Thermomechanical properties of polymer nanolithography using atomic force microscopy

Micron 42 (2011) 492–497

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Thermomechanical properties of polymer nanolithography using atomic force microscopy Te-Hua Fang ∗ , Cheng-Da Wu, Shao-Hui Kang Department of Mechanical Engineering, National Kaohsiung University of Applied Sciences, 415 Chien Kung Road, Kaohsiung 807, Taiwan

a r t i c l e

i n f o

Article history: Received 31 October 2010 Received in revised form 27 January 2011 Accepted 28 January 2011 Keywords: Nanolithography Polymer Nanotribology Friction Nanoindentation

a b s t r a c t The temperature-dependent mechanical properties of polyethylene terephthalate (PET) polymers are investigated using force–distance curves, adhesion force, and atomic force microscope (AFM) nanolithography combined the heating techniques. The results show that the width of grooves on the polymers at 20–60 ◦ C were in the range of 14–363 nm. The wear depth of the polymers increased with increasing heating temperature. A volume of 251.85–2422.66 ␮m3 at a load of 30–50 nN with heating to 30–60 ◦ C was removed, as compared to that of 26.60–70.30 ␮m3 obtained at room temperature. The contact forces of PET started increasing at 9 nN, whereas the size of the holes was average at a pressure. The results may be of importance in explaining the heating relationship among adhesion force, volume removal rate, and pressure. © 2011 Elsevier Ltd. All rights reserved.

1. Introduction Polymers are large molecules with chemical bonds between monomers, like the chain stereochemistry, and the molecular weight. Polyethylene (PE), polypropylene (PP), polymethyl methacrylate (PMMA), polyethylene terephthalate (PET), and polydimethylsiloxane (PDMS) are common examples of such materials. Polymers have many special material properties, such as light transmittance, adhesion, friction, wetting, and biological compatibility (Fang et al., 2005). Several physical properties of polymers have been investigated using a number of methods, such as differential scanning calorimetry (DSC), dynamical mechanical thermal analysis (DMTA), and X-ray scattering (Gierke et al., 1981). Atomic force microscopy (AFM) can be used for extensive analysis of the mechanical properties of polymers on the nanoscale (Fang and Chang, 2005; Fang et al., 2005). AFM has been used to investigate plastic polymer surfaces (Kajiyama et al., 1997; Boschung et al., 1994), using techniques such as force–distance curve measurements on heated samples (Hammiche et al., 2000; Hinz et al., 2004), force modulation (Ge et al., 1995; Krotil et al., 1999), friction force microscopy (Kaiiyama et al., 1997; Dinelli et al., 2000), and thermal analysis (Gorbunov et al., 1999). Seminal work was carried out by IBM on the Millipede (Binnig et al., 1999), in which the AFM tip was heated to 350 ◦ C on PMMA for data storage, with the data stored as indents in the polymer.

∗ Corresponding author. Tel.: +886 7 3814526x5336. E-mail address: [email protected] (T.-H. Fang). 0968-4328/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.micron.2011.01.010

PET is a thermoplastic polymer resin which is often used in synthetic fibers and food, beverage, and other liquid containers. It is also widely used in cardiovascular implants, such as artificial heart valve sewing rings (Struszczyk et al., 2002) and artificial blood vessels (Wang et al., 2005), because of its excellent mechanical properties, chemical stability, and biocompatibility (Hu et al., 2003). PET is composed of polymerized units of the monomer ethylene terephthalate, and repeating C10 H8 O4 units. In this study, the temperature-dependent mechanical properties of the PET polymers are investigated using force–distance curves, adhesion force, and AFM nanolithography combined with heating techniques.

