JP2009088144A - Method of setting state of switching element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of setting a state of a switching element which prevents the breakage of the element upon setting the state thereof. <P>SOLUTION: The method is for setting the state of the switching element including an ion conductive layer, a first electrode whose part in contact with the ion conductive layer contains a material that supplies metal ions to the ion conductive layer, and a second electrode whose part in contact with the ion conductive layer is made of a material that does not supply metal ions. A substrate carrying the switching element is heated to keep the substrate at a given temperature, and a voltage is applied across the first and second elements to cause the switching element to make a state transition. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a reconfigurable semiconductor integrated circuit including a reconfigurable logic integrated circuit device and an arithmetic circuit device, and a switching element state setting method used in a semiconductor memory device.

  Examples of a resistance change switch using an electrochemical reaction (hereinafter simply referred to as a switching element) are disclosed in Patent Document 1, Non-Patent Document 1, and Non-Patent Document 2. The switching elements disclosed in these documents have a configuration in which three solid materials of first electrode / ion conduction layer / second electrode are laminated.

  By applying a negative or positive voltage between the first electrode and the second electrode, the electric resistance of the ion conductive layer changes. A characteristic of this switching element is that its electrical resistance can be very small in the on-state while being small. The change in resistance occurs when the metal constituting the first electrode is deposited on the ion conductive layer or the deposited metal is dissolved. Further, the resistance value of the switching element is maintained even when the application of the voltage to the element is stopped, and the switching element also functions as a memory.

  The switching element disclosed in Non-Patent Document 2 will be described. FIG. 6 is a schematic cross-sectional view showing an example of the basic configuration of the switching element.

  As shown in FIG. 6, the switching element 13 has a configuration in which the ion conductive layer 14 is sandwiched between the first electrode 15 and the second electrode 16. The ion conductive layer 14 corresponds to a resistance change layer, and the material thereof is tantalum oxide. The material of the first electrode 15 is copper, and the material of the second electrode 16 is platinum.

  The operation of the switching element 13 shown in FIG. 6 will be described.

  In order to turn on the switching element 13, the second electrode 16 is grounded and a positive voltage is applied to the first electrode 15. At this time, copper of the first electrode 15 becomes copper ions and dissolves in the ion conductive layer 14. Then, copper ions in the ion conductive layer are deposited as copper on the surface of the second electrode 16, and a metal bridge that connects the first electrode 15 and the second electrode 16 is formed by the deposited copper. The metal bridge is a metal deposit in which metal ions in the ion conductive layer are deposited. When the first electrode 15 and the second electrode 16 are electrically connected by metal bridge, the switching element 13 is turned on.

  On the other hand, when the first electrode 15 is grounded in the ON state and a positive voltage is applied to the second electrode 16, the metal bridge copper is dissolved in the ion conductive layer 14 and a part of the metal bridge is cut. Thereby, the switching element 13 is turned off. By applying a voltage between the first electrode 15 and the second electrode 16 as described above, the switching element 13 can be repeatedly switched between the on state and the off state.

  The switching element maintains the on state and the off state even when no voltage is applied. If the on state and the off state are replaced with different information such as “1” and “0”, respectively, the switching element functions as a memory for holding information. In this case, the operation of changing the state between the on state and the off state corresponds to an information rewriting operation.

  Since the manufacturing technique of the semiconductor device can be applied to the production of the switching element, the switching element can be mounted on the semiconductor substrate together with the semiconductor integrated circuit. Therefore, the switching element can be used as a part of the semiconductor integrated circuit. In particular, it is effective to use a switching element as a switch or memory of a reconfigurable circuit of a reconfigurable semiconductor integrated circuit.

  When the switching element is used as a switch of the reconfigurable circuit, it is necessary that the state of the switch does not change during circuit operation after the circuit is configured. Even when a circuit operating voltage is applied to the switching element and an operating current flows, the switch must remain at the operating temperature (usually 85 degrees or less) for a lifetime (usually about 10 years) or longer. I must. Therefore, it is usually necessary to set the threshold voltage at which the state of the switching element changes to be sufficiently higher than the operating voltage of the circuit.

  The operating voltage of the circuit is determined by the scaling rule of the semiconductor integrated circuit, and is about 1 V in the case of an integrated circuit whose design rule is the 45 nm generation to the 90 nm generation. Therefore, in order to prevent the switching element in the off state from transitioning to the on state with the operating voltage, the threshold voltage for transitioning from the off state to the on state needs to be 1 V or more.

