Dip Pen Nanolithography
Encyclopedia
Dip Pen Nanolithography began as a scanning probe lithography
technique where an atomic force microscope
tip was used to transfer alkane thiolates to a gold surface. This technique allows surface patterning on scales of under 100 nano
meters. DPN is the nanotechnology
analog of the dip pen
(also called the quill pen), where the tip of an atomic force microscope cantilever
acts as a "pen," which is coated with a chemical compound or mixture acting as an "ink," and put in contact with a substrate, the "paper."
DPN enables direct deposition of nanoscale materials onto a substrate in a flexible manner. Recent advances have demonstrated massively parallel patterning using two-dimensional arrays of 55,000 tips. Applications of this technology currently range through chemistry
, materials science
, and the life sciences
, and include such work as ultra high density biological nanoarrays, and additive photomask
repair.
The uncontrollable transfer of a molecular 'ink' from a coated AFM tip to a substrate was first reported by Jaschke and Butt in 1995, but they erroneously concluded that alkanethiols could not be transferred to gold substrates to form stable nanostructures. A research group at Northwestern University
led by Chad Mirkin
studied the process and determined that under the appropriate conditions, molecules could be transferred to a wide variety of surfaces to create stable chemically-adsorbed monolayers in a high resolution lithographic process they termed "DPN" . Mirkin and his coworkers hold the patents on this process, and the patterning technique has expanded to include liquid "inks". It is important to note that "liquid inks" are governed by a very different deposition mechanism when compared to "molecular inks".
. In bottom-up applications, the material of interest is delivered directly to the surface via the tips.
The criticism most often directed at DPN is the patterning speed. The reason for this has more to do with how it is compared to other techniques rather than any inherent weaknesses. For example, the soft lithography
method, micro contact printing
(μCP), is the current standard for low cost, bench-top micro and nanoscale patterning, so it is easy to understand why DPN is compared directly to micro contact printing
. The problem is that the comparisons are usually based upon applications that are strongly suited to μCP, instead of comparing them to some neutral application. μCP has the ability to pattern one material over a large area in a single stamping step, just as photolithography
can pattern over a large area in a single exposure. Of course DPN is slow when it is compared to the strength of another technique. DPN is a maskless direct write technique that can be used to create multiple patterns of varying size, shape, and feature resolution, all on a single substrate. No one would try to apply micro contact printing to such a project because the it would never be worth the time and money required to fabricate each master stamp for each new pattern. Even if they did, micro contact printing would not be capable of aligning multiple materials from multiple stamps with nanoscale registry. The best way to understand this misconception is to think about the different ways to apply photolithography and e-beam lithography. No one would try to use e-beam to solve a photolithography problem and then claim e-beam to be "too slow". Directly compared to photolithography's large area patterning capabilities, e-beam lithography is slow and yet, e-beam instruments can be found in every lab and nanofab in the world. The reason for this is because e-beam has unique capabilities that cannot be matched by photolithography, just as DPN has unique capabilities that cannot be matched by micro contact printing.
It is also important to consider one of the unique characteristics of DPN, namely its force independence. With virtually all ink/substrate combinations, the same feature size will be patterned no matter how hard the tip is pressing down against the surface. As long as robust SiN tips are used, there is no need for complicated feedback electronics, no need for lasers, no need for quad photo-diodes, and no need for an AFM.
Scanning probe lithography
Scanning probe lithography describe a set of lithographic methods, in which a microscopic or nanoscopic stylus is moved mechanically across a surface to form a pattern.This type of method can be split in two different groups:...
technique where an atomic force microscope
Atomic force microscope
Atomic force microscopy or scanning force microscopy is a very high-resolution type of scanning probe microscopy, with demonstrated resolution on the order of fractions of a nanometer, more than 1000 times better than the optical diffraction limit...
tip was used to transfer alkane thiolates to a gold surface. This technique allows surface patterning on scales of under 100 nano
Nano
Nano- is a prefix meaning a billionth. Used primarily in the metric system, this prefix denotes a factor of 10−9 or . It is frequently encountered in science and electronics for prefixing units of time and length, such as 30 nanoseconds , 100 nanometres or in the case of electrical capacitance,...
meters. DPN is the nanotechnology
Nanotechnology
Nanotechnology is the study of manipulating matter on an atomic and molecular scale. Generally, nanotechnology deals with developing materials, devices, or other structures possessing at least one dimension sized from 1 to 100 nanometres...
analog of the dip pen
Dip pen
A dip pen or nib pen usually consists of a metal nib with capillary channels like those of fountain pen nibs, mounted on a handle or holder, often made of wood. Other materials can be used for the holder, including bone, metal and plastic, while some pens are made entirely of glass...
