ME 2733
Victoria Willis
April 11th 2013
Silicon Dioxide: Structure, Formation, and Applications
Abstract:
This paper presents a review on the structure, formation and applications of silicon
dioxide. Silicon dioxide is a ceramic compound commonly known as silica and is used in a
variety of industries. Silicon dioxide can exist in its many crystalline and amorphous structures at
different temperatures and pressures. The formation of silicon dioxide is a significant process
and can be synthesized by two different methods. It is one of the most abundant compounds on
the planet most commonly found in nature as sandstone, silica sand, or quartzite. Silicon dioxide
is a very versatile and simple ceramic and can be applied in building, food, optical and glass
industries.
Introduction:
Silicon dioxide is an integral part of a variety of industries due to its simple and neutral
properties. Silicon dioxide is considered to be a valuable material in industrial settings due to the
production of SiO2 layers by variable processes, including thermal oxidation or chemical vapor
deposition. SiO2 can block the diffusion of impurities while acting as an excellent insulator, and
is stable at extremely high temperatures, an attractive quality in many materials. Silicon and
oxygen are two of the most abundant elements in the earth crust and they consequently compose
the bulk of soils, rocks, clays, and sand. Silicon dioxide can exist in multiple polymorphs with
varying crystal structures and properties.
Structure:
Silicon dioxide can exist in multiple structures: quartz (α and β), tridymite (α and β),
cristobalite (α and β), stishovite, coesite, and
kaetite (synthetic). The α and β describe the
low and high of each substance, respectively.
Silicon dioxide is electrically neutral and all
atoms have stable electronic structures. This
allows multiple structures to exist at varying
temperatures and pressures. As shown in
Figure 1, alterations to temperature and
pressure can change the composition and
structure of a polymorph. Compositions on the
Figure 1 Phase diagram of SiO2 at various
line result in 2-3 solid solutions at a specific
temperatures and pressures. See reference 1.
temperature and pressure. As stated, silicon
dioxide can exist in crystalline or amorphous forms. Figure 2 describes the chemical structure of
crystalline and non-crystalline forms of SiO2. Each silicon ion is bonded to three oxygen ions in
both states. The lattice of the crystalline structure is rigid with fixed bond angles while the noncrystalline structure is much more
disordered and irregular. Silicon
dioxide has strong covalent bonds,
with two oxygen atoms bonded
around a central silicon atom.
Changes in bridge bonds between the
oxygen atoms and the silicon atom
result in changes in properties and
crystal structure. Non-bridging atoms
occur when some oxygen atoms are
only bonded to one silicon atom. If the oxygen atoms
Figure 2 Chemical structure of crystalline
are bridging, the crystalline structure, quartz, will
and non-crystalline SiO2. See reference 2.
exist. The crystal structures of these polymorphs
range from tetragonal, hexagonal, monoclinic, and orthorhombic. Bonds between silicon and
oxygen can form a variety of angles without changing the bond energies significantly. This result
of flexibility in the bridge bonds causes SiO2 to have
different crystalline structures and properties.
Formation:
In order to form SiO2, oxidation must occur.
Two methods to produce SiO2 are by thermal
oxidation and chemical vapor deposition. Thermal
oxidation is a process to turn silicon into silicon
dioxide and is characterized by high temperatures.
Figure 3 shows the process of oxidants diffusing
across the interface of an already existing oxide. A
Figure 3: Process of thermal oxidation
reaction takes place at the interface, converting Si to SiO2. of silicon dioxide. See reference 3.
Thermal oxidation of silicon is achieved by heating to
temperatures ranging from 900-1200⁰C. Pure water vapor or oxygen is typically the atmosphere
in which oxidation takes place. Both of these components can diffuse easily through the growing
layers at high temperatures. Two approaches to develop silicon dioxide is by dry oxidation and
wet oxidation. Dry oxidation describes growth using pure oxygen and is specifically based on
the chemical reaction as shown by Reaction 1.
𝑆𝑖 + 𝑂2 → 𝑆𝑖𝑂2
Reaction 1
Wet oxidation describes growth using water vapor and can be shown by Reaction 2.
𝑆𝑖 + 2𝐻2 𝑂 → 𝑆𝑖𝑂2 + 2𝐻2
Reaction 2
Dry oxidation produces a more dense oxide while wet oxidation produces a more open oxide,
𝑔
with a lower density of 2.15 𝑐𝑚3 and weaker structure. Dry oxidation produces SiO2 with a
𝑔
density of 2.25𝑐𝑚3 . Due to stoichiometric relationships in the reaction and the different densities
of silicon and silicon dioxide, about 46% of the silicon is consumed during oxidation. In other
words, for 1 um of growth, 0.46 um of silicon is
consumed. (8)
Chemical vapor deposition (CVD) is a
chemical process used to produce pure and
efficient materials. There are three types of
chemical vapor deposition: Atmospheric
pressure CVD (APCVD), low pressure CVD
(LPCVD), and plasma enhanced CVD
(PECVD.) This process is often used in the
semiconductor industry to produce thin films. It Figure 4: Overall process of CVD. Reactant gases flow over
forms a non-volatile solid film on a substrate
substrate, creating a laminar boundary layer and reacting at
interface. Byproduct gases are then removed. See reference 5.
by reaction of vapor phase chemicals as
illustrated by Figure 4. Typically, the substrate is exposed to gases and will react or decompose
the substrate surface to produce the desired deposit.[engine hb] Volatile by-products are often
formed in the process. Gases are introduced to a reaction chamber in a turbulent and well-mixed
stream and the substrate and film-forming chemical reactions occur. The shape of the substrate is
extremely important in this process so that the film thickness will be uniform throughout.
