KEYWORDS: Metrology, Process control, Semiconductors, Semiconducting wafers, Overlay metrology, Control systems, Data modeling, Computing systems, Time metrology, Modeling
Reducing overlay error via an accurate APC feedback system is one of the main challenges in high volume production of the current and future nodes in the semiconductor industry. The overlay feedback system directly affects the number of dies meeting overlay specification and the number of layers requiring dedicated exposure tools through the fabrication flow. Increasing the former number and reducing the latter number is beneficial for the overall efficiency and yield of the fabrication process. An overlay feedback system requires accurate determination of the overlay error, or fingerprint, on exposed wafers in order to determine corrections to be automatically and dynamically applied to the exposure of future wafers. Since current and future nodes require correction per exposure (CPE), the resolution of the overlay fingerprint must be high enough to accommodate CPE in the overlay feedback system, or overlay control module (OCM). Determining a high resolution fingerprint from measured data requires extremely dense overlay sampling that takes a significant amount of measurement time. For static corrections this is acceptable, but in an automated dynamic correction system this method creates extreme bottlenecks for the throughput of said system as new lots have to wait until the previous lot is measured. One solution is using a less dense overlay sampling scheme and employing computationally up-sampled data to a dense fingerprint. That method uses a global fingerprint model over the entire wafer; measured localized overlay errors are therefore not always represented in its up-sampled output. This paper will discuss a hybrid system shown in Fig. 1 that combines a computationally up-sampled fingerprint with the measured data to more accurately capture the actual fingerprint, including local overlay errors. Such a hybrid system is shown to result in reduced modelled residuals while determining the fingerprint, and better on-product overlay performance.
The semiconductor industry current standard of focus setup and control can be improved by the implementation of diffraction-based focus (DBF) marks and their applications. Determining best focus per scanner/reticle/device/layer (SRDL) combination is currently done by exposing a focus/energy matrix (FEM) wafer and examining CD features. The drawback of this process of record (POR) method is that the accuracy is greatly influenced by inter- and intra-field effects, focus step size, and machine accuracy. However, DBF marks do not suffer from these drawbacks because they are measured on product and close to the CD features. Experiments confirm that when comparing Bossung curves on each scanner, the wafer-to-wafer variation is much lower using DBF. The setup time and accuracy of new SRDL combinations can also be greatly improved using DBF. DBF uses an asymmetry signal which is translated directly to focus values and is independent of any focus settings of the scanner. After accurately determining the best focus using DBF on only one SRDL combination, the focus setting can be applied to all other combinations and scanners will be matched. Instead of exposing a FEM for each SRDL combination, best focus only needs to be determined once. Experiments using five different machine/reticle combinations show that Bossung tops can be matched with significantly more accuracy compared to POR. Experiments also show a linear relation between energy and shift in Bossung top; both DBF and CD are sensitive to energy variation. When correcting for energy differences, the Bossung top scanner-to-scanner matching accuracy can be improved further. A method using DBF for scanner best focus matching saves up to 10 hours of CD-SEM and manpower setup time per SRDL combination. When a scanner needs to be requalified, the same DBF focus setup method can be used, reducing the scanner downtime.
The National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory is a stadium-sized facility containing a 192-beam, 1.8-Megajoule, 500-Terawatt, ultraviolet laser system together with a 10-meter diameter target chamber with room for nearly 100 experimental diagnostics. Each beam line requires three different large-aperture optics made from single crystal potassium dihydrogen phosphate (KDP). KDP is used in the plasma electrode pockels cell (PEPC) and frequency doubling crystals, while deuterated KDP (DKDP) crystals are used for frequency tripling. Methods for reproducible growth of single crystals of KDP that meet all material requirements have been developed that enable us to meet the optics demands of the NIF. Once material properties are met, fabrication of high aspect ratio single crystal optics (42 × 42 × 1 cm) to meet laser performance specifications is the next challenge. More than 20% of the required final crystal optics have been fabricated and meet the stringent requirements of the NIF system. This manuscript summarizes the challenges and successes in the production of these large single-crystal optics.
The large-aperture (up to 40 cm × 80 cm) mirrors required for the National Ignition Facility have very stringent specifications. The specifications include requirements for transmitted and reflected wavefront over a wide spectral frequency, surface quality, laser resistance, spectral characteristics, etc. In order to validate optic performance, metrology tools were fielded at optic fabrication vendors to assure production control. These tools include interferometers, large-area conditioning stations, and photometers. Of the 1800 large-aperture mirrors required for the NIF, approximately 35% have been completed. This presentation will review the types of large-aperture mirrors used on NIF along with the performance of NIF optics as measured and received from our vendors.
The NIF injection laser system requires over 8000 precision optical components. Two special requirements for such optics are wavefront and laser damage threshold. Wavefront gradient is an important specification on the NIF ILS optics. The gradient affects the spot size and, in the second order, the contrast ratio of the laser beam. Wavefront errors are specified in terms of peak-to-valley, rms, and rms gradient, with filtering requirements. Typical values are lambda/8 PV, lambda/30 rms, and lambda/30/cm rms gradient determined after filtering for spatial periods greater than 2 mm. One objective of this study is to determine whether commercial software supplied with common phase measuring interferometers can filter, perform the gradient analysis, and produce numbers comparable to that by CVOS, the LLNL wavefront analysis application. Laser survivability of optics is another important specification for the operational longevity of the laser system. Another objective of this study is to find alternate laser damage test facilities.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.