Introduction

Etch is a process that removes materials that are not protected by hardened photoresist from the wafer surface to achieve the IC design requirements. Two main etching techniques are:

Wet Chemical Etching

• A buffered oxide etch is commonly used to etch windows in silicon dioxide layers. BOE is a solution containing HF and submerging the wafers in the solution performs etching.

• Etch rate is temperature dependent, and temperature is carefully monitored during the etch process.

• Wet chemical etching tends to be an isotropic process; etching takes place equally in all directions.

• Fig. 1(a) shows the result of isotropic etching of a narrow line in silicon dioxide. The etching process has etched under the resist by a distance equal to the thickness of the film. This “etch bias” becomes a serious problem in processes requiring linewidths with dimensions similar to the thickness of the film.[1]

Fig. 1 Etching profiles obtained with (a) isotropic wet chemical etching and (b) dry anisotropic etching in a plasma or reactive ion etching system. [1]

Dry Etching

• Dry etching processes are widely use in VLSI fabrication.

• Highly anisotropic etching profiles can be obtained as shown in Fig. (b).
• Plasma etching immerses the wafers in gaseous plasma created by RF excitation in a vacuum system.

• Sputter etching uses energetic noble gas ions such as Ar⁺ to bombard the wafer surface. Etching occurs by physically knocking out atoms off the surface of the wafer.

• Reactive ion etching combines the plasma and sputter etching processes.

• Plasma systems are used to ionize reactive gases, and the ions are accelerated to hit the surface. Etching occurs through a combination of the chemical reaction and momentum transfer from the etching species.[1]