N. Biswas

HfO2 gate dielectric with 0.5 nm equivalent oxide thickness

Hafnium dioxide films have been deposited using reactive electron beam evaporation in oxygen on hydrogenated Si(100) surfaces. The capacitance–voltage curves of as-deposited metal(Ti)–insulator–semiconductor structures exhibited large hysteresis and frequency dispersion. With post-deposition annealing in hydrogen at 300 °C, the frequency dispersion decreased to less than 1%/decade, while the hysteresis was reduced to 20 mV at flatband. An equivalent oxide thickness of 0.5 nm was achieved for HfO2 thickness of 3.0 nm. We attribute this result to a combination of pristine hydrogen saturated silicon surfaces, room temperature dielectric deposition, and low temperature hydrogen annealing.

Plasma enhanced metalorganic chemical vapor deposition of amorphous aluminum nitride

Plasma enhanced deposition of amorphous aluminum nitride (AlN) using trimethylaluminum, hydrogen, and nitrogen was performed in a capacitively coupled plasma system. Temp. was varied from 350 to 550 DegC, and pressure dependence of the film structure was investigated. Films were characterized by Fourier transform IR, Rutherford backscattering (RBS), ellipsometry, and x-ray diffraction (XRD). The films are amorphous in nature, as indicated by XRD. Variations in the refractive index were obsd. in ellipsometric measurements, which is explained by the incorporation of carbon in the films, and confirmed by RBS. Capacitance-voltage, conductance-voltage (G-V), and current-voltage measurements were performed to reveal bulk and interface elec. properties. The elec. properties showed marked dependence on processing conditions of the AlN films. Clear peaks as obsd. in the G-V characteristics indicated that the losses are predominantly due to interface states. The interface state d. ranged between 1010 and 1011 eV-1 cm-2. Annealing in hydrogen resulted in lowering of interface state d. values.

Supercritical carbon dioxide extraction of porogens for the preparation of ultralow-dielectric-constant films

Supercritical carbon dioxide extraction of poly(propylene glycol) porogen from poly(methylsilsesquioxane) (PMSSQ) cured to temperatures adequate to initiate matrix condensation, but still below the decomposition temperature of the porogen, is demonstrated to produce nanoporous, ultralow-dielectric-constant thin films. Both closed and open cell porous structures were prepared simply by varying the porogen load in the organic/inorganic hybrid films. 25 and 55 wt % porogen loads were investigated in the present work. Structural characterization of the samples conducted using transmission electron microscope, small angle x-ray scattering, and Fourier transform infrared spectroscopy, confirms the extraction of the porogen from the PMSSQ matrix at relatively low temperatures (⩽200 °C). The standard thermal decomposition process is performed at much higher temperatures (typically in the range of 400 °C–450 °C). The values of dielectric constants and refractive indices measured are in good agreement with the stru...

Electrical properties of fluorinated amorphous carbon films

We have studied the capacitance–voltage (C–V), conductance–voltage (G–V), and current–voltage characteristics of fluorinated amorphous carbon (a-C:Fx) films using metal/a-C:Fx/Si and metal/a-C:Fx/metal structures, respectively. Samples annealed in a vacuum were also studied. The C–V curves of the as-deposited sample are stretched about the voltage axis. Interface state density of 4.1×1011 cm−2 eV−1 at the midgap was calculated. Annealing the sample deposited on Si in a vacuum caused more frequency dispersion in the C–V and G–V curves, probably due to the diffusion of carbon into silicon. The bulk density of states for samples deposited on metal, measured by space-charge-limited current technique, decreased from 4×1018 eV−1 cm−3 for the as-deposited sample, to 7×1017 eV−1 cm−3 for the annealed sample.

