Spintronics devices are typically comprised of thin films that may be created through various vacuum deposition methods. In the present work, physical vapor deposition was used for the thermal evaporation of ferromagnetic, metallic, and oxide films using electron beam
evaporation and radiofrequency sputtering techniques. The substrate used for the deposition process consists of either an inert, rigid medium (i.e. glass, sapphire substrates, etc.) or an active material that serves a primary function in the ultimate device (i.e. semiconductor thin films or single crystals).
2.1.1 Physical Vapor Deposition
Physical vapor deposition describes a variety of processes that allow for the atomistic deposition of materials onto a suitable substrate in a vacuum environment. These processes involve the vaporization of a material from its solid or liquid form, and subsequent transportation of that gaseous matter along its mean free path. Vaporized material will condense onto a
substrate in its path and form a thin film with thicknesses ranging from nanometers to micrometers. The rate of deposition here is typically on the order of 0.1 – 10 nanometers per second. A vacuum environment with pressures on the order of 10-5 – 10-9 Torr is utilized in order to reduce scattering of the vapor material from ambient gas molecules or atoms, thus increasing its mean free path [45].
30 Figure 12. Sun Lab glove box deposition system at NC State University. (Left) Front side of the instrument with N2 filled glove box for handling air-sensitive samples. (Middle) Rear view of dual deposition chambers connected by a central load lock. (Right) Interior view of deposition chamber showing deposition sources, quartz crystal microbalance sensors, and samples stages with heating and cooling capabilities.
The Sun Lab at NC State University possesses a glovebox-integrated physical vapor vacuum deposition system with the capability of depositing a variety of metallic, organic, and ceramic materials in a controlled, inert environment (see Figure 12). The glovebox integration is particularly advantageous when working with materials that are sensitive to oxygen and humidity in ambient conditions. Two primary methods used for depositing thin films in this system are e-beam evaporation and rf sputtering, which are briefly detailed below.
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2.1.2 Electron Beam Evaporation
Figure 13. Electron beam evaporator used for deposition of ferromagnetic thin films, metallic electrodes, and oxide insulating and/or capping layers.
Electron beam, or e-beam, evaporation is a method for transferring energy from a beam of electrons into a target material causing it to evaporate or sublimate and then deposit onto a corresponding substrate. The electron beam is generated by passing electric current through a tungsten filament, resulting in resistive heating and thermal emission of electrons. A high voltage applied between the filament and target material directs the emitted electrons towards the target.
Strong magnetic fields are used to both focus the electron beam and control its path such that it impacts the bulk of the material being evaporated. As the material absorbs energy from the incident electrons it is heated and either directly sublimates or melts into a liquid before
32 evaporating. This deposition method may be used to evaporate an incredibly large library of materials ranging from refractory metals to ceramic oxides, that would be otherwise inaccessible to conventional resistive thermal evaporation.
2.1.3 RF Sputtering
Figure 14. An RF sputtering source is pictured in the bottom of the image. An adjacent QCM sensor is used for monitoring film thickness during deposition. A substrate shutter between the source and sample stage allows or prevents incoming vapor phase material hitting the substrate.
Sputtering is a physical, non-thermal vaporization process by which atoms are ejected from the surface of a target material due to the transfer of kinetic energy from bombarding, energetic ions accelerated from a nearby plasma. While reactive sputtering using a working gas
33 such as oxygen or nitrogen is sometimes used in the deposition of certain materials, typically an inert, heavy gas such as argon is used for the plasma source. A plasma is sustained by the breakdown and ionization of this gas through the application of a strong bias voltage, as in the case of dc sputtering, or an alternating electric potential, as used in rf sputtering. This plasma is usually confined close to the target material being sputtered, which in turn is negatively charged and acts as a cathode. Positively charged argon ions bombard the negatively charged target material transferring their momentum to surface atoms in the target. A fine spray of atoms are subsequently emitted from the target which may travel through the vacuum chamber along their line-of-sight before condensing on a substrate.
The main advantage of rf sputtering over dc is that traditional dc sputtering can only be performed on electrically conductive materials, like metals, whereas rf sputtering can be performed on a wide variety of materials including semiconductors and dielectrics such as aluminum oxide, silicon oxide, etc. The reason for this is that dielectric materials accumulate charge on their surfaces and over time the buildup of positively charged ions at a dielectric surface can prevent the mechanism of action for sputtering to occur. By applying a
radiofrequency alternating potential, the surface of the target material is effectively cleaned of accumulated ions at the end of each full cycle. The addition of an impedance analyzing network allows for automatic matching of the circuit impedance to a 50 Ohm load, thus providing optimal power transfer from the generator to the plasma chamber. The main drawback of this method is that deposition rates for metals are less than what can be achieved in a classic dc sputtering system.
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