Colm M. Flannery

Critical properties of nanoporous low dielectric constant films revealed by Brillouin light scattering and surface acoustic wave spectroscopy

Thin porous films with nanometer pore sizes are the subject of intense interest, primarily because of their reduced dielectric constant k. The lack of useful characterization tools and the reduction in film mechanical properties with increasing porosity have severely hindered their development and application. We show that both Brillouin light scattering and surface acoustic wave spectroscopy allow one to measure density, porosity and stiffness properties of nanoporous methylsilsesquioxane films of low-k value. Excellent correlations are observed among independent measurements of density, porosity and the Young’s modulus which show that the results obtained are reliable and reveal properties of the films which are difficult or impossible to obtain using other techniques.

Acoustic wave properties of CVD diamond

Diamond is a material which displays exceptional electronic, thermal, tribological, elastic and acoustic properties. These exceptional properties make diamond extremely difficult to characterize. In particular, diamond displays the highest acoustic velocities of any material making it an ideal candidate for acoustic wave devices. The acoustic properties of CVD diamond have not been successfully characterized by conventional acoustic techniques. This has led to a lack of process control for production of standardized CVD diamond and has hindered development of diamond devices because of a lack of knowledge of diamond acoustic properties. This paper examines recent progress in characterization of acoustic properties of CVD diamond, with a particular emphasis on the propagation of laser-generated surface acoustic waves in polycrystalline plates and what it reveals about the properties and microstructure of polycrystalline black, white and highly oriented CVD diamond samples.

POLYCRYSTALLINE DIAMOND FILMS FOR ACOUSTIC WAVE DEVICES

Abstract A laser ultrasonics technique has been used to measure acoustic wave velocities in polycrystalline CVD diamond for the first time. Lamb (plate) waves propagate with very high velocities in the range 8700–12 200 m s −1 ; the lowest values are recorded for films with the lowest crystal quality and highest non-diamond content. High-quality films with differing crystal textures [(100), (110)] or thickness (250–535 μm) show little variation. The results are discussed in terms of the fabrication of a high-sensitivity flexural plate wave (FPW) device for sensing applications in hostile environments.

Characterization of acoustic Lamb wave propagation in polycrystalline diamond films by laser ultrasonics

The propagation of acoustic Lamb waves in free standing chemical vapor deposited polycrystalline diamond has been studied using a laser ultrasonic technique. The influence of film morphology, quality, and thickness on the waves has been assessed. Acoustic waves with high velocities in the range 8700–12 200 ms−1 were observed; the lowest values were recorded for films with the lowest crystal quality and highest nondiamond content. High quality films with differing crystal textures or thickness show little variation. The influence of temperature on the dispersion characteristics of Lamb wave propagating in a 50 mm diam polycrystalline diamond wafer were also investigated. Little variation was apparent across the range studied (30–250 °C). Material parameters extracted from the dispersion chracteristics of the acoustic signal together with scanning electron microscopy studies suggested that void, microcrack, and grain boundary density most influences the propagation of low frequency Lamb waves in free standi...

Elastic constants and dimensions of imprinted polymeric nanolines determined from Brillouin light scattering

Elastic constants and cross-sectional dimensions of imprinted nanolines of poly(methyl methacrylate) (PMMA) on silicon substrates are determined nondestructively from finite-element inversion analysis of dispersion curves of hypersonic acoustic modes of these nanolines measured with Brillouin light scattering. The results for the cross-sectional dimensions, under the simplifying assumption of vertical sides and a semicircular top, are found to be consistent with dimensions determined from critical-dimension small-angle x-ray scattering measurements. The elastic constants C(11) and C(44) are found to be, respectively, 11.6% and 3.1% lower than their corresponding values for bulk PMMA. This result is consistent with the dimensional dependence of the quasi-static Youngs modulus determined from buckling measurements on PMMA films with lower molecular weights. This study provides the first evidence of size-dependent effects on hypersonic elastic properties of polymers.