Electrical properties of amorphous silicon carbide films

The interfacial properties of silicon carbide films on silicon substrate, using capacitance–voltage (C–V) and conductance–voltage techniques have been studied. The C–V characteristics observed largely depended on processing conditions. An interface state density of 2.38×1010 eV−1 cm−2 at the midgap was calculated for this sample. When CF4 was added to the precursors, the C–V curves were observed to have a “ledge” at the end of the accumulation region, indicative of the presence of additional localized defect states that respond to the applied ac signal. An interface state density of 2.11×1011 eV−1 cm−2 at the midgap was observed for this sample. However fluorination in plasma caused the etching of the weak bond, and hence a reduction in the fixed charge density and hysteresis.

Supercritical CO2 Extraction of Porogen Phase: An Alternative Route to Nanoporous Dielectrics

We present a supercritical CO{sub 2} (SCCO{sub 2}) process for the preparation of nanoporous organosilicate thin films for ultra low dielectric constant materials. The porous structure was generated by SCCO{sub 2} extraction of a sacrificial poly(propylene glycol) (PPG) from a nanohybrid film, where the nanoscopic domains of PPG porogen are entrapped within the crosslinked poly(methylsilsesquioxane) (PMSSQ) matrix. As a comparison, porous structures generated by both the usual thermal decomposition (at ca. 450 C) and by a SCCO{sub 2} process for 25 wt% and 55 wt% porogen loadings were evaluated. It is found that the SCCO{sub 2} process is effective in removing the porogen phase at relatively low temperatures (< 200 C) through diffusion of the supercritical fluid into the phase-separated nanohybrids and selective extraction of the porogen phase. Pore morphologies generated from the two methods are compared from representative three-dimensional (3D) images built from small angle x-ray scattering (SAXS) data.

Electric field and temperature-induced removal of moisture in nanoporous organosilicate films

The effects of bias-temperature-stress (BTS) or simply temperature-stress (TS) on nanoporous low-k methylsilsesquioxane films are studied. Initially, the as-given and O2 ashed/etched films exhibit physical adsorption of moisture as revealed from the electrical behavior of the samples after 15 days. The temperature stressing at 170 °C volatilized the adsorbed water but is unable to remove chemisorb and hydrophillic Si–OH groups. As a result, the TS films remain susceptible to moisture. BTS at 170 °C also removes adsorbed water. More important, the surfaces under the metal-insulator structure were dehydroxylated by breaking the chemisorb Si–OH group facilitating the formation of siloxane bonds that prevents adsorption of moisture even after 60 days.

Creating Nanoporosity by Selective Extraction of Porogens Using Supercritical Carbon Dioxide/Cosolvent Processes

This work presents a novel approach using supercritical carbon dioxide (SCCO 2 ) to selectively extract poly(propylene glycol) (PPG) porogen from a poly(methylsilsesquioxane) (PMSSQ) matrix, which results in the formation of nanopores. Nanoporous thin films were prepared by spin-casting a solution containing appropriate quantities of PPG porogen and PMSSQ dissolved in PM acetate. The as-spun films were thermally cured at temperatures well below the thermal degradation temperature of the organic polymer to form a cross-linked organic/inorganic polymer hybrid. By selectively removing the CO 2 soluble PPG porogen, open and closed pore structures are possible depending upon the porogen load and its distribution in the matrix before extraction. In the present work, two different loadings of PPG namely 25 wt.% and 55 wt.% were used. Both static SCCO 2 and pulsed SCCO 2 /cosolvent treatments were used for PPG extraction. The initial results indicate that the pulsed SCCO 2 /cosolovent treatment was more efficient. Fourier transform infrared spectroscopy (FTIR) and refractive index measurements further corroborate the successful extraction of the porogens at relatively low temperatures (2000C). For the pure PMSSQ film, the k value is 3.1, whereas it is 1.46 and 2.27 for the open and closed pore compositions respectively after the static SCCO 2 extraction and 430°C subsequent annealing. The reduction in the k-value is attributed to the formation of nanopores. The pore structure was verified from transmission electron microscopy (TEM), and from small-angle x-ray scattering (SAXS) measurements, the pore size was determined to be 1-3 nm for these films.