Vibration Characteristics of Ultrasonic Transducer in Thermosonic Flip Chip Bonding

Zhili Long, Yunxin Wu, Lei Han, Jue ZhongCentral South University, Changsha 410083, China

Email: longzhiligmail.csu.edu.cn

AbstractThermosonic flip chip bonding has been widely used in

fine pitch IC packaging for its unique features. By usingthermosonic flip chip bonding at an aluminum wire bonder,lmmxlmm chip with 8 gold bumps is bonded successfullyto Ag substrate, and the bonding strength reaches as high asabout 30gf/bump. Dynamical characteristics of ultrasonictransducer are investigated by finite element method andexperiments using impedance analysis and laser Dopplermeasurement. The FEM analysis shows that, ultrasonictransducer system vibrates in coupling of all excited modes,which results in complicated vibrations during bonding. Thethird axial vibration mode, which includes 1.5 wavelengthsand 3 nodes, is the dominant working mode. However, thedominant vibration is disturbed significantly by undesirablenon-axial modes such as flexural vibration. Moreover, thereare some other un-wanted modes parasitizing closely todominant mode. Velocities of transducer horn sampled byLaser Doppler Vibrometer show the system vibrates withseveral modes simultaneously. The impedance results usingimpedance analyzer prove there are some other undesirablefrequencies close to the working mode. All non-workingmodes of ultrasonic transducer will disturb the bondingprocess and degrade bond quality, so they must be wellcontrolled to obtain unique vibration mode and stablevibration for thermosonic flip chip bonding.

1. Introduction

In first level IC fabrication industry, wire bonding andflip chip bonding are the main microelectronic packagingway to make fine-pitch connections between chip andsubstrate. Although wire bonding holds over 80% inpackaging market nowadays, it is extremely challengedbecause the demand of ultra-compact and miniaturizedelectronic devices in the future. [1-2] Flip chip bonding,which includes C4 soldering, thermosonic flip chip,thermocompression bonding, and adhesive bonding, offersan attractive solution for high density IOs and miniaturizeddevices. Compared with other equivalent processes,thermosonic flip chip bonding offers several uniqueadvantages such as cost-effective, low temperature andlead-free material. Nowadays, thermosonic flip chipbonding has been widely used in IC packaging, such asMEMS chip, LED chip, optoelectronic modules, SAW filterand magnetic disk chip, etc.

Thermosonic flip chip bonding is a process where goldballs and substrate are joined together by ultrasonic powerand heat under certain pressure. By far, many ways ofthermsonic flip chip bonding have been developed. Kang inColorado University [3-4] built up a thermosonic flip chipplatform by assembling ultrasonic generator, transducer,and bonding tool. His platform could successfullydemonstrate to bond for 64-I/0 GaAs-silicon optoelectronicmodules, and bonding strength reached to 23gf/bump. Tanand Zhang [5-6] developed a novel longitudinal system forthermosonic flip chip, where the chip was underlongitudinal "hammering". This bonding process could

configuration. As to thermosonic flip chip bonding,ultrasonic energy is the key parameter for its softeningeffect to metal material. Transducer system plays animportant role of turning electrical energy into ultrasonicenergy and transmitting energy to bonding interface, so itsdynamics and ultrasonic energy propagation has been thesubject of much investigation. Kang [3-4] established thetransducer vibration model containing ultrasonic energyflow, which providing selection of tool length and mass foran efficient bonding system. In order to control efficientlyultrasonic energy, Tan and Brian [6] placed a layer ofpolymer to smooth the contact between bonding tool andthe chip. Sattel [7] developed a simple model treating mountbarrel as unilateral contact for longitudinal flip chip bonding.Nowadays, researches about dynamic of transducer inthermosonic flip chip are still scarce yet, and people tend touse the existing knowledge of wire bonding transducer tounderstand what happens in thermosonic flip chip [8-11].While there are distinct differences between thermosonicflip chip and wire bonding. For example, there is a chipexisting between tool bottom and substrate. As increasing ofI/O numbers for thermosonic flip chip, dynamics oftransducer system, such as vibration modes and resonancefrequencies, influence directly ultrasonic energy providedfor bonding interface. In order to obtain favorable bondingquality, it demands to control accurately the vibration of atransducer system.

Ultrasonic transducer system is the key component inflip chip bonder. In our present study, an easy and availabletechnology of thermosonic flip chip using reconstructivealuminum wire bonder is developed. Dynamics of ultrasonictransducer are investigated by finite element analysis andexperiments, and the results can be used to bonding control,transducer design, and help to understand the bondingmechanism of thermosonic flip chip.

