Fabrication of a Superconducting HotElectron Bolometer Receiver with
Micromachined Waveguide Components
Aaron Datesman
Bechtel Bettis Atomic Power Laboratory, West Mifflin, PA
With credit to Jon Schultz, Jian Zhang, and V.J. Wang-Goldfarb, Jr. (UVA),
Chris Walker and Dathon Golish (UAZ), and Jacob Kooi (CalTech)
(Stratospheric Observatory for Infrared Astronomy)
• 5 – 300 microns wavelength
• 9 first-light instruments
including GREAT and
CASIMIR
• First light winter 2005!
www.sofia.arc.nasa.gov
SOfIA’s IR Science:
• Interstellar cloud physics and
star formation in our galaxy.
• Proto-planetary disks and planet
formation in nearby star systems.
• Origin and evolution of biogenic
atoms, molecules, and solids.
• Composition and structure of
planetary atmospheres and rings,
and comets.
• Star formation, dynamics, and
chemical content of other
galaxies.
• The dynamic activity in the
center of the Milky Way.
• Ultra-luminous IR Galaxies
(ULIRGS) as a key component of
the early universe.
More than 120 molecular species identified in ISM and interstellar
gas clouds. Submillimeter wavelength regime is very rich in the
rotational transitions of these molecules.
Protostellar 4448-mm
Nisini 1999 / CFA
Mixers/Mixer Elements
Schottky diodes, SIS
junctions, and hot-electron
bolometers (HEBs)
Complementary technologies
based upon cryogenic cooling
requirements and the
superconducting energy gap
Signal
Mixer
IF Amplifier
IF Spectrum
Analyzer
Local
Oscillator
Spectral Intensity (a.u.)
Gerecht, 1998
PHEB mixers have been
installed on ground-based
telescopes
f
fIF
1
LO
fsig
850 851
Frequency (GHz)
PROJECT OVERVIEW:
(what’s new here?)
• Receiver is a 5-Element Array of HEB Mixers for 1450 GHz
Yield and Uniformity are Vital
• Focused-Ion Beam (FIB) Definition of DHEB Microbridges
Exploring the application of this tool as an alternative to EBL
• Waveguide Components Micromachined from Silicon
Inexpensive, Flexible, and Fast
• HEB Mixers on Silicon Nitride Membranes
With backshort underneath the probe, easy to make an array
• Device Passivation Scheme Using Sputtered Germanium
Niobium is a refractory metal, and thin Nb films oxidize readily.
YIELD and ESD
• Nb DHEBs are extremely susceptible to destruction by Electrostatic
Discharge (ESD).
Development of a fabrication process which suppresses or minimizes ESD
problems was a very important part of this research.
Re-ordering of the fabrication process steps, proper design of test equipment,
improved awareness about how to operate processing equipment, and reduced
handling of finished dies were all part of this effort.
The large die size, choice of substrate (Si), and passivation all may also have
helped.
• Correspondingly, DHEB fabrication yields are low
One researcher [Ganzevles] reported a yield of (a fraction of) 5%.
• To achieve a 50% yield with a 5-element array,
x5=0.5 or the individual device yield MUST EXCEED 87%!!
Bolometer
Pin
Mixer
Absorber
Heat Capacity
C
G
Tbath
C

 th

1

3 dB
G
P  I R0  PLO  2 PLO PRF e
2
0
jIF t
IF = RF - LO
Thermal Conductance
to Bath Temp.
.
V = VRFexp(jRFt) + VLOexp(jLOt)
Hot-Electron Effect
• Electrons and phonons do not interact, Tp ≠ Te
(very clean semiconductor, or very thin dirty
superconductor)
Electron Gas Bolometer

th
• Absorber is therefore only the electron gas; the
lattice does not play a role. Heat capacity C is
minimized.
Phonon-cooled HEB (PHEB)
Diffusion-cooled HEB (DHEB)
Superconducting Thin Film
1 m
I0
Normal Metal Contacts
I0
0.15 m
0.3 m
5 m
Pin
Pin
t = 3.5 nm
e
e-e

