多階記錄可擦拭型相變化光碟覆寫特性之研究

Abstract

光碟片的記錄密度和記號的大小相關，受限於光點繞射極限，當記號小於雷射光的最小聚焦光點0.6×(λ/NA)（λ為雷射光波長，NA 為物鏡的數值孔徑），讀出訊號將會隨著記號縮小而變差，因此，光碟記錄密度將受限於現有的光學系統。 新型的「多階記錄可擦拭型相變化碟片」，利用多階反射率記錄的方式，增加單一記錄區域大小所對應的位元數，提升碟片的記錄密度。其碟片結構具有多層記錄層，且皆位於相同聚焦深度之內，調變不同寫入雷射功率，可使非結晶態記號記錄在不同的記錄層上而形成多種記錄區域，不同記錄區域對應到不同的反射率值，而形成多階讀出訊號。以三層四階碟片為例，有四種不同記錄區域，產生四階訊號，若每個記錄區域對應到兩個位元，使得記錄密度提升為原先的兩倍。再者，調變不同的雷射功率，使得每個記錄層因熱擴散效應有不同溫度分佈，造成記錄層上的非結晶態記號被保留或擦拭，以形成另一個不同於原來的記錄區域，作直接覆寫的動作。 分析多階碟片的記錄機制和訊號關係，由於是利用溫差將記號記錄在不同記錄層因每層記錄層吸收率不同，以致上、下記錄層的記號大小會有差異，會產生不良訊號。針對這個問題，提出改良的方法，以三階碟片為例，即讓兩層記錄層材料採用不同的材料，上記錄層採用Ag8In13Sb49Te30記錄材料，而下記錄層採用Ge21Sb53Te26記錄材料，藉由其擁有不同熔點來控制融熔區域大小，經淬火後，上下記錄層的記號大小不會差異太大，所形成的記錄區域的品質會較好，在相同雷射波長和物鏡數值孔徑下，當記錄區域為0.4μm，其CNR仍有37dB，比起雙層記錄層皆用Ge21Sb53Te26材料的碟片，當記錄上0.4μm的記錄區域，其CNR下降至25dB。

Recording density of the erasable phase change optical disk is restricted to the optical diffraction limit. Multi-level recording has an advantage of increasing the recording density. A new multi-level phase change disk is composed of multi recording layers, which are in the same depth of focus of laser beam. By modulating the intensity of the laser pulse, marks can be chosen to be recorded in any of the recording layers, forming multi recording regions corresponding to different reflectance levels. Similarly, utilizing the modulation of laser power can directly overwrite the previous recording region to another one. For the four-level recording disk, four recording regions corresponding to four distinct reflectances can be produced. If each recording region represents two-bit, the effective mark length for one bit becomes only half of the actual mark length. Then, the recording density can be increased by the factor of two. In the thesis, the overwriting conditions have been demonstrated for three-level and four-level disk. At the laser wavelength of 660 nm and NA of 0.6, the resolvable smallest effect recording region which can be overwritten is 0.4μm. Meanwhile, the relation between the multi-level recording mechanism and multi-level readout signal has been analyzed. Due to the temperature difference of recording layers, mark can be chosen to form in any of recording layers to form multi recording regions. But the temperature difference of recording layers causes different mark sizes, which result in the distortion of readout signal. The improved mechanism is to adopt different recording material as recording layers. By utilizing the recording materials of different melting point of recording layers, the mark size in different recording layers can be controlled to enhance the signal quality. The CNR of the three-level disk with two recording layers of different composition —AgInSbTe and GeSbTe is better than that of the three-level disk with two recording layers of the same composition material—GeSbTe.