Polymer International Polym Int 53:857–862 (2004)DOI: 10.1002/pi.1383

Cationic electron-beam curing of ahigh-functionality epoxy: effect of post-curingon glass transition and conversionSK Rath,1 FYC Boey1∗ and MJM Abadie2

1School of Materials Engineering, Nanyang Technological University, Singapore2Universite Montpellier 2, Case Courrier 21, 34095 Montpellier Cedex 5, France

Abstract: This paper reports on the cationic electron-beam curing of a high-functionality SU8 epoxy resin,which is extensively used as a UV-curing negative photoresist for micro-electronics machine systems(MEMS) applications. Results show that elevated post-curing treatment significantly increased both theconversion and the glass transition. The degree of conversion and the glass transition temperature weremeasured by using Fourier-transform infrared (FTIR) spectroscopy and modulated differential scanningcalorimetry (MDSC), respectively. The glass transition temperature (Tg), which has been observed tobe dependent on the degree of conversion, reaches a maximum of 162 ◦C at 50 Mrad and post-curing at90 ◦C. The degradation pattern of the cured resin does not show much variation for exposure at 5 Mrad,but does show significant variation for 50 Mrad exposure at various post-curing temperatures. A degreeof conversion of more than 0.8 was achieved at a dosage of 30 Mrad with post curing at 80 ◦C, for the epoxyresin with an average functionality of 8 a feature simply not achievable when using UV-curing. 2004 Society of Chemical Industry

Keywords: degree of conversion; electron-beam; glass transition temperature; Modulated Differential ScanningCalorimetry (MDSC); post curing

INTRODUCTIONSU8 is a high-functionality epoxy-based negativephotoresist material that has been extensively used inthe photolithographic fabrication of micro-electronicsmachine systems (MEMS), with its curing effectedby photoinitiators in the near-UV range. Thermallycured epoxy systems are used in micro-electronicpackaging applications1,2 to protect, insulate andconceal the circuits and components of electronicdevices. However, thermally cured epoxy systemsinvariably suffer from relatively long cure timesand, for high-temperature systems, an unacceptablyhigh curing temperature may actually damage thecomponents and devices, which are expected to beprotected. Attempts to cure epoxy systems, formulatedwith photoinitiators, by UV or e-beam radiation3–8

have shown several advantages over the thermalprocess, with the most important being the reductionby at least an order of magnitude in cure time, aswell as the minimization of residual stress. This paperreports on the use of a low-energy e-beam radiationprocess to cure the epoxy system SU8 and, subsequentcharacterization of the degree of conversion by usingFourier-transform infrared (FTIR) spectroscopy. Theeffect of degree of conversion has been correlatedwith the glass transition temperature, measured by

using a Modulated Differential Scanning Calorimetry(MDSC).

Optimal use of thermoset systems precludes theability to understand its curing mechanisms andkinetics.9,10 Physically, because of the dramatic changein physical properties at this transition, Tg, is animportant material property in polymer applications,with the most important being that it becomes anupper limit for the service temperature of the material.This is simply because of the considerable drop inthe bulk resin modulus10 once the material transformsfrom glassy and rigid to a rubbery state, over theglass transition region. For this reason, one of themain objectives in the curing of thermoset resinsis to increase the glass transition temperature toits maximum possible value, while still retaining areasonably high degree of fracture toughness.

While generally the Tg increases with an increasein the degree of conversion, the relationship is notnecessarily linear. The cure reaction is typicallyassociated with significant shrinkage, due to themolecular chains being pulled closer together by thecross-links. As the degree of conversion increases,the network of cross-linked molecules also increases,subsequently impeding more cross-linking and afurther increase in Tg. Depending on the segmental

∗ Correspondence to: FYC Boey, School of Materials Engineering, Nanyang Technological University, SingaporeE-mail: fycboey@ntu.edu.eg(Received 2 January 2003; revised version received 30 May 2003; accepted 17 June 2003)Published online 10 May 2004

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SK Rath, FYC Boey, MJM Abadie

mobility of the molecules through the cross-linkednetwork, a further increase in the Tg value may ormay not be significant as the conversion reaches itsmaximum. This is true whether the process of curingis thermal or radiation-induced, since the nature of thecuring process involves the mobility of the individualgrowing chains.

