Tools and methods for moke metrology with consistent MRAM die orientation

Abstract

This invention discloses a metrology tool comprising a magnet for generating a magnetic field and a stage system for positioning a plurality of MRAM dies on an MRAM wafer within the magnetic field. The stage system includes a chuck for mounting the MRAM wafer thereon. The metrology tool further includes optics for providing and guiding a laser beam onto the respective MRAM dies positioned within the magnetic field. The metrology tool additionally includes a detector for receiving the laser beam reflected by the respective MRAM dies and measuring the rotation of the polarization of the reflected laser beam. The metrology tool can be configured to provide a common orientation of the polarization of the MRAM dies on the MRAM wafer relative to the laser beam.

Description

[Technical Field] This invention relates to the magneto-optical Kerr effect (MOKE), and more specifically, to MOKE measurement for characterizing magnetic random access memory (MRAM). [Previous Technology] The magneto-optical Kerr effect (MOKE) refers to the rotation of the polarization of light reflected from a magnetic surface. MOKE measurements can be used to characterize both continuous and patterned thin films. For example, MOKE measurements are used to characterize magnetic random access memory (MRAM) devices, where the magnetic thin film is patterned into cells. MOKE measurements can be performed before and after patterning to measure the effect of patterning on the magnetic properties of the device. MOKE measurements are also known as MOKE metrology. Figure 1 is a schematic diagram of a MOKE measurement system 100. The MOKE measurement system 100 includes a magnet 102 having a first pole piece 104-1 and a second pole piece 104-2. The magnet 102 (which is an electromagnet) also includes respective conductive coils 106-1 and 106-2 wound around the pole pieces 104-1 and 104-2. A wafer 110 is positioned in the magnet between the pole pieces 104-1 and 104-2. (When the wafer 110 is positioned between the pole pieces 104-1 and 104-2, a portion of the wafer may extend laterally beyond the bottom edges of the pole pieces 104-1 and 104-2, as shown in Figure 1). The wafer 110 may be one of an MRAM wafer (e.g., MRAM wafer 300, Figure 3) divided into a plurality of MRAM dies (e.g., MRAM die 302, Figure 3). Magnet 102 generates a magnetic field (H) 108 that is normal (i.e., perpendicular) to the surface of wafer 110. If wafer 110 is an MRAM wafer, then magnetic field 108 is perpendicular to the surface of the MRAM die on the wafer. The MOKE measurement system 100 also includes optics to provide and guide a laser beam 114 onto a wafer 110, which is positioned in a magnet between electrodes 104-1 and 104-2. The laser beam 114 is orthogonally incident on the wafer 110 (i.e., perpendicular to the surface of the wafer 110). The optics include a laser 112 for generating the laser beam 114, a polarizer 116 for polarizing the laser beam 114, and mirrors 118-1 and 118-2 for guiding the laser beam 114 onto the wafer 110. Although Figure 1 shows two mirrors 118-1 and 118-2, the number and arrangement of mirrors 118 may vary for different embodiments of the MOKE measurement system 100. For example, the laser beam 114 polarized by polarizer 116 may have a specific polarization (e.g., a specific polarization plane). For MOKE measurements, a laser beam 114 is provided and guided onto a wafer 110, which is positioned within the magnetic field 108, when a magnetic field 102 generates a magnetic field 108 (e.g., when current is applied to conductive coils 106-1 and 106-2). The laser beam 114 is reflected by the wafer 110 (e.g., by one of the MRAM arrays 304 in an MRAM die 302, FIG. 3), thereby generating a reflected laser beam 120 received by a detector 122. The polarization (e.g., polarization plane) of the reflected laser beam 120 is rotated relative to the polarization (e.g., polarization plane) of the laser beam 114 due to MOKE. The detector 122 measures this rotation. The detector 122 includes an analyzer combined with a polarizer 116 for measuring the rotation. Based on these MOKE measurements, various magnetic properties of interest (such as coercivity, saturation magnetization, exchange coupling, domain structure, spin polarization, switching curves, and / or hysteresis curves) can be determined. The MOKE measurement system 100 measures polarity MOKE, wherein the magnetic field 108 and the laser beam 114 are both orthogonal to the surface of the wafer 110. Polarity MOKE measurement is used to characterize MRAM devices with vertical magnetic tunneling junction (MTJ) stacks, such as spin torque transfer (STT) MRAM devices. Figure 2 is a perspective view of a stage system 200 conventionally used to position a wafer 110 between electrodes 104-1 and 104-2 in a magnet 102 (Figure 1). The stage system 200 includes a rotating chuck 202 on which the wafer 110 can be mounted. A motor 208 rotates the chuck 202 about a central axis of rotation of the chuck 202. The central axis is the axis of rotation of the chuck 202. The chuck 202 is coupled to a translational stage 204 (largely obstructed by the chuck in Figure 2) that can be moved (i.e. translated) along a track 206 by a motor 210. The stage system 200 thus has a single translational axis in the direction along which the chuck 202 can be translated (i.e., parallel to the track 206). By combining the rotation of chuck 202 with the translation of chuck 202 along a single translation axis, different regions of wafer 110 (e.g., individual dies on wafer 110) are continuously positioned in laser beam 114 (i.e., positioned to receive laser beam) and thus MOKE measurements of the regions (e.g., dies) are performed. Figure 3 is a hypothetical example of an MRAM wafer 300 divided into a plurality of MRAM dies 302. Each MRAM die 302 comprises a fully or partially fabricated MRAM device containing an MRAM cell array 304. (In Figure 3, the array 304 is shown for simplicity only for a single MRAM die 302.) The MRAM wafer 300 may be mounted on a chuck 202 (Figure 2) and positioned on a magnet 102 of a MOKE measurement system 100 between electrodes 104-1 and 104-2 (Figure 1). In Figure 3, the wafer 300 is positioned such that a beam spot 306 of a laser beam 114 (Figure 1) is incident on an array 304 of MRAM dies 302. The beam spot 306 may have a spot size of approximately 1 millimeter or substantially smaller (e.g., approximately several micrometers). By rotating the chuck 202 and translating it along a single translation axis of the stage system 200 (Figure 2), each MRAM die 302 can be continuously positioned so that the beam point 306 is continuously incident on the MRAM die 302. When positioned in the magnetic field 108 (Figure 1), each MRAM die 302 can receive and reflect the laser beam 114, and MOKE measurements of each MRAM die 302 can be performed accordingly. [Summary of the Invention] Using the stage system 200 (FIG. 2) and its single translation axis, each MRAM die 302 is continuously positioned in a beam spot 306 (FIG. 3) on the MRAM wafer 300, resulting in different orientations of the different MRAM dies 302 on the MRAM wafer 300 relative to the polarization of the laser beam 114 (e.g., relative to the polarization plane of the laser beam 114). Therefore, the polarization orientation of the MRAM die 302 relative to the laser beam 114 varies depending on the position of the MRAM die 302 on the MRAM wafer 300. However, the signal-to-noise ratio (SNR) measured by MOKE of the MRAM die varies depending on the polarization orientation of the MRAM die relative to the laser beam 114. This wafer position dependence of relative orientation originates from the patterning of the magnetic film on the MRAM die to form a cell, the exposed portion of the underlying layer (e.g., the bottom electrode). Some relative orientations cause a sharp decrease in the SNR. For example, the signal-to-noise ratio of some relatively oriented components can be reduced below a threshold used to perform MOKE measurements. Therefore, MOKE measurements can be limited to sampling a portion of the MRAM dies on a wafer, and some dies may have more noise than others. Therefore, a method and system are needed to measure the MOKE of individual MRAM dies (e.g., all MRAM dies) on one MRAM wafer, such that they are MRAM dies with a common orientation relative to the polarization of the laser beam used for MOKE measurement. In some embodiments, a metrology tool includes a magnet for generating a magnetic field. The metrology tool also includes a stage system for positioning a plurality of MRAM dies on an MRAM wafer within the magnetic field. The stage system includes a chuck on which the MRAM wafer is mounted. The metrology tool further includes optics for providing and guiding a laser beam to be incident on the respective MRAM dies on the MRAM wafer when each MRAM die is positioned within the magnetic field. The laser beam has a polarization. The metrology tool additionally includes a detector for receiving the laser beam reflected from the respective MRAM dies on the MRAM wafer and measuring the rotation of the polarization of the reflected laser beam. The metrology tool can be configured to provide a common orientation of the MRAM dies relative to the polarization of the laser beam when the laser beam is incident on the MRAM dies on the MRAM wafer when each MRAM die is positioned within the magnetic field. In some embodiments, a method performed in a metrology instrument includes generating a magnetic field and positioning an MRAM wafer comprising a plurality of MRAM dies within the magnetic field. With the MRAM wafer positioned within the magnetic field, a laser beam is provided incident on the MRAM wafer. The laser beam has a polarization. The respective MRAM dies on the MRAM wafer are sequentially positioned such that the laser beam is sequentially incident on each of the respective MRAM dies while they are positioned within the magnetic field. As the laser beam is incident on the MRAM dies, each of the respective MRAM dies is oriented to have a common orientation relative to the polarization of the laser beam. For each of the respective MRAM dies, the rotation of the polarization of the laser beam reflected from the MRAM die is measured. In some embodiments, a metrology tool includes a magnet for generating a magnetic field. The metrology tool also includes a stage system for positioning a plurality of MRAM dies on an MRAM wafer within the magnetic field. The stage system includes a chuck on which the MRAM wafer is mounted. The metrology tool further includes optics for providing and guiding a laser beam to be incident on the respective MRAM dies on the MRAM wafer, with each MRAM die positioned within the magnetic field. The laser beam has a polarization. The metrology tool additionally includes a detector for receiving the laser beam reflected from the respective MRAM dies on the MRAM wafer and measuring the rotation of the polarization of the reflected laser beam. The metrology tool further includes one or more processors and memory storing one or more programs to be executed by the one or more processors. The one or more programs include instructions for providing a common orientation of the MRAM die relative to one of the polarizations of the laser beam when the laser beam is incident on the MRAM die on the MRAM wafer with each MRAM die positioned in the magnetic field.

