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Preparation of damage-free glass TEM specimens
Ultramicroscopy 83 (2000) 61}66
Preparation of damage-free glass TEM specimens Bernard J. Kestel Materials Science Division, Argonne National Laboratory, 9700 S. Cass Ave., Argonne, IL 60439, USA Received 22 February 1999; received in revised form 21 October 1999
Abstract A jet polishing technique to chemically thin glass specimens for transmission electron microscopy (TEM) after a preliminary mechanical dimpling step has been developed. Slightly modi"ed commercial equipment is used with automatic optical termination of the polishing process to produce foils exhibiting large, high quality, electron transparent regions. 2000 Elsevier Science B.V. All rights reserved. PACS: 81.05.K; 61.16.B; 81.65.Ps Keywords: Glass NBS and ARM TEM specimens; Damage-free jet polishing; Automatic polishing termination
1. Introduction Glass specimens are tedious to thin for transmission electron microscopy (TEM) experiments. The use of conventional ion milling to thin such specimens has two disadvantages. First, the brittle material must be ground to a thickness of 100 lm before being mechanically dimpled which makes the specimen very fragile. Second, the ions which thin the specimen introduce damage and artifacts [1] which make detailed studies of the material almost impossible. Another method, crushing the glass, also results in damage to the thin regions most desired for experiments. Preliminary tests to develop this new technique were done on microscope slide material. Two other more complex glasses referred to as NBS and ARM were successfully thinned. The compositions of the three glasses used are shown in Table 1. The specimens needed for our experiments required a damage-free structure for in situ TEM
studies of the e!ect of various gas ion implantations into glasses. This requirement eliminated both ion milling and crushing of glass to produce thin foils. Specimens 250 lm thick were desired for easier handling and reduced breakage. When attempts were made to chemically jet thin thick specimens with a mixture of nitric and hydro#uoric acids without "rst removing some material by mechanical dimpling, a white residue gradually accumulated on the specimen, preventing the formation of a uniform dimple. The quality of the polished dimple surface was poor as well. This paper describes three key points needed to produce good glass TEM specimens using a chemical jet polishing technique. They are: (1) production of uniform dimples with a well-polished central area, (2) a reproducible method of stopping the chemical polish during "nal thinning immediately after perforation of the specimen to provide large, smooth thin areas and, (3) clean foil surfaces. Final thinning of several foils per day is possible.
0304-3991/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 9 9 1 ( 9 9 ) 0 0 1 7 3 - 4
62
B.J. Kestel / Ultramicroscopy 83 (2000) 61}66
Table 1 Composition of glasses jet thinned Type
Composition in wt%
Microscope slide NBS-1 ARM-1
Al 0 -1.1%, CaO-5.9%, K O-3.7%, MgO-3.7%, Na O-3.6%, SiO -80.1%, Trace Elements-1.9% B O -18.72%, Na O-16.67%, SiO -64.62% Al O -5.59%, B O -11.3%, BaO-0.66%, CaO-2.23%, CeO -1.51%, Cs O-1.16%, Li O-5.08%, MoO -1.57%, Na O-9.67%, Nd O -5.96%, P O -0.65%, SiO -46.5%, SrO-0.45%, TiO -3.21%, ZnO-1,46%, ZrO -1.80%.
2. Experimental details A few modi"cations to existing equipment were made to accommodate the thick specimens, protect the jet polisher from damage by strong acids, and provide a reversible method of making the glass surface opaque permitting the use of optical termination of the "nal thinning step. 2.1. Equipment modixcations 2.1.1. Mechanical dimple While discs 250 lm thick are robust and easy to handle, they cannot be dimpled with a standard 15.25 mm diameter dimpling wheel to a depth of 40 or 50 lm from both sides. Doing so would cause the dimple to exceed the specimen's 3 mm diameter, resulting in a chipped edge This problem was solved by machining and polishing a wheel down to a diameter of 11.7 mm with a curved surface pro"le. The dimpler had a depth measurement/shut-o! device with limited vertical travel that would not work until a new 5.7 mm thick specimen mounting platen was made and substituted for the standard 3.7 mm thick platen to return the wheel-to-specimen contact point to the same vertical position used with a full-sized wheel. 2.1.2. Jet polisher A jet thinning instrument was used for "nal polishing and a photo of it was published previously Model D 500 i, South Bay Technology, Inc., 1120 Via Callejon, San Clemente, CA 92692, USA. Model 550 C, South Bay Technology, Inc., 1120 Via Callejon, San Clemente, CA 92692, USA.
