Abstract:
A showerhead for a plasma process apparatus for processing substrates, comprising a showerhead body comprising a top plate and a bottom plate defining a cavity in between; a gas inlet formed in the top plate; a perforated plate positioned between the top plate and the bottom plate and dissecting the cavity into an upper gas compartment and a lower gas compartment; and, wherein the bottom plate comprises a plurality of elongated diffusion slots on its lower surface and a plurality of diffusion holes on its upper surface, each of the diffusion holes making fluid connection from the lower gas compartment to more than one of the diffusion slots.

Description:
RELATED APPLICATION 
     This application claims priority benefit from U.S. Provisional Patent Application No. 61/252,117, filed Oct. 15, 2009, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     1. Field of the Application 
     The invention relates to showerheads for plasma processing chambers, such as for Chemical Vapor Deposition (CVD), Plasma Enhanced Chemical Vapor Deposition (PECVD), and etching of films. 
     2. Related Art 
     Apparatus of plasma processing, such as, e.g., plasma enhanced chemical vapor deposition of films, utilize showerhead to introduce the gases into the chamber. Various showerheads for plasma chambers are known in the art. The showerhead delivers processing gas into the plasma processing chamber. Some of the issues considered in designing a showerhead include proper mixing of the gas, control over the distribution of the gas throughout the chamber, control over the temperature of the showerhead and gases within the showerhead, pitting of the showerhead by plasma species, rate of gas delivery, complexity and cost of production, and more. 
     The design of the showerhead assembly is critical, as it directly affects the quality of the process in the plasma chamber. For example, the design of the showerhead directly affects the uniformity of the film deposited in PECVD chambers. Uniformity of deposited film becomes more difficult to control as the size of the substrate increases, for example when processing large substrates for LCD or thin-film solar panels. Uniformity is also more difficult to control when performing batch processing, i.e., when forming film on several substrates concurrently in the same chamber. However, as is evident these days, the sizes for LCD becomes larger and larger, and recent popularity of solar panels increases the demands on systems for forming high quality films on large substrates or on multiple substrates concurrently. 
     One problem with the current state of the art showerhead is its complexity and cost of manufacturing, mainly due to the side and shape of the gas diffusion holes.  FIG. 9  illustrates a partial view of a showerhead according to the state of the art, in a cross-section view. As seen in  FIG. 9 , there are many gas diffusion holes having a complex and expensive shape to manufacture. First holes  900  of small diameter d are drilled from the top surface, generally using a small diameter dill bit that has very limited length, and must be replaced multiple times during the manufacture of a single part. Then holes  905  of larger diameter are drilled from the bottom surface. In this step it is very important that the bottom holes  905  be coaxially aligned to the top holes  900 , which complicates the fabrication of the showerhead. Also, chamfers  910  and  915  need to be fabricated, which further increases the cost of fabrication. Moreover, due to the small diameter d of holes  900 , conventional sand blasting treatment of the showerhead cannot be performed, since it clogs the holes. Therefore, special chemical cleaning must be done instead, followed by close inspection under magnification of all holes to ensure they are all open. 
     SUMMARY 
     The following summary of the invention is included in order to provide a basic understanding of some aspects and features of the invention. This summary is not an extensive overview of the invention and as such it is not intended to particularly identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented below. 
     Embodiments of the invention provide a showerhead assembly that is easier and significantly less expensive to manufacture, while having improved gas distribution. Showerheads made according to embodiments of the invention do not require the drilling of large number of small diameter holes. Embodiments of the invention also do not require complex hole shapes. Instead, control over gas distribution gradient or uniformity is done using a baffle plate. A second plate diffuses the gas into the chamber using relatively large diameter holes leading to diffusion slots. Since the diffusion slots are rather large, ionization may take place in the diffusion slots without harming the showerhead. IN such circumstances, the diffusion plate may be referred to as gas ionization plate. According to further embodiments, the showerhead assembly is coupled to the RF generator to also act as an electrode for the plasma chamber. 
     A showerhead for a plasma process apparatus for processing substrates, comprising a top plate having gas inlet; a perforated plate positioned below the top plate and distanced from the top plate so as to define an upper gas compartment with the top plate; a diffusion plate positioned below the perforated plate and distanced from the perforated plate so as to define a lower gas compartment, the diffusion plate facing the substrate to be processed; and, wherein the diffusion plate comprises a plurality of elongated diffusion slots on its lower surface and a plurality of diffusion holes on its upper surface, each of the diffusion holes making fluid connection from the lower gas compartment to more than one of the diffusion slots. 
     A showerhead for a plasma process apparatus for processing substrates, comprising a showerhead body comprising a top plate and a bottom plate defining a cavity in between; a gas inlet formed in the top plate; a perforated plate positioned between the top plate and the bottom plate and dissecting the cavity into an upper gas compartment and a lower gas compartment; and, wherein the bottom plate comprises a plurality of elongated diffusion slots on its lower surface and a plurality of diffusion holes on its upper surface, each of the diffusion holes making fluid connection from the lower gas compartment to more than one of the diffusion slots. 
