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Plastic substrates for thin film electronic devices
PLASTIC
SUBSTRATES
FOR THIN FILM ELECTRONIC
DEVICES
0. J. CAIN Dow Corning Corporation, (Received
March
Hemlock,
Mich. 48626 (U.S.A.)
28, 1968; in revised
form
August
19, 1968)
SUMMARY
Certain high temperature plastics are presently being used for substrates for solar cells and their use as substrates may be extended to other thin film devices. For this reason, it is important to determine the effect of various substrates on the semiconductor film properties. In this study, polyimide plastic substrates were compared to metal and glass substrates by observing the nucleation, grain size, surface characteristics, crystal orientation and carrier mobility for vacuum deposited cadmium sulfide thin films. INTRODUCTION
Plastics are showing promise as substrates for some electronic devices, particularly since plastics that can withstand high temperatures (200” to 500 “C) have become available. A case in point is the use of polyimide plastic film as substrates for thin film solar cells that are now under development’. Such features as spectral response, chemical stability, light weight, and flexibility, as well as the thermal stability and low cost, generate interest in these materials for such applications. Some of these features indicate advantages for use also as substrates for thin film transistors and thin film circuits. The effect of the substrate on the semiconductor films is an important consideration in such active devices. This study was directed at finding the role played by the plastic substrate on semiconductor properties such as characteristics of nucleation, growth rate in the early stages of deposition, grain size, general appearance, and degree of orientation. To accomplish this, cadmium sulfide was vacuum evaporated onto the substrate at various temperatures and the films were annealed at elevated temperatures. Their properties were then compared with films grown on more conventional substrates such as molybdenum and Pyrex. FILM GROWTH
AND ANNEAL
The process was accomplished in an Edwards 19E Vacuum Coating System Thin Solid Films, 2 (1968) 479-486
- Elsevier,
Lausanne
- Printed
in the Netherlands
480 at a residual
0. J. CAIN
pressure
of 2-8 x lo- 6 torr. The material
was evaporated
at a tem-
perature of about 1100 “C, at a rate of 0. l-l pm/minute onto substrates that were heated to between 25 and 460 “C. The substrates were heated by the standard substrate heaters provided in the microcircuit jig in the 19E evaporator. The temperature was monitored by an iron-constantan thermocouple mounted in the heater. Comparison of this temperature and the substrate itself showed good correlation after 20 minutes of heating time. The substrates were allowed to heat for this length of time before deposition was started. The CdS used in these studies was high purity, laboratory prepared material or General Electric luminescent grade that had been vacuum outgassed at 700 “C. Comparable films were deposited from both sources. After deposition, some of the samples were annealed in air for periods of 1, 5, and 25 hours at temperatures of 400 and 500 “C. The films were then investigated by optical and electron microscopy, electron diffraction, and by electrical measurements to determine film characteristics. The substrate temperature has a strong bearing on the properties of the deposited film. CdS dissociates’ during evaporation; and, of course, must reassociate at the deposition surface. If the substrate is cool, reassociation is not complete. The vapor pressure of sulfur at a given temperature is much greater than that of cadmium so the resulting material is cadmium rich. On the other hand, when the substrate is in a large volume vacuum region and is heated above 500”, the material re-evaporates and no net deposition takes place. EARLY STAGES OF FILM GROWTH
A limited study was carried out depositing CdS for one minute or less under the conditions described above with a substrate temperature of 200 “C. Figure 1 shows optical micrographs comparing early stages of growth on Pyrex and polyimide substrates. The polyimide films have many lines criss-crossing the surface which are about one micron in width. The source of these lines is unknown, but weTpresume that they arise in the manufacture of the plastic film. Before
Fig. 1. Early stages of growth for CdS deposited on (a) polyimide and (b) Pyrex (2 ym/division). Thin Solid Films, 2 (1968) 479-486
PLASTIC SUBSTRATES FOR THIN FILM ELECTRONIC DEVICES
Fig. 2. Electron micrographs
481
of thin CdS deposits on (a) molybdenum foil and (b) polyimide.
