溶膠-凝膠法應用於非揮發性奈米微晶粒記憶體元件

Abstract

本論文所探討的內容係以配製的化學溶液經由溶膠-凝膠法，來研究成長奈米微晶粒的機制。我們以四氯化鋯、四氯化鉿及四氯化矽為溶膠-凝膠溶液之前驅物，旋轉塗佈於晶圓上，經高溫退火後形成含有鋯及鉿之金屬矽氧化物。在本實驗中，溫度效應及溶劑效應是影響奈米微晶粒形成的關鍵因素。此外，我們利用溶膠-凝膠法形成之奈米微晶粒材料運用於快閃式記憶體元件中，作為電荷捕捉層(charge trapping layer)，並探討其電性表現。實驗涵概三個主題，我們將其內容摘要說明如後。 第一個主題，我們針對溫度效應做深入探討。實驗中發現溶膠-凝膠法旋轉塗佈在晶圓上形成薄膜，經600 oC高溫退火開始進入不穩定相而造成薄膜表面開始分裂，此時具有較高的表面能(interfacial energy)。當退火溫度高達900 oC，溶膠-凝膠薄膜已經過相變化分裂成完全獨立之奈米微晶粒，此時可以量測得到已達穩定態之低表面能。我們以900度退火條件製作完整的記憶體元件，測試資料保存時間為在125度的量測環境下經過10E6秒後只有少於25%的電荷流失。 第二個主題中，我們針對以溶膠-凝膠法製備快閃記憶體中之電荷捕捉層的相變化機制進行討論。溶膠-凝膠薄膜經900度高溫退火後可發現電荷陷捕層產生旋節相分離(spinodal decomposition)而形成奈米微晶粒結晶，而能否形成奈米微晶粒的關鍵在於旋轉塗佈至晶圓上的溶膠-凝膠薄膜厚度。我們選擇兩種不同的溶劑，包括乙醇及異丙醇，用以調控薄膜厚度。實驗中發現在乙醇系統中形成之奈米微晶粒呈現完全獨立、分離之形貌，然而，在異丙醇系統中形成之奈米微晶粒卻產生具有內部相連、不完全分離的狀態。根據Seol等人利用電腦模擬旋節相分離的狀態，他們推測旋節相分離後的薄膜形貌與薄膜厚度及成分相關，較薄的溶膠-凝膠薄膜經過旋節相分離能夠形成完全獨立之奈米微晶粒。由於以乙醇為溶劑能夠得到較薄之薄膜，經旋節相分離後得到獨立之奈米微晶粒，以此作為記憶體之電荷陷捕層具有良好的記憶體特性，包括資料保存時間久(在25度及85度量測條件下各只有少於5%及10%的電荷流失)，且記憶視窗可高達10V之大。 最後，我們利用整合有機與無機材料之溶膠-凝膠法，製造一完整的奈米微晶粒記憶體元件，並進行電性分析。我們成功的製作出特性良好的記憶體，此記憶體具有大的記憶視窗、資料寫入速度快、較長的資料保存時間及元件耐久力佳等等特色。在此主題中，我們針對記憶體的可靠度做一系列的分析探討。由於此奈米微晶粒記憶體元件具有相當大的記憶視窗，期待將來可應用於多位元晶胞之記憶體元件。

In this thesis, the formation mechanism of nanocrystals (NCs) from the sol-gel spin-coating method had been studied. The ZrCl4, HfCl4, and SiCl4 were used as the sol-gel precursors to form the hafnium and zirconium silicate after high temperature annealing. We evaluated the impact of temperature and preparation solvent type for the formation of sol-gel derived nanocrystals. In addition, the performance of flash memory with the nanocrystal as the charge trapping layer was also demonstrated. The aim of this study is organized in the following. At first section, we focused on the rapid thermal annealing temperature affected deposited sol-gel transformation. We detected the sol-gel film became unstable and perturbed at 600 °C annealing, and finally transformed into NCs at 900 °C annealing. A mechanism was proposed to explain the transformation of the sol-gel thin film. The morphology of sol-gel thin film at 600 °C annealing was unstable and had higher interfacial energy. The nano-crystallized process at 900 °C annealing could minimize the energy. As to the memory devices, the property of retention issue (charge preservation ability after program for nanocrystals) for 900 °C annealed sample demonstrated less than 25% charge loss after 106 sec duration under 125 °C measurement condition. Secondly, we investigated the phase separation mechanism for the charge trapping film in sol-gel derived nanocrystal memory. The nano-crystallization from spinodal phase separation was observed for sol-gel thin film at 900 oC annealing, and was strongly related to the thickness of the spin-coated thin film. We chose ethanol and IPA as the solvent in order to control the sol-gel film thickness. We found the morphology of the NCs in ethanol system existed in the isolated form, while interconnected form in IPA system. Seol et al.[1, 2] utilized computer simulation to deduce the spinodal decomposition process. They suggested the morphology of decomposed phases on initially homogeneous thin film strongly depends on the film thickness and the composition. Ethanol as preparation solvent deposited a thinner film, and formed isolated NCs after spinodal decomposition. Ethanol system sample as memory device exhibited excellent performance such as the data retention less than 5% and 10% charge loss at 25 °C and 85 °C, respectively. The ethanol system sample also demonstrated a large memory window (~10V) than IPA system (~3V). Finally, the detailed electrical properties for the sol-gel derived NC memory were measured. The obtained flash memory exhibited excellent characteristics such as large memory window, high program speed, and superior gate/drain disturbance properties. In addition, the reliability of the memory was also evaluated in this section. The sol-gel derived NC memory with large memory window from 900 oC annealed sample shows the effectiveness of device operation for future multibit application.