Abstract:
A method for forming an anti-reflective coating (ARC) includes positioning a substrate below a target and flowing a first gas to deposit a first portion of the graded ARC onto the substrate. The method includes gradually flowing a second gas to deposit a second portion of the graded ARC, and gradually flowing a third gas while simultaneously gradually decreasing the flow of the second gas to deposit a third portion of the graded ARC. The method also includes flowing the third gas after stopping the flow of the second gas to form a fourth portion of the graded ARC. In another embodiment a film stack having a substrate having a graded ARC disposed thereon is provided. The graded ARC includes a first portion, a second portion disposed on the first portion, a third portion disposed on the second portion, and a fourth portion disposed on the third portion.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims benefit of U.S. Provisional Application Ser. No. 61/887,147, filed Nov. 13, 2013 (Attorney Docket No. APPM/20741 USL01) and U.S. Provisional Application Ser. No. 61/904,437, filed Nov. 14, 2013 (Attorney Docket No. APPM/20741USL02), both of which are incorporated by reference in their entirety. 
     
    
     BACKGROUND OF THE DISCLOSURE 
       [0002]    1. Field of the Invention 
         [0003]    Embodiments of the invention generally relate to a method of forming an anti-reflective coating (ARC), and more particularly, for forming an ARC with a graded refractive index. 
         [0004]    2. Description of the Background Art 
         [0005]    Many of the materials used in manufacturing solar cells, for example silicon, have high refractive indices and result in loss of incident sunlight by reflection. Thin film materials having a series of layers of metals and dielectrics of varying dielectric constants and refractive indices, such as a graded anti-reflective coating (ARC), are used to reduce glare or reflection. 
         [0006]    Graded ARCs are often deposited by chemical vapor deposition (CVD) or plasma enhanced CVD (PECVD). However, CVD and PECVD pose a challenge because they require deposition at higher temperatures and incorporate large amounts of hydrogen, therefore resulting in impurities and low film quality. 
         [0007]    Therefore, there is a need for an improved method of forming an ARC with a graded refractive index. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention provides methods for forming an anti-reflective coating (ARC), and more particularly, for forming an ARC with a graded refractive index. 
         [0009]    In one embodiment a method for forming a graded anti-reflective ARC in a physical vapor deposition processing chamber is provided. The method includes positioning a substrate on a substrate support below a target and flowing a first gas into the processing chamber to sputter the target to deposit a first portion of the graded ARC onto the substrate. The method also includes gradually flowing a second gas into the processing chamber to deposit a second portion of the graded ARC onto the substrate. The method further includes gradually flowing a third gas into the processing chamber while simultaneously gradually decreasing the flow of the second gas into the processing chamber to deposit a third portion of the graded ARC onto the substrate. The method also includes flowing the third gas into the processing chamber after stopping the flow of the second gas to form a fourth portion of the graded ARC. 
         [0010]    In another embodiment another method for forming a graded anti-reflective ARC in a physical vapor deposition processing chamber is provided. The method includes positioning a substrate on a substrate support below a silicon target and sputtering the silicon target to deposit a first portion of the graded ARC onto the substrate. The method also includes gradually flowing nitrogen gas into the processing chamber to deposit a second portion of the graded ARC onto the substrate. The method further includes gradually flowing oxygen gas into the processing chamber while simultaneously gradually decreasing the flow of the nitrogen gas into the processing chamber to deposit a third portion of the graded ARC onto the substrate. The method also includes flowing the oxygen gas into the processing chamber after stopping the flow of the nitrogen gas to form a fourth portion of the graded ARC onto the substrate. 
         [0011]    In yet another embodiment a film stack having a substrate having a graded ARC disposed thereon is provided. The graded ARC includes a first portion, a second portion disposed on the first portion, a third portion disposed on the second portion, and a fourth portion disposed on the third portion. The first portion has a first refractive index and the second portion has a second refractive index that is less than the first refractive index. The third portion has a third refractive index that is less than the second refractive index. The fourth portion has a fourth refractive index that is less than the third refractive index. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    So that the manner in which the above recited features of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. 
           [0013]      FIG. 1  depicts a schematic cross-sectional view of a process chamber according to one embodiment of the invention. 
