Reactive ion etching of silica in a hollow cathode reactor using a CHF3/Ar gas mixture has been studied as a function of masking material, rf power, sample temperature, and O2 and CF4 additions. Etch rates in excess of 0.5 μm/min are typically obtained with a selectivity over amorphous silicon and photoresist of more than 10. The sidewall roughness for etching with an amorphous silicon mask is of the order of 0.05 μm, whereas for a photoresist mask, under similar etching conditions, the sidewall roughness is up to 0.1 μm. For the a-Si mask a further improvement in the sidewall roughness down to 0.02 μm can be obtained by adding O2 to the discharge or elevating the sample temperature, however both parameters cause lateral etching of the a-Si mask and therefore linewidth loss. Nonetheless, when using sample temperature as a control parameter, a process window was found which allows smooth sidewalls to be obtained without dimension loss. In the case of O2 additions such a process window was not found. Possible mechanisms accounting for this difference are discussed. Etching in a CHF3/Ar discharge occurs in competition with simultaneous polymer deposition. The polymer deposition rate was measured in areas shielded from ion bombardment. A phenomenological model describing the effects of polymer deposition on etch rates, sidewall slope, and roughness is proposed. This model assumes that a polymer film with different steady-state thickness can form on different etched structure surfaces, as a result of a balance between polymer etching and deposition. The model is used to explain the tendencies in etch rates, profile slope, and sidewall roughness obtained in this study.
Different mask materials (photoresist and amorphous silicon) and different sample temperatures can influence the roughness of sidewalls produced during reactive ion etching of silica. Buried-channel waveguides with different microroughness on the core sidewalls (corrugation periods less than 1 μm) have been fabricated and characterized for their propagation loss at 1.3 μm wavelength. An increase in the sidewall roughness amplitude of around 0.05 μm results in an increase in the propagation loss of 0.2 dB/cm. Sidewall roughness with a larger period appears to have smaller effect on loss.
Silica films have been deposited in a high density hollow cathode plasma deposition system from silane and oxygen gas mixtures. Additions of carbon tetrafluoride (CF4) were used to fluorine dope the silica. The deposited films were characterized by means of Fourier transform infrared (FTIR) spectroscopy, wavelength dispersive x-ray spectroscopy, chemical etch rate (P etch), stress and refractive index measurements. The pure silica films, though deposited at a high rate (over 1500 Å/min), exhibit a P-etch rate only 1.3 times that of thermal oxide. The refractive index of the as-deposited silica is higher than that of thermal oxide, but reduces to the thermal oxide value after high-temperature (1000 °C) annealing. Based on the thickness change measurements, the higher refractive index was attributed to a higher density of the deposited silica due to a smaller Si–O–Si bond angle, as supported by FTIR data. Fluorine doping results in a reduction in film stress by a factor of 4 over pure silica, as well as a reduction in OH content from about 1 at. % in pure silica to below the FTIR detection limit (0.1 at. %). The refractive index initially decreases with CF4 flow rate, concomitant with an increase in fluorine content, but then rises above the refractive index of pure silica. This increase has been found to be due to the deposition of silicon-rich oxide at the higher CF4 flow rates, which is attributed to an increasingly oxygen deficient discharge resulting from oxygen consumption by the dissociation products of CF4.
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