Doris Kang

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.

New methods to calibrate simulation parameters for chemically amplified resists

In this paper we examine new models and the indispensability of model parameters of chemically amplified resists (CAR) for their usage in predictive process simulation. Based on a careful exploration of different modeling options we calibrate the model parameters with different experimental data. Furthermore, we investigate different modeling approaches: (1) Mode of coupling between diffusion and kinetic reactions, sequence of quencher base events (Hinsberg model); (2) Mode of diffusion: Fickian and linear diffusion model; (3) Development rate model: Performance of the Enhanced Notch model. The resulting models are evaluated with respect to their performance by comparing with experimental line-width for semidense (1-2, 1-1.6, 1-1.4, 1-1.2) and dense features, the bias between different features and full resist profiles. The investigations are applied to the Shipley resist UVTM 113. Finally, a parameter extraction procedure for chemically amplified resists is proposed.

Accuracy of current model descriptions of a DUV photoresist

This paper describes the development of a lithographic model for Shipley UV6 DUV photoresist from fundamental materials constants. Parameters describing optical absorbance, acid generation, acid generation, acid diffusion, resin deprotection, and resists dissolution rate were measured and input into PROLITH/2. Initial simulations showed significant deviations from observed lithographic performance. Simulated E0 swing curves showed a large bulk effect absent in experiment data. Simulated lines showed excessive top loss and tapering which were corrected by the artificial introduction of a surface base contaminant. Lithographic process windows for lines and contacts cold be successfully simulated after considerable adjustment of the acid diffusion coefficient and development parameters, but accurate simulation of both profile and process window with a single set of parameters was only possible for contacts. The manipulations required to match simulated and experimental data did lead to some insights into resists materials design. Simulations suggest that lowering acid diffusion should increase exposure latitude and reduce film thickness loss, while suppressing the resist dissolution rate at the onset of deprotection should improve both focus and exposure latitude.

Measuring and simulating postexposure bake temperature effects in chemically amplified photoresists

The kinetic processes occurring during postexposure bake (PEB) were studied for different Shipley photoresists as a function of bake temperature with the aim of developing models to accurately simulate the observed temperature dependent responses. S7 193nm photoresist based on a high activation energy methacrylate platform and SL4000 248 nm photoresist based on a low activation energy acetal platform were investigated. Deprotection rate constants, Kamp, were measured directly by IR spectroscopy. Kamp was independent of PEB temperature for SL4000 and followed standard Arrhenius behavior for S7. Activation energies of deprotection determined from measurements of Kamp were 13.4 and 0.82 kcal/mol for S7 and SL4000, respectively, and of the same order of magnitude as the corresponding pseudo- Arrhenius activation energies, 10.3 and 3.2 kcal/mol, respectively, determined from E0 clearing doses as a matching experimental and simulated CDs of isolated lines as a function of PEB time and temperature. However, values of D could not be unambiguously determined because a finite bulk acid loss rate constant was required to accurately simulate the observed CD changes at high PEB temperature. The need for a blk acid loss rate constant at high bake temperatures suggests that the effects of base diffusion are significant under these conditions. CD changes in SL4000 were best simulated with a 1st order model, where the imaging chemistry occurs solely during the exposure step as in DNQ/Novolac photoresists.

Calibration of ESCAP resist simulation parameters from consideration of printed CD pitch bias, CD measurement offset and wafer thermal history

In this work an automate optimization routine is used to modify modeling parameters for a chemically amplified photoresist, with the goal of minimizing the error observed between lithography simulation and experimental results. It is shown that a basic tuning procedure modifying, optimizing only CD measurement offset and acid generation efficiency, improves the fit significantly. Further improvements can be made by optimization of the diffusion-deprotection kinetic parameters, in combination with the two aforementioned values. It is shown further improvement is observed if the actual temperature profile experienced in the postexposure bake process is considered and the temperature dependence of both the diffusion and the deprotection processes are optimized. This parameter values that result in this improvement infer a temporal offset in the start, and finish, of deprotection and acid diffusion.

Simulation of 193-nm photoresists based on different polymer platforms

Chemically amplified resist models for Shipley 193nm resists S6 and V2 were developed for use with commercial lithographic modeling software. S6 and V2 are based on methacrylate and vinyl ether/maleic anhydride polymer platforms, respectively, and contain an onium salt photoacid generator and proprietary base quencher. Fundamental parameters for these resists were determined experimentally and subsequently tuned to establish valid models. Current modeling algorithms appear sufficient to predict the lithographic behavior of typical features of interest. Experimental measurements that indicate that these 2 resists are similar with respect to acid photogeneration efficiency (0.04 cm2/mJ), polymer deprotection rate constant (0.05- 0.1 l/s), and developer selectivity. However, S6 exhibits greater transparency (0.35 1/micrometers vs. 0.5 1/micrometers for V2), lower acid diffusion, and greater surface inhibition. V2 exhibits considerably smoother dissolution.

