Course Overview

1. Course Overview ECE/ChE 4752: Microelectronics Processing Laboratory Gary S. May January 8, 2004

2. Outline • Introduction • Silicon Processing • History of ICs • Review of Semiconductor Devices • Conductivity and Resistivity • MOS Transistors • Hot-Point Probe • 4-Point Probe

3. Growth of Electronics Industry • Electronics industry is fundamentally dependent on semiconductor integrated circuits (ICs).

4. What do you learn in 4752? • This course deals with the fabrication of semiconductor devices and ICs. • ICs today have over 107 components per chip, and this number is growing. • Fabricating these circuits requires a sophisticated process sequence which consists of hundreds of process steps. • In this course, we’ll go through a process sequence to make complementary metal-oxide-semiconductor (CMOS) transistors.

5. Outline • Introduction • Silicon Processing • History of ICs • Review of Semiconductor Devices • Conductivity and Resistivity • MOS Transistors • Hot-Point Probe • 4-Point Probe

6. Types of Semiconductors

7. Silicon vs. Germanium Ge was used for transistors initially, but silicon took over in the late 1960s; WHY? (1) Large variety of process steps possible without the problem of decomposition (as in the case of compound semiconductors) (2) Si has a wider bandgap than Ge => higher operating temperature (125-175 oC vs. ~85 oC) (3) Si readily forms a native oxide (SiO2) • high-quality insulator • protects and “passivates” underlying circuitry • helps in patterning • useful for dopant masking (4) Si is cheap and abundant

8. Silicon Disadvantages • Low carrier mobility (m) => slower circuits (compared to GaAs) • Indirect bandgap: • Weak absorption and emission of light • Most optoelectronic applications not possible

9. Outline • Introduction • Silicon Processing • History of ICs • Review of Semiconductor Devices • Conductivity and Resistivity • MOS Transistors • Hot-Point Probe • 4-Point Probe

10. The Transistor • Bell Labs invented the transistor in 1947, but didn’t believe ICs were a viable technology • REASON: Yield • For a 20 transistor circuit to work 50% of the time, the probability of each device functioning must be: (0.5)1/20 = 96.6% • Thought to be unrealistic at the time • 1st transistor => 1 mm x 1 mm Ge

11. ICs and Levels of Integration • 1st IC: TI and Fairchild (late 50s) A few transistors and resistors => “RTL” • Levels of integration have doubled every 3-4 years since the 1960s)

12. Moore’s Law

13. SSI = small scale integration (~100 components) MSI = medium scale integration (~1000 components) LSI = large scale integration (~105 components) VLSI = very large scale integration (~105 - 106 components) ULSI = ultra large scale integration (~106 - 109 components) GSI = giga-scale integration (> 109 components) Complexity Acronyms

14. State of the Art • 1 GB DRAM • 90 nm features • 12” diameter wafers • Factory cost: ~ $3-4B => Only a few companies can afford to be in this business!

15. Outline • Introduction • Silicon Processing • History of ICs • Review of Semiconductor Devices • Conductivity and Resistivity • MOS Transistors • Hot-Point Probe • 4-Point Probe

16. Diamond Lattice • Tetrahedral structure • 4 nearest neighbors

17. Covalent Bonding • Each valence electron shared with a nearest neighbor • Total of 8 shared valence electrons => stable configuration

18. Doping • Intentional addition of impurities • Adds either electrons (e-) or holes (h+) => varies the conductivity (s) of the material • Adding more e-: n-type material • Adding more h+: p-type material

19. Donor Doping • Impurity “donates” extra e- to the material • Example: Column V elements with 5 valence e-s (i.e., As, P) • Result: one extra loosely bound e-

20. Acceptor Doping • Impurity “accepts” extra e- from the material • Example: Column III elements with 3 valence e-s (i.e., B) • Result: one extra loosely bound h+

21. Outline • Introduction • Silicon Processing • History of ICs • Review of Semiconductor Devices • Conductivity and Resistivity • MOS Transistors • Hot-Point Probe • 4-Point Probe

22. Ohm’s Law • J = sE = E/r where: s = conductivity, r = resistivity, and E = electric field • s = 1/r = q(mnn+ mpp) where: q = electron charge, n = electron concentration, and p = hole concentration • For n-type samples: s≈ qmnND • For p-type samples:s≈ qmpNA

23. Resistance and Resistivity R = rL/A

24. Outline • Introduction • Silicon Processing • History of ICs • Review of Semiconductor Devices • Conductivity and Resistivity • MOS Transistors • Hot-Point Probe • 4-Point Probe

25. MOSFET • Metal-oxide-semiconductor field-effect transistor G = gate, D = drain, S = source, B = body (substrate)

26. MOSFET Cross-Section

27. Basic Operation 1) Source and substrate grounded (zero voltage) 2) (+) voltage on the gate • Attracts e-s to Si/SiO2 interface; forms channel 3) (+) voltage on the drain • e-s in the channel drift from source to drain • current flows from drain to source

28. Hot-Point Probe • Determines whether a semiconductor is n- or p-type • Requires: • Hot probe tip (soldering iron) • Cold probe tip • Ammeter

29. Hot-Point Probe 1) Heated probe creates high-energy “majority” carriers • holes if p-type • electrons if n-type 2) High-energy carriers diffuse away 3) Net effect: a) deficit of holes (net negative charge for p-type); OR b) deficit of electrons (net positive charge for n-type) 4) Ammeter deflects (+) or (-)

30. 4-Point Probe • Used to determine resistivity

31. 4-Point Probe 1) Known current (I) passed through outer probes 2) Potential (V) developed across inner probes r = (V/I)tF where: t = wafer thickness F = correction factor (accounts for probe geometry) OR: Rs = (V/I)F where: Rs = sheet resistance (W/) => r = Rst

32. Virtual Cleanroom http://www.ece.gatech.edu/research/labs/vc/ Web site that describes entire ECE/ChE 4752 CMOS Fabrication Process!