Abstract:
A discharge electrode array for a silicon-based thin film solar cell deposition chamber is provided, relating to solar cell technologies. The discharge electrode array includes a signal feed component having a rectangular-shaped end, a flat waist corresponding to a feed-in port located in a hallowed rectangular area on a center region of a cathode plate having a shielding cover, connecting a feed-in power supply signal by surface contact. The electrode array includes at least a set of cathode plates and an anode plate, with two cathode plates sharing or surrounding one anode plate. Uniform large area and stable discharge driven by the RF/VHF power supply signal can be achieved, and the standing wave and the skin effect can be effectively removed. The production efficiency can be improved and the cost can be reduced.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to solar cell technologies and, more particularly, to a discharge electrode array in a deposition box driven by VHF (27.12 MHz˜100 MHz) power supply signals for silicon-based thin film solar cells. 
     BACKGROUND 
     Currently, silicon-based thin film solar cells often use plasma enhanced chemical vapor deposition (PECVD) to deposit single-junction or multi junction photovoltaic PIN film layers. This type of radio-frequency (RF) capacitively-coupled parallel plate reactor is commonly used in the thin film solar cell industry. The deposition process, such as plasma-enhanced chemical vapor deposition, is conducted in the reaction chamber through electrode plate components within electrode plate arrays. RF capacitively-coupled parallel-plate electrode reaction chamber is widely used in various kinds of large-area thin-film deposition of amorphous silicon, amorphous silicon germanium, silicon carbide, silicon nitride, and silicon oxide materials, etc. Industry-wide, the electrode with a supporting frame is usually called a clamping unit, a holder, or a fixture, and the plasma chemical vapor deposition apparatus with installed holders inside the vacuum chamber is often called the “deposition box,” i.e., the reaction chamber. 
     The silicon thin film solar cell sector is an important branch of the solar energy industry, and the parallel plate electrode capacitive discharge pattern is one of the core technologies of the solar cell industry. Further, 13.56 MHz RF is widely used in high-speed amorphous silicon thin film deposition with high production efficiency and low process cost. With the rising demand for silicon thin film technology, more attentions have been given to microcrystalline and nanocrystalline silicon thin film materials. 
     However, in a microcrystalline environment, plasma generated by 13.56 MHz RF may have low plasma concentration, low deposition rate, long deposition period to reach targeted film thickness, and significant background pollution. Thus, the prepared thin film often has high impurity and poor photoelectric properties, which seriously affects the quality and performance of the products. How to make high-speed deposition becomes key for crystalline silicon thin-film technology to successfully serve the industry. 
     Very high frequency (VHF) is referred to the legitimate frequency which is twice or more of 13.56 MHz. In the industry, the VHF mostly used is generally in the range of 27.12˜100 MHz. However, in the capacitive discharge model, standing wave effect and skin effect caused by VHF become very obvious, and these effects become stronger when the driving frequency increases. Professor M. A. Lieberman of University of California, Berkeley made a thorough investigation on these two effects. His research results show that the critical condition for VHF PECVD deposition of uniform thin films is that the free space wavelength of excitation frequency (λ 0 ) is much larger than the capacitive discharge electrode chamber size factor (X), and the skin depth (δ) is much larger than the thickness tolerance factor (η 0 ). For example, on 1 m 2  of discharging area and with an excitation frequency of 60 MHz, λ 0 ≈X and δ≈η. Therefore, under this excitation frequency, the skin effect and the standing wave effect become very obvious, leading to an uneven discharge on the electrode plate of 1 m 2 . Thus, how to achieve a large area of uniform discharge driven by VHF is one of the technical problems to be resolved for the crystalline silicon thin-film technology. 
     This also caused great interest in the industry. In 2003, U.S. Patent 2003/0150562A1 disclosed a method using a magnetic mirror in the capacitively-coupled discharge to improve the inhomogeneity caused by VHF. Chinese patents 200710150227.4, 200710150228.9, and 200710150229.3 disclosed three electrode designs of VHF, applying different feed-in forms of VHF signals to obtain uniform electric fields. 
     However, the following problems may still remain: 1) The electrodes in the VHF-PECVD chamber have complex design structures; 2) One reason for the continuous improvement is that the constant assembly/disassembly and cleaning of the reaction chamber and electrodes can cause abnormal deformation of the electrodes; 3) The multi-point feed-in structures disclosed in the existing patents may have a small contact surface, which requires symmetrical paths of individual feed-in points and there is no contact between the bonding conductors at the feed-in points and the cathode plate. More specifically, a shield of isolation may be needed between the bonding conductor and the cathode plate for effective discharge. These structural designs have relatively harsh actual requirements, have too many determination factors for uniform discharge, and cannot meet the actual production needs such as disassembly and cleaning. 
     Therefore, for the equipment used by the industry, a single point feed-in becomes the mainstream design. But due to the standing wave effect and the skin effect, current single-point feed-in structures cannot meet the requirement for increasing the high feed-in frequency. Thus, further development and improvement may be needed to make the existing deposition holders more practical to meet the current market demand and to reduce the cost. Meanwhile, it is also a trend to use CVD reactor system capable of processing or depositing multiple glasses. Therefore, it is of great practical significance for the industry to apply an effective VHF feed-in model to meet the demand of mass production and to enter the industrial production stage. 
     CONTENTS OF THE INVENTION 
