Alberto Cabral

Contributions to innate material roughness in resist

A method has been developed to probe the Innate Material Roughness (IMR) of resist materials. We have applied this to EUV and 248 nm resists to deconvolute the material contributions to roughness: 1) the polymer alone, 2) interaction between the polymer, photoacid generator (PAG), base quencher, and photolysis byproducts, 3) the effects of exposure, and 4) development. We studied ESCAP based resists (with more limited data on APEX polymers), an iodonium nonaflate PAG, a tetabutyl ammonium hydroxide (TBAH) base quencher, and standard tetramethylammonium hydroxide (TMAH) development.

Contribution of photoacid generator to material roughness

The authors have developed an atomic-force-microscopy-based technique to measure intrinsic material roughness after base development. This method involves performing an interrupted development of the resist film and measuring the resulting film roughness after a certain fixed film loss. Employing this technique, the authors previously established that the photoacid generator (PAG) is a major material contributor of film roughness and that PAG segregation in the resist is likely responsible for nanoscale dissolution inhomogeneities. The additional roughness imparted on a test polymer by incorporation of a series of iodonium, sulfonium, diazo, and imido PAGs was measured. The roughness was then correlated to the inhibition properties of the various PAGs. This was accomplished both through a NMR technique that measures interaction of the PAG with the polymer and by evaluating the dissolution inhibition properties of the PAG through a percolation model. Several PAGs that result in significantly lower material...

Changes in resist glass transition temperatures due to exposure

We have developed an AFM-based technique to measure intrinsic material roughness (IMR) after base development. Employing this technique we have deconstructed the resist into component parts and have shown that PAG is a major contributor to intrinsic material roughness. When PAG is exposed and thermal polymer deprotection is allowed to occur increased levels of IMR are present. The IMR of the resist is strongly dependent on the bake conditions, with increasing IMR at higher bake temperatures. This leads to the suspicion that the resist glass transition temperature (Tg) may be responsible for the changes in the level of IMR observed with both different PAGs, polymers and bake temperatures. We have measured the Tg in a series of model resists, both exposed and unexposed, and show the effect of changes in resist glass transition as a function of exposure dose and not the level of polymer deprotection. The Tg of the resists does not decrease with exposure or bake as may be expected, but instead is either unchanged or slightly increases. The change in Tg occurs due to exposure only with subsequent bake steps not affecting the resist Tg.

Deconstructing the resist to probe innate material roughness

We have developed an AFM-based technique to measure intrinsic material roughness after base development. This method involves performing an interrupted development of the resist film and measuring the resulting film roughness after a certain fixed film loss. Employing this technique, we have deconstructed the resist into component materials and established that the PAG is a major material contributor of film roughness and that PAG segregation in the resist is likely responsible for nano-scale dissolution inhomogeneities. Small differences in PAG concentration as a result of standing waves in the resist can lead to large changes in surface roughness due to PAG or PAG-photoproduct segregation and the resultant non-linear change in nano-scale dissolution rates. The temperature dependence of the PAG segregation suggests that increased mobility of the PAG occurs due to a lowering of the film Tg during the deprotection process.

Reversible Electrowetting on Dual-Scale-Patterned Corrugated Microstructured Surfaces

The ability to reversibly switch between a hydrophobic Cassie state and a hydrophilic Wenzel state is often not possible on textured surfaces because of energy barriers which result from the geometry of the microstructure. In this paper, we report on a simple microstructure geometry that allows an aqueous droplet to be reversibly switched between these states by the application of electrowetting. We demonstrate reversible electrowetting in air on microstructured surfaces consisting of parallel corrugations and show that this geometry can be engineered to produce a Cassie state and can be electrically controlled to switch to a Wenzel wetting state having high adhesion. When the electric field was removed, we observed spontaneous dewetting along the corrugations as the droplet transitioned from the Wenzel state back to a Cassie state.

Resist materials for advanced lithography

Increasing the understanding of the fundamental resist material characteristics is a necessary preamble to the development of resists with improved resolution and line edge roughness. Material characteristics will not only influence resist sensitivity and resolution, but also may influence the critical dimension control of the lithography process through its effects on line edge roughness (LER). Polymers with controlled molecular weights and polydispersities as well as several non-polymeric resist materials were prepared and studied. This entailed preparing novel derivatives of these non-polymeric materials that were compatible with photoimaging as positive acid catalyzed resists. Examples are presented where non-polymeric resist materials were isolated into single well-defined components that could be compared to mixtures of similar composition. Results are presented on materials properties such as surface roughness and resist resolution. Included in the results are examples of non-polymeric materials that are capable of sub 100-nm resolution as positive resists.