2. Experimental platform

The process and equipment of thermosonic flip chipbonding are similar to the condition of conventional wirebonding. In our experiment, an existing aluminum wirebonder with parameters of 0 40 watt ultrasonic power,30-1200gf normal pressure, and 5500ms bonding time, issuccessfully re-configurated for flip chip bonding process.As in figure 1, the tungsten carbide bonding tool is clampedinversely, which is opposite to the condition for wirebonding. Because the flat area of tool bottom directlycontacts to the chip's upside, the tool can clamp tightly thechip by the friction force under normal pressure. Thismethod of clamping chip/die is different to the conventionalvacuum adsorption.

overcome the planarity problem and simplified bonding tool

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gtoo

Fig. 1 The bonding tool is clamped inversely forthermosonic flip chip.

The reconstructive platform can fulfill the process ofthermsonic flip chip by controlling bonding parameters suchas ultrasonic energy, normal pressure and temperature. Atthis platform, a 1mm X 1mm chip with 8 gold bumps isbonded successfully to Ag substrate (Figure 2), and theshear strengths are plotted as Figure 3. It shows the bondingstrength reaches as high as 30gf/bump, which is muchhigher than the standard strength criterion of 5gf/bump. [4]Table I lists the corresponding bonding parameters.

Fig. 2 The gold chip and Ag substrate.

Table 1 Bonding parameters

Power Time Pressure Frequency(watt) (ms) (N) (kHz)

0.5-3.5 200 0.3 60

3. Wave equation

3.1. Constitutive equation ofpiezoceramicsThe basic piezoelectric action can be described by a set

of constitutive equations, which relate the intrinsic variablesstress T and electric field E with extrinsic variables strain Sand electric displacement D. The basic piezoelectricconstitutive equations for a thin piezoelectric element canbe written, in Cartesian coordinate, as: [12]

T3 = CDS3 -h33D3

E3 = -h33S3 3(1)

Where, cD is the elastic stiffness coefficient at

constant electric displacement, /QS is the impermittivity at

constant strain, and h33 is the piezoelectric coefficient.

240 /

mLU160,

120 A

0 O.5 ~1 1.5 2 2.5 3 3.5Powr (Watt)

Fig. 3 Bonding strength.3.2. Wave equation ofconcentrator

It is assumed that the concentrator horn is made upof uniform and homogeneous material, and the effect ofmechanical losses is neglected. Moreover, diameter ofconcentrator is small compared with acoustic wavelength.So wave equation of the concentrator with variable crosssection can be expressed as, [13]

a2y IasaCy 2+ + k2y 0 (2)ax2 SaX aX

Where y is particle displacement in the concentrator,

y(x, t) = y(x)ejwt. k = is wave number, where ccP

is longitudinal wave velocity in concentrator. S(x) isvariable cross section area of the concentrator.

And, the front and back slab with constant crosssectional area are treated as a special case of aboveconcentrator.3.3. Vibration equation ofbonding tool

The movement of bonding tool can be treated asvibration of a Timoshenko beam pinned at the horn end.According to vibration theory of Timoshenko beam,flexural vibration equation ofbonding tool is: [13]

a4z a2z Y a4zY2(Y) a4 +PS(y) 2 pI(y)(l+ Y Yat

+ p2I(y) 4Z oKG ay4

(3)Where, Z(y, t) is flexural displacement; I(y) is moment

of inertia; S(y) is section area of bonding tool; p is materialdensity; Y2 is Young modulus; G is shear modulus; andK' is constant parameter related to beam geometry.

4. Finite element model

The finite element model of transducer system isestablished in Ansys 9.0 (Figure 4). [14] This simulationmodel consists of piezoelectric driver, font/back slabs, innerstress bolt, tapered horn, and bonding tool. In our model,some unnecessary factors such as small holes, are ignored.Moreover, bonding tool is simplified to a cylindrical beam.Combine 40 element, which includes spring and damper, isused to simulate friction force between bonding tool andchip. The spring constant K is calculated with SSE (Sum ofSquare of Error) method, [4] as:

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.........I

SSE= _-fmin (4)fe

Where f7 and fe are the calculated and measuredfrequencies, respectively. The spring constant K can bedetermined when SSE is minimized.

