Fabrication of HfO2 patterns by laser interference nanolithography and selective dry etching for III-V CMOS application
© Benedicto et al; licensee Springer. 2011
Received: 5 November 2010
Accepted: 31 May 2011
Published: 31 May 2011
Nanostructuring of ultrathin HfO2 films deposited on GaAs (001) substrates by high-resolution Lloyd's mirror laser interference nanolithography is described. Pattern transfer to the HfO2 film was carried out by reactive ion beam etching using CF4 and O2 plasmas. A combination of atomic force microscopy, high-resolution scanning electron microscopy, high-resolution transmission electron microscopy, and energy-dispersive X-ray spectroscopy microanalysis was used to characterise the various etching steps of the process and the resulting HfO2/GaAs pattern morphology, structure, and chemical composition. We show that the patterning process can be applied to fabricate uniform arrays of HfO2 mesa stripes with tapered sidewalls and linewidths of 100 nm. The exposed GaAs trenches were found to be residue-free and atomically smooth with a root-mean-square line roughness of 0.18 nm after plasma etching.
PACS: Dielectric oxides 77.84.Bw, Nanoscale pattern formation 81.16.Rf, Plasma etching 52.77.Bn, Fabrication of III-V semiconductors 81.05.Ea
Three-dimensional multi-gate field effect transistors with integrated mobility-enhanced channel materials (i.e. GaAs, InxGa1-xAs) and high-κ gate dielectrics (i.e. HfO2, Al2O3) are considered as plausible candidates to sustain Si complementary metal-oxide-semiconductor (CMOS) performance gains to and beyond the 22 nm technology generation in the next 5 to 7 years [1, 2]. The rapid introduction of these new materials in non-planar transistor architectures will consequently have a high impact on front-end cleaning and etching processes. Cleaning processes thus need to become completely benign, in terms of substrate material removal and surface roughening. Moreover, high-κ gate etching offering high across-wafer uniformity, selectivity, and anisotropy will be essential to achieve a tight control over gate-length critical dimensions (CD) while keeping linewidth roughness low in future devices. To attain this goal, an adequate choice of photoresist type, etch bias power, and etch chemistry is necessary .
Patterning of HfO2 layers on Si substrates by means of different lithographic techniques and dry etching in F-, Cl-, Br-, CH4-, and CHF3-based plasma chemistries has been extensively investigated [4–7]. Comparatively much less attention has been paid to patterning ultrathin layers of HfO2 deposited on GaAs substrates despite its key role in the fabrication of next generation non-planar high-κ/III-V transistors. In recent papers, we have studied the nanoscale patterning of HfO2/GaAs by electron beam lithography and inductively coupled plasma reactive ion etching (ICP-RIE) using BCl3/O2 and SF6/Ar chemistries [8, 9]. Only the less-reactive F-based chemistry showed good etch selectivity of HfO2 over GaAs (i.e. 1.5) and adequate control of the etching rate. In this letter, we report on the fabrication of nanopatterned HfO2 ultrathin layers on GaAs substrates by laser interference nanolithography (LInL) and selective ICP-RIE in a CF4 plasma chemistry. The main HfO2 etching characteristics studied by a combination of atomic force microscopy (AFM), high-resolution scanning electron microscopy (HR-SEM), and high-resolution transmission electron microscopy (HR-TEM)/energy-dispersive X-ray spectroscopy microanalysis (EDS) are presented, with specific emphasis on pattern resolution; etch profile; and GaAs surface roughness and composition.
