Advances in Pulsed-Laser-Deposited AlN Thin Films for High ... · In this paper we report recent...

Advances in Pulsed-Laser-Deposited AlN Thin Filmsfor High-Temperature Capping Device Passivationand Piezoelectric-Based RF MEMSNEMSResonator Applications

SS HULLAVARAD13 RD VISPUTE14 B NAGARAJ1 VN KULKARNI1

S DHAR1 TVENKATESAN1 KA JONES2 M DERENGE2 T ZHELEVA2

MH ERVIN2 A LELIS2 CJ SCOZZIE2 D HABERSAT2

AE WICKENDEN2 LJ CURRANO2 and M DUBEY2

1mdashCenter for Superconductivity Research Department of Physics University of MarylandCollege Park MD 20742 2mdashUS Army Research Laboratory Sensors and Electron DevicesDirectorate Adelphi MD 20783 3mdashE-mail shivasquidumdedu 4mdashE-mail visputesquidumdedu

In this paper we report recent advances in pulsed-laser-deposited AlN thin filmsfor high-temperature capping of SiC passivation of SiC-based devices and fab-rication of a piezoelectric MEMSNEMS resonator on Pt-metallized SiO2Si TheAlN films grown using the reactive laser ablation technique were found to behighly stoichiometric dense with an optical band gap of 62 eV and with a sur-face smoothness of less than 1 nm A low-temperature buffer-layer approach wasused to reduce the lattice and thermal mismatch strains The dependence of thequality of AlN thin films and its characteristics as a function of processing param-eters are discussed Due to high crystallinity near-perfect stoichiometry andhigh packing density pulsed-laser-deposited AlN thin films show a tendency towithstand high temperatures up to 1600degC and which enables it to be used as ananneal capping layer for SiC wafers for removing ion-implantation damage anddopant activation The laser-deposited AlN thin films show conformal coverage onSiC-based devices and exhibit an electrical break-down strength of 166 MVcmup to 350degC when used as an insulator in NiAlNSiC metalndashinsulatorndashsemi-conductor (MIS) devices Pulsed laser deposition (PLD) AlN films grown on PtSiO2Si (100) substrates for radio-frequency microelectrical and mechanicalsystems and nanoelectrical and mechanical systems (MEMS and NEMS) dem-onstrated resonators having high Q values ranging from 8000 to 17000 in thefrequency range of 25ndash045 MHz AlN thin films were characterized by x-raydiffraction Rutherford backscattering spectrometry (in normal and oxygenresonance mode) atomic force microscopy ultraviolet (UV)ndashvisible spectro-scopy and scanning electron microscopy Applications exploiting character-istics of high bandgap high bond strength excellent piezoelectric characteristicsextremely high chemical inertness high electrical resistivity high breakdownstrength and high thermal stability of the pulsed-laser-deposited thin filmshave been discussed in the context of emerging developments of SiC powerdevices for high-temperature electronics and for radio frequency (RF) MEMS

Key words AlN SiC pulsed laser deposition MEMSNEMS passivationdopant activation

(Received August 29 2005 accepted November 22 2005)

Journal of ELECTRONIC MATERIALS Vol 35 No 4 2006 Special Issue Paper

777

INTRODUCTION

The wide bandgap Group III nitrides combinefundamental physical and chemical properties thatmake them one of the most promising semicon-ductor material systems for the fabrication of alarge variety of optical and electronic devices capa-ble of performing at extreme conditions of powerfrequency and temperature and in harsh environ-ments12 The interest in aluminum nitride (AlN)thin films has been increased over the past decadesbecause of the following specific properties thatmake it suitable for a wide variety of applications3

AlN has a wurtzite structure with lattice param-eters a 5 311 A and c 5 498 A4 almost matchableto GaN and SiC demonstrating usefulness as abuffer layer for GaN light-emitting diodes (LED)and electronic devices It exhibits excellent thermalproperties (melting temperature of 3000degC5 anddecomposition temperature 1600degC) and rela-tively high thermal conductivity6 (285 W cm1 degC1)that are useful for very high-temperature andhigh-power applications Because of its tunablerefractive index it is widely used as antireflectivecoatings with a refractive index of 21ndash22 forepitaxial 19ndash21 for polycrystalline and 18ndash19for amorphous films7 AlN has a wide bandgap(62 eV) large dielectric constant of 914 (static) at300degC8 and high electrical resistivity and break-down strength (106 Vcm) that can be useful fordielectric passivation of SiC-based devices It alsoexhibits excellent acoustic properties that makeAlN suitable candidate for resonators surfaceacoustic wave (SAW) and bulk acoustic wave(BAW) applications9 AlN along with other IIIndashVand IIndashVI compounds can be synthesized in water-based colloidal suspensions for use in colloidal semi-conductor quantum dots both as biological tagsand as structures that interact with and influencebiomolecules10

Below we discuss details of AlN thin films andtheir characteristics that have been explored in ourstudies for high-temperature surface passivationhigh-temperature dielectric and radio-frequency(RF) MEMS resonator applications

AlN Thin Films for SiC Surface Passivation

As the development in SiC for power electronicscontinues11 the requirements for better SiC mate-rials and processes become more stringent1213 Anexample of this is the need for high-quality n- andp-type SiC with low defect densities and a smoothsurface morphology for the fabrication of high-performance power devices14 Because ion implan-tation is a well-established and accepted technologyin microelectronics studies on doping and annealingof SiC are important15 However in the case of SiCdiffusion coefficients of most of the dopants in SiCare comparatively smaller than Si and hence high-temperature (1600ndash1800degC) annealing processesare required to remove the ion-implantationinduced damage and to electrically activate the

dopants16 However the high-temperature annealingprocess results in increased surface roughness anda change in surface composition of SiC due to thepreferential evaporation of silicon from the surfaceof SiC at elevated temperatures17

Due to high-temperature stability extremely lowchemical reactivity a very small lattice mismatchand similar coefficient of thermal expansion with SiC(bSiC 5 52 3 106 degC1 bAlN 5 40 3 106 degC1)AlN thin films have tremendous potential as aneffective encapsulant layer for high-temperatureannealing of ion-implanted SiC18 AlN films havebeen used successfully to impede the surface degra-dation of SiC caused by the preferential evaporationof silicon when the implanted SiC is subjected to anactivation anneal and this has resulted in an excel-lent activation of both n- and p-type dopants1920 Inorder to preserve surface morphology for high-yielddevice production and to prevent dopant loss anencapsulating layer of AlN is deposited on the SiCto act as a surface seal during annealing Figure 1shows the schematic of the AlN capping processused in dopant activation of SiC The criteria forencapsulant material are that it should withstandhigh temperatures without entering the implantedregion and that it should be easily removed withoutleaving any traces that on the SiC surface

AlN as a Passivation Layer on Side Wallsof SiC-Based Devices

SiC-based metal-oxide semiconductor field-effecttransistors (MOSFETs) and gate-turnoff (GTO)thyristors have found tremendous applications inhigh-power switching applications High-temperatureand high-power applications of these devices willrequire passivation layers that have high dielectricconstants and are stable at high temperaturesunder operating fields of at least 1 MVcm When

Fig 1 Schematic of steps involved in annealing of SiC dopant acti-vation after ion implantation Without AlN cap the surface roughensdue to preferential evaporation of Si leaving carbon on SiC

Hullavarad Vispute Nagaraj Kulkarni Dhar VenkatesanJones Derenge Zheleva Ervin Lelis Scozzie

Habersat Wickenden Currano and Dubey778

high voltage is applied to a device opposite chargesare created at the two contacts of the device Astrong electric field within the device results fromthese charged contacts which are separated by aninsulator As the voltage increases the resultingfield does so proportionally These field lines (calledlsquolsquofringingrsquorsquo lines) can go around the outside of thedevice Fringing lines can impact the device andother nearby electrical devices by causing the airbetween them to break down By depositing a thindielectric passivation layer around the sides ofthe device the fringing lines can be redirected andcontained Figure 2 shows the schematic of the thyr-istor used for AlN passivation In device-passivationtechnology the side-wall coverage and the quality oflayer are key factors in determining the final deviceperformance The use of silicon nitride and othersilicon compounds has been reported for side-walldeposition of VLSI devices21 Table I lists the impor-tant parameters of AlN and also other materialstypically used in SiC thyristor side-wall passivationapplications22

