Ion-beam sculpting
Encyclopedia
Ion-Beam scultping is a term used to describe a two-step process to make solid-state nanopore
s. The term itself was coined by Golovchenko and co-workers at Harvard
in the paper "Ion-beam sculpting at nanometer length scales." The term refers to the fact that solid-state nanopores are formed by lateral mass transport about the surface of the substrate, not simply by sputtering
which refers to the removal of material from the surface.
or a blind hole
, most commonly using an focused ion beam
(FIB). The holes are commonly ~100 nm, but can be made much smaller. This step may or may not be done at room temperature
, with a low temperature of -120 C. Next, there are three common techniques to now 'sculpt' the hole: broad area ion exposure, TEM exposure, and FIB exposure. Holes can be closed completely, but also they can be left open at a lower limit
of 1 - 10 nm
.
ion source
beam. If the hole is blind (a blind hole is a hole that has not broken through on the backside yet) the wafer (often SiN or silicon oxide
) is then turned upside down, and exposed with the argon
beam. A detector counts the amount of ions passing through the membrane (which should be zero). The process stops when ions begin to be detected. This enables for a much smaller hole to be opened than if using an FIB alone. This method of nanopore fabrication relies on the ion beam to remove (sputter) some of the material from the backside of the sample, revealing part of the hole underneath.
Alternatively, if the hole has already been milled through the substrate, the argon beam is aimed at the wafer, and by lateral mass transport atoms from elsewhere on the wafer move to the edge of the hole. It is this process of solid-state nanopore fabrication that was originally termed "ion-beam sculpting". Of paramount importance in this method is the ability to utilize a feedback controlled system to monitor nanopore fabrication in real time. A detector registers the number of ions passing through the hole as a function of time. As the hole closes from ~100 nm to its final dimension (>20 nm) the number of ions able to pass through the hole is reduced. The process is stopped when the final pore size is reached. If the current drops to zero, then the hole is closed. This process of nanopore fabrication is used by the labs of Dr. J. Li and J. Golovchenko. Recently this method has been demonstrated to occur with all the noble gas
es, not just argon.
hole). The slow method allows for great control of the hole size (since you can watch the hole decrease), but its drawback is that it takes a long time. Citation: T.Schenkel, V.Radmilovic, E.A.Stach, S.-J.Park, A.Persaud, J.Va.Sci.Tech.B 21, 2720 (2003).
, one can just image the hole (analogous to the TEM technique). The ions stimulate movement on the wafer, and also implant themselves to help close the hole. Unlike the other two methods, the holes closed in this technique are not very circular and smooth. The holes appear jagged under TEM photos. Also, it is much hard to control the size of the hole to the single nanometer regime. Another drawback to this technique is that while imaging the hole, the ion beam
is continually sputtering
membrane material away. If the beam scan area is large enough, the rate of atoms moving to close the hole will be greater than the rate of sputtering, so the hole will close. If the membrane is too thin or the scan area too small, then the rate of sputtering will win, and the hole will open up.
An alternative ion beam sculpting technique has been developed using a commercially available FIB system. This sculpting method can fabricate symmetrically circular nanopores with smooth edge, and, in addition, it can sculpt multiple nanopores of similar shape and size simultaneously. Dependent on the resolution and working condition of the instrument, this method can produce symmetrically shaped nanopores with diameters below 10 nm.
Nanopore
A nanopore is a small hole. It may, for example, be created by a pore-forming protein or as a hole in synthetic materials such as silicon or graphene....
s. The term itself was coined by Golovchenko and co-workers at Harvard
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...
in the paper "Ion-beam sculpting at nanometer length scales." The term refers to the fact that solid-state nanopores are formed by lateral mass transport about the surface of the substrate, not simply by sputtering
Sputtering
Sputtering is a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic particles. It is commonly used for thin-film deposition, etching and analytical techniques .-Physics of sputtering:...
which refers to the removal of material from the surface.
Basis
The first step in ion sculpting is to make either a through holeThrough hole
A through hole refers to a hole that is reamed, drilled, milled etc., completely through the substrate. In other words, a through hole is a hole that goes all the way through something...
or a blind hole
Hole Blind
A blind hole refers to a hole that is reamed, drilled, or milled to a specified depth, thus without breaking through to the other side of the workpiece. The etymology is that it is not possible to see through a blind hole. Other types of holes also include through holes, and clearance holes. In...
, most commonly using an focused ion beam
Focused ion beam
Focused ion beam, also known as FIB, is a technique used particularly in the semiconductor industry, materials science and increasingly in the biological field for site-specific analysis, deposition, and ablation of materials. An FIB setup is a scientific instrument that resembles a scanning...
(FIB). The holes are commonly ~100 nm, but can be made much smaller. This step may or may not be done at room temperature
Room temperature
-Comfort levels:The American Society of Heating, Refrigerating and Air-Conditioning Engineers has listings for suggested temperatures and air flow rates in different types of buildings and different environmental circumstances. For example, a single office in a building has an occupancy ratio per...
