[SiC-En-2013-22] Molding Compounds Adhesion and Influence on Reliability of Plastic Packages for SiC-Based Power MOS Devices

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Molding Compounds Adhesion and Influence onReliability of Plastic Packages for SiC-Based

Power MOS DevicesAntonino Scandurra, Giuseppe Francesco Indelli, Roberto Zafarana, Angelo Cavallaro, Emanuele Scrofani,

Jean Paul Giry, Sebastiano Russo, and Mietek Bakowski

Abstract— Adhesion and interface compositions of epoxy phe-nolic molding compounds (EMCs) for high-temperature plasticpackages are studied. Interfaces are obtained by molding twoEMCs onto aluminum oxide-hydroxide surfaces (oxide onto thinfilm of AlSiCu) and two die passivation layers consisting of fluo-rinated polyimide and cyclotene. One compound (A) is a “green”type, containing organic phosphorous-based flame retardant, andthe other compound (B) is a conventional type containing anti-mony (III) oxide and bromined resin flame retardants. A high-temperature storage test at 250 °C is employed to study chemicalmodifications occurring at the previously mentioned interfaces.A high-temperature reverse bias test at 225 °C is employed tostudy the influences of the EMC chemical formulations on thereliability of plastic packages for SiC-based power MOS devices.Green compound A shows poor adhesion onto Al oxide and highadhesion strength onto both polymer passivations. The failuremechanism is mainly cohesive on the polymer passivations. Theconventional compound B shows a high degree of delaminationbecause of poor adhesion compared with the green compound.SiC-based power MOS devices assembled in plastic packages withcompound A show better reliability under HTRB test at 225 °Ccompared with compound B.

Index Terms— Adhesion, green molding compounds, reliability,scanning acoustic microscopy (SAM), silicon carbide devices,X-ray Photoelectron Spectroscopy (XPS).

I. INTRODUCTION

DEVICES based on silicon carbide will greatly expandthe possibilities for high-temperature circuit designs [1].

These devices have the potential to operate at temperaturesranging from 300 °C to 500 °C and are suitable for harshenvironments in combustion engine applications, high-power

Manuscript received July 31, 2012; revised November 21, 2012;accepted February 1, 2013. The Large Area silicon carbide Substrates andheTeroepitaxial GaN for POWER device applications (LAST POWER)project was funded in part by the ENIAC Joint Undertaking under thesub-programme SP8 - Equipment & Materials for Nanoelectronics, underENIAC JU Grant 120218. Recommended for publication by Associate EditorT.-C. Chiu upon evaluation of reviewers’ comments.

A. Scandurra and G. F. Indelli are with Laboratorio Superficie Interfasi, Consorzio Catania Ricerche, Catania 95121, Italy (e-mail:[email protected]; [email protected]).

R. Zafarana, A. Cavallaro, E. Scrofani, J. P. Giry, and S. Russo are withSTMicroelectronics, Catania 95121, Italy (e-mail: [email protected];[email protected]; [email protected]; [email protected];[email protected]).

M. Bakowski is with ACREO AB, Kista SE-164 40, Sweden (e-mail:[email protected]).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TCPMT.2013.2247466

switching circuits, and aircraft electronics [2]. However, con-ventional plastic packaging for power electronics is not suit-able for applications even at temperatures over 175 °C–200 °C.New packaging materials are required for high-temperature,high-power electronics devices based on SiC.

Temperature is the main accelerating factor for most ofthe failure mechanisms affecting semiconductor devices usedin harsh environments [3], [4]. The limitations imposed bythe working temperature are particularly severe in plasticpackages, because of the thermal aging followed by degra-dation of the molding compound [5]. Material propertieschange dramatically during thermal aging with degradation ofmolding compounds. In addition, antimony trioxide (Sb2O3)and bromined resin (Br) flame retardants used in conventionalmolding compounds are well-known to degrade the bondinginterfaces of gold or aluminum wire onto aluminum pads athigh temperatures. Moreover, chemical constituents containingSb and Br are accompanied by traces of metallic contaminantsthat lead to further degradation mechanisms such as leakagecurrent or other undesired electrical drawbacks [6]. In addi-tion, high temperatures force the package to withstand highmechanical stress because of materials expansion mismatchduring the real life of the device. This stress in combinationwith poor adhesion at interfaces between molding compound–leadframe, molding compound–die passivation, and moistureabsorption produces cracking and delamination [7], [8]. Adhe-sion strength of the molding compound to the leadframe anddie passivation layer is an important parameter that influencesthe long-term reliability of plastic packages. Adhesion dependsmainly on the chemical structure of the respective materialsand on the parameters of the molding process.

The fabrication of power plastic packages involves vari-ous materials such as bare copper (or Ag, Ni-plated) lead-frame, wire bonds, interconnection metals, and passivationlayers. The surfaces of these materials are formed of copperoxide-hydroxide, nickel oxide-hydroxide, aluminum oxide-hydroxide, hydrogenated silicon oxide-nitride, polyimides(PIs) or other organic layers.

