Category Archives: Silicon Nitride Wet Etching Baths

Equipment Innovations Improve Silicon Nitride Etch Processing

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Equipment Innovations Improve Silicon Nitride Etch ProcessingWhen silicon nitride wet etching is used to etch masks on silicon wafers, precise control of the etch rate is important. If the etch rate varies, the mask may not be correct and the microscopic structures to be created in the silicon may not have the right shape or depth. In the silicon nitride wet etching process, the etch rate depends on both the temperature and the concentration of the etching solution. Both may vary as the process progresses and precise control becomes difficult. An ideal control strategy maintains both the concentration and the temperature at known levels. The etch rate then remains constant and the mask is etched accurately according to the applicable specification.

Silicon Nitride Mask Etching Is Effective but Hard to Handle Safely

Semiconductor manufacturers and research labs use silicon nitride wet etching because the process etches the mask material quickly and reliably. A mixture of 85 percent phosphoric acid and 15 percent deionized water is heated to the boiling point. As the mixture boils, it releases steam that reduces the amount of water in the etching solution. As the concentration of phosphoric acid rises, so does the boiling point, the temperature of the mixture and the etch rate. To keep the process under control, deionized water has to be added to bring the concentration back down.

Adding water to phosphoric acid is exothermic. To prevent a sudden increase in temperature, water has to be added in small amounts, but if too many small amounts are added to the mixture, it will stop boiling. The water then forms a thin film on top of the mixture and eventually mixes with the phosphoric acid all at once, leading to a sudden temperature spike and a possible explosion. A complex control system with variable set points can ensure safe operation but control of the etch rate is poor.

Modutek’s Silicon Nitride Etch Controls Are Simple and Precise

Modutek addresses the control issues of silicon nitride wet etching by heating the etch bath continuously. An always-on heater heats the original 85/15 percent phosphoric acid/deionized water mixture to its boiling point of 165 degrees centigrade. As the mixture loses steam and the concentration and temperature both start to rise, a thermocouple detects the slightly increased temperature and triggers the addition of a small amount of deionized water. The added water is immediately mixed into the etching solution by the boiling action. The amount added restores the concentration and keeps the temperature at the boiling point.

Modutek has refined this control concept to accurately maintain the etching solution exactly at its original concentration and at its boiling point. As a result, with both the concentration and the temperature remaining constant, the etch rate remains predictable and constant as well. Mask etching is reliable and the final mask has precisely the desired characteristics.

Because the Modutek control strategy relies on maintaining the etching solution at the boiling point, the control system has additional safety features to identify dangerous conditions that could result if the mixture stops boiling. Two thermocouples detect the presence of steam over the hot acid and acid overtemperature. If there is no steam, the mixture is no longer boiling. If the acid temperature is too high, the solution is no longer being maintained at the original boiling point. In either case, the process is shut down because equipment failure is likely and a dangerous condition may be present.

Modutek Provides a Complete line Equipment Solutions

Modutek offers a complete line of silicon wet etching equipment and works with customers to develop innovative solutions to their problems. The silicon nitride etching control system improves etching performance and results remain consistent over different batches. The number of defective products is reduced and product quality increases. When customers purchase etching systems and other equipment, Modutek will ensure they meet their requirements. Contact Modutek for a free consultation to discuss your specific process requirements.

How Process Controls Improve Silicon Nitride Wet Etching Results

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How Process Controls Improve Silicon Nitride Wet Etching ResultsThe silicon nitride wet etching process uses a phosphoric acid solution in water to etch mask material quickly and reliably but the process is hard to control. Etching speed depends on the concentration of the solution and the bath temperature. Adding water to the solution to control the concentration is dangerous and, at the same time, it can change the temperature. Controlling such an unstable solution with two interdependent variables is difficult, but in the absence of reliable controls, the etching rate is not accurately maintained and the removal of silicon nitride is not precise.

How Silicon Nitride Etching With Phosphoric Acid Works

In the silicon nitride wet etching process, the silicon wafer is masked with a silicon nitride coating that has to be selectively etched. A solution of about 85 percent phosphoric acid and 15 percent de-ionized water is used to etch the silicon nitride at an optimized temperature of 165 degrees centigrade. To speed up etching, the phosphoric acid solution is heated to its boiling point. As the solution boils, water from the solution is lost as steam and the concentration of the solution rises. The boiling point of the solution rises as the concentration rises and the etch rate increases as both the temperature and the concentration increase.

To correct the etch rate, water has to be added to the bath to bring the concentration and the temperature back down. Adding water to boiling phosphoric acid is dangerous because it can reduce the temperature of the solution to below boiling. When the phosphoric acid solution is no longer boiling, the added water can form a thin film on top of the acid instead of mixing in continuously. Eventually, the film will mix with the acid all at once, and a “bump” can result from the exothermic reaction.

