John D. Williams, Wanjun Wang Dept. of Mechanical Engineering Louisiana State University 2508 CEBA Baton Rouge, LA Producing Ultra High Aspect Ratio SU-8 Structures With Optical Lithography Crosses displayed here are 1500  m tall and range in width from 35 to 70  m

High Aspect Ratio Microfabrication  The production of mechanical systems often requires 3 dimensionality in the design.  To achieve 3-D structures, designers often transfer complex 2-D patterns deep into a substrate.  Currently there are three transfer procedures that yield significant height to width aspect ratios.  Deep x-ray lithography (aspect ratios >150:1).  Deep silicon etching ( >75:1).  SU-8 UV lithography ( >15:1).

Advantages of High Aspect Ratio Processes  Provides engineers with the ability to produce tall mechanical structures.  Allows for the development of fluidic vias and very narrow diffusers.  Provides the ability to achieve “ 3-D ” structures on the micro scale.

UV Lithography With SU-8  Optimized for producing MEMS devices.  Spun to thickness' between 10 and 1500  m.  Demonstrated aspect ratios of 25:1 using UV-lithography.  Best performer to date for thick resist processing with ultraviolet light.  Can be patterned using a common broadband contact aligner.

Advantages of SU-8 Processing for High Aspect Ratio MEMS  Lithography does not require and expensive light source.  SU-8 processing can be done using common cleanroom equipment.  3-D structures can be fabricated easily using multiple exposed layers.  Mature electroplating processes developed for LIGA processing allow for a wide choice in material selection.

Disadvantages of SU-8 Processing  Extremely difficult to define proper bake parameters.  Resist remains “soft” until after exposure.  High concentrations of stress in resist are present during traditional processing.  Solid polymer is highly self adhesive.  Exposed SU-8 is extremely difficult to selectively remove.

Current SU-8 Process Technology  Patterns are currently transferred 1500  m into resist with aspect ratios of 5:1.  25:1 aspect ratios are commonly presented in structures between 100 and 400  m tall.  Recent work demonstrates the ability to achieve 15:1 trenches in 100  m of resist and 50:1 featured patterns in 600  m of resist.

Visual Picture of the State of the Art in SU-8 UV Lithography  Dentinger et.al., Microelectronics Engineering (2002)  Lin et.al., J. Micromech. Microeng. 12 (2002)  Loechel., J. Micromech. Microeng.10 (2000)

Methodologies for Improving the Aspect Ratio of SU-8 Processes  Chemical modification of the resist.  Addition of high refractive index material between resist and mask to reduce diffraction.  Use of selective UV spectrum.  Reduces effects of diffraction.  Eliminates short wavelengths that are absorbed in the first few microns of the resist leading to pattern distortion.

Results Achieved Using Process Improvements  Wavelength filtering  Ling et.al., Proc. of SPIE (2000)  Chemical Modification  Ruhmann et.al., Proc. of SPIE (2001)  Before and after diffraction reduction w/ 365 nm light  Chuang, Tseng, lin. Microsys. Tech. 8 (2002)

Our SU-8 Process  SU-8 resist without any modifications  No specific filtering  No diffusive control by added materials between mask and wafer  Optimized spin and bake procedures  Optimized exposure conditions  Room temperature development in stagnant fluid

Issues Present in Process  How to coat SU-8 in layers greater than 800  m successful?  Multiple coats for layers over 1100  m.  Maintaining a level surface until after exposure is critical.  What are the proper bake conditions for very thick resist layers?  Approximately 50min/100  m of resist at 96 C in an oven.  Films greater than 1mm require slightly elevated temperature if hotplate is used.  Multiple coatings require extra bake time.  Stress reduction obtained by proper cooling of sample.  What is the optimal exposure dose required to achieve the pattern?  Open field structures require significantly more dose than holes and closed structures.

Experimental Results  We have greatly reduced the internal stress in SU-8 films.  We have developed a repeatable procedure for achieving 1500  m thick layers.  Have established optimal exposure doses for films 1000, 1200, and 1500  m thick.  Demonstrate the ability to produce open field structures, including cylinders, with high aspect ratios.  Demonstrate the ability to pattern holes in closed structures as deep as 1200  m.

High Aspect Ratio Features Produced in This Experiment  35 and 50  m wide crosses 1500  m tall.

1150  m Tall Cylinders With Varying ID and Wall Thickness ’  Inner diameters vary from 40  m to 200  m.  Optical image shows complete development of the cylinders.  Cylinder with wall thickness’ less than 30  m collapsed.

1150  m Tall Cylinders With Min. Wall Thickness of 50  m  Aspect ratio > 23:1.  Optical image in corner shows that the resist was completely developed away inside the cylinders.

1150  m Tall Crosses 25  m Wide  Aspect ratio 46:1.  Open field, free standing structures require higher doses than cylinders or hole patterns.

How High of an Aspect Ratio Can Be Achieved?  50:1 is easily obtainable.  Here one can see a 100:1 pattern (6  m wide and 630  m tall).  A 7  m trench is also observed from top to bottom of the features.  Required new development process.  630 um tall patterns. Numbers represent the width of the feature on the mask pattern.

Concluding Remarks  We are able to obtain high aspect ratios using a simple SU-8 lithography process that can be applied in almost any MEMS laboratory.  We demonstrate, for the first time, the ability to achieve 100:1 aspect ratios that cannot be produced using any lithographic technique other than x-ray lithography.  We believe that the exposure can be improved simply by using repeatedly published process modifications.

Acknowledgements  National Science Foundation  NSF Grant ECS-#  Louisiana Space Consortium (LaSPACE), NASA  Center for Advanced Microstructures and Devices (CAMD) at Louisiana State University