Richard A. Di Pietro

Limits to etch resistance for 193-nm single-layer resists

An important aspect of single-layer resist use at 193-nm is the inherently poor etch resistance of the polymers currently under evaluation for use. In order to provide the information necessary for resist process selection at 193 nm, we have projected the ultimate etch resistance possible in 193-nm transparent polymers based on a model we have developed. First, a data base of etch rates was assembled for various alicyclic methacrylates. This data base has been used to develop an empirical structure-property relationship for predicting polymer etch rates relative to novolac-based resist. This relationship takes the functional form normalized rate equals -3.80r3 plus 6.71r2 minus 4.42r plus 2.10, where r is the mass fraction of polymer existing as cyclic carbon. From this analysis, it appears as though methacrylate resists equal in etch resistance to deep UV resists will be possible. Early generations of methacrylate-based 193-nm resists were also evaluated in actual IC process steps, and those results are presented with a brief discussion of how new plasma etch chemistries may be able to further enhance resist etch selectivity.

193-nm single-layer positive resists: building etch resistance into a high-resolution imaging system

Our approach to the design of positive, single layer resists for 193 nm lithography is discussed. Phenolic resins, the archetype in positive photoresist materials, cannot be used at this wavelength due to optical opacity. Methacrylate polymers combine the required optical transparency at 193 nm with easily tailored properties, however. The methacrylate polymer materials used in our 193 nm resists are completely different from phenolics used in traditional (248 nm) chemically amplified resists but the imaging chemistry is very similar. With a design based on methacrylate terpolymers, we have recently developed a high resolution positive resist for 193 nm lithography with excellent imaging at both 193 and 248 nm. Our recent work has centered on gaining further insight into methacrylate polymer structure/property relationships, improving the imaging performance and finally increasing the etch resistance. Towards that end, we have developed a class of dissolution inhibitors for 193 nm resists that are combined with methacrylate polymers to provide 3-component resists. A family of B- steroid dissolution inhibitors that also increase etch resistance is described. Imaging and etch performance of four versions of our resist are disclosed. These methacrylate resists show resolution capability below 0.25 micron, etch rates 20% higher than novolak resins, good environmental stability in contrast to traditional DUV resists and dual wavelength (193/248) imaging.

Protecting groups for 193-nm photoresists

Two versions of 193-nm single layer resists based on acrylic polymer chemistry have been described previously. The version 1 resist is a tool-testing version and is based on a methacrylate terpolymer structure. Its etch resistance analogue (version 2 resist) contains alicyclic compounds attached to the acrylic backbone. Key to enabling the performance of version 2 resist are the use of steroid additives which behave principally as thermomechanical modifiers to improve the mechanical properties of these rigid polymers through plasticization. We used the tertiary-butyl ester protecting group in these resists for thermal stability and other considerations. This paper describes an investigation of the impact of acid-cleavable protecting group structure on the properties of a series of model acrylic polymers. In this investigation, factors such as thermochemical stability, reactivity to photogenerated acid, and dissolution properties of exposed films as a function of dose were examined. A new highly reactive protecting group is introduced in this study, the tetrahydrofuranyl ester (THF) of methacrylic acid. Additionally, we introduce a new polymer family (polynorbornenes) with superior etch resistance, significantly broadening the polymer chemistry available for the construction of new 193-nm photoresists.

Lithographic performance of an environmentally stable chemically amplified photoresist (ESCAP)

Improved stabilization of chemically amplified photoresist images can be achieved through reduction of free volume by film densification. When the host polymer has good thermal stability, the softbake temperature can be above or near the glass transition temperature (Tg) of the polymer. Annealing (film densification) can significantly improve the environmental stability of the photoresist system. Improvements in the photoacid generator, processing conditions, and overall formulation coupled with high NA (numerical aperture) exposure systems afford 200 nm linear resolution with excellent post-exposure delay stability. In this paper, lithographic data is shown for the improved ESCAP photoresist system (now called UVIIHS) currently under development for DRAM and logic device technology. We review the photoresist system, along with process- and formulation-related experiments on device levels and substrates demonstrating excellent 250 nm and sub-250 nm process windows.

