表面電漿波微波與光電元件之研究

Development of Surface Plasma Wave Based Microwave and Photonic Devices

謝政宏(Hsieh, Cheng Hung)

指導教授 ： 柳克強

國立清華大學/原子科學院/工程與系統科學系/博士(2015年)

摘要

表面電漿波是一種沿著電漿與介質介面傳播的表面波，其特性決於電漿中的自由電子。例如，在低溫電漿中，由於自由電子密度低，所以表面電漿波操作在微波頻段；而對於沿著金屬與介質介面傳播的表面電漿波而言，金屬中的自由電子密度高，使得此表面波操作在紅外光至可見光波段。在半導體電漿製程中，最近以表面電漿波機制設計的微波電漿密度感測器正受到廣泛注目，主要原因乃是此類感測器能以最不干擾電漿的方式來監控電漿密度。此感測器所量測的微波即是表面電漿波，它沿著電漿與介質的介面進行傳播，在量測過程中此表面波的相位變化可以因即時的電漿環境之改變而確實反應出來。另外一方面，同樣的表面電漿波機制可運用在光電子領域上；表面電漿波沿著金屬與介質的介面傳播，就是所謂的表面電漿子。對於表面電漿子應用在光電子學領域上，如表面電漿子波導與其相關之元件，由於可以突破傳統光學元件的繞射極限在奈米維度下傳遞訊號，近來十分受到重視。
在表面電漿波微波元件的研製上，本研究發展一套量測電漿密度的感測器—背脊式微帶線微波干涉儀，用以監控電漿製程中電漿的變化情況。此感測器操作在2.4 GHz的頻率下，具有體積小且材質適合大多數電漿製程設備的優點。在電磁模擬中，電漿被視為一介電質，藉由擬合此感測器所量測到的微波相位差與電漿密度的關係式，可將此式運用在量測程式之中，進行即時量測分析。雖然此感測器設置位於電漿腔壁之上，但量測結果可確實反應電漿中心密度隨操作條件而改變的情形。此感測器量測之電漿密度與電漿吸收探針之結果相比，皆隨輸入功率上升而增加，但由於量測位置不同使得兩者密度有所差異。此外，在即時監控的電漿製程中，當偏壓功率夠大(相較於輸入功率)且開啟時，電漿密度會有極明顯的增加，這是因為有額外的功率(偏壓功率)輸入，造成氣體解離形成電漿所導致。由這些結果顯示，本感測器具有發展為工業級電漿監測與回授控制系統之潛力。
在表面電漿子元件設計方面，本研究提出兩種矽基底混合式介質負載波導與微型高效能光學元件(方向耦合器、碟型共振器以及電光效應轉換器)。第一種波導為金屬置頂矽基底混合式介質負載波導，將其包覆在一層氮化矽絕緣層中可阻絕外界對波導光學特性的影響，同時又不致於影響其效能。此波導的傳播長度為350微米，模態面積為0.029微米平方。另一種波導為圓角化金屬置頂矽基底混合式介質負載波導，藉由將置頂之金屬圓角化使得金屬在邊緣與角落的損耗大幅降低，所達到的傳播長度為470微米，模態面積為0.023微米平方。據此兩波導所設計之方向耦合器，模擬結果顯示耦合長度僅分別為2.66與2.42微米，大約只佔各自傳播長度的0.76%和0.69%，此比例顯示此兩方向耦合器可以在極短的距離內有效地進行訊號的轉換；另一方面，本研究據此兩波導結構設計之碟型共振器，操作於最低損耗的TE021模態下，並以金屬結構包覆此一共振器阻絕電磁輻射損耗，可將此類共振器的品質因子提升到超過1800，比起其他共振器的最佳品質因子大了約兩倍左右，卻同時能保持在相同的尺寸大小。此兩光學元件的效能說明所設計的表面電漿子波導具有發展為其它光學元件的潛力。最後，本論文設計一方向耦合器形態的電光效應表面電漿子轉換器，其光訊號在表面電漿子波導與光學波導之間進行轉換。電光效應來自於一有機晶體(DAST)的作用，此晶體亦為表面電漿子波導的介質層。模擬結果顯示，藉由相位匹配的調變，本轉換器只需22.5伏特的電壓即可將光訊號有效率的在兩波導間進行光路轉換。此外，於最佳設計之下，訊號傳遞有超過66%的透射率與將近10dB的消光比。

關鍵字

表面電漿波 ； 電漿密度量測 ； 表面電漿子波導 ； 電光效應

並列摘要

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.

並列關鍵字

surface plasma wave ； plasma density diagnostics ； surface plasmon polarition waveguide ； electrooptic effect

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