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Fabrication of fine apertures in metal foils by photoelectrochemical milling
Fabrication of fine ape ures in metal.foils by photoelectrochem=cal milling B. Chatterjee*
The potential of a photofabrication process involving photolithography and electrochemical milling has been established for the production of accurate holes in a range of sheet materials (10-500#m thick), including molybdenum, platinum, Pt- lORh, sterling silver, carat gold and silver- and palladium-based alloys. Based on scanning electron microscopy, the new technique shows its unique capability of producing high quality components in materials which were hitherto considered to be difficult or impossible to fabricate. Furthermore, the technique does not involve the use of any highly toxic or aggressive chemicals; a non-passivating neutral solution of sodium chloride is used as the electrolyte. Details of the type, concentration and application of the electrolyte are discussed. The technique appears to be potentially attractive to the manufacturers of fine apertures and similar intricate shapes of industrial components and jewellery items. Keywords: electrochemical milling, photolithography, metal foils
With the advancement of technology, there has been a growing demand for precision apertures of fine diameter (100/~m or less) for use in electronoptical instruments, laser beam systems, fuel metering devices, as cooling holes in gas turbines, in spinnerettes and so forth. These applications frequently involve the use of difficult-to-machine exotic metals and alloys such as molybdenum, platinum group metals and gold. Figs 1 and 2 show typical pictures from a scanning electron microscope (SEM) of a molybdenum disc aperture used in an electron microscope. The characteristic sharp parallel edges and burr-free smooth, uniform profile of the disc are evident in the micrographs, which are essential for critical definition of the beam size and resolution of the instrument. In the present paper, apertures produced by several modern fabrication methods are examined. This is followed by details of an electrolytic technique which would apply to any metal, including the 'difficult' ones. Since the technique combines photolithography with an electrolytic operation, the process has been named 'photoelectrochemical milling' (PEM). Assessment of fabrication
trials
The choice of a manufacturing route depends on its application, the size and quality of aperture/shape required, and the type and thickness of material to be processed. It is thus considered essential to initially assess by SEM the quality of holes produced * Lucas CAV Limited, Materials Laboratory, PO Box 36, Warple Way, London W3 7SS, UK
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by several methods such as photochemical milling (PCM), plasma arc machining, laser beam drilling and electrodischarge machining (EDM). Only one metal, molybdenum foil (10-20 #m thick), was used for comparison purposes in all cases. In PCa, the metal foil was selectively removed by a chemical etching method. Areas to remain unetched were masked with a photoresist coating, and a suitable etchant was sprayed on the component to etch/dissolve away the bare surface. Figs 3 and 4 show milled apertures with good circular geometry but coarse profile structure. The other disadvantage of PCM is that the common etchants such as ferric chloride, cupric chloride, sodium hydroxide and potassium ferricyanide are not effective in processing 'difficult' metals. For example, in the cases of platinum and gold, a likely etchant would be a hot mixture of nitric and hydrochloric acids which are toxic, corrosive and would present environmental problems, both on the industrial shop floor and in disposal. In the plasma method (Figs 5 and 6) molydbenum foil was etched in a glow discharge where the ions reacted chemically with the material to be swept away by gas flow. The aperture profile (Fig 6) is reasonably steep and the edges are sharp; however, the profile structure does not appear to compare favourably with Figs 1 and 2. Also, the technique prevents maintenance of any satisfactory adhesion of masks during treatment and as a result, the final apertures are of unacceptable quality. Furthermore, the overall cost is high because it requires a vacuum chamber, a pumping system, and special equipment to handle the gas both before and during the etching operation.
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Chatterjee--fabrication of fine apertures in metal foils poor quality apertures and offer little control over the geometry. A low-power laser, on the other hand, offers a circular hole having too coarse a profile structure to be suitable for any precision device. In EDM, electric sparks between an electrode at high voltage and molybdenum foil eroded the metal by melting until the electrode had completely penetrated the substrate. The hole geometry is good and its edge appears sharp (Figs 11 and 12). The profile, however, is coarse, with some evidence of resolidification (Fig 12) and the quality of the aperture appears poorer than that produced by a low-power laser beam. An aperture produced by EDM was mechanically polished, whereby the quality improved somewhat, but still remained quite coarse (Fig 13 and 14).
