Device Design and ProcessConsiderations

for Mill imeter-Wave DiodeIntegrated Circuits

Eric EhlersDomingo FigueredoSigurd JohnsenMark Zurakowski

Microwave Technology Division1400 Fountain Grove Parkway

Santa Rosa, CA 95401

RF & MicrowaveMeasurementSymposiumandExhibition

Flin- HEWLETTa:~ PACKARD

www.HPARCHIVE.com

Device Design and Process Considerationsfor Millimeter-Wave Diode Integrated Circuits

2

www.HPARCHIVE.com

GaAs DIODE INTEGRATEDCIRCUITS

• Design Concepts• Fabrication Process• Characterization• Applications

COMPONENENTS AVAILABLEFOR INTEGRATION

• Schottky DiodesCjo-1.5fFlum 2

• Thin Film ResistorsRs = 50o/sq.

• Thin Film CapacitorsC = .67fFlum 2

• Beam leads

COMPOSITE MASK

3

At the front end of nearly all ofHP's new line of mm-wave instru­mentation can be found a GaAs diodeintegrated circuit developed andmanufactured at HP's MicrowaveTechnology Division in Santa Rosa,California.

The GaAs integrated diode circuitsconsist of a number of Schottkydiodes, thin film resistors andcapacitors. These components arefabricated on a small GaAs chipwith beam leads making connectionsto the outside environment.

The process technology used tofabricate these devices is beingsupported in manufacturing andprototype environments. The proto­type environment provides engineerswith the flexibility of designingtheir own customized circuit usingthe GaAs diode I.C. process. Com­posite mask sets containing severalcircuits are processed to providethe designer with samples for eval­uation and optimization.

www.HPARCHIVE.com

Device samples from composite maskruns are in sufficient quantitiesto supply the needs for the earlyphase of the instrument developmentcycle. When the device design isfinalized, it is stepped on to adedicated mask set for manufacturing.

PRODUCTION MASK

This diode integrated circuittechnology is ideal for mm-waveapplications involving frequencytranslation and other non-linearfunctions such as mixing, frequencymultiplication, detection, samplingand limiting. The ability to inte­grate diodes, resistors, capacitorsand beam leads allows on-chip diodebiasing and impedance matching.It also decreases the parts countfrom previous hybrid versions lead­ing to lower cost and higher relia­bility while achieving higherperformance.

FREQUENCY CONVERSION

SAMPLING

~Itp+~MULTIPLICATION

305 10 15 20 25

FIELD (kVfern)

-o~ 20.....Eo

ELECTRON VELOCITY

CD

o.,...->- 10 . _----------~ ---((,) - "5·g _------ IW _-> 0 L..-_-'--_-'-_--'-_--I__.L------l

o

The main building block for thesediode integrated circuits is theGaAs Schottky barrier diode. GaAsis the material of choice for mm­wave applications due to itsextremely high electron velocitycompared to silicon.

4

www.HPARCHIVE.com

..

SCHOTTKY BAND DIAGRAM

-----GaAs-----

~ELECTRONS

______Ec

- - - - - - - - - - - - - - - - - -E,

-----Ev

FABRICATION PROCESS

FOR

GaAs DIODE INTEGRATED

CIRCUITS

EPITAXIAL STRUCTURE

N-LAYERN + - LAYER

GaAs S.1. SUBSTRATE

5

A Schottky barrier is formed atthe interface between a metal anda semiconductor. The energy banddiagram for the N on N+ Schottkybarrier diode is shown here. Lowresistance and low capacitanceis due to the optimization of N+and N layers respectively. Highfrequency response is the resultof the high electron mobility inGaAs and its small charge storagecapacitance.

The fabrication process will nowbe described in sequence. Mostprocessing steps rely on standardoptical contact lithography andlift off for pattern definition.

Using liquid phase epitaxy, anN+ layer and N layer are sequen­tially grown on a semi-insulat­ing GaAs substrate. The Schottkybarrier is formed in the N layer.The N+ layer reduces the diodeseries resistance.

www.HPARCHIVE.com

A high degree of isolation isachieved in between devices byproton bombardments. A heavydose of protons is implanted athigh energies into the substrateexcept in those areas where thediodes will be fabricated.

The N layer is selectively etchedand ohmic contacts are formed byalloying the ohmic metal to theN+ epi. This provides a low re­sistance, linear contacts to thedevice.

The ohmic metalization also servesas the bottom plate of the inte­grated capacitors. The capacitoris patterned on the proton iso­lated GaAs to reduce the parasiticcapacitance.

6

PROTRON ISOLATION

/ ~II J II J

N·LAYER

N + - LAYERI---- '--

S.1. SUBSTRATE

OHMIC CONTACTS

/ \N-LAYER

N + - LAYER

BOTTOM CAPACITOR PLATE

Ohmic Metal is used forBottom Capacitor Plate

/

S.1. SUBSTRATE

..

www.HPARCHIVE.com

•

Ta N RESISTOR

S.1. SUBSTRATE

CAPACITOR DIELECTRIC

1000~ ofSilicon Nitride

/

SEMI I SUBSTRATE

SCHOTTKY CONTACT

/

Schottky ContactTi PtAu

S.1. SUBSTRATE

Tantalum nitride is deposited andpatterned by a dry etch processin the proton bombarded field toform thin film resistors. Thesheet resistivity of the Ta2N is50 ohms per square .

Silicononitride is deposited toa 1000 A thick film which is usedas the capacitor dielectric. Thedielectric is patterned by a dryetch using a photoresist mask.This dielectric has a capacitancedensity of .67 fF/sq. urn.

The TiptAu Schottky metalizationis defined on the N layer. TheSchottky forms the anode of thediode. Minimum design rulesallows for a .8 urn anode fingergeometry.

