Patent Application
20120058366
The contents of the Following Japanese patent applications and International patent application are incorporated herein by reference:
No. JP2009-134455 filed on Jun. 3, 2009,
No. JP2009-260373 filed on Nov. 13, 2009. and
No. PCT/W2010/003691 filed on Jun. 2, 2010.
1. Technical Field
The present invention relates to a film-formed article and a method for producing a film-formed article.
2. Related Art
Zinc oxide has high transmission of visible light and exhibits conductivity when doped with suitable impurities, and is therefore expected to be used as a material For transparent conductive films. Patent Document 1 discloses a method for manufacturing a zinc oxide film doped with Ga as the impurity.
Patent Document 1: Japanese Patent Application Publication No. H09-87833
However, it is difficult to achieve low resistivity for zinc oxide. One method for lowering the resistivity includes increasing the carrier density to facilitate electrical conduction, but increasing the carrier density also causes the optical absorption due to plasma vibration of the carrier to begin affecting the wavelengths of visible light from the infrared region. This is a problem when the using the zinc oxide in an optical device that utilizes visible light. When the carrier density is approximately 1×1021/cm3, the optical absorption due to plasma vibration begins affecting the visible light region from the infrared region. In order to prevent a decrease in the visible light transmission of zinc oxide, it is necessary to lower the resistivity of the zinc oxide while limiting the carrier density to the above value.
The inventors of the present invention performed a rigorous study to lower the resistivity of zinc oxide by improving the carrier mobility while limiting the carrier density to the above maximum value. According to a first aspect related to the innovations herein, provided is a manufacturing method for manufacturing a film-formed article by using sputtering to form a film including a constituent element of a target on a substrate. The manufacturing method comprises setting a distance between the target and the substrate in a range from 0.5 times to 1.5 times a mean free path of the constituent element in the sputtering gas.
The distance may be in a range from 0.8 times to 1.0 times the mean free path. The mean free path may be the mean free path occurring when zinc atoms move in argon. The distance may be set such that the substrate does not contact the sputtering gas in plasma form.
According to a second aspect related to the innovations herein, provided is a manufacturing method for manufacturing a film-formed article by using sputtering to form a film including a constituent element of a target on a substrate. The manufacturing method comprises, with d [mm] representing distance between the target and the substrate and p [Pa] representing gas pressure between the target and the substrate, forming the film under conditions where clip is in a range from no less than 600 to no more than 800.
The distance may be in a range from 150 mm to 170 mm. The gas pressure may be in a range from 0.15 Pa to 0.3 Pa. Material forming the target may have a hexagonal crystal structure. The material forming the target may be zinc oxide. Prior to forming the film on the surface of the substrate, the substrate may be irradiated with particles.
According to a third aspect related to the innovations herein, provided is a film-formed article comprising a substrate including organic material and zinc oxide formed on the substrate and having n-type conduction. The zinc oxide has electron mobility greater than or equal to 12 cm2/V.s.
According to a fourth aspect related to the innovations herein, provided is a film-formed article comprising a substrate including organic material and zinc oxide formed on the substrate and having n-type conduction. The zinc oxide has an average grain size (column diameter) greater than or equal to 30 mm.
The substrate including the organic material may be a resin substrate, and a softening point of resin of the resin substrate may be in a range from no less than 80° C. to no more than 120° C. Here, the softening point refers to the deformation temperature under load, as measured by J1S K7207 (technique A).
The summary clause does not necessarily describe all necessary features of the embodiments of the present invention. The present invention may also be a sub-combination of the features described above.
Hereinafter, some embodiments of the present invention will be described. The embodiments do not limit the invention according to the claims, and all the combinations of the features described in the embodiments are not necessarily essential to means provided by aspects of the invention.
The substrate 102 is a substrate suitable for having the film 104 formed on the surface thereof. The substrate 102 has sufficient mechanical strength to serve as a support substrate for the film 104 formed thereon. The substrate 102 may include organic material. A resin substrate is an example of such as substrate including organic material. The softening point of this resin may be between 80° C. and 120° C. Examples of the material used in the resin substrate include acryl, epoxy, polycarbonate, polyethylene, and polyester.
