R. R. Rye

Ultrahigh vacuum studies of Pd metal‐insulator‐semiconductor diode H2 sensors

Hydrogen sensitive Pd metal/insulator/semiconductor diodes provide an ambient temperature, low power electronic sensor for hydrogen as a result of hydrogen trapping at the Pd/insulator interface. Current kinetic models consider the rate limiting step to be adsorption at the Pd surface followed by rapid transport to the interface. We have obtained both steady‐state and kinetic results for diodes with clean Pd surfaces over hydrogen pressures ranging from 10−10 to 10−1 Torr. The sensitivity limit (equivalent to <1011 total H2 impacts/cm2 at the Pd surface) is set by our vacuum capabilities, and is at least seven orders of magnitude greater than that obtained for devices with contaminated surfaces. These results clearly show that the kinetic and sensitivity limitations reported for such devices are a result of surface contamination. For diodes with a clean Pd surface, analysis of the steady‐state results requires at least two binding states (9 and 6.8 kcal/mol of H relative to H2(g)) for H at the Pd/SiO2 int...

Electron spectroscopy of condensed multilayers: Line shape changes due to beam damage and excitation mode☆

Abstract Auger line-shape analysis, photoelectron spectroscopy, and thermal desorption spectroscopy have been used to study the effects of electron bombardment on condensed multilayers of (CH 3 ) 2 O, CH 3 OH, and H 2 O. The data show that electron doses as low as 5 × 10 −4 C/cm 2 (a 0.5 mm diameter, 1 μA beam for 1 sec) can cause detectable damage. New chemical species are created in the condensed layer by this electron beam interaction, and the data suggest that water and hydrocarbons are the most abundant. Auger spectra excited by X-rays and by electrons were shown to be different both before and after electron damage. This difference probably results from shake-up or shake-off processes which are sensitive to the specific mode of core-level excitation.

Interaction of hydrogen, methane, ethylene, and cyclopentane with hot tungsten: Implications for the growth of diamond films

Modulated‐beam mass spectrometry and x‐ray photoelectron spectroscopy (XPS) have been used to investigate the interaction of CH4, C2H4, C5H10, and H2 with carburized and uncarburized tungsten. It is shown that significant evaporation of C1, C2, and C3 occurs for carburized tungsten at temperatures above 1900 °C. The temperature dependence of the carbon evaporation rate was found to be similar to the temperature dependence of the diamond film deposition rate observed in chemical vapor deposition (CVD) reactors, similar to the temperature dependence for the carbon deposition rate observed in the present experiments, and similar to the expected evaporation rate of carbon from graphite and tungsten carbide. The desorption of hydrocarbon species (other than the incident gas) was not clearly observed under any conditions for methane or ethylene. In contrast, it is quite likely that cyclopentane decomposes at the surface to produce new species which are subsequently desorbed into the gas phase. The reaction of e...

Reaction of thermal atomic hydrogen with carbon

Abstract The reaction of thermal atomic hydrogen, produced at the surface of a hot tungsten filament, with evaporated carbon films leads to the production of a number of gaseous products. At least 12 have been identified with the largest number of these being cyclic compounds consistent with the structure of graphite. Methane, the major gas phase product (74% partial pressure), is produced with a stoichiometric reaction probability of 0.002. Although the relative reaction rates decrease rapidly with increasing product mass, the total rate of carbon transport is 60% greater than the rate of methane production. Activation energies obtained for 10 of the products by varying the carbon temperature are the same within experimental error. These features are discussed in terms of a highly simplified model of the reaction with the actual reaction following a complicated branching free radical mechanism.

HOT-FILAMENT-ACTIVATED CHEMICAL-VAPOR DEPOSITION OF CARBON : FILM GROWTH AND FILAMENT REACTIONS

Pure glassy carbon films [no x‐ray photoelectron spectroscopy (XPS) detectable impurities above the 0.5% level] as thick as 25 000 A have been grown on nearby silicon substrates (T≳100 °C) as a result of reactions between a hot tungsten filament and cyclopentane. Above ∼2500 °C, cyclopentane‐tungsten reactions yield a liquid W/C eutectic which limits filament operation. Below ∼2500 °C, resistance changes of the filament and XPS spectra show such reactions form carbides and graphite. It is shown that the temperature dependence of the carbon deposition rate is similar to the sublimation rate of carbon from graphite and tungsten carbide. Moreover, it is also shown that C1, C2, and C3 (carbon monomers, dimers, and trimers) are evaporated from carbarized tungsten and also from graphite. These results suggest that carbon film growth is a consequence of evaporation of carbon from the carbarized tungsten filament, with steady‐state film deposition occurring as a result of a quasisteady state in the formation and ...

Nanomechanical basis for imaging soft materials with tapping mode atomic force microscopy

The surfaces of virgin and chemically etched poly(tetrafluoroethylene) (PTFE) have been studied using scanning electron microscopy (SEM), and atomic force microscopy (AFM) in both contact and tapping modes. Contact mode AFM images of this relatively soft polymeric material are dominated by tip‐induced imaging artifacts. When subsequent, AFM imaging was performed in tapping mode these artifacts were eliminated, and comparable tapping mode AFM and SEM images were obtained for even the highly porous, unstable surface that results from sodium naphthalenide etching. Interfacial force microscopy force versus displacement, and creep experiments were performed to determine the nanomechanical nature of virgin PTFE. These experiments show that virgin PTFE is a viscoelastic material which is capable of supporting large forces on the millisecond time scale but creeps dramatically at longer times. Clearly, with scanning probe techniques which utilize constant probe force feedback, one should expect image distortions, ...