The earlier reported surfaces are the most commonly seenby the molding compound during the molding process ofplastic encapsulated devices. Epoxy phenolic molding com-pound (EMC) formulation and the specific chemical compo-sition of the surface to which the molding compound hasto adhere must be compatible. Therefore, the control of thesurface chemical composition during the fabrication processes

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represents the key to guaranteeing high device reliability. Thistask is particularly challenging and very often adhesion failureat the interfaces of molding compound chip and moldingcompound leadframe followed by moisture and migration ofadditives is the main route leading to the electrical failure ofthe microelectronic device.

Several authors have reported detailed studies on the impactof the molding compound on the electrical performance ofthe devices. Seng [9] evaluated the performance of siliconpower MOSFETs transistors encapsulated with green andnongreen EMC via autoclave stress. He proved that a greendevice indeed possesses superior electrical and physical prop-erties compared with a nongreen device. The research revealsthat a device with nongreen EMC exhibits relatively highergate-to-source leakage current (IGSS) and drain-to-source on-resistance (RDSon) as compared to a device with green EMC.The nonsteady electrical characteristic of a nongreen deviceis due to its higher content of bromide ions released fromthe flame retardant. Under a moist environment, bromide ionsform electrolytic solutions and trigger the corrosion process.

Herbsommer [10] analyzed the impact of the mold com-pound on the electrical performance of high-power, high-frequency laterally diffused metal oxide silicon transistors(LDMOS). He used the LDMOS, air-cavity ceramic packagefilled with the EMC under test. He found that the EMCproduces electrical degradations of devices such as reductionof gain and increase of parasitic capacitance between gate andsource, and between drain and source.

SiC-based MOSFETs present, with respect to the well con-solidated Si technology, the big challenge of the poor SiC/SiO2interface quality. DasGupta et al. [11] evaluated state-of-the-art, commercially available, 4H-SiC MOSFETs for stabilityunder high-temperature over-voltage and pulsed over-currentconditions. The devices show maximum vulnerability underhigh-temperature accumulation stress, demonstrating that thegate oxide is more prone to hole trapping than to electron trap-ping. The pulsed over-current operation results in degradationsimilar to electron trapping at a high temperature, presumablybecause of overheating of the device beyond its specifiedjunction temperature. Over-current degradation is more severeat a high switching frequency. Despite the recent remarkableprogress, the reliability of SiC power devices still presentseveral open issues that reflect the relatively young maturityof this technology. In particular, the actual data concerningthe field failures because of degradation of the EMC are notyet statistically significant. Epoxy molding compounds formicroelectronic devices are continuing and will continue tobe the main stay of encapsulation materials in view of theircost and productivity advantages.

For these reasons here, we perform a careful comparison ofthe adhesion and interface compositions of two EMCs moldonto aluminum oxide-hydroxide, fluorinated PI, and cyclotenetest surfaces.

Aluminum oxide-hydroxide is present on the die bondpad surfaces, whereas the polymer coatings are used as diepassivation layers. One compound is green, i.e., antimony (III)oxide and Br free, having an organic phosphorous-based flameretardant to meet the RoHS directive [12], [13]. The second is

a conventional compound with antimony oxide and brominatedresin. Both compounds are designed at the molecular levelto improve resistance to cracking and delamination, adhesionperformance, and electrical isolation and, thus, device reliabil-ity [14], [15].

The adhesion and interface compositions are studied usingthe button pull test method. The specimen is designed to allowthe delamination analysis by scanning acoustic microscopySAM, the adhesion strength measurements by pull test andthe interface chemical analysis by X-ray photoelectron spec-troscopy (XPS) [16], [17].

High-temperature storage (HTS) at 250 °C is used tostudy chemical modifications at interfaces. The HTS testis performed to determine the effect of long-term storageat elevated temperatures on devices without any electricalstresses applied. Goroll et al. [18] demonstrated that a similarmethodology based on a passive test specimen can be used atthe beginning of package development projects to investigatesuitable material combinations and production processes atthe important package interfaces molding compound–chip andmolding compound–lead frame.

The high-temperature reverse bias (HTRB) test is used tostress SiC power MOSFETs devices assembled with the twocompounds. HTRB is a type of burn-in performed on devicesthat are subjected to a reverse bias, wherein the device isgenerally in a nonconducting state. HTRB intends to bring outweaknesses within the device that will result in failures suchas excessive current leakage and breakdown voltage shifts.The test carried out on the product stresses the die and maycause junction leakage. Parametric changes may also occurbecause of gate oxide aging, ionic impurities that can bereleased onto the die surface, either from the package or thedie itself. Comparison of the existing reliability tests is beyondthe scope of this paper. Nevertheless, we may say that powercycling, i.e., exposure to repetitive voltage and current pulses,in contrast to the previous tests, causes power dissipation.This, in turn, influences the degradation of the device througha multitude of parameters that could mask the effects ofmolding compound. The resulting thermo-mechanical loadscause different failure mechanisms involving mainly the solderjoint and the metal degradation [19]. The novelty of this paperis represented by the combined employment of laboratoryspecimens as well as SiC MOSFETs in understanding theeffects of the chemical and physical properties of EMCs onthe device reliability.

II. EXPERIMENTAL

The adhesion of molding compounds as well as the interfacedelamination and chemical compositions are studied througha well-established experimental procedure in our laboratoriesand consist of the following steps [20], [21].

1) Preparation of base surfaces onto which the EMC ismolded.

