3. Experimental equipment

3.1 Sputter Deposition

In sputter deposition, commonly called 'sputtering', material is removed as atoms or molecules from a solid target by energetic ion bombardment. The target that is made of the material to be deposited is positioned in a chamber that is pumped to a pressure about 10-7 Torr. By applying a high radio frequency (RF) or direct current (DC) voltage between the target, cathode, and the substrate, anode, energetic electrons emitted from the target form ions in the process gas, typically argon at 1 to 100 mTorr pressure. Under these conditions, a plasma - an electrically approximately neutral association of electrons and positive ions - is formed.

The applied electric field accelerates Ar+ ions from the plasma's edges into the target with kinetic energies up to several hundred eV. This is comparably much higher energy than the fraction of one eV that is involved in thermally evaporating processes. The bombarding ions impact the target causing a collision cascade in the target. Some of the secondary collisions in the target propagate to the surface causing target atoms to be knocked off the target, 'sputtered'. The energy of the incident argon ion is distributed over many target atoms in the collision cascade so the sputtered atoms typically have energies of a few electron volts. Between target and substrate each ejected atom has numerous gas phase collisions with the process gas, which deflect it and lower its energy. By optimizing the distance between target and substrate, the approach angles of the target atoms to the substrate surface are so randomized that a uniform film results.

In magnetron sputtering the electrons are forced to spiral near the target surface by placing magnets below the target. This technique has many benefits. First, the electron's mean free path length in the magnetron is increased, raising its ionization probability. Second, electrons trapped by space-charge effects and magnetic fields are less likely to escape and bombard the substrate. Third, localizing the plasma confines the Ar+ ions to a volume near the target surface and keeps their impact energy high - maximizing the sputtering (and, hence, deposition) rate. The resulting films are denser, with greater adhesion to the substrate.

Compared to other conventional thin film deposition techniques, sputtering offers some important advantages:
 

  • higher kinetic energy of deposited atoms results in better film adhesion
  • no 'spitting' occurs that would leave lumps of material on the substrate
  • sputter source can be mounted in any orientation
  • since coverage is independent of line-of-sight, sputtering inherently produces uniform film coatings over non-flat surfaces
  • plasma is energetically hot, but has a small thermal capacity
  • deposit a wide variety of materials including metals, semiconductors, and ceramics, regardless of melting point or vapor pressure
  • refractory materials, elements, mixtures and alloys can be sputtered with equal facility
  • more easily automated for in-line industrial processing and can uniformly coat large area substrates in a production facility
  • minimize chemical byproducts and waste



    • 3.1.1 Sputter system

    The sputter system in use at ISML has a preparation chamber to remove and insert samples into the sputter chamber without removing the vacuum in the chamber. The sputter chamber can handle four samples at a time and have a computer driven substrate table. Rotary pumps and turbo pumps supply proper vacuum. The depositions of multilayers are performed using automatically controlled shutters and rotation of the table.
     
     




    Fig 3.2 Multi-function deposition system. Designed by Prof. T. Suzuki.


     



    The chamber contains two RF magnetron cathodes and one DC magnetron cathode. This enables the sputter chamber to contain a maximum of three different targets.  During the fabrication of the samples the Pd and SiN targets were positioned at the RF-cathodes and the TbFeCo target at the DC-cathode.


    3.2 VSM

    Maybe the most commonly used method of characterization for magnetic materials is the Vibrating Sample Magnetometer (VSM) that uses an induction technique. In a VSM the sample is mounted at the end of a rigid rod attached to a mechanical resonator which oscillates the sample, usually in a vertical direction, at a fixed frequency, . In the current VSM =80 Hz. Surrounding nearby the sample is a set of pick-up coils. As the sample moves, its magnetic field which is proportional to its magnetic moment, M, alters the magnetic flux through the coils, dM/dt. This induces a current directly proportional to dM/dt, which can be amplified and detected using lock-in amplifiers. The external magnetizing field is provided by a horizontal electromagnet. When examining magneto-optical materials, the easy-axis hysteresis loop will be obtained when the applied field is perpendicular to the plane of the sample. When rotating the sample 90 degrees and applying the field in the plane of the film it is possible to measure the deviation of Ms from the easy axis and thereby determine the magnetic anisotropy constant Ku [9].
     



    Fig 3.3 VSM


     



    It is of great importance that the design of the VSM ensures that the vibration of the sample produces no vibration of the pick-up coils relative to the magnet. If that would be the case it would result in large spurious signals. This problem is reduced if the magnet?s field is very homogeneous and thus most VSMs use large electromagnets with large pole piece diameters. The combined VSM and torque magnetometer in use at ISML, Toei VSM-5-13/TRT-2-10, can provide fields up to 15 kOe.


