Data and simulation results characterizing the capability of a DUV system to inspect EUV mask blanks and substrates
are reported. Phase defects and particles on multilayer (ML) surfaces, ARC-coated absorber, and substrate material are
considered. Phase defects on a quartz substrate surface are shown. The principle of phase detection is described.
Results demonstrating the Teron 600's readiness for meeting 32nm hp requirements for bump / pit phase defect detection
are shown. Simulations show that the 22-nm node requirement for phase defect detection should be met, assuming a
reduction in the multilayer roughness. Preliminary data on the sensitivity of SiO2 sphere detection on ML and quartz are
reported. Simulation results show relative sensitivities for detecting SiO2 spheres of different diameters on various EUV
materials.
KEYWORDS: Inspection, Extreme ultraviolet, Optical spheres, Particles, Quartz, Photomasks, Silica, Deep ultraviolet, Defect detection, Signal to noise ratio
Data and simulation results characterizing the capability of a DUV system to inspect EUV mask blanks and substrates
are reported. Phase defects and particles on multilayer (ML) surfaces, ARC-coated absorber, and substrate material are
considered. In addition to the previously reported results of inspecting phase defects on multilayer surfaces, phase
defects on a quartz substrate surface are shown. The principle of phase detection is described. Simulations show that the
22-nm node requirement for phase defect detection should be met, assuming a reduction in the multilayer roughness.
Initial inspections of deposited SiO2 spheres show sensitivities of at least 40 nm on ML and quartz; however, the
availability of calibrated spheres of smaller diameters has limited testing below this value. Simulation results show
relative sensitivities for detecting SiO2 spheres of different diameters on various EUV materials.
Line edges on masks are not perfectly smooth and straight due to writer shot placement errors and randomness in photo-resist processes. This mask roughness may affect local CD defects and CD non-uniformity on the printed wafer. We are able to measure some aspects of line edge roughness using line-space patterns and DUV light in an inspection tool. Analyzing inspection images can make visible both edge placement errors with periodic character (writer generated) and more-random, higher-spatial frequency variations (photo-resist process generated). Our technique observes relative edge placement errors of <1 nm. For example, on one mask the periodic peak-to-peak writer errors are 4 nm, the random edge noise has a standard deviation of about 1.3 nm, and there are ~7 nm steps in the edge position, about one per 200 micron mask field. These values are affected by the inspection tool lateral resolution and thus are actually higher than these values. However, this method is useful in monitoring mask relative edge quality.
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.
Alternating Phase Shift Masks (altPSM's) are an option for the production of critical layers at the 100 nm technology node and below. Successful implementation of altPSM's into a wafer manufacturing process depends upon the ability to successfully inspect, disposition and repair defects that occur during the mask manufacturing process. One technique previously described to improve phase defect contrast was the use of simultaneous transmitted and reflected light [1][2]. The previous technique provided for improved phase defect detection in altPSM's produced for the 130 nm node at a 248 nm lithographic wavelength. This work describes the results from a die-to-die inspection method that improves phase defect contrast in transmitted light for altPSM's produced for the 100 nm node at a 193 nm wavelength. The improved phase defect detection technique addresses the challenges of decreasing linewidth/pitch and reduced defect phase resulting from the decrease in lithographic wavelength relative to the inspection wavelength of light. The improved phase defect detection method also provides a method to determine whether a defect is a binary, phase bump or phase divot type of defect. Results are compared against the previous inspection methods. A test vehicle for gathering sensitivity performance data is described along with the results obtained from the inspection system.
Alternating Phase Shift Masks (altPSM’s) are an option for the production of critical layers at the 100 nm technology node and below produced at ArF lithographic wavelength. Successful implementation of altPSM’s depends upon the ability to successfully inspect, disposition and repair defects that occur during the manufacturing process.
One technique previously described to improve phase defect contrast was the use of simultaneous transmitted and reflected light. The previously described technique provided for improved phase defect detection in altPSM’s produced for the 130 nm node at a 248 nm lithographic wavelength. This work describes the results from a die-to-die inspection method that improves phase defect contrast in transmitted light for altPSM’s produced for the 100 nm node at a 193 nm wavelength. The improved phase defect detection technique addresses the challenges of decreasing linewidth/pitch and reduced defect phase resulting from the decrease in lithographic wavelength relative to the inspection wavelength of light. The improved phase defect detection method also provides a method to determine whether a defect is a binary, phase bump or phase divot type of defect. Results are compared against the previous inspection methods. A test vehicle for gathering sensitivity performance data is described along with the results obtained from the inspection system.