[2]. This unit's vertical, single jet, design with lineof-sight optical termination of the polishing process is very sensitive, reproducible, and easily modi"ed. Viewing the polishing process in situ through the magnifying lens provided in the splash guard near the specimen was important during polishing to monitor residue buildup. Several minor modi"cations to the polisher, previously used for electropolishing, were made for this project as follows. Quartz upper and lower light pipes in the jet head were replaced with sapphire ones which are resistant to etching by hydro#uoric acid and bromine. This ensured consistent termination of the polishing process when specimen perforation occurred. A PVC plastic specimen holder designed to hold 4 mm diameter mylar specimen supports was used. The polisher's stainless-steel specimen mount post and jet were also replaced with PVC parts, and an acid resistant polyethylene reservoir was used to hold the polishing solution. (These items are available from the manufacturer's chemical thinning kit.) The standard x-shaped PVC upper light pipe centering guide was removed from the nozzle and installed higher up inside the jet support tube with silicone rubber adhesive to aid smoother #uid #ow through the jet. The adhesive was cured overnight at room temperature. The standard infra-red light source in the shuto! circuit was replaced with a visible red LED whose lower intensity allowed proper control of auto shut-o! settings. 2.2. Initial specimen shaping The bulk glass used was 1.8 mm thick sheet stock. Discs 3 mm in diameter were cut with a
B.J. Kestel / Ultramicroscopy 83 (2000) 61}66
rotary abrasive slurry drill. A hollow brass tool was charged with a mixture of 30 lm diamond paste and 320 mesh boron carbide abrasives mixed in ethylene glycol. Any discs too large in diameter for the TEM holder were mounted for diameter reduction by grinding of their rims in the following manner. The wood handles of several artist brushes were ground #at at their outer end until they were about 3 mm in diameter. A dot of red Microshield lacquer was placed on an `oversizeda glass specimen. A #at-ended stick was quickly lowered onto the disc, then lifted to pick the disc up by capillary action. Light "nger pressure was applied to the specimen to squeeze out excess lacquer. After a few minutes, more lacquer was applied around the disc/stick interface for reinforcement. To protect the glass from chipping during edge grinding, a 250 lm thick, 3 mm diameter copper disc was attached to the outer surface of the glass in a manner similar to that used to mount the discs. The glass was sandwiched between the stick and the metal disc for protection and the lacquer was dried overnight at room temperature. Fine grinding of the disc rims was done on a water lubricated 200 rpm wheel covered with 30 lm alumina abrasive on adhesive backed Imperial lapping "lm using light pressure on a manually rotated mount stick. When each disc was small enough to "t into a test hole it was removed by suspending the mount stick from an overhead support with its specimen end in a dish of acetone. Discs with a "ne ground rim were more robust than those having a rough rim. Several discs can be ground thinner and then mechanically polished simultaneously on one surface in the following manner. The discs were mounted on a parallel grinding "xture with crystalbond 509 thermally melted mounting resin. Grinding
Model 350 C, South Bay Technology, Inc., 1120 Via Callejon, San Clemente, CA 92692, USA. Tolber Div., Pyramid Plastics, Inc., 220 W. 5th Street, Hope Akansas, 71801, USA. 3M Co., St. Paul, MN 55144-10000, USA. Model 150, South Bay Technology, Inc., 1120 Via Callejon, San Clemente, CA 92692, USA. Aremco Products, Inc., P.O. Box 429, Ossining, NY, 10562, USA.
63
was done on water lubricated 600 grit silicon carbide paper. When all the discs were uniformly ground, they were cleaned while still on the mounting block. Finer polishing was done sequentially on 30, 12, 9, and 3 lm water lubricated aluminum oxide abrasive "lm discs. After thermal demounting and cleaning in acetone, the specimens were inverted and remounted on the grinding "xture with the same resin used previously. Thick discs may be ground with 400 grit silicon carbide paper to a thickness of 500 lm and then with 600 grit to a thickness of 300 lm. Coarser grits should be avoided because specimens ground that way would not chemically polish well. This indicates that internal damage occurs in glass ground with coarse grit. Fine polishing was done on water lubricated 30, 12, 9, and 3 lm alumina discs to achieve a "nal disc thickness of between 200 and 250 lm. After demounting, cleaning, and drying on stainless-steel screening, which helps to aid disc cleanliness, the discs were ready for shallow mechanical dimpling.
