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Monthly Archives: May 2015

ALD of Crystalline High-K SHTO on Ge

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Alternative channel materials (ACM) such as germanium (Ge) will need to be integrated into future CMOS ICs, and one part of the integration was shown at the recent Materials Research Society (MRS) spring meeting by John Ekerdt, Associate Dean for Research in Chemical Engineering at the University of Texas at Austin, in his presentation on “Atomic Layer Deposition of Crystalline SrHfxTi1-xO3 Directly on Ge (001) for High-K Dielectric Applications.”

Strontium hafnate, SrHfO3 (SHO), and strontium titanate, SrTiO3 (STO), with dielectric constants of ~15 and ~90 (respectively) can be grown directly on Ge using atomic layer deposition (ALD). Following a post-deposition anneal at 550-590°C for 5 minutes, the perovskite films become crystalline with epitaxial registry to the underlying Ge (001) substrate. Capacitor structures using the crystalline STO dielectric show a k~90 but also high leakage current. In efforts to optimize electrical performance including leakage current and dielectric constant, crystalline SrHfxTi1-xO3 (SHTO) can be grown directly on Ge by ALD. SHTO benefits from a reduced leakage current over STO and a higher k value than SHO. By minimizing the epitaxial strain and maintaining an abrupt interface, the SHTO films are expected to reduce dielectric interface-traps (Dit) at the oxide-Ge interface.

Much of the recent conference has been archived, and can now be accessed online.

—E.K.

Bottoms-up ELD of Cobalt Plugs

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As reported in more detail at Solid State Technology, during the IEEE IITC now happening in Grenoble, imec and Lam showed a new Electroless Deposition (ELD) cobalt (Co) process that is claimed to provide void-free bottoms-up pre-filling of vias and contacts. The unit-process is intended to be integrated into flows to produce scaled interconnects for logic and DRAM ICs at the 7nm node and below. Co-incidentally at IITC this year, imec and Lam also presented on a new ELD copper (Cu) process for micron-plus-scale through-silicon vias (TSV).

The bulk resistivities of metals commonly used in IC fabrication are as follows (E-8 Ω⋅m):
Cu – 1.70,
Al – 2.74,
W – 5.3, and
Co – 5.8.
Of course, the above values for bulk materials assume minimal influence of grain sizes and boundary layers. However, in scaled on-chip interconnect structures using in today’s advanced ICs, the resistivity is dominated by grain-boundaries and interfacial materials. Consequently, the resistivity of vias in 7nm node and beyond interconnects may be similar for Cu and Co depending upon the grain-sizes and barrier layers.

The melting temperatures of these metals are as follows (°C):
Al – 660,
Cu – 1084,
Co – 1495, and
W – 3400.
With higher melting temperature compared to Cu, Co contacts/plugs would provide some of the thermal stability of W to allow for easier integration of transistors and interconnects. Seemingly, the main reason to use Co instead of W is that the latter requires CVD processing that intrinsically does not allow for bottom-up deposition.

—E.K.