Carbon Nanotubes for Interconnect Applications Franz Kreupl, Andrew P. Graham, Maik Liebau, Georg S. Duesberg, Robert Seidel, Eugen Unger Infineon Technologies Corporate Research Munich, Germany
Outline of Presentation What are carbon nanotubes (CNTs) Applications as devices & interconnects Short channel & power transistor Current density and resistance in CNTs How do nanotubes grow? Focus on integration schemes 20 nm sized nanotube vias Summary Acknowledgments
What are carbon nanotubes? Just roll up a graphene sheet and you will get a single-walled carbon nanotube (SWNT) graphene sheet Single-walled Nanotube (SWCNT) Multi-walled Nanotubes (MWCNT)
Carbon nanotube fact sheet Diameter: 0.4-50 nm Electrical conductivity: metallic or semiconducting ballistic transport mobility ~ 100000 cm 2 /Vs (Si ~ 450 cm 2 /Vs) current density > 10 9 A/cm 2 (Cu ~ 10 6 A/cm 2 ) Thermal conductivity ~ 3000 W/Km (Cu ~ 400 W/Km) Thermal, chemical, and mechanical stability excellent
Possible Applications of Carbon Nanotubes Transistors outperform silicon Source Gate Drain Catalyst Interconnects: vias & contact holes high current densities low resistance Gate Oxide CNT
Short Channel Nanotube Transistor 15 µa @ 0.4 V ds ~ 15 ma/µm @ 0.4 V I on /I off > 10 6 g = 6.75 µs/tube ~ 7900 S/m no ambipolar behavior Seidel et al., Nano Letters (2004) Ids [A] 100µ 10µ 18 nm Channel 1µ 200 mv/dec 100n 10n 1n 100p V ds = -0.4V 10p -6-4 -2 0 2 4 Vgs [V]
Nanotube Power Transistor current [A] 3m 2m 1m 2.4 ma @ 1V V ds = 1 Volt ~ 300 tubes paralleled 0-0.2 0.0 0.2 0.4 gate field [V/nm] Seidel et al., Nano Letters (2004)
Interconnect Applications of Carbon Nanotubes Current density [A/cm 2 ] 8.0M local via intermediate 6.0M via (ITRS) 4.0M 2.0M 0.0 90 80 70 60 50 40 30 20 node [nm] high current density size-effect in metals electromigration reliability use carbon nanotubes as interconnects
Contact Holes & Vias Made of Carbon Nanotubes Copper voids Copper Single Multi-walled Nanotube Via Array of Multi-walled or Single-walled Nanotubes Via
Current density and resistance of nanotubes current densities exceeding 10 9 A/cm 2 are routinely obtained in experiments resistance varies for SWCNT and MWCNT SWCNTs: smallest achievable resistance: 6.5 kohm MWCNTs: smallest achievable resistance varies between 0.3-10 kohm (depends on the number of contacted shells)
Difference in density of states SWCNT ~1nm MWCNT ~20 nm E Fermi E Fermi MWCNTs are easily doped by charge transfer additional DOS is available lower R not possible for SWCNTs
Resistance of Vias filled with CNTs resistance [ Ohm ] 1M SWCNT-Array 100k Cu-wire 700 nm 10k MWCNT 1k 100 10 0 5 10 15 20 25 30 diameter [ nm ] 0.7 nm SW-CNT-Array MW-CNT area arrays of SW-CNTs would be the best conductor MW-CNT are better than metals for < 15 nm (doping possible)
resistance [Ohm] 10000 1000 100 10 Resistance of Vias filled with 1 Single-Walled CNTs 90000 /µm^2 250000/µm^2 1E6/µm^2 10 100 via diameter [nm] densities in the order of 1/nm 2 are required growth of SWCNTs with ~ 0.1/nm 2
resistance [Ω] Length dependent resistance 100M 10M 1M 100k 10k (SWCNT) measured ballistic limit 1k 100n 1µ 10µ 100µ 1m 10m length [m] 4-6 kω/µm + contact resistance similar values for outer shell of MWCNTs
resistivity [Ωcm] Length dependence of resistivity 10-5 W (no size-effect) 10-6 Cu (no size-effect) SWCNT SWCNT exp. ballistic 10n 100n 1µ 10µ 100µ 1m 10m length [m] better than Cu for length > 1µm material parameter has length dependence use SWCNT in high aspect ratio structures CNT: free mean path of 1 µm R 500 nm = R 1000 nm for Cu ~40 nm R 20 nm = R 40 nm
Growth of Nanotubes Catalytic Growth by CVD (suitable) catalyst + energy (temperature)+ specific gas Fe, Ni, Co,...etc 500-1400 C CH4, C2H2, CH3CH2OH gas, reactant gas, reactant catalyst nanocluster nanowire nucleation nanowire growth The catalyst support, i.e., the substrate-catalyst interaction is also very important growth stops, if the particle is covered with a layer of amorphous carbon
Growth of Nanotubes Ni-catalyst ~ 5nm NATURE VOL 427 29 JANUARY 2004 Stig Helveg et al.
