Andrej Stranz

Capabilities of ICP-RIE cryogenic dry etching of silicon: review of exemplary microstructures

Inductively coupled plasma (ICP) cryogenic dry etching was used to etch submicron pores, nano contact lines, submicron diameter pillars, thin and thick cantilevers, membrane structures and anisotropic deep structures with high aspect ratios in silicon for bio-nanoelectronics, optoelectronics and nano-micro electromechanical systems (NMEMS). The ICP cryogenic dry etching gives us the advantage of switching plasmas between etch rates of 13 nm min−1 and 4 µm min−1 for submicron pores and for membrane structures, respectively. A very thin photoresist mask can endure at −75 °C even during etching 70 µm deep for cantilevers and 300 µm deep for membrane structures. Coating the backsides of silicon membrane substrates with a thin photoresist film inhibited the lateral etching of cantilevers during their front release. Between −95 °C and −140 °C, we realized crystallographic-plane-dependent etching that creates facets only at the etch profile bottom. By varying the oxygen content and the process temperature, we achieved good control over the shape of the etched structures. The formation of black silicon during membrane etching down to 300 µm was delayed by reducing the oxygen content.

Evaluation of resonating Si cantilevers sputter-deposited with AlN piezoelectric thin films for mass sensing applications

We report on a micro-machined resonator for mass sensing applications which is based on a silicon cantilever excited with a sputter-deposited piezoelectric aluminium nitride (AlN) thin film actuator. An inductively coupled plasma (ICP) cryogenic dry etching process was applied for the micro-machining of the silicon substrate. A shift in resonance frequency was observed, which was proportional to a mass deposited in an e-beam evaporation process on top. We had a mass sensing limit of 5.2 ng. The measurements from the cantilevers of the two arrays revealed a quality factor of 155–298 and a mass sensitivity of 120.34 ng Hz−1 for the first array, and a quality factor of 130–137 and a mass sensitivity of 104.38 ng Hz−1 for the second array. Furthermore, we managed to fabricate silicon cantilevers, which can be improved for the detection in the picogram range due to a reduction of the geometrical dimensions.

Pick-and-Place Silver Sintering Die Attach of Small-Area Chips

A method for high-temperature-stable die attaches based on sintering of micro and nano silver particles is described. A low-temperature (200°C) and low-pressure (3 N/mm2) process was established to ensure compatibility with conventional adhesive die attach and to avoid surface damage on the dice, respectively. A modified flip-chip bonder providing high placement accuracy (2 μm) is used for a precise pick-and-place die attach. A thermal finite element modeling simulation was performed to analyze the bonding process. Additionally, the influence of the surface properties on the adhesion of sintered silver layers was investigated. The area-selective sintering method allows combination with other standard processes for die attach. It is now possible to establish pressure-assisted silver sintering for the series production of hybrid electronic circuits, which is an option requested by the industry to expand the operation range of sensors and electronics in harsh environments (e.g., measurement while drilling).

Sintering of Copper Particles for Die Attach

First steps are taken toward a low-cost alternative to silver sintering as a highly reliable die attach technology for deep drilling applications and future power electronic modules. In this feasibility analysis, we evaluate sintering of copper particles for die attach. Particulate copper pastes are pretreated in H2 atmosphere (50 mbar) in order to gain oxide-free particles. Subsequently, particles are sintered at a pressure of 40 N/mm2 and a temperature of 350°C for 2 min. Porosity, Youngs modulus as well as electrical and thermal conductivities of sintered layers are analyzed. Moreover, shear tests at ambient temperature are performed for evaluating the adhesion of monometallic as well as Cu-Au bonds according to the American military standard for chip-substrate contacts (MIL-STD-883H, method 2019.8).

SiC-Die-Attachment for High Temperature Applications

In this paper a die-attachment technology for high temperature applications based on the Low Temperature Joining Technique (LTJT) is presented. The present challenge is to fit the thermal expansion as well as the mechanical properties of the die-attach layer to the characteristics of chip and substrate. While the classic LTJT is based on sintering a sub-micron silver paste at temperatures between 150°C and 300°C to bond an electronic device to a substrate, the modified procedure employs a powder mixture consisting of silver powder and special filling powder material. Type and amount of the filling material is dependent on the application and the used substrates. Considering a low thermal expansion and high electrical as well as thermal conductivity we chose SiC, TiC, and BN as filling materials in this work.

