To realize improved process control at high productivity, the TWINSCAN platform has been developed. This dual stage lithography system combines high throughput TWINSCAN technology with excellent dynamical performance and imaging capabilities required by sub-80nm lithography. The added value of a dual stage system is increased utilization efficiency by performing wafer measurements and other overhead in parallel with exposures thus increasing the net wafers per hour. And additionally these wafer measurements can be done more extensively and create performance advantages of a dual stage system above a single stage system through its predictive and compensation inherent capability. In the measurement position, the wafer surface height is fully mapped using a high spatial frequency measuring level sensor allowing a complete 3-dimensional wafer map to be generated. This allows the wafer surface to be placed optimally in the focal plane of the lens, minimizing the defocus and therefore delivering optimal CD control. In this paper the leveling performance advantage of a dual stage system, proven by several test cases using wafers with known/designed wafer topography, is outlined. One of these cases will address the leveling performance advantage on high topography wafers, which is presented by means of defocus and CD uniformity results. The near ideal leveling performance will be shown by comparison between experimental defocus results and theoretically best achievable defocus given the intrinsic wafer flatness and finite slit size on both inner and edge fields. In contrast to on-the-fly leveling of single stage systems, the separate measurement position eliminates the critical timing relation between the wafer height measurement and actual exposure leveling performance, independent of the exposure scan speed. Besides leveling performance advantages, the dual stage systems full wafer map capability allows in-situ metrology which results in added value, like detailed focus spot monitoring functionality covering all areas to be exposed. Measuring the wafer height before exposure also offers flexibility in the method of deriving the desired stage positioning for exposure and the possibility to extract detailed information for real time wafer flatness monitoring.
John Valley, Noel Poduje, Jaydeep Sinha, Neil Judell, Jie Wu, Marc Boonman, Sjef Tempelaars, Youri van Dommelen, Hans Kattouw, Jan Hauschild, William Hughes, Alexis Grabbe, Les Stanton
Flatness of the incoming silicon wafer is one major contributor to the ultimate focusing limitation of modern exposure tools. Exposure tools are designed to chuck wafers without creating non-flatness and then use focus control to follow as closely as possible the chucked wafer front surface topography. The smaller size of the exposure slit in a step-and-scan exposure tool, as compared to the previous generation full-field stepper tool, helps minimize the impact of chucked wafer non-flat topography. However, to maintain high throughput and improve critical dimension uniformity (CDU) at sub-wavelength line-widths requires continuous improvement in the incoming silicon wafer flatness. In this paper we report extensive experimental results that review existing wafer flatness metrics and propose the addition of a new metric. The new metric emulates the scanning motion of exposure by integrating the defocus that each point on the wafer experiences during exposure. We show that this method is in better spatial agreement with measured defocus in step-and-scan exposure tools. Simple metrics of moving average (MA) defocus prediction analysis will be defined and shown to correlate very well to post exposure defocus data. These experiments were enabled by the creation of special 300-nm wafers by MEMC. These special wafers include sites with a wide variation in flatness. Prior to exposure the wafers were measured with a high-resolution optical flatness metrology tool (WaferSight by ADE) to obtain industry standard thickness variation (flatness) data. Incoming wafer flatness data is used to predict wafer suitability for lithography at the desired device geometry node (e.g., 90 nm). The flatness data was processed and characterized using both standard metrics (SFQR) and the new MA analysis. The relationship between the industry standard metric (SFQR) and similar metrics applied to MA analysis will be presented. Full two-dimensional maps are used to present spatial correlations and permit simple physical insights into the flatness data sets. Measurements of chucked wafer flatness were made on the same wafers using ASML TWINSCAN in-line metrology. These measurements correlate very well to thickness-based flatness. Un-chucked wafer flatness metrics (SFQR and MA) are shown to correlate well to post exposure defocus data when an appropriate site size is used. This result is discussed in relationship to the industry-accepted practice of specifying un-chucked wafer flatness. Lithography performance tests were made to prove the relevance of the different flatness metrics. The same special wafers are used for lithography performance tests. These tests achieve excellent correlation between post-exposure full-wafer focus control results and predictions based on both SFQ (industry standard) and MA re-mapping of the flatness data. The relationship between measured critical dimension (CD) and defocus is also explored. Point-by-point analysis of CD residual versus measured defocus data nicely follows a Bossung curve. We also show that residual CD values predicted from defocus correlate well with measured values. These experiments confirm the application of industry standard wafer flatness measurements to step-and-scan lithography when appropriately using current metrics. They also present the potential for improved metrics based on the MA defocus prediction analysis to help drive continuous improvement of wafer flatness for advanced step-and-scan lithography.
