46

Rosetta GaAs solar cells

Tom Adams

It was all planned, like mice and men.

Sometime during the summer of 2o12, a

spacecraft named Rosetta would catch up

with the comet Wirtanen some 3o0,00o,oo0

miles from the sun. Rosetta was to travel

with the comet for a year of observations and

experiments. To meet Wirtanen, Rosetta

would have launched in early January 2003.

But the failure of Ariane 5-ESCA launch by

the European Space Agency (ESA) put

Rosetta first on hold, now on hold for a new

destination expected to be announced in

May. Whatever the outcome, Rosetta will

need the very high efficiency GaAs solar

cells which ESA chose. Their significant

advantages over silicon in this role are in

resistance to radiation and to extremes of temperature.

Rosetta GaAs solar cells (welds checl(ed) to head for a new target

Front view of the Rosetta primary structure under test

at Finavicomp, Finland, June 1999.

ESA officials record that Rosetta's flight path has

not yet been determined. Since "Rosetta can no

longer reach its original target" a launch "is not

expec ted for at least one year at the earliest."

The Rosetta team is n o w identifying alternative

targets that the spacecraf t could reach, in a

launch t imeframe of the next two-and-a-half

years, assembling a shortl ist of possible destina-

t ions .The main issues are scientific return, tech-

nical risks to the spacecraft , and the added cost

to carry out the n e w mission.

Among the possible targets repor ted are: Comet

Wild 2, (to be visited by NASA's Stardust in early

2004) Churyumov-Gerasimenko, Finlay, Howell,

and Schwassmann-Wacbmann 2.

"We have heard all these names, but we have

to actually spend t ime looking into them,"

says Paul Weissman, interdiscipl inary scientist

on Rosetta f rom the Jet Propuls ion Laboratory

CIPL). "Both Churyumov-Geras imenko and

Wild 2 are good targets. Both of those are well-

classified comets and w e k n o w quite a bit

about t hem and they would be excel lent tar-

gets; the o thers we k n o w quite a bit less about."

A final decis ion on the n e w target come t and

mission profile is e x p e c t e d for May 2003 at the

latest.

To successfully achieve its eventual mission,

Rosetta must first survive its launch and flight.

W h e n it arrives at the comet , its solar cells must

III-Vs REVIEW THE ADVANCED SEMICONDUCTOR MAGAZINE VOL ~.6 - NO 4" MAY 2003

Rosetta GaAs solar cells I N D U S T R Y FOCUS

generate sufficient power. A primary conce rn is

the welding of the solar ceils to their intercon-

nects. Previous research has s h o w n that solar

cells, and their welds, do quirk T things w h e n

exposed to space and the alterantives of dim or

ecessive light and very high or low tempera-

tures. To improve the long-term reliability of the

tiny welds, ESA abandoned previously used sol-

dering methods , wh ich tend to crack at very low

temperatures. Instead they selected an ultrason-

ic welding technique wh ich uses a sonotrode to

deliver a combinat ion of mechanical pressure

and ultrasonic energy pulses. Working at fre-

quencies from 16 to 70kHz to generate both

high temperatures and friction, the sonotrode

causes fusion b e t w e e n the solar cell and the

in te rconnect

Research into the success of the weld ing

process was carr ied out at Tecnopolis CSATA

Novus Ortus in Bari, Italy, by Dr. Giuseppe De

Liso and his colleagues. Each solar cell (40mm x

20mm x 0.35mm) is we lded to four small inter-

connec t s .The area of interest is the eight ultra-

sonic welds (each 0 .13mm x 0.20mm) on each

in te rconnect . In addit ion to optical examina-

t ion and x-ray, the former w h i c h occasionally

revealed gross defects, the team used reflection-

mode ul t rasound to examine the s t ructure of

the welds nondestructively.

The investigators w i shed to k n o w what effects

the tempera ture , pressure, and durat ion of the

weld ing p rocess might have had on the solar

cells. They were especially in teres ted in any

damage to the cells or anomalies in the we ld

itself. Both the in t e rconnec t s and the solar cells

are somewha t t ransparent acoustically, so reflec-

t ion-mode ul t rasound could be used to obtain

images of the we ld area w i thou t resort ing to

destruct ive techniques . Sonosean, the US maker

of acoustic microscopes , has had previous expe-

r ience in imaging welds on silicon solar cells

for earth-orbit ing satellites. Sonoscan and

Tecnopol is have worked toge ther before

Sonoscan cert i f ied Tecnopolis as an affiliated

acoustic micro imaging laboratory.

