Frank Laske

Registration measurement capability of VISTEC LMS IPRO4 with focus on small features

The development of the 45-nm node manufacturing process at leading edge mask shops is nearly finished. In order to reach the required registration measurement performance with a precision to tolerance value of P/T=0.25, the measurement error may not exceed 1.2 nm according to ITRS roadmap. This requires the latest generation of registration measurement tools. In addition, the demand for measuring very small features increases - for standard pattern placement measurements, as well as special engineering tasks, e.g., the position measurement of single contact holes. In this work, the error of pattern placement measurement on an LMS IPRO4 is determined using an analysis of variance methodology (ANOVA). In addition we analyze the capability as a function of the critical dimension (CD) of the registration feature. The results are compared to the previous tool generation.

First Measurement Data Obtained On The New Vistec LMS IPRO4

The development of reticles for the 65nm node is already completed and development is ongoing at the leading edge mask shops for 45nm technology node reticles. The specifications for 45 nm node reticles CD homogeneity, CD mean to target, and registration (Overlay) are much tighter than previous generation reticles. As a result of the new requirements new metrology systems will be required which provide significantly improved measurement performance compared to currently available systems. The Vistec LMS IPRO4 is the new generation registration metrology system under final development. Initial performance data of the alpha tool is reported and the planned major hardware and software improvements are described.

In-depth overlay contribution analysis of a poly-layer reticle

Wafer overlay is one of the key challenges for lithography in semiconductor device manufacturing, this becomes increasingly challenging following the shrinking of the device node. Some of Low k1 techniques, such as Double Exposure add additional burden to the overlay margin because on most critical layers the pattern is created based on exposures of 2 critical masks. Besides impact on overlay performance, any displacement between those two exposures leads to a significant impact on space CD uniformity performance as well. Mask registration is considered a major contributor to within-field wafer overlay. We investigated in-die registration performance on a critical poly-layer reticle in-depth, applying adaptive metrology rules, We used Thin-Plate-Splinefit (TPS) and Fourier analysis techniques for data analysis. Several systematic error components were observed, demonstrating the value of higher sampling to control mask registration performance.

Analysis of the Vistec LMS IPRO3 performance and accuracy enhancement techniques

Following the international technology roadmap for semiconductors the image placement precision for the 65nm technology node has to be 7nm. In order to be measurement capable, the measurement error of a 2D coordinate measurement system has to be close to 2nm. For those products, we are using the latest Vistec registration metrology tool, the LMS IPRO3. In this publication we focus on the tool performance analysis and compare different methodologies. Beside the well-established ones, we are demonstrating the statistical method of the analysis of variance (ANOVA) as a powerful tool to quantify different measurement error contributors. Here we deal with short-term, long-term, orientation-dependent and tool matching errors. For comparison reasons we also present some results based on LMS IPRO2 and LMS IPRO1 measurements. Whereas the short-term repeatability and long-term reproducibility are more or less given by the tool set up and physical facts, the orientation dependant part is a result of a software correction algorithm. We finally analyse that kind of residual tool systematics and test some improvement strategies.

Mask registration impact on intrafield on-wafer overlay performance

Improved overlay performance is one of the critical elements in enabling the continuing advancement of the semiconductor integrated circuit (IC) industry. With each advancing process node, additional sources of overlay error and new methods of reducing those errors need to be taken into account. We consider the impact of mask registration or pattern placement errors on intra-field on-wafer overlay performance. Mask registration data is typically minimally sampled and not well incorporated into the wafer fab overlay systems. In this work we consider mask-to-mask overlay and point out the importance of high density sampling as well as the potential for improved mask qualification and disposition.

