micro-transfer-printing: deterministic assembly of...

Micro-Transfer-Printing: Deterministic Assembly of Microscale Components Using Elastomer StampsC. A. Bower, M. Meitl, S. Bonafede, D. Kneeburg; X-Celeprint Ltd.

[email protected] www.x-celeprint.com

mailto:[email protected]

http://www.x-celeprint.com/

Outline

• Introduction to Micro-Transfer-Printing

• Strategies for Printable Micro Devices

• Application Examples

• Summary

C. Bower, [email protected]

Elastomer Stamp Transfer-Printing

devices are transferred

onto the stamp

devices are printed onto

non-native target substrate

Nature Mater. 5 33-38 (2006)

elastomer stamp (PDMS)

native substrate with

“printable” devices

devices are attached to stamp

by Van der Waals forces


Kinetically Controlled Adhesion

slow lift-off fast lift-off> 10 cm s-1

“The adhesion between the solid objects and the stamp is rate-sensitive owing to the viscoelastic behavior of the elastomer”

Nature Mater. 5 33-38 (2006)


Competing Adhesion

slow lift-off fast lift-off> 10 cm s-1

Slow Fast

Pick-up

Printing

Nature Mater. 5 33-38 (2006)

Stre

ngt

h o

f A

dh

esio

nStamp Velocity

“The adhesion between the solid objects and the stamp is rate-sensitive owing to the viscoelastic behavior of the elastomer”

Langmuir, Vol. 23, No. 25, 2007


Micro-Transfer-Printing (µTP) Technology

Native “Source” Wafer

Elastomer Print Head (Stamp)

Non-Native “Target” Substrate

Populated Stamp

Attributes:• manipulate objects that are too small, numerous,

fragile, or otherwise difficult to handle by other means• massively parallel high-throughput assembly• hardware (motion & optics) scales to large format

target substrates • enables re-use of native growth substrate• decouples growth (high temp, lattice constraints) and

usage (glass, plastic) of high-performance materials• tolerant to wafer size mismatch • uses device “source” wafer efficiently • placement accuracy* +/- 1.5 µm (3-sigma)• process yields* > 99%

* application dependent


Efficient Materials Usage

Non-native “Target” Substrate

1st transfer 2nd transfer

• produce devices at high density on native growth substrate• assemble microscale devices only where necessary on the target substrate

Densely packed device elements

Sparse assembled device elements

Source Wafer

Printing

“Area Multiplication”µTP allows you to spread devices from a source wafer over a much larger target area. As an example, a single 100mm InPwafer might be used to fully populate a 300mm Silicon wafer.


Transfer-Stamp Technology

i. Prepare master, e.g. by photolithography

ii. Cast prepolymer (PDMS)

iii. Cure stamp; separate from master

• Transfer stamps are fabricated by casting the elastomer (PDMS) against a microfabricated master wafer.

SEM micrograph stamp surface


Stage (x, y)

Print Head (z, θ, Tx, Ty)

Optics (x, y, z)

• A scalable process that utilizes precision motion and optics

Micro-Transfer-Printing Hardware


Stre

ngt

h o

f A

dh

esio

n

Stamp Velocity

Strategies for “printable” devices

Most materials of interest are challenging to “pick-up” in their as-grown form.

Si <111>

• The device/substrate interface is engineered to make “pickable” devices• Primary strategy is to employ MEMs-like sacrificial release processes that

are tailored for µTP

Device / Substrate Interface


The Silicon-on-Insulator (SOI) System

i. Silicon-on-insulator (SOI) wafer

ii. Photolithography and etch top silicon to expose buried oxide (blue)

iii. Etch buried oxide to undercut the structures

iv. Retrieve structures

Printable single-crystal silicon can be achieved using SOI wafers, where the buried oxide layer serves as the sacrificial layer.

anchor

tether fractures

during retrieval


Examples of Micro-Transfer-Printed Silicon

J. Micromech. Microeng. 22 (2012) 055018 (7pp)

small 2012, 8, No. 6, 901–906

Si Micro Masonry Flexible Si Solar Cells Imbricate Scales

Nature Materials 7, 907-915 (2008).

