This paper presents a common work performed by microelectronic biology and characterization teams which aims at describing the design, and the fabrication of a characterization platform dedicated to bio-inspired objects, and the first electrical measurements acquired on amyloid fibers. The design of this characterization platform takes into account biological and microelectronic constraints. Regarding biological constraints, special attention has been paid to materials choice and device fabrication in order to be biocompatible and to enable drop deposition for both wet and dry experiments. Dedicated patterns have been designed in order to measure electrical parameters such as contact resistance and resistivity of the bio-inspired objects. Regarding the fabrication step, the characterization platform has taken advantage from microelectronic technologies, especially in terms of size (nanoscale), reproducibility and robustness taken into account the specific environment. These aspects will be further discussed in this paper, as well as the first electrical measurements.
To properly understand the electrical characteristics of bio-inspired objects, one critical point is the measurement of the linear conductivity of these objects, which implies the discrimination from contact resistance. Another important point is the conduction mechanism. Table 1 presents two patterns used to investigate the conduction mechanisms. To realize a complete electrical study, we have thus decided to design specific test structures as Transmission Line Matrix (TLM) and Van der Pauw patterns, structures with a variable electrode gap, and different contact areas structures. Figure 1 shows SEM images of the electrodes fabricated with this specific layout. The inter-electrode distance has been measured at 1µm and 70µm x 70µm square pads have been obtained. The figure 2 presents a cross section view of the fabricated electrodes. Electrode thickness is equal to 200nm and the width is equal to 1.7µm. Material impact and electrochemical properties of bio-inspired objects have also been investigated. Working with biological object at the nanoscale requires further restrictions. The main constraint is the use of biocompatible material to pattern all the layers in contact with the bio-inspired object. We have decided to use (i) platinum and gold because these metals are commonly used in biological measurements, and (ii) ruthenium due to its anisotropic etching properties. Another requirement relies on back gate electrode to enable the field effect investigation of bio-inspired object. To deal with wet measurement and to control the film deposition at the desirable position we have decided to engineer further cavities.
Regarding packaging solutions, standard microelectronic metallic pads have been patterned to allow automated electrical characterization. Bioelectrical investigations as electrochemistry or control atmosphere measurements require the design of an “easy to plug” device enabling drop deposition and a very simple electrical connection adapted to electrical probers.
This “easy to plug” device has been fabricated including a patterned silicon die assembled with a conductive glue on a Side-brazed Ceramic Dual In-inline Package (SCDIP) and wire-bonding. A SCDIP has been especially chosen to be easily carried and plugged inside biological equipment such as climatic chamber and glove box. Large package have been used in order to connect the maximum of patterns but also to provide an important protein-based nanowires density on the top of the electrodes. Figure 3 presents photography picture of the characterization platform used to perform the conduction measurements in biological equipment. The figure 4 presents the first electrical signal of the bio-inspired nano object which confirms that the characterization platform has been correctly fabricated and can be able to study the electrical properties of the protein film.
This papers aims at presenting more details on (i) the design, (ii) the fabrication of silicon die in clean room (iii) the packaging adapted to the biological environment and (iv) results of electrical characterization of the bio-inspired objects at the nanoscale.
References
[1] D. V. Lim, M. M. Simpson, E. A. Kearns et al., Clin. Microbiol. Rev., vol. 18, no. 4, pp. 583–607, Oct. 2005.
[2] F. S. Ligler and J. S. Erickson, Nature, vol. 440, no. 7081, pp. 159–160, Mar. 2006.