Bonding, anodic

Favoured by the increased temperature the alkali-ions in the glass, especially Na in Pyrex-glass, become mobile and migrate under the action of the applied electrical field to the cathode (see Fig. 10.7), which eventually results in the formation of a depletion layer with high electric field strength in the contact area between the glass sheet and the silicon wafer. The resulting 

10 Joining Methods for Glass Based Microdevices - - — voltage e.g. 1000 V 

The bonded sandwich is cooled after the joining process. To minimise any thermal stresses in the bond interface between the partners the thermal expansion coefficient of the glass sheet and silicon wafer should be very similar. 

Lengtenberg et al. [328] used an atmospheric pressure chemical vapour deposition to deposit boron oxide as an intermediate layer for anodic bonding. 

Thin layers of the glasses can also be deposited onto sihcon surfaces by RF-sputtering. 

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This simple reactor concept is based on a microstructured silicon chip (Figure 3.18) covered by a Pyrex-glass plate by anodic bonding [73, 74]. The silicon microstructure comprises, in addition to inlet and outlet structures, a multi-channel array. Only the Pyrex-glass plate acts as cover and inlet and outlet streams interface the silicon chip from the rear. 

Central part of this reaction unit is a split-recombine chip micro mixer made of silicon based on a series of fork-like channel segments [32-36]. Standard silicon micro machining was applied to machine these segments into a silicon plate which was irreversibly joined to a silicon top plate by anodic bonding (Figure 4.16). 

The two plates were not manufactured via the same route and were not made of the same material [7]. Typically, rectangular channels in silicon are realized by sawing, whereas semi-circular channels are made in glass by wet-chemical etching. Such glass/silicon plates are joined by anodic bonding. 

The reaction channels were made in silicon by several photolithographic steps, followed by potassium hydroxide etching [13,14]. Silicon oxide was thermally grovm over the silicon. Nickel thin films were vapor-deposited. Pyrex was anodically bonded to such a modified micro structured silicon wafer. 

Microfabrication involves multiple photolithographic and etch steps, a silicon fusion bond and an anodic bond (see especially [12] for a detailed description, but also [11]). A time-multiplexed inductively coupled plasma etch process was used for making the micro channels. The microstructured plate is covered with a Pyrex wafer by anodic bonding. 

B. Dunn, J.M. Miller, B.C. Dave, J.S. Valentine, and J.I. Zink, Initiation toughness of silicon/glass anodic bonds. Acta Mater. 46, 737-741 (1998). 

Method //was designed for a bonded microreactor. After the channel was sealed with a Pyrex top plate using the anodic bonding technique, the liquid precursor was infiltrated into the microreactor through the outlet of the reactor under slight pressure and withdrawn. A thin film of solution remained on the walls of the microchannel. 

Attach a piece of glass as the carrying substrate using anodic bonding. The area with the cantilever is protected by a Cr layer on the glass. 

Figure 1.31 Schematic of the cross-section of the mixing chamber. An etched glass substrate is joined to a silicon wafer by anodic bonding. The silicon plate is etched from the backside to yield a diaphragm, and a PZT is attached to the oscillating diaphragm [22] (by courtesy of Elsevier Ltd.).

The channels are anisotropically etched into silicon and are separated by a thin Si02 layer, the cantilever [111]. This microstructure, open to both sides, is closed by anodic bonding to Pyrex glass wafers. 

A central demand for the mixer is the very accurate setting of the channel width. For this reason, a microfabrication technique was chosen that is known to achieve very smooth and ideally-vertical channel walls, namely the reactive ion etching (RIE) technique [114—116]. The fluidic connections are made from the bottom by RIE. Anodic bonding serves for interconnection. 

Micro fabrication was made by conventional silicon wet etching. Sealing was achieved by anodic bonding to Pyrex glass [67]. 

Channel structures are etched on two plates which are later positioned face-to-face to give the overall fluid structure [140], In the region where the channels overlap, they are separated by the separation plate defined by an etch stop layer. The channel covered by this structured plate was generated by underetching in the <100> direction through slits in the plate. The micro mixer is assembled from a silicon and a glass wafer connected by anodic bonding. 

The mixers were fabricated by deep reactive ion etching (DR1E) into silicon [159], The silicon structure was anodically bonded to a glass wafer. 

The micro mixer was fabricated from two plates by standard MEMS technology, using deep reactive ion etching (DRIE) [48]. Anodic bonding is used for sealing the plates. 

The micro channel system was fabricated by standard silicon micromachining via etching of a silicon wafer with potassium hydroxide using thermal oxide as an etch mask [6], The double mixing tee configuration consists of six micro channels. For fluid connection, an outlet hole was drilled into the silicon chip. The chip was anodically bonded to a glass slide with three inlet holes, clamped in a holder and, thereby, connected to a commercially available quench-flow instrument... 

Later, Pattekar and Kothare [21] presented a silicon reactor fabricated by deep reactive ion etching (DRIE). It carried seven parallel micro channels of 400 pm depth and 1 000 pm width filled with commercial Cu/ZnO catalyst particles (from Siid-Chemie) trapped by a 20 pm filter, which also was made by DRIE, in the reactor. The reactor was covered by a Pyrex wafer applying anodic bonding. Details of the reactor are shown in Figure 2.3. 

Tanaka et al. [73] developed another MEMS system for the catalytic combustion of butane. It is composed of a combustion chamber 8 mm wide, 14 mm long and 150 pm deep which was prepared by anisotropic wet etching of a silicon substrate. The substrate was then covered with Pyrex glass applying anodic bonding. Combustion was performed on a platinum/titania catalyst... 

MEMS-like systems are frequently sealed by anodic bonding of Pyrex glass covers [20, 73, 86], or melting of low melting-point glass frits [71], which may well be suitable for future mass production of small devices. Further details will not be discussed here. 

Anodic bonding [65], which uses electrostatic attraction to bring a glass wafer into contact with a silicon wafer and to form covalent bonds between them ... 

The wafers containing the etched trenches are anodically bonded to Pyrex glass to form closed channels. The bond strength thus formed is strong enough to withstand pressures up to 250 bar. At that point a breakdown even takes place in the mono crystalline silicon and not at the bonded interface. Clearly, the indicated structures are useful for HPLC applications. 

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