To make sure small amounts of water mix into the solution continuously, the solution has to be kept at its boiling point. Because the boiling point is variable, standard controls with a fixed temperature set point don’t work. Modutek has developed an innovative silicon nitride etching process control solution that features precise control of the temperature, concentration and etch rate while ensuring safe operation.

The Modutek Control Strategy Improves Etching Results

Modutek’s control concept uses an always-on heater to keep the 85 percent phosphoric acid solution at its boiling point. As water is lost to steam, the concentration of the solution starts to increase and the temperature starts to rise above the original value. A thermocouple senses the temperature rise and signals the system to add water. A small amount of water is added and is immediately mixed into the solution by the boiling action. The concentration is maintained at the original 85 percent phosphoric acid and 15 percent water and the temperature stays at the boiling point of 165 degrees centigrade with this concentration. The etch rate remains constant and the operation is safe.

Because the safety of the Modutek control strategy relies on the constant boiling of the phosphoric acid solution, an additional thermocouple is located above the hot acid to sense the presence of hot steam. Another thermocouple senses the temperature of the acid. If no steam is present or if the acid becomes too hot, the process is shut down. These emergency shut-downs don’t take effect during normal operation, but are an additional safety feature in case of equipment malfunction or operator error.

Modutek’s Control Strategy Delivers Improved Etching Performance

Modutek has tested and fine-tuned its control strategy with their Nb Series silicon nitride etching baths for customers using manual, semi-automated, and fully automated wet bench stations. Customers that have been 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.

A reliably-controlled and constant etch rate yields predictable etching and silicon nitride removal within product tolerances. When etching is not the same from batch to batch, product quality suffers and product lifespan can be reduced. Modutek manufactures a complete range of silicon wet etching equipment, and with the company’s innovative control system, rejection rates are reduced and facility output is improved.

 

Safely Controlling the Silicon Nitride Etching Process

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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 New Equipment Has Improved the Silicon Nitride Wet Etching Process

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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 the Silicon Nitride Wet Etching Process is Improved by Modutek

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Silicon nitride is used as a mask when etching silicon wafers during the semiconductor manufacturing process. Before the silicon wafer can be processed further, the silicon nitride has to be removed. A solution of phosphoric acid and de-ionized water etches the silicon nitride while leaving the wafer unaffected. As a result, the silicon nitride is stripped from the wafer surface and a clean wafer is left for further fabrication steps.

To optimize the silicon nitride wet etching process, the phosphoric acid solution is kept at a high temperature. This means some of the de-ionized water boils off as steam and has to be replaced. Adding water to phosphoric acid can be extremely dangerous and can result in an explosion. In addition, keeping the temperature of the solution constant is important for accurate control of the stripping process. Modutek has improved both the safety and the control of the nitride wet etching process with its Nb series silicon nitride wet etching baths.

How the Process Works

A solution of 85 percent phosphoric acid and 15 percent de-ionized water is heated to a boil at about 160 degrees centigrade. Some of the water boils off as steam and part of the steam is condensed and placed back in the solution. Some steam escapes and de-ionized water has to be added periodically. The solution is kept at the boiling point by a heater and the process temperature can be tightly controlled. With the Modutek system, nitride etch is uniform and there are no “bumps” in the chemical concentration or temperature.

How Modutek Controls the Etching Process Safely and Accurately

The two challenges faced when operating a boiling phosphoric acid bath is the safe replacement of the de-ionized water lost to steam and an accurate control of the process. If water is added to the solution too quickly, the solution may stop boiling and a film of water can accumulate on top of the viscous acid. If the water of the film then suddenly mixes with the acid, a strong reaction can occur, and an explosion is possible. Adding water in this way also makes accurate control of the process difficult.

In the Modutek system, the heater that boils the solution is always switched on. The solution is always heated up to the boiling point and further heating only increases the boiling rate but not the temperature. This control strategy results in a clearly defined temperature for the boiling solution, which will always be at its boiling point.

The boiling point of a phosphoric acid and water solution varies with the concentration, increasing as the solution loses water to steam. A thermocouple in the bath senses the increasing temperature and gives the signal to add water. De-ionized water is slowly added to the boiling solution. Because the acid is boiling rapidly, the small amounts of water are immediately mixed in and don’t form a surface film. Adding water in this way is safe and controlling the process by monitoring the concentration through the temperature rise is accurate.

Additional Safety Features

Since the silicon nitride wet etching process depends on keeping the solution at the boiling point, additional safety interlocks in Modutek’s etching baths are in place to insure no water is added if the solution is not boiling. A thermocouple above the solution senses the presence of hot steam and shuts off the water valve if steam is not present. To ensure the solution does not overheat, another thermocouple switches off the heaters when the temperature of the solution reaches 170 degrees centigrade. With its advanced control system and the additional safety interlocks, Modutek has improved the silicon nitride wet etching process with better control, accuracy and increased safety.