Transcript characterization

Wafer Characterization and Properties
Measurement
MSE-630
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
MSE-630
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+
MSE630
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
electrons will be forced in
–y direction
3. Applying a magnetic field, B, deflects
electrons: F = q(x + v x B)
MSE-630
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
→ xy = -vxBz
Define the Hall Coefficient:
RH = tVy/BzIx = 1/qn
→ n = ± 1/qRH
The “Hall mobility”, mH is
Since vx = Ix/wtn,
mH = ‫׀‬RH‫׀‬/r = ‫׀‬RH‫ ׀‬s
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
MSE-630
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
MSE-630
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
MSE-630
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 MSE-630
Clean Room and Contamination
Reduction
•Particles on wafers must be detected to <10 per wafer
•Blank wafers are scanned with a laser – particles will cause light to
be reflected
•Detects particles ~ 0.2 mm
•Computer enhancement of scanned image produces a “map” of
particles on wafers
MSE-630
Test Structures
Test structures, as shown below,
can identify shorts and opens.
A short between pads 1 and 2
indicate bridging due to a particle
or etching lines
An open between pads 1 and 4 is
a break in a continuous line,
probably due to a particle
In a MOS capacitor, a thin
insulator is grown and metal gates
are patterned.
Voltage applied to each capacitor
structure and ramped up until
dielectric breakdown occurs.
Results are plotted on a histogram
Premature failure usually due to
contamination
MSE-630
Surface Analysis Techniques
Incoming e- collide inelastically
with target e-, ejecting it from
solid. These “secondary e-” have
lowest energy and are used to
generate an SEM image
Highest energy e- “backscatter”
elastically
Intermediate energy e- release an
e- from inner bands of atom (L or
K level). An incident e- causes an
EK e- to be ejected. Then, an EL1
e- falls into the vacant EK level,
producing an x-ray. This is XES
or microprobe
Energy released in an EL1→EK
transition may b given to a 3rd e-, aka
“Auger” electron (pronounced oh-jay)
X-rays or emitted Auger e- carry a
unique signature that is element
dependent
Lighter elements produce Auger e-,
heavier elements produce x-rays
MSE-630
Surface Analysis Techniques
Incident x-ray beam can cause
an e- to be emitted (XPS) or
fluoresce (XRF) when x-ray is
emitted
Rutherford backscattering
(RBS) detects ions which are
elastically scattered by atomic
nuclei in the substrate when
bombarded by an incident ion
beam (usually He)
SIMS (secondary ion mass
spectroscopy) uses O+ or Cs+ as
bombarding ions. This is the
dominate means of determining the
doping profile
MSE-630
Method
Attributes
AES/XPS
Good depth resolution and
surface sensitivity; limited
depth
XES/XRF
Not surface sensitive, not
good depth resolution, easy
to implement on SEM
SIMS
Very sensitive, excellent
depth resolution, destructive
RBS
Good depth resolution,
sensitivity ~ 0.1 at%, poor
lateral resolution
MSE-630
Interface characterization
Need to measure
Three groups of measurements:
•Thickness
•Physical (destructive): etching and
measuring using AFM or cross
section and view on SEM or TEM
•Dielectric constant
•Index of refraction
•Dielectric strength
•Defect density
•Optical
•Electrical
MSE-630
Interface characterization – Optical
Methods
For a constant wavelength, l, incident
and reflected waves will add
constructively for certain values of l,
destructively for others
lmin,max = 2n1cocos(b)/m
m=1,2,3… for maxima
m=1/2,3/2,5/2… for minima
Conversely, b = sin-1(nosin(f1)/n1)
For a fixed f, vary l to find lmax and
lmin. Solve for co – dielectric
thickness
n1 must be known
MSE-630
Optical Characterization Methods
Elipsometry
Color Charts
•Measures thicknesses <10nm
•If white light is used to illuminate a
surface, destructive interference
causes a particular color to
emerge in the reflected light
•Works similar to reflectance
•Uses polarized light and
measures degree of change in
polarization between surface and
substrate
•n must be known, and substrate
must be transparent to l
•Color of layer corresponds to
thickness
•Resolution ~10-20 nm
•Good only for thicker samples
•Change in polarization only
depends on film thickness and n
MSE-630
Electrical Characterization Methods
4 types of charge at interface:
Qm: mobile charge in oxide, usually
metals (Na+ and K+)
Qot: oxide trapped charge between Si-OSi bonds, usually from ion implantation
Qf: fixed oxide charge – incompletely
oxidized Si with a (+) charge
Qit: interface trapped charge. Incomplete
O bonds like Qf, but may be (+), neutral
or (-). Energy levels of trapped charge
can be anywhere in the forbidden band
gap, but usually close to band edges
Charges associated with SiO2-Si
system
Density of Qf, Qit ~ 109 – 1011 cm-2 eV-1
Qot usually repaired by high temperature
anneal
MSE-630
Electrical Characterization Methods
e- to
Capacitance-Voltage Technique (CV)
In (a), + VG attracts
Si
surface. Apply small AC
signal (10 kHz – 1MHz) and
measure Cox (increase
impedance due to oxide
capacitance even though Si
acts as a resistor in series
with Cox.
