微光元件應用在 DVD-like 讀取頭

Abstract

光儲存產業隨著快速進展的電腦、多媒體、網路市場持續的成長。在這一股潮流中，對於能夠儲存更多資訊量的技術需求持續的增加。為了達到輕薄、高空間解析度、高效率，使得整個光儲存系統在市場上更具競爭力及較好的表現，光學讀取頭是最關鍵的元件。 在光學讀取頭內的光學元件中，以聚焦物鏡最為重要。本論文從DVD 的規格，如碟片的覆蓋層厚度及三光束尋軌的要求，討論物鏡的最小孔鏡。對於製作應用於物境的微小透鏡，較成熟的微元件製造方式有熱熔法，和半導體曝光顯影的灰階光罩。熱熔法所製造出來的透鏡多為球面，球面透鏡的曲面造成球差，因此要達到DVD規格的光點，熱熔方式所製造的球面並不可行。 另一種方法為灰階光罩，利用調整灰階的開孔率，我們可以製造任意複雜的曲面，因此我們建議採用灰階光罩來製造光學讀寫頭的物鏡，可採用的透鏡有下面三種：非球面透鏡，菲涅耳透鏡，和同諧透鏡 (Harmonic lens)。我們可以藉由優化時選擇適當的優化參數來設計出達到DVD要求的透鏡，透鏡的厚度為97.5 μm，但是由於受限於半導體光組塗布的厚度，對光阻AZ4620而言，最大厚度約為40 μm，因此我們必須切割鏡面來達輕薄的目的。接著我們提出非涅耳透鏡，但是因為線寬太小，製程不易。所以，我們建議同諧透鏡(Harmonic Lens)，來作為物鏡。 傳統的同諧透鏡的設計方式是先設計出一個透鏡，在對所設計出來的透鏡進行切割，切割的時候出光點的位移造成像差，因此我們提出另一種嶄新的設計方式來改善，我們在切割的時候，利用非球面來重新設計面的輪廓，並且每次都只選取不會因為切割造成像差的最外圈，我們設計出由四圈非球面所構成的同諧透鏡，透鏡的厚度為 19.5 μm，直徑 260 μm，整個透鏡的體積只有1.91□10-6 cm3，可以蒐集雷射光束直徑 210 μm，且由模擬的結果及誤差分析可以得知，藉由此方法設計出來的透鏡可以消除像差，並達到 DVD 所要求光點大小為1.08 μm。

Optical data storage industries are continually growing with rapid progress of computer, multimedia, and network markets. In this trend, technologies capable of recording more information thus become increasingly demanded. Optical pickups are key component of the recording system. To develop high-spatial-resolution, high-optical-efficiency, and small-sized optical pickup is essential to enable the system more competitive in price and performance. Among these optical devices in the optical pickup, the objective lens is the most important device. From DVD specification, such as the cover layer thickness and the three beam tracking, the minimum size of objective lens was discussed. Moreover, to fabricate the micro lens applied to objective lens, the mature processes to for micro devices are reflow process and gray-scale mask. The spherical lens made by using the reflow processes causes the spherical aberrations. Therefore, the reflow process is not feasible to make the lenses satisfied the DVD specification. The other approach to make the micro objective lenses is the gray-scale mask. By well-adjusted aperture ratio of the mask, the aspheric surface which has eliminated geometrical aberration is feasible. Therefore, we suggest using gray-scale mask to fabricate micro lenses. With this process, three kinds of lenses, namely aspheric lens, Fresnel lens, and harmonic lens are feasible. By utilizing proper optimization variables, the aspheric lenses with thickness of 97.5 μm meet the requirement of DVD-like objective lens. However, the thickness of the aspheric lens is too thick to be fabricated. Next, the Fresnel lens was proposed to overcome the thickness. Nevertheless, the 1um line width of the Fresnel lens is too fine to make the refractive profile for a lens of high NA. Thus, we propose the harmonic lens for objective lens purpose. The conventional design steps of harmonic lens include three parts. First, the refractive profile was designed. Next, the refractive profile was sliced. Finally, the sliced layers were compressed into one layer. The focal shift of slicing induces the aberrations. We proposed a novel method to overcome this issue. When slicing the lens, we employed the aspheric profile to redesign the surface of each segment and choice the outmost part with eliminated geometrical aberration. The redesigned harmonic lens obtained is the combination of several segments with aspheric profiles. The thickness of the redesigned harmonic lens is 19.5 μm. The diameter of designed harmonic lens is 260 μm to couple three-beams tracking with beam width of 210 μm. The volume of the objective lens is highly reduced to 1.91□10-6 cm3. A harmonic diffractive objective lens has been designed to be fabricated with gray-scale mask photolithography and will be integrated onto a free space optical module for a MEMS-type DVD-like pickup. The modeling and analysis show that the aberration of objective lens is negligible and the size of focused spot meets DVD specification of 1.08 μm.