JP4655021B2 - Variable resistance element - Google Patents

Variable resistance element Download PDF

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JP4655021B2
JP4655021B2 JP2006278611A JP2006278611A JP4655021B2 JP 4655021 B2 JP4655021 B2 JP 4655021B2 JP 2006278611 A JP2006278611 A JP 2006278611A JP 2006278611 A JP2006278611 A JP 2006278611A JP 4655021 B2 JP4655021 B2 JP 4655021B2
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variable resistance
resistance element
transition metal
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metal oxide
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JP2008098413A (en
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弘 宮澤
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セイコーエプソン株式会社
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  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 such as an RRAM.

The variable resistance element according to the present invention is
A pair of electrodes;
A resistor layer formed between the pair of electrodes,
The resistor layer is made of a transition metal oxide represented by Y x A 1-x O 2 (0 ≦ x ≦ 0.3),
A represents at least two kinds of transition metal elements having different work functions,
The transition metal oxide has oxygen defects.

  According to the variable resistance element of the present invention, the resistance value between the electrodes can be set to an optimum value by changing the composition ratio of the two or more transition metal elements contained in A. Therefore, the variable resistance element according to the present invention is suitably used for a resistance change type memory.

In the variable resistance element according to the present invention,
The A may be at least two of titanium (Ti), zirconium (Zr), and hafnium (Hf).

  However, when A is zirconium (Zr) and hafnium (Hf), 0 <x ≦ 0.3.

In the variable resistance element according to the present invention,
The transition metal oxide may be represented by Y x (Ti 1-a Zr a ) 1-x O 2 (0 ≦ x ≦ 0.3, 0 <a <1).

In the variable resistance element according to the present invention,
The transition metal oxide may be represented by Y x (Ti 1-b Hf b ) 1-x O 2 (0 ≦ x ≦ 0.3, 0 <b <1).

In the variable resistance element according to the present invention,
The transition metal oxide may be represented by Y x (Zr 1-c Hf c ) 1-x O 2 (0 <x ≦ 0.3, 0 <c <1).

In the variable resistance element according to the present invention,
The A can include at least titanium (Ti).

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

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 pair of electrodes 12 and 16 and a resistor layer 14 formed between the pair of electrodes 12 and 16. For example, the variable resistance element 10 includes a first electrode 12 formed on the substrate 1, a resistor layer 14 formed on the first electrode 12, and a second electrode 16 formed on the resistor layer 14. , Can be included. In the present invention, the case where the resistor layer 14 is formed between the pair of electrodes 12 and 16 is different from the case where at least one of the pair of electrodes 12 and 16 and the resistor layer 14 are different. When a layer (not shown) is formed, or when a pair of electrodes 12 and 16 are formed on one surface of the resistor layer 14 and the electrodes are physically separated (not shown) Etc.

  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.

  In order to stably operate the variable resistance element 10, it is important to appropriately select a combination of the materials of the electrodes 12 and 16 and the material of the resistor layer 14. That is, first, in the manufacturing process, a stable electrode material having a smooth surface morphology and little interatomic diffusion between interfaces is selected. Then, a material for the resistor layer 14 having an optimum band offset is selected for the selected electrode material. By doing so, the variable resistance element 10 has good characteristics. That is, if the band offset can be set variably and freely, the degree of freedom in material selection is widened, which is a great advantage.

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) for Ti (+4 valence), Zr (+4 valence), Hf (+4 valence), etc. is an example. That is, when the average valence of the transition metal site is reduced, oxygen defects are generated due to the 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 A 1-x O 2 (0 ≦ x ≦ 0.3). Y x A 1-x O 2 (hereinafter also referred to as “YAO”) can provide a thin film having a stable film thickness and resistivity, and is preferably used as the resistor layer 14.

The element A represents at least two transition metal elements having different work functions. The element A can be at least two of titanium (Ti), zirconium (Zr), and hafnium (Hf), for example. However, when the element A is zirconium (Zr) and hafnium (Hf), x is larger than 0, that is, the transition metal oxide according to the present embodiment contains yttrium (Y). The work function of Ti is 4.8 ev, the work function of Zr is 4.1 eV, and the work function of Hf is 3.9 eV. For example, when an alloy of the elements X and Y is represented by X z Y 1-z (0 ≦ z ≦ 1), the work function W of the alloy is the work function of the elements X and Y as W X and W Y , respectively. Then, it is approximately expressed by the following formula.
W = z × W X + (1−z) × W Y

  This is also true for transition metal oxides. Therefore, also in YAO according to the present embodiment, the work function of the whole YAO can be controlled by selecting a plurality of elements among Ti, Zr, Hf, and the like as the element A. That is, the work function of the whole YAO can be controlled by the composition ratio of the transition metals Ti, Zr, Hf and the like.