     The objectives of the invention include solving the non-uniformity problem of VHF power-driven discharge system, and providing a large-area VHF-PECVD deposition chamber with a uniform electric field through using a new conceptual design of the electrode array having electrode plate components, which is applied in the production of large area VHF-PECVD electrode multi-plate array. 
     Accordingly, the deposition box technology solutions in the invention include: electrode plate components and signal feed-in components. The electrode plate components having shielding covers and the signal feed-in components form an electrode array. Specifically, the electrode array includes at least one group of cathode plates and an anode plate, two signal feed-in components respectively corresponding to two working surfaces of the cathode plates by feed-in ports in surface contact with the working surfaces, and input Radio Frequency (RF) or Very High Frequency (VHF) power supply signals. 
     The feed-in port is located in a hollowed rectangular area at the center of the backside of an electrode plate component; and the signal feed-in component comprises a Z-shape feed-in belt having a copper feeding core with an outer insulating layer. One end surface of the feed-in belt is in a rectangular shape. 
     In certain disclosed technical solutions, the electrode array includes an anode plate, cathode plates, shielding covers of the cathode plates, and signal feed-in components coupled to the feed-in ports. The anode plate has two working surfaces, and the two working surfaces respectively faces effective working surfaces of two symmetrically-arranged cathode plates facing towards the two working surfaces of the anode plate. The signal feed-in component comprises a Z-shape feed-in belt having a copper feeding core with an outer insulating layer. One conductive end surface of the feed-in belt is in a rectangular shape. The electrode array can include multiple sets of signal feed-in components coupled to cathode plates with shielding covers and less than half cathode plate number of grounded anode plates to form an array of electrode plates with a certain discharge distance/space. 
     The single-side discharging cathode plate has a working surface and non-working surfaces. The shielding cover, formed by a ceramic insulating layer and a shielding layer, covers the backside and surrounding sides of the cathode plate, providing shielding of the center position of the backside of the cathode plate and the surrounding sides for feeding in RF/VHF power supply signals. 
     With respect to the signal feed-in component, one end is in a rectangular shape and connects with the feed-in port, and the other end connects with a negative output port of the RF/VHF power supply signal and a power supply matching device. 
     The solution of the present invention provides a method, in which the electrode array, formed by the electrode plate components and signal feed-in components, uses a surface feed-in mode. The one end of the signal feed-in component is of a rectangular shape and makes surface contact with a feed-in port of the electrode plate component to feed-in RF/VHF power supply signals. The feed-in port of the electrode plate component is located in a hollowed rectangular area at the center of the backside of the cathode plate with the shielding cover. 
     The disclosed electrode array includes multiple sets of signal feed-in components coupled to electrode plate components to feed-in RF/VHF power supply signals to the feed-in ports of the electrode plates in a surface feed-in mode. The signal feed-in component may be a Z-shape metal belt or strip. One end is in a rectangular shape and the outer shell is insulated and shielded. The belt has a ceramic insulating layer and a metal feeding core, which has VHF signal feeding lines. 
     The beneficial effects of the present invention, different from the slot-type cathode plate with side-feed modes, include that the present invention can achieve the higher uniformity, greater discharge area and more stable discharge performance. Further, the connection capacitance is smaller, the actual discharge power is greater, and the radio frequency interference between electrode plate arrays is smaller. Also different from cathode plate center-point-type feed mode of the single chamber deposition system, the connection capacitance is smaller, the standing wave and the skin effect is smaller or is effectively eliminated, and the integrated array type multi-chamber deposition can be obtained to greatly improve production efficiency. Therefore, through optimizing VHF power feed-in mode and the electrode plate structure, the solution of RF/VHF discharge uniformity can be obtained, and it is the premise of high efficient preparation technology for crystallized silicon thin film. The invention is applicable for any power and legitimate VHF frequencies in the range of 27.12 MHz˜200 MHz for large area uniform discharge. This structure can be applied to multiple-glasses deposition systems for greatly improving the productivity and reducing the cost of solar cells. The invention breaks through the conventional electrode design technical limitations, effectively eliminates the VHF inducing effects such as a standing wave and the skin effect, and improves uniform discharge to industrial application level. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an exemplary electrode array and embodiment 1 according to the present invention; 
         FIG. 2  is a schematic diagram of the signal feed-in component  201  of  FIG. 1 ; 
         FIG. 3  is a schematic diagram of the cathode plate  203  of  FIG. 1 ; 
         FIG. 4  is a schematic diagram of the cathode plate shield  204  of  FIG. 1 ; 
         FIG. 5  is a schematic diagram of the Embodiment 2 of the invention; 
         FIG. 6  is a schematic diagram of the Embodiment 3 of the invention; 
         FIG. 7  is a sampling distribution diagram of film thickness testing; 
         FIG. 8  illustrates thickness distributions of the microcrystalline silicon films deposited on the electrode plate by four different electrode glasses according to embodiment 2 of the present invention; 
         FIG. 9  is a typical Raman spectra of the microcrystalline silicon in Embodiment 2 of the present invention; and 
         FIG. 10  illustrates thickness distributions of the microcrystalline silicon films deposited on the electrode plate by twenty-four different electrode glasses according to embodiment 3 of the present invention. 
     
