具有氟、氮離子摻雜源/汲極低溫複晶矽薄膜電晶體之特性研究

Abstract

近年來，低溫複晶矽薄膜電晶體(LTPS TFTs)相當受到矚目，這是因為低溫複晶矽薄膜電晶體具有很高的電子移動率使然。對於液晶螢幕顯示器(LCD monitor)而言，藉此，高驅動力的電路得以整合至玻璃基板上，以實現系統電路在同一面板上(system-on-panel)。然而，對於雷射所結晶的複晶矽薄膜電晶體來說，在複晶矽晶粒邊界處、複晶矽通道層與閘極介電層接面處存在相當多的缺陷態位密度，導致複晶矽薄膜電晶體(Poly-Si TFTs)具有較高漏電流和較差的穩定度。除此之外，離子佈植後的退火製程，若採用雷射退火的方式，在源/汲極與通道接面處因溫度梯度(Temperature gradient)與繞射(Laser diffraction)緣故，產生了額外的缺陷密度(Extra-trap states)，該缺陷以”接面缺陷”(Junction defect)稱之。為了解決該接面缺陷所造成的漏電路徑，吾人使用複晶矽的缺陷鈍化(passivation)方式，利用離子佈植技術，將其所欲摻入之離子種類，諸如：氟(Fluorine)與氮(Nitrogen)，佈植於源/汲極。該劑量均為1x1014 與 1x1015 cm-2。藉由電性參數粹取方式，吾人比較對照組(不含氟與氮摻雜源/汲極)與實驗組(含氟與氮摻雜源/汲極、劑量均為1x1014 cm-2)的元件，發現實驗組具有較小的次臨限擺幅(subthreshold swings)、較高的載子移動率(Field mobility)、較低的截止電流(off-state current)與較低的臨限電壓(threshold voltage)。由以上這些參數得知，深層缺陷(deep trap state)與淺層缺陷(tail trap state)確實有被修補的效應，尤其是接面缺陷。吾人將其歸因於，在晶粒邊界處的矽斷鍵(dangling bond)，藉由氟與氮的摻雜，晶過雷射活化後，氟、氮會與矽斷鍵形成矽-氟(Si-F)、矽-氮(Si-N)鍵結，而達到鈍化(passivation)缺陷的目的。

Low-temperature polycrystalline silicon thin film transistors (LTPS TFTs) have attracted much attention due to the possibility of realizing the integration of driving circuits and pixel elements on one substrate，and the potential to accomplish the system-on-panel(SOP)。However large off-state leakage current and poor stability of polycrystalline silicon thin film transistors (poly-Si TFTs) fabricated by excimer laser annealing(ELA) due to high trap states in poly-Si grain boundaries and at the interface between poly-Si channel layer and gate dielectric are still serious problems。Recently it has been reported that residual ion implantation damage at source/drain junctions of ELA poly-Si TFTs entitled “junction defect” is also one of the problems of poly-Si TFTs。In order to eliminate the junction defects，we propose a poly-Si defect passivation technique wherein the passivating species (Fluorine and Nitrogen) is introduced via ion implantation into the source/drain region。In addition，doses of fluorine and nitrogen varying from 1x1014 to 1x1015 cm-2 were implanted in this work。The experimental results of both F- and N-passivated(dose of 1x1014 cm-2) devices showed steeper subthreshold swings，higher carrier mobility ，lower off-state current and lower threshold voltage than unpassivated devices。This indicates that a large fraction the defects have been passivated，junction defect especially。This is attributed to that F-、N-incorporated form Si-F and Si-N bond with silicon dangling bond at the grain boundary after laser annealing。