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Localised anodic oxide diffusion sources and geometry control of anodic oxide growth on silicon
NOTES Solid-State
Electronics
pp. 631-632.
Pergamon Press 1964. Vol. Printed in Great Britain
7,
Localised anodic oxide diffusion sources and geometry control of anodic oxide growth on silicon (Received
20 January 1964; in veuised form 26 February 1964)
THE use of phosphorus-doped
anodic oxide films as diffusion sources has been described.(l) The work reported here refers to the fabrication of localised diffusion patterns without the use of thermal oxide masks, and to the control of anodic oxide growth on N-type silicon by means of localised illumination. Figure 1 shows a diffused pattern produced without thermal oxide masking. The substrate, 2 Q-cm P-type silicon, was first anodised to 200 V (roughly equivalent to 1000 A.U.) in a solution producing pure SiOa films. [The purity of this anodic SiOs was tested by the absence of a junction on higher resistivity p-Si after heat treatment of an anodised sample.] A photoresist technique was used next and a resolution pattern (the negative of Fig. 1) was etched into the anodic oxide film. Distance between centers of finest lines in Fig. l(a) is 10 ,u. The photoresist was then stripped and the sample reanodised to 150 V in a solution incorporating phoshorus into the anodic oxide. Regrowth of the doped anodic oxide thus occurred in the areas previously etched. Diffusion was carried out in an atmosphere of argon for 30 min at 1175°C. All of the oxide was then stripped in HF and the diffused regions revealed by staining in HF under illumination. It can be seen that the pattern was faithfully reproduced. Figure l(a) shows only the smallest detail of a standard resolution pattern at a magnification of 200 x . The spot in the upper section of the pattern is due to a dust particle having settled on the surface during diffusion. The phosphorus diffusion sources in this instance all had equal strength, but in principle diffusion sources of different strength can be used on the same silicon chip. Figure l(b) shows the next finest detail of the resolution pattern, with the
central region representing the area covered by Fig. l(a). Growth of anodic oxide films on N-type silicon depends on the availability of minority carriers (i.e. holes) at the surface.@) No appreciable oxide growth will occur in complete darkness below the breakdown strength of the silicon/electrolyte surface barrier provided the surface recombinationregeneration velocity is low. Oxide growth can therefore be controlled by localised illumination with light of wavelength shorter than the absorption edge. Figure 2 shows an oxide pattern produced by shining a focussed white light pattern onto the dry bottom surface of a 4 mils thick 30 Q-cm N-type silicon web(a) held in a horizontal position. Anodisation was carried out to 100 V on the top surface, the web forming a natural trough for the electrolyte. The image projection system consisted of an inverted metallurgical microscope having a binocular eyepiece arrangement. The pattern was projected through one of the eyepieces while focus was ascertained by viewing through the other eyepiece. The use of a pair of crossed polarisers, one above each eyepiece, together with a quarter wave plate located near the objective allowed a high contrast image to be observed. In this arrangement the near infrared (0.7-l .l p) portion of the incident radiation contributes most to the anodic current at the top surface. Light of shorter wavelength is absorbed very near the bottom surface and largely lost due to surface recombination. With the optical system employed, the infrared focus was 5 mils farther away from the objective than the visible focus, i.e. very close to the top surface. De-focussing due to refraction in the silicon, and the fact that the two surfaces of the web were not completely parallel to each other(s) may account for some of the distortion still present in Fig. 2. The width of the lines of the ‘W’ is approximately 15-20 mils. Experiments now under way utilise front surface illumination and light from which the near infrared portion has been filtered out. Minority carriers in this case are generated mostly in the space-charge region at the silicon surface, and are swept by the field to the surface without sidewise diffusion.
631
632
NOTES
Acknozcledgements-This work was supported jointly by the Electronics Development Laboratory Aeronautical Systems Division, Wright-Patterson Air Force Base, Dayton, Ohio, and by Westinghouse Electric Corporation. Thanks are due to Dr. T. W. O’KEEFFE for help with the optical system, and to N. RONEY and J. MOYLES for help with the other experiments.
Westinghouse Research and Development Center Piftsburgh, Penn.
