Week 14a: Micro- and Nanotechnology

1. Week 14a: Micro- and Nanotechnology • Some preliminaries: • End of the propagation delay story • “Silicon Run” (video) CMOS discussion – if time • (Just for fun: “Hop On Board” -- Microlab • safety video) – if time • Energy Band View of Semiconductors if time -- • shortened version of notes handed out earlier • e. Some electronic application examples – if time • Micro- and Nanotechnology introduction • The sizes of things • Some micromachines • A bit about nanotech • What about radio? EE42/100, Prof. White

2. 1a. Propagation delay in logic circuits EE42/100, Prof. White

3. Note becomes valid one gate delay after B switches Note that becomes valid two gate delays after B&C switch, because the invert function takes one delay and the NAND function a second. No change at t =3 tD TIMING DIAGRAMS Show transitions of variables vs time A Logic state B D t C 0 t tD t tD 2tD t tD t 2tD 3tD tD EE42/100, Prof. White

4. EXAMPLE WHAT IS THE ORIGIN OF GATE DELAY? Logic gates are electronic circuits that process electrical signals Most common signal for logic variable: voltage Specific voltage ranges correspond to “0” or “1” Thus delay in voltage rise or fall (because of delay in charging internal capacitances) will translate to a delay in signal timing Note that the specific voltage range for 0 or 1 depends on “logic family,” and in general decreases with logic generations EE42/100, Prof. White

5. inside a large system VOLTAGE WAVEFORMS (TIME FUNCTIONS) Inverter input is vIN(t), output is vOUT(t) Vin(t) t EE42/100, Prof. White

6. Vin(t) 1.5 t Approximation tD tD tD GATE DELAY (PROPAGATION DELAY) Define  as the delay required for the output voltage to reach 50% of its final value. In this example we will use 3V logic, so halfway point is 1.5V. Inverters are designed so that the gate delay is symmetrical (rise and fall) Vout(t) 1.5 t EE42/100, Prof. White

7. WHAT DETERMINES GATE DELAY? 5 MODEL Vx R 2.5 II RC = 0.1ns C 5 vIN vOUT 2.5 Vx t tD = 0.069ns Example The gate delay is simply the charging of the capacitors at internal nodes. Oversimplified example using “ideal inverter, II” and 5V logic swing RC = 0.1ns so 0.069ns after vIN switches by 5V, Vx moves 2.5V EE42/100, Prof. White

8. VDD = 2V - SP is closed if VIN < VDD SP + RP VOUT VIN + + RN Input Output SN is closed if VIN > VSS + SN - - - VSS = 0V Controlled Switch Model of Inverter So if VIN is 2V then SN is closed and SP is open. Hence VOUT is zero. But if VIN is 0V then SP is closed and SN is open. Hence VOUT is 2V. EE42/100, Prof. White

9. How fast is this? Speed of light: c = 3  108 m/s Distance traveled in 57 ps is: EFFECT OF PROPAGATION DELAY ON PROCESSOR SPEED Computer architects would like each system clock cycle to have between 20 and 50 gate delays … use 35 for calculations Implication: if clock frequency = 500 MHz clock period = (5108 s1)1 Period = 2  10 9s = 2 ns (nanoseconds) Gate delay must be tD = (1/35)  Period = (2 ns)/35 = 57 ps (picoseconds) C X tD = (3x108m/s)(57x10-12) = 17 x 10-4 m = 1.7cm EE42/100, Prof. White

10. 1d. Energy Band View of Semiconductors Conductors, semiconductors, insulators: Why is it that when individual atoms get close together to form a solid – such as copper, silicon, or quartz – they form materials that have a high, variable, or low ability to conduct current? Understand in terms of allowed, empty, and occupiedelectronic energy levels and electronicenergy bands. Fig. 1 shows the calculated allowed energy levels for electrons (vertical axis) versus distance between atoms (horizontal axis) for materials like silicon. EE42/100, Prof. White

11. Fig. 1. Calculated energy levels in the diamond structure as a function of assumed atomic spacing at T = 0o K. (From “Introduction to Semiconductor Physics”, Wiley, 1964) EE42/100, Prof. White

12. In Fig. 1, at right atoms are essentially isolated; at left atomic separations are just a few tenths of a nanometer, characteristic of atoms in a silicon crystal. • If we start with N atoms of silicon at the right, which have 14 electrons each, there must be 14N allowed energy levels for the electrons. (You learned about this in physics in connection with the Bohr atom, the Pauli Exclusion principle, etc.) • If the atoms are pushed together to form a solid chunk of silicon, the electrons of neighboring atoms will interact and the allowed energy levels will broaden intoenergy bands. EE42/100, Prof. White

