Jakob Janting

Reliability of industrial packaging for microsystems

Abstract Packaging concepts for silicon-based micromachined sensors exposed to harsh environments are explored. By exposing the sensors directly to the media and applying protection at the wafer level the packaging and assembly will be simplified as compared to conventional methods of fabrication. Protective coatings of amorphous silicon carbide and tantalum oxide are suitable candidates with etch rates below 0.1 A/h in aqueous solutions with pH 11 at temperatures up to 140°C. Si-Ta-N films exhibit etch rates around 1 A/h. Parylene C coatings did not etch but peeled off after extended exposure times at elevated temperatures. The best diamond-like carbon films we tested did not etch, but delaminated due to local penetration of the etchants. Several glue types were investigated for chip mounting of the sensors. Hard epoxies, such as Epotek H77, on the one hand exhibit high bond strength and least degradation and leakage, but on the other hand introduce large sensor output drift with temperature changes. Softening of the Epo-tek H77 was observed at 70°C. An industrially attractive thin-film anodic silicon-to-silicon wafer bonding process was developed. Glass layers are deposited at 20 nm/s (1.2 μm/min) by electron-beam evaporation and bond strengths in excess of 25 N/mm 2 are obtained for bonding temperatures higher than 300°C. Through-hole electrical feedthroughs with a minimum line width of 20μm and a density of 250 wires per cm were obtained by applying electro-depositable photo-resist. Hermetically sealed feedthroughs were obtained using glass frits, which withstand pressures of 4000 bar.

SU-8 cantilever chip interconnection

The polymer SU-8 is becoming widely used for all kinds of micromechanical and microfluidic devices, not only as a photoresist but also as the constitutional material of the devices. Many of these polymeric devices need to include a microfluidic system as well as electrical connection from the electrodes on the SU-8 chip to a printed circuit board. Here, we present two different methods of electrically connecting an SU-8 chip, which contains a microfluidic network and free-hanging mechanical parts. The tested electrical interconnection techniques are flip chip bonding using underfill or flip chip bonding using an anisotropic conductive film (ACF). These are both widely used in the Si industry and might also be used for the large scale interconnection of SU-8 chips. The SU-8 chip, to which the interconnections are made, has a microfluidic channel with integrated micrometer-sized cantilevers that can be used for label-free biochemical detection. All the bonding tests are compared with results obtained using similar Si chips. It is found that it is significantly more complicated to interconnect SU-8 than Si cantilever chips primarily due to the softness of SU-8.

On the chemical diffusion in layered thin films containing amorphous Co-Zr, Ni-Zr, and Fe-Zr

The chemical diffusion in thin trilayer films of TM–TM 100− x Zr x –TM with an amorphous middle layer where TM = Co, Ni, or Fe and in amorphous Fe–Zr and Ni–Zr films with composition gradients has been investigated using Rutherford backscattering spectrometry. The growth of the amorphous layer in the trilayers, due to in-diffusion of cobalt and nickel, is initially found to be proportional to the square root of the time, t 1/2 , and subsequently found to level off before the compositions corresponding to metastable equilibria are reached. Irradiation, with 500 keV Xe + ions, is found to promote the in-diffusion. This behavior is discussed in terms of structural relaxation effects and their influence on the metastable equilibrium. In amorphous Fe–Zr the chemical diffusivity is observed to be very sluggish. Contrary to the behavior in Co–Zr and Ni–Zr trilayers, the direction of the iron diffusion in Fe–Zr trilayers suggests a broad positive peak in the Gibbs free energy at a composition around 50 at. % Zr. It is argued that the sluggish chemical diffusivity of iron is directly related to the unusual composition-dependence of the Gibbs free energy for amorphous Fe–Zr.

Chip-size-packaged silicon microphones

The first results of silicon microphones that are completely batch-packaged and integrated with signal conditioning circuitry in a chip stack are discussed. The chip stack is designed to be directly mounted into a system, such as a hearing instrument, without further single-chip handling or wire bonding. The devices are fully encapsulated and provided with a well-determined interface to the environment. The integrated microphones operate at a bias of 1.5 V and are expected to reach a sensitivity of 5 mV/Pa, an A-weighted equivalent input noise of 24 dB sound pressure level, and a power consumption of about 50 μW in the near future, thereby living up to the tight specifications of microphones for hearing instruments. Other potential applications include mobile phones, headsets, and wearable computers, in which space is constrained.

