Plasma Decapsulation of Plastic IC Packages with Copper Wire Bonds for Failure Analysis

J. Tang 1,2, H. Ye1,2, J.B.J. Schelen 3, and C.I.M. Beenakker 2

1 Materials Innovation Institute2 Delft Institute of Microsystems and Nanoelectronics (Dimes)

Laboratory of Electronic Components, Technology and Materials (ECTM)3 Electronic and Mechanical Support Division (Demo)Delft University of Technology, Delft, the Netherlands

P.O. Box 5053, 2600 GB DelftPhone: +31 15 2789428

Email: j.tang@m2i.nl

AbstractDecapsulation of plastic integrated circuit (IC) packages

with copper wire bonding is achieved by using anatmospheric pressure microwave induced plasma. A thermalmodel is built to estimate the bulk IC package temperatureunder different plasma etching conditions. Temperaturemeasurements of the plasma effluent and IC package aremade to validate the model. Due to the low heat transfer ratefrom gas to solid, the plasma effluent of 700°C raises the bulktemperature of an IC package to 150°C only. This brings agreat advantage in processing because a high temperature on afocused area where the plasma etching takes place results in ahigh etching rate, while a low IC package bulk temperatureensures minimum thermally induced damage to the internalcomponents. Recipes for three etching steps are developed.An IC package with 38 um copper wire bonds and a 2 mm *3.5 mm die is decapsulated in 20 minutes. Copper bond wires,aluminum bond pads, and structures on the die areundamaged after decapsulation.

I. IntroductionDecapsulation of plastic IC packages is the process to

remove the molding compound to expose internal componentsfor further failure analysis. A great advantage is gained whenall the wire bonds, bond pads and the die are undamagedduring the process. Moreover, due to the steady increase ofthe gold price, the IC industry has already begun to switchfrom gold wire bonding to copper wire bonding. Copper wirebonding has the additional advantage of high reliability aspurple plague formation cannot take place [1]. However, theuse of copper wire bonding also introduces a problem infailure analysis. The traditional decapsulation technique is theuse of hot nitric acid to etch away the molding compound.However, the acid easily reacts with copper thus making thistechnique unsuitable for copper wire bonded packagedecapsulation.

Plasmas with a combination of O2 and CF4 can be used toetch the molding compound. Conventional Reactive IonEtchers (RIE) use radio wave power to generate the plasma invacuum. The high etching selectivity of the plasma ensuresthat the copper bond wires remain undamaged during theetching. However, the etching rate is usually painfully lowcompared to chemical wet etching. In order to speed up thedecapsulation process, laser ablation is often used first to

remove the top thick layer of the molding compound. Thenplasma is used to remove the remaining thin layer of moldingcompound on top of the die [2,3]. However, due to the lowetching rate the last etching step still can take hours.

In previous work, it has been demonstrated that anatmospheric pressure Microwave Induced Plasma (MIP)generated by a Beenakker type microwave resonant cavity hasgreat potential in decapsulating plastic IC packages withcopper wire bonding [4,5]. An Ar plasma with an appropriateaddition of O2 and CF4 gas showed a sufficient etching rateand etching selectivity to the molding compound. Comparedto conventional RIE etchers, the etching rate of moldingcompound by this system is at least 10 times faster. Becauseof the high etching rate, laser ablation before plasma etchingis not needed. All the exposed copper bond wires and mostparts on the die appeared undamaged after decapsulation asinspected through an optical microscope.

Experimental results showed that besides plasma gascomposition, plasma effluent temperature has a majorinfluence on the etching rate of the molding compound. Inorder to reduce the processing time while preventing bondwires and die from thermally induced damage during plasmaetching, different recipes should be applied in differentetching phases.

In this paper, a thermal model of a plastic IC packageunder plasma effluent is built to simulate the etching process.Predictions of the temperature distribution inside the packageby this thermal model help to choose recipes that give bothreasonable etching rate and temperature. Thus the timeneeded for decapsulation and the processing temperature canbe optimized and controlled.

II. Experiment SetupThe Microwave Induced Plasma system consists of a

Sairem solid-state microwave generator (f=2450+-20 MHz,P=0---I80 W), a Beenakker type microwave resonant cavity[6,7], a gas discharge tube, and gas connections. Fig.I is aschematic diagram of the MIP system. Microwave power istransferred by a coaxial cable from the generator and coupledinto the cavity by an antenna loop inside the cavity. Thecavity resonates in the TMolO mode. The resonant frequency isdetermined by the inside diameter of the cavity by thefollowing equation: D == 2.405c / 1rf , where D is the inside

diameter of the cavity, f is the resonant frequency and c is the

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speed of light. For the cavity used in the experiments, D is 93mm and this results in f to be 2470 MHz.

Fig.l Schematic diagram of the MIP system

Tuning of this microwave system is achieved by acombination of antenna modification and tuning of the outputfrequency of the generator. During temperature measurementspower reflection is kept below 10%. During etchingexperiments power reflection is kept below 2%.

