Micro Contact Printing
Encyclopedia
Microcontact printing is a form of soft lithography
that uses the relief patterns on a master Polydimethylsiloxane (PDMS) stamp
to form patterns of self-assembled monolayers (SAMs) of ink on the surface of a substrate
through conformal contact. Its applications are wide ranging including microelectronics
, surface chemistry and cell biology
.
and stamp printing have been around for centuries. However, the combination of the two gave rise to the method of microcontact printing. The method was first introduced by George M. Whitesides
and Amit Kumar at Harvard University
. Since its inception many methods of soft lithography have been explored.
techniques. The master is typically created on silicon
, but can be done on any solid patterned surface. Photoresist
is applied to the surface and patterned by a photomask
and UV light. The master is then baked, developed and cleaned before use. In typical processes the photoresist is usually kept on the wafer to be used as a topographic template for the stamp. However, the unprotected silicon regions can be etched, and the photoresist stripped, which would leave behind a patterned wafer for creating the stamp. This method is more complex but creates a more stable template.
, and the stamp is poured over the master.
The PDMS stamp, in most applications, is a 10:1 ratio of silicone
elastomer
and a silicone elastomer curing agent. This mixture consists of a short hydrosilane crosslinker that contains a catalyst made from a platinum
complex. After pouring, the PDMS is cured at elevated temperatures to create a solid polymer
with elastomeric properties. The stamp is then peeled off and cut to the proper size. The stamp replicates the opposite of the master. Elevated regions of the stamp correspond to indented regions of the master.
solution either by immersion or coating the stamp with a Q-tip. The highly hydrophobic PDMS material allows the ink to be diffused into the bulk of the stamp, which means the thiols reside not only on the surface, but also in the bulk of the stamp material. This diffusion into the bulk creates an ink reservoir for multiple prints. The stamp is let dry until no liquid is visible to the eye and an ink reservoir
is created.
Deformation of the stamp can occur during removal from the master and during the substrate contacting process. When the aspect ratio
of the stamp is high buckling
of the stamp can occur. When the aspect ratio is low roof collapse can occur
Swelling of the stamp may also occur. Most organic solvents induce swelling of the PDMS stamp. Ethanol in particular has a very small swelling effect, but many other solvents cannot be used for wet inking because of high swelling. Because of this the process is limited to apolar inks that are soluble in ethanol.
. For this application inking solutions commonly consist of a solution of alkanethiol. This method uses metal substrates with the most common metal being gold
. However, silver
, copper
, and palladium
have been proven to work as well.
Once the ink has been applied to the substrate the SAM layer acts as a resist to common wet etching
techniques allowing for the creation of high resolution patterning. The patterned SAMs layer is a step in a series of steps to create complex microstructures. For example, applying the SAM layer on top of gold and etching creates microstructures of gold. After this step etched areas of gold exposes the substrate which can further be etched using traditional anisotropic etch techniques. Because of the microcontact printing technique no traditional photolithography is needed to accomplish these steps.
s has helped the advancement of biosensor
s., cell biology research ., and tissue engineering
. Various proteins have been proven to be suitable inks and are applied to various substrates using the microcontact printing technique. Polylysine
, immunoglobulin antibody, and different enzyme
s have been successfully placed onto surfaces including glass, polystyrene
, and hydrophobic silicon.
has also been done using this technique. The reduction in time and DNA material are the critical advantages for using this technique. The stamps were able to be used multiple times that were more homogeneous and sensitive than 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....
that uses the relief patterns on a master Polydimethylsiloxane (PDMS) stamp
PDMS stamp
PDMS stamps are pieces of Polydimethylsiloxane that have been patterned usually against a master to form a relief pattern used in soft lithography...
to form patterns of self-assembled monolayers (SAMs) of ink on the surface of a substrate
Substrate (printing)
Substrate is a term used in converting process such as printing and Lamination or coating as a more general term to describe the base material onto which e.g. images will be printed and to be laminated as per the packing specification required for the product...
through conformal contact. Its applications are wide ranging including microelectronics
Microelectronics
Microelectronics is a subfield of electronics. As the name suggests, microelectronics relates to the study and manufacture of very small electronic components. Usually, but not always, this means micrometre-scale or smaller,. These devices are made from semiconductors...
