Abstract:
A movable deposition box ( 02 ) for silicon-based thin film solar cell comprises an electrode array composed of at least a group of cathode plates ( 203 ) and a piece of anode plate ( 208 ) which are set in movable chamber, wherein a feeding socket ( 203 - 1 ) is positioned on a circular or semicircular concave surface in the center area on the backside of the cathode plates ( 203 ), a circular or semicircular end face ( 201 - 1 ) of a feeding component ( 201 ) which has a flat middle part contacts the signal feeding socket ( 203 - 1 ) and feeds in RF/VHF power signal, the anode plate ( 208 ) is grounded, and a shield cover ( 204 ) of the cathode plate has through-hole ( 204 - 1 ) and is insulated from the cathode plate ( 203 ).

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to solar cell technologies and, more particularly, to a deposition box driven by VHF (27.12 MHz-100 MHz) for silicon-based thin film solar cells. 
     BACKGROUND 
     Currently, silicon 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. Deposition process, such as plasma-enhanced chemical vapor deposition, is conducted in the reaction chamber through electrode plate components with electrode plate array. 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 chamber is often called the “deposition box,” i.e., the reactor. 
     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 attention has 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 aims 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 plate multi-plate array. 
     Accordingly, the deposition box technology solutions in the invention include: an electrode plate component, a signal feed-in component and the chamber, also including the shielding cover of the cathode plate, the chamber is a movable chamber with rollers, inside which installed the electrode array consisting of the electrode plates. The feed-in port is located in a hollowed circular or semicircular area at the center of the backside of the electrode plate component; the signal feed-in component connecting with the circular or semicircular-shaped feed-in port by surface contact connects with the negative electrode of the feeding Radio Frequency (RF) or Very High Frequency (VHF) power supply signals. The shielding cover of the cathode plate has a through hole. It is insulated between the cathode plate and the shielding cover, and the electrode array includes at least one set of cathode plates and one anode plate. 
     The set of cathode plates and an anode plate, mentioned in the solutions of the deposition box, refers to two effectively discharging sides of the symmetrically-arranged cathode plates facing towards two sides of the anode plate. The cathode plate is used for single-side discharge, and the shielding cover of the cathode plate includes a ceramic insulating layer and a shielding layer. The shielding cover covers entire back and side surface of the cathode plate. 
     The electrodes include multiple cathode plates with shielding covers and multiple grounded anode plates to form the electrode array with certain discharging space. 
     The shielding cover includes the shielding of the center position of the backside of the cathode plate and the surrounding sides for feeding in RF/VHF power supply signals. The signal feed-in component comprises a copper feed core, the insulating layer and the outer shielding layer. 
     The signal feed-in component comprises a waist and a head, which has a Z-shape. The waist has a high-temperature tolerant ceramic insulating layer, and the metal feeding core is a conductor of RF/VHF feeding lines. One end of the signal feed-in component is connected 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 plate component, the feed-in component and a chamber form a signal feed-in mode, and the electrode array is installed in the movable chamber with rollers. The electrode array includes at least one set of cathode plates and one anode plate. The feed-in port is located in a hollowed circular or semicircular area at the center of the backside of the electrode plate component, the feed-in component is connected with the cathode plate in the circular or semicircular concave surface by surface contact. Further, one end of the signal feed-in component is of a circular or semicircular shape and makes surface contact with a feed-in port of the electrode plate component to feed-in RF/VHF power supply signals. 
     The solution of the present invention includes that a plurality of sets of feed-in components and the electrode plates feed the RF/VHF power supply signal into electrode plate feed entrance through surface contact mode, forming an electrode array with certain discharge space. 
     The feed-in component may be a Z-shape metal belt or strip, the waist has a high temperature tolerant ceramic insulating layer and metal feeding core is a conductor of RF/VHF signal feeding lines. 
     One end of the signal feed-in component is connected with a negative output port of the RF/VHF power supply signal and a power supply matching device. 
     The beneficial effects of the present deposition box invention, different from the slot-type cathode plate with side feed modes, include that deposition which achieves the higher uniformity, greater discharge area and the stable discharge performance can be obtained in the deposition box, 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, and the integrated array type multi-chamber deposition can be obtained to greatly improve production efficiency. Therefore, through optimizing VHF power feed-in form and 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 any 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 , deposition box section view. 
         FIG. 2 , deposition box chamber diagram. 
         FIG. 3 , a schematic diagram of the signal feed-in component  201  of  FIG. 1 . 
         FIG. 4 , a schematic diagram of the cathode plate  203  of  FIG. 1 . 
         FIG. 5 , a schematic diagram of the cathode plate shield  204  of  FIG. 1 . 
         FIG. 6 , a structure sketch diagram of the embodiment 1 of the invention. 
