Journal of Mechanical Science and Technology 25 (4) (2011) 1017~1022

www.springerlink.com/content/1738-494x DOI 10.1007/s12206-011-0205-6

Sensitivity analysis of plating conditions on mechanical properties

of thin film for MEMS applications† Yoonhwan Woo and Sang-Hyun Kim*

Mechanical Systems Engineering, Hansung University, Samseon-dong 3-ga, Seongbuk-gu, Seoul, 136-792, Korea

(Manuscript Received October 14, 2010; Revised February 9, 2011; Accepted February 13, 2011)

----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Abstract In this paper, we report the detailed investigation into the effects of plating temperature and applied current density upon the mechani-

cal properties of plated nickel film such as Young’s modulus and residual stresses. This method uses the resonance method of atomic force microscope, which does not require specially microfabricated cantilevers and additional experimental set-up. Thin layers of nickel are electroplated onto the tip surface of AFM cantilevers and plating thicknesses were measured at the end of each plating step. The self-deformation of the released AFM cantilever is also measured as a function of the plated nickel thickness, which is converted into the quantitative residual stress by appropriate mechanics. The measured Young’s modulus is as high as that of bulk nickel at low plating temperature and at low applied current density, but drastically drops at high temperature or current density. The dependence of Young’s modulus on the plating thickness is negligible in thin film less than few microns. The residual stress is also a strong function of the proc-ess conditions, and decreased with the elevation of the current density and plating temperature. And the intrinsic and extrinsic stresses of plated nickel are separated from the measured residual stress, and correlated with plating conditions. Dependence of the plated thickness on Young’s modulus and residual stress is also considered.

Keywords: Atomic Force Microscope; Plating conditions; Residual stress; Resonant frequency; Young’s modulus ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 1. Introduction

Electroplated metal films have attracted wide spread inter-ests in MEMS such as microswitches, microbeams, and mi-croactuators, as it provides an easy, simple and low-cost tech-nology with comparable mechanical properties to the silicon-based material set [1-3]. Thin metal films are usually depos-ited or mounted on a substrate of different materials and the performance of MEMS devices is highly determined by the mechanical properties of electroplated thin films such as Young’s modulus and residual stresses. It is known that Young’s modulus of electroplated metal films can differ from that of bulk materials because they are highly dependent upon the composition of the plating bath along with the current and temperature [4]. Due to the thermal and lattice mismatch be-tween the deposited film and the substrate, a residual stress is generated, which subsequently induces the undesirable defor-mation and frequently degrade the reliability of MEMS. Hence, characterization of the mechanical properties of the metal films has become one of the most promising research

areas in modern measurement technology. Surprisingly, little effort has been made to clarify the mechanical properties of electroplated metal films and their relationship with process conditions.

Many different methods have been developed to determine the mechanical properties of thin film materials such as meas-uring the capacitance-voltage on bridge structure, measuring the deflection of the fabricated structures under external load or self-deformation, the optical X-rays method, and the step profiler method [5-9]. Unfortunately, most of these measure-ment methods require special microfabricated structures or special test fixtures. The step profiler method also induces the mechanical contact between the probe and the micromachined structures, which may lead to some unwanted effects such as friction during the measurement. Recently the resonance me-thod has been used to measure the mechanical properties of electroplated thin films [10, 11]. Young’s modulus and resid-ual stresses of the fabricated beam structures were determined by detecting the resonant frequencies of microbeams, which is a useful method to measure the thin film properties because of its well defined analysis theory. In all of these studies, the effects of process conditions such as plating temperature and current density have not been considered. Moreover, the effect of the plated film thickness on Young’s modulus and residual

† This paper was recommended for publication in revised form by Associate EditorYong-Tae Kim

*Corresponding author. Tel.: +82 2 760 8012, Fax.: +82 2 760 4329 E-mail address: shkim@hansung.ac.kr

© KSME & Springer 2011

1018 Y. Woo and S.-H. Kim / Journal of Mechanical Science and Technology 25 (4) (2011) 1017~1022

stress is not investigated. Thus, the present research is undertaken to experimentally

expand the results of [10] to investigate the effects of plating conditions upon Young’s modulus and residual stresses of plated nickel film by using the resonance method of AFM. Thin layers of nickel are electroplated onto the tip surface of AFM cantilevers and Young’s modulus of plated nickel film is investigated as a function of process conditions by detecting the change of the resonant frequency due to the mass change of electroplated AFM cantilever. Residual stress of plated nickel film under different process is also determined by mea-suring the self-deformation of the released AFM cantilever. And the intrinsic and extrinsic stresses of an electroplated nickel film are separated from the measured residual stress, and correlated with plating conditions. Dependence of the plated thickness on Young’s modulus and residual stress is also considered.

