**3. Lithographic patterning of OLEDs to increase resolution**

In order to realize an ultrahigh resolution display, all elements of the system (backplane, frontplane, and driving) need to provide appropriate pixel density. On the frontplane side, several options for the light source can be chosen (**Figure 2**). OLED technology currently dominates the smartphone display industry not only with performance but also with the cost structure. In this case, the colors are defined by depositing separate device stacks for each color, which is typically referred to as side-by-side, red-green-blue (RGB) array. In OLED TVs, one common white OLED stack is combined with a color filter array (CFA). The limitation of the side-by-side RGB array is the pixel density, limited by the fine metal masking (FMM) technology, which uses deposition through a metal mesh. The white OLED array can achieve very small pixel pitch, which is only limited by the backplane and CFA resolution but imposes brightness loss due to CF transmission. Patterning multicolor OLEDs by photolithography can address the needs of ultralow pixel pitch for the future AR displays by realizing side-by-side OLED stacks with extreme density.

Patterning OLEDs by photolithography is an emerging, disruptive fabrication technique. The main challenge is the extreme chemical sensitivity of OLED materials with solvent, moisture, air, and temperature exposure responsible for performance degradation. The choice of appropriate photolithography chemistry is crucial, with fluorinated [6] or non-fluorinated systems [7] as the dominant options. **Figure 3** shows the concept of using a negative-type photoresist to define patterns on top of OLED in a subtractive approach. First, the OLED stack is deposited as a plain layer over the entire substrate, on top of a pixel definition layer (PDL). This defines the active area of the light emitter. Second, photoresist is deposited on top of the entire substrate. Then, it is exposed through a lithography mask and developed to obtain the required pattern. Afterwards, the OLED layers that are not covered by the photoresist are etched away (typically with dry etching, such as reactive ion etch). In the end, the photoresist is stripped to achieve patterned OLED islands.

**Figure 2.** *Various display configuration options.*

#### **Figure 3.**

*Process flow for photolithography patterning of OLED stacks.*

Photolithography allows pattern transfer beyond 1 μm resolution, enabling high-density lines and spaces. Transfer of small islands means that, with appropriate alignment (e.g., with an i-line stepper), a pixel density of a few thousand pixels per inch (ppi) can be realized. Transfer of openings means that pixel spacing can be minimized, resulting in a high aspect ratio. This is applicable for both TFT-based flat panel displays and CMOS-based microdisplays. Tests on patterning the OLED emission layer have shown that it is possible to achieve 1 μm pitch lines and spaces (**Figure 4**). Furthermore, the photoluminescence signal of the EML is maintained proving compatibility of this process with OLED material. 1 μm presented here is not a fundamental limit of the approach but rather a limit of the lithography mask design used in the experiment.

The achievable pixel density of the frontplane is limited not only by the photoresist used but also by the critical dimension (CD) and alignment/overlay accuracy of the litho tools used. In the i-line steppers typical for flat panel

**127**

**Figure 5.**

*AMOLED Displays with In-Pixel Photodetector DOI: http://dx.doi.org/10.5772/intechopen.93016*

*patterned as lines (left) and spaces (right).*

**Figure 4.**

manufacturing, the achievable CD is 1.5 μm with an overlay between 0.25 and 0.5 μm. In contrast, CMOS fabs used for microdisplay manufacturing feature more advanced semiconductor nodes, with 248 nm KrF or 193 nm ArF light sources. Assuming a minimum PDL opening (defining the active area) of 500 nm, a 1.5 μm node imposes a density limit of 3500 ppi (for RGB) with an aperture ratio below 5%. Going to KrF steppers, the achievable density increases to 10,000 ppi while keeping the aperture ratio above 35%. This demonstrates the need of a tooling upgrade for future AR displays, both for the frontplane and the backplane. Denser and more efficient packing of pixels requires scaling down of

*OLED patterns of 1, 2, 3 and 4 μm: optical and corresponding photoluminescence pictures of red EML* 

OLED patterning by photolithography means that the deposition of the stack is interrupted (vacuum break) and the photoresist interacts with the organic materials. In the most simple case, the photolithography process is performed in a clean room in ambient atmosphere. The devices are loaded back into the glove box after the etch step for each color and after the photoresist strip when all colors are finished. This raises a serious challenge for the device lifetime. If the process is not optimized for compatibility with the stack, the current-voltage-luminance (IVL) curve shifts to the right (increased turn-on voltage) and to the bottom (reduced luminance). As a consequence, the brightness of the patterned OLED drops very fast and disappears even after a few minutes (**Figure 5**). Optimization of the photoresist system, of the OLED stack [8] and of the fabrication process, is needed to achieve OLED performance enabling implementation into devices. At imec, we demonstrated phosphorescent green OLED with T90 lifetime of >150 h at the starting brightness of 1000 nit. Efficiency remained above 85 cd/A before and after patterning. Current performance is considered an important step on the path to industrial technology readiness level,

the technology node, especially in FPD manufacturing.

estimated to be T97 of at least 1000 h (for the green stack) [8].

