TimeDomain CVD, Inc.

Silicon Nitride: Properties and Applications

Hard Stuff

"Bulk" silicon nitride, Si3N4, is a hard, dense, refractory material. It's structure is quite different from that of silicon dioxide: instead of flexible, adjustable Si-O-Si bridge bonds, the Si-N-Si structure is rendered rigid by the necessity of nitrogen forming three rather than two bonds. CVD silicon nitride is generally amorphous, but the material is much more constrained in structure than the oxide. As a consequence, nitride is harder, has higher stress levels, and cracks more readily.

The dense structure of silicon nitride does not provide the open channels found in oxide structures; thus, nitride is widely employed in electronics as a barrier material. Even hydrogen diffuses slowly in a densified nitride film, and other small positive ions (Na+ or K+) are effectively blocked by thin nitride layers. Since oxygen diffuses very slowly through nitride, deposited nitride can prevent oxidation of underlying silicon: this property is exploited in local-oxidation -of-silicon (LOCOS) transistor isolation. Nitride layers are also employed as etch stop layers both for wet etching and plasma etching.

Deposited nitrides almost always contain hydrogen, typically much more than in the comparable oxide films. The source of the hydrogen is the silane precursor and possibly also the ammonia oxidant employed in most deposition schemes, but the presence of hydrogen in the film is a consequence of the nitride structure. It is very difficult for the atoms in an amorphous but constrained film like silicon nitride to all occupy positions allowing the valence of each silicon and nitrogen atom to be filled: that is, a lot of broken bonds are present. These bonds are readily occupied by hydrogen atoms. Thus, conventional plasma nitrides can have as much as 20 atomic % hydrogen, bonded both to the Si and N atoms; thermal nitrides still have several % hydrogen even after high-temperature anneals. Note that the group Si-NH-Si is "isoelectronic" to Si-O-Si; the addition of NH groups adds flexibility to the nitride lattice. The amount of hydrogen and the bonding (Si-H or N-H) can be measured by infrared spectroscopy, and are important in characterizing the properties of plasma nitrides. The stoichiometry of nitride films also varies widely, especially in plasma deposition, so that refractive index can vary from about 1.8 to 2.2 and is another useful control parameter for nitride deposition.

In plasma deposition, mixtures of oxygen and nitrogen with silicon -- silicon oxynitrides -- can be prepared by introducing small amounts of oxidant. The properties of the films can be varied fairly continuously from those of the pure oxide to the "pure" (hydrogenated) nitride.

Let's look at some important properties of pure Si3N4:

 

density

3 - 3.3 gm/cm3

electrical conductivity

varies widely

breakdown field

typically a few 10^6 V/cm

thermal conductivity

0.15 W/cm K (bulk)

thermal diffusivity

0.07 cm2/sec (bulk)

coefficient of thermal expansion

3 ppm/ K [note Si thermal exp 2.3 ppm/K]

dielectric constant

6-8 [depends on stoichiometry]

 

 

prevarrow

Films: Table of Contents

nextarrow

Go Home