ICP-RIE of Ultrananocrystalline Diamond...Optimization of ICP-RIE, Oxford 100 NCCAVS Plasma Applications Group Meeting, San Jose, Febr 10, 2010. 300 250 200 150 100 50 0 in) 0 1 2

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ICP-RIE of Ultrananocrystalline

Diamond

UNCD

OP

NP

CO2

-e

N. Moldovan, H. Zeng, J. A. Carlisle

Ralu Divan

NCCAVS Plasma Applications Group Meeting, San Jose, Febr 10, 2010

.

Contact: N. Moldovan

[email protected]

www.thindiamond.com

(815) 905-9395

mailto:[email protected]

http://www.thindiamond.com/

Outline

• Motivation

• Preliminary thoughts on direct etching

• Optimization of diamond etching with ICP-RIE

• Discussion of tip etching results

• Conclusions


Ultrananocrystalline diamond (UNCD)


.

Smooth: 3-20 nm rms as grown, <1 nm “Horizon”

Thickness: 40 nm to 20 mm; <5% uniformity

Tunable electrical and thermal conductivities

High Young’s modulus (~900 GPa)

Biocompatibility

Up to 300 mm wafers

Most of bulk diamond mechanical properties

Deposited at 400-900 oC

HFCVD MPCVD

UNCD micro-manufacturability


.

Selective seeding (SiO2)

700-mm-long, 1.5-mm-thick,

2 to 10-mm-width

200 mm

Selective seeding through PRSelective seeding

through photoresist or SiO2 mask

O2-RIE through Al mask

Rounded edges

~1-2 mm resolution

Vertical edges, rough at the nanoscale

~ 1-2 mm UNCD

UNCD nano-manufacturability

E-beam lithography, lift off (Al, Ni), O2-RIE of UNCD, release by KOH (XeF2) etch

Molding

Tip-based nanopatterning of seeds and growth

2004

2006

NaDiaProbes, ADT

2008

2009

Why is molding not good enough?


<10nm

It can be spectacular, but…


Top look into a V-groove, showing a wedge.

Wedge-shaped tip

Double tip after oxidation-sharpening



Statistics

Best median value of wedge size: 13.3 nm

Preliminary thoughts on direct etching


.

Two ways:

• Isotropic underetching

Case of Si in XeF2

• Slope amplification by differential etching & sharpening

UNCD ICP-RIE – needs optimization


.

RIE : 1mm in 30 min (33 nm/min)O2:CF4 (50:2 sccm), 300 W, 20 mTorr

ICP-RIE: 1 mm in 10 min (100 nm/min)

O2:CF4 (50:2 sccm), ICP= 2800 W, RIE=100 W, 20mTorr

Our experience:

Literature:

O2-RIE: 30-40 nm/min

ECR O2 plasma, 400 nm/min (Pearton et.al. Electron. Lett. 28, 822 1992.)

Optimization of ICP-RIE, Oxford 100


.

300

250

200

150

100

50

0

Etc

hin

g r

ate

(n

m/m

in)

43210SF6 sccm

ICP=2500 WRIE=100WO2=50 sccmP=20 mTorr

T=20 oC

400

300

200

100

Etc

hin

g r

ate

(n

m/m

in)

25002000150010005000

ICP power (W)

RIE=100 WO2=50 sccm

T=20 oC

P=10 mTorr

500

400

300

200

100

0

Etc

hin

g r

ate

(n

m/m

in)

200150100500

RIE Power (W)

ICP= 2500 WO2=50 sccm

P= 10 mTorr

T= 20 oC

400

300

200

100

0

Etc

h R

ate

(n

m/m

in)

50403020100O2 Pressure (mTorr)

ICP=2500 WRIE=100 WO2=50 sccm

T=20 oC

Thermal oxidation of UNCD in air


.

• Diamond etches at > 600 oC in air(but slowly: 25 nm/min)

1600

1400

1200

1000

800

600

400

200

Th

ickn

ess r

em

oved

(n

m)

1000900800700600500

Temp (oC)

Oxidation in air1 hour

Stationary because entire film is etched

ICP-RIE rate with temperature


.

