Poly-HDDA microstructure fabrication using ...arindamphani.yolasite.com/resources/PHOTONICS WEST SPIE Proc Vol... · Poly-HDDA microstructure fabrication using Microstereolithography

Poly-HDDA microstructure fabrication using Microstereolithography for Micro-cantilever based sensor technology

Ankur Goswamia, Arindam Phanib, Ankit Krisnac, N.Balashanmugamc, Giridhar Madrasd, A.M.Umarji*a

aMaterials Research Centre, Indian Institute of Science, Bangalore – 560 012, India

bInstrumentation and Applied Physics, Indian Institute of Science, Bangalore – 560 012, India cCentral Manufacturing Technology Institute, Bangalore-560 022, India

dDepartment of Chemical Engineering, Indian Institute of Science, Bangalore -560012, India

ABSTRACT

The rapid development of various scanning probe methods like SFM or AFM involving microcantilever based sensor

technology has slowly enabled mechanical motion to regain its place in the field of science and engineering by

miniaturization of mechanical systems down to sub-micron dimensions. Such scaling down of dimensions of

microstructures exhibit very high sensitivity to mechanical deformations due to various induced loads. The most widely

used Optical beam deflection method (OBDM) for measuring such deflections in microcantilever based sensors is

limited by diffraction effects due to dimensional constraints of the structures involved. The use of polymer materials like

poly HDDA having very low elastic modulus has the potential to achieve high mechanical deformation sensitivity for

even moderately scaled down structures. Poly-HDDA based microcantilever sensors are being fabricated in an in house

realized Microstereolithographic system. The objective is to fabricate a double micro-cantilever structure of length

600 µm, width 60 µm and thickness 40 µm each with a gap of 100 µm between the two along the thickness dimension.

The relative deflection profile of one of the fabricated cantilevers due to induced surface stress by the self-assembly of

Alkanethiol on Gold is proposed to be measured by an optical diffraction based method. Proposed surface stress

resolution achieved in such a typical microcantilever based sensor is of the order of 1 mN/m for a deflection of 0.5 nm at

free end of one of the micro-structures subjected to self-assembly mechanism. The high thermal stability and very low

elastic modulus of Poly-HDDA enables its application as a low noise, very high sensitive sensor material for detection of

mechanical deforming agents in microcantilever based sensor technology.

Keywords: Microcantilever based sensor, Microstereolithography, Poly-HDDA, Optical diffraction profiling

*[email protected]; Ph: +91-80-2293 2944; Fax: +91-80-2360 7316

Micromachining and Microfabrication Process Technology XVI, edited by Mary Ann Maher,Jung-Chih Chiao, Paul J. Resnick, Proc. of SPIE Vol. 7926, 79260C · © 2011 SPIE

CCC code: 0277-786X/11/$18 · doi: 10.1117/12.888045

Proc. of SPIE Vol. 7926 79260C-1

Downloaded from SPIE Digital Library on 23 Feb 2011 to 203.200.35.11. Terms of Use: http://spiedl.org/terms

1. INTRODUCTION

In recent years microcantilevers (MCs) have been attributed to versatile and sensitive applications in microcantilever

based sensor technology for detection of biological and chemical agents1,2. Microcantilevers translate selective molecular

recognition events on one of the surfaces involving biological molecules and chemisorptions into nanomechanical

deflections due to induced differential surface stresses (biological or chemical events). Measurements of these nano-

metric deflections give information regarding the biological, chemical or physical phenomenon of interest. Achieving

higher deflection sensitivity of the microstructures demand scaling down of fabricated structures to an order of few

microns. The deflection (ΔZ) of a cantilever structure due to molecular recognition induced differential surface stress

(Δσ) is governed by Stoney's equation 21)1(3 ⎟⎠⎞

⎜⎝⎛⋅⋅Δ⋅−=Δ

tL

EZ σν (1)

where, E is the elastic modulus, ν is the Poisson’s ratio; t and L are the thickness and length of the microcantilever

respectively, σΔ is the induced differential surface stress and (L/t) represents the aspect ratio of the fabricated micro-

cantilever. Efforts are being made to achieve higher deflection sensitivity by scaling down the structural dimensions.

