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A new CVD technique for the preparation of transparent conducting films
Surface Scicncc X6 :..I North-tlolland
(1Y79)
I’ublisliing
230 -237 (‘ompan>
A NEW CVD TECHNIQUE CONDUCTING FILMS
FOR THE PREPARATION
OF TRANSPARENT
1. Introduction In general, such transparent conductin g tihns as SnO, and in203 are prcparcd either by j~liysi~~ll vapor ~~ep~siti~)ii (PVD), e.g., va~uu~i~ vapor deposition [I ] and [3,3], or by chemical vapor deposition (CVD) [4,S]. The films produced by the PVD method are of an excellent quality and are exclusively used in electronics. On the contrary, the fihns produced by the CVD mctlrod are rather poorer in qualit> than the former ones, particularly in appearance inspite of their better crystai structure and tougher thermd, chemical. and mechanical properties. Thcreforc, they arc seldom used in electronics. Iktwever, the PVD ruethod is at a dis~~~l~~i~t~~~~ in tm,t being ~tblc to supply the films in nmss production to meet the currently greatly increasing demancl. The C’VD method essentially has the ability both of pi-cpration of ;I large film unJ of ~iiass production. I Iencr there is B necessity to iinprovc CVD techniques
so 3s lo niect the great industrial
demml.
Fig. 1. Boundary
0. Tahata et al. /New
CC’D technique
Diftusion
Boundary Layer
1
layer above the sample
substrate
in ordinary
231
CVD.
appears partially or throughout the film. The haze, of course, reduces the optical and electrical characteristics of the films. This work proposes a new CVD technique which is able to produce high quality and haze-free transparent conducting films.
2. Cause of white haze In ordinary CVD processes, a laminar flow of inactive gas entirely envelopes the sample substrate and forms a boundary layer above the sample surface, wherein the gas flow is fairly stagnant, as shown in fig. 1. In this figure, film deposition takes place strongly depending on the diffusion of reactant gases onto the sample surface through the boundary layer, as long as plenty of reactant gases are constantly fed
Fig. 2. Pockmarks scattered on an ordinary removed from the film by a strong ultrasonic
CVD SnOz vibration.
film where flakes were struck
and later
0. Tahata et al. / ~V:ewc’1.D tcchtlirple
232
by the inactive laminar gas flow to the space above the boundary layer and at the same time surface chemical reaction continuously occurs on the sample surface at an appropriate high temperature. These CVD gas dynamics are completely different from that of PVD in which evaporated vapor molecules or sputtered target particles directly hit the sample surface and deposit there. Unfortunately, the boundary layer is heated to a high temperature by the strong irradiation from the sample substrate. Therefore, a portion of the diffusing reactant gases reacts in the space right above the sample before touching the sample surface and this results in the powders or flakes of the films. Subsequently. they fall and stick onto the depositing film. The most prevalent type of haze is caused by such sticking particles produced by an extra space reaction. Fig. 3 shows a number of pockmarks left after removing flakes from the surface of an ordinary CVD SnOz film.
3. Reflecting high speed gas stream (RGS) In this new CVD method, a high speed inactive gas stream of a V-shape, as shown in fig. 3. carries and deposits reacting materials onto a sample surface. Once the inactive gas is ejected at a high speed from a nozzle at an optimum downward angle, it is reflected by the sample surface and escapes upward into the inlet of an exhaust. The boundary layer of this V-shape gas stream is formed covering a small area at the reflecting point on the sample surface. The haze powders or flakes are produced only around this reflecting point and are removed without touching other parts of the surface. Thus, the sample surface is kept clean without contamination by dirty gas containing powders and flakes produced by the space reaction. while film deposition continuously progresses by the fresh reactant gas stream from the nozzle. This is the effect of “the V-shape” of the gas stream on haze elimination. Due to the high speed of the gas stream and the compression force by the downward momentum of the gas stream, the boundary layer becomes extremely thin at the reflecting point where the film is deposited. The decrease in boundary layer thickness directly contributes to the decrease in diffusion time of reactant gases and consequently to the decrease in powder and flake formation. In addition, the
Reflecting Point l:ig. 3. A reflecting
high speed gas stream
ERGS) on the sample surface
in a V-shape
0. Tabata et al. /New
Fig. 4. The reflecting about I .5 m/set.
CVD technique
high speed gas stream (RGS) visualized with smoke. The smoke velocity
233
is
powders or flakes produced around the reflecting point are quickly removed by the high speed of the gas stream before they can deposit onto the sample surface. These are the rewarding effects of “the high speed” of the gas stream on haze elimination. Such a V-shape gas stream is easily formed by balancing to streams: a gas stream ejected from a nozzle and a sucking stream generated by a draft head as shown in fig. 4. Fig. 4 shows a reflecting high speed gas stream (RGS) which is visualized with smoke when its velocity is about 1.5 m/set.