continuous films are formed, a build-up of material along some of these lines occurs indicating that in some cases they act as strong nucleation sites. On other lines there is no accumulation indicating that they do not form nucleation centers. When fine scratches were made on Pyrex, similar effects were seen. When thicker films are grown, anomalous growth appears on both types of substrates, apparently due to nucleation sites. On the glass, they show up as globules or crystallites of random shape, while on the polyimide film they are more frequent and usually form long narrow ridges of cadmium sulfide. In Fig. 2 electron micrographs of thin films that are 0.5 and 0.3 pm thick, respectively, for the molybdenum and polyimide substrates are shown. The electron micrographs are shadowed carbon replicas of the top surface of the films. Germanium was used for shadowing. Because of their thickness only a limited amount of information on nucleation is revealed, but nucleation site density can be inferred from the size and number of grains since they are a function of the original number of nucleation sites. The situation for Pyrex falls between molybdenum and polyimide. It is apparent from this that the more crystalline metal substrate has a greatly increased number of effective nucleation sites with the less ordered glass having fewer and the plastic having the least. This is borne out further in the thicker films as we shall see later. CHARACTERISTICS OF
1-12 MICRON THICK FILMS
As the films are grown thicker, the grains also grow in size, as one might expect. In fact, the grain size holds nearly a one to one correlation with the film thickness with the size being almost equal to the thickness, at least for the Pyrex and polyimide substrates. Figure 3 shows a comparison of the grain size of a 3.6 pm thick film on glass (3a) and a 5 pm thick film on plastic (3b). These photomicrographs are lacking in clarity, but the grain size is obviously greater in the thicker film, although part of this effect may be due to the substrate as discussed above concerning nucleation and noted below for the thicker films. Thin Solid Films, 2 (1968) 479486
Fig. 3. Comparison division).
of grain
size for deposited
CdS
on (a) Pyrex
and
(b) polyimide
(2pm/
In Fig. 4 electron micrographs are given of 8.7 pm, 10 pm and 8.2 pm thick films on molybdenum, glass and polyimide plastic, respectively. It is important here to note the relative smoothness of the films and the relative grain sizes. The smoothest film with the largest grain size is found on the plastic. These deposits were made at a substrate temperature of 100 “C. This implies that a better quality CdS film can be obtained on plastic than on the other substrates for the conditions of deposition used in these studies.
Fig. 4. Texture
of deposited
CdS on substrate
Thin Solid Films, 2 (1986) 479-486
on (a) molybdenum,
(b) Pyrex and (c) polyimide.
PLASTIC SUBSTRATES FOR THIN FILM ELECTRONIC DEVICES
Fig. 5. Electron micrographs
483
of CdS films deposited at (a) 100 “C and (b) 400 “C.
In addition to substrate influence, the substrate temperature has a strong influence on the grain size. Figure 5 illustrates this where 5a shows a deposition at a substrate temperature of 100” and the film shown in 5b is deposited at 400 “C The film deposited at the higher temperature is much smoother and is made up of larger grains Electron diffraction patterns also reveal differences in the film structure but this shows most distinctly in temperature and thickness effects rather than in the type of substrate on which the film was deposited. The 0.3 pm thin film deposited at 200 “C (Fig. 6) indicates some degree of orientation. Figure 7 shows the pattern for an 8.2 /lrn thick film deposited at 100 “C. In this case the film is well oriented in the c-axis but the grains are randomly oriented with respect to minor axes.
Fig. 6. Electron diffraction pattern of 0.3 ,um CdS tilm deposited at 200 “C. Thin Solid Films, 2 (1968) 479-486
484
0. J. CAIN
Fig. 7. Electron diffraction pattern of 8.2 ,um CdS film deposited at 100 “C.
