DE10031002A1 - Production of thin oxide or nitride layers used as tunnel barrier material for magnetoresistive sensors comprises using a plasma beam to convert the substrate into the corresponding oxide or nitride and adjusting the reaction speed - Google Patents

Abstract

Production of thin oxide or nitride layers comprises using a plasma beam produced from a high frequency plasma source (5) to convert the substrate (1) into the corresponding oxide or nitride and adjusting the reaction speed independent of the penetration depth. Preferred Features: The reaction speed is adjusted over the ion stream density of the particle stream or the degree of ionization or dissociation of the particle stream. The energy of the ions is uniform. The process is carried out at less than 10<-2> mbar pressure and at 54-150 deg C. The substrate material is aluminum, magnesium, gallium, tantalum, zinc, titanium, zirconium, yttrium, nickel, silicon, germanium, boron or carbon.

Description

The invention relates to the production of thin, highly electrically insulating films from certain oxide or nitride layers, for. B. Al ₂ O ₃ or AlN, by the plasma beam of a radio frequency excited plasma beam source is directed onto a substrate in order to convert the substrate material into the corresponding oxide or nitride.

Alumina (Al ₂ O ₃ ) is the preferred tunnel barrier material for magnetoresistive sensors today. Nitrides, in particular silicon nitride (Si ₃ N ₄ or also boron nitride (BN), are more suitable than the oxides. Due to the low reactivity of molecular nitrogen, nitride layers of sufficient quality have not been able to be produced to date with the conventional processes. The problem lies in that the usual methods require very high ion energies to initiate nitride formation, which in turn results in a very large number of defects being built into the tunnel structure, so that high-quality nitride tunnel barriers are not known.

Al ₂ O ₃ barrier layers are usually produced by applying a thin Al layer (typically 1-2 nm thick) by means of an atomization or evaporation process and then oxidizing it. Thermal, natural or plasma-assisted oxidation can be used for the oxidation process. Thermal oxidation in the atmosphere or under vacuum is very time-consuming, uncontrollable and, if necessary, as in the case of oxidation under atmosphere, the vacuum must be broken. It is rather unusable for the industrial production of oxides. For industrial production, the focus is on plasma oxidation using a standard oxygen gas discharge.

EP-A-913 830 by PS Stephen describes a magnetic tunnel contact component in which the tunnel barrier is produced by plasma oxidation of 0.5-2 nm thick Al layers. The preparation of the Al ₂ O ₃ barrier layer by plasma oxidation of Al is described in a similar manner in some technical publications (see BJJ Sun et al., Appl. Phys. Lett. 74 (1999) 448; WJ Gallagher et al., J. Appl. Phys. 81 (1997) 3741; JS Moodera et al., Phys. Rev. Lett. 74 (1995) 3273).

In the process of plasma oxidation of an Al layer to form a thin Al ₂ O ₃ layer, according to the current state of the art, the reaction rate, ie the number of aluminum-oxygen bonds per unit of time, and also the penetration depth, ie the depth of penetration of the oxygen particle, can the surface is not controlled up to the place where the chemical bond takes place. There is either an incomplete oxidation of the Al layer or an undesired partial oxidation of the ferromagnetic bottom electrode. In both cases, the magnetic resistance of the tunnel structure drops dramatically due to an increased depolarization in the transition area between the bottom electrode and the barrier layer (J.J. Sun et al., Appl. Phys. Lett. 74 (1999) 448). Since neither the mass composition nor the energy distribution of the plasma flow hitting the surface can be controlled in conventional plasma oxidation, the quality of the oxide layer cannot be optimized. For an optimal oxidation process with regard to the optimization of the material properties of the oxide, it is necessary to have an ion energy, ie the kinetic energy of the ions, which is determined by their speed, the ion current density, ie the number of ions per unit area and time , and the composition (quantitative ratio of the molecules, the ions and the atomic particles) to use well-defined or controllable oxygen plasma jet.

