7
Several major OEMs and their customers aredeveloping processes to fabricate low-k CVD filmswith dielectric constants as low as 2.5. Theseemingly simple addition of methyl groups to sil-icon dioxide to form a film with lower dielectricconstant, k, is actually not so simple when youconsider the synthesis of the gas or liquid precursor.For instance, trimethylsilane (3MS), probably themost widely integrated low-k CVD candidate at the0.13 µm node, requires a multi-step syntheticprocess. Its preparation is more complex than that oftetraethylorthosilicate (TEOS), the “baseline” CVDdielectric precursor chemical,1,2 but it is morestraightforward than the synthesis of low-k spin-ondielectrics, polymers that incorporate a much higherdegree of molecular engineering. The followingdescribes the various synthesis processes for severalCVD low-k precursors, and shows how a noveldelivery system can solve troubling chemicalhandling and transportation issues associated with3MS delivery.
The OSG structureWhen intermetal spacings in advanced ICinterconnects become smaller than ~0.18 µm, low-k
dielectric insulatorsare needed to minimize RCdelays and pourer constlmptioll. Many researchershave described motivations for integrating low-kintermetal dielectric (IMD) as a replacement forCVD-deposited SiO2.3-8 Among many low-kcandidates, however, only a few materials haveshown all the required properties needed forintegration into high-volume manufacturing pro-cesses.9 Unfortunately, the most promisingprecursors, alkyl and alkoxy silanes, do notCurrently have large-scale electronic or otherindustrial uses. For this and other reasons, thechemical synthesis of the precursors used to depositlow-k films constitutes an important set oftechnologies that directly impact the development
At a Glance
Development and process integration of CVD-depositedorganosilicate glass (OSG) materials is underway for 0.13µm device fabrication. Relative to TEOS, OSG precursors— including trimethylsilane (3MS) and tetramethylsilane(4MS) — are more complex to synthesize. This article
presents synthesis routes for 3MS, 4MS and other low-kCVD precursors, and proposes a novel method for
handling and delivering 3MS to the tool.
and manufacture of advanced ICs with copperdamascene interconnects. Several companies aredeveloping low-k CVD films using a variety ofcarbon-containing precursors. The resultingorganosilicate glass (OSG) films, also called carbon-doped oxides, have a composition of SiwCxOyHz.The implementation of CVD low-k technology canbenefit from a study of the differences between theseorganosilicon CVD precursors (with their silicon-carbon bonds) and TEOS.
The organic groups in OSGs invariably take theform of tetravalent silicon with a wide range of alkyland alkoxy substitutions. In these films, the silicon-oxygen network seen in glass is occasionallyinterrupted, in a more or less homogeneous fashion,by the presence of organic, typically methyl (CH3)groups. Hydride (H) substitution at silicon can alsobe present. The film’s lower dielectric constant, k, isdue to these changes to the SiO2 network and thereduced density of the OSG film relative to SiO2. Intropical CVD low-k films, 10% to 25% of the siliconatoms are substituted with organic groups. Agraphical representation (Figure) shows thedifferences in chemical bonding between amorphousSiO2 and amorphous OSG (Si:C = 4:1). Incorpora-tion of methyl or other groups has a significanteffect on the key physical, thermal and chemicalproperties of the resulting film.9
TEOS synthesisTEOS is an important industrial chemical used as abinder for cement casting, in high-temperature zinc-rich paints, in fabrics, and in a wide range ofindustrial films and coatings such as plastic optics.In fact, electronics applications constitute less than5% of the world’s TEOS production. The industrialroute to TEOS takes place in three steps by reacting:silica with graphite to make silicon metal; siliconmetal with chlorine to make silicon tetrachloride;and SiCl4 with ethanol to make TEOS:10
SiO2 + C → Si + CO2 (1)
Si + 2 Cl2 → SiCl4 (2)
SiCl4 + 4 CH3CH2OH → Si(OCH2CH3)4 + 4HCl (3)heat, Cu catalyst
In device manufacturing, TEOS provides lowerdeposition and oxide formation temperatures as wellas more conformal coatings than silane (SiH4).11,12
As a result, TEOS has replaced silane in many oxideCVD processes. In principal, the above reactionscould be used to manufacture semiconductor-gradeTEOS. However, the necessary removal of traces ofchloride is problematic and expensive. Instead, chlo-rine-free TEOS13 is manufactured according to:
Si + 4CH3CH2OH → TEOS + 2H2 (4)
Alkyl silanes & siloxanesOSG films require silicon-carbon bonds, of whichno known examples exist in nature. The silicon-oxygen structure in TEOS, by contrast, is commonin nature—more than 27% of Earth’s crust is silicon,all reacted with oxygen.
