應用於提升表面型微加工的光學讀寫頭功能之微光學次系統

Abstract

隨著數位資訊時代的來臨與網際網路的普及，消費性電子產品不斷朝向可攜 式化、輕薄短小化、省電、低功率輸出的方向前進。利用微機電製程技術來設計與製作相容於現今光學讀寫系統的光學元件具有體積小、重量輕及適合於批次製造的優點。而自由空間型將所有元件同時製作在一晶片上，具有在設計光罩時即完成光學元件間的對準工作的優點。然而自由空間型仍面臨許多問題，諸如多晶矽製作的光學元件僅可應用於使用近紅外光(780 nm)的CD碟片，無法應用於使用650 nm的DVD碟片及405 nm的HD-DVD碟片，而單光束讀寫碟片及分光鏡效率偏低更嚴重限制其功能。為了改善這些問題，本論文提出了以在可見光波段具有高穿透性的低應力氮化矽取代原先的多晶矽，並結合光柵與微型致動器設計出一可切換式光柵，其可發展成一可切換單/多光束的微型光學讀寫頭。此外，利用多層光學膜的原理，我們製作出可應用於微型光學讀寫頭的高效率極化分光器，以讓自由空間型的微型光學讀寫頭被更廣泛且成功地應用在光儲存領域。為進一步發展出CD/DVD/HD-DVD光碟相容的微型讀寫頭，本論文提出一種二維的微型反射系統，此系統的概念可用於驗證相容型的微型讀寫頭的可行性。 傳統上，具備可切換單/多光束功能的光學讀寫頭是以電壓控制的液晶元件或以一磁控致動器來移動多光束光柵以切換單光束寫入及多光束讀取的狀態。然此二種方式均導致讀寫頭的體積及成本大幅提高。而本論文則成功開發出「微型可切換式多光束光柵」，完全可由微機電製程製作出，除具備體積小，成本低之優點外，控制簡易。此可切換式多光束光柵是由一微型抬升臂致動器及一多光束相位光柵所組成。 而極化分光器在光學讀寫頭上主要是用來將光分成兩個極化方向互相正交的光束。並搭配四分之一波片，可同時提高去光路的穿透光效率與反射光光效率。一般的極化分光器主要是利用大體積的雙折射晶體或高成本的鍍膜玻璃來製作，均不適合整合於微型光學讀寫頭。本論文所成功開發出「微型高效率極化分光器」，利用多層膜光學的原理，我們將空氣層與低應力氮化矽薄膜層堆疊，建構出在可見光區段具有低損耗及高極化分光效率之極化分光器。 第三種元件為一種二維的反射系統，其包括一拋物形反射面鏡及一菱鏡。其中拋物形反射面鏡係在單晶矽層中蝕刻出一拋物形溝槽(trench)而成，可將經拋物面焦點的入射光以平行拋物面光軸方向反射出或將平行拋物面光軸方向的入射光反射且經拋物面焦點而出。菱鏡為一Porro(45°-90°-45°)型，主要是將入射光以平行原入射方向反射出，且入射位置與出射位置間存在一位移。藉由適當組合拋物形反射面鏡與菱鏡的位置，可使入射光以特定反射角度反射出。最大掃描角度可達傳統二維的反射面鏡的三至四倍以上，掃描時光以垂直角度進出波導，故掃描角度不因Snell’s law而縮減，光繞射損失低，反射中心於掃描期間無偏移現象。 本論文中展現了以微機電技術所製作的微型光學元件於光儲存產業的重大應用潛力，且結合微光學設計與微光機電製程的微型光學系統技術，也將在未來高科技產業中具有益形重要的地位。

In the era of multimedia, internet and mobile communication, features of portable , thin, light weight and power saving are required for consuming electronic products. Compared with conventional optical pickup systems, micromachinical optical pickup systems are smaller, lighter and can be made by a batch-fabrication technology. Among the proposed micro optical pickups, the free-space polysilicon optical bench has been realized on a single chip, on which all the optical elements can be precisely prealigned during the mask design stage. However, this device used thin-polysilicon films as optical patterns and suffered from absorption of visible light by polysilicon. Another challenge in this device was that only a single beam was used by the micro-optical pickup to read and write a disc. Moreover, the beam splitter in the device was of low efficiency. These seriously limit its performance. In this thesis, to overcome the above issues, a transparent material for visible light was derived from low stress silicon nitride and improved the light efficiency. Besides, two novel components, a switchable grating and a high efficiency polarized beam splitter, were designed to further enhance the performance of the free-space optical pickup. Conventionally, the switching function can be realized by using a voltage-controlled liquid crystal grating or a magnet-actuated glass grating. The two approaches still suffer from the difficult assembly of optical components of large size and high cost. Therefore, we propose “a micro switchable grating,” which is composed of a stress-induced curved micro actuator and a phase-type grating on the tip of the actuator. In an optical pickup, a polarized beam splitter (PBS) is used with a quarter-wave plate to improve the transmittance of the forward polarized light and the reflectance of the backward polarized light. Conventionally, the PBS is realized using a birefringence crystal or a films-coated glass. The former is large in size, while the latter is of high cost. Therefore, a micro PBS consisting of a novel stack of two silicon nitride layers separated by an air gap was developed in our research to possess the same function. Besides, a planar micro-reflective system composed of a parabolic mirror, a prism, and a thermal actuator was demonstrated to verify the feasibility of a CD/DVD/HD-DVD compatible micro-pickup. A mechanical scan angle of 38 degrees was demonstrated by applying the developed planar reflective system, which displayed more than three times the scan angle compared to that of a conventional rotary mirror. The optical simulation about the deflected beam deviating from the optical axis of the planar reflective system was also presented, and good agreement between theory and experiment is achieved. This dissertation has successfully demonstrated the great potentials of the micro switchable grating, the micro PBS, and the micro-reflective system. These sub-systems can be achieved using the surface micromachining, potentially also at very low cost. The size of them is also dramatically reduced. Further, the fabrication of these subsystems is compatible with other micro diffractive elements. This makes them feasible to build a multi-function micro-optical pickup for short wavelength optical storage applications in the near future.