Erik Byers

Etch, reticle, and track CD fingerprint corrections with local dose compensation

Meeting a specific CD uniformity roadmap becomes more and more difficult as different budget components affecting CD uniformity fail to meet their requirements. For example, reticle manufacturing is at the edge of its potential, and hotplates impact CD uniformity by design. Also, etch processes must be balanced between optimal settings for varying structures. While work continues to enhance the performance of individual budget components, applying local exposure dose compensation with a scanner can provide a near-term solution for improving CD uniformity. Within the wafer processing chain, only the scanner has the unique capability to influence the final quality across-field and field-to-field in a controlled manner, making it the most effective tool for compensation. This paper describes the subsystems required for dose compensation and presents a solution that allows full integration into an automated fabrication environment. Examples will show that both the reticle contribution as well as the process-induced across-wafer fingerprint, including etch, can be improved by up to 50 percent. This improvement is demonstrated both on test structures and on memory device layers.

Simulation-based pattern matching using scanner metrology and design data to reduce reliance on CD metrology

Scanner matching based on wafer data has proven to be successful in the past years, but its adoption into production has been hampered by the significant time and cost overhead involved in obtaining large amounts of statistically precise wafer CD data. In this work, we explore the possibility of optical model based scanner matching that maximizes the use of scanner metrology and design data and minimizes the reliance on wafer CD metrology. A case study was conducted to match an ASML ArF immersion scanner to an ArF dry scanner for a 6Xnm technology node. We used the traditional, resist model based matching method calibrated with extensive wafer CD measurements and derived a baseline scanner manipulator adjustment recipe. We then compared this baseline scanner-matching recipe to two other recipes that were obtained from the new, optical model based matching method. In the following sections, we describe the implementation of both methods, provide their predicted and actual improvements after matching, and compare the ratio of performance to the workload of the methods. The paper concludes with a set of recommendations on the relative merits of each method for a variety of use cases.

A study on the automation of scanner matching

Scanner matching based on CD or patterning contours has been demonstrated in past works. All of these published works require extensive wafer metrology. In contrast, this work extends a previously proposed optical pattern matching method that requires little metrology by adding the component requirements and the procedure for creating an automation flow. In a test case, we matched an ASML XT:1900i using a DOE to an ASML NXT:1950i scanner using FlexRay. The matching was conducted on a 4x nm process test layer as a development vehicle for the 2x nm product nodes. The paper summarizes the before and after matching data and analysis, with future opportunities for improvements suggested.

Improving aberration control with application specific optimization using computational lithography

As the industry drives to lower k1 imaging we commonly accept the use of higher NA imaging and advanced illumination conditions. The advent of this technology shift has given rise to very exotic pupil spread functions that have some areas of high thermal energy density creating new modeling and control challenges. Modern scanners are equipped with advanced lens manipulators that introduce controlled adjustments of the lens elements to counteract the lens aberrations existing in the system. However, there are some specific non-correctable aberration modes that are detrimental to important structures. In this paper, we introduce a methodology for minimizing the impact of aberrations for specific designs at hand. We employ computational lithography to analyze the design being imaged, and then devise a lens manipulator control scheme aimed at optimizing the aberration level for the specific design. The optimization scheme does not minimize the overall aberration, but directs the aberration control to optimize the imaging performance, such as CD control or process window, for the target design. Through computational lithography, we can identify the aberration modes that are most detrimental to the design, and also correlations between imaging responses of independent aberration modes. Then an optimization algorithm is applied to determine how to use the lens manipulators to drive the aberrations modes to levels that are best for the specified imaging performance metric achievable with the tool. We show an example where this method is applied to an aggressive memory device imaged with an advanced ArF scanner. We demonstrate with both simulation and experimental data that this application specific tool optimization successfully compensated for the thermal induced aberrations dynamically, improving the imaging performance consistently through the lot.

Process transfer strategies between ASML immersion scanners

A top challenge for Photolithographers during a process transfer involving multiple-generation scanners is tool matching. In a more general sense, the task is to ensure that the wafer printing results in the receiving fab will match or even exceed those of the originating fab. In this paper we report on two strategies that we developed to perform a photo process transfer that is tailored to the scanners capabilities. The first strategy presented describes a method to match the CD performance of the product features on the transferred scanner. A second strategy is then presented which considers also the down-stream process tools and seeks to optimize the process for yield. Results presented include: ASML TWINSCANTM XT:1700i and XT:1900i scanners 1D printing results from a line-space test reticle, parametric sensitivity calculations for the two scanners on 1D patterns, simulation predictions for a process-optimized scanner-matching procedure, and final wafer results on 2D production patterns. Effectiveness of the optimization strategies was then concluded.

Characterization of array CD uniformity with respect to pattern density in 193nm dry photolithography

As we move toward printing sub-100nm features using 193nm dry photolithography with high-contrast photoresists, effects of mask transmission and pattern density start to play an important role in critical dimension uniformity (CDU). With these two factors in existence, the linewidth for a dense feature block gradually increases from the center to the edge of the array of the block. This change in CD is typically observed for low-transmission reticles. In this paper, we have characterized variables, such as reticle tone and resist processing parameters, which have an effect on the CD uniformity. Use of high-contrast photoresist can increase the effect of chemical flare and can have higher CDU. We have further shown that by using a topcoat or by making changes in the resist bake temperature and time, the effect of chemical flare can be reduced. We also propose a mechanism by which resists exhibit this characteristic and show that both the photoacid generator and quencher can contribute to chemical flare.

Process margin improvement using custom transmission EAPSM reticles

Low k1 lithography poses a number of challenges to the process development engineer. Although polarization and immersion lithography will allow us to create processes at lower k1 than previous paradigms allowed, the lithographer will quickly be looking for Resolution Enhancement Techniques (RET) to push to the ultra-low k1 regime, or to extend older generation tools and avoid the aforementioned expensive options. Reticle transmission is a RET that can enable a low k1 process by increasing image contrast. With work performed in conjunction with our MP Mask facility, we have been able to obtain custom-transmission EAPSM reticles. Reticle transmission optimization can be carried out through simulation. Optimum transmission varies depending on optical parameters and feature size. Moreover, when working with 2D patterns, reticle transmission can be optimized for weaker features, without significantly sacrificing image contrast on primary features. Process improvement by optimizing reticle transmission will be explored for a variety of device types using both 248nm and 193nm lithography. Simulation, custom-transmission reticle fabrication, and empirical wafer results will be presented.