CSP Microcalorimeter Type Energy Dispersive X-Ray Spectrometers Offer New Solutions for Semiconductor Defect Analysis

Future Fab International, Issue 8, January 2000

As the semiconductor industry further shrinks feature sizes, new analytical solutions are required for elemental composition analysis. Conventional EDX analysis is limited by the obtainable energy resolution. In order to analyze extremely small impurity particles, material doping, interdiffusion effects or small chip structures, the diameter of the area hit by an electron beam and particularly, the volume of electron-surface interaction has to be accordingly reduced. This can be achieved by reducing electron beam energy. Thus, since only the low energy range - which may be crowded by multiple peaks - is accessible for X-ray analysis, X-ray spectrometers with excellent resolutions are necessary.

New, high resolution, superconducting X-ray spectrometers are becoming accessible for industrial applications. CSP Cryogenic Spectrometers GmbH, a German company based in Munich, is in the process of transferring technology that has originally been developed for astrophysical applications onto an industrial scale. This is done within the scope of the MEDEA projects and requires considerable R&D effort to shrink the usual three meter equipment to a device of weight and size suitable for mounting on an electron microscope. CSP has successfully built a cryostat cooled by liquid nitrogen and liquid helium, and equipped with an Adiabatic Demagnetization Refrigerator (ADR). The cryostat reaches temperatures below 50mK and has a high resolution, superconducting sensor working at a temperature of 80mK with an energy resolution of 15eV. In order to offer an instrument which is easy to use and is clean room compatible, CSP has developed a mechanical cooler which operates vibration free.

Like conventional EDX systems using semiconductor technology, CSP?s microcalorimeter type spectrometers are energy dispersive X-ray devices allowing for fast measurements across the entire spectral range. Further, microcalorimeter type EDX systems are based on superconducting, thin film technology and, in contrast to conventional EDX systems, they allow measurements with excellent energy resolution. There is a principle difference in the magnitude of energies required for X-ray induced electron excitation in semiconductor and microcalorimeter sensors. Since excitation energies are significantly lower for microcalorimeter type sensors, the theoretically obtainable energy resolution is at least better by a factor of 30. The improved peak to background ratio also results in a relatively high sensitivity.

The industrial development of low temperature, microcalorimeter type detector technology is just beginning. Since most of its potential is yet to be explored, many people call these detectors "detectors of the future".

The microcalorimeter detector consists of an absorber and a superconducting sensor. In order to obtain a relatively large temperature effect when a photon impinges, the heat capacity of the absorber has to be low. Due to the relationship between heat capacity and temperature, the absorber temperature has to be close to zero Kelvin, and the absorber material has to be chosen appropriately.

The microcalorimeter principle allows the determination of small temperature changes by measuring the increase of the electrical resistance of a superconducting sensor which is kept at its transition temperature. The changes in temperature and electrical resistance are proportional to the energy of the impinging X-ray quantum and enabling thus an energy dispersive measurement.

The CSP microcalorimeter type EDX detector is specified as follows:

System Specifications
Energy Resolution	15eV (FWHM) at 1.5keV
Maximum Count Rate	1000cps
Detector Area	0.1mm2
Maximum Quantum Efficiency	>98%
Data Processing	Digital Pulse Processing Elemental Analysis Software
Duty Cycle	>88%

The count rate of 1000cps is related to the photon absorption capability of the detector itself. When employed in SEMs, the number of photons actually "seen" by the detector depends on the solid angle and is usually less than 1000cps. A significant increase in solid angle can be achieved with an X-ray lens custom made for specific applications. The duty cycle of approximately 88% is determined by the need to recycle the ADR device.

The CSP microcalorimeter EDX detector system contains a cryostat with cooling capabilities down to 4K and an ADR system which makes temperatures of less than 50mK obtainable. In order to allow to access samples inside electron microscopes the temperature is transferred to a detector at the tip of a snout, with variable lengths of up to 50cm. The device has a height of 60cm, weighs 20kg and can be mounted on SEMs, FEGs and TEMs.