Etch protection questions and answers
One of our recent articles discussed the creation of through-silicon vias using ProTEK® PSB photosensitive etch protection material. In this post, we will follow up by addressing a few of the frequently asked questions regarding our etch protection materials.
What etchants can these materials withstand?
Three classes of Brewer Science® ProTEK® materials are available to provide protection against alkaline and acid etchants as well as Bosch deep reactive ion etching (DRIE) processes. ProTEK® B3 and ProTEK® PSB coatings are designed to protect against alkaline etchants. ProTEK® A3 and ProTEK® PSA materials are designed to protect against acid etchants. ProTEK® SR scratch-resistant coating resists the etch gases used in the Bosch DRIE process.
ALKALINE: ProTEK® B3 and ProTEK® PSB coatings withstand KOH, TMAH, NaOH, and NH4OH at elevated temperatures as well as some acids such as hydrochloric, sulfuric, and phosphoric acids and buffered oxide etchant (BOE) at room temperature.
ACID: BSI.P09047 and BSI.P09022 coatings are resistant to acid etchants such as nitric, liquid hydrofluoric, acetic, and hydrochloric acids as well as BOE and HNA (which is HF, HNO3, and acetic acid in a ratio of 1:2:3.2). While these materials are also resistant to alkaline solutions, they may lose adhesion to the wafer during alkaline etching.
OTHER: ProTEK® SR material is resistant to some acid etchants such as hydrochloric, sulfuric, and phosphoric acids and BOE at room temperature. It is also resistant to the fluorinated etch gases used in the Bosch DRIE process such as SF6 and C4F8.
Do these materials require a primer?
Whether or not a primer is required to use ProTEK® coatings depends on the process. Coatings used during a long alkaline etch must have a primer. Without a primer, the coatings would eventually peel off the substrate during etching. The other materials do not typically need a primer, but the need for a primer does depend on the specific application.
For applications in which a wafer is coated with an acid-resistant coating and a short alkaline etch is needed before the acid etch, a primer is not normally needed. The necessity for a primer will depend on how short the alkaline exposure is and how aggressive the etch is in terms of bath temperature and concentration.
A primer is required if BSI.P09022 coating will be used as an etch mask for liquid HF etching of glass. A single primer for this application has not yet been named, but customers have demonstrated success using a chrome-nickel or chrome-gold seed layer.
How thick must these films be to resist the wet etchant?
A thick film is not required for alkaline etch protection. As long as a continuous film is applied, an alkaline etch solution will not penetrate the coating. However, a thicker coating may be required when working with acids. The thickness will be determined by the etchant, the etchant concentration, and the etch time.
Visit our MEMS FAQs page for answers to other commonly asked questions.