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Automated ESCA augurs new era for surface analysis.

The needs of research labs for faster and more precise analyses at less cost and with fewer people are forcing dramatic changes in the design of surface analysis instruments.

The Quantum 2000 ESCA Microprobe from Physical Electronics, Eden Prairie, Minn., was completely reengineered to satisfy those needs. While the Quantum's complete enclosure hardly resembles the stainless steel jungle of conventional electron spectroscopy for chemical analysis instruments, its automated sample handling and improved spectral resolution give it increased throughput and performance capabilities.

Much of the hardware is hidden from view, with all of the optics mounted in the vacuum chamber on an "optics block." Conventional ESCA systems have virtually all hardware mounted on flanges attached to the vacuum chamber.

ESCA, one of the three major surface analysis techniques (Auger and secondary ion mass spectrometry being the others), has become the most widely used technique over the past 10 years. This is due primarily to its ease of use, quantitative results, and wide applicability to inorganic materials, such as conductors, semiconductors, and insulators.

Because ESCA's photons are generally less damaging to "fragile surfaces," it also is valuable for analyzing organic materials, such as polymers, which are subject to electron radiation damage.

ESCA is performed by analyzing the energy distribution of photoelectrons emitted from a solid sample surface exposed to a focused x-ray beam. The x-ray source consists of a coated anode, usually aluminum, which is bombarded with electrons. The x rays emitted from the anode are focused onto the sample through a single-crystal monochromator.

Smaller x-ray spot The heart of the Quantum 2000 is a redesigned quartz crystal monochromator, which produces a focused x-ray beam [is less than] 10-[micro] dia. Conventional monochromator designs generally use bent crystals and produce minimum spot sizes between 1 mm and 50 [micro] m. The Quantum's monochromator is ellipsoidal in shape, for near-perfect point-to-point focusing.

To get a focused x-ray spot from a focused electron beam, the aluminum anode is placed at one of the ellipsoidal foci and the sample at the other. When the electron beam is scanned on the anode surface, the resulting x-ray beam is scanned on the sample surface. While this is rather straightforward in theory, it required a major advance in monochromator technology to implement.

A major benefit obtained by scanning the x-ray beam over the sample surface is the ability to generate and detect a secondary electron image of the sample surface. The secondary electron image, or scanning x-ray image (SXI), is obtained in 1 to 5 sec, depending on the sample material.

Robotic handling The Quantum's sample handling system is designed to complement the x-ray generation system. Samples up to 75-mm square are loaded onto a platen and placed into a load-lock introduction chamber. Points or areas are chosen for analysis from an optical image of the sample obtained with a charge-coupled device camera while the sample platen is still in the load-lock chamber.

The sample platen is then transferred to the sample stage under full computer control and precisely positioned for analysis. The secondary electron image can then be generated by the scanning x-ray beam to enable precise location and selection of very small sample features.

The same probe beam that generates the secondary electrons also generates the x-ray photo-emitted electrons for the ESCA analysis. You don't have to worry about the aligning small features for analysis or whether your spectra came from the region of interest. The focused x-ray spot assures you of precise analysis.

The Quantum has high sensitivity at high spatial resolutions, down to 10 [micro] m in analysis spot size. Accurate chemical-state mapping is obtained with a high signal-to-damage ratio. Damage rates are low during imaging because at any given time only the pixel being analyzed is irradiated with the scanning x-ray beam. In contrast, non-scanning systems have longer x-ray exposure times per pixel.

Up to two sample-loaded platens can be placed in the chamber at a time, with one more in the load lock. The computer keeps track of each platen through a series of sensors and the video camera.

Computer automation From the moment a sample is placed in the load lock, all aspects of the Quantum's operation are controlled by the software. A workstation manages the data acquisition and data reduction, sample handling, vacuum-pumping, and automated bake-out requirements, using menu-based user interface functions.

The Quantum's automated sample handling is one factor responsible for its rapid sample analysis. Others include the software's simplified graphical user interface for selecting analysis areas and the relatively high counting rates at the small x-ray beam diameter.

To verify the instrument's capability, we analyzed a sample used as a standard for electron probe instruments. An SXI of one region of the sample revealed a set of round indium tin oxide (ITO) test pads. These pads were formed by etching a 3,000-A overlayer of chromium oxide deposited on the ITO. The pads varied in size from 60-[micro] m to 16-[micro] dia. The SXI was used to quickly select this area from among many possible test pad areas.

After selecting areas on and off a pad, microprobe spectral analysis confirmed the chemical composition of the materials. The spectrum from a pad, taken with a 10-[micro] m x-ray beam in [is less than] 10 min, confirmed the absence of contamination from the etching process. ESCA images of In and Cr confirmed the chemistry of each test pad and revealed the complementary chemical nature of the surface.

A more striking example was found in the analysis of a polyimide film covered with a photoresist. The photoresist was exposed and then etched so as to produce 100-[micro] m square pads. These pads, difficult to see optically, were located by taking a chemical-state image of the surface.

The chemistry of the pads was determined in minutes. A 10-[micro] m x-ray beam revealed that the photoresist was completely removed from the exposed polyimide.

Initial applications of the Quantum include analysis of metal-polymer adhesion in automotive tires, polymer coatings on steel beverage and foodstuff containers, bond pads on electronic circuit boards, and corrosion studies.

While the Quantum can perform spectroscopic analyses on features [is less than] 10 [micro] m anywhere on a sample, it is a single-function instrument designed to meet the applied research needs of industry-based analytical labs. It was not designed to accommodate other surface analysis techniques that might be called for in basic research.

James Burkstrand is director of marketing for Physical Electronics, Eden Prairie, Minn.
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Title Annotation:electron spectroscopy for chemical analysis
Author:Burkstrand, James
Publication:R & D
Date:Feb 1, 1995
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