By Ron Press, Mentor Graphics
Three-dimensional (3D) ICs, chips assembled from multiple vertically stacked die, are coming. They offer better performance, reduced power, and improved yield. Yield is typically determined using silicon area as a key factor; the larger the die, the more likely it contains a fabrication defect. One way to improve yield, then, is to segment the large and potentially low-yielding die into multiple smaller die that are individually tested before being placed together in a 3D IC.
But 3D ICs require some modification to current test methodologies. Test for 3D ICs has two goals: improve the pre-packaged test quality, and establish new tests between the stacked die. The basic requirements of a test strategy for 3D ICs are the same as for traditional ICs—portability, flexibility, and thoroughness. A test strategy that meets these goals is based on a plug-and-play architecture that allows die, stack, and partial stack level tests to use the same test interface, and to retarget die-level tests directly to the die within the 3D stack.
A plug-and-play approach that Mentor Graphics developed uses an IEEE 1149.1 (JTAG) compliant TAP as the interface at every die and IEEE P1687 (IJTAG) networks to define and control test access. The same TAP structure is used on all die, so that when doing wafer test on individual die, even packaged die, the test interface is through the same TAP without any modifications.
When multiple die are stacked in a 3D package, only the TAP on the bottom die is visible as the test interface to the outside world, in particular to the ATE. For test purposes, any die can be used as the bottom die. From outside of the 3D package, for board-level test for example, the 3D package appears to contain only the one TAP from the bottom die.
Each die also uses IJTAG to model the TAP, the test access network, and test instruments contained within the die. IJTAG provides a powerful means for the test strategy to adjust to and adopt future test features. It is based on and integrates the IEEE 1149.1 and IEEE 1500 standards, but expands beyond their individual possibilities.
Our test methodology achieves high-quality testing of individual die through techniques like programmable memory BIST and embedded compression ATPG with logic BIST. The ATPG infrastructure also allows for newer high-quality tests such as timing-aware and cell-aware.
For testing the die IO, the test interface is based on IEEE 1149.1 boundary scan. Bidirectional boundary scan cells are located at every IO to support a contactless test technique which includes an “IO wrap” and a contactless leakage test. This use of boundary scan logic enables thorough die-level test, partially packaged device test, and interconnect test between packaged dies.
The test methodology for 3D ICs also opens the possibilities of broader adoption of hierarchical test. Traditionally, DFT insertion and pattern generation efforts occurred only after the device design was complete. Hierarchical DFT lets the majority of DFT insertion and ATPG efforts go into individual blocks or die. Patterns for BIST and ATPG are created for an individual die and then retargeted to the larger 3D package. As a result, very little work is necessary at the 3D package-level design. Also, the DFT logic and patterns for any die can be retargeted to any package in which the die is used. Thus, if the die were used in multiple packages then only one DFT insertion and ATPG effort would be necessary, which would then be retargeted to all the platforms where it is used.
Using a common TAP structure on all die and retargeting die patterns to the 3D package are capabilities that exist today. However, there is another important new test requirement for a 3D stack— the ability to test interconnects between stacked dies. I promote a strategy based on the boundary scan bidirectional cells at all logic die IO, including the TSVs. Boundary scan logic provides a standard mechanism to support die-to-die interconnect tests, along with wafer- and die-level contactless wrap and leakage tests.
To test between the logic die and Wide I/O external memory die, the Wide I/O JEDEC boundary scan register at the memory IO is used. The addition of a special JEDEC controller placed in the logic die and controlled by the TAP lets it interface to the memory. Consequently, a boundary scan-based interconnect test is supported between the logic die and external memory. For at-speed interconnect test, IJTAG patterns can be applied to hierarchical wrapper chains in the logic die, resulting in an at-speed test similar to what is used today for hierarchical test between cores.
Finally, for 3D IC test, you need test and diagnosis of the whole 3D package. Use the embedded DFT resources to maximizes commonalities across tests and facilitate pre- and post-stacking comparisons. To validate the assembled 3D IC, you must follow an ordered test suite that starts with the simplest tests first, as basic defects are more likely to occur than complex ones. It then progressively increases in complexity, assuming the previous tests passed.
Industry-wide, 3D test standards such as P1838, test requirements, and the types of external memories that are used are still in flux. This is one reason I emphasized plug-and-play architecture and flexibility. By structuring the test architecture on IJTAG and existing IJTAG tools, you can adapt and adjust the test in response to changing requirements. I believe that test methodologies that develop for testing 3D ICs will lead to an age of more efficient and effective DFT overall.
Figure 1. The overall architecture of our 3D IC solution. A test is managed through a TAP structure on the bottom die that can enable the TAPs of the next die in the stack and so on. A JEDEC controller is used to support interconnect test of Wide I/O memory dies.
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Ron Press is the technical marketing director of the Silicon Test Solutions products at Mentor Graphics. He has published dozens of papers in the field of test, is a member of the International Test Conference (ITC) Steering Committee, and is a Golden Core member of the IEEE Computer Society, and a Senior Member of IEEE. Press has patents on reduced-pin-count testing and glitch-free clock switching.