Ina T. Martin

Comparison of pulsed and downstream deposition of fluorocarbon materials from C3F8 and c-C4F8 plasmas

Materials deposited in continuous wave (cw) and pulsed low-pressure octafluoropropane (C3F8) and octafluorocyclobutane (c-C4F8) plasmas were characterized using Fourier transform infrared spectroscopy (FTIR), x-ray photoelectron spectroscopy, static contact angle measurements, spectroscopic ellipsometry, and scanning electron microscopy (SEM). Fluorocarbon (FC) materials deposited in pulsed plasmas were less crosslinked than those deposited in cw plasmas with equivalent input powers. Within each system, higher F/C ratio materials were deposited by lowering the plasma input power/duty cycle. Using downstream depositions had a similar effect on film composition, but also resulted in decreased deposition rates. SEM analysis showed that decreases in the flexibility of the fluorocarbon films were correlated with increases in the percent of crosslinking. Additionally, the smoothness of the film surfaces suggests that polymerization processes occur on the substrate surface. Overall, films deposited in C4F8 plasm...

Angle-resolved XPS analysis and characterization of monolayer and multilayer silane films for DNA coupling to silica.

We measure silane density and Sulfo-EMCS cross-linker coupling efficiency on aminosilane films by high-resolution X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) measurements. We then characterize DNA immobilization and hybridization on these films by (32)P-radiometry. We find that the silane film structure controls the efficiency of the subsequent steps toward DNA hybridization. A self-limited silane monolayer produced from 3-aminopropyldimethylethoxysilane (APDMES) provides a silane surface density of ~3 nm(-2). Thin (1 h deposition) and thick (19 h deposition) multilayer films are generated from 3-aminopropyltriethoxysilane (APTES), resulting in surfaces with increased roughness compared to the APDMES monolayer. Increased silane surface density is estimated for the 19 h APTES film, due to a ∼32% increase in surface area compared to the APDMES monolayer. High cross-linker coupling efficiencies are measured for all three silane films. DNA immobilization densities are similar for the APDMES monolayer and 1 h APTES. However, the DNA immobilization density is double for the 19 h APTES, suggesting that increased surface area allows for a higher probe attachment. The APDMES monolayer has the lowest DNA target density and hybridization efficiency. This is attributed to the steric hindrance as the random packing limit is approached for DNA double helices (dsDNA, diameter ≥ 2 nm) on a plane. The heterogeneity and roughness of the APTES films reduce this steric hindrance and allow for tighter packing of DNA double helices, resulting in higher hybridization densities and efficiencies. The low steric hindrance of the thin, one to two layer APTES film provides the highest hybridization efficiency of nearly 88%, with 0.21 dsDNA/nm(2). The XPS data also reveal water on the cross-linker-treated surface that is implicated in device aging.

Investigation of inductively coupled Ar and CH4/Ar plasmas and the effect of ion energy on DLC film properties

Gas-phase and surface analysis techniques were utilized to investigate the effects of gas-phase species on plasma deposited diamond-like carbon (DLC) thin films. A vacuum system was built to perform Langmuir probe and energy analysis-based mass spectrometry measurements to characterize the gas-phase of low pressure, 13.56?MHz inductively coupled plasma molecular beams. Low-energy peaks contributed significantly to the total area of the ion energy distributions (IEDs) measured for Ar+ in Ar and CH4/Ar plasmas. In contrast, for all other ions in these systems, the low-energy peaks had a lower contribution to the IEDs as a result of the low probability of energy exchange via ion?neutral collisions. Hydrogenated DLC films were deposited on silicon wafers at different substrate potentials to determine the effect of ion bombardment on film properties. Films were characterized via Fourier transform infrared spectroscopy, scanning electron microscopy, atomic force microscopy and nanoindentation measurements. The hydrogen content, surface roughness and deposition rate decreased, whereas the hardness of the films increased when a negative bias voltage was applied. These results demonstrate that ion energy has a significant effect on the composition and morphology of plasma deposited DLC films.

Controlled Nitrogen Doping and Film Colorimetrics in Porous TiO2 Materials Using Plasma Processing

Nitrogen doping of TiO(2) films (N:TiO(2)) has been shown to improve the visible-light sensitivity of TiO(2), thereby increasing the performance of both photovoltaic and photocatalytic devices. Inductively coupled rf plasmas containing a wide range of nitrogen precursors were used to create nitrogen-doped TiO(2) films. These treatments resulted in anatase-phased materials with as high as 34% nitrogen content. As monitored with high-resolution X-ray photoelectron spectroscopy spectra, the nitrogen binding environments within the films were controlled by varying the plasma processing conditions. XPS peak assignments for multiple N 1s binding environments were made based on high resolution Ti 2p and O 1s XPS spectra, Fourier transform infrared spectroscopy (FTIR) data, and literature N 1s XPS peak assignments. The N:TiO(2) films produced via plasma treatments displayed colors ranging from gray to brown to blue to black, paralleling the N/Ti ratios of the films. Three possible mechanisms to explain the color changes in these materials are presented.