A resonant cantilever sensor for monitoring airborne nanoparticles

In this paper, a silicon resonant cantilever sensor is used for monitoring airborne nanoparticles (NPs) by detecting the resonant frequency shift that is directly induced by an additional NPs mass deposited on it. A piezoelectric stack actuator and a self-sensing technique using a piezoresistive strain gauge are involved in the sensor system in order to actuate and detect the oscillation of cantilever sensor, respectively. The dielectrophoresis (DEP) method is employed for trapping the airborne NPs in a stable carbon aerosol assessment. A thermal-induced frequency shift is also investigated with the purpose of observing the limitation imposed by thermal effects on the minimum detectable NPs mass. The proposed sensor reveals a mass sensitivity of 8.33 Hz/ng, a fundamental resonant frequency of 43.92 kHz, a quality factor of 1230, and a temperature coefficient of the resonant frequency (TCf) of −28.6 ppm/°C. The results demonstrate a possibility of using this resonant cantilever in mobile airborne sensor applications.

Die-attach for high-temperature applications using fineplacer-pressure-sintering (FPS)

A new joining technique called “Fineplacer-Pressure-Sintering” (FPS) for die-attach of small electronic components (e.g. LEDs and photodiodes) is described. Using a modified Flip Chip Bonder, bare dies could be bonded onto substrates with high positioning accuracy. For the FPS process a 50 tons press, which is conventionally used for pressure sintering, is no longer required. Very high average shear strengths (63 MPa) were achieved on molybdenum substrates (metallization: Ni/Au). With the help of silver powder of micro-to-nanometre grain size the electrical and mechanical properties of the compound layer could be further increased. The bond strength of metalized GaN-LEDs on Al2O3 substrates with a Ti/Pd/Au metallization is twice as high as with standard micro-powder and the process temperature could be reduced to 200° C. Finally the applicability of FPS was demonstrated by an optoelectronic module consisting of two commercial InGaN -LEDs and GaP -photodiodes on a metalized Al2O3 substrate. Successful function was found with prototype modules at temperatures up to 250° C.

Surface finish improvement of deep micro bores monitored using an active MEMS cantilever probe

Application of a novel MEMS cantilever probe is described which is designed for non-destructive profilometry with micro machined surfaces, e.g., inside high-aspect-ratio micro bores. As an actual topic of industrial production the surface finish of diesel injector nozzle spray holes is investigated for a variety of state-of-the-art nozzle types. We address the performance of the sensor (resolution, uncertainty, speed) and the limits of the method to determine the reproducibility of a micro bore fabrication process as well as its optimization.

Use of self-sensing piezoresistive Si cantilever sensor for determining carbon nanoparticle mass

A silicon cantilever with slender geometry based Micro Electro Mechanical System (MEMS) for nanoparticles mass detection is presented in this work. The cantilever is actuated using a piezoactuator at the bottom end of the cantilever supporting frame. The oscillation of the microcantilever is detected by a self-sensing method utilizing an integrated full Wheatstone bridge as a piezoresistive strain gauge for signal read out. Fabricated piezoresistive cantilevers of 1.5 mm long, 30 μm wide and 25 μm thick have been employed. This self-sensing cantilever is used due to its simplicity, portability, high-sensitivity and low-cost batch microfabrication. In order to investigate air pollution sampling, a nanoparticles collection test of the piezoresistive cantilever sensor is performed in a sealed glass chamber with a stable carbon aerosol inside. The function principle of cantilever sensor is based on detecting the resonance frequency shift that is directly induced by an additional carbon nanoparticles mass deposited on it. The deposition of particles is enhanced by an electrostatic field. The frequency measurement is performed off-line under normal atmospheric conditions, before and after carbon nanoparticles sampling. The calculated equivalent mass-induced resonance frequency shift of the experiment is measured to be 11.78 ± 0.01 ng and a mass sensitivity of 8.33 Hz/ng is obtained. The proposed sensor exhibits an effective mass of 2.63 μg, a resonance frequency of 43.92 kHz, and a quality factor of 1230.68 ± 78.67. These results and analysis indicate that the proposed self-sensing piezoresistive silicon cantilever can offer the necessary potential for a mobile nanoparticles monitor.

Silicon Nanowire Resonators: Aerosol Nanoparticle Mass Sensing in the Workplace

In this article, We focus on silicon nanowire (SiNW)-based resonators that were fabricated and employed to sense aerosol nanoparticles (NPs) by measuring resonant frequency shifts induced by the mass of stuck NPs. The fabrication of SiNW arrays was performed using inductively coupled plasma (ICP) cryogenic dry etching and multiple thermal oxidations. The SiNWs were coated with gold (Au) for contacting to the homebuilt electrostatic NP sampler to collect the flowing NPs. A piezoelectric shear actuator mounted in the frequency measurement system was used to excite the SiNW sensors into resonance. Tested in a titanium dioxide (TiO2) aerosol sampling with a total concentration of ~8,500 NPs/cm3, the sensor displayed its feasibility as a nanobalance to detect aerosol NPs in the femtogram scale with a mass sensitivity of 7.1 Hz/fg and a mass resolution of 31.6 fg. To extend the operating life of the sensor, an ultrasonic removal method was used to detach the adhered NPs.