Metal-assisted Chemical Etching

(MacEtch)

Wet etch, Dry etch, and now MacEtch

Outline

• What is MacEtch?

• Background and Mechanism

• Characteristics

• Device applications

• Inverse MacEtch (I-MacEtch)

• Magnetic-field MacEtch (h-MacEtch)

• Summary

isotropic - • most wet etches are isotropic

i.e. they etch equally in all directions

• isotropic etches result in undercutting

for an isotropic etch, radius = depth

Wet Etchhttp://fabweb.ece.illinois.edu

rate determining steps - diffusion of reactants to the surface

reaction rate at the surface

desorption of reaction products

reaction equation mA + nB reaction products

reaction rate

RT

Enm

A

eBAkr

material removed

Wet Etchhttp://fabweb.ece.illinois.edu

Dry Etch

• Goal: – enhance anisotropy

• without losing selectivity

• without causing damage

• maintain controllable etch rate

• Mechanism:– Chemical:

• plasma etch - small degree of anisotropy

– Physical: • sputtering, directional but no selectivity

• Techniques:

https://www.crystec.com/trietche.htm

http://fabweb.ece.illinois.edu

Dry Etching Issues

charging

++++

undercutting

sidewall residue

trenching

mask erosion

Surface damage

`

http://fabweb.ece.illinois.edu

SiSi

• Process Flow

– Pattern metal

– Immerse in HF/H2O2

– Etching takes

place underneath

the metal

• Contrast with

– wet vs dry etch

– bottom-up growth

MacEtch: wet etch but directional

Metal-Assisted Chemical Etching (MacEtch)

Scalable, high throughput, low cost

Metal-Assisted Chemical Etching (MacEtch)

Metal

• not stationary

• not a mask

• not consumed

Not limited to Au

• X. Li and P. W. Bohn, APL, 77, 2572 (2000).

• Patents: 4 issued and 3 pending

MacEtch mechanism

– Mechanism: metal is a catalyst

– Rate determining step: charge or mass transport

– Simplicity: no bias, no high energy ions, no light

Local electrochemical reactions:

“Metal-assisted chemical etching in HF/H2O2 produces porous silicon", X. Li and

P.W. Bohn, Appl. Phys. Lett.77, 2572 (2000).

CHARACTERISTICS

MacEtch

“Metal-assisted chemical etching of silicon: a review,” Huang Z, Geyer N,

Werner P, de Boor J, Gosele U. Adv Mater 2011; 23(2):285–308.

“Metal Assisted Chemical Etching for High Aspect Ratio Nanostructures: A

Review of Characteristics and Applications in Photovoltaics,” X. Li, Current

Opinion in Solid State & Materials Science, invited review article, 16, 71-81

(2012).

Extremely High Aspect Ratio Si nanowire array: 550 nm diameter, 51 m height

produced by Au-MacEtch in 20 mins

Nanotechnology, 23, 305304 (2012).

Scalability

MacEtch of poly and amorphous

Jpn. J. Appl. Phys. 53, 02BE09 (2014)

Metal Pattern Formation and resolution

MacEtch has been realized using metal

catalyst formed by the following techniques

• Electroless plating (e.g. from AgNO3 solution)

• Colloidal nanoparticles

• Optical lithography

• Electron beam lithography

• Nanoimprint/soft lithography

• Superionic solid state stamping

• Nanosphere lithography

• High aspect ratio shadow mask

• Tip-based lithography (AFM, STM)

Applicability to different

semiconductors

• Si

• Ge

• GaAs

• InGaAs

• AlGaAs

• InP

• GaN

1 µm

GaAs p-i-n Pillar Array by MacEtch

Heterojunctions and p-n junctions

Sidewall morphology

• No high energy ion induced sidewall

damage

• Sidewall smoothness is determined

entirely by the edge roughness of the

metal pattern

• No porosity in the pillars if the MacEtch

condition is tuned as a function of

semiconductor doping level

3D patterns formed by MacEtch

1 µm

a) b)

by periodic variation of etchant concentration

DeJarld et al. Nano Lett. 11, 5259 (2011).

