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Effect of roughness on adhesion of polymeric coatings used for microgrippers
Microelectronic Engineering 84 (2007) 1227–1230 www.elsevier.com/locate/mee
Effect of roughness on adhesion of polymeric coatings used for microgrippers N. Balabanava a
a,*
, R. Wierzbicki b, M. Zielecka c, Z. Rymuza
a
Warsaw University of Technology, Institute of Micromechanics and Photonics, Warsaw 02-525, Poland b Nascatec GmbH, Kassel 34131, Germany c Institute of Industrial Chemistry, Warsaw 01-793, Poland Available online 8 February 2007
Abstract The purpose of the work was to examine the influence of topography and materials parameters on the value of pull-off force (adhesion) with future application of obtained results to solve the adhesion problem in microhandling. The research consists of two parts: theoretical and experimental. In theoretical research the model of contact, which takes into account the roughness of the contacted bodies is presented. In experimental part polymeric films with low surface energy were investigated as a solution to decrease adhesion at the gripping surface of the micromanipulator. To introduce surface roughness into the solution of polymers the nanoball structures (nanoparticles) were mixed. In result of these modifications three polymer samples were made: with density of nanoballs in the volume 2%, 5% and 10%. In result of these studies the dependences concerning the influence of the roughness on the adhesion were obtained. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Adhesion; Microgripper; Roughness; Polymer coatings
1. Introduction The problem of handling and manipulation of microdevices is very important nowadays and the microgrippers are widely used in these processes. The essential tasks for the microgripping are controlled capture, handling and releasing of the micropart. One of the main problems that can occur is sticking of microparts to the working arms of microgripper [1]. As the gripper approaches to the part the attraction forces may cause the part to jump into the gripper, so it cannot be orientated and manipulated in the right way. When the part is placed into the surface, it may adhere better to the gripper than the substrate, preventing accurate positioning. Several techniques have been used to solve these problems: materials selection, various designs for microgrippers, precise control of environmental *
Corresponding author. Tel.: +48 505 532570. E-mail address: [email protected] (N. Balabanava).
0167-9317/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.01.183
conditions and applying additional techniques, such as sonolubrication [2], to minimize influence of adhesion forces [3]. We propose to coat the microgrippers by the polymeric films with low surface free energy. Additional suggestion is the modification of the working surface of microgrippers to vary its roughness. 2. Theoretical research In theoretical part we will examine influence of the materials properties and surface roughness on the pulloff force (adhesion) [4]. In this investigation the roughness was presented as randomly distributed asperities. The influence of such parameters as density of asperities, height distribution, Young’s modulus and surface energy was studied. Modeling of the contact of rough surfaces has been treated by several investigators since mid sixties of last century.
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N. Balabanava et al. / Microelectronic Engineering 84 (2007) 1227–1230
Difficulty in the development of the theoretical model is that the real surface has randomly distributed asperities and may be anisotropic so that stochastic models must be used. For this reason we will modify the solution JKR model used to describe adhesive contact of nominally flat elastic bodies to the model which will take into account the roughness of the surface. We will consider the contact of elastic rough surface with ideal flat rigid surface (Fig. 1). The asperities tips will be modeled by the spherical segments with the same radius b. The dependence of the total applied load from the distance d in normalized form will take a form: rffiffiffi pffiffiffi r 3=2 P ðhÞ ¼ 3Drb D KAa I 1 ðhÞ b c
ð1Þ
where K, Young’s modulus; Aa, relative real contact area; D, density of asperities; r, dispersion of asperities heights;
I 1 ðhÞ ¼
Z
1
P ðvÞp ð1Þd1;
h
h ¼ d=r; 2=3 1 9 pb2 Dc Dc ¼ ; 3Rr 8 K v ¼ d=dc ;
d ¼ rð1 hÞ;
ffi exp 12 12 – denDc– specific surface energy, p ð1Þ ¼ p1ffiffiffi 2p sity of asperities height distribution (standardized Gaussian distribution), P ðvÞ – solution of equation: 8 qffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffi 13 2 > P 1 P > < þ 1 1 þ 1 þ 1 ; ddc P 33 3 P 9 P c c d ¼ qffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffi1 dc > > : 3 P þ 1 þ 1 1 1 P þ 1 3 ; 1 6 d 6 323 Pc 9 Pc dc 1 where dc ¼ 3R spheres.
