Improving Electronic Ceramics with Niobium and Tantalum

The development of ultrahigh-purity niobium and tantalum in specific grain sizes and tight distributions is driving the advancement of high-performance electroceramics.

The electronics market has undergone remarkable changes in the past few decades. With the development and commercialization of new technologies such as mobile communication, personal computers and the Internet came the need for miniaturization and high-performance electroceramics.

Niobium and tantalum compounds have been used successfully in a variety of different applications in the electronic and electro-optic markets, including dielectric ceramics,¹ piezoelectric ceramics,² single crystals³ and ferrites.⁴ For such applications, various grades of niobium and tantalum oxide are generally used as starting materials. However, for special applications or sophisticated processes, other commercially available compounds, such as chlorides, alkoxides or oxalates, might be the raw materials of choice. Over the past several years, suppliers have invested heavily in research and development to produce niobium and tantalum oxides and compounds with the highest possible purities, small grain sizes and tight grain distributions. Their efforts will help drive the electroceramics industry to new advances in the future.

Dielectric Ceramics

Dielectric ceramics are mainly used to increase the capacitance of capacitors, which are used to store electrical energy. In its simplest form, a capacitor consists of two plates separated by space. The amount of electrical energy stored by a capacitor is given by the following equation:

C = K x A/d

The capacitance (C) depends on the surface area of the plates (A), the dielectric layer thickness (d) and the dielectric constant (K).

Due to limitations in space, attempts to increase the capacitance are made by reducing the dielectric layer thickness or by using a material with a higher dielectric constant. Table 1 provides a selection of dielectric constants for different materials.

Niobium and tantalum oxides are frequently used as both additives and main components to increase the capacitance of multilayer ceramic capacitors (MLCCs). For example, the X7R-type MLCC may vary within ±15% in capacitance between -55 and 125C. The commonly used pure barium titanate (BT), however, cannot sustain its capacitance at lower temperature ranges, and therefore requires a niobium dopant in the form of niobium oxide, niobium oxalate or niobium chloride. ⁵

A common problem in manufacturing other dielectric ceramics is the high cost of pure palladium that is used for the electrodes. Sintering at temperatures below 1000C allows the use of cheaper, silver-rich palladium electrodes, but lower temperatures can lead to a loss in capacitance. Lead magnesium niobate (PMN) can be used to provide a high dielectric constant even at lower sintering temperatures. However, the production of PMN is difficult and requires strictly controlled raw materials and processes to prevent the formation of a “pyrochlore phase” with inferior dielectric properties instead of the desired “perovskite-phase.” Recently, suppliers have developed a modified process for the production of PMN⁶ that suppresses the formation of the undesired phase. A high-quality oxide with small primary grains and excellent doping properties was also developed. ⁷

Another type of capacitor is the DRAM (dynamic random access memory), which is produced by the semiconductor industry and is used to store information. To build the capacitor, trenches or stacks are etched or drilled into Si-wafers and are coated with a dielectric material. SiO₂ is commonly used as a natural dielectric on Si with a dielectric constant of 3.6, but the push toward smaller and higher density chips with higher storage capacitance is requiring the use of new materials with higher dielectric constants. Although perovskites such as strontium bismuth tantalate (SBT) have been used because of their high dielectric constant (~200-300), difficulties in depositing homogenous layers have created a preference for binary oxides such as tantalum pentoxide (Ta₂O₅) and hafnium oxide (HfO₂).

Physical vapor deposition (PVD) and chemical vapor deposition (CVD) are common depositing methods used to produce DRAMs. A suitable Ta2O5 for the production of PVD sputter targets must posses certain physical properties, such as good sinterability and pressability. In contrast, CVD applications require high volatile Ta-compounds with high vapor pressure. Tantalum alkoxides, especially tantalum ethoxide, are suitable compounds. In addition to these properties, an extremely high purity (>99.99%) is required for semiconductor applications. Thus, suppliers have to maintain a good quality control as well as a flexible production process. Recent developments have reduced organic impurities to a minimal level and metallic impurities to the parts-per-billion (ppb) range (see Table 2 and Figure 1), allowing the production of the highest quality products.

Piezoelectric Ceramics

Most of the crystals that lack a symmetry center exhibit a property called piezoelectricity, which is the ability of a material to generate an electrical charge when mechanical stress is applied. Similarly, a dimensional deformation occurs whenever the material is exposed to an electrical charge. Piezoelectric ceramics often have a perovskite (barium titanate [BaTiO₃]) related structure, as shown in Figure 2.

Piezoelectric ceramics can be used to generate charge at high voltages, detect mechanical vibrations, apply pressure control by voltage, control frequency, and generate acoustic or ultrasonic sound. For this reason, they are often used in commercial applications ranging from buzzers, filters, igniters and ultrasonic cleaners to sonar arrays, ultrasonic imaging systems, shutters and positioners for optical systems. Since piezoelectric ceramics serve as transducers between electrical and mechanical energy, they can also theoretically compete in motor-generator applications. Recently, the automotive industry has applied piezoelectric actuators for fuel injection systems and as sensors for antilock braking systems. ⁸

Once again, niobium and tantalum compounds can be used as both additives and main components in these applications to enhance the product’s performance. Lead zircon titanate (PZT) is an effective and relatively cheap material for piezoelectric applications. However, its piezoelectric properties require some adjustments by means of doping. In the perovskite-type structure ABO₃ of PZT, A ions as well as B ions can be partially replaced by higher (donor) or lower (acceptor) charged ions. Fivefold niobium and tantalum (Nb⁵⁺ and Ta⁵⁺) is typically used to replace fourfold titanium (Ti⁴⁺), resulting in so-called “soft doping” (higher DK and higher coupling constant), while alkalines can be used to replace twofold lead (Pb²⁺), resulting in so called hard doping (lower DK and low loss). ⁵ The standard way to dope is with pure niobium or tantalum oxide. If a wet process is applied, doping can be achieved with soluble niobium and tantalum compounds, such as oxalates. However, the oxides must be highly reactive to allow a smooth reaction and a homogenous distribution of the dopant. For this reason, a high surface area and/or small grains are key requirements for such additives.

