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.