2. Materials and methods 2.1. Experimental details A commercially available scanning probe microscope system (NT-MDT SFC050L) was used, and a diagram of the experimental setup is shown in Fig. 1. The AFM measurements were obtained using a conductive probe with a tip radius of 20 nm. A constant normal force of 5 nN was maintained between the tip and the sample surface in the contact mode. In general, a plot of cantilever deflection as a function of the interaction with the sample surface along the z-axis is obtained from this process. Fig. 2(a) shows the relationships in Hooke’s model between the force, F, and the cantilever deflection. F = Ktip · z

(1)

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Fig. 1. Diagram of the experimental setup.

where spring constant Ktip is the cantilever stiffness, and z is the vertical deflection. The adhesion produced between the tip and the sample by the forces during the tip retraction is shown in Fig. 2(b). This adhesion can represent the interactive force between the PET surfaces and work of adhesion  behavior shown in Fig. 2(c). In the adhesion test, hysteresis is caused by some sort of adhesion, which moves the force curve below the zero-deflection line. Adhesion occurs between the AFM tip and the polymer sample, and an obvious adhesive force appears as the tip is pulled away. However, the source of the adhesion depends on the sample. In this experiment, a heating stage was used to heat the sample to 20–60 ◦ C. The heating stage was mounted on an exchangeable sample holder or on an exchangeable scanner, and it was used to

Fig. 3. Two spheres in contact before deformation (a), (b).

control the polymer softening temperature. When polymers are heated, their surfaces become elastic, and thus the probe pressure on the sample was dependent on temperature. The surface property between the probe-sample interactions S will be estimated in this work, and the probe pressure P on the sample can be written as (Johnson, 1989): P=

F A

(2)

where A is the contact area. The hardness, H, is determined from the maximum indentation load Fmax divided by the actual projected contact area Ac (Johnson, 1989): H=

Fig. 2. (a) Deflection–distance curve. (b) Schematic representations of the PET surface adhesion by an AFM tip.

Fmax Ac

(3)

When the AFM probe tip makes contact with the sample, elastic forces occur between the sample and the tip, which affects surface plasticity. The Hertz problem may account for deformations at the local contact of the structure inside relation to the loading force behavior (Hertz, 1882). When the tip and sample materials are isotropic, their elastic properties are represented by two parameters (Young’s moduli E, E and Poisson ratios ,  ). Fig. 3(a) shows the contact point for the undeformed parts of the sphere surface through perpendicular points orthogonal to the point. The contact structure point is described by two curvature radii with the tip (r1 , r2 ) and the sample area (r1 , r2 ). The term (a) is defined as the radius in the sense that 1/R = (1/r) + (1/r ), as in Fig. 3(b). The effective Young’s modulus of

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Fig. 4. AFM image of nanoscratched groove on the PET film.

the given two materials is (Johnson, 1989): 3 1 = K 4



1 − 2 1 −  2 + E E



(4)

The structure has light deformed through geometric relation between penetration depth h and contact circle radius a is valid (Johnson, 1989): h=

a2 r

(5)

scratching with a tip on the PET surface was produced a single groove at a 15 ␮m × 15 ␮m surface region. The widths of the grooves of the polymers at 20–60 ◦ C were in the range of 14–363 nm. The wear depth of the polymers increased along with the temperature. Fig. 5 shows AFM topography images and depth profiles of grooves prepared by nanoscratching at 20–60 ◦ C for three loads. The nanoscratch depth profiles in Fig. 5 are 14–37 nm (20 ◦ C line), 35–72 nm (30 ◦ C line), 93–153 nm (40 ◦ C line), 140–240 nm (50 ◦ C line), and 239–363 nm (60 ◦ C line). The groove depth increases with the temperature due to the softening of the polymer. The adhesion

The solution can be obtained by solving the geometry in Fig. 3(b). The equation for the Hertz problem can be written using the loading force F and the penetration depth h (Johnson, 1989): F=

3 1 Ka3 = Kh 2 R 2 R

(6)

The work of adhesion () was calculated using the Derjagin, Muller, Toropov (DMT) theory (Derjaguin et al., 1975) with the following relationship (Johnson, 1989): =−

