Stacking Signal TSV for Thermal Dissipation in Global Routing for 3D IC National Tsing Hua University Po-Yang Hsu,Hsien-Te Chen, TingTing Hwang ASPDAC’13

Outline Introduction Motivation Signal TSV Assignment and Relocation for Thermal Dissipation Experimental Result Conclusion

Introduction Three dimensional (3D) chip stacking by Through-Silicon-Via (TSV) has been identified as an effective way to achieve better performance in speed and power [2, 3]. However, such solution inevitably encounters challenges in thermal dissipation since stacked dies generate significant amount of heat per unit volume.

Introduction Temperature aware 3D global routing algorithm by inserting ”thermal vias” and ”thermal wires” to lower the thermal resistance[4] Reduces the temperature at the cost of extra area of ”thermal vias”[1,6-10] Performance and thermalaware Steiner routing algorithm to place signal TSVs to reduce temperature.[11] Does not fully utilize the outstanding thermal conductance of TSV in thermal dissipation. [12] proposed a stacked-TSV power network structure to improve thermal dissipation by fully utilizing TSVs in power network. only employs stacked-TSV structure in power network.

Motivation - Thermal model The lateral thermal resistors Rlateral are determined by heat conductance of device material

Motivation 20um

Motivation Relationship between temperature and distance of stacked signal TSV to heat source

Signal TSV Assignment and Relocation for Thermal Dissipation Overall flow of placing signal TSVs in global routing

Initial TSV Assignment

Initial TSV Assignment

Initial TSV Assignment PowDensityi,j,k : power density in grid (i,j,k) where i, j, k denotes coordinates of the grid node in x, y, z axis direction high lumped power density grid needs more signal TSVs to dissipate its heat. n : number of tiers in the design. TSVNumi,j,k : number of signal TSVs in grid (i,j,k).

Initial TSV Assignment SDi,j is defined as the stacking degree in grid (i,j), which is computed as the number of TSV stacking at grid position (i,j). Larger Gain value means higher power density, less TSVs, and more stacking signal TSVs.

Stacked-TSV Relocation Stage

Hotspot grids Identification Hotspot grid is identified by the top 10% highest thermal criticality grids. define a circle region to find its saver net.

Hotspot grids Identification Use a matching algorithm to find the overall best solution. GridDist is the summation of distance from hotspot grid to the nearest TSV of the saver net n in all tiers. wiring overhead if we stack the TSVs of saver net n close to the grid g. H S Weighted graph G = ( H∪S, E)

Hotspot grids Identification Use a matching algorithm to find the overall best solution. StackingDegree is the number of tiers that a saver net crosses. heat dissipation ability H S Weighted graph G = ( H∪S, E)

Determination of Stacking Grid Based on the matching solution, TSV of a saver net will be relocated near the hotspot grid. However, there are other factors to determine if a grid location is the best choice. Define candidate target grids which are hotspot grids and the adjacent grids nearby them to determine the best target grid location for moving signal TSV.

Determination of Stacking Grid Gain function to select our target grid to place stacked signal TSV at grid (i, j) is defined as Consider Distance between candidate target grid and hotspot grid Power density Number of TSVs Whitespace Wirelength

Determination of Stacking Grid Gain function to select our target grid to place stacked signal TSV at grid (i, j) is defined as Consider Distance between candidate target grid and hotspot grid The larger DSST the closer the distance between stacking location to the hotspot grid.

Determination of Stacking Grid Gain function to select our target grid to place stacked signal TSV at grid (i, j) is defined as Consider Power density High power density grid needs more stacked signal TSV to dissipate its heat.

Determination of Stacking Grid Gain function to select our target grid to place stacked signal TSV at grid (i, j) is defined as Consider Number of TSVs When TSVi,j,k is larger, fewer number of TSVs is in grid (i,j,k).

Determination of Stacking Grid Gain function to select our target grid to place stacked signal TSV at grid (i, j) is defined as Consider Whitespace

Determination of Stacking Grid Gain function to select our target grid to place stacked signal TSV at grid (i, j) is defined as Consider Wirelength Wirelengthi,j,k is the wirelength overhead in tier k if stacking location is at grid (i,j). smaller value of WL denotes higher wiring overhead. Move signal TSVs to the same 2D location across all tiers will change the routing topology and increase wiring overhead.

Experimental Result 2005 IWLS benchmarks [20] and industrial circuits. 3D placement results are produced by a partitioning driven placement for 3D ICs [5]. minimize the total wirelength and signal-TSV count

Experimental Result Extra hardware overhead !!! S.TSV : Total # of Stacked TSV

Conclusion A new integrated architecture, stacked signal TSV, was developed to dissipate heat. Based on this structure, a two-stage TSV locating algorithm has been proposed to construct the stacked signal TSVs and fully utilize the TSV thermal conductance to optimize the chip temperature. Compared to previous thermal-TSV insertion method, our proposed algorithm has zero hardware overhead incurred by thermal-TSV.