PHTN1432: Vacuum Systems and Thin Film Technology
Lab - Dielectric Mirror Coatings (2019W)

Purpose

WARNING:
The prelab (with unique parameters for each individual lab group section) is REQUIRED to be completed by each student and submitted at the beginning of the lab (like any other prelab). Failure to submit the prelab or submit it late (late prelab submissions are worth ZERO) will be penalized -30% on the whole lab. NO EXCEPTIONS!

eBeam Deposition System

Pre-Lab

Coating

The coating is to be applied using SPARKY, a six-target cryopumped eBeam system. When you enter the lab, the CTI cryopump will already be running (since it requires at least several hours to precool). Verify that the cryopump is ready (via the attached monitor) before proceeding.

Begin by venting the chamber then opening it. Load one substrate per student into the holders. Ensure the required target crucibles are filled (this is important since many layers will be deposited). Ensure that the crystal (XTM/2) is operating properly. Now lower the chamber, rough-pump to below 100mTorr, and open the gate valve. When the pressure reaches the low 10-5 torr range (< 2*10-5, usually about 20 minutes) deposition may begin.

Turn the CV-8 eBeam supply ON, ensuring all interlocks are closed. Before applying power, ensure both the operating voltage and the emission current controls are set to zero. Now proceed to deposit all layers in the proper order.

The deposition process is as follows:


eBeam Deposition System - Crucible The violet glow as seen through the window of the chamber. The filament is visible in the lower part of the photo and the crucible is seen in the upper part. The crucible is not visible directly, but through the periscope arrangement which ensures the window is always kept clear of the deposit. Of course, the deposited film is also deposited on the lower mirror and so the colour of the image may well change during deposition.

With MgF2 use a rate of 5 to 10 Angstroms/second and for ZnS a rate of 4 to 6 Angstroms/second. Since ZnS sublimes, be careful when controlling the rate of deposition - it may "run away" quickly. While exceedingly high rates are possible (especially since Sparky features a 13kW supply), splattering may occur resulting in poor surface quality of the coating and possible damage to the monitor crystal - be especially careful with a 'freshly' loaded crucible. The other concern is evolved gases (again, a problem with freshly loaded crucibles) which may cause a pressure increase sufficient to permit an electrical discharge (i.e. arcing) of the eBeam supply into the chamber - low rates, especially on the first few depositions, will prevent this! Remember, too, to degas each material the for the first use only by slowly heating it with the shutter closed (subsequent layers will not require this process but the shutter is still required to terminate the deposition).

Deposit six to ten alternating layers. Be prepared when you enter the lab as this is all you will have time for in the two-hour lab: each layer will take about 2 minutes to deposit plus a minute of cool-off between switching the hearth targets plus ramp-up time. Be sure to program the XTM/2 monitor for each film (simply select the new film number, all other parameters are preprogrammed including density, z-ratio, and tooling factor). Write-down the actual layer thickness deposited. More layers means higher reflectivity of the finished mirror. Be sure to also deposit the top protective layer.

When all layers are complete shut down the CV-8 power supply (remove the key for safety), close the gate valve, vent and open the chamber, and USE THE SAFETY GROUND TO DISCHARGE THE FILAMENT TERMINALS (Wherever a safety grounding probe is provided, USE IT!).

Check the XTM/2 monitor, as well, for the current tooling factor.

Analysis

Quantitative analysis requires the use of a spectrophotometer. Using either the Lambda-3B or GeneSys-20 spectrophotometer in the spectroscopy lab, determine the reflection curve for the coating (all substrates should be the same - verify this by sampling at least three from different locations in the chamber) - the reflection curve may be determined using the reflection adapter.

If using the reflectance accessory ...
The UV lamp is not required, only the VIS. Before starting the unit ensure the alignment mirror is in place on the accessory. Once ready, carefully remove the alignment mirror and replace it with your dielectric mirror (coated side down). Now, run the spectrum from 900nm to 315nm. When all runs are complete, replace the alignment mirror.

Transmission only ...
If the reflection adapter is not installed, simply run the transmission spectrum as per the previous lab and take the inverse of the transmission curve (in Excel) by subtracting 100% minus transmission for each point. Be sure to use a glass blank the same as used for the deposition as a reference.

Compare the observed results to the theoretical prediction from FilmStar (using the actual layer thicknesses deposited) and try to "fit" the FilmStar model to the observed spectrum by modifying layer thicknesses as well as changing the n value of the ZnS layer (as outlined below).

Lab Report

Prepare a condensed lab report as follows:

    COATING DESIGN:

  1. Outline how a Dielectric high-reflector coating works. Include a diagram showing each reflection at the interface between materials and the relative phase of each reflection (i.e. in-phase or out of phase with the original top reflection). Omit the protective layer in this design.
  2. PERFORMANCE:

  3. Submit a corrected FilmStar report outlining the expected performance of the mirror that was actually fabricated in the lab (using the layers and thicknesses actually deposited as read from the XTM/2 - the tooling factor given in the lab is correct). The FilmStar report must be included verbatim and without modification - sharing of FilmStar models will be considered plagiarism and will result in a mark of zero for the entire lab.
  4. Submit a copy of the spectrophotometric reflection curve for the mirror (only one required since the chamber is designed for consistency). Identify λcenter. This might be a calculated reflection based off the measured transmission of the device (see the lab).
  5. Submit several FilmStar models which attempt to match the observed results from the lab as closely as possible. This will attempt to rationalize and explain discrepancies between the theoretical curve of the FilmStar model and the actual performance of the coating. Two parameters must be modified in an attempt to "fit" the model:

    #1: Try changing the n value of the ZnS layer. In your research will will have noted that ZnS has two values of n listed, depending on the source. Try to model the filter using BOTH values to see if one is closer than the other and include both outputs to allow comparison between the predicted output in each case. Include at least ONE FilmStar model using each n value which attempts to "fit" the model to the results achieved! Explain, in each case, which value was used and ultimately which produces the best fit to observed results.
    #2: Use the n-value above which matches the closest and change the thickness of each type of layer (i.e. scale all low-index or all high-index layers simultaneously using the interactor) until a similar curve to that which was found experimentally in the lab is achieved. When the model matches the observed results as closely as possible, produce a final FilmStar model and include it in the report. Once finally optimized, compute a new tooling factor for that material/layer.

    The FilmStar reports must be included verbatim and without modification. Sharing of FilmStar models will be considered plagiarism and will result in a mark of zero for the entire lab.
    For this question, at least three FilmStar outputs are required as well as an explanation of how each was made (i.e. parameters which were changed for each model).

  6. Many datasheets mention "Stoichiometric composition" with respect to evaporation (both eBeam and sputtering techniques). One of the most interesting examples of stoichiometric change during evaporation is in the use of titanium oxides as described here. Describe (in a paragraph) HOW starting with Ti3O5 can lead to perfect deposits of TiO2 - describe the reaction that occurs and leads to the desired composition of the film.

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