Semiconductor memory device including a semiconductor film made of a material having a spontaneous polarization and method for fabricating the same

A semiconductor memory device is composed of a field effect transistor using the interface between a ferroelectric film and a semiconductor film as the channel and including a gate electrode to which a voltage for controlling the polarization state of the ferroelectric film is applied and source/drain electrodes provided on both ends of the channel to detect a current flowing in the channel in accordance with the polarization state. The semiconductor film is made of a material having a spontaneous polarization and the direction of the spontaneous polarization is parallel with the interface between the ferroelectric film and the semiconductor film.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor memory device comprising a field effect transistor having a gate insulating film composed of a ferroelectric film.

Nonvolatile memories, each using a ferroelectric material, are roughly divided into two types which are a capacitor type and a FET (Field Effect Transistor) type having a gate insulating film composed of a ferroelectric film.

The capacitor type has a structure similar to that of a DRAM (Dynamic Random Access Memory), holds charges in a ferroelectric capacitor, and distinguishes between the “0” and “1” sates of information in accordance with the polarization direction of the ferroelectric material. In the reading of the information, the stored information is destroyed so that an operation of rewriting the information is needed. As a result, each reading operation causes polarization reversal so that polarization reversal fatigue presents a problem. In addition, because polarization charges are read using a sense amplifier in the structure, an amount of charges of not less than the sensing limit (which is typically 100 fC) of the sense amplifier is necessary. Since the polarization charges per unit area of a ferroelectric material are intrinsic to the material, even when a memory cell is miniaturized, an electrode area of a given size is needed as long as the same material is used. Therefore, it is difficult to reduce the size of the capacitor in direct proportion to the miniaturization of process rules, so that the capacitor-type ferroelectric memory is unsuitable for an in crease in capacity.

By contrast, from the FET-type ferroelectric memory, information is read by detecting the conductive state of the channel which changes in accordance with the polarization orientation of the ferroelectric film. This allows non-destructive reading of the information as well as an increase in the amplitude of an output voltage through the amplifying operation of the FET. As a result, compared with the capacitor-type ferroelectric memory, the FET-type ferroelectric memory can be extremely miniaturized in dependence on the scaling law.

There has conventionally been proposed a FET-type transistor in which a ferroelectric film, serving as a gate insulating film, is formed on a silicon substrate serving as the channel. Such a structure is termed a MFS (Metal-Ferroelectric-Semiconductor) FET. However, in contrast to the capacitor-type ferroelectric memory capable of retaining data for about 10 years, data disappears from the conventional MFSFET in about several days. This is conceivably because an excellent interface is not formed between the silicon substrate and the ferroelectric film. In other words, because the ferroelectric film is formed on the silicon substrate at a high temperature, the oxidation of the surface of the silicon substrate and the diffusion of an element into silicon easily occur to prevent the formation of the excellent interface.

To solve the problem, a ferroelectric memory composed of a MFSFET using an oxide semiconductor for a semiconductor layer has been proposed (see Applied Physics Letters Vol. 68, pp. 3650-3652, Jun. 17 (1996) and Applied Physics Letters Vol. 86, pp. 162902-1 to 162902-3, April (2005)). In general, considering that a ferroelectric film is made of an oxide, an oxide layer made of an oxide such as silicon dioxide is not formed in a multilayer structure using an oxide semiconductor for a channel, in contrast to a multilayer structure using silicon for a channel. Accordingly, it can be expected to provide a stable interface state.

FIGS. 14A and 14Bare cross-sectional views each showing a typical structure of a MFSFET using an oxide semiconductor for the channel, of whichFIG. 14Ashows a structure of a back-gate MFSFET in which a gate electrode102is formed below a channel (oxide semiconductor film)104andFIG. 14Bshows a structure of a top-gate MFSFET in which the gate electrode102is formed above the channel104. In the drawings,101denotes a substrate,103denotes a ferroelectric film, and105denotes source/drain electrodes.

To keep up with the miniaturization of a semiconductor integrated circuit with an embedded memory, a structure in which a ferroelectric memory is stacked on a CMOS formed with a selection transistor is desired. In that case, a back-gate structure in which a gate electrode is disposed below a channel, as shown inFIG. 14A, is preferred to a top-gate structure in which a gate electrode is disposed above a channel, as shown inFIG. 14B. This is because, when the back-gate structure is adopted, a region for contact with the CMOS can be reduced and the area of each of memory cells can be reduced. In addition, because the ferroelectric film103and the oxide semiconductor film104can be formed continuously as a multilayer film in the back-gate structure, the achievement of a more stable interface state can be expected.

