JPH0855919A - Ferroelectric memory element - Google Patents

Abstract

PURPOSE:To provide a structually stable ferroelectric memory element that uses a fluoride ferroelectric. CONSTITUTION:A buffer layer 2 is formed between a fluoride ferroelectric 3, which is indicated by BaMF4 [M indicates the element consisting of Mg, Zn, Mn, Fe, Co and Ni], and a semiconductor single crystal substrate 1. As a buffer layer 2, a fluoride insulator layer, which is indicated by (AxB1-x)F2 (0<=x<=1) [A and B indicate the element of alkaline earth metal group consisting of Ca, Sr, Ba and Mg], or a fluoride insulator layer, consisting of two layers indicated by (AxB1-x)F2/CyD1-y)F2, (0<=x<=1) and (0<=y<=1) [A, B, C and D indicate the element of alkalkne earth metal group consisting of Ca, Sr, Ba and Mg], is used. Also, the fluoride insulator layer, indicated by AF3 [A is the element selected from the rare-earth metal group consisting of La, Nd, Ce and Er], is also used as an orientational buffer layer.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferroelectric memory device. More specifically, the present invention relates to a ferroelectric memory element that changes the amount of carrier movement in an impurity-doped region through electrostatic induction by spontaneous polarization of a ferroelectric thin film.

[0002]

2. Description of the Related Art Conventionally, ROM (Read Only) has been used as a nonvolatile semiconductor memory element used in computers and the like.
Memory), PROM (Programmable ROM), EPROM
(Erasable PROM) EEPROM (Electrically Erasa
ble PROM) and the like, and particularly EEPROM is promising because the stored contents can be electrically rewritten. In this EEPROM, MIS (Metal Insu
lator Semiconductor) The trap region or floating gate in the gate insulating film of the field effect transistor is charged by the charge injection from the silicon substrate,
A method of modulating the surface conductivity of a substrate by the electrostatic induction is known. However, in the device utilizing the tunnel effect of electrons, there is a problem that a large electric field is required at the time of injecting charges from the silicon substrate and a trap is generated in the SiO ₂ insulating film to limit the number of times of rewriting. there were.

On the other hand, a method utilizing spontaneous polarization of a ferroelectric thin film is also considered as a non-volatile memory having a method completely different from that of the EEPROM. There are two types of structures using the ferroelectric thin film, which are respectively a capacitor structure and an MFS (Metal Ferroelectric Semiconductor).
) -FET (Field Effect Transistor) structure. The capacitor structure has a structure in which a ferroelectric thin film is sandwiched between electrodes, and the presence or absence of a reversal current due to polarization reversal of the spontaneous polarization of the ferroelectric thin film is detected to read the memory contents. With the capacitor structure, the stored memory contents are destroyed at the time of reading, so there is a drawback that the operation of rewriting the memory contents must be performed after reading (rewriting operation), but the ferroelectric thin film on the platinum electrode etc. Therefore, it is easy to obtain a relatively good quality film, and currently, vigorous development is underway toward commercialization. This capacitor structure is, for example, PZT (lead zirconate titanate), PbTiO 3
So-called oxide perovskite or oxide layered perovskite ferroelectrics such as ₃ (lead titanate), BaTiO ₃ (parium titanate), Bi ₄ Ti ₃ O ₁₂ (bismuth titanate) are being investigated. The reason for this is that the oxide perovskite ferroelectric has a large spontaneous polarization value and a small coercive electric field, so that polarization reversal is sufficiently possible at the operating voltage used in the LSI, and a sufficient signal amount for determining the memory contents can be secured. Because.

On the other hand, the MFS-FET structure is MIS-F
A ferroelectric thin film is used as the gate insulating film of ET, and the conduction on the semiconductor surface is induced by the electric charges induced on the semiconductor surface so as to compensate the spontaneous polarization according to the direction and size of the spontaneous polarization of the ferroelectric thin film. The memory contents are read by utilizing the fact that the degree is modulated. With the MFS-FET structure, not only is non-destructive reading that does not destroy the memory contents at the time of reading, but DRA, which is currently highly integrated, is being advanced.
Scaling law facing a memory device composed of one transistor such as M and one capacitor
It is possible to avoid the problem of "g law", and it is considered to have excellent potential as a memory device which is expected to further increase the integration density of 1 gigabit (G bit) or more in the future. .