On the importance of ions and ion-molecule reactions to plasma-surface interface reactions

Ions are known to be key players in many plasma processes, including anisotropic etching, film deposition and surface modification. The relationship between plasma ions, film properties, and surface interactions of other plasma species is not, however, well known. Using our Imaging of Radicals Interacting with Surfaces (IRIS) technique, along with plasma ion mass spectrometry (PI-MS), and surface analysis data, we have measured the effects of ion bombardment on the surface interactions of SiF4 in SiF4 plasmas and of CF2 in C3F8 and C4F8 plasmas. SiF2 is a known product of F-atom etching of Si, and CF2 has also been cited as a product of fluorocarbon etching of Si. With both molecules, we measure surface generation when the surface is bombarded by all the plasma species. Removal of ions from the plasma molecular beam results in a net decrease in surface generation for both molecules at all powers. Results in both systems are compared with the gas-phase ion-molecule reaction data of Armentrout and coworkers. Preliminary guided-ion beam mass spectrometry results taken in the Armentrout laboratories for the Ar+ + C3F8 reaction system are also presented.

Ion effects on CF2 surface interactions during C3F8 and C4F8 plasma processing of Sia)

Surface interactions of difluorocarbene (CF2) molecules were investigated using our LIF based imaging of radicals interacting with surfaces (IRIS) apparatus. LIF data of CF2 in C3F8 and C4F8 plasma molecular beams reveal that the relative densities of CF2 increase with increasing rf power and source pressure in both plasma systems. The surface reactivity of CF2 molecules during C3F8 and C4F8 plasma processing of room temperature Si substrates was also measured over a broad rf power range and at different pressures. A scatter coefficient (S) greater than one was measured for all unperturbed systems, indicating that CF2 molecules are produced at the substrate surface during film deposition. The same systems were also studied under ion-limited conditions, yielding S∼1, clear indication that ions are partially responsible for CF2 surface production. Plasma ions were identified using plasma-ion mass spectrometry. These data indicate that higher levels of CxFy+(x>1) are produced in the C4F8 plasmas. X-ray photo...

High-Resolution X-ray Photoelectron Spectroscopy of Mixed Silane Monolayers for DNA Attachment

The amine density of 3-aminopropyldimethylethoxysilane (APDMES) films on silica is controlled to determine its effect on DNA probe density and subsequent DNA hybridization. The amine density is tailored by controlling the surface reaction time of (1) APDMES, or (2) n-propyldimethylchlorosilane (PDMCS, which is not amine terminated) and then reacting it with APDMES to form a mixed monolayer. High-resolution X-ray photoelectron spectroscopy (XPS) is used to quantify silane surface coverage of both pure and mixed monolayers on silica; the XPS data demonstrate control of amine density in both pure APDMES and PDMCS/APDMES mixed monolayers. A linear correlation between the atomic concentration of N atoms from the amine and Si atoms from the APDMES in pure APDMES films allows us to calculate the PDMCS/APDMES ratio in the mixed monolayers. Fluorescence from attached DNA probes and from hybridized DNA decreases as the percentage of APDMES in the mixed monolayer is decreased by dilution with PDMCS.

Correlating ion energies and CF2 surface production during fluorocarbon plasma processing of silicon

Ion energy distribution (IED) measurements are reported for ions in the plasma molecular beam source of the imaging of radicals interacting with surfaces (IRIS) apparatus. The IEDs and relative intensities of nascent ions in C3F8 and C4F8 plasma molecular beams were measured using a Hiden PSM003 mass spectrometer mounted on the IRIS main chamber. The IEDs are complex and multimodal, with mean ion energies ranging from 29to92eV. Integrated IEDs provided relative ion intensities as a function of applied rf power and source pressure. Generally, higher applied rf powers and lower source pressures resulted in increased ion intensities and mean ion energies. Most significantly, a comparison to CF2 surface interaction measurements previously made in our laboratories reveals that mean ion energies are directly and linearly correlated to CF2 surface production in these systems.

Dislocation-limited open circuit voltage in film crystal silicon solar cells

Carrier recombination at dislocations is a major source of efficiency loss in epitaxial film Si solar cells and significantly affects the open circuit voltage, VOC. We develop a simple empirical model that yields a logarithmic relationship between VOC and the dislocation density, which fits well to our data. Straightforward evaluation of device performance with this model provides qualitative information about the recombination activity at dislocations.