How Specialized Equipment Improves Silicon Wet Etching Process

Results from common wet etching processes, such as KOH and Piranha etch, can be improved with the use of specialized equipment.

When specialized equipment is used in a silicon wet etching process, wafer output can increase, defects can be reduced, and output quality can improve. Equipment such as temperature controls, recirculating baths, and special control strategies can optimize specific processes and positively impact facility importance.

1) KOH (potassium hydroxide) etching is a popular process for applications requiring tight control of the etch rate. The KOH speed depends on the concentration of the bath and the temperature. A base etch rate can be attained by using an appropriate concentration and the rate can then be varied by controlling the temperature.

For KOH etching, Modutek Teflon tanks are suitable as they reduce contamination and feature tight temperature control. The TI static tanks and the TFa recirculating tanks are temperature-controlled models while the TT series tanks operate at ambient temperature.

2) Piranha etching, on the other hand, uses a mixture of sulfuric acid and hydrogen peroxide to quickly remove organic residue, such as photoresist from silicon wafer surface. The mixture has to be handled carefully as it is highly exothermic when first prepared.

Modutek quartz baths are specially suitable tanks for Piranha etching, as they can withstand high temperature and can be heated. The QFa series can withstand the initial temperature spike from mixture preparation and the subsequent heating to operating temperature.

3) Buffered oxide etch uses buffered hydrofluoric acid to etch thin films of silicon dioxide or silicon nitride. The Modutek F-series sub-ambient circulating baths are specially designed for buffered oxide etch as they reduce consumption of acid and filter out contaminating particles. It delivers a safe and reliable process with low-cost operation.

4) Silicon nitride etching composes of phosphoric acid and de-ionized water and requires special control measures. The mixture usually operates at its boiling point of about 160 degrees centigrade. As the etching proceeds, de-ionized water evaporates, leaving a more concentrated acid behind. The high the concentration of the acid leads to the increase of the boiling point and the etch rate.

The concentration of the acid has to remain to the lower level to keep the process working consistently, which means adding water to it. However, doing so risks an explosive reaction when the water mixes with acid. Modutek has a control system that frequently adds small quantities of water to the silicon nitride wet etching baths whenever the temperature rises, keeping the concentration stable. The control system ensures a safe and stable operation.

You can learn more by reading our recent article, How Specialized Equipment Improves Silicon Wet Etching Processes. If you have questions or would like to set up a free consultation to discuss your needs, contact Modutek Corporation via email at sales@modutek.com or by calling 866-803-1533.

Improving Standard Clean Particle Removal in a Wet Bench Process

When the Standard Clean process doesn’t result in improved silicon wafer yields, improved particle removal can be done with advanced processing equipment.

Standard Clean, or RCA Clean, was developed by RCA company in 1965 and it has remained pretty much the same since then. While it has been successful in cleaning wafers and removing most particles, with silicon wafers and structures decreasing in size and increasing product density on wafers, even the tiniest particles can cause device defects.

The submicron particles are hard to remove completely with chemicals or by rinsing. The use of megasonic cleaning technology to generate very high-frequency sound waves in the wafer bath provides the capability to dislodge all remaining particles, allowing them to be rinsed away. Megasonic cleaning improves Standard Clean particle removal process, ensuring defect-free production of microscopic structures on the silicon wafer.

How does megasonic cleaning work to improve standard clean particle removal? Here are the two steps that make up the RCA wafer cleaning process:

•  SC1 – It uses a mixture of ammonium hydroxide and hydrogen peroxide. It removes most of the contamination from the silicon wafer surface but introduces metallic ion contaminants that can influence subsequent wafer processing steps.
• SC2 – It uses a mixture of hydrochloric acid and hydrogen peroxide. It dissolves the remaining contaminants, allowing impurities to rinsed away as they float to the surface of the cleaning mixture.
  

Megasonic cleaning can remove the majority of these particles while also dislodging the few larger particles that may still be present. The process involves adding a megasonic transducer to the tank holding the wafers. The transducer converts very high-frequency signals from a megasonic generator into sound waves that travel through the rinsing water to strike the wafer surfaces.

As the sound waves travel through water, they create microscopic cavitation bubbles that form and burst in tune with sound wave frequency. As the bubbles burst, they produce tiny jets of water. The jets from the bubbles near the wafer surfaces dislodge any remaining particles, leaving them to be rinsed away at the surface of the bath. This results in an extremely low particle count on wafers that undergo both the RCA and megasonic cleaning.

Learn more about the wafer cleaning process by reading our recent article, Improving Standard Clean Particle Removal in a Wet Bench Process. Modutek, a leading semiconductor equipment manufacturer, is available if you have questions or would like to set up a free consultation. You may call 866-803-1533 or send an email to sales@modutek.com.