1. Plasma Processing Overview
Prof. Karen K. Gleason,
Department of Chemical Engineering, MIT
© 1999 Massachusetts Institute of Technology. All rights reserved

2. (radicals, unreacted gas)
Plasma Schematic
A partially ionized gas created by application of an electric field. Positive ion/electron pairs are created by ionization reactions, maintaining overall charge neutral.
Commonly, radio-frequency (rf) at MHz is used to create the glow discharge.
Typical pressures are between 1 mtorr and 5 torr.
neutrals
(radicals, unreacted gas)
ions
electrons
glow
discharge
Sheath
(dark space)
Substrate
charged particles
are accelerated by
the electric field

3. Species Present in the Plasma
The degree of ionization is small (typically ~10-4), so the neutral gas species predominate.
ni is the ion density
ne is the electron density
Because of their charge and low mass, electrons in the plasma acquire energy from the applied electric field. AVERAGE electron energies in a plasma are 1-10 eV. Some electrons will have even higher energies, and this high energy tail is responsible for many of the reactions that occur in the plasma.
f(E) is the electron energy distribution.
The electrons have a much higher temperature than the neutral gas (T=300K ~ eV). Thus, the plasma is a non-equilibrium environment. This allows some chemical reactions to occur which would require much higher gas temperatures in the absence of the plasma.
Collisions with energetic electrons can also cause dissociation of molecules into highly reactive species known as radicals. Radical concentrations can be much higher than the concentration of charged species.

4. Reactions of Energetic Electrons
Collision of energetic electrons with neutrals can cause ionization. Typically >8 eV is required. (ex eV needed to ionize an H atom)
Collisions with less energetic electrons can produce electrically excited species. Some will give off light when they decay to their ground state. This phenomena gave rise to the term “glow discharge” being used as an synonym for plasma. The emitted light can also be used to perform optical emission spectroscopy, which can identify excited species based on the wavelength of the emitted light.
Numerous possible reactions and species (complex chemistry)

5. Gas-Phase (Homogeneous) Plasma Reactions
Dissociation: e* + AB ® A + B + e
Ionization
Atomic : e* + A ® A+ + e + e
Molecular : e* + AB ® AB+ + e + e
Excitation
Atomic : e* + A ® A* + e
Molecular: e* + AB ® AB* + e
Recombination
Atomic : e + A+ ® A
Molecular: e + AB+ ® AB
Relaxation
Atomic : A* ® A + hn
Molecular: AB* ® AB + hn
* indicates high energy species

6. Interaction of Homogeneous & Heterogeneous (Surface) Processes
Generate Reactive Species
Flowing Gas
Plasma

7. Ions
Ions can be accelerated across the sheath, producing energetic ion bombardment of the surface. The ions give rise to the directionality (anisotropy) of plasma etching processes either:
directly (sputtering, ion beam milling)
indirectly by assisting chemical attack by neutrals (ion-assisted etching)

8. Plasma Parameters n , f(E) N , t (adapted from Kay et. al., 1980)
Excitation Frequency
rf MHz
Gas Flow Rate
low- kHz
dual
µwave n
Excitation Power e
, f(E)
Reactor
Geometry
Magnetic Field
N , t
Chemical
Nature of
Pressure
Feed
Chemical
Nature of
Surface
Surface
Geometry
Consequences in
Plasma-Surface
Interaction
Substrate
Electrical
Temperature
Potential of
Surface

9. Uses of Plasma Processing in Semiconductor Industry
Chemical Vapor Deposition (CVD)
chemical reactions occur at lower temperatures in the plasma
Chamber Cleaning
able to remove film buildup from the reactor walls
Etching (Patterning)
vertical sidewalls (anisotropic etching)
Dry Cleaning
in place of wet chemical rinses to remove residues and contamination

10. Plasma-Enhanced CVD (PECVD)
Used for dielectrics like silicon dioxide and silicon nitride
High quality, defect free films
Can cover non-planar surfaces
Patterned by plasma etching
Deposition occurs on silicon wafer and on reactor walls

11. Motivation for Chamber Cleaning
Reduces particle formation
Prevents drifts of the CVD chemistry cause by interaction of the plasma with the bare vs the coated reactor walls (improves reproducibility)
Reduces equipment failure (improves production availability)

12. Etch Profiles Wet Chemical Etching: isotropic
undercut profile, increased real estate used, loss of dimensional control
Plasma Etching: anisotropic or directional
vertical sidewalls
resist
SiO2
silicon

13. Limitations of Isotropic Etching
Desired Pattern W D
W<D
impossible

14. Etching Performance Requirements
Rate
Selectivity (mask to etch layer; exposed layer to etch layer)
Profile (directionality of etch; isotropy/anisotropy)
Planarity
Uniformity (across wafer)
Minimal damage to substrates as a result of handling
Reproducibility (run-to-run)
Low particulates
Low contamination
PECVD cleaning presently uses more PFCs than wafer patterning and is the faster growing application, but wafer patterning usage is still significant.

