WO2006094821A2

WO2006094821A2 - Method for producing a thin magnesium oxide layer

Info

Publication number: WO2006094821A2
Application number: PCT/EP2006/002228
Authority: WO
Inventors: Wolfram Maass; Berthold Ocker; Jürgen LANGER
Original assignee: Singulus Technologies Ag
Priority date: 2005-03-11
Filing date: 2006-03-10
Publication date: 2006-09-14
Also published as: WO2006094821A3; DE102005011414A1

Abstract

The invention relates to a method for producing thin magnesium oxide layers on a substrate. Initially, a layer made of metallic magnesium is applied to the substrate and then the magnesium layer is transformed into a magnesium oxide layer by means of oxidation, for example, by treating plasma containing oxygen ions. The inventive method enables, in particular, TMR-layer systems to be produced, wherein the insulating barrier or intermediate layer is produced according to said method. Said method enables very thin magnesium oxide layers having small thickness tolerances, to be produced.

Description

Method for producing a thin magnesium oxide layer

The invention relates. _, a method for producing a thin magnesium oxide layer on a substrate, ^' as it is needed in particular for the production of magnetic tunnel junctions, which are an essential element of so-called. MRAM memories. But they are also known as magnetic field sensors z. B. for reading stored information in thin-film heads (TFH) of hard drives ^' used.

These tunneling contacts rely on the tunneling magnetoresistance (TMR) effect, which is based on the mechanism of spin-dependent, quantum-mechanical tunneling of conduction electrons through a thin insulator layer.

According to FIG. 1, the essential components of such a magnetic tunnel layer system are:

(A) a lower electrode 1, which in practical terms usually ^' consists of a series of individual layers and their uppermost layer. 1.1 should have a high spin polarization,

(b) an insulating barrier (intermediate) layer 2 and

(C) an upper electrode 3, which in turn ^{• in} practical embodiments is usually made as a multilayer and whose first layer applied to the barrier 2 layer 3.1 should also have a high spin polarization.

The important feature of this layer system for the applications described is that, on the one hand, a tunneling current flows through the insulating barrier when a voltage is applied between the lower and the upper electrode and, secondly, the electrical (tunneling) resistance to which this tunneling current is exposed is strongly dependent on the orientation of the magnetization of the two located below and above the barrier 2 layers 1.1 and 3.1 respectively. Depending on whether the magnetization of the layers 1.1 and 3.1 is aligned parallel or antiparallel, the resistance is smaller or larger. The size of this magneto-resistance effect is usually measured as MR ratio (MR magneto-resistance), where

is defined. To measure MR, tunnel cohesion must be produced from the layer system by means of structuring (as usual in the semiconductor industry), so that a current can flow, as it were, perpendicularly from one electrode 1 through the barrier 2 into the other electrode 3. The change in resistance is achieved by applying a magnetic field and thereby causing a change in the magnetization directions. As a normalized measure of the tunnel resistance, the product RA from the measured resistance R _{nI11 of} a given tunnel _junction and its area A is used. The value of RA is independent of the area A.

For the barrier, various materials have been tested in the past. In particular, alumina has proven to be a particularly favorable barrier material. The thickness of the aluminum oxide in tunnel layer systems determines the tunnel resistance of the system to a significant degree. This thickness is usually set between 2 nm (high resistance, RA «10 MΩ * μm ² ) and 0.5 nm (small resistance, RA« 1 Ω * μm ² ) ^' . Tunnel contacts with Al ₂ θ ₃ barriers reach MR values of more than 70% at room temperature (RT), when the barrier thickness leads to an RA> 1 kΩ * μm ^z . With thinner Al ₂ O ₃ and correspondingly smaller RA, the MR value drops sharply and only reaches values of 5% at room temperature at RA «1 Ω * μm ² . Barrier materials other than Al ₂ O ₃ have resulted in smaller MR in the past.

