JPH10303386A - Manufacture of ferroelectric thin film and ferroelectric thin film element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form a ferroelectric film for a short time at a lower temperature than the conventional annealing temperature, by forming an amorphous film containing metallic elements constituting a dielectric oxide on a substrate, processing the amorphous film in a nitrogen monoxide gas atmosphere thermally, and forming a crystalized ferroelectric thin film. SOLUTION: A target is set in a chamber and a substrate 1 in which a thermal oxide film 2, a Ta film 3 and a lower electrode layer 4 are formed successively is set to a substrate holder opposite thereto. An Ar gas is introduced into the chamber, and the substrate 1 is sputtered for 13 minutes at a sputter rate of 15 nm/min at a throw-in power of 200 W so as to form a film thereon while adjusting an amorphous film made of STB of 200 nm in film thickness to be Sr/Ta=0.4 and Bi/Ta=1.4. Then, the obtained amorphous film is heated at 550-750 deg.C for 30 minutes through RTA method for crystalization, so that a ferroelectric film 5 is obtained. A nitrogen monoxide (N2 O) gas is used as an annealing atmosphere.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for manufacturing a ferroelectric thin film and a ferroelectric thin film element. More specifically, the present invention relates to a method of manufacturing a ferroelectric thin film element used for a ferroelectric memory element, a pyroelectric sensor element, a piezoelectric element, and the like, and a ferroelectric thin film element.

[0002]

2. Description of the Related Art In recent years, by utilizing spontaneous polarization characteristics of ferroelectrics for memories in recent years, the operating speed and operating speed have been improved compared to conventional nonvolatile memories such as EEPROMs and flash memories. A ferroelectric nonvolatile memory in which the number of times of data rewriting has been dramatically improved has been realized. In addition, by using high dielectric constant characteristics,
Higher integration of semiconductor elements such as DRAMs has been achieved by reducing the size of capacitors, and gigabit-class devices have been prototyped.

As described above, in order to apply a ferroelectric material to devices such as various semiconductor elements, it is essential to develop a technique for thinning a ferroelectric material suitable for a conventional semiconductor process. In other words, the development of ferroelectric materials and their thinning technology capable of realizing desired characteristics with a thin film thickness by lowering the film forming temperature and densifying and flattening the thin film and capable of responding to fine processing and a reduction in operating voltage. Is desired.

[0004] As the ferroelectric material, conventional lead zirconate titanate _{(Pb (Zr 1-X Ti} x) O 3; PZT) has been widely used. However, PZT has problems such as a large deterioration (film fatigue) of ferroelectric characteristics due to repeated polarization inversion. The rewriting life of the data of the ferroelectric memory, when you try to guarantee 10 years as in the conventional DRAM, in 100ns cycle time of 10 ¹⁵ times rewriting degradation of the ferroelectric properties with respect to (poled) It is necessary that there is no. However, if the element of a capacitor structure using Pt which are used as electrode materials in general, endurance of PZT is insufficient as about 10 ⁸ times. Recently, it has been reported that the use of an oxide electrode such as IrO ₂ / Ir can prolong the service life and improve it to about 10 ¹² times.

On the other _{hand, Bi 2 A m-1 B} m O 3m + 3 (A is Na,
K, Pb, Ca, Sr, Ba or Bi; B is Fe, T
Among materials made of a Bi-based layered compound represented by i, Nb, Ta, W or Mo), strontium bismuth tantalate (SrBi ₂ Ta ₂ O ₉ , hereinafter) Ferroelectric materials having a layered perovskite crystal structure, such as bismuth titanate (Bi ₄ Ti ₃ O ₁₂ ) and the like, have attracted attention because of their strong resistance to film fatigue. It is being actively performed.

[0006] In particular, the SBT uses a Pt electrode,
Endurance over ¹² times has been reported (PCT / US92 / 1
0542). Conventionally, most studies on thinning of SBT have been based on a coating film forming method called a sol-gel method or a MOD method, but recently, a film forming technique such as a MOCVD method or a sputtering method has been studied.

[0007] In particular, according to the report of the sol-gel method, by shifting the composition of Sr and Bi in the raw material solution from the stoichiometric composition, the ferroelectric characteristics are improved as compared with the case of using the stoichiometric raw material solution. (Japanese Patent Laid-Open No. 8-3)
No. 19160). However, in these film forming methods, in order to finally crystallize a thin film and obtain sufficient characteristics, O
_It is necessary to perform a heat treatment at a high temperature of 800 ° C. or more in a _two- gas atmosphere for a long time of one hour or more, and it is difficult to apply it to a semiconductor device as it is. That is, since a long-term high-temperature process is required for film formation, reactions with other materials constituting the device occur during film formation, and the grain size increases, which is not suitable for fine processing. Problems arise.

For example, when a ferroelectric capacitor is applied to a non-volatile memory as a memory capacitor, a stack in which a selection transistor and a ferroelectric capacitor are connected by a contact plug using a material such as polysilicon for high integration. A type memory cell structure is desired. However, the high-temperature process causes problems such as a reaction between the polysilicon plug and the capacitor electrode, a contact failure due to oxidation of the polysilicon, and characteristic deterioration.

