Ferromagnetic resonance (FMR) scanning for characterization of magnetic films and multilayers at the wafer level

Abstract

A ferromagnetic resonance (FMR) measurement system for a wafer under test (WUT) on which one or more magnetic films are formed, comprising: (a) a waveguide transmission line (WGTL) connected to a microwave generator and mounted on a mounting plate, the WGTL being substantially U-shaped and having a first end attached to a first RF connector and a second end attached to a second RF connector, a central tip section of the WGTL being arranged below an opening in the mounting plate exposing an upper surface of the one or more magnetic films; (b) a magnet providing a magnetic field configured, together with the simultaneous application of a microwave frequency, to induce a magnetic resonance state in the one or more magnetic films;and (c) a system for detecting microwave absorption by the one or more magnetic films when a magnetic resonance condition is present, wherein the WGTL and the WUT are movable laterally and vertically relative to each other, such that the central tip section of the WGTL contacts or is positioned near the exposed upper surface of the one or more magnetic films at predetermined positions.

Description

TECHNICAL FIELDThe present disclosure relates to a system for measuring magnetic properties in magnetic films and multilayers and a method for doing so, and more particularly to a waveguide transmission line (WGTL) mounted on a platform and installed in an electrical probe station to enable fully automatic FMR testing across whole wafers without the need to cut the wafers in technical or product environments.US 2014 / 0 097 841 A1 discloses an FMR measurement system for a wafer to be tested, on which a magnetic film is formed. The FMR measurement system includes a WGTL connected to a microwave generator; a magnet that provides a magnetic field configured, along with simultaneously applying a microwave frequency, to induce a magnetic resonance state in the magnetic film; and a system for detecting microwave absorption by the magnetic film when a magnetic resonance condition exists.BACKGROUNDMagnetic thin films and multilayers play a key role in various types of magnetic storage devices, such as a magnetic hard disk drive (HDD), a magnetic random access memory (MRAM), a spin torque oscillator (STO), and magnetic domain wall devices. To develop and optimize such devices, characterization of thin film magnetic stacks is necessary. To determine all essential magnetic parameters such as crystal anisotropy, surface or interfacial anisotropy, magnetization saturation (Ms), attenuation constant (α), gyromagnetic ratio (γ), inhomogeneous broadening, resistance x area product (RA) and magnetoresistive ratio (MR), various magnetic characterization techniques must be used.FMR is an established method for measuring anisotropy fields as well as gyromagnetic ratio γ and attenuation constant α of magnetic films and multilayers in extended films having no structure or structures having a sub-micron structure. The resonance frequency f R of a ferromagnetic film is given by the so-called Kittel formula shown in the following equation (1), wherein H R is the resonance field applied perpendicular to the film plane, H K is the effective anisotropy field comprising the structural, surface and magnetostatic contributions, and γ is the gyromagnetic ratio.An FMR experiment is performed by examining the magnetic system (thin film, multilayer stack or patterned device) with a combination of microwave excitation and a quasi-static magnetic field. FMR data is obtained either by sweeping the magnetic field at a constant microwave frequency or by sweeping the frequency at a constant field. Once the ferromagnetic resonance condition is reached, this can be detected by an enhanced absorption of the microwave by the ferromagnetic probe. Thus, resonant conditions (FMR) are defined by pairs of magnetic field and microwave frequency values (H R, f R).There are various ways to subject a ferromagnetic probe to microwave excitation. In the past, FMR test conditions used tubular waveguides and probes were placed in a resonant cavity between the poles of an electromagnet. Recently, new methods have been developed with which film-shaped probes can be analyzed well. In particular, the wafer under test (WUT) is brought into contact with a non-magnetic waveguide transmission line (WGTL), which may be in the form of a grounded coplanar waveguide (GCPWG), a coplanar waveguide (CPWG), a coaxial waveguide (CWG), a strip line (SL) or a microstrip (MS). The power transmitted or reflected from the WGTL is monitored as a function of the applied magnetic field and the microwave frequency.Referring to Fig. 1a, there is shown a schematic representation in which output voltages are plotted as a function of a variable magnetic field at constant microwave frequency using five different values (f1-f5) of microwave frequency. The center and width of the Lorentz peaks are extracted from the data as a function of the excitation frequency. As mentioned above, the center field is the resonance field (H R) which refers to the excitation frequency according to the Kitzer's formula rewritten in the following equation (2) in some other form, where h is the Planck constant and μ B is the Xensch's magneton.The variation of H R with microwave frequency is shown in Figure 3. In Figure 1b, each of the points along curve 21 is derived from one of the Lorentz peaks Hr1-Hr5in Figure 1a. As indicated by equation (2), extrapolation of the data to f = 0 yields the value of the effective anisotropy field H K.The linewidth L of the resonance peak is the width at half amplitude H of the resonance peak and is related to dissipative processes involved in magnetization dynamics.The linewidth depends on the excitation frequency and the dimensionless Gilbert attenuation constant α according to the following equation (3), where L o is an inhomogeneous broadening. By adjusting H R and L with respect to the excitation frequency f R H K as well as α and γ can be derived.A network analyzer for detecting FMR in thin CoFe and CoFeB films on a coplanar waveguide is described by