A novel pad conditioner and pad roughness effects on tungsten CMP

In the BEOL (back end of line) process, tungsten (W) chemical mechanical planarization (CMP) has been commonly used to form contacts. As size of the metal line is shrunk, metal CMP will directly impacts resistance and reliability characteristic of final product. As examples, W contact recess and seam (or void) defects make resistance problems and block failure. In the W CMP Process, W is oxidized by etchant, and then removed by abrasives. Progressively, this process is cycled again until all bulk W is removed. Hydrogen peroxide (H
₂
O
₂
) and silica nanoparticles are commonly used as etchant and abrasive, respectively, to remove the material. To overcome W contact recess issue, we recently have used less selective slurry for W bulk and oxide films. To maintain such selectivity between these films during CMP, however, control of pad roughness evolution throughout the pad lifetime is the most important. Thus, proper selection of pad conditioner is demanded. Chemical etchant is essential to remove W but it also impacts on diamond grits on the pad conditioner in the manner if accelerating wear of the grits. As the pad roughness is diminishing by the wear of the grits, wear resistance of diamond is a key factor to keep desired W to oxide selectivity throughout the pad lifetime. In this experimental study, we used low selective slurry to decrease W recess and evaluated two different types of pad conditioners for keeping the process time variation as stable as possible throughout the pad lifetime. The conditioners have novel design of diamond grit pattern. Pad cut rate (PCR) of the each conditioner was evaluated as well. It was shown that minimum threshold PCR is exist to maintain the pad roughness to reduce process time variation. Finally, it is suggested that moderate surface roughness is required to form contact between pad and wafer to maintain moderate process time.