Introduction
The scanning electron microscope (SEM) is undoubtedly one of the most widely used instruments across many disciplines in basic and applied research (
Reimer 1985). There are estimated to be more than 70,000 instruments in use worldwide today. It is almost unimaginable to think how the semiconductor community would carry out research, let alone function as an industry that is heavily dependent on quality control, without the aid of the SEM. The same may, to some extent, be said about the biological and the material science communities. This instrument continues to play a similarly pivotal role in other disciplines too. Now, with the introduction of the table-top models, more SEM instruments will become accessible to much wider communities, including schools and technical colleges, but more important, venues where footprint is of concern, such as clean rooms and small laboratories.
It is important to note that the popularity of using the SEM in these applications is due to a number of features that characterize this instrument which include ease of use, where it has been automated almost exclusively in the last two decades or so; simple sample preparation, which is mostly invasive; its fast turnaround, where most SEM yield results within a few minutes from insertion of the sample to observing an image; exceptionally high spatial resolution approaching 1 nm, offered by most manufacturers employing field emission electron cathode technology; and a variety of signal detectors that extends the range of information obtainable from the SEM. The latter include, in addition to the secondary and backscattered electron modes, electron beam–induced current (EBIC), voltage contrast, backscattered electron diffraction (BSED), and energy-dispersive X-ray spectroscopy (EDS), to name only a few of the modes of operation.
However, the secondary electron (SE) and backscattered electron (BSE) emissions are by far the most widely used signals in the SEM for obtaining mostly topographic and material contrast information of the sample under study. These combined signals facillitate the instrument’s use with a length-scale, metrology type of information, particularly in the quality control applications of the semiconductor industry [where a new class of instruments is developed and referred to as critical dimension SEM (CDSEM)]. If the emitted X-rays that are generated as a result of the impinging, energetic electrons on the surface of the sample under investigation are also collected, using an energy-dispersive X-ray (EDS) detector, then further metrology information concerning the sample composition could also be obtained. It is this latter property that has greatly increased the use of the SEM in all scientific and technological areas of research and in quality control applications, such as in the semiconductor industry (where the instrument is configured in what is referred to as defect review instruments). However, with the increased research and use of materials and structures featuring dimensions of less than 100 nm (i.e., what is referred to as nanotechnology), the compositional analysis of such samples employing X-rays in the SEM is facing a big challenge due to the large excitation volume of the emitted X-rays in comparison to the small dimensions being studied or investigated.
It must be mentioned here that the improvement in the spatial resolution of the SEM and its use at much lower electron beam energies than when it was first developed is mainly due to the use of much brighter electron sources (cold and thermal field emitters alike), as well as electron optical developments, particularly in the detector technology of secondary electrons (SEs), where the detector is placed inside the lens. This configuration has allowed the user to separate the various components of this signal and hence improve the signal-to-noise ratio (SNR) of the part of the SEs that directly results from the incident primary electrons (known as
SE1). This signal contrasts with SEs collected using the conventional Everhart-Thornley (ET) type detector (
Everhart and Thornley 1960), where SE contributions induced by energetic BSEs (known as
SE2), as well as tertiary electrons (known as
SE3), are normally added together, thus lowering the SNR of the primary induced SE1 signal. The work reported here is aimed at highlighting the importance and the need to develop new electron detectors to take advantage of the information that the SE signal carries.
However, an increased use of this instrument at low voltage [with a low-voltage scanning electron microscope (LVSEM), where the incident beam energy is in the range of 100 eV to 5,000 eV] and the relatively recently developed ultra-low-voltage mode [with an ultra-low-voltage scanning electron microscope (ULVSEM), with the incident beam energy in the range of 1 eV to100 eV], is being currently witnessed (
Liu 2003). The latter mode is also known as scanning low-energy electron microscopy (SLEEM) (
Müllerová and Frank 2003). With the increased research and applications in nanotechnology, the SEM and all its associated modes of operation is likely to emerge as one of the important aids to researchers and general users alike in this growing discipline.
The use of very low and ultra-low voltages in the SEM has resulted in a new mode of operating the instrument. This is achieved by negatively biasing the sample and arranging for all the electrons emitted from the sample surface to be collected by a specially manufactured electron detector. This is the principle of the cathode lens (
Frank et al. 2007), which is employed in this method. However, while the cathode lens principle is also used in the low-electron energy microscopy (LEEM;
Bauer 1994) method, the SLEEM mode is a modification/adaptation of the conventional SEM involving a new electron detector; therefore, it can be used in a conventional, high-vacuum sample environment, which is characteristic of most SEMs. LEEM, on the other hand, is a different instrument concept and design that requires the sample under study to be placed under ultra-high vacuum (UHV) conditions, which makes it a truly surface science technique. The interest in SLEEM stems from the different, and sometimes unexpected, material contrast obtained as a function of the incident electron energy. This technique has not been widely used because of a lack of understanding of the physics behind the emitted electron signal at such low energies.
However, while the SEM may still be enjoying wide usage, such popularity brings its own challenges, too. The use of low-energy electrons has particularly opened new areas of research and resulted in SE signals that carry surface information previously unavailable to the user, as in the SE dopant contrast and most of the SLEEM reports to date. It is this pushing of the boundaries of the instrument’s use that calls for additional quantification of the signal detectors, particularly the SEM vacuum technology employed. It is miraculous that the first and very simple SE detector used in...