    
    
     In  FIG. 1-5 , the electrode plate component includes a cathode plate  203  and cathode shielding cover  204 , between which there is placed an insulation layer  207 , and anode plates  208 . The cathode shielding cover  204  and the anode plates  208  are grounded. The Z-shaped signal feed-in component  201  includes a waist having an insulation cover  202  and a rectangular-shaped signal feed-in surface  201 - 1 , which corresponding to the feed-in port  203 - 1  located at a hollowed rectangular area at the center of the backside of the cathode electrode plate. The waist is flat for easy installation and less signal feed-in loss. The electrode array discharges in the vacuum chamber  01  to deposit the P-I-N film layer on substrate  206 . The vacuum chamber  01  contains the feed-in inlet  101  of gas access system, the power supply feeding system inlet  102 , vacuum chamber door, and the feed-in inlet  105  of the vacuum system. 
     The disclosed electrode array achieves the above proposed invention task through a surface contact feed-in mode. It has overcome many problems from multi-point feed-in techniques in VHF-PECVD deposition of crystallized silicon thin film, such as that the electrode structure of reaction chamber is complex, electrode easily deforms, contact area is small, path and distance between the feed-in points require completely symmetric and fully shielded, etc. The surface feeding deposition box design of the invention solves these problems and can obtain a large area chamber discharge with uniform electric field. It especially can achieve high efficient utilization of the dual work surfaces of the anode plate. Meanwhile, for the CVD electrode array system for treating or depositing multiple pieces of glass substrates, the effective VHF surface feed-in mode is used so that the industrial production operation process is achieved and the silicon-based thin film solar cell mass production needs can be met. 
     Contribution of this invention includes providing a desired solution to uniformity and consistency problems of thin-film deposition driven by a very-high-frequency (VHF) electrical power source at a high deposition rate. The electrode array comprising anode plates  208 , cathode plates  203 , cathode plate shielding covers  204 , and signal feed-in components  201  is installed inside the vacuum chamber  01 . Feed-in port  203 - 1  at the center region of the cathode plate is rectangular-shaped. The feed-in end  201 - 1  of the corresponding signal feed-in component  201  is also rectangular-shaped. The feed-in components  201  is Z-shaped, having a flat waist for easy installation and with less feed-in signal loss. The end of the feed-in component is of a rectangular shape, connects in surface-contact mode with the electrode plate to form the electrode-plate component in the vacuum chamber with grounding setting, which all have insulating shields. 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Embodiment 1 
     The principle of this embodiment is illustrated with  FIGS. 1-4 . As shown, two cathode plates  203  surround one anode plate  208  to form two pairs of electrodes. Four pieces of substrates  206  can be deposited at the same time. More pairs of electrodes can be arranged in an array to improve efficiency of the electrode array. 
     The vapor deposition system includes vapor deposition chamber, gas system, electrical power system, vacuum system, heating system, control system, and so on (not all shown). The gas system mainly provides different gases and gas lines for vapor deposition. Electrical power system mainly provides high-frequency or very-high-frequency electrical power source to discharge plasma for film deposition. Vacuum system mainly provides vacuum pumping machines and vacuum pipelines. Heating system mainly supplies heat for vapor deposition chamber. Control system mainly controls the parameters of deposition process. Vapor deposition chamber is the apparatus for realizing thin film deposition with gases on substrate  206 . 
