退火處理對已劣化MOSFETs的恢復效應

Anneal-induced Recovery for Degraded MOSFETs

簡士宏(Shih-Hong Chien)

指導教授 ： 黃恆盛 ； 陳雙源

國立臺北科技大學/機電學院/機電整合研究所/碩士(2009年)

https://doi.org/10.6841/NTUT.2009.00201

摘要

隨著MOSFET的介電層愈來愈薄，操作的電場愈來愈大，其特性會於操作一段時間後明顯下降，主要原因一般皆推測是與介電層的陷阱(trap)有關，如在NBTI加壓(NBTI stress)測試下，Si/SiO2界面處的Si-H鍵可能會斷裂，H離子會陷入介電層，形成氧化層陷入電荷(oxide trapped charge)，同時在界面處產生Si的懸浮鍵(dangling bond or interface trap)，懸浮建會抓住通道中的電荷使介面帶電。此兩種陷入電荷會導致MOSFET性能下降。根據Katsetos研究指出經過NBTI加壓測試老化的元件經過高溫退火(anneal)後會使元件恢復(recovery)其初始的性能。為探討陷入電荷的機制，本研究使用製程退火，對已劣化MOSFET退火，其退火氣體為N2與5% H2兩種分別觀察其氧化層陷阱的變化。使用N2是為了探討高溫是否會讓已陷入在氧化層裡的電荷脫離陷阱，而H2是為了探討是否可經由外部額外的H2讓已劣化MOSFET的懸浮鍵再鈍化。
在本研究中，採用聯電0.18 μm製程，通道長度也是0.18 μm，寬度為10 μm，閘極氧化層為SiO2、厚度為32 Å之MOSFET作為實驗樣本，進行NBTI與CHC加壓實驗。NBTI實驗條件採用3T3V(亦溫度分別30、75與150°C，操作電場為7、 8、9MV/cm三種)，而CHC加壓實驗則是在常溫下做實驗。實驗中不時對元件量測I-V等特性，藉以分析其性能，並計算介面和氧化層陷入電荷的變化量。接著對已施加應力的元件，分別進行5%H2與100%N2在400°C下退火30分鐘，同樣的，再量測其特性，做後續的分析。
由實驗的結果發現，在NBTI加壓測試下MOSFET的劣化情況與文獻探討的部份近似符合。但在退火階段，無論是以NBTI或CHC加壓測試下，MOSFET皆會恢復到初始狀態，而且缺陷皆有減少的趨勢。為了更加確立溫度的恢復效應，將未加壓測試的元件分別進行N2與5%H2的退火，觀察到兩者特性並未有太大差異，5%的H2似乎不會影響實驗的結果。因此推測恢復效應只與溫度有關，外部的H2無法經由退火通過外部鈍化層與懸浮鍵產生鍵結。
層陷入電荷的變化量。接著對已施加應力的元件，分別進行5%H2與100%N2在400°C下退火30分鐘，同樣的，再量測其特性，做後續的分析。
由實驗的結果發現，在NBTI加壓測試下MOSFET的劣化情況與文獻探討的部份近似符合。但在退火階段，無論是以NBTI或CHC加壓測試下，MOSFET皆會恢復到初始狀態，而且缺陷皆有減少的趨勢。為了更加確立溫度的恢復效應，將未加壓測試的元件分別進行N2與5%H2的退火，觀察到兩者特性並未有太大差異，5%的H2似乎不會影響實驗的結果。因此推測恢復效應只與溫度有關，外部的H2無法經由退火通過外部鈍化層與懸浮鍵產生鍵結。

關鍵字

NBTI ； 氧化層陷入電荷 ； 懸浮建 ； 退火 ； 恢復

並列摘要

MOSFETs degrade apparently for operating a long time when dielectric is getting thinner and operating oxide field is much larger. It supposed that main culprit is trap. After stress, Si-H bond may be broken. Consequently, hydrogen would leave from Si/SiO2 interface to produce interface trap, is driven into oxide, called oxide trapped charge. Both interface trap and oxide trapped charge may induce MOSFETs degradation directly or indirectly. Katsetos et al. studied that the NBTI recovery depends on temperature.
In this study, we attempts to anneal degraded-device by PMA technique, including 5% H2/ pure N2, to observe trap variation. In order to research whether trapped charge would detrap by high temperature, and whether external H2 would repassivate, this study use N2 and H2 respectively.
The MOSFETs being tested are fabricated by 0.18 μm process. Their channel length is 0.18 μm, channel width is 10 μm, and insulator is SiO2 of 32 Å. For the stress conditions, we adopt NBTI stress with 3T3V (T = 25, 75, 150°C; Vstress = 2.6V (7MV/cm), 3.1V (8MV/cm), 3.6V (9MV/cm)), and CHC stress with 1T3V (30°C, Vstress=1.5Vcc, 1.75Vcc, 2Vcc). The MOSFETs’ I-V curves are measured during every stress cycle to analyze device performance and calculate trap variation; furthermore, we anneal degraded devices by both 100% N2 and 5% H2 under 400°C, 30 minute, and measure basic characteristic again.
After above experiments, we discover that NBTI degradation is similar to former researches. Degraded devices can be recovered to fresh state, and all defects are reduced after annealing. On the other hand, the external H2 seems not affecting the experimental result; It infer that recovery phenomenon depend on the temperature, and that the external H2 is hard to pass through passivation layer by annealing.

並列關鍵字

NBTI ； Oxide trapped charge ； Interface trap ； Anneal ； Recovery

參考文獻

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