Method and system for measuring thermal stability factor of magnetic tunnel junction device, semiconductor integrated circuit, and production management method for semiconductor integrated circuit

Abstract

A method and a system for measuring the thermal stability factor of a magnetic tunnel junction device, a semiconductor integrated circuit, and a production management method for the semiconductor integrated circuit, capable of measuring the thermal stability factors of individual devices in a relatively short period of time and quickly performing quality control during material development and at a production site. A meter measures change in resistance value of an evaluation MTJ for a predetermined period while causing a predetermined current to flow into the evaluation MTJ maintained at a predetermined temperature. An analyzer calculates a time constant in which a low-resistance state is maintained and a time constant in which a high-resistance state is maintained from the measured change in resistance value. A thermal stability factor of the evaluation MTJ is calculated on the basis of the calculated time constants and the predetermined current flowing into the evaluation MTJ.

Claims

1. A method for measuring a thermal stability factor of a magnetic tunnel junction device, comprising: a measurement step of measuring change in resistance value of the magnetic tunnel junction device for a predetermined period while causing a predetermined current to flow into the magnetic tunnel junction device maintained at a predetermined temperature; a first calculation step of calculating a time constant in which a low-resistance state is maintained and a time constant in which a high-resistance state is maintained from the change in resistance value measured in the measurement step, the first calculation step comprising calculating a frequency distribution N.sub.P(t1) of a period t1 in which the low-resistance state is maintained and a frequency distribution N.sub.AP(t2) of a period t2 in which the high-resistance state is maintained from the change in resistance value measured in the measurement step and calculating the time constant .sub.P in which the low-resistance state is maintained and the time constant .sub.AP in which the high-resistance state is maintained using relations of N.sub.P(t1) exp(t1/.sub.P) and N.sub.AP(t2) exp(t2/.sub.AP), respectively; and a second calculation step of calculating a thermal stability factor of the magnetic tunnel junction device based on the predetermined current and the time constants calculated in the first calculation step.

2. The method for measuring the thermal stability factor of the magnetic tunnel junction device according to claim 1, wherein the measurement step comprises measuring the change in resistance value for each of a plurality of currents of different magnitudes while supplying the plurality of currents sequentially as the predetermined current, the first calculation step comprises calculating the time constant .sub.P in which the low-resistance state is maintained and the time constant .sub.AP in which the high-resistance state is maintained for each of the currents, the second calculation step comprises calculating the thermal stability factor based on the magnitude of each of the currents and the time constants .sub.P and .sub.AP corresponding to each of the currents.

3. The method for measuring the thermal stability factor of the magnetic tunnel junction device according to claim 1, wherein the second calculation step comprises calculating a thermal stability factor .sub.0 and a threshold current value I.sub.c0, based on the predetermined current I and the time constant .sub.P in which the low-resistance state is maintained and the time constant .sub.AP in which the high-resistance state is maintained calculated in the first calculation step, using an equation of .sub.P/(.sub.P.sub.AP)=1/[1+exp{.sub.0.Math.(2I/I.sub.c0)}].

4. The method for measuring the thermal stability factor of the magnetic tunnel junction device according to claim 1, wherein the measurement step comprises measuring the change in resistance value while causing the predetermined current to flow into the magnetic tunnel junction device from a testing terminal which is electrically connected to the magnetic tunnel junction device only so that current flows into the magnetic tunnel junction device only.

5. A production management method for a semiconductor integrated circuit including a plurality of magnetic tunnel junction devices, comprising calculating a thermal stability factor of one or a plurality of the plurality of magnetic tunnel junction devices according to the method for measuring the thermal stability factor of the magnetic tunnel junction device according to claim 1; and managing a process of manufacturing the plurality of magnetic tunnel junction devices based on the calculated thermal stability factor.

6. The production management method for the semiconductor integrated circuit according to claim 5, wherein the semiconductor integrated circuit comprises: a testing terminal which is electrically connected to a single magnetic tunnel junction device so that current flows into the single magnetic tunnel junction device only.