13. When the “actual spacing” is reached, the quantum-mechanical calculation results are that: • at lowest energies very narrow ranges of energy are allowed for inner electrons (these are core electrons, near the nuclei); • a higher band of 4N allowed states exists that, at 0oK, is filled with 4N electrons; • then an energy gap, EG, appears with no allowed states (no electrons permitted!); and • at highest energies a band of allowed states appears that is entirely empty at 0oK. • Can this crystal conduct electricity? EE42/100, Prof. White

14. NO, it cannot conductor electricity at 0o K because that involves moving charges and therefore an increase of electron energy – but we have only two bands of states separated by a forbidden energy gap, EG. The (lower) valence band is entirely filled, and the (upper) conduction band states are entirely empty. To conduct electricity we need to have a band that has some filled states (some electrons!) and some empty states that can be occupied by electrons whose energies increase. EE42/100, Prof. White

15. Metals, pure silicon at 0K and 300K, and doped silicon • A. Conductors such as aluminum and gold can conduct at low • temperatures because the highest energy band is only partly • filled – there are electrons and there are empty states they can • move into when caused to move by an applied electric field. • B. Silicon at 0K – can’t conduct because the highest band containing • electrons is filled. • Pure silicon at room temp. is slightly conductive since thermal • energy can raise some electrons to the mostly empty conduction • band. • Silicon doped with donors (like P or As) can conduct (and • Become n-type) better than pure silicon at room temp. since it doesn’t • take much energy to free a valence electron so it can enter the • conduction band. • Silicon doped with acceptors (like B) can conduct (and become • p-type) at room temp. since it doesn’t take much energy to free a • valence electron and create a hole in the valence band. EE42/100, Prof. White

16. A. Metal B. Pure Si 0K C. Pure Si at 300K D. n-type Si E. p-type Si Conduction band + + + Forbidden energy band (energy gap) Donor level Acceptor level - - - Valence band EE42/100, Prof. White

17. 2. Micro- and Nanotechnology EE42/100, Prof. White

18. The Sizes of Things EE42/100, Prof. White

19. (Gordon) Moore’s Law EE42/100, Prof. White

20. Inkjet printer head: A thin-film heater behind each nozzle vaporizes the ink and ejects a droplet of ink EE42/100, Prof. White

21. Micro-guitar from Cornell University (the strings actually vibrate when plucked) EE42/100, Prof. White

22. EE42/100, Prof. White

23. Early micromachine (with dead mite) EE42/100, Prof. White

24. Laser scanning micromachine with electrostatic motors, gears, and mirror EE42/100, Prof. White

25. Micromachine: Safety lock for nuclear bomb EE42/100, Prof. White

26. Digital Light Projector (Texas Instruments) EE42/100, Prof. White

27. 16-micron-square tilting mirrors built over CMOS drive circuit EE42/100, Prof. White

28. Microstructures etched in Teflon -- useful in BioMEMS applications EE42/100, Prof. White

29. Nanotechnology Defining Dimension: Involves products and processes with significant features 100 nanometers and smaller. (To be listed in Merrill Lynch nanotechnology index, a company must indicate in a public document that nanotechnology initiatives are a significant component of their business strategy) Significant because of unusual properties: Mechanical – very strong; new devices (e.g., motor) Physics – new phenomena (e.g., metallic or semiconducting) Chemical – Reactivity, tiny hollow structures (e.g., can carry drugs into body) EE42/100, Prof. White

30. Nanotech examples Uses demonstrated: Electrostaically driven nanomotor (Alex Zettl, UCB Physics) Field effect transistors and logic devices (Ph. Avouris, Nano-Letters 16 Aug. 2001) – NOR gate, flip-flop memory cell, ring oscillator) Sensors (Nanometrix Co.) – hydrogen sensing by palladium- coated silicon nanotube operating as an FET Liquid repellent cloth Cautions (research needed): “Gray goo” – if nanotech replicating structures are made they could take over the Earth Nanostructures can pass through/around cells in the body Nanostructures may clump in liquids and have limited uses in environmental remediation, drug delivery EE42/100, Prof. White

31. C60 -- Buckeyball EE42/100, Prof. White

32. Nano structures Carbon nanotube field-effect transistor EE42/100, Prof. White

33. 3. What About Radio? EE42/100, Prof. White

34. Amplitude Modulation (AM) Radio EE42/100, Prof. White

35. Frequency Modulation (FM) Radio EE42/100, Prof. White

36. Crystal Set AM Radio Receiver – no batteries! EE42/100, Prof. White