As described above, in order to apply a ferroelectric material to an integrated circuit, it is essential to form these ferroelectric materials at a low temperature and to realize densification and thinning. Therefore, in the present invention, in the crystallization of the ferroelectric thin film, the heat treatment temperature for the crystallization is reduced and the processing time is shortened in the heat treatment process. An object of the present invention is to provide a method of manufacturing a ferroelectric thin film, a method of manufacturing a ferroelectric thin film element, and a ferroelectric thin film element that are effective for obtaining an excellent ferroelectric thin film.

[0010]

According to the present invention, after an amorphous film containing a metal element and an oxygen element constituting an oxide ferroelectric is formed on a substrate, the amorphous film is sub-oxidized. A method for manufacturing a ferroelectric thin film, comprising forming a crystallized ferroelectric thin film by heat treatment in a nitrogen gas atmosphere.

Further, a ferroelectric thin-film element obtained by the above-mentioned method for manufacturing a ferroelectric element is formed on a substrate having an integrated circuit on its surface, and is used as a part of an element constituting the integrated circuit. A ferroelectric thin film device is provided.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION In the method for producing a ferroelectric thin film of the present invention, a crystallized ferroelectric thin film can be obtained. This ferroelectric thin film is not particularly limited as long as it is a crystallized thin film having ferroelectric properties.
For example, a Bi-based ferroelectric having a layered perovskite crystal structure can be used. The Bi-based _{ferroelectric, Bi 2 A m-1 B} m O 3m + 3 (A is Na, K, Pb, Ca, Sr, Ba and Bi; B
Is selected from Fe, Ti, Nb, Ta, W, and Mo, and m is a natural number.

More specifically, Bi_FourTi_ThreeO₁₂, SrBi_Two
Ta_TwoO₉, SrBi_TwoNb_TwoO₉, BaBi_TwoNb_TwoO₉, B
aBi_TwoTa_TwoO₉, PbBi_TwoNb_TwoO₉, PbBi_TwoTa_Two
O₉, PbBi_FourTi_FourO_Fifteen, SrBi_FourTi_FourO_Fifteen, Ba
Bi_FourTi_FourO_Fifteen, PbBi_FourTi_FourO_Fifteen, Sr_TwoBi_FourTi
_FiveO₁₈, Pb_TwoBi_FourTi_FiveO₁₈, Na_0.5Bi_4.5Ti_FourO
_Fifteen, K_0.5Bi_4.5Ti_FourO_FifteenEtc., among which Sr
Bi_TwoTa_TwoO₉(SBT) is preferred.

In the present invention, the term "amorphous film containing a metal element and an oxygen element constituting a ferroelectric substance" means an amorphous film comprising two or more of the above-mentioned metal elements and an oxygen element. I do. Such an amorphous film can be formed by a known method, for example,
Sol-gel method, reactive evaporation method, EB evaporation method, sputtering method,
The film can be formed by selecting a method such as a laser ablation method, and among them, a sputtering method is preferable.

Specifically, in the sputtering method, a single composite oxide target containing a metal element and an oxygen element constituting a ferroelectric substance, a composite metal target containing these metal elements, and a ferroelectric substance are formed. An amorphous film is formed using an inert gas, for example, argon or a mixed gas of argon and oxygen, by sequentially or simultaneously using binary or ternary or more targets containing one or more of the metal elements. Method. Regardless of which target is used, the composition ratio of the target is adjusted so that the amorphous film has a desired stoichiometric composition ratio, or the crystallized ferroelectric thin film finally obtained has a desired stoichiometric composition. So that the composition ratio
It can be adjusted appropriately. In addition, various conditions such as a pressure and a sputtering rate at the time of sputtering, a composition of a mixed gas of argon and oxygen, and the like can be appropriately adjusted so as to obtain a film and a film quality having a desired composition ratio.

The sol-gel method includes a known method, for example, a method of forming an amorphous film using an alkoxy or salt of a metal element constituting a ferroelectric. EB
Examples of the vapor deposition method include a method in which a single or multiple vapor deposition source containing a ferroelectric element is heated and evaporated in a vacuum to form an amorphous film on a substrate. In general, an element constituting a ferroelectric has a high melting point, and an EB (electron beam) vapor deposition method of heating while scanning an electron beam on a vapor deposition source is generally used in order to avoid mixing of impurities. Regardless of which evaporation source is used, the evaporation rate is controlled by adjusting the electron beam acceleration voltage and thermionic current amount so that the composition of the thin film formed on the substrate has a desired composition ratio. Then, the composition ratio can be changed. Further, by firing the formed thin film in oxygen or a gas containing oxygen, an oxide ferroelectric thin film can be formed.

In the reactive evaporation method, when a thin film is formed on a substrate by the above-described EB evaporation method, an evaporation source is heated and evaporated in an atmosphere containing oxygen gas to cause an oxidation reaction on the substrate to form an oxide thin film. It is a method of forming. Also in this case, the deposition rate can be controlled and the composition ratio can be changed by adjusting the acceleration voltage of the electron beam and the current amount of thermionic electrons so that the composition of the formed thin film has a desired composition ratio. In addition, the film thickness, film quality, and the like can be appropriately adjusted depending on the oxygen concentration and the pressure at the time of film formation.