C. Bilzer et al. in "Vector network analyzer ferromagnetic resonance of thin films on coplanar waveguides: Comparison of different evaluation methods" in Journal of Applied Physics 101, 074505 (2007) and in "Open-Circuit One-Port Network Analyzer Ferromagnetic Resonance" in IEEE Trans. Magn. Vol. 44, No. 11, p3265 (2008). In these experiments, the planar WGTL is typically attached to radio frequency (RF) connections by microwave electrical probes and placed between the poles of an electromagnet. Thus, given the size of the WGTL (about 5 mm long) and the size of the gap of typical electromagnets, only small size probes (usually <1 inch in diameter) can be measured. Accordingly, wafers typically used in the microelectronics industry (having diameters of 6, 8, 12 inches or more) can only be measured using this FMR technique when cut into small coupons.Fig. 2 is reproduced from "Microwave susceptibility of thin ferromagnetic films: metrology and insight into magnetization dynamics", Claus Bilzer, Ph.D. Report, University of Paris Sud - Paris XI, 2007) and shows a conventional FMR system. The vector network analyzer 30 is connected from one terminal via a first coaxial cable 31 to a first microwave probe portion 32a attached to the coplanar waveguide (CPWG) 36. A top surface of the CPWG abuts a magnetic film 34 mounted on a substrate. When a microwave frequency from the probe 32a and an external magnetic field 33 are applied in an x-axis direction across the magnetic film, an output signal in a transmission mode enters a second microwave probe portion 32b and then passes through a second coaxial cable 35 to a second port before returning to the VTA. The magnetic film size b is typically limited to 1 inch or less, which means that the magnetic film sample 34 must be cut from an entire wafer.Since conventional FMR techniques are destructive, impractical, and time consuming, they are undesirable to the extent that prevents broad acceptance of FMR as a characterization tool in the magnetic data storage industry. What is needed is an improved FMR measurement system and method that enables fully automatic FMR measurements on entire wafers for faster throughput and results in lower cost by providing a non-invasive test that avoids slicing wafers.SUMMARYAn object of the present disclosure is to provide a fully automatic system for FMR measurements such that wafer throughput is improved and wafer damage associated with slicing wafers to produce test samples is avoided.A second object of the present disclosure is to provide a fully automatic system according to the first object, including a waveguide transmission line (WGTL) for performing the FMR test capable of characterizing multiple sites on magnetic thin films and multilayers formed on blanket or patterned whole wafers.These objects are achieved according to an embodiment of the present disclosure with an FMR test system configured around a controller (computer) connected to an electrical probe station comprising a WGTL connected at one end to an RF generator and at the other end to an RF power detector (diode), a magnetic field pole positioned over the WGTL, and a wafer chuck holding a wafer under test (WUT) under the WGTL and moving laterally in a preprogrammed direction with respect to the WGTL. An RF input cable from the RF generator is connected to a first end RF launch connector which is connected to and provides an RF input signal to a first end of the WGTL. The WGTL is connected at a second end to a second end RF launch connector that is connected to an RF output cable. Both RF launch connectors are rigidly clamped to a mounting plate and at least a portion of the WGTL projects through and under an opening in the mounting plate.According to an FMR measurement mode disclosed herein, the controller instructs the wafer chuck and the WUT to move in an x-axis and / or y-axis direction parallel to the plane of the mounting plate such that the WGTL is aligned over a predetermined (x, y) coordinate on the WUT. The WGTL is separated from the WUT by a gap distance in the z-axis direction, thereby avoiding contact. Then, the WUT is moved in the vertical direction to make contact between a WGTL bottom surface and a top surface of a magnetic film on the WUT. In other embodiments, the WUT is held stationary while the mounting plate and the WGTL are moved toward the WUT via a series of predetermined (x, y) coordinates. At certain x and y axis coordinates, which are preprogrammed in the controller, the WGTL is moved vertically to contact the surface of the magnetic film. In some embodiments, the WGTL has a substantially U-shape, while in other embodiments, the WGTL has planar top and bottom surfaces.As the WGTL contacts a portion of the surface of the magnetic film, a static magnetic field is applied from a magnetic pole piece positioned over the opening in the mounting plate and directly over the WGTL contact surface. The WGTL transmits a sequence of microwave frequencies (RF input signals) to the wafer. An FMR condition is established in the magnetic film at each applied microwave frequency, and the amount of microwave absorption by the magnetic film depends on the magnitudes of the microwave frequency, the static magnetic field and the magnetic properties of the thin magnetic film.Due to the FMR absorption by the magnetic film, a power loss occurs between the first and second ends of the WGTL detected by the RF power diode. The RF power diode, also known as an RF detector, converts each RF output signal from the WGTL into a voltage indicator which the controller uses to calculate Hkand, for example, α for the magnetic film. After each FMR measurement, the wafer chuck and the WUT are lowered to a gap distance k>0 and then moved to the next (x, y) coordinate for a subsequent FMR measurement. Thus, the FMR measurement sequence includes a "progress and repeat" pattern in which the chuck and the WUT are moved laterally (stepped) to align the WGTL over a new (x, y) coordinate on the WUT, which is raised to make contact with the WGTL, and then lowered to the post-measurement gap distance. The area of the contacted area on the magnetic film is sufficiently large to detect Hk and α for a