Päivi Sievilä

The fabrication of silicon nanostructures by focused-ion-beam implantation and TMAH wet etching.

Local gallium implantation of silicon by a focused ion beam (FIB) has been used to create a mask for anisotropic tetramethylammonium hydroxide (TMAH) wet etching. The dependence of the etch stop properties of gallium-doped silicon on the implanted dose has been investigated and a dose of 4 x 10(13) ions cm(- 2) has been determined to be the threshold value for achieving observable etching resistance. Only a thin, approx. 50 nm, surface layer is found to be durable enough to serve as a mask with a high selectivity of at least 2000:1 between implanted and non-implanted areas. The combined FIB-TMAH process has been used to generate various types of 3D nanostructures including nanochannels separated by thin vertical sidewalls with aspect ratios up to 1:30, ultra-narrow (approx. 25 nm) freestanding bridges and cantilevers, and gratings with a resolution of 20 lines microm(- 1).

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.

Focused ion beam lithography for fabrication of suspended nanostructures on highly corrugated surfaces

We propose a nanofabrication method that allows for patterning on extremely corrugated surfaces with micrometer-size features. The technique employs focused ion beam nanopatterning of ion-sensitive inorganic resists formed by atomic layer deposition at low temperature. The nanoscale resolution on corrugated surfaces is ensured by inherently large depth of focus of a focused ion beam system and very uniform resist coating. The utilized TiO₂ and Al₂O₃ resists show high selectivity in deep reactive ion etching and enable the release of suspended nanostructures by dry etching. We demonstrate the great flexibility of the process by fabricating suspended nanostructures on flat surfaces, inclined walls, and on the bottom of deep grooves.

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.

Dopant-induced stress in microfabricated silicon devices

The modification of material characteristics by introducing dopant atoms into a crystal lattice is a fundamental basis for modern micro- and nanosystems technology. In this work, the uneven distribution of dopants is shown to have a remarkable effect on the residual stress and the consequent deformation of released, mechanical silicon structures. In particular, the focus is on segregation of initial dopants inside the bulk silicon which takes place in such fabrication processes as thermal oxidation. A theoretical model based on perceiving the dopant-induced change in Si crystal lattice parameter is developed. We experimentally investigate a series of silicon-on-insulator wafers, including samples with dopant types B, P, and Sb, and concentrations in the range from 1015 to 5 × 1019 atoms cm−3. Released cantilevers are fabricated as test structures and the residual stress is determined by measuring their final curvature. Experimental results are compared with the modelled values obtained utilizing the dopan...

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})

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.

Coating on Thin-Film Cryoresistors

Optically readable silicon cantilevers have been designed, fabricated and applied to photoacoustic spectroscopy. The working principle is based on detecting the acoustic wave induced displacement of the cantilever with laser interferometry. The rectangular, 9.5 mum thick components were fabricated using two-sided anisotropic etching of silicon-on insulator (SOI) wafers. The flexible cantilever is surrounded by narrow gaps from three sides and it moves very sensitively relative to the thick frame. The cantilever sensor has been applied to detect weak pressure variations in trace gas analysis and it has been shown to improve the sensitivity of photoacoustic measurements by many orders of magnitude.