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Monday, November 27, 2017

Metal Wire Orientation (HVH or VHV)

In previous articles, we have discussed a lot about type of metal wire like Mx, My and others. We have also discussed about the Metal stack like 6 Metal layer stack : M1_3Mx_My_Mz. (If you have confusion, please refer Article "Metal Layer Stack Nomenclature").

In this article, we are going to understand another important concept: "Metal layer Orientation". Actually during routing of design, we use a terminology "Preferred Routing Direction" of Metal layer. May be you have heard HVH or VHV routing strategy. Even if not - then I am sure you have seen below fig somewhere :) .

In the above pic, you can see that M1, M3 & M5 are in Horizontal direction and M2 & M4 are in vertical direction. Still confused ??? :) Let me help you.

Metal wires in our design are at different levels. If, I assume that M1 is at first level then M2 is at second level, M3 at third and so on. Type of Metal (Mx, My, Mz) depends as per Metal Stack you are going to choose for your design. (For more detailing about the Metal stack - please refer Article "Metal Layer Stack"). By now, you should be clear that different metals present at different height with respect to substrate.

Now, only one question is remaining, how we are going to route these metals? How in the sense - in which direction or say orientation. Is/are there any standard/s behind this or user can route these metals wires as per their requirement? Very short answer of this question - There are standards or say recommendation for routing Metal wires. Timing is very critical now a days. From metal wires point of view - capacitance between them plays a very important role. (Note: Delay has relationship with RC constant - and this C is because of Capacitance between the wires). So as a designer we have to understand or say use those technique which can minimize these unwanted Capacitance (Remember - capacitance between Wires are always unwanted. It's always has side effect in negative sense :) ). To understand different type of capacitance between wires, you can refer Article "Basic of Capacitance & Resistance (from VLSI Point of view)"

Capacitance between 2 plates depends on area of plates parallel to each other OR I can say overlap area between 2 metal wires. During Routing - there are 2 extreme routing methodology.
  1. Parallel Routing Grid
  2. Cross Routing Grid

If we place metal wire on these routing grids (as an example - just picked only 2 wires of same metal layer), you can easily understand the concept of overlap area. Remember, right now we are talking about Ground cap (or Area cap) and not Coupling cap.

Parallel Metal Wire Orientation:

In the below figure, I have tried to show arrangement of capacitance between parallel metal wire between metal layers of different levels Like capacitance between M1 and M2. Since M1 and M2 both are parallel to each other 100%, Capacitance between them has dependency on width and length of the wire. This arrangement gives maximum Ground cap (Area cap).
Note: We are not discussing the capacitance between same type of metal layers (Between M1 and M1) known as Coupling Capacitance.

Cross / Perpendicular Metal Wire Orientation:

In the below figure, you can see that overlap area between M1 and M2 is only at their cross-section. This overlap area depends on their corresponding width parameter. You can see that their is no dependency on length of the wire. This arrangement gives minimum Ground cap (Area cap).
Front view and Side view of this arrangement helps you to understand this routing methodology more closely.

You can see that if we want to reduce the CAP then orientation of the Metal wire is very important. Cross (perpendicular) metal wire orientation gives minimum capacitance and that's the reason it's recommended. One of the direction is considered as Horizontal and other as Vertical. That's the reason - these orientation is known as HVH or VHV orientation (and corresponding routing strategy as HVH or VHV routing methods).

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