The addition of a small concentration of suitably chosen noble gas to a reactive plasma is shown to permit the determination of the functional dependence of reactive particle density on plasma parameters. Examples illustrating the simplicity of this method are presented using F atomic emission from plasma‐etching discharges and a comparison is made to available data in the literature.

Optical emission spectroscopy of reactive plasmas: A method for correlating emission intensities to reactive particle density

On-chip optical resonators have the promise of revolutionizing numerous fields, including metrology and sensing; however, their optical losses have always lagged behind those of their larger discrete resonator counterparts based on crystalline materials and silica microtoroids. Silicon nitride (Si3N4) ring resonators open up capabilities for frequency comb generation, optical clocks, and high-precision sensing on an integrated platform. However, simultaneously achieving a high quality factor (Q) and high confinement in Si3N4 (critical for nonlinear processes, for example) remains a challenge. Here we show that addressing surface roughness enables overcoming the loss limitations and achieving high-confinement on-chip ring resonators with Q of 37 million for a ring of 2.5 μm width and 67 million for a ring of 10 μm width. We show a clear systematic path for achieving these high Qs, and these techniques can also be used to reduce losses in other material platforms independent of geometry. Furthermore, we demonstrate optical parametric oscillation using the 2.5 μm wide ring with sub-milliwatt pump powers and extract the loss limited by the material absorption in our films to be 0.13±0.05  dB/m, which corresponds to an absorption-limited Q of at least 170 million by comparing two resonators with different degrees of confinement. Our work provides a chip-scale platform for applications such as ultralow-power frequency comb generation, laser stabilization, and sideband-resolved optomechanics.

Ultra-low-loss on-chip resonators with sub-milliwatt parametric oscillation threshold

A process module wherein radiant heating is combined with the capability for generating a plasma in proximity to the wafer face. It is useful to clamp the slice to a susceptor which is of the same material as the wafer, or at least has essentially the same thermal coefficient of expansion. The susceptor is directly heated by a radiant heater, but the rate of temperature change is slow enough that no large thermal gradients between the susceptor and the wafer develop.

Processing apparatus and method

Surface science aspects of etching reactions

Fluorine atoms etch silicon with a rate, RF(Si) = 2.91±0.20×10−12T1/2nFe−0.108 eV/kT A/min, where nF (cm−3) is the atom concentration. This etching is accompanied by a chemiluminescent continuum in the gas phase which exhibits the same activation energy. These phenomena are described by the kinetics: (1) F(g)+Sisurf→SiF2(g), (2) SiF2(g) +F(g) →SiF*3(g), (3) SiF2(g) +F2(g) →SiF*3(g) +F(g), (4) SiF*3(g) →SiF3(g) +hνcontinuum where formation of SiF2 is the rate‐limiting step. A detailed model of silicon gasification is presented which accounts for the low atomic fluorine reaction probability (0.00168 at room temperature) and formation of SiF2 as a direct product. Previously reported etch rates of SiO2 by atomic fluorine are high by a constant factor. The etch rate of SiO2 is RF(SiO2) = (6.14±0.49)×10−13nF T1/2e−0.163/kT A/min and the ratio of Si to SiO2 etching by F atoms is (4.74±0.49)e−0.055/kT.

The reaction of fluorine atoms with silicon

The plasma etching of silicon and silicon dioxide in CF4‐O2 mixtures has been studied as a function of feed‐gas composition in a 13.56‐MHz plasmagenerated in a radial‐flow reactor at 200 W and 0.35 Torr. Conversion of CF4 to stable products (CO, CO2, COF2, and SiF4) and the concentration of free F atoms ([F]) in the plasma were measured using a number of different diagnostics. The rate of etching, the concentration of F atoms, and the intensity of emission from electronically excited F atoms (3s 2 P–3p 2 P° transition at 703.7 nm) each exhibit a maximum value as a function of feed‐gas composition ([O2]); these respective maxima occur at distinct oxygen concentrations. For SiO2, the variation in etching rate with [O2] is accounted for by a proportional variation in [F], the active etchant. The etching of silicon also occurs by a reaction with F atoms, but oxygen competes with F for active surface sites. A quantitative model which takes oxygen adsorption into account is used to relate the etch rate to [F]. The initial increase of [F] with [O2] is accounted for by a sequence of reactions initiating with the production of CF3 radicals by electron impact and followed by a reaction of CF3 with oxygen. When [O2] exceeds ∼23% (under the present discharge conditions), [F] decreases due, probably, to a decrease in electron energy with an increase of oxygen in the feed gas.

C. J. Mogab

Papers

Plasma etching of Si and SiO2—The effect of oxygen additions to CF4 plasmas

Abstract: Plasma etching of Si and SiO2−The effect of oxygen additions to CF4 plasmas