JP2000103698A - Crystallographically aligned ferroelectric film usable as memory and method for crystali,ographically aligning perovskite film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a process for forming a ferroelectric film having excellent electrical characteristics and aging characteristics. SOLUTION: The objective process for forming a ferroelectric thin film comprises the following steps. A ferroelectric layer deposition step to deposit a ferroelectric layer composed of a ferroelectric material on a single crystal at an evaporation temperature to stabilize the ferroelectric material in cubic crystal form and a step to cool the deposited layer beyond a transition temperature of the ferroelectric material lower than the evaporation temperature and induce the transition of the ferroelectric material from cubic crystal form to non-cubic crystal form by cooling at a cooling rate of 10-40 deg.C/min.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates generally to ferroelectric thin films, perovskite thin films, and methods for their manufacture. More specifically, the present invention relates to a nonvolatile memory using such a ferroelectric thin film and a method for growing an oriented perovskite thin film usable in the nonvolatile memory.

[0002]

2. Description of the Related Art Ferroelectric memories have received much attention as high-density, randomly addressable, non-volatile memories. A ferroelectric memory element is essentially a capacitor consisting of two electrodes and a ferroelectric material that fills the gap between the electrodes. At least this ferroelectric material has a very high dielectric constant, which can provide an improved dynamic RAM (DRAM) capacitor.
In addition, this electrode can induce two stable polar states in the ferroelectric depending on the polarity of the applied voltage, and the ferroelectric remains in the selected state even after the voltage is removed from the electrode. maintain. Therefore, the ferroelectric memory can be made nonvolatile.

[0003]

Early ferroelectric memories relied on a relatively thick ferroelectric material opposite the location where the electrodes were formed, such as by evaporation. Such structures were not compatible with large scale integration and required significantly higher operating voltages than those used in semiconductor integrated circuits. In later ferroelectric memories, a ferroelectric thin film was deposited on an electrode on the bottom of the metal, and an upper electrode was deposited on this ferroelectric thin film. Although this semiconductor-like structure and manufacturing process have many advantages, the polycrystalline structure of the ferroelectric film resulting from the metal substrate has various problems.

[0004] More recently, US patent application Ser. No. 07 / 616,166, filed Nov. 10, 1990, entitled "Epitaxial Cuprate Superconductor / Ferroelectric Heterostructure".
Superconductor / Ferroelectric Heterostructures), S
cience, Volume 252, May 17, 1991, pp. 944-946, and "Ferroelectric PbZr _0.2 T on epitaxial Y-Ba-Cu-O"
i _{0 . 8} O ₃ thin film ”(Ferroelectric PbZr _0.2 Ti _0.8
O ₃ thin films on epitaxial Y-Ba-Cu-O), Applied Ph
ysics Letters, Vol. 59, December 30, 1991, pp. 3542-35
In 44, Ramesh et al. Disclosed a single crystal epitaxially grown ferroelectric memory. This patent application is described herein for reference. The electrode on the bottom of these memories is a single crystal thin film of high-temperature perovskite superconductor, for example,
YBa ₂ Cu ₃ O _7-x (hereinafter referred to as YBCO). The ferroelectric thin film is then epitaxially deposited on the YBCO film, and although it is cubic or nearly cubic, its crystal orientation follows epitaxially the underlying perovskite crystal orientation.

[0005] Although YBCO is a superconductor at temperatures below 92 ° K, the ferroelectric memory described above has high semiconducting or dielectric behavior, as found in most non-superconducting perovskites. Is intended to operate at room temperature, where YBCO exhibits normal conductivity.

[0006] Although the ferroelectric memory of Ramesh et al. Had some wonderful points, it had many problems. The hysteresis curve was not sufficiently square and the coercive field was low.
Such a memory element requires a fairly complex transistorized gate for writing and reading. In addition, these single crystal ferroelectric memory cells aged and fatigued fairly quickly in simulations. Typical conditions required for volatile semiconductor memory are 10
¹² is that either withstand ^{10 15} read-write cycles. Such high numbers, but seems likely unnecessary for nonvolatile ferroelectric memories, desirably at least 10 ⁸ cycles of life to the system performance to ensure its.

[0007]

In order to solve the above problems, the method according to the present invention employs the following means.

That is, a first aspect of the present invention provides a method of forming a ferroelectric thin film, comprising the steps of: forming a ferroelectric layer made of a ferroelectric material on a single crystal; Depositing a ferroelectric layer at a deposition temperature at which the dielectric material is stable in the cubic phase; and cooling the deposited layer over a transition temperature of the dielectric material below the deposition temperature;
Causing a transition that changes the ferroelectric material from the cubic phase to a non-cubic phase, wherein the cooling is from 10 ° C to 40 ° C /
Having a step performed at a cooling rate in the range of minutes.