Fig 1 SEMpicture (45 ° tilt) of aperture in molybdenum foil obtained as commercial aperture disc (170 × magnification)
Fig 3 SEMpicture (450 tilt) of aperture in molybdenum foil produced by photochemical milling (170 × magnification)
Fig 2 Profile structure of aperture in Fig 1 (2200 × magnification)
In the laser method, a laser beam was focused on to the metal until a hole had been burned through. The profile structure (Figs 7 and 8) on using a low-power laser of several watts, is coarse and shows considerable re-solidification of the metal during or after drilling. With a mediumpowered (400 W) laser, the aperture and its structure are of poor quality (Figs 9 and 10). There is a considerable amount of resolidified material on the periphery of the hole, and the hole itself is not circular. Perhaps, better quality could have resulted if all the melted metal had vaporized. It is clear from Figs 7-10 that a high-powered laser would produce
132
Fig 4 Profile structure of aperture in Fig 3 (2200× magnification)
JULY 1986 V O L 8 NO 3
Chatterjee--fabrication of fine apertures in metal foils e t ~ ~ s ) . By choosing an appropriate electrolyte, metal would be removed atom-byatom selectively at the surface roughness peaks. This should result in smoothing of the surface, with the ultimate elimination of microroughness, and followed eventually by the removal of macroroughness, ie surface unevenness. The theoretical considerations for PEMwould in many ways be similar to those for electrochemical machining (ECM) 1-3 since both processes are characterized by a high rate of anodic dissolution of the workpiece. In order however, to achieve smoothness as well as geometrical accuracy of the aperture, it is considered essential to apply photolithography to the electro-
Fig 5 SEMpicture (45' tilt) of aperture in molybdenum foil produced by plasma method (90 × magnification)
Fig 7 SEMpicture (45, tilt) of aperture in molybdenum foil produced by low power laser technique (90× magnification)
Fig 6 Profile structure of aperture in Fig 5 (1100× magnification)
Background t o PEM development It is apparent from the trial runs (Figs 3-14) that varying quality of fine apertures is produced on the metal foils by the different fabrication techniques, none comparable with the smooth profile structure of Fig 2. Thus, the objective of the present work was to develop a process which would provide precision apertures in, preferably, any metal with a smooth profile structure such as in Figs 1 and 2. An electrochemical or, more precisely, an electropolishing approach is considered, because it takes into account both a macroscopic effect (a levelling process which wears down the peaks) and a microscopic effect (a brightening process which
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Fig 8 Profile structure of aperture in Fig 7 (2200x magnification)
133
Chatterjee--fabrication of fine apertures in metal foffs lytic milling process. Details of photolithography are well known in the PCM industry 4 and are therefore not discussed here.
Experimental Type of electrolyte In selecting an electrolyte for PEM, it was considered essential to develop a solution which must be stable, safe to use, inexpensive, and pollution-free. Also, it should not attack the photoresist coating, to prevent undercutting. It is assumed, on the basis of combined theories of viscous layer and solid film
Fig 11 SEMpicture (450 tilt) of aperture in molybdenum foil produced by electrodischarge method (1 lOx magnification)
Fig 9 SEMpicture (45 ° tilt) of aperture in molybdenum foil produced by medium power laser technique (90 x magnification)
Fig 12 Profile structure of aperture in Fig 11 (1100 x magnification)
Fig 10 Profile structure of aperture in Fig 9 (1100 × magnification)
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formation for electropolishing, that satisfactory PEM w o u l d occur in the transpassive region of the polarization curve of an electrolyte. Based on the steadystate polarization studies of electrolytes commonly used in ECM, ie NaCI, NaNO 3, NaCIO3 and NaCr207, it is observed that the current density in the transpassive region for NaCI is higher than for any of the other electrolytes 5, and its use could thus result in sharp cutting of the workpiece even at low potential. Also, it has been reported 6 that a multiplicity of bright-bottomed, randomly scattered pits is known to produce a polished finish. The aggressive anions associated with pitting are usually the halides, of which the chloride ion has received the most attention. It is inexpensive, stable, and nonpassivating in the form of NaCI which has thus been used as the general purpose salt in the present work.
JULY 1986 VOL 8 NO 3
Chatterjee--fabrication of fine apertures in metal foils
Fig 13 SEMpicture (45' tilt) of aperture in molybdenum foil produced by electrodischarge followed by mechanical polishing (170 x magnification)
Alternative means of exerting turbulence were also studied, ie jetting and ultrasonic vibration of the electrolyte, but no extra advantage was gained. In jet milling, the electrolyte was forced, by compressed air, through a platinum cathode tube against the anode workpiece a few millimetres away in an enclosed splash container. Metal removal occurred directly under the cathode jet because of the large ohmic resistance of other current paths. The technique proved quite effective in eliminating uneven etching due to gas bubbles and accumulation of reaction products at the anode surface. Also, the use of a platinum cathode prevented deposit formation due to chemical reaction with the electrolyte. However, only one item per operation can be processed using a single orifice cathode and would, thus, be too slow to attract any commercial interest. A multi-orifice cathode gun, properly aligned with the workpiece, would be useful in this respect but the cost of such a platinum gun and the tooling cost for alignment proved too costly for the laboratory tests. The use of ultrasonic vibration of the electrolyte did not prove satisfactory in removing reaction products and, furthermore, the technique appeared commercially unattractive in terms of the scaling up costs.