*IVL and lifetime curves for reference OLED and the possible effects of degradation by patterning.*

*AMOLED Displays with In-Pixel Photodetector DOI: http://dx.doi.org/10.5772/intechopen.93016*

**Figure 4.**

*Liquid Crystals and Display Technology*

Photolithography allows pattern transfer beyond 1 μm resolution, enabling high-density lines and spaces. Transfer of small islands means that, with appropriate alignment (e.g., with an i-line stepper), a pixel density of a few thousand pixels per inch (ppi) can be realized. Transfer of openings means that pixel spacing can be minimized, resulting in a high aspect ratio. This is applicable for both TFT-based flat panel displays and CMOS-based microdisplays. Tests on patterning the OLED emission layer have shown that it is possible to achieve 1 μm pitch lines and spaces (**Figure 4**). Furthermore, the photoluminescence signal of the EML is maintained proving compatibility of this process with OLED material. 1 μm presented here is not a fundamental limit of the approach but rather a limit of the lithography mask

The achievable pixel density of the frontplane is limited not only by the photoresist used but also by the critical dimension (CD) and alignment/overlay accuracy of the litho tools used. In the i-line steppers typical for flat panel

**126**

**Figure 3.**

design used in the experiment.

*Process flow for photolithography patterning of OLED stacks.*

*OLED patterns of 1, 2, 3 and 4 μm: optical and corresponding photoluminescence pictures of red EML patterned as lines (left) and spaces (right).*

manufacturing, the achievable CD is 1.5 μm with an overlay between 0.25 and 0.5 μm. In contrast, CMOS fabs used for microdisplay manufacturing feature more advanced semiconductor nodes, with 248 nm KrF or 193 nm ArF light sources. Assuming a minimum PDL opening (defining the active area) of 500 nm, a 1.5 μm node imposes a density limit of 3500 ppi (for RGB) with an aperture ratio below 5%. Going to KrF steppers, the achievable density increases to 10,000 ppi while keeping the aperture ratio above 35%. This demonstrates the need of a tooling upgrade for future AR displays, both for the frontplane and the backplane. Denser and more efficient packing of pixels requires scaling down of the technology node, especially in FPD manufacturing.

OLED patterning by photolithography means that the deposition of the stack is interrupted (vacuum break) and the photoresist interacts with the organic materials. In the most simple case, the photolithography process is performed in a clean room in ambient atmosphere. The devices are loaded back into the glove box after the etch step for each color and after the photoresist strip when all colors are finished. This raises a serious challenge for the device lifetime. If the process is not optimized for compatibility with the stack, the current-voltage-luminance (IVL) curve shifts to the right (increased turn-on voltage) and to the bottom (reduced luminance). As a consequence, the brightness of the patterned OLED drops very fast and disappears even after a few minutes (**Figure 5**). Optimization of the photoresist system, of the OLED stack [8] and of the fabrication process, is needed to achieve OLED performance enabling implementation into devices. At imec, we demonstrated phosphorescent green OLED with T90 lifetime of >150 h at the starting brightness of 1000 nit. Efficiency remained above 85 cd/A before and after patterning. Current performance is considered an important step on the path to industrial technology readiness level, estimated to be T97 of at least 1000 h (for the green stack) [8].

**Figure 5.** *IVL and lifetime curves for reference OLED and the possible effects of degradation by patterning.*

**Figure 6** shows an example comparison of unpatterned and patterned OLED lifetime curve at initial brightness of 1000 nit. The performance improvement can bring the two curves closer together.

OLED photolithography was used to fabricate passive displays with a 1400 × 1400 pixel array (almost 2 megapixels). 6 μm metal lines and 10 μm line pitch with SiN pixel definition layer (PDL) were used on glass substrate. Green and red OLED stacks were deposited by thermal evaporation in ultrahigh vacuum. After deposition of the first color (until above emission layer), photoresist was spincoated, baked, exposed, and developed. Then, the OLED stack not covered by the photoresist was removed by dry etching. After that, the sample went back to the ultrahigh vacuum chamber for second color deposition, and the patterning process was repeated, this time finishing with stripping the photoresist. A semitransparent top contact stack was subsequently deposited, and the display was encapsulated with cavity glass. Both colors can be driven separately, and the PDL design allows for emission of a fixed image specified for each color (**Figure 7**). Subpixel pitch of 10 μm resulted in smooth edges and excellent feature representation. The device was tested for tens of hours with both colors on. No drop of brightness nor appearance of defects could be observed [9].