AlN-Based MEMS

AlN is being considered as a promising materialfor the fabrication of piezoelectric micro- and nano-electromechanical systems (MEMS and NEMS)devices such as sensors based on MEMS resona-

tors23 or RF microswitches24 Particularly AlN is adesirable piezoelectric material for MEMS andNEMS resonators for high-frequency filtering appli-cations even though its piezoelectric coefficients areconsiderably lower than other piezoelectric materi-als that are ferroelectric such as lead zirconate tita-nate (PZT)25 The theoretical maximum frequency ofAlN is180 GHz as opposed to60 MHz for PZT26

The Youngrsquos modulus of bulk ceramic AlN at 25degC is345 GPa and the density is 3260 kg m321 Thiscompares to a Youngrsquos modulus of 56 GPa for thin-film PZT and a density of 7600 kgm3 measured forbulk PZT27 Additionally AlN is better suited forthe integration of MEMS devices into silicon-basedelectronics due to its complete compatibility withconventional silicon technologies28 Table II liststhe parameters of AlN and also other important can-didates used in piezoelectric resonator applicationsRecently Lee at al29 have reported the bulkacoustic wave (BAW) sensors fabricated on c-axis-oriented AlN films grown by dc sputtering Theyhave studied the effect of smoothness and residualstress of a metallic layer of Mo on AlN and hence onthe device properties However in the case of abeam- or disc-type MEMSNEMS resonator thecrystalline quality of the underlying metallic elec-trode layer and of the resultant AlN thin film are

Table II Physical Constants of PZT ZnO and AlN Significant to MEMSNEMS Applications

Physical Constants

Materials

PZT ZnO AlN

k2 () (squared piezo coupling factor) 221 8 9e31 (Cm2) (piezo stress constant) 36 037 058d33 (pCN) (piezoelectric coefficient) 268 124 50e33 (dielectric constant) 1200 11 9E (GPa) (Youngrsquos modulus) 25 105 345r (kgm3) (density) 7600 5600 3255v (ms) (acoustic velocity) 4450 5000 10000Theoretical resonant frequency of a 100 mm 3 10 mm 3 1 mmcantilever f0 (kHz)

293 700 1665

Theoretical maximum frequency fmax 5 2k2(lpi3t1) fmax (GHz) 0060 115 181

Highest theoretical f0 for AlN due to optimum r and E

Table I Important Parameters of AlN and OtherMaterials Typically Used in Side Wall Passivation

of SiC-Based Devices

Material e

ECritical

(MVcm)25degC

EOperating

(MVcm)300degC

eEOperating

(MVcm)300degC

SiC 10 3 3 30SiO2 39 11 2 78AlN 85 2 2 17Si 119 04 04 48

Note The parameters EOperating and eEOperating indicate useful-ness of materials for device passivation Fig 2 Schematic of AlN dielectric passivation poised to be used in

the final SiC-based thyristors

Advances in Pulsed-Laser-Deposited AlN Thin Films forHigh-Temperature Capping Device Passivation andPiezoelectric-Based RF MEMSNEMS Resonator Applications 779

considered to be critical to the device characteris-tics The schematic of a Si-based substrate and aconfiguration for RF MEMS resonator prototypedevice configuration are shown in Fig 3

It is clear that AlN thin films have tremendousapplications in the wide bandgap semiconductorresearch while integrating with GaN SiC and Sidevice technologies Novel concepts to use AlN indevice technologies via altering the characteristicsby doping tailoring and alloying are now emergingand hence it is important to consider thin-film pro-cessing aspect while exploiting the intrinsic andextrinsic characteristics of AlN which can bedependent on the thin-film growth and processingconditions Below we review the method of fabrica-tion for AlN thin films and discuss our primaryapproach of using pulsed-laser deposition for rapidprototyping of AlN thin films addressing their appli-cations for surface passivation high-temperaturedielectric passivation and RF MEMS and NEMS

AlN Thin-Film Processing

Several methods have been reported for thegrowth of AlN films such as reactive sputtering3031

reactive molecular beam deposition (RMBD)32 epi-taxial growth33 and chemical vapor deposition(CVD)3435 Among various techniques employed forthe deposition of AlN thin films the reactive sput-tering in any of its typical configurations (DC RFmagnetron etc) has been most widely used becauseit offers several important advantages such as lowdeposition temperature fine tuning of the materialcharacteristics and low processing cost The pulsed-laser-deposition (PLD) technique has also been usedfor the growth of AlN thin films3 The quality of AlNthin films grown by PLD has been shown to be com-parable to that of metal organic chemical vapor depo-sition (MOCVD)- and molecular beam epitaxy

(MBE)-grown AlN on sapphire and silicon36 Scozzieet al37 have investigated the high-temperaturedielectric properties of PLD deposited AlN on 6H-SiC substrates in the range of 25ndash450degC It has beenshown that the PLD grown AlN on SiC-based capaci-tors can withstand dielectric field strengths of up to17 MVcm at 450degC Lelis et al38 have comparedthe currentndashvoltage leakage properties of MOCVDMBE and PLD grown AlN on SiC and showed thatthe Schottky emission at the SiCAlN interfaceappears to be the dominant high-temperature mech-anism in PLD AlN at least for the fields up to2 MVcm Zetterling et al39 in their study onleakage properties of MOCVD-deposited AlN on SiCto grain-boundary attributed conduction due to theextensive island growth occurring in the process

Pulsed-Laser Deposition and Processing

PLD is largely applied for processing of multicompo-nent ceramic thin films and multilayer structures40

The highly nonequilibrium evaporation nature of thePLD process is attractive for the synthesis of stoichio-metric thin films of various metal nitrides and oxidesfrom the corresponding bulk targets PLD appears tobe a suitablemethod for transferring stoichiometricallycomplex monolayer structures from various ceramichard-pressedsintered targets41 to highly epitaxialmdashcrystallinemdashpolycrystalline thin films on to sub-strates42 More details of the processing parametersapplied in the PLD technique for variety of materialsystems can be found elsewhere43 Experimentaldetails pertaining to the fabrication and characteriza-tion of AlN for high-temperature passivation of SiChigh-temperature dielectrics and RF MEMS andNEMS resonators are given below

AlN Thin-Film Growth

Substrates such as silicon sapphire (Al2O3) sili-con carbide and metallized SiO2Si were used forthe AlN thin-film growth experiments Appropriatesubstrate cleaning processes were developed andare discussed in the later sections Cleaned sub-strates were then transferred to the PLD chamberfor AlN deposition having a base pressure of 4 3108 torr The AlN films were deposited in the tem-perature range of 25ndash1100degC A KrF excimer laser(l 5 248 nm t 5 25 ns) fluence of 500 mJ at 10 Hzimpinged on a 2-inch diameter sintered AlN (purity9999 Plasmaterials ultra pure O2 05 Crys-tal IS) target to form the deposition plume and theevaporated species were collected at a distance of15 cm from the target For thin-film growth experi-ments an NH3 pressure of 1 3 104 Torr was usedwith the NH3 line flushed prior to deposition toeliminate excess oxygen contamination For high-temperature growth of AlN films were grown withand without buffer layers When a buffer layer wasused an AlN nucleation layer approximately 200A thick was initially deposited at 800degC followed bydeposition of AlN at 1000ndash1100degC with depositionrate of 80ndash90 Amin Details of substrate and their

Fig 3 (a) Schematic of flexural layers of PtSiO2Si (100) as a sub-strate used to deposit AlN that consists of a supporting SiO2 layer andTiPt bottom metallic layers (b) Schematic of freestanding MEMS AlNbeam resonator (layer thicknesses are not to scale in the figure)



preparation conditions necessary for our specificapplications are summarized below

AlN Film as an Encapsulation Layer on SiCto Activate the Implanted Dopants

For studying the application of AlN thin film forsurface passivation of SiC bare substrates as wellas ion-implanted SiC with device structures wereused The ion-implanted SiC junction barrier diode(JBS) substrates were cleaned with trichloroethy-lene acetone and ethanol in an ultrasonic bathand then cleaned with a 10 HF solution to removeany surface native oxide and contamination Inorder to examine the quality of the AlN films depos-ited directly on the SiC substrate one set of sampleswas grown by depositing AlN films 6000 A thick atvarious substrate temperatures from room temper-ature to 1100degC on a SiC substrate In order tooptimize the temperature and thickness of the effec-tive AlN layer to produce high-quality surface pas-sivation another set of samples were prepared byfirst depositing AlN on SiC substrates at varioustemperatures from room temperature to 1100degCwith thicknesses varying from 50 to 5000 A AnAlN film thickness of 2000 A was found to be opti-mum yielding a pinhole-free smooth and particle-free high-quality passivating layer on SiC ThePLD AlN-capped SiC samples were then annealedto 1300 1500 and 1700degC in an annealing furnacefor 30 min in an argon or nitrogen atmosphere witha flow rate of 130 SLPM (standard liter per minute)and a pressure of 400 torr Before and after anneal-ing the crystalline structure and morphology of thefilm were characterized by x-ray diffraction (XRD)Rutherford back scattering (RBS)-channeling andscanning electron microscope (SEM) techniques