Formation of copper patterns on poly(tetrafluoroethylene) via radiation controlled chemical etching and chemical-vapor deposition

Patterned copper films have been deposited on poly(tetrafluoroethylene) (PTFE) in a three‐step additive process. In the first step, a pattern is produced by cross linking the PTFE surface in selected areas by irradiation with either electrons or x rays at dose levels below those that are either visually or spectroscopically apparent. The pattern is then developed by wet chemical etching in the second step in which only the nonirradiated areas are appreciably etched with sodium naphthalenide. In the final step, chemical‐vapor deposition using the precursor (hexafluoroacetylacetonato) Cu(I) trimethylphosphine at 200 °C results in Cu deposition only on the nonirradiated areas of the surface. The Cu films are continuous with a resistivity of 4 μΩ cm, high purity as determined by Auger electron spectroscopy and x‐ray photoelectron spectroscopy, and are sufficiently adherent to survive a Scotch tape test. Patterned feature sizes as small as 35 μm can be produced.

Photolithographic metallization of fluorinated polymers

Abstract Electroless or chemical vapor deposition of copper onto commercial samples of skived poly(tetrafluoroethylene) (PTFE) that have been chemically etched with sodium naphthalenide results in Cu films sufficiently adherent that attempts to remove the Cu cause near-cohesive failure in the PTFE. Such strong Cu adhesion forms the basis for several approaches to the production of high-resolution PTFE-based printed-circuit boards. Similarly strong adhesion does not occur to melt-processed fluorocarbon polymers (Teflon-AF, FEP, Teflon-PFA, etc.) nor to samples of PTFE annealed by hot pressing or mechanical polishing, nor to radiation-crosslinked PTFE. Adhesion to etched, skived PTFE is dominated by mechanical interlocking due to penetration of Cu into the highly-crazed, porous surface produced by etching of the stressed surface caused by skiving. Patterned irradiation (electrons, X-rays, etc.) at low dose levels results in crosslinking of virgin PTFE, preventing appreciable chemical etching and subsequent metal adhesion in the irradiated areas, and resulting in a three-step process (irradiation, chemical etching, and metal deposition) with a non-optimized lateral resolution of 35 μm. Optical power absorption by an etched PTFE layer is a factor of 300 greater than for underlying (virgin) PTFE, allowing a second three-step approach to patterned metallization: chemical etching, patterned excimer-laser ablation, and metal deposition. A third approach combines chemical etching and uniform thin-metal-film deposition with standard photolithographic processing techniques, resulting in a patterned metallization process where adhesion is controlled by the initial etching step and resolution is controlled by standard lithographic technology. A non-optimized feature resolution of 17 μm for 4-μm thick Au conductors has been demonstrated with this technique.

Characterization of the copper-poly(tetrafluoroethylene) interface

Using Rutherford backscattering spectroscopy (RBS) and x-ray photoelectron spectroscopy (XPS), the authors have shown that strong adhesion of electrolessly deposited Cu to etched poly(tetrafluoroethylene) (PTFE) results from penetration of all species (tin oxide from the sensitization step, Pd from the nucleation step, and electrolessly deposited Cu) into the porous, carbon-rich, 3,000 A-deep chemically etched layer. Measurements of the deposited Cu films show a yield strength comparable to commercial Cu-clad PTFE. XPS analysis of both failure surfaces show only C(1s) and F(1s) peaks characteristic of virgin PTFE with a small amount of the C(1s) peak characteristic of etched PTFE. Near-cohesive failure occurs at a depth into the etched layer where bulk PTFE characteristics are approached but at depths greater than those shown from RBS depth profiles to be accessible to Cu penetration. Line-of-sight thermal evaporation of Cu yields Cu RBS depth profiles that are identical to those obtained from electrolessly deposited (isotropic) Cu, suggesting that the structure of the PTFE pores into which the Cu mechanically interlocks is very open, with few convolutions.

Direct deposition of patterned copper films on Teflon

Abstract Current subtractive methods yielding patterned Cu features on PTFE substrates rely on wet Cu etching processes. We have developed three variations of a new dry, additive patterning process. The mechanisms for patterning include MgKα X-ray-induced cross-linking, e-beam-induced cross-linking, or laser patterning. The X-ray and e-beam patterning processes rely on irradiation followed by selective etching of the non-irradiated areas. The laser patterning begins by chemically etching PTFE which leaves a rough surface with good adhesion characteristics. An argon-ion laser beam is then used to selectively remove the etched layer, revealing the underlying surface which has physical properties closely resembling unmodified Teflon. Typical laser patterning conditions are scan rates of 0.005–5.5 mm/s, incident powers of 40–380 mW at 514 nm, and base pressures of 10-2 Torr and at atmospheric pressure in air. In all cases, CVD from (β-diketonate)CuL compounds is used to deposit copper only on the etched regions of the sample, leaving the irradiated regions copper-free. The advantages of this procedure are: (1) subtractive Cu wet etching of copper is avoided, so no masking techniques are necessary and no liquid waste is generated; (2) ten-micron-sized features can be produced; (3) excellent adhesion is obtained.