Advances in coating technology and the optimization of existing technologies
have led to innovative new analytical instruments.
X-ray photoelectron spectroscopy (XPS) and gas chromatography (GC) are widely-used techniques for analyzing samples across many industries and areas of R&D. Although there are few breakthroughs in either of these time-tested technologies, improved results can be obtained by using technologies from other areas, such as polymers and coatings, or simply by optimizing the existing technology. Such improvements have enabled higher resolution XPS and high-temperature GC in two innovative new products.
Surface analysis
NanoESCA measurements from the laboratory of CEA-LETI, Grenoble, France, show a) bonding-state imaging, and b) an elemental map. Image: Omicron NanoTechnology
Click to enlarge.
XPS is a quantitative spectroscopic technique that can be used to analyze the chemistry of the surface of a material. XPS is also known as ESCA, electron spectroscopy for chemical analysis. It is an important analytical technique worldwide, supporting various fields of scientific research. Many scientists require detailed knowledge about the distribution of chemical elements on the surfaces of their samples.
XPS can be performed using either a commercially built XPS system or a synchrotron-based light source combined with a custom-designed electron analyzer. XPS is routinely used to determine:
- What elements and the quantity of those elements that are present within ~10 nm of the sample surface.
- What contamination, if any, exists in the surface or the bulk of the sample.
- The chemical state identification of one or more of the elements in the sample.
Conventional laboratory instruments are generally limited to approximately 3 µm resolution at best. Acquisition times as well as time for experiment setup increases unacceptably when attempting to routinely work at this resolution limit. This is especially true for those instruments acquiring each image pixel sequentially by either scanning the x-ray beam or the analysis spot.
The NanoESCA from Omicron NanoTechnology GmbH, Hessen, Germany, improves greatly on this resolution. NanoESCA is an electron spectrometer for chemical analysis with imaging capabilities better than 650 nm resolution, and it will help scientists gain a better understanding of their samples, with higher speed and better precision than conventional instruments.
Optimizing XPS
The NanoESCA combines photoemission microscopy with spectroscopy to provide both physical and chemical detail of samples.
The design of the NanoESCA combines a photo emission electron microscope (PEEM) with a new type of band pass energy filter. While the PEEM front end acts as a lens for imaging XPS electrons, it can also collect secondary electron images at a resolution better than 50 nm. The imaging double-hemispherical energy analyzer ideally filters the complete image without adding image distortions. This is accomplished by the simple but effective idea behind the double-hemispherical analyzer: aberrations introduced while passing through the first half of the analyzer cancel out when electrons pass through the second, inverted half. Thus, full chemical-state photoelectron emission spectroscopy with high energy and high lateral resolutions becomes possible.
The NanoESCA has three operation modes:
- PEEM mode: Low energy electron imaging with field of view between approximately 400 µm and 5 µm diameter is used for quick surveys with an ultimate spatial resolution of less than 50 nm (without energy filtering).
- Imaging XPS mode: Energy filtered imaging in the UV to x-ray energy range provides chemical maps with less than 650 nm lateral resolution. Local XPS spectra can be extracted from image vs energy data for selected areas.
- Quantitative small spot spectroscopy mode: Acquisition of photoelectron spectra with excellent energy resolution from selected areas smaller than 5 µm diameter is possible using a channeltron-based pulse counting detector with excellent bandwidth. The spectra can also be extracted from a series of images taken at successive energies. This allows small-spot spectroscopy from areas less than 1 µm diameter, with the area of interest being defined at any place in the image. In this mode, small quantities of localized contaminants become visible as bright spots in an image at respective binding energies, even if their concentration level is below the detection limits for an area-integrated measurement.
Improving image resolution
The NanoESCA offers a much better image resolution, faster acquisition at low resolution, small spot spectroscopy with excellent energy resolution, and overview images with 100 times better resolution than other commercial instruments.
The NanoESCA offers an improved spatial resolution by a factor of five to 10 times when compared to competitive instruments in imaging ESCA modes. A prerequisite for this improvement of the spatial resolution is improved sensitivity, yielding faster results and a better signal-to-noise ratio.
In addition to the imaging XPS capabilities, the NanoESCA offers imaging resolution below 50 nm for survey images (PEEM mode). No competitive instrument has photoelectron imaging capabilities better than a few nm resolution. In imaging XPS, significant obstacles for higher resolution are the limited x-ray brilliance in the analysis area and the small electron acceptance angle of current spectrometers.