     The vapor deposition chamber mainly comprises vacuum chamber  01  and an electrode array. Vacuum chamber  01  is used to achieve vacuum. The electrode array is used to discharge plasma and deposit P-I-N thin film layers on substrate  206 . The electrode array includes cathode plate  203 , cathode shielding cover  204 , ceramic insulating layer  207 , anode plate  208 , signal feed-in component  201 , and outside shielding layer  202 . 
     The vacuum chamber  01  includes grounded metal groove  209 , used to fix anode plate  208 , cathode plate  203 , and cathode shielding cover  204 . Anode plate  208  is directly inserted into and contacts metal groove  209 , making cathode shielding cover  204  contact metal groove  209 . Ceramic insulating layer  207  is fixed between cathode  203  and cathode shield  204  to make these two parts out of contact. 
     Anode plate  208  and cathode shielding cover  204  contact metal grooves  209 , which contacts with the vacuum chamber  01  to be grounded. A feed-in port  203 - 1  is located in a hallowed rectangular area in the middle or center of the backside of the cathode plate  203 . The signal feed-in component  201  is a Z-shaped feed-in strip. One end of the feed-in belt is in rectangular shape and in surface contact with the feed-in port  203 - 1  on the cathode plate to feed the radio-frequency/very-high-frequency signal power source to the cathode plate  203 . The cathode shielding cover  204  covers the entire back and side surfaces of the cathode plate. 
     Through-hole  204 - 1  in the middle of cathode shielding cover  204  is configured corresponding to the feed-in port  203 - 1  and makes signal feed-in component  201  coming through cathode plate  203  without touching cathode shielding cover  204 . The feed-in component is covered with outside insulating and shielding layer  202  to avoid contacting with cathode shielding cover  204 . After substrates  206  are fixed on the electrode plates, the electrode array is placed in the vacuum chamber  01 . Desired vacuum state of vacuum chamber can be achieved by using the vacuum system. Then deposition gases are added into the vacuum chamber, and thin-film vapor deposition process can be completed. 
     Embodiment 2 
     The cathode plate has a rectangular feed-in port. The feed-in component has a flat waist as the feed-in belt, one end of which is in a rectangular shape and in surface contact with the feed-in port of the cathode plate. 
     The electrode array in  FIG. 5  is similar to that in Embodiment 1. A vertical deposition box or reaction chamber is used. Two cathode plates  203  surround one anode plate  208  to form two pairs of electrodes, and 4 glass substrates  206  can be processed. In such a configuration, four substrates can be coated with thin films at the same time. Detailed processes are illustrated as follows: 
     a) Placing 4 glass substrates  206  (1640 mm×707 mm×3 mm) with 600-nm transparent conducting thin films in the substrate position in vacuum chamber  01 . Film side of the substrate faces outside, while glass side of the substrate faces toward electrode plate. 
     b) Filling the vacuum chamber with argon when vacuum reaches 5.0×10 −4  Pa. When the pressure reaches approximately 60 Pa, turning on the 40.68 MHz very-high-frequency power source, and cleaning the chamber with 400 W plasma discharge for 2 minutes. Then turning off the electrical power source. 
     c) Afterwards, pumping down the system to a high vacuum of ˜5.0×10 −4  Pa, and then wash the system with argon twice. 
     d) Adding gas mixture (silane and hydrogen) to the chamber with a flow rate of 5 slpm. Turning on the 40.68 MHz very-high-frequency power source when pressure of the chamber reaches 60 Pa to discharge with 400 W power and deposit micro-crystalline intrinsic silicon thin films for 40 minutes. 
     e) Turning off the power source, and pump down the system to high vacuum. 
     f) Filling the chamber with nitrogen gas to atmospheric pressure, open the door of the chamber, and then cool the TCO glasses at room temperature. 
     After the completion of the deposition process, 40 points on each glass substrate  206  are sampled as shown in  FIG. 7  (as location numbers) to detect the thickness of the various sampled points. The microcrystalline silicon film thickness testing results of a substrate  206  are shown in Table I: 
     
