Table-Top Evaluation Approach to Design Post-CMP Clean Chemicals
Authors: Kusano Tomohiro , Takeshita Kan 
Published: February 2020

In todays advanced integration technology, there have been tough requirements for CMP process, particularly for post CMP clean because even minute defects heavily impact on device yield. CMP slurries have small abrasive particles and many additives, which remain on post CMP wafer and cause various types of defect. However, strong bulk chemical cleaners can easily damage a polished surface of wafer. Thus how to strike right balance between high cleanability and low erodibility is major challenge for post CMP clean chemical development.

Another large difficulty to develop pCMP clean chemicals is that using full wafer-scale CMP and defect inspection tools takes too much time and cost. As a pCMP clean chemicals manufacturer, we have been trying to introduce a small coupon-scale evaluation metrology so that we can conduct screening tests for many candidate chemicals and their combinations in a quick and least cost manner. This metrology includes electrochemical analysis, surface analysis and also benchmarking test using a table-top polisher and a lab-size cleaning unit.

In this paper, we will report how we can apply this coupon-scale evaluation metrology to post CMP clean evaluation for contact plugs. In CMP process involving contact plugs between transistor and M1, great care is necessary because the finest patterns are formed in this region. Typically, tungsten (W) is employed for plug metal and acidic chemicals have been used for post CMP clean because of its strong removability of iron (Fe) that is contained in slurries as a catalyst for W film oxidation. However acidic chemicals often do not have adequate particle removability. Therefore it is one of the top issues to make Fe removability and particle removability compatible. And also it is desirable that we can rapidly evaluate Fe and particle removability in small lab-scale.

One of approach we would like to present is evaluation approach to Fe removability. Fe can be quantified by TXRF or VPD-ICP-MS after wafer polish and clean with CMP tools. But it is not practical to use every time these methods in initial screening stage due to their cost. For this purpose, we have applied electrochemical measurement approaches such as Tafel-plot and OCP (Open Circuit Potential) to estimate Fe removability for various chemicals.

Tafel plot measurement provides corrosion current (Ecorr). A cleaner that has higher Fe removability shows higher corrosion current because the cleaner solution dissolves passivation film like a Fe2O3. So this method can compare Fe removal efficiency between cleaners.

Open Circuit Potential (OCP) reflects surface passivation film condition. When passivation film is formed on surface, OCP rises because oxidation current becomes small. In contrast, when passivation film is removed, OCP drops. Therefore Fe removal speed can be compared by measuring time-dependent OCP for iron electrode with passivation layer in cleaner solution.

In this presentation we will review details of metrology and also comparison of results between coupon-size, lab-level screenings and wafer size tests.

Figure 1