Patent Publication Number: US-2018037981-A1

Title: Temperature-controlled chalcogen vapor distribution apparatus and method for uniform cigs deposition

Description:
BACKGROUND 
     The present disclosure is directed generally to an apparatus and method for depositing a metal chalcogenide material, and particularly to an apparatus and method for depositing a copper indium gallium selenide material using a selenium manifold with separate heaters. 
     A “thin-film” photovoltaic material refers to a polycrystalline or amorphous photovoltaic material that is deposited as a layer on a substrate that provides structural support. The thin-film photovoltaic materials are distinguished from single crystalline semiconductor materials that have a higher manufacturing cost. Some of the thin-film photovoltaic materials that provide high conversion efficiency include chalcogen-containing compound semiconductor material, such as copper indium gallium selenide (“CIGS”). 
     Thin-film photovoltaic cells (also known as photovoltaic cells) may be manufactured using a roll-to-roll coating system based on sputtering, evaporation, or chemical vapor deposition (CVD) techniques. A thin foil substrate, such as a foil web substrate, is fed from a roll in a linear belt-like fashion through the series of individual vacuum chambers or a single divided vacuum chamber where it receives the required layers to form the thin-film photovoltaic cells. In such a system, a foil having a finite length may be supplied on a roll. The end of a new roll may be coupled to the end of a previous roll to provide a continuously fed foil layer. 
     SUMMARY 
     According to an aspect of the present disclosure, a deposition system for deposition of a chalcogen-containing compound semiconductor material is provided. The deposition system includes a vacuum enclosure connected to a vacuum pump, a sputtering system comprising at least one sputtering target located in the vacuum enclosure, a chalcogen-containing gas source, and a gas distribution manifold having a supply side and a distribution side. The distribution side has a plurality of opening regions having independent temperature control and the supply side is connected to the chalcogen-containing gas source. 
     According to another aspect of the present disclosure, a deposition system for deposition of a copper indium gallium selenide compound semiconductor material includes a vacuum enclosure connected to a vacuum pump, a sputtering system comprising at least one copper, gallium and/or indium sputtering target located in the vacuum enclosure, a selenium evaporator, a gas distribution manifold having a supply side and a distribution side, wherein the distribution side has a plurality of opening regions and the supply side is connected to the selenium evaporator, and means for independently heating at least a first opening region of the plurality of opening regions to a different temperature than at least a second opening region of the plurality of opening regions. The means for independently heating may comprise independently controlled heaters. 
     According to another aspect of the present disclosure, a method of reactive sputter depositing a chalcogen-containing compound semiconductor material includes sputtering at least one metal component of the chalcogen-containing compound semiconductor material onto the substrate, and providing a higher chalcogen flux to ends of the substrate than to a middle of the substrate to form the chalcogen-containing compound semiconductor material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic vertical cross sectional view of a thin-film photovoltaic cell according to an embodiment of the present disclosure. 
         FIG. 2  is a schematic top view diagram of a first exemplary modular deposition apparatus that can be used to manufacture the photovoltaic cell illustrated in  FIG. 1  according to an embodiment of the present disclosure. 
         FIG. 3  is a schematic top view diagram of a second exemplary modular deposition apparatus that can be used to manufacture the photovoltaic cell illustrated in  FIG. 1  according to an embodiment of the present disclosure. 
         FIG. 4  is a schematic top view diagram of an exemplary sealing connection unit according to an embodiment of the present disclosure. 
         FIG. 5  is a schematic top view diagram of an exemplary module including a deposition system for a chalcogen-containing compound semiconductor material according to an embodiment of the present disclosure. 
         FIGS. 6A and 6B  are top view schematic diagrams of exemplary gas distribution manifolds according to an embodiment of the present disclosure. 
         FIGS. 7A-7C  are front views of exemplary sets of opening regions of a gas distribution manifold according to an embodiment of the present disclosure.  FIG. 7D  is a three dimensional perspective view of the gas distribution manifold of  FIG. 7A  located in the module of  FIG. 5 . 
         FIG. 8  is a side cross sectional view of an exemplary temperature control system for a heating element according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     As discussed above, the present disclosure is directed to an apparatus and method for depositing a copper indium gallium selenide material. The web substrate typically has a width (i.e., a height of the web substrate for a vertically positioned web substrate, which is perpendicular to the length (i.e., movement direction) of the web substrate) of at least 10 cm, and oftentimes a width of about 1 meters or more, such as 1 to 5 meters. Deposition of a film with a uniform thickness and/or composition as a function of a large web substrate width is a challenge even in a large deposition chamber. Particularly, metal chalcogen containing compound semiconductor materials such as copper indium gallium chalcogenide (e.g., CIGS) have a deposition rate that is highly sensitive to the deposition temperature. Further, the composition of such metal chalcogen containing compound semiconductor materials can vary significantly depending on rate of incorporation of a chalcogen-containing gas (e.g., evaporated selenium) during the deposition process. In one embodiment, without wishing to be bound by a particular theory, the present inventors determined that a higher chalcogen (e.g., selenium) flux is desirable at the ends (i.e., top and bottom portions of a vertically positioned web substrate) than in the middle of the vertically positioned substrate to obtain a metal chalcogen containing compound semiconductor material (e.g., CIGS) with a more uniform thickness and/or composition as a function of substrate width (i.e., height). In one embodiment, a chalcogen manifold contains independently controllable heating elements which can be independently controlled to provide the higher chalcogen (e.g., selenium vapor) flux to the ends than the middle of the web substrate. 
