Fabrication of micro heaters on polycrystalline 3C-SiC suspended membranes for gas sensors and their characteristics

This paper describes the design, fabrication, and characteristics of micro heaters mad on AlN (0.1 μm)/3C-SiC (1 μm) suspended membranes using surface micromachining technology. 3C-SiC and AlN thin films, which have a large energy band gap and very low lattice mismatch, were used for high-temperature environments. A Pt thin film was used as micro heaters and temperature sensor materials. The resistance of the temperature detector (RTD) and the power consumption of the micro heater were measured and calculated. The heater is designed for an operating temperature up to about 800 °C and can be operated at about 500 °C with a power of 312 mW. The thermal coefficient of the resistance (TCR) of fabricated Pt RTD’s is 3174.64 ppm/°C. The thermal distribution measured by IR thermovision is uniform across the membrane surface.     

The microwave properties of Li doped 0.7(Ba,Sr)TiO3-0.3MgO thick film interdigital capacitors on the alumina substrates

The crystalline and electrical properties of Li doped 0.7(Ba,Sr)TiO₃-0.3MgO thick film interdigital capacitors have been investigated. Screen printing method was employed to fabricate Li doped 0.7(Ba,Sr)TiO₃-0.3MgO thick films on the alumina substrates. (Ba,Sr)TiO₃ materials have high dielectric permittivity (>500 @ 1 MHz) and low loss tangent (0.01 @ 1 MHz) in the epitaxial thin film form. To improve dielectric properties and reduce sintering temperature, MgO and Li were added, respectively. 10 μm thick films were screen printed on the alumina substrates and then interdigital capacitors with seven fingers of 200 μm finger gap were patterned with Ag electrode. Current-voltage characteristics were analyzed with elevated temperature range. Up to 50 °C, the thick films showed positive temperature coefficient of resistivity (dρ/dT) of 6.11 × 10⁸ Ω cm/°C, then film showed negative temperature coefficient of resistivity (dρ/dT) of −1.74 × 10⁸ Ω cm/°C. From the microwave measurement, the relative dielectric permittivity of Li doped 0.7(Ba,Sr)TiO₃-0.3MgO thick films interdigital capacitors were between 313 at 1 GHz and 265 at 7 GHz.     

Effect of post annealing on the resistive switching of TiO2 thin film

The effect of annealing on the resistive switching of 35-nm-thick TiO₂ thin film deposited with magnetron sputtering system was studied. Pt and Ag were used as a top electrode (TE), and Pt was used as a bottom electrode (BE). For Pt/as-deposited TiO₂/Pt structure, both unipolar (URS) and bipolar resistive switching (BRS) were observed depending on the current compliance level. For Pt/400 °C annealed TiO₂/Pt structure, only BRS was observed regardless of the current compliance level. The increase in the work function of the TiO₂ film after annealing lowers the potential barrier height and changes the electron transfer process which was also confirmed from Ag/as-deposited TiO₂/Pt structure. Above 600 °C, the film becomes leaky with the increase in grain size and roughness and the resistive switching behavior was not observed.     

Structure and property of magnetron sputtered ternary cobalt-nickel silicide films

Ternary cobalt-nickel silicide films were prepared using magnetron sputtering from an equiatomic cobalt-nickel alloy target on Si substrate. The effect of post-deposition annealing on the phase formation, structural properties and resistivity of the resultant films has been studied. The results of XRD show that the annealing temperature and impurity level of oxygen play a crucial role in controlling the phase transformation of ternary silicide. Silicide phases are absent in the as-deposited film due to the amorphous nature. At relatively low annealing temperature, the phase of CoNi₃Si (2 2 0) and CoNiSi (2 2 0) coexist. With the increase of annealing temperature, the phase of CoNi₃Si (2 2 0) begins to transform into CoNiSi (2 2 0). At high annealing temperature (800 °C), only the phase of CoNiSi₂ (2 2 2) is formed. For Co-Ni silicide film annealed in pure argon gas ambient, two Raman peaks at 1357 cm⁻¹ and 1591 cm⁻¹ are attributed to the vibrational mode of CoSi₂ and NiSi₂ compounds. For ternary silicide annealed in atmosphere ambient, two Raman peaks located at 538 cm⁻¹ and 690 cm⁻¹ were observed and may be related to Si oxide or Co-Ni oxide. The 3D views of AFM images show that the surface roughness is relatively low when the silicidation temperature is smaller than 550 °C. After silicidation in 800 °C, the surface roughness increases abruptly. The resistance initially decreases with the increase of annealing temperature, and achieves minimum value (19 μΩ cm) in temperature ranges 500-550 °C. When the annealing temperature increases from 600 °C to 800 °C, the resistivity was found to increase slightly to 26 μΩ cm. The ternary silicide shows a temperature window for low resistivity as compared to binary NiSi.     