Figure 8 is a pattern of a 400 “C, 12.9 pm film. Here, as i ndicated by 1the spots, the grains are becoming oriented in all directions and it is beginning to look like a single crystal film, as indicated by the diffraction pat tern, in spite of the fact that there are grain boundaries. The films on metal and glass substra tes behaved much like those on plastic as far as diffraction patterns i are concern ed, indicating these substrates had no great bearing on orientation, bu t only on crys ,tal
Fig. 8. Electron diffraction pattern of 12.9 ,um CdS film deposited at 400 “C. Thin Solid Films, 2 (1968) 479486
PLASTIC SUBSTRATES FOR THIN FILM ELECTRONIC DEVICES
485
size and film texture. In most of the substrates the basal plane is tilted about 12” to that of the substrates. This is due to a 12” angle of evaporation onto the substrate due to the location of the boat with respect to the substrate. In the higher temperature deposits the angle between the basal plane and the plane of the substrate approaches zero in spite of the 12” evaporation angle.
ANNEALING EFFECTS
Annealing or heat treatment is often desirable in thin film work. In CdS solar cells for example, it is used to complete a reaction between a plated copper layer and sulfur on the CdS surface in order to create a junction. In other cases it is used for the diffusion of impurities into the film or for recrystallization of the film in order to increase grain size or improve other properties. When the films were annealed at 400-500 “C for periods up to 5 hours, the background film looked much like that seen in Fig. 5b. However, these films were annealed in air and apparently cadmium oxide precipitated on the surface. If either time or temperature relations are increased under these conditions, the plastic substrate decomposes. Small cubic crystals are seen in Fig. 9. Shalimova et aL3 have identified Cd0 under similar annealing conditions. No diffraction patterns were made of this, particular film so the presence of CdO, was not confir,med by that technique. In’reference to Shalimova’s work, it should be stated that the presence of cubic CdS was never detected in any of our deposits in contrast to his results.
Fig. 9. Electron micrograph
of CdS film annealed in air at 400 “C for 1 hour.
Thin Solid Films, 2 (1968) 479486
486 ELECTRICAL
0.
J. CAIN
PROPERTIES
Carrier mobility in a semiconducting layer can be used as a measure of the quality of the film, at least if the layer is thick enough so that scattering from the surfaces does not predominate. This can be obtained from electrical resistivity and Hall measurements. These measurements were made by cutting out about 1 cm’ areas, ultrasonicahy soldering indium contacts to the edges and then measuring them using the van der Pauw technique4. Table I gives results of such measurements for only a few of the samples with the thicker films on both plastic and glass. Based on these few measurements, there is no appreciable difference between the mobility on plastic (sample 4P) and that on the rest, which are on glass. TABLE 1 , _~.
Specimen
~~_ Resistivity (ohm cm!
Hall Mobility (cm2/v set)
4P
26
94G
360
1.5
95G
860
3.0
80G
a2
1.5
1.2
CONCLUSIONS
Semiconductor films such as cadmium sulfide can be grown on plastic film with at least comparable properties to those on glass or metal substrates. The irregularities in the surface of the plastic play a more significant role in the film characteristic because of lower number of the usual nucleation sites, but these apparently do not drastically effect even large area devices such as solar cells. The maximum temperature for long-term annealing in air is probably 400” for polyimide films. The electrical properties of the semiconductor films grown on plastic are also comparable to those on glass. ACKNOWLEDGEMENTS
I would like to acknowledge the efforts of Uldis Plies who prepared the electron micrographs and did the electron diffraction work, and of Ronald Schultz who assisted in the film growth. REFERENCES
1 Photovoltaic
Specialists Confkrence, NASA-Goddard Space Flight Center, Greenbelt, Maryland, 1965. F. A. KROGER, The Chemistry of Imperfect Crystals. Wiley, New York, 1964. K.V. SHALIMOVA,A.F.ANDRUSHKO,V. A. DIMITRIEVANDL.V.PARLOV,SOV.Phys. Cryst., 9 (1964) 340. L. J. VAN DER PAUW, Philips Tech. Rev., 20 (195819) 220. October,
2 3
4
Thin Solid Films, 2 (1968) 419486
SUBSTRATES
FOR THIN FILM ELECTRONIC
DEVICES
0. J. CAIN Dow Corning Corporation, (Received
March
Hemlock,
Mich. 48626 (U.S.A.)