     According to embodiments of the invention, a simple and cost-effective method for fabricating a gas ionization plate is disclosed, the method comprising fabricating a plate; using gang cutter to form a plurality of gas ionization slots on the lower surface of the plate; and, drilling a plurality of gas distribution holes from the top surface of the plate such that each holes reaches and opened to a plurality of the gas ionization slots. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other aspects and features of the invention would be apparent from the detailed description, which is made with reference to the following drawings. It should be appreciated that the detailed description and the drawings provides various non-limiting examples of various embodiments of the invention, which is defined by the appended claims. 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, exemplify the embodiments of the present invention and, together with the description, serve to explain and illustrate principles of the invention. The drawings are intended to illustrate features of the exemplary embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of actual embodiments nor relative dimensions of the depicted elements, and are not drawn to scale. 
         FIG. 1A  illustrates a plasma processing chamber that may incorporate a showerhead according to the invention. 
         FIG. 1B  is a schematic illustrating in a cross section showing major elements of a showerhead according to an embodiment of the invention. 
         FIG. 2  illustrates a top elevation of the perforated/gas restriction plate, according to an embodiment of the invention, while Detail D illustrates a close-up section of the perforated/gas restriction plate. 
         FIG. 3  illustrates a cross-section of a section of the showerhead according to an embodiment of the invention. 
         FIG. 4  illustrates a top view of the diffusion plate according to an embodiment of the invention. 
         FIG. 5  illustrates a bottom view of the diffusion plate according to an embodiment of the invention. 
         FIG. 6  illustrates an embodiment in which the diffusion plate is made of a single plate of metal. 
         FIGS. 7A-7D  illustrate different arrangements for the holes and slots. 
         FIGS. 8A and 8B  illustrate a method according to embodiments of the invention for cost-effectively cutting the diffusion slots. 
         FIG. 9  illustrate holes design of prior art showerhead. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1A  illustrates a plasma processing chamber  150  that may incorporate a showerhead  155  according to the invention. In this example the chamber  150  includes a loading gate  160  for loading and unloading substrates. A substrate holder  165  may be a simple holder, a holder incorporating a heater, a chuck, such as, e.g., an electrostatic chuck, etc. The substrate holder  165  may hold a single substrate or a plurality of substrates. In this example, the holder supports a plurality of substrates  170 , each of which may be lifted from the holder  165  using lift pins  175 . Also, the showerhead  155  may be coupled to the ground or RF potential of an RF generator, e.g., via an impedance match circuit (not shown). 
       FIG. 1B  is a schematic illustrating in a cross section showing major elements of a showerhead according to an embodiment of the invention. In this particular example the showerhead is rectangular, but other shape can be used, depending on the shape of the plasma chamber. Since flat panel displays and solar cells are fabricated inside a rectangular chamber, in this embodiment the showerhead is rectangular. 
     The showerhead comprises perforated plate  115 , which may act as a baffle plate restricting gas flow, and is positioned so as to divide the internal space of the showerhead into a first compartment  100  and a second compartment  110 . Due to the flow restriction action of the baffle plate, first compartment  100  has higher gas pressure than the second compartment. The bottom of the showerhead is formed by a diffusion plate  130 , which has elongated diffusion slots  120  at the lower surface, so as to face the substrate being processed. The diffusion slots may be made large enough to support gas ionization within the slots. Diffusion holes  125  connect to the diffusion slots  120 , to enable gas from the second compartment  110  to be delivered to the process region via the diffusion slots. Notably, each diffusion hole  125  connects to a plurality of diffusion slots  120 . This feature enables high conductance of the gases into the plasma chamber. 
     As is shown by the arrows in  FIG. 1B , the gas from a gas source enters the showerhead from the top plate  145  and then enters the first compartment  100 . The perforated plate  115  forces the gas to distribute into the second compartment  110  according to pressure distribution determined by the design of the perforated plate. In this manner, the flow from the first compartment to the second compartment can be controlled. For example, the size, number and distribution of the perforations can be designed to cause the gas to distribute evenly or in a pressure gradient into the second compartment, as desired by the particular design. According to embodiments of the invention the perforation can be made to have a diameter in the range of 0.006″ to 0.500″. 
     The diffusion holes  125  then distribute the gas into the diffusion slots  120 . In this embodiment the diffusion holes  125  are circular and extend vertically to form a fluid passage from the second compartment  110  to the diffusion slots  120 . Each diffusion hole  125  forms passage to several diffusion slots  120 . In this embodiment the diffusion slots  120  have rectangular cross-section and each extend horizontally the entire length of the showerhead to enable high conductance of gas. According to embodiments of the invention the diffusion holes can be made to have a diameter in the range of 0.025″ to 2.000″, while the diffusion slots can be made to have a width of 0.010″ to 1.000″, a depth of 0.010″ to 1.000″, and a slot pitch of 0.015″ to 6.000″. 