2. Experiment descriptions

Fig. 1 shows the schematic drawing of experiment: AFM cantilever (NSC16/Si3N4: MikroMasch), insoluble positive electrode (platinum wire: CH Instruments), electrochemical fluid cell (Digital Instruments) and power supply. AFM canti-lever was immersed in the electrochemical cell of AFM with all-sulfate nickel solution and the current is applied via a power supply between the AFM cantilever (working elec-trode) and the platinum wire (counter electrode). The basic constituents for all-sulfate solution include 225~410g/L of nickel sulfate (NiSO4·6H2O) as the primary source of nickel ions and 30~45g/L of boric acid (H3BO3) to stabilize the pH of the solution [12]. The applied current density was set in the range of 5~90mA/cm2 and the plating temperature was ad-justed in the range of 20~80°C. The platinum counter elec-trode had the area of 7.85×10-2cm2, which used to set of the input current of the power.

Thin layer of nickel is electroplated onto the tip side of AFM cantilever in the different plating temperature and cur-rent density, and the variance of resonant frequency due to the mass change of electroplated cantilever is monitored at the end of each plating step. The spring constant of electroplated AFM cantilever in the liquid cell is calculated by using

Sader’s method [13]. The spring constant from Sader’s method is determined from its unloaded resonant frequency and defined by

2 20.1906 ( )f f i f fk b LQρ ω ω= Γ (1)

where ρf is the mass density of the fluid, b and L are the thick-ness and length of the beam, Qf and ωf are the quality factor and beam resonant frequency in fluid, and Гi is the imaginary part of the hydrodynamic function, which depends only on the Reynolds number.

Young’s modulus of electroplated nickel varies greatly de-pending on the processing conditions. In this paper, it is ex-perimentally obtained from the spring constant k of the com-posite cantilever beam from elementary beam theory [14]:

3

23

3 ( )12

ii i i n

i

hbk E h z zL

⎡ ⎤= + −⎢ ⎥

⎣ ⎦∑ (2)

where Ei is Young’s modulus of the ith layer, zi and zn are the distances from the bottom of the beam to the center of the ith layer and the neutral axis, and hi is the thickness of the ith layer. The spring constant, the deposited film thickness and the beam dimensions were measured. Therefore, Young’s modulus of the electroplated nickel can be readily determined by evaluating Eqs. (1) and (2).

At present, the most widely adopted equation to describe the average residual stress in thin film is the Stoney equation [15], i.e.,

2

(1 )6s s

f

E hhκσ

ν=

− (3)

where Es and ν is Young’s modulus and Poisson’s ratio of the substrate cantilever, hs and hf are the substrate and nickel film thicknesses, κ is the curvature of the structure, respectively. One of the merits of this equation is that it can be conveniently used to determine the residual stress in thin films based on the measurement of curvature. The curvature can be evaluated if we measure the end deflection of nickel plated AFM cantile-ver through the optical method of AFM. In order to consider the through-thickness variation of residual stress, it is essential to obtain of Young’s modulus of each film thickness, which is obtained by evaluating Eqs. (1) and (2).

3. Results and discussion

The variation of AFM cantilever dimensions significantly affects other parameters such as the resonant frequency and spring constant, which induces the error of measured Young’s modulus. Therefore, it is important to exactly measure the dimension of each AFM cantilever. The scanning electron microscope (SEM) with the resolution of 10nm was used for measuring the cantilever dimensions in this experiment. Sam-

Fig. 1. Schematic drawing of experiment.

Y. Woo and S.-H. Kim / Journal of Mechanical Science and Technology 25 (4) (2011) 1017~1022 1019

ple cantilever was loaded in the electrochemical cell of AFM each by each and thin layers of nickel were electroplated onto the surface of the sample cantilever during the five plating steps. The current density applied to each sample cantilever was set in the range of 5~90mA/cm2. The plating temperature was adjusted in the range of 20~80°C.

