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AN ADVANCED TECHNIQUE FOR THE
PREPARATION AND TEM EXAMINATION OF FIM SPECIMENS
P. Fischione, J. Haugh, M. Burke
To cite this version:
P. Fischione, J. Haugh, M. Burke. AN ADVANCED TECHNIQUE FOR THE PREPARATION
AND TEM EXAMINATION OF FIM SPECIMENS. Journal de Physique Colloques, 1989, 50 (C8),
pp.C8-555-C8-560. �10.1051/jphyscol:1989896�. �jpa-00229995�
COLLOQUE DE PHYSIQUE
Colloque C8, Supplement au nO1l, Tome 50, novembre 1989
AN ADVANCED TECHNIQUE FOR THE PREPARATION AND TEM EXAMINATION OF FIM SPECIMENS
P.E. FISCHIONE, J.J. HAUGH* and M.G. BURKE'
?.A. Fischione Instrument Manufacturing, Pittsburgh, PA 15239, U.S.A.
Westinghouse Science and Technology Center, Pittsburgh, PA 15235, U.S.A.
ABSTRACT
A newly-developed F I M / F E M Polisher a n d T E M holder have been used t o successfully prepare and examine F I M specimens. These instruments incorporate many user-oriented features, the design aspects o f w h i c h are discussed. An example illustrating the preparation and examination o f an A533B steel specimen is presented.
INTRODUCTION
The preparation of needle-shaped specimens is an important aspect o f atom probe field-ion microscope (APFIM) and field emission studies. Electropolishing techniques.
chemical polishing procedures, and ion-milling have all been successfully employed in the production of suitable F I M Conventional electropolishing techniques are applicable t o a wide variety of metallic material, b u t generally require careful manual adjustments o f t h e specimen d u r i n g t h e polishing process and may also be tirne- consuming. In order t o reduce the time and effort involved in the specimen preparation process, a compact, self-contained polishing unit has been developed. T h i s system is based upon the conventional two-stage electropolishing technique i n which the specimen blank i s f i r s t locally thinned o r "necked", and subsequently e l e c t r o p o l i s h e d u n t i l separation occurs. The result o f this process is the production o f t w o F I M specimens.
In t h e f i r s t stage o f t h e conventional procedure, t h e specimen b l a n k i s manually reciprocated through a layer o f electrolyte (typically 75% CH3COOH
-
25% HC104) which is floated on C C I ~ . [ * * ~ ] This causes localized attack and the specimen develops a thinned region or neck. Once the neck is formed, the blank is transferred t o a second electrolyte and the polishing process i s continued. A mechanized polishing u n i t t h a t provides these functions while automatically maintaining alignment has been designed and developed. T o illustrate the use of the semi-automated unit. specimens o f an A533B steel have been prepared.INSTRUMENTATION
The equipment used in this evaluation consists o f a polishing unit, power supply.
and a T E M specimen holder for FIM needles.
Polishing Unit
T h i s i n s t r u m e n t has been designed f o r t h e c o m p l e t e p r e p a r a t i o n o f F I M specimens. It also incorporates an attachment for micro-polishing or re-polishing F I M specimens.
The basis of the polishing cell is a polyethylene sliding mechanism which contains the specimen blank material. The center portion o f the blank is reciprocated through a circular platinum cathode which is submerged in the electrolyte. The slide mechanism is easily and rapidly removed for the installation o f a specimen blank or for t h e periodic rinsing and observation of t h e specimen during t h e polishing process. T h e anodic connection is accomplished through a brass sliding contact on the back o f the slide mechanism. This eliminates the need t o connect wires directly t o the specimen, thus Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989896
preventing electrolyte from corroding the contact. The system is also configured so that there are no exposed electrical contacts.
The platinum cathode is retained in a machined PVC support component (Figure l a ) which both positions the cathode a t the proper level i n the electrolyte and prevents t h e specimen f r o m accidentally c o n t a c t i n g t h e cathode. Because o f i t s c i r c u l a r configuration, the cathode is centered in the glass beaker containing the electro!yte. The system has been designed and produced so t h a t the specimen is automatically centered in the circular cathode.
Cu contact rapid descent
beaker seat
center dwell
rapid ascent Figure l a . Schematic diagram of
P t cathode assembly.
Figure 1b. Schematic diagram of cam showing control segments.
A variable speed motor/cam assembly provides the pattern of motion. The extent o f motion is determined by t h e shape o f t h e cam as indicated i n Figure 1b. Cam dimensions can be modified t o produce a range of "neck" configurations. The complete system is shown in Figure 2.
A micro-polish attachment is employed t o repolish specimens, Figure 3. Surface tension retains a single drop o f electrolyte onto a 3 m m diameter platinum ring. The position o f the platinum ring can be easily adjusted w i t h respect t o the tip. Once this alignment has been made, the specimen/slide assembly w i l l remain centered within the platinum loop. For micropolishing. the voltage is manually pulsed as t h e t i p i s passed through the electrolyte.