    3.3 Torque magnetometer

    The torque magnetometer is based on the principle that a magnetic sample in a magnetic field, H, experiences a force. The torque, L, exerted on a sample is

    proportional to its magnetization, M. Furthermore the torque is proportional to the derivative of the anisotropy energy towards the angle of rotation. The torque measurement set-up is presented in Fig 3.4. In the figure a sample holder with a sample hanging on a thin wire in a magnetic field is shown. Because of the field the sample experiences a torque.
     
     


    Fig 3.4 Torque magnetometer


     





    At the top end of the sample-holder, a coil is mounted between a permanent magnet of a known strength. The torque on the sample can now be compensated by a torque on the coil when a current flows through this coil. Using a small mirror, a lamp and two photo diodes to detect the rotation of the sample, the current through the compensation coil is controlled. The current through the compensation coil is proportional to the torque exerted on the sample. Torque magnetometers are used for determining the anisotropy[2.2] axis in a magnetic material.

    The torque magnetometer in use to perform these experiments was a combined VSM/torque magnetometer, providing a maximum magnetic field of 15 kOe.
     


    3.3.1 Estimation of Ku through Extrapolation to Infinite Field

    One of the major problems in torque magnetometry is the fact that the anisotropy energy is determined by the angle between the magnetization, M, and the easy axis direction, , whereas we set the angle between the applied field, H, and the easy axis, . The magnetization direction is free to rotate to the equilibrium position where the torque exerted by the applied field balances the torque exerted by the anisotropy. Therefore  and  are not equal unless the applied field is much larger than the magnetic anisotropy field.
     
     



    Fig 3.5 Finite field error


     



    There are two major approaches to correct for this so called "finite field error". One is to calculate the angle  for every  using the absolute value of M. This value can be determined by for example VSM experiments. The second approach is to take torque measurements at many different values of the applied field H and to extrapolate the measured values to infinite field. Since at infinite field the torque exerted by the external field is infinitely larger than the torque exerted by the anisotropy so  and  become equal. Very often the difference between the anisotropy values obtained by both methods are very large. One of the major reasons for this is the non-uniformity of the anisotropy. Since the non-uniformity in the anisotropy is unknown beforehand, the extrapolation method is to be preferred.

    If Happlied<<Hk the system gives rise to 2 and 4 components. These are calculated from the torque curves using Fourier analysis. In order to obtain values at infinite field L2 and L4 are plotted versus 1/H.
     
     



    Fig 3.6 L2 and L4 versus 1/H


     




    Then Ku is determined using the following relation.

    When having large rotational hysteresis losses and thereby distorted torque curves the extrapolation method do not produce an accurate estimation of Ku. If that is the case it is more appropriate to estimate Ku by measurements of the rotational hysteresis loss, Wr, at different magnetic fields, H.  When plotting Wr versus 1/H a linear relationship will most likely appear. Extrapolating this linear fitted curve to zero and Hk will be obtained.
     
     




    Fig 3.8 Wr versus 1/H


     




    When Hk is obtained then Ku is easily determined by the following relation. Ms can be estimated using VSM data.


    3.4 Wide Wavelengths Magneto-optical Kerr spectroscope Apparatus

    The Wide Wavelengths Magneto-optical Kerr spectroscope Apparatus enables measurements of both wavelength dependents of the polar Kerr rotation angel and ellipticity. It is also possible to perform magneto-optical hysteresis loops. The Kerr spectra is measured in the photon energy range between 1.4 eV and 6.8 eV (=900-180 nm), in a temperature range from 80K to 600K in fields up to 20 kOe. This apparatus was originally developed at ISML by W.P. Van Drent and now it has emerged into a commercial product manufactured by Toyota Macs. Among the customers to Toyota Macs, well-known companies like Seagate can be recognized.

    The fundamental design of the magneto-optical Kerr spectroscope employs the photo-elastic modulator (PEM) principle. In a PEM, a light-transmitting material is alternately compressed and expanded in one direction by an actuator[13]. The PEM in this system is made of CaF2 and vibrates at its resonance frequency, 50 kHz. Due to the alternating compression and expansion, light with linear polarization along the vibration direction is alternately retarded or speeded up on the other hand light linear polarized orthogonal to the vibration direction passes through the PEM unchanged[14].

    To be able to measure at such a wide wavelength spectra the system utilizes two lamps as light sources, a 450 W Xe-lamp and a 150 W D2-lamp for the longer and shorter wavelengths respectively. The Xe-lamp has a wavelength spectrum from 240-2000 nm, 0.7-5.2 eV, and the D2-lamp 180-310 nm, 4.0-6.8 eV. Both lamps are focused onto its own entrance slit and are altered using a motorized mirror that reflects the light into a monochromator. Usually the lamps are switched when the photon energy is somewhere in the range of 4.0-5.0 eV. When exiting the monochromator the light passes through a 45 degrees polarizer and into the photo-elastic modulator. When the PEM is placed behind a linear polarizer, one can choose the ratio of modulated and unmodulated light.