e
e e-ph
esc
e-e
e
e
e
diff.
t = 10 nm
Superconducting Microbridge
 th  e ph
L / 2  D e  ph  0.3 m
f3dB≈10 GHz (NbN)
f3dB≈4 GHz (Nb)
Diffusion-cooled HEB Operation
• “Diffusion” refers to the flow of heat, not carrier transport
• Microbridge is a distributed, not lumped, element
e
Local Electron Temperature T (K)
7.5
 eff 
7
6.5
6
TC
5.5
5
4.5
4.2 K
4
0
0.2
0.4
0.6
x/L
0.8
1
f 3dB
2
L
 D
2
1
 2
L
Hot-Spot Mixing
•Combination of RF & LO
signals in a square-law
detector creates IF “beat”
• Diffusion of heat to contacts
creates temperature profile,
which responds to the IF
variation
• Normally conducting hot
spot grows and shrinks in
response, creating a timevarying resistance at the IF
and an IF voltage signal.
1x4 Array of Nb superconducting DHEB mixers for 850 GHz operation
Wire-Frame Assembly Drawing
• Waveguide channel (140 m x 70 m)
• Waveguide probe & mixer circuitry
• Photonic crystal junctions (PCJs)
• Backshort cavity
Mixer Circuitry
• Semicircular waveguide probe
• Diffusion-cooled HEB
• Chokes, filters, and IF lines
FIB-Sculpted Probe Transition and Finished DHEB Microbridge
• Feature could not be fabricated reliably by photolithography and liftoff
• Integral part of the HEB fabrication process using the FIB
Laser Micromachining of Silicon – Feedhorn Block Fabrication
• Fast (105 m3/sec) laser microchemical etching w/out reference to crystal planes
• Also used for backshort & PCJs
1x4 Array of Nb superconducting DHEB mixers for 850 GHz operation
Backshort Block – HEB Block – Feedhorn Block Alignment Schemes
Backshort Block – Pyramidal Stubs and Corner Compensation
• Immersive etching using a solution of KOH:IPA at 80 C with a Si3N4 mask
• 127 ± 0.3 m depth is significantly better than the design tolerance
Concave corner
Compensation scheme
Dimensions in microns
Fabrication Procedures
• Each starts with a silicon wafer covered on both sides by LPCVD Si3N4
• Dicing occurs in the middle of the HEB Block procedure, before the devices
are fabricated. Therefore, FIB processing is performed on individual dies.
Backshort Block
1. RCA Clean
2. Silicon Etch Litho.
3. Si3N4 Etch
4. Photoresist Removal
5. Backside Markers
6. Silicon Etch
7. Si3N4 Removal
8. Laser μMachining
9. Metallization
10. Dicing
HEB Block
1. Liftoff Stencil Fab.
2. Depo. & Liftoff
3. Membrane Etch Litho.
4. Si3N4 Etch
5. Membrane Etch
6. Dicing
7. Imaging Au Depo.
8. FIB1-3
9. Bridge Etch & Msr.
10.Germanium Depo.
11.Passivation Litho.
12.Ge/Nb RIE Etch
13.Packaging
HEB Block Fabrication
• Circuits rest upon silicon nitride membranes 0.75 m thick.
• Deposition occurs under a single vacuum, assuring a clean interface.
• Niobium serves both for the device layer and as an RIE etch mask.
• Silicon nitride serves as the mask for the membrane etch, which is performed
using a meniscus etching technique.
• Dicing follows the membrane etch, but still occurs in the middle of the process.
• Sputtered Ge is used as the passivation material.
Nb Etch Mask 300 Å
Gold 3000 Å
Microbridge Nb 100 Å
Focused-Ion Beam HEB Fabrication
Gallium Focused-Ion Beam (FIB)
• Beam waist 550 Å at 350 pA
• FIB1-3 process sculpts bridge kernel
with about 1000 Å of gold remaining;
removes Nb mask
• Microbridges as small as 0.15 m x
0.10 m have been fabricated
FIB3 Pattern Alignment
• Milling with gallium not selective
between Nb and Au
• Tilting stage at an angle allows
removal of just the top of the kernel and
prevents contamination
• Length of FIB3 cut determines
microbridge length
FIB View of Finished Kernel
• Geometry determines FIB3 width
• Must align and focus FIB3 pattern,
which varies from device to device
• Kernel thickness may vary as much as
200 Å
• Contamination of microbridge occurs
during FIB2, and during FIB3 focusing
Stress of Thin Nb Films Deposited on to Silicon/Si3N4 Substrates
• Compensate for target erosion
with continued use
• Devices made from films with
high stress exhibit poor
superconductive properties.
• Stress of films on Si/SiO2
substrates was controllable and
repeatable (not shown).
• Stress of films on Si/Si3N4 was
never very repeatable, but may
have been controllable prerefurbishment
• All of the devices described
