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.

Before doing any chemical process related to KOH you are obligated to read and understand the current wiki page as well as the relevant Risk Assessment.

Properties and guidelines

Physical properties

	Pellets	25-50% w/w solution
Description	White or yellowish very hygroscopic pellets, flakes	Liquid
Molecular weight	56.11 g/mol	56.1056 g/mol
Density	2.044 g/cm³	1.4 g/cm³
pH	ca 13.5 (5.6g/L)	>13
Melting point/freezing point	361°C
Boiling point	1327°C	100°C

Incompatibility

Incompatible with a.o. strong oxidizing agents, acids (violent heat generation) acid chlorides, acid anhydrides, ketones, peroxides, water (strong heat generation and can boil violently) metals while releasing hydrogen which forms explosive mixtures with air. Incompatible with animal/plant tissues, glass, plastics. Harmful to aquatic life. Absorbs carbon dioxide from the air. Do not heat above boiling point.

Storage

Must be stored tightly sealed in a well-ventilated chemical cabinet. Should not be stored higher than 160 cm due to the risk of corrosion.

Dangers and Toxicological properties

Dangers

Pellets

25-50% w/w solution

H290: May be corrosive to metals. H302: Harmful if swallowed. H314: Causes severe skin burns and eye damage. P280: Wear protective gloves/eye protection/face protection. P301+312: IF SWALLOWED: Call a POISON CENTRE/doctor/ if you feel unwell. P305+351+338: IF IN EYES: Rinse cautiously with water for several minutes. Remove contact lenses, if present and easy to do. Continue rinsing. P406: Store in a corrosion-resistant/ container with a resistant inner liner.

H314: Causes severe skin burns and eye damage. H302: Harmful if swallowed. P260: Do not breathe dust/fume/gas/mist/vapours/spray. P280: Wear protective gloves/eye protection/face protection. P305+351+338: IF IN EYES: Rinse cautiously with water for several minutes. Remove contact lenses, if present and easy to do. Continue rinsing. P301+312: IF SWALLOWED: Call a POISON CENTRE/doctor/ if you feel unwell.

Toxicological properties

Pellets	25-50% w/w solution
Inhalation: No information available Inhalation-long: No information available Skin-acute: Causes burns Skin-long: May cause allergic skin reaction Eyes: Causes burns. Ingestion: Harmful if swallowed and corrosive to the mucosa of the gastrointestinal tract.	Inhalation: Irritation. May be corrosive to the mucous membranes in the upper respiratory tract. Inhalation-long: No information available. Skin-acute: Causes burns. Skin-long: May cause allergic skin reaction. Eyes: Causes burns. Ingestion: Harmful if swallowed.

Standard Operating Procedure

Mixing KOH pellets and etching

• 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 your equipment
  • Turn on MQ 30-minute timer. Flush the MQ for 30 seconds
  • 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 ratio.
  • The KOH pellets come in units of xxx grams. Start by weighing an appropriate amount of KOH and set 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.

  Do not leave the solution unattended.

• Rinse the samples in MQ water.
• Rinse the samples in a second MQ water.
• Dry or continue with the rest of your process.
• Let the KOH solution to cool down to room temperature.
• Transfer the KOH solution to the X-base waste container.
• The MQ water that was used for rinsing can be discarded in the drain.
• Ensure that all the setup that you used (scale, hotbath, beakers) is back in their original location.

Etch rates

KOH-etching is a highly versatile and cheap way to realize micromechanical structures using Si3N4 or SiO2 masking materials.

	Temperature	Etch rate
Si (001)	60°C	0.4 μm/min
	70°C	0.7 μm/min
	80°C	1.3 μm/min
Si (110)	80°C	2.5 μm/min
Thermal SiOx	60°C	1.2 nm/min
	80°C	7.5 nm/min

First aid measures

Inhalation

The person should be taken into the fresh air, kept quiet and under supervision. If there is a risk of loss of consciousness the person should be placed in the recovery position and kept warm. If the person stops breathing, artificial respiration should be given.

Skin

Rinse thoroughly with water, remove contaminated clothing and any jewellery; caustic burns may need to be rinsed for up to 24 hours.

Eyes

Rinse with water immediately. Hold the eye wide open. Remove contact lenses. Continue to rinse with water until treatment is taken over by doctors.

If swallowed

Remove any visible particles from in and around the mouth. Immediately rinse the mouth out and drink water or milk. Do not give liquids to the unconscious. Do not cause vomiting.

If discomfort persists consult a doctor, taking along these instructions.