           [0014]      FIG. 2  depicts a cross sectional view of a filmstack having an anti-reflective coating (ARC) according to one embodiment of the invention. 
           [0015]      FIG. 3  depicts a process flow diagram for forming an ARC according to one embodiment of the invention. 
       
    
    
       [0016]    To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. 
         [0017]    It is to be noted, however, that the appended drawings illustrate only exemplary embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
       DETAILED DESCRIPTION 
       [0018]      FIG. 1  illustrates an exemplary physical vapor deposition (PVD) process chamber  100  (e.g., a sputter process chamber) suitable for sputter depositing materials according to one embodiment of the invention. One example of the process chamber  100  that may be adapted to benefit from the invention is a PVD process chamber, available from Applied Materials, Inc., located in Santa Clara, Calif. It is contemplated that other sputter process chambers, including those from other manufactures, may be adapted to practice the present invention. 
         [0019]    The process chamber  100  includes a chamber body  108  having a processing volume  118  defined therein. The chamber body  108  has sidewalls  110  and a bottom  146 . The dimensions of the chamber body  108  and related components of the process chamber  100  are not limited and are generally proportionally larger than the size of a substrate  190  to be processed. Any suitable substrate size may be processed. Examples of suitable substrate sizes include substrates with a 200 mm diameter or 300 mm diameter. 
         [0020]    A chamber lid assembly  104  is mounted on the top of the chamber body  108 . The chamber body  108  may be fabricated from aluminum or other suitable materials. A substrate access port  130  is formed through the sidewall  110  of the chamber body  108 , facilitating the transfer of the substrate  190  into and out of the process chamber  100 . The access port  130  may be coupled to a transfer chamber and/or other chambers of a substrate processing system. 
         [0021]    A gas source  128  is coupled to the chamber body  108  to supply process gases into the processing volume  118 . In one embodiment, process gases may include inert gases, non-reactive gases, and reactive gases if necessary. 
         [0022]    Examples of process gases that may be provided by the gas source  128  include, but not limited to, argon gas (Ar), helium (He), neon gas (Ne), nitrogen gas (N 2 ), fluorine gas (F 2 ), oxygen gas (O 2 ), hydrogen gas (H2), H 2 O in vapor form, methane (CH4), carbon monoxide (CO), methane (CH 4 ), and/or carbon dioxide (CO 2 ), among others. In one embodiment, a mass flow controllers (MFC) is coupled to the gas source  128  to finely and precisely control of the flow of gases. 
         [0023]    A pumping port  150  is formed through the bottom  146  of the chamber body  108 . A pumping device  152  is coupled to the processing volume  118  to evacuate and control the pressure therein. A pumping system and chamber cooling design enables high base vacuum (about 1×10 −8  Torr or less) and low rate-of-rise (about 1,000 mTorr/min) at temperatures suited to thermal budget needs, e.g., about −25 degrees Celsius to about 500 degrees Celsius. The pumping system is designed to provide precise control of process pressure which is a critical parameter for refractive index (RI) control and tuning. 
         [0024]    The lid assembly  104  generally includes a target  120  and a ground shield assembly  126  coupled thereto. The target  120  provides a material source that can be sputtered and deposited onto the surface of the substrate  190  during a PVD process. The target  120  serves as the cathode of the plasma circuit during DC sputtering. 
         [0025]    The target  120  or target plate may be fabricated from a material utilized for a deposition layer, or elements of the deposition layer to be formed in the process chamber  100 . A high voltage power supply, such as a power source  132 , is connected to the target  120  to facilitate sputtering materials from the target  120 . 
         [0026]    In one embodiment, the target  120  may be fabricated from a material containing silicon (Si), titanium (Ti), tantalum (Ta), hafnium (Hf), tungsten (W), cobalt (Co), nickel (Ni), copper (Cu), aluminum (Al), alloys thereof, or combinations thereof and the like. In one embodiment depicted herein, the target may be fabricated from silicon. 
         [0027]    The target  120  generally includes a peripheral portion  124  and a central portion  116 . The peripheral portion  124  is disposed over the sidewalls  110  of the chamber. The central portion  116  of the target  120  may have a curvature surface slightly extending towards the surface of the substrate  190  disposed on a substrate support  138 . The spacing between the target  120  and the substrate support  138  is maintained between about 50 mm to about 350 mm, for example, about 55 mm. It is contemplated that the dimension, shape, materials, configuration and diameter of the target  120  may be varied for specific process or substrate requirements. In one embodiment, the target  120  may further include a backing plate having a central portion bonded and/or fabricated by a material desired to be sputtered onto the substrate surface. The target  120  may also include adjacent tiles or segmented materials that together form the target  120 . 