In the laser ablation method, a single composite ceramic target containing a metal element constituting a ferroelectric and an oxygen element, a composite metal target containing these metal elements, and one or more of a metal element constituting a ferroelectric are provided. A binary or ternary target including two or more types is sequentially or simultaneously performed, for example, by irradiating the target with a laser in an atmosphere of a mixed gas of argon or argon and oxygen, and locally heating and evaporating the target to form a substrate. This is a technique for forming a thin film on the top. Also at this time, the composition ratio, film quality, and the like can be appropriately adjusted by laser output, atmosphere gas composition, gas pressure, and the like so that the composition of the formed thin film has a desired composition ratio.

When forming an amorphous film, there is no particular limitation as long as the film formation temperature (atmospheric temperature, substrate temperature, etc.) does not cause crystallization, but the film formation temperature is low. For example, a temperature of room temperature to about 100 ° C. is preferable. If the film formation temperature is higher than 100 ° C., a part of the metal elements constituting the ferroelectric will not be sufficiently taken into the film, and the resulting amorphous film will not have a desired composition ratio.

In the present invention, the amorphous film obtained above is heat-treated in a nitrous oxide gas atmosphere. In the nitrous oxide gas atmosphere, the nitrous oxide gas is 100%
Is preferably in an atmosphere of 90% to 100%
It is sufficient that the nitrous oxide gas is contained within the range described above. The heat treatment can be performed by a known annealing method such as an RTA method, a laser annealing method, and a furnace annealing method. The temperature at this time is not particularly limited as long as it is a temperature sufficient for crystallization, but is preferably as low as possible, for example, 750 ° C or lower, preferably 600 to 750 ° C, more preferably 600 to 700 ° C. , More preferably 600
A temperature range of ６650 ° C. The heat treatment time can be appropriately adjusted depending on the heat treatment temperature, the heat treatment method, and the like, and for example, about 1 second to 60 minutes.

The above ferroelectric thin film can be used as a ferroelectric capacitor element. In that case, for example, on a substrate provided with an electrode layer made of a conductive thin film,
The above-described amorphous film and the electrode layer may be sequentially formed and heat-treated to form the amorphous film into a crystallized ferroelectric thin film, or the above-described amorphous film may be formed on a substrate having the electrode layer. After the formation and the formation of the crystallized ferroelectric thin film by heat treatment, the electrode layer may be formed.

The substrate is not particularly limited as long as it can be generally used as a substrate for a semiconductor device or an integrated circuit. For example, a semiconductor substrate such as silicon, a compound semiconductor substrate such as GaAs, It can be selected according to the type and use of the element to be formed, such as an oxide crystal substrate such as MgO or a glass substrate. Among them, a silicon substrate is preferable.

The electrode layer provided on the substrate is formed, for example, as a lower electrode of a capacitor, and is usually formed of a conductive thin film formed as an electrode. When forming, it is not particularly limited as long as it is a material that can withstand the film forming process. For example, Ta, Ti, Pt, Pt /
Ti, Pt / Ta, and the like. The thickness of the electrode layer is not particularly limited, and can be appropriately adjusted depending on the size of an element to be formed. The electrode layer can be formed by a known method such as a sputtering method and an evaporation method. This electrode layer may be formed directly on the substrate, or may be an insulating film, a lower wiring, a desired element,
It may be formed on an interlayer insulating film or a substrate provided with a plurality thereof.

Further, an electrode layer is formed on the amorphous film or the crystallized ferroelectric film. This electrode layer is formed, for example, as an upper electrode of a capacitor, and its material, forming method, and the like are as described above. Note that a ferroelectric capacitor can be formed by performing a desired wiring step, an insulating film step, and the like on this electrode layer.

The above-mentioned ferroelectric thin film can be used for an integrated circuit as a part of the structure of a ferroelectric device or a semiconductor device in addition to a ferroelectric capacitor element. For example, an MFMIS-FET is formed by applying a ferroelectric element as a capacitance section of a nonvolatile memory and applying a ferroelectric element to a gate section of an FET and forming a gate insulating film, a source / drain region, and the like in combination. , MFS
-It can also be used as a FET or the like.

Hereinafter, a method for manufacturing a ferroelectric thin film and a ferroelectric thin film element according to the present invention will be described in detail with reference to examples. Embodiment 1 In this embodiment, a case where a ferroelectric thin film is formed by a sputtering method will be described.

FIG. 1 shows a capacitor structure element to which a ferroelectric thin film is applied. In this capacitor structure element, a thermal oxide film 2 of SiO ₂ , a Ta film 3 as an adhesive layer, and a lower electrode layer 4 of Pt are sequentially formed on a substrate 1 having a silicon single crystal (100) plane. 4 on the SBT
, And an upper electrode layer 6 made of Pt.

