How the Silicon Nitride Wet Etching Process is Improved by Modutek

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.


How the KOH Etching Process is Improved Using Modutek’s Teflon Tanks

Among the different approaches available to foundries that create intricate integrated circuitry on semiconductor chips, etching with potassium hydroxide (KOH) is frequently preferred for the error-free mass-production that it allows.

The improvement in precision that the KOH etching process brings to semiconductor fabrication is attributable in large part to the use of deionized water. When employed with high alkalinity in excess of pH 12, this process can be thermally adjusted for precise degrees of etching.

How does the use of potassium hydroxide improve the semiconductor etching process?

While dry semiconductor etching processes do exist, they tend to present challenges in process control. Semiconductor wafers etched in this way tend to suffer from quality control issues. Dry etching processes can be difficult to build and run, as well — toxic and explosive chemicals often seen as byproducts, after all.

Greater precision

Etching with potassium hydroxide offers greater precision, and lends itself to improved control, as well.

The fluids employed in the KOH etching process are stored in tanks built into the equipment. One of the most cost-effective and meaningful choices to make in KOH etching processes: to place Teflon tanks within the etching equipment in order to hold the fluids employed. Teflon tanks can make the KOH etching process safer than other options available.

It is possible to structure the KOH etching process in a way to easily create repeat manufacturing projects, as well. When such projects come in from different clients, fabrication units can focus their strengths on setting up the equipment, worrying as little as possible about safety or reliability.

Opting for fewer contaminants and greater cleanliness

Customization is one of the greatest most significant improvements that Modutek brings to the KOH etching equipment business — one where generic, mass-produced Teflon tanks are the norm. Modutek’s Teflon tanks are especially capable of bringing impurity quantities to levels that are impressively low. The result is greater reliability and consistency over an extended period of time.

Greater control

Modutek’s Teflon tanks come with highly customizable temperature settings. Temperature control is achieved either through in-line heating and cooling equipment installed, or through immersion in liquid tanks. It is precisely varied anywhere between 30°C to 100°C. Temperature changes dialed up our achieved at a fast rate of 2°C a minute, on average (although tank size does affect response rates).

Modutek brings customization to the table

Modutek designs Teflon tanks for the specific needs of each client installation. Whether a fabrication plant requires temperature controlled recirculating baths for their KOH etching processes or temperature controlled static baths, Modutek’s Teflon tanks deliver both freedom from water loss and freedom from concentration deficiency.

From dual heating Teflon tanks for fast and consistent water-based etching, to the installation of drains and valves that enable rapid cleanup, high-tech remote data interfaces, controllers and timers, Modutek’s clients have their choice of every advance in KOH etching.