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.

For planarized ILD applications, engineers at several IC man­ufacturers have reported on spin-coating solu tions of HSQ resin as a proven production technology. The forums have

typically been VMIC and DUMlC, the ULSI/VLSI Multilevel Inter­connections Conference, and the Dielectrics for ULSI/VLSI Multilevel Interconnections Conference.

Data from various reports show that oxide formation usi.ng HSQ produces greater planarization and gap fill than standard plasma processes for Si02, while providing the option to eliminate etch­back techniques [1-2]. In addition, HSQ offers a lower dielectric constant (k <3.0) than standard plasma deposited Si02, which is so crucial in reducing capacitance between adjacent metal inter­connections, 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 Row­able Oxide, is a SIlicon-based resin related to a family of ordered three-dimensional polymers explored previously [5]. The struc­ture of these siloxanes resembles a cage (Fig. Ia). but the cherni­cal 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 of HSQ resins results In b). random structures.

HSQ film properties Fourier transform infrared (FTIR) spectroscopic analysis of HSQ­based films deposited on silicon clearly identifies the H-Si-O mol­ecular bonding network. Figure 2 shows a typical IR spectrum of an oxide film deposited using an HSQ-resin solution. Assign­ments to the absorption peaks that describe the molecular struc­ture are shown in Table L Table 2 lists relevant properties of HSQ­based oxide films for applications requiring interrnetal dielectric isola tion layers.

Successful processing schemes for HSQ films control the reac­tions that result in dissociation of the Si-H bond and subse­quent molecular rearrangement and formation of Si02 bonds. The reactions are promoted by thermal decomposition and oxi­dation. 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 Penn Well

.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 lRspectrum of a HSO-deposited oxidefi lm.

Table 1. Infrared absorption assignments for oxi de fi lms deposited from HSQ-resin solutions

Absorption location (c m-1 ) Assignment

2250 Si-H bond stretch

1060-1150 Si-O-Si bond stretch

830-875 H-Si-O Ilybrid vibration [6. 7]

Table 2. Typical properties of ILD oxid e films deposited fr om HSQ resins using standard process

condit ions (1 hr, 40 0' 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 2.7-3.0

Dielectric breakdown strength >4 MV/cm

as the material is heated, the Si-H stre tch abs orpti on area decreases, and the Si-O s tretc h absorp tion a rea increases. A simple way to illustrate th is be ha vio r is to view the material as a composi te mi x­tu re of H8Si0 2-oa nd Si02 co mponen ts. As the am ou n t of Si02 bond ing in the film inc reases. the film tak es on more Si02char­acter, w hich inclu des increased pe rmit tivity a nd hyd roxyl (Si-DH and H-OH ) con ten t. Maximiz ing the H.~Si02-s content produces the lowest poss ible film dens ity, which in tu rn helps p roduce low permittivity.

Process integration For spin-coati ng p rocess es , the d epos ited HSQ film thi ckness is easily co ntro lled by man ipulating the s pin recipe a nd resi n con­centrat ion in solution . Typ ical HSQ film p roce ssing uses il com­mercial spin-on glass track system wi th integrat ed hot pla tes and either an in tegrat ed or stand-alone quartz-lined furnace. The pre­cursor material is d ispensed onto w afers usin g spin recipes that ar e op tim ized fo r plana rit y, uni for mity, an d dispens e vol u me . Th e wafer is th en passed over th ree ho t pl at es in success ion at temper atures of 150, 200, a nd 350°C, for one m inute eac h.The ho t

plates expel residual car rie r so lve nt an d initia te s tructural cha nges in the film to stabilize it pri o r to fur nace annealing .

Deposited HSQ films may be exposed to ambient m oist ure w hi le waiting fo r fu rther processing. Moisture uptak e in oxid es ha s been linked to d ielec tric re lia bi lity pro blems such as high permittivity, via poisoning, an d hot carrier injection failu res [8]. When th e film is ric h in the H-Si-O compone n t, its in terac tion wi th moisture is

w ith a va lue of x only fractionally d ifferent from 1. The reac tion is an example of so -called wa ter bloc king: The Si-H bond inter­ac ts w ith water and for ms oxi d e, an effect s imi lar to tha t d ocu ­m ented o n hyd rogenated oxid e fi lms d epo sited by e lec tron cyclotron resonance (ECR) PECVD [9].Th e small amount of wa ter absorbed from the atmosphere via hydration is d isso cia ted , thus p recl ud ing reliabil ity p robl ems. This bene ficia l cha rac ter of the HSQ -based film is reduced w hen significan t numbers o f Si-H bonds are d issociated during furnace p rocessing .

