JP3200662B2 - Coulomb blockade element and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an element formed on a silicon substrate and utilizing the Coulomb blockade phenomenon and a method of manufacturing the same.

[0002]

2. Description of the Related Art The Coulomb blockade phenomenon of electron tunneling in a small tunnel junction is a phenomenon in which a tunnel of one electron is suppressed by an increase in free energy due to charging energy accompanying the tunneling. A Coulomb blockade device utilizing such a Coulomb blockade phenomenon can control the current or charge flowing out of the device or accumulated in the device in single electron units, so that the power consumption per device is extremely low. It is characterized by its small size and extremely small device area, and is expected to achieve integration far exceeding the integration limit of existing silicon-based integrated circuits. The basic structure of this device is a single electron transistor (Single Electron Transistor).
r) and Single Electron Memory.

The conventional Coulomb blockade device mainly has a structure in which electrons are confined to a small metal island using a small tunnel junction of metal / metal oxide, or a III-V structure.
A two-dimensional electron gas formed at a heterojunction of a group III semiconductor is confined in an island shape by an electric field generated by a fine electrode or the like formed on the heterojunction to tunnel electrons between the islands. Figure 31 shows the document “Physical Review
Letter, Vol. 65, pp. 771-774, 1990 ", a bird's-eye view of the conventional Coulomb blockade device viewed from obliquely above, and FIG. 32 is an equivalent circuit diagram of the Coulomb blockade device. 71 is a substrate made of n-type GaAs, 72 is an AlGaAs layer, 73 is a GaAs layer, 7
Reference numeral 4 denotes an electrode formed on the GaAs layer 73.

In such a Coulomb blockade device, a two-dimensional electron gas is formed at the hetero interface between the AlGaAs layer 72 and the GaAs layer 73. By providing a narrowed narrow portion 75 in the horizontal direction in the electrode 74, a potential barrier is formed in this portion by a quantum size effect, and the region 76 sandwiched between these forms a conductive island for confining electric charges. Thus, the conductor island 76 and the source electrode 7
7 acts as a tunnel capacitance Cs, and a potential barrier between the conductor island 76 and the drain electrode 78 acts as a tunnel capacitance Cd.
An element having an equivalent circuit as shown in FIG.

One of the most important issues for putting such a Coulomb blockade device into practical use is the operating temperature. In order to operate the Coulomb blockade device at a practical temperature, the core of the device is required. Forming a conductive island to be an electron reservoir on the nm scale, and a few aF (1 aF is 1
It is necessary to form a tunnel barrier having an extremely small capacity of 0 ^-18 F). This is because when these become large, the charging energy of a single electron is buried in thermal energy, and the Coulomb blockade phenomenon cannot be observed. Another major issue is how to produce and arrange such an ultrafine structure with good controllability. In particular, in order to realize a new function by coupling the Coulomb blockade element, a manufacturing technique with good controllability is essential.

However, in the Coulomb blockade device shown in FIG. 31, the width of the narrow portion 75 (the left-right direction in FIG. 31) of the electrode 74, which is the smallest, needs to be sufficiently smaller than the width of the conductor island 76. If it is intended to fabricate 75 by electron beam lithography, the size of the island 76 must be much larger than the minimum dimension determined by the limit of lithography. Further, in a Coulomb blockade element using metal, there is no effective processing technology for producing a metal island, and it is difficult to produce a fine metal island with good reproducibility. Therefore, the element of any structure has 1
It will only operate at very low temperatures below K.

As a device for reducing the capacitance, it has been found that a method of fabricating a conductor island by reducing the thickness of a polycrystalline material and utilizing the fluctuation of the structure is effective (see the document "IEEE"). Trans.Electr
on Devices, 41, 1628, 1994
Year"). However, in this method, the arrangement of the conductor islands cannot be freely controlled because the fluctuation of the structure due to the polycrystal is utilized.

[0008]

As described above, in the conventional method, a technique for producing an extremely small conductor island and a technique for arranging the conductor island with good controllability have not been established. Since technology is an essential requirement for practical use of Coulomb blockade devices, there has been a problem that a Coulomb blockade device that operates at room temperature cannot be realized with good controllability and reproducibility. The present invention has been made in order to solve the above problems, and has as its object to provide a Coulomb blockade element which can be manufactured by a simple manufacturing process and can operate at a high temperature, and a method of manufacturing the same. .

[0009]

According to the present invention, on a substrate having a silicon layer formed on an insulating film, the silicon layer serves as a conductor island for confining electric charges. It has a thin wire portion and first and second electrode portions formed to be connected to both ends of the thin wire portion, the width being wider than the thin wire portion and the film thickness near the thin wire portion being thinner than the thin wire portion. . In this manner, by making the film thickness of the first and second electrode portions near the thin wire portion smaller than that of the thin wire portion, a tunnel barrier having higher energy than the thin wire portion is formed at both ends of the thin wire portion, and the conductor is formed on the thin wire portion. An island is formed.

According to a second aspect of the present invention, the silicon layer has a thin wire portion serving as a conductor island for confining electric charges and a width wider than the thin wire portion formed to be connected to both ends of the thin wire portion. A drain of a MOS transistor formed between the first and second electrode portions, which are wider and thinner in the vicinity of the thin wire portion than the thin wire portion, and a space for providing a capacitance between the thin wire portion or an insulating film; A third electrode portion serving as a source and a channel region; and a gate electrode of a MOS transistor formed on one portion of the third electrode portion via an insulating film. By providing the third electrode portion serving as the drain, source, and channel regions of the MOS transistor adjacent to the thin line portion, an element in which the Coulomb blockade element and the MOS transistor are capacitively connected can be realized.

According to a third aspect of the present invention, the silicon layer is formed so as to be connected to a first thin wire portion serving as a conductor island for confining electric charges and both ends of the first thin wire portion. The first and second electrode portions, which are wider than the first fine line portion and have a film thickness near the fine line portion smaller than the first fine line portion, and a second fine line serving as a conductive island for confining electric charges. Part and the first
Formed with a space or an insulating film for providing a capacitance between the second thin line portion and the second thin line portion, and having a width wider than that of the second thin line portion. The film thickness near the thin line portion is wider than the second thin line portion and is formed so as to be connected to the adjacent portion having a smaller thickness in the vicinity than the second thin line portion and the other end of the second thin line portion. And a fourth electrode portion which is thinner than the second thin wire portion. By causing the adjacent portion to be adjacent to the first thin line portion in this manner, the Coulomb blockade element including the first thin line portion, the first and second electrode portions, and the second thin line portion, the adjacent portion, and the fourth An element in which a Coulomb blockade element composed of the above-mentioned electrode portion is capacitively connected can be realized.

According to a fourth aspect of the present invention, on a substrate having a silicon layer formed on an insulating film, the silicon layer is converted into a thin wire portion serving as a conductor island for confining electric charges, and both ends of the thin wire portion. And a step of processing into a shape having first and second electrode portions wider than the thin line portion, and a step of thermally oxidizing the silicon layer. By such thermal oxidation, the silicon layer is oxidized from the upper surface and the lower surface, and the oxidation is suppressed at the small line portion having a small area, and the first and second wide portions are widened.
Oxidation from the lower surface is promoted in the vicinity of the thin wire portion of the electrode portion, so that a narrow portion having the smallest film thickness is automatically formed in the vicinity of the thin wire portion, and the film thickness of the first and second electrode portions is reduced. In the vicinity, it becomes thinner than the thin line portion.

According to a fifth aspect of the present invention, the silicon layer is formed so as to be a plurality of thin wire portions serving as conductor islands for confining electric charges, and to be a branch point or a bending point connecting these thin wire portions. And a connecting portion having a smaller thickness than the thin line portion. By connecting the plurality of thin wire portions at the connection portion and making the thickness of the connection portion thinner than that of the thin wire portion, a tunnel barrier having higher energy than the thin wire portion is formed at the connection portion, and the conductor is formed at the thin wire portion. An island is formed,
A connection structure of the conductor islands via the capacitance by the tunnel barrier is formed.

According to a sixth aspect of the present invention, there is provided at least one electrode portion which is formed so as to be connected to an end of the thin wire portion and which is wider than the thin wire portion. As a result, this electrode portion can be used as a single-electron introduction / extraction electrode. Further, as described in claim 7, the electrode portion is:
The film thickness near the thin line portion is smaller than that of the thin line portion. By making the film thickness of the electrode portion near the thin line portion thinner than that of the thin line portion, a tunnel barrier having higher energy than that of the thin line portion is formed near the thin line portion of the electrode portion. The thin wire portion serving as a body island is connected to the electrode portion.

According to the present invention, at least one electrode having a width wider than the thin wire portion and formed with a space or an insulating film for providing a capacitance between the thin wire portion and the end of the thin wire portion. It has a part. With such a configuration, the thin wire portion serving as a conductor island and the electrode portion are connected via the space or the capacitance of the insulating film. According to a ninth aspect of the present invention, there is provided a semiconductor device having a gate electrode formed on at least a part of the thin wire part and the connection part via an insulating film. With such a configuration, the thin line portion serving as a conductive island and the gate electrode are connected through the capacitance of the insulating film.

