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Soft Lithography, Molding, and Micromachining Techniques for Polymer Micro Devices

  • Ashis Kumar SenEmail author
  • Abhishek Raj
  • Utsab Banerjee
  • Sk Rameez Iqbal
Protocol
Part of the Methods in Molecular Biology book series (MIMB, volume 1906)

Abstract

This chapter enumerates the methods, protocol, and safety procedures of various fabrication techniques for polymer-based microfluidic devices. The polymer materials can be a solid or a liquid, and the fabrication protocol needs to be executed accordingly. Various techniques demonstrating the fabrication of microfluidic devices using solid and liquid polymers are described. Procedure for each fabrication process is delineated with detailed images. Further, dos and don’ts for all the fabrication techniques are explained in the notes of each section. This chapter will benefit those interested in the microfluidic device fabrication using polymers and guide them to avoid mistakes so as to obtain an elegant device.

The techniques are listed as follows:
  1. 1.

    Replica molding

     
  2. 2.

    Microcontact printing

     
  3. 3.

    Micro-transfer molding

     
  4. 4.

    Solvent-assisted molding

     
  5. 5.

    Hot embossing

     
  6. 6.

    Injection molding

     
  7. 7.

    CNC micromachining

     
  8. 8.

    Laser photo ablation

     
  9. 9.

    X-ray lithography

     
  10. 10.

    UV patterning

     
  11. 11.

    Plasma etching

     
  12. 12.

    Ion beam etching

     
  13. 13.

    Capillary molding

     
  14. 14.

    Micro-stereolithography

     

Key words

Etching Micromachining Molding Polymer micro devices Soft lithography 

1 Introduction

1.1 Replica Molding

Replica molding (REM) is one of the well-established soft lithography techniques, which is used for replicating the information present on an original master [1] onto a substrate. The original master can be fabricated by various processes such as photolithography and micromachining. This method of soft lithography accommodates a wider range of materials and provides a high frequency of replication [2] as compared to photolithography. Various types of materials such as PDMS, polyurethane, organic polymers, and epoxy resin can be used for replica molding. Replication against the elastomeric masters enhances the ease of separating the replica and the master, which protects the fragile structure and also minimizes the cost and damage to the master [3]. The replica molding procedure is detailed in Subheading 2.1, and the schematic for the same is shown in Fig. 1. Examples of some molds fabricated by the replica molding process have been included in Fig. 2.
Fig. 1

Process outline of the replica molding technique

Fig. 2

(ac) PDMS molds replicated from corresponding silicon masters, (df) epoxy nanopillars molded from corresponding PDMS molds (Reprinted with permission from [5]). (Copyright (2006) American Chemical Society)

1.2 Microcontact Imprinting

Microcontact printing is the only soft lithographic technique that is capable of generating chemical patterns on a surface [4]. It allows engineering of a surface with molecular-level detail. The process can provide sub-100 nm self-assembled monolayer (SAM) patterns of biomolecules over macroscopic areas [5]. This method is similar to transfer ink from an ink pad to a paper using a stamp. The mold is inked with the material (to be transferred) to the substrate (metal coated) by making contact between the substrate and the protruding features of the mold [3]. The substrates are prepared by vapor deposition methods, e.g., thermal beam evaporation and electron beam evaporation. The inks can be small biomolecules, proteins, or suspensions of cells [6]. The schematic of the process described in Subheading 2.2 is shown in Fig. 3. Representative SEM images of the master and mold fabricated by microcontact imprinting have been included in Fig. 4 for reference.
Fig. 3

Process outline of microcontact imprinting technique

Fig. 4

(ab) SEM images (at two different magnifications) of a master and the pattern of a SAM of HDT on gold formed by μCP (contact time of 10 s) with a PDMS stamp cast from this master [4] (Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission)

1.3 Micro-transfer Molding

In this technique, a liquid precursor is filled inside a PDMS microchannel; then the mold is brought into contact with a planar or non-planar substrate [1]. The liquid precursor is solidified in situ either thermally or photochemically [7]. Then the elastomer can be peeled off, leaving the desired microstructure. The technique can generate isolated and interconnected microstructures. It can generate microstructures over larger areas (~3 cm2) within a short period of time [8]. The PDMS stamps can be utilized further [9]. The schematic of the process outline is shown in Fig. 5. A representative SEM image of a micro-transfer molded sample has been included in Fig. 6 for reference.
Fig. 5

Process outline of the microcontact imprinting technique

Fig. 6

An SEM image of a fractured sample showing a pattern of isolated stars of UV-cured polyurethane (NOA 73) on Ag, fabricated by μTM [8] (Reproduced with permission from the author and publisher)

1.4 Solvent-Assisted Molding

Solvent-assisted micromolding (SAMIM) technique makes use of a solvent to wet the PDMS stamp and restructure a polymer film by swelling or dissolving the polymer [1]. This room-temperature processing is suitable for polymers whose Tg (glass transition temperature) is close to the degradation temperature and hence cannot be molded by imprinting techniques [10]. This technique avoids thermal cycling of the substrate, which can be time intensive and lead to oxidation of the substrates [3]. The solvent-assisted molding is suitable for many polymers and their precursors due to its simple procedures and high production efficiency and since it does not require special molding equipment and system [10]. The general procedure of the technique is given below, and the schematic is shown in Fig. 7. Representative SEM images of the 3D structures formed in thin films of novolac photoresist using this technique have been included in Fig. 8 [11].
Fig. 7

Schematic of the solvent-assisted molding technique

Fig. 8

SEM images of test structures formed in thin films of novolac photoresist using solvent-assisted molding, with ethanol as the solvent [12] (Reproduced with permission from the author and publisher)

1.5 Hot Embossing

Hot embossing is used for fabrication of simple microchannels. It is essentially the stamping of a mold pattern onto a polymer softened by increase in the temperature of the polymer just above its glass transition temperature, which ranges from 50 °C to 150 °C. Embossing force (0.5–2 kN/cm2) is then applied on the substrate under vacuum conditions [12]. The polymers experience two stages of deformation during embossing: stress concentration and strain hardening during heating and embossing steps and stress relaxation and deformation recovery stage during cooling and demolding steps [13]. The schematic of the process flow is shown in Fig. 9. Some representative images of hot embossed in polycarbonate (a) and PMMA (b) substrate have been included in Fig. 10 [14].
Fig. 9

Schematic of the hot embossing technique

Fig. 10

(a) Test structures in polycarbonate (PC), (b) PMMA structure made from a LIGA mold for the fabrication of a three-dimensional acceleration sensor [15] (Reproduced with permission from the author and publisher)

1.6 Injection Molding

Injection molding is a polymer molding technique in which molten polymer is injected under high pressure into a mold cavity through an opening (sprue) [15]. For amorphous thermoplastics such as polymethyl methacrylate (PMMA), polycarbonate (PC), and polysulfone (PSU), the temperature should be higher than the glass transition temperature Tg. For semicrystalline thermoplastics such as polyoxymethylene (POM) and polyamide (PA), the temperature should be higher than the crystallite melting point. The pressure applied during the process is of the order of 500–2000 bar [1]. The advantages of this technique include high production levels, high tolerance, low labor cost, and scrap cost [16, 17, 18]. The step-by-step procedure for this technique is shown in Fig. 11. An example of an injection-molded microfluidic chip has been included in Fig. 12 [19].
Fig. 11

Schematic of the injection molding technique

Fig. 12

(a) Image showing the molds and the injection-molded polymer chip, (b) molded part of the cell capture device [20] (Reproduced with permission from the author and publisher)

