Contacts, transparent conductive oxides and reflective coatings influence the performance of cutting-edge high-brightness LEDs. Each of these requires thin-film deposition, which can be realised by either using a collection of batch tools to process a set of wafers in single process steps, or a by employing a far more integrated cluster tool to handle a single wafer in a whole series of process steps. Allan Jaunzens from Evatec outlines the merits and drawbacks of both approaches.

Although MOCVD growth of active materials lies at the heart of the latest high brightness LEDs, these light emitters can only reach their full potential when additional layers have been optimised too. This includes the transparent conductive oxides that combine efficient carrier injection into the active region with minimal absorption of light generated by the device, and reflectors and anti-reflection stacks that control the output profile of the chip (see “Thin films in LEDs” on p. 38 for a  more details).

Every LED chip manufacturing facility is equipped with tools that can deposit and pattern various types of thin films on substrates with diameters of 2-inches or more. The nature of the films that are deposited, the way this is done and the materials that are used will vary from LED manufacturer to LED manufacturer, but all of these chipmakers are targeting increases in device output, higher electrical efficiency and lower manufacturing costs.

Cutting LED production costs is possible by transferring production to larger substrates. Although this move requires capital expenditure, it could pay dividends, because the LED market is currently growing at a tremendous rate and is widely tipped for further expansion throughout this decade. Equipment suppliers are aiding this transition by launching so-called ‘bridge tools’ that are compatible with 6-inch substrates and can also accommodate 8-inch production in future.

 



Typical box coater batch system – available in chamber sizes from 0.5 to 2 m

Every LED manufacturer that is looking to scale-up production will have to weigh up the relative merits of using batch or cluster tools for the processing of larger wafers. Key issues are the impact that each approach will have on the deployment of damage free processes, the possibility to match substrate throughput and handling with both upstream and downstream processes and whether this technology offers future flexibility.

Essential requirements for LED makers

Although LED producers differ in their choice of chiparchitectures and their selection of thin-film materials, theydo share several requirements for their processes. Firstand foremost, they all want processes that they currentlyuse to scale to 6-inch without any compromise in physicaluniformity, in either etch or deposition, over what isachieved currently on 2-inch or 4-inch material. Thisrequirement includes no changes in uniformity to the optical or electrical properties of the film, which is only possible by designing reactors that can retain control of substrate temperatures and internal film stresses over larger process areas.

Another common demand by LED makers is improved repeatability – in terms of batch-to-batch, wafer-to-wafer and wafer-within-wafer – when migrating to larger diameters. Currently, LED manufacturers are asking for repeatabilities of +/-3 percent or better to enable them to increase manufacturing yields and ultimately maintain profit margins.

Chipmakers also want the introduction of processes that increase chip efficiency, which is influenced by the choice of thin film materials and how they are incorporated into the device. On top of that, they are looking for faster cycle times. Etch and deposition processes tend to be fairly quick, so the greatest gains can come from reducing times associated with pumping, heating, cooling and handling a batch of wafers. Transferring processes to lower temperatures is also viewed as an attractive move, because it diminishes the likelihood of film or substrate damage and can enable the introduction of new materials technologies.

Batch processing: the pros and cons

Batch and cluster platforms with specific hardware enhancements can fulfil the deposition and etch requirements of many LED makers – their strengths and weaknesses will be discussed in turn.

Batch platforms for deposition and plasma etch are already well established in 2-inch and 4-inch LED manufacturing lines, where they are either employed to deposit films by evaporation or PECVD, or used to selectively etch away material. As the name suggests, substrates are processed in batches, traditionally with manual loading and unloading of a set of wafers. The process chamber must be pumped and vented for each cycle.

Evaporation steps in LED fabs often involve commercial ‘box coaters’, which have a pedigree in industry stretching back 50 years. They are flexible manufacturing tools that can handle a whole range of coating materials, substrate types, sizes and shapes. Coating uniformities as good as +/-1 percent over small wafers can scale up to 6-inch or 8-inch with the right know-how and larger platforms that can accommodate larger substrates sizes and boost throughput.

One great strength of evaporation is that it lends itself to fast, low-cost development of new processes – all that may be required is simply a change in evaporation materials. Increased throughput and higher yields are now possible, thanks to new hardware developments including recent advances in e-gun engineering technology that have led to a step change in the number of hearths available for evaporation, even in relatively small systems. This reduces system fill and maintenance times.

Another relatively new variant is ion-assisted evaporation. This can deliver better films than conventional evaporation where relatively low particle energies of the coating flux, in the range 0.1 to 1eV, are typical. Such energies lead to low surface mobility, which in turn causes moderate film densities (80-90 percent of the theoretical value) that can ultimately limit the stability of the film’s optical and mechanical properties.

This technology involves an ion source, which is used to produce inert and reactive ions that are accelerated towards the substrate, bombarding the growing layer during deposition. The momentum of these particles is transferred to surface atoms, improving their mobility and reducing voids associated with conventional evaporation. The upshot is a layer density that is closer to that of the theoretical values. What’s more, this superior film is formed without the need for high process temperatures, which are often used in conventional evaporation processes that improve film quality but lengthen the overall process time.

The higher quality of the film results in a reduction in shifts of optical performance with temperature and humidity. ‘Densification’ to an even higher degree is possible by turning to ‘plasma ion assist’ and ‘ion plating’ variants of this technique. In these processes the material that is being evaporated is also partly ionised and arrives at the substrate with high kinetic energies - typically 10 eV. The films that result have an extremely low degree of absorption or loss.