According to a second aspect of the present invention, there is provided the method according to the first aspect, wherein the cooling rate is in a range of 15 ° C. to 30 ° C./min.

According to a third aspect of the present invention, in the method according to the first aspect, the single crystal body is made of a metal oxide selected from a group including copper oxide and bismuth oxide, has a perovskite crystal structure, and has conductivity. Comprising a thin film, wherein the method further comprises the step of depositing the thin film on a single crystal substrate.

According to a fourth aspect of the present invention, in the method according to the third aspect, the metal oxide is made of YBaCuO.

A fifth aspect of the present invention is the method according to the fourth aspect, wherein the ferroelectric layer is made of PbZrTiO.

According to a sixth aspect of the present invention, in the method of the fifth aspect, the transition temperature is about 470 ° C.

According to claim 7, in the method of claim 1, the cooling step comprises at least 80% of the layer of the ferroelectric material.
% Is c-axis oriented.

According to claim 8, the method according to claim 1, wherein the single crystal comprises a first perovskite layer; and the step of depositing the ferroelectric layer comprises: Depositing a perovskite layer as an a- and b-axis oriented film; and further comprising depositing a second perovskite layer as an a- and b-axis oriented film on the ferroelectric layer. And

According to a ninth aspect, in the method according to the eighth aspect, the step of depositing the second perovskite layer comprises the steps of:
depositing the second perovskite layer during a gradual rise from a first temperature that promotes b-axis oriented growth to a second temperature that promotes c-axis oriented growth, and increasing the deposition temperature while continuing the second deposition step. Maintaining at the second temperature is characterized.

According to a tenth aspect of the present invention, in a method of writing and rejuvenating a ferroelectric memory, a single crystal ferroelectric material for writing data to the ferroelectric memory and the first amplitude and the second amplitude are applied to the memory. A first step of applying a voltage pulse for a first duration; and an amplitude equal to the first amplitude and longer than the first duration in the ferroelectric memory to rejuvenate the memory for the first step. A second step of applying a voltage pulse for a second duration is provided.

According to an eleventh aspect, in the method according to the tenth aspect, each electrode is formed of a perovskite thin film made of the same material and oriented in the same manner.

In a twelfth aspect of the present invention, a method of growing an oriented perovskite thin film includes the step of depositing a perovskite thin film on a crystal substrate;
forming a first portion of the thin film while gradually changing from a first growth environment promoting the formation of the b-axis alignment film to a second growth environment promoting the formation of the c-axis alignment film; the second growth environment The step of continuing the deposition in the second growth environment without interruption, forming a second portion of the thin film, and forming the second portion as an a, b-axis oriented thin film. I do.

According to a thirteenth aspect, in the method according to the twelfth aspect, the first portion and the second portion of the thin film are made of a normal superconducting material.

According to a fourteenth aspect, in the method according to the twelfth aspect, the second growth environment includes a deposition temperature higher than a deposition temperature in the first growth environment.

According to a fifteenth aspect of the present invention, in the method according to the fourteenth aspect, the changing step comprises changing the deposition temperature while depositing a plurality of unit cells of the thin film.

According to claim 16, in the method of claim 12, the depositing step deposits the thin film on a ferroelectric layer,
The ferroelectric layer is characterized in that at least 80% has a c-axis orientation.

[0024]

SUMMARY OF THE INVENTION A part of the present invention can be summarized as a ferroelectric thin film usable as a nonvolatile memory element and a method for manufacturing the same. The ferroelectric thin film is epitaxially grown on a single crystal substrate (or thin film) of a conductive perovskite material having either a-axis or c-axis orientation. The ferroelectric thin film is grown so that 80%, preferably 90%, of the ferroelectric thin film is oriented so that the c-axis is perpendicular to the electrode surface. To achieve this high degree of orientation, the ferroelectric thin film is deposited at a temperature at which it is in the cubic phase, and is cooled while being controlled through the transition temperature, below which the film is square or It becomes an orthorhombic phase. The cooling rate is in the range of 10 ° C./min and 40 ° C./min, but preferably about 20 ° C./min. The memory element is completed by depositing a top electrode on a ferroelectric and using a conductive perovskite as the bottom electrode.

The ferroelectric memory can be further improved when both electrodes are perovskite thin films made of the same material and orientation (for example, a-axis oriented YBaCuO).

Another part of the invention is a method of rejuvenating a ferroelectric memory cell from fatigue by applying a relatively long pulse of the same amplitude as the pulse used to read and write the memory. Can be summarized. Rejuvenation can be effectively implemented with the single crystal ferroelectric memory cell of the present invention.

