Metal-insulator-metal Cathodes Research Articles

Stability of deflected-beam metal–insulator–metal tunneling cathodes under high acceleration voltage

Metal–insulator–metal (MIM) tunneling cathodes have advantageous features, such as areal electron beams and robustness to surface contamination. Therefore, they are suitable for field-emission displays. This paper reports the long-term stability of MIM tunneling cathodes in display panels operating under a high acceleration voltage (anode voltage). A preliminary life test was carried out using conventional display panels, which consisted of a phosphor-coated anode plate and a cathode plate having matrix-arrayed cathodes. The life test indicated that the pace at which cathodes degrade increases significantly when the acceleration voltage exceeds a few kilovolts. This high-voltage (HV)-induced degradation is presumably attributed to cathodes being bombarded by positive ions originating from residual gases. To suppress HV-induced degradation, the authors propose the concept of a deflected-beam cathode, in which electron beams are deflected so that the recoiling ions fail to hit the cathode. This concept was implemented by introducing deflector electrodes in the cathode plate, thereby forming an electron lens in each pixel. As 6-μm high scan lines were used as the deflector electrodes in this design, the deflected-beam cathodes could be fabricated only by modifying the pattern of the scan lines. The authors also present a simulation-based design procedure using an emitter-landing diagram, which facilitates analysis of the margins for the deflected-beam cathodes. The deflected-beam cathodes thus designed were incorporated into 3.8 cm-diagonal test panels to confirm the validity of the concept. The degree of beam deflection that was observed was in excellent agreement with the results obtained from simulation. A life test extending over a 20 000-h period demonstrated that the HV-induced degradation was suppressed with the deflected-beam MIM cathodes.

Enhancing Electron-Emission Efficiency of MIM Tunneling Cathodes by Reducing Insulator Trap Density

Metal–insulator–metal (MIM) tunneling cathodes demonstrate electron-emission efficiency $(\alpha)$ of 9 $\%$ to 11 $\%$ . This threefold increase in $\alpha$ from that previously reported is achieved by reducing trap density in the cathodes' insulator, thereby enabling tunneling electrons to accelerate with fewer scatterings in the insulator. Life tests show that the high $\alpha$ is maintained for a 19 000-h operation in sealed display panels containing a ZnS-based phosphor. The reduction of trap density also suppresses long-term drifts in emission and diode currents to a satisfactory level. These emission characteristics are sufficient for realizing a large-sized field-emission display with MIM cathodes. In addition, an effective method for measuring trap density in the insulator of MIM devices is devised. This method adopts low-stress $I$ – $V$ measurement following charge-injection stress. Measurement by this method reveals that trap density is proportional to neodymium concentration in the anodized insulator. The trap density is therefore reduced by lowering the neodymium content in the precursor Al–Nd alloy film. Moreover, the robustness of the MIM cathodes to charge-injection stress is demonstrated.

Characterization of the tunneling insulator in MIM cathodes by low-stress I-V measurement

The low-stress current-voltage (I-V) measurement system was developed and applied to characterization of the insulator in metal-insulator-metal (MIM) tunneling cathodes. The amount of the total charges injected into these devices during the I-V measurement decreases by four orders of magnitude compared to that for the conventional measurement system. The developed I-V measurement, therefore, can be considered to be a damage-free characterization method. The time-reciprocal dependence of the transient current demonstrates that the low-voltage leakage current (LVLC) of an MIM cathode is a good measure of the trap density in the tunneling insulator. The LVLC measurement was used for monitoring the plasma process-induced damage and long-term degradation of MIM cathodes. The charging damage during the sputter-deposition process was reduced to an acceptable level by modifying the sputtering machine and designing the device layout pattern properly according to antenna ratio. A long-term operation experiment showed that the low-stress I-V measurement can detect the latent degradation of insulator quality. The developed I-V measurement is thus considered to be a useful tool that will lead to improvements in device performance and lifetime.

Field-emission display based on nonformed MIM-cathode array

We developed nonformed metal-insulator-metal (MIM) cathodes and demonstrated their suitability for field-emission displays (FEDs). The MIM cathodes have an emission current density of 5.8 mA/cm/sup 2/ at an operation voltage of 9 V. The cathodes are operated in the nonformed state, and they exhibit no fluctuation in the emission current, even without a ballast resistor. At a cathode-anode separation of 2 mm and an acceleration voltage of 4 kV, the divergence of the emitted electron beam is 25 /spl mu/m. These features make nonformed MIM cathodes suitable for high-voltage acceleration-type FEDs. We also fabricated a prototype 3.8-cm diagonal frit-sealed color display, and demonstrated its matrix-multiplexing operation.