Acoustic wave propagation in free standing CVD diamond: Influence of film quality and temperature

Abstract Acoustic wave propagation in high quality, free standing polycrystalline CVD diamond wafers has been studied using a laser ultrasonic technique. The influence of wafer temperature in the range 30–250°C on the propagation characteristics of Lamb waves has been assessed. Acoustic wave propagation characteristics showed little variation with temperature in the range measured suggesting that CVD diamond material of the type studied here may be suitable for the fabrication of acoustic wave devices operating at elevated temperatures. Material parameters extracted from the acoustic signal together with SEM studies strongly suggest that void, microcrack and grain boundary density rather than sp 2 content most influence the propagation of low frequency Lamb waves in free standing CVD diamond films.

Laser Ultrasound: An Inspection Tool of Soft Porous Low‐Dielectric Constant Films for Microelectronic Interconnect

The demand for miniaturization in the microelectronics industry requires that the RC (Resistance‐Capacitance) factor be lowered to reduce interconnection delay, crosstalk and power loss. The most promising way to achieve this is by introducing porosity into the dielectric film material. We show that laser‐generated surface acoustic waves can successfully and rapidly characterize porosity/density and stiffness of these films. Complementary measurements from X‐ray reflectivity and Brillouin light scattering verify our results. We discuss why nanoindentation presents difficulties.

Characterisation of free-standing polycrystalline CVD diamond films by SAW-based laser ultrasonics

Knowledge of the mechanical and elastic properties of polycrystalline diamond films necessary if one is to use these materials for mechanical, acoustic and sensing purposes. Present there is very little information available about these properties for films greater than a few microns thickness. In this work we inspect free-standing polycrystalline diamond films of thickness in the 100s of microns range by means of wideband surface acoustic Rayleigh and Lamb waves with frequencies up to 300 MHz, generated using thermoelastic excitation by a short-pulse duration laser and detected with a PVDF foil steel-wedge transducer. Measured Rayleigh wave velocities provide useful data about the properties of black diamond, white diamond and highly oriented diamond, as well as demonstrating a sensitivity to film quality. These results represent a considerable step forward in acoustic characterisation of CVD diamond, in terms of accurate and useful acoustic and elastic measurements and as a nondestructive testing tool of film quality.

Elastodynamic characterization of imprinted nanolines

The advancement of imprint lithography as a method for fabricating nanostructures is impeded by a lack of effective tools for characterizing mechanical properties and geometry at the nanoscale. This paper describes progress in establishing methods for determining elastic moduli and cross sectional dimensions of imprinted nanolines from Brillouin light scattering (BLS) measurements using finite-element (FE) and Farnell-Adler models for the vibrational modes. An array of parallel nanoimprinted lines of polymethyl methacrylate (PMMA) with widths of ~65 nm and heights of ~140 nm served as a model specimen. Several acoustic modes were observed with BLS in the low-gigahertz frequency range, and the forms of the vibrational displacements were identified through correlation with calculations using measured bulk-PMMA moduli and density as input. The acoustic modes include several flexural, Rayleigh-like, and Sezawa-like modes. Fitting of Farnell-Adler calculations to the measured dispersion curves was explored as a means of extracting elastic moduli and nanoline dimensions from the data. Some of the values obtained from this inversion analysis were unrealistic, which suggests that geometric approximations in the model introduce significant systematic errors. In forward calculations, the frequencies determined with the FE method were found to more closely match experimental values, which suggests that this method may be more accurate for inversion analysis. Initial estimates of uncertainties in the FE calculations support this conclusion.

Nanoporous polymer film properties from Brillouin light scattering and surface acoustic wave spectroscopy

Nanoporous methylsilsesquioxane polymer films, because of their low dielectric constant, have outstanding potential for application to microelectronic interconnect. Here we show that Brillouin light scattering and surface acoustic wave spectroscopy may be applied to characterise hard-to-measure properties - density/porosity, Youngs modulus - of a range of polymer films from different sources. Porosity-stiffness dependences are investigated, as well as relationship to dielectric constant. Information about pore size and Poissons ratio may also be gained, as well as showing difficulties with nanoindentation measurements.