Further, another part of the present invention comprises a, b
It can be summarized as a method of depositing an axially-oriented perovskite thin film. The perovskite thin film is deposited as a template layer on the substrate at a temperature that promotes a-axis growth and other growth parameters. Without interrupting growth
The temperature and growth environment gradually change to those that promote c-axis growth. The growth of the final perovskite thin film is
Continued at this temperature and growth environment. Nevertheless,
The final thin film is formed as an a-b-axis alignment film.

[0028]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The general structure of a first embodiment of a ferroelectric memory device according to the present invention is shown in cross-section in FIG. 1, which is in accordance with the invention of Ramesh et al.
The conductive perovskite thin film 10 is epitaxially deposited on the single crystal substrate 12 and has a c-axis orientation (or an a-axis orientation as will be described later). The ferroelectric thin film 14 is epitaxially deposited on the conductive thin film 10. The upper electrode 16 is patterned on the ferroelectric thin film 14. The conductive thin film 10 serves as an electrode on the bottom surface. The two electrodes 10 and 16 sandwiching the ferroelectric film 14 are
Construct a capacitor. The ferroelectric layer 14 has a bottom contact 18
It is patterned to expose conductive film 10 for mounting. According to the present invention, the ferroelectric film 10 must be highly oriented in the c-direction, and preferably at least 80%, and preferably 90%, of the ferroelectric material is c-axis oriented.

[0029] In the discussion below, a ferroelectric material of lead zirconate titanate or PbZr _x Ti _1-x O ₃ of formula (wherein, 0 <x <1) is assumed to be PZT having. x =
0.2, 470 ° C. or higher PZT has the cubic crystal structure shown in the perspective view of FIG. 2 and the equivalent lattice parameters listed in the table. On the other hand, at temperatures below 470 ° C. of the structural phase transition, PZT has a tetragonal crystal structure with the lattice parameters listed in the table, and the spacing of the lattice along the c-axis is slightly less than that along the a-axis or b-axis. become longer.

[0030]

A ferroelectric film grows above its transition temperature. In the structural transition during cooling, Zr or Ti ions move upward or downward along the c-axis from the center of the cubic structure. The bidirectional selection provides two stable polarities of the ferroelectric material, in which the c-axis is the preferred direction of the ferroelectric polarity.

The ferroelectric film 14 is a conductive perovskite film
This film grows on 10 and is made of a metal oxide, either copper oxide (hereinafter referred to as cuprate oxide) or bismuth oxide. Both of these materials are superconducting at low temperatures and have significant conductivity at room temperature. One of the most developed examples of oxide plates is the superconductor YBa ₂ Cu ₃ O
_7-x (YBCO), which has the orthorhombic crystal structure shown in FIG. 3 and the lattice parameters shown in the table. This material has a slight oxygen deficiency from purely stoichiometric YBa ₂ Cu ₃ O _7, which is determined by the state of deposition and annealing. Bismuth oxide resembles the crystal structure of YBCO, but is more complex and is well studied with high _Tc superconductivity.

PZT is deposited at a high temperature on c-axis oriented YBCO,
When cooled through the phase transition, the lattice matching is optimal when the PZT c-axis is parallel to the YBCO c-axis, and thus the PZT
Is mainly c-axis orientation. However, phase transitions cause considerable distortion in the ferroelectric thin film, and ferroelectrics cause a strong limiting field. Both of these induce the ferroelectric to form as a multi-domain structure in which many of the domains of the square PZT are formed with the c-axis lying in the plane of the film. In other words, a large part of the ferroelectric film contains the material having the a-axis orientation. The a-axis or equivalent b-axis appears to be perpendicular to the membrane. An excellent ferroelectric memory should have a c-axis orientation of at least 80%, preferably at least 90%.

Part of the present invention involves the discovery that, through phase transition, increasing the cooling rate to a significantly higher rate than that disclosed in the patent application by Ramesh et al. Can effectively eliminate the a-axis orientation. In. In order to obtain substantially all c-axis oriented square PZT, the cooling rate needs to be at least 10 ° C./min, preferably 20 ° C./min. Due to other considerations, the maximum cooling rate is 40 ° C /
Limited to minutes.

[0035]

Embodiment 1 The heterostructure is a substrate of [001] -oriented LaAlO ₃ .
Growed to have 12. The YBCO layer 10 was deposited to a thickness of 100 nm by pulse laser polishing using a 60 ns pulse of 248 nm light at a substrate heater temperature of 810 ° C. Ferroelectric P
The ZT layer 14 has a composition of PbZr _0.2 Ti _0.8 O ₃ , and is heated at a substrate heater temperature of 700 ° C. to 715 ° C. by pulse laser polishing.
Deposited to a thickness of 250-270 nm. For more information on evaporation, see Ramesh
And in articles in Applied Physics Letters. After PZT deposition, the chamber was pumped with oxygen to a pressure of 600 Torr and the structure was cooled from 700 ° C. to room temperature at various controlled cooling rates.

