Armin Semmler

Application of 3D EMF simulation for development and optimization of alternating phase-shifting masks

Besides halftone phase shifting masks (HTPSM) in combination with off-axis illumination alternating phase shifting masks (altPSM) are becoming more and more an important resolution enhancement technique. Their obvious benefits can only yield profit in production if certain mask properties like intensity and phase balance are controlled to a requisite extent. In order to achieve production capable masks within a reasonable time and cost frame simulation tools are of essential importance for mask development and manufacturing. Four our studies we employed solid-CMTM, a 3D EMF (electro magnetic field) simulator that handles arbitrary topographical masks. It is demonstrated by examples that these capabilities are mandatory for altPSM development. In this paper we discuss the effects of various issues relevant for development and manufacturing of altPSM on a basis of systematic 3D EMF simulations. For different balancing options sensitivity to phase errors, mask CD errors and pitch/feature size were investigated. Comparisons to 2D simulation are made for further illustration. We show the influence of certain mask errors on process window and draw conclusions for optimizing altPSM manufacturing.

Balancing of Alternating Phase Shifting Masks for Practical Application: Modeling and Experimental Verification

Alternating phase shifting masks have proven their capability to enhance the process window and to reduce the mask error enhancement factor effectively. The application of this mask type requires additional mask-properties compared to binary masks or halftone PSM. In this paper two of these mask-properties, the intensity and the phase balancing, are investigated experimentally for 4X and 5X masks at DUV and compared with simulations applying the T-Mask configuration of the SOLID-CMT program. In a first part the experimentally determined balancing results are discussed. For the measurements two independent methods are compared: Balancing measurements with an AIMS-system (MSM100) and direct optical phase and transmission measurements using a MPM-248 system. The T-Mask as a 3D Maxwell solver allows the simulation of real 3D mask topography. We compare the results of simulations with measured AIMS data. All available mask data like depth of trenches, thickness and composition of chromium/CrxOy layers, etc. are taken as input for the simulations. The comparison enables an assessment of the possibilities and limitations of 3D mask-simulation. Based on 3D mask simulations CD-sensitivity of the different balancing methods was investigated also taking the influence of proximity into account. The simulations allow an assessment of the CD-sensitivity for four analyzed mask types for feature sizes below 150nm on the wafer.

Do we need complex resist models for predictive simulation of lithographic process performance

This paper describes different simplified simulation models which characterize the behavior of the photoresist during lithography processes. The effectiveness of these models is compared with the results of more physics and chemistry containing simulators. The strengths and weaknesses of the simplified models are demonstrated for practical applications. Simplified resist model parameters are calibrated for 193nm chemically amplified resists (CAR). The results are compared with calibration of full simulation models. The validity of the simulation models under different process conditions is investigated.

Will Darwin's law help us to improve our resist models?

Calibration of resist model parameters becomes more and more important in lithography simulation. The general goal of such a calibration procedure is to find parameters and model options which minimize the difference between experimentally measured and simulated data. In this paper a multidimensional downhill simplex method and a genetic algorithm are introduced. We investigate the performance of different modeling options such as the diffusivity of the photogenerated acid and of the quencher base, and different development models. Furthermore, new objective functions are proposed and evaluated: The overlap of process windows between simulated and experimental data and the comparison of linearity curves. The calibration procedures are performed for a 248nm and for a 193nm chemically amplified resist, respectively.

New models for the simulation of post-exposure bake of chemically amplifed resists

Post exposure bake (PEB) models in the lithography simulator SOLID-C have been extended in order to improve the description of kinetic and diffusion phenomena in chemically amplified resists. We have implemented several new models and options which take into account effects such as the diffusion of quencher base, different approaches to model the neutralization between photogenerated acid and a quencher base, spontaneous loss of quencher, and arbitrary dependencies of the diffusion coefficients on acid or inhibitor, respectively. In this study, the impact of these new model options on critical phenomena like iso-dense bias, linearity and line end shortening are examined. The simulations were performed for a calibrated KrF/ArF resist models.