However, there is a tradeoff between the deflection sensitivity and the aspect ratio design of the fabricated

microstructures since they are limited by the stability of the microstructures concerned. Considering the material aspect,

E can be chosen such that small surface stress induced deflections of the microcantilevers, would be significant enough

to be applied for microcantilever based sensor technology. Recent works by Calleja et al.3 demonstrates the application

of SU-8 polymer based low noise microcantilevers as biosensors, whose elastic modulus is of the order of 5GPa. This

paper explores the possibility of Poly HDDA as a prospective material for fabricating these microcantilever structures for

microcantilever based sensor technology. Poly HDDA is a thermosetting polymer which is thermally stable up to 350oC4

with a low elastic modulus (E) of the order of 930MPa5. The thermal noise stability within a temperature range of 24oC

to 120oC for the proposed gold coated Poly-HDDA microcantilever is calculated to be of the order of 18nm for a

temperature change of 2oC. Also a novel microfabrication technique using an in house built Microstereolithography

(MSL) system is proposed for fabricating free standing microcantilever based sensor structures with achieved

dimensions of 600 μm × 60 μm × 40 μm (Length × Width × Thickness). The deflection sensitivity resolution achieved

with our fabricated microcantilever structure is of the order of 0.5 nm for a differential surface stress of the order of

1 mN/m following equation (1). A comprehensive review of Goeders et al. discussed the detection schemes of

microcantilever based sensors viz., Optical Beam Deflection Method (OBDM), interferometry, piezoresistive or

capacitive6. Among these, the applications of the latter two mechanisms are limited by the unavoidable electrical noises

involved in the measurement technique. Wherein the former two methods are the most sensitive techniques to detect

nano order deflections till date, both the techniques have a limitation regarding the resolution of measurement for highly

scaled down structures in order to achieve even higher deflection sensitivity. Evans and Craig7 discussed the key issues

related to the sensitivity and reflected signal intensity of the widely used OBDM method. Their work reports the



intensity drop of the reflected laser spot as the focused beam in OBDM is scanned along the cross-sectional length to

detect the maximum deflection at the free end. Moreover, diffraction effects due to the reduced dimension will also

decrease the signal to noise ratio (SNR) resulting in a poor resolution8 of deflection measurement for highly scaled down

structures thereby limiting the overall sensitivity of the measurement system. In an earlier paper9 a single image optical

diffraction based deflection profiling method was proposed to measure 1nm deflection of microcantilever based sensors,

where, a double MC sensor structure is employed. Diffraction of an incident planar or spherical wavefront at the double

microcantilever (MC) structure carries the signature of the structural deformation. Any nanometer order deflection of one

of the MC’s relative to the other would be reflected in the diffraction pattern by a sub-millimeter order shift of intensity

minima which can be probed by 2D image analysis. In addition a uniform deformation profile of the bent cantilever was

proposed to be achieved by analysis of the single image of the obtained diffraction pattern. A double MC structure for

sensor application as proposed in the optical diffraction based method was designed and fabricated by our MSL system

using Poly- HDDA as the polymer material with one of the beams bent relative to the other. The optical diffraction based

profiling technique was used to measure the bent profile of one of the microcantilevers relative to other.

2. FABRICATION AND MATERIAL ASPECT OF POLY-HDDA

1, 6 Hexane diol diacrylate (HDDA) is a di functional monomer which can be rapidly photo polymerized and cross-

linked to Poly HDDA under UV laser in presence of photoinitiator (PI). Here, benzophenone (BP) (purity 99%, m.p.

48oC) from Alfa Aesar, Germany and 2, 2-dimethoxy-2-phenyl acetophenone (DMPA) (99%) from Sigma Aldrich, USA

(recrystallized twice from heptane) were used as a photoinitiator to cure HDDA. In addition benzotriazole (BZT) from

S.D. Fine, India was used (as received) as a photo absorber in order to control the curing depth and width of the cured

polymer.

2.1 MSL equipment used for fabrication

Our in house built MSL system comprises of an Argon ion CW UV laser (BeamLok 2065-5S Spectra Physics, USA)

which predominantly radiate 364nm wavelength monochromatic radiation, an Acousto-Optic Modulator (AOM) (NEOS

Tech, USA), Optical Mirrors, 0.5mm diameter variable aperture (Holmarc, India), UV transparent Lens (focal length

50mm) and XYZ (25VP-XL, Newport, USA) coupled linear translational stages with XPS-C8 motion controller

(Newport, USA) computer interfaced through LABVIEW (National Instruments). The entire setup is mounted on top of

a vibration isolation optical table (Newport, USA), covered by perspex box in order to achieve a dust free environment.