1.7 CNC Micromachining

Micromachining is a subtractive manufacturing technique in which rotating cutting tools remove material from a starting workpiece. Modern milling machines use computer numerical control (CNC) for automating XYZ positioning which provide improved repeatability and precision (see Fig. 13) [20]. The CNC machine has mini-/microcomputer that acts as the controller unit of the machine to which a set of instructions are fed using a small board. All the cutting processes and dimensions of the final device are programmed into the computer. In addition to its advanced capabilities, it is also flexible since it is easy to setup and run a different program for a different device [21]. The following procedure is generally used for micromachining. An example of a microfluidic device fabricated by CNC micromachining has been included in Fig. 14 [22].
Fig. 13

Schematic of the CNC micromilling machine

Fig. 14

Image of a CNC micromilled PMMA device

1.8 Laser Photoablation

Laser photoablation is a localized, noncontact machining technique which can be used to cut, engrave, drill, mark, and texture substrates such as metal, ceramics, plastics, and wood [1]. Photoablation is the spontaneous etching of material from a polymer surface that occurs upon the absorption of a pulse of laser, whose energy is greater than the ablation threshold value (see Fig. 15) [23]. Micromachining with controlled accuracy is achievable since it is possible to remove materials in small amounts with a small heat-affected zone [1]. This helps in low-cost rapid prototyping. However, controlling the quality of the machined surface is difficult due to the redeposition of substrate material [1]. Though UV lasers are expensive than CO2 laser, the ablation is strictly confined to the area that absorbs the energy [24, 25]. As a result of UV absorption, bond breaking within the long-chain polymer molecules occurs. A shock wave and ejection of decomposed polymer fragments ensues forming of a photo-ablated cavity. The ejected material contains gas, polymer molecules, and small particulate matter [24]. Some representative structures formed by UV laser photoablation in triazene polymer and Kapton have been included in Fig. 16a, b [24].
Fig. 15

Schematic of the laser ablation technique

Fig. 16

SEM images of Siemens stars in (a) TP and (b) Kapton, both produced with five laser pulses at 308 nm [25] (Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission)

1.9 X-Ray Lithography

X-ray lithography (XRL) is a shadow printing-based technique which utilizes the patterns coated on a mask to create certain three-dimensional features in the resist material (see Fig. 17) [26]. Normally, polymethyl methacrylate (PMMA) is utilized as a resist material. Further, chemical process is incorporated to dissolve away the material volume, affected by X-ray exposure (see Note 52 ). X-rays having shorter wavelength than UV light allow this technique to increase the lateral resolution of the fabricated features by reducing the diffraction effect of the light compared to optical lithography technique [27]. Also, the higher penetrating ability of X-rays makes this technique capable of fabricating the microstructures with high aspect ratios of vertical dimensions of the order of hundreds of microns to millimeters and horizontal dimensions of few microns. Further, smooth and near 90° vertical side walls can be achieved by the technique [28]. Some representative microscopic images of device fabricated using X-ray lithography can be found in Fig. 18 [29].
Fig. 17

Schematic of the X-ray lithography setup

Fig. 18

Microfluidic PMMA device fabricated using X-ray lithography (Reproduced with the permission from the author and publisher [30])

1.10 UV Patterning

UV-patterning technique is a relatively low-cost machining process. The technique can be scaled to perform large area exposures, which makes it a good candidate for fabricating low-cost microfluidic components [30]. Moreover, this process can be accomplished using commercial grade PMMA, further lowering cost. UV light with intensity 4 mW/cm2 is utilized to expose the PMMA surface (which acts as a positive-type resist). This high-energy radiation results into the cleavage of the molecular bonds in PMMA, which is further developed using 7:3 mixture of IPA and water. In this technique, PMMA is mostly used as the substrate material. Further, the procedure of the UV-patterning technique is described below. Please refer to Fig. 19 for the flowchart of the fabrication procedure. Figure 20 shows some representative images of PMMA devices fabricated using the UV-patterning technique. Note that the exposure time should be kept optimal to achieve the desired depth of the microchannel as this depth varies with the exposure and development time as shown in Fig. 21.
Fig. 19

Schematic of the UV-patterning technique

Fig. 20

(a) PMMA substrate coated with gold film, (b) fabricated microfluidics device using UV patterning, (c) SEM image of the fabricated PMMA device using UV patterning [31] (Reproduced with permission from the author and publisher)

Fig. 21

(a) Etch depth dependence on time of exposure for UV patterning of commercial grade PMMA, (b) etch rate dependence on time of development UV patterning of commercial grade PMMA [31] (Reproduced with permission from the author)

1.11 Plasma Etching

Plasma etching is a dry etching technique in which certain types of chemical etchants in gas phase are utilized to etch substrate materials. Applying energy to the gaseous state of such etchants beyond a limit causes the existing shell of the atom to break up. Electrically charged, excited particles and molecule fragments are formed (negatively charged electrons and positively charged ions and radicals), which is called plasma or the fourth state of matter (aggregate state). The etch species in the plasma (charged or neutral particles) are utilized to bombard and etch the substrate surface. A continuous supply of the gas is maintained, and the existing gas enriched with the etched material is expelled. Although etch rate in this technique (1–10 μm/h) is much lower than the wet etching case, it provides much better control over thickness [31]. Figure 22 shows the schematic of the experimental setup. This method is suitable for polymers like POM, PTFE, PMMA, PEEK, and PDMS [32, 33]. Please refer to Fig. 23 for the layout of the procedure required for the fabrication. The device fabrication involves two stages: in the first stage, a mask is created over the polymer substrate, and in the second stage, the sample is exposed to plasma. The dependence of the etch rate and nano-roughness observed in this method on the various processing parameters has been shown in Figs. 24 and 25 [33]. Representative SEM images of a microfluidic channel fabricated using the plasma etching method have been included in Fig. 26. Typical plasma etching chemicals used for different film materials and the corresponding gaseous products are shown in Table 1 [34].
Fig. 22

Schematic of experimental setup used in plasma etching technique

Fig. 23

Flowchart of the process protocol for plasma etching of (a) PMMA and PEEK and (b) PDMS

Fig. 24

O2 and SF6 plasma ER measurements of (a) PMMA and (b) PDMS films, respectively, as a function of plasma source power and chamber pressure. (c) O2 plasma etch depth and (d) etch rate as a function of etching time of PEEK and PMMA plates are presented under etching conditions: electrode temperature of −20 °C, bias voltage of −120 V, O2 flow of 100 sccm, and chamber pressure of 0.75 Pa [33] (Reproduced with permission from the author and publisher)

Fig. 25

Plasma etching: dependence of the nano-column height of (a) PEEK, PMMA, and (b) PDMS on O2 and SF6 plasma treatment time, respectively [33] (Reproduced with permission from the author)

Fig. 26

Plasma etching for device fabrication: An SF6 etched PDMS microchannel before and after sealing (a) an O2 etched PMMA microchannel and (b) O2 etched PMMA and PEEK microchannels, before and after sealing [33] (Reproduced with permission from the author and publisher)

Table 1

Choices of plasma etching chemicals for various film materials

Film

Etchant

Typical gas compounds

Al

Chlorine

BCl3,CCl4,Cl2,SiCl4

Mo

Fluorine

CF4,SF4,SF8

Polymers

Oxygen

DF4,SF4,SF8

Si

Chlorine

Fluorine

BCl3,CCl4,Cl2,SiCl4 C SF4,SF6

SiO2

Fluorine

CF4,CHF3,C2F6,C3F8

Ta

Fluorine

CF4,CHF3,C2F6,C3F8

Ti

Fluorine

CF4,CHF3,C2F6,C3F8

W

Fluorine

CF4,CHF3,C2F6,C3F8

1.12 Ion Beam Etching

Ion beam etching is a physical process in which the ionized inert gas ions (with energy greater than 100 eV) are used to etch the material from the substrate by sputtering process [35]. Noble gases such as argon, neon, krypton, and xenon can be used as the inert gas. For the etching of light materials, neon is best suited, whereas krypton and xenon having higher masses are more suitable for removal of heavy elements [36]. This process is highly directional rather than selective. Etch rates are dependent on the sputter yield, which again is a function of the sputter ion and the material to be sputtered [35]. Further, the sputter yield is also affected by the energy of the ion and the incident angle. Initial procedure to create the mask material is similar to that used in the plasma etching technique. Once the mask is created, the material can be removed from the desired locations by applying the beam of ions over it [37]. Please refer to Fig. 27a for the schematic of the experimental setup used in this technique. An SEM image of a nanochannel fabricated using the ion beam etching method has been shown in Fig. 28.
Fig. 27

(a) Schematic of ion beam etching system and (b) cross-sectional view of an actual setup [36] (Reproduced with permission from the author and publisher)

Fig. 28

An SEM image of fabricated nanochannel [38] (Reproduced with permission from the author and publisher)

1.13 Capillary Molding

Miniaturization in chemical, analytical, and diagnostic applications has recently gained a formidable interest [38, 39]. Capillary molding is one of the techniques proven to be simple, cost-effective, and easy to fabricate for microfabrication. The technique is composed of three components—soft elastomeric mold, solid support, and fluid prepolymer—where the mold can be used to impart various patterns. This technique has several advantages over photolithography which requires two steps—forming (usually by spin coating) and patterning photoresist films by single exposure per structure. In capillary molding, forming and patterning of polymeric film can be performed simultaneously, and the master can be used several times [40]. Photolithography is limited to a special class of polymers, whereas capillary molding is applicable to most of the polymers including polymers with low viscosity. It has found various applications, such as fabrication of compact disks, encapsulation of electronic devices [41], in the semiconductor industries. The capillary molding procedure is shown as a schematic in Fig. 29 and explained below. In spite of few limitations, the simplicity, flexibility, and ease of use of this technique proved to be advantageous over other microfabrication methods [42, 43, 44]. Figure 30 shows some representative patterns of polymeric structures formed by capillary molding [45, 46].
Fig. 29

Schematic of the capillary molding technique

Fig. 30

Scanning electron micrographs (SEM) of patterned polymeric structures formed using liquid precursors to the polymers. (a) Polyurethane on Si/SiO2 using an elastomeric mold with rectangular recessed pattern. (b) Polyurethane pattern on Si/SiO2 made using a mold containing a more complex pattern (Reproduced with permission from the author and publisher)

1.14 Micro-stereolithography

The micro-stereolithography technique evolved from the rapid prototyping industry provides an ideal solution for the fabrication of complex 3D shapes with high aspect ratio in a wide variety of materials, due to its unique characteristics of high resolution, high liability, and lower cost. It uses UV laser beam or light source which is scanned on a photo-polymerizable resin, followed by the curing of the resin, layer-by-layer, and then stacking the layers. The beam scanning techniques can generally be classified into two methods: (a) scanning micro-stereolithography (or vector-by-vector micro-stereolithography) (Fig. 31) and (b) projection micro-stereolithography (Fig. 32). Micro-stereolithography being an assembly-free process can be used to fabricate components having complex structures in a single step. True 3D devices from the μm to mm scale including curvilinear and reentrant microstructures that are difficult to make using conventional micromachining can easily be fabricated. The micro-stereolithography, originally generated from conventional stereolithography process patented by Chuck Hull in 1986 [47], was first proposed by Ikuta et al. 1993 [48]. Some representative structure made using micro-stereolithography technique have been included in Fig. 33 [49].
Fig. 31

Schematic of the system used in micro-stereolithography technique

Fig. 32

Schematic of the projection micro-stereolithography system

Fig. 33

(a, b) High-resolution 3D microstructures fabricated by micro-stereolithography (Reproduced with permission from the author and publisher)

2 Materials

2.1 Replica Molding

  1. 1.

    A SiO2, Si3N4, metals, or PMMA original master patterned with the desired features.

     
  2. 2.

    Weighing balance.

     
  3. 3.

    Sylgard 184 kit (has both the silicone elastomer base and curing agent).

     
  4. 4.

    Disposable syringe.

     
  5. 5.

    Stir rod.

     
  6. 6.

    Tissue wipe.

     
  7. 7.

    Desiccator.

     
  8. 8.

    Vacuum oven.

     

2.2 Microcontact Imprinting

  1. 1.

    A patterned cross-linked PDMS stamp.

     
  2. 2.

    Ink (e.g., a thiol solution).

     
  3. 3.

    Wet etching reagent (buffered hydrofluoric acid).

     

2.3 Micro-transfer Molding

  1. 1.

    A patterned PDMS stamp.

     
  2. 2.

    Polymer or precursor liquid (polydimethylsiloxane).

     
  3. 3.

    Heating oven.

     

2.4 Solvent-Assisted Molding

  1. 1.

    Photoresist (Microposit 1813, Shipley, a positive-tone novolac resin).

     
  2. 2.

    Si wafer.

     
  3. 3.

    Spin coater.

     
  4. 4.

    A PDMS stamp.

     
  5. 5.

    Ethanol.

     
  6. 6.

    Q-tips.

     

2.5 Hot Embossing

  1. 1.

    A master stamp (silicone, SiO2, nickel) .

     
  2. 2.

    Embossing machine.

     
  3. 3.

    Polymer substrate (PMMA).

     
  4. 4.

    A heating plate with active temperature control.

     
  5. 5.

    Vacuum chamber.

     

2.6 Injection Molding

  1. 1.

    Mold inserts (hardened steel, pre-hardened steel, aluminum, and/or beryllium-copper alloy).

     
  2. 2.

    Injection molding machine.

     
  3. 3.

    Polymer pellets (e.g., made of PMMA, PC, PSU, POM, PA) .

     

2.7 CNC Micromachining

  1. 1.

    AutoCAD software (AutoCAD 2016) .

     
  2. 2.

    CNC machine.

     
  3. 3.

    CNC program created with softwares such as LazyCAM and SprutCAM.

     
  4. 4.

    Workpiece (PMMA).

     
  5. 5.

    A sacrificial material (e.g., PC, PS, PMMA) .

     
  6. 6.

    Ethanol.

     
  7. 7.

    Deionized water.

     
  8. 8.

    Compressed air.

     

2.8 Laser Photoablation

  1. 1.

    Deionized water.

     
  2. 2.

    0.1 M NaOH.

     
  3. 3.

    Compressed air.

     
  4. 4.

    Polymer substrate (PMMA , polyvinyl chloride, polyethylene terephthalate).

     
  5. 5.

    Photomask.

     
  6. 6.

    Laser.

     
  7. 7.

    Focusing lens.

     
  8. 8.

    Motorized XY stage.

     
  9. 9.

    Compressed air.

     

2.9 X-Ray Lithography

  1. 1.

    Substrate (e.g., silicon, alumina, glass).

     
  2. 2.

    Photoresist (e.g., PMMA).

     
  3. 3.

    Photomask.

     
  4. 4.

    Titanium pellet.

     
  5. 5.

    PMMA sheet.

     
  6. 6.

    Vacuum oven.

     
  7. 7.

    Methyl methacrylate.

     
  8. 8.

    Compressed air.

     
  9. 9.

    Mask holder.

     
  10. 10.

    Scanner.

     
  11. 11.

    Shims.

     
  12. 12.

    Substrate holder.

     
  13. 13.

    Synchrotron X-ray source.

     
  14. 14.

    Solvent for developing substrate (e.g., 7:3 mixture of IPA and water for PMMA substrates).

     

2.10 UV Patterning

  1. 1.

    PMMA sheet.

     
  2. 2.

    Deionized water.

     
  3. 3.

    Dishwasher gel.

     
  4. 4.

    Methanol.

     
  5. 5.

    Nitrogen gas.

     
  6. 6.

    Gold wire for sputtering gold.

     
  7. 7.

    TFA gold etchant.

     
  8. 8.

    UV light source (~254 nm).

     
  9. 9.

    Solvent for developing PMMA sheet (e.g., 7:3 mixtures of IPA and water).

     

2.11 Plasma Etching

2.11.1 For Creating a Mask over a PMMA/PEEK Substrate

  1. 1.

    PMMA/PEEK sheet .

     
  2. 2.

    Si-containing photoresist (e.g., PDMS) .

     
  3. 3.

    Photomask.

     
  4. 4.

    UV light source (~365 nm).

     
  5. 5.

    Methyl isobutyl ketone.

     
  6. 6.

    Isopropyl alcohol.

     

2.11.2 For Creating a Mask over a PDMS Substrate

  1. 1.

    PDMS sheet.

     
  2. 2.

    Aluminum wire for thermal deposition.

     
  3. 3.

    SU-8 3050 photoresist.

     
  4. 4.

    Spin coater.

     
  5. 5.

    UV light source.

     
  6. 6.

    Propylene glycol methyl ether acetate.

     
  7. 7.

    Isopropyl alcohol.

     

2.11.3 For Plasma Etching

  1. 1.

    PMMA/PEEK/PDMS workpiece .

     
  2. 2.

    Plasma etching system.

     
  3. 3.

    Vacuum pumps.

     
  4. 4.

    Oxygen gas.

     

2.12 Ion Beam Etching

  1. 1.

    Polymer substrate (PMMA/PEEK/PDMS).

     
  2. 2.

    Substrate holder.

     
  3. 3.

    Ion beam etching system.

     
  4. 4.

    Inert gas (e.g., argon).

     

2.13 Capillary Molding

  1. 1.

    A master stamp.

     
  2. 2.

    Polymer precursor (e.g., Sylgard 184 and its cross-linking agent for PDMS devices).

     
  3. 3.

    Heating oven.

     

2.14 Micro-stereolithography

  1. 1.

    CAD software.

     
  2. 2.

    UV curable photopolymer (epoxy-based photopolymer (3D systems, Valencia, CA).

     
  3. 3.

    Tank.

     
  4. 4.

    Programmable UV beam scanner.

     
  5. 5.

    Washing solvent (DBE, a mixture of dibasic esters).

     

3 Methods

3.1 Replica Molding

  1. 1.

    Obtain an original master patterned with the desired nanometer-sized features and prepared using advanced lithographic techniques.

     
  2. 2.

    Measure the mass of a plastic cup (empty) on a weighing balance and tare it.

     
  3. 3.

    Pour the required amount of polymer (Sylgard 184 Silicone elastomer base) into the cup and measure the mass.

     
  4. 4.

    Use a disposable syringe to inject the curing agent into the silicone elastomer base. Usually a mass ratio (curing agent/elastomer base) of 1:10 is recommended. After injecting, dispose the syringe into the bin.

     
  5. 5.

    Stir the mixture vigorously for 2 min using a stir rod and wipe the rod with tissue wipes after stirring (see Note 1 ).

     
  6. 6.

    Degas the mixture by placing it inside the desiccator for 50–60 min till the bubbles disappear (see Note 2 ).

     
  7. 7.

    Prepare a boat structure by wrapping a foil around the master which will restrict the PDMS to spill out of the master (see Note 3 ).

     
  8. 8.

    Now, pour the mixture (elastomer base and curing agent) gently onto the master within 10–15 min of degassing and wait for the additional bubbles (generated during pouring) to disappear.

     
  9. 9.

    Place the master into a vacuum oven at a temperature of 70 °C for 1 h for curing (see Notes 4 and 5 ).

     
  10. 10.

    After curing, peel off the PDMS stamp carefully from the master to obtain the required microstructure (see Note 6 ).

     

3.2 Microcontact Imprinting

  1. 1.

    Fabricate a patterned cross-linked PDMS stamp by replica molding (REM) (see Note 7 ).

     
  2. 2.

    Next, immerse the stamp into the ink to cover the stamp. The inking step can also be accomplished by vapor deposition and by placing the stamp above a beaker containing the ink (see Note 8 ).

     
  3. 3.

    The inked stamp should be subsequently placed in contact with the substrate to be patterned. Due to the patterned structure of the stamp, only protruded sections can make contact with the substrate (see Note 9 ).

     
  4. 4.

    Remove the PDMS stamp after the SAM formation.

     
  5. 5.

    Finally, wet etching can be performed to remove the unprotected parts.

     

3.3 Micro-transfer Molding

  1. 1.

    Prepare a PDMS mold, known as stamp, which contains the microchannels.

     
  2. 2.

    Fill the PDMS microchannels with prepolymer or precursor liquid (see Note 10 ).

     
  3. 3.

    Invert the stamp filled with the liquid polymer (see Note 11 ).

     
  4. 4.

    Make a conformal contact between the inverted stamp and the substrate to be patterned.

     
  5. 5.

    Cure the polymer at the required temperature and time, and then peel off the PDMS stamp (see Note 12 ).

     

3.4 Solvent-Assisted Molding

  1. 1.

    Prepare a thin film of photoresist on a Si wafer by spin coating at 5500 rpm for 30 s and baking at 105 °C for 3.5 min.

     
  2. 2.

    Select a suitable elastomeric mold, usually PDMS, and fabricate the desired patterns to make the master mold (see Note 13 ).

     
  3. 3.

    Wet the PDMS mold with ethanol with the help of Q-tips (see Notes 14 and 15 ). Allow the solvent to fill the recessed regions on the surface of PDMS.

     
  4. 4.

    Place the mold on top of the photoresist film such that the elastomer makes conformal contact with the resist forming microscale channels between them (see Note 16 ).

     
  5. 5.

    The compliant PDMS will adhere spontaneously with the surface and squeeze out the extra ethanol from the regions of contact. The permeability of solvent and gas through PDMS enables the uniform evaporation of solvent and escape of trapped air bubbles (see Note 17 ).

     
  6. 6.

    The remaining solvent dissolves or swells the photoresist, and a negative pattern forms in the resulting polymeric fluid (see Note 18 ).

     
  7. 7.

    Allow the PDMS mold to remain on the resist for ~5 min at room temperature until most of the solvent has dissipated [11].

     
  8. 8.

    Peel away the PDMS mold to obtain the microstructured photoresist.

     

3.5 Hot Embossing

  1. 1.

    Firstly, pattern the master stamp by silicon micromachining, LIGA process, or CNC micromachining (see Note 19 ).

     
  2. 2.

    Mount the master onto the embossing machine, which consists of a force frame that delivers the embossing force via a spindle and a T-bar to the boss.

     
  3. 3.

    Mount the master and the flat polymer substrate (see Note 20 ) on the heating plates. Circulate oil with high heat capacity through the cooling channels, which allows active cooling and isothermal heating of the plates.

     
  4. 4.

    Before operation, heat the plates in a vacuum chamber at 10−1 mbar to a temperature just above the glass transition temperature Tg of the polymer material (see Note 21 ). Vacuum is required to remove gas bubbles in the microstructures and also to prevent corrosion of the master [14].

     
  5. 5.

    Bring the master mold on the frame into contact with the substrate (see Notes 22 and 23 ), and emboss with a sensor feedback-controlled force, typically of the order of 20–30 kN [14].

     
  6. 6.

    While still applying the embossing force, cool the tool-substrate sandwich to just below Tg to stabilize the patterned microstructure on the polymer.

     
  7. 7.

    The embossing master can then be mechanically driven apart from the substrate (see Note 24 ). The polymer on the bottom plate will now have the desired pattern.

     

3.6 Injection Molding

  1. 1.

    Fabricate the mold inserts by bulk micromachining or LIGA.

     
  2. 2.

    The injection molding machine consists of a screw, an injection nozzle, a heater, and a mold insert.

     
  3. 3.

    Feed the polymer pellets through an open-bottomed container (hopper) into the screw (see Note 25 ).

     
  4. 4.

    An electric or hydraulic motor rotates the screw inside a barrel which is surrounded by heating elements as shown in Fig. 11. As the temperature is raised to a desired temperature, the polymer softens and melts (see Note 26 ).

     
  5. 5.

    The screw pushes the molten polymer into the mold cavity through its grooves at high pressure (see Note 27 ).

     
  6. 6.

    A gate before the mold cavity restricts the flow of the melt into the mold and limits backflow (see Note 28 ).

     
  7. 7.

    The screw injects the molten polymer into the mold, holds it under pressure, and adds more molten polymer to avoid formation of air gaps in the final product as a result of cooling and solidification (see Note 29 ).

     
  8. 8.

    The gate solidifies and isolates the mold from the injection cylinder (see Note 30 ).

     
  9. 9.

    Circulate a cooling liquid such as water through the small holes in the mold to cool the molten polymer inside the mold cavity. This step consumes 85% of the cycle time [16] (see Note 31 ).

     
  10. 10.

    After cooling and solidification of polymer, remove the patterned polymer mold by opening the two halves of the mold cavity holding the solidified polymer.

     
  11. 11.

    Clean any extra polymer from the mold.

     

3.7 CNC Micromachining

  1. 1.

    Design the microstructure with the help of AutoCAD/SolidWorks.

     
  2. 2.

    Choose appropriate cutting tools, speed (RPM), feed rates, and depth of cuts (see Notes 32 36 ). Prepare the CNC program with the help of softwares such as LazyCAM, SprutCAM, etc. Load the CNC program (see Note 37 ).

     
  3. 3.

    Check the levelness of the worktable with a spirit level.

     
  4. 4.

    Fix the workpiece onto the worktable (granite) with the help of clamps or adhesive tape. The schematic of the machine is shown in Fig. 13. Add a sacrificial layer (PC), polystyrene (PS), and polymethyl methacrylate (PMMA) to avoid accidental impact on the worktable [20].

     
  5. 5.

    Properly align the cutting tool to ensure accuracy. For Z alignment, lower (step) the tool toward the workpiece while the spindle is running. As soon as the tool touches the surface, a chip forms. Stop the Z movement and set it as “zero” (see Note 38 ).

     
  6. 6.

    For X and Y alignment, move the tool sideways and lower it while the spindle is running till a chip forms at the desired location. Set it as “zero” for X and Y. Check the levelness of the workpiece.

     
  7. 7.

    Start the spindle coolant and run the program (see Notes 39 and 40 ).

     
  8. 8.

    During machining, milling debris are cleared from the workpiece by blowing air and vacuuming or with a flood coolant of DI water mixed 20:1 with a synthetic coolant [20] (see Note 41 ).

     
  9. 9.

    After machining, clean the patterned polymer workpiece with 70% ethanol in DI water, and dry with compressed air [20].

     

3.8 Laser Photoablation

  1. 1.

    Clean the substrate initially with DI water, and then immerse it to a 0.1 M NaOH solution for 30 min. Rinse it with DI water, and dry with pressurized air [24] (see Notes 42 and 43 ).

     
  2. 2.

    Mount the cleaned substrate onto the stage.

     
  3. 3.

    A photomask with the desired pattern should also be mounted as shown in Fig. 15 (see Note 44 ).

     
  4. 4.

    Fire UV laser pulses (193 nm) onto the substrate through the mask and a 10:1 lens with a frequency of 50 Hz at 200 mJ/pulse [25] (see Notes 45 47 ).

     
  5. 5.

    Move the XY stage with the polymer substrate horizontally at a speed of 0.15–0.2 mm/s to desired channels of desired lengths (see Notes 48 51 ).

     
  6. 6.

    Pattern reservoirs by firing sufficient pulses to penetrate the whole polymer sheet.

     
  7. 7.

    As a result of “ejection by pressure,” each successive pulse of the laser cleans away any debris that might have accumulated in the ablated region (see Note 52 ).

     
  8. 8.

    Clean the patterned polymer using a jet of compressed air.

     

3.9 X-Ray Lithography

  1. 1.

    Select a suitable substrate for the fabrication, such as silicon, alumina, and glass.

     
  2. 2.

    Select a suitable photoresist, either positive or negative, for the technique.

     
  3. 3.

    Fabricate a mask with a material having low atomic number, and keep it ready for the exposure step (see Note 53 ).

     
  4. 4.

    Sputter coat a titanium layer onto the substrate.

     
  5. 5.

    Oxidize the titanium layer.

     
  6. 6.

    Cut PMMA sheets (with thickness ~1.5 mm) to either rectangular or circular shape depending upon the type of mask and design layout.

     
  7. 7.

    Anneal the cut PMMA sheet at 110 °C inside a vacuum oven.

     
  8. 8.

    Bond the annealed PMMA sheet over the titanium oxide side of the substrate by applying methyl methacrylate monomer on the substrate as well as substrate and pressing them using a compressed air source for overnight.

     
  9. 9.

    Normally, most of the applications utilize PMMA resist layer of thickness >100 μm. So, thin down the glued PMMA sheet by fly-cutting process (utilizing a diamond cutting tool) in two steps. In first step, remove the material from the PMMA bulk at a rate of 100 μm thickness per cycle and, in the second step, smoothen out the PMMA surface to transparency.

     
  10. 10.

    Mount the mask onto the customized holder and load into the scanner such that it is close to the water-cooled copper ring.

     
  11. 11.

    In order to maintain an optimal gap between mask and the polymer substrate, utilize shims of precise thickness. Typically, a gap of 100 μm and 1 mm is utilized for micrometer size features and millimeter size features, respectively.

     
  12. 12.

    Mount the substrate over the sample holder and fix in alignment with the mask and synchrotron X-ray source.

     
  13. 13.

    Expose the substrate to X-rays with an optimal level of intensity depending upon the feature size and thickness of the microstructure with the mask in between the substrate and X-ray synchrotron radiation source.

     
  14. 14.

    Develop the exposed substrate further using a suitable solvent. For PMMA, popular solvent used is 7:3 mixture of IPA and water.

     

3.10 UV Patterning

  1. 1.

    Cut the PMMA sheet of thickness 5 mm into desired size (mostly 3×3 in. square size) in order to proceed further with the fabrication procedure.

     
  2. 2.

    Clean the substrate with deionized water and mild dishwasher gel (see Note 54 ).

     
  3. 3.

    Immerse the sample in methanol bath for 10 min just to remove any oil residual on the surface, and dry it with N2 gas.

     
  4. 4.

    Apply a 100 nm thick layer of gold over the substrate by sputtering at 80 W (see Fig. 20a for the image of the gold-coated substrate).

     
  5. 5.

    Then pattern the gold film using optical photolithography technique. TFA gold etchant can be used for etching gold.

     
  6. 6.

    Expose the PMMA sample with non-collimated UV light having a nominal power of 4 mW/cm2 and a spectrum whose strongest peak is at 254 nm. The exposure time depends upon the depth of the microstructure required (see Notes 55 and 56 ).

     
  7. 7.

    After finishing the exposure process, transfer the sample to the development bath at 28 °C. A 7:3 mixture of IPA and water is used as the development solution. Development time of sample is dependent on the depth of the microchannel.

     
  8. 8.

    The development process is monitored continuously at a period of 10 min. Once the development is completed, the sample is dried by blowing N2 gas (see Note 55 ).

     
  9. 9.

    Once the sample is developed, the gold layer is dissolved using TFA gold etchant.

     

3.11 Plasma Etching

3.11.1 Procedure for Creating a Mask over a PMMA/PEEK Substrate [32]

  1. 1.

    Cut the PMMA/PEEK sheet to optimal size .

     
  2. 2.

    Coat a thin layer (~2 μm) of Si-containing photoresist such as PDMS or inorganic-organic hybrid polymer (ORMOCER) over the PMMA/PEEK sheet.

     
  3. 3.

    Expose the substrate with UV light broadband at 365 nm through the desired mask with the pattern which has to be replicated.

     
  4. 4.

    Remove the soluble part of the Si-containing polymer in MIBK (methyl isobutyl ketone) and IPA. This makes the sample ready for creating the masking layer plasma etching.

     

3.11.2 Procedure for Creating a Mask over PDMS Substrate [32]

  1. 1.

    Deposit a thin layer of Al over the substrate in thermal evaporator by maintaining the largest possible gap between substrate and Al target to minimize formation of wrinkles at Al-PDMS interface.

     
  2. 2.

    Spin coat photoresist SU-8 3050 over the Al layer side of the substrate.

     
  3. 3.

    Expose the SU-8 3050 layer with UV light of optimum power intensity depending upon the thickness of the photoresist layer.

     
  4. 4.

    Develop the nonexposed part of photoresist using PGMEA and IPA.

     

3.11.3 Procedure for Plasma Etching [33]

  1. 1.

    Load the sample containing the masking layer into the reaction chamber of plasma etching system.

     
  2. 2.

    Create a low pressure of approximately 0.1 mbar inside the reaction chamber with the help of vacuum pump.

     
  3. 3.

    Feed the process gas (oxygen) into the chamber, and create a working pressure of 0.1 mbar to 1 mbar.

     
  4. 4.

    Once the working pressure is achieved, switch on the generator, which ionizes the process gas converting it into the plasma state.

     
  5. 5.

    Expose the substrate to plasma for certain time duration (1 min to few hours) in order to achieve the desired etching depth (see Notes 57 59 ).

     
  6. 6.

    Switch off the generator, and evacuate the chamber to bring the chamber pressure to atmospheric condition.

     
  7. 7.

    Remove the workpiece.

     

3.12 Ion Beam Etching

  1. 1.

    Create a mask over the polymer substrate using similar procedure as explained in the plasma etching technique.

     
  2. 2.

    Load the sample onto the substrate holder.

     
  3. 3.

    Create a vacuum level of pressure ~2 × 10−4 Torr inside the chamber [35].

     
  4. 4.

    Introduce the continuous supply of inert gas (often argon) into the plasma chamber made of ceramics with a coil wound around.

     
  5. 5.

    Start the neutralizer, which emits few electrons to the plasma chamber.

     
  6. 6.

    Switch on the RF power to the coil, which sets the electron to vibration, induces the generation of more electrons and ions, and ultimately generates the plasma inside the chamber.

     
  7. 7.

    Direct the ions toward the substrate to remove the atoms from the substrate.

     
  8. 8.

    Keep the cooling systems switched on to flush away the heat generated because of machining.

     
  9. 9.

    Keep the etching system on till the etch depth is achieved (see Notes 60 62 ).

     
  10. 10.

    Switch off the RF power source, which essentially stops the generation of plasma inside the chamber.

     
  11. 11.

    Evacuate the chamber to atmospheric pressure and unload the sample carefully.

     

3.13 Capillary Molding

  1. 1.

    Firstly, fabricate a master (having a network of recessed channels of desired dimensions) using lithographic or other non-lithographic techniques [42, 43] which holds the complementary structures of the elastomeric mold.

     
  2. 2.

    Dispense the polymer on the master to fabricate the elastomeric mold. Usually, PDMS (Sylgard 184, Dow Corning, USA) with its cross-linking agent is used as the stamp material due to its high elasticity and low surface energy. The transparency of PDMS to UV light, down to 300 nm, has made it compatible with the photochemical polymerization employed for the capillary molding. Alternative materials such as polyimide [44], polyurethane, and novolac (phenol-formaldehyde) resin have also been used to prepare these stamps (see Notes 63 and 64 ).

     
  3. 3.

    Then, bring the elastomeric stamp which contains the inverse features of the master to an intimate contact with the solid substrate. Thus, the recessed microchannels on the stamp form a network of empty capillaries (see Notes 65 69 ).

     
  4. 4.

    Place a low-viscosity fluid precursor in close contact at one end, which spontaneously fills the channels by capillary action (see Notes 63 and 70 ).

     
  5. 5.

    Then, cross-link the prepolymers thermally (at 65 °C, 1–2 h for thermally cured epoxies) or photochemically (for thin PDMS films, which are optically transparent).

     
  6. 6.

    Once the polymer has cross-linked completely, peel off the elastomeric stamp from the substrate, thus leaving the patterned microstructures on the substrate surface (see Notes 71 and 72 ).

     

3.14 Micro-stereolithography

  1. 1.

    Design a 3D solid model of the product to be fabricated using CAD software.

     
  2. 2.

    After designing the 3D solid model with CAD software, slice it into a series of 2D layers of uniform thickness and convert it into a bitmap file.

     
  3. 3.

    Then, take the UV laser curable photopolymer in the tank.

     
  4. 4.

    Dip the elevator into the tank which is initially above the liquid level, and then bring it up again to ensure that a suitable thickness of liquid remains on top of it.

     
  5. 5.

    Then execute the NC codes generated from each sliced 2D file to control the UV beam scanning (see Notes 73 80 ).

     
  6. 6.

    Finally, wash the product with suitable solvents.

     

4 Notes

  1. 1.

    The mixture of base elastomer and curing agent should be stirred properly, to avoid any nonuniformity [1].

     
  2. 2.

    Degassing vacuum gauge level should go beyond 20 in./Hg for proper removal of bubbles [2].

     
  3. 3.

    Surface tension will restrict the spillage of PDMS from the master; still a proper boat structure should be prepared to avoid wastage and nonuniformity in the final microstructure [3].

     
  4. 4.

    Place the master on a flat surface; otherwise it will lead to an uneven thickness of the PDMS microstructure [3].

     
  5. 5.

    Curing temperature and time is a very crucial parameter for REM, which will decide flexibility of the final PDMS structure. Curing above the recommended time and temperature can lead to hard and brittle microstructure. Curing below the recommended time and temperature can make the structure sticky and floppy [6].

     
  6. 6.

    After peeling off the PDMS stamp, rinse it with isopropyl alcohol (IPA) to wash off the PDMS debris during peeling and cutting. Do not use any polar solvents (e.g., hexane, toluene, and methylene chloride) as these can swell up the cross-linked PDMS [50].

     
  7. 7.

    Before casting the stamp, it is very important to functionalize the master with the silane vapor so that the cast polymer does not adhere to the master.

     
  8. 8.

    During inking of the stamp, it is very important to maintain an appropriate concentration of the ink solution.

     
  9. 9.

    A gentle pressure should be applied to make a conformal contact between the inked stamp and the substrate.

     
  10. 10.

    Remove the extra liquid by scrubbing or using nitrogen stream.

     
  11. 11.

    During inversion, care has to be taken to avoid spillage of prepolymer liquid.

     
  12. 12.

    The appropriate curing temperature and time needs to be used to avoid distorted patterns.

     
  13. 13.

    Choosing appropriate solvent and mold material plays an important role in the effectiveness of this molding technique [50].

     
  14. 14.

    Many nonpolar solvents (e.g., hexane, toluene, and methylene chloride) are capable of swelling the cross-linked PDMS and therefore should not be used for the process [11].

     
  15. 15.

    The solvent should have a relatively high vapor pressure and a moderately high surface tension (e.g., methanol, ethanol, and acetone) to ensure rapid evaporation of the excess solvent and minimal swelling of the PDMS mold. Solvents with low vapor pressures (e.g., ethylene glycol and dimethyl sulfoxide) are not suitable. Solvents with high surface tension (e.g., water) wet the PDMS surface only partially and therefore cannot be used [11].

     
  16. 16.

    If the height of the pattern structure is much higher than the thickness of the resist, a hard mold can be used since the solvent can be expelled from the gap between the resist and the mold [10].

     
  17. 17.

    The distortion of PDMS mold as a result of solvent adsorption can be avoided by changing PDMS composition and surface modification and reducing surface energy [10]. Deng et al. [8] have used a coating of amorphous fluorinated polymer, a low surface energy material to avoid swelling and deformation of the PDMS. The fluoropolymer molds are easier to release from the resists and avoid swelling by solvent. However, its chemical inertness and low surface energy make functionalization and inking difficult during the microcontact printing.

     
  18. 18.

    Moderate pressure can be applied for improved engraving of pattern into the mold [10].

     
  19. 19.

    To avoid frictional forces between the mold and the polymer microstructures during de-embossing leading to defects, the master mold needs to have minimum sidewall roughness. LIGA is best suited (in place of hot embossing) for high aspect ratio channels [14].

     
  20. 20.

    Polymethyl methacrylate (PMMA) and polycarbonate (PC) are the most commonly used polymers for hot embossing. Table 2 shows the various embossing parameters for these polymers [14].

     
  21. 21.

    Glass transition temperature Tg, forming pressure, and holding time are the most important parameters for the micro-hot embossing process [1].

     
  22. 22.

    The surfaces of the mold and the substrate should have minimal chemical surface bonding sites as this will offer additional stiction force during de-embossing [14].

     
  23. 23.

    Si mold can be coated with amorphous silicon carbide, Teflon-like fluoropolymer, or self-assembled n-octadecyltrichlorosilane for reduction of adhesion between the mold and patterned polymers [14].

     
  24. 24.

    Three different configurations can be used for hot embossing: plate to plate, roll to plate, and roll to roll [1].

     
  25. 25.

    Cracking and crazing can occur due to improper reinforcement content, loading, under-curing, and resin richness. This can be solved by increasing glass and filler content, extending molding cycle time, and making sure the reinforcement is not displaced during mold closing [17].

     
  26. 26.

    Blisters on the mold can indicate faulty cooling or heating facility [18].

     
  27. 27.

    Flash (excess material on mold) may be due to over-packing of mold, high injection speed, low clamping force, or contamination of polymer. Too slow injection can lead to formation of flow lines on the mold [18].

     
  28. 28.

    Poor tool design, gate position, and high injection speed can lead to jetting by turbulent flow of material [18].

     
  29. 29.

    Voids in the mold can be caused by low pack or holding pressure, out of registration of mold halves, or improper melting of polymer [17, 18].

     
  30. 30.

    Weld lines can appear on the mold when the material or the mold temperatures are set too low or when the time between injection and holding is too low [18].

     
  31. 31.

    Warping can occur when the cooling is too short and when the cooling water is of incorrect temperature.

     
  32. 32.

    Take sufficient care while choosing the tool size, spindle speed, feed rate, and depth of cut. Incorrect input may lead to tool failure or premature tool wear and poor quality, accuracy, and surface finish.

     
  33. 33.

    Longer tools deflect more easily. Therefore, use shortest possible tool to maximize rigidity [22].

     
  34. 34.

    Ensure that the tips of the tools do not come into contact with each other in storage [22].

     
  35. 35.

    Always check for the sharpness of the tools. Replace when it loses its edge.

     
  36. 36.

    Too high spindle speed and too low feed rate can lead to friction and can generate heat. Excess heat can cause material ductility resulting in burrs, polymer melting, or tool breakage [20].

     
  37. 37.

    Writing the code manually will help in expanding the utility of the mill [20].

     
  38. 38.

    To avoid material removal during XYZ alignment, place a paper below the tool, and move it side to side while lowering the tool. At the position, where the paper gets caught between the tool and the workpiece, offset the tool with the thickness of the paper to reach the zero position [20].

     
  39. 39.

    It is advantageous to start the coolant well before starting the spindle in order to provide the coolant enough time to reach the tool [22].

     
  40. 40.

    Avoid driving the tools into deep slots and pockets except with absolutely minimal cut depth and stepover per pass [22].

     
  41. 41.

    For smaller tools, use mist instead of coolant flood to avoid deflection and breakage [22].

     
  42. 42.

    Polymers that show a photochemical ablation behavior at the irradiation wavelength would be most preferable for structuring. In these polymers, the damage of the surrounding material is minimized, and less carbonization is observed [24].

     
  43. 43.

    If the polymers do not have intrinsic absorption at the irradiation wavelength, a dopant can be added to induce the necessary absorption. The dopant can be added at the molecular level into the polymer backbone and side chains or as absorber particles at nanometer or micrometer range [24].

     
  44. 44.

    Laser machining can be done in two ways: direct writing and using a mask. In the direct writing mode, the laser beam is focused and scanned on the substrate surface to make pattern. Here, the smallest structure depends on the accuracy of the scanning system and is of the order of 25–50 mm. In the masking mode, the mask determines the detailed shape of the structure. Therefore, the minimum structure size can be brought down to twice that of the laser wavelength [1].

     
  45. 45.
    The three types of lasers that can be used for laser photoablation are [1]:
    1. (a)

      Excimer laser—UV (351, 308, 248, 193 nm)

       
    2. (b)

      Nd:YAG laser—Near infrared (1067 nm), visible (533 nm), and UV (355, 266 nm)

       
    3. (c)

      CO2 laser—Deep infrared (10.6 μm)

       
     
  46. 46.

    The choice of laser wavelength depends on the minimum structure size and the optical properties (absorption and reflection characteristics) of the substrate material. The minimum achievable focal spot diameter and the smallest structure size are about twice the laser wavelength [1].

     
  47. 47.

    The process depends on the thermal energy of the laser beam. The important parameters of photoablation are ablation rate d(F), ablation threshold fluence Fth, and effective absorption coefficient eff. The ablation process is defined as [24]

     
Table 2

Embossing parameters for PMMA and PC [15]

Material

Tg (°C)

Embossing temperature (°C)

De-embossing temperature (°C)

Hold time (s)

PMMA

106

120–130

95

30–60

PC

150

160–175

135

30–60

$$ d(F)=\frac{1}{\alpha_{\mathrm{eff}}}\ln \left(\frac{F}{F_{\mathrm{th}}}\right) $$
  1. 48.

    The cross section of the microchannel depends on the energy distribution of the laser beam, its speed, the laser power, and the thermal diffusivity of the substrate material [1].

     
  2. 49.

    The energy of the laser beam has a Gaussian distribution; thus the cross section of the channel also has a Gaussian shape [1].

     
  3. 50.

    The choice of laser power depends on the desired structure size and ablation rate. The typical ablation depths per pulse for different materials are given in Table 3 [1].

     
  4. 51.

    Short-pulsed lasers are advantageous because they create clean and accurate structures since they avoid heat flow to surrounding materials [1].

     
  5. 52.

    Depending on the chemical nature of the resist material, the X-ray exposed areas may cause cross-linking (for negative resists) or bond breaking (for positive resists). After exposure, the resist pattern on the substrate can be developed utilizing the proper solvent. The exposed areas in a positive resist will dissolve, and the unexposed areas will remain. Alternatively, the exposed areas in a negative resist will not be affected, while the unexposed areas will dissolve.

     
  6. 53.

    For the choice of the mask, the following materials as shown in Table 4 can be utilized. Please refer elsewhere for the fabrication procedure of the mask [26].

     
  7. 54.

    Instead of dishwater gel, neutral detergent solution can also be used for the cleaning purpose.

     
  8. 55.

    The depth of the microstructure varies with the exposure time and the development time as shown in Fig. 21a, b, respectively. Hence, the exposure time should be kept optimal to achieve the desired depth of the microchannel.

     
  9. 56.

    After every successive exposure time (say 10 min), the sample should be taken out of the bath and should be quenched in an ultrasonic IPA bath at room temperature (18 °C) for 10 s.

     
  10. 57.

    The main process parameters of this technique are plasma power (in W), total process pressure (in Pa), gas flow (in sccm), substrate bias voltage (in V), and substrate temperature (in °C) [33]. The etching rates of PMMA, PEEK, and PDMS materials increase with decreasing chamber pressure, increasing plasma power, and increasing bias voltage as shown in Fig. 24a, b.

     
  11. 58.

    Please refer to Fig. 24c, d for relation between etch depth, etch rate, and etching time, respectively, for PMMA and PEEK materials [33].

     
  12. 59.

    One disadvantage with this technique is the nano-roughness created at the microchannel wall surface. Figure 25a, b shows the variation of the nano-roughness level for PMMA, PEEK, and PDMS materials [33].

     
  13. 60.

    The parameters governing the ion beam etching process are the type of gas used, kinetic energy of the ions, ion flux, and the angle of incidence of the ion beam with respect to the sample surface [35].

     
  14. 61.

    Angle of incidence θ plays an important role in the yield of the process. Yield increases to 3.5 times for θ ≈ 70° and falls off rapidly at θ ≈ 80° [35].

     
  15. 62.

    Yield of the experiment increases with the increase in the incident ion energy [38].

     
  16. 63.

    The prepolymer has to be nonreactive to the soft elastomeric substrate. It should have low viscosity (<400 cP) as well. Upon curing, it is expected that the volume should change (<3%), and most importantly it should wet the support or the substrate partially or completely. UV-curable polymethyl methacrylate (SK-9, Summers Optical), ultraviolet-curable epoxies (UV 15, UV 15-7, Master Bond), heat-curable epoxies (F113, F114, TRACON), ultraviolet-curable polyurethanes (J-91 Summers Optical, NOA 60, 71, 72, 73, Norland), and heat-curable polyurethanes (NOA 133) are normally being used as liquid prepolymer for the capillary micro-molding process [41].

     
  17. 64.

    Once the master mold is made, many elastomeric stamps can be cast and used repeatedly. Clean (with compressed nitrogen gas or argon gas) the master mold every time before use, to avoid any contamination.

     
  18. 65.

    The softness of the PDMS enables the mold to make a conformal contact with the substrate which limits the lateral spreading of the prepolymer.

     
  19. 66.

    The mold and the substrates adhere without any external adhesive forces enabling reproduction of channels with high accuracy.

     
  20. 67.

    Si, SiO2 (both native and thermally grown), NiO, Ni, TiO, TiO2, platinum, glass, and gold can be used as supports in capillary molding [40].

     
  21. 68.

    The surface energy of the support is often preferred to be higher than that of PDMS to ensure selective adhesion of the polymer to the support rather than to the master.

     
  22. 69.

    The support need not be necessarily flat. For curved surfaces, a PDMS master is fabricated as a thin film (~100 μm) that can adhere to the curved substrate without losing the fidelity of the stamp [40].

     
  23. 70.

    When brought in contact with the organic solvents including chloroform, methylene chloride, toluene tetrahydrofuran, etc., PDMS stamps swell and lose the conformal contact with support [40].

     
  24. 71.

    A PDMS thin layer can be spin casted before the stamp is placed on the substrate to avoid the partial adhesion of the fabricated pattern or the channels with the support of the elastomeric stamps [46].

     
  25. 72.

    Some access holes through the PDMS stamp may be provided to improve the capillary filling into the network of capillary channels.

     
  26. 73.

    The scanning micro-stereolithography techniques work on the principle where the solid micro parts are built in a point-by-point and line-by-line approach as shown in Fig. 31. A laser beam is focused (through a dynamic lens and followed by two low-inertia galvanometric mirrors) on the surface of a resin system containing UV photo initiator, monomer, and other additives.

     
  27. 74.

    Generally, the scanning is done by line-by-line or point-by-point approach, so there can be some error associated with this. Generally, components of dimensions above 30 μm are built using this technique.

     
  28. 75.

    With the rotation of the mirrors, the focal point of the laser beam may remain slightly above or below the liquid surface, instead of remaining on top of the liquid surface. To avoid such problems, the optical setup can be fixed, while the X-Y tank can be assigned with movements.

     
  29. 76.

    To get a better resolution, the beam is focused more precisely in order to reduce the spot size to a few micrometers in diameter.

     
  30. 77.

    Projection micro-stereolithography (shown in Fig. 32) builds 3D microstructure in a layer-by-layer fashion. Here the UV laser source is replaced by the UV light source and a dynamic mask or dynamic pattern generator which is used to digitally pattern and project the incident light as an image and expose an entire cross-sectional layer at once. Feature sizes smaller than 10 μm can be fabricated as it is not limited by laser beam radius or its scan speed. After passing through a series of optical lens, the pattern light is focused on the surface of liquid photopolymer resin. Polymerization of the resin starts as soon as it comes in contact with light, and once the first layer is cured, it is repositioned such that additional resin is recoated onto the previously cured photopolymer. An image of the next cross-sectional layer is projected on top of the previous layer for curing. After the final cross-section is projected, the completed part is removed and post-processed.

     
  31. 78.

    The factors affecting the resolution of components are (a) laser intensity, (b) motion and quality of the beam, (c) photopolymer/monomer used, and (d) focusing and exposure.

     
  32. 79.

    The desired properties of the UV curable resins are (a) photosensitivity at the operating wavelength, (b) low viscosity to produce a highly smooth surface, (c) high curing speed, (d) low shrinkage during polymerization, and (e) high absorption for the low penetration of light.

     
  33. 80.

    Typically epoxy and acrylate resins are normally used, e.g., HDDA (1,6-hexanediol diacrylate) with 4% by weight of photo-initiator.

     
Table 3

Ablation depth per laser pulse for different materials [1]

Material

Depth per pulse (μm)

Polymers

0.3–0.7

Ceramics and glass

0.1–0.2

Diamond

0.05–0.1

Metals

0.1–1

Table 4

Choices of the mask for X-ray lithography technique

Mask material

Quality

Beryllium

High quality and cost (including submicron sizes)

Graphite

Intermediate quality and cost: 2 μm feature size and above

Graphite

Intermediate quality and cost: 10 μm feature size and above

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Copyright information

© Springer Science+Business Media, LLC, part of Springer Nature 2019

Authors and Affiliations

  • Ashis Kumar Sen
    • 1
    Email author
  • Abhishek Raj
    • 1
  • Utsab Banerjee
    • 1
  • Sk Rameez Iqbal
    • 1
  1. 1.Department of Mechanical EngineeringIndian Institute of Technology MadrasChennaiIndia

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