Type	Materials	Restricted Materials	Available Gases	Max RF Power	Wafer Size

Type	Materials	Restricted Materials	Available Gases	Max RF Power	Wafer Size
Wet Etching	?	?	NA	NA	?

Overview

Wet chemical etching is the preferred method for removing material when relatively low cost,

Advantages:

Simple equipment
No additional training required
Few parameters to control
High Throughput
High Selectivity

Disadvantages:

Difficult to control etch depth precisely (especially if etch rate is very high)
Chemical and disposal costs are high
Heated chemicals create adhesion problems with photoresist
Small features can suffer from blocking due to surface tension
Fully isotropic etch creates undercut

Common Problems

Materials

Aluminum (Al)

Aluminum is etched by many of the commonly used chemicals in the cleanroom, such as NaOH (AZ 351), KOH (AZ 400K), and TMAH (MF-26 A) developers at a rate around 50-100 nm/min. To avoid aluminum attack, use AZ Developer.

Intentional etching of aluminum is usually accomplished by a commercial mixture.

References:

Aluminum Etching - MicroChemicals (2013)

Aluminum Oxide (Al₂O₃ or AlOx)

ALD grown aluminum oxide (which is AlOx, but not a stoichiometric Al₂O₃ without significant tuning and annealing) will be removed in BOE. Sapphire (high quality crystalline Al₂O₃) will not etch in BOE.

Chromium Etching

Chrome withstands a wide variety of acids and bases. Etching is accomplished with commercial mixtures based on either perchloric acid (HCLO₄) or nitric acid (HNO₃).

BNC supplies CR-16, a Nitric acid based solution. SDSs for other mixtures cited commonly are seen below.

Note that for CR-16, thin Al films show no/minimal attack during chrome etching.

References:

Chromium Etching - MicroChemicals (2013)

CR-16 SDS

CR-7 SDS

CR-14 SDS

Gold (Au)

Gold can be etched in aqua regia (1:3 HNO₃:HCl) or a more Au selective iodine solution (KI and I₂).

References:

Gold Etching - MicroChemicals (2013)

Titanium (Ti)

Removed in BOE or heated (>65 C) HCl.

Silicon (Si)

Silicon may be etched in TMAH (Shikida 2001), with the etch rate depending strongly on the crystal plane. A standard etch would be 25% TMAH (supplied in the cleanroom) at 70 C.

References:

Wet Chemical Etching of Silicon - MicroChemicals (2013)

Silicon Etching in Microelectronics: A General Overview - SACHEM (2013)

Wet-Chemical Etching and Cleaning of Silicon - Virginia Semiconductor (2003)

Silicon (SiN)

Silicon nitride is removed in both HF and BOE (Knotter 2000), however these are not typically mentioned as etchants in literature due to the lack of selectivity between SiN and oxide in these environments.

Etchants

Potassium Hydroxide (KOH)

KOH is used as an anisotropic etchant of silicon, etching the <100> plane much faster than the <111> plane.

References:

KOH Etching of Silicon (UAlberta Nanofab)

KOH etching of (100) Si wafer No 1 (SEAS UPenn)

Tetramethylammonium Hydroxide (TMAH)

TMAH can be used as an anisotropic etchant of silicon, etching the <100> plane much faster than the <111> plane. It offers a metal free etch, as opposed to a KOH bath, and is available as a liquid in the cleanroon. Either nitride or oxide can be used as a mask, but photoresist will dissolve almost instantly (metal free developers are TMAH based).

References:

"Microfabrication Using Bulk Wet Etching With TMAH", Xuefeng Duan MS Thesis, McGill University

"KOH and TMAH etching of bulk silicon," K. Westra, UAlberta Nanofab.

Hydrofluoric Acid (HF)

HF is used to etch oxides, and in particular silicon dioxide.

References:

Etching with Hydrofluoric Acid - MicroChemicals (2013)

Buffered Oxide Etch (BOE)

BOE is a mixture of ammonium fluoride (NH₄F) and hydrofluoric acid (HF). The buffered HF solution gives a more stable etch rate and greater uniformity across a sample. Additionally, the higher pH of ~3-4.5 is less likely to peel/damage photoresist etching masks (Reinhardt 2008, p. 23). The etch rate of oxide is also increased due to formation of HF₂^-ions, which etches 4-5x faster than F^- ions. BOE has a relatively stable concentration of both HF₂^- and F^-, while 49% HF solution etches solely through the F^-.ion and has a less stable concentration of this ion.

Note: BOE or HF etching should be done in a plastic container (HDPE is best for any long term HF soaks, as Teflon may absorb flurorine and give insonsistent etch results in the future). BOE/HF etch glass, and the aluminum oxide present in borosilicate glass will form an insoluble product that will adhere to the wafer surface. Borosilicate glass contains 4% sodium, which can contaminate the wafer surface. The etch rate of borosilicate glass in pure HF is also much faster than silicon dioxide (~8000 nm/min vs ~23 nm/min) (Iliescu 2005).