28, 1968; in revised
form
August
19, 1968)
SUMMARY
Certain high temperature plastics are presently being used for substrates for solar cells and their use as substrates may be extended to other thin film devices. For this reason, it is important to determine the effect of various substrates on the semiconductor film properties. In this study, polyimide plastic substrates were compared to metal and glass substrates by observing the nucleation, grain size, surface characteristics, crystal orientation and carrier mobility for vacuum deposited cadmium sulfide thin films. INTRODUCTION
Plastics are showing promise as substrates for some electronic devices, particularly since plastics that can withstand high temperatures (200” to 500 “C) have become available. A case in point is the use of polyimide plastic film as substrates for thin film solar cells that are now under development’. Such features as spectral response, chemical stability, light weight, and flexibility, as well as the thermal stability and low cost, generate interest in these materials for such applications. Some of these features indicate advantages for use also as substrates for thin film transistors and thin film circuits. The effect of the substrate on the semiconductor films is an important consideration in such active devices. This study was directed at finding the role played by the plastic substrate on semiconductor properties such as characteristics of nucleation, growth rate in the early stages of deposition, grain size, general appearance, and degree of orientation. To accomplish this, cadmium sulfide was vacuum evaporated onto the substrate at various temperatures and the films were annealed at elevated temperatures. Their properties were then compared with films grown on more conventional substrates such as molybdenum and Pyrex. FILM GROWTH
AND ANNEAL
The process was accomplished in an Edwards 19E Vacuum Coating System Thin Solid Films, 2 (1968) 479-486
- Elsevier,
Lausanne
- Printed
in the Netherlands
480 at a residual
0. J. CAIN
pressure
of 2-8 x lo- 6 torr. The material
was evaporated
at a tem-
perature of about 1100 “C, at a rate of 0. l-l pm/minute onto substrates that were heated to between 25 and 460 “C. The substrates were heated by the standard substrate heaters provided in the microcircuit jig in the 19E evaporator. The temperature was monitored by an iron-constantan thermocouple mounted in the heater. Comparison of this temperature and the substrate itself showed good correlation after 20 minutes of heating time. The substrates were allowed to heat for this length of time before deposition was started. The CdS used in these studies was high purity, laboratory prepared material or General Electric luminescent grade that had been vacuum outgassed at 700 “C. Comparable films were deposited from both sources. After deposition, some of the samples were annealed in air for periods of 1, 5, and 25 hours at temperatures of 400 and 500 “C. The films were then investigated by optical and electron microscopy, electron diffraction, and by electrical measurements to determine film characteristics. The substrate temperature has a strong bearing on the properties of the deposited film. CdS dissociates’ during evaporation; and, of course, must reassociate at the deposition surface. If the substrate is cool, reassociation is not complete. The vapor pressure of sulfur at a given temperature is much greater than that of cadmium so the resulting material is cadmium rich. On the other hand, when the substrate is in a large volume vacuum region and is heated above 500”, the material re-evaporates and no net deposition takes place. EARLY STAGES OF FILM GROWTH
A limited study was carried out depositing CdS for one minute or less under the conditions described above with a substrate temperature of 200 “C. Figure 1 shows optical micrographs comparing early stages of growth on Pyrex and polyimide substrates. The polyimide films have many lines criss-crossing the surface which are about one micron in width. The source of these lines is unknown, but weTpresume that they arise in the manufacture of the plastic film. Before
Fig. 1. Early stages of growth for CdS deposited on (a) polyimide and (b) Pyrex (2 ym/division). Thin Solid Films, 2 (1968) 479-486
PLASTIC SUBSTRATES FOR THIN FILM ELECTRONIC DEVICES
Fig. 2. Electron micrographs
481
of thin CdS deposits on (a) molybdenum foil and (b) polyimide.