AlN as a Passivation Layer onSiC-Based Devices

SiC substrates were cleaned with trichloroethy-lene acetone and ethanol in an ultrasonic bath andthen cleaned with a 10 HF solution to remove anysurface native oxide and other contaminants After afinal ultrasonic bath in methanol the samples wereimmediately loaded into the vacuum chamber AlNfilms were deposited with a thickness 6000 A atvarious substrate temperatures from room temper-ature to 1100degC on a SiC substrate From the XRDanalysis the AlN films grown at 1100degC are found tobe highly crystalline and these films were used tofabricate the MIS device for studying electrical prop-erties The metalndashinsulatorndashsemiconductor (MIS)device was fabricated with 300 mm 3 300 mm Nimetal pads and a TiN metal bottom contact forIndashV and CndashV measurements The contact metalsNi and TiN were deposited by PLD at 400degC Oncethe electrical characteristics are established theSiC-based devices were passivated with PLD AlNfilm and compared with AlN layers of similar qualityfabricated using sputtering techniques

AlN for MEMSNEMS Resonator Applications

Figure 3a shows the schematic of layers of PtTiSiO2Si (100) used as a composite substrate in thepresent investigation which consists of a flexuralSiO2 layer and a TiPt bottom metal electrodelayers The flexural layer of a 02- to 1-mm-thickSiO2 film was deposited on a 2-in diameter Si waferusing either plasma-enhanced chemical vapordeposition (PECVD) that was rapid thermal an-nealed (RTA) for 1 min at 700degC in N2 or a high-temperature thermal oxide growth method Thinfilms of 250 A Ti and 1400 A Pt were then sputter-deposited at 300degC and subsequently underwentRTA for 1 min at 700degC in air The substrate wasthen transferred to the PLD chamber for AlN depo-sition having a base pressure of 4 3 108 torr Thinfilms 2000ndash5000 A thick AlN were deposited on thePt-terminated composite substrates at tempera-tures ranging from 800 to 1050degC The AlN filmswere characterized using XRD RBS Auger electronspectroscopy (AES) variable-angle spectroscopicellipsometry (VASE) and transmission electronmicroscopy (TEM) XRD analysis of the AlN filmsgrown at 1050degC indicated that they were highlyoriented in the 0001 direction with slight in-plane disorder observed in TEM analysis The c-axisorientation of these high-temperature PLD AlNfilms showed an order of magnitude improvementover typical high-quality sputtered AlN films withnegligible interfacial diffusion observed in AES andRBS analyses of the composite thin-film structureFigure 3b schematically shows the final AlN MEMSresonator after the processing which involves litho-graphic patterning for top electrode metallizationfollowed by selective reactive ion etch processing

Thin-Film and Device Characterization

The AlN films were characterized using XRD andRBS spectroscopy The quantitative analysis of thecrystalline quality composition and interface struc-ture of the films was determined by RBS and ion-channeling techniques For the RBS and ion-chan-neling measurements a 2-MeV He ion beam wasused For the RBS oxygen resonance study theuse of a 305-MeV He ion enhanced the oxygen sig-nal dramatically allowing oxygen detection in filmswith an accuracy of a few percent In the presentinvestigation we have also estimated the oxygencontent in AlN thin films using RBS in the oxygenresonance mode This chemical characterization isvery crucial in the evaluation of AlN thin films asthe oxygen impurity strongly influence the thermalstability as well as the electrical and piezoelectriccharacteristics Details of the device characteriza-tion are provided in the device sections Additionalcharacterization using scanning electron micro-scopy (SEM) atomic force microscopy (AFM) AESand TEM were performed to investigate the filmquality of the thin film surfaces and interfacesThe crystalline quality and lattice constant of theAlN films along the c-axis were measured by XRD


RESULTS AND DISCUSSION

Characteristics of Pulsed-Laser-DepositedAlN Thin Films

Figure 4 shows the x-ray diffraction of PLD AlNon an Al2O3 (0001) substrate grown at 1000degC Thediffraction pattern shows that the AlN film is highlyc-axis oriented along the normal to the substrateThe AlN films deposited at fairly lower growth tem-peratures yielded similar features except that theintegrated intensity of the AlN (0002) peak waslower by 2 orders of magnitude The rocking curvewidths of the AlN (0002) peak were found to varyfrom 007 to 012deg as a function of the growth tem-perature We have found that the growth temper-ature type of substrates (such as Al2O3 4H- and6H-SiC) and substrate surface conditions are themost critical parameters in the growth of AlN het-eroepitaxy For example one challenge of thegrowth of AlN on SiC was it had higher internalstresses in the AlN film than it did on the sapphiresubstrate Figure 5 shows the SEM images of AlNfilms grown directly on SiC substrate It can benoted that the cracks have developed in the epi-taxial AlN film grown at 1000degC (Fig 5a) and thatthe crack density has increased in the film grownat 1100degC (Fig 5b) Note that the SEM micrographclearly indicates the contrast variation due to

charge leakage along the boundaries that are paral-lel to the in-plane axes of AlN epitaxial crystallitesSuch boundaries could have formed due to thestacking mismatch associated with the miscutrelated surface steps on the surface of 8deg and 35degoff axis 4H- and 6H-SiC substrates that can impedethe smooth growth in addition to the thermal andlattice mismatch-induced stresses However whenAlN films were grown on low-temperature AlN buf-fer layers the resulting AlN films were smooth andfree from mechanical defects (Fig 5c) To investi-gate the effect of buffer layer on the quality of theAlN films systematic studies were conducted on theinternal stresses in the AlN film grown with andwithout buffer layer

Model of the AlN Growth Mechanism

The stress-induced cracking is a fundamentalissue in the heteroepitaxy of dissimilar materialsThe stresses in the epitaxial films grown on dissim-ilar materials are generally due to (i) lattice mis-match (difference in the lattice constants and thecrystal structures of the film and the substratematerials) (ii) thermal mismatch (difference in thethermal expansion coefficients between the film andthe substrate) (iii) intrinsic defects formed in thefilm during growth (defects such as vacancies inter-stitials dislocations and stoichiometry variations)and (iv) inadequate surface morphology of the sub-strate The cracks seen in the AlN films are presum-ably due to the defects as no cracking is seen inthe films when a low-temperature buffer layer isemployed Table III shows the lattice-mismatchstrain lattice stress and the thermal stress of AlNfilms grown on SiC substrates at various tempera-tures without the buffer layer and those of filmsgrown on a buffer layer at 1000degC

Figure 6a shows the XRD of AlN films depositedat varying temperatures of 800 900 1000 and1100degC with AlN buffer layer (20 nm thick grownat 600degC) on SiC All the films show only the (0002)peak of AlN indicating highly oriented AlN films onSiC Figure 6b shows the c-axis lattice parameteras a function of growth temperature The filmsdeposited in the range of 800ndash900degC have higherc-axis lattice constants than the bulk value of AlN

Fig 4 XRD of AlN films deposited on Al2O3 substrates (a) beforeannealing and (b) after annealing

Fig 5 SEM images of AlN films grown directly on SiC substrate (a) AlN film deposited at 1000degC with no buffer layer (b) AlN film deposited at1100degC with no buffer layer and (c) AlN film deposited at 1000degC with buffer layer at 800degC



