Simultaneous Removal of Particles from Front and Back Sides by A Single Wafer Backside Megasonic System
Introduction
In IC manufacturing, particle removal from a wafer’s back side (BS) has become as important as that from the front side (FS). For example, during lithography, BS particles can cause a variation on the topside surface topography. This may result in a focus-spot failure due to the reduced process window for depth of focus (DOF) . This problem increases as the feature size decreases. BS particles may cause other problems in wet benches, where BS particles can be transferred to the adjacent front side of wafers.
The typical source of BS particles is wafer handling with either an electrostatic or vacuum chuck or from plates and stages in the vacuum chamber which result in defect maps as shown in Fig. 3. For pre-lithography BS particle cleans, these particles are generated mainly during dielectric deposition, metal sputtering or implant/ash.
Because of the typical elevated temperature in the vacuum process, particles can adhere strongly to the wafer backside. Compounding the issue, the introduction of immersion lithography for advanced device fabrication brings increased concern about the presence of loose films and particles that can accumulate at the wafer edge (bevel and apex). The wafer undergoes multiple wafer processing steps in the device flow and contamination can be introduced at each step. Because of the high throughput requirements for scanners, the immersion hood water layer moves at speeds of about 0.5m/second and this exerts high capillary forces from the trailing edge of the water meniscus that can dislodge defects from the edge and re-deposit them on the front side of the wafer as shown in Fig. 4. [2] While the critical particle size for BS is larger than FS, it tends to decrease as the feature size decreases as indicated by Table I. [3]
So far most silicon wafer cleaning tools have been developed in order to remove FS particles by physical force and/or chemical reaction. Whenever BS cleans are needed, wafers are flipped over before and after cleaning or cleaned by indirect physical force transmitted through the wafer with chemicals dispensed to the back side of the wafer. But, in these cases, BS particle removal efficiency (PRE) is much lower than FS PRE. In this paper a new single wafer megasonic system was introduced and both FS and BS PREs were evaluated as a function of megasonic power/time and source of contamination.