Transcript MSE 630 - IC Processing

CZ processing
o = Cs/Co increases as ingot grows
The dopant concentration is given
Ingot diameter varies inversely with pull by:
rate:
5
1 2mTm
IL=Io(1-Vs/Vo)ko
and
V pmax 
LN
3r
Cs = -dIL/dVs = Coko(1-f)(o-1)
L = latent heat of fusion
 = Stephan-Boltzman constant
C, I and V are concentration, number
of impurities and volume when
m = thermal conductivity at Tm
o: initial
N = density
Tm = melt temperture (1417 oC for Si)
L:
liquid and s: solid
Float Zone Processing
Temperature
Liquid
Solid
Cs
Co
CL
Concentration
In Float Zone refining, solid
concentration varies with initial
concentration as follows:
 ko  x


L
Cs ( x)  Co 1  1   o  exp



4-point probe
Used to measure sheet resistivity
r = 1/q(mnn+mpp) W-cm
Outer probe forces current
through wafer; inner probes
measure voltage drop
us. n>>p or p>>n, so only one term is of
interest
r= 2ps V/I
Typically,
0.5-mm< s <1.5-mm
When s = 1.588 mm, 2ps = 1 cm
If t is not >>s, use correction factor:
r = pt/ln(2)*V/I
If xj is the dopant depth, we measure
square resistivity: rs = r/xj
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2-point probe
Useful to determine material type (n- or p-type)
•Apply two probes, one 25 – 100 oC
hotter than other
•Thermally excited electrons flow away
from hot probe, leaving holes and build
up around cold electrode
•Measure Seebeck voltage using high
impedance volt meter
•If material is p-type polarity will be
reversed
We can measure either short
circuit current or open circuit
voltage. Current for an n-type
material is:
Jn = qmnnPndT/dx
Pn is thermoelectric power,
either (-) for e- or (+) for h+
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Hall effect
Using Hall effect, we can
determine material:
•Type
•Carrier concentration
•Carrier mobility
1. Current, Ix, is forced through sample
2. Results in measurable voltage drop, Vx
r = wt/s Vx/Ix
3. Applying a magnetic field, B, deflects
electrons: F = q(x + v x B)
electrons will be forced in
–y direction
Where v = electron velocity
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Hall effect
Since no current flows in the
y-direction, an electric field e
must build to offset magnetic
force:
Fy = q(xy + vxBz) = 0
Define the Hall Coefficient:
RH = tVy/BzIx = 1/qn
→ n = ± 1/qRH
The “Hall mobility”, mH is
→ xy = -vxBz
Since vx = Ix/qwtn,
mH = ‫׀‬RH‫׀‬/r = ‫׀‬RH‫ ׀‬
xy = Bz Ix/qwtn,
Hall mobility is typically ~2 x e- or h+
mobility
or
Vy = Bz Ix/qtn
Consistent units for calculating Hall effect:
V = volts
A = Amps
length = meters
B = Tesla (1T = 104 Gauss = 1 V-s/m2)
RH = m3/C
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Typical defects in crystals
Typical defects are:
Point defects – vacancies &
interstitials
Line defects – dislocations
Volume defects – stacking faults,
precipitates
The equilibrium number of vacancies varies
with temperature:
nv = noexp(-Ev/kT)
O and C are also defects with
concentrations of 1017-1018 cm-3 and
1015-1016 cm-3
Other impurities are in the ppb range
Thermal stresses cause dislocations. Thermal stress is:  = EaDT
 = stress, E = Young’s modulus, a = thermal expansion coefficient
(mm/m/oC)
Where do defects come from?
• Naturally occurring vacancies and
interstitials
• Thermal stresses inducing dislocations
and stacking faults
• Precipitation of second phases inducing
dislocations and stacking faults
• Impurities
• Process damage (e.g, ion implantation)
The oval shaped area with a lighter contrast is the emitter of a bipolar transistor. In preferential
etching this would look similar to what was shown as an illustration for etching.
Some of these small stacking faults have a peculiar, "sailing-boat" like shape (marked by "S" in the
picture above). Below, a detailed view of a "sailing boat stacking fault":
• Oxidation of Silicon produces interstitials in supersaturation. These
surplus interstitials tend to agglomerate in discs - i.e. stacking fault
loops. The difficult part is the nucleation; it determines what will
happen. We have to consider two ways of oxidizing Si, we first
consider Surface oxidation: The surface oxidizes homogeneously by
exposing it to an oxidizing atmosphere at high temperatures. This is
the normal oxidation process. The emission of interstitials occurs at
the interface; the interstitials diffuse into the bulk; the
supersaturation decreases with the distance from the surface. There
is no easy nucleation for an interstitial type dislocation loop as long
as the interface is defect free. If defects are present, most prominent
small precipitates of metal impurities (Fe, Ni, Cu) may serve as
nucleation centers for the interstitials; a stacking fault penetrating in
a semicircular fashion into the bulk is formed. If many precipitates
are available, a large density of small stacking faults may be
observed:
The name "swirl" comes from the spiral "swirl-like" pattern observed in many
cases by preferential etching as shown on the right.
Close inspection revealed two types of etch features which must have been
caused by different kinds of defects. Lacking any information about the
precise nature of the defects (which etching can not give), they were termed
"A-" and "B-swirl defects".
Imaging Defects
Dislocations and stacking faults introduced during processing and
nucleated by oxygen, thermal stress and oxidation processes
Chemical etchants reveal density, size and location of defects. Etchants
attack areas with high chemical or strain energy and are visible with a
microscope
Etch
Composition
Sirtl
Cr2O3 (5M): HF
1:1
Secco
K2Cr2O7 (1.5M):HF or Cr2O3
(.15M):HF
1:2
Dash
HF:HNO3:acetic acid
1:3:10
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FTIR: Fourier Transform Infrared Spectroscopy
Used to measure concentrations of
O and C (down to ~1015/cm3)
Molecules absorb energy at
characteristic wavelengths
E = hn = hc/l
Si-O-Si absorbs at wave number
1106/cm
C absorbs at wave number 607/cm
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1. IR beam split & follows two separate paths to
sample and detector
2. Moving mirror causes two beams to interfere
constructively or destructively in a sinusoidal
manner
3. The Fourier transform of the signal will be a delta
function proportional to incident beam intensity
4. If the frequency of the source is swept, the FT of
the resulting transform will produce an intensity
spectrum
5. If we insert a sample, the intensity spectrum will
change because of absorption of specific
wavelengths
6. Scan of sample is compared to a baseline scan
to identify absorbed frequencies
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