17.2.2 Manufacturing Processes

Process of forming a thin film circuit on a silicon
semi-conductor - source frenchrh upenn

Photolithography is the technique commonly used to manufacture silicon and other semiconductors and is also the method used to form thin film circuits on ceramic substrates. The integration of thin film advanced ceramic material in devices has proven to be quite difficult, requiring complicated design and manufacturing processes, partly because of the incompatibility of high ceramic sintering temperatures with other materials, although thin films fire at significantly lower temperatures than bulk ceramics and are easier to densify. The ceramic thin film has to be of the correct composition, crystal structure and orientation, as microstructure problems such as with grain boundaries can affect performance. It also has to have correct dimensions, effective contact, good adhesion to the substrate and should not degrade itself or other parts over time.

The methods of depositing thin films onto the various substrates include firstly physical vapour deposition in a vacuum, secondly chemical vapour deposition, and thirdly chemical solution deposition.

An example of the first method is “sputtering” that takes place in a vacuum chamber with a small amount of an appropriate inert gas. The bulk material to be deposited forms the cathode (negative electrode) and the substrate the anode (positive). A few hundred volts (direct current) between them are enough to form a plasma discharge in the gas, similar to the functioning of a fluorescent light tube. Ions strike the cathode knocking off microscopic particles that travel to the substrate, adhere and build up the required film, but it is difficult to obtain an adhesive homogenous layer over 2 microns thick.

Another example is “vapour phase deposition” again carried out in a vacuum chamber where the target material is heated to its vaporisation temperature. The vaporised material then transfers to the substrate. This method can give thicker, even coatings of homogenous ceramic over a large (300mm) substrate, but is unidirectional, whereas sputtering can coat the sides of vias. Some methods use an electron beam to vaporise the target material, or a laser, a method called laser ablation, using a short intense pulse. The advantages of vacuum processes are that they are compatible with semiconductor processing and produce high purity films. These are successful method but have a low deposition rates, controlling the layer is difficult in multi-component ferroelectrics and it is not easy to convert to large-scale production.

Vacuum vapour deposition illustration

Vacuum electron beam vapour deposition illustration

Deposition is possible from chemical vapour, where a thermo-chemical reaction is used to produce the required deposition, for example silicon can be deposited from silane (SiH4) that decomposes to silicon and hydrogen, or molybdenum from molybdenum chloride and hydrogen producing molybdenum and HCl.

The sol-gel solution method is also extensively used and subsequent firing will remove any organic solvents used. These methods have higher deposit rates, but several coatings may be required to obtain the final thickness. They also provide good layer control, larger area high quality films, but the toxicity of some ferroelectric raw materials is a disadvantage. Recent work on lowering the sintering temperature of ceramics such as PZT uses novel forms of the sol-gel process that has achieved deposits on silicon at 440 ºC. This development could have a significant influence on the ultra miniature MEMS devices described later.