A method of manufacturing a capacitor structure element using a ferroelectric thin film will be described below. In this example,
One composite oxide target containing Sr, Bi, and Ta was used as a sputtering target. The composition of this composite oxide target was such that the excess amount of Bi was 40% with respect to the stoichiometric composition ratio SrBi ₂ Ta ₂ O ₉ , while Sr was reduced by 20%. The reason for making Bi excessive is that the deposited Bi re-evaporates due to an increase in the substrate temperature due to the collision of the secondary electrons emitted from the target, and furthermore, the evaporation temperature of Bi is higher than that of other elements. The reason for this is that Bi is insufficient in the obtained film because it is easily evaporated during annealing for crystallization because it is low. Also,
At this time, the excess amount of Bi in the thin film is effectively 0% to 40%. When the content is less than 0%, that is, when Bi is less than the stoichiometric composition, sufficient ferroelectric characteristics cannot be obtained. On the other hand, when the content is more than 40%, the leakage current increases, and This is because characteristics cannot be measured. Further, the effective amount of Sr is -20% to 0%.
If the Sr content is more than the stoichiometric composition, sufficient ferroelectric properties cannot be obtained. On the other hand, if the Sr content is less than 20%, appropriate properties such as increased leak current cannot be obtained.

First, as shown in FIG.
Was set in a chamber 8, and a substrate 1 on which a thermal oxide film 2, a Ta film 3, and a lower electrode layer 4 were sequentially formed was set in a substrate holder 7 on the opposite surface. Thereafter, the chamber 8 was evacuated to about 10 ^-5 Torr. Next, FIG.
As shown in Step 1 (S1), Ar gas was introduced into the chamber, and the main valve was adjusted to adjust the sputtering gas pressure to a desired pressure on the order of 10 ^-3 Torr. After introducing the gas, input power 200W, sputtering rate 15nm
/ Min for 13 minutes, and a film thickness of 200
nm of SBT amorphous film, Sr / Ta = 0.4,
The film was formed while adjusting it so that Bi / Ta = 1.4. The substrate temperature at this time was room temperature.

Subsequently, in order to crystallize the obtained amorphous film, as shown in Step 2 (S2) of FIG.
Heat treatment was performed by the method. The temperature at this time is 550 ° C. to 7
The temperature was 50 ° C., and the time was 30 minutes. As an annealing atmosphere, a 100% nitrous oxide (N ₂ O) gas was used. XRD patterns of the obtained ferroelectric thin film 5 are shown in FIGS. FIG. 4 to FIG. 8 show that in nitrous oxide gas,
X at annealing temperatures from 580 ° C to 660 ° C
It is a figure showing change of an RD chart. According to this, 6
A crystallization peak of SBT is observed from 00 ° C., and it can be seen that the peak showing SBT (105) particularly increases as the temperature is further increased. From this, it can be said that crystallization starts at a temperature of 600 ° C. or more in nitrous oxide. As this cause, N ₂ O is thermally decomposed, strong oxygen radicals oxidizing power, it is the oxygen ion or active oxygen, it is believed that causing crystallization even at a low temperature to act.

As a comparative example, XRD patterns obtained by annealing in an atmosphere in which oxygen gas (100%) was used instead of nitrous oxide gas and the other conditions were completely the same as above were shown in FIGS. 11 is shown. 9 to 11
FIG. 4 is a diagram showing a change in an XRD chart when an annealing temperature is from 680 ° C. to 720 ° C. in oxygen gas.
According to this, a crystallization peak of SBT was observed from 720 ° C.

As is clear from these results, it was shown that the annealing using nitrous oxide gas can lower the crystallization temperature by 100 ° C. or more as compared with the annealing using oxygen gas. Therefore, since the ferroelectric thin film can be crystallized at a very low temperature, a ferroelectric thin film having good characteristics can be formed without adversely affecting the lower electrode layer, the transistor, and the like. In addition, since the heat treatment is performed at a low temperature, the growth of crystal grains is suppressed and a dense film structure is obtained, which is very advantageous in terms of microfabrication of the device, and thus the ferroelectric material formed by the method of the present invention. It is easy to form a thin film in an integrated circuit, which is advantageous in device fabrication.

Embodiment 2 In this embodiment, shortening of the annealing time when forming a ferroelectric thin film will be described. As in Example 1, a thermal oxide film 2, a Ta film 3, and a Pt film as the lower electrode layer 4 were formed on the surface of the substrate 1, and a ferroelectric thin film 5 was formed on the substrate 1.

At this time, the ferroelectric film 5 is formed by the same sputtering method as in the first embodiment, and then annealing for crystallization is performed at 660 ° C. in a nitrous oxide gas at 660 ° C.
The annealing time was varied between minutes and 30 minutes. Thereafter, evaluation was performed using XRD in the same manner as in Example 1. The results are shown in FIGS. According to this,
Even when the crystallization annealing temperature is 660 ° C., a crystallization peak of SBT is observed from 5 minutes.

As a comparative example, annealing was performed in the same manner as above except that annealing was performed in oxygen. In this case, the SBT peak sharply decreased by shortening the time at 720 ° C. for less than 30 minutes, and the SBT peak disappeared in 20 minutes. From this result, it was found that annealing with nitrous oxide was effective not only for lowering the temperature but also for shortening the time. Further, it is considered that the cause is also the action of oxygen radicals, oxygen ions, or active oxygen, and it has been found that these crystals can be sufficiently crystallized even in a short time due to their strong oxidizing power.