According to a tenth aspect of the present invention, the silicon layer has a plurality of thin wire portions serving as conductor islands for confining electric charges, and a connecting portion serving as a branch point or a bending point connecting these thin wire portions. It has a step of processing into a shape and a step of thermally oxidizing the silicon layer. By such thermal oxidation, the silicon layer is oxidized from the upper surface and the lower surface, and the oxidation is suppressed in the thin line portion having a small area, and the oxidation from the lower surface is promoted in the connection portion having a larger area. The constriction with the smallest film thickness is automatically formed, and the film thickness of the connection portion becomes smaller than that of the thin line portion.

Further, as described in claim 11, the silicon layer before the thermal oxidation step has at least one or more electrode portions wider than the thin wire portion connected to the end of the thin wire portion. Oxidation from the lower surface is promoted near the thin line portion of the wide electrode portion due to thermal oxidation, so that a constriction having the smallest film thickness is automatically formed near the thin line portion, and the film thickness of the electrode portion is reduced near the thin line portion. And becomes thinner than the thin line portion. Claim 1
As described in 2, the silicon layer before the thermal oxidation step is connected to at least one electrode portion wider than the fine wire portion and connected to an end of the fine wire portion so that the width increases as the distance from the fine wire portion increases. Have

According to a thirteenth aspect, an insulating film containing silicon is formed on the silicon layer before the thermal oxidation step. The insulating film containing silicon is a silicon nitride film.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION

1. Embodiment 1. FIG. 1 is a sectional view showing a manufacturing process of the Coulomb blockade device according to the first embodiment of the present invention. 1 is an S in which a single crystal silicon layer is formed on an insulating film.
OI (Silicon On Insulator) substrate, 2 is the substrate silicon, 3 is the buried oxide film disposed on the substrate silicon 2, 4 is the upper silicon layer disposed on the buried oxide film 3, 5 is the SOI substrate 1 A silicon oxide film formed by oxidizing the silicon layer 4 and 6 is a gate electrode.

First, the manufacturing process of the Coulomb blockade device of the present embodiment will be described. As an SOI substrate to be used, for example, SIMOX (Separation by IMpl) in which an oxide film is formed by injecting oxygen into a single crystal silicon substrate.
anted OXygen) substrates and bonded substrates in which a silicon oxide film and a single crystal silicon layer are bonded.

First, an upper oxide film 5 is formed on the silicon layer 4 of the SOI substrate 1 including the substrate silicon 2, the buried oxide film 3, and the upper silicon layer 4 by a technique such as thermal oxidation (FIG. 1A). ). Next, when the oxide film 5 is viewed from above by reactive ion etching (RIE), FIG.
(FIG. 1 (b)).

Subsequently, the upper silicon layer 4 is etched by reactive ion etching or electron cyclotron resonance (ECR) plasma etching using the oxide film 5 as a processing mask. As a result, as shown in FIG.
The upper silicon layer 4 remains only under the upper oxide film 5. In this manner, the silicon layer 4 has the thin wire portion 10 and the source-side electrode portion 11 serving as the first electrode portion and the drain-side electrode portion serving as the second electrode portion having a width (the vertical direction in FIG. 2) wider than the thin wire portion 10. This means that the shape has been processed.

At this time, the width of the thin line portion 10 is in the order of the film thickness of the silicon layer 4 (that is, 1/10 to 1 of the film thickness).
About 0 times). Next, when such a structure is thermally oxidized in an atmosphere containing oxygen or water vapor, the upper silicon layer 4 is oxidized from the upper surface by diffusion of an oxidant (oxygen or water) through the upper oxide film 5. Further, at the pattern edge (the outer peripheral portion in FIG. 2) and its vicinity,
The diffusion of the oxidizing agent from the side surface of the pattern through the silicon layer 4 itself or the diffusion of the oxidizing agent through the buried oxide film 3 also oxidizes the side surface or the lower surface.

At this time, the oxidation from above through the upper oxide film 5 acts almost uniformly over the entire pattern. On the other hand, the amount of diffusion of the oxidant through the buried oxide film 3 is inversely proportional to the distance from the pattern edge (or the square of the distance). , The shorter the distance from the edge is, the more it is promoted.

However, at the pattern edge, the oxidizing agent concentration is reduced due to the accumulation of stress accompanying the volume expansion of the oxide film formed by thermal oxidation, so that the oxidation rate is suppressed. In particular, in a small area region such as the thin line portion 10 (the entire thin line portion can be considered as a pattern edge),
Since the silicon layer 4 is surrounded by the oxide film formed by thermal oxidation as the oxidation proceeds from both side walls, the above-described oxidation suppressing effect is extremely strongly received.

Therefore, with respect to the upper surface of the upper silicon layer 4, due to the oxidation suppressing effect, the oxidation is suppressed at the pattern edge, the oxidation is more strongly suppressed at the thin line portion 10, and the oxidation is substantially uniform at the remaining region. Act on. Similarly, the lower surface of the silicon layer 4 has
Oxidation is suppressed at the pattern edges and other pattern edges, and due to the oxidation promoting effect, oxidation is remarkably advanced in the vicinity of the pattern edges slightly away from the edges.

In this way, the oxidation rate in the vicinity of the pattern edges of the electrode portions 11 and 12 is the highest, and the silicon layer 4 in this region (the hatched portion in FIG. 2) is the thinnest. That is, when the element at this time is cut along the line II in FIG. 2, a structure as shown in a cross-sectional view in FIG. 3 is formed.

Next, the gate electrode 6 is formed of polysilicon or the like so as to cover the fine line portion 10 from above (FIG. 1D).
Therefore, oxide film 5 formed on silicon layer 4 is used as a gate oxide film. And the conventional MOS
Similarly to the transistor, a window for an electrode is opened in a part of the oxide film 5 on the source-side electrode portion 11 and the drain-side electrode portion 12,
In this portion, a source electrode and a drain electrode are formed using a metal made of aluminum, tungsten, titanium, or the like as a lead electrode. Thus, the manufacturing process of the Coulomb blockade device is completed.

As described above, if both ends of the silicon layer 4 in the thin wire portion 10 after oxidation are sufficiently thinner than the film thickness, the conduction band of the semiconductor silicon in the thinned region is quantized. As a result, the base energy becomes larger than that of the thin line portion 10. Therefore, when viewed from the electrons in the thin wire portion 10, the thin wire portion 10 is sandwiched between both ends by the energy barrier, and looks like an isolated island. FIG. 4 is an energy band diagram schematically showing this state, FIG. 5 is a schematic diagram as a Coulomb blockade element, and FIG. 6 is an equivalent circuit diagram thereof. FIG. 4 shows only the conduction band.

The extremely thin silicon region near the pattern edges of the electrode portions 11 and 12 becomes a potential barrier (tunnel barrier) as shown in FIG. 4 due to the increase in the base energy. These two potential barriers act to confine charges in the thin wire portion 10 and also act as tunnel capacitances Cs (source-side capacitance) and Cd (drain-side capacitance). Thus, the thin wire portion 10 becomes a silicon island (conductor island). The thin line portion 10 is connected to the gate electrode 6 and the capacitance for the substrate silicon 2, Cg and Cb.

The silicon layer 4 in the schematic diagram of FIG.
Is different from the actual shape of the silicon layer 4 in FIG. 3 for the following reason. In other words, the film thickness at both ends of the thin wire portion 10 becomes thinner on both sides of the upper surface and the lower surface and becomes a shape as shown in FIG. 5 (this is called a constriction). For this reason, it is necessary to be pushed up from below.

FIG. 7 is a graph showing characteristics of the Coulomb blockade device according to the present embodiment. FIG. 7 shows the conductance when the source electrode of the Coulomb blockade element and the substrate silicon 2 (back gate) are grounded, the drain voltage Vd is 1 mV, and the gate voltage Vg is changed, using temperature as a parameter. In order to make the characteristics easy to see, the characteristics other than 27K are sequentially moved upward in parallel by 0.2 μS (actually, the conductance at a gate voltage of 0 V at each temperature is almost 0 μS).

At this time, the Coulomb blockade element is:
The width of the thin line portion 10 is about 30 nm and the length (FIG. 2 left-right direction)
Is about 50 nm, the width of the electrode section 11 is about 400 nm, the width of the electrode section 12 is about 1000 nm, the thickness of the silicon layer 4 before the thermal oxidation step shown in FIG. With a thickness of about 30 nm, thermal oxidation was performed under the condition that the thin wire portion 10 was oxidized by about 10 nm or more from one side (about 90 minutes in a dry oxygen atmosphere at 1000 ° C.).

When processed under such conditions, the thin line portion 10
The thickness of the silicon layer 4 is 10 nm, the thickness of the silicon layer 4 near the pattern edges of the electrode portions 11 and 12 (hereinafter, the thinnest portion is referred to as a constriction) is 5 nm or less, and the gate capacitance Cg = 0. 3aF, drain capacitance Cd =
An element having 1 aF and back gate capacitance Cb = about 0.01 aF can be obtained. Here, the source capacitance Cs cannot be directly measured, but is smaller than Cd.

By using the structure as in this embodiment, the capacity surrounding the central silicon island can be reduced.
As shown in FIG. 7, the oscillation due to the gate voltage of the conductance can be observed even at room temperature (that is, the basic operation as a single-electron transistor is observed). This operation will be described with reference to FIG. 4. As described above, since the silicon island is surrounded by a small capacitance, the energy increase due to one electron entering the island increases, and the energy level (FIG. 4 shows only two levels above and below the Coulomb gap).