What was previously not possible is the independent variation of the ion energy and the ion current density combined with a very high proportion of atomic species in a large-area plasma beam. So far, there is no method which, over a large area, ie with at least 100-200 cm ² , delivers an almost pure atomic beam with the ion energy and current density values available here. After part of the usual method is that the dissociation of the O ₂ - or N ₂ molecule necessary for the oxidation or nitration solely by the impact interaction of the ion upon impact on the surface of the material to be oxidized or nitrided is achieved. However, in order to carry out this shock activation efficiently, the kinetic energy of the ion must be at least twice the solid-state dislocation energy, particularly in the case of nitride formation. The solid-state dislocation energy indicates the energy which is required to release an atom chemically bound in the volume of the solid from its bond and to move it from its original local position to a new position. However, if the ion energy is at least twice the solid-state dislocation energy, the ions are also able to produce undesirable defects in the oxide or nitride.

The object of the present invention is an improved manufacture of thinner High quality oxide and nitride layers for e.g. B. the application in magnetic To enable tunnel barriers.

The task is solved by using a novel plasma source, which is a dense, highly ionized, quasi-neutral plasma beam with a generates a high proportion of oxygen or nitrogen species and thereby an independent Pending control of the ion energy and the ion current density allowed. So to say neutral means that in the particle or plasma jet per unit volume exactly the same number of positive (usually ions) as well as negative charge carriers (in usually electrons) are present.

According to the method, the plasma jet is applied to the surface of thin layers of certain elements directed to them in layers of appropriate oxides or convert nitrides, being a continuous, uniform and defect-free new layer is created. The invention enables atomic for the first time Plasma rays with well-defined kinetic energy, preferably between the Surface binding energy and solid state dislocation energy, for production of oxide or nitride layers can be used. The surface binding energy is the energy that is required or must be applied in order to be able to Solid surface chemically bound atom from its chemical bond to solve.

Such a plasma source is detailed in the filed German patent registration "high-frequency plasma source" with the file number 100 08 482.6 of CCR GmbH, coating technology described, and as COPRA plasma Source 160 CF or 200 / CF in stores.

Economically interesting elements for oxidation or nitride formation are:
Al (Al ₂ O ₃ / AlN), Mg (MgO), Ga (GaN), Ta (Ta ₂ O ₅ / TaN), Ti (TiN / TiO ₂ ), Zn (ZnO), Zr (ZrN / ZrO ₂ ) , Y (Y ₂ O ₃ ), YSZ [Yttria-stabilized zirconia], Si (SiO ₂ / Si ₃ N ₄ , Ge (GeO), B (BN), C (CN _x ).

Composite materials, such as. B. SiO _x N _y can be produced, with SiO _x N _y economically very interested.

The process according to the present invention has a number of advantages over the conventional oxidation / nitridation techniques. The plasma beam source generates plasma densities of the order of 10 ¹² cm ³ at very low process pressures of less than 10 ^-2 mbar, the energy E of the ions being continuously adjustable between 10 and 1,000 eV. The ion energy distribution ΔE / E, ie the velocity distribution of the ions, is less than 10% in this plasma beam source and is therefore very narrow. The energy of the ions can thus be set very precisely. In the case of diatomic gases, such as O ₂ and N ₂ , a degree of dissociation, ie the ratio of the atomic number of particles to the absolute number of particles in the plasma, of over 80% is achieved. Conventional plasma sources achieve a maximum of 30%. In addition, a degree of ionization, ie the number of ions in relation to the total number of particles in the plasma, of at least 25% is achieved, compared to approximately 5% in conventional plasma discharges. In this respect, the process of oxidation or nitridation becomes extremely efficient through the use of atomic particles. The speed of the process is controlled by the ion current density. The reaction rate can be between 5 × 10 ¹⁴ to 5 × 10 ¹⁶ binding processes per second. The depth of the oxidation or nitridation process can be controlled by a defined selection of the ion energy. Typical penetration depths are 1 to 10 nm, whereby the penetration depth can be controlled to within ± 0.1 nm.

