plasma technology_2014

SNT5039 Nano Processing (Plasma Technology) 1

Part II: Plasma Technology

1. PLASMA PROPERTIES- PLASMA PROPERTIES- COLLISIONAL PROCESSES IN PLASMAS- FORMATION OF SPACE CHARGE REGION

2. DC AND RF DISCHARGES- DC DISCHARGES AND SPUTTERING- PARALLEL PLATE RF DISCHARGE- MATCHING NETWORK- HIGH DENSITY PLASMAS

3. PLASMA DIAGNOSTICS- OPTICAL EMISSION SPECTROSCOPY- OTHER DIAGNOSTIC TECHNIQUES


A quasi-neutral gas of charged and neutral particles exhibiting collective behavior. “Quasi-neutral” means that, except for localized regions in space, overall charge density in the plasma is zero. On average, the densities of positively and negatively charged particles

compensate each other. Weakly ionized plasma is formed by a voltage source that drives current

through a low pressure gas between two electrodes. The “collective behavior” comes about because the charged particles

interact with each other via long-range Coulomb forces.

PLASMA PROPERTIES


With the increase of temperature, solid state changes to liquidstate and liquid state changes to gas state. At a sufficiently hightemperature, the gas decomposes to atoms and further to freelymoving charged particles. This substance is defined as theplasma state

Equilibrium plasma (ne = ni = n and Te = Ti = T) Non-equilibrium plasma (ne = ni << n and Te >> Ti )

Non-equilibrium and low temperature plasma can be used forsemiconductor processing, because substrate can be processedat low temperature, while significant amount of radicals and/orhigh energy ions can be obtained

PLASMA PROPERTIES


Log10 Te(V)

HighPressure

arcs

Shocktubes

LaserPlasma

ThetaPinches

Focus

Fusionreactor

Fusionexperiments

LowPressure

GlowDischarges

AlkaliMetal

plasma

Flames

Earth’sIonos-phere

0 1 2 3 4 510-110-2

Log 1

0n

(cm

-3)

5

15

20

10

25

1De cm

34 13 Den cm

Solid Si at room temperature

PLASMA PROPERTIES


(1) Etching (Reactive ion etching)

(2) Photo-resist Stripping (Ashing)

(3) Sputter Deposition

(4) Plasma Enhanced Chemical Vapor Deposition (PECVD)

(6) Surface Modification

Application of Plasmas to Semiconductor Processes


Basic Plasma Equations (Maxwell’s equations)

Equations RelationsGauss’s law for electricity

Net electric flux vs net enclosed electric charge

Gauss’s law for magnetism

Net magnetic flux vs net enclosed magnetic charge

Faraday’s law Induced electric field vs changing magnetic flux

Ampere-Maxwell law

Induced magnetic field vs changing electric flux and current

0

ˆˆqAdE

0ˆˆ AdB

dtdsdE B

ˆˆ

idtdsdB E

000ˆˆ


Basic Plasma Equations (Other equations)

Equations Related equationsConservation(Boltzman Equation)

Particle Conservation

Momentum Conservation

Isothermal relation

Diffusion and mobility

Einstein relation:

Lorentz force law

Elecric and magnetic fields exert forces on charged particles

EnnDJ ˆˆ qkTD

)ˆˆˆ(ˆ BvEqF

LGuntn

)ˆ(0ˆˆ

fafvtf

vr

0ˆˆ umnpEqn m

nkTp


Particle density (using ideal equation)

N is a molar number, N0 is 6x1023mol-1, and R and kB are theBoltzmann constants as R=8.314J/mol K and kB=1.38x10-23J/K.

The gas density (n) in the low pressure processing plasmas of1mTorr to 1Torr is estimated to be in the range of about 3.5x1013 to3.5x1016 cm-3

Rule of thumb on gas densityng (cm-3) = 3.25 x 1013 p (mTorr)

TkP

VNN

nandTkNNNRTPVB

B 00

PLASMA PROPERTIES


Ionization ratio = ni / ng

Plasma density = Particle density x Ionization ratio

Ionization ratio is normally in the range of 10-5 to 10-2 in lowpressure processing plasmas of 1mTorr to 1Torr

The densities of ions (ni) and electrons (ne), sometimes calledplasma density, are typically in the range of 109 to 1012 cm-3

If the plasma density is higher than 1011 cm-3, in themicroelectronics processes we call it “high density plasma”

PLASMA PROPERTIES


Electron and Ion Temperatures

The average energy ischaracterized by thetemperature term (kT) inelectron volt (eV)

