JP5690635B2 - Nonvolatile semiconductor memory device and method of manufacturing the same - Google Patents

Description

  The present invention relates to a nonvolatile semiconductor memory device and a method for manufacturing the same, and more particularly to a variable resistance nonvolatile semiconductor memory device that utilizes a change in resistance value by applying a voltage and a method for manufacturing the same.

  As a new memory element, a nonvolatile semiconductor memory device called RRAM (Resistance Random Access Memory) has attracted attention. The RRAM stores a plurality of resistance states having different resistance values, uses a resistance memory element that changes its resistance state by applying an electrical stimulus from the outside, and uses, for example, information on a high resistance state and a low resistance state of the resistance memory element. By associating with "0" and "1", the memory element is used. The future of RRAM is expected because of its high potential such as high speed, large capacity, and low power consumption.

  In the resistance memory element, a resistance memory material whose resistance state is changed by application of a voltage is sandwiched between a pair of electrodes. As a typical resistance memory material, an oxide material containing a transition metal is known, and can be roughly classified into two types depending on the difference in electrical characteristics.

One is to use voltages having different polarities in order to change the resistance state between a high resistance state and a low resistance state, and SrTiO ₃ or SrZrO ₃ doped with a small amount of impurities such as chromium (Cr). Alternatively, Pr _1-x Ca _x MnO ₃ , La _1-x Ca _x MnO _3, or the like exhibiting a giant magnetic resistance (CMR) is applicable. An RRAM using such a bipolar material is described in Patent Document 1, Patent Document 2, Non-Patent Document 1, or Non-Patent Document 2, for example.

The other is to use a voltage of the same polarity to change the resistance state between a high resistance state and a low resistance state, for example a single transition metal oxide such as NiO _x or TiO _x . Etc. An RRAM using such a unipolar material is described in Non-Patent Document 3, for example.

US Pat. No. 6,473,332 JP 2005-025914 A

A. Beck et al., Appl.Phys. Lett.Vol. 77, p. 139 (2001) W. W. Zhuang et al., Tech. Digest IEDM 2002, p.193 I. G. Baek et al., Tech. Digest IEDM 2004, p. 587

In the electrode / perovskite oxide / electrode structure, the resistance change phenomenon that occurs when a voltage is applied between the upper and lower electrodes is such that the ratio of the resistance value between the high resistance state and the low resistance state is as small as an order of magnitude. However, this is disadvantageous in identifying the level of resistance during reading. In the electrode / perovskite oxide / electrode structure, the resistance value can be controlled by changing the absolute value of the voltage applied between the upper and lower electrodes or the pulse width of the applied voltage pulse. The wider the pulse width, the higher the resistance value in the high resistance state,
The resistance value in the low resistance state can be made lower, and by using this characteristic, the memory can be multi-valued. However, as described above, the fact that the ratio of the resistance value between the high resistance state and the low resistance state is small hinders multivalue application.

  In addition, when a large number of such structures are manufactured and used as a memory element, for example, when a large number of electrode / perovskite oxide / electrode structures are formed on the same wafer using photolithography or the like, the periphery and center of the wafer are formed. There may be a difference in the memory characteristics including the ratio of the high resistance and the low resistance with the located structure. A method for suppressing the variation between the elements has not been established so far, and is a factor that hinders an increase in capacity as a memory element.

  An object of the present invention is to effectively prevent a read error by providing an element structure capable of increasing a resistance value ratio between a high resistance state and a low resistance state, a method for manufacturing the element, and a method for reducing variation between elements. And multi-value application, and further to increase the capacity of the memory element.

Invention, a metal having a _{Bi 2 Sr 2 CaCu 2 O 8} + Gibbs energy of small becomes oxidized than Gibbs energy of oxidation in the perovskite oxide on one surface of the perovskite oxide consisting of _δ is provided according to the present invention according to claim 1 At the same time, the oxygen of the perovskite oxide moves from the perovskite oxide to the metal, so that the energy higher than the activation energy required for the oxidation of the metal and the reduction of the perovskite oxide to proceed is given. An oxygen-deficient layer obtained by heating is present in a region in the vicinity of the perovskite oxide in contact with the metal, and the other electrode paired via the one electrode made of the metal and the oxygen-deficient layer Non-volatile, using the hysteresis characteristics of voltage-current characteristics between electrodes To provide a semiconductor memory device. In this document, metal oxides having a perovskite type or layered perovskite type crystal structure are collectively referred to as “perovskite oxide”.

