Nakgeuon Seong

Integrating DfM components into a cohesive design-to-silicon solution (Invited Paper)

Two primary tracks of DfM, one originating from physical design characterization, the other from low-k1 lithography, are described. Examples of specific DfM efforts are given and potentially conflicting layout optimization goals are pointed out. The need for an integrated DfM solution than ties together currently parallel DfM efforts of increasing sophistication and layout impact is identified and a novel DfM-enabling design flow is introduced.

Impact of resist blur on MEF, OPC, and CD control

This paper will consider the basic concepts of resist blur in a chemically amplified resist process, and the implications of this blur to lithography. In particular, use of a double Gaussian form for the resist blur will be explored. A simple lithographic model utilizing a double Gaussian resist blur was developed and applied to the rapid calculation of lithographic CDs. A typical gate patterning problem was modeled, both with and without assist features, using several different resist blur functions. The OPC treatment was found to be profoundly affected by the resist blur, especially the long-range component. The MEF of small pitch patterns was a sensitive indicator of the short-range blur. The rapid modeling capability allowed large Monte Carlo simulations to explore CD variation at different pitches, pointing out pitches that were particularly vulnerable to CD variation.

Immersion lithography: New opportunities for semiconductor manufacturing

Immersion lithography has recently emerged as the preferred lithography solution for manufacturing the next generation of semiconductor devices (likely to address the 65, 45, and possibly the 32nm nodes). Full-field immersion scanners operating at λ=193nm with de-ionized water as the immersion fluid have been recently demonstrated. In this article we report imaging results from the AT1150i prototype, a 0.75 numerical-aperture full-field scanner from ASML. We experimentally confirm the depth-of-focus improvements that immersion enables, and explore the implications of this gain for semiconductor manufacturing. This article also highlights the challenges the technology faces before it can be successfully introduced for semiconductor manufacturing. We pay particular attention to defects, in the form of particles, bubbles, and other processing residues, and highlight evaporation as a key mechanism underpinning these challenges.

High-NA lithographic imagery at Brewster's angle

Recent advances have enabled exposure tool manufacturers to ship tools with Numerical Aperture (NA) equals 0.8, and to envision optics with even larger NA. Thus the lithography community must grapple with images formed by oblique waves close to Brewsters angle. (For a typical chemically amplified resist with index of refraction n equals 1.7, Brewsters angle is 59 degree(s), corresponding to NA equals 0.86.) This paper will consider some of the surprising phenomena that occur at such high NA. Both vector diffraction simulation results and experimental results from the IBM interferometric lithography apparatus will be discussed. One of the most interesting modeling predictions is that, near Brewsters angle, the swing curve for TM polarization is much smaller than normal, while the swing curve for TE polarization is much larger than normal, and experimental measurements verify this prediction. Special image cross sections using the Flagello decoration method will also demonstrate the loss of TM image contrast due to vector imaging effects.

Global optimization of the illumination distribution to maximize integrated process window

This paper extends our previous work on globally optimizing source shapes for lithography. A key extension is our global optimization against metrics that involve process window through focus. For example, the user can determine the particular source shape which maximizes the area of the ED window (common exposure-defocus window) across all patterns. In nominal terms, integrated process window is a highly nonlinear objective function; for example, ED window is defined in terms of fractional (i.e. percentage or relative) exposure latitude, and dose is proportional to the reciprocal of intensity, which means that when ED window is calculated the source variables appear in both numerator and denominator of a ratio of reciprocals. In addition, exposure and focus latitudes are defined in terms of the common window as bounded by all features, and the determination of which features are gating is a conditional and non-differentiable function of the source variables. Also, the focus integration should only extend to the plane where ED window first closes down to zero; this limit also depends on the variables in a nonlinear way. However, despite these complexities, it proves possible under quite benign approximations to reformulate ED window maximization as a near-linear-programming problem that can be solved globally, in polynomial time. The algorithm can be extended in several ways, e.g. to account for effects like mask linewidth errors (MEF). In some cases MEF-optimized sources can substantially reduce the sensitivity to mask error, and may differ appreciably from sources optimized for individual perturbed masks. Resist effects can be approximated by influence/diffusion kernels operating on the exposing image within the film. The area of an inscribed rectangular process band can be optimized in place of the full ED window. Source pixelation can be structured to account for finite illuminator resolution and constraints on minimum pole size. Multiple exposures can also be handled, and polarization can be selected optimally on a pixel-by-pixel basis.

