Beyond the Nines: How to Read Purity Specifications on Electronic-Grade Silicon Precursors

Every deposition precursor datasheet opens with the same number: a purity grade written in nines. A 6N gas is 99.9999% pure. An 8N liquid claims 99.999999%. It is tempting to line up quotes from several suppliers and simply pick whichever material carries the most nines. That shortcut fails more often than it works, because the headline grade says very little about how a material will behave inside your tool. Two precursors with identical grades can produce noticeably different films. The real difference sits in the composition of the remaining fraction of a percent, and that is where a specification sheet rewards careful reading.

What a Purity Grade Actually Measures

The nines convention sums the impurities that a defined set of analytical methods can detect. It does not weigh every contaminant equally, and it does not capture every contaminant class. A gas chromatography assay reports the molecular purity of the bulk material, meaning how much of the cylinder or drum is the molecule printed on the label. A trace-metal analysis run by ICP-MS reports individual metallic elements in parts per billion. These are different measurements on different lines of the same specification, and a material can post an excellent assay while still carrying metals, moisture, or oxygenates above your process limits.

This also explains why grades vary so widely across a portfolio without implying any quality gap. On our product line, gases such as high-purity silane (SiH4) and boron trichloride (BCl3) are supplied at the 6N grade, while dichlorosilane (DCS) and monochlorosilane (MCS) are supplied at 4N. The lower grade is not a compromise. It reflects how each material is produced, which analytical methods apply to it, and where it is used.

The Impurity Lines That Decide Film Quality

When you sit down with a specification, four lines deserve most of your attention.

Moisture and oxygen. Water vapor and O2 at the ppm level oxidize chamber surfaces and end up as oxygen in the deposited film. For ALD in particular, moisture absorbed into a liquid precursor or cylinder headspace during handling surfaces later as oxygen incorporation and rougher interfaces. Ask for the moisture specification explicitly; it is often not part of the headline purity number at all.

Metallic contaminants. Sodium, potassium, iron, copper, aluminum, and chromium appear in trace quantities but matter far beyond their concentration. Mobile ions such as sodium and potassium shift threshold voltages in MOS devices. Iron and copper degrade minority-carrier lifetime in silicon and create leakage paths in junctions. The ICP-MS table, not the grade, tells you whether a supplier controls these elements individually.

Hydrocarbons and amines. For liquid precursors, the synthesis route leaves fingerprints. BDEAS, for example, can carry dialkylamine by-products and straight-chain or branched-chain hydrocarbons depending on how it was made and purified. These species translate directly into carbon contamination in the deposited film.

Particles. Cylinder cleanliness, valve design, and filtration at fill determine how much particulate ships with the gas. This line is often absent from commodity datasheets, which is itself useful information.

Why Chlorosilanes Sit at 4N and Still Perform

Epitaxial growth and oxide deposition run at high temperature with high growth rates, which gives the process a wider impurity budget than a room-temperature ALD cycle. In an epi reactor running DCS or MCS, the growing silicon layer tolerates the residual impurity load of a 4N material while still meeting device requirements, which is why 4N is the industry-standard grade for chlorosilanes rather than a cost-driven downgrade. The same logic applies to disilane (Si2H6) at 4N, which serves low-temperature polysilicon and silicon-germanium deposition where its higher reactivity, not its grade, is the deciding factor. Boron trichloride, by contrast, is held at 6N because it is used for doping and etching, applications where metallic boron and chloride impurities transfer straight into the wafer.

Why ALD Liquids Push Past 7N

ALD growth is self-limiting: every cycle exposes the surface to one precursor dose, and whatever rides along with that dose adsorbs in monolayer quantities and accumulates cycle after cycle. Impurity control therefore has to be tighter than in continuous-flow CVD. That is why the ALD-oriented liquids in our range are specified above the gas family: tetramethylsilane (4MS) and trimethylsilylamine (TSA) reach 8N, while trimethylsilane (3MS), di-isopropylaminosilane (DIPAS), diiodosilane (DIS), and BDEAS are supplied at 7N. When you compare these materials across suppliers, the metals table and the moisture figure will tell you more than the extra nine on the label.

Packaging Is Part of the Purity

The grade on paper is only as good as the container that delivers it. Gas cylinders with poorly treated internals can add moisture and particles back into a 6N fill; an unpassivated valve contaminates the first several liters of flow. Our gases ship in 5-gallon and 10-gallon cylinders prepared for electronic service, and our liquid precursors are packaged in sealed stainless containers designed for bubbler delivery. Every lot should leave with a certificate of analysis showing actual measured values for assay and trace metals, not just a pass mark against the specification.

A Practical Qualification Checklist

Before qualifying a new precursor source, run through five questions:

1. Request both the GC assay and the full ICP-MS metals table, not just the headline grade.

2. Get the moisture and oxygen specifications in writing, with the analytical method named.

3. Review the packaging specification: cylinder treatment, valve type, and fill-date policy.

4. Qualify with sample lots from production equipment, not lab-scale fills, and run your own film analysis on the results.

5. Establish a second source before volume production forces the issue. Dual sourcing is cheaper than a line-down event.

Reading Between the Nines

Purity grades are a starting point for a conversation, not the end of one. Put specification sheets from competing suppliers side by side and read the individual impurity lines against what your process actually tolerates. If you would like help with that comparison, request the full specification package for any of the materials above through our contact page, and our team will match purity, packaging, and price to the films you need to grow.