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Photoresist for photochemical machining of alumina ceramic
Photoresist for p h o t o c h e m i c a l m a c h i n i n g of alumina ceramic E. Making, T. Sato and Y. Yamada* The applicability of a cyclized polybutadiene rubber as a resist material for photoetching of alumina ceramic in phosphoric acid was studied. Stencil breakdown, change in the stencil thickness during etching, and etch factor were measured. It was found that this material post-baked at temperatures as high as 300 °C for 30 min provided good resistance to severe attack by the acid at temperatures of up to 300°C. The effects of post-bake temperature, of etching temperature, of etching time, and of original slot width on etch factor are discussed. K e y w o r d s : photochemical machining, alumina ceramic
Although photochemical machining (PCM) has been developed as an industrial production technique, very little quantitative information on machining of ceramic materials is available. Because of the increasing importance of the PCM of ceramic materials, the present work was conducted. Previous work 1 dealing with an etchant system for the PCM of alumina ceramic demonstrated that boiling phosphoric acid was useful as an etchant and etch rate was of the order of several #m/min at etching temperatures of more than 250 °C. However, since etching conditions are very severe, a highly heatresistant and chemically inert material has to be used as a photoresist. For this purpose, the applicability of a cyclized polybutadiene rubber was studied.
The photoresist solution was applied to both surfaces of the alumina substrate by spinning at 800 r/min and then pre-baked at 80°C for various durations in an oven. The coating was exposed to UV light through a phototool at various exposure energies in a vacuum printing frame. After ultrasonic development in JSR developer M530, the photoresist stencil was rinsed in n-butyl acetate and post-baked at various temperatures for 30 rain in a dried nitrogen stream. The thickness of the coating after post-baking was 2.0-2.4 #m. The patterns of the phototools used in the experiments are shown in Fig 2, The mesh pattern
Substrote preporation
Experimental
details
A l u m i n a ceramic a n d photoresist stencil formation The alumina substrates were of 96 % purity and were 25 mm x 33 mm × 0,635 mm in size. The substrate was cleaned by swabbing in neutral detergent followed by rinsing in water. It was then rinsed ultrasonically in acetone for 30 min and dried at 300°C for 30 min in a dried nitrogen stream. The photoresist stencil formation process is outlined in Fig 1. Photosensitive cyclized polybutadiene rubber (Japan Synthetic Rubber Co. JSR CBR-M901) was used as a negative-working photoresist. The material is thermally more stable and exhibits higher chemical resistance than conventional resist materials. Resist flow and thermal decomposition do not occur at temperatures of up to about 300 °C and 400 °C, respectively2. * Department of Precision Engineering, Hokkaido University, Sapporo, Japan
PRECISION ENGINEERING
I 1 (Pro-ba.,og I I E,posure I I
}
1 Cleaning and drying
hotoresist coating Spinning at 800 r/rain
I At SO=C in on oven
I High pressure mercury lamp
Development
] Ultrasonically in JSR developer M-530 at 32"C for 2 rain
I
Rinsing
I Ultrasonically in n-butyl acetate for 15s
J
Post - baking
I In dried nitrogen stream for 30 rain
Fig 1 Photoresist stencil formation process
0141~359/87/030153~)5/$03.00 © 1987 Butterworth & Co (Publishers) Ltd
153
Makino et al-- photoresist for PCM of alumina ceramic
QmEimllm mnmmumnm mmmmmmmm i B m n l l i u mmmmmmmm nummnmmm mmmmmmnm
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,,(
5mm
a
b
Fig 2 Phototool patterns." (a) mesh pattern; (b) line and space pattern had 7 x 7 lines 1 mm wide with sixty-four 2 mm square apertures. The line and space patterns had about 20 lines of graded width.
