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Micro/Nanosystems Technology Wagner / Meyners 1
Micro/Nanosystems Technology
Prof. Dr. Bernhard Wagner
Dr. Dirk Meyners
Silicon Doping
Micro/Nanosystems Technology Wagner / Meyners 2
Outline
Dopant elements
Ion implantation
Concentration profiles in bulk silicon
Diffusion and annealing
Poly-Si doping
Silicon-metal contact
Micro/Nanosystems Technology Wagner / Meyners 3
Motivation for silicon doping
Modification of dopant concentration ( conductivity)
of Si substrate or Si thinfilm
Formation of electronic functionality in Si:
interconnection lines
resistors, piezoresistors
electrodes (for capacitive sensors)
diodes, transistors
Micro/Nanosystems Technology Wagner / Meyners 4
Dopants
acceptors: B, Al, Ga, In
p-doped Si (holes)
donators: P, As, Sb
n-doped Si (electrons)
III IV V
Micro/Nanosystems Technology Wagner / Meyners 5
Resistivity of c-Si vs. dopant concentration
Concentration range
typically 1015 – 1020 cm-3
Micro/Nanosystems Technology Wagner / Meyners 6
Doping methods
Doping from gas phase,
esp. in-situ poly-Si doping
Ion implantation Diffusion out of
dopant layer,
POCl3 - process
Thin film masking layer local doping of wafer substrate
Micro/Nanosystems Technology Wagner / Meyners 7
Ion implantation
most common doping method
precise generation of lateral and vertical doping profiles
generation of buried dopant layers
multiple implantation possible
Micro/Nanosystems Technology Wagner / Meyners 8
Ion implantation
Ions: B+, P+, As+, Sb+, also doubly charged
two-step process
- dopant injection (controlled number)
- dopant diffusion (drive-in to controlled depth)
wafer preparation
• thin SiO2 layer: (~30 nm scattering oxide)
to avoid channelling:
unwanted high ion range along low-index crystal planes, e.g. (100)
• masking layer: photo resist !, SiO2, Si3N4,
masking layer
Micro/Nanosystems Technology Wagner / Meyners 9
Ion implantation
Ion energy: E = 1 keV – 200 keV
Ion current: I = 0.1 – 20 mA
Ion dose: N = 1012 - 1016 ions/ cm2
Axceli
s
Micro/Nanosystems Technology Wagner / Meyners 10
Gaussian concentration profile
C concentration (atoms/cm3)
C0 peak concentration
Rp projected range
Rp standard deviation of Rp
ND0 total dose (atoms/cm2)
Ions scatter randomly in the Si crystal
statistical description of trajectory
Micro/Nanosystems Technology Wagner / Meyners 11
Real implantation profiles
light ions penetrate deeper and have broader distribution
heavy ions for shallow implantation
deviations from Gaussian profile
U = 200kV
Plummer 8-2
Rp
Micro/Nanosystems Technology Wagner / Meyners 12
Dopant range and spread
Plummer 8.3
Rp Rp
Micro/Nanosystems Technology Wagner / Meyners 13
Thermal annealing
Objective:
• dopant electrical activation:
dopant moves from interstitial site to lattice site
T > 950 °C necessary (only temperature is relevant, not time)
• diffusion (drive-in) of concentration
• silicon crystal damage repair
1. Thermal annealing
in standard diffusion furnace, several hours
diffusion (spread) of concentration
2. Rapid thermal annealing
halogen lamp heating
t = 10 - 100 sec !!
only activation, no diffusion
Micro/Nanosystems Technology Wagner / Meyners 14
Rapid thermal annealing (RTA)
= Rapid thermal processing (RTP)
profile after thermal anneal implanted profile
profile after RTA
Hilleringmann
Micro/Nanosystems Technology Wagner / Meyners 15
Diffusion
Diffusion law (3-dim): diffusion is isotropic in x, y, z
: Laplace operator
Ea activation energy
Diffusion lenght: mean diffusion distance
z
cDF
Dt2
cDtc /
2
2
z
cD
z
F
t
c
F flux (atoms/s.cm2)
c concentration (atoms/cm3)
D diffusion constant
= diffusivity (cm2/s)
z diffusion depth (cm) Diffusion law (1-dim)
Fick‘s first law
)/exp(0 kTEDD a
concentration gradient drives diffusion
Micro/Nanosystems Technology Wagner / Meyners 16
Diffusivity (diffusion constant) of dopants in c-Si
high temperature step
800 – 1200°C
fast diffusors: B, P, In
slow diffusors: As, Sb
diffusion in poly-Si can
be a factor of 100 higher!!
