Foaming of Polyimide Thin Films For Enhancing Performance of Microprocessor NJaterials : Characterization ofDielectric Behavior.~ (Mochamad Chalid)

FOAMING OF POLYIMIDE THIN FILMS FOR ENHANCINGPERFORMANCE OF MICROPROCESSOR MATERIALS:

CHARACTERIZATION OF DIELECTRIC BEHAVIORS

Mochamad ChalidDept. of Metallurgy, Faculty of Engineering -University of Indonesia,

Karnpus Barn -Ul, Depok 16424

ABSTRACT

FOAMING OF POLYIMIDE THIN FILMS FOR ENHANCING PERFORMANCE OF MICROPROCESSORMATERIALS: CHARACTERIZATION OF DIELECTRIC BEHAVIORS. The foaming of poly imide thin films gives advan-tages for dielectric applications. Because foaming creates a large interfacial area between air and polyimide phases, it generatesmany extra charge-trapping sites due to interfacial polarization, when an electrical field is applied. The SEM observations showthat the films contain closed cells. This work was focused on a study of the dielectric properties of polyimide thin films at variousporosities. The dielectric relaxation spectroscopy (DRS) results show that the porosities have a marked influence on thedielectric properties ofpolyimides. Foaming gives the reducing of dielectric constant until] .98 at 0.398, 0.861 g/cm3, 92.9 J.LID andnano-porous for air volume fraction (porosity), density, thickness and morphology, respectively. Water is able to diffuse into theinternal surface of foam in film and results in a marked influence on the dielectric properties of polyimides.

Key word\' : Porosities, closed-cells, charge transport, dielectric constant, foaming

ABSTRAK

FOAMING FILM TIPISPOLIMmA UNTUK MENINGKATKANUNJUKKERJA BAHAN MIKROPROSESOR:KARAKTERISASI KELAKUAN DIELEKTRIK. Foaming pacta film tipis polyimides memberikan keuntungan untuk aplikasidielektrik. Karenafoaming menghasilkan suatu luas antar-muka yang besar antara rasa udara dan polyimides. hal ini menghasilkantempat tambahan untuk penjebakan-muatan.dikarenakan polarisasi antar muka, saatpemberian medan listrik. Pengamatan SEMmenunjukkan bahwa film mengandung gel-gel foam yang tertutup. Kerja ini difokuskan pacta kajian sifat-sifat dielektrik film tipis

polyimides pacta berbagai porositas. Hasil pengukuran (dynamic mechanical analysis) DRS menunjukkan, bahwa porositasmemiliki suatu pengaruh yang besar terhadap sifat-sifat dielektrikpo/yimides. Foaming memberikan penurunan tetapan dielektrikum

hingga 1.98 pada 0,398.0,861 g/cm3. 92.9 ~ dan llano-porous, secara berturut-turut untukfraksi volum udara (porositas).densitas, ketebalan dan morpologi. Air pada film mampu berdifusi hingga permukaan dalam foam dan memberikan pengaruh yangbesar terhadap sifat-sifat dielektrik polyimides.

Kala kunci : Porositas, sel-sel tertutup, tetapan dielektrik, foaming

INTRODUCTION

temperature resistant polymers. They combine excellentelectrical, mechanical and thermal properties [5,6J. Thinfilms of polyimides are therefore of considerableinterest for applications as interlevel dielectrics inmicroelectronic packaging devices [1 J. Wessling and vanTumhout, et al. have developed a novel technique,based on the use of supercritical CO2, for the foanling ofthin films made ofpolyimides[2J.

In a strong electrical field, the voids in foamedfilms of polyimide block any charge transport and socan act as additional charge trapping sites due tointerfacial polarization. The phenomenon of interfacialpolarization on a nano-scale in nanofoams is of specialinterest.

This work was focussed on a study of thedielectric properties of polyimide thin films, such as the

For maximizing the speed/performance gainsthrough smaller device dimensions, the application ofinterlevel dielectrics for interconnection inmicroprocessors is in urgent need of materials with alow dielectric constant. Such materials are often alsocalled as low-k materials, and they should becompatible with device processing conditions.

