Trench Effects in Copper Electroplating Student: Oleg Gurinovich Industry: Novellus Systems Advisor: Dr. Stacy Gleixner John Kelly, Jim Stimmell, Chiu Chi Instructor: Dr. Richard Chung TFI: Bill Gadson Mate 198B Spring 2001

Research Objectives & Motivations Study variations in critical dimensions (CDs) of trenches during damascene process (lithography and etching steps) Study how variations in CDs affect electroplating performance by measuring electrical resistance of copper interconnects More companies switch to copper as a material choice for interconnects Strong correlation between to be filled feature size and electroplating bath chemistry

Why Use Copper and Electroplating? Interconnect lines density increase Need for more conductive and reliable material Relatively low cost of the process High speed of deposition High quality of the deposited film for well-optimized process Electroplated copper has good electromigration characteristics

Steps in Damascene Process Lithography Etching Diffusion Barrier Deposition (Ta, TiN, TaN) Seed Layer Deposition (Cu) Electroplating Chemical-mechanical Polishing

Electroplating Scheme CuSO 4 +H 2 O+H 2 SO 4 + additives Cathode (Wafer): Cu 2+ + e - = Cu + Cu + + e - = Cu 0 Anode: 4OH - =O 2 + 2H 2 O + 4e - 2H 2 O=O 2 + 2H + +2e -

Trench Cross Section

Electroplating Bath Additives Brighteners accelerate Cu deposition at sites where present 4,5-dithiaoctane-1,8-disulphonic acid (SCH 2 CH 2 CH 2 SO 3 H) 2 Levelers adsorb at high-charge density sites organic compounds containing nitrogen Suppressing agents create uniform diffusion layer Example: polyethylene glycol, (C 2 H 4 O) n

Trench Fillings Trenches with voids

Experimental Procedure Measure width of 0.25  m trenches after lithography and etch steps to find variance in the trench width introduced by these two processes Measure electrical resistance of copper interconnects to find how actual trench size variations affect electroplated copper characteristics

Trench Width Measurements Equipment used: Top-down SEM, Hitachi-S6780 Precision measurements were performed -random trench was selected -same trench was measured 25 times -SD was found  m

Lithography defects Normal trenches Underdeveloped trenches Overdeveloped trenches

Measurements Map After-lithography measurementsAfter-etch measurements

Measurements Data Width is distributed around mean value of  m No significant trend Trend is observed, central dies on the wafer have wider trenches after etching step

After-etch Measurements Map Four 8-inch wafers were measured

After-etching measurements

Discussions Lithography step doesn’t introduce special cause variation -data distribution does not show significant trend -mean value  m, SD is  m Etching step does have special cause variation -central area of the wafer is etched more than edges -account for this effect during electrical resistance measurements -mean trench width after etching  m, SD is Need to be aware of lithography defects while analyzing resistance data

Conclusions Etch step may affect electrical resistance of copper interconnects Based on the literature research, bath additives concentrations will affect trench fill capability Lithography step do not affect overall trench width distribution Measuring procedure (map) is developed

Future Work Measure electrical resistance of copper interconnects Anneal wafers at different temperatures after electroplating to study effect of annealing on copper grain “re-growth” Study effects of electroplating bath chemistry on copper interconnects electrical characteristics