REACTIVE ION ETCHING OF SILICA STRUCTURES FOR INTEGRATED-OPTICS APPLICATIONS

Reactive ion etching of silica in a hollow cathode reactor using a CHF 3/Ar gas mixture has been studied as a function of masking material, r f power, sample temperature, and O-2 and CF4 additions. Etch rates in excess of 0.5 mu m/min are typically obtained with a selectivity over amorphous silicon and photoresist of more than 10. The sidewall roughn ess for etching with an amorphous silicon mask is of the order of 0.05 mu m, whereas for a photoresist mask, under similar etching condition s, the sidewall roughness is up to 0.1 mu m. For the a-Si mask a furth er improvement in the sidewall roughness down to 0.02 mu m can be obta ined by adding O-2 to the discharge or elevating the sample temperatur e, however both parameters cause lateral etching of the a-Si mask and therefore linewidth loss. Nonetheless, when using sample temperature a s a control parameter, a process window was found which allows smooth sidewalls to be obtained without dimension loss. In the case of O-2 ad ditions such a process window was not found. Possible mechanisms accou nting for this difference are discussed. Etching in a CHF3/Ar discharg e occurs in competition with simultaneous polymer deposition. The poly mer deposition rate was measured in areas shielded from ion bombardmen t. A phenomenological model describing the effects of polymer depositi on on etch rates, sidewall slope, and roughness is proposed. This mode l assumes that a polymer film with different steady-state thickness ca n form on different etched structure surfaces, as a result of a balanc e between polymer etching and deposition. The model is used to explain the tendencies in etch rates, profile slope, and sidewall roughness o btained in this study. (C) 1996 American Vacuum Society.