Proximity effect (electron beam lithography)

About: Proximity effect (electron beam lithography) is a research topic. Over the lifetime, 940 publications have been published within this topic receiving 8508 citations.

The Interdependence of Exposure and Development Conditions when Optimizing Low-Energy EBL for Nano-Scale Resolution

Abstract: Electron beam lithography (EBL) is the major direct-write technique to controllably fabricate nanoscale features. A focused beam of electrons induces a chemical change in a layer of radiation sensitive material (resist), such as chain scissioning in positive tone polymethylmethacrylate (PMMA) polymer photoresist. The localized fragmented region is rendered more soluble in a suitable developer solution and removed. In negative tone resists, such as hydrogen silsesquioxane (HSQ) or calixarene, the radiation damage eventually results in bond cross-linking, generating structures locally more resistant to dissolution. Limitations of the technology are related largely with unwanted exposure of the resist away from the impact of the focused electron beam due to scattering of the primary electrons in the resist (often described as the forward scattering), generation of secondary electrons, and backscattering from the substrate (the proximity effect). The exposure and development processes have been optimized and routinely used for fabrication of submicron features. However, as requirements for lithography have progressed toward the sub-20 nm regimes, major challenges have emerged of introducing controllable radiationinduced changes at molecular-size scales, within a reasonable tradeoff with the applicability of the standard materials, as well as cost and simplicity of the processes. Due to the proximity effect, this becomes particularly demanding when dense patterns with closely positioned features must be fabricated. Achieving deep nanoscale resolutions in high density patterns at industrially-relevant throughputs requires new approaches to EBL. Novel EBL processes that would extend capabilities of the technology significantly into the deep nanoscale regime entail new approaches to resist design, exposure strategies, and development techniques (Haffner et al., 2007; Liddle et al., 2003; Ocola & Stein, 2007; Word et al., 2003). To achieve this will require a much more detailed understanding of the molecular mechanisms involved in both the electron-resist interaction and in the polymer dissolution (development) stages of the nanolithography process. Despite a significant research effort and vast literature on electron beam lithography, the detailed molecular mechanisms are still inadequately understood. Published modeling studies address

A Proximity Effect Measuring Test Chip - Design And Application

Abstract: An electrical tester has been designed for measuring proximity effect in e-beam lithography. The tester consists of a clover-shaped van der Pauw resistor for sheet resistance measurement, a four-terminal resistor for linewidth measurement, and a second four-terminal resistor of identical width but with adjacent bars for evaluating changes due to proximity exposure. The test chip is composed of a set of testers with various combinations of linewidth, bar size, and intermediate space, ranging in dimension from 0.5 μm to 10 μm. Computer software has been developed to interface a commercial computer to the wafer prober for fully automated data acquisition, statistical analysis, and graphic display. The test system yields very high precision in both the sheet resistance (3 a < 1% of nominal) and electrical linewidth (3 a < 0.01 μm). The accuracy of the linewidth data has been verified by SEM measurements. The chip can serve as a general purpose metrology tool to evaluate the efficacy of different proximity correction techniques in e-beam lithography, to complement SEM linewidth measurements which suffer from profile and threshold dependence especially for non-vertical sidewalls, and to monitor linewidth control for submicron process development. Using an e-beam exposure tool at 20kV, the chip has been delineated in GMC, a negative imaging resist, in a trilevel resist structure, on substrates of tantalum silicide and aluminum. These substrates correspond to the GATE and the METAL level substrates in a MOS integrated circuit. In addition, it has been delineated in chromium, a typical photomask substrate, using single layer resist. The extent of proximity exposure effect on each of these substrates is reported. Linewidth deviations of 0.1 μm or greater are observed for near-micron equal line and space patterns. In addition, proximity exposure increases with incident exposure dose and the atomic number of the substrate. On the basis of these results, VLSI layout constraints arising from e-beam proximity exposure are identified.

Resolution And Proximity Effect In Optical Lithography

Abstract: The question as to how accurately small object features can be reproduced in optical microlithography does not have a simple answer. It depends not only on the dimensions of the feature, but also on whether it is a line or a space, whether there are other features nearby (the proximity effect), on the resist thickness, and on whether the features are to have the dimensions of the ideal optical image or are "biased". This paper explores these topics by modeling the imaging, exposure, and development steps. In order to discover the significant dependencies we first investigate the simple model of a resist layer deposited onto a non-reflecting substrate. It shows that an isolated resist line has approximately the correct dimensions in all sizes when the imaging is done with partially coherent light. The isolated spaces, however, show deviations from the design size, which are caused by diffraction effects. For spaces (0.6 - 1.0) λ/NA wide the light intensity in the center of the space is larger than the intensity of the incident light causing the resist to develop through faster than in a very large area For spaces smaller than 0.6 λ/NA the light intensity in the image drops rapidly and the spaces can no longer be reproduced. The dependence on coherence of the light, on resist thickness, and on the degree of focus are also investigated. On real sur-faces of silicon, Si02, and aluminum reflections cause interference effects and dramatic variations in exposure with resist thickness. After these effects are taken into account, the resolution is much poorer (0.9 λ/NA in the case of silicon, worse for aluminum). Biasing of the masks does not improve the resolution capability. On the other hand, a post-bake of the re-sist pattern can cause a dramatic improvement of the imaging quality and can increase the resolution to where it is comparable to that obtained on the non-reflective substrate.

Proximity effects in low‐energy electron‐beam lithography

Abstract: In this work, the accuracy with which patterns can be delineated in electron‐beam lithography has been characterized in terms of the range of the electrons relative to the pattern size being produced. In addition, the interdependence of the minimum feature size, film thickness, and beam energy have been investigated for low‐beam energy exposures. Palladium acetate has been used as a model resist, and beam energies ranging from 1 to 30 keV have been used for exposures. The results indicate that the proximity effect can be eliminated by reducing the range of the electrons below the minimum feature width and that the thickness of the imaging layer must be less than 2/3 of the electron range for adequate exposure. The loss of resolution due to increased probe size at low beam energies has also been investigated.

Formulated surface conditioners to enhance the non-collapse window and maintain defect control: a bi-functional approach for sub-100-nm lithography

Abstract: One key challenge in sub-100 nm lithography is line pattern collapse. Pattern collapse has become an obstacle in device manufacturing processes requiring dense-high aspect ratio resist lines. In addition to pattern collapse, defect control continues to be a factor in IC manufacturing. In this study, the impact of a formulated surface conditioner, OptiPatten ® Clear, with bifunctional capabilities: improved non-collapse window and defect control, was tested using a 193 nm lithographic process. To determine pattern collapse performance, 100 nm dense lines/space (L/S) and 100 nm 1:0.9 L/S were patterned into 240 nm of resist on 200 mm wafers. The wafers were then processed with developer and a formulated surface conditioner and compared to wafers processed with developer and DI water. When analyzed, wafers processed with surface conditioner had a 33% increase in Depth-of-Focus (DOF) and a 25% increase in Critical Normalized Aspect Ratio (CNAR) compared to DI water. Optical proximity effects are often credited for having a first-order influence on pattern collapse. Trench feature data was generated using an Scanning Electron Microscope (SEM) to compare the pattern collapse performance of OptiPattern Clear to DI water. The data strongly suggests optical proximity effects are a second-order factor which OptiPattern ® Clear resolves. Defect performance for OptiPattern Clear was measured by comparison with a DI water baseline. A production reticle was used to process wafers patterned with 120 nm L/S with 240 nm of resist. The wafers processed with OptiPattern ® Clear had similar defect performance as the DI water.