Con trol of tempe rature and ambient atmosphe re ilre important for integration schemes using HSQ in the ILD stru ctures.TIle stan­d ard furnace annealing process is per formed at a temper ature of 400°C in a nitrogen atmosphere, Push-and-pull temperatures a nd ra m p rates are o p timi zed to maxim ize wafer th roughput, w hile minimizing oxid a tion of the film . Annealing temperature infl uences the mechanical stability of the film an d , more im por­tantly. th e poten tial fo r oxidation.

Figure 3 sho w s the temperature d epend en ce of the steady-state deco mposi tion of Si-H bo nd s by ox ygen. Belo w the vicin ity of 350°C there is littl e change in the stoi chiometry of the H SQ film , whi le above 360°C the am ount of Si-H bond d issociati on d ue to oxidation incr ease s very rap idl y wi th tem perature. As Si-H bo nds a re broken , the molecular stru cture begins to inclu d e Si~ bond s. Work a t Dow Corning has show n that even low con centrations of oxygen (e.g., 100 ppm) Ci1l1 oxid ize the HSQ film a t tem pera­tures above 350°C [10].

Su bsequ ent w afer p rocess ing st eps sh ou ld mi nimi ze wa fer ex posu re to temperatures high er than th e anneal temperature. This avo ids ad d itional therm al pyrolysis, w hich ten d s to increase the Si0 2co m p onen t in th e film . An H SQ-based d ie lectr ic film could be subject to a n int eractio n d uring the followi ng p rocesses:

0.7

0.6 ~ 0.5

e ~ 0.4 SiliconE

Q ....... ..:;: 0.3 • 0.2 ,

Hydrogen 0.1

J.. ~l t

0 0.001 0.002 0.003 0.004

1fT (K"1)

Figure 3. Temperature dependence of Si-Hbond decomposition by oxygen.

a)

b) ,.--­- - - - - - - - - - -----,

Figure 4. Gap fill and planarizationachieved with standard HSQ-based spin-on oxide processes for a), submicron features and b), in a multilevel metal structure. (Photo courtesy of Philips Semiconductors)

• SiOl deposition • resist deposition and patterning • via etch • chemical or plasma- ash resist strip • via metallization • metal deposition

Preserving Si-H bonding In the film throughout all subsequent wafe r processing steps is key to main taini ng a low di electric constant material. As noted previously, the Si-H bonds are bro­ken with oxidation or tem peratu re. Processes to de posit PECVD oxides, meted vias, and metal in terconnects can expose HSQ films to a highly oxidative environm ent or high temperatures (>400°0 . Both silane and TliOf-based PECVD oxide processes have been integrated with HSQ films. Optimal PECVD oxide processes applied to the HSQ film sur face should operate at 350°C, using silane to minimize therma lly initiated and oxidative reactions from changing the HSQ film [111.

An HSQ process is of ten inte grated as a replacement for a PECVD oxide pro cess. In terme tal v ia form a tion processes de ve loped for PECVD Si0 2involve s teps that can po ten tially degrade the HSQ film Plasma-etch ra tes of tile HSQ can differ from PECVD oxides and other sp in-on d ielect rics, so resist thickness should be adju sted to compensa te for HSQ etch rate. This helps minimize potenti al damage to the HSQ layer during resist strip.

HSQ users must take care in applying via formation processes that have been de veloped for PECVD-based oxide ILDs. Resist, patterning, and etch processes must be adjusted to minimize the negative effects of the strip procedure on HSQ films.Fluorine and oxygen plasma-etch chemistries are typically used to etch vias through the oxide interlayer dielectric. A critical parameter in this process is the concentration of oxygen in the plasma. Excessive amo unts of oxygen can interact with the HSQ in the via sidewa lls. Oxygen radicals attack the Si-H bond, lead ing to the formation of a porous oxide and increased hydroxyl content. As an alterna tive, neat carbo n tetra fluoride etch chemistry has been successfully used to ope n vias in HSQ-based films [12].

