Nanotech X-ray Systems: A Point of Focus
BY NICK HADLAND, X-Tek Group, Inc.
The term “nanotechnology” is no longer new to most of us, thanks, in part, to the media. Developments recently made to open-type X-ray sources and X-ray imaging detectors may provide an end-user with the ability to image features in the nanometer range.
A description published on the National Nanotechnology Initiative (NNI) website defines nanotechnology as “the understanding and control of matter at dimensions of roughly 1 to 100 nanometers, where unique phenomena enable novel applications. Encompassing nanoscale science, engineering, and technology, nanotechnology involves imaging, measuring, modeling, and manipulating matter at this length scale.”
Figure 1. Schematic of a micro-focus cabinet X-ray system.
There are no specific claims that commercially available X-ray systems actually provide a resolving capability as described by the NNI. So how can an X-ray system be assessed to prove its capability?
Figure 2a. Sealed X-ray tube.
Understanding terminology is key when trying to understand what a manufacturer is truly specifying. X-ray tube or X-ray source resolution defines the best resolution that the X-ray tube can provide. This is describing the smallest focal spot or emission point that the X-ray tube can maintain. Terms, such as feature recognition or defect detectability, describe the smallest defect or feature that the system can image. This is often described as one half of the source resolution. Nano resolution, or nano focus, is therefore a factor of two coarser in resolution than nano-feature recognition, or nano-defect detectability. Therefore, a manufacturer who specifies a focal spot of 1 µm (1000 nm) would be specifying a feature recognition or defect detectability of 0.5 µm (500 nm). For example, one X-ray source* has a focal spot of approximately 1000 nm. Systems that contain this technology can achieve feature recognition of 500 nm or better.
A modern, micro-focus cabinet X-ray system is constructed of steel with lead panels and contains the X-ray source, a manipulator, and an X-ray imaging device. Externally, the system includes an image processor, control PC, and display monitor (Figure 1).
Figure 2B. Open X-ray tube.
There are two main types of X-ray tube: sealed X-ray tubes (Figure 2a) and open X-ray tubes (Figure 2b). Sealed tubes may be described as entry-level X-ray tubes with limited kV (typically 130kV), resolution (5-10 µm), and magnification capability; relatively short life (approx. 5,000-10,000 hours); and low cost. Open tubes are considered high-end and offer high kV (typically 160kV and above), resolution (approx. 1 µm or less), and magnification capability; and provide almost limitless life. Open tubes are often higher in initial cost. However, open-tube technology is considered to be the only commercially available technology capable of producing very high resolution of 1 µm (1000 nm) or below.
High-resolution Open-type X-ray Sources
There are few “real-life” limitations to the open-tube resolution capability, although some have quoted 3-µm resolution as the practical limit. This overlooks what is currently available from some X-ray source manufacturers, and the resolving capability of the scanning electron microscope (SEM). Both have significant technology similarities. For example, some open tubes use off-the-shelf SEM filaments as their source of electrons, and SEMs make use of electromagnetic lenses in the same way as open tubes to achieve a focused point of electrons in the 1-10 -nm range.
In a modern open-type X-ray tube, a filament is housed inside a cathode assembly. Both filament and cathode assembly are electrically isolated from the ground by a vacuum inside the tube envelope and a high-voltage insulator. The cathode assembly and filament are maintained at a negative potential-to-ground (typically up to -160kV) so that electrons from the filament are accelerated toward earth potential at high speed, and focused by an electromagnetic lens on to a tungsten/beryllium X-ray target. At the point-of-focus, the high-energy electrons will be decelerated or braked through an electron (beam) to electron (target material) interaction. Energy is released in the form of X-ray photons. The electro-magnetic lens is probably the single most critical part of the tube if high resolution is desired.
Figure 3. Electron beam (a), representative focusing geometry of the magnetic lens (b), focused electron beam (c), tungsten target material (d).
