Todd Davis

Pattern collapse and line width roughness reduction by surface conditioner solutions for 248-nm lithography

In this paper, surface conditioners were applied during the post-develop process to extend the capability of 248nm lithography processing below the k1= 0.30 threshold. The interaction between surface conditioner and photoresist was found to be a critical parameter in affecting pattern collapse, line width roughness (LWR), and process latitude. Tailoring the surface interaction properties required balancing between surface conditioners that had weak interactions that improved pattern collapse only marginally, to surface conditions with strong interactions that produced a considerable reduction in LWR but provided no benefit to pattern collapse or process latitude when compared to DI water. The surface conditioners with optimized resist interactions provided significant improvement in all performance parameters including reduced pattern collapse, improved LWR, and enlarged usable process latitude.

Impact of surfactant in developer and rinse solution on 193-nm lithography performance

In this study, surfactant-formulated developer and rinse solutions were used to enhance the performance of a 193 nm lithography process. The wetting and interfacial characteristics of surfactant-formulated solutions were studied and utilized as a screening tool for optimum formulation. The selected formulation was compared to the non-formulated TMAH development and DI water rinse process. Surfactants in developer and rinse solution significantly reduced pattern collapse, enabling an 86% increase of critical normalized aspect ratio. This corresponds to an increase in the usable resist thickness for an 80 nm 1:1 feature from 179 nm to 332 nm. Additional benefit provided by surfactant formulated process was a 25% improvement on both within-wafer and wafer-to-wafer critical dimension uniformity.

DualBeam metrology: a new technique for optimizing 0.13-um photo processes

A DualBeam Metrology system was investigated for the application of obtaining 3-dimensional (3D) characterization of a 130 nm ground rule KrF photolithography process. Integrated circuit devices are 3-dimensional in structure and, hence, should be best characterized using 3-dimensional techniques to ensure adherence to the design architecture and the desired process window for manufacturing. The need for 3D metrology is further required for the characterization and monitoring of critical layer processes and equipment performance. The metrology used in this investigation is a novel technique for critical feature cross sectioning. The process for DualBeam metrology uses a focused ion beam (FIB) for milling or cutting the cross section through the photoresist or process film. An integrated scanning electron microscope (SEM) provides high-resolution imaging of the features, and a flexible automated metrology package collects and analyzes the data. To demonstrate the feasibility of the technique, critical dimension (CD) data and sidewall angle (SWA) measurements were captured from 130 nm lines and 150 nm contacts at 1:1 densities. The critical criteria for the characterization of the photolithography process window are CD control, depth of focus (DOF), exposure latitude, and feature sidewall angle or profile. Using the DualBeam technique, 2D and 3D data are captured on a single machine platform using a cut, look, and measure routine. A further benefit is the availability of high-resolution cross-sectional SEM images that can be used qualitatively to validate the quantitative data. The results presented here show the performance of this 130 nm ground rule process and the benefits of utilizing this efficient characterization technique.

Precision process calibration and CD predictions for low-k1 lithography

Leading resist calibration for sub-0.3 k1 lithography demands accuracy <2nm for CD through pitch. An accurately calibrated resist process is the prerequisite for establishing production-worthy manufacturing under extreme low k1. From an integrated imaging point of view, the following key components must be simultaneously considered during the calibration - high numerical aperture (NA>0.8) imaging characteristics, customized illuminations (measured vs. modeled pupil profiles), resolution enhancement technology (RET) mask with OPC, reticle metrology, and resist thin film substrate. For imaging at NA approaching unity, polarized illumination can impact significantly the contrast formation in the resist film stack, and therefore it is an important factor to consider in the CD-based resist calibration. For aggressive DRAM memory core designs at k1<0.3, pattern-specific illumination optimization has proven to be critical for achieving the required imaging performance. Various optimization techniques from source profile optimization with fixed mask design to the combined source and mask optimization have been considered for customer designs and available imaging capabilities. For successful low-k1 process development, verification of the optimization results can only be made with a sufficiently tunable resist model that can predicate the wafer printing accurately under various optimized process settings. We have developed, for resist patterning under aggressive low-k1 conditions, a novel 3D diffusion model equipped with double-Gaussian convolution in each dimension. Resist calibration with the new diffusion model has demonstrated a fitness and CD predication accuracy that rival or outperform the traditional 3D physical resist models. In this work, we describe our empirical approach to achieving the nm-scale precision for advanced lithography process calibrations, using either measured 1D CD through-pitch or 2D memory core patterns. We show that for ArF imaging, the current resist development and diffusion modeling can readily achieve ~1-2nm max CD errors for common 1D through-pitch and aggressive 2D memory core resist patterns. Sensitivities of the calibrated models to various process parameters are analyzed, including the comparison between the measured and modeled (Gaussian or GRAIL) pupil profiles. We also report our preliminary calibration results under selected polarized illumination conditions.

Impact of surfactant in developer on CD performance

Surfactant-formulated developers were utilized to enhance the CD performance for 365nm (I-line), 248nm (DUV) and 193nm resist processing. From one generation to the next, the resist surface becomes more and more hydrophobic, creating the need for enhanced surface wetting. Contact angle measurement of surfactant-formulated developers on different generations of resist surfaces, from 365nm to 157nm resist surfaces, indicated improved wetting. On-wafer testing showed significant improvement on CD uniformity with surfactant-formulated developers for 365nm, 248nm and 193nm processing. Faster development rates were also observed for chemically amplified resist systems, including 248nm, 193nm and 157nm.