Chapter 2 Extraordinary Transmission (EOT)
2.1 Fabrication
2.1.5 Metal Deposition
Two options of metal deposition were used for this work: sputtering and evaporation.
Both worked well for the two choices of metals in this work, Au or Al, depending on the structures intended to be made.
Sputtering offers the benefit of conformal coating, which is especially useful if the nanostructures are 3D-sculpted during the pseudo Bosch etch. It is also a good option when the nanostructures are sparse enough for thermal reflow of gold to be successful; more discussion on the reflow process can be found in Section 3.1.1. The sputtering of Au was done at a home-built machine at KNI maintained by the Scherer Group, nicknamed McGill, with an argon flow of 100 sccm. First, an adhesion layer of Ti was sputtered at 30 mTorr and 175 W for about 2 minutes, which deposits ~5 nm of titanium as a wetting layer for the subsequent Au sputtering. Gold was then sputtered at 10 mTorr and 80 W for a few minutes, depending on the desired final thickness. In this work the Au thickness ranged from 150 to 200 nm approximately. Aluminum can also be sputtered by either the McGill system or the
other one made by a company called TES. A typical recipe is 100 sccm of Ar at a pressure of 10 mTorr and a power setting of more than 300 W.
Evaporation, on the other hand, has very good directionality and sidewall deposition can be minimized; this is a very desirable feature when thermal annealing could not drive the metal satisfactorily away from the sidewalls when the pattern is too dense, or when the nanostructures are of a shape that cannot be mechanically broken well without specialized apparatus. The process of mechanical snapping will be discussed in the next section. In this work e-beam evaporators were used for the deposition, although thermal evaporators can also be an option if the chamber can accommodate two targets so that Ti and Au can be evaporated sequentially without breaking the vacuum.
To obtain the desirable nano-apertures for extraordinary transmission, however, additional factors have to be taken into consideration than in sputtering. The angular distribution in evaporation, though small, is still nonzero and can result in intolerable sidewall deposition that makes subsequent processing more difficult. One prominent parameter is the height of nanostructures before the evaporation. If the structures are too tall, they will “shadow” the regions around the base by blocking the small angular contribution from the other side that could have otherwise reached the spot. The result would be shapes of slightly upward-facing funnels for the fabricated apertures, which may not be desirable in some applications. Figure 2.6 shows the different results from the same deposition run in the CHA machine on two chips of different etching depth. In Figs. 2.6 (C) and (D), the structures are much taller than those in (A) and (B), and the shadowing effect is therefore much more obvious since higher structures are more efficient in blocking; the sidewalls also collected more deposition along the way.
Figure 2.6 Results of evaporation from the CHA machine on chips of different etching depth. The structures in (A) and (B) were shorter than those in (C) and (D), while the thickness of the Au deposition was the same.
Due to its high directionality, evaporation can achieve something that cannot be done in sputtering: angled deposition by tilting the chip orientation. This is another issue to consider, however, when the sidewall deposition has to be minimized. At KNI there are two e-beam evaporators, one manufactured by the TES and the other by CHA Industries, Inc. The CHA model has a domed wafer loader that conforms to the angular distribution of evaporated metal, so that the metal vapor will impinge on the chips at a right angle. The wafer loader of the TES machine, however, is planar in shape. As a result, unless the chip sits right above the crucible of the TES machine and the table rotation is turned off during the deposition, more sidewall deposition is anticipated from this machine. This is indeed the case, as seen in Fig.
2.7.
Figure 2.7 Results of evaporation from the TES machine; the sidewall deposition was more serious than in CHA.
If for any reason the gold layer has to be removed for another deposition, the gold etchant TFA from Transene does a good job without affecting the nanostructures since it is based on potassium iodide (KI). For extreme purists who are concerned about the thin adhesion layer of Ti, a very quick dip in BHF would be sufficient since BHF attacks Ti very fast [55].
Another option is aqua regia, which is a mixture of nitric acid and hydrochloric acid with an optimal molar ratio of HNO3 : HCl = 1 : 3. It should be noted that since commercial nitric acid and hydrochloric acid are commonly available at a w/w concentration of 70% and 37%, respectively, this translates to a volume ratio of approximately 1 : 4. The specifications of commercially-available HNO3 and HCl are listed in Table 2.1. Aqua regia does not attack silicon dioxide, and will leave the nanostructures intact.
Acid Molecular Weight Density (g/cm3) Concentration w/w
HNO3 63.01 1.42 68 - 70%
HCl 36.46 1.19 36.5 - 38.0%
Table 2.1 Specifications of commonly-available nitric acid and hydrochloric acid.