High-resolution 248-nm bilayer resist

Bilayer thin film imaging is one approach to extend 248 nm optical lithography to 150 nm regime and beyond. In this paper, we report our progress in the development of a positive-tone bilayer resist system consisting of a thin silicon containing imaging layer over a recently developed crosslinked polymeric underlayer. The chemically amplified imaging layer resist is based on a novel dual-functional silicon containing monomer, tris(trimethylsilyl)silylethyl methacrylate, which in addition to providing etch resistance, also functions as the acid sensitive functionality. The stabilization of (beta) -silyl carboncation by silicon allows this moiety to serve as an acid sensitive protecting group. Thus high silicon content and high resist contrast are achieved simultaneously. Lithographic evaluation of the bilayer resist with a 0.63 NA and a 0.68 NA 248 nm exposure tool has demonstrated resolution down to 125 nm equal line/space features with a dose latitude of 16 percent and depth of focus (DOF) of 0.6 um. The dose latitude and DOF for 150 nm equal line/space features are 22 percent and 1.2 um, respectively. Finally, residue-free, ultra-high aspect ratio resist features have been obtained by O2 or O2/SO2 reactive ion etching using a high-density plasma etch system. The resist design, deprotection chemistry, lithographic and etch characteristics of the top layer, as well as the design of the new underlay, will be discussed.

Positive bilayer resists for 248- and 193-nm lithography

We have designed and developed new silicon containing methacrylate monomers that can be used in bilayer resist systems. New monomers were developed because the commercially available silicon monomers were found to be unsuitable for our applications. During the course of the investigation we determined that these monomers were acid labile. We have developed a high resolution DUV bilayer resist system based on these monomers. Although most of our work was concentrated on 248 nm lithography, we have demonstrated that this chemistry can be extended to 193 nm applications.

Design of an etch-resistant cyclic olefin photoresist

In the quest for a high performance 193 nm photoresist with robust plasma etching resistance equivalent to or better than the DUV resists of today, we have focused on the use of cyclic olefin polymers. In this paper, we will discuss monomer synthesis, polymerization approaches, polymer properties and early lithographic results of 193 nm photoresists formulated from cyclic olefin polymeric materials made from a metal-catalyzed addition polymerization process. The goal of this work is to produce a 193 nm photoresist with excellent imaging performance and etch resistance exceeding DUV resists, and in fact approaching novolak-based photoresists.

Role of photoacid structure on the performance of 193-nm resists

The impact of photoacid generator (PAG) structure has been largely ignored for 193 nm single layer resists. Most published work to date has involved the use of triflic or metallic (antimonate or arsenate) photoacids. Many PAGs used in DUV (248 nm) resists are inefficient when formulated with (non-phenolic) polymers used in 193-nm resists, presumably due to the lack of electron transfer sensitization. In this paper, we document the negative consequences of triflic acid on 193- nm resist performance, including data on acid volatility and the impact of apparent diffusion. Acid generators which combine high reactivity, low photoacid volatility, and improved resolution are described.

Approaches to etch-resistant 193-nm photoresists: performance and prospects

We have investigated three substantially different routes to 193nm single layer resists. This paper will attempt to shed light on the strengths and weaknesses of each approach. Design principles, polymer synthesis and properties, and resist properties will be discussed for the three main branches of 193nm resists.

193-nm positive-tone bilayer resist based on norbornene-maleic anhydride copolymers

We have designed and developed a high resolution 193 nm bilayer resist system based on alternating copolymers of silane substituted norbornene and maleic anhydride. We have utilized a combination of acid labile silane functionalities and acid stable silicon groups in this resist development.