probe storage – concepts and challengesprojects.exeter.ac.uk/protem/images/bhaskaran ibm...

Zurich Research Laboratory

WIND/IMST © 2008 IBM Corporation

Probe Storage – Concepts and Challenges

Harish BhaskaranAnd the IBM Probe Storage Team

IBM Almaden, BurlingtonUniversity of WisconsinUniversity of PennsylvaniaUniversity of Patras, GreeceUniversity of Ulm, Germany

Collaborating partners:

IBM Portable Data Center


© 2008 IBM CorporationWind/ IMST 20082/39

Single Lever Write / Read Principle

Probe-Based Data StorageProbe-Based Data Storage



Highly Parallel Recording

Parallel Probe Data StorageParallel Probe Data Storage



Probe Storage: State-of-the-art not neededDon Eigler, IBM AlmadenXe atoms on Cu, Low Temp.~1000 Tb/in2

H. F. Hamann et al., IBM WatsonPhase Change Media (Ge2Sb2Te5)3.3Tb/in2

Cho et al., Tohoku UniversityFerroelectric Media (LiTaO3) 1.5 Tb/in2

IBM ZurichPolymer Media2Tb/in2 & 3Tb/in2

R.Bennewitz et al. UW MadisonAu atoms on Si, Room Temp.250Tb/in2

Density Depends on:

•Tip Shape-does not require state of the art lithography

•Media – Read/Write Mechanism-fundamental limit at atomic/molecular scale

•Positioning System Resolution-<1nm resolution demonstrated withMEMS based nano-positioners

Silicon tip produced with conventional lithography(3 micron min. feature size)

Tip Radius ~ 3nm

1990

2002

2006

2005

2002



Different Probe Storage Concepts

•Erasing not yet demonstrated• Low read back speed• Bit stability not proven in probe recording• Tip Wear

•Read back slow and complex •Samsung technique looks promising•Extreme Tip Wear!

• Low read back speed (thermal method)• Power consumption• Tip Wear

Issues

•Storage density, write speed, power

consumption very attractive• High readback contrast between states

•Storage density write speed, and power consumption very

attractive

•Storage density, write speed very attractive• Good readback sensitivity• Good polymer reusability• Low cost medium

Advantages

Similar to write process

Various forms of capacitive sensing or piezoresponsemicroscopy

Conducting Probe: apply voltage to tip and change polarization state of medium locally (4ns write time demonstrated)

Ferroelectric

Similar to write and 10 4

cycles demonstrated (IBM)

Monitor resistance change of microheater (IBM) or piezoresistive sensing (LG)

Thermal probe: apply voltage to heat tip and force tip into polymer (~1us write time demonstrated)Write-energy ~10nJ

Thermo-mechanical

Difficult – complex schemes required (Nanochip)Erase

Simple resistance change measurement – potentially very fast

Read

Conducting Probe: Apply voltage to conducting tip to change phase of medium (1nJ power and 50ns theoretically possible)

Write

Phase Change



scandirection

polymersubstrate

resistiveheater

Thermomechanical Writing



writingcurrent

scandirection

polymersubstrate

resistiveheater




scandirection

polymersubstrate

resistiveheater

writingcurrent




scandirection

polymersubstrate

resistiveheater

“1”




Less coolingby substrate

∆∆∆∆R/R ~ 10-4 per nm

=> T => RMore coolingby substrate

sensingcurrent

Thermomechanical Reading



Coil Magnet Scanner

Base Plate InterconnectSpacerBonding Pad

LeverElectronicCell

CMOS ChipLever

Interconnect

Storage Device Concept – Mobile Applications



Linearity: +/- 60 microns, Resolution: < 2nm

Fundamental Challenges - Nanopositioning

(a)

(b)

T↓ → R ↓

Microscanner with nm-scale accuracy and high;y linear operation demonstrated (see Lantz et. al., Nanotechnology, 2004)



F

Indentation phase diagram

t

- glass temperature- cross-linking- chemistry- film thickness- ….