The average kinetic energy ofelectrons becomes very high onthe order of 1eV (1-5eV)

The average energy of the ionsis generally close to but slightlyhigher than that of the neutralsin the range of 0.03eV to0.1eV

Q) Compare plasma density and electrontemperature between conventional plasmaand high density plasma

PLASMA PROPERTIES

+

ELECTRONWork Done

Acceleration

Acceleration

ION

Work Done

FtpmeEt

mp

ee

,22

22

ee meE

mF

im

eEt2

2

imeE

E


The average speed of particles in a Maxwell-Boltzmann gas

dnc

ncor

mkTc 18 2/1

sec/107sec/105 52 mxcandmxcc ein

Electrons move much faster and also respond more quickly to an electrical disturbance than ions f = Ee/m, the lighter a particle is, the more it is accelerated and the greater velocity (current) is acquired

PLASMA PROPERTIES

Overall kinetic energy of each gas molecule (root mean square speed)2/1

22 323

21

mkTcandkTcm


Collision Cross Sections N(x): Number of electrons having made no collision, along the path x

N(x+dx) = N(x)(probability that an electron does not collide between x and x+dx)

That is, N(x+dx) = N(x)(1-ndx)

nxNdxxdN )()(

N(x) = N0 exp (- n x)

dxdxdN

Ndxxp

0

1)( Introducing dimensionless parameterp(x)dx as a probability for collision tooccur between x and x + dx )exp()( nxnxp

COLLISIONAL PROCESSES IN PLASMAS


Mean free path

The average distance an electron traveled before collision

0

)( dxxxp

Integrating the above using

1)( n

)exp()( nxnxp

** Estimate mean free path of the electrons in the plasma of 10mtorr that is considered as an ideal gas!! ( = r2 where r is the atomic radius about 0.3nm)



Elastic collision

Total kinetic energy of the particles is conserved

Elastic scattering: e + Ar e + Ar

Inelastic collision

Part of the kinetic energy is converted into potential energy

Ionization (conversely, recombination) : e + Ar 2e + Ar+

Excitation (conversely, relaxation) : e + Ar e + Ar*


orbit.Bohr first in theelocity electron v theas m/s,2.19x104π

e ve wher

n

v vis velocity Here,

radius)(Bohr m5.29x10me

4πa

),anafor 1(n levellowest For thenmva

momentumangular theof multiples theremomentumsatheir way thatain orbits thelimitsn descriptio quantumA

amv

a4πe

is,That force. lcentrifuga outward theand force ticelectrosta inward thebetween balance by the orbitscircular ain move electons n,descriptio classical aIn

6

0

2

at

atn

112

20

0

02

n

2

20

2

Inelastic Collision

Inelastic Collision

1)n when atom H of potentialn (Ionizatio 13.6V4π

e2emE

nEE and (V),eE(J) WapplyingBy

a4πemv

21W

energy, potential and kinetic theof sum theisenergy electron The

s2.42x10va

w1t

is timescalesticcharacteri The

2

0

2

at

atnnn

n0

22nn

17

at

0at


Scattering cross section

An area that scattering takes placewhen the electron passes through anatom

An effective total cross sectionalarea for scattering between anelectron and an atom, includingcontributions from collisionalprocesses such as elastic scattering,ionization, excitation

The cross section is a strong function of electron energy


electrons

atoms


Q) Explain why cross sections decrease with increasing electron energyQ) Explain why threshold energy for excitation is lower than that of ionization


1 10 100

(c

m2 )

Electron Energy (eV)

E + Ar

Elastic

Ionization

Excitation

10-15

10-16

10-17


(1) Ion-Atom Collisions with energy exchange: Ar++Ar Ar++Ar or Ar+Ar+

(2) Radiative relaxation: An excited atom returns to the ground state, or to the lower energy-level excited state by emitting a photon:

Ar* Ar + photon In the etching plasma, various radiative relaxation processes occur and

this generates optically visible glow that can be used for monitoring processes

(3) Recombination: e- + Ar+ ArThis process hardly occurs due to energy conservation. But, if the third body participates in the collision, this process can take place in the following ways

e- + Ar++ Ar Ar + Ar ore- + Ar++ wall Ar + Ar

The second process has higher probability and occurs on the reactorwalls. This is the main path the ions are lost in processing plasmas ofsputtering, CVD, and Reactive Ion Etching

COLLISIONAL PROCESSES IN PLASMAS (Collisional Processes Involving Atoms)