[0005]

However, this MFS
There are the following difficulties in producing a stable element with a -FET structure.

(1) Since the ferroelectric thin film is formed directly on the semiconductor, the interface state density becomes large.

(2) Since an oxide film is formed on the semiconductor surface during the process of forming the ferroelectric thin film, the crystallinity and surface morphology (surface morphology: morpholog) of the ferroelectric thin film are formed.
y) is deteriorated and the ferroelectric properties are impaired.

(3) Ferroelectrics have a high relative permittivity (200 to 100
In the case where a low dielectric constant layer such as silicon oxide is formed, the effective voltage applied to the ferroelectric capacitor portion becomes very small due to the presence of (0).

(4) When the voltage applied to the ferroelectric capacitor portion is increased, a large voltage is applied to the capacitor formed by the low dielectric constant layer such as silicon oxide, causing dielectric breakdown.

(5) When the voltage applied to the ferroelectric capacitor portion is increased, the operating voltage is increased.

The problems (2) to (5) are solved by avoiding the formation of silicon oxide and having a small relative permittivity (ε _r ) (ε _r <1.
0) It is considered that this can be avoided by using a ferroelectric substance.
That is, a non-oxide ferroelectric substance having a small relative permittivity may be adopted.

For these reasons, BaMF ₄ (M = Mg, Zn, Mn, Fe, C), which is a fluoride-based ferroelectric substance, is used.
o, Ni) is promising as a material for MFS-FET (the relative permittivity of BaMF ₄ is ε _r <10). However, it has been known that the following problems occur when forming a BaMF ₄ ferroelectric on silicon (Si), particularly on a Si (100) single crystal substrate used as a memory element.

(6) The orientation of the BaMF ₄ ferroelectric thin film in the polarization direction cannot be obtained on the Si (100) substrate.

(7) The thermal expansion coefficient of BaMF ₄ ferroelectric is 20
Since ppm / ° C. is higher than the thermal expansion coefficient of silicon (about 3 ppm / ° C.), stress due to thermal strain accumulates and cracks occur.

The polarization axis of BaMF ₄ is [100],
The crystal system is an orthorhombic system. However, when epitaxially grown on a Si (100) substrate, the orientation is (011), so the polarization direction is greatly inclined with respect to the film surface.
It is difficult to modulate the conductivity of the semiconductor surface due to the reduced available polarization charge density. Also, Si (10
0) Since the BaMF ₄ film naturally oriented on the substrate exhibits b-axis orientation, it has the same problem. Further, there is a problem that cracks are likely to occur due to thermal strain and the conditions for forming a thin film are difficult.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a structurally stable ferroelectric memory element using a fluoride-based ferroelectric.

[0017]

In order to achieve the above object, the present invention provides a ferroelectric memory device having the following formula (I) BaMF ₄ (I) [where M is Mg, Zn, Mn or Fe]. , An element selected from the group consisting of Co and Ni], and a buffer layer was formed between the fluoride-based ferroelectric and the semiconductor single crystal substrate.

As the fluoride-based ferroelectrics, there are materials listed in the above formula (I). Among them, BaMgF ₄ whose constituent element is an alkaline earth metal element, and BaZnF having the largest spontaneous polarization value. It is preferable to use ₄ .

As the semiconductor substrate, Si (100) or Si
(111) is preferably used.

For the buffer layer, the following formula (II) (A _x B _1-x ) F ₂ (0≤x≤1) (II) [where A and B are Ca, Sr, Ba and Mg, respectively]
An element selected from the group of alkaline earth metals consisting of], or a fluoride insulator layer represented by the following formula (III) (A _x B _1-x ) F ₂ / (C _y D _{1- y} ) F ₂ (II
I) (0 ≦ x ≦ 1, 0 ≦ y ≦ 1) [where A, B, C and D are Ca, Sr and Ba, respectively]
And an element selected from the group of alkaline earth metals consisting of Mg].

In the formula (III), “/” represents a laminated structure, the right side of “/” is the substrate side, and the left side is the ferroelectric film side. That is, as a structure, the ferroelectric film / (A _x B
The _{1-x) F 2 / (} C y D 1-y) F 2 / substrate.

