2-Photon Polymerization for

Laser Direct-Write Target Fabrication*

O. Stein, N. Petta

Schafer CorporationYing Liu, Lijia Jiang, Yongfeng Lu

University of Nebraska, Lincoln

*Work Supported under DOE contract No:

DE-NA0001430

Outline:

• Introduction- 2 photon polymerization process

• The hardware

• Motivation- Schafer’s developmental effort

• Target fabrication for NRL

• Experimental conditions and characterization

• Future outlook

Schafer 2PP effort started in 2011 and subsequently

launched collaborations with universities and DOE Labs:

• Initial work (2011-2014) focused on demonstrating 2PP as a possible

means to make:

o Fill tubes (UC Berkeley- Bernat et al. FS&T 70(2) Aug 2016)

o Foam-like structures complex surface shapes (with UNL)

• Jiang et al. FS&T 70(2) Aug 2016

• Jiang et al. ICALEO proc. 2015

• Jiang et al. MRS Bulletin 41 (2016) 975

• Our initial work led to a number of follow-on collaborations:

o LLNL in support of foam targets (UNL, Park, Biener, Oakdale)

o LLE custom target support structures (Harding et al.)

o LANL- Nanoscribe training at UNL, joint characterization of parts etc.

(Oertel, Munchehausen, Patterson, Herman, Grote)

• Our colleagues at LLNL recently published on UV post treatment to improve

mechanical properties (Oakdale et al. Opt. Exp. 24, 24 Nov 2016 27077)

Photo- Induced Polymerization

C. LaFratta et al., Angew. Chem. Int. Ed. 2007, 46, 6238 – 6258

Hg Lamp Ti: Sapphire

• The process involving the absorption of two photons scales as I2

(where I is the laser intensity)

• Low average power and in wavelengthsfar from absorption prevent

single photon process

Typical voxel :

a/b ~2.5 (a~1000nm, b~400nm)

Threshold

𝒅𝑰 𝒛

𝒅𝒛= − 𝛽𝑰 𝒛 2

𝒅𝑰 𝒛

𝒅𝒛= −𝑰 𝒛 − 𝛽𝑰 𝒛 2

0

2-photon absorption

𝒅𝑰 𝒛

𝒅𝒛= −𝑰 𝒛

Beer-Lambert law

α, absorption coefficient.

One photon absorption

Cut-away side view (highly schematic)

* Collaboration with Prof Yongfeng Lu’s group at Univ. of Neb

• Relatively high scan

velocity

• High resolution

scanning

in low area

• Proprietary resins

• Vast stitching defects.

200μm 30μm

Stitch defect

Currently we are printing structures using a commercial

2PP system: Nanoscribe Photonics GT

Although accessible, this machine isn’t optimized for R&D and target fabrication

• Laser direct write can serve as a complementary process to conventional

sol-gel process for low density material (LDM) synthesis

• Complex architectures can be realized in micron scale

• Short turnover time form CAD model design to fabrication

In 2011 Schafer’s effort began through collaborations with Universities

Fill tube1 Fill tube+support2,3 Low density Material AM-FOAM FY16 shot on OMEGA2

0.2g/cc1 prof. Costas Grigoropoulos, Mechanical Engineering, UC Berkeley2 prof. Yongfeng Lu, Electrical and Computer Engineering UN, Lincoln3 University of Rochester, LLE

A B0.4mm

2.0mm

1.5mm

The Motivation: 2PP could serve as an efficient tool to

directly print complex structures

300 mm250 mm

2000 mm

15 mm

aa

c

• 15μm thick fully polymerized resin top layer

• 2.0X0.25X0.3mm logpile LDM ~0.10g/cc

• AM foam similar to DVB foam structure

Y. Aglitskiy PRL 103, 085002 (2009)

Laser

300 mm

Foam 10%

of solid

Holder

former design,

casted DVB

Holder

a=5μmc=8μm

3m

m g

ap

2PP has successfully been used to fabricate “ DVB like” low density foam

targets for NRL HED shock propagation experiments

• While attached to the substrate the

structures were intact

• After liftoff, severe deformation took

place

Stitching defects

Non-uniform shrinkage and collapse of pore structure

during development were observed

Failure seems to be a combination of buckling

and shear

• Our logpile design consists of a typical

pore size of d=4-5μm

• The applied pressure due to solvent evaporation

is estimated as ~1.4psi

• Compared to the mechanical properties

of the resin these forces are low and practically negligible

At pore sizes of 4-5 microns we would not expect Capillary

forces to play a role and were calculated to be negligible

d

𝑃 = 2𝛿 ∙ 𝑐𝑜𝑠 𝜃 /𝑑

for typical rinsing fluid θ->0˚

δIPA=23mN·m-1

50μm

A

40μm

C

20μ

m

D

20μm

A,C- Air

Dried

40μm

B

B, D- Supercritically

dried

CAD model of the structure

Drying method have an

extensive effect on the

structural integrity outcome.

The effect of solvent removal

made us reconsider the

effect of capillary forces.

