1 WIREBONDING CHARACTERIZATION AND OPTIMIZATION ON THICK FILM SU-8 MEMS STRUCTURES AND ACTUATORS LIGA and Biophotonics Lab NTHU Institute of NanoEngineering and MicroSystem Speaker ： Wen Cheng Yang

2 Outline Introduction Fabrication Testing Thermal actuator operation Conclusion LIGA and Biophotonics Lab

3 Outline Introduction Fabrication Testing Thermal actuator operation Conclusion LIGA and Biophotonics Lab

4 Introduction Although polymers for MEMS applications are becoming more common as an alternative to silicon for low temperature and microfluidic applications, few techniques are available to electrically integrate them into existing packaging technologies The epoxy based photoresist SU-8 has quickly become one of the most common polymers used in the MEMS field because of its ability to easily produce high aspect ratio structures with near-UV lithography LIGA and Biophotonics Lab

5 Introduction LIGA and Biophotonics Lab

6 Outline Introduction Fabrication Testing Thermal actuator operation Conclusion LIGA and Biophotonics Lab

7 Fabrication Cross-section of an SU-8 bond pad coated in metal and electrically isolated from substrate. Step 1 ： RIE 15 sccm O 2 and 5 sccm of CF 4 at 80W and 68mtorr Step 2 ： Metal deposition All hardbaking steps are done on a ramping hotplate while constraining SU-8 structures in-plane to avoid out-of-plane stress. LIGA and Biophotonics Lab

8 Fabrication Maximum wirebond yield onto 1 µm thick SU-8 hardbaked at 200 ºC with no surface treatments LIGA and Biophotonics Lab

9 Outline Introduction Fabrication Testing Thermal actuator operation Conclusion LIGA and Biophotonics Lab

10 Testing Effect of a 30 minute hardbake on wirebond yield to SU-8 of different thicknesses. Metal layer is 50/300 nm Cr/Au and no surface treatments are used. SU-8 can exhibit significant visco- elastic effects at higher temperatures, with a lower Young’s modulus and higher energy absorption When bonding at temperatures close to the Tg value, plastic deformation of the SU-8 under the ball bond reduces bond strength and yield. LIGA and Biophotonics Lab

11 Testing Wirebonds from 100 µm thick gold coated SU-8 test structures to DIP. LIGA and Biophotonics Lab

12 Testing Failure force of bonds on 16 µm thick SU-8 for different activation times and bonder temperatures. Bonding to 16µm thick SU-8 structures at 123 ºC resulted in yields above 95% for all but the 20 minute treatment time. Despite the high yields, there were significant differences in the mechanical strength of the bonds. LIGA and Biophotonics Lab

13 Testing Average failure force with standard deviation and ratio of failure modes of ball bonds on 16 µm thick SU-8. An activation time of 10 minutes produces maximum bond strength for all bonding temperatures and only wire failures when bonded at 123 ºC LIGA and Biophotonics Lab

14 Outline Introduction Fabrication Testing Thermal actuator operation Conclusion LIGA and Biophotonics Lab

15 Thermal actuator operation Performance of a thermal actuator (insert), powered through direct wirebonding. The hot-cold arm thermal actuators were 40 µm thick, 1 mm long, and had been activated with O 2 /CF 4 for 20 minutes prior to being coated with 50/300 nm Cr/Au. For these actuators, a maximum input current of 63 mA was used at 2V without any signs of failure at the ball bonds. LIGA and Biophotonics Lab

16 Outline Introduction Fabrication Testing Thermal actuator operation Conclusion LIGA and Biophotonics Lab

17 Conclusion Maximizing metal adhesion to SU-8 is the most critical step for successfully wirebonding onto SU-8 with thin metal films and optimized conditions can result in yields near 100%. SU-8 test structures up to 100 µm thick have been electrically connected,and thermal actuators have been successfully connected to standard DIPs for operation. LIGA and Biophotonics Lab

18 LIGA and Biophotonics Lab Thanks for your attendance.