AVS2011 Session TF1-ThM: Post-Deposition Processing and Characterization of Thin Films

The processes controlling shape transformations during post-deposition treatments attract great interest due to their technological implications. As an example, residual stress relaxation during thermal annealing could influence component lifetime and performance in applications ranging microelectronics to mechanical coatings, affecting their functional properties [1]. In this work, we investigate the morphology evolution during thermal annealing at 0.2 T_melting in polycrystalline Au(111) films using atomic force [scanning electron] (AFM [SEM]) microscopies and x-ray diffraction in order to follow both shape and structural transformations of surface features for different annealing times. Before annealing, a high density of round-like surface grains ―which are surface components of an underlying competitive columnar microstructure―, is observed. During annealing, surface grains merge together into “multigrain” structures that expand laterally up to a saturation size, suggesting a size-dependent phenomenon controlling surface recrystallization. By comparison with Au polycrystalline growth fronts for the same temperature range [2] (T zone of the zone models [1]), we can relate the annealing–generated multigrain structures to the incipient formation of extended sub-micrometric-sized plates that are observed for thinner films. They contain many small surface grains with low-angle or no inner grain boundaries [2]. The small amount of material to be recrystallized for each component favors the multi-component extension of recrystallized zone. Interestingly, results here reported concerning the recrystallization phenomenon are discussed on the basis of local interactions between surface grains triggering plausible elastic/plastic mechanisms of stress accommodation (grain zipping and shear strain) and relaxation by surface diffusion processes [1,3], which in principle seems not be very different from those expected in the coalescence stage at the T zone.

[1] L. B. Freund and S. Suresh, Thin Film Materials: Stress, Defect Formation and Surface Evolution (Cambridge University Press, Cambridge, England, 2003) and references therein.

[2] C. Munuera et al., J. Vac. Sci. Technol. A 22, 1767 (2004)

[3] A. González-González et al., “Morphology evolution of thermally annealed polycrystalline thin films” (submitted)

Acceptor doping of III-Nitrides has been the subject of many studies due to the relativity low hole concentration (~10¹⁷-10¹⁸ cm^-3) material commonly grown in contemporary devices. This limitation is thought to be one of the contributing factors to efficiency droop in LED’s. Recently, extremely high hole concentration (p>4x10¹⁹ cm^-3) material with very high (>50%) ionization efficiency (see Fig. 1) has been demonstrated by metal modulated epitaxy (MME), an application of MBE where surface chemistry is controlled via shuttering of the source material [J. Appl. Phys. 106, 014905]. The samples in this study are characterized by temperature dependent Hall measurement to elucidate the reduction in the acceptor activation energy. Electron paramagnetic resonance spectroscopy (EPR) was used to show the microscopic view of the Mg acceptor without influence of the surrounding crystal as well as the effect of annealing temperature on the EPR signal. It is found that the primary transport mechanism is likely due to impurity band conduction, consistent with a distributed acceptor band as opposed to an isolated acceptor energy level as is generally observed in p-type GaN. Likewise, the interaction of the Mg with hydrogen shows different annealing behavior when compared to MOCVD p-type GaN.

The samples analyzed by EPR were annealed at various temperatures in forming gas (H₂:N₂). Fig. 2 shows the relative intensity from the EPR signal of the neutral Mg acceptor as a function of forming gas anneal temperature. It is clear that there is a sharp decrease in signal for the MME grown samples (doped >10²⁰ Mg as seen in fig. 1) at 525 °C as compared to the drop observed at 750 °C for the MOCVD grown samples (doped ~10¹⁹ Mg). The exact nature of this temperature dependent decrease in signal is not fully understood, however it is suggested the different growth kinetics and resulting surface leads to a change in H₂ transport. A second mechanism may be the formation of Mg-H complexes with varying activation energies.

Preliminary temperature dependent Hall results (Fig. 3) show an as-grown activation energy of 70 meV in the MME samples as well as 5x10¹⁸ cm^-3 holes remaining at cryogenic temperatures. This extremely low activation energy and lack of carrier freeze out is evidence of impurity band conduction and shows the degenerative nature of the material. The full effect of the annealing temperature on this degenerative material will be presented in further detail as it pertains to device processing.

