nsf/src engineering research center for environmentally benign semiconductor manufacturing gleason,...

Gleason, et al. 1NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing

PFC Replacement Chemistries

Prof. Karen K. Gleason, Department of Chemical Engineering, MIT

Source materials contributed by :Mr. Simon Karecki &

Prof. Rafael ReifDepartment of Electrical Engineering & Computer Science,

MIT

© 1999 Massachusetts Institute of Technology. All rights reserved


Outline

Potential applications

Selection guidelines and tradeoff (performance and ESH)

Broad view for alternative processes


Potential Applications for PFC Replacement Chemistries

Dielectric materials – chamber cleaning of CVD reactors for oxides & nitrides– etching (patterning) of oxides & nitrides

dielectic for device isolation and insulating metal lines corrosion and mechanical protection mask against dopants, impurities and oxidation planarization (smooth out topography)

– fluorine is required (SiF4 etch product) 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


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.


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


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


PFC Characteristics

Gas Atm. Lifetime(years)

GWP(100 ITH)

BoilingPoint (°C)

CF4 50,000 6,500 -128C2F6 10,000 9,200 -79C3F8 5,600 6,950 -36.7SF6 3,200 23,900 -50.6NF3 740 13,100 -128.9

CHF3 264 11,700 -84.4

Gas Tc (K) Pc (atm)CF4 132.9 34.5C2F6 292.8 -SF6 318.7 37.1NF3 234 44.7

Critical Point Data


Estimating Vapor Pressure, Pvp

Vapor pressure is a function of temperature, T Theory of corresponding states (based on critical point

data) 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(Pvp ) Tb

Tc

ln Pc

1 Tb

Tc

1 Tc

T

from “The Properties of Gases and Liquids”R.C. Reid, J.M. Prausnitz & T.K. SherwoodMcGraw-Hill, 1977, p. 182


Halogenated Compounds: A Tradeoff

Stable(high long-term

environmental impact)

Reactive(high health/safety

impact)


Trade-off example: TFAA

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


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.


Other Fluorine-Based Chemistries

Hydrofluorocarbons (HFCs) Iodofluorocarbons (IFCs) Unsaturated Fluorocarbons Chlorine and Bromine containing replacements have

been ruled out because of their high ozone depletion potential (ODP)


Hydrofluorocarbons (HFCs)

CxFyHz Line Formula Halocarbonnumber

Flamm. Toxicity GWP100 BoilingPoint

Lifetime Used asEtchant?

CF2H2 32 Y toxic 580 -51.7 ºC 6 yrs. Y

C2F5H CF3-CF2H 125 N slight 3200 -48.5 ºC 36 yrs. Y

C2F4H2 CF2H-CF2HCF3-CFH2

134134a

N slight 12001300

-19.7 ºC-26.5 ºC

11.9 yrs.14 yrs.

Y

C3F7H CF2H-CF2-CF3

CF3-CFH-CF3

227ca227ea

N slight ?3300

-16.3 ºC-15.2 ºC

?41 yrs.

C2F3H CF2=CFH 1123 Y N/A ? -51.0 ºC ?

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


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


Iodofluorocarbons

CxFyIz Line Formula Flamm. Toxicity Vapor Pres.@ 20 ºC

BoilingPoint

Used asEtchant?

CF3I N irritant 85 psi -22.5 ºC Y

CF2I2 N

C2F5I CF3-CF2I N irritant 35 psi 12-13 ºC

C2F4I2 CF2I-CF2I N irritant N/A 112-113 ºC

CF3-CFI2 N/A N/A

C3F7I CF2I-CF2-CF3 N N/A N/A 40 ºC

CF3-CFI-CF3 7.1 psi 38 ºC

C2F3I CF2=CFI N irritant N/A 30 ºC


Unsaturated Fluorocarbons

CxFy Line Formula Name Flamm. Toxicity BoilingPoint

Used asEtchant?

C2F4 CF2=CF2 tetrafluoroethylene Y none -76.3 ºC Y

C3F6 CF3-CF=CF2 hexafluoropropylene N moderate -29.5 ºC Y

C4F6 CF3-CC-CF3 hexafluoro-2-butyne ? irritant -24.6 ºC

C4F6 CF2=CF-CF=CF2 hexafluoro-1,3-butadiene N slight 6.0 ºC

c-C4F6 CF2-CF2-CF=CF- hexafluorocyclobutene N high 3-5.5 ºC

C4F8 CF3-CF=CF-CF3 octafluoro-2-butene N slight 1.2 ºC


Screening Strategy

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.


Design of Experiments

35

55

75

95

115

Pressure (mTorr)

0

20

40

60

80

B-field (Gauss)

0

5

10

15

20

O2 Flow (sccm)

0

5

10

15

20

O2 Flow (sccm)

Center Point+ 5 Replicates

Test Points

Several commercial software packages are available for generatingexperimental protocols and analyzing the resulting data set.


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.

Summary of Potential Replacement Chemistries


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.)


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)


Example of a New Material: Low- Dielectrics

The performance of integrated circuits is becoming “interconnect limited”– The RC time constant is given by R C = m0L2/(tmtd)

– To reduce this delay lower m (resistivity): Al --> Cu lower(dielectric constant)

Passivation

Intermetal Dielectric

IM DielectricInterlayer Dielectric

Metal

Metal

Metal

Si

td

tm


Future “Low-” Dielectric Materials

SIA Roadmap– predicts lower is required– does not specify material beyond evolutionary

change to fluorinated oxides

Year 1995 1998 2001 2004 3.9 2.9 2.3 < 2.0

Material SiO2 Fluorinated SiO2 Polymers


Potential Low-Materials

5.0 - 3.9 TEOS based SiO2

3.7 - 3.0 FxSiOy

3.9 - 2.9 Polyimides 2.8 - 2.3 Fluorinated polyimides 2.9 Hydrogen silesquioxane SiRO1.5

2.7 - 2.3 Hydrocarbon polymers (polyethylene, polystyrene) 2.6 - 2.4 Fluorinate polyarylene ether(FLARE) 2.3 Parylene-F 2.2 - 1.8 Fluoropolymers (teflon) 1.7 - 1.3 Porous polymers (aero-gels, foams) 1.2 - 1.0 Air bridges 1.0 Vacuum

Fluorine is found in many of these materials


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 (avoid PFCs deposition gases and by-products) chamber cleaning requirements?


CVD precursor for “teflon-like” ILD Hexafluoropropylene oxide

Deposited filmshave =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 propertiesthe ESH issues cast doubt on its commercialization

Evaluating Unknown Risks

CF2----CF---CF3

OCF2 + CF---CF3

Oenergy

polymerizes

nsf/src engineering research center for environmentally benign semiconductor manufacturing gleason,...

Documents

pfc replacement chemistries

rights reserved slide

fluorinebased chemistries

nonfluorine chemistries

chamber cleaning requirements

potential applications

potential use

point ability