According to the second aspect of the present invention, a metal having an oxidation Gibbs energy smaller than the oxidation Gibbs energy in the perovskite oxide is provided on one surface of the perovskite oxide, and then the perovskite oxide is transferred from the perovskite oxide to the metal. An inert gas such as Ar or He or N ₂ is provided so that an energy higher than the activation energy necessary for the oxidation of the metal and the reduction of the perovskite oxide to proceed due to the movement of oxygen in the oxide. By heating in gas , an oxygen-deficient layer is formed in the vicinity of the perovskite oxide in contact with the metal, and one electrode made of the metal and the other paired via the oxygen-deficient layer A method for manufacturing a nonvolatile semiconductor memory device is provided.

The invention according to the present invention claimed in claim 3, prior Symbol metal is either Al, Cu, Ti, Ni, Ta, V, Cr, Sb, Mn, Fe, Co, Zn, Hf, Nb, Mo, Sn, of Si The nonvolatile semiconductor memory device according to claim 1, wherein the nonvolatile semiconductor memory device is a material selected from one or an alloy thereof.

In the invention according to claim 4, the perovskite oxide may be Bi ₂ Sr ₂ CuO _{6 + δ} , Bi ₂ Sr ₂ CaCu ₂ O _{8 + δ} , Bi ₂ Sr ₂ Ca ₂ Cu ₃ O _{10 + δ} , YBa. _{2 Cu 3 O 7-δ,} La 2-x Ba x CuO 4, TlBa 2 Ca n-1 Cu n O 2n + 3 (n = 1,2,3,4,5), Tl 2 Ba 2 Ca n- _{1 Cu n O 2n + 4 (} n = 1,2,3,4), HgBa 2 YCa n-1 Cu n O 2n + 2 + δ (n = 1,2,3,4,5), La 2- _{x Sr x CuO 4, Nd 2} -x Ce x CuO 4, La is _2-x Sr x from any one selected substance of CuO _4, wherein the metal, Al, Cu, Ti, Ni , Ta, The substance according to claim 2, wherein the substance is selected from V, Cr, Sb, Mn, Fe, Co, Zn, Hf, Nb, Mo, Sn, Si, or an alloy thereof. To provide a method of manufacturing a nonvolatile semiconductor memory device.

  The present invention is a non-volatile semiconductor memory device having the above-described configuration, which can increase the ratio of high resistance to low resistance as compared with the conventional resistance change type semiconductor memory device, and therefore, the read reliability is high. Improved and advantageous for multi-value applications.

  Further, materials can be selected according to the purpose of use of the storage device. Furthermore, since the present apparatus can be manufactured by heat treatment, variation between elements in the semiconductor wafer surface is reduced, and as a result, the capacity can be increased.

(a), (b), (c), (d) is sectional drawing for demonstrating the manufacturing method of the principal part of the apparatus in one Example of this invention. (a) is a figure which shows the current-voltage characteristic of the apparatus obtained by the Example same as the above, (b) is a figure which shows the current-voltage characteristic of another apparatus hung up for comparison. It is a figure which shows the energy level for demonstrating this invention. (a), (b), (c) is a figure for demonstrating the effect of this invention. It is a figure which shows the condition from which resistivity changes with the grade of the oxidation and reduction | restoration of a specific perovskite oxide. It is a figure which shows the structure of the principal part of the apparatus in the other Example of this invention.

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

First, as shown in FIG. 1A, an Al electrode and a Pt electrode are formed on both surfaces of Bi-2212 (copper oxide superconductor Bi ₂ Sr ₂ CaCu ₂ O _{8 + δ} ) which is an example of a perovskite oxide. Each 100 nm film was formed to produce an Al / Bi-2212 / Pt structure. Sputtering was used to form Al and Pt. The film forming method may be a vapor deposition method or a CVD method.

1A was heat-treated at 100, 200, 300, 400, and 500 ° C. for 1 hour in an Ar atmosphere (FIG. 1B). Here, an inert gas such as He may be used instead of Ar. The main reason for using an inert gas is to prevent electrode oxidation. Therefore, the gas is not limited to the inert gas, and N ₂ gas (nitrogen gas) may be preferable. That is, when Ti is used as the electrode metal, oxidation proceeds on the side where the metal is in contact with Bi-2212, and nitride is formed on the metal surface, but Ti nitride is more stable than using Ti as it is. It can be used as a simple electrode.

  Through the steps (a) and (b), the Al electrode takes Al from Bi-2212 to form AlOx, and the region in contact with the Al electrode of Bi-2212 has an oxygen deficiency (reduction). A layer is formed (FIG. 1 (c)).

  Next, as shown in FIG. 1 (d), the Pt electrode is grounded and the bias voltage V is applied to the Al electrode, thereby measuring the IV characteristics of the element obtained in the steps (a) to (c). did.

  FIG. 2 (a) shows the IV characteristics measured in FIG. 1 (d). As the annealing temperature was increased in the process of FIG. 1 (c), the hysteresis opening became larger, and the ratio of high resistance to low resistance (= high resistance / low resistance) increased.