For the oxide semiconductor film104, Non-Patent document 1 uses tin oxide (SnO2) and Non-Patent Document 2 uses indium tin oxide (ITO). In the case of using SnO2, an On/Off ratio of 60 is obtained. In the case of using ITO, an On/Off ratio of 104is obtained. However, in either case, a long-term data retention characteristic has not been obtained.

Because zinc oxide (ZnO) has an electron mobility higher than that of another oxide semiconductor, when ZnO is used for the channel of a MFSFET, a large on-current is obtained to increase the On/Off ratio. Accordingly, the enlargement of the read margin of a memory can be expected. However, in an actual situation, even when a ZnO film was used for a channel, the obtained On/Off ratio was only about 90 and the obtained retention time was only not more than 104seconds (Japanese Journal of Applied Physics, Vol. 48, pp. L1266-L1269, December (2006).

SUMMARY OF THE INVENTION

As described above, even when ZnO having a high electron mobility is used as an oxide semiconductor, the obtained On/Off ratio is not so large and the retention time thereof is also short. As a result of conducting a dedicated study on the cause thereof, the prevent inventors have noticed the following problem resulting from a characteristic specific to ZnO.

That is, because ZnO is a polar semiconductor having a wurtzite-type crystal structure and has a spontaneous polarization (about 5 μC/cm2) in the c-axis direction (<0001> direction) of the crystal thereof, uneven charge distribution occurs in the crystal. As a result, as shown inFIG. 15, a (0001) surface perpendicular to the c-axis, i.e., a C surface (a surface terminated with an oxygen surface is termed a −C surface and a surface terminated with a zinc surface is termed a +C surface) becomes a polar surface so that a spontaneous polarization oriented in a direction perpendicular to the −C surface is formed.

On the other hand, in the MFSFET with a back-gate structure shown inFIG. 14A, the oxide semiconductor film104is formed by epitaxial growth on the ferroelectric film103. The ferroelectric film103represented by lead zirconium titanate (Pb(Zr1−x,Tix)O3where 0≦x≦1 is satisfied or PZT) is typically formed on the electrode102made of platinum (Pt), iridium, or strontium ruthenate (SrRuO3or SRO) and having a (111) orientation that can be controlled easily. When the PZT film103is grown on the electrode102made of any of the materials shown above, the PZT film103having the (111) orientation is easily obtainable. When the ZnO film104is epitaxially grown on the PZT film103having the (111) orientation, the ZnO film104having a (0002) orientation (c-axis orientation) is obtained, as shown by the X-ray diffraction ofFIG. 16. A conceived reason for this may be that, because the atomic arrangement in the (111) surface of the PZT film103has an equilateral triangular period, the ZnO film104is also likely to have the (0002) orientation in which the atomic arrangement also has an equilateral triangular period. When the ZnO film104having the c-axis orientation is formed on the oxide substrate, the ZnO104film having the −C surface on the film surface side is normally formed. As a result, the spontaneous polarization oriented toward the surface side of the ZnO film104is formed.

When the polarization thus oriented is formed, uneven charge distribution occurs in the ZnO film104so that electrons are reduced at the interface with the ferroelectric film103. That is, due to the polarization of the ferroelectric film103, the spontaneous polarization of the ZnO film104acts in a direction which reduces charges (electrons) localized in the vicinity of the interface when the electrons are induced at the interface between the ferroelectric film103and the semiconductor film104(On state). As a result, it is considered that the on-current decreases and the retention time decreases.

The present invention has been achieved in view of the foregoing finding and a primary object thereof is to provide a semiconductor memory device comprising a MFSFET having a large On/Off ratio and an improved retention characteristic by reducing the influence of the spontaneous polarization of a semiconductor film.

To attain the object, the semiconductor memory device according to the present invention adopts a structure using a semiconductor film having a non-polar surface orientation for the channel of a field effect transistor using the interface between a ferroelectric film and the semiconductor film as the channel.

Specifically, a semiconductor memory device according to the present invention comprises a field effect transistor using an interface between a ferroelectric film and a semiconductor film as a channel, wherein the field effect transistor comprises: a gate electrode to which a voltage for controlling a polarization state of the ferroelectric film is applied; and source/drain electrodes provided on both ends of the channel to detect a current flowing in the channel in accordance with the polarization state, wherein the semiconductor film is made of a material having a spontaneous polarization and a direction of the spontaneous polarization is parallel with the interface between the ferroelectric film and the semiconductor film.