Thermal annealing is a critical but often neglected step in semiconductor production. It has been routinely used to activate the ion implanted dopant and at the same time, to restore the single crystal structure. However, it is also a powerful tool in repairing damages in bulk films as well as at film-film interfaces, which are caused by the plasma-related deposition or etching process. In addition, the thermal annealing step can change film characteristics, which affects the subsequent process result. In this talk, the author will review some of his recent work related to thermal annealing, which involves three types of solid-state devices and thin films. First, an example on repairing the RIE damaged a-Si:H TFT with a low temperature thermal annealing process will be given. This step reduced defects in the bulk a-Si:H and SiNx layers as well as at the gate-dielectric interface. This result has been interpreted in all TFT LCD productions around the world. In the second example, it will be shown that the thermal annealing step changed the grain size of the sputter deposited copper thin film. It resulted in the change of the copper-Cl reaction mechanism and therefore, the copper consumption rate. This is critical to a new plasma-based copper etch process that has been interpreted in large-area TFT LCDs and BiCMOS chips. In the third example, it will be shown that the post deposition annealing (PDA) condition is the key to the realization of a new type of nonvolatile memory device, i.e., the nanocrystals embedded high-k capacitors or MOSFETs. Furthermore, an addition low temperature annealing step removed the defects generated in the gate and back-contact sputtering processes. In summary, the thermal annealing effects are non-negligible because they are directly involved in product reliability and yield.

(1) Y. Kuo, “TFT and ULSIC – Competition or Collaboration,” Jpn. J. Appl. Phys., 47(3), 1845-1852 (2007).

(2) G. Liu, Y. Kuo, S. Ahmed, D. N. Buckley, and T. Tanaka-Ahmed, “Grain Size Effect on Plasma-based Copper Etch Process,” J. Electrochem. Soc., 155(6) H432-H437 (2008).

(3) Y. Kuo, “Status Review of Nanocrystals Embedded High-K Nonvolatile Memories,” ECS Trans. 35(3), 13-31(2011).

Here we discuss the use of coherent acoustic phonon spectroscopy as a noninvasive and nondestructive measurement tool for semiconductor thin films. Specifically we emphasize its usefulness in studying the optoelectronic structure of materials with various types of structural defects. We discuss in-depth studies of ion-implanted GaAs, demonstrating that the technique may be used to quantitatively determine depth profiles of lattice disorder. Our optoacoustic measurements are shown to be 2-3 orders of magnitude more sensitive in defect concentration than channeling techniques, and establish a quantitative dependence between the change in optical response and defect concentration between 10¹⁸-10²¹ defects/cm³. Further, we demonstrate the entire range over which the coherent acoustic phonon technique is applicable in defect studies, and show results ranging from no noticeable change in optical response to complete damping of the phonon wave. We also discuss the electronic nature of the CAP response, which can provide insight into the interplay between lattice disorder and electronic structure.

ZrO₂ is a material used as thin films in numerous applications. One of the applications with the highest added value is its use as ionic conductor in, for example, fuel cell devices. Nevertheless, only the tetragonal and cubic polymorphs of ZrO₂ exhibit these ionic conduction properties. Therefore, effort have to be made in order to synthesis phase pure tetragonal or, ideally, cubic ZrO₂ thin films. It has been demonstrated that these polymorphs are generated when about 10% of oxygen vacancies are introduced in the material lattice. Recently, some works have demonstrated that it was possible to synthesize, by reactive high power impulse magnetron sputtering, cubic HfO₂ (a compound very close to ZrO₂) containing, as for cubic ZrO₂, 5% of oxygen vacancies when working in the transition region of the poising curve. This implementation of this strategy was possible thanks to the smoothness of this transition using this peculiar sputtering technology.

In this work, we aim to apply such a strategy to grow tetragonal (or cubic) ZrO₂ by reactive DC magnetron sputtering. Due to the very sharp metallic-to-poisoned mode transition, we used a plasma monitoring system (PEM) to work in stable conditions in that region.

A pure Zr metallic target is sputtered at constant current (0.2 A) at 10 mTorr in Ar/O₂ mixtures using a DC magnetron sputtering source with an unbalanced magnetic field configuration. A PEM system is implemented in order to grow films in the transition region. The Zr line at 340 nm is monitored in real time. The deposited films were characterized by grazing angle X-Ray diffraction (GAXRD) and X-Ray photoelectron spectroscopy (XPS).

Discharge parameters (voltage, current) and XPS data reveal that the transition occurs for 4% < %O₂ < 6%. For %O₂ < 4%, the films are under-stoichiometric with a metallic character. In the transition, we measure a stoichiometry of about ZrO_1.8. Finally, %O₂ > 6%, the films are stoichiometric. In term of phase constitution, it has been demonstrated that working in the poisoned mode, the monoclinic phase is synthesized while in the transition, for which the compound is under-stoichiometric, the tetragonal (and maybe cubic) phase is grown. These data support, for the first time, the theoretical assumption about the oxygen vacancies-generated tetragonal (or cubic) phase of ZrO₂. Finally, the thermal stability of the coatings has been studied: the as-grown samples have been annealed up to 1200°C. For annealing temperature up to 600°C, no modification of the phase constitution is observed while for an annealing temperature of 1200°C, the films experience a phase transition towards the monoclinic structure.

