Nitran Series, Nitride/Oxide Tanks

Nitride/Oxide Etch Bath of Fluoropolymer
Auto • DI • Water • Injection

Designed specifically for Silicon Nitride/Oxide Etch processes, the Nitran Series offers excellent process control. Incorporated with each Nitran is a tightly-sealed condensing head with cover and an automatic DI water replenishment system for maintaining important H₃PO₄ / H₂O ratios. The Nitran process module features a high wattage fluoropolymer heater for fast temperature ramp-up. Capacities are available for 3" to 8" wafer boats. The Nitran Process Control Module includes a network of safety features. Each Nitran is supplied as a "complete" system and includes all necessary components such as a control module, process thermocouple, liquid level sensor, fluoropolymer tank, condensing flange, etc.

STANDARD FEATURES

• Tightly Sealing Condensing Head / Cover & Automatic DI Water Replenishment System - Maintain important H₃PO₄/H₂O ratios.
• Fast Ramp-Up and Temperature Recovery - Faster heat-up and recovery is achieved as the process heater directly contacts the process solution.
• Temperature Stability and Uniformity - The Nitran heater takes up the entire bottom of the tank minimizing temperature gradients. Temperature range up to 180°C, depending on chemistry, concentration, etc.
• Minimal Downtime - The Nitran heater exhibits long life, however when replacement is necessary it can be done more quickly, on-site and less expensively than other styles of baths.
• More Durable - The fluoropolymer process tank is very durable and is virtually unbreakable.
• Warranty - Standard one year.

OTHER FEATURES

Process Module

• PTFE fluoropolymer heater with patented N₂ purge feature (patent numbers 4,553,024 - 4,158,764) 110 or 208 VAC, single phase, wattage dependent on model ordered.
• Fluoropolymer sleeved J-type process thermocouple.
• Over-temperature thermocouple.
• Liquid level sensor (capacitance type).

Process Control Module

• Digital PID based temperature controller.
• Heater purge controls.
• All operational switches.
• Kydex enclosure.

THE PROCESS

The etching of Silicon Nitride in phosphoric acid, using silicon dioxide as a mask, may be achieved by boiling the liquid and "refluxing" (condensing) the resultant vapors. Refluxed, boiling phosphoric acid at 180°C has been found to provide an etch rate of up to 100 Å per minute for silicon nitride films. Deposited silicon dioxide has an etch rate of about 10 Å per minute ( a range of 0-25 Å per minute depending upon temperature and preparation). Elemental silicon has an etch rate of about 3 Å per minute. According to experiments from 140-200°C, etch rates increase with temperature.

Water content of the phosphoric acid plays an important role in etching of silicon nitride and silicon dioxide. At a constant temperature, addition of water increases the etch rate of silicon nitride and decreases the etch rate of silicon dioxide. The difference in etch rates between silicon nitride, deposited silicon oxide, and elemental silicon allows etching contact holes in silicon nitride using deposited silicon dioxide as a mask. In such a manner, the making of transistors can be successful using silicon nitride over silicon dioxide as a junction seal. The maximum possible temperature at atmosphere pressure for a fixed water content is always realized by boiling the acid and "refluxing" the vapor phase. The NITRAN Series nitride etch bath has been designed especially for this process.

GENERAL PROCESS/PRODUCT OPERATION

• All Nitran wetted surfaces are constructed of fluoropolymer and PVDF. Typical Nitride etch/strip operating temperature are 160-180°C. Temperature is controlled by a Digital PID based process control module. Temperature control is +/- 1°C. Solution temperature is sensed directly by a fluoropolymer encapsulated J-type thermocouple.
• Heaters are sized to provide rapid 30 to 45 minute heat up rates. Heaters are protected from over-temperature by a resetable over-temperature OTCD. It consists of a special thermocouple installed inside the heater. Further protection from permeating contaminants in the heater is provided by an inert gas purge and alarm/cutoff system.
• A flowmeter is used to regulate the introduction of DI water. Typically, drip rates are set between 0 and 4.7 mi/min. Tests indicate an optimum 2.0 mi/min. rate for 160°C operation. Many users find that 1 drip per second is ideal. DI drip pressure is usually 15-20 PSI, (40 PSI maximum).
• City water (typical temperature 18-22°C) is used as the "coolant" for the condensing coil. Flow rate of 25-30 GPH (50 GPH maximum), at a pressure of 20-40 PSI.
• In addition to controlling process temperature, the Process Control Module automatically actuates the DI water drip and flow of city water to the condensing coil located in the condensing head. Both the DT water drip and condensing coil flow may be controlled manually for times during start-up or operation when operators may wish to more actively control process solution concentration.

The NITRAN Process Control Module allows the operator to control key process parameters. Safety alarm annunciators enable the operator to immediately verify process status and any safety shutdowns that may have occurred.

OPERATIONAL SWITCHES

• "Power" on/off: energizes the control module and safety circuits.
• "Start/Reset": turns on the heater.
• "Stop": turns off the heater.
• "Drip": allows manual override (on / off) of DI water drip for situations such as when low liquid level has occurred and a slow, controlled replenishment is desired.
• "Cooling": allows manual override (on / off) of the cooling water flow to the cooling coil.

SAFETY ANNUNCIATOR LIGHTS

• Low liquid level in the tank.
• Low purge pressure in the heater.
• Over-temperature shutdown.
• High heater temperature alarm from the process temperature controller.

The interaction of constant or automatic DI drip and an efficient (up to 8 sq. ft. coil) condensation head tends to maintain minimum fluid concentration gradients. Uniformity of temperature throughout the tank is enhanced by the particular spacing of the several rows of heater coils in the patented Nitran heater. This design of the heater allows uniform convection flow of the bath fluids. The combination of constant fluid concentration and uniform temperature in the Nitran assures a high etch rate uniformly across the wafers being processed.

In a number of domestic installations customers have determined that the Nitran system offers noticeably higher, as well as uniform, etch rates on all wafers. While the details of the processes are proprietary and cannot be divulged, listed below are some comparisons of etch rates and temperatures achieved while using the Nitran.

OPTIONS

• Drain valve.
• Hinged lid.
• Hinged lid (rear venting).
• Microprocessor controller.
• Timer.

ELECTRICAL SERVICE

• Heater: 110 or 208 VAC, single phase 50/60 HZ.
• Three phase, custom wattages and voltages available upon request.

For more information, prices and availability please visit our contact page.