Microstructures on a dummy substrate are bonded and transferred to the main substrate in a transfer process. The novel transfer process is developed using a new release layer on the dummy substrate. The release layer consists of a TiO 2 layer and a poly(methyl methacrylate) (PMMA) film. Although the dummy substrate sticks to the main substrate after the bonding, the dummy substrate can be separated from the main substrate without using a pulling force by the UV exposure of the release layer. The force-free release can be obtained by PMMA film degradation by UV exposure. A cantilever with long beams (up to 1000 mm) and a wide base (5 Â 3 mm 2 ) is successfully fabricated by the new transfer process.
A new transfer process for micro-electro-mechanical systems (MEMS) is developed. The process consists of two sequential transfers using two dummy substrates, and MEMS structures are fabricated on a main substrate. Ni cantilevers are also fabricated. All the processes for Ni cantilever pattern fabrication are performed on the first dummy substrate. The second dummy substrate is used for force-free release transfer. Ni cantilevers with long beams and a wide area are fabricated. Two methods are developed to fabricate Ni cantilever patterns without lithography. 1) A silicon template with a cantilever pattern step is used. 2) Selective evaporation is used. Ni cantilevers are successfully fabricated on the main substrate.
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 ptm and the beam lengths are varied from 100 pim to 1000 pim [1]. Note that a long beam is damaged easily by the adhesion. The SU8 film of about 4,um 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°C, 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 tha...
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