In this case, it is desirable to form the layer in consideration of the matching of the lattice constants between the fluoride insulator layer and the substrate, and as a result, the strain due to the mismatch of the lattice constants can be alleviated. It is possible to realize that the ferroelectric film of (1) has a good surface morphology (surface morphology).

For example, CaF ₂ has the best lattice constant matching with Si (100), and CaF ₂ → SrF
Matching of lattice constants goes away in the order of ₂ → BaF ₂ . Therefore, when viewed from the substrate side, CaF ₂ → SrF
By forming the double-layered buffer layer in the order of ₂ → BaF ₂ , it becomes possible to relax the strain due to the mismatch of the lattice constants.

Further, in consideration of diffusion of the constituent elements in the film from the ferroelectric film in contact with the buffer layer, it is desirable that the buffer layer in contact with the film is composed of the constituent elements of the ferroelectric film.

An example of a fluoride insulator layer composed of two layers is shown below.

BaF ₂ / CaF ₂ , BaF ₂ / (Sr _X
Ca _1-X ) F ₂ , (Ba _X Sr _1-X ) F ₂ / CaF ₂ ,
(Ba _X Sr _1-X ) F ₂ / (Sr _X Ca _1-X ) F ₂ , M
gF ₂ / (Ba _x Ca _1-x ) F ₂ and the following formula (IV) AF ₃ (IV) [where A is an element selected from the group of rare earth metals consisting of La, Nd, Ce and Er] The represented fluoride insulator layer can also be used as the orientational buffer layer.
Especially for Si (111) substrates, the use of this fluoride insulator layer is suitable.

[0027]

In the ferroelectric memory element of the present invention, the buffer layer of the fluoride insulator is formed between the fluoride-based ferroelectric and the semiconductor single crystal substrate, so that the stress due to the thermal strain is alleviated and the crack It is possible to prevent the generation and form a dense ferroelectric film. Moreover, by using the buffer layer, it is possible to control the orientation of the ferroelectric thin film to a-axis orientation or random orientation, and obtain a polarization charge density sufficient to modulate the conductivity of the semiconductor surface.

By using the two-layered fluoride insulator layer represented by the above formula (III), the influence of the mismatch of the lattice constant between the ferroelectric thin film and the silicon substrate can be mitigated, In addition, the orientation direction of the ferroelectric thin film can be controlled.

In addition, usually, BaF ₂ is directly deposited on Si (100).
When a film of SrF ₂ or SrF ₂ is formed, a polycrystalline film is obtained.
By inserting CaF ₂ between i and BaF ₂ or SrF
The crystallinity of the _second buffer itself can be improved.

[0030]

Example 1 MFIS (Metal Ferroe) using the fluoride ferroelectric material BaMgF ₄ and the fluoride buffer layer BaF _{2 according} to the present invention.
A manufacturing example of a so-called varactor structure and an example of examining this structure will be described with reference to FIGS. 1 and 2.

Single crystal Si (100) with p type and resistivity of 50 Ωcm
The substrate 1 is treated with BHF (buffered fluoric acid solution),
The native oxide film on the surface was removed. Then, this substrate is mounted on the substrate holder in the vacuum chamber of the EB vapor deposition apparatus as quickly as possible, and vacuum exhaust is started. After reaching a vacuum degree of 1 × 10 ⁻⁴ Pa or less, heating of the substrate was started. The substrate temperature was 600 ° C. BaF ₂ (purity 4N-granule) and MgF ₂ (purity 4N-granule) are used as vapor deposition sources, and the two independent controls are performed, whereby a BaMgF ₄ film can be produced with good reproducibility. .

First, as the buffer layer 2, BaF ₂ having a thickness of 10 nm is used.
It was manufactured to a film thickness of. The vapor deposition rate is 2Å / sec. After that, as the ferroelectric film 3, BaF ₂ and Mg are formed in the same vacuum.
Deposition rate of F ₂ is 1.8Å / sec and 1.0Å / sec respectively
Then, BaMgF ₄ having a film thickness of 140 nm was produced by simultaneous vapor deposition. The vapor deposition rates are BaF ₂ and MgF _{2 respectively.}
The supply molar ratio of is set to 1: 1. Then, the film was allowed to cool to room temperature and then taken out to examine the surface morphology (morphology) and orientation of the film.

