TREA

Magnetic memory and method of operation thereof

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Information

  • Patent Grant
  • 6812537
  • Patent Number
    6,812,537
  • Date Filed
    Wednesday, May 8, 2002
    22 years ago
  • Date Issued
    Tuesday, November 2, 2004
    19 years ago
Information
Abstract
A magnetic memory according to the present invention comprises: a single magnetic memory cell having at least first to third magnetic layers, a first tunnel insulating layer between the first and second magnetic layers and a second tunnel insulating layer between the second and third magnetic layers. The resistance between the first and third magnetic layers when magnetization of the first and second magnetic layers are in opposite directions is different from the resistance between the second and third magnetic layers when magnetization of the second and third magnetic layers are in opposite directions. Multiple data are therefore stored into the memory cell.
Description




BACKGROUND OF THE INVENTION




1. Field of the Invention




The present invention relates to a magnetic memory device and more particularly to a magnetic memory device which is constructed to store data of multiple bits in a single memory cell.




2. Description of Related Arts




Magnetic Random Access Memory (hereinafter referred to as MRAM) is known as one of the semiconductor memories storing data in a nonvolatile manner. In MRAM, ferromagnetic films are integrated. Magnetization or magnetic vector of the ferromagnetic material is set to correspond to either “1” or “0”, and, in the case of MRAM, digital data is stored in a nonvolatile manner. That is, vectors in magnetic layers represent two states of magnetization within cell.




Technology for storing multivalued information in a single memory cell of magnetic memory is being developed in an attempt to expand memory capacity. Such magnetic memory has been disclosed in Japanese Patent Application Laid-open No. Hei 11-176149 though this publication teaches GMR memory cell which can be set multiple value.

FIG. 16

shows the cross sectional structure of the magnetic memory. The magnetic memory comprises a substrate


101


. Upon substrate


101


, there is formed, in order, a lower electrode


102


, a first magnetic layer


103


, a first non-magnetic spacer layer


104


, a second magnetic layer


105


, a second non-magnetic spacer layer


106


, a third magnetic layer


107


, and an upper electrode


108


. As shown in FIGS.


17


through


19


, magnetization direction of the multiple magnetic layers is controlled individually. The resistance value between the lower electrode


102


and upper electrode


108


varies in accordance with the combination of magnetization directions. That is, the combination has three type, one of which is that all layers


103


,


105


and


107


are magnetized in the same direction (

FIG. 17

, all left direction), another of which is that the magnetized direction of adjacent layers


103


,


105


,


107


is only one different (FIG.


19


:


103


, left magnetized direction,


105


right magnetized direction and


107


right magnetized direction), the other of which is that the magnetized direction of adjacent layers is all different (FIG.


18


). It is thus possible to store three information in a single memory cell. If number of magnetic layers is n (n>=3) in the solo memory cell, the cell can store n value.




SUMMARY OF THE INVENTION




According to the present embodiments, Magnetic memory as claimed in the present invention comprises: first through n-th tunnel insulating layers (n being a natural number of 2 or greater) and first through (n+1)-th magnetic layers, each respectively having first through (n+1)-th magnetization. The i-th tunnel insulating layer (i being an arbitrary integer of 1 or greater but no greater than n) of the first through n-th tunnel insulating layers are provided between the i-th magnetic layer and the (i+1)-th magnetic layer of the first through (n+1)-th magnetic layers Here, the i-th resistance between the i-th magnetic layer and (i+1)-th magnetic layer is R


i


when the i-th magnetization and the (i+1)-th magnetization of the first through (n+1)-th magnetization are in the same direction, and the i-th resistance is R


i


+ΔR


i


when the i-th magnetization and the (i+1)-th magnetization are in opposite directions. This makes it possible to store at least n+1 values or more as data in this magnetic memory.




At this point, it is preferable that each of ΔR


1


, ΔR


2


, . . . , ΔR


n


be different. In this magnetic memory, there can be obtained at least {(n


2


+n+2)/2} resistance values differing with each other between the first magnetic layer and the (n+1)-th magnetic layer depending on the direction of the first through (n+1)-th magnetization. In this magnetic memory, at least {(n


2


+n+2)/2} values or more can be stored as data.




In this magnetic memory, it is preferable that the sum of any progression generated from elements if the group consisting of, ΔR


1


, ΔR


2


, . . . , ΔR


n


be different from each other. This makes it possible in this magnetic memory for the resistance value between the first magnetic layer and the (n+1)-th magnetic layer to beat least 2


n


different values depending on the direction of the first through the (n+1)-th magnetization. In this magnetic memory, at least 2


n


values or more can be stored as data.




Also, it is preferable that each of ΔR


1


, ΔR


2


, . . . , ΔR


n


be substantially equal to any of ΔR, ΔR/2, ΔR/2


2


, . . . , ΔR/2


n−1


given that ΔR is a predetermined resistance. This causes the resistance value between the first magnetic layer and the (n+1)-th magnetic layer to vary linearly depending on the direction of the first through (n+1)-th magnetization in this magnetic memory.




At this point, it is preferable that the magnetic material comprising the first through (n+1)-th magnetic layers be selected so as for each of the above mentioned ΔR


1


, ΔR


2


, . . . , ΔR


n


to be substantially equal to any of ΔR, ΔR/2, ΔR/2


2


, . . . , or ΔR/2


n−1


.




In addition, it is preferable that the film thickness of the first through n-th tunnel insulating layers be set so as for each of these ΔR


1


, ΔR


2


, . . . , ΔR


n


to be substantially equal to any of ΔR, ΔR/2, ΔR/2


2


, . . . , ΔR/2


n−1


.




In addition, it is preferable that the material comprising the first through n-th tunnel insulating layers be set so as for each of the above mentioned ΔR


1


, ΔR


2


, . . . , ΔR


n


is substantially equal to either one of ΔR, ΔR/2, ΔR/2


2


, . . . , ΔR/2


n−1


.




In addition, it is preferable that the area where the i-th tunnel insulating layer comes into contact with the i-th magnetic layer and the area where the i-th tunnel insulating layer comes into contact with the (i+1)-th magnetic layer be set so as for each of these ΔR


1


, ΔR


2


, . . . , ΔR


n


to be substantially equal to any of ΔR, ΔR/2, ΔR/2


2


, . . . , ΔR/2


n−1


.




In addition, it is preferable that the film quality of the first through n-th tunnel insulating layers be set so as for each of the these ΔR


1


, ΔR


2


, . . . , ΔR


n


to be substantially equal to any of ΔR, ΔR/2, ΔR/2


2


, . . . , ΔR/2


n−1


.




In addition, it is preferable that each of these ΔR


1


, ΔR


2


, . . . , ΔR


n


be substantially equal to any of ΔR/n, 2ΔR/n, 3ΔR/n, . . . , (n−1) ΔR/n, ΔR given that ΔR is a predetermined resistance. This makes it possible for {(n


2


+n+2)/2} values to be stored as data in this magnetic memory depending on the direction of first through (n+1)-th magnetization. Moreover, in this magnetic memory, the resistance value between the first magnetic layer and the (n+1)-th magnetic layer varies linearly.




In addition, it is preferable that the direction of the first magnetization be fixed.




At this point, it is preferable that the magnetic memory further comprise an antiferromagnetic layer connected to the first magnetic layer and that the direction of the first magnetization be fixed due to the interaction with the antiferromagnetic layer.




As to aspect of the embodiments of the present invention, a magnetic memory as claimed in the present invention comprises:




(a) a step of directing in a first direction the j-th though the (n+1)-th magnetization (j being an integer of 1 or greater but no greater than n) of the first through (n+1)-th magnetization; and




(b) a step of directing a (j+1)-th through (n+1)-th magnetization of the first through (n+1)-th magnetization, after the (a) step, either in the first direction or a second direction, which is substantially opposite the first direction.




As to another aspect of the embodiments of the present invention, a magnetic memory comprises a memory cell having at least a first magnetic layer, a second magnetic layer and third magnetic layer. In this magnetic memory, the resistance between the first magnetic layer and second magnetic layer when the magnetization or magnetic vector of the first magnetic layer and magnetization of second magnetic layer are aligned in opposite directions or antiparallel orientation, and the resistance between the second magnetic layer and third magnetic layer when the magnetization of the second magnetic layer and the magnetization of the third magnetic layer are in opposite directions, are different.











BRIEF DESCRIPTION OF THE DRAWINGS





FIG. 1

is a cross-sectional view showing a structure of a magnetic memory cell according to a first embodiment;





FIG. 2

is cross-sectional views for explaining magnetic vectors of the cell shown in

FIG. 1

;





FIG. 3

is a cross-sectional view showing a magnetic memory cell according to a second embodiment;





FIG. 4

is a plan view showing the magnetic memory cells according to the second embodiment;





FIGS. 5A and 5B

show the respective resistance values of tunnel insulating layers in the film thickness direction;





FIGS. 6A

to


6


B show the correspondence between resistance value R


read


between bit line and word line, and the condition of memory cell;





FIGS. 7A

to


7


C show method of adjusting contact area of tunnel insulating layers with fixed magnetic layer


4


and free magnetic layers;





FIGS. 8A

to


8


D show operation to write data into memory cell of the magnetic memory according to the second embodiment;





FIG. 9

shows a magnetic memory cell according to a third embodiment;





FIG. 10

shows a magnetic memory cell according to a fourth embodiment;





FIG. 11

is a cross-sectional view showing a magnetic memory structure according to a fifth embodiment;





FIGS. 12A and 12B

show the respective resistance values of tunnel insulating layers in the film thickness direction;





FIGS. 13A-13C

to shows an operation to write data into memory cell


30


of the magnetic memory according to the fifth embodiment;





FIG. 14

shows a magnetic memory cell according to a sixth embodiment;





FIG. 15

shows a magnetic memory cell according to a seventh embodiment;





FIG. 16

shows a magnetic memory cell of a related art;





FIG. 17

shows the operation of the magnetic memory cell in

FIG. 16

;





FIG. 18

shows the operation of the magnetic memory cell in

FIG. 16

; and





FIG. 19

shows the operation of the magnetic memory cell in FIG.