When the gate voltage Vg is changed, this energy level rises and falls while maintaining a constant gap due to the capacitive connection between the gate electrode 6 and the island. Then, when the source-drain voltage Vd is smaller than this Coulomb gap, if a source or drain level enters the gap, a blockade state is formed in which no current flows between the source and drain.

When one of the levels of the silicon island enters between the energy levels of the source and the drain, a current flows from the source to the drain via this level.
Therefore, when the gate voltage Vg is changed, these two states appear alternately, so that the conductance between the source and the drain pulsates.

At a temperature other than absolute zero, the pulsation of the conductance is blurred by thermal energy, and is observed as a smooth vibration as shown in FIG. And, the smaller the total capacity surrounding the silicon island,
Since the Coulomb gap of the silicon island becomes large, the oscillation of this conductance can be observed up to a high temperature.

As a forming condition for realizing such a Coulomb blockade element, it is preferable that the dimension of the thin wire portion 10 before thermal oxidation is about several tens nm or less for both film thickness, width and length. . Either of the two ends of the thin wire portion 10 may be the source electrode.
The thinner the film thickness is, the more remarkable the width of the silicon layer 4 is narrow to some extent. This is based on the principle of oxidation of the silicon layer 4 that the oxidizing agent diffuses in the lateral direction through the buried oxide film 3 and is oxidized from the back surface side. Also, the supply amount of the oxidizing agent to the back surface increases as the number of voids located at a certain distance is short. It is because it increases.

However, if the width is too small, the concentration of the stress on the edge of the silicon layer 4 during the oxidation causes the effect of lowering the oxidation speed, so that a certain width is required. This width depends on the film thickness and oxidation conditions,
m to 200 nm or more is required. Regarding the upper limit of the width, it is necessary to pay attention that the effect of increasing the oxidation rate at the end of the thin line is obtained even if the width is infinitely wide, but the effect is reduced if the width is too wide.

The most effective promotion of oxidation depends on the oxidation temperature and the conditions of the oxidation atmosphere.
When oxidation in an atmosphere of dry oxygen is used, about 40
Oxidation from the back side is most promoted when the thickness is about 0 nm.
In the structure of the element having the characteristics shown in FIG.
Since it is as narrow as 0 nm, as shown in FIG. 3, the thickness of the silicon layer 4 on the electrode section 11 side is smaller than that on the electrode section 12 side.

Therefore, the confinement potential due to the increase in the ground energy accompanying the thinning of the silicon layer 4 is larger on the source side (11) than on the drain side (12) as shown in FIG. Here, as the potential increases, the distance between the electrode and the silicon island effectively increases, so that the tunnel capacitance decreases on the source side.

That is, by giving the widths of the electrode portions 11 and 12 asymmetry, the magnitude relation can be set. Of course, the width can be symmetrical. As described above, the width and thickness of the electrode portions 11 and 12 and the thermal oxidation conditions (oxidation temperature and oxidation time) can control the formation of the constriction that makes the silicon layer 4 the thinnest. That is, since the size of the confinement potential and the size of the tunnel capacitance can be adjusted, the capacitance surrounding the silicon island can be reduced.

Since the thermal oxidation technique is particularly excellent in controllability and reproducibility among silicon LSI processing techniques, the structure of the Coulomb blockade element of the present embodiment can be realized with good controllability and reproducibility. Further, the horizontal dimension of the silicon island, that is, the width and length of the thin line portion 10 can be set irrespective of the size of the constriction, and the silicon island can be formed at the limit size of lithography. Furthermore, since the silicon of the fine wire portion 10 is thinned by performing the thermal oxidation, the silicon island can be made smaller than the limit of lithography, and the total capacity of the island can be effectively reduced.

Before the thermal oxidation, using an HF aqueous solution or the like,
If the buried oxide film 3 is slightly etched (about several nm to several tens nm) from the slit portion (the portion where the upper silicon is etched), it is effective to promote oxidation from the back surface of the upper silicon 4. is there. However, if the etching is performed too much, oxidation from the back surface occurs uniformly in a wide range, and it becomes impossible to make the upper silicon layer 4 thin in a narrow region, and it becomes difficult to form a tunnel barrier.

In this case, the gate electrode 6 does not necessarily have to overlap the thin line portion 10, but is located next to the thin line portion 10 (FIG. 2).
It is needless to say that the substrate silicon 2 on the back side can be used instead of the above. Fine line part 1
When it is formed beside 0, a silicon oxide film formed by the thermal oxidation in the step of FIG. 1C is formed beside the thin line portion 10, so this film is used as a gate oxide film. In this structure, since the wide electrode portion attached to the silicon island functions as an electrode for transmitting voltage and current, the wider the electrode portion, the lower the resistance.

Embodiment 2 In the first embodiment, the structure of the simple Coulomb blockade device shown by the equivalent circuit in FIG. 6 and the manufacturing method thereof have been described. Next, a simple structure for providing a memory function will be described as an example of developing this structure. FIG. 8 is a plan view of a Coulomb blockade device showing another embodiment of the present invention. However, FIG. 8 shows only the upper silicon layer and the gate electrode corresponding to the silicon layer 4 in FIG.

In FIG. 8, reference numeral 14 denotes a third electrode portion serving as a drain, a source, and a channel region of a MOS transistor formed with a space for providing a capacitance between the thin wire portion 10 and an insulating film, and reference numeral 15 denotes This is the gate electrode of the MOS transistor. Next, a manufacturing process of such a Coulomb blockade device will be described.

First, as in the first embodiment, the upper silicon layer of the SOI substrate is divided into a thin line portion 10 and a first electrode portion 11 and a second electrode portion 12 having a width of about 100 nm or more.
And processed into a shape having At the same time, the upper silicon layer adjacent to this structure is connected to the adjacent portion 13 adjacent to the thin line portion 10.
And a third electrode portion 14 having a width of about 100 nm or more.

A space 1 is provided between the thin line portion 10 and the adjacent portion 13.
6 or an insulating film such as a silicon oxide film may be inserted. Further, since the adjacent portion 13 forms a part of the electrode portion 14, the adjacent portion 13 is not provided. The electrode portion 14 may be directly adjacent to the thin wire portion 10.
That is, the condition at this time is that the capacitance (C16 described later) formed between the thin wire portion 10 and the electrode portion 14 is
The order of the tunnel capacity Cs, Cd (ie, Cs,
It should be formed so as to be 1/10 to 10 times Cd).

Then, this silicon layer is used in the first embodiment.
When the thermal oxidation is performed in the same manner as described above, the silicon layer in the hatched portion in FIG. 8 becomes thinnest, a tunnel barrier is formed in this region, and a silicon island is formed in the thin wire portion 10. Next, a first gate electrode 15 made of conductive polysilicon or the like is formed so as to cover a part of the electrode portion 14. When the structure at this time is cut along the line II-II in FIG. 8, a structure as shown in the sectional view in FIG. 9 is formed. In FIG. 9, reference numeral 7 denotes a silicon oxide film formed by the thermal oxidation (including an upper oxide film formed on a silicon layer as in the first embodiment).

Further, after an insulating film such as a silicon oxide film is entirely formed thereon, a second gate electrode (not shown) is formed of polysilicon or the like so as to cover the confinement potential forming regions at both ends of the fine wire portion 10. I do. Needless to say, the second gate electrode can be formed at the same time as the first gate electrode 15.

Next, using the first and second gate electrodes as a mask, a high-concentration impurity such as phosphorus or arsenic is introduced into a region to serve as a conductive region to form a low-resistance silicon region. Then, similarly to the conventional MOS transistor, the electrode portion 11,
A window for an electrode is formed in a part of the silicon oxide film on 12 and a source electrode and a drain electrode made of aluminum, tungsten, titanium, or the like are formed in these parts, respectively. Similarly, the drain electrode of the MOS transistor is formed on the electrode portion 14 (here, the region above the gate electrode 15 in FIG. 8).

Thus, the manufacturing process of the Coulomb blockade device according to the present embodiment is completed, and the region above the gate electrode 15 in FIG. 8 is the drain region, and the region below (the adjacent portion 13 side)
Region as a source region, the electrode portion 14 and the gate electrode 15
Is defined as a channel region 17 and the gate electrode 1
5 means that a MOS transistor capable of controlling the opening and closing of the channel 17 is formed in the electrode portion 14.

The above-mentioned second gate electrode is used to cover the confining potential forming regions at both ends of the fine wire portion 10 in order to avoid introducing a high concentration of impurities. Accordingly, it is possible to prevent the silicon in the region from being polycrystallized and changing its shape or becoming metallic, so that a sufficient potential barrier is not formed. By adjusting the voltage applied to the second gate electrode, the range in which the conductance changes can be controlled as shown in FIG.

Since the range in which the conductance changes can be controlled by the back gate voltage applied to the substrate silicon 2, the second gate electrode becomes unnecessary when the amount of impurity introduced is small or depending on the method of introduction.

