UV lithography process for ultra-thick high aspect-ratio SU

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Transcript UV lithography process for ultra-thick high aspect-ratio SU

Producing Ultra High Aspect Ratio SU-8 Structures With Optical Lithography

John D. Williams, Wanjun Wang Dept. of Mechanical Engineering Louisiana State University 2508 CEBA Baton Rouge, LA 70808

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  Lin et.al.,

J. Micromech. Microeng



(2002) 590-597.

 Loechel.,

J. Micromech. Microeng



(2000) 108-115.

 Dentinger et.al.,


Engineering. 61-62 (2002) 1001-1007.

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) 1019-1027.

  Before and after diffraction reduction w/ 365 nm light Chuang, Tseng, lin.


Tech. 8 (2002) 308-313.

  Chemical Modification Ruhmann et.al.,

Proc. of SPIE



(2001) 502-510.

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-#0104327  Louisiana Space Consortium (LaSPACE), NASA  Center for Advanced Microstructures and Devices (CAMD) at Louisiana State University