4. Experimental The arrangement of nozzle and draft head for the formation of a reflecting high speed gas stream on the heating disk is shown in fig. 5. The nozzle has a flat bill of 50 mm in width, 30 mm in length, and 0.5 mm in thickness on the inside and is fixed at a height of -30 mm above the heating disk, having an inclination of -60”. Facing this, the rectangular draft head is set in a position such that the lower side of the opening is about 10 mm higher than the heating disk, the opening being bent downward about 45”. The size of the opening is 90 mm X 40 mm. The heating disk is made of quartz and is about 200 mm in diameter and 5 mm thick. The chemical vapor reaction represented below was used for the synthesis of undoped transparent conducting SnO, films: (CH3)2SnCl,
+ O2 + SnOz + 2 CH3Cl .
(1)
A 6 l/min total flow of reactant gases is ejected from the flat bill and forms a beltlike reflecting gas stream of about 50 mm in width on the glass substrate placed on the quartz heating disk. The gas stream flows 1 to 5 m/set in reality. The composi-
234
Fig. 5. Schematic
representation
of the laboratory
type RGSCVD
apparatus
tested hcrc.
tion of the ejected high speed gas is 4 l/min Ar as carrier gas. 1 limin Ar as pickup gas of a (CH3)2SnC1, vapor at 100°C. and 1 l/min 02. The heating disk rotates at rates of 5 to 10 rpm while heating the glass substrates to around 500°C. The SnOz film deposition experiments were carried out successively at the reflecting point.
5. Results
In this experiment, a typical growth rate of SnOz films is about 1500 A/min at a deposition temperature of 500°C. The optical transparency of over 90% of the Ghn up to a film thickness of 5000 A is substantially high and white haze is not visible as shown in fig. 6. Fig. 6 compares film appearance in PVD (a) and in this RGS-CVD (b) together with the striking white haze in ordinary CVD (c). Combined with an integration sphere, a spectrophotometer provides tninute evaluation
Fig. 6. Appearances of In203 film (a) by PVD and of SnO2 film (b) by this RGSCVD with the striking white haze of SnOz film (c) by ordinary CVD.
together
0. Tabata et al. /New
0
Fig. 7. Logarithmic
dependence
2 4 Thickness
of the haze factor
CVD technique
6
235
8 10 xld( % )
of CVD
SnOz films on their thicknesses: (a) and (c) SnOl films by ordinary CVD. light of 550 nm.
an InzOa film by PVD, (b) SnOz films by RGS-CVD, The haze was measured
by monochromatic
of haze as shown in fig. 7. The white haze caused by film structure, not by the sticking of powders or flakes, gradually increases with film thickness. This tendency appears both in ordinary CVD films and in this RGS-CVD films as seen in the lines (b) and (c). However, while ordinary CVD films have large haze factors of 4 to 10% even in the range of film thickness of 1000 to 3000 8, the haze factor of RGS-CVD ftims remains below 1% in film thicknesses up to about 5000 ,& after deducting approximately 0.55% as the value of glass substrate itself, and it tightly approaches
b
Fig. 8. Surface structures of a PVD In203 by scanning electron microscopy.
film (a) and a RGSCVD
SnOz
film (b) as revealed
the haze factor of PVD films at around a thickness of 1600 A, being still higher by several tenths of a percent. A scanning electron microscopy revealed the close similarity in their surface structure as shown in fig. 8.