Observation of the surface of the film with a scanning electron microscope revealed that, when BaMgF ₄ was directly formed on the silicon substrate, cracks were generated, but the film was formed via the BaF ₂ layer. It was found that the BaMgF ₄ film had a dense morphology. This is due to the fact that the BaF ₂ layer was able to alleviate the stress caused by the large difference in thermal expansion coefficient between the silicon substrate and BaMgF ₄ .

Further, when the orientation of the ferroelectric film was examined by the X-ray diffraction method (XRD), it was found that the diffraction pattern showing the b-axis preferential orientation was obtained on Si (100) in the past. As a BaMgF ₄ film formed through the BaF ₂ layer, a randomly oriented film was obtained. This indicates that the BaF ₂ layer is effective as a base that makes it possible to obtain an alignment pattern different from that of the silicon substrate. this,
The random alignment film is expected to provide sufficient polarization charge density to modulate the conductivity of the semiconductor surface.

The film thus formed has a film thickness of 100 n
m of aluminum was vapor-deposited to form a 100 μm square upper electrode 4, and an electrode 5 was arranged on the back surface of the substrate 1 to fabricate an MFIS structure having a schematic cross section shown in FIG. This M
The C-V characteristics were measured in the FIS structure, and the dielectric constant ε, the dielectric loss tan δ, and the threshold voltage shift ΔVth were obtained.

The measurement was performed by applying a DC bias from -2.5 to 2.5 V to a sine wave having an amplitude of 10 mVrms and 1 MHz. The bias was swept under the conditions of ΔV = 0.1 V and Δt = 100 msec. First, apply a voltage of -5 V to the gate,
When the dielectric constant and the dielectric loss were measured while the storage layer was sufficiently formed, the values of ε = 7.7 and tanδ = 0.01 were obtained. Next, when C-V measurement was performed under the above-described measurement conditions, a C-V hysteresis loop corresponding to the D-E hysteresis loop due to polarization of the ferroelectric substance was observed. When the measurement bias is ± 2.5 V, the threshold voltage shift is calculated from this C-V hysteresis, and ΔVth =
It was 1.6 V (Fig. 2).

Thus, the fluoride ferroelectric BaMgF ₄
And by using the fluoride buffer layer BaF ₂ , the stress due to thermal strain is relieved, the occurrence of cracks is prevented,
It becomes possible to form a dense ferroelectric film. Also,
By using the polycrystalline BaF ₂ film as the buffer layer, the ferroelectric thin film can be randomly oriented, and a polarization charge density sufficient to modulate the conductivity of the semiconductor surface can be obtained. Then, by using a fluoride insulator film that forms a good interface as the buffer layer, a good C-V
The characteristics were obtained. Furthermore, since the ferroelectric layer and the buffer layer both have a low dielectric constant, it is possible to operate in the MFIS structure at a low voltage.

Although the present embodiment is an example of fabrication by the EB vapor deposition method, the film forming method may of course be the MBE method which is a more advanced vapor deposition method. Further, other physical film forming methods such as a sputtering method and a laser ablation method, or a chemical film forming method typified by a MOCVD method or a sol-gel method may be used.

Example 2 MFIS using the fluoride ferroelectric material BaMgF ₄ and the fluoride buffer layer (Ba _X Ca _1-X ) F _{2 according} to the present invention.
An example of manufacturing the structure and an example of examining the structure will be described.

Using a Si (100) substrate, the substrate was prepared by the same procedure as in Example 1. BaF ₂ (purity 4
N- granules), CaF ₂ (purity 4N- granules), by performing control to binary independently using MgF ₂ (purity 4N- granules), (Ba _X C
An a _{1 -X} ) F ₂ film and a BaMgF ₄ film were prepared. The substrate temperature was 600 ° C. Here, by changing the vapor deposition rates of BaF ₂ and CaF _2, respectively, X = O.2, 0.5,
Three kinds of (Ba _X Ca _1-X ) F ₂ buffer films of 0.8 were prepared.