Next, the crystal structure of this heterostructure was tested by X-ray diffraction. FIG. 4 shows the diffraction pattern of the heterostructure cooled at 5, 20, and 100 ° C./min. The figure also shows the expected positions of the (002) and (200) peaks of PZT, which show the square PZT of c-axis and a-axis orientation, respectively, and the expected position of the (006) peak of YBCO is c-axis orientation. Oblique Y
Indicates BCO. At a cooling rate of 5 ° C./min, significant amounts of both a-axis and c-axis oriented PZT were observed, but the peaks were relatively broad. (006) The YBCO peak was strong but broad. At a cooling rate of 20 ° C./min, the a-axis oriented PZT substantially disappeared. (006) The YBCO peak remained strong and narrowed further. At a cooling rate of 100 ° C./min, the a-axis orientation was further suppressed. However, (0
06) In YBCO, both slippage and reduction occurred. The relatively fast cooling rate was thought to prevent YBCO from absorbing enough oxygen to achieve the low temperature oblique phase required for superconducting YBCO.

Several single crystal ferroelectric devices were used for 5, 10, 2
Produced at different cooling rates of 0, 30, 40, 60, 100 ° C / min. As shown in FIG. 5, the memory capacitor was formed by vapor deposition on the upper surface of the PZT layer 14 such that two platinum / gold electrodes 20 were separated by 20 μm. The YBCO layer 10 substantially functioned as a ground plane. After production, this device has residual polarity, fatigue,
Aging and other parameters were tested. The apparatus was fatigued by applying an AC bipolar 5V pulse signal of 8.6 μs at a constant frequency between the electrodes 20 at a repetition rate of 40 kHz. Polarity characteristics were measured intermittently by applying a 5 V, 2 ms bipolar pulse and monitoring the integrated current directly related to polarity.

The residual polarity pulse signal is a polar remaining after returning to zero P _R is measured before the large aging and fatigue, the results are shown in the scale of the line 24 and the left ordinate connecting square I have. Thereafter, the fatigue state of the device was measured. The line 26 connecting the circles indicates the period of fatigue, which is indicated by the scale on the right, after which the differential residual polarity δ
P decreased to half of the original value. Differential residual polarity is the difference in polarity between a switching pulse and a non-switching pulse. These data indicate that a cooling rate of 20 ° C. is optimal during testing. Satisfactory results are obtained in the range 10 ° C to 40 ° C, but preferably 15 ° C to 30 ° C
Is desirable. The line 28 connecting the triangles represents the fraction c / (c + a) of the amount of PZT with c-axis orientation, expressed as a percentage on the left ordinate. This fraction was obtained by combining the intensities of the (002) and (200) X-ray diffraction peaks of PZT similar to that shown in FIG. 4 to determine the values of c and a. Orientation data is 80% or more at 15 ° C./min, 20%
It shows an a-axis orientation of 90% or more at ° C./min.

It is believed that a very slow cooling rate causes the formation of multiple domains, which allows the formation of a-axis oriented PZT. Very high cooling rates promote c-axis orientation, which also leads to interfacial defects, ultimately leading to degradation in YBCO.

The fatigue data shows that the optimum cooling condition is about 10 ⁸
It shows the useful life of the cycle, which is very good and probably satisfactory for many applications using non-volatile memory. However, in the case of a volatile memory, the operation condition is often 10 ¹⁵ cycles. However, it has been discovered that the useful life of a PZT / YBCO memory can be extended by rejuvenating it with longer pulses of the same amplitude as the fatigue pulse. After the memory device of FIG. 5 is a tired period of 10 ^9, 2 seconds, and applying a DC pulse of 5V to the memory device. Thus, the hysteresis polarity characteristics of the memory element returned to almost the initial values. More detailed data on other devices is shown in FIG.
Before fatigue, the device is in the initial state 30, which is 27μ
It could switch the polarity of C / cm ² . 1. At 5V
After 7 × 10 ⁹ cycles, the device exhibited a fatigue state of 32, where the differential residual polarity δP was reduced to 4 μC / cm ² . Thereafter, as illustrated by the recovery line 34, the device could be rejuvenated logarithmically by the 5V DC pulse to a degree dependent on the polling time, ie the duration of the DC pulse. In about 1-2 seconds, the device was completely rejuvenated. This recovery can be greater than the initial degradation. This effect is not fully understood, but was thought to occur because all measurements of switched polarity used a bipolar pulse sequence of 2 ms in length. Bipolar poling increases recovery, and if one of the electrodes is a polycrystalline metal other than cuprate, the polarity of unipolar poling is important. For symmetrical electrodes, such as those with YBCO at the top and bottom, the recovery is also symmetrical.

