Alvise Bagolini

Stress characterization of electroplated gold layers for low temperature surface micromachining

In this work, the stress of electroplated gold films has been analyzed versus plating current density and bath temperature. Two different plating solutions have been adopted, one being based on cyanide-gold salt, the other on sulfite-gold. Gold surface quality was investigated in the experimented range of plating temperature and current density, in order to control the limit conditions for plating: surface roughness and non-uniformity appear whenever deposition parameters are brought to the limit (typically below 2 mA/cm/sup 2/ and above 5 mA/cm/sup 2/). Plated gold stress measurement was carried out by wafer curvature comparison, before and after deposition, using Stoneys formula for thin films/sup 1,2/. A current density range between 1.5 and 6 mA/cm/sup 2/ and temperature range between 50 and 70/spl deg/C was investigated. Stress analysis was also carried out on a Cr-Au multilayer, which actually is the structural layer employed for gold microstructures: the multilayer consists of a chromium adhesion layer, a PVD gold seed layer and a plated gold layer, with thickness respectively 10 nm, 150 nm and 1500 nm. A range of stress was obtained, varying from tensile to compressive: cyanide bath yielded stress from -30 MPa to about 0 MPa, and sulfite bath showed stress between -90 MPa and 110 MPa. Stress variation induced by thermal treatments after deposition was also investigated, by examining the effect of photoresist sacrificial etching on the internal stress of chromium-gold structural layers: the final stress was about 180 MPa tensile for all samples, regardless the as-deposited stress, with a variation ranging from about 80 MPa to more than 200 MPa.

Slim edges in double-sided silicon 3D detectors

Minimization of the insensitive edge area is one of the key requirements for silicon radiation detectors to be used in future silicon trackers. In 3D detectors this goal can be achieved with the active edge, at the expense of a high fabrication process complexity. In the framework of the ATLAS 3D sensor collaboration, we produced modified 3D silicon sensors with a double-sided technology. While this approach is not suitable to obtain active edges, because it does not use a support wafer, it allows for a new type of edge termination, the slim edge. In this paper we report on the development of the slim edge, from numerical simulations to design and testing, proving that it works effectively without increasing the fabrication complexity of silicon 3D detectors, and that it could be further optimized to reduce the insensitive edge region to less than 100 μm.

Development of Double-Sided Full-Passing-Column 3D Sensors at FBK

We report on the main design and technological characteristics related to the latest 3D sensor process developments at Fondazione Bruno Kessler (FBK, Trento, Italy). With respect to the previous version of this technology, which involved columnar electrodes of both doping types etched from both wafer sides and stopping at a short distance from the opposite surface, passing-through columns are now available. This feature ensures better performance, but also a higher reproducibility, which is of concern in medium volume productions. In particular, this R&D project was aimed at establishing a suitable technology for the production of 3D pixel sensors to be installed into the ATLAS Insertable B-Layer. An additional benefit is the feasibility of slim edges, which consist of a multiple ohmic column termination with an overall size as low as 100 μm. Eight batches with two different wafer layouts have been fabricated using this approach, and including several design options, among them the ATLAS 3D sensor prototypes compatible with the new read-out chip FE-I4.

PECVD low stress silicon nitride analysis and optimization for the fabrication of CMUT devices

Two technological options to achieve a high deposition rate, low stress plasma-enhanced chemical vapor deposition (PECVD) silicon nitride to be used in capacitive micromachined ultrasonic transducers (CMUT) fabrication are investigated and presented. Both options are developed and implemented on standard production line PECVD equipment in the framework of a CMUT technology transfer from R & D to production. A tradeoff between deposition rate, residual stress and electrical properties is showed.The first option consists in a double layer of silicon nitride with a relatively high deposition rate of ~100 nm min−1 and low compressive residual stress, which is suitable for the fabrication of the thick nitride layer used as a mechanical support of the CMUTs. The second option involves the use of a mixed frequency low-stress silicon nitride with outstanding electrical insulation capability, providing improved mechanical and electrical integrity of the CMUT active layers. The behavior of the nitride is analyzed as a function of deposition parameters and subsequent annealing. The nitride layer characterization is reported in terms of interfaces density influence on residual stress, refractive index, deposition rate, and thickness variation both as deposited and after thermal treatment. A sweet spot for stress stability is identified at an interfaces density of 0.1 nm−1, yielding 87 MPa residual stress after annealing. A complete CMUT device fabrication is reported using the optimized nitrides. The CMUT performance is tested, demonstrating full functionality in ultrasound imaging applications and an overall performance improvement with respect to previous devices fabricated with non-optimized silicon nitride.

