Metal Layout

This is a very important question for the Layout design engineer, Please don’t ignore this question, its a very basic but very complicated question.

Let’s see in-depth,

To determine which case is better from a layout perspective, we need to consider various factors such as manufacturing constraints, performance requirements, and design goals. Here are some considerations for both cases:

Case 1: One Metal with 2um Width and 10um Length

1. Simplicity: Having a single metal with uniform dimensions (2um width, 10um length) simplifies the layout, making it easier to design and manufacture.
2. Manufacturability: Larger feature sizes, such as the 2um width, may be easier to manufacture with higher yield, as they are less sensitive to manufacturing variations.
3. Resistance and Current Handling: A wider metal may have lower resistance, which could be advantageous for certain applications that require higher current-carrying capacity.

Case 2: Two Metals, Each with 1um Width and 10um Length

1. Routing Flexibility: Having two separate metals with 1um width each allows for more flexibility in routing and can be beneficial in scenarios where specific routing constraints or signal integrity considerations are present.
2. Signal Isolation: If the two metals are used for different signals or purposes, having them separated can provide better isolation and reduce interference.
3. Customization: Using different metals with distinct widths allows for customization based on specific design requirements for each metal.

What about Resistance in both cases?

The resistance of a conductor is given by the formula:

where:

• R is the resistance,
• ρ (rho) is the resistivity of the material,
• L is the length of the conductor,
• A is the cross-sectional area of the conductor.

Case 1: One Metal with 2μm Width and 10μm Length

• W = 2μm (width)
• L = 10μm (length)
• A = A

Case 2: Two Metals, Each with 1μm Width and 10μm Length

• W = 1μm (width, for each metal)
• L = 10μm (length, for each metal)
• A = 2A

Assuming the same material (same resistivity ρ) for both cases, we can compare the resistances.

So for the case, Resistance will be the same.

What about Electromigration (EM) in both cases?

Electromigration is the process by which metal atoms are displaced from the metal conductor due to the movement of electrons through the conductor. This can lead to voids or gaps in the metal, which may result in device failure over time. Narrower conductors are generally more susceptible to electromigration because the electron current density is higher in a smaller cross-sectional area.

• In Case 1, where you have a single metal with a 2um width, the wider metal may be less prone to electromigration compared to narrower metals.
• In Case 2, if the two metals are running partially and each has a 1um width, the susceptibility to electromigration may be higher compared to Case 1.

What about IR Drop in both cases?

Infrared drop, or voltage drop, refers to the reduction in voltage along a conductor due to its inherent resistance. Excessive voltage drop can lead to degraded circuit performance and may impact the functionality of the device. Higher resistance in a conductor leads to a higher voltage drop for a given current flowing through it.

• In Case 1, the wider metal (2um width) may have lower resistance, resulting in a lower IR drop for a given current compared to narrower metals.
• In Case 2, while you have two metals, the narrower width (1um) may lead to higher resistance and potentially higher IR drop.

What about Density-related DRC violations in both cases?

Density-related Design Rule Checking (DRC) violations are concerned with the spacing and density of features on a semiconductor layout. These violations can affect manufacturability and yield. Let’s analyze the two cases in terms of density-related DRC violations:

1. Case 1: One Metal with 2um Width and 10um Length:
• Having a single metal with a width of 2um may result in a lower density of features compared to Case 2, where there are two metals with 1um width each.
• Larger feature sizes generally allow for more generous spacing between features, reducing the likelihood of density-related violations.
2. Case 2: Two Metals, Each with 1um Width and 10um Length:
• Having two metals with a width of 1um each may increase the overall density of features on the layout.
• Higher feature density can potentially lead to violations related to minimum spacing requirements, especially if the design rules specify certain minimum distances between adjacent features.

From a density-related DRC perspective, Case 1 with a single metal of 2um width may be more favorable. The larger feature size typically allows for more relaxed spacing constraints, reducing the likelihood of density-related violations.

What about Capacitance in both cases?

The capacitance of a metal interconnect in a semiconductor device is influenced by its geometry, specifically the width and spacing between adjacent conductors. Capacitance plays a crucial role in determining the electrical performance of the circuit, affecting factors such as signal propagation delay and power consumption. Let’s evaluate the impact of capacitance for the two cases you provided.

Case 1: One Metal with 2um Width and 10um Length:

• The wider metal (2um width) in Case 1 may result in lower capacitance compared to narrower structures.
• Wider conductors generally have lower capacitance per unit length because there is more separation between the metal and any nearby conductive structures.

Case 2: Two Metals, Each with 1um Width and 10um Length:

• The narrower metals (1um width) in Case 2 may lead to higher capacitance due to closer proximity between conductors.
• Having two metals in close proximity may also result in parasitic capacitance between them.

From a capacitance perspective, Case 1 with a single metal of 2um width may be more favorable if lower capacitance is desired. The wider conductor typically leads to reduced capacitance per unit length.

In Short

Case 1 (Single Metal with 2um Width)

Pros:

• Potentially lower resistance due to wider metal.
• Lower susceptibility to electromigration.
• Potentially lower IR drop.
• May have fewer density-related DRC violations.
• Lower capacitance per unit length.

Cons:

• Less routing flexibility compared to Case 2.

Case 2 (Two Metals, Each with 1um Width):

Pros:

• More routing flexibility.
• Potential for better signal isolation.
• Customization for different signals or purposes.

Cons:

• Higher susceptibility to electromigration.
• Potential for higher IR drop.
• May have more density-related DRC violations.
• Higher capacitance per unit length.

Conclusion

• If simplicity, lower resistance, lower susceptibility to electromigration, lower IR drop, fewer density-related DRC violations, and lower capacitance per unit length are critical factors for your design, Case 1 (single metal with 2um width) may be preferable.
Share.
error: Content is protected !!