The theoretical investigation of the magnetic tunneling effect concluded that the use of monocrystalline magnesium oxide as a barrier material between monocrystalline iron should lead to extremely high MR values of over 1000% (J. Mathon, A. Umerski, Phys. Rev. B 63 (2001) 220403R; WH Butler, X. -G. Zhang, TC Schulthess, JM MacLaren, Phys. Rev. B 63, (2001) 054416). Since this prediction several groups have attempted to produce a tunnel layer system with a Fe / MgO / Fe structure which shows the expected high MR values. First Successes have been found in the construction of the tunneling layer system by molecular beam epitaxy (Faure-Vincent, C. Tiusan, E. Jouguelet, F. Canet, M. Sajieddine, C. Bellouard, E. Popova, M. Hehn, F. Montaigne, A. Schuhl, Applied Physics Letters, 83 (2003) 4507-4509, S. Yuasa, A. Fukushimal, T. Nagahamal, K. Ando, Y. Suzuki, Jap. Journal of Applied Physics, 43, (2004) L588-L590, S. Yuasa, T. Nagahama, A. Fukushima, Y. Suzuki, K. Ando, Nature Materials-advance online publication, October 31, 2004), achieving up to 180% MR at room temperature. However, this method has the disadvantage that it can hardly be used under production conditions.

In another method, the entire layer system by sputtering (sputtering) was prepared, in particular the preparation of the MgO layer with the addition of oxygen to the process gas argon was carried out (reactive process) in (S. S- P. Parkin, C. Kaiser, A Panchula, PM Rice, B. Hughes, M. Samant, S.H. Yang, Nature Materials, Vol. 3, December 2004, pp. 862-867). In complete conformity of the manufacturing conditions and, in particular, by setting a strong bcc (100) texture of the lower electrode MR values were achieved for ^'220% up by an annealing process. The disadvantage of this method is that generally reactive ^' sputtering processes for the production of oxides are difficult to control, especially if the reactant has a high oxygen affinity and the desired layer thicknesses are very small.

Another well-known method for producing thin MgO layers has been the RF sputtering of the material from a MgO target. In this case, an approximately monocrystalline MgO layer is likewise produced by an elevated temperature and / or by subsequent tempering of the substrate with a suitable layer substrate. This method has meanwhile also been used for the production of tunnel layer systems. The layer systems produced show an MR of up to 230% at room temperature, provided that for the above and below the barrier lying layers a CoFeB alloy was used. Investigations by means of transmission electron microscopy (TEM) on layer cross sections show that the MgO is monocrystalline after appropriate temperature treatment, as a prerequisite for the measured high MR values. A disadvantage of this method is that. the required MgO targets must have a very high purity and density in order to achieve the required quality of the MgO layers. For production purposes is also a disadvantage that these targets are difficult to produce as large individual pieces.

On the other hand, the object of the invention is to inexpensively provide thin MgO layers of high quality and uniform thickness on a substrate, in particular for the production of TMR elements. This object is achieved with the features of the claims.

In the solution of the invention is based on the idea, on the substrate, or on the lower electrode of the tunnel layer system, first metallic Mg, z. B. by means of direct current (DC) -Kathodenzerstäubung of a metallic Mg target, apply. Subsequently, the metallic Mg layer - if necessary in a separate process chamber - by an appropriate oxidation procedure treated ^'to the MgO layer form.

In a suitable oxidation process, the metallic Mg layer z. B. in a separate low pressure or vacuum chamber with a low pressure plasma of z. B. 10 ^{~ 4} to 10 ^{~ 2} hPa containing atomic oxygen ions. It is expedient that the energy of the ions present in the plasma of the Mg layer thickness to be oxidized can be adjusted. For very small layer thicknesses of z. B. 0.5 nm, this energy is usually in a range of some ten eV (eg., 10 to 30 eV), for very thick Mg layers of z. 2 nm to 4 nm, up to 150 eV or more may be required. In the practical embodiment of the oxidation device, it has proved to be advantageous if said plasma with the aid of a so-called. ECWR ion source (Electron Cyclotron Wave Resonance (ECWR)) is generated. By treatment with the plasma, the Mg is oxidized to MgO.

In order to further optimize the energy of the ions impinging on the thin metallic Mg layer, it may be useful in the practice of the oxidation chamber for the plasma to be spatially separated from the substrate with the Mg layer by a metallic disk; this disc preferably has holes or is formed in the form of a grid.

In another suitable oxidation process, the oxidation of the metallic Mg layer z. B. in an oxygen atmosphere at a pressure of preferably 0.01 hPa to 100 hPa.

Furthermore, it may be advantageous according to the invention that in the production of thicker MgO layers, the total layer is not manufactured with a single implementation of the two process steps, but that several times a thinner ^■ Mg layer is applied and then oxidized until the desired total layer thickness is reached , Thus, for example, from a layer thickness of 1.0 to 1.5 nm, a corresponding multi-stage treatment may be advantageous, wherein the number of stages and the partial thickness can be optimized in each case via the source parameters, such as pressure and / or power, and / or over the duration of treatment.

