Transcript No Slide Title

Plasma Processing Overview
Prof. Karen K. Gleason,
Department of Chemical Engineering, MIT
© 1999 Massachusetts Institute of Technology. All rights reserved
Gleason
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
1
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, radiofrequency (rf) at 13.56
MHz is used to create the
glow discharge.
Typical pressures are
between 1 mtorr and 5
torr.
glow
discharge
neutrals
(radicals, unreacted gas)
ions
Sheath
(dark space)
electrons
Substrate
charged particles
are accelerated by
the electric field
Gleason
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
2
Species Present in the Plasma

The degree of ionization is small (typically ~10-4), so the neutral gas species
predominate.
–
–

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.
–


ni is the ion density
ne is the electron density
f(E) is the electron energy distribution.
The electrons have a much higher temperature than the neutral gas (T=300K ~
0.025 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.
Gleason
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
3
Reactions of Energetic Electrons



Collision of energetic electrons with neutrals can cause
ionization. Typically >8 eV is required. (ex. 13.6 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)
Gleason
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
4
Gas-Phase (Homogeneous) Plasma
Reactions
Dissociation:
Ionization
Atomic :
Molecular :
e* + AB  A + B + e
e* + A A+ + e + e
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
Gleason
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
5
Plasma
Flowing Gas
Generate Reactive Species
Interaction of Homogeneous &
Heterogeneous (Surface) Processes
1/2
homogeneous
gas phase
reaction
creates
reactive
precursor
gas phase
diffusion
adsorption
transports
precursor
to surface
surface
diffusion
desorption &
diffusion of
unreacted
precursor
desorption &
diffusion of
volatile reaction
products
*
heterogeneous
surface
reactions
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
*
Gleason
6
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)
Gleason
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
7
Plasma Parameters
(adapted from Kay et. al., 1980)
Excitation Frequency
rf-13.56 MHz
low- kHz
dual
µwave
Gas Flow Rate
Excitation Power
ne , f(E)
Magnetic Field
N , t
Chemical
Nature of
Feed
Surface
Geometry
Reactor
Geometry
Pressure
Consequences in
Plasma-Surface
Interaction
Chemical
Nature of
Surface
Electrical
Potential of
Surface
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
Substrate
Temperature
Gleason
8
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
Gleason
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
9
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
Gleason
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
10
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)
Gleason
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
11
Etch Profiles
Wet Chemical Etching: isotropic
undercut profile, increased real estate used, loss of dimensional control
resist
SiO2

silicon
Plasma Etching: anisotropic or directional
vertical sidewalls

Gleason
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
12
Limitations of Isotropic Etching
Desired Pattern
W
D
W<D
impossible
Gleason
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
13
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.
Gleason
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
14
Perfluorocompound (PFC) Usage
Use
Chemicals
Silicon Dioxide Etching CF4, SF 6, NF3, CHF3
Silicon Nitride Etching
SF6, NF3, CF4, CHF3
Chamber Cleaning :
SiO 2 and SiN x
C2F6, NF3, SF 6, CF4


Dielectric film processes constitute bulk of PFC usage.
Dielectric film processes presently rely exclusively on PFCs.
Gleason
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
15
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.
Gleason
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
16
Competitive Etching and Deposition
tends to consume
the F etchant to
form HF
H2 addition
C2F4
Bias Applied to Surface (volts)
tends to consume
the CF radicals
form CO, CO2
O2 addition
C2F6
CF4
-200
Etching
-100
Polymerizing
0
0
1
2
3
4
Fluorine to Carbon Ratio (F / C)
of Gas Phase
Gleason
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
17
Possible F etching mechanisms
surface
F
F
bulk
Si
Si
F
Si
F
+ 2F
FF F
F Si
F
Si
Si
F
+ 2F
F
Si
F F
F
Si
+ SiF4
Gleason
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
18
Flow Rate & Pumping Speed
Considerations
Q : gas flow rate
sccm=standard cm3/min
standard conditions
are 0°C & 1 atm
P : reactor pressure
S : pumping speed
V : reactor volume
t: average residence time
S = Q*1atm/P
for gas in the reactor
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.
Gleason
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
19
Mass Balance on Fluorine Atoms:
Maximum Etch Rate
CzHyFx


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
–
–
–

SiF4
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 1022 Si/cm3
for SiO2 : 2.66 x 1022 Si/cm3

Emax =
( x*Q* Nsccm )/(4 * A * NSi)

Utilization Factor = Eobserved/Emax
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
Gleason
20
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.
E
Q

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.
Gleason
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
21
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
Gleason
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
22
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. 539-585 (also p. 627-646 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).
Gleason
NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing
23