MAKING GOLD PADS

1. On the spinner, rinse silicon dioxide wafer with acetone and isopropanol.

2. Spin LOL2000 @ 3000 rpm (3s acceleration and deceleration).

This gives about 200 nm thickness of the undercut layer according to the figure below.

     Picture taken from the SNL webpage. http://fy.chalmers.se/assp/snl/

3. Bake the sample @ 180 °C for 10 mins.

4. Spin Shipley UV5Ô DUV photoresist @ 4000 rpm (3s acceleration and deceleration).

This gives about 630 nm thickness of the top photoresist layer according to the figure below.

Picture taken from the SNL webpage. http://fy.chalmers.se/assp/snl/

5. Bake the sample @ 130 °C for 1 min and 30 secs.

6. Expose on Mask Aligner KS-MJB3 DUV for 7-8 seconds.

By default, the intensity is set at 1.6 mJ/cm². Keep in mind that the product of the intensity (measured using the intensity meter) and the exposure time should be equal to the energy per unit area, which is 3.8 mJ/cm².

7. Bake the sample @100 °C for 1 min.

8. Develop using MF319.

An indication when it is ready would be when the color in the region of the pattern changes plus about 10-15 secs.

9. Rinse in DI water thoroughly, and blow dry carefully.

10. Check in the optical microscope if the pattern is acceptable.

11. Ash at 50 W, 250 mTorr oxygen for 30 secs.

12. Evaporate the contact pads.

Cr is used for good adhesion between silicon and gold. A thin layer of Cr would suffice. Then Au is evaporated. Finally, Pd is added for protection. When Al is evaporated on top of gold, Au-Al intermetallic bonds may be formed across the contact area, which increases the contact resistance. This is undesirable for our purposes. Intermetallic compounds have differing structures, crystal lattice constants and volume expansion coefficients. These differences form cracks along the intermetallic phase boundary stress surfaces. To avoid these faults, an intermediate metal is formed to prevent physical contact between Au and Al.

13. Spin Shipley S1813 photoresist @4000 rpm for 2-5 minutes.

The purpose of the photoresist in this case is to protect the sample from getting dirty during wafer cutting. So the parameters are not really important.

14. Bake sample @ 100 °C for 2 minutes.

15. Cut wafer.

16. Remove photoresist in Microposit Remover 1165 bath.

It may be good to have an extra beaker with remover or acetone to remove residual photoresist.

17. Rinse thoroughly in isopropanol bath, and blow dry carefully.

18. Check in the optical microscope if the pattern is acceptable.