7. A system for measuring a thermal stability factor of a magnetic tunnel junction device, comprising: a temperature controller that maintains the magnetic tunnel junction device at a predetermined temperature; a meter that measures change in resistance value of the magnetic tunnel junction device for a predetermined period while causing a predetermined current to flow into the magnetic tunnel junction device; and an analyzer configured to: calculate a time constant in which a low-resistance state is maintained and a time constant in which a high-resistance state is maintained from the change in resistance value measured by the meter and calculates a thermal stability factor of the magnetic tunnel junction device based on the time constants and the predetermined current; and calculate a frequency distribution N.sub.P(t1) of a period t1 in which a low-resistance state is maintained and a frequency distribution N.sub.AP(t2) of a period t2 in which a high-resistance state is maintained from the change in resistance value measured by the meter and calculate the time constant .sub.P in which the low-resistance state is maintained and the time constant .sub.AP in which the high-resistance state is maintained using relations of N.sub.P(t1) exp(t1/.sub.P) and N.sub.AP(t2) exp(t2/.sub.AP), respectively.

8. The system for measuring the thermal stability factor of the magnetic tunnel junction device according to claim 7, wherein the meter is configured to measure the change in resistance value for each of a plurality of currents of different magnitudes while supplying the plurality of currents sequentially as the predetermined current, and the analyzer is configured to calculate the time constant .sub.P in which the low-resistance state is maintained and the time constant .sub.AP in which the high-resistance state is maintained for each of the currents and calculate the thermal stability factor based on the magnitude of each of the currents and the time constants .sub.P and .sub.AP corresponding to each of the currents.

9. The system for measuring the thermal stability factor of the magnetic tunnel junction device according to claim 7, wherein the analyzer is configured to calculate a thermal stability factor .sub.0 and a threshold current value I.sub.c0, based on the predetermined current I and the time constant .sub.P in which the low-resistance state is maintained and the time constant .sub.AP in which the high-resistance state is maintained calculated in the first calculation step, using an equation of .sub.P/(.sub.P+.sub.AP)=1/[1+exp{.sub.0.Math.(2I/I.sub.c0)}].

10. The system for measuring the thermal stability factor of the magnetic tunnel junction device according to claim 7, wherein the system comprises a testing terminal which is electrically connected to the magnetic tunnel junction device only so that current flows into the magnetic tunnel junction device only, and the meter is configured to measure the change in resistance value while causing the predetermined current to flow into the magnetic tunnel junction device from the testing terminal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a perspective view illustrating a semiconductor integrated circuit according to an embodiment of the present invention.

(2) FIG. 2 is a flowchart illustrating a process of manufacturing the semiconductor integrated circuit according to the embodiment of the present invention.

(3) FIG. 3 is a perspective view illustrating an evaluation MTJ of the semiconductor integrated circuit according to the embodiment of the present invention and is a block diagram illustrating a system for measuring a thermal stability factor of the magnetic tunnel junction device according to the embodiment of the present invention.

(4) FIG. 4(a) is a plan view in which evaluation MTJs are arranged at corners of a memory chip of the semiconductor integrated circuit according to the embodiment of the present invention, and FIG. 4(b) is a plan view in which evaluation MTJs are arranged in respective sub-arrays in the memory chip.

(5) FIG. 5 is a perspective view illustrating an MTJ array structure of the semiconductor integrated circuit according to the embodiment of the present invention.

(6) FIG. 6(a) illustrates an example of an observed waveform of a resistance value of an evaluation MTJ observed by an oscilloscope according to a method for measuring the thermal stability factor of a magnetic tunnel junction device according to an embodiment of the present invention, FIG. 6(b) is a graph illustrating a frequency distribution of a period in which a low-resistance state is maintained and a period in which a high-resistance state is maintained and an example of a curve obtained by fitting, and FIG. 6(c) is a graph illustrating a relation between a current flowing into the evaluation MTJ and the time constants for the low-resistance state and the high-resistance state and an example of a curve obtained by fitting.

DETAILED DESCRIPTION OF THE INVENTION

(7) Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

(8) FIGS. 1 to 6 illustrate a method and a system for measuring a thermal stability factor of a magnetic tunnel junction device, a semiconductor integrated circuit, and a production management method for the semiconductor integrated circuit according to an embodiment of the present invention.

(9) Semiconductor Integrated Circuit Manufacturing Process

(10) As illustrated in FIG. 1, a semiconductor integrated circuit 10 of an embodiment of the present invention is formed of an MRAM (magnetoresistance memory) having a number of MTJs (magnetic tunnel junction devices) 11 and is manufactured according to a manufacturing process illustrated in FIG. 2.

