連續波固態綠光雷射退火之面板型類磊晶矽電晶體

Abstract

本論文主要探討與展示用連續波綠光雷射退火於非矽基基材上製作類磊晶電晶體之電特性與可靠度增益特性，這對於主動式開關電路、非揮發性記憶體、影像感測器與光偵測放大器等各式面板及與光積體線路具非常大之貢獻。本論文主要探討以下四個研究部分。 首先，研究發現連續波綠光雷射退火所製作類單晶之薄膜電晶體電性特徵與缺陷密度非常相關。隨雷射能量的提高其通道結晶性也隨之增加，結果發現通道缺陷密度隨著雷射退火能量提高而降低至3×1019 eV-1cm-3，這符合通道缺陷密度降低而提高場效電子遷移率至284cm2/Vs，然而，提高雷射能最初降低深態位密度(deep-state density)，但隨後升至3×1016eV-1cm-3。深態位密度的反轉是受雷射結晶增加其表面粗糙度而產生額外表面態缺陷所導致，其亦造成次臨界斜率提高與臨界電壓具飽和狀的降低趨勢。 下一步，我們利用連續波固態綠光雷射退火製作類單晶之高電洞載子遷移率之薄膜電晶體並探討其熱載子效應行為。當通道層變厚，雷射退火導致通道結晶性增加，然而也增加其表面粗糙度。由於此類磊晶之複晶矽薄膜有較厚通道時，雖然具較少通道缺陷密度，然而增及其表面粗糙度。在這樣類磊晶複晶矽基材，厚通道雖具較低之通道缺陷但也有差的表面品質，在熱載子效應作用下，造成較多之電荷陷阱(charge trapping)進而產生更多之深態位之通道缺陷密度(deep-state density of grain traps)，因此，薄通道 (50 nm) 之薄膜電晶體，因具有平整之表面，使得熱載子難以劣化元件特性。 第三，利用背向連續波固態雷射活化源極汲極區來獲得高電洞載子遷移率與可靠度之類磊晶之電晶體與於玻璃基板上。綠光雷射能量是均勻的跨過源、汲極與通道之介面，並不受閘極層之影響，並企圖達到超可見光側向活化。電特性效能的提升在於側向活化可以連續修補汲/源極與通道之介面，雷射活化能量亦可以降低通道缺陷密度且幾乎不增加表面缺陷密度，因此本研究也獲得超高之P型電晶體遷移率達403cm2/Vs，為傳統快速熱退火活化之兩倍大。 第四，探討正向連續波固態綠光雷射活化並製作面板電晶體。對於在自對準複晶矽閘極之薄膜電晶體結構，由於綠光雷射在複晶矽中具有長的穿透深度因此雷射能量將能有效的穿透複晶矽閘極。綠光雷射能量能不易受閘極所影響而均勻，且於次毫秒時間內，側向活化通道與源、汲極區，反之亦然。在綠光雷射活化的薄膜電晶體中，這樣快速的綠光雷射退火可獲得低外部阻抗與近乎連續的改善複晶矽結構來降低通道中之缺陷。利用連續波固態雷射正向退火活化技術可於相同雷射退火結晶之100nm之高結晶通道上製造出一個具有非常可觀的電子遷移率為530cm2/Vs與陡峭的次臨界斜率為120mV/dev之電晶體。正向連續波雷射活化方式也可成功製作具氮化鈦金屬閘極結構之電晶體，其電子遷移率為230cm2/Vs。在金屬閘極面板電晶體中，正向連續波綠光雷射活化，由於雷射光受金屬反射，因而可選擇性的活化源汲極區，使金屬閘極下之材料僅受微小之熱損傷，因此能確保先進面板及光積體線路中之電晶體元件內嵌入複合物、奈米結構化、功能性介電層或複晶矽材料之發展。 未來可將這些連續波綠光雷射退火製造電晶體引入至面板光感應器、記憶體與奈米光積體線路元件上。

This dissertation explores and demonstrates enhanced electrical characteristics and reliability of continuous-wave green laser fabricated epi-like silicon transistors on non-silicon substrates, which greatly impacts on active-matrix no/off circuits, nonvolatile memories, linear image sensors, and photo-detector amplifiers for various panels and photonic circuits. The main focus of this dissertation can be divided into four parts. First, electrical characteristics of continuous-wave (CW) green laser-annealed single-grainlike silicon thin-film transistors in relation to trap-state densities were characterized. As laser power increases, highly crystalline channels form, reducing tail-state densities to as low as 3x1019 eV-1cm-3. This occurrence is responsible for high field-effect electron mobility of 284 cm2/Vs. In contrast, increasing laser power initially reduces the deep-state density and then increases it to 3x1016 eV-1cm-3. This reversal in deep-state density, and thus in the subthreshold slope, as well as a saturating reduction in threshold voltage are associated with the formation of extra interface defects caused by laser-crystallization-enhanced surface roughness. Next, stability of high hole-mobility thin-film transistors (TFTs) on single-grainlike silicon channels formed by CW laser-crystallization (CLC) during hot-carrier stressing (HCS) was studied. As channel layers become thicker, laser-mediated channel crystallinity increases, increasing channel roughness. On such epi-like polycrystalline silicon (poly-Si) substrates, the poorer interface quality for thicker channels, even those with lower tail-state densities of grain traps, is responsible for the extensive charge trapping and creation of deep-state densities in the fabricated TFTs due to HCS. Hence, on a thin channel with a thickness of 50 nm and ultra-smooth surfaces, HCS hardly degrades the electrical parameters of the devices. Third, the hole-mobility and reliability of green CW laser-crystallized epi-like Si transistors on glass panel substrates were enhanced by source/drain activation by back-side green laser-irradiation. Green laser-energy was scanned uniformly across junctions, since the gate structures included no interrupt, in an attempt to conduct super visible-laser lateral-activation. The enhancement was thus explained by the formation of continuous improved epi-like Si microstructures with reduced grain defects and with a barely increased number of interface defects over the entire channel/junction. The hole-mobility in such laser-activated devices was as high as 403 cm2/V.s – doubles that of thermally activated devices. Fourth, panel transistors were activated by front-side CW green laser irradiation. In self-aligned poly-Si TFTs, significant laser-energy penetrates through poly-Si gates owing to the considerably long penetration depth of green light in poly-Si. Green laser-energy was thus uniformly scanned laterally from channels to source/drain regions and, vice versa, in under a millisecond, hardly affected by gate structures. Such spike green-laser annealing yields low parasitic source/drain resistance and quasi-continuous improved poly-Si microstructures in green laser-activated TFTs, with reduced grain defects over the entire channel/junction. Electron-mobility and sub-threshold slope for such transistors that were fabricated on CLC channels of 100 nm, were remarkable values of 530 cm2/V.s and 120 mV/dec, respectively. In gate structures of TiN/SiO2, laser-activated panel transistors that were fabricated on CLC channels of 100 nm, also revealed electron-mobility as high as 230 cm2/V.s. In metal gated panel transistors, front-side CW green laser irradiation intrinsically activates source/drain regions selectively, because of light reflection by metal gates, causes little thermal damage on materials underneath metal gates, which endorses advanced panel or photonic transistors with compound, nanostructured, and functionalized gate dielectrics or polycrystalline materials. In future, those CW green laser-fabricated transistors will be routines for the development of panel photo-sensors and memories, as well as nano-photonic circuits.