Benchmarking of available rigorous electromagnetic field (EMF) simulators for phase-shift mask applications

Abstract It is well known that accurate simulation of phase-shifting masks in optical lithography requires representation as full 3D objects, as opposed to binary masks. To simulate these objects, the electromagnetic field in the neighbourhood of the mask must be calculated. The additional step of solving Maxwell’s equations tends to consume a large amount of CPU time compared to conventional simulation in which an infinitely thin mask is assumed. Therefore, the question of how much accuracy is gained using EMF solvers for the additional cost in terms of CPU time is relevant for its application in advanced lithography. The paper first addresses accuracy: comparisons from a Kirchhoff-type simulator to the newly developed 3D Maxwell solver, SOLID-CM are made, followed by a juxtaposition of an exact solution, comparisons among existing finite-difference, time-domain (FDTD) Maxwell equation solvers, as well as a comparison to a rigorous coupled wave analysis (RCWA). Finally, benchmarks with focus on CPU consumption will be given in 2D (no material variation in one dimension) compared to two other simulators, both well-known FDTD-based solvers, called VendorA and VendorB. Finally, benchmarks are extended to full 3D geometries using SOLID-CM and VendorA.

Actinic aerial image measurement tool for 157-nm mask qualification

The challenge to achieve an early introduction of 157 nm lithography requires various advanced metrology systems to evaluate the 65 nm node lithography performances, equipments and processes. Carl Zeiss AIMS tool based on the Aerial Image Measurement Software is the most promising approach to evaluate the mask quality in terms of aerial image properties, in order to assess post repair quality. Selete has joint activities with Carl Zeiss, International SEMATECH and Infineon to accelerate the development of an AIMS tool operating at the 157 nm wavelength. The alpha tool phase of the project has been completed, and beta tools are currently being built. This paper is discussing the results from measurements on the alpha tool of some 157 nm attenuated phase shift masks (Att-PSM). Resolution results and CD evaluation with respect to these measurements will be presented.

Improved manufacturability by OPC based on defocus data

The paper describes the advantages of optical proximity correction (OPC) based on defocus data instead of best focus data. By additionally acepting asymmetric variations of the dimension of different patterns e.g. for an isolated line that can become wider than its nominal width this method can deliver structures much more robust against opens and shorts than in the standard OPC approach which is based on data taken at best process conditions. The differences of both OPC methods are compared based on simulations and checked against experimental data of characteristic IC patterns.

Influence of e-beam-induced contamination on the printability of resist structures at 157-nm exposure

A CD-SEM was used to contaminate specific areas in dense line test structures on a 10x chrome on glass binary mask with e-beam induced deposition of hydrocarbons. Different degrees of contamination have been realised by varying the exposure time. Additionally, styrene was deposited with focussed ion beam (FIB) at different doses. The impact of the transmission loss caused by the deposited carbon was investigated by printing the manipulated test structures using Sematechs 157nm Exitech Micro Stepper and an experimental 157nm resist. Resist line width variations revealed the impact of the mask deposits on imaging. Additionally, simulations have been performed using 2D Kirchoff approximations in order to predict changes of the process windows and CD.

Alternating phase-shifting mask application: effect of width and geometry of shifters, 3D EMF simulation and experimental verification

Besides assist features in combination with HTPSM (half-tone phase shifting mask} and off-axis illumination altPSM (alternating phase shifting mask} is the major resolution enhancement technique to extend optical lithography to low k1. AltPSM in addition has the potential of superior CD control. However to achieve this in production altPSM has to fullfil a number of specifications with respect to phase and transmission. Another important aspect to obtain maximum CD control and overlapping process window for all kinds of structures at different pitches is that the phase shifters need to be optimized. Optimizing shifters by means of simulation results provides valuable input for both setting up design rules for altPSM application and for development of OPC strategies and software. Therefore various systems with different widths of lines and shifters were studied with special focus on basic asymmetric cases. We applied Solid-CM TM, a 3D EMF (electro magnetic field) simulator for our studies. Some results obtained from simulation were experimentally verified by wafer printing results (SEM imaging and CD measurement}. In addition, comparison to 2D simulation results clearly allows the determination of cases in which 3D effects have to be taken into account. The effect of varying shifters is monitored by pattern placement and process window analysis. We apply this investigation to develop solution strategies and to optimize shifter dimensions.