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Cost of Ownership Implications of a Novel CMP Pad Conditioning Device

Amit Inamdar¹, Michael A. Fury¹, Dan Towery¹, Albert B. Stubbmann², and Jerry W. Zimmer³

¹AlliedSignal Advanced Microelectronic Materials, Sunnyvale, CA
²DIAMONEX, Allentown, PA
³sp3 Corporation, Mountain View, CA

Executive Summary

Conditioning of CMP polishing pads is a process required to establish and maintain stable and acceptably high removal rates for ILD planarization. It is typically accomplished by applying a diamond-impregnated nickel disk to the pad surface using a controlled down force and sweep rate. Commonly used conditioners have a diamond particle size in the range of 100-200 grit. The nickel base precludes the use of these conditioning devices with acidic and oxidizing slurries. The grit size determines the polishing removal rate at which the system stabilizes, but the grit also mechanically abrades the pad surface. This pad wear rate has been characterized and contributes to the end of pad life through one of two mechanisms: thinning of the primary pad, which changes its mechanical properties and leads to degradation of uniformity; and gross removal of the primary pad, exposing the underlying adhesive and base pad. Extension of pad life is beneficial not only for reducing overall pad costs, but for eliminating pad changes and requalifications which impair throughput.

This paper discusses the cost of ownership (CoO) implications of a novel pad conditioning device which is capable of direct replacement of the traditional nickel-diamond conditioner without requiring process parameter changes. The DIABOND^® pad conditioner consists of diamond grit epitaxially bonded to a CVD diamond film on a non-metallic substrate such as a silicon or silicon carbide wafer. Polishing data in a fab production environment indicates a 50% reduction in pad wear, reducing pad costs and the number of both pad changes and process requalifications. Data also indicates up to a 75% increase in conditioner life, reducing the lost production time for conditioner requalifications. Removal rate for ILD and BPSG processes is stable and is also seen to increase up to 30% following optimization of the process parameters for the DIABOND^® conditioner. The value of these attributes is presented in a SEMATECH-based cost of ownership model comparing the DIABOND^® conditioner with nickel-diamond conditioners.

Extended Abstract

Preliminary reports from fab customers indicate a realization of productivity benefits which had been hypothesized to result from the use of the DIABOND^® conditioner. Using this information, a cost of ownership model was constructed using baseline parameters indicated in Table 1. These parameters are based on ILD planarization processes using a Rodel IC-1000™ pad and conventional silica slurry. One reported attribute of DIABOND^® conditioning is extended pad life, so a model was constructed to show pad life up to 2000 wafers. Figure 1 shows that CoO levels off at about 600 wafers, with a step function due to a tool utilization break point. Therefore, subsequent comparisons were done comparing a conservative pad life of 300 wafers using a conventional conditioner to a pad life of 600 wafers using a DIABOND^® conditioner. Another reported attribute is an increase in the removal rate at which the polishing system stabilizes. Using 2000 /min as a baseline removal rate, models were created showing the CoO benefits of removal rate increases of 10%, 20% and 30%.

The overall CoO behavior is shown in Figure 2. The upper set of curves, based on a removal rate of 2000 /min, shows that the cost of pad conditioning is a constant value of about $0.10 per wafer over the hypothetical (and unrealistic) case of no pad conditioning. The DIABOND^® conditioner contributes approximately the same cost of ownership as the conventional conditioner despite its higher unit cost, due to its reportedly longer life time. CoO improvement is realized from an increase in pad life, reading horizontally in Figure 2. If the polishing process parameters are optimized to take advantage of the higher removal rates reported with this conditioner, then larger CoO savings are realized through reduced slurry consumption and increased throughput, reading vertically in Figure 2. The break point between 400 wafers and 450 wafers pad life at 2400 /min is due to a decrease in unit tool requirements, and is common in CoO modeling. The decrease in consumables cost alone, without the throughput contribution, is shown in Figure 3. In this case, consumables cost includes pads, slurry, and pad conditioners. Using the case of 2000 /min and 300 wafer pad life with a conventional conditioner as a base line, the percentage reduction in total CoO is shown in Figure 4 as a function of percentage improvement in pad life and percentage improvement in removal rate. Similarly, the percentage reduction in consumables cost is shown in Figure 5.

The model data presented indicate that total CoO savings of 10% to 25% are achievable under a variety of conservative scenarios, and that consumables savings can range from 4% to 30%. Additional reported benefits not included in this model are the stabilization of polishing uniformity, a reduction in pad break-in time, and a reduction in defect levels due to pad conditioning. The materials of construction of the DIABOND^® pad conditioner enable the implementation of pad conditioning for acidic slurry processes such as tungsten, aluminum and copper CMP. In doing so, it also enables the use of IC-1000™-type pads for metal CMP under a wider variety of process conditions. The implications of this capability remain to be examined.

References

Parameter	Value	Parameter	Conventional Conditioner	DIABOND^®Conditioner
Fab starts/week	10,000 wafers	Unit cost	$180	$600
Tool	IPEC 472	Conditioner life	3,000 wafers	6,000 wafers
ILD Pad cost	$270 each	Pad life	300 wafers	600 wafers
ILD Slurry cost	$13/gallon	Conditioner size	2" diameter	2" diameter
Oxide removed	8,000
Throughput @ 2000 /min	12 wafers/hr.
Slurry flow rate	100 ml/min.
Conditioner change & requalification	2 hr.

Table 1. Baseline Parameters for CoO Model Cases.

Figure 1. Stabilization of CoO as a function of pad life.

Figure 2. Total Process CoO for Pad Conditioning (for legend see Fig. 3).

Figure 4. Total Process CoO Reduction Potential (for legend see Fig. 5).

Figure 3. Consumables CoO for Pad Conditioning.

Figure 5. Consumables CoO Reduction Potential.

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