3RP 23 c
K
; P c ¼ 32 pRDc; R – radius of the contact
Fig. 1. Profile of the rough surface.
Fig. 2. Dependence of pull-off force on materials and topography properties.
N. Balabanava et al. / Microelectronic Engineering 84 (2007) 1227–1230
Given equation describes the dependence of the depth d from the applied load P, which was obtained in the JKR models solution for the adhesive contact of the flat elastic spheres [4]. According to our model and Eq. (1) the force needed to separate the contact to the level h > hc can be found from rffiffiffi Z 1 33=2 r 3=2 Dc KAa P sep ðhÞ ¼ Drb P ðyÞpðy þ hÞdy: ð2Þ b 4 L where L = min{h hc, Dc}, hc – minimal deformation level. For pull-off force (adhesion) the maximum repulsing force during the contact separation (h ! 1): P ad ¼ P sep ðh Þ ¼ max jP sep ðhÞj:
ð3Þ
To determine the pull-off force we need to calculate Pad for the set of values of h in the neighborhood of supposed maximum of Psep. The dependences of Pad from the such mechanical and material properties as Young’s E, surface energy Dc, dispersion of asperities heights r and asperities density D were obtained and plotted in Fig. 2. 3. Investigation of polymeric coatings First part of experiments deals with research on polymeric films. The microgrippers (Fig. 3) were coated with four types of polymeric films (Table 1). Electrostatic comb-driven, single crystal silicon microgrippers have been used for evaluation of coatings functionality [5]. The grippers’ structure has been fabricated with DRIE process. Gripping surfaces (as well as all sidewalls of the structure) according to the fabrication process are slightly rippled (periodic surface corrugations with amplitude at level of few tens of nm and period at level of 200 nm). The thickness of the structure is 30 lm, the width of gripping tips is 4 lm. The sticking effect were examined and compared with the case of uncoated microgripper. In second part of experiments we examined influence of the roughness of the surface on the value of pull-off force (adhesion). For this purpose the polymer solutions were modified. Into the solution of polymers the nanoball structures were mixed. The nanoballs were made from SiO2, the
1229
Table 1 Tested samples with various configurations of resin and solution composition and curing treatment Sample Nr.
Resin
Solution
1 10 2 20
MS MS + C MPS MPS + C
ALH ALH ALH ALH
& & & &
ARH ARH ARH ARH
Cure temperature (°C)
Cure time (h)
180 20 180 20
2 24 2 24
MS, methylsilicone resin, MPS, methylphenylsilicone resin, C, catalyst improving adhesion to the substrate, ALH, aliphatic hydrocarbons, ARH, aromatic hydrocarbons.
average diameter of the piece is 170 nm. In result of these modifications three samples for each polymer solution were made: with density of nanoballs in the volume 2%, 5% and 10%. The pull-off force was measured using AFM techniques. The special cantilever was used in these experiments. The cantilever was made from beryllium bronze; upper side is polished very precisely and covered with 300 nm of gold. Length of cantilever is 6 mm, width 1 mm, thickness 50 lm. On the bottom side the steel ball with diameter 0.7 mm is glued. In the result of the experiment for each polymer solution the dependence of change of the pull-off force (%) from density of the nanoballs in polymer was obtained. Obtained results are presented in Fig. 4. For 100% the value of pull-off force of the clean silicon was taken. 4. Results and discussion Dependences received in the result of theoretical research give a possibility to predict the adhesive behavior of the material. Proposed model takes into account such parameter as roughness of the surface which allows to obtain more reliable data. Made investigations give us an opportunity to predict behavior of the surfaces in micropart–microgripper contact and as a fact to choose materials with suitable mechanical and topographical parameters to minimize action of adhesion. According to experimental research the presence of roughness decrease the influence of surface forces. In the case of the tested samples, in which content of the nanoballs in polymer solution was 10%, the value of pull-off force has decreased down to 60% (Fig. 5).