First, as a buffer layer (Ba _X Ca _1-X )
An F ₂ film was formed to a film thickness of 20 nm, and then BaF ₂ and MgF ₂ were simultaneously deposited as a ferroelectric film in the same vacuum at vapor deposition rates of 1.8 Å / sec and 1.0 Å / sec, respectively. BaMgF ₄ with a thickness of 130 nm (Ba _X Ca _1-X )
It was prepared on the F ₂ film. Then, it was left to cool to room temperature and taken out to examine the surface morphology and orientation of the film.

Observation of the surface of the film with a scanning electron microscope revealed that BaMgF formed through the BaF ₂ buffer layer shown in Example 1 in all cases of X = 0.2, 0.5 and 0.8.
_It showed a more compact morphology than the _four films. When X = O.2, the density was the highest, and when X = 0.5 and 0.8, the density was similar. This is (Ba _X Ca _1-X ) F ₂
Not only did the buffer layer relieve the stress caused by the difference in the coefficient of thermal expansion, but also because it was a mixed crystal with the CaF ₂ layer, which had a small lattice constant mismatch of about 0.6% with the silicon substrate, BaMgF _It is considered that this was due to the fact that the effect of lattice constant mismatch between the ₄ -silicon substrates could be mitigated.

Further, when the orientation of the ferroelectric film was examined by XRD, it was found that BaMgF formed through the BaF ₂ layer.
_A randomly oriented film was obtained in the same manner as the _four films. Using the film thus produced, an MFIS structure similar to that shown in Example 1 was produced, C-V characteristics were measured under similar measurement conditions, dielectric constant ε, dielectric loss tan δ, threshold value The voltage shift ΔVth was obtained.

When the dielectric constant and the dielectric loss were measured, X =
In all cases of O.2, 0.5, 0.8, ε = 7-8, tanδ =
A value of 0.01 was obtained. Next, when C-V measurement was performed under the above measurement conditions, when the measurement bias was ± 2.5 V, the shift of the threshold voltage was calculated from this C-V hysteresis, and ΔVth = 1.4 to 1.5 V. there were.

Thus, the fluoride ferroelectric material BaMgF ₄
And the fluoride buffer layer (Ba _X Ca _1-X ) F ₂ not only alleviate the stress due to thermal strain, but also alleviate the effect of the lattice constant mismatch, resulting in a more precise ferroelectric It becomes possible to form a body membrane.

Here, the case of the combination of BaF ₂ and CaF ₂ (X = 0.2, 0.5, 0.8) is shown, but SrF ₂
It may also be a combination with MgF ₂ or MgF ₂ . Further, as the composition range, it is possible to select a range that is easy to produce depending on the film forming conditions such as the substrate temperature and the vapor deposition rate.

Example 3 An example of manufacturing the MFIS structure using the fluoride ferroelectric material BaMgF ₄ and the fluoride buffer layer BaF ₂ / CaF _{2 according} to the present invention and an example of examining this structure will be described.

Using a Si (100) substrate, the substrate was prepared in the same procedure as in Example 1. BaF ₂ (purity 4
It is possible to produce the buffer layer BaF ₂ / CaF ₂ in the same vacuum with good reproducibility by controlling the binary independently using N-granule) and CaF ₂ (purity 4N-granule). is there.

First, CaF ₂ was formed to a thickness of 10 nm as a buffer layer. The deposition rate is 1Å / sec. Then, BaF ₂ was continuously used as a buffer layer in the same vacuum.
It was formed to a film thickness of 10 nm. The deposition rate is 1.0Å / sec. After that, as a ferroelectric film, BaF ₂ and MgF ₂ were deposited in the same vacuum at vapor deposition rates of 1.8 Å / sec and 1.0 Å
BaMg with a film thickness of 130 nm by simultaneous vapor deposition at 1 / sec.
F ₄ was produced. Other manufacturing conditions are the same as those described in the first embodiment.

When the surface of the film was observed with a scanning electron microscope, it was found that the BaM film formed through the BaF ₂ / CaF ₂ layer was formed.
The gF ₄ film showed a dense morphology. Not only was the buffer layer alleviated the stress caused by the difference in the coefficient of thermal expansion, but a CaF ₂ layer with a small lattice constant mismatch with silicon of about 0.6% was also provided between the BaF ₂ layer and the silicon substrate. It is considered that the use thereof could alleviate the effect of mismatch in lattice constant between the BaMgF ₄ -silicon substrate.