Traditionally, high-volume LED manufacturing has involved the manual handling of wafers. Some chipmakers are now looking to eliminate all possibe sources of handling errors and yield losses, and are turning to cassette-to-cassette systems that involve robot wafer loading of the coating dome within a controlled environment. Such systems can also fully track wafer position for the coating process, thereby enabling the highest levels of traceability and statistical process control. Another deposition technology that is compatible with batch processing is PECVD, a well established technology that is capable of depositing thin and thick SiO2 layers on 2 inch or 4-inch substrates on typical batch platforms of 300 mm. The purpose of the layer dictates the required film quality, but modern PECVD reactors offer LED makers sophisticated possibilities, such as control of internal film stress and the film refractive index in addition to the usual deposition rate.

 



Robot loading of substrates into an evaporation batch system involves: Front end cassette load / unload, substrate pick, dome load including metrology substrates, and dome transfer to coater

 

It is also possible to eliminate manual cleaning of PECVD reactors by purchasing a tool that features in-situ plasma cleaning. Although more complex than evaporation, PECVD can be scaled without compromise in deposition uniformity. 500 mm reactors are now available that can either handle larger wafers or significantly increased batch quantities for 2-inch and 4-inch wafer sizes. A major limiting factor for larger tool sizes is the handling and loading times relative to deposition times, which can dominate in the case of thinner films.

 





In addition to film deposition, etching is often used to bolster LED output according to substrate type. This step can pattern substrates and increase extraction efficiencies, and it can also be used to process the epitaxial films. Depending on the material and process requirements, either reactive ion etching or plasmaenhanced etching are routinely used by LED chipmakers. Just like PECVD, reactor technologies can be scaled up without compromising in etch uniformity

 



e-gun with ‘digital control’ for ITO or metals evaporation processes

A case for cluster tools?

The major alternative to batch processing is to use a cluster tool. It is an approach that is widely used in the silicon industry, which already processes wafers up to 12 inches in diameter.

Unlike typical batch tools, cluster tools integrate many different process types on a single platform. Process chambers remain under vacuum continuously, resulting in excellent performance stability and the opportunity to perform a sequence of steps without intermediate exposure to air.

The cluster-based approach involves cassette-to-cassette operation, and 2-inch or 4 inch ‘mini-batches’ can already be handled conveniently. As the LED industry migrates to 6-inch, the approach will change to direct wafer handling, which is also well proven for silicon processing

Up until recently, cluster tools incorporated a series of ‘single process’ modules, with the wafer being passed between them by a central handler. However, some equipment manufacturers are now starting to integrate batch modules into their tools, a step that blurs the distinction between the traditional definition of batch and cluster tools.

Although more complex to manage that a traditional batch tool, a cluster tool architecture brings the flexibility to add new modules or technologies as production demands change or grow. Traditionally, one downside of these tools has been a lack of customisation for LED manufacturing that offer increased process control flexibility, but that shortcoming is now being addressed.

New features for cluster tools include adjustable source and substrate geometries that simplify development and optimisation of new processes; sputter source technologies for rapid film prototyping in thin-film alloys, which enable simultaneous deposition from several sources; and integrated metrology facilities, such as stress measurement, plasma and optical monitoring for stress control and accurate end-point termination of optical layers.

Cluster tools are well suited to the integration of typical PECVD and ICP etch processes that have already been described for batch tools. What’s more, it is possible to address some of the handling limitations for batch tools by integrating the same processes on a cluster platform. And last but by no means least, thanks to their load lock configuration, cluster tools are also ideal for sputtering, a high-energy coating technologies that is becoming more mainstream in LED manufacturing.

 



Complex cluster configuration with separate load, unload stations, central handler, a batch module for multiple sputter cathodes and three additional single process modules

 



Simple cluster configuration with load lock, central handler and single process modules for sputter, etch

 

Although the sputtering process is more complex to scale than well established evaporation processes, the high kinetic energies (10-20 eV) and the increased mobilities that result enable formation of dense, stable single layers and also the deposition of dielectric stacks for antireflection coatings in reflector and contact layers. One of the biggest advantages of sputtering is that it can yield films with outstanding mechanical and optical layer performance, which can be formed without substrate heating, a usual requirement when using conventional evaporation. Eliminating this heating step slashes process and handling time, while opening up more possibilities to process materials that degrade at elevated temperatures. The most recent developments also enable deposition of high-quality, transparent conductive oxides, such as indium tin oxides layers with customized transmission and resistivity requirements without any damage to plasma sensitive sub-layers and also eliminates the post-deposition, high-temperature anneal often required after evaporation.

Horses for courses?

It is widely expected that cluster tool sales to the LED industry will increase. However, that does not mean that any single approach will become dominant to the exclusion of all others. While the cluster-based approach is undeniably attractive, converting existing fabs from batch tools and processes to cluster-based alternatives is costly and time-consuming.

In addition, the competition from cluster tools is certain to spur further improvements to existing batch processes – that is not surprising, given that manufacturers of evaporation tools have been doing this for years and years so that their technology continues to play a leading role in many industries.

For some chipmakers, there are also concerns that failure of a cluster tool could be far more disastrous than that of a batch tool, impacting the progress of several processing steps. However, some LED manufacturers will not be put off by this, but rather be won over by the appeal of automated handling and process flexibility safe in the knowledge that these tool types are proven in 6-inch, 8- inch and even 12-inch silicon wafer fabs. In short, the debate between batch tools and their cluster-based rivals is by no means over.



Plasma sources enable enhanced evaporation at lower process temperatures

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