2) Substrate characterization by means of XPS.3) Molding the EMC truncated cone on the substrate.4) Nondestructive analysis of the interface by means of

SAM.


SCANDURRA et al.: MOLDING COMPOUNDS ADHESION AND INFLUENCE ON RELIABILITY OF PLASTIC PACKAGES 3

Fig. 1. Schematic experimental procedure used to study EMCs adhesion.

Fig. 2. (a) View of pull test specimen used in this paper: molding compoundis attached to aluminum stud for fixture to head of INSTRON. (b) Specimenafter pull test.

5) Pulltest and measurements of the adhesion strength.6) XPS analysis of the fractured surface (resin and metal

side) obtained after pull test.Fig. 1. shows experimental procedure.A photo of a test specimen is shown in Fig. 2(a). The EMC

sample has a truncated cone geometry 3 mm height with basediameters of 8 and 10 mm (base on the metal substrate) moldedonto the substrate (see Fig. 4). An aluminum stud is attachedto the resin for the measurement of adhesion strength using acyanoacrylate adhesive. Fig. 2(b) shows a photo of a specimenafter pull test. The adhesive attachment must be stronger thanthe EMC to substrate. Rarely does this attachment fail, evenwhen the EMC adhesion is very high.

The test surfaces are as follows.1) Al(98.5) Si(1) Cu(0.5) (wt. %) metal deposited onto 3-in

diameter SiC wafers. The complete stack consists ofback side metallization (BSM)/SiC/Ti (80 nm)/AlSiCu(3 μm) (hence named Al oxide). The function ofTi layer is to improve AlSiCu metal adhesion. Dice12.7×12.7 mm are defined by photolithography. Scribelines 120-μm wide, which prevent film peeling duringwafer sawing, are formed by a sequence of wet andCF4/O2 plasma etchings of the AlSiCu and Ti films.

2) A polymer passivation layer of fluorinated PI, HDMicrosystems 8820, is deposited onto a 3-in SiCwafer. The complete stack consists of BSM/SiC/TEOS(1.6 μm)/PI (7 μm) (hence named PI). Dice12.7×12.7 mm are defined by photolithography. Scribe

(a)

(b)

(c)

Fig. 3. Cross-sectional illustration of layers in substrates (not to scale).(a) AlOx . (b) PI. (c) Cyclotene.

Fig. 4. (a) Copper-based plate before die attach. (b) Copper-based plate afterdie attach of SiC die. (c) Specimen after molding.

lines 120-μm wide, are formed by a sequence of wetand CF4/O2 plasma etchings of the PI and TEOS film.

3) A polymer passivation layer of Cyclotene 3022-46 DowChemical Co. is deposited onto a 3-in SiC wafer. Thecomplete stack consists of BSM/SiC/Cyclotene (4 μm)(hence named passivation C). This wafer is not pat-terned. Fig. 3 shows the cross-sectional (not to scale)illustration of all the layers involved in the completespecimens.

The wafers are mechanically sawed to obtain12.7 × 12.7 mm dice. The dice are attached to 25 × 25 mmcopper plates using Pb-5%Sn (wt. %) soft solder in an H2furnace. Fig. 4 shows the optical pictures of the copper plate,dice, and the complete specimen. Fig. 5 shows the picturesof the specimens preparation phases. The yield after thedie-attach process is 15 specimens for Al oxide, 18 specimensfor PI dice, and ten specimens for C passivation dice.

The molding compounds used in this paper are of thecommercial type. In particular, compound A is a green (i.e., Brand antimony oxide free) EMC having organic phosphorous-based flame retardant.

Compound B is a commercial EMC formulated with Br andantimony flame retardant additives. The main chemical andphysical properties of both compounds A and B are reportedin Table I.

The molding process is done at 175 °C and 1.38 MPa. Themolding is followed by postcuring of 8 h at a temperatureof 175 °C. As some substrates are broken during molding,



TABLE I

MAIN CHEMICAL AND PHYSICAL PROPERTIES OF COMPOUNDS A AND B

Compound Properties Unit A BMatrix — Epoxy–phenolic Epoxy–phenolic

Flame retardant — “Organic” phosphorous Br + Sb2O3Flammability UL-94 — V-0 V-0

Glass transition temperature °C 145 150CTE α1 (ppm °C−1) 16 18CTE α2 (ppm °C−1) 55 67

Thermal conductivity (25 °C) W m−1 K−1 0.8 0.6Flexural strength (20 °C) MPa 125 100Flexural modulus (20 °C) GPa 14 12

Volume resistivity at 150 °C � cm 2 × 1012 1 × 1012

Conductivity of extracted water mS m−1 7.5 5pH of extracted water pH 6 4.5Concentration of Na+ ppm 6 10Concentration of Cl− ppm 10 10

TABLE II

SPECIMEN YIELDS FOR EACH SURFACE–EMC COMBINATION

Molding CompoundA B

AlSiCu 8 7PI passivation 9 8C passivation 5 5

Fig. 5. Specimens preparation procedure.

the final yield is 15 for Al oxide, 17 for PI passivation,and ten specimens with C passivation. Table II shows thesample sizes for each combination of test surface and moldingcompound. To enhance the aging effects on EMC interfaces,several specimens are submitted to HTS at 250 °C andthen studied following the previously reported experimentalprocedure starting from Step 4.