Therefore, since the ferroelectric thin film can be crystallized at a very low temperature in a short time, it is possible to form a ferroelectric thin film having good characteristics without adversely affecting the lower electrode layer and the transistor. it can. In addition, since the heat treatment is performed at a low temperature, the growth of crystal grains is suppressed and a dense film structure is obtained, which is very advantageous in terms of microfabrication of the device, and thus the ferroelectric material formed by the method of the present invention. It is easy to form a thin film in an integrated circuit, which is advantageous in device fabrication.

Embodiment 3 In this embodiment, the electrical characteristics of a device using a ferroelectric thin film will be described. As shown in FIG. 1, a thermal oxide film 2, a Ta film 3, and a Pt film serving as a lower electrode layer 4 are formed on the surface of a substrate 1.
A ferroelectric thin film 5 and a Pt film serving as the upper electrode layer 6 were formed to form an element having a capacitor structure. First, as shown in Step 1 (S1) in FIG. 16, the composition was formed on the substrate 1 on which the thermal oxide film 2, the Ta film 3, and the Pt film as the lower electrode layer 4 were formed in the same manner as in Example 1. The ratio is Sr / Ta
= 0.4, Bi / Ta = 1.2, film thickness 200n
m of amorphous film was formed.

Subsequently, step 2 (S
As shown in 2), the upper electrode layer 6 of Pt was mask-deposited by EB (Electric Beam) evaporation. The size of the electrode depends on the diameter of the mask, and an electrode layer of 100 μmφ was used as an electrode size for evaluating ferroelectric characteristics. Then, FIG.
As shown in Step 3 (S3) in Step 6, annealing for crystallization and stabilization with the electrode interface is performed.
The capacitor structure was completed. Annealing is 550 ° C
Performed in a temperature range of ７750 ° C. for 30 minutes. The atmosphere at this time was nitrous oxide (100%).

With respect to the capacitor obtained above, FIG.
In the Sawyer Tawa bridge circuit as shown in Fig. 7, the applied voltage
Is changed between 1 and 12 V, using an oscilloscope.
The hysteresis curve of the ferroelectric thin film was displayed. Oscilloscope
The voltage applied to the ferroelectric thin film is displayed on the horizontal axis terminal of the scope.
Voltage V obtained by dividing V_XIs entered. Ferroelectric capacity
Capacitor C connected in series with_RIs the reference key
It is Japashita. Here, the polarization surface charge density is P, the true charge
When the area density is D, (P + ε_OE) × A, that is, D ×
A and charge C stored in the reference capacitor_RV_XAre both Q
Since the voltage is proportional to D,_Y(DA /
C_R) Is entered. In ferroelectrics, P is ε_OCompared to E
Since they are all sufficiently large, it can be assumed that D = P. This
V_Y-V _XThe curve is a known amount of film thickness t, partial pressure ratio, electrode
Area A, capacitance C of reference capacitor_RScale using
If corrected, a DE or PE hysteresis curve is obtained.
It is. From there, the residual spontaneous polarization (Pr) and the coercive electric field (E
The value of c) was read.

FIG. 18A shows the annealing temperature dependence of Pr obtained by applying 3V. According to this,
For the sample annealed at 750 ° C., Pr = 6.5 μC / in both oxygen and nitrous oxide atmospheres.
The value is about cm ² . However, for samples annealed at 650 ° C., oxygen does not show ferroelectricity,
With N ₂ O, a value of about Pr = 5.5 μC / cm ² was obtained. This is due to the effect of lowering the crystallization temperature.

In oxygen annealing, Pr is about 2.2 at 700 ° C., whereas in annealing using nitrous oxide, the same value can be realized at 600 ° C. Therefore, also from the viewpoint of electrical characteristics, the annealing temperature of 10
A low temperature of 0 ° C. was realized. This is due to the action of active oxygen, oxygen ions or oxygen radicals, which have a strong oxidizing power due to thermal decomposition from nitrous oxide, and is effective not only in reducing the crystallization temperature but also in improving the electrical characteristics. Do you get it.

Next, the leakage current density will be described.
When the power is turned off as a memory device, there is a non-volatility peculiar to the ferroelectric memory. In the case of an NVDRAM or the like which operates as a DRAM during a normal operation, there is a problem that a large leak current shortens a refresh time.
If the leakage current can be reduced by several orders of magnitude while keeping the accumulated charge constant, the refresh time during DRAM operation can be lengthened and the element characteristics can be greatly improved. Also,
When the leakage current increases, the electric field applied to the ferroelectric thin film decreases, and problems such as insufficient polarization inversion occur.

When the leak current densities shown in FIG. 18B were compared, when annealing was performed at 750.degree. C., the values were in the same range of ^{10.sup.-8 in} both oxygen and nitrous oxide atmospheres. However, when annealing at 650 ° C., annealing in oxygen increases the leakage current value by two orders of magnitude, while annealing in nitrous oxide atmosphere maintains almost the same value as 750 ° C. Was.

