ElectroHolography Laboratory

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====18 Channel Prototyping Protocol:====

1. ==Initial Preparation==

1.1) Note: begin with a new X-cut lithium niobate wafer. It should be optical grade, 1mm thick, clean, with nothing deposited on the surface, both sides polished, and the top side marked.  

1.2) Using an Electron Beam Evaporator or equivalent machine at a vacuum of 50 µtorr, evaporate 200nm of aluminum on the wafer at 5Å per sec. To replicate the presented results, position the wafer constellation 65 cm above the Aluminum crucible. 

1.3) Spin on 30 drops of AZ3330 photoresist at 3000 rpm for 60 sec. For a detailed description of the mechanics of spinning polymer films see the work by C. J. Lawrence21.

1.4) Softbake the resist at 90°C for 60 sec.

1.5) Using the mask supplied as a .dxf file in the appendix, expose the wafer using a mask aligner with a 350W mercury bulb or its equivalent for 10 sec as per machine specifications. The wafer should be aligned so that the waveguides are parallel to the y-axis. 

1.6) Develop the resist in a solution of MIF 300 for 60 sec. 

1.7) Hard bake the wafer for 60 sec at 110°C. 

1.8)  Etch away the exposed aluminum completely by submerging it for 2 min in a 1L solution aluminum etch heated to 50°C.

1.8.1) CAUTION: Aluminum etch is toxic, corrosive and harmful. See MSDS for proper handling and storage of this chemical. Use proper personal protective equipment for acid when handling this chemical. 

1.9) Remove the photoresist mask with a rinse of Acetone then IPA.

1.10) Using a 0.016in thick diamond blade with an exposure depth of 0.165in on an automatic dicing saw, cut the wafer into 10x15mm devices with the long dimension parallel to the y-axis.

1.10.1) The blade will not cut all the way through the substrate. To separate each device, simply stress each scored line cut by the dicing saw.

1.10.2) Note: Each 10x15mm device will individually go through the remaining steps of the protocol.

2. ==Proton Exchange==

2.1) Place an individual device in a test tube with a small hole cut in the bottom to allow interaction between the device and all liquid baths. 

2.2) Proton exchange the device by immersing it in a 1L melt of 99% pure benzoic acid at 240°C.  In order to achieve the target depth of 0.4504µm, use an immersion time of 10min and 10sec.

2.2.1) Note: The proton exchange immersion time is dictated by the diffusion coefficient, D, which for the authors’ melt is currently D=0.2993. The proton exchange immersion time is calculated using the relation T=d^2⁄(4D). Where T is the exchange time in hours, d is the waveguide depth in microns and D is the diffusion coefficient. For a detailed description of the mechanics of proton exchange see the work by J. L. Jackel15.

2.3) Remove the device and allow to cool for 5 min or until cool to the touch.

2.4) Clean off any benzoic acid residue with a rinse of Acetone then IPA.

3. ==Anneal==

3.1) Place device in a regular test tube and wrap the tube in aluminum foil. 

3.2) Place the tube in a muffle furnace for 45 min at 375°C.

3.3) Remove the device and allow to cool for 5 min or until cool to the touch.

4. ==Clean==

4.1) Clean the Aluminum mask from the device using Aluminum etch for roughly 2 min at 50°C. 

4.2) Clean the device in Acidic Piranha etch to remove any organic residues.

4.2.1) CAUTION: Acidic Piranha etch is toxic, corrosive and harmful. See MSDS for proper handling and storage of these chemicals. Use proper personal protective equipment for acid when handling these chemicals.

4.3) Rinse the device in Acetone, then IPA, and dry with compressed nitrogen.

5. ==Waveguide Measurements==

5.1) Using any commercial waveguide analyzer measure the characteristics of the proton exchanged waveguide. A good device will have 2 guided modes using a 633nm laser. See Figure 3 for an example of desired results.