Concentration of electrolyte A high concentration of active anions is essential for consideration of transpassive-polishing criteria (Hoar diagram). An increase in the number of active ions in a strong electrolyte would provide a high dissolution current and, hence, high dissolution kinetics. A concentrated electrolyte would also prevent any large undesirable drop in inter-electrode resistance. It is important, however, to remember that the electrical conductivity of most electrolytes, including NaCI, increases with concentration but only to a level controlled mostly by the solubility limit. With an increase in electrolyte strength, the film thickness may increase so a compromise is usually required in the film-former/film-attacker ratio and in the overall concentration. Total concentrations of between 4 M and 5 M are presently employed (Table 1).
Details of PEM assembly A schematic diagram of the test rig layout is shown in Fig 15. The assembly consisted of a power source, an electrolyte circulation system with continuous filtration facility, and a gas extraction system for removing hydrogen evolved from the cathodes. The electrode arrangements were such that each anode reciprocated between two cathodes. A narrow gap (up to 5 mm) was maintained between the cathode sheets of platinized titanium and the workpiece. The direct current for the apparatus was
Application of electrolyte A high rate of anodic dissolution is an essential requirement for PEM,and can be achieved by minimizing concentration polarization due to gas bubbles at the electrodes and ensuring removal of reaction products from the electrode surface. Both these criteria can be met by strong agitation of the electrolyte (Reynolds number >/2000). A high turbulence was achieved by applying pressurized recirculation (about 4 I/min) of continuously filtered electrolyte and using anode movement. The turbulence also prevented boiling of the electrolyte from Joule heating; this is important because, even though the interelectrode gap was small, a finite resistance leading to electrical heating developed due to the high current densities used.
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Fig 14 Profile structure of aperture in Fig 13 (1700 x magnification)
135
Chatterjee--fabrication of fine apertures in metal foils Table 1 Processing parameters for photoelectrochemical milling of various materials 10-500/~m thick at ambient temperature Current density, A/cm 2
Typical milling rate, l~m/min
Material
Electrolyte
Type of power supply
Mo Pt, Pt-10Rh Pd-base alloy (Pd-30Ag-1 5Cu-10Pt-10Au)* Both pure and carat golds Sterling silver (Ag-7Cu) Ag-20Cu-1 5Pdt Stainless steel (EN 58J)
2 M NaCI + 2 M NaNO3 5 M NaCI
DC AC
100--125 20--30
40 10
5 M NaCI 5 M NaCI
AC DC
20--30 20--25
10 40
3 M NaNO3 5 M NaCI
DC DC
20--25 25--35
60 10
5 M NaCI
PC
30
44
* Typical electrical contact alloy; ttypical brazing alloy
volve, C 0 th0de~'~"""'~'--~
~
/DC power / supply r-~--]---1 ~ I~ 1 / A.ode II .+ 111 / workpiece i~---f--~l ~ /Photoresist
I-L.L---4--~-,41 ~ I II I
Electrolyte
I
I
~F'~ " / ]
I I t l I,_. ~ - ' ~ . . I,
I I
'-/~
coot,og
r./"/EC I~?t r ° chetrli c ° I
I
J
LV
Fig 15 Schematic diagram of laboratory set-up for PEM
supplied by a Westinghouse transformer-rectifier system, where the voltage could be varied between 0 and 25 V with a maximum current of 40 A. The filter between the pump and PEM cell consisted of closely woven polypropylene material, with a proportionately large surface area. The flow rate of the electrolyte was estimated to be about 4 I/min.
Experimental procedure The initial requirement was to produce an outline of the specific design (ie an aperture) on a metal foil 10-500/~m thick in the form of a photoresist coating which would be resistant to the subsequent processing operation. The unprotected metal of the photoresist stencil was then electrochemically dissolved away at ambient temperature. The applied voltage, typically 10-15 V, was adjusted to give the desired current density; it was preferable that the operating current be attained as quickly as possible, ie within seconds of operation. For fabrication of platinum group metals, a voltage of 20 V AC at 50 Hz frequency was applied using a platinum counterelectrode and the current strength was regulated by an autotransformer.
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Materials processed During electrochemical milling of molybdenum, large quantities of anode products were produced (oxide/hydroxide of Mo 3+ ions) which inhibited the process. The addition of ammonia as a complexing agent overcame the problem, but the photoresist (Kalle type T225 or Robertson's CM 2000) became unstable in the high pH solution. The problem was resolved by replacing ammonia with an oxidizing agent like sodium nitrate which oxidized molybdenum into soluble Mo 6~ ions. The milling of platinum and its alloys, unlike other materials, did not proceed smoothly by DC because of polarization leading to passivation. The depolarizing effect of AC in chloride solutions has been reported elsewhere for platinum 7, thus AC, either alone or superimposed on PC, was used to process platinum group metals. Electromilling of silver in sodium chloride solution was inhibited by the formation on the silver surface of highly insoluble silver chloride. An electrolyte of sodium nitrate was used, producing soluble silver nitrate as the reaction product. A number of other precious metal alloys were successfully milled using NaCI electrolyte. Samples were also produced in stainless steel; although stainless steel can be routinely processed by PCM,the dissolution rate was faster by an order of magnitude using the present PEM technique.