Indiffusion of cobalt in amorphous Co–Zr

Abstract By use of trilayers consisting of ∼ 35 nm cobalt ∼ 45 nm amorphous CoZr, and ∼ 35 nm cobalt, the indiffusion of cobalt into the amorphous structure was studied by means of Rutherford Backscattering Spectroscopy (RBS). At temperatures between 650 K and 690 K, an initially high diffusion essentially stopped after 20–40 min annealing before equilibrium was reached. It is proposed that the effect is related to the diffusional asymmetry in the amorphous phase. The relatively immobile zirconium atoms cannot relax their configuration to permit cobalt indiffusion up to the equilibrium value.

Solution-Mediated Annealing of Polymer Optical Fiber Bragg Gratings at Room Temperature

In this letter, we investigate the response of poly(methylmethacrylate) (PMMA) microstructured polymer optical fiber Bragg gratings (POFBGs) after immersion in methanol/water solutions at room temperature. As the glass transition temperature of solution-equilibrated PMMA differs from the one of solvent-free PMMA, different concentrations of methanol and water lead to various degrees of frozen-in stress relaxation in the fiber. After solvent evaporation, we observe a permanent blue-shift in the grating resonance wavelength. The main contribution in the resonance wavelength shift arises from a permanent change in the size of the fiber. The results are compared with conventional annealing. The proposed methodology is cost-effective as it does not require a climate chamber. Furthermore, it enables an easy-to-control tuning of the resonance wavelength of POFBGs.

Ion‐induced modification of graphite coatings

Burnished graphite powders and physical vapor‐deposition (PVD) coatings of layered crystalline materials such as MoSx have often been used as solid lubricants. This letter will report on a novel ion‐induced modification of a graphite‐powder coating on a silicon surface. Even at very low ion doses, the bombardment results in structural modifications observed as a considerable reduction in x rays reflected from the (002) sliding planes, which indicates an amorphization process. Transmission electron microscopy (TEM) studies have confirmed almost complete amorphization with 200 keV Ar ions at 1016 ions cm−2. A commercial scanning tunneling microscope (STM) was used as an advanced profilometer to study ion‐induced changes in the surface morphology at the powder surface. The mechanical properties of the combined graphite‐coating/silicon system were studied by nanoindentation technique, and the perspectives in ion‐beam burnishing of graphite coatings will be discussed.

Polymer Optical Fiber Compound Parabolic Concentrator Fiber Tip-Based Glucose Sensor: In Vitro Testing

We present in vitro sensing of glucose using a newly developed efficient optical fiber glucose sensor based on a compound parabolic concentrator (CPC)-tipped polymer optical fiber (POF). A batch of nine CPC-tipped POF sensors with a 35-mm fiber length is shown to have an enhanced fluorescence pickup efficiency with an average increment factor of 1.7 as compared with standard POF sensors with a plane cut fiber tip. in vitro measurements for two glucose concentrations (40 and 400 mg/dL) confirm that the CPC-tipped sensors can efficiently detect both glucose concentrations.

Evidence for phase separation in amorphous Fe-Zr from chemical diffusion

Abstract The chemical diffusion of iron in layered films of amorphous Fe-Zr and b.c.c. Fe and in amorphous Fe-Zr films with a composition profile have been investigated using Rutherford backscattering spectrometry. The compositional changes after annealing observed in these films provide evidence for a broad positive peak in the Gibbs free energy of amorphous Fe-Zr at around 50 at% Zr consistent with phase separation of the amorphous state.

Friction and wear characteristics of ion-beam modified graphite coatings

Abstract Solid lubricated surfaces are now widely used in the tool industry, and the new concept of ‘soft tools’ recently introduced has emphasized low-friction surfaces. The present paper deals with a novel ‘burnishing’ process based on ionic bombardment of powder graphite coating/substrate systems. This process may influence both the coating and the coating/substrate interface, and it is effective for improving lubrication even at low doses of bombarding ions. The present study will discuss the friction and wear properties of graphite-powder coatings on a silicon wafer bombarded with 200 keV ion beams of argon, nitrogen and hydrogen ions, the last two as molecular ions. The coefficients of friction and wear rates of the coatings were found to be strongly dependent on the ion-bombarding species and ion dose. The argon ion bombardment increased the coefficient of friction and wear rate of the powder coating. However, at the interface of the silicon substrate, the ion-induced burnishing improved the tribological behaviour of the silicon material. Bombardment with nitrogen and hydrogen ions showed a marked improvement in the tribological properties of the graphite powder coating. Thus a reduction in wear rate by three orders of magnitude was observed in the case of nitrogen, and for both ions it was noted that ion-beam burnished graphite was lubricating in a dry environment, which has not been reported previously. The perspectives of ion bombardment as a burnishing process will be discussed and the observed effects will be qualitatively explained in the context of the theory for ionic penetration into solids.