Air-channel fabrication for microelectromechanical systems via sacrificial photosensitive polycarbonates

JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 12, NO. 2, APRIL 2003 147

Air-Channel Fabrication for MicroelectromechanicalSystems via Sacrificial Photosensitive Polycarbonates

Joseph Paul Jayachandran, Hollie A. Reed, Hongshi Zhen, Larry F. Rhodes, Clifford L. Henderson,Sue Ann Bidstrup Allen, and Paul A. Kohl, Member, IEEE

Abstract—This research involves the fabrication of encapsulatedair-channels via acid-catalyzed degradation of photosensitive poly-carbonates (PCs). There is a need for lower-temperature, degrad-able polymeric materials to fabricate buried air-channels for mi-croelectromechanical systems (MEMS), microfluidic devices, andmicro-reactors. Some polycarbonates undergo thermolytic degra-dation in the temperature range of 200 to 350 C. These polycar-bonates are also known to undergo acid-catalyzed decompositionin the presence of catalytic amounts of acid. A small percentage ofan acid in the polycarbonate formulation can greatly reduce theonset of decomposition temperature to the 100 to 180C temper-ature range. The photoacid and thermalacid induced degradationbehavior of PCs and its use as a sacrificial material for the for-mation of air-gaps have been studied in this work. The decompo-sition of several polycarbonates with the aid ofin situ generatedphoto-acid has been demonstrated and applied to the fabricationof micro air-channels. Based on FT-IR, mass spectrometry, andthermogravimetric analysis (TGA), a degradation mechanism wasproposed. [849]

Index Terms—Degradation, electrical interconnects, MEMS,micro air-channels, microfluidics, photosensitive, polycarbonates,sacrificial materials.

I. INTRODUCTION

A WIDE spectrum of microelectronic and microelectrome-chanical systems (MEMS) applications has increased the

need for lower-temperature, thermally decomposable sacrificialmaterials. This includes fabrication of air-gaps in electricalinterconnects, MEMS, microfluidic devices, and micro-reactors.The formation of air-gaps is important in electrical interconnectsbecause it lowers the effective dielectric constant of the matrix[1]–[5]. The fabrication of buried air-channels is useful for thecreation of vias in multi-level wiring boards, micro-displayboards with high resolution, and ink-jet printer heads. In MEMStechnology, the fabrication of micro air-cavities may alleviatethe stress associated with thermal expansion of materials andalso can act as a temperature-activated release material. Mi-crofluidic devices and microreactors, fabricated with air-gaptechnology can be used for miniature-scale chemical syntheses,medical diagnostics, and micro-chemical analysis and sensors[9], [10]. These applications require the formation of buriedmicrochannels in several different materials at a variety oftemperatures. Recently, Harnettet al.[6] demonstrated the use of

Manuscript received April 10, 2002; revised October 1, 2002. Subject EditorW. N. Sharpe, Jr.

J. P. Jayachandran, H. A. Reed, H. Zhen, C. L. Henderson, S. A. Bid-strup Allen, and P. A. Kohl are with the School of Chemical Engineering,Georgia Institute of Technology, Atlanta, GA 30332-0100 USA (e-mail:[email protected]).

L. F. Rhodes is with the Promerus LLC, Brecksville, OH 44141 USA.Digital Object Identifier 10.1109/JMEMS.2003.809963

polycarbonates as a sacrificial material in fabricating nanofluidicdevices by electron beam lithography. Lee and Gleason reported[7] the fabrication of air-gaps using the hot-filament chemicalvapor deposition of polyoxymethylene as a sacrificial layer.Also, Moore et al. used highly structured dendritic material,specifically, hyperbranched polymers as a dry-release sacrificialmaterial in the fabrication of cantilever beam [8]. Previous workby Kohl et al.has demonstrated the fabrication of air-gaps usingnonphotosensitive sacrificial polymers that decompose in therange 250–425C [1], [4]. Fig. 1 shows the previously proposedmethod for the buried air-cavity formation using a hard mask,photoresist, and reactive ionetching forpatterning.Theproposedprocessing methods use photosensitive polycarbonates for thefabrication of air-gaps. This greatly lowers the decompositiontemperature of the sacrificial material and also reduces thenumber of processing steps required for fabrication, as shownin Fig. 2. In Fig. 2(a) and (b), the simplified process sequencesusing photosensitive (positive tone) sacrificial material wereshown. Upon exposure to ultraviolet (UV) irradiation, thepolymer becomes more thermally unstable, allowing selectivedecomposition of the exposed areas. These areas can be removedprior to encapsulation (see Fig. 2-Method 1) or following encap-sulation (see Fig. 2-Method 2). The fabrication method chosenresults in a different final structure. Thus, the photoacid-inducedthermal decomposition of the polymer allows a greater varietyof structures to be built than with the nonphotosensitive process,for a wider range of applications and encapsulating materials.

