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  • 1. Microfluidics and Lab-on-a-Chip for biomedical applications Chapter 4 : Micromanufacturing. By Stanislas CNRS Université de Lyon, FRANCE Stansan International Group
  • 2. CONTENT Chapter 1: Introduction. Chapter 2 : Basic principles of Microfluidics. Chapter 3 : Basis of molecular biology and analytical tools. Chapter 4 : Micromanufacturing. Chapter 5 : Lab-on-a-Chip & applications. Chapter 6 : Cancer diagnostics and monitoring.
  • 3. Device Fabrication
  • 4. Materials
  • 5. Fabrication Technologies Outline
  • 6. Building Microstructures
  • 7. Summary : Microsystem fabrication • Microsystem technology is inspired by silicon microelectronics technology • Main Microsystem techniques – Bulk micromachining – Surface micromachining – LIGA and variations – Wafer bonding Dedicated to Microsystems Additive & Subtractive Techniques (often spatially resolved)
  • 8. Interconnects in an Integrated Circuit
  • 9. Patterning 1. Lithography a) Photolithography b) Electron beam lithography c) Ion beam lithography d) X-ray lithography 2. Soft Lithography 3. Dedicated Spatially resolved Additive & Subtractive techniques
  • 10. Microsystem Design Methodology
  • 11. Examples of Microsystem (from Sandia Laboratories)
  • 12. Examples of a Microfluidic System (from Sandia Laboratories) World's Smallest Microsteam Engine Water inside of the compression cylinder is heated by a flow of electric current and vaporizes, pushing the piston out. Capillary forces then retract the piston when the current is not flowing.
  • 13. Photolithography Photolithography process involves the use of an optical image (on the photolitographic mask) and a photosensitive film (on the substrate) to produce desired patterns on the substrate. Photolithographic mask: A quartz plate with Chromium patterns through which light can not pass. Methods of mask fabrication : leaser writing, electron beam writing. The mask is then placed above the top-face of a silicon substrate coated with thin film of photoresistive materials. Then, the photoresist is exposed through the mask to an UV light, X-rays or E-beam (depending on the type of the photoresist and the process requirements).
  • 14. Application of photoresists ● The process begins with securing the substrate wafer onto the top of a vacuum chuck. ● A resist puddle is first applied to the center portion of the wafer from a dispenser. ● The wafer is then subjected to high speed spinning at a rotational speed from 1500 to 8000 rpm for 10 to 60 seconds. The speed is set depending on the type of the resist, and on the desired thickness. ● The centrifugal forces applied to the resist puddle cause a uniform spread of the fluid over the entire surface of the wafer. ● Typically the thickness is between 0.5 – 10 µm with ±5 nm variation. ● Pre-bake (e.g. 90°C, 60 min). PR Dispenser ● Exposure through the mask. ● Develop in appropriate solution. ● Post bake (e.g. 90°C, 30 min). Wafer Chuck Spin Xrpm
  • 15. The two main kinds of photoresists Positive resist – destroy bonds, the exposed area is soluble Negative resist – crosslinking, the exposed area is NOT soluble
  • 16. Light sources Photoresist materials used in micro fabrication are sensitive to light with wavelength ranging from 300 to 500 nm. Most popular light source for photolithography is the mercury vapor lamps. This light source provides a wavelength spectrum from 310 to 440 nm. Deep UV (ultra violet) light has a wavelength of 150-300 nm. In special applications for extremely high resolutions, x-ray is used. The wavelength of x-ray is in the range from 4 to 50 Angstrom. (an Angstrom, = 0.1 nm or 10-4 µm).
  • 17. Direct Write Hardware
  • 18. Subtractive & Additive Processes
  • 19. Etching
  • 20. Silicon
  • 21. Pure Silicon wafers Pure silicon boules of 300 mm diameter and 30 ft long, can weigh up to 400 Kg.
