CHARACTERIZATION OF OPTICAL PROPERTIES OF SU-8 AND

FABRICATION OF OPTICAL COMPONENETS

Om Prakash Parida

1 and Navakant Bhat

2

DOI, DRDL, Kanchanbagh, Hyderabad-581, Dept. of ECE, IISc. Bangalore-560012

2

omprakash.273bc@gmail.com1, navakant@ece.iisc.ernet.in

2

Abstract: SU-8 is a negative tone, near UV resist. Due to its unique properties, it is widely used as a structural

material in diverse fields of MEMS. Its excellent optical transparency beyond 400nm makes it a preferred

material for the fabrication of large variety of optical components and systems. UV-Visible spectroscopy and

Spectroscopic ellipsometry were carried out for characterizing the optical properties of SU-8. Process parameters

were optimized for the fabrication of different types of optical components like photonic bandgap structures,

optical waveguides, splitters, directional couplers and gratings using Laser and electron beam lithography.

1. INTRODUCTION SU-8 is a relatively new polymer material in the

field of MEMS introduced by IBM in 1989. It is an

epoxy based chemically amplified, negative tone,

near UV photo-resist as well as a low cost X-ray and

e-beam resist [1]-[3]. It is a highly functionalized

molecule with 8-epoxy groups, polymerized by

cationic photo-polymerization which is induced by

Lewis acid during UV illumination. Subsequent

heating during post-exposure baking (PEB) activates

cross-linking and regenerates the acid catalyst which

causes significant enhancement of the resist

sensitivity and results in high mechanical strength

and thermal stability of the structure. Chemical

structure of SU-8 monomer and standard SU-8 film

processing sequence are presented in figure-1.

Figure-1: (a) Structure of SU-8 monomer and

(b) basic SU-8 processing sequence

Cross-linked SU-8 films are highly resistant to a

large number of acids, bases and solvents. Due to its

unique physical, chemical, mechanical,

electromagnetic and bio-compatible properties, it has

been explored as an important material in diverse

fields of MEMS manufacturing not only as a resist

material but also as the core structural material [4].

Because of its excellent optical transparency and

favorable optical parameters, SU-8 has been

preferred as a suitable material for the fabrication of

optical waveguides, gratings, Bragg filters and

reflectors, photonic bandgap structures, on-chip light

sources, optical pressure sensors, opto-fluidic

systems and MOEMS [5]-[8]. Optical properties and

parameters specified in the datasheets are limited and

obtained under specific processing conditions, hence

cannot be straightway acceptable for a different

processing environment. So extensive characteri-

zations for optical properties of SU-8 under different

processing conditions were carried out. ‘Ultraviolet-

Visible spectroscopy’ and ‘Spectroscopic

ellipsometry’ [9] - [10] of SU-8 films were carried

out to evaluate the absorption characteristics and

refractive index of SU-8 respectively. Process flow

was optimized for the fabrication of optical

components like 2-D photonic bandgap structures,

optical waveguides and splitters, 4-port directional

couplers and optical gratings.

2. OPTICAL CHARACTERIZATION

2.1 Absorption characteristics of SU-8

Absorption characteristics of SU-8 were studied

by carrying out ‘Ultraviolet-Visible (UV-Vis.)

Spectroscopy’ of SU-8 films using ‘Cintra 40 UV-

Visible Spectrometer’. A new formulation was

prepared by thoroughly mixing SU-8 2150 and SU-8

thinner (Microchem, USA) in an optimized ratio and

spin coated on a 2.5cm X 2.5cm clean glass substrate

to get 10µm SU-8 film. The sample was then soft

baked (SB) on a flat leveled hot plate at 650C for 3

minutes followed by 8 minutes at 950C and then

exposed to UV radiation for 45 seconds using

‘MJB-3 single-sided mask aligner’. After exposure,

the sample was post-exposure-baked (PEB) on the

hot plate at 950C for 9 minutes. During UV-Vis.

spectroscopy, the PEB SU-8 sample was exposed to

radiations varying from λ=190nm to 900nm.

