Micro/Nanosystems Technology · Wagner / Meyners Micro/Nanosystems Technology 8 Ion implantation...

Micro/Nanosystems Technology Wagner / Meyners 1

Micro/Nanosystems Technology

Prof. Dr. Bernhard Wagner

Dr. Dirk Meyners

Silicon Doping


Outline

Dopant elements

Ion implantation

Concentration profiles in bulk silicon

Diffusion and annealing

Poly-Si doping

Silicon-metal contact


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


Dopants

acceptors: B, Al, Ga, In

p-doped Si (holes)

donators: P, As, Sb

n-doped Si (electrons)

III IV V


Resistivity of c-Si vs. dopant concentration

Concentration range

typically 1015 – 1020 cm-3


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


Ion implantation

most common doping method

precise generation of lateral and vertical doping profiles

generation of buried dopant layers

multiple implantation possible


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


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


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


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


Dopant range and spread

Plummer 8.3

Rp Rp


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


Rapid thermal annealing (RTA)

= Rapid thermal processing (RTP)

profile after thermal anneal implanted profile

profile after RTA

Hilleringmann


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


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


Solution of diffusion differential equation:

Case1: Doping profile for constant surface concentration c0:

Example: infinite dopant source

erfc-profile

Complementary error function

Widmann


Case2: Diffusion of existing gaussian doping profile:

Example:drive-in process after implantation

Broadening of gaussian profile

Reduction of peak concentration

Widmann


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


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


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


Electrical properties of Poly-Si

Resistivity of low-doped poly-Si is significantly higher compared to c-Si

Ph Boron

c-Si


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


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


Metal - Silicon contact


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


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


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


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


Literature

Plummer Ch. 7.2

Widmann Ch. 6

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