Abstract
Surface plasma wave (SPW) is a surface wave, propagating along the interface of plasma and dielectric, determined by the free charges (electrons) in the plasma. For example, in the gas discharge, such as low temperature plasma, the SPW operates at the microwave frequency while the infrared and visible frequency would be employed to sustain the SPW as the wave exists on the interface of metal and dielectric. In the plasma based semiconductor processes, for decades, the microwave based plasma density sensor, according to the properties of sur-face plasma wave, has attracted plenty of interest in the monitoring of plasma condition be-cause of its minimal perturbation to the plasma. The plasma density can be measured by the variations of the phase of surface plasma wave due to the environmental plasma conditions. On the other hand, the same physical mechanism of surface plasma wave can be employed in the photonics. The surface plasma wave propagating along the interface of metal and dielectric is so-called surface plasmon polariton (SPP). For the photonics based on SPPs, the corre-sponding SPP devices have increased potential to nanoscale transmission due to breaking the diffraction criteria of light guiding. In this study, a novel sensor, ridged microstrip microwave interferometer (RMMI), based on the characteristics of SPW, is developed for monitoring of plasma density in plasma pro-cessing tools. The sensor is designed to operate at 2.4 GHz microwave frequency, with a compact size and materials that are compatible with most plasma processing tools. 3D EM simulations, where plasma is treated as a dielectric medium having a plasma permittivity de-termined by plasma density and microwave frequency, are employed to determine the phase shift/plasma density relation of this sensor. Measurement results show that plasma density measured by the sensor, although placed at the chamber wall, does reflect the variations of the plasma density near the chamber center. Compared with the measurement by plasma absorp-tion probe (PAP), the difference of plasma density measured by RMMI and PAP is due to the position of sensors. In real-time plasma based process, the temporal result shows that the plasma density obviously increases as the bias power is turned on and large enough, compara-ble to the source power. With this capability, the RMMI can be used for real-time feedback control of plasma density in plasma processing tools. The second topic in this study is to design SPP devices, such as SPP waveguides (SPPWGs), SPPWG based directional coupler / optical resonator / switch. We present low loss (rounded) top metal silicon (Si) hybrid dielectric-loaded plasmonic waveguides (TM-SiHDLW/ RTM-SiHDLW) and the associated compact high performance optical devices, e.g., directional coupler, optical disk resonator. Simulation analysis using finite element meth-od is employed for the design of the SPP based devices. For the design of the TM-SiHDLW, we investigate the effect of a thin (10 nm) silicon nitride (SiNx) layer covering the waveguide which was added for minimizing uncertainties on optical properties of SiHDLW resulting from high density of dangling bonds on Si surface. The resulting propagation length is 0.35 um and the mode area is around 0.029 um^2. In the case of the RTM-SiHDLW, it adopted rounded corners for reducing Ohmic loss around stripe edges/corners, and thus, a propagation length of 0.47 um is obtained by numerical simulation, an increase of ~ 30%, at a similar mode area, compared to conventional TM-SiHDLW. The directional couplers based on the two SPPWGs we proposed here show comparable coupling length, 2.66 and 2.42 um, which is only ~ 0.76% and ~ 0.69% of the propagation length, demonstrating high efficiencies of light coupling. The low loss TE021 optical disk resonators, also built by the two SPPWGs, are also designed to operate at the 1550 nm wavelength. A metal enclosure is employed for re-ducing the radiation loss. Simulation results show that, for both resonators, quality factors of > 1800, more than twice the results in previous works, could be obtained with a comparable resonator size. Finally, a compact high performance electro-optic (E-O) plasmonic switch con-structed in a “directional coupler” like structure, including a SPPWG (RTM-DLW), similar to RTM-SiHDLW proposed above, and an optical waveguide, is designed and operated around the 1550 nm wavelength. An organic crystal, DAST, is adopted to serve as both the E-O ma-terial of the switch and the dielectric in the SPPWG. The variation of phase matching between the two waveguides is achieved by applying a voltage as low as 22.5 V on the E-O material, so that the optical wave can be efficiently switched between the two output ports. For the op-timized dimensions, a transmittance up to 66% and an extinction ratio nearly 10 dB are achieved.