Etching process and measurements All of the etching experiments were conducted with boiling phosphoric acid at atmospheric pressure and at temperatures ranging from 260 °C to 320 °C. The etching apparatus is shown schematically in Fig 3. Commercial 85 % orthophosphoric acid was contained in a 500 ml borosilicate glass vessel and was heated with an electric heating mantel. Evaporating water vapour was constantly condensed and returned to the boiling acid. Under these refluxing conditions, the acid remained at a constant
temperature and composition, independent of heating 1. The alumina substrate covered with the photoresist stencil was held in the glass holder horizontally and immersed directly into the boiling acid after its temperature had become constant by refluxing, and then etched for 1 5-60 min. Following etching, the substrates were rinsed in water and blow-dried. Etch factors were measured at 7 positions along 7 slots each of 1 mm wide in the mesh pattern. The undercuts (U) were determined from measurements of the widths of slots in the resist stencils and the widths of the etched slots at the substrate surface with an optical microscope. The depths of etch (D) were measured by scanning with a surface roughness tester (Tokyo seimitsu, Surfcom-300B). With these measured values, the etch factors were calculated as the ratio D/U. The changes in the thickness of the photoresist stencil during etching were determined by measuring the stencil thickness before and after etching by the surface tester. The breakdown ratios of the photoresist stencil were determined by counting the number of square stencils broken down during etching to sixty-four 2 mm square stencils formed with the mesh pattern. The dependences of etch depths and undercuts on original line widths were measured on the line and space pattern.
Results and discussion Fig 4 shows the influence of pre-baking conditions and exposure energies on photoresist image Pre-bake
time,
rain ( a t 8 0 ° C )
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40
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Fig 3 Schematic diagram of etching apparatus: A glass reaction vessel (500 ml); B phosphoric acid; C boiling aids; D heating mantle; E sample; F glass sample holder; G stopcock," H aspirator; I air-cooled condenser; J water-cooled condenser; K flow meter, L glass-encased thermocouple; M SCR unit; N temperature controller; 0 recorder
154
(~) G o o d s t e n c i l 0
Stencil
z~ S c u m m e d x
image
surface
Insufficient
attack
image development
Fig 4 Influence of pre-baking and exposure on resist image formation
JULY 1987 VOL 9 NO 3
Makino et al--photoresist for PCM of alumina ceramic formation. Over-pre-baking and over-exposure led to an insufficient development on the unexposed area of the resist. On the other hand, underexposure led to occurrence of attack by the developer on the exposed area of the resist. Good resist images without scum were obtained under conditions of pre-bake time of less than 30 min at 80 °C and exposure energies of 5-10 mJ/cm2. The etch rates and the stencil breakdown ratios as a function of etching temperature are shown in Fig 5. These values were measured after a 30 min etching. The etch rate increased with an increase in etching temperature, and it was about 6 #m/min at 320 °C. From this result, boiling phosphoric acid at temperatures of more than about 250 °C was found to be a convenient etchant for alumina ceramic. However, as etching conditions became severe with an increase in etching temperature, the breakdown ratio of the stencil increased. Especially, the ratios were very high at post-bake temperature as low as 1 50 °C. Increasing the post-bake temperature reduced the breakdown ratio, because adhesion of the resist stencil to the substrata was improved. At post-bake temperatures of more than 250 °C, stencil breakdown did not occur at the etching temperatures of up to 300 °C. The changes in the stencil thickness during etching are shown in Fig 6. It is found that the stencil did not dissolve in boiling phosphoric acid. As the etching temperature was raised and as the post-bake temperature was reduced, the thickness of the stencil increased. XPS and FT-IR studies of the stencil revealed that chemical compounds between the resist material and the phosphoric acid were produced, resulting in an increase in the stencil thickness. Although the stencil thickened by chemical reactions, it showed superior resistance for prolonged etching of the alumina substrate. -"~J
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260
280
300
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Etching temperature,QC
Fig 6 Change in the stencil thickness during etching versus etching temperature. Conditions as for Fig 5 Etch factors as a function of etching temperature are shown in Fig 7. These values are the mean of 7 measurements at different positions in the mesh pattern. Etch factors increased with an ///#
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Fig 5 Etch rate and stencil breakdown ratio versus
etching temperature. Conditions: pre-baking 80°C, 20 min; exposure energy 8.3 mJ/cm2; post-bake time 30 min; etching time 30 min
PRECISION ENGINEERING
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Etching temperature ,=C
Fig 7 Etch factor versus etching temperature. Conditions as for Fig 5
155
Makino et al--photoresist for PCM of alumina ceramic !
