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

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

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

What do you learn in 4752? This course deals with the fabrication of semiconductor devices and ICs. ICs today have over 10 7 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.

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

Types of Semiconductors ElementalCompound Si GaAs, InP (III-V) Ge CdS, CdTe (II-VI)

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 ( o C vs. ~85 o C) (3) Si readily forms a native oxide (SiO 2 )   high-quality insulator   protects and “passivates” underlying circuitry   helps in patterning   useful for dopant masking (4) Si is cheap and abundant

Silicon Disadvantages Low carrier mobility (  ) => Low carrier mobility (  ) => slower circuits (compared to GaAs) Indirect bandgap: Indirect bandgap:  Weak absorption and emission of light  Most optoelectronic applications not possible Material Mobility (cm 2 /V-s) Si  n = 1500,  p = 460 Ge  n = 3900,  p = 1900 GaAs  n = 8000,  p = 380

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

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

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)

Moore’s Law

Complexity Acronyms SSI = small scale integration (~100 components) MSI = medium scale integration (~1000 components) LSI = large scale integration (~10 5 components) VLSI = very large scale integration (~ components) ULSI = ultra large scale integration (~ components) GSI = giga-scale integration (> 10 9 components)

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!

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

Diamond Lattice Tetrahedral structure Tetrahedral structure 4 nearest neighbors 4 nearest neighbors

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

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

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 -

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 +

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

Ohm’s Law J =  E = E /  where:  = conductivity,  = resistivity, and E = electric field  = 1/  = q(  n n+  p p) where: q = electron charge, n = electron concentration, and p = hole concentration For n-type samples:  ≈ q  n N D For p-type samples:  ≈ q  p N A

Resistance and Resistivity R =  L/A

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

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

MOSFET Cross-Section

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

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

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 (-)

4-Point Probe Used to determine resistivity

4-Point Probe 1) Known current (I) passed through outer probes 2) Potential (V) developed across inner probes  = (V/I)tF where: t = wafer thickness F = correction factor (accounts for probe geometry) OR: R s = (V/I)F where: R s = sheet resistance (  /) =>  = R s t

Virtual Cleanroom Web site that describes entire ECE/ChE 4752 CMOS Fabrication Process!