16


.











DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS




Various embodiments of the present invention will now be described in detail with reference to a number of drawings.




Referring to

FIG. 1

, a magnetic cell according to the present invention is shown. The cell includes a pinned magnetic layer


3


, free magnetic layers


5


-


1


and


5


-


2


. Tunnel insulating layers


4


,


6


-


1


are inserted between magnetic layers


3


,


5


-


1


,


5


-


2


. Lower conductive layer


2


as electrode is formed under the pinned magnetic layer


3


and upper conductive layer


6


-


2


is formed on the free magnetic layer


5


-


2


. Substrate


1


is formed under the lower electrode


2


. Coercive force of the free magnetic layer


5


-


1


is different from that of the free magnetic layer


5


-


2


. For example, coercive force of the free magnetic layer


5


-


1


is stronger than that of the free magnetic layer


5


-


2


. That is, the free magnetic layer


5


-


2


easily changes its magnetization or magnetic vector with weaker magnetic field, compared with the free magnetic layer


5


-


1


. The magnetic vector of this multi-layer magnetic cell is aligned along one axis, for example, x-axis. The resistance value between the upper and lower electrodes


2


,


6


-


2


is established, based on the direction (+x or −x) of each magnetic vector of the free magnetic layers


5


-


1


and


5


-


2


.





FIG. 2

illustrates resistance values between the electrodes


2


,


6


-


2


of the cell under four conditions. When reading out, the upper electrode


6


-


2


as a word line is supplied to a current. The current flows from the upper electrode


6


-


2


to the lower electrode


2


through layers


5


-


2


,


6


-


1


,


5


-


1


,


4


and


3


. The magnetic vector of each magnetic layer controls the current volume. First condition corresponds to that magnetic vectors of the layers


3


,


5


-


1


,


5


-


2


are the same. The resistance value is defined in 2R under the first condition. Second condition corresponds to that magnetic vectors of the layers


3


,


5


-


1


are the same and the magnetic vector of the layer


5


-


2


is the opposite. The resistance value is defined in 2R+ΔR/4 under the second condition. Third condition corresponds to that magnetic vectors of the layers


5


-


1


,


5


-


2


are the same and the magnetic vector of the layer


3


is the opposite. The resistance value is defined in 2R+2ΔR/4 under the third condition. Fourth condition corresponds to that magnetic vectors of the layers


3


,


5


-


2


are the same and the magnetic vector of the layer


5


-


1


is the opposite. The resistance value is defined in 2R+3ΔR/4 under the fourth condition.




Under the above, by the location of the magnetic layer, the resistance value changes. This is able to achieve by changing materials every magnetic layer or changing materials, forming method, thickness, size of every tunnel insulating layers.




This embodiment can store 4 value in the solo cell. On the other hand, the related art can store 3 value in the single cell. If number of the sum of the pinned and free magnetic layers is n, the embodiment can store 2


n−1


(n>=3) value in the single cell. That is, if number of the free magnetic layers is n, this embodiment can store 2


n


(n>=2) value in the single cell. On the other hand, when number of the sum of the pinned and free magnetic layers is n, the related art can merely store n value (n>=3) in the single cell. In other words, when number of the free magnetic layer is n, the related art can store n+1 (n>=2) value in the single cell. This embodiment is therefore superior to the related art.





FIG. 3

is a sectional view showing a magnetic memory structure according to a second embodiment of the present invention.

FIG. 4

is a plan view showing the structure of the memory in FIG.


3


.

FIG. 31

is a sectional view of the cross section,


2


A—


2


A, in FIG.


4


. Symbol A


1


in FIG.


1


and symbol A


2


in

FIG. 2

show a predetermined coordinate system.




This magnetic memory is provided with a bit line


2


and a word line


9


. The bit line


2


is formed on a substrate


1


. On the upper surface of bit line


2


, there is formed, in order, an antiferromagnetic layer


3


, a fixed magnetization (pined ferromagnetic) layer


4


, a tunnel insulating layer


5


-


1


, a free magnetization (free ferromagnetic) layer


6


-


1


, a tunnel insulating layer


5


-


2


, a free magnetic layer


6


-


2


, a tunnel insulating layer


5


-


3


, and a free magnetization


6


-


3


. Memory cell


10


is constructed from the layers


3


,


4


,


5


-


1


to


5


-


3


and


6


-


1


to


6


-


3


. The bit line


2


is formed with a conductor such as aluminum or copper. Electric current is flowed in bit line


2


in either the +y axis direction or −y axis direction. The Fixed magnetic layer


4


, free magnetic layer


6


-


1


, free magnetic layer


6


-


2


, and free magnetic layer


6


-


3


are respectively formed from a ferromagnetic material. The magnetization of fixed magnetic layer


4


is fixed or pinned upon interaction with the antiferromagnetic layer


3


. On the other hand, the magnetization of free magnetic layer


6


-


1


, free magnetic layer


6


-


2


, and free magnetic layer


6


-


3


is reversible. The direction of the magnetization of free magnetic layer


6


-


1


, free magnetic layer


6


-


2


, and free magnetic layer


6


-


3


is in either the +x axis direction or −x axis direction upon application of a magnetic field. 8 (=2


3


) values can be stored as data in memory cell


10


corresponding to the magnetization directions of free magnetic layer


6


-


1


, free magnetic layer


6


-


2


, and free magnetic layer


6


-


3


.




Magnetization of free magnetic layer


6


-


1


, free magnetic layer


6


-


2


, and free magnetic layer


6


-


3


is reversed with the magnetic field produced by the electric current flowing to bit line


2


. When electric current flows to bit line


2


in either the +y axis direction or the −y axis direction, a magnetic field is respectively applied to free magnetic layer


6


-


1


, free magnetic layer


6


-


2


, free magnetic layer


6


-


3


in either the +x axis direction or the −x axis direction. Magnetization of free magnetic layer


6


-


1


, free magnetic layer


6


-


2


, and free magnetic layer


6


-


3


is reversed by the magnetic field in either the +x axis direction or the −x axis direction.




Free magnetic layer


6


-


1


, free magnetic layer


6


-


2


, and free magnetic layer


6


-


3


are formed so that closer to bit line


2


the higher the coercive force is. In other words, free magnetic layer


6


-


1


, free magnetic layer


6


-


2


, and free magnetic layer


6


-


3


are formed so that the coercive force of free magnetic layer


6


-


3


is lower than the coercive force of free magnetic layer


6


-


2


, and also the coercive force of free magnetic layer


6


-


2


is lower than the coercive force of free magnetic layer


6


-


1


. The coercive force of free magnetic layer


6


-


1


, free magnetic layer


6


-


2


, and free magnetic layer


6


-


3


is adjustable with the type and film thickness of magnetic material comprising free magnetic layer


6


-


1


, free magnetic layer


6


-


2


, and free magnetic layer


6


-


3


.




Tunnel insulating layer


5


-


1


, tunnel insulating layer


5


-


2


, and tunnel insulating layer


5


-


3


are formed with an insulator such as alumina (Al


2


O


3


) or hafnium oxide. The film thickness of tunnel insulating layer


5


-


1


, tunnel insulating layer


5


-


2


, and tunnel insulating layer


5


-


3


are thin enough to allow tunnel current to flow in the direction of the film thickness.




Above bit line


2


and memory cell


10


, inter-layer insulating film


7


is formed to coat bit line


2


and memory cell


10


. Contact hole


8


is formed in inter-layer insulating film


7


. Moreover, on the upper surface of inter-layer insulating film


7


, word line


9


is formed. Word line


9


is connected to free magnetic layer


6


-


3


via contact hole


8


. Electric current flows to word line


9


in the −x axis direction. When electric current flow to word line


9


, a magnetic field is applied to free magnetic layer


6


-


1


, free magnetic layer


6


-


2


and free magnetic layer


6


-


3


in the −y axis direction of.




As shown in

FIG. 4

, bit lines


2


and word lines


9


are provided in plural. Bit lines


2


are extended in x axis direction, and word lines


9


are extended in the y axis direction. At the parts where bit lines


2


and word lines


9


intersect, memory cell


10


is provided respectively between bit lines


2


and word lines


9


.




The resistance of tunnel insulating layer


5


-


1


, tunnel insulating layer


5


-


2


, and tunnel insulating layer


5


-


3


to the direction of the film thickness varies depending on the direction of the magnetization of free magnetic layer


6


-


1


, free magnetic layer


6


-


2


, and free magnetic layer


6


-


3


due to the tunnel magnetic resistance result (TMR) shown by the ferromagnetic body.





FIGS. 5A and 5B

show the resistance of tunnel insulating layer


5


-


1


, tunnel insulating layer


5


-


2


, and tunnel insulating layer


5


-


3


in the direction of the film thickness.




