利用介電層熔斷崩潰之電阻式記憶單元設計新穎的內嵌式可重寫單次寫入唯讀記憶體

Abstract

在物聯網快速興起的時代，市場對內嵌式記憶體的需求大幅增加。在過去，單次寫入唯讀記憶體(One-time-programming, OTP)最被看好由於低成本及容易與晶片整合等優勢，但寫入後不能修改伴隨著誤寫的風險。然而，多次寫入記憶體(Multi-time-programming, MTP)雖然可以重複寫入，但資料的保存能力不夠優秀是一個隱憂。針對上述特點，我們設計了一種新穎的可重寫單次寫入唯讀記憶體(Rewritable, RW-OTP)，結合了重複寫入以及卓越的資料保存能力兩項優點，成為最有潛力的記憶體之一。 首先，我們會先介紹雙層電阻式記憶體，利用它的電阻轉換特性在低阻態(LRS)和高阻態(HRS)之間切換，達到儲存資訊的目的。接著，為了探討尚未被證實的導通機制，我們從介電層的軟性崩潰開始著手，在其中觀察到許多隨機電報雜訊(RTN)訊號，代表這些缺陷是造成阻值轉換的重大因素。透過量測隨機電報雜訊，可以收集到訊號的捕捉時間和發射時間，進而萃取出缺陷的位置以及能階。隨著時間的演變，收集不同時間點的缺陷位置及能階，我們就可以描繪出在介電層內完整的崩潰路徑。我們也將這個方法詳細的運用在雙層電阻式記憶體的不同狀態中，可以發現在低阻態時有完整的崩潰路徑連通上電極和下電極，形成短路使電阻變低；而在高阻態時，崩潰路徑會在靠近上電極附近斷開，形成斷路使電阻變高。另外還有熔斷機制的發現與探討等。 其次，根據上述的現象，我們設計出了一種新穎的可重寫單次寫入唯讀記憶體，操作上一開始可以依照用戶的需求隨意修正資料，ㄧ旦確認要寫入的資料後，將會利用熔斷的特性永久保存資料，達到不可更改的保密需求以及極佳的保存能力。在電路架構上，每個記憶單元會串聯一個選擇器來抵擋潛洩電流(sneak current)，以此為單位擴展成廣大的陣列。透過相關實驗，我們證明了可重寫單次寫入唯讀記憶體具有大的視窗(window)、干擾免疫(disturb immunity)、良好的資料保存力及耐力(endurance)，充分展現了它的潛力。

In the coming Internet-of-Things (IOT) era, the market have huge demand for embedded memory. In the past, One-Time-Programing memory are the most promising one because of low cost, easy-integrated on chip and excellent data retention. But the OTP can be only programmed once, which has the risk of miss-coding. However, although Multi-Time-Programming memory can program and erase lots of times, data retention is a serious problem. For the above characteristics, we design a novel rewritable OTP, RW-OTP, which combines both the multi-time programming capability and excellent data retention.

First, we will introduce the bilayer RRAM, which is an important element as a storage cell by utilizing the switching characteristic between low resistance state (LRS) and high resistance state (HRS). In order to explore the physical mechanism of RRAM, we designed an experiment to locate the soft-breakdown path by the RTN measurement technique, from which we can extract the trap position and energy level by collecting the capture and emission time. We then locate the generation of traps as a function of time and complete the conductive path for three different conditions, i.e., the paths during forming, set, and reset. Results showed that, in the set path, filament links top-electrode and bottom-electrode and forms a low resistance state (LRS). In the reset path, filament is broken near the top-electrode and became a low resistance state (HRS). Moreover, the interesting characteristics of dielectric-fuse breakdown are also examined.

Next, according to the above understandings on the filament formation, we design a new type of OTP which can modify the data at first and store the data permanently when users confirm it is of no problem. The fuse characteristics have excellent retention ability for security requirement. At the circuit level, the one selector and one RRAM (1S1R) array has been designed to resist the sneak current. Other experiments shows that this new type of RW-OTP has the ability of large window, disturb immunity, excellent data retention, and great endurance.