ICE 4010: Micro Electro Mechanical Systems: Silicon & Crystal Growth

1. Dr. S.Meenatchi Sundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
ICE 4010: MICRO ELECTRO
MECHANICAL SYSTEMS (MEMS)
Lecture #03
Silicon & Crystal Growth
Dr. S. Meenatchi Sundaram
Email: meenasundar@gmail.com
1

2. MEMS Materials and Their Preparation
2Dr. S.Meenatchi Sundaram, Department of Instrumentation & Control Engineering, MIT, Manipal

3. • Atoms within a mineral are arranged in an ordered geometric
pattern which determine its "crystal structure".
• A crystal structure will determine as its symmetry, optical
properties, cleavage and geometric shape.
• The recipe or mixture of these compounds becomes the
blueprint for how the crystal will grow. This growth pattern is
called as crystal's “habit”.
• The "unit cell" is the smallest divisible unit of a mineral with
symmetrical characteristics unique to a crystalline structure.
• A structure's "unit cell" is a spatial arrangement of atoms
which is tiled in three-dimensional space to form the crystal.
• The unit cell is determined by its lattice parameters, the length
of the cell edges and the angles between them, while the
positions of the atoms inside the unit cell are described by the
set of atomic positions (xi,yi,zi) measured from a lattice point.
Crystal Structures
3Dr. S.Meenatchi Sundaram, Department of Instrumentation & Control Engineering, MIT, Manipal

4. • The crystal system is a grouping of crystal structures that
are categorized according to the axial system used to
describe their "lattice".
• The seven unique crystal systems, listed in order of
decreasing symmetry, are: 1. Isometric System, 2.
Hexagonal System, 3. Tetragonal System, 4. Rhombohedric
(Trigonal) System, 5. Orthorhombic System, 6. Monoclinic
System, 7. Triclinic System.
Crystal System
4Dr. S.Meenatchi Sundaram, Department of Instrumentation & Control Engineering, MIT, Manipal

5. Why Silicon?
5Dr. S.Meenatchi Sundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
• Most abundant material on earth.
• Mechanically stable and can be integrated into electronics.
• Same Young’s modulus as steel (2 x 105 MPa) but as light as
aluminum (2.3 g/cm3).
• Melting point at 1400 0C, which is about twice as high as that of
aluminum.
• Thermal coefficient is about 8 times smaller than that of steel
and more than 10 times smaller than that of aluminum.
• Virtually no mechanical hysteresis.
• Silicon wafers are extremely flat and accept coatings and
additional thin-film layers.
• Greater flexibility in design and manufacture.
• Treatments and fabrication processes for silicon substrates are
well established and documented.

6. Doping
6Dr. S.Meenatchi Sundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
• All Group V atoms will donate electrons if they substitute for host
atoms in crystals of Group IV elemental semiconductors (n-type
semiconductor ) where as Group III atoms will generate a hole (p-type
semiconductor ).

7. Basically, the technique used for silicon crystal growth from
the melt is the Czochralski technique.
The technique starts when a pure form of sand (SiO2) called
quartzite is placed in a furnace with different carbon-releasing
materials such as coal and coke. Several reactions take place
inside the furnace and the net reaction that results in silicon is
SiC + SiO2 ----> Si + SiO (gas) + CO (gas)
The silicon so produced is called metallurgical-grade silicon
(MGS), which contains up to 2 percent impurities.
Subsequently, the silicon is treated with hydrogen chloride
(HCl) to form trichlorosilane (SiHCl3):
Si + 3HCl ----> SiHCl3 (gas) + H2 (gas)
Silicon Crystal Growth from the Melt
7Dr. S.Meenatchi Sundaram, Department of Instrumentation & Control Engineering, MIT, Manipal

8. SiHCl3 is liquid at room temperature. Fractional
distillation of the SiHCl3 liquid removes impurities, and
the purified liquid is reduced in a hydrogen atmosphere to
yield electronic grade silicon (EGS) through the reaction
SiHCl3 + H2 ----> Si + 3HCl
EGS is a polycrystalline material of remarkably high
purity and is used as the raw material for preparing high-
quality silicon wafers.
Silicon Crystal Growth from the Melt
8Dr. S.Meenatchi Sundaram, Department of Instrumentation & Control Engineering, MIT, Manipal

9. The Czochralski technique uses the
apparatus shown in Figure called the
puller.
The puller comprises three main
parts:
1. A furnace that consists of a fused-silica
(SiO2) crucible, a graphite susceptor, a
rotation mechanism, a heating
element, and a power supply.
2. A crystal pulling mechanism, which is
composed of a seed holder and a
rotation mechanism.
3. An atmosphere control, which includes
a gas source (usually an inert gas), a
flow control, and an exhaust system.
Silicon Crystal Growth from the Melt
9Dr. S.Meenatchi Sundaram, Department of Instrumentation & Control Engineering, MIT, Manipal

10. In crystal growing, the EGS is placed in the crucible and the
furnace is heated above the melting temperature of silicon.
An appropriately oriented seed crystal (e.g. [100]) is
suspended over the crucible in a seed holder.
The seed is lowered into the melt.
Part of it melts but the tip of the remaining seed crystal still
touches the liquid surface.
The seed is then gently withdrawn.
Progressive freezing at the solid-liquid interface yields a large
single crystal.
A typical pull rate is a few millimeters per minute.
After a crystal is grown, the seed and the other end of the
ingot, which is last to solidify, are removed.
Next, the surface is ground so that the diameter of the
material is defined.
Silicon Crystal Growth from the Melt
10Dr. S.Meenatchi Sundaram, Department of Instrumentation & Control Engineering, MIT, Manipal

11. After that, one or more flat regions are ground along the
length of the ingot.
These flat regions mark the specific crystal orientation
of the ingot and the conductivity type of the material.
Finally, the ingot is sliced by a diamond saw into wafers.
Slicing determines four wafer parameters: surface
orientation, thickness, taper (which is the variation in
the wafer thickness from one end to another), and bow
(i.e. surface curvature of the wafer, measured from the
centre of the wafer to its edge).
Typical diameter of silicon wafers are 100mm, 150mm,
200mm.
Silicon Crystal Growth from the Melt
11Dr. S.Meenatchi Sundaram, Department of Instrumentation & Control Engineering, MIT, Manipal

12. Silicon Crystal Growth from the Melt
12Dr. S.Meenatchi Sundaram, Department of Instrumentation & Control Engineering, MIT, Manipal

13. Silicon Crystal Growth from the Melt
13Dr. S.Meenatchi Sundaram, Department of Instrumentation & Control Engineering, MIT, Manipal