T h e specimen area i s b a c k l i t w i t h a v a r i a b l e i n t e n s i t y l a m p t o f a c i l i t a t e observation during the polishing process. A l l materials used in the construction o f the polishing cell are electrolyte resistant.
Power Control
A dedicated power supply was designed t o provide complete electronic support for the Polisher. It controls the speed o f reciprocation, lamp intensity, and polishing voltage. Electropolishing can be performed in one of the following three modes: AC. DC, or DC micro-polish. The voltage and current ranges for the various modes are 0-10V a t 1 amp (max) for AC polishing, and 0-30V a t 2 amps (max) f o r D C polishing and micropolishing. A single potentiometer controls the voltage level i n each mode. For micro-polishing, a momentary push-button switch pulses the applied voltage. The pulse duration is determined by the t i m e the switch i s depressed by t h e operator. Digital meters continuously indicate voltage and current levels.
TEM Holder
A specimen holder was specially developed for the examination o f F I M specimens i n the transmission electron microscope (TEM). It is compatible with the Philips E M and CM series TEMs. This simplified holder has been designed with few parts for easy loading and position adjustment o f the specimen. The t i p o f the specimen holder is fabricated from phosphor bronze, whereas the body of the holder is brass.
The specimen (mounted in a standard copper stub) is fitted into a brass interim holding block (3.05 m m by 3.05 m m by 10.0 m m ) and i s held i n place by spring tension. T h e holding block i s shown i n Figure 4. T h i s block i s secured i n t h e specimen holder by a single screw clamp. The large open area surrounding the specimen facilitates loading, removing, and positioning the specimen. A marker line in the body of the specimen holder provides a reference for locating the t i p in the center o f the T E M field of view.
PREPARATION OF F I M SPECIMENS
A sample of A533B steel was prepared using the instrumentation described herein.
T h e specimen b l a n k h a d a 0.7 m m square c r o s s - s e c t i o n . T h e f i r s t s t a g e o f electropolishing was performed using a 25% perchloric acid
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75% acetic acid solution a t 12.5 VDC. T h e specimen blank was automatically reciprocated through t h e cathode submerged in the electrolyte u n t i l the necked diameter o f the blank was reduced t o approximately 0.2 mm. As the specimen thinned, the polishing current decreased from -450 mA t o approximately 50 mA. It is important t o note that CC14 was not required in order t o obtain the "neck"; a significant advantage o f t h e system design was the elimination o f the layer electrolyte configuration. Final polishing was conducted using an electrolyte of 2% perchloric acid in 2-butoxyethanol a t 13 VDC. The specimen remained stationary in the solution, b u t was manually moved on occasion t o induce preferred thinning. This process was continued until separation occurred.A series o f micrographs illustrating t h i s sequence o f thinning are presented in Figure 5. Subsequent examination i n a Philips EM4OOT analytical electron microscope revealed t h a t t h e radius o f t h e as-polished t i p was approximately 50 nm. Figure 6.
Periodically. during all o f the polishing processes, the specimen was removed f r o m the polishing cell, rinsed, and examined in an optical microscope. No recentering operations were necessary when the specimen was replaced in the polishing cell.
The micro-polishing attachment was tested using a bent t i p in the 2% perchloric acid-butoxyethanol s o l u t i o n . A f t e r m i c r o p o l i s h i n g , t h e specimen t i p radius was approximately 35 nm.
SUMMARY
T h e use o f t h e F I M / F E M M i c r o P o l i s h i n g s y s t e m g r e a t l y s i m p l i f i e d t h e preparation of F I M tips. The compact, self-contained system produced APFIM-ready specimens without the use of inert solvents. Because the system closely controls all of the polishing parameters. samples can be produced consistently. T h e T E M holder enables all tips t o be examined rapidly and without difficulty.
ACKNOWLEDGEMENTS
The authors would like t o thank Dr. M.K. Miller and Ms. K.F. Russell o f Oak Ridge National Laboratory for very helpful discussions. and E.A. Fischione and R.J. Spelic for instrument design.
REFERENCES
1. K.M. Bowkett and D.A. Smith. Field-Ion Microscopy (North-Holland Publishing Co., 1970).
2. S.S. Brenner and M.K. Miller. J. Metals. 35 (1983) p. 54.
3. M.K. Miller, lnt. Mat. Rev.. 32 (1987) p. 221.
4. M . K . M i l l e r a n d G.D.W. S m i t h . A t o m Probe M i c r o a n a l y s i s : P r i n c i p l e s a n d Applications, (MRS. 1989) in press.
5. K.B. Alexander, P. Angelini and M.K. Miller, this proceedings.
Figure
2.
Polishin]~d
power supply.Fig ;ure 3. Micro-polishing attachment for t i p resharpening
Figure 4. Holding block with spring clamp for retaining F I M specimen.
Figure 6. T E M micrograph of sample prepared using the polishing unit. The sample is mounted in the new T E M holder.
Figure
