鋯鈦酸鉛鐵電電容器之可靠度研究

Abstract

非揮發性鐵電記憶體由於具有低功率消耗、低寫入偏壓、高速寫入、及較佳的操作容忍度等特點，有可能取代現階段的主流記憶體，包括快閃式記憶體及可寫入可抹除記憶體(EEPROM)，這些優點使得鐵電記憶體適用於呼叫器、行動電話，以及個人辨識卡(smart card)等應用中。即使如此，我們仍然非常關心鐵電電容器中的鐵電性及它可能擁有的可靠度問題。 本篇論文的主旨將在於探討鐵電電容器中的可靠度問題。除了一些熟知的問題如鐵電疲勞(fatigue)、鐵電印象(imprint)及老化(aging)，我們特別著重於極化電荷鬆弛及極化偏壓所造成的可靠度問題。在研究極化電荷鬆弛對可辨識訊號的影響時，我們提出一種適用於電路模擬的新模型，在此模型中我們加入了一個電壓源來模擬極化電荷鬆弛效應，藉著這個方法，我們發現了極化電荷鬆弛在不同的架構中有不同的影響：在2T/2C架構中，極化電荷造成可辨識到的電壓大幅度降低；反之在1T/1C架構中，我們發現由電容值不平衡所影響的可辨識訊號，反而是在不考慮極化電荷鬆弛的情況下最小。 在研究極化偏壓所造成的效應方面，我們發現了在鋯鈦酸鉛(PZT)鐵電記憶體中，一種於低偏壓及高速操作時所造成的極化電荷喪失，這種極化電荷喪失的情形不僅會降低可辨識訊號的大小，也同時在鐵電記憶體中，特別是低偏壓操作的先進鐵電記憶體中，造成嚴重的可靠度的問題。我們發現這種極化電荷喪失是由於電極及鐵電材料中形成了蕭特基能障，此蕭特基能障在界面產生了能帶彎曲，進而使得極化速度變慢。在本研究中，為了消除並改善這種極化電荷喪失，我們也提出一種解決方法並加以證實。

Non-volatile ferroelectric random access memories (FeRAMs) have the potential to replace current state-of-the-art non-volatile memories such as flash and EEPROMs because of their lower power consumption, lower writing voltage, faster writing speed, and better endurance. Such attributes make this technology very attractive for applications such as pagers, cellular phones, and smart cards. However, the ferroelectric properties and their reliability are still of major interest. The main theme of this work will focus on the reliability concerns in ferroelectric capacitors. Besides reviewing the most investigated subjects such as fatigue, imprint and aging, we intensively investigate the mechanisms for the polarization loss including polarization relaxation and polarized pulse effect. To evaluate the relaxation effect on bitline sense voltage, a SPICE compatible polarization relaxation model has been developed. A relaxation voltage source, responsible for polarization relaxation effects, is incorporated into the conventional ferro-capacitor model. The sense voltage in a 2T/2C cell is significantly reduced from the relaxation model. The relaxation effect on minimum sense voltage attributed to capacitance mismatch in a 1T/1C cell is also evaluated. As opposed to the result in the cell structure of 2T/2C, the worst case occurs when polarization is not relaxed at all. Polarization charge loss in PZT ferroelectric memory under low-voltage and high-speed operation is observed. This effect significantly reduces the sensing margin and causes severe reliability issue in advanced ferroelectric memory, particularly for low-voltage applications. This polarization loss is attributed to slowing-down of polarization caused by band bending from Schottky potential at the electrode/ferroelectric interface. A solution to eliminate the polarization loss is proposed and verified.