     The drawings are not drawn to scale. Multiple instances of an element may be duplicated where a single instance of the element is illustrated, unless absence of duplication of elements is expressly described or clearly indicated otherwise. Ordinals such as “first,” “second,” and “third” are employed merely to identify similar elements, and different ordinals may be employed across the specification and the claims of the instant disclosure. As used herein, a first element located “on” a second element can be located on the exterior side of a surface of the second element or on the interior side of the second element. As used herein, a first element is located “directly on” a second element if there exist a direct physical contact between a surface of the first element and a surface of the second element. 
     Referring to  FIG. 1 , a vertical cross-sectional view of a photovoltaic cell  10  is illustrated. The photovoltaic cell  10  includes a substrate, such as an electrically conductive substrate  12 , a first electrode  20 , a p-doped semiconductor layer  30 , an n-doped semiconductor layer  40 , a second electrode  50 , and an optional antireflective (AR) coating layer (not shown). 
     The substrate  12  is preferably a flexible, electrically conductive material, such as a metallic foil that is fed into a system of one or more process modules as a web for deposition of additional layers thereupon. For example, the metallic foil of the conductive substrate  12  can be a sheet of a metal or a metallic alloy such as stainless steel, aluminum, or titanium. If the substrate  12  is electrically conductive, then it may comprise a part of the back side (i.e., first) electrode of the cell  10 . Thus, the first (back side) electrode of the cell  10  may be designated as ( 20 ,  12 ). Alternatively, the conductive substrate  12  may be an electrically conductive or insulating polymer foil. Still alternatively, the substrate  12  may be a stack of a polymer foil and a metallic foil. In another embodiment, the substrate  12  may be a rigid glass substrate or a flexible glass substrate. The thickness of the substrate  12  can be in a range from 100 microns to 2 mm, although lesser and greater thicknesses can also be employed. 
     The first or back side electrode  20  may comprise any suitable electrically conductive layer or stack of layers. For example, electrode  20  may include a metal layer, which may be, for example, molybdenum. Alternatively, a stack of molybdenum and sodium and/or oxygen doped molybdenum layers may be used instead, as described in U.S. Pat. No. 8,134,069, which is incorporated herein by reference in its entirety. In another embodiment, the first electrode  20  can include a molybdenum material layer doped with K and/or Na, i.e., MoK x  or Mo(Na,K) x , in which x can be in a range from 1.0×10 −6  to 1.0×10 −2 . The electrode  20  can have a thickness in a range from 500 nm to 1 micron, although lesser and greater thicknesses can also be employed. 
     The p-doped semiconductor layer  30  can include a p-type sodium doped copper indium gallium selenide (CIGS), which functions as a semiconductor absorber layer. The thickness of the p-doped semiconductor layer  30  can be in a range from 1 microns to 5 microns, although lesser and greater thicknesses can also be employed. 
     The n-doped semiconductor layer  40  includes an n-doped semiconductor material such as CdS, ZnS, ZnSe, or an alternative metal sulfide or a metal selenide. The thickness of the n-doped semiconductor layer  40  is typically less than the thickness of the p-doped semiconductor layer  30 , and can be in a range from 30 nm to 100 nm, although lesser and greater thicknesses can also be employed. The junction between the p-doped semiconductor layer  30  and the n-doped semiconductor layer  40  is a p-n junction. The n-doped semiconductor layer  40  can be a material which is substantially transparent to at least part of the solar radiation. The n-doped semiconductor layer  40  is also referred to as a window layer or a buffer layer. 
     The second (e.g., front side or top) electrode  50  comprises one or more transparent conductive layers  50 . The transparent conductive layer  50  is conductive and substantially transparent. The transparent conductive layer  50  can include one or more transparent conductive materials, such as ZnO, indium tin oxide (ITO), Al doped ZnO (“AZO”), Boron doped ZnO (“BZO”), or a combination or stack of higher resistivity AZO and lower resistivity ZnO, ITO, AZO and/or BZO layers. The second electrode  50  contacts an electrically conductive part (e.g., a metal wire or trace) of an interconnect, such as an interconnect described in U.S. Pat. No. 8,912,429, issued Dec. 16, 2014, which is incorporated herein by reference in its entirety, or any other suitable interconnect that is used in photovoltaic panels. 
     Referring now to  FIG. 2 , an apparatus  1000  for forming the photovoltaic cell  10  illustrated in  FIG. 1  is shown. The apparatus  1000  is a first exemplary modular deposition apparatus that can be used to manufacture the photovoltaic cell illustrated in  FIG. 1 . The apparatus  1000  includes an input unit  100 , a first process module  200 , a second process module  300 , a third process module  400 , a fourth process module  500 , and an output unit  800  that are sequentially connected to accommodate a continuous flow of the substrate  12  in the form of a web foil substrate layer through the apparatus. The modules ( 100 ,  200 ,  300 ,  400 ,  500 ) may comprise the modules described in U.S. Pat. No. 9,303,316, issued on Apr. 5, 2016, incorporated herein by reference in its entirety, or any other suitable modules. The first, second, third, and fourth process modules ( 200 ,  300 ,  400 ,  500 ) can be under vacuum by first, second, third, and fourth vacuum pumps ( 280 ,  380 ,  480 ,  580 ), respectively. The first, second, third, and fourth vacuum pumps ( 280 ,  380 ,  480 ,  580 ) can provide a suitable level of respective base pressure for each of the first, second, third, and fourth process modules ( 200 ,  300 ,  400 ,  500 ), which may be in a range from 1.0×10 −9  Torr to 1.0×10 −2  Torr, and preferably in range from 1.0×10 −9  Torr to 1.0×10 −5  Torr. 
