3-Dimensional IC Fabrication Dominic DelVecchio Bradley Hensel

Outline What is 3D IC? Why? Theoretical Process Benefits Technical Challenges Conclusion References Concepts

What is 3D IC? 3D IC fabrication is a process that involves building transistors on an IC chip in such a way that you have a vertical structure instead of a simple 2D structure.

Why 3D Fabrication? As transistors grow smaller and smaller to fit more on a wafer, the precision of equipment needed to reliably manufacture IC chips grows vastly in cost. Currently we are at a point where the cost to manufacture smaller transistors into an 2D IC chip is not economical in any regard. Thus IC chip manufacturers have begun to produce 3D structures on wafers allowing for more transistors per chip without the incurred cost of nanofabrication level machinery. This trade off for more transistors in the vertical as compared with the horizontal comes with both great benefits and challenges.

3D IC Fabrication 1. A crystalline structure such as another silicon wafer must be place on top of the first circuit level and joined through the process of plasma-activated lower temperature wafer bonding. Seeded Crystallization may also be used.

3D IC Fabrication 2. Construction of the second or above layers on an IC is accomplished through a process of laser annealing. Laser annealing as opposed to more conventional methods allow for the first circuitry layer to remain at almost room temperature while the second or above layer gets hot enough to perform the needed fabrication steps. This is extremely useful as you will not negatively affect the doping concentrations of previous layers.

Completed (2 layer) IC

Benefits (Power and Bandwidth) 3D fabrication allows circuit components to be located physically closer to each other. This allows circuits to operate with less power consumption and at a higher bandwidth - two qualities that are of utmost importance in today's technology (especially in memory).

Benefits (Cost and Material Type) By producing layers individually and testing before combining them, cost can be reduced. If a layer is defective, only that portion of the circuit is discarded and not the entire unit. Furthermore, layers can be produced of different semiconductor materials to combine the benefits of various materials into the same chip

Benefits Summary Reduced power consumption Reduced cost Higher bandwidth over many components Ability to combine materials ALSO (extrapolating from these benefits) Reduced footprint Greater design flexibility

Technical Challenges (Heat) In shifting from a planar to a volumetric structure, heat buildup inside becomes an issue. To mitigate this issue, heat must be considered in the design phase and kept to the outside of the structure.

Technical Challenges (Yield) Producing the 3D chips requires more manufacturing steps than producing 2D chips. Since some units are generally lost at each step, yield may be reduced. Contamination between layers is much more severe than standard 2D chip contamination Alignment, imagine trying to align something in the nanometers of length...one mistake and it's all over Layered testing may help offset yield related issues.

Technical Challenges (Design Complexity) 3D chips are inherently more complex and difficult to visualize, and are thus more difficult to design for. 3D CAD tools need to be developed to make the design process manageable and economic.

Technical Challenges (Wire Density) Current processes state the interlevel interconnect density of 3D circuits obtained by aligned bonding of pre-fabricated circuit levels is currently limited to about 10^6 wires/mm^2 With more advanced, in development, processes such as the one described above can reach well into the millions of wires/mm^2

Conclusion Massive benefits are to be had from 3D IC chips, however many challenges must be overcome to employ 3D fabrication processes on a large scale, in more than the currently limited industries. It's likely that 3D IC production will serve an important role in memory and cache memory based applications in the near future.

References Rajendran, Bipin. "Sequential 3D IC Fabrication – Challenges and Prospects." Chomsky Stanford. Department of Electrical Engineering, Stanford University, n.d. Web. 27 Apr Patti, Robert. "Impact of Wafer-Level 3D Stacking on the Yield of ICs." Future Fab International. Tezzaron Semiconductor, 07 Sept Web. 27 Apr "EDA's Big Three Unready for 3D Chip Packaging." EDA's Big Three Unready for 3D Chip Packaging. EE Times, n.d. Web. 27 Apr Dally, William J. "Future Directions for On-Chip Interconnection Networks." OCIN Workshop. 7 Dec Web.

Concepts Main Concepts: 1. Not economical to build smaller?, build vertical. 2. Designing in 3D is inherently more difficult, and new software needs to be developed 3. Lower yield process due to complexity and vulnerability between layers. 4. Higher bandwidth capability due to reduced interconnection length. 5. Temperature considerations must be dealt with.