Deposition technology for TFT-LCD production savings
03/01/2008
EXECUTIVE OVERVIEW
A flat panel deposition technology, which utilizes a rotary target arrangement instead of the traditional planar and is also applicable beyond Gen 8.5, is described. In addition to enabling manufacturing and materials cost reductions in pre-sputter qualification, target consumption is reduced and data is presented showing that thickness uniformity, sheet resistivity, and target erosion profiles comply with Gen 8.5 TFT requirements.
The ubiquitous nature of large area TFT-LCD (thin film transistor-liquid crystal display) televisions are truly one of the major success stories of consumer electronic products for the 21st century. They have become an affordable option for the majority of consumers because of the industry’s ability to lower prices over the years. TFT-LCD equipment manufacturers have played no small part in these cost reduction efforts.
Take, for example, plasma-enhanced chemical vapor deposition (PECVD). By increasing the capacity to process ever larger size substrates, PECVD equipment makers have significantly reduced the unit production costs per area of substrate (usually measured in meters squared) by 80% from Gen 4 size equipment (680mm × 880mm), to Gen 8.5 size equipment (2200 × 2500mm). Other equipment used in the array process such as physical vapor deposition (PVD) and etch have achieved similar cost reductions.
Using a 40-in. vertically aligned TFT-LCD, high definition (HD) resolution panel as an example, DisplaySearch reported in 2007 that the equipment cost per panel now represents only 11% of the total manufacturing cost of the panel. Seventy-six percent of the total manufacturing cost comes from final panel materials such as glass, chemicals and targets, polarizers, liquid crystals, driver ICs, backlights, and inverters.
Reducing manufacturing costs
For TFT-LCD TV manufacturers to continue to lower the cost of manufacturing and therefore reduce prices for new TVs, they will have to focus on reductions in material costs. The cost reduction contribution of equipment manufacturers whose focus has been to increase substrate size area manufacturing capability will also have to focus efforts on reduction in material costs and overall cost of ownership (CoO).
The large area processing equipment is required to achieve high cost performance, low CoO and excellent film properties. The requirements for PVD array systems are complex.
 Gen 8.5 rotary target array: one array comprises 12 rotary targets, each 2.7m long. |
High throughput with low operating cost is expected, and the system architecture needs to be flexible to maximize the performance in both single layer and three-layer deposition. The layer thickness varies from 250Å to 5000Å with various metal materials as well as indium tin oxide (ITO). The system needs sputtering deposition capability under pure argon (Ar) gas or reactive process.
A new concept TFT array sputtering system has been developed to further reduce the CoO of large-size TV panel manufacturing as well as material costs of production. The new design supports large size substrates beyond Gen 8.5 and is used for gate and source drain conductive layers and pixel ITO deposition.
Reducing material costs in manufacturing
More specifically, the focus for material cost reduction in large area TFT-LCD sputtering systems is in the incorporation of low cost target materials and rotary target design enabling higher target utilization, and new pre-sputtering technologies.
Typical target materials required by TFT-LCD manufacturers include molybdenum (Mo), titanium (Ti), molybdenum-alloy, and ITO. Less expensive alternatives are thick aluminum (Al) and copper (Cu). In addition to the intrinsic value saving in the cost of the material, the Mo, Mo-alloy and ITO require the target to be bonded to the backing tube or backing plate in regards to planar targets. In the case of Al, Cu, and Ti, the target manufacturing costs are reduced by utilizing a monolithic target design which requires no backing tube eliminating bonding with the high price of in-solder being avoided in the process.
Generally, the surface of the sputtering target (especially aluminum) is initially covered with a layer of oxide; AlO in the case of aluminum targets. The newly installed targets must by cleaned in a pre-sputtering process before deposition begins. Typically, up to 50 sheets of glass are used and deposited to clean an Al target surface using planar cathodes. These are 50 glass substrates that cannot be used for production and are discarded.
As expensive target materials like Mo, Ti, and ITO are in use, rotary targets have intrinsic design advantages over their planar counterparts. Target lifetime is defined as sputtering power times sputtering time (kilowatt-hours), or as the total thickness of material deposited on the substrates. The change in geometry and design from planar to rotary cathodes results in increased target utilization-from 20-40% for planar targets to >80% for rotary targets. In addition, rotary targets have a 5× greater lifetime than planar targets when measured in kilowatt-hours. The rotary target has a re-deposition-free target surface as the target is continuously turning during sputtering. The performance of the rotary targets has already been proven at Gen 6 size systems.
The cathodes are arranged in a semi-circle for best layer uniformities and to overcome the shadowing effect of the edge exclusion (mask), and improve uniformity. Utilizing rotary cathodes enables the turning of the magnetic bar within the cathode toward a water-cooled pre-sputter shield. The target surface cleaning process can take place continuously and is not interrupted by loading new glass. Only one substrate is necessary for this burn-in (pre-sputtering) process so the system running costs are typically reduced by up to 49 Gen 8.5 substrates. Typically, the dummy glass for pre-sputter can be reused a maximum of five times.
Process data
The new deposition technologies described above, while enabling cost savings to end users, will be implemented only if the overall standards for film deposition performance are maintained for up to Gen 8.5 substrates. In this case, performance measured by film thickness uniformity, resistivity and defect ratio.
 Figure 1. Thickness uniformity of Al on a Gen 8.5 substrate using Al rotary targets. |
All the process data are based on experiments done in our laboratories and do not reflect any data results related to specific customers. The uniformity data (Fig. 1) conducted on Gen 8.5 equipment utilizing the alumium rotary targets as described in this paper shows aluminum to have thickness uniformity of <±15%, well within existing specifications (Fig. 2).
 Figure 2. Sheet resistivity measurement for an Al rotary target on a Gen 8.5 glass substrate. |
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 Figure 3. Target erosion profile for a Gen 8.5 rotatable AlMgSi target (35 kW). |
Figure 3 shows the target erosion profile for Gen 8.5 Al rotatable target measures erosion in terms of kW-hrs or sputtering power times sputtering time. The graph shows erosion as measured along the corresponding position on the rotary target positioned immediately below the profile chart with corresponding kW-hr usages. The target utilization associated with the erosion profile shown is up to 88%.
Conclusion
- For the TFT-LCD industry to maintain a high level of success and continue to gain market share overall for color television, all sectors of the industry-from manufacturers to component suppliers to equipment suppliers-share in the responsibility of managing the reduction in material costs and overall CoO of the final products, while achieving process requirements. New solutions for TFT-LCD array sputtering contribute to both of these goals.
Kerry Cunningham is a product marketing engineer at Applied Materials, 3101 Scott Blvd, Santa Clara, CA 95054 USA; ph 408/235-6134, e-mail [email protected].