4. Fabrication and characterization

The fabrication of the microgrippers is based on a three mask process. The OmniCoat stripper (MicroChem) is used in order to completely release the final structures [21–24].

#### 4.1. First fabrication process

A silicon wafer of any orientation was used after a typical chemical cleaning. A thin layer of 40 nm thickness of Omnicoat was deposited on the silicon wafer by spin-coating and baked at 200C on a hotplate for 1 minute. Then the SU-8 2015 (MicroChem) polymer was deposited on the wafer using a spinner at 4000 rpm in order to obtain a thickness of 9 μm. The wafer was soft-baked at 65C and at 95C for 1 minute and 3 minutes, respectively. The SU-8 layer was then patterned using the first mask in order to obtain the microgripper configuration. After the exposure, the wafer was post-baked at 65C and at 95C for 1 minute and 2 minutes, respectively and then developed using mr-Dev 600 developer. The polymer structure was hardbaked at 185C for 15 minutes in order to complete cross-linking of the SU-8 polymer. The metal layer consists of a sandwich of Cr/Au/Cr films of 10 nm/300 nm/10 nm thicknesses. The metals were evaporated and the heater and pads were obtained using a lift-off process based on AZ4562 photoresist. The second SU-8 2015 layer was obtained using the same settings as for the first layer. In this step the access to the metallic pads was created using the third mask for SU-8. The final thermal process of the polymer in this step was the hard-baking at 195C for 30 minutes for cross-linking of the SU-8 polymer. To release the microgripper structures the Omnicoat layer was developed (Figure 6). The SU-8 and the metallic layers are well patterned (Figure 7). Figure 8 (a) shows an optical picture of the fabricated microgripper before releasing.

#### 4.2. Second fabrication process

The simulated in-plane and the out-of-plane deflections of the microgripper tips were presented in order to evaluate the opening and the displacements of the gripper arms (Figure 5). The simulation results demonstrate that the tips deflect no more than 0.12 μm in the out-of-plane

Figure 5. FEM coupled electro-thermo-mechanical simulations results: (a) the in-plane deflections at 22 mA [22]; (b) the

out-of-plane displacements of the tips vs. electrical current (Coventorware 2014 simulation).

Figure 4. Simulations and thermal measurements results: (a) FEM coupled electro-thermo-mechanical simulations results of the temperatures distribution in the microgripper at 22 mA (Coventorware 2014 simulation) [22]; (b) the radiation

distribution in the microgripper (IR thermography measurements).

The simulation results show that the gripper can work up to a temperature of 165C for a complete closing tips. The gripper can continue to work up to 205C in order to obtain a higher displacement or a higher pressure on the griped micro-object. The results indicate that the polymeric micromanipulators can work at low operation temperatures of the tips and with high in plane displacement. A displacement of 25 μm for each microgripper polymeric arm was obtained at a temperature value of 165C and for a current value of 25 mA. At the tips the temperatures remain of 30–35C close to the settings performed for the initial temperature of the media. The capable manipulating size range of the simulated microgripper is from 1 to 50 μm. If

direction (Figure 5(b)).

30 Actuators

A thin layer of Omnicoat was deposited on a silicon wafer by spin-coating as in the first fabrication process. The SU-8 polymer was deposited on the wafer using a spinner in order to

Figure 6. Schematic cross section of the actuator arm after fabrication and release.

Figure 7. SEM image of the fabricated microgripper using the Cr/au/Cr films for the heater.

obtain a thickness of ~10 μm. The wafer was soft-baked in the same conditions and the SU-8 layer was then exposed using the first mask. After the exposure, the wafer was post-baked and then developed. The polymer structure was hard-baked in order to complete cross-linking of the SU-8 polymer. The heaters and the pads were obtained using a lift-off process based on AZ4562 photoresist. An O2 plasma treatment was performed for a couple of seconds in order to clean and increase the adhesion of the substrate [24]. Then, a metal layer of a gold thin film with 300 nm thickness was evaporated on the wafer. The second SU-8 layer was obtained using the same conditions as for the first layer. In this step the access to the metallic pads was created using the third mask for SU-8. The final thermal process of the polymer in this step was the hard-baking at 195C for 30 minutes for cross-linking of the SU-8 polymer. To release the microgripper structures the Omnicoat layer was developed. Figure 8 (b) shows an optical picture of the released fabricated microgripper. A released chip with 4 structures is presented in Figure 8 (c).

values of the line equation parameters which fit the resistance graphs, the TCR was determined for each microgripper. For the Cr/Au/Cr thin films microgrippers the measured TCR is 0.001569/C. For the thin gold film microgripper the TCR was found to be 0.00314/C using the same microgripper configuration. We notice that for the gold microgripper the measured TCR value is very close to the bulk gold values which are between 0.0034 and 0.0037/C, while for

Figure 8. Optical microscope picture of the fabricated electrothermal SU-8 microgripper: (a) the microgripper with Cr/au/ Cr films [22]; (b) the fabricated microgripper using the gold film for the heater; (c) a released chip with 4 structures with

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These measured TCR values were used to determine the heater temperatures when the

The in-plane deflection change with drive current was observed with an optical microscope and a camera with the associated viewing software. For each actuation step, the displacements of the gripper tips were measured using the optical images. Figure 10 shows the first and the last stage of the opening-closing tips of the microgripper, while the Figure 11 proves a good

the Cr/Au/Cr thin films microgrippers the measured TCR is half of the bulk value.

microgripper is actuated.

au film used for the heater.