High numerical aperture lithographic imagery at the Brewster angle

Recent advances have enabled exposure tool manufacturers to ship tools with numerical aperture (NA) = 0.8, and to envision optics with even larger NA. Thus the lithography community must grapple with images formed by oblique waves close to the Brewster angle. (For a typical chemically amplified resist with index of refraction n = 1.7, the Brewster angle is 59°, corresponding to NA = 0.86.) In this paper we will consider some of the surprising phenomena that occur at such high NA. Both vector diffraction simulation results and experimental results from the IBM interferometric lithography apparatus will be discussed. One of the most interesting modeling predictions is that, near the Brewster angle, the swing curve for transverse magnetic (TM) polarization is much smaller than normal, while the swing curve for transverse electric (TE) polarization is much larger than normal, and experimental measurements verify this prediction. Special image cross sections using the Flagello decoration method will also demonstrate the loss of TM image contrast due to vector imaging effects.

Global optimization of masks, including film stack design to restore TM contrast in high NA TCC's

We provide an expanded description of the global algorithm for mask optimization introduced in our earlier papers, and discuss auxiliary optimizations that can be carried out in the problem constraints and film stack. Mask optimization tends inherently to be a problem with non-convex quadratic constraints, but for small problems we can mitigate this difficulty by exploiting specialized knowledge that applies in the lithography context. If exposure latitude is approximated as maximization of edge slope between image regions whose intensities must print with opposite polarity, we show that the solution space can be approximately divided into regions that contain at most one local minimum. Though the survey of parameter space to identify these regions requires an exhaustive grid search, this search can be accelerated using heuristics, and is not the rate-limiting step at SRAM scale or below. We recover a degree of generality by using a less simplified objective function when we actually assess the local minima. The quasi-binary specialization of lithographic targets is further exploited by searching only in the subspace formed by the dominant joint eigenvectors for dark region intensity and bright region intensity, typically reducing problem dimensionality to less than half that of the full set of frequency-domain variables (i.e. collected diffraction orders). Contrast in this subspace across the bright/dark edge will approximately reflect exposure latitude when we apply the standard fixed edge-placement constraints of lithography. However, during an exploratory stage of optimization we can define preliminary tolerances which more explicitly reflect constraints on devices, e.g. as is done with compactor codes for design migration. Our algorithm can handle vector imaging in a general way, but for the special case of unpolarized illumination and a lens having radial symmetry (but arbitrary source shape) we show that the bilinear function which describes vector interference within the film stack can be expressed in terms of three generic radial functions, enabling rapid numerical evaluation of the Hopkins kernel. By inspection these functions show that one can in principle recover classical scalar-like imaging even at high NA by exposing a very thin layer spaced above a reflective substack. The reflected image largely restores destructive interference in TM polarized fringes, if proper phasing is achieved. With an ideal reflector, the first-order azimuthal contrast loss term vanishes in all TCC components, and complete equivalence to scalar imaging is obtained in classical two-beam imaging.

Image fidelity improvement through optical proximity correction and its limits

Lack of image fidelity, such as corner rounding and line end foreshortening, can have adverse effects on semiconductor devices and circuits, and its magnitude is of interest to lithography integration into the device flow. Yet corner rounding is rarely quantified. The question arises which fraction of the problem can be corrected by optical proximity correction, and which fraction cannot be corrected because of the spatial frequency limitation of the image transfer process. Image fidelity problems typically get worse with highly coherent illumination settings that are used for alternating phase shifting masks, so it is important to investigate corner rounding in connection with such masks. Equally, it is important to understand the impact of numerical aperture on corner rounding. Because of its simple shape, a corner lends itself to simulated and experimental evaluation. We propose a metrology algorithm for corner rounding and investigate it with simulation and experiment. We study the impact of optical settings, mask parameters, and serifs on corner rounding and discuss the impact on optical proximity correction.

Assessing the impact of intrinsic birefringence on 157-nm lithography

Birefringence can be represented using the matrix generated by multiplying together Jones matrices for the separate lens elements. Conventional vector imaging methods, which use orthogonal electric field components in resist combined to yield the intensity, can be extended to handle this matrix representation of the optical pupil. The mean amplitude ratio of the off-diagonal elements in the matrix pupil is shown to correspond quite well to the birefringence-induced CD error.

Full field imaging with a 157-nm scanner

157 nm has been explored as a lithographic technology for several years on small field imaging tools with encouraging results. Significant progress has occurred in tool platform design, resist performance, and optical material quality. However, a major test of a new lithography comes when full field, scanned images can be produced as this becomes a crucial test of system performance and uniformity. We report on early results from a 22 mm x 26 mm (slot x scan) field Micrascan VII 157 nm lithography scanner obtained using a binary reticle. In addition, a full field alternating phase shift reticle was fabricated on modified fused silica1 and used to extend the imaging capability. Resolution and uniformity data from both reticles will be presented. The lithographic performance will also be compared to simulations using predicted performance from the scanner.