Ion-Beam sculpting is a two-step process to make solid-state

nanopore A nanopore is a pore of nanometer size. It may, for example, be created by a pore-forming protein or as a hole in synthetic materials such as silicon or graphene. When a nanopore is present in an electrically insulating membrane, it can be used as ...

s. The term itself was coined by Golovchenko and co-workers at Harvard in the paper "Ion-beam sculpting at nanometer length scales." In the process, solid-state nanopores are formed by lateral mass transport about the surface of the substrate, not simply by

sputtering In physics, sputtering is a phenomenon in which microscopic particles of a solid material are ejected from its surface, after the material is itself bombarded by energetic particles of a plasma or gas. It occurs naturally in outer space, and ...

, which is the removal of material from the surface.

Basis

The first step in ion sculpting is to make either a through hole or a

blind hole A hole is an opening in or through a particular medium, usually a solid body. Holes occur through natural and artificial processes, and may be useful for various purposes, or may represent a problem needing to be addressed in many fields of en ...

, most commonly using a

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. A FIB setup is a s ...

(FIB). The holes are commonly ~100 nm, but can be made much smaller. This step may or may not be done at

room temperature Colloquially, "room temperature" is a range of air temperatures that most people prefer for indoor settings. It feels comfortable to a person when they are wearing typical indoor clothing. Human comfort can extend beyond this range depending on ...

, 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 The following are examples of orders of magnitude for different lengths. __TOC__ Overview Detailed list To help compare different orders of magnitude, the following list describes various lengths between 1.6 \times 10^ metres and 10 ...

Broad area ion exposure

This technique uses a broad area

argon Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as a ...

ion source An ion source is a device that creates atomic and molecular ions. Ion sources are used to form ions for mass spectrometers, optical emission spectrometers, particle accelerators, ion implanters and ion engines. Electron ionization Electro ...

beam. If the hole is blind (a

is a hole that has not broken through on the backside yet) the

wafer A wafer is a crisp, often sweet, very thin, flat, light and dry biscuit, often used to decorate ice cream, and also used as a garnish on some sweet dishes. Wafers can also be made into cookies with cream flavoring sandwiched between them. They ...

(often

SiN In a religious context, sin is a transgression against divine law. Each culture has its own interpretation of what it means to commit a sin. While sins are generally considered actions, any thought, word, or act considered immoral, selfish, ...

or silicon/

silicon oxide Silicon oxide may refer to either of the following: *Silicon dioxide Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many part ...

) is then turned upside down, and exposed with the

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 The noble gases (historically also the inert gases; sometimes referred to as aerogens) make up a class of chemical elements with similar properties; under standard conditions, they are all odorless, colorless, monatomic gases with very low che ...

es, not just argon.

TEM exposure

A through hole in a wafer can be closed down by a

transmission electron microscope Transmission electron microscopy (TEM) is a microscopy technique in which a beam of electrons is transmitted through a specimen to form an image. The specimen is most often an ultrathin section less than 100 nm thick or a suspension on a gr ...

. Due to hydrocarbon buildup, the electrons stimulate hole closure. This method is very slow (taking over an hour to close a

100 nm The following are examples of orders of magnitude for different lengths. __TOC__ Overview Detailed list To help compare different orders of magnitude, the following list describes various lengths between 1.6 \times 10^ metres and 10 ...

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 FIB, 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 harder 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 An ion beam is a type of charged particle beam consisting of ions. Ion beams have many uses in electronics manufacturing (principally ion implantation) and other industries. A variety of ion beam sources exists, some derived from the mercu ...

is continually

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.

References

{{DEFAULTSORT:Ion-Beam Sculpting Nanotechnology

Basis

Broad area ion exposure

TEM exposure

FIB exposure

See also

References