In this layerwise preparation of the MgO barrier or intermediate layer, it is particularly advantageous in the invention to leave the first and / or in particular the last sub-layer of this multilayer interlayer first in the metallic state, that is not to oxidize, and only after application of the final covering layer (upper electrode) by a thermal treatment of this layered body by suitable diffusion of oxygen from intermediate layers already present as MgO which are not yet oxidized, consisting of metallic magnesium Oxidize boundary layers. By this method, the quality of the interfaces between the total barrier layer or total intermediate layer on the one hand and the two ferromagnetic electrodes is advantageously influenced; this leads to a further improvement of the magnetoelectronic properties of the finished product and of the TMR element produced according to the invention.

The proposed method has a number of advantages compared with the prior art, especially when it comes to the production of a MgO layer on large substrates, as commonly used in the semiconductor industry or in the hard disk drive hard disk industry. On the one hand, very thin metal layers with a very high layer thickness uniformity can easily be produced by DC sputtering of metallic targets. The layer thickness of the Mg can be adjusted with great precision with a resolution of up to 0.01 nm. The exact adjustment of the layer thickness is necessary because it determines the RA value of the tunnel contacts to be produced after completion of the entire tunnel layer package. Performing the oxidation as described above can also be achieved with very high uniformity over the entire substrate area, especially when using a large diameter ECWR source.

Another advantage is that the stoichiometry of the magnesium oxide can be set very precisely by the proposed method.

From the point of view of production technology, it is also advantageous that it is no problem to obtain large targets of very high purity metallic magnesium.

Initial attempts to produce MgO barrier tunnel layer systems using the method described herein have already yielded MR values in the range of 200%. CoFeB layers were also used below and above the MgO layer. The measurements of MR and RA were performed on unstructured layer systems using the DC Worledge, PL Troilloud, Applied Physics Letters, 83 (2003) pages 84-86.

To achieve the high MR values shown, the layer systems were subjected to a magnetic field of e.g. For example, 1 Tesla was annealed for 1.5 hours using as described by Parkin et al. described, a temperature of about 36O ⁰ C proved to be optimal.

The invention will be explained below with reference to the accompanying drawings. Show it:

1 shows a schematic cross section of the layer structure of a TMR element produced according to the invention and

Fig. 2 is a schematic representation of the oxidation chamber with a plasma source.

According to FIG. 1, the TMR element mentioned at the outset comprises a lower electrode 1 optionally made up of a plurality of layers, a magnesium oxide layer 2 formed thereon, and an upper electrode 3 formed on the magnesium oxide layer 2, which optionally also has a multilayer structure. Adjacent to the existing of magnesium oxide barrier or intermediate layer 2 is located as part of the lower electrode 1, a sub-layer 1.1 with high spin polarization and as part of the upper electrode 3, the sub-layer 3.1 with also high spin polarization. The thicknesses of these layers are only a few nm. Changes in the magnetization directions of the ferromagnetic partial layers l. ^' l and 3.1 can cause the above-mentioned relative changes in resistance by antiparallel or parallel orientation of the polarization.

2 shows a schematic representation of a treatment chamber for the oxidation of the magnesium layer by treatment by means of an oxygen-ion-containing plasma. Within the schematically indicated treatment chamber 10 there is a plasma source 11 for producing a plasma 12 containing oxygen ions.

Opposite the opening of the plasma source 11, the substrate 1 is arranged so that the metallic magnesium layer 2 'faces the opening of the plasma source. The from the plasma source 11th leaking oxygen ions migrate to the magnesium layer 2 'and are deposited there, thus forming the magnesium oxide layer 2.

In a preferred embodiment, a perforated plate or grid 13 is provided at the opening of the plasma source 11 to ensure the inclusion of the plasma 12 in FIG. 2 below the grid 13 so that substantially only oxygen atoms pass upwardly through the grid 13 and back the impact on the magnesium layer 2 'convert this into a magnesium oxide layer 2.

The invention has been described above in connection with the preparation of thin MgO layers.

However, the ^' invention also covers the production of other compound consisting of two or more elements compound layers: for example, in process step (a) as the first component instead of metallic Mg other metals or non-metals such as aluminum, iron, sulfur, etc., and in the process step (B) for the second component, the plasma treatment by means of oxygen ions, nitrogen ions or other ions are used which form a desired compound with the first component.