plurality of thousands of magnetic memory devices within the contacted area.According to one embodiment, a commercial WGTL having a flat, rectangular shape is modified to provide sufficient flexibility that allows the WGTL to be bent without breakage.The present disclosure also includes an embodiment in which the RF input and output cables are attached to first and second end launch RF connectors, respectively, which in turn are attached to a top surface of a flat WGTL. The WGTL bottom is below the mounting plate and is unloaded when contacting a magnetic film top during an FMR measurement. Preferably, the end-of-launch RF connectors should not extend below the contact plane between the WGTL and the magnetic film to allow contact or close proximity between the WGTL and the WUT.Because a single controller can be used to manage all aspects of testing, including movement of the wafer chuck and mounting plate, RF signal processing, magnetic field generation, and collection of test data, throughput is optimized so that the system and FMR measurement method can be easily implemented in an engineering or production environment.BRIEF DESCRIPTION OF THE DRAWINGSFigure 1a shows a schematic description of the typical series of Lorentzian shaped peaks derived from ferromagnetic resonance measurements for microwave frequencies from 24 GHz to 48 GHz. Figure 1b shows a plot of the resonance field (H R) as a function of the microwave frequencies used in Figure 1a. FIG. 2 is a diagram illustrating a prior art FMR measurement scheme. FIG. 3 is a diagram illustrating the various components of a fully automatic FMR measurement system according to a first embodiment of the present disclosure, illustrating a transmission mode for performing FMR measurements. FIG. 4 is a diagram illustrating the various components of a fully automated FMR measurement system according to a second embodiment of the present disclosure, illustrating a reflection mode for performing FMR measurements. FIG. 5 is a cross-sectional view of an FMR measurement system according to the present disclosure, wherein a bent WGTL is attached to a mounting plate and the wafer under test is held on a movable chuck during the FMR measurements. FIG. 6 is an oblique view of the embodiment of FIG. 5 with the curved WGTL suspended over an opening in a mounting plate and a central portion of the WGTL below the plane of the mounting plate. FIGS. 7 aand 7 bare a top view and a cross-sectional view, respectively, of a flat WGTL with back end Launch RF connector mounts according to another embodiment of the present disclosure. FIG. 8 is a flow chart showing various steps for performing FMR measurement according to a method of the present disclosure. FIG. 9 shows the FMR measurement results of H K as a function of the x and y axis coordinates of a perpendicular magnetic tunnel junction (pMT) stack with a nominal uniform thickness throughout the wafer and obtained according to a method described herein. FIG. 10 shows FMR measurement results for the attenuation parameter α as a function of the x and y axis coordinates of a pMT stack having a nominally uniform thickness throughout the wafer obtained according to a method of the present disclosure. FIG. 11 ashows the FMR measurement results of H K as a function of the x- and y-axis coordinates for a magnetic thin film with variable thickness on a wafer according to an embodiment of the present disclosure. Figure 11b is a plot of H K versus thickness taken from data obtained during the FMR measurements used to generate Figure 11a.DETAILED DESCRIPTIONThe present disclosure is a scanning FMR system designed to measure magnetic properties including H K and α for magnetic films or multilayers on an entire wafer at a plurality of locations identified by their (x, y) coordinates. The FMR scanning system is fully automatic and has a mounting plate with a central opening in which a waveguide transmission line (WGTL) is suspended at two ends which are fastened to RF connectors which are rigidly held on a mounting plate. It should be understood that the term "RF connector" includes various types of RF connectors, including end-launch RF connectors mentioned in the exemplary embodiments. However, other embodiments may include other types of RF connectors used in the prior art, but not described herein. The x-axis and y-axis coordinates on the wafer under test (WUT) lie in a plane that is parallel to and below the plane of the mounting plate. Each FMR measurement location on a magnetic film has a coordinate pair (x n, y n) where n is an integer. The z-axis direction in the drawings is orthogonal to the plane of the WUT. The present disclosure also includes an FMR test method for measuring magnetic properties of one or more films on patterned or non-patterned wafers.Referring to FIG. 3, an embodiment of an FMR measurement system according to the present disclosure is illustrated in a diagram showing the layout of the main components. There is a computer, hereinafter referred to as the control unit 40, for managing the movement of the WGTL 45 and the mounting board 46 to which the WGTL is attached. The magnetic field pole 83 can also be moved by the control unit to maintain a position over the WGTL and the test location of a magnetic film 43 fon the WUT 43, as will be explained later. It will be understood that the term "magnetic film" may include one or more magnetic films formed in a stack of layers. In some embodiments, the WUT is held by a vacuum on an upper surface of a wafer chuck 42 and the wafer chuck and the WUT are moved in a programmed manner while the magnetic field pole, the WGTL, and the mounting plate are held in a stationary position. In preferred embodiments, a magnetic field is applied to a predetermined (x n, y n) coordinate on the WUT while a signal path layer of the WGTL contacts a top surface 43 tof the magnetic film at the (x n, y n) - coordinate. Due to the simultaneous application of a microwave frequency by the WGTL and a magnetic field of up to 3 Tesla from the magnetic pole piece, a ferromagnetic resonance condition (FMR) is established in the magnetic film near the predetermined (x, y) coordinate on the WUT. The WGTL may be a