The Tg is uniquely related to the conversion fora thermosetting system. For a single-cure-reactionresin system, this relationship has been shown to bemodelled by the well-known DiBenedetto equation,as follows:11

Tg − Tgu

Tgu= (ε∞/ε0 − C∞/C0)x

1 − (1 − C∞/C0)x(1)

where Tgu is the glass transition temperature ofthe uncrosslinked polymer, x the cross-link density,defined as ‘‘the fraction of all segments that are cross-linked’’, ε the lattice energy, c the segmental mobility,and the subscripts ‘0’ and ‘∞’ refer to the uncross-linked and fully cross-linked polymers, respectively.The segmental mobility, c, is the phenomenologicalconstant used to express the measure of mobility ofthe molecules during cure. Although shown as havingsimply an initial and a final value in the DiBenedettoequation, this, of course, should be a variable that isa function of the molecular structure at that point intime during the cure process. As such, Equation (1) isa simplification of the actual process. This is the mainlimitation of this model, which becomes apparentwhen used for highly cross-linked systems. The Tg-conversion data for highly cross-linked systems, suchas a multifunctional epoxy novolac system, have beenshown previously to have the Tg increase rapidly inthe later stages of cure. The cure process for theresin appears to have a considerable change in theconfigurational entropy caused by the act of cross-linking itself, which is not reflected in the physicalbasis of the model. This deviation has been shown toresult from both a high cross-linking density and fromsteric restrictions on the freedom of chain segments inthe region around the junctions.

By assuming an idealized system consisting of amixture of a fully cured network and an unreacted

monomer phase, a more commonly used version of theDiBenedetto equation can be derived from entropicconsiderations.12 Based on these thermodynamicconsiderations, it can be shown that:

Tg − Tg0

Tg∞ − Tg0= λα

1 − (1 − λ)α(2)

where α is the degree of conversion, λ an adjustable,structure-dependent parameter, and Tg0 and Tg∞the values of Tg at zero and 100 % conversion,respectively. Boey and Qiang10 have shown how thederived value of λ can indicate a higher rate of increasefor the Tg value for lower conversion values than forhigher ones, or a more consistent increase in the Tg

value whatever the value of conversion.

EXPERIMENTALThe epoxy resin used was a commercial UV-curablesystem based on SU8 and was obtained from theMicroChem Corporation (USA) in the form of aviscous solution. It contains the epoxy resin SU8,γ -butyrolactone, cationic photoinitiators (based onmixed triarylsulfonium hexafluoroantimonate salts)and propylene carbonate (percent composition: 69,28, 2 and 1 %, respectively, of total weight). The epoxyresin SU8 is of a high functionality (f = 8) and itsstructure is represented in Fig 1. Potassium bromide(KBr) and dichloromethane (CH2Cl2) were obtainedfrom the Nicolet Corporation, USA and E. Merck,Germany, respectively. An Electrocurtain ElectronBeam System Model CB175 from Energy ScienceInc., USA was used with an accelerating voltage rangebetween 150 and 175 keV and a beam-current rangeof 0–10 mA. The dose per pass of the sample was5 Mrad at an accelerating voltage of 175 keV and aconveyor speed of 19.3 m min−1. For higher dosages,samples were trented using multiple passes.

A TA Instruments (. . ...) modulated differentialscanning calorimeter (MDSC 2910 with a refrig-erated cooling system (RCS)) was used for measuringthe glass transition temperature of all of the curedsamples. A Perkin Elmer thermogravimetric analyzer

O

O

CH2

CH2CH

O

CH2CH CH2

CH2CH CH2

O

O

CH2CH

O

CH2

CH2

O

O

O

CH2

CH2CH CH2

O

CH2CH CH2

O

O

O

CH2CH CH2

O

CH2CH CH2

O

CH2

O

Figure 1. Chemical structure of the epoxy monomer, SU8.