When the orientation of the ferroelectric film was examined by XRD, it was found that BaMgF formed through the BaF ₂ layer.
_A randomly oriented film was obtained as in the case of the ₄ films, but the (120) peak intensity was further increased in the film using the BaF ₂ / CaF ₂ layer as the buffer layer. This is CaF
_It is considered that the crystallinity of the BaF ₂ layer itself was improved by the _two layers, and therefore the crystallinity of the BaMgF ₄ film deposited thereon was improved. Therefore, an increase in the polarization charge density used to modulate the conductivity of the semiconductor surface is expected.

Using the film thus produced, an MFIS structure similar to that shown in Example 1 was produced, C-V characteristics were measured under the same measurement conditions, and the dielectric constant ε and dielectric loss t
An δ and the shift ΔVth of the threshold voltage were obtained respectively.

When the dielectric constant and the dielectric loss were measured, ε =
A value of 7.5 and tan δ = 0.01 was obtained. Next, CV measurement was performed under the above measurement conditions. When the measurement bias was ± 2.5 V, the threshold voltage shift was calculated from this CV hysteresis, and ΔVth = 1.58 V. .

Thus, the fluoride ferroelectric material BaMgF ₄
By using the fluoride buffer layer BaF ₂ / CaF ₂ and the stress due to thermal strain, it is possible to prevent the occurrence of cracks and form a dense ferroelectric film. In addition, CaF that can obtain a single crystal film on Si (100)
_{By using the two} layers between the BaF ₂ layer and the silicon substrate to mitigate the mismatch of the lattice constant and improve the crystallinity of the BaF ₂ film, the crystallinity of the ferroelectric thin film is improved and the polarization charge density is increased. It was realized.

[0055] Example 4 is the invention fluoride ferroelectric BaMgF ₄ and two layers fluoride buffer layer _{(Ba X Sr 1-X)} F 2 / (Sr y C
An example of manufacturing an MFIS structure using a _1-y ) F ₂ and an example of examining this structure will be described.

Using a Si (100) substrate, the substrate was prepared by the same procedure as in Example 1. CaF ₂ (purity 4
N-granule), SrF ₂ (purity 3N-granule), BaF ₂ (purity 4N-granule), MgF ₂
(Purity 4N-granule) is used to control each of the two independently, so that the buffer layer (Ba _X Sr _{1-
X) F 2 / (Sr y} Ca 1-y) F 2 film and BaMgF ₄
Membranes were made in the same vacuum. The substrate temperature was 600 ° C.
Here, by changing the vapor deposition rates of CaF ₂ and SrF ₂ , and SrF ₂ and BaF _2, respectively, (1) BaF ₂
/ (Sr _0.1 Ca _0.9 ) F ₂ (x = 1.0, y = 0.1),
(2) (Ba _0.9 Sr _0.1 ) F ₂ / CaF ₂ (x = 0.
9, y = 0), (3) (Ba _0.9 Sr _0.1 ) F ₂ / (S
_{r 0.1 Ca 0.9) F 2 (} x = 0.9, y = 0.1), 3 kinds of the _{(Ba X Sr 1-X)} F 2 / (Sr y Ca 1-y) F 2
A buffer film was prepared.

First, the bottom buffer layer was formed to a thickness of 10 nm, and then the second buffer layer was similarly formed to a thickness of 10 nm in the same vacuum. After that, as a ferroelectric film, BaF ₂ and MgF ₂ were simultaneously vapor-deposited at vapor deposition rates of 1.8 Å / sec and 1.0 Å / sec, respectively, to form BaMgF ₄ having a film thickness of 130 nm. The manufacturing conditions are the same as those described in the first embodiment.

Observation of the film surface with a scanning electron microscope revealed that in any of the above cases (1), (2) and (3),
The morphology was more dense than that of the BaMgF ₄ film formed via the BaF ₂ buffer layer shown in Example 1. In either case, the mismatch of the lattice constant with the silicon substrate is as small as about 0.6%, and CaF ₂ or (Sr _0.1 Ca
_0.9 ) F ₂ is placed in the bottom layer, and CaF ₂ → SrF ₂ → B
By using a buffer layer composed of aF ₂ in this order, Ba
It is considered that the influence of the mismatch of the lattice constant between the MgF ₄ -silicon substrate could be alleviated.

