Cantilever Fabrication by Force Free Release Transfer Process

Abstract

Summary form only given. In the transfer process microstructures which are fabricated on the dummy substrate are bonded on the main substrate (Fig. 1). The process is very attractive for MEMS fabrication because movable microstructures can be fabricated without a sacrificial layer [1]. However, the following difficult requirements should be satisfied in the transfer process. The microstructures have to be adhesive on the dummy substrate in the fabrication step. On the other hand they have to be easily removed from the dummy substrate in the release step. It is very important to control the adhesion between the microstructures and the dummy substrate [2]. Especially it is difficult to release the microstructures whose one end is free such as a cantilever, because the free end is not fixed to the main substrate and the microstructures can be easily damaged as shown in Fig. 1(d). In this paper a new release layer is developed and the nickel cantilevers are fabricated. The nickel cantilevers are fixed on the dummy substrate in the fabrication step and can be released from the dummy substrate with very small release force by use of the new release layer. The process flow is shown in Fig. 2. A PMMA film with TiO2 nanopowder is coated on a quartz dummy substrate as the release layer (Fig. 2(a)). TiO2 is activated by the UV exposure. The nickel cantilever pattern is fabricated by use of the conventional photolithography and the nickel electro plating (Fig. 2(b)). The beam width is 100 mum and the beam lengths are varied from 100 mum to 1000 mum [1]. Note that a long beam is damaged easily by the adhesion. The SU8 film of about 4 mum thickness is patterned on the silicon main substrate as the adhesive layer (Fig. 2(c)). The dummy substrate is pressed to the main substrate (Fig. 2(d)). The press conditions are 8.0 MPa, 190 degC, 15 min. After the press the dummy substrate is bonded to the main substrate. The UV light is exposed to the samples from the backside of quartz dummy substrate (Fig. 2(e)).. The UV light source is the low pressure mercury lamp of 110 W. The TiO2 nanopowder in the release layer is activated by the UV exposure and the PMMA film is damaged. The dummy substrate is separated from the release layer by a very small release force (force free release) and the nickel cantilever is transferred to the main substrate. (Fig. 2(f)). Two methods are used in order to make the PMMA film with TiO2 nanopowder. One is that TiO2 thin layer is coated on the quartz substrate by the commercial spray and PMMA film is spin coated on the TiO2 layer (spray method) [3]. The other is that TiO2 nanopowder is mixed to the PMMA solution and the PMMA solution containing TiO2 nanopowder is spin coated (mix method). Figure 3 shows the exposure time for the force free release. The force free release is possible for the PMMA film without TiO2. However, the exposure time for the PMMA film with TiO2 nanopowder by the spray method is reduced to one third of that without TiO2 nanopowder. Moreover, the exposure time by the mix method can be reduced to about 60% of that by the spray method. Although the exposure time is not short at the present time, it will be decreased by improving the experimental conditions. The examples of the fabricated cantilever are shown in Figs. 4. When the release layer is not used, that is, the nickel cantilever pattern is directly fabricated on the quartz dummy substrate, the result is shown in Fig. 4(a). The release force greatly depends on the sample. The release force normalized by the total cantilever area is varied from 15 kPa to 80 kPa. The cantilever beams greatly bend to upwards. When the PMMA/TiO2 release layer made by the spray method is used, the result is shown in Fig. 4(b). The release layer thickness is 3 mum. It is clear that the bending of the cantilever beam can be greatly reduced. The usefulness of the new release layer is demonstrated by this results.