Armen Kroyan

Modeling the effects of excimer laser bandwidths on lithographic performance

In many respects, excimer lasers are almost ideal light sources for optical lithography applications. Their narrow bandwidth and high power provide tow of the main characteristics required of a light source for high- resolution imaging. However, for deep-UV lithography projection tools with no chromatic aberration in the imaging lens, even the very narrow bandwidth of an excimer laser may lead to image degradation.

Understanding chromatic aberration impacts on lithographic imaging

Recent development of high-precision aberration measurement techniques has enabled in situ characterization of the aberration response to wavelength offset. These measurements show that majority of the reconstructed Zernike terms exhibit some degree of sensitivity to wavelength. Although this dependence diminishes with the increasing order of Zernike polynomial, we consider the cumulative contribution of five Zernike terms, which have the strongest wavelength dependence ( Z2, Z4, Z6, Z8, and Z11 ). The imaging impacts of KrF laser wavelength and spectral bandwidth are investigated using aerial image simulation; the behavior of the process window, mask error enhancement factor (MEEF), image placement, proximity effect, and sidelobe intensity is quantified. In this model, the chromatic aberrations are experimentally measured in a 0.68-NA KrF step-and-scan exposure system using the LITEL aberration test (InspecStep interferometer manufactured by LITEL Instruments, Inc., San Diego, California). The illumination spectrum input is characterized by spectroscopic measurement of a 2-KHz KrF laser source. In the lithography model, it is important to incorporate all of the wavelength-sensitive terms due to the additive contribution to the overall lens aberration balance. As shown previously, the longitudinal and lateral chromatic aberrations (image height and magnification) are the most sensitive to shift in center wavelength and have the strongest contribution to the aerial image modulation. Simulation results show several imaging changes for isolated lines and contact holes with changes in illumination spectrum. However, the rates of change are shown to decrease as bandwidth is reduced well into the subpicometer level. In the case of isolated contacts, the depth of focus (DOF) increases with the increase in bandwidth, however, at the expense of reduced exposure latitude. This suggests that engineering the spectral output of the laser can provide some process enhancement, although careful compromise is needed to utilize any DOF enhancement, since other image metrics including MEEF, side-lobe intensity, and image placement are also affected.

Effects of 95% Integral vs. FWHM Bandwidth Specifications on Lithographic Imaging

Bandwidth of a laser spectrum is generally specified in terms of the full-width-at-half-maximum (FWHM) metric. Another bandwidth specification is based on the 95% integral energy (E95%) of the spectrum. While providing a more complete information about the spectral shape, E95% bandwidth is very sensitive to small changes in spectral background intensity. In this work, both bandwidth specifications and their effects on aerial image properties are evaluated using computer simulations. Also, in order to obtain a more comprehensive understanding of illumination spectrum effects on lithographic imaging, aerial image sensitivity to the shift of central wavelength and to the change of spectral background intensity is investigated. Results show that the overall shape of the laser spectrum is critically important, and that the E95% metric is more suitable for bandwidth specification.

Contribution of polychromatic illumination to optical proximity effects in the context of deep-UV lithography

In this paper, various optical proximity effects are evaluated as a function of spectral properties of excimer laser illumination. Sensitivity of linewidth biasing and line-end pullback to spectral bandwidth and its variations is investigated using computer simulations based on PROLITH software. Studies are performed for isolated and dense lines ranging in size from 150nm to 130nm using projection lens numerical aperture of 0.7 and KrF illumination. Results show that a non-linear, through-pitch critical dimension sensitivity to laser bandwidth variation introduces additional feature biasing, which can not be compensated with optical proximity correction techniques, and can result in an additional shift of the iso-dense bias. Also, line-end pullback of isolated lines exhibits a non-linear response to bandwidth resulting in up to 7nm of pullback per 0.1pm of bandwidth change.