After each plating step, we measured the plating thickness of the sample cantilevers using a surface profiler at three dif-ferent positions (center and two sides of the groove). The measured plating thicknesses after each plating step of each sample cantilever were slightly different along the cantilever and the variations are less than 4.6%. The resonant frequen-cies of each cantilever were also measured at the end of each plating step. Effective Young’s modulus of the electroplated nickel was determined by using the measured resonant fre-quencies and the plated layer thickness of each cantilever. Fig. 2 presents the evolution of effective Young’s modulus of nickel films along the plating thickness under the various ap-plied current density. At a given plating current density, the effective Young’s modulus after each plating step changed slightly with the maximum variation of 6.35%, which implies the plating thickness effect is negligible in thin films less than a few micron.

The dependence of Young’s modulus of nickel films on the applied current density in the range of 8.6~86mA/cm2 at T=40°C is plotted in Fig. 3. Young’s modulus of the plated nickel decreased linearly from a near bulk value of 202 to 87GPa as increasing the applied current density. Fig. 4 shows the effect of plating temperature upon Young’s modulus of plated nickel for a constant current density of J=8.6mA/cm2. The error bars in Figs. 3 and 4 indicate the maximum varia-tions of the measured Young’s modulus (two sample cantile-vers and five measurement of each sample cantilevers after each plating step).

Young’s modulus was approximately 190GPa at a tempera-ture of less than 40°C and 203GPa at a temperature between 40~60°C, which was close to that of bulk nickel. However, Young’s modulus dropped dramatically to 110GPa above the temperature of 70°C. The reduction of Young’s modulus at high plating temperature was repeated in other sample cantile-

ver, which confirms that it is not due to the experimental error. The standard deviation of Young’s modulus in the range of 40~60°C was less than 5GPa. However, the standard devia-tions of Young’s modulus at 30°C and below, and at 70°C and above were 17.73 and 12.2GPa, which were beyond experi-mental scatter. That is why the plating temperature of all-sulfate nickel solution is recommended in the range of 40~60°C in the literature [12].

Experiment results imply that the plating rate is low at low current density. Because there is a sufficient time for nickel ions to form a dense coating at the interface between nickel solution and sample cantilever, the plated nickel film has the bulk value of Young’s modulus. As the plating current density increases, the plating rate rises. Therefore, the limited time for nickel ion in the solution to migrate to the surface of the sam-ple cantilever induces less dense plated nickel film with pores or gabs, whose Young’s modulus drops about half of the bulk value. The reduction of Young’s modulus at high temperature may be attributed to the formation of sulfate and ammonium ions [16]. An elevation in plating temperature to 70°C causes the inclusion of nonmetallic species due to hydrolysis and leads to a weaker and more compliant nickel film. Experimen-tal results are closed to those values in the literatures. One study reported a Young’s modulus of 156GPa at 70mA/cm2

Fig. 2. Evolution of effective Young’s modulus of nickel films alongthe plating thickness under the various applied current density.

Fig. 3.Young’s modulus of nickel films as a function of the applied current density at T=40°C.

Fig. 4. Young’s modulus of nickel films as a function of the plating temperature at J=8.6mA/cm2.

1020 Y. Woo and S.-H. Kim / Journal of Mechanical Science and Technology 25 (4) (2011) 1017~1022

and 131GPa at 200mA/cm2 in the nickel sulfate bath [6]. An-other study concluded that Young’s modulus decreased from 182 to 168GPa when the current density increased from 10 to 20mA/cm2 in the nickel sulphamate plating condition [8].

The deflections of electroplated AFM cantilevers were in situ measured as a function of the plated thickness to obtain the residual stresses. End deflections (δ) with the length of L can then be converted into curvatures using the equation of κ=2δ/L2. Finally, average value of the residual stress in nickel films was determined by using Eq. (3). Fig. 5 presents the dependence of the average residual stress in nickel films on the applied current density at a temperature of 40°C along the plating thickness. The residual stress in the given plating thickness increased as increasing current density, which is consistent with the result of other experiments [17, 18]. It has been reported that the residual stress increased 160 to 220MPa when the current density increased 2 to 5mA/cm2 in the nickel sulfamate plating condition at 50°C [17]. However, those re-ports did not investigate the effect of the plating thickness on the residual stress. Fig. 5 also shows the residual stress in the thin film is much greater than that of the thick film at a fixed plating condition. Average residual stress decreased from 122.79 to 60.28MPa when the plating thickness increased from 0.27 to 1.35m at the current density of 86mA/cm2. A decrease of the residual stress implies that the maximum re-

