Photolithography Identify the basic steps of a photolithographic process Define and calculate depth of focus and explain how it is related to resolution in projection printing Describe the differences between positive and negative photoresist Define the terms Explain why photolithography requires a clean environment Dose and dose to clear, Classify cleanrooms using both ISO and US FED standards Light intensity, Resist sensitivity, and Describe the process of a RCA clean Contrast Describe the process of applying resist via spinning and calculate the resulting resist thicknesses Calculate sensitivity and contrast for both positive and negative resist Describe, compare and contrast the exposure processes of Explain the shape of resist profiles and calculate the slopes of resist layer Contact printing Proximity printing Explain the need for and use of alignment marks Projection printing Explain how photo masks are produced Calculate the resolutions of the above processes and explain what they depend on and why

Reminder of the photolithography steps in the μ-machining process Unexposed photoresist removed by developer Exposed photoresist becomes less soluable Mask SiO2 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? Opaque region Silicon wafer Glass plate Spin on photoresist

Reminder of the photolithography steps in the μ-machining process 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? Note the pattern is the opposite of that on the mask. This is true for negative photoresist.

 Photolithography must be done is a very clean environment. Keeping it clean Photolithography can be the “bottle neck” in terms of how small you can make a MEMS structure. Dust particles on masks behave as extra opaque regions (lugares oscuros) and transfer unwanted patterns.  Photolithography must be done is a very clean environment. 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?

Clean rooms Clean rooms are classified based on how many particles of a certain six exist within a certain volume: In the EE. UU. Class 1  less than 1 particle > 0.5 μm/ft3 Class 10  less than 10 particles > 0.5 μm/ft3 Class 100  less than 100 particles > 0.5 μm/ft3 Class 1000  less than 1000 particles > 0.5 μm/ft3 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? Outside the US (ISO 14644-1) ISO Class 1  less than 10 particles > 0.1 μm/m3 ISO Class 2  less than 100 particles > 0.1 μm/m3 ISO Class 3  less than 1000 particles > 0.1 μm/m3

Comparison of cleanroom standards 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?

Equivalency of cleanroom classifications Comparison of cleanroom standards Cn = 10N (0.1 / D)2.08 Equivalency of cleanroom classifications ISO 14644-1 US FED STD 209E ISO 1 - ISO 2 ISO 3 Class 1 ISO 4 Class 10 ISO 5 Class 100 ISO 6 Class 1000 ISO 7 Class 10,000 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? ISO = log(US) + 3 US = 10(ISO - 3)

¿Por qué parece tan amalliro? Clean room etiquette and requirements ¿Por qué parece tan amalliro? “Bunny suits” required (main source of airborne dust is human skin) Not constructed near sources of pollution Floors are conductive for electrostatic discharge. Only certain types furniture are allowed Specially designed paper (pens no pencils) No eating and drinking Perfume, cologne and makeup are discouraged. 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? Rose-Hulman students in the clean room facility at RHIT

Wafer cleaning RCA clean Developed by Werner Kern in 1965 while working at RCA Laboratories 1:1:5 to 1:1:7 by volume solution of NH4OH : H2O2 : H2O is used to remove organic contaminants and heavy metals HCl : H2O2 : H2O in a 1:1:5 to 1:2:8 volume ratio is used to remove aluminum, magnesium, and light alkali ions 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? Both steps approximately 20 minutes while gently heating to 75-85°C on a hot plate There are other cleaning techniques, such as “piranha clean”

Usually sensitive to a narrow band of ultraviolet (UV) light Photoresist Photoresist is the “stuff” of photolithography Often called “resist” Three (3) components: a base resin, which is a polymer: gives the resist structure photoactive compound (PAC): The light- sensitive component solvent. Comes in two varieties Positive resist Negative resist Usually sensitive to a narrow band of ultraviolet (UV) light 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?

