When the four-probe resistance tester measures the square resistance of a thin layer of material, the distance from the edge of the probe to the edge of the material is much greater than the probe spacing, which is generally more than 10 times; And the distance between the probe heads should be equal, otherwise an equal proportion of the test error will be generated. The Qiushan Instrument Four-Probe Resistance Tester fully considers these factors, and the probe head standard of the Qiushan Instrument Four-Probe Resistance Tester is: Probe Spacing: 100 mm；Probe Material: Tungsten Carbide; Probe pressure (optional): 5 7 N; Mechanical Wandering: < 03%；Gemstone bearing bore and probe distance: < 6 m
Square resistance, also known as film resistance, is a measurement value used to indirectly characterize the thermal infrared performance of vacuum coatings on samples such as thin film coatings and glass coating layers, and the magnitude of this value can be directly converted into thermal infrared emissivity. The size of the square resistance is independent of the sample size, and its unit is SIEMENTS SQ, and then the ohmic SQ characterization method is added, which is directly translated as the square resistance or surface resistance, which is used for film measurement, also known as film resistance.
For thin layer materials, sheet resistance (also known as surface resistance) is commonly used to characterize its conductivity. Square resistance is defined as the resistance between the opposite sides of a thin layer of conductive material in a square, as shown in the figure.
According to the definition of resistance,
where l is the length of the side of the square, a is the area of the side of the square, and since a = l · t, it can be obtained.
Simplify. 
From the above equation, it can be seen that the square resistance measurement of any size of square film material is the same, regardless of whether the side length is 1 meter or 01 meter, their square resistance is the same, because the block resistance is only related to the thickness of the conductive film and the resistivity of the material. The larger the measured value of the block resistance, the worse the thermal infrared isolation performance, and the smaller the measured value of the block resistance, the better the thermal infrared isolation performance.
According to the resistivity of the thin layer material (see the article Four-Probe Principle Resistivity Calculation of Thin Layer Samples).
The expression for the thin layer of square resistance is obtained.
The value of LN2 is about 4532, i.e., the coefficient between the square resistance of the thin sample and the resistance value displayed by the ohmic meter is 4532。The resistivity of a thin layer can also be derived from the square resistance and thickness of the thin layer, ie.
Theoretically, the smaller the point of contact between the probe tip and the conductive film, the better. However, in practical applications, the needle-shaped electrode is easy to destroy the conductive film material being tested, so a round probe tip is generally used. The probe head of the Qiushan Instruments Four-Probe Resistance Tester borrows from the manufacturing process of a mechanical clock movement, using a ruby bearing to guide the tungsten carbide probe to ensure high mechanical accuracy and long service life.