       FIG. 2  illustrates a top elevation of the perforated plate  115 , according to an embodiment of the invention, while Detail D illustrates a close-up section of the perforated plate  115 . The perforated plate  115  has many small holes to serve as a baffle plate, so that gas is delivered controllably from the first compartment  100  to the second compartment  110 . This feature ensures the controlled uniform or gradient distribution of the gas in spite of the high conductance of the diffusion plate. For example, in some applications a uniform gas distribution may be needed, while in other applications center high or center low gas flow may be needed. 
       FIG. 3  illustrates a partial cross-section of showerhead according to an embodiment of the invention.  FIG. 3  shows the upper gas compartment  100 , the baffle (or flow restriction) plate  115 , lower gas compartment  110 , the gas distribution holes  125 , and the gas distribution slots  120 . As shown, in this embodiment each gas hole  125  connects to several gas slots  120 . Also, in this embodiment the holes are staggered such that each row of holes is offset from the rows that are immediately next on each side of it. As explained above, in this embodiment the holes are circular and are vertical, such that each hole forms a passage from the second compartment  110  to several diffusion slots  120 . Of course, other hole shapes can be used. The diffusion slots  120  are horizontal and each diffusion slot intersects all diffusion holes  125  in one row. Also, as will be shown in other embodiments, each slot  120  may intersect holes belonging to more than one row. 
       FIG. 4  illustrates a top view of the diffusion plate  130 , i.e., the view from inside the second compartment  110 , looking down towards the plasma processing chamber.  FIG. 4  shows the round gas holes  125  arranged in rows, wherein each hole  125  leads to several diffusion slots  120  (the slots are shown in broken lines).  FIG. 4  also shows how each successive row of diffusion holes is offset from its immediate neighbor rows, such that the holes are shifted.  FIG. 5  illustrates a bottom view of the diffusion plate  130 , i.e., looking from inside the plasma processing chamber towards the second compartment  110 .  FIG. 5  shows the slots  120  in solid lines and round gas holes  125  in broken lines, behind gas slots  120 . Each of the holes is leading to several diffusion slots and each diffusion slot intersect all of the holes in one row of holes. 
     According to the embodiment illustrated in  FIG. 6 , the diffusion plate is made of a single plate of metal. The bottom surface of the plate is machined to have diffusion slots  620 , while the top surface of the plate is drilled to have the rows of holes  625 , such that each drilled hole intersects several diffusion slots. In one embodiment, each hole intersect three diffusion slots, one of which passes in the center of the hole, while the other two pass at the opposing edge of the hole, as shown in  FIG. 6 . Since the rows of holes are staggered, each hole has one diffusion slot passing at its center and two passing at its opposing edges. 
     As noted above, other arrangements for the gas distribution holes and gas distribution slots can be used. For example,  FIG. 7A  illustrate an arrangement wherein multiple rows of holes are provided, wherein each hole in each row intersect three slots and wherein each slot intersect all of the holes in a row. The row of holes are aligned, i.e., not staggered. That is, all of the holes  700   a  of one row, are aligned with the holes  700   b  of the neighboring row of holes. In this respect, the rows are defined as the collection of holes that are aligned in the slots direction.  FIG. 7B , on the other hand, illustrates the staggered rows arrangement, as shown in  FIGS. 3-5 . That is, holes  700   d  of one row are shifted from the holes  700   c  of its neighboring row. The shift is such that each holes  700   d  is aligned exactly midway between two holes  700   c  of the neighboring row.  FIG. 7C  illustrates a case wherein the row of holes are aligned (i.e., not staggered) but are shifted in a direction perpendicular to the slots, such that certain slots intersect holes from two neighboring rows of holes. That is, holes  700   f  of one row are exactly aligned with holes  700   e  of its neighboring row, but they are shifted in a direction perpendicular to the slots, such that holes from one row overlaps slots that are connected to holes from another row.  FIG. 7D  illustrates the case wherein the holes in a neighboring row are staggered and shifted. That is, holes  700   h  from one row are positioned midway between holes  700   g  of its neighboring row, and the holes  700   h  overlap over slots that are also connected to holes  700   g  from the neighboring row. 
       FIG. 8A  illustrates a cross section along line A-A in  FIG. 6 , to enable understanding of a fabrication method according to an embodiment of the invention. As shown in  FIG. 8A , a circular saw  800 , also referred to as gang cutter, is used to machine the gas distribution slots  120  in the lower surface of the gas distribution plate  130 . As shown in FIG.  8 B, a gang cutter having multiple blades can be used to cut several slots  120  at a single pass. Drill  805  is used to drill the gas distribution holes  125  from the upper surface of the gas distribution plate. 
     It should be understood that processes and techniques described herein are not inherently related to any particular apparatus and may be implemented by any suitable combination of components. Further, various types of general purpose devices may be used in accordance with the teachings described herein. It may also prove advantageous to construct specialized apparatus to perform the method steps described herein. 
     The present invention has been described in relation to particular examples, which are intended in all respects to be illustrative rather than restrictive. Those skilled in the art will appreciate that many different combinations of hardware, software, and firmware will be suitable for practicing the present invention. Moreover, other implementations of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.