Apple's patent background begins by acknowledging that active matrix organic light emitting diode (AMOLED) displays have become more mainstream due to their better contrast ratios when compared to conventional liquid crystal displays (LCDs).

According to Apple, AMOLED displays are self-emissive devices, and do not require backlights. AMOLED displays may also provide more vivid colors and a larger color gamut than the conventional LCDs. Further, AMOLED displays can be made more flexible, thinner, and lighter than a typical LCD.

An OLED generally includes an anode, one or more organic layers, and a cathode. A microcavity is formed between the anode and the cathode. The AMOLEDs often use the microcavity to enhance color purity of a display. In the microcavity, an optical path length between the anode and the cathode may be tuned to enhance the aforementioned color purity.

AMOLEDs can either be a bottom emission type or a top emission type. In bottom emission OLEDs, the light is extracted from an anode side. In such embodiments, the anode is generally transparent, while a cathode is generally reflective. The pixel area is shared between the OLED and a backplane driving circuit that includes thin film transistors (TFT), metal routing and capacitors. The driving circuit may contain a few TFTs, for example, three to five TFTs, with few control signals, and one or two capacitors. As a result, the area available for OLED is limited such that an OLED aperture is small.

In a top emission OLED, light is extracted from a cathode side. The cathode is optically transparent, while the anode is reflective. This top emission OLED normally enables a larger OLED aperture than a bottom emission OLED.

Apple's invention generally relates to active matrix organic light emitting diode (AMOLED) displays. More specifically, certain embodiments relate to processes for realizing a variable optical path length for one or more microcavities in an AMOLED display.

One aspect of Apple's invention relates to AMOLED displays having a different optical path length for each sub-pixel emitting light of a different wavelength, such that each sub-pixel has a microcavity tuned for each respective wavelength. The different optical wavelengths are achieved by including a tuning layer between the anode and the cathode. The tuning layer has different thicknesses for each of the sub-pixels.

Apple's invention also provides methods for achieving different thicknesses of the tuning layer without using extra masking operations or with only one extra masking operation, when compared to conventional fabrication techniques. For example, a shadow mask may be used for depositing the OLED layer. Methods described herein may use the same shadow mask for depositing the tuning layer. This method is much simpler than a conventional process which generally requires four extra masks in order to deposit a second ITO layer having different thicknesses for different sub-pixels.

Apple's patent FIG. 1A noted above illustrates a perspective view of a sample electronic device such as an iPad which includes a touch screen display which incorporates a cover glass 106 and an AMOLED display behind the cover glass. Apple's patent FIG. 4F shows a cross-sectional view of a process architecture including light emitting layer deposition and common electrode deposition for the top emission OLED display.

If you're interested in the finer details of Apple's complicated OLED related invention, see patent application 20140203245.

Patent Credits

Apple credits Vasudha Gupta, Young Bae Park and Shih Chang Chang (yes double Chang) as the inventors of this patent application which was originally filed in Q1 2013. Considering that this is a patent application, the timing of such a product to market is unknown at this time.

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