CHAPTER 7. CONCLUSIONS AND PERSPECTIVES
7.2. Next Generation of the Reticle
The current version of the reticle is limited to measuring spatial resolution down to 30 µm. Next generation of the reticle will require sub-micron features to quantify resolution in the
state of the art MS instruments. Working towards this goal fabrication tools within the VINSE clean room were used to fabricate features below 10 µm. The new version of the reticle is designed to have features ranging from 1 µm to 100 µm divided into 3 segments: 1 – 10 µm, 10 – 50 µm, and 50 – 100 µm. The 50 – 100 µm regions consisted of features in 5 µm increments, and each feature had five replicates to enable confident identification (see Figure 7.1). The second region from 10 to 50 µm also consisted of 5 lines for each dimension but the increments were 2 µm. Thus, within this range, the accuracy will be +/- 2 µm. The third region contained features from 1 to 10 µm in increments of 1 µm.
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Figure 7.1. Photomask design for the next generation of the reticle contains three regions for testing: 1-10 µm, 10-50 µm, and 50-100 µm. The 1-10 µm region contains features in 1 µm increments, the 10-50 µm region contains features in 2 µm increments, and the 50-100 µm region contains features in 5 µm increments. For each feature, five replicates are included to enable confident measurement of spatial resolution. In the figure, features smaller than 20 µm are not apparent. White areas in the photomask represent transparent regions that allow UV light transmission through the photoresist.
The photomask design was printed onto a chrome-coated substrate as shown in Figure 7.2. Heidelberg laser writer (µPG 101) with a 405 nm diode laser was used to irradiate the photoresist, and the pattern was developed in MF319 solution to remove the exposed photoresist.
The substrate was then inserted in etchant 9030 for 1 min to remove the exposed chrome.
Finally, oxygen plasma was used to remove the remaining photoresist.
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Figure 7.2. Process for designing the chrome mask. A glass substrate is coated with chrome, and a AZ1518 photoresist layer is patterned using a Heidelberg Laser Writer. The exposed photoresist is developed in MF319 solution. Etchant 9030 is used to remove the exposed chrome followed by an O2 plasma to remove remaining photoresist.
The fabricated photomask is shown in Figure 7.3. Figure 7.3a shows the 1” x 1” region on the photomask that was patterned. Figure 7.3b-c shows the 1 µm features were barely resolved on the photomask whereas 2 µm features were clearly resolved. In both cases, the line width was enlarged.
Figure 7.3. Photomask fabricated using a Heidelberg laser writer. (a) The substrate measured 5”
x 5” and a small region is used for printing (1” x 1”) (b) An expected 1 µm line width was enlarged to 1.8 µm, and two of the lines coalesced together. (c) Expected 2 µm lines were enlarged to about 2.4 µm. The tan color corresponds to non-transparent chrome layer, and the darker color is due to a black background.
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Photolithography process was implemented using the new mask with features down to 2 µm in size (see Figure 7.4a). As the UV light is transmitted through the mask it diffracts
producing features that are larger than intended sizes. As shown in Figure 7.4d, the broadening of the feature can cause individual lines to merge together. The angle of diffraction is directly dependent on the size of the aperture and the extent of line broadening can be calculated by the product of the diffraction angle and the thickness of the photoresist using the small angle approximation. The employed photoresist (SU-8 3025) was 25 µm thick when spin-coated onto the substrate. Using this photoresist, distinct features down to 8 µm can be produced as shown in Figure 7.4. To enable fabrication of smaller features, a photoresist with lower viscosity should be employed to generate thinner coating. For example, SU-8 3005 will lead to coatings that are 5 µm in thickness so that the process can be scaled down to 2 µm features.
Figure 7.4. Fabrication of the next generation of the reticle. (a) Photolithography process that exposes the photoresist through a photomask. (b) Generated 10 µm features.
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Figure 7.5 demonstrates the image of crystal violet pattern produced using the developed stamp. Line with widths of 9 µm or smaller coalesced together suggesting that the gap between the lines was insufficient to ensure distinct features. The density of crystal violet shows gradient corresponding to the size of the features. As the feature size increases, the thickness of crystal violet increases. Due to these reasons adhesive/ diffusive printing is recommended for dimensions below 10 µm.
Figure 7.5. Crystal violet lines produced using the next generation stamp. Lines with widths of 9 µm or smaller coalesce together suggesting that the gap between the lines was insufficient. 10 µm lines were isolated. The thickness of the lines increases as the width of the line increases.
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Three major improvements were made in this new version of the reticle. First, the smallest feature on this version was 10 µm in size compared to 25 µm in the previous version.
Secondly, 5 replicates were included for each dimension enabling a higher degree of confidence in our measurements whereas previously only two or three replicates existed. Thirdly, finer increments were employed in this version allowing for a higher precision in the measurement of resolution. The fabrication process has to be tailored to the size range of the features precluding the possibility of including the full range of features sizes on a single reticle slide. The fabrication process has to be separately optimized for three set of ranges: 100 nm – 1 µm, 1 - 10 µm, 10 - 200 µm.