The silicon substrate and adding to it—Part 2 Describe the processes of Oxidation, both dry oxidation and wet oxidation Evaporation, both resistive reheating and e-beam Sputtering, DC, RF, reactive, and magnetron Chemical vapor deposition (CVD) Electrodeposition Spin casting Wafer bonding Calculate relative thicknesses of added oxide layers to original wafer thickness, and oxide thickness as a function of time and vice versa Compare and contrast the advantages and disadvantages of evaporation versus sputtering Give the relative advantages and disadvantages of CVD compared to PVD Adding layers to the silicon substrate thin film thin film Bulk micromachining Surface micromachining Adding layers to the substrate Many different methods • Epitaxy—growing an additional crystalline layer of Si on top of an existing wafer − Has same crystalline orientation of underlying Si (unless it is on top of an amorphous substrate, in which case it is polycrystalline) − Has different dopant type and concentration − Uses? • Oxidation—chemical reaction of Si with O2 to form layer of amorphous silicon dioxide (SiO2) Physical vapor deposition • Evaporation (PVD) • Sputtering • Chemical vapor deposition (CVD) • Electrodeposition • Spin casting • Wafer bonding Oxidation Chemical reaction of Si with O2 to form layer of amorphous silicon dioxide (SiO2) • • • • • Called “oxide layer” or just “oxide” Uses? Thin layers < 100 nm Thick layers 100 nm – 1.5 μm Use of furnaces at high temperatures, ~800°-1200°C Oxidation furnaces A schematic diagram of a typical oxidation furnace “Bubblers” (bubble = burbuja) are used for wet oxidation. Wet oxidation vs. dry oxidation Oxidation can be dry or wet. Dry oxidation: Si + O2 → Dry oxidation creates a very high quality (de calidad alta) oxide, but it takes a long time. Wet oxidation: Si + H2O → Wet oxidation creates a lower quality (de menos calidad) oxide, but it is fast. ¿Cuál se usaría para estructuras? ¿para “sacrificial layer? ¿por qué? Oxidation xadd xox Te toca a ti A 150-mm (6 inch) diameter silicon wafer requires a 0.8-μm thick layer of oxide as a sacrificial layer. If the wafer is originally 650 mm thick, how much thicker is the wafer after oxidation? How much of the wafer has been “used up” (se ha sido gastado) to create the oxide later? xadd 0.54 xox Respuesta: • 0.43 μm thicker (total thickness = 650.43 μm) • 0.37 μm of wafer “used up” Oxide thickness How can you tell how thick your oxide layer is? (www.filmetrics.com) Look at the color! (onlinelibrary.wiley.com) (www.cleanroom.byu.edu) Oxidation kinetics The Deal-Grove model of oxidation kinetics is the most widely used model to predict oxide thickness as a function of time. A 4B xox 1 (t ) 1 2 2 A A and B depend on • Temperature • Wafer type; i.e., (100) or (111) Depends on native oxide thickness What do you think the model is based on? Te toca a ti • Sketch (don’t plot) the general shape of the oxide thickness as a function of time. Why does it look this way? • Approximate what the function is for very long times. (Es decir, t >> τ) xox B(t ) The Deal Grove model Short time approximation comes from a series expansion: short time approx. B xox (t ) A Linear rate constant long time approx. Deal Grove model for wet oxidation of (100) Si at 1000°C ¡Más te toca a ti! • Approximate how long it takes to grow 1 μm of oxide at 1000°C for (100) silicon using wet oxidation. • Compare your result to the long time approximation. Respuestas: 4.47 hr, 3.13 hr ¡Aun más te toca a ti! Repeat the last problem for (111) Si. That is, • Approximate how long it takes to grow 1 μm of oxide at 1000°C for (111) silicon using wet oxidation. • Compare your result to the long time approximation. Respuestas: 3.93 hr, 3.13 hr Oxidation for (111) Si is faster: ( B / A) (111) ( B / A) (100 ) Why? 1.68 ¡Mucho tiempo significa muchísimo tiempo! Physical vapor deposition Physical vapor deposition (PVD) − a purified, solid material is vaporized and then condensed onto a substrate in order to form a thin film. Evaporation PVD shadow Sputtering thin film target source PVD is called a line-of-sight method. Shadowing Vacuums PVD requires the use of a vacuum. Write down some reasons why you think a vacuum is necessary for PVD. • Vaporized atoms do not run into other gas atoms • Need a vacuum to create a vapor out of the source material • Vacuum helps keep contaminants from being deposited on the substrate Vacuum fundamentals Vacuum means pressure less than atmospheric pressure. Standard unit is a torr: 1 atm = 1.01325×105 Pa = 760 torr Pressure ranges for various vacuum regions Creating a vacuum Vacuum pumps Pressure ranges for various vacuum regions A rotary vane pump High vacuum pumps Turbopump Diffusion pump Cryopump Vacuum systems In what order would you operate the pumps and open and close valves to create a high vacuum in the vacuum chamber? Typical vacuum system setup in a PVD system 1. Close Hi-vac and foreline valves 2. Run the “rough pump” to lower chamer to low vacuum 3. Close rough valve 4. Open foreline valve 5. Open Hi-vac valve 6. Run Hi-vac pump Vacuum theory and relationships The ideal gas equation PV Nk b T Boltzmann´s constant Mean free path kb = 1.381×10-23 J/K V kbT 2 N 2P σ is the interaction cross section. • ~ probability of interaction between particles • dimensions of area Te toca