JP3403507B2 - Ferroelectric memory element and method of manufacturing the same - Google Patents

Description

Detailed Description of the Invention

The present invention relates to a ferroelectric thin film element applied to a memory element and the like, and a method for manufacturing the same , and more specifically, to a transfer amount of carriers in an impurity region via electrostatic induction by spontaneous polarization of a ferroelectric substance. ferroelectric memory element to change your
And a manufacturing method thereof .

[0002]

2. Description of the Related Art Conventionally, a ROM (Read Only) has been used as a semiconductor element of a nonvolatile memory used in a computer or the like.
Memory), PROM (Programmable ROM), EPRO
M (Erasable PROM), EEPROM (Electrically
EPROM) and the like, but EEPROM is particularly promising because the stored contents can be electrically rewritten. In this EEPROM, MIS (Metal-In
(Sulator-Semiconductor) field effect transistor
Trap area in the insulating film or floating gate
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.

A characteristic of this element is that it has a two-transistor structure, so that rewriting in byte units can be performed with a single 5V power supply, and rewriting is easy on board. However, since erasing an arbitrary cell requires a selection transistor, the cell size must be reduced in order to increase the degree of integration, which limits the degree of integration of the device.

Further, recently, in order to overcome the limitation of the degree of integration in the EEPROM, a flash memory has been developed which realizes a one-transistor / one-cell structure by collectively performing erasing. The feature of this element is that EEPRO
Although there is an advantage that the element area can be reduced to about 1/4 of M, there are problems that an arbitrary cell cannot be erased and that a high voltage of 10 to 12 V is required for writing.

On the other hand, there is a ferroelectric non-volatile memory utilizing the spontaneous polarization of a ferroelectric as a non-volatile memory whose operating principle is completely different from that of the conventional semiconductor device. The ferroelectric non-volatile memory is roughly classified into the following two types.

[0006] One of them is a memory element of a system which detects a change in the amount of electric charge stored in a dielectric capacitor, and a typical example of this system is a capacitor + transistor memory in which a selection transistor is added to a ferroelectric capacitor. There is a cell (capacitor structure). A memory of this system is manufactured by sandwiching a thick interlayer insulating film on a CMOS layer and providing a ferroelectric capacitor on it.

The other is a memory element of a system that detects a resistance change of a semiconductor due to spontaneous polarization of a ferroelectric substance. A typical example of this system is an MFS (Metal Ferroelectric).
Semiconductor) -FET (Field Effect Transistor)
There is a structure. This MFS-FET structure is MIS-FE
The gate insulating film of T is a ferroelectric film, and the electric charge induced on the semiconductor surface according to the direction and the magnitude of the spontaneous polarization of the ferroelectric material induces a charge on the semiconductor surface to The content of the memory is read by utilizing the conductivity modulation. This type of memory element is capable of non-destructive read and is therefore excellent in improving the number of times of rewriting.

However, among the conventional ferroelectric non-volatile memory elements described above, the one using the capacitor structure is a destructive read in which the data is read once to destroy the data. Fatigue occurs, the residual polarization of the film becomes small, and the function as a memory is hindered.

Further, in the MFS-FET structure, nondestructive reading is possible, but since the ferroelectric substance is formed directly on the semiconductor, the following process problems occur. For example, when an oxide ferroelectric such as PZT (lead zirconate titanate), which is currently being actively developed as a capacitor structural material, is directly formed on Si, a silicon oxide film or the like is formed at the ferroelectric / Si interface. Unnecessary film is formed. Then, the operating voltage of the element increases, and charges are injected into the film due to the generation of trap levels, which causes adverse effects such as canceling the charges due to remnant polarization. Further, if the film forming temperature in the device manufacturing process is high, the component elements of the ferroelectric substance may diffuse into Si, which may deteriorate the FET characteristics.

Non-oxide ferroelectric materials such as BaMgF ₄ have been studied to overcome the drawbacks of oxide ferroelectrics in such FET structures (S. Sinharoy et.al., J.
Vac.Sci.Technol.A9 (3) .p.409, 1991 etc.). However, even in such a non-oxide ferroelectric material, S
Considering the matching of i such as the lattice constant and the coefficient of thermal expansion, S
It is very difficult to form it directly on i (hereinafter referred to as “formation directly on Si”).