       
         
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                 The unit of the film thickness is nm, the numbers in 
               
               
                 the parentheses are the location numbers in FIG. 7 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 697 
                 717 
                 721 
                 724 
                 719 
                 718 
                 704 
                 710 
               
               
                  (1) 
                  (2) 
                  (3) 
                  (4) 
                  (5) 
                  (6) 
                  (7) 
                   8) 
               
               
                 719 
                 741 
                 741 
                 754 
                 740 
                 735 
                 727 
                 721 
               
               
                  (9) 
                 (10) 
                 (11) 
                 (12) 
                 (13) 
                 (14) 
                 (15) 
                 (16) 
               
               
                 727 
                 737 
                 740 
                 740 
                 739 
                 740 
                 738 
                 724 
               
               
                 (17) 
                 (18) 
                 (19) 
                 (20) 
                 (21) 
                 (22) 
                 (23) 
                 (24) 
               
               
                 715 
                 730 
                 737 
                 739 
                 734 
                 736 
                 727 
                 731 
               
               
                 (25) 
                 (26) 
                 (27) 
                 (28) 
                 (29) 
                 (30) 
                 (31) 
                 (32) 
               
               
                 717 
                 728 
                 729 
                 735 
                 730 
                 729 
                 730 
                 724 
               
               
                 (33) 
                 (34) 
                 (35) 
                 (36) 
                 (37) 
                 (38) 
                 (39) 
                 (40) 
               
               
                   
               
             
          
         
       
     
