Coulomb-blockade element and method of manufacturing the same

Abstract: A Coulomb-blockade element includes a silicon layer (4) formed on a substrate (2) through an insulating film (3). The silicon layer includes a plurality of narrow wire portions (30-32) and a connecting portion (33). The narrow wire portion serves as a conductive island for confining a charge. The connecting portion is formed as a branching or bending point for coupling the narrow wire portions and has constrictions on at least one of the upper and lower surfaces thereof, which make a portion near the narrow wire portion thinner than the narrow wire portion.

Description: Background of the Invention The present invention relates to an element using the Coulomb-blockade phenomenon and formed on a silicon substrate, and a method of manufacturing the same. The Coulomb-blockade phenomenon in electron tunneling at a small tunnel junction is a phenomenon in which tunneling of one electron is suppressed as free energy based on charging energy accompanying the tunneling increases. A Coulomb-blockade element using such Coulomb-blockade phenomenon can control currents and charges flowing out from or stored in the element in units of electrons. For this reason, the power consumption per element is very small. In addition, the device area is also very small. Owing to these characteristics, an integration degree much higher than the integration limits of the existing silicon-based integrated circuits is expected. As the basic structure of this element, a single electron transistor or a single electron memory has been proposed. Most of conventional Coulomb-blockade elements have a structure in which electrons are confined in a small metal island by using a small tunnel junction of a metal/metal oxide or a structure in which a two-dimensional electron gas formed at a heterojunction of a III-V compound semiconductor is confined in the form of an island by using an electric field generated by a narrow electrode or the like formed on the junction. FIG. 18 shows the conventional Coulomb-blockade element disclosed in "Single-Electron Charging and Periodic Conductance Resonances in GaAs Nanostructures", U. Meirav et. al., Phys. Rev. Lett., Vol. 65, No. 6, pp. 771 - 774, 1990. FIG. 19 shows an equivalent circuit of this Coulomb-blockade element. Reference numeral 71 denotes a substrate consisting of n-type GaAs; 72, an AlGaAs layer; 73, a GaAs layer; and 74, an electrode formed on the GaAs layer 73. In such a Coulomb-blockade element, a two-dimensional electrode gas is formed at the. heterointerface between the AlGaAs layer 72 and the GaAs layer 73. Constrictions 75 which are constricted in the horizontal direction are formed on the electrode 74 to form potential barriers. A region 76 between these barriers becomes a conductive island for confining charges. The potential barrier between the conductive island 76 and a source electrode 77 serves as a tunnel capacitance Cs. The potential barrier between the conductive island 76 and a drain electrode 78 serves as a tunnel capacitance Cd. As a result, an element having an equivalent circuit like the one shown in FIG. 19 is formed. One of the most important subjects in putting such a Coulomb-blockade element to practical use is an operating temperature. In order to operate the Coulomb-blockade element at a practical temperature, a conductive island serving as an electron reservoir, which is the core of the element, must be formed on the nm scale, and a tunnel barrier having a very small capacitance of several aF (1 aF = 10 -18 F) must be formed. This is because the charging energy of a single electron is buried in thermal energy as the sizes of these portions increase, and no Coulomb-blockade phenomenon can be observed. Another important subject is how to manufacture and arrange such minute structures with good controllability. In order to realize a new function by coupling Coulomb-blockade elements, in particular, a manufacturing technique with good controllability is required. In the Coulomb-blockade element shown in FIG. 18, the width (in the lateral direction in FIG. 18) of each constriction 75, where the gap in the electrode 74 is the smallest, must be sufficiently smaller than the width of the conductive island 76. For this reason, if the constriction 75 is to be manufactured by electron beam lithography, the size of the island 76 inevitably becomes much larger than the minimum dimensions determined by the lithography limit. With regard to a Coulomb-blockade using a metal, since no effective working technique of forming a metal island is available, it is difficult to form a small metal island with good reproducibility. Either of the elements having the above structures, therefore, can operate only at a very low temperature of 1 K or less. A method of forming a conductive island by using fluctuations of a structure made of a thin polycrystalline material is known as an effective means for attaining a small capacitance (K. Yano et al., "Room-Temperature Single-Electron Memory", IEEE Trans. Electron Devices, Vol. 41, p. 1628, 1994). In this method, however, the position and the size of a conductive island cannot be arbitrarily controlled because fluctuations caused by a polycrystalline structure are used. As described above, in the conventional methods, a technique of manufacturing a very small conductive island and a technique of positioning the conductive island with good reproducibility have not been established. These techniques are indispensable for putting a Coulomb-blockade element to practical use. Therefore, a Coulomb-blockade element which can operate at room temperature cannot be realized with good controllability and good reproductibility. A further element is disclosed in IEEE IEDM, 11-14 Dec, pages 438-440. Summary of the Invention It is, therefore, a principal object of the present invention to provide a Coulomb-blockade element which can be manufactured by a simple manufacturing process and can operate at high temperatures, and a method of manufacturing the same. In order to achieve the above object, according to an aspect of the present invention, there is provided a Coulomb-blockade element as claimed in claim 1 comprising a silicon layer formed on a substrate through an insulating film, the silicon layer including a plurality of narrow wire portions serving as conductive islands for confining charges; and a connecting portion which is formed as a branching or