Safely Controlling the Silicon Nitride Etching Process

Safely controlling the Silicon Nitride Etching ProcessSilicon nitride etch removes silicon nitride from silicon wafers during the fabrication process of semiconductor components. A solution of phosphoric acid in water etches silicon nitride rapidly and consistently as long as the temperature of the solution and the concentration of phosphoric acid are kept constant. Maintaining consistent process conditions during the silicon nitride wet etching process is difficult because adding water to phosphoric acid can result in an energetic explosive reaction. The accurate monitoring of the solution is extremely important for safe control of the process.

The Silicon Nitride Wet Etching Process

Silicon nitride is used as a mask to produce micro-structures and connections in semiconductor manufacturing. In most etching applications, the etch rate can be varied by changing the temperature or chemical concentration, but silicon nitride etch is best controlled at its boiling point and at a concentration of 85 percent phosphoric acid in a de-ionized water solution.

The phosphoric acid etching solution is a viscous liquid that is heated until it boils at about 160 degrees centigrade. The high temperature means some of the water will boil off and be lost as steam, increasing the acid concentration of the remaining liquid. As the concentration increases, the boiling point of the solution rises and water has to be added to keep the process variables constant.

The addition of water to the solution is dangerous because, if too much water is added at once, the solution stops boiling and the added water collects as a film above the viscous acid. As the temperature rises again and the acid starts boiling, the large quantity of water from the film mixes with the acid and may cause an explosive reaction.

Instead, the control system has to ensure that only small amounts of water are added at a time and these small amounts are immediately mixed with the remaining acid solution. Such a control strategy results in constant process characteristics, a safe operation and a high quality output.

How Modutek’s Nb Series Wet Etching Baths Ensure Safe Operation

Modutek has developed a bath control system that combines consistency with safe operation. For the Nb series etching baths, the phosphoric acid solution is kept boiling with a constantly-on heater that maintains the solution at its boiling point. As water evaporates and the acid concentration rises, the boiling point increases and the solution temperature goes up. The temperature rise is detected by a thermocouple and a small amount of water is added to the solution to bring the concentration back down.

Because the solution is constantly boiling, the small amount of water is immediately mixed in with the rest of the acid. The amount of water is too little to stop the solution from boiling and the heater is powerful enough to always maintain a vigorous boiling condition. To ensure that water is added only when the solution is boiling, a second thermocouple senses the presence of steam above the bath liquid and blocks the addition of water when no steam is present. A third thermocouple monitors the bath temperature to switch off the heater if the liquid overheats.

The Benefits of the Modutek Silicon Nitride Etch Bath

The advanced control system of Modutek’s Nb series baths allows semiconductor manufacturers and research labs to safely implement the silicon nitride wet etching process to achieve optimum consistency characteristics. The temperature and concentration of the etching bath remain within tight limits due to the two-level control, monitoring the temperature to correct the acid concentration.

Modutek offers the Nb series baths in their fully automatic, semi-automatic and manual wet bench stations as part of its complete line of wet process equipment. The company constantly works with customers to continuously develop improvements in wet process technology. Contact Modutek for a free consultation to discuss your specific process requirements.

How Quick Dump Rinsers Improve Silicon Wet Etching Results

Manufacturing lines in semiconductor fabrication facilities and research labs use corrosive chemicals to clean and etch silicon wafers. Depending on the semiconductor product, a silicon wafer may undergo multiple steps in baths containing chemicals such as hydrochloric acid or hydrogen peroxide. When the cleaning or etching process in a particular fabrication step is complete, the chemicals must be rinsed from the wafer before the silicon wafer can be processed further. Quick Dump Rinsers provide a quick and effective way of thoroughly rinsing the wafers without introducing new contaminants.

How Quick Dump Rinsers Work

Once wafers are placed into the rinser, powerful jets spray de-ionized water over the wafers to remove all traces of chemicals. As the rinse tank fills up, chemical residue and contaminating particles are flushed out and rise to the surface. A nitrogen gas bubbler system serves to agitate the de-ionized water further, removing additional contaminants from the surface of the wafers. An overflow weir allows the surface water to flush the chemicals and particles out of the tank. When the rinsing process is complete, the quick dump door at the bottom of the tank opens and the water drains out in a matter of seconds.

The key characteristics of a Quick Dump Rinser are rapid operation, complete removal of contaminants and avoiding the addition of new contamination. Rapid operation helps reduce the use of de-ionized water and saves process time. If traces of chemicals are not completely removed, etching of the wafer might continue and result in defective or low quality products. The same is true if particles are not eliminated or are added during the rinse. Since rinsing has to be carried out at the end of many process steps, Rinser performance is critical for product output quality.

Modutek’s DR Series Quick Dump Rinser Features

Modutek’s Quick Dumps Rinsers are designed to meet the needs of advanced semiconductor fabrication. Wet process semiconductor manufacturing facilities and research labs can use the Modutek rinsers as stand-alone units or integrated into a wet process manufacturing line. The rinsers work quickly and deliver completely clean wafers. They incorporate the following features:

  • Contoured vessel design
  • Nitrogen bubbler on all models
  • Natural polypropylene or PVDF (option) tanks, nozzles and fittings
  • Large machined dump door without gaskets or seals
  • 360 degree overflow weir

Modutek’s design minimizes de-ionized water consumption and reduces rinse times while ensuring effective rinsing. The tanks require a de-ionized water supply, a source of pressurized nitrogen and a 120 or 24 V AC power supply. Options include a reclaim system, Teflon nozzles, fittings and valves, and special process cover configurations. The rinsers are available in a variety of sizes.

How Modutek’s DR Series Rinsers Improve Silicon Etching Results

Semiconductor manufacturing makes use of ever smaller microstructures and circuit connections. During wet etching of the silicon wafers, even a tiny particle can block a circuit path or deform microstructure design. Modutek’s rinsers are designed and constructed to avoid particle contamination.

The contoured vessel and machined trap door avoid entrapment of particles in corners or in cracks around seals and gaskets. The nitrogen bubbler helps dislodge any particles remaining on the surfaces of the silicon wafers. The 360 degree overflow weir gets rid of contaminants in the surface water quickly and the polypropylene material avoids the addition of metallic particles from fittings.

Modutek’s DR Series Quick Dump Rinsers are state-of-the-art process rinsing modules that work quickly and rinse completely. Used together with wet bench stations from Modutek’s extensive line of silicon wet etching equipment they can deliver exceptional performance for semiconductor manufacturing facilities and research labs. Contact Modutek for a free consultation on selecting the best equipment that will meet your manufacturing requirements.