As shown in

FIG. 5A

, when the respective magnetization of fixed magnetic layer


4


and free magnetic layer


6


-


1


are in the same direction, the resistance between fixed magnetic layer


4


and free magnetic layer


6


-


1


caused by tunnel insulating layer


5


-


1


is R


1


. On the other hand, as shown in

FIG. 5B

, when the respective magnetization of fixed magnetic layer


4


and free magnetic layer


6


-


1


are in opposite directions, the resistance between fixed magnetic layer


4


and free magnetic layer


6


-


1


caused by tunnel insulating layer


5


-


1


is R


1


+ΔR


1


.




Also, as shown in

FIG. 5A

, the resistance between free magnetic layer


6


-


1


and free magnetic layer


6


-


2


caused by tunnel insulating layer


5


-


2


is R


2


when the respective magnetization directions of free magnetic layer


6


-


1


and free magnetic layer


6


-


2


are the same. On the other hand, as shown in

FIG. 5B

, the resistance between free magnetic layer


6


-


1


and free magnetic layer


6


-


2


caused by tunnel insulating layer


5


-


2


is R


2


+ΔR


2


when the respective magnetization directions of free magnetic layer


6


-


1


and free magnetic layer


6


-


2


are opposite.




In addition, as shown in

FIG. 5A

, the resistance of tunnel insulating layer


5


-


3


in the film thickness direction is R


3


when the magnetization of free magnetic layer


6


-


2


and free magnetic layer


6


-


3


are in the same direction. On the other hand, as shown in

FIG. 5B

, when the magnetization of free magnetic layer


6


-


1


and free magnetic layer


6


-


2


are in opposite directions, the resistance of tunnel insulating layer


5


-


2


in the direction of the film thickness is R


3


+ΔR


3


.




At this point, the above ΔR


1


, ΔR


2


, and ΔR


3


are determined so as to satisfy the following relational equations given that the predetermined resistance value is ΔR:






Δ


R




1




=ΔR/


2


0


(=Δ


R


),








Δ


R




2




=ΔR/


2


1










Δ


R




3




=ΔR/


2


2


  (1)






By having ΔR


1


, ΔR


2


, and ΔR


3


satisfy relational equations (1), memory cell


10


with three free magnetic layers can store 8 (=2


3


) values as data.





FIGS. 6



a


and


6


B show the corresponding relationship of the resistance between bit line


2


and word line


9


, and the respective directions of magnetization of free magnetic layer


6


-


1


, free magnetic layer


6


-


2


, and free magnetic layer


6


-


3


. As shown in

FIG. 6A

, memory cell


10


can be under any of 8 conditions: condition


1


through condition


8


. Condition


1


is a condition where the magnetization of all of free magnetic layer


6


-


1


, free magnetic layer


6


-


2


, and free magnetic layer


6


-


3


are in the +x axis direction. Condition


2


is a condition where the magnetizations of free magnetic layer


6


-


1


and free magnetic layer


6


-


2


are in the +x axis direction, and the magnetization of free magnetic layer


6


-


3


is in the −x axis direction. Condition


3


is a condition where the magnetization of free magnetic layer


6


-


1


is in the +x axis direction, and magnetizations of free magnetic layer


6


-


2


and free magnetic layer


6


-


3


is in the −x axis direction. Condition


4


is a condition where magnetizations of free magnetic layer


6


-


1


and free magnetic layer


6


-


3


is in the +x axis direction, and magnetization of free magnetic layer


6


-


2


is in the −x axis direction. Condition


5


is a condition where the magnetizations of all of free magnetic layer


6


-


1


, free magnetic layer


6


-


2


, and free magnetic layer


6


-


3


is in the −x axis direction. Condition


6


is a condition where the magnetization of free magnetic layer


6


-


3


is in the +x axis direction, and the magnetizations of free magnetic layer


6


-


1


and free magnetic layer


6


-


2


is in the −x axis direction. Condition


7


is a condition where the magnetizations of free magnetic layer


6


-


2


and free magnetic layer


6


-


3


is in the +x axis direction, and the magnetization of free magnetic layer


6


-


1


is in the −x axis direction. Condition


8


is a condition where the magnetization of free magnetic layer


6


-


2


is in the +x axis direction, and the magnetizations of free magnetic layer


6


-


1


and free magnetic layer


6


-


3


is in the −x axis direction.




As shown in

FIG. 6B

, when memory cell


10


is in condition i of condition


1


through condition


8


(i being an integer 1 or greater but no greater than 8), resistance R


read


between bit line


2


and word line


9


is:








R




read




=R




const


+(


i−


1)×(Δ


R/


2


2


).






Here,








R




const




=ΔR




1




+ΔR




2




+ΔR




3


.






In this manner, resistance R


read


between bit line


2


and word line


9


becomes one of the respectively different 8 (=2


3


) values depending on the magnetization direction of free magnetic layer


6


-


1


, free magnetic layer


6


-


2


, and free magnetic layer


6


-


3


. Therefore, based on resistance R


read


between bit line


2


and word line


9


, it is possible to determine which condition, of condition


1


through condition


8


, memory cell


10


is under. In this manner, memory cell


10


can store 8 (=2


3


) values as data, and also, the data stored can be read out from memory cell


10


.




The above ΔR


1


, ΔR


2


, and ΔR


3


are represented in the following equations:








ΔR




1




=R




1


×γ


1


,










ΔR




2




=R




2


×γ


2


,










ΔR




3




=R




2


×γ


3


.






Here, γ


1


, γ


2


, and γ


3


denote the MR ratio. R


1


is, as described earlier, the resistance of tunnel insulating layer


5


-


1


in the film thickness direction when magnetization of fixed magnetic layer


4


and free magnetic layer


6


-


1


are in the same direction; R


2


is the resistance of tunnel insulating layer


5


-


2


in the film thickness direction when the magnetization of free magnetic layer


6


-


1


and free magnetic layer


6


-


2


are in the same direction; and R


3


is the resistance of tunnel insulating layer


5


-


3


in the film thickness direction when the magnetization of free magnetic layer


6


-


2


and free magnetic layer


6


-


3


are in the same direction. γ


1


, γ


2


, γ


3


, R


1


, R


2


, and R


3


are set so that ΔR


1


, ΔR


2


, and ΔR


3


satisfy equations (1).




γ


1


, γ


2


, and γ


3


can be adjusted in conformity with the type of magnetic material comprising fixed magnetic layer


4


, free magnetic layer


6


-


1


, free magnetic layer


6


-


2


, and free magnetic layer


6


-


3


. γ


1


is fixed in conformity with the types of magnetic material comprising fixed magnetic layer


4


and free magnetic layer


6


-


1


. γ


2


is fixed in conformity with the types of magnetic material comprising free magnetic layer


6


-


1


and free magnetic layer


6


-


2


. γ


3


is fixed in Conformity with the types of magnetic material comprising free magnetic layer


6


-


2


and free magnetic layer


6


-


3


. The types of magnetic material comprising fixed magnetic layer


4


, free magnetic layer


6


-


1


, free magnetic layer


6


-


2


, and free magnetic layer


6


-


3


is selected appropriately so that ΔR


1


, ΔR


2


, and ΔR


3


satisfy equations (1).




Generally, the highr the magnetic polarizability x of the magnetic material, the highr the effect of tunnel magnetic resistance result and is the highr the MR ratio. As for a ferromagnetic body with a low magnetic polarizability x, Ni


0.8


Fe


0.2


(magnetic polarizability: 37%) is known, and as for a ferromagnetic body that has a high magnetic polarizability x, Co (magnetic polarizability: 42%) and Fe (magnetic polarizability: 42 to 46%.) are known. Generally, the highr the magnetic polarizability x is, the greater the coercive force of the magnetic body. Adjustment of ΔR


1


, ΔR


2


, and ΔR


3


, by adjusting γ


1


, γ


2


, and γ


3


, through the type of magnetic material, is advantageous in that resistance can be adjusted through the material itself, with no need for fine tuning such as through film thickness control, etc.




R


1


, R


2


, and R


3


are adjustable through:




(a) the film thickness of tunnel insulating layer


5


-


1


, tunnel insulating layer


5


-


2


, and tunnel insulating layer


5


-


3


;




(b) the film quality and forming method of tunnel insulating layer


5


-


1


, tunnel insulating layer


5


-


2


, and tunnel insulating layer


5


-


3


;




(c) the material comprising tunnel insulating layer


5


-


1


, tunnel insulating layer


5


-


2


, and tunnel insulating layer


5


-


3


; and




(d) The area of tunnel insulating layer


5


-


1


, tunnel insulating layer


5


-


2


, and tunnel insulating layer


5


-


3


.




The highr the respective film thickness of tunnel insulating layers


5


-


1


, tunnel insulating layer


5


-


2


, and tunnel insulating layer


5


-


3


are, the highr R


1


, R


2


, and R


3


respectively become. The respective film thickness of tunnel insulating layer


5


-


1


, tunnel insulating layer


5


-


2


, and tunnel insulating layer


5


-


3


are selected appropriately so that ΔR


1


, ΔR


2


, and ΔR


3


satisfy equations (1). For instance, the film thickness of tunnel insulating layer


5


-


1


may be set to 18 Å; film thickness of tunnel insulating layer


5


-


2


to 14 Å; and film thickness of tunnel insulating layer


5


-


3


to 12 Å. This situation is advantageous in that ΔR


1


, ΔR


2


, and ΔR


3


can be adjusted while using the same material for tunnel insulating layer


5


-


1


, tunnel insulating layer


5


-


2


, and tunnel insulating layer


5


-


3


. It is preferable that the same material be used for tunnel insulating layer


5


-


1


, tunnel insulating layer


5


-


2


, and tunnel insulating layer


5


-


3


since the amount of equipment required to produce magnetic memory can be cutback or simplified.