In addition, the breakdown voltage of the sample annealed in a nitrous oxide atmosphere does not cause dielectric breakdown up to about 20 V, whereas the sample performed only with oxygen has a breakdown voltage of 7 V.
Leakage current gradually increased from around V, causing dielectric breakdown at 9V. As described above, annealing in a nitrous oxide atmosphere improved both the leak characteristics and the voltage resistance. It is considered that the reason for this is that the crystal grains were sufficiently oxidized and the crystal grains became dense, and the amorphous region of the grain boundary was reduced.

In the above results, the crystallization annealing was performed after the electrodes were formed. However, the same results were obtained when the crystallization annealing was performed before the electrodes were formed and the electrodes were formed thereafter.

Embodiment 4 (Ferroelectric Nonvolatile Memory Element) In this embodiment, an example in which a ferroelectric thin film formed by the method used in the above embodiment is applied to an actual electronic device will be described. First, an embodiment in which the above-mentioned ferroelectric thin film is used in a nonvolatile memory having a capacitor structure is shown in FIG.
FIG. 19A shows an equivalent circuit thereof. The nonvolatile memory having this capacitor structure is composed of one capacitor 30.
And one selection transistor 36.
Here, the capacitor 30 includes a ferroelectric thin film 38 and a pair of upper and lower electrode layers 33 and 32 sandwiching the ferroelectric thin film 38. The transistor 36 is connected to the source region 35 connected to the bit line 35a and the word line 34a. And a drain region 37 connected to the Al wiring layer 31. The Al wiring layer 31 is also connected to the upper electrode layer 33 of the capacitor 30.

A method for manufacturing a nonvolatile memory having the above-described capacitor structure will be described. First, a field oxide film 39 for element isolation is formed on an n-type Si substrate 1. Thereafter, the source region 3 is formed by a known MOS fabrication process.
5 and a drain region 37, and further, a gate insulating film and a gate electrode 34 are formed to form a MOS transistor. Further, the surface of the substrate on which the transistor is formed is covered with a PSG (silicate glass) film 40 as an interlayer insulating film, and then flattened by a reflow process.

After forming the lower electrode layer 32 thereon, a ferroelectric thin film 38 and an upper electrode layer 33 are sequentially formed. Then, after covering again with PSG and performing reflow, contact holes are formed on the drain region 37 and the upper electrode layer 33, and finally, the Al wiring layer 31 is provided. Hereinafter, the operation of the nonvolatile memory having the capacitor structure will be described.

When writing information "1", the bit line 3
From 5a, a negative voltage pulse equal to or higher than the coercive electric field is applied to the ferroelectric thin film 38 via the selection transistor 36. Thereby, the ferroelectric thin film 38 is polarized, and the direction of spontaneous polarization is directed to the lower electrode layer 32 side of the capacitor 30. on the other hand,
When writing information "0", a positive voltage pulse higher than the coercive electric field is applied to the ferroelectric thin film 38 from the bit line 35a via the transistor. As a result, the ferroelectric thin film 38 is polarized, and the direction of spontaneous polarization is
To the upper electrode layer 33 side.

In order to read such information, the polarization direction of the ferroelectric thin film 38 is detected. When a positive voltage pulse is applied to the lower electrode layer 32 in a state where the information “1” is written, the direction of the spontaneous polarization of the ferroelectric thin film 38 is reversed.
At this time, an inversion current accompanying the polarization inversion flows through the capacitor 30. On the other hand, even if a positive voltage pulse is applied to the lower electrode layer 32 in a state where the information “0” is written, the spontaneous polarization direction of the ferroelectric thin film 38 does not change. The reversal current is smaller than the above reversal current. Therefore, the current flowing through the ferroelectric thin film 38 when a voltage pulse is applied is detected by a sense amplifier (not shown) connected to the bit line, and a threshold is provided between the magnitude of the inversion current and the magnitude of the non-inversion current. Thus, it can be determined whether the written information is “1” or “0”.

Since the spontaneous polarization state is maintained in the ferroelectric even when the electric field is cut off, the operation as a nonvolatile memory becomes possible. It should be noted that it is possible to operate the DRAM as a DRAM by using only the high dielectric constant characteristic of the ferroelectric substance with the same structure. As a result, it is possible to normally perform the DRAM operation and the nonvolatile operation only when the power is turned off.

Embodiment 5 (FET type ferroelectric nonvolatile memory) In this embodiment, non-destructive reading is possible by using a ferroelectric thin film as a gate insulating film of a field effect transistor (FET). MFMIS (Metal Ferroelect
ric Metal Insulater Semiconductor) structure will be described.

FIG. 20 shows an FET type ferroelectric nonvolatile memory in this embodiment. This ferroelectric nonvolatile memory has a structure in which a ferroelectric thin film gate is provided between a source region 43 and a drain region 44. Here, the ferroelectric thin film gate includes a gate insulating film (SiO ₂ ) 45 in contact with the channel portion of the silicon substrate 1, a floating gate 46, and a ferroelectric thin film (S) formed thereon.
rBi ₂ Ta ₂ O ₉ ) 38, a control gate 47 and an Al wiring layer 42 connected to the control gate 47.