Desorption and removal of gaseous by-products occurs and the process is continued. Source
gases may often include silane (𝑆𝑖𝐻4 ) and oxygen, nitrous oxide (𝑁2 𝑂) and dichlorosilane
(𝑆𝑖𝐶𝑙2 𝐻2), and tetraethylorthosilicate (𝑆𝑖(𝑂𝐶2 𝐻5 )4). These reactions are shown by Reactions 3,
4, and 5.
𝑆𝑖𝐻4 + 𝑂2 → 𝑆𝑖𝑂2 + 2𝐻2
Reaction 3
𝑆𝑖𝐶𝑙2 𝐻2 + 2𝑁2 𝑂 → 𝑆𝑖𝑂2 + 2𝑁2 + 2𝐻𝐶𝑙
Reaction 4
𝑆𝑖(𝑂𝐶2 𝐻5 )4 + 6𝑂2 → 𝑆𝑖𝑂2 + 10𝐻2 𝑂 + 8𝐶𝑂2
Reaction 5
Each of the source gases will deposit at various
temperatures. Silane deposits between 300-500⁰C,
dichlorosilane deposits near 900⁰C, and
tetraethylorthosilicate (TEOS) deposits between 650-750⁰C.
Reaction 4 occurs by PECVD while Reaction 3 and 5 both
take place by APCVD. APCVD operates at atmospheric
pressure and results in high deposition rates, however the
film is not always uniform. Figure 5 shows the deposition
films with APCVD of Reactions 4 and 5. Both have
conformal characteristics and the film is in thicker in (a) but
is more uniform in (b). LPCVD occurs at a thin boundary
layer and reactants easily diffuse into the film. PECVD
produces denser films and improve deposition rates. The
deposition rate slowly increases with an increasing
temperature when at already high temperatures. When this
occurs, the reaction rate increases and diffusion decreases.
The process of chemical vapor deposition will overall
Figure 5 Film depositions on Si using
produce a lower quality oxide compared to thermal
SiH4 (a) and TEOS (b). See reference .
oxidation. There is a higher efficiency in the produced film
thickness and usage of reactants CVD is a thermally driven
reaction.
Applications:
Silicon dioxide can be used in a variety of industries involving glass, food,
microelectronics, and building manufacturing. Quartz glass provides the basis for manufacturing
lenses and other optical components as well as temperature resistant equipment for the chemical
industry.[4] Silica is an important material for use as a refractory in the production of glass.[5] The
addition of sodium oxide will decrease the softening temperature and the viscosity of SiO2,
allowing the glass to be worked at a lower temperatures. In the food and drug industry,
amorphous forms of SiO2 are added to salts and spices to prevent caking and act as carriers in
drug capsules. They are additionally added to flavoring and colors of foods and are considered
safe because they cannot be absorbed by the human body. In microelectronic mechanical systems
(MEMS), silicon dioxide is a key material in the production of electronic devices where it acts as
a supporting or insulating layer. It is a desirable material to use due to its high thermal stability
and abrasiveness. SiO2 acts as an intermetal dielectric where it provides electrical insulation and
prevents stress and cracking of other materials. In the building industry, silica consists 19 to 23%
of portland cement, a fine powder used for concrete. SiO2 is also found in the compositions of
many building materials including plaster, tile and cement. The structure and properties of SiO2
make it an attractive material due to its chemical stability and willingness to react with other
compounds.
Conclusion:
This paper described the structure, formation, and applications of silicon dioxide. Silicon
dioxide can exist in a crystalline form or in many amorphous forms in which the bridge bonds
between atoms describe the resulting crystal structure. Formation of SiO2 can be completed by
thermal oxidation or chemical vapor deposition. Silicon dioxide can be applied to various
industries varying from glass, food, microelectronic mechanical systems, and construction.
Further research can be constructed for additional properties for silicon dioxide and its
crystalline and amorphous structures and their applications in industry.
References:
1. “Tectosilicates, Carbonates, Oxides, & Accessory Minerals” available by
http://www.tulane.edu/~sanelson/eens211/tectosilictes&others.htm [Accessed 04/09/2013]
2. “Lessons for Cryonics from Metallurgy and Ceramics” available via
http://www.benbest.com/cryonics/lessons.html [Accessed 04/07/2013]
3. “Principles of the Oxidation Process” available via
http://www.iue.tuwien.ac.at/phd/hollauer/node12.html [Accessed 04/07/2013]
4. “Silicon Dioxide” available via
http://www.nanopartikel.info/cms/lang/en/wissensbasis/siliciumdioxid [Accessed 04/07/2013]
5. “Chemical Vapor Deposition- Rice University” available via
http://www.precisionfab.net/tutorials02.aspx [Accessed 04/08/2013]
6. “Chemical Vapor Deposition” available via
http://www.engineershandbook.com/MfgMethods/cvd.htm [Accessed 04/09/2013]