JP4538577B2 - Zinc oxide thin film deposition method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a zinc oxide thin film and a film forming method thereof.
[0002]
[Prior art]
As the transparent electrode of the display element, an ITO film, a SnO ₂ film, a ZnO film, or the like is used.
[0003]
Among these, the ITO film has a small specific resistance of about 2 to 3 × 10 ⁻⁴ Ω · cm or less, and is therefore widely used in liquid crystal display devices and the like.
[0004]
However, the ITO film has low chemical stability compared to other materials. For this reason, since the ITO film is reduced in hydrogen plasma and causes a blackening phenomenon, a process of forming amorphous Si by CVD is used as a post-process of ZnO film formation, such as a solar cell manufacturing process. In this case, the ITO film cannot be used as an electrode. In addition, In, one of the raw materials for the ITO film, is expensive and rare in quantity.
[0005]
On the other hand, the SnO ₂ film has a large specific resistance of about 10 ⁻³ Ω · cm, and thus is not suitable for a film that requires high conductivity.
[0006]
On the other hand, the ZnO film is usually produced by a sputtering method. In this case, the specific resistance is about 5 to 6 × 10 ⁻⁴ Ω · cm, which is smaller than the SnO ₂ film, and is more chemically than the ITO film. Therefore, it is employed as an electrode of a solar cell using the above-described amorphous Si film. In addition, Zn, which is a raw material for the ZnO film, is inexpensive and has abundant resources.
[0007]
The ZnO film is conventionally formed by various methods such as a sol-gel method, a spray method, an MOCVD method, and an RF sputtering method in addition to the above sputtering method.
Of these film forming methods, the sputtering method and the sol-gel method have an advantage of high productivity.
[0008]
[Problems to be solved by the invention]
However, a ZnO film obtained by a film forming method such as the sputtering method or the sol-gel method described above cannot always obtain good crystallinity because a deep level exists in the band gap.
[0009]
On the other hand, it has been reported that a ZnO film having good crystallinity can be obtained by using a method such as MBE or pulse laser deposition. However, these methods are not practical because the film formation rate is extremely slow and the film formation area is limited to a small one.
[0010]
The present invention has been made in view of the above problems, and a method for forming a zinc oxide thin film capable of forming a ZnO (zinc oxide) film having good crystallinity over a large area at a high film formation rate. And it aims at providing the zinc oxide thin film formed into a film by this method.
[0011]
[Means for Solving the Problems]
A method of forming a zinc oxide (ZnO) thin film according to the present invention is a method of forming a zinc oxide thin film by an ion plating method, wherein zinc oxide is a main component in a hearth arranged as an anode in a film forming chamber. An evaporation material to be disposed is disposed, a magnetic field control member disposed around the hearth is used to control an upper magnetic field close to the hearth, and a high-density plasma beam by arc discharge is supplied to the evaporation material to thereby form an evaporation material. Evaporated and ionized, the vapor deposition material is attached to the surface of the substrate disposed opposite the vapor deposition material, and a voltage is applied so that the hearth has a potential of +29.3 to +60 V with respect to the film formation chamber. by applying, especially to obtain a thin film exhibiting no peaks from the peak of the band edge emission in the photoluminescence spectrum measured at room temperature or more deep levels 300meV To.
[0012]
Here, the hearth refers to a member on which the vapor deposition material is disposed, and includes not only the hearth of the embodiment described later but also a crucible.
[0013]
Thereby, the zinc oxide thin film which has favorable crystallinity can be uniformly formed over a large area at a high film formation rate.
[0014]
In this case, if the Haas potential is lower than +29.3 V, the migration of the deposited particles on the surface of the substrate is insufficient, and the crystallinity of the zinc oxide thin film is lowered. On the other hand, the Haas potential is higher than +60 V. If it is high, the deposition material having high energy collides with the substrate to generate crystal defects, and the crystallinity of the obtained zinc oxide thin film is lowered, which is not preferable.
[0015]
Further, in this case, the vapor deposition material is (1) Group 1 element, (2) Group 3 element, (3) Group 4 element, (4) Group 5 element, (5) Group 7 element, (6) Group 1 Element oxide, (7) Group 3 element oxide, (8) Group 4 element oxide, (9) Group 5 element oxide, (10) Group 7 element oxide One may be included.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
A preferred embodiment of a zinc oxide thin film and a method for forming the same according to the present invention (hereinafter referred to as this embodiment) will be described below with reference to the drawings.
[0020]
First, a film forming apparatus suitable for carrying out a method for forming a zinc oxide thin film according to this embodiment (hereinafter simply referred to as a film forming method) will be described with reference to FIG.