EMC-substrate adhesion strength is measured by tensilepull test at 24 °C and a relative humidity of 50%, using theINSTRON 4501 dynamometer equipped with a 1-kN load cell,for which accuracy is ±0.002% of load-cell capacity. In thistest, carried out at a crosshead speed of 16.7 μm s−1, the loadis applied perpendicular to the resin–metal interface plane.Fig. 6 shows a picture of the pull test fixture with a specimenmounted to the INSTRON for the adhesion measurement.The INSTRON instrument has two axially aligned grips. Thebottom grip is rigid with the instrument frame, whereas theupper grip is mechanically coupled to the INSTRON crossheadto maintain axial alignment during the pull test. The bottomgrip is used to mechanically fix the pull test fixture that iscomposed mainly of two plates. The lower plate is fixed tothe bottom instrument grip, orthogonally. The upper plate

Fig. 6. Picture of pull test fixture with specimen mounted to INSTRON foradhesion measurement.

is parallel to the lower plate and is milled to permit theinsertion of the aluminum stud. The top grip is used to securethe aluminum stud of the sample for the measurement. Thesample substrate is not constrained in an axially rigid stateduring the crosshead travel until it reaches the upper plateand is blocked. At this point the load increases until thesample breaks. The load-displacement curve is recorded. Themaximum load at break is used to determine the adhesionstrength. This alignment and fixture scheme prevent torqueand peel forces when the sample is pulled.

The fractured surfaces of the substrate and the EMC areanalyzed by XPS using an AXIS-HS Kratos Analytical spec-trometer equipped with a Mg and Al dual anode X-ray source.Here, the Mg kα1,2 of 1253.6 eV is used.

SAM is used to analyze the delamination of the moldingcompound–substrate interfaces. This analysis is performedusing a SONIX instrument equipped with a piezoelectrictransducer that acts both as an ultrasonic wave pulser workingat a frequency of 50 MHz and as a receiver with sampling rateof 200 MHz. For the analysis, the sample and the transducerare acoustically coupled to a water bath. Delaminated inter-faces are characterized by the phase inversion of the acousticwave. Here, delaminated areas are colored in red, whereasareas colored in grey are nondelaminated. The interfaces areconsidered delaminated assuming a delamination threshold of



Fig. 7. Upper part: XPS survey spectrum of AlSiCu metal. Lower part: Al2pspectral region.

96% i.e., when the intensity of the signal with inverted phaseis 96% with respect to the intensity of the reflected power.

The effects of molding compound formulations on thereliability of devices are studied employing SiC-based powerMOS devices encapsulated in TO220 plastic packages withcompounds A and B. The devices are subjected to HTRB testat VS = 960 V, Ta = 225 °C, up to 1000 h [22].

III. RESULTS AND DISCUSSION

A. Substrate Surfaces Characterization

Fig. 7 (upper part) shows the survey spectrum of oxidizedAlSiCu metal that shows the Al, Si, Ti, C, and O signals. Fluo-rine signals are not visible in the survey spectrum, although itssurface atomic concentration is ∼2% (also see Table III). Thepeak of F1s (not shown) is centered at 686.2 eV of bindingenergy that is assigned fluorine bonded to aluminum oxidein the form of Al(OF)x or AlFx [23], [24]. Fluorine is oftenpresent on aluminum pad surfaces of devices. It originatesfrom the fluorocarbon gas plasma that is used to selectivelyetch the oxide or nitride passivation. Fluorine comes from theprocesses used to define the scribe lines. Fig. 7 (lower part)shows the spectral region of Al2p that shows the correspondingpeak, centered at 74.7 eV, assigned to Al oxide [23]. Themetallic component Al° is not visible in the spectrum. Weattributed this to masking by the oxide layer, which is >7.2-nmthick (more than three times the inelastic mean free path ofAl2p electrons into Al2O3) [23]–[26]. Further investigations(not shown) based on Ar+ 5-keV ion beam sputter depthprofiling, indicate an oxide thickness of 15 nm. Moreover, Tiand Si are also in the form of oxides: TiO2 and SiO2.

Fig. 8 (upper part) shows the XPS survey spectrum of the PIpassivation layer showing the C, F, O, N, and Si, signals. Fig. 8(lower part) shows the C1s spectral region that can be deconvo-luted, in a first approximation, using five Gaussian components

TABLE III

SURFACE COMPOSITIONS (% ATOMS) OBTAINED BY XPS ANALYSES

C N O F Al Si Ti AuAlSiCu 15.5 — 55.5 1.9 17.5 8.2 1.5 —

PI passivation 64.4 2.7 14.8 16 — 2 — —C passivation 69.9 — 20.3 6.9 — 1.6 1 0.2

Fig. 8. Upper part: XPS survey spectrum of PI passivation. Lower part: C1sspectral region.

