Passive circuits, such as waveguides, antennas and resonators, are important components in microwave systems. With increasing frequency, the wavelength reduces, and these components become smaller and more sensitive to the dimensional accuracy. For example, the cross section of typical rectangular waveguide at 100 GHz is2.54 mm by 1.27 mm. At 300 GHz, the size shrinks to 0.864 mm, to 0.432 mm and at 700 GHz to 0.381 mm by 0.191 mm. For components such as filters structures in the waveguides are much smaller than these dimensions. For very small features micromachining can be used.
We have developed an SU8 fabrication process to fulfil the requirements of these terahertz passive circuits. SU8 is a UV sensitive polymer used as a base material to construct the desired waveguide structure, and it has a precise micro-fabrication process derived from mature microelectronics manufacturing techniques. SU8 was chosen because it offers excellent flexibility to build a wide range of devices. In order to adapt the SU8 microfabrication process, the passive circuits need to be designed in a certain way. Figure 2 shows the structure of a typical waveguide designed for SU8 fabrication. Four layers are used with the waveguide slot in two of them. Two right angle bends are embedded at the waveguide ends to couple with the measurement ports. Thus, for such waveguide, the entire device consists of 4 equally thick SU8 pieces which can be fabricated individually. The EDT group has demonstrated other types of devices, i.e. filters, antennas and resonators using the SU8 process following the same principle. In addition to waveguide components ultra-miniature low loss coaxial cable devices have been made.
The SU8 fabrication process is shown in figure 3. Starting as a liquid photoresist, SU8 is spin-coated on 100 mm diameter silicon wafer, followed by soft bake which evaporates out the solvent and turns the liquid into solid. The wafer carrying SU8 is then exposed to ultraviolet (UV) light through a photomask which has the desired layout of the microwave circuit(s). The UV light induces the photo acids needed for cross-linking of the polymer molecular chains in SU8 during the following post-exposure bake step. However, before this a second layer of SU8 is spun onto the first wafer and another UV exposure through a mask is performed, this gives a more complex two-layer structure. The wafer is developed and the parts of the SU8 not exposed to the UV are dissolved. The cross-linked SU8 is then released from the wafer and metallised with silver or gold. The microwave circuit can then be formed by assembling the metallised SU8 pieces in two brass plates with the guide of precise alignment pins (figure 4).
The advantages of the SU8 fabrication process include:
- High accuracy – The dimension of the terahertz structure is defined by the very accurate photolithography process where the features can be as small as a few microns depending on the thickness of individual layer. The tolerance of the 2D-dimension on the photomask is of the order of 1 µm.
- High flexibility – The SU8 can form any shape of structures with very little limitation as long as a standard photomask is made according to the design.
- Low cost – The process is derived from long-matured photolithography technology and the equipment needed includes standard spinner, mask aligner, hot plate and metal evaporator which are cost-effective compared with the state-of-the-art expensive CNC work station.