#### 5. Experimental testing

In order to validate the model, the experiments were performed in air. For the tests we used the microgrippers with 50 μm the initial opening. Each structure was fixed manually on a silicon substrate (Figure 9) and the electrical contacts were placed directly on the metallic pads.

The dimensions of the microheaters were measured using a miscroscope. The TCR (temperature coefficient of resistance) measurements were carried out using a small chamber where the microgripper were fixed one a hotplate [21, 25]. A thermocouple based temperature sensor was used. Then, the heater resistance was measured at different temperatures. Based on the An SU-8 Microgripper Based on the Cascaded V-Shaped Electrothermal Actuators: Design, Fabrication, Simulation… http://dx.doi.org/10.5772/intechopen.75544 33

obtain a thickness of ~10 μm. The wafer was soft-baked in the same conditions and the SU-8 layer was then exposed using the first mask. After the exposure, the wafer was post-baked and then developed. The polymer structure was hard-baked in order to complete cross-linking of the SU-8 polymer. The heaters and the pads were obtained using a lift-off process based on AZ4562 photoresist. An O2 plasma treatment was performed for a couple of seconds in order to clean and increase the adhesion of the substrate [24]. Then, a metal layer of a gold thin film with 300 nm thickness was evaporated on the wafer. The second SU-8 layer was obtained using the same conditions as for the first layer. In this step the access to the metallic pads was created using the third mask for SU-8. The final thermal process of the polymer in this step was the hard-baking at 195C for 30 minutes for cross-linking of the SU-8 polymer. To release the microgripper structures the Omnicoat layer was developed. Figure 8 (b) shows an optical picture of the released fabricated microgripper. A released chip with 4 structures is presented

Figure 7. SEM image of the fabricated microgripper using the Cr/au/Cr films for the heater.

In order to validate the model, the experiments were performed in air. For the tests we used the microgrippers with 50 μm the initial opening. Each structure was fixed manually on a silicon substrate (Figure 9) and the electrical contacts were placed directly on the metallic pads.

The dimensions of the microheaters were measured using a miscroscope. The TCR (temperature coefficient of resistance) measurements were carried out using a small chamber where the microgripper were fixed one a hotplate [21, 25]. A thermocouple based temperature sensor was used. Then, the heater resistance was measured at different temperatures. Based on the

in Figure 8 (c).

32 Actuators

5. Experimental testing

Figure 8. Optical microscope picture of the fabricated electrothermal SU-8 microgripper: (a) the microgripper with Cr/au/ Cr films [22]; (b) the fabricated microgripper using the gold film for the heater; (c) a released chip with 4 structures with au film used for the heater.

values of the line equation parameters which fit the resistance graphs, the TCR was determined for each microgripper. For the Cr/Au/Cr thin films microgrippers the measured TCR is 0.001569/C. For the thin gold film microgripper the TCR was found to be 0.00314/C using the same microgripper configuration. We notice that for the gold microgripper the measured TCR value is very close to the bulk gold values which are between 0.0034 and 0.0037/C, while for the Cr/Au/Cr thin films microgrippers the measured TCR is half of the bulk value.

These measured TCR values were used to determine the heater temperatures when the microgripper is actuated.

The in-plane deflection change with drive current was observed with an optical microscope and a camera with the associated viewing software. For each actuation step, the displacements of the gripper tips were measured using the optical images. Figure 10 shows the first and the last stage of the opening-closing tips of the microgripper, while the Figure 11 proves a good agreement between the simulation results and the measured openings and the temperatures of the microgripper arms. The out-of-plane displacement was not observed in the experiments while the simulation results provided an out-of-plane displacement less than 100 nm. Currents larger than 27–28 mA make the SU-8 softer and the device will be damaged due to the increased temperature over 210C.

Figure 9. The fixed structures on a silicon substrate for experiments tests.

