Introduction to microfabrication sami franssila pdf free download

A microvoid causes a laser scatterometry signal similar to a particle. Vacancy clusters were therefore classified as particles, and were given the name COP, for Crystal Originated Particles today, advanced multiangle scatterometry tools can distinguish voids from particles. It was the fact that the number of COPs did not decrease in cleaning and it could in fact increase! Haze is defined as light scattering from surface defects, for example, scratches, surface roughness or crystal defects.

Calculate an estimate for silicon lattice constant from atomic mass and density. Consider an Olympic swimming pool filled with golf balls and one squash ball. If the golf balls represent silicon atoms, and the squash ball represents a phosphorous atom, what would be the resistivity of a silicon piece with such a doping concentration? Electronic grade polysilicon is available with 0. What is the highest ingot resistivity that can be pulled from such a starting material?

If 50 kg of ultrapure polysilicon is loaded into a CZcrystal puller, how much boron should be added if the target doping level of the ingot is 10 ohm-cm? If the wafer-resistivity specifications are 5 to 10 ohm-cm phosphorus , calculate the fraction of the ingot that yields wafers within this specification. If the neck in a CZ-ingot is 2 mm in diameter, what is the maximum ingot size that can be pulled before the silicon yields catastrophically?

Silicon 45 Fischer, A. Green, M. Hull, R. Jenkins, T. Petersen, K. IEEE, 70 , Reprinted in W. Trimmer ed. Shimura, F. Thin films have roles as permanent parts of finished devices, but they are also used intermittently during wafer processing as protective films, sacrificial layers and etch and diffusion masks. Metallic, semiconducting and insulating films are employed Table 5. Films are often used, however, not because of their metallic, semiconducting or dielectric properties, but for other features.

For example, doped single-crystalline silicon carbide is a semiconductor, but amorphous SiC thin films are insulators for all practical purposes. Similarly, silicon is used not only for its electronic properties but also for its mechanical strength micromechanics , optical absorption in visible wavelengths solar cells, photodetectors , low absorption in infrared waveguides for 1.

Silicon nitride is used for free-standing thin membranes as etch and oxidation mask, as an etch-stop and polish-stop layer and as a passivation material that protects from mechanical and chemical damage.

For narrow lines, two dimensions are small, and for dots all three dimensions are small. The size scale for quantum effects is estimated by Debye lengths, which are of the order of 10 to nm at room temperature. The density of thin films is often very low compared to bulk materials. Thin films are often porous, which results in long term instability: humidity can be absorbed in the film, and high surface-area porous films oxidize and corrode readily.

Deposition processes have profound effects on all film properties as shown in Table 5. The films have been deposited in different sputtering systems under slightly different process conditions.

Structure depends on film thickness, and it may be that thick films are polycrystalline even though thinner depositions result in amorphous structure. This is shown in Figure 5. X-ray diffraction XRD peaks indicative of crystallinity only appear for thicker films. Films prepared by different sputtering systems are different, and films prepared by two completely different deposition processes will differ even more.

Copper Thin-film Materials and Processes 49 films made by sputtering, evaporation, electroplating or chemical vapour deposition CVD can have a factor of 2 differences in resistivity or grain size.

When an amorphous film is annealed at high temperature, it will crystallize. But its crystal size and crystal orientation, and surface roughness will be different from a film that was initially polycrystalline, even though the films received identical anneals.

Very thin films are discontinuous and the thickness required for continuous films is process- and materialdependent. Shutter blades enable very accurate and abrupt interfaces to be made, almost at the atomic thickness limit. The general idea of PVD is material ejection from a solid target material and transport in vacuum to the substrate surface Figure 5.

Atoms can be ejected from the target by various means. Atoms arrive at thermal speeds, which results in basically room-temperature deposition.

There are very few parameters in evaporation that can be used to tailor film properties. There is no bombardment in addition to thermalized atoms themselves, which bring very little energy to the surface. Substrate heating is possible, but because of high vacuum requirement, there is the danger of outgassing of impurities from heated system parts.

In high vacuum, the atoms do not experience collisions, and therefore they take a line-of-sight route from source to substrate. Mean free path MFP is the measure of collisionless transport, and below ca. To get uniform film thickness, the substrate direction relative to the beam is important, and substrate rotation is used to ensure uniformity.