bending point for coupling the narrow wire portions to each other, the connecting portion having constrictions on at least one of upper and lower surfaces thereof, which make a portion near the narrow wire portion thinner than the narrow wire portion. According to another aspect of the present invention, there is provided a method of manufacturing a Coulomb-blockade element as claimed in claim 6 comprising the step of processing a silicon layer formed on a substrate through an insulating film into a shape including a plurality of narrow wire portions serving as conductive islands for confining charges, and a connecting portion serving as a branching or bending point for coupling the narrow wire portions to each other, and the thermal oxidation step of thermally oxidizing the silicon layer, the connecting portion being formed to be thinner than the narrow wire portion at positions near the narrow wire portion. Brief Description of the Drawings Figs. 1A to 1C are sectional views showing the steps in manufacturing a Coulomb-blockade element according to the first embodiment of the present invention; Fig. 2 is a plan view showing the pattern of a silicon layer; Figs. 3A and B are sectional views taken along lines A-A and B-B of Fig. 2 illustrating the element after thermal oxidation in Fig. 1C; Fig. 4 is an equivalent circuit diagram of the Coulomb-blockade element in Fig. 1; Fig. 5A to 5C are views illustrating the energy diagram for a single electron between narrow wire portions and electrode portions; Figs. 6A to 6F are plan views and equivalent circuit diagrams for explaining the formation of silicon islands and control-of the degree of coupling between the silicon islands; Figs. 7A to 7F are plan views and equivalent circuit diagrams for explaining control of the forms of the junctions between electrodes; Figs. 8A to 8C are plan views showing Coulomb-blockade elements according to another embodiment of the present invention; Fig. 9 is a plan view showing a Coulomb-blockade element according to still another embodiment of the present invention; Fig. 10 is an equivalent circuit diagram of the Coulomb-blockade element in Fig. 9; Fig. 11 is a plan view showing a Coulomb-blockade element according to still another embodiment of the present invention; Fig. 12 is an equivalent circuit diagram of the Coulomb-blockade element in Fig. 11; Figs. 13A to 13D are views showing the energy diagram for a single electron between narrow wire portions and electrode portions; Figs. 14A to 14B are graphs showing the waveforms of voltages applied to electrode portions for single-electron transportation control in Fig. 11; Fig. 15 is a plan view showing a Coulomb-blockade element according to still another embodiment of the present invention; Fig. 16 is an equivalent circuit diagram of the Coulomb-blockade element in Fig. 15; Figs. 17A to 17D are timing charts showing the waveforms of voltages applied to electrode portions for single-electron transportation control in Fig. 15; Fig. 18 is a perspective view showing a conventional Coulomb-blockade element when viewed obliquely from above; and Fig. 19 is an equivalent circuit diagram of the Coulomb-blockade element in Fig. 18. Description of the Preferred Embodiments First Embodiment As shown in Figs. 1A to 1C, in order to manufacture the Coulomb-blockade element of the first embodiment, an upper silicon layer 4 may be processed into a shape like the one indicated by the solid lines in Fig. 2, which includes narrow wire portions 30 to 32, a connecting portion 33 serving a branching point for coupling these narrow wire portions, electrode.portions 34 and 35, and an electrode portion 36 formed to be spaced apart from the narrow wire portion through a space for forming a capacitances (Fig. 1C), and the silicon layer may be thermally oxidized in the same manner as in the first embodiment of the parent EP-Patent Application 95 25 0256.5. The silicon layer 4 (the hatched portions in Fig. 2) is thinnest near the pattern edges of the electrode portions 34 to 36. The connecting portion 33 has an area larger than that of each of the narrow wire portions 30 50 32, and the influence of a stress on the connecting portion 33 is small. For this reason, the silicon layer 4 is thin near the connecting portion 33 as well as the edges. When the element is cut along a line A - A in Fig. 2, a structure like the one shown in Fig. 3 is obtained. When the element is cut along a line B - B in Fig. 2 , a structure like the one shown in Fig. 3B is obtained. In this manner, oxidation is made to progress at the hatched portions in Fig. 2 , and volume expansion occurs. As a result, as shown in Figs. 3A and 3B, these portion protrude in the direction of thickness. Referring to Fig. 3B, reference numeral 37 denotes a region in which an oxide film is formed between the narrow wire portion 32 and the electrode portion 36 by thermal oxidation. In this embodiment, the gap between the narrow wire portion 32 and the electrode portion 36 is completely filled with the side gate oxide film. However, some space may be left between these portions. Finally, windows for electrodes are formed in portions of an upper layer oxide film 5 on the electrode portions 34, 35, and 36, and electrodes are respectively formed in these windows. With this step, the manufacturing process for the Coulomb-blockade element is completed. Fig. 4 shows an equivalent circuit of the Coulomb-blockade element in Fig. 1. A thin silicon region (a hatched portion in Fig. 2) near the pattern edge of the electrode portion 34 becomes a tunnel barrier owing to an increase in ground energy state. As a result, this region serves as the tunnel capacitance C31. Similarly, a thin silicon region near the pattern edge of the electrode portion 35 becomes a tunnel barrier and serves as a tunnel capacitance C34. In addition, thin silicon regions formed at the connecting portion 33 become tunnel barriers and serve as tunnel capacitances C32 and C33. The narrow wire portions 30 to 32 become a silicon island adjacent to the tunnel barriers. A side gate capacitance C35 consisting of the side gate oxide film 37 is connected to the narrow wire portion 32. As will be described later, by changing the junction shape between a narrow wire portion and an electrode portion, the electrode can be formed into an electrode portion for single-electron injection/extraction (current injection/extraction) or an electrode portion for single-electron transportation