The effect of super-critical point drying

We took a closer look at causes of capillary forces

8μm

700nm

Under higher magnification SEM nanometer

sized cracks are evident throughout the

structure

Estimating the capillary pressure according to

the pore dimension gives a conservative

value of

~65psi

Cracks indicate imperfect polymerization

during illumination and

stage scanning.

Jiang et al. Opt. Lett. (2014) 39 3034

𝐷. 𝐶. = 1 −

𝑃𝐶=𝐶𝑃𝐶=𝑂

𝑀𝐶=𝐶

𝑀𝐶=𝑂

𝑋100

600μm

1μm

A

C

700nm

B

UV cured

UV cured2PP

• A drop of resin was UV cured and imaged

with SEM. Image A

• Most of the areas look crack free either on

the surface of the sample or within its

volume. Image C

• Polymerization via 2 photon absorption of

the same resin reveals high density of

cracks. Image B.

UV Cured resin is fully polymerized with no cracks

Low density structure bowing- 2nd type of

deformation

• All of the structures containing a top fully densified layer buckle

to some extent

• Low density structures without top layer retain their form

Side View, taken on a laser confocal microscope Fully densified layer, 15μm thick.

uniform linear Shrinkage of 7.5% occurs during the development process

1.86mm (2.0)

1.85mm (2.0)

1.76mm (2.0)

1.92mm (2.0)B

A

Perspective view

0.25mm

0.3mm

2.0mm

0.015mm

stitching defects

0.25mm

0.3mm

2.0mm

3D rendering of the structure

(bottom side up)It is possible to fit a circle to the

deformed shape

R=3.08mm

Blue curve: circular foam deflection

𝑦 = 3.08 − 3.082 − 𝑥2

Orange curve: the load that develops

as a function of position

Bowing deformation analysis-cont.

𝑞 𝑥 = 𝐸𝐼𝑑4𝑦 𝑥

𝑑𝑥4

(a) (b)

(c) (d)

Mechanical deformation of Low density structure

cont.

No top layer With top layerInitial work done at Nebraska

university analyzing the force

acting to deform the structure

is done via finite element

analysis (FEA)

• The final deformation

depends on the existence

of a top fully densified layer

• Bottom substrate acts as

an anchor, preventing

deformation close to it

• Understanding the forces involved

will help guide the design

AM foam mounted on the target

holder. In this case foam dimensions:

2000X400X250µm with 10µm top fully

densified polymer film

Density of the structure: ~0.15gr/cm3

Pore size: ~ 4μm

15 foam targets were supplied to NRL at the summer of

2016.

Analysis of data and development of

better target design are ongoing

Preliminary results- 2PP structure pyrolysis

8μm40μm

2μm20μm

It’s possible to

pyrolyze the structure

in an inert atmosphere

• significant size reduction

• Carbon rich 2PP foam

• Low Z, low density

material structure

• electrically conducting

Resin (CHNO) Carbon

Summary

• 15 Additive Manufacturing foam targets were supplied to

NRL to be shot on Nike laser (original shot date was in late 2016)

• Super-critical drying was proven to be crucial in preserving the

foam’s internal structure

• analysis of foam bending is performed via FEA

• Additional designs are constantly examined in order to reduce

global deformation of the target

• Demonstration of 2PP structure carbonization lifted off the

substrate

Acknowledgements

• Ying Liu, Jack Campbell, Lijia

Jiang, Yongfeng Lu

• Yefim Aglitskiy

• Tom Bernat, Jared Hund, Jon

Streit

• Brian Patterson

• David Harding

Additional slides

Bowing deformation analysis.

According to Euler- Bernoulli beam theory, the developed load on

a beam as a function of deflection:

𝑞 𝑥 = 𝐸𝐼𝑑4𝑦 𝑥

𝑑𝑥4E is Young’s modulus, I is the second moment

of the area (~moment of inertia)

𝐼 = 𝑧2𝑑𝑥𝑑𝑦 (rotation around z axis)

In the simple case of a rectangular beam profile, this is a function of cross section dimensions.

x

y

z

Iz>>Ix

Extreme shrinkage

Pyrolyzing the structure

results in a 60% reduction

in longitudinal dimension.

It is unclear if the structure is 100%

carbon

Stitch defect

Fully polymerized

layer

Foam Buckling, cont.

Shrinkage of ~8% occurs during the development process.

1.86mm (2.0)

1.85mm (2.0)

Adding a rigid layer and braking the structure’s symmetry causes the

deformation.

1.76mm (2.0)

1.92mm (2.0)

Fully densified layer,

15μm thick.

Photopolymerization of resists

Resins are generally comprised of two main parts:

1. Photo Initiator or sensitizer/initiator (<1wt%)

2. Monomer plus Oligomer units of the desired polymer

• Monomer: M

• Oligomer: (M)n where n is a few

• Monomer/oligomer ratio can be adjusted to give desired viscosity

and, in some cases, modify physical properties

• Other components (e.g. quenchers) can also be added as needed

Ihυ/ 1/2hn

X + R.

R. + M RM. + M RMM. + M …. RMn

I*