Tunnel Oxide Gets a Boost From Ozone

Laura Peters, Senior Editor -- Semiconductor International, 9/1/2006

The effectiveness of the pre-oxidation wet clean in flash memory devices is important because, although they do not scale as rapidly as logic gates, they require lower leakage performance and generally operate at higher voltages than logic devices. A recent benchmark study explored the effect of preclean properties on oxide quality using non-contact and traditional electrical testing. Conducted by engineers at Semitool (Kalispell, Mont.) and MVC (San Jose), the study compared three cleaning processes on different process tools and showed that an ozone-based clean produced oxides with lower interface trap density and higher oxide resistivity, indicating a lower leakage interface. Three wet cleaning tools were used to perform the processing on 200 mm wafers: Semitool’s Raider system, Vendor A’s batch cleaning tool, and Vendor B’s spray processing tool. The engineers chose to use the same cleaning chemistries as those used in production for the respective tools. The Semitool system used dilute HF (20:1) together with HydrOzone and FluorOzone chemistries. Vendor A tools used traditional RCA chemistry (SPM, 100:1 HF, SC-1), with the SC-1 of 1:2:20 NH₄OH/H₂O₂/H₂O. Vendor B system used SPM, 500:1 HF, SC-1 and SC-2, with SC-1 of 1:1:20 NH₄OH/H₂O₂/H₂O and SC-2 of 1:1:20 HCl/H₂O₂/H₂O.

After the preclean splits, the engineers measured the particle counts (>0.06 µm) and oxide thicknesses. A Quantox tool from KLA-Tencor (San Jose) was used to measure interface charge density (D_it), total oxide charge and oxide resistivity. Some wafers were further processed with a patterned, doped polysilicon gate to provide electrical test capacitors (200 × 450 µm). Then an Agilent 4155 analyzer measured the hard breakdown characteristics (Figure). Three wafers were measured from each preclean group, covering five sites per wafer and six capacitors per site, giving 90 breakdown measurements per split group.

The wafers in the ozone-based process showed lower particle counts and a slightly thicker oxide (~2 Å) than those processed in the more traditional RCA cleaning chemistries. The engineers observed that the impact of using higher-concentration chemistries became more evident as oxide thicknesses decreased with shrinking device geometries. The active oxide quality can be significantly impacted by the preoxidation clean. With scaling, the interface between the silicon and oxide layer becomes a larger percentage of the device structure, and the properties of the interface are largely dictated by the preclean process.

The average breakdown for the ozone clean is 19.4 V, 18.2 V for the Vendor A clean, and 17.7 V for the Vendor B clean. (Source: Semitool)

The results (Tables 1 and 2 ) indicate that the ozone-based process led to oxides with lower interface trap density and higher oxide resistivity. It can be inferred from this data that the quality of the interface is better. The higher average breakdown levels could be caused by a decrease in oxide defects, either from a reduction in particles, a better terminated oxide interface (as indicated by the interface charge density and oxide resistivity), or a combination of the two.

Since tolerance at high operating voltages (~20 V) is required for flash memory devices, high breakdown characteristics are warranted and low leakage is needed to retain information. The characteristics of lower particle levels, lower interface charge density and higher resistivity (lower oxide leakage) contribute to better oxide quality and reliability in this important application.