Microelectronics Processing

Transcription

1 Microelectronics Processing Oxidation 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 Thermal SiO 2 Properties Thermal SiO2 Properties (cont.) (7) Amorphous material Structure of silicon dioxide Thermal Oxidation A method for growing a film of SiO 2 from a singlecrystal silicon (SCS) wafer or a polysilicon thin film High temperature process ( C) Used extensively in commercial ICs and MEMS Thermal oxidation by far is the most important method for growing a SiO 2 thin film in contrast several other methods : PECVD and electrochemical process. One of the major reasons for the popularity of silicon ICs is that silicon forms an excellent oxide, SiO 2 Page A. Kolodny 1

2 SiO 2 for IC and Surface Micromachining Functions of Oxide Layers (1) Passivation Physically protects wafers from scratches and particle..contamination Traps mobile ions in oxide layer Masking During Diffusion, Ion Implantation, and Etching SiO 2 Oxides as Dopant Masks SiO 2 Masks for B and P SiO 2 can provide a selective mask against diffusion at high temperatures. Oxides used for masking are ~ μm thick. Dopants B Ga P As Sb Diffusion Constants at 1100 o C(cm 2 /s) Function of Oxide Layers (2) Insulating Material Gate region - Thin layer of oxide Functions and Thickness of Oxide Layers Dielectric Material Insulating material between electrodes - Field Oxide Page A. Kolodny 2

3 Goal of Oxidation Process The goal of oxidation is to grow a high quality oxide layer on a silicon substrate Oxidation Process Oxidation Techniques Thermal Oxidation Rapid Thermal Oxidation Thermal Oxidation Techniques Wet Oxidation Si (solid) + H 2 O SiO 2 (solid) + 2H 2 Dry Oxidation Si (solid) + O 2 (gas) SiO 2 (solid) Dry Thermal Oxidation Process Dry Thermal Oxidation Characteristics Oxidant is dry oxygen. Used to grow oxides less than 1000Å thick. Slow process, Å / hour Thin Oxide Growth Thin oxides grown (<150Å) for features smaller than 1..micrometer - 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) 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 Wet Thermal Oxidation Techniques Wet Thermal Oxidation Techniques Flash System Bubbler Page A. Kolodny 3

4 Wet Thermal Oxidation Techniques Si Oxidation System Dryox System Oxidation Furnace 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 Thermal oxidation in practice Oxidation Gas Flow Sequence 1. Clean the wafers (RCA) 2. Put wafers in the boat 3. Load the wafers in the furnace 4. Ramp up the furnace to process temperature in N2 5. Stabilize 6. Process (Wet or Dry Oxidation) 7. Anneal in N2 8. Ramp down 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 Page A. Kolodny 4

5 Thermal Silicon Oxide Methods Additional (Chemical) Oxidation Processes 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 Anodic Oxidation Characteristics Oxidation reaction occurs at the surface of the oxide - Silicon atoms move to top of oxide layer during oxidation Additional Oxidation Processes Rapid Thermal Oxidation Equipment Additional Processes - Thermal Nitridation 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 Oxidation Processes 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 Page A. Kolodny 5

6 Thickness Change during Si Oxidation Effect of X i on Wafer Topography Local Oxidation Local Oxidation of Si (LOCOS) Kinetics of SiO 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 Oxide Growth Mechanism (1) 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 Page A. Kolodny 6

7 Oxide Growth Mechanism (2) 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 Deal-Grove model: Basic Diagram C o = concentration of oxidizing species at oxide surface (cm - 3 ) C s = concentration of oxidizing species at Si surface (cm -3 ) d = oxide thickness F s = fluxes (cm -2 s -1 ) Deal-Grove model: Flux Flux dc D( Co Cs ) F1 = D dx x F2 = κc s D = diffusion coefficient of oxidizing species x = thickness of existing oxide layer κ = surface reaction rate constant How will the concentration profiles look like? At steady-state, F 1 = F 2 = F, so: DCo F = x + ( D / κ ) Limiting cases in Si oxidation Deal-Grove model: Growth rate (a) (b) a) Interface reaction is the rate limiting step b) Limited by oxidant transport through the SiO 2 rate Page A. Kolodny 7