microstrip, a grounded coplanar waveguide, a coplanar waveguide, a strip line, or a coaxial waveguide.The RF detector may be a power diode 55 which detects an RF output signal from the WGTL corresponding to an RF power loss caused by the FMR state in which a certain amount of microwave power is absorbed and the magnetic film is excited to a resonant state. After the FMR measurement is performed at the (x n, y n) - coordinate, the wafer chuck and the WUT are lowered via a signal through the connection 41c at 50d to restore a gap under the WGTL. Subsequently, the wafer chuck and the WUT are moved to another predetermined (x n, y n) coordinate and the chuck is raised at 50u to allow the WGTL to contact the magnetic film surface 43t for further FMR measurement.The control unit 40 has a first connection 41a to a power generator 47 which is responsible for providing an appropriate magnetic field from the magnetic field pole 83 so that an applied microwave frequency and the applied field induce an FMR state in the magnetic film 43f when the probe chuck 42 and the WUT 43 are raised at 50u to allow contact of the magnetic film with a WGTL ground surface (not shown) suspended through and below an opening in the mounting plate 46. Meanwhile, the control unit has established a second connection 41b to an RF generator 49 which supplies a plurality of microwave frequencies to one end of the WGTL via the RF input cable 51a.In a preferred mode of operation, the applied magnetic field is varied (swept from minimum to maximum) at constant microwave frequency. The FMR measurement may be repeated for a plurality of different microwave frequencies. The RF power diode converts the output power into a voltage signal which is transmitted to the control unit via an output cable 51b. Thereafter, the control unit H calculates k and α based on each pair of applied magnetic field values and applied microwave frequencies used for establishing an FMR condition and voltage output data from the RF power diode for each (x n, y n) coordinate used in the FMR measurement sequence.The present disclosure contemplates that arrangements other than that shown in FIG. 3 may be employed provided to generate the microwave excitation (FMR condition) of a magnetic film sample and detect the absorption therein. For example, a vector network analyzer (VNA) similar to the VNA 30 shown in FIG. 2 may be used as the RF output generator and the RF input analyzer. In another embodiment related to pulsed microwave inductive magnetometer (PIMM), a pulse generator and time-resolved oscilloscope may serve as an RF source and analyzer, respectively. In another embodiment, a lock-in amplifier detection technique known to those skilled in the art may be employed to amplify the FMR output signal (power loss).According to various embodiments of the present disclosure, FMR measurements are performed by analyzing an RF signal transmitted by a WGTL as described with reference to FIGS. 3 and 5-6, or by measuring an RF signal reflected from a waveguide terminated by an open circuit or a short circuit. An embodiment showing a reflectivity mode of operation is illustrated in Figure 4 where a directional coupler 75 is inserted between the RF generator 49 and the WGTL 45 to transmit an RF input signal to the WGTL and receive an RF output signal reflected from the WGTL. Alternatively, a power divider or a bias T-device may be used in place of the directional coupler, as is known to those skilled in the art. Microwave frequencies are input from the RF generator to the directional coupler via a first input cable 51a1 and then to the WGTL via a second input cable 51a2. All other components of the transmit mode layout in FIG. 3 are retained.In some embodiments, the RF input and output signals pass through an end-of-the-launch RF connector (not shown) positioned between the second input cable and the WGTL. More specifically, the directional coupler receives reflected RF signals (output signals) from the WGTL via cable 51a2 and then passes the signals to RF power diode 55 where conversion to a voltage output signal is made for each pair of applied magnetic field and applied microwave field frequency (H R, f R) as previously described. The output signal is then transmitted to the control unit 40 via the output cable 51b.Referring to FIG. 5, a cross-sectional view of an FMR measurement system based on RF signal transmission through a WGTL according to a first embodiment of the present disclosure is shown and includes the magnetic pole piece 83, the bent WGTL 45, and the magnetic film 43 fon the WUT 43. In the exemplary embodiment, the WGTL has a first end attached to an end Launch RF input connector 54 aand a second end connected to an end Launch RF output connector 54 bsuch that both ends Launch RF input / output connectors are above an upper surface. The WGTL is bent substantially in a U-shape and protrudes through the opening 46 asuch that a central portion 45 b, which will be referred to as a tip portion below a bottom surface 46 bof the mounting plate. According to one embodiment, the mounting plate, along with the WGTL and the WUT, are installed on a commercial electrical probe station available from various suppliers.During intervals in which FMR measurements are not performed, the tip portion 45 bis spaced a gap distance k>0 from the upper surface 43 tof of the magnetic film 43 fon the WUT 43. As previously stated, when the FMR system is programmed to perform an FMR measurement at a particular (x n, y n) coordinate, the wafer chuck is raised so that k=0 until the FMR measurement is complete. In an alternative embodiment, k is near O and is sufficiently small, preferably less than 100 microns, to allow efficient transmission of microwaves from the WGTL to the magnetic film. It is important that the magnetic pole tip 83b be located over the aperture 46a and be aligned over the (x n, y n) coordinate where the tip portion 45b contacts the top surface 43t so that the same area near the (x n, y n) coordinate is exposed to both the magnetic field and the microwave frequency.Referring to FIG. 6, there is shown an oblique view of the mounting plate 46 and the WGTL 45 of FIG. 5 with the magnetic pole piece