3. Mechanical dimpling For dimpling, a glass disc was mounted in the center of a new `thicka platen, previously described, using Aremco 509 resin. The platen was mounted in an adapter equipped with x}y centering screws. The loaded adapter was then mounted on the dimpler's vertical specimen mount rotation shaft. The disc was dimpled for a few seconds using 1 lm diamond paste lubricated with Hyprez #uid. Coarser abrasive should not be used at this stage to avoid causing damage deep inside the specimen. A weight of 100}200 g and a 200 rpm wheel speed were used for about 30 s of dimpling. Inspection of the dimple location revealed centering adjustments required at this time. Dimpling to a depth of 40 lm left a #at rim around the dimple to retain specimen strength and to provide an annular surface for coating with acid resistant Microshield lacquer before the chemical jet polish-
Engis Corp., 8035 Austin Ave., Morton Grove, Il 60053, USA.
64
B.J. Kestel / Ultramicroscopy 83 (2000) 61}66
ing step described in Section 4. After the `"rst sidea dimple reached a depth of 40 lm, the specimen was removed, cleaned, and remounted in an inverted position. The second side was also dimpled to a depth of 40 lm. After removal from the platen and cleaning, the disc was ready for chemical jet polishing.
4. Initial chemical polish of the dimple surface Mechanical dimpling was necessary before the chemical jet polishing step because chemical polishing for long times produced a passivating residue which inhibited the polishing process. Adding bromine to the polishing solution reduced the amount of residue produced, but did not eliminate it. Therefore, mechanical dimpling was used to shorten the chemical polishing time and thus minimize the formation of a white, powdery residue, limiting its accumulation to the outer circumference of the dimple. When the chemical polishing time was less than 2 min, the dry residue did not cover the polished central portion of the dimple, and could be removed from the disc's rim with an artist's brush. In the initial chemical thinning step, a dimpled disc was mounted on a 4 mm diameter mylar disc, (available from South Bay Technology, Inc.), by lowering the disc onto a fresh dot of Microshield lacquer on the mylar support. Slight downward pressure on the surface of the glass disc forced any excess lacquer outward. When the lacquer was dry, more was painted around the rim of the specimen leaving only the upper dimple exposed. Slight dilution of some lacquer with acetone for this step results in a thinner layer next to the dimple which is less likely to trap any residue produced during chemical polishing. The mylar disc carrying the specimen was mounted on the PVC jet polisher holder shown in the cross sectional view of the specimen polishing area in Fig. 1. The specimen and its support were retained by a standard 38 lm thick polyethylene diaphragm having a 2.1 mm central hole designed for thinning 3.0 mm diameter discs. A standard jet polisher o-ring retainer, lubricated with alcohol, was manually installed over the diaphragm to tighten it, securing the specimen on the holder. This assembly was placed on the jet
Fig. 1. Schematic cross-section showing the jet, loaded specimen holder, and optical shut-o! detector arrangement.
polisher's PVC mount post for thinning. To reduce the volume of acids used, which required mixing fresh daily, a liquid-displacing block of high-density polyethylene was made to sit in the bottom of the polyethylene reservoir, surrounding the liquid pump. The `pumpa mode was used to polish the `"rst sidea dimple. The composition of the solutions and conditions used are given in Table 2. Each disc, while still mounted on mylar, was rinsed in several ethyl alcohol baths to remove all the hydro#uoric acid, thereby preventing the etching of the optical microscope lenses during specimen inspection. Each dimple's surface was checked optically at a magni"cation of 200;, and when adequately smooth, the specimen was removed from it's mylar support by soaking the assembly in acetone.
