Molecular
transport and morphological change were examined in films
of amorphous solid water (ASW). A buried N2O4 layer absorbs pulsed 266 nm radiation, creating heated fluid. Temperature
and pressure gradients facilitate the formation of fissures through
which fluid travels to (ultrahigh) vacuum. Film thickness up to 2400
monolayers was examined. In all cases, transport to vacuum could be
achieved with a single pulse. Material that entered vacuum was detected
using a time-of-flight mass spectrometer that recorded spectra every
10 μs. An ASW layer insulated the N2O4 layer from the high-thermal-conductivity MgO substrate; this was
verified experimentally and with heat-transfer calculations. Laser-heated
fluid strips water from fissure walls throughout its trip to vacuum.
Experiments with alternate H2O and D2O layers
reveal efficient isotope scrambling, consistent with water reaching
vacuum via this mechanism. It is likely that ejected water undergoes
collisions just above the film surface due to the high density of
material that reaches the surface via fissures, as evidenced by complex
temporal profiles extending past 1 ms. Little material enters vacuum
after cessation of the 10 ns pulse because cold ASW near the film
surface freezes material that is no longer being heated. A proposed
model is in accord with the data.
Thin films comprised of 400 -500 monolayers (ML) of either amorphous solid water (ASW) or ASW/CO 2 mixtures are grown atop a MgO(100) substrate under ultrahigh vacuum conditions. Samples are irradiated at an infrared frequency of 3424 cm -1 , which lies within the broad OH stretch band of condensed water. Ablation is achieved using 10 ns pulses whose energy (< 2.7 mJ) is focused to a beam waist of approximately 0.5 mm. By using a time-of-flight mass spectrometer to monitor ablated material, excellent single-shot detection is demonstrated. This capability is essential because, in general, the first infrared pulse can induce irreversible changes throughout the irradiated volume. With ASW/CO 2 samples, CO 2 is released preferentially. This is not surprising in light of the metastability of the samples. Indeed, repeated irradiation of the same spot can rid the sample of essentially all of the CO 2 in as little as a few pulses, whereas only 10-20 ML of H 2 O are removed per pulse. The influence of the substrate is profound. It cools the sample efficiently because the characteristic time for heat transfer to the substrate is much less than the infrared pulse duration. This creates temperature gradients, thereby quenching processes such as explosive boiling (phase explosion) and the heterogeneous nucleation of cavities that take place at lower depths in significantly thicker samples, i.e., with sufficient inertial confinement. This efficient quenching accounts for the fact that only 10-20 ML of H 2 O are removed per pulse. The presence of small protonated water cluster ions in the mass spectra is interpreted as evidence for the trivial fragmentation mechanism examined assiduously by Lewis and coworkers. Mixed samples such as ASW/CO 2 , where species segregation plays a pivotal role, add interesting and potentially useful dimensions to the ablation phenomenon.
This work describes the use of Single Particle Inductively Coupled Plasma Mass Spectrometry (spICP-MS) to measure ceria particle number concentrations and compare changes in size distributions to silicon dioxide wafer removal rates from different chemical mechanical planarization (CMP) processes. Particle number concentrations were measured for the 21 to 559 nm size range at 1 nm size resolution. Changes in the ceria particle size distribution after CMP included a decrease in large (>130 nm) particles, an increase in small (<40 nm) particles, an increase in the total number of particles, and a decrease in median particle size. The decrease in median size was as high as 7% and influenced by flow rate, pressure and pad type. A novel microreplicated CMP pad was used which requires no pad conditioning to ensure consistent pad surface features, and the effect of different pad types on removal rate and particle size was isolated. A decrease in the median particle size correlated with higher silicon dioxide removal rates (R2 = 0.96) for a series of pad types with unique combinations of chemistry and surface features. This new combination of nano particle metrology and control of pad surface features is an innovative tool set for modeling advanced CMP processes.
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