Smoothly does it

Conventional chemical mechanical planarization techniques can cause problems at the 65nm device node. One promising alternative is electrochemical mechanical planarization. Alain Duboust, Yan Wang, Feng Liu and Wei-Yung Hsu of Applied Materials reports.

Electrochemical mechanical planariz-ation (Ecmp) is planarization and process control by electric charge. It is a highly efficient, high removal rate process that operates independent of downforce. The inherently low downforce (0.3psi) of this approach is 5-10x lower than conventional CMP; minimal downforce is essential to minimise stress and enable true low-k materials at the 65nm device node and below. Since Ecmp utilises a removal rate directly controlled by the applied voltage (voltage directly controls the charge), this proportional and precise relationship means that Ecmp technology can provide superior and repeatable planarity.

The near-perfect planarization of the Ecmp polishing step has several benefits. For bulk copper removal, it halves the necessary total plated thickness from 2x the trench depth to about 1.2x trench depth. This in turn reduces via voiding due to stress voiding/stress migration. It also enhances electrochemical plating (ECP) throughput, especially at upper metal layers, where plating thickness often exceeds 2µm.

Besides its technical benefits, Ecmp is significantly cheaper and more productive than conventional CMP. Its consumables cost (a huge cost factor in CMP) around 30% less than that for conventional CMP. Throughput, especially for upper metal layers, is also substantially increased due to shorter bulk polishing time.

A new generation of CMP

The limitations of conventional slurry-based CMP become evident at the 65nm node. Applying a typical process downforce (~2-3psi) causes too much dishing and erosion, resulting in high electrical resistance across various pattern densities. High downforce-dependent processes also tend to cause mechanical failures in fragile low-k dielectrics.

To overcome these challenges, Applied Materials developed a new planarization technology that uses an electrolyte chemistry instead of abrasive slurry and features high planarization efficiency at very low downforces (<0.5psi). As the copper removal is greatly optimised by Ecmps precise control and electrolyte chemistry, the selection of barrier slurry provides a further means to reduce dishing, erosion, metal loss, etc, and enhance overal planarization performance. Additional benefits include increased process stability and repeatability, at a lower operating cost due to a 30% reduction in consumables cost.

Multi-step process

Copper CMP has traditionally been a multi-step process, because of the need to remove thick bulk copper overburden, as well as a thin barrier layer (usually TaN), to the level of the interlayer dielectric. The Ecmp process flow is similar to the conventional multi-platen process flow in that it uses separate steps for bulk copper removal, final copper/barrier removal and finish barrier/dielectric removal. However, at the first step, bulk copper is removed by electrochemical mechanical polishing (Ecmp) combined with precision charge endpoint and electrochemical profile control. The very thin, completely planar copper film and barrier layer remaining after Ecmp step one are removed using conventional platens two and three respectively, which operate with high-precision, low downforce (~0.6psi) polishing conditions.

Before Ecmp begins, the incoming thickness profile of each wafer is automatically measured to accurately control bulk copper removal and detect endpoint. The incoming profile data is automatically fed into the polishing recipe for accurate and repeatable profile and endpoint control.

After measuring the incoming wafer profile, the charge control algorithm sets a post-profile model of the desired remaining copper thickness. This algorithm uses a physical model of the process cell and current measurements to trigger the endpoint. Then, the algorithm computes the optimal charge for the multiple electrochemical zones. A process recipe sets the voltages and times for the process. This control technique provides a tight, repeatable and reliable control of endpoint, independent of incoming film variations.

The Ecmp process

Ecmp is driven by electrochemical reactions and is practically independent of mechanical downforce. High removal rate (>6,000Å/min) is achieved with nearly no shear force being applied to the wafer. The removal rate is controlled by the electric charge applied to the wafer, enabling very high rates with full control.

The Ecmp technology is designed for use on dual damascene copper/low k structures created with PVD TaN barrier/seed films. The Ecmp process and profile control technology is used for the bulk copper removal step.

