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 artificial membrane, membra ...

s. The term itself was coined by Golovchenko and co-workers at

Harvard Harvard University is a private Ivy League research university in Cambridge, Massachusetts, United States. Founded in 1636 and named for its first benefactor, the Puritan clergyman John Harvard, it is the oldest institution of higher lear ...

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 c ...

, 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 (not penetrating completely), 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 sc ...

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

room temperature Room temperature, colloquially, denotes the range of air temperatures most people find comfortable indoors while dressed in typical clothing. Comfortable temperatures can be extended beyond this range depending on humidity, air circulation, and ...

, with a low temperature of -120 C. Next, three common techniques can be used to 'sculpt' the hole: broad area ion exposure, TEM exposure, and FIB exposure. Holes can be closed completely, or left open at a lower limit of 1 -

10 nm 10nm and 10 nm may refer to: * 10 nm process In semiconductor fabrication, the International Technology Roadmap for Semiconductors (ITRS) defines the "10 nanometer process" as the MOSFET technology node following the "14 nm" node. Since at ...

Broad area ion exposure

This technique uses a broad area

argon Argon is a chemical element; it has 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 abu ...

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 Elect ...

beam. If the hole is blind (a blind hole 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 biscuit, often used to decorate ice cream, and also used as a garnish on some sweet dishes. They frequently have a waffle surface pattern but may also be patterned with insignia of the foo ...

(often

SiN In religious context, sin is a transgression against divine law or a law of the deities. 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 ...

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 , commonly found in nature as quartz. In many parts of the world, silica is the maj ...

) 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 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 back 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 about 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, the hole is closed. This process of nanopore fabrication is used by the laboratories of J. Li and J. Golovchenko. As of 2006 this method was shown to be applicable with all the

noble gas The noble gases (historically the inert gases, sometimes referred to as aerogens) are the members of Group (periodic table), group 18 of the periodic table: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn) and, in some ...

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 hole; this has the advantage that the hole can be watched as it reduces, allowing good control of the hole size, but 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, the hole can be imaged as with the TEM technique. The ions stimulate movement on the wafer, and also implant themselves to help close the hole. Unlike for the other two methods, the holes closed in this technique are not smooth or accurately circular; they appear jagged under TEM photos. Also, it is much harder to control the size of the hole to the single nanometer regime. Another drawback is that while imaging the hole the

ion beam An ion beam is a beam of ions, a type of charged particle beam. Ion beams have many uses in electronics manufacturing (principally ion implantation) and other industries. There are many ion beam sources, some derived from the mercury vapor ...

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 can also 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