JP4655019B2 - Variable resistance element - Google Patents

Description

  The present invention relates to a variable resistance element.

In recent years, RRAM (Resistance Random Access Memory) has attracted attention as one of nonvolatile memories capable of high-speed operation, high integration, and low power consumption. RRAM generally utilizes the fact that the resistance of a film reversibly changes when a pulse voltage is applied to the film of a metal oxide or the like. That is, the RRAM can hold data in a non-volatile manner by setting the resistance value of the variable resistance element by changing the polarity and magnitude of the applied pulse voltage. As a material of the resistor layer constituting such an RRAM, for example, an oxide containing manganese (Mn) is disclosed (see Patent Document 1).
JP-A-8-133894

  An object of the present invention is to provide a novel variable resistance element applicable to a resistance change type memory.

The first variable resistance element according to the present invention is:
A first electrode;
A resistor layer formed above the first electrode;
A second electrode formed above the resistor layer,
The resistor layer is made of a transition metal oxide represented by Y _x Ti _1-x O ₂ (0 ≦ x <1),
The transition metal oxide has oxygen defects.

  The variable resistance element according to the present invention can provide a high resistance change rate. Therefore, the variable resistance element according to the present invention is suitably used for a resistance change type memory such as an RRAM.

  In the description according to the present invention, the word “upward” refers to, for example, “another specific thing (hereinafter referred to as“ B ”) formed“ above ”a specific thing (hereinafter referred to as“ A ”)”. Etc. In the description of the present invention, in the case of this example, there are a case where B is directly formed on A and a case where B is formed on A via another. The word “above” is used as included.

In the variable resistance element according to the present invention,
It can be 0 ≦ x ≦ 0.3.

In the variable resistance element according to the present invention,
It can be 0.03 ≦ x ≦ 0.15.

The second variable resistance element according to the present invention is:
A first electrode;
A resistor layer formed above the first electrode;
A second electrode formed above the resistor layer,
The resistor layer is made of a transition metal oxide having oxygen defects,
The resistance changes due to the movement of the oxygen defects.

In the variable resistance element according to the present invention,
The transition metal oxide may have an oxygen defect in an amount that maximizes the rate of change in resistance.

In the variable resistance element according to the present invention,
The transition metal oxide _is represented by _{Y x Ti 1-x O 2} ,
The x may be a value that maximizes the rate of change of the resistance.

In the variable resistance element according to the present invention,
It can be used for a resistance change type memory.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.

  1. First, the variable resistance element 10 according to the present embodiment will be described. FIG. 1 is a cross-sectional view schematically showing a variable resistance element 10 according to the present embodiment.

  The variable resistance element 10 is formed on the base 1. The variable resistance element 10 includes a first electrode 12 formed on the base 1, a resistor layer 14 formed on the first electrode 12, and a second electrode 16 formed on the resistor layer 14. Including.

  As the substrate 1, different substrates can be used depending on the apparatus to which the variable resistance element 10 is applied. For example, when the variable resistance element 10 is applied to an RRAM, a semiconductor substrate on which a MOS transistor or the like is formed can be used as the base 1 as will be described later.

Examples of the material of the first electrode 12 include platinum group metals such as Pt, Ir, and Ru, alloys containing the platinum group metals, oxides of the platinum group metals, SRO (SrRuO ₃ ), and LSCO ((La, Sr ) Conductive oxides such as CoO ₃ ) and LaNiO ₃ can be used. As the material of the second electrode 16, the same material as that of the first electrode 12 can be used.

  The resistor layer 14 is made of a transition metal oxide having oxygen defects. The transition metal oxide having oxygen defects is formed, for example, by replacing a part of the transition metal in the crystal with a transition metal element having a lower valence. For example, Y (+3 valence) with respect to Ti (+4 valence) is an example. That is, when the average valence of the transition metal site is reduced, oxygen defects are generated due to elimination of oxygen atoms based on the principle of charge neutrality. At this time, the system is stable while maintaining insulation.

The transition metal oxide according to the present embodiment is represented as Y _x Ti _1-x O ₂ (0 ≦ x <1). Y _x Ti _1-x O ₂ (hereinafter also referred to as “YTO”) represents a transition metal oxide in a state having no oxygen defects. YTO can provide a thin film having a stable film thickness and resistivity, and is suitably used as the resistor layer 14.

In addition, when x = 0 in the above-described YTO, that is, when the transition metal oxide according to the present embodiment is titanium oxide (TiO ₂ ), the transition metal oxide is already in a stable state at room temperature. It can have a large number of oxygen defects. This is because the ionic radius of titanium (Ti) is smaller than that of zirconium (Zr) or the like, so that the lattice constant of the titanium oxide crystal is smaller than that of zirconium oxide (ZrO ₂ ) or the like. That is, since the lattice constant of titanium oxide is smaller than that of zirconium oxide or the like, it is difficult for oxygen atoms to enter the lattice during film formation. Therefore, titanium oxide is also preferably used as the resistor layer 14.