Moreover, the film quality of tunnel insulating layer


5


-


1


, tunnel insulating layer


5


-


2


, and tunnel insulating layer


5


-


3


respectively affects R


1


, R


2


, and R


3


. At this point, film quality of tunnel insulating layer


5


-


1


, tunnel insulating layer


5


-


2


, and tunnel insulating layer


5


-


3


can be changed depending on the method of forming tunnel insulating layer


5


-


1


, tunnel insulating layer


5


-


2


, and tunnel insulating layer


5


-


3


respectively. For instance, a case can be considered where alumina (Al


2


O


2


) film is used for tunnel insulating layer


5


-


1


, tunnel insulating layer


5


-


2


, and tunnel insulating layer


5


-


3


. As for the method of forming the alumina film, a method, which first forms the aluminum thin film, and then oxidizes the aluminum thin film can be employed. Here, if the respective methods oxidizing the aluminum thin films differ, the resistance in the film thickness direction of the alumina film formed through oxidation also varies. As for the method of oxidizing the aluminum thin film, there are three methods known: the natural oxidation method, the plasma oxidation method, and the radical oxidation method. The natural oxidation method is a method of oxidizing aluminum thin film by leaving the aluminum thin film in an oxidation atmosphere at an ordinary temperature. The plasma oxidation method is a method of oxidizing aluminum thin film by exposing the aluminum thin film in an oxygen plasma. The radical oxidation method is a method of oxidizing the aluminum thin film by exposing the aluminum thin film in an oxidation atmosphere that includes oxygen radicals. An oxidation atmosphere which includes oxygen radicals can be produced by generating oxygen plasma and electrically naturalizing the oxygen ions included in the oxygen plasma. An alumina film formed by oxidizing the aluminum thin film with the natural oxidation method is different in quality from an alumina film formed by oxidizing an aluminum thin film with the radical oxidation method or plasma oxidation method, and resistance in the film thickness direction is low. The film quality of tunnel insulating layer


5


-


1


, tunnel insulating layer


5


-


2


, and tunnel insulating layer


5


-


3


and method of forming tunnel insulating layer


5


-


1


, tunnel insulating layer


5


-


2


, and tunnel insulating layer


5


-


3


are selected appropriately so that ΔR


1


, ΔR


2


, and ΔR


3


satisfy equations (1).




Moreover, the material comprising tunnel insulating layer


5


-


1


, tunnel insulating layer


5


-


2


, and tunnel insulating layer


5


-


3


affects R


1


, R


2


, and R


3


. For instance, R


1


, R


2


, and R


3


are adjustable depending on whether alumina or hafnium oxide is used as the material comprising tunnel insulating layer


5


-


1


, tunnel insulating layer


5


-


2


, and tunnel insulating layer


5


-


3


.




Moreover, the area of tunnel insulating layer


5


-


1


, tunnel insulating layer


5


-


2


, and tunnel insulating layer


5


-


3


affects R


1


, R


2


, and R


3


, respectively. As shown in

FIG. 7B

, the area of tunnel insulating layer


5


-


1


in contact with fixed magnetic layer


4


and free magnetic layer


6


-


1


, area of tunnel insulating layer


5


-


2


in contact with free magnetic layer


6


-


1


and free magnetic layer


6


-


2


, and the area of tunnel insulating layer


5


-


3


in contact with free magnetic layer


6


-


2


and free magnetic layer


6


-


3


can be adjusted by appropriately setting the masking pattern during the etching treatment on tunnel insulating layer


5


-


1


, tunnel insulating layer


5


-


2


, and tunnel insulating layer


5


-


3


Moreover, as shown in

FIG. 7C

, the area of tunnel insulating layer


5


-


1


in contact with fixed magnetic layer


4


and free magnetic layer


6


-


1


, the area of tunnel insulating layer


5


-


2


in contact with free magnetic layer


6


-


1


and free magnetic layer


6


-


2


, and the area of tunnel insulating layer


5


-


3


in contact with free magnetic layer


6


-


2


and free magnetic layer


6


-


3


can be adjusted by adjusting the side angle of tunnel insulating layers


5


-


1


through


5


-


3


and free magnetic layers


6


-


1


through


6


-


3


during the batch etching treatment on tunnel insulating layers


5


-


1


through


5


-


3


and on free magnetic layers


6


-


1


through


6


-


3


. The area of tunnel insulating layer


5


-


1


in contact with fixed magnetic layer


4


and free magnetic layer


6


-


1


, the area of tunnel insulating layer


5


-


2


in contact with free magnetic layer


6


-


1


and free magnetic layer


6


-


2


, and area of tunnel insulating layer


5


-


3


in contact with free magnetic layer


6


-


2


and free magnetic layer


6


-


3


are selected appropriately so that ΔR


1


, ΔR


2


, and ΔR


3


satisfy equations (1).




Adjusting ΔR


1


, ΔR


2


, and ΔR


3


through the area of tunnel insulating layer


5


-


1


, tunnel insulating layer


5


-


2


, and tunnel insulating layer


5


-


3


is preferable in that it is possible to form tunnel insulating layer


5


-


1


, tunnel insulating layer


5


-


2


, and tunnel insulating layer


5


-


3


while keeping the film thickness and material of tunnel insulating layer


5


-


1


, tunnel insulating layer


5


-


2


, and tunnel insulating layer


5


-


3


the same.




As described above, ΔR


1


, ΔR


2


, and ΔR


3


are adjusted so as to satisfy equations (1) through:




(A) the types and film thickness of magnetic material comprising fixed magnetic layer


4


, free magnetic layer


6


-


1


, free magnetic layer


6


-


2


, and free magnetic layer


6


-


3


;




(B) the film thickness of tunnel insulating layer


5


-


1


, tunnel insulating layer


5


-


2


, and tunnel insulating layer


5


-


3


;




(C) the quality of film and forming method of tunnel insulating layer


5


-


1


, tunnel insulating layer


5


-


2


, and tunnel insulating layer


5


-


3


;




(D) the material comprising tunnel insulating layer


5


-


1


, tunnel insulating layer


5


-


2


, and tunnel insulating layer


5


-


3


; and




(E) the area of tunnel insulating layer


5


-


1


, tunnel insulating layer


5


-


2


, and tunnel insulating layer


5


-


3


.




This enables memory cell


10


to store 8 (=2


3


) values as data, and also, it becomes possible for the stored data to be read from memory cell


10


.




Memory cell


10


with such configuration has data written therein in the following manner.




Electric current


11


flows to word line


9


, which is connected to memory cell


10


whereto data is to be written. In the following explanation, as shown in

FIG. 4

, it is given that electric current


11


flows to word line


9


-


2


. As electric current


11


flows to word line


9


-


2


in the −x axis direction, as shown in

FIG. 4

, the memory cell connected to word line


9


-


2


of memory cell


10


is applied magnetic field


12


in the −y axis direction. As described earlier, the magnetization directions included in free magnetic layer


6


-


1


, free magnetic layer


6


-


2


, and free magnetic layer


6


-


3


are fixed in either the +x axis direction or the −x axis direction, and the magnetization does not reverse even if magnetic field


12


is applied to memory cell


10


in the −y axis direction. However, the magnetic field in the x axis direction required to reverse magnetization of free magnetic layer


6


-


1


, free magnetic layer


6


-


2


, and free magnetic layer


6


-


3


becomes low due to the asteroid characteristics exhibited by the magnetization.




Moreover, electric current


13


flows to bit line


2


, which is connected to memory cell


10


whereto data is to be written. In the following explanation, as shown in

FIG. 2

, it is given that electric current


13


flows to bit line


2


-


2


. As electric current


13


flows to bit line


2


-


2


, a magnetic field is applied in either the in the +x axis direction or the −x axis direction to memory cell


10


connected to bit line


2


-


2


.




As electric current


11


flows to word line


9


-


2


, and electric current


13


flows to bit line


2


-


2


, reversal of the polarization is performed selectively only in the memory cell


10


connected to word line


9


-


2


and bit line


2


-


2


. At this point, the magnitude of electric current


13


flowing to bit line


2


-


2


is suppressed to a level which does not invoke polarization in the memory cells


10


not connected to word line


9


-


2


. This processing causes data to be written only in the memory cell


10


where to the data is to be written.




The direction and magnitude of electric current


13


is controlled in time sequence as described below.

FIGS. 8A through 8D

, which show the style of controlling the direction and magnitude of electric current


13


, are cross-sectional views of


2


A—


2


A in FIG.


4


. However, word lines


9


-


2


, etc. are not illustrated. To begin with, as shown in

FIG. 8B

, electric current


13


is made to flow in a direction, where a magnetic field develops which directs free magnetic layer


6


-


1


in a the desired direction. As shown in

FIG. 8B

, when magnetization of free magnetic layer


6


-


1


is desired to be directed in the −x axis direction, electric current


13


is made to flow in the −y axis direction. At the same time, electric current


11


is made to flow to word line


9


-


2


in the direction shown in FIG.


4


. At this time, the magnitude of electric current


13


and electric current


11


are set so that synthetic magnetic field H


1


, which is high enough to reverse the magnetization of free magnetic layer


6


-


1


, which has the highest coercive force among free magnetic layer


6


-


1


, free magnetic layer


6


-


2


, and free magnetic layer


6


-


3


is applied to free magnetic layer


6


-


1


. At this time, the coercive force of free magnetic layer


6


-


2


and free magnetic layer


6


-


3


is lower than the coercive force of free magnetic layer


6


-


1


, so, as shown in

FIG. 8B

, magnetization of free magnetic layer


6


-


1


, free magnetic layer


6


-


2


, and free magnetic layer


6


-


3


are all in the same direction. In the example shown in

FIG. 8B

, magnetization of free magnetic layer


6


-


1


, free magnetic layer


6


-


2


, and free magnetic layer


6


-


3


are all in the −x axis direction.