P. F. SCHMIDT J. OROSHNIK C. C. HARD~IAN
References 1. P. F. SCHhlIDT and A. E. OWEN, Extended Abstract No. 42, Electronics Division, Pittsburgh, 1963 Spring Meeting of the Electrochemical Society; J. Electrochem. Sot. June (1964). 2. W. H. BRATTAIN and C. G. B. GARRET, Bell Syst. Tech. J. 34, 129 (1955); P. F. SCHMIDT and W. MICHEL, J. Electrochem. Sot. 104, 230 (1957). 3. S. DERMATIS and J. W. FAUST, Inst. Elect. Engrs Trans. Comm. Electron. 82, 94 (1963) ; S. O’HARA, J. Appl. Phys. 35,409 (1963).
Solid-State Electronics pp. 632-633.
Pergamon Press 1964. Printed in Great Britain
Vol. 7,
Generation of phosphorus pentoxide directly in diffusion furnaces (Receiaed
24 January
1964)
PHOSPHORUS pentoxide
(PzOs) has found widespread usage for the production of n-type regions in silicon by diffusion. In a typical process a container of PzOs powder is inserted into a hot portion
RED
P- /
of a furnace tube containing silicon wafers. The evaporated PzOs is carried to the wafers by a stream of inert gas. As any user of this or related processes will testify, it is both messy and difficult to control. Particles of material tend to drop out on the surfaces of the wafers, giving uneven junctions. A sticky mixture deposits in various regions of the diffusion tube. These difficulties are primarily due to the presence of water in the PzO5 creating phosphoric acid which has a much lower vapor pressure. Because PzO5 is one of the best desiccants known, absorption of water is virtually impossible to avoid in at least one stage of its handling. In order to avoid these difficulties, we have generated PsOs directly in the diffusion tube by combustion of red phosphorus, as shown in Fig. l.* The phosphorus is contained in a small tube which can be inserted to various temperature levels of the furnace, thus regulating the rate of combustion. In the present experiments a gas of 100,; 02 and 9096 Nz was used. The gas flow was varied from 1.4 to 5 SCFH, with no apparent effect on the combustion. Weights of red P ranging from 0.09 to 0.74 g were burned at rates varying from 0.0009 to 0.1 g/min. At the lower burning rates no particulate matter was visible in the furnace exhaust. At 0.1 g/min, however, the exhaust w-as a white smoke. In order to test this process an emitter diffusion was made at 1010°C for 70 min. Slow combustion * Patent applied for.
FURNACE
FIG. 1.
Electronics
pp. 631-632.
Pergamon Press 1964. Vol. Printed in Great Britain
7,
Localised anodic oxide diffusion sources and geometry control of anodic oxide growth on silicon (Received
20 January 1964; in veuised form 26 February 1964)
THE use of phosphorus-doped
anodic oxide films as diffusion sources has been described.(l) The work reported here refers to the fabrication of localised diffusion patterns without the use of thermal oxide masks, and to the control of anodic oxide growth on N-type silicon by means of localised illumination. Figure 1 shows a diffused pattern produced without thermal oxide masking. The substrate, 2 Q-cm P-type silicon, was first anodised to 200 V (roughly equivalent to 1000 A.U.) in a solution producing pure SiOa films. [The purity of this anodic SiOs was tested by the absence of a junction on higher resistivity p-Si after heat treatment of an anodised sample.] A photoresist technique was used next and a resolution pattern (the negative of Fig. 1) was etched into the anodic oxide film. Distance between centers of finest lines in Fig. l(a) is 10 ,u. The photoresist was then stripped and the sample reanodised to 150 V in a solution incorporating phoshorus into the anodic oxide. Regrowth of the doped anodic oxide thus occurred in the areas previously etched. Diffusion was carried out in an atmosphere of argon for 30 min at 1175°C. All of the oxide was then stripped in HF and the diffused regions revealed by staining in HF under illumination. It can be seen that the pattern was faithfully reproduced. Figure l(a) shows only the smallest detail of a standard resolution pattern at a magnification of 200 x . The spot in the upper section of the pattern is due to a dust particle having settled on the surface during diffusion. The phosphorus diffusion sources in this instance all had equal strength, but in principle diffusion sources of different strength can be used on the same silicon chip. Figure l(b) shows the next finest detail of the resolution pattern, with the