Development of Micro-Grippers for Tissue and Cell Manipulation with Direct Morphological Comparison

Although tissue and cell manipulation nowadays is a common task in biomedical analysis, there are still many different ways to accomplish it, most of which are still not sufficiently general, inexpensive, accurate, efficient or effective. Several problems arise both for in vivo or in vitro analysis, such as the maximum overall size of the device and the gripper jaws (like in minimally-invasive open biopsy) or very limited manipulating capability, degrees of freedom or dexterity (like in tissues or cell-handling operations). This paper presents a new approach to tissue and cell manipulation, which employs a conceptually new conjugate surfaces flexure hinge (CSFH) silicon MEMS-based technology micro-gripper that solves most of the above-mentioned problems. The article describes all of the phases of the development, including topology conception, structural design, simulation, construction, actuation testing and in vitro observation. The latter phase deals with the assessment of the function capability, which consists of taking a series of in vitro images by optical microscopy. They offer a direct morphological comparison between the gripper and a variety of tissues.

Development of active and slim edge terminations for 3D and planar detectors

We report novel solutions for the edge termination in silicon detectors. In the framework of a project aimed at the optimization of 3D detectors with active edge, we have developed both active edges using a single sided process with support wafer, and slim edges using a double sided process without support wafer. TCAD simulations and experimental tests have been carried out to validate and compare the proposed approaches. While active edges can provide a better sensitivity up to a few microns from the physical edge, slim edges can simplify the fabrication technology while limiting the dead area at the edge to about 50 µm. The main design and technological issues are reported in this paper, along with selected results from TCAD simulations and electro-optical tests performed on these devices.

Fabrication of Novel MEMS Microgrippers by Deep Reactive Ion Etching With Metal Hard Mask

The fabrication of a novel class of microgrippers is demonstrated by means of bulk microelectromechanical systems (MEMS) technology using silicon on insulator wafer substrates and deep reactive ion etching. Hard masking is implemented to maximize the selectivity of the bulk etching using sputtered aluminum and aluminum–titanium thin films. The micro-roughness problem related to the use of metal mask is addressed by testing different mask combinations and etching parameters. The O2 flow, SF6 pressure, wafer temperature, and bias power are examined, and the effect of each parameter on micro-masking is assessed. Sidewall damage associated with the use of a metal mask is eliminated by interposing a dielectric layer between silicon substrate and metal mask. Dedicated comb-drive anchors are implemented to etch safely both silicon sides down to the buried oxide, and to preserve the wafer integrity until the final wet release of the completed structures. A first set of complete devices is realized and tested under electrical actuation. [2017-0039]

Optimization of double-side 3D detector technology for first productions at FBK

We report on the optimization of the technology aimed at the production of Double-Sided, Double-Column 3D detectors with full passing columns (3D-DTTC) at FBK (Trento, Italy). This R&D project is aimed at establishing a suitable technology for the production of 3D pixel sensors to be installed into the ATLAS IBL. We describe the main process modifications adopted on more recent 3D batches, to overcome the limitations affecting the first 3D batch, as arisen from its electrical characterization.

Development of modified 3D detectors at FBK

We report on the main design and technological issues related to a modified 3D-DTTC (Double-side, Double-Type-Column) detector process at FBK (Trento, Italy). With respect to the previous versions of this technology, which involved columnar electrodes of both doping types etched from both wafer sides and stopping at a short distance from the opposite surface, passing-through columns are now available. This is expected to enhance the performance but most of all to make it more reproducible, having in mind medium volume productions. An additional benefit is the feasibility of slim edges, which consist of a multiple ohmic column termination with an overall size in the order of 200 μm. Two batches of detectors have been fabricated at FBK using this modified 3D-DDTC approach, and with two different wafer layouts including several design options, among them the ATLAS 3D sensor prototypes compatible with the new read-out chip FE-I4. Selected results from the characterization of test structures from the first processed wafer are reported.

New MEMS Tweezers for the Viscoelastic Characterization of Soft Materials at the Microscale

As many studies show, there is a relation between the tissue’s mechanical characteristics and some specific diseases. Knowing this relationship would help early diagnosis or microsurgery. In this paper, a new method for measuring the viscoelastic properties of soft materials at the microscale is proposed. This approach is based on the adoption of a microsystem whose mechanical structure can be reduced to a compliant four bar linkage where the connecting rod is substituted by the tissue sample. A procedure to identify both stiffness and damping coefficients of the tissue is then applied to the developed hardware. Particularly, stiffness is calculated solving the static equations of the mechanism in a desired configuration, while the damping coefficient is inferred from the dynamic equations, which are written under the hypothesis that the sample tissue is excited by a variable compression force characterized by a suitable wave form. The whole procedure is implemented by making use of a control system.