         [0028]    The lid assembly  104  may further comprise a full face erosion magnetron cathode  102  mounted above the target  120  which enhances efficient sputtering materials from the target  120  during processing. The full face erosion magnetron cathode  102  allows easy and fast process control and tailored film properties while ensuring consistent target erosion and uniform deposition of films, such as SiO x N y , across the wafer for a variety of values of x and y ranging from 0% to 100%. Examples of the magnetron assembly include a linear magnetron, a serpentine magnetron, a spiral magnetron, a double-digitated magnetron, a rectangularized spiral magnetron, among others. 
         [0029]    The ground shield assembly  126  of the lid assembly  104  includes a ground frame  106  and a ground shield  112 . The ground shield assembly  126  may also include other chamber shield member, target shield member, dark space shield, and dark space shield frame. The ground shield  112  is coupled to the peripheral portion  124  by the ground frame  106  defining an upper processing region  154  below the central portion of the target  120  in the processing volume  118 . The ground frame  106  electrically insulates the ground shield  112  from the target  120  while providing a ground path to the chamber body  108  of the process chamber  100  through the sidewalls  110 . The ground shield  112  constrains plasma generated during processing within the upper processing region  154  and dislodges target source material from the confined central portion  116  of the target  120 , thereby allowing the dislodged target source to be mainly deposited on the substrate surface rather than chamber sidewalls  110 . In one embodiment, the ground shield  112  may be formed by one or more work-piece fragments and/or a number of these pieces bonding by processes known in the art, such as welding, gluing, high pressure compression, etc. 
         [0030]    A shaft  140  extending through the bottom  146  of the chamber body  108  couples to a lift mechanism  144 . The lift mechanism  144  is configured to move the substrate support  138  between a lower transfer position and an upper processing position. Bellows  142  circumscribe the shaft  140  and are coupled to the substrate support  138  to provide a flexible seal there between, thereby maintaining vacuum integrity of the chamber processing volume  118 . 
         [0031]    The substrate support  138  provides an electro-static chuck (ESC)  180 . The ESC  180  uses the attraction of opposite charges to hold both insulating and conducting substrates  190  for PVD processes and is powered by a DC power supply  181 . The ESC  180  comprises an electrode embedded within a dielectric body. The DC power supply  181  may provide a DC chucking voltage of about 200 volts to about 2000 volts to the electrode. The DC power supply  181  may also include a system controller for controlling the operation of the electrode by directing a DC current to the electrode for chucking and de-chucking the substrate  190 . 
         [0032]    The ESC  180  performs in the temperature range required by the thermal budget of the device integration requirements formed by the substrate  190 . For example, the temperature range for: (i) a detachable ESC  180  (DTESC) is about minus 25 degrees Celsius to about 100 degrees; (ii) a mid-temperature ESC  180  (MTESC) is about 100 degrees Celsius to about 200 degrees Celsius; (iii) a high temperature or high temperature biasable or high temperature high uniformity ESC  180  (HTESC or HTBESC or HTHUESC) is about 200 degrees Celsius to about 500 degrees Celsius, to ensure fast and uniform heating of the substrate  190 . Additionally, any of the ESCs may be used without being heated, i.e., at room temperature. 
         [0033]    After the process gas is introduced into the process chamber  100 , the gas is energized to form plasma. A plasma is commonly formed from an inert gas, such as argon, before a reactive gas is introduced into the process chamber  100 . An antenna  176 , such as one or more inductor coils, may be provided adjacent the process chamber  100 . An antenna power supply  175  may power the antenna  176  to inductively couple energy, such as RF energy, to the process gas to form plasma in a process zone in the process chamber  100 . Alternatively, or in addition, process electrodes comprising a cathode below the substrate  190  and an anode above the substrate  190  may be used to couple RF power to generate plasma. The operation of the power source  175  may be controlled by a controller that also controls the operation of other components in the process chamber  100 . 
