1. LIGA & Soft Lithography Presented by: Kanchan Singh Richa Yadav Poonam Yadav

2. LIGA LIGA is a German acronym for Lithographie, Galvanoformung, Abformung (Lithography, Electroplating, and Molding) that describes a fabrication. technology used to create high-aspect-ratio microstructures. The LIGA consists of three main processing steps 1)lithography 2)electroplating 3)molding There are two main LIGA-fabrication technologies 1) X-Ray LIGA:-which uses X-rays produced by a synchrotron to create high-aspect ratio structures.. 2)UV LIGA:-a more accessible method which uses ultraviolet light to create structures with relatively low aspect ratios.

3. Characteristics of LIGA Any lateral shape is possible. Structural height is above 1mm. Smallest lateral size is 0.2 μm. Aspect ratios can range up to 500. Surface roughness is typically small ~ 30nm. Materials other than Si can be used.

4. Processes of LIGA A sacrificial layer pattern is deposited on the substrate. Substrate used are Ti, Al, Cu, or Cr

5. Processes of LIGA The structure is covered with a thin layer of plating Ni or sputtered Ti/Ni is commonly used.

6. Processes of LIGA A thick layer of photoresist called PMMA is added to the surface. A mask is aligned and X- ray or UV light from a synchrotron radiation source is exposed to the photoresist

7. Processes of LIGA The area exposed by X- ray or UV light is filled with electroplated metal. Ni is commonly used The remaining photoresist is removed

8. Processes of LIGA The plating base is removed

9. Lithography It is a process used in microfabrication to selectively remove parts of a thin film or bulk of substrate It uses light to transfer a geometric pattern from photomask to a light sensitive chemical photoresist on the substrate Steps of lithography 1.Cleaning 2.Preparation 3.Photoresist application 4.Exposure and developing 5.Etching 6.Photresist removal

10. Electroplating In the electroplating step, nickel, copper, or gold is plated upward from the metalized substrate into the voids left by the removed photoresist.

11. Taking place in an electrolytic cell, the current density, temperature, and solution are carefully controlled to ensure proper plating. In the case of nickel deposition from NiCl 2 in a KCl solution, Ni is deposited on the cathode (metalized substrate) and Cl 2 evolves at the anode. Difficulties associated with plating into PMMA molds include voids, where hydrogen bubbles nucleate on contaminates; chemical incompatibility, where the plating solution attacks the photoresist; and mechanical incompatibility, where film stress causes the plated layer to lose adhesion. These difficulties can be overcome through the empirical optimization of the plating chemistry and environment for a given layout.

12. Molding Molding is of three types-  Liquid Resin  Injection Molding  Compression Molding

13. INJECTION MOLDING- Injection molding is based on heating a thermoplastic material until it melts,thermostatting the mold parts,injecting the melt with a controlled injection pressure into the mold cavity&cooling the manufactured goods. The mold insert in IM must have smooth walls to be able to demold the plastic structures. It is of two types- 1.Injection compression molding 2.Thin-wall injection molding

14. In this,the injection starts while the mold is closing & stops 1mm before the mold is closed completely. The method is used for thin- wall structures with medium replication demands and low cost. 1.Injection compression-

15. 2. thin-wall injection molding- This is basically high speed injection molding. The polymer melt can quickly be injected into the mold to fill all the detail structure of the cavity before solidification occurs.

16. ProsCons Good for small structures with low aspect ratios,e.g CD & DVD Only low molecular weight polymers(may reduce mechanical & thermal strength) Good for large, high-aspect-ratio & 3D features More expensive equipments Excellent dimensional controlCyclic process only Short cycle time(as low as 10 s)High stress on master High productivityHigh residual stresses on molded parts

17. X-ray LIGA X-Ray LIGA is a fabrication process in microtechnology that was developed in the early 1980s. LIGA was one of the first major techniques to allow on- demand manufacturing of high-aspect-ratio structures (structures that are much taller than wide) with lateral precision below one micrometer. The LIGA technique's unique value is the precision obtained by the use of deep X-ray lithography. The technique enables microstructures with high aspect ratios and high precision to be fabricated in a variety of materials (metals, plastics, and ceramics). Many of its practitioners and users are associated with or are located close to synchrotron facilities.

18. In the process, an X-ray sensitive polymer photoresist, typically PMMA, bonded to an electrically conductive substrate, is exposed to parallel beams of high-energy X- rays from a synchrotron radiation source through a mask partly covered with a strong X-ray absorbing material. Chemical removal of exposed (or unexposed) photoresist results in a three-dimensional structure, which can be filled by the electrodeposition of metal. The resist is chemically stripped away to produce a metallic mold insert. The mold insert can be used to produce parts in polymers or ceramics through injection molding.

19. UV LIGA UV LIGA utilizes an inexpensive ultraviolet light source, like a mercury lamp, to expose a polymer photoresist, typically SU-8. Because heating and transmittance are not an issue in optical masks, a simple chromium mask can be substituted for the technically sophisticated X- ray mask. These reductions in complexity make UV LIGA much cheaper 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.

20. Applications of LIGA Fabrication of actuators and mechanisms Large structural heights allow actuators to generate more torque. Precision of LIGA allow shafts, contacting parts,bearings, and gears to have with minimal friction. Also produces micro-sized sensors. Surface area dependent structures such as mass spectrometers and switches have better sensitivity. Height of coils and springs allows greater stiffness and rigidity

21. Advantages and Disadvantages of LIGA Advantages Allows material other than Si to be used. Better precision and low surface roughness. Can fabricate arbitrary cross sections. Mass production of parts is possible. Can use polymers and ceramics. Disadvantages Synchrotron radiation source is not widely available. Results may vary depending on the deposition method used. (e.g. thermal evaporation vs. plasma sputtering

22. Applications of LIGA Fabrication of spinnerets Spinnerets have smooth surfaces and fixed geometries to produce textile fiber efficiently. High aspect ratio allows micro-sized spinneret capillaries to function properly

23. Soft Lithography Lithography in Greek means “to write in stone” It is called “soft” because it is typically used on elastomeric materials such as PDMS (polydimethylsiloxane) The technique underwent rapid expansion from 1995-2005 It uses photolithography to make a mold for PDMS, which gets permanently bonded onto glass to make a microfluidic device.

24. Why PDMS???  Certain properties make it suitable as a stamp material. Low interfacial free energy Chemically inert Non hygroscopic Passes gas easily Good thermal stability Optically transparent Isotropic and homogeneous.

25. Step 1: Create PDMS Mixture PDMS stands for Polydimethylsioloxane 1.Tape around the edges of the mask 2.Follow a 10:1 weight ratio of Silicon Elastomer Base to Silicon Elastomer Curing Agent roughly 30g of base and 3g of curing agent 3. A pipette is used to achieve desired weight of Silicon Elastomer Curing Agent 4. Mix the PDMS gel vigorously to ensure equality throughout. It should turn from a clear to a white color

26. Step 2: Pour onto Mold and De-Gas Pour the mixture onto the photoresist mold The mask was premade Made by photolithography. The PDMS gel contained air bubbles which must be removed through a vacuum or/and through just letting it sit. de-gas PDMS to protect integrity

27. Step 3: Cure the PDMS Cure the PDMS by heating it on a hot plate at 100C for roughly 10 min. Curing at a lower temperature, such as 60C, for much longer durations is ideal To help ensure equal heating, cover molds with tin foil to create a convection oven-like affect. Once cured, pull it off of hot plate and let the PDMS cool

28. Step 4: Cut and Mount PDMS After PDMS solidified and cooled, take a sharp knife and cut out PDMS parts. The desired part will be a negative of the mold. Mount the part to a glass slide, punch holes into desired location for fluid injection. The process is then complete

29. Advantages/Disadvantages/Applications Advantages – Low cost – Fast production – Easily accessible – Does well with small details Disadvantages – PDMS is a soft structure – PDMS has a high thermal expansion Applications – MEMS Devices – Sensors – Micoreactors – Microfluidics