Richard H. Stulen

Hydrogen chemisorption and the structure of the diamond C(100)-(2 × 1) surface

Abstract Upon heating to greater than 1300 K, diamond C(100)-(1 × 1) reconstructs, exhibiting a low energy electron diffraction (LEED) pattern with two domains of (2 × 1) symmetry. No evidence for higher order reconstructions (e.g., c(4 × 2)) is observed, ruling out correlated buckled dimers as a possible structure for the reconstruction. Hydrogen is found to play an important role in the transformation of the (1 × 1) surface during reconstruction. The presence of chemisorbed hydrogen on the (1 × 1) surface is monitored by electron-stimulated desorption time-of-flight spectroscopy (ESD-TOF) as a function of anneal temperature. Two distinct H+ ESD velocity distributions, fast and slow, are observed in the TOF spectra on the polished (1 × 1) surface. Upon heating, the fast component disappears with the onset of the (2 × 1) reconstruction. The slower of the two features remains even after annealing at temperatures of up to 1530 K, demonstrating that some hydrogen remains on the reconstructed (2 × 1) surface. Temperature-programmed desorption (TPD) of H2 from the diamond C(100)-(1 × 1) surface also correlates with the surface reconstruction and the loss of the fast proton peak in the ESD spectra. For the “as polished” surface, the TPD results yielded an integrated hydrogen desorption flux of up to ten monolayers (based on two hydrogen atoms per surface carbon atom, e.g. 3.14 × 1015 H cm−2) indicating that the near-surface region for the as-polished diamond C(100)-(1 × 1) contains a large quantity of absorbed hydrogen. At low hydrogen coverage, the desorption process appears to be first order with an activation energy of ∼ 37 kcal mol and a first order pre-exponential of ∼ 3 × 105s−1. At higher coverages the apparent order increases and the activation energy decreases. Ultraviolet photoelectron spectroscopy (UPS) of the diamond C(100)-(1 × 1) does not reveal any occupied states in the band gap from the valence band maximum (VBM) to the Fermi level (EF). In contrast, UPS for diamond C(100)-(2 × 1) exhibits occupied states in the band gap from the VBM to 1.5 eV above the VBM. No empty states could be found from 1.2 to 5.5 eV above EF for this surface using two-photon photoemission. By analogy to the Si and Ge(100) surfaces, the proposed model for the diamond C(100)-(2 × 1) surface is based on the formation of dimer pairs. The sharp two domain (2 × 1) LEED pattern, the presence of hydrogen on the reconstructed surface, and the observation of occupied surface states in the band gap suggest that symmetric dimers with monohydride termination at each carbon atom of the pair is the most likely structure.

Extreme ultraviolet lithography

Current microlithography used in high-volume integrated circuit manufacturing employs some form of optical projection technology. The most advanced tools use deep-ultraviolet (DUV) radiation having a wavelength of 248 nm and are used to print 250-nm features. These tools will likely be extended for use at the 180-nm generation and perhaps below. New DUV tools using 193-nm radiation are actively under development and are expected to be used for 130-nm generation and perhaps even 100-nm generation. Extending these DUV optical projection tools for manufacturing in the 100-200-nm region will be paced by the development of new high numerical aperture imaging systems and highly complex phase shift masks. For future generations of integrated circuits with minimum feature sizes below 100 nm, 193-nm tools will have great difficulty meeting all manufacturing requirements. This paper describes an alternate optical approach, for sub-100-nm generations, based on extreme ultraviolet radiation at around 13 nm, called extreme ultraviolet lithography (EUVL). This approach uses a laser-produced plasma source of radiation, a reflective mask, and a 4/spl times/ reduction all-reflective imaging system. The technology is currently in the engineering development phase for an alpha machine. This paper reviews its current status and describes the basic modules or building blocks of a generic EUVL exposure tool.

