1 Microelectronics Processing Course - J. Salzman - Jan Microelectronics Processing Oxidation

2 Microelectronics Processing Course - J. Salzman - Jan Content  Properties of SiO 2  Oxidation Process  Functions of SiO 2  Equipment for Si Oxidation  Mechanism of Si Oxidation  Factors affecting oxidation  Doping  Substrate Orientation  Pressure  Chlorine addition  Dopant Redistribution  Polysilicon Oxidation  Additional Oxidation Processes

3 Microelectronics Processing Course - J. Salzman - Jan Thermal SiO 2 Properties

4 Microelectronics Processing Course - J. Salzman - Jan Thermal SiO 2 Properties (cont.) (7) Amorphous material

5 Microelectronics Processing Course - J. Salzman - Jan Oxidation Techniques Thermal Oxidation Rapid Thermal Oxidation Oxidation Process Thermal Oxidation Techniques Wet Oxidation Si (solid) + H 2 0 SiO 2 (solid) + 2H 2 Dry Oxidation Si (solid) + O 2 (gas) SiO 2 (solid)

6 Microelectronics Processing Course - J. Salzman - Jan Conceptual Si Oxidation System Thermal Oxidation Heat is added to the oxidation tube during the reaction..between oxidants and silicon ,200  C temperature range - Oxide growth rate increases as a result of heat Used to grow oxides between 60-10,000Å

7 Microelectronics Processing Course - J. Salzman - Jan Thermal Oxidation Process Wafers are placed in wafer load station Dry nitrogen is introduced into chamber - Nitrogen prevents oxidation from occurring Nitrogen gas flow shut off and oxygen added to chamber - Occurs when furnace has reached maximum temperature - Oxygen can be in a dry gas or in a water vapor state Nitrogen gas reintroduced into chamber - Stops oxidation process Wafers are removed from furnace and inspected Dry Thermal Oxidation Characteristics Oxidant is dry oxygen Used to grow oxides less than 1000Å thick Slow process Å / hour

8 Microelectronics Processing Course - J. Salzman - Jan Dry Thermal Oxidation Process Thin Oxide Growth Thin oxides grown (<150Å) for features smaller than 1..million - MOS transistors, MOS gates, and dielectric components Additional of chemical species to oxygen decreases..oxide growth rate (only in special cases) - Hydrochloric acid (HCI) - Trichloroethylene (TCE) - Trichloroethane (TCA) Decreasing pressure slows down oxide growth rate

9 Microelectronics Processing Course - J. Salzman - Jan Wet Thermal Oxidation Wet Thermal Oxidation Characteristics Oxidant is water vapor Fast oxidation rate - Oxide growth rate is Å / hour Preferred oxidation process for growth of thick oxides

10 Microelectronics Processing Course - J. Salzman - Jan The goal of oxidation is to grow a high quality oxide layer on a silicon substrate Goal of Oxidation Process

11 Microelectronics Processing Course - J. Salzman - Jan Passivation Physically protects wafers from scratches and particle..contamination Traps mobile ions in oxide layer Functions of Oxide Layers (1)

12 Microelectronics Processing Course - J. Salzman - Jan Masking During Diffusion, Ion Implantation, and Etching Function of Oxide Layers (2) SiO 2

13 Microelectronics Processing Course - J. Salzman - Jan Insulating Material Gate region - Thin layer of oxide - Allows an inductive charge to pass between gate metal and silicon Function of Oxide Layers (3)

14 Microelectronics Processing Course - J. Salzman - Jan Dielectric Material Insulating material between metal layers - Field Oxide Function of Oxide Layers (4)

15 Microelectronics Processing Course - J. Salzman - Jan Dielectric Material Tunneling oxide - Allows electrons to pass through oxide without resistance Function of Oxide Layers (5)

16 Microelectronics Processing Course - J. Salzman - Jan Functions and Thickness of Oxide Layers

17 Microelectronics Processing Course - J. Salzman - Jan Projections for Si Technology

18 Microelectronics Processing Course - J. Salzman - Jan Oxidation occurs in tube furnace - Vertical Tube Furnace - Horizontal Tube Furnace Thermal Oxidation Equipment