a ti Estimate the number of molecules of air in a 1 cm3 volume at room temperature and the two pressures given. Also calculate the mean free path. a. P = 1 atmosphere b. P = 1×10-7 torr. Take the interaction cross section to be σ = 0.43 nm2 Useful information: − kb = 1.381×10-23 J/K − 1 atmosphere = 760 torr − torr = 133 Pa Respuestas: a. 2.50×1019 molecules, 66 nm b. 3.29×109 molecules, 500 m Now estimate how many molecules are in a thumbprint. . Thermal evaporation substrate Flux, F: (molecules leaving source)/(area×time) F Pv (T ) 2Mkb T Requirements for evaporated materials: • Pv must be > background vacuum pressure, ~ < 10-2 torr < Pv < 1 to vacuum pump source • Elements or simple oxides of elements • 600°C < T < 1200°C • Examples Al, Cu, Ni, ZiO • No heavy metals; e.g. Pt, Mo, Ta, and W Resistive heating vs. e-beam evaporation evaporant resistive heater Evaporation by resistive heating e-beam evaporation Shadowing shadow thin film target source Shadowing Arrival rate A, (incident molecules)/(area×time) cos cos A F 2 d Compare to view factors in radiation heat transfer (radiación térmica) 1 cos 1 cos 2 F 12 dA2 dA1 2 A1 A1 A2 d Shadowing cos cos A F 2 d t1 cos 1 t 2 cos 2 Step coverage Te toca a ti Aluminum is evaporated onto a silicon substrate at a rate of 0.5 nm/s according to the evaporator. For the geometry shown in the figure, estimate the thickness of aluminum on surfaces (1), (2), (3) , and (4) after one hour. Respuestas: t1=1.56 μm, t2=1.64 m (1) (2) 54.7° source (3) (4) 30° Shadowing How do you think you might reduce shadowing and therefore increase step coverage? • Rotate the wafers as the deposition is taking place planetary wafer rotators • Heat the wafers to allow the deposited material to flow • Or don’t! Sometimes you can use shadowing to make structures you want Lift-off Sputtering substrate substrate Ar+ source Evaporation Sputtering source DC sputtering Source is not a “point” but a parallel plate. Source material must be conductive Typical DC sputtering configuration Other sputtering techniques RF (radio frequency) Sputtering Reactive sputtering Magnetron sputtering • Applies an AC voltage to target at frequencies > 50 Hz • Target does not need to be conductive • Chamber walls also sputtered • Reactive gas (such as O2) added to chamber • Reacts with target, products forming the deposited materials • Products can be deposited on surfaces other than the substrate • Reduction in sputtering rates typically seen • Addition of magnets behind target keep electrons from travelling too far • Increased ionization at cathode • Leads to higher yields Magnetron sputtering F q ( E v B) Magnetron principle Comparison of evaporation and sputtering Evaporation • Limited to lighter elements and simple compounds • Low energy ions/atoms (~0.1 eV) • High purity thin films • Less dense films, large grain size, adhesion problems (problemas de pegar) • Requires a high-vacuum • Directional can use for lift-off • Components evaporate at different rates composition of deposited film is different than source Sputtering • Virtually anything can be sputtered • High energy ions/atoms (~1-10 eV) • Gas atoms implanted in films lower purity • Dense films, smaller grain size, good adhesion • Can use a low vacuum ~10-2 to 10-1 torr • Poor directionality good step coverage • Components deposited at similar rates Chemical vapor deposition Chemical Vapor Deposition (CVD) Common way to deposit polycrystalline silicon thin films (often called simply “poly” Using silane: SiH4 → Si + Using trichlorosilane: Basic chemical vapor deposition process HSiCl3 → Si + Chemical vapor deposition Silicon dioxide (SiO2) thin films Using silane: SiH4 + O2 → Using dichlorosilane and nitrous oxide: SiCl2H2 + N2O → SiO2 Uses? Silicon nitride (Si3N4) thin films • Insulator • Structural layer • Chemical barrier Using silane: SiH4 + NH3 → Si3N4 Using dichlorosilane: SiCl2H2 + NH3 → Si3N4 + Comparison of PVD and CVD PVD • Evaporation is limited to certain materials. Sputtering has yield problems. • Generally no hazardous byproducts • Lower temperatures • Requires a high-vacuum • Directional can use for lift-off CVD • Preferred method for • polysilicon layers and • silicon nitride • Hazardous byproducts • Often requires high temperatures (~500°850°C) Cannot deposit on top of many metal layers • Requires a high-vacuum (LPCVD is most common) • Poor directionality good step coverage Other deposition methods Electrodeposition (electroplating) - + Cu Metal e- SO42- Cu2+ • Often used to deposit metals and magnetic materials • Inexpensive and easy (barato y fácil) • Surface quality usually worse than PVD (higher roughness) • Uniformity can be an issue Spin casting Material is • dissolved in solution, • poured onto wafer, and • the wafer is spun to distribute the solution across surface • Wafer is then baked to remove the solvent, leaving behind the thin film. Also called simply “spinning” Used for polymers, piezoelectric materials, and is the standard method of applying photoresist. Wafer bonding Most commonly used in packaging rather than in creating MEMS structures themselves. • Use of adhesives and solders • Thermal bonding • Anodic bonding Generic anodic bonding setup Thermally induced stress can be an issue, leading to fracture.
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