800

600

400

200

0

Etc

hin

g r

ate

(n

m/m

in)

200150100500-50-100

Temperature (oC)

ICP= 2800 WRIE= 200 WO2= 50 sccm

SF6= 0.5 sccm

P= 9 mTorr

• Almost no temperature dependence

• Isotropy unlikely in this regime

• Machine tripped above 300 oC (cooling problems)

• Do the isotropic etching by thermal oxidation sequentially

Investigating isotropy: test mask


• 100 mm x 100 mm field

• 3.5 mm UNCD/350 nm PECVD SiO2

• Exposed with Raith 150, (30 keV, 60 um aperture, 1554 pA, dose 400 mC/cm2)

• Resist: ma-N 1405 (MicroChem) @ 2000 rpm, baked at 100oC for 90 sec, thickness 350 nm, developed 30 sec in mD533S

• SiO2 etched by ICP-RIE (CHF3-Ar plasma)

• UNCD etched half way for rate measurement, then continued to full removal

Etched structures on 3.5 mm films

EIPBN 2009 Marco Island, FL.

Etched structures: best conditions for isotropy


Sample etched incompletely at 300oC ICP=2500W, RIE=50 W, P=50 mTorr , O2=50 sccm

• No sign of underetching

• No sign of isotropy

• Ultra-sharp “grass” originating from SiO2 in crevasses (~50 nm) on the surface, generating spikes by differential etching mechanism

• Mostly vertical walls, except the top of structures, where edges are curved by differential etching when the oxide vanishes

Thermal oxidation after ICP-RIE


Before After 675oC 30 min, air

Oxidation proceeds by roughing the surface and generation of cavities

Boosting differential etching


• 6.5 mm of UNCD

• 350 nm of SiO2• ICP= 2800W, RIE= 200 W, P= 9 mTorr

SF6= 0.5 sccm, O2= 50 sccm, T= 20 oC,

R<10 nm

From

F = 3 mm circle

12 min

Pleasant surprises: Nanowire ending tips

Are not that rare!

14%?

Statistics rules!


1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

Oc

cu

ren

ce

pro

ba

bil

ity

10009008007006005004003002001000

Wedge size (nm)

Molded tips, circular precursors

ICP-RIE, 3 mm circle precursors

Molded tips, circular precursors (test mask)

Molded tips, square precursors

Median tip radii 13.3 nm 23 nm 48.5 nm120 nm

• Tips considered: from the 6.5 mm batch, 3 mm circles

• ICP-RIE tips are somewhat worse than the best molded tips

• ICP-RIE tips statistics improved by the NW-ending tips

ICP-RIE: etching of pores*


100-nm-diameter pores 80 nm pores

1 mm low stress UNCD

Same ICP-RIE recipe

*to be presented at EIPBN 2010, O.Makarova et.al.

Conclusions


.

• Sharp tips can be obtained by ICP-RIE of UNCD, R<10 nm

• Isotropic underetching does not work

• Differential etching and sharpening work

• Post-RIE Oxidation sharpening does not work – roughness

• ICP-RIE was optimized: DRIE of UNCD?

• Maximum rate (674 nm/min) with ICP=2800W, RIE=200W, P = 9 mTorr, O2 = 50 sccm, SF6 = 0.5 sccm, T = 20 oC

• Technique is good for arrays of tips (field emission?), but still inferior to molding for AFM probes

•Some AFM probes may benefit from feathery shape – DPN?

ACKNOWLEDGMENTSUse of the Center for Nanoscale Materials, Argonne National Laboratory was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357

Q&A


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Documents

ICP-RIE of Ultrananocrystalline Diamond...Optimization of ICP-RIE, Oxford 100 NCCAVS Plasma Applications Group Meeting, San Jose, Febr 10, 2010. 300 250 200 150 100 50 0 in) 0 1 2