introduction - esa.int · introduction welcome to the eighth issue of eurocomp, the newsletter of...

IntroductionWelcome to the eighth issue of Eurocomp, the newsletter of the Space Components Steering Board (SCSB).

The Components Technology Board (CTB) is one of the primary organisational bodies in the SCSB. Its majorobjective is to formulate strategic programmes and work plans for technology research and development. In previousissues we have highlighted the technology dossiers defined by the CTB. In this issue we spotlight the internalworkings of the CTB.

-1eurocomp no. 8, winter 2005

In this issue...• Tantalum capacitors

• ESCIES: ESCC and ESCIES websites

• ESCC-Sponsored Technology‘Round Tables’:

– ESA-CTB Round Table Consultation on WideBand Gap Technologies for MicrowaveApplication

– ESA-CTB DC-DC Converter Round TableConsultation

tantalum capacitors. And yes, that is a remnant of anexploded solid tantalum capacitor captured on film,above.

A regular feature of Eurocomp is the European SpaceInformation Exchange System, or ESCIES, which canbe reached at http://escies.org/. Its goal is to serve as acentral point for collaboration and data exchange withinthe Space Components community. In addition toproviding standard and legacy information that greatlyimproves our concurrent engineering efforts, it providesa portal for the collaborative workings of the ESCCgroups at https://spacecomponents.org/. The diversegroups can rely on ESCIES for informed interactionleading to formulation of policies and technologyprogrammes, as well as the administration of all ESCCspecifications. A brief overview is presented here, givingus a picture of a busy site dedicated to meeting its goal.

Finally, the 7th issue of the European Preferred PartsList (EPPL) is now available online at https://escies.org/public/eppl/. We reported in the very firstissue of Eurocomp that the EPPL is a result ofcollaborative efforts involving and on behalf of allpartners of the ESCC. It is a standardisation tool fordesigners to apply type reduction and thereby achievecost effective and efficient component procurement.The commitment of the Technical Authority tomaintaining this list for our users is appreciated. •

In order to identify and anticipate the needs of thecommunity, the CTB sponsors ‘round table meetings’to improve research coordination. Two such roundtables were held this year, co-sponsored by theEuropean Space Agency (ESA) at the European SpaceResearch and Technology Centre (ESTEC) inNoordwijk, The Netherlands. The round tables on wideband gap technologies and DC-DC converters helpedto consolidate the needs of academia, industry,government and satellite operators. The outcome ofthese round tables is briefly detailed in twoaccompanying articles. In the next volume ofEurocomp, we hope to bring you results of anotherround table directed at micro/nano technologies forspace applications.

Also in this issue, Paul Collins from ESA’s ComponentTechnology Section at ESTEC asks “…with all thereported reliability concerns associated with thistechnology, particularly in low impedance circuits, whywould designers want to use solid tantalum capacitors inESA flight applications at all?”

Indeed, solid tantalum capacitors have been viewedwith some wariness for many years in the power supplycommunity, particularly in their filtering applications.Our feature article presents a synopsis of a riskmitigation investigation involving surface mount solid

Eurocomp8.qxp 1/10/06 6:00 PM

2eurocomp no. 8, winter 2005

Tantalum capacitors

1. IntroductionSince its first appearance in the mid 1950s, the solidtantalum capacitor has enjoyed increasing popularitywith circuit designers, mainly due to the hugeadvantages the SMD styles offer in volumetricefficiency over other capacitor types available. Thispopularity has not always been shared in the spacesector, leading to some restrictions in its application,notably in power supply filtering circuits, where highcurrent-handling capability and low impedance areprerequisite. These concerns were largely due to thefact that the tantalum capacitor industry itself had setan application requirement for a minimum of 3 Ohms/Volt to be present in series with eachcapacitor in circuit. ESA PSS-01-301 ‘Deratingrequirements applicable to EEE components for ESAspace systems’ (last issue circa 1992) formalised thecapacitor industry’s concerns and banned the use ofsolid tantalum capacitors in power supply filterscompletely. This directive has not been rigidlyadhered to in recent years, but appeared more thanjustified after the occurrence of several high profilefailures in flight equipment. The ensuinginvestigation, performed with the support of AVX Tantalum Division, the ESA-qualified source forthis technology, revealed some critical processimprovements, which led to a relaxation of the PSS-01-301 directive. Thus the use of SMD solidtantalum capacitors in power supply filterapplications is now allowed, provided that certain riskmanagement activities are carried out. This articlegives an insight into the manufacturing processesused, the reasoning behind the relaxation inapplication rules, and the risk mitigation activities.

