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LTPO Could Replace LTPS for OLED Backplanes
September 03, 2018

 
IHS Markit predicted that Apple could shift from using LTPS TFT backplanes to usingLTPO (Low Temperature Polycrystalline Oxide)backplanes in future iPhones to conserve battery life.Of course, since Apple doesn’t make displays, they would have to convince, Samsung, LG Display, Sharp, or JDI to develop the technology.IHS believes that LTPO TFT could save between 5-15 percent in power consumption versus LTPS by combine both the technical strengths of c-IGZO and LTPS on a single array backplane. C-IGZO used for the pixel switch TFT have low leakage current and therefore, low power consumption. Experiments have shown good quality of crystal grain size and clear crystallization properties of IGZO using high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) patterns.  The manufacturing process steps as described by AUO[1]include:
 

• Top gate IGZO (TG-IGZO) TFTs:  Amorphous silicon crystallized into poly silicon film using excimer laser annealing, 
• TG-IGZO active layer formed on the same buffer layer with TG-LTPS. 
• The active layers of IGZO and LTPS are on the same plane of buffer layer top surface. 
• The remaining processes are similar to LTPS TFT process after poly silicon layer. 
  • Bottom gate IGZO (BG-IGZO) TFTs: After deposition of the inter layer dielectric (ILD), the BG-IGZO active layer is deposited
  • The active layers of IGZO and LTP are on the different planes. 
  • The remaining processes are similar to LTPS TFT process after Source/Drain (SD) metals layer.
    • The single protective layer SiOx was deposited using PECVD with precursor SiH4 and oxygen donor N2O gases. For a better film quality, the higher ratio of Si-O/Si-N was focused under process optimization and the conditions include different spacing and temperature of PECVD. 
    • To eliminate unnecessary glass substrate features, the deposition of protective layer SiOx was performed on a Silicon wafer sticking to a glass substrate. 
  • After the growth processes, the samples were peel off and analyzed by X-ray photoelectron spectroscopy (XPS) and Fourier Transform Infrared spectroscopy (FTIR).
  • The IGZO TFT electrical stability is improved by adjusting the thickness and post-treatment of the IGZO active layer, those adjustments are qualified by positive gate bias temperature stress (PBTS) measurements. 
  • The post-treatment of the IGZO active layer used different PECVD plasma power of N2O gas. The electronic transfer characteristics and positive gate bias temperature stress (PBTS) of TFTs were measured by HP-4156C semiconductor parameter analyzer. And finally the improved process conditions were apply on the 1.4” inch circular LCD to test high temperature operation (HTO) (60°C) and temperature humidity- bias (THB) (60°C and 90% humidity) under operating.

 
In this AUO experiment, the mobility was relatively low at 5.7 cm2/Vsec, but the point was to demonstrate a working prototype.  The process includes the entire excimer laser, ion doping steps for p-Si and PVD for amorphous oxide, so the capex is likely to be more than LTPS. What wasn’t clear from the AUO report was whether they were able to reduce the number of masks or the excimer laser shots required to achieve high uniformity of the grains. 
 
IHS’s claim that LTPO would reduce the power consumption by 10%-15% was not verified and the reduction apparently only relates to the power consumed by the backplane.
 
Figure 1: (a) Schematic cross-section of IGZO and LTPS hybrid TFTs with top gate IGZO TFTs (b) Schematic cross-section of IGZO and LTPS hybrid TFTs array technology with bottom gate IGZO TFTs

​(a)

Source: AUO

(b)

Source: AUO

Figure 3: LTPO Power Reduction

Source: IHS