Digital VLSI design. Lecture 2: Complementary Metal Oxide Semiconductor (CMOS) Chips

Transcription

1 Digital VLSI design Lecture 2: Complementary Metal Oxide Semiconductor (CMOS) Chips

2 What will we learn? How integrated circuits work How to design chips with millions of transistors Ways of managing the complexity Use of tools to speed up the design process Identifying performance bottlenecks Ways to speeding up circuits Making sure the designs are correct Making the chips testable after manufacturing Other issues: effect of technologies, reducing power consumption etc.

3 Learning general principles Chip design involves optimization, tradeoffs Need the ability to work as part of a group Technology changes fast, so it is important to understand the general principles which would span technology generations Systems are implemented using building blocks (which may be technology-specific) Lot of work in course, but we will learn a lot

4 Agenda How to build your own simple CMOS chip CMOS transistors Building logic gates from transistors Transistor layout and fabrication (Self-study) Rest of the course: How to build a good CMOS chip Note: This course will not cover semiconductor physics We will design VLSI circuits knowing the electrical behavior of the transistor 4

5 licon Lattice Transistors are built on a silicon substrate licon is a Group IV material Forms crystal lattice with bonds to four neighbors 5

6 Dopants licon 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) As B - 6

7 p-n Junctions A junction between p-type and n-type semiconductor forms a diode. Current flows only in one direction p-type n-type anode cathode 7

8 nmos Transistor Four terminals: gate, source, drain, body Gate oxide body stack looks like a capacitor Gate and body are conductors O 2 (oxide) is a very good insulator Called metal oxide semiconductor (MOS) capacitor Even though gate is no longer made of metal* Source Gate Drain Polysilicon O 2 * Metal gates are returning today! n+ Body n+ p bulk 8

9 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 Source Gate Drain Polysilicon O 2 n+ p n+ bulk S 0 D 9

10 nmos Operation Cont. 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 Source Gate Drain Polysilicon O 2 n+ p n+ bulk S 1 D 10

11 pmos Transistor milar, but doping and voltages reversed Body tied to high voltage (V DD ) Gate low: transistor ON Gate high: transistor OFF Bubble indicates inverted behavior Polysilicon Source Gate Drain O 2 p+ p+ n bulk 11

12 Power Supply Voltage GND = 0 V In 1980 s, V DD = 5V V DD has decreased in modern processes High V DD would damage modern tiny transistors Lower V DD saves power V DD = 3.3, 2.5, 1.8, 1.5, 1.2, 1.0, 12

13 Transistors as Switches We can view MOS transistors as electrically controlled switches Voltage at gate controls path from source to drain g = 0 g = 1 nmos g d d OFF d ON s s s d d d pmos g ON OFF s s s 13

14 CMOS Inverter A Y V DD A 01 OFF ON Y ON OFF A Y GND 14

15 CMOS NAND Gate A B Y OFF ON OFF ON A 1 0 ON OFF Y B 10 ON OFF 15

16 CMOS NOR Gate A B Y A B Y 16

17 3-input NAND Gate Y pulls low if ALL inputs are 1 Y pulls high if ANY input is 0 A Y B C 17

18 CMOS Fabrication CMOS transistors are fabricated on silicon wafer Lithography process similar to printing press On each step, different materials are deposited or etched Easiest to understand by viewing both top and cross-section of wafer in a simplified manufacturing process 18

19 Inverter Cross-section Typically use p-type substrate for nmos transistors Requires n-well for body of pmos transistors A GND Y V DD O 2 n+ diffusion n+ n+ p+ p+ p+ diffusion p substrate n well polysilicon metal1 nmos transistor pmos transistor 19

20 Well and Substrate Taps Substrate must be tied to GND and n-well to V DD Metal to lightly-doped semiconductor forms poor connection called Shottky Diode Use heavily doped well and substrate contacts / taps GND A Y V DD p+ n+ n+ p+ p+ n+ p substrate n well substrate tap well tap 20

21 Inverter Mask Set Transistors and wires are defined by masks Cross-section taken along dashed line A Y GND V DD substrate tap nmos transistor pmos transistor well tap 21

22 Detailed Mask Views x masks n-well Polysilicon n+ diffusion p+ diffusion Contact Metal n well Polysilicon n+ Diffusion p+ Diffusion Contact A Y GND VDD Metal substrate tap nmos transistor pmos transistor well tap 22

