Silicon oxide has steadily been replaced by hafnium oxide as gate oxide in MOSFETS. In the mid-2000s, size of transistors continued to reduce leading to this change [i]. Hafnium oxide, a high-k dielectric material, is one of the few binary oxides that are thermodynamically stable with silicon. Thus, it can be naturally integrated in logic and memory devices. For example, in 2007, Intel announced the addition of hafnium-based high-k metal gates in their processors [ii]. This new transistor formulation assured lower power consumption in combination with reduced electricity leaks, and the delivery of a performance that comes with downscaling.
Limitations Faced in the Characterization of Hafnium Oxide Thin Films
In 2011, the ferroelectricity of hafnium oxide was revealed [iii]. As all bulk phases of hafnium oxide have centrosymmentric crystal structures, they do not exhibit ferroelectricity. Hafnium oxide is monoclinic at room temperature and tetragonal above 2050 K. Nevertheless, the formation of thin films under mechanical encapsulation (“capping”) which occur when doped with silicon oxide, hafnium oxide forms crystalline phases that are non-centrosymmetric.
Consequently, it is expected to be ferroelectric [iv]. This ferroelectric phase is orthorhombic, and is formed by the inhibition of the tetragonal to monoclinic transition by mechanical confinement. It is possible that ferroelectricity of hafnium oxide can lead to fascinating devices taking advantage of silicon/ferroelectric junctions. For example, ferroelectric transistors (FeFETS), show potential outcomes to be as ultra-fast, low-power non-volatile memory, that may soon compete with current flash technology [v, vi, vii].
A difficulty in the characterization of hafnium oxide thin films, is the property of ferroelectric materials; weak piezoelectric response. Piezoresponse force microscopy (PFM) often needs to be completed at the contact resonance frequency in order to gain benefits from resonance enhancement of the signal [viii]. However, the frequency of the contact resonance changes, and can introduce topographic crosstalk that can lead to an obscurity of the piezoelectric response.
Dual Amplitude Resonance Tracking
Asylum Research’s proprietary Dual Amplitude Resonance Tracking or DART™ mode tracks contact resonance shifts, and minimizes the effect of topography on the measurement. Some examples of DART™-PFM images of Si:HfO2 thin films can be seen below, clearly revealing poled piezoelectric domains. The sample is a 10 nm thin film in its initial state after crystallization, before wake-up cycling. These data were collected during the installation of a new Cypher S system.
DART amplitude (left) and phase (right) data overlaid on topographic surface of 10 nm Si:HfO2 thin film (3 µm scan size).
DART amplitude (left) and phase (right) images of 10 nm Si:HfO2 thin film (1.5 µm scan size), with line sections across piezoelectric domains of opposite polarity.
[i] Zhu, H., C. Tang, L. R. C. Fonseca, and R. Ramprasad. "Recent progress in ab initio simulations of hafnia-based gate stacks." Journal of Materials Science 47, no. 21 (2012): 7399-7416.
[ii] Intel News Release: “Intel's Fundamental Advance in Transistor Design Extends Moore's Law, Computing Performance: Sixteen Eco-Friendly, Faster and 'Cooler' Chips Incorporate 45nm Hafnium-Based High-k Metal Gate Transistors” (https://www.intel.com/pressroom/archive/releases/2007/20071111comp.htm)
[iii] Böscke, T. S., J. Müller, D. Bräuhaus, U. Schröder, and U. Böttger. "Ferroelectricity in hafnium oxide thin films." Applied Physics Letters 99, no. 10 (2011): 102903.
[iv] Polakowski, Patrick, and Johannes Müller. "Ferroelectricity in undoped hafnium oxide." Applied Physics Letters 106, no. 23 (2015): 232905.
[v] NamLab (Nanoelectronic Materials Laboratory) Website: “Hafnium Oxide Based Ferroelectric Memory (http://www.namlab.de/research/reconfigurable-devices/hafnium-oxide-based-ferroelectric-memory)
[vi] Dünkel, S., M. Trentzsch, R. Richter, P. Moll, C. Fuchs, O. Gehring, M. Majer et al. "A FeFET based super-low-power ultra-fast embedded NVM technology for 22nm FDSOI and beyond." In Electron Devices Meeting (IEDM), 2017 IEEE International, pp. 19-7. IEEE, 2017.
[vii] Trentzsch, M., S. Flachowsky, R. Richter, J. Paul, B. Reimer, D. Utess, S. Jansen et al. "A 28nm HKMG super low power embedded NVM technology based on ferroelectric FETs." In Electron Devices Meeting (IEDM), 2016 IEEE International, pp. 11-5. IEEE, 2016.
[viii] Rodriguez, Brian J., Clint Callahan, Sergei V. Kalinin, and Roger Proksch. "Dual-frequency resonance-tracking atomic force microscopy." Nanotechnology 18, no. 47 (2007): 475504.
This information has been sourced, reviewed and adapted from materials provided by Asylum Research - An Oxford Instruments Company.
For more information on this source, please visit Asylum Research - An Oxford Instruments Company.