Thursday, October 23, 2014
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Photoresist Adhesion

Photoresist Adhesion PromotionYES introduced a process now commonly known in the semiconductor industry as vacuum bake/vapor prime, a key step in the front end processing of silicon chips. The process dramatically improves the application of hexamethyldisilizane (HMDS) as a surface priming treatment, which is used to enhance adhesion of photoresist on a wafer surface.

Early application of HMDS used immersion or spraying of liquid chemical onto the surface of the wafer in a room atmosphere, but wet HMDS application processes have limited surface longevity. Process engineers must apply photoresist within a few hours of HMDS treatment or the surface modification affects of the HMDS are degraded or lost.

Investigation into the limited surface longevity revealed the culprit was water vapor, which is always entrained on surfaces exposed to atmosphere. Effectively, the HMDS was bonding to a microscopic water layer rather than to the wafer surface.

In order to extend the lifespan, first, wafers must be completely dehydrated—not only surface moisture, but chemically bound water molecules as well. To achieve this, Yield Engineering Systems developed a process combining high temperature with low pressure. YES vacuum ovens remove the moisture that is chemically bound to the surface so the silane/substrate bond isn’t impaired.

Then, when chemical is applied, a superior bond is formed that is stable even after exposure to atmospheric moisture. Wafers properly treated will last for weeks with no change to surface adhesion.

The Vacuum Bake/Vapor Prime process offers several process advantages over liquid HMDS application:

  • Chemical Deposition Uniformity
  • Contact Angle Control Within +/- 3 Degrees
  • Moisture Resistant Surface Modification
  • Increased Time Available Between Process Steps
  • Angstrom-Level Thickness Control
  • Reduced Chemical Usage

YES-TA Series vacuum bake/vapor prime ovens are ideally suited for HMDS deposition.

Having a photoresist adhesion issue? Check out our Troubleshooting Photoresist Adhesion Checklist.

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