The acoustic micro imaging system used at

Tecnopolis has a single scan head which , as it

travels back and for th above the we ld area,

alternately transmits very high f requency ultra-

sound into the sample and receives the re turn

echoes . W h e n ul t rasound is beamed into the

sample, it may e n c o u n t e r three classes of inter-

nal features:

ROSETTA ORBITER PLASMA CONSORTIUM (RPC) (5 Sensors & joint instrument control, space- craft interface, power management unit (PIU))

• Physical properties of the nucleus • Inner coma structure, dynamics, aeronomy

\

• Development of cometary activity • Solar-wind interaction i } }

• Formation and evolution ~ / n ~ g m of plasma tail /(MAGI

"Solar-wind asteroid l hi: K.JH. Gl~k~r~r I~IG)

interaction ~ " B Langmulr Probo (UU =) 8" Pl: R. BcetrOrn (S) |

Mutunl Impedance Probe (MIP) PI: J.G. Trotignon (F)

*" Ion Electron SensOr (IF.~) Ph J.L. Burch (USAI

!

Ion Compo=ltion Analylmr (ICA)

PI: R. Lundln (S) Co-lnvesb'gators France Sweden UK USA . ~ Hungary

ESA

• the bulk of the in te rconnect and of the solar

cell, through which ultrasound travels easily.

• the interfaces in the sample, including the top

surface and the interface b e t w een the inter-

connec t and the solar cell. Some ultrasound

is ref lected back to the scan head by inter-

faces, enabling those interfaces to be imaged.

Each tiny weld also presents its own interface

and is imaged.

• any internal defects. Gap-type defects (delami-

nations, disbonds, cracks) reflect all of the

ultrasound back to the scan head, and are

therefore very strongly imaged.

Ref lect ion-mode acoustic micro imaging sys-

tems use ultrasonic f requencies b e t w e e n

10MHz and 200MHz. As the f requency goes up,

the pene t ra t ing ability of ul t rasound decreases,

but the spatial resolut ion increases. The 10

MHZ frequency, for example, is used to image

Ideas behind Rosetta intend- ed to be tested tracking com- ment Wirtanen will first travel to Mars in the Mars Express. Europe's first planetary mis- sion, will be launched in June 2003 from Baikonur, Kazak- hstan. This takes advantage of a once in 17 years minimal distance time windo~z Built by prime contractor Astfiurn, involving more than 20 European companies, it was built for half the costs of simi- lar, previous missions. The industrial team used off- the-shelf equipment and technology already developed for the Rosetta mission. New project management and more responsibility at the ini- tial stages o f the collabora- tion with the European Space Agency, successfully kept the project within the allocated time limits and budgets.

The solar power 30kg Beagle 2 is the smallest, most heavi- ly instrumented soft landing spacecraft ever produced. The Mars Express lander unit is intended to conduct a full suite of life detection experiments on Mars.

III-Vs REVIEW THE ADVANCED SEMICONDUCTOR MAGAZINE VOL ~6 - NO 4" MAY 2003 47

Rosetta GaAs solar cells

4 8

ESA's first 35-metre deep- space ground station, is situ- ated at New Norcia, 140 km north of Perth, Australia. The

630 tonne antenna will be used to track Rosetta and

Mars Express, as well as other missions in deep space. The

massive antenna is over 600t and over 40 metres high. This is the first of a series

deep space ground stations that ESA intends to build to

make a European deep space network.

Construction of the ~28m project began April 2000. The last six months of 2002 were

devoted to testing the electronic and communication

equipment. The station will be controlled remotely from

ESA's Space Operations Centre in Darmstadt,

Germany and the Perth International Telecoms

Centre at Gnangara.

PV cells currently being used are dual junction and triple junction GaAs cells, with two or three layers.

The solar cells are a sufficiently recent development that the European Space Agency ESA) has yet to use them in space. The first EU space application will be at the end of next year when the SMART-1 lunar mission is launched in July. SMART-1 will also test a solar powered ion engine.

RWE Solar GmbH (formerly ASE), is Europe's largest producer of solar cells for space. It is now preparing for large-scale production of triple-junction GaAs based solar cells for space applications, at its plant in Heilbronn, Germany. The ESA will use them in spacecraft such as PROBA-2 for technology develop- ment, GOCE for chart- ing the Earth's gravity, and Herschel and Planck for astronomy.

t he ent i re th ickness of relatively robus t plast ic

1C packages, whi l s t a t h i n n e r IC package, such

as aTSOR migh t be imaged at 30 MHz. Because

of the relat ive t h inness of the solar cell assem-

bly, De Liso and his col leagues we re able to use

the h igh acoust ic f requenc ies of 50 MHz and

100 MHz.

The opera to r of the system usually 'gates ' the

re tu rn echoes electronically to limit in ternal

imaging to a specific level or layer wi th in the

sample. He can do this because the r e tu rn

echoes are separated in time; by set t ing his gate

properly, the opera tor p roduces the acoustic

image only from those echoes re turn ing f rom the

level of interest . Echoes f rom o the r levels in the

sample are ignored.

