Of flexible circuitry

The early printed circuit was a pattern of strips of copper, where components like transistors and resistors could be soldered in. Then came the integrated circuit, which is a whole circuit built on a slice of silicon crystal.

The IC soon grew complex, with hundreds, thousands and millions of components on the crystal slice, but the limitation is the need for the silicon crystal, which is difficult to build and needs to be carefully encased. 

Scientists at Tsukuba, Japan and the University of Tokyo have come up with a long-awaited alternative, a method to spray on the crystalline material on which circuits could be built. This would sidestep the need to grow large silicon single crystals and also allow electronic circuits to exist in rolled sheets or other convenient shapes and spaces.

The transistor
This marvel of electronics depends on the properties of semiconductor materials for its action. Materials conduct electricity because electrons in the atoms of conducting materials can move within the mass of the material somewhat freely. Metals, which have just a few electrons in their loosely bound outer shell of electrons, are the best conductors.     

In non-metals, which are usually insulators, the outer shell electrons are short of the ideal number of eight, and atoms of these materials need to borrow electrons, not let go of what they have. Silicon, with four electrons in the outer shell, is neither a conductor nor an insulator. But it can be made into a conductor if an impurity, of atoms that have five or three outer shell electrons, is added to the crystal structure. The extra, or one short electron (which is called a hole) in the lattice is not bound within the structure and can move, just like a free electron in a conductor! 

Now, when one part of a crystal of silicon is doped with an extra electron impurity and the adjoining part with the electron short impurity, the junction stays a conductor, but allows current to pass only in one direction – it becomes a one-way street. And again, if a part of doped silicon has oppositely doped silicon on either side, the part in the middle can act as a gate, which can be opened or closed by applying a voltage. Such a three-way bit of silicon, where the middle terminal controls the current that flows between the other two, is the transistor.

This small, light, inexpensive and versatile device set off a revolution in electronics, pocket radios, cheap TV sets, a host of control mechanisms and best of all, the modern digital computer.

Integrated circuits
The next step was to build many transistors on a single chip of silicon crystal, with connections by depositing layers of metal. The first of such devices had five transistors built on the same silicon chip. The technology developed, and with deposit of material on the silicon chip in layers, adding components like resistors became feasible.

Progressively, Large Scale Integration and Very Large Scale Integration combined hundreds and thousands and the current Ultra Large Scale Integration combines millions of components on a single, albeit large, slice of silicon crystal. The growth of the personal computer and super computers is entirely thanks to whole computer processors now being built on single silicon chips with this technology. 

But the limitation of integrated circuits is that it needs large single crystals of silicon. Growing large diameter crystals to create such slivers, or chips is difficult and expensive. Crystals almost 30 cm across are now being grown, but at astronomical cost. While large crystals are unwieldy, costly and scarce, small crystal dimensions drive the need to miniaturise electronics. High currents in such crowded circuitry create heat and need heat sinks and cooling arrangements. The effect, overall, is that the need for single crystals is a roadblock in the way of integrated circuits.

Alternative
The alternative, so far a dream, is to create a silicon substrate not by growing it from ground up, but by spraying it on. Regular printing technologies, of depositing material on a surface, are not effective because semiconductor material rapidly forms islands of micro-crystals, with interfaces that the electron or electron hole carriers of charge cannot traverse. In effect, using inks that have semiconductor material in solution or in dispersed form, or even spraying molten semiconductors to condense on a surface, have not been successful in creating the thin films with the required crystal properties.

The team of scientists in Tsukoba and Tokyo has now found a way out. What they have used is not semiconductor material that crystalises out of solution by evaporation of the solvent, for instance, but material that is rapidly thrown out of solution using an outside agent. A supersaturated solution, which is an unstable solution with more of the solute than normal, is first sputtered on the surface. Next, specks of a complementary anti-solvent are added. An antisolvent is a liquid in which the material does not dissolve.

The addition of the antisolvent brings about immediate and large scale release of the solute, which settles on the surface in crystal form. The growth of material by evaporation of a solvent results in crater formation because material crowds the periphery when the solution is in the form of a drop. But when deposit is by antisolvents, the solute is released at the surface of the solvent which is in contact with air, where the solute spreads out as an orderly crystalline layer.

The film settles and fixes to the substrate as the solute evaporates and the film formed is uniform and smoothed out. X ray studies show that the films are in the form of single crystals, over long distances.

The work was done with an organic semiconductor called BTBT, using solvent and anitsolvent that easily mix and evaporate together. The technique is generic and shows the way for creating thin, crystalline films of many useful semiconductor materials. The work, the authors say, is “in principle, a useful new way of producing transistor arrays on top of plastic substrates, which is indispensable for realising large-area, light-weight and high-speed electronic products”.