R.SINDHU, Assistant Professor, Department Of ECE.

The Electron beam heating technique is based in the heat produced byhigh energy electron beam bombardment on the material to be deposited. The electron beam is generated by an electron gun, which uses thethermionic emission of electrons produced by an incandescent filament. Emitted electrons are accelerated by a high voltage potential (kilovolts). A magnetic field is often applied to bend the electron trajectory, allowingthe electron gun to be positioned below the evaporation line. As electrons can be focalized, it is possible to obtain localized heating on thematerial to evaporate, with a high density of evaporation power. This allows controlling the evaporation rate, from low to very high values,and best of all, the chance of depositing materials with high melting point(W, Ta, C, etc.).

Advantages of vacuum evaporation: High-purity films can be deposited from high-purity source material. Source of material to be vaporized may be a solid in any form and purity. The line-of-sight trajectory and "limited-area sources" allow the use ofmasks to define areas of deposition on the substrate and shutters between thesource and substrate to prevent deposition when not desired. Deposition rate monitoring and control are relatively easy. It is the least expensive of the PVD processes.

Disadvantages of vacuum evaporation: Many compounds and alloy compositions can only be deposited withdifficulty. Line-of-sight and limited-area sources result in poor surface coverage oncomplex surfaces unless there is proper fixturing and movement. Few processing variables are available for film property control. Source material use may be low. Large-volume vacuum chambers are generally required to keep anappreciable distance between the hot source and the substrate.

SPUTTER DEPOSITION Sputter deposition are methods of depositing thin films by sputtering. They involve ejecting material from a “target” that is a source onto a“substrate” such as a silicon wafer. Sputtered atoms ejected from the target have a wide energy distribution,typically up to tens of eV. The sputtered ions (typically only a small fraction — order 1% — of theejected particles are ionized) can ballistically fly from the target in straightlines and impact energetically on the substrates. The sputtering gas is often an inert gas such as argon. For efficient momentum transfer, the atomic weight of the sputtering gasshould be close to the atomic weight of the target, so for sputtering lightelements neon is preferable.

Magnetron sputtering is the most commonly used method for asputter deposition. It usually utilizes a strong electric and magnetic fields to trapelectrons close to the surface of the magnetron, which is known as thetarget. The electrons follow helical paths around the magnetic field linesundergoing more ionizing collisions with gaseous neutrals near thetarget surface than would otherwise occur. The extra argon ions created as a result of these collisions leads to ahigher deposition rate. It also means that the plasma can be sustainedat a lower pressure. The sputtered atoms are neutrally charged and so are unaffected bythe magnetic trap.

Advantages of sputter deposition: Elements, alloys and compounds can be sputtered and deposited. The sputtering target provides a stable, long-lived vaporization source. In some configurations, the sputtering source can be a defined shapesuch as a line or the surface of a rod or cylinder. In some configurations, reactive deposition can be easily accomplishedusing reactive gaseous species that are activated in plasma. There is very little radiant heat in the deposition process. The source and substrate can be spaced close together. The sputter deposition chamber can have a small volume.

Disadvantages of sputter deposition: Sputtering rates are low compared to those that can be attained in thermalevaporation. In many configurations, the deposition flux distribution is non-uniform,requiring moving fixturing to obtain films of uniform thickness. Sputtering targets are often expensive and material use may be poor. Most of the energy incident on the target becomes heat, which must beremoved. In some cases, gaseous contaminants are "activated" in the plasma, makingfilm contamination more of a problem than in vacuum evaporation. In reactive sputter deposition, the gas composition must be carefullycontrolled to prevent poisoning the sputtering target.

Sputter deposition is widely used to deposit thin film metallization on semiconductor material, coatings on architectural glass, reflective coating on polymers, magnetic films for storage media, transparent electrically conductive films on glass and flexible webs,dry-film lubricants, wear resistant coating on tools and decorativecoatings.

Ion beam sputtering Ion beam sputtering utilizes an ion source to generate a relatively focused ion beamdirected at the target to be sputtered. The ion source consists of a cathode and anode with a common central axis.Applying a high voltage field of 2-10 kV to the anode creates an electrostatic fieldinside the ion source, confining electrons around a saddle point in the center of thesource. When argon gas is injected into the ion source, the high electric field causes the gasto ionize, creating a plasma inside the source region. The ions are then accelerated from the anode region to the exit aperture (cathode)creating a “collimated” ion beam. The resulting ion beam impinges upon a target material and, via momentum transferbetween the ion and the target, sputters this material onto the sample.

LITHOGRAPHYSelectively remove oxide from those areas in which dopantatoms are to be introduced is accomplished using a processcalled photolithography or optical lithography.Major steps in the lithography process are1. The top surface of the wafer is first coated with an ultraviolet (UV)light sensitive material called photoresist.Liquid photoresist is placed on the wafer, and the wafer is spun athigh speed to produce a thin, uniform coating.After spinning, a short bake at about 90 C is performed to drivesolvent out of the resist.

FIGURE Major steps in the lithography process: (a) application of resist; (b) resist exposure through a mask and anoptical reduction system; (c) after development of exposed photoresist; and (d) after oxide etching and resist removal.