The sample was then hard-baked (HB) on hot plate at

2000C for 30 minutes and again UV-Vis.

spectroscopy was carried out. Absorption spectra of

the PEB and HB samples are shown in figure-2. It

can be observed from the absorption spectra that

absorption starts increasing sharply below 350-

360nm. Absorbance peaks around 290nm for the

PEB film whereas for the HB film it peaks around

315nm. The HB film also shows a little higher

absorption than the PEB film. This red shift in the

absorption peak and enhanced absorption in case of

the HB film may be attributed to a structural

rearrangement most probably by the removal of

dangling bonds and/or formation of conjugation in

the structure because of relatively longer heating of

the film at a higher temperature. From both the

spectra, it can be observed that beyond 400nm,

absorption is very low and go on decreasing with

higher wavelengths. This indicates that PEB and HB

SU-8 films exhibit excellent optical transparency

above 400nm. This makes SU-8 an excellent polymer

material for a variety of optical applications. Optical

bandgap of 3.4eV for SU-8 was evaluated from the

absorption spectrum of the HB film.

200 300 400 500 600 700 800 9000

0.2

0.4

0.6

0.8

1

Wavelength in nm

Normalized Absorbance

Absorption spectrum of SU-8 films

PEB film

HB film

Figure-2: Absorption spectra of post-exposure baked

(PEB) and hard-baked (HB) SU-8 films.

2.2 Refractive index of SU-8

Refractive index (n) of SU-8 was evaluated by

carrying out spectroscopic ellipsometry of SU-8

films. 2cm X 2cm Silicon wafer pieces were piranha

cleaned and dehydration baked on hot plate at 2000C

for 10 minutes. 60nm and 1.2µm films were spin

coated on the Si substrate and soft baked (SB) on hot

plate at 650C for 1 minute followed by for 2 minutes

at 950C and exposed to UV radiation for 15 seconds

using MJB-3. After exposure, the samples were

baked (PEB) on hot plate at 650C for 1 minute and at

950C for 1 more minute. The samples were then

hard-baked (HB) on hot plate at 2000C for 30

minutes. After each stage of processing i.e. SB, PEB

and HB, the samples were put for spectroscopic

ellipsometry and refractive index ‘n’ as function of

wavelength was evaluated. It can be observed from

figure-3 that for the PEB SU-8 film, refractive index

monotonically decreases with increase in wavelength

and the thicker (1.2µm) SU-8 film has marginally

higher refractive index than the thinner (60nm) film.

Similar trend was observed for the SB and HB SU-8

films. Variation in the refractive index among the

SB, PEB and HB cases is much smaller for the thin

film than for the thick film (figure-4 & 5).

300 400 500 600 700 8001.55

1.6

1.65

1.7

1.75

1.8

1.85

1.9

Wavelength in nm

Refractive index

Plot of refractive index of thin and thick PEB SU-8 films

Thin (60 nm) film

Thick (1.25um) film

Figure-3: Refractive index (n) of PEB SU-8 film as

function of wavelength.

300 400 500 600 700 8001.55

1.6

1.65

1.7

1.75

1.8

1.85

1.9

Wavelength in nm

Refractive index

Plot of refractive for thick (1.2um) SU-8 film

SB film

PEB film

HB film

Figure-4: Refractive index (n) of 1.2µm thick SU-8

film as function of wavelength. Large variation in ‘n’

can be observed for different processing conditions.

300 400 500 600 700 8001.55

1.6

1.65

1.7

1.75

Wavelength in nm

Refractive index

Plot of refractive index for thin (60nm) SU-8 film

SB film

PEB film

HB film

Figure-5: Refractive index (n) of 60nm thin SU-8

film as function of wavelength. Variation in ‘n’ is

relatively small for different processing conditions.

Table-1: Values of refractive index ‘n’ at 365nm and

620nm for SB, PEB and HB SU-8 films

‘n’ /

Sample

365nm

(60nm

film)

365nm

(1.2µm

film)

620nm

(60nm

film)

620nm

(1.2µm

film)

SB 1.6410 1.6649 1.5776 1.5976

PEB 1.6396 1.7720 1.5755 1.6950

HB 1.6339 1.6387 1.5696 1.5726

Values of ‘n’ at 365nm and 620nm are presented in

table-1. It can be observed that at 365nm, ‘n’ varies

from 1.63 to 1.77 and at 620nm; it varies from 1.56

to 1.69 due to variations in film thickness and film

processing conditions. The extinction coefficient (k),

which is the imaginary part of the complex refractive

index and gives a measure of the fraction of light lost

due to scattering and absorption per unit distance in

the film, was found to be decreasing with wavelength

and very close to zero beyond 400nm (figure-6).

