1. Crystal Growth, Wafer Fabrication and
Chapter 3
Crystal Growth, Wafer Fabrication and
Basic Properties of Silicon Wafers
Dr. Wanda Wosik
ECE 6466, F2012

2. Basic Crystal Lattice
Silicon in microelecronics: Single Crystal Polycrystalline Amorphous periodic small crystals no long range
arrangements order between of atoms atoms
Crystal lattice is described by a unit cell with a base vector (distance between atoms)
Types of unit cells
Body Centered Cube
Face Centered Cube

3. Directions and Planes in Crystals
Directions (vector components: a single direction is expressed as [a set of 3 integers], equivalent directions (family) are expressed as < a set of 3 integers >
Planes: a single plane is expressed as (a set of 3 integers h k l = Miler indices) and equivalent planes are expressed as {a set of 3 integers}
Miler indices: take a,b,c (multiple of basic vectors ex. x=4a, y=3a, z=2a)
reciprocals (1/4, 1/3, 1/2)-> common denominator (3/12, 4/12, 6/12) -> the smallest numerators (3 4 6)
lattice constant

4. Wikipedia

5. Silicon Crystal Structure
Diamond lattice (Si, Ge, GaAs)
Two interpenetrating FCC
structures shifted by a/4 in
all three directions
All atoms in both FCCs
covalent bonding
Diamond
Atoms inside one FCC come from
the second lattice
(100) Si for devices
(111) Si not used oxide charges

8. • Silicon has the basic diamond crystal structure - two merged
FCC cells offset by a/4 in x, y and z.
See 3D models
• Various types of defects can exist
in crystal (or can be created by
processing steps. In general these
are detrimental to device
performance.

9. Defects in Crystals
Point defects, line defects, volume defects

10. Dislocation Formation by Point Defects Agglomeration
Intrinsic point defects in a crystal
Nv and NI increase with T
Agglomeration of Interstitials
collapse
After Shimura

11. Stacking Faults and Grain Boundary
Perfect stacking
OISF
Induced by oxidation
Missing (111) plane
SFs bound by dislocations
After Campbell
After Shimura

12. Types of Dislocations Dislocation line Screw Dislocations
Start the contour here
Dislocation line | | Burger vector - screw
Burger vector - edge b
Edge Disclocation
inserted plane b
In Si 60° dislocations are formed in processing
After Shimura

13. Silicon Crystal w/o Dislocations
Process induced dislocations deteriorate devices
Grown Si crystal is dislocation-free
Dangling bonds of a half plane affect physical and electrical properties (here 60°)
Pure edge dislocation
After Shimura

14. Propagation of Dislocations by Climb Motion
Shift
After Shimura

15. Motion of Dislocations by Glide
Stress induced by
T gradient T
Mismatch of thermal expansion coeff., layers, precipitates
Easy motion of dislocations T
After Wolf&Tauber

16. Raw Material and Purification
After Wolf&Tauber

17. Purification and Preparation of Electronic Grade Semiconductor
MGS EGS Si Crystal
2SiHCl3+H2→2Si+6HCL
gas
solid
MSG
Refined quartzite (SiO2)
ppb purity of EGS for CZ or FZ
300 °C MGS+HCl →SiHCl3
98% pure
Grind to powder
Liquid ar RT
After Wolf&Tauber

18. Crystal Growth
• Si used for crystal growth is purified from SiO2 (sand) through refining,
fractional distillation and CVD.
• The raw material contains < 1 ppb impurities. Pulled crystals contain O
(≈ 1018 cm-3) and C (≈ 1016 cm-3), plus any added dopants placed in the melt.
• Essentially all Si wafers used for ICs today come from Czochralski grown crystals.
• Polysilicon material is melted, held at close to 1417 ˚C, and a single crystal seed is used to start the growth.
• Pull rate, melt temperature and rotation rate are all important control parameters.
→ Introduces SiO2 in CZ; Oi≈ cm-3
→ C ≈ cm-3
Ar ambient

19. Czochralski Growth EGS 100 kg seed •Load EGS+Impurities P, B, As
•pump-out, •seed down,
•pull fast, •pull slow
EGS
100 kg
Neck confines
dislocations
Crystal solidifies
increases -
pull rate decreases
seed
(More information on crystal growth at
Also, see animations of
(Photo courtesy of Ruth Carranza.))