The details of the setup can be referred in an upcoming paper to be communicated shortly. Keeping the laser beam path

fixed, a thin monomer layer on top of the linear translational stage (automated according to a CAD design interfaced

through LABVIEW) is subjected to linear predefined scanning (based on design structure). The thin monomer layer gets

cured line by line following the translational stage movement according to the design. A detailed working principle of

scanning MSL system can be referred elsewhere10.



2.2 Fabrication of the dual microcantilevers Our fabrication involves polymerization of an initial base block to act as a base support for the microcantilever

structures. A 3D base block of dimension of 5mm × 5mm × 1.5mm (Length ×Width × Height) was cured from a thin

monolayer of 150μl solution of HDDA (activated with 0.5g 1-1 concentration of DMPA and 2g l-1 BZT) wetted on top of

a 10mm × 10mm silicon wafer following a layer by layer curing approach. Silicon wafer reduces scattering of UV

radiation allowing us to achieve an initial focus spot diameter of the order of 2 – 5μm with controlled radiation power of

the order of 5 – 7mW. For subsequent layers the focus spot diameter changes to an order of 10 – 15μm due to scattering

from cured Poly-HDDA layer. The achieved curing depth per layer and the minimum curing width was optimized by

varying the type11 and concentration of photoinitiator12, radiation power and linear stage translation rate. For fabricating

the microcantilevers of desired length (in this case curing depth) monomer solution (activated with 0.5 g l-1 BP and 0.005

g l-1 BZT) was used. Optimized curing following a vertical layer by layer approach allowed us to achieve cantilever

dimensions of the order 60μm (width) × 40μm (thickness, in this case minimum curing width) and 600μm (height, in this

case curing depth) with a gap of 100μm between the two structures along the thickness dimension from center to center.

This structure is subjected to an incident planar or spherical wavefront as shown in Fig 2a. The SEM image of our

obtained structure is shown in Fig 2b.

Fig 2a. Schematic diagram showing achieved dimension of double MC beams and proposed optical diffraction profiling methodology

Fig 2b. SEM image of obtained double MC structure



Since, the whole MC system including the 3D block and the dual microcantilevers is fabricated using the same polymer

having same thermo-physical properties, the designed sensor will show a lesser effect to thermal noises during

measurement due to small fluctuations of temperature. The fabricated microstructures can act as a double slit aperture

mask to an incident planar or spherical wavefront giving a distinctive diffraction pattern. Carefully designed aspect ratio

following equation (1) would give mechanically stable microstructures to be effectively used for microcantilever based

sensor technology. A very low value of Young’s modulus of the order of 930MPa5 increases the deflection sensitivity of

the active sensor beam subjected to molecular recognition induced surface stresses achieving deflection resolution of the

order of 0.5nm for a differential surface stress resolution of 1mN/m.

2.3 Surface functionalization of one of the microcantilever

To obtain a bent profile of one of the microcantilevers for studying its applicability as a viable sensor for microcantilever

based sensor technology, cantilever surface along the cross sectional width of one of the microcantilevers is proposed to

be functionalized by gold sputtering allowing the other surface to remain passive to any chemical or biological

interaction. The second uncoated microcantilever simply would act as reference beam for our deflection measurement.

On top of the sputtered gold surface of the MC, a solution of dodecane thiol can be applied carefully which leads to

selective adsorption induced surface stress, generated on the active cantilever surface by self-assembly mechanism of

thiol-gold chemistry13. The induced differential surface stress with respect to the passive surface of the active beam will

lead to bending of the active MC with respect to the other fixed reference beam. The surface stress induced bent profile

is proposed to be obtained by the optical diffraction based profiling technique9. The achieved temperature sensitivity of

our gold coated Poly-HDDA cantilevers is comparable to the temperature sensitivity of SU-8 on Fluro Carbon (FC)

structures as reported by Calleja et.al3. The calculated bending compares well according to the Timoshenko formula

given by equation (2), considering the dimensions and material properties of the microcantilevers, summarized in Table

(1).

