Polymer Micromachining Packaging of MEMS Sensors Mikrosensorer

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Polymer Micromachining Packaging of MEMS Sensors Mikrosensorer Mikrosensorer Polymer Micromachining Packaging of MEMS sensors Project meeting 1 Sensor Time Accelerometer 18/4 , 9.15 Pressure sensor 18/4 , 10.15 Flow sensor 18/4, 11.15 Polymer Micromechanics Polymer Micromachining Cheaper materials Low cost fabrication Allow single use ”thermal recycling” Rapid prototyping Very flexible electronic components (design & state) Polymer Micromechanics • Thin Film Litography • Hot Embossing • Injection Moulding • Laser Fabrication • 3D-printing Polymer classes Plastic: materials that can be formed into shapes. Thermoplastic: materials that can be shaped more than once. Thermosetting plastic: material that can only be shaped once. Elastomer: material that is elastic in some way. If a moderate amount of deforming force is added, the elastomer will return to its original shape. Useful for fibers. Properties of Polymers • Chain length • Structure, orientation of chains • Identity of side groups • Degree of cross‐linking – Cross linking with covalent bonds formed between chains make the polymer stiffer; more crystalline Thermoplastics Melting (Tm) Rubbery flow OSTE(+) Carlborg et al Lab on a chip 2011 Saharil, F., et al µTAS 2012 Particle doped polymers • Magnetic particles • Silver • Carbon / graphene Hot Embossing Nano Imprint Litography (NIL) Thin Films Higher Temperatures Curable Polymers Limit in aspect ratio rather than linewidth Very high resolution Injection Moulding High throughput massfabrication Expensive initial cost Limited resolution (10 µm) 3D printing Stereolitography • Polymerisation by multi photon absorption • High energy density achieved by fs- pulses • Resolution down to 120nm • Completely arbitrary 3D -structures Microfluidic filters Cell Gym Adv. Mater. 23 (2011) Microrobots Adv Materials 24, 2012 Laser Ablation • Evaporation – Long wavelength (~1µm) – Gaussian profile – Condensation ”bumps” Laser Ablation • Ionisation – Short wavelength (200 – 300 nm) – High power bursts – Smaller spot size (5 µm) – High aspect ratio Materials • Metals • Ceramics • Polymers • Crystalline materials • Semiconductors • Delicate materials • Glass • Flammables/explosives Cutting Drilling Structuring in 2.5 D Packaging • MEMS Packaging Issues • MEMS Packaging Approaches • Electrical connections • Sealing Recommended Literature Handbook of silicon based MEMS Materials & technologies Author: Lindroos, Veikko. Available as eBook on http://www.lub.lu.se/en/search/lubsearch.html Part V: Encapsulation of MEMS Components Packaging • One of least explored MEMS components • Litterature is scarce • No unique and generally applicable packaging method for MEMS • Each device works in a special environment • Each device has unique operational specs Design Issues in MEMS packaging • Up to and exceeding 80% of total cost • Sensors need direct access to the environment • Package must be specifically designed for device • Reliability • Media compatibility • Modularity • Small quantities Example Accelerometer • Key Issues - Free standing microstructures - Temperature sensitive microelectronics - Hermetic sealing - Alignment Example Pressure Sensor Key Issues – Exposure to external pressure – Housing for harsh environment – Interface coating Example Microfluidic Device Key Issues – Micro-to-Macro interconnections – Good sealing – Temperature sensitive materials – Optical access Packaging serves two main functions • Protection from environment – Electrical isolation from electrolytes and moisture – Mechanical protection to ensure structural integrity – Optical and thermal protection to prevent undesired effects on performance – Chemical isolation from harsh chemical environment Packaging serves two main functions • Protection from device – Material selection to eliminate or reduce host response – Device operation to avoid toxic products – Device sterilization – Size and contacts Major Issues in MEMS packaging • Release and stiction • Die handling and dicing • Stress • Outgassing • Testing • Electrical contacts • Encaptulation / Hermetic seals • Integration Die Packaging Operations • Die separation (dicing) • Preseal inspection • Die pick • Packaging and Sealing (c) • Die attach (a) • Plating • Inspection • Lead trim • Wire Bonding (b) • Final Tests Packaging levels • Wafer • Die • Device • System Wafer Level Packaging • To adopt IC packaging processes as much as possible • Stay in Batch process as long as possible • Includes both interconnections and Encapsulations Electrical Contacts Wire bonding • Wire bonding – Most common method – Uses variety of metals depending on bondpad Ball bond Wedge bond Electrical Contacts Wire bonding Wedge bonding – Aluminium or Gold wire – Aluminium is ultrasonically bonded at room temperature Electrical Contacts Wire bonding Ball bonding – Gold or Copper (Need inert atmosphere) – Ball is formed with high voltage arc or