Implementation Method

Claims

1. A measuring instrument comprising: A magnet is used to generate a magnetic field; A stage system for positioning an MRAM wafer containing a plurality of magnetic random access memory (MRAM) chips in a magnetic field, the stage system comprising: a chuck for mounting the MRAM wafer thereon, the chuck being rotatable about a rotation axis; a first translation stage coupled thereto to cause the chuck to translate along a first translation axis; and a second translation stage coupled thereto to cause the chuck and the first translation stage to translate along a second translation axis; and an optical device for providing and guiding a laser beam to incident on the MRAM wafer when the MRAM wafer is positioned in the magnetic field, the optical device comprising a... A polarizer is used to polarize the laser beam with a specific polarization, wherein the stage system is used to: rotate the chuck to orient the plurality of MRAM dies to a common orientation having one of the specific polarizations relative to the laser beam; and, with the chuck rotated to orient the plurality of MRAM dies to the common orientation, translate the chuck along the first translation axis and the second translation axis to continuously position each of the plurality of MRAM dies to receive the laser beam; and a detector is used to receive the laser beam reflected by each of the plurality of MRAM dies and to measure rotation of one of the polarizations of the reflected laser beam relative to the specific polarization.

2. The tool as claimed in claim 1, wherein the stage system further includes: A first motor is used to move the first translation stage along the first translation axis; A second motor for moving the second translation stage along the second rotation axis; and a third motor for rotating the chuck about the rotation axis.

