Editing Chemical vapor deposition (section)

=== Silicon dioxide ===
Silicon dioxide (usually called simply "oxide" in the semiconductor industry) may be deposited by several different processes. Common source gases include [[silane]] and [[oxygen]], [[dichlorosilane]] (SiCl<sub>2</sub>H<sub>2</sub>) and [[nitrous oxide]]<ref>Proceedings of the Third World Congress of Chemical Engineering, Tokyo, p. 290 (1986)</ref> (N<sub>2</sub>O), or [[tetraethylorthosilicate]] (TEOS; Si(OC<sub>2</sub>H<sub>5</sub>)<sub>4</sub>). The reactions are as follows:<ref name="cao:2011">{{cite book|last1=Cao|first1=Guozhong|last2=Wang|first2=Ying|title=Nanostructures and Nanomaterials -- Synthesis, Properties and Applications|date=2011|publisher=World Scientific Publishing|isbn=978-981-4322-50-8|page=248|doi=10.1142/7885}}</ref>

:SiH<sub>4</sub> + O<sub>2</sub> → SiO<sub>2</sub> + 2 H<sub>2</sub>

:SiCl<sub>2</sub>H<sub>2</sub> + 2 N<sub>2</sub>O → SiO<sub>2</sub> + 2 N<sub>2</sub> + 2 HCl

:Si(OC<sub>2</sub>H<sub>5</sub>)<sub>4</sub> → SiO<sub>2</sub> + byproducts

The choice of source gas depends on the thermal stability of the substrate; for instance, [[aluminium]] is sensitive to high temperature. Silane deposits between 300 and 500&nbsp;°C, dichlorosilane at around 900&nbsp;°C, and TEOS between 650 and 750&nbsp;°C, resulting in a layer of ''low- temperature oxide'' (LTO). However, silane produces a lower-quality oxide than the other methods (lower [[dielectric strength]], for instance), and it deposits non[[conformal film|conformally]]. Any of these reactions may be used in LPCVD, but the silane reaction is also done in APCVD. CVD oxide invariably has lower quality than [[thermal oxidation|thermal oxide]], but thermal oxidation can only be used in the earliest stages of IC manufacturing.

Oxide may also be grown with impurities ([[alloy]]ing or "[[doping (semiconductor)|doping]]"). This may have two purposes. During further process steps that occur at high temperature, the impurities may [[atomic diffusion|diffuse]] from the oxide into adjacent layers (most notably silicon) and dope them. Oxides containing 5–15% impurities by mass are often used for this purpose. In addition, silicon dioxide alloyed with [[phosphorus pentoxide]] ("P-glass") can be used to smooth out uneven surfaces. P-glass softens and reflows at temperatures above 1000&nbsp;°C. This process requires a phosphorus concentration of at least 6%, but concentrations above 8% can corrode aluminium. Phosphorus is deposited from phosphine gas and oxygen:

:4 PH<sub>3</sub> + 5 O<sub>2</sub> → 2 P<sub>2</sub>O<sub>5</sub> + 6 H<sub>2</sub>

[[Glass]]es containing both boron and phosphorus (borophosphosilicate glass, BPSG) undergo viscous flow at lower temperatures; around 850&nbsp;°C is achievable with glasses containing around 5 weight % of both constituents, but stability in air can be difficult to achieve. Phosphorus oxide in high concentrations interacts with ambient moisture to produce phosphoric acid. Crystals of BPO<sub>4</sub> can also precipitate from the flowing glass on cooling; these crystals are not readily etched in the standard reactive plasmas used to pattern oxides, and will result in circuit defects in integrated circuit manufacturing.

Besides these intentional impurities, CVD oxide may contain byproducts of the deposition. TEOS produces a relatively pure oxide, whereas silane introduces hydrogen impurities, and dichlorosilane introduces [[chlorine]].

Lower temperature deposition of silicon dioxide and doped glasses from TEOS using ozone rather than oxygen has also been explored (350 to 500&nbsp;°C). Ozone glasses have excellent conformality but tend to be hygroscopic – that is, they absorb water from the air due to the incorporation of silanol (Si-OH) in the glass. Infrared spectroscopy and mechanical strain as a function of temperature are valuable diagnostic tools for diagnosing such problems.

==== Silicon nitride ====
Silicon nitride is often used as an insulator and chemical barrier in manufacturing ICs. The following two reactions deposit silicon nitride from the gas phase:

:3 SiH<sub>4</sub> + 4 NH<sub>3</sub> → Si<sub>3</sub>N<sub>4</sub> + 12 H<sub>2</sub>

:3 SiCl<sub>2</sub>H<sub>2</sub> + 4 NH<sub>3</sub> → Si<sub>3</sub>N<sub>4</sub> + 6 HCl + 6 H<sub>2</sub>

Silicon nitride deposited by LPCVD contains up to 8% hydrogen. It also experiences strong tensile [[stress (physics)|stress]], which may crack films thicker than 200&nbsp;nm. However, it has higher [[resistivity]] and dielectric strength than most insulators commonly available in microfabrication (10<sup>16</sup> [[ohm|Ω]]·cm and 10 M[[volt|V]]/cm, respectively).

Another two reactions may be used in plasma to deposit SiNH:

:2 SiH<sub>4</sub> + N<sub>2</sub> → 2 SiNH + 3 H<sub>2</sub>

:SiH<sub>4</sub> + NH<sub>3</sub> → SiNH + 3 H<sub>2</sub>

These films have much less tensile stress, but worse electrical properties (resistivity 10<sup>6</sup> to 10<sup>15</sup> Ω·cm, and dielectric strength 1 to 5 MV/cm).<ref>{{cite book|page = 384| title = Semiconductor devices: physics and technology|author = Sze, S.M. | publisher = Wiley-India| year = 2008|isbn =978-81-265-1681-0}}</ref>