Uniformity is very much fixed when the chamber geometry is frozen, whereas in gas flow systems such as CVD, uniformity is very much processdependent. Low melting-point metals, such as gold and aluminium, can easily be evaporated, but refractory metals require more sophisticated heating methods. Localized heating by an electron beam can vaporize even tungsten melting point K , but deposition rates are, however, very low, of the order of angstroms per second.

Additionally, X-rays will be generated, which can damage sensitive devices. It is possible that the molten metal reacts with the crucible because temperatures are very high, even though it is being minimized by use of refractory materials for crucibles: Mo, Ta, W, graphite, BN, SiO2 and ZrO2. If a misaligned electron-beam hits the crucible, crucible material will be evaporated and incorporated in the deposited film.

Molecular beam epitaxy MBE is a variant of evaporation. Instead of an open crucible, the source material is heated in an equilibrium source known as the Knudsen cell. An atomic beam in the molecular flow regime, therefore the name MBE exits the cell through an orifice that is small compared to the source size.

Such equilibrium sources are much more stable than open sources, be they heated resistively or by an electron beam. Alloy evaporation results in a film of a different composition than the source material because of vapour pressure differences of the elements. Compound evaporation is also difficult because most compounds do not evaporate as a molecular species, but are decomposed.

Some oxides e. The use of multiple sources is a standard solution to multicomponent films. Evaporated metal films are usually under tensile stress, in the range of MPa to 1 GPa. Nonmetals are found in both tensile and compressive stresses, but the values are smaller than for metals.

More discussion on thin-film stresses can be found in Chapter 7. The ejected target atoms will be transported to the substrate wafers in vacuum Figure 5. Because sputtering pressures are quite high, 1 to 10 mTorr three to five orders of magnitude higher than evaporation pressures , sputtered atoms will experience many collisions before reaching the substrate.

In a process called thermalization, the high-energy sputtered particles 5 eV corresponds to ca. Thermalization also occurs to other species present in the plasma, the reflected neutrals some argon ions are neutralized upon target collision.

These neutrals provide energy to the substrate. Thermalization reduces the energy of particles reaching the substrate Matching network Reproduced from Ohring, M. Lower flux means a lower deposition rate, but lower energy leads to less re-sputtering of the film.

This re-sputtering can sometimes be very useful, and it will be discussed in the context of bias sputtering in Chapter In contrast to evaporation, the energy flux to the substrate surface can be substantial.

This has both beneficial and detrimental effects: loosely bound atoms film-forming atoms as well as unwanted impurities will be knocked out, improving adhesion and making the film denser. But too high energies can cause damage to the film, the substrate and underlying structures thin oxide breakdown because of high voltages.

There will always be some argon trapped in the film but no effect is seen in the first approximation. Sputtering yield Y is a number of target atoms ejected per incident ion. Sputtering yields of metals range from ca. Refractory metals have low sputtering yields, which is the fundamental reason for lower deposition rates. In practice, there is another reason that further lowers the deposition rate: refractory metals tend to have higher resistivity and thus lower thermal conductivity, which means that high sputtering powers cannot be applied to refractory sputtering targets.

For heavy metals like tungsten and tantalum, sputtering yields are higher with xenon and krypton: these heavy gases transfer energy more efficiently to similar mass target atoms. However, argon is almost exclusively used. In alloy sputtering, the flux is enriched in the component with higher yield yields from alloys are even less accurately known than yields from elemental solids; elemental solid yields are used as approximations.

A steady state situation develops and composition remains unchanged. Gaseous by-products are pumped away, as shown schematically in Figure 5. There are various possible CVD reaction types. CVD reaction rates obey Arrhenius behaviour, that is, exponentially temperaturedependent.

CVD processes are also complex from the point of view of fluid dynamics. CVD of silicon on a single crystalline silicon wafer can result in a single-crystalline film. This is termed epitaxy and it is an important special case of thinfilm deposition. The next chapter is devoted to epitaxial deposition. Most deposition processes lead to amorphous or polycrystalline films.

