Silicon anodic bonding

M Esashi, A Nakano, S Shoji and H Hebiguchi, Low-temperature silicon-to-silicon anodic bonding with intermediate low melting point glass. Sens Actuators A21-A23 (1990) 931-934... 

AHanneborg, M.Nese and P.Ohlckers, Silicon-to-silicon anodic bonding. Pro. of MME 90, Tech. Dig., (1990), 100-107... 

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). 

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. 

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 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... 

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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