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Fig 8 Variation of depth of etch and etch factor with time. Conditions: pre-baking 80°C, 20 rain; exposure energy 7.3 mJ/cm2; post-baking 300 °C, 30 min; etching temperature 280 °C increase in etching temperature, as well as with an increase in post-bake temperature, and an etch factor around 2 could be obtained. Concerning an increase Qf etch factor with an increase in etching temperature, it appears that quite complex processes may be occurring. As mentioned previously, I
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increasing etching temperature,caused an increase in the stencil thickness, implying a consumption of reactive species for etching alumina substrate near the stencil. This consumption of reactive species appears to retard lateral etching. Moreover, as the etching temperature was increased and the stencil softened, unsupported flaps of the stencil might hang down on side walls of the slots. This appears to retard replenishment of fresh etchant to the side walls, resulting in retarding lateral etching. Such effects would seem the cause for observation of high etch factors at raised etching temperature. But too high etching temperature and post-bake temperature resulted in reduction of etch factors. It seems that the stencil could not keep good adhesion to the substrate at the etching temperature of 320 °C and the stencil might be brittle under the postbaking at 350 °C for 30 min. Variation of the depth of etch and the etch factor with etching time is shown in Fig 8. The depth of etch increased linearly with time, resulting in a constant etch rate of about 3.2/~m/min. This is due to the composition of the boiling phosphoric acid being kept constant by refluxing. Moreover, the etch factor kept constant during etching. Fig 9 shows the influence of orginal slot width on the depth of etch and the undercut. Above a critical slot width of 0.2 mm, the depth of etch and the undercut were constant. Fig 10 shows photographs illustrating a sample
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Original slot width, m m
Fig 9 Influence of original slot width on depth of etch and undercut. Conditions as for Fig 8 except etching temperature, which was varied; etching time 30 min
156
Fig 10 Photograph of alumina ceramic sample etched at 300°C for 30 min: (a) line and space profile; (b) cross-section of slots
JULY 1987 VOL 9 NO 3
Makino et al--photoresist for P C M of alumina ceramic
profile and its cross-section etched at 300 °C for 30 min. Since the stencil edge was lifted off by the etchant during etching, roughening of etched slot edge occurred, and deviation from straightness (peak-to-valley value) of etched slot was about 20/~m. Corner radii were about 0.28 mm for an inside corner and about 0,08 mm for an outside corner.
Conclusions In the work described here, applicability of cyclized polybutadiene rubber as a resist material for photochemical machining of alumina ceramic was studied. The resist stencils formed by this material showed superior resistance to chemical attack by a boiling phosphoric acid etchant. The higher the etching temperature, the higher was the etch factor.
PRECISION ENGINEERING
When etching at 300 °C, an etch factor of about 2 was obtained. High post-bake temperature of about 300 °C was required to obtain a high etch factor without stencil breakdown.
Acknowledgements The authors wish to thank Mr K. Kakizaki (Toshiba Corp.) for supplying the alumina ceramic, and Mr Y. Ueda (Hirai Seimitsu Corp.) for providing the phototools.