         [0034]    A shadow frame  122  is disposed on the periphery region of the substrate support  138  and is configured to confine deposition of source material sputtered from the target  120  to a desired portion of the substrate  190  surface. A chamber shield  136  may be disposed on the inner wall of the chamber body  108  and have a lip  156  extending inward to the processing volume  118  configured to support the shadow frame  122  disposed around the substrate support  138 . As the substrate support  138  is raised to the upper position for processing, an outer edge of the substrate  114  disposed on the substrate support  138  is engaged by the shadow frame  122  and the shadow frame  122  is lifted up and spaced away from the chamber shield  136 . When the substrate support  138  is lowered to the transfer position adjacent to the substrate transfer access port  130 , the shadow frame  122  is set back on the chamber shield  136 . Lift pins (not shown) are selectively moved through the substrate support  138  to list the substrate  190  above the substrate support  138  to facilitate access to the substrate  190  by a transfer robot or other suitable transfer mechanism. 
         [0035]    A controller  148  is coupled to the process chamber  100 . The controller  148  includes a central processing unit (CPU)  160 , a memory  158 , and support circuits  162 . The controller  148  is utilized to control the process sequence, regulating the gas flows from the gas source  128  into the process chamber  100  and controlling ion bombardment of the target  120 . The CPU  160  may be of any form of a general purpose computer processor that can be used in an industrial setting. The software routines can be stored in the memory  158 , such as random access memory, read only memory, floppy or hard disk drive, or other form of digital storage. The support circuits  162  are conventionally coupled to the CPU  160  and may comprise cache, clock circuits, input/output subsystems, power supplies, and the like. The software routines, when executed by the CPU  160 , transform the CPU into a specific purpose computer (controller)  148  that controls the process chamber  100  such that the processes are performed in accordance with the present invention. The software routines may also be stored and/or executed by a second controller (not shown) that is located remotely from the process chamber  100 . 
         [0036]    During processing, material is sputtered from the target  120  and deposited on the surface of the substrate  190 . The target  120  and the substrate support  138  are biased relative to each other by the power source  132  to maintain a plasma formed from the process gases supplied by the gas source  128 . The ions from the plasma are accelerated toward and strike the target  120 , causing target material to be dislodged from the target  120 . The dislodged target material and reactive process gases together form a layer on the substrate  190  with desired compositions. RF, DC or fast switching pulsed DC power supplies or combinations thereof provide tunable target bias for precise control of sputtering composition and deposition rates. 
         [0037]      FIG. 2  depicts a cross sectional view of a filmstack  200  having a graded ARC  204  according to one embodiment of the invention.  FIG. 3  depicts a process flow diagram for forming the graded ARC  204 . Referring to  FIGS. 1-3 , a method  300  for forming the graded ARC  204  begins at block  302  by positioning the substrate  190  on the substrate support  138  and below the target  120 . In one embodiment, the substrate  190  has one or more layers formed thereon, e.g., a silicon substrate having a photodiode  202 , and the substrate support  138  includes an HTESC  180 . The substrate  202  is set about 55 mm away from the target  120 , and the target  120  is fabricated from silicon. In one embodiment, the process chamber  100  pressure may be set to a low pressure, e.g., less than about 100 mTorr, or about 10 mTorr at room temperature. In one embodiment, the DC power supply  181  is pulsed to provide less than about 20 kW, for example about 6 kW, at a frequency of about 100 kHz and a duty cycle of about 97%. 
         [0038]    At block  304 , one or more sputtering gasses from the gas source  128  are flowed into the process chamber  100  to sputter the silicon target  120  to form a first portion  206  of the graded ARC  204  having silicon onto the substrate  202 . In one embodiment, the sputtering gas is argon gas flowed at about 30 sccm. 
         [0039]    At block  306 , one or more reactive gasses from the gas source  128  are flowed into the process chamber  100  to react with the silicon target  120 . In one embodiment, the reactive gas is selected from a group comprising nitrogen gas (N 2 ), nitrogen dioxide (NO 2 ), fluorine gas (F 2 ), oxygen gas (O 2 ), hydrogen gas (H2), H 2 O in vapor form, methane (CH4), carbon monoxide (CO), methane (CH 4 ), and carbon dioxide (CO 2 ). For example, in one embodiment, nitrogen gas is gradually introduced into the process chamber  100  to form nitrogen plasma. The nitrogen gas is gradually introduced until it reaches about 100 sccm to form a second portion  208  of the graded ARC  204  having silicon nitride (SiN x  wherein x is between about 0% to about 100%). As noted by the phantom lines in  FIG. 2 , the change in the composition of the graded ARC  204  from silicon in the first portion  206  to silicon nitride in the second portion  208  is gradual, i.e., no distinct layers in each portion of the graded ARC  204 . 