When the orientation of the ferroelectric film was examined by XRD, it was found that BaMgF formed through the BaF ₂ layer.
_A film having a random orientation was obtained as in the case of the _four films, but the (120) peak intensity was further increased in the film using this buffer layer. The (120) peak intensity was the strongest in the buffer structure of (2), followed by (1) and (3) in that order. This is because the lowermost buffer layer containing CaF ₂ improved the crystallinity of the second buffer layer itself mainly composed of BaF ₂ and thus the crystallinity of the BaMgF ₄ film deposited thereon. Conceivable. Therefore, an increase in the polarization charge density used to modulate the conductivity of the semiconductor surface is expected.

Using the film thus produced, an MFIS structure similar to that shown in Example 1 was produced, C-V characteristics were measured under the same measurement conditions, and the dielectric constant ε and dielectric loss t
An δ and the shift ΔVth of the threshold voltage were obtained respectively.

When the dielectric constant and the dielectric loss were measured, ε = 7-8, in any of the above cases (1), (2) and (3).
A value of tan δ = 0.01 was obtained. Next, when CV measurement was performed under the above measurement conditions, measurement bias = ±
At 2.5 V, the shift of the threshold voltage was obtained from the C-V hysteresis, and it was ΔVth = 1.5 to 1.6 V.

Thus, the fluoride ferroelectric BaMgF ₄
And a fluoride buffer layer (Ba _X Sr _1-X ) F ₂ / (S
By using r _y Ca _1-y ) F ₂ , it becomes possible to relieve stress due to thermal strain, prevent the occurrence of cracks, and form a dense ferroelectric film. In addition, the lattice constant mismatch between silicon substrates is made possible by the double-layer fluoride buffer layer structure having CaF ₂ or a (Sr _0.1 Ca _0.9 ) F ₂ that is equivalent to it as a bottom layer, which can obtain a single crystal film on Si (100). By improving the crystallinity of the second buffer layer itself, which can mitigate the influence of the above, the crystallinity of the ferroelectric thin film is improved, the polarization charge density is increased, and a sufficient threshold voltage shift is achieved. I was able to get it.

Here, BaF ₂ , SrF ₂ and SrF _{2 are used.}
Although the case of a combination of CaF ₂ and CaF ₂ is shown, a combination with MgF ₂ or the like may of course be used. In addition, the composition range can be selected such that it can be easily produced depending on the film forming conditions such as the substrate temperature and the vapor deposition rate.

Example 5 A description will be given of an example of manufacturing an MFIS structure using the fluoride ferroelectric material BaMgF ₄ and the fluoride buffer layer LaF _{3 according} to the present invention and an example of investigating this structure.

Using a Si (111) substrate, the substrate was prepared by the same procedure as in Example 1. LaF ₃ (purity of 4
N-granule) as a buffer layer
_Three films were formed with a film thickness of 10 nm. Vapor deposition rate is 1Å / sec
Is. After that, BaF ₂ and MgF ₂ were simultaneously vapor-deposited at the vapor deposition rates of 1.8 Å / sec and 1.0 Å / sec, respectively, in the same vacuum to prepare a BaMgF ₄ having a film thickness of 140 nm. Other manufacturing conditions are the same as those described in the first embodiment.

When the surface of the film was observed with a scanning electron microscope, the BaMgF ₄ film formed via the LaF ₃ layer showed a dense morphology. This is due to the buffer layer
It is considered that the stress caused by the difference in thermal expansion coefficient was alleviated.

Further, when the orientation of the ferroelectric film was examined by XRD, it was found that BaMgF formed through the LaF ₃ layer.
_{The four} films were (120) epitaxial alignment films. Conventionally, a BaMgF ₄ film is (120) epitaxially grown on a Si (111) substrate by ultra-high vacuum deposition such as MBE, but it is known that a film naturally oriented by vapor deposition or the like exhibits a b-axis orientation. There is. Therefore, it is shown that the orientation of the BaMgF ₄ film can be controlled by the LaF ₃ layer in the present invention also by the vapor deposition method.

