Bulk Silicon Micromachining Bulk micromachining is one of the micromachining technologies that are used to fabricate MEMS MicroElectronicMechanical Systems structures Micromachining is the technique used to sculpt or machine silicon on a microelectronic scale In bulk micromachining structures are shaped by etching a large single crystal substrate2 The two types of etches that are associated with bulk micromachining are isotropic etch and anisotropic etch Each type of etch is defined by their selectivity or the way that the material in the substrate are etched away and the type of etchant used Isotropic vs Anisotropic etch There are some differences between isotropic and anisotropic etch used in bulk micromachining In general isotropic etch etches the substrate faster than anisotropic etch while anisotropic etch offers a more precise and defined profile on the substrate Isotropic etch is characterized by its uniform profile and undercutting of the masking material at the surface The etching process can be done with either wet chemistry or more modern techniques such as vapor and plasma etching1 Isotropic wet etching may be preferred over plasma because of the high quality edge definition it offers Some factors affecting isotropic wet etching include the etchant used agitation methods temperature dopant level and crystal defects2 For silicon the most commonly used isotropic etch is a solution of nitric acid hydrofluoric acid and acetic acid HNO3 HF CH3COOH The etch rate depends on the ratio of the solution For example a solution with low HF and high HNO3 will results in higher etch rate that is dependent on diffusion limitation For high HF and low HNO3 a slower etch rate with a bigger influence from temperature will result2 This example was interpreted from the following Iso etch curves In anisotropic etch crystal orientation of the silicon is important since the etch attack certain crystal planes much more rapidly than others This etch rate selectivity can be used to create various cavity and groove structures The etch rate is orientation dependent in the crystal2 For the case of silicon 100 and 110 crystal plane etches much faster than the 111 plane For example a solution of KOH water and alcohol can etch the 100 110 and 111 planes at a relative rate of 40 30 1 These planes refer to the miller indices and are visualized in the picture below The etch rate of anisotropic etch is slower than isotropic etch with typical etch rate of 1um min or less The reaction rate is controlled depending on the etch used KOH NaOH LiOH etc The ratio that describe the etch rate for each plane to the other is called Anisotropy ratio AR It is given by AR hkl 1etch rate hkl 2 etch rate For isotropic etching AR 1 and maybe 400 200 1 for 110 100 111 planes for a 50 50 ratio solution of KOH H2O in anisotropic etch2 Finally to control the etching and ensure that it stops at a specific plane etch stops are used Boron doped regions are etched very slowly and therefore commonly used as etch stops In addition to wet etchant used in isotropic wet etch such as HNO3 HF CH3COOH solution and anisotropic wet etchant which includes inorganic alkaline solutions KoH LiOH NaOH organic alkaline solutions EDP dry etching techniques or plasma etching can be used in both isotropic and anisotropic etching process In reactive ion etching RIE a dry etch method sulfur hexfluouride SF6 gas is utilized in plasma systems to etch the substrate Fluorine free radials formed by dissociation in the plasma produce etch rate comparable to we chemistry with isotropic profiles In some plasma systems polymers can be simultaneously deposited on the sidewalls of etched regions to reduce the etch rate resulting in anisotropic plasma etching1 Other type of dry etch techniques include sputter etching and vapor phase etching Processes For single crystal silicon bulk micromachining in the main is an anisotropic etch The end results depend on the orientation of the mask features with crystal planes so alignment with the flats on the wafer s crystal planes is critical For example given a 100 wafer if straight 90 walls are desired then the mask needs to be oriented in the 100 direction i e the secondary flat If however the mask is oriented in the 110 direction i e the primary flat then 54 74 walls are obtained which is the leaning angle of the 111 diagonal plane To facilitate the precise alignment of the mask with the crystal planes a fan shaped test pattern can be etched in the surface of the wafer The flats are accurate only to 1 This wet chemical etching is complimented by other manufacturing processes Electroformation of Porous Silicon LIGA HEXSIL and Hinged Polysilicon The following list is some of the application of bulk silicon micromachining Porous Silicon Porous silicon is made by electrochemically etching a silicon wafer in HF solution A current is maintained between the silicon wafer and an opposing electrode By varying the current density it is possible to change the porosity percentage of void space of the film Multilayer porous silicon films are simply created by periodically varying the current density in time Etching proceeds preferentially at the pore tips so layers previously etched will not be modified as subsequent layers are formed LIGA LIthographie Galvanoformung Abformung LIGA is a technique to build high aspect ratio structures in metal or plastic using an X ray light source It is capable of fabricating very high structures up to 1 mm with sub micron lateral accuracy The substrate has to be a conductor e g steel or copper or non conductors metallized on top The top layer is roughened for good adhesion of the resist PMMA A thick layer of PMMA is spread over the substrate exposed and developed PMMA is squeezed on in a tool Once PMMA etched metal is electroplated in the cavities PMMA substrate PMMA thickne HEXSIL Hexagonal Silicon structures This is a millimeter scale molding process where the silicon substrate is deep etched to the desired geometry and used in subsequent steps as a tool to deposit polysilicon First a sacrificial oxide layer is grown on the surface of the trenches and holes Then the actual parts are CVD deposited and the oxide layer is removed The parts are released and the mold is ready for another round of the process Hinged Polysilicon One way to achieve high aspect ratios is to use surface micromachining to build large flat structures and then stand them up vertically like cards References 1 Introduction to