Shottky Barrier Diode Advances in SiC Material and Technology for (Invited) Schottky Diodes Applications T. Billon CEA/LETI, France Development of 600 V/8A SiC Schottky Diodes with Epitaxial Edge Termination F. Templier[1], T. Billon[1], E. Collard[2], A. Lhorte[2], L. Di Cioccio[1], P. Ferret[1] [1]CEA/LETI, France; [2]ST Microelectronics, France Performance of SiC Bipolar (PiN) and Unipolar (SBD) Power Rectifiers in Current-Voltage-Frequency Parameter Space D. T. Morisette, J. A. Cooper Purdue University, USA 4H-SiC MPS Diode Fabrication and Characterization in an Inductively Loaded Half-bridge Inverter up to 100 Kw P. Alexandrov[1], J. H. Zhao[2], B. Wright[1], M. Pan[1], M. Weiner[1] [1]United Silicon Carbide, Inc., USA; [2]Rutgers University, USA High-Voltage Pulse Instabilities in SiC Schottky Diodes with Implanted Resistive Edge Terminations D. T. Morisette, J. A. Cooper Purdue University, USA

Technical Digest oflnt 7conf' on SiCandReiated Mate'iais -IcscRM200]-, Tsuk~ba, Jap"n, 200] WeA3-l(Invited)

AdVanceSin SiC material and technOIOgyfOr

Schottky diOdes applicatiOns

T. Billon

CEA-LETl, 17 rue des Martyrs, 38054Grenoble, FranceContact author : Tel: +33438783680- Fax : +33438789456,email:Thierry,billon@cea.fr

Keywords: SiC - Schottky - diode - epitaxy - ion implantation - defect

Even when sampling and mass-production of Silicon Carbide Schottky diodes have.just

started, manystudies are canied out on the "next generation" of devrces TheseR&Dfields

mainly concern the crystal quality improvement of large diameters 4H-SiC wafers but also

the availability of accurate processes for reliable, Iost cost and efficient devices. Therefore,

the aim of this paper is to makea state of the art of the present research activities.

For the "bulk material", including the epitaxial growth, strong developments are dedicated to

the reduction of structural defects. The electrical effects of specific defects like micropipes,

scratches and dislocations were already published, showing either stro~g correlations

between active area of devices and defect densities, or no influence on the electrical

behaviour. At the present time, defect densities in the range of 100 and 103 cm~ are obtained

for micropipes and dislocations, respectively. Many researchs are also focused on the

reduction of the low surface mosaicity. Somecorrelations have been madebetween white

beamX-rays topographies, FWHMrocking curves and electrical characteristics of Schottky

diodes. Finally, accurate parameters are neededin epitaxial growth to get powerful devices.

The key points concern the wayto obtain reproducible and homogeneousdoping level; this

last parameter having a strong influence on the reverse characteristics andpowerlosses.

For the device processing developments, the three main researchs are :

-the wayto get reliable and efficient metallization processes; which are mainly basedon Ti

and Ni for Schottky and ohmic contacts, respectively. With Ti, the crucial point is the

control of a stable barrier height that must be as low as possible to combine low forward

drop voltage and low reverse leakage current. Additionally, the quality of the epitaxial

layers strongly influences the metallization process.

- howto obtain powerful edge termination process; thus ensuring the lowest influence onthe device; the goal is to get localized P-type regions. Beside the epitaxial approach, manyworks are based on ion implantation. Improved processes combine targeted doping levels

and low surface roughness, even after high temperature post-annealing treatment.

- the control of efficient and high reliability passivations. Strong works concern the control

of a good Si02/SiC interface. For Schottky diodes, high charge rates and/or high surface

potential fluctuations can induce long term instabilities; nowadays, oxide charges in the

range of 1011 cm~ can be obtained. Using secondary passivation like polyimide on the top

of the primary oxide layer, 600VSchottky diodes have demonstrated yields in the 50-75%

range andpromising aging features, even after 1000h@250'C.

All these R&Dfields allow the manufacturing of reliable and powerful SiC Schottky diodes,

which will supersede present Si bipolar diodes.

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Technical Digest oflnt 'I Conif on SiCandRelated Materials -ICSCRM2001-,Tsukuba, Japan, 2001 WeA3-2

DevelOpmentof 600V/8A SiC Schottky Diodeswith epitaxial edge termination.

F. Temlierl, E. Collard2, L. Di Ciocciol, P. Ferretl, A. Lhorte2, T. BillonlICEA-LETI, 17 rue des Martyrs, 38054Grenoble, France

2STMicroelectronics, 16 rue Pierre et Marie Curie, 37071 Tours, France.