control (voltage application). According to this embodiment, the electrode portions 34 and 35 serve as electrode portions for single-electron injection/extraction, and the electrode portion 36 serves as an electrode portion for single-electron transportation control. The equivalent circuit of Fig. 4 corresponds to a single-electron turnstile as a coupling structure type Coulomb-blockade element, which includes a plurality of silicon islands. The operation of a Coulomb-blockade element having such an equivalent circuit will be described next. Figs. 5A to 5C illustrate the relationship between the narrow wire portions 30 to 32, the electrode portions 34 and 35, and the energy of a single electron. Fig. 5A shows a state wherein a constant voltage is applied between the electrode portions 34 and 35 (the electrode 35 is set to be positive; and the electrode 34, negative). The energy level of the silicon island 32 is lower than that of each of the silicon islands 30 and 31 for the following reason. The silicon island 30 is surrounded by the tunnel capacitances C31 and C32, and the silicon island 31 is surrounded by the tunnel capacitances C33 and C34. In addition, as will be described later, the narrow wire portions 30 and 31 are formed to be minimized in size. In contrast to this, the capacitance around the silicon island 32, including the side gate capacitance C35, is larger than the silicon islands 30 and 31. When a voltage is applied to the electrode portion 36 while a constant voltage is applied between the electrode portions 34 and 35, as shown in Fig. 5A, the levels of the silicon islands 30 to 32 rise and fall because of connection through the capacitance C35. More specifically, when a positive voltage is applied to the electrode portion 36, the levels of the silicon islands 30 to 32 fall. When the level of the silicon island 30 becomes equal to or lower than that of the electrode 34 as the positive voltage increases, a single electron e in the electrode portion 34 moves to the silicon island 30 and to the silicon island 32 having a lower level, as shown in Fig. 5 B. When a negative voltage is applied to the electrode portion 36, the levels of the silicon islands 30 to 32 rise. When the level of the silicon island 31 becomes equal to or lower than that of the silicon island. 32 as the negative voltage increases, the single electron e in the silicon island 32 moves to the silicon island 31 and to the electrode portion 35 having a lower level, as shown in Fig. 5 C. By applying an AC voltage to the electrode portion 36 in this manner, one electron can be transported from the electrode portion 34 to the electrode portion 35 through the silicon islands 30, 32, and 31. Formation conditions for realizing such a coupling structure type Coulomb-blockade element will be described next. In order to realize the function of the above single-electron turnstile at room temperature, the narrow wire portions 30 and 31 must be minimized, and tunnel barriers must be effectively formed. The dimensions of the narrow wire portions 30 and 31 before thermal oxidation, including thickness, width (in the vertical direction in Fig. 2 ), and length (in the lateral direction in Fig. 2 ), are preferably set to be about several tens nm or less. In order to most effectively form a tunnel barrier at the connection portion 33, the connecting portion 33 needs to have a certain area, in consideration of the oxidation progressing effect and the oxidation suppressing effect described in the first embodiment of said parent EP Patent Application. The formation of a tunnel barrier with the most effective progression of oxidation of the connecting portion 33 is dependent on conditions such as oxidation temperature and oxidation atmosphere. When, for example, the silicon layer 4 having a thickness of about 20 nm before oxidation is to be oxidized in a dry oxygen atmosphere at 1,000°C, both the width (in the lateral direction in Fig. 2 ) and length (in the vertical direction in Fig. 2 ) of the narrow wire portion 32 before oxidation may be set to be about 100 to 200 nm. The formation of tunnel barriers with the most effective progression of oxidation of the electrode portions 34 and 35 is dependent on conditions such as oxidation temperature and oxidation atmosphere. If the same conditions as those for the connecting portion 33 are to be set, the widths (in the vertical direction in Fig. 2) of the electrode portions 34 and 35 may be set to be about 400 nm. As is apparent, the dimensional design must be optimized in accordance with oxidation conditions and the function of each Coulomb-blockade element. A method of controlling the formation of silicon islands and the degree of coupling (tunnel capacitance) between the silicon islands, as formation conditions for realizing the Coulomb-blockade element, will be described next. Figs. 6A to 6F show a Coulomb-blockade element to explain this control. The same reference numerals in Figs. 6A to 6 F denote the same parts as in Figs. 1A to 1C. Figs. 6A to 6F, however, show only the silicon layer corresponding to the upper silicon layer 4. Referring to Figs. 6A, 6C, and 6E, the hatched portion indicates a region in which tunnel barriers are formed, and the dotted portions (to be referred to as satin portions hereinafter) indicate regions in which silicon islands are formed. As shown in Fig. 6A, when the silicon layer is processed into a T shape with the narrow wire portion 32 having a large width W2, and the silicon layer is thermally oxidized, oxidation of the narrow wire portion 32 and the connecting portion 33 is accelerated because their areas are sufficiently large and the effect of stress thereon is small. In this region, therefore, a tunnel barrier is formed, and no silicon island is formed at the narrow wire portion. As a result, the equivalent circuit of the element in Fig. 6A becomes a circuit like the one shown in Fig. 6 B, in which the two silicon islands 30 and 31 are connected to each other through the tunnel capacitance C36. As shown in Fig. 6 C, when the silicon layer is processed into a T shape with the narrow wire portion 32 having a relatively small width W2, and the silicon layer is thermally oxidized, oxidation of the narrow wire portion 32 is suppressed, as well as oxidation of the narrow wire portions 30 and 31, owing to stress, and a silicon island is formed. At this time, a tunnel barrier is formed at the connecting portion 33. However, since the