2. Capacitance and the Dielectric Constant – Volumetric Efficiency

“…with all the reported reliability concerns associatedwith this technology, particularly in low impedancecircuits, why would designers want to use solid tantalumcapacitors in ESA flight applications at all?”

First some important volumetric considerations.

The effects of capacitance were first discovered circa1746, during Muschenbroeck’s Leyden Jarexperiments. The fundamental principle investigatedwas that electrodes separated by a non-conductivedielectric material will store a charge when a potentialdifference is applied. Capacitance is defined as thecharge stored per unit potential difference, i.e.

[1] C = QV

where: C = capacitance in FaradsQ = charge in CoulombsV = potential difference in Volts

Dielectric materials are graded relative to thedielectric properties of free space, which is said tohave a relative dielectric constant of 1 and apermittivity of 8.85 x 10-12 F/m. The total capacity ofa sample of dielectric material is a function of thedielectric constant, is proportional to the total surfacearea of the electrodes or plates, and is inverselyproportional to the distance between the plates or thedielectric thickness. i.e.

[2] C = (ε o ε r A) / d

where: εo = The dielectric constant for free space,8.85 x 10-12 F/m

εr = Relative dielectric constant (alsoreferred to as K)

A = Area of the electrode plates (dielectric area)

d = Dielectric thickness

From [2] it is clear that in order to design a capacitorwith a relatively high value, in excess of say 1µF,whilst at the same time keeping the physicaldimensions within practical limits for electronicassemblies, i.e. minimising the dielectric area and thusincreasing volumetric efficiency, it is necessary tochoose a dielectric whose properties are such that itcan exist in a thickness of less than 1 µm whilstsupporting a realistic working voltage of, say,5 to 50 V. Within the solid tantalum capacitor,electrochemically formed tantalum pentoxide(Ta2O5) dielectric provides one such solution. Ta2O5dielectric formed to a thickness of approximately200nm will support a working voltage of at least 50 V. However, the dielectric constant of Ta2O5 is just27, which, from equation [2] above, dictates that arelatively large electrode surface area is required inorder to provide a capacity that is attractive for thedesigner to use.

The required electrode surface area is derived fromthe formation voltage (and hence the formation ratioof the dielectric material) as follows:



3.1 Tantalum Powder Formation

Solid tantalum capacitors are manufactured fromhigh-purity tantalum powders obtained from minedraw tantalum using a hydrofluoric acid extractionprocess. Particle sizes are normally processed andshaped to range from 2 to 10 µm, the exact size usedbeing dependent upon the working voltage andcapacitance requirements of the finished product (CVvalue). Typical powders are shown in Figure 1 below,with their capacitance voltage per product gram(CV/g) values.

3.2 Anode Construction

Anode manufacture involves tantalum powder beingmixed with a binder and pressed at high pressurearound a tantalum wire, known as a riser, forming ananode pellet. The binder also acts as a lubricant toensure an even flow into the press tool. After removalfrom the press, the green anode assembly is sinteredunder vacuum at a temperature of between 1500° and2000°C. This process also burns off the binder, alongwith many other impurities, and fuses the pressedtantalum anode pellet into a sponge-like structure.The pellet at this stage has very good mechanicalproperties. Normally a sample of sintered anodepellets is taken at this time and anodised (see thefollowing section), and a wet capacitance check is thenmade, allowing verification of the expectedcapacitance value.

Figure 1 – Typical Tantalum Powders (Courtesy of AVX)

Figure 2 – SEM Image of Tantalum Anode Pellet, Pre-Sinterleft, Post-Sinter right (Courtesy of AVX)

[3] VF = RF ✕ VW

where: VF = formation voltageRF = formation ratioVW = working voltage

[4] d = VF ✕ dielectric growth rate

where: d = dielectric thickness

Hence substituting in equation [2] gives:

[5] Dielectric surface area A = C d / (ε o ε r)

The dimensions of an ESA-qualified TAJ-type 22 µF,35 V capacitor from AVX are 4.3 x 4.5 x 7.5 mm. Thisequates to an overall volume of 0.145 cm3. However,the capacitive element (slug) only occupies about onethird of the overall case volume, 0.048 cm3. So,incredibly, the required 164 cm2 of dielectric surfacearea is formed in a pellet with a volume ofapproximately 0.048 cm3. This clearly illustrates theadvantages of using this technology in applicationswhere mass and volume budgets are a majorconsideration.

