Lloyd Litt

Improved prediction of across chip linewidth variation (ACLV) with photomask aerial image CD metrology

Critical dimension (CD) metrology is an important process step within the wafer fab. Knowledge of the CD values at resist level provides a reliable mechanism for the prediction of device performance. Ultimately tolerances of device electrical performance drive the wafer linewidth specifications of the lithography group. Staying within this budget is influenced mainly by the scanner settings, resist process and photomask quality. At the 65nm node the ITRS roadmap calls for sub-3nm photomask CD uniformity to support a sub-3nm wafer level CD uniformity. Meeting these targets has proven to be a challenge. What can be inferred from these specifications is that photomask level CD performance is the direct contributor to wafer level CD performance. With respect to phase shift masks, criteria such as phase and transmission control are also tightened with each technology node. A comprehensive study is presented supporting the use of photomask aerial image emulation CD metrology to predict wafer level Across Chip Linewidth Variation (ACLV). Using the aerial image can provide more accurate wafer level prediction because it inherently includes all contributors to image formation such as the physical CD, phase, transmission, sidewall angle, and other material properties. Aerial images from different photomask types were captured to provide across chip CD values. Aerial image measurements were completed using an AIMSTMfab193i with its through-pellicle data acquisition capability including the Global CDU MapTM software option for AIMSTM tools. The through-pellicle data acquisition capability is an essential prerequisite for capturing final CD data (after final clean and pellicle mounting) before the photomask ships or for re-qualification at the wafer fab. Data was also collected on these photomasks using a conventional CD-SEM metrology system with the pellicles removed. A comparison was then made to wafer prints demonstrating the benefit of using aerial image CD metrology.

High transmission mask technology for 45nm node imaging

The lithography prognosticator of the early 1980’s declared the end of optics for sub-0.5μm imaging. However, significant improvements in optics, photoresist and mask technology continued through the mercury lamp lines (436, 405 & 365nm) and into laser bands of 248nm and to 193nm. As each wavelength matured, innovative optical solutions and further improvements in photoresist technology have demonstrated that extending imaging resolution is possible thus further reducing k1. Several authors have recently discussed manufacturing imaging solutions for sub-0.3k1 and the integration challenges. The requirements stated in the ITRS roadmap for current and future technology nodes are very aggressive. Therefore, it is likely that high NA in combination with enhancement techniques will continue further for aggressive imaging solutions. Lithography and more importantly “imaging solutions” are driven by economics. The technology might be extremely innovative and “fun”, however, if its too expensive it may never see the light of scanner. The authors have investigated and compared the capability of high transmission mask technology and image process integration for the 45nm node. However, the results will be graded in terms of design, mask manufacturability, imaging performance and overall integration within a given process flow.

Tunable transmission phase mask options for 65/45nm node gate and contact processing

Today the industry is filled with intensity-balanced c:PSM and much more focus is being placed on innovative approaches such as CPL (and in conjunction with IML for Contacts) and tunable transmission embedded attenuating phase shift mask (TT-EAPSM). Each approach has its own merits and demerits depending on the manufacturing strategy and lithography performance required. Currently the only commercially available photomask blanks are different chrome thickness binary and 6% attenuating blanks using molybdenum-silicide, making the accessibility to alternate transmissions much more challenging. This paper investigates the mask manufacturability of a tunable transmission embedded attenuating phase shift mask. New film materials that are used in the mask blank manufacture are modeled, deposited and characterized to determine its ability to meet performance requirements. Sputtering models, by rate and gas component, determines film stacks with tunable transmissions and thicknesses. Chemical durability, etch selectivity and thickness are a few parameters of the films that have been characterized to enhance the manufacturability and process reliability of the masks. Lithography simulation models using measured optical properties were developed and test masks that include actual device designs were fabricated. Analysis of CD variation, pattern fidelity and process margin was performed using 3D mask simulation to understand the impact on 65nm design rules. Feasibility and performance of tunable transmission photomasks for use in design and lithography are verified. Moreover, the mask manufacturability and lithography performance is compared to other enhancement techniques and their merits presented.

The impact of illumination on feature fidelity for CPL mask technology

Various types of line ends have been evaluated for either straight CPL mask or hybrid type builds. The authors will focus on image line end shortening and the impact of through dose and focus performance for very high NA ArF imaging. Simulations on test structures have been calculated along with in photoresist simulations to predict the impact on process window capability. Test structures have been designed and fabricated into a functional test for evaluation. Process evaluations have been completed and exposure-defocus window calculated.

Strategies of optical proximity correction dedicated to chromeless phase lithography for 65 and 45 nm node

This paper shows the capability of chromeless phase lithography (CPL) and is particularly focused on different strategies for optical proximity corrections (OPC). A chromeless phase database is easily obtained from the original layout by changing the chromium pattern into a phase pattern. However, a specific optical proximity correction has to be applied due to the phase effect and the high transmission of the mask. Mask Error Enhancement Factor (MEEF) and process window for CPL technology have been estimated through wafer exposures. Moreover, various optical proximity correction strategies have been explored through a comparison between phase and chromium features such as hammerhead, zebra and scattering bars 1,2. Indeed, depending on the density of the pattern, we can improve the contrast and the process window by changing the local transmission. The transmission can be controlled by the addition of sub resolution chromium feature such as zebra chromium transverse features on the line for dense pattern, or chromium scattering bars in the space for a sparse pattern, or chromium patches on the line end. From 65 nm node measurements and 45 nm node simulations, the authors will then present the most effective sub resolution pattern to implement.