Back Side Reticle Cleaning
As geometries continue to shrink the requirements for mask cleanliness in terms of both haze and particulate matter become evermore stringent. Furthermore, in the case of the most advanced lithography tools the backside cleanliness can degrade more rapidly than the imaged and Pellicilized side. Heretofore in order to clean the backside it is necessary to remove the Pellicle and clean both sides and then remount the pellicle; thus creating more wear and tear on the image side and requiring a rather lengthy cleaning process that can require more reticle sets than might otherwise be necessary. It is clear that a tool that can clean the back side without in any way degrading the front (Pellicilized side) would be a means of substantially lowering costs of Reticles by enabling rapid turn around of cleaned Reticles to a production role.
The issues are:
- Clean the back side of haze
- Clean the back side of particles
- Do not endanger the image with possible ESD
- Do not endanger the image side by contact
- Do not endanger the pellicle
- Wet process and Dry the backside while maintaining the image Pellicilized side in a dry ambient.
The approaches to resolve the issue are in the order above
- Ozonated DIW for haze removal
- Dual brush types for energetic and dynamic particle removal, the entire back side is addressed by these motorized brushes.
- A patent applied for Reticle chuck the contacts the reticle only at points, ensuring minimal contact.
- A patent applied for Reticle chuck that bathes the Pellicilized side in ionized CDA to protect the pellicle and the image.
- Ozonated DIW removes thin organic films that would render the Reticle surface hydrophobic. By removing such films the surface is Hydrophilic and will therefore spin dry without any “water spots”.
As mask shops and wafer fabs often dispose the pellicle up or down the tool can be provided with a Reticle “flipper”. All material handling is done at edge only with point contact for cleanliness. All Reticle handling mechanisms are “fail safe” so that in event of a facility failure the Reticle is maintained in a safe state.