Abstract:
Testing a TMR element includes a step of measuring initially a resistance value of the TMR element to provide the measured resistance value as a first resistance value, a step of measuring a resistance value of the TMR element after continuously feeding a current through the TMR element for a predetermined period of time, to provide the measured resistance value as a second resistance value, and a step of evaluating the TMR element depending upon a degree of change in resistance of the TMR element. The degree of change in resistance is determined based upon the first resistance value and the second resistance value.

Description:
PRIORITY CLAIM 
     This application claims priority from Japanese patent application No. 2004-062031, filed on Mar. 5, 2004, which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and apparatus for testing a tunnel magnetoresistive effect (TMR) element such as a TMR head element using the tunnel magnetoresistive effect or a magnetoresistive random access memory (MRAM). 
     2. Description of the Related Art 
     An evaluation test is in general performed for determining whether or not a thin-film magnetic head with a magnetoresistive effect (MR) head element is a non-defective product, during or after manufacturing the magnetic head. This evaluation test includes a stability test for confirming that any random telegraph noise (RTN) such as Barkhausen noise (BHN) is not produced from the MR head element, and a reliability test for confirming that a breakdown voltage of the MR head element is sufficiently high to withstand an extended period of use. 
     Actually, for all the MR head elements manufactured, performed is the stability test by using for example a dynamic performance (DP) tester to confirm that no reproduced output of the head elements exceeds a threshold level due to the RTN for a predetermined period of time. However, because the RTN is a burst noise, it may not be generated during the predetermined period of time. Even if the RTN is generated, it may not be detected due to its level lower than the threshold. 
     Japanese patent publication No. 2000-260012 discloses a testing method of a stability of a normal MR head element but not a TMR head element. In this method, change in the reproduced output from the MR head element is evaluated by repeatedly executing a step of applying both AC record current and external DC magnetic field to the MR head element and a step of measuring the reproduced characteristics after that application. 
     However, this testing method not only additionally needs a magnetic field generation device for applying the external DC magnetic field to the MR head element but also adversely affects a bias magnetic field of the MR head element due to the application of the external magnetic field. On top of that, it is difficult to detect the burst noise RTN with reliability even by this testing method. 
     As will be noted, the conventional reliability test of measuring a breakdown voltage of the magnetic head cannot be performed for all the magnetic heads because it breaks the magnetic head during the breakdown voltage measurement. 
     BRIEF SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a method and apparatus for testing a TMR element, whereby stability and reliability of the TMR element can be extremely easily confirmed without destroying the element. 
     According to the present invention, a method for testing a TMR element includes a step of measuring initially a resistance value of the TMR element to provide the measured resistance value as a first resistance value, a step of measuring a resistance value of the TMR element after continuously feeding a current through the TMR element for a predetermined period of time, to provide the measured resistance value as a second resistance value, and a step of evaluating the TMR element depending upon a degree of change in resistance of the TMR element. The degree of change in resistance is determined based upon the first resistance value and the second resistance value. 
     With respect to a plurality of TMR elements, a distribution of degrees of change in their resistances is determined based upon the first resistance value (initial resistance value) and the second resistance value (resistance value just after continuously feeding a current through the TMR element for a predetermined time period), that is a distribution of rates of change in resistance values or changed amount of resistance values, polarizes two groups. The first group is a set of the TMR elements with large rates of change in resistance values or large changed amount of resistance values, whereas the second group is a set of the TMR elements with small rates of change in resistance values or small changed amount of resistance values. Many of the TMR elements in the first group generate RTN, whereas few of the TMR elements in the second group generate RTN. Also, breakdown voltages of the TMR elements in the first group are lower than those of the TMR elements in the second group. Therefore, it is possible to extremely easily evaluate non-defective/defective with respect to a TMR element by confirming stability and reliability of the TMR element depending upon changed amount from the first or initial resistance value to the second resistance that is the resistance value just after continuously feeding a current through the TMR element for a predetermined time period. Moreover, because this evaluation test can be performed without destroying the TMR element, 100% inspection of the manufactured TMR elements becomes possible. 
     It is preferred that the step of evaluating includes evaluating the TMR element by using a rate of change R 2 /R 1 , where R 2  represents the first resistance value and R 1  represents the second resistance value. 
     It is also preferred that the evaluating step includes judging that the TMR element is non-effective product when the rate of change R 2 /R 1  is larger than a predetermined threshold. In this case, preferably, the predetermined threshold is a predetermined value within a range of 97.5–98.5(%). 
     It is preferred that the predetermined period of time is a predetermined time period of two to three minutes. 
     It is also preferred that the TMR element is a TMR head element or a MRAM. 