distance between the silicon islands 30 and 31 is sufficiently larger than the distance between the silicon islands 30 and 32 or between the silicon islands 31 and 32, the frequency of electron tunneling between the silicon islands 30 and 31 is low, and the corresponding tunnel capacitance C37 becomes small. As a result, the effective equivalent circuit of the element in Fig. 6C becomes a circuit like the one shown in Fig. 6 D, in which the silicon islands 30, 32, and 31 are connected in series. This structure corresponds to the Coulomb-blockade element in Fig. 1 C. If thermal oxidation is performed while the width W2 of the narrow wire portion 32 is set to be small as shown in Fig. 6 E, the junction between the silicon islands 30 and 31 cannot be neglected. As a result, the equivalent circuit of the element in Fig. 6E becomes a circuit like the one shown in Fig. 6F, in which the three silicon islands 30 to 32 are connected to each other through the capacitances C32, C33, and C37. The actual width W2 corresponding to each case is dependent on conditions such as oxidation temperature and oxidation atmosphere. Assume that a silicon layer has a thickness of about 20 nm before oxidation, the width W1 of the narrow wire portions 30 and 31 is about 30 to 40 nm, and length L1 of the narrow wire portion 32 is about 100 nm. In this case, when this sample is oxidized in a dry oxygen atmosphere at 1,000°C, width W2 = about 100 nm or less corresponds to the case in Fig. 6E; with W2 = 100 nm to several 100 nm, the case in Fig. 6 C; and width W2 larger than several 100 nm, the case in Fig. 6 A. The value of W2 as the boundary between the structures in Figs. 6 A and 6 C is strongly dependent on the value of the length L1 of the narrow wire portion 32 (if, for example, L2 is about 100 nm as in the above case, W2 = about several 100 nm), and the value of W2 as the boundary decreases with an increase in the length L1. As described above, by changing the shape or dimensions of a narrow silicon line portion, the manner of the formation of a silicon island or the degree of coupling between silicon islands (the magnitude of a tunnel capacitance) can be controlled. The connecting portion 33 described in this embodiment is a region having width W1 in the vertical direction and width W2 in the lateral direction in Figs. 6 A to 6 F. A method of controlling the junction form of electrodes will be described as a formation condition for realizing a Coulomb-blockade element. Figs. 7 A to 7 F show only one silicon layer corresponding to the upper silicon layer 4 in Figs. 1A to 1C. As shown in Fig. 7A, when the silicon layer is processed into a shape including a narrow wire portion 38 and an electrode portion 39a having a tapered region whose width decreases toward the narrow wire portion 38, and the silicon layer is thermally oxidized, tunnel barriers are formed near the edges of the electrode portion 39a which are indicated by the hatched portions, but no tunnel barrier is formed near the narrow wire portion 38. The reason for this is that oxidation is suppressed by stress because the area of the portion in contact with the narrow wire portion 38 is small. The narrow wire portion 38 and the electrode portion 39a are therefore connected to each other without the mediacy of a tunnel barrier (tunnel capacitance), as shown in Fig. 7B. The electrode portion 39a having such a form of a junction is used as an electrode for single-electron injection/extraction (current injection/extraction). Note that a tunnel barrier having a low potential may be formed at the tapered region depending on the shape of the tapered region. If, however, this tunnel barrier can be neglected in effect upon application of a voltage, no problem will be posed. Alternatively, the tunnel barrier may be used, as in the case shown in Fig. 7 C, which will be described later. As shown in Fig. 7 C, the silicon layer is processed into a shape having the narrow wire portion 38 and an electrode portion 39b, and the silicon layer is thermally oxidized (corresponding to the portions between the narrow wire portion 30 and the electrode portion 34 and between the narrow wire portion 31 and the electrode portion35 in Fig. 1), a tunnel barrier having a sufficiently high potential is formed near the edge of the electrode portion 39b. The narrow wire portion 38 and the electrode portion 39b are therefore connected to each other through the tunnel capacitance C38, as shown in Fig. 7D. The electrode portion 39b having such a form of a junction is used as an electrode for single-electron injection/extraction (current injection/extraction). As shown in Fig. 7 E, when the silicon layer is processed into a shape including an electrode portion 39c formed to be spaced apart from the narrow wire portion 38 through a space (corresponding to the portion between the narrow wire portion 32 and the electrode portion 36 in Fig. 1), since the space is filled after the oxidation process, a junction which allows no tunneling is formed. As shown in Fig. 7 F, the narrow wire portion 38 and the electrode portion 39c are connected to each other through a capacitance C39. The electrode portion 39c having such a form of a junction is used as an electrode for controlling the transportation of a single electron by applying a voltage. In addition, the magnitude of the capacitance C39 can be controlled by changing the size of the space. In this case as well, a tunnel barrier having a low potential may be formed near the tapered region. The function of an electrode portion can be changed by changing the form of the junction in this manner. This function can be obtained by using either the shape in Fig. 7C or the shape in Fig. 7A. As is apparent from Figs. 7A to 7F, there are two different equivalent circuits corresponding to a case wherein a tunnel capacitance is to be added (the structure in Fig. 7C) and a case wherein no tunnel capacitance is added (the structure in Fig. 7 A). One of the structures which is more effective than the remaining ones is selected in accordance with the magnitude of each constituent capacitance (determined by the size of each portion), operation conditions (the set values of voltage and current), and the machine to which the selected structure is to be applied. In addition, the shapes of the above silicon islands and tunnel barriers and the potentials of the tunnel barriers are controlled by the above formation conditions and are also dependent on conditions (oxidation temperature and oxidation time) for an oxidation process. Since the oxidation suppressing effect based on stress becomes more conspicuous as the oxidation temperature drops, the difference in silicon film thickness between a region in which a silicon island is formed and a region in which a tunnel barrier is formed can be increased. Second Embodiment In the first embodiment, the electrode , portion 36 formed as a single-electron transportation control electrode by using the upper silicon layer. However, a gate electrode may be formed by using a polysilicon or the like to cover all or some of the narrow wire portions 30 to 32 and the connecting portion 33 through a gate insulating film. This gate electrode may be used as a single-electron transportation control electrode in place of the electrode portion 36. In this case, the upper oxide film 5 is used as a gate insulating film (corresponding to the capacitance C35). Third Embodiment Figs. 8A to 8C show a Coulomb-blockade element according to still another embodiment of the present invention. The hatched portions indicate regions in which tunnel barriers are formed, and the satin portions indicate regions in which silicon islands are formed. As shown in Fig. 8 A, when a silicon layer is processed into L shape including two narrow wire portions 40a and a connecting portion 41a as a bending point where the two narrow wire portions are connected to each other, and the silicon layer is thermally oxidized, a tunnel barrier is formed at the connecting portion 41a and silicon islands are formed on the two narrow wire portions 40a. As shown in Fig. 8B, when a silicon layer is processed into radial shape including a connecting portion 41b and six narrow wire portions 40b radially extending from the connecting portion 41b, and the silicon layer is thermally oxidized, a tunnel barrier is formed at the connecting portion 41b, and silicon islands are formed on the narrow wire portions 40b. Alternatively, a silicon layer may be processed into matrix shape including narrow wire portions 40c and connecting portions 41c are arranged as shown in Fig. 8C, and the silicon layer may be thermally oxidized. Although no electrode portions connected to the ends of the narrow wire portions 40a to 40c are described in this embodiment, a closed state like the narrow wire portion 32 in the first embodiment may be formed, or electrode portions may be connected as shown in Figs. 7A to 7F. Alternatively, a gate electrode may be formed as in the second embodiment. As is apparent, the structures shown in Figs. 8A to 8C can be coupled to each other to form a new array. Fourth embodiment In the first to third embodiments, although all the patterns are constituted by straight lines, the patterns may be constituted by curves. By using curves, the degree of freedom in the design of the shapes of narrow wire portions and the like further increases. As a result, variations of coupling structures which can be put to practical use further increase. Fifth Embodiment Fig. 9 shows a Coulomb-blockade element according to still another embodiment of the present invention. The same reference numerals in Fig. 9 denote the same parts as in Figs. 1 A to 1 C. However, Fig. 9 shows only one 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 like the one indicated by the solid lines in Fig. 9, and the silicon layer may be thermally oxidized in the same manner as in the first embodiment of said parent EP Patent Application 95 25 0256.5-2203. With this process, tunnel barriers are formed in the hatched regions, and silicon islands are formed in the regions indicated by the satin portions. Windows are formed in portions of oxide films on electrode portions 34, 36, 43, and 44, and extraction electrodes are respectively formed in the windows. Note that no extraction electrode is formed on an adjacent portion 35a connected to a single-electron transistor (to be described later) through a space, because the adjacent portion 35a serves as a silicon island used as a single-electron memory cell. In this manner, the electrode portions 34, 43, and 44 become single-electron injection/extraction electrodes, and the electrode portion 36 becomes a single-electron transportation control electrode. Fig. 10 shows an equivalent circuit of the Coulomb-blockade element in Fig. 9. In this Coulomb-blockade element, a single-electron turnstile Q31 is connected, as a single-electron write line similar to the one in the first embodiment of said parent EP-Patent Application, to a silicon island formed on the adjacent portion 35a and serving as a single-electron memory cell, and a single-electron transistor Q32 constituted by a narrow wire portion and the electrode portions 43 and 44 is also connected, as an electrode read line, to the silicon island. In this Coulomb-blockade element, when the electrode portion 34 is set at an appropriated negative voltage, and an AC voltage is applied to an electrode portion 36, electrons can be injected one by one into the silicon island formed on the adjacent portion 35a. The number of electrons in this silicon island can be read as a change in the conductance between the electrode portions 43 and 44 in the single-electron transistor Q32. Sixth Embodiment Fig. 11 shows a Coulomb-blockade element according to still another embodiment of the present invention. However, Fig. 11 shows only the silicon layer corresponding to the upper silicon layer 4 in Figs. 1 A to 1 C. In order to manufacture such a Coulomb-blockade element, the silicon layer may be processed into a shape like the one indicated by the solid lines in Fig. 11, and the silicon layer may be thermally oxidized in the same manner as in the first embodiment of the parent EP Patent Application 95 25 0256.5. With this process, tunnel barriers are formed in the hatched regions, and silicon islands are formed in the regions indicated by the satin portions. Windows are formed in portions of oxide films on electrode portions 45 and 48 to 50, and electrodes are respectively formed in the windows. As a result, the electrode portions 45 and 48 serve as single-electron injection/extraction electrodes, and the electrode portions 49 and 50 serve as single-electron transportation control electrodes. Fig. 12 shows an equivalent circuit of this Coulomb-blockade element. This coupling structure type Coulomb-blockade element is called a single-electron pump. The operation of the Coulomb-blockade element having such