Ossi Hahtela

Fabrication and characterization of an ultrasensitive acousto-optical cantilever

A cantilever-type silicon device for sensing changes in pressure has been designed, fabricated and characterized. The microfabrication process is based on two-sided etching of silicon-on insulator (SOI) wafers. The rectangular cantilevers are 9.5 µm thick, and cover an area of a few square millimeters. The cantilevers are surrounded by thick and tight frames, since on the three free sides there are only narrow, micrometer sized air gaps between the cantilever and the frame. This design and excellent mechanical properties of single crystal silicon enable sensitive detection of time-dependent gas pressure variations, i.e. acoustic waves. The mechanical properties of the cantilever have been characterized by analyzing its dynamic behavior. The resonance frequency and the mechanical vibrational mode patterns have been determined using finite-element method (FEM) simulations and laser interferometry. These results are found to be in good agreement with each other. Initially this mechanical door-like cantilever was designed to be used in ultra-high sensitivity photoacoustic gas sensing, but it can also be applied quite generally in various kinds of sound wave detection schemes.

Atomic layer deposited alumina (Al2O3) thin films on a high-Q mechanical silicon oscillator

In this paper, the influence of the atomic layer deposited alumina (Al2O3) thin films on the dynamics of a high-Q mechanical silicon oscillator was experimentally studied. The resonance frequency and Q value of uncoated oscillators used in this work were about f0 = 27 kHz and Q = 100 000 at p < 10−2 mbar and T = 300 K. Deposited alumina film thicknesses varied from 5 to 662 nm. It is demonstrated that the resonance frequency of the mechanical oscillator increases with the film thickness because the added alumina films effectively stiffen the oscillator structure. In addition, it is shown that alumina thin films with thickness up to 100 nm can be deposited on microfabricated mechanical resonant structures without degrading the initially high quality (Q value) of the resonance. The resonance frequency of the silicon oscillator was less sensitive to the changes in ambient temperature with thicker alumina coatings. The reflectivity of silicon at 633 nm was reduced from RSi = 0.35 to RAR = 0.035 by coating the silicon oscillator with an alumina film whose thickness corresponds to the quarter of the optical wavelength serving as a single-layer anti-reflection coating.

Position measurement of a cavity mirror using polarization spectroscopy

A high-reflectivity coated mechanical silicon oscillator with high quality value of the mechanical resonance was employed as a planar rear mirror in a Fabry–Perot interferometer. Active stabilization of the interferometer improves the stability of the resonance and makes it possible to perform sensitive interferometric measurements. The frequency locking of lasers to optical cavities requires typically the generation of an error signal with a typical steep slope at resonance. The Hansch–Couillaud locking method utilizes polarization spectroscopy by monitoring changes in the polarization of the light field reflected from the cavity. A polarization analyser detects dispersion shaped resonances which give the error signal for the electronic servo loop. The error signal contains information about the changes in the cavity length and thus the motion of the mechanical oscillator can be observed. The error signal was detected with the use of a spectrum analyser. The noise floor of the interferometer response indicates that the sensitivity of the optomechanical sensor or the minimum observable displacement in the mechanical oscillator position is Δxmin = 1.7 × 10−14 m. This gives high enough sensitivity to observe the Brownian motion of the oscillator at room temperature.

Non-tilting out-of-plane mode high-Q mechanical silicon oscillator

A single-crystal silicon oscillator with a non-tilting out-of-plane vibrational mode and high-quality factor for the mechanical resonance was designed, fabricated and characterized. The finite-element method (FEM) was utilized before the fabrication process to simulate the oscillator behavior and give guidance in optimizing the design. At low pressure p =10 −3 mbar and at room temperature, the resonance frequency and Q value were measured to be f0 = 26 526 Hz and Q = 100 000, respectively. The measured resonance frequency was in a good agreement with the simulated one, f0,FEM = 26 787 Hz. The actual mode pattern was verified by measurements and compared with the simulation result. An interferometric laser beam was scanned over the oscillator surface and position-dependent oscillation amplitudes were stored with the phase-sensitive detection. The oscillation was proved to occur effectively in a pure non-tilting out-of-plane mode. We propose to use this kind of micromechanical probe in various measurement schemes, where one needs to approach the surface with a single non-torsional plane. In addition, such an oscillator can be utilized as an optical mirror so that the optical mode can be kept the same when moving the mirror.

