimizing image contrast. We have found that image contrast varies markedly with inspection wavelength, and that the inspection wavelength must be considered in the design of NGL masks if optimum defect sensitivity is to be obtained. This research was sponsored in part by NIST-ATP and KLA-Tencor Cooperative Agreement #70NANB8H44024.

Impact of EUV mask quality on optical inspection sensitivity

EUV masks are exposed at a wavelength of 13.4 nm, but patterned mask inspection will be in the wavelength range of 157 nm to 257 nm. This large mismatch in wavelength raises questions as to whether the defects that are found in inspection will be the defects that print in a EUV exposure tool. This paper addresses part of this question by considering how small certain nuisance defects must be in order to not limit the optical inspection tool’s sensitivity. That is, the tool must be capable of finding critical printing defects and must not find nonprinting defects. A nuisance defect is considered to be one that the inspection tool may be sensitive to, but will not print on a wafer. We have used a 3D Maxwell equation simulator to simulate the inspection images obtained for a variety of nuisance defects of different types and sizes. We have done these calculations assuming that the EUV lithography will be performed at mask dimensions of 200 nm lines and spaces with a 4X mask, so the features would print at 50 nm lines and spaces. We have determined the critical size of such nuisance defects to be 40nm or larger, depending on defect type. Nuisance defects larger than about 40 nm square may limit the inspection tool’s sensitivity to printing defects. The ITRS roadmap specification for patterned defects at the 50 nm node is 40 nm. Therefore, the limit in size for such nuisance defects is not more stringent than the limits that must be met to match the patterned defect size specification. This work should provide guidance in developing a EUV mask specification that ensures that inspection tools will be able to meet the needs of EUV lithography. This work has been sponsored in part by NIST-ATP Cooperative Agreement #70NANB8H44024.