, surface chemistry and cell biology
Cell biology
Cell biology is a scientific discipline that studies cells – their physiological properties, their structure, the organelles they contain, interactions with their environment, their life cycle, division and death. This is done both on a microscopic and molecular level...
.
History
Both lithographyLithography
Lithography is a method for printing using a stone or a metal plate with a completely smooth surface...
and stamp printing have been around for centuries. However, the combination of the two gave rise to the method of microcontact printing. The method was first introduced by George M. Whitesides
George M. Whitesides
George M. Whitesides is an American chemist and professor of chemistry at Harvard University. He is best known for his work in the areas of NMR spectroscopy, organometallic chemistry, molecular self-assembly, soft lithography, microfabrication, microfluidics, and nanotechnology...
and Amit Kumar at Harvard University
Harvard University
Harvard University is a private Ivy League university located in Cambridge, Massachusetts, United States, established in 1636 by the Massachusetts legislature. Harvard is the oldest institution of higher learning in the United States and the first corporation chartered in the country...
. Since its inception many methods of soft lithography have been explored.
Preparing the Master
Creation of the master, or template, is done using traditional photolithographyPhotolithography
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...
techniques. The master is typically created on silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
, but can be done on any solid patterned surface. Photoresist
Photoresist
A photoresist is a light-sensitive material used in several industrial processes, such as photolithography and photoengraving to form a patterned coating on a surface.-Tone:Photoresists are classified into two groups: positive resists and negative resists....
is applied to the surface and patterned by a 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:...
and UV light. The master is then baked, developed and cleaned before use. In typical processes the photoresist is usually kept on the wafer to be used as a topographic template for the stamp. However, the unprotected silicon regions can be etched, and the photoresist stripped, which would leave behind a patterned wafer for creating the stamp. This method is more complex but creates a more stable template.
Creating the PDMS Stamp
After fabrication the master is placed in a walled container, typically a petri dishPetri dish
A Petri dish is a shallow glass or plastic cylindrical lidded dish that biologists use to culture cells or small moss plants. It was named after German bacteriologist Julius Richard Petri, who invented it when working as an assistant to Robert Koch...
, and the stamp is poured over the master.
The PDMS stamp, in most applications, is a 10:1 ratio of silicone
Silicone
Silicones are inert, synthetic compounds with a variety of forms and uses. Typically heat-resistant and rubber-like, they are used in sealants, adhesives, lubricants, medical applications , cookware, and insulation....
elastomer
Elastomer
An elastomer is a polymer with the property of viscoelasticity , generally having notably low Young's modulus and high yield strain compared with other materials. The term, which is derived from elastic polymer, is often used interchangeably with the term rubber, although the latter is preferred...
and a silicone elastomer curing agent. This mixture consists of a short hydrosilane crosslinker that contains a catalyst made from a platinum
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
complex. After pouring, the PDMS is cured at elevated temperatures to create a solid polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
with elastomeric properties. The stamp is then peeled off and cut to the proper size. The stamp replicates the opposite of the master. Elevated regions of the stamp correspond to indented regions of the master.
Inking the Stamp
Inking of the stamp occurs through the application of a thiolThiol
In organic chemistry, a thiol is an organosulfur compound that contains a carbon-bonded sulfhydryl group...
solution either by immersion or coating the stamp with a Q-tip. The highly hydrophobic PDMS material allows the ink to be diffused into the bulk of the stamp, which means the thiols reside not only on the surface, but also in the bulk of the stamp material. This diffusion into the bulk creates an ink reservoir for multiple prints. The stamp is let dry until no liquid is visible to the eye and an ink reservoir
Reservoir
A reservoir , artificial lake or dam is used to store water.Reservoirs may be created in river valleys by the construction of a dam or may be built by excavation in the ground or by conventional construction techniques such as brickwork or cast concrete.The term reservoir may also be used to...
is created.
