fabrication | featured | news

Bosch polymer removal comparison

Aaron Hryciw

2020-05-13

What is the best technique to remove sidewall polymer after DRIE?

fabrication | featured | news

Deep reactive ion etching (DRIE) of silicon via the Bosch process is well known for its ability to produce features with vertical sidewalls and high aspect ratios, due to the protection of vertical surfaces etched during each cycle by a Teflon-like fluoropolymer. While essential to achieving the highly anisotropic etch, however, this Bosch sidewall polymer is infamously difficult to remove completely, as it is largely chemically inert and adheres very well to the underlying Si. Typical removal techniques include oxygen plasma ashing, wet chemical processing (piranha, hydroxylamine-based strippers, RCA clean), high-temperature anneals, or combinations of these processes.

Bosch etch process. The isotropic etch and fluoropolymer steps are repeated in each cycle.

Part of the difficulty lies in determining conclusively whether the polymer has indeed been removed after any stripping process: as it coats the sidewalls with high conformality and is typically very thin (<100 nm), the polymer is nearly indistinguishable from a bare Si sidewall when imaged via SEM. Furthermore, the polymer coating the photoresist sidewalls generally tears off during the resist strip process, such that there is no tell-tale “crown” of polymer surrounding the etched features, even if polymer still remains on the Si sidewalls.

To overcome this issue and make it plain whether any sidewall polymer remains after various removal techniques, we developed the following process flow:

Initial substrates: 150 mm diameter Si wafers coated with ~3.5 µm AZ 1529 photoresist.
DRIE: Perform 100 cycles of the Bosch High Rate recipe on all wafers, using the Oxford Estrelas ICPRIE.
Strip: Cleave wafers into quarters, then strip resist and attempt to remove sidewall polymer using one of the following methods:
- Branson Barrel Etcher – 30 min O₂ plasma
- Branson – 60 min O₂ plasma
- Branson – 30 min O₂ plasma + Middle Minibrute Furnace – 1 hr anneal at 700 °C in flowing N₂
- Branson – 30 min O₂ plasma + 30 min soak in ECK265 (heated to 60 °C)
- Trion RIE – 30 min O2 stripping recipe
- Oxford NGP80 RIE – 30 min Descum high recipe
- NGP80 – 30 min Descum PE mode recipe
- Piranha – 30 min in self-heated 3:1 H₂SO₄:H₂O₂
DRIE: Perform 50 cycles of the Bosch Smooth Sidewalls recipe (Oxford Estrelas), with wafer quarters mounted to 150 mm carriers using Crystalbond 555HMP.
N.B. If any sidewall polymer remains after the stripping step, it will be exposed by this second etch, making it easy to image via SEM.

Bosch polymer removal testing process flow

SEM imaging after the second DRIE step was done using the Zeiss Sigma FESEM:

Branson (30 min) - 10 µm posts

Stripping process: Branson Barrel Etcher - 30 min O₂ plasma Structure: 10 µm posts — **Stripping process:** Branson Barrel Etcher - 30 min O₂ plasma
**Structure:** 10 µm posts

Branson (30 min) - 10 µm vias

Stripping process: Branson Barrel Etcher - 30 min O₂ plasma Structure: 10 µm vias — **Stripping process:** Branson Barrel Etcher - 30 min O₂ plasma
**Structure:** 10 µm vias

Branson (30 min) - ovals

Stripping process: Branson Barrel Etcher - 30 min O₂ plasma Structure: oval trenches — **Stripping process:** Branson Barrel Etcher - 30 min O₂ plasma
**Structure:** oval trenches

Branson (60 min) - 10 µm posts

Stripping process: Branson Barrel Etcher - 60 min O₂ plasma Structure: 10 µm posts — **Stripping process:** Branson Barrel Etcher - 60 min O₂ plasma
**Structure:** 10 µm posts

Branson (60 min) - 10 µm vias

Stripping process: Branson Barrel Etcher - 60 min O₂ plasma Structure: 10 µm vias — **Stripping process:** Branson Barrel Etcher - 60 min O₂ plasma
**Structure:** 10 µm vias

Branson (60 min) - ovals

Stripping process: Branson Barrel Etcher - 60 min O₂ plasma Structure: oval trenches — **Stripping process:** Branson Barrel Etcher - 60 min O₂ plasma
**Structure:** oval trenches

Branson + EKC - 10 µm posts

Stripping process: Branson Barrel Etcher - 30 min O₂ plasma + 30 min soak in EKC265 Structure: 10 µm posts — **Stripping process:** Branson Barrel Etcher - 30 min O₂ plasma + 30 min soak in EKC265
**Structure:** 10 µm posts

Branson + EKC - 10 µm vias

Stripping process: Branson Barrel Etcher - 30 min O₂ plasma + 30 min soak in EKC265 Structure: 10 µm vias — **Stripping process:** Branson Barrel Etcher - 30 min O₂ plasma + 30 min soak in EKC265
**Structure:** 10 µm vias

Branson + EKC - ovals

Stripping process: Branson Barrel Etcher - 30 min O₂ plasma + 30 min soak in EKC265 Structure: oval trenches — **Stripping process:** Branson Barrel Etcher - 30 min O₂ plasma + 30 min soak in EKC265
**Structure:** oval trenches