References 1 Makino E., Sato T. and Yamada Y. Photochemical machining of aluminaceramic in phosphoricacid. J. Photo Chemical Machining Inst., 1987, (27), 4-6
2 Watanabe T. Denshi Zairyo (Electronic Parts and Materials), 1978, 17(8), 119-124
157
Although photochemical machining (PCM) has been developed as an industrial production technique, very little quantitative information on machining of ceramic materials is available. Because of the increasing importance of the PCM of ceramic materials, the present work was conducted. Previous work 1 dealing with an etchant system for the PCM of alumina ceramic demonstrated that boiling phosphoric acid was useful as an etchant and etch rate was of the order of several #m/min at etching temperatures of more than 250 °C. However, since etching conditions are very severe, a highly heatresistant and chemically inert material has to be used as a photoresist. For this purpose, the applicability of a cyclized polybutadiene rubber was studied.
The photoresist solution was applied to both surfaces of the alumina substrate by spinning at 800 r/min and then pre-baked at 80°C for various durations in an oven. The coating was exposed to UV light through a phototool at various exposure energies in a vacuum printing frame. After ultrasonic development in JSR developer M530, the photoresist stencil was rinsed in n-butyl acetate and post-baked at various temperatures for 30 rain in a dried nitrogen stream. The thickness of the coating after post-baking was 2.0-2.4 #m. The patterns of the phototools used in the experiments are shown in Fig 2, The mesh pattern
Substrote preporation
Experimental
details
A l u m i n a ceramic a n d photoresist stencil formation The alumina substrates were of 96 % purity and were 25 mm x 33 mm × 0,635 mm in size. The substrate was cleaned by swabbing in neutral detergent followed by rinsing in water. It was then rinsed ultrasonically in acetone for 30 min and dried at 300°C for 30 min in a dried nitrogen stream. The photoresist stencil formation process is outlined in Fig 1. Photosensitive cyclized polybutadiene rubber (Japan Synthetic Rubber Co. JSR CBR-M901) was used as a negative-working photoresist. The material is thermally more stable and exhibits higher chemical resistance than conventional resist materials. Resist flow and thermal decomposition do not occur at temperatures of up to about 300 °C and 400 °C, respectively2. * Department of Precision Engineering, Hokkaido University, Sapporo, Japan
PRECISION ENGINEERING
I 1 (Pro-ba.,og I I E,posure I I
}
1 Cleaning and drying
hotoresist coating Spinning at 800 r/rain
I At SO=C in on oven
I High pressure mercury lamp
Development
] Ultrasonically in JSR developer M-530 at 32"C for 2 rain
I
Rinsing
I Ultrasonically in n-butyl acetate for 15s
J
Post - baking
I In dried nitrogen stream for 30 rain
Fig 1 Photoresist stencil formation process
0141~359/87/030153~)5/$03.00 © 1987 Butterworth & Co (Publishers) Ltd
153
Makino et al-- photoresist for PCM of alumina ceramic
QmEimllm mnmmumnm mmmmmmmm i B m n l l i u mmmmmmmm nummnmmm mmmmmmnm
I,
,,(
5mm
a
b
Fig 2 Phototool patterns." (a) mesh pattern; (b) line and space pattern had 7 x 7 lines 1 mm wide with sixty-four 2 mm square apertures. The line and space patterns had about 20 lines of graded width.
Etching process and measurements All of the etching experiments were conducted with boiling phosphoric acid at atmospheric pressure and at temperatures ranging from 260 °C to 320 °C. The etching apparatus is shown schematically in Fig 3. Commercial 85 % orthophosphoric acid was contained in a 500 ml borosilicate glass vessel and was heated with an electric heating mantel. Evaporating water vapour was constantly condensed and returned to the boiling acid. Under these refluxing conditions, the acid remained at a constant
temperature and composition, independent of heating 1. The alumina substrate covered with the photoresist stencil was held in the glass holder horizontally and immersed directly into the boiling acid after its temperature had become constant by refluxing, and then etched for 1 5-60 min. Following etching, the substrates were rinsed in water and blow-dried. Etch factors were measured at 7 positions along 7 slots each of 1 mm wide in the mesh pattern. The undercuts (U) were determined from measurements of the widths of slots in the resist stencils and the widths of the etched slots at the substrate surface with an optical microscope. The depths of etch (D) were measured by scanning with a surface roughness tester (Tokyo seimitsu, Surfcom-300B). With these measured values, the etch factors were calculated as the ratio D/U. The changes in the thickness of the photoresist stencil during etching were determined by measuring the stencil thickness before and after etching by the surface tester. The breakdown ratios of the photoresist stencil were determined by counting the number of square stencils broken down during etching to sixty-four 2 mm square stencils formed with the mesh pattern. The dependences of etch depths and undercuts on original line widths were measured on the line and space pattern.
