Environmental scanning electron microscope

Starting with Manfred von Ardenne,[1] early attempts were reported of the examination of specimens inside "environmental" cells with water or atmospheric gas, in conjunction with conventional and scanning transmission types of electron microscopes.

[8] Spivak et al. reported the design and use of various environmental cell detection configurations in an SEM including differential pumping, or the use of electron transparent films to maintain the specimens in their wet state in 1977.

In 1974, an improved approach was reported by Robinson[10] with the use of a backscattered electron detector and differential vacuum pumping with a single aperture and the introduction of water vapor around 600 Pa pressure at the freezing point of temperature.

Starting work with Robinson in 1978 at the University of New South Wales in Sydney, Danilatos undertook a thorough quantitative study and experimentation that resulted in a stable operation of the microscope at room temperature and high pressures up to 7000 Pa, as reported in 1979.

[12][13][14][15] These early works involved the optimization of the differential pumping system together with backscattered electron (BSE) detectors until 1983, when he invented the use of the environmental gas itself as a detection medium.

The decade of 1980 closed with the publication of two major works comprehensively dealing with the foundations of ESEM[16] and the theory of the gaseous detection device (GDD).

The company placed an emphasis on the secondary electron (SE) mode of the GDD[19] and secured the monopoly of the commercial ESEM with a series of additional key patents.

With the expiration of key patents and assistance by Danilatos, new commercial instruments have been later added to the market by LEO[24] (succeeded by Carl Zeiss SMT).

The specimen chamber sustaining the high-pressure gaseous environment is separated from the high vacuum of the electron optics column with at least two small orifices customarily referred to as pressure-limiting apertures (PLA).

A schematic diagram shows the basic ESEM gas pressure stages including the specimen chamber, intermediate cavity and upper electron optics column.

Additional pumping stages may be added to achieve an even higher vacuum as required for a LaB6 and field emission type electron guns.

This is achieved with an orifice made on a thin plate and tapered in the downstream direction as shown in the accompanying isodensity contours of a gas flowing through the PLA1.

This is a quantitatively vivid demonstration of a first principle that enables the separation of the high-pressure specimen chamber from the low pressure and vacuum regions above.

Initially, the amount of electron scattering is negligible inside the intermediate cavity, but as the beam encounters an increasingly denser gas jet formed by the PLA1, the losses become significant.

By this second principle of electron beam transfer, the design and operation of an ESEM is centered on refining and miniaturizing all the devices controlling the specimen movement and manipulation, and signal detection.

The problem then reduces to achieving sufficient engineering precision for the instrument to operate close to its physical limit, corresponding to optimum performance and range of capabilities.

In lieu of this, the environmental gas itself has been used as a detector for imaging in this mode: In a simple form, the gaseous detection device (GDD) employs an electrode with a voltage up to several hundred volts to collect the secondary electrons in the ESEM.

The principle of this SE detector is best described by considering two parallel plates at a distance d apart with a potential difference V generating a uniform electric field E = V/d, and is shown in the accompanying diagram of the GDD.

Notably, in this design and the associated gaseous electron amplification, the product p·d is an independent parameter, so that there is a wide range of values of pressure and electrode geometry which can be described by the same characteristics.

The consequence of this analysis is that the secondary electrons are possible to detect in a gaseous environment even at high pressures, depending on the engineering efficacy of any given instrument.

[17] As is further explained below, backscattered electrons can also be detected by the signal-gas interactions, so that various parameters of this generalized gaseous detector must be controlled to separate the BSE component out of the SE image.

[41] Clearly, ESEM is more powerful and meaningful under this detection mode than SEM, since the natural surface of any specimen can be examined in the imaging process.

[42][43][44][45] These methods involve spot masking, or the extrapolation technique by varying the pressure and calibrating out the effects of skirt, whereby considerable improvement has been achieved.

This appears as charging artifacts on the image, which are eliminated in the SEM by depositing a conductive layer on the specimen surface prior to examination.

That is because the resolving power of the instrument is determined by the electron beam diameter which is unaffected by the gas over the useful travel distance before it is completely lost.

[31] This has been demonstrated on the commercial ESEMs that provide the finest beam spots by imaging test specimens, i.e. customarily gold particles on a carbon substrate, in both vacuum and gas.

Therefore, the practical resolution depends on the original specimen contrast of a given feature, on the design of the instrument that should provide minimal beam and signal losses and on the operator selecting the correct parameters for each application.

The main disadvantage arises from the limitation of the distance in the specimen chamber over which the electron beam remains usable in the gaseous environment.

The ESEM can also be used in transmission mode (TESEM) by appropriate detection means of the transmitted bright and dark field signals through a thin specimen section.

[55] The generally low accelerating voltages used in ESEM enhance the contrast of unstained specimens while they allow nanometer resolution imaging as obtained in transmission mode especially with field emission type of electron guns.