     According to the present invention, also, an apparatus for testing a TMR element includes a unit for measuring initially a resistance value of the TMR element to provide the measured resistance value as a first resistance value, a unit for measuring a resistance value of the TMR element after continuously feeding a current through the TMR element for a predetermined period of time, to provide the measured resistance value as a second resistance value, and a unit for evaluating the TMR element depending upon a degree of change in resistance of the TMR element. The degree of change in resistance is determined based upon the first resistance value and the second resistance value. 
     As aforementioned, with respect to a plurality of TMR elements, a distribution of degrees of change in their resistances is determined based upon the first resistance value (initial resistance value) and the second resistance value (resistance value just after continuously feeding a current through the TMR element for a predetermined time period), that is a distribution of rates of change in resistance values or changed amount of resistance values, polarizes two groups. The first group is a set of the TMR elements with large rates of change in resistance values or large changed amount of resistance values, whereas the second group is a set of the TMR elements with small rates of change in resistance values or small changed amount of resistance values. Many of the TMR elements in the first group generate RTN, whereas few of the TMR elements in the second group generate RTN. Also, breakdown voltages of the TMR elements in the first group are lower than those of the TMR elements in the second group. Therefore, it is possible to extremely easily evaluate non-defective/defective with respect to a TMR element by confirming stability and reliability of the TMR element depending upon changed amount from the first or initial resistance value to the second resistance that is the resistance value just after continuously feeding a current through the TMR element for a predetermined time period. Moreover, because this evaluation test can be performed without destroying the TMR element, 100% inspection of the manufactured TMR elements becomes possible. 
     It is preferred that the evaluating unit includes a evaluation unit for evaluating the TMR element by using a rate of change R 2 /R 1 , where R 2  represents the first resistance value and R 1  represents the second resistance value. 
     It is also preferred that the evaluation unit includes a unit for judging that the TMR element is non-effective product when the rate of change R 2 /R 1  is larger than a predetermined threshold. In this case, preferably, the predetermined threshold is a predetermined value within a range of 97.5–98.5(%). 
     It is preferred that the predetermined period of time is a predetermined time period of two to three minutes. 
     It is also preferred that the TMR element is a TMR head element or a MRAM. 
     Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a view schematically illustrating a configuration of testing a TMR head element as a preferred embodiment according to the present invention; 
         FIG. 2  is a flow chart illustrating a testing procedure in the embodiment of  FIG. 1 ; 
         FIG. 3  is a sectional view, seen along a direction orthogonal to an air bearing surface (ABS) of the head element, illustrating an example structure of each TMR head element in the embodiment of  FIG. 1 ; 
         FIG. 4  is a sectional view, seen from the ABS, illustrating the TMR head element of  FIG. 3 ; 
         FIG. 5  is a graph illustrating the measured result of initial resistances R 1  and resistances after the passage of current R 2  with respect to many TMR head elements; 
         FIG. 6  is a graph illustrating the measured result of initial resistances R 1  and resistances after the passage of current R 2  with respect to many TMR head elements; 
         FIG. 7  is a graph illustrating the measured result of nonoccurrence of RTN and occurrence of RTN with respect to many TMR head elements; 
         FIG. 8  is a graph illustrating the measured result of breakdown voltages by increasing applied voltages to the TMR head elements contained in groups A and B; 
         FIG. 9  is a graph illustrating the measured result of breakdown voltages by increasing applied voltages to the TMR head elements contained in groups A and B; 
         FIG. 10  is a view schematically illustrating a partial configuration of testing a TMR head element as another embodiment according to the present invention; and 
         FIG. 11  is a view schematically illustrating a partial configuration of testing a TMR head element as further embodiment according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  schematically illustrates a configuration of testing a TMR head element as a preferred embodiment according to the present invention. 
     In the figure, reference numeral  10  denotes a bar member or bar block with a plurality of aligned TMR heads that are not individually separated yet, and  11  denotes a testing apparatus of the TMR head element. 
     The bar member  10  is provided by forming a large number of TMR heads arranged in matrix on a wafer according to thin-film technology, by cutting the wafer into bar shaped members so that each member has the aligned magnetic heads, and by grinding the ABS of the head member  10  so as to adjust gap-depth (MR height) of the heads. Each TMR head  10   a  of the bar member  10  is provided with a TMR read head element, an inductive write head element, a pair of terminal pads  10   b  electrically connected to the TMR read head element, and a pair of terminal pads  10   c  electrically connected to the inductive write head element. 
