The silicon substrate and adding to it—Part 2 Describe the processes of Calculate Oxidation, both relative thicknesses of added oxide layers to original wafer thickness, and dry oxidation and wet oxidation oxide thickness as a function of time and vice versa Evaporation, both Compare and contrast the advantages and disadvantages of evaporation versus sputtering resistive reheating and e-beam Give the relative advantages and disadvantages of CVD compared to PVD Sputtering, DC, RF, reactive, and magnetron Chemical vapor deposition (CVD) Electrodeposition Spin casting Wafer bonding

Adding layers to the silicon substrate thin film thin film Bulk micromachining Surface micromachining

Adding layers to the substrate Many different methods Evaporation Epitaxy—growing an additional crystalline layer of Si on top of an existing wafer Sputtering Chemical vapor deposition (CVD) Has same crystalline orientation of underlying Si (unless it is on top of an amorphous substrate, in which case it is polycrystalline) Electrodeposition Spin casting Wafer bonding 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 (PVD) Ask students for uses of epitaxy (ans: create p-n junctions, create piezoresistitity, etc. Ask students to give uses of evaporation and sputtering. Why is it called PVD? What is electrodeposition? What substance that you have already seen is spin casting used for? Why might you want to nond wafers to each other?

Called “oxide layer” or just “oxide” Uses? Thin layers < 100 nm 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 Use as a hard mask, electrical insulation (isolator), sacrificial layer, structural layer

Oxidation furnaces A schematic diagram of a typical oxidation furnace “Bubblers” (bubble = burbuja) are used for wet oxidation.

Oxidation can be dry or wet. Wet oxidation vs. dry oxidation Oxidation can be dry or wet. Dry oxidation: Si + O2 → Wet oxidation: Si + H2O → Dry oxidation creates a very high quality (de calidad alta) oxide, but it takes a long time. Wet oxidation creates a lower quality (de menos calidad) oxide, but it is fast. Have students balance chemical reactions: Si + O2  SiO2 Si +2H2O  SiO2 + 2H2 ¿Cuál se usaría para estructuras? ¿para “sacrificial layer? ¿por qué?

xadd xox Oxidation Te toca a ti Respuesta: 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? Respuesta: 0.43 μm thicker (total thickness = 650.43 μm) 0.37 μm of wafer “used up”

How can you tell how thick your oxide layer is?  Look at the color! Oxide thickness How can you tell how thick your oxide layer is?  Look at the color! (www.filmetrics.com) (onlinelibrary.wiley.com) (www.cleanroom.byu.edu)

Depends on native oxide thickness 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 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? Native oxide thickness is ~25 nm. Deal Grove does not work well for under 25 nm. τ found by setting xox = xi with t=o and solving for τ. 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)

Short time approximation comes from a series expansion: The Deal Grove model Short time approximation comes from a series expansion: short time approx. Linear rate constant long time approx. ¡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. Go to BYU site and show the online calculator. Deal Grove model for wet oxidation of (100) Si at 1000°C Respuestas: 4.47 hr, 3.13 hr

Repeat the last problem for (111) Si. That is, ¡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. Oxidation for (111) Si is faster: Why? Respuestas: 3.93 hr, 3.13 hr ¡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. Spray paint Evaporation PVD shadow thin film Sputtering target Why is it called “physical”? 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 A rotary vane pump Pressure ranges for various vacuum regions A rotary vane pump

High vacuum pumps Turbopump Diffusion pump Cryopump In turbopumps a gas molecule will randomly enter the turbo pump and be trapped between a rotor and a stator. When the gas molecule eventually hits the spinning underside of the rotor, the rotor imparts momentum to the gas molecule, which then heads towards the exhaust. Cryopump

Typical vacuum system setup in a PVD system In what order would you operate the pumps and open and close valves to create a high vacuum in the vacuum chamber? Close Hi-vac and foreline valves Run the “rough pump” to lower chamer to low vacuum Close rough valve Open foreline valve Open Hi-vac valve Run Hi-vac pump Typical vacuum system setup in a PVD system