5.1.1) Note: If the device shows more than two guided modes for red illumination then the exchange time in step 2.2 should be reduced. Likewise if the device shows less than two guided modes the exchange time should be increased.

6. ==Add Resist==

6.1) Spin on 4 drops of a Lift Off Resist (LOR) at 3000 rpm for 60 sec and then bake at 200°C for 1 hr. 

6.2) Remove and allow the device to cool for 5 min or until cool to the touch.

6.3) Spin on 4 drops of 3:1 PMMA:Anisole and spin at 3000 rpm for 60 sec and then bake at 150°C for 15 min.

6.4) Remove and allow the device to cool for 5 min or until cool to the touch. 

6.5) Spin on 2 drops of a conductive polymer at 1000 rpm for 60 sec, then 6000 rpm for 4 sec to remove any excess. 

7. ==Pattern==

7.1) Use an electron microscope enhanced with a beam blanker to enable writing or an equivalent machine to expose the device.

7.1.1) Under a vacuum of 50 µtorr, expose the conductive layer to an electron beam with an area dose of 30 µC/cm2 that scans the pattern of the interdigital transducers. To replicate the results presented in this paper use a measured beam current of 410 pA.

7.1.2) Write the pattern from a .dxf or equivalent file on the electron microscope as per machine specifications.

7.1.3) Note: For a detailed description of the E-Beam lithography process see the work done by R. E. Fontana22.

8. ==Develop==

8.1) Remove the conductive layer by rinsing the device in a continuous stream of deionized water for 5 sec. 

8.2) Remove the exposed PMMA by dipping the device into 1:3 solution of MIBK:IPA for 45 sec. 

8.2.1) Remove from the 1:3 solution of MIBK:IPA and rinse with IPA for 5 sec.

8.2.2) Dry the device with compressed nitrogen.

8.2.3) Repeat steps 8.2-8.2.2 as necessary to fully develop the PMMA. However expose the device to the solution of MIBK:IPA  in 5 sec increments only.  Complete development should reveal the LOR underneath the PMMA and can be identified by uniform coloration throughout the developed area surrounded by crisp edges and corners.

8.2.3.1) Note: Over development of PMMA leads to small feature blowout and can completely erase the interdigital transducer fingers, leaving a single large developed block. Likewise under development leaves non-uniform residues that will decrease the effectiveness of the liftoff process that follows. 

8.3) Remove LOR in exposed region by dipping the device into a 1:1 solution of MIF 300:H2O for 25 sec.

8.3.1) Remove from the 1:1 solution of MIF 300:H2O and rinse with IPA for 5 sec.

8.3.2) Dry with compressed nitrogen.

8.3.3) Repeat steps 8.3-8.3.2 as necessary to fully develop the LOR. However expose the device to the solution of MIF 300:H2O in 2 sec increments only. Complete development should reveal the surface of the substrate underneath the LOR and can be identified by uniform white coloration throughout the developed area maintaining its crisp edges and corners. Failure to develop the LOR properly also leads to the problems discussed in 8.2.3.1. See Figure 4 for an example LOR development process.

8.3.3.1) Note: switching to a lower dose of MIF 300: H2O such as 1:2 or 1:3 is helpful as the device nears complete development to allow the fine features to develop without blowing out the device. However, it is not advantageous to start with these doses as total time increases and exceeds optimal time in MIF 300.

9. ==Deposit Aluminum== 

9.1) Using an Electron Beam Evaporator or equivalent machine at a vacuum of 50 µtorr, evaporate 200nm of aluminum on the wafer at 5Å per sec.

10. ==Liftoff Aluminum==

10.1) Fill a large glass dish with 750mL of water on a hot plate at 90°C.

10.2) Insert a plastic buffer into the water dish.
10.3) In a separate small glass container submerge the device in 100mL a solution of NMP.

10.4) Place the container of NMP solution containing the device onto the plastic buffer insuring that the water level does not exceed the height of the container of NMP.