Magnetic-Field Guidance (h-MacEtch)

• Use Au/Fe/Au Tri-Layer catalyst

• Guidance

• Programmability

• a – c: Increasing External magnetic field guidance

Inverse MacEtch: InP

• Metal edge roughness no longer limits sidewall smoothness

• Limited aspect ratio (height/width) because of the inverse nature

“Inverse Metal-Assisted Chemical Etching Produces Smooth High Aspect Ratio InP Nanostructures,” S. H. Kim, P. K. Mohseni, Y. Song, T. Ishihara, and X. Li, Nano Lett. 15 (1), pp 641–648 (2015).

Examples of semiconductor

nanostructures produced by MacEtch

d) e) f)Sub-15 nm InP Fins

100/1 aspect ratio Si NWs n-GaAs pillars

f)Curvy-linear Si Holey Si from SOI

GaAs p-i-n pillars

MacEtch vs wet and dry etch

Wet Etch Dry Etch MacEtch

Directionality Isotropic Anisotropic Anisotropic

Aspect Ratio Low Medium High

Ion Induced Damage None Mild to Severe None

Crystal-Orientation

Dependence

Some Weak Weak

Etch Rate Fast Slow Fast

Sidewall Smoothness Smooth Not Smooth Smooth or Rough

Chemical Selectivity Good Poor Depends

Cost Low High Low

“Metal Assisted Chemical Etching for High Aspect Ratio Nanostructures: A

Review of Characteristics and Applications in Photovoltaics,” X. Li, Current

Opinion in Solid State & Materials Science, 16, 71-81 (2012).

APPLICATIONS

MacEtch

• Solar cells

• LEDs

• Photonic Crystals

• Thermoelectrics

• Transistors

GaAs p-i-n Pillar Based LED by MacEtch

“III-As pillar array-based light emitting diodes fabricated by metal-assisted chemical etching,”

P. K. Mohseni, S. H. Kim, X. Zhao, K. Balasundaram, J. D. Kim, L. Pan, J. A. Rogers, J. J. Coleman, and X. Li,

J. Appl. Phys. 114, 064909 (2013).

GaAs p-i-n Pillar Based LED by MacEtchGaAs p-i-n Pillar Based LED by MacEtch

Mohseni et al. Appl. Phys. 114, 064909 (2013).

• Pillar showed stronger emission at all injection levels

• Enhancement is stronger at higher I

MacEtched Si NW PV cell

Shin et al. IEEE J. Photovoltaics, 2, 129 (2012).

Efficiency limited by surface area

Shin et al. IEEE J. Photovoltaics, 2, 129 (2012).

It is all about the surfaces!

MacEtched Ph.C. Reflectors

I-MacEtch of InP

• Aspect ratio > 40:1

• MOSFETs – excellent offstateperformance

Song et al. unpublished.

Publications on MacEtch from Illinois

• “Inverse Metal-Assisted Chemical Etching Produces Smooth High Aspect Ratio InP Nanostructures,” S. H. Kim, P. K. Mohseni, Y. Song, T.

Ishihara, and X. Li, Nano Lett. 15 (1), pp 641–648 (2015).

• “Fabrication of Arbitrarily-Shaped Silicon and Silicon Oxide Nanostructures Using Tip-based Nanofabrication,” Huan Hu, Parsian K.

Mohseni, Lei Pan, Xiuling Li, Suhas Somnath, Jonathan Felts, Mark A Shannon, and William P. King, J. Vac. Sci. Tech. B, 31(6), 06FJ01

(2013).

• ''Photonic crystal membrane reflectors by magnetic field-guided metal-assisted chemical etching,' K. Balasundaram, P. K. Mohseni, Y-C

Shuai, D. Zhao, W. Zhou, and X. Li, Appl. Phys. Lett, 103, 214103 (2013).

• ''GaAs pillar array-based light emitting diode fabricated by metal-assisted chemical etching'', J. Appl. Phys, 114, 064909 (2013).

• "Silicon nanowires with controlled sidewall profile and roughness fabricated by thin-film dewetting and metal-assisted chemical etching,

"Bruno Azeredo, Jyothi Sadhu, Jun Ma, Kyle Jacobs, Junhwan Kim, KunHyuck Lee and James Eraker, Xiuling Li, Sanjiv Sinha, Nicholas

Fang, Placid Ferreira and Keng Hsu, Nanotechnology, 24, 225305 (2013).

• “Sub-100 nm Si nanowire and nano-sheet array formation by MacEtch using a non-lithographic InAs Nanowire Mask,” Jae Cheol Shin,

Chen Zhang and Xiuling Li, Nanotechnology, 23, 305305 (2012).

