Assoc. Prof. Dr. Mohamed Ragaa Balboul

ELCT 705 :Semiconductor Technology

Lecture 05: Wafer Fabrication (FZ Crystal Growth) and Basic Properties of Si Wafers

Department of Electronic and Electrical Engineering

Crystal growth, Wafer fabrication and Basic Properties of Silicon Wafers

Measurement methods

Crystal structure (silicon) and defects

Atomic Packing Factor (APF)Miller Indices

Czochralski single crystal growth.Growth rate and dopant incorporation for CZ method.

Float zone single crystal growth and doping.Dopant incorporation for FZ method.

Wafer fabrication

Float-Zone (FZ) Process

A seed crystal is brought into contact with the top end of the rod.

The seed crystal determines the crystal orientation of the boule.

A small RF coil provides power, which generates large currents in the silicon and locally melts it.

The "floating" melt zone is about 2 cm wide.

Surface tension and levitation due to RF field keep the system stable.

Atoms from the liquid phase bond to the single-crystal solid material plane by atomic plane (As in the CZ process).

No crucible is used, which reduces impurity levels. Sin

gle

Seed

RF

Coil

Single

Crystalline

Si

Polysilicon

Ingot

Float-Zone (FZ) Process

Melt is not held in a container, it is “float”, thus the name “float

zone”.

FZ material is used primarily today in applications which require high resistivities, low oxygen content, such as detectors and power devices (switches or rectifiers).

Dopant Incorporation during Crystal Growth

Doping is required in CMOS process with specified type (P or N) and concentration.

Dopants are incorporated into the crystal during growth simply by adding dopants to the melt.

It is very important to be able to predict the dopant concentration in the pulled crystal. This is not as straightforward as might be because of segregation.

The impurities segregate between the liquid and solid phase at the interfacebetween them.

The concentrations of the impurity are CS in the solid and CL in the liquid, A segregation coefficient kO is defined as:

Segregation occurs due to different solubilities of impurity in two phases.

L

SO

C

Ck

Segregation Coefficients for Si

kO values are normally experimentally measured for a particular impurity in a particular system at a particular temperature.

For most impurities in silicon CZ growth, kO 1. This means that these impurities prefer to be in the liquid phase.

Both CL and CS are functions of time during the growth and will increase if kO 1.

Modeling Dopant Behavior During CZ Crystal Growth

VO, IO, CO VL, IL, CL

VS, CS

We define VO, IO and CO to be, respectively, the initial volume, number of impurities and impurity concentration in melt, VL, IL and CL to be, respectively, the volume, number, and concentration of impurities in the melt during growth.

While, VS and CS to be the corresponding quantities in the solid crystal.

Note that VL, IL, CL ,VS and CS will all be functions of time if kO 1.

Modeling Dopant Behavior During CZ Crystal Growth

If during the growth process, an additional volume of melt dV freezes, it will remove from the melt a number (I) of impurities given by

dVVV

IkdVCkdI

SO

LOLO

L

O

SI

I

V

o SO

O

L VV

dVk

I

dI

ok

O

S

O

L

V

V

I

I

1loglog

ok

O

SOL

V

VII

1

Which gives the number of impurities in the melt as a function of how much of the melt has been frozen (Vs/Vo). We are interested in the impurity concentration in the solid crystal (CS).

)(

)/(1

SO

k

OSO

L

LL

VV

VVI

V

IC

O

))/(1(

)/(1

OSO

k

OSOL

VVV

VVIC

O

Modeling Dopant Behavior During CZ Crystal Growth

If during the growth process, an additional volume of melt dV freezes, it will remove from the melt a number (I) of impurities given by

dVVV

IkdVCkdI

SO

LOLO

L

O

SI

I

V

o SO

O

L VV

dVk

I

dI

ok

O

S

O

L

V

V

I

I

1loglog

ok

O

SOL

V

VII

1

Which gives the number of impurities in the melt as a function of how much of the melt has been frozen (Vs/Vo). We are interested in the impurity concentration in the solid crystal (CS).

1

1

Ok

O

SOOS

V

VkCC

o

SL

k

CC

Dopant Behavior During CZ Crystal Growth

For dopants like antimony where kO 1, the doping concentration increasesdramatically along the length of the pulled crystal.

The common P-type dopant boron, produces a much flatter profile because is closer to 1.

1

1

Ok

O

SOOS

V

VkCC

VS /V0

Antimony

Boron

Phosphorus, Arsenic

CO impurity concentration in melt and CS impurity concentration in solid crystal.

Segregation in FZ ProcessIf the molten zone (L) moves upwards by a distance dx, the number of impurities in the liquid zone (I) will change since some will be dissolved into the melting liquid (KOCL dx) at the top and some will be lost to the freezing solid (CO dx) on the bottom. Thus

where I is the number of impurities in the liquid. But CL = I/L. Substituting and integrating, we find that

dxCkCdI LOO )(

x I

I OOO

L

IkC

dIdx

0

where Io is the number of impurities in the zone when it is first formed at the bottom. Noting that IO = COL and CS = kOI/L,

L

xk

OOS

O

ekCxC )1(1)(L

xk

eIk

LC

k

LCI

0

0

0

0

0

0

Segregation in FZ Process

In the float zone process, dopants and other impurities tend to stay in the

liquid.

