|
[ Pobierz całość w formacie PDF ]
.Those techniquesborrowed from the integrated circuit processing industry are essentially two dimen-sional, and control over parameters in the third dimension is only achieved by stack-ing a series of two-dimensional layers on the workpiece, which is usually a siliconwafer.There are practical and economic limits, however, to the number of layersthat can be managed in such a serial process, and therefore, the expansion of devicesinto the third dimension is restricted.Micromachining techniques enable structuresto be extended further into the third dimension; however, it has to be understoodthat these structures are simply either extruded two-dimensional shapes or are gov-erned by the crystalline properties of the material.True three-dimensional process-ing would allow any arbitrary curved surface to be formed, and this is clearly notpossible with the current equipment and techniques.An important aspect of MEMSis to understand the limitations of the micromachining techniques currently avail-able.Although the range of these techniques is continually being expanded, thereare some core techniques that have been part of the MEMS toolkit for many years.This chapter deals mainly with these core techniques, but also with those processtechniques borrowed from integrated circuit manufacturing.2.2 Materials2.2.1 Substrates2.2.1.1 SiliconJust as silicon has dominated the integrated circuit industry, so too is it predominantin MEMS.There are a number of reasons for this: (1) pure, cheap, and well-characterized material readily available; (2) a large number and variety of mature,easily accessible processing techniques; and (3) the potential for integration withcontrol and signal processing circuitry.In addition to these reasons, the mechanicaland physical properties of silicon give it a powerful advantage for its use in mechani-cal sensors, and therefore, this book deals mainly with devices fabricated in bulksilicon and silicon on insulator (SOI).Crystalline silicon has a diamond structure.This is a face-centered cubic latticewith two atoms (one at the lattice point and one at the coordinates ¼, ¼, ¼7 8 Materials and Fabrication Techniquesnormalized to the unit cell) associated with each lattice point.The crystal structureis shown in Figure 2.1.The crystal planes and directions are designated by Millerindices, as shown in Figure 2.2.Any of the major coordinate axes of the cube can bedesignated as a direction, and planes perpendicular to these are designatedas {100} planes.The {111} planes are planes perpendicular to the directions,which are parallel to the diagonals of the cube.Bulk silicon from material manufac-turers is usually either {100} or {111} orientation, although other orientations canbe obtained from specialist suppliers.This orientation identifies the plane of the topsurface of the wafer.The wafers are cut at one edge to form a primary flat in a {110}plane.A secondary flat is also cut on another edge to identify the wafer orientationand doping type, which is either n- or p-type.The doping is done with impurities togive a resistivity of between 0.001 and 10,000 &!cm.For mainstream integrated cir-cuit processing wafers are typically of the order of 10 to 30 &!cm corresponding toan impurity level of
[ Pobierz całość w formacie PDF ]
zanotowane.pldoc.pisz.plpdf.pisz.plhanula1950.keep.pl
|