Photolytically or thermally labile polymers and compositesfind numerous applications in the fields of coatings and resistmaterials due to their rapid change in properties when exposedto UV irradiation or heat. Polycarbonates undergo thermolyticdegradation upon heating to 200 to 350C. Introducing aphoto-acid generator (PAG) such as, diphenyliodonium ortriphenylsulphonium salts into the polycarbonates, results ina UV sensitive material. Upon irradiation to UV light, thetriphenylsulphonium or diphenyliodonium salts undergo aphotolysis reaction. The organic cation of the PAG decomposesand a strong Brφnsted acid is generated [11]. The protonic acidcatalyzes the degradation of polycarbonate in the subsequentprocess at a lower temperature than the acid-free polymer.Taylor reported [12] the elimination of carbonates through apolar transition state. Higher reaction rates are based on themore polar transition state which occur in the following orderprimary secondary tertiary. Also, the presence of ahydrogen in the carbonate group facilitates the elimination ofcarbon dioxide. Inoue and Tsuruta reported [13] the synthesisand thermal degradation of carbon dioxide-epoxide copolymers

1057-7157/03$17.00 © 2003 IEEE

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Fig. 1. Process for the fabrication of air-channels using a nonphotosensitive sacrificial polymer.

Fig. 2. Process of fabrication of air-channels using photosensitive polycarbonates.

JAYACHANDRAN et al.: AIR-CHANNEL FABRICATION FOR MEMS VIA SACRIFICIAL PHOTOSENSITIVE POLYCARBONATES 149

and proposed a thermal degradation mechanism. Frechetet al.reported [14] the acid-catalyzed thermolytic decompositionof polycarbonates using photoactive triarylsulphonium saltsand demonstrated photopatterning using these materials.Narang and Attarwala synthesized a variety of polycarbonatesincorporating tertiary diols as photoresist materials for thedeep UV and mid UV microlithography in the presence ofdiaryliodonium and triarylsulphonium salts [15].

The sacrificial polymeric material and surrounding encapsu-lating material must meet certain requirements for the buried air-cavity fabrication process. In the case of sacrificial materials, forburied air-channels, the PAG in the formulation must be able toproduceanaciduponirradiation toUVlight.Thesacrificialmate-rialshoulddecomposeinanarrowtemperaturerangeleavinglittleornosolid residueeither fromthepolymeror thePAG.Thedegra-dation products of the sacrificial material need to be volatile, andtheproductsshouldhavetheability topermeate through the fully-densified overcoat material. The thermal decomposition temper-ature (nonirradiated) of the PAG should be much higher than thephoto-induced acid-catalyzed decomposition temperature of thepolymer material so that the UV exposed area preferentially de-composes. The sacrificial materials must also have sufficient ad-hesion to substrates. The overcoat material must provide goodmechanical strength to span the air-gap without sagging duringor after fabrication. The curing or deposition temperature of theovercoat material must be below the decomposition temperatureof the formulated sacrificial material. Also, the overcoat materialmust have adequate permeability to the volatile decompositionproducts from the sacrificial material.

We have recently reported the use of polycarbonates (PC) asthermally decomposable sacrificial materials to form air-gapsin dielectric materials which are available from Promerus LLCas Unity 200P Sacrificial Polymers [3]. This paper reports thedecomposition behavior of a variety of polycarbonates with theaid of PAGs. A simple demonstration of the formation of air-gapsusing a photosensitive polypropylene carbonate formulation(polypropylene carbonate DPI-TPFPB or TTBPS-TPFPB)material has been performed using Avatrel EPM and Avatrel2000P (Promerus LLC) as the encapsulating materials. Mech-anisms for the photo-acid catalyzed and thermal-acid induceddecomposition of PCs have been proposed.

II. EXPERIMENTAL

A. Reagents and Instruments

Polypropylene carbonate (PPC) with a weight-averagemolecular weight of 50 000 was obtained from AldrichChemical Company. Polyethylene carbonate (PEC) andpolycyclohexanepropylene carbonate (PCPC) were ob-tained from Q-PAC polymers and the structures wereconfirmed by FT-IR and NMR spectroscopy. Polycyclo-hexane carbonate (PCC) and polynorbornene carbonate(PNC) were synthesized at Promerus LLC (Brecksville,OH 44141). Cyclohexene oxide and 2-(3,4-epoxycyclo-hexyl)-ethyl-trimethoxysilane were purchased from Aldrichand Fluka, respectively. Instrument grade (4.0) carbon dioxidewas purchased from Praxair. The commercially available

PAGs used in this work include tetrakis(pentafluorophenyl)bo-rate-4-methylphenyl[4-(1-methylethyl)phenyl iodonium (DPI-TPFPB), tris(4-t-butylphenyl)sulfonium tetrakis-(pentafluo-rophenyl)borate (TTBPS-TPFPB) and tris(4-t-butylphenyl)sul-fonium hexafluorophosphate (TTBPS-HFP). The anisole andpropylenecarbonate solvents were obtained from AldrichChemical Company and used without further purification.

