Jeff D. Byers

Nonconstant diffusion coefficients: short description of modeling and comparison to experimental results

Chemical changes within a resist material (for example, resulting from the exposure and subsequent chemical reactions during post exposure bake) will in general, result in a change in diffusivity of components within that material. In the case of positive chemically amplified resists, the diffusivity of the photo-generated acid changes as a function of the extent of polymer deprotection. The deprotection reaction leads to the generation of small reaction product molecules, some of which are volatile. The liberation of these reaction products causes an increase in the free volume and changes in the chemical behavior in the exposed area. These changes, primarily the increase in free volume, results in an increase in the diffusivity of the acid. Low exposure areas have lower acid diffusivity, leading to a lower efficiency of reaction. This results in a contrast enhancement of the latent image due to the concentration dependent diffusivity of the acid. In this paper, a concentration dependent diffusivity expression is incorporated into a lithography simulator to explore these effects on lithographic performance. Using the assumption of free volume, suitable expressions for the diffusivity are examined and compared to experimentally measured values. The experimental work consists of XP-9402 positive acting, chemically amplified resist that was imaged using different thermal doses.

157 nm resist materials: Progress report

Many semiconductor device manufacturers plan to make products with 157 nm lithography beginning in 2004. There is, at this time, no functional photoresist suitable for 157 nm exposure. Developing resist materials for 157 nm lithography is particularly challenging since water, oxygen, and even polyethylene are strongly absorbing at this wavelength. A modular approach to the design of a single layer resist for 157 nm has been undertaken. In this approach, the resist has been conceptually segmented into four functional modules: an acidic group, an acid labile protecting group, an etch resistant moiety, and a polymer backbone. Each of these modules has an assigned function and each must be transparent at 157 nm. Progress has been made toward finding candidate structures for each of these modules. We have demonstrated that acidic bistrifluoromethylcarbinols are very transparent at 157 nm and function efficiently in chemically amplified resists with both high and low activation energy protecting groups. Judicious incorporation of fluorine in acrylates and alicyclics has provided etch resistant polymers with greatly improved transparency at 157 nm. In particular, esters of poly(α-trifluromethylacrylic acid) are far more transparent than their protio analogs. The Diels–Alder adducts derived from reaction of these and other fluorinated alkenes with cyclopentadiene offer a route to a wide range of alicyclic monomers that show great promise as transparent, etch resistant platforms for the design of 157 nm resists. Polymers of this sort with absorbance below 2 per micrometer are reported.

New model for focus-exposure data analysis

The paper introduces an improved, physics-based function for fitting lithographic data from focus-exposure matrices. Unlike simple polynomial functions, the coefficients of this equation offer physical insight into the meaning and nature of the data. Derivation of this equation from first principles of the physics of lithographic imaging is presented. Examples based on typical experimental data are shown and the advantages of using a physics-based fitting function is described based on improved fitting and noise filtering.

Calibration of chemically amplified resist models

Chemically amplified (CA) resists are expected to provide the majority of the lithography capability for 0.25 micron feature sizes. The sensitivity of CA resists to processing conditions makes implementation of a resist system very dependent on the tool set (stepper, track, etc.) used. Modeling of these systems can be useful in optimizing a process for a particular tool set. However, the modeling parameters are also sensitive to the target tool set. Variations in dose calibration between different steppers, the differing temperature ramps found in contact and proximity bakes, and batch to batch variations between resist materials are examples why resist model parameters require calibration to each tool set. An ideal calibration procedure would entail in-situ measurement techniques at each processing step. The techniques would analytically determine chemical, physical, and kinetic quantities relevant to the resist system and processing conditions. Methods previously used have included interferometric measurement of photobleaching, FTIR measurement of the deprotection extent, in-situ DRM measurements, etc. Unfortunately, few if any fabs are equipped with the necessary in-situ techniques for complete model calibration.

