PHYSICS NEWS UPDATE
A digest of physics news items by Phillip F. Schewe, American Institute of
Physics
Number 186  July 5, 1994                   physnews@aip.org

NEAR-FIELD OPTICAL MICROSCOPES can achieve a spatial
resolution of a fraction of a light wavelength by positioning the
optical source only tens of nm from the sample.  By working in this
"near field," scientists can greatly reduce the resolution-limiting
effects of diffraction.  Physicists at AT&T Bell Labs were able to
image dye molecules with a resolution of less than 100 nm using laser
light with a wavelength of 600 nm brought to the sample in an optical
fiber tapered at the tip to only 20 nm. (Physics Today, May 1994.)
Using the incident light to excite photoluminescence in a sample, the
microscope becomes a spectrometer with potentially high spatial and
energy resolution.  In this way, the Bell Labs researchers are
beginning to resolve individual centers of luminescence in quantum
wells, structures in which electrons are confined to an essentially 2-
dimensional GaAs region sandwiched between AlGaAs layers.  The
study of quantum wells in such fine (spatial) detail is important since
they play a key role in certain high-tech lasers and transistors.  (H.F.
Hess et al., Science, 17 June 1994.)

THE EVIDENCE FOR TOP QUARK PRODUCTION, announced by
the Collider Detector at Fermilab (CDF) collaboration in April, is
now officially published in Physical Review Letters (PRL).  An
abbreviated form of a much longer article that will appear later in the
journal Physical Review D, the PRL account does not much add to
the salient facts established in April, namely the observation of 12
events consistent with top production, with an estimated cross section
of 13.9 picobarns and an estimated top mass (based on analysis of 7
of the 12 events) of 174 GeV.  (F. Abe et al., Phys. Rev. Lett., 11
July.)

INTEGRATED CIRCUITS ARE MOSTLY TWO-DIMENSIONAL,
whereas nature more efficiently works in three dimensions. The
human retina, for example, is a massively-parallel, 3D imaging
system consisting of a layer of sensing cells (rod and cone cells), two
layers of processing cells (bipolar and ganglion) and two layers of
interconnection cells.  As for manmade circuits, one method for
stacking two chips is to flip one over and attach it to the other using
raised metal bumps which serve as a support and as electrical
connectors.  Such "flip chips" have been used in military infrared-
detecting "smart Pixel" arrays.  Some stacks with more than two
layers have been made using metal interconnections that go all the
way through the silicon wafer.  Efforts are also underway to link up
several stacked circuit layers with optical signals that pass through the
wafers.  One of the problems here is the lattice mismatch between
light-emitting materials and the silicon substrate.  One solution may
be the use of "epitaxial liftoff" (ELO), a technique in which a
specially-grown "epilayer" can be separated from an underlying
growth substrate by etching away an intermediate sacrificial layer.
The microns-thick epilayer can then be transferred to a different host
substrate for further processing.  (Optics & Photonics News, April
1994.)

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