858 Polym Int 53:857–862 (2004)

Electron-beam curing of a high-functionality epoxy

(TGA 7) was used to study the degradation pat-tern of the cured samples. The infrared spectrometerused in this study was a Nicolet Magna 560 FTIRmachine with a DTGS KBr detector and a XT–KBrbeamsplitter.

The viscous epoxy formulation was coated ontoglass plates by using gravure rods, so as to obtain awet film thickness of 50 µm, followed by vacuum ovendrying at 90 ◦C for solvent removal. Subsequently,the plates were exposed to the electron-beamaccelerator at dosages of 5, 10, 20, 30 and 50Mrad, respectively. Immediately after exposure, allof the exposed samples were uniformly post-cured for45 min before placing in a refrigerator at −4 ◦C toprevent further polymerization. Four different post-curing conditions were tried, namely 25, 70, 80and 90 ◦C. The glass transition temperature (Tg) ofthe cured samples was measured by subjecting thesamples to a heating programme of 0–250 ◦C at5 K min−1, with a modulation of ±1 K per 60s. Thetemperature corresponding to the specific heat (Cp)change at half height was accepted as the Tg.13 Typicalsample sizes were 10–20 mg. For measurement ofthe degradation temperature, the cured samples wereheated from 30 to 600 ◦C at a heating rate of10 K min−1. In another experiment, a dilute solutionof the epoxy formulation was made from the originalformulation by using methylene chloride. Pellets ofKBr (13 mm diameter) were produced and coatedwith thin films of the resin solution (ca. 10 µm) anddried in a vacuum oven, in a similar manner tothat described above. The resin-coated KBr pelletswere passed through the electron-beam acceleratorto expose the samples at different dosages (5, 10,20, 30 and 50 Mrad). The samples, after post-curing under identical conditions, were stored ina refrigerator at −4 ◦C. The FTIR spectroscopicmeasurements were carried out by placing the resin-coated KBr pellets directly in the path of the IR beamin transmission mode. All of the spectra were collectedat a resolution of 4 cm−1 and a total of 100 scanswere signal-averaged to obtain an enhanced signal-to-noise ratio. The wavenumber range scanned was from400–8000 cm−1, embracing the mid-IR region andpart of the near-IR region. Normalized peak heightswere determined at constant peak positions andbaselines by using the quantitative analysis software‘TQ Analyst’ (OMNIC). The percent conversion wascalculated by using the following relationship:

Conversion, α(%) = 1 − (Apeak)E(Aref )0

(Aref )E(Apeak)0(3)

where (Apeak)0 and (Apeak)E refer to the heights of theactive functional groups, and (Aref )0 and (Aref )E tothe heights of the reference peaks, before and afterexposure to the e-beam, respectively.

RESULTS AND DISCUSSIONThe generalized mechanism for the decompositionof the photoinitiator in the presence of Bronsted

protogenic impurities, to produce Bronsted acids,which initiate the cationic curing process, may bepresented as follows:6,14

Ar3S+X− e∗−−−→

2mAr2S + Ar• + X− (4)

Both the mid-IR15 and near-IR16–19 regions of thespectra have been explored for finding the degreeof conversion of the epoxy resin. The small-ringepoxy compounds usually absorb at three positionsin the mid-IR region: a medium-intensity symmetricstretching vibration in the range 1280–1230 cm−1, astrong asymmetric band between 950 and 815 cm−1,and a strong band20 between 880 and 750 cm−1.Most of the studies pertaining to epoxy resins hasbeen carried out in the mid-IR region by followingthe disappearance of the asymmetric C–Hstr peakof the epoxy group at 910–915 cm−1. Other studiesfollowed the disappearance of the peak at 4530 cm−1

(epoxy combination band–stretching and bending) inthe near-IR region.19 In this case, an attempt wasmade to evaluate the degree of conversion from theepoxy peaks at 912 and 860 cm−1 in the mid-IR rangeand 4530 cm−1 in the near-IR range. Three differentpeaks were evaluated as internal reference peaks:the Ar–C–Ar vibration at 1180 cm−1, the aromaticC=C stretching vibration at 1508 cm−1 and the –CH2

stretching vibration at 2964 cm−1. It was observed thatthe epoxy functional absorption at 912 cm−1 and the–CH2 stretching vibration at 2964 cm−1 produced themost consistent results.

