ECE 261: CMOS VLSI Design Methodologies

1 1 1 ECE 261: cmos vlsi Design Methodologies Prof. Krishnendu (Krish) Chakrabarty Dept. Electrical and Computer Engineering Room 2513 CIEMAS Ph: 660-5244 E-mail: URL: ~krish Course URL: ~krish/ Google group: ECE261_Fall2011 2 Course Objectives Introduction to cmos vlsi Design Methodologies Emphasis on full-custom Design Circuit and system levels Extensive use of Mentor Graphics CAD tools for IC Design , simulation, and layout verification Specific techniques for designing high-speed, low-power, and easily-testable circuits 2 4 Introduction Integrated circuits: many transistors on one chip. Very Large Scale Integration ( vlsi ): bucketloads! Complementary Metal Oxide Semiconductor Fast, cheap, low power transistors Today: How to build your own simple cmos chip cmos transistors Building logic gates from transistors Transistor layout and fabrication Rest of the course: How to build a good cmos chip 3 5 Designing for vlsi Designing a system on a chip Craft components from silicon rather than selecting catalog parts ICs (chips) are batch fabricated Inexpensive unit cost Bugs are hard to fix!

2 Extensive Design verification needed 6 vlsi Design : Overview vlsi Design is system Design Designing fast inverters is fun, but need knowledge of all aspects of digital Design : algorithms, systems, circuits, fabrication, and packaging Need to bridge gap between abstract vision of digital Design and the underlying digital circuit and its peculiarities Circuit-level optimization, verification, and testing techniques are important Tall thin approach does not always work Today s designer is fatter , but well-versed in both high-level and low-level Design skills 4 7 vlsi : Enabling Technology Automotive electronic systems A typical car has over 100 ICs (stereo systems, display panels, fuel injection systems, smart suspensions, antilock brakes, airbags) Signal Processing (DSP chips, data acquisition systems) Transaction processing (bank ATMs) PCs, workstations, servers, consumer electronics Medical electronics (artificial eye, implants) Avionics, space applications Networking hardware: Routers and switches.

3 The Semiconductor Industry 53% compound annual growth rate over 50 years No other technology has grown so fast so long Driven by miniaturization of transistors Smaller is cheaper, faster, lower in power! Revolutionary effects on society 8 5 9 Design Complexity Transistor counts and IC densities continue to grow! Moore s Law-The number of transistors on an IC doubles every years Intel x486: 1 million transistors (1989), PowerPC: 2-3 million transistors (1994), Pentium: million transistors (1994), DEC Alpha: 10 million transistors (1995)-9 million in SRAM, Pentium IV (2001): 42 million transistors, Xeon (8 cores): billion transistors Memory (DRAM) is the technology driver 256 Mbits DRAM (1998), 512 Mb DRAM (2003), 1GB, 2 GB,.. 10 vlsi Technology cmos : Complementary Metal Oxide Silicon Based on voltage-controlled field-effect transistors (FETs) Other technologies.

4 Bipolar junction transistors (BJTs), BiCMOS, gallium arsenide (GaAs) BJTs, BiCMOS, ECL circuits are faster but cmos consumes lower power and are easier to fabricate GaAs carriers have higher mobility but high integration levels are difficult to achieve in GaAs technology 6 11 Transistor Types Bipolar transistors npn or pnp silicon structure Small current into very thin base layer controls large currents between emitter and collector Base currents limit integration density Metal Oxide Semiconductor Field Effect Transistors nMOS and pMOS MOSFETS Voltage applied to insulated gate controls current between source and drain Low power allows very high integration 12 IC Manufacturing Some manufacturing processes are tightly coupled to the product, Buick/Chevy assembly line IC manufacturing technology is more versatile cmos manufacturing line can make circuits of any type by changing some basic tools called masks The same plant can manufacture both microprocessors and microwave controllers by simply changing masks Silicon wafers: raw materials of IC manufacturing Test structure Wafer IC 7 13 Chip Designer s Lexicon Boston geometry: masks that use only curves In contrast to Manhattan and Euclidean geometries Dog and pony show: A presentation to management Hit by a truck: Losing a key technical person at a crucial point in a project (especially common in the IC Design industry today!)

5 Infant mortality: failure of ICs during the first few hours of operation 14 Silicon Lattice Transistors are built on a silicon substrate Silicon is a Group IV material Forms crystal lattice with bonds to four neighbors 8 15 Dopants Silicon is a semiconductor Pure silicon has no free carriers and conducts poorly Adding dopants increases the conductivity Group V: extra electron (n-type) Group III: missing electron, called hole (p-type) 16 p-n Junctions A junction between p-type and n-type semiconductor forms a diode. Current flows only in one direction 9 17 nMOS Transistor Four terminals: gate, source, drain, body Gate oxide body stack looks like a capacitor Gate and body are conductors SiO2 (oxide) is a very good insulator Called metal oxide semiconductor (MOS) capacitor Even though gate is no longer made of metal 18 nMOS Operation Body is usually tied to ground (0 V) When the gate is at a low voltage: P-type body is at low voltage Source-body and drain-body diodes are OFF No current flows, transistor is OFF 10 19 nMOS Operation (Contd.)