http://rogers.matse.illinois.edu

http://rogers.matse.illinois.edu/


Printable Microscale Integrated Circuits

• The BOX (buried oxide) can be used as the sacrificial layer • First demonstration was done using XFAB’s XT06 SOI-CMOS process

sacrificial layer


Microscale ICs on Glass

Printed µICs

50µm

• Over 2 million printed µICs• Process yield > 99.9%• Print accuracy +/- 1.5um 3σ

R.S. Cok, J. W. Hamer, C. A. Bower, E. Menard, S. Bonafede, “AMOLED

displays with transfer-printed integrated circuits,” Journal of the Society

of Information Display (JSID) 19, 335 (2011)


Printable Microscale Compound Semiconductors

Growth Substrate

Epitaxial Devices Layers(InGaAs, InGaP, GaAs, InP, etc…)

Selective Release Layer (Al>0.6GaAs, InGaAs, InAlP, etc…)

1. Epitaxial Growth (MOCVD, MBE, etc…)

2. Patterning of material down to substrate

3. Formation of anchor and removal of release layer

anchor

“ready to print”

Epitaxial compound semiconductors are very practical systems for µTP because lattice-matched sacrificial layers can be introduced under the device layers.

growth substrate can be re-used after printing


Microscale Multi-Junction III-V Solar Cells

43% efficient solar cell printed to as-fired ceramic. Cell contains InGaP, InGaAs, GaAs.

600um x 600um x 10um

microcells printed onto ceramic

surface mountable microcell

• Microscale solar cells1 for high efficiency high-concentration photovoltaics (HCPV)

• Over one million printed microcells in the field generating power

• Semprius opened its first factory in 20122

1. X. Sheng, C.A. Bower, M. Meitl et al. "Printing-based Assembly

of Quadruple-Junction Four-Terminal Microscale Solar Cells

and Their Use in High-Efficiency Modules," Nature Materials

13, 593-598 (2014).

2. http://semprius.com/wordpress2/wp-

content/uploads/2014/08/SEM-Facility-Opening.pdf

http://semprius.com/wordpress2/wp-content/uploads/2014/08/SEM-Facility-Opening.pdf


Microscale III-V Lasers on Silicon

• Applications include:• Next generation hard drives: Heat-assisted magnetic recording (HAMR)• Silicon Photonics & Optical Interconnects• Sensors

J. Justice, C. A. Bower, M. Meitl, M. B. Mooney, M. A. Gubbins, B. Corbett, “Wafer

scale integration of III-V lasers on silicon using transfer printing of epitaxial layers,”

Nature Photonics, Volume 6, Issue 9, pp. 612-616 (2012)

direct print to silicon

Micro-Transfer-Printed

GaAs laser


Microscale LEDs

InGaN

(111) Si

Tether

Anchor InGaN µLED on plastic

Rogers, J. A., et al. (2011). Unusual strategies for using indium gallium nitride grown on silicon (111) for solid-state lighting, PNAS

• The <111> substrate is used as the release layer for printable GaN-based devices

• GaAs-based red µLEDs printed on plastic

Applications include:

• Solid-state lighting

• Display backlights and displays

• Indicator lights


Summary

• Micro-Transfer-Printing (µTP) with engineered elastomer stamps provides a cost-effective approach for the deterministic assembly of microscale devices

• µTP enables the integration of high-performance microscale devices onto non-native substrates such as plastics, glass or other semiconductors

• mature high-performance device technologies (Si ICs,GaAs and GaN) are compatible with µTP

• many emerging technologies can benefit from a cost-effective approach to micro-assembly


Acknowledgements• The X-Celeprint technical team

• Professor John Rogers and his group at the University of Illinois at Urbana-Champaign

• Semprius

• XFAB

• Tyndall National Institute

• Seagate

• RTI International

Thank you! Please don’t hesitate to contact us with your questions!

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