The "hole story" for FED flat-panel displays

Field emitter displays (FEDs) offer improved image quality and brightness, as well as lower power consumption, over conventional flat-panel displays that use liquid-crystal display (LCD) technology. FEDs are not new. In fact, the Spindt cathode, the basis for FED design, was developed more than 30 years ago (see box on next page). The difficulty of converting success in the laboratory into practical, large-sized FEDs that can be mass produced, however, has hampered efforts to bring these products to market. Such devices are finally ready to compete with active-matrix LCDs as the industry's technology of choice.

FEDs are three times brighter and four times more energy efficient than their LCD counterparts. They permit viewing from any angle up to almost 180°. A response time of 20 ns allows image changes that are fast enough for the most demanding motion applications. In addition, expected costs will be 40–60% less than for a comparable product with an LCD.

Manufacturing FEDs, however, present formidable challenges. One such challenge involves inspecting holes fabricated as part of FED construction and precisely measuring their diameters.

Figure 1 represents the cross-section of an FED tube. On a superficial level, FEDs look like cathode-ray tubes (CRTs) in disguise. That is, electrons emitted from a cathode are accelerated through a vacuum by a voltage differential between cathode and anode before striking a red, green, or blue phosphor coating on the anode with sufficient velocity to produce light.

There the similarities end. A CRT, for instance, is bell-shaped and generally several inches to a foot or more deep. In contrast, an FED is flat and typically only 2.0–4.0 mm thick. Instead of a single beam steered by a deflection system, FED technology employs a matrix—rows and columns with millions of cone-shaped emitters about 1 micron in diameter at the base. All cone tips are positioned in the same plane at the center of holes microfabricated in a metal plate. An insulating material is sandwiched between the plate and the conductive cathode. The holes, called "gates," are approximately the diameter of the base of the cones—the exact dimensions depend on each manufacturer's design.

By applying a voltage differential between cone and hole, the cone tip produces electrons, creating an electron "cloud." The high electric field (typically 1 kV) between cathode and anode generates an electron stream between the emitters and the anode's phosphor coating. Unlike with CRTs, however, the electrons do not travel far. Cathode-to-anode distances can be as little as 200 microns.

Both the number of emitters—more than 200,000 for a 4-in. square of silicon (the cathode material)—and the 1-micron hole diameter challenge the processes of manufacture and inspection. Gates are micromachined in the metal using a resist to selectively etch the holes through the plate material. Checking for out-of-roundness of the holes, as well as ensuring that gates have been machined to the specified diameter, requires an inspection technique that offers high precision and high repeatability.

Various optical systems, including conventional microscopes or lenses, can measure linewidths down to 1 micron or less. Measuring hole dimensions to that level, however, demands additional features. We have developed a system that includes image-analysis software as well as auto-focus and auto-illumination capability. It also incorporates a CCD camera to digitally transfer microscope images to computer memory and to a CRT screen. The screen permits strain-free operator viewing of the hole being measured, and it allows the system software to overlay the analysis results onto the image.

Measuring FED hole diameters with this system encompasses a number of steps. In the first step, the system roughly centers the gate plate on the x-y platform within the microscope's field of view and then activates the autoillumination and autofocus functions to achieve the desired illumination and bring the image into perfect focus.

Before measuring a circle, the inspection system must locate it by analyzing the pixels within the measurement window (the red-outlined box in Figure 2). To do so, the system identifies the centroid of the group of pixels demonstrating gray-scale brightness different from the background and then builds a radial-intensity profile by calculating 360 radial scan lines (one for each degree in the circle). As the profile in the lower left of Figure 2 shows, the lowest brightness values represent the actual hole.

The circle may show multiple edges (concentric rings of various diameters), depending on which portion is being measured. The operator can define the desired edge with the measurement setup file or can select it manually with a mouse click near an edge on the intensity profile screen display. The system stores the resulting setup file so users can reload it for subsequent hole measurements.

The measuring system calculates the threshold (gray-scale location) for the selected edge between a minimum and maximum determined from the intensity profile. The threshold can be thought of as the brightness value at which the circle edge occurs. Areas above the threshold value (brighter) are outside the circle, while areas below the threshold value (darker) lie inside the hole.

Using the predetermined threshold level, the inspection system finds a circle edge point on each of the radial scan lines. The software collects the edge points into a data set and then applies a best-fit circle algorithm to determine the diameter and center. To further improve precision, the system discards outliers (edge points that deviate from the circle by a predetermined amount) and calculates a new best-fit circle. Finally, the system produces a new intensity profile, repeating the measurement until a specified number of edge pixels meet established criteria for deviation from the best-fit circle.

In production, this technique has produced results faster, more accurately, and with higher repeatability than a scanning electron microscope. Micromachined hole measurements showed accuracies better than ±0.01 microns and repeatability better than ±0.005 microns.