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 Silicon Wet Etching Processes Are Improved with Specialized Equipment

how-silicon-wet-etching-processes-are-improved-with-specialized-equipmentWhile Modutek’s wet bench equipment supports all common wet etching processes, the continuous development of new wet bench technologies allows the company to offer specialized equipment that can improve performance for specific applications. Key factors for high output quality, fast processing, high yields and reliable outcomes are cleanliness, repeatability, tight control and precise dosages. Modutek’s standard line of silicon wet etching equipment already scores highly on these factors but the company’s specialized equipment can further improve performance for certain processes.

Buffered Oxide Etch (BOE)

The BOE process requires tight and precise temperature control while filtering lowers the particulate count and reduced acid consumption results in lower costs. Modutek’s F-series sub-ambient circulation bath is specially designed for BOE applications and addresses these factors. The temperature controller is accurate to less than plus/minus one degree centigrade in a range from 10 to 60 degrees centigrade with sub-ambient capability. The circulating bath includes 10.0 to 0.2 micron filtration and is designed for low acid consumption. Using a wet bench system specialized for the BOE process can reduce costs and defects in products while providing safe and reliable operation.

Piranha Etching

The sulfuric acid/hydrogen peroxide mixture of the Piranha solution is extremely corrosive, and the mixture is exothermic when first prepared. The Modutek QFa Series of Quartz High Temperature Recirculating Baths are ideal for withstanding the high temperatures, first of the initial heating after mixing and then during the process that is heated to preserve mixture reactivity. The baths have heaters that can raise the temperature of the mixture by two degrees Centigrade per minute and the temperature controller can maintain the temperature at plus/minus one degree centigrade over a range of 30 to 180 degrees centigrade. These Modutek quartz baths are characterized by the high degree of safety, reliable operation and low cost of ownership.

Potassium Hydroxide (KOH) Etching

Modutek Teflon tanks are ideal for use in KOH etching because they can be custom fitted to any wet bench configuration and come in three models to satisfy common heating and recirculating requirements. The TFa and TI series of Teflon tanks are temperature controlled to an accuracy of plus/minus 0.5 degrees centigrade over a range of 30 to 100 degrees centigrade. The TFa series are overflow re-circulating models while the TI series are static. The TT series are static ambient temperature models. All Teflon tanks are made from PFA Teflon with special welding techniques to minimize the presence of undesirable by-products in the process and to reduce contamination. Modutek’s Teflon tanks improve KOH process control, increasing reliability and reproducibility of process conditions.

Silicon Nitride Etching

Etching silicon nitride with a phosphoric acid deionized water mixture is challenging because the concentration changes as the deionized water evaporates and because adding water to the process can cause an explosion. Modutek’s silicon nitride wet etching bath is specially designed to address these issues and provide reliable and safe process control. The mixture is kept at the boiling point, which changes as the concentration increases. Depending on the temperature of the mixture, small amounts of deionized water are added to the boiling mixture to maintain the concentration at the required level. Using the boiling point temperature as an indication of the concentration and allowing the boiling mixture to immediately distribute small amounts of water lets Modutek control the process while maintaining safe operation.

With its extensive experience in silicon wet etching process technology, Modutek can deliver specialized equipment suited to a particular process and help customers select the right equipment for their applications. With equipment specially designed for the process used, customers obtain superior results while often saving money as well. Contact Modutek for a free consultation or quote on using the right silicon wet etching equipment for your application.


Improvements to the Silicon Wet Etching Process

Improvements to the Silicon Wet Etching ProcessAs a result of improvements to equipment design and materials,  silicon wet etching process controls have improved to produce higher wafer yields with reduced defects. Modutek provides silicon wet etching equipment engineered to incorporate such improvements that are designed for reliability, safety, and low cost of ownership. The wet etching products from Modutek include features that offer specific advantages and deliver improved results. Process engineers and fabrication facility managers can order customized equipment to meet their needs and to ensure they receive the process improvements they expect.

Modutek’s QFa Series High-Temperature Recirculating Quartz Baths and the Qa Series Constant Temperature Quartz Baths are designed to set the quality standard in quartz baths. Used for etching, stripping, and heating, the baths feature fast, even heating to improve process yield and minimize damage from runaway temperatures. The high-quality internally flame-polished virgin fused quartz process vessels reduce downtime and increase reliability due to longer vessel life.

Although Modutek’s Teflon tanks come in standard carrier sizes, the company will build custom sizes to match any requirement. Tank configurations are Temperature Controlled Recirculating Baths, Temperature Controlled Static Baths and Teflon Ambient Baths. Inline heating or immersion heating are available. Modutek can help design the optimal configuration for both new as well as existing systems you may already have.

Modutek’s silicon wet etching baths deliver excellent process control while retaining flexibility in installation and operation. The control system monitors the temperature of the bath to determine when adding de-ionized water is required to maintain the acid-to-water ratio. As water is lost to evaporation, the boiling point of the bath increases and the control system adds small amounts of water to the boiling liquid, ensuring that the water mixes in immediately. This method tightly controls the boiling of the chemicals and avoids “bumping.” The availability of remote metering means that the product is easy to install and Modutek can help find the best solution for any process station.

Modutek’s line of Quick Dump Rinsers can increase process efficiency by reducing rinse times and DI water consumption. The Rinsers feature state-of-the-art engineering to eliminate particle entrapment and minimize rinsing times. Dual overhead spray manifolds, a contoured vessel design and a large machined dump door ensure efficient and rapid operation.

For facilities that could benefit from using KOH etching, Modutek offers customized solutions manufactured according to customer specifications. KOH etching is a repeatable process that delivers fast etching at a low cost. Modutek’s solutions use the company’s Teflon tanks with advanced welding techniques and all-PFA material. The products can be designed to particular specifications or to be compatible with your existing wet bench equipment. Once the wet etching process is established, continual adjustments are not necessary. Modutek can integrate the new products into an existing line to deliver process improvements and increased efficiency.

Making use of improved designs or higher-quality materials can generate substantial benefits from reduced costs and better throughput. The increased ability to manufacture customized products that are optimized for a particular application can further improve silicon wet etching results. For any questions about silicon wet etching products or for help selecting the right manufacturing equipment for your requirements, call Modutek at 866-803-1533.