Fixed boundary except for axial direction is applied tothe mounting barrel of transducer. So, finite element modelincludes 19,716 3-D structure solid elements and 38,905nodes. And, Block Lanczos method is used to computevibration mode shape and resonant frequency of ultrasonictransducer.

AN

Fig. 4 Finite element model of ultrasonic transducer.

FEM results show that, there are 54 naturalfrequencies and vibration modes of transducer systemwithin 100 kHz. The vibration mode shapes can becategorized as axial mode, flexural modes, torsion modes,and coupling modes. When driven by electrical signal withappropriate frequency, those vibration modes would beexcited.4.1. Dominant vibration mode

The axial vibration modes of transducer are the mostdesirable mode in flip chip bonding because it causes the tipof bonding tool to move in a direction parallel to the chip,which can successfully bond the chip to substrate. Therefore,these axial modes are mostly worth investigating. Resultsfrom FEM calculation show there are four axial vibrationmodes, which are 24.149 kHz, 44.291 kHz, 59.383 kHz and81.157 kHz, respectively. As to thermosonic flip chip, allthese axial modes can be regarded as working modes inbonding. However, the third axial vibration mode with59.383 kHz resonance frequency is in a good agreementwith the designed frequency band of 58.0-61.0 kHz, so it isthe dominant working mode in practical flip chip bondingprocess.

Figure 5 shows the dominant mode shape of transducerand its relative displacement distributions. For axial (X)direction, the transducer has 3 nodes (node A, B, C). That is,the length of the whole transducer is about 1.5 wavelength,where the PZT driver and the concentrator correspond to 0.5and 1 wavelength, respectively. And node B, where axialamplitude is zero, can be used for mounting transducersystem for least loss of ultrasonic energy. Axial amplitudeof transducer at horn end is amplified markedly forproviding possible largest energy to the chip and bondinginterface. On the other hand, it is found that relativeamplitude of Y and Z direction which is non-axial

components is not zero. It suggests there be some otherexcited modes besides the axial vibration. Althoughamplitudes of these non-axial components are small, theywill badly disturb the axial desirable vibration and degradebonding quality. So, FEM results suggest that, in practicalbonding process, when locked at about 60 kHz signal byPLL (Phase Locked Loop) of the ultrasonic generator, thetransducer system vibrats at a complex state, which couplesthe axial with non-axial components. The non-axialundesirable components should be reduced or controlledbecause of its bad disturbance.4.2. Vibration modes near the dominant one

From the result ofFEM analysis, it is found that thereare many kinds of undesirable vibration modes parasitizingclosely to the 59.383 kHz dominant mode, which includevertical flexural modes (56.054 kHz), horizontal flexuralmodes (56.422 kHz), and torsional modes (57.331 kHz).Figure 6 shows these mode shapes with frequencies from 56kHz to 63 kHz. Because these un-wanted mode shapes arevery close to the dominant mode and may be insidegenerator PLL frequency ranges, some of these modes willbe excited. Once these parasitical modes are excited, theywill cause undesirable motion of the bonding tool, whichlowers the bonding strength and worst destroy bondinginterface, which results to unsuccessful bonding. Therefore,these undesirable modes should be well controlled inbonding process and must be considered carefully intransducer design.

5. Experiment

5.1. Frequency characteristicsFrequency and impedance characteristics of ultrasonic

transducer system are measured using an impedanceanalyzer and results are shown in figure 7. The scanningfrequency range is from 20 kHz to 100 kHz with a step of5Hz. The results show that the resonant frequencies oftransducer system are very complicated. The strongestmaximum resonant frequency is observed at about 60.289kHz. This is the dominant frequency which is locked by thePLL ultrasonic generator in practical bonding. In addition,other three resonant frequencies of axial vibration can befound. As shown in Table 2, there is a good agreementbetween FEM simulation and experimental results, and theerror of simulation may be originated from the materialparameters and structure sizes, or the simplified model.

I_ dd~~~A

(a)

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0 0 40 60 80 100Length (mi)

(b)Fig. 5 Vibration mode (a) and its corresponding

displacement distributions (b) for the working mode.

-_t-_3 -9w9bZ w44.594 '17450 ib ;14a

(a)AN

-7 947 E MI A4 LI

.'5 lE.:.si2 ', jS *iM.

(b)Fig. 6 Undesirable vibration modes near the dominant one.(a) Vertical flexural mode and (b) Horizontal flexural mode.