(also called the quill pen), where the tip of an atomic force microscope cantilever
Cantilever
A cantilever is a beam anchored at only one end. The beam carries the load to the support where it is resisted by moment and shear stress. Cantilever construction allows for overhanging structures without external bracing. Cantilevers can also be constructed with trusses or slabs.This is in...
acts as a "pen," which is coated with a chemical compound or mixture acting as an "ink," and put in contact with a substrate, the "paper."
DPN enables direct deposition of nanoscale materials onto a substrate in a flexible manner. Recent advances have demonstrated massively parallel patterning using two-dimensional arrays of 55,000 tips. Applications of this technology currently range through chemistry
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
, materials science
Materials science
Materials science is an interdisciplinary field applying the properties of matter to various areas of science and engineering. This scientific field investigates the relationship between the structure of materials at atomic or molecular scales and their macroscopic properties. It incorporates...
, and the life sciences
Life sciences
The life sciences comprise the fields of science that involve the scientific study of living organisms, like plants, animals, and human beings. While biology remains the centerpiece of the life sciences, technological advances in molecular biology and biotechnology have led to a burgeoning of...
, and include such work as ultra high density biological nanoarrays, and additive photomask
Photomask
A photomask is an opaque plate with holes or transparencies that allow light to shine through in a defined pattern. They are commonly used in photolithography.-Overview:...
repair.
The uncontrollable transfer of a molecular 'ink' from a coated AFM tip to a substrate was first reported by Jaschke and Butt in 1995, but they erroneously concluded that alkanethiols could not be transferred to gold substrates to form stable nanostructures. A research group at Northwestern University
Northwestern University
Northwestern University is a private research university in Evanston and Chicago, Illinois, USA. Northwestern has eleven undergraduate, graduate, and professional schools offering 124 undergraduate degrees and 145 graduate and professional degrees....
led by Chad Mirkin
Chad Mirkin
Chad A. Mirkin is an American chemist. He is the George B. Rathmann Professor of Chemistry, Professor of Medicine, Professor of Materials Science and Engineering, and Director of the International Institute for Nanotechnology and Center for Nanofabrication and Molecular Self-Assembly at...
studied the process and determined that under the appropriate conditions, molecules could be transferred to a wide variety of surfaces to create stable chemically-adsorbed monolayers in a high resolution lithographic process they termed "DPN" . Mirkin and his coworkers hold the patents on this process, and the patterning technique has expanded to include liquid "inks". It is important to note that "liquid inks" are governed by a very different deposition mechanism when compared to "molecular inks".
Molecular inks
Molecular inks are typically composed of small molecules that are coated onto a DPN tip and are delivered to the surface through a water meniscus. In order to coat the tips, one can either vapor coat the tip or dip the tips into a dilute solution containing the molecular ink. If one dip-coats the tips, the solvent must be removed prior to deposition. The deposition rate of a molecular ink is dependent on the diffusion rate of the molecule, which is different for each molecule. The size of the feature is controlled by the tip/surface dwell-time (ranging from milliseconds to seconds) and the size of the water meniscus, which is determined by the humidity conditions (assuming the tip's radius of curvature is much smaller than the meniscus).- Water meniscus mediated (exceptions do exist)
- Nanoscale feature resolution (50 nm to 2000 nm)
- No multiplexed depositions
- Each molecular ink is limited to its corresponding substrate
Liquid inks
Liquid inks can be any material that is liquid at deposition conditions. The liquid deposition properties are determined by the interactions between the liquid and the tip, the liquid and the surface, and the viscosity of the liquid itself. These interactions limit the minimum feature size of the liquid ink to about 1 micrometre, depending on the contact angle of the liquid. Higher viscosities offer greater control over feature size and are desirable. Unlike molecular inks, it is possible to perform multiplexed depositions using a carrier liquid. For example, using a viscous buffer, it is possible to directly deposit multiple proteins simultaneously.- 1-10 micrometre feature resolution
- Multiplexed depositions
- Less restrictive ink/surface requirements
- Direct deposition of high viscosity materials
Examples
- Protein, peptide, and DNA patterning
- Hydrogels
- Sol gels
- Conductive inks
- Lipids
Applications
In order to define a good DPN application, it is important to understand what DPN can do that other techniques can't. Direct-write techniques, like contact printing, can pattern multiple biological materials but it cannot create features with subcellular resolution. Many high-resolution lithography methods can pattern at sub-micrometre resolution, but these require high-cost equipment that were not designed for biomolecule deposition and cell culture. Micro contact printing can print biomolecules at ambient conditions, but it cannot pattern multiple materials with nanoscale registry.Industrial applications
The following are some examples of how DPN is being applied to potential products.- Biosensor Functionalization - Directly place multiple capture domains on a single biosensorBiosensorA biosensor is an analytical device for the detection of an analyte that combines a biological component with a physicochemical detector component.It consists of 3 parts:* the sensitive biological element A biosensor is an analytical device for the detection of an analyte that combines a biological...
