Delcom sheet resistance meters are most often used to measure the active layer of Low-E glass. This includes both metal coatings such as Silver (Au) and Gold (Ag) and semiconductive coatings such as Indium Tin Oxide (ITO) and Fluorine-doped tin oxide (FTO).

Figure 1: Possible Low E Glass cross-section

Delcom sensors are used extensively in the Low-E industry because sheet resistance approximates emissivity.
Assuming wavelengths more than 3 micrometers (i.e λ > 3 µm) and a sufficiently low sheet resistance, we can say that normal emissivity approximates four times sheet resistance divided by the impedance of vacuum.

Figure 2: Equation for emissivity

Knowing that the impedance of vacuum is 377 Ohms, the formula can be further simplified resulting in the statement that emissivity approximates 0.0106 times sheet resistance.

Figure 3: Equation for emissivity as a function of sheet resistance

Of course, each researcher/production manager has their own constant that they are comfortable with. Delcom software allows for users to designate their own constant and observe readings on the Delcom meter in units of emissivity.

It is important to note that because eddy current meters rely on a magnetic field to achieve their reading, they have a number of advantages over four-point probes to include:

• Reliably approximates emissivity
• Is non-destructive
• Reads through insulating layers
• Measures moving material
• Provides nearly instantaneous readings
• Provides real-time process inspection

Low-E glass manufacturing faces a number of challenges to include:

• Problems caused by curing, annealing, temperature change, and other processes changing the sheet resistance over the course of production that needs to be carefully monitored
• Cross-web consistency born of using multiple targets or other segmented deposition techniques

For those customers manufacturing Passive Low-E glass, a hard coat pyrolytic process of applying a tin oxide or silver layer (Chemical Vapor Deposition when performed in vacuum) is employed.  In such a process, a Delcom sensor can be deployed before or after annealing.  Regardless of placement, efforts must be made to shield the sensor from the high radiant heat of the glass and equipment.  In addition to shielding, the cooling line option might be needed to ensure the Delcom sensor does not overheat.

Figure 4: Possible sensor locations for hard coating process

For those producing solar control Low-e and passive Low-E glass, an off-line soft coating process is often used.  This process occurs after the glass ribbon has been cut into stock sheets, occurs at room temperature under vacuum, and employes Magnetron Sputter Vacuum Deposition (MSVD) to apply a thin transparent coating of tin oxide or silver.  If a Soft Coat manufacturing process is being employed, any Delcom vacuum-ready sensor can be employed in nearly any location.

Figure 5: In-situ monitoring for a soft coating process (including possible positions for Delcom Sensors):

In general, Delcom sensor deployment strategies can include one or more of the following deployment tactics.

Deployment Strategy	Image	Advantages
Single sensor single spot		Most cost-effective option on the market One single fixed-point Achieve instantaneous process feedback
Two sensors monitoring cross-web		Monitor cross-web uniformity
Two sensors monitoring downstream		Monitor downstream consistency
Multi-channel monitoring cross-web		Monitor cross-web uniformity Movable sensor positions Add sensor positions at a later time

Delcom recommends the following sensors based on the user’s material, stage of development, and application.

Use case	Image	Recommended Sensor	Use case
Benchtop		RD200	For material up to 12 mm thick For materials up to 200 mm x 200 mm
Benchtop		RS200	For material more than 12 mm thick For materials up to 200 mm x 200 mm
Benchtop		RD300	For material more than 12 mm thick For materials up to 300 mm x 300 mm
Benchtop		RS300	For material more than 12 mm thick For materials up to 300 mm x 300 mm
Benchtop		RD450	For material up to 12 mm thick For materials up to 450 mm x 450 mm
Benchtop		RS450	For material more than 12 mm thick For materials up to 450 mm x 450 mm
Inline		20J3 Hybrid	Most cost-effective inline solution 300 mm reach and 9 mm gap
Inline		20J3 Inline	Most cost-effective inline solution 300 mm reach and 9 mm gap
Inline		30C9 Hybrid	Most cost-effective inline solution 300 mm reach and 9 mm gap
Inline		30C9 Inline	Most cost-effective inline solution 300 mm reach and 9 mm gap
Inline		S3 Hybrid	Most cost-effective inline solution 300 mm reach and 9 mm gap
Inline		S3 Inline	Most cost-effective inline solution 300 mm reach and 9 mm gap
Inline		OEM	Variable position, gap, and spot size Customer decides where and how to mount
Inline		Multisensor	Monitor cross-web uniformity Variable position, gap, and spot size Add channels anytime

Delcom sensors measure sheet resistance in Ohms/square, therefore ranges of our instruments stated in terms of emissivity require multiplication by the user’s preferred constant.  Below are Delcom sensor ranges restated in units of emissivity assuming a constant of 0.0106.

Figure 6: Delcom sensors are available in the following ranges

Low E Glass products typically have a target sheet resistance of between 1 and 8 ohms/square.  Therefore, the recommended instrument range for this application is “x1”.