In (b), -VG repels e- from
surface creating a depleted
region. Donor atoms have
net + charge after losing eand act to balance negative
VG
(b), continued Capacitance of
depletion region: CD = xs/xD
where xs = permittivity of Si,
11.9
Measured capacitance now
CD in series with Cox
accumulation
‫׀‬QD‫׀=׀‬QG ‫ = ׀‬NDxD
where ND = doping in
substrate, xD is depletion
region depth, QD, QG: charge
on gate fro donor atoms,
#/cm2
depletion
(c) for larger values of VG,
surface layer inverts from n-type
to p-type. Negative VG attracts
minority carrier holes in
substrate to the surface to form
an inversion layer of p-type
carriers. Occurs at Vth, xD stops
expanding, becomes xDmax
The gate charge must always
be balanced by the substrate
charge: QG = NDxD+QI
Once QI forms, QD stops
expanding
inversion
MSE-630
Interface Capacitance, Continued
Oxide prevents current flow
(+) voltage in accumulation region
places EF closer to Ec, at the surface.
e- population is higher at surface
than in bulk
Negative gate voltage in depletion
region causes bands to bend
upward. This creates depletion
region with depth xD. EF is far from
EC and EV, and n and p are small,
i.e., mobile carrier concentration <<
ND.
In the inversion layer, EV is close to EF, resulting in holes building up in EC.
Hole population is significant at surface – inversion has occurred and material
is p-type. Now, increasing VG tries to move EV closer to EF, but since #p’s
increase exponentially, QI offsets QG and QD = constant at xDmax
MSE-630
Interface Capacitance, Continued
In inversion region, we
superimpose high frequency
voltage on VG, causing QG to
fluctuate. To balance charge, QI or
QD must change. But, if we
modulate faster than QI can
respond, only QD changes.
Charge balancing occurs between
gate and bottom of depletion
region, DQD = DQG. For any –VG in
the inversion region xD = XDmax and
CD = CDmin
At low frequency (<1Hz) QI can
follow changes in QG and
measured capacitance is just Cox
because the effective tops and
bottoms of the capacitor are at the
interface.
Deep depletion occurs when rate
of change in VG exceeds ability of
QI to respond
MSE-630
Charges at interfaces cause shifts in the CV curves
DC are “charge traps”. Because
HF traps cannot charge and
discharge, but at LF they can, DC
is proportional to trap density
DC are “charge traps”. Because HF traps cannot charge and
discharge, but at LF they can, DC is proportional to trap density
As VG shifts, EF goes from EC to EV, and QIT traps fill and empty
as EF moves through their energy levels, distorting CV curve.
QF is the fixed positive charge in the oxide. This induces a mirror
negative charge in the Si, making Si more n-type at the surface
and harder to invert to p-type.
Result: lateral shift by the amount qQF/Cox
Additional lateral shift from work function, fMS. fMS is known and
known for any experiment.
