Working with KOH

KOH belongs to the family of anisotropic Si-etchants based on aqueous alkaline solutions. The anisotropy stems from the different etch rates in different crystal directions. The {111}-planes are almost inert whereas the etch rates of e.g. {100}- and {110}-planes are several orders of magnitude faster.

At NBI cleanroom, we use as standard a 28% w/w KOH solution. The etch rate and the selectivity towards a SiO2 mask depends on the temperature. We normally use a temperature of 80°C but it may reduce it to, e.g., 60 °C or 70°C for a higher-precision timed etch.

Si(001) temperature	Etch rate (μm/min)
60°C	0.4
70°C	0.7
80°C	1.3
Si(110) temperature	Etch rate (μm/min)
80°C	2.5
Thermal SiOx temperature	Etch rate (μm/min)
60°C	0.0012
80°C	0.0075

Standard Operating Procedure

Mixing KOH (with pellets)

• Book the HF fumehood in CR1.

It is recommended to use a setup where you can run your process undisturbed

• Gather all necessary materials and equipment:
  • IKA hotplate with 2L beaker
  • Scale from to shelves
  • Beakers for measuring and mixing
  • KOH pellets
• Set up equipment
  • Fill 2L beaker with water to create a hotbath and add a magnetic stirring pill
  • Balance the scale
• Calculate the portions of water and solid KOH pellets that you would like to mix:
  • You are aiming to make a 28% w/w solution

There is no shame on using an online calculator to figure out the ratios.

• • The KOH pellets come in units of xxx grams. Start by weighing an appropriate amount of KOH and store it to the side.
  • Calculate the respective amount of water and fill a beaker with it.

• Slowly add the KOH pellets to the water a few at a time while stirring gently.
• Continue stirring until the pellets dissolve completely
• Place your solution in the hotbath
• Place a second beaker with MQ next to it
• Submerge the probe heater into the MQ beaker
• Start the stirring and set the temperature.
• Once you reach the desired temperature, you can start etching your sample