2. The next step is to expose the resist through a photomask and a high-precision reduction (for example 5 to 1 reduction) lens system using UVlight as illustrated in Fig. 3–5b.The photomask is a quartz photoplate containing the patterns to beproduced.Opaque regions on the mask block the UV light.Regions of the photoresist exposed to the light undergo a chemicalreaction that varies with the type of resist being employed.In negative resists, the areas where the light strikes become polymerizedand more difficult to dissolve in solvents. When placed in a developer(solvent), the polymerized regions remain, while the unexposed regionsdissolve and wash away.

The net result after development is pictured on the righthand side of Fig. 3–5c. Positive resists contain a stabilizer that slows down the dissolution rate ofthe resist in a developer. This stabilizer breaks down when exposed tolight, leading to the preferential removal of the exposed regions as shownon the left-hand side of Fig. 3–5c. Steps (a) through (c) make up thecomplete lithography process. To give a context for the purpose of lithography, step (d) for oxide removalincluded . Buffered hydrofluoric acid (HF) may be used to dissolveunprotected regions of the oxide film. Lastly, the photoresist is removed in a step called resist strip. This isaccomplished by using a chemical solution or by oxidizing or “burning”the resist in an oxygen plasma or a UV ozone system called an asher.

PATTERN TRANSFER—ETCHING After the pattern is formed in the resist by lithography, the resistpattern is often transferred to an underlying film, is removed with HF,this etching method is called wet etching. Since wet etching is usually isotropic (meaning without preference indirection, and proceeding laterally under the resist as well asvertically toward the silicon surface), the etched features are generallylarger than the dimensions of the resist patterns as shown in Fig. 3–8a Dry etching technique can overcome this shortcoming and is thedominant etching technology.

In dry etching, also known as plasma etching or reactive-ion etching orRIE, the wafer with patterned resist is exposed to a plasma, which is analmost neutral mixture of energetic molecules, ions, and electrons that isusually created by a radio frequency (RF) electric field The energetic species react chemically with the exposed regions of thematerial to be etched, while the ions in the plasma bombard the surfacevertically and knock away films of the reaction products on the wafersurface. The latter action is directional so that the etching ispreferentially vertical because the vertical surfaces can be covered withfilms of the reaction products. Hence the etch rate is anisotropic.

FIGURE 3–8 Comparison between (a) isotropic etching and (b) anisotropicetching.DOPINGThe density profile of the dopant atoms in the silicon is generallydetermined in two steps. First, the dopant atoms are placed on ornear the surface of the wafer by ion implantation, gas-sourcedoping, or solid-source diffusion. This step may be followed by anintentional or unintentional drive-in diffusion that transports thedopant atoms further into the silicon substrate.

Ion Implantation In ionimplantation, an impurity is introduced into the semiconductor bycreating ions of the impurity, accelerating the ions to high energies rangingfrom subkiloelectronvolt to megaelectronvolt, and then literally shooting theions onto the semiconductor surface (Fig. 3–10). The ions themselves do not necessarily come to rest on lattice sites. A follow-up anneal (heating) of the wafer is therefore necessary for damageremoval and for dopant activation (placing the dopant atoms on lattice sites asshown in Fig. 1–6) so that implanted impurities behave as donors andacceptors.FIGURE 3–10 In ion implantation, a beam of high-energy ions penetrates into the unprotected regions of the semiconductor .

MICROMACHINING Fabrication of MEMs device involves the basic IC fabricationmethods along with the micromachining process involving theselective removal of silicon or addition of other structural layersSteps of MEMs Fabrication using Bulk Micromachining:

Step1: The first step involves the circuit design and drawing of the circuiteither on a paper or on using software like PSpice or Proteus. Step 2: The second step involves simulation of the circuit and modelingusing CAD( Computer Aided Design). CAD is used to design thephotolithographic mask which consists of the glass plate coated withchromium pattern. Step 3: The third step involves photolithography. In this step, a thin film ofinsulating material like Silicon Dioxide is coated over the silicon substrateand over this a organic layer, sensitive to ultra violet rays is deposited usingspin coating technique. The photolithographic mask is then placed in contactwith the organic layer.

The whole wafer is then subjected to UV radiation, allowing the pattern mask tobe transferred to the organic layer.The radiation either strengthens the photoresist or weakens it.The uncovered oxide on the exposed photoresist is removed using Hydrochloricacid. The remaining photoresist is removed using hot Sulphuric acid and theresultant is an oxide pattern on the substrate, which is used as a mask.Step 4: The fourth step involves removal of the unused silicon or etching. Itinvolves removal of a bulk of the substrate either using wet etching or dryetching. In wet etching the substrate is immersed in a liquid solution of achemical etchant, which etches out or removes the exposed substrate eitherequally in all directions(isotropic etchant) or in a particular direction(anisotropicetchant).Popularly used etchants are HNA (Hydrofluoric acid, Nitric acid and Aceticacid) and KOH(Potassium Hydroxide).