3. Construction and the Manufacturing Process

“…but exactly how is this dielectric surface arearequirement achieved within such a small volume?”

The manufacture of a solid tantalum capacitor is asurprisingly complex process and contains manystages. These stages can loosely be grouped as:

1. Tantalum powder formation2. Anode construction3. Dielectric formation4. Cathode formation5. Interconnection6. Packaging and test

These are described in more detail in the followingsections.

Example

For a 22 µF, 35 V capacitor, this becomes

VF = 3 ✕ 35 V = 105 V

d = 1.7 ✕ 10-9 m/V = 105 ✕ 1.7 ✕ 10-9 m = 178.5 ✕ 10-9 m

A = (22 ✕ 10-6 ✕ 178.5 ✕ 10-9) / (8.85 ✕ 10-12 ✕ 27) = 0.0164 m2



In addition to the verification of the wet capacitancevalue, an SEM image is used to verify sintering. Inaddition a density check is also performed. Figure 2shows the difference in SEM images between sinteredand unsintered green pellets.

It is the density in combination with the high porositythat results in the very large surface area required toform the dielectric. It should be noted that the surfacearea for dielectric formation is the whole of the pellet,from the outer shell right through to the central anoderiser wire.

A Teflon washer is added to the anode riser wire toprevent possible short circuits after formation of themanganese dioxide cathode contact.

3.3 Dielectric Formation

The dielectric, tantalum pentoxide (Ta2O5), is formedon the exposed tantalum surfaces within the pellet,using electrochemical anodisation. The pellet is dippedin a weak acidic solution, normally phosphoric acid, anda forming voltage applied to the anode riser wire.

The oxide forms with about two thirds growing on thesurface and one third growing into the tantalum byapproximately 1.7 nm/V. The choice of formationvoltage is dependent upon the working voltagerequirement of the finished capacitor.A higher workingvoltage obviously requires the formation of a thickerdielectric, which in turn requires a higher voltage forformation. However, it should be noted that thickerdielectrics mean lower capacitance, larger anodes, andhence lower volumetric efficiency. The formationvoltage is usually set at between 3 and 4 times that ofthe required working voltage, and it is applied from a

power supply initially in constant current mode. Asdielectric starts to form, the voltage rises and currentdecays as the forming dielectric offers more resistance.This process continues until the pre-set formationvoltage value is reached and the current becomeslimited to that of dielectric leakage only. At this pointthe power supply is switched from current to voltagemode until formation is complete. Figure 3 shows asection of the formed dielectric, using SEM imagery.

During formation, unwanted semiconductingtantalum oxide forms between the tantalum metaland the tantalum pentoxide dielectric. It is thisphenomenon that polarises tantalum capacitors. Thetantalum oxide is kept to a minimum by interruptingthe formation process and removing the anode fromthe formation bath when 90% of the formationvoltage is reached, followed by a bake-out atapproximately 400°C.

The importance of selecting the correct powder size,sinter temperature and formation voltage for therequired CV characteristic of the finished productnow becomes very clear. The dielectric contains a lowPPM count of evenly-distributed impurity sites, whichare responsible for the leakage current characteristic. Itis possible to partially isolate these sites by increasingthe forming voltage. However, this is limited by therequired thickness and hence the capacitance of thedielectric.

3.4 Cathode Formation

The cathode is formed using Manganese Dioxide(MnO2) in a process called manganising, and this isachieved by the pyrolysis of manganese nitrate. Theslug is submerged in a nitrate slurry and then baked atapproximately 250°C to produce a dioxide coat. This

Figure 3 – SEM Image of Tantalum Pentoxide Dielectric Layer(Courtesy of AVX)

Figure 4 – SEM Image of Manganese Dioxide CathodeContact Layer (Courtesy of AVX)

ManganeseDioxide

Tantalum metal

TantalumPentoxide



3.6 Packaging and Test

The slug and lead frame assembly is moulded into anepoxy resin case which is laser marked with themanufacturer’s identification, value, polarity and datecode. In-line electrical testing is performed, normallyconsisting of capacitance, ESR, tan delta and leakagecurrent tests. Additional tests and screening can beperformed as per customer requirements. Finally thecapacitor assembly is packaged ready for delivery.

4. Failure Modes and Self Healing

“…OK, but what of the known failure modes andreliability issues?”