This effect is described by Scott et al.
Raman spectroscopy II of NO ₃ film. Fatigue and space discharge effects '' (Rama
n spectroscopy of submicron NKO3 film. II.Fatigue
andspace-charge effects) Journal of Applied Physi
cs, Vol. 64, 1988, pp. 1547-1551, similar to the pulse polling, but the researchers found that the polling had a much larger amplitude than the fatigue pulse, typically about three times the fatigue voltage. Pulse, which is typically 3 to
There is no compatibility with a 5V semiconductor power supply. Comparable devices were fabricated on platinum electrodes by conventional sol-gel processing to make the ferroelectric polycrystalline. The device only partially recovered using a 5V, 2 second polling pulse.

The apparatus of the present invention also shows recovery with a short polling pulse when the amplitude of the polling pulse is large. For example, a 14.5V polling pulse is
Complete recovery in 2ms. However, this experiment showed that substantially the same polling and data write / probe voltages could be used, and that the memory element was periodically rejuvenated by only increasing the applied voltage time rather than applying a special voltage.

The use of the upper metal electrode of FIG. 1 is disadvantageous because it results in an asymmetric device. Therefore, it is advantageous to manufacture a ferroelectric memory element having a symmetric structure as shown in FIG. For more details, PZ the upper YBCO layer 40
Formed epitaxially on the T layer 14, metal contacts 18 and 16 are formed on the YBCO layers 10 and 40. This structure
It is similar to the Josephson junction device of US Pat. No. 5,087,605 disclosed by Hegde et al. However, this structure can be disadvantageous if both YBCO layers 10 and 40 are manufactured to have a c-axis orientation. High quality c-axis oriented YBCO
Is formed epitaxially at an elevated temperature at which the lead in the PZT layer 14 sublimes (or otherwise leaves the alloy). Moreover, these elevated temperatures may not be compatible with CMOS processing in silicon / ferroelectric integrated circuits. Therefore, it is disadvantageous to form the upper YBCO layer 40 to have the c-axis orientation. However, a US patent by Inam et al. (Serial number 07 / 531,255, registered May 3, 1990
1 day, this document described), and "a-axis oriented epitaxial _{YBa 2 Cu 3 O 7-x} -PrBa 2 Cu 3 O 7-y heterostructures" for reference _{(a-axis oriented epitaxial YBa 2} Cu 3 O 7- _x-
PrBa ₂ Cu ₃ O _7-y heterostructures) Applied Physic
It is well known that a-axis oriented YBCO is formed at somewhat lower temperatures, as disclosed in s Letters, Vol. 57, 1990, pp. 2484-2486.

[0044]

Embodiment 2 Therefore, the lower YBCO layer 10 is deposited as an a-axis alignment film by maintaining the substrate heater temperature at 700 ° C., and the PZT layer 14 is deposited as a c-axis alignment film at 640 ° C. Laser polishing was used for this. The upper YBCO layer 40
As the thickness of this layer 40 increases, the substrate heater temperature
The deposition was started at 640 ° C. and increased to 700 ° C. Although the sublimation of lead was prevented at the lower temperature portion, the higher temperature portion resulted in high quality a-axis oriented YBCO.

These devices were electrically tested after placing the contacts as shown in FIG. 10 ^{12 at} 5V
After the bipolar cycle, the differential residual polarity decreased by 15%. This loss could be recovered by pulse polling. Some devices, aging was carried out immediately after the 10 ¹² fatigue cycles. They exhibited excellent aging properties with 2% attenuation per 10 years.

In the above example, c-axis oriented PZT was grown on a-axis oriented YBCO. A comparison of the crystal structures of FIGS. 2 and 3 and the lattice parameters in the table show that this heterostructure is not epitaxial in the usual sense, and that the long lattice spacing of YBCO is at least two times more effective than the short lattice spacing of PZT. It shows that it will be a nice template. YBCO
Lattice spacing comparisons show that a perovskite
This indicates that it is very difficult to distinguish between the axis and the b-axis orientation, both of which are described in the description of the a-axis orientation in this document, but which may sometimes be referred to as the a-b-axis orientation. Sometimes called.

The growth of a- and b-axis-oriented YBCO using a temperature gradient followed a method for improving and simplifying the oriented growth of a high Tc superconductor disclosed in a patent application by Inam et al. The researchers found that PrBa ₂ C at low temperatures promoted a-axis oriented growth.
depositing a layer of _{u 3 O 7-y (PrBCO} ), after the growth was suspended until the temperature that promotes c-axis growth (15 minutes normally), YBCO
Resumed growth. Despite the high temperature, the PrBCO template layer forcibly grown the overlying YBCO in a- and b-axis orientation, and had higher quality than the a, b-axis oriented film grown at low temperature. However, the use of PrBaCuO is disadvantageous when the current needs to pass through the template layer because PrBCO is semiconducting.

