System Bits: Feb. 24

New solder for semis
A research team led by the University of Chicago has demonstrated how semiconductors can be soldered and still deliver good electronic performance by working out new chemistry for a broad class of compositions relevant to semiconductors.

The compounds that the team developed can be used to join pieces of semiconductor, which researchers have longed struggled with.

The researchers said if two pieces of semiconductor are put next to each other, each joint will be unique, and in most cases, will block transport of charges. A reasonably good electronic circuit can’t be made by simply taking different semiconductor pieces and pressing them against each other as with metals.

The UChicago team along with colleagues from Argonne National Laboratory and the Illinois Institute of Technology created the compounds of cadmium, lead and bismuth that can be applied as a liquid or paste to join two pieces of a semiconductor by heating them to several hundred degrees Celsius, which is mild by industry standards.

Semiconductor devices like this one now can be joined electronically with a new solder developed by a UChicago research group. (Source: University of Chicago)

They said the paste converts cleanly into a material that will be compositionally matched to the bonded parts, and that required development of new chemistry. To do this, they designed special molecules that fulfill this requirement so that they do not contaminate the material. After application as a liquid or paste, they decompose to form a seamless joint.

Spintronics compound
According to researchers at the University of Michigan, a new semiconductor compound could bringing fresh momentum to the field of spintronics.

The new compound is what the researchers said is the first to build spintronic properties into a material that’s stable at room temperature, is easily tailored to a variety of applications, and was created from a unique low-symmetry crystal structure. It could eventually be used as the base material for spintronic processors and other devices, much like silicon is the base for electronic computing devices.

The team reminded that spintronics use both the “on” or “off” electrical charge and the “up” or “down” magnetic spin of electrons to store information, compared to today’s electronics that use only electrical charge. Spin-based circuits can be smaller than charge-based circuits, enabling device makers to pack more of them onto a single processor. This is a key advantage, since traditional electronics are approaching their physical size limits.

An electronic circuit can only be made so small before the charge of an electron becomes erratic, but the spin of electrons remains stable at much smaller sizes, so spintronic devices open the door to a whole new generation of computing.

Since spintronic semiconductors require precise levels of both magnetism and conductivity, researchers have struggled to create one that can be easily tuned to the levels required and that maintains its properties over a range of temperatures. The root of the problem lies in the crystalline structure that makes up semiconductors.

Today’s semiconductors are made of crystals with simple, symmetrical patterns, like a microscopic lattice that repeats over and over. The properties of those semiconductors are controlled by adding atoms of different elements to the holes in that lattice. For example, bismuth can be added to increase conductivity, or iron to increase magnetism.

Juan Lopez, MSE PhD Student, constructs a semiconductor used for spintronics. The semiconductor has a novel “low symmetry” crystal structure that allows for researchers to build spintronic properties into material that is stable and room temperature and can be used for a variety of applications. (Image credit: Joseph Xu, Michigan Engineering)

To make spintronic semiconductors, atoms of different sizes have to be added, and flexibility is needed in where those atoms are placed. But in most commonly used crystals, the holes are all similarly sized and regularly spaced, which gives a very limited amount of control.

The team created an entirely new crystal structure in its work. They used a mixture of iron, bismuth and selenium to create a complex crystal that offers much greater flexibility. Their low-symmetry crystal has holes of varying size placed at varying distances in multiple, overlapping layers that allows atoms to be arranged in many different combinations in order to manipulate conductivity and magnetism independently.

The team has only created and tested the new compound in powder form. The next step is to manufacture it in the thin film that would be required for a spintronic device.

Fiber-based electronics
Scientists have known how to draw thin fibers from bulk materials for decades but a new approach by MIT researchers to that old method could lead to a new way of making high-quality fiber-based electronic devices.

According to MIT, this idea grew out of a long-term research effort to develop multifunctional fibers that incorporate different materials into a single long functional strand, which until now could only be created by arranging the materials in a large block or cylinder called a preform, which is then heated and stretched to create a thin fiber that is drastically smaller in diameter, but retains the same composition.

Now, fibers created through this method can have a composition that’s completely different from that of the starting materials — an advance that the team refers to as a kind of “alchemy,” turning inexpensive and abundant materials into high-value ones.

The fibers are made from aluminum metal and silica glass, commonly used to make windows and window frames. The aluminum metal and silica glass react chemically as they are heated and drawn, producing a fiber with a core of pure, crystalline silicon — the raw material of computer chips and solar cells — and a coating of silica.

The researchers said the initial discovery was a complete surprise: In experiments designed to test the possibility of incorporating metal wires inside fibers, they tried a variety of metals, including silver, copper, and aluminum — and in the latter case, the result was not what they expected.

When they looked at the fiber, instead of a shiny metallic core, they observed a dark substance which turned out to be very pure crystalline silicon.

It turned out that the chemical reaction in the fiber was a well-known one: At the high temperatures used for drawing the fiber, about 2,200 degrees Celsius, the pure aluminum core reacted with the silica, a form of silicon oxide. The reaction left behind pure silicon, concentrated in the core of the fiber, and aluminum oxide, which deposited a very thin layer of aluminum between the core and the silica cladding.

Now, this can be used to make electrical devices, like solar cells or transistors, or any silicon-based semiconductor devices, that could be built inside the fiber. Many teams have tried to create such devices within fibers, but so far all of the methods tried have required starting with expensive, high-purity silicon.

This uses an inexpensive metal and gives a new approach to generating a silicon-core fiber.

The researchers want to use this technique to generate not only silicon inside, but also other materials. They are also working to produce specific structures, such as an electrical junction inside the material as it is drawn by putting in other metals like gold or copper to make a real electrical circuit.

They expect this technology could open up new possibilities for electronics — including solar cells and microchips — to be incorporated into fibers and woven into clothing or accessories.

An illustration explaining the changes happening in the aluminum-core preform to silicon-core fiber drawing process.
(Source: MIT)