liftoff
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  • 1. Liftoff Process Using Positive Photoresist March 6, 2003 Biren Patel, Akil Sutton, Leslie George Advisor: Dr. James Zhou
  • 2. Ideal Liftoff Process Substrate Spin Photoresist Pattern Photoresist Deposit Metal Remove Photoresist
  • 3. Problems 1. If sidewalls are not perpendicular, the metal is likely to be continuous. There is no separation between the undesired and desired metal. 2. Metal on substrate doesn’t fall perfectly flat. Parts can also pile up and cling to the resist sidewalls. After the lift, unpredictable results will occur.
  • 4. Solution Create an overhang in the resist profile using chlorobenzene. Soaking resist in chlorobenzene makes the resist harder, decreasing the development rate. Only the surface of the wafer is exposed to the chlorobenzene causing just the top layer of the resist to have a reduced development rate.
  • 5. Important Points Increasing exposure the development rate. Chlorobenzene slowsinsure overhang. piling up against the Soak long enough to timeif the also speed development rate. resist walls, the metal will Even if not continuous, will metal is fall back on the substrate in an unpredictable location after removing the resist. It will also make the angle time to the substrate Increase your developmentbetweencompensate. and sidewalls closer to 90°. Overhang needs to reach over the base of the resist wall.
  • 6. Important Points Even if not continuous, if the metal is piling up against the resist walls, the metal will fall back on the substrate in an unpredictable location after removing the resist.
  • 7. Important Points Even if not continuous, if the metal is piling up against the resist walls, the metal will fall back on the substrate in an unpredictable location after removing the resist. This is true even for perfectly isotropic development.
  • 8. Resist Height Except for conditions of very isotropic development, it is generally better to use a thicker resist such as Shipley 1827 instead of 1813. Using a thicker resist lowers the chances of the metal being continuous. However, if your development is more isotropic, then using a thicker resist requires a longer overhang. Thick Thin Isotropic profile with untreated resist. Not enough clearance between metal on substrate and metal on resist. Anisotropic profile with chlorobenzene treated resist. Overhang is not long enough.
  • 9. Final Process Recipe 1000 Å of oxide thermally grown on 4” silicon test wafers using iniox.003. Spin and expose using Shipley 1813 photoresist. Speed: 5000 rpm Acceleration: 2500 rps2 Time: 30 seconds Intensity: 16.5 mW/cm2 Wavelength: 405 nm Soak in Chlorobenzene for 20 Note: Chlorobenzene is minutes after exposure. dangerous to inhale, keep well under fume hood.
  • 10. Final Process Recipe Develop using MF-319 developer for 70 seconds or until visually complete. 1. 200 Å Ti at at 4.0 Å/s (Bottom) Deposit metal by evaporation. 2. 4000 Å Au at 3.0 Å/s (Middle) 3. 50 Å Ti at 4.0 Å/s (Top) Perform liftoff using a photoresist Use AZ400T PR Stripper at 80° C solvent/stripper. or acetone at room temperature, then rinse off carefully using acetone. No agitation is necessary if a good resist profile was achieved. AZ400T attacks resist faster than acetone.
  • 11. Results Visually, the 5 µm (left) as well as the 2 µm (right) lines are clean and continuous over most of the wafer. The key to the successful liftoff was achieving a good overhanging resist profile.
  • 12. Results A resist profile such as this one will: 1. Prevent the deposited metal from being continuous. 2. Prevent the metal from piling up on the edges of the resist walls.
  • 13. Special Thanks: James Zhou Akil Sutton Leslie George Fredrick L’Hereec Demetris Geddis Christophe Courcimault Reference: Campbell, Stephen. The Science and Engineering of Microelectronic Fabrication. Oxford University Press; 2nd edition.