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.
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.
Chemical properties
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 |
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 (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 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
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.