Probe station solutions for wafer-level testing of MEMS

For wafer-level tests, one can choose between manual or automated probe stations. While the wafer is placed on the chuck, the optical alignment microscope on wafer probe stations is essential to have an unrestricted view of the device under test (DUT) and align the probe needles to the test pads. Electrical probing can be done in various ways, from DC to high-frequency RF signals. Also, the number of contacts can differ from just two manual probe positioners placed on the probe platen above the wafer up to many probe needles requiring a needle probe card.

In some cases, it is necessary to place different instrumentation above the probing area. For optical testing, this can include an integrating sphere, available in different sizes, for the collection of device-emitted light. If the device is light sensitive, stimulation can be achieved via a specific light source. Or non-optical components can be excited using a magnet.

Optical analysis tools can measure the mechanical properties and the behaviour of the DUT, such as 3D motion or topography. MEMS is an excellent example of such devices where an electrical stimulus causes an automatic movement. On manual probe stations, the standard alignment microscope can easily be replaced by the measurement-related microscope if the probe tips and pads can also be seen.

For automated probe systems, the microscope is used in combination with a camera for automatic wafer alignment, which is essential for automatic wafer testing. What all these scenarios have in common is that at first, a microscope is needed to align the wafer correctly and match the probe needles to the pads. After that, when the probe station automatically steps over the wafer, the microscope needs to be swapped with different instrumentation for the measurement.

MPI provides two solutions for automated wafer testing, namely a swappable microscope bridge and an off-axis alignment camera.

The swappable microscope bridge has a modular design and can be swapped between two positions to support the wafer alignment or measurement with the required instrumentation. Changing positions can be done either manually or in an automated fashion. The latter is required for fully automated test capability when testing several wafers from a cassette. The exceptional stability of the microscope bridge enables it to carry the two X, Y, Z linear drive units and additional instrumentation. Different standard microscope drive units—from a simple X, Y, Z stage to a heavy-duty, motorised scope support—can be placed on the bridge depending on requirements. (Figure 4a)