CSCE 613: Fundamentals of VLSI Chip Design Instructor: Jason D. Bakos

Fund. of VLSI Chip Design 2 Topics for this Lecture Semiconductor theory in a nutshell MOSFET devices as switches Transistor-level logic Logic gates IC fabrication SCMOS design rules Cell libraries

Fund. of VLSI Chip Design 3 Elements

Fund. of VLSI Chip Design 4 Semiconductors Silicon is a group IV element (4 valence electrons, shells: 2, 8, 18, …) –Forms covalent bonds with four neighbor atoms (3D cubic crystal lattice) –Si is a poor conductor, but conduction characteristics may be altered –Add impurities/dopants (replaces silicon atom in lattice): Makes a better conductor Group V element (phosphorus/arsenic) => 5 valence electrons –Leaves an electron free => n-type semiconductor (electrons, negative carriers) Group III element (boron) => 3 valence electrons –Borrows an electron from neighbor => p-type semiconductor (holes, positive carriers) forward bias reverse bias P-N junction

Fund. of VLSI Chip Design 5 MOSFETs Diodes not very useful for building logic Metal-oxide-semiconductor structures built onto substrate –Diffusion: Inject dopants into substrate –Oxidation: Form layer of SiO 2 (glass) –Deposition and etching: Add aluminum/copper wires body/bulk GROUND NMOS/NFETPMOS/PFET channel shorter length, faster transistor (dist. for electrons) body/bulk HIGH positive voltage (Vdd) negative voltage (rel. to body) (GND) (S/D to body is reverse-biased) current

Fund. of VLSI Chip Design 6 FETs as Switches NFETs and PFETs can act as switches pull-up OFF pull-up ON pull-down OFF Z (floating) 1 pull-down ON 0smokin’! bulk node not shown CMOS logic CMOS: assuming PU and PN network are perfect switches and switch simultanously, no current flow and no power consumption! “and structure”“or structure”

Fund. of VLSI Chip Design 7 Logic Gates inv NAND2 NAND3 NOR2 DeMorgan’s Law CMOS: complimentary in form and function NMOS devices (positive logic) form pull-down network PMOS devices (negative logic) form pull-up network Implication: CMOS transistor-level logic gates implement functions where may the inputs are inverted (inverting gates) Add inverter at inputs/outputs to create non- inverting gate

Fund. of VLSI Chip Design 8 Compound Gates Combine parallel and series structures to form compound gates –Example: –Use DeMorgan’s law to determine complement (pull- down network): Y A A B B C C D D

Fund. of VLSI Chip Design 9 Pass Transistors/Transmission Gates NMOS passes strong 0 (pull-down) PMOS passes strong 1 (pull-up) Pass transistor:Transmission gate:

Fund. of VLSI Chip Design 10 Tristates

Fund. of VLSI Chip Design 11 Multiplexer Transmission gate multiplexer Inverting multiplexer

Fund. of VLSI Chip Design 12 Multiplexer 4-input multiplexer

Fund. of VLSI Chip Design 13 Latches Positive level-sensitive latch

Fund. of VLSI Chip Design 14 Latches Positive edge-sensitive latch

Fund. of VLSI Chip Design 15 IC Fabrication Inverter cross-section field oxide

Fund. of VLSI Chip Design 16 IC Fabrication Inverter cross-section with well and substrate contacts (ohmic contact)

Fund. of VLSI Chip Design 17 IC Fabrication Chips are fabricated using set of masks –Photolithography Inverter uses 6 layers: –n-well, poly, n+ diffusion, p+ diffusion, contact, metal Basic steps –oxidize –apply photoresist –remove photoresist with mask –HF acid eats oxide but not photoresist –pirana acid eats photoresist –ion implantation (diffusion, wells) –vapor deposition (poly) –plasma etching (metal)

Fund. of VLSI Chip Design 18 IC Fabrication Furnace used to oxidize ( C) Mask exposes photoresist to light, allowing removal HF acid etch piranha acid etch diffusion (gas) or ion implantation (electric field) HF acid etch

Fund. of VLSI Chip Design 19 IC Fabrication Heavy doped poly is grown with gas in furnace (chemical vapor deposition) Masked used to pattern poly Poly is not affected by ion implantation

Fund. of VLSI Chip Design 20 IC Fabrication Metal is sputtered (with vapor) and pla s ma etched from mask

Fund. of VLSI Chip Design 21 Layout Design Rules Design rules define ranges for features –Examples: min. wire widths to avoid breaks min. spacings to avoid shorts minimum overlaps to ensure complete overlaps –Measured in microns –Required for resolution/tolerances of masks Fabrication processes defined by minimum channel width –Also minimum width of poly traces –Defines “how fast” a fabrication process is Lambda-based (scalable CMOS) design rules define scalable rules based on (which is half of the minimum channel length) –classes of MOSIS SCMOS rules: SUBMICRON, DEEP SUBMICRON

Fund. of VLSI Chip Design 22 Layout Design Rules

Fund. of VLSI Chip Design 23 Layout Design Rules Transistor dimensions are in W/L ratio –NFETs are usually twice the width –PFETs are usually twice the width of NFETs Holes move more slowly than electrons (must be wider to deliver same current)

Fund. of VLSI Chip Design 24 Layout 3-input NAND

Fund. of VLSI Chip Design 25 Design Flow Design flow is a sequence of steps for design and verification In this course: –Describe behaviors with VHDL/Verilog code –Simulate behavioral designs –Synthesize behaviors into cell-level netlists –Simulate netlists with cell-delay models –Place-and-route netlists into a physical design –Simulate netlists with cell-delay models and wire-delay models Need to define a cell library: –Function –Electrical characteristics of each cell –Layout

Fund. of VLSI Chip Design 26 Cell Library (Snap Together) Layout