Microelectronics Devices

Transcription

1 Microelectronics Devices Yao-Joe Yang 1 Outline Basic semiconductor physics Semiconductor devices Resistors Capacitors P-N diodes BJT/MOSFET 2

2 Type of Solid Materials Solid materials may be classified as follows: Amorphous no ordered atomic arrangement Polycrystalline short range atomic order usually in small crystalline grains (10 Å - few µm) Crystalline long range, ordered, atomic arrangement, repeating unit cell All important semiconductor devices are based on crystalline materials (Si especially) because of their reproducible and predictable electrical properties 3 Amorphous Structure 4

3 Polycrystalline Structure Grain Boundary Grain 5 Single Crystal Structure 6

4 What Are Semiconductors? Group IV and Group III-V compounds Silicon(Si), Germanium(Ge), Gallium arsenide (GaAs) Covalent bond, no free electrons their energy gaps (~ 1 ev) are not too high free electrons are generated under light and thermal agitation after electrons escape, holes are formed and can be treated as positive electrons these electrons and holes provides certain electrical conductivity the conductivity becomes higher as temperature increases 7 Terminology Intrinsic semiconductor: undoped semiconductor electrical properties are native to the material extrinsic semiconductor: doped semiconductor electrical properties controlled by the added impurity donor impurity atom that increases the electron concentration group V acceptor impurity atom that increases the hole concentration group III 8

5 n-type material: Terminology semiconductor containing more electrons than holes in thermal equilibrium p-type material: semiconductor containing more holes than electrons in thermal equilibrium majority carrier: in n-type material: electrons in p-type material: holes minority carrier: in n-type material: holes in p-type material: electrons 9 Intrinsic Silicon Perfect covalent bond Some bonds will be free at temperature T, create free electrons and holes concentration of free electrons/hole is a function of temperature n n i law of mass action 16 3/ eV / kt 3 i = T e cm (300K) np = n i = cm 10

6 Properties of Crystalline Silicon Crystal structure: diamond or double FCC cm -3 (density = 2.33) Cubic structure 3 material constants E: Gpa ν: Energy gap: 1.1 ev valance band to conduction band Dielectric constant: 11.7 Resistivity of pure silicon at RT = Ω.cm 11 Crystalline Silicon Other mechanical properties expansion coefficient 2.6µm/mK melting point 1412 C fracture toughness ~ 1MPa m brittle-ductile transition point ~ 550 C Strength strongly depends on surface quality Poly silicon has similar elastic constant and mechanical properties as crystalline silicon. However, residual strength, toughness, and electrical properties are quite different 12

7 Extrinsic Semiconductors In all important electronic devices, dopant are purposely added to control the electronic properties n-type semiconductor add phosphorus or arsenic to provide excess electron carriers p-type semiconductor add boron, gallium, or indium into silicon to provide additional vacancies or holes The mass-action law is still valid 2 np = n i 13 Doping All semiconductor devices are fabricated LOCALLY introducing controlled number of n- and p-type dopant 14

8 n µ Semiconductor Conductivity The conductivity((ω.cm) -1 ) is determined by the mobility and concentration of both electrons and holes p µ σ = q n + q p where µ is the mobility, for silicon µ n = 1350 cm 2 /V.s µ p = 480 cm 2 /V.s q = C temperature decreases, conductivity increases 15 Resistivity Vs. Doping Concentration Resistivity = 1/conductivity 16

9 Semiconductor Device Overview VLSI are consisted by many transistors, capacitors, diodes, and resistors. However, the transistor fabrication can cover the other three One need to know the basic definition, working principle, and fabrication routes for these basic elements 17 Resistors A resistor can be defined as a device in which the applied electric potential and measured current exhibit a certain relationship, i.e., V = f(i) For linear device, we have V = RI, where R is called the resistance of the resistor Consider a resistor with length L and crosssectional dimension W and d, R can be expressed as R = ρ L Wd ρ = d L W L W d 18

10 Diodes A diode is a device made of p-n junction Can be used for rectification Mathematical model of diodes at forward bias I V D T = I S ( e V D T = / ηv T 1) η~ 2 for silicon 19 p-n Junction Forward bias reduce the junction barrier and eliminate the depletion zone Reverse bias enhance the junction barrier and increase the depletion zone 20

11 Capacitors Capacitor is a device in which the charge and electric potential can be defined, i.e., V = f(q). In linear element, we can express the above relationship as Q=CV. Where C is the capacitance of the capacitor. For parallel plate, C = εa/d. Where ε is the dielectric constant of dielectric, A is the overlapped area and d is the separation of two parallel plates. 21 Transistors Transistors are widely used for switching and amplification replace vacuum tubes Two major transistors Bipolar Junction Transistor (BJT) collector, emitter, base current controlled Field Effect Transistor (FET) source, drain, gate voltage controlled 22

12 Bipolar Junction Transistors (BJT) 23 Field Effect Transistors (FET) FET is the most popular transistor at this moment incorporate with MOS process Can be divided into two catalog MOSFET depletion enhancement JFET 24

13 Symbols of FET FETs are unipolar devices for switch operation, usually we use NMOS or CMOS technology to further reduce power consumption and increase the device density 25 CMOS IC n + Source/Drain Gate Oxide p + Source/Drain Polysilicon p-si STI Balk Si n-si USG 26

14 CMOS Chip with 4 Metal Layers Tantalum barrier layer Passivation 2, nitride Passivation 1, USG Metal 4 Lead-tin alloy bump Copper FSG Tungsten plug Tungsten local Interconnection Metal 3 Copper FSG FSG Metal 2 Copper FSG M 1 Cu Cu FSG FSG PSG Tungsten STI n + n + USG p + p + P-well N-well P-epi P-wafer Nitride etch stop layer Nitride seal layer Tantalum barrier layer T/TiN barrier & adhesion layer PMD nitride barrier 27 layer From Basic Elements to a IC Chips Analog basic devices (transistors, resistors ) to OPAMP OPAMP to analog circuit The designer may start from basic devices Digital basic devices to basic logic elements, e.g., NAND gate from basic logic element to logic devices, e.g., Flip- Flop from logic device to logic circuit e.g., register, memory, adder,. 28

15 Fab Cost Fab cost is very high, > $1B for 8 fab Clean room Equipment, usually > $1M per tool Materials, high purity, ultra high purity Facilities People, training and pay 29 Wafer Yield Y = W Wafers Wafers good total 30

16 Die Yield Y = D Dies Dies good total 31 Packaging Yield Y = C Chips Chips good total 32

17 Overall Yield Y T = Y W Y D Y C Overall Yield determines whether a fab is making profit or losing money 33