  On the other hand, the threshold voltage for transitioning the switching element from the on state to the off state may be 1 V or less. This is because only a sufficiently smaller voltage than the operating voltage is applied to the switch in the low resistance state during circuit operation. However, the threshold voltage for making the transition from the on state to the off state needs to be larger than the sufficiently small voltage.

  In order to transition from the off state to the on state, a voltage higher than the threshold voltage is applied to the switching element. However, when a high voltage is applied to the switching element, the element may be significantly deteriorated and the element may be destroyed. For this reason, it is desirable that the threshold voltage is low when transitioning the state in order to prevent deterioration of the element. Considering the case where the state is held, the threshold voltage is set to prevent the state from transitioning with the operating voltage. Higher is desirable.

  Non-Patent Document 3 discloses that a threshold voltage for transitioning a state between an on state and an off state varies with the environmental temperature of the switch. It has been known so far that when the switching element is set to a temperature lower than room temperature, the threshold voltage for transitioning the switching element from the off state to the on state or from the on state to the off state increases with temperature. The reason for the dependence on temperature is that the threshold voltage depends on the diffusion coefficient of metal ions in the ion conductive layer, and the diffusion coefficient of the metal has temperature dependence. Therefore, when the switching element is set to a temperature higher than room temperature, the threshold voltage is predicted to decrease.

As a method of changing the environmental temperature of the switch, Patent Document 2 discloses a method of providing a heating device in the vicinity of the switch inside the semiconductor integrated circuit. Patent Document 2 describes that the voltage for changing the state can be changed by heating the switch with a heating device. One switch is provided with one heating device, and by dividing the heating device group into a heating device and a non-heating device, it is possible to individually rewrite specific switches in a large number of switch groups. . Further, it is described that the threshold voltage can be lowered by changing the environmental temperature of the switch, and the deterioration of the element can be prevented.
JP 2006-319028 A Applied Physics Letter, 85, 5655 pages (2004) VLSI Symposium Technical Digest, p38, 2007. Japanese Journal of Applied Physics, Volume 45, 4B, 3666 pages (2006) JP 2006-222420 A

  In order to change the resistance state of the switching element, a large current of several tens of mA or more may flow through the switching element. The current value of several tens of mA is a very large value as compared with a current value normally used in a semiconductor integrated circuit. The reason why a large current is required is that the resistance of the switching element is kept low and the threshold voltage for changing the state is large.

  For example, when the on-resistance is 100Ω or less and the threshold voltage from the off state to the on state is 2 V or more, a current flowing through the switching element of 20 mA or more is required. For this purpose, it is necessary to increase the area of the drive circuit for supplying current to the switch, or to increase the area of the transistor inserted in series with the switching element. As a result, the area of a peripheral circuit such as a drive circuit or a transistor increases, and the advantage that the size of the switch itself is small cannot be utilized.

  As described in the background art, when a large voltage is applied to the switching element, the ion conductive layer between the first electrode and the second electrode may break down, and in this case, the element is broken. According to the experiments by the inventors, it is often observed that the ion conduction layer breaks down when set to the off state. In order to prevent element destruction, there is a method of lowering the threshold voltage by providing a heating mechanism in the vicinity of the switching element. However, as described above, the advantage that the switch itself is small cannot be utilized.

  In order to reduce the peripheral circuits such as the driving circuit and the transistor and prevent the dielectric breakdown of the ion conductive layer, it is necessary to make the current flowing through the switching element as small as possible. Furthermore, if a heating mechanism for changing the environmental temperature of the switching element is provided for each switch in the semiconductor integrated circuit, it is impossible to take advantage of the small size of the switch.

  The present invention has been made to solve the above-described problems of the technology, and an object of the present invention is to provide a switching element state setting method that prevents destruction of the element during state setting.

In order to achieve the above object, the switching element state setting method of the present invention includes an ion conductive layer, and a first electrode including a material that supplies a metal ion to the ion conductive layer at a portion in contact with the ion conductive layer. A method of setting a state of a switching element having a second electrode that is a material that does not supply the metal ions at a portion in contact with the ion conductive layer,
Heating the substrate on which the switching element is mounted to maintain the substrate at a constant temperature;
A voltage is applied between the first electrode and the second electrode to change the state of the switching element.