Silicon dioxide can be deposited by many reactions. Gaseous reactants form a solid film on the wafer and gaseous by-products are pumped away. The precursor name TEOS has become synonymous with the resulting oxide film; it should be obvious which meaning is used. The use of N2 O laughing gas instead of oxygen is preferred because silane reaction with oxygen is spontaneous and oxide particles are produced everywhere in the system and they float around in the reactor and deposit sporadically on wafers.

CVD is not limited to simple compounds: films can be doped during deposition. CVD oxide can be doped by adding phosphine PH3 gas to the source gas flow. Phosphorus oxide is formed by CVD and intermixed with silicon dioxide. Higher doping levels lead to porous, hygroscopic material. Phosphorus getters mobile ions like sodium and potassium, and makes PSG a more efficient barrier against the ambient than undoped CVD oxide which is sometimes known as USG, for undoped silica glass.

PSG etch rate is much faster than that of undoped oxide, and PSG is a popular sacrificial layer in micromechanics. CVD tungsten is deposited in two steps. In PVD processes, deposition rate depends primarily on target excitation energy. CVD processes are chemical processes, and their rates obey Arrhenius behaviour. The activation energy Ea can be extracted from the Arrhenius formula when the deposition rate has been determined at several temperatures.

The magnitude of the activation energy gives hints to possible reaction mechanisms. Two temperature regimes can be found for most CVD reactions Figure 5. The reaction is then in the surface reaction—limited regime. This is compensated by the fact that deposition takes place on up to wafers simultaneously. When the temperature increases, the surface reaction rate increases exponentially, and above a certain temperature, all source gas molecules react at the surface. The fluid dynamics of the reactor then plays a major role in deposition uniformity and rate.

Process temperatures are often severely limited: for instance, after an aluminum—silicon interface has been formed, the maximum allowed temperature is ca.

When aluminum has to be coated by an oxide or nitride layer, plasma activation is usually employed. There is a thermal CVD process for depositing oxide on aluminium at ca.

Most often plasma activation is employed. Instead of thermal decomposition of the source gases, a glow discharge is utilized. Much lower temperatures can be used: plasma activation ensures enough reactive species even at low temperatures, typically at ca.

PECVD deposition rate is only mildly temperaturedependent. Wafers are placed on a heated bottom electrode, the source gases are introduced from the top, and pumped away around the bottom electrode. Operating frequency is often kHz, which is slow enough for ions to follow the field, which means that heavy ion bombardment is present. At In thermal CVD, pressure, temperature, flow rate and flow rate ratio are the main variables.

In advanced PECVD reactors, RF power can be applied to both electrodes, and the two power sources can supply different frequencies, duty cycles and power levels. The ratio of This can cause device instability later on if hydrogen diffuses into the devices. Mixture of silane, nitrous oxide and ammonia will result in oxynitride, SiOx Ny , with varying ratios of nitrogen and oxygen, covering the whole range of compositions and material properties between oxide and nitride.

Fluorinedoped oxide, SiOF can be deposited, but film instability limits the usable fluorine range to ca. Amorphous carbon, a-C:H and related materials resemble diamond in many but not all respects, and they are known as diamond-like carbon DLC.

For most polycrystalline and amorphous CVD and PVD films, however, stresses build up to unacceptable levels for thicker films, limiting thicknesses to a few micrometres. Liquid phase deposition methods include a wide variety of techniques that are unrelated physico-chemically.

A beaker is enough for electroless deposition with an optional hot plate. Add a current source and an electroplating system is ready. Liquid phase methods are widely used in printed wiring board industry, thin-film head fabrication and in MEMS, and they are being introduced in IC fabrication, for deposition of copper and for inter-metal dielectric layer deposition. But as is usual with other deposition technologies, film properties will be strongly influenced by subsequent annealing steps.

Copper deposition chemistries traditionally use sodium hydroxide in the plating bath, but this has to be eliminated if copper is used in IC metallization. The electroless deposition set-up is extremely simple and no electrical connection needs to be made to the wafers. Selectivity, however, is difficult to maintain. Hydrogen evolution and incorporation into the film is a problem because hydrogen is mobile, and carbon incorporation is another problem.