Contact author .' F. Templierl, Telephone .' +33438784387,fax .' +33438789456, email.'FTemplier@ceafr

Thanks to its excellent properties such as high breakdown voltage and high thermalconductivity, 4H-SiC has emergedas the material of choice for power devices. Siliconcarbide Schottky diodes are nowcoming on the market and can replace bipolar Si diodes forapplications such as PFCconverters, providing superior switching characteristics resultingfrom the absenceof minority camiers.

Edge terminations, which consist of local p-type areas, are needed for all devices such asdiodes operating over one hundred volts. Theycan be madeby two ways. The first way is toperform localized ion implantation: this process requires very high temperature annealing toprovide activation of the dopants (typically 1500 to 1700 'C for acceptor dopants in SiC).Processing at these temperatures is very critical due to the evaporation of SiC which causessurface damage.The other way is to deposit a p-type epilayer on top of the n- Iayer, and toetch this layer locally to provide the Schottky contact. Our paper will describe thedevelopment of high performance 600 V SiC Schottky diodes using epitaxial edgetermination.

Process issues will be discussed in the paper. A11 devices are developed on 50.8 mmn-typesubstrates from Cree. Thestructure of the device is shownon figure I .

Epitaxy: As shownon figurel, the stacking consist of three epilayers: N+buffer layer; N-drift layer, and P-type edge termination layer. Epilayers are deposited at CEA/LETI, in anEpigress VP508hot wall reactor.

Device processing: The fabrication of the devices required the development of key processsteps such as:- Patterning of Mesastructures with vertical slope using high density dry etching (fig. 2);

- Developing of specific lithography process compatible with the presence of the Mesastructures;

- Passivation process, using conventional silicon-technology materials (fig, 3), andproviding high reverse voltage with high reliability.

Electrical performance: figure 4showstypical reverse characteristics obtained on the devices.Moredetailed electrical results will be given, including reliability tests. Yield results will alsobe discussed.

As a conclusion, wehave developed a process for manufacturing high performance 600 VSiC Schottky diodes. This process uses epitaxial p-type edge termination, which avoidspenalizing process steps such as high temperature (1500-1700 'C) annealing required whenusing implanted p-type edge termination.

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Ptype epi layer

N+ type epi layer

Schottky metallization + connexion pad for Al bonding

ANODEPassivation

Metauization for backside brazing CATHODE ohmic contact metauization

Figure I : structure of 600VSchottky diode using epitaxial edgetermination.

Figure 2: SEMcross sectional view of test pattern for

Mesavertical etching.Figure 3: SEMcross sectional view showing

passivation layers.

Figure 4: typical reverse bias characteristic of the diodes

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Technical Digest oflnt 'l Conf on SiCandRelated Materials -ICSCRM2001-,Tsukuba, Japan, 2001 WeA3-3

Performanceof SIC Blpolar (PIN) andUnipolar (SBD)PoWerRectifiers in Current-Voltage-Frequency ParameterSpace

D. T. Morisette and J. A. Cooper, JrSchool of Electrical and ComputerEngineering

PurdueUniversity, WestLafayette, Indiana, USA47907-1285Phone: (765) 775-4556 Fax: (765) 775-4557 morisett@purdue.edu

To select the optimal device for a particular application, power circuit designers must carefullyanalyze the tradeoffs between competing devices. Recent progress in SiC power rectifiers hasresulted in the demonstration of high-voltage and high-current PiN and Schottky barrier diodes(SBDs) [1-3]. With both technologies maturing, power electronics engineers face the task ofselecting between these two devices. Until recently, the choice was simple, since silicon SBDSareonly available for relatively low voltage applications. The choice is not as clear whenconsideringSiC diodes, and guidelines for determining the proper application of each are needed. This paperprovides such guidelines, basedon a simple, yet complete analysis of the tradeoffs involved.

The selection criteria used in this analysis can be summarizedas follows: An application is

defined as a unique combination of forward current density JF, reverse voltage VR

,and switching

frequency f. Theappropriate choice of rectifier is the device which requires the least area f '

or a grvenapplication. Therefore the device that carryies the highest current density JF Without exceeding the

maximumpowerdissipation allowed by the package is the proper choice. Weassumeboth PiN andSchottky diodes are independently optimized for each application, specified by a particular set ofperformance parameters (JF, VR, j). This requires independent optimization of the blocking layerthickness and doping of both diodes, and appropriate adjustment of the ambipolar lifetime TA mthePiN diode for each reverse voltage considered.

