Understandillg hydrogen silsesquioxane-based …

1 DEPOSITION Understandillg hydrogen silsesquioxane - based dielectric film processing Mark J. Loboda, George A. Toskey, Dow Coming Corp., Midland, Michigan hydrogen silsesquioxane (HSQ) resin has demonstrated unique performance as a precursor for the formation of interlayer dielectrics (ILDs) used in manufacturing ICs with multilevel metallization schemes. Commercially available HSQ- based films routinely provide dielectric constants lower than PECVD silicon dioxide films in submicron devices, with high degrees of planarization. Understanding HSQ film properties is key to successful integration of this material into current and future wafer processing lines. Fo r planarized ILD applications, engineers at several IC man ufacturers have reported on spin-coating solu tio n s of HSQ resin as a proven production technology.

2 The forums have typically been VMIC and DUMlC, the ULSI/VLSI Multilevel Inter connections Conference, and the Dielectrics for ULSI/VLSI Multilevel Interconnections Conference. Data from various reports show that oxide formation HSQ produces greater planarization and gap fill than standard plasma processes for Si02, while providing the option to eliminate etch back techniques [1-2]. In addition, HSQ offers a lower dielectric constant (k < ) than standard plasma deposited Si02, which is so crucial in reducing capacitance between adjacent metal inter connections, paving the way to decreased electrical delay and higher information processing rates on ICs [3-4]. Chemical structure HSQ, which is commercially available in solution as FOx Row able Oxide, is a SIlicon- based resin related to a family of ordered three-dimensional polymers explored previously [5].

3 The struc ture of these siloxanes resembles a cage (Fig. Ia). but the cherni cal reactions that produce HSQ resins do not fully form these cages, resulting in random structures of various sizes (Fig. Ib). Standard siloxane spin-on glasses and organic materials used as spin-on dielectrics contain silicon-carbon or carbon-carbon bonds. I-ISQresins do not have carbon bonds. HSQ exhibits good solubility in most hydrocarbon and siloxane solvents, with the exception of alcohols and water, which can promote gelation. __-0----<.. Ha) b) o Figure 1. HSQ polymers are three-dimensional molecules whose structure resembles a) a cage: however, formation ofHSQ resins results In b). random structures. HSQ film properties Fourier transform infrared (FTIR) spectroscopic analysis of HSQ based films deposited on silicon clearly identifies the H-Si-O mol ecular bonding network.

4 Figure 2 shows a typical IR spectrum of an oxide film deposited using an HSQ-resin solution. Assign ments to the absorption peaks that describe the molecular struc ture are shown in Table L Table 2 lists relevant properties of HSQ based oxide films for applications requiring interrnetal dielectric isola tion layers. Successful processing schemes for HSQ films control the reac tions that result in dissociation of the Si-H bond and subse quent molecular rearrangement and formation of Si02bonds. The reactions are promoted by thermal decomposition and oxi dation. Furnace temperature and oxygen concentration are two key processing variables that influence these reactions. Changes in the film during processing are easily tracked using infrared spectroscopy. Dow Corning research shows that Reprinted from the May 1998 edition of SOLID STATE TECHNOLOGY Copyright 1998 by PennWell.

5 C Si-O stretch Si-H stretchu '" = -e'" c:::> '" <t 4000 3500 3000 2500 2000 1500 1000 500 Wave number (em") Figure 2. A typical lR spectrumof a HSO-depositedoxidefilm. Table 1 . Inf rared absorption a ssignments f or oxide fi lms deposited from HSQ-resin solutions Absorpti on location (c m-1 ) Assignment 2250 Si-H bond stretch 1060- 1150 Si-O-Si bond st retch 830-875 H-Si-O Ilybrid vibration [6. 7] Table 2. Typica l properties of ILD oxide films deposit e d fr om HSQ resins using standa rd process condit ions (1 hr, 400 ' C, N2 ambient) Property Value hydrogen content 18 atom percent Silicon content 30 atom percent Oxygen content 52 atom percent film stress 50---S0 MPa (tensile) Relative permittivity 0 dielectric breakdown strength >4 MV/cm as thematerial is hea ted, the Si-Hs tre tch absorpti on areadecreases, andthe Si-Os tretc h abso r ptiona rea increas es.

