ECSE-6300 IC Fabrication Laboratory Lecture 4: Dielectrics and Poly-Si Deposition. Lecture Outline

Transcription

1 ECSE-6300 IC Fabrication Laboratory Lecture 4: Dielectrics and Poly-Si Deposition Prof. Rensselaer Polytechnic Institute Troy, NY Office: CII-6229 Tel.: (518) s: /index.html 4-1 Lecture Outline Dielectrics and Poly Si Film Deposition Processes (CVD) Reactor Configurations Gas Safety Poly Si Deposition and Doping SiO 2 Deposition and Properties Si 3 N 4 Deposition and Properties Note: The lecture slides were prepared based on the original materials written by Profs. T.P. Chow and J.-Q. Lu 4-2

2 Dielectric and Poly-Si Films Most commonly used films Poly Si, SiO 2, Si 3 N 4 and SiN x Most commonly deposition methods APCVD, LPCVD, PECVD, PVD Most common applications Doped poly Si as MOS gates SiO 2 as interlevel dielectric (ILD) Si 3 N 4 as diffusion and sodium barrier SiN x as chip passivation layer 4-3 Deposition Reaction Processes Silane (SiH4) is flammable in air, colorless, toxic 4-4

3 Advantages: High throughput Good uniformity Handle large wafers Disadvantages: Fast gas flows High particulate count Needs frequent cleaning APCVD Reactor CVD: Chemical Vapor Deposition APCVD: Atmosphere Pressure CVD SiO2 4-5 LPCVD Reactor Advantages: Excellent uniformity Large load size Hold large wafers Uniform step coverage Precise control of composition & Structure Disadvantages: Low deposition rates Toxic, corrosive or flammable gases Medium throughput (vacuum) Poly-Si, SiO2, Si3N4 4-6

4 Advantages: Low deposition temperature Disadvantages: Limited capacity Individual wafer loading Easily contaminated by loosely adhering deposits falling PECVD Reactor SiO2, SiNx:H 4-7 Physical Vapor Deposition (PVD) Advantages: Low deposition temperature Good purity Gas Safety Disadvantages: Poor step coverage High vacuum / Limited capacity Limited materials (mostly for metals) Cryo pump is preferred over diffusion pump for cleanliness (oil back diffusion in the latter) 4-8

5 Gas Safety Tetraethoxysilane (TEOS) Si(OC2H5)4: liquid, stable, flammable, toxic (tetra-ethyl-ortho-silicate) 4-9 Poly-Si Deposition: LPCVD - Pyrolyze Silane at o C SiH 4 Si + 2H 2 Arrhenius Equation: R = A exp(- qe a /kt) E a ~ 1.7eV (40Kcal/mol) for poly-si At high temperatures, R constant * Mass-transport limited * Dependent on reactant conc., reactor geometry and gas flow Deposition Rate: nm/min 4-10

6 Poly-Si Deposition: LPCVD At low T, the deposition is surface reaction limited (the rate of reaction is slower than the rate of reactant arrival) Surface reaction limited regime gives films with excellent thickness uniformity Deposition rate calculation: log (R 1 /R 2 ) = (qe a /k) (T 1 T 2 )/(T 1 T 2 ) If R 1 = 10 nm/min at 600 o Cwith E a = 1.7eV, R 2 = 2.5 nm/min at 550 o C 4-11 Poly-Si Deposition: LPCVD Reactor pressure can be controlled by Pumping speed Nitrogen flow Total gas flow with constant ratio Deposition reproducibility is best when the pressure is controlled by pumping speed 4-12

7 Poly-Si Deposition: LPCVD Sequence of surface process: Effect of silane concentration (non linearity) SiH 4(g) SiH 4(ad) SiH 4(ad) SiH 2(ad) + H 2(ad) SiH 2(ad) Si + H 2(ad) H 2(ad) H 2(g) Rate = K 1 p 1/2 s / (1 + K 2 p 1/2 s ) p s is the partial pressure (g: gas, ad: adsorb) Surface reaction limited regime gives films with excellent thickness uniformity 4-13 Poly-Si Deposition: LPCVD Poly Si films deposited below 580 C is amorphous Poly Si films deposited above 625 C is polycrystalline and has a columnar structure Poor quality at >650 o C Grains formed after annealing at 700 o C 4-14