hydrogen torch Electrical Contacts • Flip chips – Solder bumps used to attach flipped chip – Quick universal connection – Allows individual chip optimization – Connect dissimilar materials Anisotropic Conductive Film • Polyester film with 10µm Particles of Gold coated polymer Sealing Methods • Hermetic – Soldering, Brazing, Welding (Metals) – Anodic bonding, Glass frit (Glass) – Wafer bonding (Silicon) • Nonhermetic – Epoxy molding – Blob top (polymers) Sealing Methods Issues • Thermal expansion • Permeability • Surface Roughness Package Encapsulation • Protection from corrosion, mechanical damage • Moisture is one of the major sources of corrosion Metal sealing methods Soldering and Brazing • Soldering – Tin-Lead solder (indium and silver are sometimes added) – Tin-Lead oxidizes easily and should be stored in nitrogen • Brazing – Eutectic Au-Sn (80:20) at 280oC – 350oC for stronger, more corrosion-resistant seal and the use of flux can be avoided Eutectic Bonding • Formed by heating two materials (Au and Si) so they diffuse together. • The resulting alloy composition melts at a lower temperature than the base materials (97Au - 3Si eutectic melts at 363°C). Eutectic Bonding • Benefits: • Limitations: • Good thermal conductivity • High stresses on Si chip due to • Electrically conducting CTE mismatch on larger dies • Good fatigue/creep • Relatively high processing resistance temperatures • Low contamination • Die back metallisation may be • 'High' process/operating required temperature capability. • Rework is difficult. Metal sealing methods Glass Sealing • Hermetic glass-to-metal seals or glass-ceramic seal • Chemical inertness, oxidation resistance, electrical insulation, impermeability to moisture and other gasses, wide choice of thermal characteristics • Soft glass sealing are made by lead-zinc-borate glasses below 420oC ->low water content, good chemical durability, thermal expansion closely matched to that of the ceramic Glass Sealing Disadvantages: • low strength and brittleness • Water is absorbed on glass network and may get released into the sealed cavity Anodic bondning Anodic Bonding • Sodium-rich glass (Pyrex) • Operation temperature is well below the melting temperature of glass • Surface roughness < 1 µm • Native oxide on Si must be thinner than 0.2 µm • Bonding temperature below 500oC or the thermal properties of materials begin to deviate seriously Glass Frit Bonding • Low melting point glass (lead-glass, 430C) • Screen printed as grained glass paste • Burn-out (melting to real glass) • Bonding (Melting) • Excellent Hermetic sealing to most materials Silicon Fusion Bonding • Clean surface, roughness < 4 nm • Activated (Hydrated) in warm sulfuric acid • Weak Hydrogen bond • Dehydration in 1000oC • Forms stable silicondioxide bond • Possible to do hydrophobic bond with weak H-F binding Low temperature Si bonding • Plasma Activation Based Low- Temperature Bonding • UHV Low-Temperature Hydrophobic Bonding • Direct Bonding of CVD Oxides Wafer Bonding Processes • Anodic Bonding – Temperature ~450oC, voltage ~1000 volts – Silicon (metal) to glass • Glass Frit Bonding – Temperature ~450oC, voltage – Silicon (metal) to glass • Fusion Bonding – Temperature ~1000oC – Silicon to silicon (glass, oxide) • Eutectic Bonding – Silicon to metal (silicon-to-gold ~363oC) LPCVD encapsulation (a) Standard surface micromachining process (b) Additional thick PSG (phosphosilicate glass) deposition to define encapsulation regions (c) Additional thin PSG deposition to define etch channels LPCVD encapsulation (d) Nitride shell deposition; etch hole definition (e) Removal of all sacrificial PSG inside the shell; supercritical CO2 drying; global LPCVD sealing CVD Chemical Vapor Deposition • Chemical reaction in vapor phase forms a solid film • Pressure and temperature dependent • Activation energy (heat, radiation, plasma) Polysilicon, Nitrides, Oxides, Semiconductors (III - V) Metals, Polymers, Diamond CVD Chemical Vapor Deposition Critical deposition temperature of niobium as a function of NbCl5 initial pressure. CVD Chemical Vapor Deposition • Atmospheric-pressure CVD (APCVD) • Low-pressure CVD (LPCVD) • Plasma-enhanced CVD (PECVD) • Photo-enhanced CVD (PHCVD) • Laser-induced CVD (PCVD) • Metalorganic CVD (MOCVD) Polymer Sealing • Advantages: – Low bonding temperature – No metal ions – Elastic property of polymer can reduce bonding stress • Disadvantages: – Not a good material for hermetic sealing – High vapor pressure – Poor mechanical properties • Examples: – Silicone (Blob top) – UV-curable encapsulant resins – Thick ultraviolet photoresists such as polyimides, AZ-4000, and SU-8 Thermal bonding of polymers Melting (Tm) Rubbery flow The substrates are heated above Tg and pressed together Laser bonding of polymers Other bonding methods • UV Curable Materials • Photoresists • Adhesives (Glues, Silicones) • Waxes • Chemical Bonding • Hydrophilic bond Adhesive application on structured surfaces .
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