     Each neighboring pair of process modules ( 200 ,  300 ,  400 ,  500 ) is interconnected employing a vacuum connection unit  99 , which can include a vacuum tube and an optional slit valve that enables isolation while the substrate  12  is not present. The input unit  100  can be connected to the first process module  200  employing a sealing connection unit  97 . The last process module, such as the fourth process module  500 , can be connected to the output unit  800  employing another sealing connection unit  97 . 
     The substrate  12  can be a metallic or polymer web foil that is fed into a system of process modules ( 200 ,  300 ,  400 ,  500 ) as a web for deposition of material layers thereupon to form the photovoltaic cell  10 . The substrate  12  can be fed from an entry side (i.e., at the input module  100 ), continuously move through the apparatus  1000  without stopping, and exit the apparatus  1000  at an exit side (i.e., at the output module  800 ). The substrate  12 , in the form of a web, can be provided on an input spool  110  provided in the input module  100 . 
     The substrate  12 , as embodied as a metal or polymer web foil, is moved throughout the apparatus  1000  by input-side rollers  120 , output-side rollers  820 , and additional rollers (not shown) in the process modules ( 200 ,  300 ,  400 ,  500 ), vacuum connection units  99 , or sealing connection units  97 , or other devices. Additional guide rollers may be used. Some rollers ( 120 ,  820 ) may be bowed to spread the web (i.e., the substrate  12 ), some may move to provide web steering, some may provide web tension feedback to servo controllers, and others may be mere idlers to run the web in desired positions. 
     The input module  100  can be configured to allow continuous feeding of the substrate  12  by adjoining multiple foils by welding, stapling, or other suitable means. Rolls of substrates  12  can be provided on multiple input spools  110 . A joinder device  130  can be provided to adjoin an end of each roll of the substrate  12  to a beginning of the next roll of the substrate  12 . In one embodiment, the joinder device  130  can be a welder or a stapler. An accumulator device (not shown) may be employed to provide continuous feeding of the substrate  12  into the apparatus  1000  while the joinder device  130  adjoins two rolls of the substrate  12 . 
     In one embodiment, the input module  100  may perform pre-processing steps. For example, a pre-clean process may be performed on the substrate  12  in the input module  100 . In one embodiment, the substrate  12  may pass by a heater array (not shown) that is configured to provide at least enough heat to remove water adsorbed on the surface of the substrate  12 . In one embodiment, the substrate  12  can pass over a roller configured as a cylindrical rotary magnetron. In this case, the front surface of substrate  12  can be continuously cleaned by DC, AC, or RF sputtering as the substrate  12  passes around the roller/magnetron. The sputtered material from the substrate  12  can be captured on a disposable shield. Optionally, another roller/magnetron may be employed to clean the back surface of the substrate  12 . In one embodiment, the sputter cleaning of the front and/or back surface of the substrate  12  can be performed with linear ion guns instead of magnetrons. Alternatively or additionally, a cleaning process can be performed prior to loading the roll of the substrate  12  into the input module  100 . In one embodiment, a corona glow discharge treatment may be performed in the input module  100  without introducing an electrical bias. 
     The output module  800  can include an output spool  810 , which winds the web embodying the photovoltaic cell  10 . The photovoltaic cell  10  is the combination of the substrate  12  and the deposited layers ( 20 ,  30 ,  40 ,  50 ) thereupon. 
     In one embodiment, the substrate  12  may be oriented in one direction in the input module  100  and/or in the output module  800 , and in a different direction in the process modules ( 200 ,  300 ,  400 ,  500 ). For example, the substrate  12  can be oriented generally horizontally in the input module  100  and the output module  800 , and generally vertically in the process module(s) ( 200 ,  300 ,  400 ,  500 ). A turning roller or turn bar (not shown) may be provided to change the orientation of the substrate  12 , such as between the input module  100  and the first process module  200 . In an illustrative example, the turning roller or the turn bar in the input module can be configured to turn the web substrate  12  from an initial horizontal orientation to a vertical orientation. Another turning roller or turn bar (not shown) may be provided to change the orientation of the substrate  12 , such as between the last process module (such as the fourth process module  500 ) and the output module  800 . In an illustrative example, the turning roller or the turn bar in the input module can be configured to turn the web substrate  12  from the vertical orientation employed during processing in the process modules ( 200 ,  300 ,  400 ,  500 ) to a horizontal orientation. 
     The input spool  110  and optional output spool  810  may be actively driven and controlled by feedback signals to keep the substrate  12  in constant tension throughout the apparatus  1000 . In one embodiment, the input module  100  and the output module  800  can be maintained in the air ambient at all times while the process modules ( 200 ,  300 ,  400 ,  500 ) are maintained at vacuum during layer deposition. 
     Referring to  FIG. 3 , a second exemplary modular deposition apparatus  2000  is illustrated, which can be used to manufacture the photovoltaic cell illustrated in  FIG. 1 . The second exemplary modular deposition apparatus  2000  includes an alternative output module  800 , which includes a cutting apparatus  840  instead of an output spool  810 . The web containing the photovoltaic cells  10  can be fed into the cutting apparatus  840  in the output module  800 , and can be cut into discrete sheets of photovoltaic cells  10  instead of being rolled onto an output spool  810 . The discrete sheets of photovoltaic cells are then interconnected using interconnects to form a photovoltaic panel (i.e., a solar module) which contains an electrical output. 
