ICMCTF1998 Session H3: Chemical Mechanical Polishing

Post-CMP wafer cleaning has become a key step in successful CMP processing. The cleanup strategy includes post polishing buffing, wet wafer transfer, and wet wafer cleaning. Typically post-CMP cleaning uses a combination of double-sided brush scrubbing, megasonic cleaning, and spin-rinse drying to remove polishing residues from wafer surfaces. This paper investigates the importance of these processes by evaluating IPEC Planar's Avanti 9000 post-CMP cleaner, and infers the mechanisms of particle removal during these processes.

Previously polished and cleaned W sheet wafers were used for this study. These substrates were dipped in diluted CMP slurry before being cleaned by the Avanti 9000 with the megasonic tank off-line. Diluted SC-1 and DI water were used as the cleaning solutions in the brush scrubbing and spin-rinse drying units, respectively.

Our results show that the number of brush cycles, the brush pressure, and the brush rotating speed are key parameters in the removal of particles with diameters greater than 0.21 microns from the wafer. Two factor interactions between: 1) the number of brush cycles and the brush pressure, and 2) the number of brush cycles and the brush rotating speed, were also important to the process performance. Our results also show that the extent of particle removal is relatively independent of the operation of the spin-rinse dryer over the range studied. Thus, after some minimum level of SRD speed and number of cycles (in our studies 500 rpm and 1), operating the SRD for longer does not improve cleaning effectiveness. This suggests that particle removal occurs only during the brushing process, and that the SRD serves only to transport previously dislodged particles off the wafers and to facilitate drying, not to actually remove bound particles from the wafers. Based on these results we will present physical and theoretical arguments for the mechanisms of particle transport and removal during post-CMP cleaning.

As device geometries have continued to shrink and circuit complexity has continued to grow, device isolation has become a major factor limiting circuit density. Shollow trench isolation technology (STI) has been developed and applied to deep-sub-micro process. Poly Si or CVD Si₂N₄ or SiO₂ were deposited on trench and planarized by CMP. However, the wafer surface after processing CMP is contaminated with particles and metallic impurities in slurry. The silica particles will defect VLSI patterns, and metallic impurities even if it is a bit of quanity, will induce many crystal defects in Si wafers during furnace processing. Therefore, the post CMP cleaning very important for STI process. However, the wafers for poly Si surface is hydrophobic, siO₂ surface is hydrophilic and Si₃N₄ films is easy to charge up. The defect is difficult to remove them by conventional cleaning technique.

In this study, we propose to use a modify multi-chemicals cleaning for post STI CMP clean. We use the modified heated (75-90) °C) ammonia/peroxide mix (APM) clean with 5 sec NH₄OH pre-soak time in order to pre-wetting the polished surface and then the NH₄OH helps to remove slurry particles from the wafer surface through including a negative zeta potential between the silica particles and polished surface. There is a clear advantage of etching a thin layer of hydrous oxide with dilute HF, since the films had been damaged during CMP process. The metallic ions can be "trapped" in the damage film and can be very difficult to remove without a light etching. Also, the hydrochloric/peroxide mix (HPM) clean with high temperature (75-80 °C) can help to remove all the metallic ions. This cleaning method of improving the solubility is the formation of soluble complexes.