All experiments described here were performed on 10-nm-thick HfO2 layers grown by atomic layer deposition (Cambridge NanoTech Inc., Cambridge, MA, USA) on a 2-in.-diameter GaAs (001) wafer (Wafer Technology Ltd., Milton Keynes, UK), where a 400-nm-thick GaAs buffer layer had been previously deposited by metal-organic vapour phase epitaxy. Nanostructuring of the HfO2 thin film was carried out by Lloyd's mirror LInL using a commercial system (Cambridge NanoTools LLC, Somerville, MA, USA) and a He-Cd laser (λ = 325 nm) as the light source. Prior to exposure to the laser source, the HfO2/GaAs substrates were first spin coated with a 210-nm-thick antireflective coating (ARC), then covered by a 20-nm-thick SiO2 layer grown by plasma-enhanced chemical vapour deposition, and finally spin coated with a negative photoresist (OHKA PS4, Tokyo OHKA Kogyo Co., Japan). The ARC has the adequate refractive index to suppress 325-nm reflections from the substrate. The SiO2 layer acts as a mask and improves the pattern transfer from the photoresist to the ARC. Subsequently, a stripe pattern was transferred to the photoresist by LInL. The samples were then introduced in an ICP reactive ion etcher (PlasmaLab80Plus-Oxford Instruments, Oxfordshire, UK) to transfer the pattern to the HfO2 layer through a series of successive etching steps aimed to selectively remove the exposed areas of SiO2, ARC, and HfO2. An initial CF4 plasma-etching step was used to transfer the pattern from the resist to the SiO2 layer. This was followed by O2 plasma etching to transfer the pattern from the SiO2 to the ARC. During this step, the resist layer is completely eliminated. Finally, the HfO2 was patterned in a CF4 plasma using a radio-frequency power of 100 W. The nanostructured HfO2/GaAs samples were then exposed to a second treatment with O2 plasma to eliminate all organic residues from the surface. Finally, a dip in a 1:1 HCl/H2O solution followed by a D.I. H2O rinse was applied to clean the exposed GaAs bottom trenches.
The surface morphology of the patterned HfO2/GaAs samples was examined with an AFM microscope (5500 Agilent, Santa Clara, CA, USA) working in the dynamic mode. Si cantilevers (Veeco, Plainview, NY, USA) with a nominal radius of 10 nm were used. An SEM microscope (FEI NovaNanoSEM 230, FEI Co., Hilsboro, OR, USA) was used for HR-SEM sample examination. Cross-sectional specimens suitable for HR-TEM were prepared using a focused ion beam (FIB) FEI Quanta FEG dual-beam system (FEI Co.). In order to protect the surface of interest from milling by the Ga+ ion beam during sample preparation, a Pt layer was deposited in the FIB on the HfO2/GaAs nanopatterns. This common procedure is accomplished by introducing an organometallic gas in the vacuum chamber, where it decomposes on the sample surface upon interaction with the ion beam. HR-TEM/EDS compositional maps were acquired using a Philips Tecnai 20 FEG TEM (FEI Co.) operating at 200 keV.
Results and discussion
The main characteristics of the nanostructuring process were investigated by a combination of AFM, HR-SEM, HR-TEM, and EDS. In particular, we studied the resolution and anisotropy of the HfO2-etched nanostructures as well as the roughness and compositional integrity of the underlying GaAs surface.
We have demonstrated the fabrication of HfO2/GaAs patterns with nanoscale resolution using He-Cd laser interference lithography and dry etching using a combination of CF4 and O2 plasmas. The etched GaAs trenches formed by this process were found to be residue-free and atomically smooth after plasma etching. Strong sidewall passivation during HfO2 selective etching and wet cleaning with an HCl/H2O solution results in the formation of tapered HfO2 etch profiles. Optimisation of the CF4 plasma composition and etch bias power to lessen the re-deposition of non-volatile by-products, in combination with the use of more benign cleaning solutions than HCl/H2O, are some of the future improvements to be introduced in the current process to reach the approximately 30 nm HfO2 gate lengths and CD control better than 2 nm required for the fabrication of III-V-based CMOS.
This work was funded by MICINN (Spain) under projects TEC2007-66955 and FIS2009-12964-C05-04, by Comunidad de Madrid under projects S2009/MAT1585 (Estrumat) and S2009/PPQ-1642, (AVANSENS), and by the EU FP7 MAT ERA-Net "ENGAGE" project, with local support provided by Enterprise Ireland and Fundación Madrid. The use of LInL at FideNa (Pamplona, Spain), the FIB system at CEIT (San Sebastian, Spain), and TEM at Universidad Carlos III (Madrid, Spain) is gratefully acknowledged.
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