Improved image resolution requires higher magnification, which is proportional to the square of the acquisition time (an adverse effect): the smaller the diameter of a particle that needs to be resolved, the fewer x-ray photons will hit it, and consequently fewer photoelectrons are emitted for the detector to see.
To overcome this fundamental problem, it is helpful to both increase the photon flux and optimize the photoelectron detection efficiency. Further improvement can only be gained by massively parallel detection. By optimizing all three factors, the NanoESCA enables a substantially improved resolution without extending the acquisition time.
The NanoESCA can optionally be used with synchrotron radiation. Due to the higher photon brightness compared to laboratory photon sources, imaging XPS with less than 120 nm is achievable, depending on the synchrotron beamline characteristics.
With a polarized photon source (synchrotron, UV, or laser), it is possible to image magnetic domains and combine magnetic and chemical information. The NanoESCA has been used with excellent success for the imaging of copper grains with different workfunctions, a topic which is of high importance for the analysis of copper interconnect semiconductor devices.
GC gets hot
In the field of GC, not much has changed in the past 15 years. Scientists have resigned themselves to the fact that if they want to do high-temperature applications, they will have to use metal columns. A new series of GC columns changes all of that. The Zebron ZB-1HT and -5HT Inferno columns from Phenomenex, Inc., Torrance, Calif., are the world’s first capillary GC columns that provide stable performance at temperatures greater than 400°C. They are coated with a high temperature stable polyimide coating which enables their use at these high temperatures.
The Zebron ZB-1HT and -5HT Inferno columns will enable research in a wide variety of industrial areas, such as petrochemical, environmental, and toxicology, to identify specific compounds within a complex mixture. A very common technique is the separation of pesticides from foods to ensure that the fruits and vegetables purchased at the local grocery store are safe.
In many other industries, such as environmental or food testing, the extended temperature limits will allow scientists to use high-temperature bakeouts to remove harmful contaminants that might otherwise decrease the lifetime of their GC column.
Stability of high temperature
The NanoESCA combines photoemission microscopy with spectroscopy to provide both physical and chemical detail of samples. Image: Omicron NanoTechnology
The basic GC system consists of an inert carrier gas that pushes the sample through the system, a heated injection port that volatizes the sample, a chromatography column that separates the mixture, a GC oven that can be used to help separate the compounds, and a detector that measures the compounds eluting from the column. The ability to separate the individual components within the mixture is achieved primarily based on the GC column stationary phase and the oven program.
Making a GC that is stable past 370°C is difficult because both the polyimide resin and the GC column stationary phase become instable at high temperatures.
The two major advances in the Zebron ZB-1HT and -5HT Inferno columns are in the polyimide coating and the liquid stationary phase. The combination of a new resin material and new bonding technology for polymers enables the high-temperature stability in the columns.
Polyimide coating
Capillary GC columns are externally coated with a thermoplastic polymer known as polyimide resin. It stabilizes the internal glass capillary tubing and allows the columns to be flexible even at elevated temperatures. Standard polyimide resin pyrolyzes at temperatures above 370°C, making the tubing unstable. The new Zebron ZB-1HT and -5HT Inferno columns utilize a polyimide resin material that shows minimal thermal degradation, even at programmed temperatures up to 430°C. The new resin material is also the reason for the graphite black appearance of the capillary tubing. However, unlike traditional tubing that darkens due to oxidation, this tubing will provide long lifetime, even at 430°C.
Liquid stationary phase
Column bleed is the loss of lower molecular weight stationary phase pieces that are either the result of impurities in the starting polymer or the decomposition of the phase at elevated temperatures. Traditional bonding processes result in columns that have excessive bleed at temperatures greater than 350°C. Engineered self cross-linking (ESC) bonding technology reinforces the stationary phase by incorporating ladder bridges into the polymer backbone. The process begins with a careful fractionation of the base polymer, which eliminates low molecular weight impurities and enhances coating efficiencies. The columns are then cross-linked and surface bonded using an aggressive catalyst forming an interpenetrating network, which provides enhanced column durability and extremely low bleed levels—even at programmed 430°C temperatures.
Industrial benefits
By optimizing existing technologies and incorporating improvements in other technological areas, more precise measurements can be made, as well as measurements that could not previously be made due to failure at high temperatures. Many industries will benefit from these new measurements, including semiconductor, organic chemistry, biodiesel, polymers/plastics, environmental, and the food and beverage arena.
