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Low Temperature Magnetic Field SensitivityLow Temperature Magnetic Field Sensitivityof 2DEG based GaAs/AlGaAs Hall Sensorsof 2DEG based GaAs/AlGaAs Hall Sensors
andand
Temperature Correction to Magnetic Sensitivity Temperature Correction to Magnetic Sensitivity of a Hall Magnetic Sensor fabricated on an of a Hall Magnetic Sensor fabricated on an
implanted GaAs waferimplanted GaAs wafer
Low Temperature Magnetic Field SensitivityLow Temperature Magnetic Field Sensitivityof 2DEG based GaAs/AlGaAs Hall Sensorsof 2DEG based GaAs/AlGaAs Hall Sensors
andand
Temperature Correction to Magnetic Sensitivity Temperature Correction to Magnetic Sensitivity of a Hall Magnetic Sensor fabricated on an of a Hall Magnetic Sensor fabricated on an
implanted GaAs waferimplanted GaAs wafer
Ch. Ravi Kumar, T. S. Abhilash, G. Rajaram andB. Uday Bhaskar
School of Physics, University of Hyderabad, HyderabadAnd
Rita Saha, L.S.Vaidyanathan, K.GireesanSuperconductivity & Applications Section,
Materials Science Division, IGCAR, Kalpakkam
Presentation Overview
• Objectives• Specifications• Materials used, Mask Design & Fabrication• Results on 2DEG based Hall Sensor • Results of Sensor fabricated on Ion implanted wafer
•Correction for temperature dependence of sensitivity• Summary
This work is part of a project whose objectives were:
Fabrication and characterization of Hall Sensor Arrays useful for 2 applications
• Magnetic phase diagram studies on small superconductor single crystals (Sensor I)
• Non destructive testing (Sensor II)
Sensor I
Sensor II
Superconductor Magnetic phase diagram studies
•Crystals of size ~1mm are placed on the sensor.
•Should be able to measure Bz with 1μT resolution.
•Low Temperature operation T<100K.
•Sensitivity should not vary with T, T<100K.
•7-10 sensors with 100 m separation
•10 m x10 m active area (~100 vortices)
Specifications required for Application 1
Specifications required for Application 2
Non-Destructive Testing (NDT)•Magnetic methods•Flux leakage, eddy current
Nominal specifications:•Sensors need to be rugged•~Active area 50 m x 50 m •Ability to operate at RT or higher
Sensitivity of Hall Sensor
Sensitivity: KH= VH /IB =RH /t = 1/nse
CannotCannot be arbitrarily increased by decreasing ns, since R increases with the consequences:
For a maximum allowed power dissipation, maximum possible excitation current decreases.
Noise increases thus limiting the lowest measurable field. Hence increase KH by using materials with larger mobility H.
Choice of Hall Sensor Materials
Y. Sugiyama J. Vac. Sci. Technol. B13 (1995) 1075
2DEG Wafer Structure
ns ~ 5.3x1011cm-3,
H(295K) ~ 7326 cm2/V.s
H(77K) ~ 93190 cm2/V.s
•Supply Layer is degenerately doped- •Carriers available down to low temperatures
5 mask processMask I - Alignment marks
Mask II - EtchMask III - Alloy
Mask IV - Interconnect/padsMask V - Protection
Fabrication
No Ni under-layers to be used for improving metal film adhesion to GaAs. Cr or Ti are OK but not as good.
GaAs/AlGaAs Wafer with 2DEG layer
Etch(~1000Ao)
• Deposition: AuGe(88:12) or AuGe/Cr/Au
Alloy Ohmic Contact
• Anneal ~400oC, 30 s
Alloy Ohmic Contact
• Metallization Cr(20nm)/Au(200nm)
Interconnect
Protection Layer: Photoresist
Hard bake
Mask #4
1. Scribe wafer to cut 15mm x 15mm pieces (die + 2 to 3 ~10mm2
piece for etch-test) 2. Cleave: sandwich between filter paper and roll a (1") solid rod
on unscribed side. 3. Clean wafer: in acetone with ultrasonic power for ~10 min, dip
in methyl alcohol, dip in water, blow-off immediately, dry (100C, 5 min.)