Several studies have been carried out to reducethe dielectric constant to below 2.5 in order to lower thecrosstalk noise and propagation delay [1]. Foaming ofthin films of polymers has been introduced in order tocreate porous materials to obtain low dielectric constantmaterials from polymers that have a high thermalstability [2,3]. The foaming technique creates also other

significant advantages [1,4].Polyimides form a family of high performance, high

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phenomena of interfacial polarization, at variousporosities.

For this reason, the C=O group forms a dipole andcauses the polar properties of the material [8]. Solidpolymers might be macro-structurally arranged in asemi-crystalline structure with two different phases, viz.the amorphous and the crystalline phase [9-11].Examination of the physical and optical propertiesrevealed that the polyimide films are anisotropic [3].

MATERIALS

Within this work, materials used were thin filmsof nanoporous polyimide (PI) films developed byWessling and van Turnhout, et al [2]. The materials wereavailable in different types, thicknesses, densities and

air volume fractions or porosities «1>.), as shown inTable 1.

THEOREllCAL BACKGROUND

Thermal Behavior of Polymers

Table I. Micro & nano-porous polymers used (Tg forCP I and Matrimid are 2$0 .C and 314 .C respectively)

Glass Transition

The glass transition temperature can be definedas that temperature where e.g. the entropy changesmarkedly. One of theories describing the glass transitionis the free-volume model [9-11]. The free volume of amaterial is the volume that is not occupied by atoms ormolecules. The free volume is produced by the motionof atoms, group of atoms and molecules. If a liquid iscooled down from its melting temperature it starts tobecome solid. During this transition the free volume isreduced. The fraction of the resulting free volume alsodepends on the symmetry of the molecules involvedand the viscosity of the liquid. For certain values ofviscosity, symmetry, and cooling rate, part of the freevolume remains within the solid, which is then called asupercooled liquid. If the symmetry of the molecules islow, very rapid cooling is necessary to reach thesupercooled state. In the case of polymers, the moleculesusually do not show a high symmetry and also theviscosity of the molten polymer is high. In order to get aspecific amount of free volume, the cooling process hasto be well defined. Otherwise a polymer can havedifferent amounts of free volume at a given temperature,and this may cause differences in the physical propertiesof the polymer.

The LaRC~-CP 1 and Matrirnid@ 5218 were patented bySRS Technologies, Huntsville, USA and Ciba SpecialtyChemicals Ltd., Basel, Switzerland, respectively. Thebasic properties of polyimides are described in theliterature [I, 7J. And the chemical structure of twopolyimides is shown in Figure 1

rQ

+4

0 0n n'1f:jr c 'rQC; ; ,--<Q)-~~ !

it0

(i) Molecular Relaxations

(ii)

Figure 1. Molecular structure of (i) Matrimid(!) 5218and (ii) LaRC<B>-CPl [2,7).

During cooling from above the glass transition,parts of the liquid lose their ability to move, theybecome frozen in their current position. This positionwill be a non-equilibrium state for the material. Thus, ifthe material is heated up again parts of the frozen liquidbecome mobile at certain temperatures. Thesetemperatures characterize relaxations towards theequilibrium state within the material. There can beseveral relaxations originating from different physicalprocesses. They are classified as [12-14] :1. a-relaxation: joint motions of side groups and

segments of the main chains,2. ~-relaxation: motions of side groups,3. y-relaxation: motion within side groups,4. p-relaxation: motion of space charges.

The multiplicity of interactions possible betweenand within polymer chains and their fine structure

As shown in Figure I, Matrimid@ andLaRC@-CPI consists molecularly of long chains madeup in part of imide derivates. Apart from the C=O groups,the main chain is generally symmetric. Since theelectro-negativity of oxygen is higher than that ofcarbon, the C=O bond is polar. This means that theelectron density is shifted towards the oxygen atom.

58

the availability of free charge catriers and their mobility[21]. Both terms depend on the trapping states in the

polymer. Charge transport in polymer films is generallydivided into inter-chain and intra-chain. The inter-chaintransport occurs at low frequencies and is determinedby hopping conduction. And the intra-chain transportis determined by band conduction at high frequencies

[22].

behavior in relation to dielectric relaxations and electricalconduction in viscoelastic polymers was described by

Das-Gupta. [15].