The images shown in this section header illustrateexactly what can happen when a tantalum capacitorfails. The explosive nature of the failure, which usuallyresults in a short circuit condition, has understandablybeen the cause of much concern in the space industry.There have been many articles and papers written bythe tantalum capacitor industry, giving in-depthexplanations of failure mechanisms and preventativeapplication restrictions. The fact that so manydocuments exist, written over many years, must be anindication of the magnitude of the problem, and actsas a historical record of the problems encountered byusers of these devices in certain applications.

The fundamental problem, which has been welldocumented over the years, is that of irregularities orweaknesses in the delicate constituent layers, resultingin high-leakage current paths introduced during eitherthe manufacturing process, the mounting orapplication. Irregularities can be formed by impuritysites that are not totally eradicated during reform,

process has been continually improved and now consists of several stages with differingconcentrations of nitrate slurry, allowing maximumpenetration into the anode structure. This in turn givesgreater dielectric contact and results in a reduction inthe all-important series resistance characteristic.

All tantalum capacitors have many fault sites in thedielectric layer, resulting in high leakage current paths.In order to remove these sites during production, theassembly is dipped into an acetic acid bath with avoltage applied of approximately half the formingvoltage. This process removes manganese from thefault sites and forms a dielectric which literally plugsthe gap. This process is known as ‘reform’.

3.5 Interconnection

The next stage is to make the cathode and anodeconnections to the lead frame. The slug is dipped intoa graphite solution, transferred to an oven and bakedto assure adhesion. This process is then repeated witha silver solution which forms the final layer forconnection of the cathode. The graphite layer isrequired in order to separate the manganese dioxidefrom the silver. Any contact between these twomaterials would result in a chemical reaction leadingto the silver becoming oxidised and forming high-resistance silver oxide. Additionally, the manganesedioxide would be reduced to a higher-resistancemanganese oxide.

The cathode connection is made to the lead framewith silver epoxy whilst the tantalum anode riser wireis laser-welded into position.

Figure 5 – Section Through a Completed SMD TantalumCapacitor (Courtesy of AVX) Figure 6 – High Leakage Current Through Fault Site

Silver loaded Epoxy

Current evenlydistributed throughout

dielectric

More current flowsthrough impurity

degraded dielectricManganese

Dielectric

Tantalum


6-eurocomp no. 8, winter 2005

dielectric cracks, delaminations, thinning of either thedielectric or manganese dioxide layers, voltagebreakdown, or popcorning of the manganese dioxide(caused by moisture ingression during storage, boilingoff during solder reflow). This list is by no meansexhaustive.

High leakage current paths result in localised heating,which will cause the following chemical reaction tooccur in the Manganese Dioxide layer if thetemperature rise exceeds approximately 450°C:

2MnO2 + Energy → Mn2O3 + 1/2 O2

MnO2 has relatively poor conduction characteristics(2 to 6 Ohms/cm), but this is generally overlooked dueto a more valuable property that allows conversionfrom the semi-conductive state (MnO2) to the highlyresistive state (Mn2O3) with the appropriate energyavailable. The Mn2O3 formed literally plugs the highleakage path, a process which is referred to as ‘selfhealing’. Without this ability to ‘self heal’, the solidtantalum capacitor would not be a viable proposition,due to severely degraded reliability and yield.

From the above it would be easy to assume that theself healing reaction is a ‘cure all’ and that thereshould be no further need for discussion. However,note the use of the term ‘with the appropriate energyavailable’. There are two further scenarios wherein thecapacitor’s ability to self heal will not necessarilyprevent failure:

• Insufficient Energy

If a solid tantalum capacitor is used in a circuit witha high series resistance, it may be the case thatinsufficient energy (current) is available for a selfhealing reaction to take place. Hence any fault sitewill remain ‘leaky’, and the capacitor could exhibita high leakage current failure mode. This isparticularly problematic and noticeable in timingcircuit failures.

• Excessive Energy

The self-healing process described generatesoxygen and Mn2O3. As with all reactions, a finiteamount of time is required for this to occur. Should

excessive energy be available, the production ofoxygen will occur faster than it can recombine,causing combustion and a thermal runawaycondition with explosive results. This is the type offailure associated with low impedance powersupply filter applications. This type of failure isunpredictable and could occur either after manyhours of operation or after the application of a hightransient current surge at switch-on or -off.

In all of these scenarios, energy levels are notpredictable, hence the need for the tantalumindustry’s historical 3 /V series resistancerecommendation, later reduced to 1 /V followed by 0.1 /V and below for some products, as processimprovements were made.

5. Process improvements made since the PSS-01-301 directive circa 1992

“…so what exactly has the tantalum capacitor industrydone to improve the overall performance of their productsince the 1992 issue of the ESA derating directive, andwhy are certain types now being considered as acceptablefor use in power supply filter applications?”