         [0040]    At block  308 , one or more reactive gasses from the gas source  128  is again flowed into the process chamber  100  to react with the silicon target  120 . In one embodiment, oxygen gas is gradually introduced into the process chamber  100  to form oxygen plasma, while the flow of nitrogen gas is gradually decreased in the process chamber  100 . The oxygen gas is gradually introduced until it reaches to about 50 sccm to about 100 sccm, for example about 50 sccm or about 100 sccm, to form a third portion  210  of the graded ARC  204  having SiN x O y , wherein x and y are between about 0% to about 100%. As discussed above, the change in the composition of the graded ARC  204  from silicon nitride in the second portion  208  to SiN x O y  in the third portion  210  is gradual, i.e., no distinct layers in each portion of the graded ARC  204 . 
         [0041]    At block  310 , while the flow of oxygen gas is gradually increasing, the nitrogen gas is gradually reduced to 0 sccm to form a fourth portion  212  of the graded ARC  204  having silicon oxide SiO 2 . The oxygen gas continues to flow after the flow of nitrogen gas stops. Beneficially, the gradual flow changes in nitrogen gas and oxygen gas prevents the plasma from being extinguished in the process chamber  100 . As discussed above, the change in the composition of the graded ARC  204  from SiN x O y  in the third portion  210  to SiO 2  in the fourth portion  212  is gradual, i.e., no distinct layers in each portion of the graded ARC  204 . In one embodiment, an optional buffer laying having oxide or nitride may be deposited over the graded ARC  204  to form a filmstack  202  configured for a complementary metal-oxide-semiconductor (CMOS) image sensor device. 
         [0042]    Advantageously, as the graded ARC  204  has no distinct layers in each portion  206 ,  208 ,  210  and  212  of the graded ARC  204 , the refractive index of the graded ARC  204  (at about 633 nm) can be tuned from over about 2.0 to about 1.47. In one embodiment, as the flow of the oxygen reaches about 50 sccm, the refractive index of the ARC  204  is tuned to about 1.47. For example, in one embodiment: (i) at about 0 sccm of oxygen gas, the graded ARC  204  has a refractive index between about 2.0 and about 2.5, for example, about 2.0 or about 2.1; (ii) at about 25 sccm of oxygen gas, the graded ARC  204  has a refractive index of between about 1.5 and about 2.0, for example about 1.75; (iii) at about 50 sccm of oxygen gas, the graded ARC  204  has a refractive index of between about 1.0 and about 1.5, for example about 1.47; (iv) at about 75 sccm of oxygen gas, the graded ARC  204  has a refractive index of between about 1.0 and about 1.5, for example about 1.48 or about 1.49; and (v) at about 100 sccm of oxygen gas, the graded ARC  204  has a refractive index of between about 1.0 and about 1.5, for example about 1.47. 
         [0043]    Additionally, as the flow of the oxygen increases to about 100 sccm the graded ARC  204  has a low compressive stress level, i.e., the stress of the graded ARC  204  is tunable with different levels of oxygen gas. For example, in one embodiment: (i) at about 0 sccm of oxygen gas, the graded ARC  204  has a stress level of −599 MPa; (ii) at about 25 sccm of oxygen gas, the graded ARC  204  has a stress level of −276 MPa; (iii) at about 50 sccm of oxygen gas, the graded ARC  204  has a stress level of −144 MPa; (iv) at about 75 sccm of oxygen gas, the graded ARC  204  has a stress level of −157 MPa; and (v) at about 100 sccm of oxygen gas, the graded ARC  204  has a stress level of −119 MPa. Therefore, the graded ARC  204  provides an ARC that gradually controls the refractive index of the filmstack  202  without having to control the thickness of the graded ARC  204 , and has low stress levels. 
         [0044]    While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.