Example from Wang group Work of Jiasheng Lu. Starting material.
Starting material and wafer production
Transcript of Starting material and wafer production
-
8/12/2019 Starting material and wafer production
1/20
WAFER PRODUCTIONWAFER PRODUCTIONWAFER PRODUCTIONWAFER PRODUCTION
-
8/12/2019 Starting material and wafer production
2/20
Raw materials: Sand,
coke, coal, wood chips
Metallurgical grade silicon
(used in making steel)
Electronic grade
silicon(polysilicon)
Single crystal boule
Single crystal
wafers
Finished wafers
Semiconductor materials used: Silicon
GaAs (only for high frequency applications)
Aim: Obtaining extremely pure silicon to be able to control doping to
control electrical properties accepted level of impurities < 1 over
109 atoms of silicon
Obtaining very regular silicon crystals (defects in the
crystallographic building worsen conduction characteristics of the
semiconductor)
Obtaining a monocrystalline structure rather than a polycrystalline
one (a set of different crystallographic buildings with different
orientations polysilicon): in a polycrystalline structure the
carriers mobility is reduced because of discontinuities betweendifferent crystals !!!
Monocrystals Growth for IntegratedMonocrystals Growth for IntegratedMonocrystals Growth for IntegratedMonocrystals Growth for Integrated
Circuits TechnologyCircuits TechnologyCircuits TechnologyCircuits Technology
-
8/12/2019 Starting material and wafer production
3/20
Starting material for silicon is relatively pure quartz based sand (SiO2 orsilica)
Silicon Refining Sand is reduced in an arc furnace with coal, coke, and
wood chips
SiO2 + 2 C Si (solid) + 2CO (gas) @ 2000C for ~ 8 days
This forms metallurgical grade silicon (MGS
): purity ~ 98%
Major impurities (metals):
Fe 0.8%, Al, 0.3%, Cr 0.04%, Ti 0.03%, Mn & V 0.02%
Mg & Ni 0.01%
MGS crushed and reacted with HCl in a fluidized bed to obtain
trichlorosilane (TCS)
3HCl (gas) + Si (solid) --> HSiCl3 (gas) + H2 (gas) @ 325C
Reaction is exothermic, must cool to keep at 325 C to minimize byproduct formation
B and P impurities present in HSiCl3 HSiCl3 is a liquid at room temperature (boiling point at 32C) impurities are
eliminated through fractioned distillation of the liquid
Monocrystals GrowthMonocrystals GrowthMonocrystals GrowthMonocrystals Growth Starting MaterialsStarting MaterialsStarting MaterialsStarting Materials
for Siliconfor Siliconfor Siliconfor Silicon
-
8/12/2019 Starting material and wafer production
4/20
Purified TCS is reduced with H2 to obtain electronic-grade silicon (EGS)
HSiCl3 (gas) + H2 (gas) 3HCl (gas) + Si (solid) Reaction (Chemical Vapor Deposition) occurs in a cold chamber of a reactor
containing an heated Si rod which acts as crystalline seed
The result is an extremely pure (~ 1 impurities over 109 atoms) polycrystalline silicon,
which will be the raw material for the production of the monocrystalline one with the
Czochralski (CZ) or with the Float-Zone (FZ) process
The fabrication of slices from pure silicon is based on a large set of steps that are
presented in the following. The approach consists first to fabricate a crystal, precisely
an ingot, which will be further cut in slices that will be transformed afterwards in
wafers.
Monocrystals GrowthMonocrystals GrowthMonocrystals GrowthMonocrystals Growth EGS SiliconEGS SiliconEGS SiliconEGS Silicon
-
8/12/2019 Starting material and wafer production
5/20
-
8/12/2019 Starting material and wafer production
6/20
CzochralskiCzochralskiCzochralskiCzochralski ProcessProcessProcessProcess Crucibles & PullersCrucibles & PullersCrucibles & PullersCrucibles & Pullers
EvolutionEvolutionEvolutionEvolution
-
8/12/2019 Starting material and wafer production
7/20
Parts of Ingot DefinitionParts of Ingot DefinitionParts of Ingot DefinitionParts of Ingot Definition
-
8/12/2019 Starting material and wafer production
8/20
Doping From The meltDoping From The meltDoping From The meltDoping From The melt
Add dopants to melt to pull doped crystals
dopants added in the form of doped polysilicon
for concentration control.
Concentration of dopant in crystal is not the
same as that in the melt.
segregation coefficient
k=C s
CLCS = Concentration of dopant in solid. [wt dopant/wt solid]
CL = Concentration of dopant in melt. [wt dopant/wt melt]
-
8/12/2019 Starting material and wafer production
9/20
Doping from the meltDoping from the meltDoping from the meltDoping from the melt
Since k in general is less than 1, thedopant becomes increasingly more
concentrated in the melt. Dopant concentration changes along the
length of the crystal.
Another problem is oxygen doping due
to the oxygen presents in the crucible(generally made of SiO2)
Dopant P As Bk 0.35 0.3 0.8atomic
weight
30.97 74.92 10.81
-
8/12/2019 Starting material and wafer production
10/20
Float Zone (FZ)Float Zone (FZ)Float Zone (FZ)Float Zone (FZ) ProcessProcessProcessProcess
Crystallization is performed by controlling the growth of the seed fixed
at an extremity thanks to a moving of a "fused-zone" (noted "FZ").
This zone is heated at a temperature just under the fusion point by a
high frequency power supply via a coil surrounding the ingot.