The thin line portion 10 and the adjacent portion 13 are not affected even if they are made of metal. However, in order to avoid a large change in the shape due to polycrystallization, the thin line portion 10 and the adjacent portion 13 are protected by the second gate electrode. Good. In this case, it is necessary to adjust the second gate voltage so that electric charges can be accumulated in the thin line portion 10 and the connection portion 13 (to enable the Coulomb blockade element to operate). However, it is not necessary to apply the back gate voltage to the substrate silicon 2 for adjustment.

Needless to say, the role of the second gate electrode can be replaced by another insulating mask film. At this time, a region that should function as a conductive region may be covered with the mask film, but in that case, the voltage may be adjusted by a back gate voltage. Also, without introducing the impurities described above, the back gate voltage was set to a large positive value,
It is also possible to substitute by exciting the charge as a whole.

FIG. 10 is an equivalent circuit diagram of the Coulomb blockade element according to the present embodiment, where C16 is the capacitance of the space 16 (or insulating film) between the thin wire portion 10 and the adjacent portion 13, and Q1 is the electrode. This is a MOS transistor formed in the portion 14. Hereinafter, the operation principle of this element will be described. First, when writing to this element, the voltage V1 is applied to the first gate electrode 15 to turn on the channel of the MOS transistor Q1.

At this time, the source voltage Vs and the drain voltage Vd are fixed to potentials close to zero potential, and
When the voltage V2 applied to the (drain of the transistor Q1) is made negative, an amount of electrons corresponding to the magnitude of the voltage V2 is accumulated in the adjacent portion 13. Here, the voltage V1 is changed and M
When the channel of the OS transistor Q1 is turned off, the electrons are held in a state accumulated in the adjacent portion 13.

Since the adjacent portion 13 is coupled to the silicon island formed in the thin wire portion 10 by the capacitance C16, the difference in electric charge accumulated in the adjacent portion 13 causes a capacitance around the silicon island. When a single-electron transistor having a Coulomb blockade element formed of Cs, Cd, and C16 as a basic structure operates and a voltage Vd is applied to the drain (electrode portion 12), the conductance appears as a change.

That is, an output corresponding to the amount of charge accumulated in the adjacent portion 13 is applied between the source and the drain (the electrode portions 11 and 12).
(Interval) Can be extracted as conductance. Therefore, if the amount of charge is digitally changed, the Coulomb blockade device can function as a normal digital memory device.

If a function capable of measuring the amount of charge up to an analog amount is used, the analog amount can be stored in memory. The analog memory of this example is a useful memory device for constructing a neuro device (or a neural network). This utilizes the feature that the degree of coupling between neurons (using conductance) can be changed by the amount of charge stored in the memory.

When a large number of the elements shown in FIG. 10 are used as memory cells arranged vertically and horizontally, a memory element capable of storing a large amount of data can be obtained. FIG. 11 schematically shows one example of this. In the memory element of FIG. 11, since the gate electrode of the MOS transistor Q1 of each cell arranged in the horizontal direction is used as a common electrode, the first gate electrode 15 of FIG.
Is extended in the horizontal direction to form a common gate,
Each cell arranged in the horizontal direction can be connected.

In such a configuration, it is necessary to insert a MOS transistor Q2 serving as a switch for reading data into the drain of each memory cell. For this purpose, an electrode 18 similar to the first gate electrode 15 is connected to the structure shown in FIG.
May be formed so as to vertically cross the electrode portion 12 as shown by the broken line portion (this electrode can be formed simultaneously with the gate electrode 15). Thus, a transistor Q2 similar to the MOS transistor Q1 can be formed. If the electrode 18 is formed as a common electrode extending in the vertical direction, the cells arranged in the vertical direction can be connected.

When writing data to this memory element, a desired line out of WA1, WA2,... Is turned on, and data is supplied from the electrodes A1, A2,. When reading data, WR1, WR2,.
・ Select a desired line among R1, R2,.
• A voltage is applied to the terminal to read out the value of the conductance.

Embodiment 3 In the second embodiment, the MOS transistor is used as a switch for confining the accumulated charge. Next, a description will be given of a structure using a Coulomb blockade element instead of the MOS transistor by changing this structure. FIG. 12 is a plan view of a Coulomb blockade device showing another embodiment of the present invention, and shows only a silicon layer as in FIG.

Reference numeral 22 denotes an adjacent portion, which is formed with a space for providing a capacitance or an insulating film between the first thin wire portion 10 and the first thin wire portion 10 and is connected to one end of a second thin wire portion to be described later. It is formed to be wider than the second thin line portion, and the film thickness near the thin line portion is smaller than that of the thin line portion. Reference numeral 23 denotes a second thin line portion for confining charges, and 24 denotes a thin line portion formed wider than the thin line portion 23 so as to be connected to the other end of the second thin line portion 23. The fourth electrode portion is thinner.

Next, the manufacturing process of such a Coulomb blockade device will be described. First, Embodiment 1
Similarly to the above, the upper silicon layer of the SOI substrate is made to have a width of about 100 nm or more (the vertical direction in FIG. 12)
It is processed into a shape having a first electrode portion 11 and a second electrode portion 12 having. At the same time, the upper silicon layer adjacent to this structure is divided into an adjacent portion 22 adjacent to the fine line portion 10 and having a width of about 100 nm or more (the horizontal direction in FIG. 12), a second fine line portion 23, and a width of about 100 nm or more ( (Fig. 12 left-right direction)
And a fourth electrode portion 24 having

In FIG. 12, a narrower portion 21 is provided below the adjacent portion 22, but the adjacent portion 22 may be directly adjacent to the thin line portion 10 without providing the portion 21. That is, the condition at this time is that the thin wire portion 10 and the connection portion 2
2 (C21 described later) is in the order of the tunnel capacitances Cs and Cd (ie, Cs and Cd).
(Approximately 1/10 to 10 times).

When this silicon layer is thermally oxidized in the same manner as in the first embodiment, a tunnel barrier is formed in the shaded region, and the thin wire portions 10 and 23 become silicon islands having both ends sandwiched by the tunnel barrier. Then, the electrode parts 11, 12, 2
A window for an electrode is opened in a part of the silicon oxide film on 4, and an electrode is formed in this part. FIG. 13 is an equivalent circuit diagram of the Coulomb blockade device of the present embodiment,
Reference numeral 1 denotes a capacitance due to the space between the thin wire portion 10 and the adjacent portion 22, and C 22 and C 24 denote tunnel capacitances due to tunnel barriers formed at both ends of the thin wire portion 23.

The device according to the present embodiment is obtained by replacing the MOS transistor Q1 in FIG. 10 with a Coulomb blockade device. The feature of this device is that the silicon island formed in the thin line portion 23 is transferred by the Coulomb blockade effect. Of the electric charge (the number of electrons: in other words, the potential of the island) that is accumulated in the conductive island of the adjacent portion 22 when the voltage V3 is applied from the electrode portion 24. Is to do. That is, the adjacent portion 22 (including the narrow portion 21) can function as a single-electron memory cell. That is, the voltage V3
Is applied to accumulate electric charges in the silicon islands of the fine wire portion 23, the electric charges in the silicon islands are maintained even when the voltage of the electrode portion 24 is set to zero.

Therefore, a switching transistor as in the case of the second embodiment is not necessarily required. Further, in order to operate the Coulomb blockade element including the thin line portions 10 and 23 well, a voltage may be applied to the back gate (substrate silicon).
It is preferable to form a gate electrode that covers the top of the gate electrode 23 and adjust the Coulomb blockade element with the potential of the gate electrode (in this case, the portion other than the gate electrode is reduced in resistance by introducing impurities or the like). It is good to put it).

Embodiment 4 FIG. 14 is a sectional view showing a manufacturing process of a Coulomb blockade device according to another embodiment of the present invention. In order to manufacture the Coulomb blockade device of the present embodiment, the upper silicon layer 4 is
0 to 32, a connection portion 33 serving as a branch point connecting these thin wire portions, electrode portions 34 and 35, and an electrode portion 36 formed with a space for providing a capacitance between the thin wire portions 32 and 14 (see FIG. 14).
(C), thermal oxidation may be performed as in the first embodiment.

Thus, as in the first embodiment, the silicon layer 4 near the pattern edges of the electrode portions 34 to 36 is formed.
(The hatched portion in FIG. 15) is the thinnest. Also, the connection unit 33
Has a larger area than the thin line portions 30 to 32,
Since the influence of the stress is small, the silicon layer 4 becomes thin similarly to the vicinity of the edge.

That is, the element at this time is referred to as AA in FIG.
When cut along the line, a structure like the cross-sectional view of FIG. 16A is formed, and when cut along the line BB of FIG.
The structure as shown in the sectional view of FIG. 6B is formed. As described above, oxidation is promoted in the hatched portions in FIG. 15 and volume expansion occurs. As a result, these portions rise in the film thickness direction as shown in FIGS. 16 (a) and 16 (b).

In FIG. 16B, reference numeral 37 denotes a region where an oxide film is formed between the fine wire portion 32 and the electrode portion 36 by thermal oxidation. In the present embodiment, the space between the fine line portion 32 and the electrode portion 36 is completely buried by the side gate oxide film 37, but a certain amount of space may be provided.

Finally, a window for an electrode is formed in a part of the upper oxide film 5 on the electrode parts 34, 35, 36, and an electrode is formed in each of these parts. Thus, the manufacturing process of the Coulomb blockade device is completed.