Therefore, in order to solve various problems of forming directly on Si, a silicon oxide film established as a conventional semiconductor process technology is used as a gate, and a floating gate type electrode is provided on the silicon oxide film. The MFMIS (Metal
Ferroelectric Semiconductor) -FET structure has been proposed (see JP-A-49-131646). As shown in FIG. 7, the structure of the memory element disclosed in Japanese Patent Laid-Open No. 49-131646 is such that a source region 103 is formed on a Si substrate 101.
And a drain region 103 'are formed, a metal floating gate 104 and a bismuth titanate film 105 are formed between them with a silicon oxide film 102 interposed therebetween, and a gate electrode 106, a source electrode 107, a drain electrode 107', and an ohmic electrode 108 are formed. Are formed respectively.

With such a structure, the conventional silicon oxide film (silicon oxide film 1 in FIG. 7) is used as the gate insulating film.
02) can be used. Further, since the ferroelectric substance can be formed on the metal as the floating gate (the metal floating gate 104 in FIG. 7), the underlying layer of the ferroelectric substance can be used as it is as the floating gate.

[0013]

However, in the ferroelectric memory device having the structure disclosed in Japanese Patent Laid-Open No. 49-131646, the most problematic one is the bismuth titanate film 105 and the silicon oxide film 102. Therefore, the voltage applied to the ferroelectric substance composed of the bismuth titanate film 105 becomes small because the multilayer capacitor structure in which each of them is a dielectric film is formed. Therefore, if the applied voltage is increased to give a sufficient electric field to the ferroelectric substance, a high voltage is applied to the silicon oxide film (silicon oxide film 102 in FIG. 7), which may cause dielectric breakdown.

In the case of the multilayer capacitor structure, the film thickness of the ferroelectric film, the relative permittivity of the ferroelectric film, and the voltage applied to the ferroelectric film are t _F , ε _F , and V _F , respectively, and the silicon oxide film When the film thickness, the relative dielectric constant of the silicon oxide film, and the voltage applied to the silicon oxide film are t _OX , ε _OX , and V _OX , respectively, when the voltage V is applied to the gate, the voltage V _F applied to the ferroelectric film and The relationship of the voltage V _ox applied to the silicon oxide film can be described as the following Expression 1.

[0015]

[Equation 1]

From this equation 1, in order to increase the ratio V _F / V _OX between the voltage applied to the ferroelectric film and the voltage applied to the silicon oxide film, the film thickness t _F of the ferroelectric film is increased to It is necessary to reduce the relative permittivity ε _F of the dielectric film and the thickness t _OX of the silicon oxide film. However, if the silicon oxide film is too thin, there is a risk that the dielectric breakdown limit of the silicon oxide film will be exceeded when an electric field required for the ferroelectric film is applied.

The present invention has been made in order to solve the above problems and has a MFSMIS-FET structure, which can be driven at a low voltage and causes a dielectric breakdown of a gate insulating film such as a silicon oxide film. It is an object of the present invention to provide a ferroelectric memory element which can obtain good element characteristics such as being hard to occur.

[0018]

In order to solve the above problems, according to the present invention, a substrate made of either a p-type or n-type conductive semiconductor material and a substrate formed on the surface of the substrate are provided. Are at least two impurity regions of opposite conductivity type, a dielectric film formed on the substrate between the at least two impurity regions, a lower electrode formed on the dielectric film, and a lower electrode thereof. In a ferroelectric memory element comprising a ferroelectric film formed on the ferroelectric film and an upper electrode formed on the ferroelectric film, a ferroelectric containing fluoride is used as the ferroelectric film. , Platinum is used as the lower electrode, and the lower electrode, the ferroelectric film and the upper electrode have the same shape.
To form a ferroelectric film and a dielectric film between the upper electrode and the substrate.
To form a multilayer capacitor.

Further, according to the present invention, in the above ferroelectric memory element, a silicon substrate is used as the substrate and a silicon oxide film is used as the dielectric film.

Further, in the present invention, a ferroelectric film made of BaMgF ₄ is used as the above ferroelectric film. Also,
In the present invention, the substrate is heated by using the high vacuum deposition method.
State, form a ferroelectric containing fluoride on the lower electrode
It has a process.