As the growth temperature increases the c-axis latticeparameter approaches the bulk value As alreadymentioned the films grown above 1000degC show evi-dence of the formation of cracks What this impliesis that the stresses in the epitaxial films increasebeyond the critical limit and result in the fracturingof the film by which the stress is relaxed Interest-ingly the film deposited at 1000degC with a low-temperature buffer layer also has a c-axis latticeparameter close to that of the bulk indicating aminimal stress in the film without any formationof cracks or mechanical failure Quantitative analy-sis of the defects in the AlN epilayers on SiC withand without buffer layers was carried out usingRBS and ion-channeling techniques The RBS chan-neling indicated a minimum yield of 5ndash8 forepilayers grown with buffer layers as compared to10ndash15 for AlN layers grown without buffer layersRBS studies also indicate that the interface of theAlNSiC (without buffer layer) had a significantamount of defects such as threading dislocationsand low-angle grain boundaries Our ion channelingresults also indicated that the films were poorlyaligned with the substrate when they were grownbelow 850degC The film alignment and hence epi-taxy improved with an increase of growth temper-ature However due to a high nucleation densityand 3D growth mechanisms at higher tempera-

tures defects at the interface are inevitable Wehave also investigated the growth properties ofPLD AlN using AFM Figure 7 shows the surfacemorphology of the AlN films deposited at 1000degCwith and without a low-temperature AlN bufferlayer It is clearly seen that the film deposited at1000degC on unbuffered SiC exhibits mixed two-dimensional (2D) and three-dimensional (3D orisland) growth features and a rough surface mor-phology with a root mean square (RMS) roughnessof about 200ndash250 A (Fig 7a) However the filmdeposited at 1000degC on a 200-A AlN buffer layergrown at 600degC shows 2D growth (or a flat surface)and has a very smooth surface morphology with anRMS roughness of about 1 nm

SEM AFM XRD and RBS clearly demonstratethe role of the growth temperature and buffer layeron the stresses in the heteroepitaxially grown AlNfilms on SiC Although the total amount of the stressdepends on the lattice and thermal mismatches inthe case of AlN epitaxy with SiC mechanical failureor fracturing largely depends on the intrinsicstresses that are associated with the growth mech-anism and defect formation which depends primar-ily on the substrate surface conditions surfacechemistry and termination Figure 8 shows thegrowth mechanisms of AlN films on SiC with andwithout a buffer layer In the case of AlN growth

Table III Lattice and Thermal Strains between the AlN Film Grown on SiC at Different Temperatures

DepositionTemp (degC)

c-AxisLattice

Parameter(A)

LatticeMismatch

()LatticeStrain

LatticeStress(MPa)

ThermalStress (MPa)

800 4994 09 298 3 103 18628 1917900 4992 1 271 3 103 16945 21651000 497 13 779 3 104 +4870 24131000 with LT-AlN BufferLayer

497 12 241 3 105 151 2413

Fig 6 (a) XRD of AlN films deposited on SiC substrate at varying temperatures of 800 900 and 1000degC without the buffer layer and that of AlNfilm deposited at 1000degC on 20-nm-thick AlN buffer layer grown at 600degC (b) c-axis lattice parameter as a function of growth temperature


without the buffer layer planar-type defects areintroduced due to misalignment of the stackingsequence between 6H-SiC and 2H-AlN (Fig 8a)Yamada et al44 while depositing AlN on as-receivedSiC substrates by CVD process have observed simi-lar mechanisms These misalignments are known toproduce incoherent mismatched grain boundariesThese boundaries could pin the dislocations and maycreate compressive strain Also the growth can bepseudomorphic which results in strained films andis consistent with our XRD results To prevent sur-face defects a low-temperature AlN buffer layerwas employed The low-temperature grown AlN buf-fer layer was smooth and covers the entire SiC sur-face (Fig 8b) At growth temperatures of 1000ndash1200degC the crystalline quality of buffered AlNimproves as also reported by Akasaki et al45 Therecrystallized layers usually have minimal straindue to solid-phase epitaxial regrowth46 Further thegrowth of AlN on the recrystallized buffer shouldhave minimal strain because

(a) the epilayer growth mode could be 2D (thelow-temperature thin AlN buffer layer actsas a nucleation layer for 2D growth)

(b) the in-plane growth rate of AlN on the bufferlayer could be significantly higher than that ofwithout the buffer layer surface (due to a low-energy interface between AlNndashAlN growth vsAlNndashSiC interface) and

(c) the homogeneous nucleation of AlN occurs onthe entire surface of the buffer layer

Our results with buffer layers clearly indicatethat the AlN films are relaxed have smooth surfacemorphology and have no cracks

Chemical Analysis Using RBS OxygenResonance Techniques

RBS spectroscopy was also used in the oxygenresonance mode to detect the level of oxygen con-tamination in the AlN films In this technique the

Fig 8 Growth mechanisms of AlN films on SiC (a) without and (b) with buffer layer

Fig 7 AFM images of the AlN films on SiC grown at 1000degC (a) as-grown and (b) with low-temperature buffer layer



energy of incident a particle used to probe the filmis so chosen that it produces resonance with 16O anoxygen isotope By this method oxygen levelspresent even in minor quantities can be detectedFigure 9 shows the oxygen resonance in RBS spec-tra for two AlN films deposited under the sameconditions but from two different AlN targets asdiscussed in the experimental section The spectrawith open and solid circles indicate the AlN filmswith oxygen and without any traces of oxygenrespectively An oxygen level 5 is detected inthe films deposited from the conventional polycrys-talline (Plasmaterials Livermore CA) AlN targetThe oxygen is completely eliminated from the filmby employing the ultrapure AlN target procuredfrom Crystal Is A low level of oxygen impurity inthe AlN films is crucial for electronic applicationssuch as SiC device passivation capping and AlNpiezoelectric MEMS The presence of oxygen impur-ities in AlN has shown to degrade the thermal con-ductivity of AlN47 oxygen impurities in IIIndashVsemiconductors have also been shown to affect theelectrical and optical characteristics48 Additionallyoxygen introduced as interstitials in wurtzite AlNlattices may cause additional compressive stressesThese compressive stresses can only be relievedby diffusion of the oxygen ions from the latticebut this does not take place easily because of thehigh affinity of oxygen to aluminum49

Optical Characterization

Figure 10 shows the optical transmission spectrafor the PLD AlN films on an Al2O3 substrate Thefilms exhibit 80ndash85 optical transmission in thevisible and UV range The PLD film shows a sharpdrop in the optical transmission at 200 nm corre-sponding to a bandgap of 62 eV which is close tothe bulk AlN valueBelow we discuss the applications of PLD grown

AlN films for MEMS encapsulation layers for dop-

ant activation in SiC and high-temperature dielec-tric passivation for SiC power devices

AlN as Encapsulation Layer on SiC to Activate theImplanted Dopants

In the case of p-type doping implantation of boronand aluminum acceptors is a more difficult problemActivation levels for acceptors are frequentlyreported to be less than 10 and are often below1 Compounding the problem of poor activation ofacceptor implants are the unacceptably low inversion-layer mobilities of electrons in many MOS devicesand hence high on-resistances in SiC power MOS-FET devices which otherwise reveal outstandingblocking voltage characteristics Roughening of thesurface region following implant activation anneal-ing may be one of the causes for the low mobilitiesTemperatures in excess of 1650ndash1700degC arerequired to reach high activation levels for boronand aluminum implants in 4H-SiC and their com-plete activation requires a temperature of 1750degCAnnealing ion-implanted SiC in an Ar atmospheredoes cause the implanted surfaces to roughen Caplayers on B- or Al-doped SiC will be more effectivefor maintaining a planar surface In this context wehave examined processing variables that are domi-nant in achieving high activation levels for bothboron and aluminum implants into SiC and weare investigating how processing under differentconditions affects the surface morphology We havedeveloped two approaches for capping single layerand multilayer In the single-layer approach 2000-A-thick PLD AlN film was used Whereas in themultilayer approach the first layer was deliberatelyleft with a thin film of AlN so that the wet chemicaletching feature of AlN can be exploited to removemultilayer capping and second layer was eitherBN When the capping approach was not used Sifrom the top layers of SiC was found to evaporateleaving behind particles of carbon and making the

Fig 10 UVndashvisible spectra of AlN film on Al2O3 substrate depositedat 1100degC

Fig 9 RBS oxygen resonance spectra of oxygen free (solid circle)and oxygen content (open circle) AlN films to determine the oxygenconcentration in the films


device not worthy of any operation Below we discussdetails of capping annealing and surfaces afterannealing SiC as well as structural propertiesSmooth thin films of AlN BN and their multilayerswere grown on n-type and p-type doped SiC

Figure 11a shows the AFM of as-received ion-implanted SiC Figures 11b and 11c show AFMpictures of an implanted SiC surface after annealingat 1600degC in Ar for 30 min with AlN and carboncaps respectively Note that the sacrificial capping

layer was removed by chemical etching as dis-cussed earlier Figure 12 shows the SEM of JBSdiodes when annealed with AlN cap (Fig 12a) andin an SiH4 overpressure to suppress Si evaporation(Fig 12b) These results show a remarkable differ-ence in the surface morphologies clearly indicatingthe usefulness of AlN capping We have noted thatthe thermal stability of AlN degrades at annealingtemperatures above 1600degC We explored otherhigh-temperature materials to meet this require-ment Although there are many materials andalloys available to surpass the 1700degC limit theyare difficult to etch The most suitable structures inthe multilayer form was found to be BNAlN thatare useful for p-type dopant activation

