Introduction:
In integrated circuit (IC) design, power and signal lines are essential for the proper functioning of the circuit. These lines are responsible for supplying power and transmitting signals between different components on the chip. Various layers are used in IC fabrication, and each layer serves a specific purpose.

Explanation:
The layer used for power and signal lines in IC fabrication is the metal layer. This layer is typically made of aluminum or copper and is used to create interconnects between different components on the chip.

Reasons for using the metal layer:

1. Conductivity: Metal layers have high electrical conductivity, making them ideal for carrying power and signals across the integrated circuit. This ensures efficient transmission of electrical current without significant loss.

2. Low resistance: Metal layers have low resistance, which helps in minimizing power losses and signal degradation. Low-resistance interconnects are crucial for maintaining the integrity of the power supply and signal quality throughout the chip.

3. Easy fabrication: Metal layers are relatively easy to fabricate during the IC manufacturing process. The metal material can be deposited onto the substrate using techniques like sputtering or chemical vapor deposition (CVD). The metal layer can then be patterned using photolithography and etching processes to create the desired interconnects.

4. Capability for multi-level interconnects: Metal layers can be stacked on top of each other to create multiple levels of interconnects. This allows for complex routing and efficient use of chip space. Different metal layers can be connected using vertical vias, enabling connections between different circuit elements.

5. Compatibility with other layers: Metal layers can be easily integrated with other layers in IC fabrication, such as the polysilicon layer or the diffusion layers. This allows for the creation of complex circuit structures involving transistors, resistors, capacitors, and other components.

Conclusion:
In IC design, the metal layer is used for power and signal lines due to its high conductivity, low resistance, ease of fabrication, capability for multi-level interconnects, and compatibility with other layers. The metal layer plays a crucial role in ensuring reliable power supply and signal transmission within the integrated circuit.