The substrate 102 may be a quartz glass substrate, an aluminum substrate, a SiC substrate, a silicon substrate, a GaAs substrate, or a metal substrate, for example. The substrate 102 may be made of single-crystal, polycrystalline, or amorphous material, For example, if the compound film 104 to be formed on the substrate 102 is zinc oxide, the zinc oxide enables easy growth of a film in which the c-axis of a hexagonal crystal is oriented perpendicular to the substrate surface, even for amorphous substrates, and therefore polycrystalline or amorphous material may be used for the substrate 102. In the method for producing the compound film described below, a film is formed using target ions, which have high energy and uniform incidence directions, and therefore a compound film 104 with good crystallinity can be formed even on a polycrystalline or amorphous substrate.
The film 104 may be formed contacting the surface of the substrate 102. The film 104 may be zinc oxide having n-type conduction. The zinc oxide may have electron mobility greater than or equal to 12 cm2/V.s, The zinc oxide may have an average grain size (column diameter) greater than or equal to 30 nm.
The deposition chamber 202 is a vacuum apparatus that deposits the film 104 on the substrate 102. The deposition chamber 202 may be connected to a load-lock chamber, not shown. The insertion and removal of the substrate before and after the deposition may be performed by a load-lock system. By adopting the load-lock system, the deposition chamber 202 is kept in a constant vacuum state, thereby greatly reducing the chances of impurities unintentionally mixing with the film.
The substrate holder 204 holds the substrate 102 during deposition. The substrate 102 is arranged on the substrate holder 204 such that the surface of the substrate on which the film 104 is to be grown faces down. The substrate holder 204 may rotate on a Z-axis, which passes vertically through the center of the substrate holder 204, during deposition. As a result of this rotation, the film 104 can be formed uniformly over the surface of the substrate 102.
The substrate holder 204 may be raised and lowered in a prescribed range, shown by h in
The rotary pump 242, the valve 244, the turbomolecular pump 246, the valve 248, the leak valve 252, the valve 254, the leak valve 256, and the vacuum gauge 258 form a vacuum system for controlling the pressure p within the deposition chamber 202. The vacuum gauge 258 may be a Baratron vacuum gauge. In the initial stage of the vacuum drawing, the valve 254 is opened and the rotary pump 242 is used to perform the vacuum drawing. After a prescribed vacuum degree is reached, the valve 254 is closed, the valve 244 and the valve 248 are opened, and the turbomolecular pump 246 is used to continue the vacuum drawing. During the deposition process, the opening amount of the valve 248 is adjusted to control the pressure p within the deposition chamber 202.
The cathode 212 and the cathode 216 are arranged in the lower portion of the deposition chamber 202. The target 214 and the target 218 are respectively fixed to the cathode 212 and the cathode 216.
The material forming the target 214 or the target 218 may have a hexagonal crystal structure. Material having a hexagonal crystal structure enables easy growth of a film in which the c-axis of the hexagonal crystal is oriented perpendicular to the substrate surface, and this results in the film having large grain size and high carrier mobility. Furthermore, the growth rate is high for the same reason. The material forming the target 214 or the target 218 may be zinc oxide. The target 214 or the target 218 may supply impurity elements. The impurity elements may be Al, Ga, or In, for example.
The valve 222, the mass flow controller 224, the valve 226, the gas container 228, the valve 232, the mass flow controller 234, and the valve 236 form the system supplying the sputtering gas. The sputtering gas supply system can supply the sputtering gas near each of the targets. The sputtering gas flow rate can be controlled as desired by the mass flow controller 224 and the mass flow controller 234. The mass flow controller 224 and mass flow controller 234 can modulate the sputtering gas flow rate over time, according to a control signal. In
During deposition, when power is supplied to the targets while the sputtering gas is being introduced to the deposition chamber 202, glow discharge occurs on the targets, thereby generating particles from the target elements in a variety of states, such as ions, neutral atoms, clusters, and macroparticles. These particles are supplied to the substrate 102 to form the film 104.