Effect of surface temperature on the sorption of hydrogen by Pd(111)

Temperature programmed desorption (TPD) subsequent to various hydrogen exposure conditions indicates the formation of chemisorption, solid solution, and hydride phases of hydrogen in the near surface region of Pd(111). Variation of the sample exposure temperature (Te) between 80 and 300 K has a strong effect on the subsequent TPD spectra. At Te = 80 K a single desorption peak, β, appears at 310 K. Coverage variation of the β peak is consistent with second-order recombinative desorption of chemisorbed hydrogen. For Te between 90 and 140 K a slight enhancement of the β peak occurs and a new peak, α, appears initially near 170 K. It does not saturate, exhibits near-zeroth-order desorption kinetics, and is assigned to the decomposition of a near surface palladium hydride phase. Population of the α peak is thermally activated with a maximum at Te ≈ 115 K. For Te, greater than 140 K, α disappears while the total amount of absorbed hydrogen increases significantly. At these temperatures, the concentration of absorbed hydrogen decreases significantly if the sample is held in vacuo at Te after completion of the hydrogen exposure. At all exposure temperatures there is also a broad desorption feature near 800 K which is enhanced by higher Te and is associated with hydrogen in solid solution.

The role of hydrogen on the diamond C(111)−(2 × 1) reconstruction

Abstract The correlation between hydrogen adatom coverage and the diamond C(111)-(1 × 1) to (2 × 1) structural phase transition has been investigated with electron-stimulated desorption time-of-flight (ESD-TOF) mass spectrometry and low energy electron diffraction (LEED). In addition, electron-stimulated desorption ion angular distributions (ESDIAD) have been recorded to investigate C-H bonding on the hydrogen-terminated (1 × 1) surface. The H+ desorption angular distribution is found to be strongly peaked along the surface normal, consistent with H termination of carbon dangling bonds oriented along the normal. A sequence of thermal annealing treatments induced the reconstruction of the (1 × 1) surface to the (2 × 1) surface. The emergence of half-order diffraction beams occurred only after heating to 1275 K; at this temperature a fivefold decrease in the H+ ESD signal intensity was also observed. Further annealing produced a sharpening and a doubling of the half-order spot intensities but only a slight further decrease in the H+ ESD signal intensity. When compared to model calculations, these measurements indicate that hydrogen loss from the surface is necessary but not sufficient to induce the formation of the ordered (2 × 1) structure: annealing after hydrogen desorption is required for the complete reconstruction.

Electron-stimulated desorption and thermal desorption spectrometry of H2O on nickel (111)

The adsorption of H2O at low temperature on clean and oxygen dosed nickel (111) has been studied using thermal desorption spectrometry (TDS) and electron-stimulated desorption (ESD). Both the single and multilayer H2O regimes have been investigated. For first layer chemisorbed water, the thermal desorption data can be modeled well assuming first order kinetics with a desorption energy of about 9.8 kcalmol. For ice multilayers, a leading edge analysis gives a desorption energy of 11.5 ± 0.5 kcalmol. ESD measurements detected both H+ and H2O+ ions, and several types of ESD measurements were made: determination of ion yield versus electron beam energy, determination of ion kinetic energy distribution curves (lEDCs), and determination of the temperature dependence of the ion yield (TDIY) at fixed excitation energy. The latter measurements show a correlation with the thermal desorption data: TDIY of H2O+ during desorption of multilayer H2O films gives 11.0 ± 0.5 kcalmol for the desorption energy, in excellent agreement with the TDS results. Additional features are resolved in the temperature dependence of the ESD signal which are absent in the TDS spectra. Characteristic ion yield and ion kinetic energy distribution curves are given for adsorbed layers on the clean surface in both the single and ice multilayer regime. For the oxygen predosed surface, TDS was used to isolate a H2O desorption state at higher temperature which is due to dissociative recombination, and ESD was used to characterize the ion yields and IEDCs of H+ from hydroxyl groups adsorbed on this type of surface. Comparison of the data for H+ ions from adsorbed water and hydroxyls indicates that ESD in this case is sensitive primarily to the local parent O-H bond, and insensitive to secondary chemical bonds.