(11) As illustrated in FIGS. 1 and 2, first, a wafer 12 serving as a substrate is loaded (step 21), a CMOS 13 is fabricated on a surface thereof (step 22), and a multi-layered intermediate wiring 14 (for example, M1-M4 wiring) for connection to the MTJs 11 is formed on the fabricated CMOS 13 (step 23). Generally, the manufacturing process up to this step 23 takes approximately two months. Subsequently, a magnetic film is formed by PVD (physical vapor deposition) on an upper part of the intermediate wiring 14 (step 24), and a film property of the magnetic film is examined (step 25). After that, an MTJ pattern is formed by lithography (step 26) and the shape of the lithography is examined (step 27). After that, the MTJs 11 are formed by etching (step 28), and the shape of the MTJs 11 is examined (step 29). Generally, the MTJ manufacturing process of steps 24 to 29 takes approximately one week.

(12) After the shape of the MTJs 11 is examined, an upper wiring 15 is formed on the MTJs 11 (step 30). Generally, this step 30 ends in two weeks. After the upper wiring 15 is formed, electrical characteristics of single MTJs 11 are examined (step 31), and the characteristics of an MTJ array are examined (step 32). Examples of the examined electrical characteristics of the MTJs 11 include a resistance value, thermal stability, a writing current, an error rate, and the like. Examples of the examined characteristics of the MTJ array include a bit error rate, a resistance distribution, and the like.

(13) In step 30, as illustrated in FIG. 3, a testing terminal block 17, which includes the intermediate wiring 14, a connection region (VIA) 16, and the upper wiring 15, is formed so as to be electrically connected to an evaluation MTJ 11a only without being electrically connected to the CMOS 13 illustrated in FIG. 1 or the intermediate wiring 14 connected to the CMOS 13. In the example illustrated in FIG. 3, the testing terminal block 17 is made up of a pair of terminals. One terminal is formed of the upper wiring 15 formed in an upper part of a single evaluation MTJ 11a, and the other terminal includes an intermediate wiring (BASE) 14 connected to a lower part of the evaluation MTJ 11a, the connection region 16 formed on the intermediate wiring 14, and the upper wiring 15. The testing terminal block 17 is configured to supply current to only the single evaluation MTJ lla electrically connected thereto and can be used when examining the electrical characteristics in step 31.

(14) One to several testing terminal blocks 17 may be disposed at a corner of a memory chip as a single TEG (test element group) as illustrated in FIG. 4(a), and one to several testing terminal blocks 17 may be arranged in respective sub-arrays in the memory chip as illustrated in FIG. 4(b). The disposed one to several testing terminal blocks 17 have a pad 17a connected to the upper wiring 15, which is completely independent from a pad group for realizing external electrical connection to bit lines and word lines of a memory array to be described later. A probe such as a prober for realizing electrical connection to various electrical measurement apparatuses, for example, comes into contact with this pad 17a to electrically measure the characteristics of the evaluation MTJ 11a. On the other hand, the pad group for realizing external electrical connection to bit lines and word lines of a memory array is connected to pins of a package via wires, for example. Therefore, it is not possible to measure the electrical characteristics of the single evaluation MTJ 11a using these pads.

(15) In this manner, the semiconductor integrated circuit 10 made up of MRAM memory chips can be manufactured according to the manufacturing method illustrated in FIG. 2.

(16) As illustrated in FIG. 5, the manufactured semiconductor integrated circuit 10 excluding the evaluation MTJ 11a is formed of MRAMs in which a plurality of bit lines 18 are wired as the upper wiring 15 of the MTJs 11 and a plurality of word lines 19 are wired in the upper part of the CMOS 13. The bit lines 18 are arranged in parallel to each other and are connected to a bit-line selection circuit 18a. The word lines 19 are arranged in parallel to each other and are connected to a word-line selection circuit 19a.

(17) In this semiconductor integrated circuit 10, when data is written to an arbitrary MTJ 11, the bit-line selection circuit 18a and the word-line selection circuit 19a apply a voltage to a predetermined bit line 18 and a predetermined word line 19, respectively, on the basis of a writing bit number. Moreover, when data is read from an arbitrary MTJ 11, the bit-line selection circuit 18a and the word-line selection circuit 19a select a predetermined bit line 18 and a predetermined word line 19, respectively, on the basis of a reading bit number and connect the selected bit line and word line to a sense amplifier 20. The examination of the characteristics of the MTJ array in step 32 of FIG. 2 can be performed using the circuit illustrated in FIG. 5.

