Material tuning for ion implantation.

How the properties of graphite and refractory metals improve system reliability and the lifetime of ion source and beamline replacement components.

During implantation, dopant ions are accelerated and typically implanted into a monocrystalline silicon substrate so as to influence its bulk properties. For example, creating buried layers for isolation, increasing local conductivity, or straining the lattice to increase electron mobility. Dopant materials are typically boron, arsenic, phosphorous, indium or germanium.

The energy, direction, and uniformity of the ion beam must be repeatable over many process changes and not fluctuate even if it remains in constant operation for many hours. Process stability and life are strongly impacted by component shape and material properties; i.e. the longer that components retain their shape and properties, the more stable and repeatable the process will be.

Many components in the implanter system are in contact with process gases and the ion beam over long periods of time. Powerful chemical and electro-mechanical forces within the system can cause erosion, re-deposition, and transport of component materials along the beamline. The loss of material from such components can reduce process precision and accelerate equipment failures. Components must be replaced early enough to preclude damage to the wafer and predictably enough to prevent unplanned production stops. Such unplanned stops can cost high volume factories several tens of thousands of dollars per event.

Graphite and refractory metals are the primary materials used in ion implanter beamlines. Their chemical, thermal, electrical, and mechanical properties are particularly important in increasing the service life of the system. It takes a thorough knowledge of these material properties to choose the perfect material for each replacement part and this is precisely where the materials experts from Plansee come in with more than 90 years of experience in the field of refractory metals and more than 30 years of expertise with graphite and ion implantation processes. "More than just replacement parts": True to this motto, Plansee improves not only the design of OEM parts, but also the materials used in them.

Three graphite quality classes. A wealth of possibilities.

 

Other manufacturers may use the same material quality for all graphite replacement parts in the ion implanter. Plansee offers three different and perfectly matched quality classes to meet the different demands in an ion implanter. These matched graphite classes include: Purified Structural Graphite, Aperture Graphite, and Specialty Graphite.

So why do we need three graphite quality classes? Because graphite has to meet a range of different requirements in the beamline. For example, many apertures require high mechanical stability with medium to high thermal conductivity. For components in the source, thermal conductivity can be crucial because the temperature throughout the source should remain as constant as possible.

The difference between the three quality classes of graphite lies primarily in the particle size, hardness, and strength. Purified Structural Graphite is graphite with a particle size of 5-10 µm. This material is used where the beam will not strike the material under normal operating conditions. For example, it is used in mounting frames and covers.

As its name implies, Aperture Graphite is used primarily for apertures. Aperture Graphite is graphite with a particle size of ~5 µm and is around 50% harder and stronger than Purified Structural Graphite. Aperture Graphite is particularly homogeneous. In cases where material loss cannot be avoided, the homogeneity of this material means that such loss is more even than with Purified Structural Graphite. As a result, the implantation process is considerably more stable.

Specialty Graphite represents the ultimate in homogeneity. With an ultra-fine particle size of 1-2 µm, the material is some 50% harder and stronger than Aperture Graphite. The particular fields of application for this graphite are apertures used for extraction optics and in mass resolution where the shape of the electrostatic lens is critical or where resistance to erosion is important. With specialty graphite, material loss is so minimal that it can even be used after mass analysis without leading to a higher level of particle formation.

The three graphite quality classes differ not only in terms of their mechanical properties, but also in terms of their electrical and chemical properties. In addition to its particularly smooth surface, certain Specialty Graphites can also boast good electrical conductivity. In extraction aperture systems, Specialty Graphite can reduce arcing effects known as glitching. In some applications it may be possible to eliminate the glitching effect completely. Aperture Graphite possesses particularly good thermal conductivity that allows it to ensure a uniform temperature distribution in an ion source. All three graphite quality classes have a common and high degree of purity. The maximum impurity level of 5 ppm demanded by the semiconductor industry is guaranteed in all cases. In practice, actual values achieved are typically an ultra-pure 2 ppm.

There are few suppliers of graphite implanter components that meet Plansee's stringent quality requirements and Plansee makes no secret of what material is used for which component. Dr. Thomas Werninghaus, responsible for the development of new materials for ion implanters at Plansee: "For us,transparency goes without saying when dealing with a process as sensitive as ion implantation. In the documentation for each one of our replacement parts, we identify which of our graphite quality classes has been used."

