1. Field of the Invention
This invention relates to magnetic sensors of the spin-valve type using giant magnetoresistive elements (or GMR elements) and manufacturing methods therefor.
This application claims priority on Japanese Patent Application No. 2003-320089, the content of which is incorporated herein by reference.
2. Description of the Related Art
Conventionally, various types of magnetic sensors using magnetoresistive elements such as giant magnetoresistive elements (or GMR elements) have been developed and put into practical uses.
In
In order to accurately detect an external magnetic field whose magnitude is relatively small, the magnetic sensor 100 should maintain the magnetization direction of each segment of the free layer 53 to match a prescribed direction (hereinafter, referred to as an initial direction) in a stable manner when no external magnetic field is applied thereto. In
For this reason, the free layer made of a thin film is generally formed in a rectangular shape in plan view, wherein the long side (or longitudinal axis) is forced to match the initial direction, which is called “segment magnetization”, whereby by using the so-called “form anisotropy” in which the magnetization direction substantially matches the longitudinal direction, the magnetization direction of each segment of the free layer 53 is forced to match the initial direction. When the external magnetic field disappears, it is necessary for the magnetization direction of each segment of the free layer 53 to be restored to the initial direction, which should be maintained for a long time in a stable manner. Therefore, the bias magnetic layers 52 made of permanent magnet films are arranged on both ends of the free layer 53 in the longitudinal direction, whereby a certain magnetic field is applied to the free layer 53 in the initial direction. This is disclosed in Japanese Patent Application Publication No. H10-91920 (see paragraph [0004]), for example.
The formation of the bias magnetic layer 52, which is used to establish the segment magnetization with respect to each segment of the free layer 53, is continuously performed so as to produce a required magnitude of a bias magnetic field (i.e., coercive force and residual magnetism) after an embedded layer 51 made of a non-magnetic film for mounting the bias magnetic layer 52 is produced with a prescribed thickness. In order to control the bias magnetic field, it is necessary to adequately change the film composition adapted to the bias magnetic layer 52 or to change the thickness of the bias magnetic layer 52. This is disclosed in Japanese Patent Application Publication No. 2000-137906 (see paragraph [0272]), for example.
Normally, when a sputtering method is performed to form the bias magnetic layer 52, it is necessary to change the composition of a target, thus changing the film composition thereof. Herein, a vacuum chamber incorporating the target is temporarily opened to the atmospheric air so as to change the target; then, the chamber is decompressed to a certain degree of vacuum that allows the film formation to progress therein. This requires a relatively large amount of work, cost, and time, which increases the overall manufacturing cost. When the free layer and pinned layer are changed in structures, it is necessary to change a bias magnetic field in response to the magnetic property, whereby it is necessary to change the composition of the target at each time when the bias magnetic field is changed in structure.
In the manufacture conditioning, the composition of the target differs from the film composition of the bias magnetic layer 52 so that an unwanted deviation may occur in the composition of the target. For this reason, it is necessary to provide various types of targets having different compositions in order to determine the prescribed composition of the target. Even when the composition of the target is determined, an erosion form occurring in the target may vary in time as the used time period thereof becomes longer; therefore, the substantial used time of the target should be limited to a relatively short time period because the film composition may be easily varied.
By changing the thickness of the bias magnetic layer 52, it may be possible to change the magnetic property of the bias magnetic layer 52, which is represented by two factors, i.e., the coercive force Hc and the residual magnetism Mr shown in
It is an object of the invention to provide a magnetic sensor and a manufacturing method therefor, wherein it is possible to obtain desired magnetoresistive (MR) characteristics that can be maintained for long periods in a stable manner, regardless of a disturbance magnetic field.
It is another object of the invention to provide a magnetic sensor and a manufacturing method therefor, wherein it is possible to freely change the combination of coercive force and residual magnetism, which represent the magnetic property of a bias magnetic layer, without changing the composition of a target used for producing the bias magnetic layer.
This invention is directed to a magnetic sensor comprising a spin-valve type magnetoresistive element that is arranged on a substrate, wherein bias magnetic layers composed of permanent magnet films are connected with both ends of the magnetoresistive element, by which the magnitude of a magnetic field is detected. Herein, the bias magnetic layer is formed on an embedded layer that is composed of a non-magnetic material and that has the laminated structure in which a thick first layer and a thin second layer are sequentially subjected to deposition.
