奈米尺度應力下薄閘極氧化層之退化及崩潰特性

Degradation and breakdown characteristics of thin gate oxide under nano-scaled stresses

林式庭(Shi-Tin Lin)

指導教授 ： 吳幼麟

國立暨南國際大學/科技學院/電機工程學系/博士(2008年)

https://doi.org/10.6837/NCNU.2008.00139

摘要

本論文旨在探討奈米尺度應力下的薄閘極氧化層退化與崩潰特性。近年來，有不少研究出氧化層的退化與崩潰是非常侷限性的行為，傳統以MOS元件量測的方式僅能夠得到閘極下方氧化層退化與崩潰的平均資訊，因此在本論文中，我們使用傳導式原子力顯微鏡對氧化層進行奈米尺度之應力並分析氧化層的退化與崩潰特性。為了加強傳導式原子力顯微鏡的電性量測能力，我們將Agilent 4156 C 半導體參數分析儀與傳導式原子力顯微鏡結合，此結合後的量測系統之電流量測範圍大幅增加並可施加奈米尺度之定電壓應力(CVS)以及定電流應力(CCS)於閘極氧化層上，如此，我們可取得更多氧化層退化的相關資訊。在我們施加步階式電壓應力於閘極氧化層上並觀察其退化行為時，我們發現氧化層在承受應力後因為介面處的斷鍵而產生負電荷累積及等效厚度降低的兩種效應。我們也發現在氧化層發生崩潰後，氧化層崩潰後的I – V特性曲線在特定電壓範圍內隨RVS次數增加而震盪，這是因為氧化層經施加RVS崩潰後會形成的不穩定崩潰電流傳導路徑。為了解釋不穩定的崩潰後傳導行為，在本論文中我們提出兩個崩潰後電流傳導模型 – 能帶調變模型及雙缺陷輔助穿遂模型，這兩個模型皆能解釋崩潰後I – V特性曲線曲線不穩定的行為，但雙缺陷輔助穿遂模型能夠更進一步解釋崩潰後I – V特性曲線中的彎曲特性。另外，我們也將閘極氧化層置於Co60γ-ray輻射環境中，我們發現在隨後施加的奈米尺度RVS作用下，閘極氧化層會出現偏壓退火的效應。藉由計算輻射及RVS作用前、後的電流波峰與介面能位障，我們發現氧化層在輻射照射過後會有缺陷產生於氧化層內，而在RVS過後有電流波峰減少且能位障增加的偏壓退火效應。在施加奈米尺度之CVS與CCS方面，由觀察得知氧化層的崩潰後傳導特性與崩潰點的崩潰後電流限制條件有極大的關係，氧化層崩潰後的電流傳導呈現potential (power law)與F–N穿遂兩種傳導行為，且崩潰後限制電流較低的大多呈現F – N穿遂傳導，此研究結果可有效釐清氧化層崩潰後傳導模式的爭議。此外氧化層崩潰程度低的崩潰後傳導行為也與傳統量測中的SILC傳導行為相當類似，因此我們認為傳統量測所得的SILC是崩潰程度很低的崩潰後傳導行為。在分析氧化層施加CVS與CCS後所得的崩潰時間(tBD)及崩潰電荷(QBD)的韋伯分佈後，我們發現在奈米尺度應力下量測所得的韋伯斜率β與傳統量測相仿，這表示奈米尺度下觸發的崩潰機制與傳統量測相同，而在QBD的統計結果方面，QBD 隨量測面積減少而增加，且呈現power law的關係，這表示我們在奈米尺度下得到的結果較為接近單一崩潰點所需的電荷注入量。

關鍵字

閘極氧化層、退化、崩潰、可靠度、傳導式原子力顯微鏡

並列摘要

The main purpose of this thesis is to understand the degradation and breakdown characteristics of thin gate oxide under nano-scaled electrical stresses. Recently, a number of research results published in the literature indicate that the mechanism of oxide degradation and breakdown is an extremely localized phenomenon which has been confirmed to occur in the range of 10-14 ~10-12 cm2. Thus, conventional oxide reliability tests can only provide us average information under the gate area for oxide degradation and breakdown because the gate area of the MOS devices used for conventional tests is rarely less than 10-8 cm2. Since the contact area between the tip of conductive atomic force microscopy (C-AFM) and the sample surface is in the range of 10-12 ~ 10-10 cm2 which is close to the oxide breakdown area, the C-AFM becomes a powerful tool for investigating the degradation behaviors of a single degradation and breakdown spot of oxide. In order to extend the current measurement range as well as facilitating the capabilities of applying constant voltage stress (CVS) and constant current stress (CCS) to the oxide, the semiconductor analyzer (Agilent 4156 C) has been connected to the C–AFM system in this work.

Repetitive ramped voltage stress (RVS) was applied to the oxide samples and we found two parallel oxide degradation mechanisms, bond-breaking and negative charge accumulation near the SiO2/Si interface as well as oxide thinning which results in an effective barrier height increase at the SiO2/Si interface, as determined from the measured current-voltage (I-V) characteristics and surface topographies. Breakdown does not actually occur until permanent damage is formed within the oxide during the after several times of repetitive RVS. The permanent damage produced inside the oxide film is in the form of traps, which will cause the bending as well as current oscillation in I-V curves. An energy band modulation model and a two-trap-assisted tunneling (TTAT) model were proposed to explain the post-breakdown I-V behaviors in this work. Both of the two models can explain the current oscillation occurred in the post-breakdown I – V curve, and the TTAT model can illustrate the bended I – V behavior. We also investigated the reliability of thin gate oxide subjected to irradiation followed by nano-scale stress. By taking advantage of a small contact area we report the nano-scale post-irradiation bias annealing effect in thin SiO2 film using C-AFM. Based on the number of fluctuating current peaks appearing in the I-V curves of the pre- and post-treatment oxide films as well as the calculated effective barrier height from the Fowler-Nordheim (F–N) tunneling theory, we found that the trapped charge in the oxide films caused by Co60γ-ray irradiation, can be effectively annealed out by a post-irradiation ramped voltage. Nano-scaled constant voltage stress (CVS) and constant current stress (CCS) were also applied to the gate oxide in this work. We found that the CVS-triggered breakdown I-V characteristics obtained by C-AFM were dependent on the current compliance during measurement. Two different post-breakdown conduction modes in thin SiO2 films, one follows the F–N tunneling and the other obeys the power law, were observed. Furthermore, the post-breakdown conduction follows F–N tunneling which is dominant when the current compliance is low, while power law is dominant if the current compliance is high. This result clarifies the previous controversy about the oxide post-breakdown conduction discussed in the literature. We also found that the post-breakdown conduction after nano-scaled CVS with a low current compliance is highly similar to the SILC conduction measured by conventional oxide breakdown tests. Base on our observation, the SILC conduction can be recognized as a light breakdown conduction behavior. In this work, we also investigated the Weibull plots of time-to-breakdown tBD and charge-to-breakdown QBD obtained under nano-scaled CVS and CCS. It is found that the Weibull slope ?? obtained in this work is in consistent with those in the conventional stress tests, which states that the breakdown triggering mechanism under nano-scaled stress is the same as the one obtained in conventional oxide breakdown tests. As to QBD distribution under nano-scaled stress, an opposite trend to the results obtained in conventional tests was observed. We believe that QBD obtained under nano-scaled stress is more close to the actual QBD value of a single breakdown spot.

並列關鍵字

Thin gate oxide, degradation, breakdown, irradiation effect, conductive - atomic force microscopy.

參考文獻

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