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Method Uses Blurry Images

Photonics.com
Nov 2008
GAITHERSBURG, Md., Nov. 3, 2008 -- A novel technique under development uses a relatively inexpensive optical microscope and a set of blurry images to quickly and cheaply analyze nanoscale dimensions with nanoscale measurement sensitivity. Termed "through-focus scanning optical microscope" (TSOM) imaging, the method has potential applications in nanomanufacturing, semiconductor process control and biotechnology.
Nanodimension.jpg
Using an optical microscope, several images of a 60-nm gold particle sample are taken at different focal positions and stacked together. This computer-created image shows the resultant TSOM (through-focus scanning optical microscope) image. (Images: NIST)
The technique is being developed at the National Institute of Standards and Technology (NIST) in Gaithersburg using an optical microscope. Optical microscopes are not widely considered for checking nanoscale (below 100 nanometers) dimensions because of the limitation imposed by wavelength of light -- you can’t get a precise image with a probe three times the object’s size. NIST researcher Ravikiran Attota gets around this, paradoxically, by considering lots of “bad” (out-of-focus) images.

“This imaging uses a set of blurry, out-of-focus optical images for nanometer dimensional measurement sensitivity,” he said. Instead of repeatedly focusing on a sample to acquire one best image, the new technique captures a series of images with an optical microscope at different focal positions and stacks them one on top of the other to create the TSOM image. A computer program Attota developed analyzes the image.

While Attota believes this simple technique can be used in a variety of applications, he has worked with two. The TSOM image can compare two nanoscale objects such as silicon lines on an integrated circuit. The software “subtracts” one image from the other. This enables sensitivity to dimensional differences at the nanoscale—line height, width or side-wall angle. Each type of difference generates a distinct signal.
TSOMscope.jpg
This schematic shows how a TSOM image is acquired. Using an optical microscope, several images of a 60 nanometer gold particle sample (shown in red) are taken at different focal positions and stacked together.
TSOM has also been theoretically evaluated in another quality control application. Medical researchers are studying the use of gold nanoparticles to deliver advanced pharmaceuticals to specific locations within the human body. Perfect size will be critical. To address this application, a TSOM image of a gold nanoparticle can be taken and compared to a library of simulated images to obtain “best-match” images with the intent of determining if each nanoparticle passes or fails.

This new imaging technology requires a research-quality optical microscope, a camera and a microscope stage that can move at preset distances.

“The setup is easily under $50,000, which is much less expensive than electron or probe microscopes currently used for measuring materials at the nanoscale,” Attota said. “This method is another approach to extend the range of optical microscopy from microscale to nanoscale dimensional analysis.”

So far, sensitivity to a 3-nm difference in linewidths has been demonstrated in the laboratory.

The work appeared last month in Optics Letters.

For more information, visit: www.nist.gov


GLOSSARY
camera
A light-tight box that receives light from an object or scene and focuses it to form an image on a light-sensitive material or a detector. The camera generally contains a lens of variable aperture and a shutter of variable speed to precisely control the exposure. In an electronic imaging system, the camera does not use chemical means to store the image, but takes advantage of the sensitivity of various detectors to different bands of the electromagnetic spectrum. These sensors are transducers...
electron
A charged elementary particle of an atom; the term is most commonly used in reference to the negatively charged particle called a negatron. Its mass at rest is me = 9.109558 x 10-31 kg, its charge is 1.6021917 x 10-19 C, and its spin quantum number is 1/2. Its positive counterpart is called a positron, and possesses the same characteristics, except for the reversal of the charge.
light
Electromagnetic radiation detectable by the eye, ranging in wavelength from about 400 to 750 nm. In photonic applications light can be considered to cover the nonvisible portion of the spectrum which includes the ultraviolet and the infrared.
microscope
An instrument consisting essentially of a tube 160 mm long, with an objective lens at the distant end and an eyepiece at the near end. The objective forms a real aerial image of the object in the focal plane of the eyepiece where it is observed by the eye. The overall magnifying power is equal to the linear magnification of the objective multiplied by the magnifying power of the eyepiece. The eyepiece can be replaced by a film to photograph the primary image, or a positive or negative relay...
nanoparticle
A small object that behaves as a whole unit or entity in terms of it’s transport and it’s properties, as opposed to an individual molecule which on it’s own is not considered a nanoparticle.. Nanoparticles range between 100 and 2500 nanometers in diameter.  
optical
Pertaining to optics and the phenomena of light.
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
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