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.
This paper discusses the challenges to alternating phase shift mask defect inspection and new approaches for phase defect detection using multiple illumination methods in conjunction with defect detection algorithm modifications. Die-to-die inspection algorithms were developed for the KLA-Tencor 365UV-HR (APS algorithm) and TeraStar SLF27 (TeraPhase algorithm) inspection systems based upon the use of simultaneous transmitted and reflected light signals. The development of an AltPSM programmed test vehicle is described and defect sensitivity characterization results from programmed phase defect reticles are presented. A comparison of the two approaches used for the different inspection systems is discussed. A comparison of TeraPhase to transmitted light only results from a programmed phase defect test mask shows improved phase defect detection results.
Alternating phase shift masks (altPSM) are gaining importance as a reticle enhancement technique to meet the ITRS Litho Roadmap sub-130 nm node line widths. AltPSM fabrication usually involves etching of the quartz substrate in order to form the phase shift structures. Defects can arise during the quartz-etching step from imperfections in the resist image thereby causing various forms of phase shifting defects on the reticle. These reticle phase shift defects can result in printable defects on the wafer. In order to prevent wafer yield loss from occurring, it is necessary to detect and repair the reticle defects. A die-to-die inspection algorithm using simultaneous transmitted and reflected light signals was developed for the KLA-Tencor TeraStar SLF27 inspection system. The algorithm processes the transmitted and reflected light signals in parallel to detect both phase and chrome defects at high speed. One of the several challenges in the use of reflected light for pattern defect detection on alternating phase shift masks is to ignore lithographically insignificant mask process artifacts such as bright chrome 'halos' which may exhibit significant differences between adjacent die. This paper discusses the inspection challenges of alternating phase shift masks. Defect sensitivity characterization results from programmed phase defect reticles are presented.
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.
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.
A laser scanning system designed for inspection of patterned wafers is described. This system addresses the inspection needs for 64 Mb (0.35 micrometers ) and 256 Mb (0.25 micrometers ) DRAM process technologies. The system is capable of detecting contaminant particles and planar pattern defects on memory and logic devices. The throughput of the system is designed for 30 wafers (200 mm in diameter) per hour. The beam at 488 nm is brought to a focal spot and is scanned on the wafer surface using an acousto-optic deflector (AOD). The entire wafer is scanned under oblique illumination in narrow strips in a serpentine fashion. The specular beam is collected and processed in, what we have named, the autoposition sensor (APS) to servo- lock the height position of the wafer during the scan. The system utilizes multiple independent collection channels positioned around the scan line and it is possible to select the polarization of the collected light for enhanced signal-to-background ratio. The engineering tradeoffs for realizing a system with high throughput and sensitivity are formulated and discussed. Calculations ilustrating scattering from submicron size particles under various polarization conditions are shown. These results lead to optimum design for collection optics. The APS channel is described and illustrated by results indicating that it is possible to keep the surface height of the wafer constant to within 0.4 micrometers in the presence of large changes in topography and wafer reflectivity. Results obtained from a range of production wafers demonstrating detection of 0.1 micrometers anomalies on bare wafer, 0.3 micrometers on memory devices, and 0.4 micrometers on random logic structures are presented.
This paper describes a broad range of design issues that influence the performance of optical equipment for in-line inspection of random (logic) and repetitive (memory) patterns. In particular, we describe the angular distribution of signals from defects on a patterned wafer illuminated by a focused optical beam. We analyze the configuration of both illumination and collection optics to maximize the signal to background ratio for the detection of submicron defects on pattern. In addition, we analyze the distribution of the scattered light as a function of pattern periodicity and orientation with respect to the illuminating beam. The advantages of polarization selection and spatial filtering techniques are explored to enhance the detection sensitivity on repetitive and random pattern wafers. From these results we have developed a new patterned wafer inspection system that offers increased sensitivity and improved defect capture.
Visible and ultraviolet light reflectometry provides a fast, convenient, and nondestructive method of characterizing multilayer film structures that include polycrystalline silicon. Reflectance measurements of silicon wafers containing such films have provided information as to the roughness of the poly surface, the thickness of the films, and the optical properties of the poly.
Refractive index variations of film materials are measured using a spectral micro-reflectometer, the Tencor®
TF-1. The principles of thickness and refractive index determination are discussed. An effective medium model of film
materials is applied to calculating refractive indices and their wavelength dependence. Refractive indices for typical
poly-crystalline silicon are given. Compositional and structural inhomogeneities cause refractive index variations.
Neglecting these index variations leads to misinterpretation of film thickness measurements.
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