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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
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Device Design and Process Considerationsfor Millimeter-Wave Diode Integrated Circuits
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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
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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.
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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.
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..
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.
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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.
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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
..
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•
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.
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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
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•
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
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The device characterization processincludes dc and rf tests.
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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.
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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.
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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.
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GaAs DIODE IeAPPLICATIONS
• Mixers• Multipliers• Samplers
SPECTRUM ANALYZER mm-WAVE MIXER
INTEGRATED MIXER DIODE
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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
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The integrated sampler has twodiodes, four resistors andtwo capacitors on a single chipwith beam leads.
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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.
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5954-1230
Flin- HEWLETT~~ PACKARD
April 1985
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Printed in U.S.A.