         FIG. 7 , a structure sketch diagram of the embodiment 2 of the invention. 
         FIG. 8 , a structure sketch diagram of the embodiment 3 of the invention. 
     
    
    
     In  FIG. 1-8 , deposition box  02  is composed of signal feed-in component  201 , insulating shielding layer  202 , cathode plate  203 , cathode plate shielding cover  204 , substrate  206 , insulating strip  207 , anode plate  208 , grounding metal guide groove  209 , bottom back-door plate  211 , upper back-door plate  212 , gas cavity  214 , a front door panel  215 , the side frame  216 , wheels or rollers  218 , gas pipeline  220 , and a bottom base plate  221 , etc., to process gas deposition in a vacuum chamber  01 . Vacuum chamber  01  contains the feed-in inlet  101  of gas access system, the power supply feeding system inlet  102 , vacuum chamber door  103 , track  104 , and the feed-in inlet  105  of vacuum system. 
     The invention of the deposition box 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 deposition box system for treating or depositing more pieces of glasses, the effective VHF surface feed-in mode is applied so that the industrial production operation process is achieved and can meet the silicon-based thin film solar cell mass production needs. 
     Contribution of this invention includes providing a substantial 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. Deposition box  02  is placed in vacuum chamber  01 , and the deposition box  02  comprises electrode plates, signal feed-in component, chamber and cathode plate shielding cover  204 . Parallel electrode plates include cathode plate  203  and anode plate  208 . Feed-in port  203 - 1  of cathode plate is circular shaped. Signal feed-in component  201  is of a stair shape and comprises a waist section and a semi-circular end  201 - 1 , which connects with circular feed-in port  203 - 1  sunken in the middle of cathode plate  203  with shielding cover  204 . The waist of the component is flat for easy installation and with less feed-in signal loss. The other end  201 - 3  of the component connects with the negative port of RF/VHF electrical power source and a power matching device (not depicted in figures), and is of the shape of stairs. The end of the component is of a semi-circular shape, connects in surface-contact mode with electrode plate to constitute the electrode-plate component in the deposition box with grounding setting, which all have insulating shields (not depicted in figures). 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Embodiment 1 
     Electrode plates are vertically placed. Cathode plates have circular feed-in ports, and feed-in components have flat waists and semi-circular feed-in interfaces. 
     The principle of this embodiment is illustrated with  FIGS. 1-6 . In deposition box  02 , two cathode plates  203  surround one anode plate  208 . PECVD deposition system consists of vapor deposition chamber, gas system, electrical power system, vacuum system, heating system, control system, and so on. 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 deposition box  02  with scroll wheels  218  and grounding setting. Vacuum chamber  01  is used to achieve vacuum. Deposition box  02  is used to discharge plasma and deposit P-I-N thin film layers on substrate  206 . Deposition box  02  includes cathode plate  203 , cathode shield  204 , insulating strip  207 , anode plate  208 , signal feed-in component  201 , shielding layer  202 , bottom plate  221 , gas chamber  214 , grounding metal groove  209 , front-door plate  215 , upper back-door plate  212 , bottom back-door plate  211 , side frame  216 , and wheel  218 . Side frame  216  is a quadrangle frame made of welded stainless steel slip, which has rectangular hook ear  216 - 4 . Side frame  216  connects with gas chamber  214  on top side and with bottom plate  221  on bottom side to form an integral body. Metal groove  209 , located on the counterpart side of gas chamber  214  and bottom plate  221 , can fix anode plate  208 , cathode plate  203 , and cathode shield  204 . Anode plate  208  is directly inserted into and contacts metal groove  209 , making cathode shield  204  contact metal groove  209 . Insulating slip  207  is fixed between cathode  203  and cathode shield  204  to make these two parts separate. 
     Anode plate  208  and cathode shield  204  contact metal grooves  209 , which connects with bottom plate  221  to acquire grounding. A circular feed-in port  203 - 1  is located in the middle or center area of cathode plate and sunken from the surface of the plate. Waist and head of signal feed-in component  201  form the shape of Z. Further, signal feed-in component  201  has a semi-circular end  203 - 1  to connect sunken circular port in the middle of cathode plate with radio-frequency/very-high-frequency signal power source. Through hole  204 - 1  in the middle of cathode shield  204  is set corresponding the feed-in port  203 - 1  and makes signal feed-in component  201  coming from cathode plate  203  without touching cathode shield  204 . Another end of signal feed-in component  201  connects with electrical power supply port  205  through hole  201 - 3 . Waist of the feed-in component is covered with high-temperature tolerant ceramic insulating layer  202  to avoid contacting with cathode shield  204 . Signal feed-in component is made from copper with good conductivity. Front door plate  215  can make deposition box  02  to form a relatively enclosed space by hanging hook  215 - 2  on hook ear  216 - 1  on side frame  216  and inserting bottom side into Z-shaped socket, after substrates  206  are placed in deposition box  02 . Deposition box  02  can be pushed into vacuum chamber  01  through track  104 . Thus, top inlet of gas pipe line  220  on deposition box  02  connects with inlet of gas system  101  of vacuum chamber  01  by inserting into the pipe of vacuum chamber. Desired vacuum state of vacuum chamber can be achieved after movable vacuum door  103  in vacuum chamber  01  is closed. Then gases are added into vacuum chamber, and thin-film vapor deposition proceeds. 