  According to the present invention, when the state of the switching element is set, the voltage applied to the switching element is reduced, the current flowing through the switching element is suppressed, and the breakdown of the switching element can be suppressed.

  The configuration of the switching element of this embodiment will be described. The basic configuration of the switching element of the present embodiment is a configuration in which the ion conductive layer 14 is sandwiched between the first electrode 15 and the second electrode 16 as shown in FIG.

  FIG. 1 is a schematic cross-sectional view showing a configuration example of the switching element of this embodiment. As shown in FIG. 1, the switching element is provided on a silicon substrate 20 covered with silicon oxide 28.

  A silicon oxide 28 is formed on the silicon substrate 20, and a platinum electrode 26, which is an electrode made of platinum, is provided thereon. A silicon oxide 27 having an opening 22 is provided on the platinum electrode 26. A tantalum oxide 24 covering the opening 22 is provided on the silicon oxide 27. On the tantalum oxide 24, a copper electrode 25, which is an electrode made of copper, is provided. The tantalum oxide 24 corresponds to the ion conductive layer 14 shown in FIG. 6, the copper electrode 25 corresponds to the first electrode 16, and the platinum electrode 26 corresponds to the second electrode 16.

  The tantalum oxide 24 is in contact with the platinum electrode 26 through the opening 22. The opening 22 defines the contact area between the tantalum oxide 24 and the platinum electrode 26. The plane pattern of the copper electrode 25 is larger than the opening 22. The planar pattern here is a pattern of a plane parallel to the element formation surface of the silicon substrate 20.

  Next, a method for manufacturing the switching element shown in FIG. 1 will be described.

  A silicon oxide 28 having a thickness of 300 nm is formed on the surface of the silicon substrate 20. Subsequently, platinum having a thickness of 100 nm is formed on the silicon oxide 28 by a sputtering method. The formed platinum film is used as a platinum electrode 26.

  A silicon oxide layer 27 having a thickness of 100 nm is formed on the platinum electrode 26 by a sputtering method. The opening 22 is formed in the silicon oxide 27 using a lithography technique and an etching technique. Subsequently, a tantalum oxide 24 having a film thickness of 15 nm is formed on the silicon oxide 27 so as to cover the opening 22 by sputtering. Further, copper having a thickness of 100 nm is formed on the tantalum oxide 24 by a sputtering method. Thereafter, the copper electrode 25 is formed by processing the copper using a lithography technique and an etching technique.

  In the present embodiment, platinum is used as the material of the second electrode 16, but the material of the second electrode 16 is not limited to platinum, and any material that does not allow metal ions to elute into the ion conductive layer 14 may be used. Examples of materials other than platinum include tungsten, tantalum, and titanium.

  Next, an experiment performed on the switching element shown in FIG. 1 will be described. FIG. 2 is a schematic diagram for explaining the heating method of the switching element of this embodiment.

  A switching element is manufactured on the semiconductor substrate 31 by the above-described manufacturing method. A plurality of switching elements may be formed on the semiconductor substrate 31. In addition, a heating device (not shown) for heating the semiconductor substrate 31 is prepared. The heating device includes a heating stage 30 for mounting the semiconductor substrate 31, a temperature sensor (not shown) for detecting the temperature of the heating stage 30, a heater (not shown) for heating the heating stage 30, and the heating stage 30. A temperature control unit (not shown) for controlling the temperature.

  The heater is provided on the surface of the heating stage 30 opposite to the surface on which the semiconductor substrate 31 is mounted. Since the material of the heating stage 30 is a metal having good thermal conductivity, the temperature of the heater and the heating stage 30 is substantially equal. The temperature control unit is connected to the temperature sensor via a signal line, and is connected to the heater via a power supply line. The temperature control unit monitors the detection value of the temperature sensor and supplies power to the heater so that the heating stage 30 maintains a constant temperature.

  The semiconductor substrate 31 on which the switching element of this embodiment is formed is placed on the heating stage 30 and the heating device is operated to keep the temperature of the heating stage 30 constant, and the switching element is changed from the off state to the on state. In addition, the transition from the on state to the off state was performed. The temperature of the heating stage 30 was set to three cases of 20 ° C. (room temperature), 85 ° C., and 150 ° C. When the state of the switching element was changed, the relationship between the voltage applied to the electrode and the current flowing between the two electrodes was examined. Note that when the heating stage 30 is kept at 20 ° C., room temperature is sufficient, and thus control by a heating device is not necessary.

