DE102005011414A1 - Method for producing a thin magnesium oxide layer by means of plasma oxidation - Google Patents

Abstract

Disclosed is a method for producing thin magnesium oxide layers on a substrate by first depositing a layer of metallic magnesium on the substrate and then converting the magnesium layer into a magnesium oxide layer by treatment in an oxygen ion-containing plasma. In particular, TMR layer systems in which the insulating barrier or intermediate layer is produced by the aforementioned method have been produced by this method. The method described allows very thin magnesium oxide layers with small thickness tolerances.

Description

The The invention relates to a method for producing a thin magnesium oxide layer on a substrate, as in particular for the production of magnetic Tunnel contacts needed which are an essential element of so-called MRAM memories. But you will too as magnetic field sensors z. For reading stored information in thin-film heads (TFH) of hard drives used.

These Tunnel contacts are based on the tunnel magnetoresistance (TMR) effect, the on the mechanism of spin-dependent, quantum mechanical Tunneling of conduction electrons is based on a thin insulator layer.

• (a) eine untere Elektrode 1, die in praktischen Ausführungen meist aus einer Reihe von Einzelschichten besteht und deren oberste Schicht 1.1 eine hohe Spinpolarisation aufweisen sollte,
• (b) eine isolierende Barriere-(Zwischen)-Schicht 2 und
• (c) eine obere Elektrode 3, die in praktischen Ausführungen wiederum meist als Mehrfachschicht hergestellt ist und deren erste auf der Barriere 2 aufgebrachte Schicht 3.1 ebenfalls eine hohe Spinpolarisation aufweisen sollte.

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

• (a) a lower electrode 1 , which in practical terms usually consists of a number of individual layers and their top layer 1.1 should have a high spin polarization,
• (b) an insulating barrier (intermediate) layer 2 and
• (c) an upper electrode 3 , which is usually made in practical versions mostly as a multi-layer and the first on the barrier 2 applied 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 both below and above the barrier 2 located layers 1.1 respectively. 3.1 depends. Depending on whether the magnetization of the layers 1.1 and 3.1 aligned parallel or anti-parallel, the resistance is smaller or larger. The size of this magneto-resistance effect is usually measured as MR ratio (MR magneto-resistance), wherein MR = (R Max - R min ) / R min is defined. To measure MR, tunnel contacts must be produced from the layer system by means of structuring (as is customary in the semiconductor industry), so that a current is, as it were, perpendicular to an electrode 1 through the barrier 2 into the other electrode 3 can flow. 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 _{min 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 O ₃ barrier achieve MR values of more than 70% at room temperature (RT), when the barrier thickness leads to an RA ≥ 1 kΩ · μm ² . With thinner Al ₂ O ₃ and correspondingly smaller RA, the MR value drops sharply and, at RA ≈ 1 Ω · μm ^2, only reaches sizes around 5% at room temperature. Barrier materials other than Al ₂ O ₃ have resulted in smaller MR in the past.

The Theoretical investigation of the magnetic tunnel effect came to light the result that the use of single-crystalline magnesium oxide as a barrier material between single crystal iron to extremely high MR values of over 1000 to lead J. Mathon, A. Umerski, Phys. Rev. B 63 (2001) 220403 R; W.H. Butler, X.-G. Zhang, T.C. Schulthess, J.M. MacLaren, Phys. Rev. B 63, (2001) 054416). Since this prediction was made by various Groups attempted a tunnel layer system with monocrystalline Fe / MgO / Fe Structure that shows the expected high MR values. First Successes resulted in the production of the tunnel layer system by means of Molecular Beam Epitaxy (J. Faure-Vincent, C. Tiusan, E. Jouguelet, 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 hardly under Apply production conditions.

at In another method, the entire layer system was Sputtering (also Sputtering), in particular the preparation of the MgO layer with the addition from oxygen to the process gas Argon took place (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). By closely observing the manufacturing conditions, and in particular by setting a pronounced bcc (100) texture of the lower electrode by a tempering process, MR values of up to 220 could be achieved. The disadvantage of this method is that in general 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.

One more for a long time known method for the production of thin MgO layers is the RF sputtering of the material from a MgO target. It is through an increased Temperature and / or by subsequent annealing of the substrate with suitable layer substrate also an approximately monocrystalline MgO layer generated. This procedure has meanwhile also been used for the production of tunnel layer systems used. The layer systems produced show at room temperature an MR of up to 230% provided that and above the barrier layers, a CoFeB alloy was used. Investigations by transmission electron microscopy (TEM) at layer cross sections show that the MgO after appropriate Temperature treatment is monocrystalline, 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 to the required quality to reach the MgO layers. For production purposes is also of Disadvantage that this Targets difficult as big Single pieces are to produce.

the across from The invention is based on the object cost-effective thin MgO layers high quality and more uniform Thickness on a base, in particular for the production of TMR elements provide. 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 - possibly in a separate vacuum chamber - with a low-pressure plasma of z. B. 10 ^-4 to 10 ^-2 hPa treated 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 is generated by means of a so-called. ECWR ion source (electron cyclotron wave resonance (ECWR)). By treatment with the plasma, the Mg is oxidized to MgO.

Around the energy of the thin Metallic Mg layer to further optimize incident ions It may be in the practical execution the oxidation chamber be useful that the plasma spatially from the substrate with the Mg layer separated by a metallic disc is; this disc preferably has holes or is in the form of a grid educated.

Further it may be advantageous according to the invention that at the production of thicker MgO layers does not affect the overall layer with a single implementation the two process steps is manufactured, but that several times each a thinner one Mg layer is applied and then oxidized until the desired Total layer thickness is reached. For example, from a Layer thickness of 1.0 to 1.5 nm, a corresponding multi-stage treatment be advantageous, wherein the number of stages and the sub-layer thickness each over the source parameters such as pressure and power can be optimized.