8 PVD Poly-Si by Ionized Magnetron Sputtering The target, upon bombardment by Ar ions, ejects atoms of the target material, which then float and condense on a substrate PVD Poly-Si by Ionized Magnetron Sputtering Gas flow: Ar:H 2 =10:6 T (Substrate): 250 o C Deposition rate: ~10 nm/min Poly Si/Glass: columnar structure Grains: nm in diameter J. Joo, Low-temperature polysilicon deposition by ionized magnetron sputtering, JVST A 18, 2006 (2000) 4-16

9 Poly-Si Doping Effect of dopants on poly Si deposition 3 doping methods for poly Si Increase 600 o C Decrease Dopants: Boron, Arsenic, Phosphorus Phosphorus doped Poly Si 4-17 Poly-Si Doping Diffusion doping by POCl 3 or PH 3 ( o C) 5POCl 3 P 2 O 5 + 3PCl 5 4PCl 5 + 5O 2 2P 2 O Cl 2 2 P 2 O 5 + 5Si 5SiO 2 + 4P In situ doping for poly Si Phosphorus 4PH 3 + 5O 2 2P 2 O 5 + 6H 2 Difficult to dope poly Si moderately, either very lightly doped or heavily doped Grain size is dependent on the doping method used Boron 4-18

10 SiO 2 Deposition APCVD SiH 4 + O 2 SiO 2 +2H 2 4PH 3 + 5O 2 2P 2 O 5 + 6H 2 Phosphosilicate glass (PSG) APCVD silane oxygen reaction at 350 o C 4-19 SiO 2 Deposition LPCVD LPCVD TEOS Oxide LPCVD Si(OC 2 H 5 ) 4 SiO 2 + 4C 2 H 4 (g) + 2H 2 O (g) 4-20

11 PECVD TEOS SiO 2 Deposition Plasma energy to reduce the deposition T: o C at pressure of 2 10 torr (suitable for ILD). O 2 :TEOS in 10:1 to 20:1 to minimize C & N Conformal Unlike thermal TEOS CVD (no H detectable), PECVD TEOS oxide has 2 9% H Si(OC 2 H 5 ) 4 + O 2 SiO 2 + by-products RPI P5000 PE-TEOS SiO 2 TEOS/O2, o C / 9 Torr / RF 350 W R ~ 500 nm/min SiO 2 Step Coverage (a) TEOS at 700 C (b) Silane-O 2 at 450 C and low P (c) Silane-O 2 at 480 C and Atm. P Dependent on reactant surface migration and mean free path 4-22

12 Properties of Deposited Oxides PECVD O2:TEOS SiO 2 (2-9% H) conformal stable <1.5 T2C SiCl 2 H 2 + 2N 2 O SiO 2 + 2N 2 + 2HCl C: compressive; T: tensile 4-23 Si 3 N 4 Deposition: LPCVD 3SiH 4 + 4NH 3 Si 3 N H 2 APCVD: 700 to 900 C 3SiCl 2 H 2 + 4NH 3 Si 3 N 4 + 6HCl + 6H 2 LPCVD: 700 to 800 C 2.01 E a ~ 1.8 ev (41Kcal/mol) Nitride films can contain up to 8% of hydrogen 4-24

13 PECVD-Nitride Deposition Concentration of Hydrogen Groups LPCVD Si 3 N 4 LPCVD Si 3 N 4 RPI PlasmaTherm PECVD 300 o C / 900 mtorr / RF Power 25 W 2% SiH 4 /N sccm / He 2250 sccm R ~ 5 nm/min Properties of Deposited Nitrides LPCVD nitride is stoichiometric and an excellent barrier against oxidation and sodium diffusion PECVD nitride contains a large amount of hydrogen (can be etched with HF) and serves as a passivation layer 4-26

14 Comparison of Different Deposition Methods (PETEOS) (PVD) ( ) (SiO2) (RT 300) (Poly-Si) (ILD) (Low) (Conformal) (Few) (Good) (Yes) (Metal gate, TFT) (Low) (Poor) (Few) (Good) (Yes) 4-27 Guidelines for IC FabLab Report Cover with group names and responsibility Acknowledgement Abstract Chapter 1: Technical Background Introduction Device Physics Process Considerations Basic NMOS Processing Processing Modeling T-SUPREM Test Devices Testing Techniques Chapter 2: Processing Procedures Detailed process flows with comments/suggestions Inspection results with photos Chapter 3: Electrical Test Results and Discussions I-V, C-V, Device variations, etc., with tables/plots Chapter 4: Summary & Conclusions References & Appendix 4-28