**8. Casting of three-dimensional micro-structures and formation of suspended or hollow parts**

Once the embossing stamp and the SU-8/impurity are available, we can proceed to replicate the desired 3D micro-structures and fabricate suspended and hollow parts. This process is illustrated in **Figure 2a–h**.

In step 1, as shown in **Figure 2a**, several height-defining blocks of equal height are fabricated onto the corners of the wafer. These height-defining blocks are used to ensure the SU-8 monomer to be deposited on the wafer becomes even and accurate in thickness.

In step 2, as shown in **Figure 2b**, the SU-8/impurity composite is deposited onto the surface of the substrate until its thickness slightly exceeds the height of the height-defining blocks. The wafer is then heated so that its temperature is slightly above about 70–80°C. The SU-8/impurity composite is heated at this temperature for a prolonged period until the SU-8/impurity composite is void of any solvent.

In step 3, as shown in **Figure 2c**, the top surface of the SU-8/impurity is pressurized and ironed flat with a glass slice. Then, the heat source is removed.

In step 4, as shown in **Figure 2d**, patterns defining the hollow or suspended regions are metalized. This metallization process can be carried out by painting with metal ink followed by etching. The purpose of this step is to create an embedded mask which allows selective UV exposure in the later step. After the UV exposure step, the embedded mask created in this step will become a sacrificial layer to be removed in the final step.

In step 5, as shown in **Figure 2e**, the embossing stamp fabricated in the previous stage is manually aligned with a mask-aligner and pressed downwards slowly. This step not only casts the conical pattern on the right of **Figure 2e**, but it also forms hemispherical solids by pressing the metal layers downwards. Since the top surface of the semi-molten SU-8/impurity compo‐ site has its own surface tension, this step will ensure that a smooth surface with a spherical profile is formed on the top of the hemispherical package.

In step 6, as shown in **Figure 2f**, the embossing stamp is removed from the wafer. The wafer is then cooled down for about 3 hours until the 3D patterns on the SU-8/impurity composite become fully solidified. This step further eliminates the surface adhesion on the top of the wafer.

In step 7, as shown in **Figure 2g**, areas requiring hollow or suspended parts are selectively exposed to a UV light. This step can be realized by two methods. We can expose the wafer to an ordinary UV light through a photomask that defines the patterns of hollow or suspended parts. Since the SU-8/impurity composite has been mixed with a UV-opaque impurity, the UVexposed surface of the SU-8/impurity composite can be polymerized into a polymeric layer. This polymeric layer can be easily thickened by increasing the duration of the UV exposure. This polymeric layer can be as thin as a membrane or as thick as the application demands. The SU-8/impurity underneath the UV-exposed surface will remain uncured and become remov‐ able by melting.

In step 8, as shown in **Figure 2h**, the wafer is baked at 110°C. This heat temperature not only hardens the polymerized surface from step 7 but also melts the uncured SU-8/impurity mixture underneath the UV-exposed surface, into a liquid. Traces of the SU-8/impurity not removable by melting can be further removed by developing in an appropriate SU-8 developer.

The fabricated component on the top left corner of **Figure 2h** is a hemispherical package used to protect RF-MEMS devices against humidity. This hemispherical package has been glued onto a metallic washer which serves as a radiofrequency ground for a radiofrequency-printed circuit board. The package is intended to be capped on the substrate of a printed circuit board on which the RF-MEMS devices are mounted. The interface between the hemispherical package and the substrate can be further sealed with PDMS.

The fabricated component on the top middle **Figure 2h** is a spherical hollow object with a diameter equal to approximately 500 μm. It is realized by gluing together two hemispherical shell elements fabricated using the same process as illustrated in **Figure 2a–h**. Gluing the two hemispherical shell elements together involves manual alignment under a microscope.

Fabrication of Three-Dimensional Concave or Convex Shell Structures with Shell Elements at Micrometer Resolution in SU-8 http://dx.doi.org/10.5772/62405 9

The component in the top right corner of **Figure 2h** is a conical funnel with base diameter equal

#### to approximately 50 μm.

In step 3, as shown in **Figure 2c**, the top surface of the SU-8/impurity is pressurized and ironed

In step 4, as shown in **Figure 2d**, patterns defining the hollow or suspended regions are metalized. This metallization process can be carried out by painting with metal ink followed by etching. The purpose of this step is to create an embedded mask which allows selective UV exposure in the later step. After the UV exposure step, the embedded mask created in this step

In step 5, as shown in **Figure 2e**, the embossing stamp fabricated in the previous stage is manually aligned with a mask-aligner and pressed downwards slowly. This step not only casts the conical pattern on the right of **Figure 2e**, but it also forms hemispherical solids by pressing the metal layers downwards. Since the top surface of the semi-molten SU-8/impurity compo‐ site has its own surface tension, this step will ensure that a smooth surface with a spherical

In step 6, as shown in **Figure 2f**, the embossing stamp is removed from the wafer. The wafer is then cooled down for about 3 hours until the 3D patterns on the SU-8/impurity composite become fully solidified. This step further eliminates the surface adhesion on the top of the

In step 7, as shown in **Figure 2g**, areas requiring hollow or suspended parts are selectively exposed to a UV light. This step can be realized by two methods. We can expose the wafer to an ordinary UV light through a photomask that defines the patterns of hollow or suspended parts. Since the SU-8/impurity composite has been mixed with a UV-opaque impurity, the UVexposed surface of the SU-8/impurity composite can be polymerized into a polymeric layer. This polymeric layer can be easily thickened by increasing the duration of the UV exposure. This polymeric layer can be as thin as a membrane or as thick as the application demands. The SU-8/impurity underneath the UV-exposed surface will remain uncured and become remov‐

In step 8, as shown in **Figure 2h**, the wafer is baked at 110°C. This heat temperature not only hardens the polymerized surface from step 7 but also melts the uncured SU-8/impurity mixture underneath the UV-exposed surface, into a liquid. Traces of the SU-8/impurity not removable

The fabricated component on the top left corner of **Figure 2h** is a hemispherical package used to protect RF-MEMS devices against humidity. This hemispherical package has been glued onto a metallic washer which serves as a radiofrequency ground for a radiofrequency-printed circuit board. The package is intended to be capped on the substrate of a printed circuit board on which the RF-MEMS devices are mounted. The interface between the hemispherical

The fabricated component on the top middle **Figure 2h** is a spherical hollow object with a diameter equal to approximately 500 μm. It is realized by gluing together two hemispherical shell elements fabricated using the same process as illustrated in **Figure 2a–h**. Gluing the two hemispherical shell elements together involves manual alignment under a microscope.

by melting can be further removed by developing in an appropriate SU-8 developer.

package and the substrate can be further sealed with PDMS.

flat with a glass slice. Then, the heat source is removed.

8 Lab-on-a-Chip Fabrication and Application

will become a sacrificial layer to be removed in the final step.

profile is formed on the top of the hemispherical package.

wafer.

able by melting.

**Figure 2.** (a–h) Cross-sectional view illustrating the process flow of the technology for fabricating hollow or suspended micro-structures.


**6. References**

<sup>\*</sup> Corresponding Author

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