Due to the laminated structure of the embedded layer for mounting the bias magnetic layer, it is possible to control the coercive force of the bias magnetic layer in a broad range by adjusting the thickness of the second layer while the thickness of the bias magnetic layer is maintained at a certain value in order to provide prescribed residual magnetism. That is, the magnetic sensor of this invention has the bias magnetic layer that allows the coercive force and residual magnetism to be independently controlled and to be freely selected in combination without substantially changing the composition of a target, which is used for the film formation. This allows both of the coercive force and residual magnetism to be maintained at prescribed values, respectively. Unlike the conventional magnetic sensor, the magnetic sensor of this invention does not necessarily sacrifice one of two factors (i.e., coercive force and residual magnetism) of magnetic characteristics when the other is set to a desired value.
In the magnetic sensor of this invention, the bias magnetic layer is hardly influenced by a disturbance magnetic field; therefore, it is possible to maintain the coercive force of the bias magnetic layer in a range that does not cause difficulties in magnetizing the bias magnetic layer; it is possible for the pinned layer of the GMR element not to be easily deteriorated over a long period of use; and it is possible to control the residual magnetism of the bias magnetic layer in a range that does not cause difficulties in performing segment magnetization on the free layer of the GMR element. By adequately controlling the thickness of the second layer of the embedded layer, it is possible to eliminate all the problems that the conventional technology suffers when exchanging a target for use in the formation of the bias magnetic layer. That is, the manufacturing method of this invention is preferably applied to mass production of magnetic sensors because it demonstrates remarkable cost-saving effects in manufacturing.
As the material for use in the embedded layer, it is preferable to use Cr or a Cr alloy, for example. When the thickness of the second layer of the embedded layer is less than 2 nm, it is difficult to form the second layer having the uniform thickness. In that case, it may be possible to omit the second layer, whereas no substantial change can be observed with respect to magnetic characteristics of the bias magnetic layer even though the embedded layer comprises the first layer only. In addition, the coercive force of the bias magnetic layer is not substantially increased even when the second layer of 10 nm or more is formed in the embedded layer. That is, it is preferable that the thickness of the second layer range from 2 nm to 10 nm.
In the aforementioned magnetic sensor, the first and second layers are composed of crystal grains forming columnar structures, which are mutually discontinuous, wherein it is preferable that the average diameter of crystal grains forming the first layer is larger than the average diameter of crystal grains forming the second layer. In addition, it is preferable that the first layer composed of crystal grains forms a columnar structure, and the second layer has an amorphous structure.
The embedded layer for mounting the bias magnetic layer has a double-layered structure in such a way that the second layer is formed after the formation of the first layer. The first layer has a relatively large thickness that allows crystal grains to grow at a high pace, thus forming the columnar structure. In contrast, the second layer has a relatively small thickness that allows crystal grains thereof to form a relatively fine structure, or it may be stopped in the initial stage of crystal growth so as to form a non-crystal structure or an amorphous structure. Sizes of crystal grains and their structures can be observed using a scanning electron microscope (SEM) or a transmission electron microscope (TEM), wherein it is possible to measure the average diameter of crystal grains by use of the known measurement method(s) allowing visual confirmation of crystal grains through image processing. The magnetic sensor of this invention is designed such that the bias magnetic layer is formed on the second layer having the aforementioned structure, wherein the embedded layer is designed such that the second layer is composed of crystal grains whose sizes are relatively small compared with sizes of crystal grains forming the first layer. This allows the bias magnetic layer to grow on the second layer so that magnetic characteristics of the bias magnetic layer varies in proportion to sizes of crystal grains forming the second layer. Thus, even though the free layer and pinned layer of the GMR element are changed in specifications and dimensions, it is possible to produce a magnetic sensor that is not influenced by a disturbance magnetic field and that can be maintained in desired MR characteristics in the long time in a stable manner.
The manufacturing method of this invention adapted to the aforementioned magnetic sensor comprises two steps, namely, step A in which a thick first layer is deposited, and then, a thin second layer is continuously deposited on the first layer so as to form the embedded layer for mounting the bias magnetic layer, and step B in which the bias magnetic layer is deposited on the second layer of the embedded layer.
In the above, step A is contributed to “continuous” formation or deposition realized on the embedded layer, thus avoiding intervention of impurities and oxygen on the interface between the first and second layers. This allows crystal grain growth with respect to the “extremely thin” second layer so as to realize uniform crystal orientation. In step B, the bias magnetic layer is deposited on the embedded layer having the double-layered structure, wherein it is possible for the bias magnetic layer to have extremely high crystal orientation.