An ideal application for SiC rectifiers is as a replacement for the silicon fly-back rectifiers ininductive load switching circuits, such as electric motor drives. Thecomponentsof powerdissipationassociated with the fly-back rectifier can be divided into two general categories: static and dynamicpower. Static power includes powerdissipated by the diode whenin either the forward conduction orreverse blocking state. Dynamic power includes any energy dissipated by the circuit duringswitching events that can be directly attributed to the presence of the diode. Since these energy losses

occur each time the circuit switches, the dynamic power dissipation is proportional to switchingfrequency. All powerdissipation componentsare expressed as powerdensities, in units of W/cm.

Thedesired switching frequency determines which device technology provides the best solutionfor a given application. To illustrate the general framework for our comparison, Fig. 1(a) plots the

maximumallowable current density JF(MAX),

subject to the constraint that the total powerdissipationequals the packagelimit, as a function of switching frequency for both types of devices. Since powerdissipation in SBDSis essentially independent of frequency, JFrMAXJcan remain almost constant as fisincreased. However, power dissipation in a PiN diode increases with f, so the static loss must bereduced as f is increased to satisfy the packagepower limit. This requires increasing the area of thediode, which reduces the forward current density JF(MAX).

For each application there is a unique crossover of the PiN and SBDJF(FMAX)vs. f characteristics.

Above this frequency any PiN, however well optimized, is less efficient than an optimized SBD.This crossover frequency can be determined for a range of operating voltages VR

.Figure I (a) can be

thought of a constant-voltage slice in three-dimensional (JF, VR,j) space, as illustrated in Fig. 1(b),

Theresulting data can be best presented by projecting these crossover points onto the f - VRPlane.

- 304-

f~wteh

_.- v~

- f*~**.*

f~-

(a) (b)

Figure 1: General framework for comparing bipolar (PiN) and unipolar (SBD)devices. Wefirst find the frequency

fMAXat which the maximumJF of PiNs andSBDSare equal, subject to the powerdissipation limit of the package (a),

and then werepeat for several voltages VR, andproject the resulting locus onto the f vs. VRPlane (b).

Theresult of this analysis, using apackagedissipation limit of I kW/cm,is shownin Fig. 2. The

bold curve is the frequency fMAXat which a PiN diode has the sameefficiency as a Schottky. Forapplications requiring switching frequencies above this curve, the PiN requires more area than the

SBDand thus the SBDis preferred. For applications requiring lower frequencies, the PiN is

preferred.

2.0 1.2

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Area (PiN)

lOOO

~~;hc' 100

~~:

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l

o o1.O

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1.O

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ReverseVoltage (kV)

Figure 2: Contour plot of the area ratio for a4H-SiCPiN diode relative to a4H-SiCSBDas a function of VRandf ,

assumingapackagelimit of I kW/cm, a50%duty cycle, and a 509if. voltage derating factor,

Referring to Fig. 2, the following conclusions can be drawn from this analysis: Re ion I : PlNsrequire morearea than SBDSfor any VR 1.4 kV. Re ion 2: PiNs require morearea than SBDSfor

f> 70 kHz, regardless of voltage rating VR. (However, for VR> 4kV the difference is small, andother factors may determine the appropriate choice.) Re ion 3: SBDSare highly preferred at

VR 4kV for high frequency operation (> 300 kHz). Rion 4: PiNs are preferred for VR> 2kVand

f 30 kHz. Thesegeneral conclusions apply for other choices of packagepowerdissipation limit,

although the boundaries between the different regions change. These guidelines, with slight

modifications, also apply to the analysis of other bipolar and unipolar devices, such as BJTSvs.

powerMOSFETS.Moredetails of the analysis will be presented at the conference.

This work is supported by the Office of Naval Researchunder MURIgrant NOO014-95-1-1302.

[1] L. RumyantsevandR. Singh, IEEETrans. on Electron Devices, 46, 2188 (1999).

[2] R. Singh and J. W.Palmour, "High Temperature Characteristics of 8.6 kV 4H-SiCPiN Diode", Proc. HighTemperatureElectronics Conf

,Albuquerque, NM,June 13 -

15, 2000.[3] H. M. McGlothlin, et al., "4 kV Silicon Carbide Schottky Diodes for High-Frequency Switching

Apphcauons" IEEEDevlce ResearchConf Santa Barbara CAJune 28 - 30, 1999.

