Patterning Photoresist for the Etching Process

Etching is not only suitable to work with resolution smaller than microns, but can also deal with very thin surface without damaging the substrate [Lei, 2012]. In order to pattern gaps or lines smaller than 20 µm, the use of etching is considered. The conventional method of patterning electrodes using etching is to use a metal or plastic mask with the printed patterned on the photoresist, which allows the illumination of UV light to cure the photoresist and protect the pattern under the photoresist. Once the photoresist is cured under the UV light, etching is used to remove the unprotected part of the metal surface and leave the wanted patterns. However, making masks with high resolution takes time to produce, therefore, a method to directly print photoresist on the surface without using masks is developed and tested.

3.2.3.1 Introduction of Microplotter

GIX microplotter manufactured by Sonoplot was considered to be a good alternative method to replace the masking process which can print the photoresist directly on the copper with the required patterns. The microplotter is a leading-edge tool which is used for microarrays, polymer electronics, and other applications which require precise and accurate dispensing of extremely small liquid volumes, such as a few microns [Lei, 2012]. This system can deposit discrete spots or continuous traces such as lines or arcs onto any planar surface with a wide range of solutions [Lei, 2012].

Figure 3. 3 GIX Microplotter

This microplotter consists of a positioning system, which contains printhead and optics system already mounted, iMac control computer with keyboard and

a wireless mouse, and control electronics, as shown in Figure 3.3. The positioning system can create features in the area as small as 5µm wide. And it can print liquid with viscosity of less than 450cP, and anything having more viscosity will not be printed from the glass capillary.

3.2.3.2 Experimental Results

In order to test whether microplotter is suitable to use for patterning structures, both small and large size structures, such as several microns tens and tens of millimetre are designed and tested. The photoresist S1818 from Shipley is mixed with its solvent PGMEA from Sigma Aldrich is chosen to meet the viscosity requirement to use, and this mixture is used to draw the patterns directly on the copper. The processes of using Microplotter are as follows:

1. The structures are drawn with Sono draw, which is the build-in software of microplotter.

2. The drawn structures are imported into the Sono guide, which is the software to control the microplotter. After loading the structures, the proper position to absorb solutions (photoresist mixtures) and the position of the board to draw are set for the microplotter.

3. With the pre-set data, the printing job can be finished automatically. 4. The printed patterns is then etched using the method stated in 3.2.4

and checked with microscope and Atomic Force Microscope (AFM).

Three types of the filters and resonators structures are designed and drawn with Microplotter, and Figure 3.4 and 3.5 are the designed structures and the correponding structures after etching. The dimensions for the three structures excluding the feed lines shown in Figure 3.4(a), (b) and (c) are 14mm×14mm, 18mm×18mm and 15mm×15mm. Compared the (a) in Figure 3.4 and 3.5, the electrodes are not precisely patterned, where the patterned central plates are not parallel and not constant width. Figure 3.5(b) shows a discontinuity gap in the line and different width of the lines, which are not what they designed in Figure 3.4(b). While Figure 3.4(c) shows different gap width and different line width, and the corner of the microstrips are not perpendicular. Based on the

comparison between the three structures, microplotter is not suitable to use for structures with tens of millimeters.

(a) (b) (c)

Figure 3. 4 Designed patterns (a) ELC resonator (b) edge coupled filter (c) split ring

(a) (b) (c)

Figure 3. 5 Patterned electrodes using Microplotter (a) (b) (c) are the correponding strctures in Figure 3.4

As millimeter scale structures are not suitable to print with the microplotter, small scale structures, such as several microns, are also designed and tested to find out whether it is suitable for several structures with microns. Seperate lines and meander line structure, as shown in Figure 3.6, are designed, patterned and etched. The etched pattern are then checked with Atomic Force Microscopy (AFM) to measure whether the width matches the width designed.

(a) (b)

(a) _(b)

Figure 3. 7 AFM results of printed patterns (a) Separate lines (b) meander lines

Figure 3.7 shows the AFM results for separate lines and meander lines. As can be seen from Figure 3.7 (a), the results shows that the average distance between the two peaks is 50.89µm, which is slightly larger than the designed value of 50 µm. It is believed that the reason why the distance is larger than 50µm is the coffee ring effect, which is that after droplet evaporation, the particles are highly concentrated along the original edge of the drop [Yunker, 2011]. Figure 3.7 (b) indicates that for the meander structure, the average width for the line is 49.73µm, and the average width of the space is 19.79µm. The proposed width for the line is 50 µm and the space drawn between two lines is 20 µm, and the results shows the printed width and space are slightly less than the proposed width and space. The discrepancies between designed and patterned structures are very small, which are around 2%, therefore, the discrepancies are acceptable.

Based on the experiment results shown above, the microplotter is not suitable for structures of tens of millimetres, but it can be used for small scale devices, such as several microns. However, due to the un-stability and high failure rate to print the whole small scale structures, microplotter is not recommended to use. Therefore, microplotter is not suggested to use to replace the mask procedure, and the traditional process of using mask is continued to use in the etching process.