Thus, it is possible to control the coercive force of the bias magnetic layer in a broad range by merely adjusting the thickness of the second layer while the thickness of the bias magnetic layer is maintained at a prescribed value in order to provide prescribed residual magnetism.
In particular, it is preferable for step A to perform deposition of the second layer on the first layer before an oxide film is formed on the first layer. Thus, even when the second layer is very thin, it is possible to maintain high crystal orientation in a stable manner. A sputtering method is preferably used for the deposition of the embedded layer and the bias magnetic layer. This is because it has an advantage in that high adhesion can be secured even in the formation of an extremely thin film.
Due to the sequential and continuous deposition of the first and second layers forming the embedded layer for mounting the bias magnetic layer, it is possible to independently control the coercive force and residual magnetism of the bias magnetic layer by changing the thickness of the second layer and the thickness of the bias magnetic layer respectively. Thus, it is possible to produce the magnetic sensor that has a strong resistance to a disturbance magnetic field and that demonstrates a highly stable magnetoresistive effect over a long period of use.
In the manufacturing method of this invention, it is possible to produce magnetic sensors each having the aforementioned characteristics in a stable manner by adequately changing the thickness of the second layer and the thickness of the bias magnetic layer. This eliminates the necessity of performing exchanging operation of a target (i.e., a base material for the film formation), which is required in the conventional manufacturing method in order to change the composition of the bias magnetic layer. This realizes a remarkable reduction of the manufacturing time, which is conventionally increased due to the exchanging operation of a target. Thus, this invention can demonstrate cost-saving effects in manufacturing because it brings remarkable reductions in the manufacturing time and working labor.
These and other objects, aspects, and embodiments of the present invention will be described in more detail with reference to the following drawings, in which:
This invention will be described in further detail by way of examples with reference to the accompanying drawings.
A magnetic sensor 1 shown in
According to the graph of
With respect to the basic range α, as long as the thickness of the first layer 11a is 200 Å or more, the coercive force may have a certain value (i.e., 650 Oe) without depending on the thickness of the first layer 11a. With respect to the first variation range β, when the bias magnetic layer 12 is 1000 Å or more, the coercive force tends to be decreased in response to an increase of the thickness of the bias magnetic layer 12. With respect to the second variation range y, the coercive force tends to be increased when the thickness of the second layer 11b is increased in a range from 20 Å to 100 Å.
From the results shown in
(1) By arranging the second layer 11b whose thickness is 20 Å, it is possible to increase the coercive force by 20 Oe to 30 Oe, regardless of the thickness of the bias magnetic layer 12.
(2) When the second layer 11b is designed to have the thickness of 30 Å, it is possible to noticeably increase the coercive force by 200 Oe or so, regardless of the thickness of the bias magnetic layer 12.
(3) When the second layer 11b is designed to have the thickness of 50 Å, it is possible to further increase the coercive force by 150 Oe or so, compared with the coercive force produced using the second layer 11b of the 30 Å thickness.
(4) When the second layer 11b is designed to have the thickness of 70 Å, it is possible to further increase the coercive force by 100 Oe or so, compared with the coercive force produced using the second layer 11b of the 50 Å thickness.
(5) When the second layer 11b is designed to have the thickness of 100 Å, it is possible to further increase the coercive force by 20 Oe to 30 Oe, compared with the coercive force produced using the second layer 11b of the 70 Å thickness.
(6) When the second layer 11b is designed to have the thickness of 200 Å, substantially no increase is recognized with respect to the coercive force, which is substantially identical to the coercive force produced using the second layer 11b of the 100 Å thickness.
Based on the aforementioned results, it is possible to conclude that by controlling the thickness of the second layer 11b in a range from 20 Å to 100 Å (i.e., from 2 nm to 10 nm), it is possible to adequately adjust the coercive force within a range of 500 Oe. In addition, it is possible to detect an increase in the coercive force when the thickness of the bias magnetic layer 12 is 100 Å or less, regardless of the thickness of the second layer 11b. By using such a tendency, it is possible to actualize minor adjustment with respect to the coercive force.
