Cryogenic silicon etching has been commonly used in microelectromechanical systems (MEMS) research in order to etch high aspect ratio structures and deep, large features in silicon due to its high etch rate and high selectivity to resist and oxide masks.
Cryogenic Silicon Etching
Etching is performed with SF6 and O2 at temperatures ranging between -90°C and -130°C. Oxford Instruments Plasma Technology has developed a cryogenic silicon etch process with which it is possible to etch shallow and high aspect ratio nanoscale features below 50 nm that can be used in nano-imprint and electron beam lithography pattern transfer into silicon with the use of polymer resists. With shrinking feature sizes, profile and critical dimension control tolerance is reduced and appropriate processes must be developed.
Advantages of Cryogenic Silicon Etching
Cryogenic silicon etching using SF6-O2 offers a number of benefits over other SF6- C4F8 or heavier halogen based processes such as the Bosch and the HBr processes that include the following:
- The passivation layer (SiOxFy) is very thin in the cryogenic etch process, of the order of 2-5 nm, and hence only a low energy ion bombardment is needed to remove the passivation layer from the bottom and continue the vertical component of the etch.
- The low ion bombardment means that the selectivity to soft masks that include photoresist can be high while still maintaining a quick etch rate.
- The passivation layer is thin, which results in a smooth sidewall in narrow trenches. The weak dependency on ions also reduces problems when etching narrow trench sizes associated with ion interactions at the sidewall which can cause less than ideal profiles.
- Sidewall contamination is minimal eliminating critical dimension (CD) variations due to etch residue cleaning.
Examples of Cyrogenic Silicon Etching
The main challenges to create a suitable cryogenic SF6-O2 process for 50 nm features and below are optimizing the passivant to remove undercut and reduce the etch rate enough to control the process. With a detailed study of important parameters affecting the etching process, 45 nm and 20 nm wide trenches were etched at a 7:1 aspect ratio with a vertical profile as shown in Figures 1 and 2.
Figure 1. Electron-beam lithography patterned 45 nm wide trenches etched to a depth of 300 nm using an electron beam resist mask and an Oxford Instruments PlasmaLab 100 ICP-RIE system.
Figure 2. Silicon trenches 22-nm wide etched a depth of 155 nm in silicon. The trenches were patterned using electron-beam lithography and the mask is ZEP-520A electron beam resist.
Features as small as 12-14 nm were also etched to a 3:1 aspect ratio using resist masks as shown in Figure 3. All cryogenic SF6-O2 based silicon etching processes were evaluated using an Oxford Instruments Plasmalab 100 using a Cobra inductively coupled plasma (ICP) source and cryogenic cooled electrode. Sub-100 nm features were patterned both with electron beam lithography and nanoimprint lithography techniques using polymer resists. Resist selectivity is high in both cases: from 10:1 to 15:1, and vertical and smooth sidewalls are obtained.
Figure 3. 13-nm wide trenches etched to a depth of 36 nm in silicon. The trenches were patterned with electron-beam lithography and resist.
For nanoscale-sized trenches in MEMS and nanoelectronics, a well controlled etch/passivate process, which produces vertical sidewalls and smooth surfaces is essential. As features shrink, more oxygen flow is required to avoid bowing and undercutting and bowing. A delicate balance between the SF6:O2 ratio, temperature, and RF power is requiredto control the profile. The increase in oxygen flow along with the reduced exposed silicon area also can reduce the etch rate down to 100 nm/min or less, making it much easier to control for shallow etches. One thing to note is that it is difficult to etch trenches that greatly differ in size using a single process as the larger features will be overpassivated and have a positive slope. However, for pattern transfer of sub-50 nm trenches for nanophotonics, nanofluidics, and nano- imprint lithography templates using soft masks, the cryogenic silicon etch process is an excellent alternative.
This information has been sourced, reviewed and adapted from materials provided by Oxford Instruments Plasma Technology.
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