View source for IBD: Calibrating Optical Thickness

Basic method for calibrating optical thickness, for [https://en.wikipedia.org/wiki/Distributed_Bragg_reflector DBR]/multi-layer optical coatings (2 alternating films only).  Commonly used for SiO2/TaO DBR mirrors/filters.

*Get dep rate & refractive index (RIX) of individual SiO and TaO films, using single-deps and [[Ellipsometer (Woollam)|J.A. Woolam Ellipsometer]] or equivalent tool. Approx. rate from previous user is also acceptable.

:*Get RIX at the target wavelength, using "Derived Params" on JAW or Cauchy equation A/B/C params.  For example, if targeting a DBR centered at 1550nm (target λ=1550nm), you will want to know the RIX at 1550nm specifically.

*Calculate approximate dep. time to achieve a 1/4-wave thickness at the target wavelength, for each film (SiO and TaO).

:*For example, if the dep. rate of SiO2 measured at 5.2nm/min and RIX is n<sub>1550</sub> = 1.494, then
::::''SiO 1/4λ thickness: d<sub>1/4λ</sub> = 1550nm / 4 / 1.494 = 259.4nm''
::::'''''SiO 1/4λ time''': t<sub>1/4λ</sub> = 259.4nm ÷ 5.2nm/min = 49.88min = '''2993.077 sec'''''
::::(and then do the same for TaO)

*Deposit a fabry-perot 1/2-λ of one film (SiO or TaO) cavity onto a Silicon piece, using the above 1/4-λ deposition <u>times</u> (aka. "SiO/4" or "TaO/4" in the following), to calibrate that film's optical-thickness.

:*eg. for SiO Fabry-Perot cavity (aka. SiO-FP):
::::''SiO/4 + TaO/4 + SiO/4 + TaO/4 + SiO/4 + TaO/4 + <u>('''SiO/4 + SiO/4''')</u> + TaO/4 + SiO/4 + TaO/4 + SiO/4 + TaO/4''
::*Here we used only 3 periods of DBR on either side of the 1/2-λ cavity, to speed up the deposition.

*Measure the reflectivity on [[Optical Film Spectra + Optical Properties (Filmetrics F10-RT-UVX)|Filmetrics F10-RT]].
*Locate the wavelength of the minimum (dip) in the optical spectrum. Spectrum will typically be very broad, due to omitting many of the surrounding DBR layers for speed.

:*For example, the trough might show a minimum at 1600nm instead of the targeted 1550nm.

*Correct the film's optical thickness as so:

:::t<sub>1/4λ</sub> * λ<sub>target</sub> / λ<sub>measured</sub> = ''corrected'' t<sub>1/4λ</sub>
:::2993.077 sec * (1550nm / 1600nm) = '''2899.543 sec''' for SiO/4 layers
::*Use this corrected time for all SiO/4 layers.
::*You only need to edit the One "SiO2_dep" step in the IBD recipe, which will also change all "SiO2_dep" steps in the recipe.
:*Here is an example of a Fabry-Perot cavity that was targeting a 1050nm center-wavelength, but measured a 1100nm trough:[[File:IBD SiO-FP - long 1100nm - Reflectivity Spectrum (EMpy).png|alt=Simulated plot of SiO-FP reflectivity, for a 1050nm target but measured trough at 1100nm|none|thumb|Example: Targeting a 1050nm SiO-FP, the measured trough location is at 1100nm. ]]In this case, one would apply a correction to the ''SiO/4 time'' by multiplying by (''1050÷1100)''.

*Do the same Fabry-Perot correction for the other film, in this case do an "TaO-FP", and apply the new TaO/4 time to the recipe.
*Perform a test-DBR deposition onto Silicon, eg. 9 periods (less than full, which could be 15 periods or more), and measure on [[Optical Film Spectra + Optical Properties (Filmetrics F10-RT-UVX)|Filmetrics F10-RT]], to confirm that center wavelength is in the right spot.   An example DBR test-dep targeting 1050nm looks like this:[[File:IBD 9-period DBR - Reflectivity Spectrum (EMpy).png|alt=Example reflectivity spectra of the 9-period DBR test-dep, showing 1050nm center-wavelength in the middle of the DBR high-reflectivity spectrum.|none|thumb|Example reflectivity spectra of the 9-period DBR test-dep.]]
*Perform full-DBR deposition onto production parts.  Include a flat Silicon witness for measuring the final DBR reflectivity spectrum.<br />

*Sources of error: thick, stressy films exhibit the stress-optic effect, in which compressed films (closer to the substrate) will often show a reduction in RIX. In going from a 9-period DBR to a 18-period DBR, you might see a ~10-20nm blue-shift.  Some users for whom such a shift is outside the device tolerance will do a full DBR dep, then apply a % reduction to all dep times to further dial in the DBR reflectivity band.
*The same method can be used to calibrate optical-thickness for arbitrary multi-layer optical filters.


Note, you can verify/better understand the above method using any electromagnetic thin-film simulator.  For example, [https://github.com/lbolla/EMpy EMpy] has a simple transfer-matrix example for doing this, along with RIX models contributed by [[Demis D. John|Demis]].  Demis created the above simulations using EMpy.

<br />
 Developed by Demis D. John, Bob Farrell, Dustin Kleckner, ~2008-2010. This is the same method used by UCSB VCSEL groups years earlier, for calibrating VCSEL MOCVD/MBE growths.  Please consider our [[Frequently Asked Questions#Publications acknowledging the Nanofab|<u>publication policy</u>]] if you publish papers using this information.
 [[category:Process]]