The Ecmp liquid chemical delivery system (LCDS) mixes electrolyte chemicals as needed and delivers them to the wafer at a low flow rate. Within the Ecmp platen, the electric charge is applied to multiple independent electrochemical zones, controlling within-wafer removal rates to adjust for variations in topography, creating a planar, uniform removal profile across the wafer.

Ecmp utilises a specially designed pad in conjunction with an electrolyte solution instead of slurry. Material removal is localised in areas where the pad contacts the wafer, such as overburden or humps over dense arrays. Areas of no contact (recessed areas such as in large features) undergo virtually no removal, reducing pattern dependence compared to conventional CMP (Figure 1).

A chemical passivation mechanism, in which a layer of passivation forms almost instantly on non-contact areas, is the key to the process. This passivating file is composed of polymers which have excellent chemical stability but poor mechanical stability, and so lend themselves to be removed easily by the application of a very small shear force (<0.5psi). In high areas of the wafer that contact the pad, the thin passivating layer is removed and electrical dissolution (removal) of the copper occurs. The passivation layer has a much higher electrical resistance than the passivation-free areas contacted by the pad, allowing copper removal to be localised to the pad contact zone. At the same time, this mechanism suppresses removal in the recessed areas.

Topographic control

The absence of material removal in protected recessed areas in the Ecmp polishing process eliminates device pattern dependence, providing uniform planarization across both dense arrays and large open areas or trenches (regardless of low k dielectric films). The key effect of this pattern independence is a significant reduction of dishing and erosion (Figure 2).

In addition, the subsequent fine polishing steps are rendered much easier and more uniform due to the highly uniform profile of the Ecmp layer. Figure 3 shows the post-polishing thickness that remains over a range of dense and wide features. Sheet resistance distribution across dense and wide patterns is extremely uniform and independent of pattern effects (Figure 4).

The conventional bulk removal step has typically operated at 2-3psi, which generates significant shear forces that can damage the low k dielectric. In order to successfully integrate ultra-low k materials (k~2.5), it is necessary to minimise downforce for the bulk polishing step. Ecmp employs just enough downward pressure (0.3psi) to ensure adequate contact of wafer to pad. This dramatic downforce reduction also effectively avoids the peeling that plagues conventional processes (Figure 5) and reduces particulates and other types of CMP damage, such as microscratches.

Line resistance

Electrical results (Figure 6a) show that the isolated line resistance for advanced low-k dielectric wafers polished with Ecmp is ~130% lower than typical CMP, indicating improved erosion performance. The Van Der Pauw results (Figure 6) indicate ~20% reduction in sheet resistance compared to typical CMP.

Environmental advantages

With Ecmp, disposing of CMP slurries is no longer a major issue. Although the polishing chemicals in the slurry are not difficult to neutralise, the presence of large amounts of abrasive particulates in the slurry complicates the disposal. Many larger fabs employ expensive concentrators to reduce the slurry volume for disposal; however, smaller fabs often have to ship the used slurry to disposal facilities at considerable expense.

The electrolyte used in the Ecmp process dramatically reduces disposal complexity and cost. The electrolyte chemicals are easily neutralised and are no more difficult or expensive to dispose of than many other industrial chemicals.

Integration and productivity

Using Ecmp for planarization, copper ECP deposition can be theoretically as thin as just filing the trench features, with a bare minimum of copper overburden. This capability of Ecmp can potentially boost the productivity of ECP systems in the fab and even reduce the number of such systems needed. Ecmps enhanced process control (planarization efficiency, profile control, endpoint detection) and increased copper removal rate during the bulk removal step also reduce the copper residue removal time on platen two, increasing system throughput.

Conclusion

A new CMP technology, Ecmp offers capabilities needed for cost-effective and superior planarization at and beyond the 65nm device generation. Its virtual absence of shear force removes a key roadblock to using fragile, ultra low k dielectrics. At the same time, its highly effective global planarization, independent of pattern density, maintains low line resistance. By reducing the need for total plated thickness to about 1.2x trench depth, Ecmp can enhance throughput of both the CMP and ECP systems. Replacement of expensive copper slurry with a low-cost electrolyte lowers the cost of consumables by >30%.