photolithography identify the basic steps of a photolithographic process describe the differences...

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

Reminder of the photolithography steps in the -machining process Silicon wafer SiO 2 Spin on photoresist Glass plate Opaque region Unexposed photoresist removed by developer Mask Exposed photoresist becomes less soluable

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

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.

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 m/ft 3 Class 10 less than 10 particles > 0.5 m/ft 3 Class 100 less than 100 particles > 0.5 m/ft 3 Class 1000 less than 1000 particles > 0.5 m/ft 3 Outside the US (ISO 14644-1) ISO Class 1 less than 10 particles > 0.1 m/m 3 ISO Class 2 less than 100 particles > 0.1 m/m 3 ISO Class 3 less than 1000 particles > 0.1 m/m 3

Comparison of cleanroom standards

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

Clean room etiquette and requirements 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. Rose-Hulman students in the clean room facility at RHIT Por qu parece tan amalliro?

Wafer cleaning RCA clean Developed by Werner Kern in 1965 while working at RCA Laboratories 1.1:1:5 to 1:1:7 by volume solution of NH 4 OH : H 2 O 2 : H 2 O is used to remove organic contaminants and heavy metals 2.HCl : H 2 O 2 : H 2 O in a 1:1:5 to 1:2:8 volume ratio is used to remove aluminum, magnesium, and light alkali ions Both steps approximately 20 minutes while gently heating to 75-85C on a hot plate There are other cleaning techniques, such as piranha clean

Photoresist Often called resist Three (3) components: 1.a base resin, which is a polymer: gives the resist structure 2.photoactive compound (PAC): The light- sensitive component 3.solvent. Comes in two varieties 1.Positive resist 2.Negative resist Photoresist is the stuff of photolithography Usually sensitive to a narrow band of ultraviolet (UV) light

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

Positive versus negative resist

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. Positive resistNegative resist Mask Side view of wafer

Applying resist 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) Three steps 1.A pre-bake to reduce water (water can the reduce adhesion of resist) 2.Spin on the resist 3.A post-bake to remove the solvent

Applying resist T is the film thickness C is polymer concentration, is viscosity, is rotational spin speed, and K, , , and are experimentally determined constants that vary from system to system. Coating step (proceso de cubrirlo) poses the largest danger from a dust contamination. An empirical relation often used to predict the film thickness is given by 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 0.7m. How would you create a resist layer a. 0.8 m thick? b. 1.0 m?

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

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

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

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

Projection printing Photolithography is an optical printing process ff s s'

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? ff s s' Respuesta: 50 mm

Dose, dose to clear, and arial image latent image Dose to clear -Dose required to expose the resist so that it completely develops -D p for positive resist (completely develops away) -D g 0 for negative resist (completely remains) D -Dose -Optical energy per unit surface area -D = It exposure I -Light intensity -Optical power per unit surface area -[W/m 2 ]

Development and post-treatment 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 Developer

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 Positive resist Wet stripping usually used Chemical solvent such as acetone or methylethylketone (C 4 H 8 O) Often requires T ~ 80C Can ignite with O 2 ! Safety important Negative resist Harder to remove Use of acids and/or chlorinated hydrocarbons; e.g., H 2 SO 4 and H 2 O 2 at 150C (piranha clean) Sometimes a plasma ash required. Plasma ashing

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

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

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

Resolution Relationship of numerical aperture to other quantities Numerical aperture NA = nsin( max ) = D/2F where F = f/D (effective f number) 0.16 < NA < 0.6 0.3 < k 1 < 1.1

Resolution Contact/proximity How do you increase resolution? Projection 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 small Make numerical aperture big Big lenses and costly optics Other trade-offs Make small

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

ResolutionDepth of focus Depth of focus: the distance across which the aerial image is in focus. resist aerial image

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

Sensitivity and resist profiles Positive resistNegative resist sidewalls not vertical

Sensitivity and resist profiles Positive resist 0 50 100 dose, D (mJ/cm 2 ) 10 DpDp 100 Dp0Dp0 (+) DpDp -Dose to clear -Lithographic sensitivity Contrast High p sharp images (ms claro)

Sensitivity and resist profiles Negative resist 0 50 100 dose, D (mJ/cm 2 ) 10 DgxDgx 20 Dg0Dg0 DgiDgi For negative resist -dose to clear sensitivity -sensitivity = D g x

Modeling of resist profiles For positive resist: -, wavelength -a and are related to resist absorbance -D p, dose to clear -k 2, process dependent contrast (same as in ) -k, depends on coherence of light To get vertical sidewalls: Small wavelength Small absorbance Small dose to clear Small k 2 (small ) Large NA

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

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

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

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

photolithography identify the basic steps of a photolithographic process describe the differences...

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