With the most dramatic developments driven by thetelecomms boom of the 1990s, a general policy ofcontinual improvement in raw material processing,chemical formation, packaging and parametric testinghas led to a greatly enhanced product with vastimprovements in electrical characteristics, reliabilityand volumetric efficiency.

In discussions with the tantalum capacitor industry, itwas made clear that supply chain developmentbecame an essential element in their quest for rangeextension, miniaturisation and improved reliability.Powder formation and shaping is one of the key areasin solid tantalum capacitor construction and has beenimproved steadily over the past ten to fifteen years.Powders are now available with CV/g valuesapproaching an order of magnitude higher andimpurity levels an order of magnitude lower than waspossible at the beginning of the 1990s.The overall effect has been to assure the productionof a more homogeneous anode with far fewer high-leakage fault sites.

Much development effort has been concentrated onthe reduction of ESR (Effective Series Resistance),a key parameter in power supply filter applications.ESR is of great importance because, primarily inorder for the filter to function efficiently, thecapacitive element should offer very little resistance(ESR) to ground at switching frequency. Low ESR

Figure 7 – Self Healing

Fault site Mn2O3

Mn O2

Ta2O5

Ta



will not only enhance the filter’s electricalcharacteristics, but will also reduce joule heating andhence increase the capacitor’s long term reliability. Inall capacitors, ESR is a combination of the real seriesresistance of all the interconnects (DC component)and the hysteresis losses in the dielectric (ACcomponent). In order to reduce ESR, manufacturershave increased the MnO2 cathode contact area in anumber of ways, including angular shaping of theanode pellet, multi-anode construction, andoptimisation of the cathode manganising process aspreviously discussed. In addition, laser welding hasbeen introduced at the interconnect of the anoderiser wire to the lead frame, which significantlyreduces the DC resistance component. The resultscan clearly be seen by reviewing current data sheets.Certain types now such as the AVX TPS III and TPMproduct exhibit ESRs of only a few mOhms, someorders of magnitude better than types available inthe early 90s. These products have been specificallydesigned for power supply filter applications, andequivalents are available from all majormanufacturers including Kemet and Firadec.

With the reduction in ESR and subsequent use inlower impedance circuits, came an increase in theprobability of high transient surge current failure. Therisks associated with surge currents meant thatprocess improvements were necessary to increase thecapacitors’ reliability under surge conditions. For lowESR types, dielectric formation was optimised andmade thicker at the outer anode region. Consideringthat capacitance drops with frequency, an effectlargely caused by current density migration to theouter region of the anode, it follows that a highfrequency event such as surge causes very highcurrent density in this outer region. Hence the outerdielectric region was thickened, which resulted in amuch better surge handling capability.

Several other improvements have been made whichhave had a very positive effect on reliability, includingthe addition of a moisture barrier layer, in-linetransient surge current screening, and in-line solder

reflow screening. The industry’s addition of solderreflow screening highlights a major concern, and theimportance of remaining within recommended solderprofiles during mounting.

6. Risk Mitigation and Additional Activities

To complement the in-depth investigation into thedevelopment activities outlined above, ESA hasundertaken a series of evaluation test campaignsspecifically targeted at the use of solid tantalumcapacitors in power supply filter applications. Thetesting has included both single and parallel bankconfigurations in real power supply filter circuits.Results of these tests have been very positive, and apaper has been written outlining the test conditionsused and results obtained. The paper, “Investigationon Tantalum Capacitors in Parallel Configuration”,written jointly by Paul Collins, Olivier Mourra, JohnHopkins and Ferdinando Tonicello, was presented atCartes Europe in October 2005.

In the future, a study into the criticality of appliedsolder profiles is planned, but at the time of writingno start date has been fixed.

The relevant section of the new derating document,ECSS Q30-11, reflects the findings of theinvestigation and evaluation testing to date with aseries of application and procurement directives.

For further information please contact:[email protected]

References

[1] “Basic Tantalum Capacitor Technology”,John Gill, AVX Ltd.

[2] “Tantalum and Niobiam Technology Roadmap”,T. Zednicek, B.Vrana, AVX Czech Republic.

[3] “Tantalum in Power Supply Applications”,John Prymark, Kemet •

ESCIES: ESCC and ESCIES websitesESCIES, the European Space ComponentsInformation Exchange System website, is a system byand for the space components community; it has beenhosted and facilitated by ESA for the last five years.The system has run successfully with approximately99.9% availability over this time. Any unavoidable

planned ‘downtime’ is always announced in advance onthe website.