At step S310 of performing preliminary settings, the target 214 and target 218 are arranged on the cathode 212 and cathode 216. The target 214 and the target 218 may be arranged respectively on the cathode 212 and the cathode 216. For example, if zinc oxide including Al is used as the impurity is used, a target made of zinc oxide may be arranged on the cathode 212 and a target made of Al may be arranged on the cathode 216.
The substrate holder 204 is raised and lowered to adjust the distance d. The distance d may be within a range from 0.5 times to 1.5 times the mean free path of the particles of the constituent elements of the targets in the sputtering gas. As another example, the distance d may be within a range from 0.8 times to 1.0 times the mean free path. The mean free path may be the mean free path occurring when zinc atoms move within argon. The distance d is such that the substrate 102 is not in contact with the sputtering gas in plasma form. The distance d may be within a range from 150 mm to 170 mm.
If film-formed articles 100 are repeatedly manufactured using the same target material and the same distance d, step 5310 can be omitted in the repetitions of the manufacturing, from when the target 214 and the target 218 are first set until these targets are completely depleted.
In the manner described above, when forming zinc oxide having n-type conduction, the visible light transmission of the zinc oxide is impaired, and therefore there is a limit on how much the resistivity can be lowered by increasing carrier density. Therefore, the present inventors considered lowering the resistivity of zinc oxide by improving carrier mobility. The mobility of carrier electrons in n-type zinc oxide is affected by scattering of the electrons by the impurities and scattering of the electrons by the grain boundaries. The present inventors attempted to increase the carrier mobility by increasing the grain size of the zinc oxide film and decreasing the grain boundary scattering.
During deposition, the glow discharge generates particles of the target elements having a variety of energies. Among these particles, there are those that reach the substrate 102 but do not have enough energy to sufficiently diffuse in the film 104 being grown. These low-energy particles can be thought of as impeding the crystallization of the film. Therefore, the present inventors thought that, by separating out these particles such that deposition occurs using only the high-energy particles, the film can be formed with high crystallinity, large grain size, and uniform high-quality, thereby, increasing the carrier mobility of the film and decreasing resistivity. This result was achieved by increasing the distance d to be greater than the distance usually used in sputtering.
By increasing the distance d to be set near the mean free path of the particles, the low-energy particles repeatedly collide with the other particles causing energy loss and preventing the low-energy particles from reaching the substrate 102, thereby excluding these low-energy particles from the deposition. Only the high-energy particles avoid colliding with other particles within the range of the mean free path and reach the substrate 102 white maintaining high energy, and therefore these high-energy particles are used in the deposition. The high-energy particles can sufficiently diffuse within the film 104 during growth, resulting in uniform crystallization of the film 104.
By increasing the distance d, the substrate 102 is prevented from contacting the plasma, as shown in
Normally, during deposition, the substrate can be heated to encourage crystallization of the film. In this case, however, a substrate that is easily deformed when heated, such as a resin substrate, cannot be used. This is a significant impediment when trying to lower the cast and improve the flexibility of the device. By increasing the distance d as described above, the present embodiment can realize a film with high crystallinity, large grain size, and uniform high-quality, without heating the substrate. Furthermore, since the substrate does not come into contact with the plasma, the substrate is almost completely unaffected by the plasma. Therefore, in the present embodiment, a resin substrate that is weak to heat and easily deformed by plasma can be used without problems.
At step S320 of vacuum drawing of the deposition apparatus, the deposition chamber 202 is vacuum drawn. First, the valve 254 is opened and the rotary pump 242 is used to perform the vacuum drawing. After a prescribed vacuum degree is reached, the valve 254 is closed, the valve 244 and the valve 248 are opened, and the turbomolecular pump 246 is used to continue the vacuum drawing. The opening amount of the valve 248 is adjusted to control the internal pressure p within the deposition chamber 202.
