Stewart A. Robertson

Comparison of simulation approaches for chemically amplified resists

Lithography simulators have become a standard tool in industrial and governmental research and development departments. IN contrast to the modeling approaches for the optical system and for the lithographic performance of i- line resists, there is still no consensus on the modeling of chemically amplified resist (CAR). Existing models differ in the description of the kinetics and the diffusion phenomena during post exposure bake and in the specification of the development rate. A modeling approach was established, that combines the light induced generation of photoacid, in- and out-diffusion of acid or base components, a generalized deprotection kinetics, Fickian and non-Fickian diffusion of resist components and an arbitrary development rate model. Existing models such as the effective acid model and a standard deprotection model for CAR can be considered as special cases of the implemented model. To evaluate the importance of certain options of the model and of the model parameters we have evaluated the performance of the model by comparing simulated CD data and resists profiles with experimental data.

Effects of Mask bias on the Mask Error Enhancement Factor (MEEF) of contact holes

The effects of mask bias on the Mask Error Enhancement Factor (MEEF) of 180 nm contact holes is studied through lithographic simulation using commercial software and a DUV (248 nm) ESCAP photoresist model. Dense contacts show higher MEEF than isolated or semi-dense contacts. However, dense features exhibit a minimum in MEEF at a single negative mask bias (CD on reticle > 180 nm). Aerial image simulations indicated that low MEEF correlates approximately with high normalized aerial image log-slope (NILS). Hence, factors that affect NILS, such as numerical aperture, partial coherence, and wavelength, also influence MEEF, although without altering the optimum mask bias for minimum dense MEEF. Numerical aperture and wavelength of exposure have the greatest influence on MEEF. For 180 nm contact holes worst case MEEF values below 2 can be achieved by increasing NA to 0.8 at 248 nm or by decreasing (lambda) to 193 nm at 0.6 NA. Resist identity has little influence on the magnitude of MEEF but was the only factor affecting the mask bias setting for minimum dense MEEF.

Resist pattern prediction at EUV

Accurate and flexible simulation methods may be used to further a researchers understanding of how complex resist effects influence the patterning of critical structures. In this work, we attempt to gain insight into the behavior of a state-of-the-art EUV resist through the use of stochastic resist simulation. The statistics of photon and molecule counting are discussed. A discrete, probabilistic ionization and electron scattering simulator for acid generation at EUV is discussed. At EUV, acid generators are hypothesized to be activated by secondary electrons yielded by ionization of the resist upon absorption of photons. Model fit to experimental data of mean CD and LWR for a state-of-the-art EUV resist is shown.

Statistical simulation of resist at EUV and ArF

Requirements of resist modeling strategies for EUV and low-k1 ArF nanolithography continue to become more stringent. Resist designers are consistently faced with the task of reducing exposure dose and line roughness while simultaneously improving exposure latitude, depth-of-focus and ultimate resolution. In this work, we briefly discuss a next-generation resist model for the prediction of statistical resist responses such as line-edge roughness, line-width roughness and CD variability, as well as base lithographic responses such as exposure latitude. The models parameterized fit to experimental data from a state-of-the art polymer-bound PAG resist irradiated at ArF and EUV will be shown. The probabilistic computation of acid generation at ArF and EUV will be discussed. The factors influencing the hypothesized primary cause of resist roughness, acid shot noise, are discussed.

Mechanistic Simulation of Line-Edge Roughness

Physically-based photoresist models, such as those in PROLITH, have been very successful in describing photolithography from a continuum standpoint. These models allow engineers to accurately predict the final resist CD on the wafer and to analyze process robustness. However, as the critical dimension continues to shrink, yield-limiting phenomena are observed that are related to the molecular nature and reaction kinetics of photoresist materials. An example of these phenomena is line-edge roughness (LER). In this paper, the origin of LER is hypothesized to be caused by fluctuations occurring in the initial position of the reactants, fluctuations during the exposure process (shot noise) and fluctuations occurring during thermally-induced reaction-diffusion (post-exposure bake). We have developed a lattice-based mechanistic simulator to better understand the stochastic nature of reactant initial position, the exposure step, the importance of the discrete nature of the reactants, the coupling to the deprotection kinetics and the deep complexity evident in the diffusion-limited acid-quencher reaction.