In accordance with this portion of the present invention, as shown in FIG. 9, the lower perovskite epitaxially grown on crystal substrate 40 at a temperature or other growth environment that promotes the growth of the a, b axis orientation. There is a layer 42. The crystal substrate 40 includes
Various materials such as a bulk substrate of a general perovskite thin film such as SrTiO ₃ and LaAlO ₃ , a c-axis oriented perovskite thin film, and a square ferroelectric layer can be used. Next, the perovskite transition layer 44 is epitaxially grown on the lower perovskite layer 42 without greatly interrupting the growth. The temperature (or other growth environment) during the deposition of the transition layer 44 gradually increases from a temperature (growth environment) that promotes a- and b-axis oriented growth to a temperature that promotes c-axis oriented growth. When the c-axis growth temperature is reached, the upper perovskite layer is deposited at that temperature. Nevertheless, it is formed epitaxially as an a, b axis alignment film.

[0049]

Example 3 A series of such a- and b-axis oriented heterostructures were grown on a LaAlO ₃ substrate 40 by pulsed laser polishing. All growth rates were about 0.01 nm / sec. YB
The lower perovskite layer 42, either CO or PrBCO, was deposited at a substrate heater temperature of about 700 ° C. to a thickness of 30-50 nm. This temperature promotes the growth of the a-axis oriented perovskite film. Next, the transition layer 42 made of YBCO
The substrate temperature gradually and continuously from 700 ℃ to about 800 ℃
Deposition was performed while rising at a rate of 6-7 ° / min. High temperatures promote the growth of c-axis oriented perovskite films.
If the lower perovskite layer 42 is YBCO, the deposition is uniformly continuous, and in the case of PrBCO, the deposition is changed from PrBCO to YBCO without interruption. With a growth rate of 0.01 nm / sec, the transition layer 4
2 had a thickness of about 10 nm, ie, a plurality of unit cells, and a growth temperature gradient of about 10 ° C./nm. During growth of the upper perovskite layer 46 of YBCO, the temperature was maintained at 800 ° C.

X-ray experiments demonstrated that at least 95% of the upper perovskite layer 46 was formed in a, b-axis orientation, despite growing at a temperature that promoted c-axis orientation growth. That is, the perovskite layer 42 below the a-axis orientation functions as a template. According to Rutherford backscattering, the orientation interface showed that the heterostructure was less extreme than when grown without the transition layer 42, ie, when growth was interrupted during the temperature change. The results were similar whether the lower perovskite layer 42 was YBCO or PrBCO.

The superconducting transport properties of heterostructures grown at gradual temperature transitions have shown significant improvements. The superconducting transition temperature of YBCO remained above 92 ゜ K, but the transition width became much sharper. In the plane of the YBCO film 46, the critical current density increased from about 10 times to more than 10 ⁶ A / cm ² . Microwave surface resistance and flux quantum noise were similar to those of the c-axis oriented film.

The uninterrupted growth between the perovskite layers 42 and 46 is due to the formation of impurity layers (probably Ba and Cu moving upward during the long interruptions needed to raise the temperature in the prior art). Was thought to interfere.
The a- and b-axis alignment films were expected to grow in multiple domains, with the long c-axis oriented perpendicular to adjacent domains. The impurity layer diffuses the boundary between the a- and b-axis alignment domains, and deteriorates the link between the domains. Hence, the uninterrupted growth of the present invention prevents the formation of weak links in the YBCO plane, thus improving superconducting transport properties. This part of the invention is not limited to laser polishing, but can be extended to other forms of deposition and other growth environments that promote growth of different orientations.

Commercial ferroelectric memory arrays should be fabricated on semiconductor substrates to integrate support electronics and reduce costs. Such an array shown in FIG. 10 is formed on a single crystal silicon substrate 50. Buffer layer 52
Is deposited on the silicon substrate 50. It is well known in the superconductor art that a buffer layer grows a single crystal perovskite film on a silicon substrate. Examples of the material of the buffer layer include yttria-stable zirconia (YSZ) and MgO. A lower YBCO layer 54 (preferably a-axis oriented) is deposited on buffer layer 52, and a c-axis oriented ferroelectric layer 56 is deposited on lower YBCO layer 54. Upper YBCO layer
58 is preferably symmetrically oriented with the underlying YBCO layer 54, but is deposited on the ferroelectric layer 56 and patterned as a separate YBCO electrode 58. The side portions are etched down to the lower YBCO layer 54. Metal contacts 60 are deposited on the lower YBCO layer 54, and individual metal contacts 62
It is deposited on the electrode 58. Of course, the contacts shown are only one example of metal leads used in silicon / ferroelectric integrated circuits.