Next, the operation of the FET type nonvolatile memory will be described. First, information is written by applying a positive or negative pulse voltage to the control gate 47 and setting the spontaneous polarization direction of the ferroelectric thin film 38. Since the gate insulating film 45 is also dielectrically polarized by the spontaneous polarization of the ferroelectric thin film 38, there are two states in which a depletion layer is generated on the semiconductor surface immediately below the gate according to the direction of the spontaneous polarization. That is, the current flowing between the source region 43 and the drain region 44 can be turned on and off depending on the direction of spontaneous polarization of the ferroelectric thin film 38. This makes it possible to read out the direction (information) of the spontaneous polarization by measuring the current between the source and the drain to determine whether the current is in the ON state or the OFF state.

For example, when the control gate 47 is in the zero bias state, it is assumed that the floating gate 46 side of the ferroelectric thin film 38 is polarized so as to have a negative polarity. In this case, the gate insulating film 45 is dielectrically polarized, the surface in contact with the substrate 1 becomes negative, the surface of the substrate 1 in contact with the gate insulating film 45 becomes positive, and the drain region 43 and the source region are not connected (OFF Status).

On the other hand, a positive voltage larger than the coercive electric field of the ferroelectric thin film 38 is applied to the control gate 47. As a result, the polarization direction of the ferroelectric thin film 38 is reversed, and the floating gate 46 is polarized so as to have a positive polarity. In this case, the gate insulating film 45 is dielectrically polarized, and the surface in contact with the substrate 1 becomes a positive electrode. The surface of the substrate 1 that is in contact with the gate insulating film 45 has a negative polarity, and the drain region 43 and the source region 44 are connected (ON state).

In this case, since the read operation does not involve polarization inversion, the read operation is nondestructive. Further, since the spontaneous polarization is maintained even when the gate voltage is turned off, a nonvolatile memory operation becomes possible.

[0058]

According to the present invention, after an amorphous film containing a metal element and an oxygen element constituting a ferroelectric substance is formed on a substrate, the amorphous film is heat-treated in a nitrous oxide gas atmosphere. Thus, the crystallized ferroelectric thin film is formed, so that the ferroelectric thin film can be formed in a short time within a temperature range lower than the conventional annealing temperature. Therefore, a ferroelectric thin film with good characteristics can be formed without adversely affecting an element such as an electrode layer and a transistor formed with the ferroelectric film.

Further, the crystallized ferroelectric thin film is made of Bi _{2 Am-1.}
B _m O _{3m + 3} (A is Na, K, Pb, Ca, Sr, Ba and Bi; B is selected from Fe, Ti, Nb, Ta, W and Mo, and m is a natural number) In particular, when the compound has a layered perovskite crystal structure represented by the following formula, low-temperature crystallization is effective. Further, the amorphous film is heated at 100 ° C.
When formed on a substrate having the following substrate temperature by a dry process sputtering film forming method using a sputtering gas composed of an argon gas or a mixed gas of argon and oxygen, carbon as an organic component in a conventional coating film formation is used. Such impurities can be avoided, and the reproducibility of the film composition can be improved by forming the film at a low temperature.

Further, the amorphous film is made of an alloy containing Bi, Sr and Ta elements or a raw material target containing these oxides, and the Sr / Ta ratio is 0.4 to 0.5 and the Bi / Ta ratio is 1.0 to 1.4, and SrBi ₂ Ta ₂ O
_In the case of forming an amorphous film deviating from the stoichiometric composition of No. _9, the composition of the amorphous film can be arbitrarily deviated from the stoichiometric composition, and the ferroelectric characteristics can be controlled.

Further, using a substrate on which an electrode layer made of a conductive thin film is formed in advance, and further forming an electrode layer made of a conductive thin film on the formed crystallized ferroelectric thin film,
After forming an amorphous film using a substrate on which an electrode layer made of a conductive thin film has been formed in advance, an electrode layer is further formed on the amorphous film, and the obtained substrate is heat-treated to crystallize ferroelectric substance. When a body thin film is formed, a ferroelectric thin film element having more stable ferroelectric characteristics can be formed according to the oxidation resistance of an electrode layer formed thereon.

Further, by applying the ferroelectric thin film obtained by the present invention to various elements, the ferroelectric thin film is given to other integrated circuit element parts by lowering the crystallization temperature and shortening the crystallization temperature. Influence (heat damage) can be reduced,
Various devices with high reliability can be realized.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view showing an example in which a ferroelectric thin film obtained by a method of manufacturing a ferroelectric thin film of the present invention is applied to an element.

FIG. 2 is a schematic diagram of a sputtering apparatus for forming a ferroelectric thin film.

FIG. 3 is a flowchart showing steps of manufacturing a ferroelectric thin film of the present invention.

FIG. 4 is a diagram showing an XRD pattern of a ferroelectric thin film when crystallized at an annealing temperature of 580 ° C. in one embodiment of the method of the present invention.

FIG. 5 is a diagram showing an XRD pattern of a ferroelectric thin film when crystallized at an annealing temperature of 600 ° C. in one embodiment of the method of the present invention.

