GALLIUM-ARSENIDE SURFACE-CHEMISTRY AND SURFACE DAMAGE IN A CHLORINE HIGH-DENSITY PLASMA ETCH PROCESS

In an effort to monitor ion-driven surface chemistry in the high densi ty plasma etching of GaAs by Cl-2/Ar plasma chemistries, we have appli ed mass spectrometry and careful substrate temperature control. Etch p roduct chlorides were mass analyzed while the substrate temperature wa s monitored by optical bandgap thermometry and as pressure (neutral fl ux), microwave power (ion flux) and rf bias of the substrate (ion ener gy) were varied. By ensuring that the substrate temperature does not d eviate during process variations, the changes in product mass peak int ensities are a direct measure of changes in the ion-assisted surface c hemistry which promotes anisotropic etching. Experimental results show that ion-assisted surface chemistry is optimum when sufficient Cl and Cl+ are present in the incident plasma near, These conditions are met at low coupled microwave powers (<300 W) and low total process pressu res (<1.0 mTorr) for input gas mixtures of 25% Cl-2 in Ar. Three mecha nistic regions are identified for surface chemistry as a function of i ncident ion energy: 1) largely thermal chemistry for <50 eV; 2) ion-as sisted chemistry for 50-200 eV; and 3) sputtering for >200 eV, Photore flectance measurements of the surface Fermi level show significant dam age for ion energies >75 eV. However, in situ and ex situ surface pass ivations can recover the surface Fermi level for up to 200 eV ion ener gies, in good correlation to the onset of sputtering and subsurface da mage. Thus, anisotropic, low damage pattern transfer is possible for i on energies between 50 and 200 eV.