5. Application of an opaque 5lm on chemically polished dimples Final chemical jet thinning requires that an opaque layer be present on the lower surface of the transparent specimen to facilitate automatic optical termination of thinning upon specimen perforation. Several discs at a time, with their chemically polished dimple face up, were placed on a microscope slide and loaded into a di!usion pumped vacuum system equipped with a liquidnitrogen cold trap. The evaporation of 99.99% pure copper from a tungsten wire basket was
B.J. Kestel / Ultramicroscopy 83 (2000) 61}66
65
Table 2 Chemical solutions and conditions for thinning glass specimens Glass type
Solution content
Microscope slide
125 ml hydro#uoric acid 325 ml nitric acid 30 ml acetic acid 175 ml hydro#uoric acid 15 ml butyl cellosolve l00 ml methanol 1 ml bromine 325 ml nitric acid 30 ml acetic acid 60 ml hydro#uoric acid 160 ml nitric acid 200 ml methanol 15 ml butyl cellosolve 1 ml bromine
NBS-1
ARM-1
performed next. About 5 min were used to heat the basket and evaporate a 200 nm thick "lm onto the specimens. The slow evaporation rate reduced the number density and size of pinholes in the copper "lm produced. Coated discs were stored in a purged, inert gas atmosphere to prevent oxidation of the copper "lm until "nal chemical thinning was done. The "lm's thickness where a disc `shadoweda the slide from evaporated copper was measured with an optical interference microscope. At this stage of preparation, some specimens were implanted with gas ions through the copper "lm and then stored for chemical back-thinning later. Others were thinned from the back toward the copper "lm as control specimens or implanted with gas ions as thin foils.
6. Final jet polishing to perforation For this step, the discs were again mounted on mylar supports. Care was taken not to damage the fragile copper "lm when the coated surface of each disc was lowered onto a fresh dot of lacquer on mylar. Applying lacquer to the coated dimple with a brush would damage the copper "lm. With the specimen mounted on the polisher and the acid solution pumped gently upon the
Temp. (3C)
Removal rate (lm/min)
20
30
20
25
!10
6
`as-dimpleda surface of the specimen, the appropriate sensitivity of the unit was adjusted. The detector bias knob on the rear of the controller was set to shut the polisher's pump o! at a setting of about 70% on the front sensitivity control. The front sensitivity control was then adjusted to a lower setting of 30% or 40% to reduce the brightness of the visible red LED `shut-o!a light source. This setting allowed the instrument to polish continuously until the acid polishing mixture perforated the glass and quickly etched a hole in the copper "lm on it's rear surface. The increased light passing through this hole triggered the detector to shut o! the pump quickly enough to preserve a large, electron transparent area of glass. Final polishing of the NBS glass took about 5 min while the ARM glass was perforated in 12}15 min. After perforation, while still on the polisher, the specimens were gently rinsed with acetic acid, then ethyl alcohol to preserve their polished surface. Removal of the copper "lm was done aided by a stereo microscope. With the "lm side facing upward, each disc was placed in a petri dish of distilled water. Then 70% concentrated nitric acid was dispensed one drop at a time above the copper "lm. The high density of the acid caused it to sink through the water toward the specimen. After the
66
B.J. Kestel / Ultramicroscopy 83 (2000) 61}66
Fig. 2. Optical view at the edge of a hole chemically jet polished through an NBS-1 glass specimen. Bright"eld interference fringes indicate the presence of large electron transparent areas.
copper "lm dissolved, the specimen was transferred to a petri dish of acetone. This dissolved any remaining lacquer and also removed most of the residue from the chemical polish. Finally, the specimens underwent at least two ethyl alcohol rinses followed by drying in a stream of warm air. An optical photograph of a glass specimen thinned with this process is shown in Fig. 2. The wide interference fringes indicate the presence of large, electron transparent regions which were later con"rmed by TEM examination. A micrograph of features seen during TEM examination of a specimen implanted with gas ions is shown in Fig. 3.
7. Conclusions When large electron transparent regions free of artifacts caused by the thinning of glass specimens are required, this method is superior to either crushing pieces of material or ion milling it. Slightly di!erent conditions may be needed for glasses of di!erent compositions, but should be possible after some experimentation. This method produces specimen discs with thick rims making the normally fragile specimens more robust, reducing the chance of breaking a specimen after expensive gas im-
Fig. 3. A TEM image of an NBS-1 glass specimen after implantation with 50 KeV xenon ions to a dose of 6.72;10 15th ions/cm. The dark, round features are bubbles of Xenon gas.
plantation, etc. Any chemical polishing residue present was seldom in the thin central region of the disc and could be removed with a small artist's brush.
Acknowledgements The author wishes to thank Dr. L. Rehn and Dr. R. Birtcher for their support and encouragement during this project. Work supported by the US Department of Energy, Basic Energy Sciences-Materials Sciences, under contract C W-31-109-ENG38. References [1] D.B. Williams, B. Carter, Transmission Electron Micros. I (1996) 162. [2] B.J. Kestel, in: R. Anderson (Ed.), Improved Methods and Novel Techniques for Jet Electropolishing of TEM Foils, Materials Research Society Proceedings Vol. 199, p. 51, 1990.