6. Discussions
From figs. 6, 7. and 8, it is evident that the undoped hate-free transparent conducting films. being as good as those produced by I’VD, are prepared in the atmosphere. not in a vacuum. This means that the reflecting high speed gas stream successfully effects the elimination of obstinate white haze by decreasing both the existing area of boundary layer on a sample surface and the thickness of the layer. In ordinary CVD. the deposition of films progresses through such a boundary layer as described in section 2. The average thickness of a boundary layer, I?, over the whole sample substrate can be easily derived by referring to ref. [6] as follows: 15= 3.1 (UL/U)1’2 ,
(1)
where u is the kinematic viscosity of the gas, L the length of the sample substrate, and U the velocity of the gas flow. Under usual conditions of ordinary CVD, ;i is calculated to be of the order of unit centimeter by eq. (2). For example, when v = 0.705 cm*/sec at 500°C for Ar, L = 10 cm, CI = 10 cm/set, and 8 = 2.6 cm. In RGS-CVD, the length of a reflecting gas stream in the forward flowing direction at the reflecting point is about unit centimeter and the velocity of the gas is about 1 .5 m/set in this experiment. Thus 6 is estimated to be only 2 mm. Such a small value of 6 corresponds to l/10 or less of that of ordinary CVD. Consequently, the diffusion time of reactant gases in the boundary layer is also decreased to l/IO or less and it is expected to decrease haze generation right next to the sample surface, to the same order as the decreased diffusion time. Hence it could be suggested that the haze reduction of roughly 10%’ between the curves (b) and (c) in fig. 7 results from the decreasing rate of boundary layer thickness of about I/IO. 6.2. Nozzle
chzracteristics
The flat bill nozzle used here performed satisfactorily producing a beltlike gas jet stream. In this experiment, the critical haze velocity at which haze vanishes from films appeared to be in the vicinity of 1 ~/SW. The critical uniform velocity which makes the velocity distribution uniform over a uniformity of 90%~is about 1 m/set. Such a uniform velocity distribution is necessary for obtaining the uniformity of film thickness. Thus the use of a long slit nozzle enables one to produce a large film and a number of films successively.
0. Tahata et al. / Nc~v CI’D tcchrkpre
231
7. Conclusions (1) A reflecting high speed gas stream (RGS) provides haze-free transparent conducting films, as good as those produced by the PVD method, in open air, not necessitating a vacuum. (2) Undoped transparent conducting Sn02 films produced by the RGS-CVD method possess a transparency of over 90% and a haze factor of less than 1%~.
References [ 11 [2] [3] [4]
J.C. Manifacicr, M. dc Murcia and J.P. Fillard, Thin Solid Films 41 (1977) 127. A.G. Sabnis and L.D. F&cl, J. Vacuum Sci. Tcchnol. 14 (1977) 685. D.B. Fraser and H.D. Cook, J. Electrochem. Sot. 119 (1972) 1368. 0. Tabata, in: Proc. 5th. Intern. Conf. CVD (Electrochemical Society, Inc., Princeton, 1975) 681. [S] R.N. Ghoshtagorc, J. Elcctrochcm. Sot. 125 (1978) 110. 161 E.R.G. Eckcrt, Heat and hlnss Transfer, 2nd cd. (McGraw-Hill, New York. 1959) 140.
(1Y79)
I’ublisliing
230 -237 (‘ompan>
A NEW CVD TECHNIQUE CONDUCTING FILMS
FOR THE PREPARATION
OF TRANSPARENT
1. Introduction In general, such transparent conductin g tihns as SnO, and in203 are prcparcd either by j~liysi~~ll vapor ~~ep~siti~)ii (PVD), e.g., va~uu~i~ vapor deposition [I ] and [3,3], or by chemical vapor deposition (CVD) [4,S]. The films produced by the PVD method are of an excellent quality and are exclusively used in electronics. On the contrary, the fihns produced by the CVD mctlrod are rather poorer in qualit> than the former ones, particularly in appearance inspite of their better crystai structure and tougher thermd, chemical. and mechanical properties. Thcreforc, they arc seldom used in electronics. Iktwever, the PVD ruethod is at a dis~~~l~~i~t~~~~ in tm,t being ~tblc to supply the films in nmss production to meet the currently greatly increasing demancl. The C’VD method essentially has the ability both of pi-cpration of ;I large film unJ of ~iiass production. I Iencr there is B necessity to iinprovc CVD techniques
so 3s lo niect the great industrial
demml.
Fig. 1. Boundary
0. Tahata et al. /New
CC’D technique
Diftusion
Boundary Layer
1
layer above the sample
substrate
in ordinary
231
CVD.
appears partially or throughout the film. The haze, of course, reduces the optical and electrical characteristics of the films. This work proposes a new CVD technique which is able to produce high quality and haze-free transparent conducting films.
2. Cause of white haze In ordinary CVD processes, a laminar flow of inactive gas entirely envelopes the sample substrate and forms a boundary layer above the sample surface, wherein the gas flow is fairly stagnant, as shown in fig. 1. In this figure, film deposition takes place strongly depending on the diffusion of reactant gases onto the sample surface through the boundary layer, as long as plenty of reactant gases are constantly fed
Fig. 2. Pockmarks scattered on an ordinary removed from the film by a strong ultrasonic
CVD SnOz vibration.
film where flakes were struck
and later
0. Tahata et al. / ~V:ewc’1.D tcchtlirple
232
by the inactive laminar gas flow to the space above the boundary layer and at the same time surface chemical reaction continuously occurs on the sample surface at an appropriate high temperature. These CVD gas dynamics are completely different from that of PVD in which evaporated vapor molecules or sputtered target particles directly hit the sample surface and deposit there. Unfortunately, the boundary layer is heated to a high temperature by the strong irradiation from the sample substrate. Therefore, a portion of the diffusing reactant gases reacts in the space right above the sample before touching the sample surface and this results in the powders or flakes of the films. Subsequently. they fall and stick onto the depositing film. The most prevalent type of haze is caused by such sticking particles produced by an extra space reaction. Fig. 3 shows a number of pockmarks left after removing flakes from the surface of an ordinary CVD SnOz film.