     The testing apparatus  11  has a pair of probes  11   a  that can be electrically can be electrically contact to the pair of terminal pads  10   b  of the TMR head element, a constant voltage supply circuit  11   b  electrically connected to the pair of probe  11   a , for supplying a constant voltage to the TMR head element, a current measurement circuit  11   c  electrically connected to the pair of probe  11   a , for measuring current flowing through the TMR head element, an analog to digital (A/D) converter  11   d  electrically connected to the current measurement circuit  11   c , for converting its analog output into digital signals, and a digital computer lie electrically connected to the A/D converter  11   d  and the constant voltage supply circuit  11   b . The digital computer  11   e  sequentially receives the digital signals from the A/D converter  11   d  and calculates an initial resistance R 1  and a resistance after the passage of current R 2  so as to judge whether or not the TMR head element is defective. Also, the digital computer  11   e  controls the operations of the constant voltage supply circuit  11   b  and the A/D converter  11   d.    
       FIG. 2  illustrates a testing procedure of the testing apparatus  11  in this embodiment. 
     As shown in the figure, first, the pair of probes  11   a  are brought into electrical contact with the terminal pads  10   b  of the TMR head element to be tested in the bar member  10 , and under this state application of a constant voltage of 150 mV for example to the TMR head element from the constant voltage supply circuit  11   b  is started (Step S 1 ). In this case, the voltage should be applied so that the current certainly flows upward in the laminating direction (from the substrate side to the upper side in the laminating direction) of the TMR head element irrespective of the laminated order of TMR multi-layers. 
     Then, a value of the current flowing through the TMR head element is measured and the measured current value is input into the computer  11   e  to calculate a resistance value of the TMR head element (Step S 2 ). The resistance value can be easily calculated from the applied constant voltage, for example 150 mV, and from the measured current value. The calculated resistance value is stored in the computer  11   e.  Particularly, initially obtained resistance value is stored as a resistance R 1 . 
     Then, it is judged whether or not a predetermined period of time such as for example 2–3 minutes (2 minutes and 15 seconds in this embodiment) has lapsed from the start of the application of the constant voltage (Step S 3 ). If not, namely the predetermined period has not lapsed yet, the measurement and calculation process at Step S 2  is repeatedly executed. If it is yes, namely the predetermined period has lapsed, a process of next Step S 4  is executed. 
     At Step S 4 , a resistance value is calculated based upon the last measured current value, the calculated resistance is stored in the computer  11   e  as a resistance R 2 , and then the application of the constant voltage is finished. 
     As a result, the resistance value just after the constant voltage application is started is stored as the resistance R 1 , and the resistance value just before the constant voltage application is terminated is stored as the resistance R 2 . The resistance R 1  corresponds to an initial resistance value of the TMR head element before a current flows, and the resistance R 2  corresponds to a resistance value of the TMR head element after a current continuously flows through the TMR head element for a predetermined period of time. 
     Thereafter, whether or not a rate of change in resistance (R 2 /R 1 )×100(%) is larger than a predetermined threshold in a range of 97.5–98.5(%) is judged (Step S 5 ). 
     If yes, namely the rate of change in resistance is larger than the threshold, it is evaluated that the TMR head element is non-defective product (Step S 6 ). If not, namely the rate of change in resistance is not larger than the threshold, it is evaluated that the TMR head element is defective product (Step S 7 ). 
     Then, the remaining TMR head elements in the bar member  10  are sequentially evaluated in the same manner. 
       FIGS. 3 and 4  illustrate an example structure of each TMR head element in this embodiment.  FIG. 3  shows a section seen along a direction orthogonal to the ABS of the TMR head element and  FIG. 4  shows is a section seen from the ABS. 
     As shown in these figures, a TMR film of the TMR head element has a multilayered structure of an anti-ferromagnetic layer (PtMn 15 nm)  32 , a pin layer (CoFe 2 nm/Ru 0.8 nm/CoFe 3 nm)  33 , a barrier layer (Al 0.575 nm−Ox)  34 , a free layer (CoFe 2 nm/NiFe 3 nm) and a cap layer (Ta)  36  sequentially laminated on a lower shield layer (NiFe)  30  through a buffer layer (Ta/NiFe)  31 . On the TMR film, a metal gap layer  37  and an upper shield layer  38  are laminated. Bias layers  39  are formed in lateral regions located in the track-width direction of the TMR film. In modifications, the order of lamination of the TMR multi-layers may be in inverse. 
     Hereinafter, why the procedure shown in  FIG. 2  is possible to evaluate non-defective/defective with respect to the TMR head element will be described. 
       FIGS. 5 and 6  illustrate the measured result of initial resistances R 1  and resistances after the passage of current R 2  with respect to many TMR head elements. The lateral axis of  FIG. 5  indicates an initial resistance R 1  (Ω) and the longitudinal axis thereof indicates a rate of change in resistance R 2 /R 1  (%) that is (resistance after the passage of current)/(initial resistance). The lateral axis of  FIG. 6  indicates a rate of change in resistance R 2 /R 1  (%) and the longitudinal axis thereof indicates its frequency (%). 