(18) Measurement of Thermal Stability Factor

(19) Next, a method and a system for measuring the thermal stability factor of the magnetic tunnel junction device according to the embodiment of the present invention, for measuring the thermal stability factor among the electrical characteristics of the single evaluation MTJ 11a examined in step 31 of FIG. 2 will be described.

(20) As illustrated in FIG. 3, a system 40 for measuring the thermal stability factor of the magnetic tunnel junction device according to the embodiment of the present invention includes a temperature controller (not illustrated), a meter 41, and an analyzer 42.

(21) The temperature controller is formed of a prober capable of controlling temperature and has a wafer 12 including the evaluation MTJ 11a mounted thereon so that the evaluation MTJ can be maintained at a predetermined temperature. The meter 41 includes a pair of probes 41a of the prober of the temperature controller, a voltage pulse generator 41b, a reference resistance 41c, and a voltage meter 41d, for example. The voltage pulse generator 41b is connected to the respective probes 41a to apply a voltage pulse between the probes 41a. The reference resistance 41c is connected in series between one probe 41a and the voltage pulse generator 41b. The voltage meter 41d is formed of a meter such as an oscilloscope capable of measuring a voltage and is connected in parallel to the reference 41c to measure a voltage generated in the reference resistance 41c. For example, a DC current source and a trigger mechanism for designating a timing for generating a current from the DC current source may be used instead of the voltage pulse generator 41b.

(22) The meter 41 is configured such that the probes 41a are brought into contact with the testing terminal blocks 17 of the evaluation MTJ 11a, a predetermined voltage pulse is generated by the voltage pulse generator 41b and is applied across both ends of the evaluation MTJ 11a to cause a predetermined current to flow into the evaluation MTJ 11a. Moreover, a voltage generated between both ends of the reference resistance 41c generated due to the current is monitored and measured by the voltage meter 41d for a predetermined period to measure change in resistance value of the evaluation MTJ 11a.

(23) The analyzer 42 is formed of a computer and is connected to the voltage meter 41d to receive the measured values. The analyzer 42 calculates a time constant in which a low-resistance state is maintained and a time constant in which a high-resistance state is maintained from the change in resistance value measured by the voltage meter 41d. Moreover, the analyzer 42 calculates a thermal stability factor of the evaluation MTJ 11a on the basis of the calculated time constants and the current flowing into the evaluation MTJ 11a.

(24) The method for measuring the thermal stability factor of the magnetic tunnel junction device according to the embodiment of the present invention can be ideally performed by the system 40 for measuring the thermal stability factor of the magnetic tunnel junction device illustrated in FIG. 3. That is, in the method for measuring the thermal stability factor of the magnetic tunnel junction device according to the embodiment of the present invention, first, in a state in which the evaluation MTJ 11a is maintained at a predetermined temperature by the temperature controller, the meter 41 measures change in resistance value of the evaluation MTJ 11a for a predetermined period while causing a predetermined current to flow into the evaluation MTJ 11a. A specific measurement example in the voltage meter 41a formed of an oscilloscope in this case is illustrated in FIG. 6(a). A measurement temperature was 240 C. The measurement period in FIG. 6(a) was 500 msec. As illustrated in FIG. 6(a), it can be ascertained that the measured change in resistance value includes a low-resistance state and a high-resistance state.

(25) Subsequently, the analyzer 42 calculates a frequency distribution N.sub.P(t) of a period in which a low-resistance state is maintained and a frequency distribution N.sub.AP(t) of a period in which a high-resistance state is maintained from the measured change in resistance value and calculates a time constant .sub.P in which a low-resistance state is maintained and a time constant .sub.AP in which a high-resistance state is maintained from the decaying states of the frequency distributions using the relations of N.sub.P(t) exp(t,.sub.P) and N.sub.p(t) exp(t/.sub.AP), respectively, by fitting such as the least-squares method. The frequency distributions N.sub.P(t) and N.sub.AP(t) of the period in which a low-resistance state is maintained and the period in which a high-resistance state is maintained obtained by actual measurement and an example of a curve obtained by fitting are illustrated in FIG. 6(b).

(26) The meter 41 measures change in resistance value for each of a plurality of currents of different magnitudes while supplying the plurality of currents sequentially and the analyzer 42 calculates the time constant .sub.P in which the low-resistance state is maintained and the time constant .sub.AP in which the high-resistance state is maintained, corresponding to each of the currents.