Results and discussion The scanning electron photomicrographs (at 45 ° tilt) of the apertures in molybdenum and platinum foils shown in Figs 16-19 reveal typical structures produced by PEa. It has been possible to process both precious and refractory metals by PEM using appropriate conditions (Table 1). The extremely smooth profile structures with parallel edges of Figs 16-19 appear to be the characteristic structures obtainable by PEM,and compare favourably with Figs 1 and 2.
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Chatterjee--fabrication of fine apertures in metal foils The milling process can be accelerated by increasing the rate of metal removal by increasing the current density and maintaining the closest possible anode-cathode gap ( < 5 mm), thereby minimizing the ohmic drop. Any increase in current density is, however, limited by the cathodic reaction controlled by the hydrodynamic conditions of the electrolyte and the interelectrode gap. Too high a current density could result in the production of electric sparks on the cathode.
Tolerances In considering the potential of PEM,the limitations inherent in both photolithography and electro-
Fig 16 SEMpicture (45" tilt) of aperture in molybdenum foil produced by present photoelectrochemical technique (550 x magnification)
Fig 18 SEMpicture (45' tilt) of aperture in platinum foil produced by present photoelectrochemical technique (670 x magnification)
Fig 17 Profile structure of aperture in Fig 16 (2200 × magnification) Distribution of current A nonuniform distribution of current is expected on the anode surface due to the relatively smaller anode area and as a result, metal is likely to be removed more from the edges than from the centre of the anode stencil. However, no such nonuniform profiling was produced in the present work, probably due to the sudden surge of high current at the start of the operation. It was also considered important that the anode stencil be located centrally, facing the larger cathode surfaces, so that the current could flow equally and freely from the top and bottom of the stencil to the facing cathodes.
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Fig 19 Profile structure of aperture in Fig 18 (1300 x magnification)
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Chatterjee--fabrication of fine apertures in metal foils chemical milling must be recognized. It must be appreciated that, during milling, there would be some lateral dissolution of the metal resulting in a final hole larger than the size on the stencil, the situation becoming more apparent with thicker foil. However, the dimensional tolerances in the present work have generally been within + 10% of the metal thickness. In cases where the tolerances are critical, the original photomasks must be reduced in size to allow lateral dissolution. On keeping the working parameters constant, a high degree of precision fabrication can be reproduced. Furthermore, a statistical analysis of the hole diameters produced by PEM shows no evidence of ovality.
Capabilities of PEM The technique is governed by Faraday's laws of electrolysis and depends on the electrochemical equivalent of the metal, not on its hardness. Thus, with most modern alloys becoming harder than before, PEM will find its application in the cutting of such hard exotic alloys more competitive against conventional techniques. With suitable design modifications, it is now possible to fabricate fine apertures or any intricate shapes in metals, including precious and refractory metals for use in the industrial and jewellery fields.
Conclusions A comparative assessment of various fabrication methods of producing apertures clearly reveals the superior quality of the finished product resulting from the present PEM technique. An important feature of PEM is the use of nonpollutant, nonaggressive electrolytes, thus eliminating limitations on the type of photoresists used. The development of the present method ensures removal of any metal, including 'difficult' ones with no burrs, and processinduced stresses on the product, and also no tool wear. Furthermore, the operation is unaffected by
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the mechanical or thermochemical treatment of the metal. It is, thus, now possible to fabricate apertures of any design in any metal foil. For simpler alloys like stainless steel, the metal removal rate by PEM is estimated to be about an order of magnitude faster than by PCMoThe versatility and simplistic approach of PEM are likely to attract commercial interest in manufacturing jewellery items, as well as industrial components such as aperture discs, electrical contacts and brazing preforms made from simple to exotic metals and alloys.
Acknowledgements The author wishes to thank Mr R.B. Worsnop for his photographic assistance and the following organizations for providing materials and fabrication facilities used in the present work: Culham Laboratory (UKAEA, Oxfordshire), Engelhard Ind. (Surrey), Philips Research Laboratory (Surrey), Tecan Components (Dorset), Unicastex (Berkshire) and Veco (Surrey).