160

14

40

FEM and impedance measurement

Mode Computed Measured Errorindex frequency (kHz) frequency (kHz)

6 24.149 23.604 2.3%14 44.291 45.770 3.2%26 59.383 60.289 1.5%39 81.157 79.360 2.3%

Moreover, it is found that there are a number ofweaker resonant frequencies close to the dominantfrequency. They are the frequencies of undesirable non-axial vibration modes which parasitize nearly to thedominant vibration. If these undesirable vibration modes areexcited, they will waste ultrasonic energy and causeundesirable motion. So, observation from impedanceanalyzer agrees well with above FEM result.

5.2. Velocity measurements oftransducer horn

Velocity measurement of marked points on transducerhorn is carried out by using a laser Doppler vibrometer fromGermany Polytec Corp., as shown in figure 8.

Transducer Laser S ource

Digital ComputerOscill ograph

Fig. 8 Laser Doppler vibrometer measurement platform.

x

A

Fig. 9 Measured marked points.

At normal bonding power from 0.2 to 2.5 watt,velocities of points 'C', 'F' and 'E' of the horn end, 'A' and'K' of tool tip shown in figure 9 are sampled by aboveexperimental platform. Where,

VC and VA: axial velocities (X direction);

VE: vertical velocity (Z direction);Frequency (kHz)

Fig. 7 Impedance characteristics of transducer system.

Table 2 Axial resonant frequencies comparisons between

UF and UK: horizontal velocities (Y direction).

The ratios of SEC = VE /VC , (7FC UF /VC and

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40

il

oKA UK/IVA are used to denote the effect of non-axial onaxial vibration. The measurement results are shown infigure 10 and 11. It is found that, (a) velocities of axialvibration ( VC and VA ) are dominant among all directionvibrations, which ensures the successful bonding because itis the working vibration mode; (b) velocities of non-axialcomponents (VE, VF, and VK) are not zero, which indicatesthat non-axial undesirable vibration modes have beenexcited in practical bonding, and (c) the vertical vibrationvelocity (VE) among all non-axial modes influences mostsignificantly the axial vibration (VC and VA), and ratio of

SEC = VE/V can reach as high as 7000. The measurement

results by laser Doppler vibrometer show that, in bondingprocess of flip chip, ultrasonic transducer vibrates in acoupling mode, where the axial mode is dominant, butdisturbed by other non-axial vibrations such as verticalflexure mode. Those non-axial vibrations will degradebonding quality and must be well controlled.

t*ir..rUB |)

K'

U

t--.;;,

~arm-Y V3

304tt....ji,,._ _

I * - ; - fl_-x

(a)....................... *... Fr (Waft)(b)

Fig.11 Vibration velocities of tool end, (a) velocities ofmarked points, (b) ratio of non-axial to axial vibration.

6. Conclusions

Thermosonic flip chip bonding technology has beenregarded as a packaging process achieving integrated chipwith high I/0 numbers nowadays. An easy and availabletechnology of thermosonic flip chip is introduced in thispaper. By the similarity between thermosonic flip chip andwire bonding, process of thermosonic flip chip has beendeveloped successfully at a conventional Al wire bonder,where a 1mm X 1mm chip with 8 gold bumps is bondedsuccessfully to Ag substrate. The finite element method andlaser Doppler Vibrometer measurement are used to analyzethe dynamics of ultrasonic transducer system. The FEManalysis shows that transducer system works with its thirdaxial vibration mode including 1.5 wavelengths and 3 nodes,where the vibration amplitude is amplified and ultrasonicenergy is concentrated to the bonding interface. However,the axial mode is disturbed by other non-axial componentssuch as flexural vibration. Moreover, there are some otherundesirable vibrations parasitizing closely to the axial mode.The measurement of impedance and laser Dopplervibrometer is in a good agreement with the results of FEManalysis. Those research results can be benefit to control thebonding process, design transducer system, and help tounderstand the bonding mechanism of thermosonic flip chip.

0O 0.5 1 1,5 2 Z.5Power (Watt(b)

Fig.10 Vibration velocities ofhorn end, (a) velocities ofmarked points, (b) ratios of non-axial to axial vibration.

)

_.x~ ~~

Power (Waft)(a)

Acknowledgements

This work was supported by the National ScienceFoundation of China under Contract 50390064 and50605064; the Ph.D. Program Foundation of Ministry ofEducation of China under Contract 20060533068; the ChinaDepartment of Science & Technology Program 973 underContract 2003CB716202.

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