     As shown in these figures, the distribution of the rate of change in resistance R 2 /R 1  that is the changed ratio of resistance after the passage of current with respect to the initial resistance of the TMR head element polarized into groups A and B. Also, from these figures, it was revealed that the threshold between the groups A and B would be a predetermined value of the rate of change in resistance R 2 /R 1  within a range of R 2 /R 1 =97.5–98.5(%). 
     Then, stability and reliability of the TMR head elements in each group were confirmed. 
     First, operation test using a DP tester was performed for the TMR head elements in each group so as to measure reproduced outputs from the TMR head elements and to confirm whether or not any RTN was contained in the reproduced outputs. The measured and confirmed result is shown in  FIG. 7 . In the figure, the lateral axis indicates an initial resistance R 1  (Ω) and the longitudinal axis indicates a rate of change in resistance R 2 /R 1  (%). As will be noted from  FIGS. 5 and 7 , most of the RTN-nonoccurred and thus stable TMR head elements were contained in the group A of  FIG. 5 , and most of the RTN-occurred and thus unstable TMR head elements were contained in the group B of  FIG. 5 . 
     Also, breakdown voltages were checked for the TMR head elements A 1 –A 5  contained in the group A and the TMR head elements B 1 –B 5  contained in the group B by gradually increasing the applied voltages and measuring their resistances. The checked result is shown in  FIGS. 8 and 9 . In  FIG. 8 , the lateral axis indicates an applied voltage (mV) and the longitudinal axis indicates a resistance (Ω). In  FIG. 9 , the lateral axis indicates a rate of change in resistance R 2 /R 1  (%) and the longitudinal axis indicates a breakdown voltage (mV). As will be noted from these figures, all the TMR head elements A 1 –A 5  contained in the group A had breakdown voltages higher than those of the TMR head elements B 1 –B 5  contained in the group B, and thus had higher degree of reliability. 
     Therefore, it can be evaluated that TMR head elements having rates of change in resistance R 2 /R 1 , namely changed ratios of resistances after the passage of current with respect to the initial resistances, higher than a predetermined threshold that is within a range of 97.5–98.5(%), for example of 98.0(%), are highly stable and-reliable, and thus are non-defective products. Contrary to this, it can be also evaluated that TMR head elements having rates of change in resistance R 2 /R 1  lower than the predetermined threshold that is within a range of 97.5–98.5(%), for example of 98.0(%), are unstable and unreliable because pin holes may be formed in their barrier layers, and thus are defective products. 
     As will be understood, according to this embodiment, confirmation of stability and reliability of the TMR head element and therefore evaluation test of non-defective and defective products can be extremely easily performed. Also, according to this embodiment, because this evaluation test can be performed without destroying the TMR head element, 100% inspection of the manufactured TMR head elements becomes possible. 
     In the aforementioned embodiment, in order to know an initial resistance and a resistance after the passage of current, a constant voltage is applied to the TMR head element and a current flowing through the TMR head element is measured. However, according to the present invention, the same can be obtained by feeding a constant current through the TMR head element and by measuring a voltage across the TMR head element. 
       FIG. 10  schematically illustrates a partial configuration of testing a TMR head element as another embodiment according to the present invention. 
     In the embodiment of  FIG. 1 , the test of the TMR head element is conducted for the MR-height adjusted bar member with the aligned TMR heads not individually separated. However, in this embodiment, the test is performed for an individual magnetic head slider  100  separated from a bar member by electrically contacting a pair of probes  101   a  to a pair of terminal pads  100   b  of the TMR head element. Another configuration of the testing apparatus, operations and advantages of this embodiment are substantially the same as those of the embodiment of  FIG. 1 . 
       FIG. 11  schematically illustrates a partial configuration of testing a TMR head element as further embodiment according to the present invention. 
     In this embodiment, the test is performed for a head gimbal assembly (HGA) consisting of a suspension  112  and a magnetic head slider  110  mounted on the suspension  112  by electrically contacting a pair of probes  111   a  to a pair of connection pads  112   a  that are electrically connected to a TMR head element. Another configuration of the testing apparatus, operations and advantages of this embodiment are substantially the same as those of the embodiment of  FIG. 1 . 
     Although the aforementioned embodiments concern the method and apparatus for testing the TMR head element, it is apparent that the present invention can be similarly applied to a test of an MRAM. 
     Many widely different embodiments of the present invention may be constructed without departing from the spirit and scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.