Polymer Dielectric Properties

Polarization and conduction

Polymers often contain dipoles, which usuallyconsist of polar molecules or parts of them. Dependingon the electronegativity of the atoms and the structureof the molecule, a dipole can be formed by shiftingelectrons in a certain direction. Then the moleculecarriers nonuniformly distributed charges of positiveand negative sign and forms a dipole. If an electric fieldis applied the dipoles try to orient themselves in thedirection of the field. This process, called as the

polarization, depends on the viscosity of the matrixmaterial and on the electric field.

A polymer usually is an insulator due to its largeband gap (4-10 eV) [16,17,19], as indicated inFigure 2(i). The states within the conduction and thevalence band are delocalized. There are no transitionsof thermally activated band-to-band across the largegap. Localized states are shown as regions between theconduction and the valence band. These are traps for

negative charges close to the conduction band andtraps for positive charges close to the valence band.Porosities can lead a polymer to store a higher chargedensity and provides a higher concentration of deeptrap [20]. An electric field tends to move the chargecarriers between the localized states.

Figure 2 ( i ) -Model of energy band'! in a polymer;T. electron traps, T b hole traps, and ( ii ) -density ofstates in a polymer; traps (localized states) are shaded,E, and Ev are the mobility edges [16,18). Usuallyboth shallow and deep traps are present.

The storage can happen at the surface as well asin the bulk. At the surface, the traps can be caused bychemical impurities, broken chains, adsorbed moleculesand other mechanisms [16-18]. In the bulk, structuraltrapping and trapping at impurities can take place [17, 18].In Figure 2 (ii), the localized states constitute the traps.Delocalized states are energetically located near thebottom of the conduction band or at the top of thevalence band. Polymer conductivity is established by

When a static electrical field is applied to amaterial, two types of interaction might occur. Firstly,the material can absorb field energy that is reversiblyregained when the field is removed. Secondly, thematerial can dissipate field energy. The first one is acapacitive effect, due to the polarizability of the material,while the second one is a resistive effect due to theelectrical conductivity of the material. Both interactionscan be converted into the real part of the permittivity(s') and the dielectric loss constant (s"). Theircombination gives tan 8 (s" Is ') and the complex dielectricconstant (s*).

Several mechanisms of polarization provoked byan electric field can be distinguished [13,23]I. Atomic polarization: displacement of atoms out of

their equilibrium position.2. Electronic polarization: displacement of bound

electrons out of their equilibrium position.3. Dipole orientation: alignment of permanent dipoles

by the electrical field.4. Macroscopic or Maxwell-Wagner-Sillars (MWS)

polarization: build up of space charges near interfacesin the material.

In the first two mechanisms a dipole moment isinduced on a molecular scale. The dipole moment in thethird process is permanent, while the dipole moment inMWS polarization is created on a macroscopic scale. Incase of a heterogeneous material like foamed polyimides,interfacial polarization appears because of the quitedissimilar conductivities of air and matrix. This differencein conductivity between the phases causes anaccumulation or trapping of migrating charges near theboundaries, which results in the formation of a

macro-dipole.Near the glass transition temperature, segments

of the main chains within the polymer become mobile. Ifthe material has been polarized before and no electricfield is applied during heating, the alignment of thedipoles will gradually disappear. The glass transitionnot only remobilizes the dipoles of the material, chargesare also released, depending on their activation energy.The charge movement inside the sample will result in achange (a drop) of the surface charge density.

The interfacial charges can be destroyed;because the local field in the dissimilar parts of thematerial is reversed so that neutralizing charges ofopposite sign are conveyed to the interfaces by ohmicconduction.