FIG. 17 is an equivalent circuit diagram of the Coulomb blockade device of FIG. The thin silicon region (hatched portion in FIG. 15) near the pattern edge of the electrode portion 34 becomes a tunnel barrier due to an increase in base energy, and acts as a tunnel capacitance C31. Similarly, the thin silicon region near the pattern edge of the electrode portion 35 also functions as a tunnel barrier and acts as a tunnel capacitance C34. Further, the thin silicon region formed in the connection portion 33 also functions as a tunnel barrier and acts as tunnel capacitances C32 and C33. Then, the thin line portions 30 to 32 become silicon islands adjacent to the tunnel barrier.

The thin line portion 32 is connected to a side gate capacitance C 35 formed by a side gate oxide film 37. As will be described later, by changing the joint shape between the thin wire portion and the electrode portion, the electrode can be used for introducing and deriving a single electron (introducing a current).
(For derivation) or single-electron transport control (for voltage application), but in the present embodiment, the electrode portions 34 and 35 are for single-electron introduction / derivation, and the electrode portion 36 is for single-electron use. For transport control. The equivalent circuit of FIG. 17 corresponds to a single-electron turn style, which is one of the Coulomb blockade devices having a connection structure having a plurality of silicon islands.

Next, the operation of the Coulomb blockade device having such an equivalent circuit will be described. FIG. 18 is a diagram schematically showing a relationship between single electron energies in the thin wire portions 30 to 32 and the electrode portions 34 and 35. FIG. 18 (a)
A state where a constant voltage (the electrode 35 is positive and the electrode 34 is negative) is applied between the electrode portions 34 and 35 is shown.

The energy level of the silicon island 32 is lower than that of the silicon islands 30 and 31. This is because the silicon island 30 is surrounded by the tunnel capacitors C31 and C32,
Are surrounded by tunnel capacitances C33 and C34, and the thin wire portions 30 and 31 themselves are formed to be as small as possible as described later, whereas the capacitance surrounding the silicon island 32 is connected to the side gate capacitance C35. The reason is that it is larger than the silicon islands 30 and 31 including the fact that they are present.

Then, as shown in FIG.
While a constant voltage is applied between 4 and 35, the electrode section 36
, The level of the silicon islands 30 to 32 rises and falls due to the connection via the capacitor C35. That is, when a positive voltage is applied to the electrode portion 36, the silicon islands 30 to 32
At the time when the level of the silicon island 30 becomes equal to or lower than the level of the electrode portion 34 due to the increase of the positive voltage, FIG.
As shown in (b), the single electron e in the electrode portion 34 is
0, and further to the silicon island 32 having a lower level.

Subsequently, when a negative voltage is applied to the electrode portion 36, the levels of the silicon islands 30 to 32 rise, and the level of the silicon island 31 becomes lower than the level of the silicon island 32 due to the increase in the negative voltage. At this point, as shown in FIG. 18C, the single electron e of the silicon island 32 moves to the silicon island 31 and further moves to the electrode portion 35 having a lower level. Thus, the electrode section 36
By applying an AC voltage to the silicon islands 30, 3 in one cycle of the AC voltage,
It becomes possible to transport to the electrode part 35 via 2, 31.

Next, the forming conditions for realizing such a coupled structure type Coulomb blockade element will be described. In order to realize the above-described single-electron turn-style function in a room temperature environment, it is necessary to make the thin wire portions 30 and 31 as small as possible and effectively form a tunnel barrier.

The dimensions of the thin line portions 30 and 31 before oxidation are film thickness, width (vertical direction in FIG. 15), and length (horizontal direction in FIG. 15).
In both cases, the thickness is preferably about several tens nm or less. In consideration of the oxidation promoting effect and the oxidation suppressing effect described in the first embodiment, the connection portion 33 needs a certain area in order to form the tunnel barrier in the connection portion 33 most effectively.

The reason why the oxidation is most effectively promoted at the connection portion 33 and the tunnel barrier is formed depends on the oxidation temperature and the conditions of the oxidation atmosphere. For example, the thickness of the silicon layer 4 is about 20 nm before oxidation. When the sample is oxidized in a dry oxygen atmosphere at 1000 ° C., the dimension of the thin line portion 32 before oxidation is set to about 100 to 200 nm in both width (horizontal direction in FIG. 15) and length (vertical direction in FIG. 15). I just need.

The reason why oxidation is most effectively promoted in the electrode portions 34 and 35 to form a tunnel barrier depends on the oxidation temperature and the conditions of the oxidation atmosphere. Width of electrode parts 34 and 35 (vertical direction in FIG. 15)
Should be about 400 nm. These dimensional designs are
Needless to say, it is necessary to optimize in accordance with the oxidation conditions and the function of each Coulomb blockade element circuit.

Next, as a forming condition for realizing the Coulomb blockade element, a method of controlling the formation of silicon islands and the degree of connection between silicon islands (tunnel capacitance) will be described. FIG. 19 is a plan view of a Coulomb blockade device for explaining this control and its equivalent circuit diagram.
The same parts as those in FIG. 14 are denoted by the same reference numerals. However, FIG. 19 shows only a silicon layer corresponding to the upper silicon layer 4.

In FIGS. 19 (a), (c) and (e),
The hatched portion indicates a region where a tunnel barrier is formed, and a dotted portion (hereinafter referred to as a Riko ground portion) indicates a region where a silicon island is formed.

As shown in FIG. 19A, when the silicon layer is processed into a T-shape in which the width W2 of the thin wire portion 32 is large and thermal oxidation is performed, the area of the thin wire portion 32 and the connection portion 33 are sufficiently large. Oxidation is promoted due to less stress effect. Therefore, a tunnel barrier is formed in this region, and the thin line portion 3 is formed.
2 cannot have a silicon island. As a result, FIG.
The equivalent circuit of the element is a circuit as shown in FIG. 19B in which two silicon islands 30 and 31 are connected by a tunnel capacitance C36.

Further, as shown in FIG. 19C, when the silicon layer is processed into a T-shape in which the width W2 of the thin wire portion 32 is small to some extent, thermal oxidation is performed. 3
Also in the case of No. 2, oxidation is suppressed by stress, and a silicon island is formed. At this time, a tunnel barrier is formed in the connection portion 33, but the silicon islands 30, 31 are compared with those between the silicon islands 30, 32 or between the silicon islands 31, 32.
Since the distance between the silicon islands 30 and 31 is sufficiently small, the frequency of electron tunneling between the silicon islands 30 and 31 is low, and the tunnel capacitance C37
Becomes smaller.

As a result, an effective equivalent circuit of the element shown in FIG. 19C is a circuit as shown in FIG. 19D in which silicon islands 30, 32, and 31 are connected in series. This structure corresponds to the Coulomb blockade element in FIG. Further, as shown in FIG. 19E, the width W2
When the thermal oxidation is performed with a small value, the junction between the silicon islands 30 and 31 cannot be ignored. As a result, an equivalent circuit of the element of FIG. 19E is obtained by connecting three silicon islands 30 to 32 with tunnel capacitances C32, C33, and C37 in FIG.
The circuit is as shown in FIG.

The actual size of the width W2 corresponding to each case depends on the oxidation temperature and the conditions of the oxidation atmosphere. For example, the thickness of the silicon layer is about 20 nm before oxidation, , 31 have a width W1 of about 30 to 40 nm,
For a sample in which the length L1 of the thin line portion 32 is about 100 nm,
When oxidation is performed in a dry oxygen atmosphere at 1000 ° C., the width W
2 is about 100 nm or less, and becomes the case of FIG.
The case of FIG. 19C is from 100 nm to several 100 nm, and the case of FIG. However, the value of W2 at the boundary between FIGS.
2 also strongly depends on the value of the length L1 (for example, when L1 is about 100 nm as described above, W2 is about 100 nm), and as the length L1 increases, the boundary W2 value decreases.

As described above, by changing the shape or size of the silicon fine wire, the method of forming the silicon island,
Degree of connection between silicon islands (size of tunnel capacity)
Etc. can be controlled. Note that the connection part 33 described in the present embodiment corresponds to W1 in the vertical direction in FIG.
And has a size of W2 in the left-right direction.

Next, a method for controlling the bonding mode of the electrodes will be described as a forming condition for realizing the Coulomb blockade element. FIG. 20 is a plan view of a thin line portion and an electrode portion for explaining this control and an equivalent circuit diagram thereof. However, FIG. 20 shows only a silicon layer corresponding to the upper silicon layer 4 of FIG.

As shown in FIG. 20A, the silicon layer is processed into a shape having a thin line portion 38 and an electrode portion 39a having a tapered region whose width becomes narrower as approaching the thin line portion 38, and thermal oxidation is performed. And the tunnel barrier is the electrode 39
a is formed in the vicinity of the edge indicated by the hatched portion,
8 is not formed in the vicinity. This is because, since the area of the portion connected to the thin wire portion 38 is small, oxidation is suppressed by stress. Therefore, the thin wire portion 38 and the electrode portion 39
a is connected without a tunnel barrier (tunnel capacitance) as shown in FIG.

The electrode portion 39a having such a bonding form is used as an electrode for introducing / leading out single electrons (for introducing / leading out current). In addition, depending on the shape of the tapered region,
In some cases, a tunnel barrier having a low potential is formed in the tapered region. However, a tunnel barrier having a low potential can be effectively ignored by applying a voltage. It may be used for.