Figure 13 shows the RBS spectra of annealed SiCsamples after the removal of the capping layer Thisstudy investigates the crystal recovery and surfacequality as it undergoes the high temperature annealas a function of implanted doses Table IV shows theRBS xmin for the as-implanted samples and thesamples annealed at 1400 1500 1600 1650 or1700degC for 30 min Although the BNAlN combina-tion works very well for annealing and eventualdopant activation in SiC other important issuesneed to be addressed The most important is thecrystal quality and persistent defects any inducedby the implantation process Jones at al50 have dis-cussed in detail the nature of defects due to ionimplantation and subsequent annealing They haveshown that the extended defect density such as dis-location loops increases The EPR measurements inconjunction with RBS measurements confirm thatthe implanted Al acceptor does not behave in thesame way as one in an epitaxial film does and anew peak is associated with an extended defect asit has little anisotropy51 One possible explanation isthat the point defects created by the implantationprocess coalesce to form more complex structuressuch as dislocation loops Ohno et al52 have shownthat they can be created by room-temperature ionimplantation and can combine to form fewer largerdislocation loops when the sample is annealed Usovet al53 also showed that the dislocation loops can beformed by implantation at elevated temperatures theycan form when the dose is as low as 1 3 1014 cm2and the size of the loops increases with the dose Thusan alternate approach to fabricate high-quality dopedSiC materials is necessary

Selective area growth can be an alternative forthe ion implantation approach In this context weare developing masking layers for selective areagrowth of p-type doped SiC As the growth temper-atures for epi-SiC layers are very high (in the rangeof 1400ndash1800degC) stable masks operating in areducing ambient such as H2 are necessary Thesurface chemistry of the mask material is also animportant factor in determining the selectivity asadsorption decomposition and desorption processeson the mask surface are quite different for the dif-ferent mask materials Our approach has been toinvestigate high-temperature materials such as

Fig 11 AFM of SiC samples (a) as-received (b) after annealing at1600degC in Ar for 30 min and the AlN sacrificial film stripped out(c) after removal of the carbon cap layer post-annealing at 1600degCfor 30 min



TaC BN AlN and their multilayer structures forthis application The family of carbides is suitablefor this application due to their stability in varioustemperature ranges and gas ambient TaC particu-larly seems to be an attractive material due to itshigh-temperature stability in a hydrogen ambientStudies on TaC masking are underway and theresults will be published elsewhere54

AlN as a Passivation Layer on Side Walls ofSiC-based Devices

We have studied AlN as a dielectric passivationlayer on SiC devices The AlN film quality in thecontext of electrical leakage currents and break-down strength is assessed and also as a prelude toimplement AlN for side-wall passivation of SiCdevices37

The electrical leakage current in NiAlNSiC het-erostructures was tested through IndashV measure-ments We measured the capacitance to determinethe dielectric constant at frequencies varying from

100 KHz to 1 MHz of the devices with an HP4194Aimpedancegain-phase analyzer and the currentwith a Kiethley 2400 source meter We observedboth accumulation and depletion behavior of thedevice Capacitance was steady and constant at9 pF for all frequencies scanned in the accumulationmode An empirical value of the dielectric constantfor the AlN film was determined to be in the range of72ndash85 Figure 14 shows the IndashV characteristics ofNi based AlNp-SiCTiN MIS capacitor structuresThe device schematic is shown in the inset of Fig14 The device leakage currents are very low on theorder of 20ndash30 pA at 100 V (not shown in the figureas it lies in the background noise level of the IndashVinstrument used to characterize the device) indicat-ing a significantly higher breakdown strength ofmore than 166 MVcm After completing the room-temperature evaluation we measured the leakagecurrent of MIS capacitors as a function of temper-ature raising the temperature in increments of50degC from 100ndash350degC At elevated temperature theleakage current starts to increase slowly on the

Fig 13 (a) RBS spectra of annealed SiC samples after the removal of capping layer (b) xmin of channeling verses the annealing temperature

Fig 12 SEM of JBS diodes when annealed (a) with AlN cap and (b) with no capping but annealed in SiH4 to suppress Si evaporation Resultsshow a remarkable difference in the cap dependent surface morphologies


order of 100 pA and reaches a maximum of 100 nAat temperature of 350degC at 100 V

The current densities derived from the IndashV char-acteristics in Fig 14 for the dielectric fields ei of 10and 166 MVcm are plotted as a function of inversetemperature in Fig 15 Note that the maximumleakage current densities at our highest measurementtemperature 350degC for dielectric fields ei of 10 and166 MVcm are only 4 3 106 and 1 3 105 Acm2respectively These leakage currents are orders ofmagnitude lower than those that have been previ-ously reported55 for thin films of AlN5657

Figure 16 gives Schottky plots58 for the leakagecurrent densities of Fig 15 for ei of 10 and 166MVcmAll curves give exceedingly linear relationshipsalthough the slope is slightly greater for the caseof 1 MVcm Values for the Richardson constant orcurrent density prefactor for Schottky emissioncan be determined from intercepts of the plots inFig 16 Current prefactors were extracted from thedata that are in remarkable agreement with thetheoretical value of 45 Acm2K2 for the Richardsonconstant for 4H-SiC that is calculated using an effec-

tive hole mass from the formula

A frac14 4pqk2m

h3

where m is the hole mass q is the electroniccharge k is Boltzmannrsquos constant and h is Plankrsquosconstant The hole effective mass mm for 4H-SiCis calculated to be 024 which is in agreement withvalues reported elsewhere to be in the range of022ndash05259 Figure 17 summarizes the comparisonof device yields at various current densities for a100-V dc bias on n- and p-SiC substrates

To study the conformal deposition characteristicsof the pulsed-laser deposition process AlN filmswere deposited on vertically etched patterned Si(for optimization) and then patterned SiC samples60

Figure 18 shows cross-sectional SEM micrographsof the pulsed-laser-deposited AlN samples as afunction of background NH3 pressure Figure 18ashows the AlN film grown under 1 3 104 torr ofNH3 The cross-sectional SEM image clearly indi-cates the deposition of AlN on the basal plane ofthe substrate and no evidence of deposition on thevertical side wall This indicates that the laser depo-sition under 1 3 104 torr is a line-of-sight process

Table IV RBS xmin for As-Grown As-Implanted SiC Samples and Samples Annealed at 1300 1400 1500 16001650 or 1700 degC for 30 min after Removal of the AlN Cap Layer

Sample

xmin xmin

Al 1020

ions cm2AlC 1020

ions cm2Al 1020

ions cm2AlSi 1020

ions cm2

As-Grown 19As-Implanted 61 68 210 166Annealed at 1300degC 43 50 mdash mdashAnnealed at 1400degC 43 43 45 435Annealed at 1500degC 41 45 44 435Annealed at 1600degC mdash 51 mdash 98Annealed at 1650degC 62 77 56 63Annealed at 1700degC mdash mdash 79 78

Fig 15 NiAlNp-SiCTiN capacitor leakage current density as afunction of inverse temperature from 27ndash350degC for several gate fields

Fig 14 Currentndashvoltage characteristics for NiAlNp-SiCTiN capaci-tor over the temperature range 275ndash350degC



which is a typical characteristic of physical vapordeposition As the process pressure of NH3 isincreased to the range of (1ndash7) 3 101 torr out-growth on the vertical wall is clearly seen in Figs18b and 18c The occurrence of the out-growth fea-tures is associated with the morphology on theetched surface on the Si vertical wall Only processpressure parameter was found to play a critical roleon the AlN film coverage on the front and side wallsThe thickness of coverage for three NH3 partialpressures of 104 101 and 7 3 101 torr are084 101 and 104 mm respectively For smoothside walls of SiC complete coverage of AlN layeris seen in Fig 18dIt is interesting to note that the aspect ratio (cov-

erage of the side wall film to the base film) under thehigh-pressure laser-deposition process is close to12 This indicates that the thin-film depositioncan take place if the ablated species inside the laserplume have a significant velocity component per-pendicular to their strike direction Thus thegrowth on the side wall is due to the higher processpressure of (1ndash7) 3 101 torr where the mean freepath of the NH3 molecules is considerably lowerresulting in a high impact probability with theablated particles of the AlN target For the atomicspecies predominantly present in the laser plumethese impacts result in a Brownian-like motion ofthe particles leading to deposition on the verticalwall For better coverage optimization of the PLDAlN film by the tilt-substrate deposition methodmay yield a better aspect ratio61