Investigation of RF sputtered PSG films for MEMS and semiconductor devices

In this work, we report the preparation of phospho-silicate-glass (PSG) films using RF magnetron sputtering process and its application as a sacrificial layer in surface micromachining technology. For this purpose, a 76 mm diameter target of phosphorus-doped silicon dioxide was prepared by conventional solid-state reaction route using P₂O₅ and SiO₂ powders. The PSG films were deposited in a RF (13.56 MHz) magnetron sputtering system at 200-300 W RF power, 10-20 mTorr pressure and 45 mm target-to-substrate spacing without external substrate heating. To confirm the presence of phosphorus in the deposited films, hot-probe test and sheet resistance measurements were performed on silicon wafers following deposition of PSG film and a drive-in step. As a final confirmatory test, a p-n diode was fabricated in a p-type Si wafer using the deposited film as a source of phosphorus diffusion. The phosphorus concentration in the target and the deposited film were analyzed using energy dispersive X-rays (EDAX) tool. The etch rate of the PSG film in buffered HF was measured to be about 30 times higher as compared to that of thermally grown SiO₂ films. The application of RF sputtered PSG film as sacrificial layer in surface micromachining technology has been explored. To demonstrate the compatibility with MEMS process, micro-cantilevers and micro-bridges of silicon nitride were fabricated using RF sputtered PSG as a sacrificial layer in surface micromachining. It is envisaged that the lower deposition temperature in RF sputtering (<150 °C) compared to CVD process for PSG film preparation is advantageous, particularly for making MEMS on temperature sensitive substrates.     

Study of ultrathin vanadium nitride as diffusion barrier for copper interconnect

Ultrathin Vanadium nitride (VN) thin film with thickness around 10 nm was studied as diffusion barrier between copper and SiO₂ or Si substrate. The VN film was prepared by reactive ion beam sputtering. X-ray diffraction, Auger electron spectroscopy, scanning electron microscopy and current-voltage (I-V) technique were applied to characterize the diffusion barrier properties for VN in Cu/VN/Si and Cu/VN/SiO₂ structures. The as-deposited VN film was amorphous and could be thermal stable up to 800 °C annealing. Multiple results show that the ultrathin VN film has good diffusion barrier properties for copper.     

Solid phase epitaxial growth of Dy-germanide films on Ge(0 0 1) substrates

Dy thin films are grown on Ge(0 0 1) substrates by molecular beam deposition at room temperature. Subsequently, the Dy film is annealed at different temperatures for the growth of a Dy-germanide film. Structural, morphological and electrical properties of the Dy-germanide film are investigated by in situ reflection high-energy electron diffraction, and ex situ X-ray diffraction, atomic force microscopy and resistivity measurements. Reflection high-energy electron diffraction patterns and X-ray diffraction spectra show that the room temperature growth of the Dy film is disordered and there is a transition at a temperature of 300-330 °C from a disordered to an epitaxial growth of a Dy-germanide film by solid phase epitaxy. The high quality Dy₃Ge₅ film crystalline structure is formed and identified as an orthorhombic phase with smooth surface in the annealing temperature range of 330-550 °C. But at a temperature of 600 °C, the smooth surface of the Dy₃Ge₅ film changes to a rough surface with a lot of pits due to the reactions further.     

