Chromatron in Moscow manufactures a variety of color shadow mask cathode ray tubes 12-23" in diameter. While a new production line was almost finished at the time of the WTEC visit, the panelists saw no CRT that would function in a high- quality computer monitor.
Erotron in L'viv, Ukraine, builds shadow mask CRTs for airborne and land vehicle systems. These have extra vibration and shock resistance, and can operate in high magnetic fields and in intense levels of ambient illumination.
Platan builds a large variety of CRTs, from small units (0.5" diameter) for helmet- mounted applications to high brightness devices for avionics applications, as well as oscilloscope CRTs (to 10 GHz bandwidth). Another unusual CRT uses multiple beams in a single electron gun to improve the brightness. By applying a time- delayed signal to the spatially separate electron beams (as many as eight have been built), the video signal appears in the same spot a multiple number of times, which allows the phosphor to cool down between pulses to minimize thermal saturation. (This improves the phosphor efficiency compared to providing the same total current in a single spot.) Alternatively, the total current can be increased with no loss in spot size. This technique can also be used to reduce the video bandwidth if the multiple beams are scanned in separate rows (i.e., parallel Z-axis input).
Monochrome CRTs for projection TV are also manufactured at Platan, including a 625-line version that uses the multibeam addressing technique. Platan also has an active interest in developing CRTs for head-mounted displays. Erotron builds projection CRTs for aircraft heads up displays; low bandwidth oscilloscope (ó200 MHz) CRTs; and small diameter, high-resolution (e.g., 2,000,000 pixels with a 2.5 cm diagonal) monochrome CRTs.
The Radiotechnical Devices Department of L'viv Polytechnica State University has developed a high resolution CRT for photorecording that has an electron beam spot size of only 10-20 microns (50-100 lines/mm), and is working on other improvements, both inside and outside the CRT.
Erotron produces many different models of photoregistering CRTs, and intends to have available in 1995 a unit with 140 TV lines/mm resolution (compared to the commonly used 115 TV lines/mm) that uses a single crystal phosphor screen. Another device to be used as the input signal to a spatial light modulator has a 40 mm screen diameter, 70 TV lines/mm resolution, fiber-optic faceplate, and emission in the red and green wavelengths. Erotron has a particular interest in designing high-resolution CRTs for these types of applications.
The field emission display is a flat cathode ray tube that uses a matrix-addressed cold-cathode to produce light from a cathodoluminescent phosphor screen, and has received considerable attention in the last few years. Promising full color at low power consumption in a form factor that is compatible with laptop computers, its proponents anticipate that it will be an attractive alternative to the active matrix liquid crystal display. The technical feasibility of the FED has been demonstrated, and various companies and institutions are in the process of developing cost-effective prototypes. Figure 4.1 is a cross section of a low-voltage ( aprox. 500 V) FED using Spindt cathodes, which are evaporated metal cones in a one micron diameter hole. The spacing between the substrates would be typically 0.1-0.2 mm, and the display would be row and column addressed, one row at a time.
Figure 4.1. Field emission display cross section.
The Institute of Crystallography at the Russian Academy of Sciences is growing very sharp silicon tips that may be of use in a field emission display. Using a vapor-liquid-solid (VLS) growth technique, needles are formed in regular arrays by applying a pattern of gold dots to a silicon substrate using standard photolithography. The growth is performed at 900-1,000 degrees centigrade. After the sharpening procedure, tips are formed which have radii of curvature of a few nanometers. As an additional enhancement, Dr. E. Givargizov has grown polycrystalline diamond particles on the tips of the needles from an H2-CH4 mixture (see Figure 4.2). Single crystal diamond particles also can be grown on the tips (Figure 4.3). The deposition process is not yet controlled enough to place a diamond particle on each tip. Another approach is to grow a nearly continuous layer of coalescent diamond particles (Figure 4.4). This latter process is more reproducible.
Figure 4.2. Polycrystalline diamond particles on silicon tips.
Figure 4.3. Single crystal diamond particle on silicon tip.
Figure 4.4. Continuous coating of silicon tip with diamond particles.
Field emission plots for the diamond-coated Si tips are shown in Figure 4.5. Dr. Givargizov reports [Oct. '94 - ed.] that considerable currents have been obtained at moderate voltages from very blunt emitters with radii of curvature ranging from 0.3-3 þm. Values of the effective work function, determined from the emission plots, are estimated to be between 0.3 and 1.1 eV. This presents an exciting possibility for achieving low voltage field emission.
The Volga R&D Institute has developed vacuum fluorescent displays (VFDs) for Reflector, and is now developing cold-cathode field emission sources for low-voltage cathodoluminescent displays. The Volga scientists' main path is to use evaporated Mo tips as pioneered by C. Spindt et al., at SRI International, although they are also evaluating graphite edge-emitters with a 1 micron anode-cathode separation. The WTEC team saw a working 4- inch square FED (monochrome green). Dr. Boris Gorfinkel would like to find development partners to support work in the following areas: (1) low-voltage phosphors; (2) a field emission cathode using a thin carbon sheet as an edge emitter; and (3) FED packaging, including sealing and vacuum processing, for example, a device with dimensions of 100 mm x 100 mm x 2 mm, with an internal gap of 0.2 mm supported by spacers.
Figure 4.5. Field emission (Fowler-Nordheim) plots for diamond- coated silicon tips.
At the Zelenograd Research Institute of Physical Problems, Dr. Vladimir Makhov has been working on field emission since 1973. The WTEC team examined a mechanical sample of a 3-year-old FED. The display area was about 2" x 2" on a 4" x 5" glass substrate. The pixel count was 128 x 128 and a monochrome ZnO phosphor was used. With 40 V on the accelerating electrode and 75 V on the phosphor screen, a brightness of 3,000 cd/m2 was obtained with a phosphor efficacy of 3 lm/W. The substrates were 20 þm apart and the operating pressure was 5 x 10-4 Pa. Wedge-shaped silicon cathodes with a packing density of 106/mm2 were used, yielding about 103 emitters per pixel. (See Figure 4.6)
The Moscow Institute of Physics and Technology has been building field emission displays based upon carbon fiber emitters. Dr. Evgenij Sheshin believes that carbon emitters are the best candidates for practical field emission cathodes due to their self-healing characteristics, which lead to uniformity of emission, (106 emission sites/fiber are possible). In addition, he believes that carbon cathodes will work at pressures of 10-6 Torr, and that Spindt cathodes will need 10-8 Torr for good emission and lifetime. He has some evidence that carbon cathodes will tolerate 10-4 Torr, and that a current density of 105 A/cm2 is possible.
Figure 4.6. Sample of a field emission display (Zelenograd).
One facility heavily involved in FED development that the team could not visit due to its remoteness was the Vacuum Microelectronics Laboratory at the Polytechnical Institute in Krasnoyarsk, Russia. Dr. Leonid Karpov presented his work at the 1993 Vacuum Microelectronics Conference in Newport, RI ("Some Ways to Increase Brightness Stability of Flat Displays Based on Field Emission"), and he is now proposing to build the design shown in Figure 4.7. His telephone number is (391-2) 45-78-9, and his Fax number is (391-2) 43-06-92.
Figure 4.7. FED design at Krasnoyarsk.