Branson + Minibrute - 10 µm posts

Stripping process: Branson Barrel Etcher - 30 min O₂ plasma + 60 min anneal in Minibrute Furnace at 700 °C Structure: 10 µm posts — **Stripping process:** Branson Barrel Etcher - 30 min O₂ plasma + 60 min anneal in Minibrute Furnace at 700 °C
**Structure:** 10 µm posts

Branson + Minibrute - 10 µm vias

Stripping process: Branson Barrel Etcher - 30 min O₂ plasma + 60 min anneal in Minibrute Furnace at 700 °C Structure: 10 µm vias — **Stripping process:** Branson Barrel Etcher - 30 min O₂ plasma + 60 min anneal in Minibrute Furnace at 700 °C
**Structure:** 10 µm vias

Branson + Minibrute - ovals

Stripping process: Branson Barrel Etcher - 30 min O₂ plasma + 60 min anneal in Minibrute Furnace at 700 °C Structure: oval trenches — **Stripping process:** Branson Barrel Etcher - 30 min O₂ plasma + 60 min anneal in Minibrute Furnace at 700 °C
**Structure:** oval trenches

NGP80 (RIE) - 10 µm posts

Stripping process: Oxford NGP80 (RIE) - 30 min Descum high recipe Structure: 10 µm posts — **Stripping process:** Oxford NGP80 (RIE) - 30 min *Descum high* recipe
**Structure:** 10 µm posts

NGP80 (RIE) - 10 µm vias

Stripping process: Oxford NGP80 (RIE) - 30 min Descum high recipe Structure: 10 µm vias — **Stripping process:** Oxford NGP80 (RIE) - 30 min *Descum high* recipe
**Structure:** 10 µm vias

NGP80 (RIE) - ovals

Stripping process: Oxford NGP80 (RIE) - 30 min Descum high recipe Structure: oval trenches — **Stripping process:** Oxford NGP80 (RIE) - 30 min *Descum high* recipe
**Structure:** oval trenches

NGP80 (PE) - 10 µm posts

Stripping process: Oxford NGP80 (PE) - 30 min Descum PE mode recipe Structure: 10 µm posts — **Stripping process:** Oxford NGP80 (PE) - 30 min *Descum PE mode* recipe
**Structure:** 10 µm posts

NGP80 (PE) - 10 µm vias

Stripping process: Oxford NGP80 (PE) - 30 min Descum PE mode recipe Structure: 10 µm vias — **Stripping process:** Oxford NGP80 (PE) - 30 min *Descum PE mode* recipe
**Structure:** 10 µm vias

NGP80 (PE) - ovals

Stripping process: Oxford NGP80 (PE) - 30 min Descum PE mode recipe Structure: oval trenches — **Stripping process:** Oxford NGP80 (PE) - 30 min *Descum PE mode* recipe
**Structure:** oval trenches

Trion RIE - 10 µm posts

Stripping process: Trion (RIE) - 30 min O2 stripping recipe Structure: 10 µm posts — **Stripping process:** Trion (RIE) - 30 min *O2 stripping* recipe
**Structure:** 10 µm posts

Trion RIE - 10 µm vias

Stripping process: Trion (RIE) - 30 min O2 stripping recipe Structure: 10 µm vias — **Stripping process:** Trion (RIE) - 30 min *O2 stripping* recipe
**Structure:** 10 µm vias

Trion RIE - ovals

Stripping process: Trion (RIE) - 30 min O2 stripping recipe Structure: oval trenches — **Stripping process:** Trion (RIE) - 30 min *O2 stripping* recipe
**Structure:** oval trenches

Piranha - 10 µm posts

Stripping process: 30 min in self-heated 3:1 H₂SO₄:H₂O₂ piranha Structure: 10 µm posts — **Stripping process:** 30 min in self-heated 3:1 H₂SO₄:H₂O₂ piranha
**Structure:** 10 µm posts

Piranha - 10 µm vias

Stripping process: 30 min in self-heated 3:1 H₂SO₄:H₂O₂ piranha Structure: 10 µm vias — **Stripping process:** 30 min in self-heated 3:1 H₂SO₄:H₂O₂ piranha
**Structure:** 10 µm vias

Piranha - ovals

Stripping process: 30 min in self-heated 3:1 H₂SO₄:H₂O₂ piranha Structure: oval trenches — **Stripping process:** 30 min in self-heated 3:1 H₂SO₄:H₂O₂ piranha
**Structure:** oval trenches

Based on these SEM images, the sidewall polymer removal techniques are ranked as follows, from best to worst:

Branson + EKC265
Piranha (very similar to Branson + EKC)
Branson 60 min
Branson 30 min
Branson 30 min + Minibrute anneal
NGP80 RIE
Trion RIE (similar to NGP80 RIE)
NGP80 PE

We therefore recommend either a 30-minute piranha, or a 30-minute ash in the Branson Barrel Etcher followed by a 30-minute soak in EKC265, to remove Bosch sidewall polymer most completely. A combination of 30 min Branson followed by 30 min piranha, while not tested in this study, is also highly recommended to remove especially tenacious sidewall polymer.

For more information on this study, as well as general DRIE and plasma etching questions, please contact Scott Munro.