Fabrication of Transformers

This section describes the fabrication processes of the transformer on a capillary and the MEMS transformer on a wafer.

2.2.1 Fabrication of Transformer on a Capillary

The transformer on a capillary is simply made from the winding of enamel-insulated wire and a capillary used as the container of magnetic core. The air, bulk Fe3O4 and Fe3O4 nanofluids of 0.25M, 0.5M, 0.75M and 1M are applied as the

magnetic core. The bulk Fe3O4 is made by repeatedly dehydrating and baking the ethanol-based Fe3O4 nanofluid in the capillary. Figure 2.4 shows the structure of the transformer on a capillary which carries the oil-based Fe3O4 nanofluid. The diameter of applied enamel-insulated wire of #26 is 0.45mm, and the thickness of the enamel layer is near 0.05mm. Both the primary and secondary windings have 20 turns. The outer and inner diameters of capillary are respectively 3.2mm and 2.3mm, and the capacity of capillary is 100μl.

2.2.2 Fabrication of MEMS Transformer on a wafer

The MEMS transformer on a wafer is a solenoid type transformer, fabricated by the MEMS process. The fabrication process of the MEMS transformer is shown in Fig.

2.5. The detail procedures are described as follow:

1. Wafer cleaning: The surface of wafer is cleaned and coarsened by the reactive ion etch (RIE) with 50W power for 60 second. This step is to remove the impurities on the wafer and increase the adhesion between the wafer and materials.

2. Seed layer sputtering: The Cr layers of 0.05 μm thickness and the Cu layer of 3 μm thickness are sputtered sequentially on the wafer by a physical vapor deposition (PVD)

3. Line patterning: The surface of the metal layer is coated with the photoresist

EPG-512 by using a spin coater. The coating rotation speed and time are shown in Fig. 2.6(a). After spin coating, the photoresist layer of 1.5 μm thickness is baked under 110℃ for 1 minute. Then the photoresist is exposed with the mask 1 shown in Fig. 2.7(a) by the aligner with 150 mJ exposing power and developed by the TMAH solution of 2.38% concentration. The processed wafer after line patterning is shown in Fig. 2.8.

4. Wet etching and photoresist removing: Firstly, the Cu layer is etched by the etchant.

The etchant consists of sulfuric acid and hydrogen peroxide solution. Then the Cr layer is etched by the potassium ferricyanide solution. After etching, the photoresist is removed by the remover. The processed wafer after metal etching is shown in Fig.

2.9

5. Electroplating: In order to reduce the resistance, the 6 μm Cu layer is electroplated on the seed layer with dipping in the CuSO4 solution bath. And the 1 μm Au layer is electroplated with dipping in the potassium gold cyanide solution bath. The processed wafer after Cu and Au electroplating is shown in Fig. 2.10

6. Isolation layer patterning: In order to increase the adhesion between polyimide and wafer, the adhesive VM652 is coated on the wafer and baked under 120℃ for 2 minutes. The coating rotation speed and time are shown in Fig. 2.6(b). And the light-sensitive polyimide of 12 μm thickness is coated on the wafer and baked under

120℃ for 3 minutes. The coating rotation speed and time are shown in Fig. 2.6(c).

Then the polyimide is exposed with the mask 2 shown in Fig. 2.7(b) by the aligner with 600 mJ exposing power, developed by the TMAH solution of 2.38%

concentration and finally baked under 250℃ in the oxygen-deprived oven.

7. Dry film patterning: In order to increase the adhesion, RIE with 100W power is used to coarsen the surface of polyimide for 60 seconds. Two layers of dry film are attached on the wafer by the dry film machine. The temperature of roller of dry film machine is controlled at 105℃. Then the dry film is exposed with the mask 3 shown in Fig. 2.7(c) by the aligner with 150 mJ exposing power and developed by the sodium carbonate solution of 1% concentration. The processed wafer after dry film patterning is shown in Fig. 2.11.

8. Via electroplating and Die sawing: The 200 μm height metal pillars are electroplated with dipping in the CuSO4 solution bath. The electroplated wafer is shown in Fig.

2.12. Then the wafer is cut according to the cutting line shown in Fig. 2.13. Finally, the protrudent Cu pillars are ground into flats.

9. Wire bonding: The wires are bonded on the wafer according to the Fig. 2.14. Then the wafer is bonded on a printed circuit board (PCB). The final testing sample is shown in Fig. 2.15. The oil-based Fe3O4 nanofluid will be loaded in the channel of wafer as a magnetic fluid core.

In order to verify the insulation of circuits under a high voltage, the insulativity of dry film is investigated. A self-made high voltage output device is used to test the insulativity of dry film. Figure 2.16 shows the circuit diagram of high voltage output device. The switching pulse width module (PWM) IC chip TL494 and two power MOSFET transistors IRF510 are used to cut the DC source into 56 kHz pulse. Then the voltage of pulse is stepped up by a transformer, rectified to DC and stepped up again by the voltage doublers. Figure 2.17 shows the photo of high voltage output device. The testing sample is a glass wafer coated the Cr/Cu layer and the dry film. The insulativity of dry film is investigated by applying a high voltage as shown in Fig. 2.18. The breakdown voltage of dry film with 100 μm thickness is over 5 kV, which indicates that the insulativity of dry film is high enough to endure the high surge.