JPH02232974A - Semiconductor device - Google Patents

Abstract

PURPOSE:To improve a ferroelectric film in stability and residual polarized charge density by a method wherein the ferroelectric film is formed in a multilayered hetero-superlattice structure which comprises at least one or more ferroelectric layers of perovskite crystal structure. CONSTITUTION:A high impurity concentration region 102 is formed on an Si substrate 101, and a buffer layer 103 of MgAl2O4 is formed thereon. Then, a PZT-MgAl2O4 superlattice 104 is formed. It is desirable that the final growth layer of the superlattice 104 is an MgAl2O4 108 of cubic system. Lastly, a metal 107 of Al or the like is sputtered thereon, and an electrode is provided to the region 102 and the metal 107 respectively.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an electrically rewritable nonvolatile memory using a ferroelectric material.

[Summary of the Invention] The present invention provides a nonvolatile memory semiconductor device using a ferroelectric material, in which a ferroelectric thin film is a heterogeneous material having a multilayer structure having at least one ferroelectric layer having a perovskite crystal structure. A ferroelectric thin film with excellent crystallinity is obtained as a superlattice structure.

[Prior Art] As a conventional semiconductor non-volatile memory, an MIS type transistor uses a phenomenon in which the surface potential of a silicon substrate is modulated by injecting charge from the silicon substrate into a trap or floating gate in an insulated gate. is generally used and has been put into practical use as K'FROM (ultraviolet erasable nonvolatile memory), KKFROM (electrically rewritable nonvolatile memory), and the like.

[Problems to be Solved by the Invention] However, these nonvolatile memories have a high voltage for rewriting information, usually around 2 CIV, and a very long rewriting time (for example, in the case of EEFROM, it takes about 1'rL
sec). Further, the number of times information can be rewritten is about 105 times, which is a very small number of times, and K has many problems when used repeatedly.

Using a ferroelectric material whose self-polarization can be electrically reversed,
Non-volatile memory has the potential to become an ideal non-volatile memory because the write time and read time are basically the same, and the polarization is maintained even when the power is turned off. Regarding nonvolatile memories using such ferroelectric materials, for example, there is a structure in which a capacitor made of ferroelectric material is integrated on a silicon substrate as in U.S. Pat. Nonvolatile memories and the like have been proposed in which a ferroelectric film is placed in the gate portion of an S-type transistor.

However, it has not yet been put into practical use because ferroelectric films lack stability and are not suitable for integration.

Therefore, the present invention aims to solve these problems, and its purpose is to provide a nonvolatile memory in which the stability of the ferroelectric film and the residual polarization charge density are improved. .

[Means for Solving the Problems] The semiconductor device of the present invention is characterized in that the ferroelectric thin film thereof has a hetero superlattice structure, which is a multilayer structure including at least one ferroelectric layer with a perovskite crystal structure. do.

[Example 1] FIG. 1 is a sectional view of an example of a semiconductor device using a ferroelectric thin film according to the present invention.

The semiconductor device of the present invention will be described below with reference to FIG. Here, for convenience of explanation, an example in which an 81 substrate is used and PZT is used as the material of the hetero superlattice will be described.

Reference numeral 101 is an N- (or P-) Si substrate, and a heavily impurity concentration region N+ (or P+,
Depends on the impurity type of the substrate. ) is formed by, for example, an ion implantation method.

Next, MgAt has good lattice matching with both S1 and PZT.
, O, are used as the buffer layer 103 and are formed epitaxially using the ovD method or the like. For example, it grows by about 3 μrrL.

Next, using an epitaxial growth method such as the optical OVD method, P
A ZT-MgAl2O4 superlattice 104 is produced. During this epitaxial growth, it is necessary to impart zero-axis orientation by applying high frequency to the Si substrate.

In this superlattice, as is often seen in conventional PZT films grown on MgAt. can be prevented from being absorbed. Therefore, it is preferable to make the period of the superlattice as small as possible. For example, the final growth layer of the 30A-5OA superlattice is a cubic MgAt
It is better to set it to t04108.

Finally, a metal 107 such as At is formed by sputtering or the like,
By attaching electrodes to the N (P) region on the 81 substrate and the metal portion, a structure of an embodiment of the semiconductor device of the present invention is obtained.

In conventional nonvolatile memories using ferroelectric films, the ferroelectric film is formed in the form of ceramic by sputtering or the like. As described above, when looking at the characteristics of non-volatile memory, for example, focusing on the information retention characteristics, it lasts only about eight months (converted to room temperature). When the cause of this was investigated in detail, it was found that the deterioration progressed at the grain boundaries of the ferroelectric material. Therefore, when a strong electric thin film is used as in the present invention, a single crystal with almost 0-axis orientation can be obtained, so there are almost no grain boundaries.
(converted to room temperature).

In addition, one of the reasons is that the remanent polarization charge density has become thicker because the orientation axes of the crystals are aligned. The structure is not only the structure explained in Fig. 1, but also the OMOS structure, the structure using bibolar transistors, and the structure using bibolar transistors.
OMOS (7) S1 structure and also as a substrate
It goes without saying that a compound semiconductor such as GaA° C. may also be used as long as both the ferroelectric material and the substrate in the puffer layer are made of materials with lattice matching.

It goes without saying that this method is effective for all materials of the velopskite crystal system, and is also effective for semiconductor devices in which a high-temperature superconducting film is formed.

[Brief explanation of the drawing]

FIG. 1 shows a semiconductor device (
FIG. 2 is a main cross-sectional view of a capacitor. 101...S1 substrate 102...High impurity concentration region 103... ...MgAt,O, buffer layer 1 0
4 = − M g A t t O , − P
Z T superlattice 105...PzT layer 106...MgAl2O4 layer 107...At electrode 108...MgAl2O4 }Up layer or above [Effect of the invention or above] As described above, in the semiconductor device of the present invention, since the characteristics of the ferroelectric film are improved, a nonvolatile memory using the ferroelectric film can be manufactured.

Claims (2)

[Claims] (1) In a semiconductor device using a ferroelectric thin film, the ferroelectric thin film has a hetero superlattice structure consisting of a multilayer structure having at least one ferroelectric layer having a perovskite crystal structure. Characteristic semiconductor devices. (2) The main components of the perovskite crystal structure ferroelectric material constituting the ferroelectric thin film are PbTiO_3, PZT(P
bTiO_3/PbZrO_3), PLZT(La/P
2. The semiconductor device according to claim 1, wherein the material forming a heterojunction therewith is MgAl_2O_4.