Manufacturing Bits: May 13

Telling a FIB; stretchable molecular electronics; flexible device process.


Telling a FIB
The National Institute of Standards and Technology (NIST) has built the first low-energy focused ion beam (FIB) microscope that uses a lithium ion source.

Still in the R&D stage, the FIB microscope from NIST could be used to examine adjacent materials that are chemically different and identify the elements that make them up. The FIB microscope uses an ion source based on photoionization of laser-cooled lithium atoms. The microscope has various probe sizes and beam energies from 500 eV to 5 keV.

The NIST focused lithium ion beam microscope traps and cools a gas of lithium atoms to just a few millionths of a degree above absolute zero. (Source: NIST).

The resolution doesn’t quite match a scanning electron microscope (SEM) or a helium ion microscope (HIM). But the new FIB-based system can image nonconductive materials better than high-energy SEMs and FIBs. In fact, the beam energies in the new FIB are much lower than the typical operating energies of HIMs or gallium focused ion beam systems.

In operation, the new FIB-based system first cools a gas of neutral lithium atoms to a temperature of about 600 microkelvins. This is done by using lasers and a magneto-optical trap (MOT) to hold the atoms. Another laser ionizes the atoms and then electric fields accelerate them.

In one application, the tool could help solve a problem in nanoimprint lithography. “Before manufacturers can etch the silicon, they have to make sure the spaces are free of chemical residue,” said Jabez McClelland of NIST on the agency’s Web site. “Commonly, they use a process called plasma etching to clean that residue off, but they have to be careful not to overdo it or they can damage the substrate and ruin the chip. Our FIB scope could check to see if the plasma has done its work without damaging the chip. A scanning electron microscope couldn’t do this because it’s difficult to see the thin residue, and the high-energy beam is likely to charge up and/or melt the stencil and make the problem worse.”

Another potential application is to see how lithium batteries work. Using the microscope, researchers will inject lithium ions into the materials and will see how they affect the behavior of the batteries. “This new form of microscopy we’ve developed promises to provide a new tool for nanotechnology with good surface sensitivity, elemental contrast and high resolution,” McClelland said. “The applications range from nanofabrication process control to nanomaterial development and imaging of biomaterials.”

Stretchable molecular electronics
Flexible electronic devices will enable a new class of applications, such as displays, wearable sensors, RFIDs and other products.

The University of California at San Diego has devised a new class of molecular materials for use in flexible electronic devices. The molecular structures permit deformation without the loss of electronic function.

Molecularly stretchable electronics (Source: UC San Diego).

This technology is called molecularly stretchable electronics, which are superior over today’s flexible electronics. Today’s flexible electronics enable a new generation of wearable sensors and other products, but they use hard composite materials that limit their elasticity.

In contrast, stretchable electronics make use of molecular materials, based on non-composite conductors and semiconductors. These materials accommodate strain intrinsically by the rational design of their chemical structures.

“We are developing the design rules for a new generation of plastic–or, better, rubber–electronics for applications in energy, biomedical devices, wearable and conformable devices for defense applications, and for consumer electronics,” said U.C. San Diego Jacobs School of Engineering professor Darren Lipomi on the university’s Web site. “We are taking these design rules and doing wet chemistry in the lab to make new semiconducting rubber materials.”

One of the applications for these materials is a flexible solar tarp. Another goal is to produce electronic polymers for medical applications, such as implantable biomedical devices and prosthetics.

In fact, researchers recently devised a series of poly(3-alkylthiophene)s or P3ATs, a class of materials for use in stretchable electronics. P3ATs with longer alkyl side chains (n≥8) have high elasticity and ductility, but they have poor electronic performance, according to researchers. P3ATs with shorter chains (n≤ 6) exhibit the opposite characteristics.

Researchers have also devised a separate high-performance, low-bandgap elastic semiconducting polymer using a new synthetic technology. The material is PDPP2FT, an alternating copolymer. It comprises of an N-alkylated diketopyrrolopyrrole (DPP) unit flanked by two furan rings (2F) alternating with thiophene (T). In the modified polymer, PDPP2FT-seg-2T, the DPP is exchanged for a tail-to-tail coupled unit of two 3-hexylthiophene rings (bithiophene, 2T) in an average of one of approximately five repeat units.

“The tensile modulus of the segmented polymer, 0.93 ± 0.16 GPa, is lower than that of the homopolymer, 2.17 ± 0.35 GPa,” according to researchers. “When blended with PC61BM, the segmented material produces devices with power conversion efficiencies of 2.82 ± 0.28%, which is similar to that of PDPP2FT, 2.52 ± 0.34%. These results suggest that it is possible to increase the mechanical resiliency of semiconducting polymers for solar cells without having a deleterious effect on the photovoltaic properties.”

Flexible device process
Making sensors or electronic circuits on ultra-thin polymeric films is a difficult task using conventional semiconductor processes.

One group claims to have made a major breakthrough. The University of Tokyo and the Japan Science and Technology Agency have recently developed ultra-thin, soft organic thin-film transistors and integrated circuits. This includes a 2D array of amplifiers on polymeric films with a thickness of only 1.2μm.

Researchers manufactured organic TFTs on 1.2-μm-thick polyethylenenaphthalate (PEN) films. The structures weigh 3 g/m2. This was possible using a technique to form a 19-nm-thick insulating layer on the rough surface of the polymeric film.

“First, Al gate layers are deposited on the base film by the vacuum system with a shadow mask. Gate dielectric layers comprise anodic aluminium oxide and a phosphonic acid self-assembled monolayer (SAM), according to researchers.

“A 30-nm-thick dinaphtho[2,3-b:29,39-f]thieno[3,2-b]thiophene (DNTT) layer is deposited as air-stable organic semiconductor by vacuum evaporation,’’ according to researchers. “Finally, Au layers are deposited as source/drain electrodes by vacuum evaporation. The channel length and width are 40μm and 500μm, respectively. The transistors exhibit a saturation mobility of 3 cm2/ Vs.”