In such a structure, the spontaneous polarization of the semiconductor film is oriented in a direction perpendicular to a surface of the ferroelectric film. As a result, it is possible to suppress a reduction in the polarization of the ferroelectric film due to the spontaneous polarization and thereby obtain the semiconductor memory device comprising a MFSFET having a large On/Off ratio and an improved retention characteristic.

In a preferred embodiment, the semiconductor film is formed on the ferroelectric film by epitaxial growth and the direction of the spontaneous polarization is controlled by a crystal orientation of the ferroelectric film.

With such a structure, it is possible to easily control the direction of spontaneous polarization of the semiconductor film and bring the interface between the ferroelectric film and the semiconductor film into an excellent state. As a result, the semiconductor memory device with a further improved retention characteristic can be obtained.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, the embodiments of the present invention will be described hereinbelow. Throughout the drawings shown below, components having substantially the same functions will be denoted by the same reference numerals for the sake of simple illustration. It is to be noted that the present invention is not limited to the following embodiments.

FIG. 1is a cross-sectional view schematically showing a structure of a semiconductor memory device according to the first embodiment of the present invention.

As shown inFIG. 1, the semiconductor memory device according to the present embodiment is composed of a field effect transistor using the interface between a ferroelectric film3and a semiconductor film4as a channel and comprising a gate electrode2to which a voltage for controlling the polarization state of the ferroelectric film3is applied and source/drain electrodes5and6for detecting a current flowing in the channel in accordance with the polarization state. The semiconductor film4is made of a material having a spontaneous polarization, in which the direction of the spontaneous polarization is parallel with the interface between the ferroelectric film3and the semiconductor film4.

In the present embodiment, the semiconductor film4is composed of a ZnO film having a wurtzite-type crystal structure. The crystal orientation of the semiconductor film4is controlled such that the <11-20> direction or <1-100> direction thereof is perpendicular to the principal surface of the semiconductor film4. As a result, the direction of spontaneous polarization of the semiconductor film4is parallel with the interface between the ferroelectric film3and the semiconductor film4, and therefore a reduction in the polarization of the ferroelectric film3due to the spontaneous polarization can be suppressed.

That is, as shown inFIGS. 2A and 2B, when the direction of spontaneous polarization of the semiconductor film4is the same as the direction of polarization73of the ferroelectric film3(polar surface orientation), the polarization73of the ferroelectric film3attenuates in a depletion state shown inFIG. 2A, while electrons74as accumulated charges disappear in an accumulation state shown inFIG. 2B. By contrast, when the direction of spontaneous polarization of the semiconductor film4is parallel with the interface between the ferroelectric film3and the semiconductor film4(non-polar surface orientation), the polarization73of the ferroelectric film3does not attenuate even in the depletion state shown inFIG. 2C, and the electrons74as the accumulated charges are held even in the accumulation state shown inFIG. 2D.

Next, a description will be given to a specific structure of the semiconductor memory device according to the present embodiment with reference to step cross-sectional views shown inFIGS. 3A to 3D.

First, as shown inFIG. 3A, the gate electrode2made of strontium ruthenate (SrRuO3or SRO) having a thickness of about 30 nm is formed by a PLD (Pulse Laser Deposition) method on a substrate1made of strontium titanate (SrTiO3or STO) cut out to have a (100) surface in a state where the substrate temperature is set at 700° C. Further, an annealing process is performed with respect to the SRO film2in an oxygen atmosphere at 700° C. and 1 atmospheric pressure in an oven. Then, the ferroelectric film3made of PZT having a thickness of about 450 nm is formed by a PLD method on the SRO film2at a substrate temperature of 700° C.

The composition of a sintered body having a composition ratio of Pb:Zr:Ti=1:0.52:0.48 is used as the target of PLD. In the PZT film3having the composition, a tetragonal phase and a rhombohedral phase normally coexist. However, since the present embodiment uses the cubic-system STO substrate1and the peak of the tetragonal phase cannot be separated from that of the rhombohedral phase, it can be conceived that the PZT film3is composed of the tetragonal phase and oriented in the <001> direction. Further, as shown inFIG. 4A, the respective in-plane crystal orientations of the STO substrate1, the SRO film2, and the PZT film3were observed by an electron back scattering diffraction (EBSD) method. As a result, 4-fold symmetrical pole figures (corresponding to31,32, and33ofFIGS. 4B to 4D) each having orientations in an equal direction in a plane were obtained. From this, it will be understood that the PZT film3is an excellent epitaxial film.