later in this talk were fabricated
post-refurbishment.
Region Implanted
by FIB2 & FIB3
Top Surface in Contact
with Kernel Gold
Actual Orientation of
Gallium Ion Flux at Angle
to Exposed Surface
Region Implanted
by FIB2
Bottom Surface in
Contact with Substrate
Gallium Ion Flux Assumed
Normal Incidence to Exposed
Edge of Microbridge
Gallium Contamination
• 30 keV Ga in Nb: Rlong = 125 Å, Rlat = 49 Å
• 10 x 1018 cm-3 threshold:
FIB2: 200 Å (60% free)
FIB3: 330 Å (* variable *)
• Peak dose 1021 cm-3 (2%) from FIB2
Further Materials Issues
• Gold misbehaves. Niobium is OK even though it oxidizes and reacts with Ge.
Melty & clumpy
Non-uniform bridge etch
Lengths are short
• Gold is removed from the microbridge
kernel with a low-power argon RIE etch
which is selective to niobium.
• Because the bridge etch is non-uniform,
the devices must be significantly overetched. (Also, wider microbridges etch
more slowly than narrow ones.)
• The HEB microbridges respond robustly
to this treatment (left, 300 Volts)
Devices Fabricated with a 300 Volt Argon RIE Bridge Etch
HEB08
HEB10T – HEB14T (pictured)
• FIB2 gap 54 pixels, FIB3 0.2 m
• FIB2 gap 28 pixels, FIB3 0.3 m
• 0.12 m long x 0.32 m wide
• 0.22 m long x 0.17 m wide
• 45 min. etch in 3 stages
• 75 W target resistance (50 W LHe)
• 31 ± 3 W RT resistance (83 W/□)
• Undiced, not membrane etched
Devices Fabricated with a 300 Volt Argon RIE Bridge Etch, cont’d
Results:
•Uniformity ± 26%, often better
• “Yield” 37/38
• 4 sets within 20% of 75 W RT
Variation Between Sets:
• Two not etched in stages
• Uncontrolled thin film stress
• Magnification calibration
• RIE chamber conditioning
Variation Within a Set:
• Width ±0.01 m – 6%
• Kernel thickness (200 Å) – 5%
Devices Fabricated with a 150 Volt Argon RIE Bridge Etch
HEB19 & HEB21
• FIB2 gap 22 pixels, FIB3 0.30 m
• 0.19 m long x 0.12 m wide
• FIB3 depth 0.15 m
• Etch times 90 – 220 minutes
• “Yield” 12/13; not enough devices
were made for statistical validity
• Single dies, membrane etched,
greased down on a clean glass slide
• Pictured device 32 W/□ at LHe
• RRR = 1.40 – 1.70
Resistive Transitions
• “A” – Silicon nitride, no membrane, oxidized 13 hours then passivated (105 W)
• “B” – Quartz, oxidized 16 hours (121 W)
• “C” –Silicon nitride, membrane, passivated immediately (62 W)
Current – Voltage Characteristics
• “A” – Silicon nitride, no membrane, oxidized 13 hours then passivated (105 W)
• “B” – Quartz, oxidized 16 hours (121 W)
• “C” –Silicon nitride, membrane, passivated immediately (62 W)
Device “C”, I-V vs. Temp.
Similar to other published results, except:
• No negative differential resistance (NDR)
• No hysteresis (OK)
WHY? Normally conducting edges
Transition temperature gradient
Device “C”, I-V w/10 GHz RF
• Absorbs 10 GHz RF radiation,
which drives the microbridge
resistive
• Suggests device should act as a
terahertz hot-spot mixer
CONCLUSIONS:
• Use and limitations of FIB fabrication were explored.
Inconsistent magnification calibration
Non-uniform microbridge widths
• “Yield” of >90% was observed
ESD suppression
Passivation with sputtered germanium
• Devices near the 10 K target resistance were manufactured
150 Volt etch not superior to 300 Volt bridge etch (50 W/□)
Device resistance uniformity of ±25% or better
• Negative differential resistance was not observed
Complicated graded structure exists in both dimensions
• Finished 1x5 arrays have been sent to UAZ.
Gallium Implantation Experiments
• 30 keV Ga+ into 100 Å Nb/100 Å Au film
• Reduction in TC and increasing resistance
with increasing dose
• Idea: FIB trimming of finished HEBs!
Anomalous Increase of the Contact Pad Transition Temperature
Curve
Dose (1019 cm-3)
T (K)
R (Ω)
10 K R (Ω)
DR (Ω)
0
0
n/a
n/a
18.2
n/a
A
2.7
6
5.5
19.5
14
B
10.6
5.5
8.1
22.3
14.2
C
21.4
5
11.5
26
14.5
Additional Slides
Demonstrating that gold etches off of wider microbridges more slowly, and that
uncontrolled stress may be responsible for the variation between sets (HEB10)
Suggesting that an inconsistent magnification calibration may be to blame for some
non-uniformity, especially for very narrow microbridges
Gallium Contamination Curve
Residual Resistance Ratio