23 Fabrication Chips are built in huge factories called fabs Contain clean rooms as large as football fields Courtesy of International Business Machines Corporation. Unauthorized use not permitted. 23

24 Fabrication Steps Start with blank wafer Build inverter from the bottom up First step will be to form the n-well Cover wafer with protective layer of O 2 (oxide) Remove layer where n-well should be built Implant or diffuse n dopants into exposed wafer Strip off O 2 p substrate 24

25 Oxidation Grow O 2 on top of wafer C with H 2 O or O 2 in oxidation furnace O 2 p substrate 25

26 Photoresist Spin on photoresist Photoresist is a light-sensitive organic polymer Softens where exposed to light Photoresist O 2 p substrate 26

27 Lithography Expose photoresist through n-well mask Strip off exposed photoresist Photoresist O 2 p substrate 27

28 Etch Etch oxide with hydrofluoric acid (HF) Seeps through skin and eats bone; nasty stuff!!! Only attacks oxide where resist has been exposed Photoresist O 2 p substrate 28

29 Strip Photoresist Strip off remaining photoresist Use mixture of acids called piranah etch Necessary so resist doesn t melt in next step O 2 p substrate 29

30 n-well n-well is formed with diffusion or ion implantation Diffusion Place wafer in furnace with arsenic gas Heat until As atoms diffuse into exposed Ion Implanatation Blast wafer with beam of As ions Ions blocked by O 2, only enter exposed O 2 n well 30

31 Strip Oxide Strip off the remaining oxide using HF Back to bare wafer with n-well Subsequent steps involve similar series of steps p substrate n well 31

32 Polysilicon Deposit very thin layer of gate oxide < 20 Å (6-7 atomic layers) Chemical Vapor Deposition (CVD) of silicon layer Place wafer in furnace with lane gas (H 4 ) Forms many small crystals called polysilicon Heavily doped to be good conductor Polysilicon Thin gate oxide p substrate n well 32

33 Polysilicon Patterning Use same lithography process to pattern polysilicon Polysilicon Polysilicon Thin gate oxide p substrate n well 33

34 Self-Aligned Process Use oxide and masking to expose where n+ dopants should be diffused or implanted N-diffusion forms nmos source, drain, and n-well contact p substrate n well 34

35 N-diffusion Pattern oxide and form n+ regions Self-aligned process where gate blocks diffusion Polysilicon is better than metal for self-aligned gates because it doesn t melt during later processing n+ Diffusion p substrate n well 35

36 N-diffusion cont. Historically dopants were diffused Usually ion implantation today But regions are still called diffusion n+ n+ n+ p substrate n well 36

37 N-diffusion cont. Strip off oxide to complete patterning step n+ n+ n+ p substrate n well 37

38 P-Diffusion milar set of steps form p+ diffusion regions for pmos source and drain and substrate contact p+ n+ n+ p+ p+ n+ p substrate n well 38

39 Contacts Now we need to wire together the devices Cover chip with thick field oxide Etch oxide where contact cuts are needed Contact p+ n+ n+ p+ p+ n+ Thick field oxide p substrate n well 39

40 Metalization Sputter on aluminum over whole wafer Pattern to remove excess metal, leaving wires Metal p+ n+ n+ p+ p+ n+ Metal Thick field oxide p substrate n well 40

41 Layout Chips are specified with set of masks Minimum dimensions of masks determine transistor size (and hence speed, cost, and power) Feature size f = distance between source and drain Set by minimum width of polysilicon Feature size improves 30% every 3 years or so Normalize for feature size when describing design rules Express rules in terms of l = f/2 E.g. l = 0.3 mm in 0.6 mm process 41

42 mplified Design Rules Conservative rules to get you started 42

43 Inverter Layout Transistor dimensions specified as Width / Length Minimum size is 4l / 2l, sometimes called 1 unit In f = 0.6 mm process, this is 1.2 mm wide, 0.6 mm long 43

44 Summary MOS transistors are stacks of gate, oxide, silicon Act as electrically controlled switches Build logic gates out of switches Draw masks to specify layout of transistors Now you know everything necessary to start designing schematics and layout for a simple chip! 44

45 CMOS Gate Design Activity: Sketch a 4-input CMOS NOR gate A B C D Y 45