1. Field of the Invention
The present invention relates to a plasma film-forming method and a plasma chemical vapor deposition (CVD) device capable of forming a thin film on a substrate by using a capacitively-coupled plasma (CCP) CVD technology, in particular, to a film-forming method and a film-forming device for forming a nitride film used in a reflective film of a crystalline solar cell.
2. Description of Related Art
Film-forming devices capable of fabricating a thin film on a substrate already exist. Such a film-forming device is a plasma CVD device, which is used for fabricating thin films for solar cells and various types of semiconductors like a thin-film transistor (TFT) array in a liquid crystal display (LCD).
An in-line plasma CVD device has a load chamber, a reaction chamber, and an unload chamber. In the device, a substrate loaded on a susceptor, such as a tray, is sequentially moved among the load chamber, the reaction chamber, and the unload chamber while being processed, so as to form a thin film thereon. A lamp heater etc. is disposed in the load chamber, and the lamp heater is used for heating the substrate when the substrate is loaded on the tray. Afterwards, the substrate is delivered into a vacuum chamber and then delivered into a film-forming chamber. In the film-forming chamber, one or more types of chemical gases containing elements for composing a thin-film material is introduced and decomposed by using a high-frequency electric-power CCP, so as to form a thin film on the substrate through CVD. Finally, the substrate with the thin film formed thereon is removed from the unload chamber.
FIG. 9 is a diagram exemplifying a configuration of a CVD device having a film-forming chamber and a heating chamber in a vacuum chamber. A CVD device 101 in FIG. 9 includes a preheating chamber 102 (102A, 102B), a film-forming chamber 103 (103A, 103B), and an unload chamber 104 arranged in line. In the device, a substrate 120 loaded on a tray 109 is delivered while being preheated, so as to form a film thereon. Heaters 107 are disposed in the preheating chamber 102 and the film-forming chamber 103.
After a film is formed on the substrate 120 in the film-forming chamber 103, the substrate 120 is guided out of the unload chamber 104, unloaded from the tray 109, and delivered for subsequent process (not shown). Meanwhile, the tray 109 with the substrate 120 unloaded there-from is returned to a substrate transfer device 106 via a tray return belt 105, so that the tray 109 is once again loaded with an unprocessed substrate 120 by the substrate transfer device 106. The loaded tray 109 is then guided into the preheating chamber 102 for the substrate 120 to be processed by the film-forming process.
A gas is introduced into the film-forming chamber 103, and a radio-frequency (RF) electric power that has been impedance-matched in a match box 103c is supplied from an RF power source 103a to a high-frequency electrode 103d, so that a plasma is formed. Previously, the RF power source continuously supplies an RF electric power to the high-frequency electrode.
During the film-forming process of the plasma CVD device, in order to raise the deposition rate, the supplied electric power needs to be increased. However, the increase of the supplied electric power may result in a problem of an increased probability of an abnormal arc discharge.
In the prior art, the following method is mainly adopted to suppress the arc discharge. In order to protect an RF power source, the power source is cut off when an arc discharge occurs, and is restarted after a specified time period has been elapsed. FIGS. 10(a) and 10(b) are diagrams of arc discharge suppression in the prior art. FIG. 10(a) shows a state of an RF reflected power returned from a load to an RF power source, and FIG. 10(b) shows a state of an RF input power input from the RF power source to the load.
Once an arc discharge occurs, the RF reflected power returned from the load to the RF power source is increased. The RF power source detects the RF reflected power, compares the detected RF reflected power with a predetermined threshold, and detects the occurrence of the arc discharge when the RF reflected power exceeds the predetermined threshold (indicated by A in FIG. 10(a)). Upon detecting the occurrence of the arc discharge, the RF power source controls the electric power in the following manner. The RF electric power is stopped from being supplied to the load (indicated by B in FIG. 10(b)) and the electric power is cut off for a specified time period (for example, 0.01 sec) (indicated by C in FIG. 10(b)), so that the arc discharge disappears. Afterwards, the input electric power is gradually increased to reach the predetermined threshold (indicated by D, E in FIG. 10(b)), and the supply of the electric power is thereby resumed.
However, the above control method of detecting the occurrence of arc discharge by monitoring the RF reflected power and cutting off the electric power after the occurrence of the arc discharge has the following problems. As long as an arc discharge occurs at least once, it is impossible to suppress the later occurring arc discharge if the arc discharge does not occurred again; and when an arc discharge occurs again, the electric power control must be performed to cut off the electric power at any time. However, the occurrence of a micro-arc discharge that cannot be observed as a reflected power may not be suppressed.
Furthermore, it has been proposed that waveforms other than an alternating current (AC) waveform may be used in the electric power supply process of the plasma CVD device. For example, it has been proposed that a pulse capable of pulsing a voltage and shortening a voltage rising time is used to replace the AC waveform, so as to facilitate the generation of the plasma.
Moreover, it has been proposed that a supplied pulse voltage may be intermittently cut off before an arc discharge is about to occur, instead of continuously supplying the RF electric power. Hence, the arc discharge is avoided and a stable glow discharge is maintained. Besides, it has been proposed that an arc discharge is prevented by setting an intermittent power supply in which a voltage is stopped from being applied at a proper time interval (referring to Patent Document 1).
