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Pre-oxidation cleaning

The first step in oxidizing a surface of silicon is the removal of the native oxide which forms due to exposure to open air. This may seem redundant to remove an oxide only to put on another, but this is necessary since uncertainty exists as to the purity of the oxide which is present. The contamination of the native oxide by both organic and inorganic materials (arising from previous processing steps and handling) must be removed to prevent the degradation of the essential electrical characteristics of the device. A common procedure uses a H 2 O-H 2 O 2 -NH 4 OH mixture which removes the organics present, as well as some group I and II metals. Removal of heavy metals can be achieved using a H 2 O-H 2 O 2 -HCl mixture, which complexes with the ions which are formed. After removal of the native oxide, the desired oxide can be grown. This growth is useful because it provides: chemical protection, conditions suitable for lithography, and passivation. The protection prevents unwanted reactions from occurring and the passivation fills vacancies of bonds on the surface not present within the interior of the crystal. Thus the oxidation of the surface of silicon fulfills several functions in one step.

Thermal oxidation

The growth of oxides on a silicon surface can be a particularly tedious process, since the growth must be uniform and pure. The thickness wanted usually falls in the range 50 - 500 Å, which can take a long time and must be done on a large scale. This is done by stacking the silicon wafers in a horizontal quartz tube while the oxygen source flows over the wafers, which are situated vertically in a slotted paddle (boat), see [link] . This procedure is performed at 1 atm pressure, and the temperature ranges from 700 to 1200 °C, being held to within ±1 °C to ensure uniformity. The choice of oxidation technique depends on the thickness and oxide properties required. Oxides that are relatively thin and those that require low charge at the interface are typically grown in dry oxygen. When thick oxides are required (>0.5 mm) are desired, steam is the source of choice. Steam can be used at wide range of pressures (1 atm to 25 atm), and the higher pressures allow thick oxide growth to be achieved at moderate temperatures in reasonable amounts of time.

Horizontal diffusion tube showing the oxidation of silicon wafers at 1 atm pressure.

The thickness of SiO 2 layers on a Si substrate is readily determined by the color of the film. [link] provides a guidline for thermal grown oxides.

Color chart for thermally grown SiO 2 films observed under daylight fluorescent lighting.
Film thickness (μm) Color Film thickness (μm) Color
0.05 tan 0.63 violet-red
0.07 brown 0.68 "bluish"
0.10 dark violet to red-violet 0.72 blue-green to gree
0.12 royal blue 0.77 "yellowish"
0.15 light blue to metallic blue 0.80 orange
0.17 metallic to light yellow-green 0.82 salmon
0.20 light gold 0.85 light red-violet
0.22 gold 0.86 violet
0.25 orange to melon 0.87 blue violet
0.27 red-violet 0.89 blue
0.30 blue to violet blue 0.92 blue-green
0.31 blue 0.95 yellow-green
0.32 blue to blue-green 0.97 yellow
0.34 light green 0.99 orange
0.35 green to yellow-green 1.00 carnation pink
0.36 yellow-green 1.02 violet red
0.37 green-yellow 1.05 red-violet
0.39 yellow 1.06 violet
0.41 light orange 1.07 blue-violet
0.42 carnation pink 1.10 green
0.44 violet-red 1.11 yellow-green
0.46 red-violet 1.12 green
0.47 violet 1.18 violet
0.48 blue-violet 1.19 red-violet
0.49 blue 1.21 violet-red
0.50 blue green 1.24 carnation pink to salmon
0.52 green 1.25 orange
0.54 yellow-green 1.28 "yellowish"
0.56 green-yellow 1.32 sky blue to green-blue
0.57 "yellowish" 1.40 orange
0.58 light orange to pink 1.46 blue-violet
0.60 carnation pink 1.50 blue

Questions & Answers

what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
Daniel
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
Maciej
characteristics of micro business
Abigail
for teaching engĺish at school how nano technology help us
Anassong
Do somebody tell me a best nano engineering book for beginners?
s. Reply
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
Virgil
is Bucky paper clear?
CYNTHIA
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Harper
Do you know which machine is used to that process?
s.
how to fabricate graphene ink ?
SUYASH Reply
for screen printed electrodes ?
SUYASH
What is lattice structure?
s. Reply
of graphene you mean?
Ebrahim
or in general
Ebrahim
in general
s.
Graphene has a hexagonal structure
tahir
On having this app for quite a bit time, Haven't realised there's a chat room in it.
Cied
what is biological synthesis of nanoparticles
Sanket Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
China
Cied
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
Porter
many many of nanotubes
Porter
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Yasmin
what is the function of carbon nanotubes?
Cesar
I'm interested in nanotube
Uday
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
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what is system testing?
AMJAD
preparation of nanomaterial
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how to synthesize TiO2 nanoparticles by chemical methods
Zubear
how did you get the value of 2000N.What calculations are needed to arrive at it
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Source:  OpenStax, Chemistry of electronic materials. OpenStax CNX. Aug 09, 2011 Download for free at http://cnx.org/content/col10719/1.9
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