In the past period, data contribution from registeredorganisations was actively encouraged. Presentations toparts of the ESA Science Directorate have led to



valuable feedback. The preparation of a data providers’kit is in its final stages: the kit should be available bythe time you read this article.

As we have seen the usage of and contribution toESCIES increase steadily over the last five years, it issafe to say that ESCIES is meeting its goal of serving asa central point for collaboration and data exchangewithin the Space Components community. You willfind a more detailed description of the growth below.

The parent of ESCIES, spacecomponents.org, is theESA ESCC website. ESCIES is defined as an ESCCproduct; what is produced on the ESCC website willbe published in ESCIES. The ESCC site provides ameans for working groups to collaborate in private,prior to publishing papers.

Both sites hold a large amount of information (severalgigabytes). They have fine-grained access control: auser has to belong to a ‘group’ to see a link to adocument on either website, e.g. the ESCIES privatearea or MEMS. If a user does not belong to a group andhappens to know the link to a page, then a second layerof security prevents the user from accessing the page.There are further levels of security at lower layers,beyond the scope of this article.

Growth of ESCIES

June 2005 was the busiest month ever for ESCIES.Initially, several years ago, the increase in traffic wasclose to exponential; now it is close to linear but stillincreasing. That may just be a sign of a slowdown inrate of increase in Internet connections, which nolonger follows Moore’s Law.

WEEK ONE, JUNE 2000 4000 hits

WEEK 144, YEAR 2003 The first week withmore than 30000 hits.

YEAR 2005 The trend continues with 1.5million requests so far thisyear, 6000 per day(excluding search engines).



Wide band gap semiconductor materials havefundamental properties which make them highlysuitable for a variety of RF and microwaveapplications. These electrical properties aresummarised in Table 1 and include a band gap that ismuch higher than Si or GaAs (e.g. 3.4eV for GaNcompared with 1.4eV for GaAs), a large breakdownfield, and a high saturated electron velocity [1]. Thehigh electron mobility of the AlGaN/GaN materialsystem, along with the ability to grow deviceheterostructures such as high electron mobilitytransistors (HEMTs), means that it is the preferredchoice for frequency operation above 8GHz [2].

GaN devices offer significant potential for realisingmicrowave components with an order of magnitudeimprovement in output power capability compared toGaAs or Si, with frequency of operation up to at least100GHz [3]. The wide band gap means that thematerial is radiation hard and that there is the potentialto operate devices at temperatures of 200ºC or higher,with significant reduction in the size, mass andcomplexity of cooling systems.

These performance benefits are of significant interestfor realising next-generation space-based payloads.

ESA has been undertaking research into wide band gapsemiconductors for several years. Ongoing researchactivities are listed in Table 2. In addition, nationaldefence agencies have been undertaking research intowide band gap technologies across Europe, which hasculminated in the recent launch of KORRIGAN (a

multi-nation European R&D project aimed atdeveloping wide band gap technology for defenceapplications).

To help improve research coordination across Europe,ESA held a round table meeting to promote andfacilitate information exchange on wide band gaptechnologies for microwave applications. The eventtook place on 17/18 March at ESTEC in Noordwijk,The Netherlands. Over 60 people attended the roundtable, from academia, industry and governmentresearch establishments (including members of theKORRIGAN consortium). 23 presentations weregiven over the two-day period, covering aspects suchas device physics, material growth, transistor electricalperformance and disruptive applications of GaNtechnology.

ESCC-Sponsored Technology ‘Round Tables’ESA-CTB Round Table Consultation on Wide Band Gap Technologies for Microwave Application

Si GaAs 4H-SiC GaN

Bandgap (eV) 1.12 1.43 3.26 3.4

Breakdown field(V/µm)

30 30 250 250

Thermalconductivity(W/cmK)

1.5 0.5 4 1.5

Sat. velocity(cm/s)

1E7 1E7 2E7 1.5-2.7E7

Elec. mobility(cm2/Vs)@2E17cm-3

600 4000 400 1000-2000

Table 1: Comparison of material electrical properties

In other words, ESCIES now receives approximatelyas many requests per day as it did per week during itsfirst year of operation.

