The fabrication steps are illustrated in Figure 4-4, and they are analogous to the steps illustrated in Section 2.6.1 and 2.6.2. The silicon frame is fabricated first. Thin silicon wafers, 200 m thick wafers are used as substrates. A first layer of parylene C, 0.5
m thick, is deposited by LPCVD, and molten at 350 C in a nitrogen environment. The molten parylene acts as an adhesion layer for a second layer of parylene C, 1 m thick.
Platinum is deposited by e-beam evaporation, and it is patterned by liftoff. The adhesion of platinum to parylene is sufficient for the fabrication of the devices. The
(a) (b)
force
adhesion of platinum to parylene in different conditions was examined. Platinum adhesion to molten parylene was found to be very poor, which is the reason why the second parylene C layer is deposited on top of the molten parylene. The higher crystalline fraction of molten parylene compared to as-deposited parylene is suspected to be the cause of the scarce adhesion. Furthermore, the thickness of the second parylene layer seems to affect the platinum adhesion, with thicker layers providing better adhesion. The crystalline structure of molten parylene layer may act as a seed for the parylene layer above, causing the crystalline structure to propagate through the second layer. If the second layer is thick enough (1 or 2 m), the crystalline structure may be lost, with parylene returning to an amorphous condition that provides better platinum adhesion.
Parylene is patterned by RIE in oxygen plasma. A through-wafer DRIE step releases the devices. Thin wafers (200 m thick, instead of the conventional 500 m) were chosen to shorten the DRIE etching step, without having to thin down the wafers. Figure 4-5a shows a finished frame, with close-ups on the spring and electrodes.
The reflow of the photoresist that acts as a mask for DRIE causes some difficulties. The reflow is caused by the baking step preceding DRIE, which makes the photoresist assume a rounded profile. This is particularly evident for the photoresist that masks narrow features, such as the spring beam (50 m wide). The photoresist is thinner at the edges of a feature, making it unable to withstand the entire DRIE etch there. As a consequence, the rounded profile is transferred, to a certain extent, to the etched silicon structures. This effect is reduced by making the photoresist thicker.
Processing 200 m thick 4-inch diameter wafers requires special care, due to the fragility of the substrate. Mounting a backing wafer on the back of a wafer is very
convenient to avoid breaking the wafers during the fabrication process, even though processing without the backing wafers is possible. Photoresist is used as the adhesive between the device wafer and the backing wafer. Photoresist has the advantage of being dissolvable at the end of the process to release the devices from the substrate. The backing wafer can be mounted only after the parylene-melting step, as photoresist cannot be exposed to a temperature of 350 C. Therefore, the first part of the process must be done without the support of the backing wafer. The backing wafer and the steps required to mount it are not shown in the process steps of Figure 4-4.
After the frame is completed, the balloon fabrication is analogous to the process developed illustrated in Section 2.6.2. The inchworm is based on disc-less balloons with parylene walls, fabricated with a sacrificial photoresist process. The silicon frame is mounted on a glass slide, for ease of handling. Sacrificial photoresist (AZ4620) is dispensed over the frame to define the balloon shape (Figure 4-5d). A commercial fluid dispenser (EFD 2000XL) with a 33-gauge needle is used. Some photoresist is dispensed over the pads as well, so they can be reopened after the parylene deposition. The photoresist is baked on a hotplate at 70 C. It is important to make sure that no gas bubbles are trapped in the photoresist, as they may burst in vacuum during the following parylene deposition. A layer of parylene is then deposited (typically 10 m thick). A sharp needle is then used to poke a hole in the parylene of each balloon and over the pads. The device is immersed in acetone for releasing the photoresist. After the photoresist is completely released (Figure 4-5e), and the acetone has completely evaporated, the device is immersed in an electrolyte solution inside a vacuum chamber,
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Figure 4-5. Fabricated first-generation inchworm devices. a) Silicon frame. Frames with different dimensions were fabricated, and the number on the frame identifies them. b) Close-up of the spring. c) Close-up of the electrodes. d) Frame with sacrificial photoresist. Due to photoresist reflow, the shape tends to become spherical. e) Frame with parylene balloons, after release of the sacrificial photoresist.