JP2003023190A - Method for fabricating magnetic tunnel element and element fabricated by that method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for fabricating a magnetic tunnel element exhibiting high cost performance and productivity by forming a good magnetic tunnel film through a plasma reforming process, and an element fabricated by that method. SOLUTION: In the method for fabricating a magnetic tunnel element comprising two ferromagnetic layers and a nonmagnetic layer located between, the nonmagnetic layer is formed on a first ferromagnetic layer and then reformed by ultrashort wave plasma of 20 MHz or above. The process for reforming the nonmagnetic layer is oxidation and the not yet reformed nonmagnetic layer may be a metal film or an insulating film. The ultrashort wave is preferably in the range of 20-300 MHz.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a magnetic tunnel element and an element manufactured by the method, and more specifically, a method of manufacturing a magnetic tunnel element by a process of modifying a nonmagnetic layer to be a tunnel film with plasma. And an element manufactured by the method.

[0002]

2. Description of the Related Art In recent years, a giant magnetoresistive effect (Giant Magneto-
Resistance) was discovered, and magnetic sensors and memory devices (hereinafter referred to as MRAM: Magn) that utilize this giant magnetoresistive effect.
etic Random Access Memory) is attracting attention. In particular, a magnetic tunnel (hereinafter, TMR: Tunnel Magneto-Resistance) element using a non-magnetic layer as an insulator such as Al ₂ O ₃ can obtain a high magnetoresistive ratio, so that it is developed for practical use of an MRAM or a magnetic head. It is accelerating.

In order to use it for an MRAM or a magnetic head, it is necessary to reduce the resistance of the TMR element to about 1 to 1000 Ωμm ² . Regarding the method of manufacturing the insulator that will be the tunnel film of the TMR element, the method of forming a base metal film by sputtering or the like and then making it an insulating film by post-treatment is the mainstream. Aluminum is often reported as the base metal of the tunnel film, and oxidation and nitridation are considered as the insulating film formed by the post-treatment, but there are many reports of oxidation. Therefore, the most reported example is Al ₂ O ₃ . As a post-treatment process, natural oxidation, plasma oxidation, radical oxidation, ozone oxidation, UV oxidation and the like have been proposed.

To reduce the resistance of the TMR element, natural oxidation, which can be expected to be mild oxidation, is considered to be good.
However, natural oxidation requires a standing time of several minutes to several hours. Considering productivity, a faster process is needed.

Plasma oxidation and radical oxidation are one of the fast process candidates. High productivity can be obtained because desired characteristics can be obtained within a processing time of several seconds to several hundred seconds.

There are known examples of substrate bias plasma and inductively coupled plasma (ICP) for plasma oxidation, and known examples of ECR plasma for radical oxidation. For example I
As a known example of CP plasma, there is JP-A-2000-0999.
No. 22 publication, and as a known example of ECR plasma, the Japan Society for Applied Magnetics, 23, 1277-1280 (1
999).

[0007]

[Problems to be Solved by the Invention] However, 13.
Since the 56 MHz RF plasma has a large self-bias, there is a problem that the post-treatment with the RF plasma causes damage to the tunnel film due to ion irradiation. The tunnel film has a thickness of about 1 nm and is a very thin film of several to several tens of atoms. For this reason, if the kinetic energy of the irradiated ions is high, the ions easily penetrate through the tunnel film. The point that penetrates becomes a problem because it becomes a leak source.

ICP plasma is usually 13.56 MHz
RF is used. JP-A 2000-099922 does not describe the frequency, but in ICP plasma, high frequency power is applied to the coil introduced into the vacuum chamber, so that etching damage does not occur as compared with the case where RF is applied to the element side. I am trying. However, there is a problem that ion penetration damage occurs when a milder oxidation is intended or when a thinner base film is used.

ECR plasma has a merit that plasma energy and ion accelerating voltage can be controlled independently because plasma is generated by microwave discharge and ions are extracted by an ion accelerating electrode. However, to induce electron cyclotron resonance, 2
Since a strong magnetic field of 875 Gauss is required in the case of 450 MHz, and a waveguide is required for microwave transmission, there is a problem that the device becomes large in size and costly.

