I. T. Wade, 'Spectroscopic Studies of the Reactive Ion Etching of GaAs in CCl2F2 and CCl3F Plasmas', PhD Thesis, 1989.

This investigation is concerned with the reactive ion etching of GaAs in plasmas of CCl₂F₂.and CCl₃F. These processes are preeminent in GaAs device technology. Studies were performed on a commercial single wafer reactive ion etcher (Electrotech Special Research Systems, Minstrel chamber) at various process gas flow rates and RF power densities. RF frequency was also varied from 3.75 MHz to 24 MHz, but the majority of the studies used 13.56 MHz. The gas pressure was held constant at 60mtorr.

Optical emission spectra were recorded in the wavelength range 2100-8600Å for CCl₂F₂ and for CCl₃F plasmas. Excited states of Cl and CF₂ were observed in both plasmas. Product emission from Ga, GaF and GaCl was observed when etching GaAs (100) substrates in both plasmas. No ionic species were detected in either plasma, and no As species were observed.

The optical work suggests that GaAs is etched predominantly by Cl atoms. A chemical model is presented which accounts for the observed emission from GaCl a³π (a-triplet pi) in terms of ion-assisted desorption of the product molecule GaCl_x (x=1,2) from the substrate surface. A model is also presented which accounts for emission from Ga ²D (doublet-D) in terms of electron impact dissociation of GaCl_x (x=1,2)(g) and ion sputtering of Ga from the substrate surface.

The variation in etch rate of GaAs with power, flow rate, frequency and etchant gas was determined. Etch rate was found to: increase with power and flow rate, decrease at lower RF frequencies, and decrease when CCl₃F replaced CCl₂F₂.

A mass spectroscopic study was made of the CCl₂F₂ plasma during etching. Some species not detected optically were observed mass spectroscopically, i.e. Cl₂ and the product species As, AsCl_x (x=1-3) and AsF_y (y=1-3). No Ga species were detected.

An XPS investigation of CCl₂F₂ reactive ion etched GaAs indicated preferential removal of As. F and Cl are adsorbed at the substrate surface, and F penetrates into the crystal lattice to greater depths than does Cl: very little Cl is present at depths greater than ~2500Å. Cl is bonded to both Ga and As, whereas F is bonded only to Ga. A polymer deposit identified as (CF₂)_n (where n is large) was detected on the substrate surface and to depths > 1000Å. Al was detected to depths > 2500Å below the surface and results from ion sputtering of the lower electrode.

Finally an Appendix is included which contains a study of the etching of silicon nitride in a CF₄/0₂ plasma using optical emission spectroscopy.