Implications of Impurities in Sputter Targets and Evaporation Sources
All sputter targets and evaporation sources contain impurities. It is neither practical nor economically feasible to manufacture 100% pure materials. Understanding how pure a material is, how to interpret material impurities quoted, and which impurities are important to a particular application is critical in selecting materials and for ensuring processed stability over multiple lots.
Determining True Purity Value
Many targets and sources are shipped with a certificate of analysis listing only a handful of elements that were tested. The true value of the overall impurity concentration is therefore unknown. The reason for this is pragmatic - assaying all elements is expensive. So to control costs, vendors limit the assays to elements known to be important to their customer applications.
When an overall purity is quoted, it is essential to understand how purity is defined when specifying materials. For example, when a material is quoted as being 99.99% pure (or 4N), the true value of the concentration of impurities is often not the same as the declared value. This is largely because, depending upon the specifications, manufacturers may not be required to test all possible elemental impurities. A common distinction in the chemical industry is to distinguish between the absolute purity and the common metals basis purity.
For example on the absolute scale, 99.999% means that only 10 ppm impurity is allowed with respect to all elements on the periodic table. On the other hand, a metals basis purity typically would not include: H, C, N, O, F, S, Cl, Br, At. These may be listed separately, or may not be tested for at all. For many applications this is acceptable, while for other applications, additional element specifications may need to be included.
In addition, for some materials certain elements are often so-called “blanked” or not counted as impurities in the overall assay. This may occur because they are extremely difficult to separate from the main constituent, or because for many applications they are not regarded as chemically distinct. A classic example is the Hafnium content in Titanium materials.
Materion Laboratory Ensures Accurate Assays
For many semiconductor applications, it is important to know the percent of purity as well as to precisely identify any impurities. Particularly in alloys, it is crucial to ensure that the quoted assay is from the actual target or source and not estimated by summing the impurities of the raw material constituents. To ensure that the assays are accurate and representative of Materion’s manufactured materials, we maintain one of the most comprehensive analytical laboratories in the marketplace.
Our in-house laboratory is equipped with a full suite of analytical tools, consisting of: Inductively Coupled Plasma Spectrometers (ICP), including for Optical Emission Spectroscopy (ICP-OES) and Mass Spectroscopy (ICP-MS), and Leco analyzers for the analysis of finished product and incoming raw material. Our state of the-art Glow Discharge Mass Spectroscopy (GDMS) performs analysis of inorganic materials including elemental coverage of the whole periodic table. It is considered the industry standard for determining minor-to-trace elements in high purity materials, especially semiconductor materials. With the GDMS, reliable results can be obtained at a concentration significantly below 1 part per billion (ppb) for most elements with the ultimate limits of detection around 10 parts-per-trillion (ppt) in the solid.
The Lab can assay incoming raw materials as well as completed final products. This ISO 17025 certified laboratory allows Materion to ensure that individual targets or evaporation sources are of the highest quality and conform to customer specifications from batch-to-batch.
Semiconductor Applications Require High Purity
For many semiconductor applications extremely tight purity specifications are required. Even small amounts of trace elements can dramatically affect device performance. For instance, alkali metal elements such as Na and K exhibit high diffusivities and can readily migrate into semiconductor devices and degrade them. For these elements, it is common to limit their maximum contents to 0.1 ppm each or less.
In addition, radioactive elements such as U and Th emit alpha rays which can cause errors in semiconductor components. Thus, materials for critical components are required to contain less than 0.001 ppm (1 ppb) of these elements. The Buffalo GDMS has detected both Th and U down to 15 ppt (parts per trillion.) Transition metal elements such as Fe, Ni, and Cr can also degrade interfacial properties, and their proportions should be 1 ppm or less. If the target material is being used to fabricate conductive layers, traces of metallic impurities must be reduced to less than 1 ppm, and gaseous ingredients such as carbon and oxygen kept to 5 ppm or less. To meet this criteria, a copper sputtering target is required to exhibit an overall purity (excluding the gaseous ingredients) of at least 99.999%.
Meeting Customer Purity Needs
Obviously, manufacturing and certifying a high purity material can be more expensive, but it is warranted by the performance requirements of a particular application. Materion has the capability to produce sputtering targets and evaporation sources to precise customer purity specifications. For further details concerning our products and impurity analyses, please contact Dr. David VanHeerden, Applications Engineer, David.VanHeerden@materion.com.