1 A Lithography-friendly Structured ASIC Design Approach By: Salman Goplani* Rajesh Garg # Sunil P Khatri # Mosong Cheng # * National Instruments, Austin, TX # Department of ECE, Texas A&M University, College Station, TX

2 Outline Motivation Motivation Mask costs increasing Mask costs increasing Systematic process variations increasing Systematic process variations increasing Previous Work Previous Work Our Approach Our Approach NAND2 based circuit implementation methodology NAND2 based circuit implementation methodology Experimental Results Experimental Results Conclusions Conclusions

3 Motivation – Mask Costs Process (microns) Single Mask Cost ($K) # of Masks Mask Set cost ($K) A full set of lithography masks can cost between $1-3M. A full set of lithography masks can cost between $1-3M. Roughly 25% reduction in ASIC design starts in past 7 years. [Sematech Annual Report 2002], [ A. Sangiovanni-Vincentelli “The Tides of EDA”, keynote talk, DAC 2003]. Roughly 25% reduction in ASIC design starts in past 7 years. [Sematech Annual Report 2002], [ A. Sangiovanni-Vincentelli “The Tides of EDA”, keynote talk, DAC 2003]. Need an approach in which different designs share a set of masks Need an approach in which different designs share a set of masks

4 Motivation - Variations Process variations can be classified as Process variations can be classified as Random variations Random variations Systematic variations Systematic variations Random variations are unpredictable Random variations are unpredictable Caused by random fluctuations such as number of dopant atoms Caused by random fluctuations such as number of dopant atoms Systematic variations Systematic variations Predictable variation trends across a chip Predictable variation trends across a chip Caused by spatial dependencies during device processing Caused by spatial dependencies during device processing Chemical and mechanical polishing (CMP) Chemical and mechanical polishing (CMP) Optical proximity effects (OPE) Optical proximity effects (OPE) Changes in poly shapes translates into channel length variations Changes in poly shapes translates into channel length variations Impacts circuit performance more severely compared to metal variations Impacts circuit performance more severely compared to metal variations

5 Motivation – Structured ASICs Standard cell based design approach (ASIC) Standard cell based design approach (ASIC) Severely affected by OPEs due to lack of regularity in design Severely affected by OPEs due to lack of regularity in design Optical proximity correction (OPC) is performed to deal with OPEs Optical proximity correction (OPC) is performed to deal with OPEs OPC needs to be performed on all layers for each new ASIC design OPC needs to be performed on all layers for each new ASIC design Computationally expensive process Computationally expensive process Need a circuit design approach that Need a circuit design approach that Allows us to share a majority of fabrication masks across different designs Allows us to share a majority of fabrication masks across different designs Allows us to share the OPC computation for some layers, across different designs Allows us to share the OPC computation for some layers, across different designs Our approach achieves these goals Our approach achieves these goals

6 Previous Work Jayakumar et. al proposed a structured ASIC approach using a network of fixed (medium) sized PLAs Jayakumar et. al proposed a structured ASIC approach using a network of fixed (medium) sized PLAs Large delay (area) overhead of ~260% (~240%) Large delay (area) overhead of ~260% (~240%) Gulati et. al reported a pass transistor logic (PTL) based structured ASIC approach Gulati et. al reported a pass transistor logic (PTL) based structured ASIC approach Delay and area overheads are ~50% and ~240% Delay and area overheads are ~50% and ~240% Pillegi et. al reported that FPGAs are typically ~25X slower than ASICs Pillegi et. al reported that FPGAs are typically ~25X slower than ASICs Our approach provides a structured ASIC solution with small area (~10%) and delay (~35%) overheads Our approach provides a structured ASIC solution with small area (~10%) and delay (~35%) overheads

7 Our Solution Use a regular array of 2-input NAND cells as the underlying circuit structure, and customize only METAL and VIA masks Use a regular array of 2-input NAND cells as the underlying circuit structure, and customize only METAL and VIA masks NAND2 is functionally complete NAND2 is functionally complete Stock such arrays pre-processed until metallization step Stock such arrays pre-processed until metallization step Or, use previously generated masks for all other layers and use new masks for only METAL, VIA layers Or, use previously generated masks for all other layers and use new masks for only METAL, VIA layers To create an ASIC for a given design – technology-map this design to the smallest available NAND2 array To create an ASIC for a given design – technology-map this design to the smallest available NAND2 array Only METAL and VIA masks require changes Only METAL and VIA masks require changes Easier to fix bugs, since only METAL and VIA masks change Easier to fix bugs, since only METAL and VIA masks change Optimize poly layer mask for maximum yield Optimize poly layer mask for maximum yield Perform aggressive OPC on the poly layer Perform aggressive OPC on the poly layer Required to be done only once Required to be done only once Beneficial since performance highly sensitive to channel length variations Beneficial since performance highly sensitive to channel length variations