Investigation of cross-field wavefront aberrations of KrF lithography exposure systems as a function of excimer laser bandwidth

Quantification of projection lens aberrations in lithographic exposure systems has gained significant importance due to more stringent critical dimension control and image fidelity requirements. As linewidths shrink, the impacts of wavefront aberrations on imaging become more pronounced. Therefore, minimization of the wavefront aberrations across the image field is desired and has led to the development of a number of measurement approaches. The proposed techniques have been evaluated extensively for characterization and specification of lens systems, adjustments, matching, and periodic control and monitoring of lithography systems for volume production. In this study, we discuss the contribution of excimer laser bandwidth towards lens aberrations. We carry out simulations of the effects of image contrast on conventional projection patterning, to evaluate the degree of aberation-induced linewidth changes depending on image contrast level. Also, experiments have been conducted to measure the response of wavefront error as a function of spectral bandwidth for a 0.6NA stepper and scanner. Depending on the field location, a positive relationship is observed between the measured aberration level and bandwidth. We propose a formalism to correlate the aberration measurement with aberration response to wavelength offset, presented elsewhere.[2] The wavefront error, in this work, is measured using a commercially available in-situ interferometric technique, whose response is largely insensitive to focal plane changes and partial coherence.

Illumination spectral width impacts on mask error enhancement factor and iso-dense bias in 0.6-NA KrF imaging

In this study, process latitude, mask error enhancement factor and iso-dense bias have been experimentally measured as a function of the KrF excimer laser bandwidth. The experiment results are in agreement with photoresist simulations over a range of imaged nominal feature sizes from 120nm to 300nm at 0.6/0.75 NA/(sigma) . The mask error enhancement factor (MEEF) is shown to vary by approximately 2.3 percent for 160nm and 3.3 percent for 150nm isolated lines per 0.1pm of excimer-laser bandwidth, characterized by the full width at half maximum (FWHM). The 180nm line iso-dense bias exhibits a shift of approximately 2nm per 0.1pm FWHM. Under the given process conditions, linear empirical relationships are derived for the dependency of MEEF and iso-dense offset on FWHM excimer-laser spectral width for a range of imaged CDs. Such considerations can be used to augment the existing predictive CD-control estimation and model-based optical proximity correction.

Image-blur tolerances for 65-nm and 45-nm node IC manufacturing

The deployment of 157nm lithography for manufacturing of integrated circuits is faced with many challenges. The 65 and 45nm ITRS nodes, in particular, require that the lithographic imaging technology be pursued to its theoretical limits with full use of the strongest resolution enhancement techniques. Stringent demands are therefore placed on the quality of the imaging optics to attain the optimal image fidelity for all critical IC device structures. Besides aberrations and light scatter in projection optics, image quality is also strongly influenced by the dynamics of the wafer and reticle stage. The tradeoffs involved in increasing scan speeds and exposure slit-widths, to achieve the ever-important productivity improvements as well as aberration, distortion, and pulse-energy averaging, must be carefully gauged against the image quality impacts of scan-induced errors. In this work, we present a simulation methodology, based on incoherent image superposition, for treatment of the general aerial image effects of transverse image-blur in two dimensions. Initial simulations and experimental results from state-of-the-art 193nm scanner exposures are discussed. The requirements for the transverse image stability during a step-and-scan exposure are defined in the context of 193nm and 157nm lithography, based on generalized image contrast and process window criteria. Furthermore, careful consideration of actual mask layout (post resolution enhancement and optical proximity correction) is necessary in order to understand the implications on CD control. Additionally, we discuss the contributors to transverse image blur in scan-and-repeat lithography, and show that the fading requirements for 65nm and 45nm node imaging notably differ from predicted exposure set-up and process contributions in manufacturing. The total fading budget, or tolerance, for the 65nm node is 15nm, and less than 10nm for the 45nm node given the present imaging strategy assumptions. This work concludes that image-blur contributors must be well controlled, and as such are enablers of 65nm and 45nm lithographic imaging.