Positive resist Negative resist Positive versus negative resist Positive resist Exposure degrades the PAC Becomes more soluble to the developer after exposure Unexposed regions of the resist are left behind after development Developed resist pattern is identical to the mask pattern. Alkalis such as NaOH or KOH used as developers Very sensitive to UV light with wavelength of 365 nm, called the I-line of the mercury spectrum Negative resist Exposure increases MW of resist or creates new insoluble products Becomes less soluble to the developer after exposure Unexposed regions of the resist are removed after development Developed resist pattern is the opposite of the mask pattern. Organic solvents such as benzene used as developers Very sensitive to UV light with wavelength of 405 nm, called the H-line of the mercury spectrum ~ 10 times more sensitive than positive resist 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?

Positive versus negative resist 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?

Te toca a ti The mask shown in the figure is used to transfer a pattern to a silicon wafer. Sketch the resulting pattern on the wafer after exposure and development for both positive and negative resist.. Also sketch the profile from the side of the wafer. Mask 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? Side view of wafer Positive resist Negative resist

Applying resist Three steps A pre-bake to reduce water (water can the reduce adhesion of resist) Spin on the resist A post-bake to remove the solvent Pour it onto wafer Spin wafer to distribute the solution across surface Photoresist can also be sprayed (which may or may not get around uniformity issues)

Applying resist T is the film thickness C is polymer concentration, An empirical relation often used to predict the film thickness is given by T is the film thickness C is polymer concentration, η is viscosity, ω is rotational spin speed, and K, α, β, and γ are experimentally determined constants that vary from system to system. Coating step (proceso de cubrirlo) poses the largest danger from a dust contamination. 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? Spinning creates a slight vacuum along the plane of the wafer entraining airborne dust particles

Te toca a ti With a given spinner and photoresist formulation, a spin speed of 4000 rpm gives a resist thickness of 0.7μm. How would you create a resist layer a. 0.8 μm thick? b. 1.0 μm? 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?

Exposure and pattern transfer Two basic types of machines ¿Por qué parece tan amalliro? Contact aligner Projection printer 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? Contact aligner from University of Alabama Center for Materials for Information Technology Projection “stepper” from Cornell NanoScale Science and Technology Facility

Ultraviolet light spectrum Photoresist made to be sensitive to ultraviolet (UV) light. Hg-Xe lamps often used as light source. Ultraviolet light regions 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? Emission spectrum for an Hg-Xe lamp

Contact and proximity printing 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? Proximity printing Contact printing Contact printing and proximity printing create features in the photoresist that more or less look just like the mask.

Projection printing 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? A generic lithographic projection system In projection printing the size of the features on the resist can be different that on the mask itself.

Photolithography is an optical printing process Projection printing Photolithography is an optical printing process s s' f f 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?

Te toca a ti A projection printing system is to reduce the size of mask features by a facture of 5. If the objective lens is kept 60 mm from the surface of the wafer, what should the focal length of the objective be? Respuesta: 50 mm f s s' 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?

Dose, dose to clear, and I D Light intensity Dose Optical power per unit surface area [W/m2] D Dose Optical energy per unit surface area D = Itexposure Dose to clear Dose required to expose the resist so that it completely develops Dp for positive resist (completely develops away) Dg0 for negative resist (completely remains) 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? arial image latent image

Development and post-treatment Developer Positive resist Exposure degrades the PAC Becomes more soluble to the developer after exposure Unexposed regions of the resist are left behind after development Developed resist pattern is identical to the mask pattern. Alkalis such as NaOH or KOH used as developers Negative resist Exposure increases MW of resist or creates new insoluble products Becomes less soluble to the developer after exposure Unexposed regions of the resist are removed after development Developed resist pattern is the opposite of the mask pattern. Organic solvents such as benzene used as developers 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?

