How Long Does A Four Point Probe Head Last?
We are often asked, “How long does a probe head last?” This is a difficult question to answer. Jandel Engineering offers four point probe heads built with tungsten carbide only. The previously offered 50% Osmium tips had too many limitations including durability. Tungsten carbide is extremely hard, measuring 8.5 – 9.0 on the Moh Scale. The following discussion only pertains to probes built with the much more commonly used and much more durable tungsten carbide needles.
Probe life is dependent on several factors including:
- The amount of usage
- The hardness and texture of the material being probed
- The needle spring loads and tip radii
- The ability of the material to maintain measurement integrity as the probe gradually wears
- The possibility that a human error could be made in handling the probe
Usage
Often, customers will purchase a probe and then not return to purchase a replacement for many years. Other customers will have several probes, and every few months will purchase replacements since some have worn out. In silicon wafer testing, the probes are put into an automatic stepper which will probe cassettes of wafer with 49 or 121 locations (or more) per wafer in a production environment. These systems are often run three shifts per day. The most commonly used probe tip specifications for silicon wafer testing are 100 micron radii with 100 gram loads. Contrast this with a probe that has tip radii of 300 micron radii and 60 gram spring loads which is used in a university setting where it is probing only a few dozen sites per week.
Texture of Material Measured
Materials with an abrasive texture will wear out probe tips more quickly. Smooth materials allow for longer probe tip life. ITO (Indium tin oxide) on glass, for example, is very hard yet its smooth surface allows probes tips to last a relatively long time. Four point probe stepper systems used in silicon foundries are sometimes used to perform probe tip “conditioning” in which the probe tips are brought into contact several times with an alumina (ceramic) substrate of a given roughness for the purpose of making microscopic asperities in the tip surfaces. This procedure is designed to improve ohmic contact when measuring higher resistivity wafers. This procedure is normally only performed on automatic four point probe stepper systems. Conditioning is something that should only be performed as needed, and over-conditioning will unnecessarily shorten probe life.
Needle Spring Loads and Tip Radii
The probe tip specification for probing bare silicon wafers includes relatively sharp tips (40 micron radii), and strong spring loads of 200 grams per tip. This combination naturally leads to quicker tip wear than a probe with blunter tips and lighter loads. The range of probe tip specifications for use in measuring silicon wafers runs from 40 micron radii up to 500 micron. Spring loads are generally 100 grams per tip, with 200 grams and 40 micron radii specified for bare silicon wafers, ingots, and potscrap. Conductive polymers sometimes require 500 micron tip radii and 60 gram loads to make successful measurements. Other conductive polymers can be successfully measured using the very common probe tip specifications of 100 micron radii and 100 gram loads which are considered “general purpose” and are capable of measuring the widest range of materials.
Material Sensitivity to Probe Tip Specifications
Probes used in an automatic system or probes used over a long period of time in a manual system, will eventually develop small “flats” on the tips which can be a source of measurement irregularities. Some materials are more sensitive to the flats than others. As tips wear, the contact area increases which causes a decrease in tip pressure per a given area. The needles also gradually become shorter which effectively reduces the spring load. Some materials can be successfully measured with a wide range of probe tip specifications from blunt to sharp tips and from strong to light springs. If a material is easily measured with a wide range of probe tip specifications, a wise probe tip specification recommendation may still be the “general purpose” 100 micron radii with 100 grams loads so that the probe will be able to measure a broad range of materials as needs develop in the future. If a probe is going to be dedicated to one particular material, and if blunter tips and lighter springs work well, those specifications may be selected for longer tip life and to reduce unnecessary marking on the material. Ideally, the marks left by a four point probe should be only microscopic in nature.
The Human Error Factor
Probe life can be lengthened by taking steps to avoid having the current flowing as the needles touch-down onto the material. This can cause sparking which can damage the tips. Jandel’s probe stands are equipped with a microswitch at the base of the lever so that when properly setup, the current does not flow until after the needles have made contact to the material. To prevent breaking a probe tip, care must be taken to avoid moving the wafer or material while the tips are in contact. Tungsten carbide is four times stiffer than steel, so care must be taken not to impact the needles from the side as can happen if dropped on the floor.
Measurement Degradation and Probe Tip Cleaning
There is no standard procedure for checking the condition of a four point probe head other than watching for degradation of the measurement data. Sometimes measurements can be improved by cleaning the substrate and cleaning the probe tips. A cotton swab dipped in either isopropyl alcohol or acetone can be used to clean the tips occasionally to insure that the needles are not contaminated with some insulating dust or other material. Canned air can also be used to blow away contaminates.
Jandel has no hard data regarding expected tip life based upon variables. However, they have always contended that if the needles do not wear out, the precision made jeweled probe mechanisms should last for hundreds of thousands of tip retractions. Jandel has a customer located in the Netherlands that is an equipment manufacturer producing an automated resistivity stepping system which incorporates the Jandel Cylindrical probe head. They ordered a probe head for their in-house system, and after some period of time finally started seeing anomalies in their data. They were kind enough to share with Jandel that they had a counter mounted on the system which showed that the probe had made more than 900,000 contacts before the measurement data began to degrade. This was the only time that Jandel has received data based upon a feedback system which actually counted each time the probe made contact. Although this example of probe life might be exceptional, Jandel has no way of knowing whether this length of probe usage occurs often or not.
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