     Referring to  FIG. 4 , an exemplary sealing connection unit  97  is illustrated. The unit  97  may comprise the sealing unit described in U.S. Pat. No. 9,303,316, issued on Apr. 5, 2016, incorporated herein by reference in its entirety, or any other suitable sealing unit. The sealing connection unit  97  is configured to allow the substrate  12  to pass out of a preceding unit (such as the input unit  100  or the last processing chamber such as the fourth process module  500 ) and into a subsequent unit (such as the first process module  200  or the output unit  800 ), while impeding the passage of gasses such as atmospheric gasses or processing gasses into or out of the units that the sealing connection unit  97  is adjoined to. The sealing connection unit  97  can include multiple isolation chambers  72 . The staged isolation chambers  72  can be configured to maintain internal pressures that graduate from atmospheric on a first side of the sealing connection unit  97  (such as the side of the input module  100  or the output module  800 ) to a high vacuum on the second side of the sealing connection unit  97  opposite of the first side (such as the side of the first process module  200  or the last process module  500 ). Multiple isolation chambers  72  can be employed to ensure that the pressure difference at any sealing surface is generally less than the pressure difference between atmospheric pressure and the high vacuum inside the process module. 
     The substrate  12  enters the sealing unit  97  between two external nip rollers  74 . Each of the isolation chambers  72  of the sealing connection unit  97  can be separated by an internal divider  78 , which is an internal wall among the isolation chambers  72 . A pair of internal nip rollers  76 , similar in function and arrangement to that of the external rollers  74 , may be provided proximate to the internal dividers  78  between some of the neighboring internal chambers  72 . The passage between the internal rollers  76  is generally closed off by rolling seals between the internal rollers  76  and the substrate  12 . The internal dividers  78  may include curved sockets or contours that are configured to receive internal rollers  76  of a similar radius of curvature. The passage of gasses from one isolation chamber  72  to a neighboring, lower pressure internal chamber  72  may be reduced by a simple surface to surface contact between the internal roller  76  and the divider  78 . In other embodiments, a seal such as a wiper seal may be provided for some or all of the internal rollers  76  to further reduce the infiltration of gasses into neighboring isolation chambers  72 . The internal rollers  76  may be freely spinning rollers, or may be powered to control the rate of passage of the substrate  12  through the sealing connection unit  97 . Between other chambers  72 , the passage of gasses between neighboring chambers  72  may be limited by parallel plate conductance limiters  79 . The parallel plate conductance limiters  79  are generally flat, parallel plates that are arranged parallel to the surface of the substrate  12  and are spaced apart a distance slightly larger than the thickness of the substrate  12 . The parallel plate conductance limiters  79  allow the substrate to pass between the chambers  72  while limiting the passage of gasses between chambers  72 . 
     In one embodiment, the sealing connection unit  97  may also include inert gas purge at the in-feed nip. In one embodiment, the sealing connection unit  97  may also include optional reverse crown or spreading rollers. The difference in pressure between neighboring chambers may deform the internal rollers  76 , causing them to deflect or crown towards the chamber with a lower pressure. The reverse crown rollers are placed such that they correct for vacuum-induced deflection of the internal rollers  76 . Thus, other than the slight deformation corrected by the reverse crown rollers, the sealing connection unit  97  is configured to pass the web substrate without bending or turning or scratching the web substrate  12 . 
     Referring back to  FIGS. 2 and 3 , each of the first, second, third, and fourth process modules ( 200 ,  300 ,  400 ,  500 ) can deposit a respective material layer to form the photovoltaic cell  10  (shown in  FIG. 1 ) as the substrate  12  passes through the first, second, third, and fourth process modules ( 200 ,  300 ,  400 ,  500 ) sequentially. 
     Optionally, one or more additional process modules (not shown) may be added between the input module  100  and the first process module  200  to sputter a back side protective layer on the back side of the substrate  12  before deposition of the first electrode  20  in the first process module  200 . Further, one or more barrier layers may be sputtered over the front surface of the substrate  12  prior to deposition of the first electrode  20 . Alternatively or additionally, one or more process modules (not shown) may be added between the first process module  200  and the second process module  300  to sputter one or more adhesion layers between the first electrode  20  and the p-doped semiconductor layer  30  including a chalcogen-containing compound semiconductor material. 
     The first process module  200  includes a first sputtering target  210 , which includes the material of the first electrode  20  in the photovoltaic cell  10  illustrated in  FIG. 1 . A first heater  270  can be provided to heat the web substrate  12  to an optimal temperature for deposition of the first electrode  20 . In one embodiment, a plurality of first sputtering targets  210  and a plurality of first heaters  270  may be employed in the first process module  200 . In one embodiment, the at least one first sputtering target  210  can be mounted on dual cylindrical rotary magnetron(s), or planar magnetron(s) sputtering targets, or RF sputtering targets. In one embodiment, the at least one first sputtering target  210  can include a molybdenum target, a molybdenum-sodium, and/or a molybdenum-sodium-oxygen target, as described in U.S. Pat. No. 8,134,069, incorporated herein by reference in its entirety. 
     The portion of the substrate  12  on which the first electrode  20  is deposited is moved into the second process module  300 . A p-doped chalcogen-containing compound semiconductor material is deposited to form the p-doped semiconductor layer  30 , such as a sodium doped CIGS absorber layer. In one embodiment, the p-doped chalcogen-containing compound semiconductor material can be deposited employing reactive alternating current (AC) magnetron sputtering in a sputtering atmosphere that includes argon and a chalcogen-containing gas at a reduce pressure. In one embodiment, multiple metallic component targets  310  including the metallic components of the p-doped chalcogen-containing compound semiconductor material can be provided in the second process module  300 . 
     As used herein, the “metallic components” of a chalcogen-containing compound semiconductor material refers to the non-chalcogenide components of the chalcogen-containing compound semiconductor material. For example, in a copper indium gallium selenide (CIGS) material, the metallic components include copper, indium, and gallium. The metallic component targets  310  can include an alloy of all non-metallic materials in the chalcogen-containing compound semiconductor material to be deposited. For example, if the chalcogen-containing compound semiconductor material is a CIGS material, the metallic component targets  310  can include an alloy of copper, indium, and gallium. More than two targets  310  may be used. 
