Transcript Fabrication - Shahadat Hussain Parvez

FABRICATION
PROCESSES
FABRICATION PROCESSES
Presentation on ‘FABRICATION PROCESSES’
Course no : EEE 453
Course by : Mohiuddin Munna
Before Starting the Fabrication
It starts with making silicon
Before Starting the Fabrication
 Ingots get cut into wafers, which are 1-2mm thick, and
upto 12” in diameter
 Entire wafers are processed, and then cut into chips
when needed
Fabrication Processes
Six main process steps
1.
2.
3.
4.
5.
6.
Oxidation
Diffusion
Ion implantation
Lithography
Thin film deposition
Epitaxy
Oxidation
 the first step in semiconductor device fabrication
involves the oxidation of the wafer surface in order to
grow a thin layer of silicon dioxide (SiO2). This oxide is
used to provide insulating and barrier to diffusion
layers.
 The most common method of oxidation is thermal,
and can be classified as either "dry" or "wet" oxidation.
Wafers are loaded into quartz boats and slid into a
furnace heated to approximately 1200ºC.
Oxidation
 In dry oxidation, thin oxide layers are grown in an
environment containing oxygen and hydrogen
chloride near atmospheric pressure
Oxidation
 Thicker oxide layers require higher pressures and the
use of steam (wet oxidation). Wet oxidation is
performed by exposing the wafer to a mixture of
oxygen and hydrogen in the furnace chamber. Water
vapor is formed when the hydrogen and oxygen react
Oxidation Process (Schematic)
Oxidation Process
 Standard oxidation temperature 800-1200 C
 Heat is produced by resistance heating
 Coil like heating elements are arranged in 3 controlled
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zone
Outer zones operates at higher power to compensate heat
loss
Simply Oxygen is fed for dry oxidation
Carrier gas like Ar on N2 is used in wet oxidation along with
heated water or burning O2 and H2 at input of tube
Required time in furnace depends on temperature and
desired thickness
Whole system is automated
Oxidation Process
Fig: Typical Thickness Vs
Oxidation time for 100
crystal compared for dry
and wet oxidation
Diffusion process
 Process of doping
 Si wafer is exposed to solid, liquid or gaseous source
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containing desired impurity
A reaction at wafer surface establishes a supply of dopant
atoms immediately adjacent to Si crystal
At elevated temperature atoms difuse in the region Si is not
protected by oxide
Surface doping concentration is up to 1021 / cm3
Diffusion in SiO2 is relatively low
SiO2 protects Si for a limited time depending on oxide
thickness ,temperature and background doping.
Diffusion process
Diffusion process (Schematic)
Ion Implantation
 It’s an alternative process of introducing dopants
 Dopant ion is accelerated in high energy range from 5 keV
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to 1MeV then shooted into semiconductor
Ions displace Si atoms along their path into crystal
follow-up heating binds ions with crystel
But before that automatic scanning is performed
automatically to determine total number of ions / cm3
Si wafer can be masked using thin flims of SiO2 ,Si3N4 and
photoresist .
Ion Implantation (schematic)
Advantage of Ion Implantation
 Lower temperature process
 Implantation is performed in room temperature
 Follow-up heating is done in 600 C
 Gives precise control over impurity
 Ideally suited for a number of modern device
structures requiring extremely shallow junctions
 damage from implantation can be annealed by
heating the wafer in a furnace to T > 900 C.
Doping by Ion Implantation
 Dose = ion beam flux (# cm-2 s-1) x time for implant ...
units # cm-2
Doping by Ion Implantation
 SiO2 film masks the implant by preventing ions from
reaching the underlying silicon (assuming it’s thick
enough)
 after implantation, the phosphorus ions are confined to a
damaged region near the silicon surface
Doping by Ion Implantation
 Annealing heals damage and also redistributes the
ions (they diffuse further into the silicon crystal)
Doping by Ion Implantation
Fig: Computed phosphorus
Implantation profile assuming a
constant dose of 1014 /cm
Lithography
Process of selectively removing
SiO2 and other masking material
covering wafer surface.
Transfers circuit diagram on wafer
Lithography Process
 At first Si wafer is coated with UV light sensitive
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photoresist in a thin uniform coating.
Wafer is pre baked at 80-100 C
Exposing wafer to UV light through a mask
Mask here is carefully prepared with glass or quartz photo
pale containing a copy of pattern to be transferred to SiO2
Exposed photoresist parts undergo chemical changes
depending on photo resist
In negative photo resist exposed parts form polymer like
structures and unexposed parts dissolves after developing
Lithography
Fig: majos steps in
lithography
a) Apply resist
b) Expose resist through
mask
c) After developing
d) After oxide etching and
resist removal
Thin Film deposition
Three different Techniques
 Evaporation
 Sputtering
 Chemical Vapor deposition
Thin Film deposition(Evaporation)
Thin Film deposition(Sputtering)
Chemical Vapor Deposition(CVD)
 1.Atmospheric Pressure CVD
 2.Low Pressure CVD
 3.Plasma Enhanced CVD
Epitaxy
Epitaxy is a special type of thin layer deposition.
 Whereas deposition described in the previous yields
either amorphous or polycrystalline layer, it yield a
crystalline layer