Figure 2a shows the spectrum of the resin withabsorptions labelled in the mid-IR region, while thenear-IR absorptions of the epoxy group are shown inFig 2b. Figure 3 shows the combined spectra of theresin cured at different electron-beam dosages, butunder similar post-curing condition. Both the peaks at912 and 860 cm−1 in the mid-IR range (Figure 3a) andthe one at 4530 cm−1 in the near-IR region disappearwith increasing electron-beam dosage and hence anincreased extent of curing.

Figure 4 shows plots of the degree of conversion asa function of the electron-beam dosage for differencepost-cure temperatures, with the former calculated byusing the peak at 912 cm−1. The results for the post-cure at 25 ◦C shows the conversion as increasing withdosage up to a limiting maximum of slightly less than50 % conversion for 30 Mrad and maintaining thisvalue even up to 50 Mrad. Figure 4 also shows thatsubsequent higher temperature post-curing appearedto significantly increase the degree of curing. Post-curing at all of the three elevated temperaturesincreased the limiting maximum conversion to 85 %.Initial curing involves the gelation of the SU 8liquid system to form longer but initially linearchains, thus resulting in an increase in viscositybut not an entanglement of the growing chains.However, as the curing proceeds beyond gelation,the entanglements increase significantly to isolate the

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SK Rath, FYC Boey, MJM Abadie

0.550.500.450.400.350.300.250.200.150.100.050.00

4000 3500 3000 2500 2000 1500 1000

913 cm–1

Wavenumber (cm–1)

Abs

orba

nce

Wavenumber (cm–1)

Abs

orba

nce

0.013

(a)

(b)

0.012

0.011

0.010

0.009

0.008

0.007

0.006

5400 5200 5000 4800 4600 4400 4200

4530 cm–1

4068 cm–1

Figure 2. Spectra of the uncured epoxy resin: (a) mid-IR spectrum;(b) near-IR spectrum.

Increasing electron-beam dosage

1.21.11.00.90.80.70.60.5

Abs

orba

nce

0.40.30.2

1050 1000 950 900 850 800

4530 cm–1

Increasing electron-beam dosage0.040

0.035

0.030

0.025

Abs

orba

nce

0.020

0.015

0.010

0.005

4800 4700 4600 4500

Wavenumber (cm–1)

Wavenumber (cm–1)

4400 4300

(a)

(b)

Figure 3. Variations in (a) mid-IR and (b) near-IR epoxy absorptionswith increasing electron-beam dosage carried out under similarpost-curing conditions (80 ◦C).

reactive epoxy groups within the matrix, together withsome of the residual protons and any other reactivespecies. At room temperature, such entanglements

0

20

40

60

80

100

0 10 20 30 40 50 60

E-beam dosage (Mrad)

Deg

ree

of c

onve

rsio

n (%

)

25°C 70°C80°C 90°C

Figure 4. Variation of degree of conversion with increasingelectron-beam dosage under various post-curing conditions (from theepoxy function at 912 cm−1).

may be sufficient to impede further increases in thecross-linking, so resulting in a limiting conversionvalue. However, when the samples are heated underpost-cure conditions, the matrix becomes more flexibleand the reaction propagates once again. To thisextent, the higher maximum conversion reached canbe attributed to the increase in the reaction rate at thisincreased temperature.