[0021]

In the ferroelectric memory element of the present invention, fluoride is used as the material of the ferroelectric film. For example, BaMgF ₄ which is a fluoride ferroelectric material has a relative dielectric constant of about 9, which is about twice that of a silicon oxide film used as a dielectric, which has a relative dielectric constant of about 4. . Therefore, when a multilayer capacitor is composed of a ferroelectric film and a dielectric film, it is better to use BaMgF ₄ as the ferroelectric film as compared with PZT having a relative dielectric constant of 500.
The voltage applied to the ferroelectric film can be remarkably increased.

The polarization axis of BaMgF ₄ is [100].
In addition, the spontaneous polarization characteristic of the bulk state is 7.7 μC / cm ² (M.Eib).
schutz et.al., Phys. Lett. vol. 29A.p.409, 1969).
In the case of a ferroelectric memory element having an FET structure, it is sufficient if there is sufficient remanent polarization to form an inversion layer in the channel portion.
Such a fluoride ferroelectric material can be used because it can sufficiently operate even with a small residual polarization of C / cm ² or less.

Further, the above-mentioned fluoride ferroelectric film is
If formed directly on a substrate such as silicon, good characteristics cannot be obtained due to cracks in the film due to differences in lattice constant and thermal expansion coefficient, but by using an electrode made of platinum as the lower electrode, Such a problem can be solved.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a cross-sectional view of essential parts showing the basic structure of a ferroelectric memory element according to an embodiment of the present invention. As shown in FIG. 1, a source region 3 and a drain region 3'which are p ⁺ -type impurity regions are formed on the surface of an n-type Si (silicon) semiconductor substrate 1, and the source region 3 and the drain region 3'are formed. A silicon oxide film 2, a platinum lower electrode 4, a fluoride ferroelectric film 5 and an upper electrode 6 made of platinum or the like are sequentially formed on the Si semiconductor substrate between them. An ohmic electrode 7 and an ohmic electrode 7 ′ made of Al or the like are formed on the source region 3 and the drain region 3 ′, respectively.
The structure is such that an ohmic electrode 8 made of, for example, is formed.

The operation principle of the ferroelectric memory element shown in FIG. 1 will be described. When a voltage is applied between the upper electrode 6 and the ohmic electrode 8, the fluoride ferroelectric film 5 and the silicon oxide film 2 are charged, and the electrostatic induction thereof causes the surface of the Si semiconductor substrate 1 below the silicon oxide film 2. The electric conductivity of is modulated. Therefore, the current flowing through the source region 3 and the drain region 3'can be controlled by this. After that, even after the voltage application between the upper electrode 6 and the ohmic electrode 8 is stopped, electrostatic induction occurs in the silicon oxide film 2 due to the remanent polarization of the fluoride ferroelectric film 5, and when the voltage is applied. The value of the electric conductivity of the surface of the Si semiconductor substrate 1 is retained. This is applied to a switching memory.

Next, an example of a method for manufacturing the ferroelectric memory element shown in FIG. 1 will be described with reference to FIG. First, as shown in FIG. 2A, the n-type Si semiconductor substrate 11
A 10 nm silicon oxide film 12 is formed on the surface by using a thermal oxidation method under the condition of a temperature of 1000 ° C. next,
As shown in FIG. 2B, the silicon oxide film 12 is etched by photolithography to obtain a gate width of 1 μm.
The gate insulating film 12 ′ having a gate length of 100 μm is formed by m. Then, on the surface of the Si substrate 11, 150 kV,
Boron is injected under the condition of 1 × 10 ¹⁶ ions / cm ² ,
By applying an annealing treatment at a temperature of 1000 ° C,
As shown in FIG. 2C, the source region 13 and the drain region 1 which are p ⁺ -type impurity regions having a depth of 0.35 μm.
To form 3 '. Next, a platinum film 14 is formed on the surface of the Si substrate 11 on which the gate insulating film (silicon oxide film) 12 'is formed by a sputtering method, as shown in FIG. On this, as shown in FIG. 2E, a BaMgF ₄ film 15 which is a fluoride ferroelectric is formed.

Here, the method of forming the BaMgF ₄ film will be described in detail. In this example, a high vacuum vapor deposition method was used. First, set the background vacuum pressure to 1 x
Evacuate to 10 ^-10 Torr and raise the substrate temperature to 600 ° C.
Hold on. Then, using BaF ₂ and MgF ₂ as vapor deposition sources, the vapor deposition rate was 0.2 nm / sec. Then, vapor deposition was performed for 25 minutes. In this way, as shown in FIG. 2E, a BaMgF ₄ film 15 having a film thickness of 200 nm was formed.