continuous films are formed, a build-up of material along some of these lines occurs indicating that in some cases they act as strong nucleation sites. On other lines there is no accumulation indicating that they do not form nucleation centers. When fine scratches were made on Pyrex, similar effects were seen. When thicker films are grown, anomalous growth appears on both types of substrates, apparently due to nucleation sites. On the glass, they show up as globules or crystallites of random shape, while on the polyimide film they are more frequent and usually form long narrow ridges of cadmium sulfide. In Fig. 2 electron micrographs of thin films that are 0.5 and 0.3 pm thick, respectively, for the molybdenum and polyimide substrates are shown. The electron micrographs are shadowed carbon replicas of the top surface of the films. Germanium was used for shadowing. Because of their thickness only a limited amount of information on nucleation is revealed, but nucleation site density can be inferred from the size and number of grains since they are a function of the original number of nucleation sites. The situation for Pyrex falls between molybdenum and polyimide. It is apparent from this that the more crystalline metal substrate has a greatly increased number of effective nucleation sites with the less ordered glass having fewer and the plastic having the least. This is borne out further in the thicker films as we shall see later. CHARACTERISTICS OF
1-12 MICRON THICK FILMS
As the films are grown thicker, the grains also grow in size, as one might expect. In fact, the grain size holds nearly a one to one correlation with the film thickness with the size being almost equal to the thickness, at least for the Pyrex and polyimide substrates. Figure 3 shows a comparison of the grain size of a 3.6 pm thick film on glass (3a) and a 5 pm thick film on plastic (3b). These photomicrographs are lacking in clarity, but the grain size is obviously greater in the thicker film, although part of this effect may be due to the substrate as discussed above concerning nucleation and noted below for the thicker films. Thin Solid Films, 2 (1968) 479486
Fig. 3. Comparison division).
of grain
size for deposited
CdS
on (a) Pyrex
and
(b) polyimide
(2pm/
In Fig. 4 electron micrographs are given of 8.7 pm, 10 pm and 8.2 pm thick films on molybdenum, glass and polyimide plastic, respectively. It is important here to note the relative smoothness of the films and the relative grain sizes. The smoothest film with the largest grain size is found on the plastic. These deposits were made at a substrate temperature of 100 “C. This implies that a better quality CdS film can be obtained on plastic than on the other substrates for the conditions of deposition used in these studies.
Fig. 4. Texture
of deposited
CdS on substrate
Thin Solid Films, 2 (1986) 479-486
on (a) molybdenum,
(b) Pyrex and (c) polyimide.
PLASTIC SUBSTRATES FOR THIN FILM ELECTRONIC DEVICES
Fig. 5. Electron micrographs
483
of CdS films deposited at (a) 100 “C and (b) 400 “C.
In addition to substrate influence, the substrate temperature has a strong influence on the grain size. Figure 5 illustrates this where 5a shows a deposition at a substrate temperature of 100” and the film shown in 5b is deposited at 400 “C The film deposited at the higher temperature is much smoother and is made up of larger grains Electron diffraction patterns also reveal differences in the film structure but this shows most distinctly in temperature and thickness effects rather than in the type of substrate on which the film was deposited. The 0.3 pm thin film deposited at 200 “C (Fig. 6) indicates some degree of orientation. Figure 7 shows the pattern for an 8.2 /lrn thick film deposited at 100 “C. In this case the film is well oriented in the c-axis but the grains are randomly oriented with respect to minor axes.
Fig. 6. Electron diffraction pattern of 0.3 ,um CdS tilm deposited at 200 “C. Thin Solid Films, 2 (1968) 479-486
484
0. J. CAIN
Fig. 7. Electron diffraction pattern of 8.2 ,um CdS film deposited at 100 “C.