Mechanisms of electron-stimulated desorption of protons from water: Gas, chemisorbed and ice phases☆

Abstract The stimulated desorption of ions from gas phase and condensed phase H 2 O on Ni(111) has been examined theoretically and experimentally for the near threshold excitation region, 15 to 40 eV. The excited state potential energy curves have been calculated using configuration interaction for H 2 O and a restricted Hartree-Fock (RHF) approach for a variety of small clusters including (H 2 O) 5 and NiH 2 O. Both proton yield and kinetic energy distributions have been measured for chemisorbed, ice phase, and gas phase water and are discussed in terms of specific electronic excitations corresponding to possible desorption pathways. For condensed phase water, the major proton desorption threshold occurs at 20–21 eV and is due to surface predissociation. The final state potential energy curves reached in this process are, in general, described by two electron excitations from the ground state and are thus not dipole allowed. At threshold, these potential energy curves correspond to the excited states of the neutral rather than the ionized molecule. Above 28–29 eV, predissociation or shake-up involving excitations from the O 2s orbital contributes to the ion yield and can give rise to protons of high (7–8 eV) kinetic energy.

Diffraction-limited soft-x-ray projection imaging using a laser plasma source

Projection imaging of 0.1-microm lines and spaces is demonstrated with a Mo/Si multilayer coated Schwarzschild objective and 14-nm illumination from a laser plasma source. This structure has been etched into a silicon wafer by using a trilevel resist and reactive ion etching. Low-contrast modulation at 0.05-microm lines and spaces is observed in polymethylmethacrylate.

System integration and performance of the EUV engineering test stand

The Engineering Test Stand (ETS) is a developmental lithography tool designed to demonstrate full-field EUV imaging and provide data for commercial-tool development. In the first phase of integration, currently in progress, the ETS is configured using a developmental projection system, while fabrication of an improved projection system proceeds in parallel. The optics in the second projection system have been fabricated to tighter specifications for improved resolution and reduced flare. The projection system is a 4-mirror, 4x-reduction, ring-field design having a numeral aperture of 0.1, which supports 70 nm resolution at a k1 of 0.52. The illuminator produces 13.4 nm radiation from a laser-produced plasma, directs the radiation onto an arc-shaped field of view, and provides an effective fill factor at the pupil plane of 0.7. The ETS is designed for full-field images in step-and-scan mode using vacuum-compatible, magnetically levitated, scanning stages. This paper describes system performance observed during the first phase of integration, including static resist images of 100 nm isolated and dense features.

First lithographic results from the extreme ultraviolet Engineering Test Stand

The extreme ultraviolet (EUV) Engineering Test Stand (ETS) is a step-and-scan lithography tool that operates at a wavelength of 13.4 nm. It has been developed to demonstrate full-field EUV imaging and acquire system learning for equipment manufacturers to develop commercial tools. The initial integration of the tool is being carried out using a developmental set of projection optics, while a second, higher-quality, projection optics is being assembled and characterized in a parallel effort. We present here the first lithographic results from the ETS, which include both static and scanned resist images of 100 nm dense and isolated features throughout the ring field of the projection optics. Accurate lithographic models have been developed and compared with the experimental results.

Hydrogen desorption from chemical vapor deposited diamond films

Temperature programmed desorption was used to measure the desorption kinetics of hydrogen and its isotopes from chemical vapor deposited diamond surfaces. The desorption spectra are surprisingly simple considering the polycrystalline nature of the sample, exhibiting a single peak at ∼1300 K for a heating rate of 6 K/s. There is no isotope effect to the desorption, and neither the position of the peak maximum nor the peak width change with increasing hydrogen coverage. The maximum surface coverage achieved is approximately one monolayer. The spectra can be represented by a single peak first order desorption model, yielding kinetic parameters of Ea=51 kcal/mol and ν=5×107 s−1. An alternate model of multiple desorption sites with a Gaussian‐distributed population gives kinetic parameters of Ea,mean=82 kcal/mol, ν=9×1012 s−1, and σ (the width of the Gaussian distribution)=3 kcal/mol. A comparison to desorption from low‐index natural diamond surfaces is presented.