Rapid Thermal Processing

Campbell, Chapter 6

• why RTP?

• radiation and heat transfer

• optical heat sources

• temperature measurement

• thermally-induced stress

Why RTP?

• Quartz-tube furnaces are still (and will continue to be) important

in semiconductor processing

BUT…

• Thermally-activated diffusion can kill small device structures

• Some implant-generated defect structures require high

temperatures to anneal out, but conventional furnace

annealing would be too slow

• Rapid heating + tube furnace = disaster!

• Single-wafer processing is much more amenable to advanced

(robotic) processing

• Basic RTP methods are adaptable to a wide variety of advanced

processing technologies

Fundamental aspects - heating

For most materials of interest, t 0

t , , ,T T T 1“emissivity”

• heat can be brought to a surface by thermal conduction, thermal

convection (natural flow transfer of heat via transport of gas or liquid),

forced flow (pumping/injection of heated gas or liquid) and radiation

• radiation is the principle tool for high-temperature semiconductor

materials processing

• When light strikes a surface it may be reflected (), transmitted (t) or

absorbed ():

Rapid Thermal Processing (RTP)

RTP PhysicsHeat Flow Mechanisms can be related to temperature rise by:

•

dT=

q(T )

Where Cp is the specific heat (a measure of how much energy a material can absorb before it manifests in

a temperature rise), r is the gram/cm3 density, and q-dot is the heat flow density (W/cm2) Note your book is

inconsistent on how it uses q-dot.

Temperature ramp rate can be enormous!!!!!

dt (C p )x(ρ )x(thickness)

cm2 Second cm2q(T) =

Watts=

Joules •

Rapid Thermal Processing (RTP)

Types of RTP1.) Adiabatic: Excimer laser pulses (<uS) anneal the thin skin of

material.=>huge vertical temperature gradients 2.) Thermal flux: rastering a

focused beam (electron or laser) across a wafer. =>huge vertical and lateral

temperature gradients

3.) Isothermal: Broad area optical illumination. => minimal temperature gradients.

RTP Physics3 types of Heat Flow Mechanisms:

1.) Conduction: Flow of heat between two bodies in intimate contact.

Heat flow per unit area in a solid is expressed in terms of a solids thermal

conductivity, k(T), as,xWhere k(T) has units of Watts/(cm-K) and x is the thickness measured between the two temperatures. Note this is

different from your book.

2.) Convection: Flow of heat between two bodies through an intermediate medium (a gas in our case)

For a gas with effective heat transfer coefficient, h with units (Watts/cm2-K) is,

•

q(T ) = h(Twafer − T∞ )

Notice that both of these expressions are linear in temperature

q(T ) = k(T ) ∆T

•

Rapid Thermal Processing (RTP)

3.) Radiation: Flow of heat between two bodies through

radiation and absorption of light. We can use the spectral

radiant exitance= the radiated power per area per unit

wavelength,

emissivity.where c = 3.7142x10−16 W − m2 , c = 1.4388x10−2 m − K and ε(λ) is the wavelength dependent

1 2

If ε(λ) is independent of λ, then thetotal power radiatedper unit area, the total

exitance,

M (T ) = q T ) = ε σ T 4

where σ = 5.6697x108 W/m2 K 4 is the Stefan - Boltzmann constant.NOTE:1). The unit change to meters and 2) The radiated power depends on temperature to the forth while

conduction and convection depend on temperature linearly. Thus, radiation is the dominate mechanism at high

temperature while conduction and convection dominate heat flow at lower temperatures.

The emissivity is related to the absorbance by Kirchoff’s law of conversation of power which states that in

steady state at (constant temperature and absorbed and emitted power), the power absorbed by a wafer must be

equal to the power emitted.

.

M Tc

c

T

1

5 2 1exp

M ()= C1 -5 (exp c2/ T - 1)-1 dMλ/d = 1 C1

[(exp c2/T - 1)]-1 (-5-6)

- -5 (exp c2/T - 1)-2 (exp c2/T) (-c2/ 2T)]

=0

when 5-1 = (exp c2/ T-1)-1exp(c2/ T)(c2/ 2T)

(c2/ 2T) for 1 nm, T=500ºC

max=(c2/5)/T = 0.2898 cm.K/T

i.e. Temperature T ---> characteristic color

• heat will be lost by (diffusive) thermal conduction

and by thermal convection and forced flow

• radiation from the substrate will be negligible at “low”

substrate temperatures

Fundamental aspects -- cooling

q1

q2

2q T 1q T

Variations on radiant heating

• adiabatic

– an entire surface is illuminated with high power (~107 watts cm-2) short time (~nanosecond) laser pulses (LASIK)