Emission current enhancement of MIM cathodes by optimizing the tunneling insulator thickness

The relationship between the thickness of the anodized Al/sub 2/O/sub 3/ tunneling insulator and the transfer ratio was investigated for metal-insulator-metal (MIM) cathodes to optimize the thickness in terms of a high transfer ratio and emission current. Combining ellipsometry, X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM), we determined the accurate thickness of an anodized Al film less than 20 nm-thick. With the knowledge of accurate thickness, we found that the transfer ratio increases as the insulator thickness increases from 5.2 nm to 10.6 nm, but saturates at 13.3 nm and decreases slightly at 20.1 nm. Optimizing the thickness of the insulator to 13.3 nm raised the transfer ratio of 0.1% for our previous work (Kusunoki and Suzuki, IEEE Trans. Electron Devices, vol. 47, pp. 1667-1672, 2000) to 0.7%. A high emission current of 14 mA/cm/sup 2/ was thus obtained. The existence of an optimal thickness for the anodized Al/sub 2/O/sub 3/ insulator was also clarified from a theoretical simulation. This is the result of a trade-off, as thickness increases, between the decreasing probability of cut-off at the surface workfunction barrier of the Ir-Pt-Au top electrode and the increased scattering of hot electrons inside the Al/sub 2/O/sub 3/ insulator and top electrode. The relationship is discussed on the basis of the absolute distribution of energy of the hot electrons, which we determined by simulating inelastic scattering driven by electron-optical phonon interaction in the Al/sub 2/O/sub 3/ insulator.

Increasing emission current from MIM cathodes by using an Ir-Pt-Au multilayer top electrode

We investigated the effect of the top electrode materials on the electron emission and durability of metal-insulator-metal (MIM) cathodes. The durability is improved when high sublimation-enthalpy material, such as Ir, Mo, or W, are used; however, the emission current, and the transfer ratio, decrease. The material dependence of the transfer ratio is shown to be dominated by the scattering probability of hot electrons in the metal. The scattering probability was estimated from the metal's density-of-states, or more simply, from the number of d-electrons. We found that the multilayer top electrode consisting of Ir, Pt, and Au provides the best top electrode combination for MIM cathodes. The high sublimation-enthalpy Ir layer, which is in contact with the insulator, acts as a barrier metal and improves the durability, whereas the surface Au layer maintains the transfer ratio at a high value. With this top electrode structure, emission current density is increased to 5.8 mA/cm/sup 2/, which is sufficient for field-emission displays. We demonstrated a display consisting of a 30/spl times/30-dot MIM cathode-array with the multilayer top electrodes.

MIM cathodes, their application to electron lithography and characteristics of the emitted electrons

Abstract A brief account of the possibility of applying metal-insulator-metal (MIM) cathodes to projection electron lithography (a 1:1 scale) is given, together with a description of the measurement of the energy distribution and angular distribution characteristics of the electrons emitted from MIM cathodes.

A 1:1 electron stepper

A new approach to the design of a 1:1 electron stepper has been suggested. A metal-insulator–metal (MIM) cathode is used in place of the photoemission one. The mechanism of the electron emission from the MIM cathode is markedly different from that of photoemission. In the MIM cathode the electrons from the base metal layer move through the thin dielectric layer, in which a strong electric field is generated (about 7×108 V m−1), into the outer thin metal layer. Those electrons that have enough energy remaining after scattering in the dielectric and outer thin metal layer (to overcome the work function barrier of the outer metal layer) will escape into the vacuum. Compared to photoemission, the described mechanism has some advantages if considered for use in electron projection lithography. For example, an emission current density of the order of 10−4 A cm−2; the cathode can be easily and quickly switched on and off with the image of the mask and independently of the orientation marks; and finally, the possibility of preparing cathodes outside the vacuum chamber of the stepper and their storing. To verify the advantages of the MIM cathode and experimental 1:1 electron stepper has been constructed and orientation experiments have been evaluated.