Applications > MEMS

Cantilevers

Cantilevers became popular with the invention of the atomic force microscope (AFM) in 1986. Micro cantilevers are used in microcalorimetry, mass detection with resonating devices, and magnetic force microscopy. As an example, in biosensing, changes of surface stress or the selective attachment of various molecules induce static bending of the cantilever. The simple mechanical behavior of cantilever-based sensors allows straightforward translation of mechanical forces into displacement. Various applications of SU-8 cantilevers with the thicknesses of 2 –10 µm have been described in the literature.

SU-8: Benefits/Attributes

• Highly chemical resistant
• Low Young’s Modulus
• Relatively easy fabrication
• Surface can be modified

PMGI Material Attributes/Properties

• Spin-coatable from 10nm to 6nm in a single coat
• No intermixing with imaging resist
• High thermal stability <300 C (Tg~189C)
• Strippable in NMP and aqueous-based developers
• Resistant to conventional semiconductor solvents
• Excellent adhesion to various substrates
• DUV, E-beam and x-ray sensitive
• High etch rate in O₂ plasma
• Excellent conformal or planarizing formulations available

Process Flow

1. Coat release layer according to supplier recommendations.

2. Coat SU-8 layer according to the datasheet recommendations.

3. Expose and PEB SU-8 layer. Do not develop.

4. Coat second SU-8 layer according to datasheet recommendations.

5. Expose and PEB second SU-8 layer.

6. Develop SU-8 layers.

7. Release SU-8 layers from the substrate.

Applications > MEMS

LIGA

LIGA is a German acronym for Lithographie, Galvanoformung, Abformung (Lithography, Electroplating, and Moulding) that describes a fabrication technology used to create high-aspect-ratio (HAR) microstructures. UV LIGA utilizes an inexpensive ultraviolet light source, like a Hg lamp, to expose a polymer photoresist, typically SU-8 or PMMA. Because heating and transmittance are not an issue in optical masks, a simple Cr mask can be substituted for the technically sophisticated X-ray mask. These reductions in complexity make UV LIGA much less expensive and more accessible than its X-ray counterpart. However, UV LIGA is not as effective at producing precision molds and is thus used when cost must be kept low and very high aspect ratios are not required.

SU-8: Benefits/Attributes

• High aspect ratio imaging
• Excellent chemical & temperature resistance
• Coatings to hundreds of microns

SU-8 Microgear B. Loechel, et. al., Abstracts HARMST 2003, 55 (2003)	50µm x 2.5mm SU-8 LIGA gear mold. Courtesy of ZG Ling, CAMD, Louisiana State Univ. (2003)

Source: Univ. of Alabama, Huntsville

Applications > MEMS

Sacrificial Via Protection for TSV and MEMS Applications

The rapid expansion of 3D-WLP (wafer-level-packaging) and MEMS is accelerating the growth of high topography features and their integration with 3D-TSV technologies. A large number of technologies have been demonstrated with the common goal to create more functionality while occupying less real-estate.

One of the preferred processes for creating deep TSV’s (through-silicon-via’s) is the Via’s first approach. This allows the TSV’s to be etched before any expensive and critical CMOS processing. This method can impact later critical process stages. As a result the use of sacrificial thick planarizing and protection materials can be used to return a flat starting substrate that’s pre-etched with TSV’s.

Benefits/Attributes

• Materials specially designed as a spin-on polymer for sacrificial TSV filling and planarization
• Silicon Via’s protected against subsequent processes
• Easily removed using industry standard wet and dry chemistries
• Single coat with void free filling
• Thermally stable
• Options for etch back process, wet, dry or CMP.

Applications > MEMS

Microwell Arrays

Microwell arrays* are widely used for microfluidic diagnostics. SU-8 is being used to lithographically fabricate addressable low cost high density microwell arrays for disposable biochips. Rapid diagnostic tests can be done with luminescent based assays using dyed SU-8 fiber optic faceplates. Reducing optical cross talk improves detection sensitivity. Applications include bio-mems for genomics, proteomics, and other bio determinations.