Thick conductor layers High aspect ratio metallization Selective metallization Photoresists Thick polymer layers Spin-on-glasses Porous dielectrics Thick, complex materials 5. The counterelectrode is either passive, like platinum, or made of the metal to be deposited. Metal deposition takes place as a result of metal ion reduction. The surface needs to be suitable for electroless deposition and this is achieved by exposing the surface to a catalyst, such as PdCl2.

This reducing agent starts the reduction reaction, which then continues locally. Selective deposition is thus possible. Many of the metals used in microfabrication, aluminum, titanium, tungsten, tantalum and niobium, do not have practical electroplating processes. Three transport processes are active during electrochemical deposition ECD : diffusion at electrodes due to local depletion of reactant via deposition, migration in the electrolyte and convective transport in the plating bath.

The latter is connected to electrochemical cell design, and it is affected by factors such as stirring, heating, recirculation and hydrogen evolution. Macroscopic current distribution is determined by the plating bath electrode arrangement and wafer and bath conductivity.

Electrical contact to the wafer also needs careful consideration. Microscopic local current distribution depends on pattern density and pattern shapes. The third scale in ECD is the feature scale: potential gradients inside structures are important especially when high aspect ratio structures are filled.

In practice, the plating solutions are complex mixtures of electrolytes, salts for conductivity control, modifiers for film uniformity and morphology improvement as well as surfactants. Many plating solutions are proprietary.

Plating baths are rather aggressive solutions, and photoresist leaching into plating bath or adhesion loss are real concerns for reproducible plating.

Accelerators brighteners are additives that modify the number of growth sites. Suppressors are additives for surface diffusion control. Taken together, these additives increase the number of nucleation sites, and keep the size of each nucleation site small, which drives smooth growth. Pulsed plating can also be used in balancing nucleation and grain growth: high overpotential and low surface diffusion favour nucleation, and the opposite conditions favour grain growth.

Damascene plating Figure 5. Polishing is needed to remove excess metal. Metal remains in the grooves and recesses of the wafer, and the wafer surface remains planar. Electroplating can also be done in resist grooves, and more plating applications will be presented in Chapters 23 and It is now a method to deposit films that will remain as structural parts of finished devices.

Spinning is a simple process for viscous materials deposition. Spinners, with typical speeds up to 10 rpm, are found in every microfabrication laboratory. The main parameters for film thickness control are viscosity, solvent evaporation rate and spin speed. Spin-coated film thicknesses range from 0. The coating of thick spin films will discussed in Chapter 10 in connection with thick photoresists. Dispensing can be in static mode, or slow rotation of ca.

Depending 56 Introduction to Microfabrication Resist dispensing a few millilitres Acceleration resist expelled Final spinning rpm partial drying via evaporation Figure 5. Acceleration to ca. Half of the solvent can evaporate during the first few seconds, so rapid acceleration is a must because viscosity changes with solvent content, and radially non-uniform thickness will result from viscosity differences. Spin speed can be controlled to ca. Turbulence both from the spin process itself and from cleanroom airflows and ambient humidity which is affected by exhaust from the spinner bowl and the cleanroom environmental control affect evaporation rate, and consequently, film thickness.

Pinhole defects in spin-coated films are thickness-dependent: thinner films are more defective. Pinholes can be caused by particles on the wafers, and also by particles in the dispensed fluid, even though all chemicals in microfabrication have been filtered with submicron filters. Air bubbles formed during dispensing caused by e.

Spin-coated films fill cavities and recesses because they are liquids during spin coating. This is advantageous for gap filling and smoothing, but if uniform thickness over the topography is desired, spinning is not ideal. A gel is 3D solid network that forms in a colloidal liquid.

A typical sol—gel process uses metal alkoxides M— O—CH3 n in organic solvents. Compositional variation by changing alkoxides ratios is easy.

Thickness can be tailored not only by spin speed but also by chemical modifications in the organic side chain R. Film thicknesses of hundreds of micrometres are possible for both glassy SiO-type materials and ceramics like lead—zirconium titanate PZT. Drying of gel leads to drastic volume shrinkage easily by a factor of 10 , and the resulting material is known as xerogel. Application of these materials as structural parts in microdevices will be difficult, but as sacrificial materials they could be easily removable.