Suitable for crystal purification reduction of impurities after each

pass. Suitable for crystal purification

Oxygen Contamination in Silicon During CZ Growth Process

Oxygen Contamination in SiliconOxygen is the most important impurity found in silicon.

It is incorporated in silicon during the CZ growth process as a result of dissolutionof the quartz crucible in which the molten silicon is contained.

Oxygen in silicon is always present at concentrations of ~10-20 ppm (5x1017-1018/cm3) in CZ silicon.

Oxygen has three principal effects in the silicon crystal.

1. The oxygen is incorporated primarily as dispersed single atoms OI occupying interstitial positions in the silicon lattice. The oxygen atoms thus replace one of the normal Si-Si covalent bonds with a Si-O-Si structure. Oxygen atoms improve the yield strength of silicon by as much as 25%, making silicon wafers more robust in a manufacturing facility.

Oxygen Contamination in SiliconOxygen is the most important impurity found in silicon.

It is incorporated in silicon during the CZ growth process as a result of dissolutionof the quartz crucible in which the molten silicon is contained.

Oxygen in silicon is always present at concentrations of ~10-20 ppm (5x1017-1018/cm3) in CZ silicon.

Oxygen has three principal effects in the silicon crystal.

2. The formation of oxygen donors. A small amount of the oxygen in the crystal forms SiO4

complexes which act as donors. As many as 1016/cm3 donors can be formed, which is sufficient to significantly increase the resistivity of lightly doped P-type wafers.

Oxygen Contamination in SiliconOxygen is the most important impurity found in silicon.

It is incorporated in silicon during the CZ growth process as a result of dissolutionof the quartz crucible in which the molten silicon is contained.

Oxygen in silicon is always present at concentrations of ~10-20 ppm (5x1017-1018/cm3) in CZ silicon.

Oxygen has three principal effects in the silicon crystal.

3. The tendency of the oxygen to precipitate under normal device processing conditions, forming SiO2 regions inside the wafer. The precipitation arises because the oxygen was incorporated at the melt temperature and is therefore supersaturated in the silicon at process temperatures.

Oxygen PrecipitationThe solubility of oxygen in silicon increases at higher temperatures and is given by

Oxygen’s diffusivity in silicon is given by

320* 89.0exp105.5

cm

kT

eVCO

12 sec53.2

exp13.0

cm

kT

eVDO

There is an optimum temperature at which SiO2 precipitates can be nucleated at 700 oC.

At lower temperatures the diffusivity of oxygen is too low to provide an appreciable probability of nuclei forming.

Once the nuclei are formed, they continue to grow as long as the wafer is at process temperature; optimum growth temperature is 1000 oC .

*

OC

Future Trends in TechnologiesMagnetic field during CZ growth process help in controlling oxygen concentrations.

Surrounding the crucible containing the molten silicon with magnetic field results in suppression of the thermal convection currents in the melt.

The direction of the magnetic force tends to reduce the currents flowing in the silicon conductor due to thermal convection.

This produces a more uniform ingot diameter and resistivity because the temperature fluctuations are smaller, and it produces lower and more uniform oxygen concentrations.

Vacuum system operating at above 1400 °C offers

The application of DC magnetic field in CZ Si growth.

Crystal growth, Wafer fabrication and Basic Properties of Silicon Wafers

Measurement methods

Crystal structure (silicon) and defects

Atomic Packing Factor (APF)Miller Indices

Czochralski single crystal growth.Growth rate and dopant incorporation for CZ method.

Float zone single crystal growth and doping.Dopant incorporation for FZ method.

Wafer fabrication

Hot Point Probe

Vm

HotCold

e-

n-type wafer

The hot probe technique is used to determine the type of dopant in a wafer N or P.

A voltmeter placed across the probes will measure a potential difference whose polarity indicates whether the material is N or P type.

For N-type sample

25 -100 oChotter

As the electrons diffuse away from the hot probe, they leave behind the positively charged, while, the cold probe will be negatively charged.

The current that flows due to the majority carrier is given by

dx

dTpqnJ nnn

Pn is thermoelectric power, negative for electrons, positive for holes.

At the hot probe, the thermal energy of the electrons is higher than at the cold probe so the electrons will tend to diffuse away from hot probe.

-

Sheet Resistance

V

I

St

d

The four point probe method measures the resistance/resistivity of a wafer.

Using values of carrier mobility, one can calculates the carrier concentration.

Use four points (rather than two) to eliminate the effect of contact resistance.

If we assume that the semiconductor dimensions are large compared to the probe spacing, then the resistivity r is simply

cmpqnq pn

r1

cmI

Vs r 2

where I is the current driven through the two outer probes and V is the potential difference measured between the inner two.

Hall Effect Measurements This technique can determine the material type, carrier concentration and carrier mobility separately. Silicon is placed inside a magnetic field (Bz).

The Hall coefficient RH is defined as The sign of RH gives (n or P).

The Hall mobility is given as where r is the Hall scattering factor.

The carrier concentration is given as

XZ

HH

IB

tVR

heHH rR ,

HqRnp

1,