The use of conventional physical vapor depositionprocesses such as RF magnetron sputtering inwhich deposition is spatial in nature can be morebeneficial Figure 19 shows cross-sectional SEMmicrographs of the RF-sputtered AlN samples onthe side walls of a SiC device Typical growth con-ditions for RF reactive sputtering were 102 torrtotal gas pressure (Ar 1 N2) with 75 N2 150-Wdischarge power and a 400degC substrate tempera-

ture The aspect ratio is close to 1 We have testedthe dielectric properties of the sputtered AlN thinfilms grown under similar conditions and found thatthe leakage currents are an order of magnitudehigher than the best PLD films these observationswill be published elsewhere62


For MEMS and NEMS high-quality AlN films of5000 A thickness were deposited on to PtTiSiO2Sisubstrates at growth temperatures of 1100degC aftera 200-A-thick buffer layer had been grown at 800degCThe MEMSNEMS processing required the films tobe grown on 2-in diameter Si substrates Ellipso-metric analysis of the PLD AlN film showed 4thickness nonuniformity across the 2-in wafer

XRD and RBS analyses were performed on Pt-terminated composite thin-film structures withand without PLD AlN deposition at 1100degC Thecrystalline quality of the AlN films is of significantimportance in the resonant functionality of the result-ing MEMS or NEMS devices Figure 20 shows XRDspectra of PLD AlN grown on PtTiSiO2Si layersThe data (Fig 20a) shows that the AlN film is highlyoriented along the c-axis normal to the substrateFigure 20b shows the x-ray rocking curve of theAlN film The full width at half-maximum (FWHM)of the AlN peak is 025deg indicating a highly orderedfilm in the 0001 axis that represents an order ofmagnitude improvement over high-quality sput-tered AlN films deposited in the same configurationHowever the AlN film is not aligned in-plane due tothe random in-plane orientation of the underlyingPt layer itself Initial analysis of the films usingcross-section transmission electron microscopy(XTEM) also suggested in-plane disorder ie mis-alignment of the AlN grains about the 0001 axisSubsequent in-plane XRD analysis of the 0001-oriented PLD AlN film showed no evidence of in-plane orientation Further analysis revealed that

Fig 16 Schottky plots for NiAlNp-SiCTiN capacitor leakage forgate fields of eI values of 1 125 14 and 166 MVcm

Fig 17 Comparison of device yield at various current densities for a100-V dc bias on n-SiC and p-SiC substrates


although the 111 Pt layer is found to be well-oriented normal to the silicon wafer surface it hasno registry with the underlying SiO2 film

During AlN growth studies for MEMS we noticedthat the thermal stability of the thermally grownSiO2 is significantly better than the PECVD-grownSiO2 Figure 21 shows the RBS spectrum of PLD-grown AlN deposited on PtTiSiO2Si in which theSiO2 was thermally grown at 1000degC using con-ventional oxidation methods RBS analysis of thecomposite structure after high-temperature AlNdeposition was virtually identical to that of the asreceived PtTiSiO2Si substrate and demonstratedsharp interfaces which suggested that no signifi-cant diffusion of Pt Ti or SiO2 occurred duringthe high-temperature PLD process Ti and O alloy-ing in the Pt layer was observed in both the controland PLD sample The edges for Pt from Pt layer andAl from AlN layer are clearly visible in the spectraA small amount of Ti which is used as an adhesionlayer for the Pt film is also visible

We have also studied the effect of PECVD grownSiO2 on the growth of AlN thin films We observedthat the PECVD oxide is highly unstable in PLD-optimized AlN growth conditions (NH3 partialpressure of 104 torr and growth temperature of1000degC) At about 900degC the substrate was ob-served to have bubbles appearing randomly on thesurface Figure 22a shows the optical micrograph of

the AlNPtPECVD-SiO2Si structure formed duringdeposition of AlN The microscopic images have verydistinctive areas of dark and diffused concentricregions distributed over the sample surface Weinvestigated the different regions on the surfaceusing RBS to assess the nature of the interface inthe context of interdiffusion and stoichiometryFigure 23a shows the RBS spectrum of the AlNPtPECVD-SiO2Si structure formed during the

Fig 18 SEM cross-sectional images of PLD AlN side-wall coverages on the vertical wall of the Si and SiC etched samples at NH3 processpressures of (a) 104 (b) 101 and (c) 7 3 101 torr (d) SiC side-wall coverage at 7 3 101 torr

Fig 19 SEM image of RF-sputtered AlN film on vertical wall of theSiC device indicating an aspect ratio close to 1



deposition of AlN The open circles indicate the RBSspectrum taken on the dark spot and the solidcircles indicate the RBS on the diffused concentricrings around the dark spot Interestingly it appearsthat in a region of 20 mm the interface seems brokendown drastically as shown in the RBS spectrumThe Pt signal on the surface (open circles) has com-pletely intermixed with the Ti and the Ti with thethick SiO2 layer (solid circles) However a similarstudy conducted on the same structure with thermallygrown SiO2 shows better stability and no intermix-ing of successive interfaces when raised to similartemperatures during AlN deposition (Figs 22b and23b) This result is of significant interest as it pro-vides feedback to having a suitable type of oxidelayer The stability of the thermal oxide has its ori-gins in the growth temperature of 1000degC as com-pared to 250degC for the PECVD oxide The otherfactor is the stress that is associated with the natureof the oxideSi substrate For thermally grown

oxide there is a compressive stress of 250 MPawhile for the PECVD oxide it has a tensile stressof 0ndash50 MPa depending on the hydrogen contentintroduced inadvertently during the growth Al-though the thermal oxide has a large compressivestress it has excellent thermal stability which makesit an ideal flexural layer substrate in the high-temperature growth of any material in generaland specifically to a piezoelectric material that hasdirect relevance to the stress and Youngrsquos modulus

Figure 24 shows the SEM micrograph of a free-standing AlN MEMS beam resonator device fabri-cated by the standard RIE etching process to definethe beam of dimensions 300 mm in length and 1 mmin width More details about the device processingare found elsewhere28 The quality factor Q of theresonator device was measured for the MEMS AlNresonator and it is defined as Q 5 f0(fmax ndash fmin)3dBwhere f0 is the resonant frequency and fmax and fmin

define the bandwidth of the frequency response TheQ values for AlN fixed-fixed type resonator aremeasured to be 17400 at f0 5 044 MHz and 8000for f0 5 25 MHz Other independent studies haverevealed a striking contrast in the microstructuraldependence on the resonator beam vibrational char-acteristics There seems to be a direct correlationbetween the stresses induced in the film whichmay be process and technique dependent Wicken-den et al28 have shown the buckling effects of theresonator beams in which AlN is grown by a reac-tive sputtering technique and compared the per-formance with AlN films grown by PLD

We have also fabricated AlN NEMS resonatorbeams on the modified flexural layers of SiO2 withthickness 800 A an adhesion layer Ti 100ndash200 Athick and a top metallic layer of Pt 700 A thickThe reduced dimensions of the individual substratelayers are in accordance with the simulation of thelayer thickness and the beam dimensions FinallyAlN thin films of thickness 1500 A were deposited

Fig 20 XRD spectra of PLD AlN grown on PtTiSiO2Si layers (a) The pattern AlN film is highly oriented with (0002) along the c-axis normal tothe substrate (b) X-ray rocking curve of the AlN film

Fig 21 RBS spectrum of PLD-grown AlN deposited on PtTiThermal-SiO2Si


by PLD The NEMS resonator beam 10 mm in lengthand 250 nm thick were fabricated by an electronbeam lithographic technique The details of devicethe processing and characterization are foundelsewhere63

CONCLUSIONS

High-quality epitaxial PLD AlN thin films grownon 4H- as well as 6H-SiC with low-temperature buf-fer layers exhibit structural properties band gapsand dielectric constants close to those of bulk AlNThese thin films have been exploited for surfaceand device passivation of SiC for high-temperatureapplications and RF resonators AlN film in combi-nation with BN film is shown to be a high-temper-ature encapsulation layer and can protect the SiCsurface from dissociation when annealed at a tem-perature up to 1700degC to activate the implanteddopants in SiC No chemical interfacial reactionwas observed indicating excellent material forhigh-temperature capping of SiC We have also

investigated the bulk dielectric properties of 6000-A-thick PLD AlN thin films on p-type 4H-SiC sub-strates from room temperature up to 350degC We havemeasured a leakage current density of 20 mAcm2

at 350degC for a 166 MVcm dielectric field which isorders of magnitude lower than any previousvalues reported for thin films of AlN on p-type 4H-SiC We have studied the PLD and RF magnetronsputtering for the conformal growth of AlN filmsfor side-wall passivation of SiC devices Comparingthe aspect ratios for the conformal coatings it appearsthat the sputtering technique produces better cover-age than the PLD however the leakage currents arean order of magnitude higher than the PLD films