#### **Figure 6.**

*Lifetime curves of a phosphorescent green OLED at 1000 nit starting brightness for unpatterned and patterned stack.*

#### **Figure 7.**

*Passive 1250 ppi patterned OLED display with 1400 × 1400 pixels, 10 μm subpixel pitch, and independent color driving: general view (left) and detailed view for different color drivings (right).*

**129**

**Figure 8.**

*AMOLED Displays with In-Pixel Photodetector DOI: http://dx.doi.org/10.5772/intechopen.93016*

**4. Increased resolution by pixel driving techniques**

2T1C pixel circuit, can achieve a much higher pixel density.

for a full-color display.

This fabrication process is compatible with both CMOS backplanes and flexible TFT backplanes. The frontplane can thus be implemented in an active matrix display. Of course, photolithography can be used several times to realize more colors

To increase the display resolution, not only the technology (backplane and frontplane) but also the pixel driving techniques should be optimized. The OLED light output is dependent on the drain current of the driving TFTs of the AMOLED displays. Due to inherent variations in AMOLED displays, some compensation methods to the drain current of the TFTs are required to achieve uniform brightness. This can be implemented through either in-pixel compensation [10] or external compensation [11, 12]. Since in-pixel compensation schemes typically require more transistors inside the pixel, external compensation methods are preferred for high-resolution applications. **Figure 8** shows pixel circuits and a possible layout for in-pixel compensation, using an 8T1C [10] pixel, and external compensation using a 3T2C [11] and, respectively, a 2T1C [12] pixel. For all these layouts, the same design rules were used. It is clear from this figure that a display with external compensation, especially the

The achievable pixel density depends on both the pixel circuit and the design rules imposed by the technology, such as the critical dimension (CD) of the lithography tool. **Figure 9** compares the achievable resolutions for different CDs for the 8T1C, the 3T2C, and the 2T1C pixel circuit. Although the CD of 1.5 μm, as currently achievable with typical i-line steppers, only yields a maximum pixel density of 565 ppi for the 8T1C pixel circuit, the same CD already yields a significant improvement for the pixel circuits using external compensation, namely, 847 ppi for the 3T2C pixel

*Pixel circuits and corresponding layouts for (left) 8T1C, (middle) 3T2C, and (right) 2T1C pixels.*

#### *AMOLED Displays with In-Pixel Photodetector DOI: http://dx.doi.org/10.5772/intechopen.93016*

*Liquid Crystals and Display Technology*

bring the two curves closer together.

appearance of defects could be observed [9].

**Figure 6** shows an example comparison of unpatterned and patterned OLED lifetime curve at initial brightness of 1000 nit. The performance improvement can

OLED photolithography was used to fabricate passive displays with a 1400 × 1400 pixel array (almost 2 megapixels). 6 μm metal lines and 10 μm line pitch with SiN pixel definition layer (PDL) were used on glass substrate. Green and red OLED stacks were deposited by thermal evaporation in ultrahigh vacuum. After deposition of the first color (until above emission layer), photoresist was spincoated, baked, exposed, and developed. Then, the OLED stack not covered by the photoresist was removed by dry etching. After that, the sample went back to the ultrahigh vacuum chamber for second color deposition, and the patterning process was repeated, this time finishing with stripping the photoresist. A semitransparent top contact stack was subsequently deposited, and the display was encapsulated with cavity glass. Both colors can be driven separately, and the PDL design allows for emission of a fixed image specified for each color (**Figure 7**). Subpixel pitch of 10 μm resulted in smooth edges and excellent feature representation. The device was tested for tens of hours with both colors on. No drop of brightness nor

**128**

**Figure 7.**

**Figure 6.**

*patterned stack.*

*Passive 1250 ppi patterned OLED display with 1400 × 1400 pixels, 10 μm subpixel pitch, and independent* 

*Lifetime curves of a phosphorescent green OLED at 1000 nit starting brightness for unpatterned and* 

*color driving: general view (left) and detailed view for different color drivings (right).*

This fabrication process is compatible with both CMOS backplanes and flexible TFT backplanes. The frontplane can thus be implemented in an active matrix display. Of course, photolithography can be used several times to realize more colors for a full-color display.