  • 22. Silicon Crystal Structure ● Single silicon crystals are basically of “facecubic-center” (FCC) structure. ● Total no. of atoms in a single silicon crystal = 18. ● The unsymmetrical distribution of atoms within the crystal make pure silicon anisotropic in its mechanical properties.
  • 23. Planes of a Cubic Crystal
  • 24. Anisotropic Etching of Silicon
  • 25. Wet anisotropic etchants for Silicon and Silicon compounds
  • 26. Bulk micromachining of Si Anisotropic Etching
  • 27. Anisotropic Etching
  • 28. Isotropic Etching of Silicon 1. Isotropic Silicon etch Isotropic etching of silicon is typically performed using an etchant consisting of HNO3 and HF. 6HF + HNO3 + Si H2SiF6 + HNO2 + H2O + H2 2. Isotropic Silicon etch HNA (160 ml acetic acid, 60 ml nitric acid, 20 ml hydrofluoric acid) This is an extremely aggressive acidic mixture which will vigorously attack silicon. It is an isotropic wet etchant which etches silicon at a rate of approximately 1-3 microns per minute. Substrate must be clean with hard nitride mask. No photoresist.
  • 29. Microsystem's manufacturing technologies : Bulk and Substrate micromashining Bulk micromachining Bulk micromachining is the oldest paradigm of silicon based MEMS. The whole thickness of a silicon wafer is used for building the micro-mechanical structures. Silicon is machined using various etching processes. Anodic bonding of glass plates or additional silicon wafers is used for adding features in the third dimension and for hermetic encapsulation. Bulk micromachining has been essential in enabling high performance pressure sensors and accelerometers that have changed the shape of the sensor industry in the 80's and 90's.
  • 30. Bulk micromachining makes thick structures
  • 31. Microsystem's manufacturing technologies : Bulk and Substrate micromashining Surface micromachining Surface micromachining uses layers deposited on the surface of a substrate as the structural materials, rather than using the substrate itself. Surface micromachining was created in the late 1980s to render micromachining of silicon more compatible with planar integrated circuit technology, with the goal of combining MEMS and integrated circuits on the same silicon wafer. The original surface micromachining concept was based on thin polycrystalline silicon layers patterned as movable mechanical structures and released by sacrificial etching of the underlying oxide layer. Interdigital comb electrodes were used to produce in-plane forces and to detect in-plane movement capacitively. This MEMS paradigm has enabled the manufacturing of low cost accelerometers for e.g. automotive air-bag systems and other applications where low performance and/or high granges are sufficient. Analog Devices have pioneered the industrialization of surface micromachining and have realized the cointegration of MEMS and integrated circuits.
  • 32. Surface Micromachining substrate Important issues: • selectivity of structural, sacrificial and substrate materials • stress of structural material • stiction
  • 33. Surface Micromachining Most commonly used materials for surface micromachining: • substrate: silicon • sacrificial material: SiO2 • structural material: polysilicon, Si3N4 Alternative materials : Substrates Sacrificial Structural Glass Plastic metals Polymer Metals silicon nitride Thin film silicon (a-Si:H, c-Si) silicon nitrides Silicon carbide Metals polymers bilayer composites
  • 34. Surface micromachining on glass Sacrificial Layer D eposition and Patterning Structural Layer D eposition and Patterning Sacrificial Layer R oval em d=1 m; h=500 nm; b=10 m Lmax(bridge) ~ 60 m ; Lmax(cantilever) ~ 30 m
  • 35. Surface micromachining makes thin structures
  • 36. Etching with BHF of SiO2 or glass Buffered oxide etch (BOE), also known as buffered HF or BHF, is a wet etchant used in microfabrication. Its primary use is in etching thin films of silicon dioxide (SiO2) or silicon nitride (Si3N4). It is comprised of a mixture of a buffering agent, such as ammonium fluoride (NH4F), and hydrofluoric acid (HF). Concentrated HF (typically 49% water) etches silicon dioxide too quickly for good process control. Buffered oxide etch is commonly used for more controllable etching. Some oxides produce insoluble products in HF solutions. Thus, HCl is often added to BHF solutions in order to dissolve these insoluble products and produce a higher quality etch. A common buffered oxide etch solution comprises a 6:1 volume ratio of 40% NH4F in water to 49% HF in water. This solution will etch thermally grown oxide at approximately 2 nanometres per second at 25 degrees Celsius.