The proposed fabrication sequence is shown in Fig. 2 usingtwo methods. Method 1 required six steps.

1) The PC: PAG (12–20 wt% PC; 3–5 wt% PAG based onpolymer mass only) mixture in solvent was spin-coatedonto a silicon wafer.

2) The wafer was then soft-baked on a hotplate at 110Cfor 10 min and the thickness of the polymer film wasmeasured using a Tencor Alphastep profilometer.

3) The deep UV exposures (240 nm) were performed usinga Karl Suss MJB 3 mask aligner. Exposure doses weremeasured by an UVX radiometer with a 240 nm probe.

4) The film was baked at 110C for 1–10 min to decomposethe exposed area.

5) This was followed by the encapsulation of the sacrificialmaterial with Avatrel dielectric polymers (PROMERUS,LLC.).

6) The unexposed area was decomposed to form the air-gapsby heating at 170C for 1–10 h in a nitrogen-purged Lind-berg tube furnace.

Method 2 required five steps.

1) The PC: PAG mixture in solvent was spin-coated onto asilicon wafer.

2) The wafer was soft-baked on a hotplate at 110C for 10min and the thickness of the film was measured using aTencor Alphastep profilometer.

3) The film was UV irradiated (240 nm) using a Karl SussMJB 3 mask aligner.

4) The Avatrel overcoat material was then spin-coated ontothe wafer.

5) The exposed areas were selectively decomposed to formair-gaps in a tube furnace at 110C for 30 min. The un-exposed PC remained as part of the final structure.

A wide range of aspect ratios and air-gap sizes can be fabri-cated [3], [4]. Features ranging from 10 nm to mm have beenfabricated. Generally, longer decomposition times are requiredfor larger and thicker air-gap structures. The fabrication processhas been used with a variety of glasses, polymers, and metals[3], [4].

Thermogravimetric analysis was performed on PC films ina nitrogen atmosphere using a Seiko TG/DTA 320. DynamicTGA experiments were performed at the rate of 1C/minfrom 30 to 450 C. Fourier Transform Infrared Spectroscopy(FT-IR) spectra were recorded with a Nicolet 520 spectropho-tometer (at room temperature with 128 scans) to study thedecomposition behavior of the polypropylene carbonate. Gaschromatography-mass spectrometry (GC-MS) experimentswere performed with a 70-SE mass spectrometer (VG Instru-ments) to investigate the acid-induced thermal decompositionof polypropylene carbonate.H NMR analyses were per-formed with a Bruker AMX 500 NMR spectrometer. Size

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Fig. 3. Polycarbonates studied in this research work.

exclusion chromatography data were obtained on a Waters SECSystem. Glass transition measurements were recorded using aPerkin-Elmer DSC7 instrument.

B. Synthesis of Poly(cyclohexenecarbonate) (PCC)

The catalyst difluorophenoxide Zn THF usedfor the polymerization reaction of PCC was prepared following amethod in the literature [16]. 1.0 g of this Zn catalyst was mixedwith 175.0 g of cyclohexene oxide under nitrogen. The mixturewas then transferred to a 500 mL stainless steel pressure reactor.CO was introduced and the pressure was increased to 780 psi.The reaction mixture was heated to 80C and allowed to reactfor 48 h. The mixture was then cooled, drained from the reactor,diluted with THF, and poured into methanol to precipitatethe polymer resulting in a yield of 128 g. The chemical shiftvalues, (ppm) from the proton nuclear magnetic resonancespectra (H NMR (500 MHz, CDCl ) are as follows: 1.00–2.30(m, C ), 4.63 (s, C ), 4.67 (s, C ). A small amount of anether linkage (8%) was apparent from the broad peaks between3.40–3.70 ppm. The infrared spectrum exhibited a vC Oband at 1750 cm . SEC (THF, polystyrene standard) gave aweight average molecular weight of 125 000Mw Mn .DSC analysis determined that the glass transition temperature(Tg) was 115 C.

C. Synthesis of Poly(norbornenecarbonate) (PNC)

The precursor namely, spiro[bicyclo[2.2.1hept-5-ene-2,5-[1,3]dioxan]-2-one was prepared according to literature

procedure described elsewhere [17]. It was hydrogenated(70 psi) using [(1,5-cyclooctadiene)Ir (tricyclohexy-lphosphine)(pyridine)]PF. The product was further puri-fied by recrystalization from methylcyclohexane. Completehydrogenation of the precursor was confirmed byH NMR.The chemical shift values, (ppm) from the proton nuclearmagnetic resonance spectra are as follows: 1.00–1.85 (m, 8H),2.28 (s, 1H), 2.37 (s, 1H), 4.00–4.40 (m, 4H).