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EXPERIMENTAL

Sample Preparation

The all samples were dried at an elevatedtemperature of 150 °C for about one hour. The sampleswere stored by putting them in a desicator. Toinvestigate the influence of moisture, samples of similarMatrimid types were prepared without drying. Fordielectric relaxation spectroscopy, squared samples weresputtered on both sides with circular Au electrodes of30 mm in diameter to provide good electrical contact tothe parallel plate electrodes and to ensure a well-defined electrically active area. The morphology of thesamples is observed by SEM.

be perfonned withoutoperator interaction. The computerstarts the measurements, collects the data from theinstruments, calculates the dielectric properties of thematerials, plots the results on screen and stores themon a mass storage medium. The amplitude accuracy ofthe spectrometer is about to 3% and the phase accuracyup to IMHz:!:O.OOO5rad[24].

By performing dielectric measurements the dy-namic electrical response of the material is studied. Inthe frequency range scanned, the response of the atomicand electronic polarization mechanisms are instanta-neous, while the dipolar and macroscopic, interfacialmechanisms need some time to build up polarization.The instantaneous polarization raises the dielectric con-stant to a level of BOO.

Dielectric Relaxation Spectroscopy

RESULTS AND DISCUSSIONS

Observation of Sample Micrographs

Micrographs of samples used within this workwere observed by SEM. And the results are shown inFigure 4.

Dielectric measurements below 100 MHz areusually performed by inserting the material underinvestigation between the plates of a plane-platecapacitor and measuring the complex admittance of thesample cell as a function of frequency and temperature.

If a material with a complex dielectric constant 8*(oo'? T) is inserted in a capacitor with a vacuum capacityCo= 8??AJd, in which A [m2] is the surface area of thecapacitor plates and d [m] the distance between theplates, the cell admittance Yequals [24,25]:

Y=i.oo.C(oo,1)+G(oo,1) (1)Using these relations the dielectric properties of thematerial can be calculated by measuring the capacity Cand conductance G

(i)

Figure J. Experimental set up of dielectricrelaxation spectroscopy

The sample is placed in a custom-made ovensystem, which can be cooled an~ heated with a flow ofnitrogen gas to temperatures between 25 °C and 270 °C.Both measurement systems can be automaticallyconnected to the sample cell via a computer-controlledrelay-box as shown in Figure 3.1. All equipment iscontrolled by a PC, which controls the oven and bothspectrometer systems. A complete measurement seriesover a broad range of temperatures and frequencies can

FigJlre 4. SEM micrographs of cross sections ofthe microstructure for (i) foamed Matrimid(!) 5218(+ =0.126) and (iii) foamed LaRC<B>-CP1

(+:=0.141)

60

Foaming of Polyimide Thin Films For Enhancing Performance of Microprocessor Materials: Characterization ofDielectric Behaviors (Mochamad Chalid)

'..".,

"",

"f:l

1~'..," I.' ~.IiJ'-

.:,,--~~;-"~;;~)'

(III}

Figure 5. Permittivity (left side) and (right side) loss dielectric spectra ofCP-l at 0.000; 0.156 and 0.285 in air volumefraction displayed from top to bottom. In Fig. 4-2(i), the spectra above frequency of 1 Hz are not considered due toexperimental error

Prosiding Perlemuan llmiah lbnu Pengetahuan dan Teknologi Bahan 2002Serpong, 22 -23 Oktober 2002 ISSN 1411-2213

These figures display cross sections of themicrostructureofMatrimid@ 5218 and LaRC@-CPl foamon a 0.2 and 2 J.I11l scale respectively, as indicated in eachfigure. The dark regions in the micrographs representthe air and the light part is the polyimide. Furthermore,the Figure reveals that the samples are foams with closedcells.

Molecular Behavior of Polyimides

A significant increase in the pennittivity, emergesaround 240 °C in the foams. It may be an indication ofthe interfacial polarization due to the voids. When thefrequency of the applied electric field is low enough, thefree charges can follow the changes in the electric fieldeaSily. The charge carriers can migrate over distancesthrough the material at high temperatures.

An interfacial polarization builds up when themotion of these charges is impeded [26]. The chargesare trapped at the interfaces between air and thepolyimide phase. The presence of mobile chargesincreases the overall capacitance of a material, whichappears as an increase in s'. This condition is illustratedin Figure 7. Unfortunately the final temperature of theDRS scan was not high enough. We therefore could notobserve the collapse of the foams in the samples.