an equivalent circuit will be described next. Fig. 13 illustrates the relationship between the narrow wire portions 46 and 47, the electrode portions 45 and 48, and the energy of a single electron. Figs. 14A and 14B respectively show the waveforms of voltages to be applied to the electrode portions 49 and 50. In this Coulomb-blockade element, a constant voltage need not be applied between the electrode portions 45 and 48, unlike in a single-electron turnstile. Fig. 13A shows an initial state (time t1 in Figs. 14A and 14B). When the positive voltage applied to the electrode portion 49 rises at time t2 in Fig. 14A, a single electron e in the electrode portion 45 moves to the silicon island 46, as shown in Fig. 13B. When the voltage applied to the electrode portion 49 approaches 0 at time t3, the electron e in the silicon island 46 tries to return to the electrode portion 45. However, as shown in Fig. 14B, since the positive voltage applied to the electrode portion 50 rises, the single electron e in the silicon island 46 moves to the silicon island 47, as shown in Fig. 13 C. Finally, when the voltage applied to the electrode portion 50 becomes 0 at time t4, and the voltage applied to the electrode portion becomes negative, the single electron e moves to the electrode portion 48. As described above, by applying AC voltages, which are out of phase, to the electrode portions 49 and 50, one electron can be transported from the electrode portion 45 to the electrode portion 48 through the silicon islands 46 and 47. Seventh Embodiment Fig. 15 shows a Coulomb-blockade element according to still another embodiment of the present invention. Fig. 15 shows only the silicon layer corresponding to the upper silicon layer 4 in Figs. 1A to 1 C. In order to manufacture such a Coulomb-blockade element, the silicon layer may be processed into a shape like the one indicated by the solid lines in Fig. 15, and the silicon layer may be thermally oxidized in the same manner as in the first embodiment of the parent EP Patent Application 95 25 0256.5. With this process, tunnel barriers are formed in the hatched regions, and silicon islands are formed in the regions indicated by the satin portions. Windows are formed in oxide films on electrode portions 51 to 58, and electrodes are formed in the respective windows. The electrode portions 51, 52, 57, and 58 become single-electron injection/extraction electrodes, and the electrode portions 53 to 56 become single-electron transportation control electrodes. Fig. 16 shows an equivalent circuit of this Coulomb-blockade element. The coupling structure type Coulomb-blockade element is a single-electron transportation circuit having two inputs (electrode-portions 51 and 52).and two outputs (electrode portions 57 and 58). The operation of the Coulomb-blockade element having this equivalent circuit will be described next. Assume that a single electron is to be transported from the electrode portion 51 to the electrode portion 58. Figs. 17 A to 17 D show the waveforms of voltages applied to the electrode portions 53 to 56. When the positive voltage applied to the electrode portion 53 rises at time t1 in Fig.17 A, a single electron e in the electrode portion 51 moves to a narrow wire portion 60 through a silicon island-formed on a narrow wire portion 59, as indicated by the broken lines in Fig. 16. When a negative voltage is applied to the electrode portion 53 at time t2, and at the same time, a positive voltage is applied to the electrode portion 56 as shown in Fig. 17D, the single electron e moves from the silicon island on the narrow wire portion 60 to a silicon island formed in a narrow wire portion 61. Finally, when negative voltages are applied to the electrode portions 53 to 56 at time t3, the single electron e moves to the electrode portion 58. By applying AC or pulse-like voltages to the electrode portions 53 to 56 with proper phases or at proper timings with respect to the input voltages to the electrode portions 51 and 54, transportation of single electrons from the electrodes 51 and 52 to the electrodes 57 and 58 can be realized.The function of an output with respect to an input can be controlled by controlling the magnitude, phase, and timing of AC or pulse-like voltages applied to the electrode portions 53 to 56, respectively. In order to realize this function, four silicon islands 59 and 62 to 64 which are in contact with the electrode portions 51, 52, 57, and 58 must be formed to be smaller than other silicon islands. Eighth Embodiment Each embodiment described above uses an SOI substrate having single crystalline silicon formed on a silicon oxide film. The same principle is established when an amorphous or polysilicon layer is formed on a silicon oxide film. The same effects as those described above can be obtained by using such a substrate as long as techniques and structures similar to those in the first to 7th embodiments are used. Ninth Embodiment In each embodiment described above, thermal oxidation is performed while an upper oxide film used as a mask in forming a pattern of an upper silicon layer. As is apparent, however, if a resist film or the like is used as another mask film in place of the upper oxide film, since the mask film can be selectively removed after the silicon layer is processed, the upper oxide film need not be used. In each embodiment described above, an upper oxide film having a thickness of about 30 nm is formed on an upper silicon layer. If, however, the thickness of this upper oxide film is increased to about 60 nm, an oxidant passing through the upper oxide film decreases in amount to suppress oxidation from above, and oxidation from below, which is based on the diffusion of the oxidant in the lateral direction, becomes dominant. Therefore, tunnel barriers are formed at the two ends of a narrow wire portion while oxidation from the upper side of a flat portion serving as an electrode (the upper silicon layer spaced apart from an edge) is suppressed (almost no oxidation occurs from below because of the long distance from the edge). That is, since oxidation of the silicon layer from above is suppressed, the degree of oxidation and the silicon layer to be finally left can be set easily. In addition, since the upper oxide film is thick, stress accompanying oxidation increases. This allows effective formation of a constriction. Similarly, if a silicon nitride film is used in place of an upper oxide film as a mask film or a