19 Microelectronics Processing Course - J. Salzman - Jan Bubbler Wet Thermal Oxidation Techniques

20 Microelectronics Processing Course - J. Salzman - Jan Flash System Wet Thermal Oxidation Techniques

21 Microelectronics Processing Course - J. Salzman - Jan Dryox System Wet Thermal Oxidation Techniques

22 Microelectronics Processing Course - J. Salzman - Jan Thickness of Si consumed during oxidation

23 Microelectronics Processing Course - J. Salzman - Jan Kinetics of Si0 2 Growth - Oxide Growth Mechanism 1.Oxidant (O 2 ) reacts with silicon atoms 2.Silicon atoms are consumed by reaction 3.Layer of oxide forms on silicon surface

24 Microelectronics Processing Course - J. Salzman - Jan Linear Parabolic Model Linear (first) Stage of Oxidation - Chemical reaction between silicon and oxidants at wafer surface - Reaction limited by number of silicon atoms available to react with oxidants - During the first 500Å of oxide growth, the oxide grows linearly with time - Growth rate begins to slow down as oxide layer grows Oxide Growth Mechanism (1)

25 Microelectronics Processing Course - J. Salzman - Jan Linear Parabolic Model Parabolic Stage - Begins when 1,000Å of oxide has been grown on silicon - Silicon atoms are no longer exposed directly to oxidants - Oxidants diffuse through oxide to reach silicon - Reaction limited by diffusion rate of oxidant Oxide Growth Mechanism (2)

26 Microelectronics Processing Course - J. Salzman - Jan Deal-Grove Model (1)

27 Microelectronics Processing Course - J. Salzman - Jan Deal-Grove Model (2)

28 Microelectronics Processing Course - J. Salzman - Jan Deal-Grove Model (3)

29 Microelectronics Processing Course - J. Salzman - Jan Deal-Grove Model (4)

30 Microelectronics Processing Course - J. Salzman - Jan Deal-Grove Model (5)

31 Microelectronics Processing Course - J. Salzman - Jan Deal-Grove Model (6)

32 Microelectronics Processing Course - J. Salzman - Jan Deal-Grove Model (7)

33 Microelectronics Processing Course - J. Salzman - Jan Deal-Grove Model (8)

34 Microelectronics Processing Course - J. Salzman - Jan Deal-Grove Model (9)

35 Microelectronics Processing Course - J. Salzman - Jan Limiting cases in Si oxidation (a) (b) a) Interface reaction is the rate limiting step b) Limited by oxidant transport through the SiO 2 rate

36 Microelectronics Processing Course - J. Salzman - Jan Deal-Grove Model Parameters

37 Microelectronics Processing Course - J. Salzman - Jan Deal-Grove model (10) - Effect of temperature on the rate constants B, and B/A B(T)=B o exp(-E A /kT) (B/A)(T)=(B/A) o exp(-E A /kT)

38 Microelectronics Processing Course - J. Salzman - Jan Values for the coefficients D o and E A Each of the coefficients B, and B/A has an Arrhenius relationship of the type: D=D 0 exp(-E A /kT)

39 Microelectronics Processing Course - J. Salzman - Jan Diffusivities of some materials in silicon glass

40 Microelectronics Processing Course - J. Salzman - Jan Examples

41 Microelectronics Processing Course - J. Salzman - Jan Effect of X i on Wafer Topography (1)

42 Microelectronics Processing Course - J. Salzman - Jan Effect of X i on Wafer Topography (2)

43 Microelectronics Processing Course - J. Salzman - Jan Factors that Affect Oxidation

44 Microelectronics Processing Course - J. Salzman - Jan High Doping concentration effect Dopants in silicon Dopants increase oxide growth rate - During Linear Stage of oxidation N-type dopants increase growth rate Dopants cause differential oxidation - Results in the formation of steps - Affects etching process

45 Microelectronics Processing Course - J. Salzman - Jan High Doping concentration effect

46 Microelectronics Processing Course - J. Salzman - Jan Growth Rate Dependence on Si Substrate Orientation Wafer Orientation Oxide grows faster on wafers - more silicon atoms available to react with oxidant Affects oxide growth rate during Linear Stage

47 Microelectronics Processing Course - J. Salzman - Jan Origin of Substrate Orientation Effect

48 Microelectronics Processing Course - J. Salzman - Jan Substrate Orientation Effect - Oxidation Charts (a) (b) Growth of SiO 2 on and oriented Si wafers: (a) dry oxygen; (b) steam.