From the aforementioned results, the magnetic sensor 1 of the present embodiment in which in which the bias magnetic layer 12 is formed on the embedded layer 11 having the laminated structure comprising the ‘thick’ first layer 11a and the ‘thin’ second layer 11b is advantageous in that the coercive force can be adjusted in a relatively wide range by adjusting the thickness of the second layer while the thickness of the bias magnetic layer 12 is substantially maintained at a prescribed value in order to produce the prescribed residual magnetism. Therefore, it is possible to produce a variety of combinations of magnetic characteristics (represented by combinations of the coercive force and residual magnetism), which can be applied to magnetoresistive elements, without substantially changing the film composition of the bias magnetic layer.
The aforementioned description is made in the case where the embedded layer 11 is composed of Cr. The same tendency as described in the aforementioned embodiment can be recognized with respect to the other composition of the embedded layer 11, which can be made of a Cr alloy composed of Cr—Mn, Cr—Mo, and Cr—Ta, for example. In addition, the aforementioned description is made in the case where the bias magnetic layer 12 is made of a CoCrPt alloy, whereas the bias magnetic layer 12 does not necessarily depend upon certain film composition as long as it is made of a so-called inter-plane magnetic film composed of a Co alloy that shows inter-plane orientation when arranged on the embedded layer made of a Cr film.
1. Control of Residual Magnetism
Experimentally, when the residual magnetism is less than 0.0040 emu, the free layer cannot be subjected to segment magnetization; hence, the so-called “spin-valve operation” becomes unstable, which makes it impossible to produce a desired magnetoresistive effect in a stable manner. On the other hand, when the residual magnetism is greater than 0.0055 emu, the free layer may not precisely respond to the external magnetic field, which makes it impossible to produce a relatively large MR ratio and which may eventually cause a reduction in the MR ratio in the long use because it becomes very difficult to maintain the magnetization direction of the fixedly magnetized layer being perpendicular to the magnetization direction of the free layer (see
2. Control of Coercive Force
Normally, in the production of the magnetoresistive element, a magnetization process using a ‘bulk’ permanent magnet is performed in order to align and coordinate all of the magnetization directions of the bias magnetic layers. When the coercive force of the bias magnetic layer is greater than 1100 Oe so that a relatively large magnetic field beyond the coercive force of the permanent magnet is required for the magnetization of small regions of the bias magnetic layer, it is very difficult to magnetize the bias magnetic layer. On the other hand, when the coercive force of the bias magnetic layer is less than 800 Oe, the bias magnetic layer may be easily magnetized with a relatively intense magnetic field applied to the magnetoresistive element, wherein the magnetization direction of the free layer becomes unstable, which makes the spin-valve operation become unstable. In summary, in order to control the coercive force within the aforementioned range from 800 Oe to 1100 Oe, it is necessary to change the thickness of the embedded layer in conformity with the thickness of the bias magnetic layer.
The production method of this invention is adapted to a magnetic sensor comprising a spin-valve type magnetoresistive element that is formed on a substrate so as to detect the magnitude of an external magnetic field, wherein both ends of the magnetoresistive element are connected with bias magnetic layers made of permanent magnet films. Specifically, it comprises step A in which a ‘thick’ first layer is deposited, then, a ‘thin’ second layer is continuously deposited on the first layer so as to form an embedded layer on which the bias magnetic layer is arranged, and step B in which the bias magnetic layer is deposited on the second layer of the embedded layer.
As described above, step A is performed first so as to activate the deposition of the thick first layer, and then, step B is performed so as to continuously deposit the thin second layer on the first layer.
In the case of the structure of
Specifically,
Furthermore, even when the thickness of the second layer 11b is increased by 100 Å to 200 Å, substantially no change can be observed with respect to the coercive force, compared with the coercive force that is produced using the second layer 11b having 70 Å thickness. In summary,
According to this invention, through the comparison between the results of
As described heretofore, this invention provides a magnetic sensor and a manufacturing method therefor in which the magnetic sensor has a relatively strong resistance against a disturbance magnetic field and provides desired MR characteristics that are maintained for a long time in a stable manner. Therefore, the magnetic sensor of this invention is preferably applied to portable electronic devices such as cellphones and navigation systems used in automobiles, which require highly stable characteristics to be maintained even when environmental conditions successively change. In addition, the manufacturing method of this invention is advantageous in that the combination of the coercive force and residual magnetism in the bias magnetic layer can be changed in a flexible manner by merely controlling the film thickness. This contributes to the stable and cost-saving manufacturing for magnetic sensors.
As this invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, the present embodiments are therefore illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalents of such metes and bounds are therefore intended to be embraced by the claims.
Number | Date | Country | Kind |
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2003-320089 | Sep 2003 | JP | national |