3. Reflecting high speed gas stream (RGS) In this new CVD method, a high speed inactive gas stream of a V-shape, as shown in fig. 3. carries and deposits reacting materials onto a sample surface. Once the inactive gas is ejected at a high speed from a nozzle at an optimum downward angle, it is reflected by the sample surface and escapes upward into the inlet of an exhaust. The boundary layer of this V-shape gas stream is formed covering a small area at the reflecting point on the sample surface. The haze powders or flakes are produced only around this reflecting point and are removed without touching other parts of the surface. Thus, the sample surface is kept clean without contamination by dirty gas containing powders and flakes produced by the space reaction. while film deposition continuously progresses by the fresh reactant gas stream from the nozzle. This is the effect of “the V-shape” of the gas stream on haze elimination. Due to the high speed of the gas stream and the compression force by the downward momentum of the gas stream, the boundary layer becomes extremely thin at the reflecting point where the film is deposited. The decrease in boundary layer thickness directly contributes to the decrease in diffusion time of reactant gases and consequently to the decrease in powder and flake formation. In addition, the
Reflecting Point l:ig. 3. A reflecting
high speed gas stream
ERGS) on the sample surface
in a V-shape
0. Tabata et al. /New
Fig. 4. The reflecting about I .5 m/set.
CVD technique
high speed gas stream (RGS) visualized with smoke. The smoke velocity
233
is
powders or flakes produced around the reflecting point are quickly removed by the high speed of the gas stream before they can deposit onto the sample surface. These are the rewarding effects of “the high speed” of the gas stream on haze elimination. Such a V-shape gas stream is easily formed by balancing to streams: a gas stream ejected from a nozzle and a sucking stream generated by a draft head as shown in fig. 4. Fig. 4 shows a reflecting high speed gas stream (RGS) which is visualized with smoke when its velocity is about 1.5 m/set.
4. Experimental The arrangement of nozzle and draft head for the formation of a reflecting high speed gas stream on the heating disk is shown in fig. 5. The nozzle has a flat bill of 50 mm in width, 30 mm in length, and 0.5 mm in thickness on the inside and is fixed at a height of -30 mm above the heating disk, having an inclination of -60”. Facing this, the rectangular draft head is set in a position such that the lower side of the opening is about 10 mm higher than the heating disk, the opening being bent downward about 45”. The size of the opening is 90 mm X 40 mm. The heating disk is made of quartz and is about 200 mm in diameter and 5 mm thick. The chemical vapor reaction represented below was used for the synthesis of undoped transparent conducting SnO, films: (CH3)2SnCl,
+ O2 + SnOz + 2 CH3Cl .
(1)
A 6 l/min total flow of reactant gases is ejected from the flat bill and forms a beltlike reflecting gas stream of about 50 mm in width on the glass substrate placed on the quartz heating disk. The gas stream flows 1 to 5 m/set in reality. The composi-
234
Fig. 5. Schematic
representation
of the laboratory
type RGSCVD
apparatus
tested hcrc.
tion of the ejected high speed gas is 4 l/min Ar as carrier gas. 1 limin Ar as pickup gas of a (CH3)2SnC1, vapor at 100°C. and 1 l/min 02. The heating disk rotates at rates of 5 to 10 rpm while heating the glass substrates to around 500°C. The SnOz film deposition experiments were carried out successively at the reflecting point.
