Deposited AlN Films Research Articles

Highly textured growth of AlN films on sapphire by magnetron sputtering for high temperature surface acoustic wave applications

Piezoelectric aluminum nitride films were deposited onto 3 in. [0001] sapphire substrates by reactive magnetron sputtering to explore the possibility of making highly (002)-textured AlN films to be used in surface acoustic wave (SAW) devices for high temperature applications. The synthesized films, typically 1 μm thick, exhibited a columnar microstructure and a high c-axis texture. The relationship between the microstructures and process conditions was examined by x-ray diffraction (XRD), transmission electron microscopy, and atomic force microscopy analyses. The authors found that highly (002)-textured AlN films with a full width at half maximum of the rocking curve of less than 0.3° can be achieved under high nitrogen concentration and moderate growth temperature, i.e., 250 °C. The phi-scan XRD reveals the high in-plane texture of deposited AlN films. The SAW devices, based on the optimized AlN films on sapphire substrate, were characterized before and after an air annealing process at 800 °C for 90 min. The frequency response, recorded after the annealing process, confirmed that the thin films were still strong in a high temperature environment and that they had retained their piezoelectric properties.

Sputtering of (001)AlN thin films: Control of polarity by a seed layer

The authors report on the ability to control the polarity of sputter deposited AlN(001) thin films using seed layers. Reactive sputter deposition leads to N-polarity on any substrate hitherto applied, i.e., Si(111), sapphire, SiO2, and polycrystalline metals such as Pt(111), Mo(110), and W(110). A site-controlled polarity allows for an efficient excitation of shear modes of surface, bulk, and Lamb waves by interdigitated electrodes. The authors were able to introduce the Al-polarity through a metal-organic chemical-vapor deposition seed layer. By subsequently patterning the substrate surface, it was possible to define the desired film polarity of sputter deposited AlN film. Polarities were determined by selective etching with KOH solutions and by piezoresponse force microscopy.

Detection of carcinoembryonic antigen using AlN FBAR

In this study, we investigated AlN-based film bulk acoustic resonator (FBAR) for the detection of carcinoembryonic antigen(CEA). CEA is a glycoprotein most often associated with breast, colorectal and lung cancer and used to monitor patients with this type of cancer. AlN thin film was deposited on Mo (200 nm)/Ti (10 nm)/Si 3N 4 (300 nm)/Si (300 µm)/Si 3N 4 (300 nm) substrate by reactive RF magnetron sputtering method. Deposited AlN films showed strongly c-axis preferred orientation with main (002) peak as well as the high intensity of 2.50° for FWHM in rocking curve. The FBAR has a resonant frequency of 2.477 GHz and a sensitivity of 3514 Hz cm 2/ng. To investigate optimal condition of self-assembled monolayer (SAM) and anti-CEA concentration for detecting CEA, protein A and protein G as SAM and anti-CEA concentration from 0.2 to 1.0 mg/ml were adopted.

Microstructure and chemical wet etching characteristics of AlN films deposited by ac reactive magnetron sputtering

The influence of the surface morphology of a molybdenum underlayer on the crystallinity and etchability of reactively sputtered c-axis oriented aluminum nitride thin films was investigated. Atomic force microscopy, scanning electron microscopy, transmission electron microscopy, high resolution x-ray diffraction, and defect selective chemical etching were used to characterize the microstructure of the Mo and AlN films. 1000nm thick films of AlN with a full width at half maximum (FWHM) of the x-ray rocking curve ranging from 1.1° to 1.9° were deposited on 300nm thick Mo underlayers with a FWHM of around 1.5°. The Ar pressure during the Mo deposition had a critical effect on the Mo film surface morphology, affecting the structure of the subsequently deposited AlN films and, hence, their wet etching characteristics. AlN films deposited on Mo sputtered at a relatively high pressure could not be etched completely, while AlN films deposited on low pressure Mo etched more easily. Postdeposition etching of the Mo surface in Ar rf discharge prior to deposition of the AlN film was found to influence the formation of AlN residuals that were difficult to etch. Optimal rf plasma etching conditions were found, which minimized the formation of these residuals.

AlN Epitaxial Film Growth Using MOCVD For a GHz-band FBAR

We present the fabrication of a film-bulk-acoustic resonator (FBAR) with a highly c-axisoriented AlN film on Mo/SiO2/Si (100) by using metal-organic chemical-vapor deposition (MOCVD). The resonant frequency and the anti-resonant frequency of the fabricated resonator were 3.219 GHz and 3.249 GHz, respectively. The quality factor and the effective electromechanical coupling coefficient (keff ) were 24.7 and 2.65%, respectively. The conditions of AlN deposition were a substrate temperature of 950 ◦C, a pressure of 20 Torr, and a V-III ratio of 25000. We successfully grew highly c-axis-oriented AlN film with 4 × 10−5 Ωcm resistivity for the Mo bottom electrode. The full widths at half maximum (FWHM) for the AlN (0002) deposited on Mo/SiO2/Si (100) and Mo/SiO2/Si (111) were 4◦ and 3.8◦, respectively. The FWHM value of the deposited AlN film meets the RF band-pass-filter specification for a GHz-band wireless local area network.