OSG films can be prepared from both substitutedsilanes and siloxanes. As shown in the forthcomingreactions, the industry uses fundamentally differentsynthetic processes for low-k OSG precursors thanthose used for TEOS.
The substituted silanes are most commonlyalkylated silicon derivatives (molecules) whereinone to four of the hydrogen atomshave been replaced with an organicgroup—typically, but not necessarily,a methyl (CH3) group. As shownthrough the series of chemicalreactions that follow, the ubiquitouspresence of the methyl group inorganosilicon chemistry canultimately be traced to the availabilityof methanol from very large-scalepetroleum processes.
Siloxanes are silicon derivatives that have Si-O-Si linkages. As is the case with alkylated silanes, themethyl group is the most common organicsubstituent in siloxanes. Hydrogen substitution onsilicon (Si-H) is often present and plays an importantrole in both types of OSG precursors.
The chemistry of organically substituted siliconhas been comprehensively researched.1,2 The Tablelists several key properties of representative OSGprecursors.
The industry knows and has investigated all thealkylated (methylated) derivatives of silane (SiH4)
for use in CVD low-k film deposition. These includemethylsilane (CH3)SiH3, dimethylsilane (CH3)2SiH2,trimethylsilane (CH3)3SiH and tetramethylsilane(CH3)4Si. While all of these methylated silanes are inprinciple useful in the deposition of CVD low-kfilms, trimethylsilane (3MS) and tetramethylsilane(4MS) are the derivatives that have received themost attention.
Trimethylsilane, 3MSTrimethylsilane’s most significant industrial use isin the semiconductor industry. Companies are using3MS to deposit both low-k OSG films (k<3.0) and
low-k barrier films (k~5) as areplacement for silicon nitride (k~7).Fabricators use a multi-step syntheticroute based on dimethylchlorosilane(CH3)2Si(H)(Cl) to make 3MS. Thisroute is problematic and ratherexpensive because dimethylchloro-silane is itself acquired as a very minorcomponent resulting from the reactionof methyl chloride with silicon:
CH3Cl + Si → (CH3)2Si(H)(Cl) (≤~ 1% of mixture) (5)
where (CH3)SiCl3, (CH3)2SiCl2 and (CH3)3SiCl arethe primary products. Thus, the desireddimethylchlorosilane must be distilled from thereaction mixture using large, specialized distillationtowers.
Then the trimethylsilane is prepared via themethylation of dimethylchlorosilane using theGrignard methylating reagent derived from methylchloride. The synthesis of methyl chloride (which isalso a reactant in reaction 5) starts with methanol
(CH3OH). Methanol forms the basis for mostorganically substituted silicon chemicals, and issynthesized from petroleum byproducts in largeprocess chemical volumes. Methyl chloride isconverted to the required Grignard reagent through:
CH3Cl + Mg → CH3MgCl (6)
Finally, combination of the Grignard reagent withdimethylchlorosilane yields trimethylsilane:
(CH3)2Si(H)(Cl) + CH3MgCl → (CH3)3SiH + MgCl2 (7)
Reaction 7 is carried out under quite high-dilutionconditions, typically in tetrahydrofuran (THF), tooptimize yield. This in turn adds cost andcomplexity. Finally, it is important to remove alltraces of MgCl2 and other sources of chloride for usein low-k semiconductor processes.
By analogy to the use of a chloride-free processin the synthesis of TEOS, one can use a secondapproach to 3MS that employs a chloride-freechemistry developed by Gelest. In the first step,Gelest treats hexamethyldisilazane (HMDS) with analcohol to form an alkoxy derivative, also called asilicon ester:
HN[Si(CH3)3]2 + 2ROH → 2(CH3)3Si(OR) + NH3 (8)
Then they directly reduce the ester to 3MS according to:
(CH3)3Si(OR) + reducing agent → (CH3)3SiH + byproducts (9)
This method features high yield and overall purity,and it results in a chloride-free 3MS product.Disadvantages include the relatively high cost ofboth the silicon ester and suitable reducing agents.
