Yao-Ting Wang

Application of alternating phase-shifting masks to 140-nm gate patterning: II. Mask design and manufacturing tolerances

In this paper we present the results of experimental patterning 140 nm poly gates with double-exposure alternating phase-shifting masks (PSM) using a Nikon EX-1 (KrF, 0.42NA) stepper. We show that: systematic intrafield line width variations can be controlled within 10 nm (3(sigma) ), interfield variations across the wafer to within 6 nm (3(sigma) ), and total variation across the wafer held to within 15 nm (3(sigma) ), with a target k1 factor of k1 equals 0.237 (140 nm target gate lengths). We also present the results of studies addressing several issues related to the production application of alternating PSMs, including mask manufacturing tolerances and full chip PSM design capabilities. We show that, in comparison to conventional binary masks, alternating PSMs reduce the criticality of mask line width control and reduce the sensitivity to mask defects. Furthermore tolerance to PSM phase errors can be significantly improved by placing a chrome regulator between phase-shifters. Automatic, high-speed full chip design of alternating strong PSM is now possible.

Application of alternating phase-shifting masks to 140-nm gate patterning: linewidth control improvements and design optimization

In this paper we show that the problem of intrafield line width variations can be effectively solved through a novel application of alternating phase-shifting mask (PSM) technology. To illustrate its advantages, we applied this approach to produce 140 nm transistor gates using DUV (248 nm wavelength, KrF) lithography. We show that: systematic intrafield line width variations can be controlled to within 10 nm (3 (sigma) ), and variations across the wafer held to within 15 nm (3 (sigma) ), with a target k1 factor of K1 equals 0.237 (140 nm target gate lengths).

Automated design of halftoned double-exposure phase-shifting masks

In our earlier work, we have proposed an efficient algorithm for phase- shifting mask design of an arbitrary IC pattern. Experimental results have shown that the algorithm works well for patterns such as gratings, contact-holes, U-patterns, etc.. In this paper, we use the concept of moment expansion to give a rigorous proof of the well-known fact that two-phase masks such as alternating phase-shifting masks or attenuated phase-shifting masks possess a better depth of focus; thus, our proposed double-exposure masks have depth-of-focus enhancement property. In addition, we also investigate the quantization problems related to our algorithm and develop a new quantization scheme based on the so-called halftoning techniques, which is commonly used in the printing industry to render the illusion of the continuous-tone images on binary output devices such as laser printers. Simulation results have shown that the new quantization scheme performs very well even for much larger and complicated patterns. Currently, empirical verification is underway in cooperation with HP ULSI Research Laboratory, Palo Alto, CA.

Practicing extension of 248-nm DUV optical lithography using trim-mask PSM

It is becoming increasingly clear that semiconductor manufacturers must rise to the challenge of extending optical microlithography beyond what is forecast by the current SIA roadmap. Capabilities must be developed that allow the use of conventional exposure methods beyond their designed capabilities. This is driven in part by the desire to keep up with the predictions of Moores law. Additional motivation for implementing optical extension methods is provided by the need for workable alternatives in the event that manufacturing capable post-optical lithography is delayed beyond 2003. Major programs are in place at semiconductor manufacturers, development organization, and EDA software providers to continue optical microlithography far past what were once thought to be recognized limits. This paper details efforts undertaken by Motorola to produce functional high density silicon devices with sub-eighth micron transistor gates using DUV microlithography. The preferred enhancement technique discussed here utilizes complementary or dual-exposure trim-mask PSM which incorporates a combined exposure of both Levenson hard shifter and binary trim masks.

Depth of focus and the moment expansion.

We introduce the so-called moment expansion of a defocused image as a tool for analyzing and improving the depth of focus in optical imaging. It is shown that a number of previously noted defocus phenomena can be readily derived or explained in terms of moment expansion. Some potential applications of the moment expansion to phase-shifting mask and pupil filter design for optical lithography are also briefly noted.

Phase-shifting masks: automated design and mask requirements

In this paper we present a computationally viable algorithm for the rapid design of phase- shifting masks for arbitrary two-dimensional patterns. Our approach is based on the construction of a class of optimal coherent approximations to partially coherent imaging systems described by the Hopkins model. We show that for partially coherent imaging systems with coherence factor (sigma) <EQ 0.5, the associated approximation error in the image is quite small (< 10%). A fast iterative algorithm is used to generate (suboptimal) phase- shifting masks using the approximate imaging system model. The computational effort required per iteration is O(N log N), where N is the number of discrete image points considered. Analytical results related to practical requirements for phase-shifting masks are also presented. These results address questions related to the number of discrete phase levels required for arbitrary patterns, and provide some insight into alternative phase-shifting strategies. A number of phase-shifting mask design examples are also discussed.

Systematic design of phase-shifting masks

In this paper, we present a systematic method for phase-shifting mask design in coherent optical lithography using approaches based on approximation theory, phase-retrieval, and image extrapolation. This method also provides some insight into the design of enhanced- resolution masks for incoherent and partially coherent systems and suggests a possible strategy for pupil filter design. The optical lithography system is divided into three subsystems: (1) a softlimiter, (2) a squarer, and (3) a bandlimiter. Thus, the phase-shifting mask design is approached via three corresponding subproblems: (1) bandlimited approximation, (2) phase- retrieval, and (3) extrapolation. Using this method, we demonstrate the possibility of composing a large and complicated phase-shifting mask by abutting smaller and simpler masks. Simulation results of phase-shifting design examples are provided to illustrate the method and ideas described here.

Pattern recognition of trench width using a confocal microscope

A pattern recognition algorithm is developed to estimate the top width and bottom width of trenches of semiconductor wafers simultaneously. A confocal microscope is used in this study because of its excellent depth resolution. The signals from the confocal microscope are compared to a set of a priori waveforms with known trench dimensions. An optimal estimate is chosen. Principle component decomposition of the set of waveforms is used to achieve reduced storage and improved computational speed. An interpolation method is used to improve the accuracy of the estimate. Experimental results of deep trenches are provided to demonstrate the approach.