  • 37. Example : etchnig of microfluidic channels in soda-lime-silica glass Technologie optimized in our group : xº‚ x#Û#¸{"O¸¥GB�Yø####opendo 1) Substrat of soda-lime-silica glass of Corning. 2) Cleaning in « piranha mixture » : perhydrol : H2SO4 3) Deposition (by sputtering) of Cr mask and of a photoresist 4) Photolithography. 5) Selective etching of the Cr layer(commercial chromium etchant). 6) Etching of glass substrate in BHF + HCl 7) Etchnig of remaining Cr layer.
  • 38. Example : etchnig of microfluidic channels in soda-lime-silica glass Etching in BHF Etching in BHF + HCl Channel width : 70 µm
  • 39. Dry Etching Dry etching involves the removal of substrate materials by gaseous etchants without wet chemical or rinsing. Dry etching is one of the core processes in microsystem's technology. Compared to wet etching RIE enables new possibilities such as etching of vertical structures independent of the crystal structure in the material. The dry etching also leaves you with the possibility to etch delicate structures without exposing them to a liquid that might ruin the structures by breakage or sticking particles. Several dry etching techniques : 1) The etching of silicon with XeF2 vapor (no plasma) 2) Plasma Etching (PE) 3) Reactive Ion Etching (RIE) 4) Deep Reactive Ion Etching (DRIE) 5) Inductive Coupled Plasma (ICP) 6) Focused Ion Beam (FIB) or Ion Milling
  • 40. Potential distribution in the Plasma Reactor The etching mechanisms can be influenced by ion bombardment. Ion bombardment is caused by positive ions which are accelerated by the negative DC voltage towards the LOWER electrode and the wafer placed on this electrode. The vertical etch rate is increased, while the horizontal etch rate remains constant. For this reason, it is possible to obtain an anisotropic etching with plasmas, even for noncrystalline structures, We go from « PE etchning conditions » to «RIE » etching conditions. However, the most common mechanism to obtain an anisotropic vertical etch process is through the use of a passivation layer at the vertical surfaces (DRIE process).
  • 41. Example of a capacitively coupled plasma (CCP) reactor Capacitively coupled RF plasmas are still the most common plasmas used in dry etching. A typical reactor chamber is shown above. The power is applied to the lower or the upper electrode (or in some special cases to the reactor walls ). In general the frequency of the applied power is 13.56 MHz.
  • 42. Schematic of an ICP System
  • 43. (PE) Plasma Etching in practice standard method for low damage isotropic etch available as large batch system substrate electrode on ground potential, cooled top electrode RF driven (13.56 MHz) shower head gas inlet (in the top electrode) parameter: gas flows, pressure, RF power typical process pressure: 200 - 1.000 mtorr low ion energies ( 3 - 20 eV) Typical Applications : - isotropic low damage - photoresist stripping - isotropic SiN removal - plasma cleaning
  • 44. Reactive Ion Etching (RIE) In reactive ion etching (RIE), the substrate is placed inside a reactor in which several gases are introduced. A plasma is struck in the gas mixture using an RF power source, breaking the gas molecules into ions. The ions are accelerated towards, and react with, the surface of the material being etched, forming another gaseous material. This is known as the chemical part of reactive ion etching. There is also a physical part which is similar in nature to the sputtering deposition process. If the ions have high enough energy, they can knock atoms out of the material to be etched without a chemical reaction. It is a very complex task to develop dry etch processes that balance chemical and physical etching, since there are many parameters to adjust. By changing the balance it is possible to influence the anisotropy of the etching, since the chemical part is isotropic and the physical part highly anisotropic the combination can form sidewalls that have shapes from rounded to vertical. Another versionn of RIE can be deep and its name will be Deep RIE or DRIE Deep reactive ion etching (DRIE)
  • 45. Deep Reactive Ion Etching (DRIE) DRIE is a highly anisotropic etch process used to create deep holes and trenches in wafers, with aspect ratios of 30:1 or more. It was developed for microelectromechanical systems (MEMS), which require these features, but is also used to excavate for creating through wafer via's in advanced 3D wafer level packaging technology. The primary technology is based on the so-called "Bosch process", which can fabricate truly vertical walls. The Bosch process, also known as pulsed or time-multiplexed etching, alternates repeatedly 3 modes to achieve nearly vertical structures. What also distinguishes DRIE from RIE is etch depth: Practical etch depths for RIE is limited to around 10 µm at a rate up to 1 µm/min, while DRIE can etch features much greater, up to 1000 µm or more with rates up to 20 µm/min or more in some applications Mostly we are doing RIE of dielectric films and DRIE of silicon.
