Using Refractory Metals In LED Production

Interview conducted by G.P. ThomasDec 17 2013

AZoM talks to Heike Larcher, Application Group Manager at Plansee, about the importance of high purity refractory metals in the LED industry.

Could you please give a brief overview of the products Plansee delivers to the LED industry?

Plansee is a very diverse company and so we play an important role in the LED industry, which incorporates a lot of different steps.

This starts with the growing of sapphire crystals. As this occurs at more than 2000^oC, this is a very high temperature process, meaning heaters and crucibles must be created from refractory metals.

The MOCVD (metalorganic chemical vapour deposition) process is also a high temperature process, so we can also supply the heaters for this because molybdenum and tungsten both have good high temperature strength.

We also supply evaporation boats and coils for applying the reflecting layer on the back of the sapphire and wafers on the base of the LED Chip. These can be made of molybdenum or a molybdenum-copper alloy which has the same CTE as the sapphire itself.

We also have a daughter company that produces tungsten powder phosphorous, namely Global Tungsten & Powders Corp. And a part of our company produces components for the LED chip itself: Heat spreaders from tungsten-copper or tungsten-molybdenum-copper laminates. So we are integrated into the whole LED process in a lot of the different steps.

Could you go into a little bit more detail about why refractory metals are widely used in the production steps of LEDs?

I think one of the most important properties of these metals is the high temperature resistance. When you come to the first step of sapphire production, which is the melting of the sapphire, this high temperature resistance is certainly required because you have temperatures above 2000^oC.

This high temperature strength is needed to ensure a longer life time of the refractory metal hot zones.

Regarding the components for MOCVD, this is exactly the same. They need high strength with no creep, and using refractory metals ensures a longer lifetime.

For the wafers we have developed a special molybdenum-copper alloy that has exactly the right CTE to fit perfectly to the base.

Another important factor is the chemical resistance of our materials. This is apparent in the melting process of sapphire - our material is completely inert against aluminum oxide – the prematerial of sapphire, so there is no contamination in the sapphire.

Going back to the first step of LED production, what exactly do you produce for sapphire growing?

You first need a kind of crucible in which the aluminum oxide is molten. These crucibles are always made out of molybdenum and tungsten because of their high melting temperature and chemical resistance to the aluminum oxide melt.

Plansee are specialists in high temperature furnaces for crystal growth. In high temperature processes, you often use hot zones, and heaters made of refractory metals. The heaters are made out of tungsten at this high temperature and the shieldings also need layers of tungsten and molybdenum.

The whole furnace can be made out of refractory metals and there are almost no alternatives, because above 2000 Degrees there are almost no other materials to use. The only exception is graphite, but this reacts with the crucibles and influences the purity of the material. Plansee provides a product to all major sapphire producers across the globe.

Metal hot zone from Plansee. Image credit: Plansee

What different technologies are used to produce these sapphire crystals?

There are a lot of different technologies on the market, but essentially you always need a temperature of more than 2100^oC, to melt the batch in the crucible, which is then cooled down slowly to get the single crystal.

It is always the same basic process and it takes 15 to 20 days per single crystal you grow. The main processes which are frequently used are the KY process, Etch Defined Growth , and the HEM technique which uses the well-known ASF furnaces.

What are the main challenges when producing sapphires with these processes?

I think the main challenge for our products is that they need to be very durable, with a long life time at very high temperatures. This means a very good quality refractory METAL product is required.

For instance, when using the KY process the crucibles, with thick tungsten walls, need to be able to last for at least a year. Therefore, it is absolutely necessary that they have the right density and purity.

Similarly, for hot zones and heaters it’s necessary to design them in the right way and use the right material in the right area of the furnace.

Why is the purity of the material in the crucible so important and how do you achieve this?

Essentially, if you have small impurities coming from the crucible material, it means the crystal might not be usable in the LED industry because a very high purity is required.

What are Plansee’s main achievements as a material specialist?

Our main achievement as a company is that we are not only a supplier, but we also try to give our experience and knowledge to the customers.

Regarding crucibles for example, we found out that an important goal is uniform density in order to have a longer lifetime, because otherwise the crucibles start to re-sinter, which leads to a shorter lifetime. In the same way, we offer alternative designs for the hot zones depending on the customer’s needs.

So our main goal is to have a very positive TCO for the company meaning we don’t just find a cheap solution, but try to find the solution which works well and gives other benefits to the customers.

For example, we provide hot zones which have low energy consumption. This means you may spend a little bit more money to begin with, but in the end savings will be made.

For the heater itself, it the same – a standard heater for the KY process at the moment may have a lifetime of half a year. Plansee produces these with better, stronger materials, and our WVM W quality, leading to a lifetime which is much more than a year. This longer lifetime in turn leads to a much better TCO for the customer.

Tungsten crucibles from Plansee. Image credit: Plansee

Apart from the LED industry, who else buys from Plansee?

Plansee we also work directly with the OEMs that are producing high temperature furnaces for the LED industry, as well as other industries. We work with these companies from the beginning, so we have joint development regarding the design of the furnace and the usage of materials.

We have a design team in Austria of 10 engineers, but we also have a global designing team in Europe, USA, Japan and China, working together to produce solutions for the customers. They have extensive knowledge of the correct materials to use in various diverse applications, and in-house modelling can also be used directly for hot zones, furnaces and processes.

Where do you see the LED industry expanding and how do you see Plansee being a part of this?

I think that the LED industry will continue to grow, especially with regarding to general lighting, which I believe will lead to a new era for the industry. For this to happen there needs to be a cost reduction for the producers and for our customers, especially with regards to energy consumption.

I think currently the LED industry is not focused enough on the TCO approach- they want to buy cheap, but a cheap product is not always the best product.

This is the message we want to bring to the customers - in working together with experts they can have a better TCO and therefore they can lower their costs, meaning the whole LED industry will be able to supply to LEDs cheaper. This will be the gateway to the general lighting sector in my opinion.

About Heike Larcher

Heike Larcher is Application Group Manager Single Crystal Growing in the Market Unit Thermal Processes. She is responsible for the global sales and marketing of PLANSEE’s product for this industry.

This includes Silicon as well as Sapphire single crystal growing process. She is also acting as Key Account Manager in this application group.

Her background is in chemistry and chemical engineering. After 18 years in R&D in chemical industry in Germany she relocated after her marriage in 2001 to Tyrol and started in PLANSEE in the R&D department.

Since 2003 she is working in PLANSEE sales, first as Product manager and since 2009 as Application Group Manager.