Using the film thus produced, an MFIS structure similar to that shown in Example 1 was produced, C-V characteristics were measured under the same measurement conditions, and the dielectric constant ε and dielectric loss t
An δ and the shift ΔVth of the threshold voltage were obtained respectively.

When the dielectric constant and the dielectric loss were measured, ε =
A value of 7.8, tan δ = 0.01 was obtained. Next, CV measurement was performed under the above measurement conditions. When the measurement bias was ± 2.5 V, the shift of the threshold voltage was calculated from this CV hysteresis, and ΔVth = 1.64 V.

Thus, the fluoride ferroelectric BaMgF ₄
And by using the fluoride buffer layer LaF ₃ , stress due to thermal strain is mitigated, cracks are prevented from occurring,
It becomes possible to form a dense ferroelectric film. Also,
It was possible to obtain a BaMgF ₄ (120) oriented film on Si (111), and an increase in polarization charge density was realized.

[0071]

According to the present invention, stress due to thermal strain can be relieved, cracks can be prevented, and a dense ferroelectric film can be formed. Further, by using the buffer layer, it becomes possible to control the orientation of the ferroelectric thin film to be a-axis orientation or random orientation, and obtain a polarization charge density sufficient to modulate the conductivity of the semiconductor surface. Furthermore, by using a fluoride insulator film that forms a good interface as the buffer layer, good CV characteristics and transistor characteristics can be obtained. Since the band gap of BaMF ₄ / buffer is large, a structure in which charge injection is small, leak current density is small and breakdown voltage is large can be obtained, and excellent operating speed, reliability and power consumption of the device can be realized. Further, it is possible to realize a field-effect type ferroelectric memory element having a low dielectric constant in both the ferroelectric layer and the buffer layer and capable of nondestructive read corresponding to a power supply voltage of 5 Vp-p required as a memory device.

The present invention makes it possible to realize a ferroelectric memory element that is excellent in principle of nondestructive read-out from the viewpoint of a device, and can be highly integrated from the viewpoint of a process. It is of great value.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view of an MFIS structure according to the present invention.

FIG. 2 is a diagram showing CV characteristics obtained in the MFIS structure according to the present invention.

[Explanation of symbols]

1 semiconductor substrate (p-type silicon substrate) 2 buffer layer (BaF ₂ film) 3 ferroelectric film (BaMgF ₄ film) 4 electrode (aluminum) 5 back electrode

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location C30B 29/12 9261-4G H01L 27/10 451 27/108 21/8242

Claims (8)

[Claims] 1. A fluoride-based ferroelectric represented by the following formula (I): BaMF ₄ (I) [wherein M is an element selected from the group consisting of Mg, Zn, Mn, Fe, Co and Ni]. A ferroelectric memory element comprising a buffer layer formed between a semiconductor single crystal substrate and a semiconductor single crystal substrate. 2. A buffer layer having the following formula (II) (A _x B _1-x ) F ₂ (0 ≦ x ≦ 1) (II) [wherein A and B are Ca, Sr, Ba and Mg, respectively]
2. The ferroelectric memory element according to claim 1, which is a fluoride insulator layer represented by an element selected from the alkaline earth metal group consisting of 3. The ferroelectric memory element according to claim 2, wherein the fluoride insulator layer is made of BaF ₂ . 4. The buffer layer has the following formula (III) (A _x B _1-x ) F ₂ / (C _y D _1-y ) F ₂ (II
I) (0 ≦ x ≦ 1, 0 ≦ y ≦ 1) [where A, B, C and D are Ca, Sr and Ba, respectively]
And an element selected from the group of alkaline earth metals consisting of Mg], which is a fluoride insulator layer composed of two layers. 5. The fluoride insulator layer is BaF ₂ / CaF ₂
The ferroelectric memory element according to claim 4, comprising: 6. A fluoride insulator in which the buffer layer is represented by the following formula (IV) AF ₃ (IV) [where A is an element selected from the group of rare earth metals consisting of La, Nd, Ce and Er]. The ferroelectric memory element according to claim 1, wherein the ferroelectric memory element is a layer. 7. The ferroelectric memory element according to claim 6, wherein the fluoride insulating layer is LaF ₃ . 8. The ferroelectric memory element according to claim 1, wherein the semiconductor single crystal substrate is silicon.