Other niobate or tantalite compounds that can be used as dopants include potassium niobate, nickel niobate and cobalt niobate. Potassium niobate combines the advantages of hard and soft doping.

In some cases, niobates and tantalates themselves posses excellent piezoelectric properties. For example, lead magnesium niobate (PMN) shows attractive electrostrictive properties that could be used in high precision applications (see Figure 3). The optical performance of the Hubble space telescope, for instance, was enhanced by using PMN actuators. ⁹

Single Crystals

Of the various niobates and tantalates that are grown as single crystals (potassium niobate, lead magnesium niobate, etc.), lithium niobate and lithium tantalate are certainly the most remarkable cases. Crystals grown from these materials are used for two types of commercial applications: integrated optics and surface acoustic wave (SAW) devices.

Integrated Optics. ¹⁰ With the need for high-speed data transmission, new developments involving optical transmission have emerged, such as DWDM (dense wavelength division multiplexing) filters and sophisticated integrated optical circuits (IOCs).

Signal transmission by light has the ability to transmit millions of calls simultaneously through a single glass fiber. However, optical modulation is necessary to encode transmitted signals. Lithium niobate (LN) modulators interrupt the laser signal to ensure a good quality, long-distance transmission. ¹¹

Surface Acoustic Wave Devices. SAW devices allow the manipulation of acoustic signals in various ways. They are used in consumer electronics, such as TV sets, video games and video recorders, as well as in military applications, such as encoders/decoders. ¹² In recent years, mobile phones have also greatly contributed to the market growth of lithium tantalate (LT) and lithium niobate.

LN and LT single crystals are produced by the Czochralski method. ¹³ Niobium or tantalum oxide and lithium carbonate are mixed and prereacted to form pellets. These pellets are fed into a platinum crucible, which is surrounded by a refractory housing, and are melted by radio frequency induction heating. A rotating seed crystal with a given orientation is lowered into the melt and then slowly withdrawn, forming a single crystal boule. The crystal boule is then sliced into wafers, polished and sectioned into the desired shape.

Here too, special raw material requirements are desired to secure reliable products in high yields. Next to high chemical purity (>99.99%), physical properties like high reactivity and good mixing behavior are necessary to achieve a pure and homogenous lithium niobate/tantalite phase. To meet this need, suppliers have developed special oxide qualities, referred to as LT and LN grades, with the desired purity. Their physical properties, such as agglomerate sizes, etc., can be adjusted according to the manufacturer’s processes (see Figure 4).

Ferrites

Ceramics with magnetic properties are called ferrites. They can be generally categorized as hard (permanent magnetization) or soft (nonpermanent magnetization). Soft ferrites are mainly based on the material groups MnZn and NiZn. Classical hard ferrites are based on the material system MFe₁₂O₁₉, where M might be Ba, Sr or Pb. Modern hard ferrites are based on rare earth metal alloys, such as SmCo₅ or Nd/Fe/B. Major applications for ferrites include transformers and inductors used in telecommunications, power conversion and interference suppression. ¹⁴

Modern ferrites have to fulfill high demands with respect to efficiency. Additives such as niobium and tantalum oxide are needed to improve magnetic properties such as power loss, electric resistance and permeability. It is generally thought that such additives influence the grain growth and grain density of the doped ferrites. ¹⁵ Adding niobium and tantalum compounds to soft ferrites ensures the high quality that is needed.

Raw materials used as additives in these applications don’t have to be of the highest purity. More important is an even distribution in the basic ceramic formulation, resulting in a homogenous coating of the ferrite particles. Therefore, if a conventional mixed oxide formulation is used, the primary grains of the additives have to be significantly smaller than the primary grains of the ferrites. This can easily be achieved through a wet process using soluble compounds such as tantalum and niobium oxalates, chlorides or alkoxides. The choice of the proper starting material depends on the solvent. Water-based processes require the use of oxalates, while processes based on organic solvents are suitable for chlorides or alkoxides.

Advancing Applications

Ceramics used in electronic applications must increasingly become smaller and higher performing. To meet these demands, manufacturers require raw materials that are often different from those used in conventional ceramic markets. Next to ultrahigh purity, in some cases in the ppb range, tailor-made physical features are also sometimes necessary to meet the increasing trend toward miniaturization. Suppliers are helping to push the industry toward new advances with the development of higher purity tantalum and niobium compounds; improved physical properties, such as grain size distribution and specific surface area; and new soluble compounds, such as alkoxides. Suppliers are also working closely with the high tech industries (automotive, nanotechnology and semiconductor), as well as ensuring close cooperation between their R&D and production staff, to achieve a high level of quality at a reasonable cost. With these advances in materials, ceramics are likely to increase their market share in the high-tech electronics industry.

For More Information

For more information about niobium and tanatalum developments, contact H.C. Starck Inc., P.O. Box 25 40, D-38615 Goslar, Germany; (49) 5321-751145; fax (49) 5321-751193; or e-mail Bettina.essmann.be@hcstarck.de. (U.S. contact: Jonathan Margalit, 45 Industrial Place, Newton, MA 02461; (617) 630-4879; e-mail jonathan.margalit.b@bayer.com.)