sidual stress happens on the interface when two different ma-terials are connected. Fig. 6 presents the dependence of the residual stress of nickel films on the plating temperature of 20~70°C at a fixed current density of 8.6mA/cm2 along the plating thickness. As similar to the current density, the resid-ual stress in the plated film increased with increasing plating temperature, but decreased as increasing with the plated thick-ness, which well agree with other results [19].

The measured residual stress can be classified as an extrin-sic and intrinsic stresses. The origin of the extrinsic stress of the plated nickel film on the silicon AFM cantilever is mostly due to the mismatch of thermal expansion coefficient between plated nickel film (12.7×10-6/°C) and silicon cantilever (2.5×10-6/°C), and also temperature changes. The generated thermal stress is given by

0( )th fT T Eσ α= −∆ − (4) where ∆α is the difference in thermal expansion coefficient of the nickel and silicon cantilever, and T0 is the room tempera-ture. The intrinsic stress may be generated due to the lattice mismatch between the substrate and deposited film, annihila-tion of excess impurities and defect as well as phase transfor-mations during and after the film deposition. The magnitude of the intrinsic stress is dependent on the relative kinetics of the various processes and is difficult to quantify. After the extrinsic thermal stress of plated nickel film was measured, the intrinsic stress can be calculated by extracting the thermal stress from total film stress.

Fig. 7 presents the evolution of the total and intrinsic stress of nickel films under the various applied current density at a temperature of 40°C. As mentioned above, the magnitude of total residual stress increased with the increasing current den-sity. The dot line is the intrinsic stress in the film after extract-ing the thermal tensile stress, which also increased toward more tensile direction as increasing current density. When the current density is high, fine-grained microstructures are ob-tained, and the non-equilibrium growth of the crystallites is favored. Thermal stress is tensile and linearly related to the plating temperature by Eq. (4). However thermal stress de-creased with increasing current density because Young’s modulus decreased as increasing current density, as shown in Fig. 3. Fig. 8 presents the evolution of the total and intrinsic stress of nickel films under the various temperatures at a fixed current density of 8.6mA/cm2. The intrinsic stress decreased with increasing plating temperature because the higher tem-perature lowers the overvoltage of deposition and favors the equilibrium crystal growth. Low plating temperature produces a compressive stress, while a high plating temperature pro-duces a tensile stress. Obviously a zero stress regime therefore exists. The intrinsic stress is nearly zero when it is plated around 45°C. As the current density increases, the temperature to make low-stress film increases.

In this experiment, the fabricated cantilever beams were bent upward due to the residual stress and the measured reso-

Fig. 5. Dependence of the residual stress of nickel films on the applied current density at a temperature of 40°C along the plating thickness.

Fig. 6. Dependence of the residual stress of nickel films on the plating temperature at a fixed current density of 8.6mA/cm2 along the platingthickness.

Y. Woo and S.-H. Kim / Journal of Mechanical Science and Technology 25 (4) (2011) 1017~1022 1021

nance frequencies may be shifted from those of the ideal, flat beams due to beam deformation. One study compared the deviation of the frequency between the curled beam and stress-free flat beam by using ABAQUS software [20]. The maximum deflection reaches about 15% of the beam length and the relative errors of frequencies were within 1%. Another study reported the similar result for silicon nitride cantilevers bent downward [21]. Therefore, the maximum deflection in this experiment was less than 5% and the deviation of reso-nance frequencies between the curled and stress-free flat beam was not considered.

4. Uncertainty analysis

The uncertainty of the Young’s modulus of electroplated material is associated with the spring constant of multilayered cantilever which depends on many variables such as dimen-sional and material parameters, which is derived in Eq. (2). The combined relative uncertainty attributed to the spring constant of electroplated cantilever based on Eq. (1) is given by

2 2 222 2 22f f fiQk b L

f f i f

u u uuu u uk b L Q

ρ ω

ρ ωΓ⎛ ⎞ ⎛ ⎞ ⎛ ⎞⎛ ⎞⎛ ⎞ ⎛ ⎞= + + + + +⎜ ⎟ ⎜ ⎟ ⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟ ⎜ ⎟ ⎜ ⎟Γ⎝ ⎠⎝ ⎠ ⎝ ⎠⎝ ⎠ ⎝ ⎠ ⎝ ⎠

(5) where u is the uncertainty of individual parameters.