The conside rations that apply to the via etch process will also apply to the resist strip process . Even though most integrat ion schem es protect the HSQ layer with an oxide overcoat, oxygen­plasma ashing must be performed with care to prevent exces­sive heatin g of the wa fer and to minimize the material modifica­tion of the via sidewall.Wet chemicalstripping solutions containing ami ne-based solven ts such as aminoethoxy ethano l and hydrox­ylamine will result in chemical attack of the Si-H bond in the HSQ­film sidewall. The chemical reaction between the amine-based solvent and the Si-H bond will brea k the bond and incorporate hydroxyl contamination into the material adjacent to the via side­wa ll. This con tamination can degrade via performance through oxidat ion of the via metal during deposi tion, a p rocess known as via poisonin g.

Via fill processing should be tailored to minimize tem pera­ture excursion, thus suppressing undesirable changes in the HSQ oxid e. The v ia me tallization process is usually performed a t temperatures in excess of 450°C, using aluminum or tungs ten. A degas procedure is often used beforehand to remove weak ly bound moistu re in the oxide lLD, which would otherw ise escape from the via sidewa llsduring metal dep osition.This moisture has bee n linked with h igh resistance vias if not eliminated prior to metal de posi tion. Via etch and s trip processes that have not been op timized for HSQ mater ials can result in significant ou t­gassing of water and hydr ogen from the HSQ film d uring this stage of device fab rication. At metal de position temp era tures >400°C, further hyd rogen loss from the HSQ layer is also likely, but shoul d not degrade via-fill processes. Minimizing exposure of the HSQ film to temperatures greater than 400°C in subsequent processes will help to mainta in a low dielectric constant and high film integrity.

Performance and cost-of-ownership Figures 4a and 4b illustrate typical gap-fill and planarization per­formance in a mu ltilevel IC design. Key pe rformance features includ e etch back elimination and reduced interconnect capa ci­tance relative to SiOI [1 ,3, 13]. Cus tomer testing has shown reli­ability of d ielectric interlayers de posited using HSQ ma terials to be comparable with PECVD processes, includ ing standard and fluorinated silicon oxide films [14, 15].

In a previous issue of SolidStateTechnology, cost-of-ownership (COO) analysis of HSQ films and other spin-on, low-k dielectr ic film technologies has been reported at $12- 514/ layer [161 . COO results can vary conside rably, de pending on specific inputs to the mod el used. For HSQ processes, a COO reduction to $7- $9/layer is achi evable through mo re efficien t ma terial usage, such as optimized d ispensing volumes, as well as film processing (e.g.. improved anneal methods and maximized coater throughput) . Forthcoming technologies such as closed-cup spin-coaters show promise for addi tional COO benefits.

Conclusion HSQ-b ased sp in-o n diel ectrics offer improvements in the pro­cess ing and electr ical characteristics associated with multilevel metal IC des igns. The HSQ film process is easily integrated as a su bs titu te for a traditional oxide process, and the ben efits have been regularly demon strated in high-volume IC manufactur­ing. For the LIse of HSQ-based films as dielectr ic interla yers, an ove ra ll process strategy has been described that helps minimize oxida tion of the HSQ film during post-deposition process ing steps, ensur ing optimal performance . •

Acknowledgments HSQ-based FOx Flowable Oxide is a registered trademark of Dow Corning Corp.

References 1. B.Y. Ahlburn. et al., "Hydrog n Silsesquioxane·based Rowable Oxide as n

Element in tile In erlevel Dielec tnc for 0 .5 Micron ULSI Circuits," Proc. of 1st Dielectrtcs for ULSIN L.SI Multilevellnlerconnec tions Conference. p. 36, 995.

2 . B .K. Hwang, et al., "New pm-on Glass Baseo on Hydrog en Silsesqu lox­ane lor lnter-me tal Pia mizat i n," Prcc. 2m ULSINLSI Mullllew;." tmercon ­nec tions Con/erence. p. 113, 1995.

3. .-P . J eng , et ai . , "Hig hly Po rous lntertayer D ielec tr ic for Interconnec t Capacitance Reduction," VLSI TechnOlogy Svmposium Digest, . 6 1, 1995.

4. M .K. Jain , et aI., "A Novel Hiqh Periormance Integration Scheme USing Ru ­orinated SiO, and Hydrc gen Silsesuuioxane for Capacl tan e Redu ction," Proc. 13th ULSII\ILSI Multi/eve/ Interco nnec tions Conference, p. 23, 1996.