Also, as shown in FIG.
When the silicon layer is processed into a shape having the electrode portion 39b and thermal oxidation is performed (between the thin line portion 30 and the electrode portion 34 in FIG.
Between the thin wire portion 31 and the electrode portion 35), the electrode portion 39b
A tunnel barrier having a sufficient potential height is formed in the vicinity of the edge of. Therefore, the thin wire portion 38 and the electrode portion 39
b is connected via a tunnel capacitance C38 as shown in FIG. The electrode portion 39b having such a bonding form
Is used as an electrode for introducing and deriving a single electron (for introducing and deriving a current) through a tunnel capacitor.

Also, as shown in FIG.
When the silicon layer is processed into a shape having an electrode portion 39c formed with a space between the silicon layer and the silicon layer (corresponding to a portion between the thin line portion 32 and the electrode portion 36 in FIG. 1), the space is filled by the oxidation process, Impossible joining will result.

Therefore, the thin wire portion 38 and the electrode portion 39c are connected via the capacitor C39 as shown in FIG. The electrode portion 39c having such a bonding form is used as a single-electron transport control electrode for controlling single-electron transport by applying a voltage. Further, the size of the capacitor C39 can be controlled by the size of the space. In this case as well, a tunnel barrier having a low potential may be formed in the tapered region. By changing the bonding mode in this manner, the role of the electrode portion can be changed.

Such an electrode structure has the same shape as the tip of the adjacent portion 13 to the thin line portion 10 in FIG. 8 of the second embodiment, or the width of the adjacent portion 22 of the third embodiment in FIG. It can be applied to the connection shape of the narrow portion 21. In either embodiment, the operation is performed using either the shape shown in FIG. 20 (c) or (e). However, as can be seen from FIG. 20, the equivalent circuit includes a case where the tunnel capacitance is added (the structure of FIG. 20C) and a case where the tunnel capacitance is not added (FIG. 20).
(E), which is different from one another, so which one should be selected depending on the size of each constituent capacitor (determined by the size of each part), operating conditions (set values of voltage and current), and the machine. Is effective.

The silicon island, the shape of the tunnel barrier, and the potential height of the tunnel barrier as described above are controlled not only by the above formation conditions but also by the conditions (oxidation temperature and oxidation time) during the oxidation process. . As the oxidation temperature becomes lower, the effect of suppressing the oxidation due to the stress becomes more remarkable, so that the difference in silicon film thickness between the region where the silicon island is formed and the region where the tunnel barrier is formed can be increased.

5. Embodiment 5 In the fourth embodiment, the electrode portion 36 formed of the upper silicon layer is used as the single-electron transport control electrode. However, the gates are formed so as to cover all or a part of the thin wire portions 30 to 32 and the connection portion 33 from above. The electrode may be formed of polysilicon or the like, and this may be used as a single-electron transport control electrode instead of the electrode section 36. In this case, the upper oxide film 5 is used as a gate insulating film (corresponding to the capacitor C35).

Sixth Embodiment Further, in the fourth embodiment, the T-shaped thin line portion including the thin line portions 30 to 32 and the connecting portion 33 is used. However, the T-shaped thin line portion may not be used. FIG. 21 is a plan view of a Coulomb blockade device according to another embodiment of the present invention, in which a hatched portion indicates a region where a tunnel barrier is formed, and a Riko ground portion indicates a region where a silicon island is formed.

As shown in FIG. 21A, two thin line portions 40
When a silicon layer is processed into an L-shape having a connection portion 41a serving as a bending point connecting them and subjected to thermal oxidation, a tunnel barrier is formed at the connection portion 41a, and a silicon island is formed at the two thin wire portions 40a. It is formed. FIG.
As shown in (b), when the silicon layer is processed into a radial type having a connection portion 41b and six thin wire portions 40b radially extending from the connection portion 41b and thermal oxidation is performed, a tunnel barrier is formed at the connection portion 41b, and the thin wire portion is formed. A silicon island is formed at 40b.

Further, the thin wire portion 40c and the connection portion 41c are formed as shown in FIG.
The thermal oxidation may be performed by processing the silicon layer into a matrix type arranged as shown in FIG. In this embodiment, the electrode portions connected to the ends of the thin wire portions 40a to 40c are not shown. However, the electrode portions are closed as in the thin wire portion 32 of the first embodiment, or as shown in FIG. , Or a gate electrode may be formed as in the fifth embodiment. Needless to say, it is possible to create a new sequence by connecting each of FIGS. 21 (a) to 21 (c).

Seventh Embodiment Also, in the fourth to fourth embodiments.
In FIG. 6, all the patterns are constituted by straight lines, but may be constituted by curves. By using the curve, the degree of freedom in designing the shape of the thin line portion and the like is further increased, and the variation of the connection structure that can be realized is further increased.

8. Embodiment 8 FIG. 22 is a plan view of a Coulomb blockade device showing another embodiment of the present invention, and the same parts as those in FIG. 14 are denoted by the same reference numerals.
However, FIG. 22 shows only a silicon layer corresponding to the upper silicon layer 4. In order to manufacture such a Coulomb blockade element, the silicon layer may be processed into a shape as shown by the solid line in FIG. 22, and thermal oxidation may be performed as in the first embodiment.

As a result, a tunnel barrier is formed in the hatched area, and a silicon island is formed in the Rikochi area. Then, a window is opened in a part of the oxide film on the electrode parts 34, 36, 43, and 44, and a lead electrode is formed in each of these parts. Note that the adjacent portion 35a connected to a single-electron transistor described later with a space therebetween functions as a silicon island serving as a single-electron memory cell, and thus no extraction electrode is formed. Thus, the electrode portions 34, 43, 44
Is an electrode for introducing and deriving a single electron, and the electrode section 36 is an electrode for controlling single electron transport.

FIG. 23 is an equivalent circuit diagram of the Coulomb blockade device of FIG. In this Coulomb blockade element, a single-electron turn style Q31 similar to that of the first embodiment is connected as a single-electron write line to a silicon island serving as a single-electron memory cell formed in the adjacent portion 35a, A single-electron transistor Q32 comprising a portion 42 and electrode portions 43 and 44 is connected as an electron read line.

In such a Coulomb blockade device, an appropriate negative voltage is set to the electrode portion 34 and the electrode portion 3
By applying an AC voltage to 6, it becomes possible to introduce electrons one by one into the silicon island formed in the adjacent portion 35a. The number of electrons in the silicon island can be read as a change in conductance between the electrode portions 43 and 44 in the single-electron transistor Q32.

9. Embodiment 9 FIG. 24 is a plan view of a Coulomb blockade device showing another embodiment of the present invention. However, FIG. 24 shows only a silicon layer corresponding to the upper silicon layer 4 in FIG. In order to fabricate such a Coulomb blockade device, a silicon layer is formed as shown in FIG.
Then, thermal oxidation may be performed in the same manner as in the first embodiment.

As a result, a tunnel barrier is formed in the hatched area, and a silicon island is formed in the Rikochi area. Then, windows are opened in a part of the oxide film on the electrode parts 45 and 48 to 50, and electrodes are formed in these parts, respectively. Thus, the electrode portions 45 and 48 become single-electrode introduction / derivation electrodes, and the electrode portions 49 and 50 become single-electron transport control electrodes. FIG. 25 is an equivalent circuit diagram of the Coulomb blockade element. This connection type Coulomb blockade element is called a single electron pump.

Next, the operation of the Coulomb blockade device having such an equivalent circuit will be described. FIG. 26 is a diagram schematically showing the energy relationship of single electrons in the thin wire portions 46 and 47 and the electrode portions 45 and 48. FIGS. 27A and 27B show the voltage waveforms applied to the electrode portions 49 and 50, respectively. FIG. In the case of this Coulomb blockade element, unlike the case of the single electron turn style, the electrode portions 45 and 48 are different.
A constant voltage may or may not be applied during.

FIG. 26A shows the initial state (time t1 in FIG. 27). Then, at time t2 in FIG.
When the positive voltage applied to the electrode portion 49 increases as shown in FIG. 26, the single electron e in the electrode portion 45 moves to the silicon island 46 as shown in FIG. Subsequently, when the voltage applied to the electrode portion 49 approaches zero at time t3, the electrons e in the silicon island 46 try to return to the electrode portion 45 again, but as shown in FIG. Since the positive voltage applied to the portion 50 increases, the single electron e of the silicon island 46 moves to the silicon island 47 as shown in FIG.

Finally, at time t4, when the voltage applied to the electrode portion 50 becomes zero and the voltage applied to the electrode portion 49 becomes negative, the single electron e moves to the electrode portion. Thus,
By applying an AC voltage having a phase shift to the electrode portions 49 and 50, one electron is transferred from the electrode portion 45 to the silicon island 46,
It becomes possible to transport to the electrode section 48 via 47.

10. Tenth Embodiment FIG. 28 is a plan view of a Coulomb blockade device showing another embodiment of the present invention. However, FIG. 28 shows only a silicon layer corresponding to the upper silicon layer 4 in FIG. In order to fabricate such a Coulomb blockade device, the silicon layer is formed as shown in FIG.
8 and then thermal oxidation may be performed in the same manner as in the first embodiment.

