Testing Simple Capacitors on the PNDS
Rev A
Joe T. Evans Jr.
Radiant Technologies, Inc.
Albuquerque, NM
December 7, 2015
Radiant Technologies, Inc.
Summary
• Radiant’s Precision Nano-Displacement System targets the
measurement of piezoelectrically active samples.
• The PNDS samples must be driven electrically by a Radiant tester
independent of the PNDS operation.
• Electrical connection can be made to a sample in one of two ways:
– The sample capacitor has bond pads so it can be mounted in open packages and
connections can be made through the pins of the package.
– The sample capacitor does not have bond pads and its top electrode must be
contacted by a conductive cantilever.
• This application note describes how to mount simple capacitors on
dedicated printed circuit boards supplied by Radiant so the samples
can be tested with the PNDS.
Radiant Technologies, Inc.
AFM Reference Capacitors
•
Radiant supplies dedicated piezoelectric capacitors for use in verifying the
proper operation of Atomic Force Microscopes (including the PNDS) when
measuring piezoelectric displacement.
•
The AFM Reference consists of two capacitors fabricated on a single die with
bond pads.
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•
100,000µ2 Platinum/PNZT capacitor clamped to the substrate
100,000µ2 Platinum/PNZT membrane capacitor released from the substrate
The die is attached to a TO-18 header and the capacitors are connected to the
package pins using gold wire bonds.
Radiant Technologies, Inc.
AFM Reference Capacitors
•
A special printed circuit board with a TO-18 socket holds the header and
mounts into most commercial AFMs as well as the PNDS.
•
The board is long enough to fit across large AFM stages but it also has side
tabs that can be broken off to make the sample board smaller.
•
The board has mounting holes to allow it to be bolted down to the PNDS
plastic chuck for testing in the PNDS. A completed board on the chuck is
below.
Radiant Technologies, Inc.
Simple Capacitors
•
A simple capacitor is the typical device fabricated in most research programs.
•
It is defined here as a global bottom electrode, global deposition of the nonlinear material under study, and dots of top electrode material.
•
Typically, part of the substrate is etched back to expose the bottom electrode.
 Radiant has created a simple printed circuit board for mounting such devices
in the PNDS.
 The PCB bolts to the top of the PNDS plastic chuck in the same manner as the AFM
Reference board.
Radiant Technologies, Inc.
Capacitor Geometry
•
Standard AFM cantilever tips have a diameter at their points of approximately
40 nanometers, giving the tips an area of ~1300 square nanometers.
•
A 4µm x 6µm [ 24µm2 ] ferroelectric capacitor executing a 5-volt hysteresis
loop in 1ms requires 1µA peak current during switching.
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The current density through a 40nm diameter conductive AFM tip will be ~80kA/cm2 .
This is just below the 100kA/cm2 threshold for conductor damage.
•
Testing larger capacitors requires cantilever tips with larger diameters.
•
Radiant recommends the App Nano “plateau” tip with a 3-micron diameter
coated with 1000Å of platinum.
–
•
Current density for a 300µ-diameter capacitor would be 40kA/cm2. (2.8mA maximum)
A 300µm diameter is the limit for a ferroelectric capacitor using a 3µm-wide
conductive cantilever on the PNDS. Materials with low dielectric constant like
PVDF or AIN may be tested up to 1 millimeter diameter!
Radiant Technologies, Inc.
PNDS Blank Sample Board
•
The sample board has four solder pads that lead to solder holes from which
wires can make connection to the sample.
•
The board has a space 8mm x 18mm in the center between the countersunk
bolt holes on which to mount samples. Samples may also be mounted along
the outer edges of the board.
•
The pads are arranged to keep one side of the board clear of wires. The AFM
cantilever should approach the sample from that direction to prevent
entanglement.
•
The center of the board is clear of metal traces so that mounting surface is
perfectly flat.
Radiant Technologies, Inc.
Bottom Electrode Connection
•
To test simple capacitors, a single connection is made by wire to the bottom
electrode. A conductive cantilever electrically connected to the tester makes
contact with the capacitor top electrode.
•
Connection to the bottom electrode can be made by conductive epoxy or by
soldering.
Clearing only the corner of the BE
leaves the most capacitors for test but
makes it difficult to connect the BE wire.
Clearing a large BE area makes it easy
to connect the BE wire but leaves less
area to test.
Radiant Technologies, Inc.
Shadow Mask Samples
•
•
A shadow mask will suffice to make top electrodes for test on the PNDS.
Procedure:
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Start with a substrate coated with bottom electrode
Deposit the dielectric material of interest
Etch off some of the dielectric material to expose the bottom electrode.
Deposit the top electrode metal using a shadow mask or lift-off.
The etch stop is optional. If the dielectric material is not removed, place a
large area top electrode square over part of the film. It is possible that a defect
in the film will short it to the bottom electrode. If no short occurs, the large
TE contact can be contacted as if it were the bottom electrode. If this contact
has a much larger area than the small top electrode contacts, it will form a
capacitor to the bottom electrode but the voltage drop to the bottom electrode
will be very small.
BE
Radiant Technologies, Inc.
Demonstration Assembly
Note the wire soldered to the board for
electrical connection to the BE.
Radiant Technologies, Inc.
Another Example
Note: The orientation of the
sample is different on this
mounting.
Note: The cantilever comes
in from the right. The BE
wires are on the left.
Note: If the bottom electrode
material is a noble metal like
platinum and a solder contact is
to be made, the soldering iron
must be very hot to heat the
metal locally before the singlecrystal substrate and the PCB
can carry away the heat.
Radiant Technologies, Inc.
Tester Connections
•
When an AFM cantilever is inserted into the cantilever holder of the PNDS, it
makes electrical connection to the copper spring holding it in place.
•
The copper spring is connected by twisted-pair wire and coax cable to the
BNC on the rear panel of the PNDS.
Theoretically, the RETURN input of the
Radiant tester should be connected to
the conductive cantilever through the
BNC on the rear panel of the PNDS
because this path provides more
shielding for the signal.
However, the user should try the
DRIVE connected to the cantilever as
well and then select the best cable
order for lowest noise.
Radiant Technologies, Inc.
Conclusion
•
The PNDS provides an inexpensive path to measure the piezoelectric
performance of thin piezoelectric and ferroelectric films.
•
This technique would not be successful without a more robust conductive
cantilever capable of driving the large areas typical of top electrodes fabricated
with shadow masks. The metal-coated large-diameter plateau cantilever tip
solves this problem.
•
The mounting process is fast and simple, requiring only the PNDS Blank
Sample Board, wire, and conductive epoxy or a soldering iron.
•
Sample capacitors over global bottom electrode can be made with shadow
masks or lift-off of the top electrode.
 Use 300 micron diameter dots for ferroelectric ceramics and 1mm dots for low
polarization materials.
 Create a bottom electrode contact by etching or with a large top electrode contact.
Radiant Technologies, Inc.