How do I determine the most appropriate choice of probe tips specifications (tip spacing, spring loads, tip material, tip radii) for a given material?
Having a sample of your material tested by the probe manufacturer is by far the best way to determine the optimum choice of probe tip specifications for a given material. This is the only way that a four point probe manufacturer can guarantee that a certain set of probe tip specifications will work satisfactorily in measuring a particular material. Regarding the testing procedure that Jandel Engineering follows when testing customer’s samples, Jandel wrote:
“Initial choices of probe head are based upon various factors including over 40 years experience in making such measurements. When possible, we test samples on our CDE ResMap for which we have types A, B, C, D, E, F, G and H. In addition, we have approximately 12 Cylindrical probe heads for use in our own equipments which cover a broad range of characteristics. When we measure conductive polymers, we commonly start with the lowest pressure and then progress to higher pressures to see if we can get a better result. We start with a 500 micron radius, i.e. the 500 micron profile is over the whole diameter of the needle (usually 0.40mm diameter). We work the opposite way with hard samples, i.e., highest pressure first, followed by lower and lower pressures, especially if the subject is a shallow layer or film.”
Following are some of the considerations involved in selecting probe tip specification for a given material:
Probe Tip Spacing
Tip spacing is the spacing between any two adjacent needles. Jandel Engineering builds probes with tip spacing in the range from 0.5mm (~20 mils) up to 1.591mm (~62.6 mils). Tip spacings from 0.635mm up to 1.591mm are sold at the standard price, however, probes with 0.5mm tip spacing cost more due to the difficulty in manufacturing 0.5mm “close spacing” probes. Info regarding close spacing probes can be found here:
Close Probe Tip Spacing
When measuring thin films, the tip spacing is not taken into consideration in calculating sheet resistance, however, it is sometimes important for the following reasons:
- If the material is very small, close tip spacing is sometimes required simply to get the tips to all fit onto the material. When measuring small samples, a correction factor must be applied if one wants to determine the true value in ohms-per-square. Information about sample size and correction factors can be found here:
Correction factors for four point probe measurements - When measuring wafers in production, closer tip spacing is sometimes desired so that the tips can be placed closer to the edge of the wafer before a correction factor is required.
- When measuring materials that are small enough in size such that a correction factor of slightly more than 1% may be required to arrive at a true value, using closer needle spacing will sometimes make the difference between requiring or not requiring to apply a correction factor.
- When measuring multicrystalline wafers, it is normally prudent to use a 0.635mm tip spacing as it will be easier to land all four points on a single crystal (if required).
The probe tip spacing of 1mm is commonly used to measure thin films. Other common probe tip spacings includes 0.5mm, 0.635mm, 1.27mm, and 1.591mm. Theoretically, a probe with a larger tip spacing such as 1.591mm as opposed to tip spacing of 0.635mm should give more accurate data, however, in practice the difference is not measurable.
The probe tip spacing of 1.59mm is traditionally used to measure “volume” or “bulk” resistivity, expressed as ohms-cm, of thick materials such as ingots. Modern four point probe electronics include software to calculate volume resistivity by inputting the probe spacing, and in the case of thin films, inputting the thickness of the film. However, using 1.591mm tip spacing simplifies the mathmetics if one is manually calculating volume resistivity of thick materials such as ingots.
Resistivity = 2 x pi x s x V/I (where s is the probe spacing in cm).
If one uses a probe head with tip spacing of 1.591mm (62.6 mils), since 1.591mm is 1/(2 x pi)cm, it cancels out to V/I
Resistivity = 1 x V/I
Therefore, if a probe with 1.59mm tip spacing is used with an input current of 1mA, (assuming that the material can be successfully measured with an input current of 1mA), it will give a mV reading that is a numerically equivalent to the resistivity in ohm-cm. More regarding this application can be found HERE.
Uniform tip spacing is important in making an accurate four point probe measurement. Jandel hand grinds their probes to a given radii using a watchmaker’s lathe. Since it is possible, but not likely, that the center of the contact area (the tip of the probe) may not be exactly in the center of the probe shaft, Jandel uses an optical interferometer to verify the tip spacings. The needles are pressed onto a sapphire flat (making probe contact marks) and interference rings are viewed using an optical interferometer. A micron graduated reticle is used to ensure that the distances between the centers of the rings are accurate within a 10 micron tolerance.
Spring Loads and Probe Tip Radii
Probe tip radii and probe tip spring loads go hand-in-hand in making the correct combination of probe tip contact area and tip pressure for a given material.
Spring Loads
Spring loads on Jandel probes can range from 10 to 200 grams per tip with 250 grams being an option which is rarely required or recommended due to the increase in probe tip wear. Each of the four probe tips in a Jandel four point probe are independently adjusted to the correct spring load to within a 1% tolerance using a custom electronic force gauge. High pressure probes are typically required to penetrate an oxide layer on a silicon wafer, or for measuring ingots. KLA-Tencor and CDE machines use their type “E” type probe with 40 micron tip radius and 200g pressure for these applications. The Four Dimensions system uses the Cylindrical probe which can be adjusted up to 150g – hence they use 25 micron radii tip set at 90 grams. Lighter spring loads are required when the substrate is soft. More information regarding the Cylindrical probe can be found here: Jandel Cylindrical Probe
Probe Tip Radii
Some high resistance materials require blunt tips to make more surface area contact. Softer materials will often require blunter tips to prevent unnecessarily marking the material. Ideally, probe tip marks should be microscopic in nature. Some materials require sharp tips to dig-in and make better ohmic contact or to punch through an oxide layer. A chart showing probe tip specifications for various silicon wafer applications is shown here: Four Point Probe Tip Specifications & Applications Chart
The chart shows the range of tip radii and spring loads that are recommended for measuring various silicon wafer types, but it only serves as a guide. The chart was developed many years ago and does not take into account some modern types of wafer such as PV wafers and multicrystalline wafers. Although Jandel occasionally builds probes with 12.5 micron tip radii, 25 micron radii tips are normally the sharpest probes made. The bluntest available is 500 micron radii.
Probe tip radii of 40 microns are the most common type used in silicon wafer measurements. However, if considering all materials that four point probes are used to measure, the most common probe tip specifications are 100 micron tip radii with 100 gram spring loads. The 100µ/100g probe works well in measuring a very wide range materials, and Jandel refers to this combination as the “general purpose probe tip specifications”. In situations where samples are not available to test, especially if a probe is to be used to measure a wide range of materials, Jandel will recommend the 100µ/100g general purpose probe tip specifications. However, they cannot guarantee that it will work successfully for any particular material without first testing the material.
Probe Tip Material
The De facto standard material for use in making four point probes tips is tungsten carbide. Jandel Engineering offers an osmium alloy material as well, but it is rarely justified and does not wear as well. More information about the merits of using tungsten carbide tips can be found here: The relative advantages of tungsten carbide tips vs. osmium alloy tips
Once an order has been placed, Jandel can test samples to determine the optimum choice of probe tip specifications. This is the only reliable way to insure the successful measurement of specific materials.
Four-Point-Probes is a division of Bridge Technology. To request further information please call Bridge Technology at (480) 219-9007 or send e-mail to Joshua Bridge at: sales@bridgetec.com