  • 46. DRIE - Bosch Process
  • 47. Deep Reactive Ion Etching (DRIE) EXAMPLE DRIE is a high aspect ratio, deep trench silicon etching process (Bosch process). The principle of the deep trench silicon etching process is an alternating fluorine based etching and passivation of the structures. Masking layers can be made of photo resist or silicon oxide. Main Benefits of DRIE - etch rate of up to 10 µm/min - aspect ratio up to 40:1 - selectivity to positive resist > 75:1 - selectivity to silicon oxide >150:1 - etch depth capability 10 to 675 µm (through wafer etching) - sidewall profile 90°±1° - eature size 1 to >500 µm
  • 48. ICP Deep Plasma Etching of Polymethylmethacrylate (PMMA) to a depth of 120 microns
  • 49. Materials and Gas Systems in Plasma Etching
  • 50. Focused Ion Beam (FIB) Focused ion beam (FIB) systems have been produced commercially for approximately twenty years, primarily for large semiconductor manufacturers. FIB systems operate in a similar fashion to a scanning electron microscope (SEM) except, that FIB systems use a finely focused beam of ions (usually gallium) that can be operated at low beam currents for imaging or high beam currents for site specific sputtering or deposition. Why ions ? In summary, ions are positive, large, heavy and slow, whereas electrons are negative, small, light and fast. The most important consequence of the properties listed above is that ion beams will remove atoms from the substrate and because the beam position, and size are so well controlled it can be applied to remove material locally in a highly controlled manner, down to the nanometer scale.
  • 51. The principle of FIB
  • 52. Focused ion beam (FIB) apparatus
  • 53. FIB – Example of application
  • 54. Silicon Compounds
  • 55. Thermal oxidation process
  • 56. Silicon carbide (SiC) & Silicon nitride (Si3N4)
  • 57. Evaporation Vacuum thermal evaporation (VTA): Uses resistive heating, laser heating, or magnetic induction to elevate the source temperature. Electron beam evaporation (EBE): The electron beamis focused on the target material, which locally melts.
