Electron microscopy in the 1990s
by Tim Palucka
The 1990s produced several corporate mergers in the electron microscope industry. Carl Zeiss and Leica joined to form LEO Electron Microscopy, Inc. In 1996 Philips bought Electroscan, the developer of the environmental SEM in the 1980s, to form Philips Electroscan. The following year Philips Electron Optics and a company called FEI merged under the name FEI to continue manufacturing electron microscopes. Hitachi and JEOL remained independent entities.
The resolution of TEMs had already reached its theoretical limit (the best possible resolution predicted by calculations), so the 1Å resolution obtained using field emission gun (FEG) electron sources remained the standard. Medium voltage range instruments up to 300 kV were common, although 100 kV instruments still kept their long lasting popularity.
SEMs with resolutions of 1.0 nm (10Å) were now being manufactured by JEOL in their analytical models with FEGs, though 3.0 nm (30Å) was more standard for an average, research grade microscope using a tungsten or LAB6 electron source.
Environmental SEMs, sometimes called variable-pressure SEMs, gained in popularity. FEI, formerly Electroscan, the pioneer in the field, was joined by Hitachi and LEO Electron Microscopy, Inc. in this area.
Computers were now a vital part of every electron microscope, with graphical user interfaces (GUIs) being the norm. They were involved in both the control of the instrument and the processing of data, including post-analysis enhancement of micrographs using contrast-enhancing software.
TEMs and AEMs
JEOL offered TEMs with maximum accelerating voltages of 120, 200, and 300 kV. The 120 kV model JEM1230 had a resolution of 0.2 nm (2Å). The JEM-2010 F FasTEM (200 kV) and the JEM-3000 F FasTEM (300 kV) both used FEG sources and achieved resolutions of 0.1 nm (1.0 Å).
Hitachi produced 3 conventional TEMs ( the H-7500, H-7600, and H-9000 models, 2 FEG TEMs (the HF-2200 and HF-3000 models) with resolutions of 0.1 nm (1Å), and one STEM unit, the HD-2000.
FEI decided to use market specialization to differentiate its TEMs. The Technai G2 12 was designed for advanced life science applications, the Technai G2 30 for general materials research, the Technai G2 F20 for analytical materials research, structural biology, and IC applications, and the Technai G2 F30 for ultrahigh atomic resolution studies.
JEOL's SEMs now had standardized on a maximum 30 kV accelerating voltage. They produced 3 models of research grade instruments with resolutions in the 3.0 to 3.5 nm (30 to 35Å) range, 3 low-vacuum instruments with the same resolution, and 3 analytical FEG SEMs capable of 1.0 nm (10Å) resolution. JEOL did not enter the environmental SEM market.
Hitachi produced 9 SEMs, 3 each in the categories of standard, variable pressure, and FEG SEMs. The standard SEMs achieved resolutions of around 4 nm (40Å) at 25 kV, the variable pressure units 3.0 nm (30Å) at 25 kV, and the FEG units 1.5 nm (15Å) at 15 kV. Amazingly, this same FEG microscope could achieve 5nm resolution at an acceleration voltage of only 1 kV.
FEI introduced its ESEM Explorer model environmental SEM in 1992. This instrument had a pressure range of up to 20 Torr. By 1994, the ESEM Model 2020 could achieve 50 Torr in the sample chamber. These environmental SEMs now allowed the researcher to heat his sample up to 1500 C, cool it to as low as 190 C using a cryogenic cooling unit, and examine it under humid conditions or variable pressures of different gas atmospheres. They also permitted insulating materials, which used to require a thin metal coating to dissipate surface charge, to be examined without such a coating.
This page was updated on 19 July 2002 by Arne Hessenbruch.