Equipment Innovations Improve Silicon Nitride Etch Processing

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 Advanced Ozone Cleaning Reduces Costs and Improves Wafer Yields

How Advanced Ozone Cleaning Reduces Costs and Improves Wafer YieldsWhen ozone is used to clean silicon wafers, it reduces the use of aggressive chemicals and it can decrease the wafer particle count. Modutek’s advanced ozone cleaning process can clean more quickly than many chemical-based processes and it delivers other benefits. Ozone is used to remove organic contaminants from wafer surfaces in either the Coldstrip sub ambient process or the Organostrip process. Either process can reduce overall wafer fabrication costs and improve manufacturing facility performance.

Advanced Ozone Cleaning Outperforms Chemical Stripping

With increased regulation of the use of dangerous chemicals and pressure on businesses to make their operations more environmentally friendly, Modutek developed the advanced ozone cleaning processes to help semiconductor manufacturers and research labs reduce chemical use. Ozone is introduced in Modutek’s DryZone system in compact units using the Coldstrip or Organo strip processes. Both cleaning methods also feature a reduced particle count compared to chemical cleaning methods.

In the Coldstrip process, wafers are first rinsed with deionized water to remove soluble non-organic contaminants. The process operates at four to ten degrees centigrade. After the rinsing, ozone is introduced into the chamber and combines with the carbon of the organic compounds on the wafer surfaces. The ozone-carbon reaction forms carbon dioxide, leaving wafers clean and almost free of particles.

The Organostrip process also uses ozone but operates at ambient temperature. Ozone is introduced into the process dissolved in acetic acid, a mild solvent in which ozone has a very high solubility. The wafers are rinsed with the ozone-acetic acid solution and the high level of ozone rapidly oxidizes the organic contaminants. The waste products of the process do not require special treatment. Both processes work quickly and feature excellent cleaning performance with low particle counts.

Changing to the Advanced Ozone Cleaning Process Delivers Substantial Benefits

The major benefits of changing from chemical cleaning to an advanced ozone cleaning process is a reduction in the use of toxic chemicals and improved cleaning performance with a lower particle count. These two factors are at the root of additional benefits resulting in cost savings, better yields and improved safety for employees.

Reduced Use of Chemicals

When the use of toxic chemicals is reduced, there are cost reductions in addition to savings resulting from fewer chemical purchases. Other savings include reduced costs for chemical storage, handling and disposal. Employees benefit from lower exposure to harmful chemicals and from increased workplace safety because there is less danger of spills or accidents.

Regulatory compliance costs are also reduced. As environmental regulations become more onerous, costs rise and compliance becomes more difficult. Reduced chemical use results in lower compliance costs, and a more environmentally friendly operation helps improve community relations and the environmental reputation of the business.

Lower Particle Contamination

Lower particle counts are the result of better cleaning performance. Particle counts play ang increasingly important role in wafer structures that are smaller and more tightly packed. Even a single particle can block a conducting path or the etching of a tiny detail. Fewer particles means a lower rate of defective products and higher yields. Product quality and longevity may also be improved.

Improved Cleaning Performance and Wafer Yields with Less Equipment

In addition to a lower particle count, the improved cleaning performance of the advanced ozone cleaning process results in shorter cleaning times within a smaller footprint. Modutek’s DryZone units used for the ozone cleaning process are compact and take up less space than the corresponding chemical cleaning stations. Cleaning is more rapid resulting in higher throughput. As a result of these benefits, the overall performance of the silicon wafer fabrication facility can improve substantially.

Modutek Provides Advanced Equipment for Manufacturers

As one of the leading semiconductor equipment manufacturers, Modutek can advise customers on how to implement a change to ozone cleaning. Modutek offers a free consultation and quote on their equipment, and ensures their equipment will support processes that meet customer requirements.