Density of various particles in the low pressure processing plasma


PLASMA PROPERTIES

In the low pressure processing plasma, densities ofpositively and negatively charged particles are aboutthe same, but electrons are much more mobile thanpositive ions (ni = ne, ci < ce),resulting in disturbance in charge neutrality (je >> ji)

14

14

e e e

i i i

j en c

j en c

+

+--

+

+--

+

+--

+--

+

+-- 23

4

6.6 1020 293 1/ 404.0 10 / sec

m gT C k eVc cm

Neutrals

23

4

6.6 10500 0.04

5.2 10 / sec

i

i

i

m gT k eVc cm

Ions28

7

9.1 1023200 2

9.5 10 / sec

e

e

e

m gT k eVc cm

Electrons1/ 28( )kTc

m


iiieee cenjcenj41,

41

To maintain the chargeneutrality, electron flux shouldbe equal to ion flux in theplasma, resulting in potentialdifference near the electrode

Plasma potential (Space potential)Potential obtained in the bulk of the plasma when the densities of ionsand electrons are equal, and therefore electric potential is constant.

Floating potentialPotential obtained when satisfying condition that electron flux is equalto ion flux. It is negative with respect to the plasma potential,

PLASMA PROPERTIES

+

+--

+

+--

+-

-

+

+

-

-

4

4

4

e e

i i

n c

nc

n c


ie

eiefp Tm

Tme

kTVV ln

2

• Calculate Vp-Vf for an Ar plasma with electron and ion temperatures of kTe=2eV and kTi=0.1eV !

Sheath (Dark space)- The region that has an excess of ions and,

there charge neutrality does not hold.- The positive space charge shields the

floated body from the bulk of the plasma.- Ions are accelerated towards the

electrode due to the electric field,bombarding the surface.

FORMATION OF SPACE CHARGE REGION


Debye Length:

• A characteristic thickness of the space charge sheath around the grid.

)exp()( 0D

xx

a way to quantify sheath thickness

• Expressed as a function of electron density and electron temperature


21

20

enkT

i

eD


Debye length

as a function of electron density and electron temperature

Q) Fill in the Debye length under conditions given in the table.

2/1

20

enkT

i

eD

)( 3cmne )(eVkTe1 3 10

1 x 1010 75mm 130mm

1 x 1011 24mm

1 x 1012

Q) Compare the Debye length between cases of conventional plasma and high density plasma.



(1) Two parallel electrodes are grounded at the both ends: V(x) = 0 at x = 0 and x = l.(2) Ion density is constant at ni across the plasma: ni(x) = ni for 0 ≤ x ≤ l. (3) Electron density is 0 across the plasma: ne(x) =0 for 0 ≤ x ≤ l.

(Q1) Using the Poisson equation,

find expressions on the potential V(x), and the electric field, in the plasma.

(Q2) Plot V(x) and E(x) for 0 ≤ x ≤ l.

(Q3) Using the result in Q1, estimate potential at the center, V(x=l/2), assuming that the distance between the parallel electrodes is 10 cm and ion density is 1010 cm-3.

(Q4) According to the electrical measurement results using a Langmuir probe on usual processing plasmas having a reactor size of l ~ 10 cmand an ion density of ni ~ 1010 cm-3, plasma potential is in the range of 10 – 50V. Determine whether the result in Q3 is reasonable or not. If it is unreasonable, explain the reason why the unreasonable result was obtained.

Poisson Equation in Plasmas

02

2 )(

ei nnedxVd

dxdVxE )(


Plasma Technology





The most common reactor geometry in the DC and RF dischargesis a parallel electrode type where the plasma is mostly confinedbetween two planar electrodes

After studying a parallel plate DC discharge which is widely usedfor sputter deposition, we review RF discharges that are used formost of the plasma etching applications as well as PEVCD

In the DC discharges, the voltage applied between the electrodesis constant in time, whereas in the RF discharges the appliedvoltage varies with time.

The time-dependent behavior of an RF discharge needs to bediscussed, although its time-averaged behavior resembles that ofthe DC discharge

PARALLEL PLATE DC DISCHARGES (DC AND RF PLASMAS)


The plasma is generated by

applying a high E to a gas so as to

result in electrical breakdown

Once a steady state is reached,

the externally applied power gives

rise to an electrical current

flowing through the plasma

PARALLEL PLATE DC DISCHARGES (DC PLASMA)

Argon Gas

Target

Substrate

Vacuum Chamber

DCPower Supply

+

_

Argon Gas

Target

Substrate

Vacuum Chamber

DCPower Supply

+

_


Assume that the bulk of the plasma between two electrodes is uniform,

By using the equation about ion flux,

The ion current density at the cathode sheath edge is estimated to be 0.15mA/cm2.