8 NAND2 Cell Array NAND2 cells are placed NAND2 cells are placed to create rectangular array of cells array of cells Some space is left between Some space is left between two rows of NAND2 cells two rows of NAND2 cells Used for routing Used for routing

9 NAND2 Cell Size- 1.6  m X 2.6  m Size- 1.6  m X 2.6  m Input/output pins on Metal1 Input/output pins on Metal1 Symmetrical along vertical axis up to poly layer Symmetrical along vertical axis up to poly layer Placer can map to original or flipped cell orientation, thereby reducing area Placer can map to original or flipped cell orientation, thereby reducing area Poly and diffusion layers unchanged if a cell is flipped, hence same masks used for either orientation. Poly and diffusion layers unchanged if a cell is flipped, hence same masks used for either orientation. Layout of NAND2 cell is lithography- friendly Layout of NAND2 cell is lithography- friendly No bends in poly No bends in poly Poly on a fixed pitch (as required in more recent fabrication processes) Poly on a fixed pitch (as required in more recent fabrication processes) Good for manufacturability reasons Good for manufacturability reasons

10 Circuit Mapping to NAND2 Array Library L consists of 1X, 2X, 3X and 4X NAND2 cells Library L consists of 1X, 2X, 3X and 4X NAND2 cells 2X, 3X and 4X NAND2 cells are implemented by connecting 2, 3 and 4 NAND2 cells in parallel 2X, 3X and 4X NAND2 cells are implemented by connecting 2, 3 and 4 NAND2 cells in parallel Combination circuit N in blif format Place N2 using QPLACE -SEDSM and Route using WROUTE Technology indep. opt. of N Map N * with L for area or delay N*N* N1 Replace all 2X, 3X or 4X NAND2 cells in N1 by 2, 3 or 4 1X NAND2 cells N2

11 Characterization of NAND2 Array Delay ( D ) is obtained using the sense package in SIS Delay ( D ) is obtained using the sense package in SIS Sense reports the largest sensitizeable delay of the circuit (excludes any false paths) Sense reports the largest sensitizeable delay of the circuit (excludes any false paths) We use gate netlist N1 with 1X, 2X, 3X and 4X NAND2 We use gate netlist N1 with 1X, 2X, 3X and 4X NAND2 Power - dynamic power of a circuit is Power - dynamic power of a circuit is f (= 1/ D ) is the operating frequency of circuit f (= 1/ D ) is the operating frequency of circuit C eff is the total switching capacitance C eff is the total switching capacitance where: C k is the capacitance of the node k where: C k is the capacitance of the node k is the probability of transition of the node k is the probability of transition of the node k

12 Characterization of NAND2 Array Transition probability of the node k is given by Transition probability of the node k is given by where: p k is the probability that node k is at logic “1” Probability p k is obtained using the approach of Gulati et. al Probability p k is obtained using the approach of Gulati et. al p k = 0.5 for primary inputs p k = 0.5 for primary inputs For any node, obtain p k by propagating input probabilities based on node functionality For any node, obtain p k by propagating input probabilities based on node functionality Area is obtained by placing and routing N2 using SEDSM tools from Cadence Area is obtained by placing and routing N2 using SEDSM tools from Cadence All benchmark circuits are routed using up to 4 Metal layers All benchmark circuits are routed using up to 4 Metal layers