Tetramethylsilane, 4MSTetramethylsilane has historically been used as aninternal standard in nuclear magnetic resonance(NMR) spectroscopy. Thus it had a minorapplication, largely as a research chemical, prior toits introduction as a CVD low-k precursor. In recentinvestigations into the environmental abatement of4MS in the CVD low-k process, ATMI demon-strated that the chemical is more kinetically stable,and thus more resistant to decomposition, than other
alkylated silanes. We can synthesize tetramethyl-silane through the methylation of trimethylchloro-silane:
(CH3)3Si(Cl) + CH3MgCl → (CH3)4Si + MgC12 (10) THF
Because trimethychlorosilane is a product ofreaction 5, one can prepare 4MS using a routesimilar to the traditional synthesis of 3MS. In bothpreparations, the fabricator must take great care toremove traces of chloride and other impurities.
DimethyidimethoxysilaneSiloxanes, both acyclic and cyclic, can be used in thefabrication of OSG films. We show examples ofboth, starting with dimethyldimethoxysilane,(CH3)2Si(OCH3)2. Its use in CVD film fabricationhas been described.14 Dimethyldi-methoxysilane isprepared though the reaction of dimethyl-dichlorosilane (from reaction 5) with methanol:
(CH3)2SiCl2 + H3COH → (CH3)2Si(OCH3)2 + HCl (11)
As with other precursors, one must remove alltraces of chloride from the dimethyldimethoxysilane.Unfortunately, highly toxic methanol is releasedwhen the CVD low-k precursor is used.
TMCTSThe cyclic 1,3,5,7-tetramethylcyclotetrasiloxane(TMCTS) may also be used as a low-k CVDprecursor:15–17
TMCTS is prepared through hydrolysis ofmethyldichlorosilane to firstly form a linear siloxanepolymer that is end-capped with trimethylsilylgroups (derived from trimethylchlorosilane)according to:
H |
(CH3)Si(H)(Cl2) + (CH3)3SiCl → (CH3)3Si-O-[Si-O]n-Si(CH3)3H2O |
CH3 (12)
The linear siloxane polymer is subsequently“backtracked”to form a combination of cyclic silox-anes, predominantly 6-, 8- and 10-membered rings:
H |
(CH3)3Si-O-[Si-O]n-Si(CH3)3 → TMCTS + | other cyclic compounds (13) CH3
The company then distills the product to separate theTMCTS from other products of thedepolymerization.
OSG packaging, storage, deliveryAs with most traditional CVD materials, the newlow-k precursors are either gases (e.g., methylsilaneand dimethylsilane) or liquids—includingtetramethysilane (4MS) and alkoxy-substitutedsilanes and siloxanes. As such, the majority of theselow-k precursors are amenable, with only slightmodification, to the chemical packaging and CVDdelivery equipment utilized with traditional CVDdielectric precursors. However, the physicalproperties of one important CVD low-k precursor,trimethylsilane, introduce significant challenges tochemical transport and delivery in the fabenvironment.
Trimethylsilane is a highly flammable, liquefiedcompressed gas with a relatively low vapor pressureof ~8 psig at 21°C, and a boiling point of 6.7°C.CVD low-k processes require flow rates as high as2.0 slpm per chamber or greater. While personnelcan transport 3MS through the fab as a liquid (likeTEOS), adsorption of inert “push gases” wouldresult. Conversely, the use of traditional, positivepressure, gas phase distribution methods createsconditions that support the liquefication (condensa-tion) of 3MS in distribution tubing. Furthermore,3MS’s relatively low vapor pressure and high heat ofvaporization (5.8 kcal/mole) readily cause the sourcevessel to drop below atmospheric pressure, genera-ting flow perturbations. These fluctuations can causemass flow controller errors and contribute to CVDtool downtime.
Since neither traditional liquid nor conventionalgas delivery schemes appeared suitable for 3MS,ATMI developed an approach using a “sub-atmospheric fab-wide storage and deliverysystem.”18 The delivery system incorporates anintegral vacuum actuated cylinder19 for the fab-wide3MS source. The system further uses an adsorbent-based, sub-atmospheric, point-of-use reservoir20 forlocal storage and immediate availability of 3MS anda sub-atmospheric gas control system.21 The useravoids condensation problems by withdrawing the3MS directly from the source vessel outlet as a sub-atmospheric vapor. By positioning the sub-atmospheric 3MS reservoir at or near the CVD tool,3MS flow perturbations are eliminated. Finally, inaddition to providing an uninterrupted supply of3MS to multiple CVD tools, the 3MS deliverysystem increases safety by removing liquefiedcompressed gas cylinders and change-outs from thesub-fab. This dramatically reduces the capital andinstallation costs of the delivery system.