An example from the USA shows how the use of higher-quality materials can pay for itself. In this case, the Plansee team optimized the component and assembly design as well as the choice of materials in an implanter flight tube. The one size fits all graphite strategy of the OEM paved the way for extending service life through selection of materials based on their expected level of beam strike. The result: Thanks to the measures taken by Plansee, it was possible to double the service life of the flight tube.

In order to save costs while delivering high-quality products, Plansee relies on a combination of graphites for its customers: Top-quality graphite for the most vulnerable parts and standard quality for peripheral parts. As Dr. Thomas Werninghaus so fittingly remarks: "You need a Ferrari for racing, but a family saloon is perfectly adequate for a shopping trip".

For example, Plansee supplements one-piece graphite apertures made from inexpensive Purified Structural Graphite with inserts made from higher quality Aperture or Specialty Graphite. They are used where hardness and strength are critical, namely in the vicinity of the ion beam. This reduces wear and increases the service life of the aperture.

The fact that Plansee understands the properties of graphite down to the finest detail also pays dividends in engineering and manufacturing where even the most complex geometrical shapes can be manufactured or where complicated, multi-part assemblies, can be simplified to facilitate installation and reduce component costs.

Alloyed metals for top performance.

 

Purity and material properties are also top priorities for tungsten and molybdenum components in the ion implanter. If the source of the material changes frequently, it is probable that the qualities of the material used will also fluctuate and result in process instabilities, unpredictable downtime, or uneven component usage. The reason for this is simple, as material properties change, so does the complex interaction between the component and the demanding process environment (high energy, temperature, and chemical reactivity). Plansee has been proactive in ensuring its supply of raw materials: The sister company Global Tungsten Powders and significant investments in Molymet, the world’s largest molybdenum processor, guarantee long-term supply security and consistent quality.

Pure, secure, and consistent raw materials are not the end of Plansee’s expertise in materials. For Plansee, the challenges in an ion implanter also involve the question of how materials are combined. At Plansee a dedicated research group develops new alloys and composite materials with the aim of improving implanter performance and component life. One particularly successful composite material is tungsten lanthanum-oxide (WL) which many Plansee customers are already using successfully to reduce the effects of halogen cycling in the ion source. But what is halogen cycling?

Halogen containing gases such as Boron Triflouride (BF3), Germanium Tetraflouride (GeF4), and others, are commonly used to deliver required dopants such as Boron and Germanium to the ion source. These molecules are split and ionized within the plasma of the ion source. For instance, production of B+ from BF3 yields BF2, BF, B, F, and several of their ions.

The separated fluorine atoms react with available tungsten to form tungsten halides. The halide molecules are eventually broken by thermal energy which is highest at the surface of the cathode. Flourine is returned to the plasma and tungsten is deposited on the hottest areas of the source. This process, which is known as the halogen cycle, causes cooler tungsten components in the ion source to erode rapidly and leads to heavy tungsten deposits in other areas of the source. This results in shorting and can even cause the source to fail.

In order to reduce halogen cycling, Plansee has introduced a new material as well as an improved design.

If tungsten is replaced by the composite material WL in the cooler regions of the ion source, the halogen cycling is significantly reduced. Combined with the right graphite quality in the right places, this material ensures a homogeneous temperature distribution in the chamber. "Our customers are extremely satisfied with our WL components. Initial figures from the field indicate that the use of WL increases the lifetime of components by up to 150% in the right applications," explains Werninghaus.

Plansee High Performance Materials

 

Plansee High Performance Materials is an expert in the field of molybdenum, tungsten, tantalum, niobium and chromium components. Alloys and composite materials from Plansee come into their own in electronics, coating technology or high-temperature furnaces - wherever traditional materials are stretched beyond their limits.

Plansee manufactures thousands of different replacement parts from tungsten, molybdenum, tantalum, graphite and ceramics for ion implantation and other semiconductor applications, operating two centers of excellence in the United States and Japan. Plansee manufactures products precisely according to the OEM standards of all major manufacturers worldwide, with a primary focus on the design of improved and enhanced performance systems. Marketed under the "Plansee Advanced Standard" brand, upgrade solutions from Plansee are known throughout the semiconductor industry for their extended service life, simplified handling, lower maintenance expenditure, and reduced costs.

Plansee’s global sales force and international team of product engineers and designers are available for on-site consultation, to provide solutions for your ion implantation needs.Find out more about our graphite and metal components for ionimplantation.