Fig. 3. Mechanical microgripper by Nascatec company (Germany). (1) Uncoated microgripper; (2) coated microgripper.
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N. Balabanava et al. / Microelectronic Engineering 84 (2007) 1227–1230
Fig. 5. Dependence of the pull-off force (in %) on density of nanoballs structures in the polymer solutions.
5. Conclusion Coating techniques look to be very promising to use in microtechnology. Application of thin films can solve a majority of problems occurring in microdomain. It can improve optical, wear, friction and adhesive properties of microstructures. Made experiments show that the topographical properties of objects are very important in microdomain. Moreover, the controlled surface roughness gives a possibility to control adhesive properties of microobjects. The idea to use modified with nanoparticles polymeric coating to decrease action of adhesion forces turn out to be very effective and easy way to solve sticking problems. Acknowledgement The work was supported by ASSEMIC training network project under number 504826. References
Fig. 4. Dependence of the pull-off force (in %) from the density of nanoballs structures in the polymers solutions. Compared with the pull-off force of clean silicon (taken as 100%).
[1] K.F. Bohringer, R.S. Fearing, K.Y. Goldberg, The Handbook of Industrial Robotics, Chapter Microassembly, 1998. [2] N. Balabanava, V. Chygurinau, M. Zielecka, Z. Rymuza, Journal Czasopismo Techniczne 6-M (2006) 11. [3] S. Bou, A. Almansa, N. Balabanava, Z. Rymuza, in: Proceedings of 2005IEEE/ASME International Conference on Advanced Intelligent Mechatronics, pp. 384–389. [4] A.I. Sviridenok, A.S. Chizhik, M.I. Petrokovets, Mechanics of Discontinuous Friction Iinteraction, Tekhnika, Minsk, 1990 (in Russian).. [5] B.E. Volland, H. Heerlein, I.W. Rangelow, Microelectronic Engineering (2002) 1015–1023.
Effect of roughness on adhesion of polymeric coatings used for microgrippers N. Balabanava a
a,*
, R. Wierzbicki b, M. Zielecka c, Z. Rymuza
a
Warsaw University of Technology, Institute of Micromechanics and Photonics, Warsaw 02-525, Poland b Nascatec GmbH, Kassel 34131, Germany c Institute of Industrial Chemistry, Warsaw 01-793, Poland Available online 8 February 2007
Abstract The purpose of the work was to examine the influence of topography and materials parameters on the value of pull-off force (adhesion) with future application of obtained results to solve the adhesion problem in microhandling. The research consists of two parts: theoretical and experimental. In theoretical research the model of contact, which takes into account the roughness of the contacted bodies is presented. In experimental part polymeric films with low surface energy were investigated as a solution to decrease adhesion at the gripping surface of the micromanipulator. To introduce surface roughness into the solution of polymers the nanoball structures (nanoparticles) were mixed. In result of these modifications three polymer samples were made: with density of nanoballs in the volume 2%, 5% and 10%. In result of these studies the dependences concerning the influence of the roughness on the adhesion were obtained. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Adhesion; Microgripper; Roughness; Polymer coatings
1. Introduction The problem of handling and manipulation of microdevices is very important nowadays and the microgrippers are widely used in these processes. The essential tasks for the microgripping are controlled capture, handling and releasing of the micropart. One of the main problems that can occur is sticking of microparts to the working arms of microgripper [1]. As the gripper approaches to the part the attraction forces may cause the part to jump into the gripper, so it cannot be orientated and manipulated in the right way. When the part is placed into the surface, it may adhere better to the gripper than the substrate, preventing accurate positioning. Several techniques have been used to solve these problems: materials selection, various designs for microgrippers, precise control of environmental *
Corresponding author. Tel.: +48 505 532570. E-mail address: [email protected] (N. Balabanava).
