John D. Williams, Wanjun WangDept. of Mechanical Engineering

Louisiana State University

2508 CEBA

Baton Rouge, LA 70808

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. 61-62 (2002) 1001-1007.

Lin et.al., J. Micromech. Microeng. 12 (2002) 590-597.

Loechel., J. Micromech. Microeng.10 (2000) 108-115.

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. 3999

(2000) 1019-1027.

Chemical Modification Ruhmann et.al., Proc. of SPIE.

4345 (2001) 502-510.

Before and after diffraction reduction w/ 365 nm light

Chuang, Tseng, lin. Microsys. Tech. 8 (2002) 308-313.

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. Numbersrepresent 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-#0104327

Louisiana Space Consortium (LaSPACE), NASA

Center for Advanced Microstructures and Devices (CAMD) at Louisiana State University