Our studies have also indicated that PLD AlNfilms grown on PtSiO2Si substrates can be usedfor RF MEMS applications It is demonstrated thatthe resulting MEMS resonators showed high Qvalues of 8000ndash17000 for a frequency range of 04ndash250 MHz The high Q values are attributed in partto the highly c-axis oriented AlN films In the cur-rent study we have also addressed the thermal

Fig 22 Optical micrographs of (a) AlNPtPECVD-SiO2Si and (b) AlNPtthermal-SiO2Si samples after deposition of AlN

Fig 23 RBS spectrum of (a) AlNPtPECVD-SiO2Si and (b) AlNPtthermal-SiO2Si samples taken on the surfaces indicated in Fig 22



stability of SiO2 used as a supporting layer duringthe growth of PLD AlN processing conditions andhave shown that the thermally grown SiO2 was sta-ble than PECVD grown SiO2

ACKNOWLEDGEMENTS

This research was funded by the US ArmyResearch Laboratory under the Power and EnergyElectronics Research Program (Contract DAAD17-99-2-0078) Authors also acknowledge financialassistance from the Collaborative Technology Alli-ance program (Contract CTA DAAD19-01-2-00-10)The Rutherford back-scatteringchanneling workreported herein was done using the PelletronShared Experimental Facility (SEF) supported bythe NSF-MRSEC (Grant DMR-00-80008)

REFERENCES

1 S Strite and H Morkoc J Vac Sci Technol B 10 1237 (1992)2 RF Davis Proc IEEE 79 702 (1991)3 RD Vispute H Wu and J Narayan Appl Phys Lett 67

1549 (1995)4 S Iwama K Hayakawa and T Arizumi J Cryst Growth

56 265 (1982)5 JB MacChesney PM Bridenbaugh and PB OrsquoConnor

Mater Res Bull 5 783 (1970)6 GA Slack and SF Bartram J Appl Phys 46 89 (1975)7 WJ Meng Properties of Group III Nitrides ed J H Edgar

EMIS Data Reviews Series no N11 (London INSPEC1994) pp 22ndash29

8 AT Collins EC Lightowlers and PJ Dean Phys Rev158 833 (1967)

9 H Okano N Tanaka Y Takahashi T Tanaka K Shibataand S Nakano Appl Phys Lett 64 166 (1994)

10 D Alexson et al J Phys Condens Matter 17 R637 (2005)11 TP Chow and R Tyagi IEEE Trans Electron Devices 41

1481 (1994)12 Silicon Carbide A Review of Fundamental Questions and

Applications To Current Device Technology Vols I and IIed WJ Choyke HM Matsunami and G Pensi (BerlinAkademic Verlag 1998)

13 Properties of Silicon Carbide ed G Harries Emis DataReview Series Vol 13 (London INSPEC 1995)

14 JJ Sumakeris JR Jenny and AR Powell MRS Bull 30280 (2005)

15 RF Davis G Kelner M Shur JW Palmour and JAEdmond Proc IEEE 79 677 (1991)

16 HL Dunlap and OJ Marsh Appl Phys Lett 15 311 (1969)17 S Seshadri GW Eldridge and AK Agarwal Appl Phys

Lett 72 2026 (1998)18 KA Jones K Xie DW Eckart MC Wood V Talyansky

RD Vispute T Venkatesan K Wongchotigul andM Spencer J Appl Phys 83 8010 (1998)

19 KA Jones MA Derenge TS Zheleva KW KirchnerMH Ervin and MC Wood J Electron Mater 29 262(2000)

20 KA Jones MA Derenge MH Ervin PB Shah JAFreitas Jr and RD Vispute Phys Status Solidi 201A486 (2004)

21 S Osono Y Uchiyama M Kitazoe K Saito M HayamaA Masuda A Izumi and H Matsumura Thin Solid Films430 165 (2003)

22 CRC Materials Science and Engineering Handbook 63rd ed(Boca Raton FL CRC Press 1982ndash1983)

23 F Xu RA Wolf T Yoshimura and S Trolier-McKinstryProceedings of 11th International Symposium on Electretsed RJ Fleming (Piscataway NJ IEEE 2002) pp 386ndash396

24 GM Rebeiz RF MEMS Theory Design and Technology(Hoboken NJ John Wiley amp Sons Inc 2003)

25 AN Cleland M Pophristic and I Ferguson Appl PhysLett 79 2070 (2001)

26 A Ballato JG Gualtieri and JA Kosinski Proc NinthIEEE Intl Symp Appl Ferroelectrics 674 (1994)

27 TS Low and W Guo J MEMS 4 230 (1995)28 AE Wickenden LJ Currano T Takacs J Pulskamp

M Dubey SS Hullavarad and RD Vispute Integr Ferroe-lectr 54 565 (2003)

29 SH Lee J Lee and K Yoon J Vac Sci Technol A 211 (2003)

30 CR Aita J Appl Phys 53 1807 (1982)31 S Muhl JA Zapien JM Mendez and E Andrade J Phys

D 30 2147 (1997)32 S Yoshida S Misawa and Y Fujii J Vac Sci Technol 16

990 (1979)33 MT Duffy CC Wang and GD OrsquoClock J Electron Mater

2 359 (1973)34 AH Khan MF Odeh and JM Meese J Mater Sci 29

4314 (1994)35 JK Liu KM Lakin and KL Wang J Appl Phys 46 3703

(1975)36 RD Vispute J Narayan H Wu and K Jagannadham

J Appl Phys 77 4724 (1995)37 CJ Scozzie AJ Lelis FB McLean RD Vispute S

Choopun A Patel RP Sharma and T Venkatesan J ApplPhys 86 4052 (1999)

38 AJ Lelis CJ Scozzie FB McLean BR Geil RDVispute and T Venkatesan Mater Res Forum 338ndash3421137 (2000)

39 CM Zetterling M Ostling K Wongchotigul MG SpencerX Tang CI Harris N Nordell and SS Wong J ApplPhys 82 2990 (1997)

40 Pulsed Laser Deposition of Thin Films ed DB Chrisey andGK Hubler (New York John Wiley amp Sons 1994)

41 IN Mihailescu E Gyorgy and T Asakura (ed) PulsedLaser Deposition An Overview Springer Series in Opti-cal Science (New York Springer-Verlag 1999) pp 201ndash214

42 SS Hullavarad RD Vispute B Varughese S DharI Takeuchi and T Venkatesan J Vac Sci Technol A 23982 (2005)

43 RD Vispute J Narayan and JD Budai Thin Solid Films299 94 (1997)

44 S Yamada J Kato S Tanaka I Suemune A AvramescuY Aoyagi N Teraguchi and A Suzuki Appl Phys Lett 783612 (2001)

45 I Akasaki H Amano Y Koide K Hiramatsu andN Sawaki J Cryst Growth 98 209 (1989)

46 RD Vispute A Patel RP Sharma T Venkatesan TZheleva and KA Jones Mater Res Soc Symp 587O741 (2000)

Fig 24 SEM of freestanding AlN MEMS resonator beam fabricatedby a standard RIE etching process to define the beam of dimensions300 mm in length and 1 mm in width


47 FS Ohuchi and RH French J Vac Sci Technol A 6 1695(1988)

48 BC Chung and M Gershenzon J Appl Phys 72 651 (1992)49 K Jagannadham AK Sharma Q Wei R Kalyanraman

and J Narayan J Vac Sci Technol A 16 2804 (1998)50 KA Jones PB Shah TS Zheleva MH Ervin MA

Derenge JA Freitas S Harmon J McGee and RD VisputeJ Appl Phys 96 5613 (2004)

51 KA Jones MH Ervin PB Shah MA Derenge RDVispute and T Venkatesan and JA Freitas AIP ConfProc 680 694 (2003)

52 T Ohno H Onose Y Sugawara K Asano T Hayashi andT Yatsuo J Electron Mater 28 180 (1999)

53 IO Usov AA Suvorova VV Sololov YA Kudryavtsevand AV Suvorov J Appl Phys 86 6039 (1999)

54 SS Hullavarad RD Vispute T Venkatesan and KAJones lsquolsquoGrowth of TaC on SiC and AlNSiCrsquorsquo (in preparation)

