Glow-discharge decomposition apparatus

0001The present invention relates to a glow-discharge decomposition apparatus, and the preparation of a semiconductor film by employing the glow-discharge decomposition apparatus.

0002In a conventional glow-discharge decomposition apparatus, a substrate 4 shown in Fig. 1 is horizontally provided on a ground electrode 3, wherein a RF-electrode 1, heater 5, and the like are arranged in a way as illustrated in Fig. 1. The numeral 10 is a port from which the gases enter into the chamber. The numeral 11 is another port through which gases are evacuated by an evacuator. In such an apparatus, a semiconductor film can be widely deposited on a substrate, but an excess discharge takes place at the rear side of the RF-electrode. To avoid the discharge at the rear side, a shield 7 must be prepared as in Fig. 1, but such a shielding results in an unstability of the glow-discharge.

0003Furthermore, a multi-planar electrode plasma etching device is know (US-A-4 381 965) which is provided for a plurality of planar thin-film semiconductor wafers. With this device patterning of the wafers is obtained by etching off unnecessary portions of the wafers while the thickness of the necessary portions of the wafers are not influenced by the etching process. RF power is used, and parallel electrodes are provided which are electrically insulated from each other, but the RF power supply is connected only to one electrode while the opposing electrode is grounded.

0004A further conventional apparatus shown in Fig. 2 is arranged for two-side deposition and is free from the shielding. In the apparatus, substrates 41 and 42 are vertically provided on ground electrodes 31 and 32, and a RF-electrode 1 is vertically placed in the center between the ground electrodes. Glow-discharge takes place over both sides of the RF-electrode 1. If necessary, the substrates 41 and 42 are heated by heaters 51 and 52. In such an apparatus, only one RF-electrode 1, one RF-power supply 9 and one matching circuit 8 can be provided, although two substrates are placed. However, the apparatus represents a certain handicap since the deposition rate on the substrate cannot be independently controlled when the required deposition rates on both sides are much different from each other.

0005To control the individual power to the RF-electrodes, there can be designed an apparatus in which two substrates are placed in the center of a chamber, two RF-electrodes 11 and 12 are faced to them, respectively, and two independent RF-power supplies 91 and 92 and matching circuits 81 and 82 are provided as in Fig. 3. Individual control of the discharge is realized by adjusting each electric output power, so that a control of deposition rates is realized. However, there might be an interference between RF-power supplies which supply RF-power to each electrode. Furthermore, two RF-power supplies and two matching circuits are needed, so that the apparatus as disclosed above, due to its complexity, costs too much and establishes some limitations.

0006In a similar known apparatus (US-A-4 287 851) opposite to one RF electrode two grounded electrodes supporting substrates for depositing an amorphous film are provided.

0007The object of the present invention is to provide a glow-discharge decomposition apparatus which gives different or same depositing rate on each substrate by an individual control of RF-powers supplied to each electrode with one RF-power supply and one matching circuit.

0008This object is solved by an apparatus and a method comprising the features of claims 1 and 13. Preferred improvements and embodiments are subject of the subclaims.

0009The present invention provides a glow-discharge decomposition apparatus which comprises a RF-power supply, matching circuit and controlling circuit having at least one electric element. The matching circuit is connected to accept a RF-power from the RF-power supply, and the controlling circuit is connected to accept RF-powers from the matching circuit. The outputs of the controlling circuit are connected to supply RF-power to the RF-electrodes.

0010There can be deposited films at a different or same rate on each substrate in the glow-discharge decomposition apparatus by controlling plasmas over each RF-electrode. The control is carried out by adjusting the controlling circuit which supplies RF-power to each of the RF-electrode.

0011The glow-discharge decomposition apparatus of the present invention makes it possible to deposite films on each substrate at a different or same rate by means of one RF-power supply, one matching circuit and one controlling circuit. <ul id="ul0001" list-style="none"><li>Fig. 1 is a diagram of a conventional horizontally arranged glow-discharge decomposition apparatus arranged for one-side deposition;</li><li>Fig. 2 is a diagram of a conventional two-substrate type glow-discharge decomposition apparatus arranged for two-side deposition;</li><li>Fig. 3 is a diagram of a conventional glow-discharge decomposition apparatus including two independent RF-systems;</li><li>Fig. 4 is a diagram of an embodiment of a glow-discharge decomposition apparatus in accordance with the present invention;</li><li>Figs. 5, 6, 7 and 8 are diagrams, respectively, illustrating a controlling circuit;</li><li>Fig. 9 is a diagram of electric elements of RF-conducting plates;</li><li>Fig. 10 is a diagram illustrating a matching circuit; and</li><li>Figs. 11 and 12 are explanatory drawings as to an arrangement of an apparatus having plural sets of RF-electrode, substrate and heater.</li></ul>

