Electron micrograph of a 3D diamond structure in silicon (outlined). The scale bar in the top right corner represents a distance of 2 um. The object on the upper left-hand side is a dust particle that settled onto the crystal after the etching procedure. |
Researchers at the University
of Twente’s MESA+ research institute, together with ASML,
TNO (the Netherlands Organisation for Applied Scientific Research) and TU/e
(Eindhoven University of Technology) have developed a method for etching 3D structures
in silicon. These structures behave as photonic crystals (semiconductors for
light), making it possible to manipulate light in all sorts of novel ways. For
instance, you can use them to “trap” light, or to create zones that are
impenetrable to light of specific wavelengths. This method brings the optical
computer one step closer. The researchers give details of their method in two
articles that will be published in Advanced
Functional Materials and the Journal
of Vacuum Science and Technology B.
Moore’s Law states that the
number of transistors that can be mounted on a computer chip will double every
two years. However, it is becoming increasingly difficult to reduce the size of
transistors any further. So, given the limitations of current technology, Moore’s Law will not be
applicable for very much longer. One way around this problem is to stack the
transistors in three dimensions. Researchers at the University of Twente
have developed a method for making 3D structures out of silicon. These
structures behave as semiconductors for light, making it possible to manipulate
light in all sorts of different ways. For instance, these structures make it
possible to trap light, or to create zones that are impenetrable to light of
specific wavelengths.
These
structures are produced using standard equipment developed for the manufacture
of computer chips. This has various practical benefits. For example, the
structures are easier to produce and they can be integrated with electronic
components on silicon chips.
Method
The
method consists of two steps. In the first step, millions of tiny holes are
etched into the upper surface of a wafer of silicon. Just 300 nm in diameter
and less than 8 um deep, these holes are too small to see, even with the aid of
an optical microscope. The second step involves the truly innovative aspect of
this method, and is therefore the toughest. The wafer is tilted and millions of
tiny holes are then etched into the side of the silicon, in the same way as before.
In order to obtain the requisite structure, the second structure has to be
aligned extremely accurately relative to the first structure. The maximum
permissible deviation is just 30 nm and half a degree. The 3D structures
created in this way have tiny pores that intersect at an angle of 90 degrees.
The material has a structure resembling that of a diamond crystal, but larger
by a factor of 2,000.
