Vladan Blahnik

Speckle in optical lithography and its influence on linewidth roughness

In recent years speckle in optical projection microlithography received increasing interest because of its potential contribution to linewidth roughness (LWR). Speckle is a light interference effect that causes the dose delivered to the reticle to be nonuniform, causing a linewidth variation of the patterns imaged in the resist. The contrast of the speckle pattern is shown to be caused by a combination of temporal and spatial coherence effects of the light. The temporal part, determined by the bandwidth of the laser light and the duration of the laser pulse, is found to be the dominant contributor to speckle in todays ArF optical lithography. The spatial distribution of the speckle pattern depends on the intensity distribution of the light in the pupil. Consequently, the spatial frequencies of the LWR induced by speckle depend on the illumination condition, which is confirmed experimentally by exposing wafers with different amounts of speckle contrast. The experiments demonstrate that the amplitude of the LWR induced by speckle is consistent with theory and simulations. Its amplitude is small compared to other sources of LWR, but it is clearly present and should not be ignored when extending ArF optical lithography into future technology nodes.

Lithographic performance of a dual-stage 0.93NA ArF step and scan system

This paper presents lithographic performance results obtained from the newest member of ASMLs TWINSCAN platform-based step & scan systems, the TWINSCAN XT:1400. The system has been designed to meet the semiconductor industrys aggressive requirements on CD control, overlay and productivity at and below the 65 nm node. This dual stage 193 nm lithographic system combines the worlds highest NA, with excellent overlay and CD control at high throughput on both 200 and 300 mm wafers and is intended for use in volume production environments. Advances in stage technology have enabled further extension of stage scan speeds and an associated increase in tool productivity. However, maximizing the number of yielding die per day also requires stringent overlay and Critical Dimension (CD) control. Tight CD control at improved resolution is supported by the Starlith 1400 projection lens and the extended sigma capabilities of the new AERIAL-E illumination system. Focus control is improved in line with the stringent requirements posed by low-k1 imaging applications, taking full advantage of the unique dual-stage TWINSCAN system architecture.

Radiometric consistency in source specifications for lithography

There is a surprising lack of clarity about the exact quantity that a lithographic source map should specify. Under the plausible interpretation that input source maps should tabulate radiance, one will find with standard imaging codes that simulated wafer plane source intensities appear to violate the brightness theorem. The apparent deviation (a cosine factor in the illumination pupil) represents one of many obliquity/inclination factors involved in propagation through the imaging system whose interpretation in the literature is often somewhat obscure, but which have become numerically significant in todays hyper-NA OPC applications. We show that the seeming brightness distortion in the illumination pupil arises because the customary direction-cosine gridding of this aperture yields non-uniform solid-angle subtense in the source pixels. Once the appropriate solid angle factor is included, each entry in the source map becomes proportional to the total |E|^2 that the associated pixel produces on the mask. This quantitative definition of lithographic source distributions is consistent with the plane-wave spectrum approach adopted by litho simulators, in that these simulators essentially propagate |E|^2 along the interfering diffraction orders from the mask input to the resist film. It can be shown using either the rigorous Franz formulation of vector diffraction theory, or an angular spectrum approach, that such an |E|^2 plane-wave weighting will provide the standard inclination factor if the source elements are incoherent and the mask model is accurate. This inclination factor is usually derived from a classical Rayleigh-Sommerfeld diffraction integral, and we show that the nominally discrepant inclination factors used by the various diffraction integrals of this class can all be made to yield the same result as the Franz formula when rigorous mask simulation is employed, and further that these cosine factors have a simple geometrical interpretation. On this basis one can then obtain for the lens as a whole the standard mask-to-wafer obliquity factor used by litho simulators. This obliquity factor is shown to express the brightness invariance theorem, making the simulators output consistent with the brightness theorem if the source map tabulates the product of radiance and pixel solid angle, as our source definition specifies. We show by experiment that dose-to-clear data can be modeled more accurately when the correct obliquity factor is used.

Speckle in optical lithography and the influence on line width roughness

In recent years the topic of speckle in optical projection microlithography received increasing interest because of its potential contribution to line width roughness (LWR). Speckle is a light interference effect that causes the dose delivered to the reticle to be not uniform. This will cause a line width variation of the patterns imaged in the resist. The contrast of the speckle pattern is shown to be caused by a combination of temporal and spatial coherence effects of the light. The temporal part, determined by the bandwidth of the laser light and the duration of the laser pulse, is found to be the dominant contributor to speckle in todays ArF optical lithography. The spatial distribution of the speckle pattern depends on the intensity distribution of the light in the pupil. Consequently the spatial frequencies of the LWR induced by speckle will depend on the illumination condition, which is confirmed experimentally by exposing wafers with different amounts of speckle contrast. The experiments demonstrate that the amplitude of the LWR induced by speckle is consistent with theory and simulations. Its amplitude is small compared to other sources of LWR, but it is clearly present and should not be ignored when extending ArF optical lithography into future technology nodes.

Evaluation and analysis of chromatic aberrations in images

Optical designers typically use aberration diagrams based on ray tracing to analyze the chromatic aberration performance of their optical system. However to evaluate the impact of chromatic aberrations on color fringes in images optical imaging simulations are necessary including the effect of spectral response of the sensor or film, exposure time, Gamma corrections, etc. We have analyzed the correspondence of classical chromatic aberration measures versus analysis based on image simulations. We propose a metric directly linked to color fringes in images which can be used to constrain and specify the chromatic aberration performance of an optical system.