Development of New FET-Type Ferroelectric Memory with an Intermediate Electrode

1.为了降低读电压V_R，我们在铁电PZT薄膜的上电极和下电极采用了不同的电极材料。底部电极为Ir的中间电极，顶部电极为IrO_2、RuO_2和PtO_x。PZT薄膜的磁滞回线按Ir-O_2、RuO_2和PtO_x的顺序向正电场E方向移动，说明PZT薄膜中存在一个内电场E。虽然这个E降低了V_R，但它有助于改变剩余极化，即存储器状态，这对非易失性存储器是不利的。这种存储器有一个问题，即由于读取时通过失态写入场效应管的漏电流而改变了剩余极化安全，从而改变了存储器的状态。针对这一问题，我们提出了一种将电中间点连接到读取场效应管的漏极区域的方法。通过使用这种方法，减少了泄漏电流，使读数比传统连接方式提高了约一个数量级。在写入电压为5 V， VR为7 V的条件下，我们在硅片上验证了我们的新存储器集成电路的工作原理。然而，两种存储状态之间的输出电压差异仅为15 mV，并且非常小。这是因为YSZ薄膜的厚度永远不会变薄，而PZT薄膜的介电常数很大。同时，栅极漏电流也很大。针对这些问题，我们提出了一种解决方案，即使用与硅界面性能良好的SiO_2薄膜代替YSZ，同时使用介电常数较低的铁电材料。

1.Reduction of Reading Voltage V_RWe used different electrode materials for the top and bottom electrodes of the ferroelectric PZT film in order to reduce e V_R. The bottom electrode is an intermediate electrode of Ir and the top electrodes are IrO_2, RuO_2 and PtO_x. The hysteresis loop of the PZT film was shifted toward positive electric field (E) in order of Ir-O_2, RuO_2 and PtO_x, which means that an inner E exists in the PZT films. Although this E reduces V_R, it assists the change of the remanent polarization, i.e., memory state, which is unfavorable for non-volatile memory.2.Increase of Reading NumberThis memory has one problem that the memory state is changed by reading because the remanent polarization sate is changed due to the leakage current through the off-state writing FET for reading. For this problem, we proposed one method that the electric intermediate point is connected to the drain region of the reading FET. By using this, the leakage current is reduced so that the reading number is increased by about one order than the conventional connection.3.Fabrication of the Integrated circuitsWe confirmed the principle operation of the integrated circuits of our new memory on an Si wafer under the conditions that the writing voltage was 5 V and the VR was 7 V. However, the difference on output voltage between two memory states was only 15 mV and very small. This is because the thickness of the YSZ film can be never thinned and the dielectric constant of the PZT film is large. Also, the gate leakage current flows largely. For these problems, we suggest one solution to use not only SiO_2 film with good interface property to Si instead of YSZ and but also ferroelectric materials with lower dielectric constant.