⎟⎟⎠

⎞⎜⎜⎝

⎛+⎟⎟

⎠

⎞⎜⎜⎝

⎛++⎟⎟

⎠

⎞⎜⎜⎝

⎛+

⎟⎟⎠

⎞⎜⎜⎝

⎛+

+Δ−

=

11

2222

21

22

11

2

2

1

2

2

1

21

212

113

1)()(3

EtEt

tt

EtEt

tt

tt

ttTL

ZT

αα

(2)

where, α1 and α2 are the temperature expansion coefficients of the two materials, E1 and E2 the corresponding Young’s

modulus and ΔT is the temperature variation. The thermal stability gained by the use of polymer materials is significant

in decreasing the noise in microcantilevers to be used as sensors for bio/chemical sensing. Developing a dedicated

surface functionalization protocol for Poly-HDDA based microcantilevers, fabricated by the proposed MSL system, can

altogether remove the thermal noises, for it would no longer involve two separate materials as in gold coated Poly-

HDDA microcantilevers used to demonstrate generation of surface stresses due to self assembly chemistry of Thiol-

Gold.



Table 1. Dimension and material properties of the cantilevers together with the corresponding calculated deflections (ZT) for a

temperature change ΔT of 2OC.

Materials Length (μm)

Thickness (μm)

E (GPa)

α (10-6/K)

ZT (nm)

FC SU-8

Poly-HDDA

Au

200

600

0.02 4.5

40

0.02

8 5.4

0.930 7.5

34-102 52

100 14.2

6 – 16

18

3. OPTICAL DIFFRACTION BASED DEFLECTION PROFILING TECHNIQUE

The fabricated double microcantilever structures acts as a periodic grating function9 given by

∏∏ ⎥⎥

⎦

⎤

⎢⎢

⎣

⎡ +−

⎥⎥

⎦

⎤

⎢⎢

⎣

⎡ −−=

a

bx

a

bxxg 221)( ( 3 )

where, ∏ is a rectangular slit function, a is the width of each structure (40μm) and b is the distance between the two

structures centre to centre (100μm ). A simple optical setup gives a spherical diverging wavefront from a pinhole and

microscope objective placed in the path of a laser of wavelength 670nm. The fabricated micro structure is placed in the

path of the diverging wavefront such that the propagation direction of the wavefront is perpendicular to cross sectional

thickness dimension of the dual microbeam structure. Fraunhofer diffraction pattern is obtained on a screen placed at a

distance D of 100cm following the condition of Fresnel number F<<1, where F is given by

λD

aF2

= ( 4 )

The intensity of the obtained diffraction pattern follows the expression

⎟⎠⎞

⎜⎝⎛

⎥⎦

⎤⎢⎣

⎡⎟⎠⎞

⎜⎝⎛−= f

DbCosf

DaSincIffI s λ

πλπδ 2

2

0)()( ( 5 )

where, I0 is the unperturbed intensity, D is the distance of the screen from the slits and f is the spatial frequency. A

simulated intensity profile of diffraction pattern of our double MC structure of achieved dimension is shown in Fig 3.



Fig 3. Plot of Simulated Intensity pattern [ Is(f) ]

Minima intensities occur when argument of the cosine function in equation (5) equals a multiple of π

πλπ mfDb

= [m=0, 32,1, ±±± ] (6)

Measuring the distance between the first order minimas at any cross section along the length gives a measure of the

distance between the two structures (centre to centre) at that cross section.

Fig 4. Cropped diffraction pattern of central and first order minima of a designed and fabricated bent cantilever and corresponding

pattern obtained after filtering and transformation to gray scale.



Determination of distance B between the first order minimas were carried out for each cross section along the cross

sectional length from the filtered diffraction pattern in Matlab after appropriate low pass filtering to remove edge

diffraction effects of the edge of the base block on which the microcantilever structures stand as shown in Fig 4.

The spatial separation b between the two microstructures (centre to centre) along the thickness dimension for each cross

sectional length can be obtained from the equation

bDfB λ22 1 == (7)

where, B is the spatial separation between the first order minimas at any cross section, λ=670nm is the wavelength of

light used, D=100cm is the distance of the imaging screen from the object. The shift in intensity minima for two cross

sectional lengths is shown in Fig 5. The corresponding bends (Δb) of the fabricated bent beam relative to the other

straight beam is calculated from the intensity minima shifts with respect to the fixed end of the MC’s using Equation (7)

as tabulated in Table 2.

Fig 5. Intensity minima shift at cross sectional lengths 191.97μm & 311.23μm relative to the base of the cantilever structures.