Hybrid SU-8 on glass biochip

• Precise wells – diameter, depth, pitch
• Highly addressable
• Spectrally specific absorbing chemistry
• Optical cross-talk eliminated with dye
• Low cost fabrication
• Wide variety of analysis, sensing
• Sensing at nano-levels for life science and homeland security

SU-8 Benefits/Attributes

• Excellent adhesion to glass
• Many shapes, channels possible
• High aspect ratio
• Chemically resistant to diagnostic chemistries
• Coating method can greatly reduce material cost
• Photolithography compatible with widely used IC processes and equipment

Bottom of wells/glass surface SU-8 Magenta Resist is shown

* This application has been jointly developed by MicroChem and Incom. Incom is a global leader in fused fiber optic technology. The fiber optic face plate used in this application was developed by and is manufactured by Incom.

Applications > Specialty Displays

Permanent Epoxy Photoresist for Pixel Grids in Electrowetting

In electrowetting liquid surface tensions are modified by applying a field potential. With no voltage applied colored oil lies flat between water and the electrode within a defined well. When a voltage is applied between the electrode and water, the tension changes and the water then forces the oil aside.

In electrowetting displays (EWD) the use of an active matrix TFT back-plane can control each pixel allowing for high speed and video content. Because electrowetting is low power this is an attractive technology for many applications. Electrowetting displays can also be used in both reflective and transmissive modes and offer higher brightness compared with other reflective technologies.

Electrowetting is also used for a wide range of microfluidic lab-on-chip applications.

SU-8: Benefits/Attributes

• Highly crosslinked system with high structural strength
• High contrast with vertical sidewalls
• High aspect ratio imaging
• High fill factor pixel walls
• Ability to adhere to fluoropolymers
• Fast photospeeds for high volume manufacture

SU-8 Pixel Walls with oil shown in the on, mid and off state   Transmissive display using an SU-8 Grid

Courtesy of University of Cincinnati, Plastics Electronics 2010

Applications > III-Vs

T-Gate Fabrication

T-shaped gates improve high electron mobility transistors (HEMTs) performance. T-gate structures are usually fabricated in high electron mobility heterostructures like InGaAs. Sub 50nm gate length result in reduced capacitance, high electron mobility, and ultra high RF characteristics. Tri-layer resist structures with a PMGI layer (aqueous developer) in between two PMMA layers (solvent developer) are being used for precise control of the T-gate shape to optimize device electrical properties.

PMGI/LOR: Benefits/Attributes

• Sacrificial layer to assist in fabrication of higher gate stem heights
• Excellent adhesion to III-V substrates
• Suitable for multi-layer processing
• Suitable for use as a protective separation layer between incompatible layers
• Allows gate head and gate foot to be written separately for improved resolution
• Can be used in a resist stack with low intermixing

PMMA: Benefits/Attributes

• Positive tone; E-beam imageable
• Wide range of film thicknesses
• Excellent adhesion to most substrates

Process Flow	Definition of Gate Stem 　 E-beam Litho. On Gate Stem 　 　 Courtesy of Seoul Nat University	Pattern Transfer to SiN Final Trip Gate
		Courtesy of Ratheon
	Learn More
	PMGI/LOR Data Sheet PMMA Data sheet Application Technical References: T-gates

Applications > III-Vs

Airbridges

Airbridges use air as the dielectric between two conductors to extend the operating frequency. Fabricating microelectromechanical systems (MEMS) where airbridges or cantilevers are required has been done for many years using polymethylglutarimide (PMGI) as a sacrificial release layer.

PMGI: Benefits/Attributes

• Spin-coatable from 10nm to 6µm in a single coat
• No intermixing with imaging resist
• High thermal stability <300 C (Tg~189C)
• Strippable in NMP, DMSO and aqueous-based developers
• Resistant to conventional semiconductor solvents
• Excellent adhesion to various substrates

Process Flow

Wafer Thinning

The wafer’s front surface containing the micro devices must be protected during the harsh wafer thinning/grinding and polishing of the back surface. PMMA coatings are useful as a protective bond and release layer for the front surface in wafer thinning. PMMA also affords protection during the subsequent dicing and handling steps. Its thermal and mechanical stability as well as the ease of application, i.e. spin coating, and later removal in solvent make PMMA a good candidate for this application.

