Semiconductor Applications
Particle Removal
Resist Strip
Masks
Post-CMP Cleaning
Particle removal
The EHD process generates uniquely-small, sub-micron-sized nanodroplets that are not obtainable with other spray technologies. Upon striking particles, these inbound droplets transfer their momentum to the particle, overcoming adhesive forces and removing the particle from the substrate. More efficient momentum coupling is realized when the particle and droplet are closer in size.
This particle removal process is a green technology, significantly reducing chemical usage and disposal. EHD may run at rates under 10 microliters per minute.
Because EHD offers additional "knobs" due to more input parameters, the process window is larger. This allows an EHD system to be optimized for a number of diverse applications.
Below are before and after defect maps of a bare silicon wafer that has been contaminated with a slurry of <50nm silicon nitride particles. Even under non-optimized conditions, particle removal efficiency of 95% is realized.
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| Bare wafer coated with <50nm silicon nitride slurry | Same wafer after EHD clean showing 95% particle removal efficiency (PRE) |
Resist Strip
After its use in imaging, photo resist must be removed in a step known as a resist strip. One extremely difficult resist strip application is removal after High Dose Implant Strip (HDIS), where it is exposed to high-intensity ion beams during the implant of impurities into the silicon crystalline structure. During ion implant, the top surface of the photo resist is severely damaged by the energy of the ion beam, forming a carbonized crust. This crust is difficult to remove without heated acids and other chemicals. EHD has demonstrated its ability to break up the crust with physical energy, and then remove the underlying bulk resist, as shown below.
Masks
Photolithography masks are used to create patterns, most often in photo resist. Advanced masks, such as those for extreme ultraviolet (EUV) exposure, do not have protective barriers, or pellicles, found on older masks because of the pellicle’s light absorption properties. Particle removal is more difficult because EUV mask blanks and patterned masks are sensitive to the chemicals typically used in cleaning. EHD holds potential for addressing this problem.
Post-CMP Cleaning
As copper interconnect technology advances, smaller via and line geometries require better defect removal after chemical mechanical planarization (CMP). This is made difficult by the incompatibility of low-k dielectric materials with traditional wet cleaning methods. EHD has been used on post-CMP cleaning demonstrations on wafers treated with a 50-nm silica particle KOH slurry.