DREM: Infinite etch selectivity and optimized scallop size distribution with conventional photoresists in an adapted multiplexed Bosch DRIE process

Flemming Jensen,Bingdong Chang,Henri Jansen,Pele Leussink,Jörg Hübner

doi:10.1016/j.mee.2018.01.034

Flemming Jensen, Bingdong Chang + Show 3 more

Open Access

https://doi.org/10.1016/j.mee.2018.01.034

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Abstract

The quest to sculpture materials as small and deep as possible is an ongoing topic in micro- and nanofabrication. For this, the Bosch process has been widely used to achieve anisotropic silicon microstructures with high aspect ratio. Reactive ion etching (RIE) lag is a phenomenon in which etch rate depends on the opening areas of patterns, aspect ratio of the trenches and other geometrical factors. The lag not only gives a non-uniform distribution of scallop size, but it also sets a limit for the maximum achievable aspect ratio. The latter since the mask suffers from persistent erosion. While different kinds of hard masks have been suggested to ensure a longer total etch time, here we report a correctly tuned 3-steps Bosch process – called DREM (Deposit, Remove, Etch, Multistep) – without mask erosion. The erosion-free feature is independent of the type of mask. For example, an aspect ratio of more than 50 is achieved for trenches with 1μm linewidths, while no erosion is observed for 360nm thin standard photoresists. The mechanism behind this extraordinary infinite selectivity is mainly due to the depletion of C4F8 deposition species. Furthermore, a linear ramping of the etch step duration is performed to achieve a uniform distribution of scallop sizes along the etch profile. This outperforms the usual broad scallop size distribution when no parameter ramping is performed and improves the straightness of the etch profile considerably, in addition, it allows an easier way to control periodic sidewall shaping. e.g., a sausage-chain-like feature is demonstrated with an almost perfect periodicity.

Highlights

Etching silicon is one of the key technologies in mainstream semiconductor industry and dry plasma etching has become indispensable to transfer patterns anisotropically
By performing a pulsed process in so-called deep reactive ion etching (DRIE; e.g. the Bosch sequence), the sidewall of trenches can be protected during an etch process and the etch depth can be significant with minimal undercut
This means that the selectivity drops drastically when the aspect ratio increases and the etching should be halted before the mask is totally eroded away

Summary

Introduction

Etching silicon is one of the key technologies in mainstream semiconductor industry and dry plasma etching has become indispensable to transfer patterns anisotropically (i.e. directionally). The main reason is that, when etch depth increases, the silicon etch rate slows down due to the notorious reactive ion etching (RIE) lag [1], while the mask erosion continues at the same speed. This means that the selectivity (etch rate of silicon divided by the erosion rate of mask) drops drastically when the aspect ratio increases and the etching should be halted before the mask is totally eroded away. The transfer of resist patterns into the hard mask will increase fabrication complexity

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