In addition, it has been proposed that a pulse electric field is applied between opposite electrodes, so that a raw gas is introduced to form a glow discharge plasma. Thus, in an environment containing no component such as a helium gas that lasts for a long time from a plasma discharge state to an arc discharge state, the discharge is stopped before the occurrence of arc discharge and then the discharge is restarted, so as to generate a stable discharge plasma through such a discharge cycle. Preferably, a rise time (when the voltage (absolute value) continuously rises) and a fall time (when the voltage (absolute value) continuously drops) of the pulse electric field are smaller than or equal to 100 μs; and one pulse duration is smaller than or equal to 200 μs (referring to Patent Document 2).    Patent Document 1: Japanese Patent Publication No. 2004-14494 (Paragraph 0005, Paragraph 0011, and Paragraph 0014)    Patent Document 2: Japanese Patent Publication No. 2002-110587 (Paragraph 0011, Paragraph 0051, and Paragraph 0055)
FIG. 11 is an energy distribution diagram of an electron temperature of a glow discharge plasma. Referring to FIG. 11, for a high-density low-electron-temperature plasma, electrons are distributed at high density in areas of a low electron temperature; whereas for a low-density high-electron-temperature plasma, electrons are distributed at a low density in areas of a high electron temperature. In this case, if frequencies of 13.56 MHz and 250 KHz are being compared, a high-density low-electron-temperature plasma is formed by a plasma CVD device using an RF power source of 13.56 MHz, and a low-density high-electron-temperature plasma is formed by a plasma CVD device using an RF power source of 250 KHz.
In the plasma CVD device using an RF power source having a common industrial frequency, that is, 13.56 MHz, for the glow discharge plasma, even if the electric power is cut off, the plasma may not disappear instantly. Instead, the plasma state is maintained for a time period of about 100 μsec to 150 μsec. Generally, the reason is that, due to the recombination of ions with electrons in the plasma, the disappearance of the diffusion of electrons to the wall and the combining of electrons with neutral gas molecules in the plasma, the plasma density is gradually reduced until it is impossible to maintain the plasma, and thus the plasma disappears.
If the energy supply is cut off, the vibration energy of the electrons instantly disappears, so that energy in the micro amount of high-temperature electrons that possibly causes the arc discharge disappears within a time period much shorter than the reducing time of the overall plasma density.
Therefore, as for the high-density low-electron-temperature plasma, during most of the time within 150 μsec after the electric power is cut off, the low-temperature electrons take up a large proportion in the residual plasma, while the density of the high-temperature electrons is low. Hence, it is difficult to cause an arc discharge in this state.
It is known to all that, when the plasma CVD thin-film forming device is used, an arc discharge is sometimes caused by high-temperature electrons. Therefore, when an RF power source having a frequency greater than or equal to 13.56 MHz is adopted to form a high-density low-electron-temperature plasma, as described in the prior art, the electric power may be cut off to eliminate the high-temperature electrons in the plasma; thus, an arc discharge may be precluded.
When a low-frequency RF power source of about 250 KHz is adopted, the plasma is a low-density high-electron-temperature plasma, so that the electron temperature is distributed at a high temperature range, which may easily cause an arc discharge.
The CCP obtained by using a low-frequency RF power source of about 150 KHz to 600 KHz is generally a low-density high-electron-temperature plasma.
In a CCP-CVD device used for foaming a nitride film of a crystalline solar cell, a CCP for supplying an RF electric power provided by a low-frequency RF power source of about 150 KHz to 600 KHz to an electrode for forming a plasma is adopted. The plasma generally has a low density but a high electron temperature. Referring to FIG. 11, the low-density high-electron-temperature plasma contains high-temperature electrons having high energy, so that an arc discharge may easily occur in this state.
Furthermore, when the nitride film of the crystalline solar cell is formed, the temperature of the substrate in the CVD process is greater than or equal to a high temperature of 400° C., and the obtained nitride film is an insulation film. Further, the electrode has a large area, which is greater than or equal to the area of a square having a side length of 1 m. Hence, an arc discharge is easily resulted during the CVD process.
Therefore, when a CCP device using a low-frequency RF power source is adopted to form the nitride film of the crystalline solar cell, it is considered to be in an environment in which an arc discharge is extremely likely to occur.
During the CVD process, if an arc discharge occurs on the surface of the electrode, on the substrate, near the substrate, inside the film-forming chamber or on the wall of the chamber, it is impossible to produce a uniform thin film. Besides, a large number of particles are generated due to the arc discharge, and the generated particles not only lead to defects of the substrate, but also interrupt the operation of the CVD process itself.
The occurrence of an arc discharge is detected, and the electric power is cut off each time when an arc discharge is detected, so that the arc discharge disappears. In such an arc discharge suppression method in the prior art, the generation of the particles is sometimes inevitable, and the substrate is damaged due to the arc discharge. Moreover, the arc discharge may occur partially, so that the uniformity of the plasma is destroyed, and the uniformity of the thin film cannot be guaranteed. Furthermore, the occurrence of a micro-arc discharge that cannot be observed as a reflected power may not be suppressed.
In addition, as disclosed in Patent Document 1, the arc discharge is prevented by setting an intermittent power supply in which a voltage is stopped from being applied at a proper time interval, but how to select an intermittent time interval for the intermittent power supply is not exemplified. Besides, as exemplified in Patent Document 2, a rise time (when the voltage (absolute value) continuously rises) and a fall time (when the voltage (absolute value) continuously drops) are made to be smaller than or equal to 100 μs. In an example, a certain voltage is applied in a time range of 3 mμs to 200 mμs, but the references for determining the cut-off time or the time for applying a certain voltage are not explicitly described with respect to specific values.
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