Donald C. Hofer

High temperature polymer foams

Abstract A means of generating high temperature polymer foams with pore sizes in the nanometre range has been developed. Foams were prepared by casting block copolymers comprising a thermally stable block as the matrix and a thermally labile material as the dispersed phase. Upon thermal treatment the thermally unstable block underwent thermolysis, leaving pores with a size and shape dictated by the initial copolymer morphology. Nanopore foam formation is shown for triblock copolymers composed of a poly(phenylquinoxaline) (PPQ) matrix with either poly(propylene oxide) (PO) or poly(methyl methacrylate) (PMMA) as the thermally labile coblocks. Upon decomposition of these blocks, a 10–20% reduction in density was observed, consistent with the initial PO or PMMA composition, and the resulting PPQ foams showed dielectric constants of about 2.4, substantially lower than that of PPQ (2.8). Small-angle X-ray scattering and transmission electron microscopy showed pore sizes of approximately 100 A.

Photoresists for 193-nm lithography

Photolithography using 193-nm light appears to be a viable route for the extension of optical lithography to the dimensions required for the manufacture of 1Gb DRAM and advanced CMOS microprocessors with 180-140-nm minimum feature sizes. In this paper, we discuss the origin of resist technology for 193-nm lithography and the current status of 193-nm photoresists, focusing on single-layer resist materials. We emphasize the photoresist design approaches under investigation, compare these with deep-UV (DUV) (248-nm) resist design and materials, and consider possible future lithography processes employing 193-nm lithography. Research and development on 193-nm photoresists by the lithography group at the IBM Almaden Research Center is highlighted.

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.

Polysilane Bilayer uv Lithography

Polysilanes are a class of Si-Si backbone polymers that have been demonstrated to function as high resolution positive resists with excellent uv sensitivity. These materials have a unique photochemistry with high quantum yields and nonlinear bleaching. Polysilanes serve as excellent RIE barriers for bilevel resist applications because a protective layer of SiO2 is formed during exposure to an oxygen plasma. Aliphatic polysilanes have been applied to full wafer mid-uv lithography with 0.75 μm resolution.

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.

Nanopore foams of high-temperature polymers

A method of generating high-temperature polymer foams with pore sizes in the nanometer regime was developed. The nanofoams were prepared by casting a block copolymer composed of a thermally stable and a thermally labile block, followed by a subsequent thermal treatment to degrade the labile material and generate the pores. The morphology of the block copolymer film was made up of a high-temperature polymer matrix, with the labile component as the dispersed phase. Thermolysis of the labile block affords pores where the size and shape of the pores are dictated by the initial copolymer morphology. Nanopore foam formation is described for triblock copolymers of poly(phenylquinoxaline), with poly(propylene oxide) as the labile block. Foam formation led to a 10-15% reduction in density, consistent with the poly(propylene oxide) composition, and a dielectric constant of 2.3. SAXS and TEM (transmission electron microscopy) measurements indicated pore sizes of approximately 10 nm.<<ETX>>

Dissolution/swelling behavior of cycloolefin polymers in aqueous base

Polycycloolefins prepared by addition polymerization of norbornene derivatives are quite different from hydroxystyrene-based polymers in terms of their interaction with aqueous base. Their dissolution kinetics monitored on a quartz crystal microbalance is not a smooth function of the ratio of the polar to nonpolar functionalities in polymer but abruptly changes from very fast dissolution to massive swelling within a narrow range of composition. The maximum swelling is a function of thickness and the entire film thickness can swell in a few seconds at > 3,000 angstroms/sec or at immeasurably fast rates. The initial concentration of a pendant carboxylic acid in polymer has to be selected to minimize swelling and the concentration of an acid-labile group to induce fast dissolution in the exposed area. Furthermore, swelling which occurs in the partially- exposed regions must be minimized by incorporating a third monomer unit or by adding a dissolution modifying agent (DMA) such as t-butyl cholate. However, the function of DMA which is also acid-labile is quite complex; depending on the matrix polymer composition and its dissolution/swelling behavior, DMA could function as a swelling suppressor or promoter and a carboxylic acid generated by acidolysis of DMA as a dissolution or swelling promoter. Photochemically generated sulfonic acid could also affect the dissolution/swelling behavior. Base hydrolysis of anhydride during development is controlled by the polarity (carboxylic acid concentration) in polymer film, which has been demonstrated in an unequivocal fashion by IR spectroscopy under the condition strongly mimicking the development process and thus could boost development contrast but could hurt performance as well. Thus, incorporation of carboxylic acid in the form of methacrylic acid, for example, in radical copolymerization of norbornene with maleic anhydride must be handled carefully as it would increase the susceptibility of the anhydride hydrolysis and could introduce heterogeneity in the polymer as methacrylic acid is rapidly consumed, producing a terpolymer containing a different molar concentration of norbornene and maleic anhydride (a proof against the commonly believed charge transfer polymerization mechanism).

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.

Modeling Planarization With Polymers

When a patterned surface is coated with a polymer solution and dried or cured, the resulting surface is not completely flat (contrary to most drawings illustrating the planarization layer in multilayer resist schemes.) In order to understand the thickness variation of the polymer film, it is useful to define local planarization as the reduction in height of a single step and global planarization as the long range effects of different pattern densities. It has been determined that the efficiency of planarization depends on the thickness of the polymer coating, the shrinkage of the polymer solution, the flow properties of the polymer during the cure cycle, and the dimensions of the patterns on the surface. The degrees of both local and global planarization can be predicted by straightforward calculations.