Silicon Nitride Etching Baths
Silicon Nitride Etching Process
The silicon nitride etching process is used in integrated circuit manufacturing to selectively remove silicon nitride from silicon wafer surfaces. Silicon nitride is used as a mask on silicon wafers, and later manufacturing steps require its removal. A solution containing phosphoric acid and deionized water etches the silicon nitride with minimal oxide etch while leaving the rest of the wafer unaffected. As a result, the silicon nitride is effectively stripped from the wafer surface, allowing additional fabrication steps on a clean wafer.
How the Process is Improved
A solution of 85% phosphoric acid and 15% deionized water is heated to a boil at about 165 degrees centigrade. To improve the silicon nitride wet etching process, the phosphoric acid solution must be maintained at high temperatures. This means some deionized water will boil off as steam and must be replaced. Adding deionized water directly to phosphoric acid is extremely dangerous and can cause an explosion. Keeping the solution at a constant temperature is also important, as it provides precise control of the stripping process. Modutek’s Nb Series silicon nitride etching baths improve and optimize safety and control in the nitride etching process.
How Modutek's Nb Series Controls It
Controlling the Etching Process Safely and Accurately
The two main challenges when operating a boiling phosphoric acid bath are safely replacing the de-ionized water lost to steam and accurately controlling the process. Adding water too quickly can trigger the same surface-film reaction described above, so the Nb Series eliminates manual dosing.
Within Modutek’s Nb Series bath, the heater that boils the solution is always on. The solution temperature is always maintained at the boiling point, and any further heating only increases the boiling rate, not the temperature. This control strategy ensures a clearly defined temperature for the boiling solution, which is always maintained at its boiling point.
The boiling point of the phosphoric acid and water solution varies with concentration, increasing as the solution loses water to steam. A built-in thermocouple in the bath senses the rising temperature and signals the system to add water. The system then slowly adds de-ionized water to the boiling solution. Because the acid is boiling rapidly, the small amounts of water added mix immediately, and no surface film of water forms. Adding water this way is safe, and the process is carefully controlled because monitoring the concentration via the temperature rise is accurate.
Additional Safety Features Built into Modutek’s Nb Series Bath
Because the silicon nitride wet etching process depends on maintaining the solution at the boiling point, Modutek’s Nb series etching bath uses additional safety interlocks to ensure no water is added when the solution is not boiling. A thermocouple above the solution senses the presence of hot steam (from the deionized water) and shuts off the water valve if steam is not present. To prevent overheating, another thermocouple switches off the built-in heaters if the solution temperature rises to 170 degrees centigrade. With this advanced process control system and additional safety interlocks, Modutek’s Nb Series bath improves silicon nitride etching with precise control, accuracy, and increased safety.
More Information
Benefits
- Uniformity of Nitride Etch
- Process control of Nitride Etching
- Controls chemical boiling (no “bumping”)
- Remote DI water metering system
Features
- Vapor thermocouple
- Etched-foil resistance heater
- Semiconductor-grade quartz process vessel
- PVDF or Teflon refluxor with safety cover
- Ease of installation and repair of new existing wet station configuration
- Uniform heat transfer
Specifications
- Application: silicon nitride etch
- Chemical usage: H20, H3PO4
- Operating temperature range: 30 – 180º C
- Process temperature control: ± 0.5º C
- Heat-up rate: 2-3º C per minute (rate affected by the size of system)
- Vapor thermocouple for chemical boiling control
- PVDF refluxor with Teflon® cooling coils
- One year warranty
Facility Requirements
- Electrical: 208 VAC, 50/60 Hz, 1 Ø, 15 amps
- Nitrogen: 15 PSI, 0.25 cfm
- D.I. water: 10-15 psi, 0.5 gpm
- City water (or chilled water): 10-15 psi, 2 gpm
Construction Materials
- Process Vessel: Semiconductor Grade Quartz
- Housing (plastic enclosure): flame retardant stress-relieved polypropylene UL-94 V-O rated
- Process thermocouple: J-type Teflon® encapsulated
Options
- Auto cover (pneumatically actuated)
- Aspirator valve system
- Teflon® gravity drain system
- Remote operation timer switches
- Model C1815a process controller/timer
- RS232 Interface
Nb Series – Dimensions
- Tolerance ± .06 (1.5)
Model nomenclature: model code, number of cassettes, and size of cassettes. Eg, Nb26 (Nb series 2-6″)
1 Actual vessel opening is less than 0.25″ to accommodate the refluxor.
2 Vessel depth does not include the refluxor.
3 Measure with refluxor lid open.
4 Measured without drain plumbing.
Silicon Nitride Etching Bath FAQs
There’s no fixed interval. The replenishment rate depends on the boil rate, tank size, and how continuously the bath runs, since steam loss drives the need for makeup water, not a schedule. Modutek’s Nb Series tracks this through the boiling point control system rather than a manual timer, which is what the remote DI water metering feature is built around.
Request a Quote for Your Silicon Nitride Etching Bath
Modutek designs and builds its Nb Series silicon nitride etching baths in-house at its San Jose, California facility, with safety interlocks, temperature control, and DI water metering built around the specific demands of running boiling phosphoric acid in a production environment.
For help specifying an Nb Series bath for a new or existing wet station, contact us and a member of the Modutek team will follow up.