0167-9317/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2007.01.183
conditions and applying additional techniques, such as sonolubrication [2], to minimize influence of adhesion forces [3]. We propose to coat the microgrippers by the polymeric films with low surface free energy. Additional suggestion is the modification of the working surface of microgrippers to vary its roughness. 2. Theoretical research In theoretical part we will examine influence of the materials properties and surface roughness on the pulloff force (adhesion) [4]. In this investigation the roughness was presented as randomly distributed asperities. The influence of such parameters as density of asperities, height distribution, Young’s modulus and surface energy was studied. Modeling of the contact of rough surfaces has been treated by several investigators since mid sixties of last century.
1228
N. Balabanava et al. / Microelectronic Engineering 84 (2007) 1227–1230
Difficulty in the development of the theoretical model is that the real surface has randomly distributed asperities and may be anisotropic so that stochastic models must be used. For this reason we will modify the solution JKR model used to describe adhesive contact of nominally flat elastic bodies to the model which will take into account the roughness of the surface. We will consider the contact of elastic rough surface with ideal flat rigid surface (Fig. 1). The asperities tips will be modeled by the spherical segments with the same radius b. The dependence of the total applied load from the distance d in normalized form will take a form: rffiffiffi pffiffiffi r 3=2 P ðhÞ ¼ 3Drb D KAa I 1 ðhÞ b c
ð1Þ
where K, Young’s modulus; Aa, relative real contact area; D, density of asperities; r, dispersion of asperities heights;
I 1 ðhÞ ¼
Z
1
P ðvÞp ð1Þd1;
h
h ¼ d=r; 2=3 1 9 pb2 Dc Dc ¼ ; 3Rr 8 K v ¼ d=dc ;
d ¼ rð1 hÞ;
ffi exp 12 12 – denDc– specific surface energy, p ð1Þ ¼ p1ffiffiffi 2p sity of asperities height distribution (standardized Gaussian distribution), P ðvÞ – solution of equation: 8 qffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffi 13 2 > P 1 P > < þ 1 1 þ 1 þ 1 ; ddc P 33 3 P 9 P c c d ¼ qffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffi1 dc > > : 3 P þ 1 þ 1 1 1 P þ 1 3 ; 1 6 d 6 323 Pc 9 Pc dc 1 where dc ¼ 3R spheres.
3RP 23 c
K
; P c ¼ 32 pRDc; R – radius of the contact
Fig. 1. Profile of the rough surface.
Fig. 2. Dependence of pull-off force on materials and topography properties.
N. Balabanava et al. / Microelectronic Engineering 84 (2007) 1227–1230
Given equation describes the dependence of the depth d from the applied load P, which was obtained in the JKR models solution for the adhesive contact of the flat elastic spheres [4]. According to our model and Eq. (1) the force needed to separate the contact to the level h > hc can be found from rffiffiffi Z 1 33=2 r 3=2 Dc KAa P sep ðhÞ ¼ Drb P ðyÞpðy þ hÞdy: ð2Þ b 4 L where L = min{h hc, Dc}, hc – minimal deformation level. For pull-off force (adhesion) the maximum repulsing force during the contact separation (h ! 1): P ad ¼ P sep ðh Þ ¼ max jP sep ðhÞj:
ð3Þ
To determine the pull-off force we need to calculate Pad for the set of values of h in the neighborhood of supposed maximum of Psep. The dependences of Pad from the such mechanical and material properties as Young’s E, surface energy Dc, dispersion of asperities heights r and asperities density D were obtained and plotted in Fig. 2. 3. Investigation of polymeric coatings First part of experiments deals with research on polymeric films. The microgrippers (Fig. 3) were coated with four types of polymeric films (Table 1). Electrostatic comb-driven, single crystal silicon microgrippers have been used for evaluation of coatings functionality [5]. The grippers’ structure has been fabricated with DRIE process. Gripping surfaces (as well as all sidewalls of the structure) according to the fabrication process are slightly rippled (periodic surface corrugations with amplitude at level of few tens of nm and period at level of 200 nm). The thickness of the structure is 30 lm, the width of gripping tips is 4 lm. The sticking effect were examined and compared with the case of uncoated microgripper. In second part of experiments we examined influence of the roughness of the surface on the value of pull-off force (adhesion). For this purpose the polymer solutions were modified. Into the solution of polymers the nanoball structures were mixed. The nanoballs were made from SiO2, the
1229
Table 1 Tested samples with various configurations of resin and solution composition and curing treatment Sample Nr.
