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The radio frequency microelectromechanical system (RF MEMS) Materials

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RF technologies. Besides RF MEMS technology, III-V compound semiconductor (GaAs, GaN, InP, InSb), ferrite, ferroelectric, silicon-based semiconductor (RF CMOS, SiC and SiGe), and vacuum tube …

RF technologies. Besides RF MEMS technology, III-V compound semiconductor (GaAs, GaN, InP, InSb), ferrite, ferroelectric, silicon-based semiconductor (RF CMOS, SiC and SiGe), and vacuum tube technology are available to the RF designer. Each of the RF technologies offers a distinct trade-off between cost, frequency, gain, large-scale integration, lifetime, linearity, noise figure, packaging, power handling, power consumption, reliability, ruggedness, size, supply voltage, switching time and weight.

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  • 1. Materials for MEMS andMicrosystemsPresented by: Jitendra Jangid
  • 2. MATERIALS FOR MEMS ANDMICROSYSTEMSThis chapter will cover the materials used in “silicon-based”MEMS and microsystems. As such, silicon will be theprincipal material to be studied.Other materials to be dealt with are silicon compounds suchas: SiO2,SiC, Si3N4 and polysilicon.Also will be covered are electrically conducting of siliconpiezoresistors and piezoelectric crystals forelectromechanical actuations and signal transductions.An overview of polymers, which are the “rising stars” to beused as MEMS and microsystems substrate materials, willbe studied too.
  • 3. Silicon – an ideal substrate material forMEMS Silicon is a host of materials commonly used in thesemiconductor integrated circuit industry. Normally deposited as thin films, they include siliconoxides, silicon nitrides, and silicon carbides, metals suchas aluminum, titanium, tungsten, and copper, andpolymers such as photoresist and polyimide. It is mechanically stable and it is feasible to beintegrated into electronics on the same substrate (b/c it isa semiconducting material). Electronics for signal transduction such as the p or n-type piezoresistive can be readily integrated with the Sisubstrate-ideal for transistors.
  • 4. It has a melting point at 1400oC, which is abouttwice higher than that of aluminum. This highmelting point makes silicon dimensionally stableeven at elevated temperature.Its thermal expansion coefficient is about 8 timessmaller than that of steel, and is more than 10 timessmaller than that of aluminum
  • 5. Single-Crystal Silicon For silicon to be used as a substrate material inintegrated circuits and MEMS, it has to be in apure single-crystal form. The most commonly used method of producingsingle-crystal silicon is the Czochralski (CZ)method. The Czochralski method for producing single-crystal silicon
  • 6. The Czochralski method for producing single-crystal siliconEquipment: a crucible and a “puller”.Procedure:(1) Raw Si (quartzite) + coal, coke, woodchips)are melted in the crucible.(2) A “seed” crystal is brought to be in contactwith molten Si to form larger crystal.(3) The “puller” slowly pulls the molten Si upto form pure Si “boule” after thesolidification.(4) The diameters of the “bologna-like” boulesvary from 100 mm (4”) to 300 mm (12”) indiameters.Chemical reaction for the process: SiC + SiO2 → Si + CO +SiO
  • 7. Pure silicon wafers Pure silicon boules of 300 mm diameter and 30ft long, can weigh up to 400 Kg. These boules are sliced into thin disks (wafers)usingdiamond saws. Standard sizes of wafers are:100 mm (4”) diameter x 500 μm thick.150 mm (6”) diameter x 750 μm thick.200 mm (8”) diameter x 1 mm thick300 mm (12”) diameter x 750 μm thick(tentative).
  • 8. Single Silicon Crystal Structure Single silicon crystals are basically of “face-cubic-center” (FCC) structure. The crystal structure of a typical FCC crystal isshown below:
  • 9. •Single crystal silicon, however has 4 extra atoms inthe interior.•The situation is like to merge two FCC crystalstogether as shown below:
  • 10. •Total no. of atoms in a single silicon crystal =18.•The unsymmetrical distribution of atoms withinthe crystal make puresilicon anisotropic in its mechanical properties.• In general, however, we treat silicon as anisotropic material.
  • 11. Silicon Compounds There are 3 principal silicon compounds used inMEMS and microsystems: Silicon dioxide (SiO2), Silicon carbide (SiC) andsilicon nitride (Si3N4) – each has distinctcharacteristic and unique applications.