Under a nitrogen atmosphere, 0.5 mL of secondary butyllithium (1.3 M in cyclohexane) was added to 15 g of spiro[bi-cyclo[2.2.1]heptane-2,5-[1,3]dioxan]-2-one in 200 mL oftoluene at 0C. After addition, the reaction mixture was slowlywarmed to ambient temperature and stirred overnight. Thepolymer was then precipitated into MeOH and dried undervacuum. Yield: 11.0 g. The chemical shift values,(ppm) fromthe proton nuclear magnetic resonance spectra are as follows:0.93 (b1H), 1.13 (b 1H), 1.26 (d, 1H), 1.42 (b, 2H),1.45–1.80 (m,3H), 2.14 (d, 1H), 2.27 (s, 1H), 3.80–4.30 (m, 4H). The weightaverage molecular weight was 51 000 and MwMn .

III. RESULTS AND DISCUSSION

A. Acid-Catalyzed Decomposition of Polycarbonates UnderDifferent Conditions

Acid-catalyzed decomposition of five polycarbonates avail-able as Unity 200P Sacrificial Polymers has been studied viaTGA. Fig. 3 shows the chemical structures of the five PCs. Fig. 4shows the different PAGs used in this work. In order to be used

JAYACHANDRAN et al.: AIR-CHANNEL FABRICATION FOR MEMS VIA SACRIFICIAL PHOTOSENSITIVE POLYCARBONATES 151

Fig. 4. Photoacid generators used in the present study.

for the fabrication of air-gap structures, the degradation temper-ature of the PC is critical. Table I shows the half-decompositiontemperatures, T (T : temperature at the 50% weight loss)of PCs under different experimental conditions. Thin layers ofPCs containing sulphonium or iodonium salts were irradiatedwith a 1 J cm (240 nm) UV light or thermally activated withoutUV exposure. The UV exposure resulted in the photolytic de-composition of the PAG to generate a protic acid in the polymermatrix. The activated (photolytically or thermally) PC samplewas analyzed by dynamic TGA, and the corresponding Tis reported in Table I. T for the photo-induced acid cat-alyzed decomposition of the PC (PIAD), the thermally inducedacid catalyzed decomposition of the PC (TIAD) (i.e., PC andPAG mixture without UV exposure), and the thermal decom-position of the PC (TD) are shown in Table I. In all cases, pho-tolytic PC:PAG decomposition results in a lower decompositiontemperature. For TIAD, the acid was produced thermolyticallyin the polymer matrix as the decomposition temperature of thePAG was reached, resulting in the acid catalyzed decompositionof the polymer. It is clear from the Table I that the PAG-induceddecomposition significantly lowers the decomposition temper-ature of the PC for the photolytic decomposition of the PAG,and sometimes for the thermolytic decomposition of the PAG.The T was lower for the photolytic case than the thermolyticcase because the acid catalyzed PC decomposition temperaturewas lower than the thermolytic PAG decomposition tempera-

ture. Further manipulation of the decomposition temperaturescould occur by controlling the concentration of the PAGs.

B. Thermogravimetric Analysis of Polypropylene CarbonateFilm

Fig. 5 illustrates the dynamic TGA results specificially for aPPC:PAG mixture (12 wt% PPC). The samples were preparedby spin-coating the polymer in anisole onto a silicon wafer,soft baking on a hotplate at 110C for 10 min to evaporatethe solvent and then exposing, where applicable, to 1 Jcm of240 nm UV irradiation. The samples were then removed fromthe silicon and analyzed by dynamic TGA ramping from 30 to450 C at the rate of 1C under a nitrogen atmosphere. Fig. 5shows the TGA thermograms for (a) PPC:PAG after UV irra-diation, (b) PPC:PAG without UV irradiation, (c) PPC withoutPAG, and (d) PPC without PAG, but with UV irradiation. Asshown in Fig. 5(a), the onset temperature for the photo-acid in-duced decomposition was found to be 80C, and the T was100 C. The decomposition was complete at 230C with lessthan 3 wt% residue in the TGA pan. When the polycarbonatematerial was not UV irradiated [see Fig. 5(b)], decompositionoccurred via the thermolytically induced PAG acid generationonce the temperature reached the decomposition temperature ofthe PAG. The decomposition occurred in a narrow temperaturerange from the onset at 168C. At 230 C, the thermal-acidinduced decomposition was complete with less than 1.5 wt%

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TABLE IHALF DECOMPOSITIONTEMPERATURES(T ) OF POLYCARBONATE FORMULATIONS UNDER DIFFERENTCONDITIONS, DETERMINED USING DYNAMIC TGA.