Table 1 presents the relative uncertainty of each parameter. The effective Young’s modulus of the AFM cantilever with Cr/Au layer is 168.08GPa and its thickness is 7.08µm±10nm. The nominal value of Young’s modulus of nickel is 200GPa and its thickness is assumed as 1.0µm±10nm. The nominal spring constant of composite AFM cantilever with the electro-plated nickel layer is 75.9122N/m. The uncertainty of spring constant is 1.9434N/m and the spring constant of the compos-ite cantilever ranges between 75.0476 and 76.7768N/m. The Young’s modulus of the nickel considering the uncertainty of spring constant ranges between 191.74 and 208.41GPa, and its uncertainty is 8.41GPa. Therefore, the relative uncertainty of the Young’s modulus of electroplated nickel is 4.21%.

5. Summary

In summary, we addressed a relatively simple method to in-vestigate the sensitivity of Young’s modulus and residual stress to the plating conditions by using the resonance method of AFM and Stoney equation. Young’s modulus of nickel films was as high as that of bulk nickel at the plating tempera-ture between 40~60°C and at low current density (J=8.6 mA/cm2), but dramatically decreased with increasing plating temperature or current density. The dependence of Young’s modulus on the plating thickness was negligible in thin film less than few microns. The residual stress was also a strong function of the process conditions, and decreased with the elevation of the current density and plating temperature. The tensile extrinsic stress appeared because of the CTE mismatch between the plated nickel film and silicon AFM cantilever. The intrinsic stress was also temperature-dependent. At low temperature the plated films was tensile stressed and became progressively more compressive with increasing temperature, changing from tensile to compressive at a temperature of ~45°C. The residual stress decreased as increasing with the plated nickel thickness.

Acknowledgment

This research was financially supported by Hansung Uni-versity.

Table 1. Relative uncertainty of each parameter.

Parameter u Nominal value

Relative uncertainty

air density 0.004 1.1845 0.34 %

Air viscosity 0.006 1.1866 0.32 %

resonant frequencey 1~2 170 ~1.0 %

width 0.05 40 0.125 %

length 0.05 230 0.022 %

Q 1.43 %

Reynolds number 1.13 %

hydrodynamic function 0.0034 0.5719 0.59 %

Spring constant 2.56%

Fig. 7. Evolution of the total and intrinsic stress of nickel films under the various applied current density at a temperature of 40°C.

Fig. 8. Evolution of the total and intrinsic stress of nickel films under the various temperatures at a fixed current density of 8.6mA/cm2.

1022 Y. Woo and S.-H. Kim / Journal of Mechanical Science and Technology 25 (4) (2011) 1017~1022

References

[1] W. H. Teh, J. K. Luo, M. R. Graham, A. Pavlov and C. G. Smith, Near-zero curvature fabrication of miniaturized mi-cromechanical Ni switches using electron beam cross-linked PMMA, Journal of Micromechanics and Microengineering, 13 (2003) 591-598.

[2] K. Kataoka, S. Kawamura, T. Itoh, K. Ishikawa, H. Honma and T. Suga, Electroplating Ni micro-cantilevers for low contact-force IC probing, Sensors and Actuators A, 103 (2003) 116-121.

[3] W. M. Huang, Q. Y. Liu, L. M. He and J. H. Yeo, Micro NiTi–Si cantilever with three stable positions, Sensors and Actuators A,114 (2004) 118-122.

[4] W. N. Sharpe, MEMS Handbook, CRC Press, Boca Raton, FL (2002).

[5] Y. C. Hu and W. H. Tu, Nonlinear and linearized algorithms for the Young's modulus extraction of thin films through the capacitance-voltage measurement of microstructures, Jour-nal of Applied Physics, 98 (2005) 104504.