The plasma treatment can also be carried out with a mixture of ions of different elements, so that the finished after the plasma treatment layer consists of a compound of three or more elements.

As stated above, the oxidation of the metallic Mg layer instead of by plasma treatment z. B. by so-called. "Natural" oxidation, ie by the action of an oxygen atmosphere at a gas pressure in the range of 0.01 hPa to 100 hPa or by diffusion of oxygen through the substrate or the lower electrode 1 and / or through the upper electrode. 3 respectively.

Claims

claims

1. A method for producing a thin magnesium oxide layer (2) on a substrate (1) by the following method steps:

(a) applying a layer of magnesium on the substrate (1) and

(b) Oxidation of the magnesium layer to form the magnesium oxide layer (2).

2. The method of claim 1, wherein the thickness of the magnesium layer before the oxidation of 0.3 nm to 5.0 nm.

3. The method of claim 1 or 2, wherein in step (b) the oxidation takes place in an oxygen atmosphere.

4. The method of claim 3, wherein the pressure of the oxygen atmosphere is in the range of 0.01 hPa to 100 hPa.

5. The method of claim I ₁ 2, 3 or 4, wherein for adjusting the stoichiometry of the magnesium oxide, the operating parameters, such as the gas pressure and / or the treatment time, during the oxidation are adjustable.

6 / Method according to claim 1 or 2, wherein in step (b) the oxidation takes place by treatment in a plasma containing oxygen ions.

7. The method according to claim 6, wherein in step (b) the treatment is carried out in a low-pressure plasma, preferably at 10 ^"4 to 10 ^{" 2} hPa.

8. The method of claim 6 or 7, wherein the energy of the ions present in the plasma is adapted to the thickness of the magnesium layer to be oxidized.

9. The method of claim 8, wherein the energy of the plasma ions in. Range from 10 eV to 250 eV is adjustable.

10. The method of claim 6, 7, 8 or 9, wherein an ECWR ion source is used to generate the plasma.

11. The method according to any one of claims 6 to 10, wherein for adjusting the stoichiometry of the magnesium oxide, the operating parameters of the plasma source, such as the gas pressure and / or the power of the plasma source and / or the treatment time, are adjustable.

12. The method according to any one of claims 6 to 11, wherein between the substrate (1) with the magnesium layer and the plasma (12) a metallic disc (13), preferably a grid or a perforated disc, is arranged.

13. The method according to any one of claims 1 to 12, wherein the steps (a) and (b) are repeated at least once in accordance with the desired final thickness of the magnesium oxide layer.

14. The method according to claim 13, wherein in the repetition of the process steps (a) and (b) the oxidation of the magnesium in the respective step (b) takes place according to the same or a different oxidation process.

15. The method according to any one of claims 1 to 14, wherein before the first-time execution of step (b) either (i) a second magnesium layer is deposited and oxidized the first time step (b) is carried out, or (ii) the magnesium layer is only partially oxidized from the free side and the oxidation of the remaining magnesium layer is effected by diffusion of oxygen atoms by thermal treatment of the coated substrate.

16. The method according to any one of claims 1 to 15, wherein after the oxidation step (b) or the last oxidation step (b) a further magnesium layer and thereon a cover layer or cover layer group is applied and the oxidation of this further magnesium layer by diffusion of oxygen atoms by thermal treatment of the coated substrate.

17. The method according to any one of claims 1 to 16, wherein the magnesium layer by sputtering, ion beam sputtering or by precipitation in the vapor phase (VPD = vapor phase deposition) is formed.

18. A method for producing a TMR layer system consisting of a first layer or layer group

(1) having high spin polarization, an interlayer insulating layer (2) and a second high spin polarization layer or layer group (3) stacked in this order, wherein the intermediate layer (2) is formed on the first layer or layer group (1 ), the method according to any one of claims 1 to 17 is applied, and then the second layer or layer group (3) is applied to the free surface of the intermediate layer (2) in a conventional manner.

19. The method according to claim 18, wherein the intermediate layer (2) closest sublayer (1.1, 3.1) of the first and the second layer group has a high spin polarization.

20. The method of claim 19, wherein the high spin polarization layer is a ferromagnetic material.

21. TMR layer system consisting of a first layer or layer group (1) with high spin polarization, an insulating intermediate layer (2) and a second layer or layer group (3) with high spin polarization, which are stacked in this order, wherein the intermediate layer ( 2) can be produced by the method according to one of claims 1 to 17.