Atomic-Layer-Deposited Alumina

Metal-alloy (Ni-Cr-Cu-Al-Ge) thin-film resistors were coated with alumina (Al2O3) using the atomic layer deposition (ALD) technique. The electrical properties of the thin-film resistors were studied in the temperature range of 4.2-300 K. It was experimentally demonstrated that the protective dielectric alumina coating improves the long-term stability and repeatability of high-value, thin-film resistors (100-500 kOmega). The drift rate of the resistance due to the native oxidation at room temperature was reduced from -2.45 times10-6 h-1 for a nonaged uncoated resistor to 0.03 times10-6 h-1 for an alumina-coated resistor. It was shown that the additional 15-nm-thick alumina coating does not significantly change the thermoelectrical properties of the metal-alloy, thin-film resistors.

(\hbox{Al}_{2}\hbox{O}_{3})

Thin film resistors, based on the NiCr alloys, have been fabricated and studied at temperatures down to 50 mK. Resistivity of the films varied within (7-11) Ohm*square depending on additions of Cu, Al and Ge. The resistors demonstrate Kondo minimum at 20 K - 30 K. Power dependence measurements at sub Kelvin temperatures showed an electron overheating at the power level above 10 pW for the Ni75Cr20Cu2.5Al2.5 (Evanohm) 500 kOhm film resistor.

Coating on Thin-Film Cryoresistors

Cryo resistors based on Pd thin films were designed and investigated in temperature range 50 mK – 300 K. The resistors in the range 100 kΩ – 1.3 MΩ were fabricated using thermal evaporation technique. Minimum temperature coefficient, −0.4 (37)·10{−6}/K, in temperature range 50 mK – 100 mK has been obtained for 20 nm thin film 560 kΩ resistor. Binary and decimal configuration of connection of resistive elements were used for flexible adjustment of resistance value.

NiCr based thin film cryo resistors

Cryoresistors based on Pd thin films were designed and investigated in the temperature range 50 mK-300 K. The resistors in the range 100 kΩ-1.3 MΩ were fabricated by thermal evaporation technique. Resistivity of Pd film at 293 K varies in (21-53) · 10 8 Ωm range for 15-40 nm films. Temperature co efficient of resistance at low temperatures depends on the thickness of the film. The lowest temperature coefficient in the range 3 K-4 K of about few parts in 106/K was obtained for 20-nm-thick film. Current coefficient measured on 20-nm-thin film 1.002-MΩ resistor does not exceed 2 · 10 5/μA in 0.7 K-1 K temperature range.

Temperature dependence of Pd thin film cryo resistors

Ni-Cr-based thin-film resistors have been fabricated and studied at temperatures of down to 50 mK. The resistivity of the films varied within (14-25)Omega*sq, depending on the additions of Cu, Al, Ge, and Mn. The minimum temperature coefficient (TC) at 4.2 K (TC = -50 middot 10 -6/K) is obtained for Ni75Cr20CU2.5AI2.5 (Evanohm alloy) doped with 2.5% Ge. At the 50-150 mK range, the TC of the alloy increases to -4.15 middot 10 -6/mK. The resistors demonstrate the Kondo minimum at 20-30 K. The power coefficient of the 560-kOmega sample, which was measured at 4.2 K, was found to be les -0.008 middot 10 -6/muW. Power dependence measurements at subkelvin temperatures showed an electron overheating at the power level of above 10 p W for a 500-k film resistor.

Temperature Dependence of Pd Thin-Film Cryoresistors

Metal alloy (NiCrCuAlGe) thin film resistors were coated with atomic layer deposited (ALD) alumina (Al2O3) in order to improve the stability and repeatability of the high value resistors (100 kOmega - 500 kOmega) in the temperature range from 4.2 K to 300 K. The drift rate of the resistance due to the native oxidation at room temperature was reduced from -2.45 ppm/h for an uncoated resistor to 0.03 ppm/h for an alumina-coated resistor. It was shown that the additional alumina coating does not significantly change the thermoelectrical properties of the metal alloy thin film resistors.