Naoki Izumi

Development of Low Dielectric Constant Ferroelectric Materials for the Ferroelectric Memory Feild Effect Transistor

In this paper we discuss ferroelectric materials suitable for a metal-ferroelectric-metal- insulator-semiconductor feild effect transistor (MFMIS FET). It is important for a ferroelectric material to have a low dielectric constant to enable the application of sufficient electric field to a ferroelectric layer. Films of Sr 2 Nb 2 O 7 and Sr 2 (Ta 1-x Nb x ) 2 O 7 were prepared by the sol-gel method on Pt/IrO 2 electrodes for an MFMIS FET. The ferroelectricities of Sr 2 (Ta 1-x Nb x ) 2 O 7 films were confirmed to be in the range of x = 0.1-0.3. In the case of x = 0.3, the largest remanent polarization was obtained in the hysteresis loop. The values of the remanent polarization and the coercive field are 0.5 μC/cm 2 and 44kV/cm, respectively. The film has a low dielectric constant (e r = 53). The characteristics of Sr 2 (Ta 1-x Nb x ) 2 O 7 thin films are suitable for MFMIS FET.

Evaluation of transistor property variations within chips on 300-mm wafers using a new MOSFET array test structure

A new test structure has been designed to evaluate fluctuations of transistor properties, both within a chip and across a 300-mm wafer. The evaluation system was established with a conventional parametric tester and dc power supplies suitable for application on production lines. It was observed that threshold voltage (V/sub th/) variations increased with the reduction of the channel area. A difference was also observed in the standard deviation (/spl sigma//sub vt/) between NMOS and PMOS. From statistical evaluations, controlling CDs and improving rolloff characteristics were found to be important to reduce V/sub th/ variations.

FABRICATION TECHNOLOGY OF FERROELECTRIC MEMORIES

Two types of ferroelectric memories have been proposed. One is a 1 transistor and 1 capacitor (1T1C) type and another is an FET (field effect transistor) type. The fabrication technique for fabrication ferroelectric capacitors on polycrystalline Si (poly-Si) was very effective for high density 1T1C ferroelectric memory and metal-ferroelectric-metal-insulator-semiconductor (MFMIS) FET with stable properties. MFMIS FET is one type of FET ferroelectric memory which we proposed. To obtain good ferroelectric capacitors on poly-Si, we used Ir based materials as electrodes of the capacitors. By using Ir based electrodes, the characteristics of Pb(Zi,Ti)O3 (PZT) capacitors on poly-Si were equivalent to those on SiO2. Moreover, the PZT capacitors with Ir based electrodes showed dramatic improvement in fatigue and imprint properties.

Application of Sr2Nb2O7 Family Ferroelectric Films for Ferroelectric Memory Field Effect Transistor

The compounds of the Sr2Nb2O7 (SNO) family are suitable for use as ferroelectric materials for ferroelectric memory field effect transistors (FETs), because these substances have a low dielectric constant, low coercive field and high heat-resistance. In this study, we succeeded in preparing Sr2(Ta,Nb)2O7 (STN) capacitors on polycrystalline silicon (poly-Si). From SIMS profiles, no interdiffusion in the STN metal ferroelectric metal insulator semiconductor (MFMIS) structure was confirmed. C–V and ID–VG hysteresis curves which were dependent on ferroelectric polarization were obtained. These capacitors were applied to floating gate type ferroelectric random access memory (FFRAM) cells. The degradation in ferroelectricity of STN capacitors was not observed during FFRAM cell fabrication process. We succeeded in operating FFRAM cells with a lower voltage than that required for PZT and confirmed the drain current difference of 1 or 2 orders at the 30 s after applying write pulses of ±5 V or ±10 V.

Sr2(Ta, Nb)2O7 ferroelectric thin film for ferroelectric memory FET

Abstract Sr2Nb2O7 (SNO) family are suitable as ferroelectric materials for ferroelectric memory FETs, because they have low dielectric constant, low coercive field E c and high heat-resistance. In this study, we succeeded to prepare the Sr2(Ta, Nb)2O7 (STN) capacitors on the poly-Si. These capacitors were applied to FFRAM (Floating gate type Ferroelectric Random Access Memory) cells. The STN thin films were prepared by the sol-gel method. After several times spin coating and preannealing, these films were crystallized at about 950°C in oxygen for 30 sec by RTA (Rapid Thermal Annealing). When the Nb/(Ta+Nb) ratio was between 0.1 and 0.3, the ferroelectricities were confirmed. The maximum remanent polarization P r showed 0.4μC/cm2 when the ratio was 0.3. The FFRAM cell using STN thin films showed good memory characteristics.