6 A simplewayto illustrat e th is behavior i s to vie w th e material as a composite mix tu re of H8Si0 2-o a n d Si02 components . As the am oun t of Si02 bond ing in the film increases. th e film takes o n more Si02char acter , w hich inclu d esincr eas ed permittivitya nd hyd roxyl(Si -D H a nd H-OH ) con te n t. Ma xim iz ing the H.~Si02-s content produces th e lowestposs iblefilm dens ity, whichin turn helpsp roducelow permittivity. Process integration Fo r spin-coati ng p ro cesses , the de pos ited HSQ film thickness is ea sil y con tro lled by m an ipulating the s pi n recipe a nd resin con centrat ion in solutio n . Ty p ical H SQ film proce ssing u ses il com mercial spin-on glass tra ck sys te mwithinteg rat ed hotp la tes and eith er an in tegrat ed or stand-alonequartz-lined furnace.

7 Thep re cursor mat e rial is d is p e nsed on to w a fers u sin g sp in recipes th at ar e o p tim ized fo r plana rit y,unifor m it y,and disp e nse volu me . The wafer i s th en p assedo v er three hot plat es in succe ss ionat temper atures of150, 200, a nd 350 C,fo r o ne min ut e each . T he hot plat esexpe l residual car rie rsolven t and ini tia tes tructural cha nges in the film to stabiliz eit prio rto furna ce annealing . Deposited HSQ films m ay be exposed to ambient m o ist ure w hi le waiting fo r fu rther processing . Moisture uptak e in oxides has be e n lin ke d to dielec tric re lia bi lity problems such a s h igh permittiv ity, via po isoning, an d hot carrie r injec tion failu res [8]. When th e film is rich in the H -Si-O compo ne n t, its in terac tion wi th mo isture is w ith a value of x o nl yfractio na llyd iffe re nt fro m 1.

8 T he reaction is an exampleo f so-ca lled wate r blockin g: TheSi-H bond inter acts w ith water andforms oxid e,an effects imi lar to th a t do cu mented o n hyd rogenated o xi d e fi lms d e p o sited by e lec tron cyclotron resonance (ECR) PECVD [9].The sm all amount of wa ter absorbed from the a tmos phere via hydration is disso cia ted, thus p recl uding reliabil ity p robl ems. This be ne ficia lc ha rac ter of th e HSQ - based filmis reducedw h e n significantnumberso f Si-H bo nd s ar e dissociated during furnace p ro cessing . Co n tro l of tempe ratureandambientatmosphere ilre important for integr ation sc he mes using HSQin the ILDs tructures. TIle stan d a rd furnaceanne a lingpro cessis per formeda t a temperature of 400 C in a nitr og en atmosph ere, Push-and-pull temperatures a nd ra m p rates areo p ti mizedto m a xim ize wafer th roughput, w hile minimizingo xida tio n of thefilm.

9 Annealing temp erature infl u ences the mechanical s tabili ty of the film and , more impor ta n tly. th e poten tial fo r oxidation. Figure3 show s th e temperature d e pend en ce of th e steady-s ta te deco mpositio n of Si-H bon d s by oxyg e n. Below th e vi cin it y of 350 C the re is little change in the stoichiometry ofthe HSQ film , whileabove360 Cth e amounto f Si-H bondd issociati on due to oxid ation increases veryra p idl y with te m perature. As Si-H bo nds a re broken ,the moleculars tru cturebegins to inclu d e Si~ bo nd s. Worka t Do w Corninghas shownthateven lo w concentrations o f oxygen (e .g., 100 ppm) Ci1l1 o xid ize th e H SQ film a t tempe ra tures a bove 350 C [10]. Subs e q u ent wafer processing st e p s sh o u ld mi nimize wa fe r exposu r e to temperatures hi ghe r than th e anneal tempe rature.

10 T his avoids add itio na l th e rm alp yr ol ysis, which ten d sto increase theSi0 2 com p onent in the fil m . An H SQ-base d die le ctr ic film could bes u bject to a n inte ra ctio n d urin g the foll o wi ng pro cess es : ~ e ~ SiliconE Q .. :;: , hydrogen ~l t 0 1fT (K"1) Figure 3. Temperature dependence of Si-Hbond decomposition by oxygen. a) b) ,.-- ---------------, Figure 4. Gap fill and planarizationachieved with standa rd HSQ- based spin-on oxideprocesse s for a), submicron features and b), in a multilevel metal stru ct ur e. (Photo courtesy of Philips Semi co nductors) SiOl deposition resis td epo sitio n and patterning via etch chem ical or plasma-ash resist strip via metallization metal deposition Preservin g Si-HbondingIn the film throu gh out all subsequent wafer p ro cessing steps is k ey to maintaini ng a low di electric constant material.