ConclusionsAt the 0.13 µm device generation, leading-edgecompanies are beginning to incorporate low-kdielectric films in their copper damasceneinterconnect structures. From the standpoint ofcomplexity, the synthesis, purification and handlingof a low-k CVD precursor is more difficult toprepare than TEOS, but is considerably less complexthan spin-on low-k polymeric films. These newCVD low-k films are being prepared using a widerange of organically substituted silicon-basedchemicals. We showed the various synthesis tech-niques, noting the advantages and disadvantagesassociated with each route. To handle and deliver3MS, ATMI developed a chemical storage anddelivery system that helps move 3MS into theproduction fab environment. •
References1. B. Arkles, Silicon Compounds (Silanes), Kirk-OthmerEncyclopedia of Chemical Technology, Vol. 22, Wiley,1997, p. 38.2. B. Arkles, Silicon Compounds (Ethers and Esters),Kirk-Othmer Encyclopedia of Chemical Technology, Vol.22, Wiley, 1997, p. 69.3. O.S. Nakagqwa et al., Proc. of the U.C. BerkeleyAdvanced Metalization and Interconnect Systems forULSI Applications Cont, Oct. 1995, Portland, Ore.4. C.H. Ting and T.E. Seidel, MRS Proc., Vol. 381, p. 3.5. K.R. Carter et al., ibid, p. 197.6. S.-P. Jeng et al., ibid., p. 197.7. P.L. Pai and C.H. Ting, Proc. IEEE VMIC Conf, 1989,p. 258.8. J. Ida et al., Digest of Symp. on VLSI Technology, 1994,p. 59.9. N. Hendricks, Proc. of the 6th International Dielectricsfor ULSI Multilevel Interconnection Conf (DUMIC),Santa Clara, Calif., Feb. 2000, p. 17.10. J . Von Ebelman, Analytical Chem istry, Vol . 57,1946, p . 319.11. R.M. Levin and K. Evans-Lutterodt, J. Vac. Scienceand Technology, Vol. B1, No. l, Jan-Mar 1983, p. 54.12. R.S. Rosier, Solid State Technology, June 1991, p. 67.13. G. Kreuzberg, A. Lenz, and W. Rogher (to DynamitNobel), U.S. Patent 4,113,761, Sept. 12, 1978.14. N. Matsuki, U.S. Patent 935283, Aug. 11, 1999, toASM Japan.15. K. Gleason et al., U.S. Patent 6,045,877, April 2000.16. Rose et al., U.S. Patent 6,068,884, May 2000.17. A. Grill et al., MRS Spring Proceedings, 1999.18. L.P. Wang and T. Tabler, patent pending.19. U.S. Patents 6,101,816 and 6,089,027 and otherpatents pending. 20. U.S. Patent 6,027,547, U.S. Patent5,985,008, and related patents and patents pending.21. L.P. Wang, J. Dietz and T. Tabler, patent pending.
Ravi K. Laxman is global product manager for low-kproducts at ATMI Materials. He has worked as atechnologist in the processing of low-k dielectric films atNovellus Systems Inc. (San Jose), and as a seniorprincipal research chemist at Air Products and ChemicalsInc. Allentown, Pa.), where he developed commercialsilicon nitride and low-k dielectric CVD precursors.Laxman received his Ph.D. in chemistry from theUniversity of Missouri (St. Louis)[email protected]
Neil H. Hendricks is the chief technologist of ATMI, whichhe joined in August 1999. Since 1991, he has focused hisresearch on low-k dielectrics for IC intermetal dielectricapplications. He received his Ph.D. in chemistry fromStanford University. Hendricks has over 40 publicationsand 15 U.S. patents in the areas of low-k dielectrics andother advanced [email protected]
Barry Arkles is the president and founder of Gelest Inc., amanufacturer of organosilicon and organometallicchemicals. He co-founded and was president of PetrarchSystems (now Sivento). Arkles received his B.S. degreeand Ph.D. from Temple University. He received the LeoFriend Award from the American Chemical Society. Hehas published more than 100 papers in the areas ofpolymer and organometallic materials [email protected]
Terry A. Tabler manages new product and marketdevelopment efforts for ATMi Materials’ gas operationsand previously was VP and general manager of ATMl’ssensor business, EcoSys, which acquired TeloSense in1999. He is a 20-year veteran of the specialty gasindustry, has served in various production management,sales and marketing positions, and was responsible forstarting up Praxair's site gas management business.Tabler received his B.A. in chemistry from WittenbergUniversity (Springfield, Ohio)[email protected]