4. Spin-coat photo-resist (eg. AZ1300-31), bake (85C, 20 min) 5. Expose with align mask- Mask #1 6. MCB dip (90 s), take out, blow-off 7. Develop (eg. in #2300 MIF), stop, blow-off, dry 8. Deposit 20 nm Cr, 200 nm Au 9. Strip photo resist, clean wafer 10. Clean wafer 11. Spin-coat photo resist. Bake. 12. Cut 2-3 test pieces (~3mm x 4mm) from wafer for etch process
calibration 13. Expose wafer with etch mask. 14. Expose test pieces with any part of any mask involving a step.15. Develop, stop, blow-off 16. Prepare etch solution: in beaker measure out and mix
Phosphoric acid, H2O2(30% soln), water (volume ratio: 3: 1 : 50) [or HNO3+HF+CH3OH at 40 : 1 : 15]
17. Dip test piece #1 for 60 s, test piece #2 for 90 s, test piece #3for 120 s etc(-100nm/min may be the etch rate). Use water as stop solution.
18. Interpolate to find time required for desired etch depth (-60 s for 100nm, 10 min for 1 micron)
19. Etch wafer, stop, clean thoroughly in (flowing, if possible) water 20. Clean, dry. 21. Strip photo-resist, Clean wafer 22. Spin-coat photo resist, bake 23. Expose with alloy mask. Mask #3 24. MCB dip (90 s)
GaAs Hall sensor arrays– Process StepsGaAs Hall sensor arrays– Process Steps25. Develop, stop, blow-off, dry 26. Deposit 100 nm Au(88%)Ge(12%)27. Strip photo-resist, clean wafer 28. Anneal: Atmosphere: flowing N2 ramp to 400C @ 250C/min, for ~30 seconds29. Clean wafer 30. Spin-coat photo-resist, bake 31. Expose with Au-interconnect mask. 32. MCB dip 33. Develop, stop, blow-off, dry 34. Deposit 20 nm Cr, 200 nm Au 35. Strip photo-resist, clean wafer 36. Clean wafer 37. Spin-coat photo-resist, bake 38. Expose with protection mask 39. Develop, stop 40. Hard bake photoresist (200C, 30 min) 41. Scribe to separate devices.42. Cleave 43. Bond to holder using silver paste (press with a finger) 44. Cure -150C, 30 min (depends on curing characteristics of silver paste) 45. Bond wires (25- 50 micron) 46. Test
Materials: Appropriate Photo resist, developer, MonoChloroBenzene, H3PO4, H2O2(30%), Au, Cr, Au(88%)Ge(12%) alloy
Mask #2
Mask #5
Scribing and cleaning
Force: 0 excess weight (scriber: Suss HR100)Cleaving: roll light rod, blow-off splinters, cleanCleaning: acetone (mild ultrasonic), propanol, water, flowing water, N2 blow off
Etch Parameters:Etch Solution: H3PO4, H2O2(30% soln), H2Ovolume ratio 3 : 1 : 50Etch depth planned: 90 nm (should be in the range 65nm-100nm)Etch time, etch-depth obtained: 80s, 80-90 nm
Alloy Ohmic ContactFilm: Au (100nm)/Cr(20nm)/AuGe(100nm)Ion-Cleaning: 5 min. (Ar 0.05 mbar, 1A primary)AuGe deposition: Resistive heating using tungsten boat (VS&P, UoH)Cr deposition: Electron Beam evaporation (VS&P, UoH) at 6kV, 20 mA; Vacuum: 3x10-6 mbarAu deposition: Resistive heatingAnneal after lift-off: 400 0C (30 s), Heating rate: 250 C/min, flowing N2
Lithography Parameters:
Aligner: Karl Suss MJB3Photoresist: S-1400-31 Shipley; spin-coating 5000 rpm for 30sIntensity & exposure time: Contact mode, 6 mW/cm2 and 15 sDevelopment: Normal: MF312 60s stop: water; Lift-off: MCB 90s, MF312 90s, water
Metal deposition: (align mask)Method: RF sputtering (Nordiko)Starting vacuum: 9x10-7 torrPlasma etch of substrate: Ar gas 6.2x10-3 torr, flow 77scc/min 4min. etch after 20min flowSputter etch of targets: Inc. RF 56W/refl RF 1W/DC bias 700V; Ar 14.2mT flow rate 76scc/m Duration 45minChromium deposition: 112W/2W/1200V; 9mT/81scc/m/200s, thickness ~ 5nmGold deposition: 52W/2W/180V; 4mT/81scc/m/195s, thickness ~ 50nm
