R&D Profile: Silicon on Ceramics - A New Concept for Micro-Nano-Integration on Wafer Level
One of the challenges of using nano effects and patterns in semiconductor devices is the realization of an intelligent and robust connection to the macro world.
LTCC (low temperature cofired ceramics) are established materials for “System in Package” solutions due to the integration of passive elements, such as capacitors or resistors, associated with short development times as well as simple and cheap processing. The advantages of reliability, thermal stability and chemically inert packages offered by ceramic interconnect devices are combined with thin film precision by means of a smart wafer level packaging process. Tough mechanical, electrical or fluidic coupling of nano elements without affecting their functionality is guaranteed by a fully silicon-ceramic wafer compound material. The method is based on a bonding procedure between a nano patterned silicon surface (modified Black Silicon) and LTCC. A LTCC tape with adapted TCE to silicon is joined with a silicon wafer by lamination and pressure assisted firing. This manufactured “Silicon-On-Ceramic”-substrate enables a wide range of design solutions, in witch several, unfired ceramic layers are prepared with vias, wirings and fluidic channels using standard LTCC-technologies. After sintering, the ceramic acts as a carrier system with electrical and fluidic properties. To ensure the electronic functionality of MEMS devices only a thin silicon layer is necessary. Fig. 1 illustrates the scheme of this new concept for micro-nano-integration.
The uniformly distributed silicon nano structure is generated by a self organized reactive ion etching process (Fig. 2a). During the lamination, the nano patterned wafer surface penetrates into the ductile, unfired LTCC tape. Due to the highly increased surface area, a form-fit bonding and a material connection between the glass phase of BGK and the silicon is generated during the firing step (Fig. 2b).The separation of silicon areas can easily be done by standard silicon etching processes such as DRIE or RIE, in which the ceramic works as a natural etching barrier (Fig. 2c).
Currently, we are able to fabricate a tight and high strength crack free silicon ceramic substrate compound in a standard 4 inch wafer format. Using this novel integration concept electrical contacts as well as fluidic components from nm to mm scale can be fabricated in one batch.
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H. Bartsch de Torres
Embossing of LTCC – A Technological Enlargement of the new Integration Concept
One method to arrange fluidic channels directly at the interface between silicon and ceramic (Fig. 3) is offered by embossing. This cost effective technique is derived from polymer processing and adapted to LTCC green tapes. It stands out against classic LTCC processing as a result of its high precision and surface quality. Small channels with smooth surfaces are replicated into the ceramic green body. During the subsequent pressure assisted sintering, only the channels height shrinks while the lateral dimensions are kept constant. The method allows for the arrangement of silicon nanostructures directly inside of an easy-to-produce ceramic package with fluidic and electric interface and also enables the use of nanostructures as functional elements in microfluidic devices.
New Integration Possibilities for Fused SilicaK. Lilienthal
The mechanical interlocking of self-organizing nano-structures enables a novel joining process with advantages to gluing and bonding. This way of bonding is free of additive materials and solvents, durable against ultra violet rays and can be done at any desired temperature. Up to now hybrids play a great role in MEMS or MOEMS, which incorporate different technologies, control modules and/or sensors at the same time in one device. `Black Silicon´ uses self-organization processes to generate nanostructure needle surfaces, which create the possibility of interlocking with itself (silicon) or other materials like polymers or special ceramics.
Another important material class beside silicon are glasses. Borosilicate glasses are adapted to the thermal expansion coefficient of silicon and thus enable anodic bonding. But the properties of these boron containing glasses are far behind fused silica. Fused silica is a superior material for many applications in microsystem technologies, in micro-optics because of its optical transparency over an important spectrum range without any auto fluorescence. Outstanding mechanical stability and durability make it an ideal construction material for fluidic devices beside the high electrical insulating character and low thermal conductance. The excellent biocompatibility of pure silicon dioxide is necessary for many biomedical systems and cell studies. In all those fields of applications fused silica has great advantages to silicon or polymers, but due to its brittle character precision machining and interconnecting to other materials is still a big challenge.
Self-organization processes during dry etching processes are under intense investigation since about 20 years. Materials like silicon, gallium-bonds or boron containing glasses show interesting effects that allow the replacement of expensive and time intensive nano-lithography or similar nano-masking-steps. Nevertheless efforts to utilize the unwanted side effect of the later so called `Black Silicon´ took a long time resulting in several mechanical applications for silicon nanostructures.
We are developing dry etching processes for fused silica (Fig. 4) which are showing analogue properties to `Black Silicon´ and investigating it by a parameter study to search for new usable structures and hybrids. This innovative starting point allows the transfer of `Black Silicon´ technologies and its applications to fused silica.
Michael Fischer, Heike Bartsch de Torres and Katharina Lilienthal are researchers in the MacroNano® group, a multidisciplinary team at the Institute for Micro- and Nanotechnologies, Technische Universität Ilmenau, Germany. They are working in the field of Biosensors and Functionalized Packaging with focus on multimaterial fluidic systems for biological and electronic micro devices. The close cooperation with the local partner HITK (Hermsdorfer Institut für Technische Keramik) with focus on industrial ceramics and composites facilitated the current work on the silicon-ceramic wafer compound material.