Preparation of damage-free glass TEM specimens Bernard J. Kestel Materials Science Division, Argonne National Laboratory, 9700 S. Cass Ave., Argonne, IL 60439, USA Received 22 February 1999; received in revised form 21 October 1999
Abstract A jet polishing technique to chemically thin glass specimens for transmission electron microscopy (TEM) after a preliminary mechanical dimpling step has been developed. Slightly modi"ed commercial equipment is used with automatic optical termination of the polishing process to produce foils exhibiting large, high quality, electron transparent regions. 2000 Elsevier Science B.V. All rights reserved. PACS: 81.05.K; 61.16.B; 81.65.Ps Keywords: Glass NBS and ARM TEM specimens; Damage-free jet polishing; Automatic polishing termination
1. Introduction Glass specimens are tedious to thin for transmission electron microscopy (TEM) experiments. The use of conventional ion milling to thin such specimens has two disadvantages. First, the brittle material must be ground to a thickness of 100 lm before being mechanically dimpled which makes the specimen very fragile. Second, the ions which thin the specimen introduce damage and artifacts [1] which make detailed studies of the material almost impossible. Another method, crushing the glass, also results in damage to the thin regions most desired for experiments. Preliminary tests to develop this new technique were done on microscope slide material. Two other more complex glasses referred to as NBS and ARM were successfully thinned. The compositions of the three glasses used are shown in Table 1. The specimens needed for our experiments required a damage-free structure for in situ TEM
studies of the e!ect of various gas ion implantations into glasses. This requirement eliminated both ion milling and crushing of glass to produce thin foils. Specimens 250 lm thick were desired for easier handling and reduced breakage. When attempts were made to chemically jet thin thick specimens with a mixture of nitric and hydro#uoric acids without "rst removing some material by mechanical dimpling, a white residue gradually accumulated on the specimen, preventing the formation of a uniform dimple. The quality of the polished dimple surface was poor as well. This paper describes three key points needed to produce good glass TEM specimens using a chemical jet polishing technique. They are: (1) production of uniform dimples with a well-polished central area, (2) a reproducible method of stopping the chemical polish during "nal thinning immediately after perforation of the specimen to provide large, smooth thin areas and, (3) clean foil surfaces. Final thinning of several foils per day is possible.
0304-3991/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 9 9 1 ( 9 9 ) 0 0 1 7 3 - 4
62
B.J. Kestel / Ultramicroscopy 83 (2000) 61}66
Table 1 Composition of glasses jet thinned Type
Composition in wt%
Microscope slide NBS-1 ARM-1
Al 0 -1.1%, CaO-5.9%, K O-3.7%, MgO-3.7%, Na O-3.6%, SiO -80.1%, Trace Elements-1.9% B O -18.72%, Na O-16.67%, SiO -64.62% Al O -5.59%, B O -11.3%, BaO-0.66%, CaO-2.23%, CeO -1.51%, Cs O-1.16%, Li O-5.08%, MoO -1.57%, Na O-9.67%, Nd O -5.96%, P O -0.65%, SiO -46.5%, SrO-0.45%, TiO -3.21%, ZnO-1,46%, ZrO -1.80%.
2. Experimental details A few modi"cations to existing equipment were made to accommodate the thick specimens, protect the jet polisher from damage by strong acids, and provide a reversible method of making the glass surface opaque permitting the use of optical termination of the "nal thinning step. 2.1. Equipment modixcations 2.1.1. Mechanical dimple While discs 250 lm thick are robust and easy to handle, they cannot be dimpled with a standard 15.25 mm diameter dimpling wheel to a depth of 40 or 50 lm from both sides. Doing so would cause the dimple to exceed the specimen's 3 mm diameter, resulting in a chipped edge This problem was solved by machining and polishing a wheel down to a diameter of 11.7 mm with a curved surface pro"le. The dimpler had a depth measurement/shut-o! device with limited vertical travel that would not work until a new 5.7 mm thick specimen mounting platen was made and substituted for the standard 3.7 mm thick platen to return the wheel-to-specimen contact point to the same vertical position used with a full-sized wheel. 2.1.2. Jet polisher A jet thinning instrument was used for "nal polishing and a photo of it was published previously Model D 500 i, South Bay Technology, Inc., 1120 Via Callejon, San Clemente, CA 92692, USA. Model 550 C, South Bay Technology, Inc., 1120 Via Callejon, San Clemente, CA 92692, USA.