  FIG. 3 is a graph showing the current-voltage characteristics when the switching element of this embodiment transitions from the off state to the on state. The horizontal axis represents the voltage applied to the copper electrode 25 corresponding to the first electrode 15, and the vertical axis represents the current flowing between the two electrodes. In addition, the measurement current of the measuring device is limited so that the measurement current does not exceed 10 μA.

  In the case where the temperature of the substrate is 20 ° C., when the applied voltage is increased from 0 V to around 6 V, the current increases with the change. The current at this time is a leak current flowing through the tantalum oxide 24 that is an insulating film. When the applied voltage is around 6 V, the current increases rapidly, and the high-resistance off state transitions to the low-resistance on-state. The resistance value in the ON state is about 20Ω.

  On the other hand, when the substrate temperature is 85 ° C., the applied voltage transitions from the off state to the on state in the vicinity of 4V. When the substrate temperature is 150 ° C., the applied voltage transitions from the off state to the on state when the applied voltage is around 2.5V. It can be seen that when the substrate temperature is 85 ° C. and 150 ° C., the voltage for transition from the off state to the on state is smaller than when the temperature is 20 ° C.

  FIG. 4 is a graph showing the current-voltage characteristics when the switching element of this embodiment transitions from the on state to the off state. In the switching element in the ON state, the measurement result when the copper electrode 25 corresponding to the first electrode 15 is grounded and a positive voltage is applied to the platinum electrode 26 corresponding to the second electrode 16 is shown. The horizontal axis represents the voltage applied to the platinum electrode 26, and the vertical axis represents the current flowing between the two electrodes.

  When the temperature of the substrate is 20 ° C., if the voltage applied to the platinum electrode 26 is increased from 0V, the current increases in proportion to the voltage up to about 0.4V. It can be seen that this slope is the on-resistance, and the resistance in the on-state is about 20Ω. When the applied voltage is further increased, a current of about 20 mA flows in the vicinity of 0.42 V, and when the applied voltage is increased above 0.42 V, the current decreases, and it can be seen that the state changes to a high resistance state.

  On the other hand, when the temperature of the substrate is 85 ° C., the voltage transitioning from the on state to the off state is around 0.3V. The current that flows immediately before the transition to the off state is about 13 mA, which is lower than 15 mA. When the temperature of the substrate is 150 ° C., the voltage transitioning from the on state to the off state is around 0.15V. The current that flows immediately before the transition to the off state is about 7 mA, which is less than 10 mA. From the graph, it can be seen that the substrate temperature is 100 to 120 ° C. and the current value is about 10 mA.

  When the substrate temperature is 85 ° C. and 150 ° C., the voltage for transition from the on state to the off state is smaller and the maximum current is also smaller than when the temperature is 20 ° C. Thus, it can be seen that by increasing the environmental temperature, the current required to turn off also decreases in proportion to the change in the threshold voltage.

  When the temperature of the substrate is room temperature, the dielectric breakdown of the ion conductive layer was observed due to the maximum current flowing through the switching element when transitioning from the on state to the off state. However, the frequency decreased as the temperature of the substrate was increased, and dielectric breakdown could be completely suppressed at a temperature of 150 ° C. This is because the breakdown voltage has a temperature dependency similar to the threshold voltage, but the temperature dependency of the breakdown voltage is weaker than the threshold voltage, and at a high temperature, the breakdown voltage becomes sufficiently larger than the threshold voltage. It is.

  Next, the state setting method of the present embodiment will be described for the switching element shown in FIG. FIG. 5 is a flowchart showing the procedure of the state setting method of this embodiment.

  As shown in FIG. 5, the semiconductor substrate 31 on which the switching element is mounted is heated by the heating stage 30 to keep the substrate at a constant temperature (step 101). While maintaining the semiconductor substrate 31 at a constant temperature, a voltage is applied between the copper electrode 25 corresponding to the first electrode and the platinum electrode 26 corresponding to the second electrode, and the switching element is switched from the on state to the off state, or A transition is made from the off state to the on state (step 102).

