Presentation for Advanced VLSI Course presented by:Shahab adin Rahmanian Instructor:Dr S. M.Fakhraie Major reference: 3D Interconnection and Packaging: Impending Reality or Still a Dream? By:Eric Beyne December 2004

Overview Introduction : Why 3D-interconnects? Classification of 3D-technologies 3D-SIP 3D-SOC 3D-IC Conclusion

Introduction : Why 3D? Drivers for 3D interconnects & packaging : Size reduction: minimal area/volume of an electronic system Solving the “interconnect bottleneck” : Long interconnects are too slow Long interconnects consume too much power Hetero-integration: “Seamless” mixing of different microelectronic technologies at the wafer level

The Interconnect Bottleneck Driven by technology scaling, SOC devices are partioned in functional blocks (“tiles”) Within a “tile”: Operations can be performed within a single clock cycle Interconnects : Local and intermediate interconnect levels on the die The interconnect between the tiles : “Global” interconnect levels : longest on chip interconnect lines, significantly less interconnects than on local & intermediate levels Speed-limiting factor on the chip : require repeaters Significant source for area & power consumption.

The Interconnect Bottleneck If the functional “tiles” on the chip could be stacked in the 3rd dimensions, the chip area would be reduced, resulting in much shorter global interconnect lines.

Logic & Memory 2D interconnect Long lines between Logic & Memory Through bus SOC solution: Large die Large size Memory cells 3D interconnect Short, direct lines Between Logic & Memory banks

3D-Interconnect technology Current developments in packaging technology are delivering the key enabling technologies for building true 3D stacked devices: Thinning of wafers, below 50 µm, as thin as the active layer. Wafer-to-wafer bonding, up to 300 mm diameter wafers. Die-to-wafer bonding: singulated “top” die are bonded to “bottom” die on a base wafer Wafer-through-hole technologies: realization of electrically isolated connections through the silicon substrate. Many of these technologies were originally developed in the field of MEMS technology and are now finding their application in IC packaging technologies.

Different 3D-interconnect “flavors” 3D-SIP : 3D-”System-in-a-Package” Stacking of multiple die in a single package Stacking of multiple SIP-packages 3D-interconnects at the traditional chip pin-out level 3D-SOC : 3D-”System-on-a-Chip” Stacking of wafers or die-to-wafer with 3D-global interconnectivity at the “tile”-level 3D-IC : Stacking of wafers with interconnectivity at local level (gate or transistor level)

3D-SIP Die stacking in a single package Assembly by wire bonding of stacked die in a single package has been shown by several packaging vendors. Main limitation : interconnectivity interposer substrate does not allow for complex rerouting among the die. Only possible for specific applications, such as standard memory stacking, or die with specific I/O pad rerouting. Requires “Known-good-die”(KGD)

3D-SIP Stacking of chip-scale SiP-packages Improving the Yield and manufacturability of 3D-SIP by stacking of 2D-SIP “sub-systems” Each layer is an SIP package Each layer has the same size Each layer is fully tested before final assembly Many different technologies may be used for each individual layer Results in : Generic 3D technology Best yield and manufacturability Limitations : Relatively low 3D interconnectivity Lack of standardization of package sizes

3D-SIP visionary application: E-cube E-Cube: distributed, fully autonomous system for realizing “ambient intelligent” systems.

3D-SIP application Example 3D-SIP bluetooth rf radio, measuring 7mmx7mmx2.5mm, Rf-front-end CSP stacked on a digital base band CSP.

3D-SOC Stacking of die at the wafer level Reduced critical global interconnect lengths by stacking 3D-interconnectivitiy at the “tile” level Requires a significantly higher 3D-wiring density than 3D-SiP Technology components: “Die-to-wafer” transfer & bonding : different chip sizes allowed Uses very thin Si : 50 µm and below

Ultra Thin Chip Stacking, UTCS Objective: To Realize 3D-VLSI structures, based on the integration of thinned standard die, in a modified multilayer thin film technology :

3D Wiring scheme UTCS 3D interconnectivity around the perimeter of the die: > 100 connections per mm 2 and per layer

Wafer thinning 200 mm wafer CMOS wafer, thinned down to 50 µm thickness.

Thinning of standard CMOS wafers, down to µm

Embedded test die 15 µm thin Si-die, transferred to a host substrate and electrically connected to that substrate

Why ultra thin die & thin dielectrics ? Thermal : thermal resistance dielectrics between die in the stack result in an increase of the thermal resistance 1 µm BCB dielectric =10 µm SiO2 =1 mm Si 10 µm BCB between two 1 cm 2 die : 0.5 K/W thermal resistance Interface thermal resistance Die/BCB : for 1 cm 2 die : 0.3 to 0.75 K/W (experimental) Mechanical : thermo-mechanical stress limits the maximum height of the stack to 3 layers.

Why ultra thin die & thin dielectrics ? Electrical: thin dielectrics allow for a tighter interconnect line pitch for the same cross-talk level

Impact of wafer thinning on the electrical performance Test chip: 20x20 mm, IMEC 0.35 µm CMOS process development test reticle. Processing: Mechanical wafer thinning down to 50 µm Face-down bonding test wafer to a dummy carrier wafer Plasma thinning test die down to µm Singulation die Transfer of thinned die to a carrier substrate Measurements: Transistor parameters before and after thinning.

Electrical measurements Vtlin versus Ldes for CMOS transistors, measured before and after thinning and stacking on a host Si wafer.

3D-IC 3D-local interconnects at “gate-level” “Scaling-driven” technology: integrates more transistors per unit area. Differences with 3D-SOC : 3D-interconnects are local interconnects: several orders of magnitude more connections required than 3D-SOC. 3D-interconnects block substantial areas for transistor logic : lower effective integration density No solution for long, global interconnect lines Requires “wafer-to-wafer” bonding : equal size stacked die. Highly complex technology, questionable yield & economics

Conclusions In order to be successful, 3D-Interconnect and packaging technologies need to become manufacturable technologies (high yield) : high degree of parallel processing of individual layers, testing of these intermediate layers stacking of “known-good-devices”. This goal is likely to be reached first for 3D-SIP, followed by 3D-SOC. This is confirmed by the emergence of such technologies in actual products. It is much further out for 3D-IC technologies.

References [1] Eric Beyne “3D Interconnection and Packaging: Impending Reality or Still a Dream?”ISSCC 2004 Pictures from Reference [1] [2] G.Carchon et.al., IEEE-CPMT, vol. 24, pp , 2001.