[2]. This unit's vertical, single jet, design with lineof-sight optical termination of the polishing process is very sensitive, reproducible, and easily modi"ed. Viewing the polishing process in situ through the magnifying lens provided in the splash guard near the specimen was important during polishing to monitor residue buildup. Several minor modi"cations to the polisher, previously used for electropolishing, were made for this project as follows. Quartz upper and lower light pipes in the jet head were replaced with sapphire ones which are resistant to etching by hydro#uoric acid and bromine. This ensured consistent termination of the polishing process when specimen perforation occurred. A PVC plastic specimen holder designed to hold 4 mm diameter mylar specimen supports was used. The polisher's stainless-steel specimen mount post and jet were also replaced with PVC parts, and an acid resistant polyethylene reservoir was used to hold the polishing solution. (These items are available from the manufacturer's chemical thinning kit.) The standard x-shaped PVC upper light pipe centering guide was removed from the nozzle and installed higher up inside the jet support tube with silicone rubber adhesive to aid smoother #uid #ow through the jet. The adhesive was cured overnight at room temperature. The standard infra-red light source in the shuto! circuit was replaced with a visible red LED whose lower intensity allowed proper control of auto shut-o! settings. 2.2. Initial specimen shaping The bulk glass used was 1.8 mm thick sheet stock. Discs 3 mm in diameter were cut with a
B.J. Kestel / Ultramicroscopy 83 (2000) 61}66
rotary abrasive slurry drill. A hollow brass tool was charged with a mixture of 30 lm diamond paste and 320 mesh boron carbide abrasives mixed in ethylene glycol. Any discs too large in diameter for the TEM holder were mounted for diameter reduction by grinding of their rims in the following manner. The wood handles of several artist brushes were ground #at at their outer end until they were about 3 mm in diameter. A dot of red Microshield lacquer was placed on an `oversizeda glass specimen. A #at-ended stick was quickly lowered onto the disc, then lifted to pick the disc up by capillary action. Light "nger pressure was applied to the specimen to squeeze out excess lacquer. After a few minutes, more lacquer was applied around the disc/stick interface for reinforcement. To protect the glass from chipping during edge grinding, a 250 lm thick, 3 mm diameter copper disc was attached to the outer surface of the glass in a manner similar to that used to mount the discs. The glass was sandwiched between the stick and the metal disc for protection and the lacquer was dried overnight at room temperature. Fine grinding of the disc rims was done on a water lubricated 200 rpm wheel covered with 30 lm alumina abrasive on adhesive backed Imperial lapping "lm using light pressure on a manually rotated mount stick. When each disc was small enough to "t into a test hole it was removed by suspending the mount stick from an overhead support with its specimen end in a dish of acetone. Discs with a "ne ground rim were more robust than those having a rough rim. Several discs can be ground thinner and then mechanically polished simultaneously on one surface in the following manner. The discs were mounted on a parallel grinding "xture with crystalbond 509 thermally melted mounting resin. Grinding
Model 350 C, South Bay Technology, Inc., 1120 Via Callejon, San Clemente, CA 92692, USA. Tolber Div., Pyramid Plastics, Inc., 220 W. 5th Street, Hope Akansas, 71801, USA. 3M Co., St. Paul, MN 55144-10000, USA. Model 150, South Bay Technology, Inc., 1120 Via Callejon, San Clemente, CA 92692, USA. Aremco Products, Inc., P.O. Box 429, Ossining, NY, 10562, USA.
63
was done on water lubricated 600 grit silicon carbide paper. When all the discs were uniformly ground, they were cleaned while still on the mounting block. Finer polishing was done sequentially on 30, 12, 9, and 3 lm water lubricated aluminum oxide abrasive "lm discs. After thermal demounting and cleaning in acetone, the specimens were inverted and remounted on the grinding "xture with the same resin used previously. Thick discs may be ground with 400 grit silicon carbide paper to a thickness of 500 lm and then with 600 grit to a thickness of 300 lm. Coarser grits should be avoided because specimens ground that way would not chemically polish well. This indicates that internal damage occurs in glass ground with coarse grit. Fine polishing was done on water lubricated 30, 12, 9, and 3 lm alumina discs to achieve a "nal disc thickness of between 200 and 250 lm. After demounting, cleaning, and drying on stainless-steel screening, which helps to aid disc cleanliness, the discs were ready for shallow mechanical dimpling.