Continuing, as shown in

FIG. 8C

, electric current


13


is made to flow in a direction where a magnetic field develops which directs magnetization of free magnetic layer


6


-


2


is in a desired direction. As shown in

FIG. 8C

, when magnetization of free magnetic layer


6


-


2


should be in the +x axis direction, electric current


13


flows in the +y axis direction. At the same time, electric current


11


flows to word line


9


-


2


in the −x axis direction. At this time, the magnitude of electric current


13


and electric current


11


are set so that synthetic magnetic field H


2


, which is high enough to reverse the magnetization of free magnetic layer


6


-


2


but not to reverse the magnetization of free magnetic layer


6


-


1


, is produced. The magnetization of free magnetic layer


6


-


1


does not reverse, but maintains the present direction. At this time, the coercive force of free magnetic layer


6


-


3


is lower than the coercive force of free magnetic layer


6


-


2


, so the magnetization of free magnetic layer


6


-


3


and the magnetization of free magnetic layer


6


-


2


are in the same direction.




Continuing, as shown in

FIG. 8D

, electric current


13


is made to flow so that the magnetization of free magnetic layer


6


-


3


is in the desired direction. As shown in

FIG. 8D

, when the magnetization of free magnetic layer


6


-


2


is to be directed in the −x axis direction, electric current


13


is flowed in the −y axis direction. At the same time, electric current


11


is flowed to word line


9


-


2


in the −x axis direction. Here, the magnitude of electric current


13


is set to produce magnetic field H


3


, which is high enough to reverse the magnetization of free magnetic layer


6


-


3


but not to reverse the magnetization of free magnetic layer


6


-


1


and free magnetic layer


6


-


2


. The magnetization of free magnetic layer


6


-


1


and free magnetic layer


6


-


2


does not reverse but maintains the present direction. The above operation directs the magnetization of free magnetic layer


6


-


1


, free magnetic layer


6


-


2


, and free magnetic layer


6


-


3


in the desired direction and completes writing of desired data.




In order to read out data that has been written, predetermined electric current is flowed to bit line


2


and word line


9


connected to memory cell


10


from which data is read out. At this time, as described earlier, resistance between bit line


2


and word line


9


is either one of 8 values: R


const


, R


const


+R/4, . . . , R


const


+7·ΔR/4 depending on the magnetization direction of free magnetic layer


6


-


1


, free magnetic layer


6


-


2


, and free magnetic layer


6


-


3


. Data stored in memory cell


10


is distinguished based on the resistance between bit line


2


and word line


9


.




As described above, magnetic memory according to this embodiment is capable of storing 2


3


values in memory cell


10


as data depending on the direction of the magnetization of free magnetic layer


6


-


1


, free magnetic layer


6


-


2


, and free magnetic layer


6


-


3


, and in addition 2


3


values as data can be read out of memory cell


10


.




In the embodiment, ΔR


1


, ΔR


2


, and ΔR


3


are not always limited to ΔR/2


0


, ΔR/2


1


, and ΔR/2


2


.




In this embodiment, ΔR


1


, ΔR


2


, and ΔR


3


can be any value if at least two of them are different. This enables memory cell


10


to store at least 4 or more values as data.




However, it is preferable that ΔR


1


, ΔR


2


, and ΔR


3


be different from each other. This enables memory cell


10


to store at least 7 values or more as data.




Moreover, when ΔR


1


, ΔR


2


, and ΔR


3


are different from each other, it is preferable that ΔR


1


, ΔR


2


, and ΔR


3


be set so that each sum of any progression generated from set {ΔR


1


, ΔR


2


, and ΔR


3


} be different. In other words, it is preferable that ΔR


1


, ΔR


2


, and ΔR


3


be set so that ΔR


1


, ΔR


2


, ΔR


3


, ΔR


1


+ΔR


2


, ΔR


2


+ΔR


3


, ΔR


3


+ΔR


1


, and ΔR


1


+ΔR


2


+ΔR


3


become different values from each other. This makes resistance value, R


read


, between bit line


2


and word line


9


to be one of 8 different values:








R




const


(=


R




1




+R




2




+R




3


),








R


const


+ΔR


1


,








R


const


+ΔR


2


,








R


const


+ΔR


3


,








R


const


+ΔR


1


+ΔR


2


,








R


const


+ΔR


2


+ΔR


3


,








R


const


+ΔR


3


+ΔR


1


, and








R


const


+ΔR


1


+ΔR


2


+ΔR


3


,






and makes it possible to store 8 values as data.




However, as described above, it is preferable that ΔR


1


, ΔR


2


, and ΔR


3


be any of ΔR/2


0


, ΔR/2


1


, and ΔR/2


2


values. This causes resistance value R


read


between bit line


2


and word line


9


to change linearly depending on the magnetization direction of tunnel insulating layers


5


-


1


through


5


-


3


, and makes it easier to distinguish data stored in memory cell


10


from resistance value, R


read


.




At this time, provided that each of ΔR


1


, ΔR


2


, and ΔR


3


are values different from each other, they can be any of the value either ΔR/2


0


, ΔR/2


1


, or ΔR/2


2


. As described above, ΔR


1


, ΔR


2


, and ΔR


3


are not always set to satisfy equations (1) For instance, ΔR


1


, ΔR


2


, and ΔR


3


can be set as








R




1




=ΔR/


2


2


,










R




2




=ΔR/


2


1


,










R




3




=ΔR/


2


0


.






In addition, in this embodiment, the number of tunnel insulating layers and free magnetic layers are not limited to 3. The number of tunnel insulating layers and free magnetic layers can be any numbers as long as they are plural. For instance, as shown in

FIG. 9

, tunnel insulating layer


5


-


4


, free magnetic layer


6


-


4


, and so on through to, tunnel insulating layer


5


-n, and free magnetic layer


6


-n can be formed in that order on the upper surface of free magnetic layer


6


-


3


. In other words, as for the number of set of tunnel insulating layers and free magnetic layers, n sets can be built up in that order provided that n is a natural number of 2 or greater.




In this case, the resistance of tunnel insulating layers


5


-


1


through


5


-n in the film thickness direction can be set in the following manner. In the explanation below, the resistance of tunnel insulating layer


5


-


1


in the film thickness direction is R


1


when the magnetization of fixed magnetic layer


4


beneath tunnel insulating layer


5


-


1


and free magnetic layer


6


-


1


on the upper surface of tunnel insulating layer


5


-


1


are in the same direction, and the resistance is R


1


+ΔR


1


when in opposite directions. In addition, the resistance of tunnel insulating layer


5


-i in the film thickness direction is R


i


when magnetization of free magnetic layer


6


-(i−1) beneath tunnel insulating layer


5


-i (i being an integer of 2 or greater but no greater than n) amongst tunnel insulating layer


5


-


2


through tunnel insulating layer


5


-n and free magnetic layer


6


-i thereupon are in the same direction; and when opposite, the resistance of tunnel insulating layer


5


-i is R


i


+ΔR


i


.




At this point, ΔR


1


through ΔR


n


, can be any value if at least two of them are different. This enables memory cell


10


to store at least n+1 or more values as data.




However, similar to the case of n=3 described above, it is preferable that ΔR


1


through ΔR


n


be set so that each of them are different from each other. This enables memory cell


10


to store at least {(n


2


+n+2)/2} or more values as data.




At this time, similar to the case of n=3 described above, it is preferable that ΔR


1


through ΔR


n


be set so that the sum of any progression generated from set {ΔR


1


, ΔR


2


, . . . , ΔR


n


} is different from each other. This enables memory cell


10


to store 2


n


values as data. In the magnetic memory according to this embodiment, the number of values that can be stored in memory cell


10


increases in the order of 2 to the n-th power (2


n


) for the number of free magnetic layers, n.




Moreover, as with the case of n=3 described above, it is preferable that ΔR


1


through ΔR


n


be set to any one of ΔR, ΔR/2


1


, ΔR/2


2


, . . . , ΔR/2


n−1


.




As shown in

FIG. 10

, it is possible that antiferromagnetic layer


3


cannot be provided as a third embodiment of the present invention. In this case, fixed magnetic layer


4


is provided directly upon the surface of bit line


2


. Since the structure of memory cell


10


can be simplified, it is preferable that antiferromagnetic layer


3


be not provided. However, in this case, the coercive force of fixed magnetic layer


4


and the magnitude of electric current


13


and electric current


11


, which are respectively made to flow to bit line


2


and word line


9


, must be controlled so as not to reverse the magnetization of fixed magnetic layer


4


when tunnel insulating layer


5


-


1


is reversed.





FIG. 11

shows memory cell


20


of a magnetic memory according to a fourth embodiment of the present invention. This magnetic memory comprises a structure wherein memory cell


10


in the magnetic memory according to the first embodiment is replaced by memory cell


20


.




Memory cell


20


comprises TMR element


21


-


1


, TMR element


21


-


2


, and buffer conduction layer


22


. TMR element


21


-


1


and TMR element


21


-


2


are electrically connected with buffer conduction layer


22


. TMR element


21


-


1


is electrically connected in series with TMR element


21


-


2


between bit line


2


and word line


9


.