[0054]

Example 4 A ferroelectric memory capacitor on a silicon substrate was fabricated by pulsed laser polishing and tested electrically. The layered structure is as shown in FIG. The buffer layer 52 was 100 nm perfectly stable (9 mol% Y) YSZ on a (001) oriented silicon substrate 50. Both YBCO layers
The a-axis orientation was performed by a temperature orientation method. The structure was laterally unpatterned, but was in contact with two gold electrodes on the upper YBCO layer as in FIG. Fatigue data is LaAlO
It was very similar to that of a similar device using _three substrates.

[0055]

Example 5 Another silicon / ferroelectric heterostructure with a-axis orientation YBCO was fabricated, which was fabricated using a 30 nm lower Y
680 ° C so that BCO layer 10 always becomes a-axis oriented template
Grown in. Next, YBCO growth was performed continuously without interruption while gradually increasing the temperature to 800 ° C. in 4 minutes. Thereafter, a further 70 nm of the lower YBCO layer 10 was grown. Subsequently, a PZT layer 14 and an upper a-axis oriented upper YBCO layer 40 were grown as described above. X-ray diffraction showed that less than 5% of YBCO was c-axis oriented and the PZT layer was completely c-axis oriented.

[0055]

Example 6 A silicon / ferroelectric heterostructure was created such that both electrodes had c-axis oriented YBCO. The lower YBCO layer was grown at 810 ° C and the PZT layer was grown at 640 ° C. Next, YBCO 6 with 500 laser pulses without interruption
Deposited at 40 ° C. The growth was then interrupted for 4 minutes until the temperature rose to 810 ° C. for the deposition of the upper YBCO layer. 10 ¹² bipolar this device (i.e., ± 5V) after the fatigue is applied with a period, a differential residual polarity were only slightly decreased 10-15%, which could be recovered by the pulse polling. Therefore,
a-axis oriented YBCO is desirable but not essential. However, symmetric heterostructures are highly desirable.

[0056]

EXAMPLE 7 A series of ferroelectric memory capacitors of various structures were fabricated, the ferroelectric layer comprising Pb _0.9 La
_It consisted of _0.1 Zr _0.2 Ti _0.8 O ₃ . Lanthanide alloys were used to increase the resistance of the ferroelectric layer. These devices showed some reduced residual polarity, but could be fatigued over 10 ¹² cycles. Importantly, the overall resistance of the ferroelectric layer increased by a factor of 20 to 50.

[0057]

Example 8 An additional ferroelectric memory capacitor was fabricated.
This ferroelectric layer is PbZr_0.5Ti _0.5O₃Consists of
Was something. These were not tested electrically,
Visual inspection shows that the surface quality is excellent,
This was indicative of high electrical quality. This ferroelectric
Alloys are commonly used in polycrystalline ferroelectric memories
You.

The above two embodiments show that the present invention relates to the alloy Pb
_1-y La _y Zr _x Ti _1-x O ₃ (where 0 ≦ y ≦ 1 and 0 ≦ x
<1. However, the general upper limit of y is 10%, and the general upper limit of x is 65
%) Has proven to be readily applicable to a wide range of applications. However, other ferroelectrics can be used, as described in the Ramesh et al. Patent. For example, Bi ₄ Ti ₃ O
₁₂ , PbTiO ₃ , LiNbO _{3 and the} like.

The perovskite conductor can be said to be one of cuprate or bismuth oxide of the type exhibiting superconductivity. The oxidized cuprate contains not only YBCO but also other oxides that replace Y by lanthanide rare earth metals, ie, from Ce to Lu. Although PrBCO is generally poor in conductivity, it is known as an excellent epitaxial template and is therefore useful unless a metal oxide layer is used as an electrode. Bismuth oxide is Bi ₂ Sr ₂ Ca
_{n-1 Cu n O 4 +} 2n + x ( where, n is a positive integer) contains. Bismuth oxide has been well studied in high _Tc superconductivity.

The present invention provides a ferroelectric memory cell having excellent electric characteristics and aging characteristics. The large coercive field and square hysteresis allow the ferroelectrics to make direct contact in a matrix-type addressing. That is, the row electrodes pass parallel below the ferroelectric layer, the column electrodes pass vertically above the ferroelectric layer, and both electrodes directly contact the ferroelectric layer, reducing the need for a transistor gate. Can be removed. Such a structure can be very dense.

The present invention also provides a more flexible method for growing an a-axis oriented perovskite film. Such films exhibit excellent superconducting properties and can be advantageously used in devices other than ferroelectric memories.