III. Thermal ModelHeat can be transferred via three paths: conduction,

convection and radiation. The flow of plasma effluent ontothe IC package sample surface causes the sample temperatureto increase. Heat can be transferred from the effluent to thepackage by convection. Inside the package, heat is transferredby conduction. Radiation takes place on the surface of thepackage. From a rough estimation the temperature of thepackage should be much lower than the plasma effluenttemperature because heat transfer from gas to solid is notefficient [8].

A thermal model is built in Fluent software to find out thetemperature distribution inside the package under differentplasma etching conditions. A better understanding of theetching process can help to choose suitable recipes fordifferent etching steps from a thermal point of view.

Model defineThe schematic diagram of the IC package sample under

the plasma effluent in the thermal model is depicted in Fig.2.This graph is not drawn to scale in order to show allcomponents. An alumina gas discharge tube is placed on topof the package. The plasma effluent flows downwards to thesurface of the IC package. The package with a size of 20.7mm in length, 7.6 mm in width, and 2.45 mm in height hasthree components. The bulk material is molding compound.Inside the package a silicon die with a size of 3.4 mm inlength, 2.0 mm in width, and 0.4 mm in height is located inthe center. The copper lead frame inside the package islocated beneath the die and is simplified to a strip in themodel. Two simplified copper lead fingers extend out of thepackage. The parameters used in the model are set upaccording to an IC package sample being etched by plasma.

Fig.2 Schematic diagram of the thermal model

Simulation resultsThe temperature distribution along a cross section through

the center of the 3D thermal model is depicted in Fig.3.Plasma effluent flows to the surface of IC package.Temperature of the effluent on top of the package isinfluenced by the total gas flow and the initial gastemperature. As shown in the graph, under this configurationthe effluent temperature at 1 cm on top of the package is 688K (415°C). The temperature of the IC package is 372 K(99°C) near the edge, which is much lower than the effluenttemperature.

Fig.3 Cross sectional view of temperature distribution

The simulation results show that heat transfer from gas tosolid is not efficient. The temperature inside the IC package isincreased from around 100°C to 150°C when the effluenttemperature at 1 cm from the top of the package is increasedfrom around 400°C to 700°C. In etching experiments, it isfound that without changing plasma gas recipes a highermolding compound etching rate always associates with ahigher plasma effluent temperature. At effluent temperaturebelow 300°C the etching rate is too low to measure. Ateffluent temperature around 600°C the etching rate can be ashigh as 2.5 mm3/min. When etching the thick moldingcompound, a recipe with a high plasma effluent temperaturecan be used to achieve high etching rate. In the meantime theplasma effluent does not raise the temperature inside the ICpackage to above 150°C.

For the same configuration, the velocity vector of gas flowis depicted in Fig.4. Arrows indicate the flow direction. It isseen that the effluent flows from the tube opening to the ICpackage surface and then spreads toward the edges of thepackage. The flow velocity is lower at the edge of the

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package than at the center of the package. When measuringthe IC package surface temperature, a thermal couple shouldbe placed at a place where the gas flow velocity iscomparatively low in order to reduce misreading caused bythe effluent.

Fig.4 Velocity vectors of gas flow

IV. Temperature MeasurementThe temperature of the plasma effluent and an IC package

is measured by a thermal couple. To measure the effluenttemperature, a thermal couple is placed in the center of theeffluent flow path from the alumina tube. The thermal coupleis placed at 1 cm above the package. To measure the packagesample temperature, a thermal couple is glued by thermal glueto the left and right comer of the package where the gas flowvelocity is comparatively low. By gluing a thermal couple tothe bottom side of the package, the bottom temperature of thepackage is also measured.Verification of the thermal model

Temperature of the plasma effluent and the correspondingpackage temperature under different input microwave powerlevels are measured. During the measurements, gas flow andgas composition are kept constant. Ar is used as plasmacarrier gas, sufficient 02 is added into the plasma so that theplasma filament does not extend out of the gas discharge tubeand the microwave leakage is low. Absorbed microwavepower is calculated by the output power and reflected powerreadings from the microwave generator.

As it is shown in Fig.5, the plasma effluent temperatureincreases from 380°C to 670°C when absorbed microwavepower is increased from 40 W to 90 W. The correspondingpackage temperature is much lower than the effluenttemperature. Temperature at the comer of the packageincreases from 85°C at 40 W to 150°C at 90 W. These valuesare in good agreement with the simulation results. Packagetemperature on the right comer and left comer are almost thesame. Package temperature on the bottom side is 10°C to20°C lower than temperature on either side of the comers.

Fig.5 Effluent and package temperature versus absorbedmicrowave power

Influence of total gas flow rate to plasma effluenttemperature

From simulation, it is found that the total gas flow rate andinitial gas temperature influence the plasma effluenttemperature on top of the IC package surface. The initial gastemperature can be controlled by varying the absorbedmicrowave power. To study the influence of total gas flow,plasma with a constant Ar/02 ratio of 35: 1 is used.

