Gökhan Perçin

Metrology for stepper illumination pupil profile

A technique for measuring the profile of the illumination in the pupil of a lithography projector is presented. The technique is based on exposing pinhole patterns on a wafer at different dose and defocus settings, and processing the scanning electron microscopy (SEM) images of the printed pinholes. The latent image intensity at the edges of the resist patterns equals the dose-to-clear. This establishes a multitude of equations, each of which states that the latent image intensity at a particular field location, dose, and defocus is known. The intensity distribution in the pupil of the illuminator is obtained by solving a large system of such equations, subject to the constraint that the intensity distribution is non-negative. An image processing algorithm based on nonlinear diffusion is used for finding coordinates of points on the edges of resist in SEM images. The results of the inversion for 193-nm stepper with 0.55/0.85 annular illumination and numerical aperture of 0.75 at five exposure field locations are presented.

Building a computational model for process and proximity compensation

Computational models used in process proximity correction require accurate description of lithography and etch processes. We present inversion of stepper and photoresist parameters from printed test structures. The technique is based on printing a set of test structures at different dose and defocus settings, and processing the CD-SEM measurements of the printed test structures. The model of image formation includes: an arbitrary pupil illumination profile, defocus bias, flare, chromatic aberrations, wavefront errors and apodization of the lens pupil; interaction of vector EM waves with the stack of materials on the wafer; and molecular diffusion in photoresist. The inversion is done by minimizing a norm of the differences between CDs calculated by the model and CD-SEM measurements. The corresponding non-linear least square problem is solved using Gauss-Newton and Levenberg-Marquardt algorithms. Differences between the CD measurements and the best fitting model have an RMS error of 1.63 nm. An etch model, separate from the lithography model, is fitted to measurements of etch skew.

Pattern-based full-chip process verification

This paper discusses a novel pattern based standalone process verification technique that meets with current and future needs for semiconductor manufacturing of memory and logic devices. The choosing the right process verification technique is essential to bridge the discrepancy between the intended and the printed pattern. As the industry moving to very low k1 patterning solutions at each technology node, the challenges for process verification are becoming nightmare for lithography engineers, such as large number of possible verification defects and defect disposition. In low k1 lithography, demand for full-chip process verification is increasing. Full-chip process verification is applied post to process and optical proximity correction (OPC) step. The current challenges in process verification are large number of defects reported, disposition difficulties, long defect review times, and no feedback provided to OPC. The technique presented here is based on pattern based verification where each reported defects are classified in terms of patterns and these patterns are saved to a database. Later this database is used for screening incoming new design prior to OPC step.

Process and proximity correction, and verification for extreme ultraviolet lithography

Extreme ultra-violet (EUV) lithography has been planned for high-volume manufacturing (HVM) in 2014 for critical layers of advanced nodes in the semiconductor industry. Process and proximity correction (PPC) and verification is necessary in order to compensate various optical and other process effects in EUV lithography. Since the long-range flare, mask shadowing effect, and lens characteristics all vary throughout the whole mask range, position dependent PPC and verification may be needed for accurate mask pattern synthesis. In this paper, we will study the PPC accuracy. The PPC flow uses a single PPC kernel set and a full-mask flare map for long-range flare correction. The lithography model is calibrated in accordance with this PPC flow. The lithography model is used to perform full-mask correction for the 10nm node test chip mask for BEOL/FEOL short loop flow development. The optimized full-mask patterns were placed on the mask and printed using a 0.25 NA EUV scanner at various focus and dose conditions. Printed wafers were measured by a CD-SEM and compared to post-PPC verification results.

Inverting pupil illumination from resist-based measurements

Computational models used in process proximity correction require accurate description of the pupil illumination function of the lithography projector. Traditional top-hat approximation for pupil illumination function is no longer sufficient to meet stringent CD control requirements of low-k1 applications. The pupil illumination profile can change across the exposure field, contributing to across-field linewidth variation. We present a measurement of the pupil illumination based on exposing pinhole patterns on a wafer at different dose and defocus settings, and processing SEM images of patterns printed in photoresist. The fundamental principle of the method is Abbes formulation of image formation: the intensity-image formed in resist is an incoherent, linear superposition of images each one of which is formed by illuminating the photomask by a single plane-wave. A single plane-wave that is incident on the photomask maps to a single point in the Fourier-transform aperture of the illuminator. The pupil-fill of the illuminator is obtained from SEM images by a model-based method consisting of these steps: First, resist edges in the SEM images are detected by an edge detection algorithm based on Perona-Malik diffusion. Coordinates of the points on the resist edge are obtained with respect to a reference ruler. The image intensity at any resist edge is equal to the dose-to-clear. This provides an equation for the image intensity at each point on the edge of a pinhole image. Multiple values of dose and defocus, and multiple points on each resist edge provide a large system of equations. The result of the inversion for a 193nm 0.75 NA stepper with σ = 0.55/0.85 annular illumination at five exposure field locations is presented. The CD difference between the nominal top-hat illumination and the inverted illumination was up to 1.8 nm for 1:1 line and space features ranging from 100nm to 300nm. Variation of the illumination along the long-dimension of the slit of the scanner caused 0.6 nm of CD variation for the same 1:1 dense lines.