Step 5: The fifth step involves the joining of two or more wafers to produce amulti layered wafer or a 3 D structure. It can be done using fusion bondingwhich involves direct bonding between the layers or using anodic bonding. Step 6: The 6th step involves the assembling and integrating the MEMs deviceon the single silicon chip. Step 7: The 7th step involves packaging of the whole assembly to ensureprotection from outer environment, proper connection to the environment,minimum electrical interference. Commonly used packages are metal canpackage and ceramic window package. The chips are bonded to the surfaceeither using wire bonding technique or using flip chip technology where thechips are bonded to the surface using an adhesive material which melts onheating, forming electrical connections between the chip and the substrate.

MEMs Fabrication using Surface MicromachiningManufacturing of Cantilever Structure using Surface Micromachining

First step involves deposition of the temporary layer (an oxide layer or anitride layer) on the silicon substrate using low pressure chemical vapordeposition technique. This layer is the sacrificial layer and provideselectrical isolation. Second step involves deposition of the spacer layer which can be aphosphosilicate glass, used to provide a structural base. Third step involves subsequent etching of the layer using dry etchingtechnique. Dry etching technique can be reactive ion etching where thesurface to be etched is subjected to accelerating ions of the gas or vaporphase etching.

Fourth step involves chemical deposition of phosphorus dopedpolysilicon to form the structural layer. The fifth step involves dry etching or removal of the structurallayer to reveal the underlying layers. The 6th step involves removal of the oxide layer and the spacerlayer to form the required structure. The rest of the steps are similar as in the bulk micromachiningtechnique.

MEMs fabrication using LIGA Technique It is a fabrication technique which involves lithography, electroplating andmolding on a single substrate. LIGA is the german acronym forlithography, electroplating and moulding (Lithographie, Galvanik undAbformung).

1st step involves deposition of layer of Titanium or copper or Aluminumon the substrate to form a pattern. 2nd step involves deposition of thin layer of Nickel which acts as theplating base. 3rd step involves addition of a X-ray sensitive material like PMMA(polyMethyl metha acrylate). 4th step involves aligning a mask over the surface and exposing thePMMA to x-ray radiation. The exposed area of PMMA is removed andthe remaining one covered by the mask is left. 5th step involves placing the PMMA based structure into anelectroplating bath wherein the Nickel is plated on the removed PMMAareas. 6th step involves removal of the remaining PMMA layer and the platinglayer, to reveal the required structure.

Advantages of LIGA LIGA is a versatile process – it can produce parts by several differentmethods High aspect ratios are possible (large height to- width ratios in thefabricated part) Wide range of part sizes is feasible - heights ranging from micrometers tocentimeters Close tolerances are possibleDisadvantages of LIGA LIGA is a very expensive process– Large quantities of parts are usually required to justify its application LIGA uses X-ray exposure– Human health hazard

MEMS Accelerometers MEMS accelerometers are widely used in various application fields,such as consumer electronics, automobiles, medical, structural healthmonitoring, and inertial navigation. According to the displacement or force transduction mechanisms,MEMS accelerometers can be categorized as different types, includingpiezoelectric, capacitive , piezoresistive, tunneling, resonant,optical, thermal , and electromagnetic accelerometers. Capacitive transduction is most commonly used technologies forhigh-performance MEMS accelerometers– since it takes advantage of simple structure, low noise, low powerconsumption, cost-effectiveness, and reliability

The MEMS accelerometer consists of a silicon-based acceleration-sensitive spring-mass structure which is sandwiched by the upper andlower glass cover plates, as illustrated in Figure 1. The in-plane motion of the proof mass is sensed capacitively between thearray of parallel-plate electrodes on the proof mass and the matchingarray of electrodes on the upper glass plate which is separated by a fixedgap above the proof mass.

Figure 2. Structure of the spring-mass system.The spring-mass system of the accelerometer transforms theexternal accelerations to displacements of the proof mass. Inorder to measure accelerations, it is necessary to convert thedisplacement into other measurable quantities. By applyingcapacitive displacement sensing technology, the displacementcan be transduced into capacitance changes which can thenbe converted to voltage or current through the signal

An area-variation periodic array capacitive displacement transducer is introducedwith the schematic illustrated in Figure 4. Each set of the periodic array capacitive transducer consists of two drive electrodesplated on the movable proof mass and one pickup electrode plated on the upperglass cover plate with a gap of d.The overlapping length of the drive and pickup electrodes is le.The width of both drive electrodes is defined as aand the separation between each drive electrode is g1.While the width of the pickup electrode is b andthe separation between each other is g2.

When the electrodes pairs are located in the null position wherethe pickup electrode is right in the middle of two drive electrodes,the overlapping width of the pickup electrode to each driveelectrode is x0. In this case, the sensing capacitors C1,constructed by the positive drive electrod

JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY ANANTAPUR B. Tech III-ISem. (ECE) L T P C 3 1 0 3 15A04506 -MEMS & MICRO SYSTEMS (MOOCS-I) UNIT I Introduction:Introduction to MEMS & Microsystems, Introduction to Microsensors, Evaluation of MEMS, Microsensors, Market Survey, Application of MEMS, MEMS Materials, MEMS