Optical microscopes, medical x-rays, and even CAT (computed axial tomography) scans are familiar items for most of us. Optical microscopes use glass lenses to bend (refract) light so that tiny objects can be magnified, allowing us to see their structure and detail in high resolution.
Like visible light, x-rays are a form of electro-magnetic energy, but with much shorter wavelength and much higher energy. Because of their short wavelength, x-rays do not reflect or refract easily and are invisible to the human eye. Their high energy, however, means they have the ability to penetrate most objects, allowing images to be created that see the internal structure of those objects. Medical x-rays use large-field x-ray sources to illuminate relatively large areas of the body to look for internal features of interest, such as bone fractures, tumors and other phenomena.
Standard medical x-rays are two dimensional. They can locate the position of an internal feature in a sample both vertically and horizontally, but no information is available on the depth of that feature within the sample. CAT scans use computers to reconstruct highly detailed 3-dimensional images, by merging hundreds or thousands of individual x-ray images into a single, three-dimensional “block” that can then be studied from any angle and viewed in virtual “slices” one layer at a time, in any direction.
X-ray microscopes combine the magnifying power of optical microscopy with the penetrating power of x-rays, to generate highly detailed two-dimensional images of very small features, including their internal structure. X-ray microscopes capable of full 3D tomography, such as those manufactured by Xradia, combine the imaging principles and computing power of a CAT scan, with the magnifying power of a high-resolution x-ray microscope, thereby providing high-resolution 3D images of microscopic and macroscopic objects.
History
Kirkpatrick-Baez x-ray reflecting optics
As noted above, x-rays do not lend themselves to the conventional focusing capabilities that glass lenses bring to visible light, so how are x-ray microscopes possible?
In 1948 Paul Kirkpatrick of Stanford University, and his graduate student Albert Baez (father of singer Joan Baez) developed the first x-ray microscope using reflective optics and are credited with co-inventing the x-ray microscope. By bouncing x-rays off of parabolic curved mirrors at very high angle of incidence (i.e. grazing angle) they were able to bend and focus x-rays onto a small spot, creating the first microscopic images using x-ray energy. Today, Xradia sells Kirkpatrick-Baez (KB) mirror systems to x-ray microscopists using Synchrotron radiation soft x-ray sources.
The most common form of commercial x-ray microscope in use today owes its origins to Sterling Price Newberry of General Electric. While working on an alternative approach to reflective optics for microscopy, which can create blurry images due to diffraction, a simple test performed by a technician to detect the presence of x-rays in a machine, gave him the inspiration to locate samples or specimens very close to a point-source of x-rays and then place a photographic plate at a relatively large distance. The resulting projection image was a reasonably high resolution, magnified view of the x-ray shadow of the sample.
X-ray Microscopy – The Basics:
In a projection-based conventional x-ray microscope, magnification is achieved by positioning the sample very close to a point-source x-rays. The ratio of the distance between the source and the detector, to the distance from the source to the sample, yields the geometric magnification:
In order for the magnified image to also provide reasonable resolution, the source must be as small as possible. The larger the source, the more blurring occurs at the edges of the image, as illustrated in Figure 1.
The resolution limit of projection-based x-ray microscopy is thus largely determined by the size of the point source (some as small as 1µm).
High resolution and 3D tomography:
By their very nature, projection-based x-ray microscope systems have two important limitations. The first is that spatial resolution (the ability to separate or "resolve" two objects that are very close together) is limited by the point size of the x-ray source. It is impossible to achieve resolution better than the source size. Thus, typical 2D projection-based commercial x-ray systems have a resolution limit on the order of several microns to tens of microns. The second limitation is in the positioning of the sample close to the x-ray source. 3D tomography requires the collection large numbers of 2D x-ray images on all sides of the sample, so the sample must be free to rotate fully in front of the x-ray source. The placement of the sample immediately in front of the source places a physical limitation on the ability to execute free rotation. Depending on the geometry of the sample there is occasionally enough space to allow for a partial rotation, which can yield so-called quasi-3D images, but complete 3D reconstructions cannot be achieved.
SEM image of Fresnel zone plate lens
Both of these problems are overcome by the introduction of x-ray optical elements into the design of x-ray microscope systems. An alternative method to reflective optics for focusing x-rays, is the use of Fresnel zone-plate lenses. These are tiny (~80µm diameter) circular diffraction gratings comprised of gold, copper or nickel rings deposited on a silicon nitride substrate.
Today, Xradia is the world's leading commercial producer of zone-plate lenses for x-ray microscopy and is the only company with commercial systems capable of full 3D tomography with sub 100nm resolution. The introduction of zone-plate optics allows for the positioning of the sample roughly in the middle of the space between source and detector, and spatial resolution is no longer limited by the spot size of the source. Xradia produces zone plate lenses today, which provide spatial resolution better than 50nm, and this performance continues to improve.
While the resolution of x-ray microscopes lies between that of optical microscopes and scanning electron microscopes (SEM), x-ray microscopes have a fundamental advantage over SEM systems based on their ability to non-destructively image the inside of complex structures, such as advanced integrated circuits. Biological samples can also be imaged in their natural state, since x-ray microscopes do not require sample preparation or a high-vacuum sample chamber.
In essence, because of their different features and advantages, optical, x-ray and electron microscopes are complimentary technologies, rather than competitive technologies. Each in turn, has capabilities that the others cannot duplicate. Nevertheless, the ability to non-destructively image the internal structure of complex samples gives high-resolution 3D x-ray transmission microscopes critical advantages in applications such as failure analysis, process development, quality control, metrology and fundamental research, in industries ranging from biotech and life sciences to semiconductor manufacturing.
For a more in-depth treatise on high-resolution 3D x-ray microscopy, click on the link below to download our Whitepaper: “High Resolution 3D Tomography for Advanced Package Failure Analysis”