Development and post-treatment After exposure, a mild oxygen plasma can be used to remove leftover exposed/unexposed resist. A post-bake follows, hardening resist even more. After the resist has done what it needs to do (act as a mask for doping, or for the etching of the layer below, e.g., ) resist needs to be removed completely  stripping Plasma ashing Positive resist Wet stripping usually used Chemical solvent such as acetone or methylethylketone (C4H8O) Often requires T ~ 80°C Can ignite with O2! Safety important Negative resist Harder to remove Use of acids and/or chlorinated hydrocarbons; e.g., H2SO4 and H2O2 at 150°C (“piranha” clean) Sometimes a plasma ash required. 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?

Diffraction in contact and proximity printing Resolution Diffraction in contact and proximity printing http://www.colorado.edu/physics/phys2020/phys2020_sum98/lab_manual/Lab5/Image2088.gif 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? https://webspace.utexas.edu/cokerwr/www/index.html/double_slit.gif Constructive  dsin(θ) = Destructive  dsin(θ) = m·λ (m + ½ λ) http://www.colorado.edu/physics/phys2020/phys2020_sum98/lab_manual/Lab5/Image2091.gif

Resolution Resolution: smallest distinguishable feature size of a transferred pattern Contact and proximity printing z 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? intensity position

Resolution Want a wide lens close to the image plane Projection printing Want a wide lens close to the image plane 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?

Relationship of numerical aperture to other quantities Resolution Numerical aperture NA = nsin(θmax) = D/2F where F = f/D (effective f number) 0.16 < NA < 0.6 0.3 < k1 < 1.1 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? Relationship of numerical aperture to other quantities

Resolution Contact/proximity ◄ How do you increase resolution? ► Projection 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? Make s small (s0 is contact printing  short mask life) Make resist layer thin Too thin and will not serve as good ion implantation mask, etc. develop away Make λ small Make numerical aperture big Big lenses and costly optics Other trade-offs Make λ small

¡Eso es la razón que usamos luz ultravioleta! Resolution smaller wavelengths  better resolution ¡Eso es la razón que usamos luz ultravioleta! Ultraviolet light regions 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? Emission spectrum for an Hg-Xe lamp

Resolution—Depth of focus Depth of focus: the distance across which the aerial image is in focus. resist aerial image 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? δ δ

Resolution—Depth of focus Variations in the height of surface features must be less than the depth of focus. For the best results surfaces should be planar (flat). structural or sacrificial layer δ 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? resist

Sensitivity and resist profiles Positive resist Negative resist 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? sidewalls not vertical

High γp  sharp images (más claro) Sensitivity and resist profiles Positive resist 100 50 Dp Resist remaining (%) Dp0 (+) 10 100 dose, D (mJ/cm2) 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? Dp Dose to clear Lithographic sensitivity Contrast High γp  sharp images (más claro)

Sensitivity and resist profiles Negative resist 100 Dgx 50 Resist remaining (%) Dg0 10 20 Dgi dose, D (mJ/cm2) 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? For negative resist dose to clear ≠ sensitivity sensitivity = Dgx

Modeling of resist profiles For positive resist: λ, wavelength a and α are related to resist absorbance Dp, dose to clear k2, process dependent contrast (same as in δ) k, depends on coherence of light 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? To get vertical sidewalls: Small wavelength Small absorbance Small dose to clear Small k2 (small δ) Large NA

Mask alignment also called registration alignment mark Good to use asymmetry with alignment marks Arrows (flechas) are often used to make alignment marks easier to find 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? alignment mark Processed wafer Mask

Phase shift masks are used to enhance resolution Mask types Phase shift masks are used to enhance resolution Make thicker or thinner Alternating phase shift Attenuating phase shift 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? Binary mask Phase shift mask Make less transparent

Mask materials Emulsion is photosensitive. glass Emulsion is photosensitive. Mask produced by photolithography High resolution emulsion (gelatin) or Thin (l000-2000 Å) metal, usually Cr Glass first coated with metal. Photoresist coated on top and patterned. Metal etched through photoresist mask and resist is stripped. 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?

Creating masks Typically both processes use a projection printing machine called a stepper. 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?