20. Details of Czochralski Growth
Rotation of crucible and crystal in opposite
Directions improves growth and doping
uniformity Ar
Oxygen incorporation - important for
intrinsic gettering cm-3
Carbon contributes to native defects cm-3
After Shimura

21. Oxygen Concentrations in CZ Silicon
Si melts - C-Si growth
Role of temperature
After Campbell

22. Requirement for Larger Crystals
12 inches
After Wolf&Tauber

24. Wafer Preparation and Specification
Mark wafer earlier (laser process) to track their process flow
Grind crystal to a diameter (200mm750µm) … 850µm thick
Grind flats (the primary and secondary)
Saw of the boule into wafers
Lapping, etching (batch process in acids etching Si) 20 µm, polishing (chemical-mechanical) 25µm removes damage and improves flatness ±2µm
SiO2 10nm in NaOH/DI
Suspension Al2O3
CMP
3Si +4HNO3+18HF 3H2SiF6+4NO+8H2O

25. Orientation of ICs on Silicon Wafers
Si cleaves along {111}
For (100) the {111} planes are along <110>
Primary and Secondary Flats
plane
After Shimura

26. Float Zone Method for Crystal Growth
• An alternative process is the float
zone process which can be used for
refining or single crystal growth.
No crucible - no impurities
High resistivity Si
Add Dopants (gas) PH3 B2H6
EGS
ESG

27. Crystal Growth and Wafers Fabrication
• After crystal pulling, the
boule is shaped and cut into
wafers which are then
polished on one side.

28.
After Wolf&Tauber

31. Modeling CZ Crystal Growth
• We wish to find a relationship
between pull rate and crystal
diameter.
• Freezing occurs between
isotherms X1 and X2.
• Heat balance (A B C):
latent heat of crystallization +
heat conducted from melt to crystal
= heat conducted away.
C=Radiation
B=conduction in solid
A=Heat of crystallization
Freezing interface
Liquid to solid → HEAT 
(1)
For crystal uniformity T uniformity is important
Vpmax ~1√r
Large crystal requires slow pull rates

32. • The rate of growth of the crystal is
(2)
where vP is the pull rate and N is the density.
(3)
• Neglecting the middle term in Eqn. (1) we have:
• In order to replace dT/dx2, we need to consider the heat transfer processes.
• Heat radiation from the crystal (C)
is given by the Stefan-Boltzmann law
(4)
• Heat conduction up the crystal is
given by
(5)

33. • Differentiating (5), we have
(6)
(7)
• Substituting (6) into (4), we have
• kS varies roughly as 1/T, so if kM is the
thermal conductivity at the melting point,
(8)
(9)
• Solving this differential equation, evaluating it at x = 0 and substituting the
result into (3), we obtain (see text):
(10)
• This gives a max pull rate of ≈ 24 cm hr-1 for a 6” crystal (see text). Actual values
are ≈ 2X less than this.

34. Dopant Segregation During Crystal Growth
“k” affects doping uniformity 1
After Wolf&Tauber

35. Modeling Dopant Behavior During Crystal Growth
• Dopants are added to the melt to provide a controlled N or P doping level in the wafers.
• However, the dopant incorporation process is complicated by dopant segregation.
Segregation Coefficients of Various Impurities in Silicon
• Most k0 values are <1 which means the impurity prefers to stay in the liquid.
• Thus as the crystal is pulled, NS will increase.

36. Solid Solubility
After Campbell

37. Dopant Incorporation During CZ Growth
• If during growth, an additional volume dV freezes, the impurities incorporated into dV are given by
solidified
(12)
Removed →
from the melt
(13)
(14)
Initial in melt
During the growth
• We are really interested in the impurity level in the crystal (CS), so when incremental volume freezes
(15)
(16)
where f is the fraction of the melt frozen (Vs/V0).

38. Uniformity of Crystal Doping
tail
seed
ko=0.8
• Plot of Eq. (16).
• Note the relatively flat profile produced by boron with a kS close to 1.
• Dopants with kS << 1 produce much more variation in doping concentration along the crystal.
k0=0.35
where f is the fraction of the melt frozen.
k0=0.023

39. After Wolf&Tauber

40. Radial Doping Nonuniformity
After Shimura

41. Zone Refining and FZ Growth
Segregation of Impurities Between Solidus and Liquidus (in FZ Growth)
RF -> melt=zone
moving
Poly-Si
Crystal
C0 original concentration in the rod
I - the number of impurities
in the liquid
dI=(C0-k0CL)dx
• In the float zone process, dopants and other impurities tend to stay in the liquid and therefore refining can be accomplished, especially with multiple passes
• See the text for models of this process.
Distribution of a dopant along the crystal

42. Float Zone Growth; Removal of Impurities
For one pass only → k0 small gives better refining
Better crystal purity

43. Modeling Point Defects in Silicon
• Point defects (V and I) will turn out to play fundamental roles in many process
technologies.
• The total free energy of the crystal is minimized when finite concentrations of
these defects exist.
(17)
Sf entropy & Hf enthalpy of defect formation
• In general and both are
strong functions of temperature.
• Kinetics may determine the concentration
in a wafer rather than thermodynamics.
• In equilibrium, values for these concentrations are given by:
(18)
(19)
Smaller concentrations than those of dopants/carriers(hard to CI0≈1012cm-3 and CV0≈5x1013 cm-3