Growth of ESCC

The ESCC website, spacecomponents.org, has grownrapidly with the addition of new data and applications.When you log in, you will see only a small fraction ofwhat is available, depending on your working groupmembership. This site is developing into an on-lineapplication for workflow management, rather than thetraditional website envisaged at its conception. •



Highlights of the workshop included results presentedby IMEC on novel interlayer growth techniques thatcan allow high quality, low dislocation density growthof GaN/AlGaN layers on host substrates. This isimportant since GaN is not currently available insingle crystal boule form. Accordingly, GaN and itsalloys have traditionally been grown epitaxially ondissimilar substrates, in particular sapphire, SiC andmore recently Si. There is a large lattice mismatch of~3% on SiC and ~12% on sapphire, so a very highdensity of dislocations in the structures is required toaccommodate the mismatch (typically 108 – 1010 cm-2

of threading dislocations compared with 103 cm-2 onGaAs). IMEC have shown that they are able to growhigh quality GaN/AlGaN device layers using AlNinterlayers that reduce formation of threadingdislocations, as illustrated in Figure 1.

Impressive results were also presented by TNO andIAF for the first ever reported European Ka-band GaNMMIC power amplifier (Psat 2W at 27GHz). Aphotograph of this state of the art MMIC is shown inFigure 2.

Impressive noise figure and overdrive measurementresults were also presented by two research groups(QinetiQ and Thales), illustrating that there isexcellent potential for realising highly robust receiverfront-ends using GaN HEMTs, but without the needfor expensive and bulky limiters.

Overall the round table event was well received andshowed that excellent headline performance resultsare now being obtained in Europe.

Table 2: ESA Research Programmes on Wide Band Gap Semiconductors

ESA research contract Objectives Industrial partner

Microwave frequencycapability of wide bandgap semiconductors

Investigation of the microwave and mm-wave frequencypotential of GaN HEMTs. Device demonstration at 30GHz,60GHz and 94GHz.

TNO (NL) and IAF (D)

Athena Development of high quality epitaxial growth techniques. IMEC (B)

Noise assessment of wideband gap semiconductors(two consortia, workrecently completed)

Assessment of the low frequency and RF noiseperformance of wide band gap devices, including receiveroverdrive measurements, transmit/receive modulesimulation, oscillator phase noise evaluation.

(1) Thales (F), IEMN(F), TNO (NL) (2) QinetiQ (UK), Astrium (UK),phConsult (UK), IXL (F)

X-band TT+C SSPA Development of a hermetically packaged transistor familyfor realisation of a 30W, X-band power amplifier.

TESAT (D) and FBH (D)

Thermal management andpackaging

Investigation and demonstration of novel thermalmanagement techniques.

Inasmet (S),CNM (S), UPM (S),AMS (I)

Figure 2: First 2W Ka-band GaN power MMIC in Europe(Courtesy of TNO)

Figure 1 : Reduction of threading dislocations at theAlGaN/GaN interface (Courtesy of IMEC)



DC-DC converter technology is essential to the spaceuser: it appears on every spacecraft and the interfacebetween the power system and the analogue ordigital equipment. The types of DC-DC convertertopologies and the voltage supplied for Europeanmissions vary significantly, and even thoughconverters are used consistently and extensively, theyare generally considered by systems people as anafterthought to the equipment. The types ofconverters used to date in European missions includecustom-designed PCB based units, 3D modules, andoff-the-shelf hybrids.

The driving factor for the procurement of a DC-DCconverter is usually the efficiency, which is typicallybetween 65% and 85%, and for EPC applications it is»85%. Other factors include power rating, outputsignal quality, synchronisation to allow multipleconverters to be coupled, and a design mitigation thatensures that the spacecraft power bus is notcompromised if there is a short circuit condition.

ESA has long recognised that DC-DC convertertechnology is very complex and that for Europeanmissions it is one of the last items considered,resulting in a custom design solution. Consequently,European DC-DC converters often carry long leadtimes, high non-recurring engineering costs and largeunit costs. In addition, the technology often chosenin the converter is subject to the application of ITARrestrictions or export license, or procured through acontrolled supply chain; in particular the powerMOSFET, the driver and the pulse width modulator(PWM).

The US market is of interest to European users withthe hybrid DC-DC converter solutions reportedlyoffered to Mil38534 QML system. However, theseoff-the-shelf items often do not have the assurancelevels required for the space user, or they are subjectto export restriction. Additionally, the European userhas no visibility of the design, the testing or theheritage of the product, all of which is an unsuitable

ESA-CTB DC-DC Converter Round Table Consultation

The key conclusions were as follows:

• Good RF output power performance and highpower densities are being achieved by severalcompanies (e.g. 7W/mm at 2GHz, 6W/mm at10GHz, 3-4 W/mm at 30GHz)

• NORSTEL AB announced it is setting up a newcommercial facility for manufacture of SiC wafers.This will allow Europe to establish an independentsupply route without having to rely solely on USsuppliers such as Cree.