0012The apparatus of the present invention is explained referring to the diagram of Fig. 4, wherein RF-electrodes 11 and 12 are standing in parallel and insulated from each other by an insulator 2. A controlling circuit having a variable capacitor 61 and a fixed capacitor 62 is connected to the RF-electrodes 11 and 12 outside a reacting chamber. RF-power is generated by a RF-power supply 9 and divided into two through a matching circuit 8, thereby, the divided RF-power is supplied to the RF-electrodes 11 and 12, respectively. In parallel to the RF-electrodes 11 and 12, ground electrodes 31 and 32 are placed. Substrates 41 and 42 are provided on the ground electrodes 31 and 32, respectively. Heaters 51 and 52 can be used to heat the substrates 41 and 42. With respect to the RF-electrodes 11 and 12, the insulator 2 can be replaced by another member as far as it fixes and insulates the RF-electrodes.

0013In the embodiment as described in Fig. 4, the rate of deposition is individually controlled by adjusting the capacitors which are respectively connected to the RF-electrodes.

0014Examples of methods of adjusting the capacitor are e.g. a method of checking the thickness of the film after a testing deposition, a method of detecting the strength of the glow-discharge by eye, a method of detecting the strength of the glow-discharge by OES (Optical Emission Spectroscopy), and the like. In the last case, the adjustment is carried out by a manual operation or an automatic operation. The automatic operation is carried out by an automatic-mechanism combined with a detector and a servo-motor. The adjustment by the use of the automatic-mechanism is the most preferred in those methods.

0015The controlling circuit includes electric elements, such as the series capacitors in Fig. 4, a couple of capacitors or inductors which are connected in series to the RF-electrodes or in parallel to the ground. The explanatory diagrams of those connections are illustrated in Figs. 5, 6, 7 and 8. Another combination of the electric elements can be designed. For instance, only one element is connected to one of the RF-electrodes, and the other RF-electrode is directly connected to a branch of the matching circuit.

0016In conclusion, the controlling circuit having electric elements is capable of being adjusted to control RF-power derived from the matching circuit and to supply the RF-power to the RF-electrodes.

0017The inductor in the invention is, for example, a spiral coil, or a RF-conducting plate having inductance due to its prescribed shape. Fig. 9 shows the latter case, wherein the inductor is a pair of RF-conducting copper plates. The inductance can be varied to some extent depending upon the shape or length of the plate.

0018When the adjustment of the capacitance or inductance is carried out to vary or equalize the strength of the plasma, electrical parameters of the whole circuit are also changed. Thus, the matching cirucit must be tuned again under the RF-power condition in use. The matching circuit includes a variable capacitor in most cases. Fig. 10 shows an illustrative diagram of a typical matching circuit.

0019In spite of the explanation of the disclosure, "a matching circuit" can also be defined so as to mean a circuit including both the controlling circuit and the matching circuit as described above.

0020The distance between the RF-electrodes 11 and 12 which are insulated by the insulator 2 is optionally determined, for example, 1 to 200 mm under ordinary conditions. The distance between the RF-electrode and the substrate is preferably 5 to 50 mm and more, preferably 10 to 30 mm from a viewpoint of stability and uniformity of the plasma.

0021The area of the RF-electrode is preferably not more than 1 m², and when a wider area is required, a plurality of RF-electrodes, each of which is limited within 1 m², can be used. The arrangement of such RF-electrodes is shown in Fig. 11. Such an arrangement allows a deposition onto a wide area, wherein each of the RF-electrodes is electrically connected in series. As pointed out in the foregoing, the area of each RF-electrode is limited within 1 m² in the parallel-plate electrodes arranged as shown in Figs. 1 to 12.

0022The apparatus can have a substrate carrying means that transfers substrates before, during or after the deposition. The substrates can be carried in one way or right and left keeping a distance to the RF-electrode. The substrates face in parallel to the RF-electrode during movement. Also, the substrates can be moved in one direction keeping a distance to the RF-electrode. An example of the embodiment is a multi-chambered depositing apparatus, wherein the substrates in one chamber are transfered to another chamber. The movement in one direction is preferable when a film is successively deposited on a long continuous substrate which is moving from one chamber to another.

0023Uniformity of the thickness of the film can be preferably obtained by shifting the substrate right and left as mentioned above. The RF-electrodes and substrates can be vertically, horizontally or obliquely arranged as far as they are set in parallel to each other. However, a vertical arrangement permits a deposition of an excellent film since the vertical arrangement prevents dusts from falling onto the substrates.

