Citric Acid Etching

Written by NTS, Dec 2012. Modified NTS 20130920, NTS 20140409, NTS 20160804, NTS20170922.

The basic idea is to have patterned III-V samples (before acid prep), prepare the citric acid solution, and perform the etch. There are two acid solutions differentiated by citric acid solution and Etch solution.

Prepare Citric Acid Solution

• Prepare samples (litho) if not already prepared

• Clean workspace and set out equipment

• Weigh citric acid crystals (III-V bay scale preferred, Escali scale in metals box OK)

  • Record weighing beaker weight, and tare the scale

  • Pour crystals into a weighing dish first then carefully into the weighing beaker

  • Record target and actual crystal weight (aim for w/in 0.5%). Do not reset scale

• Measure and record target and actual DI H2O volume in H2O2 graduated cylinder

• Dissolve citric acid crystals in DI (ratio 1 g : 1 ml) in etch beaker (citric acid solution)

  • A stir bar will be used to assist the mixing of the crystals into the DI H2O

  • Cover the etch beaker, put in foil boat on hot plate, turn on stir bar slowly to 350 rpm

  • Move beaker around to make sure all crystals clumps (at edge) are mixed in

  • Stir til done and clear, or at least 20 minutes

• Record weighing beaker weight for leftover crystals (will be some due to stickiness)

• Add more crystals if necessary, or acid trash the extra (in the weighing dish)

• Record volume of dissolved citric acid crystal and DI water solution (measure CAS)

• Later, Add desired volumetric ratio X of H2O2 (30-31%) ~16 minutes before etching

Lithography

• See lithography procedure. Should be done before making Etch solution.

  • You can use any ‘spare’ mask with features. I recommend Laser 2011 Mask 1 and 2. Do not use Laser 2013 masks.

Etch

• Add desired volumetric ratio X of H2O2 (30-31%) ~16 minutes before etching, e.g. 2 mg : 2 ml citric acid solution to 1 ml of H2O2 (X = 2). (Etch solution)

  • Use old Citric acid + H2O2 beaker in general acid bin to catch extra H2O2

• Triple rinse any used H2O2 equipment before putting away

• Prepare acid bench for etching, make sure it’s clean, safe, and has some wipes set up

• Set out Etch solution and prepare at ~20C

• Etch for Y minutes

• DI rinse 2 minutes and blow dry with N2

• Repeat as needed

• Remove PR using Kwikstrip heated to ~90 C

• Dektak to determine etch rate

  • Dektak several times at multiple points of a feature to find averages and variances

  • Record the sample ID and dish location, Etch # and time, mask type and orientation

  • Record the etch depth, the target feature width, actual horizontal width of the top and bottom of features, location on sample, and general notes about the sample and scan

  • Describe or draw sketches of the scan, esp. if it has noteworthy features

• Record data in notebook and later put into spreadsheet.