Next, as shown inFIG. 3B, the semiconductor film4made of n-type ZnO having a thickness of about 30 nm is formed in the state where the substrate temperature is set at 400° C. in the same chamber of a PLD system. When it is assumed herein that the thickness of the semiconductor film4is not more than 60 nm (more preferably, not more than 30 nm), it is possible to form the semiconductor film4without degrading the crystallinity thereof. Accordingly, the carrier concentration can be adjusted to be not more than 1×1017cm−3(not more than 1×1016cm−3). Since a film which is extremely low in carrier concentration for the semiconductor film4can be thus obtained, an intrinsically high resistance value is obtainable. As a result, when the transistor is operated, an off-current can be reduced, and therefore the achievement of a high On/Off ratio can be expected.

By observing the crystal orientation by an X-ray diffraction method, it can be seen that the ZnO film4formed by the foregoing method is oriented in the <11-20> direction under the influence of the orientation of the (0001) surface of the PZT film3, as shown inFIG. 5. The crystal orientation is tilted by 90 degrees from the c-axis direction (<0001> direction) having a polarity, which indicates that a non-polar surface orientation is provided. Further, as a result of observing the cross-sectional structure using a cross-section transmission electron microscope (cross-section TEM), the interface had no disturbance, as shown inFIG. 6. From this, it will be understood that the crystal lattice of the PZT film3matches that of the ZnO film4.

Moreover, in a diffraction image43(FIG. 7C) obtained by synthesizing the respective transmission electron diffraction images of the PZT region41and the ZnO/PZT region42, the <100> direction of the PZT film3and the <1-102> direction of the ZnO film4are equally oriented. From this, it will be understood that the ZnO film4has epitaxially grown with respect to the PZT film3.

Next, as shown inFIG. 3C, the ZnO film4is removed by etching except for the portions thereof located in isolation regions. Then, a resist film22is patterned to form a Ti film5having a thickness of about 30 nm and a Pt film6having a thickness of about 60 nm on the substrate1by an electron beam vapor deposition method.

Finally, as shown inFIG. 3D, the source/drain electrodes each composed of a multilayer film consisting of the Ti film5and the Pt film6are formed on the ZnO film4by removing the resist film22, so that the semiconductor memory device shown inFIG. 1is completed.

According to the present embodiment, by controlling the crystal orientation of the ferroelectric film3, the <11-20> direction or <1-100> direction of the semiconductor film4epitaxially grown thereon can be controlled to be perpendicular to the principal surface of the semiconductor film4. As a result, it is possible to easily control the spontaneous polarization direction of the semiconductor film4and suppress a reduction in the polarization of the ferroelectric film3due to spontaneous polarization.

In addition, since the semiconductor film4is epitaxially grown with respect to the ferroelectric film3, there is no grain boundary in the semiconductor film4. As a result, the scattering of carriers by grain boundaries is suppressed, and the on-current can be increased. This increases the On/Off ratio and allows the enlargement of the read margin of the memory.

The temperature at which the ZnO film4is formed is typically in the range of 400 to 1,000° C., while the temperature at which the PZT film3is formed is typically in the range of 500 to 700° C. Therefore, by selecting formation temperatures, it is possible to obtain the ferroelectric film3and the semiconductor film4having an excellent interface therebetween and each having an excellent crystallinity.

By controlling the <001> direction when the PZT film3is composed of a tetragonal phase or controlling the <100> direction when the PZT film3is composed of a rhombohedral phase such that the controlled direction is perpendicular to the substrate1, the polarization direction of the PZT film is oriented perpendicular to the film surface, and the difference in remnant polarization of the ferroelectric film3can be increased to a large value of about 2Pr=40 to 70 μC/cm2. As a result, a large number of charges (e.g., about a surface charge density of 2×1014cm−2) can be induced, and an increase in on-current can be expected.

When the <11-20> direction of the ZnO film4having a wurtzite-type structure is perpendicular to the substrate, not only the ZnO film4has a non-polar surface orientation, but also the lattice matching property with the PZT film3is improved. This allows a reduction in surface states, and therefore an increase in on-current.