centered at 284.7, 286,4, 288.7, 290.6, and 292.5 eV of bindingenergy. The components are assigned to the functional groupsC–C/C–H, C–O–C, (C* = O)–N, to shake up and to CFx,respectively [23], [27], characteristic of fluorinated PI. Fig. 9(upper part) shows the XPS survey spectrum of the cyclotenepassivation layer that shows the C, F, O, Si, Ti, and Ausignals. Fig. 9 (lower part) shows the C1s spectral regionthat can be deconvoluted, in first approximation, using threeGaussian components centered at 284.9, 287.5, and 290 eV ofbinding energy. The components are assigned to the functionalgroups C–C/C–H, –C*H2–CHF/R–CH2–OH, R–(C* = O),(C* = O)–O, –CHF, respectively [23], [27]. Cyclotene isa resin based on divinylsiloxane-bisbenzocyclobutene (DVS-bisBCB) polymer [28]. Significant amount of detected oxygen,compared with that expected from the polymer stoichiometry(3.8% atoms), is because of light surface oxidation, whereasfluorine is a contaminant. The source of Ti and Au is likelythe backside metal. Table III shows the surface compositions(% atomic) obtained by XPS analyses.

B. Interface Analysis by SAM

Fig. 10(a) and (b) shows the SAM pictures of speci-mens for Al oxide–compound A and Al oxide–compoundB not aged, e.g., as molded. Both compounds exhibitthe presence of large delamination (red colored regions).So, Al oxide-based specimens are not subjected to an HTS test.



Fig. 9. Upper part: XPS survey spectrum of C passivation. Lower part: C1sspectral region.

Fig. 10(c) and (d) shows the SAM pictures of specimensprepared with PI passivation. Compound A shows the absenceof delamination in all specimens, whereas compound Bexhibits three-delaminated and five-not delaminated pieces.The behavior of the specimens made with compound Bis due to adhesive-type fractures involving, in addition tothe PI–molding compound, the TEOS–PI interface (see alsoSection III-D and Fig. 11(i)–(n). Fig. 10(e) and (f) showsthe SAM pictures of specimens prepared with C passivation.Compound A shows the absence of delamination, whereas thespecimens prepared with compound B are fully delaminated.Fig. 10(g)–(i) shows the SAM pictures of PI–compound A,C passivation–compound A, and PI–compound B after 200 hof HTS at 250 °C, respectively. Only the system formed ofcompound A–PI shows a partial delamination, whereas theother two systems show large delamination.

C. Adhesion Strength Results

Table IV shows the adhesion strength of compound A andcompound B onto Al oxide (not aged). The adhesion strengthfigures (including the mean value and standard deviation)are reported in the same numerical order as reported inFig. 8(a)–(f) (from top-left to down-right). Some specimensshow the fracture of SiC substrate or at the interface betweenTEOS and polymer passivations. The adhesion strength ofcompound A is higher than compound B. The significantspread in data for these measurements may be due to sampleto sample variation. Sample-to-sample variation is producedby segregation of releasing agent at the interface between themolding compound and the substrate. Moreover, inhomogene-ity in the concentration of the adhesion promoter at interfacescould occur. Releasing agent and adhesion promoter are twoimportant EMC additives in competition [15]. Additionally,the spread in data is caused by the fractures occurring at the

interfaces involving the underlying film rather than the EMC(see Fig. 3 and Section III-DD).

Table V shows the adhesion strength of compounds A and Bonto PI passivation. High spread is observed also for the PI testsurface. The untested specimens are used for the HTS test. Theadhesion strength of compound A is higher than compound B.

Table VI shows the adhesion strength of compound A andcompound B onto passivation C. Even here, the untestedspecimens are used for the HTS test. The adhesion strength ofcompound A is higher than that of compound B. In particular,compound B onto passivation C shows very low adhesionstrength.

A good correlation can be observed between the level ofdelaminated interfaces (roughly the percentage of delaminatedarea) and the adhesion strength in all specimens.

Table VII shows the adhesion strength of compounds A andB onto passivation PI and C after 200 h of HTS at 250 °C.According to the delamination data, only the combination ofPI–A maintains significant adhesion strength after an HTStest, whereas the systems PI–B and C–A show a significantdecrease in the adhesion strength.

Table VIII shows the data reported in Tables V–VII. Theadhesion strengths of the systems PI–A, PI–B, and C–A arenot aged and after performing 200 h of HTS at 250 °C arecompared. The last column of the table shows the percentageof adhesion loss as consequence of degradation during thestress test. The significant adhesion loss observed after 200 hof HTS at 250 °C indicates the severe nature of this test. Inparticular, according to Goroll and Pufall [18], the adoptedtest condition is roughly ten times more accelerated than testcondition of 150 °C that has a typical duration of 1000 h.

D. Fracture Surface Analysis

Fig. 11(a) and (b) shows the XPS survey spectra of thesubstrate side and the resin side, respectively, of the delami-nated Al oxide–A system (pull strength 10.8 N). The spectrumof the substrate side shows the C, O, Ti, Si, and Al signals.The spectrum of resin side shows the C, O, N, Si, and Psignals. Fluorine detected on the pristine Al oxide surface actsas a catalyst in the presence of water, so that hydrates andhydroxides on the surface form hydrofluoric acid, etching thealuminum and creating additional hydrates, oxides, and oxyflu-orides [24]. According to the chemical species detected on thefractured surfaces, the Al oxide–compound A specimens showan adhesive-type failure mode.

