Transcript No Slide Title

PFC Replacement Chemistries
Prof. Karen K. Gleason,
Department of Chemical Engineering, MIT
Source materials contributed by :
Mr. Simon Karecki &
Prof. Rafael Reif
Department of Electrical Engineering & Computer Science, MIT
© 1999 Massachusetts Institute of Technology. All rights reserved
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Outline



Potential applications
Selection guidelines and tradeoff (performance and
ESH)
Broad view for alternative processes
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Potential Applications for PFC
Replacement Chemistries

Dielectric materials
– chamber cleaning of CVD reactors for oxides & nitrides
– etching (patterning) of oxides & nitrides




–
fluorine is required (SiF4 etch product)

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dielectic for device isolation and insulating metal lines
corrosion and mechanical protection
mask against dopants, impurities and oxidation
planarization (smooth out topography)
Other halogens (Cl, Br, I) are not effective etch species
Currently F is generated from PFCs
Other materials (tungsten, polysilicon)
–
can be etched in non-fluorine chemistries
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Replacement Chemistries

Chamber cleans have been targeted first.
– utilize most of the gas
– have less stringent process requirements than the dielectric etching
– higher probability for finding a “drop-in” replacement

Replacing one PFC by another may not positively impact global
warming.
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Selection Guidelines - ES&H

Desire alternative chemistries with no long term environmental
impact (i.e., with low atmospheric stability)
– low global warming potential (GWP)
– low ozone depletion potential (ODP)


Ease of handling and use
Exclude chemistries with high health hazards
– mutagenic
– teratogenic
– carcinogenic
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Selection Guidelines-Performance

Chamber cleaning
– vapor pressure (boiling point)
– ability to generate etchant (fluorine)
– rate (minimize gas volume & increase throughput)

Etching
In addition to the chamber cleaning requirements:
– ability to form some polymer (anisotropic etching to achieve
desired profile)
– selectivity
– uniformity
– reproducibility
– avoid particulates
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PFC Characteristics
Gas
Atm. Lifetime
(years)
50,000
10,000
5,600
3,200
740
264
CF4
C 2 F6
C 3 F8
SF6
NF3
CHF3
GWP
(100 ITH)
6,500
9,200
6,950
23,900
13,100
11,700
Boiling
Point (°C)
-128
-79
-36.7
-50.6
-128.9
-84.4
Critical Point Data
Gas
CF4
C 2 F6
SF6
NF3
Tc (K)
132.9
292.8
318.7
234
Pc (atm)
34.5
37.1
44.7
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Estimating Vapor Pressure, Pvp

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Vapor pressure is a function of temperature, T
Theory of corresponding states (based on critical point data)
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Tc is the critical temperature (units of absolute temperature, K)
Pc is the critical temperature (will give Pvp in the same units)
critical point data is tabulated for many compounds
critical point data can be estimated for the others
Empirical correlation also requires the boiling temperature, Tb






ln Pc  Tc 
 Tb


ln( Pvp )  
1 


T
 c 1  Tb  T 

T 


c
from “The Properties of Gases and Liquids”
R.C. Reid, J.M. Prausnitz & T.K. Sherwood
McGraw-Hill, 1977, p. 182
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Halogenated Compounds: A Tradeoff
Stable
(high long-term
environmental impact)
Reactive
(high health/safety
impact)
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Trade-off example: TFAA
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Triflouroacetic anhydride TFAA (CF3COOCCF3)
Potential use for chamber cleaning
Reacts readily with water to form trifluoroacetic acid TFA
(CF3COOH)
Atmospheric lifetime of TFAA < 30 minutes (GPW~0)
TFA degraded by microbes
But TFA has known, and potentially unknown, health & safety
hazards
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Excerpts from MSDS for TFA

Inhalation: Material is extremely destructive to mucous membranes
and upper respiratory tract. Symptoms of exposure may include
burning sensation, coughing, wheezing, laryngitis, shortness of breath,
headache, nausea and vomiting. Inhalation may be fatal as a result of
spasm, inflammation and edema of the larynx and bronchi, chemical
pneumonitis and pulmonary edema.

