Vertical Divider
Musing in Shanghai at the International OLED Summit
LTPO for Low Power
January 20, 2020
One of the latest innovations in pixel design is the use of a combination of IGZO and LTPO for the pixel circuit. This approach takes advantage of the low leakage current of oxide TFTs, while still using the smaller, higher mobility p-Si TFTs to permit the required TFTs to fit in high pixel density implementations. The benefit is achieved when an image is relatively static , for example the time on a watch. For a smart watch, when only the time is shown in minutes the update cycle need only be 1Hz if there is no leakage. At 1 HZ, the display power consumption is theorectically 1/60thof the normal image. This approach was originally designed by Apple and was implemented in the Apple Watch 4 and 5. The 5 takes advantage of the capability by offering an always on time. The downside if the approach is the increased cost of the backplane process, which requires an increase in the # of masks from 9 for LTPS to 20 for LTPO and the additional tool set.
Figure 1: Implementing LTPO
January 20, 2020
One of the latest innovations in pixel design is the use of a combination of IGZO and LTPO for the pixel circuit. This approach takes advantage of the low leakage current of oxide TFTs, while still using the smaller, higher mobility p-Si TFTs to permit the required TFTs to fit in high pixel density implementations. The benefit is achieved when an image is relatively static , for example the time on a watch. For a smart watch, when only the time is shown in minutes the update cycle need only be 1Hz if there is no leakage. At 1 HZ, the display power consumption is theorectically 1/60thof the normal image. This approach was originally designed by Apple and was implemented in the Apple Watch 4 and 5. The 5 takes advantage of the capability by offering an always on time. The downside if the approach is the increased cost of the backplane process, which requires an increase in the # of masks from 9 for LTPS to 20 for LTPO and the additional tool set.
Figure 1: Implementing LTPO
Source: APS Holdings
Source: APS Holdings
Source: APS Holdings
Dr. Kim also addressed one of the major bottlenecks in the OLED deposition process, the current method of producing FMMs. FMM’s are relatively expensive costing around US$1m for reach display design and the current pixel density is ~600 ppi.
Figure 2: RGB Patterning
Figure 2: RGB Patterning
Source: APS Holdings
Figure 3: Current Wet Etch Process for Making FMMs
Source: APS Holdings
Currently, there is no source for invar sheets larger than 1M in width, which limits the FMM and therefore the VTE system to 925 x 1500m or 1⁄2 6G. Mask sag contributes to mask limitations. If this limitation is overcome, full 6G evaporation systems could result in increased productivity and reduced panel costs.
Figure 4: Two Reasons for Poor Yield, No HR FMM
Figure 4: Two Reasons for Poor Yield, No HR FMM
Source: APS Holdings
An option is to use the stick and frame approach.
Figure 5: FMM Stick + Frame
Figure 5: FMM Stick + Frame
Source: APS Holdings
Figure 6: Thin Rolled Invar Stick Process and Thickness Process Control (TCP) from APS
Source: APS Holdings
Figure 7: Laser Ablation Process
Source: APS Holdings
Figure 8: 800 -ppi Invar Stick and Frame Example
Source: APS Holdings
Figure 9: FMM Multi-head Laser Processing System Concept
Source: APS Holdings
Figure 10: 6G HR Pilot Line
Source: APS Holdings