Fabrication of AlN films at low temperature by CS‐MBE technique

AbstractThe low‐temperature deposition of AlN films by compound‐source molecular‐beam epitaxy (CS‐MBE) technique was studied. AlN powder was used as the source and no additional nitrogen source was introduced during the deposition. X‐ray photoelectron spectroscopy (XPS) results show that Al‐N bonds were formed in the deposited films. It was clarified that the nitrogen atoms were supplied to the substrate from the AlN source and that the oxygen concentration in the deposited AlN films depended on the residual oxygen concentration in the AlN powder used as the source. The breakdown electric field of the AlN film was over 4.2 MV/cm, which indicates that the films deposited at a low temperature of less than 300 °C can be applied as the insulator in GaN‐based electroluminescent devices. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Relationships between material properties of piezo-electric thin films and device characteristics of film bulk acoustic resonators

The relationships between the material properties of piezo-electric thin films (such as ZnO and AlN) and the frequency response characteristics of film bulk acoustic resonator (FBAR) devices based on those films are examined. ZnO and AlN films are deposited by RF magnetron plasma reactive sputtering. Issues in film growth are focused on electric bias application, oxygen incorporation, and two-step deposition. The XRD spectra, AFM topographies, and SEM images measured from all the deposited films present the optimal condition for both ZnO and AlN films to have excellent piezo-electric properties. For ZnO films, excessive injection of oxygen deteriorates the crystal quality and increases the surface roughness. Applying negative d.c. bias during deposition contributes to the improvement of crystal quality of ZnO films as well as the enhancement of acoustic coupling capability of ZnO-based FBAR devices. By adopting the two-step method to deposit AlN films, the films reveal the highly (002)-preferred growth nature and the RMS surface roughness values are drastically reduced, which also leads to the remarkable improvement in the acoustic coupling coefficient of AlN-FBAR devices.

LOW TEMPERATURE GROWTH OF SPUTTERED ALN FILMS FOR LAYERED STRUCTURE SAW DEVICES

ABSTRACT AlN films with c-axis oriented perpendicular to the surface were deposited on silicon substrates by reactive RF magnetron sputtering method, at various temperatures (without heating −400°C). The structural, morphological and optical properties of AlN films were investigated by X-ray diffraction, scanning electron microscope, atomic force microscopy and Fourier transform infrared absorbance spectroscopy. It was found that the AlN films showed the same highly (002) preferred orientation with low full with of half maximum of rocking curve, which is about 2° for all the deposited films. The surface roughness of AlN films determined by AFM is less than 1nm for the film grown at low temperature. This result is very important and means that films with good crystalline quality, low surface roughness can be processed at low temperature. Elastic properties of deposited AlN films were evaluated by realization and characterization of AlN/silicon SAW device.

Growth of the AlN nano-pillar crystal films by means of a halide chemical vapor deposition under atmospheric pressure

Preparation of AlN thin film has been examined by halide chemical vapor deposition technique using AlI3 and NH3 as starting materials under atmospheric pressure (AP-HCVD). The structural analysis of the deposited AlN films prepared on a Si(1 0 0) substrate by the AP-HCVD technique were carried out by the X-ray pole figure analysis. They consist of the hexagonal AlN nano-pillar crystals. It was found that the nano-pillar crystals, which have random rotation around the 〈1 0 0〉 axis, were grown at an angle of 80–90° to the substrate.

Growth of the AlN nano-pillar crystal films by means of a halide chemical vapor deposition under atmospheric pressure

Preparation of AlN thin film has been examined by halide chemical vapor deposition technique using AlI 3 and NH 3 as starting materials under atmospheric pressure (AP-HCVD). The structural analysis of the deposited AlN films prepared on a Si(1 0 0) substrate by the AP-HCVD technique were carried out by the X-ray pole figure analysis. They consist of the hexagonal AlN nano-pillar crystals. It was found that the nano-pillar crystals, which have random rotation around the 〈1 0 0〉 axis, were grown at an angle of 80–90° to the substrate.