removed. According to an embodiment, the opening 46 aincludes two parallel sides 46 d, which are oriented in an x-axis direction and may be substantially orthogonal to the mounting plate sides 46 s 1, 46 s 2. At each end of the opening 46a where the sides 46d terminate is an end Launch RF connector. An "outer" end 54a1 of a first end Launch RF connector is attached to the RF input connector 52a and a second "inner" end 54a2 is connected to the end 45e of the WGTL. Meanwhile, an inner end 54 b 2 of a second end Launch RF connector is attached to the end 45 fof the WGTL, and an outer end 54 b 1 thereof is attached to the RF output connector 52 b. Thus, microwave frequencies in the range of 1-100 GHz are transmitted from the RF generator (not shown) via the input cable 51a to the WGTL via the first end Launch RF connector, and RF output signals are transmitted via the second end Launch RF connector to the RF diode 55. According to one embodiment, the WGTL 45 is bent to be compatible with suspension through the opening 46 ain the mounting plate structure and to provide a small contact area (several square millimeters) with the magnetic film on the underlying WUT.The entire WGTL assembly (WGTL and End Launch RF connectors) is secured to the mounting plate 46 by two removable and retractable terminals 56. The aperture 46a may be formed therein by a conventional method and has an x-axis dimension of sufficient length to allow a bent WGTL to be suspended between the two clamps and have a central tip portion below the plane of the mounting plate.The present disclosure also includes a second embodiment of an FMR measurement system similar to the scheme illustrated in FIG. 6, except that the WGTL is not bent and has planar top and bottom surfaces. In order for a bottom surface of a flat WGTL to contact a magnetic film under the opening 46a in the mounting plate, a different end Launch RF connector is required.Referring to the top view of the second embodiment of Figure 7a, a key feature required in addition to the flat WGTL 45' is a second type of end-launch RF connector 54c, 54d which is secured to an upper surface of a rigid substrate 63 near the ends 45e' and 45f', respectively, of the flat WGTL by screws 58. An RF input cable (not shown) is connected to the RF connector 52f of the end-launch connector 54c near the end 45e', and an RF power diode (not shown) is connected to the RF connector 52f of the end-launch connector 54d near the end 45f. Each end-to-end connector has a pin 54p on an inner side contacting an upper conductor layer 61t. The screws preferably extend through an opening (not shown) in the rigid substrate to securely hold the WGTL, particularly for lateral movements in the (x, y) plane and for vertical movements in a z-axis direction required for FMR measurements. Plane 70- 70 is oriented along the x-axis direction and bisects the flat WGTL.Figure 7b shows a cross-sectional view of the flat WGTL 45' and end-to-end tap connector in plane 70-70 in Figure 7a. As a result of the attachment of the end-to-end connectors 54 cand 54 dwith a rigid substrate top surface 63 twhich is a top layer in the WGTL, the bottom surface including the signal path layer 61 bis free to make contact with an underlying magnetic film 43 fduring FMR measurement. According to an embodiment, at least a bottom portion of the flat WGTL including the signal path layer is suspended through an opening in the mounting plate. Alternatively, the flat WGTL may be suspended below the opening. Dielectric layer 60 bseparates the signal path layer from rigid substrate 63 and upper ground path layer 62 a. The gap k is indicated during a stationary position between FMR measurements when there is no contact between the WGTL and the magnetic film. The upper conductor layer 61 tis connected to the signal path layer by a first via 61 v 1 that may be positioned below a first pin 54 pat the end launch connector 54 cand by a second via 61 v 2 that may be positioned below the second pin 54 pat the end launch connector 54 d. Ground path and signal path layers are typically made of a conductive metal.During FMR measurement, when a magnetic field is applied to a region around a (x, y) coordinate of the magnetic film contacted by the WGTL, a microwave frequency passes through the end Launch connector 54 cand the upper conductor layer 61 t, and then through the via 61 v 1 and into the signal path layer 61 b. A portion of the microwave power is absorbed by the magnetic film 43f during an FMR state, so that the RF output signal passed through the via 61v2 and the end launch connector 54d has reduced power compared to the RF input signal. The RF power diode (not shown) converts the RF power output signal for each (H R, f R) pair into a voltage measurement which is passed to the controller.Referring to FIG. 8, an FMR measurement method is illustrated in a flowchart according to an embodiment of the present disclosure, and may be used for both embodiments related to a bent WGTL or a flat WGTL. In step 110, the controller commands (described earlier with respect to a transmit mode in FIG. 3 or a reflection mode in FIG. 4 ) to contact a WGTL via the link 41 c, for example, a first location represented by an (x, y) coordinate on a magnetic film to be tested. Contact is achieved by lowering the WGTL and the mounting plate or by raising the wafer chuck and overlaying the wafer under test with an exposed top surface of the magnetic film (see motion 50 uin FIGS. 3, 4 ). Next, in step 111, the control unit instructs an RF generator via another link 41b to transmit a microwave frequency F1 to the WGTL while an overlying magnetic pole piece applies a magnetic field at the selected (x, y) coordinate.By applying the microwave frequency F1 and the magnetic field simultaneously, the magnetic film reaches an FMR state and absorbs a part of the microwave power depending on the magnetic properties of the magnetic film F1 and the applied field (H R).Accordingly, the RF power diode detects a reduced power value in the RF output signal compared to the value specified by the controller in the RF input signal of step 111 in step 112.In step 113, the RF power diode converts the RF output signal