55 TOuisseHPD Schenk SKarmann andUKaiser inProceed-ingsof theInternationalConferenceonSiliconCarbideIII-Nitridesand Related Materials-1997 Vols 264ndash268 Materials Science

Forum ed G Pensl H Morkoc B Monemar and E Janzen(Utikon Zurich Switzerland Trans Tech 1998) p 1389

56 C Kim IK Robinson J Myoung KH Shim and K KimJ Appl Phys 85 4040 (1999)

57 TW Weeks Jr DM Bremser KS Alley E CarlsonWG Perry and RF Davis Appl Phys Lett 70 2735(1997)

58 SM Sze Physics of Semiconductor Devices 2nd ed (NewYork Wiley 1981) p 402

59 Chanana et al Appl Phys Lett 77 2560 (2000)60 R Bathe et al Thin Solid Films 398 575 (2001)61 N Reeves E Muzio SS Hullavarad RD Vispute T

Venkatesan B Geil A Lelis D Habersat and CZScozzie lsquolsquoAlN Film on SiC Device Side Wallsrsquorsquo PEER-MERIT(University of Maryland College Park MD 2002)

62 B Nagaraj SS Hullavarad RD Vispute T VenkatesanA Lelis D Habersat and CJ Scozzie lsquolsquoDielectric Propertiesof Sputtered AlN Thin Films on SiC-Based Devicesrsquorsquo (underpreparation)

63 LJ Currano AE Wickenden M Dubey IEEE-NANO 20032 778 (2003)



INTRODUCTION

The wide bandgap Group III nitrides combinefundamental physical and chemical properties thatmake them one of the most promising semicon-ductor material systems for the fabrication of alarge variety of optical and electronic devices capa-ble of performing at extreme conditions of powerfrequency and temperature and in harsh environ-ments12 The interest in aluminum nitride (AlN)thin films has been increased over the past decadesbecause of the following specific properties thatmake it suitable for a wide variety of applications3

AlN has a wurtzite structure with lattice param-eters a 5 311 A and c 5 498 A4 almost matchableto GaN and SiC demonstrating usefulness as abuffer layer for GaN light-emitting diodes (LED)and electronic devices It exhibits excellent thermalproperties (melting temperature of 3000degC5 anddecomposition temperature 1600degC) and rela-tively high thermal conductivity6 (285 W cm1 degC1)that are useful for very high-temperature andhigh-power applications Because of its tunablerefractive index it is widely used as antireflectivecoatings with a refractive index of 21ndash22 forepitaxial 19ndash21 for polycrystalline and 18ndash19for amorphous films7 AlN has a wide bandgap(62 eV) large dielectric constant of 914 (static) at300degC8 and high electrical resistivity and break-down strength (106 Vcm) that can be useful fordielectric passivation of SiC-based devices It alsoexhibits excellent acoustic properties that makeAlN suitable candidate for resonators surfaceacoustic wave (SAW) and bulk acoustic wave(BAW) applications9 AlN along with other IIIndashVand IIndashVI compounds can be synthesized in water-based colloidal suspensions for use in colloidal semi-conductor quantum dots both as biological tagsand as structures that interact with and influencebiomolecules10

Below we discuss details of AlN thin films andtheir characteristics that have been explored in ourstudies for high-temperature surface passivationhigh-temperature dielectric and radio-frequency(RF) MEMS resonator applications

AlN Thin Films for SiC Surface Passivation

As the development in SiC for power electronicscontinues11 the requirements for better SiC mate-rials and processes become more stringent1213 Anexample of this is the need for high-quality n- andp-type SiC with low defect densities and a smoothsurface morphology for the fabrication of high-performance power devices14 Because ion implan-tation is a well-established and accepted technologyin microelectronics studies on doping and annealingof SiC are important15 However in the case of SiCdiffusion coefficients of most of the dopants in SiCare comparatively smaller than Si and hence high-temperature (1600ndash1800degC) annealing processesare required to remove the ion-implantationinduced damage and to electrically activate the

dopants16 However the high-temperature annealingprocess results in increased surface roughness anda change in surface composition of SiC due to thepreferential evaporation of silicon from the surfaceof SiC at elevated temperatures17

Due to high-temperature stability extremely lowchemical reactivity a very small lattice mismatchand similar coefficient of thermal expansion with SiC(bSiC 5 52 3 106 degC1 bAlN 5 40 3 106 degC1)AlN thin films have tremendous potential as aneffective encapsulant layer for high-temperatureannealing of ion-implanted SiC18 AlN films havebeen used successfully to impede the surface degra-dation of SiC caused by the preferential evaporationof silicon when the implanted SiC is subjected to anactivation anneal and this has resulted in an excel-lent activation of both n- and p-type dopants1920 Inorder to preserve surface morphology for high-yielddevice production and to prevent dopant loss anencapsulating layer of AlN is deposited on the SiCto act as a surface seal during annealing Figure 1shows the schematic of the AlN capping processused in dopant activation of SiC The criteria forencapsulant material are that it should withstandhigh temperatures without entering the implantedregion and that it should be easily removed withoutleaving any traces that on the SiC surface

AlN as a Passivation Layer on Side Wallsof SiC-Based Devices

SiC-based metal-oxide semiconductor field-effecttransistors (MOSFETs) and gate-turnoff (GTO)thyristors have found tremendous applications inhigh-power switching applications High-temperatureand high-power applications of these devices willrequire passivation layers that have high dielectricconstants and are stable at high temperaturesunder operating fields of at least 1 MVcm When

Fig 1 Schematic of steps involved in annealing of SiC dopant acti-vation after ion implantation Without AlN cap the surface roughensdue to preferential evaporation of Si leaving carbon on SiC




AlN-Based MEMS





Physical Constants

Materials

PZT ZnO AlN


293 700 1665





Material e

ECritical

(MVcm)25degC

EOperating

(MVcm)300degC

eEOperating

(MVcm)300degC

SiC 10 3 3 30SiO2 39 11 2 78AlN 85 2 2 17Si 119 04 04 48












































c-AxisLattice

Parameter(A)

LatticeMismatch

()LatticeStrain

LatticeStress(MPa)

ThermalStress (MPa)


497 12 241 3 105 151 2413












































A frac14 4pqk2m

h3





Sample

xmin xmin

Al 1020

ions cm2AlC 1020

ions cm2Al 1020

ions cm2AlSi 1020

ions cm2

































CONCLUSIONS










ACKNOWLEDGEMENTS


REFERENCES



































































AlN-Based MEMS





Physical Constants

Materials

PZT ZnO AlN


293 700 1665





Material e

ECritical

(MVcm)25degC

EOperating

(MVcm)300degC

eEOperating

(MVcm)300degC

SiC 10 3 3 30SiO2 39 11 2 78AlN 85 2 2 17Si 119 04 04 48












































c-AxisLattice

Parameter(A)

LatticeMismatch

()LatticeStrain

LatticeStress(MPa)

ThermalStress (MPa)


497 12 241 3 105 151 2413












































A frac14 4pqk2m

h3





Sample

xmin xmin

Al 1020

ions cm2AlC 1020

ions cm2Al 1020

ions cm2AlSi 1020

ions cm2

































CONCLUSIONS










ACKNOWLEDGEMENTS


REFERENCES










































































































c-AxisLattice

Parameter(A)

LatticeMismatch

()LatticeStrain

LatticeStress(MPa)

ThermalStress (MPa)


497 12 241 3 105 151 2413












































A frac14 4pqk2m

h3





Sample

xmin xmin

Al 1020

ions cm2AlC 1020

ions cm2Al 1020

ions cm2AlSi 1020

ions cm2

































CONCLUSIONS










ACKNOWLEDGEMENTS


REFERENCES











































































































A frac14 4pqk2m

h3





Sample

xmin xmin

Al 1020

ions cm2AlC 1020

ions cm2Al 1020

ions cm2AlSi 1020

ions cm2

































CONCLUSIONS










ACKNOWLEDGEMENTS


REFERENCES





























































































CONCLUSIONS










ACKNOWLEDGEMENTS


REFERENCES



































































ACKNOWLEDGEMENTS


REFERENCES


































































Advances in Pulsed-Laser-Deposited AlN Thin Films for High ... · In this paper we report recent...

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