The pressure p during deposition may be in a range from 0.15 Pa to 0.3 Pa. When the distance d is expressed in mm and the pressure p is expressed in Pa, the ratio dip during deposition may be in a range from no less than 600 to no greater than 800. Lowering the pressure p lengthens the mean free path of the particles, which results in more high-energy particles reaching the substrate for the same distance d, thereby enabling formation of a film with high crystallinity.
At step S330 of arranging the substrate in the deposition apparatus, the substrate 102 is arranged in the substrate holder 204. If a load-lock system is used, the substrate 102 can be inserted and removed while maintaining the vacuum degree of the deposition chamber 202. The substrate holder 204 is rotated.
At step S340 of introducing the sputtering gas, a sputtering gas such as Ar is introduced. The sputtering gas supplied near the target 214 is introduced from the gas container 228 while the valve 222 and the valve 226 are open and the mass flow controller 224 controls the flow rate. The sputtering gas supplied near the target 218 is introduced from the gas container 228 while the valve 232 and the valve 236 are open and the mass flow controller 234 controls the flow rate.
At step S350 of electrical discharge, power is supplied to the targets to cause glow discharge on the targets. The glow discharge causes particles to be generated from the target elements in a variety of states, such as ions, neutral atoms, clusters, and macroparticles. These particles are supplied to the substrate 102 to form the film 104. As described above, the distance d and the pressure p are adjusted such that only high-energy particles are used in the deposition, and therefore the film 104 can be formed with high crystallinity and large grain size. As a result, the carrier mobility of the film 104 can be increased and the resistivity can be decreased. The control of the pressure p in the deposition chamber 202 may be performed by adjusting the opening degree of the valve 248.
At step S360 of stopping the discharge, the supply of power to the target 214 and the target 218 is stopped, and the valve 222 and the valve 236 are closed to stop the supply of the sputtering gas.
At step S370 of removing the substrate, the film-formed article 100, which is the film 104 and the substrate 102 on which the film 104 is formed, is removed from the deposition chamber 202. If a load-lock system is used, the film-formed article 100 can be removed while maintaining the vacuum degree of the deposition chamber 202. At this point, the process of manufacturing the film-formed article 100 is finished. If a following film-formed article 100 manufacturing process is to be performed, the above manufacturing process may be repeated from step S330 of arranging the next substrate 102 in the deposition apparatus.
As another example, the film 104 may be aluminum nitride. Aluminum nitride also has a hexagonal crystal structure. In this case as well, an aluminum nitride film can be formed with high crystallinity by setting the distance d to be the mean free path of aluminum.
Using a deposition apparatus 200 having the configuration shown in
The distance d was changed in a range from 105 mm to 190 mm when forming the zinc oxide film, and the relationship between the electron mobility and the distance d was analyzed. Furthermore, the pressure p during deposition was changed in a range from 0.22 Pa to 0.5 Pa when forming the zinc oxide film, and the relationship between the electron mobility and the pressure p was analyzed.
A prescribed distance d was set, the composite quartz glass wafer was arranged in the substrate holder 204, and the substrate holder 204 was rotated. The deposition chamber 202 was vacuum drawn, and the vacuum gauge 258 was used to confirm a pressure within the deposition chamber 202 of no more than 1×104 Pa. After this, a power of 200 W and a power of 22.5 W were simultaneously supplied to the ZnO target 214 and the Al target 218 respectively, from a high-frequency power source having a frequency of 13.56 MHz, to form the film via electrical discharge. In previous testing, it was discovered that the electrical resistivity of the ZnO layer is extremely low when the Al doping is performed with the above ratio for the supplied power, and this was used as the ideal doping condition in the apparatus: The sputtering gas was Ar gas supplied at 10 seem near the target 214 and the target 218, using the mass flow controller 224 and the mass flow controller 234. By adjusting the conditions during the deposition, a film was formed for each test condition to have a thickness of 200 nm.
The mean free paths of the target elements in an Ar atmosphere were calculated. When there are A particles and B particles, the mean free path λΛ of the A particles can be calculated as shown below.
