Spin on Glass Defect Inspection

Pre Spin Coating

Exaggerated View of Large Particulate and Improper Dehydration Bake

1. Particulate/dust contaminating liquid SOG.

2. Explanation: If disruptions to the surface tension of the liquid SOG are seen before spinning, then some type of particulate is in the SOG and this warrants restarting the test. There are three possible causes of this…

   1. Dust landed onto the wafer after the SOG was applied

   2. Dust already existed on the wafer from improper cleaning

   3. Some SOG has hardened inside the bottle creating fine flakes of SiO2. These flakes then get sucked up by the pipette and deposited onto the wafer with the liquid SOG (Example picture above). This warrants preparing a new bottle of SOG based on this document.

Post Spin Coating

Radial Variation

1. Particulate/dust contaminating wafer surface.

   1. Often, after spin coating disruptions to the SOG evenness/thickness can be seen in localized spots on the wafer. There are possible causes of this…

   2. Some SOG has hardened inside the bottle creating fine flakes of SiO2. These flakes then get sucked up by the pipette and deposited onto the wafer with the liquid SOG. These flakes are clear and sometimes not visible pre spinning, and are then only visible after spinning.

2. Particulate Landed on the wafer during spinning. Sometimes the spin coater can create turbulent air and kick up dust from inside the spin coater which then lands onto the wafer.

3. The wafer had a preexisting scratch or mark big enough to cause surface tension issues which result in the liquid SOG “un-wetting” in that spot and exposing the substrate. These scratches are often caused by scratches that occur during cleaving, slipping across the wafer when using hard tip tweezers, or dropping the wafer.

1. Radial variation in thickness/diffraction color

   1. Sometimes after spin coating, there may be a radial pattern of changing color on the wafer. The varying colors indicate variation in thickness since SiO2 is translucent so thin films diffract and appear different colors based on thickness. These variations are usually on the order of a few hundred Angstroms (A) (film thickness ~2000 A) and may not warrant restarting.

During Spin Coating

1. Wafer flies off the chuck while spinning

   1. Sometimes either due to bad two sided tape adhesion (regular spin coater) or improper sealing onto the vacuum chuck (vacuum spin coater). The chip may fly off the chuck due to inertial forces. This usually warrants restarting since when the chip flies off it hits the side causing uneven spreading of the SOG or lands onto the SOG.

Post-Anneal

Identifying “defects” in the deposited SOG

Pinholes

Example 1

Example 2

Example 3

Possible Causes

1. Scratches or dust particles on the wafer Cause the liquid SOG to “un-wett” in some spots. This essentially creates a dip in the SOG layer that extends to the wafer surface below the SOG.

Particulate resting on top of the SOG

Possible Causes:

1. Dust coming into the fume hood and landing onto the wafer after the SOG layer has solidified.