into a voltage measurement which is transmitted to the controller and which indicates microwave absorption by the magnetic film for the applied microwave frequency F1 and the applied magnetic field H R.Step 114 includes repeating steps 111-113 except that the RF input signal has a second frequency F2 that is applied to the magnetic film at the previously selected (x, y) coordinate after F1 is applied. In some embodiments, steps 111- 113 are repeated multiple times at each (x, y) coordinate used for the FMR measurement method. In other words, a third frequency F 3 different from F 1 and F 2 may be applied during a time interval after F 2 and so on up to an "nth" frequency Fn after F 3. It should be noted that the applied magnetic field sweeps the band from a minimum to a maximum value when each frequency F 1 to Fn is applied according to a preferred embodiment. Thus, at each applied frequency, an FMR state occurs and each FMR state has a unique microwave absorption that is converted by the RF power diode to a corresponding voltage signal. In an alternative embodiment, the applied magnetic field is maintained constant while the microwave frequencies are varied (swept) from F1 to Fn to establish an FMR state.Referring to step 115, the controller commands the wafer chuck and the overlying magnetic film to be lowered (motion 50 din FIG. 3 ), thereby preventing the WGTL from making contact with the first location. In an embodiment in which the wafer chuck and the wafer perform all movements in the x and y axis directions, the controller instructs the wafer chuck to move to a second location where the WGTL tip portion is aligned over a different (x, y) coordinate with respect to the initial coordinate (x, y). The wafer chuck and the magnetic film on the WUT are raised to make contact with the WGTL again.Thereafter, steps 111-114 are repeated to complete an FMR measurement at the second location. It should be noted that each location may comprise a contact area of multiple square millimeters and each (x, y) coordinate may be a midpoint in the area contacted by the WGTL.Depending on the size of the magnetic film area to be contacted by the WGTL and the number of different positions desired for FMR measurement, step 116 indicates that steps 111- 115 may be repeated multiple times to obtain multiple FMR measurements. These are "m" different (x, y) coordinates each having a plurality of (H R, f R) pairs, where f R is one of F1 to Fn. In step 117, a decision is made as to whether or not all the preselected (x, y) coordinates on the magnetic film have been tested. If "no", another FMR measurement is performed on another (x, y) coordinate. If "yes", step 118 indicates that the FMR measurement process is completed.As mentioned above, the control unit may determine the magnetic properties in the magnetic film at any location corresponding to a different (x, y) coordinate. Each FMR measurement yields one or more pairs (frequency, field), also referred to in equation (1) as (H R, f R) - pairs, that correspond to each FMR condition. The controller uses FMR measurement data and one or more of equations (1)-(3) described above to determine Hk,α, and in some cases γ and inhomogeneous broadening (L o).An experiment was performed using the FMR system with a curved WGTL as shown in Figures 5-6 and shows how effectively the perpendicular magnetic anisotropy field (Hk) is monitored across an unstructured film consisting of a stack of magnetic tunnel junction layers (MTJ). In this example, 43 regions each having a different (x, y) coordinate were tested on an uncut 8-inch diameter wafer by applying microwave frequencies between 10-40 GHz and magnetic fields up to 1 Tesla (10000 Oe) according to an FMR measurement method of the present disclosure. Each region has a rectangular shape having a length of about 10 mm along the x-axis direction and a width of about 9 mm in the y-axis direction. The FMR measurement at each (x, y) coordinate required about two minutes of process time for a total FMR measurement time of about 90 minutes for the entire wafer.Figure 9 shows a plot of the H K- results and Figure 10 shows a plot of the attenuation coefficient α result for each of the 43 regions on the 8 inch wafer as a function of their x-axis and y-axis coordinates. H K varies between 9760 Oe in the upper left corner and 9670 Oe in the lower right corner of the wafer. The MTJ stack used for this experiment was a CoFeB / MgO / CoFeB-based MTJ structure. The results show that the MTJ stack has very high perpendicular magnetic anisotropy (PMA) and substantially uniform thickness across the wafer.In a second experiment, the same MTJ stack of layers from the first experiment was retained. However, the free layer thickness was intentionally varied from 17 angstroms in the upper left portion of the wafer by small increments to 18.5 angstroms in the lower right corner to demonstrate how the FMR measurement system and method of the present disclosure can be advantageously used as a tool for examining the H K- dependence on the free layer thickness. The experimental results are depicted in FIG. 11 a, and show how H K varies from 10500 Oe in the upper left portion of the wafer to 8760 Oe in the lower right corner of the wafer. Figure 11b shows the results of H K as a function of thickness along plane 95-95 in Figure 11a. More specifically, a substantially linear plot is formed indicating that H K is steadily increasing as the thickness of the free layer is gradually decreased from 18.5 angstroms to 17 angstroms.RF connectors, probe station (platform), RF generator, RF power diode, input and output cables, as well as alternative RF devices such as vector network analyzers, pulse generators, and time-resolved oscilloscopes are commercially available. A suitable magnet comprising a magnetic pole piece that provides the required magnetic field for inducing magnetic resonance in accordance with the present disclosure is also commercially available or can be custom manufactured by one of ordinary skill in the art.While this disclosure has been particularly shown and described with reference to the preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of this disclosure.