Next, BaMg formed as described above
A platinum film 16 is formed on the F ₄ film 15 by the sputtering method to obtain the structure shown in FIG. Next, FIG. 2 (G)
As shown in FIG. 1, a resist 21 is formed by photolithography, and ion-milling is performed in the direction of an arrow 22 to form a platinum film 1 at a portion where the resist 21 is not formed.
4, unnecessary portions of the BaMgF ₄ film 15 and the platinum film 16 are removed by etching. In this way, FIG.
As shown in (H), platinum lower electrode 14 ', BaMgF
_{The four} films 15 'and the platinum upper electrode 16' are formed in the same shape as the gate insulating film (silicon oxide film) 12 ', respectively.

Finally, as shown in FIG. 2I, an Al film 1 having a thickness of 0.5 μm is formed on each of the source region 13, the drain region 13 ′ and the back surface of the Si semiconductor substrate 11.
7, Al film 17 ′, and Al film 18 were formed. The Al film 17, the Al film 17 ', and the Al film 18 act as electrodes, and all of them are ohmic electrodes. As described above, the ferroelectric memory element according to the present invention can be manufactured.

Evaluation of the BaMgF ₄ film formed on the platinum film as described above will be described with reference to FIGS. 3 and 4. The X-ray diffraction pattern obtained by X-ray observation is shown in FIG.
As shown in FIG. 3, a random alignment pattern is obtained from FIG. 3, and the components ((120), (201), (211)) in the a-axis direction, which is the polarization axis, are included. Recognize. The result of observing the BaMgF ₄ film on this platinum film with a reflection electron microscope is shown in an electron microscope photographic image (SEM image) of FIG. According to this FIG.
It can be seen that a very flat film without cracks is obtained. As a result of evaluating the hysteresis characteristic of the BaMgF ₄ film on the platinum film, the remanent polarization Pr was 2 μC /
The cm ² and the coercive electric field value Ec became 56 kV / cm. Thus, the fluoride ferroelectric film according to the present invention has excellent characteristics for use in a ferroelectric memory element.

Next, the operation characteristics of the ferroelectric memory element according to the present invention manufactured as described above will be described with reference to FIGS. 5 and 6. C-V of this ferroelectric memory element
The characteristics were measured under the conditions of ± 2.5 V and 1 MHz, and the capacitor area was measured as a gate area of 100 (μm) ^2. As a result, the results are as shown in FIG. 5, and a threshold of about 2.1 V when ± 2.5 V is applied. A shift in value voltage was obtained.

The drain voltage V when the ferroelectric memory element is in the "ON" state and when it is in the "OFF" state
A characteristic curve showing the relationship between _D and the drain current I _D is shown in FIG. According to FIG. 6, it can be seen that a drain current peculiar to the field effect type flows in the “ON” state and no drain current flows in the “OFF” state. Further, the characteristics as shown in FIG. 6 are very stable, indicating that stable operation as a memory element is possible.

Although the n-type Si semiconductor substrate is used as the semiconductor substrate in the above embodiment, the semiconductor substrate is not limited to this, and is made of either p-type or n-type conductive semiconductor material. Any substrate will do. However,
When a p-type semiconductor substrate is used, the impurity regions to be the source region and the drain region must be n-type. For that purpose, the impurity regions can be formed by implanting phosphorus or arsenic. it can.

Further, in the above embodiment, the silicon oxide film having a film thickness of 10 nm is used as the insulating film formed on the semiconductor substrate, but the film thickness and the material of the insulating film are not limited to this. In the case of a silicon oxide film, the film thickness is 10
With a thickness of up to 50 nm, sufficiently good device characteristics can be obtained. As a material, in addition to a silicon oxide film, Si ₃
Good characteristics can be obtained even when N ₄ or the like is used. Also, the method of forming the insulating film is not limited to the above embodiment, and a CVD method, a sputtering method, or the like can be used.

Further, in the above embodiment, the platinum film is formed on the silicon oxide film, but in order to improve the adhesion between the silicon oxide film and the platinum film, Ti film is formed between these films.
Good characteristics can be obtained even if a (titanium) film or a Ta (tantalum) film is interposed.

