Once the active devices and regions are fabricated they must be electrically connected to each other to make circuits. They must also be connected to the outside world through their inputs and outputs on bonding pads. Making these connections is the job of contacts, vias and interconnects. Separating the interconnects from each other is the job of dielectric layers.

All of these components are part of the “metallization” or “backend” structure. The figure below is a schematic diagram showing these components in a typical integrated circuit structure. In recent times the relative importance of the backend structures has greatly increased, and will likely to continue as integrated circuit technology progresses.

Interconnects can either be global or local. In general, local interconnects are the first, or lowest, level of interconnects. They usually connect gates, sources and drains in MOS technology, and emitters, bases, and collectors in bipolar technology. In MOS technology a local interconnect, polycrystalline silicon, also serves as the gate electrode material. Silicided gates and silicided source/drain regions also act as local interconnects. In addition, TiN, a by-product of a silicided gate process, can be used as a local interconnect, and W is sometimes used as well. Local interconnects can afford to have higher resistivities than global interconnects since they do not travel very long distances. But they must also be able to withstand higher processing temperatures.

Global interconnects, mostly made of Al, are generally all of the interconnect levels above the local interconnect level. They often travel over large distances, between different devices and different parts of the circuit, and therefore are always low resistant metals. Local and global interconnects are shown in the multilevel schematic diagram.

In modern ICs as the complexity of interconnects is increasing an additional level, semiglobal interconnects between local and global has been introduced. The hierarchy of interconnects is as follows:

• Local interconnects – used for very short interconnects at the device level.

• Semiglobal interconnects – Used to connect devices within a block.

• Global interconnects – Used to connect long interconnects between the blocks, including power, ground and clocks.

As the device dimensions are scaled down so are that of the interconnects. The scaling trends of the height and width of the interconnects are shown below.

Ohmic contacts connect an interconnect with active regions or devices in the silicon substrate. A high resistivity dielectric layer, usually silicon dioxide, separates the active regions from the first level global interconnect, and electrical contact is made between the interconnect and the active regions in the silicon through openings in that dielectric layer.

At the same time in the processing, contact can be made between the first level global interconnect and the local interconnects, since they are also separated by that same dielectric layer. This is shown in Figure 1, with contacts from the lowest level global interconnect to: 1. a silicided source region in the silicon: 2. a TiN local interconnect: and 3. a silicided gate. Connections between two levels of global interconnects are usually given a different name: vias. They are made through openings in the different levels of the intermetal dielectrics. These are also illustrated in the figure. Whether they are contacts or vias, the goal of them is to electrically connect different levels of conducting layers, and to contribute as little as possible to the total electrical resistance.

Separating interconnects from each other and from the active areas and devices are dielectric materials. Those dielectric layers separating one global interconnect level from another are called intermetal dielectrics, or IMD. (Some call these interlevel dielectrics, or ILD.) Vias connect interconnects through these layers.

The layer separating global interconnects from both the substrate and local interconnects, through which the contacts are made, are given a variety of names: first level dielectric, dielectric-1, poly-gate dielectric, pre-metal dielectric, etc. - anything but intermetal dielectric. We will usually use the term “first level dielectric”. Examples of both kinds - intermetal dielectrics and first level dielectrics - are shown in Figure 1.