Design of a cost-effective multiwavelength development rate monitoring tool

Understanding the development rate of resists is critical for the characterization of photoresist formulations and accurate modeling of the photolithographic process. Most commercial development rate monitors (DRMs) are based on the optical interference of a single wavelength of light. (Perkin-Elmer DRM5800; Litho-tech Japan RDA-790). DRMs based on the interference across a broad spectrum of wavelengths, known as multi wavelength DRMs (MW-DRM), were first reported by Konnerth1,2 and have also been used for photolithographic research3,4. This technique has been applied to commercial DRMs (SC Technology Inspector), but the high cost of these tools has made them inaccessible to most research and development facilities. This paper describes the development of a new cost-effective, scaleable, multi-channel DRM that allows collection and calculation of multiple development rate curves using MW-DRM technology. Techniques are presented for collection of multi wavelength data at rates exceeding 80 Hz, which in turn allows the study of photoresists that develop at rates in excess of 5 microns per second. The algorithms necessary to analyze this data are presented. The use of these algorithms for the extraction of development rate curves is demonstrated with resists that exhibit surface inhibition and standing waves. The use of multi-layer algorithms to collect development rate information in films between 0 and 200 nm thick is also shown. Finally, the use of these techniques for characterization of deprotection in chemically amplified photoresists, is presented.

Novel polychromatic measurement technique for determining the dissolution rate of very thin resist films

Conventional optical development rate measurement techniques are generally unsuitable for monitoring the dissolution very tin resist films. Monochromatic systems have inadequate thickness resolution to capture the details of surface and standing wave effects, while traditional polychromatic techniques are generally unable to measure thicknesses below 250 nm. The failure of polychromatic analysis methods occurs when there is an absence of turning points int eh relative reflection spectrum. The exact thickness at which this happens is a function of the wavelength range utilized and the resist materials optical characteristics. A novel measurement method is introduced which allows a polychromatic DRM system to measure any resist thickness. Rather than placing the film under analysis directly on a reflecting substrate, it is spun on a wafer that has a relatively thick transparent film on its surface. The transparent film induces turning points in the relative reflection spectrum. The position of these turning points is modified by the presence of thin resist films in a predictable way, allowing accurate measurement of the resist film, providing the optical and thickness details of the intermediate film are known. Experimental results are presented demonstrating the capability of the technique to measure the dissolution rates of films with initial thickness ranging from 56 nm to 4400 nm. The ability of the method to resolve fine dissolution detail, such as standing waves and surface effects is also presented.

Verification of the effect of mask bias on the mask error enhancement factor of contact holes

The mask error enhancement factor for contact holes is experimentally determined for 180 nm features under a variety of exposure conditions. Since its magnitude depends, in part, upon the slope of the aerial image, the value is calculated as a function of binary and phase shift masks, mask bias, and conventional and quadrupole illumination. The primary purpose is to compare experimental results to a simulation study and determine which simulation trends are accurately predicted. The results show that isolated contacts have lower MEEF than dense contacts but that dense contacts do not necessarily have the largest error factor. The magnitude of MEEF and the optimal bias that minimizes it are show to be accurately predicted.

Manufacturing considerations for MEEF minimization and process window optimization for 180-nm contact holes

This study examines through simulation the effects of mask bias and illumination settings on the MEEF and process window of 180nm contact holes. Previous work has shown that application of a global mask bias of -40 or -60 nm collectively minimizes MEEF for 180 nm contacts of varying pitches printed simultaneously with binary mask or 6 percent transmittance attenuated phase shifting mask respectively. Simulations in the present work show that in addition to reducing MEEF, negative mask bias lowers sizing energy and reduces sidelobe formation in patterns printed with 6 percent AttPSM. However, increased film loss from dense contacts and slightly reduced process window also result from the use of negative mask bias. These drawbacks can be partly mitigated by optimizing the illumination parameters. Higher (sigma) , higher NA, and shorter wavelength of exposure all reduce or eliminate top loss and increase overall exposure latitude, while higher (sigma) also increases focus latitude at low NA. At higher NA, a tradeoff exists between lower MEEF with negative mask bias and loss of focus latitude with 6 percent AttPSM.