By Pete Singer, Editor-in-Chief, Solid State Technology
Applied Materials introduced three new tools for the display market aimed at metal oxide thin film transistors. The tools, one of which is CVD and the other two PVD, employ new hardware designs and process technology that enable better film uniformity with fewer defects, and are designed for use with next generation IGZO-based thin film transistors (TFTs). The display industry is quickly switching to metal oxide TFTs and IGZO (indium gallium zinc oxide) is the material of choice.
Higher resolution LCD displays, greater than 300 dpi, require a switch from amorphous silicon designs to either metal oxide transistors or low-temperature polysilicon (LTPS), which offer higher-mobility in a smaller area (Figure 1). They also operate at lower power levels, which is important in mobile devices. Another problem with larger transistors is that they block too much of the light in the display.
LG has already begun shipping 55-inch OLED TVs using metal oxide backplanes and by 2014, all major LCD and LED display makers will have begun the switch over to metal oxide TFTs.
The advantage of metal oxide transistors over LTPS transistors is that they consume less power and are more easily scaled.
The layers in an IGZO transistor are deposited by both PVD and CVD, according to Max McDaniel, Applied Materials’ director and chief marketing officer for its display business. Figure 2 shows a cross-section of the device. “You use PVD to deposit the metal gate material (on the glass substrate), then you have an insulator over the top of the gate (GI = gate insulator in the figure). That’s deposited by PECVD. On top of that, you’ve got the active layer, which is the IGZO. This is deposited by PVD. Then you’ve an etch stop layer (ESTL in the figure) and that’s a PVD layer. Then you’ve got the source/drain, which is a metal deposited by CVD. Finally, you’ve got the passivation on the top which is a CVD layer,” McDaniel said. He noted that these interfaces between the CVD layers and the IGZO are critical. “We want to reduce the hydrogen as much as we can, so that’s what our technology helps the customer to do,” he said, adding that Applied Materials has the capability to build transistors in house and test them. “We’re able to solve some of these integration challenges before we deliver it to the customer.”
This time last year, Applied Materials introduced two new products. One offers a new design for depositing IGZO films for TFTs; the other handles bigger substrates of low temperature polysilicon (LTPS) films to help lower manufacturing costs.
The three new products now being introduced are the Applied AKT-PiVot 55K DT PVD, Applied AKT-PiVot 25K DT PVD and Applied AKT 55KS PECVD. The 55k nomenclature is a reference to the Gen 8.5 size panesl the system can handle, which are 2.2m x 2.5 m, or 55,000 cm2. DT stands for “dual track” which is new.
The AKT-PiVot DT PVD system.
One of the key changes in the 55KS PECVD system include is related to how process gas is distributed the substrate surface. “The hundreds of thousands of holes that the gas is distributed out of – you have to customize them across the whole area of the chamber to compensate for the shape of the plasma,” McDaniel said. “It’s not just the diameter of the holes, it’s the depth of them.” A new gas deflector pre-distributes the gas before it goes into the diffuser, and support structures were added to achieve a higher degree of flatness over the 2.5 wide area.
New hardware provide better gas distribution and better uniformity.
On the PVD side, the new systems are designed specifically for IGZO. “Unlike our prior Pivot PVD system, where you want to have lots of chambers and be able to run multiple materials in different chambers, customers really want a system that just deposited the IGZO,” McDanield said. “It gets the substrates in and out quickly, so this is a compact, efficient platform that’s designed for depositing the IGZO.” The 25K system is targets displays for mobile applications. “We’re entering a whole new segment,” McDanield added.
The Pivot employs a set of rotary cathodes and targets, which act quite differently than conventional planar targets. Planar targets don’t get consumed uniformly and there can be redeposition of the material back onto the target. This redeposited material can spall off as particles. “Our technology is different,” McDaniel said. “The target is an array of rotating targets/cathodes. As they are being bombarded and consumed, you’re actually rotating the tubes in a circle and consuming them evenly throughout the deposition. The other benefit is this is a reactive process so you also have to introduce oxygen gas into the reaction. With the planar cathode, you have to introduce the gas from around the sides of the planar target. It’s hard to get it evenly over the substrate. With this array of tubes, you can introduce the process gas in between the tubes and get it uniformly distributed over the substrate,” he said. The rotary cathode employ magnets inside the tubes for uniformity enhancement.
Old-style planar (left) vs new-style rotary (right).
Material can redeposit onto planar cathodes (left) but that doesn't happen on rotray cathodes (right).
McDaniel added that presently everyone who is doing metal oxide IGZO use the etch stop (ES) structure (Figure, right), but would like to eliminate the etch stop and use a back channel etch (BCD) directly (Figure, left). “The IGZO material is very sensitive to hydrogen. What you’re trying to do is not expose it to the etching chemistry,” he said. “You put an etch stop layer on top of the IGZO, which is a CVD SiO2 process, and that protects it while you’re etching the source and drain. That adds an extra mask and extra process step. The panel makers would like to get rid of that etch stop layer and go to a back channel etch (BCE). This is where you etch the source drain directly down all the way to the IGZO and it’s unprotected. We’re not there yet, but the industry would like to see that structure developed. That’s on the roadmap for the industry.”
The display industry hopes to use a back channel etch (left), but presently uses an etch stop layer (right), which adds an extra mask and process step.
Looking forward, the holy grail for the display industry might just be the flexible display. McDaniel said flex displays will not likely be based on LCDs, but OLEDs. “For flexible OLED, you want to deposit on a flexible, non-glass substrate and then you need to encapsulate the OLEDs with something other than rigid glass.” This could require numerous thin films, which is good news for a supplier of tool deposition systems. He added that they would probably require an alternative to ITO (a commonly used transparent conductor). “There are a number of ITO replacement materials that are being looked at now, so as metal mesh, nanowires and even carbon nanotubes,” he said.