Atomic Layer Deposition of Aluminum Nitride Thin films from Trimethyl Aluminum (TMA) and Ammonia

AbstractAluminum nitride (AlN) thin films were deposited from trimethyl aluminum (TMA) and Ammonia (NH3) by thermal atomic layer deposition (thermal ALD) and plasma enhanced atomic layer deposition (PEALD) on 200 mm silicon wafers. For both thermal ALD and PEALD, the deposition rate increased significantly with the deposition temperature. The deposition rate did not fully saturate even with 10 seconds of NH3 pulse time. Plasma significantly increased the deposition rate of AlN films. A large number of incubation cycles were needed to deposit AlN films on Si wafers. 100% step coverage was achieved on trenches with aspect ratio of 35:1 at 100 nm feature size by thermal ALD. X-ray diffraction (XRD) data showed that the AlN films deposited from 370 °C to 470 °C were polycrystalline. Glancing angle X-ray reflection (XRR) results showed that the RMS roughness of the films increased as the film thickness increased.

Effects of bottom electrodes on the orientation of AlN films and the frequency responses of resonators in AlN-based FBARs

Effects of bottom electrode materials, included in film bulk acoustic resonators (FBARs), on the orientation of piezoelectric AlN thin films and the frequency response characteristics of AlN-resonators are investigated. AlN films are deposited using RF reactive sputtering on various bottom metals, such as Al, Cu, Ti and Mo. Measurements of X-ray diffraction, field-emission scanning electron microscopy and atomic force microscopy show that the AlN film deposited on the Mo electrode exhibits highly desirable properties, namely, a texture coefficient value of ∼93%, crystallite size of ∼40 nm and surface roughness of ∼8.5 nm. The AlN film deposited on the Mo electrode reveals a relatively dense and well-textured columnar structure with fairly uniform grains, while the films deposited on the other electrode metals exhibit a granular type of structure with mixed small and large grains. FBAR devices employing an Al (top)/AlN/metal (bottom)/Si configuration were also fabricated and their frequency response characteristics ( S 11) were measured. The resonator using the Mo electrode was found to have a superior performance (average return loss ⩾10 dB at 3 GHz). It is concluded that the positive role of the Mo electrode in achieving the high-quality AlN films and the high-performance FBAR devices may be attributed to the smaller lattice mismatch as well as the similarity of the thermal expansion coefficient between the deposited AlN film and the Mo electrode substrate.

Effects of bottom electrodes on the orientation of AlN films and the frequency responses of resonators in AlN-based FBARs

Effects of bottom electrode materials, included in film bulk acoustic resonators (FBARs), on the orientation of piezoelectric AlN thin films and the frequency response characteristics of AlN-resonators are investigated. AlN films are deposited using RF reactive sputtering on various bottom metals, such as Al, Cu, Ti and Mo. Measurements of X-ray diffraction, field-emission scanning electron microscopy and atomic force microscopy show that the AlN film deposited on the Mo electrode exhibits highly desirable properties, namely, a texture coefficient value of ∼93%, crystallite size of ∼40 nm and surface roughness of ∼8.5 nm. The AlN film deposited on the Mo electrode reveals a relatively dense and well-textured columnar structure with fairly uniform grains, while the films deposited on the other electrode metals exhibit a granular type of structure with mixed small and large grains. FBAR devices employing an Al (top)/AlN/metal (bottom)/Si configuration were also fabricated and their frequency response characteristics (S11) were measured. The resonator using the Mo electrode was found to have a superior performance (average return loss ⩾10 dB at 3 GHz). It is concluded that the positive role of the Mo electrode in achieving the high-quality AlN films and the high-performance FBAR devices may be attributed to the smaller lattice mismatch as well as the similarity of the thermal expansion coefficient between the deposited AlN film and the Mo electrode substrate.

Crystalline Structure and Surface Morphology of AlN Films Sputtered on Rotated Y-Cut Quartz (ST-Quartz)

Highly c-axis-oriented and fine structural AlN films were successfully prepared on Rotated Y-cut q uartz (ST-quartz) by rf magnetron sputtering. The crystalline structure of the films was determined by X-ray diffraction (XRD) and the surface microstructure of films was quantitatively investigated using an atomic force microscope (AFM). Different rf powers (250 W, 350 W and 450 W) were used to deposit the films. The films prepared in the rf power range of 250–450 W were all polycrystalline hexagonal AlN and had fine surface morphology. The experimental results demonstrate that AlN films deposited in this research were fine structural piezoelectric films. The optimal AlN films prepared at 250 W had small average grain sizes (13.31 nm), uniformity, dense structures and a very smooth top surface (Ra=1.838 nm). It is suitable to use low power to deposit AlN films on ST-quartz.