Fundamental Challenges – Media Design Space

Dilemma: Little prior art & huge parameter space



scandirection

polymersubstrate

resistiveheater

Tip cannot go into previously written bit

Fundamental Challenges - Tip Wear

New bits are larger



Probes in archival storage

FP6 funded ProTeM (Probe-based Terrabit Memory) with objectives of using probe storage technology for ultra-high capacity, non-volatile, low power, low-cost, write-once and rewritable memories*

Address the needs of data storage in the domain of digital archiving, as per laws that govern these requirements

– the Sarbanes-Oxley Act

– Basel II

– IFRS/IAS

Tip wear and endurance even more important, since data is most likely stored by a sharp tip and will have to be read-out reliably –reliability in data retrieval is the key for archival storage

Probe-Storage is a very interesting and compelling emerging technology for archival and back-up

* http://www.protem-fp6.org/



scandirection

polymersubstrate

resistiveheater

Tip cannot go into previously written bit

But…..

New bits are larger

A problem for all forms of probe storage, and is especially exacerbated in phase change probe storage because of the high forces and hard media



Tip Requirements

Must preserve integrity (maintain dia<20nm) for 1010 read cycles – 10s of km

Additional requirements for probe technologies based on conduction –must also conduct reliably for this distance!

Coatings, if present cannot exceed in thickness the required final diameter – this is a problem

Must be mass manufacturable – no piece by piece assembly for each probe possible for manufacturing arrays of 1000s of probes

Issues with presently available tips

Silicon tips are worn through tribo-chemical wearSi-O-Si + H2O Si-OH +Si-OH

Silicon Tips do not conduct reliably (oxide formation) – an issue for electrical probe storage

Conducting tips using coatings not an option –other tips too blunt – 100s of nm tip diameter



Si-DLC Tips Integrated on Si Cantilevers

DLC Si

SiO2



SOI wafer with thermal oxide Pattern and etch anchor

Mold Sharpening

Mold Etching

Deposit Si-DLC Define and Etch DLC

Etch Cantilever Back-Side RIE Release of cantilever

Si SiO2 Si-DLC

Process Flow

Circle



Diamond-like-Carbon Ultra-sharp tips (r<5nm)

200nm

H. Bhaskaran et. al., Proc. MNE 2008



Existing state-of-the-art

Epinosa, Auciello at. Al, Novel UltrananocrystallineDiamond Probes for High-Resolution Low-Wear Nanolithographic Techniques, Small (2005)

Si doped DLC Tips

Dia ~ 9 nm


UNCD Tips



The Wear Test

Wear of a 22nm diameter DLC tip on thermally grown SiO2 (thk. 400nm)

Sliding wear is carried out at an applied normal loading force of 25 nN and at 0.25mm/s in a specialized home-made AFM

Wear is monitored in-situ as a function of increasing tip diameter, and correspondingly the adhesion. Adhesion is recorded after every 800 um of sliding

Spring constant of cantilever used is 65 mN/m, and was individually calibrated using Sader’s method



Adhesion curves on SiO2

0 500 1000 1500 2000

50

100

150

200

250

Sliding Distance (mm)

Adh

esio

n (n

N)

Silicon TipDLCDrops in Adhesion are probably

chunks of Si breaking off

DLC Curve looks even, and shows

very little wear



Wear Volumes

Si Tip

Wear Volume ~ 66x106 nm3

DLC Tip

Wear Volume ~17x103 nm3

Wear of DLC on SiO2 < 3000 times Si on SiO2

Evidence for oxide on Si-DLC: Junho Choi et. al. Depo sition of Si-DLC film and its microstructural, tribological and corrosion properti es, Microsys. Techn. (2007)




Platinum Silicide Tip Apexes

Not a coating… preserves tip geometry

Hard - Vicker’s hardness is 1761 GPa

(Hv (Si)= 1089 and Hv (Poly. diamond) = ~2000)

PtSi is an ohmic contact to Si –important for good conduction

Pt being a noble metal reduces probability of oxide at tip

PtSi can be easily formed ONLY at the tip by a single mask layer, and this process is completely compatible with standard MEMS processing

Fabricate tip Deposit

10nm Pt using a mask Anneal at 700oC.