As a result, a tunnel barrier is formed in the hatched area, and a silicon island is formed in the Rikochi area. Then, windows are opened in a part of the oxide film on the electrode parts 51 to 58, and electrodes are formed in these parts. Thus, the electrode portions 51, 52, 57, and 58 become single-electron introduction / derivation electrodes, and the electrode portions 53 to 56 become single-electron transport control electrodes. FIG. 29 is an equivalent circuit diagram of this Coulomb blockade element. The Coulomb blockade element of this connection type has two inputs (electrodes 51 and 52) and two outputs (electrodes 57,
58) is a single electron transport circuit.

Next, as an operation of the Coulomb blockade device having such an equivalent circuit, a case where a single electron is transported from the electrode portion 51 to the electrode portion 58 will be described. FIG.
0 (a) to (d) are diagrams showing voltage waveforms applied to the electrode units 53 to 56, respectively. When the positive voltage applied to the electrode unit 53 increases at time t1 in FIG. 30A, the single electron e in the electrode unit 51 is changed to the silicon formed in the thin line unit 59 as shown by the broken line in FIG. It moves to the silicon island formed in the thin line portion 60 via the island.

At time t2, a negative voltage is applied to the electrode portion 53 and a positive voltage is applied to the electrode portion 56 as shown in FIG.
Move from the silicon island of the thin line portion 60 to the silicon island formed in the thin line portion 61. Finally, at time t3, a negative voltage is applied to the electrode portions 53 to 56 to thereby generate a single electron e.
Moves to the electrode section 58.

In this way, by applying an AC or pulse-like voltage to the electrode portions 53 to 56 at an appropriate phase or timing with respect to the input voltage to the electrode portions 51 and 52, the electrodes 51 and 52 are applied to the electrodes 57 and 58. Transport of single electrons to the substrate can be realized. Therefore, the electrode portions 53 to 56
The magnitude or phase of the AC or pulsed voltage applied to
By controlling the timing, the function of the output with respect to the input can be controlled. In order to realize such a function, it is necessary to manufacture the four silicon islands 59, 62 to 64 in contact with the electrode portions 51, 52, 57, 58 particularly smaller than other silicon islands. .

Embodiment 11 In the above embodiment,
SOI with single crystal silicon formed on silicon oxide film
An example using a substrate has been described. The same principle holds when an amorphous silicon or polycrystalline silicon layer is formed on a silicon oxide film. Therefore, even if these are used, if a method and structure similar to those of the above-described embodiments 1 to 10 are used, The same effect can be obtained.

Embodiment 12 Further, in the above embodiment, the thermal oxidation is performed while leaving the upper oxide film used as a processing mask at the time of forming the pattern of the upper silicon layer. Since the silicon layer can be selectively removed after processing, it goes without saying that the upper oxide film is not required.

In the above embodiment, the upper oxide film having a thickness of about 30 nm is formed on the upper silicon layer. However, if the upper oxide film is thickened, for example, to about 60 nm, the upper oxide film becomes thinner. Since the amount of oxidizing agent passing therethrough is reduced, oxidation from the upper side is suppressed, and oxidation from the lower side due to lateral diffusion of the oxidizing agent becomes dominant, so that a flat portion serving as an electrode (the upper silicon layer away from the edge) is formed. Since the oxidation from the upper side is small (the oxidation from the lower side is far from the edge and hardly occurs), a tunnel barrier is formed at both ends of the thin wire portion, that is, since the silicon layer from the top is less reduced, It is easy to set the amount of oxidation and the silicon layer that is finally left. Further, since the thickness of the upper oxide film is large, the stress accompanying the oxidation is further increased, so that the constriction can be effectively formed.

In the same sense, when a silicon nitride film is used instead of the upper oxide film serving as a mask film or a silicon nitride film is laminated on the upper oxide film, the next step of thermal oxidation is prevented. When performed, oxidation from the upper side through the nitride film is suppressed, so that only the oxidation from the side wall of the upper silicon layer and the buried oxide film side is mainly performed, and there is almost no decrease in the silicon layer from the upper side. It is more effective than a method using only a film. Further, since the stress due to the silicon nitride film is large, the stress due to oxidation is also large, and the reduction in the thickness of the constriction is emphasized with a small amount of oxidation, which is effective for obtaining the structure of the above embodiment. .

[0127]

According to the present invention, by making the film thickness of the first and second electrode portions near the thin wire portion smaller than that of the thin wire portion, a tunnel barrier having higher energy than the thin wire portion is formed at both ends of the thin wire portion. Since the conductor island is formed in the thin wire portion, a Coulomb blockade element operating at a high temperature can be easily realized. In addition, M of conventional silicon
Since the OS structure can be used, the manufacturing process technology of the silicon-based integrated circuit can be used, the Coulomb blockade element can be mounted on the same substrate as the conventional silicon-based integrated circuit, and a large-scale circuit can be realized. .

Further, by providing a third electrode portion serving as a drain, a source, and a channel region of the MOS transistor adjacent to the thin line portion, a Coulomb blockade device and a MOS transistor are capacitively connected to form a memory device. A functioning Coulomb blockade element can be easily realized.

By making the adjacent portion adjacent to the first thin line portion, a Coulomb blockade element comprising the first thin line portion, the first and second electrode portions, and the second thin line portion, the adjacent portion, A Coulomb blockade element functioning as a memory element, which is capacitively connected to a Coulomb blockade element comprising the fourth electrode portion, can be realized.

Further, the silicon layer is formed by the thin wire portion and the first and second portions.
By processing the silicon layer into a shape having an electrode part and thermally oxidizing the silicon layer, a constriction with the thinnest film thickness is automatically formed near the thin line part of the electrode part, and a tunnel barrier is formed near this thin line part. The conductor island is formed in the thin wire portion. Since the horizontal dimension of the conductor island can be set independently of the size of the constriction, the conductor island can be reduced, and the conductor island can be further reduced by thermal oxidation. Therefore, a Coulomb blockade device that operates at a high temperature can be realized by a simple manufacturing process similar to the conventional silicon integrated circuit manufacturing process technology. The formation of the constriction can be controlled by the pattern size of the electrode part and the thermal oxidation conditions. The thermal oxidation technology is particularly excellent in controllability and reproducibility among silicon LSI processing technologies. It can be realized with good controllability and reproducibility.

By providing a plurality of thin wire portions and connecting portions and making the thickness of the connecting portions thinner than that of the thin wire portions, a tunnel barrier is formed at the connecting portions, and a conductor island is formed at the thin wire portions. Since the connection structure of the conductor islands is formed via the capacitance by the tunnel barrier, a Coulomb blockade element of a connection structure type that operates at a high temperature can be easily realized. If the thin wire portion and the connection portion are designed to be spread on a two-dimensional plane, a Coulomb blockade element of a connection structure in which conductor islands are arranged in the two-dimensional plane can be manufactured with high flexibility. Also, since the manufacturing process of silicon MOS can be used,
A coulomb blockade element of a connection structure type can be mounted on the same substrate as a conventional silicon integrated circuit.
By hybridizing with a circuit using an OS, a large-scale and high-performance circuit can be realized.

By providing at least one electrode portion wider than the thin wire portion formed to be connected to the end of the thin wire portion, this electrode portion can be used as a single electron introduction electrode.

Further, by making the film thickness near the thin line portion of the electrode portion smaller than that of the thin line portion, a tunnel barrier is formed near the thin line portion of the electrode portion. The thin wire portion can be connected to an electrode portion serving as a single-electron introduction electrode.

By providing at least one electrode portion wider than the thin wire portion and having a space for providing a capacitance between the thin wire portion and the insulating film or an insulating film, the electrode portion is provided. Can be used as a single-electron transport control electrode, and the thin wire portion serving as a conductor island can be connected to the electrode portion via a space or a capacitance of an insulating film.

Also, at least one of the thin wire portion and the connection portion
By providing a gate electrode formed with an insulating film in the portion, the thin wire portion serving as a conductor island and the gate electrode serving as a single-electron transport control electrode can be connected through the capacitance of the insulating film.

Further, by processing the silicon layer into a shape having a plurality of fine wire portions and connection portions and thermally oxidizing the silicon layer, a constriction having the smallest film thickness is automatically formed at the connection portion. Since a tunnel barrier is formed in the thin wire portion and a conductive island is formed in the thin wire portion, and a connecting structure of the conductive island is formed through the capacitance by the tunnel barrier, a Coulomb blockade element having a connecting structure operating at a high temperature is formed. The present invention can be realized by a simple manufacturing process similar to the conventional silicon integrated circuit manufacturing process technology. The formation of the constriction can be controlled by the pattern size and the thermal oxidation conditions, and the thermal oxidation technology is particularly excellent in controllability and reproducibility among silicon LSI processing technologies. Can be realized with good controllability and reproducibility.

Further, at least one electrode portion wider than the thin line portion connected to the end of the thin line portion is provided on the silicon layer before the thermal oxidation step, and the silicon layer is thermally oxidized to thereby form the electrode portion. A constriction with the thinnest film thickness is automatically formed in the vicinity of the thin line portion, and a tunnel barrier is formed in the vicinity of this thin line portion. Therefore, the connection structure between the conductor island and the electrode portion through the tunnel capacitance is controlled. It can be realized with good reproducibility.

