Amorphous carbon films grown with fluorohydrocarbons can be grown to have dielectric constant values around 2.0. The behavior of these films when subjected to thermal excursion is studied. We have investigated material deposited in an ECR plasma, and find that the F:H ratio of the gas mixture is a good guide to material properties. Films deposited at 5°C were placed in a vacuum chamber at 400°C as long as 60 minutes. The film thickness, dielectric constant, and infrared absorption spectrum change with the F:H ratio of the incoming gas and thermal cycling. It was found that the dielectric constant and loss tangent decrease upon heating and that there is an apparent increase in C=C groups. As expected, as the F:H ratio increases, the dielectric constant and thermal stability decrease. Good thermal stability is shown for F:H ratios of 1.5, which result in films with a dielectric constant of ∼2.4 after heating.
The change in sheet conductance of a thin, highly doped layer of GaAs is measured after exposure to inert gas plasmas (He, Ar, and Xe) and to molecular gas plasmas (CCl2F2, CF4, SF6, and O2) in a parallel-plate rf discharge. In order to compare these data, the change in sheet conductance is converted to a damage depth scale. A different linear relationship is found for the damage dependence on the rf-induced dc bias for each plasma. An inverse-mass relationship is derived from the data for He, Ar, and Xe plasma exposures. Using this, two models are tested for their ability to predict the damage from the molecular gas plasmas. For one model, the assumption is that damage is caused by molecular ions that remain intact upon impact. For the other model, which is quite successful at predicting the measured damage, the assumption is that molecular ions fragment completely upon impact. This interpretation indicates that Cl+ and/or F from CF+ in CCl2F2 plasma, F from CF+ in CF4 plasma, and S from SF+ in SF6 plasma are responsible for the measured damage effect. Neither model adequately predicts the low level of damage from O2 plasma. Helium ions caused the greatest amount of damage; when mixed with CCl2F2, the measured damage was a factor of three lower. Using optical emission spectroscopy, quenching of He ions was observed when molecular gases were introduced into a He plasma. Quenching was also observed in other mixed gas plasmas and this indicates that ions with ionization potentials substantially higher than those of other ions in the plasma will not be formed in typical rf glow discharges.
We have studied the role of aluminum in the formation of an etch barrier at the GaAs/ Alx Ga1−x As interface during reactive ion etching in CCl2 F2 plasma. The minimum Alx Ga1−x As thickness needed to form the barrier is Al mole fraction dependent and was determined with etching experiments monitored by optical emission spectroscopy. Effective Alx Ga1−x As layers for forming an etch barrier are 275 Å for x=0.02, 22 Å for x=0.10, 15 Å for x=0.15, 12 Å for x=0.20, and 9 Å for x=0.30. For all Al mole fractions except x=0.02, these thicknesses correspond to a sheet dose equivalent to 3/4 of a monolayer of Al in the original Alx Ga1−x As layer. Barrier layers for x=0.02, 0.10, 0.25, and 0.30 were examined without air exposure by angle-dependent x-ray photoelectron spectroscopy. For samples that are not overetched, the surface is covered with ∼20 Å of AlF3 intermixed with a gallium halide containing chlorine and fluorine and is depleted of arsenic. For substantially overetched barriers, a 30 Å layer is formed with gallium halide present at the surface, AlF3 found farther in, and arsenic depletion throughout the barrier. During extreme overetch, barrier layers on the order of tens of Å in thickness were not etched away and yet did not completely prevent very slow etching of underlying GaAs. Barrier layers on the order of 60 Å in thickness did prevent etching of underlying GaAs. Collectively the data suggest that the role of Al is formation of AlF3 exclusively and that only this compound is responsible for stopping the GaAs etch.
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