xps/ups and efm - university of florida · xps/ups and efm brent gila xps/ups – ryan davies efm...

XPS/UPS and EFM

Brent GilaXPS/UPS – Ryan Davies

EFM – Andy Gerger

A 21st Century Approach to Reliability

XPS/ESCA

X-ray photoelectron spectroscopy (XPS) also called ElectronSpectroscopy for Chemical Analysis (ESCA) is a chemical surface analysismethod. XPS measures the chemical composition of the outermost 100 Å of a sample.Measurements can be made at greater depths by ion sputter etching to remove surfacelayers. All elements (except for H and He) can be detected at concentrations above 1.0atom %. In addition, chemical bonding information can be determined from detailedanalysis.

30 25 20 15Binding Energy (eV)

Ga(GaOX)

Ga(GaN) A Ga 3d peak with 2 curves fitted.Can determine relativeconcentration of Ga-O and Ga-N


XPS/UPS

XPS/ESCA Perkin-Elmer PHI 5100 ESCA System - MAIC

1. X-ray source – Mg or Al anode2. Sample3. Lens and aperture4. Hemisphere energy analyzer5. Position Sensitive Detector6. Data Collection - Computer


0 5 10 15

co

un

ts

sputter cycles

as-rec 12.5 min ozone 25 min ozone

Oxygen concentration vs.ozone exposure time

(depth)

Used XPS to determine the surfacecomposition/contamination and nature ofthe chemical bonding.

Oxygen surface conc. increases with ozoneexposure time, however, oxygen depthdoes not. Surface converted to anoxynitride. (sample temperature~50°C)

Surface carbon concentration reduced 50%by conversion to CO and CO2.

Optimize the ex-situ cleaning recipe

Past XPS use on HEMT passivation optimization


540 536 532 528

Binding Energy (eV)

OxynitrideonAlGaN(23%)cappedHEMT

80 76 72 68

Binding Energy (eV)

O1sAl2p

30 25 20 15

Binding Energy (eV)

as received 2 min sputter

Ga3d

80 76 72 68

Binding Energy (eV)24 20 16

Binding Energy (eV)540 536 532 528

Binding Energy (eV)

as-rec1st sputter

Native Oxide(0.7eV surface charge)

UV-ozone Oxide(0.6eV surface charge)

as-rec1st sputter2nd sputter3rd sputter4th sputter



OxynitrideonGaNcappedHEMT

24 20 16

Counts

Binding Energy (eV)536 532 528

Counts

Binding Energy (eV)

540 536 532 C

ou

nts

Binding Energy (eV)

O 1sGa 3d

28 24 20 16 12

Counts

Binding Energy (eV)

as-rec1st sputter

Native Oxide(0.6eV surface charge)

UV-ozone Oxide(4.8eV surface charge)

as-rec1st sputter2nd sputter3rd sputter4th sputter




Ga3dfromGaNcappedHEMTAs-ReceivedGaNSurface

Ga(GaOX)

Ga(GaN)

NoticethedecreaseintheGaOXportion(correspondingwithsputtertime)andtheshiftintheGa3dsignalassurfacechargeisreduced.

Past XPS use on HEMT passivation optimizationSPU

TTER TIM

E

0min

2min

4min

6min

8min

Ga 3d

GaOx

GaN



GaN

X-RAY

e

e

XPS signal

GaON

As x-rays enter the material,photo-electrons are generatedfrom different depths.Thick (2nm) ozone GaON isnot conductive;

Large surface charge (4.8eV) isindicated from XPS spectra shift.

28 24 20 16 12

Cou

nts

Binding Energy (eV)

Ga 3d

electrons leavingthe surface are not replenishedfrom the bulk.

+ + + + + + + + +Surface charge forms



CurrentCollapseMeasurements

GaNcappedHEMTsshowbetterpassivationthatAlGaNcappedHEMTs

IncreaseinAl%=increaseinsurfacereactivity=lesspassivation

HEMT structure As-received UV-Ozone

treatment Sc2O3

passivation

AlGaN cap20-23%

50% of DC 35% of DC 80% of DC

GaN cap3nm 40% of DC 40% of DC 95% of DC



Effects on Isolation Currentmesa to mesa leakage measurements

0 10 20 30 4010

-10

10-9

10-8

10-7

10-6

Before UV Ozone 25 mins UV Ozone UV Ozone + 3Days Effect

Isola

tion

Cu

rren

t (A

)

Voltage (V)

1 2 3 4 5 610

-9

10-8

10-7

10-6

1x10-5

1. Unpassivated2. O

3 + Sc

2O

3

3. O3 + MgO

4. O3 + SiN

X

5. O3 + NH

4OH + SiN

X

6. Ashing + NH4OH + SiN

X

Iso

lati

on

Cu

rren

t (A

)Passivation Method

Must have isolation for realization of HEMT circuits.

Large scale features to aid in optimization


Passivation Reliability

0 10 20 30 40 50 60 70

1.4

1.5

1.6

1.7

200°C age100°C agepassivateas-rec

Sheet C

arr

ier

Conc. (x

10

13

cm-2

)

Day

control HEMT (no age) aged HEMT (RT) aged HEMT (ele.temp.)

Hall effect samples (5mm x 5mm) areprocessed to determine the passivationeffect of the dielectric films. Samplesare aged at elevated temperatures todetermine effectiveness.

5mm Hall Effect sample

5mm

Hall samples are large enough for furtheranalysis (AFM, AES, XPS) for post agingstudies.

Hall sample w/ 10nm MgO passivation, a 15%increases in sheet carrier density post passivation

Large scale features to aid in optimization


Small spot XPS/UPS

Custom XPS/UPS on Nitride MBE system allows for sampletransfer without exposure to atmosphere, 3” wafer capable.Intermediate chamber available for additional processing or testing.