300 400 500 600 700 800-0.04

-0.03

-0.02

-0.01

0

0.01

0.02

0.03

0.04

Wavelength in nm

Extinction coff.

Plot of extinction coff. of SU-8 films

PEB film

HB film

Figure-6: Plot of extinction coff. (k) of 1.2µm PEB

and HB SU-8 films as function of wavelength. ‘k’ is

negligibly small beyond 400nm.

3.FABRICATION OF OPTICALCOMPONENTS

3.1 Photonic band-gap structures

Photonic band-gap structures or photonic crystals

(PC) are periodic structures (1D, 2D or 3D) in which

periodic spatial modulation in the refractive index

leads to coherent scattering of light and alters the

modes of propagation of light of wavelengths

commensurate with length scale of periodicity. A

PBS is characterized by its band gap where light with

certain wavelength cannot propagate through it.

Figure-7: Photonic bandgap structures made of

circular SU-8 micropillars

2D-PCs of different shapes and dimensions were

fabricated with SU-8. 2µm of SU-8 film was spin

coated on a 2cm X 2cm silicon substrate. The sample

was soft baked on hot plate at 650C for 1 minute

followed by for 2 minutes at 950C. The photonic

bandgap structure pattern was directly written on the

sample by ‘Microtech LW 405 Laser Writer’. As this

Laser writer operates at 405nm where sensitivity of

SU-8 is low, 6 times writing of the pattern (at an

optimized setting of the Laser writer parameters) was

carried out to ensure proper cross-linking. The

sample was then post-exposure baked on hot plate at

650C for 1 minute followed by at 95

0C for 1 minute.

For thicker films longer baking times and more

number of Laser writings were required. Figure-7

shows the microscopic image of a 2D PC consisting

of an array of circular micropillars of height 2µm,

diameter 2µm and inter-pillar spacing of 5µm.

3.2 Optical waveguides, splitters and 4-port

directional coupler

Excellent optical transparency of SU-8 makes it

suitable for fabricating optical components like

optical waveguides, splitters and directional couplers.

When surrounded by suitable cladding medium,

SU-8 core can allow light to propagate through it by

total internal reflection.

Figure-8: Single mode SU-8 optical waveguides of

width 5µm, height 1µm and spacing 90µm.

(a) (b)

(c) (d)

Figure-9: (a) Symmetric Y-splitter, (b) asymmetric Y-

splitter, (c) T-splitter, (d) 4-port directional coupler.

1µm of SU-8 (SU-8 2002) was spin coated on an

oxidized Si substrate (0.9µm of SiO2 was thermally

grown by wet oxidation). The sample was soft baked

on hot plate and the patterns were directly written

(6 times) on the sample by the ‘LW-405 Laser

writer’. Figure-8 shows the microscopic images of

single mode optical waveguides of width 5µm and

thickness 1µm. As refractive index of SiO2 is around

1.45 whereas that of SU-8 is around 1.56 (at 620nm),

light in the SU-8 core is guided by the SiO2 layer

below and air (‘n’=1) at the other 3 sides acting as

the cladding layers. Figure-9 shows symmetrical and

asymmetrical Y-splitters, T-splitter and a 4-port

directional coupler fabricated with SU-8 by direct

laser writing following the above process flow.

3.3: Optical gratings

Grating is an optical component with a regular

pattern which splits light into several beams

travelling in different directions depending upon the

spacing of grating and the incident light so that it acts

as a dispersive element. Optical gratings of varying

periodicity were fabricated with SU-8 by electron

beam lithography (EBL). 1.5cm X 1.5cm glass

substrate was properly cleaned in chromic acid and

10nm of Au was sputter deposited on it to avoid

drifting during EBL. A new SU-8 formulation was

spin coated on the glass sample to get 150nm of

SU-8 film. The sample was then soft baked on hot

plate at 650C for 1 minute followed by at 95

0C for

1 minute. The grating patterns were directly written

on the SU-8 film by ‘Raith-e-Line’ electron beam

lithography machine with an optimized setting of the

EBL parameters. Figure-10 shows the scanning

electron microscopic image of the 150nm SU-8

grating with a spacing of 50nm.