[PIAD—PHOTO-INDUCED ACID DECOMPOSITION; TIAD—THERMAL-INDUCED ACID DECOMPOSITION; TD—THERMOLYTIC DECOMPOSITION W/OACID]

Fig. 5. Thermogravimetric plot of (a). PPC+PAG after 240 nm UV irradiation;(b) PPC+ PAG without UV irradiation; (c) PPC+ NO PAG without UV; (d)PPC+ NO PAG on 240 nm UV irradiation.

residue. The decomposition behavior of PPC without PAG isshown in Fig. 5(c), and the T was found to be 210C. At230 C, 19 wt% of the mass remained in the pan. The decompo-sition was complete at 287C with 3 wt% residue. At 350C,the weight percentages of the residues were 1.79%, 0.12% and0.37% for (a), (b), and (c), respectively, under the same exper-imental conditions. The decomposition behavior of PPC filmwithout PAG but exposed to UV irradiation [see Fig. 5(d)] wasfound to be similar to that of the non-UV irradiated PPC film.This shows that no change occurred in the PPC upon UV ir-radiation without PAG. Thus, the photo-acid induced decom-position of the polycarbonates significantly lowers the decom-position temperatures; however, it leaves more residue than thethermolytically induced acid-catalyzed decomposition or ther-molytic decomposition of polypropylene carbonate alone. Theresidue level can be brought to less than 1 wt% by lowering the

Fig. 6. Olympus microscopic pictures of positive images of 12%PPC+ DPI-TPFPB formulation.

percentage of PAG level in the formulation for the acid catalyzeddegradation.

C. Photopatterning and Fabrication of Air-Gaps UsingPhotosensitive Polycarbonates

Fig. 6 shows positive-tone patterns obtained using PPC (12wt%):DPI-TPFPB (5 wt% of PPC) formulation in anisole. Thesolution was spin-coated onto a silicon and soft-baked on a hot-plate at 110C for 10 min to achieve a thickness of 1.58m.The film was UV irradiated through a clear-field quartz maskwith a dose of 1 Jcm (240 nm). The film was dry-developedby post-exposure baking at 110C on a hotplate for 3 min. At110 C, the UV generated acid induced the decomposition of

JAYACHANDRAN et al.: AIR-CHANNEL FABRICATION FOR MEMS VIA SACRIFICIAL PHOTOSENSITIVE POLYCARBONATES 153

Fig. 7. SEM micrograph of the lithographic images of PPC-TPFPB system.

the PPC. Fig. 7 shows SEM micrographs of lithographic imagesproduced using a dark-field mask. The process was repeated asabove except a PPC (20 wt%):DPI-TPFPB (5 wt % of PPC) inanisole formulation was used to achieve a 5.45m thick film.The circles and squares are the areas in which the PPC has beendecomposed and removed.

Air-gaps were fabricated via the process flow using Method1 described in Fig. 2 using the photosensitive polycarbon-ates as the sacrificial polymeric material. Two differentencapsulating materials were used: Avatrel EPM and Avatrel2000P (PROMERUS, LLC.). First, the photosensitive PPC (12wt%):DPI-TPFPB (5 wt% of PPC) formulation was spin-coatedonto a silicon wafer and softbaked on a hotplate at 100C for 10min to achieve a thickness of 5.45m. The photosensitive filmwas photopatterned using a clear-field mask having lines/spacepattern with 70 m wide lines and 35 m wide spaces. Thiswas followed by a postexposure bake at 110C for 1–10min to decompose the UV-irradiated area. The Avatrel EPMencapsulating material was then spin-coated and softbaked ona hotplate at 80 C for 5 min. The unexposed area was laterdecomposed in a Lindberg horizontal tube furnace at 150Cfor 4 h under nitrogen. Fig. 8 shows a cross-sectional scanningelectron micrograph (SEM) image of the 70m wide resulting

Fig. 8. Air-channel fabricated using 20% PPC+ DPI-TPFPB formulationencapsulated in Avatrel EPM dielectric polymer.

Fig. 9. Air-channel fabricated using 20% PPC+ DPI-TPFPB formulationencapsulated in Avatrel 2000P dielectric polymer.

Fig. 10. FT-IR spectra of polypropylene carbonate and DPI-TPFPBformulation.

air-channel structure encapsulated by 3.9m Avatrel EPMdieletric material. A similar fabrication process sequence wasfollowed using Avatrel 2000P as the encapsulating material.However, after spin-coating Avatrel 2000P, the film was UV

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Fig. 11. Selected MS scans for UV irradiated PPC� DPI-TPFPB. (a). Scan showing the evolution of acetone. (b). The scan at 110C correspond to propylenecarbonate. (c). The scan showing the peak at 168 m/z assigned for pentafluorobenzene.

irradiated with 1 Jcm at 365 nm and postexposure baked inan oven at 110C for 30 min. This was necessary to crosslinkthe Avatrel 2000P polymer. The resulting thickness of theovercoat was 9.3m. The unexposed sacrificial polycarbonatewas then decomposed in a Lindberg horizontal tube furnace at170 C for 1 h under nitrogen. Fig. 9 shows an SEM image ofthe resulting 70 m wide buried air-channel in Avatrel 2000Pencapsulant. The air-channels were clean from visible debris,which shows the permeability of the volatiles through theencapsulating polymeric material.