Results and discussion Fig 4 shows the influence of pre-baking conditions and exposure energies on photoresist image Pre-bake
time,
rain ( a t 8 0 ° C )
I0
20
50
40
50
60
3.3
o
o
o
o
/
/
6.6
(~)
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~)
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x
x
=~
9.9
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x
a
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x
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=:
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Fig 3 Schematic diagram of etching apparatus: A glass reaction vessel (500 ml); B phosphoric acid; C boiling aids; D heating mantle; E sample; F glass sample holder; G stopcock," H aspirator; I air-cooled condenser; J water-cooled condenser; K flow meter, L glass-encased thermocouple; M SCR unit; N temperature controller; 0 recorder
154
(~) G o o d s t e n c i l 0
Stencil
z~ S c u m m e d x
image
surface
Insufficient
attack
image development
Fig 4 Influence of pre-baking and exposure on resist image formation
JULY 1987 VOL 9 NO 3
Makino et al--photoresist for PCM of alumina ceramic formation. Over-pre-baking and over-exposure led to an insufficient development on the unexposed area of the resist. On the other hand, underexposure led to occurrence of attack by the developer on the exposed area of the resist. Good resist images without scum were obtained under conditions of pre-bake time of less than 30 min at 80 °C and exposure energies of 5-10 mJ/cm2. The etch rates and the stencil breakdown ratios as a function of etching temperature are shown in Fig 5. These values were measured after a 30 min etching. The etch rate increased with an increase in etching temperature, and it was about 6 #m/min at 320 °C. From this result, boiling phosphoric acid at temperatures of more than about 250 °C was found to be a convenient etchant for alumina ceramic. However, as etching conditions became severe with an increase in etching temperature, the breakdown ratio of the stencil increased. Especially, the ratios were very high at post-bake temperature as low as 1 50 °C. Increasing the post-bake temperature reduced the breakdown ratio, because adhesion of the resist stencil to the substrata was improved. At post-bake temperatures of more than 250 °C, stencil breakdown did not occur at the etching temperatures of up to 300 °C. The changes in the stencil thickness during etching are shown in Fig 6. It is found that the stencil did not dissolve in boiling phosphoric acid. As the etching temperature was raised and as the post-bake temperature was reduced, the thickness of the stencil increased. XPS and FT-IR studies of the stencil revealed that chemical compounds between the resist material and the phosphoric acid were produced, resulting in an increase in the stencil thickness. Although the stencil thickened by chemical reactions, it showed superior resistance for prolonged etching of the alumina substrate. -"~J
I
I
I
/J
I
l
E. 2.0
=
i
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Post-bake temperature 200 =C~O
..= o_
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=, .E .
0.5
0
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i
1
1
260
280
300
320
Etching temperature,QC
Fig 6 Change in the stencil thickness during etching versus etching temperature. Conditions as for Fig 5 Etch factors as a function of etching temperature are shown in Fig 7. These values are the mean of 7 measurements at different positions in the mesh pattern. Etch factors increased with an ///#
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20 6 15
E E
m
2.0
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/./
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-280
-300
520
0
Etching temperature, °C
Fig 5 Etch rate and stencil breakdown ratio versus
etching temperature. Conditions: pre-baking 80°C, 20 min; exposure energy 8.3 mJ/cm2; post-bake time 30 min; etching time 30 min
PRECISION ENGINEERING
,.,,~/
I 260
I 280
| :300
I $20
Etching temperature ,=C
Fig 7 Etch factor versus etching temperature. Conditions as for Fig 5
155
Makino et al--photoresist for PCM of alumina ceramic !