0024If necessary, the substrates can be heated by a heater. A temperature of the substrates is selected depending on the composition of the film or the using purpose for the deposited film. In an ordinary condition, the temperature is preferably employed in the range of 50^o to 400^oC.

0025The apparatus of the invention consists of RF-electrodes, substrates and heaters as described above. In such an apparatus, one RF-electrode, one substrate and one heater make up a fundamental set for deposition. The apparatus may have a pluarity of those sets as in Fig. 12. The number of those sets can be 1 to 100, preferably 1 to 10 in the invention.

0026In an apparatus for depositing a film by glow-discharge plasma, any type of apparatus can be utilized. However, when a multi-chambered apparatus is used, a film can be prepared by the present invention in each of the chambers, wherein p, i and n-layers of the semiconductor are successively deposited. In the embodiment, the multi-chambered apparatus has slits or gate valves on walls for dividing chambers so as to enter the substrates into the neighboring chamber. Gas in the chamber can be exhausted by a differential evacuator. The differential evacuator is a means to successively evacuate gas in each chamber during a substrate in one chamber moving into the next chamber. When a differential evacuator is provided in the multi-chambered apparatus, the substrates can be continuously transferred from chamber to chamber without opening or shutting the gate valves. Such a function in the multi-chambered apparatus yields an increase of productivity.

0027The process for preparation of amorphous-semiconductor films by the apparatus of the invention is as follows: A glow-discharge takes place in an atmosphere of gases containing silicide, carbonate, nitride, dopant gases and inert gases under 1,3 to 666,6 Pa (0.01 to 5 Torr), at a RF-frequency of 1 to 100 MHz, and under a RF-power density for the deposited area of 0.003 to 0.2 W/cm² (The RF-power is 0.1 to 5 W/cm² when microcrystalline film is desired), and a film of 0.005 to 100 µm is deposited on the substrate.

0028In such a process of the invention, a uniform and homogeneous deposition over a wide area of substrate can be performed. Moreover, owing to the stability of the glow-discharge plasma in the apparatus, an excess discharge is avoided, so that the RF-power can be efficiently utilized.

0029In the arrangement of the invention, the use of RF-power is fully improved. A film of an electric device such as p-i-n diode, p-n diode, solar cells having heterojunction or homojunction, sensor, TFT (Thin Film Transistor) and CCD (Charge Coupled Device) can be prepared according to the arrangement of the invention. And a sensitized film for electrophotography, LSI passivation film, insulating film for printed-circuit, or the like, is also preferably provided. Especially, owing to the stability of the plasma in the apparatus, an amorphous silicon solar cell having a high efficiency of more than 10 % can be prepared over a wide area by the process of the invention.

0030Examples according to the present invention are described in the followings.

Example 1

0031A film was prepared by the glow-discharge decomposition apparatus shown in Fig. 4.

0032The RF-electrodes (500 mm X 560 mm) were insulated by an insulator of 4 mm thickness. The RF-power was supplied to the capacitors via the matching circuit. The RF-frequency was 13.56 MHz and the capacitances were 250 pF in the fixed capacitor and 500 pF (maximum) in the variable capacitor. A p-i-n semiconductor film was prepared on a transparent ITO/SnO₂-glass substrate of 40 cm². The temperature of the substrate was about 200^oC.

0033First, a p-layer was deposited in 10nm (100 Å) under a mixed gas of SiH₄ (50 % by mole) and CH₄ (50 % by mole) including 0.05 % by mole of B₂H₆. Next, an i-layer was deposited in 600nm (6000 Å), and at last a n-layer was deposited in 50nm (500 Å) under the mixed gas including 0.2 % by mole of PH₃. The capacitances of the fixed and variable capacitors were 250 pF and 350 pF, respectively.

0034Thereon, a layer of aluminum in 100 nm (1000 Å) thickness was deposited as a backing electrode by electron-beam-evaporation. The conversion efficiencies of the solar cells prepared by the above-described manner were respectively measured by means of a AM-1 solar simulator of 100 mW/cm². The efficiencies were distributed at the average of 11 %, the maximum of 11.7 % and the minimum of 10.4 %. The deposition rate was 1nm (10 Å) per second. The thickness of the obtained films on the two substrates was identical.

Example 2

0035Films were prepared in the same manner as in Example 1, but the capacitance of the variable capacitor was varied from 10 to 500 pF. The deposition rates of the obtained films prepared on the both substrates are shown in Table 1. <tables id="tabl0001" num="0001"><img file="EP0165618B1_D0001.tif" /></tables>