Etching silicon wafers to make colorful Rugate optical filters (porous silicon)

Passing an electrical current through a silicon wafer in a special acid etchant will create a porous layer with a variable index of refraction. I describe how this process works, and how the Fourier transform relates filter design to electrical etch waveform and resulting spectral response.

Notes: 1. The Rugate filters look especially good in polarized light because the reflection from the silicon wafer is reduced, but the reflection from the filter remains strong. I noticed this while holding the wafer in front of my computer monitor. Later, when showing it to friends, the color intensity was poor. Their monitor must have been horizontally polarized, so holding the wafer low, and tilting it upward didn't work! Some monitors are horizontally polarized, and some are vertical.

2. The magnet used to hold down the PTFE cup to the wafer may have a very slight impact on the etch process. The dramatic shift in filter performance at the periphery is due to O-ring restricting the conductive etchant to a sharp edge, creating an electrical field concentration.

Outstanding visual Fourier series tutorial:
http://www.jezzamon.com/fourier/index.html

LR-1 spectrometer: http://www.aseq-instruments.com/LR1.html

p-type wafers on eBay. (You have to hunt around and check the photos for info on the label attached to the box of wafers. Be sure to get wafers less than 0.01 ohm-cm) eg https://www.ebay.com/itm/25-silicon-wafer-P-type-150mm-100-sumco/263441166009 n-type wafers are photosensitive during the etch process.

Online graphing calculator page from this video: https://www.desmos.com/calculator/hiju8zdqfz

Original Desmos page that I used: https://www.desmos.com/calculator/qpnz9celzf

Code for Keithley 2450 control and processing.org waveform generation:
https://github.com/benkrasnow/Porous_Silicon_Optics

Fourier transform to understand optical coatings:
http://www.willeyoptical.com/pdfs/92_180.pdf

Porous silicon refs:
https://www.rp-photonics.com/rugate_filters.html
https://www.intechopen.com/books/porosity-process-technologies-and-applications/porous-silicon
https://sci-hub.se/https://doi.org/10.1016/j.mee.2011.03.143
https://sci-hub.se/https://doi.org/10.1080/10408436.2010.495446
https://sci-hub.se/https://doi.org/10.1007/978-3-319-71381-6_2
https://sci-hub.se/https://doi.org/10.1364/AO.44.005415
https://sci-hub.se/10.1364/OE.16.015531
https://sci-hub.se/https://doi.org/10.1002/adma.19940061214
https://sci-hub.se/https://doi.org/10.1063/1.2906337

Silicon wafer identification flats:
https://www.tf.uni-kiel.de/matwis/amat/elmat_en/kap_5/illustr/i5_2_4.html


Applied Science on Patreon:
https://www.patreon.com/AppliedScience

Shooting an electron beam through air

A special 100nm thick window allows 25 KeV electrons to pass from a vacuum tube to the atmosphere where they hit a fluorescent screen -- a CRT in air!

Shielded GoPro goes through a powerful electron beam: https://www.youtube.com/watch?v=yPMpAR9w-L0

More powerful amateur electron beam in air: https://fusor.net/board/viewtopic.php?t=7828

Deep technical resource on dielectric charging via electron bombardment:
https://apps.dtic.mil/dtic/tr/fulltext/u2/a172204.pdf

Tons of information on industrial e-beam processing: http://iiaglobal.com/uploads/documents/Industrial_Radiation_eBeam_Xray.pdf


KF25 to glass tube quick adapter: https://www.idealvac.com/SWIFT-SEAL-KF-to-Compression-Port-Adapter/pp/P103772

100nm silicon nitride windows: https://www.tedpella.com/grids_html/silicon-nitride-x-ray-windows.htm

More windows: http://www.temwindows.com/category_s/22.htm

Tritium light sources (eBay removed most): https://usa.banggood.com/search/tritium.html

E-beam crosslinking: https://ebeamservices.com/polymer-crosslinking/services/plastic-parts/

KF25 cross $18 on Amazon: https://www.amazon.com/gp/product/B07CKS3H99

Lightbulb sockets: https://www.amazon.com/gp/product/B07SSYN83Y/

Hysol 1C: https://www.amazon.com/Loctite-HL1373425-1373425-Hysol-1C/dp/B000B631G8

Applied Science on Patreon: https://www.patreon.com/AppliedScience

Ultrasonic soldering bonds glass, titanium, stainless steel, ceramics, tungsten, nichrome...

Technical details and how to build an ultrasonic soldering iron. This technique can bond difficult-to-solder metals such as titanium as well as glass and ceramics.

Cerasolzer technical info: http://cerasolzer.com/cerasolzer/basic_info_gb.html

S-bond technical info: https://www.s-bond.com/solutions-and-service/ultrasonic-soldering/ultrasonic-solder-materials/

Overview of active soldering process: https://sci-hub.tw/10.5772/intechopen.82382

Another good overview: https://www.intechopen.com/books/recent-progress-in-soldering-materials/recent-advances-in-solderability-of-ceramic-and-metallic-materials-with-application-of-active-solder

Discussion of Sn - La solders: https://www.hindawi.com/journals/amse/2015/269167/

Sn - Ti phase diagram: https://sci-hub.tw/https://doi.org/10.1007/s11669-010-9663-2

This patent has the key list of ingredients (not just broad ranges) listed at paragraph 45: https://patents.google.com/patent/US20160204303A1/en

https://www.patreon.com/AppliedScience

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