Kristopher A. Lavery

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.

Thin-film solid-state proton NMR measurements using a synthetic mica substrate: Polymer blends

Solid-state proton nuclear magnetic resonance (NMR) measurements are performed successfully on polymer blend thin films through the use of synthetic mica as a substrate. When used as a substrate, synthetic fluorophlogopite mica with its proton-free, diamagnetic character, allows for adequate measurement sensitivity while minimally perturbing the proton thin-film spectra, especially relative to more commonly available natural micas. Specifically, we use multiple-pulse techniques in the presence of magic-angle spinning to measure the degree of mixing in two different polymer blend thin films, polystyrene/poly(xylylene ether) and poly(1-methyladamantyl methacrylate) (PMAdMA)/triphenylsulfonium perfluorobutanesulfonate (TPS-PFBS), spin-coated onto mica substrates. Our earlier studies had focused on bulk systems where NMR signals are stronger, but may not be representative of thin films of the same systems that are relevant to many applications such as photoresist formulations in the electronics industry. The superiority of synthetic over natural paramagnetic mica is demonstrated by the maintenance of resolution and spinning sideband intensities (relative to bulk samples) for the synthetic mica samples. In contrast, degraded resolution and large spinning sidebands are shown to typify spectra of the natural mica samples. This approach can be applied to many other proton measurements of solid thin films, thereby greatly extending the types of systems to be investigated. Magnetic susceptibility measurements are also reported for all micas used.

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.

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 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.

Lateral uniformity in chemical composition along a buried reaction front in polymers using off-specular reflectivity.

Off-specular neutron reflectometry was applied to characterize the form and amplitude of lateral compositional variations at a buried reaction-diffusion front. In this work, off-specular neutron measurements were first calibrated using off-specular x-ray reflectivity and atomic force microscopy via a roughened glass surface, both as a free surface and as a buried interface that was prepared by spin coating thin polymer films upon the glass surface. All three methods provided consistent roughness values despite the difference in their detection mechanism. Our neutron results demonstrated, for the first time, that the compositional heterogeneity at a buried reaction front can be measured; the model system used in this study mimics the deprotection reaction that occurs during the photolithographic process necessary for manufacturing integrated circuits.

Off‐Specular X‐ray and Neutron Reflectometry for the Structural Characterization of Buried Interfaces

Off‐specular reflectivity or diffuse scattering is sensitive to lateral compositional variations at surfaces and interfaces. It is particularly well‐suited as a means of measuring the form and amplitude of surface roughness, as well as separating contributions from physical roughness and gradients in material density. The roughness and lateral correlation lengths of model rough surfaces were cross‐correlated using neutron and x‐ray off‐specular reflectivity (OSNR and OSXR, respectively), and using power spectral density (PSD) analysis of atomic force microscopy (AFM) data. These experiments highlight the advantages of the technique for the investigation of buried interfaces while illustrating how x‐ray and neutron techniques work complementarily to measure interfacial roughness.