Figure 10. Optical images of the actuated microgrippers with the tips in the open and close stage: (a) the initial stage of the microgripper tips with Cr/au/Cr films used for the heater and with the initial opening of 50 μm [22]; (b) closing tips stage at 24 mA for the microgripper with Cr/au/Cr films used for the heater [22]; (c) the initial stage of the microgripper tips with au film used for the heater and with the initial opening of 50 μm; (b) closing tips stage at 24 mA for the microgripper with au film used for the heater;

In order to demonstrate the microgripper capability, different micro-elements were used in order to pick and place and manipulate them with the gripper arms of a similar SU-8 fabri-

Figure 12. Optical images of an SU-8 microgripper manipulating a polymeric micro-object: (a) gripping the object; (b)

Figure 11. Experimental and simulation results: (a) measurements and simulation results of the jaw displacement versus electrical current; (b) measurements and simulation results of the maximal values of the temperatures in the microgripper

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versus electrical current; (c) optical image of the damaged SU-8 microgripper at 28 mA [22].

cated microgripper (Figure 12).

placing the object in the final position.

An SU-8 Microgripper Based on the Cascaded V-Shaped Electrothermal Actuators: Design, Fabrication, Simulation… http://dx.doi.org/10.5772/intechopen.75544 35

agreement between the simulation results and the measured openings and the temperatures of the microgripper arms. The out-of-plane displacement was not observed in the experiments while the simulation results provided an out-of-plane displacement less than 100 nm. Currents larger than 27–28 mA make the SU-8 softer and the device will be damaged due to the

Figure 10. Optical images of the actuated microgrippers with the tips in the open and close stage: (a) the initial stage of the microgripper tips with Cr/au/Cr films used for the heater and with the initial opening of 50 μm [22]; (b) closing tips stage at 24 mA for the microgripper with Cr/au/Cr films used for the heater [22]; (c) the initial stage of the microgripper tips with au film used for the heater and with the initial opening of 50 μm; (b) closing tips stage at 24 mA for the

increased temperature over 210C.

34 Actuators

microgripper with au film used for the heater;

Figure 9. The fixed structures on a silicon substrate for experiments tests.

Figure 11. Experimental and simulation results: (a) measurements and simulation results of the jaw displacement versus electrical current; (b) measurements and simulation results of the maximal values of the temperatures in the microgripper versus electrical current; (c) optical image of the damaged SU-8 microgripper at 28 mA [22].

Figure 12. Optical images of an SU-8 microgripper manipulating a polymeric micro-object: (a) gripping the object; (b) placing the object in the final position.

In order to demonstrate the microgripper capability, different micro-elements were used in order to pick and place and manipulate them with the gripper arms of a similar SU-8 fabricated microgripper (Figure 12).

#### 6. Conclusions

In this paper, a complete work regarding an SU-8 electro thermally actuated microgripper based one a cascaded V-shaped configuration was presented. The gripper were designed, fabricated and investigated experimentally and numerically. Two kind of fabrication were presented, using only a gold thin layer for the heater avoiding the deposition of an adhesion metal, like chromium and using Cr/Au/Cr films for the heaters. The optimized design consists of three material layers symmetrically disposed. The metallic layer for the heater is implanted between two SU-8 based structure layers with the same thickness. From numerical simulation, the out-of-plane displacement of the tips was found to be always lower than 100 nm during the operation process.

Author details

Romania

References

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Rodica-Cristina Voicu

Address all correspondence to: rodica.voicu@imt.ro

Recent Patents on Mechanical Engineering. 2013;6(2)

National Institute for Research and Development in Microtechnologies-IMT Bucharest,

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Therefore, the fabrication processes can be used in the fabrication of different SU-8 based MEMS devices actuated electrothermally with the in-plane deflection.

The results show that the microgripper can work in air in his maximal stage for an electrical current up to 25–26 mA and a temperature up to 165C. A 50 μm jaw gap can be obtained for 24–25 mA. The temperature of the microgripper SU-8 tip remains below 35C. Our experimental and the simulation results demonstrate that our microgripper fulfills the design requirements having a thickness of less than 20 μm and the out-of-plane displacement almost eliminated.

A comparison between the simulations results and the measurements was presented regarding the displacements/opening of the arms and the maximal temperatures reached in the structure. The simulation results are in good agreement with the measurements.

Over 26–28 mA the device is damaged due to the SU-8 transformations over the glass transition temperature reached in the structure.

#### Acknowledgements

This work was supported by a grant of the Romanian National Authority for Scientific Research and Innovation, CCCDI – UEFISCDI, project number 22/2016, within PNCDI III, ERA-MANUNET-II-Robogrip project "Microgrippers as end-effectors with integrated sensors for microrobotic applications (ROBOGRIP)" (2016-2017).

#### Thanks

I thank to Muaiyd H. Al-Zandi and C. Wang (Heriot-Watt University, Edinburgh) for the help with the electrical measurements, to R. Muller for the support, to C. Tibeica for the discussions and electrical measurements, to R. Gavrila for the nanoindentation measurements and to D. Varsescu for the thermal measurements.