Etch remaining Pt in 3HCl:1HNO3

Complete processing to fabricate cantilevers



Assessment of Wear and Conduction

The force of adhesion is used as a measure of wear

The sample we use is ta-C (for wear)

For conduction we use 200nm Au on SiO2

Tests were done on Si and PtSi tips fabricated on the same wafer, with the same spring constant

– For 40nN load we used a 100 µm long cantilever with k = 0.26N/m

– For 100nN load we used a 50 µm long cantilever with k = 1N/m

All tests done in ambient conditions (typically 22-25oC and 28-34% RH)



0 1000 2000 3000 4000 5000 60000

50

100

150

200

250

300

Sliding Distance (mm)

Adh

esio

n (n

N)

Si (k=1.13N/m)Pt (k=1.13N/m)Si (k=0.26N/m)Pt (k=0.26N/m)

Adhesion vs. Sliding Distance

PtSi

Si

100nN Loading force

40nN Loading forceSi

PtSi



Si-PtSi Comparison

0 0.2 0.4 0.6 0.8 1 1.2 1.4-20

0

20

40

60

80

100

120

Voltage (V)

Cur

rent

(µA

)

Si at ≈ 500 nNPtSi at ≈ 56 nN

H. Bhaskaran et. al., IEEE Trans. Nanotech. (2008)



Conduction Measurements (high current)

4.05 4.1 4.15 4.20

500

1000

1500

2000

Z position (µm)

Res

ista

nce

(kΩ

)

4.05 4.1 4.15 4.2-50

0

50

100

Z position (µm)

Def

lect

ion

(nm

)

Measurement done with a 10kΩ Series Resistor – Current through tip ~ 100µA



Encapsulated conducting tips

Conducting PtSi

Oxide encapsulation

Simultaneous measurement of deflection (bottom) and conduction (top) of encapsulated tip on TiN (commonly used as an electrode for PCM). Blue indicates approach and red

indicates retraction.

Chip with 4 cantilevers

Figure showing sustained conduction of encapsulated tip. The red indicates a conduction image during the first 1.6 mm

of scanning. The black indicates conduction image during the last 1.6 mm of a

2000mm scan

The voltage drop across the tip is the measure used for this figure.



Progress on tip endurance

We verify that PtSi is 3-4 times better than Si in terms of wear resistance

We verify that PtSi tips are much superior to standard doped silicon tips for conduction

A process to make conducting encapsulated probes with PtSi tips has been developed

Encapsulated tips have been shown to have much superior wear resistance and high adhesion force

DLC Tips of ~5nm radius have been fabricated

Their integration into standard silicon microfabrication has been demonstrated

Systematic wear tests pending, but initial results confirm that wear is significantly lower in these tips – potential for use in thermo-mechanical probe technology

Significant progress has been made in tip endurance enhancement, but future work must continue to concentrate on this aspect to improve

reliability



Conclusion

Probe storage offers an interesting alternative to compete in the archival storage sector

Many existing challenges (nanopositioning, media design and tip wear) are being actively addressed.

Tip wear seems to be a resolvable problem through: -

– Use of new materials

– Mass manufacturing methods compatible with standard microprocessing

– Superior scanning techniques to minimize tip wear (e.g. tapping)

Future challenges – Density Scaling and Cost

IBM Research GmbH

University of Exeter

ST Microelectronics S.r.l.

CEA

Fraunhofer

RWTH Aachen UT

Plasmon Data Systems Ltd.

Arithmatica Limited

Alma Consulting Group.