Direct Contact
Applying the stamp to the substrate is easy and straightforward which is one of the main advantages of this process. The stamp is brought into physical contact with the substrate and the thiol solution is transferred to the substrate. The thiol is area-selectively transferred to the surface based on the features of the stamp. During the transfer the carbon chains of the thiol align with each other to create a hydrophobic self-assembling monolayer (SAM).Other Application Techniques
Printing of the stamp onto the substrate, although not used as often, can also take place with a rolling stamp onto a planar substrate or a curved substrate with a planar stamp.Advantages
Microcontact Printing has several advantages including:- The simplicity and ease of creating patterns with micro-scale features
- Can be done in a traditional laboratory without the constant use of a cleanroomCleanroomA cleanroom is an environment, typically used in manufacturing or scientific research, that has a low level of environmental pollutants such as dust, airborne microbes, aerosol particles and chemical vapors. More accurately, a cleanroom has a controlled level of contamination that is specified by...
(cleanroom is needed only to create the master). - Multiple stamps can be created from a single master
- Individual stamps can be used several times with minimal degradation of performance
- A cheaper technique for fabrication that uses less energy than conventional techniques
Technical Problems with Microcontact Printing
After this technique became popular various limitations and problems arose, all of which affected patterning and reproducibility.Stamp Deformation
During direct contact one must be careful because the stamp can easily be physically deformed causing printed features that are different than the original stamp features. Horizontally stretching or compressing the stamp will cause deformations in the raised and recessed features. Also, applying too much vertical pressure on the stamp during printing can cause the raised relief features to flatten against the substrate. These deformations can yield submicron features even though the original stamp has a lower resolution.Deformation of the stamp can occur during removal from the master and during the substrate contacting process. When the aspect ratio
Aspect ratio
The aspect ratio of a shape is the ratio of its longer dimension to its shorter dimension. It may be applied to two characteristic dimensions of a three-dimensional shape, such as the ratio of the longest and shortest axis, or for symmetrical objects that are described by just two measurements,...
of the stamp is high buckling
Buckling
In science, buckling is a mathematical instability, leading to a failure mode.Theoretically, buckling is caused by a bifurcation in the solution to the equations of static equilibrium...
of the stamp can occur. When the aspect ratio is low roof collapse can occur
Substrate Contamination
During the curing process some fragments can potentially be left uncured and contaminate the process. When this occurs the quality of the printed SAM is decreased. When the ink molecules contain certain polar groups the transfer of these impurities is increased.Shrinking/Swelling of the Stamp
During the curing process the stamp can potentially shrink in size leaving a difference in desired dimensions of the substrate patterning.Swelling of the stamp may also occur. Most organic solvents induce swelling of the PDMS stamp. Ethanol in particular has a very small swelling effect, but many other solvents cannot be used for wet inking because of high swelling. Because of this the process is limited to apolar inks that are soluble in ethanol.
Ink Mobility
Ink diffusion from the PDMS bulk to the surface occurs during the formation of the patterned SAM on the substrate. This mobility of the ink can cause lateral spreading to unwanted regions. Upon the transfer this spreading can influence the desired pattern.Applications
Depending on the type of ink used and the subsequent substrate the microcontact printing technique has many different applicationsMicromachining
Microcontact printing has great applications in micromachiningMicromachining
Micromachining may refer to:*Superfinishing, a metalworking process for producing very fine surface finishes*Various microelectromechanical systems**Bulk micromachining**Surface micromachining**High-aspect-ratio microstructure technologies-See also:...
. For this application inking solutions commonly consist of a solution of alkanethiol. This method uses metal substrates with the most common metal being gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
. However, silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
, copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
, and palladium
Palladium
Palladium is a chemical element with the chemical symbol Pd and an atomic number of 46. It is a rare and lustrous silvery-white metal discovered in 1803 by William Hyde Wollaston. He named it after the asteroid Pallas, which was itself named after the epithet of the Greek goddess Athena, acquired...
have been proven to work as well.