References 1 Wilson J.F. Practice and theory of electrochemical machining. Wiley Interscience, New York, 1971 2 McGeough J.A. Principles of electrochemical machining. Chapman and Hall, London, 1974 3 DeBarr A.E. and Oliver D.A. Electrochemical machining. Macdonald and Co. Ltd, London, 1975 4 Allen D.M. et al. Production of spring steel camera shutter blades by photoetching. Precis. Eng., 1979, 1 (1), 25 5 Hoare J.P. and LaBoda M.A. Electrochemical machining, in Comprehensive treatise of electrochemistry (ed. Bockris, J.O'M.) vol 2. Plenum Press, New York, 1981, chapter 8, 399520 6 Evans J . i . and Boden P.J. Surface finish produced during electrochemical machining on nickel and nimonic 80A in chloride electrolytes, in Principles of electrochemical machining (ed. Faust C.L.). Electrochemical Society Inc., Princeton, New Jersey, 1971, 40-62 7 Llopis J. and Sancho A. Electrochemical corrosion of platinum in hydrochloric acid solutions. J. Electrochem. Soc., 1961, 108 (8), 720
JULY 1986 VOL 8 NO 3
The potential of a photofabrication process involving photolithography and electrochemical milling has been established for the production of accurate holes in a range of sheet materials (10-500#m thick), including molybdenum, platinum, Pt- lORh, sterling silver, carat gold and silver- and palladium-based alloys. Based on scanning electron microscopy, the new technique shows its unique capability of producing high quality components in materials which were hitherto considered to be difficult or impossible to fabricate. Furthermore, the technique does not involve the use of any highly toxic or aggressive chemicals; a non-passivating neutral solution of sodium chloride is used as the electrolyte. Details of the type, concentration and application of the electrolyte are discussed. The technique appears to be potentially attractive to the manufacturers of fine apertures and similar intricate shapes of industrial components and jewellery items. Keywords: electrochemical milling, photolithography, metal foils
With the advancement of technology, there has been a growing demand for precision apertures of fine diameter (100/~m or less) for use in electronoptical instruments, laser beam systems, fuel metering devices, as cooling holes in gas turbines, in spinnerettes and so forth. These applications frequently involve the use of difficult-to-machine exotic metals and alloys such as molybdenum, platinum group metals and gold. Figs 1 and 2 show typical pictures from a scanning electron microscope (SEM) of a molybdenum disc aperture used in an electron microscope. The characteristic sharp parallel edges and burr-free smooth, uniform profile of the disc are evident in the micrographs, which are essential for critical definition of the beam size and resolution of the instrument. In the present paper, apertures produced by several modern fabrication methods are examined. This is followed by details of an electrolytic technique which would apply to any metal, including the 'difficult' ones. Since the technique combines photolithography with an electrolytic operation, the process has been named 'photoelectrochemical milling' (PEM). Assessment of fabrication
trials
The choice of a manufacturing route depends on its application, the size and quality of aperture/shape required, and the type and thickness of material to be processed. It is thus considered essential to initially assess by SEM the quality of holes produced * Lucas CAV Limited, Materials Laboratory, PO Box 36, Warple Way, London W3 7SS, UK
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by several methods such as photochemical milling (PCM), plasma arc machining, laser beam drilling and electrodischarge machining (EDM). Only one metal, molybdenum foil (10-20 #m thick), was used for comparison purposes in all cases. In PCa, the metal foil was selectively removed by a chemical etching method. Areas to remain unetched were masked with a photoresist coating, and a suitable etchant was sprayed on the component to etch/dissolve away the bare surface. Figs 3 and 4 show milled apertures with good circular geometry but coarse profile structure. The other disadvantage of PCM is that the common etchants such as ferric chloride, cupric chloride, sodium hydroxide and potassium ferricyanide are not effective in processing 'difficult' metals. For example, in the cases of platinum and gold, a likely etchant would be a hot mixture of nitric and hydrochloric acids which are toxic, corrosive and would present environmental problems, both on the industrial shop floor and in disposal. In the plasma method (Figs 5 and 6) molydbenum foil was etched in a glow discharge where the ions reacted chemically with the material to be swept away by gas flow. The aperture profile (Fig 6) is reasonably steep and the edges are sharp; however, the profile structure does not appear to compare favourably with Figs 1 and 2. Also, the technique prevents maintenance of any satisfactory adhesion of masks during treatment and as a result, the final apertures are of unacceptable quality. Furthermore, the overall cost is high because it requires a vacuum chamber, a pumping system, and special equipment to handle the gas both before and during the etching operation.
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Chatterjee--fabrication of fine apertures in metal foils poor quality apertures and offer little control over the geometry. A low-power laser, on the other hand, offers a circular hole having too coarse a profile structure to be suitable for any precision device. In EDM, electric sparks between an electrode at high voltage and molybdenum foil eroded the metal by melting until the electrode had completely penetrated the substrate. The hole geometry is good and its edge appears sharp (Figs 11 and 12). The profile, however, is coarse, with some evidence of resolidification (Fig 12) and the quality of the aperture appears poorer than that produced by a low-power laser beam. An aperture produced by EDM was mechanically polished, whereby the quality improved somewhat, but still remained quite coarse (Fig 13 and 14).