diffusion along grain boundaries
diffusion in SiO2 is much lower
=> SiO2 diffusion mask
Plummer
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Solution of diffusion differential equation:
Case1: Doping profile for constant surface concentration c0:
Example: infinite dopant source
erfc-profile
Complementary error function
Widmann
Micro/Nanosystems Technology Wagner / Meyners 18
Case2: Diffusion of existing gaussian doping profile:
Example:drive-in process after implantation
Broadening of gaussian profile
Reduction of peak concentration
Widmann
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Multiple diffusions
Total thermal budget is relevant
Effective diffusion dominated by highest temperature step
i
i
ieff
effeff
tDDt
Dt
)(
)(2
Example:
Drive-in at T= 1100°C for t = 12h
D (B, P) 1.5∙10-13 cm2/s (B, P) = 1.6 µm
D (As, Sb) 3∙10-14 cm2/s (As, Sb) = 0.72 µm
Micro/Nanosystems Technology Wagner / Meyners 20
Doping induced stress in single-crystal silicon
• Low doped crystalline Si-layers are stress free
• High doping leads to residual stress
doping with atoms of different covalent radius lattice mismatch
r(B) = 0.088 nm; r(Si) = 0.117 nm; r(Ge) = 0.122 nm
Heuberger 219
B-doping tensile stress Ge-doping compressive stress
stress-free Si-layers
can be achieved
by B-Ge co-doping
5µm thick Si-Epi-layers
plastic deformation
Micro/Nanosystems Technology Wagner / Meyners 21
Polysilicon layer doping methods
1. In-situ doping during LPCVD-deposition
gaseous sources:
phosphine PH3, diborane B2H6, arsine AsH3
2. POCl3-doping
• grow an additional phosphorsilicate glass (PSG) on poly-Si
from POCl3 vapor source
• diffuse P into poly-Si (drive-in)
• glass removal
3. Implantation
Micro/Nanosystems Technology Wagner / Meyners 22
Electrical properties of Poly-Si
Resistivity of low-doped poly-Si is significantly higher compared to c-Si
Ph Boron
c-Si
Micro/Nanosystems Technology Wagner / Meyners 23
Resistivity of Poly-Si
Grain boundaries
represent potential barriers
reduce carrier mobility
trap dopants
-> higher resistivity for smaller grains
~ 1/<a>
Mean grain size <a>
conduction band
valence band
Micro/Nanosystems Technology Wagner / Meyners 24
Metal - Silicon contact
Ohmic contact to p-doped Si (low work function B)
Schottky diode contact to n-doped Si (high B):
forward biased with „+“-terminal at metal
counter-measure: reduction of barrier width by high n-doping (n+-Si doping)
tunneling contact
ohmic behavior for ND > 6*1019 cm-3 in case of n-Si - Al contact
metal n-Si
metal n+ -Si
tunneling contact
Schottky contact
Micro/Nanosystems Technology Wagner / Meyners 25
Metal - Silicon contact
Micro/Nanosystems Technology Wagner / Meyners 26
Diffusion of non-doping materials (contaminants) in Si
very fast diffusion of Au, Ni, Cu, Fe
and also alkali metals Na, K (similar to Fe)
due to interlattice diffusion mechanism
already at moderate temperatures
Example:
Copper diffusion at room temperature
D = 2.4·10-7 cm2/s
mean diffusion distance after 1 hour
= 588 µm ! Dt2
H. Bracht, Materials Science in Semicond. Process. 7 (2004) 113–124
Micro/Nanosystems Technology Wagner / Meyners 27
Contaminants are lifetime killers
Contaminants act as recombination
centers:
undesired charge carrier recombination
through intermediate energy levels in Si
bandgap
maximum recombination probability for trap
energy level ET in the center of bandgap
(deep level traps)
Counter-measures: - trapping materials have to be separated
from diffusion processes!
- introduce diffusion barrier layer for
silicon to metal (Au, Cu) contact:
e.g. TiN, TiW layer
Plummer 1-27
trap
level
Micro/Nanosystems Technology Wagner / Meyners 28
Energy levels of materials in the Si bandgap
Allowed in IC-technology: Al, W, Ta, Ti, Mo
Not allowed in IC-technology: Au, Cu, Ni, Fe, Ag, Pt, Cr, Na, K
Ruge 4.1
band center
Micro/Nanosystems Technology Wagner / Meyners 29
Summary
modification of dopant concentration of Si substrate or layer
dopant dose and profile can be precisely controlled
ion implantation is preferred
annealing step necessary for dopant diffusion and activation
contaminants can be life-time killers in semiconductor components
Micro/Nanosystems Technology Wagner / Meyners 30
Literature
Plummer Ch. 7.2
Widmann Ch. 6