Air-channels were also fabricated via the process flow usingMethod2described inFig.2usingaPPC:TTBPS-TPFPBformu-lation as the sacrificial material. The PAG, TTBPS-TPFPB, waschosen because it thermally decomposes at a higher temperature

than the DPI-TPFPB. The higher decomposition temperaturePAG is needed so that the unexposed region of the sacrificialmaterial is left intact while the exposed area is selectively decom-posed. The decomposition temperature of TTBPS-TPFPB wasfound to be 190 C from the differential scanning calorimetry(DSC). Buried air-channels in Avatrel EPM encapsulant werefabricated using the process sequence described as Method 2of Fig. 2. A PPC (20 wt%):TTBPS-TPFPB (5 wt% of PPC)formulation was spin-coated onto a silicon wafer and softbakedon a hotplate at 110C, resulting in a thickness of 5.45m.The photosensitive PPC film was irradiated (J cm ; 240 nm)through a clear-field mask having with a line/space pattern of100 m wide lines and 240m wide spaces. The Avatrel EPMencapsulantwasthenspin-coatedandsoftbakedat80Cfor5min

JAYACHANDRAN et al.: AIR-CHANNEL FABRICATION FOR MEMS VIA SACRIFICIAL PHOTOSENSITIVE POLYCARBONATES 155

Fig. 12. Selected MS scans for PPC� DPI-TPFPB without UV irradiation. (a) Scan showing the evolution of acetone. (b) This scan showing the peak at m/z102 assigned for propylene carbonate.

to remove any solvent from the encapsulating layer. The exposedarea was then selectively decomposed by heating at 110C for30 min in a tube furnace under nitrogen to form air-gaps.

D. FT-IR Analysis of the Decomposition of PolypropyleneCarbonate Film

The acid catalyzed decomposition of PPC (12 wt%):DPI-TPFPB PAG (5 wt% of PPC) was studied by FT-IR. Thepolycarbonate solution was spun onto a NaCl plate and softbaked on a hotplate at 110C for 10 min. The thickness of thefilm was measured to be 1.45m. The FT-IR spectrum of theunexposed film was recorded as shown in Fig. 10(a). The filmwas then exposed to UV light (1 Jcm ; 240 nm) and scannedagain [see Fig. 10(b)]. The film was postexposure baked on ahotplate at 110 C for 2 h [see Fig. 10(c)]. In Fig. 10(a), theabsorptions at 2990 cm, 1470 cm and 1250 cm wereassigned to C-H stretches, C-H bending and C-C stretching ofthe PPC, respectively. A strong absorption band at 1750 cmcorresponds to C O stretch of the PPC. On examining the

UV-exposed PPC film [see Fig. 10(b)], no specific transfor-mation in the chemical structure has occurred, except that thephotosensitive acid generator was activated during irradiation.The intensities of all peaks were nearly zero after the final bakeat 110 C [see Fig. 10(c)]. This is due to the decomposition ofthe PPC into volatile products.

E. Decomposition of Polypropylene Carbonate Monitored byMass Spectrometry

Mass spectrometry (MS) using electron impact ioniza-tion was employed to detect the evolved species during thedepolymerization and volatilization process. Three sampleswere analyzed to determine the nature of the chemical speciesthat were produced during degradation. In the first case, thePPC (12 wt%): DPI-TPFPB (5 wt% of PPC) formulation wasspin-coated onto a silicon wafer, soft-baked on a hotplate at110 C for 10 min, then irradiated with 1 Jcm UV irradiationat 240 nm. In the second case, the sample was prepared asabove, but was not UV irradiated. In the third case, to study

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Fig. 13. Mass spectrum scan for PPC without PAG showing the formation of propylene carbonate and its related fragmentation products.

Fig. 14. The other volatiles that evolved from the degradation of PC formulation.

the volatiles evolved from the polypropylene carbonate withoutan acid generator, a sample was made from the solution thatcontained only PPC. This PPC film was not UV irradiated. Thefilms were removed from the silicon and analyzed by GC-MSto study the evolution of volatiles under different conditions.The samples were ramped at 5C/min from 30 to 110 C andheld for 30 min followed by a ramp at 15C/min to 300 Cand a hold for 20 min.

The mass spectrum data for the decomposition ofPPC:DPI-TPFPB formulation with UV exposure showsthe ion current for a mass-to-charge ratio (m/z) of 58 and 102which correspond to acetone and propylene carbonate, respec-tively. Scans of the UV irradiated photosensitive polycarbonatesample are shown in Fig. 11(a)–(c). These scans at differentintervals of time during the degradation process reveal theevolution of these two prominent species. Fig. 11(a) also shows

other species at m/z 43.1, 40.1 and 36.1 which corresponds tothe fragmentation products of acetone. The m/z 58 peak mayalso correspond to the formation of CHCH CHO. Fig. 11(b)shows other high intensity volatiles corresponding to themolecular weights 43, 57, and 87, which are the fragmentationproducts of the propylene carbonate and may be assignedto ethylene oxide, propylene oxide and ethylene carbonate,respectively. The mass spectrum at 63C is presented inFig. 11(c). The high intensity peak at 168 m/z can be assignedto a decomposition fragment of the PAG. This matches themass of pentafluorobenzeneHC F which is the derivativefragment of the anion of the photoacid, tetrakis(pentafluo-rophenyl)borateB C F . This was confirmed by pyrolysingthe photoacid (dissolved in anisole) which exhibited a strongmass peak at 168 m/z at 67C. Toba et al. have also re-ported [18] that the tetrakis(pentafluorophenyl)borate anion