Fig. 11(c) and (d) shows the XPS survey spectra of thesubstrate side and the resin side, respectively, of the nonde-laminated PI–A system (pull strength 363.6 N). The spectrumof the substrate side shows the C, O, N, Si, and P signals. Thespectrum of the resin side shows the C, O, N, Si, and P signals.According to the chemical species detected on the fracturedsurfaces, the PI–compound A specimens show a cohesive-typefracture in the molding compound. This result is in agreementwith the high adhesion strength of the system.

Fig. 11(e) and (f) shows the XPS survey spectra ofthe substrate side and the resin side, respectively, ofthe nondelaminated C–A system (pull strength 239.5 N).



(f)

(b)

(c)

(d)

(e)

(a)

(g)

(h)

(i)

Fig. 10. (a) SAM pictures of specimen Al oxide-compound A not aged. (b) SAM pictures of specimen Al oxide-compound B not aged. (c) SAM picturesof specimen PI passivation-compound A not aged. (d) SAM pictures of specimen PI passivation-compound B not aged. (e) SAM pictures of specimen Cpassivation-compound A not aged. (f) SAM pictures of specimen C passivation-compound B samples not aged. (g) SAM pictures of specimen PI passivation-compound A, after 200 h HTS at 250 °C. (h) SAM pictures of specimen C passivation-compound A, after 200 h HTS at 250 °C. (i) SAM pictures of specimenPI-compound B, after 200 h HTS at 250 °C.

The spectrum of the substrate side shows the C, O, andSi signals. The spectrum of the resin side shows the C, O,and Si signals. According to the chemical species detectedon the fractured surfaces, the C passivation–compoundA specimens show a cohesive-type fracture in the passivationlayer.

Fig. 11(g) and (h) shows the XPS survey spectra of thesubstrate side and the resin side, respectively, of the delam-inated Al oxide–B system (pull strength close to zero). Thespectrum of the substrate side shows the C, O, Ti, Si, and Alsignals. The spectrum of the resin side shows the C, O, andSi signals. According to the chemical species detected on the



Fig. 11. XPS survey spectra of fracture surfaces obtained by pull test. (a) Al oxide side-A. (b) Al oxide-A side. (c) PI side-A. (d) PI-A side. (e) C passivationside-A. (f) C passivation-A side. (g) Al oxide side-B. (h) Al oxide-A side. (i) PI side-B delaminated. (l) PI-B side delaminated. (m) PI side-B not delaminated.(n) PI-B side not delaminated. (o) C passivation side-B. (p) C passivation-B side.

fractured surfaces, the Al oxide–compound B specimens showan adhesive-type fracture.

Fig. 11(i) and (l) shows the XPS survey spectra of the sub-strate side and the resin side, respectively, of the delaminated

PI–B specimen (pull strength 17.1 N). The spectrum of thesubstrate side shows the C, O, N, and Si signals. The spectrumof the resin side shows the C, F, O, and N signals. Accordingto the chemical species detected on the fractured surfaces, the



TABLE IV

ADHESION STRENGTH (NEWTON) OF COMPOUND A AND COMPOUND B ONTO Al OXIDE (NOT AGED)

Surface AlOxSpecimen Number* Compound A Specimen Number* Compound B

1 65.5 1 Not measurable**2 70.7 “cold resin” 2 4.73 9.6 3 79.1 die fractured4 142.9 4 5.95 133.3 5 3.76 44.9 die fractured 6 14.8 die fractured7 10.8 7 6.78 84.9 — —

Mean value 70.3 19.1Standard deviation 49.7 29.6

TABLE V

ADHESION STRENGTH (NEWTON) OF COMPOUND A AND COMPOUND B ONTO PI (NOT AGED)

Surface PISpecimen Number* Compound A Specimen Number* Compound B

3 363.6 3 72.14 297.3 6 279.3 (PI–TEOS interface)5 226.4 7 1.99 113.4 (die fractured) 8 17.1 (PI–TEOS interface)

Mean value 250.2 92.6Standard deviation 107 128

TABLE VI

ADHESION STRENGTH (NEWTON) OF COMPOUND A AND COMPOUND B ONTO PASSIVATION C (NOT AGED)

Surface CSpecimen Number* Compound A Specimen Number* Compound B

4 258.2 1 1.85 239.5 2 2.4

3 Not measurable**4 21.55 Not measurable**

(passivation–substrateinterface)

Mean value 248.8 8.6Standard deviation 13.2 11.2*Referred to the numbering of SAM pictures [Fig. 8 (a)–(f)].**Adhesion strength close to 0.

TABLE VII

ADHESION STRENGTH (NEWTON) OF COMPOUND A AND COMPOUND B ONTO PASSIVATION PI AND C AFTER 200 h OF HTS AT 250 °C

Surface PI

Specimen Number* Compound A Specimen Number* Compound B

1 53.7 1 16.62 127.6 2 25.03 107.4 — —4 208.4 — —5 186.8 — —

Mean value 136.8 20.8Standard deviation 62.2 5.9

Surface C

1 0 — —2 24.1 — —3 19.2 — —

Mean value 14.4 — —Standard deviation 12.7 — —

*Referred to the numbering of SAM pictures [Fig. 8 (g)–(i)].

PI–B specimens show adhesive-type fractures involving bothTEOS–PI and PI–molding compound interfaces.

