Via filling

Figure C3.2.15. Schematic diagram showing (A) electron hopping between electron reservoirs via empty states of an intervening bridge, (B) tunnelling, and (C) hole hopping via filled states of an intervening bridge. From...

The electromigration problem has led to the investigation of other electrical-conductor materials, such as tungsten (presently used in contact and via fills) and more recently copper. 

This reaction proceeds at a much faster rate than the hydrogen reduction of WFg. The result is erosion of the silicon substrate causing encroachment and tunnel defects. The use of a different precursor, such as tungsten carbonyl, W(CO)g, may solve this problem. CVD tungsten is presently limited mostly to multilevel-via-fill applications. 

W deposition for via filling. Proceedings of the Second International IEEE VLSI Multilevel Interconnection Conference (1985), p. 343. 

Printed circuit boards Connectors, via filling and embedded functions. Under development. 

Y. Harada, K. Fushimi, S. Madokoro, H. Sawai, and S. Ushio, The characterization of via-filling technology with electroless plating method, J. Electrochem. Soc. 133, 2428, 1986. 

In this chapter we will focus on contact and via fill using the blanket tungsten approach. In chapter 5 we will discuss another application of blanket tungsten, namely, that of tungsten as the interconnect material. 

The main application of CVD-W in the immediate future is contact or via fill. We have seen that both selective and blanket tungsten can give plugged contacts and vias. Therefore, we need to investigate what process will be the first choice for a given situation. Two aspects are important the feasibility and the costs of the contact/via fill process. In addition, a very important criteria will be at what time will the fill process be needed in production. Let us first focus on the feasibility and process requirement aspects. 

In table 4.1 a comparison is made between selective and blanket tungsten for contact and via fill. In the following we will discuss each of these process requirements. 

Process limitations for blanket and selective contact/via fill... 

Based on the experience so far, blanket tungsten is the only production proven solution for contact or via fill. It is expected that rapid general acceptance in the next two years will occur. This implies that it took blanket W plug and interconnect technology about 5 years to become accepted which is surprisingly fast. 

The first opportunity of selective tungsten to become incorporated in IC manufacturing will be for a via fill application. In order to achieve this a very close and extensive cooperation between equipment vendors and major IC manufacturers is essential. Only in this way will a production compatible process be developed. 

Figure 2.39 Tracking the interface mid-height position during via filling provides a comparison between simulation and experiment (Figure 2.38). The inset shows the simulated bottom-up feature filling, with contours colorized to reflect the local 0Sps coverage (source Ref. [12]).

Improving electroless Cu via filling with optimized Pd activation was reported by Lau et al.39 In this work a proper cleaning of the TiN surfaces involving HF and then further activation with PdC was recommended. Other reports on electroless deposition of copper on silicon40 or on TiSiN41 are available in the literature. 

In the IC industry electroless metal deposition can be used for contact filling, via filling, and conductor patterns. In micromachining electroless deposition can be used for all of the same purposes but also to make structural microelements from a wide variety of metals, metal alloys, and even composite materials. 

Figure 14.8.7. Left Misprinted PCB, Right Via filled with solder paste.

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