1. Lithographic Process For High-Resolution Metal Lift-Off
Randy Redd
Motorola Semiconductor Technologies - ATL, 2100 E. Elliot Rd., Tempe, AZ 85284
Mark Spak, John Sagan, and Ralph Dammel
AZ Electronic Materials, 70 Meister Ave., Somerville, NJ 08876

2. INTRODUCTION
Lift-off processing is an attractive additive metalization process that has no alternative in cases where subtractive etch processes will lead to surface damage that will impair the device function.
Practical application of lift-off processing has been hampered by the fact that there was no easy, highly reproducible process to generate the require re-entrant slopes or overhangs.
This paper reports on an optimization of a developer-induced surface inhibition process including solvent-containing developers and flood exposures. This process is simple, can be carried out with standard resists, and generates highly reproducible overhangs .

3. Metal Lift-Off Process Sequence
*: may optionally be carried out
at reduced temperature before soak

4. Chemistry of Developer-Induced Inhibition Layer
D T
The developer soak generates basic TMA+ phenolates on the resist surface.
These basic sites catalyze azocoupling which partially crosslinks the resist.

5. AZ®7900 Resist - Developer Soak, No Flood Exposure
Focus -0.2 µm
Focus 0.0 µm
Focus 0.2 µm
Focus 0.4 µm
DOF,
0.40 µm
Top CD
Focus 0.6 µm
Focus 0.8 µm
Focus 1.0 µm
Coat : AZ® um Emax Dip : AZ® 23°C w/ 120sec spray/puddle SB: °C / 60sec (contact) Exposure : Nikon 0.54 NA -i line PEB: °C / 40sec (contact) Flood Exposure: mJ/cm² Develop : AZ® °C for 120sec continuous spray

6. AZ®7900 Resist - Developer Soak, 50 mJ/cm² Flood Exposure
Focus +0.30
Focus +0.10
Focus -0.10
Focus -0.20
Focus -0.80
Focus -0.40
Focus -0.60
Focus -1.0
Coat : AZ® um Emax Dip : AZ® 23°C w/ 120sec spray/puddle SB: °C / 60sec (contact) Exposure : Nikon 0.54 NA -i line
PEB: °C / 40sec (contact) Flood Exposure: mJ/cm² Develop : AZ® °C for 120sec continuous spray
DOF, 0.50 µm
Top CD

7. AZ®7900 Resist - Developer Soak, 75 mJ/cm² Flood Exposure
Focus +0.10
Focus +0.30
Focus -0.10
Focus -0.20
Focus -0.40
Focus -0.60
Focus -0.80
Focus -1.0
Coat : AZ® um Emax Dip : AZ® 23°C w/ 120sec spray/puddle SB: °C / 60sec (contact) Exposure : Nikon 0.54 NA -i line PEB: °C / 40sec (contact) Flood Exposure: mJ/cm² Develop : AZ® °C for 120sec continuous spray
DOF, 0.50 µm
Top CD

8. Bossung Plot For AZ®7900 Lift-Off Process (0.5 µm features)
Nikon® 0.54 NA I-line stepper

9. AZ®6200 Resist: Bossung Plot For 0.50 µm isolated space
Nikon® 0.54 NA i-line stepper

10. AZ®6200 Resist: Exposure Latitude For 0.50 µm Top CDs, 1.0 µm Spaces
ASML® NA = 0.48 i-line stepper

11. AZ®6200 Linearity For Lift-Off Process

12. AZ®6200: Exposure Latitude For Top and Bottom CD, 0
AZ®6200: Exposure Latitude For Top and Bottom CD, 0.50 µm Top CD Target

13. Solvent Addition To Developer For Soak
AZ®7900 resist, 70° SB,
80 mJ/cm² flood exposure
On GaAs, some resists show adhesion failure in large circular areas with the soak process if no SB is performed.
The effect is believed to be caused by developer penetrating to the surface through pinholes.
A low-temperature SB (60-90 °C) heals the pinholes and removes the defects, but hardens the resist too much for efficient TMAH uptake, giving too little overhang.
The required lift-off profile can be regenerated by addition of small concentrations of solvents to the developer used in the soak step.
0.005% PGME, 2 min
0.5% PGME, 2 min

14. Metallization Results Prior To Lift-Off

15. Metallization Results After Lift-Off
lines/spaces
0.8 µm
lines/spaces
0.6 µm
lines/spaces

16. CONCLUSION
We have demonstrated stable, highly reproducible half-micron lift-off processes with a developer treatment of the photoresist prior to exposure for two i-line resists.
Overhangs of up to 0.2 µm can be routinely achieved by developer soak treatment alone of AZ®6200 and by combined developer soak plus flood exposure and solvent-containing soak developers for AZ®7900.
While this study used point source evaporation, it is anticipated that the developer soak process can also be applied to sputtered metals if the resist thickness is reduced to the appropriate range.