W CMP process

Fayolle M, Sicurani E, Morand Y. W CMP process integration consumables evaluation—electrical results and end point detection. Microelectron Eng 1997 37(38) 347-352. 

Similar to oxide CMP, a basic W CMP process uses a polisher equipped with a polishing pad, a cleaning unit, and a slurry distribution system. Owing to the need for an oxidizer, the slurry distribution system also includes a premixing sequence. As the lifetimes of these oxidizers are usually short, the... 

The slurry distribution and mixing system operates in the background of the actual wafer processing. However, a slurry with consistent physical and chemical properties is critical for maintaining a repeatable W CMP process with steady yield. Two of the most important parameters to monitor are etchant (oxidizer) and solid (abrasive particle) concentrations. 

The post-W CMP cleaning with ammonia in the past has caused wafer surface defect issues. Is it possible for you to determine alternative cleaning chemistries for the post-W CMP process Explain why you think they will provide a more efficient process. 

A typical W CMP process consists of several polishing materials W, W liner (Ti and/or TiN), and Oxide [45-47]. In some unique W CMP applications, the process may involve the removal or sloping of other materials such as for SiN and poly-Si. The focus of this section will be on the popular stacks (W, Ti, TiN, and oxide) and the common defects left on the oxide surface after CMP and post-CMP cleaning [48,49]. 

On the large structures, a W CMP process may leave a very rough W surface behind as a result of insufficient W filling before CMP. The CMP process simply did not reach down to these recessed areas. Normally, the W structures... 

Because the problems associated with W plasma etchback process are inherited in its chemistry and process design, it was difficult to resolve them [2-5]. Therefore, an alternative process - metal chemical mechanical polishing - has been proposed and studied extensively in the last several years [6-12]. With the maturing equipment and consumable sets, the W CMP process is proven to be better in terms of defectivity [6], controlling the plug recess, and high... 

Unlike W plasma etch back process, the typical W CMP process usually removes the adhesion layer such as Ti/TiN or TiN during the primary polish. As a result, during the over polish step there is some oxide loss. Since the oxide deposition, planarization CMP (oxide CMP), and tungsten CMP steps are subsequent to each other, the oxide thickness profile could become worse further into the process flow. Therefore, the across-wafer non-uniformity of the oxide loss during W CMP process is one of the very important process parameters needs to be optimized. To determine the effect of the process and hardware parameters on the polish rate and the across-wafer uniformity, designed experiments were run and trends were determined using analysis of variance techniques. Table speed, wafer carrier speed, down force, back pressure, blocked hole pattern, and carrier types were examined for their effects on polish rate and across-wafer uniformity. The variable ranges encompassed by the experiments used in this study are summarized in Table I. 

Fig. 3. Heavy slurry residue accumulated in the recess area (alignment mark) after standard Fe(N03) 3 W CMP process.

Table 2 shows a summary of DOE data for normalized values of P [represented by the down-force variable, and relative velocity V between the platen and carrier (represented by the variables, the platen speed, and the wafer carrier speed)], with the corresponding normalized removal rate for the W CMP process. Fig. 5 shows a good fit between Preston s model for the process and empirical data with normalized polish rate for each of the values shown in Table 2. 

During polishing, metal films are hard to remove by abrasive because of their inertness and hardness. With the addition of a suitable oxidizer, a metal can form an oxide layer on its surface, which can be easily removed during CMP. Since Kaufman et al. first proposed a model of the W CMP process using K3(Fe(CN)6) (Kaufman et al., 1991), there have been studies on metal slurries containing various oxidizers... 

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