In this way, only the layer to be processed is treated with very high precision, and influencing the layers underneath is avoided. The low process gas pressure and the short process time ensure that practically no harmful impurities are built into the barrier during layer growth. By adding argon to the process gas (O ₂ or N ₂ ) favorable conditions are created that interfaces between the ferromagnetic electrodes and the insulating layer with reduced surface roughness arise. This process is fully compatible with existing production systems and eliminates the breaking of the vacuum.

The production of the oxides or nitrides according to the invention is controlled and efficient with high precision and reproducibility. One of the possible Applications is the manufacture of magnetic resistors in tunnel structures in order ensure the insulation between the ferromagnetic electrodes. Further Possible applications are the creation of diffusion barriers Generation of insulation layers in the manufacture of integrated semiconductors circuits or the generation of insulation layers for capacitive construction elements such as B. capacitors.

The following embodiment uses the drawings to describe the production of magnetoresistive sensors (also called tunnel structure) from aluminum oxide (Al ₂ O ₃ ), for example the production of thin, highly electrically insulating layers in accordance with the present invention.

Fig. 1a shows (schematically) the oxidation process in the manufacture of magnetoresistive sensors or tunnel structures;

Fig. 2 shows the change of the relative conductivity of a 10 nm thick aluminum layer;

Fig. 3 shows the dependence of the oxidation depth of the ion energy;

Fig. 4 shows the dependence of the process speed on the ion current density.

FIG. 1a shows the oxidation process using the example of a tunnel structure. A ferromagnetic electrode ( 2 ) and an aluminum layer ( 3 ) with a defined thickness (about 1-2 nm) were applied to the silicon substrate ( 1 ). The aluminum layer ( 3 ) was applied as usual by sputtering or electron beam evaporation. The aluminum layer ( 3 ) is then oxidized by being exposed to the particle beam ( 4 ) from the plasma source COPRA ( 5 ). FIG. 1b shows the basic construction of a finished tunnel structure: silicon substrate (1), lower ferromagnetic electrode (2), oxidized aluminum layer (6) and the applied subsequent to the oxidation process upper ferromagnetic electrode (7). The higher the proportion of atomic oxygen and the greater the ion current density in the particle beam, the faster the oxidation process. The depth of oxidation, ie the mean penetration depth, can be set and controlled very well via the ion energy. Since the particle beam has a very narrow energy distribution of less than 10% of the mean energy, any ion energy between the surface binding energy and the surface activation energy, ie between about 10 and 50 eV, can be set precisely. Damaged areas are avoided and good layer qualities are ensured. Argon can usually be added to the process gas in order to reduce the surface roughness of the oxide layer. After completion of the oxidation, a thin stoichiometric Al ₂ O ₃ layer is obtained.

Fig. 2 shows the advantage of a mainly consisting of atomic species (O or N) particle beam over a treatment with molecular oxygen. The molecular oxygen causes practically no oxidation or change in conductivity within a treatment period of 10 min. An atomic oxygen or nitrogen particle beam, on the other hand, causes a considerable drop in the relative conductivity, since the metal layer is efficiently oxidized or nitrided.

The method described above can be used as nitrogen Process gas can also be used to form thin AlN layers. As well can further layers such. B. NiO, MgO or BN.

The thickness of the desired layer is set via the ion energy, the speed of the oxidation or nitration via the ion current density. FIGS. 3 and FIG. 4 illustrate this relationship at the example of the oxidation of aluminum. The desired ion energy is set by a control integrated in the plasma source. The ion current density is varied via the fed-in high-frequency power.

An ion with a certain kinetic energy loses after entering the solid, d. H. on the way down, continuously in energy. To Walking through a certain distance is its entire original one Energy is used up and it comes to rest. Where it comes to rest, there is a chemical bond. The maximum range, i.e. H. the depth of penetration of a Ions is therefore a function of the kinetic energy which the ions Have entry into the surface. The conversion of a thin aluminum  layer in an aluminum oxide layer then proceeds as follows: the surface is the aluminum layer under continuous bombardment by an ion beam, so Depending on their kinetic energy, the oxygen ions first penetrate into the depth and oxidize the aluminum there completely. Are on At this point, all possible binding sites are occupied, so the oxygen atoms move back towards the surface and go to the next free binding burst a chemical bond. This happens until the entire waiter surface layer was converted into oxide. The oxidation therefore proceeds from the Depth out to the surface. The same applies to the nitriding process.

