Sharadha Sambasivan

Effect of self-assembled monolayer film order on nanofriction

Self-assembled monolayers have increasingly been explored as potential protective films in devices against friction and adhesion. However, detailed characterization of the monolayer film structure is difficult. This article utilizes a combination of near edge x-ray absorption fine structure (NEXAFS) spectroscopy and Fourier transform infrared (FTIR) spectroscopy to determine the film structure in order to explain the observed nanofriction measurement results. A series of n-alkyltrichlorosilane self-assembled monolayer films with various chain lengths (C5–C30) was prepared on silicon (100) surfaces. Nanofriction measurements were conducted using an atomic force microscope. Results showed that the lowest friction was obtained with a C12 film with higher friction values observed for C5 and C30 films. To explain these observations, the x-ray absorption technique NEXAFS was used to quantitatively measure the surface molecular orientation (order) of these films. It was observed that C12, C16, and C18 films were...

Surface and bulk chemistry of chemically amplified photoresists: segregation in thin films and environmental stability issues

The performance of chemically amplified photoresists, including next generation thin film 157 nm fluorinated copolymers and blends, is affected by such phenomena as polymer/substrate and polymer/air interfacial (surface energy) effects, blend miscibility, small molecule diffusion in thin films, permeability of airborne contaminants, and interactions with products from the deprotection reaction. Using near edge x-ray absorption fine structure (NEXAFS) spectroscopy, it is possible to simultaneously probe the surface and bulk chemistry of chemically amplified photoresists to determine possible causes of pattern degradation, including post exposure delay induced material failure, blend component and small molecule diffusion/segregation to the photoresist surface, and interactions between components of the photoresist formulation and developer. The surface and bulk chemistry of model photoresists were analyzed in the NEXAFS vacuum chamber, equipped with in situ processing capabilities for exposure, controlled dosing of a model contaminant gas (NMP or water vapor), and heating, to quantify component segregation and identify surface phenomena that may be responsible for pattern degradation. For model 157 nm blend films, it was found that there is segregation of one component to the surface of the photoresist film, in excess of the composition of that component in the blend. For polymer blends the more hydrophobic or lower surface tension species will typically wet the film surface when heated in air. Segregation of photo-acid generator has also been demonstrated and the effect of reducing film thickness investigated. As photoresist film thickness continually decreases and the photoresists become increasingly sensitive to environmental contaminants, the interfacial and surface regions dominate the behavior of the material and it is crucial to understand both their physical and chemical nature.

Near edge x-ray absorption fine structure measurements of surface segregation in 157 nm photoresist blends

The surface and bulk chemistry of photoresist blends for use at the 157 nm node were analyzed using near edge x-ray absorption fine structure spectroscopy to quantify component segregation and identify surface phenomena that may impact pattern formation. Spectral combinations of the constituent polymers are used to fit the spectra of the blend films. Significant segregation of one component to the surface of the photoresist film was found, in excess of the composition of that component in the blend. The bulk data were consistent with initial blend compositions. As expected, the more hydrophobic or lower surface tension component wets the film surface even under typical photoresist processing conditions.

Effect of Cross-Linking Ultrahigh Molecular Weight Polyethylene: Surface Molecular Orientation and Wear Characteristics

Molecular orientation at the surface layer of cross-linked ultrahigh molecular weight polyethylene (UHMWPE) has been examined. Molecular orientation has been shown to affect the wear resistance and surface mechanical properties of UHMWPE under biomechanical loading conditions. This study utilizes a nondestructive synchrotron based soft x-ray technique; near edge x-ray absorption fine structure at the carbon K-edge to examine the degree of surface molecular orientation of UHMWPE subjected to various cross-linking/sterilization techniques as a function of stress and wear. UHMWPE samples prepared under gamma irradiation, ethylene-oxide (EtO) treatment, and electron beam irradiation were worn in a wear tester systematically. Results suggest that the cross-linking resists surface orientation when the samples were under tensile and biomechanical stresses. The molecular orientation in the C–C chains in the polymer showed a monotonic decrease with an increase in gamma irradiation dosage levels. EtO sterilized sam...

Near Edge X-Ray Absorption Fine Structure Measurements of the Interface between Bottom Antireflective Coatings and a Model Deprotected Photoresist

The interface between bottom anti-reflective coatings (BARCs) and a model deprotected photoresist, poly(4-hydroxystyrene) (PHS), was investigated using near edge x-ray absorption fine structure spectroscopy to identify mechanisms responsible for pattern degradation at the BARC/photoresist interface. Interactions at this interface can lead to pattern deviations such as footing, undercut, and pattern collapse. It was found that a residual layer is only formed when the bilayer is subject to ultraviolet exposure. The spectra of the BARC surfaces after photoresist processing and development show a combination of spectral features from both PHS and the BARC formulations. The data suggest that the residual layer results from interactions between crosslinker and photoresist that occur during normal photoresist processing.