(27) After the time constants .sub.P and .sub.AP for the plurality of currents of different magnitudes are calculated, a thermal stability factor .sub.0 and a threshold current value I.sub.c0 are calculated by fitting such as the least-squares method using Equation (4). A relation between each current and the left side .sub.P/(.sub.P+.sub.AP) of Equation (4) obtained by actual measurement and an example of the curve obtained by fitting are illustrated in FIG. 6(c). In the specific example illustrated in FIG. 6(c), the thermal stability factor .sub.0 was 24.69 and the threshold current value I.sub.c0 was 175.44 A when the temperature of the evaluation MTJ 11a was 240 C. The thermal stability factor at a room temperature is estimated to be 24.69(240+273)/(25+273)=43.5. In this manner, it is possible to calculate the thermal stability factor at all operating temperatures including a room temperature easily from measurements performed at high temperatures.

(28) In the embodiment, an example in which the change in resistance value for each of a plurality of currents of different magnitudes is measured while supplying the plurality of currents sequentially, and the analyzer 42 calculates the time constant .sub.P in which a low-resistance state is maintained and the time constant .sub.AP in which a high-resistance state is maintained, corresponding to each of the currents, has been described. However, a DC current sufficient for monitoring a resistance state may be used as the current, the time constant .sub.P in which the low-resistance state is maintained and the time constant .sub.AP in which the high-resistance state is maintained may be calculated for each magnetic field while changing a magnetic field pulse (intensity H), and the thermal stability factor .sub.0 and a threshold magnetic field H.sub.C0 may be calculated using Equation (5) below, for example.
.sub.P/(.sub.P+.sub.AP)=1/[1+exp{.sub.0.Math.(4H/H.sub.C0)}](5)

(29) In the measurement method which uses the current or the magnetic field, the temperature may be changed while fixing the current or magnetic field value. In this case, by changing the value of the temperature T in .sub.0=E.sub.b/k.sub.BT of Equation (4) or (5), E.sub.b can be calculated from the measurement result of different T.

(30) As described above, according to the method and the system 40 for measuring the thermal stability factor of the magnetic tunnel junction device according to the embodiment of the present invention, it is not necessary to generate a magnetic field during measurement and measurement is performed on the single evaluation MTJ 11a only rather than the entire chip having a number of devices. Therefore, it is possible to measure the thermal stability factor .sub.0 of the single evaluation MTJ 11a in a relatively short period of time. Moreover, since the value of the energy barrier E.sub.b does not change during measurement like the magnetic field pulse method and the current pulse method, it is possible to calculate an accurate thermal stability factor .sub.0 of the single evaluation MTJ 11a.

(31) Production Management of Semiconductor Integrated Circuit

(32) Next, a production management method for the semiconductor integrated circuit according to the embodiment of the present invention will be described.

(33) When the thermal stability factor of one or a plurality of evaluation MTJs 11a on the memory chip is calculated by the method and the system 40 for measuring the thermal stability factor of the magnetic tunnel junction device according to the embodiment of the present invention in step 31 of FIG. 2, management of the MTJ manufacturing process of steps 24 to 29 is performed on the basis of the thermal stability factor. For example, when a desired thermal stability factor is not obtained, the thermal stability factor may be fed back to step 24 and the conditions for manufacturing the MTJ 11 may be changed.

(34) According to the production management method for the semiconductor integrated circuit according to the embodiment of the present invention, it is not necessary to feed the management information based on the measurement result of the evaluation MTJ 11a back to the initial stage (step 21) of the chip manufacturing process. Due to this, it is possible to save a period from the start of the chip manufacturing process to the preceding stage (steps 21 to 23) of manufacturing the MTJ 11 as compared to a case of feeding the management information back to the initial stage of the chip manufacturing process. For example, in the manufacturing process illustrated in FIG. 2, it is possible to save two months for steps 21 to 23 and quickly perform quality control and production management during material development and at a production site.

REFERENCE SIGNS LIST

(35) 10: Semiconductor integrated circuit 11: MTJ 12: Wafer 13: CMOS 14: Intermediate wiring 15: Upper wiring 16: Connection region 17: Testing terminal block 17a: Pad 18: Bit line 18a: Bit-line selection circuit 19: Word line 19a: Word-line selection circuit 20: Sense amplifier 40: System for measuring thermal stability factor of magnetic tunnel junction device 41: Meter 41a: Probe 41b: Voltage pulse generator 41c: Reference 41d: Voltage meter 42: Analyzer