Session Tu-C4

Materials for MEMS and CMOS

Chair: Asim Ray, Brunel University

Tu-C4.1 16:10–16:30

Deposition and Characterization of AlN Thin Films by r.f. Reactive Magnetron Sputtering

Marcus V. Pelegrini, Maria A. Alvarado, Marco Alayo, and Inés Pereyra

Universidade do Estado de São Paulo, São Paulo, SP, Brazil

Aluminum Nitride films have received increasing attention of the scientific community because they have interesting properties such as a wide bandgap, high electrical resistivity, high resistance to breakdown voltage, high thermal conductivity, chemical stability and others, what make this material very interesting for applications in surface acoustic wave (SAW) devices, insulating layer and MEMS [1,2].

In this work, we report on the deposition of Aluminum Nitride films (AlN) deposited by reactive r.f. (13.56 MHz) sputtering of a pure (99.999%) Aluminum (Al) target in a Nitrogen (N2) and Argon (Ar) mixture atmosphere. In previous work we have obtained AlN films with strong c-axis (002) orientation [2], which is the one with the biggest piezoelectric response. These films presented high compressive residual stress (1 GPa), not allowing the fabrication of structures such as bridges and diaphragms. In this work we deposited a series of samples changing the r.f. power, process pressure and target-substrate distance maintaining fixed the argon/nitrogen ratio in order to study the changes in chemical and physical characteristics, and reduce the residual stress of the films. These characteristics are measured by FTIR, RBS, X-Ray diffraction and rocking curve for the different films deposited. With the knowledge of these properties we believe that we will be able to produce MEMS using Aluminum Nitride films.

[1] J. Hwang, W. Schaff, B. Green, H. Cha, and L. Eastman, "Effects of a molecular beam epitaxy grown AlN passivation layer on AlGaN/GaN heterojunction field effect transistors", Solid-State Electron. 48, 363 (2004)

[2] M. V. Pelegrini and I. Pereyra, "Characterization of AlN films deposited by r.f. reactive sputtering aiming MEMS applications", Phys. Status Solidi C 3, 840 (2010)

Keywords: aluminum nitride, MEMS, piezoelectricity

Tu-C4.2 16:30–16:50

AlN Pedestal-type Optical Waveguide: Fabrication and Characterization

Maria A. Alvarado, Marcus V. Pelegrini, Inés Pereyra, and Marco I. Alayo

Universidade de São Paulo, São Paulo, SP, Brazil

Aluminum nitride is a III–V family compound that has gained ground in the semiconductor industry because of its remarkable electrical, mechanical, piezoelectric [1] and optical properties. Some of these notable properties are high thermal conductivity, wide bandgap (6.2 eV), high SAW velocity, high temperature stability, high resistivity, birefringence and high refractive index [2,3], which make it suitable for varied applications such as surface acoustic wave (SAW) devices, MEMS filters, and optical waveguides [4].

In this paper we present the fabrication and characterization of pedestal-type optical waveguides [5] using AlN as the core layer. AlN thin films were first optical and physically characterized before the optical waveguide fabrication. The thin films were obtained by reactive RF magnetron sputtering using a 99.999% purity aluminum (Al) target, and Argon (Ar) and nitrogen (N2) were used as sputtering and reactive gas. A related work has found the best parameters to achieve the minimal intrinsic stress, which were used in the present work. However, this paper considered the target to substrate distance as a variable parameter (70 and 53 mm) in order to study the effect of ion bombardment on the optical characteristics of the optical device [6]. Thin films were examined by X-ray diffraction (XRD) measurements to determine their crystalline structure and the refractive index was determined by the ellipsometry technique. Additional characterization techniques such as FTIR spectroscopy, rocking curve, and residual stress calculated by Stoney's equation were also used to characterize the samples.

For the optical waveguide fabrication, the pedestal technique was used. This technique consists in etching the silicon oxide lower cladding before depositing the core layer. Thus, the geometrical definition of the optical waveguide is simplified because it is no longer necessary to perform an etching of the AlN film. In this manner aluminum nitride films with thickness of 1 and 0.5 μm were deposited on silicon dioxide thermally grown on a silicon (100) substrate. The pedestal profile, defined on SiO2 film, was obtained using conventional photolithography procedures, followed by plasma etching using CHF3 and O2 as reactive gases. Optical propagation losses are presented for different pedestal heights (1 and 0.5 μm) and widths (1–10 μm).