Effect of a thin W, Pt, Mo, and Zr interlayer on the thermal stability and electrical characteristics of NiSi

It is reported that the thermal stability of NiSi is improved by employing respectively the addition of a thin interlayer metal (W, Pt, Mo, Zr) within the nickel film. The results show that after rapid thermal annealing (RTA) at temperatures ranging from 650 °C to 800 °C, the sheet resistance of formed ternary silicide Ni(M)Si was less than 3 Ω/□, and its value is also lower than that of pure nickel monosilicide. X-ray diffraction (XRD) and raman spectra results both reveal that only the Ni(M)Si phase exists in these samples, but the high resistance NiSi₂ phase does not. Fabricated Ni(M)Si/Si Schottky barrier devices displayed good I-V electrical characteristics, with the barrier height being located generally between 0.65 eV and 0.71 eV, and the reverse breakdown voltage exceeding to 40 V. It shows that four kinds of Ni(M)Si film can be considered as the satisfactory local connection and contact material.     

Properties of transparent conductive ZnO:Al thin films prepared by magnetron sputtering

Middle-frequency alternative magnetron sputtering was used to deposit transparent conductive ZAO (ZnO:Al) thin films with ZAO (98 wt%ZnO+2 wt%Al₂O₃) ceramic target on glass and Si wafers. The influences of the various deposition parameters on the structural, optical and electrical performances of ZAO films have been studied. The structural characteristics of the films were investigated by the X-ray diffractometer and atomic force microscope, while the visible transmittance, carrier concentration and Hall mobility were studied by UV-VIS and the Hall tester, respectively. The lowest resistivity obtained in the work was 4.6×10⁻⁴ Ω cm for the film with average transmittance of 90.0% within the visible wavelength range and sheet resistance of 32 Ω, which was deposited at 250 °C and 0.8 Pa.     

The effect of Si addition and Ta diffusion barrier on growth and thermal stability of NiSi nanolayer

Formation and thermal stability of nanothickness NiSi layer in Ni(Pt 4 at.%)/Si(1 0 0) and Ni_0.6Si_0.4(Pt 4 at.%)/Si(1 0 0) structures have been investigated using magnetron co-sputtering deposition method. Moreover, to study the effect of Si substrate in formation of NiSi and its thermal stability, we have used Ta diffusion barrier between the Ni_0.6Si_0.4 layer and the Si substrate. Post annealing treatment of the samples was performed in an N₂ environment in a temperature range from 200 to 900 °C for 2 min. The samples were analyzed by four point probe sheet resistance (R_s) measurement, X-ray diffraction (XRD) and atomic force microscopy (AFM) techniques. It was found that the annealing process resulted in an agglomeration of the nanothickness Ni(Pt) layer, and consequently, phase formation of discontinuous NiSi grains at the temperatures greater than 700 °C. Instead, for the Ni_0.6Si_0.4(Pt)/Si structure, 100 °C excess temperature in both NiSi formation and agglomeration indicated that it can be considered as a more thermally stable structure as compared with the Ni(Pt 4 at.%)/Si(1 0 0) structure. XRD, AFM and R_s analyses confirmed formation of a continuous NiSi film with R_s value of 5 Ω/□ in a temperature range of 700−800 °C. Use of Ta diffusion barrier showed that the role of diffusion of Ni atoms into the Si substrate is essential in complete silicidation of a NiSi layer.     

Properties of ZnO/Cu/ZnO multilayer films deposited by simultaneous RF and DC magnetron sputtering at different substrate temperatures

ZnO/Cu/ZnO transparent conductive multilayer films are prepared by simultaneous RF sputtering of ZnO and DC sputtering of Cu. The properties of the multilayer films are studied at different substrate temperatures. Sheet resistance of the multilayer film decreased initially with increase of substrate temperature and increased further with increase of substrate temperature beyond 100 °C. However, transmittance of the multilayer film increased with increase of substrate temperature. Good transparent conductive film of sheet resistance 9.3 Ω/sq and transmittance of 85% was found at a substrate temperature of 100 °C. The performance of the multilayer film was evaluated using a figure of merit. The observed property of the multilayer film is suitable for the application of transparent conductive electrodes.     

