3D ICs and Chip Stacking Technologies

What Are 3D ICs?

3D ICs (Three-Dimensional Integrated Circuits) are advanced chips where multiple layers of circuits (called dies) are stacked on top of each other, instead of placing them side by side (as in 2D chips).

This technique helps:

• Save space

• Increase performance

• Reduce power consumption

Just like stacking floors in a building instead of spreading out over land, 3D ICs stack layers to fit more power into a smaller area.

 Why Use 3D ICs?

Traditional chips (2D ICs) are reaching their limits in terms of speed, size, and power. 3D ICs offer:

• Smaller size: Takes less space on a board

• Faster communication: Layers are closer, so signals travel faster

• Lower power: Less distance means less energy is needed

• Better performance: More functions in a compact chip

 Chip Stacking Technologies

There are different ways to stack chips in 3D ICs:

1. Die-to-Die Stacking

• Separate dies are placed on top of each other.

• Connected using wire bonding or through-silicon vias (TSVs).

2. Die-to-Wafer Stacking

• A single die is stacked on a full wafer (a big round silicon disc with many chips).

3. Wafer-to-Wafer Stacking

• Entire wafers are stacked together and then cut into individual 3D ICs.

What Are TSVs?

TSVs (Through-Silicon Vias) are tiny vertical holes filled with metal that connect one layer of the chip to another. They are like “elevators” in a building, moving data up and down between floors (chip layers).

Benefits of TSVs:

• Fast data movement between layers

• Lower power usage

• Higher bandwidth

 2.5D vs 3D ICs – What’s the Difference?

• 2.5D ICs: Chips are placed side by side on an interposer (a small chip that connects them). Not stacked.

• 3D ICs: Chips are actually stacked on top of each other using TSVs.

 Where Are 3D ICs Used?

• Smartphones: More power in a small space

• High-performance computing (HPC): Faster processing

• Memory (like HBM – High Bandwidth Memory): Faster data transfer

• AI & Graphics Chips: Better performance in smaller sizes

 Challenges of 3D ICs

While 3D ICs are powerful, they come with some challenges:

• Heat: Multiple layers make it harder to cool the chip

• Cost: More expensive to manufacture

• Testing: Harder to test individual layers once stacked

• Complex design: Needs special tools and planning

 Summary

• 3D ICs stack chips vertically to save space and boost performance.

• Use TSVs for fast, vertical connections.

• 2.5D ICs place chips side-by-side using an interposer.

• Used in modern phones, AI, memory, and computing devices.

• They offer better speed and lower power but need advanced cooling and design.