When the element A is Ti and Zr, the transition metal oxide according to this embodiment is Y x (Ti 1-a Zr a ) 1-x O 2 (0 ≦ x ≦ 0.3, 0 <a <1). By increasing the Zr composition a, the resistance value between the electrodes of the variable resistance element 10 can be increased. Further, when the element A is Ti and Hf, the transition metal oxide according to the present embodiment is Y x (Ti 1-b Hf b ) 1-x O 2 (0 ≦ x ≦ 0.3, 0 <B <1). By increasing the Hf composition b, the resistance value between the electrodes of the variable resistance element 10 can be increased. Further, when the element A is Zr and Hf, the transition metal oxide according to the present embodiment is Y x (Zr 1-c Hf c ) 1-x O 2 (0 <x ≦ 0.3, 0 <C <1). By increasing the Hf composition c, the resistance value between the electrodes of the variable resistance element 10 can be increased.

  In addition, when x = 0 in the above-described YAO, that is, when the transition metal oxide according to the present embodiment does not contain yttrium (Y), the transition metal oxide includes It can already have a large number of oxygen defects in a stable state at room temperature. This is because the ionic radius of titanium (Ti) is smaller than that of zirconium (Zr), hafnium (Hf), etc., and the lattice constant of the transition metal oxide crystal is smaller than that of the crystal that does not contain Ti in A. is there. Therefore, YAO in which x = 0 in YAO and the element A contains at least Ti 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, for example, by increasing the amount of substitution of A in the oxide of element A (AO 2 ) 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 the present embodiment, that is, the range of the composition x of Y in YAO: Y x A 1-x O 2 is 0 <x ≦ 0.3, preferably 0.03 ≦ x ≦ 0.15. is there. When the composition x of yttrium is in this range, a high resistance change rate can be obtained. When the variable resistance element 10 is applied to, for example, an RRAM, the higher the resistance change rate, the better. Therefore, it is preferable that the oxygen defect is 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. Note that the above Y x A 1-x O 2 represents a transition metal oxide in a state having no oxygen defects. The transition metal oxide according to this embodiment in a state having oxygen defects is based on the principle of charge neutrality, and Y x A 1-x O 2- (x / 2) -α (0 ≦ x ≦ 0.3, 0 <α <2). However, alpha is an oxide of an element A (AO 2) represents the amount of oxygen defects included in a stable state in the 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. The composition ratio of the target can be the ratio of each composition of yttrium (Y) and element A (for example, Ti, Zr, Hf, etc.) in the desired Y x A 1-x O 2 .

When using the sol-gel method, the raw material solution is mixed so as to have a desired composition ratio, and the prepared solution is applied onto the substrate 1 and then heat-treated to form the resistor layer 14. be able to. The composition ratio of the prepared solution can be a ratio of each composition of yttrium (Y) and element A (for example, Ti, Zr, Hf, etc.) in desired Y x A 1-x O 2 .

  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, the resistance value between the electrodes can be set to a desired value by changing the composition ratio of two or more transition metal elements contained in the element A in YAO. . The reason is considered as follows.

In YAO, changing the composition ratio of transition metal elements (Ti, Zr, Hf, etc.) having different work functions changes the band gap of YAO. Specifically, the more transition metal elements having a high work function, the smaller the band gap of YAO tends to be. Thereby, it is estimated that the band offset between the electrodes of the variable resistance element 10 changes and the resistance value changes. Specifically, the greater the band gap of YAO, the greater the band offset between the electrodes of the variable resistance element 10, and the greater the resistance value. The band gap of the transition metal oxide (YAO) used as the resistor layer 14 is mainly determined by the work function of the transition metal oxide. That is, the work function of an element having an atomic orbit that constitutes the bottom of the conduction band of the resistor layer 14 determines the band gap. This band gap corresponds to the band offset between the resistor layer 14 and the electrodes 12 and 16. The band gap of TiO 2 is 3.2 eV, the band gap of ZrO 2 is 5.0 eV, and the band gap of HfO 2 is 5.6 eV. For example, when an alloy of the elements X and Y is represented by X z Y 1-z (0 ≦ z ≦ 1), the band gap E of the alloy is the band gap of the elements X and Y as E X and E Y , respectively. Then, it is approximately expressed by the following formula.
E = z × E X + (1−z) × E Y

  This is also true for transition metal oxides. Therefore, also in YAO according to the present embodiment, the band gap of the entire YAO can be controlled by selecting a plurality of elements among Ti, Zr, Hf, and the like as the element A. That is, the overall band gap of YAO can be controlled by the composition ratio of the transition metals Ti, Zr, Hf, and the like.

  For example, when the variable resistance element 10 is applied to a resistance change type memory such as an RRAM, if the resistance value is too low, the power consumption increases. Conversely, if the resistance value is too high, the current value decreases and the SN ratio increases. Decreases. As described above, according to the variable resistance element 10 according to the present embodiment, the resistance value can be set to an optimum value. Therefore, the variable resistance element 10 according to the present embodiment is preferably used for a resistance change type memory. .