Table. 2. Measured deflections along cross sectional length

Cross-section along length (μm)

Δb (μm)

0 (base) 191.97 311.23

0 2.58

0.69204



4. SUMMARY

Poly-HDDA with a relatively low Young’s modulus of the order of 930 MPa and a high thermal stability parameter

fulfills the criteria of a prospective material for microcantilever based sensor technology. The proposed fabrication

technique by microstereolithography using optimized parameters can fabricate microcantilevers with desired dimensions

achieving very high deflection sensitive microcantilever structures. Optimization involves concentration of photoinitiator

and photoabsorber, photoinitiator type, linear stage rate, incident laser power and laser spot diameter. Also, developing a

definite surface functionalization protocol for Poly-HDDA can remove thermal noise effects totally in single polymer

material microcantilever structures fabricated by the proposed technique. Achieved surface stress resolution in our

moderately scaled down structure of 600 μm length, 60 μm width and 40 μm thickness is of the order of 1 mN/m for a

deflection of 0.5 nm at the free end. Further scaling down of dimension can achieve even better surface resolutions of the

order of few μN/m. The demonstrated optical diffraction based profiling technique can effectively detect the nanometric

deflection profiles for further scaled down structures.

ACKNOWLEDGEMENTS

The authors would like to acknowledge Prof. R.M. Vasu from Instrumentation & Applied Physics, IISc for his valuable

suggestions and guidance at various stages of the work. The authors would also like to thank Center of Excellence in

Nanoelectronics (CEN), IISc for funding this research. Mr. Arindam Phani gratefully acknowledges the financial aid

from Department of Science & Technology – Govt. of INDIA and permission from Indian Institute of Science,

Bangalore for attending this conference.

REFERENCES

[1] Raiteri, R., Grattarola, M., Butt, H.J and Skládal, P., “Micromechanical cantilever-based biosensors,” Sensors and Actuators B 79, 115-126 (2001). [2] Lavrik, N.V., Sepaniak, M.J and Datskos, P.G., “Cantilever transducers as a platform for chemical and biological sensors,” Rev. Sci. Instrum. 75(7), 2229-2253 (2004). [3] Calleja, M., Tamayo, J., Nordström, M and Boisen, A., “Low-noise polymeric nanomechanical biosensors,” Appl. Phys. Lett. 88, 113901, (2006). [4] Goswami, A., Umarji, A.M. and Madras, G., “Degradation kinetics of Poly-(HDDA-co-MMA),” J. Appl. Polym. Sci. 117, 2444-2453 (2010). [5] Manias, E., Chen, J., Fang, N. and Zhang, X., “Polymeric micromechanical components with tunable stiffness,” Appl. Phys. Lett. 79(11), 1700-1702, (2001). [6] Goeders, K.M., Colton, J.S and Bottomley, L.A., “Microcantilevers: Sensing chemical interactions via mechanical motion,” Chem. Rev. 108, 522-542 (2008).



[7] Evans, D.R. and Craig, S.J., “Sensing cantilever bending by optical lever technique and its application to surface stress,” J. Phys. Chem. B 110, 5450-5461 (2006). [8] Putman, C.A.J., de Grooth, B.G., Van Hulst, N.F. and Greve, J., “A detailed analysis of the optical beam defection technique for use in atomic force microscopy,” J. Appl. Phys. 72(1), 6-12 (1992). [9] Phani, A., “A non-contact measurement technique to measure micro surface stress and obtain deformation profiles of the order of 1nm in micro-cantilever based structures by single image optical diffraction method,” Proc. SPIE 7750, 77502C (2010). [10] Varadan, V.K., Jiang, X and Varadan, V.V., [Microstereolithography and other Fabrication Techniques for 3D MEMS], John Wiley & Sons, Ltd., West Sussex, Chap. 4 (2001). [11] Ravve, A., [Light-Associated reactions of synthetic polymers], Springer Science+Business Media, New York, 28-30 (2006). [12] Zhang, X., Jiang, X.N. and Sun, C., “Micro-stereolithography of polymeric and ceramic microstructures,” Sens. Actuators A 77, 149-156 (1999). [13] Berger, R., Delamarche, E., Lang, H.P., Gerber, C., Gimzewski, J.K., Meyer, E. and Güntherodt, Hans-J., “Surface stress in self-assembly of alkanethiols on gold,” Science 276, 2021-2024 (1997).



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Poly-HDDA microstructure fabrication using ...arindamphani.yolasite.com/resources/PHOTONICS WEST SPIE Proc Vol... · Poly-HDDA microstructure fabrication using Microstereolithography