Figure 5a shows the results obtained in themodulated DSC analysis for SU8 samples exposed

Glasstransition

–0.10 0.1

0.0

0.20

0.15

0.10

0.05

–0.1

–0.2

–0.15

Rev

ersi

ble

heat

flow

(w

g–1

)

–0.20

–0.25

–0.30–50 0 50 100

Temperature (°C)150 200 250

Tota

l hea

t flo

w (

w g

–1)

Non-reversibleheat flow (w g–1)

–0.10

–0.15

–0.20

–0.25

–0.30–50 0 50 100

Temperature (°C)

150 200 2500.06

0.08–0.10

–0.05

0.10

0.12

0.140.00

0.050.16

0.18

Rev

ersi

ble

heat

flow

(w

g–1

) Total heat flow (w g-1)N

on-r

ever

sibl

e he

at fl

ow (

w g

–1)

(a)

(b)

Exo

ther

mE

xoth

erm

Figure 5. (a) Typical MDSC plots (20 Mrad, post-cured at roomtemperature) showing total heat flow and its two components, iereversible and non-reversible heat flow. Note that the glass transitionregion, which is very clear in the reversible heat flow component, isoverlapped by the non-reversible heat flow component and would notbe observed in the total heat flow curve, ie the one usually measuredby conventional DSC. (b) MSDC plots of the sample examined in (a),re-run under similar conditions. Note the near total disappearance ofthe exothermic peaks in the total and non-reversible heat flow curves.

860 Polym Int 53:857–862 (2004)

Electron-beam curing of a high-functionality epoxy

to 20 Mrad dosage and post-cured at 25 ◦C. Theglass transition temperature, which is a reversiblephenomenon, is clearly observed as a step change onthe reversible component of the heat-flow curves. Onthe other hand, the non-reversible component showeda large exotherm peak arising from the simultaneousthermal curing. Curing was evidently initiated by thee-beam radiation but was subsequently arrested bycooling the sample in a refrigerator. The exothermthus represented the residual uncured phase leftin the sample. This is corroborated by the resultsshown in Fig 5b, obtained after the sample had beenheated again. The exotherm observed has becomesignificantly smaller, since curing has essentially beencompleted in the first run itself. It may be noted thatthe concurrence of the positions of both the glasstransition and the enthalpic peak would not have beendetected under conventional DSC conditions, sincethe exotherm would have obscured it completely.

Figure 6 shows the plots of the measured glass tran-sition temperatures of the samples cured at varyingdosages and for different post-curing temperaturesof 25, 70, 80 and 90 ◦C. Increases in the dosagesincreased the Tg values to a limiting maximum value,between 20 and 30 Mrad. However, the limiting max-imum value reached was significantly increased at allof the three elevated post-curing temperatures. Forexample, post-curing at 90 ◦C, after 20 Mrad expo-sure, resulted in a Tg value of about 165 ◦C, comparedto post-curing at 25 ◦C, which gave only 60 ◦C.

An interesting observation may be made about thevalues of the glass transition temperatures obtainedand the post-curing temperatures used. It is clear thatthe Tg values are always higher (about 40–70 ◦C)than the post-curing temperatures. In a closely relatedstudy, Barton et al21 have reported the effect ofdifferent thermal cure schedules on the final propertyrelationships of a commercial epoxy resin usingmodified imidazole curing agents. The Tg of theepoxy formulation cured at different scanning rates(curing in dynamic scan mode), decreases with theincreasing rate of heating. Similarly, the Tg of thefinal cured product decreases with increasing curingtemperature, for samples cured at different isothermalcuring temperatures.

0

30

60

90

120

150

180

0 10 20 30 40 50 60

Tg

(°C

)

E-beam dosage (Mrad)

25 °C80 °C

70 °C90 °C

Figure 6. Variation of the glass transition temperature of the epoxyresin under various post-curing conditions.

As expected, the Tg increases with increasing extentof cure. At the same time, the extent of cure is alsoaffected by the relative rate of approach of the gelationstage. Chemically, the curing of epoxies occurs via twosteps, ie initiation and propagation. However, in thelater stages of curing, the system may experience chaintransfer as well as chain termination. In addition,at higher curing and post-curing temperatures, thehydroxyl groups (formed during the initial reaction)may undergo etherification. The final Tg of the curedproduct is therefore a function of several complexparameters, ie initial monomer functionality, degreeof conversion, relative rate of different reaction paths,relative rate of approach of the gelation stage, and thevariation of the free volume of the system, during thecourse of the cure reaction. Both physical effects aswell as chemical kinetics would appear to influencethe network structure and hence the glass transitiontemperature, Tg.