• United Monolithic Semiconductors (UMS)announced it has included GaN on its developmentroadmap, with MMIC production planned for2009-2010.

• More work is required on fully understanding thesystem drivers and implementation benefits ofusing GaN. In addition a wider range of devicefunctions (e.g. switches, mixers, multipliers, diodesand oscillators) should also be considered.

• Component reliability is felt to be the number oneproblem to resolve, with device output powertypically fading after only a few hundred hours ofoperation. As a result, significant research and

development effort is still needed within Europe toensure this problem is overcome and that GaNtechnology is available for use in space-basedsystems by 2009-2010.

ESA is currently using the key findings from the roundtable to better focus its future research programmeson wide band gap technologies. One of the outcomeswill be to ensure that there is coordination andinformation exchange between the ESA andKORRIGAN research activities, to avoid duplicationand ensure rapid transition of the technology fromresearch to manufacturing industry.

For more information on ESA wide band gap researchactivities contact: [email protected]

References

[1] R J Trew et al, “Microwave AlGaN/HFETs”,IEEE Microwave Magazine, March 2005

[2] S McGrath and T Rodle, “Moving past the hype:real opportunities for wide band gapcompound semiconductors in RF power markets”, GaAs Mantech digest”, 2005

[3] Compound Semiconductor, 29th March 2005,“152GHz GaN transistor” •



Published by: ESA Publications Divisionc/o ESTEC, PO Box 299, 2200 AG Noordwijk, The NetherlandsTel. (31) 71 565 3400 - Fax (31) 71 565 5433

Editor: Karen Fletcher

Layout: Eva Ekstrand

To change your subscription details, please contact one of the membersof the editorial board.

Editorial BoardJohn Källberg, ESA/ESTEC (NL)[email protected]

Jean-Claude Tual, Astrium Vèlizy Villacoublay (F)[email protected]

Philippe Lay, CNES Toulouse (F)[email protected]

John Wong, ESA/ESTEC (NL)[email protected]

state of affairs for the high-reliability spacemanufacturer. ESA therefore hosted a round tablemeeting at ESTEC on 13 April 2005, held inconjunction with the Component Technology Boardand the ESCC, to understand the space users’ needsand determine European industry’s level of interest indeveloping a series of off-the-shelf, reliable convertersthat are short lead time and low cost.

The meeting consisted of DC-DC convertermanufacturers, space users and other interestedparties, and suggested that while some DC-DCconverter manufacturers are indeed producingradiation-tolerant devices, they are, as predicted, highcost and long lead time. However, what is of interest isthat when European ‘components’ are compared withtheir U.S. equivalent converters, the pricing is of thesame order and yet the performance and qualityassurance provisions are as good as or in some casesbetter than the US products. The meeting alsodiscussed off-the-shelf converter products, which areprovided by several European manufacturers asprototype DC/DC converter items with a tailored up-screening process that also assures the off-the-shelfconstituent parts being used in the design. In addition,the range of these products consists of those withalternative packaging methods, for example the typesannounced by the company 3D-Plus. The price ofthese ‘commercial’ units is cited as very low, howeverthey are not yet qualified and the risks to date areunquantified.

The ESA round table meeting provoked manydiscussions. The presentations of the attendees arelisted in the ESCIES conferences section onhttps://escies.org/, while the conclusions of themeeting may be summarised as follows:

• Innovative topological solutions are not evident,therefore the use of commercial components mayhave to be evaluated.

• The price of Hi-Rel components can account for 60or even 70% of the recurring price

• At present, Hi-Rel MFET drivers and PWMcontrollers are only available from USmanufacturers, and these may be subject to ITARrestrictions in the near future.

• It seems urgent and appropriate to develop basicEuropean components to be used in DC-DCconverters (e.g. Hi-Rel, possibly radiation-hardMFET drivers and PWM controllers).

• The availability of the basic power conversionbuilding blocks must be guaranteed before thedevelopment of the hosting equipment.

• Existing European DC/DC converter products andsolutions are not always known to externalpotential users (PIs, etc)

• It is necessary to advertise what EU industry canoffer!

• A handbook or datasheet file of available offeringsmay be one way of doing this.

The DC-DC converter user’s (payload developer’s)point of view needs to be further investigated.

For more information on ESA DC-DC converteractivities, contact [email protected] •


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