5. C.L. Frye. W .T. Collins , "The Oligomeric Silsesquioxanes." J. Amer. Cnem. Soc 92. p. 5586, 1970.

0 . P. Bornnauser, G. Calzaierri. "Ring Openir 9 Vibrations f Spherosiloxanes,' J. Phys. Cnen«, 100. p. 2035, 1995.

7 G.uxovs v. et al., "Oxygen-Bending Enwon,nenls 1 G1ow -Dsclwge-Deposoted Amoronous Slicon-HyeJrog n Alloys," Phys. Rev. B. 28 (6), p . 3225, 1983 .

8. C. Chiang , et al.. "Detects tudy on Spin-On Glass Plananzancn Technology," Proc . 11th UL.S/NLSIMulti/e'l el ln terconneclions Conference, p. 404 . 19S7,

9 . N . Shim oyarna. et al. , "Enll ancecl Ho t Carrier Degradation Due to Water Related Compo nents in TEOS/O l Oxide and Waler B!ocking with EGR·SiO Film ." IEEE Trans. on Elect. DeViceS, 40 , pp. 1682- 1687,1 993.

10 . J.N. Bremmer, Y. Liu, K.G. Gruszynski, F.G. Dall, "Cure of FOx Flowab le Oxide lor Intenn etal Dielec tric Apphcations," Proc. 3rd Die/eelncs for ULSIN LSI Mul­ti/evel lnterconnec tions Coater nee. p. 333, ; 997.

11. M.J . Loboda. I. Goswami, "PECVD Oxide Gap Integration for Op timal HSO Performan ce ," Proc. 14th ULS/I\ILSI Mu/lilevel lnierconnevtions Conference. p. 632, 1997

12. M .J. Loboca . et al.. "Chip Scale Packaging wit h High Reliability lo r MCM Applications ," Proc. of tre Filth ISHM Inn ConI. on Multicfll{J Modules, op. 257- 262 , 1996 .

13. J . Waelerloos , et al.. "Integrating a Hydrogen Sitsesquioxane Spin-On Dielec ­tric in a Quar ter-Micron Technology," Di tecuics for ULSi Multilevel inter­connections Conference. p. 31 0, 1997.

14 . D.B. Nguyen, et al., "Reliability Evaluat ion of Low-k Dielec tncs for Sub -Micro n Interconnection Appncations," 2nd tnt '! S vtnp. on Low and High Dielectnc Constant Ma terials. 1997 Spnng Meet of the Elec troc hem Soc , Montreal.

15. E. Sabin, G. Albrech t, "Reliability Concerns Integrating Hyd rogen Silsesquiox­ane Into a SiCM OS Process." 2nd int '! Symp , on Low and High Dielectric Constant ,vlalenals, 1997 Sp ring Mee t o f tile Elec! rcc hem Soc , Montreal,

16. E. Korczynsk i. 'Low-x Dielectric Integration Cost Mod eling ," Solid Stale Teets­nology, pp . 123- 128, October 1997 ,

MARK J. LOBODA rece ived his BS degree in physics in 1983 a nd MS de g ree in app lied p hys ics in 1985 from De Paul Uni vers ity. In 1989 , he joined Do w Co rn ing, w here he wo rks o n sp in-o n a nd CV D dielect rics, process integra tion and characterization of th in film s. He has obta ined six US patents and has pu blis hed more than 30 techni­ca ) papers in the arms of sp ect roscopy, frequency control, and thin film mate rials science. Dow Corning Corp., 2200 W. Salzbllrg Rd., Mid­uuui, M/48686; pi, 517/496-6249, fax 517/496-5121.

GEORGE A. TOSKEY received his BS d egree in electrica l engi nee r­ing from Mich igan Sta te Uni versity in 1982, and his MBA degree from Un iversity of Michigan-Flint in 1988. In 1982, he join ed Dow Co rn ­ing , serving as an appl icat ion engineer and en gin eering ll1il[1Clger for silic on -based dielectric materials . He is now ma rket manilger of IC in te rconnect in the Semiconductor Fabri cation Materials departme nt.

DOW CORNING

Dow Co rni ng Co rpo ra tio n P.O . Bo x. 0994

Mid laud . MI 4H6H6-0994 (S17) 496 -6000

Form no. 10-851-98