    After exiting the photo-elastic modulator the light is reflected against the sample. The reflected light is then fed through another polarizer to the detector. The detector can be either a photo multiplier or a germanium detector. The entire light path is kept in nitrogen gas, to avoid ozone formation and to make transmission above 6 eV possible. It is therefor necessary to subtract the Faraday rotation of the nitrogen gas from the measured data. The Faraday rotation is easily determined by measuring a non-magnetic sample like fcc Pd.
     
     


    Fig 3.9 Wide Wavelengths Magneto-optical Kerr spectroscope Apparatus


     




    3.5 Polar Kerr loop tracer

    The polar Kerr loop tracer, or Kerr looper, enables determination of the magneto-optical polar Kerr hysteresis loop with a maximum applied field of 15 kOe at a wavelength of = 830nm. The measurements can be performed at different temperatures in a range between ambient and 200C.

    For a given wavelength, the Kerr rotation and ellipticity are proportional to the magnetization of the sample. The rotation and ellipticity becomes zero when the magnetization of the sample is zero. The value of rotation at maximum positive field is equal in magnitude, but opposite in sign at maximum negative field. This is also true for the ellipticity. Any remaining rotation or ellipticity at zero magnetization is due to some optical constants of the sample, or failure during the alignment of the measurement system. Therefore the magneto-optical polar Kerr hysteresis loop is assumed to be equivalent to the M-H hysteresis loop.

    The polar Kerr and Faraday effects changes sign at the compensation point and can be used to help locate the compensation point.

    3.6 Alternating Gradient Field Magnetometer - AGFM

    An AGFM is a highly sensitive measurement system, capable of measuring hysteresis properties on a wide range of sample types and strengths. It uses a modified technique to conventional vibrating sample magnetometry. Traditionally, a sample placed in a magnetic field is vibrated at a fixed frequency via an electro-mechanical transducer[3.2]. In an AGFM, however, an alternating gradient field is utilized to exert a periodic force on a sample placed within a variable or static DC field. The force is proportional to the magnitude of the gradient field and the magnetic moment of the sample. The force deflects the sample and this deflection is measured by a piezoelectric sensing element mounted on the probe arm. The output signal from the piezoelectric element is synchronously detected at the operating frequency of the gradient field. Operating near the mechanical resonant frequency of the assembly enhances the signal from the piezoelectric element. The AGFM in use at ISML, 2900 MicroMag, utilizes a software function that automatically determines the mechanical resonance and sets the appropriate operating frequency for the sample under study and the electromagnet can provide fields up to 20 kOe.


    3.7 XRD

    A x-ray diffractometer is a versatile instrument that measures the intensities of a reflected x-ray beam from a small area. The intensities follow Bragg's law[15].

                                                                                               (3.4)

    Results provide direct evidence for the atomic level spacing within the crystal lattice of the specimen. This information can tell details of the crystal structure for the substance, which is formed from the chemical components in the specimen. Where different phases with identical compositions occur, these can be distinguished by XRD. In addition, finer details of the crystal structure, such as the state of atomic order, also can be derived. Also less delicate details like multilayer thickness can be detected.


    3.8 SPM

    Scanning Probe Microscopy (SPM) which includes Atomic Force Microscopy and Magnetic Force Microscopy (AFM and MFM) provides quantitative, three-dimensional images and surface measurements with spatial resolution of a few micrometers to below 10 Angstroms. SPM can image a wide range of materials, including magnetic media, silicon wafers, semiconductors, laser optics and polymers.

    3.8.1 AFM
    The basic working mode of an Atomic Force Microscope is the following: a very sharp tip (not more than some micron large) scans the surface of the sample to be analyzed. The interaction forces occurring between the tip and the atoms of the scanned sample surface, in the order of nanonewtons, promote a deflection of the cantilever where the tip is mounted. When there is a change in the surface topography a change in the cantilever deflection is detected by the position of a laser beam. From a scan over the sample surface a three-dimensional image with accuracy of the order of 0.1 nm along the vertical scale can be obtained. This technique permits the observation of the samples at a nanometric scale, it can be performed in air, it is not destructive and do not requires complicated sample preparation. Atomic Force Microscopy is very suitable for the analysis of wafers, magnetic media and compact disks.

    3.8.2 MFM
    In a Magnetic Force Microscope (MFM) a magnetic tip is used to probe the magnetic stray field above the sample surface. The magnetic tip is mounted on a small cantilever that translates the force into a deflection, which can be measured. The microscope can sense the deflection of the cantilever that will result in a force image (static mode) or the resonance frequency change of the cantilever, which will result in a force gradient image. The sample is scanned under the tip that results in a mapping of the magnetic forces or force gradients above the surface.

    The fact that no sample preparation is necessary and that a lateral resolution below 50 nm can be reached make it a powerful tool for investigation of submicron magnetization patterns. Since it is possible to apply external magnetic fields during the measurement, the field dependence of domain structures and magnetic reversal processes can be observed. Methods to separate topography and magnetic features allow pure magnetic images to be achieved. Topographic and magnetic details from the same scan can be related to each other.