central region representing the area covered by Fig. l(a). Growth of anodic oxide films on N-type silicon depends on the availability of minority carriers (i.e. holes) at the surface.@) No appreciable oxide growth will occur in complete darkness below the breakdown strength of the silicon/electrolyte surface barrier provided the surface recombinationregeneration velocity is low. Oxide growth can therefore be controlled by localised illumination with light of wavelength shorter than the absorption edge. Figure 2 shows an oxide pattern produced by shining a focussed white light pattern onto the dry bottom surface of a 4 mils thick 30 Q-cm N-type silicon web(a) held in a horizontal position. Anodisation was carried out to 100 V on the top surface, the web forming a natural trough for the electrolyte. The image projection system consisted of an inverted metallurgical microscope having a binocular eyepiece arrangement. The pattern was projected through one of the eyepieces while focus was ascertained by viewing through the other eyepiece. The use of a pair of crossed polarisers, one above each eyepiece, together with a quarter wave plate located near the objective allowed a high contrast image to be observed. In this arrangement the near infrared (0.7-l .l p) portion of the incident radiation contributes most to the anodic current at the top surface. Light of shorter wavelength is absorbed very near the bottom surface and largely lost due to surface recombination. With the optical system employed, the infrared focus was 5 mils farther away from the objective than the visible focus, i.e. very close to the top surface. De-focussing due to refraction in the silicon, and the fact that the two surfaces of the web were not completely parallel to each other(s) may account for some of the distortion still present in Fig. 2. The width of the lines of the ‘W’ is approximately 15-20 mils. Experiments now under way utilise front surface illumination and light from which the near infrared portion has been filtered out. Minority carriers in this case are generated mostly in the space-charge region at the silicon surface, and are swept by the field to the surface without sidewise diffusion.
631
632
NOTES
Acknozcledgements-This work was supported jointly by the Electronics Development Laboratory Aeronautical Systems Division, Wright-Patterson Air Force Base, Dayton, Ohio, and by Westinghouse Electric Corporation. Thanks are due to Dr. T. W. O’KEEFFE for help with the optical system, and to N. RONEY and J. MOYLES for help with the other experiments.
Westinghouse Research and Development Center Piftsburgh, Penn.
P. F. SCHMIDT J. OROSHNIK C. C. HARD~IAN
References 1. P. F. SCHhlIDT and A. E. OWEN, Extended Abstract No. 42, Electronics Division, Pittsburgh, 1963 Spring Meeting of the Electrochemical Society; J. Electrochem. Sot. June (1964). 2. W. H. BRATTAIN and C. G. B. GARRET, Bell Syst. Tech. J. 34, 129 (1955); P. F. SCHMIDT and W. MICHEL, J. Electrochem. Sot. 104, 230 (1957). 3. S. DERMATIS and J. W. FAUST, Inst. Elect. Engrs Trans. Comm. Electron. 82, 94 (1963) ; S. O’HARA, J. Appl. Phys. 35,409 (1963).
Solid-State Electronics pp. 632-633.
Pergamon Press 1964. Printed in Great Britain
Vol. 7,
Generation of phosphorus pentoxide directly in diffusion furnaces (Receiaed
24 January
1964)
PHOSPHORUS pentoxide
(PzOs) has found widespread usage for the production of n-type regions in silicon by diffusion. In a typical process a container of PzOs powder is inserted into a hot portion
RED
P- /
of a furnace tube containing silicon wafers. The evaporated PzOs is carried to the wafers by a stream of inert gas. As any user of this or related processes will testify, it is both messy and difficult to control. Particles of material tend to drop out on the surfaces of the wafers, giving uneven junctions. A sticky mixture deposits in various regions of the diffusion tube. These difficulties are primarily due to the presence of water in the PzO5 creating phosphoric acid which has a much lower vapor pressure. Because PzO5 is one of the best desiccants known, absorption of water is virtually impossible to avoid in at least one stage of its handling. In order to avoid these difficulties, we have generated PsOs directly in the diffusion tube by combustion of red phosphorus, as shown in Fig. l.* The phosphorus is contained in a small tube which can be inserted to various temperature levels of the furnace, thus regulating the rate of combustion. In the present experiments a gas of 100,; 02 and 9096 Nz was used. The gas flow was varied from 1.4 to 5 SCFH, with no apparent effect on the combustion. Weights of red P ranging from 0.09 to 0.74 g were burned at rates varying from 0.0009 to 0.1 g/min. At the lower burning rates no particulate matter was visible in the furnace exhaust. At 0.1 g/min, however, the exhaust w-as a white smoke. In order to test this process an emitter diffusion was made at 1010°C for 70 min. Slow combustion * Patent applied for.
FURNACE
FIG. 1.