JAYACHANDRAN et al.: AIR-CHANNEL FABRICATION FOR MEMS VIA SACRIFICIAL PHOTOSENSITIVE POLYCARBONATES 157

Fig. 15. Mechanism for the acid-catalyzed decomposition of poly(propylene carbonate).

can decompose on heating to yield pentafluorobenzene andtri(pentaflurophenyl)borate. The reaction includes the volatilesof particular interest that evolved from the degradation of thepolycarbonate formulation. The suspected reaction productsare shown in Fig. 14. The biphenyl derivative (m/z 210) and thepentafluorobenzene (m/z 168) resulted from the decompositionof the PAG, DPI-TPFPB. The other fragments having m/zvalues 137, 116, and 98 are the degradation products of thePPC. At higher temperatures, small amounts of higher molec-ular weight fragments were observed. These may have resultedfrom more complex fragmentation patterns.

The mass spectrum of the PPC formulation without UV ex-posure is shown in Fig. 12 and shows similar fragmentation pat-terns with that of PPC formulation with UV irradiation. Specif-ically, m/z peaks at 58 and 102 were observed, correspondingto acetone and propylene carbonate, respectively. The peak atm z may also correspond to CHCH CHO. This con-firms that the acid catalyzed PPC decomposition is the same for

photolytic or thermal activation of the PAG. The mass spectrumof the PPC without acid (and without UV irradiation) is shownin Fig. 13. The mass peak at mz at 145 C can be as-signed to propylene carbonate (as with the acid catalyzed PPC).The other mass peaks at 87, 57, and 43 can be assigned to thepropylene carbonate fragments: ethylene carbonate, propyleneoxide, and ethylene oxide, respectively.

F. Mechanism for the Acid-Catalyzed Decomposition ofPolypropylene Carbonate

In our present study, the MS data shows that the degradationof polypropylene carbonate is initiated by the in-situ acidgenerated from the PAG either photolytically or by thermalheating. Inoueet al.studied the thermal degradation behavior ofpolypropylene carbonate via pyrolysis-GC [13]. They suggestthat the degradation of PPC may take place in two stages: thescission reaction of the polymer chain followed by the unzip-ping reaction. Under pyrolysis at 180C, they observed that

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Fig. 16. Aldol condensation products evolved during the acid-catalyzeddegradation of PPC.

polypropylene carbonate gave propylene carbonate. Further,the propylene carbonate was fragmented as COand propyleneoxide. In this investigation, the evolution of the two prominentspecies, acetone and propylene carbonate from the degradationof polypropylene carbonate formulations (with and withoutUV irradiation) led us to propose a dual pathway degradationmechanism.

Fig. 15(a) describes the decomposition of the DPI-TPFPBPAG. On exposure to 240 nm UV irradiation, the cation partof the PAG decomposes to produce a proton that pairs withthe complex anion to form a protic acid. In this mechanisticscheme, RH is a donor of protons either from the solvent orthe polymer itself. The decomposition of iodonium salts can beachieved by either UV exposure or by thermal break down of thecation/anion pair of the iodonium salt. Thus, the iodonium saltcan act as a photo-acid generator as well as a latent thermal-acidgenerator. It was also reported by Crivelloet al.[19] that iden-tical reaction products were obtained when onium salts were de-composed either photolitically or thermally. Fig. 15(b) describesthe proposed mechanism for the acid catalyzed decompositionof polypropylene carbonate. During post-bake, the Hfrom thegenerated acidH X protonates the carbonyl oxygen andfurther rearranges the polar transition state leading to the for-mation of unstable tautomeric intermediates-, [A] and [B]. Fromthe mass spectrum we observed a strong mass peaks at 58 m/zand 102 m/z which was assigned for acetone and propylene car-bonate, respectively. The acetone may be formed as the inter-mediate [A] (Path 1) rearranges and fragmenting as acetone andCO . The formation of propylene carbonate may be attributedto the intramolecular attack of the anion of the intermediate[B] (Path 2a) leading to the formation of the cyclic propylenecarbonate. This further breaks down thermally into propyleneoxide and CO. The mass peaks at 43 m/z and 57 m/z confirmthe formation of ethylene oxide and propylene oxide, respec-tively. It is also reasonable to propose an alternative route forthe formation of propylene carbonate in this highly acidic envi-ronment. The H may activate the terminal double bond (Path2b) leading to the formation of a cyclic transition state which onfurther rearrangement and abstraction of a proton by the counterion X , may yield propylene carbonate. Further confirmation ofthe degradation via Path 1 is the formation of aldol condensation

products. The mass peaks at 116 m/z and 98 m/z may be due toproducts that were produced due to the aldol condensation (seeFig. 16).

