負電容場效電晶體之操作速度及可靠度探討

Abstract

本篇論文中，我們嘗試從元件操作速度與元件可靠度的角度切入來探討負電容場效電晶體在實際應用上的可能性。我們在負電容場效電晶體上，使用不同速度掃描閘極電壓，測試不同掃瞄速度是否會影響負電容場效電晶體的元件特性；另外我們也在鐵電電容上面，施加不同的方波時間長度，量測時間長短對鐵電電容翻轉的對應量。然而，在負電容場效電晶體上面，我們的速度量測操作受限於內部延伸閘極造成的電容，而鐵電電容則受限於電阻電容延遲。隨後，我們在鐵電電容上測試定方波寬度及電壓翻轉電流時，都發現存在一種較慢的翻轉偶極，透過方波循環操作後，發現缺陷幫助穿隧效應造成的電流提高，而緩慢翻轉偶極造成的電流則逐漸下降。而在放大率的實驗當中也發現放大率會隨著操作次數增加而減少，表示負電容場效電晶體在操作多次之後，有可能會造成次臨界斜率的劣化。

In this thesis, we investigated the operational speed and stress induced reliability issues in both the Negative Capacitance FET (NCFET) device and the ferroelectric capacitor. The tested NCFET has a floating internal gate between the ferroelectric HfZrO2 and HfO2 which can be electrically probed. Sub-60 mV/decade subthreshold swing in NCFET is observed. However, subthreshold swing depending on the Vg sweep rate is “fake” due to the capacitance associated with the internal gate pad. Instead, we measured the polarization switching speed in the ferroelectric capacitor. Nevertheless, the maximum polarization switching speed is limited by RC delay. We found that the dipole switching time has a wide distribution, which results in a variable negative capacitance at different frequencies, indicating that the subthreshold swing in NCFET may vary with the operational frequency. In NCFET, we observed that there was a strong correlation on the internal voltage amplification and the polarization, meaning that the subthreshold swing may degrade after on/off cycling.