3. Mechanical dimpling For dimpling, a glass disc was mounted in the center of a new `thicka platen, previously described, using Aremco 509 resin. The platen was mounted in an adapter equipped with x}y centering screws. The loaded adapter was then mounted on the dimpler's vertical specimen mount rotation shaft. The disc was dimpled for a few seconds using 1 lm diamond paste lubricated with Hyprez #uid. Coarser abrasive should not be used at this stage to avoid causing damage deep inside the specimen. A weight of 100}200 g and a 200 rpm wheel speed were used for about 30 s of dimpling. Inspection of the dimple location revealed centering adjustments required at this time. Dimpling to a depth of 40 lm left a #at rim around the dimple to retain specimen strength and to provide an annular surface for coating with acid resistant Microshield lacquer before the chemical jet polish-
Engis Corp., 8035 Austin Ave., Morton Grove, Il 60053, USA.
64
B.J. Kestel / Ultramicroscopy 83 (2000) 61}66
ing step described in Section 4. After the `"rst sidea dimple reached a depth of 40 lm, the specimen was removed, cleaned, and remounted in an inverted position. The second side was also dimpled to a depth of 40 lm. After removal from the platen and cleaning, the disc was ready for chemical jet polishing.
4. Initial chemical polish of the dimple surface Mechanical dimpling was necessary before the chemical jet polishing step because chemical polishing for long times produced a passivating residue which inhibited the polishing process. Adding bromine to the polishing solution reduced the amount of residue produced, but did not eliminate it. Therefore, mechanical dimpling was used to shorten the chemical polishing time and thus minimize the formation of a white, powdery residue, limiting its accumulation to the outer circumference of the dimple. When the chemical polishing time was less than 2 min, the dry residue did not cover the polished central portion of the dimple, and could be removed from the disc's rim with an artist's brush. In the initial chemical thinning step, a dimpled disc was mounted on a 4 mm diameter mylar disc, (available from South Bay Technology, Inc.), by lowering the disc onto a fresh dot of Microshield lacquer on the mylar support. Slight downward pressure on the surface of the glass disc forced any excess lacquer outward. When the lacquer was dry, more was painted around the rim of the specimen leaving only the upper dimple exposed. Slight dilution of some lacquer with acetone for this step results in a thinner layer next to the dimple which is less likely to trap any residue produced during chemical polishing. The mylar disc carrying the specimen was mounted on the PVC jet polisher holder shown in the cross sectional view of the specimen polishing area in Fig. 1. The specimen and its support were retained by a standard 38 lm thick polyethylene diaphragm having a 2.1 mm central hole designed for thinning 3.0 mm diameter discs. A standard jet polisher o-ring retainer, lubricated with alcohol, was manually installed over the diaphragm to tighten it, securing the specimen on the holder. This assembly was placed on the jet
Fig. 1. Schematic cross-section showing the jet, loaded specimen holder, and optical shut-o! detector arrangement.
polisher's PVC mount post for thinning. To reduce the volume of acids used, which required mixing fresh daily, a liquid-displacing block of high-density polyethylene was made to sit in the bottom of the polyethylene reservoir, surrounding the liquid pump. The `pumpa mode was used to polish the `"rst sidea dimple. The composition of the solutions and conditions used are given in Table 2. Each disc, while still mounted on mylar, was rinsed in several ethyl alcohol baths to remove all the hydro#uoric acid, thereby preventing the etching of the optical microscope lenses during specimen inspection. Each dimple's surface was checked optically at a magni"cation of 200;, and when adequately smooth, the specimen was removed from it's mylar support by soaking the assembly in acetone.