  • 58. Sputtering
  • 59. Chemical Vapor Deposition ● Chemical vapor deposition (CVD) is the most important process in Microfabrication. ● It is used extensively for producing thin films by depositing many different kind of foreign materials over the surface of silicon substrates, or over other thin films that have already been deposited to the silicon substrate. ● Materials for CVD may include: (a) Metals: Al, Ag, Au, W, Cu, Pt and Sn. (b) Other materials: Al2O3, polysilicon, SiO2, Si3N4, piezoelectric ZnO... ● There are three (3) available CVD processes in microfabrication: (a) APCVD: (Atmospheric-pressure CVD) (b) LPCVD (Low-pressure CVD), (c) PECVD (Plasma-enhanced CVD). ● CVD usually takes place at elevated temperatures
  • 60. Working principle of AP CVD and LP CVD
  • 61. Plasma Enhanced CVD (PECVD) ● Both APCVD and LPCVD operate at elevated temperatures, which often damage the silicon substrates. High substrate surface temperature is required to provide sufficient energy for diffusion and chemical reactions. ● The operating temperatures may be avoided if alternative form of energy supply can be found. ● CVD using plasmas generated from high energy RF (radio-frequency) sources is one of such alternative methods. ● This popular deposition method is called “Plasma Enhanced CVD” or PECVD. ● A typical PECVD reactor is shown here :
  • 62. Comparison of 3 CVD Processes
  • 63. Evaporation Examples of evaporated materials : Resistance Heaters or Electron Beam Heated Source. Electron Beam Heated Source
  • 64. Electroplating
  • 65. The LIGA Process
  • 66. The LIGA Process
  • 67. The LIGA Process
  • 68. The LIGA Process
  • 69. Polymers
  • 70. Polymers
  • 71. Polymers Polymethylmethacrylat – PMMA Cycloolefincopolymer – COC Polydimethylsiloxane – PDMS SU - 8 Polyoxymethylen – POM Polyamid – PA Polycarbonat – P Polyetheretherketon – PEEK Polytetrafluoroethylene – PTFE (teflon) Fluorinated Ethylene Propylene – FEP (teflon)
  • 72. Polymer microfabrication Polymer applications Photoresists (high aspect ratio, X-ray litography) Sacrificial layers Parts of microsystems (in particular for microfluidics) Wafer bonding
  • 73. Polymer structuring methods Casting Casting is a manufacturing process by which a liquid material is poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process. A typical casting material used in microfluidics is PDMS. Hot embossing Hot embossing is a technique to produce microstructures in thermoplastic polymers. During the process the substrate is heated slightly above the glass transition point. Then a structuring tool is pressed on the polymer. This tool is a stamp that has the microstructure that will be transferred on the substrate. After the polymer has cooled down, the tool is removed and the structure stays in the substrate. Injection moulding Injection moulding is a manufacturing process for producing parts from both thermoplastic and thermosetting plastic materials. Material is fed into a heated barrel, mixed, and forced into a mold cavity where it cools and hardens to the configuration of the mold cavity.
  • 74. Polymer structuring methods
  • 75. Polymethylmethacrylat – PMMA - Often use as an alternative to glass - Easily scratched - Not malleable - It can come in the form of a powder mixed with liquid methyl methacrylate (MMA), which is an irritant and possible carcinogen Fabrication of high aspect ratio structures requires the use of a photoresist able to form a mold with vertical sidewalls. Thus the photoresist should have a high selectivity between the exposed and the unexposed area in the developer. It should be relatively free from stress when applied in thick layers necessary to make high aspect ratio structures. PMMA is the photoresist of choice in the LIGA process, mainly for its ability to hold vertical sidewalls for tall structures. Part of microsystems, in particular in microfluidics and micro-optics.. In polymer-based microfabrication techniques, microinjection molding is most popular and generally used for micromolding in the industry. However, compared to the microinjection molding, hot embossing provides several advantages such as a relatively low-cost for embossing tools, simple operation and higher accuracy in the replication of small features.
  • 76. Hot embossing proces of PMMA
  • 77. PMMA cell culture chip
  • 78. Cycloolefincopolymer – COC Cyclic olefin copolymer (COC) draws attention as a primary substrate material for the microfluidic lab on a chip (LOC) applications based on its various advantages with regards to physical and chemical properties. For the replication of small structures (especially less than 10 μm), hot embossing is preferable and provides the simple fabrication process compared to the injection molding technique. However, as far as productivity is concerned, an injection molding process is the most effective replication technique for the polymer based LOC which has microstructures with feature size greater than 10 μm. For the injection molding of the COC LOC, the fabrication process of nickel mold inserts is introduced based on UVphotolithography and subsequent electroplating process.