Claims

A ferromagnetic resonance (FMR) measurement system for a wafer under test (WUT) on which one or more magnetic films are formed, comprising: (a) a waveguide transmission line (WGTL) connected to a microwave generator and mounted on a mounting plate, wherein the WGTL is substantially U-shaped and has a first end attached to a first RF connector and a second end attached to a second RF connector, a center tip portion of the WGTL is disposed below an opening of the mounting plate exposing an upper surface of the one or more magnetic films; (b) a magnet providing a magnetic field configured, along with a simultaneous application of a microwave frequency, to induce a magnetic resonance condition in the one or more magnetic films; and (c) a system for detecting microwave absorption by the one or more magnetic films when a magnetic resonance condition exists, wherein the WGTL and the WUT are laterally and vertically movable relative to each other such that the central tip portion of the WGTL contacts or is disposed proximate the exposed top surface of the one or more magnetic films at predetermined positions.The FMR measurement system of claim 1, wherein the WGTL and the WUT are installed in an electrical probe station.The FMR measurement system of claim 1, wherein the WGTL is movable with respect to the WUT and the one or more magnetic films that are held stationary.The FMR measurement system of claim 1, wherein the WUT and the one or more magnetic films are movable with respect to the WGTL being held stationary.The FMR measurement system of claim 1, wherein the WGTL is disposed proximate the WUT such that the vertical distance between the WGTL and the WUT during an FMR measurement is less than 100 micrometers.The FMR measurement system of claim 1, wherein the WGTL is a microstrip.The FMR measurement system of claim 1, wherein the WGTL is a coplanar waveguide or a grounded coplanar waveguide.The FMR measurement system of claim 1, wherein the WGTL is a strip line.The FMR measurement system of claim 1, wherein the WGTL is a coaxial waveguide.A scanning ferromagnetic resonance (FMR) measurement system comprising: (a) a controller having a first connection to an RF generator, a second connection to a power generator connected to a magnet, and a third connection to one or both mounting plates on which a bent waveguide transmission line (WGTL) is mounted and a wafer chuck is configured to hold an overlying whole wafer under test (WUT) with a magnetic film having an exposed top surface, wherein the mounting plate and / or the wafer chuck are laterally and vertically movable in a stepwise and repeated manner; (b) the RF generator configured to provide a sequence of RF input signals each having a different microwave frequency to the bent WGTL via an RF input cable; (c) the bent WGTL having a center tip portion configured to contact a predetermined area of the exposed top surface of the magnetic film during an FMR measurement to sequentially transmit the different microwave frequencies thereto; and (d) the magnet having a magnetic pole piece aligned over the bent WGTL center tip portion and providing a magnetic field through a mounting plate opening on the predetermined area of the magnetic film so that an FMR state is established in the predetermined area, causing unique microwave absorption for each different microwave frequency (f R) resulting in an RF power loss for each combination of H R and f R and being determined from an RF output signal passed from the WGTL to an RF detector, the RF detector configured to send a voltage signal as an FMR measurement to the controller such that one or more magnetic properties for the magnetic film are determined.The scanning FMR measurement system of claim 10, wherein the bent WGTL and the mounting plate are installed in an electrical probe station.The sensing FMR measurement system of claim 10, further comprising a first RF connector connecting the RF input cable to the bent WGTL and a second RF connector connecting the bent WGTL to the RF detector, and wherein the first and second RF connectors do not contact the exposed top surface of the magnetic film and do not extend beyond a contact plane between the bent WGTL and the magnetic film.The scanning FMR measurement system of claim 12, comprising a transmission mode for performing the FMR measurement, wherein the RF input signal is transmitted via the first RF connector and the RF output signal is transmitted via the second RF connector.The scanning FMR measurement system of claim 12, wherein the first and second RF connectors are end-launch RF connectors.The scanning FMR measurement system of claim 10, further comprising a directional coupler, a power divider, or a bias T component that receives the RF input signal from the RF generator and then transmits the RF input signal to the WGTL via the RF input cable, wherein the directional coupler, the power divider, or the bias T component also receives a reflected RF output signal from the WGTL and transmits the reflected RF output signal to the RF detector in a reflection mode for performing the FMR measurement.The scanning FMR measurement system of claim 10, wherein the curved WGTL has a substantially U-shape.The scanning FMR measurement system according to claim 10, wherein the mounting plate and the bent WGTL are movable with respect to the WUT and the magnetic film held stationary.The scanning FMR measurement system according to claim 10, wherein the WUT and the magnetic film are movable with respect to the WGTL and the mounting plate that are held stationary.The scanning FMR measurement system of claim 10, wherein the bent WGTL and the magnetic film are separated by a vertical movement after the FMR measurement to establish a gap therebetween before a lateral movement is performed to align the bent WGTL over another predetermined area in the magnetic film.The scanning FMR measurement system of claim 10, wherein the FMR condition is established at each of the different microwave frequencies by sweeping the magnetic field from a minimum value to a maximum value.The scanning FMR measurement system of claim 10, wherein the FMR condition is established by maintaining the magnetic field constant and sweeping at a plurality of different microwave frequencies applied in a sequential order.A scanning ferromagnetic resonance (FMR) measurement system comprising: (a) a controller having a first connection to an RF generator, a second connection to a power generator connected to a magnet, and a third connection to one or both mounting plates to which a flat waveguide transmission line (WGTL) is attached and a wafer chuck is configured to hold an overlying entire wafer under test (WUT) with a magnetic film having an exposed top