For comparison, FIG. 2 (b) shows the IV characteristics measured when Pt is used for both the upper and lower electrodes. Almost no hysteresis was observed from as-prepared (as-made) to 400 ° C annealed samples, and similar IV characteristics were observed regardless of annealing temperature. It should also be noted that when Al / Bi-2212 / Pt structure is heat-treated at 500 ° C. for 1 hour in an Ar atmosphere, Al oxidation proceeds too much and AlO _x becomes Al ₂ O _3. Therefore, the function as a memory element was not achieved due to high insulation.

  In short, the heat treatment is performed by transferring the oxygen of the Bi-2212 (perovskite oxide) from Bi-2212 (perovskite oxide) to Al (metal), thereby oxidizing the Al (metal) and the Bi-2212 ( It is necessary to heat so that an energy higher than the activation energy necessary for the reduction of the perovskite oxide) to proceed. In addition, excessive oxidation of Al (metal) increases the resistance of the oxidized region compared to the resistance of the oxygen-deficient layer of Bi-2212 (perovskite oxide), and voltage application to Bi-2212 (perovskite oxide) It is also practically important that the energy is less than that which prevents the resistance change from high resistance to low resistance or low resistance to high resistance in Bi-2212 (perovskite oxide).

Fig. 3 shows Al-TE (TE means the upper electrode, the same shall apply hereinafter) / Bi-2212 interface Al + Bi ₂ Sr ₂ CaCu ₂ O _{8 + δ1} (α) and Al ₂ O ₃ + Bi ₂ Sr ₂ Indicates the energy level of CaCu ₂ O _{8 + δ2} (β). In order to transition from the state α to β where the oxygen-deficient layer is formed, the activation energy E _a needs to be exceeded, and the reaction rate v = Aexp (−E _a / k _B T) depends on the annealing temperature T . On the other hand, at the Pt-TE / Bi-2212 and Bi-2212 / Pt-BE (BE means the lower electrode, the same shall apply hereinafter) interface, Pt is difficult to oxidize, and oxygen migration is unlikely to occur. The resistance of as-prepared specimens of Al-TE / Bi-2212 / Pt-BE structure is higher than that of Pt-TE / Bi-2212 / Pt-BE structure. Sputtering energy during Al deposition (plasma radiant heat and sputtering) This is because the reaction of α ⇒ β partially progressed due to the kinetic energy of the particles.

  Fig. 4 shows the reason for the formation of an oxygen-depleted layer (Oxygen-depleted layer) and the resulting high resistance switching by applying a positive bias to Al and applying a negative bias to Al. Indicates.

  First, by depositing a metal (Al) with low Gibbs energy on a Bi-2212 single crystal, Al takes oxygen from Bi-2212 and forms an oxygen-deficient layer, resulting in HRS (high resistance state) ( Fig. 4 (a)).

  Furthermore, the annealing treatment promotes the reduction of the oxygen-deficient layer by Al, and the resistance increases. Subsequently, when a positive voltage is applied to Al, oxygen ions in the Bi-2212 single crystal are attracted to the Al side by Coulomb force, the oxidation of the oxygen-deficient layer proceeds, and the oxygen-deficient layer is repaired. (Fig. 4 (b)). When a negative voltage is applied to Al again, oxygen ions move to the BE side, and an oxygen-deficient layer is formed again on Bi-2212, resulting in HRS (FIG. 4 (c)).

Therefore, it is considered that resistance switching of ReRAM using a Bi-2212 single crystal is caused by oxidation / reduction of Bi-2212 near the Al electrode due to the movement of oxygen ions. The oxygen concentration in the oxygen-deficient layer decreases as the annealing temperature increases, increasing the resistance in the high resistance state. Therefore, R _{HRS /} R _LRS increases with increasing annealing temperature.

As a supplementary explanation, it is stated that Bi ₂ Sr ₂ CaCu ₂ O _{8 + δ} has a large change in resistivity due to oxidation / reduction, that is, a change in δ.

FIG. 5 shows the δ dependence of the resistivity of Bi ₂ Sr ₂ CaCu ₂ O _{8 + δ} reported by Watanabe et al. The resistivity increases as δ decreases.

  In FIG. 4 (a), an oxygen-deficient (reduced) layer, ie, a decrease in δ occurs in Bi-2212, so that the region has a high resistance and a high resistance state is realized. On the other hand, in FIG. 4 (b), by applying a positive bias to the Al electrode, oxygen ions in the Bi-2212 single crystal are attracted to the Al side by Coulomb force, and the oxidation of the oxygen-deficient layer proceeds. Since it is repaired, that is, δ recovers to some extent, it becomes LRS. Subsequently, in FIG. 4 (c), when a negative voltage is applied to Al, oxygen ions move to the BE side, and an oxygen-deficient layer is formed again in Bi-2212, that is, δ decreases. It becomes.