TMR element


21


-


1


comprises antiferromagnetic layer


23


-


1


, fixed magnetic layer


24


-


1


, tunnel insulating layer


25


-


1


, and free magnetic layer


26


-


1


. Antiferromagnetic layer


23


-


1


is formed on the upper surface of bit line


2


. Fixed magnetic layer


24


-


1


is formed on the upper surface of antiferromagnetic layer


23


-


1


. Fixed magnetic layer


24


-


1


is formed with ferromagnetic material. The magnetization direction of fixed magnetic layer


24


-


1


is fixed through interaction with antiferromagnetic layer


23


-


1


. Tunnel insulating layer


25


-


1


is formed on the upper surface of fixed magnetic layer


24


-


1


. Tunnel insulating layer


25


-


1


is formed with an insulator such as alumina, or hafnium oxide. The thickness of tunnel insulating layer


25


-


1


is thin to the degree that the tunnel current can flow in the film thickness direction. Free magnetic layer


26


-


1


is formed on the upper surface of tunnel insulating layer


25


-


1


. Free magnetic layer


26


-


1


is formed with ferromagnetic body. The magnetization of free magnetic layer


26


-


1


is reversible, and the magnetization direction of free magnetic layer


26


-


1


is either in the +x axis direction or in the −x axis direction depending on the direction of magnetic field applied.




Buffer conduction layer


22


is formed on the upper surface of free magnetic layer


26


-


1






TMR element


21


-


2


comprises antiferromagnetic layer


23


-


2


, fixed magnetic layer


24


-


2


, tunnel insulating layer


25


-


2


, and free magnetic layer


26


-


2


. Antiferromagnetic layer


23


-


2


is formed the upper surface of buffer conduction layer


22


. Fixed magnetic layer


24


-


2


is formed on the surface of antiferromagnetic layer


23


-


2


. Fixed magnetic layer


24


-


2


is formed with ferromagnetic material. The magnetization of fixed magnetic layer


24


-


2


is fixed through interaction with antiferromagnetic layer


23


-


2


. Tunnel insulating layer


25


-


2


is formed on the upper surface of fixed magnetic layer


24


-


2


. Tunnel insulating layer


25


-


2


is formed with insulator such as alumina, or hafnium oxide. Thickness of tunnel insulating layer


25


-


2


is thin to the level that tunnel current can be applied in the film thickness direction. Free magnetic layer


26


-


2


is formed with ferromagnetic material. The magnetization of free magnetic layer


26


-


2


is reversible, and the magnetization direction of free magnetic layer


26


-


2


is either in the +x axis direction or in the −x axis direction depending on the direction of magnetic field applied. At this time, coercive force of free magnetic layer


26


-


2


is lower than coercive force of free magnetic layer


26


-


1


.




The resistance of tunnel insulating layer


25


-


1


and tunnel insulating layer


25


-


2


in the film thickness direction varies depending on the direction of the magnetization of free magnetic layer


26


-


1


and free magnetic layer


26


-


1


by the tunnel magnetism resistance effect (TMR) exhibited by ferromagnetic body.

FIGS. 12A and 12B

show resistance of tunnel insulating layer


25


-


1


and tunnel insulating layer


25


-


2


in the film thickness direction.




As shown in

FIG. 12A

, when the magnetization of fixed magnetic layer


24


-


1


and free magnetic layer


26


-


1


are in the same direction, resistance between fixed magnetic layer


24


-


1


and free magnetic layer


26


-


1


due to tunnel insulating layer


25


-


1


is R


1


□. On the other hand, as shown in

FIG. 12B

, when the magnetization of fixed magnetic layer


24


-


1


and free magnetic layer


26


-


1


are in the opposite direction, resistance between fixed magnetic layer


24


-


1


and free magnetic layer


26


-


1


due to tunnel insulating layer


25


-


1


is R


1


□+ΔR


1


.




Similarly, as shown in

FIG. 12A

, the resistance between free magnetic layer


26


-


1


and free magnetic layer


26


-


2


due to tunnel insulating layer


25


-


2


is R


2


□ when the magnetization of free magnetic layer


26


-


1


and free magnetic layer


26


-


2


are in the same direction. On the other hand, as shown in

FIG. 12B

, when the magnetization of free magnetic layer


26


-


1


and free magnetic layer


26


-


2


are in the opposite direction, the resistance between free magnetic layer


26


-


1


and free magnetic layer


26


-


2


due to tunnel insulating layer


25


-


2


is R


2


□+ΔR


2


□.




At this point, the above ΔR


1


□ and ΔR


2


□ are set so as to satisfy the following relational equations:








R




1




□=ΔR□/


2


0


(=Δ


R


□)










R




2




□=ΔR□/


2


1


(=Δ


R


□/2)  (2)






R□ is a predetermined resistance value. By ΔR


1


□ and ΔR


2


□ satisfying the equations (2), the resistance value between bit line


2


and word line


9


is any one of four values: R


const


□ (=R


1


+R


2


), R


const


+ΔR□/2, R


const


+ΔR□, or R


const


+3ΔR□/2. The magnetization direction of free magnetic layer


26


-


1


and free magnetic layer


26


-


2


can be distinguished with the resistance value between bit line


2


and word line


9


. Memory cell


20


can store 4 (=2


2


) values as data.




Here, similar to the case of the first embodiment, ΔR


1


□ and ΔR


2


□ are adjusted so as to satisfy equations (2) through:




(A) the types and film thickness of the magnetic material comprising fixed magnetic layer


24


-


1


, free magnetic layer


26


-


1


, fixed magnetic layer


24


-


2


, and free magnetic layer


26


-


2


;




(B) the film thickness of tunnel insulating layer


25


-


1


and tunnel insulating layer


25


-


2


;




(C) the quality of film and forming method of tunnel insulating layer


25


-


1


and tunnel insulating layer


25


-


2


;




(D) the material comprising tunnel insulating layer


25


-


1


and tunnel insulating layer


25


-


2


; and




(E) the area of tunnel insulating layer


25


-


1


and tunnel insulating layer


25


-


2


.




The operation to write data in memory cell


20


is the same as that in the first embodiment.




As with the first embodiment, electric current is made to flow to word line


9


, which is connected to memory cell


20


where to data is written in the −x axis direction. Magnetic field is flowed in the −y axis direction to memory cell


20


, which is connected to word line


9


whereto electric current is flowed. Due to the asteroid characteristics exhibited by the magnetization, the magnitude of the magnetic field in the x axis direction required to the reverse the magnetization of free magnetic layer


26


-


1


and free magnetic layer


26


-


2


of memory cell


20


, which is connected to word line


9


whereto electric current is flowed, becomes low.




Moreover, as shown in

FIGS. 13A

to


13


C, electric current


13


flows to bit line


2


either in the +y axis direction or in the −y axis direction. The direction and magnitude of electric current


13


is controlled in time sequence as described in the following. To begin with, as shown in

FIG. 13B

, electric current


13


is flowed in a direction so as to direct the magnetization of free magnetic layer


26


-


1


in the desired direction. As shown in

FIG. 13B

, when the magnetization of free magnetic layer


26


-


2


is to be directed towards the −x axis direction, electric current


13


is made to flow in the −y axis direction. At the same time, electric current


11


flows to word line


9


. At this point, the magnitude of electric current


13


and electric current


11


is controlled so that synthetic magnetic field H


1


, which has a magnitude high enough to reverse the magnetization of free magnetic layer


26


-


1


, which has the highr coercive force out of free magnetic layer


26


-


1


and free magnetic layer


26


-


2


. At this point, the magnetization of free magnetic layer


26


-


1


and free magnetic layer


26


-


2


are in the same direction because the coercive force of free magnetic layer


26


-


2


is lower than the coercive force of free magnetic layer


26


-


1


. The direction of the magnetization of fixed magnetic layer


24


-


1


and fixed magnetic layer


24


-


2


does not reverse but maintains the present direction due to interaction with antiferromagnetic layer


23


-


1


and antiferromagnetic layer


23


-


2


, respectively. In the example shown in

FIG. 13B

, the magnetization of both free magnetic layer


26


-


1


and free magnetic layer


26


-


2


is in the −x axis direction.




Then, as shown in

FIG. 13C

, electric current


13


is flowed so as to direct the magnetization of free magnetic layer


26


-


2


in the desired direction. As shown in

FIG. 13C

, electric current


13


is flowed in the +y axis direction when the magnetization of free magnetic layer


26


-


2


is to be directed in the +x axis direction. Moreover, electric current


11


flows to word line


2


. At this point, the magnitude of electric current


13


and electric current


11


are set so as to generate synthetic magnetic field H


2


, which has a magnitude high enough to reverse the magnetization of free magnetic layer


26


-


2


but not to reverse the magnetization of free magnetic layer


26


-


1


. The magnetization of free magnetic layer


26


-


1


does not reverse, but remains in the present direction. The magnetization direction of fixed magnetic layer


24


-


1


and fixed magnetic layer


24


-


2


do not reverse but maintain the present direction due to interaction with antiferromagnetic layer


23


-


1


and antiferromagnetic layer


23


-


2


, respectively. With the above operation, the respective magnetizations of free magnetic layer


26


-


1


and free magnetic layer


26


-


2


are directed to the desired direction and writing of desired data is completed.