8 Growth Rate Deal-Grove model: Growth rate dx dt = F C DCo / C1 = x + ( D / ) 1 κ where: C 1 = # molecules of oxidizing species/unit volume = cm -3 for O 2 = cm -3 for H 2 O 2 where: τ ( d 0 + 2Dd 0 / κ ) C1 / 2DC0 Deal-Grove model: Growth rate Compact Form Thickness depends on square root of time! Deal-Grove Model Temperature and orientation dep. Short times (reaction rate-limited): B x = ( t + τ ) Linear Regime A Long time (diffusion-limited): x 2 = B(t + τ) Parabolic Regime Discuss: 1) Should B/A depend on temperature? 2) Should B/A depend on orientation of the substrate? 3) Should B depend on temperature? 4) Should B depend on orientation of the substrate? Page A. Kolodny 8

9 Values for 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) Temperature Variation Example A silicon sample is oxidized in dry O 2 at 1200 o C for one hour. (a) What is the thickness of the oxide grown? For dry O 1200 o C B/A = 0.04 μm, B = μm 2 /h, τ = h SOLUTION: Using these parameters, we obtain an oxide thickness of x = μm Example (cont.) (b) How much additional time is required to grow 0.1 μm more oxide in wet O 2 at 1200 o C? For wet O 2 at 1200 o Care A = 0.05 μm, B = 0.72 μm 2 /H SOLUTION: Since d 0 = μm from the first step, 2 d0 + Ad0 τ = = h B The final desired thickness is x = d μm = μm. Using these parameters, we obtain an additional time of t = 0.76 h = 4.53 min Page A. Kolodny 9

10 Oxidation thickness--experiments Examples Dry vs. Wet Oxides Wet oxides are usually used for masking SiO 2 growth rate is much higher when water is the oxidant. Dry oxidation results in a higher quality oxide that is denser and has a higher breakdown voltage (5 10 MV/cm). Thin gate oxides in MOS devices are usually formed using dry oxidation. Trace amounts of water Diffusivity of water in SiO 2 <D for O2, but the parabolic rate constant B is larger for wet oxidation than for dry because B is proportional to C*(bulk concentration) which is much larger for H 2 O:3x10 19 and 5.2x10 16 /cm 3 Silicon dioxide has a more open structure when water is present during growth H 2 O increases parabolic rate constant B. The linear rate constant also increases when H 2 O present but more gradually. 2 X 0 = k λ / n Profilometry Requires a step feature Accurate for thicknesses in 100 nm 0.5 μm range Page A. Kolodny 10

11 High Doping concentration effect Factors that Affect Oxidation 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 High Doping concentration effect Growth Rate Dependence on Si Substrate Orientation Wafer Orientation Oxide grows faster on <111> wafers - more silicon atoms available to react with oxidant Affects oxide growth rate during Linear Stage Effect of High Pressure Oxidation High Pressure Oxidation 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 Page A. Kolodny 11

12 Chlorine added with Oxidants Oxidation With Cl Bearing Gas Chlorine species - Anhydrous chloride (CI 2 ) - Anhydrous hydrogen chloride (HCI) - Trichloroethylene TCE - Trichloroethane TCA Addition of Cl bearing species to oxidation ambient lowers interface state density, stabilizes surface potential, enhances dielectric strength of oxide, and increases both linear and parabolic rate constants Oxide growth rate increases Oxide cleaner Device performance is improved Effect of HCl on Oxidation Rate Oxide Charge Definitions 1. Interface trapped charge (Q it ): located at Si/SiO 2 interface 2. Fixed oxide charge (Q f ): positive charge located within 3nm of Si/SiO 2 interface 3. Oxide trapped charges (Q ot ): associated with defects in the SiO 2 4. Mobile ionic charges (Q m ): result from contamination from Na or other alkali ions Oxide Charge Locations Dopant Redistribution During Thermal Oxidation (1) Dopant concentration Dopant concentration Page A. Kolodny 12

13 Dopant Redistribution During Thermal Oxidation (2) Dopant Redistribution During Thermal Oxidation (3) 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 (4) Dopant Redistribution During Thermal Oxidation (5) a) boron b) boron with hydrogen ambient c) Phosphorus d) gallium Thin Oxide Growth Page A. Kolodny 13