13 Characterization of NAND2 Array OPC and lithographical simulations OPC and lithographical simulations Used Calibre tool from Mentor Graphics Used Calibre tool from Mentor Graphics We used optical model with = 193nm We used optical model with = 193nm Constant threshold resist model was used Constant threshold resist model was used We perform OPC on poly and metal layers (referred to as M) of the placed and routed N2 design. Resulting layers are referred to as M OPC We perform OPC on poly and metal layers (referred to as M) of the placed and routed N2 design. Resulting layers are referred to as M OPC Lithographical simulations are then performed on all layers in M OPC to obtain resulting layers M SIM Lithographical simulations are then performed on all layers in M OPC to obtain resulting layers M SIM Error is the area of layer E M which is given by Error is the area of layer E M which is given by E M = XOR(M, M SIM ) E M = XOR(M, M SIM )

14 Experimental Results Designed NAND2 cells library L using 100 BPTM with VDD = 1.2V Designed NAND2 cells library L using 100 BPTM with VDD = 1.2V Also implemented standard cell library L STD Also implemented standard cell library L STD L contains 1X, 2X, 3X and 4X NAND2 cells L contains 1X, 2X, 3X and 4X NAND2 cells L STD consists of INV and NAND, NOR, AND & OR gates (with 2 and 3 inputs) L STD consists of INV and NAND, NOR, AND & OR gates (with 2 and 3 inputs) Implemented several ISCAS and MCNC benchmark circuits using our approach and ASIC approach Implemented several ISCAS and MCNC benchmark circuits using our approach and ASIC approach We mapped these designs for both area and delay optimality We mapped these designs for both area and delay optimality

15 Area, Delay and Power Average results for several circuits implemented using our NAND2 structured ASIC approach and traditional ASIC approach Average results for several circuits implemented using our NAND2 structured ASIC approach and traditional ASIC approach Detailed results in paper Detailed results in paper Performance Parameter Area Mapped Delay Mapped Ratio (NAND2/ASIC) Area Delay Power

16 Lithography Simulation Ratio of lithographical error for poly and Metal1-4 layers for both approaches Ratio of lithographical error for poly and Metal1-4 layers for both approaches Errors on poly and Metal1 for our approach is lower than ASIC approach Errors on poly and Metal1 for our approach is lower than ASIC approach Poly error translates into channel length variations Poly error translates into channel length variations Sheet resistivity of Metal1 is higher than Metal2-4 Sheet resistivity of Metal1 is higher than Metal2-4 Wires in these layers is largely restricted to within the cell alone Wires in these layers is largely restricted to within the cell alone Our approach uses more wiring on Metal2-4 due to an overall area increase, resulting in an increase in error on these layers Our approach uses more wiring on Metal2-4 due to an overall area increase, resulting in an increase in error on these layers EPEPEPEP E M1 E M2 E M3 E M4 Area Mapped Delay Mapped

17 Conclusions With increasing cost of masks and process variations With increasing cost of masks and process variations Need to implement circuits using regular structures Need to implement circuits using regular structures We presented a new structured ASIC approach We presented a new structured ASIC approach Implements circuits using regular array of 2-input NAND gates Implements circuits using regular array of 2-input NAND gates Our approach has small overheads compared to standard cell (ASIC) based design approach Our approach has small overheads compared to standard cell (ASIC) based design approach Area - 12% Area - 12% Delay - 40% Delay - 40% Power - 7% Power - 7% Lithographical errors of our approach are lower on poly and Metal1 layers by 7% and 24% compared to ASIC approach Lithographical errors of our approach are lower on poly and Metal1 layers by 7% and 24% compared to ASIC approach Our approach is lithography friendly Our approach is lithography friendly

18 `Thank You!!

19 Backup Slides Backup Slides

20 AREA

21 Delay

22 Power

23 Lithographical Error

24 Implementing Sequential Circuits Flip Flop can be implemented using NAND2 gates as shown Flip Flop can be implemented using NAND2 gates as shown

25 Circuit Mapping to NAND2 Array Library L - 1X, 2X, 3X and 4X NAND2 cells Library L - 1X, 2X, 3X and 4X NAND2 cells 2X, 3X and 4X NAND2 cells are implemented by connecting 2, 3 and 4 NAND2 cells in parallel 2X, 3X and 4X NAND2 cells are implemented by connecting 2, 3 and 4 NAND2 cells in parallel Circuit mapping Circuit mapping Combination circuit N in blif format SIS Mapped Circuit N2 using only 1X NAND2 Technology Indep. Opt. of N Map N * with L for Area and Delay N*N* N1 Replace all 2X, 3X and 4X NAND2 cells by 2, 3 and 4 1X NAND2 Cells N2