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

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 2 Outline l Potential applications l Selection guidelines and tradeoff (performance and ESH) l Broad view for alternative processes

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 3 Potential Applications for PFC Replacement Chemistries l Dielectric materials –chamber cleaning of CVD reactors for oxides & nitrides –etching (patterning) of oxides & nitrides l dielectic for device isolation and insulating metal lines l corrosion and mechanical protection l mask against dopants, impurities and oxidation l planarization (smooth out topography) – fluorine is required (SiF 4 etch product) l Other halogens (Cl, Br, I) are not effective etch species l Currently F is generated from PFCs l Other materials (tungsten, polysilicon) – can be etched in non-fluorine chemistries

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 4 Replacement Chemistries l 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 l Replacing one PFC by another may not positively impact global warming.

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 5 Selection Guidelines - ES&H l 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) l Ease of handling and use l Exclude chemistries with high health hazards –mutagenic –teratogenic –carcinogenic

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 6 Selection Guidelines-Performance l Chamber cleaning –vapor pressure (boiling point) –ability to generate etchant (fluorine) –rate (minimize gas volume & increase throughput) l Etching In addition to the chamber cleaning requirements: –ability to form some polymer (anisotropic etching to achieve desired profile) –selectivity –uniformity –reproducibility –avoid particulates

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 7 PFC Characteristics Critical Point Data

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 8 Estimating Vapor Pressure, P vp l Vapor pressure is a function of temperature, T l Theory of corresponding states (based on critical point data) l T c is the critical temperature (units of absolute temperature, K) l P c is the critical temperature (will give P vp in the same units) l critical point data is tabulated for many compounds l critical point data can be estimated for the others l Empirical correlation also requires the boiling temperature, T b from The Properties of Gases and Liquids R.C. Reid, J.M. Prausnitz & T.K. Sherwood McGraw-Hill, 1977, p. 182

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 9 Halogenated Compounds: A Tradeoff Stable (high long-term environmental impact) Reactive (high health/safety impact)

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 10 Trade-off example: TFAA l Triflouroacetic anhydride TFAA (CF 3 COOCCF 3 ) l Potential use for chamber cleaning l Reacts readily with water to form trifluoroacetic acid TFA (CF 3 COOH) l Atmospheric lifetime of TFAA < 30 minutes (GPW~0) l TFA degraded by microbes l But TFA has known, and potentially unknown, health & safety hazards

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 11 Excerpts from MSDS for TFA l 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. l Extremely destructive to eyes l Extremely destructive to skin (corrosive - causes severe burns) l To the best of our knowledge, the chemical, physical, and toxicological properties have not been thoroughly investigated.

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 12 Other Fluorine-Based Chemistries l Hydrofluorocarbons (HFCs) l Iodofluorocarbons (IFCs) l Unsaturated Fluorocarbons l Chlorine and Bromine containing replacements have been ruled out because of their high ozone depletion potential (ODP)

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 13 Hydrofluorocarbons (HFCs) CF 2 H 2 : Acute and chronic heart damage, narcotic effect, prolonged skin exposure can cause defatting and dermatitis C 2 F 5 H and C 2 F 4 H 2 : Very large doses may cause CNS depression, heart irregularities, dizziness, anesthetic effect

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 14 HFCs - A Conservative Approach l CF 3 H is not a candidate (GWP=12,100) l HFCs are mostly not toxic, or at least, not acutely toxic l Sizable but finite lifetimes

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 15 Iodofluorocarbons

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 16 Unsaturated Fluorocarbons

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 17 Screening Strategy l Consult literature for physical property and MSDS data, experts on atmospheric chemistry. l Generic experiments on large number of chemistries, both etching and cleaning processes l Detailed experiments on smaller subset of chemistries (i.e., those most likely to perform well) l Use Design of Experiments to minimize laboratory testing.

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 18 Design of Experiments Several commercial software packages are available for generating experimental protocols and analyzing the resulting data set.

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 19 l It may not be possible to find viable etchants as safe and easy to handle as PFCs. l No magic bullets (that is drop-in replacement) l It may be possible to identify effective etchants which carry acceptably low health/safety risks. l Alternatives for chamber cleaning may be easier to develop because of less stringent process requirements. Summary of Potential Replacement Chemistries

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 20 Broader Issues l 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 l Greenhouse gas production is associated with energy used in abatement schemes l 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.)

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 21 Longer Range Issues: New Materials l Represents the biggest opportunity in designing for the environment l Design a process which does not require abatement l Environmental benefit is achieved for entire life cycle of the process l More difficult to evaluate ESH evaluation of revolutionary processes rather than evolutionary ones –unknown data and issues l flow rates l by-products l toxicology of new chemistry l equipment cost) l unanticipated issues (material interaction)

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 22 Example of a New Material: Low- Dielectrics l The performance of integrated circuits is becoming interconnect limited –The RC time constant is given by R C = m 0 L 2 /(t m t d ) –To reduce this delay lower m (resistivity): Al --> Cu lower (dielectric constant) Passivation Intermetal Dielectric IM Dielectric Interlayer Dielectric Metal Si tdtd tmtm

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 23 Future Low- Dielectric Materials l SIA Roadmap –predicts lower is required –does not specify material beyond evolutionary change to fluorinated oxides

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 24 Potential Low- Materials TEOS based SiO F x SiO y Polyimides Fluorinated polyimides 2.9Hydrogen silesquioxane SiRO Hydrocarbon polymers (polyethylene, polystyrene) Fluorinate polyarylene ether(FLARE) 2.3Parylene-F Fluoropolymers (teflon) Porous polymers (aero-gels, foams) Air bridges 1.0Vacuum Fluorine is found in many of these materials

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 25 Processes for Applying Low-k Materials l Spin-on processes (analogous to photoresist applications) generates waste solution potential for worker exposure to hazardous solvents l 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?

NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing Gleason, et al. 26 l CVD precursor for teflon-like ILD l Hexafluoropropylene oxide Deposited films have =1.9 l MSDS Dupont May, 1995 no acceptable information is available to confidently predict the effects of excessive human exposure to this compound l 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 Evaluating Unknown Risks CF CF---CF 3 O CF 2 + CF---CF 3 O energy polymerizes