device - Nanoscale Sensor Fabrication - Small, high-value sensors that can detect multiple targets
- Nanoscale Protein Chips - High-density protein arrays with increased sensitivity
Cell engineering
DPN is emerging as a powerful research tool for manipulating cells at subcellular resolution- Stem cell differentiation
- Subcellular drug delivery
- Cell sorting
- Surface gradients
- Subcellular ECM protein patterns
- Cell adhesion
Rapid prototyping
- Plasmonics and Metamaterials
- Cell and tissue screening
Direct write
DPN is a direct write technique so it can be used for top-down and bottom-up lithography applications. In top-down work, the tips are used to deliver an etch resist to a surface, which is followed by a standard etching processEtching (microfabrication)
Etching is used in microfabrication to chemically remove layers from the surface of a wafer during manufacturing. Etching is a critically important process module, and every wafer undergoes many etching steps before it is complete....
. In bottom-up applications, the material of interest is delivered directly to the surface via the tips.
Unique advantages
- Directed Placement - Directly print various materials onto existing nano and microstructures with nanoscale registry
- Direct Write - Maskless creation of arbitrary patterns with feature resolutions from as small as 50 nm and as large as 10 micrometres
- Biocompatible - Subcellular to nanoscale resolution at ambient deposition conditions
- Scalable - Force independent, allowing for parallel depositions
Direct comparisons to other techniques
Soft lithography
200px|right|thumb|Figure 1 - "Inking" a stamp. PDMS stamp with pattern is placed in Ethanol and ODT solution200px|right|thumb|Figure 2 - ODT from the solution settles down onto the PDMS stamp. Stamp now has ODT attached to it which acts as the ink....
method, micro contact printing
Micro Contact Printing
Microcontact printing is a form of soft lithography that uses the relief patterns on a master Polydimethylsiloxane stamp to form patterns of self-assembled monolayers of ink on the surface of a substrate through conformal contact...
(μCP), is the current standard for low cost, bench-top micro and nanoscale patterning, so it is easy to understand why DPN is compared directly to micro contact printing
Micro Contact Printing
Microcontact printing is a form of soft lithography that uses the relief patterns on a master Polydimethylsiloxane stamp to form patterns of self-assembled monolayers of ink on the surface of a substrate through conformal contact...
. The problem is that the comparisons are usually based upon applications that are strongly suited to μCP, instead of comparing them to some neutral application. μCP has the ability to pattern one material over a large area in a single stamping step, just as photolithography
Photolithography
Photolithography is a process used in microfabrication to selectively remove parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical "photoresist", or simply "resist," on the substrate...
can pattern over a large area in a single exposure. Of course DPN is slow when it is compared to the strength of another technique. DPN is a maskless direct write technique that can be used to create multiple patterns of varying size, shape, and feature resolution, all on a single substrate. No one would try to apply micro contact printing to such a project because the it would never be worth the time and money required to fabricate each master stamp for each new pattern. Even if they did, micro contact printing would not be capable of aligning multiple materials from multiple stamps with nanoscale registry. The best way to understand this misconception is to think about the different ways to apply photolithography and e-beam lithography. No one would try to use e-beam to solve a photolithography problem and then claim e-beam to be "too slow". Directly compared to photolithography's large area patterning capabilities, e-beam lithography is slow and yet, e-beam instruments can be found in every lab and nanofab in the world. The reason for this is because e-beam has unique capabilities that cannot be matched by photolithography, just as DPN has unique capabilities that cannot be matched by micro contact printing.
Connection to Atomic Force Microscopy
DPN evolved directly from AFM so it is not a surprise that people often assume that any commercial AFM can perform DPN experiments. In fact, DPN does not require an AFM, and an AFM does not necessarily have real DPN capabilities. There is an excellent analogy with scanning electron microscopy (SEM) and electron beam (E-beam) lithography. E-beam evolved directly from SEM technology and both use a focused electron beam, but no one would ever suggest that one could perform modern E-beam lithography experiments on a SEM that lacks the proper lithography hardware and software requirements.It is also important to consider one of the unique characteristics of DPN, namely its force independence. With virtually all ink/substrate combinations, the same feature size will be patterned no matter how hard the tip is pressing down against the surface. As long as robust SiN tips are used, there is no need for complicated feedback electronics, no need for lasers, no need for quad photo-diodes, and no need for an AFM.
See also
- NanolithographyNanolithographyNanolithography is the branch of nanotechnology concerned with the study and application of fabricating nanometer-scale structures, meaning patterns with at least one lateral dimension between the size of an individual atom and approximately 100 nm...
- NanoInk, Inc. DPN Instrumentation
- DPN based protein assay kits
- NanoProfessor, Hands-on nanotech education