Fig 1 SEMpicture (45 ° tilt) of aperture in molybdenum foil obtained as commercial aperture disc (170 × magnification)
Fig 3 SEMpicture (450 tilt) of aperture in molybdenum foil produced by photochemical milling (170 × magnification)
Fig 2 Profile structure of aperture in Fig 1 (2200 × magnification)
In the laser method, a laser beam was focused on to the metal until a hole had been burned through. The profile structure (Figs 7 and 8) on using a low-power laser of several watts, is coarse and shows considerable re-solidification of the metal during or after drilling. With a mediumpowered (400 W) laser, the aperture and its structure are of poor quality (Figs 9 and 10). There is a considerable amount of resolidified material on the periphery of the hole, and the hole itself is not circular. Perhaps, better quality could have resulted if all the melted metal had vaporized. It is clear from Figs 7-10 that a high-powered laser would produce
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Fig 4 Profile structure of aperture in Fig 3 (2200× magnification)
JULY 1986 V O L 8 NO 3
Chatterjee--fabrication of fine apertures in metal foils e t ~ ~ s ) . By choosing an appropriate electrolyte, metal would be removed atom-byatom selectively at the surface roughness peaks. This should result in smoothing of the surface, with the ultimate elimination of microroughness, and followed eventually by the removal of macroroughness, ie surface unevenness. The theoretical considerations for PEMwould in many ways be similar to those for electrochemical machining (ECM) 1-3 since both processes are characterized by a high rate of anodic dissolution of the workpiece. In order however, to achieve smoothness as well as geometrical accuracy of the aperture, it is considered essential to apply photolithography to the electro-
Fig 5 SEMpicture (45' tilt) of aperture in molybdenum foil produced by plasma method (90 × magnification)
Fig 7 SEMpicture (45, tilt) of aperture in molybdenum foil produced by low power laser technique (90× magnification)
Fig 6 Profile structure of aperture in Fig 5 (1100× magnification)
Background t o PEM development It is apparent from the trial runs (Figs 3-14) that varying quality of fine apertures is produced on the metal foils by the different fabrication techniques, none comparable with the smooth profile structure of Fig 2. Thus, the objective of the present work was to develop a process which would provide precision apertures in, preferably, any metal with a smooth profile structure such as in Figs 1 and 2. An electrochemical or, more precisely, an electropolishing approach is considered, because it takes into account both a macroscopic effect (a levelling process which wears down the peaks) and a microscopic effect (a brightening process which
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Fig 8 Profile structure of aperture in Fig 7 (2200x magnification)
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Chatterjee--fabrication of fine apertures in metal foffs lytic milling process. Details of photolithography are well known in the PCM industry 4 and are therefore not discussed here.
Experimental Type of electrolyte In selecting an electrolyte for PEM, it was considered essential to develop a solution which must be stable, safe to use, inexpensive, and pollution-free. Also, it should not attack the photoresist coating, to prevent undercutting. It is assumed, on the basis of combined theories of viscous layer and solid film
Fig 11 SEMpicture (450 tilt) of aperture in molybdenum foil produced by electrodischarge method (1 lOx magnification)
Fig 9 SEMpicture (45 ° tilt) of aperture in molybdenum foil produced by medium power laser technique (90 x magnification)
Fig 12 Profile structure of aperture in Fig 11 (1100 x magnification)
Fig 10 Profile structure of aperture in Fig 9 (1100 × magnification)
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formation for electropolishing, that satisfactory PEM w o u l d occur in the transpassive region of the polarization curve of an electrolyte. Based on the steadystate polarization studies of electrolytes commonly used in ECM, ie NaCI, NaNO 3, NaCIO3 and NaCr207, it is observed that the current density in the transpassive region for NaCI is higher than for any of the other electrolytes 5, and its use could thus result in sharp cutting of the workpiece even at low potential. Also, it has been reported 6 that a multiplicity of bright-bottomed, randomly scattered pits is known to produce a polished finish. The aggressive anions associated with pitting are usually the halides, of which the chloride ion has received the most attention. It is inexpensive, stable, and nonpassivating in the form of NaCI which has thus been used as the general purpose salt in the present work.
JULY 1986 VOL 8 NO 3
Chatterjee--fabrication of fine apertures in metal foils
Fig 13 SEMpicture (45' tilt) of aperture in molybdenum foil produced by electrodischarge followed by mechanical polishing (170 x magnification)
Alternative means of exerting turbulence were also studied, ie jetting and ultrasonic vibration of the electrolyte, but no extra advantage was gained. In jet milling, the electrolyte was forced, by compressed air, through a platinum cathode tube against the anode workpiece a few millimetres away in an enclosed splash container. Metal removal occurred directly under the cathode jet because of the large ohmic resistance of other current paths. The technique proved quite effective in eliminating uneven etching due to gas bubbles and accumulation of reaction products at the anode surface. Also, the use of a platinum cathode prevented deposit formation due to chemical reaction with the electrolyte. However, only one item per operation can be processed using a single orifice cathode and would, thus, be too slow to attract any commercial interest. A multi-orifice cathode gun, properly aligned with the workpiece, would be useful in this respect but the cost of such a platinum gun and the tooling cost for alignment proved too costly for the laboratory tests. The use of ultrasonic vibration of the electrolyte did not prove satisfactory in removing reaction products and, furthermore, the technique appeared commercially unattractive in terms of the scaling up costs.