250
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'
150
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0.5
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30
45
60
o
Etching time, rain
Fig 8 Variation of depth of etch and etch factor with time. Conditions: pre-baking 80°C, 20 rain; exposure energy 7.3 mJ/cm2; post-baking 300 °C, 30 min; etching temperature 280 °C increase in etching temperature, as well as with an increase in post-bake temperature, and an etch factor around 2 could be obtained. Concerning an increase Qf etch factor with an increase in etching temperature, it appears that quite complex processes may be occurring. As mentioned previously, I
I
!
!
increasing etching temperature,caused an increase in the stencil thickness, implying a consumption of reactive species for etching alumina substrate near the stencil. This consumption of reactive species appears to retard lateral etching. Moreover, as the etching temperature was increased and the stencil softened, unsupported flaps of the stencil might hang down on side walls of the slots. This appears to retard replenishment of fresh etchant to the side walls, resulting in retarding lateral etching. Such effects would seem the cause for observation of high etch factors at raised etching temperature. But too high etching temperature and post-bake temperature resulted in reduction of etch factors. It seems that the stencil could not keep good adhesion to the substrate at the etching temperature of 320 °C and the stencil might be brittle under the postbaking at 350 °C for 30 min. Variation of the depth of etch and the etch factor with etching time is shown in Fig 8. The depth of etch increased linearly with time, resulting in a constant etch rate of about 3.2/~m/min. This is due to the composition of the boiling phosphoric acid being kept constant by refluxing. Moreover, the etch factor kept constant during etching. Fig 9 shows the influence of orginal slot width on the depth of etch and the undercut. Above a critical slot width of 0.2 mm, the depth of etch and the undercut were constant. Fig 10 shows photographs illustrating a sample
I
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e
m
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u
c
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I 0.2
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I 0.6
I 0.8
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Original slot width, m m
Fig 9 Influence of original slot width on depth of etch and undercut. Conditions as for Fig 8 except etching temperature, which was varied; etching time 30 min
156
Fig 10 Photograph of alumina ceramic sample etched at 300°C for 30 min: (a) line and space profile; (b) cross-section of slots
JULY 1987 VOL 9 NO 3
Makino et al--photoresist for P C M of alumina ceramic
profile and its cross-section etched at 300 °C for 30 min. Since the stencil edge was lifted off by the etchant during etching, roughening of etched slot edge occurred, and deviation from straightness (peak-to-valley value) of etched slot was about 20/~m. Corner radii were about 0.28 mm for an inside corner and about 0,08 mm for an outside corner.
Conclusions In the work described here, applicability of cyclized polybutadiene rubber as a resist material for photochemical machining of alumina ceramic was studied. The resist stencils formed by this material showed superior resistance to chemical attack by a boiling phosphoric acid etchant. The higher the etching temperature, the higher was the etch factor.
PRECISION ENGINEERING
When etching at 300 °C, an etch factor of about 2 was obtained. High post-bake temperature of about 300 °C was required to obtain a high etch factor without stencil breakdown.
Acknowledgements The authors wish to thank Mr K. Kakizaki (Toshiba Corp.) for supplying the alumina ceramic, and Mr Y. Ueda (Hirai Seimitsu Corp.) for providing the phototools.
References 1 Makino E., Sato T. and Yamada Y. Photochemical machining of aluminaceramic in phosphoricacid. J. Photo Chemical Machining Inst., 1987, (27), 4-6
2 Watanabe T. Denshi Zairyo (Electronic Parts and Materials), 1978, 17(8), 119-124
157