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CHAPTER 2:
System Overview
Model CRX-6.5K Probe Station
2.5.6 Planarization
Another concern with microwave probes is that the probe must be rotated to ensure
that the three points of the probe (ground, signal, and ground) are in the same plane
as the sample; this is referred to as planarization.
FIGURE 2-36 shows the S-parameters for a probe station before and after the
contacts are properly planarized and good contacts established.
2.6 Contact
Quality
The movable probe tip contacts that make probe stations such flexible tools can also
lead to poor measurement repeatability if contact quality is poor. Low resistance,
ohmic contacts are the goal for most electrical measurements. The following topics
should be considered when establishing contacts and testing their quality.
2.6.1 Contact Material
The most repeatable probe contacts are formed between the metal probe tip and a
metal pad patterned on the sample. Contacting other materials like bulk
semiconductors requires special considerations not covered in this manual. Gold
plated metal is the most common pad material used in probe station applications,
but any conductive metal that resists oxidation or reaction with the tip metal can be
used to form low resistance contacts. Lake Shore offers three probe tip materials that
are compatible with different probing applications (see section 2.3.2.1).
2.6.2 Contact Area
In addition to contact material, contact area is a major factor in the ultimate contact
resistance. Lake Shore offers probe tips with a variety of radii. In general, a larger tip
radius will create a larger contact area, but this may not translate to lower contact
resistance, as several factors dictate how much surface is actually in contact.
Focusing on the metal to metal interface, the true nature of the surfaces is not
smooth, but rough. It would not be unusual for this roughness to be 1 µm or 2 µm.
Surface roughness causes the actual contact area to be much smaller than the
physical contact area because conduction is through a few asperities (high points).
The use of soft pad materials and steady contact pressure can minimize the effect of
surface roughness.
Probe contamination is another factor that can reduce contact area. Any foreign
material picked up on the probe tip will prevent metal to metal contact. Probes
should always be handled with gloves and stored in their original shipping containers
when not in use to prevent contamination. Probes also cold weld pad metal to
themselves after repeated landings. Nonconductive materials are frequently
attached to the pad material, causing contamination. Probe tips should be cleaned
regularly to remove contamination. Cleaning instructions are given in section 6.2.7
and section 6.2.8.
FIGURE 2-36
Left: Improperly planarized test with poor contact—uncalibrated response;
Right: Improvement shown in the S-parameters after proper planarization and quality contact—uncalibrated response
