Silicon Nitride Etching Baths -Nb Series

The Silicon Nitride Etching Process

The silicon nitride etching process is used in the manufacture of integrated circuits to selectively remove silicon nitride from the surface of silicon wafers. Silicon nitride is used as a mask on silicon wafers and further manufacturing process steps require that the silicon nitride be removed. A solution that contains phosphoric acid and de-ionized water etches the silicon nitride with a minimal oxide etch while leaving the rest of the wafer unaffected. As a result, the silicon nitride is effectively stripped from the wafer surface so additional fabrication steps can be done on a clean wafer.

How the Process is Improved

A solution of 85% phosphoric acid and 15% de-ionized water is heated to a boil at about 160 degrees centigrade. In order to improve the silicon nitride wet etching process, the phosphoric acid solution needs to be maintained at high temperatures. This means some of the de-ionized water will boil off as steam and will need to be replaced. Adding de-ionized water directly to phosphoric acid is extremely dangerous and may result in an explosion. Keeping the solution at a constant temperature is also important, which provides precise control of the stripping process. The safety and control of the nitride etching process have been improved and optimized with Modutek’s Nb Series silicon nitride etching baths.

Modutek’s Silicon Nitride Etching Bath

Modutek’s Nb Series silicon nitride wet etching bath is engineered to provide unparalleled process control, safety, and flexibility. This is accomplished by creating a two-tiered diagnostic system that monitors the temperature while maintaining the acid-to-water ratio through de-ionized water supplementation. With our new remote metering system, the Nb series bath is easier to install and service in new or existing process stations.

Controlling the Etching Process Safely and Accurately

The two main challenges faced when operating a bath of boiling phosphoric is the safe replacement of the de-ionized water that is lost to steam and accurate control of the process. If water is added to the solution too quickly, the solution will stop boiling and a thin film of water may form on top of the viscous acid. If the film of water suddenly mixes with the acid, a strong reaction can occur, which can produce an explosion. Adding water using this method also makes accurate control of the etching process difficult.

Within Modutek’s Nb Series bath, the heater that boils the solution is always on. The temperature of the solution is always maintained at the boiling point and any further heating will only increase the boiling rate but not the temperature. This control strategy will ensure a clearly defined temperature for the boiling solution, which will always be maintained at its boiling point.

The boiling point of the phosphoric acid and water solution will vary depending on the concentration, increasing as the solution loses water to steam. A built-in thermocouple within the bath senses the increasing temperature and provides a signal for the system to add water. De-ionized water is then slowly added to the boiling solution. Since the acid is boiling rapidly, the small amounts of water that are added will immediately mix in and no surface film of water will form. Adding water using this procedure is safe and the process is carefully controlled since monitoring the concentration from the temperature rise is accurate.

Additional Safety Features Built into Modutek’s Nb Series Bath

Since the silicon nitride wet etching process depends on maintaining the solution at the boiling point, additional safety interlocks within Modutek’s Nb series etching bath are used to ensure no water is added whenever the solution is not boiling. A thermocouple located above the solution senses the presence of hot steam (from the de-ionized water) and will shut off the water valve if steam is not present. In order to ensure that the solution does not overheat, another thermocouple is used to switch off the built-in heaters if the temperature of the solution rises to 170 degrees centigrade. With this advanced process control system and the additional safety interlocks, Modutek’s Nb Series bath improves the silicon nitride etching process with precise control, accuracy as well as increased safety.

More Information

• Nb SERIES THEORY OF OPERATION

Benefits Include:

• Uniformity of Nitride Etch
• Process control of Nitride Etching
• Controls chemical boiling (no “bumping”)
• Remote DI water metering system

Features Include:

• 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

System 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

Options:

• Auto cover (pneumatically actuated)
• Aspirator valve system
• Teflon® gravity drain system
• Remote operation timer switches
• Model C1815a process controller/timer
• RS232 Interface

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

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.