In addition to the above-mentioned methods, direct sputtering of the insulator can be considered as a method of manufacturing the insulator which will be the tunnel film of the TMR element. Occurs. Therefore, it is necessary to supplement the deficient element by performing reactive sputtering or exposing it to plasma in the post-treatment. However, in the RF plasma of 13.56 MHz, the efficiency of decomposing the introduced gas is low, so that there is a problem that the active radicals and ions are not sufficiently generated, and thus the replenishment is not sufficiently performed.

The present invention has been made in view of various problems of the above-mentioned conventional techniques, and the first object of the present invention is to perform plasma modification for producing a good tunnel film. To provide the process.

A second object of the present invention is to provide a plasma reforming process with high cost performance and high productivity.

[0013]

[Means for Solving the Problems] As a result of extensive studies on means for solving such problems, the following method was confirmed to be excellent.

That is, a first aspect of the present invention is a method of manufacturing a magnetic tunnel element comprising two ferromagnetic layers and a non-magnetic layer located between them, wherein a non-magnetic layer is laminated on the first ferromagnetic layer. After that, the non-magnetic layer is modified by ultra-high frequency plasma of 20 MHz or more.

A second aspect of the present invention is that, in the first aspect, the process for modifying the nonmagnetic layer is oxidation.

A third aspect of the present invention is that, in the second aspect, the non-magnetic layer before modification is a metal film.

A fourth aspect of the present invention is that, in the second aspect, the nonmagnetic layer before modification is an insulating film.

A fifth aspect of the present invention is that the ultrashort wave is in the range of 20-300 MHz in the third or fourth aspect.

A sixth aspect of the present invention is a magnetic tunnel element manufactured by the method for manufacturing a magnetic tunnel element according to any one of the first to fifth aspects.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention utilizes plasma generated by ultrashort waves having a frequency higher than RF and lower than microwaves to obtain a high-speed plasma reforming process while controlling ion damage. It is a thing.

In the present invention, "RF" is 13.
Industrial frequency of 56MHz, "ultra high frequency" is the frequency of 20MHz or more and less than 1GHz in the megahertz (MHz) band,
“Microwave” is a frequency in the gigahertz (GHz) band of 1 GHz or more and less than 1 THz, and “high frequency” is kilohertz (k).
Hz) and above.

In high-frequency plasma, electrons having a light mass move at high speed between the cathode and the anode following the electric field, whereas ions stay in the discharge space with almost no movement, so that a large electric field is not generated. Is maintained.
Therefore, the plasma space potential generated for sustaining the discharge tends to become lower as the power supply frequency becomes higher.
Saturates around Hz. The accelerating electric field in which ions plunge into the substrate is defined by a value obtained by subtracting the substrate potential from the plasma space potential, and converges to almost the same value at 300 MHz or higher.
That is, plasma with less ion damage is obtained at a frequency of 20 MHz or higher, which is higher than 13.56 MHz.

Since the effect of obtaining the plasma with less ion damage is established for the plasma having a frequency of 20 MHz or more, the 2GH used for the ECR plasma is used.
The same holds true for microwaves in the z band. However, this effect saturates in the vicinity of 300 MHz, and ion damage does not decrease even if the frequency is further increased. Therefore, by performing the plasma modification process using the frequency range of 20 to 300 MHz specified in the present invention,
2GHz microwave E
An equivalent effect can be obtained without using CR.

Further, the present invention does not have to be bound by the method of coupling with plasma. Therefore, not only general capacitively coupled plasma but also ICP plasma can be applied. That is, by generating an ICP plasma using a frequency in the range of 20 to 300 MHz and performing a reforming process using the ICP plasma, the merit of the coupling method can be enjoyed.

Further, if the frequency of the power source for generating the plasma is increased, the frequency with which electrons collide with the ions and the raw material gas increases, and the ionization degree of the gas increases. Therefore, not only the raw material gas is decomposed well, but also a plasma containing a large amount of highly reactive ions and radicals is generated, so that the plasma reforming process proceeds faster than natural oxidation. Therefore, by using the technique of the present invention, the gas can be decomposed with high efficiency, so that the plasma reforming process can be executed without increasing the input power.