     Embodiment 2 
     Electrode plates are vertically placed. Cathode plates have circular feed-in ports. Feed-in components have flat waists and semi-circular feed-in interface. Cathode plates are insulated from shields and through holes are set in the shields of cathode plates. 
     Deposition box in  FIG. 7  is the same as that in Embodiment 1, in which 8 glass substrates  206  can be processed at the same time. Two cathode plates  203  surround one anode plate  208 , and four pairs of electrodes can be formed by two anode plates  208  and four cathode plates  203 . In such a configuration, eight substrates can be coated with thin films at the same time. Detailed processes are illustrated as follows: 
     a) Place 8 glass substrates (1640 mm×707 mm×3 mm) with 600-nm transparent conducting thin films in the substrate position in deposition box  02 . Film side of the substrate faces outside, while glass side of the substrate faces toward electrode plate. 
     b) Open the active door  103  of the vacuum chamber, and push the deposition box  02  along the track  104  into the vacuum chamber  01 . Then close the active door  103  of the vacuum chamber  01 . 
     c) Fill the chamber with argon when vacuum reaches 5.0×10 −4  Pa. Turn on the 40.68 MHz very-high-frequency power source, and clean the chamber with 400 W plasma discharge for 2 minutes. Then turn off the electrical power source. 
     d) Afterwards, pump down the system to a high vacuum of ˜5.0×10 −4  Pa, and then wash the system with argon twice. 
     e) Add gas mixture (silane and hydrogen) to the chamber with a flow rate of 5 slpm. Turn on the 40.68 MHz very-high-frequency power source when pressure of the chamber reaches 60Pa. Glow the plasma discharge with 400 W power, and deposit micro-crystalline intrinsic silicon thin films for 40 minutes. 
     f) Turn off the power source, and pump down the system to high vacuum. 
     g) Fill the chamber with nitrogen gas to atmospheric pressure, and open the active door  103  of the chamber. Push deposition box  02  out of the chamber, and then cool the TCO glasses at room temperature. 
     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 
     Electrode plates are vertically placed. Cathode plates have circular feed-in ports. Feed-in components have flat waists and semi-circular feed-in interface. Cathode plates are insulated from shields and through holes are set in the shields of cathode plates. 
     Deposition box in  FIG. 8  is the same as that in Embodiment 1, in which 24 glass substrates  206  can be processed at the same time. Two cathode plates  203  surround one anode plate  208 , and twelve pairs of electrodes can be formed by six anode plates  208  and twelve cathode plates  203 . In such a configuration, twenty four substrates can be coated with thin films at the same time. Detailed processes are illustrated as follows: 
     a) Place 24 glass substrates  206  (1640 mm×707 mm×3 mm) with 600-nm transparent conducting films in the substrate position in deposition box  02 . Film side of the substrate faces outside, while glass side of the substrate faces toward electrode plate. 
     b) Open the active door  103  of the vacuum chamber, and push the deposition box  02  along the track  104  into the vacuum chamber  01 . Then close the active door  103  of the vacuum chamber  01 . 
     c) Fill the chamber with argon when vacuum reaches 5.0×10 −4  Pa. Turn on the 40.68 MHz very-high-frequency power source, and clean the chamber with 400 W plasma discharge for 2 minutes. Then turn off the electrical power source. 
     d) Afterwards, pump down the system to a high vacuum of ˜5.0×10 −4  Pa, and then wash the system with argon twice. 
     e) Add gas mixture (silane and hydrogen) to the chamber with a flow rate of 5 slpm. Turn on the 40.68 MHz very-high-frequency power source when pressure of the chamber reaches 60 Pa. Glow the plasma of 400 W, and deposit micro-crystalline intrinsic silicon thin films for 40 minutes. 
     f) Turn off the electrical power, and pump down to high vacuum. 
     g) Fill the chamber with nitrogen gas to atmospheric pressure, and open the active door  103  of the chamber. Push deposition box  02  out of the chamber, and then cool the TCO glasses at room temperature. 
     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%. 
     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.