silicon nitride film is stacked on the upper oxide film, downward oxidation through the nitride film can be suppressed when thermal oxidation as the next step is performed. For this reason, only oxidation from a side wall of the upper silicon layer and from the buried oxide film side mainly occurs, and reduction in the silicon layer by the downward oxidation does almost not occur. This method is therefore more effective than the above method of using only a silicon oxide film. In addition, since stress based on the silicon nitride film is large, stress accompanying oxidation increases. This method is therefore effective in obtaining the structure in each embodiment described above. According to the present invention, portions of first and second electrode portions which are near a narrow wire portion are made thinner than the narrow wire portion by forming constrictions on the upper and/or lower surfaces of the electrode portions. As a result, tunnel barriers having a higher energy than that of the narrow wire portion are formed at the two ends of the narrow wire portion, and a conductive island is formed on the narrow wire portion. Therefore, a Coulomb-blockade element which operates at high temperature can be easily realized. In addition, since a conventional silicon MOS structure can be used, a manufacturing process technique for a silicon-based integrated circuit can be used. For this reason, a Coulomb-blockade element can be mounted on the same substrate on which the conventional silicon-based integrated circuit is mounted, thereby realizing a large-scale circuit combining the MOS and Coulomb-blockade devices. By forming a third electrode portion serving as the drain, source, and channel regions of a MOS transistor to be adjacent to a narrow wire portion, a Coulomb-blockade element serving as a memory element, with the Coulomb-blockade element being capacitively connected to the MOS transistor, can be easily realized. By forming an adjacent portion adjacent to a first narrow wire portion, a Coulomb-blockade element constituted by the first narrow portion and first and second electrode portions is capacitively coupled to a Coulomb-blockade element constituted by a second narrow wire portion, an adjacent portion, and a fourth electrode portion, thereby easily realizing a Coulomb-blockade element serving as a memory element. By processing a silicon layer into a shape including a narrow wire portion and first and second electrode portions, and thermally oxidizing the silicon layer, a constriction having minimum thickness is automatically formed at a portion of each electrode portion which is near the narrow wire portion. As a result, tunnel barriers are formed near the narrow wire portion, and a conductive island is formed on the narrow wire portion. Since the size of the conductive island in the lateral direction can be set regardless of the size of each constriction, the conductive island can be reduced in size. In addition, the conductive island can be further reduced in size by thermal oxidation. Therefore, a Coulomb-blockade element which operates at high temperature can be realized by a simple manufacturing process similar to a conventional manufacturing process technique for a silicon-based integrated circuit. Furthermore, the formation of each constriction can be controlled in accordance-with the dimensions of a pattern of each electrode portion and the shape of each constriction. Since a thermal oxidation technique of the silicon LSI process techniques exhibits good controllability and good reproducibility, a Coulomb-blockade element can be realized with good controllability and good reproducibility. By forming a plurality of narrow wire portions and a connecting portion, and making the connecting portion thinner than each narrow wire portion, a tunnel barrier is formed at the connecting portion and a conductive island is formed on each narrow wire portion, thereby forming a structure in which the conductive islands are coupled to each other through a capacitance based on the tunnel barrier. Therefore, a coupling structure type a Coulomb-blockade element which operates at high temperature can be easily realized. If narrow wire portions and a connecting portion are designed on a two-dimensional plane, a coupling structure type Coulomb-blockade element having conductive islands arranged within the two-dimensional plane can be manufactured with a high degree of freedom. In addition, since a manufacturing process for a silicon MOS can be used, the coupling structure type Coulomb-blockade element can be mounted on the same substrate on which a conventional silicon integrated circuit is mounted. Therefore, a large-scale, high-performance circuit can be realized by hybridizing a Coulomb-blockade element and a conventional MOS circuit. When at least one electrode portion wider than a narrow wire portion is formed to be connected to an end of the narrow wire portion, this electrode portion can be used as a single-electron injection electrode. Since a tunnel barrier is formed at a portion of an electrode portion which is near a narrow wire portion by making the portion of the electrode portion which is near the narrow wire portion thinner than the narrow wire portion, the narrow wire portion as a conductive island can be connected to the electrode portion serving as the single-electron injection/extraction electrode through a capacitance based on the tunnel barrier. When at least one electrode portion having a smaller width than that of a narrow wire portion, which is formed to be spaced apart from the narrow wire portion through a space or an insulating film for forming a capacitance, this electrode portion can be used as a single-electron transportation control electrode, and the narrow wire portion serving as a conductive island and the narrow wire portion can be connected to each other through a capacitance based on a space or an insulating film. By forming a gate electrode on at least part of a narrow wire portion and a connecting portion through an insulating film, the narrow wire portion serving as a conductive island can be connected to the gate electrode serving as a single-electron transportation control electrode through a capacitance based on the insulating film. By processing a silicon layer into a plurality of narrow wire portions and a connecting portion, and thermally oxidizing the silicon layer, a constriction having the minimum thickness is automatically formed