The silicon wet etching process is used in the manufacture of integrated circuits to selectively remove silicon nitride with respect to silicon oxide. Engineers and process specialists in the industry who work with semiconductor equipment manufacturers owe a great deal of their expertise to the quality of the etching equipment at their disposal today.

Background

Integrated circuit fabrication processes include selectively removing silicon nitride using wet or dry etching. Selectivity between materials removed and not removed should be high. However, selectivity is not the only consideration. In a manufacturing environment, a process engineer needs consistent and repeatable results from high-quality and reliable equipment.

Field Oxides Provide Mask Layer

Field oxides deposit a dielectric mask layer over silicon using a patterning mask layer exposing the silicon or oxide, and growing the thick oxide called field oxide. The preferred choice of dielectric for the mask layer is silicon nitride. This process is LOCOS, Localized Oxidation of Silicon. A polysilicon layer is frequently present beneath the nitride. This is a Poly Buffered LOCOS stack. Thus, both silicon nitride and polysilicon must be removed without affecting the silicon oxide.

Aging Effects and High Particle Count

Wet etching techniques are used for stripping nitride and silicon when oxides are present. Silicon nitride is etched in phosphoric acid that is boiling. Amounts of nitric acid may be in the bath. These baths are susceptible to aging and have higher particle counts because the silicate concentration in the bath increases. Others use separate baths for nitride and polysilicon. This increases the number of baths required and can lead to pitting of the pad oxide, which engineers overcome by using a bath with phosphoric, hydrofluoric, and nitric acids. Bath life is extended by hydrofluoric and nitric acids. Nitric acid is required to oxidize silicon. The initial etching selectivity of nitride to oxide may be undesirably low, with selectivities of 9:1 observed, and depends on fluorine concentration. Higher initial etch selectivities are preferred.
Before etching silicon nitride or polysilicon to silicon oxide, you should add silicon to the etch process bath. The silicon enhances initial etch selectivity. Fluorosilicates also can be used. These compounds dissolve readily in phosphoric acid without particles. The compounds increase the etch selectivities to 40:1 and 33:1 for silicon nitride and polysilicon. The additional silicon stabilizes may be in the bath from initial use until its use is terminated. Variations of this procedure are tested and reviewed by engineers and researchers.

Equipment Innovations

IC equipment suppliers provide manufacturers and researchers with leading-edge products required for producing smaller geometry semiconductors, thin-film heads, and MEMS, and also need to offer service, support, and retrofitted systems. These products are usually designed to meet the specific requirements of manufacturers.

Silicon Nitride Etch Baths that Provide Safety, Control, and Flexibility

The Nb Series Operation Process used with Modutek’s silicon nitride etch baths guarantee process control, safety, and flexibility with a two-level diagnostic system monitoring temperature while maintaining an acid-to-water ratio through D.I. water supplementation. By using a new remote metering system, the Nb Series is easy to install and service in new or existing stations. Process engineers know that this kind of equipment needs to be manufactured and supported by a company that:

• Has extensive experience and expertise;
• Uses the latest technology;
• Meets specific customer requirements;
• Provides high-quality and reliable equipment;
• Offers excellent technical support and onsite repairs

Modutek has over 30 years of experience providing high-quality and reliable equipment to support the silicon nitride etch process and is a leader among semiconductor equipment manufacturers. If you need to discuss your process requirements or would like a free quote, call Modutek at 866-803-1533 or visit www.modutek.com.