In order to read out data that has been written, a predetermined electric current is made to flow to bit line


2


and word line


9


, which are connected to memory cell


20


wherefrom data is to be read out. At this point, as described earlier, the resistance between bit line


2


and word line


9


is any one of four resistance values: R


const


□, R


const


□+ΔR□/2, R


const


□+ΔR□, or R


const


□+3·ΔR□/2, each of which are dependent on the magnetization direction of free magnetic layer


26


-


1


and free magnetic layer


26


-


2


. Data stored in memory cell


20


is distinguished based on the resistance between bit line


2


and word line


9


.




As described above, magnetic memory of this embodiment is capable of storing 2


2


values as data in memory cell


20


depending on the magnetization direction of free magnetic layer


26


-


1


and free magnetic layer


26


-


2


, and further, is capable of reading 2


2


values as data from memory cell


20


.




Moreover, the respective directions of the magnetization of fixed magnetic layer


24


-


1


and fixed magnetic layer


24


-


2


are fixed so as to stabilize the operation of the magnetic memory.




In this embodiment, ΔR


1


□ and ΔR


2


□ can take any arbitrary value as long as they are different from each other. As described above, ΔR


1


and ΔR


2


are not exclusively limited to any of the values: ΔR/2


0


, ΔR/2


1


, and ΔR/2


2


. If ΔR


1


□ and ΔR


2


□ are different from each other, resistance value R


read


between bit line


2


and word line


9


may be any one of four resistance values: R


const


□, R


const


□+ΔR


1


□, R


const


+ΔR


2


□, or R


const


□+ΔR


1


□+ΔR


2


□. In this manner, memory cell


20


can store four values as data by having ΔR


1


□ and ΔR


2


□ different from each other.




However, as described above, it is preferable that ΔR


1


□ and ΔR


2


□ be either ΔR□/2


0


or ΔR□/2


1


. This changes R


read


linearly depending on the magnetization direction of free magnetic layers


26


-


1


through


26


-


3


, and it becomes easier to distinguish data stored in memory cell


20


from R


read


.




At this time, it is arbitrary which respective value ΔR


1


and ΔR


2


will take from either ΔR/2


0


or ΔR/2


1


. In other words, it is also possible to set:








R




1




=ΔR/


2


1


,










R




2




=ΔR/


2


0


.  (2)□.






Moreover, as a fifth embodiment, as shown in

FIG. 14

, it is possible to connect n-number of TMR elements


21


-


1


through


21


-n in series between bit line


2


and word line


9


. In other words, the number of TMR elements is not limited to two. Here, TMR element


21


-i (i being an integer of 1 or greater but no greater than n) out of elements


21


-


1


through


21


-n comprises, deposited in order, antiferromagnetic layer


23


-i, fixed magnetic layer


24


-i, tunnel insulating layer


25


-i, and free magnetic layer


26


-i. TMR element


21


-i□ (i□ being an integer of 1 or greater but no greater than n) and TMR element


21


-(i□+1) are connected to each other by buffer conduction layer


22


-i□.




In this case, the resistance of tunnel insulating layers


25


-


1


through tunnel insulating layer


25


-n in the film thickness direction are set as follows. In the following explanation, when the magnetization of fixed magnetic layer


24


-i beneath tunnel insulating layer


25


-i and free magnetic layer


26


-i on tunnel insulating layer


25


-i are in the same direction, it is given that the resistance of tunnel insulating layer


25


-i in the film thickness direction is Ri□, and when in opposite direction, Ri□+ΔR


1


□.




At this point, ΔR


1


□ through ΔR


n


□ are set so that at least two of them are different. This enables memory cell


20


to store at least {(n


2


+n+2)/2} values as data.




Moreover, it is preferable that the sum of any progression generated from set {ΔR


1


□, ΔR


2


□, . . . , ΔR


n□


} be different from each other. This makes it possible for memory cell


20


to store 2


n


values as data. The number of values that can be stored in a single memory cell


20


increases by the order of 2 to the n-th power (2


n


) in the magnetic memory as claimed in this embodiment.




However, similar to the first embodiment, it is preferable that ΔR


1


□ through ΔRn□ be set as any of ΔR□, ΔR□/2


1


, ΔR□/2


2


, . . . , ΔR□/2


n−1


, respectively.




Moreover, in the fourth and fifth embodiments, it is arbitrary whether the fixed magnetic layer or free magnetic layer comprising a single TMR element is positioned on the substrate side. For instance, as shown in

FIG. 15

, free magnetic layer


26


-


2


can be located on the substrate side and connected to buffer conduction layer


22


, and antiferromagnetic layer


23


-


2


and fixed magnetic layer


24


-


2


be on the opposite side as the substrate and connected to word line


9


, as a sixth embodiment.




As a seventh embodiment, its magnetic memory has the structure similar to the magnetic memory according to first embodiment shown in

FIG. 1

or FIG.


3


. However, the resistance of tunnel insulating layer


5


-


1


, tunnel insulating layer


5


-


2


, and tunnel insulating layer


5


-


3


in the film thickness direction is set to be different from the first embodiment.




In the seventh embodiment, unlike the first embodiment, the above described ΔR


1


, ΔR


2


, and ΔR


3


are set to satisfy the following relational equations wherein it is given that the predetermined resistance value is ΔR:








ΔR




1




=ΔR,












ΔR




2


=2


ΔR/


3,






 


ΔR




3




=ΔR/


3,  (3)




Here, the resistance between fixed magnetic layer


4


and free magnetic layer


6


-


1


due to tunnel insulating layer


5


-


1


is R


1


when the magnetization of fixed magnetic layer


4


and free magnetic layer


6


-


1


is in the same direction, and the resistance is R


1


+ΔR


1


when the magnetization of fixed magnetic layer


4


and free magnetic layer


6


-


1


is in the opposite direction. Moreover, the resistance between free magnetic layer


6


-


1


and free magnetic layer


6


-


2


due to tunnel insulating layer


5


-


2


is R


2


when the magnetization of free magnetic layer


6


-


1


and free magnetic layer


6


-


2


is in the same direction, and the resistance is R


2


+ΔR


2


when the magnetization of free magnetic layer


6


-


1


and free magnetic layer


6


-


2


is in the opposite direction. The resistance between free magnetic layer


6


-


2


and free magnetic layer


6


-


3


by tunnel insulating layer


5


-


3


is R


3


when the magnetization of free magnetic layer


6


-


2


and free magnetic layer


6


-


3


is in the same direction. Further, resistance between free magnetic layer


6


-


1


and free magnetic layer


6


-


2


due to tunnel insulating layer


5


-


3


is R


3


+ΔR


3


when the magnetization of free magnetic layer


6


-


2


and free magnetic layer


6


-


3


is in the opposite direction.




By setting ΔR


1


, ΔR


2


, and ΔR


3


so as to satisfy the equations (3), the resistance, R


read


, between bit line


2


and word line


9


becomes any one of seven values: R


const


, R


const


+ΔR/3, R


const


+2ΔR/3, . . . , R


const


+5ΔR/3, and R


const


+6ΔR/3. This makes it possible for R


read


to change linearly depending on the magnetization direction of free magnetic layers


6


-


1


through


6


-


3


and easier to distinguish data stored in memory cell


10


from R


read


.




As shown in

FIG. 9

, when n sets of tunnel insulating layer and free magnetic layer are built up in that order, ΔR


1


, ΔR


2


, . . . , and ΔR


n


are set so that each of ΔR


1


, ΔR


2


, . . . , and ΔR


n


are different and ΔR


1


, ΔR


2


, . . . , and ΔR


n


are any of n-number of values: ΔR/n, 2ΔR/n, 3ΔR/n, . . . , (n−1) ΔR/n, and ΔR respectively. Here, it is given that the resistance of tunnel insulating layer


5


-


1


in the film thickness direction is R


1


when the magnetization of fixed magnetic layer


4


beneath tunnel insulating layer


5


-


1


and free magnetic layer


6


-


1


upon tunnel insulating layer


5


-


1


are in the same direction, and the resistance is R


1


+6ΔR


1


when in opposite direction. Moreover, resistance of tunnel insulating layer


5


-i in the film thickness direction is set to R


i


when the magnetization of free magnetic layer


6


-(i−1) beneath tunnel insulating layer


5


-i (i being an integer of two or greater but no greater than n) among tunnel insulating layer


5


-


2


through tunnel insulating layer


5


-n and free magnetic layer


6


-i on the upper surface of tunnel insulating layer


5


-i are in the same direction, and the resistance of tunnel insulating layer


5


-i is set to R


i


+ΔR


1


when in opposite directions.




In this case, the resistance, R


read


, between bit line


2


and word line


9


becomes any one of {(n


2


+n+2)/2}-number of values: R


const


, R


const


+ΔR/n, R


const


+2ΔR/n, R


const


+3ΔR/n, . . . , R


const


+{n (n+1)/2}·ΔR/n. In other words, memory cell


10


is capable of storing {(n


2


+n+2)/2}-number of values as data. In the magnetic memory according to the third embodiment, the number of values that can be stored in a single memory cell increments by the order of n-squared (n


2


) for the number, n, of free magnetic layers.




It is noted that, in the magnetic memory connected in series to n-number of TMR elements shown in

FIG. 14

, the resistance of the tunnel insulating layer in the film thickness direction can be determined in the same way. In the following description, it is given that the resistance of tunnel insulating layer


25


-i in the film thickness direction is R


i


□ when the magnetization of fixed magnetic layer


24


-i beneath the lower surface of tunnel insulating layer


25


-i and free magnetic layer


26


-i on the upper surface of tunnel insulating layer


25


-i are in the same direction, and the resistance is R


i


□+ΔR


i


when in opposite direction. ΔR


1


□, ΔR


2


□, . . . , and ΔR


n


□ are set so that each of ΔR


1


□, ΔR


2


□, . . . , and ΔR


n


□ is different from each other, and ΔR


1


□, ΔR


2


□, . . . , and ΔR


n


□ is any of n-number of values: ΔR□/n, 2ΔR□/n, 3ΔR□/n, . . . , (n−1) ΔR□/n, and ΔR□, respectively. This makes it possible to store {(n


2


+n+2)/2}-number of values in memory cell


20


as data.




