Microprobing

Microprobing, or simply probing, is a failure analysis technique used to achieve electrical contact with or access to a point in the active circuitry of the die. It employs a special piece of equipment known as a microprobing station, which is also commonly referred to as a 'probe station' (see Fig. 1).

Electrical contact is made by dropping fine-tipped probe needles directly on the point of interest, or on an area to which the point of interest is connected. The size of the tip of the needle used is chosen based on the electrical contact needed and on how large the probing area is. Each of these needles is held by a micromanipulator (see Fig. 2), which is the accessory controlled by the analyst to land the needle on the die precisely.

Fig. 1. Examples of Microprobing Stations

During circuit analysis by microprobing, the analyst employs the same thought process as when troubleshooting a full-size circuit. Microprobing is only a tool for the analyst to access critical nodes on the microscopic die circuit while analyzing the behavior of the various parts of the circuit. The process of electrically pinpointing the failure site is known as failure isolation, which requires the analyst to identify abnormal voltages and/or currents on the die.

Voltage and current measurements are performed by the electrical measurement instruments attached to the probe needle through the micromanipulator. Common instruments attached to the probe station are voltmeters, curve tracers, oscilloscopes,and the like. Circuit excitation from voltage supplies, waveform generators, and the like may also be supplied to the die circuit in the same manner.

Fig. 2. Examples of Micromanipulators

The glassivation on top of the die surface to be probed is often removed by reactive ion etching prior to probing. Probing a die surface with an intact glassivation is difficult because the glass layer is hard to penetrate with a probe needle. Furthermore, broken glass tends to make the probing area messy, and may even make the electrical contact intermittent, especially when small or narrow metal lines are involved. As an alternative, glass openings may be made on prospective probing points by blasting the glass accurately with a laser cutter.

Fig. 3. Photo of a stand-alone laser cutter

Note: a laser cutter is more often installed atop the microscope of the probing station since laser cutting is almost indispensable to microprobing

Microprobing almost always requires a schematic diagram and a die lay-out of the device to be effective and efficient. The analyst needs these diagrams to immediately pinpoint critical nodes for probing to isolate the failure site.

During failure isolation, the goal of the analyst is to 'zero in' on the bad component(s). This is difficult, if not impossible, to do without the ability to isolate circuits from each other. The laser cutter is an indispensable tool for this purpose, allowing metal lines to be burned open for convenient isolation of nodes from one another.

If the analyst is not so familiar with the circuit and how it works, microprobing can still be performed as long as a correlation or known good unit is available. Probing in this case proceeds by comparing the voltages or signals at critical nodes of the sample with those of the good unit.

A novice failure analyst must never make his first microprobing attempt on a real sample, since microprobing requires a high degree of skill. Eye and hand coordination, as well as experience, is needed to locate the area of interest, choose an appropriate probing spot, land the needles (see Fig. 4), and exert the correct pressure for good electrical contact. Die scratches and even chip-outs result if probing is not done correctly.

Fig. 4. Photo of a microprobe needle on the die surface

Decapsulation; Microthermography; LEM; Die Deprocessing; SEM/TEM

FA Lab Equipment; Basic FA Flows; Package Failures; Die Failures

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