In this embodiment, the Al electrode is made of the following materials (including Al): Al, Cu, Ti, Ni, Ta, V, Cr, Sb, Mn, Fe, Co, Zn, Hf, Nb, Mo, Sn Any one of Si and an alloy thereof may be used.

  Further, the Pt electrode may be any one of the following materials (including Pt), that is, any one of Pt, Au, Ag, Pd, and Ir, or an alloy thereof.

Further, Bi-2212 may be the following materials (including Bi-2212). That is, Bi ₂ Sr ₂ CuO _{6 + δ} , _Bi2 Sr ₂ CaCu ₂ O _{8 + δ} , Bi ₂ Sr ₂ Ca ₂ Cu ₃ O _{10 + δ} , YBa ₂ Cu ₃ O _7-δ , _La2-x Ba _x CuO _{4 , TlBa 2 Ca n-1 Cu} n O 2n + 3 (n = 1,2,3,4,5), Tl 2 Ba 2 Ca n-1 Cu n O 2n + 4 (n = 1,2,3, _{4), HgBa 2 YCa n-} 1 Cu n O 2n + 2 + δ (n = 1,2,3,4,5), La 2-x Sr x CuO 4, Nd 2-x Ce x CuO 4, La _2-x Sr _x CuO ₄ , Pr 1-xCax MnO ₃ (PCMO), SrTiO ₃ .

A combination of metals having an oxidation Gibbs energy smaller than the oxidation Gibbs energy in the perovskite oxide employed based on FIG. 3 is selected from these materials, and heated to a temperature equal to or higher than E _a to obtain Al / Bi2212. / Same function as Pt.

  Regarding the positional relationship of the electrodes, the Al electrode and the Pt electrode, that is, the oxide reducing electrode and the electrode inactive to the oxidation reaction do not necessarily need to be arranged on the front and back of the oxide. For example, as shown in FIG. 6, it may be arranged on the same plane of the oxide.

Claims (4)

With the metal having a Bi ₂ Sr ₂ CaCu Gibbs energy of small becomes oxidized than Gibbs energy of oxidation in the perovskite oxide on one surface of the perovskite oxide consisting of ₂ O 8 _{+ δ} is provided, to the metal from the perovskite oxide There is an oxygen-deficient layer obtained by heating so that an energy higher than the activation energy necessary for the oxidation of the metal and the reduction of the perovskite oxide to proceed as the oxygen of the perovskite oxide moves. Hysteresis characteristics of voltage-current characteristics between the one electrode made of the metal and the other electrode paired via the oxygen-deficient layer so as to exist in the vicinity of the perovskite oxide in contact with the metal A nonvolatile semiconductor memory device characterized by using the above. After providing a metal having an oxidation Gibbs energy smaller than the oxidation Gibbs energy in the perovskite oxide on one surface of the perovskite oxide, oxygen of the perovskite oxide moves from the perovskite oxide to the metal The perovskite is heated by heating in an inert gas such as Ar or He or N ₂ gas so that energy higher than the activation energy necessary for the oxidation of the metal and the reduction of the perovskite oxide to proceed is given. An oxygen-deficient layer is formed in a region in the vicinity of the oxide in contact with the metal, and one electrode made of the metal and the other electrode paired via the oxygen-deficient layer are provided. For manufacturing a conductive semiconductor memory device. Before SL metals, Al, Cu, Ti, Ni , selected Ta, V, Cr, Sb, Mn, Fe, Co, Zn, Hf, Nb, Mo, Sn, from any one or an alloy of Si The nonvolatile semiconductor memory device according to claim 1, wherein the nonvolatile semiconductor memory device is a substance. The perovskite oxides are Bi ₂ Sr ₂ CuO _{6 + δ} , Bi ₂ Sr ₂ CaCu ₂ O _{8 + δ} , Bi ₂ Sr ₂ Ca ₂ Cu ₃ O _{10 + δ} , YBa ₂ Cu ₃ O _7-δ , La _{2 -x Ba x CuO 4, TlBa 2} Ca n-1 Cu n O 2n + 3 (n = 1,2,3,4,5), Tl 2 Ba 2 Ca n-1 Cu n O 2n + 4 (n = _{1,2,3,4), HgBa 2 YCa n-} 1 Cu n O 2n + 2 + δ (n = 1,2,3,4,5), La 2-x Sr x CuO 4, Nd 2-x A material selected from one of Ce _x CuO ₄ and La _{2 -x} Sr _x CuO ₄ , wherein the metal is Al, Cu, Ti, Ni, Ta, V, Cr, Sb, Mn, Fe, 3. The method of manufacturing a nonvolatile semiconductor memory device according to claim 2, wherein the nonvolatile semiconductor memory device is a material selected from any one of Co, Zn, Hf, Nb, Mo, Sn, and Si or an alloy thereof. .