Pulsed laser ablation of aluminum in the presence of nitrogen: Formation of aluminum nitride

We report on the pulsed laser ablation of aluminum in the presence of nitrogen gas using a 1.06 μm wavelength of Nd:YAG laser. A prominent band of aluminum nitride corresponding to the (0-0) band of the system belonging to a π3−π3 transition was observed at 507.8 nm. An attempt is made to identify the ionized states of aluminum and nitrogen contributing to formation of the AlN band. AlN films were deposited at room temperature and characterized using x-ray diffraction. A direct correlation between the laser ablated aluminum plasma and the deposited AlN film is reported.

Extensional piezoelectric coefficients of gallium nitride and aluminum nitride

Measurements of piezoelectric coefficients d33 and d31 in wurtzite GaN and AlN using an interferometric technique are presented. We report on the clamped values, d33c of these coefficients found in GaN and AlN thin films, and we derive the respective bulk values, d33b. The clamped value of d33c in GaN single crystal films is 2.8±0.1 pm V−1 which is 30% higher than in polycrystalline films grown by laser assisted chemical vapor deposition. The value of d33b in bulk single crystal GaN is found to be 3.7±0.1 pm V−1. The value of d33c in plasma assisted and laser assisted chemical vapor deposited AlN films was 3.2±0.3 and 4.0±0.1 pm V−1, respectively. The bulk value estimate of d33b in AlN of 5.6±0.2 pm V−1 was deduced. The values of d31, both clamped and bulk, were calculated for wurtzite GaN and AlN. We have also calculated the values of d14 in cubic phase film and bulk GaN and AlN. Interferometric measurements of the inverse piezoelectric effect provide a simple method of identifying the positive direction of the c axis, which was found to be pointing away from the substrate for all films.

Effect of lead resistance on spin-dependent tunneling junction with ion-beam deposited AlN barrier

Directly ion-beam deposited AlN films were used as insulating barriers for spin-dependent tunneling junctions, and the effect of the lead resistance on the measured Δ R/ R was investigated. `Bow-tie’ junctions with AlN thicker than 15 Å showed non-linear I– V characteristics with Δ R/ R of 1.4%. Junctions with AlN films thinner than 12 Å showed small `negative’ resistance with Δ R/ R of −14%. The finite-element analysis was used to simulate the current flow and the measured resistance of the `bow-tie’ structure with different resistance ratios of the tunneling junction to the lead. The result of simulation shows when the ratio is smaller than 0.077, the measured resistance becomes negative, and Δ R/ R is significantly enhanced by the lead effect.

Investigation of d.c.-reactive magnetron-sputtered AlN thin films by electron microprobe analysis, X-ray photoelectron spectroscopy and polarised infra-red reflection

Thin films of AlN were deposited at a total gas pressure of 7×10 −4 mbar and a nitrogen partial presure of 9×10 −5 mbar by reactive d.c. magnetron sputtering. Low-carbon steel 08KP, Si (100) wafers and monocrystal KCl were used as substrates. Electron microprobe analysis (EMA), X-ray photoelectron spectroscopy (XPS), polarised infra-red reflection and infra-red (PIRR) transmission measurements were carried out to characterise these films. The EMA analysis confirms the microcrystalline structure of our films. From an XPS spectrum of the deposited AlN films, we have demonstrated that AlN was formed at these values of total and partial pressures. The PIRR measurements indicate the presence of a phonon at about 680 cm −1, typical for AlN. The deposited films have a high transmittance (about 80%) in the wavelength region between 2.5 and 12 μm.

Study of reactive DC magnetron sputtering deposition of AlN thin films

AlN thin films, 326–366 nm thick, were deposited by reactive d.c. magnetron sputtering on glass and monocrystal KCl substrates. The deposition parameters (nitrogen partial pressure, total sputtering pressure, discharge voltage and deposition time) were correlated and their influences on the structural and optical properties of the films were determined by means of X-ray diffraction, scanning electron microscopy and spectrophotometry. The experimental results confirmed the polycrystalline hexagonal wurtzite structure of AlN thin films. The c-axis orientation increase with decreasing of N 2 concentration. The deposited AlN films at lower N 2 partial pressure possess better optical constants—;higher value of refractive index and lower value of coefficient of extinction. All the deposition films are transparent in the visible region with average transmittance about 83%.

Low temperature growth of reactive partially ionized beam deposited AlN films

We have grown aluminum nitride thin films on quartz and Si(111) substrates by reactive partially ionized beam deposition, using Al as evaporation material and N 2 as reactive gas, respectively. We show the results of film characterization by Fourier transform infrared spectroscopy, ultraviolet-visible spectroscopy, X-ray photoelectron spectroscopy and X-ray diffraction, which establish the growth of aluminum nitride thin films at the substrate temperature of 200°C. X-ray diffraction results show that the acceleration voltage is a major parameter influencing the crystal structure of the films.