By using to the equation about electron flux,

The thermal electron current density is 28mA/cm2 and it is much larger than 0.15mA/cm2 (je >> ji)

The positive current density at the cathode must be compensated by thenegative current density at the anode. It follows that there must be smallsheath at the anode to reduce the electron current, where the sheathpotential will be equal to the plasma potential, Vp, for grounded anode

eVkTandcmnnn eie 110 3100

2/10 )/( iei mkTenj

2/100 )2/(

41

eee mkTencenj

PARALLEL PLATE DC DISCHARGES


Vtj

dAC o

For continuous plasma, RF frequency higher than about 100kHz is required.

(Ex)-1 mm thick SiO2-1000V DC Bias, 1 mA/cm2 ion current

This requires 130kHz RF for continuous plasma

At low frequency, plasma is short-lived (ON and OFF).

PARALLEL PLATE RF DISCHARGES


RF generator voltage (Va)At low frequency, targetvoltage (Vb) changes withion bombardment andelectron current

>> Steady state with zeronet current

PARALLEL PLATE RF DISCHARGES


Development of Self DC Bias (VDC)PARALLEL PLATE RF DISCHARGES


Development of Self DC Bias (VDC) and Space Charge RegionPARALLEL PLATE RF DISCHARGES


The I-V characteristic of the electrode is determined by the large difference inmobility of electrons and ions

In the negative potential regime, electrons are repelled from but ions areattracted to the electrode, resulting in small net positive current. As the positivepotential increases, electrons become attracted but ions become repelled fromthe electrode, resulting in large net negative current. This is due to high mobilityof the electrons compared to the ions

Apply this to the time-dependent RF discharge where the average currentshould be zero. Net current for one RF cycle becomes largely negative and thismust be corrected for the net current to be zero

If the average value of the RF oscillation is shifted to some negative value, VDC,the net current can be zero. Here, VDC, is called DC self bias, and it has veryimportant implications in determining various characteristics of ion bombardmentin the sputtering and reactive ion etching processes

As a result of negative DC self bias, the sheath of the positive space charge isformed, as we have seen in the DC discharges

PARALLEL PLATE RF DISCHARGES: Development of Self DC Bias (VDC)


MATCHING NETWORK

Purpose of RF matching network

To increase the power dissipation in the discharge and to protect the RF

generator.

(1) Analysis using DC power source

Maximum power is obtained at dP/dR = 0, i.e , R=r.

Therefore, we need to match the resistance of the load to the resistance

of the power supply.

Power dissipated in the variable resistor R,

2

2

)( RrREP


MATCHING NETWORK

(2) Analysis using RF power source• Most common L-type matching network

(shown) consists of a shunt capacitor having susceptance of BM=ωCM and a series inductor having a reactance XM=ωLM.

• Time averaged power flowing into discharge,

22

2

)(~

21

DDT

DT XRR

RVP

0

DXP 0

DRP• Maximum power transferred to the load can be obtained when and .

Therefore, XD=0 , RD=RT and T

T

R

VP

2

max

~

41

• By further circuit analysis, the inductance and capacitance in the matching network for

maximum power transfer can be determined.


Why High Density Plasma?- High utilization of power for high ionization efficiency

(Capacitively coupled plasma (CCP): 1-3%, partially for ionization. Most power is spent in sheaths for CCP, but it is spent in the bulk of the plasma for HDP.)

- Low bias (low damage to devices, high selectivity)- High processing rates (High density of ions and radicals)

High Density Plasma

Table Range of Parameters for Rf Diode and High-Density DischargesParameter Rf Diode High-Density Source

Pressure p(mTorr) 10-1000 0.5-50

Power p (W) 50-2000 100-5000

Frequency f (MHz) 0.05-13.56 0-2450

Volume V (L) 1-10 2-50

Cross-sectional area A (cm2) 300-2000 300-500

Magnetic field B (kG) 0 0-1

Plasma density n (cm-3) 109-1011 1010-1012

Electron temperature Te (V) 1-5 2-7

Ion acceleration energy εi (V) 200-1000 20-500

Fractional ionization Xiz 10-6-10-3 10-4-10-1


How to Generation HDP?- Magnetic enhancement (ME)- Inductive coil- etc

High Density Plasma (Supplementary slide)

B

e

RF power RF power

N NS

Be

E × B

RF power

BERF

RF power

B

MERIE ICPEF = q · r × B E plasma = -dΦB/dt


PARALLEL PLATE RF DISCHARGES (High Density Plasma)

Generation of HDP:

- Magnetic enhancement (ME)- Inductive coil (ICP)- Electron Cyclotron Resonance (ECR)


Faraday’s law:“A changing magnetic field produces an electric field. E = - dFB/dt”In the ICP system, RF power source using coils generates a magnetic field that induces an electric field in the plasma reactor. The efficient power transfer by the inductive coil produces a high density plasma. Schematics on the generation of electric fields from two RF coils are shown below.

