ASTM F390 Standard Test Method for Sheet Resistance of Thin Metallic Films With a Collinear Four-Probe Array

Get Certified ASTM F390 Testing for Thin-Film Electrical Performance 

ASTM F390 testing services are offered by accredited laboratories to determine the sheet resistance of thin metallic films. These services provide full analytical reports with sheet resistance data, measurement current and potential data, any correction factors used, and documents of compliance. For semiconductor, microelectronic, and coating applications, manufacturers specify the performance of thin metallic films in terms of their electrical properties, and certified testing ensures these properties. Select a testing partner that holds ISO/IEC 17025 accreditation to ensure that your films comply with sheet resistance requirements. Laboratories have a collinear four-probe array, precision current source, voltage measurement devices, and a temperature-controlled environment.

The scope of ASTM F390 includes: 

• Material type: Thin metallic films (Aluminum, Copper, Gold, Platinum, Titanium, conductive alloy metallizations) on insulating substrates (Glass, Silicon dioxide, Ceramics, Polymers). 
• Purpose: To measure electrical sheet resistance in Ohms per square (Ω/□), quantitatively. 
• Properties evaluated: It includes sheet resistance, film resistivity (when the thickness is known), electrical uniformity on substrate surfaces, and film continuity. 
• Testing method: The collinear four-point probe method uses a controlled DC current through the outer probes and measures the voltage drop across the inner probes. 
• Results: The method calculates sheet resistance mathematically from the substrate geometry, probe spacing, and edge proximity. 
• Test limitations: The method applies only to thin metallic films. The film thickness must be much smaller than the spacing between the probes. Uninsulated conductive substrates or non-planar geometries require specific boundary corrections. 
• Use conditions: The test requires a controlled laboratory room temperature to avoid changes in resistivity due to temperature changes. 
• Applications: semiconductor metallization quality control, contact optimization for solar cells, coatings for touch displays, manufacturing of smart glass, and thin-film sensors. 

 What are the Uses of ASTM F390Testing?

ASTM F390 testing helps laboratories determine thin metallic films’ resistance per square (sheet resistance) with a collinear four-probe array. The results are used by manufacturers, engineers, and quality control professionals for material characterization, process control, and quality assurance. This standard-

• Measures thin metal films’ sheet resistance for electrical characterization.
• Determines the properties of conductive coatings and semiconductor layers.
• Helps with quality control in thin film production processes.
• Evaluates thin metal deposits of microelectronic circuits.
• Allows for the non-destructive electrical characterization of films.
• Minimizes contact resistance errors using the 4-point method.
• Ensures process control in coating and deposition processes

What Materials Can Be Tested Under ASTM F390? 

It tests thin metallic films such as aluminum, copper, gold, silver, and alloys on substrates like silicon wafers, glass, or flexible polymers, allowing for the precise measurement of electrical properties in microelectronics and flexible electronics.

Why is ASTM F390 Important? 

For high-performance applications, the standard is critical as it provides a standardized method to measure sheet resistance for the reliable functionality of thin metallic films and ensures that there is no deficiency in material selection and process optimization, thereby minimizing defects

ASTM F390 Testing Process and Data Collection

The analyst checks instrument zero, calibration, and probe tip condition. The technician holds the specimen in place on the insulating stage. The 4 probe tips are spring-loaded and gently come down on the film surface, making a low-stress contact without penetrating the thin film. The outer probes provide a precise DC source, and the instrument records the voltage across the inner probes. The instrument cancels thermal offset voltage by automatically switching current polarity. Technicians can map the sheet resistance in multiple spatial locations along the surface of the substrate and measure the percent non-uniformity for the entire wafer/panel. 

Common Challenges and Troubleshooting

The probe can puncture thin soft metallic films (e.g., gold or aluminum) when the force is too great, which causes spikes in local resistance or exposes the underlying substrate. Spring-loaded probe heads calibrated to specific forces can prevent puncturing the film. Probing at the edges of small substrates causes edge proximity effects; geometric correction factors based on the size and distance between the samples and the probe eliminate errors caused by the edges. Temperature differences between the tip and the sample develop thermal voltages in the probe tips, and the technician eliminates these voltages by using a mean of the forward- and reverse-going currents. 

• The laboratory reports the sheet resistance, measured in units of ohms per square (Ω/□), which is the resistance of the film per square of its surface area. The measurement range is from 10⁻² to 10⁴ Ω/□.
• The analysis takes into account correction factors related to the geometry of the specimen and the probe spacing. These factors are used to determine the accurate sheet resistance values based on the measured current and potential data.
• An analog circuit with a known standard resistor and other resistors that represent the resistance at the film surface and the tips of the probes is used to check the accuracy of the equipment in the laboratory.
• The analyst measures in a controlled temperature environment (typically 23°C), and current reversal techniques minimize thermoelectric EMF.
• The report compares test results to relevant product specifications or customer requirements and states whether the tested film conforms to the specifications.
• To evaluate the film’s suitability for the application, engineers compare the sheet resistance result to the specifications of the product the film will be used for.

Link to ASTM F390