6 When the gate is at a high voltage: Positive charge on gate of MOS capacitor Negative charge attracted to body Inverts a channel under gate to n-type Now current can flow through n-type silicon from source through channel to drain, transistor is ON 20 pMOS Transistor Similar, but doping and voltages reversed Body tied to high voltage (VDD) Gate low: transistor ON Gate high: transistor OFF Bubble indicates inverted behavior 11 21 Power Supply Voltage GND = 0 V In 1980 s, VDD = 5V VDD has decreased in modern processes High VDD would damage modern tiny transistors Lower VDD saves power VDD = , , , , , , .. 22 Annual Sales >1019 transistors manufactured in 2008 1 billion for every human on the planet 12 23 1970 s processes usually had only nMOS transistors Inexpensive, but consume power while idle 1980s-present: cmos processes for low idle power MOS Integrated Circuits Intel 1101 256-bit SRAM Intel 4004 4-bit Proc [Vadasz69] 1969 IEEE.

7 Intel Museum. Reprinted with permission. 24 Moore s Law: Then 1965: Gordon Moore plotted transistor on each chip Fit straight line on semilog scale Transistor counts have doubled every 26 months Integration Levels SSI: 10 gates MSI: 1000 gates LSI: 10,000 gates vlsi : > 10k gates [Moore65] Electronics Magazine 13 25 And 26 14 27 Feature Size Minimum feature size shrinking 30% every 2-3 years 28 Corollaries Many other factors grow exponentially Ex: clock frequency, processor performance 15 Cost of Fabs 29 30 Some History Basic principles of MOS: Lilienfield (1925), Heil (1935) Problem: Fabrication, materials processing BJT transistors were designed at Bell Labs in the late forties and fifties (sixty years of the transistor!) MOS planar process: 1960, Weiner ( cmos flip-flops:1962), Wanlass (inverter, NOR, NAND gates: 1963) 16 31 History 1958: First integrated circuit Flip-flop using two transistors Built by Jack Kilby at Texas Instruments 2010 Intel Core i7 processor billion transistors 64 Gb Flash memory > 16 billion transistors Courtesy Texas Instruments [Trinh09] 2009 IEEE.

8 32 Invention of the Transistor Vacuum tubes ruled in first half of 20th century Large, expensive, power-hungry, unreliable 1947: first point contact transistor John Bardeen and Walter Brattain at Bell Labs See Crystal Fire by Riordan, Hoddeson AT&T Archives. Reprinted with permission. 17 33 The First Computer 34 ENIAC - The first electronic computer (1946) 18 35 Intel 4004 Micro-Processor 1971, 2300 transistors, 10 microns process 36 Intel Pentium (II) microprocessor 1997, M transistor, micron processs 19 Newer Microprocessors Intel Pentium 4 2000, 42 M transistors, V, micron process Core i7 4 physical cores 2008, 45 nm process, 781 M transistors, GHz max Intel Xeon server chip 8 cores 2009, billion transistors, 45 nm process 37 38 Design Abstraction Levels 20 39 Coping with Complexity How to Design System-on-Chip?

9 Many millions (even billions!) of transistors Tens to hundreds of engineers Structured Design Design Partitioning 40 Structured Design Hierarchy: Divide and Conquer Recursively system into modules Regularity Reuse modules wherever possible Ex: Standard cell library Modularity: well-formed interfaces Allows modules to be treated as black boxes Locality Physical and temporal 21 41 Design Partitioning Architecture: User s perspective, what does it do? Instruction set, registers MIPS, x86, Alpha, PIC, ARM, .. Microarchitecture Single cycle, multcycle, pipelined, superscalar? Logic: how are functional blocks constructed Ripple carry, carry lookahead, carry select adders Circuit: how are transistors used Complementary cmos , pass transistors, domino Physical: chip layout Datapaths, memories, random logic 42 Gajski Y-Chart 22 43 MIPS Architecture Example.

10 Subset of MIPS processor architecture Drawn from Patterson & Hennessy MIPS is a 32-bit architecture with 32 registers Consider 8-bit subset using 8-bit datapath Only implement 8 registers ($0 - $7) $0 hardwired to 00000000 8-bit program counter 44 MIPS Microarchitecture Multicycle architecture ( [Paterson04], [Harris07] ) 23 45 Multicycle Controller 46 Logic Design Start at top level Hierarchically decompose MIPS into units Top-level interface 24 47 Block Diagram 48 Hierarchical Design 25 49 HDLs Hardware Description Languages Widely used in logic Design Verilog and VHDL Describe hardware using code Document logic functions Simulate logic before building Synthesize code into gates and layout Requires a library of standard cells 50 Verilog Example module fulladder(input a, b, c, output s, cout); sum s1(a, b, c, s); carry c1(a, b, c, cout); endmodule module carry(input a, b, c, output cout) assign cout = (a&b) | (a&c) | (b endmodule 26 51 Circuit Design How should logic be implemented?)