| AVS 58th Annual International Symposium and Exhibition | |
| MEMS and NEMS Group | Friday Sessions |
| Session MN-FrM |
| Session: | Characterization of Materials and Structures at the Micro- and Nano-scale |
| Presenter: | S. Azimi, University of Tehran, Iran |
| Authors: | Z. Sanaee, University of Tehran, Iran S. Azimi, University of Tehran, Iran M. Poudineh, University of Tehran, Iran S. Mohajerzadeh, University of Tehran, Iran A. Sandoughsaz, University of Tehran, Iran |
| Correspondent: | Click to Email |
We report the formation of ultra-high aspect ratio and three dimensional features on silicon substrates using a novel low-density capacitive-coupled plasma reactive ion etching (13.56MHz). The etching process is based on using three gases of hydrogen/oxygen and SF6 in two sub-sequences called as passivation and etching sub-cycles. All three gases are used in the passivation step and SF6 in the etching step. Unlike Bosch process no polymer is used for passivation. By controlling the passivation sub-cycle, one is able to allow desired under-etching followed by “recovery” of the formerly under-etched features to make unique three-dimensional structures directly on silicon substrates [1].
Cleaned silicon samples are placed in an e-beam evaporation unit to deposit a 40nm chromium layer as the mask for the subsequent processing steps. The masking layer is patterned using precision projection lithography to achieve desired features between 100nm and 20um. For ultra-high aspect ratio and scallop-free etching while keeping the etch-rate of 1um/min, it is necessary to include trace values of H 2/O 2 during the etching step. Typical flows for H 2/O 2 and SF 6 are 200/200 and 5 sccm in the passivation step while the etching is mainly practiced with SF 6 (35 sccm). The plasma power is set at 250 W for the passivation and 130 W for the etching sub-cycle. By controlling these important parameters, we have realized three-dimensional features where the vertical structures have serpentine surfaces with desired recessions of 10um. Moreover, we have been able to realize arrays of nano-metric 3-D features using Si/SiO2 structures with a diameter of 2-3um and features of the order of 100nm.
We realized 9-10um high and 90nm wide nano-wires where the mask undercut is 30nm and the surface of the wires is almost free of “scallop”. Scallop is side-effect of time-multiplexed processes where the periodic track of the etching step is seen on side-walls. To avoid this, while obtaining high-rates we have included H2/O2 gases during the etching sub-cycle. Normally H2/O2 gases act as the passivation layer, however the trace value of these gases does not affect the etching. Instead a slight passivation is formed on side-walls while the etching proceeds, prohibiting further lateral-etching of walls. We have studied the passivation layer using XPS and Ellipsometry. Thickness of the passivation layer is 2-3nm and it is mainly SiOF bonds (XPS). Field-emission SEM has been used to compare the results. Using this process we obtained high etch-rates of 0.8-1.1um/min for features around 100nm. The height of the nano-wires is around 10um, with an aspect ratio of 100 and more. This process uses low-density plasma with rapid steps and apart from MEMS/NEMS applications it can be used for “solar-cells” where nano-wires can significantly affect the efficiency and cost.
[1]. S. Azimi, A. Sandoghsaz, B. Amirsolaimani, J. Naghsh-Nilchi, S. Mohajerzadeh, "Three-dimensional etching of silicon substrates using a modified deep reactiveion etching technique", J. Micromech. Microeng. No. 21, 074005, (2011).