, with a low temperature of -120 C. Next, there are three common techniques to now 'sculpt' the hole: broad area ion exposure, TEM exposure, and FIB exposure. Holes can be closed completely, but also they can be left open at a lower limit
Limit superior and limit inferior
In mathematics, the limit inferior and limit superior of a sequence can be thought of as limiting bounds on the sequence...
of 1 - 10 nm
1 E-8 m
To help compare different orders of magnitude this page lists lengths between 10−8 and 10−7 metres .Distances shorter than 10 nanometres*10 nm = 10 nanometres = 10−8 metres*10 nm — lower size of tobacco smoke...
.
Broad area ion exposure
This technique uses a broad area argonArgon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...
ion source
Ion source
An ion source is an electro-magnetic device that is used to create charged particles. These are used primarily to form ions for mass spectrometers, optical emission spectrometers, particle accelerators, ion implanters and ion engines.- Electron ionization :...
beam. If the hole is blind (a blind hole is a hole that has not broken through on the backside yet) the wafer (often SiN or silicon oxide
Silicon oxide
Silicon oxide may refer to either of the following:*Silicon dioxide, SiO2, very well characterized*Silicon monoxide, SiO, not very well characterized...
) is then turned upside down, and exposed with the argon
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...
beam. A detector counts the amount of ions passing through the membrane (which should be zero). The process stops when ions begin to be detected. This enables for a much smaller hole to be opened than if using an FIB alone. This method of nanopore fabrication relies on the ion beam to remove (sputter) some of the material from the backside of the sample, revealing part of the hole underneath.
Alternatively, if the hole has already been milled through the substrate, the argon beam is aimed at the wafer, and by lateral mass transport atoms from elsewhere on the wafer move to the edge of the hole. It is this process of solid-state nanopore fabrication that was originally termed "ion-beam sculpting". Of paramount importance in this method is the ability to utilize a feedback controlled system to monitor nanopore fabrication in real time. A detector registers the number of ions passing through the hole as a function of time. As the hole closes from ~100 nm to its final dimension (>20 nm) the number of ions able to pass through the hole is reduced. The process is stopped when the final pore size is reached. If the current drops to zero, then the hole is closed. This process of nanopore fabrication is used by the labs of Dr. J. Li and J. Golovchenko. Recently this method has been demonstrated to occur with all the noble gas
Noble gas
The noble gases are a group of chemical elements with very similar properties: under standard conditions, they are all odorless, colorless, monatomic gases, with very low chemical reactivity...
es, not just argon.
TEM exposure
A through hole in a wafer can be closed down by a transmission electron microscope. Due to hydrocarbon buildup, the electrons stimulate hole closure. This method is very slow (taking over an hour to close a 100 nm1 E-7 m
To help compare different orders of magnitude this page lists lengths between 10−7 and 10−6 m .Distances shorter than 100 nm*100 nm — greatest particle size that can fit through a surgical mask...
hole). The slow method allows for great control of the hole size (since you can watch the hole decrease), but its drawback is that it takes a long time. Citation: T.Schenkel, V.Radmilovic, E.A.Stach, S.-J.Park, A.Persaud, J.Va.Sci.Tech.B 21, 2720 (2003).
FIB exposure
This is the easiest of the techniques, but the least useful. After a hole is milled with an FIBFib
Fib may refer to:* A form of lying that is usually forgiven because it is not intended to deceive.* Fib , a form of poetry similar to haiku* Fib, a Pictish kingdom, which went on to become Fife, in modern Scotland...
, one can just image the hole (analogous to the TEM technique). The ions stimulate movement on the wafer, and also implant themselves to help close the hole. Unlike the other two methods, the holes closed in this technique are not very circular and smooth. The holes appear jagged under TEM photos. Also, it is much hard to control the size of the hole to the single nanometer regime. Another drawback to this technique is that while imaging the hole, the ion beam
Ion beam
An ion beam is a type of charged particle beam consisting of ions. Ion beams have many uses in electronics manufacturing and other industries. A variety of ion beam sources exist, some derived from the mercury vapor thrusters developed by NASA in the 1960s.-Ion beam etching or sputtering:One type...
is continually sputtering
Sputtering
Sputtering is a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic particles. It is commonly used for thin-film deposition, etching and analytical techniques .-Physics of sputtering:...
membrane material away. If the beam scan area is large enough, the rate of atoms moving to close the hole will be greater than the rate of sputtering, so the hole will close. If the membrane is too thin or the scan area too small, then the rate of sputtering will win, and the hole will open up.
An alternative ion beam sculpting technique has been developed using a commercially available FIB system. This sculpting method can fabricate symmetrically circular nanopores with smooth edge, and, in addition, it can sculpt multiple nanopores of similar shape and size simultaneously. Dependent on the resolution and working condition of the instrument, this method can produce symmetrically shaped nanopores with diameters below 10 nm.