Influence of rapid thermal annealing temperature on structure and electrical properties of high permittivity HfTiO thin film used in MOSFET

HfTiO thin films were prepared by r.f. magnetron co-sputtering on Si substrate. To improve the electrical properties, HfTiO thin films were post heated by rapid thermal annealing (RTA) at 400 °C, 500 °C, 600 °C and 700 °C in nitrogen. It was found that the film is amorphous below 700 °C and at 700 °C monoclinic phase HfO₂ has occurred. With the increase of the annealing temperature, the film becomes denser and the refractive index increases. By electrical measurements, we found at 500 °C annealed condition, the film has the best electrical property with the largest dielectric constant of 44.0 and the lowest leakage current of 1.81 × 10⁻⁷ A/cm², which mainly corresponds to the improved microstructure of HfTiO thin film. Using the film annealed at 500 °C as the replacement of SiO₂ dielectric layer in MOSFET, combining with TiAlN metal electrode, a 10 μm gate-length MOSFET fabricated by three-step photolithography processes. From the transfer (I_DS–V_G) and output (I_DS–V_DS) characteristics, it shows a good transistor performance with a threshold voltage (V_th) of 1.6 V, a maximum drain current (I_ds) of 9 × 10⁻⁴ A, and a maximum transconductance (G_m) of 2.2 × 10⁻⁵ S.     

Surface acoustic wave characteristics of AlN thin films grown on a polycrystalline 3C-SiC buffer layer

In this study, AlN thin films were deposited on a polycrystalline (poly) 3C-SiC buffer layer for surface acoustic wave (SAW) applications using a pulsed reactive magnetron sputtering system. AFM, XRD and FT-IR were used to analyze structural properties and the morphology of the AlN/3C-SiC thin film. Suitability of the film in SAW applications was investigated by comparing the SAW characteristics of an interdigital transducer (IDT)/AlN/3C-SiC structure with the IDT/AlN/Si structure at 160 MHz in the temperature range 30-150 °C. These experimental results showed that AlN films on the poly (1 1 1) preferred 3C-SiC have dominant c-axis orientation. Furthermore, the film showed improved temperature stability for the SAW device, TCF = −18 ppm/°C. The change in resonance frequency according to temperature was nearly linear. The insertion loss decrease was about 0.033 dB/°C. However, some defects existed in the film, which caused a slight reduction in SAW velocity.     

A cost-effective flexible MEMS technique for temperature sensing

A simplified, cost-effective flexible micro-electronic-mechanical systems (MEMS) technology has been developed for realizing a temperature-sensing array on a flexible polyimide substrate. The fabrication technique utilized liquid polyimide to form flexible film on the rigid silicon wafer using a temporary carrier during the fabrication. The platinum thin film is employed as temperature sensitive material and 8×8 temperature-sensing arrays were micromachined on the polyimide, from which the silicon wafer carrier was removed at the end of fabrication. The platinum thin film temperature sensor exhibits excellent linearity and its temperature coefficient of resistance reaches 0.00291 °C⁻¹. Because of the effective thermal isolation, the flexible temperature sensors show a high sensitivity of 1.12 Ω/°C at 10 mA to the constant drive current. The flexible MEMS technology based on liquid polyimide enables the development of flexible, compliant, robust, and multi-modal sensor skins for many other important applications, such as robotics, biomedicine, and wearable microsystems.     

Effects of post-thermal treatment on the properties of rf reactive sputtered ITO films

Indium tin oxide (ITO) thin film prepared by rf sputtering at various Ar-O₂ mixtures, were annealed at several temperatures. The electrical, optical and structural properties of the film were systematically investigated before and after post-thermal treatment. The influence of a reactive gas (O₂) on the sputtering rate of a metallic (indium/tin) alloy target was also investigated. The films were characterized by X-ray diffraction (XRD) measurement, scanning electron microscopy, and transmittance as a function of wavelength. The resistivity of 8.3×10⁻⁴ Ω cm has been achieved for the film thickness of 250 nm, deposited in pure Ar at room temperature (RT).     