     At least one chalcogen-containing gas source  320  (such as a selenium evaporator) and at least one gas distribution manifold  322  can be provided on the second process module  300  to provide a chalcogen-containing gas into the second process module  300 . The chalcogen-containing gas provides chalcogen atoms that are incorporated into the deposited chalcogen-containing compound semiconductor material. While  FIGS. 2 and 3  schematically illustrate a second process module  300  including two metallic component targets  310 , a single chalcogen-containing gas source  320 , and a single gas distribution manifold  322 , multiple instances of the chalcogen-containing gas source  320  and/or the gas distribution manifold  322  can be provided in the second process module  300  as illustrated in  FIG. 5 . 
       FIG. 5  illustrates an exemplary configuration for the second process module  300 . In this configuration, the second process module  300  is provided with three sets of chalcogen-containing compound semiconductor material deposition units  302 . Each chalcogen-containing compound semiconductor material deposition unit  302  may be a separate deposition chamber or different portions of the same deposition chamber. Each deposition unit includes a separate chalcogen-containing gas source  320  (e.g., selenium evaporator) and one or more separate targets  310 . For example, in one embodiment illustrated in  FIG. 5 , each distribution unit  302  includes a dedicated second vacuum pump  380 , one or more (e.g., pair) of dedicated second heaters  370 , one or more (e.g. four) dedicated metallic component targets  310  (e.g., CIG targets), a dedicated chalcogen-containing gas source  320 , one or more (e.g., five) gas distribution manifolds  322 , and a dedicated connection manifold  324  that connects the chalcogen-containing gas source  320  to the gas distribution manifolds  322 . As many chalcogen-containing compound semiconductor material deposition units  302  can be provided along the path of the substrate  12  as is needed to achieve the target thickness and composition (e.g., Cu, In and/or Ga composition gradient) for the p-doped chalcogen-containing compound semiconductor material (e.g., for the CIGS absorber  30  shown in  FIG. 1 ). The number of second vacuum pumps  380  may, or may not, coincide with the number of the deposition units  302 . The number of second heaters  370  may, or may not, be commensurate with the number of the deposition units  302 . Likewise, the number of manifolds  322  and targets  310  may be different in different deposition units  302 . Likewise, the composition of each target  310  may be different in different deposition unit  302  (e.g., the Cu:(Ga+In) or Ga:In ratio in the targets  310  in different deposition units  302  may vary to obtain a CIGS absorber with a graded composition). 
     The chalcogen-containing gas source  320  includes a source material for the chalcogen-containing gas. The species of the chalcogen-containing gas can be selected to enable deposition of the target chalcogen-containing compound semiconductor material to be deposited. For example, if a CIGS material is to be deposited for the p-doped semiconductor layer  30 , the chalcogen-containing gas may be selected, for example, from hydrogen selenide (H 2 Se) and selenium vapor. In case the chalcogen-containing gas is hydrogen selenide, the chalcogen-containing gas source  320  can be a cylinder of hydrogen selenide. In case the chalcogen-containing gas is selenium vapor, the chalcogen-containing gas source  320  can be a selenium evaporator, such as an effusion cell that can be heated to generate selenium vapor. Each second heater  370  can be a radiation heater that maintains the temperature of the web substrate  12  at the deposition temperature, which can be in a range from 400° C. to 800° C., such as a range from 500° C. to 700° C., which is preferable for CIGS deposition. 
     The chalcogen incorporation during deposition of the chalcogen-containing compound semiconductor material determines the properties and quality of the chalcogen-containing compound semiconductor material in the p-doped semiconductor layer  30 . When the chalcogen-containing gas is supplied in the gas phase at an elevated temperature, the chalcogen atoms from the chalcogen-containing gas can be incorporated into the deposited film by absorption and subsequent bulk diffusion. This process is referred to as chalcogenization, in which complex interactions occur to form the chalcogen-containing compound semiconductor material. The p-type doping in the p-doped semiconductor layer  30  is induced by controlling the degree of deficiency of the amount of chalcogen atoms with respect the amount of non-chalcogen atoms (such as copper atoms, indium atoms, and gallium atoms in the case of a CIGS material) deposited from the metallic component targets  310 . 
     In one embodiment, each metallic component target  310  can be employed with a respective magnetron (not expressly shown) to deposit a chalcogen-containing compound semiconductor material with a respective composition. In one embodiment, the composition of the metallic component targets  310  can be gradually changed along the path of the substrate  12  so that a graded chalcogen-containing compound semiconductor material can be deposited in the second process module  300 . For example, if a CIGS material is deposited as the chalcogen-containing compound semiconductor material of the p-doped semiconductor layer  30 , the atomic percentage of gallium of the deposited CIGS material can increase as the substrate  12  progresses through the second process module  300 . In this case, the p-doped CIGS material in the p-doped semiconductor layer  30  of the photovoltaic cell  10  can be graded such that the band gap of the p-doped CIGS material increases with distance from the interface between the first electrode  20  and the p-doped semiconductor layer  30 . 
     In one embodiment, the total number of metallic component targets  310  may be in a range from 3 to 20. In an illustrative example, the composition of the deposited chalcogen-containing compound semiconductor material (e.g., the p-doped CIGS material absorber  30 ) can be graded such that the band gap of the p-doped CIGS material varies (e.g., increases or decreases gradually or in steps) with distance from the interface between the first electrode  20  and the p-doped semiconductor layer  30 . For example, the band gap can be about 1 eV at the interface with the first electrode  20 , and can be about 1.3 eV at the interface with subsequently formed n-doped semiconductor layer  40 . 