     The differences of the microcrystalline silicon film thickness of the four glass substrates is shown in  FIG. 8 , and the typical microcrystallization degrees of the films are shown in  FIG. 9 . 
     Based on the above data, it can be determined that, with this feed-in configuration, uniform electric field driven by 40.68 MHz very-high-frequency power source can be achieved, and micro-crystalline silicon thin films can be deposited on 1640 mm×707 mm (length×width) TCO glass with a uniformity of ˜5% and adjustable micro-crystallization degree. 
     Embodiment 3 
     The cathode plate has a rectangular feed-in port. The feed-in component has a flat waist as the feed-in belt, one end of which is in a rectangular shape and in surface contact with the feed-in port of the cathode plate. 
     The electrode array in  FIG. 6  is similar to that in Embodiment 1. A vertical deposition box or reaction chamber is used. Twelve cathode plates  203  and six anode plates  208  form twelve pairs of electrodes, with two cathode plates  203  coupled to or surrounding one anode plate  208  to form two pairs of electrodes, and 24 glass substrates  206  can be processed. In such a configuration, twenty four substrates can be coated with thin films at the same time. 
     a) Placing 24 glass substrates  206  (1640 mm×707 mm×3 mm) with 600-nm transparent conducting thin films in 24 substrate position in vacuum chamber  01  arranged from left to right. Film side of the substrate faces outside, while glass side of the substrate faces toward electrode plate. 
     b) Filling the vacuum chamber with argon when vacuum reaches 5.0×10 −4  Pa. When the pressure reaches approximately 60 Pa, turning on the 40.68 MHz very-high-frequency power source, and cleaning the chamber with 400 W plasma discharge for 2 minutes. Then turning off the electrical power source. 
     c) Afterwards, pumping down the system to a high vacuum of ˜5.0×10 −4  Pa, and then wash the system with argon twice. 
     d) Adding gas mixture (silane and hydrogen) to the chamber with a flow rate of 5 slpm. Turning on the 40.68 MHz very-high-frequency power source when pressure of the chamber reaches 60 Pa to discharge with 400 W power and deposit micro-crystalline intrinsic silicon thin films for 60 minutes. 
     e) Turning off the power source, and pump down the system to high vacuum. 
     f) Filling the chamber with nitrogen gas to atmospheric pressure, open the door of the chamber, and then cool the TCO glasses at room temperature. 
     After the completion of the deposition process, 40 points on each glass substrate  206  are sampled as shown in  FIG. 7  to detect the thickness of the various sampled points. The microcrystalline silicon film thickness testing results of a substrate  206  are shown in Table II: 
     
       
         
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE II 
               
               
                   
               
               
                 The unit of the film thickness is nm, the numbers in 
               
               
                 the parentheses is the location numbers in FIG. 7 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                  982 
                 1007 
                 1019 
                 1034 
                 1029 
                 1020 
                 1015 
                  997 
               
               
                  (1) 
                  (2) 
                  (3) 
                  (4) 
                  (5) 
                  (6) 
                  (7) 
                   8) 
               
               
                  991 
                 1021 
                 1035 
                 1065 
                 1050 
                 1025 
                 1027 
                 1011 
               
               
                  (9) 
                 (10) 
                 (11) 
                 (12) 
                 (13) 
                 (14) 
                 (15) 
                 (16) 
               
               
                 1027 
                 1040 
                 1039 
                 1057 
                 1045 
                 1040 
                 1028 
                 1006 
               
               
                 (17) 
                 (18) 
                 (19) 
                 (20) 
                 (21) 
                 (22) 
                 (23) 
                 (24) 
               
               
                 1025 
                 1030 
                 1037 
                 1049 
                 1034 
                 1039 
                 1034 
                 1021 
               
               
                 (25) 
                 (26) 
                 (27) 
                 (28) 
                 (29) 
                 (30) 
                 (31) 
                 (32) 
               
               
                 1013 
                 1006 
                 1009 
                 1035 
                 1030 
                 1029 
                 1025 
                 1019 
               
               
                 (33) 
                 (34) 
                 (35) 
                 (36) 
                 (37) 
                 (38) 
                 (39) 
                 (40) 
               
               
                   
               
             
          
         
       
     
     The differences of the microcrystalline silicon film thickness of the twenty four glass substrates is shown in  FIG. 10 . 
     Based on the above data, it can be determined that, with this feed-in configuration, uniform electric field driven by 40.68 MHz very-high-frequency power source can be achieved, and micro-crystalline silicon thin films can be deposited on 1640 mm×707 mm (length×width) TCO glass with a uniformity of ˜4.8% and adjustable micro-crystallization degree. 
     Above descriptions illustrate embodiments of this invention in details together with the accompanying figures. However, the present invention is not limited to the above embodiments, especially with respect to the shape of the feed-in components. Those with ordinary skills in the art can make different changes to this invention without departing the principles of the present invention.