[0021]
The film forming apparatus includes a vacuum container 10 that is a film forming chamber, a plasma gun 30 that is a plasma source that supplies a plasma beam PB into the vacuum container 10, and a plasma beam PB that is disposed at the bottom of the vacuum container 10. And an anode member 50.
[0022]
The vacuum vessel 10 is grounded.
[0023]
The plasma gun 30 is a pressure gradient type plasma gun disclosed in Japanese Patent Application Laid-Open No. 9-194232 and the like, and its main body portion is mounted on a cylindrical portion 12 provided on the side wall of the vacuum vessel 10. The main body portion is composed of a glass tube 32 whose one end is closed by a cathode 31. In the glass tube 32, a cylinder 33 made of molybdenum is fixed to the cathode 31 and arranged concentrically with the glass tube 32, and in the cylinder 33, a disk formed of lanthanum hexaboride (LaB ₆ ) 34 and a pipe 35 made of tantalum are incorporated. Between the end of the glass tube 32 opposite to the cathode 31 and the end of the cylindrical portion 12 provided in the vacuum vessel 10, first and second intermediate electrodes 41 and 42 are coaxially arranged in series. Has been. An annular permanent magnet 44 for converging the plasma beam PB is built in the first intermediate electrode 41, while an electromagnetic coil 45 for converging the plasma beam PB is also provided in the second intermediate electrode 42. Built in. In addition, a steering coil 47 that guides the plasma beam PB generated at the cathode 31 and drawn to the first and second intermediate electrodes 41 and 42 into the vacuum vessel 10 is provided around the cylindrical portion 12. .
[0024]
The operation of the plasma gun 30 is controlled by a gun driving device (not shown). Thereby, the power supply to the cathode 31 can be turned on / off or the supply current can be adjusted, and the power supply to the first and second intermediate electrodes 41 and 42, the electromagnetic coil 45 and the steering coil 47 can be adjusted. can do. As a result, the intensity and distribution state of the plasma beam PB supplied into the vacuum vessel 10 can be controlled.
[0025]
The pipe 35 arranged on the innermost side of the plasma gun 30 introduces a carrier gas made of an inert gas such as argon (Ar), which is the source of the plasma beam PB, into the plasma gun 30 and thus the vacuum vessel 10. And is connected to an inert gas source 90 via a flow meter 93 and a flow rate control valve 94.
[0026]
The anode member 50 disposed at the bottom of the vacuum vessel 10 includes a hearth 51 that is a main anode for guiding the plasma beam PB downward, and an annular auxiliary anode <magnetic field control member> 52 disposed around the hearth 51. Become.
[0027]
The hearth 51 is formed of a conductive material and is supported by the grounded vacuum vessel 10 through an insulator (not shown). The hearth 51 is controlled to an appropriate positive potential by the anode power supply device 80, and the plasma beam PB emitted from the plasma gun 30 is guided by the hearth 51 and the auxiliary anode 52 to reach the hearth 51 disposed below. To do.
[0028]
The hearth 51 is a vapor deposition material arrangement portion, and has a through hole TH at a central portion where the plasma beam PB is incident, and a columnar or rod-shaped material rod 53 that is a vapor deposition material is loaded into the through hole TH. The material rod 53 is a tablet formed by adding a metal such as Al to ZnO or ZnO and shaping it into a predetermined shape. The material rod 53 is heated and sublimated by the current of the plasma beam PB to generate vapor of a vapor deposition material. The material supply device 58 provided in the lower part of the vacuum vessel 10 has a structure in which the material rod 53 is successively loaded into the through hole TH of the hearth 51 and the loaded material rod 53 is gradually raised. Even if the upper end of 53 evaporates and wears, the upper end can always protrude from the through hole TH of the hearth 51 by a certain amount.
[0029]
The auxiliary anode 52 is configured by an annular container disposed around the hearth 51 concentrically with the hearth 51. An annular permanent magnet 55 made of ferrite or the like and a coil 56 laminated concentrically with the permanent magnet 55 are accommodated in the annular container. The permanent magnet 55 and the coil 56 are magnetic field control members, and form a cusp-like magnetic field directly above the hearth 51. Thereby, the direction of the plasma beam PB incident on the hearth 51 can be corrected.
[0030]
The coil 56 of the auxiliary anode 52 constitutes an electromagnet and is supplied with power from the anode power supply 80 to form an additional magnetic field that has the same direction as the magnetic field on the center side generated by the permanent magnet 55. That is, by changing the current supplied to the coil 56, the direction of the plasma beam PB incident on the hearth 51 can be finely adjusted.
