Kwang-Woo Choi

Band offsets of atomic-layer-deposited Al2O3 on GaAs and the effects of surface treatment

The metal gate/high-k dielectric/III-V semiconductor band alignment is one of the most technologically important parameters. We report the band offsets of the Al/Al2O3/GaAs structure and the effect of GaAs surface treatment. The energy barrier at the Al2O3 and sulfur-passivated GaAs interface is found to be 3.0±0.1 eV whereas for the unpassivated or NH4OH-treated GaAs is 3.6 eV. At the Al/Al2O3 interface, all samples yield the same barrier height of 2.9±0.2 eV. With a band gap of 6.4±0.05 eV for Al2O3, the band alignments at both Al2O3 interfaces are established.

Lateral length scales of latent image roughness as determined by off-specular neutron reflectivity

A combination of specular and off-specular neutron reflectometries was used to measure the buried lateral roughness of the reaction-diffusion front in a model extreme ultraviolet lithography photoresist. Compositional heterogeneities at the latent reaction-diffusion front has been proposed as a major cause of line edge roughness in photolithographic features. This work describes the experimental observation of the longitudinal and lateral compositional heterogeneities of a latent image, revealing the buried lateral length scale as well as the amplitude of inhomogeneity at the reaction-diffusion front. These measurements aid in determining the origins of line edge roughness formation, while exploring the material limits of the current chemically amplified photoresists.

Effect of photo-acid generator concentration and developer strength on the patterning capabilities of a model EUV photoresist

Current extreme ultraviolet (EUV) photoresist materials do not yet meet requirements on exposure-dose sensitivity, line-width roughness (LWR), and resolution. Fundamental studies are required to quantify the trade-offs in materials properties and processing steps for EUV photoresist specific problems such as high photoacid generator (PAG) loadings and the use of very thin films. Furthermore, new processing strategies such as changes in the developer strength and composition may enable increased resolution. In this work, model photoresists are used to investigate the influence of photoacid generator loading and developer strength on EUV lithographically printed images. Measurements of line width roughness and developed line-space patterns were performed and highlight a combined PAG loading and developer strength dependence that reduce LWR in a non-optimized photoresist.

Exposure dose effects on the reaction-diffusion process in model extreme ultraviolet photoresists

The effect of exposure dose on the latent image deprotection profile in a model extreme ultraviolet (EUV) photoresist polymer, poly(hydroxystyrene-co-d9-tert-butyl acrylate), is measured with neutron reflectometry. As the photoacid concentration is increased by exposure dose, the spatial extent of propagation increases but eventually becomes self-limited by the products of the reaction. A long-range deprotection path occurs with diffusion length between 10 and 100A, while an additional subnanometer short-range deprotection length scale proceeds monotonically with dose. These measurements show that the photoacid diffusion length into unexposed regions of a photoresist is limited even in the absence of base quencher additives. These fundamental data can be used to highlight materials effects on photoresist processing and to improve quantitative models for EUV photoresists needed at the sub-32‐nm half pitch lithography.

Hunting the Origins of Line Width Roughness with Chemical Force Microscopy

Chemical force microscopy, a variant of atomic force microscopy, is being developed to image the spatial distributions of chemical species at different stages of the lithography process. By imaging the chemical changes that occur in the resist during the lithographic process we gain insight into the fundamental physical and chemical processes that determine the fidelity of pattern replication in the resist. We have examined several stages of the lithography process including the homogeneity of resist after spin casting, latent image formation, and segregation during development to look for possible contributions to line width roughness in lithographic patterns. We have obtained good chemical contrast in a variety of systems that provide insight into the underlying fundamentals. We also discuss work to develop the metro logical underpinnings of chemical force microscopy.

CD-SAXS measurements using laboratory-based and synchrotron-based instruments

Critical dimension small angle X-ray scattering (CD-SAXS) is a metrology platform capable of measuring the average cross section and line width roughness (LWR) with a sub-nm precision in test patterns with line widths ranging from 10 to 500 nm. The X-ray diffraction intensities from a collimated X-ray beam of sub-Angstrom wavelength were collected and analyzed to determine line width, pitch, sidewall angle, LWR, and others structural parameters. The capabilities of lab-scale and synchrotron-based CD-SAXS tools for LWR characterization were tested by measuring a set of identical patterns with designed roughness amplitude and frequency. These test patterns were fabricated using EUV lithography with sub-50 nm linewidths. To compensate for the limited photon flux from the lab-based X-ray source, the incident beam of the lab system was collimated to a less extent than the synchrotron beam-based tool. Consequently, additional desmearing is needed to extract information from data obtained from lab-based equipment. We report the weighted nonlinear least-squares algorithm developed for this purpose, in addiiton to a comparison between the results obtained from our lab system and the synchrotron beam-based tool.

