Goals:

The goal of this work is to image the slurry layer between the polishing pad and a wafer during chemical mechanical polishing (CMP).  At high image resolution the darkest pixels represent regions of pad-wafer contact. We can determine the percent pad-wafer contact by examining the dark extreme of the image histograms.

Outcomes:

The imaging area is 1.30x1.74 mm with a resolution of 2.5 ?m/pixel. At this magnification, some regions imaged contain contact, whereas others do not. For the contact regions discussed in this paper, contact percentage varies from 0.07% to 0.27% on a Cabot Microelectronics D100 polishing pad. The asperity contact area increases with applied load, which was varied from 0.28 to 3.1 psi.

Polishing Setup:

A Struers RotoPol-31 table top polisher unit was is fitted with a Mitsubishi Freqrol frequency modulator that allows the platen to rotate at speeds ranging from 20 rpm to 300 rpm. A simple aluminum shaft is driven by the same Dayton ½ HP DC motor. The shaft system is support by 10 series 80/20 aluminum 2”X4” beams. A weighted lever arm is mounted on the top of the shaft to achieve variable applied wafer pressures.

For a complete description of the setup click here.

Optical Setup:

Standard silicon wafers can not be used for imaging because they are opaque.  Instead we use an optically transparent borosilicate glass disk (BK-7 glass).  For slurry imaging we use a technique called dual emission laser induced fluoresce (DELIF). Cab-O-Sperse SC-1 slurry from Cabot Microelectronics is mixed with a fluorescent dye, Calcien.  LabVIEW® software provides an interface through which Nd/YAG laser pulses and the cameras can be timed and controlled for slurry film imaging. The cameras were fitted with a Nikon EL-Nikkor zoom lens for high spatial resolution images.