Semiconductor Process Technology

1. Semiconductor Process Technology io Lecture II Basic of Semiconductor Materials & Devices School of Microelectronic Engineering

2. Semiconductor Process Technology Objectives ` School of Microelectronic Engineering

3. Semiconductor Process Technology ` Semiconductor Materials School of Microelectronic Engineering

4. Semiconductor Process Technology What Is a Semiconductor? ` • Semiconductors are materials with electrical conductivity between conductors and insulators. • The most commonly used semiconductor materials are silicon and germanium. • Some compounds, such as GaAs, SiC and SiGe. • Most important property is its conductivity can be controlled by adding certain impurities in the process called doping. School of Microelectronic Engineering

5. Semiconductor Process Technology ` School of Microelectronic Engineering

6. Semiconductor Process Technology ` School of Microelectronic Engineering

7. Semiconductor Process Technology Periodic Table of The Elements School of Microelectronic Engineering

8. Semiconductor Process Technology Periodic Table of The Elements School of Microelectronic Engineering

9. Semiconductor Process Technology Elementary semiconductor Silicon Germanium ` II-IV SiGe Electron mobility Optical properties Compound Semiconductor III-V GaAs, InP etc II-VI CdS, ZnTe etc Offer high performance devices (optical characteristics, higher frequency and power) Binary SiGe, GaAs Ternary AlxGa1-xAs Quaternary InxGa1-xAsyP1-y School of Microelectronic Engineering

10. Semiconductor Process Technology Band Gap • Atom is basic building block of all materials • Classical mechanics – every atom has it own orbit structure. • Electron orbits are called shells. • The outermost shell is called valence shell. • When e leaves the valence shell, it becomes a free electron and can conduct electric current. ` School of Microelectronic Engineering

11. Semiconductor Process Technology • When 2 or more identical atoms bond together to form solid materials, their orbit overlap and form so called energy bands. Can be represented by the energy band diagram • The bottom of conduction band is called Ec, and the top of the valence band is called Ev. • Eg = Ec – Ev • Eg is defined as the energy required to break a bond in semiconductor to free an e to cond band and leave the hole in the valence band. • Electrons in conduction band are free to move and can conduct electric current • Electrons in the valence band are bonded with nuclei and connot move freely, therefore cannot conduct electric current ` School of Microelectronic Engineering

12. Semiconductor Process Technology Resistivity • Resistivity is the capability of a material resisting electric current. • A good conductor has a very low resistivity and a good insulator has a very high reistivity. • Unit: Ohm.cm ` School of Microelectronic Engineering

13. Semiconductor Process Technology Resistivity and Band Gap • For most metals, conduction and valence bands almost overlap or very small band gap. Electron can easily jump from valence to conduction band. Therefore the conduction band has a lot of e. • For insulators, the band gap is so large that electrons cannot jump across it. ` School of Microelectronic Engineering

14. Semiconductor Process Technology ` Crystal Properties of Semiconductors School of Microelectronic Engineering

15. Semiconductor Process Technology Classification of Solids (Based on Atomic Arrangement) • Amorphous • Single Crystal • Poly Crystal ` School of Microelectronic Engineering

16. Semiconductor Process Technology Crystal Structures ` School of Microelectronic Engineering

17. Semiconductor Process Technology Amorphous Structure ` School of Microelectronic Engineering

18. Semiconductor Process Technology Polycrystalline Structure ` School of Microelectronic Engineering

19. Semiconductor Process Technology Single Crystal Structure ` School of Microelectronic Engineering

20. Semiconductor Process Technology Doping Semiconductors School of Microelectronic Engineering

21. Semiconductor Process Technology Two types of Semiconductor Materials • Intrinsic Semiconductor • Extrinsic Semiconductor School of Microelectronic Engineering

22. Semiconductor Process Technology Intrinsic Semiconductor • Pure semiconductor materials with no impurity atoms and no lattice defect. • At T=0 K, all energy states in valence band are filled with electrons, states in conduction band are empty. School of Microelectronic Engineering

23. Semiconductor Process Technology Electrical Conduction in Intrinsic Semiconductor Si Si Si Si Si Si Si Si Si Si Si Si e- Si Si Si Si Silicon covalence bonding at T=0K School of Microelectronic Engineering

24. Si Si Si Si e Si Si Si Si Si Si Si Si e- Si Si Si Si Semiconductor Process Technology Conduction band e- Eg + Valance band • As the temperature increase above 0K, a few valence bond electrons may gain enough thermal energy to break the bond and jump into the conduction band. • As temperature increase further, more bonds broken, more electrons jump to the conduction band and more “empty states or holes” created in the valence band. School of Microelectronic Engineering

25. Semiconductor Process Technology • In intrinsic material, electrons and holes are created in pairs by thermal energy. So the number of electrons in conduction band is equal to the number of holes in the valence band • Electron concentration = hole concentration ni = pi and nipi = ni2 (MASS ACTION LAW) – the product of n p is always a constant for a given semiconductor material at given temperature For Silicon School of Microelectronic Engineering

26. Semiconductor Process Technology Extrinsic Semiconductor • Extrinsic s/c is defined as a semiconductor in which controlled amounts of specific dopant or impurity atoms have been added so that the thermal equilibrium electron and hole concentration are different from the intrinsic carrier concentration. Si Si Si Si Si Si Si Si e Si P Si Si Si Si Si Si Si Si Si Si Si Si Si Si e- e- Si Si Si Si Si Si Si Si Intrinsic silicon lattice Extrinsic silicon lattice School of Microelectronic Engineering