Fundamentals of the reaction-diffusion process in model EUV photoresists

More demanding requirements are being made of photoresist materials for fabrication of nanostructures as the feature critical dimensions (CD) decrease. For extreme ultraviolet (EUV) resists, control of line width roughness (LWR) and high resist sensitivity are key requirements for their success. The observed LWR and CD values result from many factors in interdependent processing steps. One of these factors is the deprotection interface formed during the post-exposure bake (PEB) step. We use model EUV photoresist polymers to systematically address the influence of exposure-dose on the spatial evolution of the deprotection reaction at a model line edge for fixed PEB time using neutron reflectivity. The bilayer consists of an acid feeder layer containing photoacid generator (PAG) and a model photoresist polymer, poly(hydroxystyrene-co-tert-butylacrylate) with perdeuterated t-butyl protecting group. The deuterium labeling allows the protection profile to be measured with nanometer resolution. The evolution of two length scales that contribute to the compositional profile is discussed.

Characterization of the Photoacid Diffusion Length

The photoacid diffusion length is a critical issue for extreme ultraviolet (EUV) lithography because it governs the critical dimension (CD), line-edge-roughness (LER), and line-width-roughness (LWR) of photoresist materials. Laboratorybased experimental methods that complement full lithographic testing would enable a rapid screening of materials and process conditions. This paper provides an approach to characterize the photoacid diffusion length by applying a bilayer stack technique. The method involves quantitative measurements of the deprotection kinetics as well as film thickness at each process step: radiation exposure, post-exposure bake, and development. Analogous to a contrast curve, by comparing the film thickness of the bilayer before and after development, the photoacid diffusion length was deduced in a commercial EUV photoresist and compared to EUV lithography. Further, by combining the experiments with kinetics modeling, the measured photoacid diffusion length was predicted. Lastly, based upon the measured kinetics parameters, a criterion was developed that next-generation resists must meet to achieve a 16 nm photoacid diffusion length. These guidelines are discussed in terms of correlations and contributions from the photoacid and resist properties. In particular, the trapping kinetics of the photoacid provides a route to reduce LER and the CD at low dose.

Characterization of the latent image to developed image in model EUV photoresists

Current extreme ultraviolet (EUV) photoresist materials do not yet meet performance requirements on exposure-dose sensitivity, line-width roughness, and resolution. In order to quantify how these trade-offs are related to the materials properties of the resist and processing conditions, advanced measurements and fundamental studies are required that consider EUV-resist specific problems. In this paper, we focus on the correlations between the latent image and developed image in EUV exposed line/space features. The latent images of isolated lines produced by EUV lithography are characterized by atomic force microscopy through the change in topology caused by change in film thickness that occurs upon deprotection. The resulting latent-image deprotection gradient (DGL), based on line cross-sections, and latent-image line-width roughness (LWRL) provide metrics and insight into ways to optimize the lithographic process. The results from a model poly(hydroxystyrene-co-tert-butylacrylate) resist and a model calix[4]resorcinarene molecular glass type resist show the general applicability of the metric before development.

Linewidth roughness and cross-sectional measurements of sub-50 nm structures with CD-SAXS and CD-SEM

Critical dimension small angle X-ray scattering (CD-SAXS) is a measurement platform that is capable of measuring the average cross section and sidewall roughness in patterns ranging from (10 to 500) nm in pitch with sub-nm precision. These capabilities are obtained by measuring and modeling the scattering intensities of a collimated X-ray beam with sub-nanometer wavelength from a periodic pattern, such as those found in optical scatterometry targets. In this work, we evaluated the capability a synchrotron-based CD-SAXS measurements to characterize linewidth roughness (LWR) by measuring periodic line/space patterns fabricated with extreme ultraviolet (EUV) lithography with sub-50 nm linewidths and designed with programmed roughness amplitude and frequency. For these patterns, CD-SAXS can provide high precision data on cross-section dimensions, including sidewall angle, line height, line width, and pitch, as well as the LWR amplitude. We also discuss the status of ongoing efforts to compare quantitatively the CD-SAXS data with topdown critical dimension scanning electron microscopy (CD-SEM) measurements.