Fig. 11(m) and (n) shows the XPS survey spectra of thesubstrate side and the resin side, respectively, of the nondelam-

inated PI–B specimen (pull strength 279.3 N). The spectrumof the substrate side shows the C, F, O, and Si signals. Thespectrum of the resin side shows the C, F, O, and N signals.According to the chemical species detected on the fractured



TABLE VIII

COMPARISON OF ADHESION AT t = 0 AND AFTER 200 h OF HTS AT 250 °C. PERCENTAGE OF ADHESION LOSS IS REPORTED

t = 0 HTS 200 h at 250 °C % of Adhesion LossSurface–Compound

Combination

PI–A 250 ± 107 136.8 ± 62.2 45

PI–B 92.6 ± 128 20.8 ± 5.9 77.5

C–A 248 ± 13.2 14.4 ± 12.7 94

Fig. 12. Typical XPS C1s spectra. (a) Compound A of nondelaminatedspecimen. (b) Compound A of delaminated specimen.

surfaces, the PI–compound B specimens show adhesive-typefractures involving both TEOS–PI and PI–molding compoundinterfaces.

A high fluorine concentration is detected on the resinfracture surfaces in both delaminated and nondelaminatedspecimens, PI–B. The fluorine comes from the PI film (F1speak is centered at 688.3 eV) that detaches from the TEOSlayer and remains bonded to the resin. As the TEOS–PIinterface is not involved in the specimens based on compoundA, we may conclude that this interface is influenced by thechemical composition of compound B.

Fig. 11(o) and (p) shows the XPS survey spectra of thepassivation side and the resin side, respectively, of the delam-inated C passivation–B specimen (pull strength close to zero).The spectrum of the passivation side shows the C, O, andSi signals. The spectrum of the resin side shows the C, O,and Si signals. According to the chemical species detected onthe fractured surfaces, the C passivation–B specimens showadhesive type fracture at passivation-SiC and passivation EMCinterfaces.

Polar chemical species at interfaces are identified study-ing the XPS C1s spectra. Fig. 12(a) shows the XPS C1sspectra of compound A of a nondelaminated specimen. Thespectrum can be deconvoluted in a first approximation, using

Fig. 13. Typical XPS C1s spectrum of compound B of delaminated specimen.

three Gaussian components centered at binding energies of284.8, 286.5, 291 eV assigned to C–C/C–H, C–O–C andshake up components, respectively [22], [26]. The spectrumof delaminated specimen [Fig. 12(b)] can be deconvolutedin a first approximation, using four Gaussian componentscentered at binding energies of 284.8, 286.5, 289, and 291 eVand assigned to C–C/C–H, C–O–C, R–(C* = O)–OR′ andshake up components, respectively [23], [26]. The additionalcomponent R–(C* = O)–OR′ can be due to the segregationof releasing agents (waxes) that are often responsible forthe interface delamination. Fig. 13 shows a typical XPSC1s spectrum of compound B of a delaminated specimenthat can be deconvoluted using five components centeredat 284.8, 286.5, 288, 289, and 291 eV assigned to C–C/C–H, C–O–C, R(C* = O), R–(C* = O)–OR′ and shake upcomponents, respectively. The higher intensity of componentsassigned to polar oxygenated chemical groups in the spectrumof compound B of Fig. 13 with respect to the spectra ofcompound A of Fig. 10(a) and (b) [29], [30].

Fig. 14(a)–(f) shows the XPS survey spectra of fracturedsurfaces obtained by tensile pull testing of the specimensubjected to storage of 200 h at 250 °C: 1) PI–A, substrateside; 2) PI–A, resin side; 3) PI–B, substrate side; 4) PI–B,resin side; 5) C–A, substrate side; and 6) C–A, resin side.

The elements detected on the fractured surfaces and theirconcentrations are reported in Table IX. In particular, thesystems based on PI show the presence of fluorine on theresin side indicating a partial fracture in the substrate layer.Ionic fluorine, which indicates PI degradation, is not detected.The system PI–B, in addition, exhibits the segregation ofantimony and Br at the interface. Moreover, both compounds,even in absence of delamination, exhibit the segregation of apolar additive at the interface, as shown in the example of theXPS C1s spectrum of Fig. 15.



TABLE IX

TYPICAL SURFACE COMPOSITIONS (% OF ATOMS) OF FRACTURE SURFACES AFTER 200 h OF HTS AT 250 °C

C O Si F N Sb Br P (Organic)PI–A resin side 67.8 20.3 3.5 4.8 2.7 — — 0.9

PI–A substrate side 67.9 20.5 3.5 3.8 2.5 — — 1.7PI–B resin side 79 8.9 — 7.4 3.9 0.2 0.5 —

PI–B substrate side 75.7 12.2 2.3 6.8 1.8 0.4 0.7 —C–A resin side 75.4 16.6 0.9 1.3 4.2 — — 1.7

C–A substrate side 73.4 19.5 1.9 0.8 3.0 — — 1.4

Fig. 14. XPS survey spectra of fracture surfaces obtained by pull test ofspecimen after 200 h at 250 °C. (a) PI-A substrate side. (b) PI-A resin side.(c) PI-B substrate side. (d) PI-B resin side. (e) C-A substrate side. (f) C-Aresin side.

