Authors: M. David Henry*, Andrew Homyk, Sameer Walavalkar, & Axel Scherer, Applied Physics Department, California Institute of Technology
Achieving nanoscale features requires a plasma etch utilizing passivation and a good etch mask determined by the etch chemistry selected.
Silicon etching at nanoscale dimensions requires quality and precision from both the etching system and the etch mask. The etching system must be clean and deliver a well tuned mixed-mode etch incorporating a continuous etch/passivate chemistry. Two such chemistries are the cryogenic silicon etch, utilizing the SF6 and O2 chemistry, and the 'Waveguide Etch' referred to here as pseudo Bosch, utilizing SF6 and C4F8 chemistry. Both of these etches were performed here using Plasmalab System 100 ICP-RIE 380s equipped with a cryogenic electrode. The cryogenic etch, performed typically at -120 C, provides high selectivity for most etch masks with vertical sidewall control capable of features with dimensions of hundreds of nanometers. However to achieve structures with lateral dimensions below 100 nanometers, a slower etch becomes more useful.
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25 nanometer silicon pillars etched 650 nanometers using an alumina etch mask (still on) using Pseudo Bosch silicon etch in an Oxford Instruments PlasmaLab 100 ICP-RIE 380. |
Rows of 100, 75, and 50 nanometer silicon pillars (from left to right) etched 650 nanometers using an alumina etch mask (still on) using Pseudo Bosch silicon etch in an Oxford Instruments PlasmaLab 100 ICP-RIE 380.
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Using the pseudo Bosch etch at 15 C, slows the etch rate down from microns per minute, typical in cryogenic etching, to 200-300 nanometers per minute. The cost of slowing the etch rate down using this chemistry is that the etch mask selectivity also decreases. For example, where the cryogenic etch achieves selectivity values better than 75:1 for resist etch masks, pseudo Bosch achieves closer to 3:1.
To compensate for the reduced selectivity, etch masks which do not significantly add to the complexity of patterning nanoscale features are required. Although metal masks, such as nickel, offer the increased selectivity desired, they also increase anisotropic undercutting directly below the masks due largely to electrostatic interactions. Thus, smaller scale structures are possible to define with such masks, but achieving deep silicon etches at low bias voltages is not. A good etch mask, then, needs to be hard and chemically inert to the etch gas, like some metals are, yet electrically insulating as well. Insulating etch masks, such as silicon nitride and silicon dioxide do improve nanoscale etches using cryogenic chemistry by improving selectivity to around 200:1. For the Freon gas chemistry in pseudo Bosch etches, silicon dioxide and silicon nitride rapidly lose their selectivity and etch with rates closely resembling that of silicon etching.
A less common etch mask is sputtered alumina. Thin alumina layers can achieve selectivity of greater than 65:1 for the pseudo Bosch and 5000:1 for cryogenic etching. Alumina etch masks are also easily removed without etching the silicon by using buffered hydrofluoric acid or RCA-1 cleaning chemistry, NH4OH with H2O2. Using this etch mask combined with the pseudo Bosch etch chemistry, we have etched silicon nanopillars with aspect ratios of 60:1 with diameters down to 20 nanometers.
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57 nanometer diameter silicon pillar 2.8 microns using an alumina etch mask using Pseudo Bosch silicon etch in an Oxford Instruments PlasmaLab 100 ICP-RIE 380. |
Tungsten probe contacting a 60 nanometer diameter silicon pillars, 2.5 microns tall. Etched using an alumina etch mask and Pseudo Bosch etch in an Oxford Instruments PlasmaLab 100 ICP-RIE 380. |
Summary
Choosing the right mask depends on the required etch dimensions and etch chemistry. If the desired structures are hundreds of nanometers in dimensions, etch depths of microns, and a clean process is desired then choosing cryogenic etch chemistry with resist or silicon dioxide is ideal. If the desired structures are tens to hundreds of nanometers in dimensions, etch depths of a few hundred nanometers, and simple etch mask fabrication is desired, than using pseudo Bosch with organic resist is appropriate. But if etch dimensions are tens of nanometers or if extremely high aspect ratios are required, then using alumina etch masks and pseudo Bosch etch chemistry becomes the appropriate choice.
*MDH gratefully acknowledges the support of the John & Fannie Hertz Foundation.