Our results show that foaming significantly

+++++++++++

POIYimide<f;~~:~~' ~ ElectrodeVoid 1-( ~ IJ

Figure 7. Representation of a air cell between twoelectrodes showing interfacial occurring in a foam.

decreases the dielectric constant of polyimide thin films.The experimental data related to this decrease are givenin Table 2.

Study of the relaxation processes as a functionof temperature is a necessity for gaining insight in theelectret behavior of polyimide thin films. The e~rimentalresults of Dielectric Relaxation Spectroscopy (DRS)show that the temperature detennines to a great extentthe intra- and inter-molecular bonding motions of

polyimides.At relatively low temperatures, we notice that

the molecular motions are restricted due to the low freevolume. This is indicated by the relatively low dielectricconstant (Figure 5). When the temperature increasesthe free volume increases. The polyimide moleculestherefore show an increase in the mobility, which isdisplayed by gradually increasing 8'.

The first transition is a ~-relaxation occurring dueto the contribution of side group's motions. Thisrelaxation is attributed to a rotation of part of thepolyimide. The DRS results of Figures 5(iv); (v) and (vi)show this behavior. The second one is the a-relaxationresulting from the cooperative motion of side groupsand main chain segments.

The possible interaction among the moleculesrelated to a-peak is the hydrogen bond, which is formedbetween the phenyl and the carbonyl groups. Theincrease of the air volume fraction reduces the dielectricconstant.

Table 2. Dielectric constants in variouspolyimide samples measured at 10' Hz and25 .C

Influence of Porosity

Based on the DRS measurement, condition ofthe foamed thin film is indicated in Figure 6.

Electrodes

-~\.'~"' Within this work, the foaming is able to reducedielectric constant of poly imide film from 2.8 at densecondition until 1.98 at 0.398 in air volume fraction.Compared the other one, void distribution of this thinfilm has the smallest size in void (nanoporous). It givesthe biggest interfacial area between air and polyimidematrix. This condition therefore results in the greate~tinterfacial polarization and lowest dielectric constant.

j:WiPolymide Ail

Influence of MoistureFigure 6. A thin film of a foamed polyimide withclosed cells in a DRS measurement

The presence of moisture in PI films given anextra contribution to the dielectric permittivity and thedielectric loss, as can be seen in Figure 8. Figures 8(i) to(iii) show that during heating the water undergoesdesorption via evaporation. This process goes on up to

Interestingly, a comparison of the permittivitybehavior as shown in Figures 5 (i); (iii) and (iv) at tem-peratures close to the glass transition, informs us aboutthe influence of the air volume fraction in polyimides.

62

Foaming of Polyimide Thin Fibns For Enhancing Performance of Microprocessor Materialr : Characterization ofDielectric Behavior.\' (Mochamad Chalid)

a relatively high temperature due to hydrogen bondingbetween the water molecules and polar groups in the polyimide.

The water desorption is thus move rapid infoamed Matrimide, so the unfoamed film needs more

energy (a high temperature) for the water to diffiIse out.At cooling back to room temperature, the permittivitykeeps on decreasing. This indicates that all water hasleft the film during the heating run.

I ,',~

CONCLUSIONS

2.

3.

4.

5.

Foaming, which was observed on a micro scale byScanning Electron Microscopy (SEM), has a markedeffect on the dielectric properties, offeringadvantages. An increase in air volume fractionreduces the dielectric constant.For obtaining more advantages, investigation ofvoid influences such a modeling is a necessity toreveal a correlation between void shape in a thinfilm and dielectric constant (interfacial polarization).The temperature influences the mobility of the

polyimide molecules; a higher temperature will thusspeed up the decay processes of charges in amaterial.Moisture has quite an influence on the dielectricproperties of polyimides. Because of its highconductivity, water can cause an enhancement ofthe capacitive and conductive effects in dielectricmeasurements. This is not a desirable effect for theapplications. It is expected however that fluorinatedPI will be much less sensitive to moisture.And a investigation related to mechanical andelectret properties of thin films is needed to verifythe feasibility of polyimide thin films in

microprocessor application.

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