Electron microscopy in the 1970s

by Tim Palucka

The 1970s were a time of rapid development on all fronts in the electron microscope industry. Further improvements in TEM came from brighter electron sources (lanthanum hexaboride and field emission guns). The addition of scanning coils and analytical detectors to the TEM resulted in the STEM and the Analytical Electron Microscope (AEM). The terms STEM and AEM are frequently used interchangeably, since both are scanning instruments loaded with analytical detectors. (See here for a technical description of an analytical electron microscope.) Another trend toward low-cost and easy-to-use instruments emerged. Bigger specimen stages were provided for researchers working on large samples. On the business side, Cambridge Instruments was purchased by Leica in 1975.

TEMs

The resolution of the TEM was pushed to 0.2 nm (2 Å) in the 1970s, with better results reported in some cases for lattice imaging resolutions; Hitachi claimed a 1.4 Å lattice resolution for its 1975 model H-500 TEM, and JEOL claimed the same resolution for its 1973 model JEM-100C. Accelerating voltages of 100 kV maximum had become the norm.

Low cost, easy-to-use TEMs

Philips 1970 model EM 201 was one of the simplified, easy-to-use TEMs on the market. The gun alignment was more automated, and the microscope stayed in focus despite changes in voltage and magnification. This was for the user who did not need optimum performance, and instead preferred an instrument that would require little training to operate. The JEM-T8 from JEOL was the last in the T-series, which was started in 1955 to provide low-cost alternatives to researchers on a budget. JEM-T8 had a maximum acceleration voltage of 60 kV, with a remarkably high resolution of 6.9 Å. The T-series was replaced by the JEM-100S Analytical in 1974, a low-cost AEM with 3.4 Å resolution.

High resolution, high-performance TEMs and AEMs

In contrast to the low cost instruments, Philips 1972 model EM 301 TEM was designed for high performance and versatility for the skilled operator who had the time to coax the best results from his instrument. The EM 400 introduced in 1975 used a LAB₆ electron gun, which was ten times as bright as the standard tungsten filament at the time. On the down side, the reactivity of lanthanum hexaboride required an ultra-clean vacuum system of 10^-6 Torr. In 1977 Philips introduced accessories for the EM 400, including a secondary electron detector for topographical studies and a field emission gun (FEG)– a single crystal tungsten tipped filament that emits electrons from a very localized region of the tip to produce narrow, bright electron beams. FEGs can have100 to 1,000 times the brightness of a LAB₆ filament, with electron beam diameters as small as 1 nm. Vacuum requirements for these FEGs is 10^-10 Torr.

Hitachi contributed the HU-12 TEM in 1970, with a maximum voltage of 125 kV and a lattice resolution of 2 Å, the H-500 in 1975 mentioned above with its 1.4 Å lattice resolution, and the H-700, also in 1975, which extended the maximum voltage to 200 kV.

JEOL dedicated its TEM development work in the 70s to the Analytical Electron Microscope. Basically a combination of an SEM and a TEM, the AEM scans a fine electron beam in a raster pattern across the surface of a thin specimen. The sample is located in the objective lens, just as in the TEM. A fluorescent screen at the bottom of the column shows the standard diffracted image of the sample, while SE and BSE detectors located above the sample send their signals to a CRT, as in the SEM. EDS and WDS detectors located near the sample provide elemental analysis of the sample down to atomic number 5, while an EELS detector located at the bottom of the column picks up the lower atomic number elements.

JEOL started with the JEM-100B Analytical model in 1970, which added scanning ability and an EDX x-ray spectrometer to the TEM. This was improved upon by the JEM-100C in 1973, with its 1.4 Å resolution, and further upgraded by the JEM-100CX Analytical model in 1976, which added an ultraclean vacuum system and a LAB₆ electron gun.

SEMs

Low cost, easy-to-use SEMs

Philips SEM 501, introduced in 1977, had a maximum voltage of 30 kV and a resolution of 7 nm (70 Å). The 1979 SEM 501B gave cost-conscious users both EDS and WDS x-ray analysis capabilities for the first time.

Akashi's Mini-SEM MSM2 (1972) had a maximum voltage of 15 kV. Its successor, the 1973 Mini-Rapid-Scan was a slightly modified version of the Mini-SEM. The Super-Mini-SEM SMS-1, introduced in 1974, raised the voltage to 25 kV, had a resolution of 100 Å, and could accommodate samples up to 32 mm in diameter. The 1975 SMS-2 increased the sample chamber size to 3 inches in diameter, and the 1977 ISI-100 could handle 4-inch-diameter specimens. It was also equipped with a LAB₆ electron gun.

High-performance SEMs

Hitachi entered the SEM market with its model HSM-2A in 1971, a 3-40 kV instrument with a modest resolution of 150 Å, but fully equipped with both (EDS) and WDS x-ray analytical capabilities. Their 1972 model HFS-2 was the first SEM to use a field emission gun as an electron source; this instrument had a resolution of 30 Å. The 1975 model S-500 SEM had two 8 inch CRT displays, and featured automatic recording of image data (including magnification, acceleration voltage, and film number) directly on the micrograph.  When researchers began to demand larger specimen stages so they could look at larger samples, Hitachi responded with the S-450 SEM in 1977 that could accept samples up to 4 inches.

Philips' first SEM, the PSEM 500 introduced in 1972, had a resolution of 10 nm (100Å) with an acceleration voltage of 1 to 50 kV.  The 1973 model PSEM 500M was designed for larger samples, accommodating specimens with dimensions of 153 mm x 146 mm x 84 mm. The PSEM 500X (1976) provided simultaneous EDS and WDS x-ray microanalysis.

JEOL marketed 18 different SEMs in the 1970s, including the JRSM-50A (1973), which included a hot stage for examining samples at elevated temperatures, and the JFSM-30 (1974), an ultrahigh resolution (3nm) SEM with a field emission gun. Specialized units for the IC industry, like the JSM-IC25S, were also among their offerings.

Ultrahigh Voltage EMs

In the ultrahigh voltage EM market, The Hitachi 3MeV HU-3000 was installed at Osaka University in 1970. This accelerating voltage was the highest ever for an electron microscope. A resolution of 4.6 Å was reported for this instrument. The 1976 model H-1250 had a maximum voltage of 1250 kV, but a superior resolution of 2.04 Å.

• Electron microscopy in the 1960s
• Electron microscopy in the 1970s
• Electron microscopy in the 1980s
• Electron microscopy in the 1990s