Conductors: Resistivity is the main consideration: aluminum and copper are main choices for most applications, and gold is often used in RF devices, like inductor coils, to minimize resistive losses. Doped silicon and polycrystalline silicon can be used as a conductor, but its resistivity is very high compared to metals. Thin-film Materials and Processes 57 Contacts to semiconductors: ohmic metal-like and Schottky diode-like contacts are possible.

Aluminum, itself p-type dopant in silicon, makes good ohmic contact to p-type silicon. Platinum silicide is one candidate for silicon Schottky contacts. Capacitor electrodes: Capacitor electrodes need not be highly conductive. The most important capacitor electrode, the MOSFET gate, is chosen to be polycrystalline silicon because its interface with silicon dioxide is stable, and its lithography and etching properties are good.

Plug fills: When vertical holes need to be filled with a conducting material, CVD tungsten and electrodeposition of copper are employed. Resistors: Doped semiconductors, metals, metal compounds and alloys can be used as resistors. Heating resistors can be made of almost any material, but precision resistors are difficult to make.

Barriers: Barriers are needed to prevent unwanted reactions between thin films. Mechanical materials: Aluminum and nickel are materials for micromechanical free-standing beams and cantilevers, in, for example, micromirrors and resonators. Films such as TiN can be used as mechanical stiffening layers to prevent mechanical changes in the underlying softer films, like aluminum. Optical materials: Transparent conductors like indiumdoped tin oxide ITO; Inx Sny O2 are needed in displays and light-emitting devices.

In image sensors, metals act as light shields, and in many micro-optical devices, as mirrors. TiN is often deposited on top of aluminum to reduce reflectivity, because lithography is difficult on highly reflecting surface. Magnetic materials: Nickel and nickel alloys, Ni:Fe, are used in magnetic microactuators. Cores of microtransformers are also made of these materials, which are usually deposited by electroplating.

Catalysts and chemically active layers: Chemical sensors often use films such as palladium and platinum as catalysts. Electron emitters: Vacuum microemitter tips are often made of molybdenum because of its high melting point and low work function.

Infrared emitters and other IR components: Heated wires emit infrared, and porous metallic films, like aluminum black, act as IR absorbers. Metallic meshes act as IR filters. Sacrificial layers: Many devices require free-standing structures. These must be fabricated on solid films, which will subsequently be etched away. Copper is often used as a sacrificial material under nickel or gold.

Protective coatings: Sometimes the role of the topmost layer is simply to protect the underlying layers from the ambient: from etching agents or environmental stressors. Nickel and chromium are used as masks for etching. X-ray components: Masks for X-ray lithography require high atomic mass materials that effectively block Xrays. Tungsten, gold and lead are prime candidates. X-ray mirrors are made by alternating layers of heavy tungsten, molybdenum and light materials carbon or silicon of X-ray wavelength thicknesses.

The deposition process greatly influences the choice of metals. Not all materials are amenable to all deposition methods, and the resulting film properties resistivity, phase, texture, adhesion, stress, surface morphology are closely connected with the details of the deposition process, and may well be idiosyncratic with the equipment.

Reproducing results that have been obtained with another piece of equipment can be a nightmare. Thinfilm resistivity is often much higher than bulk resistivity. Aluminum, copper and gold thin-film resistivities are close to bulk values; for most others, thin films resistivities are factor of 2 higher. Guide prepares to order.

When in various other time you will certainly need more days to obtain the book, in this article the soft documents that we will supply will be directly done. Checking out, for some people come to be a demand that is to do every day such as spending time for eating. Now, what regarding you? Do you prefer to review an e-book?

When reading this book, you can get one point to always remember in every reading time, even detailed. The volume contains a large number of figures and illustrations for easy understanding by the readers. It will also be useful to researchers and professionals looking for an introduction to the topic. Fortunately a large percentage of the neural cells connected to the photoreceptors remain viable, and electrical stimulation of these cells has been shown to result in visual perception.

These findings have generated worldwide efforts to develop a retinal prosthesis device, with the hope of restoring vision. Advances in microfabrication, integrated circuits, introduction to microfabrication sami franssila pdf free download , and wireless technologies provide the means to reach this challenging goal.