Toward a universal resist dissolution model for lithography simulation

In lithography simulation dissolution rate equations are used to map development rate to the resist latent image. This work examines the quality of fit of four rate equations to experimental dissolution data for a wide variety of different resists ranging from medium contrast i-line novolak/DNQ materials to the state-of-the-art 248nm and 193nm chemically amplified photoresists. Three of the rate equations are routinely used for modeling: the Mack rate equation, the Enhanced Mack rate equation, and the Notch rate equation. The fourth is the recently developed Enhanced Notch model. Although each class of photoresist can be fitted reasonably well by one of the conventional rate equations, the Enhanced Notch model yields the best fit to the experimental data in all cases.

Improved notch model for resist dissolution in lithography simulation

The use of experimental development rate information is used to demonstrate various deficiencies in the dissolution rate equations commonly employed in commercial lithography simulation programs. An improved version of the Notch dissolution rate equation, incorporating one new parameter, is proposed, which addresses the observed deficiencies. Simulation work comparing the new equation to the standard Notch model reveals significant differences in process window and exposure margin, yet negligible changes in feature profile and iso-dense bias at best focus and exposure.

Stochastic simulation of resist linewidth roughness and critical dimension uniformity for optical lithography

The physical processes that underpin a recently developed commercial stochastic resist model are introduced and the model details discussed. The model is calibrated to experimental data for a commercially available immersion chemically amplified photoresist using basic physical information about the resist and an iterative fitting procedure. This data comprises CD (critical dimension) and LWR (linewidth roughness) measurements through focus and exposure for three separate line-type features on varying pitches: dense, semidense, and isolated. A root mean square error (RMSE) of 2.0 nm is observed between the calibrated model and the experimental CD data. The ability of the calibrated model to predict experimentally observed CD uniformity distributions is tested for a variety of 1-D and 2-D patterns under fixed focus and exposure conditions. The subnanometer RMSE obtained between experiment and simulation suggests that the calibrated stochastic model has excellent predictive power for a variety of applications.

Negative tone development: gaining insight through physical simulation

A simple analysis of aerial image quality reveals that negative tone imaging is superior to positive tone for small dimension contacts and trenches. Negative Tone Development (NTD) of positive chemically amplified (de-protecting) photoresist is currently the favored method for realizing such images on the wafer. When experimental process windows are determined for NTD systems, it is apparent that the results far exceed the upper limit predicted using current physical modeling. Since real data transcends the capabilities of the current model to predict, some important physical process is clearly missing. In this work, we explore whether resist shrinkage during PEB can account for the observed discrepancies. Two very simple shrinkage models are developed and tested. Results show that shrinkage in the vertical direction explains some profile artifacts observed in actual NTD processes but has negligible impact on conventional positive tone processes. The horizontal shrinkage model reveals that this type of phenomenon can significantly increase the exposure latitude of a negative tone process but has marginal impact on positive tone exposure latitude only introducing a small CD offset. While horizontal shrinkage does enhance exposure latitude appreciably, the effect does not seem large enough on its own to account for the entire increase observed in the experimental results. Further work is ongoing to investigate other potential mechanisms for observed behavior.

Simulation of optical lithography in the presence of topography and spin-coated films

Experimental results on etched silicon wafers show that after two consecutive spin-coat processes the upper material surface achieves near planar flatness. This was observed for three separate dual layer BARC systems and the case of photoresist over a single layer BARC. The wafer topography step height (60 nm) and the thicknesses of the organic films (20 nm - 100 nm) were typical for state-of-the-art IC manufacturing lithography processes. A lithographic proximity effect driven by wafer topography pitch was experimentally observed for a single layer BARC system. The response was reproduced with good quantitative accuracy using rigorous wafer plane EMF simulations incorporating ideal etched wafer topography, a planarizing resist film and a simple spin-coat approximation of the BARC coverage, as observed by x-section SEM. In contrast, simulations assuming the limiting cases of a perfectly conformal BARC and a perfectly planarizing BARC failed to predict any meaningful proximity effect.