  • 79. COC for microfluidics COC microfluidic components
  • 80. Polydimethylsiloxane – PDMS - Silicon-based, organic polymer - Non-toxic - Non-flammable - Gas permeable - Most organic solvents can diffuse and cause it to swell PDMS is commonly used as a stamp resin in the procedure of soft lithography, The process of soft lithography consists of creating an elastic stamp, which enables the transfer of patterns of only a few nanometers in size onto glass, silicon or polymer surfaces. With this type of technique, it is possible to produce devices that can be used in the areas of microsystems for optic telecommunications or biomedical research. The resolution depends on the mask used and can reach 6 nm. PDMS is commonly used as a material for fast prototyping of microfluidic devices.
  • 81. Microfluidic Device from Teflon
  • 82. SU-8 Photoresists
  • 83. SU-8 realization example
  • 84. Polymer microfluidic Chips
  • 85. Stereolithography Stereolithography is a common rapid manufacturing and rapid prototyping technology for producing parts with high accuracy and good surface finish. Stereolithography is an additive fabrication process utilizing a vat of liquid UV-curable photopolymer "resin" and a UV laser to build parts a layer at a time. On each layer, the laser beam traces a part cross-section pattern on the surface of the liquid resin. Exposure to the UV laser light cures, or, solidifies the pattern traced on the resin and adheres it to the layer below. After a pattern has been traced, the elevator platform descends by a single layer thickness, typically 0.05 mm to 0.15 mm (0.002" to 0.006"). Then, a resin-filled blade sweeps across the part cross section, re-coating it with fresh material. On this new liquid surface the subsequent layer pattern is traced, adhering to the previous layer. A complete 3-D part is formed by this process. After building, parts are cleaned of excess resin by immersion in a chemical bath and then cured in a UV oven.
  • 86. Structuring methods Laser photo ablation Laser photo ablation is a direct micromachining method. It is based on the removal of polymer material by using intense UV or infrared irradiation provided by a laser. The radiation wavelength used affects the removal mechanism. If infrared lasers are used, the irradiated material is heated and decomposes, leaving a void in the polymer material. If UV radiation is used, the irradiated polymer decomposes, presumably of a mixture of two mechanisms: thermal and direct bond breaking. Thermal bond breaking is induced by heat, as with infrared irradiation. In direct bond breaking, polymer molecules directly absorb ultraviolet photons, often absorbing enough energy so that the chemical bonds within the polymer chains are broken. Micro-milling Micro milling is a mechanical way to structure a substrate. It works like milling, but on a microscopic scale. The material is removed by a fast rotating cutter. By moving this tool, the structure is created on the substrate. Micro-sandblasting Sandblasting is a generic term for the process of smoothing, shaping and cleaning a hard surface by forcing solid particles across that surface at high speeds. Micro sandblasting uses air to create structures on a micrometric scale.
  • 87. Laser Micromachining of Microfluidic Devices in Glass Substrates SEM image of a laser machined microfluidic pattern in glass. Left Bar denotes 1mm, Right bar denotes 0.1mm
  • 88. Example of metalization technique using a fs laser
  • 89. Nanopatterning Nanopatterning can be defined as nanolitography techiques that do not use mask or direct radiation writing to expose a resist layer, but imprint a nanoscale patern by means of mechanical tools. When the physical action is based on preasure assisted by heat or light, the techniques are called NIL (Nano-Imprint-Litography). Examples :
  • 90. Soft Litography, Microcontact Printing (µCP) Contrary to NIL, the µCP stamp is "inked" with a product which form a self-assembled monolayers.Self-assembly is ubiquitous in nature. Self-assembly is the spontaneous organization of molecules into stable, well defined structires by noncovalent forces. Another difference between NIL and µCP is that in the case of µCP the material is deposited from the stemp on the substrate. Furthermore, this material can be chosen over a wide range of materials, which can not be "imprinted" by NIL. It is conceivable to modify a NIL machine and make it working for µCP. Soft Lithography was pioneered by George Whitesides at MIT and David Beebe at University of Wisconsin.
  • 91. Self-Assembled Monolayers
  • 92. Alkanethiolates CH3(CH2)nS- on Gold
  • 93. µCP - Principle (Whitesides)
  • 94. µCP – Fabrication of microfluidic Channels in glass substrates µCP is performed using patterned PDMS stamp inked in alkanethiols (e.g. Hexadecanethiol - HDT), that forms self-assembled monolayers (SAMs) on glass substrates possessing on the top layers of Cr and Ag (our own experience).