Figure 8 is a pattern of a 400 “C, 12.9 pm film. Here, as i ndicated by 1the spots, the grains are becoming oriented in all directions and it is beginning to look like a single crystal film, as indicated by the diffraction pat tern, in spite of the fact that there are grain boundaries. The films on metal and glass substra tes behaved much like those on plastic as far as diffraction patterns i are concern ed, indicating these substrates had no great bearing on orientation, bu t only on crys ,tal
Fig. 8. Electron diffraction pattern of 12.9 ,um CdS film deposited at 400 “C. Thin Solid Films, 2 (1968) 479486
PLASTIC SUBSTRATES FOR THIN FILM ELECTRONIC DEVICES
485
size and film texture. In most of the substrates the basal plane is tilted about 12” to that of the substrates. This is due to a 12” angle of evaporation onto the substrate due to the location of the boat with respect to the substrate. In the higher temperature deposits the angle between the basal plane and the plane of the substrate approaches zero in spite of the 12” evaporation angle.
ANNEALING EFFECTS
Annealing or heat treatment is often desirable in thin film work. In CdS solar cells for example, it is used to complete a reaction between a plated copper layer and sulfur on the CdS surface in order to create a junction. In other cases it is used for the diffusion of impurities into the film or for recrystallization of the film in order to increase grain size or improve other properties. When the films were annealed at 400-500 “C for periods up to 5 hours, the background film looked much like that seen in Fig. 5b. However, these films were annealed in air and apparently cadmium oxide precipitated on the surface. If either time or temperature relations are increased under these conditions, the plastic substrate decomposes. Small cubic crystals are seen in Fig. 9. Shalimova et aL3 have identified Cd0 under similar annealing conditions. No diffraction patterns were made of this, particular film so the presence of CdO, was not confir,med by that technique. In’reference to Shalimova’s work, it should be stated that the presence of cubic CdS was never detected in any of our deposits in contrast to his results.
Fig. 9. Electron micrograph
of CdS film annealed in air at 400 “C for 1 hour.
Thin Solid Films, 2 (1968) 479486
486 ELECTRICAL
0.
J. CAIN
PROPERTIES
Carrier mobility in a semiconducting layer can be used as a measure of the quality of the film, at least if the layer is thick enough so that scattering from the surfaces does not predominate. This can be obtained from electrical resistivity and Hall measurements. These measurements were made by cutting out about 1 cm’ areas, ultrasonicahy soldering indium contacts to the edges and then measuring them using the van der Pauw technique4. Table I gives results of such measurements for only a few of the samples with the thicker films on both plastic and glass. Based on these few measurements, there is no appreciable difference between the mobility on plastic (sample 4P) and that on the rest, which are on glass. TABLE 1 , _~.
Specimen
~~_ Resistivity (ohm cm!
Hall Mobility (cm2/v set)
4P
26
94G
360
1.5
95G
860
3.0
80G
a2
1.5
1.2
CONCLUSIONS
Semiconductor films such as cadmium sulfide can be grown on plastic film with at least comparable properties to those on glass or metal substrates. The irregularities in the surface of the plastic play a more significant role in the film characteristic because of lower number of the usual nucleation sites, but these apparently do not drastically effect even large area devices such as solar cells. The maximum temperature for long-term annealing in air is probably 400” for polyimide films. The electrical properties of the semiconductor films grown on plastic are also comparable to those on glass. ACKNOWLEDGEMENTS
I would like to acknowledge the efforts of Uldis Plies who prepared the electron micrographs and did the electron diffraction work, and of Ronald Schultz who assisted in the film growth. REFERENCES
1 Photovoltaic
Specialists Confkrence, NASA-Goddard Space Flight Center, Greenbelt, Maryland, 1965. F. A. KROGER, The Chemistry of Imperfect Crystals. Wiley, New York, 1964. K.V. SHALIMOVA,A.F.ANDRUSHKO,V. A. DIMITRIEVANDL.V.PARLOV,SOV.Phys. Cryst., 9 (1964) 340. L. J. VAN DER PAUW, Philips Tech. Rev., 20 (195819) 220. October,
2 3
4
Thin Solid Films, 2 (1968) 419486