Process for preparing ferroelectric thin films

Details Process for preparing ferroelectric thin films Process for preparing ferroelectric thin films

1999-12-03

2002-05-14

Bowers, Charles (Department: 2813)

Semiconductor device manufacturing: process

Having magnetic or ferroelectric component

C438S763000, C438S792000, C438S909000

Reexamination Certificate

active

06387712

ABSTRACT:

This invention relates to a film structure having a ferroelectric thin film, an electronic device comprising the film structure, and a recording medium utilizing the film structure. It also relates to a process for preparing a ferroelectric thin film.
The film structure finds use as electronic devices having a ferroelectric film incorporated in a semiconductor device including non-volatile memories, infrared sensors, optical modulators, optical switches, and opto-electronic integrated circuits (OEIC) as well as recording media of the type wherein information is recorded by inducing polarization reversal in ferroelectric material by means of an atomic force microscope (AFM) probe etc.
BACKGROUND OF THE INVENTION
Electronic devices have been elaborated which are fabricated by forming dielectric films on silicon substrates or semiconductor crystal substrates, followed by integration. Studies have been made to fabricate LSIs having a higher degree of integration and dielectric isolated LSIs relying on SOI technology, by combining semi-conductors with dielectrics. Ferroelectrics which are one class of dielectrics can be applied to non-volatile memories by utilizing their polarization reversal phenomenon. Also, infrared sensors, optical modulators, optical switches, OEIC (opto-electronic integrated circuits) or the like can be fabricated using ferroelectrics. Active research works have been made on the ferroelectric thin film material. It has also been investigated to apply the ferroelectric thin film to recording media of the type wherein polarization reversal by an AFM probe etc. is utilized to record information. In order to establish the non-volatile memories and recording media utilizing the polarization reversal of ferroelectrics, ferroelectric thin film materials having a sufficient residual polarization value to withstand repetitive record and retrieval cycles are necessary.
One of non-volatile memories devised heretofore is a memory of the structure using ferroelectric material in the gate of FET. As described in a technical report issued by the Japanese Electronic Information Communication Society, SDM 93-136, ICD 93-130, (1993-11), page 53, the memory using ferroelectric material in the gate has not reached the practically acceptable level because there remain many outstanding problems associated with their manufacture and the physical properties of ferroelectric thin films. For this type of memory, it is ideal, but difficult to implement a metal-ferroelectric-semiconductor (MFS) structure in the memory cell and therefore, a metal-ferroelectric-insulator-semiconductor (MFIS) structure or metal-ferroelectric-metal-insulator-semiconductor (MFMIS) structure must be fabricated. In order that the ferroelectric material undergo polarization reversal to ensure storage operation for this structure, an electric field of sufficient strength must be applied across the ferroelectric material. Since the ferroelectric material and insulator in the MFIS and MFMIS structures become equivalent to a serial connection of capacitors, it is necessary to take appropriate measures for lowering the dielectric constant of ferroelectric material and raising the dielectric constant of insulator in order that a sufficient electric field be applied across the ferroelectric material.
Among ferroelectric thin film materials, investigations have heretofore been made on lead family oxides such as PbTiO
3
, PZT (PbZrO
3
-PbTiO
3
system), and PLZT (PbZrO
3
-PbTiO
3
system having La
2
O
3
added thereto) and bismuth family oxides such as Bi
2
Ti
2
NbO
9
for the reason that they exhibit good polarization characteristics.
The PZT and PLZT, however, exhibit a dielectric constant as high as about 1,000 when they are formed into thin film. If they are used as a ferroelectric thin film in the above-mentioned MFIS and MFMIS structures, it is difficult to apply sufficient voltage.
On the other hand, PbTiO
3
has a dielectric constant of lower than about 100 at room temperature in bulk form, a spontaneous polarization value of 80 &mgr;C/cm
2
as theoretically calculated in bulk crystal form which is outstandingly greater than materials of other compositions, and a Curie temperature as high as 500° C. Namely, the physical data of PbTiO
3
found in the literature are most ideal when considered as the ferroelectric material for memories. Even in thin film form, PbTiO
3
has a dielectric constant as low as about 500. Nevertheless, the recent research and development works to form PbTiO
3
as a thin film to construct electronic devices revealed many problems. First, the voltage Ec at which polarization reversal occurs is as high as 85 kV/cm. Secondly, crystal defects and semiconductor areas can cause leakage in the thin film. Thirdly, fatigue or repetition properties of polarization reversal are poor. Specifically, the material deteriorates after about 1,000 cycles of polarization reversal.
The aforementioned lead and bismuth family oxides must be crystallized for their thin films to exhibit ferroelectric characteristics. The material can be crystallized by heating at a temperature of higher than 600° C. during thin film formation or by annealing a thin film at a temperature of higher than 600° C. as disclosed in Jpn. J. Appl. Phys., 31, 3029 (1992), Jpn. J. Appl. Phys., 33, 5244 (1994), and Mat. Res. Soc. Sympo. Proc., 243, 473 (1993). Since lead and bismuth, however, have a high vapor pressure in both elemental and oxide forms, they evaporate during heat treatment at elevated temperatures, incurring a compositional deviation. Composition control is thus difficult.
It is generally desired to use a single crystal form of ferroelectric material in order to ensure optimum device characteristics and reproducibility thereof. Polycrystal-line material is difficult to provide satisfactory device characteristics due to the disturbance of physical quantities by grain boundaries. This is also true for thin film materials, and a ferroelectric epitaxial film which is as close to a complete single crystal as possible is desired. The same applies to ferroelectric thin films for use in non-volatile memories of the above-mentioned MFIS or MFMIS structure, and a dielectric epitaxial film which is as close to a complete single crystal as possible is desired. Also for media (usually of the MFIS or MFMIS structure) wherein information is recorded using an AFM or STM probe, there is a demand for a ferroelectric epitaxial film which is as close to a complete single-crystal as possible because such a film enables to write a high density of bits. In order to form a ferroelectric epitaxial film in the MFIS or MFMIS structure, a metal thin film and a ferroelectric thin film must be epitaxially grown on a silicon substrate which is a semiconductor substrate. No one has succeeded in this epitaxial growth.
Insofar as lead family ferroelectric materials are concerned, a thin film which is free of a compositional deviation and approximate to a single crystal has not been formed on a semiconductor substrate. The high reactivity of lead family ferroelectric materials with silicon serving as the substrate allows for diffusion of Pb into the silicon substrate, which has serious influence on the characteristics of integrated circuits built in the silicon substrate.
SUMMARY OF THE INVENTION
An object of the invention is to provide a film structure having a ferroelectric thin film featuring a relatively low dielectric constant, great residual polarization, a low polarization reversing voltage, and minimal deterioration upon repetitive polarization reversal.
Another object of the invention is to provide a process for preparing a lead family ferroelectric thin film of a consistent composition and high crystallinity which could not be formed by conventional processes.
The present invention is directed to a film structure comprising a substrate and a ferroelectric thin film formed on one surface of the substrate.
In a first aspect of the invention, the ferroelectric thin film contains a rare earth element R
1
, lead, titanium, and oxygen in an atomic ratio

Affiliated with

Also associated with

Rating

Process for preparing ferroelectric thin films does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Process for preparing ferroelectric thin films, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for preparing ferroelectric thin films will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2908573