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Thread: 3D IC Wiki

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    Admin Daniel Nenni's Avatar
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    3D IC Wiki

    A three-dimensional integrated circuit (3D IC ) is a chip in which two or more layers of active electronic components are integrated both vertically and horizontally into a single circuit. The semiconductor industry is hotly pursuing this emerging technology in many different forms, as a result the full definition is still somewhat fluid.

    3D IC Wiki-3dic.jpg

    Flip chip, memory on logic
    The first approach to talk about is flipping one chip and attaching it to the top of another. This is done by creating bonding areas on each chip, growing (usually copper) microbumps to create die-die interconnect at a pitch of approximately 50um. The big user of this technology is in digital camera chips. The CCD image sensor is actually thinned to the point that it is transparent to light and then attached to the image processing chip. The light from the camera lens passes through the silicon to the CCD unobstructed by interconnect etc which is all on the other side of the sensor.
    3D IC Wiki-logmem.jpg

    This approach is also used for putting a flipped memory chip onto a logic chip (see picture). It is not well-known, but the Apple A4 chip is built like this, with memory on top of the processor/logic chip. There are now standardization committees working on the pattern of microbumps to use for DRAMs (analagous to standard pinout for DRAMs) so that DRAM from different manufacturers should be interchangeable. Unlike in the picture, the bumps are all towards the center of the die so that the pattern is unaffected by the actual die size which may differ between manufacturers and between different generations of design.

    Although this technology is formally 3D, since there are two chips, it doesn’t require any connections through any chips and is a sort of degenerate case.

    Through Silicon Vias (TSVs)
    The key technology for real 3D chips is the through-silicon-via (TSV). This is a via that goes from the front side of the wafer (typically connecting to one of the lower metal layers) through the wafer and out the back. TSVs vary in diameter from 1 to 10um, with a depth from 5 to 10 times the width. A hole is formed into the wafer, lined with an insulator, and then filled with copper or tungsten. Finally the wafer is thinned to expose the backside of the TSVs. Note that this means that the wafer itself ends up merely 10-100um thick. Silicon is brittle, so one of the challenges is handling wafers this thin both in the fab and when they have to be shipped to an assembly house. The wafer must be bonded to some more robust substrate (glass or silicon) before thinning and, in some cases, may be separated again during assembly. The wafer is thinned using CMP (chemical mechanical polishing, similar to how planarization is done between metal layers in a normal semiconductor process) until the TSVs are almost exposed. More silicon is then etched away to reveal the TSVs themselves.
    3D IC Wiki-samsungtsv.jpg
    The picture to the above (click for a bigger image) shows Samsung’s approach. FEOL (which means front-end of line which means transistors and is nothing to do with front-end design) is done first. So the transistors are all created. Then the TSVs are formed. Then BEOL (which means back-end of line which means interconnect and is nothing to do with back-end design). After the interconnect is done then the microbumps are created. The wafer is glued to a glass carrier. The back is then ground down, a passivation layer is applied, this is etched to expose the TSVs and then micropads are created. This approach is known as TSVmiddle since the TSVs are formed between transistors and interconnect. There is also TSVfirst (build them before the transistors) and TSVlast (do them last and drill them through all the interconnect as well as the substrate).

    There are two design issues with TSVs. First is the exclusion area around them. The via comes up through the active area and usually through some of the metal layers. Due to the details of manufacturing, quite a large area must be left around the TSV so that it can be manufactured without damaging the layers already deposited. The second problem is that the manufacturing process stresses the silicon substrate in a way that can alter the threshold values of transistors anywhere nearby, thus altering the performance of the chip in somewhat unpredictable ways.

    2˝D interposers
    There are two classes of true 3D chips which are being developed today. The first is known as 2˝D where a so-called silicon interposer is created. The interposer does not contain any active transistors, only interconnect (and perhaps decoupling capacitors), thus avoiding the issue of threshold shift mentioned above. The chips are attached to the interposer by flipping them so that the active chips do not require any TSVs to be created. True 3D chips have TSVs going through active chips and, in the future, have potential to be stacked several die high (first for low-power memories where the heat and power distribution issues are less critical).

    3D IC Wiki-25dchip.jpg
    The active die themselves do not have any TSVs, only the interposer. This means that the active die can be manufactured without worrying about TSV exclusion zones or threshold shifts. They need to be microbumped of course, since they are not going to be conventionally wire-bonded out. The picture above shows (not to scale, of course) the architecture.

    The image shows two die bonded to a silicon interposer using microbumps. There are metal layers of interconnect on the interposer, and TSVs to get through the interposer substrate to be able to bond with flip chip bumps to the package substrate. Flip-chip bumps are similar to micobumps but are larger and more widely spaced.

    Production interposer: Xilinx Virtex-7
    Xilinx is using this technology for their Virtex-7 FPGAs. They call the technology “stacked silicon interconnect” and claim that it gives them twice the FPGA capacity at each process node. This is because very large FPGAs only become viable late after process introduction when a lot of yield learning has taken place. Earlier in the lifetime of the process, Xilinx have calculated, it makes more sense to create smaller die and then put several of them on a silicon interposer instead. It ends up cheaper despite the additional cost of the interposer because such a huge die would not yield economic volumes.
    3D IC Wiki-xilinx3d.jpg
    The Xilinx interposer consists of 4 layers of 65um metal on a silicon substrate. TSVs through the interposer allow this metal to be connected to the package substrate. Microbumps allow 4 FPGA die to be flipped and connected to the interposer. See the picture above. An additional advantage of the interposer is that it makes power distribution across the whole die simpler. This seems to be the only design in high volume production today.

    1. Wikipedia
    2. GSA 3D IC Meeting Minutes
    3. 3D IC's An overview of where we are
    4. All images from the presentations from 2010 3D architectures for semiconductor integration and packaging conference.

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  2. #2
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    3D Integration--The vertical integration of thinned and bonded silicon circuit with vertical interconnects between the IC Layers.

    Major Markets--1.Pixel arrays for Imaging. 2. Memory 3. Microprocessors 3.FPGAs

    Basic Principle 1 Vias----a)Via First-done at foundary,lowest cost. b)Via last--after wafers are made,3rd party vendors.

    2.Bonding Options---a)Mechanical bond only,electrical connections later(i)Oxide to Oxide (ii)Adhesive such as BCB.
    b)Mechanical & Electric together (i)CuSn Eutectic.(ii)CuCu Fusion (iii)Direct Bond Interconnect.
    Posted by Manoj Nagesh

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    Capacitive Coupling

    Here's a post about capacitive coupling between chips -

    Sun Procures Early Capacitive Coupling Technology from Polychip Inc.

    - anybody know what's happened to it?

    It seems to me that multiple serial signals capacitively coupled would be more reliable than thousands of solder bumps as wide buses.

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