Properties of Interconnect Materials

4.1 Properties of Interconnect Materials

The full functionality of an IC cannot be reached with the use of a single isolated device. It requires an engineered interconnected network, which enables the proper communication between the different components (e.g. resistors, capacitors, diodes, transistors, and more). The interconnections should behave akin to an ideal wire and minimize the disruption of the signal flux inside the circuit.

Interconnections may be composed by conductive lines, vias, wires, pads, and joints [65]. Each of these components can be formed by different materials. Metals and theirs alloys are usually the best choice for building interconnections, due to their high electrical conductivity. In the very beginning of the semiconductor industry aluminum and copper were the most common metals, but with the increasing demand for performance, integration, and reliability a wider variety started to be employed, such as tin, tungsten, zinc, gold, and nickel [66].

The introduction of metal structures inside semiconductor devices creates several problems. For instance, metals have a large CTE when compared to silicon, therefore during thermal variation, stresses arise due to the thermal mismatch between the materials. Furthermore, the manufacturing process is also very challenging. Metals are usually deposited by employing various methods, such as Chemical Vapor Deposition (CVD), Physical Vapor Deposition (PVD), and their variations [38][66]. Each method has its particularities regarding material quality, adhesion, and mechanical properties, which also must be considered when attempting to generate reliable devices[38].

To overcome all the challenges of an interconnection fabrication, the implemented materials and their interactions with the silicon must be understood. Each part of an interconnection demands a specific approach. In this work the focus stays with vias and the materials most frequently found in a via structure.

Vias are the elements responsible for the vertical connections in a device. They make the bridge between the different metal lines and, in the case of TSVs, between dies. Aluminium, copper, and tungsten are the most common conduction metals, but there is a large assortment of other materials required to support via fabrication. Some of these materials and their purpose are present in Table 4.1.

Table 4.1.:

Melting point and electrical resistivity of common interconnection materials. The materials are grouped by their function within an interconnect structure.

Function	Material	Melt. point	Electr. resist.	Ref.
		( $∘$ C)	( $μΩ$ /cm)

	Al	1084	1.70	[67]
Metal	Cu	660	2.70	[44]
	W	3410	5.65	[43]


	PtSi	1229	25-35	[68]
	TiSi $2$	1540	13-16	[69]
Silicides	WSi $2$	2165	50	[70]
	CoSi $2$	1326	18-20	[70][71]
	NiSi	992	14-20	[70][72]


Barriers,	TiN	2950	25	[47]
glues and	Ti $x$ W $1− x$	220	NA
passivations	N+ polysilicon	1410	NA	[73]

The function of each type of material can be summarized in the following:

Silicides are used as a conductive material only in local regions of the structure, which have to be exposed to high temperatures or oxidizing environment.
Barriers are applied to prevent diffusion of metal atoms toward silicon, minimizing the risk of contamination and current leakage.
Glues are needed to improve the adhesion of metal films in the silicon surface, avoiding delamination problems which could lead to a via failure.
Passivation materials are protective coating against the hazards of the ambient.

As seen in Table 4.1, copper is the most suitable material regarding electrical conductivity, but it was not used in large scale production as the conductive material in a via until 1997. Several challenges were present which limited copper deposition, such as patterning difficulties and silicon contamination. Volatile copper compounds were unknown and the technology at the time required such compounds for metal deposition [74]. The problem was overcome through the invention of the dual damascene process which patterns the silicon itself by etching and filling the created trenches and vias with copper [74][75][76].

Silicon contamination was solved by the application of barriers metals (e.g. Ta, TaN), which must prevent copper diffusion without drastically reducing the conductivity, otherwise there is no benefit for copper utilization[75][76][77].

Before 1997, tungsten (vias) and aluminium (lines) were the dominant interconnection materials [66]. The processing of Al-W interconnections is easier in comparison to the processes required to generate copper based vias [66]. They do not face the severe problems of silicon contamination and volatile compounds are known and available. Moreover, aluminium and tungsten mechanical properties are more compatible with silicon, i.e., aluminium and tungsten enjoy a lower CTE mismatch and higher melting point than copper.

The effort required to incorporate high conductivity metals is justified only for some industrial applications. The high conductivity metal allows for the creation of smaller and more energy efficient devices, a very important feature in general purpose processors.

The idea behind a TSV is not different from the traditional via. Both are vertical connections, but the dimension of the TSVs are much larger in comparison to the metal line vias. Therefore, mechanical properties become much more relevant than before. The manufacturing of TSVs demand a more careful design to manage the increasing mechanical instability of the structure.

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