BEOL Al & Cu

Overview Review BEOL Why is it important? Copper vs Al Technology Process technology to achieve the final product Some Integration issues and work-arounds

Chip manufacturing: Snap shot Electrical Chip Design R C Physical “Layout” Design Blue Print- Photo “negative” Creating the chip Review Testing “Print” Quality Control

Creating the chips Review Front End of the Line FEOL Back End of the Line BEOL Creating the devices (transistors, capacitors, resistors) Connecting the devices (wiring) Review Metal Contact Silicon Wafer Device 1 Device 2 Device 3 Device 4 Packaging

Chip - Simplified Schematic Silicon Wafer Device 1 Device 2 Device 3 Device 4 Metal Contact Review What if you want to connect Device 3 to another device 5 just at the back of Device 3?

Chip - Simplified Schematic Level 1 Level 2 Via Device 1 Device 2 Device 3 Device 4 Silicon Wafer Review Many layers of metal are necessary for current Chips (typically 4 to 5) Up to 8 metal layers are manufacturable in the top class manufacturing units (called fabs)

Intel 7 metal SEM (90 nm node)

BEOL: Importance Importance has been increasing recently and will continue to do so For older generation, BEOL was an important factor in the yield of the chip. The speed of the chip was determined by the transistors (FEOL) What is yield? More layers than FEOL => more likely to fail. Hence the yield impact For newer generation, BEOL may be important in both yield and speed Transistors may switch fast, but the signal has to go through wires!

Estimates on delay © www.eedesign.com

BEOL Processes Review Photo Lithography Deposition (Chemical Vapor Deposition, Physical Vapor deposition, Electrochemical Deposition) Removal (Chemical Mechanical Polishing, Etching) Anneal Review

Overview Review BEOL Why is it important? Copper vs Al Technology Aluminum Product : Details Aluminum/ W Process Flow (partial) Litho, Dep, Removal Aluminum process flow (complete) Cu Process Flow Process technology to achieve the final product Some Integration issues and work-arounds

Relevant Conductor Properties Some of the important & relevant properties Resistivity (micro ohm-cm) Litho Process compatibility eg. Will it react with photo resist? Melting point Thermal Expansion coefficient Diffusivity in silicon di oxide (or any other insulator) Adhesion to the insulator Inter-atomic distances (stress) Process Crystal orientations, grain sizes Step coverage

BEOL Materials: Al vs Cu Conductor: Copper OR Aluminum Aluminum : Older generation (mostly up to 0.18 um) Copper: 0.18 um and below Insulator: Oxide (silicon-di-oxide) or Low-K materials What is Low-k? Low di-electric constant Why? Capacitance Metal Metal Via Via Via

BEOL Materials: Al vs Cu Low-K: Being introduced now Will not be discussed in detail in the class Few pointers, in the Process Integration chapter Aluminum and Copper products Al metal lines are connected by Tungsten Via Copper lines are connected by copper via Aluminum technology Copper technology Al W Cu

Aluminum Process: General Aluminum technology Al W Aluminum Resistivity: 2.6 m ohm-cm Copper Resistivity: 1.7 m ohm-cm W Resistivity: 5.7 m ohm-cm W Via: Why? Easier to Process W is higher resistivity, but via is short ==> Lower impact on the overall resistance

Aluminum Process: General Aluminum technology Al W Other relevant information: Both W and Al need “Liners” Ti/TiN liners Diffusion Barriers Adhesion enhancement Stress Reduction (==> better reliability)

Aluminum Process: Detail (a bit) Aluminum technology Aluminum technology

Aluminum Process: General Oxide deposition - CVD Shape Definition - Litho Oxide Removal -Dry etch with plasma Ti/TiN Deposition - PVD /CVD W Deposition -CVD W Removal - CMP Ti/TiN Deposition- PVD /CVD Al Deposition PVD Ti/TiN Deposition - PVD/CVD Ti/TiN/Al Removal - Dry etch with plasma

Digression: Intel 7 metal SEM (90 nm node) Pitch = space+width Normally pitch means minimum pitch in a layer Usually M1 pitch is tighter than poly Note M7 is very large (power lines) © Intel

Digression: Metal line widths M1 is usually very small (eg 200 nm) M1 pitch is typically tighter than poly pitch M2, M3, etc are slightly larger (eg 250 nm) Last metal (and the last but one) can be very large (eg 400 nm) Why? After lot of layers are built, the photo margin tends to be low for small line width/spaces Last metal tend to carry lot of current and you need the width, to reduce resistance Fewer wires are needed. Space is available. So why not?

Aluminum Process: Details (more) CMP Oxide Deposition Tunsten CMP Tunsten Dep Al Dep Resist Strip Photo resist coat Litho-Develop Litho-expose Liner Dep (PVD/CVD) Liner Dep Etch Post CMP Clean Densification Other steps: Later on

Copper Process-General Lower resistivity Reduced power consumption, reduced heating, longer battery life... More reliable connection (long term) Lower processing cost with Dual Damascene At least, that is the theory! Faster diffusion Difficult to etch/ Process not as mature as Al process Damascene Process Single Damascene vs Dual Damascene

Damascene Method Initiated by IBM (practiced in ancient times for pottery designs)

Damascene Method Example of Single Damascence Process Single, because one layer is created in one damascene step Polishing of Copper makes this possible Polish Dep Etch Dep

Damascene Method Example of Dual Damascence Process Dual, because two layer are created in one damascene step Polish Dep Etch Etch

CMP Tool: Schematic Controlled Pressure Rotation Rate Controlled slurry flow rate and temp Cu Disk Pad Controlled Rotation Rate Removal is mechanical (abrasive particles) and chemical (dissolution) Inhibitors added for controlled removal

CMP removal mechanism Copper No Removal in this region Removal in these regions Pad ==> non planar surface --> planar surface

CMP: Basics Pressure, Velocity - Removal Rate Chemical Dissolution Controlled Removal ==> Dissolution Inhibitors Smooth Surface ==> Surfactants

BEOL Process-General Cu or Al process: Liners are present Cu - Ta/TaN liners Also has PVD ‘seed’ layer of copper Al - Ti/TiN liners Contacts from Transistors to the M1 Always W CMP in Cu newer challenges Main deposition is Electrochemical Grain Size (resistivity, electro migration) void free fill

BEOL Summary Focus chiefly on conductor (insulator options & processes not discussed in detail) Al vs Cu processes Al -etch / Cu Damascene (CMP) Al -via is W / Cu- via is Cu Al & Cu: Contact is W Not all the details of Al vs Cu discussed now Overall, the processes involved are Litho, PVD, CVD, Electrochem Dep, Etch, Anneal Comparison: Production of a chemical: Flow sheet, unit operations