Film quantum yields of EUV& ultra-high PAG photoresists

Base titration methods are used to determine C-parameters for three industrial EUV photoresist platforms (EUV- 2D, MET-2D, XP5496) and twenty academic EUV photoresist platforms. X-ray reflectometry is used to measure the density of these resists, and leads to the determination of absorbance and film quantum yields (FQY). Ultrahigh levels of PAG show divergent mechanisms for production of photoacids beyond PAG concentrations of 0.35 moles/liter. The FQY of sulfonium PAGs level off, whereas resists prepared with iodonium PAG show FQYs that increase beyond PAG concentrations of 0.35 moles/liter, reaching record highs of 8-13 acids generated/EUV photons absorbed.

Improving the performance of 193-nm photoresists based on alicyclic polymers

This paper reports our work on a series of alicyclic polymer-based photoresist platforms designed for 193 nm lithography. The polymers described here were prepared from derivatives of norbornene and appropriate co-monomers by either free radical or ring opening metathesis polymerization methods. A variety of techniques were explored as a means of enhancing the lithographic, optical, dissolution, and mechanical properties of photoresists formulated from these alicyclic polymers. Recent studies designed to improve the lithographic performance of photoresists formulated with these materials are described.

3D lumped parameter model for lithographic simulations

Simplified resist models are desired for fast simulation of resist profiles over large mask areas. The Lumped Parameter Model was originally developed as one such model. However, the LPM model has been limited to 2D resist simulations of 1D aerial image slices with positive tone resists. In this paper we present a modified Lumped Parameter Model applicable to 3D resist simulations of both positive and negative tone resists. In addition several new LPM parameters are introduced that further improve accuracy. The derivation of the 3D LPM model, rationale for including the new parameters, and simulation results using the new model are given.

Polymers for 157-nm photoresist applications: a progress report

Finding materials that offer the all of the characteristics required of photoresist matrix resin polymers while trying to maintain a high level of transparency at 157 nm is a daunting challenge. To simplify this task, we have broken the design of these polymers down into subunits, each of which is responsible for a required function in the final material. In addition, we have begun collecting gas-phase VUV spectra of these potential subunits to measure their individual absorbance contributions. Progress on developing materials for each of these subunits are presented along with plans for future studies.

New single-layer positive photoresists for 193-nm photolithography

New series of chemically amplified, single layer, positive tone photoresists for 193 nm lithography have been developed. These resists were formulated from a series of cycloaliphatic co- and terpolymers of 2-methyl propyl bicyclo(2.2.1)hept-2- ene-5-carboxylate (carbo-tert-butoxynorbornene), bicyclo(2.2.1)hept-2-ene carboxylic acid (norbornene carboxylic acid), 8-methyl-8-carboxy tetracyclo(4,4,0.12,5,17,10)dodec-3-ene (methyltetracyclododecene carboxylic acid), norbornenemethanol, and maleic anhydride, which were synthesized by free radical, vinyl addition and ring opening metathesis polymerization techniques. The polymers derived from ring opening metathesis polymerization have bee successfully hydrogenated to provide yet another member of this group of materials. The cycloaliphatic polymer backbones provide etch resistance, mechanical properties and stability to radiation. The lithographic function is provided by carefully tailored pendant groups, which include an acid functionality that is masked by protecting groups that undergo acid catalyzed thermolysis as well as polar groups that influence the adhesion, wetability and dissolution properties of the polymer. The polymers are soluble in common organic solvents and have glass transition temperatures ranging from less than 60 degrees Celsius to higher than 250 degrees Celsius depending on their specific structure and mode of polymerization. They are at least as transparent at 193 nm as the corresponding acrylics. Their dry etch resistance varies with the formulation, but the base polymers etch more slowly than novolac under conditions typically used to pattern polysilicon. Upon exposure and baking, the resists have demonstrated high sensitivities (9-25 mJ/cm2), and 0.16 micrometer features have bean resolved.

Automatic calibration of lithography simulation parameters

A method is presented for automatically adjusting the input parameters of a lithography simulator to more accurately match a given set of experimental conditions. Using contrast curves, swing curves or focus-exposure matrices, simulation parameters are automatically modified in a search to minimize the difference between the simulated results and the experimental data. The algorithms used are described, as well as their robustness and sensitivity to experimental noise. Results of these tuning procedures are presented and the tuned set of parameters is shown to give good quantitative agreement of simulation to experiment.