5. Application of an opaque 5lm on chemically polished dimples Final chemical jet thinning requires that an opaque layer be present on the lower surface of the transparent specimen to facilitate automatic optical termination of thinning upon specimen perforation. Several discs at a time, with their chemically polished dimple face up, were placed on a microscope slide and loaded into a di!usion pumped vacuum system equipped with a liquidnitrogen cold trap. The evaporation of 99.99% pure copper from a tungsten wire basket was
B.J. Kestel / Ultramicroscopy 83 (2000) 61}66
65
Table 2 Chemical solutions and conditions for thinning glass specimens Glass type
Solution content
Microscope slide
125 ml hydro#uoric acid 325 ml nitric acid 30 ml acetic acid 175 ml hydro#uoric acid 15 ml butyl cellosolve l00 ml methanol 1 ml bromine 325 ml nitric acid 30 ml acetic acid 60 ml hydro#uoric acid 160 ml nitric acid 200 ml methanol 15 ml butyl cellosolve 1 ml bromine
NBS-1
ARM-1
performed next. About 5 min were used to heat the basket and evaporate a 200 nm thick "lm onto the specimens. The slow evaporation rate reduced the number density and size of pinholes in the copper "lm produced. Coated discs were stored in a purged, inert gas atmosphere to prevent oxidation of the copper "lm until "nal chemical thinning was done. The "lm's thickness where a disc `shadoweda the slide from evaporated copper was measured with an optical interference microscope. At this stage of preparation, some specimens were implanted with gas ions through the copper "lm and then stored for chemical back-thinning later. Others were thinned from the back toward the copper "lm as control specimens or implanted with gas ions as thin foils.
6. Final jet polishing to perforation For this step, the discs were again mounted on mylar supports. Care was taken not to damage the fragile copper "lm when the coated surface of each disc was lowered onto a fresh dot of lacquer on mylar. Applying lacquer to the coated dimple with a brush would damage the copper "lm. With the specimen mounted on the polisher and the acid solution pumped gently upon the
Temp. (3C)
Removal rate (lm/min)
20
30
20
25
!10
6
`as-dimpleda surface of the specimen, the appropriate sensitivity of the unit was adjusted. The detector bias knob on the rear of the controller was set to shut the polisher's pump o! at a setting of about 70% on the front sensitivity control. The front sensitivity control was then adjusted to a lower setting of 30% or 40% to reduce the brightness of the visible red LED `shut-o!a light source. This setting allowed the instrument to polish continuously until the acid polishing mixture perforated the glass and quickly etched a hole in the copper "lm on it's rear surface. The increased light passing through this hole triggered the detector to shut o! the pump quickly enough to preserve a large, electron transparent area of glass. Final polishing of the NBS glass took about 5 min while the ARM glass was perforated in 12}15 min. After perforation, while still on the polisher, the specimens were gently rinsed with acetic acid, then ethyl alcohol to preserve their polished surface. Removal of the copper "lm was done aided by a stereo microscope. With the "lm side facing upward, each disc was placed in a petri dish of distilled water. Then 70% concentrated nitric acid was dispensed one drop at a time above the copper "lm. The high density of the acid caused it to sink through the water toward the specimen. After the
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Fig. 2. Optical view at the edge of a hole chemically jet polished through an NBS-1 glass specimen. Bright"eld interference fringes indicate the presence of large electron transparent areas.
copper "lm dissolved, the specimen was transferred to a petri dish of acetone. This dissolved any remaining lacquer and also removed most of the residue from the chemical polish. Finally, the specimens underwent at least two ethyl alcohol rinses followed by drying in a stream of warm air. An optical photograph of a glass specimen thinned with this process is shown in Fig. 2. The wide interference fringes indicate the presence of large, electron transparent regions which were later con"rmed by TEM examination. A micrograph of features seen during TEM examination of a specimen implanted with gas ions is shown in Fig. 3.
7. Conclusions When large electron transparent regions free of artifacts caused by the thinning of glass specimens are required, this method is superior to either crushing pieces of material or ion milling it. Slightly di!erent conditions may be needed for glasses of di!erent compositions, but should be possible after some experimentation. This method produces specimen discs with thick rims making the normally fragile specimens more robust, reducing the chance of breaking a specimen after expensive gas im-
Fig. 3. A TEM image of an NBS-1 glass specimen after implantation with 50 KeV xenon ions to a dose of 6.72;10 15th ions/cm. The dark, round features are bubbles of Xenon gas.
plantation, etc. Any chemical polishing residue present was seldom in the thin central region of the disc and could be removed with a small artist's brush.
Acknowledgements The author wishes to thank Dr. L. Rehn and Dr. R. Birtcher for their support and encouragement during this project. Work supported by the US Department of Energy, Basic Energy Sciences-Materials Sciences, under contract C W-31-109-ENG38. References [1] D.B. Williams, B. Carter, Transmission Electron Micros. I (1996) 162. [2] B.J. Kestel, in: R. Anderson (Ed.), Improved Methods and Novel Techniques for Jet Electropolishing of TEM Foils, Materials Research Society Proceedings Vol. 199, p. 51, 1990.