       FIGS. 6A and 6B  are cross-sectional views of a portion of the second process module  300  that illustrates a configuration of an exemplary gas distribution manifold  322 . The exemplary gas distribution manifold  322  may be employed for any, or each, of the gas distribution manifolds  322  illustrated in  FIGS. 2, 3, and 5 . The cross-sectional view of  FIG. 6A  is along a plane that is perpendicular to the direction of movement of the substrate  12  within the second process module (i.e., the substrate moves in or out of the page in  FIG. 6A ). 
     The second process module  300  includes a deposition system for deposition of a chalcogen-containing compound semiconductor material for forming the p-doped semiconductor layer  30 . As discussed above, the deposition system includes a vacuum enclosure attached to a vacuum pump (such as at least one second vacuum pump  380 ), and a sputtering system comprising at least one sputtering target (such as the at least one metallic component target  310 , for example a CIG target) located in the vacuum enclosure and at least one respective magnetron. The sputtering system is configured to deposit a material including at least one component of a chalcogen-containing compound semiconductor material (i.e., the non-chalcogen metallic component(s) of the chalcogen-containing compound semiconductor material) over the substrate  12  in the vacuum enclosure. In other words, the module  300  is a reactive sputtering module in which the chalcogen gas (e.g., selenium vapor) from gas distribution manifolds  322  reacts with the metal (e.g., Cu—In—Ga) sputtered from the targets  310  to form the chalcogen-containing compound semiconductor material (e.g., CIGS) layer  30  over the substrate  12 . 
     The gas distribution manifold  322  has a supply side that is connected to a chalcogen-containing gas source  320  directly or indirectly. The supply side may face the chalcogen-containing gas supply  320  and/or an optional connection manifold  324  that connects the chalcogen-containing gas supply  320  to the gas distribution manifold  322 . At least one supply side opening  361  is provided in the gas distribution manifold  322  (e.g., between manifolds  322  and  324 ). Each supply side opening  361  is a path through which the chalcogen-containing gas is provided into the gas distribution manifold  322 . 
     The gas distribution manifold  322  has a distribution side, which is a different side than the supply side. In one embodiment, the distribution side can be the opposite side of the supply side. The distribution side has at least one set of opening regions  369  facing the substrate  12  and having independent temperature control. The gas distribution manifold  322  includes a manifold enclosure  360  that extends from the at least one supply side opening  361  to the opening regions  369 . 
     In one embodiment, the gas distribution manifold  322  may optionally include at least one branching connections, which may include, for example, bifurcating connections, trifurcating connection, quadrifurcating connections, etc. In case multiple branching connections are employed, the multiple branching connections may be connected in a cascading configuration. In an illustrative example, N stages of bifurcating connections can provide a set of 2 N  distinct opening regions  369  that are connected to the internal volume of the vacuum enclosure adjacent to the front surface of the substrate  12 . In this embodiment, the manifold enclosure  360  may contain a plurality of branching conduits  362  which connect the at least one supply side opening  361  to the opening regions  369 , as shown in  FIG. 6A . In another embodiment, the manifold enclosure  360  may contain an open space which connects the at least one supply side opening  361  to the opening regions  369 , as shown in  FIG. 6B . 
     In one embodiment, the deposition system can be configured to continuously move the substrate  12  along a first direction (such a the horizontal direction in and out of the page in  FIG. 6A ) from an input port (which faces the first processing module  200 ) of the vacuum enclosure to an output port (which faces the third processing module  400 ) of the vacuum enclosure of the second processing module  300 . In one embodiment, the deposition system can be configured to maintain the front surface of the substrate  12  within a plane that extends along the first direction and a second direction d 2  that is perpendicular to the first direction and is along the widthwise (i.e., height) direction of the substrate  12 , as shown in  FIG. 7D . In one embodiment, each set of opening regions  369  can be configured to be at a substantially same distance from the front surface of the substrate  12 . In other words, for any given set of opening regions  369  on the same gas distribution manifold  322 , the distance between each opening region  369  and the substrate  12  can be substantially the same. As used herein, distances are substantially the same if the differences in the distances do not exceed 20% of the average of the distances. In one embodiment, the differences in the distances can be less than 1% of the average of the distances. In an alternative embodiment, different sets of opening regions  369  can have different distances between the respective set of opening regions  369  and the front surface of the substrate  12  on which the chalcogen-containing compound semiconductor material is deposited. 
     Each individual opening region  369  corresponds to an independently temperature-controlled region provided with a respective temperature-controlled heating element H#, in which # represents the numeral assigned to each temperature-controlled heating element. For example,  FIG. 6A  illustrates hollow (e.g., quasi-cylindrical or cylindrical) temperature-controlled heating elements H 01 -H 16  corresponding to sixteen discrete opening regions  369  located within a single gas distribution manifold  322 . While sixteen temperature-controlled heating elements H 01 -H 16  (and correspondingly sixteen opening regions  369 ) are illustrated in  FIGS. 6A, 6B and 7A-7D , it is understood that the number of temperature-controlled heating elements may be selected as needed to provide optimal film uniformity of the deposited chalcogen-containing compound semiconductor material. In general, the number of temperature-controlled heating elements may be in a range from 2 to 100. In one embodiment, the number of temperature-controlled heating elements may be in a range from 5 to 20. 
