Nested potassium hydroxide etching and protective coatings for silicon-based microreactors

We have developed a multilayer, multichannel microfabricated reactor that uses elemental fluorine as a reagent and generates hydrogen fluoride as a byproduct. Nested potassium hydroxide etching (using silicon nitride and silicon oxide as masking materials) was developed to create a large number of channels (60 reaction channels connected to individual gas and liquid distributors) of significantly different depths (50–650 m) with sloped walls (54.7o with respect to the (100) wafer surface) and precise control over their geometry. The wetted areas were coated with thermally grown silicon oxide and electron-beam evaporated nickel films to protect them from the corrosive fluorination environment. Up to four Pyrex layers were anodically bonded to three silicon layers in a total of six bonding steps to cap the microchannels and stack the reaction layers. The average pinhole density in as-evaporated films was 3 holes/cm2. Heating during anodic bonding (up to 350 C for 4 min)did not significantly alter the film composition. Upon fluorine exposure, nickel films (160 nm thick) deposited on an adhesion layer of Cr (10 nm) over an oxidized silicon substrate (up to 500 nm thick SiO2) led to the formation of a nickel  Biologics Process Development, Manufacturing Sciences & Technology, Bristol-Myers Squibb Company, 38 Jackson Road, Devens, MA 01434, United States (current affiliation).  Corresponding author. E-mail: [email protected]. Fax: (617) 258-8992. 2 fluoride passivation layer. This microreactor was used to investigate direct fluorinations at room temperature over several hours without visible signs of film erosion.