Vacuum theory and relationships The ideal gas equation Boltzmann´s constant kb = 1.381×10-23 J/K Mean free path σ is the interaction cross section. ~ probability of interaction between particles dimensions of area

Take the interaction cross section to be σ = 0.43 nm2 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. P = 1 atmosphere 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: 2.50×1019 molecules, 66 nm 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) Requirements for evaporated materials: Pv must be > background vacuum pressure, ~ < 10-2 torr < Pv < 1 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 to vacuum pump source

Evaporation by resistive heating Resistive heating vs. e-beam evaporation evaporant resistive heater Evaporation by resistive heating e-beam evaporation

Shadowing shadow thin film target source Spray paint Why is it called “physical”? target source

Shadowing Arrival rate A, (incident molecules)/(area×time) Compare to view factors in radiation heat transfer (radiación térmica)

Shadowing 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. (1) (2) (3) Note that the angle w, surface 4 is negative & therefore the thickness is O. Also note that we have ignored the d’ term assuming that the deposition rate given in the problem would be for9 surface that looks like surface I in the first figure. 54.7° (4) source 30° Respuestas: t1=1.56 μm, t2=1.64 m

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 source Evaporation

Source material must be conductive 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 Applies an AC voltage to target at frequencies > 50 Hz Target does not need to be conductive Chamber walls also sputtered Reactive sputtering 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 Magnetron sputtering Addition of magnets behind target keep electrons from travelling too far Increased ionization at cathode Leads to higher yields

Magnetron sputtering Magnetron principle

Comparison of evaporation and sputtering Limited to lighter elements and simple compounds Virtually anything can be sputtered Low energy ions/atoms (~0.1 eV) High energy ions/atoms (~1-10 eV) High purity thin films Gas atoms implanted in films  lower purity Less dense films, large grain size, adhesion problems (problemas de pegar) Dense films, smaller grain size, good adhesion Requires a high-vacuum Can use a low vacuum ~10-2 to 10-1 torr Directional Poor directionality  can use for lift-off  good step coverage Components evaporate at different rates  composition of deposited film is different than source 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: HSiCl3 → Si + SiH4 → Si + 2H2 HSiCl3 → Si + HCl + Cl2 Basic chemical vapor deposition process

Chemical vapor deposition Silicon dioxide (SiO2) thin films Using silane: SiH4 + O2 → Using dichlorosilane and nitrous oxide: SiCl2H2 + N2O → SiO2    Silicon nitride (Si3N4) thin films Using silane: SiH4 + NH3 → Si3N4 Using dichlorosilane: SiCl2H2 + NH3 → Si3N4 + Uses? Insulator Structural layer Chemical barrier SiH4 + O2 → SiO2 + 2 H2 SiCl2H2 + 2 N2O → SiO2 + 2 N2 + 2 HCl 3 SiH4 + 4 NH3 → Si3N4 + 12 H2 3 SiCl2H2 + 4 NH3 → Si3N4 + 6 HCl + 6 H2

Comparison of PVD and CVD Evaporation is limited to certain materials. Sputtering has yield problems. Preferred method for polysilicon layers and silicon nitride Generally no hazardous byproducts Hazardous byproducts Lower temperatures Often requires high temperatures (~500°- 850°C)  Cannot deposit on top of many metal layers Requires a high-vacuum Requires a high-vacuum (LPCVD is most common) Directional Poor directionality  can use for lift-off  good step coverage

Electrodeposition (electroplating) Other deposition methods Electrodeposition (electroplating) - + Cu 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 Metal e- Cu2+ SO42-

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.

Use of adhesives and solders Thermal bonding Anodic bonding Wafer bonding Most commonly used in packaging rather than in creating MEMS structures themselves. Use of adhesives and solders Thermal bonding Anodic bonding Thermally induced stress can be an issue, leading to fracture. Generic anodic bonding setup