Ion sources liquid metal

Computers will be integrated more and more into commercial SEMs and there is an enormous potential for the growth of computer supported applications. At the same time, related instruments will be developed and extended, such as the scanning ion microscope, which uses liquid-metal ion sources to produce finely focused ion beams that can produce SEs and secondary ions for image generation. The contrast mechanisms that are exhibited in these instruments can provide new insights into materials analysis. 

Fig. 3.2. Schematic diagram ofthe design and operation of a liquid-metal ion source (LMIS) [3.7] (a) metal ions (b) extractor (c) liquid-metal film (d) capillary tube (e) liquid metal (f) needle.

Element mapping with non-resonant laser- SNM S can be used to investigate the structure of electronic devices and to locate defects and microcontaminants [3.114]. Typical SNMS maps for a GaAs test pattern are shown in Fig. 3.43. In the subscript of each map the maximum number of counts obtained in one pixel is given. The images were acquired by use of a 25-keV Ga" liquid metal ion source with a spot size of approximately 150-200 nm. For the given images only 1.5 % of a monolayer was consumed -"static SNMS". 

As to expected performance, Figure 8.3 shows some (conservative) values for lateral and in-depth resolutions for various physical methods of analysis. Recent developments already allow better performance, e.g. a lateral resolution of about 0.1 irn with liquid metal ion sources in SIMS and small beams of 150 xm in diameter in [xXPS. 

Figure 4.3. Diagram of the extraction region of a liquid metal ion source.

SPMs such as AFM, FFM, and SSPM were performed with a SEIKO SPA-300 unit together with an SPI-3700 control station. Details for fluorescence microscopy by SNOAM based on a modified SEIKO SPA-300 unit with an SPI-3700 control station were reported previously [27-33]. Conventional fluorescence microscopy was carried out with a Nikon XF-EFD2 fluorescence microscope [40]. SIMS was performed with a Perkin Elmer PHI model 6600 SIMS system with a Ga liquid metal ion source (beam diameter ca. 80 nm). For mapping of F negative secondary ions, a width of ca. 50 pm was scanned with 256 lines. 

A practical application coming out of field ion emission is the liquid metal ion source. Ion sources of a wide variety of chemical elements, most of them low melting point metals, can be produced by using either liquid metals131,132 or liquid alloys.133 The idea of extracting charged droplets out of liquid by application of an electrostatic field is perhaps older than field ion microscopy. But the development of liquid metal ion sources from liquid capillaries, from slit shaped emitter modules and from wetted field emission tips, etc., as well as the understanding of the mechanisms of ion formation in terms of field evaporation and field ionization theories,... 

The mechanisms of ion formation are usually studied with measurements of mass separated ion energy distributions using a magnetic sector mass spectrometer.144,145 For a liquid metal ion source, both atomic ions and cluster ions of all sizes are emitted. If the total ion current is large, neutral atoms and small droplets may also be emitted. There is little question that most of the atomic ions in a liquid metal ion source at low... 

Fig. 5.21 Two commonly used configurations of liquid metal ion sources.

Liquid metal ion source - - Extraction electrode Beam defining aperture... 

TOFSIMS analyses were performed on a Kratos PRISM instrument. It was equipped with a reflectron-type time-of-flight mass analyzer and a pulsed 25 kV liquid metal ion source of monoisotopic 69Ga ions with a minimum beam size of 500 A. Positive and negative spectra were obtained at a primary energy of 25 keV, a pulse width of 10-50 ns, and a total integrated ion dose of about 10" ions/cm2. This is well below the generally accepted upper limit of 5 x 1012 ions/cm2 for static SIMS conditions in the analysis of organic materials [12], The mass resolution at mass 50 amu varied from M/AM= 1000 at 50 ns pulse width to about 2500 at 10 ns pulse width. 

The liquid metal Ion source could help dramatically In the area of+negatlve secondary Ion mass spectrometry where bombarding with Cs ions has proven very beneficial (19). This is due to the large Increase In negative ion emission from a cesiated, low work function surface. Unfortunately, the sources used to produce cesium ions are of the surface ionization type (20) which has a low brightness. If the liquid metal field emission Ion source can be developed to operate reliably with liquid cesium, then small, high-current density ion beams could be formed. Currently, however, the reactive nature of the metal makes the liquid metal field emission cesium Ion source more of a research project than a routine analytical technique. 

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