Extremely destructive to eyes

Extremely destructive to skin (corrosive - causes severe burns)

To the best of our knowledge, the chemical, physical, and toxicological
properties have not been thoroughly investigated.
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Other Fluorine-Based Chemistries

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Hydrofluorocarbons (HFCs)
Iodofluorocarbons (IFCs)
Unsaturated Fluorocarbons
Chlorine and Bromine containing replacements have been ruled
out because of their high ozone depletion potential (ODP)
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Hydrofluorocarbons (HFCs)
C xF yH z
Line Formula
CF2H2
Halocarbon Flamm. Toxicity GWP100 Boiling
number
Point
32
Y
toxic
580
-51.7 ºC
Lifetime
6 yrs.
Used as
Etchant?
Y
C2F5H
CF3-CF2H
125
N
slight
3200
-48.5 ºC
36 yrs.
Y
C2F4H2
CF2H-CF2H
CF3-CFH2
CF2H-CF2-CF3
CF3-CFH-CF3
CF2=CFH
134
134a
227ca
227ea
1123
N
slight
slight
Y
N/A
-19.7 ºC
-26.5 ºC
-16.3 ºC
-15.2 ºC
-51.0 ºC
11.9 yrs.
14 yrs.
?
41 yrs.
?
Y
N
1200
1300
?
3300
?
C3F7H
C2F3H
CF2H2: Acute and chronic heart damage, narcotic effect,
prolonged skin exposure can cause defatting and dermatitis
C2F5H and C2F4H2: Very large doses may cause CNS depression,
heart irregularities, dizziness, anesthetic effect
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HFCs - “A Conservative Approach”

CF3H is not a candidate (GWP=12,100)

HFCs are mostly not toxic, or at least, not acutely toxic

Sizable but finite lifetimes
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Iodofluorocarbons
CxFyIz Line Formula Flamm. Toxicity Vapor Pres.
@ 20 ºC
CF3I
N
irritant
85 psi
CF2I2
Boiling
Point
-22.5 ºC
N
C2F5I
CF3-CF2I
N
irritant
35 psi
12-13 ºC
C2F4I2
CF2I-CF2I
N
irritant
N/A
112-113 ºC
N/A
N/A
N/A
40 ºC
7.1 psi
38 ºC
N/A
30 ºC
CF3-CFI2
C3F7I
CF2I-CF2-CF3
N
N/A
CF3-CFI-CF3
C2F3I
Used as
Etchant?
Y
CF2=CFI
N
irritant
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Unsaturated Fluorocarbons
CxFy
Line Formula
C2 F4
CF2=CF2
C3 F6
C4 F6
C4 F6
c-C4F6
C4 F8
Name
Flamm. Toxicity
tetrafluoroethylene
Y
none
Boiling
Point
-76.3 ºC
CF3-CF=CF2
hexafluoropropylene
N
moderate
-29.5 ºC
CF3-CC-CF3
hexafluoro-2-butyne
?
irritant
-24.6 ºC
CF2=CF-CF=CF2 hexafluoro-1,3-butadiene
N
slight
6.0 ºC
CF2-CF2-CF=CF- hexafluorocyclobutene
N
high
3-5.5 ºC
CF3-CF=CF-CF3 octafluoro-2-butene
N
slight
1.2 ºC
Used as
Etchant?
Y
Y
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Screening Strategy

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Consult literature for physical property and MSDS data, experts
on atmospheric chemistry.
Generic experiments on large number of chemistries, both
etching and cleaning processes
Detailed experiments on smaller subset of chemistries (i.e., those
most likely to perform well)
Use Design of Experiments to minimize laboratory testing.
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Design of Experiments
20
20
15
15
Center Point
+ 5 Replicates
10 O2 Flow (sccm)
O2 Flow (sccm) 10
5
5
0
0
Test Points
115
80
95
60
B-field (Gauss)
75
40
Pressure (mTorr)
55
20
0
35
Several commercial software packages are available for generating
experimental protocols and analyzing the resulting data set.
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Summary of Potential Replacement
Chemistries