at the connecting portion. As a result, a tunnel barrier is formed at the connecting portion, and conductive islands are formed on the narrow wire portions, thereby forming a structure in which the conductive islands are coupled to each other through a capacitance based on the tunnel barrier. Therefore, a coupling structure type Coulomb-blockade element which operates at a high temperature can be realized by a simple manufacturing process similar to a manufacturing process technique for a conventional silicon-based integrated circuit. In addition, the formation of a constriction can be controlled in accordance with pattern dimensions and thermal oxidation conditions, and a thermal oxidation technique of the silicon LSI process techniques exhibits good controllability and good reproducibility. Therefore, a coupling structure type Coulomb-blockade element can be realized with good controllability and good reproducibility. By forming, on a silicon layer before a thermal oxidation step, at least one electrode portion connected to an end of a narrow wire portion and having a width smaller than that of the narrow wire portion and by thermally oxidizing the silicon layer, a constriction having minimum thickness is automatically formed at a portion of the electrode portion which is near the narrow wire portion, and a tunnel barrier is formed near the narrow wire portion. Therefore, a structure in which the conductive island is connected to the electrode portion through a tunnel capacitance can be realized with good controllability and good reproducibility. At least one electrode portion which is wider than a narrow wire portion and connected to an end of the narrow wire portion such that the width of the electrode portion increases as it is separated from the narrow wire portion is formed on a silicon layer before a thermal oxidation step, and the silicon layer is thermally oxidized. As a result, an electrode portion having no constriction can be realized, and a structure in which the conductive island is coupled to the electrode portion without any tunnel capacitance can be realized with good controllability and good reproducibility. When an insulating film containing silicon is formed on a silicon layer before a thermal oxidation step, stress accompanying oxidation changes in accordance with the thickness of this insulating film. Therefore, the formation of a constriction can be controlled. By using a silicon nitride film as an insulating film containing silicon, a silicon layer can be oxidized only from below. As a result, the oxidation amount and the thickness of the remaining silicon film can be designed easily. In addition, the formation of a constriction is accelerated because of stress caused by the silicon nitride film.

Claims: A Coulomb-blockade element comprising: a substrate (2), an insulating film (3) on said substrate and a silicon layer (4) formed on said insulating film, said silicon layer including a plurality of narrow wire portions (30 - 32) serving as conductive islands for confining charges, wherein a connecting portion (33) also included in said silicon layer is formed as a branching or bending point for coupling said narrow wire portions to each other, said connecting portion having constrictions on at least one of the upper and lower surfaces thereof, which make a portion of said silicon layer close to said narrow wire portions thinner than the silicon layer in said narrow wire portions so as to form a tunnel barrier in said connecting portion adjacent to said conductive islands of said narrow wire portions. Element according to claim 1, further comprising at least one electrode portion (34, 35) which is formed to be connected to an end of said narrow wire portion (30, 31) and is wider than said narrow wire portion. Element according to claim 2, wherein said electrode portion (34, 35) has constrictions on at least one of upper and lower surfaces thereof to make said electrode portion thinner than said narrow wire (30, 31) portion at a position near said narrow wire portion. Element according to claim 1, further comprising at least one electrode portion (36) which is formed to be spaced apart from said narrow wire portion (32) through a space or an insulating film for forming a capacitance and is wider than said narrow wire portion. Element according to claim 1, further comprising a gate electrode formed on at least portions of upper surfaces of said narrow wire portions and said connecting portion through an insulating film. Method of manufacturing a Coulomb-blockade element comprising: the step of processing a silicon layer (4) formed on an insulating film (3) deposited on a substrate (2) into a shape including a plurality of narrow wire portions (30 - 32) serving as conductive islands for confining charges, and a connecting portion (33) serving as a branching or bending point for coupling said narrow wire portions to each other; and the thermal oxidation step of thermally oxidizing said silicon layer, which, in said connecting portion, is thereby formed to be thinner than said narrow wire portion so as to form a tunnel barrier in said connecting portion adjacent to said conductive islands of said narrow wire portions. Method according to claim 6, wherein said silicon layer (4) before the thermal oxidation step includes at least one electrode portion (39b) which is connected to an end of said narrow wire portion (38) and is wider than said narrow wire portion, said electrode portion being formed by said thermal oxidation stop to be thinner than said narrow wire portion at a position adjacent to said narrow wire portion. Method according to claim 6, wherein said silicon layer before the thermal oxidation step includes at least one electrode portion (39a) which is wider than said narrow wire portion (38) and connected to one end of said narrow wire portion such that said electrode portion becomes wider with an increase in distance from said narrow wire portion. Method according to claim 6, further comprising forming an insulating film (5) containing silicon on said silicon layer (4) before the thermal oxidation step. Method according to claim 9, wherein said insulating film is a silicon nitride film.

Inventors: FUJIWARA AKIRA, NAGASE MASAO, TAKAHASHI YASUO, FUJIWARA, AKIRA, NAGASE, MASAO, TAKAHASHI, YASUO
Applicants: NIPPON TELEGRAPH & TELEPHONE, NIPPON TELEGRAPH AND TELEPHONE CORPORATION
Application year: 1995
Main classification: H01L29
IPC codes: H01L29