10.5) Cover and let sit 3 to 4 hours or until Aluminum liftoff is complete. 

10.6) Remove the device from NMP.

10.6.1) Note: it is advantageous to clean off large sections of aluminum from the device by squirting the device using a pipette filled with NMP while still submerged in the NMP bath. 

10.7) Rinse the device in IPA and dry with compressed nitrogen.

10.8) Under a microscope, verify that liftoff is complete.

10.8.1) If unwanted residual Aluminum remains, wet the device with acetone and brush very carefully with a cleanroom swab coated in acetone to remove.

10.8.2) Rinse in IPA, dry with compressed nitrogen, and recheck under the microscope.

10.8.3) Repeat 10.8.1 and 10.8.2 as needed.

11. ==Polish the End==

11.1) Coat the device in a protective film such as a layer of AZ3330 photoresist.

11.2) Clamp the device so that the end with the transducers is exposed for polishing.

11.3) Using proper polishing procedures, slowly polish the end of the device to a surface roughness of less than 100nm, so that no surface defects interfere with the light exiting the device.

11.4) Remove the device from the clamp and clean off the protective film.

11.4.1) If photoresist was used as a protective film, a generous rinse in acetone and then IPA will remove it.

11.4.2) Dry the sample as necessary with compressed nitrogen.

12. ==Mount on a Breakout Board==

12.1) If any assembly is required for the RF breakout board, assemble the breakout board according to its specifications.

12.2) Build, out of glass slides, a mounting platform to hold firmly both the RF breakout board and the device.

12.3) Mount the device to the top of the mounting platform with double sided tape. Make sure the end of the device overhangs the end of the mounting platform so that the mounting platform does not interfere with light exiting the end of the device.

12.4) Mount the RF breakout board to the mounting platform so that it is not in the beam path of the light exiting the device. A simple way to do this is to elevate the breakout board with thick tape so that the bottom of the breakout board is above the top of the device.

12.5) Wire bond the pads on the device to their respective locations on the RF breakout board.

12.6) Use a 27nH series inductor to impedance match each transducer to the breakout board inputs. 

13. ==Prism Coupling==

13.1) Select a rutile prism to couple light into the device. The polarization of the light (TE) should be parallel to the optical axis of the rutile the optical axis (Z-axis) of the X-cut lithium niobate.  

13.2) Clean the contact surfaces of both the device and the prism thoroughly with IPA.

13.3) Position the prism so that it is centered on the channel to be tested.

13.4) Press the bottom of the prism firmly against the top of the device with a clamping mechanism.

13.5) If successful, a wet spot will appear. A wet spot is a region of frustrated total internal reflection at the interface between the prism and the sample. For an example of proper prism coupling see Figure 5.

14. ==Mount in the Characterization Apparatus==

14.1) Mount the device on the rotating platform of the Frequency division color characterization apparatus for anisotropic leaky mode light modulators discussed by A. Henrie4.

14.1.1) A schematic of the Characterization Apparatus is supplied in Figure 6.

15. ==Align in the Characterization Apparatus==

15.1) Turn on the laser. To replicate the results presented in this paper use 5V for 638 nm, 5.5V for 532 nm, and 6.5V for 445 nm.

15.2) Attenuate the beam for safe alignment.

15.3) Verify the laser polarization.

15.3.1) Place the polarizer in the beam path after the half wave plate so that it blocks horizontally polarized light.

15.3.2) Rotate the half wave plate to achieve maximum attenuation of the laser light.

15.3.3) Remove the polarizer.

15.4) Rotate the platform to the proper entrance angle for the wavelength and mode to be measured. See Table 1.

15.5) Using the linear translation stages, align the coupling edge of the prism so that it lies in the focal point of the laser.

15.5.1) At this point, light should be coupling into the device which can be verified either by the characteristic streak of light caused by scattering in the waveguide or by the characteristic mode lines coming out of the end of the device23. See Figure 7.