In Fig. 3 the linear relationship between penetration depth and the ion energy is shown. The maximum thickness of the oxidized layer can thus be set very precisely by varying the ion energy.

The ion current density determines how many oxygen atoms per unit time and per unit area hit the surface of the material to be oxidized or nitrided. In the case of an atomic plasma beam with an ion current density of 1 mA / cm ² , this means that approximately 6 × 10 ¹⁵ atoms / cm ² s strike the surface and are therefore available for oxidation or nitration. How quickly the total dose of oxygen atoms necessary for the complete oxidation or nitration of a certain volume is reached is thus determined by the ion current density.

As shown in Fig. 4, there is a linear relationship between the process speed and the ion current density.

The adjustability of the ion energy and the ion current density thus enable the inventive production of particularly high quality, d. H. defect-free layers using the process of dynamic deep oxidation or deep nitriding.

If one takes into account that the conversion into an oxide or a nitride takes place from the depth to the surface, it is obvious to adjust the penetration depth of the reactive species (N or O) in time to the growth rate. With constant kinetic energy and constant current density, a volume range ΔN in the depth is completely converted after the time t ₀ . Further penetration of O or N atoms in this area can only have a disadvantageous effect on the formation of a high-quality oxide or nitride. If further bombardment of an already fully oxidized, nitrided area occurs, defects can thus occur in this area. An inferior oxide or nitride is formed.

The dynamic nitriding or oxidation process according to the invention then proceeds as follows: First, the desired layer thickness is determined via the depth of penetration by the choice of the appropriate energy. This energy is considered the initial value and determines the maximum process depth. The choice of a specific current density determines the rate of expansion or growth of the oxide or nitride region towards the surface. The ion energy is now continuously reduced to the extent that the penetrating atoms always only penetrate to a depth in which oxygen or nitrogen atoms are still required for chemical bonding. For the oxidation of an aluminum layer z. B. at an energy of 40 eV and a current density of 0.24 mA / cm ^2, an output depth (maximum process depth) of 1 nm. The growth rate is 0.2 nm per sec. The energy is continuously reduced from 40 to 0 eV within 5 seconds.

Claims (12)

1. A method for producing thin oxide or nitride layers by means of a plasma jet from a high-frequency plasma radiation source in order to convert the substrate material into the corresponding oxide or nitride, characterized in that the reaction rate can be set independently of the depth of penetration. 2. The method according to claim 1, characterized in that the reaction speed set via the ion current density of the particle beam becomes. 3. The method according to claim 1 or 2, characterized in that the Rate of reaction over the degree of ionization or dissociation of the Particle beam is adjusted. 4. The method according to claim 1 to 3, characterized in that the Penetration depth is set via the ion energy of the particle beam. 5. The method according to claim 1 to 4, characterized in that the energy the ions of the particle beam are uniform. 6. The method according to any one of claims 1 to 5, characterized in that the energy of the ions of the particle beam between the surface binding energy and the solid state dislocation energy. 7. The method according to any one of claims 1 to 6, characterized in that the plasma jet contains more than 70% of atomic particles. 8. The method according to any one of claims 1 to 7, characterized in that one works at a process pressure of less than 10 ^-2 mbar. 9. The method according to any one of claims 1 to 8, characterized in that the treatment takes place at temperatures from 54 ° C to 150 ° C.   10. The method according to any one of claims 1 to 9, characterized in that the process gas from O ₂ or N ₂ argon is added. 11. The method according to any one of claims 1 to 10, characterized in that the substrate material to be converted into the corresponding oxide or nitride Aluminum, magnesium, gallium, tantalum, titanium, zinc, zirconium, yttrium, Is nickel, silicon, germanium, boron or carbon. 12. The method according to any one of claims 1 to 11, characterized in that the ion energy, starting from a value for the maximum desired depth of penetration, reduced to the extent that the oxygen or Only penetrate nitrogen atoms to such a depth that they still reach the chemical bond are needed.