Growth of Nanotubes Ni-catalyst ~ 5nm walls of a MWCNT NATURE VOL 427 29 JANUARY 2004 Stig Helveg et al. this process needs to happen at the bottom of a via or contact hole
Growth of Nanotubes bottom growth tip growth
Wafer level growth dense growth at lithographically defined locations
How to create a CNT via or contact hole? Via in oxide catalyst SiO 2 Ta/Cu How to bring a fertile catalyst at the bottom of a via? 3 different groups (Infineon, Fujitsu, NASA), 2 approaches
Bottom up approach (NASA) carbon fibers grown by plasma-enhanced growth bias voltage creates perpendicular oriented fibers Merkulov et al. APL 80, 2002
Bottom up approach (NASA) deposit catalyst first bias voltage creates perpendicular oriented fibers Intermetal dielectric deposition later metal deposition catalyst patterning fiber growth Top Metal deposition CMP TEOS CVD
Bottom up approach (NASA) Li et al., APL(82)15,(2003)
Bottom up approach (NASA) + catalyst first + successful CNT growth + lithographical definition - carbon fibers (no tubes) low sheet alignment - low density - low precision (tilt fibers) - low overlay accuracy - high resistance ~300 kohm per tube Li et al., APL(82)15,(2003) Meyyappan et al., Plasma Sources Sci. Technol. 12 (2003) 205 216
Buried catalyst approach (Infineon, Fujitsu) resist via definition by resist oxide catalyst metal
Buried catalyst approach resist via etch stop on catalyst oxide problem: etch stop on catalyst catalyst metal
Buried catalyst approach resist strip oxide catalyst resist strip metal
Buried catalyst approach nanotube nanotube growth Infineon: CVD Fujitsu: PECVD oxide catalyst metal
Buried catalyst approach 2µm vias and contact holes (Fujitsu) resistance: 135 kohm per tube Nihei et al., IITC 2004, San Francisco
Buried catalyst approach 400 nm vias and contact holes resistance: 10 kohm per tube Kreupl, Microel. Eng. 64, 2002
Quality of the tubes number of shells and shell alignment α = 0 α 0 graphitic planes direction of current flow α catalyst catalyst tube axis ρ(α)= ρ a sin 2 (90 - α) + ρ c cos 2 (90 - α) Zhang et al,apl, 2004
Quality of the tubes 10 shells α = 18 7 shells α = 0 25 shells α = 0
20 nm vias and contact holes ~20 nm 150 20 nm etch via stop on catalyst particle formation nanotube growth single MWCNT in 20 nm via at lithographically defined location
20 nm vias and contact holes a nanotube protrudes from every hole
Current [µa] 20 nm vias and contact holes 150 100 50 0-50 -100 resistance and current densitiy resistance [Ohm] 1M 100k 10k -150-1.0-0.5 0.0 0.5 1.0 Voltage [V] 1k 0 300 600 900 Annealing [ C] Resistance approx. 8kΩ Current Densities > 5 10 8 A/cm 2
resistance [kω] 70 60 50 40 30 20 10 0 20 nm vias and contact holes statistics for contacts with Pd or Ti top contacts Pd-Contacts 1 2 3 4 5 6 7 8 9 10 v i a n u m b e r resistance [kω] 500 400 300 200 100 0 Ti-Contacts 10 20 v i a n u m b e r resistance is dominated by top and bottom contact & the ability to contact all shells
20 nm nanotube via chain reliability test for 2 nanotube via in series resistance [kohm] 200 100 0 current density: > 10 7 A/cm 2 0 12 24 36 48 60 time [h] 26 kohm per tube
Summary Properties of Carbon Nanotubes Motivation: Interconnects & transistors of the future Short channel transistor & Power transistor length dependence of the resistance & specific resistivity current density and resistance Growth of carbon nanotubes Integration issues Bottom up approach (NASA) Buried catalyst approach ( IFX, Fujitsu) first end-of-the-roadmap sized interconnects made of MWCNTs 20 nm via chain main problem: density of tubes & integration issues