The amount of oxygen defects in the transition metal oxide according to the present embodiment can be increased by increasing the amount of Ti in titanium oxide (TiO ₂ ) replaced with yttrium (Y). If the amount of oxygen vacancies is too small, the rate of change in resistance of the variable resistance element 10 cannot be increased. Conversely, if the amount is too large, the transition metal oxide crystal is disturbed, so the rate of change in resistance is low. The amount of substitution by Y in this embodiment, that is, the range of the composition x of Y in YTO: Y _x Ti _1-x O ₂ is preferably 0 ≦ x ≦ 0.3, more preferably 0.03 ≦ x ≦ 0. .15. When the composition x of yttrium is in this range, a high resistance change rate can be obtained. Since the resistance change rate of the variable resistance element 10 is preferably as high as possible, the oxygen defect is preferably in an amount that maximizes the resistance change rate. This can be achieved, for example, by adjusting the composition x of Y to a value that maximizes the resistance change rate. In addition, the transition metal oxide according to the present embodiment in the state having oxygen defects is Y _x Ti _1-x O _{2-(x / 2) -α} (0 ≦ x <1, 0 <α <2). Here, α represents the amount of oxygen defects that titanium oxide (TiO ₂ ) has in a stable state at room temperature.

  2. Next, a method for manufacturing the variable resistance element 10 according to the present embodiment will be described.

  (1) First, the first electrode 12 is formed on the substrate 1 by sputtering or the like. Next, a resistor layer 14 made of a transition metal oxide having oxygen defects is formed on the first electrode 12. The resistor layer 14 is formed by, for example, a sputtering method, a sol / gel method, or the like. When the sputtering method is used, the resistor layer 14 can be formed by forming a film in an oxygen atmosphere using a target having a desired composition ratio. In the case of using the sol-gel method, the raw material solution is mixed so as to have a desired composition ratio. Can be formed.

  Next, the second electrode 16 is formed on the resistor layer 14 by sputtering or the like. Next, if necessary, the second electrode 16, the resistor layer 14, and the first electrode 12 can be patterned using a lithography technique and an etching technique, respectively.

  3. According to the variable resistance element 10 according to the present embodiment, a high resistance change rate can be provided. Therefore, the variable resistance element 10 of this embodiment is suitably used for a resistance change type memory such as an RRAM.

  In the variable resistance element 10 according to the present embodiment, it is estimated that the resistance is reversibly changed for the following reason. That is, for example, when oxygen defects in the crystal move to the vicinity of the electrode by an external voltage, the band offset near the electrode interface decreases, increasing the amount of electrons injected by tunneling and reducing the electrical resistance. Guessed. Conversely, if the oxygen vacancies in the crystal move away from the vicinity of the electrode due to an external voltage, the band offset near the electrode interface increases, and the amount of injected electrons due to tunneling decreases and the electrical resistance increases. Is done.

The oxygen vacancies in the transition metal oxide effectively behave as positive ions and move to the negative electrode side. On the other hand, the oxygen atom itself behaves effectively as a negative ion and moves to the positive electrode side. In the transition metal oxide, since the transition metal and oxygen are bound by ionic bonds, for example, oxygen defects and oxygen atoms are more likely to move than when bound by covalent bonds. The external voltage for moving oxygen defects and oxygen atoms has a threshold value V ₀ , and when a voltage equal to or higher than the threshold value V ₀ is applied, the oxygen defects and oxygen atoms move toward the respective electrodes. For example, it is possible to record the signal information by applying a threshold value greater than or equal _to V ₀ of the voltage V _W. Further, the threshold value V ₀ is lower than the voltage V _R, the oxygen defects and the oxygen atom does not move. For example, it is possible to read out signal information by measuring the resistance value at this voltage V _R. Further, for example, by the V _W and V _R which applies a reverse voltage -V _E, accumulation of oxygen defects are eliminated in the one side of the electrode side, it is possible to reset the recorded information. However, V _E > V ₀ is preferable.

Signal initialization / recording / erasing can be performed by applying a voltage pulse from a pulse generator between the electrodes of the variable resistance element 10, for example. The resistance value is obtained by measuring the IV characteristic with a parameter analyzer. First, initialization pulse voltage that changes between + _{V I} and -V _I to the variable resistance element 10 (e.g., pulse width 100 nsec, a duty ratio of 50%) was added to initialize the signal. Next, to measure the resistance value before the signal recorded in the DC voltage V _R. Next, a forward pulse voltage _VW is applied to record a signal. Next, to measure the resistance value after the signal recorded in the DC voltage V _R. Next, a reverse pulse voltage -V _E is applied between the electrodes to erase the signal.

Examples of each voltage are as follows. The signal initialization voltage _{V I} 4.0V, the signal writing voltage _{V W} is 3.0 V, the signal readout voltage _{V R} is 0.8 V, the signal erasing voltage _{V E} is 3.0 V. The reference voltage at the time of signal writing and erasing is 0V. Each voltage pulse shape has, for example, a pulse width of 50 nsec, a duty ratio of 50%, and a time length of 1 μsec. Note that the initialization of the signal can be performed in 1 sec.

