紫外光照射高分子光波導元件之研製

Abstract

本論文探討以248 nm KrF準分子雷射直接照射的方式製作通道式BCB高分子光波導元件，並首次針對其製程步驟、波導特性、波導設計、材料折射率變化分析、及相關應用與優勢，作一系統性的研究與討論。 由於BCB的特殊光學性質，本研究以紫外光照射方式改變BCB高分子的折射率，進而應用於光波導元件製作。在製程特性方面，此操作方式簡便省時，可避免傳統製程曝光顯影等步驟，完成之波導結構也沒有脊形波導側壁粗糙之問題；並且能藉由控制雷射光照射發數來調整波導折射率變化，對BCB造成1×10-3∼4×10-3之折射率差。在波導特性方面，於波長1550 nm下量得之傳播損耗為0.6 dB/cm，經計算求得波導之折射率為近階梯式分佈；並且在製程上能夠藉由有彈性而可調整的特性，得到不同半高寬的輸出光場能量分佈，使得元件間耦合達到最佳效率，能有效應用於積體光學元件之整合上。 在材料分析方面，以電子束顯微鏡與原子力顯微鏡觀察經雷射照射之BCB波導表面，得到材料破壞產生之臨界條件與適合元件之製程範圍。再以傅立葉轉換紅外線光譜儀分析材料內部官能基鍵結之情形，深入探討材料受雷射照射後分子的鍵結改變，並以X射線光電子能譜分析原子含量，驗證並解釋BCB薄膜折射率上升之原因。 在元件的應用上，本研究首先以此新興方式製作方向耦合器，量得兩組耦合長度值為2073 µm與2502 µm，與模擬數值相當接近，最小誤差為0.20%，並搭配雷射照射可調之製程特性，達到大幅度輸出光場之分光比例調整，為方向耦合器提供一個更具有彈性的製程選擇；其次，本研究也實際製作三組1×2多模干涉功率分離器，最佳結果為折射率差3.3×10-3之分光元件，於作用長度850 µm具有最高傳輸率92.03%，與模擬相比誤差僅為0.52%，元件分光之不等分率僅0.06 dB，可以大幅提高積體光學元件製程的準確度，極有應用價值。

In this dissertation, buried-type benzocyclobutene (BCB) optical waveguides and devices fabricated by ultraviolet (UV) pulsed-laser illumination are investigated, particularly in the fabrication process, waveguide characteristics, material analysis, and advantages of related applications for the first time. UV pulsed-laser illumination is used to change the refractive index of BCB for optical waveguide fabrication due to its unique optical properties. Experimental results show the proposed fabrication process is greatly simplified as compared to that of the conventional method. The measured refractive index change of BCB can be varied from 1x10-3 to 4x10-3 by controlling the number of laser shots. The measured propagation loss is 0.6 dB/cm and the refractive index distribution is step-like, which is verified by the inverse Wentzel-Kramers-Brillouin method. As the fabrication is adjustable, single-mode waveguides with different mode sizes can be tailored for efficient coupling. Furthermore, chemical properties of surface damage threshold, root mean square roughness, changes of chemical functional groups, and changes of atom percentages are characterized by scanning electron microscopy, atomic force microscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy for the analysis of the chemical mechanism behind the UV-illumination and the process parameters suitable for the fabrication of waveguides. For practical applications, directional couplers with coupling lengths of 2073 µm and 2502 µm are successfully fabricated. Compared to the simulated results, the measured coupling lengths have a minimum error of 0.2%. With the controllable refractive index, a wide range of output power ratio can be achieved, which is much more flexible for the fabrication of directional couplers. Moreover, 1×2 multimode interference power splitters are also fabricated. The best device has a refractive index change of 3.3×10-3 and an interaction length of 850 µm, which exhibits a transmission of as high as 92.03% and an imbalance of only 0.06 dB. Various integrated optical devices can be fabricated by the proposed method, which will be of great interest for future study.