System based on a PHI 5400 XPS - Area resolution is <500µm,2500mm2 viewing area.

Aging of samples by both thermal stress (800°C sample heater)and bias stress (electrical feedthru to sample). Surface chemicaldesorption experiments and real-time diffusion data possiblewith sample heater.

Ultraviolet He arc discharge source installed for UPS


What UPS adds to the package

Mainly used for studies of the valence electron density of states. TheVUV (10-200nm) discharge covers a useful range from 10 to 50 eVphoton energy. The line width of excitation is generally a few meV,excellent for resolving molecular vibrational structures.

UV excited photoelectron spectra could be used for a comparison withband structure calculations and (in comparison with XPS) providing someadditional fingerprint information in surface chemical analysis.(ex. Determine Al2O3, AlPO4, Al4(P4O12)3 on a sample)

Provides a high resolution valance band spectra compared to XPS fordetermining valance band on surfaces.


Reliability of dielectrics

Reliability (aging) of the dielectric can be determined through boththermal stress and bias stress to determine method of failure. (largearea features)

The two regions for dielectric coverage have significantly differentsurface chemistries and structure.

Identify key interface species/reactions that lead to stablepassivation, isolation or field plates dielectric.

AlGaN

GaN

AlGaNRegion 2

Region 1

dielectric


Metal contact failure mechanism

0 40 80 1200.0

5.0k

10.0k

15.0k

20.0k

25.0k

Al

N

Ga

Au

O

Ni

Co

un

ts

Sputter cycles

Ni/Au gate on HEMT after thermal failure

-2 -1 0 1 2

-100.0n

-50.0n

0.0

50.0n

100.0n

Cu

rre

nt

Voltage

Using AES (area = micron to submicron) onecan easily determine certain metal contactfailure mechanisms.

Example: Ni/Au gate contactThe Ni diffuses to the surface,

leaving Au in contact with the AlGaN.

Fairly straight forward.


Metal contact failure mechanism

0 1000 2000 30000

10

20

30

40

50

60

70

80

90

100

Sputter Depth (Å)

N

Ato

mic

Co

nc

en

tra

tio

n (

%)

Au Ir

GaTi

Al

O

Ir-Control

0 1000 2000 3000 40000

10

20

30

40

50

60

70

80

90

100

Sputter Depth (Å)

N

Ato

mic

Co

nc

en

tra

tio

n (

%)

Au

Ir

Ga

Ti

Al

O

Ir-900oC

Good Ohmic Contact (Ir based)

Failure of Ohmic contacts not as easilyunderstood using AES.

Using AES one cannot determine thebonding state of the element, only theamount and depth position.

Example: A low resistivity ohmic contact isa blending of the different metals. Cannotdetermine if TiN interface formed.

Must use XPS.


280 285 290 295

Counts

/sec

Binding energy (eV)

acetone ipa copolymer mibk ipa

Identification of contamination

525 530 535 5400

5000

10000

15000

20000

25000

30000

Counts

/sec

Binding energy (eV)

acetone ipa copolymer mibk ipa

C 1s

O 1s

Use of XPS can identify differentsources of carbon contamination thatare left behind from deviceprocessing.

Characterization enhanced with UPS

AES can only determine that C andO are present, but not the species orsource.


XPS/UPSProjected MilestonesYear 1- Characterize operational and failed devices in the mesa isolationregion. Determine changes in surface chemistry and work function.Correlate chemical/electrical changes.

Year 2- Design test structures to be placed in the die cut lines. Apply teststructures to the cut lines and investigate variations in surface chemistryfrom post field stressing.

Year 3- Design in-situ bias stressing fixture for XPS-UPS. Characterize cutline test structures under bias stress using XPS-UPS. Identify degradationmechanism. Investigate reactions of semiconductor/passivation interface.

Year 4- Real-time observation of near-surface changes during biasstressing.

Year 5- Small spot size XPS capability for direct analysis on die


Electrostatic force microscopy - EFM

- - --- --- --

- -- + + ++ +

-- -

--

----

-

sample

tip

EMF trace

Image of high aspect ratio EFM tipavailable from Veeco.

Scan area in AFM Tappingmode, then apply a bias to the tipand scan in EFM mode.

Veeco Dimension 3100 with Nanoscope V controller iscurrently installed in the MAIC facility



Applied to pre and post aged devices to scanfor variations of the electric field on the devicesurface.

Examine defects propagating to the surfaceduring testing, electromigration of gate metalin the gate-drain region, and increases in thesurface potential from UV illuminationemptying traps.

Identify area for TEM/LEAP.


Projected MilestonesYear 1 – Map the device surface of operational devices varying substratebias and tip bias. Establish scan parameters for various compoundsemiconductor surfaces.

Year 2 – Map the device surface of failed devices and establish trends offailure in this region. Design probe cards for device biasing during scanning.

Year 3 – Correlate the failure mode to evolution of defects observed byTEM/LEAP. Incorporate defect generation and field profile into simulators.

Year 4- Real-time imaging of devices during stressing

Year 5-Establish limits of predictive forecast of reliability using short bais-aging cycles.



xtra


High resolution XPS/UPS

State of the art tools have <9um spot size and can build chemicalmaps of the surface – Physical Electronics & Omicron

High intensity monochromatic x-ray sourcesscan the surface.

Sphera – 60µm NanoESCA – 650nm


Imaging Mode XPS/UPS

Imaging at high kinetic electron energies.Ag evaporated on Cu, 35 µm field of view,20 min acquisition time on Ag 3d5/2.

xps/ups and efm - university of florida · xps/ups and efm brent gila xps/ups – ryan davies efm...

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