  • 12. Silicon dioxide (SiO2):•It is least expensive material to offer goodthermal and electrical insulation.•Also used a low-cost material for “masks” inmicro fabrication processes such as etching,deposition and diffusion.•Used as sacrificial material in “surfacemicromachining”.•Above all, it is very easy to produce:- by dry heating of silicon: Si + O2 → SiO2- or by oxide silicon in wet steam: Si + 2H2O →SiO2 + 2H2
  • 13. Silicon carbide (SiC) Its very high melting point and resistance to chemical reactions makeit ideal candidate material for being masks in micro fabricationprocesses. It has superior dimensional stability.Silicon nitride (Si3N4) Produced by chemical reaction:3SiCl2H2 + 4NH3 → Si3N4 + 6HCL + 6H2 Used as excellent barrier to diffusion to water and ions. Its ultra strong resistance to oxidation and many etchants make it asuperior material for masks in deep etching. Also used as high strength electric insulators.
  • 14. Polycrystalline silicon It is usually called “Polysilicon”. It is an aggregation of pure silicon crystals withrandomly orientations deposited on the top ofsilicon substrates:
  • 15. •These polysilicon usually are highly dopedsilicon.•They are deposited to the substrate surfaces toproduce localized “resistors” and “gates fortransistors”•Being randomly oriented, polysilicon is evenstronger than single silicon crystals.
  • 16. Silicon Piezoresistors Piezoresistance = a change in electricalresistance of solids whensubjected to stress fields. Doped silicon arepiezoresistors (p-type or n-type). Relationship between change of resistance {ΔR}and stresses {σ}:{ΔR} = [π] {σ}where {ΔR} = { ΔRxx ΔRyy ΔRzz ΔRxy ΔRxzΔRyz}T represents the change of resistances in
  • 17. an infinitesimally small cubic piezoresistive crystalelement with corresponding stress components:{σ} = {σxx σyy σzz σxy σxz σyz}T and [π] =piezoresistive coefficient matrix.
  • 18. Gallium Arsenide (GaAs) GaAs is a compound simiconductor with equalnumber of Ga and As atoms. Because it is a compound, it is more difficult toprocess. It is excellent material for monolithic integrationof electronic and photonic devices on a singlesubstrate. GaAs is also a good thermal insulator. Low yield strength (only 1/3 of that of silicon) –“bad”.
  • 19. A comparison of GaAs and silicon as substrate materials in micromachining:
  • 20. Quartz Quartz is a compound of SiO2. The single-unit cell is in shape of tetrahedron: Quartz crystal is made of up to 6 rings with 6silicon atoms.
  • 21. Quartz is ideal material for sensors because ofits extreme dimensionalstability.● It is used as piezoelectric material in manydevices.● It is also excellent material for microfluicssystems used in biomedicalapplications.● It offers excellent electric insulation inmicrosystems.● A major disadvantage is its hard in machining.It is usually etched inHF/NH4F into desired shapes.
  • 22. Piezoelectric Crystals Piezoelectric crystals are solid ceramic compounds thatproduce piezoelectric effects:
  • 23. •Natural piezoelectric crystals are: quartz,tourmaline and sodium potassium tartrate.•Synthesized crystals are: Rochelle salt, bariumtitanate and leadzirconate.
  • 24. Piezoelectric coefficients:
  • 25. Polymers What is polymer?Polymers include: Plastics, adhesives,Plexiglass and Lucite. Principal applications of polymers in MEMS:Currently in biomedical applications andadhesive bonding.New applications involve using polymers assubstrates with electric conductivity madepossible by doping.
  • 26. Molecular structure of polymers: It is made up of long chains of organic(hydrocarbon) molecules. The molecules can be as long as a fewhundred nm.Characteristics of polymers: Low melting point; Poor electricconductivity Thermoplastics and thermosets arecommon industrial products Thermoplastics are easier to form intoshapes. Thermosets have higher mechanicalstrength even at temperature up to 350oC
  • 27. Polymers as industrial materials Polymers are popular materials used for manyindustrial products for the following advantages: Light weight Ease in processing Low cost of raw materials and processes for producingpolymers High corrosion resistance High electrical resistance High flexibility in structures High dimensional stability
  • 28. Packaging Materials Unlike IC packaging in which plastic or ceramic areextensively used as encapsulate materials for thedelicate IC circuits, MEMS packaging involve a greatvariety of materials-varying from plastic andpolymers to stainless steel, as can be seen in aspecially packaged micropressure sensor: