Thin film ferrites were originally produced by “liquid phase epitaxy”, where a hot substrate is dipped in the molten ferrite (typically at 1000 ºC), which allows the ferrite to grow onto the substrate. After a few minutes the layer of ferrite will be 1 to 4 microns thick. The first use was for “bubble memories” by growing a garnet as a thin film on a suitable non-magnetic substrate. These fell out of use in favour of better performing semiconductor memory. This technology was later used for magneto-optic devices.

Single crystal sapphire (alumina) is used as a substrate on which a thin film of single crystal silicon is deposited to produce some semiconductor devices. It produces integrated circuits with very good isolation as the silicon can be accurately selectively etched, and sapphire also has good thermal conductivity.

Some ceramics such as PZT cannot be deposited directly on to silicon as it reacts with it, so a conducting layer such as iridium oxide is used as a buffer between them. In practice several different thin films are used as barriers making highly sophisticated structures.

17.2.3 Applications

Because of their reliability, thermal resilience and stability, thin film ceramic packages are used extensively in the military and automotive industry. Individual thin film chips are produced in 100’s of millions each year.

Magnetic Applications - Thin film ferrites are used extensively for recording media, microwave devices in integrated circuits and coatings for microwave shielding. By far the largest application of thin film ferrites is in recording media. Magnetic tape and discs are made up of layers of single domain particles deposited on a plastic substrate. A balance has to be made in its coercive force so it is high enough to resist demagnetisation but low enough to write on. The material used was initially ferrous oxide but this was then replaced by barium ferrite. The magnetic media is written on and read by a magnetically soft recording “head” by way of its magnetic fringing field that aligns the domains to store the data, and reads the subsequent magnetic flux density changes. Ferrite heads are preferred as they are very resistant to tape wear. The heads can be polycrystalline (manganese zinc ferrite), but single crystal permits narrower track widths. Hard discs are contactless and are supported by airflow from the rotation.

Diagram of read/write mechanism

To form an inductor, a thin metallic meander pattern can be sputtered onto an epitaxial ferrite film such as YIG on the surface of a semiconductor. This can then have an overlay of epitaxial YIG applied to increase its inductive value. 

Planar inductors - sources Murata and amdl
Shinshu

Pyrometric Applications - Thin film pyrometers can have very small thermal capacity so have a good response time when used for thermal imaging. Linear and 2D arrays of PZT devices typically 100 microns thick can be deposited on thin silicon nitride membranes over the silicon semiconductor signal processing substrate. By very accurate selective etching, thin microbridges of ferroelectric and electrodes are produced on narrow legs above the silicon surface. The devices are used for thermal imaging for fire fighters, the detector being attached to their helmets.

Capacitive Applications - There are many uses for thin film capacitors. Three examples are firstly in DRAM memory cells, secondly integrated onto silicon chips in their final process stages for decoupling direct and ac currents, and thirdly in multichip modules where the capacitors are discrete devices.