How New Equipment Has Improved the Silicon Nitride Wet Etching Process

how-new-equipment-has-improved-the-silicon-nitride-wet-etching-processThe silicon nitride wet etching process is difficult to control safely but is a key component of semiconductor wafer manufacturing. The process uses phosphoric acid to remove silicon nitride masks from silicon wafers to allow the clean wafers to undergo further fabrication steps. The hot phosphoric acid and water mixture is unstable and requires periodic addition of small quantities of water, but adding water to phosphoric acid may produce a bump. Modutek has developed a new approach to control the nitride etching process safely and accurately while achieving high etching rates.

How the Silicon Nitride Wet Etching Process Works

The etching of silicon nitride masks with phosphoric acid is optimized at a temperature of 160 degrees centigrade for a mixture of 85 percent acid and 15 percent deionized water. At this temperature, the mixture boils and some water is lost as steam. The lost water has to periodically be replaced, but adding water to phosphoric acid can be dangerous. The water may not immediately mix with the acid, instead forming a film on top of the mixture. If the film suddenly mixes with the acid, introducing a large amount of water into the mixture at once, a bump can result.

In order to optimize etching performance small amounts of water are added periodically which immediately mix with the acid to maintain the temperature at the ideal 160 degrees centigrade. If too much water is added, the temperature of the mixture may drop and the mixture may stop boiling, allowing the water film to form and create a dangerous situation. If too little water is added, the temperature of the mixture will increase and even more water will be lost. Modutek has new equipment provides an innovative control strategy that addresses these issues and delivers excellent etching performance.

Modutek’s Nb Series Silicon Nitride Wet Etching Bath

Modutek’s new Nb series bath provides tight control of the bath temperature while ensuring a safe operation and superior etching of the silicon nitride masks. In addition to the new control strategy, the series Nb baths offer additional safety features that make sure the dangerous condition in which large amounts of water are mixed with hot phosphoric acid is avoided.

In their new silicon nitride wet etching equipment, Modutek uses the acid mixture concentration as a reference value for adding small amounts of deionized water. At normal operation, the bath heater is on and the mixture is always boiling at the normal mixture boiling point of 160 degrees centigrade.

The boiling point of the mixture varies with the concentration. As water is lost and the mixture concentration rises, the boiling point increases as well and the mixture temperature starts to rise above 160 degrees centigrade. This rise triggers the addition of a small amount of deionized water. Since the mixture is always boiling because the heater is always on, the deionized water is immediately mixed in with the boiling acid, the concentration of the acid is reduced and the mixture boiling point decreases back down to 160 degrees centigrade.

To guard against the addition of water when the mixture is not boiling, a thermocouple sensor above the boiling mixture detects the presence of steam and closes the water valve when no steam is present. This additional safety feature ensures that a dangerous water film cannot form even under abnormal conditions such as heather failure.

Modutek has been testing and fine tuning the new system with Nb silicon nitride wet etching baths to customers in manual, semi automated and fully automated wet bench stations. Semiconductor fabrication facilities and research centers using the new equipment have achieved an average etch rate of 65 angstroms per minute while limiting oxide etch as the controls ensure safe operation at a consistent and optimum operating point.



How to Safely Use Solvents for Wafer Processing

how-to-safely-use-solvents-for-wafer-processingAt semiconductor research centers and foundries, solvent wet bench stations provide a critical role in the semiconductor fabrication process. These stations are designed for operations that include substrate cleaning, photoresist stripping and liftoff pattern transfers. Since the solvents are often highly inflammable, wet bench equipment needs to be designed with stringent compliance to safety codes.

Equipping for safety

Precautions related to fire safety and electrical safety is among the most basic requirements in wet bench station design. Fire suppression can involve the application of spark-resistant electrical design all around, and the provision of extinguisher equipment. Solvent wet bench stations must be designed to meet Class 1 Div 2 certifications, and incorporate a carbon dioxide (CO2) fire system.

Safety alarms: Exhaust, CTA and N2 safety interlocks should monitor for use within acceptable ranges. Safety doors on the head case and elsewhere should be monitored during use and should be able to shut down operation if they are opened during use. Float switches should detect liquid spills on the floor of the bench. Differences in pressure between the exhaust on the bench and the rest of the room should be monitored through the use of photohelic gauges (low readings indicating incorrect operation should sound an alarm).

Carboy collection: Since solvent waste should never be sent down the drain, carboys help collect waste for containment. Multiple containment volume sizes are available.  All come with current safety interface.

Heated solvent baths: Temperature control is achieved in many ways, and heated baths are one of them. They should be used to control the temperature of the solvents being used. Temperature controllers should be used both to raise and lower temperatures and to monitor temperatures.

Process timers: These vital pieces of equipment should be mounted to the head case to ensure that personnel are able to keep track of how long each process is to run.

Restrictions on use: A login system should ensure that only authorized individuals are able to access the bench.

In many cases, safety comes from simpler approaches — the use of auto lids made of Teflon or stainless steel to help seal off chemical containers.

Modutek’s Solvent Wet Bench Equipment

Available in manual, dry-to-dry, semi-and fully-automated designs, Modutek’s stainless steel solvent stations are constructed all through using 304 stainless steel. Solvent applications require the use of this material. From photoresist to IPA, and acetone, these stations are suitable for critical operations in the wafer processing activities. From solid works simulation to professional flow simulation, many critical flow characteristics are available. Improvements to productivity are achieved through the use of Modutek’s advanced software.

Modutek’s safety design for solvent processing involves every required feature. With wiring specified to NFPA 70 & 79 codes, emergency power off design for every safety interlock, lip exhausts on all solvent tanks, N2 head case purge design, photohelic EPO interlocks, safety shields, Teflon N2 guns, and continuous flow of the water manifolds, Modutek’s designs are among the safest of all wet stations.

With a chemical management interface, a PCL with touchscreen GUI, a rear access design, Teflon plumbing throughout, certification to the CE Mark, and seismic certification, Modutek’s equipment incorporates features beyond legally required safety measures.

Modutek’s Wet Bench Design and Build Process

Wet bench construction is a complicated process, and Modutek provides systems that are designed to meet the requirements of specific applications of individual customers. With 35 years in the business, Modutek understands that acid and solvent processing for micro fabrication requires customized solutions. Modutek’s designs and builds all their wet bench stations in house to ensure every part of the system provides reliable performance. If you need a solvent wet bench station designed and built to meet your requirement s contact Modutek for a free consultation or quote.