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Technical Digest of'Int 'l Conf, on SiCandRelated Materials -ICSCRM20()1-, Tsukuba, Japan, 200l WeA3-4

4H-SiCMPSDiode Fabrication and CharacterizationIn an Inductively LoadedHalf-bridge Inverter up to 100kW

P. Alexandrovl, B. Wrightl, M. Panl, M. Weinerl, K. Tone~and J. H. Zha02IUnited Silicon Carbide, Inc., 100 Jersey Ave., Building D, NewBrunswick, NJ08901

, USASiCLAB,Dept. ofECE. Rutgers University. Piscataway, NJ08854. USA,

Telephone: (732) 445-5240, Fax: (732) 445-5240, Email: jzhao@ece.rutgers.edu

In this work, 600V4H-SiC MPSdiodes were designed, fabricated, packaged, and tested.

The cross sectional view of the MPSdiode is shownin Fig.1. The starting material used in thediode fil~bric3ation were 4H-SiC wafers purchased from Cree Research, Inc, with a 10um2.1xlO cm~ or 6um I.4xl016cm~3 doped n~ epilayer on n+ substrate. Multiple energy Alimplantation wasdone to create simultaneously the p-region rings in the device active area andthe p region for multi-step junction termination extension (MJTE). Implanted samples wereannealed at I,550'C for 30 min. TheMJTEwasformed by thinning the implanted region at thedevice periphery using dry etching. Thenual oxide with thickness 600Aand I umLPCVDoxide

were grownfor surface passivation, Ni wassputtered on the substrate andannealed at 1050'C for

1Omin in Ar forming gas to form ohmic contact. Contact windowsin the oxide were openedbywet etching and Ti Schottky contact metal was then deposited by sputtering and defined bystandard lift-off process. Finally Au layers were deposited over the contacts and multiple Imnldiameter cells were packagedin single packages, forming multi-cell high current capacity MPSdiodes. Thepackageddevices were tested under DCconditions at temperatures up to 250*C. Thetermination technique used was shown to ensure excellent blocking voltages. The targetedblocking voltage was600V, howeverthe best blocking voltage for Immsingle-cell MPSdiode

wasmorethan 900V. Thehighest blocking voltage measuredon a small area control diode was1550Vand IOOOV,respectively, for the IOand 6umstructures. They correspond to parallel-

plane electric fields of 2.9MV/cmand 3. IMV/cm, respectively.

The diodes reverse voltage I-V characteristics (Fig.2) showexcellent suppression of theSchottky reverse leakage current at temperatures up to 250~C. Forward I-V curve shows291A/cm2at Vf=1.5V and 1493A/cm2at Vf=4V at room temperature. The highest forwardcurrent capability of a packaged600Vdiode is 140Aat 4V (Fig.2 (a)). SiC MPSdiodes turn-offcharacteristics were tested from room temperature up to 250"C and compared to thecharacteristics of similarly rated commercially available ultrafast Si PlN diodes (Fig.3).

Switching results of SiC MPSdiode in an inductively loaded half-bridge inverter will bepresented for up to 100 kWat RT, and 400V- 145Aat 250~C. The substantial reduction of thediode reverse recovery current leads to a substantial decrease in the diode dissipated switching

energy. At roomtemperature the diode energy loss reduction is 470/0, and at 200~Cthe reductionis 840/0. Our packaged SiC MPSdiodes, as opposed to Si diodes, can be safely operated at

ternperatures around 250~Cwith energy losses comparable to the losses of Si diodes at roomtemperature. The use of SiC MPSdiodes reduces not only the diode reverse recovery losses butalso the losses in the IGBTswitch. The IGBTenergy loss reduction whena SiC MPSdiode is

used is 150/0 at room temperature and 450/0 at 150~C. It is clear that even using the existing Si

IGBTswitches the substitution of Si diodes with SiC diodes could significantly improve theoverall perfonuance of motor control inverters. Comparison will be madeamongdifferent

designs andkey issues for high performance will be discussed.

- 306-

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Technical Digest oflnt'/ Conf' on SiCandRelated Materials - ICSCRM2001-,Tsukuba, Japan, 200] WeA3-5

High-Voltage Pulse Instabilities in SiC Schottky DiodesWith Implanted Resistive EdgeTerminations

D. T. Morisette and J. A. Cooper, Jr

School of Electrical and ComputerEngineeringPurdueUniversity, WestLafayette, Indiana, 47907-1285 USA

Phone: (765) 775-4556, Fax: (765) 775-4557, morisett@purdue,edu

Several reports have recently been published on pulse measurementsof SiC PiN diodes [1, 2, 3].

Suchmeasurementshave beenused to determine the temperature coefficient of breakdownvoltage and to

reveal breakdown voltage instabilities. High-voltage (HV) pulse characterization is performed byapplying a fast (- 5-50 ns risetime) reverse bias pulse to the device under test, and by monitoring the

resulting transient voltage and current waveforms. Ablock diagram of the system used in this work is

shownin Fig. l.

