Wetting Behaviour of Glasses on Nanostructured Silicon Surfaces

Abstract

Abstract:The behavior of molten glass on nanostructured silicon surface is of essential importance for the fabrication of a strong bond interface between glass (or glass-based ceramic tapes) and silicon. It was found that typical glasses do not wet the silicon surface that is always coated with a thin silica layer. It is shown that the high surface tension of molten glasses at high temperatures in combination with the dewetting surface of the structured silicon prohibits the formation of an interlocking bond between the two substrates. The theory of wetting can be applied to molten glasses, too. As a consequence, a similar solution as for liquids is investigated: the surface has to be chemically modified to become wettable. Investigations with sputtered metals on the nanostructured silicon improve wetting of the surface and result in a better bond homogeneity of the SiCer compound during sintering with low pressure.

Thermal analysis of the ceramic material and evaluation of the bonding behavior of silicon-ceramic composite substrates

“…The purpose of these nanorods is the utilization of titanium not only as a wetting layer but also as a nanostructured compliant layer [10] to reduce residual stresses originating from the slight remaining mismatch of thermal expansion coefficient between the LTCC, the titanium film and the silicon. In addition, the nanorods should be better able to compensate asperities at the silicon-LTCC interface, supporting lower sintering pressures [6]. The resulting titanium nanorods were not intentionally oxidized but a native oxide can be assumed.…”

“…After dry oxidation (Tempress Junior, Tempress Systems, Vaasen, Netherlands), a 50 nm thick TiO 2 layer is obtained. Based on previous reports [6] and the known dependency of the wetting behavior on surface roughness [8], titanium was furthermore, deposited by electron beam evaporation at an oblique angle (OAD: oblique angle deposition), which leads to oblique titanium nanorods [9]. SEM images of the deposited titanium nanorods are shown in figures 2(a) and (b).…”

“…Due to the initially required relatively high sintering pressures in the range of 800 kPa, investigations on wetting promoting layers were triggered to lower the required pressure. Intermediate TiO 2 thin layers appear, for instance, to reduce the needed sintering pressure dramatically [6] leading the so-called pressureassisted sintering with pressure of as low as 3 kPa [3]. The benefits of SiCer substrates are, for instance, demonstrated in radio frequency micro-electro-mechanical systems (RF-MEMS) applications [2,7].…”

Innovative Silicon-Ceramic (SiCer) Technology for High-Strength Pressure Sensor Applications Using Different Manufacturing Methods

“…Black silicon can be applied additionally onto the interface-side to enhance the bond strength [6]. To improve wetting [7] of the bondinterface a ~25 nm titanium is applied on the Si-wafer and thermally oxidized to ~50 nm thick titanium oxide. [4], [6] The LTCC process begins with cutting the tape in the required processable size (115 mm x 115 mm) with consideration of the casting direction.…”

“…If required further pre-processing steps like screen-printing, via filling or laser structuring can be carried out. After preprocessing each double tape, the layers are stacked on a metal plate with stacking pins using alignment vias (7). A planar plate is placed on top of the ceramic to protect the cavities from deformation, followed by a silicone mat to smoothen sharp edges and thus allows an equal pressure distribution as well as prevents a tearing of the vacuum-sealed bag during lamination.…”