3. The tool of claim 1, wherein the first translation axis and the second translation axis are perpendicular.

4. The tools as requested in item 1, wherein: The magnet is configured to generate the magnetic field perpendicular to one surface of the MRAM wafer when the MRAM wafer is positioned in the magnetic field; and the optical devices are configured to guide the laser beam to be orthogonally incident on each of the plurality of MRAM wafers when each MRAM die is sequentially positioned to receive the laser beam.

5. The tools as requested in item 1, wherein: The optical devices further include a mirror for manipulating the laser beam to be incident on the MRAM wafer; and the detector includes an analyzer.

6. A method for metrology, the method comprising, in a metrology instrument: generating a magnetic field; positioning a magnetic random access memory (MRAM) wafer in the magnetic field, the MRAM wafer comprising a plurality of MRAM chips; with the MRAM wafer positioned in the magnetic field, providing a laser beam incident on the MRAM wafer, the laser beam having a polarization, wherein providing the laser beam includes generating the laser beam using a laser and polarizing the laser beam using a polarizer; translating the MRAM wafer along a single translation axis and rotating the MRAM wafer about a rotation axis to continuously rotate the MRAM chips of the plurality of MRAM chips. Positioning to receive the laser beam; rotating the laser and the polarizer about their respective longitudinal axes by an equal amount to change one polarization of the laser beam, providing common orientation of each MRAM die relative to one polarization of the laser beam when each MRAM die is positioned to receive the laser beam; receiving the laser beam reflected by each MRAM die of the plurality of MRAM dies at a detector, including rotating the detector by an amount equal to the equal amount used for the laser and the polarizer; and using the detector to measure the rotation of one polarization of the reflected laser beam relative to the polarization of the laser beam received by each MRAM die.

7. As in request item 6, where: Providing the laser beam includes guiding the laser beam orthogonally incident on each of the plurality of MRAM dies when each MRAM die is successively positioned to receive the laser beam; and generating the magnetic field includes configuring the magnetic field orthogonal to one surface of the MRAM wafer when the MRAM wafer is positioned in the magnetic field.

8. The method of claim 6, further comprising mounting the MRAM wafer on a chuck coupled to a single translation stage, wherein: Positioning the MRAM wafer in the magnetic field includes positioning the chuck on which the MRAM wafer is mounted in the magnetic field; rotating the MRAM wafer includes rotating the chuck; and translating the MRAM wafer includes translating the chuck along a single translation axis using a single translation stage.

9. A measuring instrument comprising: A magnet is used to generate a magnetic field; A stage system for positioning an MRAM wafer containing a plurality of magnetic random access memory (MRAM) chips in a magnetic field, the stage system comprising: a chuck for mounting the MRAM wafer thereon, the chuck being rotatable about a rotation axis; and a single translation stage coupled thereto to the chuck to allow the chuck to translate along a single translation axis; and optics for providing and guiding a laser beam to be incident on the MRAM wafer while it is positioned in the magnetic field, the optics including a laser and a polarizer, the laser for generating the laser beam and the polarizer for polarizing the laser beam, wherein: the stage system translates the chuck along the single translation axis and about the rotation axis The chuck is rotated to continuously position each of the plurality of MRAM dies to receive the laser beam; and the laser and the polarizer are rotatable about their respective longitudinal axes by an equal amount to change one polarization of the laser beam, providing common orientation of each of the plurality of MRAM dies relative to the polarization of the laser beam when each MRAM die is positioned to receive the laser beam; and a detector is used to receive the laser beam reflected by each of the plurality of MRAM dies on the MRAM wafer and to measure the rotation of one polarization of the reflected laser beam relative to the polarization of the laser beam received by each MRAM die, wherein the detector is rotatable by an amount equal to that used for the laser and the polarizer.

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