The invest igat ing t eam first imaged a re fe rence

we ld sample w h i c h had no defects. A br igh t

rec tangle is the i n t e r connec t ; e ight dark struc-

tures are the welds. Each individual we ld is very

dis t inct and m a r k e d by a dark out l ine ( the

acoust ic 'edge effect ' ) and a very b r igh t cen t re

a rea .The b r igh t cen t r e area indica tes posi t ive

polar i ty in this reg ion - tha t is, the u l t r asound

t ravel led f rom a reg ion of lower acoust ic veloc-

ity ( the i n t e r c o n n e c t ) to a reg ion of h ighe r

acoust ic veloci ty ( the weld) . In a 100 MHz

acoust ic image of a defect ive we ld area

a l t hough the locat ions of the individual welds

are vaguely visible, t he ve ry s t rong re f lec t ion

f rom this area indica tes one large and two

smaller de lamina t ions w h i c h do no t e x t e n d

b e y o n d the we ld area proper .

The same solar cell was subsequen t ly imaged

w i t h Scanning Optical Microscopy (SOM), us ing

a Nd:YAG laser to w h i c h GaAs is t ransparent .

A l though SOM c a n n o t image good welds, it

can d e t e r m i n e w h e t h e r the we ld ing p roces s

has c rea ted a r ipple in the gold layer on the

GaAs, and w h e t h e r the crystal i tself has b e e n

damaged. The SOM image s h o w e d a d i s to r t ion

in the gold layer c o r r e s p o n d i n g nicely to the

acoust ic image of the de laminat ion . In addi t ion,

small dark areas s h o w e d w h e r e the same

excess sono t rode p ressure damaged the GaAs

crystal.

The 50 MHz acoust ic image of a different

we ld area shows no de lamina t ion is visible.

But two welds at lower r ight appear, acoustical-

ly, to be miss ing .The two welds at lower left

are of subs t anda rd size. To invest igate this

anomaly, r esea rchers sec t ioned the solar cell

and i n t e r c o n n e c t t h r o u g h the lower row of

we lds .The results in the 200x opt ical micro-

scope image s h o w e d a crack exis ted b e t w e e n

the i n t e r c o n n e c t and the solar cell, p robab ly

caused by i m p r o p e r p ressu re of the sonot rode ,

w h i c h p r e v e n t e d welds f rom be ing made in

this area.

In charac te r i s ing the we ld ing p roces s to be

used w i t h the solar cells, Dr. De Liso and his

g roup have e n c o u n t e r e d these and o t h e r types

of anomalies . The k n o w l e d g e t hey are ga in ing

a b o u t t e c h n i q u e s to we ld the cells and the i r

i n t e r c o n n e c t s successful ly will be p u t to work

w h e n the Roset ta spacecraf t final mee t s it

comet .

For now, Roset ta is s to red away, "safely and

cleanly, unt i l it is called upon." Engineers will

r emove all bat ter ies , take off the l ander har-

poons , and drain the fuel tanks. "The same care

tha t w e n t in to bu i ld ing the spacecra f t will n o w

be app l ied to s tor ing it and making sure tha t it

will be in per fec t shape for us to l aunch it

w h e n the date comes," says J o h n Ellwood,

Roset ta ' s Projec t Manager.

The cost of g round ing the miss ion was some-

w h e r e b e t w e e n C50 -100m.

Bibliography: 1. L.W. Kessler, "Acoustic Microscopy, 'ASM

Internat ional , Metals Handbook, Nin th Edition,

Vol. 17, Nondes t ruc t ive Evaluation and Quality

Control, pp. 465-482, 1989.

2. M. Cavone, G. De Liso, Michele Muschitel lo,

G. D 'Accol t i , "Charac te r iza t ion of Ul t rasonic

Welding Techn ique Appl ied to GaAs Solar

Cells,' P roceed ings of The 1st In te rna t iona l

Acoust ic Micro Imaging Society (IAM1S),

San Diego, California, J anuary 29-30, 1996,

pp . 51-57.

III-Vs REVIEW THE ADVANCED SEMICONDUCTOR MAGAZIRE VDL 16 - NO 4 - MAY 2003