testing simple caps on the pnds - accunetsecure.com · • the pnds samples must be driven...

Radiant Technologies, Inc.

Testing Simple Capacitors on the PNDS Rev A

Joe T. Evans Jr.


Albuquerque, NM

December 7, 2015


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.


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.

– 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.


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.


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 non-

linear 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.


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.

– 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!


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.


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.


Shadow Mask Samples • A shadow mask will suffice to make top electrodes for test on the PNDS.

• Procedure:

– 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


Demonstration Assembly

Note the wire soldered to the board for

electrical connection to the BE.


Another Example

Note: The orientation of the

sample is different on this

mounting.

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 single-

crystal substrate and the PCB

can carry away the heat.

Note: The cantilever comes

in from the right. The BE

wires are on the left.


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.


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.

testing simple caps on the pnds - accunetsecure.com · • the pnds samples must be driven...

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