  • 95. µCP – Fabrication of microfluidic Channels in glass substrates Top view of the glass etched channel cross-junction and reservoir (optical microscope). (our own
  • 96. Rapid « Multiplication » of glass microfluidic devices (our experience) own
  • 97. µCP – experimental set-up (our own development)
  • 98. Micro-contact Printing (µCP) View of the Microcontact Printer. The microscope slide is placed in the substrate holder, which has three degrees of freedom (YZX) to position and align the pattern of the stamp on the substrate. The red PDMS stamp, placed in a stamp holder with two degrees of freedom (YZX), is made parallel to the substrate. The stamp is inked by means of the miniaturized pulverization head and is guided toward the substrate by a linear slide actuated with a pneumatic jack. The load on the stamp during the contact with the substrate is controlled via air pressure from the piston. The stamp is released from the substrate by reversing the direction of airflow in the piston (our own experience).
  • 99. µCP of antibodies in the microfluidic channel (our own development)
  • 100. Wafer bonding techniques
  • 101. Bonding Adhesive bonding Adhesive Bonding is a modern joining process in which a liquid or semi liquid substance is applied to adjoining work pieces to provide a long lasting bond. This process is highly useful in bonding dis-similar materials that can not be welded. Materials that have the ability to be bonded together are virtually unlimited. Adhesives used in bonding can exist in many forms and be made from various natural and/or artificial compounds. A hindrance to this process is that adhesive bonds are not instantaneous such as welding or nailing. Adhesive bonds take more time to process, in order to allow the adhesives to cure. Anodic bonding Anodic bonding is the bonding of two substrates , usually glass and silicon, by an electrical potential. The substrates are placed between two electrodes and at temperatures around 400° C a high potential (around 1 kV) is applied to the substrates. This forces sodium ions in the glass to move away from the bonding surface. Therefore the surface is highly reactive and bonds easily to the other substrate
  • 102. Bonding Direct bonding The substrates are first exposed to a chemical treatment, for example with a mixture of hydrogen peroxide and sulphuric acid. This makes them hydrophilic what is essential for the bonding. By a high contact force, two substrates (e.g two silicon wafers) are compressed. The materials get so close, that molecular adhesive forces begin to act. The material then is annealed at high temperature so that the direct bonding is strong enough to keep the substrates together. Plasma bonding Plasma activation has to be done to improve adhesion properties of surfaces prior to coating. Weakly ionised oxygen plasma is used. Plasma removes surface layers with the lowest molecular weight, at the same time it oxidises the uppermost atomic layer of the polymer. Oxygen radicals help break up bonds and promote the three dimensional bonding of molecules. Oxidation of the polymer is responsible for the increase in polar groups which is related to the adhesion properties of the polymer surface. After having exposed the substrates to the oxygen plasma they are pressed together and then heated to a temperature slightly below the glass transition temperature.
  • 103. Bonding Roll laminating In roll laminating, two polymer foils are bonded. Usually one of them is structured. Roll laminating is a continuous process and therefore suitable for mass production. The rolls of foil are unwound during the process and pressed together. High temperature and pressure are usually necessary but variable. Optionally, chemical agents may be added. Thermal bonding Thermal bonding exploits the fact that thermoplastic polymers become soft at elevated temperatures close to the glass transition temperature. Therefore the substrates to be bonded are heated and then pressed together. As the polymer is soft, bindings between the two layers establish. Care must be taken to choose the right temperature to bond the substrate without damaging the microstructure.
  • 104. One can say : Micro-fabrication is DIFFICULT: Many of them are still vibrant fields of research… Micro-fabrication is EXPENSIVE at developing stage... Micro-fabrication is expected to be EASY and CHEAP at high volume production...
  • 105. THANK YOU FOR YOUR ATTENTION Any question ?

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