The proposed method has in comparison with the prior art a number of advantages, especially when it comes to manufacturing a MgO layer on large Substrates work as they usually do in the semiconductor industry or in the hard disk industry for thin film heads use Find. On the one hand can be by DC sputtering of metallic Targets easily thin metal layers produce with a very high layer thickness uniformity. It can the layer thickness of the Mg with large precision with a resolution of up to 0.01 nm. The exact adjustment of the layer thickness is required since they are complete after completion of the tunnel bundle package the RA value of determined to be produced tunnel contacts. The implementation of Oxidation, as described above, is also of very high uniformity over the entire substrate surface especially when using a large diameter ECWR source is used.

One Another advantage is that by the proposed method the stoichiometry of Magnesium oxide can be set very accurately.

From the point of view of production technology, it is also advantageous that it is not a problem, big To get targets of metallic magnesium with very high purity.

First Try to use tunnel layer systems with MgO barrier with the help of here already have MR values in the Range of 200 results. CoFeB layers also became below and above the MgO layer used. The measurements of MR and RA were unstructured Layer systems using the method described in D.C. Worledge, P.L. Troilloud, Applied Physics Letters, 83 (2003) pages 84-86 carried out.

For reaching The high MR values shown were the layer systems under a magnetic field from Z. B. 1 Tesla for annealed for 1.5 hours using as described by Parkin et al. described a temperature of about 360 ° C proved to be optimal.

The Invention will be described below with reference to the accompanying drawings approaches explained. Show it:

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

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

According to 1 For example, the TMR element mentioned at the beginning has a lower electrode, which may consist of several layers 1 , a magnesium oxide layer formed thereon 2 and one on the magnesium oxide layer 2 formed upper electrode 3 on, which is optionally also formed multi-layered. Adjacent to the magnesium oxide barrier or interlayer 2 is located as part of the lower electrode 1 a partial layer 1.1 with high spin polarization and as part of the upper electrode 3 the sub-layer 3.1 with likewise high spin polarization. The thicknesses of these layers are only a few nm. Changes in the magnetization directions of the ferromagnetic sublayers 1.1 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 a plasma containing oxygen ions. Within the schematically indicated treatment chamber 10 there is a plasma source 11 for generating an oxygen ion-containing plasma 12 ,

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

In a preferred embodiment, the plasma source is at the opening 11 a perforated plate or a grid 13 provided to the inclusion of the plasma 12 in 2 below the grid 13 ensure that essentially only oxygen atoms through the grid 13 emerge upwards and after hitting the magnesium layer 2 ' this into a magnesium oxide layer 2 convert.

The The invention is above in connection with the production of thin MgO layers described.

The However, the invention also covers the production of other two or more elements existing compound layers: so, for example in process step (a) as the first component instead of metallic Mg also other metals or non-metals such as aluminum, iron, sulfur etc., and in step (b) for the second component the Plasma treatment by means of oxygen ions, nitrogen ions or other ions are used, which make a desired connection with the first Component enter.

The Plasma treatment can also with a mixture of ions of different Elements are made so that the finished after the plasma treatment Layer consists of a compound of three or more elements.

Claims (14)

Method for producing a thin magnesium oxide layer ( 2 ) on a substrate ( 1 ) by the following steps: (a) applying a layer of magnesium on the substrate ( 1 ) and (b) converting the magnesium layer into the magnesium oxide layer ( 2 ) by treatment in an oxygen ion-containing plasma. Process according to claim 1, wherein in step (b) the treatment is carried out in a low-pressure plasma, preferably at 10 ^-4 to 10 ^-2 hPa. The method of claim 1 or 2, wherein the energy the ions present in the plasma to the thickness of the magnesium layer to be oxidized is adjusted. The method of claim 3, wherein the energy of the Plasma ions in the range of 10 eV to 250 eV is adjustable. The method of claim 1, 2, 3 or 4, wherein the Generation of the plasma is used an ECWR ion source. Method according to one of claims 1 to 5, wherein for adjusting stoichiometry of magnesium oxide the operating parameters of the plasma source, such as the gas pressure, the power of the plasma source and the duration of treatment, are adjustable. Method according to one of claims 1 to 6, wherein the steps (a) and (b) at least once according to the desired Final thickness of the magnesium oxide layer can be repeated. Method according to one of claims 1 to 7, wherein the thickness the magnesium layer before the oxidation is from 0.3 nm to 5.0 nm. Method according to one of claims 1 to 8, 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. Method according to one of claims 1 to 9, wherein the magnesium layer by sputtering, ion beam sputtering or by precipitation in the vapor phase (VPD = vapor phase deposition) is formed. Method for producing a TMR layer system consisting of a first layer or layer group ( 1 ) with high spin polarization, an insulating barrier or intermediate layer ( 2 ) and a second layer or layer group ( 3 ) with high spin polarization, which are stacked in this order, wherein for the preparation of the barrier layer ( 2 ) on the first layer or layer group ( 1 ) the method according to one of claims 1 to 10 is applied, and then the second layer or layer group ( 3 ) on the free surface of the barrier layer ( 2 ) is applied in a conventional manner. The method of claim 11, wherein the barrier layer ( 2 ) nearest sublayer ( 1.1 . 3.1 ) of the first and the second layer group has a high spin polarization. The method of claim 12, wherein the layer comprises high spin polarization consists of a ferromagnetic material. TMR layer system consisting of a first layer or layer group ( 1 ) with high spin polarization, an insulating barrier or intermediate layer ( 2 ) and a second layer or layer group ( 3 ) with high spin polarization, which are stacked in this order, wherein the barrier or intermediate layer according to the method of one of claims 1 to 10 can be produced.