Concentration of electrolyte A high concentration of active anions is essential for consideration of transpassive-polishing criteria (Hoar diagram). An increase in the number of active ions in a strong electrolyte would provide a high dissolution current and, hence, high dissolution kinetics. A concentrated electrolyte would also prevent any large undesirable drop in inter-electrode resistance. It is important, however, to remember that the electrical conductivity of most electrolytes, including NaCI, increases with concentration but only to a level controlled mostly by the solubility limit. With an increase in electrolyte strength, the film thickness may increase so a compromise is usually required in the film-former/film-attacker ratio and in the overall concentration. Total concentrations of between 4 M and 5 M are presently employed (Table 1).
Details of PEM assembly A schematic diagram of the test rig layout is shown in Fig 15. The assembly consisted of a power source, an electrolyte circulation system with continuous filtration facility, and a gas extraction system for removing hydrogen evolved from the cathodes. The electrode arrangements were such that each anode reciprocated between two cathodes. A narrow gap (up to 5 mm) was maintained between the cathode sheets of platinized titanium and the workpiece. The direct current for the apparatus was
Application of electrolyte A high rate of anodic dissolution is an essential requirement for PEM,and can be achieved by minimizing concentration polarization due to gas bubbles at the electrodes and ensuring removal of reaction products from the electrode surface. Both these criteria can be met by strong agitation of the electrolyte (Reynolds number >/2000). A high turbulence was achieved by applying pressurized recirculation (about 4 I/min) of continuously filtered electrolyte and using anode movement. The turbulence also prevented boiling of the electrolyte from Joule heating; this is important because, even though the interelectrode gap was small, a finite resistance leading to electrical heating developed due to the high current densities used.
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Fig 14 Profile structure of aperture in Fig 13 (1700 x magnification)
135
Chatterjee--fabrication of fine apertures in metal foils Table 1 Processing parameters for photoelectrochemical milling of various materials 10-500/~m thick at ambient temperature Current density, A/cm 2
Typical milling rate, l~m/min
Material
Electrolyte
Type of power supply
Mo Pt, Pt-10Rh Pd-base alloy (Pd-30Ag-1 5Cu-10Pt-10Au)* Both pure and carat golds Sterling silver (Ag-7Cu) Ag-20Cu-1 5Pdt Stainless steel (EN 58J)
2 M NaCI + 2 M NaNO3 5 M NaCI
DC AC
100--125 20--30
40 10
5 M NaCI 5 M NaCI
AC DC
20--30 20--25
10 40
3 M NaNO3 5 M NaCI
DC DC
20--25 25--35
60 10
5 M NaCI
PC
30
44
* Typical electrical contact alloy; ttypical brazing alloy
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supplied by a Westinghouse transformer-rectifier system, where the voltage could be varied between 0 and 25 V with a maximum current of 40 A. The filter between the pump and PEM cell consisted of closely woven polypropylene material, with a proportionately large surface area. The flow rate of the electrolyte was estimated to be about 4 I/min.
Experimental procedure The initial requirement was to produce an outline of the specific design (ie an aperture) on a metal foil 10-500/~m thick in the form of a photoresist coating which would be resistant to the subsequent processing operation. The unprotected metal of the photoresist stencil was then electrochemically dissolved away at ambient temperature. The applied voltage, typically 10-15 V, was adjusted to give the desired current density; it was preferable that the operating current be attained as quickly as possible, ie within seconds of operation. For fabrication of platinum group metals, a voltage of 20 V AC at 50 Hz frequency was applied using a platinum counterelectrode and the current strength was regulated by an autotransformer.
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Materials processed During electrochemical milling of molybdenum, large quantities of anode products were produced (oxide/hydroxide of Mo 3+ ions) which inhibited the process. The addition of ammonia as a complexing agent overcame the problem, but the photoresist (Kalle type T225 or Robertson's CM 2000) became unstable in the high pH solution. The problem was resolved by replacing ammonia with an oxidizing agent like sodium nitrate which oxidized molybdenum into soluble Mo 6~ ions. The milling of platinum and its alloys, unlike other materials, did not proceed smoothly by DC because of polarization leading to passivation. The depolarizing effect of AC in chloride solutions has been reported elsewhere for platinum 7, thus AC, either alone or superimposed on PC, was used to process platinum group metals. Electromilling of silver in sodium chloride solution was inhibited by the formation on the silver surface of highly insoluble silver chloride. An electrolyte of sodium nitrate was used, producing soluble silver nitrate as the reaction product. A number of other precious metal alloys were successfully milled using NaCI electrolyte. Samples were also produced in stainless steel; although stainless steel can be routinely processed by PCM,the dissolution rate was faster by an order of magnitude using the present PEM technique.
Results and discussion The scanning electron photomicrographs (at 45 ° tilt) of the apertures in molybdenum and platinum foils shown in Figs 16-19 reveal typical structures produced by PEa. It has been possible to process both precious and refractory metals by PEM using appropriate conditions (Table 1). The extremely smooth profile structures with parallel edges of Figs 16-19 appear to be the characteristic structures obtainable by PEM,and compare favourably with Figs 1 and 2.
