How Process Controls Improve Silicon Nitride Wet Etching Results

 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 wet 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 their control strategy with their Nb Series silicon nitride wet 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.


How Wet Bench Stations Are Designed for Ergonomics and Safety

How Wet Bench Stations Are Designed for Ergonomics and SafetyTo etch and clean silicon wafers, wet bench stations use aggressive and toxic chemicals that have to be handled carefully. Ergonomic design ensures that employees don’t have to perform risky or strenuous maneuvers in normal operation of the facility. To load trays, control processes and dispose of waste, employees should not have to lift especially heavy weights, stretch to the limits of their reach or perform repeated strenuous actions. The Semiconductor Equipment and Materials International (SEMI) S8-0712 – Safety Guidelines for Ergonomics Engineering of Semiconductor Manufacturing Equipment can help manufacturers design equipment for comfortable and safe operation.

Key safety aspects of semiconductor equipment manufacturing include safe chemical handling and fire prevention. The FM 4910 specification and the Cleanroom Materials Flammability Test Protocol help manufacturers avoid using flammable materials in their equipment. Wet bench stations that adhere to these guidelines provide a better and safer working environment.

How Modutek Design Wet Bench Stations for Ergonomic and Safe Operation

Modutek designs its wet bench chemical stations according to the SEMI S8 guidelines and minimizes the use of flammable materials in its designs. Examples of safe and ergonomic

design measures applied by Modutek include the following:

  • Wafer holding trays are easy to load
  • Operator interface is on a swing arm
  • All stations are built around your ergonomic requirements

Ergonomic and safe design means that operators must be able to load equipment without strain. The wafer holding trays on Modutek’s wet benches can move to the front on robot arms that position the trays for optimal loading and unloading. Operators don’t have to stretch and can perform this often repeated action with a minimum of effort.

Operators have to be able to see the operator interface while monitoring the process. The interface monitor is usually off to the side of the wet bench station. The interface monitor is mounted on a swing arm that allows operators to adjust the position where they can view it while monitoring the process. The interface mounting arm can rotate and tilt to an ideal position and then can be locked with one hand. The operator can continue working while adjusting the interface monitor and doesn’t have to assume any unnatural positions.

All of Modutek’s wet bench stations are built and designed around your ergonomic and process requirements. Modutek does not sell stations which customers order from a catalog. Wet bench stations are built based on a customer’s specifications and incorporate features for ergonomic safety and repeatability. Besides ease of use Modutek also designs each station around your facility requirements. When the station is delivered all facility connections are pre-approved so your system will be up and running in a very short time. All of these added features save customers time and money.

In addition to these specific examples, Modutek applies ergonomic design principles and safety features across its full product line of wet process equipment. All its wet bench chemical stations satisfy safety regulations and are designed to keep employee physical effort to a minimum.

Equipment That Meets Ergonomic and Safety Guidelines

Modutek’s extensive line of wet process equipment and wet bench chemical stations are all designed according to the latest applicable ergonomic and safety guidelines. When customers need special equipment, Modutek can design customized systems to satisfy specific requirements. In that case, ergonomic and safety features are customized as well to make sure the special equipment meets or exceeds the guidelines.

Older equipment is often not compliant with new regulations. When that is the case, it’s a good time to purchase new or upgraded equipment. Whether new or an upgrade, the equipment comes with improved ergonomics and safety features. When employees work with reduced physical stress and feel safe in the workplace, employee morale and job satisfaction increases. If you need help selecting a wet bench station for your process requirements and also all ergonomic and safety regulations, contact Modutek for a free quote or consultation at 866-803-1533 or email [email protected].