Joerg Froemel

A fast and low actuation voltage MEMS switch for mm-wave and its integration

A novel mm-wave MEMS single pole single throw (SPST) switch has been developed, which is driven by 5.0 V in 10.3 µs. The insertion loss and the isolation at 60 GHz were 1.2 dB and 18 dB, respectively. A two metal layer silicon interposer technology was also developed. We designed single pole double throw (SPDT) switch module, in which two SPST switch are accommodated on the silicon interposer chip. It consists of 3 µm thick Al wires and 12 µm thick low-k benzocyclobutene (BCB) interlayer dielectrics. They enabled sufficient signal integrity for 60 GHz and higher frequencies.

Multi-wafer bonding technology for the integration of a micromachined Mirau interferometer

The paper presents the multi-wafer bonding technology as well as the integration of electrical connection to the zscanner wafer of the micromachined array-type Mirau interferometer. A Mirau interferometer, which is a key-component of optical coherence tomography (OCT) microsystem, consists of a microlens doublet, a MOEMS Z-scanner, a focusadjustment spacer and a beam splitter plate. For the integration of this MOEMS device heterogeneous bonding of Si, glass and SOI wafers is necessary. Previously, most of the existing methods for multilayer wafer bonding require annealing at high temperature, i.e., 1100°C. To be compatible with MEMS devices, bonding of different material stacks at temperatures lower than 400°C has also been investigated. However, if more components are involved, it becomes less effective due to the alignment accuracy or degradation of surface quality of the not-bonded side after each bonding operation. The proposed technology focuses on 3D integration of heterogeneous building blocks, where the assembly process is compatible with the materials of each wafer stack and with position accuracy which fits optical requirement. A demonstrator with up to 5 wafers bonded lower than 400°C is presented and bond interfaces are evaluated. To avoid the complexity of through wafer vias, a design which creates electrical connections along vertical direction by mounting a wafer stack on a flip chip PCB is proposed. The approach, which adopts vertically-stacked wafers along with electrical connection functionality, provides not only a space-effective integration of MOEMS device but also a design where the Mirau stack can be further integrated with other components of the OCT microsystem easily.

Fabrication of a multilayer spiral coil by selective bonding, debonding and MEMS technologies

For the further miniaturization of integrated circuits, the integration of passive components on the chip is one approach. In DC-DC converter applications, the integration of the inductor with high inductivity is one problem. This paper addresses this problem by proposing a new technique for fabricating a multilayer spiral coil that is also useful as part of electromagnetic MEMS (Micro-Electro-Mechanical Systems) actuators. The multilayer coil is made by stacking separately fabricated coil layers and joining them with a selective bonding and debonding technique.

Investigation of Surface Pre-Treatment Methods for Wafer-Level Cu-Cu Thermo-Compression Bonding

To increase the yield of the wafer-level Cu-Cu thermo-compression bonding method, certain surface pre-treatment methods for Cu are studied which can be exposed to the atmosphere before bonding. To inhibit re-oxidation under atmospheric conditions, the reduced pure Cu surface is treated by H2/Ar plasma, NH3 plasma and thiol solution, respectively, and is covered by Cu hydride, Cu nitride and a self-assembled monolayer (SAM) accordingly. A pair of the treated wafers is then bonded by the thermo-compression bonding method, and evaluated by the tensile test. Results show that the bond strengths of the wafers treated by NH3 plasma and SAM are not sufficient due to the remaining surface protection layers such as Cu nitride and SAMs resulting from the pre-treatment. In contrast, the H2/Ar plasma–treated wafer showed the same strength as the one with formic acid vapor treatment, even when exposed to the atmosphere for 30 min. In the thermal desorption spectroscopy (TDS) measurement of the H2/Ar plasma–treated Cu sample, the total number of the detected H2 was 3.1 times more than the citric acid–treated one. Results of the TDS measurement indicate that the modified Cu surface is terminated by chemisorbed hydrogen atoms, which leads to high bonding strength.

Solid Liquid Inter-Diffusion bonding at low temperature

Solid Liquid Inter-Diffusion (SLID) bonding using the systems Cu/Ga and Au/In have been investigated regarding the bonding parameters and their influence on shear strength. Especially temperature dependence and composition of interface have been focused on.

Bonding-Based Wafer-Level Vacuum Packaging Using Atomic Hydrogen Pre-Treated Cu Bonding Frames

A novel surface activation technology for Cu-Cu bonding-based wafer-level vacuum packaging using hot-wire-generated atomic hydrogen treatment was developed. Vacuum sealing temperature at 300 °C was achieved by atomic hydrogen pre-treatment for Cu native oxide reduction, while 350 °C was needed by the conventional wet chemical oxide reduction procedure. A remote-type hot-wire tool was employed to minimize substrate overheating by thermal emission from the hot-wire. The maximum substrate temperature during the pre-treatment is lower than the temperature of Cu nano-grain re-crystallization, which enhances Cu atomic diffusion during the bonding process. Even after 24 h wafer storage in atmospheric conditions after atomic hydrogen irradiation, low-temperature vacuum sealing was achieved because surface hydrogen species grown by the atomic hydrogen treatment suppressed re-oxidation. Vacuum sealing yield, pressure in the sealed cavity and bonding shear strength by atomic hydrogen pre-treated Cu-Cu bonding are 90%, 5 kPa and 100 MPa, respectively, which are equivalent to conventional Cu-Cu bonding at higher temperature. Leak rate of the bonded device is less than 10−14 Pa m3 s−1 order, which is applicable for practical use. The developed technology can contribute to low-temperature hermetic packaging.

Electroplating of neodymium iron alloys

Aim is to show how to make neodymium iron alloys available for MEMS process by electroplating used for permanent magnetic material. Because of the low electrochemical potential of neodymium, three different deposition methods were investigated: Electroplating from aqueous solvent, from non-aqueous solvent and molecular plating. All results shows as brittle deposited film. Due to the high oxidation behavior of neodymium a high amount of oxygen could be observed, which indicates a complete oxidized film.