Section 8: Metallization

1. Section 8: Metallization Jaeger Chapter 7 EE143 – Ali Javey

2. Multilevel Metallization EE143 – Ali Javey

3. Multilevel Metallization Components FOX Si substrate ( InteMetal Oxide e.g. BPSG. Low-K dieletric) (e.g. PECVD Si Nitride) EE143 – Ali Javey

4. Interconnect RC Time Delay Interconnect Resistance RI =R/L =  / (WAlTAl) Interconnect-Substrate Capacitance CVC/L = WAlox / Tox Interconnect-Interconnect Capacitance CL C/L = TAlox / SAl * Values per unit length L EE143 – Ali Javey

5. low ohmic resistance interconnects material has low resistivity low contact resistance to semiconductor device reliable long-term operation Interconnect Requirements EE143 – Ali Javey

6. Resistivity of Metals Commonly Used Metals Aluminum Titanium Tungsten Copper Less Frequently Utilized Nickel Platinum Paladium EE143 – Ali Javey

7. Aluminum to p-type silicon forms an ohmic contact [Remember Al is p-type dopant] Aluminum to n-type silicon can form a rectifying contact (Schottky barrier diode) Aluminum to n+ silicon yields a tunneling contact Ohmic Contact Formation EE143 – Ali Javey

8. Heavily doped Semiconductor surface Metal Contact Area Contact Resistance Rc For a uniform current density flowing across the contact area Rc = c / (contact area ) c of Metal-Si contacts ~ 1E-5 to 1E-7 W-cm2 c of Metal-Metal contacts < 1E-8W-cm2 EE143 – Ali Javey

9. - 1 ¶ æ ö J r º ç ÷ ¶ c at V~0 è ø V Contact Resistivity Specific contact resistivity é ù æ ö f e * 2 m ç ÷ r = ê ú exp s B ç ÷ c h N ê ú è ø ë û Bis the Schottky barrier height N = surface doping concentration c = specfic contact resistivity in ohm-cm2 m = electron mass h =Planck’s constant = Si dielectric constant Approaches to lowering of contact resistance: 1) Use highly doped Si as contact semicodnuctor 2) Choose metal with lower Schottky barrier height EE143 – Ali Javey

10. Silicon absorption into the aluminum results in aluminum spikes Spikes can short junctions or cause excess leakage Barrier metal deposited prior to metallization Sputter deposition of Al - 1% Si Aluminum Spiking and Junction Penetration EE143 – Ali Javey

11. Alloying of Contacts EE143 – Ali Javey

12. Electromigration High current density causes voids to form in interconnections “Electron wind” causes movement of metal atoms EE143 – Ali Javey

13. Copper added to aluminum to improve lifetime (Al, 4% Cu, 1% Si) Heavier metals (e. g. Cu) have lower activation energy Electromigration EE143 – Ali Javey

14. Sputtering has been the technique of choice high deposition rate capability to deposit complex alloy compositions capability to deposit refractory metals uniform deposition on large wafers capability to clean contact before depositing metal CVD processes have recently been developed (e.g. for W, TiN, Cu) better step coverage selective deposition is possible plasma enhanced deposition is possible for lower deposition temperature Metal Deposition Techniques EE143 – Ali Javey

15. Metal CVD Processes • TiN • used as barrier-metal layer • deposition processes: EE143 – Ali Javey

16. Electroplating EE143 – Ali Javey

17. Dual Damascene Process EE143 – Ali Javey

18. Self-Aligned Silicide on silicon and polysilicon Often termed “Salicide” Salicides EE143 – Ali Javey

19. Low-K Dielectrics EE143 – Ali Javey

20. Porous low-k dielectric examples EE143 – Ali Javey