Passivation

Generally speaking, this has lead to devices being coated directly by a thin film of either silicon dioxide or, more commonly, silicon nitride. These are usually deposited at relatively low temperatures using a technique known as plasma enhanced chemical vapour deposition (PECVD), since high temperatures may affect components already on the device, or induce unnecessary mechanical stress.

Silicon nitride is commonly used as it is resistant to wear and provides a good barrier to sodium ions in the environment, which penetrate into oxide layers and destroy their insulating properties. However, under some circumstances a nitride layer alone is not suitable. For instance, in physiological saline solution, under an applied electric field such as may result from active components on a chip, nitride rapidly degrades. So sometimes multiple layers of oxide and nitride are used: the oxide insulating the nitride from current flow, and the nitride protecting the oxide from sodium ions.

More recently, it has become possible to deposit films of diamond. Diamond has excellent resistance to wear, is a good electrical insulator, and a good conductor of heat. However, the deposition process for these films is relatively high temperature (around 700 to 900 Celsius), the films are polycrystalline (made of many small crystals), and are relatively difficult to machine.

An alternative to diamond is so-called diamond like carbon (DLC). DLC films can be deposited using PECVD techniques, and the deposition can be controlled to produce films with different qualities. These, like diamond, are also biocompatible.

It would appear that diamond like carbon will be an excellent material for passivation of future microsystems and devices.