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Chatterjee--fabrication of fine apertures in metal foils The milling process can be accelerated by increasing the rate of metal removal by increasing the current density and maintaining the closest possible anode-cathode gap ( < 5 mm), thereby minimizing the ohmic drop. Any increase in current density is, however, limited by the cathodic reaction controlled by the hydrodynamic conditions of the electrolyte and the interelectrode gap. Too high a current density could result in the production of electric sparks on the cathode.
Tolerances In considering the potential of PEM,the limitations inherent in both photolithography and electro-
Fig 16 SEMpicture (45" tilt) of aperture in molybdenum foil produced by present photoelectrochemical technique (550 x magnification)
Fig 18 SEMpicture (45' tilt) of aperture in platinum foil produced by present photoelectrochemical technique (670 x magnification)
Fig 17 Profile structure of aperture in Fig 16 (2200 × magnification) Distribution of current A nonuniform distribution of current is expected on the anode surface due to the relatively smaller anode area and as a result, metal is likely to be removed more from the edges than from the centre of the anode stencil. However, no such nonuniform profiling was produced in the present work, probably due to the sudden surge of high current at the start of the operation. It was also considered important that the anode stencil be located centrally, facing the larger cathode surfaces, so that the current could flow equally and freely from the top and bottom of the stencil to the facing cathodes.
PRECISION ENGINEERING
Fig 19 Profile structure of aperture in Fig 18 (1300 x magnification)
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Chatterjee--fabrication of fine apertures in metal foils chemical milling must be recognized. It must be appreciated that, during milling, there would be some lateral dissolution of the metal resulting in a final hole larger than the size on the stencil, the situation becoming more apparent with thicker foil. However, the dimensional tolerances in the present work have generally been within + 10% of the metal thickness. In cases where the tolerances are critical, the original photomasks must be reduced in size to allow lateral dissolution. On keeping the working parameters constant, a high degree of precision fabrication can be reproduced. Furthermore, a statistical analysis of the hole diameters produced by PEM shows no evidence of ovality.
Capabilities of PEM The technique is governed by Faraday's laws of electrolysis and depends on the electrochemical equivalent of the metal, not on its hardness. Thus, with most modern alloys becoming harder than before, PEM will find its application in the cutting of such hard exotic alloys more competitive against conventional techniques. With suitable design modifications, it is now possible to fabricate fine apertures or any intricate shapes in metals, including precious and refractory metals for use in the industrial and jewellery fields.
Conclusions A comparative assessment of various fabrication methods of producing apertures clearly reveals the superior quality of the finished product resulting from the present PEM technique. An important feature of PEM is the use of nonpollutant, nonaggressive electrolytes, thus eliminating limitations on the type of photoresists used. The development of the present method ensures removal of any metal, including 'difficult' ones with no burrs, and processinduced stresses on the product, and also no tool wear. Furthermore, the operation is unaffected by
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the mechanical or thermochemical treatment of the metal. It is, thus, now possible to fabricate apertures of any design in any metal foil. For simpler alloys like stainless steel, the metal removal rate by PEM is estimated to be about an order of magnitude faster than by PCMoThe versatility and simplistic approach of PEM are likely to attract commercial interest in manufacturing jewellery items, as well as industrial components such as aperture discs, electrical contacts and brazing preforms made from simple to exotic metals and alloys.
Acknowledgements The author wishes to thank Mr R.B. Worsnop for his photographic assistance and the following organizations for providing materials and fabrication facilities used in the present work: Culham Laboratory (UKAEA, Oxfordshire), Engelhard Ind. (Surrey), Philips Research Laboratory (Surrey), Tecan Components (Dorset), Unicastex (Berkshire) and Veco (Surrey).
References 1 Wilson J.F. Practice and theory of electrochemical machining. Wiley Interscience, New York, 1971 2 McGeough J.A. Principles of electrochemical machining. Chapman and Hall, London, 1974 3 DeBarr A.E. and Oliver D.A. Electrochemical machining. Macdonald and Co. Ltd, London, 1975 4 Allen D.M. et al. Production of spring steel camera shutter blades by photoetching. Precis. Eng., 1979, 1 (1), 25 5 Hoare J.P. and LaBoda M.A. Electrochemical machining, in Comprehensive treatise of electrochemistry (ed. Bockris, J.O'M.) vol 2. Plenum Press, New York, 1981, chapter 8, 399520 6 Evans J . i . and Boden P.J. Surface finish produced during electrochemical machining on nickel and nimonic 80A in chloride electrolytes, in Principles of electrochemical machining (ed. Faust C.L.). Electrochemical Society Inc., Princeton, New Jersey, 1971, 40-62 7 Llopis J. and Sancho A. Electrochemical corrosion of platinum in hydrochloric acid solutions. J. Electrochem. Soc., 1961, 108 (8), 720
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