The electromagnetic lens can be pictorially represented by a simple optical lens (Figure 3). The optical lens is actually an electromagnetic field that influences the electron beam. Current passed through a wound copper coil induces a magnetic field. The strength of the magnetic field varies the lens focusing power. The internal geometries and manufacturing process are key to the focusing capability of the lens. Small inconsistencies in the magnetic lens, stray magnetic fields, fluctuations in coil current, and fluctuations in the high-voltage supply all affect the consistency of the lens’ focus point.
The most common X-ray imaging system comprises an X-ray image intensifier coupled to a camera. The image intensifier is a large-area, X-ray-to-light converter and multiplier. An X-ray shadow of varying energy photons representing the sample is projected onto the detector. The X-ray detector will convert the differing energy photons into visible light, and present the image to the camera. The camera will convert the visible image into an electrical signal, which is processed and visually displayed on a monitor. A basic system may use a low-cost image intensifier and an 8-bit analog CCD camera. This provides an image displayed through a PC video card and monitor in up to 255 individual levels, from white to black.
Figure 4. Die attach inspected with a 12-bit digital-camera-based system.
Some X-ray system manufacturers use higher quality and sensitivity image intensifiers (such as beryllium image intensifiers), greater-bit-depth digital cameras, and may even use beryllium X-ray source windows that attenuate X-rays to a lesser extent. The camera signal will comprise up to 1024 (10 bit) or 4096 (12 bit) individual steps. Providing more gradations between black and white can translate into better determination of differences in materials. Improving the sensitivity of the imaging system is essential when attempting to image minute features. Features of a few microns or less will only slightly attenuate the X-ray beam, and are more difficult to image because subtle changes in the X-ray energy are harder to detect. Figure 4 shows a die-attach image inspected with a 12-bit digital-camera-based system.
New Imaging Technology
Amorphous silicon detectors are optional on most high-end X-ray inspection systems, replacing both the image intensifier and camera. These detectors are large-area, amorphous silicon arrays manufactured with thousands of individual light-sensitive photodiodes. A scintillator material is mounted inside the detector unit in front of the amorphous array. X-ray photons collide with the scintillator, emitting light photons which the photodiodes capture. When read out or addressed, the photodiode signal represents the amount of light it has captured and will ultimately represent a grey level at a pixel location on a viewing monitor.
Figure 5. SEM of a sample test piece.
Amorphous detectors provide an output signal of 12, 14, or even 16 bits; and provide high dynamic range. Such detectors often contain over 1 million photodiodes, providing a very high resolution output. The arrays are almost perfectly flat, providing distortion-free images. There are drawbacks however; the detectors are typically not capable of a real-time readout (25-30 frames per second), and do not provide a smooth continuous image display. The detectors are not yet produced in sufficient volume to be supplied at a cost even approaching image intensifiers (typically 5-10 times the cost of an image intensifier).
A simple test piece (Figure 5) consists of a star pattern lithographically etched into a 1-µm-thick, gold-on-silicon-nitride substrate. With this pattern positioned close to the source, the feature recognition of the X-ray system can be visually assessed by looking at the projected image displayed on the system’s monitor. The projected image will appear as a series of radial convergent lines. As the lines approach the center of the starburst, the spacing and width become tighter. The resolvability of the line/space as they narrow towards the center indicates better resolution. This simple test will not only qualify the focal spot from 5 µm down to 0.5 µm, but will also check for inconsistency in focal-spot circularity. Because the test pattern is manufactured as a thin gold layer that X-ray photons pass through with only limited attenuation, the sensitivity and dynamic range of the detector system can also be assessed.
Achieving ultra-high resolution requires a stable, high-resolution X-ray tube, and sensitive, high-contrast, low-noise X-ray imaging system. Amorphous silicon detectors are desirable option, but not essential to imaging small features if the right combination of image intensifier and digital camera are provided. Standardized test devices exist to prove a system’s capabilities and substantiate what a manufacturer means by “nano.”
* NanoTech by X-Tek.
Contact the author for a complete list of references.
NICK HADLAND, president, may be contacted at X-Tek Group, Inc., P.O. Box 374, Tyngsboro MA 01879; 978/649-6333; E-mail: firstname.lastname@example.org