Fadh 2R

(7)

where Fadh is the adhesion force and R is the tip radius. The stiffness of a body is a measure of the resistance offered by an elastic body to deformation. The stiffness can analyze the elastic modulus on the indent sample, S = dF/dh, such the slope of the initial place of the unloading curve. Where S is the stiffness, F is force and h is the distance. The average surface roughness (Ra ) of the PET film was 2–5 nm obtained by the AFM. 3. Results and discussion The nanoscratch technique was used to study the mechanical properties of the surface of PET polymers. The topographies of the scratched polymers for loads of 30, 40, and 50 nN at 20–60 ◦ C and their groove depths along the z-direction are shown in Fig. 4. AFM

Fig. 5. AFM profile data for 30–50 nN loads prepared by AFM tip at 20–60 ◦ C.

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Fig. 6. Relationship between the volume removal rate and the scratch loads.

differences between the surface and the tip leading to the hysteresis energy of the heated polymer were investigated. Fig. 6 shows the relationship between the volume removal rate and the scratch load with heating of the polymers. A volume of 251.85–2422.66 ␮m3 at a load of 30–50 nN with heating to 30–60 ◦ C was removed, as compared to that of 26.60–70.30 ␮m3 obtained at room temperature. When temperature rises from 20 to 60 ◦ C, the volume removal rate increases (the ratio of the volume removal rate at temperature of 60–20 ◦ C) to 31, 36, and 28 times for a load of 30, 40, and 50 nN, respectively. The volume of the removed material significantly increased with the applied load and temperature. This result is consistent with the previous observation that can easier processed on polymers of nanomachining furrow is deeper through the loading with heating. Fig. 7(a) shows the adhesion force at various loads at temperatures from 20–60 ◦ C, and it can be seen that the force increased along with the temperature. Curves obtained with a maximum adhesion forces ranging from 0.7 to 5.6 nN. Fig. 7(a) shows that polymers was heating with loading 30 nN of (20 ◦ C) 0.7 nN, (30 ◦ C) 1.4 nN, (40 ◦ C) 4.7 nN, (50 ◦ C) 5.6 nN, and (60 ◦ C) 15.8 nN at 20–60 ◦ C, which adhesion force is increased, so that means more hysteresis energies is certain remnant deformation of the polymer. The plastic regime of deformations of adhesion is always proportional to the square of the permanent plastic deformation, which depends on both the load and temperature (Cappella and Stark, 2006). Fig. 7(b) and (c) shows the force–distance curves of the PET samples. The curves exhibit a sharp contact point to heat the PET and a pull-off point in the retracing cycle. Fig. 8(a) shows the interaction between the work of adhesion and hardness. Analysis of variance for the temperature shows that there were works of adhesion  differences. The work of adhesion  = 1.4–11.3 nJ and the hardness is 6–75 MPa at temperatures of 20–60 ◦ C. When temperature increases from 20 to 60 ◦ C, the hardness rapidly reduces to 8% left relative to the hardness value at 20 ◦ C. The stiffness of the PET surface decreased when plastic deformation occurred at a high temperature in Fig. 8(b). Compared with Figs. 6 and 8, the results shows that the volume removal rate increases, and the hardness and the stiffness decrease with temperature. This also indicates that the volume removal rate decreases with increasing the hardness and the stiffness. The polymer film surface became sufficiently soft at the high temperature, at which point

Fig. 7. (a) Relationship between the adhesion and the scratch loads. (b) AFM force distance curves at 30 ◦ C. (c) AFM force distance curves at 50 ◦ C.

the adhesion and friction behavior changed significantly (Kim et al., 2008). Fig. 9(a) shows AFM images of nanopatterns processed by indentation for loads of 5–45 nN at 30 ◦ C. The heating can increase the puncture ability on PET surfaces. For examples of PET application are used a cantilever with capacitive voltage process technology between the sample and cantilever was used to apply load forces

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Fig. 9. (a) AFM image of the fabrication of a nanopatterned PET film with loads 5–45 nN at 30 ◦ C and (b) profile image.