Resist deconstruction as a probe for innate material roughness

We developed an atomic force microscopy (AFM)-based technique to measure intrinsic material roughness after base development. This method involves performing an interrupted development of the resist film and measuring the resulting film roughness after a certain fixed film loss. Employing this technique, we have deconstructed the resist into component materials and established that the photoacid generator (PAG) is a major material contributor of film roughness and that PAG segregation in the resist is likely responsible for nanoscale dissolution inhomogeneities. Small differences in PAG concentration as a result of standing waves in the resist can lead to large changes in surface roughness due to PAG or PAG-photoproduct segregation and the resultant nonlinear change in nanoscale dissolution rates. The temperature dependence of the PAG segregation suggests that increased mobility of the PAG that occurs may be due to a lowering of the film Tg during the deprotection process.

Quantum efficiency of PAG decomposition in different polymer matrices at advanced lithographic wavelengths

The Dill ABC parameters for optical resists are typically determined by measuring the change in the intensity of transmitted light at the wavelength of interest as a function of incident energy. The effectiveness of the experiment rests with the fact that the resist optical properties change with exposure and that the optical properties are directly related to the concentration of PAG compound. These conditions are not typically satisfied in CA resists and thus C is unobtainable by this method. FT-IR spectroscopy can directly measure changes in the photoactive species by isolating and measuring absorbance peaks unique to the photoactive species. We employed the ProABC software, specially modified to allow FT-IR absorbance input, to extract ABS parameters through a best fit of the lithography model to experimental data. The quantum efficiency of PAG decomposition at 157-, 193-, and 248-nm was determined for four diazomethane type PAGs in four different polymer matrices. It was found that both the Dill C parameter and the quantum efficiency for all PAGs increased as wavelength decreased, but that the magnitude of the increase was strongly dependent on the polymer matrix.

Fast, electrically tunable filters for hyperspectral imaging

Tunable, narrow-wavelength spectral filters with a ms response in the mid-wave/long-wave infrared (MW/LWIR) are an enabling technology for hyperspectral imaging systems. Few commercial off-the-shelf (COTS) components for this application exist, including filter wheels, movable gratings, and Fabry-Perot (FP) etalon-based devices. These devices can be bulky, fragile and often do not have the required response speed. Here, we present a fundamentally different approach for tunable reflective IR filters, based on coupling subwavelength plasmonic antenna arrays with liquid crystals (LCs). Our device operates in reflective mode and derives its narrow bandwidth from diffractive coupling of individual antenna elements. The wavelength tunability of the device arises from electrically-induced re-orientation of the LC material in intimate contact with antenna array. This re-orientation, in turn, induces a change in the local dielectric environment of the antenna array, leading to a wavelength shift. We will first present results of full-field optimization of micron-size antenna geometries to account for complex 3D LC anisotropy. We have fabricated these antenna arrays on IR-transparent CaF2 substrates utilizing electron beam lithography, and have demonstrated tunability using 5CB, a commercially available LC. However, the design can be extended to high-birefringence liquid crystals for an increased tuning range. Our initial results demonstrate <60% peak reflectance in the 4- 6 μm wavelength range with a tunability of 0.2 μm with re-orientation of the surface alignment layers. Preliminary electrical switching has been demonstrated and is being optimized.

Switchable electrowetting of droplets on dual-scale structured surfaces

The authors report on the development of surfaces containing artificially fabricated structures of dual nanometer and micrometer surfaces that allow an aqueous droplet to be reversibly switched by electrowetting from a Cassie state with low adhesion to a Wenzel state with high adhesion. A variety of geometries were fabricated to study parameters that affect switchable wetting–dewetting. Nanometer parallel corrugations, posts, and holes were fabricated and combined with micrometer features consisting of parallel corrugations, streets, and checkerboard patterns of varying widths and pitches. It was observed that many combinations of the dual-textured surfaces produced superhydrophobic wetting states and aqueous droplets on these surfaces could be electrically controlled to switch from a Cassie state to a Wenzel state. Reversible switching between these wetting states occurred on specific combinations of surface geometries, namely surfaces that had parallel corrugations.