Preferably, the <100> direction in a plane of the tetragonal PZT film3generally coincides with the <1-102> direction in a plane of the ZnO film4. Alternatively, the <001> direction in a plane of the rhombohedral PZT film3generally coincides with the <1-102> direction in a plane of the ZnO film4. This provides the in-plane crystal orientations in equal directions and can reduce the surface states. Because of the resulting epitaxial growth, there is no grain boundary. As a result, it is possible to suppress scattering by grain boundaries and therefore increase the on-current.

As the PZT film3, a PZT film doped with an element such as lanthanum (La), niobium (Nb), vanadium (V), tungsten (W), praseodymium (Pr), or samarium (Sm) may also be used. By doping the PZT film3with another element, the crystallization temperature can be reduced. This allows low-temperature formation and also achieves the effect of reducing repeated polarization reversal fatigue. As the ZnO film4, a ZnO film doped with another element such as magnesium (Mg), gallium (Ga), or aluminum (Al) may also be used. This allows free control of the carrier concentration as well as control of a switching state. As the semiconductor film4, a film made of gallium nitride (GaN), indium nitride (InN), or a mixed crystal thereof may also be used instead of the ZnO film. As the method for depositing the SRO film2, the PZT film3, and the ZnO film4, a metal organic chemical vapor deposition (MOCVD) method, a sputtering method, a molecular beam epitaxial (MBE) method, or the like may also be used instead of the PLD method.

A multilayer electrode consisting of the Ti film5and the Pt film6was formed directly on the PZT film3formed by the fabrication method according to the present embodiment and the polarization characteristic of the PZT film3was examined. As a result, the difference in remanent polarization value obtained by applying voltages of ±10 V to the SRO film2and between the electrodes was 59 μC/cm2. Since the crystal orientation in the thickness direction of the PZT film3is in the <001> direction, which is the polarization direction, a large remanent polarization value has been obtained.

Next, a description will be given to the subthreshold characteristic of the semiconductor memory device according to the present embodiment with reference toFIGS. 8 and 9.

As shown inFIG. 8, a drain current Idwas measured by applying a gate voltage Vgto the terminal51of the gate electrode2, grounding the terminal52of the source electrode5or6, and applying a drain voltage of Vd=1 V to the terminal53of the drain electrode6or5.FIG. 9is a graph showing the result of the measurement. The locus (hysteresis) exhibited by the drain current when Vgwas scanned from −10 V to +10V is different from that exhibited by the drain current when Vgwas scanned from +10 V to −10V. Each of the drain currents when Vg=0 V was satisfied was not more than 1 nA and not less than 1 μA, so that the obtained current ratio was not less than three orders of magnitude.

The reason for the difference in current value produced even in the state where the application of the voltage to the gate electrode2was cut off is that the depletion/accumulation of interface charges is held by the remanent polarization of the ferroelectric film3. That is, when a negative voltage was applied to the gate electrode2as shown inFIG. 10A, the polarization of the ferroelectric film3was oriented downward to repel carriers away, so that the whole semiconductor film4(channel) was depleted and high in resistance. By contrast, when a positive voltage was applied to the gate electrode2as shown inFIG. 10B, the polarization of the ferroelectric film3was oriented upward to induce carriers at a concentration corresponding to the polarization density at the interface, so that the semiconductor film4was in a charge accumulation state and low in resistance.

The actual carrier concentration of the ZnO film measured by hole measurement was 4.7×1018cm−3and, when the thickness of the ZnO film4was 30 nm, the surface charge density was 1.5×1012cm−2. Since the value is sufficiently small compared with the surface charge density (2×1014cm−2) corresponding to the remanent polarization value of the PZT film, the ZnO film is completely depleted. By further associating the larger drain current value and the smaller drain current value with binary data “1” and “0”, the memory function is implemented. Moreover, because the remanent polarization of the ferroelectric film is stored even in the sate where the voltage is cut off, a nonvolatile memory can be realized.

FIG. 11is a graph showing the result of measuring the retention time of the On/Off ratio. The curve61shows the retention time in the semiconductor memory device according to the present embodiment, while the curve62shows the retention time in the conventional semiconductor memory device. On/Off ratios were measured by applying pulse voltages of +10 V and −10V each having a pulse width of 500 nanoseconds to the gate electrodes, subsequently applying a drain voltage of 0.1 V while keeping the gates in a zero-bias state, and measuring the drain currents. As shown by the curve61ofFIG. 11, the On/Off ratio of not less than four orders of magnitude was maintained even after the semiconductor memory device according to the present embodiment was allowed to stand for 16 hours at a room temperature. From this, it can be seen that, compared with the conventional semiconductor memory device, the retention characteristic of the semiconductor memory device according to the present embodiment has been significantly improved.