5. Results
In this experiment, a typical growth rate of SnOz films is about 1500 A/min at a deposition temperature of 500°C. The optical transparency of over 90% of the Ghn up to a film thickness of 5000 A is substantially high and white haze is not visible as shown in fig. 6. Fig. 6 compares film appearance in PVD (a) and in this RGS-CVD (b) together with the striking white haze in ordinary CVD (c). Combined with an integration sphere, a spectrophotometer provides tninute evaluation
Fig. 6. Appearances of In203 film (a) by PVD and of SnO2 film (b) by this RGSCVD with the striking white haze of SnOz film (c) by ordinary CVD.
together
0. Tabata et al. /New
0
Fig. 7. Logarithmic
dependence
2 4 Thickness
of the haze factor
CVD technique
6
235
8 10 xld( % )
of CVD
SnOz films on their thicknesses: (a) and (c) SnOl films by ordinary CVD. light of 550 nm.
an InzOa film by PVD, (b) SnOz films by RGS-CVD, The haze was measured
by monochromatic
of haze as shown in fig. 7. The white haze caused by film structure, not by the sticking of powders or flakes, gradually increases with film thickness. This tendency appears both in ordinary CVD films and in this RGS-CVD films as seen in the lines (b) and (c). However, while ordinary CVD films have large haze factors of 4 to 10% even in the range of film thickness of 1000 to 3000 8, the haze factor of RGS-CVD ftims remains below 1% in film thicknesses up to about 5000 ,& after deducting approximately 0.55% as the value of glass substrate itself, and it tightly approaches
b
Fig. 8. Surface structures of a PVD In203 by scanning electron microscopy.
film (a) and a RGSCVD
SnOz
film (b) as revealed
the haze factor of PVD films at around a thickness of 1600 A, being still higher by several tenths of a percent. A scanning electron microscopy revealed the close similarity in their surface structure as shown in fig. 8.
6. Discussions
From figs. 6, 7. and 8, it is evident that the undoped hate-free transparent conducting films. being as good as those produced by I’VD, are prepared in the atmosphere. not in a vacuum. This means that the reflecting high speed gas stream successfully effects the elimination of obstinate white haze by decreasing both the existing area of boundary layer on a sample surface and the thickness of the layer. In ordinary CVD. the deposition of films progresses through such a boundary layer as described in section 2. The average thickness of a boundary layer, I?, over the whole sample substrate can be easily derived by referring to ref. [6] as follows: 15= 3.1 (UL/U)1’2 ,
(1)
where u is the kinematic viscosity of the gas, L the length of the sample substrate, and U the velocity of the gas flow. Under usual conditions of ordinary CVD, ;i is calculated to be of the order of unit centimeter by eq. (2). For example, when v = 0.705 cm*/sec at 500°C for Ar, L = 10 cm, CI = 10 cm/set, and 8 = 2.6 cm. In RGS-CVD, the length of a reflecting gas stream in the forward flowing direction at the reflecting point is about unit centimeter and the velocity of the gas is about 1 .5 m/set in this experiment. Thus 6 is estimated to be only 2 mm. Such a small value of 6 corresponds to l/10 or less of that of ordinary CVD. Consequently, the diffusion time of reactant gases in the boundary layer is also decreased to l/IO or less and it is expected to decrease haze generation right next to the sample surface, to the same order as the decreased diffusion time. Hence it could be suggested that the haze reduction of roughly 10%’ between the curves (b) and (c) in fig. 7 results from the decreasing rate of boundary layer thickness of about I/IO. 6.2. Nozzle
chzracteristics
The flat bill nozzle used here performed satisfactorily producing a beltlike gas jet stream. In this experiment, the critical haze velocity at which haze vanishes from films appeared to be in the vicinity of 1 ~/SW. The critical uniform velocity which makes the velocity distribution uniform over a uniformity of 90%~is about 1 m/set. Such a uniform velocity distribution is necessary for obtaining the uniformity of film thickness. Thus the use of a long slit nozzle enables one to produce a large film and a number of films successively.
0. Tahata et al. / Nc~v CI’D tcchrkpre
231
7. Conclusions (1) A reflecting high speed gas stream (RGS) provides haze-free transparent conducting films, as good as those produced by the PVD method, in open air, not necessitating a vacuum. (2) Undoped transparent conducting Sn02 films produced by the RGS-CVD method possess a transparency of over 90% and a haze factor of less than 1%~.
References [ 11 [2] [3] [4]
J.C. Manifacicr, M. dc Murcia and J.P. Fillard, Thin Solid Films 41 (1977) 127. A.G. Sabnis and L.D. F&cl, J. Vacuum Sci. Tcchnol. 14 (1977) 685. D.B. Fraser and H.D. Cook, J. Electrochem. Sot. 119 (1972) 1368. 0. Tabata, in: Proc. 5th. Intern. Conf. CVD (Electrochemical Society, Inc., Princeton, 1975) 681. [S] R.N. Ghoshtagorc, J. Elcctrochcm. Sot. 125 (1978) 110. 161 E.R.G. Eckcrt, Heat and hlnss Transfer, 2nd cd. (McGraw-Hill, New York. 1959) 140.