λΛ=1/[π·DΛB2·□B[1+{mA/mB}]1/2] Expression 1
D
ΛB=(DΛ+DB)/2 Expression 2
n
B
=p/k
B
T Expression 3
DAB: Average diameter of the A particles and B particles
DA: Diameter of the A particles
DB: Diameter of the B particles
nB: Density of the B particles
mA: Mass of the A particles
mB: Mass of the B particles
p: Pressure during deposition
kB: Boltzmann constant
With the A particles being zinc ions, which are a metal component forming a target, and the B particles being the argon of the sputtering gas, the following data can be substituted into Expressions 1 to 3 to calculate the mean free path of the zinc ions.
Zn2+ ion diameter: DZn=1.5×10−10 m
Ar diameter: DAr=1.4×10 −10 m
Zn2+ ion mass: mZn=1.1×10−25 kg
Ar mass: mAr=6.6×10−26 kg
Pressure p during deposition: p=0.22 Pa
Absolute temperature T: T=300 K
Based on the calculations, the mean free path λZn of the zinc ions is 0.175 m (175 mm).
Instead of zinc ions, when the A particles are oxygen ions, which are a non-metallic component for forming a target, the following data can be substituted into Expressions 1 to 3 to calculate the mean free path of the oxygen ions.
O2− ion diameter: DO=2.8×10−10 m
O2− ion mass: mO=2.6×10−26 kg
Based on the calculations, the mean free path λO of the oxygen ions is 0.114 m (114 mm).
When the A particles are aluminum ions, which are used for impurity doping when forming a target, the following data can be substituted into Expressions 1 to 3 to calculate the mean free path of the aluminum ions.
Al3+ ion diameter: DAl=1.1×10−10 m
Al3+ ion mass: mAl=4.5×10−26 kg
Based on the calculations, the mean free path λal of the aluminum ions is 0.296 m (296 mm).
X-ray analysis was performed on the zinc oxide films, and crystallinity of the zinc oxide films was evaluated from the half-width value of the rocking curve of the ZnO (002) lattice plane. Evaluation was performed on zinc oxide films formed while the pressure was fixed at 0.3 Pa during deposition and the distance d was respectively set to 105 mm, 135 mm, 160 mm, 170 mm, 180 mm, and 190 mm. It was found that, when the distance d is 170 mm, the half-width value of the rocking curve is at a minimum and the crystallinity of the formed film is at a maximum. This distance considerably matches the above value calculated for the mean free path of the Zn ions thought to maximize crystal growth of the ZnO. In other words, if the distance d is set to be near the mean free path of the Zn ions, a zinc oxide film with good crystallinity can be expected to be formed.
The carrier mobility increases as the distance d increases from 105 mm, but reaches a maximum value of 17.4 cm2/V.s at d=160 mm. When the distance d increases beyond this point, the carrier mobility gradually decreases. In other words, for a prescribed pressure p, there is an optimal distance d at which the carrier mobility has a maximum value. When d is set near this optimal distance, a zinc oxide film with high carrier mobility can be formed without heating the substrate 102.
The reason for this phenomenon is believed to be as follows. At a certain pressure p, increasing the distance d results in low-energy particles being unable to reach the substrate and in a lower ratio of particles being incident to the substrate at an angle, Therefore, the majority of particles actually used in the deposition are high-energy particles that are perpendicularly incident to the substrate, and so the carrier mobility and the crystallinity of the film is increased. However, when the distance d significantly exceeds the mean free path determined by the pressure p, the rate at which the energy of the particles used in the deposition decreases due to collisions with other particles increases. As a result, the diffusion distance into the film of the particles reaching the substrate is decreased, which causes lower crystallinity of the film and lower carrier mobility. Accordingly, there is an optimal relationship between the two parameters of distance d and pressure p.
