Fabrication Process

(1) Forming Seed Layer (Ni)

(2) Dispensing PR SU8-5

(3) Coating PR SU8-5

(4) Soft bake

(5) Exposure

(6) Post Exposure Bake

(7) Develop

(8) Electroplating Ni Structure

(9) Stripping PR SU8-5

Figure 22. Steps for processing SU8-5.

In this work, the employed substrate is a silicon wafer with 4” diameter, and the environment temperature for processing the SU8-5 photoresist is 24 .℃

The procedure to pattern the micromold, comprises following steps shown in Figure 22:

(1) Dispense the SU-8 photoresist onto the substrate

Distribute enough SU8-5 manually and to form a uniformly round region covered with SU8 on the silicon substrate from its center to about 0.6 to 0.8 times diameter, in order to coat films with thickness within the range from 5 to 80 μm. However, a poor SU8 distribution is happened over the entire substrate surface after spin coating, if dispensed insufficient or not uniform SU8 on the substrate.

(2) Spin coating

Spin speed and duration time are two major concerns for coating SU8 film to a demanded thickness on the wafer. The thickness of the SU8 film is not only related to an adopted spin speed but also its duration time. In general, under the same spin speed, the SU8 film becomes thinner as the duration time is taken longer. However, for the identical spinning duration, the film thickness is inversely proportional to the spin speed.

Additionally, the uniformity of the film’s thickness is also affected by the duration time.

That is, an insufficient spinning duration will cause SU8 transiently distributed on the substrate. Thereby, a thickness variation of the coated SU8 film appears. Figure 23, lists the relation between the SU8-5 film thickness and the spin speed, which is similar with that recommended by MCC. However, it also indicates that, it is difficult to coat precisely a SU8-5 film with thickness over 60 μm, since the curve slope of the film thickness versus the spin speed changes abruptly for spin speed less than 300 rpm. In

addition to spin speed and duration time, environment temperature while processing the SU8-5 is another factor has to be concerned. Since the viscosity of SU8 is decreased as the environment temperature increases. The thickness of the coated SU8 film is also decreased in higher temperature. It is easy to realize that the relation shown in Figure 2 will be deviated under different environment temperature.

Spin Speed v.s. Film Thickness

0 20 40 60 80 100

0 500 1,000 1,500 2,000 2,500 3,000

Spin Speed (rpm)

Film Thickness (micron)

Figure 23. Spin speed and film thickness.

(3) Soft bake

Although, MCC has recommended the baking temperature of 90 ~ 95 to cure SU8. ℃ However, the time required to soft bake SU8 film with different thickness related to the adopted spin speed is not available. In our work, the soft bake temperature used in this experiment is kept at 90 . The SU8℃ -5 is just soft baked by hotplate, no forced convection surrounding the SU8 film adopted to facilitate the solvent evacuation.

Further, for avoiding the heat shock, a filter paper is spaced between wafer and hot plate for about 3 minutes after the initial contact. In addition, the hotplate has to be well leveled, otherwise, a tilted hotplate makes the SU8 photoresist to redistribute during soft

regarding the thickness versus required time for soft bake is shown in Figure 24, which including the using the filter paper as a spacer for the first 3 minutes. It shows that, for properly curing the SU8 film, there is a linear tendency relation between the thickness and the soft bake time. In this work, a tweezers is used to touch the cured surface of the SU8 film to inspect the film solidity. If the surface of the cured SU8 film is little deformed by tweezers in gentle touch, the SU8 is cured properly. Otherwise, there has un-negligible residual solvent in the baked SU8. It is found that, an insufficient soft baking cannot properly cure the SU8, the residual solvent will affect the absorption of the exposure energy for the SU8 in the exposure process. On the other hand, if the SU8 is over baked, the cracks appeared at the interface between the substrate and the SU8 film will degrade the adhesiveness. That will make the patterned SU8 film to be lifted off during or after development process.

Soft Bake Time v.s. Film Thickness

0

Figure 24. Soft bake time and film thickness.

(4) Exposure

To construct the vertical sidewall on the pattern made of SU8, it has to supply enough exposure energy. Otherwise, insufficient exposure dose cannot offer enough

exposed SU8 film cannot endure the encroachment from the developer during development and thereby deforms. Contact printing is adopted for exposure to alleviate the diffraction effect, and a longer exposure time or higher exposure dose is preferred to process the SU8 film to achieve a well defined structure with vertical sidewall and sharp corner. Similar to the typical negative photoresist, the relation between the exposure time (dose) and the thickness of the SU8-5 film tends to a slightly nonlinear, as shown in Figure 25. The curve in Figure 25 indicates the minimum exposure dose required for the SU8-5 film with different thickness. To achieve a successful exposure, it has to supply exposure dose higher than that shown in Figure 25 to pattern the SU8. However, an overdose makes the stripping of the SU8 mold more difficult, after electroplated the metal structure.

Exposure Time v.s. Film Thickness

0

Figure 25. Exposure time and film thickness.

(5) Post Exposure Bake (PEB)

Dose of post exposure bake is critical for successfully defining the pattern on the SU8-5 film. For negative photoresist SU8, PEB is to facilitate the crosslink between

enough to withstand the encroachment applied by the developer. However, an overdosed PED makes a low selectivity between an exposed and an unexposed region on the SU8 film during development, thereby distort and misshape the pattern of the SU8 film. To define a sharp and clear pattern, Figure 26 shows the adopted PEB time at 90 fo℃ r the SU8-5 film. A break of 3 ~ 5 minutes between PEB and development is necessary to allow a chemical reaction for crosslinking the exposed polymer in the SU8.

PEB Time v.s. Film Thickness

0

Figure 26. PEB time and film thickness.

(6) Development

Development is to chemically remove the unexposed portions of the SU8 film. By gentle shaking, the exposed portion of the SU8 film on the wafer immersed in the developer lifts off completely from the substrate during a primary development. Fresh developer is used to rinse the wafer has been primary developed for less than 30 seconds to clean the residual scum on the wafer. Thereafter, compressed dry air or nitrogen is used to blow the residual developer away from the substrate. However,

similar to the typical negative photoresist, SU8 absorbs developer and swells during development. Especially, when the solvent in the SU8 has not been adequately removed by soft bake, or the develop time is taken too long. That is, the developer will keep going to encroach on the SU8, and the residual solvent makes the SU8 absorbing more developer and swelling worse. Thereby, the SU8 pattern is distorted.

The purple or deep blue scum resulting from the encroached SU8 is appeared on the substrate after development. Figure 27 shows the time adopted for developing the SU8 film, which including the rinse time after the primary development.

Development Time v.s. Film Thickness

0

Figure 27. Development time and film thickness.