[0031]
The container of the auxiliary anode 52 is also made of a conductive material, like the hearth 51. The auxiliary anode 52 is attached to the hearth 51 via an insulator (not shown). By changing the voltage applied from the anode power supply device 80 to the auxiliary anode 52, the electric field above the hearth 51 can be controlled in an auxiliary manner. That is, when the potential of the auxiliary anode 52 is made the same as that of the hearth 51, the plasma beam PB is attracted to the auxiliary anode 52 and the supply of the plasma beam PB to the hearth 51 is reduced. On the other hand, when the potential of the auxiliary anode 52 is lowered to the same level as that of the vacuum vessel 10, the plasma beam PB is attracted to the hearth 51 and the material rod 53 is heated.
[0032]
The substrate W is held by the holding member 60 at a position facing the hearth 51 in the upper space of the vacuum vessel 10.
[0033]
The oxygen gas supply device 71 and the nitrogen gas supply device 72 supply (introduce) appropriate amounts of oxygen (O ₂ ) gas and nitrogen (N ₂ ) gas as atmospheric gases to the vacuum vessel 10 at appropriate timing, respectively. Gas supply means. Supply lines from an oxygen gas source 71a for storing oxygen gas and a nitrogen gas source 72a for storing nitrogen gas are connected to the vacuum vessel 10 via flow control valves 71b and 72b and flow meters 71c and 72c, respectively.
[0034]
The hearth gas supply device 73 is for supplying an appropriate amount of inert gas such as argon to the hearth 51 at an appropriate timing. The inert gas branched from the inert gas source 90 is guided to a gas introduction port (not shown) provided in the hearth 51 via the flow rate control valve 73b and the flow meter 73c of the hearth gas supply device 73, and the material rod 53 It is discharged around.
[0035]
The exhaust device 76 appropriately exhausts the gas in the vacuum container 10 through the vacuum gate 76a by the exhaust pump 76b.
[0036]
The vacuum pressure measuring device 77 can measure the partial pressure of oxygen gas, nitrogen gas, etc. in the vacuum vessel 10, and the measurement result is monitored by the atmosphere control device 78.
[0037]
The atmosphere control device 78 adjusts the supply amounts of oxygen gas, nitrogen gas, and argon gas into the vacuum vessel 10 via the oxygen gas supply device 71, the nitrogen gas supply device 72, the hearth gas supply device 73, and the like. be able to. The atmosphere control device 78 controls the operations of the oxygen gas supply device 71, the nitrogen gas supply device 72, the hearth gas supply device 73, the exhaust device 76, and the like based on the measurement result of the vacuum pressure measuring device 77, The partial pressures of oxygen gas, nitrogen gas and argon gas in the vacuum vessel 10 can be controlled to target values.
[0038]
The film forming apparatus configured as described above is an ion plating apparatus, and operates as follows.
[0039]
First, the material rod 53 is attached to the through hole TH of the hearth 51 disposed at the lower part of the vacuum container (film formation chamber) 10. The vapor deposition material which comprises the material rod 53 has ZnO as a main component. Note that when the vapor deposition material further includes a Group 3 element such as Al, Ga, In, or B, a Group 5 element such as N, or an oxide of these elements, the specific resistance of the film can be further reduced. Further, a group 1 element, a group 4 element, a group 7 element, or an oxide of these elements may be included. On the other hand, a glass substrate W, for example, is disposed at an opposing position above the hearth 51.
[0040]
Next, when ZnO is deposited on (deposited with) the substrate W, oxygen gas is introduced into the vacuum vessel 10 in advance.
[0041]
A negative voltage is applied to the cathode 31 of the plasma gun 30 and a positive voltage is applied to the hearth 51 with the vacuum vessel 10 at the ground potential interposed therebetween, thereby generating a plasma beam PB. The voltage of the hearth 51 in a stable discharge state is preferably in the range of +30 to +60 V with respect to the vacuum vessel 10 (wall of the vacuum vessel 10). The voltage of the hearth 51 can be adjusted by changing conditions such as the magnetic field, the pressure of the vacuum vessel 10 and the discharge current.
[0042]
The plasma beam PB reaches the hearth 51 by being guided by a magnetic field determined by the steering coil 47 and the permanent magnet 55 in the auxiliary anode 52. At this time, since the argon gas is supplied around the material rod 53, the plasma beam PB is easily attracted to the hearth 51.