• “Porosity control in metal assisted chemical etching of degenerately doped silicon,” Karthik Balasundaram, Jyothi S. Sadhu, Jae Cheol

Shin, Bruno Azeredo, Debashis Chanda, Mohammad Malik, Keng Hsu, John A. Rogers, Placid Ferreira, Sanjiv Sinha, and Xiuling Li.,

Nanotechnology, 23, 305304 (2012).

• “Metal Assisted Chemical Etching for High Aspect Ratio Nanostructures: A Review of Characteristics and Applications in Photovoltaics,” X.

Li, Current Opinion in Solid State & Materials Science, invited review article, 16, 71-81 (2012).

• “Experimental Study of Design Parameters in Periodic Silicon Micropillar Array Solar Cells Produced by Soft Lithography and Metal

Assisted Chemical Etching,” J.C. Shin, D. Chanda, W. Chern, K.J. Yu, J.A. Rogers, and X. Li, IEEE J. Photovoltaics, 2, 129-133 (2012).

• “Formation of High Aspect Ratio GaAs Nanostructures with Metal Assisted Chemical Etching,” M. T. DeJarld, J. C. Shin, W. Chern, D.

Chanda, K. Balasundaram, J. A. Rogers, and X. Li, Nano Lett., 11, 5259-5263 (2011).

• “Nonlithographic Patterning and Metal-Assisted Chemical Etching for Manufacturing of Tunable Light-Emitting Silicon Nanowire Arrays,” W.

Chern, K. Xu, I. Chun, B. P. de Azeredo, N. Ahmed, K-H. Kim, J. Zuo, N. Fang, P. Ferreira, and X. Li, Nano Lett. 10 (5), pp 1582–1588

(2010).

• "Nanoscale three dimensional pattern formation in light emitting porous silicon," I.S. Chun, E. Chow, and X. Li, Appl. Phys. Lett. 92, 191113

(2008).

• "In-plane Bandgap Control in Porous GaN through Electroless Wet Chemical Etching," X. Li, Y.-W. Kim, P. W. Bohn, and I. Adesida, Appl.

Phys. Lett., 80 980-982 (2002).

• “Metal-assisted chemical etching in HF/H2O2 produces porous silicon", X. Li and P.W. Bohn, Appl. Phys. Lett.77, 2572 (2000).

http://mocvd.ece.illinois.edu

MacEtch Patent Portfolio

• TF00067 metal-assisted chemical etching to produce porous

silicon US 6790785

• TF00154 metal assisted chemical etch to produce group III-V

porous material and porous silicon US 6762134

• TF08145 method of forming nanoscale three-dimensional

patterns in a porous material US 8486843

• TF09098 method of forming an array of high aspect ratio

semiconductor nanostructures US2013/0052762

• TF11074 metal assisted chemical etching to produce III-V

semiconductor nanostructures US2013/0280908

MacEtch: metal-assisted chemical etching

Defying text definition of wet etch, MacEtch is an anisotropic wet etching method that could potentially replace, improve dry etch for various electronics, photonics, and energy applications.

Current group members:

• Dr. Parsian Mohseni

• Kevin Bassett

• Chen Zhang

• Jeong Dong Kim

• Kyooho Jung

• Yi Song

• Karthik Balasunderam

• Paul Froeter

• Wen Huang

• Kelson Chabak

• Ryan Kim

• Xiang Zhao

• Wonsik Choi

• Moyang Li

• Clarence Chan

Funding sources

• NSF ECCS

• NSF DMR

• DARPA

• DOE

• INTEL etc.

• ONR YIP

• I2CNER

Acknowledgement

Collaborators

http://mocvd.ece.illinois.edu

• P. Braun, J. J. Coleman, P. Ferreira, M.

Gillette, S. Gong, L. Lee, J. Lyding, J. A.

Rogers, P. Ye, K. J. Hsia, W. King, E. Pop,

J. Schutt-Aine, S. Sinha, D. Cahill, D. Ruzic,

J. Williams, W. Wilson, J. ZuoFormer group members:

• Dr. Ik Su Chun, Dr. Ryan Dowdy, Dr. Xin Miao, Dr. Jae Cheol Shin, Dr. Xin Yu, Lei Pan, Matt

Dejarld, Seth Fortuna, Winston Chern, Archana Challa, Mohammad Malik, Aiyin Liu