The protective PMMA layer does not contaminate the active devices during mechanical processing nor leave residues or organic contamination prior to a final passivation.

PMMA: Benefits/Attributes

• Clean de-bonding
• Ease and efficiency of removal vs. wax
• Dual functionality: bonding and release layer
• Capable of withstanding various back-side processes:
  - Lithography
  - Etch
  - CMP
  - Metal Deposition:(Sputtering or Plating)

Passivation

Passivation is the process of depositing a thin inert film onto the surface of a micro device in order to greatly improve its electrical characteristics. SU-8 passivation enhances device performance of small area diodes which are susceptible to surface area effects. Surface leakage currents due to abrupt termination of crystal structure, dangling bonds, inversion layer, and interfacial traps at the device/air interface can be overcome by using SU-8 passivation.

HgCdTe and newer type-II InAs/GaSb superlattice (T2SL) IR imagers benefit from SU-8 passivation. GaN Light Emitting Diodes (LEDs) performance is also greatly enhanced.

SU-8: Benefits/Attributes

• Easily integrates into fabrication process
• Improves device performance by reducing dark current density
• High aspect ratio imaging
• 0.5 to >200µm in a single coat
• Superb chemical and temperature resistance
• Optical transparency
• Hydrophobic surface after cross-linking
  

Microlenses

Microlens arrays (MLAs) are being used more and more in optical and lighting systems in applications such as fiber optics, image systems and illumination. Various fabrication methods for MLAs have been described. These include the reflow process, molds, laser-aided fabrication, lithography with a gray-scale mask, etching, and assembly by surface properties. Among these processes, molding has more advantages such as fast, high precision and mass product. MCC’s PMMA, SU-8, and PMGI have all been used for microlens fabrication

Any material in liquid state tends to exist in its lowest surface free energy, resulting in the least surface area, and spherical shape. The polymer particles on the smooth surface of a substrate will shrink into a hemispherical lens due to the surface tension when heated above their softening temperature. The key is to make sure of regular arrangement of lenses in a two dimensional plane. SU-8 can also be used as the micro-mold or stamp to form PDMS or PMMA microlenses.

PMMA, SU-8, PMGI Lenses: Benefits/Attributes

• Excellent adhesion
• Optical transparency
• Spin-coatable

Waveguides

Waveguides used at optical frequencies are typically a dielectric structure, e.g. SU-8 or PMMA, with high transmission and high index of refraction surrounded by a material with lower %T. The structure guides optical waves by total internal reflection.

Integrated optics/photonics is becoming more pervasive as devices communicate with one another. Low optical loss is an important consideration for any waveguide fabrication technique. Optical losses in waveguides arise due to the material absorption and scattering losses from the sidewalls. The former is intrinsic to a particular material while the latter is attributed to the lithographic process. The refractive indices of the waveguide core, cladding and substrate are essential input parameters necessary to predict system behavior. Above a wavelength of 400 nm, the transmission of SU-8 is greater than 95%. SU-8 is optically transparent at 632.8 nm as well as at the telecommunications wavelengths of 1330 nm and 1550 nm. SU-8 is therefore a suitable material for optical waveguides. A sensor/detector waveguide stripe interferometer can be formed from SU-8 with a reactive clad coating that changes optical properties upon interaction with the substance to be detected.

PMMA and SU-8: Benefits/Attributes

• Excellent adhesion
• Optical transparency
• Excellent chemical resistance
• Spin coatable
• Material compatibility
  

Sensor structure – waveguide Mach-Zehnder interferometer. Made of SU-8 photolithography deposited stripe waveguides. Source: M. Bednorz, Silesian U Molecular and Quantum Acoustics vol. 27, (2006)