David C. Gray

Phenomenological modeling of ion‐enhanced surface kinetics in fluorine‐based plasma etching

A multiple beam apparatus has been constructed to facilitate the study of ion‐enhanced fluorine chemistry on undoped polysilicon and silicon dioxide surfaces by allowing the fluxes of fluorine (F) atoms and argon (Ar+) ions to be independently varied over several orders of magnitude. The chemical nature of the etching surfaces has been investigated following the vacuum transfer of the sample dies to an adjoining x‐ray photoelectron spectroscopy facility. The etching ‘‘enhancement’’ effect of normally incident Ar+ ions has been quantified over a wide range of ion energy through the use of Kaufman and electron cyclotron resonance‐type ion sources. The increase in per ion etching yield of fluorine saturated silicon and silicon dioxide surfaces with increasing ion energy (Eion) was found to scale as (Eion1/2−Eth1/2), where Eth is the etching threshold energy for the process. Simple near‐surface site occupation models have been proposed for the quantification of the ion‐enhanced etching kinetics in these syste...

Plasma–surface interactions in fluorocarbon etching of silicon dioxide

The major species present in a fluorocarbon plasma environment were simulated and independently controlled using radical and ion beams in an ultrahigh‐vacuum apparatus. The beams used in this study were chosen to determine the importance of CFx radicals in a CF4 plasma; the beams included F and CF2, with a beam of Ar+ to simulate energetic ion bombardment. Both CF2 and F enhance the etching yield of SiO2 under energetic Ar+ bombardment; however, the enhancement with F is twice that seen with CF2 at similar fluxes. When CF2 and F fluxes are used simultaneously, F dominates and the CF2 flux has little effect on the overall etching yield. Combined with previous work on Si substrates, these results are consistent with qualitative theories for SiO2/Si selectivity in fluorocarbon plasmas. Possible elementary steps in the ion‐enhanced etching process are proposed and reduced to a two‐parameter model which represents the process as ion‐enhanced neutral adsorption followed by ion‐induced reaction to form volatile ...

Redeposition kinetics in fluorocarbon plasma etching

The redeposition kinetics of plasma etching products have been measured as a function of ion and free‐radical fluxes which are representative of the fluorocarbon etching environment. Silicon and SiO2 surfaces were exposed in a multibeam etching tool to energetic ions (Ar+), etchant radicals (F), and depositing carbonaceous species (CF2), the relative fractions of which were independently varied to alter the etching product distributions. The rate of product redeposition (i.e., the deposition rate on nonbombarded surfaces such as trench or via sidewalls) was measured using a quartz crystal microbalance (QCM) which could be rotated around the etching sample face. While redeposition rates of Ar+ sputtering products from Si and SiO2 were characteristically high, the addition of F suppressed redeposition by nearly an order of magnitude. The mechanisms for this reduction involve the passivation of the nonbombarded QCM surface by atomic F, the chemical etching of the redeposited material, and the production of v...

Quantification of surface film formation effects in fluorocarbon plasma etching of polysilicon

A multibeam apparatus has been constructed which allows synthesis of the primary fluxes to a wafer surface during fluorocarbon plasma etching. These species include CF2 as a polymer forming precursor, F as the primary etchant, and Ar+ or CF+x for energetic ion bombardment. The individual fluxes can be adjusted to within an order of magnitude of those encountered in typical plasma etching processes, and can be manipulated independently to ascertain the effects of radical‐to‐ion flux ratio on fluorocarbon film formation and etching yields. Direct sticking of CF2 radicals on undoped polysilicon films can account for suppression of silicon MONITOR sputtering yields on the order of 70%, but cannot completely stop the net removal of silicon by Ar+ at energies above 150 eV. Studies of the competitive interaction between CF2 and F radicals on undoped silicon surfaces showed up to 30% suppression of Ar+/F etching yields under flux conditions representative of low pressure CF4 reactive ion etching (RIE).

Influence of reactant transport on fluorine reactive ion etching of deep trenches in silicon

The reactive ion etching of silicon by fluorine in high‐aspect ratio features was modeled to assess the relative importance of reactant transport on etching rate at the bottom of rectangular trenches. The flux of ions to the feature bottom was found by summing two components: ions arriving directly from the plasma and ions reflected from the sidewalls before reaching the bottom. The transport of neutral reactants within the feature was modeled with diffuse scattering and reaction rates following the kinetics reported by Gray et al. [J. Vac. Sci. Technol. B 11, 1243 (1993)] at all surfaces. The etching rate was found to depend most strongly upon the ion flux under typical process conditions, because of relatively low fluorine reaction probability and low reactant depletion within the feature. Reactant transport limitations are expected to be more important under conditions of low fluorine to ion flux ratio, high‐substrate temperature, and high‐ion energy.

Design considerations for high-flux collisionally opaque molecular beams

Flux distributions and fractional interceptions for high‐flux straight‐tube sources have been calculated in the collisionally opaque gas flow regime. Previous theoretical work in this regime has been assimilated to provide a useful design guide similar to that provided by Campbell and Valone [J. Vac. Sci. Technol. A 3, 408 (1985)] for transparent regime molecular‐beam sources. Several of these large throughput straight‐tube sources have been constructed in the laboratory for use in etching studies of semiconductor materials. Reasonable agreement has been found between the available theories and measurements of peak beam fluxes and angular dispersions. While angular dispersion is a function of aspect ratio only (tube diameter‐to‐length ratio) in the transparent regime theory discussed by Campbell and Valone, additional parameters are required in the opaque regime since gas‐phase collisions effect the beam directionality. The dimensionless group η=a/λtip, the ratio of tube diameter to gas mean free path at ...

Photochemical Dry Stripping of Silicon Nitride Films

UV photochemical processes have been developed for rapidly stripping films of LPCVD Si 3 N 4 in a dry reaction environment, free of plasma or plasma effluents. These processes are carried out in a vacuum reactor which allows simultaneous exposure of a substrate wafer to a polyatomic halogen gas and UV radiation. Si 3 N 4 stripping rates approaching 1000 A/min have been demonstrated for fluorine-based processes, while maintaining the bulk wafer temperature below 250°C. It has been shown that the mechanism for photochemical Si 3 N 4 etching requires both photolytic production of gas-phase F atoms and direct photon exposure of the etching surface. Selectivities between Si 3 N 4 , SiO 2 , and silicon films are controlled through UV lamp exposure, substrate temperature, and with additions of N 2 diluent and various halogen-containing gases. Selectivities for Si 3 N 4 -to-Sio 2 etching of greater than 30 can be achieved for the stripping of Si 3 N 4 LOCOS mask layers in the presence of field oxide and pad oxide layers.

Photochemical Dry Etching of Doped and Undoped Silicon Oxides

The photochemical dry etching of doped and undoped silicon oxides with ClF 3 and F 2 gas has been investigated. Etching rates of several hundred angstroms per minute have been achieved with bulk substrate temperatures below 300°C. Borophosphosilicate glass-to-thermal oxide etching rate selectivity is near 1-to-1 over a wide range of process conditions. The addition of Cl 2 gas is found to significantly reduce the etching rates of both doped and undoped silicon oxide under these conditions. The photochemical dry cleaning of a 10 to 20 A chemical oxide layer from trench and via bottoms is discussed.