[6] N. M. Zhou, Y. Zhou, C. S. Yang, J. A. Chen, G. F. Ding, W. Ding, M. J. Wang and Y. M. Zhang, The evaluation of Young's modulus and residual stress of nickel films by mi-crobridge testings, Measurement Science and Technology, 15 (2004) 2389-2394.

[7] J. K. Luo, A. J. Flewitt, S. M. Spearing, N. A. Fleck and W. I. Milne, Young’s modulus of electroplated Ni thin film for MEMS applications, Materials Letters, 58 (2004) 2306-2309.

[8] S. Ballandras, S. Basrour, L. Robert, S. Megtert, P. Blind, M. Rouillay, P. Bernede and W. Daniau, Microgrippers fabri-cated by the LIGA technique, Sensors and Actuators A, 58 (1997) 265-272.

[9] M. W. Denhoff, A measurement of Young's modulus and residual stress in MEMS bridges using a surface profiler, Journal of Micromechanics and Microengineering, 13 (2003) 686-692.

[10] S.-H. Kim and J. G. Boyd, Analytical and experimental study of mismatch strain-induced microcantilever behavior during deposition, Journal of Mechanical Science and Tech-nology, 21 (2007) 415-420.

[11] S.-H. Kim and J. G. Boyd, Intrinsic stress, mismatch strain, and self-assembly during deposition of thin films subjected to an externally applied force, Journal of Mechanical Sci-ence and Technology, 22 (2008) 2048-2057.

[12] M. Schlesinger and M. Paunovic, Modern Electroplating, Wiley, New York, USA (2000).

[13] J. E. Sader, Calibration of rectangular atomic force micro-scope cantilever, Review of Scientific Instruments, 70 (1999) 3967-3969.

[14] R. F. Gibson, Principles of Composite Material Mechanics, McGraw-Hill, New York, USA (1994).

[15] G. G. Stoney, The tension of metallic films deposited by electrolysis, Proceedings of the Royal Society of London A, 82 (1909) 172-175.

[16] S. A. Watson: Ni Dev. Inst. Tech. Ser. Rep. 10047 (1998). [17] S.-H. Kim, J.-Y. Kim, J. Yu and T. Y. Lee, Residual stress

and interfacial reaction of the electroplated Ni-Cu alloy un-der bump metallurgy in the flip-chip solder joint, Journal of Electronic Materials, 33 (2004) 948-957.

[18] S. Basrour and L. Robert, X-ray characterization of residual stresses in electroplated nickel used in LIGA technique, Ma-terials Science and Engineering, 288 (2000) 270-274.

[19] M. Qin, Young’s modulus measurement of nickel silicide film on crystal silicon by a surface profile, Journal of Mate-rials Science Letters, 19 (2000) 2243-2245.

[20] C.-W. Baek, Y.-K. Kim, Y. Ahn and Y.-H. Kim, Meas-urement of the mechanical properties of electroplated gold thin films using micromachined beam structures, Sensors and Actuators A, 117 (2005) 17-27.

[21] L. Kiesewetter, J. M. Zhang, D. Houdeau and A. Stecken-born, Determination of Young’s moduli of micromechanical thin films using resonance method, Sensors and Actuators A, 35 (1992) 153-159.

Yoonhwan Woo is an assistant profes-sor in the department of mechanical systems engineering in Hansung Univer-sity, Seoul, Korea. Formerly, he was a software engineering researcher in the ACIS development team at Spatial Cor-poration in the USA. He received a PhD degree in Mechanical Engineering from

Colorado State University, a MS degree in Mechanical and Aerospace Engineering from Illinois Institute of Technology, and a BS degree in Precision Mechanical Engineering from Hanyang University, Korea. His research interest includes CAD, geometric and solid modeling, and computer-aided process planning.

Sang-Hyun Kim obtained his B. S. (1993) and M.S. (1995) degrees in Aero-space Engineering from Pusan Nation University and Ph.D. (2005) in Aero-space Engineering from Texas A&M University. After getting Ph.D., he joined Micro Systems Lab. at Samsung Advanced Institute of Technology

(SAIT) and worked in the field of MEMS designs, especially microsensors, microactuator and inkjet print head. He is cur-rently working as a faculty member in Mechanical Systems Engineering at Hansung University. His research interest lies in design, fabrication and test of MEMS/NEMS. He is also interested in the development of design platform of multi-physics and multiscale phenomena.