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Vapour-deposited bismuth layers for unconventional photography
Thin Solid Films, 90 (1982) 199-202
199
ELECTRONICS AND OPTICS
VAPOUR-DEPOSITED BISMUTH LAYERS F O R U N C O N V E N T I O N A L PHOTOGRAPHY* P. DE MAAYER, G. VERBEKE AND H. SNEYERS
Research Laboratories, Agfa-Gevaert, Mortsel (Belgium) (Received August 14, 1981 ; accepted September 22, 1981)
The photolithographic principle and know-how of controlling the optical characteristics of thermally evaporated pure bismuth layers have been combined in the development of a new silverless metal/photoresist composite film for professional photography. A correlation exists between the spectral distribution of the specular reflection density of these films and the packing density of the evaporated layers. The photographic performance profile is also discussed.
1. INTRODUCTION
The instability of the silver price and the ever-present menace of a silver shortage have triggered the search for alternative imaging layers and systems. New functional requirements in the technology of image registration and information handling moreover have clearly shown the limits of "classical" silver halide photography. New materials, such as thin vacuum-deposited metal films, have been shown to be suitable as substitutes for complex AgX emulsions in such applications as ablative heat-mode recording L 2 for high density information storage. It has previously been shown that simple metallic layers 3 and bismuth films in particular 4 can be made perfectly black rather than mirror like by controlling the deposition parameters. As such, they approach quite closely the typical appearance of a developed AgX material. On the basis of these ideas, new imaging layers have been developed for use in graphic arts and in various reprographic fields. 2. PREPARATION AND PROPERTIES OF THIN BISMUTH LAYERS
2.1. Film deposition technique and choice of material The imaging layer of this film consists of a thin bismuth film with a total coating weight of approximately 0.1mgcm -2. The upper or winding chamber of a differentially pumped double-section roll-coater is used to degas and clean the * Paper presented at the Fifth International Thin Films Congress, Herzlia-on-Sea, Israel, September 21-25, 1981.
0040-6090/82/0000-0000/$02.75
© ElsevierSequoia/Printedin The Netherlands
200
P. DE MAAYER, G. VERBEKE, H. SNEYERS
polyester base by moving it through a high energy glow discharge. In the lower section pure bismuth is thermally deposited from a stationary indirectly heated vapour source, the thickness and optical characteristics of the film being controlled through the evaporation rate, the web speed and the system pressure. The material then passes a series of photometers which check that its optical or transmission density is maintained at about 4.5. The choice of bismuth as an optimal material for this particular application has been dictated by a number of considerations such as availability and price, its nontoxic nature, its low melting point and high vapour pressure and further, as will be appreciated later, by its ease of etching in acids and by the occurrence of "black" bismuth. 2.2. Optical characteristics of "coloured" bismuth layers
The properties of particular interest for photographic applications are the transmission density D t and the reflection density D,, which are defined as Dt =
log
(1)
= log
(2) where I0, It and lr are the intensities of incident, transmitted and reflected light respectively. Figure 1 demonstrates how, for a given transmission density D t (about 4.75), the reflection density D, is a direct function of the porosity or packing density of the bismuth layer. Data for Dr were taken at a wavelength 2 of 600 nm. The packing density is defined here as the ratio of the film thickness calculated from the known coating weight to the thickness measured directly by means of a diamond stylus profiler (Rank Taylor Hobson Talystep). Widely varying porosities could be induced in the films through careful control of the evaporation rate, the glow discharge power density and the system pressure. The last parameter was used to generate our data, all others being kept constant. 0.9
i
i
0.8
.~ 0.7
~ ° ° ° ~ o Q.
O.e
I 0.5
i I t I I i a I 1.0 SPECULAR REFLECTION DENSITY (D,)
i
1.5
Fig. 1. Specular reflection density Dr at 2 - 6 0 0 n m a s a function ofthefilm packing density for thermally evaporated bismuth layers.
VAPOUR-DEPOSITED
Bi
LAYERS FOR U N C O N V E N T I O N A L P H O T O G R A P H Y
201
Figure 2 shows, again for a constant D t , how the spectral distribution of the specular reflection density can be altered from a perfect black (curve a) to a goldbrown (curve d) by a simple decrease in the partial pressure in the evaporation chamber. Eventually at a low deposition rate and/or a low system pressure films with metallic lustre can be generated (curve e). The transmission density D t was kept at a practical level for photographic purposes at all times. It should be stated, however, that this is also a function of morphology and light scattering in the film and of course of film thickness and coating weight.
"1 L_
i i26 '
J /
i
400
450
500 550 WAVELENGTH
600 ~. (nm)
650
Fig. 2. S p e c u l a r reflection densities D r as a function of the w a v e l e n g t h at the c o n s t a n t t r a n s m i s s i o n density of D t = 4.75: c u r v e a, p = 4 × 10-1 P a ; c u r v e b, p = 1.5 × 10-1 Pa; c u r v e c, p = 2 x 1 0 - 2 P a ; c u r v e d , p = 3 x 1 0 - 3 P a ; c u r v e e , p = 8 × 1 0 - 5 Pa.