NEXAFS Measurements of the Surface Chemistry of Chemically Amplified Photoresists

Near edge x‐ray absorption fine structure (NEXAFS) spectroscopy was used to quantify the surface composition profile (top 1 nm to 6 nm) of model chemically amplified photoresists with various photo‐acid generators. These materials are prone to interfacial and surface chemical changes that cause deviations in the desired lithographic pattern such as T‐topping and closure. If interfacial excess or depletion of the photo‐generated acid occurs, either from atmospheric contamination, evaporation, or segregation within the film, the resulting compositional heterogeneity will affect the interfacial photoresist structure, composition, and deprotection kinetics. A significant technical challenge lies in measuring the surface composition and extent of reaction with depth resolution at interfaces. Electron yield NEXAFS allows measurement of the surface chemical composition, particularly for carbon, fluorine, oxygen, and nitrogen. When exposed to vacuum ultraviolet x‐rays (soft x‐rays), the top surface of the materia...

BARC-resist interfacial interactions

With the increasing drive towards smaller feature sizes in integrated circuits and the consequent use of shorter exposure wavelengths, the imaging resist layer and underlying bottom anti-reflective coating (BARC) layer are becoming thinner. At this scale, the performance of chemically amplified resists can be adversely affected by the BARC-resist interfacial interactions. These interactions can cause distortion of resist profiles and lead to footing, undercut, or pattern collapse. BARC components can immensely influence the deprotection and dissolution properties of the resist. A thorough understanding of the physico-chemical interactions at these interfaces is essential to design and develop new material platforms with minimal adverse interactions and maximum compatibility between BARC and resist. Results are reported from studies of (A) surface versus bulk chemistry of BARC materials as a function of cure temperature, (B) the dependence of the thickness and composition of the residual layer (resist material remaining on the surface of the BARC after development) on BARC components, as determined by formulating the BARC or resist with an excess of various BARC components, and (C) the dependence of the residual layer thickness on crosslink density, exposure does, and resist bake temperature. The BARC thin films and the interphase between BARC and resist were characterized with near edge x-ray absorption fine structure (NEXAFS) spectroscopy. Surface chemical properties of BARC films were derived from contact angle measurements of various liquids on these thin films. Preliminary results from these studies indicate that some BARC components may migrate to the BARC-resist interphase and act as dissolution inhibitors. Similarly, small molecule additives in the resist may migrate into the BARC layer, causing chemical modifications.

X-ray absorption spectroscopy to probe interfacial issues in photolithography

We utilize near edge X-ray absorption fine structure spectroscopy (NEXASFS) to provide detailed chemical insight into two interfacial problems facing sub-100 nm patterning. First, chemically amplified photo-resists are sensitive to surface phenomenon, which causes deviations in the pattern profile near the interface. Striking examples include T-topping, closure, footing, and undercutting. NEXAFS was used to examine surface segregation of a photo-acid generator at the resist/air interface and to illustrate that the surface extent of deprotection in a model resist film can be different than the bulk extent of deprotection. Second, line edge roughness becomes increasingly critical with shrinking patterns, and may be intimately related to the line edge deprotection profile. A NEXAFS technique to surface depth profile for compositional gradients is described with the potential to provide chemical information about the resist line edge.

Focusing multilayer mirror detection system for carbon K edge soft x-ray absorption spectroscopy (invited)

Fluorescence yield carbon K edge soft x-ray absorption measurements of mixed element samples are impeded by background scattered x-rays and fluorescence from noncarbon atoms within the sample induced by the incident x-ray beam (first and higher orders). We describe a high efficiency near-normal incidence focusing multilayer mirror detection system for carbon K edge fluorescence yield soft x-ray absorption spectroscopy. A spherical focusing multilayer mirror collects nearly 60% of the available solid angle with a reflectivity of 6% and a resolution of 2% (dE/E) for carbon K radiation in combination with a high efficiency proportional counter detector. The focusing multilayer system is able to effectively discriminate the background fluorescence and scattered light signals originating at the sample during a carbon K edge soft x-ray absorption experiment. A signal-to-background ratio in excess of 100 is possible for very dilute carbon samples, enabling a whole new class of in situ photon-in photon-out soft x...