[0026]

EXAMPLES The present invention will be described in detail below with reference to examples.

(Embodiment 1) In order to evaluate the plasma reforming process using plasma, it is necessary to examine the characteristics of the plasma together with the actually produced insulating film. An experiment was conducted to examine how the power supply frequency changes the plasma characteristics.

FIG. 2 is a schematic view of a vacuum device used to confirm the effects of the present invention. The vacuum apparatus of FIG. 2 includes a vacuum chamber 201, a cathode 202 for supplying ultrashort waves,
It is composed of an exhaust system 203 connected to a vacuum pump, a substrate 204 located on the anode, a microwave power supply 205, a matching box 206, a gas supply system 207, a plasma diagnostic device 208, and a plasma diagnostic probe 209. There is.

In the vacuum apparatus of the embodiment, the pressure in the vacuum chamber 201 is reduced by using the exhaust system 203, and after reaching the ultimate pressure of 10 ⁻⁶ Pa, the gas supply system 207 is controlled to generate a small amount of plasma generating gas. It is introduced into the vacuum chamber 201. At this time, when the output of the microwave power supply 205 is supplied to the cathode 202 and the matching box 206 is adjusted, plasma is generated between the cathode 202 and the substrate 204 on the anode. The various plasma amounts of the plasma thus generated were measured by the plasma diagnostic probe 209 and analyzed by the plasma diagnostic device 208.

The plasma generating gas was Ar, which does not react with the materials forming the anode and cathode 202, the plasma diagnostic probe 209 and the like. Matters not related to the essence of the present invention are omitted. For example, the actual exhaust system 2
Reference numeral 03 is composed of a turbo molecular pump, a dry pump, and a plurality of valves, but these notations are omitted.

The output of the ultra high frequency power supply 5 is changed from 10 MHz to 30
3 to 5 show the results of measuring the electron density, plasma space potential, and self-bias when the frequency was changed to 0 MHz. The pressure is 0.1 Pa, the Ar gas flow rate is 30 sccm, and the power supply voltage is 200 V.

The electron density shown in FIG. 3 increases from the 8th power of 10 to the 9th power up to about 100 MHz in accordance with the increase of the power supply frequency.
It tends to saturate at higher levels. Since the electron density indicates the amount of gas ionized as plasma, it is clear that plasma having a higher electron density produces active ions and radicals. That is,
At frequencies higher than 13.56 MHz, more reactive plasma is obtained.

The plasma space potential shown in FIG. 4 drops sharply from 45 V to around 20 V by 30 MHz as the power supply frequency increases, but thereafter it changes at the same level. In plasma with a high frequency, electrons with a light mass move at high speed following the electric field, while ions stay in the space with almost no movement, so that the discharge is maintained without generating a large electric field. Therefore, the plasma space potential generated for sustaining the discharge is low. If the substrate were placed on the anode, it would rush from the low plasma space potential to the anode at ground potential. Since the plasma space potential is lower as the power supply frequency is higher, the energy of rushing ions is lower on the high frequency side. That is, 13.56 MH
At frequencies higher than z, plasma with less ion damage is obtained.

The self-bias shown in FIG. 5 decreases from about 25 V to about 1 V up to about 90 MHz as the power supply frequency increases, and thereafter changes at the same level. The self-bias functions as an accelerating electric field in which ions are accelerated by an ion sheath located outside the region where electrical neutrality is maintained as plasma and plunge into the cathode. Therefore, when the substrate is placed on the cathode , it is clear that the plasma having a higher self-bias has a larger ion damage because the energy that rushes into the substrate is higher. In addition, since a high self-bias is often synonymous with a high plasma space potential, the same relationship holds even when the substrate is on the anode.
That is, in the above case, the higher the power supply frequency is, the smaller the self-bias is, so that the plasma with less ion damage is obtained at the frequency higher than 13.56 MHz.

From the above three points, the power supply frequency is set to 1
By setting the frequency to 20 to 300 MHz higher than 3.56 MHz, it was revealed that the plasma characteristics have characteristics suitable for the plasma reforming process.