Fig. 8. (a) Hardness and the work of adhesion of PET films. (b) Stiffness variation of PET films.

(Pantazi et al., 2008; Despont et al., 2000), while the data can store as indents in polymer media. Fig. 9(b) shows the 5 nN indentation loads profile at a temperature of 30 ◦ C. The PET was loaded of 1–50 nN show different holes and pile-up in the indentation part of the loading, while the holes are relatively at a pressure average is 0.64 MPa. For pressures up to 1.86 MPa, the depth of holes is between 23 and 90 nm. The pile-up height of holes is about 1.2 times relative to its depth for a load of 5 nN. For nanoscratch process, the pile-up height is 10–50 nm, 15–60 nm, and 10–75 nm as temperature increases from 20 to 60 ◦ C for a load of 30, 40, and 50 nN, respectively. The results indicate that the pile-up height increases with increasing load and temperature; however, the ratio of pile-up height to the depth of holes significantly decreases, as shown in Fig. 5. When the cantilever and the sample are in contact, elastic forces start to act, giving rise to sample and tip deformations, which can affect the acquired pressure. The pressure increases with applied load, and indentation depth is up to 9 nN for PET. The high pressure was found on the grooves of PET at loads of 10–50 nN. Suppose that the Hertz problem solution from the dependence of the penetration depth (horizontal axis) upon the loading force (vertical axis) for positive F ∼ h3/2 , but the PET forcedepth present interaction is F ∼ h3/5 at 30 ◦ C because heating bring

Fig. 10. Relationship between force and penetration depth.

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soften surface and work of adhesion effect to decrease F − h rate in Fig. 10. This behavior is due to the material being crushed as a result of plastic deformation. Higher pressure can damage the tip during indentation, and the volume of the removed material can be used to evaluate the pressure and wear on PET using the scratch technique. 4. Conclusions The adhesion force, indentation, and nanomachining on PET films were analyzed at various temperatures using AFM. When the PET films were heated up to 60 ◦ C, the surface became sufficiently soft. The widths of grooves on the polymers at 20–60 ◦ C were in the range of 14–363 nm. The wear depth of the polymers increased with heating temperature, while the volume of the removed material increased in proportion to the increase in load. The probe forces between PET surfaces started increasing at 9 nN, whereas the holes were relatively at a pressure average was 0.64 MPa. These results indicate that the heating temperature at 30 ◦ C was able to easier be processed and low pressure and comfortable hysteresis energy. Acknowledgments This work was partially supported by the National Science Council of Taiwan under grant NSC 099-2811-E-151-002. The authors would also like to thank Cintyia Wang (UT-Austin). References Binnig, G., Despont, M., Drechsler, U., Haberle, W., Lutwyche, M., Vetriger, P., 1999. Ultra high-density AFM data storage with erase capability. Appl. Phys. Lett. 74, 1329–1331. Boschung, E., Heuberger, M., Dietler, G., 1994. Energy dissipation during nanoscale indentation of polymers with an atomic force microscope. Appl. Phys. Lett. 64, 3566–3568. Cappella, B., Stark, W., 2006. Adhesion of amorphous polymers as a function of temperature probed with AFM force–distance curves. J. Colloid Interface Sci. 296, 507–514. Derjaguin, B.V., Muller, V.M., Toporov, Y.P., 1975. Effect of contact deformation on adhesion of particles. J. Colloid Interface Sci. 53, 314–326. Despont, M., Brugger, J., Drechsler, U., Haberle, W., Lutwyche, M., Rothuizen, H., Stutz, R., Widmer, R., Binnig, G., Rohrer, H., Vetriger, P., 2000. VLSI-NEMS chip for parallel AFM data storage. Sensors Actuat. A 80, 100–107.

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