The resistance change rate of the variable resistance element 10 according to the present embodiment is obtained by the following equation.
Resistance change rate (%) = | (resistance value after signal recording) − (resistance value before signal recording) | / (resistance value before signal recording) × 100

4). Next, an experimental example in which Y _x Ti _1-x O ₂ is used as a transition metal oxide constituting the resistor layer 14 will be described.

In this experimental example, it is shown that the resistance change rate described above changes depending on the composition x of yttrium in YTO. The following were used as experimental samples. As the substrate 1, a silicon substrate having a silicon oxide layer on the surface was used. As the variable resistance element 10, an element having a first electrode 12 made of platinum having a thickness of 200 nm, a resistor layer 14 made of YTO having a thickness of 50 nm, and a second electrode 16 made of platinum having a thickness of 100 nm was used. A 150 W DC sputtering method was used to form the first electrode 12 and the second electrode 16. A 200 W RF sputtering method was used to form the resistor layer 14. Argon was used as the sputtering gas, and the gas pressure was 2 × 10 ⁻³ Torr.

  In this experimental example, a plurality of samples were formed by changing the composition x of yttrium in YTO, and the resistance change rate of each sample was measured. The results are shown in Table 1.

From Table 1, it was confirmed that the composition x of yttrium (Y) is preferably 0 ≦ x ≦ 0.3, more preferably 0.03 ≦ x ≦ 0.15. In the range of 0.03 ≦ x ≦ 0.15, a high resistance change rate of 100% or more is obtained. It was also confirmed that a sufficient resistance change rate of 40% can be obtained even when the yttrium composition x = 0, that is, when the transition metal oxide constituting the resistor layer 14 is TiO ₂ . . The resistance change rate (40%) in the case of TiO ₂ is 8 times higher than the resistance change rate in the case where the resistor layer 14 is made of ZrO ₂ (comparative example) was 5%. Yes.

  5). Next, a resistance change type memory 100 to which the variable resistance element 10 according to the present embodiment is applied will be described. FIG. 2 is a cross-sectional view schematically showing the resistance change type memory 100.

  The resistance change type memory 100 includes a base 1 and a variable resistance element 10 formed on the base 1. A plurality of variable resistance elements 10 can be arranged to constitute a memory cell array.

  The base 1 includes, for example, a semiconductor substrate 20, an interlayer insulating layer 24 formed on the semiconductor substrate 20, an insulating layer 29 formed on the interlayer insulating layer 24, and the like.

  As the semiconductor substrate 20, for example, a silicon substrate can be used. On the semiconductor substrate 20, a drive circuit and peripheral circuits for the variable resistance element 10 are formed. For example, circuit elements such as an element isolation region 22 and a MOS transistor 30 are formed on the semiconductor substrate 20. The MOS transistor 30 includes a gate insulating layer 32, a gate electrode 34, and impurity layers 36 and 38 constituting source / drain regions. For example, a silicon oxide layer can be used as the interlayer insulating layer 24. A contact portion (plug) 26 connected to the impurity layers 36 and 38 is formed in the interlayer insulating layer 24. A wiring layer 28 is formed on the contact portion 26. On the interlayer insulating layer 24, for example, an insulating layer 29 having an oxygen barrier property, a hydrogen barrier property, a high adhesion property and the like is formed. As the insulating layer 29, for example, a titanium oxide layer can be used. A plurality of variable resistance elements 10 are formed in the memory cell region on the insulating layer 29.

  In the resistance change memory 100 according to the present embodiment, for example, a signal (information) is recorded (written), read, and erased by applying a voltage to the variable resistance element 10 by the method described above and measuring the resistance value. It can be carried out.

  6). Although the embodiments of the present invention have been described in detail as described above, those skilled in the art will readily understand that many modifications are possible without substantially departing from the novel matters and effects of the present invention. Accordingly, all such modifications are intended to be included in the scope of the present invention.

FIG. 3 is a cross-sectional view schematically showing the variable resistance element according to the embodiment. 1 is a cross-sectional view schematically showing a resistance change type memory according to an embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Base | substrate, 10 Variable resistance element, 12 1st electrode, 14 Resistor layer, 16 2nd electrode, 20 Semiconductor substrate, 22 Element isolation region, 24 Interlayer insulation layer, 26 Contact part, 28 Wiring layer, 29 Insulation layer, 30 Transistor, 32 gate insulating layer, 34 gate electrode, 36, 38 impurity layer, 100 resistance change type memory

Claims (2)

A first electrode;
A resistor layer formed above the first electrode;
A second electrode formed above the resistor layer,
The resistor layer is made of a transition metal oxide represented by Y _x Ti _1-x O ₂ ( 0.06 ≦ x ≦ 0.15 ),
The transition metal oxide has a variable resistance element having an oxygen defect. In claim 1,
A variable resistance element used in a resistance change type memory.

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