27. Semiconductor Process Technology Doping of Semiconductors • The purpose of doping is to alter the conductivity of semiconductor materials. • Two types of dopant; p-type (B), n-type (P, As) • N-type dopants provide an electron in s/c materials, hence called donors. • P-type dopants provide a hole in s/c materials, hence called acceptor. School of Microelectronic Engineering

28. Semiconductor Process Technology N-type Dopant • P and As have 5 electron valens • When doped into Si, 4 electrons used to form the covalence bond with Si • 1 extra electron is left in the outermost shell and will occupy a new energy level called Donor Energy. School of Microelectronic Engineering

29. conduction band - - - Ec Ec Ed Ed + + + Ev Ev valence band Semiconductor Process Technology • Energy required to elevate donor electron is less than that for electron involved in covalence bonding. • With small thermal energy, donor electron is elevated to the conduction band • This process add electron to the conduction band without creating holes in the valence band. • The resulting material is referred as n-type semiconductor. School of Microelectronic Engineering

30. Semiconductor Process Technology P-type Dopant • B have 3 electron valens • When doped into Si, one empty state is created in the covalence bond • This empty state will occupy a new energy level called Acceptor Energy. School of Microelectronic Engineering

31. Semiconductor Process Technology Si Si Si Si Si Si Si Si Si B Si Si B Si Si Si Si Si Si Si Si Si Si Si e- e- Si Si Si Si Si Si Si Si + Extrinsic silicon lattice doped with B Hopping of valence electron creating hole movement • Some valence electron gain a small amount of energy to move around the crystal lattice. • This electron would occupy the “empty” position associated with B atom. • The vacated electron position is considered as holes. School of Microelectronic Engineering

32. Semiconductor Process Technology conduction band Ec Ec - - - Ea Ea Ev Ev + + + valence band • This process generate holes in the valence band without creating electrons in the conduction band. • The resulting material is referred as p-type semiconductor School of Microelectronic Engineering

33. Si Si Si Si Si Si Si Si e Si Si Si Si Si Si Si Si + Si Si Si Si Si Si Si Si e- e- Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si + + + Si Si Si Si Si Si Si Si e- e- Si Si Si Si Si Si Si Si Semiconductor Process Technology School of Microelectronic Engineering

34. The product of n0 and p0 is a constant for a particular material and temperature, even the doping is varied. Therefore We also know that Thus

35. Problem A Silicon sample is doped with 1017 As atom per cm3. What is the equilibrium hole concentration p0at 300 K? ni at room temperature is approximately 1.5 x 1010 cm-3 Since Nd >> ni, we can approximate that no = Nd

36. Semiconductor Process Technology Dopant Concentration and Resistivity School of Microelectronic Engineering

37. Semiconductor Process Technology Dopant Concentration and Resistivity WHY? School of Microelectronic Engineering

38. Semiconductor Process Technology Basic Semiconductor Devices School of Microelectronic Engineering

39. Semiconductor Process Technology Basic Semiconductor Devices School of Microelectronic Engineering

40. Semiconductor Process Technology Resistor • The simplest electronic device. • In the IC fabrication, patterned doped silicon normally used to make resistors with resistance determined by the length, linewidth, junction depth and dopant concentration. • Poly silicon also used a resistor. School of Microelectronic Engineering

41. Semiconductor Process Technology Example 1 • Many people use polysilicon to form gates and local interconnect. Resistivity of polysilicon is determined by dopant concentration, about 1022 cm-3, and ρ = 200 Ω.cm. Assume polysilicon gate and local interconnect line width, height, and legth are 1m, 1m and 100m respectively. Calculate the resistance. R = ρ l / wh = 200 Ω.cm x (100 x 10-4) cm / [(1x10-4 cm) x 1X10-4cm) = 2 x 108Ω = 200 Ω School of Microelectronic Engineering

42. Semiconductor Process Technology Capacitor • One of the most important IC components • When two conducting materials are separated by a dielectric, a capacitor is formed. h l 0 C = d 0 • Absolute permittivity of vacuum (8.85 x 10-12 F/m  • Dielectric constant School of Microelectronic Engineering

43. Semiconductor Process Technology

44. Semiconductor Process Technology

45. Semiconductor Process Technology • Unwanted (parasitic) capacitor, as result of dielectric sandwiched between 2 metal layers. This will result in the RC delay of the IC circuit. • Major limitation for current IC device speed. • This application required low k dielectric and better conduction metal

46. Semiconductor Process Technology Example 2 • Calculate the capacitance for a capacitor shown with h = l = 10 m. Assume the dielectric between the 2 conducting plates is silicon dioxide, with k=3.9 and d=1000 Å. h l 0 C = d 10x10-6 x 10x10-6 = 3.9 x 8.85x10-12 x 1000x10-10 = 3.45x10-14 F

47. Semiconductor Process Technology Example 3 • Most IC chips use aluminum-copper alloy metal interconnection. The resistivity ρ = 3.2 Ω.cm, metal line geometry width w, height h, length l, and line spacing d are 1 m, 1 m, 1 m (1 million transistors connected by one metal line at 1 m between each transistor), and 1 m respectively. CVD silicon oxide lies between the metal line, with dielectric constant k = 4. Calculate the time delay. h l 0 C = d R = ρ l / wh Answer = 1.13 x 10-8 sec

48. Semiconductor Process Technology Diode • P-N junction • Allow electric conduction only in one way (positively biased)

49. Semiconductor Process Technology • When p-type and n-type semiconductors join together, they form a p-n junction diode. • Holes in p-type region will diffuse to the n-type region, and electrons in n-type region will diffuse to the p-type region (at thermal equilibrium, without applied bias). • The area dominated by minority carriers is called the transistion region. • The voltage across the transistion region given by; For Si at room temperature, V0 ~ 0.7 V I-V curve for diode

50. Semiconductor Process Technology MOSFET