15.5.1.1) If using mode lines to verify coupling, it is helpful to remove the power meter from the beam path. Instead insert a uniformly scattering object, such as a sheet of white paper, into the beam path.

15.5.2) If no coupling is detected, slowly rotate the device while maintaining the coupling edge of the prism in the focal point of the laser. If after five degrees rotation in either direction no coupling can be detected, remove the device from the rotating platform, remove the prism and return to step 13.

15.6) Once coupling is detected, fine tune the rotational platform and linear translation stages to maximize the coupled light into the mode that will be tested. 

16. ==Attach the RF Input and Enclose the Device==

16.1) If the power meter was removed during alignment, replace it now. Also remove any obstructions to the beam path used for alignment purposes.

16.2) Attach the RF input to the device breakout board and turn on the RF signal generator.  Make sure the amplifier is powered.

16.2.1) Note: the electrical power of the signal reaching the device should not exceed 1W.

16.3) Remove any attenuation used for safety during alignment. The laser should now be at the optical power levels used for testing.

16.4) Enclose the entire system in an optically isolating box.

17. ==Run the Provided LabView Testing Program.==

17.1) Download and open the LabView file AutomatedDeviceCharacterization.vi provided in the appendix.

17.2) Insert all user parameters into the testing software on the control computer. Fields in Figure 8 boxed in yellow must be updated before each automated test is run in order for the Analytical program to run properly in step 19.

17.2.1) Note: to replicate the results presented in this paper use the following testing parameters: Initial Frequency: 100 MHz, Final Frequency: 800 MHz, Frequency Step: 10, Rough Initial Position: 0, Rough Final Position: 25, and Position Step: 1.

17.2.2) Make sure the Output to File button is pressed.

17.3) Run the testing program.

17.3.1) Note: The program drives a power meter along a linear track at user defined intervals. At each position the RF input signal is swept through a set of chosen frequencies and power measurements are made. A measurement is also made with the RF input at its lowest frequency setting and lowest output power which has been experimentally determined as equivalent to no input signal4. These measurements are then graphed in real time in a 3D interactive chart.

17.3.2) Note: The testing program creates four output files: *config.csv describes the experiment, *data.csv contains the power reading at each frequency, *no_stim.csv contains the background noise reading, and *graph.jpeg contains a copy of the graph on the user interface of the LabView program as it was when the program ended. See Figure 9.

18. ==Repeat steps 15-17 for each wavelength and the TE1 mode described in Table 1.==

19. ==Analyze the Frequency and Angular Output Profiles.==

19.1) Download the CompareWDMmodes.m MATLab code provided in the appendix.

19.2) In the folder where CompareWDMmodes.m is located, create a subfolder, “Sample Number” Where “Sample Number” is the device identificationn numder entered into the LabView file.

19.3) In this folder, “Sample Number,” create three subfolders. Each should be named in the format “Sample Number”_“Color”_M1_“Transducer”, where the names in “bold and italics” are values entered into the LabView file by the user. (e.g. A16_BLUE_M1_T1, C5_RED_M1_T13, D35_GREEN_M1_T18).

19.4) Copy into each subfolder the four files created by the LabView software that correspond with that particular wavelength, mode, and transducer.  

19.5) Open CompareWDMmodes.m and change the user defined variables at the top of the MATLab code to reflect the user LabView values.

19.5.1) Note: If, the user defined Labview values are “Sample Number”=A16, “Guided Mode”=1, “Transducer”=1 the MATLab code would be modified to the following:
% User Defined Variables
series='A';
sample=16;
modes=[1];
transducer='T1';

19.6) Run CompareWDMmodes.m in the MATLab software.

19.6.1) Note: Among other things, CompareWDMmodes.m creates a figure that compares the normalized frequency response and the angular output for red, green, and blue light. The file it creates is located in the “Sample Number” subfolder. See Figure 10.
ElectroHolography Laboratory

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