In the first embodiment, the crystal orientation (the direction of spontaneous polarization) of the semiconductor film4is controlled by the crystal orientation of the ferroelectric film3. However, to determine the crystal orientation of the ferroelectric film3, it is necessary to use a substrate having a specified crystal structure and a specified crystal orientation. The second embodiment of the present invention provides a method for fabricating a semiconductor memory device comprising a semiconductor film having a crystal orientation thereof controlled in a predetermined direction without being restricted by the type of the substrate.

FIGS. 12A to 13Dare step cross-sectional views schematically illustrating the method for fabricating the semiconductor memory device according to the present embodiment;

First, as shown inFIG. 12A, a MgZnO film82having a thickness of 20 nm and a ZnO film83having a thickness of 30 nm are continuously grown by a PLD method on a ZnO substrate81cut out to have a (11-20) surface (non-polar surface) in the state where the substrate temperature is set at 800° C. The crystal growth of each of the films is controlled to be in the same direction as the orientation surface of the substrate81.

Next, as shown inFIG. 12B, a resist film91is patterned, and then a Pt film84having a thickness of 50 nm is formed by an electron beam vapor deposition method. Thereafter, the resist film91is removed so that source/drain electrodes84are formed at predetermined positions on the ZnO film83, as shown inFIG. 12C.

Next, as shown inFIG. 12D, a PZT film85having a thickness of 450 nm is formed on the ZnO film83to cover the source/drain electrodes84at a substrate temperature of 630° C. without opening a PLD chamber to an atmosphere. The composition of a sintered body as the target of PLD is Pb:Zr:Ti=1:0.30:0.70. At the composition, PZT is completely composed of a tetragonal phase, and the lattice mismatch between the (1-102) surface of the ZnO film83and the (001) surface of the PZT film85is smaller than in the case where a sintered body having a composition ratio of Pb:Zr:Ti=1:0.52:0.48 is used as the target of PLD, so that surface states are reduced.

Next, as shown inFIG. 12E, the resist film92is patterned on the PZT film85, and then a Pt film86having a thickness of 60 nm is formed by an electron beam vapor deposition method. Thereafter, the resist film92is removed so that a gate electrode86is formed at a predetermined position on the PZT film85, as shown inFIG. 12F. The gate electrode86is formed to overlap a part of each of the source/drain electrodes84.

Next, as shown inFIG. 13A, an AlN film87serving as a protective film is deposited by 300 nm by using a sputtering method. Because the AlN film does not permit the transmission of hydrogen therethrough, it can prevent the degradation of the PZT film85and the ZnO film83. Thereafter, as shown inFIG. 13B, the AlN film87is planarized by chemical mechanical polishing (CMP) till the gate electrode86is exposed.

Next, as shown inFIG. 13C, a conductive supporting substrate88is bonded to the exposed surface of the gate electrode86using a wafer bonding system.

Finally, as shown inFIG. 13D, the ZnO substrate81is scraped down till the MgZnO film82is reached and, by further etching a part of each of the MgZnO film82and the ZnO film83, contact windows reaching the source/drain electrodes84are formed.

By using such a method, the supporting substrate88can be freely selected. Since wafer bonding is used, even when a memory portion is formed directly on a CMOS, the degradation of the CMOS due to a heat budget does not occur. As a result, it becomes possible to embed the memory even in a miniaturized process node of not more than 65 nm which uses nickel silicide as the silicide material of a CMOS contact portion. In addition, the MgZnO film62serves as a stopper when the substrate81is removed and also as an electron barrier layer with respect to the ZnO film83. As a result, when the channel (ZnO film83) is composed of a thin film having a thickness of not more than 30 nm, the effect of confining electrons within the channel can be expected, and therefore a loss in the current flowing in the channel can be suppressed.

The supporting board88may also be an insulating substrate made of glass, a plastic material, or the like as long as the gate electrode86can be connected to a driving circuit. Alternatively, the supporting board88may also be a substrate formed with a CMOS. This allows the realization of a merged logic/memory configuration.

As the substrate81, it is also possible to use a ScAlMgO4(SCAM) substrate, a SiC substrate, or a sapphire substrate cut out to have a non-polar surface, such as a (11-20) surface or a (1-100) surface.

Although the present invention has been described thus far by using the preferred embodiments thereof, it will easily be appreciated that the description is not restrictive and various changes and modifications can be made to the present invention.