Die Stacking



Die Stacking is the process of mounting multiple chips on top of each other within a single semiconductor package. Die stacking, which is also known as 'chip stacking', significantly increases the amount of silicon chip area that can be housed within a single package of a given footprint, conserving precious real estate on the printed circuit board and simplifying the board assembly process. Aside from space savings, die stacking also results in better electrical performance of the device, since the shorter routing of interconnections between circuits results in faster signal propagation and reduction in noise and cross-talk. 


Early applications of stacked die involved the stacking of two memory chips on top of each other, such as Flash and SRAM devices.  Today, however, die stacking technology has advanced beyond 'mere' memory chip stacking, and may now involve six or more chips of varying function or technology, e.g., logic, analog, mixed-signal, etc. Indeed, die stacking is now synonymous with 'vertical integration', or the integration of circuits in vertical fashion instead of the traditional horizontal or planar approach.


Die stacking naturally started out with a pyramid style of piling up smaller die on top of larger ones.  The technology has advanced into something that finds no limit in the sizes of chips to stack. It is now common to see a stack of equal-size die, or even a larger die on top of a smaller one. One technique developed is to place a spacer (a dummy layer of silicon) between two die, so that the bond wires are not crushed even if the top die is larger than the bottom die. Unfortunately, the use of spacers between die add to the total package thickness. 


Tessera Inc. pioneered the technique of folding stacked die to eliminate the need for spacers between them, calling the process 'folded/stacked' technology. The die are produced side by side and then folded over so that the bond pads are independent of each other.  A relieving layer is placed between the chips to alleviate thermomechanical stresses.


The interconnection of the stacked die within a package presents an even more daunting challenge, especially if wirebonding is employed. Aside from the mechanical intricacies involved in managing the complex lay-out of hundreds of microscopic wires subject to loop profile restrictions, cross-talk during device operation must likewise be avoided. At times, such as when digital, analog, and RF circuits need to be integrated together, the solution would require the use of two interconnection technologies (wirebonding and flipchip bonding) to get the required results.


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Figure 1.  Wirebonding complexity required by die stacking;

as reconstructed from photos posted in www.ap.pennnet.com


Stacked die may be interconnected using wirebonding alone, or by a combination of wirebonding and flipchip assembly. The use of wirebonding as the exclusive means of interconnection is somewhat restrictive, since the number of stacked die that may be wirebonded may be limited to only three. Nonetheless, a common technique employed for wirebonding stacked die is to wirebond each die individually to the substrate.  The conductor patterns on the substrate take care of interconnecting the die to each other and to the outside world. 


Die that are wirebonded to the substrate must have a 0.5-1 mm shelf or exposed area around its periphery to allow the formation of the necessary loops during wirebonding. Die-to-die wirebonding is also done, but this requires the bottom die to be sufficiently larger than the top die, to allow enough room for the wirebond connections.  Wirebonding of stacked die could call for  loop heights that are less than 100 microns, which are much more challenging than loop heights of 150-175 microns commonly seen in conventional wirebonding of unstacked die.


Die stacking is fraught with challenges other than those of wirebonding. One of these is the need to keep the stack thermally and mechanically stable on the substrate.  At the same time, the resulting package must be as thin as possible, with die interconnections that are electrically good and reliable. Of course, the final thickness of the package depends on the number of die in the stack. As an example, current technology would generally require a 1.4-mm chip scale package (CSP) to accommodate a six-die stack while a four-die stack can fit within a 1.2-mm CSP.


Wafer thinning, thin-wafer handling, and thin die attach are essential elements of successful die stacking. Wafer thinning still involves conventional wafer backgrinding, but it must be followed by a polishing step that relieves stresses imparted by the backgrind process to the wafer.  Wafers intended for die stacking can be thinned to just 3-6 mils, depending on the use and the wafer size. Wafers that are this thin are already inherently weak, and require special handling and transport systems to ensure their proper support at all times. Die attach of very thin die, in particular, can be very challenging.  The application of preformed tape epoxy on the wafer backside prior to sawing is one technique that facilitates die attach of very thin die. 



Figure 2. Side view of wirebonded stacked die;

Photo source: www.kns.com



Another challenge in die stacking is the ability to pick known good die (KGD) from a wafer. The inadvertent use of defective die in die stacking will result in yield losses and higher costs. Unfortunately, wafer-level testing is often not enough to ensure that only KGD's will be picked for die stacking, especially if the device involved is a complex circuit. Thus, poorly yielding wafers that are difficult to test at wafer level are not good candidates for die stacking.


Substrate thickness is also an important factor in die stacking. The thickness of the substrate adds to the over-all package thickness.  This means that for a given package height, increasing the substrate thickness will decrease the number of die that can be stacked on it.  Stacked die that involve complex devices may require complex substrate routing, which in turn would require additional layers or laminates within the substrate. The core thickness and the number of laminate layers define the over-all substrate thickness. Die stacking should therefore involve some form of substrate engineering to keep the required number of substrate layers and their thicknesses to a minimum.


Die stacking becomes less attractive as the number of die to be stacked increases and as the die involved become more expensive or complex.  In such cases, engineers are more inclined to employ package stacking instead of die stacking.


Primary Reference: http://www.elecdesign.com



See Also:  System in a PackageWafer BackgrindDie Attach Wirebond




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