2.3. Imageformation andphotographicperformance After the initial deposition of bismuth, a suitable photoresist is applied in order to "sensitize" the film. Exposure to UV radiation through a transparent original results in an image-wise differentiation of alkali solubility of the resist, which is either positive or negative depending on the photoresist system chemistry used. The metal film which appears after development is then etched away, thus forming a high contrast image. The black surface of the imaging layer, which makes it an almost perfect antihalation layer, and the thinness of both the metal film (approximately 1500/~ at Dt = 4.75 and Dr = 1.2) and the non-scattering photoresist (1.5 rtm) result in a modulation transfer with useable response up to more than 500 line pairs m m - 1 (Fig. 3). The modulation transfer function (MTF) is given in terms of frequency v by
Mr(v) (3) mo(v) where Mo(v) and Mr(v)are the ratios of the signal amplitude to its average value for MTF(v) -
an original sinusoidal signal and its reproduction at a given frequency v.
202
P. DE MAAYER,G. VERBEKE,H. SNEYERS
Finally, the size of the dots in screened originals can be reduced without loss of density in the remaining portion (Fig. 4), something which c a n n o t be achieved with silver halide films. This is possible because the presence of the photoresist causes the etching agent to undercut, preventing its access to the central part and allowing accurate dot size reduction.
td
W
•
k
LL I.- I0
L [
.......
i;2
.......
LINE PAIRS/turn
;°3
(a)
|
,
1
i
I
(b)
Fig. 3. Modulation transfer functions for negative (- - -) and positive ( ) working versions. Fig. 4. Dot size reduction by controlled undercutting from (a) a dot covering 40% of the surface area to (b) a dot covering 12%. 3. CONCLUDING REMARKS By extending the state of the art of photolithographic techniques and by controlling the spectral dependence of the specular reflection density of thermally evaporated pure bismuth layers, a new silverless p h o t o g r a p h i c film for professional use was developed. Its high intrinsic acuity and the limitation in undercutting to less than 1 ~tm by current chemistry renders these metal/photoresist composite films in m a n y aspects far superior to other films now on the market for copying and duplicating of screen work. REFERENCES 1 D. Maydan, BellSyst. Tech. J.,50(1971) 1761. 2 R.A. Bartolini, H. A. Weakliem and B. F. Williams, Opt. Eng., 15 (1976) 99. 3 Y. Mizushima, Z. Naturforsch., 16a (1961) 1260. 4 L. Fritsche, F. Wolf and A. Schaber, Z. Natur/brsch., 16a (1961) 31.
199
ELECTRONICS AND OPTICS
VAPOUR-DEPOSITED BISMUTH LAYERS F O R U N C O N V E N T I O N A L PHOTOGRAPHY* P. DE MAAYER, G. VERBEKE AND H. SNEYERS
Research Laboratories, Agfa-Gevaert, Mortsel (Belgium) (Received August 14, 1981 ; accepted September 22, 1981)
The photolithographic principle and know-how of controlling the optical characteristics of thermally evaporated pure bismuth layers have been combined in the development of a new silverless metal/photoresist composite film for professional photography. A correlation exists between the spectral distribution of the specular reflection density of these films and the packing density of the evaporated layers. The photographic performance profile is also discussed.
1. INTRODUCTION
The instability of the silver price and the ever-present menace of a silver shortage have triggered the search for alternative imaging layers and systems. New functional requirements in the technology of image registration and information handling moreover have clearly shown the limits of "classical" silver halide photography. New materials, such as thin vacuum-deposited metal films, have been shown to be suitable as substitutes for complex AgX emulsions in such applications as ablative heat-mode recording L 2 for high density information storage. It has previously been shown that simple metallic layers 3 and bismuth films in particular 4 can be made perfectly black rather than mirror like by controlling the deposition parameters. As such, they approach quite closely the typical appearance of a developed AgX material. On the basis of these ideas, new imaging layers have been developed for use in graphic arts and in various reprographic fields. 2. PREPARATION AND PROPERTIES OF THIN BISMUTH LAYERS
2.1. Film deposition technique and choice of material The imaging layer of this film consists of a thin bismuth film with a total coating weight of approximately 0.1mgcm -2. The upper or winding chamber of a differentially pumped double-section roll-coater is used to degas and clean the * Paper presented at the Fifth International Thin Films Congress, Herzlia-on-Sea, Israel, September 21-25, 1981.