Measurement tool influence on CD results on photolithographic masks

The precise targeting of critical dimension (CD) features on photolithographic masks is an essential part of the mask production process. It is straight forward that the usual decrease of specification numbers can only be achieved using cutting edge CD Metrology tools. That also implies that the most advanced CD tool might change from node to node and over time mask houses accommodate a small variety of different tools. Therefore, it is an important task of current mask metrology to ensure accurate matching and calibration and also to transfer these standards precisely over time. Here, we investigate the influences of the photolithographic mask material and the resist type on critical dimension measurements utilizing one Atomic Force Microscope (AFM), two CD-Scanning electrical microscopes (CD-SEM) by different suppliers and one optical CD tool. Simulating usual mask house strategies we defined one CD tool as golden tool and measured a 700 A chrome mask on it. This reference measurement was then repeated on all other tools and each of them was matched to the golden tool using standard procedures. Once matching was achieved 5 other masks were measured on all tools with exactly the same settings as the reference measurements. In all we varied the material COG, Mosi193, Mosi248, Chrome thickness 700A and 1000A and different resists. We do observe that calibration within the CD SEM tool class works very well for linearity, but with detectable offset in the range of a couple of nm for different reference masks used. Cross-calibration on the other hand from optical CD to CD SEM tools shows significant differences for process variations, layer thickness and different materials. These findings strongly point out that first of all cross calibration is extremely difficult with current metrology tools and can not be utilized for high end products with the necessary precision. And secondly, even matching within tool classes is material dependent which has to be considered for accurate tool to tool matching.

Evaluation of KLA-Tencor LMS IPRO5 beta system for 22nm node registration and overlay applications

Using various technical tricks, 193nm lithography has been pushed for the 22nm logic node. For optical and EUV lithography, the International Technology Roadmap for Semiconductors (ITRS [1]) requests a registration error below 3.8 nm for masks for single-patterning layers. Double patterning further reduces the tolerable pattern placement error to < 2.7 nm for each mask of a pair that forms one layer on the wafer. For mask metrology on the 2x node, maintaining a precision-to-tolerance (P/T) ratio of 0.25 will be challenging. The total measurement uncertainty has to be significantly below 1.0nm. In this work, results obtained during the LMS IPRO5 beta system evaluation are presented. LMS IPRO5 beta system evaluation is part of the CDUR32 project, funded by the German Federal Ministry of Education and Research. A major improvement to previous LMS IPRO generations is the new laser illumination system, which significantly improves optical resolution and contrast (especially on EUV substrates). Therefore, optical resolution and measurement capability are evaluated using standard registration targets, in-die wafer overlay targets, and arbitrary shaped features on different substrates comprising EUV and binary MoSi masks. Position measurement uncertainty for the new center of gravity (CofG) measurement algorithm, important for in-die measurement capability, is evaluated. The results are compared with results obtained using the traditional edge detection algorithm.

Experimental test results of pattern placement metrology on photomasks with laser illumination source designed to address double patterning lithography challenges

Double Patterning Lithography techniques place significantly greater demand on the requirements for pattern placement accuracy on photomasks. The influence of the pellicle on plate bending is also a factor especially when the pellicle distortions are not repeatable from substrate to substrate. The combination of increased demand for greater accuracy and the influence of pellicle distortions are key factors in the need for high resolution through-pellicle in-die measurements on actual device features. The above requirements triggered development of a new generation registration metrology tool based on in-depth experience with the LMS IPRO4. This paper reports on the initial experimental results of DUV laser illumination on features of various sizes using unique measurement algorithms developed specifically for pattern placement measurements.

Mask contribution to intra-field wafer overlay

Shrinking wafer overlay budgets raise the importance of careful characterization and control of the contributing components, a trend accelerated by multi-patterning immersion lithography [1]. Traditionally, the mask contribution to wafer overlay has been estimated from measurement of a relatively small number of standard targets. There are a number of studies on test masks and standard targets of the impact of mask registration on wafer overlay [2],[3]. In this paper, we show the value of a more comprehensive characterization of mask registration on a product mask, across a wide range of spatial frequencies and patterns. The mask measurements will be used to obtain an accurate model to predict mask contribution to wafer overlay and correct for it.

In-die mask registration for multi-patterning

193nm immersion lithography is the mainstream production technology for the 20nm and 14nm logic nodes. Considering multi-patterning as the technology to solve the very low k1 situation in the resolution equation puts extreme pressure on the intra-field overlay, to which mask registration error is a major error contributor. The International Technology Roadmap for Semiconductors (ITRS) requests a registration error below 4 nm for each mask of a multi-patterning set forming one layer on the wafer. For mask metrology at the 20nm and 14nm logic nodes, maintaining a precision-to-tolerance (P/T) ratio below 0.25 will be very challenging. Mask registration error impacts intra-field wafer overlay directly and has a major impact on wafer yield. We will discuss a solution to support full in-die registration metrology on reticles.