The degree of conversion and the glass transitiontemperatures of the cured samples were modelledby employing the modified Di Beneditto equation(Equation (2)), using a non-linear regression analysismethod. The results for the statistical parametersobtained from the regression analysis are presentedin Table 1. The resulting model curve is shown inFig 7, together with the experimental data. The modelis seen to give a reasonable correlation to the modifiedDi Beneditto equation, giving a Tg of 21 ◦C at zeroconversion, and 142 ◦C as the theoretical maximumTg at 100 % conversion. The λ value obtained wasclose to the maximum value of 1, in contrast to lowervalues in the range of 0.3 to 0.64 obtained for thermalcuring of other epoxy systems.10 It has been indicatedin an earlier paper10 that the value of λ represents thedegree to which the curve is ‘skewed’. A lower valueindicates that the increase in the Tg lags significantlybehind the increase in the degree of conversion. Avalue close to unity indicates that the increase in Tg

Table 1. Regression analysis parameters (from Equation (2))

λ Tg0 (◦C) Tg∞ (◦C) R2a

0.88 21 142 0.84

a R2, coefficient of correlation.

160

140

120

100

80

60

40

20

00 10 20 30 40 50 60 70 80 90 100

Tg

(°C

)

Degree of conversion (%)

25 °C 70 °C80 °C 90 °C

Figure 7. Modelling of the degree of conversion and glass transitiontemperature using non-linear regression analysis (from Equation (2)).

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follows more closely and consistently with the increasein the conversion value.

Figure 8 shows the results of thermogravimetricanalysis for the samples cured using a 5 Mrad dosage,and post-cured at two different temperatures (roomtemperature and 90 ◦C). Interestingly, the initialdegradation temperature,—at about 398 ◦C, for thetwo post-cured samples—did not differ significantlywhen curing using a 5 Mrad dosage. This, coupledwith the fact that the degree of conversion fordifferent post-curing temperatures does not increaseappreciably, establishes that the cured samples donot possess a sufficiently high cross-link density andhence resulted in a low thermal stability. Figure 9shows that there is a significant amount of residueleft for samples exposed at 50 Mrads, but post-curedat different temperatures. This is because of the highcross-link density obtained from the high extent ofconversion. This increased cross-link density increasesthe thermal stability.

105100

90

80

70

60

50

40

30

20

10

030 100 150 200 250 300 350 400 450 500 550 600 650

Am

ount

(wt%

)

Degradation temperature (°C)

Figure 8. Thermogravimetric analysis curves of samples exposed to5 Mrad dosage and post-cured at two different temperatures: (1)room temperature; (2) 90 ◦C.

105100

90

80

70

60

50

40

30

20

10

0

Am

ount

(wt%

)

30 100 150 200 250 300 350 400 450 500 550 600 650

Degradation temperature (°C)

12

34

Figure 9. Thermogravimetric analysis curves of samples exposed to50 Mrad dosage and post-cured at various temperatures: (1) roomtemperatures; (2) 70 ◦C; (3) 30 ◦C; (4) 90 ◦C.

CONCLUSIONSThe maximum attained conversion for electron-beam-cured SU 8 is a function of post-curing temperature,with a higher post-curing temperature inducing a highconversion of >80 %, in contrast to a sample leftat room temperature which attained less than 50 %conversion for the same exposure dosage. The glasstransition temperature, as expected, is a function ofthe degree of conversion and is higher for samplespost-cured at elevated temperatures. The highest glasstransition temperature obtained for samples post-cured at room temperature is 64 ◦C, whereas thesame for a sample cured at 90 ◦C is 162 ◦C, underthe conditions used in this study. Thermogravimetricanalysis shows that samples exposed at 50 Mrad, andpost-cured at different temperatures, have a higherthermal stability when compared to those exposed at5 Mrad.

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