     Each set of opening regions  369  includes one or more opening regions  369  arranged along the second direction d 2  as illustrated in  FIGS. 7A, 7B, 7C and 7D . In one embodiment, each set of opening regions  369  can include a respective set of one or more discrete openings  369 A that are spaced from one another at least along the second direction d 2  as illustrated in  FIGS. 7A, 7B and 7D . In one embodiment illustrated in  FIGS. 7A and 7D  each set of opening regions  369  may include a one-dimensional array of discrete openings  369 A that extend along the second direction d 2 . In another embodiment illustrated in  FIG. 7B , each set of opening regions  369  may include a two-dimensional array of discrete openings  369 A that extend along the first direction d 1  and along the second direction d 2 . In this e embodiment, each set of opening regions  369  can include a respective set of discrete openings  369 A that are spaced from one another at least along the first direction d 1  and along the second direction d 2  as illustrated in  FIG. 7B . In one embodiment, the two-dimensional array of discrete opening can extend farther along the second direction d 2  than along the first direction d 1  as illustrated in  FIG. 7B . 
     In another embodiment, the opening regions  369  can include a single continuous opening  369 B as illustrated in  FIG. 7C . In this case, each opening region  369  is defined by an associated heating region that is independently temperature-controlled. In other words, each region of independent temperature control within the single continuous opening constitutes an opening region  369 . 
     Referring collectively to  FIGS. 6A, 6B and 7A-7D , each of the opening regions  369  can comprise a subset of a volume of the gas distribution manifold  322  bounded, at least in part, by a respective subset of surfaces of the gas distribution manifold  322 . For example, each of the opening regions  369  can comprise a subset of a volume of the gas distribution manifold  322  bounded by a respective subset of surfaces of the distribution manifold  360  that are laterally surrounded by a respective heating element illustrated in  FIGS. 6A and 6B . Locations of a first temperature-controlled heating element H 01  and a last temperature-controlled heating element H 16  are illustrated in  FIGS. 7A-7C . 
     In one embodiment, each heating element can be independently controlled to heat the respective subset of the surfaces of the gas distribution manifold  322  to the same or different temperature from other surfaces of the manifold  322 . Referring to  FIG. 8 , each of the opening regions  369  can be configured to be independently temperature-controlled by a respective combination of a temperature measurement device  356  and a temperature controller  358  providing a controlled power output to a respective heating element  350  (which may comprise any one of heating elements H 01  to H 016  of  FIG. 6A or 6B ). For example, the heating element  350  may be a resistive or inductive heating element. The resistive heating element  350  can include a resistive heating wire  352 , while the inductive heating element can include an inductor, such as a inductor coil (not shown). An electrical cable  354  can be employed to transmit a controlled voltage output from the temperature controller  358  to the heating element  350 . 
     In one embodiment, the temperature measurement device  356  can be a thermocouple configured to measure temperature of the respective subset of the surfaces of the gas distribution manifold  322 , which can be a subset of the surfaces of the manifold enclosure  360 . In one embodiment, each subset of the volume of the gas distribution manifold  322  corresponding to an opening region  369  can be bounded by a respective surface of the gas distribution manifold  322 , which may be a hollow (e.g., quasi-tubular or tubular) surface of the conduit  362  in the manifold enclosure  360  which terminates in the opening(s)  369 A or  369 B, as illustrated in  FIGS. 6A, 6B, 7A and 7B . In another embodiment, multiple opening regions  369  may be adjoined among one another to form a single continuous opening  369 B having multiple temperature-controlled regions as illustrated in  FIG. 7C . In this case, different opening regions  369  of the continuous opening  369 B may be heated to different temperatures by the independently controlled heating elements  350 . 
     In an illustrative example, the chalcogen-containing compound semiconductor material can comprise a copper indium gallium selenide, and the at least one sputtering target (i.e., the metallic component targets  310 ) can comprise materials selected from copper, indium, gallium, and alloys thereof (e.g., Cu—In—Ga alloy, CIG). In one embodiment, the chalcogen-containing gas source  320  can be configured to supply a chalcogen-containing gas selected from gas phase selenium and hydrogen selenide (H 2 Se). In one embodiment, the chalcogen-containing gas can be gas phase selenium, i.e., vapor phase selenium, which is evaporated from a solid source in an effusion cell. 
     While the present disclosure is described employing an embodiment in which metallic component targets  310  are employed in the second process module  300 , embodiments are expressly contemplated herein in which each, or a subset, of the metallic component targets  310  is replaced with a pair of two sputtering targets (such as a copper target and an indium-gallium alloy target), or with a set of three supper targets (such as a copper target, an indium target, and a gallium target). 
     Generally speaking, the chalcogen-containing compound semiconductor material can be deposited by providing a substrate  12  in a vacuum enclosure attached to a vacuum pump  380 , providing a sputtering system comprising at least one sputtering target  310  located in the vacuum enclosure and at least one respective magnetron located inside a cylindrical target  310  or behind a planar target (not explicitly shown), and providing a gas distribution manifold  322  having a supply side and a distribution side. The distribution side has at least one set of opening regions  369  facing the substrate  12  and having independent temperature control, and the supply side is connected to a chalcogen-containing gas source  320 . The chalcogen-containing compound semiconductor can be deposited by sputtering a material including at least one component (i.e., the non-chalcogen component) of a chalcogen-containing compound semiconductor material onto the substrate  12  while flowing a chalcogen-containing gas (e.g., Se vapor) into the vacuum chamber through the gas distribution manifold  322  and while providing a non-uniform temperature profile within a set of opening regions  369  that is selected from the at least one set of opening regions  369 . 
     Each set of opening regions  369  may have an independent temperature profile (i.e., independently controlled temperature), such that a temperature of a given opening region  369  is either the same and/or different from the temperature of one or more other opening regions  369  in the same manifold  322  at a given time. 
