PELCO Dimpler
Precision polishing system for TEM specimen thinning and site-specific analysis.
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The PELCO® Dimpler™ provides Automated Precision Thinning of Specimens for TEM Analysis, for deprocessing and failure analysis, and similar applications where targeted fine polishing is required. It is a precision electro-mechanical metallographic lapping instrument that will continuously monitor and control dimpling parameters and accurately terminate at a preset specimen thickness. This electronic and mechanical integration has added repeatability and reliability to the dimpling process, as well as automation.
Active monitoring of the Z-position, feedback-controlled damping, and automated controls for determining termination of processing work together to provide repeatable, reliable results in sample preparation.
Z Position
The Z position is continuously monitored via a non-contact sensor. The digital display indicates the dimple depth in microns and is updated once every revolution. This negates any errors due to tool run out, especially significant when using padded polishing tools. The termination depth can be set using the Z-position, allowing processing to stop when a specified depth of material is removed.
Termination
Process termination can be determined using the built-in timer, or by the Z Position. When using Z Position termination, the timer counts up to measure the total time elapsed until the Z Position is reached. This is useful for creating process recipes for various sample types. Four factors improve the overall stability during processing, and the termination accuracy:
- Non-Contact Position Sensor
- Tool Phase Sensor
- No external measuring device
- Magnetically coupled arm damping
The Non-Contact Position Sensor measures the location of the fulcrum arm which corresponds precisely to the thickness of the specimen. This sensor has an accuracy of better than one micron. Since it is non-contacting there are no detrimental effects on the action of the fulcrum arm. The reaction time of the sensor is fast enough to terminate the dimpling process the first time the Tool Phase Sensor indicates that the Z termination set point has been reached. Specimen thickness is measured directly on the PELCO Dimpler with the tool surface as the measuring point. The dimple depth and the Z termination set point are measured with the same reference. This prevents any measurement error from being translated directly to a termination error.
Damping
Damping of the arm is accomplished by using position information from the Non-Contact Position Sensor, processing it and feeding it back to the arm motor, which controls the up/downforce of the arm. The position sensor and the processing circuit is fast enough to actively dampen any vibration the arm may experience. Since the damping force is magnetically coupled to the arm there is no mechanical play in the system, allowing damping response on the sub-micrometre level. Damping is adjustable from zero damping to stiff damping.
Damping offers three significant attributes:
- Vibrations that can crack thin specimens are greatly reduced.
- Keeps tools round, negating excess impacts, providing more accurate Z position control.
- Tool gently contacts the specimen when the arm is lowered.
Automated Controls
Automated features make the dimpling process more repeatable and easy to use. Tools are raised and lowered at the touch of a button; dimpling force, damping force, fine balance, and Z offset (termination point) all are set from the front panel.
Once dimpling has begun no monitoring is necessary. After the initial specimen thickness has been measured, the specimen is mounted and the user simply selects a dimple depth and begins dimpling. The precise Z position of the tool/surface interface is continuously displayed while dimpling with any tool. When the selected dimple depth is reached the PELCO Dimpler controls will automatically raise the arm then stop the tool rotation, specimen rotation, and clock timer.
Working Interface
The working interface for the unpadded tools shows very high stability in terms of the ΔZ reading (change in the offset value), with any deviation usually the result of tilt in the mounted sample. Research has indicated that for padded tools, the working diameter is within the nap of the pad. In this case, the ΔZ between the working interface and the specimen is a function of dimpling load (force set point), tool speed, damping, and pad compliance. After the dimpling load, tool speed, and damping are selected, the working diameter will stabilise at the working interface of the pad. This working interface is monitored on the Z display and used as a reference location from which a specimen is to be dimpled further.
Specimen Mounting
Different mounts are available depending on the size and type of sample. TEM specimens are generally mounted on optically smooth sapphire flats. The low thermal mass of the flat reduces mounting and dismounting times and the smooth surface allows the dimpled specimens to be easily removed. The hardness of sapphire limits damage if specimens are accidentally perforated due to operator error. The retainer ring which secures the sapphire flat also serves as a slurry reservoir. This platen set is used with the traditional Dimpler tools. For larger specimens for surface analysis or delayering a separate, shorter magnetic base and platen set is used with larger diameter tools. These were developed for delayering and deprocessing.
A wide range of tools and accessories are available for use with the PELCO Dimpler, please see our accessories page for more information.
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