Method for driving semiconductor memory

• US 6,421,268 B2
• Filed: 07/09/2001
• Issued: 07/16/2002
• Est. Priority Date: 07/13/2000
• Status: Expired due to Term

• Current Assignee: Matsushita Electric Industrial Company Limited (Panasonic Holdings Corporation)
• Original Assignee: Matsushita Electric Industrial Company Limited (Panasonic Holdings Corporation)
• Inventors: Kato, Yoshihisa, Shimada, Yasuhiro
• Primary Examiner(s): Elms, Richard
• Assistant Examiner(s): NGUYEN, HIEN N
• Application Number:
  US09/899,839
• Publication Number:
  US 20020006052A1
• Time in Patent Office: 372 Days
• Field of Search: 365/145, 365/189.06, 365/149, 365/189.08
• US Class Current: 365/145
• CPC Class Codes: G11C 11/22 using ferroelectric elements

Abstract

A multi-valued data is written in a ferroelectric capacitor, which stores a multi-valued data in accordance with displacement of polarization of a ferroelectric film thereof, by applying a relatively high first writing voltage or a relatively low second writing voltage between a first electrode and a second electrode of the ferroelectric capacitor. Next, a potential difference induced between the first and second electrodes is removed. Then, the multi-valued data is read by detecting the displacement of the polarization of the ferroelectric film by applying a reading voltage between the second electrode and a substrate where a reading FET for detecting the displacement of the polarization of the ferroelectric film is formed. The reading voltage has the same polarity as the first writing voltage and is set to such magnitude that, in applying the reading voltage, a first potential difference induced between the gate electrode of the reading FET and the substrate when the multi-valued data is written by applying the first writing voltage is smaller than a second potential difference induced between the gate electrode and the substrate when the multi-valued data is written by applying the second writing voltage.

Claims

1. A method for driving a semiconductor memory including a ferroelectric capacitor for storing a multi-valued data in accordance with displacement of polarization of a ferroelectric film thereof and a reading field effect transistor that is formed on a substrate and has a gate electrode connected to a first electrode corresponding to one of an upper electrode and a lower electrode of said ferroelectric capacitor for detecting the displacement of the polarization of said ferroelectric film, comprising:

a first step of writing a multi-valued data in said ferroelectric capacitor by applying a relatively high first writing voltage or a relatively low second writing voltage between said first electrode and a second electrode corresponding to the other of said upper electrode and said lower electrode of said ferroelectric capacitor;

a second step of removing a potential difference induced between said first electrode and said second electrode; and

a third step of reading the multi-valued data by detecting the displacement of the polarization of said ferroelectric film by applying a reading voltage between said second electrode and said substrate,

wherein said reading voltage has the same polarity as said first writing voltage and is set to such magnitude that, in applying said reading voltage, a first potential difference induced between said gate electrode and said substrate when the multi-valued data is written by applying said first writing voltage is smaller than a second potential difference induced between said gate electrode and said substrate when the multi-valued data is written by applying said second writing voltage.

2. The method for driving a semiconductor memory of claim 1,

wherein said semiconductor memory has a switch for equalizing potentials of said first electrode and said second electrode of said ferroelectric capacitor, and

the second step includes a sub-step of removing the potential difference by equalizing the potentials of said first electrode and said second electrode with said switch.

3. The method for driving a semiconductor memory of claim 1,

wherein the third step includes a sub-step of detecting the displacement of the polarization of said ferroelectric film by detecting whether a potential difference induced between said gate electrode and said substrate owing to division of said reading voltage in accordance with a ratio between capacitance of said ferroelectric capacitor and gate capacitance of said reading field effect transistor is relatively high or relatively low.

4. A method for driving a semiconductor memory including a plurality of successively connected ferroelectric capacitors each for storing a multi-valued data in accordance with displacement of polarization of a ferroelectric film thereof, and a reading field effect transistor that is formed on a substrate and has a gate electrode connected to one end of said plurality of successively connected ferroelectric capacitors for detecting the displacement of the polarization of said ferroelectric film of each of said successively connected ferroelectric capacitors, comprising:

a first step of writing a multi-valued data in one ferroelectric capacitor selected from said plurality of ferroelectric capacitors by applying a relatively high first writing voltage or a relatively low second writing voltage between an upper electrode and a lower electrode of said selected ferroelectric capacitor;

a second step of removing a potential difference induced between said upper electrode and said lower electrode of said selected ferroelectric capacitor; and

a third step of reading the multi-valued data by detecting the displacement of the polarization of said ferroelectric film of said selected ferroelectric capacitor by applying a reading voltage between another end of said plurality of successively connected ferroelectric capacitors and said substrate,

wherein said reading voltage has the same polarity as said first writing voltage and is set to such magnitude that, in applying said reading voltage, a first potential difference induced between said gate electrode and said substrate when the multi-valued data is written by applying said first writing voltage is smaller than a second potential difference induced between said gate electrode and said substrate when the multi-valued data is written by applying said second writing voltage.

5. The method for driving a semiconductor memory of claim 4,

wherein said semiconductor memory has a plurality of switches each for equalizing potentials of an upper electrode and a lower electrode of each of said plurality of ferroelectric capacitors, and

the second step includes a sub-step of removing the potential difference by equalizing the potentials of said upper electrode and said lower electrode of said selected ferroelectric capacitor with said switch.

6. The method for driving a semiconductor memory of claim 4,

wherein the third step includes a sub-step of detecting the displacement of the polarization of said ferroelectric film of said selected ferroelectric capacitor by detecting whether a potential difference induced between said gate electrode and said substrate owing to division of said reading voltage in accordance with a ratio between capacitance of said selected ferroelectric capacitor and gate capacitance of said reading field effect transistor is relatively high or relatively low.