[0043]
The material rod 53 exposed to the plasma is gradually heated. When the material rod 53 is sufficiently heated, the material rod 53 is sublimated, and a vapor deposition substance such as ZnO is evaporated (emitted). The vapor deposition material is ionized in the high-density plasma beam PB just above the hearth 51. Since the potential immediately above the hearth in the plasma beam PB is substantially the same as the potential of the hearth 51, the positively ionized vapor deposition material (the element of the vapor deposition material) has a potential substantially equal to the potential of the hearth 51, that is, the potential. Have energy.
[0044]
If the pressure of the vacuum vessel 10 is appropriately selected, the ionized vapor deposition material has a kinetic energy substituted from the potential energy without losing energy because there is less chance of colliding with particles such as Ar during movement, and the substrate has a kinetic energy. It reaches W and adheres (incidents). For example, if the pressure in the vacuum vessel 10 is too high, the ionized vapor deposition material collides with particles such as Ar and loses kinetic energy to heat it. That is, the kinetic energy of the ionized vapor deposition material decreases to a level corresponding to the average temperature thermal energy.
[0045]
ZnO or the like deposited on the substrate W reacts with oxygen gas to form a ZnO thin film on the substrate W.
[0046]
As described above, the potential of the hearth 51 is adjusted in the range of +30 to +60 V, and the pressure of the vacuum vessel 10 is appropriately selected so that the kinetic energy of the ionized deposition material is within the optimum range for ZnO crystal growth. By doing so, a ZnO thin film having good crystallinity can be obtained. In this case, if the kinetic energy of the ionized vapor deposition material is too low, the surface migration of the particles of the vapor deposition material that has reached the substrate W will be insufficient. On the other hand, the kinetic energy of the ionized vapor deposition material will be low. If it is too high, since the film growth proceeds while breaking the structure of the already formed film after crystal growth, none of the ZnO thin films having good crystallinity can be obtained.
[0047]
At this time, the amount of oxygen defects in the ZnO thin film can be controlled by adjusting the atmospheric pressure of the oxygen gas. In addition, an oxide containing an element of Group 1, Group 3, Group 4, Group 5, Group 7, or Group 1, Group 3, Group 4, Group 5, Group 7 is added to the vapor deposition material or a gas. By supplying in the form, the additive element in the ZnO thin film can be controlled.
[0048]
The ZnO thin film obtained by the ZnO thin film forming method according to this embodiment has few crystal defects and good crystallinity. Moreover, according to the ZnO thin film forming method according to the present embodiment, a ZnO thin film having good crystallinity can be uniformly formed over a large area at a high film forming rate.
[0049]
【Example】
The present invention will be further described with reference to examples. In addition, this invention is not limited to the Example demonstrated below.
Example 1
A ZnO film was prepared using an ion plating method which is a film forming method of the present invention.
[0050]
A non-alkali glass substrate having a thickness of 0.7 mm was used as the substrate. The alkali-free glass substrate was kept at a temperature of 200 ° C. ZnO was used as the vapor deposition material constituting the material rod. The deposition conditions were arc discharge with a discharge voltage of 80V, a hearth potential of + 40V, and a discharge current of 120A. The film forming pressure was 0.1 Pa, and the atmospheric gas conditions were argon: oxygen = 30 sccm: 10 sccm. Thereby, a zinc oxide thin film having a thickness of 200 nm was obtained.
[0051]
FIG. 2 shows a PL spectrum obtained by performing photoluminescence (hereinafter referred to as PL) measurement on the obtained ZnO film. In addition, for comparison, a PL spectrum of a ZnO film formed by a sputtering method is shown.
[0052]
Note that the PL measurement was performed using a 325 nm line of a He—Cd laser under two conditions of 77 K (FIG. 2A) and room temperature (RT: FIG. 2B). A ZnO film formed by the plating method, and (B) is a ZnO film formed by the sputtering method, the vertical axis indicates the PL intensity, the horizontal axis indicates the photon energy, and the shape of the PL spectrum is substantially the same. 2B, the width of the vertical axis of the PL spectrum waveform of the sputtering method of (B) is enlarged 10 times for FIG. 2A and 25 times for FIG. 2B.
[0053]
The ZnO film formed by the sputtering method has an emission band (A peak in FIG. 2) considered to be due to a defect level of 2.2 eV in a band gap of 3.3 eV, and a band edge emission (NBE in FIG. 2). The intensity of the PL peak (B peak in FIG. 2) on the low energy side of 0.1 eV is large. On the other hand, the ZnO film formed by the ion plating method has a light emission band (A peak in FIG. 2) considered to be due to a defect level of 2.2 eV and a PL of 0.1 eV low energy side of band edge emission. It can be seen that the peak (B peak in FIG. 2) disappears, the emission at the band edge is dominant, and the ZnO thin film is a high-quality film having few crystal defects and high crystallinity.
