Back end of line (BEOL) metal-insulator-metal capacitors (MIMCAPs) have become a core passive component in modern integrated circuits. International Technology Roadmap for Semiconductors (ITRS) projections for scaling of analog/mixed-signal MIMCAP applications require simultaneously increasing capacitance density while maintaining low leakage current density and low voltage nonlinearity (characterized by the quadratic voltage coefficient of capacitance, αVCC). In addition to these conflicting performance requirements, BEOL processing allows for temperatures of no more than 400°C.
In this work, atomic layer deposition (ALD) of both dielectrics and metals have been investigated to develop complementary multi-insulator MIMCAPs to meet future ITRS requirements. Initially Al2O3/SiO2 bilayers are assessed for targeting the ITRS 2020 node. These oxides are attractive due to their large metal-insulator barrier heights, high dielectric breakdown strength, and common usage in IC fabrication. SiO2 is one of only a few materials to exhibit a negative αVCC, which in combination with the positive αVCC of Al2O3 enables ultra-low device αVCC through the "canceling" effect. ALD for these ultra-thin insulators has become the preferred deposition method due to the inherent low deposition temperatures, precise film thickness control, and excellent film quality.
Next, to support scaling beyond the 2020 node, novel ALD processes are developed for bismuth oxide (Bi2O3), ruthenium oxide (RuO2), and ruthenium metal (Ru). RuO2 is a promising electrode material due to its high work function of ~5.1 eV and ability to template the high-κ rutile phase of TiO2. Rutile TiO2 is known to exhibit a negative αVCC with a high-κ of ~100, which makes it a potential replacement for SiO2 and a complementary material to Al2O3. Thus, using RuO2 as the lower electrode, TiO2/Al2O3 multi-insulator MIMCAPs are demonstrated to significantly enhance capacitance density while maintaining low leakage current density and relatively low αVCC.
Finally, various low enthalpy of oxide formation (ΔHox) metals are investigated as a function of ALD Al2O3 and HfO2 dielectric thickness (dox) to examine the mechanism of the influence from the top metal electrode on αVCC, in the absence of an interfacial oxide layer. It is found for each low ΔHox metal that a different αECC, quadratic electric field coefficient of capacitance, value is measured for an otherwise identical device structure. Differences between the metals become more pronounced as the dox decreases, which indicates an interaction at the metal/dielectric interface. To explain these differences, we propose interacting stresses due to applied bias and edge dislocations from lattice mismatch, which modulate the voltage nonlinearity. This new understanding of the impact from metal electrodes on nonlinearity should aid in rapid scaling optimization of low αVCC MIMCAPs.
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