  Furthermore, according to the variable resistance element 10 according to the present embodiment, a high resistance change rate can be provided. From this, it can be said that the variable resistance element 10 of the present embodiment is suitable 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 that moves 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. It can be recorded 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. It can be read 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 one 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, experimental examples will be described.

  4.1. First, a first experimental example will be described.

In this experimental example, Y x (Ti 1-a Zr a ) 1-x O 2 (hereinafter also referred to as “YTZO”) was used as the transition metal oxide constituting the resistor layer 14. This experimental example shows that the resistance value between the electrodes of the variable resistance element 10 varies depending on the composition a of zirconium (Zr) in YTZO. 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 YTZO having a thickness of 50 nm, and a second electrode 16 made of platinum having a thickness of 100 nm was used. X in YTZO was set to 0.10. 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. The planar shapes of the first electrode 12, the resistor layer 14, and the second electrode 16 were 10.0 μm × 10.0 μm.

  In this experimental example, a plurality of samples were formed by changing the Zr composition a in YTZO, and the resistance value of each sample was measured. The results are shown in Table 1. The resistance value (%) is described with the resistance value at the time of Zr composition a = 0 as a reference (100%).

  From Table 1, it was confirmed that the resistance value increased in proportion by increasing the Zr composition a. Therefore, for example, in YTZO, it was confirmed that the resistance value can be freely controlled by changing the Zr composition a.

  4.2. Next, a second experimental example will be described.

In this experimental example, Y x (Ti 1-b Hf b ) 1-x O 2 (hereinafter also referred to as “YTHO”) was used as the transition metal oxide constituting the resistor layer 14. This experimental example shows that the resistance value between the electrodes of the variable resistance element 10 varies depending on the composition b of hafnium (Hf) in YTHO. Regarding this experimental sample, it was the same as the first experimental example described above except that the resistor layer 14 was made of YTHO.

  Table 2 shows the measurement results of the resistance value of each sample in which the composition b of Hf in YTHO was changed. The resistance value (%) is described with the resistance value at the time of Hf composition b = 0 as a reference (100%).

  From Table 2, it was confirmed that the resistance value increased in proportion by increasing the Hf composition b. Therefore, for example, in YTHO, it was confirmed that the resistance value can be freely controlled by changing the Hf composition b.

  4.3. Next, a third experimental example will be described.

In this experimental example, Y x (Zr 1-c Hf c ) 1-x O 2 (hereinafter also referred to as “YZHO”) was used as the transition metal oxide constituting the resistor layer 14. This experimental example shows that the resistance value between the electrodes of the variable resistance element 10 varies depending on the composition c of hafnium (Hf) in YZHO. Regarding this experimental sample, it was the same as in the first experimental example described above except that the resistor layer 14 was made of YZHO.

  Table 3 shows the measurement results of the resistance values of the respective samples in which the composition c of Hf in YZHO was changed. The resistance value (%) is described with the resistance value when the Hf composition c = 0 as a reference (100%).

  From Table 3, it was confirmed that the resistance value increased in proportion by increasing the Hf composition c. Therefore, for example, in YZHO, it was confirmed that the resistance value can be freely controlled by changing the Hf composition c.

  4.4. Next, a fourth experimental example will be described.

In this experimental example, an experimental example in which Y x (Ti 1-a Zr a ) 1-x O 2 : YTZO is used as the 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 yttrium composition x in YTZO. In this experimental example, the Zr composition a in YTZO was set to 0.50, and the Y composition x was changed from 0 to 0.35 to form a plurality of experimental samples. Other points regarding the experimental sample were the same as in the first experimental example described above. Table 4 shows the measurement results of the resistance change rate of each sample.

  From Table 4, it was confirmed that the composition x of yttrium (Y) is preferably 0 <x ≦ 0.3, more preferably 0.03 ≦ x ≦ 0.15. Further, when the Y composition x = 0, that is, when the transition metal oxide constituting the resistor layer 14 does not contain yttrium (Y), the transition metal oxide contains Ti. It was confirmed that the resistance change rate can be obtained.

  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, an insulating layer 29 having, for example, an oxygen barrier property, a hydrogen barrier property, and a high adhesion property 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)

  1. A pair of electrodes;
    A resistor layer formed between the pair of electrodes,
    The resistor layer is made of a transition metal oxide represented by Y x (Ti 1-a Zr a ) 1-x O 2 (0.06 ≦ x ≦ 0.15, 0 <a <1) ,
    The transition metal oxide has a variable resistance element having an oxygen defect.
  2. In claim 1,
    A variable resistance element used in a resistance change type memory.
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JP5251349B2 (en) * 2008-08-08 2013-07-31 富士通株式会社 Resistance change element and resistance change element manufacturing method
CN102782846B (en) * 2010-06-10 2015-05-20 松下电器产业株式会社 Non-volatile memory element and non-volatile memory device equipped with same
KR101283767B1 (en) 2011-11-17 2013-07-08 재단법인대구경북과학기술원 Non-volatile resistance memory device

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JP2006245322A (en) * 2005-03-03 2006-09-14 Sony Corp Memory element and memory
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