– the surface is heated & cooled w/out heat transfer to the ambient

– difficult to monitor/control temperature; large thermal gradients

• thermal flux

– an intense spot source (electron beam or laser) is scanned across the surface

– lateral temperature gradients generate defects

• isothermal

– a broad (incoherent) beam heats the entire wafer for several seconds

– the wafer is maintained in thermal isolation from the surroundings

– most commercial RTP systems operate in this mode

Properties of electromagnetic radiation

• monochromatic – single wavelength

• polychromatic – many wavelengths

– discrete spectrum

– continuous spectrum

• coherence – the degree of correlation

between phases of monochromatic

radiation (random phase relations are

incoherent)

• EM radiation will transport and deposit

energy

• radiometry is the science of the

measurement of EM radiation

Properties of electromagnetic radiation

• radiant energy (Q) – Joules

• radiant energy density (w) – J/m2

• radiant flux () – Watts (=J/s)

• radiant flux density (Watts/m2)

– emitted: radiant exitance

– incident: irradiance

Blackbody radiation

• A blackbody is an ideal absorber –

all radiation falling on a blackbody

(irrespective of wavelength or

angle) is completely absorbed

• As a result, the blackbody is a

perfect emitter

• No body at the same temperature

can emit more radiation at any

wavelength or into any direction

than a blackbody

• A blackbody can be approximated

by placing a tiny aperture in a

radiating cavitygreybody: same radiant exitance as a

blackbody except for a lower emissivity

Properties of optical emitters (1)

• The spectral radiant exitance

is the amount of power

radiated by a body into a

perfect absorber per unit area

of the emitter per unit

wavelength of the radiation:

M Tc

c

T

1

5 2 1exp

where

= the emissivity ( =1 for a blackbody)

c1 = 2phc2 = 3.74510-16 W-m2

c2 = hc/k = 1.438810-1 m-K blackbody radiant exitance as a function

of wavelength and temperature

Properties of optical emitters (2)

• The spectral radiant exitance increases with temperature at all wavelengths

• The peak shifts to shorter wavelengths as the temperature increases

• The variation of max with the temperature can be found by differentiating M(T) w.r.t. and setting = 0

• The spectral exitance can be integrated over all wavelengths:

• High temperatures radiation dominates heat transfer

• Low temperatures thermal conduction dominates heat transfer

M T T 4 = Steffan-Boltzmann constant

(5.669710-8 W m-2 K-4)

3

max 2.88 10 m-K5

hcT

k

Properties of optical emitters (3)

• The net power transfer between two bodies depends on their

relative orientations and is given by:

s q q

T T A F

T Tr

dA dA

A A

A A

cos cos

,

1 2 2 1

1 1

4

2 2

4

1 1 2

1 1

4

2 2

4 1 2

21 21 2

p

12r

A1

A2 “view factor”

Optical sources for RTP

• Tungsten-halogen lamps

– tungsten filament in a quartz

envelope

– halogenated gas (PNBr2)

– gas reacts with deposited W and redeposits on filament

– 0.8 m to 4.0 m emission; total exitance ~ 2.5 kW

spectral emission from a 100-W quartz-halogen lamp

spectral emission from a xenon compact arc lamp

• Noble gas discharge lamps

– fused silica tube containing

Kr or Xe and two electrodes

– a DC discharge (~ 2 kV/cm)

ionizes the gas

– produces both continuous and discrete wavelength spectrum

Optical sources for RTP (2)

A comparison of furnace heating vs. RTP

• Thermal budget refers to the allowed time at elevated temperature

that can be tolerated to control dopant impurity diffusion and oxide

growty

Sources of non-uniform heating

• View factor differences

(different parts of the wafer

“see” the optical sources

differently)

• Edge losses -- the edges are

blocked from the incoming

radiation

• Gas convection/conduction --

edges are more effectively

cooled by the gas ambient

RTP uniformity from four-point probe

resistivity measurements

750ºC, 15 S + 1000 ºC, 20 S

= 604 W/sq, = 1.26%

800ºC, 10 S + 1100 ºC, 10 S

= 290 W/sq, = 2.72%

Temperature measurement

• Accurate temperature measurement remains one of the most

difficult aspects of RTP

• Temperature control requires temperature feedback:

time

tem

per

atu

re

“set

point” Control via temperature feedback

requires three parameters:

• proportional band

• integral signal

• derivative signal

The “P-I-D” values will be

characteristic of a given furnace

Proportional temperature control

time

tem

per

atu

re

set

point

• Proportional band (gain) – a temperature band expressed in % full scale

or degrees within which the controller’s proportional action takes place

• Integral (reset) – adjusts the proportional band to correct for offset

(“droop”) from the setpoint

• Derivative (rate) – senses the rate of rise/fall of the system temperature and

automatically adjusts the proportional band to minimize over/undershoot

time

tem

per

atu

re

proportional band too large

and/or insufficient reset control

proportional band too small

and/or insufficient rate control

Temperature measurement (1)