This dissertation describes the development of innovative silicone-based microfabrication techniques for producing an implantable microelectrode array. The microelectrode array is a component of an epiretinal prosthesis being developed by a multi-laboratory consortium. This array will serve as the interface between an electronic imaging system and the human eye, directly stimulating retinal neurons via thin film conducting traces.

Because the array is intended as a long-term implant, vital biological and physical design requirements must be met. A retinal implant poses difficult engineering challenges due to the size of the intraocular cavity and the delicate retina.

Not only does it have to be biocompatible in terms of cytotoxicity and degradation, but it also has to be structurally biocompatible, with regard to smooth edges and high conformability; basically mimicking the biological tissue. This is vital to minimize stress and prevent physical damage to the retina. Also, the device must be robust to withstand the forces imposed on it during introduction to microfabrication sami franssila pdf free download and implantation.

In order to meet these biocompatibility needs, the use of non-conventional microfabrication materials such as silicone is required. This mandates the enhancement of currently available polymer-based fabrication techniques and the development of new microfabrication methods.

Through an iterative process, devices were designed, fabricated, tested and implanted into a canine eye. Metal traces were embedded within a thin substrate fabricated using poly dimethyl siloxane PDMSan inert biocompatible elastomeric material with high oxygen permeability and low water permeability.

Due to its highly conformable nature, PDMS contacted the curved retinal surface uniformly. Fundamental material characteristics were examined to develop reliable and repeatable fabrication processes. Author : Thomas M. Multiple field-tested laboratory exercises are included, designed to facilitate student learning about the fundamentals of microfabrication processes, introduction to microfabrication sami franssila pdf free download.

References, suggested reading, review questions, and homework problems are provided at the close of each chapter. Introductory MEMS: Fabrication and Applications is an excellent introduction to the subject, with a tested pedagogical structure and an accessible writing style suitable for students at an advanced undergraduate level across academic disciplines. A simple one-dimensional simulator will do. It is often enough to understand if a process can be done in seconds, minutes or hours; or whether resistance range is milliohms, ohms or kiloohms.

You must learn to make simplifying assumptions, and to live with uncertain data. Searching the Introduction to microfabrication sami franssila pdf free download for answers is no substitute to simple calculations that can be done in minutes because the simple estimates are often as accurate or inaccurate as answers culled from Internet.

With thin film materials properties are very much deposition process dependent, and different workers have measured widely different values for such basic properties as resistivity or thermal conductivity. Polymeric materials, too, exhibit large variation in properties and processing. There are also calculations of economic aspects of microfabrication: wafer cost, chip size and yield.

A bit of memory costs next to nothing, but the fabs fab is short for fabrication facility that churn out these chip are enormously expensive. Comments and hints to selected homework problems are given in Appendix A. In Appendix B you can find useful physical constants, silicon material properties and unit conversion factors. Acknowledgements Writing a book takes a lot of time, and numerous people have contributed their time and effort at various stages of this project.

Their interest in both details and overall structure is much appreciated. A far larger group of people have contributed to selected parts of the book by providing me with data, micrographs and photos; they have led me to useful sources, pointed out gaps and corrected my text.

Charlotta Tuovinen has provided assistance with computers on countless occasions. My students and teaching assistants Tuuli Juvonen, Antti Niskanen, Santeri Tuomikoski, Esa Tuovinen and Seppo Marttila have introduction to microfabrication sami franssila pdf free download guinea pigs for the reading of the text and exercises. Introduction to microfabrication sami franssila pdf Gregor the overlander audiobook youtube, Introduction to Microfabrication, Second Edition.

Post a Comment. Machinerys handbook 30th edition pdf download Uploader: Akmirad Date Added: Thursday, September 9, Introduction to microfabrication sami franssila pdf free download. Introduction to microfabrication sami franssila pdf free download Uploader: Dzhastin Date Added: They have lived to tell the tale!

Read More Introduction to microfabrication sami franssila pdf free download Introduction to microfabrication sami franssila pdf Gregor the overlander audiobook youtube, Introduction to Microfabrication, Second Edition. Labels: