Open main menu

Direct writing (also known as maskless lithography) refers to any technique or process capable of altering the chemistry, depositing, removing, dispensing, or processing various types of materials over different surfaces following a predetermined layout or pattern.
There are various means to achieve the desired patterns and they can be broadly classified into additive and subtractive techniques. Additive techniques such as ink jet printing, dip pen nano-lithography (DPN), and micropens add material to a substrate based on a CAD layout. Subtractive techniques such as focused-ion beam (FIB), and laser micromachining selectively remove material from a substrate using ion beam and laser source, respectively.
Other techniques such as electron beam lithography, maskless lithography, thermal scanning probe lithography, and nanoimprint lithography use methods to chemically or physically alter the composition of thin films to pattern various geometries.

Direct writing
Direct Write.png
Technology Details
Other Names Direct Write, Maskless Lithography
Technology Lithography
Equipment Dimatix Inkjet Printer
Heidelberg µPG 501 Mask Maker
JEOL JBX-6300FS Electron Beam Lithography System
Materials Inks, Resists, Thin Films, Polymers, Inkjet Cartridge
Only the techniques that are available at LNF will be discussed in detail here.

Contents

Equipment

Below is a general description of the direct writing equipment at the LNF.

Dimatix MP-2831 Inkjet Printer

The Dimatix MP-2831 allows the deposition of fluidic materials on an 8x11 inch or A4 substrate, utilizing a disposable piezo inkjet cartridge. This printer can create and define patterns over an area of about 200 x 300 mm and handle substrates up to 25 mm thick with an adjustable Z height.

Heidelberg µPG 501 Maskless Lithography

The μPG 501 is a micro pattern generator for direct writing applications and low volume mask making. The system offers a raster-scan and vector exposure mode for 2D patterns and in addition it is also possible to create complex 3D (grey scale) structures in thick photoresist in a single pass. This system is able to pattern on most photoresists used in Optical lithography.

JEOL JBX-6300FS Electron Beam Lithography System

The JEOL JBX-6300FS is an electron beam lithography system equipped with a thermal field emission electron gun with a ZrO/W emitter and a Vector Scan Method for beam deflection. This instrument can be used to fabricate patterns down to 8 nm on substrates from 5 mm X 5 mm pieces up to 200 mm wafers on E-beam resists.

Nanonex NX2000 Nanoimprinter

The Nanoimprinter creates patterns by physical deformation of imprint resists by molds on which pressure is applied. The imprint resist is typically polymeric material that is cured by heat or UV light during the imprinting process. Adhesion between the resist and the mold is controlled to allow proper release of the mold from the resist.

The Nanonex NX2000 Nanoimprinter in the LNF cleanroom belongs to Prof. L. Jay Guo and is used by his research group exclusively.

Method of operation

Inkjet Printing

In inkjet printing, a MEMS piezoelectric driven cartridge is used to generate drops of ink. By placing electrodes on the surface of the piezoelectric material, a section of the material can be made to move without affecting the surrounding material. By applying a voltage to the center electrode, an electric field is created between the center electrode and the ground electrodes. This creates the shear response in the piezoelectric material between the electrodes[1]. By coupling the piezoelectric material to a pumping chamber that communicates with a nozzle, an ink drop is formed. This is also called drop on demand (DOD)[2].The actual motion of the piezoelectric material is approximately one millionth of an inch.

Piezo DOD inkjet offers a variety of advantages over other materials deposition methods. As a non-contact deposition technology, it avoids contamination or damage of substrates. Rather than flooding a surface with functional fluids it is precise and purely additive, able to deposit the exact amount of material at the exact locations where it is needed without waste.

Dip Pen Nanolithography (DPN)

Dip Pen Nanolithography (DPN) is a direct write , AFM tip-based lithography technique capable of multi-component deposition of a wide variety of materials with nanosclae registry. DPN allows the user to create user-defined patterns with feature size as small as 50nm and as large as 10um on substrates such as glass, plastic, gold and silicon, etc. DPN has the ability to design, deposit and characterize a wide variety of features and sizes on a benchtop, without the need of a clean room, master stamp or photomask. DPN was invented in 1999 by the Mirkin Group[3], and it can be used to deposit molecules and materials on surfaces with sub-50 nm resolution. This method employs an atomic force microscope (AFM) probe “pen” coated with a molecule- or materials-based “ink” that, upon contact with a surface, deposits the ink by diffusion through a water meniscus that forms under ambient conditions between the tip and substrate.

Maskless Lithography

In maskless lithography, a LED or Laser beam is shined onto micromirrors in the Digital Micromirror Device (DMD). This light is broken up into millions of square-shaped beams corresponding to every mirror. The pattern to be exposed is formed by the on/off function of each and all the mirrors in the DMD array. This light is then exposed onto a substrate that has been previously coated with photoresist. The stage onto which the substrate is fixed moves by an incremental amount in a direction determined by the layout and the next exposure takes place. This sequence is repeated till all the area to be patterned undergoes exposure.

Electron Beam Lithography

In electron beam lithography an accelerated and focused beam of electrons is used to pattern features down to sub-10 nm on substrates. The substrates are previously coated with a polymer that is sensitive to electron irradiation, known as e-beam resist. The e-beam can be moved to different areas of the substrates by deflector coils as well as stage movement. Exposure to the electron beam changes the solubility of the resist, enabling selective removal of either the exposed or non-exposed regions of the resist by immersing it in a developer.

The primary advantage of electron beam lithography is that it can write custom patterns with sub-10 nm resolution. This form of direct writing has high resolution and low throughput, limiting its usage to photomask fabrication, low-volume production of semiconductor devices, and research & development.

Nanoimprinting

Nanoimprint lithography (NIL) is a lithographic technique for high-throughput patterning of polymer nanostructures with great precision and at low cost. NIL relies on direct mechanical deformation of the resist material by a hard mold to which pressure is applied and can therefore achieve resolutions beyond the limitations set by light diffraction or beam scattering that are encountered in conventional lithography techniques. The resist material can be a thermal plastic, thermal setting, or low-viscosity precursor that can be cured either thermally or by UV light. The molds or stamps are normally made in silicon, dielectric materials (e.g, silicon dioxide or silicon nitride), metals (e.g., nickel), or polymeric materials that have a sufficient Young modulus.[4]

Parameters

Inkjet Printing

The physical-chemical characteristics of the ink will or will not allow the jetting of fluids. For example, viscosity and surface tension will determine if the the fluid can flow through the micron size nozzle. If the ink has solid, the size of the particles should be less than 0.2 µm to avoid clogging of the microchannels. In addition pH is also important.

Dip Pen Nanolithography

In this technique the chemical affinity between the ink and the substrate materials is critical. The technique is based on the transfer of material from tip (source) to the substrate via the meniscus formed between them. The local chemistry on the substrate must be such that allows the molecules transfer from the tip. For example, alkane thiolate inks (sulfur terminated hydrocarbons) can be easily deposited on gold surfaces as silanes (hydrogen-silicon compounds) can write on silicon. Therefore the surface chemistry of the substrate and the chemistry of the ink are the first two considerations.

Heidelberg Maskless Lithography

The light source for the Heidelberg mask maker puts up a constant power at 375 nm wavelength. The exposure parameters used will depend on the photoresist sensitivity. The exposure time (also called dwell time) will determine the energy deposited by the LED per unit area of the photoresist. The Heidelberg also has 2 write modes; high throughput and high accuracy. High accuracy mode is used for patterning features with critical dimensions 2 µm or below. For all other cases, high throughput mode is used.

Electron Beam Lithography

Electron beam lithography systems expose resists one pixel at a time. The energy deposited onto the resist by the e-beam (known as dose) is expressed in µC/cm². The dose depends on the beam current, beam diameter, sensitivity of the resist, and the spacing between neighboring pixels. For high throughput, it is desirable to have a large beam current. But increasing the beam current also increases the beam diameter which decreases resolution. For a given resist, the parameters that can be varied are beam diameter, beam current, and the spacing between pixels.

Nanoimprinting

This is not an LNF supported tool.

Applications

Inkjet Printing

This technique is convenient to use when a suitable ink needs to be deposited on a flat or relatively flat substrate and features larger that 30 microns are desired. This technique is not advisable for features less than 30 microns. There are two designs of jetting head available, one for pico liter size drop and another for um liter size drop. Smaller features can be obtained, with same materials from the smaller volume cartridge, but the size of the features that can be obtained, still depends on the surface chemistry of the substrate and the nature of the ink. In general it is recommended the ink physical characteristics (vapor pressure, viscosity) resembles that of water, though toluene is widely used as an ink.

The materials these unique systems can “print” range from UV-curable light-emitting polymers, liquid silver and conductive fluids to enzymes and DNA and other “organic inks” on all types of surfaces whose dimensions often must be controlled to within a few ten-millionths of a meter. The deposition of these fluids, from adhesives, masking inks, anti-scratch/ anti-glare compounds to UV-curable light-emitting polymers and conductive and electronics displays for life sciences, chemistry, 3-D mechanics, optics and photovoltaics, to name a few.

Dip Pen Nanolithography

This technique has been used to write with small organic molecules, such as alkanethiols (octadecanethiol (ODT) and mercaptohexadecanoic acid (MHA)) onto gold substrates. These molecules can form self-assembled monolayers (SAMs). It can also de used to write or template many different types of molecules and materials on a variety of surfaces (including metals, semiconductors and insulators) by controlling various experimental parameters such as ambient humidity, writing speed, and dwell time. These materials include polymers, colloidal nanoparticles (e.g. magnetic nanocrystals, carbon nanotubes), sol-gel precursors, small organic molecules, biomolecules (proteins and oligonucleotides) and even single virus particles and bacteria.

Heidelberg Direct Writing

The Heidelberg µPG 501 is used for mask patterning for contact and projection lithography as well as direct writing on substrates that have been coated with a photoresist. Applications include research & development and small volume production in areas such as MEMS, micro-fluidics, micro-optics, electronics and all other areas where optical lithography is required. This system can also be used for grey-scale or 3-D lithography on thick photoresists such as SU8.

Electron Beam Lithography

The ability to pattern structures down to a few nm scale makes electron beam lithography an extremely capable technique for a fabrication of quantum devices such as quantum tunneling devices and spintronics. Apart from that, it is widely used in the fabrication of metasurfaces, photonic crystals, ultra-high density storage media, micro and nanofluidic channels. The lower throughput of electron beam lithography compared to projection lithography at the expense of higher resolution is advantages in the small volume production of devices and sensors. Certain types of electron beam lithograpy systems are also used in the manufacture of high quality photomasks for projection lithography.

Nanoimprinting

Nanoimprinting provides a cost-effective pathway for much more precise control of surface optical properties and has been exploited for numerous applications such as fabrication of active photovoltaic layer in solar cells, control of polarization, color, media for hard-disk drives. As well as being used in biological applications include sensing, nanofluidic devices for DNA stretching, tissue engineering.[5]

Materials

Inkjet Cartridge

Main article: Inkjet Cartridge

Dip Pen Nanolithography Ink

Photoresist

Main article: Photoresist

Developer

Main article: Developer

E-Beam Resist

Main article: Photoresist

Nanoimprint Resist

Main article: Photoresist

See also

References

  1. http://www.fujifilmusa.com/products/industrial_inkjet_printheads/technology/piezoelectric/index.html
  2. http://www.fujifilmusa.com/shared/bin/Dimatix-Inkjet-Helps-Nano-Mfg-Gain-Scale.pdf
  3. Richard D. Piner, Jin Zhu, Feng Xu, Seunghun Hong, and Chad A. Mirkin, “‘Dip-Pen’ Nanolithography,” Science, vol. 283, no. 5402, pp. 661–663, 29/1/1999.
  4. Guo, L. J, “Nanoimprint Lithography: Methods and Material Requirements,” Advanced Materials, vol. 19, no. 4, pp. 495–513, 2/2007.
  5. https://www.azonano.com/article.aspx?ArticleID=5008

Further reading

  1. How Dimatix Works - A Movie
  2. Direct Writing in the Heidelberg - June 29, 2018, Vishva Ray: Video recording and complete slides
  3. E-beam Lithography (Part 1 of 2) - April 10, 2020, Vishva Ray: Video recording and complete slides