FIG. 6 is a view showing an XRD pattern of a ferroelectric thin film when crystallized at an annealing temperature of 620 ° C. in one embodiment of the method of the present invention.

FIG. 7 is a view showing an XRD pattern of a ferroelectric thin film when crystallized at an annealing temperature of 640 ° C. in one embodiment of the method of the present invention.

FIG. 8 is a view showing an XRD pattern of a ferroelectric thin film when crystallized at an annealing temperature of 660 ° C. in one embodiment of the method of the present invention.

FIG. 9 shows, as a comparative example, the X of the ferroelectric thin film when crystallized in an oxygen atmosphere at an annealing temperature of 680 ° C.
It is a figure showing an RD pattern.

FIG. 10 is a diagram showing an XRD pattern of a ferroelectric thin film when crystallized at an annealing temperature of 700 ° C. in an oxygen atmosphere as a comparative example.

FIG. 11 is a diagram showing an XRD pattern of a ferroelectric thin film when crystallized at an annealing temperature of 720 ° C. in an oxygen atmosphere as a comparative example.

FIG. 12 is a diagram showing an XRD pattern of a ferroelectric thin film when crystallized with an annealing time of 1 minute in another embodiment of the method of the present invention.

FIG. 13 is a diagram showing an XRD pattern of a ferroelectric thin film when crystallized with an annealing time of 5 minutes in another embodiment of the method of the present invention.

FIG. 14 is a diagram showing an XRD pattern of a ferroelectric thin film when crystallized with an annealing time of 10 minutes in another embodiment of the method of the present invention.

FIG. 15 is a diagram showing an XRD pattern of a ferroelectric thin film when crystallized with an annealing time of 30 minutes in another embodiment of the method of the present invention.

FIG. 16 is a flowchart showing another process for manufacturing the ferroelectric thin film of the present invention.

FIG. 17 is a circuit diagram of a saw tower.

FIG. 18 is a graph showing changes in residual polarization Pr and leak current density with respect to an annealing temperature when annealing is performed in a nitrous oxide atmosphere and an oxygen atmosphere in the method of the present invention.

FIG. 19 is a schematic sectional view and a circuit diagram of a main part showing a nonvolatile memory element having a capacitor structure which is a ferroelectric element of the present invention.

FIG. 20 is MFMIS which is a ferroelectric thin film element of the present invention.
FIG. 3 is a schematic cross-sectional view of a main part showing an FET.

[Explanation of symbols]

 Reference Signs List 1 substrate 2 thermal oxide film 3 Ta film 4, 32 lower electrode layer 5, 38 ferroelectric thin film 6, 33 upper electrode layer 7 substrate holder 8 chamber 9 target 30 ferroelectric capacitor 31, 42 Al wiring layer 34 gate electrode 34a Word line 35, 37, 43, 44 Source / drain region 35a Bit line 36 Transistor 39 Field oxide film 40 PSG film 45 Gate insulating film 46 Floating gate 47 Control gate

Continued on the front page (51) Int.Cl. ⁶ Identification code FI H01L 29/792

Claims (7)

[Claims] An amorphous film containing a metal element and an oxygen element constituting a ferroelectric substance is formed on a substrate, and the amorphous film is heat-treated in a nitrous oxide gas atmosphere to form a crystalline ferroelectric substance. A method for producing a ferroelectric thin film, comprising forming a body thin film. 2. A crystallized ferroelectric thin film, wherein _{Bi 2 A m-1 B m} O 3m + 3 (A is Na, K, Pb, Ca, Sr, Ba and Bi; B
Is selected from Fe, Ti, Nb, Ta, W and Mo, and m is a natural number), and has a layered perovskite crystal structure. 3. An amorphous film is formed on a substrate having a substrate temperature of 100 ° C. or lower by a sputtering film forming method using a sputtering gas composed of argon gas or a mixed gas of argon and oxygen. 3. The method for producing a ferroelectric thin film according to 1 or 2. 4. An amorphous film is formed by using a raw material containing Bi, Sr and Ta elements, an alloy containing these elements, or a raw material target containing oxides of these elements, and a Sr / Ta ratio of 0.4.
0.5, there Bi / Ta ratio in the range of 1.0 to 1.4, and claim 1 which comprises forming a amorphous film deviated from the stoichiometric composition of SrBi ₂ Ta ₂ O ₉ 4. The method for producing a ferroelectric thin film according to any one of items 1 to 3. 5. The method according to claim 1, further comprising using a substrate on which an electrode layer made of a conductive thin film is formed in advance, and further forming an electrode layer made of a conductive thin film on the formed crystallized ferroelectric thin film. 5. The method for producing a ferroelectric thin film according to any one of items 1 to 4. 6. An amorphous film is formed using a substrate on which an electrode layer made of a conductive thin film is formed in advance, and an electrode layer is further formed on the amorphous film, and the obtained substrate is subjected to a heat treatment. And forming a crystallized ferroelectric thin film by heating.
The method for producing a ferroelectric thin film according to any one of the above. 7. A ferroelectric thin film element wherein the ferroelectric thin film according to claim 5 formed on a substrate having an integrated circuit on the surface is used as a part of an element constituting the integrated circuit.