The present invention provides a magnetic memory capable of storing more data in a single memory cell.




In addition, the present invention provides a magnetic memory capable of storing high amounts of data in a single memory cell and further with simple structure.




In addition, the present invention provides a magnetic memory capable of storing high amounts of data in a single memory cell and further operates in stable manner.



Claims
  • 1. A magnetic memory cell, comprising:a plurality of free magnetic layers, each of said free magnetic layers having different coercivities, the magnetization of adjacent magnetic layers of said plurality of free magnetic layers being equally likely to be in the same direction as in opposite directions; and a pinned magnetic layer not arranged between said free magnetic layers.
  • 2. The cell as claimed in claim 1, whereinsaid free magnetic layers are arranged in that order on one side of said pinned magnetic layer and wherein the coercive force of each of said free magnetic layers weakens with increasing distance from said pinned magnetic layer.
  • 3. The memory as claimed in claim 1, said memory further comprising a plurality of tunnel insulating layers, each of said plurality of tunnel insulating layers being provided between the adjacent two free magnetic layers.
  • 4. A magnetic memory comprising:a memory cell including: a plurality of free magnetic layers, each of the plurality of free magnetic layers having different coercivities to be capable of setting at least three values therein based on the direction of the magnetization of each of the plurality of free magnetic layers, and a pinned magnetic layer not arranged between the plurality of free magnetic layers; and a plurality of conductive layers arranged to sandwich said memory cell and access said memory cell.
  • 5. The memory as claimed in claim 4, said memory further comprising a plurality of tunnel insulating layers, each of said plurality of tunnel insulating layers being provided between the adjacent two free magnetic layers.
  • 6. A magnetic memory, comprising:first through n-th tunnel insulating layers (n being a natural number of 2 or greater); and first through (n+1)-th magnetic layers having a first through (n+1)-th magnetization, respectively, said first magnetic layer being a pinned magnetic layer, said second to (n+1)-th magnetic layer being a free magnetic layer, said second to (n+1)-th magnetic layer each being arranged on one side of said first magnetic layer, no pinned magnetic layer being between said free magnetic layers; an i-th tunnel insulating layer (i being an arbitrary integer of 1 or greater but no greater than n) of said first through n-th tunnel insulating layers being provided between the i-th magnetic layer and the (i+1)-th magnetic layer of said first through the (n+1)-th magnetic layers; an i-th resistance between said i-th magnetic layer and said (i+1)-th magnetic layer being Ri when the i-th magnetization and said (i+1)-th magnetization at said first through (n+1)-th magnetization are in the same direction; and said i-th resistance being R1+ΔR1 when said i-th magnetization and said (i+1)-th magnetization are in opposite directions; and at least two of said ΔR1, ΔR2, . . . , ΔRn being different in value.
  • 7. The magnetic memory as claimed in claim 6, wherein:each of said ΔR1, ΔR2, . . . , ΔRn is different.
  • 8. The magnetic memory as claimed in claim 7, wherein;each of said ΔR1, ΔR2, . . . , ΔRn is substantially equal to any one of ΔR, ΔR/2, ΔR/22, ΔR/2n−1, given that ΔR is a predetermined resistance.
  • 9. The magnetic memory as claimed in claim 8, wherein:the magnetic material comprising said first through n-th magnetic layers is selected so that each of said ΔR1, ΔR2, . . . , ΔRn is substantially equal to any of ΔR, ΔR/2, ΔR/22, . . . , or ΔR/2n−1.
  • 10. The magnetic memory as claimed in claim 8, wherein:the film thickness of said first through n-th tunnel insulating layer is set so that each of said ΔR1, ΔR2, . . . , ΔRn is substantially equal to any of ΔR, ΔR/2, ΔR/22, . . . , or ΔR/2n−1.
  • 11. The magnetic memory as claimed in claim 8, wherein:the film quality of said first through n-th tunnel insulating layer is set so that each of said ΔR1, ΔR2, . . . , ΔRn is substantially equal to any of ΔR, ΔR/2, ΔR/22, . . . , or ΔR/2n−1.
  • 12. The magnetic memory as claimed in claim 8, wherein:the area where said i-th tunnel insulating layer (i being an arbitrary integer of 1 or greater but no greater than n) in contact with said i-th magnetic layer and the area where said i-th tunnel insulating layer in contact with said (i+1)-th magnetic layer are set so that each of said ΔR1, ΔR2, . . . , ΔRn is substantially equal to any of ΔR, ΔR/2, ΔR/22, . . . , or ΔR/2n−1.
  • 13. The magnetic memory as claimed in claim 8, wherein:the insulating material comprising said first through n-th tunnel insulating layer is set so that each of said ΔR1, ΔR2, . . . , ΔRn is substantially equal to any of ΔR, ΔR/2, ΔR/22, . . . , or ΔR/2n−1.
  • 14. The magnetic memory as claimed in claim 7, wherein:each of said ΔR1, ΔR2, . . . , ΔRn is substantially equal to any of ΔR/n, 2ΔR/n, 3ΔR/n, . . . , (n−1) ΔR/n, or ΔR, given that ΔR is a predetermined resistance.
  • 15. The magnetic memory as claimed in claim 22, wherein the sum of any progression generated from elements of the group consisting of, ΔR1, ΔR2, . . . , ΔRn is different from each other.
  • 16. The magnetic memory as claimed in claim 6, wherein:the direction of said first magnetization is pinned.
  • 17. A magnetic memory, comprising:a memory cell including a first magnetic layer, a second magnetic layer and a third magnetic layer, the first, second, and third magnetic layers including a plurality of free magnetic layers, the memory cell being capable of storing at least three values; wherein: the resistance between said first magnetic layer and said second magnetic layer of when magnetization of said first magnetic layer and magnetization of said second magnetic layer are in opposite directions is different from the resistance between said second magnetic layer and said third magnetic layer of when magnetization of said second magnetic layer and magnetization of said third magnetic layer are in opposite directions, the magnetization of said second magnetic layer and the magnetization of said third magnetic layer being equally likely to be in the same direction as in opposite directions.
  • 18. The magnetic memory as claimed in claim 17, wherein:the respective materials of said first to third magnetic layers are different from each other.
  • 19. The magnetic memory as claimed in claim 17, wherein:the respective film thickness of said first to third magnetic layers are different from each other.
  • 20. The memory as claimed in claim 17, whereinsaid second and third magnetic layers are arranged on one side of said first magnetic layer.
  • 21. The memory as claimed in claim 20, wherein said first to third magnetic layers are arranged in that order, a coercive force of said second magnetic layer being larger than that of said third magnetic layer.
  • 22. A magnetic memory, comprising:a memory cell including at least a first magnetic layer, a second magnetic layer and a third magnetic, a first non-magnetic layer formed between said first and second magnetic layers, and a second, non-magnetic layer formed between said second and third magnetic layers, a current being able to flow from said first magnetic layer to said third magnetic layer through said first and second non-magnetic layers and said second magnetic layers, volume of said current being controlled by resistance between adjacent magnetic layers, the resistance being changeable by individual direction of the magnetization of said first to third magnetic layers, the resistance corresponding to data to be read out from said memory cell; wherein: the difference of resistance when the direction of the magnetization of said first magnetic layer and the magnetization of said second magnetic layer are reversed from being in the same direction to being in opposite directions is different from the difference of resistance between said second magnetic layer and said third magnetic layer when the magnetization of said second magnetic layer and the magnetization of said third magnetic layer are reversed from being in the same direction to being in opposite directions, the magnetization of said second magnetic layer and the magnetization of said third magnetic layer being equally likely to be in the same direction as in opposite directions.
  • 23. The magnetic memory as claimed in claim 22, wherein:the respective film thickness of said first and second non-magnetic layers are different from each other.
  • 24. The magnetic memory as claimed in claim 22, wherein:the respective areas of said first non-magnetic layers are different from each other.
  • 25. The magnetic memory as claimed in claim 22, wherein:respective methods of forming said first and second non-magnetic layers are different from each other.
  • 26. The memory as claimed in claim 22, whereinsaid second and third magnetic layers are arranged on one side of said first magnetic layer.
  • 27. The memory as claimed in claim 26, wherein said first to third magnetic layers are arranged in that order, a coercive force of said second magnetic layer being larger than that of said third magnetic layer.
  • 28. The memory as claimed in claim 22, wherein each of said first and second non-magnetic layers is a tunnel insulating film.
Priority Claims (1)
Number Date Country Kind
2001-139382 May 2001 JP
US Referenced Citations (7)
Number Name Date Kind
5953248 Chen et al. Sep 1999 A
5959880 Shi et al. Sep 1999 A
6081446 Brug et al. Jun 2000 A
6166948 Parkin et al. Dec 2000 A
6269018 Monsma et al. Jul 2001 B1
6404673 Matsui Jun 2002 B1
20020036331 Nickel et al. Mar 2002 A1
Foreign Referenced Citations (1)
Number Date Country
11-176149 Jul 1999 JP
Non-Patent Literature Citations (1)
Entry
Korean Office Action dated Apr. 28, 2004 with Translation