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It may not be possible to find viable etchants as safe and easy to
handle as PFCs.
No “magic bullets” (that is “drop-in” replacement)
It may be possible to identify effective etchants which carry
acceptably low health/safety risks.
Alternatives for chamber cleaning may be easier to develop
because of less stringent process requirements.
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Broader Issues

Risk evaluation of unknown hazards
– toxicology and atmospheric behavior of replacement compounds
and the by-products they form may be unknown and are expensive
to evaluate


Greenhouse gas production is associated with energy used in
abatement schemes
Consider optimizing dielectric deposition process to reduce
need for chamber clean
– how to weight this ESH requirement relative to performance for
dielectric deposition (film quality, gap fill, rate etc.)
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Longer Range Issues: New Materials




Represents the biggest opportunity in designing for the
environment
Design a process which does not require abatement
Environmental benefit is achieved for entire life cycle of the
process
More difficult to evaluate ESH evaluation of revolutionary
processes rather than evolutionary ones
– unknown data and issues





flow rates
by-products
toxicology of new chemistry
equipment cost)
unanticipated issues (material interaction)
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Example of a New Material: Low-k
Dielectrics

The performance of integrated circuits is becoming
“interconnect limited”
– The RC time constant is given by R C = rmke0L2/(tmtd)
– To reduce this delay


lower rm (resistivity): Al --> Cu
lower k(dielectric constant)
Passivation
Intermetal Dielectric
IM Dielectric
Interlayer Dielectric
Metal
Metal
Metal
tm
td
Si
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Future “Low-k” Dielectric Materials

SIA Roadmap
– predicts lower k is required
– does not specify material beyond evolutionary change to
fluorinated oxides
Year
k
Material
1995
3.9
SiO2
1998
2.9
Fluorinated SiO2
2001 2004
2.3
< 2.0
Polymers
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Potential Low-kMaterials
5.0 - 3.9
3.7 - 3.0
3.9 - 2.9
2.8 - 2.3
2.9
2.7 - 2.3
2.6 - 2.4
2.3
2.2 - 1.8
1.7 - 1.3
1.2 - 1.0
1.0
TEOS based SiO2
FxSiOy
Polyimides
Fluorinated polyimides
Hydrogen silesquioxane SiRO1.5
Hydrocarbon polymers
(polyethylene, polystyrene)
Fluorinate polyarylene ether(FLARE)
Parylene-F
Fluoropolymers (teflon)
Porous polymers (aero-gels, foams)
Air bridges
Vacuum
Fluorine is found in many of these materials
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Processes for Applying Low-k Materials

Spin-on processes (analogous to photoresist applications)
 generates waste solution
 potential for worker exposure to hazardous solvents

Chemical Vapor Deposition (CVD), potentially plasma
enhanced




solventless
low waste
potential toxic precursors/effluents
Fluorinated oxides, fluorinated polymers
gases and by-products)
 chamber cleaning requirements?
(avoid PFCs deposition
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Evaluating Unknown Risks


CVD precursor for “teflon-like” ILD
Hexafluoropropylene oxide
polymerizes
O
CF2----CF---CF3
energy
O
CF2 + CF---CF3
Deposited films have k=1.9

MSDS Dupont May, 1995
“no acceptable information is available to confidently predict the effects of excessive
human exposure to this compound”

Hexafluoroacetone impurity (<0.3%)

– potential developmental abnormalities
– not indicated on the MSDS for HFPO in 1994 but does appear on 1995 version
Even though processes deposits films with desirable properties
the ESH issues cast doubt on its commercialization
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