All of this is made possible by a confluence of technological developments in silicon chip capability and display technology. On the silicon side, ever-decreasing feature sizes have led to a high level of functional integration on a single chip. In displays, technologies have progressed both in terms of pixel addressing (from passive to active matrix) and operating voltage levels — from about 40 V in the mid-1980s down to less than 3 V today for LCD technologies. In addition, the introduction of a low-voltage emissive technology based upon organic light emitters has provided an alternative to liquid-crystal-based displays.

As with flat-panel displays, the targets for image characteristics are set by the need to visually match the CRT or, more precisely, the computer monitor color CRT, which has significantly better capability than a TV CRT. This means that luminance should be at least 100 cd/m2 and contrast greater than 100:1.

Typically, 256 gray levels are desired for each primary color, and the color gamut, or range, should match or exceed that of a standard color CRT. To match the CRT for showing video information, the rise and fall times for the microdisplays must be less than a few milliseconds.

Efficiency is always a concern for displays, but it is especially important for microdisplays that are to be used in handheld appliances or headsets for wearable products. For example, a white OLED emitter with an efficiency of one lumen per watt will result in a filtered white color microdisplay that dissipates about 0.25 W (assuming a chip active area of 1 cm2 and a surface luminance of 150 cd/m2. Control and driving circuitry can add another 150 mW, depending upon the level of functional integration, leading to a total of 0.4 W of power for the microdisplay subsystem.

The best match to the range of microdisplay requirements is achieved by the combination of OLEDs on silicon. OLEDs are efficient Lambertian emitters that operate at voltage levels (3 to 10 V) accessible with relatively low-cost silicon. They are capable of extremely high luminances (>100,000 cd/m2), a characteristic that is especially important for use in the helmets of military pilots, even though most of the time they would be operated at much lower levels. Luminance is directly linear with current, so gray scale is easily controlled by a current-control pixel circuit. OLEDs are very fast, with faster response than liquid crystals, an important feature for video displays. Fabrication is relatively straightforward, consisting of vacuum evaporation of thin organic layers, followed by thin metal layers and a transparent conductor oxide layer. A whole wafer can be processed at one time, including a process for sealing, before the wafer is sawed into individual displays. Up to 750 small displays (QVGA format using a 12-mm color pixel pitch and a die size of 7 mm x 5 mm) can be produced from a single 8-inch wafer, including interface and driver electronics embedded in the silicon.

Last spring eMagin announced the availability a full-color, video-capable OLED microdisplay for use as a computer or video display. When viewed through magnifying optics, the SVGA+ high-resolution microdisplay creates virtual images similar to the real images of a computer monitor or large TV screen. The display has over 1.5 million potential color subpixel elements (600 x 3 x 852 pixels) and stores all the color and luminance value information at each of the pixel elements in the display array, eliminating the flicker or color breakup seen by most other high-resolution display technologies.

There are two basic approaches to matrix display operation: passive and active modes. The passive mode relies on data and select drivers to provide the desired information at the correct location in the pixel array. The amount of time spent at each pixel converting the electrical information into a light modulation is therefore inversely proportional to the number of pixels in the array. More precisely, because of the use of parallel addressing techniques, the addressing time is inversely proportional to the number of rows in the matrix. So this addressing mode cannot practically be used for video-rate flat-panel displays with high-information content (those of formats greater than QVGA color).

Popularized by LCD flat panels, the active addressing mode places a storage element at each pixel cell. The benefits of this approach have already been demonstrated with notebook (and now desktop) flat-panel displays. While its implementation requires additional substrate processing, it is now an accepted standard and its price premium over passively addressed displays is no longer the barrier it once was. Combined with a line buffer and a vertical-stripe color pixel arrangement, this mode maximizes not only the time a pixel has to convert electrical data to light-modulated information, but also the time it takes to transfer the source information to the pixel storage element. The net result is reduced bandwidth requirement as well as widened tolerance of substrate technology performance.

CMOS is the preferred technology for high-information-content microdisplays designed for portable systems. It can accommodate analog or digital inputs or both, offers a well-established infrastructure that is accessible and does not require the display developers to make extraordinary capital investments. The advances in CMOS processes far exceed the requirements of the display technologies themselves. Sometimes this may be a drawback because of the fast obsolescence of older (three- to five-year-old processes are considered ancient) and higher-voltage processes. Indeed, most display technologies lag behind advances in integrated circuits with respect to voltage levels. This has placed constraints on material developments, such as a reduced voltage swing for LCD microdisplays, that have delayed products.

Critical to the performance of the microdisplay chip is the circuit that controls the luminance of each pixel. To understand the electrical requirements here, we need to review some of the basic facts surrounding OLEDs.

Organic light-emitting diodes were invented by C. W. Tang and S. A. Van Slyke of Kodak, who found that p-type and n-type organic semiconductors could be combined to form diodes, in complete analogy to the formation of p-n junctions in crystalline semiconductors. Moreover, as with gallium arsenide and related III-V diodes, the recombination of injected holes and electrons produced light efficiently. In contrast to the difficult fabrication of III-V LEDs, where crystalline perfection is essential, organic semiconductors can be evaporated as amorphous films, for which crystallization may be undesirable.

The prototypical Kodak OLED, which is used in consumer products today by Pioneer Corp., is a down-emitting stack, with light coming out through a transparent glass substrate.

For use on top of an opaque silicon chip, we modify the stack, starting with a metal anode with a high work function and ending with a transparent cathode, followed by a layer of transparent indium tin oxide. We also change the active layer to make it a white-light emitter. We do this by using a diphenylene-vinylene type blue-green emitter, which is co-doped with a red dye to yield a white spectrum.

Even though an OLED microdisplay on silicon may have millions of subpixels, the OLED formation can be rather simple, because the complexity is all in the substrate. For each subpixel, corresponding to a red, green or blue dot, there is a small electrode pad, possibly 3.5 x 13.5 microns, attached to an underlying circuit that provides current. The OLED layers can be deposited across the whole active area, using shadow masking in the evaporator. This includes the cathode, which is common for all pixels. This simple approach is made possible by the fact that the materials are not good lateral conductors, so the very thin organic films cannot shunt current from one subpixel to another. With such thin structures, light also does not leak into adjacent pixels, so contrast is maintained even between neighboring pixels.

The preferred way of producing color in microdisplays is to use color filters or color conversion materials (CCMs). The filters, which are identical to those that are used in liquid crystal displays, can be used in conjunction with the white emitter described above to give a well-balanced color display. Each filter, of course, absorbs about two-thirds of the light, so a more efficient approach is to use CCMs, whereby light from a blue emitter can be converted to green or red by absorption and re-emission. In this system, less than half of the light is lost — provided the CCMs are efficient.

OLED devices are very sensitive to moisture, which attacks the cathode materials; to a lesser degree, they are sensitive to oxygen, which can potentially degrade the organics. For this reason they must be sealed in an inert atmosphere after fabrication and before being exposed to ambient environmental conditions. OLEDs on silicon can be sealed with a thin film seal, developed by eMagin. This thin film seal has been critical to the development of full color OLED microdisplays, since microscopic color filters can be processed on top of a white OLED using photolithography after passivation with the thin film seal.

In a basic active-matrix OLED pixel cell, the current through the OLED is controlled by an output transistor and all emitting devices share a common cathode to which a negative voltage bias is applied. This bias is set to allow the full dynamic range. Since the output transistor has a limited voltage capability, depending upon the silicon process used, it is important to have a steep variation in the OLED of luminance versus voltage so that a large variation in luminance — for example, 100:1 — can be achieved for a voltage swing of 4 V or less.