The effluent temperature measured at different Ar gasflow rates under different absorbed microwave power levelsis shown in Fig.6. At the same absorbed power level, theplasma effluent temperature increases sharply when the Arflow rate is increased from 500 seem to 800 seem, When theAr flow rate is further increased from 800 seem to 1400 seem,the increase of effluent temperature gradually slows down.The increase of temperature tends to saturate when the Ar gasflow rate approaches 1700 seem, When the total gas flow isincreased, more heat from the plasma in the center of themicrowave cavity can be transferred out of the discharge tube.At a constant input microwave power level, the amount ofheat in the plasma that can be carried out is limited. As aresult the effluent temperature tends to a constant value whenthe total gas flow is further increased.

Fig.6 Ar gas flow rate versus plasma effluent temperature

At a higher Ar flow rate, the increase of effluenttemperature caused by the increase of absorbed power is more

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Fig.ll Optical picture of the die and bond wires

optical microscope. Fig. 14 and 15 show SEM pictures of theundamaged Cu bond wires and Al bond pads. However, dueto the nature of the plasma the SbN4 passivation layer isremoved.

obvious. When absorbed power is increased in steps of 10 W,under 1400 seem Ar flow condition the effluent temperatureincreases 60°C for each step, while under 800 seem Ar flowcondition the effluent temperature only increases 35°C foreach step. From these results, in order to tune the plasmaeffluent temperature by only changing the microwave powerlevel it is more suitable to use 1400 seem than 800 seem Arflow rate because the former provides a broader variationrange.

V. Decapsulation of Plastic IC Package with Cu WireBond

To reduce both the total processing time and the thermallyinduced damage, the decapsulation process is carried out indifferent steps. Initially more aggressive recipes with higherpower and higher effluent temperature can be used to increasethe etching rate. When reaching the bond wires and the die,less aggressive recipes should be used to provide a moderateprocessing temperature. A suitable recipe is a balancebetween etching rate, effluent temperature and controllability.

Plastic IC packages with 38 um Cu bond wires and a 3.4mm * 2.0 mm * 0.4 mm die inside are decapsulated by theplasma. O2 and CF4 are added as etchants to the Ar carriergas. Fig.7 shows the package before decapsulation. As shownin Fig.8, during the first step of plasma etching a recipe with590°C effluent temperature is used to remove the 0.8 mmthick molding compound layer. Cu bond wires can be seenafter this etching step. Exposing Cu bond wires to a relativelyhigh temperature plasma for a short time or to a relatively lowtemperature plasma for a long time will both cause severeoxidization. During the second etching step, a recipe with530°C effluent temperature is used. Fig.9 shows that themolding compound around the bond wires is etched. Asshown in Fig.l0, during the third etching step the remainingcompound left on top of the die is removed by a plasma with460°C effluent temperature.

Fig.12 Magnified detailsof the wire bond

Fig.14 SEM pictureof the Cu bond wires

Fig.13 Magnified detailsof the structures on the die

Fig.15 SEM pictureof the wire bond

From the simulation results, the temperature inside thepackage during decapsulation process is below 150°C. Theduration of the three etching steps in total is 20 minutes.Fig.ll shows an optical picture of the die and bond wiresafter decapsulation. The cavity opening is sufficient for circuitaccess and the die is clean. Fig.12 and 13 show magnifieddetails of the wire bond and structures on the die under the

Fig.7 Package beforedecapsulation

Fig.9 Package afterthe second etching step

Fig.8 Package afterthe first etching step

Fig.l0 Package afterthe third etching step

ConclusionsA thermal model of the plasma etching process is built and

validated by temperature measurements. The heated plasmaeffluent raises the bulk temperature of the IC package duringetching. When the temperature of the plasma effluentincreases from 380°C to 670°C the IC package temperatureraises from 85°C to 150°C. High plasma temperatures andlow package temperatures ensure high etching rates and lowthermally induced damage. The total gas flow rate is found toaffect the plasma effluent temperature. A 1400 seem Ar flowrate is chosen to provide a broader variation range of plasmaeffluent temperatures under the same input power range.Molding compound on a plastic IC package is etched by threeconsecutive steps in 20 minutes. Optical and SEM picturesshowed the copper bond wires, aluminum bond pads, andstructures on the die are undamaged after decapsulation. Thusit is demonstrated that the main advantage of this technology

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for failure analysis is high speed and suitability for copperwire bonded packages.

AcknowledgmentsThis research was carried out under project number

M21.9.SE2Ab in the framework of the Research Program ofthe Materials innovation institute M2i and co-funded byENIAC Joint Undertaking and Agentschap NL under GrantAgreement number 120009 as part of the project "SE2A"("Nanoelectronics for Safe, Fuel Efficient and EnvironmentFriendly Automotive Solutions" ). The authors would like tothank H. Pomp at NXP Semiconductors and Dimes colleaguesA. Akhnoukh, A. van den Bogaard, C. C. G. Visser, and R. P.van Viersen for their help on experiments.

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