Resin
Solution
1 10 2 20
MS MS + C MPS MPS + C
ALH ALH ALH ALH
& & & &
ARH ARH ARH ARH
Cure temperature (°C)
Cure time (h)
180 20 180 20
2 24 2 24
MS, methylsilicone resin, MPS, methylphenylsilicone resin, C, catalyst improving adhesion to the substrate, ALH, aliphatic hydrocarbons, ARH, aromatic hydrocarbons.
average diameter of the piece is 170 nm. In result of these modifications three samples for each polymer solution were made: with density of nanoballs in the volume 2%, 5% and 10%. The pull-off force was measured using AFM techniques. The special cantilever was used in these experiments. The cantilever was made from beryllium bronze; upper side is polished very precisely and covered with 300 nm of gold. Length of cantilever is 6 mm, width 1 mm, thickness 50 lm. On the bottom side the steel ball with diameter 0.7 mm is glued. In the result of the experiment for each polymer solution the dependence of change of the pull-off force (%) from density of the nanoballs in polymer was obtained. Obtained results are presented in Fig. 4. For 100% the value of pull-off force of the clean silicon was taken. 4. Results and discussion Dependences received in the result of theoretical research give a possibility to predict the adhesive behavior of the material. Proposed model takes into account such parameter as roughness of the surface which allows to obtain more reliable data. Made investigations give us an opportunity to predict behavior of the surfaces in micropart–microgripper contact and as a fact to choose materials with suitable mechanical and topographical parameters to minimize action of adhesion. According to experimental research the presence of roughness decrease the influence of surface forces. In the case of the tested samples, in which content of the nanoballs in polymer solution was 10%, the value of pull-off force has decreased down to 60% (Fig. 5).
Fig. 3. Mechanical microgripper by Nascatec company (Germany). (1) Uncoated microgripper; (2) coated microgripper.
1230
N. Balabanava et al. / Microelectronic Engineering 84 (2007) 1227–1230
Fig. 5. Dependence of the pull-off force (in %) on density of nanoballs structures in the polymer solutions.
5. Conclusion Coating techniques look to be very promising to use in microtechnology. Application of thin films can solve a majority of problems occurring in microdomain. It can improve optical, wear, friction and adhesive properties of microstructures. Made experiments show that the topographical properties of objects are very important in microdomain. Moreover, the controlled surface roughness gives a possibility to control adhesive properties of microobjects. The idea to use modified with nanoparticles polymeric coating to decrease action of adhesion forces turn out to be very effective and easy way to solve sticking problems. Acknowledgement The work was supported by ASSEMIC training network project under number 504826. References
Fig. 4. Dependence of the pull-off force (in %) from the density of nanoballs structures in the polymers solutions. Compared with the pull-off force of clean silicon (taken as 100%).
[1] K.F. Bohringer, R.S. Fearing, K.Y. Goldberg, The Handbook of Industrial Robotics, Chapter Microassembly, 1998. [2] N. Balabanava, V. Chygurinau, M. Zielecka, Z. Rymuza, Journal Czasopismo Techniczne 6-M (2006) 11. [3] S. Bou, A. Almansa, N. Balabanava, Z. Rymuza, in: Proceedings of 2005IEEE/ASME International Conference on Advanced Intelligent Mechatronics, pp. 384–389. [4] A.I. Sviridenok, A.S. Chizhik, M.I. Petrokovets, Mechanics of Discontinuous Friction Iinteraction, Tekhnika, Minsk, 1990 (in Russian).. [5] B.E. Volland, H. Heerlein, I.W. Rangelow, Microelectronic Engineering (2002) 1015–1023.