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Here are the answers to some of the questions we've received over the years concerning our resistivity calibration standards. If you have questions about VLSI's calibration standards that you'd like answered, please forward them to sales.support@vlsistandards. We'll add them to our list of FAQs so they're available to all customers.
Why does VLSI Standards only carry 76.2 mm or 200 mm diameter wafers only for the
Resistivity Standard product line? Can you manufacture 100 mm, 150 mm, or 300
mm Resistivity Standards?
This fact has to do with the history of the product line. For many years in the
industry, electrical property measurement tools were not available with
automated handling equipment. At the time VLSI Standards only produced 76.2 mm
(3”) versions of the Resistivity Standards since these were readily available
and less expensive for the user. Users could easily load this wafer diameter
manually. When automated tools became more prevalent in the industry 200 mm
fabrication facilities were the most common at that time. There were fewer 100
mm and 150 mm fabs being built. VLSI Standards is currently investigating the
possibility of 300 mm Resistivity Standards.
Why does VLSI Standards certify both sheet resistance and resistivity?
Most contact probes (as the one VLSI uses) output measurement data in sheet
resistance (ohms/sq.). Resistivity (ohm.cm) is the inherent electrical property
of the whole material. It is sometimes called Specific Resistance. Sheet
resistance is the resistance measured only at the probe/surface interface of
the material. In order to calculate resistivity, VLSI must first certify the
sheet resistance and the wafer thickness. VLSI makes all 3 certified values
available on the certificate.
The Resistivity range that I need to calibrate or monitor cannot be purchased from VLSI Standards.
Why doesn’t VLSI Standards provide other resistivity values?
VLSI Standards tries to match the resistivity values that are available as NIST master values whenever possible. This limits the scope of our offerings.
Why are some Resistivity Standards as-lapped and some polished?
Simply put, the as-lapped version is for use with contact tools (i.e., 4-point probes)
and the polished version is for use with non-contact tools (i.e., eddy current
gauges). As-lapping the surface allows contact tools to make better contact /
penetration of the surface and prolongs the life of the standards. This textured surface is not required for
non-contact tools.
Why do some Resistivity Standards have a clear, designated certified area and others do not?
With the as-lapped versions of the product the wafer is not patterned. Due to this
surface texturing it would be difficult to grow a silicon dioxide film uniform
enough to create a center measurement area aperture. VLSI Standards certifies
only a small area (10 mm) in the center of the wafer in order to keep
uncertainties from being dominated by radial resistivity variation inherent to
the production of silicon wafers. All polished versions of the 200 mm
Resistivity standards do come with a clearly defined measurement area 32 mm
in diameter.
Are both sides of my standard certified? Can I use both
sides to extend the life of my standard?
For the as-lapped versions of the standards, yes. You should
observe the same certified values using either side of the wafer. Since the
polished versions only have a patterned measurement aperture on one side, only
this one side (front) is certified.
What method/tool does VLSI use to certify the Resistivity Standards?
VLSI Standards uses contact methods in measuring its Resistivity Standards. For 3” as-lapped wafers VLSI
uses ASTM Method F-84 which calls for measurement with a 4-point probe correcting for temperature and probe spacing. For VLSI Standards 200 mm versions we follow ASTM Method F1529 using an
In-Line Four-Point Probe with the Dual-Configuration Procedure. See http://www.astm.org for more information on
these procedures.
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