[Brief description of the drawings]

FIG. 1 is a sectional view of a ferroelectric memory device according to the present invention.

FIG. 2 is a perspective view of a PZT lattice structure that is either cubic or square.

FIG. 3 is a perspective view of an oblique lattice structure.

FIG. 4 is a graph of X-ray data showing a crystal orientation as a cooling rate from a deposition temperature.

FIG. 5 is a sectional view of a test device used for evaluation of the present invention.

FIG. 6 is a graph showing the dependence of crystal orientation and two polar properties on cooling rate.

FIG. 7 is a graph of rejuvenation data.

FIG. 8 is a sectional view of a symmetric ferroelectric memory device according to the present invention.

FIG. 9 is a cross-sectional view of a-axis oriented heterostructure growth according to the present invention.

FIG. 10 is a cross-sectional view of a single crystal ferroelectric memory array manufactured on a silicon substrate.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification FI FI Theme Court ゛ (Reference) H01L 21/8242 21/8247 29/788 29/792 (72) Inventor Enum, Arun United States, 07701 New Jersey 28, Riverside Ave, Apartment, Red Bank, State 11 (72) Inventor Ramesh, Ramamoosie United States, 07701 Tainton Falls, Palomino Place 29, Tainton Falls

Claims (16)

[Claims] 1. A method for forming a ferroelectric thin film, comprising the following steps: forming a ferroelectric layer made of a ferroelectric material on a single crystal body, wherein the ferroelectric material has a cubic phase. Depositing a ferroelectric layer at a stable deposition temperature; and cooling the deposited layer above the transition temperature of the dielectric material below the deposition temperature to remove the ferroelectric material from the cubic phase. Causing a transition to change to a non-cubic phase, wherein the cooling is performed at a cooling rate in the range of 10 ° C. to 40 ° C./min. 2. The method according to claim 1, wherein said cooling rate is one.
Those in the range of 5 ° C to 30 ° C / min. 3. The method according to claim 1, wherein the single crystal is
Consisting of a metal oxide selected from the group including copper oxide and bismuth oxide, having a perovskite crystal structure,
Providing a conductive thin film, and wherein the method further comprises depositing the thin film on a single crystal substrate. 4. The method of claim 3, wherein said metal oxide is Y
What consists of BaCuO. 5. The method of claim 4, wherein said ferroelectric layer comprises PbZrTiO. 6. The method of claim 5, wherein said transition temperature is about 470 ° C. 7. The method of claim 1, wherein said cooling step causes at least 80% of said layer of said ferroelectric material to be c-axis oriented. 8. The method of claim 1, wherein the single crystal comprises a first perovskite layer; and the step of depositing the ferroelectric layer comprises the step of: depositing the first perovskite layer on another single crystal;
and e. depositing a second perovskite layer on the ferroelectric layer as an a- and b-axis alignment film. The method further comprises depositing a second perovskite layer on the ferroelectric layer. 9. The method of claim 8, wherein the step of depositing the second perovskite layer comprises gradually increasing the deposition temperature from a first temperature that promotes a- and b-axis oriented growth to a second temperature that promotes c-axis oriented growth. Depositing said second perovskite layer while rising, and maintaining said deposition temperature at said second temperature while continuing said second deposition step. 10. A method of writing and rejuvenating a ferroelectric memory, comprising the following steps: a single crystal ferroelectric material for writing data to a ferroelectric memory;
A first step of applying a voltage pulse at a first amplitude and a first duration to said memory consisting of two electrodes; and
A second step of applying a voltage pulse to the ferroelectric memory with an amplitude equal to the first amplitude and a second duration longer than the first duration to rejuvenate the memory for the step. 11. The method according to claim 10, wherein each electrode is made of a perovskite thin film of the same material which is similarly oriented. 12. A method of growing an oriented perovskite thin film, comprising the steps of: depositing a perovskite thin film on a crystalline substrate; forming an a- and b-axis oriented film during the deposition. Forming a first portion of the thin film while gradually changing from a first growth environment for promoting the formation of a c-axis oriented film to a second growth environment for promoting the formation of a c-axis oriented film; Forming the second portion of the thin film, and forming the second portion as an a, b-axis oriented thin film. 13. The method of claim 12, wherein said first and second portions of said thin film comprise a conventional superconducting material. 14. The method of claim 12, wherein said second growth environment includes a deposition temperature higher than a deposition temperature in said first growth environment. 15. The method of claim 14, wherein the changing step changes the deposition temperature while the depositing step deposits a plurality of unit cells of the thin film. 16. The method of claim 12, wherein said depositing step deposits said thin film on a ferroelectric layer, said ferroelectric layer having at least 80% c-axis orientation.