• Thermocouple

– operates on the Seebeck effect – when a circuit is by a

junction of two dissimilar materials and the junctions are held

at different temperatures a current flows

– if the circuit is broken the net open circuit voltage (the

Seebeck voltage) is a function of the junction temperature

and the composition of the two metals

metal B

metal A metal A

metal B

metal A

AB

Temperature measurement (2)

• thermocouple (continued…)

– a wide range of standard thermocouple types exist

• J – iron/constantan (Cu-Ni alloy)

• K – chromel (Cr-Ni)/ alumel (Al-Ni)

• R – Pt/Pt-13%Rh

– true vs. measured temperature difference may be large

– possibility of metallic contamination

• thermopile

– an arrangement of thermocouples

in series such that the thermoelectric

voltage is amplified (up to 200 junctions)

– usually used as infrared detectors in

pyrometry

Temperature measurement (3)

• semiconductor detectors

– photon or quantum detectors generate charge carriers by

absorption or infrared radiation

– carriers are detected as a photovoltaic or photoconductive

diode

– typical semiconductors include Si, Ge, PbS, PbSe, HgCdTe

and InAs

• response time is fast (~10 sec) but their spectral range is small

• thermopiles are slow (~10 msec) but they are sensitive over a

broad spectral range and are inexpensive

Temperature measurement (4)

• Pyrometry

– a pyrometer measures radiant energy in a band of

wavelengths using either a thermopile, a semiconductor

detector, or both

– converts to temperature using the Stefan-Boltzman relation

– requires knowledge of the emissivity of the wafer

– requires an unobstructed “view” of the wafer

– don’t look right at the heater!

– emissivity of the sample may change during time (deposition)

or with temperature (bandgap) and doping concentration

– “two color” pyrometers are more accurate, but much more

expensive

Thermoplastic stresses

• Thermal gradients thermal stresses

• The radial stress component:

• The angular (tangential) stress component:

R r

r rdrrTr

rdrTR

Er0 022

11

R r

s rTrdrrTr

rdrTR

Er0 022

11

linear thermal

expansion coeff.Young’s modulus temperature at

radial position r

rs

Thermoplastic stresses (2)

• If the stress is high enough the wafer can exhibit

plastic deformation:

kTEe

eA a

n

o

yield exp

1

“reference” strain rate

(Si: 10-3 sec-1)

actual strain rate

• Compare with “power law creep”:

n

aa

n

K

eAkTEkTE

AKe

1

expexp

A 3630Pa, E 1.073eV, and n 2.45

X-ray topographic images of silicon wafers

low strain sensitivity imaging --

shows oxygen “swirl” (A) and

slight quartz boat damage (B)

high strain sensitivity imaging --

shows extensive boat damage (C)

and slip lines (D)

DCAB

Application -- implant anneal

• Implant activation anneals were the “driving force” for the

development of RTP

– a major advantage of ion implant is the ability to produce

well-controlled dopant profiles long anneals can “wash

out” a profile by diffusion

– severe damage may require high temperatures (~1100°C)

that can only be applied for a short time

– high dose implants can exceed the solid solubility, but RTP

does not require thermodynamic equilibrium

Rapid thermal growth of dielectrics

• RTP has several applications in the growth or deposition of thin

dielectric layers

– RTO using dry O2, H2O and N2O

– rapid thermal growth of nitrides and oxynitrides in N2 and NH3

– rapid thermal chemical vapor deposition (RTCVD) of oxides,

nitrides, oxynitrides and doped glasses

– reflow of phosphosilicate (PSG) and borophosphosilicate

(BPSG) glasses

– post-oxidation high temperature annealing (RTA) of charges

and traps at the Si/SiO2 interface

Equipment issues in RTO

• The system must be compatible with the gases used for RTO,

RTN and RTA – O2, H2O, HCl, NH3, N2O

• The system must be ultra-clean

• Gas handling must be vacuum-capable and be able to switch

rapidly from one gas to another

• The process chamber must have a low thermal mass and a

minimum thermal memory

• Heating must be uniform in both static and dynamic conditions

• Temperature measurement and control must be fast, accurate,

and non-contact

RTP of silicides

• Silicides (usually WSi2, CoSi2or TiSi2) are widely used to

establish low-resistance

contacts to source, drain and

gates in MOS devices

• The self-aligned silicide

(“salicide”) process reacts a

blanket metal with exposed

silicon regions

• Reactions of metals occur

rapidly at relatively low

temperatures, making

silicides a “tailor-made”

application of RTP

Other applications -- RTA of GaAs

• as in silicon, the ion implantation of GaAs requires a post-implant

activaition anneal

• problem – the high vapor pressure of arsenic causes loss of

stoichiometry at the surface at temperatures >600°C

• arsenic loss can be limited by several methods

– “capped anneal” -- Si3N4 or SiOxNy

– “proximity cap” -- a sacrificial GaAs wafer is place on or near

the wafer being annealed

– arsenic overpressure

– “capless” anneal is possible if the anneal time is very short

(less than 5 to 10 seconds)

Typical RTP system configurations