(Example 2) A TMR element was manufactured by using a frequency of 20 MHz or more of the present invention as a power supply frequency, and a limit film thickness at which a metal base film before forming a tunnel film can be thinned is compared to obtain a tunnel. The film quality of the film was evaluated. Plasma oxidation was performed as the plasma reforming process.

FIG. 1 is a schematic view showing an apparatus used for manufacturing an element. The device used for element production is a vacuum chamber 101.
An anode 102 on which a sample is placed, an exhaust system 103 connected to a vacuum pump, a substrate 104 located on the anode, a target electrode 105, and a DC power source 106.
A gas supply system 107, a matching box 108,
111, an ultrashort-wave ring cathode 109, a bias power supply 110, an ultrashort-wave power supply 112, and an RF power supply 113. By using this apparatus, the sputtering film formation and the plasma reforming process can be processed in the same apparatus without breaking the vacuum. Substrate bias sputtering can also be performed by using the matching box 108 and the RF power source 113. Matters that are not related to the essence of the present invention are omitted. For example, in an actual device, there are a plurality of target electrodes and their associated power supplies in order to form a multilayer film. Further, a shutter mechanism, a vacuum gauge, a current introduction terminal and the like for controlling the film formation time and stabilizing the film quality are omitted.

The element was processed by photolithography and lift-off.

FIG. 6 shows a machining process. In FIG. 6A, the lower electrode P2 and the first magnetic layer P are formed on the substrate P1.
3, the nonmagnetic layer P4 is formed by sputtering. Next, the plasma modification process of the present invention is performed to modify the nonmagnetic layer P4 into a tunnel film. Next, after reaching the ultimate pressure, the second magnetic layer P
5, the antiferromagnetic layer P6 and the cap layer P7 are formed to form a magnetoresistive film. FIG. 6B shows a state in which the film formation so far is completed. The fine processing for defining the tunnel junction is performed, and the insulating film P8 is formed and processed by self-alignment using the same mask. FIG. 6C shows the insulating film P8.
In the figure, the state A is completed, and the central portion A corresponds to the tunnel junction portion, and both sides correspond to the pad portions for taking out the electrodes. The tunnel junctions B and C of the pad portion are penetrated by the probe when evaluating the device characteristics. Finally, the upper electrode P9 is formed and lifted off to manufacture the device.
FIG. 6D shows the completed state.

The substrate P1 is a 4-inch Si wafer having a crystal orientation of (1,0,0) and a thermal oxide film of 1 μm formed thereon. Lower electrode P2, first magnetic layer P3, non-magnetic layer P
4, second magnetic layer P5, antiferromagnetic layer P6, cap layer P
Table 1 shows the materials and film thicknesses of 7, the insulating film P8, and the upper electrode P9.

[0041]

[Table 1] The plasma modification process was performed according to the following procedure. Substrate P1
On the other hand, the lower electrode P2 and the first magnetic layer P3 were sequentially formed by sputtering, and then Al was sputtered by 1 nm as a metal to be the base film of the nonmagnetic layer P4. Next, 30 sccm of O ₂ gas was introduced into the vacuum chamber 101 to maintain the pressure at 0.5 Pa, and a bias of 1 V was applied to the anode 102 on which the substrate was placed. 10W, 30 for ring cathode 109
The plasma reforming process by the plasma oxidation was completed by adding the ultra short wave of the second to generate the plasma.

The limit film thickness at which the resistance is short-circuited was measured by fixing other conditions with the film thickness of Al serving as the base film and the frequency as parameters. Considering the change in resistance value due to process variations, samples of 10 points or more were prepared for each film thickness, and a short circuit rate of 1 Ω or less exceeding 50% was regarded as a short circuit. The results are shown in Table 2.
When oxidized at a higher frequency than 13.56 MHz, the limiting film thickness could be made thinner.

[0043]

[Table 2] (Embodiment 3) In order to confirm the effect of modifying the tunnel film by direct sputtering of the insulator, 20MH of the present invention was used.
A tunnel resistance was compared by making a TMR element that was plasma-modified and an element that was not plasma-modified using a frequency of z or higher. Plasma oxidation was performed as the plasma reforming process.