44. Point Defects
• V and I also exist in charged states with
discrete energies in the Si bandgap.
• In N type Si, V= and V- will dominate; in
P type, V+ and V++ will dominate.
• Shockley and Last (1957) first described
these charged defect concentrations (see text).
Note: • The defect concentrations are always
<< ni. ( doping EF point defect
concentrations)
• As doping changes, the neutral point
defect concentrations are constant.
• However, the charged defect
concentrations change with doping.
\ the total point defect concentrations
change with doping.
(20)
(21)

45. Point Defects
I and V increase with T & are very mobile and affect many various processing
Very difficult to measure
Concentrations of I and V are different (surface generation & recombination= sinks, defects)
Equilibrium concentrations change instantaneously with T
Non-equilibrium concentrations possible (after implantation, CZ growth - freeze leads to swirls, oxidation)
These concentrations can be measured (ex. RBS)
@1000°C
1012 cm-3
5x1013 cm-3
ni≈7.14x1018cm-3
Point Defects are very fast:
Diffusivities of V and I >> dopants’ diffusivities

47. Neutral – do not depend on F-level
Fermi-Dirac
distribution
not sharp at high T
Neutral – do not depend on F-level
Low doped
Highly doped
Still extrinsic semiconductor so F-level in the upper half.
interstitials
vacancies

48. Continue the Example
• At 1000 ˚C, the P region will
be intrinsic, the N region is
extrinsic.
Note:
• ni relative to doping in the
two regions.
• V0 is the same in the two
regions.
• Different charge states
dominate in the different

49. Oxygen and Carbon in Silicon
10-20 ppm (5x cm-3)
Interstitial oxygen improves crystal strength (≈25%)
Oxygen (thermal 450°C) donors TD (1016cm-3) affect  resistivity; TD dissolve T>500°C
Precipitations weakens strength, getter impurities,
contribute for dislocation and SF formation
volume expansion (stress)
alleviated by V and I
grow
Si-Si → Si-O-Si
Growing precipitates consume V release I
Solubility of oxygen
1.2x1018cm-3 in melt
30 Å embryos °C
precipitation at T
heterogenous or
homogeniuos
Co*1000°C=1017cm-3
Carbon CC ≈ 1016 cm-3 is usually substitutional in Si, affects SiO2 precipitates and high T thermal donors ( °C), interacts with point defects and some dopants
Processes that generate V cause SiO2 precipitates’ growth
I cause smaller growth
compensate stress

50. Measurements of the Grown Crystal
Resistivity
xj<<t r
>>s
Average resistivity
Sheet resistance

51. Measurements of the Grown Crystal
Conductivity Type
Seebeck voltage
25-100°C hotter
Electron current
Electrons move - Donors  stay
Pn is the thermoelectric power
Sign of the measured voltage V tells what is the conductivity type

52. Hall Effect Measurement
E field
Hall Effect Measurement
Measurements of majority carrier concentrations (and type) and their mobility
w/o B field
Test structure •w
av. carriers concentrations
Electrons at the bottom

53. Fourier Transport Infrared Spectroscopy (FTIR)
For Interstitial Oxygen incorporated during CZ growth and Substitutional Carbon (detection limit O- 2x1015cm-3, C- 5x1015cm-3 )
Transmittance
Fourier transform
Sweep the wavelength of incident energy  look for absorption by specific molecules to identify their presence (ex. Wavenumbers of Oxygen as SiO2 is 1106 cm-1, C it is 607cm-1)

54. Electron Microscopy: SEM and TEM
SEM keV e-beam (primary)
Secondary or backscattered e are collected
and displayed on CRT -> up to 300,000X
TEM keV e-beam passes through
a sample -> atomic resolution

55. Defects Etches
Defect free crystals are grown: no dislocations, no stacking faults!
Crystals contain Native Defects (Swirls - role of oxygen and carbon) due to
point defects’ agglomeration and metal contaminants (seen as Saucer Pits after annealing)

56. Limits and Future Trends in Technologies and Models
Crystal growth *magnetic field to reduce thermal convection - more uniform doping and size
*double crucible
Wafers *epi-layers (N/N+ or P/P+)
*SOI
*SIMOX (Separation by IMplanted OXygen)
*BESOI (Bonded and Etch-Back Technology)
*Smart-cut
*ELO (Epitaxial Lateral Overgrowth)

57. Summary of Key Ideas
• Raw materials (SiO2) are refined to produce electronic grade silicon with a
purity unmatched by any other commonly available material on earth.
• CZ crystal growth produces structurally perfect Si single crystals which can then
be cut into wafers and polished as the starting material for IC manufacturing.
• Starting wafers contain only dopants, O, and C in measurable quantities.
• Dopant incorporation during crystal growth is straightforward except for
segregation effects which cause spatial variations in the dopant concentrations.
• Point, line, and volume (1D, 2D, and 3D) defects can be present in crystals,
particularly after high temperature processing.
• Point defects are "fundamental" and their concentration depends on temperature
(exponentially), on doping level and on other processes like ion implantation
which can create non-equilibrium transient concentrations of these defects.
• For more information see