The above measurement results indicate that the carrier mobility is closely related to the distance d and the pressure p, and can have a maximum value for each of these characteristics. Therefore, the present inventors expressed the relationship between the carrier mobility and dip, which is the ratio between the distance d and the pressure p, as shown in FIG. 6. In FIG. 6, the horizontal axis represents d/p and the vertical axis represents the carrier mobility. As shown in FIG. 6, the carrier mobility tends to increase monotonically as d/p increases, until reaching d/p=730 mm/Pa, at which point the carrier mobility has a maximum value of 17.4 cm2/V.s. The d and p values at this maximum value are d=160 mm and p=0.22 Pa. When dip increases further, the carrier mobility slowly decreases. Therefore, a zinc oxide film with maximum carrier mobility can be formed by setting the distance d such that the substrate does not contact the plasma and controlling the pressure p such that dip is near 730 mm/Pa.
Combining the above with the results regarding carrier mobility shown in
Using the deposition apparatus 200 having the configuration shown in
The raw material target 214 for the film was ZnO, and the impurity target 218 was Al. The sputtering gas was Ar gas.
If the resin substrate were put into a vacuum as-is and used for deposition, dust on the surface of the substrate and degassing from the substrate in the vacuum would lower the adhesive strength of the film formed on the resin substrate. Therefore, in order to prevent a drop in the adhesive strength of the film, the substrate was pre-processed in the manner described below.
First, the substrate 102 was washed in ethanol. The washed substrate 102 was then placed in a separate preparation chamber, in which a reverse sputtering process was performed on the substrate 102. Specifically, in a vacuum state with a pressure of approximately 1×10−4 Pa, a high-frequency power source with a frequency of 13.56 MHz was used to apply a power of 50 W to the substrate 102 and cause a discharge, and this reverse sputtering was performed for 30 minutes. During this process, the energy particles generated by the discharge irradiate the substrate 102 as ions, for example. This irradiation not only removes dust affixed to the surface of the substrate 102, but also actively encourages degassing of the substrate in the vacuum, thereby restricting the effect of degassing during deposition. The preparation chamber in which the reverse sputtering is performed may be the load-lock chamber of the deposition apparatus 200.
The substrate 102 processed in the above manner was placed in the deposition chamber 202 and deposition was performed. The distance d and pressure p during deposition were set to be 170 mm and 0.22 Pa, which result in a film having high mobility and minimum electrical resistivity according to the results described in the first embodiment. The other deposition conditions were the same as in the first embodiment. The thickness of the film was set to 200 nm by adjusting the deposition time.
Upon analyzing the characteristics of the zinc oxide film formed in this way, it was found that the electric resistivity was 1.8×10−3 Ω.cm, the carrier density was 2.69×1020 cm−3, and the carrier mobility was 12.5 cm2/V.s. These results indicate that the zinc oxide film formed on the acrylic resin substrate has a high carrier mobility above 12 cm2/V.s, in the same manner as the zinc oxide film formed on the composite quartz glass substrate in the first embodiment. In other words, the results prove that the present embodiment can form a film that has high carrier mobility even on an acrylic resin substrate. Furthermore, these results suggest that a film with the same high carrier mobility could be formed on substrates made of other resin materials whose softening point is above 100° C., such as epoxy, polycarbonate, polyethylene, and polyester.
While the embodiments of the present invention have been described, the technical scope of the invention is not limited to the above described embodiments. It is apparent to persons skilled in the art that various alterations and improvements can be added to the above-described embodiments. It is also apparent from the scope of the claims that the embodiments added with such alterations or improvements can be included in the technical scope of the invention.
The operations, procedures, steps, and stages of each process performed by an apparatus, system, program, and method shown in the claims, embodiments, or diagrams can be performed in any order as long as the order is not indicated by “prior to,” “before,” or the like and as long as the output from a previous process is not used in a later process. Even if the process flow is described using phrases such as “first” or “next” in the'claims, embodiments, or diagrams, it does not necessarily mean that the process must be performed in this order.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009-134455 | Jun 2009 | JP | national |
| 2009-260373 | Nov 2009 | JP | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2010/003691 | Jun 2010 | US |
| Child | 13296826 | US |