WaveformGenerator

HVPowerSupply

ADigital Oscilliscope

~]..

V+HVPulse Generator

1OO*probe

r-- -lI II 50 ~ I

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Figure 2: Nominal diode I-V characteristics

Thestudy is conducted on 30 - 200umdiameter circular Schottky diodes fabricated on an n-type

4H-SiC substrate with a 10 umn-type epilayer doped I x 1016cm~3with nitrogen. A resistive edgetermination is formed by implanting I x 1015 cm~2boron atoms at 30 keV in 30 umrings surrounding

each diode. The implants are activated at 1050'C to remove lattice damagewithout activating the

dopants [4]. Nickel Schottky contacts are deposited by E-beamevaporation and pattemed by liftoff.

Eachdiode is prescreened by measuring its static I-V characteristics, and only those that accurately

reproduce the nominal characteristics shown in Fig. 2 are used in subsequent pulse testing. Thereverse-bias measurementsare limited to 1200V to prevent damage.

Repetitive pulses similar to that shownin Fig. 3(a) are possible at low pulse amplitudes. Dueto

the limited resolution of the experimental configuration, reverse current cannot be measuredfor pulse

amplitudes below approximately 200V. As the pulse amplitude is increased above this threshold,

the repetition rate must be progressively reduced to prevent damageto the devices. Reverse current

waveforms at pulse amplitudes below -300 V are relatively stable and repeatable. At pulse

amplitudes between 300V and 400V the reverse current tends to increase with time and it is

necessary to reduce the repetition rate to prevent device failure. Above400V it is impossible to

reduce the repetition rate sufficiently, and single-pulse measurementsmust be used in order to

capture useful data. Single-pulse capture is used for each measurementpresented in this paper to

insure repeatable results.

- 308-

o

300

200"E

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•300o 200 400 800 IoOO600

o

*""" "' '"' oo' so' "" "",,.+,, ,""',,

Ti~* (*~) Ti~. (**) '=~" (~*)

(a) (b) (c)

Figure 3. SiC Schottky barrier diode HVpulse characteristics

(a) 350 Vpulse; (b) comparison of two 350 Vpulses offset by a 200 VDCbias; (c) 600Vpulse

Figure 3(a) shows a typical measuredpulse at an amplitude of approximately 350V, and is

characteristic of devices tested up to -500 V. The magnitude of the pulse current (-50 AJcm2) is

many orders of magnitude higher than the static leakage current at the same bias (less than

1uA/cm2). This is the first of several indications that these devices do not behaveproperly under the

application of fast high-voltage pulses. However, notice that the pulse current does showa negative

temperature coefficient, indicating that at least at this amplitude the leakage mechanismis thermally

stable. As illustrated in Fig. 3(b), the magnitude of the pulse current is dependent not on the

maximumreverse bias, but on the amplitude of the pulse. This test is repeated for DCbiases from100V to 1000Vusing several different pulse amplitudes with the sameresult. Whensubjected to

pulse amplitudes above 500 V, as illustrated in Fig. 3(c), the pulse current increases with time,

indicating a thermally unstable condition. Every device subjected to single-shot pulses with

amplitudes above - 500V suffers catastrophic failure and is destroyed, regardless of the duration of

the pulse.

Webelieve that the HVpulse instability illustrated above is caused by the deep-level traps

introduced into the termination ring by implantation. The implanted boron impurities, which areintentionally not activated, produce a highly resistive surface layer. Shunt leakage through this layer

under reverse bias results in a lateral voltage drop which reduces field crowding at the edge of the

metal contact and increases the breakdownvoltage of the device. However,whena high-speed pulse

is applied to the device, the charge in these deep-level traps maynot emit rapidly enoughto follow

the applied signal. The device then reacts as an unterminated device, breaking downbetween200-

400V. The instability reported in this paper may limit the safe-operating-area (SOA) of SiCSchottky diodes fabricated with this termination method, or mayrequire the use of an alternative

edge termination.

This work is supported by the Office of Naval Researchunder MURIgrant NOO014-95-5-1302.

[ll P. G. Neudeckand C. Fazi, J. Appl. Physics, 80, 1219 (1996).

[2] P. G. Neudeck,and C. Fazi, IEEEElectron Device Lett., 18, 96 (1997).

[31 A. Torres, et al. Mat'Is Science Forum, 338-342, 1215 (2000).

[4] A. Itoh, et al., Proc. 6th Int'l Conf. on Silicon Carbide and Related Materials, 685 (1996).

200

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- 309-