The Metal Layers

Note: 

1. Layout Rules for all the layers that are going to be discussed will be part of a individual post. 
2. All layers referred here are as per the MOSIS Rules. 

In this post, we will discuss Metal layers used in layout development, which eventually will be used in fabrication process. Following will be discussed,

• Bonding Pads , Probe Pads 
• metal layer (metal1 and metal2) 
• via for contacts  

Note: Metals normally used is either aluminium or copper. 

Metal1, Metal2 and Via1 Layers:  

• metal2 layer will always be placed above metal1 layer. If metal3 layer is to be used then this layer will sit above metal2 layer and so on. 
• via1 connects metal1 and metal2 layers.
• Similarly as the first point,if you were to use more than one metal layers in the fabrication,
  then via2 will connect metal2 and metal3 layers.
• vias' are normally fabricated using tungsten.
• Following figure illustrates the points mentioned before, 

• Parasitics:
•  Let us consider the figure below to understand the parasitcs, 
• Capacitance:Fringe(end/border/perimeter) and Plate capacitance exists.
  Total Capacitance = Fringe Capacitance + Plate Capacitance
  Table below illustrates capacitance's associated with the metal layers
• Resistance:
  There is a finite resistance associated with the metal layers and contact vias.

Bonding Pads :

• are at the interface between the die and the package/outside world. 
• Probe pads are similar to bonding pads with lesser dimensions.
• Final size of the pads are the only part of a layout that doesn't scale as process
  dimensions shrink.