IV. CONCLUSION

The fabrication of air-gaps via acid-catalyzed degrada-tion of polypropylene carbonate and PAG (DPI-TPFPB orTTBPS-TPFPB) formulations has been demonstrated. Basedon the FT-IR and mass spectral studies, a detailed mechanismfor the polypropylene carbonate and the DPI-TPFPB systemhas been proposed. The decomposition behavior of severaldifferent polycarbonate formulations has also been studiedusing TGA. The low decomposition temperatures provide amechanism to tailor the decomposition temperature throughPAG concentration or polycarbonate structure. It also providesa processing method to selectively decompose areas throughphotoexposure and makes these materials a promising can-didate for various microelectronic, microfluidic and MEMSapplications. In particular, the insolubility of polycarbonatesin the solvents used for Avatrel dielectric materials makes thecombination especially valuable.

ACKNOWLEDGMENT

The intellectual and material contributions of Dr. E. Elce,Dr. R. Shick (Promerus. LLC, Brecksville, OH), and C. White(Georgia Institute of Technology, Atlanta) are gratefullyacknowledged.

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JosephPaul Jayachandranreceived the Ph.D. de-gree from the University of Madras, Chennai, India,in 1997.

From 1994 to 1997 he was a Council of Scientificand Industrial Research—Senior Research Fellowat the Department of Physical Chemistry, Universityof Madras, where he has carried out research inthe field of Phase Transfer Catalysis. From 1997to 2000, he was a Fellow of the National ScienceCouncil of Taiwan. Currently, he is a ResearchScientist at the School of Chemical Engineering,

Georgia Institute of Technology, Atlanta. His research is focused on thedevelopment of new sacrificial polymeric materials and its application in thefield of microelectronics, microfluidics, and microelectromechanical systems.

Hollie Reed received the B.S. degree in chemicalengineering from Youngstown State University,Youngstown, OH, and is currently pursuing thePh.D. degree in chemical engineering from theGeorgia Institute of Technology, Atlanta.

Her research is focused on sacrificial polymers forapplications in microelectronics and microfluidics.

Hongshi Zhen received the B.S. and M.S. degreesfrom the Zhongshan University and Nankai Univer-sity, respectively. He received the Ph.D. degree fromthe University of Southern California, Los Angelesin 1999.

After receiving the Ph.D. degree, he became aPostdoctoral Researcher with Prof. M. Chishoimat Indiana University, Bloomington, and then OhioState University, Columbus. In 2001, he joinedPromerus, LLC, as a Visiting Scientist from theGeorgia Institute of Technology, Atlanta. His

research interests include catalysis, polymer synthesis, and photoimagablematerials.

Larry F. Rhodes received the B.S. degree from theUniversity of North Carolina, Chapel Hill, in 1980and the Ph.D. degree in inorganic chemistry at In-diana University, Bloomington, in 1984.

After postdoctoral study at the ETH-Zurich,Switzerland, and the Ohio State University,Columbus, he joined the BFGoodrich Companyin 1998. In 2001, he joined Promerus, LLC. Hisresearch interests include organometallic chemistry,polymer synthesis and photosensitive materials.

Clifford L. Henderson received athe Bachelor ofChemical Engineering degree from the GeorgiaInstitute of Technology, Atlanta, in 1994 and theM.S. and Ph.D. degrees from The University ofTexas at Austin in 1996 and 1998, respectively.

He currently holds the position of AssistantProfessor in the School of Chemical Engineering atthe Georgia Institute of Technology. His researchinterests include polymer thin films, photoresistmaterials, microlithography, and materials andprocesses for microfabrication.

Sue Ann Bidstrup Allen received the S.B. degree inchemical engineering from the Massachusetts Insti-tute of Technology (MIT), Cambridge, in 1981 andthe Ph.D. degree from the University of Minnesota,Minneapolis, in 1986.

She then spent two years as a PostdoctoralAssociate in the Electrical Engineering Departmentat MIT. She currently holds the position of Professorin the School of Chemical Engineering, the GeorgiaInstitute of Technology, Atlanta. Her area of researchis focused on the application of polymeric materials

in microelectronics.

Paul A. Kohl (M’92) received the B.S. degree fromBethany College in 1974 and Ph.D. degree from TheUniversity of Texas, Austin, in 1978.

He worked at AT&T Bell Laboratories, MurrayHill, NJ, from 1978 to 1989. He is currentlyRegents’ Professor of Chemical Engineering atthe Georgia Institute of Technology, Atlanta. Hisresearch interests include materials and processingfor microelectronic devices and electrochemicalengineering.