Although the BaMgF ₄ film of 200 nm is used as the fluoride ferroelectric film in the above embodiment, the film thickness and material of the ferroelectric film are not limited to this, and the film thickness is 100 nm. Sufficient characteristics have been obtained if the thickness is up to 500 nm, and the material is BaM.
nF ₄ , BaFeF ₄ , BaCoF ₄ , BaNiF ₄ , Ba
Fluoride-based ferroelectric materials such as ZnF ₄ have similar results to those of the above-mentioned embodiment. Further, the method for forming the fluoride ferroelectric film is not limited to the above embodiment, and a sputtering method, a laser ablation method or the like can be used in addition to the high vacuum vapor deposition method.

[0037]

As described above, according to the ferroelectric memory element of the present invention, since the fluoride ferroelectric film such as BaMgF ₄ is formed on the platinum film, the randomly oriented fluoride ferroelectric film is formed. It is possible to obtain a body film, and it is possible to secure a polarization charge sufficient to modulate the electric conductivity of the semiconductor surface. Furthermore, B
Fluoride ferroelectrics such as aMgF ₄ exhibit a lower relative permittivity than conventional ferroelectrics such as PZT, so ensure a sufficiently large effective voltage applied to the ferroelectric capacitor portion. You can Therefore, according to the ferroelectric memory element of the present invention, 5V _PP which cannot be obtained by the conventional one is used.
The non-destructive reading corresponding to the power supply voltage becomes possible.

Further, according to the ferroelectric memory element of the present invention, the fluoride ferroelectric substance such as BaMgF ₄ exhibits a lower relative permittivity than the conventional ferroelectric substance such as PZT. Since a large electric field is not applied to a dielectric film such as an oxide film, the risk of dielectric breakdown can be significantly reduced.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of essential parts showing a configuration of a ferroelectric memory element according to an example of the present invention.

FIG. 2 is a diagram illustrating a method of manufacturing the ferroelectric memory element according to the present embodiment.

FIG. 3 is a fluoride ferroelectric film B produced in this example.
It is a diagram showing an X-ray diffraction pattern of AMgF ₄ film.

FIG. 4 is a photograph showing the result of observing the surface of the thin film (BaMgF ₄ film which is a fluoride ferroelectric film) manufactured in this example with an electron microscope (SEM).

FIG. 5 is a diagram showing the results of measuring the CV characteristics of the ferroelectric memory element of this example.

6 is a diagram showing the results of measuring the V _D -I _D characteristic of the ferroelectric memory device of this embodiment.

FIG. 7 is a cross-sectional view showing the structure of a conventional ferroelectric memory element.

[Explanation of symbols]

1,11 n-type silicon semiconductor substrate 2,12 'silicon oxide film (gate insulating film) 3,3', 13,13 'p ⁺ -type impurity region 4,14' platinum lower electrode 5,15 'fluoride ferroelectric Membrane 6, 16 'Upper electrode 7, 7', 8, 17, 17 ', 18 Ohmic electrode

─────────────────────────────────────────────────── ─── Continuation of front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) H01L 29/78 371

Claims (4)

(57) [Claims] 1. A substrate made of a p-type or n-type conductivity type semiconductor material, at least two impurity regions of a conductivity type opposite to the substrate and formed on the surface of the substrate, A dielectric film formed on the substrate between two impurity regions, a lower electrode formed on the dielectric film, a ferroelectric film formed on the lower electrode, and a ferroelectric substance. in configured ferroelectric memory element and an upper electrode formed on the film, the ferroelectric film is made of a ferroelectric containing fluorides, the lower electrode Ri consists of platinum, the
The lower electrode, the ferroelectric film and the upper electrode have the same shape.
Forming a ferroelectric substance between the upper electrode and the substrate.
Forming a multilayer capacitor consisting of a film and the dielectric film
Ferroelectric memory device characterized Rukoto have. 2. The ferroelectric memory element according to claim 1, wherein the substrate is a silicon substrate, and the dielectric film is a silicon oxide film. 3. The ferroelectric memory element according to claim 1, wherein the ferroelectric film is made of BaMgF ₄ . 4. A method of manufacturing the ferroelectric memory element according to claim 1.
A method of manufacturing, wherein the substrate is applied using a high vacuum deposition method.
In a heated state, the fluoride was included on the lower electrode.
Ferroelectric device characterized by having a step of forming a ferroelectric film
Method for manufacturing electric storage element.