7

www.HPARCHIVE.com

The Schottky metal is also usedto form the top capacitor plateand the interconnection for thecapacitor, diode and resistorcomponents.

INTEGRATED STRUCTURE

The diode I.C. is passivated witha thick layer of polyimide. Viaholes are defined in the polyi­mide to make electrical contactto the diode I.C. components.

Schottky

\

VIA HOLES

r----~- VIAS

POLIMIDE

capacito\

Beam leads are plated through thevia holes to provide electricalcontact and bonding capability.

BEAM LEADS

/~ BEAM LEADS~

-----, ,-----

S.1. SUBSTRATE

8

www.HPARCHIVE.com

•

SEPARATION

~BEAMS~

--.. ,----

The wafer is thinned to 50 urn, andthe field in between the devicesis etched away leaving a smallGaAs pellet with beam leads extend­ing out from the side.

'~~~~:_-------->;;;S.1. SUBSTRATE

GaAs pellet/

DEVICE BONDED ON+ REL. TEST CIRCUIT

AIA~

The device is now completed andready for assembly.

DEVICE

CHARACTERIZATION

9

The device characterization processincludes dc and rf tests.

www.HPARCHIVE.com

Accurate current vs. voltage plotscan be made with the HP4l45 Semi­conductor Parameter Analyzer.Incremental resistance (6V/6 I)is also plotted on the graph.

IF(rnA)

20.00

2.000Idlv

VF .1500/dlv I VI

RS(0 I

100.0E+OO

10.00Idlv

The ideal diode equation definesthe following key diode para­meters:

10 = saturation currentRs = series resistance

n = ideality factor

A plot of log current vs. voltageis often useful. Extrapolationto zero volts yields the satura­tion current which can be usedto calculate the barrier height.The barrier height for GaAsSchottky-barrier diodes istypically .7 to .8 volts.

10

www.HPARCHIVE.com

IDEAL DIODE EQUATION

{q (V - IRs) }

10 [eXp -1]nkT

10 = saturation currentRs = series resistance

n = ideality factor

[hp]****** GR~~~\1~EPLOT ******IF

( AI CURSOR ( .5029V. 195. BnA •

o

26.5 GHz TEST PACKAGE

~ Co - planar test circuit

~oo

Small-signal characterization ofdiodes at microwave frequenciesis most easily accomplished bymeasuring S-parameters. For thismeasurement, beam lead diodes canbe conveniently mounted on micro­strip or co-planar transmissionline test circuits. This co-planartest circuit and fixture has beenused successfully to 26.5 GHz.

DEVICE ON COPLANAR SUBSTRATE

FINLINE TEST PACKAGE

00 waveguide housingo

o

o

enlargement showingbeam lead diode

11

This Scanning Electron Micrographshows a sinqle GaAs Schottky­barrier diode with beam leadsbonded upside down on a co-planartransmission line test circuit.

A wavequide test fixture is moreconvenient for measurinq scatter­inq parameters at frequenciesabove 26.5 GHz. This unilateralfinline test circuit is usefulfor measurinq beam lead diodes.

www.HPARCHIVE.com

Three applications of GaAs diodeI.C.'s are mixers, multipliers,and samplers.

The frequency range of microwavespectrum analyzers can be extendedto mm-waves with the use of externalmixers. An "anti-parallel" pairof GaAs Schottky-barrier diodeshas been used successfully in thisapplication as an even-harmonicmixer.

The two diodes in the mm-wavemixer have well-matched charac­teristics to minimize odd harmon­ic mixing and the beam leadsserve the dual purpose of minimiz­ing inductance and simplifyingassembly in the waveguide.

12

GaAs DIODE IeAPPLICATIONS

• Mixers• Multipliers• Samplers

SPECTRUM ANALYZER mm-WAVE MIXER

INTEGRATED MIXER DIODE

www.HPARCHIVE.com

mm· WAVE MULTIPLIER

fo 3 fo""I-- ~---- ---~ "'-I-- 40-60

LPF HPF 6Hz

BIRS

Diode multipliers can be used toextend the frequency range ofmicrowave sweepers. One exampleis this tripIer, designed foran output of 40 to 60 GHz.

INTEGRATED MULTIPLIER DIODEThe two diodes for the triplerhave well-matched characteristicsto suppress even harmonics atthe output and the three beamlead connections allow separatebiasing of the diodes.

SAMPLER FOR FREQUENCY COUNTER

Integrating diodes, resistors andcapacitors onto one chip canimprove performance by providingmatched components and inter­connections that are electricallyshort in length and reproducible.Instrument cost can also be re­duced by decreasing the number ofseparate components that must beassembled. As an example, allof the components shown insidethe dashed line of this samplercircuit have been integratedon to a single GaAs chip.

C3

~PUTC4

R4

R3

01

C2

02INPUT I

III R2

II 4t _

----- -------,I 11 C1II R1I

MICROUAV[ :

o---,.--+----l

13

www.HPARCHIVE.com

The integrated sampler has twodiodes, four resistors andtwo capacitors on a single chipwith beam leads.

14

www.HPARCHIVE.com

INTEGRATED SAMPLER

References:

1. R.J. Matreci and F.K. David, "Un-biased Sub-harmonic Mixers for Millimeter Wave Spectrum

Analyzers", in 1983 IEEE MITT-S International Microwave. Symposium Sigest, pp. 130-132

2. S. Gibson, "GaAs I.C. Adds Power to Low-cost Frequency Counters", Microwaves & RF,

Jan. 1985, pp.127-132

3. Sze, Physics of Semiconductor Devices, 2nd edition.

15

www.HPARCHIVE.com

5954-1230

Flin- HEWLETT~~ PACKARD

April 1985

www.HPARCHIVE.com

Printed in U.S.A.