The Properties of Mfmos and MFOS Capacitors with High K Gate Oxides for one Transistor Memory Applications
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THE PROPERTIES OF MFMOS AND MFOS CAPACITORS WITH HIGH k GATE OXIDES FOR ONE TRANSISTOR MEMORY APPLICATIONS TINGKAI LI, SHENG TENG HSU, HONG YING, AND BRUCE ULRICH Sharp Laboratories of America, Inc, 5700 NW Pacific Rim Blvd. Camas, WA 98607
ABSTRACT MFMOS and MFOS (M: Metal, F: Ferroelectrics, O: Oxide, S: Silicon) capacitors with high k gate oxides, such as ZrO2, HfO2 thin films, have been fabricated for one transistor memory applications. Experimental results showed that ZrO2 and HfO2 have no serious reaction or diffusion into silicon substrate. Due to their high dielectric constant, the operation voltages of MFMOS capacitors are reduced. . The MFMOS capacitor exhibits 2V memory window. For lead germanium oxide (PGO) on ZrO2 and PGO on HfO2 MFOS memory cells the memory windows are 1.8 V and 1.6 V, respectively, which are large enough for one-transistor memory applications. The basic mechanism for one-transistor memory applications was also discussed. INTRODUCTION Ferroelectric-gate controlled devices, such as metal-ferroelectric-silicon (MSF) FET, were studied as early as 1950s1-4. There are several groups of investigators actively research on ferroelectric-gate controlled memory transistors in recent years5-9. The typical device structures are Metal–Ferroelectric–Insulator–Silicon (MFIS) FET, Metal–Ferroelectric–Metal–Silicon (MFMS) FET, and Metal–Ferroelectric– Metal–Oxide–Silicon (MFMOS) FET. These devices have very small memory cell size and non-destructive read out (NDRO). Therefore, fatigue is not a major issue for these devices. However, there is no reliable memory device fabricated yet. This is because the difficulty in selecting appropriate ferroelectric and dielectric materials in addition to the complicated integration processes for one transistor memory device fabrication. The basic operation principal for MFMOSFET and MFOSFET memory transistors are depicted in Fig. 1 and Fig. 2. When the ferroelectric material is poled towards the gate electrode, positive charges are induced at the channel. The threshold voltage of the transistor is very large. The transistor is programmed to the "off" state ("0" state). Similarly negative charges are induced at the channel when the ferroelectric material is poled toward the channel. The threshold voltage of the transistor is low and the device is programmed to the "on" state ("1" state). During read operation, the sense amplifier detects the state of the MFMOSFET. If there is a current, it is “1”; if there is no current, it is “0”. This is the basic operation mechanism of MFMOS and MFOS one-transistor memory devices. The operation voltage and the memory window are important properties of a memory transistor. The operation voltage of MFMOS and MFOS is VOP, where VOP = VO + VF and VO and VF are the voltages across oxide and ferroelectric capacitor, respectively. In order to reduce operation voltage, VO should be as small as possible. Since VO = Qo/Co, where QO and CO are the electrical charge and capacitance of the oxide capacitor, decreasing the thickness or increasi
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