Once the ink has been applied to the substrate the SAM layer acts as a resist to common wet etching
Wet etching
Wet etching may refer to:*Industrial etching*Etching...
techniques allowing for the creation of high resolution patterning. The patterned SAMs layer is a step in a series of steps to create complex microstructures. For example, applying the SAM layer on top of gold and etching creates microstructures of gold. After this step etched areas of gold exposes the substrate which can further be etched using traditional anisotropic etch techniques. Because of the microcontact printing technique no traditional photolithography is needed to accomplish these steps.
Patterning Proteins
The patterning of proteinProtein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s has helped the advancement of biosensor
Biosensor
A 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...
s., cell biology research ., and tissue engineering
Tissue engineering
Tissue engineering is the use of a combination of cells, engineering and materials methods, and suitable biochemical and physio-chemical factors to improve or replace biological functions...
. Various proteins have been proven to be suitable inks and are applied to various substrates using the microcontact printing technique. Polylysine
Polylysine
Polylysine is a small natural homopolymer of the essential amino acid L-lysine that is produced by bacterial fermentation. ε-Poly-L-lysine is used as a natural preservative in food products.-Chemical structure & function:...
, immunoglobulin antibody, and different enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
s have been successfully placed onto surfaces including glass, polystyrene
Polystyrene
Polystyrene ) also known as Thermocole, abbreviated following ISO Standard PS, is an aromatic polymer made from the monomer styrene, a liquid hydrocarbon that is manufactured from petroleum by the chemical industry...
, and hydrophobic silicon.
Patterning Cells
Microcontact printing has been used to advance the understanding of how cells interact with substrates. This technique has helped improve the study of cell patterning that was not possible with traditional cell culture techniques.Patterning DNA
Successful patterning of DNADNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
has also been done using this technique. The reduction in time and DNA material are the critical advantages for using this technique. The stamps were able to be used multiple times that were more homogeneous and sensitive than other techniques.
Technique Improvements
To help overcome the limitations set by the original technique several alternatives have been developed.- High-Speed printing: Successful contact printing was done on a gold substrate with a contact time in the range of milliseconds. This printing time is three orders of magnitude shorter than the normal technique, yet successfully transformed the pattern. The process of contact was automated to achieve these speeds through a piezoelectric actuatorActuatorAn actuator is a type of motor for moving or controlling a mechanism or system. It is operated by a source of energy, usually in the form of an electric current, hydraulic fluid pressure or pneumatic pressure, and converts that energy into some kind of motion. An actuator is the mechanism by which...
. At these low contact times the surface spreading of thiol did not occur, greatly improving the pattern uniformity - Submerged Printing: By submerging the stamp in a liquid medium stability was greatly increased. By printing hydrophobic long-chain thiols underwater the common problem of vapor transport of the ink is greatly reduced. PDMS aspect ratios of 15:1 were achieved using this method, which was not accomplished before
- Contact Inking: as opposed to wet inking this technique does not permeate the PDMS bulk. The ink molecules only contact the protruding areas of the stamp that are going to be used for the patterning. The absence of ink on the rest of the stamp reduces the amount of ink transferred through the vapor phase that can potentially affect the pattern. This is done by the direct contact of a feature stamp and a flat PDMS substrate that has ink on it.[
- New Stamp Materials: In order to create uniform transfer of the ink the stamp needs to be both mechanically stable and also be able to create conformal contact well. These two characteristics are juxtaposed because high stability requires a high Young’s Modulus while efficient contact requires an increase in elasticityElasticityElasticity may refer to:*Elasticity , continuum mechanics of bodies that deform reversibly under stressNumerous uses are derived from this physical sense of the term, which is inherently mathematical, such as used in Engineering, Chemistry, Construction and variously in Economics:*Elasticity , the...
. A compositeComposite materialComposite materials, often shortened to composites or called composition materials, are engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct at the macroscopic or...
, thin PDMS stamp with a rigid back support has been used for patterning to help solve this problem.