Fig. 15. Typical XPS C1s spectrum of compound A after 200 h of HTS at250 °C.

E. HTRB Results

Table X shows the results of the HTRB test of SiC powerMOS devices assembled in plastic packages with compounds

TABLE X

RESULTS OF HTRB TEST ON SiC POWER MOS DEVICES. NUMBER OF

REJECTED DEVICES WITH RESPECT TO TESTED DEVICES. TEST

CONDITIONS: Vs = 960 V, Ta = 225 °C

Molding Compound Time (h)

168 500 1000

A 0/8 0/8 2/8

B 0/9 0/9 8/9

A and B. The devices that are encapsulated employing mold-ing compound containing flame retardant based on organicphosphorus (A) performed better than devices encapsulatedwith molding compound containing flame retardant based onBr and antimony (B). In particular, at 1000 h the rejecteddevices with respect to the number tested are 2/8 and 8/9 forcompounds A and B, respectively. This result is in agreementwith the physical and chemical features of molding com-pounds, earlier described, which indicate superior performanceof compound A with respect to B. Previous work shows thatdevices assembled in TO220 packages with the organic phos-phorous flame retardant component in the molding compoundperformed even better than devices assembled in hermetic TO3metal packages [31].

Our findings about device reliability can be explainedconsidering how the physical and chemical properties ofEMCs can influence the leakage current and other electricalparameters in microelectronics devices. The most importantmolding compound parameters that need to be consideredare concentration of ions (both as contaminants as well aschemical reaction byproducts) and the presence of polarchemical species at the EMC–die interface such as waxes,polyoxyalkylenethers, and siloxanes having the functions ofreleasing and kneading agents, respectively [20]. The presenceof ions and polar chemical species at the EMC–die interfacecan be measured by performing XPS, utilizing the meth-ods reported or after mechanical detachment of real plasticpackages [30]. The reported results clearly show the minorconcentration of polar additives at interfaces of compound Acompared with compound B.

Important sources of halogen and metallic ions are repre-sented by the presence of brominated resin and antimony (III)oxide. Br also can act as catalyst for metal corrosion, partic-ularly aluminum, which in turn produces additional ions anddevice failure. Aluminum metal passivation is a good remedyagainst corrosion [25], [26]. Adhesion strength that influencesthe delamination, moisture, and ionic migration at interfaces



is also found to be a critical attribute of molding compounds.Again, compound A shows better adhesion performance thancompound B. Molding compounds having a flame retardantbased on organic phosphorous usually exhibit low ioniccontaminants in comparison with the other EMCs. Moreover,the phosphorous containing groups also act as ion catchers.This leads to overall better reliability in comparison with theother class of flame retardants.

IV. CONCLUSION

The adhesion and interface compositions of two EMC(a “green” compound A and a conventional compound B)molded onto aluminum oxide-hydroxide surfaces (oxide grownonto AlSiCu) and onto two die passivation layers, consistingof fluorinated PI and Cyclotene, were studied.

This research was done by correlating interface chemicalcompositions (XPS data) with delamination (measured bySAM) and adhesion strength.

Both compounds showed poor adhesion onto the Al oxidesubstrate with adhesive fracture. Mainly, poor adhesion maybe ascribed to the presence of fluorine that was bonded toaluminum oxide as oxyfluoride Al(OF)x or fluoride AlFx.

Compound A, for which formulation produced a low con-centration of polar additives at interfaces, showed high adhe-sion strength onto both passivation layers that correspondedto an absence of delamination. Compound B showed pooradhesion onto all the substrates that corresponded to interfacedelamination.

Fractures under pull test occur on the basis of differentmechanisms. In the polymer passivation layers, the green com-pound showed cohesive fracture consistent with high adhesionstrength. Compound B showed adhesive fractures for the EMCto both top layer interfaces (PI and cyclotene) and/or theinterface between passivation and the underlying layer. HTSat 250 °C was used to study chemical modifications occurringat the earlier interfaces. The combination of green compoundA with PI substrate showed the best resistance against aginginduced by the HTS test.

HTRB test of SiC power MOS devices encapsulated withthe compounds (A) showed better reliability performancein comparison with devices encapsulated using the moldingcompounds (B). This result was in good agreement with thephysical and chemical molding compounds properties.

The results reported here indicated promising use of thecompound A for plastic packages operating at high tempera-ture (up to 225 °C).

ACKNOWLEDGMENT

The authors would like to thank M. Saggio and E. Zanetti(STMicroelecronics) for providing some of the substrates usedin this paper.

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Antonino Scandurra photograph and biography are not available at the timeof publication.

Giuseppe Francesco Indelli photograph and biography are not available atthe time of publication.

Roberto Zafarana photograph and biography are not available at the timeof publication.

Angelo Cavallaro photograph and biography are not available at the time ofpublication.

Emanuele Scrofani photograph and biography are not available at the timeof publication.

Jean Paul Giry photograph and biography are not available at the time ofpublication.

Sebastiano Russo photograph and biography are not available at the time ofpublication.

Mietek Bakowski photograph and biography are not available at the time ofpublication.

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[SiC-En-2013-22] Molding Compounds Adhesion and Influence on Reliability of Plastic Packages for SiC-Based Power MOS Devices