A Phase-Grating Focus Monitor (PGFM) is used to assess the focus control of a state-of-the-art lithography scanner (TWINSCAN AT:1100) over substrate topography. The starting wafer flatness quality is found to be critical in minimizing the overall defocus distribution. In fact, on nearly all wafers, the most significant contributor to defocus across the wafer was the small-scale topography. Results obtained over programmed topography, created by etching various patterns into silicon, are found to agree well with the simulated defocus behavior based on the measurement of the wafer surface obtained on the scanner metrology stage. Finally, we report on preliminary focus control results over realistic device-type substrate topography, involving thin-film and polish effects.
Rian Rubingh, Youri van Dommelen, Sjef Tempelaars, Marc Boonman, Roger Irwin, Edwin van Donkelaar, Hans Burgers, Guustaaf Savenije, Bert Koek, Michael Thier, Oliver Roempp, Christian Hembd-Soellner
To realize high productivity at the 100-nm node, ASML developed the TWINSCAN™ AT:1100B. This dual-stage 193-nm lithography system combines high throughput TWINSCAN™ technology for 300-mm wafers, excellent dynamical performance, and low-aberration 0.75-NA Starlith™ 1100 projection optics. The system is equipped with a 20-W 4-kHz ArF laser and the AERIAL™ II illuminator, enabling high intensity off-axis and multipole QUASAR™ illumination. Important process control requirements for the 100-nm technology node are CD variation across the chip and across the wafer. Full wafer leveling, including dies on the edge of the wafer, and CD uniformity performance on 300-mm wafers with and without topology are presented, showing full wafer CD uniformity numbers as low as 6.3 nm 3σ for 100-nm isolated lines with assisting features. Imaging performance of dense, fully isolated lines plus dense and isolated contact holes is shown. Also, printing of critical customer structures is discussed. With these results it is demonstrated that the TWINSCAN™ AT:1100B 300-mm ArF Step & Scan system meets the requirements for the 100-nm node.
Rian Rubingh, Youri van Dommelen, Sjef Tempelaars, Marc Boonman, Roger Irwin, Edwin van Donkelaar, Hans Burgers, Guustaaf Savenaije, Bert Koek, Michael Thier, Oliver Roempp, Christian Hembd-Soellner
To realize high productivity at the 100 nm node, ASML developed the TWINSCANTM AT:1100B. This dual stage 193 nm lithography system combines high throughput TWINSCANTM technology for 300 nm wafers, excellent dynamical performance, and low aberration 0.75 NA StarlithTM1100 projection optics. The system is equipped with a 20 W 4 kHz ArF laser and the AERIALTM II illuminator, enabling high intensity off axis and multi-pole QUASARTM illumination. Important process control requirements for the 100 nm technology node are CD variation across the chip and across the wafer. Full wafer leveling, including dies on the edge of the wafer, and CD uniformity performance on 300 mm wafers with and without topology are presented, showing full wafer CD uniformity numbers as low as 6.3 nm 3(sigma) for 100 nm isolated lines with assisting features. Imaging performance of dense, fully isolated lines, plus dense and isolated contact holes is shown. Also printing of critical customer structures is discussed. With these results it is demonstrated that the TWINSCANTM AT:1100B 300 mm ArF Step & Scan system meets the requirements for the 100 nm node.
ASML's recently announced TWINSCAN$TM) lithography platform is specifically designed to meet the specific needs of handling and processing 300 mm substrates. This new platform, already supporting a family of Step & Scan lithography systems for I-line and 248 nm DUV, is designed to further support optical lithography at its limits with systems for 193 nm and 157 nm. The conflicting requirements associated with higher productivity on one side, and more extensive metrology on the other, have led to the development of a platform with two independent wafer stages operating in parallel. The hardware associated with exposure, and the hardware and sub-systems required for metrology, are located in two separate positions. While a wafer is exposed on one stage, wafer unload/load and measurements of the horizontal and vertical wafer maps are done in parallel on the second stage. After the two processes are completed, where the exposure sequence typically is the longest, the two stages are swapped. The process is continued on the second stage, while the first stage unloads the exposed wafer and starts the process again.
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