  In step 102, when the switching element is changed from the off state to the on state, the platinum electrode 26 is grounded and a positive voltage is applied to the copper electrode 25. If the environmental temperature of the switching element is in the range of 85 to 150 ° C., the absolute value of the applied voltage may be 4 to 2.5V. Conversely, when the switching element is transitioned from the on state to the off state, the copper electrode 25 is grounded and a positive voltage is applied to the platinum electrode 26. If the environmental temperature of the switching element is in the range of 85 to 150 ° C., the absolute value of the applied voltage may be 0.45 to 0.2V.

  When setting the state of the switching element, the temperature of the semiconductor substrate 31 is preferably 85 ° C. or higher from the above experimental results. On the other hand, in the state setting method of this embodiment, the entire semiconductor integrated circuit is heated, so that the transistors included in the integrated circuit are also placed in a high temperature environment. Therefore, the environmental temperature at the time of setting the state needs to be lower than the temperature at which the operation of the transistor is guaranteed. The transistor is subjected to an environmental resistance test at 150 ° C., and when the transistor does not operate at 150 ° C., it is treated as a failure. Therefore, it is desirable that the environmental temperature at the time of setting the state does not exceed 150 ° C.

  In this embodiment, when switching the switching element from the ON state to the OFF state or from the OFF state to the ON state, the state is changed while the substrate on which the switching element is mounted is heated and maintained at a constant temperature. . This promotes diffusion and electrochemical reaction of metal ions in the ion conductive layer. As a result, the threshold voltage for state transition and the current required for state transition can be reduced.

  The effect improves more that the environmental temperature at the time of the state setting of a switching element is the range of 85-150 degreeC. When the environmental temperature is set to 150 ° C., the current flowing through the switching element becomes smaller than 10 mA or less, which is a standard value of the current value used in the semiconductor integrated circuit, and it is possible to prevent the switch from being destroyed during the setting of the state.

  ADVANTAGE OF THE INVENTION According to this invention, the voltage applied to a switching element reduces at the time of the state setting of the switching element using an electrochemical reaction, the electric current which flows into a switching element is suppressed, and destruction of a switching element can be suppressed.

  Further, since the voltage applied to the switching element and the current flowing through the switching element can be small, it is possible to prevent the peripheral circuit supplying the voltage and current from becoming large. Furthermore, the state setting method of the present invention can provide a useful effect for switching elements used in semiconductor memory devices and reconfigurable semiconductor integrated circuits.

  In addition, if the semiconductor integrated circuit mounted on the semiconductor substrate includes a plurality of switching elements, the state setting method of the present invention is useful for setting the states of the plurality of switching elements at once.

  In the present embodiment, the first electrode is made of copper and the first electrode is made of a material that supplies metal ions. However, at least the portion of the first electrode that contacts the ion conductive layer is an ion. The material which supplies a metal ion to a conductive layer should just be included. In addition, platinum, tungsten, tantalum, and titanium are given as examples of the material of the second electrode, and the second electrode is made of a material that does not supply metal ions. However, the second electrode is in contact with at least the ion conductive layer. Any material that does not supply metal ions may be used.

It is a cross-sectional schematic diagram which shows one structural example of the switching element of a present Example. It is a schematic diagram for demonstrating the heating method of the switching element of a present Example. It is a graph which shows the current-voltage characteristic at the time of the switching element of a present Example changing from an OFF state to an ON state. It is a graph which shows the current-voltage characteristic at the time of the switching element of a present Example changing from an ON state to an OFF state. It is a flowchart which shows the procedure of the state setting method of the switching element of a present Example. It is a cross-sectional schematic diagram which shows an example of the basic composition of the switching element using an electrochemical reaction.

Explanation of symbols

14 Ion conductive layer 15 First electrode 16 Second electrode

Claims (3)

A first electrode including a material that supplies a metal ion to the ion conductive layer, a portion that contacts the ion conductive layer, and a material that does not supply the metal ion. A switching device state setting method having a second electrode,
Heating the substrate on which the switching element is mounted to maintain the substrate at a constant temperature;
A switching element state setting method in which a voltage is applied between the first electrode and the second electrode to change a state of the switching element.   The switching element state setting method according to claim 1, wherein the constant temperature is in a range of 85 ° C. or more and 150 ° C. or less. When the switching element transitions from an off state to an on state, the second electrode is grounded and a positive voltage is applied to the first electrode,
The switching element state setting method according to claim 1 or 2, wherein, when the switching element transitions from an on state to an off state, the first electrode is grounded and a positive voltage is applied to the second electrode.

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