2004-Apr-07 JaeYeong Kim

1. Chap. 8 Chemical Mechanical Polishing (CMP) 2004-Apr-07 JaeYeong Kim

2. Tstepi Tstepf Definition of Planarization • No Planarization • Smooth only • Smoothing and partial planarization • Complete Local Planarization • Complete Global Planarization Tstepf = Tstepi Tstepf Tstepf < Tstepi Tstepf =0

3. Need for Dielectric Planarization • Metal thinning by non-planar surface by poor step coverage • Etch stringers cause shorts of metal interconnection • DOF (Depth of Focus) margin limitation of submicron optical lithography Tools • Additive effect on upper level metal layers worsen these effect • planar surface is essential for multilevel metallization • Inter-metal dielectric and/or plug metal planarization, or Metal dual damascene structure

4. Planarization • Thermal Reflow of Oxide Layers • Spin-on-glass (SOG) • Sacrificial resist etchback • Deposition and Etchback

5. History of CMP • 1983 : Proprietary development initiated at IBM • 1988 : Oxide CMP in production at IBM • IBM share some CMP data to improve Intel’s interconnect technology for Intel microprocessor • IBM sold CMP process technology to Micron Technology for 4M DRAM • 1989 : Tungsten CMP in production at IBM Aluminum-copper CMP in production at IBM SEMATECH CMP programs initiated • 1992 : Copper CMP process announces by IBM All production firsts were implemented using existing polishing consumables A consequence of proprietary CMP development

6. Pressure Carrier Diamond disk Conditioner Slurry wafer Pad Table Rotation Principle of CMP • Mounting wafer face down on a carrier • The carrier is pressed against a moving platen containing a polishing pad • The carrier is rotated • An abrasive-containing aqueous slurry is dripped onto the table and centrifugal force distributes the slurry across the pad • This forms a thin colloidal layer of slurry which saturate the pad • The combination of mechanical effect and chemical reactions results in material removal from the surface of the wafer

7. Advantages of CMP • High manufacturing yields and high device speeds with multilevel interconnects • Increase depth of focus  smaller critical dimension available without yield reduction • Reducing defect density

8. ΔT Tpre-CMP Tpost-CMP RR = ΔT/t Terminology Used to Characterize CMP Process • CMP Removal-Rate (RR) • Measured by performing a CMP process on a planarized, unpatterned wafer for a specific time • Determining how much material has been polished away RR = ΔT/t ,ΔT : thickness of material removed in time t • Degree of Planarization (DOP) • Quantity that describes how well the CMP process meets ideal planarization %DOP = [(Hdi-Hdf)/Hdi]X100% ,Hdi : the height of feature being polished prior to carrying out the CMP step Hdf : the height of the feature after the completion of the CMP step If %DOP=100%  complete global planarization %DOP = [(Hdi-Hdf)/Hdi]X100% Hdi Hdf maxi Before Polish Measurement Post Polish Measurement mini maxf minf

9. Terminology Used to Characterize CMP Process • Within-Die Non-Uniformity (WIDNU) • WINDU = σWID/xWID , σWID : standard deviation of film thickness within a die xWID : average value of all “n” film thickness measurements within the die • Pattern dependent polarization : Narrow, isolated-features tend to be more planarized than wide isolated-feature or narrow, densely-spaced feature

10. Terminology Used to Characterize CMP Process • Within-Wafer Non-Uniformity (WIWNU) • WIWNU = σWIW/xWIW , σWIW : standard deviation of film thickness within a wafer xWIW : average value of all “n” film thickness measurements on same wafer • Origin of WIWNU • Non-uniform thickness of substrate itself • Relationship between the angular rotation velocities of the wafer carrier and the polishing pad platen.

11. Terminology Used to Characterize CMP Process • Wafer-to-Wafer Non-Uniformity (WTWNU) • WTWNU = σWTW/xWTW , σWTW : standard deviation of film thickness from wafer to wafer xWTW : average value of all measurements of film thickness taken over all “n” wafers • Origin of WIWNU • Pad aging • Slurry inconsistency from batch to batch

12. Terminology Used to Characterize CMP Process • Efficiency of Planarization (EOP) • %EOP = [(Tr-Td)/Tr]X100% , Tr : dielectric thickness removed by CMP process on top of feature Td : oxide thickness loss at center of wide space between metal lines • Ideal case, %EOP = 100% • In other expression %EOP = [Hmetal/(Ttotal-Tcov)]X100% , Hmetal : Thickness of metal lines being covered by oxide Ttotal : total amount of oxide film deposited over metal line prior to CMP Tcov : target amount of oxide that should remain over the metal line  Ttotal = (Hmetal/EOP) + Tcov • Generally, Tcov is closely related with device speed by RC delay. So Tcov is suggested by IC designer or fixed for technology generation. But Ttotal is closely related with EOP, so generally determined by CMP process engineer for least-expensive process. • Ex) %EOP = 50%, Hmetal=0.5um, Tcov=0.5um •  Ttotal = 1.5um • but %EOP=100%, Ttotal = 1.0um and %EOP=80% then Ttotal = 1.1um

13. Preston’s equation • Preston’s Equation R = H/△t = K(L/A)(△s/△t ) , H/△t : changes in height (=removal rate) L/A : total load/area (= pressure) △s/△t : velocity of a polishing point relative to pad K : Preston’s eq. Coefficient → K is process dependent such as pad property(Young’s modulus), film hardness and basically derived from pure mechanical aspect.

14. H2O due to pressure imposed by pad OH O Si OH Si hydrated O Si Si O Silica abrasive Si Silica abrasive Si OH Si O Si H2O Si O OH OH Si Si redeposited Chemical – Slurry Removal mechanism • Cook’s Model for Oxide CMP Oxide forms hydroxyls(Si-OH) in aqueous solution. • Hydrogen bond is formed between the abrasive hydroxyl in the slurry and the oxide surface of wafer • Si-O bond (siloxane bond in case of silica abrasive) is formed. • As the abrasive moves away, siloxane linkage of oxide is broken resulting in the removal of oxide.

15. Passivating Film Metal Insulator Passivating Film Metal Insulator Passivating Film Metal Insulator Metal Insulator Chemical – Slurry Removal mechanism • Kaufman Model for Metal(W) CMP Removal of passivating film by mechanical action : chemical oxidation of metal surface Passivation Reaction: W+6Fe(CN)63-+3H2O WO3+6Fe(CN)64-+6H • Wet etch of unprotected metal by chemical action (1) : Dissolution of abraded species Etching(dissolution) Reaction : W+6Fe(CN)63-+4H2O WO42-+6Fe(CN)64-+8H+ • Passivating film reforms (2) • Passivation : Better planarity but if passivation effect is dominant, slower removal rate • Dissolution : Chemical dissolution enhances removal rate but poor selectivity • Planarization by repetitive cycles of (1) & (2)