Process Parameters SummaryProcess Parameters Summary
Chip Holder & Devices
Room Temperature Response of 2DEG Sensor
5
10
15
20
25
30
35
40
45
0 20 40 60 80 100Magnetic Field(Oe)
VH/I
(V/A
)
CH111(v1)_20plc
CH113(v3)_20plc
CH114(v4)_20plc
ch111(v1)_100plc
ch113(v3)_100plc
ch114(v4)_100plc
2DEG Sensor response as function of Temperature
• Hall Voltage at H=0 as function of temperature (offset)• Hall voltage at H=Ho as function of temperature (80 Oe)• For purposes of interpolation, fit function.• we used Butterworth functions:
443
210 )0,(
TP
PPHorHTV H
C
• Measured Field is
Typical calibration procedure
)0,(),(
)0,()(0
HTVHHTV
HTVTVH
CHa
CH
CHH
Sensitivity and Offset of 2DEG Sensor
Field Response of Sensor fabricated on Ion implanted wafer
Hall Voltage as a function of Temperature
Magnetic Sensitivity
•If temperature of sensor is known the procedure that has been outlined before can be used.
• However in applications such as NDT, temperature sensor may not be an integral part of the test system.
• The sensor itself may not be at the same temperature as the as the temperature sensor.
• Sensor resistance itself as temperature sensor?
Correction for temperature dependence of sensitivity
Sensor Resistance: Temperature dependence
Procedure for Temperature correction to the Magnetic Sensitivity
RbaR ooOS
R
RRR b
aRTTbaR
When an unknown magnetic field ‘B’ is applied then by measuring ‘RH’ and ‘R’, one can obtain the magnetic field as
H
OSH
K
RBRB
)(
Offset Resistance as a function of temperature
Calculated temperature using sensor resistance
RbaRH 11 Hall resistance as a function of temperature
CAL
OSCALHH B
RRRBRK
)(),( Sensitivity calculated using calibration field BCAL
Measured Values are ‘RH’ and ‘R’
Temperature Correction to the Magnetic Sensitivity of Implanted GaAs Sensor
2.0E-03
2.2E-03
2.4E-03
2.6E-03
2.8E-03
3.0E-03
3.2E-03
3.4E-03
3.6E-03
3.8E-03
4.0E-03
30 40 50 60 70 80 90 100Temperature(C)
Mag
net
ic F
ield
(T)
measured field using RTcalibration
Set field
measured field withtemperature correction
Effect of Sensor Magnetoresistance
Change in sensor magnetoresistance is ~7 for a mag. field change of 10mT. 1oC change in temperature Magnetoresistance change of 7.. hence neglected but requires correction when sensor operates at higher fields.
Summary2DEG based Micro Hall Sensors Were fabricated and calibrated. High sensitivity was obtained ~1300V/AT. Constant sensitivity at low temperatures (<100K),Useful
for studies on magnetic phase diagrams of superconducting samples.
Sensors fabricated on Ion Implanted wafer Sensitivity variation with temperature is ~4% / oC from
30C to 100C. Sensors resistance was used for temperature sensing
and to correct for the temperature dependence of the sensitivity successfully.
Acknowledgements
We gratefully acknowledge M. P. Janwadkar, Y. Hariharan and T.S.Radhakrishnan MSD, IGCAR for useful discussions and encouragement.We acknowledge Geetha Kumari MSD, IGCAR for help in preparation of AuGe alloy.
We thank BRNSBRNS for their support through research grants.
Acknowledgements
We gratefully acknowledge M. P. Janwadkar, Y. Hariharan and T.S.Radhakrishnan MSD, IGCAR for useful discussions and encouragement.We acknowledge Geetha Kumari MSD, IGCAR for help in preparation of AuGe alloy.
We thank BRNSBRNS for their support through research grants.