A European Project supported within the sixth framework program

5.3M5.3M€€(2006(2006--10)10)



Back-up slides



Probe Storage Technologies

Probe-Storage

State-of-the-art Nanofabrication

technology

New advances in polymer science and

chemistry

Understanding of nanoscale tip wear

Nanoscale heat transport measurements

Engineering complicated

control systems

Noise in NEMS

The probe storage device is the first true Micro/ Nano Electro Mechanical SYSTEM – precedent of future devices



Thermomechanical Probe Storage on Polymers(IBM, LGE)

Write: Thermal probe: apply voltage to heat tip and force to push tip into polymer(~1us write time demonstrated)Write-energy ~10nJ

Read: monitor resistance changes due to changingthermal conductance from topographyData-rate 30 Kb/s demo, 100 Kb/s possibleSNR ~ 10 dB, BER ~ 10-4

Erase: similar to write, lower power>104 erase cycles demonstrated

Advantages• Storage density, write speed very attractive• Good readback sensitivity, SNR• Good polymer reusability• Low cost medium

Issues to resolve• Low read back speed (thermal method)• Tip Wear• Bit stability potential, needs to be demonstrated• Power consumption

1.2 Tb/in2

SNR 9.2 dB1.2 Tb/in2

Erasing a subfield



LG: Polymer / Thermomechanical

Write: thermomechanical: write indentations using a resistively heated tip mounted on a cantilever

Advantages• Storage density, write speed very attractive• Low cost medium• Advanced fabrication and integration stage

Read: contact mode topography imaging using piezo electric sensor built into cantilever

Erase: not demonstrated yet, but should be like Millipede

Piezo-electric read

Transferred Cantilever array (CMOS + Cantilever)

Lever design

128x128 probe array

Issues to resolve• SNR/BW of readback signal may be limited• Low readback speed• Tip wear



Ferroelectric Probe Storage(Samsung, HP, Fuji, Pioneer, >10 Universities/Institutes)

Advantages•Storage density write speed, and power consumption very attractive

•Strong dependence on quality of material •Single crystals, epitaxial materials best, •poly-crystaline materials more problems

Write: apply voltage pulse to conductingtip in contact with ferroelectric media changes electrical polarization state(4ns write time demonstrated!)

Read: various forms of capacitance sensingor piezo response microscopy (slow)Samsung: tip with built in FET

Erase: similar to write process

717Gb/in2

5µs

4.7kb/s

-12V10ms

Issues to resolve•Read back slow and complex •Samsung technique looks promising•Extreme Tip Wear! •Bit stability/fatigue unproven, conflicting results, no data for elevated temps or > 1month at RT•Role of water unclear in imaging mechanism



Probe Storage on Phase-change media(Nanochip, IBM, Matsushita, LETI, Tohoku Univ., Exeter Univ.)

Advantages• Storage density, write speed, power

consumption very attractive• High readback contrast between states

Write: apply voltage pulse to conductingtip in contact with phase-change media amorphous to crystalline state(~1us write time demonstrated,~50ns theoretically possible)

Ultra-low write-energy ~1nJ

Read: conductance images: monitor currenton application of bias voltage betweenprobe and bottom electrodeRead power ~100nW/tip shown

Erase: re-amorphization by heating abovemelting temp. and rapid quenchingNOT demonstrated

Issues to resolve• Erasing not yet demonstrated• Low read back speed• Tip Wear• Bit stability not proven in probe recording

20 nm pitch 1.6 Tb/in2

IBM YKT14 nm pitch3.3 Tb/in2

Nanochiparray/actuator



Other Important MediaFerroelectric Probe Storage Phase Change Probe Storage

Reading in contact mode at high forces (50-200nN, typically 125nN)

severely reduces tip lifetime



Archival Requirements

Source: Plasmon Data Storage Limited



Archival Technologies

TCO ($/GB, 10 years)

Pow

er

Con

sum

ptio

n

Tape

UDO

probe storage – concepts and challengesprojects.exeter.ac.uk/protem/images/bhaskaran ibm...

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