15. Perfluorocompound (PFC) Usage
Dielectric film processes constitute bulk of PFC usage.
Dielectric film processes presently rely exclusively on PFCs.

16. PFC plasmas
Electrical energy (usually rf or microwave) dissociates PFC to free fluorine and CFx radicals.
F is the dominate etch species.
CFx lead to polymer formation.
Ion bombardment sputters away fluorocarbon polymer at bottom of trench.
Fluorocarbon polymer remains on sidewalls, resulting in anisotropic etching.

17. Competitive Etching and Deposition
tends to consume
the F etchant to
form HF
H addition
O addition
tends to consume
the CF radicals
form CO, CO2 2 2
C2F4
C2F6
CF4
-200
Etching
Bias Applied to Surface (volts)
-100
Polymerizing
Fluorine to Carbon Ratio (F / C)
of Gas Phase

18. Possible F etching mechanisms
surface
bulk
+ 2F F Si F F F Si F F Si Si
+ 2F F F F F Si Si
+ SiF4

19. Flow Rate & Pumping Speed Considerations
P : reactor pressure
V : reactor volume
t : average residence time
for gas in the reactor
Q : gas flow rate
S : pumping speed
sccm=standard cm3/min
S = Q*1atm/P
standard conditions
are 0°C & 1 atm
t = V/S
Pressure can be adjusted in two ways:
a) Fix Q, vary S. This varies t.
b) Fix S, vary Q. This fixes t.
The etch rate as a function of pressure can depend on the method
used to vary the pressure.

20. Mass Balance on Fluorine Atoms: Maximum Etch Rate
CzHyFx
SiF4
silicon or silicon dioxide
Highest etch rate is when all fluorine leaves the reactor as SiF4.
Flux of Fluorine in (#atoms per min)= x*Q* Nsccm
x= number of fluorine atoms per etchant molecule
Q = inlet flow rate of etchant molecules in sccm
Nsccm = 2.69 x 1019 molecules/min
Flux of Fluorine out = 4 * E * A * NSi
4 = the number of fluorine atoms per SiF4
E = etch rate (cm/min)
A = area being etched (substrate for pattering or chamber wall for cleaning) (cm2)
NSi = number of silicon atoms per cm3
for Si : 5 x Si/cm3
for SiO2 : 2.66 x Si/cm3
Emax = ( x*Q* Nsccm )/(4 * A * NSi)
Utilization Factor = Eobserved/Emax

21. Etch Rate and Flow
Maximum etch rate is proportional to flow rate. At low flow rate, the etch rate can be limited by the rate at which reactants are supplied.
At high flow rates, the reactants are pumped out before before etching can occur.
These two competing factors lead to a maximum in etch rate.
Increasing the utilization factor will use less gas. However, at utilization factors > 0.1, non-uniform etch rates often occur across the wafer. Thus, there is a trade-off between using less resources and process performance. E Q

22. Process Optimization to Improve Conversion Efficiency
In situ process monitoring - end point detection
insures run to run reproducibility
over topography, thickness variation in dielectric films to be etched. Thus some areas will be etched before others.
optical emission spectroscopy
laser interferometry and laser reflectance
Effluent characterization
mass spectroscopy
fourier transform infrared spectroscopy
Process parameter variation
Many process variables
Many non-linear interactions
Design of Experiments (DOE) desirable

23. Recommended Reading
S. Wolf and R.N. Tauber, “Silicon Processing for the VLSI Era: Volume 1-Process Technology” (Lattice Press, Sunset Beach, CA, 1986). p (also p for DOE)
B.N. Chapman, T.A. Hansen and V.J. Minkiewicz, “The implication of flow-rate dependencies in plasma etching”, J. Appl. Phys. 51, 3608 (1980).