一種量測快閃式記憶體元件中微量漏電流的新式方法

Abstract

在此篇論文中, 我們提出一套新的兩階段式次臨界電流測量方 法及解析模式用以量測快閃式記憶體元件中因應力所引起之微量漏電流, 並計算在 read-disturb 退化現象中相關的穩態以及暫態特性. 此外, 我 們亦利用兩種不同的抹除方式設計出一套特別的實驗來研究氧化層電荷對 read-disturb 退化現象所造成的影響. 此種量測方法包含兩個相互變 換的階段, 其一是 read-disturb 的階段, 另一個則是測量次臨界電流的 階段. 藉著改變 read-disturb 階段中閘極的偏壓, 我們可以得到漏電流 及 read-disturb 機制與電場的相依關係. 另外, 我們推導出一套解析模 式, 建立起次臨界電流與穿隧氧化層及浮動閘極中電荷變化的相關性. 由 於在此種模式中次臨界電流和氧化層及浮動閘極中的電荷呈現自然指數的 相依性, 因此, 和傳統的臨界電壓法比較, 這種方法能提供更好的解析 度. 在這項實驗中, 我們同時利用到傳統的抹除方式以及改良式的抹 除方式. 我們的研究結果顯示漏電流的暫態部份是由於氧化層電荷充放電 所造成, 而其穩態部份則是受到缺陷輔助所產生的連續穿隧效應所影響. 我們可以分別量測出暫態以及穩態部份對於臨界電壓所帶來的變化. 在 read-disturb 偏壓條件下, 雖然在開始時有極大的暫態電流, 但穩態部 份仍是造成臨界電壓改變的主要因素. 然而, 若是在一低偏壓操作的應用 情形下, 則暫態部份有可能成為造成 read-disturb 退化現象的主要機 制. 我們的研究證明了利用改良式的抹除方式, 可以有效地抑制漏電流中 的暫態部份. 此外, 藉由此種兩階段式的次臨界暫態電流量測技術, 我們 可以量得一極低的漏電流 (約 1E-19 安培). In this thesis, we proposed a novel two-phase subthreshold transient current measurement method and an analytical model to characterize the stress induced low-level leakage current (SILC) in a flash EEPROM cell. The associated steady-state and transient characteristics of the read-disturb degradation was evoluated. In order to investigate the oxide charge effect on the read-disturb degradation, a specially designed experiment with twoerase methods were performed. The measurement method consists of two alternating phase; one is the read-disturb phase and the other is the subthreshold current measurement phase. By varying the gate bias in the read-disturb phase, the field dependence of the SILC and the read-dsiturb mechanism can be obtained. An analytical model relating a subthreshold current transient to the charge variations in the tunnel oxide and in the floating gate has been derived. Since in the model the subthreshold current exhibits an exponential dependence on oxide charge and floating gate charge, this method can provide a better resolution than the traditional threshold voltage method. Two erase methods, a conventional method and a modified method, were employed in this work. Our study reveals that the transient component of the SILC is attributed to oxide charge trapping/ detrapping while the steady-state component of the SILC is due to trap-assisted sequential tunneling. Threshold voltage shifts resulting from these two components were characterized separately. During read-disturb, in spite of the large initial transient, thethreshold voltage shift is mainly affected by the steady-state component.However, in the application of a low- voltage operation, the transient component may appear to be a dominant read-disturb degradation mechanism. Our study shows that the transient component can be significantly suppressed by using a modified erase method. Furthermore, by using the two- phase subthreshold transient current technique, an extremely low leakage current (about 1E-19 Amp) can be measured.