Figure-10: SU-8 gratings of width, height 150nm and

spacing 50nm, fabricated on glass substrate by EBL.

4. CONCLUSION

Process parameters were optimized for the

fabrication of SU-8 films. From UV-Visible

spectroscopy of SU-8 films (10µm thick), it was

observed that optical absorption increases drastically

below 350-360nm and peaks around 290nm and

310nm for the PEB and the HB films respectively.

Beyond 400nm very low absorption was observed.

Thus SU-8 exhibits excellent optical transparency

above 400nm and hence becomes suitable for

fabrication of optical components. Spectroscopic

ellipsometry of SU-8 films indicated that irrespective

of the film processing conditions, thicker film

(1.2µm) exhibits higher refractive index than the

thinner film (60nm) and at any wavelength, the

variation in the refractive index under different

processing conditions (SB, PEB and HB) is less for

the thin film than for the thicker film. Further

research can be taken up to study the exact nature of

thickness dependency of refractive index of SU-8

films in the nano-scale. In the wavelength range of

365nm to 620nm, refractive index of SU-8 was found

to be varying from a minimum of 1.56 (for a 60nm

HB film at 620nm) to 1.77(for 1.25µm PEB film at

365nm). Extinction coefficient was found to be very

small (maximum 0.007 at 300nm) and decreases very

close to zero beyond 400nm. Processing sequences

were established and relevant process parameters

were optimized for the fabrication of a variety of

SU-8 based optical components like 2D photonic

bandgap structures, waveguides, symmetric and

asymmetric Y-splitters, T-splitters, 4-port directional

couplers and gratings by Laser writing and electron

beam lithography. Optical characterization of these

components is in the process. Further research can be

taken up to utilize these components in optical

sensors, MOEMS, OICs and nano-photonics.

ACKNOWLEDGEMENT

We thank MCIT for their support through Centre

of Excellence in Nanoelectronics. We also thank

Prof. S. A. Shivsankar, Prof. G. Hegde, Prof. V.

Venkatraman, Prof. M. Ashokan, and Tamilarson of

IISc. for their support and valuable suggestions at

crucial points of this work.

REFERENCES

[1] H. Lorenz, M Despont et al., “SU-8: A low cost

negative resist for MEMS”, J. Micromech.

Microengg., 7, 121-124, (1997) .

[2] A Pepin et al., “Exploring the high sensitivity of

SU-8 resist for high resolution electron beam

patterning.”, Microelectronic engg., 73-74,

233-237, (2004).

[3] Websites: SU-8 homepage (www.geocities.com),

SU-8: Thick photoresist for MEMS

(www.memscyclopedia.org), SU-8: A versatile

material for MEMS (www.semi.org/mems), and

SU-8 datasheets (www.microchem.com).

[4] Patrick Abgrall et al., “Review: SU-8 as a

structural material for lab-on-chips and MEMS”,

Electrophoresis, 28, 4539-4551, (2007)

[5] Maria Nordström et al., “Single-Mode

Waveguides With SU-8 Polymer Core and

Cladding for MOEMS Applications”, Journal of

Lightwave Technology, Vol.25, 5, May (2007)

[6] Shen-Qi xie et al., “A nanimprint lithography for

fabrication of SU-8 gratings for near-infrared to

deep-UV applications”, Microelectronic engg.,

85, 914-917, (2008)

[7] A Ghaffari et al., “Analysis of photonic crystal

power splitters with different configurations,”

J. of Applied Sciences, 8, 1416-1425, (2008)

[8] Ronald W. Waynant, M N. Ediger, “Electro-

Optics handbook” (2e/d, Mc-Graw hill, 2000)

[9] C. Richard Brundle, et al., “Encyclopedia Of

Materials Characterization: Surfaces, Interfaces,

Thin Films”, (Butterworth Heinemann, 1992)

[10] Débora Gonçalves et al., “Fundamentals and

applications of spectroscopic ellipsometry,”

Quim. Nova, Vol. 25, No. 5, 794-800, (2002)