surface, wherein the mounting plate or wafer chuck is laterally and vertically movable in a stepwise and repeated manner; (b) the RF generator configured to provide a sequence of RF input signals each having a different microwave frequency to the flat WGTL via an RF input cable; (c) the flat WGTL mounted to the mounting plate by a first RF connector mounted to an upper surface of the WGTL proximate a first end thereof and by a second RF connector mounted to an upper surface of the WGTL proximate a second end thereof, the flat WGTL having a bottom surface below a mounting plate opening and configured to contact a predetermined area on the exposed upper surface of the magnetic film and sequentially transmit the plurality of microwave frequencies thereto; (d) the magnet having a magnetic pole piece aligned over the flat WGTL and providing a magnetic field through the mounting plate aperture on the predetermined area of the magnetic film such that an FMR state is established in the predetermined area, thereby causing unique microwave absorption for each frequency (f R) that results in an RF power loss for each combination of H R and f R and is detected from an RF output signal transmitted from the WGTL to an RF power diode, the RF power diode configured to convert the output signal to a voltage signal and transmit the voltage signal to the controller; and, so that one or more magnetic properties are determined for the magnetic film.The scanning FMR measurement system of claim 22, wherein the flat WGTL and the mounting plate are installed in an electrical probe station.The scanning FMR measurement system of claim 22, wherein the vertical movement of the mounting plate or wafer chuck causes the bottom surface of the flat WGTL to contact the predetermined area of the exposed top surface of the magnetic film.The scanning FMR measurement system of claim 22, wherein the first and second RF connectors do not contact the exposed top surface of the magnetic film or extend below a contact plane between the flat WGTL and the exposed top surface of the magnetic film.The scanning FMR measurement system according to claim 22, wherein the mounting plate and the flat WGTL are movable with respect to the WUT and the magnetic film held stationary.The scanning FMR measurement system according to claim 22, wherein the WUT and the magnetic film are movable with respect to the flat WGTL and the mounting plate that are held stationary.The scanning FMR measurement system of claim 22, wherein the flat WGTL and the magnetic film are separated by a vertical movement after the FMR measurement to establish a gap therebetween before a lateral movement is performed to align the flat WGTL over another predetermined area in the magnetic film.The scanning FMR measurement system of claim 22, wherein the flat WGTL comprises an upper conductor layer, a rigid substrate layer, a lower ground plane layer having a signal layer, and a dielectric layer between the rigid substrate layer and the lower ground plane layer.The scanning FMR measurement system of claim 22, wherein the FMR measurement is performed with a transmission mode in which the RF input signal passes through the first RF connector and the RF output signal passes through the second RF connector.The scanning FMR measurement system of claim 22, further comprising a directional coupler, a power divider, or a bias T component that receives the RF input signal from the RF generator and then transmits the RF input signal to the WGTL via the RF input cable, wherein the directional coupler, the power divider, or the bias T component also receives a reflected RF output signal from the WGTL via the RF input cable and transmits the reflected RF output signal to the RF detector in a reflection mode for performing the FMR measurement.The scanning FMR measurement system of claim 22, wherein the first and second RF connectors are end-to-end connectors.A method of performing a scanning ferromagnetic resonance (FMR) measurement on an entire wafer under test (WUT) having a magnetic film, comprising: (a) providing a wafer chuck on which the WUT is supported and which has a first connection to a controller; (b) providing a waveguide transmission line (WGTL) mounted on a mounting plate, the WGTL being substantially U-shaped and having a first end attached to a first RF connector and a second end attached to a second RF connector, a center tip portion of the WGTL being disposed below an opening of the mounting plate that exposes the magnetic film on the WUT and configured to contact or be disposed proximate a first location on the exposed magnetic film; (c) applying a sequence of RF input signals from an RF generator having a second connection to the controller, each RF input signal passing through the WGTL and having a different microwave frequency sequentially transmitted through the center tip portion of the WGTL to the first location; (d) applying a magnetic field from an overlying magnetic pole piece simultaneously with each different microwave frequency to establish an FMR state, thereby providing an RF output signal for each pair of an applied microwave frequency value and an applied magnetic field value; (e) transmitting a sequence of RF output signals from the WGTL to an RF detector configured to convert each RF output signal to a voltage signal and transmit each voltage signal to the controller, the controller determining one or more characteristics in the exposed magnetic film.The method of claim 33, wherein the WGTL and the mounting plate are installed in an electrical probe station.The method of claim 33, wherein the contact between the WGTL and the exposed magnetic film is achieved by a vertical movement of the mounting plate or a vertical movement of the wafer chuck.The method of claim 33, wherein the WGTL is secured to the mounting plate at two ends.The method of claim 33, further comprising performing a second FMR measurement at a second location on the exposed magnetic film after performing steps (a) - (e).The method of claim 37, wherein performing the second FMR measurement comprises: (a) separating the WGTL and the exposed magnetic film using vertical movement of the mounting plate or the wafer chuck; (b) performing lateral movement of the mounting plate or the wafer chuck such that the WGTL and the magnetic pole piece are aligned over the second location; and (c) performing vertical movement of the mounting plate or the wafer chuck such that the WGTL contacts or is moved proximate to the exposed magnetic film.The method of claim 33, wherein the different microwave frequencies are between 1 and 100 GHz.The method of claim 33, wherein the strength of the magnetic field is up to 3 Tesla.

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