[1] K. Tonisch, V. Cimalla, Ch. Foerster, H. Romanus, O. Ambacher, and D. Dontsov, "Piezoelectric properties of polycrystalline AlN thin films for MEMS application", Sensors and Actuators A: Physical, Volume 132, Issue 2 (2006)

[2] Kuan-Hsun Chiu, Jiann-Heng Chen, Hong-Ren Chen, and Ruey-Shing Huang, "Deposition and characterization of reactive magnetron sputtered aluminum nitride thin films for film bulk acoustic wave resonator", Thin Solid Films, Volume 515, Issue 11, Pages 4819–4825 (2007)

[3] M. Garcia-Mendez, S. Morales-Rodriguez, R. Machorro, and W. De La Cruz, "Characterization of AlN thin films deposited by DC reactive magnetron sputtering", Revista Mexicana De Fisica 54(4), Pages 271–278 (2008)

[4] Siddhartha Ghosh and Gianluca Piazza, "Photonic microdisk resonators in aluminum nitride", J. Appl. Phys. 113, 016101 (2013)

[5] Daniel O. Carvalho and Marco I. Alayo, "Pedestal anti-resonant reflecting optical waveguides", Proc. SPIE 7940, Oxide-based Materials and Devices II, 794017 (2011)

[6] Jyoti Prakash Kar and Gouranga Bose, "Aluminum Nitride (AlN) Film Based Acoustic Devices: Material Synthesis and Device Fabrication, Acoustic Waves - From Microdevices to Helioseismology", Prof. Marco G. Beghi (Ed.) (2011)

Keywords: optical waveguide, aluminum nitride, piezoelectricity

Tu-C4.3 16:50–17:10

(PE)ALD TaCN Film Nucleation and Growth on TiNx: Influence of Nitrogen Content

F. Piallat (1,2,3), R. Gassilloud (2), P. Caubet (1), B. Pélissier (3), and C. Vallée (3)

1. STMicroelectronics, 850 rue Jean Monnet, 38920 Crolles, France

2. CEA, LETI, CAMPUS Minatec, 17 rue des Martyrs, F-38054 Grenoble Cedex 9, France

3. LTM/CNRS/UJF/CEA-Leti, 17 rue des Martyrs, F-38054 Grenoble Cedex 9, France

Tantalum alloys are currently investigated as possible gate electrode materials on high-k dielectrics for advanced CMOS nodes. Alternatively, due to its high affinity with oxygen, leading to a possible reduction of other materials, TaCN has been proposed as a capping layer on TiN deposited on HfO2, especially when process air breaks are involved. It has been shown in a previous work [1], that TiN gets highly oxidized and that TaCN deposition leads to a reduction of this oxygen, with an increase of N content in TiN layer. TaCN deposition parameters (temperature, reactant gas, plasma gas, plasma) were investigated to highlight the influence of each on the reduction of oxygen in TiN. It appeared that only H2 plasma (PEALD deposition) and TaCN precursor (both PEALD and ALD deposition) are at the origin of modifications in Ti chemical bonding. Thanks to the oxygen migration further away from HfO2 and SiO2 materials, the regrowth of interfacial layer (IL) SiO2 is controlled, leading to low Equivalent Oxide Thickness (EOT) and good current leakage gain at low thermal budget.

To further apprehends these interactions, in particular at TiN / TaCN interface, a study of TaCN / TiN / HfO2 / SiO2 / Si stack, with (PE)ALD TaCN deposited on TiNx, x ∈ [0.5 ; 1.5], was done by X-Ray Reflectivity (XRR), X-Ray Diffraction (XRD) and X-Ray Photoelectron Spectroscopy (XPS) analysis. Two main issues were investigated, first the impact of TiN stoichiometry and second the importance of anneal, with a budget corresponding to gate last integration scheme.

XRR measurements revealed clear alteration of TiN after TaCN deposition, with increase of density and reduction of thickness. XRR thickness measurements indicate that TiN stoichiometry has an influence on the deposition rate of TaCN, thus influence the nucleation of TaCN. Furthermore, XPS analysis confirmed the decrease of O content in TiN, after TaCN deposition, no matter if plasma was used for the deposition of TaCN neither the N content of TiN. Other interactions, as N increase in TiN layer, N migration in HfO2 and possible Ta-Ti bonding are also observed by XPS.

Low budget annealing (420°C) led to several modifications of the stack. First, there is an evolution of the Si2p XPS spectra exhibiting a regrowth of the SiO2 IL, regrowth which is influenced by the amount of N present in the TiN layer. Then, unexpectedly no Si-N formation was observed after anneal, which suppose that N is not migrating down to the SiO2 layer. On the other hand, HfO2 chemical environments show important changes, with the creation of new O-Hf-N or Hf-N bonds. Thus, low budget anneal is sufficient to activate the migration of N from TiN to HfO2.

Finally, a mechanism of interaction between the two materials is proposed to explain the observed phenomena happening after TaCN deposition and supposition on the migration mechanisms at low budget anneal are proposed for N.

[1] F. Piallat, et al., MRS Spring 2013 San Francisco (California, USA)

Keywords: metal, interfacial reactions, deposition mechanism