Further, the silicon layer before the thermal oxidation step is provided with at least one electrode portion wider than the fine wire portion and connected to the end of the fine wire portion so that the width increases as the distance from the fine wire portion increases. By thermally oxidizing the silicon layer,
An electrode portion in which no constriction is formed can be realized, and a connection structure between the conductor island and the electrode portion without a tunnel capacitor can be realized with good controllability and reproducibility.

Further, by forming an insulating film containing silicon on the silicon layer before the thermal oxidation step, the stress accompanying oxidation changes depending on the thickness of the insulating film, so that the formation of constriction can be controlled. .

Further, by using the silicon-containing insulating film as the silicon-containing insulating film, the silicon layer can be oxidized only from the lower side, and the amount of oxidation and the remaining silicon film thickness can be easily designed. In addition, the stress due to the silicon nitride film is added, and the formation of the constriction is accelerated.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a process of manufacturing a Coulomb blockade device according to a first embodiment of the present invention.

FIG. 2 is a plan view of a pattern of a silicon layer as viewed from above.

FIG. 3 shows the device after thermal oxidation of FIG.
It is sectional drawing cut | disconnected by the line.

FIG. 4 is an energy band diagram schematically showing the principle of the Coulomb blockade device of FIG.

FIG. 5 is a schematic diagram of the Coulomb blockade device of FIG. 1;

FIG. 6 is an equivalent circuit diagram of the Coulomb blockade device of FIG. 1;

FIG. 7 is a diagram showing characteristics of the Coulomb blockade device of FIG. 1;

FIG. 8 is a plan view of a Coulomb blockade device showing another embodiment of the present invention.

9 is a sectional view taken along line II-II in FIG.

FIG. 10 is an equivalent circuit diagram of the Coulomb blockade device of FIG.

11 is a circuit diagram of a memory device using a plurality of Coulomb blockade devices of FIG.

FIG. 12 is a plan view of a Coulomb blockade device showing another embodiment of the present invention.

FIG. 13 is an equivalent circuit diagram of the Coulomb blockade device of FIG.

FIG. 14 is a cross-sectional view showing a step of manufacturing a Coulomb blockade element according to another embodiment of the present invention.

FIG. 15 is a plan view of the pattern of the silicon layer as viewed from above.

FIG. 16 is a cross-sectional view of the device after thermal oxidation of FIG. 14C, taken along line AA and line BB in FIG.

17 is an equivalent circuit diagram of the Coulomb blockade device of FIG.

18 is a diagram schematically showing a relationship between single-electron energies in a thin line portion and an electrode portion in FIG.

FIG. 19 is a plan view for explaining formation of a silicon island and control of the degree of connection between the silicon islands, and an equivalent circuit diagram thereof.

FIG. 20 is a plan view and an equivalent circuit diagram for explaining control of a bonding mode of electrodes.

FIG. 21 is a plan view of a Coulomb blockade device showing another embodiment of the present invention.

FIG. 22 is a plan view of a Coulomb blockade device showing another embodiment of the present invention.

FIG. 23 is an equivalent circuit diagram of the Coulomb blockade device of FIG. 22.

FIG. 24 is a plan view of a Coulomb blockade device showing another embodiment of the present invention.

FIG. 25 is an equivalent circuit diagram of the Coulomb blockade device of FIG. 24.

26 is a diagram schematically showing the relationship between the energy of a single electron in the thin line portion and the electrode portion in FIG.

FIG. 27 is a diagram showing a voltage waveform applied to the single electron transport control electrode unit of FIG. 24.

FIG. 28 is a plan view of a Coulomb blockade device showing another embodiment of the present invention.

FIG. 29 is an equivalent circuit diagram of the Coulomb blockade device of FIG. 28.

30 is a diagram showing a voltage waveform applied to the single electron transport control electrode unit of FIG. 28.

FIG. 31 is a bird's-eye view of a conventional Coulomb blockade element viewed from obliquely above.

FIG. 32 is an equivalent circuit diagram of the Coulomb blockade device of FIG. 31.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 SOI substrate 2 substrate silicon 3 buried oxide film 4 upper silicon layer 5 upper oxide film 10 and 2
3, 30-32, 38, 40a-40c, 42, 46,
47, 59 to 64: Thin line portion, 33, 41a to 41c: Connection portion, 11, 12, 34, 35, 39a, 39b, 4
3, 44, 45, 48, 51, 52, 57, 58... Electrodes for single electron introduction / extraction, 36, 39c, 49, 50,
53 to 56: Electrodes for controlling single electron transport.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-8-306904 (JP, A) IEDM94, (1994), pp. 1-26. 938-940 (58) Field surveyed (Int. Cl. ⁷ , DB name) H01L 29/00-29/96 JICST file (JOIS)

Claims (14)

(57) [Claims] 1. On a substrate having a silicon layer formed on an insulating film, the silicon layer is formed so as to be connected to both ends of a thin wire portion serving as a conductor island for confining electric charges and both ends of the thin wire portion. A Coulomb blockade element having first and second electrode portions that are wider than the thin line portion and have a film thickness near the thin line portion smaller than the thin line portion. 2. On a substrate having a silicon layer formed on an insulating film, the silicon layer is formed so as to be connected to both ends of a thin wire portion serving as a conductive island for confining electric charges and both ends of the thin wire portion. The first and second electrode portions, which are wider than the thin line portion and have a smaller film thickness in the vicinity of the thin line portion, are formed with a space or an insulating film for providing a capacitance between the thin line portion and the first and second electrode portions. A third electrode portion serving as a drain, source, and channel region of the MOS transistor, and a gate electrode of the MOS transistor formed via an insulating film on a portion of the third electrode portion. Coulomb blockade element. 3. On a substrate having a silicon layer formed on an insulating film, the silicon layer has a first thin line portion serving as a conductive island for confining electric charges, and a first thin line portion at both ends of the first thin line portion. Formed to connect,
A first and a second electrode portion which are wider than the first thin wire portion and have a smaller film thickness in the vicinity of the thin wire portion than the first thin wire portion; and a second thin wire portion which becomes a conductor island for confining electric charges. A width for forming a capacitor between the first thin line portion and an insulating film, and a width smaller than that of the second thin line portion formed to be connected to one end of the second thin line portion. A second portion formed so as to be connected to an adjacent portion which is wider and thinner in the vicinity of the thin line portion than the second thin line portion, and to the other end of the second thin line portion;
And a fourth electrode portion having a width wider than the thin line portion and a film thickness near the thin line portion being thinner than the second thin line portion. 4. On a substrate in which a silicon layer is formed on an insulating film, the silicon layer is formed by forming a thin line portion serving as a conductor island for confining electric charges, and a narrower line portion at both ends of the thin line portion than the thin line portion. A step of processing the silicon layer into a shape having first and second electrode portions; and a step of thermally oxidizing the silicon layer.
The method of manufacturing a Coulomb blockade device, wherein the film thickness of the electrode portion is formed to be thinner in the vicinity of the thin wire portion than in the thin wire portion. 5. On a substrate in which a silicon layer is formed on an insulating film, the silicon layer has a plurality of thin wire portions serving as conductor islands for confining electric charges, and a branch point or bend connecting these thin wire portions. A Coulomb blockade element having a connection portion formed to be a point and having a thickness smaller than a thin line portion. 6. The Coulomb blockade device according to claim 5, further comprising at least one electrode portion formed so as to be connected to an end of the thin wire portion and having a width wider than the thin wire portion. Coulomb blockade element. 7. The Coulomb blockade device according to claim 6, wherein the electrode portion has a smaller film thickness in the vicinity of the thin wire portion than in the thin wire portion. 8. The Coulomb blockade device according to claim 5, wherein at least one wider than the thin wire portion is formed with a space for providing a capacitance between the end of the thin wire portion and an insulating film. A Coulomb blockade device having the above electrode portion. 9. The Coulomb blockade device according to claim 5, further comprising a gate electrode formed on at least a portion of the thin line portion and the connection portion via an insulating film. 10. On a substrate on which a silicon layer is formed on an insulating film, a plurality of thin wire portions serving as conductor islands for confining electric charges, and branch points or bending points connecting these thin wire portions are formed on the silicon layer. A step of processing the silicon layer into a shape having a connection portion, and a step of thermally oxidizing the silicon layer, wherein the thickness of the connection portion is formed to be thinner than that of the thin wire portion. A method for manufacturing a blockade element. 11. The method for manufacturing a Coulomb blockade device according to claim 10, wherein the silicon layer before the thermal oxidation step has at least one or more electrode portions wider than a thin wire portion connected to an end of the thin wire portion. A method of manufacturing a Coulomb blockade element, wherein the film thickness of the electrode portion in the vicinity of the thin wire portion is formed to be thinner than that of the thin wire portion. 12. The method of manufacturing a Coulomb blockade device according to claim 10, wherein the silicon layer before the thermal oxidation step is connected to an end of the thin wire portion so that the width increases as the distance from the thin wire portion increases. A method for manufacturing a Coulomb blockade device, comprising at least one electrode portion wider than a portion. 13. The method for manufacturing a Coulomb blockade device according to claim 4, wherein an insulating film containing silicon is formed on the silicon layer before the thermal oxidation step. A method for manufacturing a blockade element. 14. The method for manufacturing a Coulomb blockade device according to claim 13, wherein the insulating film is a silicon nitride film.