     In one embodiment, without wishing to be bound by a particular theory, the present inventors determined that a higher chalcogen (e.g., selenium) flux is desirable at the ends (i.e., top and bottom portions of a vertically positioned web substrate in direction d 2  in  FIG. 7D ) than in the middle of the vertically positioned substrate to obtain a metal chalcogen containing compound semiconductor material (e.g., CIGS) with a more uniform thickness and/or composition as a function of substrate width (i.e., height) in direction d 2 . In one embodiment, the manifold  322  contains independently controllable heating elements  350  (e.g., H 01  to H 16 ) which can be independently controlled to provide a lower temperature at the end opening regions  369  than the middle opening regions  369  of the same manifold. In other words, the end heating elements (e.g., H 01 , H 02 , H 15  and H 16 ) are maintained at a lower temperature than the middle heating elements (e.g., H 07 , H 08 , H 09  and H 10 ) at a given time. It is believed that the lower heating element temperature results in a higher selenium vapor flux from the respective opening region. Thus, a higher selenium flux is provided from the end opening regions  369  than the middle opening regions  369  of the same manifold  322  at the same time, as shown in by the length of the arrows in  FIGS. 6A and 6B . The higher chalcogen (e.g., selenium vapor) flux to the ends than the middle of the web substrate  12  can enhance the uniformity of the composition and thickness of the deposited chalcogen-containing compound semiconductor material (e.g., the p-CIGS absorber layer  30 ). Thus, the non-uniform temperature profile of the opening regions  369  (e.g., lower temperature of the end regions than middle regions) can compensate for the non-uniformity of the substrate temperature and the non-uniformity of the local reactant composition to provide a CIGS absorber layer  30  by reactive sputtering with improved compositional uniformity and thickness uniformity without using moving parts, such as valves, to control relative selenium flux. 
     Thus, in one embodiment, first respective heating elements  350  located adjacent to opening regions  369  at the ends of the gas distribution manifold  322  are configured to be at a lower temperature than second respective heating elements  350  located adjacent to opening regions  369  at a middle of the gas distribution manifold  322  to provide a higher selenium flux from the opening regions at the ends of the gas distribution manifold than from the opening regions at the middle of the gas distribution manifold. 
     In one embodiment, the substrate  12  can be provided as a vertically oriented moving web having the middle located below the first end and above the second end. The substrate  12  can be continuously moved along a first direction d 1  from an input port on the vacuum enclosure to an output port on the vacuum enclosure. In one embodiment, the front surface of the substrate  12  can be maintained within a plane that extends along the first direction d 1  and a second direction d 2  that is perpendicular to the first direction d 1  and is along a widthwise direction of the substrate  12 . In one embodiment, each set of opening regions  369  can be configured to be at a substantially same distance from the front surface of the substrate  12 . In other words, the plane of the front surface of the substrate  12  can be maintained at a uniform distance from all opening regions  369  for each set of opening regions  369 . 
     The selenium vapor flows to the substrate  12  through the gas distribution manifold  322  having a plurality of opening regions  369  facing the substrate  12 . First opening regions  369  located at ends of the gas distribution manifold  322  and facing the ends of the substrate  12  are maintained at a lower temperature than second opening regions  369  located at a middle of the gas distribution manifold  322  and facing the middle of the substrate  12  to provide a higher selenium flux to the ends of the substrate  12  than to the middle of the substrate  12 . 
     Referring back to  FIGS. 2 and 3 , the portion of the substrate  12  on which the first electrode  20  and the p-doped semiconductor layer  30  are deposited is subsequently passed into the third process module  400 . An n-doped semiconductor material is deposited in the third process module  400  to form the n-doped semiconductor layer  40  illustrated in the photovoltaic cell  10  of  FIG. 1 . The third process module  400  can include, for example, a third sputtering target  410  (e.g., a CdS target) and a magnetron (not expressly shown). The third sputtering target  410  can include, for example, a rotating AC magnetron, an RF magnetron, or a planar magnetron. 
     The portion of the substrate  12  on which the first electrode  20 , the p-doped semiconductor layer  30 , and the n-doped semiconductor layer  40  are deposited is subsequently passed into the fourth process module  500 . A transparent conductive oxide material is deposited in the fourth process module  500  to form the second electrode comprising a transparent conductive layer  50  illustrated in the photovoltaic cell  10  of  FIG. 1 . The fourth process module  500  can include, for example, a fourth sputtering target  510  and a magnetron (not expressly shown). The fourth sputtering target  510  can include, for example, a ZnO, AZO or ITO target and a rotating AC magnetron, an RF magnetron, or a planar magnetron. A transparent conductive oxide layer  50  is deposited over the material stack ( 30 ,  40 ) including the p-n junction. In one embodiment, the transparent conductive oxide layer  50  can comprise a material selected from tin-doped indium oxide, aluminum-doped zinc oxide, and zinc oxide. In one embodiment, the transparent conductive oxide layer  50  can have a thickness in a range from 60 nm to 1,800 nm. 
     Subsequently, the web substrate  12  passes into the output module  800 . The substrate  12  can be wound onto the output spool  810  (which may be a take up spool) as illustrated in  FIG. 2 , or can be sliced into photovoltaic cells using a cutting apparatus  840  as illustrated in  FIG. 3 . 
     While sputtering was described as the preferred method for depositing all layers onto the substrate, some layers may be deposited by MBE, CVD, evaporation, plating, etc. It is to be understood that the present invention is not limited to the embodiment(s) and the example(s) described above and illustrated herein, but encompasses any and all variations falling within the scope of the appended claims. For example, as is apparent from the claims and specification, not all method steps need be performed in the exact order illustrated or claimed, but rather in any order that allows the proper formation of the photovoltaic cells of the present invention.