Equipment

Spinner ^[1], Hot Plate,MJB3 Mask Aligner^[2], Metal electron Beam Deposition System ^[3], and Panasonic E620 Etcher ^[4], Keithley 6221 AC/DC voltage/current source

Pre-cleaning

1. Solvent clean pieces of SiC to be electroplated using Toulene, Acetone, methanol/IPA each for 5 minutes.
2. Use Plasmatech to etch the samples for 1 minute using the following parameters; RF power 100 W, SF6 Flow rate 50 sccm, DC bias 100 W and pressure 50 mTorr. This will roughen the surface to ensure the seed layer sticks without peeling off from SiC.
3. Deposit the seed layer of Titanium 50 nm and Gold 70 nm using electron beam evaporation system.

Lithography

1. Spin AZ9260 at 3000 rpm for 80 seconds (an initial spreading step with a 500 rpm for 10 seconds may be used)
2. Soft bake the samples at 120 °C for 3 minutes then cool it down for 1 minute.
3. Spin a second layer of AZ9260 as in step 1 (A total of approximately 18 µm should be obtained)
4. Soft bake the samples for 8 minutes at 120 °C and let them cool down.
5. Expose the samples using mask aligners for 120 seconds at 10 mW/cm2.
6. Develop the sample using AZ 400 developer: Water = 1:3 until it all the photoresist is removed from the channels then stop the development in water and dry with nitrogen gas.
7. Postbake the samples in the oven for 30 minutes at 90 °C to harden the photoresist.
8. Use Plasmatech with the parameters, RF 100 W, pressure 50 mTorr, the argon flow rate 50 sccm for 1 minute to de-scum the channels.
9. Remove photoresist from the region of the sample on which the electroplating electrode will be attached using Q-tips and acetone.
10. Cover the front side of the wafers with a clear adhesive tape to protect it from getting electroplated and to protect devices from getting into contact with the electroplating solution. (do not use the blue dicing tape)

Preparing the electroplating solution

1. Dissolve 350 g/ liter of Nickel Sulfamate solution in water stirred at 40 °C. Add 30 g/liter of Boric acid and stir until it dissolves. Then add 30 g/liter of Nickel Carbonate and stir for 1 hour. Switch off the heat and let the solution settle down for 24 hours.
2. Sieve the clear solution and dispose of the solute that settles at the bottom of the beaker.
3. Filter the solution by using Whatman’s filter papers to filter out large particles still in the solution.
4. Measure the PH and ensure that it is between 3.5 and 5.
5. If the PH is too high, lower it by adding 10 ml of sulfamic acid and re-measure after 5 minutes. Repeat till the correct PH is achieved. If it is too low, add 5 grams of sodium carbonate and stir well(You may need to filter the solution to remove large particles after raising the PH). Measure to check the level.

Electroplating

1. Set the bath at 40 °C and stir continuously
2. Use the AC/DC voltage/current source (Keithley 6221 or any other) to draw a constant current density of 10 mA/cm^2. (Estimate the Area to be electroplated and multiply by the current density to find the current to use.
3. Use either inert electrode (Titanium coated platinum) or a Nickel plate and connect it to the positive terminal and insert into the solution.
4. The negative electrode should be connected to the sample to be electroplated. Before inserting it into the solution ensure that it is wet with water to avoid forming bubbles on the surface to be electroplated hence preventing Nickel deposition.
5. Ensure that the clipped parts of the cathode(sample) and the anode are kept out of the solution.
6. Press the output button and ensure that the LED shows a steady light and not blinking. If it is blinking, check the connection and ensure that it is correctly done.
7. The deposition rate can be several microns per hour. Electroplate to get the desired thickness of the metal. ( A typical deposition rate at 10 mA/cm^2 on a 4-inch wafer is 1.6 – 2.9 microns/hour)
8. Once done, disconnect the power supply, rinse the sample and dry with nitrogen gas.
9. Remove the clear plastic cover protecting the front side by exposing it to the UV lamp (located adjacent to the dicing saw) for 20 seconds. The adhesive changes to liquid and can then be peeled off easily.
10. Soak the sample in PRS 2000 for 20 minutes to dissolve all the photoresist on the regions that were protected from electroplating.
11. Continue to clear all the remaining photoresist by soaking the sample in acetone heated to 70 °C for 5 minutes followed by IPA rinse for a further 5 minutes.
12. Etch the gold seed layer by soaking the sample in potassium iodide +Iodine solution till all the gold is etched.
13. Rinse in water and dry in nitrogen and check on the microscope to ensure that all the Au seed layer is etched away.
14. The sample is then ready for ICP/ RIE etching.

Figures

Figure 1 Figure 2

Figure 1 shows the process flow for the electroplating process described above. It also includes the etching step flow diagram described elsewhere. Figure 2 shows an S4800 image of a nickel plated 4H-SiC piece after the photoresist is dissolved but before trench etching. The thickness of nickel deposited is approximately 15 microns.

Sources for further information

1. ^ //www.purdue.edu/discoverypark/birck/facilities/equipment/Fabrication/Patterning/Coat-and-Develop/Desk-Top%20Precision%20Spin%20Coating%20Systems.php

2. ^ //www.purdue.edu/discoverypark/birck/facilities/equipment/Fabrication/Patterning/Optical-Align-Expose/Suss%20MJB-3%20Wafer%20Aligner.php

3. ^ http://www.purdue.edu/discoverypark/birck/facilities/equipment/Fabrication/Deposition/Evaporator/Airco%20E-beam%20Evaporator.php

4. ^ http://www.purdue.edu/discoverypark/birck/facilities/equipment/Fabrication/Etching/DRIE/Panasonic%20E620%20Etcher.php

Created on 05 Jan 2018, Last modified on 16 Aug 2018