1. MIT Lincoln Laboratory kickoff-1 PWW 4/7/2000 Via Topology for 3-D Integration 7 April 2000 Peter W. Wyatt and Paul V. Davis wyatt@LL.mit.edu 781-981-7882 voice 781-981-7889 fax

2. MIT Lincoln Laboratory kickoff-2 PWW 4/7/2000 Via Topology Introduction Interconnect reticle set is being designed to test four options for vertical vias connecting three wafers –Explore variations of design rules –Optimize process for high yield –Minimize reticle cost by using the same reticles for all three wafers to the extent possible Bottom wafer is always upright during assembly Top two wafers may be either inverted or upright –Initial work used inverted wafers Vias may be either offset or concentric –Initial work used offset vias

3. MIT Lincoln Laboratory kickoff-3 PWW 4/7/2000 Offset Via Topology with Inverted Circuits -- The Original All layers are the same on all 3 circuits except deep via and Metal 4 Mask count: Remove, M1, V12, M2, V23, M3, SV, DV12, DV23, M4W2, M4W3, Passivation = 12 masks Cannot have a transistor lined up with a shallow or deep via Si wafer Bonding layer M1 M2 M3 M1 M3 M2 M1 M3 M4 Deep vias Shallow vias W1 W2 W3 Passivation cut Oxide

4. MIT Lincoln Laboratory kickoff-4 PWW 4/7/2000 Chip Topology with Inverted Assembly Use a single reticle set for all three wafers to save cost Inverted assembly creates two copies of each structure One is useful, the other is not Does not occur with upright assembly Not an issue for real circuits Cell 1Bottom Cell 2Bottom Cell 2Top Cell 1Top Cell 1Bottom Cell 2Bottom Cell 2Top Cell 1Top Bottom wafer, uprightTop wafer, inverted Cell 1Bottom Cell 2Bottom Cell 2Top Cell 1Top Cell 1Bottom Cell 2Bottom Cell 2Top Cell 1Top Both wafers the same through metal 3

5. MIT Lincoln Laboratory kickoff-5 PWW 4/7/2000 Offset Via Topology with Inverted Circuits -- Chain of Deep Vias Deep vias and metal 4 are different on each wafer, but all other layers are the same on all three Shallow via chain is easy, the same on wafers 2 and 3 M4W3 M4W2 M3 DV12 DV23 Si wafer

6. MIT Lincoln Laboratory kickoff-6 PWW 4/7/2000 Offset Via Topology with Inverted Circuits -- Chain of Both Shallow and Deep Vias Deep vias and metal 4 are different on each wafer, but all other layers are the same on all three Creates extraneous floating metal, but that does no harm A problem: these chains occupy large area because –They are long -- many thousand vias –Multiple copies are needed with design rule variations M4W3 M4W2 M1 M3 DV12 DV23 SV Si wafer One period, 4 deep vias and 6 shallow vias Extraneous metal

7. MIT Lincoln Laboratory kickoff-7 PWW 4/7/2000 Offset Via Topology with Inverted Circuits -- Making Chains Reusable Make the same structure work for multiple cases –Left and right instances of inverted –Inverted and upright To save space on the reticle given the need for long chains Requires M3W1 different than M3W2 => mask count = 13 Si wafer DV12 DV23 M1 M3 M1 M3 M1

8. MIT Lincoln Laboratory kickoff-8 PWW 4/7/2000 Offset Via Chain with Upright Circuits -- Same Layout as Inverted Allows transistor below shallow or deep via Si wafer DV12 DV23 M1 M3 M1 M3 M1

9. MIT Lincoln Laboratory kickoff-9 PWW 4/7/2000 Si wafer Bonding layer M3 M1 M2 M3M3 (same as W3) Bonding layer Concentric Via Topology with Inverted Circuits M1 and M3 must both be different on W2 and W3 M1 is the same on W1 and W2; M3 is the same on W1 and W3 One more mask => count = 14 Masks if this were the only type of via: Remove, M1W1&2, M1W3, V12, M2, V23, M3W1&3, M3W2, DV12, DV23, Passivation; count = 11 Passivation cut M3 DV12 M3 DV23 M1 M1 (same as w2)

10. MIT Lincoln Laboratory kickoff-10 PWW 4/7/2000 Concentric Via Topology with Upright Circuits M3 is different on W1 and W2; W1 and 3 can be the same M1 is the same on all wafers No added mask Masks if this were the only type of via: Remove, M1, V12, M2, V23, M3W1&3, M3W2, DV12, DV23, Passivation; count = 10 Si wafer Passivation cut M3 M1 M3 DV12 M1 DV23 M3 M1 M3 M1

11. MIT Lincoln Laboratory kickoff-11 PWW 4/7/2000 Si wafer Concentric Via Topology with Inverted Circuits Complete Chain M1 is the same on W1 and W2; M3 is the same on W1 and W3 M2 is the same on all three wafers Left and right instances are both good Cannot make inverted and upright with the same layout –Possible with offset vias because M4 is patterned after assembly DV12 DV23 DV12 DV23 M3 M1 M3 M1 M3 DV12 DV23 DV12 DV23 M3 M1 M3 M1 M3

12. MIT Lincoln Laboratory kickoff-12 PWW 4/7/2000 Via Topology -- Status Four types of via have been evaluated –Most reticles can be used for all three wafers –All four types of via can be fabricated with 14 reticles –Many different via chains must be laid out –Some layouts can produce more than one type, saving space Layout of offset via chains is in progress Concentric via layout will follow