-
8/12/2019 Starting material and wafer production
11/20
This FZ technique is also involved for purifying the CZ ingot; the principle is to
take benefit of the difference of diffusion of species at high temperature in a
crystal. In this case, up to three coils are set in the system to save time.
Several passes are usually performed for purification.
This technique allows fabricating very lightly doped ingots, very useful for
high power devices or high voltage integrated circuits (quasi-intrinsic zones
allowing high voltage strength).
Float Zone (FZ)Float Zone (FZ)Float Zone (FZ)Float Zone (FZ) ProcessProcessProcessProcess
Technique more expensive than CZ:
Used really very seldom (about 2% of the wafer market are produced by FZ process)
Only for applications where very pure silicon is required
-
8/12/2019 Starting material and wafer production
12/20
IIIIIIIIIIII----V Monocrystals GrowthV Monocrystals GrowthV Monocrystals GrowthV Monocrystals Growth
LIQUID ENCAPSULATED CZ (LEC): CZ technique modification for the growth of GaAs
GaAs has a low boiling point and evaporation could result in toxic fumes and non-
uniform crystal growth
Solution: to pressurize the chamber and melt B2O3 (about 1 cm thick) on top to seal and
suppress evaporation
B2O3 does not react with GaAs during growth
Ga melt at 30C, B2O3 melt at 500C and seal the crucible, while Ga and As startreacting at 800C to produce GaAs
-
8/12/2019 Starting material and wafer production
13/20
IIIIIIIIIIII----V Monocrystals GrowthV Monocrystals GrowthV Monocrystals GrowthV Monocrystals Growth
LIQUID ENCAPSULATED CZ (LEC):
High pressure As atmosphere in the reactor to avoid Asevaporation (very volatile element)
Crystalline seed introduction and monocrystal growth
Slow cooling down (30-80C/h)
Pyrolitic BN crucibles do not react with GaAs
-
8/12/2019 Starting material and wafer production
14/20
Ingot inspection
Undersize
Control of the resistivities on the top and bottom faces of the ingot: due to the variation
of the doping concentration during the pulling, the final resistivity varies in function of
location. A check of resistivity and an agreement with specifications are needed. A four
probe method equipment makes these measurements.
Check of the crystallographic orientation of the ingot
~ 50% rejected
Shaping of ingot
Make it round and of the correct diameter: during the pulling, due to the very large set of
physical parameters to control, the diameter of the ingot slightly varies. That creates
some waves at the ingot surface. To get slices of calibrated diameter suitable for
automatic equipment, a cylindrical polishing is needed.
From Boule to WafersFrom Boule to WafersFrom Boule to WafersFrom Boule to Wafers Ingot cropping
This operation consists in cutting, or cropping,the extremities of the ingot, which are the high
defect concentration regions with a variable
diameter
-
8/12/2019 Starting material and wafer production
15/20
From Boule to WafersFrom Boule to WafersFrom Boule to WafersFrom Boule to Wafers
Flat(s) grinding
One or two flat zones on the edge of the ingot areprocessed to get a crystallographic orientation reference
for the wafer fabrication. This reference will be used during
the wafer process (orientation of conducting zones,
crystallographic axes for the die cutting).
Ingot sawing
This sawing is proceeded with a diamond tooth saw (creation of a thickness of powder
equivalent to the saw thickness)
-
8/12/2019 Starting material and wafer production
16/20
From Boule to WafersFrom Boule to WafersFrom Boule to WafersFrom Boule to Wafers
-
8/12/2019 Starting material and wafer production
17/20
From Boule to WafersFrom Boule to WafersFrom Boule to WafersFrom Boule to Wafers
Edge grinding
After sawing, some "peaks of matter" remain on theperipheral zones of the slices. One has to remove them. In
addition, to make easier the manipulation of the wafers
during the IC's fabrication process, a circle shaped edge is
proceeded; this avoids the degradation of the wafer transfer
equipment but also creating cracks, or dislocations in thecrystal, which lead to definitive brakes.
Wafer lapping and grinding
The thicknesses after sawing can be significantly
different. To decrease the cost, the quantity of matter
to grind has to be minimized. Usually, the wafers aresorted by thickness range of ten micrometers (10
m).
In order to improve their surface quality, the slices
are polished using a mixture that contains alumina or
diamond grains for which the size is about severalmicrometers (final roughness < 2 m).
-
8/12/2019 Starting material and wafer production
18/20
From Boule to WafersFrom Boule to WafersFrom Boule to WafersFrom Boule to Wafers
Wafer polishing
This polishing can be mechanical or chemico-mechanical based. This operation is performed with an
equipment similar to the lapper, but the polishing
solution is less agressive with a mixture containing
smaller grains of alumina or diamond (the grain
diameters can be as low as 0.1 m) and acid or basicchemical agents.
Mirror finishing is necessary for lithographic steps.
-
8/12/2019 Starting material and wafer production
19/20
From Boule to WafersFrom Boule to WafersFrom Boule to WafersFrom Boule to Wafers
Wafer cleaning
This step consists in removing the abrasive species and contaminants by ultra puredesionized water rinsing.
Laser marking for ID
The writting of the lots mentionning ingot number, date, etc.. , is performed
through a laser beam scanning. These indications will allow controllingeach wafer during the fabrication process of circuits and devices.
-
8/12/2019 Starting material and wafer production
20/20
Finished WaferFinished WaferFinished WaferFinished Wafer
300 mmdiameter
wafer
200 mm
diameterwafer
450 mm
diameter
wafer