(Example 2)
Next, using the ion plating method which is the film forming method of the present invention, ZnO films were prepared by changing only the Hearth potential among the manufacturing conditions of the above-described examples.
[0054]
FIG. 3 shows the result of PL measurement performed at 77K, and FIG. 4 shows the result of PL measurement performed at room temperature. In FIG. 3 and FIG. 4, the numerical value of the voltage assigned to each PL spectrum is the Haas potential. In addition, each PL spectrum is adjusted so that the waveform becomes substantially the same in each PL spectrum by magnifying the vertical axis at the magnification given thereto.
[0055]
When the PL spectrum measured at room temperature in FIG. 3 is observed, the ZnO film manufactured at a Hearth potential of 28.3 V has a defect level in the vicinity of 2.2 eV, and the peak of band edge emission is small. On the other hand, as the Haas potential is increased, the defect level in the vicinity of 2.2 eV disappears, and the peak of band edge emission increases. It can be seen that when the Haas potential is 30 V or higher, a ZnO film having no deep level of at least 300 meV, in other words, having only band-edge light emission and few crystal defects and good crystallinity can be obtained.
[0056]
The same tendency as in FIG. 3 is also observed for the PL spectrum measured at 77K in FIG.
[0057]
【The invention's effect】
According to the method for forming a zinc oxide thin film according to the present invention, a method for forming a zinc oxide thin film by an ion plating method, comprising zinc oxide as a main component in a hearth disposed as an anode in a film forming chamber. The deposition material is arranged, the magnetic field control member arranged around the hearth is used to control the upper magnetic field close to the hearth, and a high-density plasma beam by arc discharge is supplied to the deposition material to ionize the deposition material. In order to obtain a thin film by depositing a vapor deposition substance on the surface of the substrate disposed opposite to the vapor deposition material, a zinc oxide thin film having good crystallinity can be uniformly formed over a large area at a high film formation rate. it can.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view of an ion plating apparatus used in a method for forming a zinc oxide thin film according to an embodiment of the present invention.
FIG. 2 is a diagram showing a PL spectrum of the zinc oxide film of Example 1, where (a) is a result measured at a temperature of 77 K, and (b) is a result measured at room temperature.
FIG. 3 is a diagram showing a result of measuring a PL spectrum of a zinc oxide film of a second example at room temperature.
FIG. 4 is a diagram showing the result of measuring the PL spectrum of the zinc oxide film of the second example at 77K.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 Vacuum container 30 Plasma gun 50 Anode member 51 Hearth 53 Material rod 60 Holding member 71 Oxygen gas supply apparatus 73 Nitrogen gas supply apparatus 78 Atmosphere control apparatus

Claims (3)

A method for forming a zinc oxide thin film by ion plating,
Arrange the deposition material mainly composed of zinc oxide in the hearth arranged as the anode in the film formation chamber,
The upper magnetic field close to the hearth is controlled by a magnetic field control member disposed around the hearth,
A high-density plasma beam by arc discharge is supplied to the vapor deposition material to evaporate and ionize the vapor deposition material, and attach the vapor deposition material to the surface of the substrate disposed opposite to the vapor deposition material,
By applying a voltage so that the hearth has a potential of +29.3 to +60 V with respect to the film formation chamber, a peak appears at a deep level of 300 meV or more from the peak of the band edge emission in the photoluminescence spectrum measured at room temperature. A method for forming a zinc oxide thin film, comprising: A method for forming a zinc oxide thin film by ion plating,
Arrange the deposition material mainly composed of zinc oxide in the hearth arranged as the anode in the film formation chamber,
The upper magnetic field close to the hearth is controlled by a magnetic field control member disposed around the hearth,
A high-density plasma beam by arc discharge is supplied to the vapor deposition material to evaporate and ionize the vapor deposition material, and attach the vapor deposition material to the surface of the substrate disposed opposite to the vapor deposition material,
By applying a voltage so that the hearth has a potential of +40.1 to +60 V with respect to the film formation chamber, a peak appears at a deep level of 300 meV or more from the peak of the band edge emission in the photoluminescence spectrum measured at 77K. A method for forming a zinc oxide thin film, comprising:   The vapor deposition material is a group 1 element, a group 3 element, a group 4 element, a group 5 element, and a group 7 element, and an oxide of a group 1, element, group 3, element, group 4, element, or group 7 element. 3. The method for forming a zinc oxide thin film according to claim 1, comprising at least one of the above.

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