0040-6090/82/0000-0000/$02.75
© ElsevierSequoia/Printedin The Netherlands
200
P. DE MAAYER, G. VERBEKE, H. SNEYERS
polyester base by moving it through a high energy glow discharge. In the lower section pure bismuth is thermally deposited from a stationary indirectly heated vapour source, the thickness and optical characteristics of the film being controlled through the evaporation rate, the web speed and the system pressure. The material then passes a series of photometers which check that its optical or transmission density is maintained at about 4.5. The choice of bismuth as an optimal material for this particular application has been dictated by a number of considerations such as availability and price, its nontoxic nature, its low melting point and high vapour pressure and further, as will be appreciated later, by its ease of etching in acids and by the occurrence of "black" bismuth. 2.2. Optical characteristics of "coloured" bismuth layers
The properties of particular interest for photographic applications are the transmission density D t and the reflection density D,, which are defined as Dt =
log
(1)
= log
(2) where I0, It and lr are the intensities of incident, transmitted and reflected light respectively. Figure 1 demonstrates how, for a given transmission density D t (about 4.75), the reflection density D, is a direct function of the porosity or packing density of the bismuth layer. Data for Dr were taken at a wavelength 2 of 600 nm. The packing density is defined here as the ratio of the film thickness calculated from the known coating weight to the thickness measured directly by means of a diamond stylus profiler (Rank Taylor Hobson Talystep). Widely varying porosities could be induced in the films through careful control of the evaporation rate, the glow discharge power density and the system pressure. The last parameter was used to generate our data, all others being kept constant. 0.9
i
i
0.8
.~ 0.7
~ ° ° ° ~ o Q.
O.e
I 0.5
i I t I I i a I 1.0 SPECULAR REFLECTION DENSITY (D,)
i
1.5
Fig. 1. Specular reflection density Dr at 2 - 6 0 0 n m a s a function ofthefilm packing density for thermally evaporated bismuth layers.
VAPOUR-DEPOSITED
Bi
LAYERS FOR U N C O N V E N T I O N A L P H O T O G R A P H Y
201
Figure 2 shows, again for a constant D t , how the spectral distribution of the specular reflection density can be altered from a perfect black (curve a) to a goldbrown (curve d) by a simple decrease in the partial pressure in the evaporation chamber. Eventually at a low deposition rate and/or a low system pressure films with metallic lustre can be generated (curve e). The transmission density D t was kept at a practical level for photographic purposes at all times. It should be stated, however, that this is also a function of morphology and light scattering in the film and of course of film thickness and coating weight.
"1 L_
i i26 '
J /
i
400
450
500 550 WAVELENGTH
600 ~. (nm)
650
Fig. 2. S p e c u l a r reflection densities D r as a function of the w a v e l e n g t h at the c o n s t a n t t r a n s m i s s i o n density of D t = 4.75: c u r v e a, p = 4 × 10-1 P a ; c u r v e b, p = 1.5 × 10-1 Pa; c u r v e c, p = 2 x 1 0 - 2 P a ; c u r v e d , p = 3 x 1 0 - 3 P a ; c u r v e e , p = 8 × 1 0 - 5 Pa.
2.3. Imageformation andphotographicperformance After the initial deposition of bismuth, a suitable photoresist is applied in order to "sensitize" the film. Exposure to UV radiation through a transparent original results in an image-wise differentiation of alkali solubility of the resist, which is either positive or negative depending on the photoresist system chemistry used. The metal film which appears after development is then etched away, thus forming a high contrast image. The black surface of the imaging layer, which makes it an almost perfect antihalation layer, and the thinness of both the metal film (approximately 1500/~ at Dt = 4.75 and Dr = 1.2) and the non-scattering photoresist (1.5 rtm) result in a modulation transfer with useable response up to more than 500 line pairs m m - 1 (Fig. 3). The modulation transfer function (MTF) is given in terms of frequency v by
Mr(v) (3) mo(v) where Mo(v) and Mr(v)are the ratios of the signal amplitude to its average value for MTF(v) -
an original sinusoidal signal and its reproduction at a given frequency v.
202
P. DE MAAYER,G. VERBEKE,H. SNEYERS
Finally, the size of the dots in screened originals can be reduced without loss of density in the remaining portion (Fig. 4), something which c a n n o t be achieved with silver halide films. This is possible because the presence of the photoresist causes the etching agent to undercut, preventing its access to the central part and allowing accurate dot size reduction.
td
W
•
k
LL I.- I0
L [
.......
i;2
.......
LINE PAIRS/turn
;°3
(a)
|
,
1
i
I
(b)
Fig. 3. Modulation transfer functions for negative (- - -) and positive ( ) working versions. Fig. 4. Dot size reduction by controlled undercutting from (a) a dot covering 40% of the surface area to (b) a dot covering 12%. 3. CONCLUDING REMARKS By extending the state of the art of photolithographic techniques and by controlling the spectral dependence of the specular reflection density of thermally evaporated pure bismuth layers, a new silverless p h o t o g r a p h i c film for professional use was developed. Its high intrinsic acuity and the limitation in undercutting to less than 1 ~tm by current chemistry renders these metal/photoresist composite films in m a n y aspects far superior to other films now on the market for copying and duplicating of screen work. REFERENCES 1 D. Maydan, BellSyst. Tech. J.,50(1971) 1761. 2 R.A. Bartolini, H. A. Weakliem and B. F. Williams, Opt. Eng., 15 (1976) 99. 3 Y. Mizushima, Z. Naturforsch., 16a (1961) 1260. 4 L. Fritsche, F. Wolf and A. Schaber, Z. Natur/brsch., 16a (1961) 31.