XeF2 Recipes

Typical recipe of XeF2 etcher

• The number of cycles: 1~60
• Etch time: 60 sec
• The pressure of XeF2: 3.0 Torr.
• The pressure of N2: 2.0 Torr

To optimize the etch rate, the pressure of XeF2 is chosen from 2.8 to 3.2 Torr (from the log data).

Note

• For high etch rates, the manufacturer recommends maximizing the XeF2 pressure and running 60 second cycles.
• Since Si vs SiO2 selectivity of XeF2 is high (> 50:1), SiO2 works well as a masking material during XeF2 etching. On the other hand, it may take 30~60 min to remove SiO2 residue left after patterning by XeF2 etching. Therefore, when you start etching, expect to add extra cycles to etch through native oxide, and monitor the etch rate as you proceed.
• Photoresists are very good masking materials. However, selectivity to photoresist is NOT infinite. Tests conducted in 2009 showed it to be greater than 40:1. Extended etches using baked SPR220 demonstrated degradation of resist after 10 hours of etching. Any minor flaws, cracks, or pits in the resist will aggravate this problem, giving lower apparent selectivity.
• DO NOT set up the XeF2 pressure more than the practical upper limit (~3.6 Torr). If do so, the etch process will get stuck in the pre-etch cycles (press Stop to cancel).
• Wafers should be kept evenly on the support surface to prevent gas from traveling around to the back side of the wafer and reacting with (etching) the back side rather than the top surface.
• The surface must be dry.
• Do not introduce solvents such as acetone into the XeF2 etcher. These can cause serious safety hazards.
• The etching is isotropic and will roughen the Si surfaces. The etch rate may be nonuniform. Larger, more open features are expected to etch faster than smaller features, due in part to confined circulation of the etchant gas around small channels and pockets.
• If the XeF2 gas pressure cannot reach 3 Torr, notify the staff.
  • Since the XeF2 gas is slowly depleted during the process, the actual pressure might be smaller than the set-up operating pressure. In fact, etch rates of another Xactix XeF2 etcher with an operating pressure of 3.5 torr were close to those of the UPenn machine running at 3.1 torr.