The manufacturing process of the device was performed in the same manner as in Example 2 along with FIG. The plasma modification process was performed according to the following procedure. For the substrate P1, the lower electrode P2 and the first electrode
After sequentially forming the magnetic layer P3 by sputtering, Al ₂ O ₃ was sputtered by 1.5 nm as a metal to be the base film of the non-magnetic layer P4. Then, the vacuum chamber 101 is filled with O ₂ gas for 30 s.
The pressure was maintained at 0.5 Pa by introducing ccm, and a bias of 1 V was applied to the anode 102 on the substrate. Ultrashort waves of 100 MHz, 10 W, and 30 seconds were applied to the ultrashort-wave ring cathode 109 to generate plasma, and the plasma modification process by plasma oxidation was completed.

The resistance was compared with and without the plasma modification process. The results are shown in Table 3. RA is a resistance value normalized by area, and its unit is Ωμm ² . Since the tunnel junction size of the sample used for measurement is 10 μm square, for example RA
If = 1 MΩμm ² , the element resistance is 10 kΩ. The resistance value was higher in the case of plasma oxidation than in the case of no plasma oxidation. It is considered that the oxidation was performed uniformly and the traps contained in the direct sputtered film disappeared. The tunnel film could be modified without causing ion damage.

[0046]

[Table 3]

[0047]

INDUSTRIAL APPLICABILITY According to the present invention, a plasma reforming process for producing a good tunnel film can be obtained by utilizing plasma generated by ultrashort waves having a frequency higher than RF and a frequency lower than microwave. . In addition, a plasma reforming process with high cost performance and high productivity can be obtained.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an apparatus for producing a tunnel element of the present invention.

FIG. 2 is a configuration diagram of an apparatus used for investigating plasma characteristics of a plasma reforming process using an ultra-short wave plasma for producing a tunnel element of the present invention.

FIG. 3 is a graph showing the correlation between frequency and electron density in the experimental results of Example 1.

FIG. 4 is a graph showing the correlation between frequency and plasma space potential in the experimental results of Example 1.

FIG. 5 is a graph showing the correlation between frequency and self-bias in the experimental result of Example 1.

FIG. 6 is a cross-sectional view showing a method for manufacturing a tunnel element of the present invention.

[Explanation of symbols]

101, 201 vacuum chamber 102 anode 103, 203 exhaust system 104, 204 substrate 105 target electrode 106 DC power supply 107, 207 gas supply system 108, 111, 206 matching box 109 ring cathode 110 bias power supply 112, 205 ultrashort wave Power source 113 RF power source 202 Cathode 208 Plasma diagnostic device 209 Plasma diagnostic probe P1 Substrate P2 Lower electrode P3 First magnetic layer P4 Nonmagnetic layer P5 Second magnetic layer P6 Antiferromagnetic layer P7 Cap layer P8 Insulating film P9 Upper electrode arrow A Arrows indicating the position of the tunnel junction Arrows B and C Arrows indicating the tunnel junction located at the electrode pad portion

Claims (6)

[Claims] 1. A method of manufacturing a magnetic tunnel element comprising two ferromagnetic layers and a non-magnetic layer located between the two ferromagnetic layers, wherein the non-magnetic layer is stacked on the first ferromagnetic layer and then 20 MHz.
A method for manufacturing a magnetic tunnel element, which comprises modifying the non-magnetic layer with the above ultra-short wave plasma. 2. The method for manufacturing a magnetic tunnel element according to claim 1, wherein the process for modifying the nonmagnetic layer is oxidation. 3. The method of manufacturing a magnetic tunnel element according to claim 2, wherein the non-magnetic layer before modification is a metal film. 4. The method of manufacturing a magnetic tunnel element according to claim 2, wherein the nonmagnetic layer before modification is an insulating film. 5. The ultrashort wave according to claim 3 or 4,
A method for manufacturing a magnetic tunnel element, which is in the range of −300 MHz. 6. A magnetic tunnel element manufactured by the method for manufacturing a magnetic tunnel element according to claim 1.