DIT: interface state density # traps/cm2 eV
MSE-630
Bias Temperature Stressing
(BTS)
Purpose: to characterize Qm – the mobile charge density of the insulator
Mobile charges typically due to Na+ or K+
1. Make initial HF measurement at room temperature
2. Heat to 200oC with VG applied. Na+ and K+ are
highly mobile at this temperature and migrate up and
down
3. Keep under bias and cool to room temperature. Take
second CV measurement
4. Curve will shift laterally as it did with QF. The extent
of shift is due to QM
5. Two tests with opposite bias can be conducted to get
total QM
MSE-630
Breakdown Voltage Test
1. Ramp V until dielectric breakdown. Thick oxides
usually break down at ~15MV/cm in high quality
SiO2.
2. Same process can be used to measure tunneling
currents in thin oxides (<10 nm)
3. If DC currents are forced through a MOS capacitor
for a period of time and CV measurements are
taken before and after, shift in CV curve are due to
QOT. A series of experiments with varying time will
produce a plot of QOT vs time
4. Since xD depends on doping, CV can be used to
extract NA or NP vs. depth
MSE-630
Measure Concentration vs.
Depth to get:
•Channel doping profile
•Source-drain profile
•Well profiles
•field region profiles
May be unreliable in thin
regions. Primary ion beam
energies typically 200eV to 5
keV. To do surface profiles to
identify contaminants over first
50-nm, use lower beam energy
and adjust the beam angle
Use SIMS to measure primary doping
profile and chemical concentration of
dopants
Usually large squares, 200 x 200-mm,
are incorporated in layout and SIMS is
performed on these
Sputtered ions are accelerated, mass
analyzed and counted to get depth profile
Dopant profile resolutions are 10161017/cm3
n-type dopants (As, P, Sb) use Cs beam,
p-type dopants (B, In) use O
MSE-630
Spreading Resistance
Purpose: to obtain doping
profiles
•Technique: measure
resistivity on beveled
sample and compare to
standard
•Measurements made with
2 probes on beveled
samples at 2-10-mm
increments
MSE-630
Mechanical Measurements
Differential stresses occur from:
•
Thermal expansion
•
Intrinsic stress from
Ion bombardment
Lattice constant mismatch
Gas or inclusions on film
s in interconnects lead to failures from:
•Cracking
•Loss of Adhesion
•Void and hillock formation
MSE-630
Residual thermal stress
Residual thermal stress
can be calculated from:
sf = (as-af)DT Yf/(1-nf)
Thermal stresses can
also be measured using
x-ray diffraction to
determine the strain in
the lattice:
sf = -Yf/2nf·(ao-as)/as
A third way to measure
residual stress is from the
curvature of the film:
sf = 1/6R·Ysxs2/(1-ns)xf
Film may lose adhesion due to stresses. Adhesion
often measured using the tape test. A more
quantitative method to measure adhesion is to epoxy
a pin onto the film, then pull on the pin with a
calibrated weight or at a calibrated rate. A third
method is to grow a 3rd layer on top of the
film/substrate until adhesion is lost. In this method
you must know top layer’s stress as a function of
thickness.
MSE-
630
Electrical Characterization Methods
Electrical measurements are used to
measure:
•Sheet resistance
•Contact resistance on interconnects
•Dielectric breakdown voltage
•Contact reliability
Contact resistance, RK,
measured from V/I using the
configuration at right.
RK = V/I (W)
then calculate contact
resistance:
RK = rc/l2
Note: Average resistance may also be measured in a series of
interconnected contacts on test sheets.
MSE-630
Hillocks, Valleys and Median
Time to Failure
Hillocks and valleys lead to shorts and
open circuits. They are due to residual
thermal stresses, formation of
second phases, and electromigration.
Electromigration is current-induced
hillock and void formation.
H&V formation is measured using high
T’s (~200oC) and I’s (1-2 MA/cm2)
Voltage and resistance are monitored; a
20% change in resistance indicates
failure
Here,
Median time to failure, t50, is the time
at which 50% cumulative failure occurs.
J = current density
MTTF = A·J-nexp(EA/kT)
Extrapolate to room temperature
A = constant
n ~ 2 typical (1-3)
EA = Activation energy for
migration, us. 0.5-0.8 eV
Goal: 0.1% fail in not less than 10-20 years MSE-630
MSE-630