Characterization of direct patterned Ag circuits for RF application

We investigated the effects of sintering temperature on microstructural evolution and electrical characteristics of screen printed Ag patterns. A commercial conducting paste containing Ag nanoparticles was screen printed onto a Si substrate passivated with SiO₂ and sintered under a sintering temperature range from 150 °C to 300 °C. Four point probe method was used to measure the DC resistance, while a network analyzer and Cascade’s probe system in the frequency range from 10 MHz to 30 GHz were employed to measure the S-parameters of the sintered Ag conducting patterns. The resistivity under the application of a DC decreased from 398 μΩ cm to 9 μΩ cm with increasing sintering temperature from 150 °C to 300 °C. From the measured S-parameters, the electrical losses in high frequencies also decreased with increasing sintering temperature (about 1.2 dB at 30 GHz) due to the formation of an interparticle neck after heat treatment at high temperatures.     

Electronic properties of GST for non-volatile memory

Impacts of annealing temperature and film thickness to the resistivity of Ge₂Sb₂Te₅(GST) have been studied. The resistivity of GST drops when the annealing temperature reaches 180 °C, rises above 360 °C and the thicker film crystallized more easily. Electronic device of phase change memory also has been fabricated with metal sidewall technology using 5 μm lithographic technology. The device was successfully programmed by 100 ns of 5 V pulse for SET and 10 ns of 10 V pulse for RESET. More than 100 times on/off ratio has been reached.     

Temperature behavior and modeling of ohmic contacts to Si implanted n-type GaN

The behavior of an ohmic contact to an implanted Si GaN n-well in the temperature range of 25-300 °C has been investigated. This is the sort of contact one would expect in many GaN based devices such as (source/drain) in a metal-oxide-semiconductor transistor. A low resistivity ohmic contact was achieved using the metal combination of Ti (350 Å)/Al (1150 Å) on a protected (SiO₂ cap) and unprotected samples during the post implantation annealing. Sheet resistance of the implanted layer and metal-semiconductor contact resistance to N⁺ GaN have been extracted at different temperatures. Both, the experimental sheet resistance and the contact resistance decrease with the temperature and their characteristics are fitted by means of physical based models.     

Effects of substrate temperature on electrical and structural properties of copper thin films

This paper addresses the effects of substrate temperature on electrical and structural properties of dc magnetron sputter-deposited copper (Cu) thin films on p-type silicon. Copper films of 80 and 500 nm were deposited from Cu target in argon ambient gas pressure of 3.6 mTorr at different substrate temperatures ranging from room temperature to 250 °C. The electrical and structural properties of the Cu films were investigated by four-point probe and atomic force microscopy. Results from our experiment show that the increase in substrate temperature generally promotes the grain growth of the Cu films of both thicknesses. The RMS roughness as well as the lateral feature size increase with the substrate temperature, which is associated with the increase in the grain size. On the other hand, the resistivity for 80 nm Cu film decreases to less than 5 μΩ-cm at the substrate temperature of 100 °C, and further increase in the substrate temperature has not significantly decreased the film resistivity. For the 500 nm Cu films, the increase in the grain size with the substrate temperature does not conform to the film resistivity for these Cu films, which show no significant change over the substrate temperature range. Possible mechanisms of substrate-temperature-dependent microstructure formation of these Cu films are discussed in this paper, which explain the interrelationship of grain growth and film resistivity with elevated substrate temperature.     

Improved properties of Al-doped ZnO film by electron beam evaporation technique

High-quality Al-doped ZnO (AZO) thin films have been fabricated by electron beam evaporation technique. The effect of the growth temperature on the optical and electrical properties of the electron-beam (e-beam) evaporated AZO film is investigated. X-ray diffraction measurements have shown that e-beam evaporated films are highly c-axis oriented at appropriate growth temperature. Transmittance measurement showed that the best optical and structural quality of the e-beam evaporated AZO film occurred at 200 °C. The scanning electron microscope images have shown that the surfaces of the e-beam evaporated AZO became smoother for the growth temperature at and above 200 °C. Finally, the maximum electrical resistivity of 2.5×10⁻⁴ Ω cm and optical transmittance of more than 85% has been found at 200 °C growth temperature, which explains its relation with the crystal quality of the film.