The unique physical structure of boron-rich carbides, based on an extended molecular network of covalently bound icosahedral cages, has distinguished this material with an exceptional set of thermal, electrical, and mechanical properties. Technologically, boron carbide has generated interest for applications in solid-state neutron detectors, interlayer low-k dielectrics for ultra-large-scale integrated circuits, and high-temperature thermoelectric power convertors. A method that has proven amenable to thin-film heterostructure device fabrication is the plasma-enhanced chemical vapor deposition (PECVD) of high-resistivity amorphous hydrogenated boron carbide (a-B_xC:H_y; x ≈ 2–5) from the single-source precursor orthocarborane (C₂B₁₀H₁₂). However, although the physical structure of bulk crystalline boron carbide (e.g., B_4.3C) is nowadays well-understood, the short-range physical structure of the hydrogenated material (e.g., the number and types of atoms/bonds) has remained unsatisfactorily characterized, likewise for the intermediate-range physical structure of the amorphous lattice (e.g., how molecular subunits are bound together and arranged on a short sub-nm length scale)—structural modifications which have important consequences on the properties of the a-B_xC:H_y films. Herein, we investigate the short- and intermediate-range physical structure of a-B_xC:H_y films using solid-state magic angle spinning nuclear magnetic resonance (MAS-NMR) and Fourier transform infrared (FTIR) spectroscopies, backed by density functional theory (DFT) molecular structure calculations. The comparison of experimentally observed spectral features with theoretical predictions for model molecular compounds provides valuable insight into the different local chemical environments and intermediate-range networks that make up the a-B_xC:H_y films. We demonstrate how applying these combined analyses provides an important stepping stone to understanding and optimizing the chemical, electrical, and mechanical properties of a-B_xC:H_y films for next-generation device fabrication.

Diffusion barrier layers with a uniform thickness and good step coverage in narrow lines are needed to enable the continuing scaling of Cu interconnects. Since physical vapor deposition (PVD) processes have limitations in respect of conformality, atomic layer deposition (ALD) and chemical vapor deposition (CVD) might become the preferred alternatives. Among others, Mn-based CVD barriers have been proposed [1], but up to now, Cu barrier properties for such films have not been proven electrically.

In this study, MnOx layers were deposited by CVD, on top of an O3/TEOS SiO2 layer. Deposition was done at two different temperatures: 200 °C and 350 °C. The effect of a post-plating anneal of one hour at 300 °C or 430 °C was studied. A test structure based on planar capacitors was used, in which first wide areas were patterned, followed by the deposition of the oxide and CVD MnOx layers, metallization, and CMP, respectively [2]. After passivation, voltage ramp (at 25 °C and 100 °C) and TDDB measurements (at 100 °C) were conducted. The electrical data were compared with those from a known good TaN/Ta-barrier reference system.

Voltage ramp measurements at 25 °C show a similar behaviour for all Mn-based films under study, except for the one deposited at 350 °C, with a post-plating anneal at 430 °C. For the latter a leakage current of about 1 order of magnitude lower is found, comparable to the leakage current of our TaN/Ta reference. For the film deposited at 200 °C, and annealed at 430 °C, voltage ramp measurements only showed shorts.

TDDB lifetimes were in all cases higher than for a reference without barrier, but lower than for a TaN/Ta reference. For the Mn-based films deposited at 350 °C, a post-plating anneal at 430 °C clearly improved the reliability properties. In this case, the extrapolated lifetime at user conditions (using the E-model), is above 10 years. A γ value of -3.4 cm/MV was found. However, for the films deposited at 200 °C, an anneal at 300 °C already degraded the reliability properties.

In conclusion, voltage ramp and TDDB measurements on planar capacitors structures show that, with optimised processing, CVD Mn-based barriers are promising candidates as Cu barriers in advanced interconnects.

[1] K. Neishi, S. Aki, K. Matsumoto, H. Sato, H. Itoh, S. Hosaka, J. Koike, Appl. Phys. Lett. 93 (2008) 032106

[2] L. Zhao, Zs. Tökei, G. Giai Gischia, M. Pantouvaki, K. Croes, G. Beyer, IEEE International Reliability Physics Symposium, 2009, 848-850