Induction of Electric Field in Inductively Coupled Plasma


Density of ICP:

- 1011 to 1012 cm-3,- CCP < ICP < helicon, ECR

In ICP, power is transferred from the electric fields to theplasma electrons within “skin depth” near the surface bycollisional (ohmic) dissipation and by collisionless heatingprocess in which bulk plasma electrons collide with theoscillating inductive electric fields within the layer.

In the collisionless heating process, electrons are acceleratedand subsequently thermalized like the heating process incapacitive RF sheaths.

Inductively Coupled Plasma


(1) In low pressure and high density plasmas, the common feature isthat “RF power is coupled to the plasma across the dielectricwindow or wall” rather than by direct connection to an electrode, asfor the capacitive discharge

(2) That is, non-capacitive power transfer is the key to achieve lowvoltage, normally 20-40V, across the sheath at the dielectric or thewall

(3) To control the ion energy, the substrate can be independently drivenby capacitively coupled RF source. Hence, independent control ofthe ion flux and ion energy is possible

(4) RF coil results inCCP at low plasma density and ICP at high plasma density

Inductively Coupled Plasma


Plasma Technology





Radiative relaxation: An excited atom returns to the ground state, orto the lower energy-level excited state by emitting a photon:

Ar* Ar + photon

* In the etching plasma, various radiative relaxation processesoccur and this generates an optically visible glow that can be usedfor plasma monitoring and end point detection.

OPTICAL EMISSION SPECTROSCOPY (OES)


An excited atom returns to the ground state, or to the lower energy-levelexcited state by emitting a photon: Ar* Ar + photon (Optical Emission)* In the etching plasma, various radiative relaxation processes occur and thisgenerates optically visible glow that can be used for monitoring processes

OXYGEN NITROGEN CHLORINE0

20

40

60

80

100 4p' 2F

4s' 2D2p2 3s 4P

2p2 3p 4Do

3s 3S

3p 3p

3p5 2Po2p3 4So

904nm869nm845nm

210nm211nm

ENER

GY

(cm

-1)

226nm

2p 3p



1. Detection of plasma species- used to estimate etching rate and determine the amount of over etching

2. Endpoint detection- the shape of optical emission curve can be used to detect endpoint


200 300 400 500 600 700 8000

2000

4000

6000

8000

10000

12000

SiGe etching by products

Si264.5nm

Inte

nsity

Wavelength (nm)

Ge264.5nm

0 20 40 60 80 100 1200

500

1000

1500

2000

2500

3000

Inte

nsity

Time (s)

HBr Main Etching

Substrateetching

HfO2

Si etchingbyproducts: 405 nm

Cl2breakthrough

Si-substrate

HfO2

Poly Si

Mask


For determination of ion density (ni) and electron temperature (Te)

OTHER PLASMA DIAGNOSTICSELECTRICAL PROBES (LANGMUIR PROBES)


e

ppreeee kT

VVemkTAenI

)(exp)2/( 2/1

iieii imkTAenI 2/1)2/( 2/1)

)((15.1

e

ppri kT

VVei

OTHER PLASMA DIAGNOSTICS (LANGMUIR PROBES)


OTHER PLASMA DIAGNOSTICS (LANGMUIR PROBES)


OTHER PLASMA DIAGNOSTICS Mass Spectroscopy


(1) The species in the ionizer should represent the species in the plasmas orspecies near the substrate wafer. Ionization rates can be much different in QMSdue to pressure difference from plasma

(2) Ambiguous identification, for example, both N2 and CO have m=28amu.(3) QMS is more useful for qualitative analysis than for quantitative analysis.

OTHER PLASMA DIAGNOSTICS :Mass Spectroscopy

plasma technology_2014

Documents

plasma technology1

low temperature plasma

weakly ionized plasma

t nonequilibrium plasma

forsemiconductor processing

gas state

gas density n

low pressure gas