Service Manuals, User Guides, Schematic Diagrams or docs for : Keithley Appnotes 2871 Probe Card WP
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WHITE
PA P E R
Probe card considerations when migrating to new testers
Yang Pan
Keithley Instruments, Inc.
Introduction
For on-wafer automatic parametric testers, the probe card is the interface
between the test head and the device pads on the wafer, so it is key to achieving
a successful electrical measurement. The probe cards specified by each of the
two dominant parametric tester companies, Agilent and Keithley Instruments,
are incompatible with the other's testers. Therefore, when fabs change to a
new tester type, such as from an Agilent 4000 Series tester to a Keithley S600
Series tester, they must also migrate to new probe card types.
To ensure good (i.e., correlated) parametric tests, a minimal, repeatable,
and reliable pin-to-pad contact resistance is a necessary prerequisite. Test
system specifiers must select a suitable probe card to fulfill these requirements.
This White Paper addresses two major considerations: 1) how to select and
specify a suitable probe card, and 2) how to use the probe card properly. Many
measurement results are presented. For the sake of simplicity, we will assume
that the new tester will be used to probe the same wafer types with the same
tests as the old one was.
Probe card selection
Obviously, the new card's probe pin count, pad pitch, leakage current, test
temperature, and frequency limit specifications should be the same as those of
the previous probe card. In addition to these basic aspects, the following list
of parameters should be considered. All the listed parameters affect contact
resistance, current handling capability, and bond pad scrub characteristics. Card
Keithley Instruments, Inc.
28775 Aurora Road
Cleveland, Ohio 44139
(440) 248-0400
Fax: (440) 248-6168
www.keithley.com
specifiers should specify the same parameters for the new card as the old probe card to ensure
proper correlation of old and new tester results.
1. Needle material. The needle metallurgy must be compatible with the device pad
metallurgy. Is long needle life more desirable than lower contact resistance? Is probe
pad damage an issue with multiple probing touchdowns at a site (i.e., is a softer needle
material required for a given prober Z over-drive)? For example, rhenium-tungsten
(ReW) material is hard and durable, but its contact resistance is higher than beryllium-
copper (BeCu), which is soft and wears out more easily but has lower contact
resistance. It may be advisable to contact probe card manufacturers for expert advice
on needle and pad metallurgy.
2. Tip shape. Flat or semi-radius (hemispherical)? Flat needle tips are low cost and offer
good performance. The semi-radius tip is a little more expensive, but it provides better
performance in terms of contact resistance and pad damage.
3. Tip diameter. What dimension? Larger diameter tips offer better contact resistance, but
have more difficulty in breaking through pad oxide. Conversely, it's easier for smaller
tip diameters to break through pad oxide, but they have poorer contact resistance. Also,
the larger the tip diameter, the more difficult it becomes to keep the tip completely
within the (steadily shrinking) pad area as the needle scrubs the pad surface. (This is
a requirement for maintaining measurement repeatability, as well as to preventing any
possible device damage.)
4. Alignment accuracy. This measures the distance of the probe tip at over-drive with
respect to the center of the bond pad.
5. Planarity. This parameter defines the difference in vertical distance between the
highest and the lowest probe tips in the Z dimension. Planarity is the most important
needle placement parameter.
6. Over-drive. The wafer prober pushes the wafer up against the probe tips at a test site.
The initial probe-pad contact Z height is the zero level, and the prober then moves
higher by a specified distance called over-drive. This action scrubs the needles on the
probe pad, breaking through pad oxide and ensuring good contact between the needle
tip and the device pad. Probe card vendors' specification sheets define the over-drive
used to qualify the probe card. Vendors also refer to this term as overtravel.
7. Probe-force rate. This parameter is also called "probe-spring rate" or "probe-force"
for short. This is the rate of downward tip force per unit over-drive distance, defined
as grams/mil of over-drive. If the probe-force rate is too low, it needs larger over-drive
to obtain a good contact; however, this increases the risk of the probe tip sliding out of
the bond pad laterally. On the other hand, if probe-force rate is too high, it can damage
the bond pad and increase the tip's wearout. Needle material and construction directly
affect the spring constant of the needle, and therefore the probe-force rate as well.
Probe card application
Contact resistance behavior
To speed and simplify the tester migration process, probe card specifiers should understand
the basics of probe card contact. This section discusses how contact resistance works and what
an acceptable contact condition is. The following equation describes the relationship between
probe-force and over-drive. Probe-force is linearly proportional to over-drive in normal range
and it follows Hooke's Law.
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