What is a low–k dielectrics ? ■ Define : A low-k dielectric material with a dielectric constant less than the dielectric constant of silicon dioxide, k<4. The propagation delay of a signal through an
interconnect layer is related to the resistance and capacitance of the conductor. Low-k dielectric materials surrounding the conductor reduce capacitance and propagation delay.
Continuing decrease in feature size of microelectronic devices cause unexpected results such as “cross talk” and RC delay
Therefore, it is necessary to decrease resistance of metal wiring (Al Cu) and decrease dielectric constant (k) of interlayer dielectrics (ILD)
What is a low–k dielectrics ?
Interconnect Structure Roadmap
Candidate Low-k materials
Material
Silicon dioxide (SiO2) Fluorosiicate glass(FSG) Organosilicate glass(SiCOH or SiCOH) Fluorinated amorphous carbon (a-C:F) Parylene-N Parylene-F Polyimides Fluorinated polyimides Benzocyclobutene polymers (BCB) Hydrogen-silsesquioxane (HSQ) Methyl-silsesquioxane (MSQ) Poly(arylene ether) Aromatic thermosets(SiLK) Teflon AF Porous silica(aerogel and xerogel)
Dielectric constant (k)
3.9-4.5 3.2-4.0 2.7-3.3 2.4 2.7 2.4-2.5 3.1-3.4 2.5-2.9 2.7 2.9-3.2 2.6-2.8 2.8-2.9 2.6-2.7 2.1 1.1-2.4
Deposition method
PECVD
CVD
Spin-on
Advanced Materials, 12,1769 (2000)
General Requirements for Low-k
■ Low dielectric constant ■ Low water absorption ■ Good chemical and thermal stability, high glass transition temperature, Tg ■ High gap-fill and planarization capability, as required for the process ■ Low film stress. ■ Good electrical properties - low dielectric loss - positive effect to the hot carrier degradation effect - low shift of the threshold-voltage caused by mobile charges - low leakage current - low power consumption
PECVD Systems
Reactor Shower Ring Power Supply Substrate Ar,He (99.999%)
Bubbler and Temperature Control System
MFC
Turbo-molecular Pump (TMP) Heating Tape To Exhaust Molecular Sieve Trap Rotary Pump Gas Tank
□ Sample Preparation
○ First step : Deposition
• Plasma Power : 15 W • Deposition Pressure : 0.6 ~ 1.5 Torr • Precursors : DMCPSO (75oC) + He (40 sccm) CHex • Temp. : 400oC • Time : 60 min • Apparatus : N2-furnace (45oC) + He (20 sccm)
Low-k thin film n-Si(100) • FT-IR FT• Hardness & Modulus V Al Low-k thin film
Metallic P - Si(100) or n-Si(100)
++
○ Second step : Annealing
□ Measurements
Ag glass • C-V measurement (MIM) • I-V measurement (MIS)
Capacitance : Agilent 4287A LCR meter Thickness : α-step profilometer , FESEM Refractive index (n) : gaertner L116 ellipsometer Leakage current : Keithley 6517A electrometer Chemical structure : Fourier Transform Infrared Spectroscopy Hardness and Modulus : Nano-indentor
Structure of Silicon oxide based low-k dielectric materials
Alfred Grill, Journal of Applied Physics, Vol. 94, 15. Nov. 2003
Diagram of networked structure of annealed SiCOH film deposited by PECVD -> SiCOH (k > ~2.7)
Diagram of porous cage networked structure of annealed SiCOH film deposited by PECVD -> porous SiCOH (k < ~2.4)
Si-O(str) CHx(str)
(a) PPDMCPSO:CHex, k = 2.76
Si-CH3(b) FTIR spectra of both (a) and (b) samples did not show OH and Si-OH peaks
(b) PPDMCPSO:CHex, k = 2.05