49 Microelectronics Processing Course - J. Salzman - Jan Atmospheric pressure - Slow oxide growth rate An increase in pressure increase oxide growth rate Increasing pressure allows temperature to be..decreased - Oxide growth rate remains the same - For every 10atm of pressure the temperature can be reduced 30°C Dry Thermal oxidation - Pressure in oxidation tube increased Wet Thermal oxidation - Steam pressure introduced into oxidation tube Effect of High Pressure Oxidation

50 Microelectronics Processing Course - J. Salzman - Jan Effect of High Pressure Oxidation

51 Microelectronics Processing Course - J. Salzman - Jan High Pressure Oxidation

52 Microelectronics Processing Course - J. Salzman - Jan Chlorine species - Anhydrous chloride (CI 2 ) - Anhydrous hydrogen chloride (HCI) - Trichloroethylene – TCE - Trichloroethane – TCA Oxide growth rate increases Oxide cleaner Device performance is improved Chlorine added with Oxidants

53 Microelectronics Processing Course - J. Salzman - Jan Oxidation With Cl Containing Gas

54 Microelectronics Processing Course - J. Salzman - Jan Effect of HCl on Oxidation Rate

55 Microelectronics Processing Course - J. Salzman - Jan Local Oxidation of Si (LOCOS)

56 Microelectronics Processing Course - J. Salzman - Jan Local Oxidation

57 Microelectronics Processing Course - J. Salzman - Jan Dopant Redistribution During Thermal Oxidation (1) Dopant concentration

58 Microelectronics Processing Course - J. Salzman - Jan Dopants affect device performance - The change in dopant location and concentration during oxidation can affect the device operation - N-type dopants move deeper into silicon so high concentration at the silicon/silicon dioxide interface - P-type dopants move into the silicon dioxide and deplete the silicon layer Dopant Redistribution During Thermal Oxidation (2)

59 Microelectronics Processing Course - J. Salzman - Jan Dopant Redistribution During Thermal Oxidation (3)

60 Microelectronics Processing Course - J. Salzman - Jan Dopant Redistribution During Thermal Oxidation (4)

61 Microelectronics Processing Course - J. Salzman - Jan Dopant Redistribution During Thermal Oxidation (5) a) boron b) boron with hydrogen ambient c) Phosphorus d) gallium

62 Microelectronics Processing Course - J. Salzman - Jan Thin Oxide Growth

63 Microelectronics Processing Course - J. Salzman - Jan Structure of SiO 2 -Si Interface

64 Microelectronics Processing Course - J. Salzman - Jan Thin Oxide Tunneling Current Comparison

65 Microelectronics Processing Course - J. Salzman - Jan Polycrystalline Si Oxidation

66 Microelectronics Processing Course - J. Salzman - Jan Polysilicon Oxidation

67 Microelectronics Processing Course - J. Salzman - Jan Oxide inspection techniques Surface Inspection Oxide Thickness Oxide Cleanliness

68 Microelectronics Processing Course - J. Salzman - Jan Anodic Oxidation Process Wafer is attached to a positive electrode Wafer is immersed in bath of potassium nitrate..(KNO 3 ) Immersion tank contains a negative electrode Oxygen produced when current is applied Reaction between silicon and oxygen occurs Additional (Chemical) Oxidation Processes

69 Microelectronics Processing Course - J. Salzman - Jan Anodic Oxidation Characteristics Oxidation reaction occurs at the surface of the oxide - Silicon atoms move to top of oxide layer during oxidation Used to grow oxide on wafers that will be tested for..dopant location and concentration Additional Oxidation Processes

70 Microelectronics Processing Course - J. Salzman - Jan Rapid Thermal Oxidation Equipment Additional Oxidation Processes

71 Microelectronics Processing Course - J. Salzman - Jan Thermal Nitridation Characteristics Alternative method to Oxidation Oxidant is nitrogen - Pure ammonia gas (NH 3 ) - Ammonia plasma Reaction produces silicon nitride (Si 3 N 4 ) - Reaction occurs at the gas/silicon nitride interface - Silicon atoms diffuse through silicon nitride layer during process Silicon nitride is a good substitute for silicon dioxide - Silicon nitride is denser than silicon dioxide - Silicon nitride has a higher dielectric rating Additional Processes - Thermal Nitridation

72 Microelectronics Processing Course - J. Salzman - Jan Thermal Nitridation Disadvantage Process puts high level of strain on wafer - Thermal expansion rate of silicon nitride is 2 times greater than silicon dioxide - High temperature processing techniques ( °C) results in wafer strain Additional Oxidation Processes