Abstract:
A system and method for improved atomic layer deposition. The system includes a top showerhead plate, a substrate and a bottom showerhead plate. The substrate includes a porous microchannel plate and a substrate holder is positioned in the system to insure flow-through of the gas precursor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. Provisional Application 61/761988, filed Feb. 7, 2013, and is incorporated herein by reference in its entirety. 
     
    
     STATEMENT OF GOVERNMENT INTEREST 
       [0002]    The United States Government has certain rights in this invention pursuant to Contract No. W-31-109-ENG-38 between the United States Government and The University of Chicago and/or pursuant to Contract No. DE-AC02-06CH11357 between the United States Government and UChicago Argonne, LLC representing Argonne National Laboratory. 
     
    
     FIELD OF THE INVENTION 
       [0003]    This invention relates to an improved system and method for ALD/CVD deposition for coating porous substrates. More particularly, the invention relates to a system and method for coating porous substrates, such as capillary glass arrays, using an ALD reactor having a showerhead inlet section which provides a uniform flux of precursor reactants across a surface of the porous substrate, substrate fixturing to provide a flow-through geometry in which precursor reactants and carrier gas are forced to flow-through pores of the porous substrate, and a showerhead outlet section to ensure that the flow remains perpendicular to the porous substrate throughout the coating process, to minimize deposition nonuniformities and enhance the efficiency of precursor purging. 
       BACKGROUND 
       [0004]    High surface area, porous substrates such as capillary glass arrays are challenging to coat efficiently by atomic layer deposition (ALD) which is well known for highly self-limiting conformal deposition. Using conventional, cross-flow ALD reactors, the upstream portion of the substrate will always experience a larger precursor flux while the downstream portion of the substrate will experience a larger flux of the reaction byproducts. Non-idealities in the ALD process, coupled with these non-uniform fluxes, can produce non-uniform hereinafter (“NU”) coatings along the flow axis. In addition, the non-uniform consumption of precursor across the surface of the substrate in the direction perpendicular to the flow can be another source of NU in cross-flow reactors. An additional problem with porous substrates is that molecules such as H 2 O which physisorb strongly to surfaces can continue to outgas for long periods of time. When there is excess H 2 O precursor or the H 2 O is a reaction product o in a porous substrate, the H 2 O must diffuse out of the pore in order to become entrained in the purge gas. Because the purge gas flow is normal to the axis of the pores, there is a high probabliltiy that the H 2 O will diffuse back into a downstream pore and again physisorb. The net result of multiple physisorption events is a trapping effect, and this introduces additional NU to the coatings along the flow axis, particularly when the H 2 O encounters the metal precursor (such as trimethyl aluminum) from the subsequent ALD cycle. This trapping effect can be mitigated somewhat by increasing the purge times of the ALD cycles, but this will decrease the throughput and increase the fabrication cost. 
       SUMMARY OF THE INVENTION 
       [0005]    An improved ALD reactor is provided for coating substrates, particularly porous substrates having an aspect ratio, defined as pore length divided by pore diameter, of greater than about 10-1000, and a high surface area by virtue of the porosity. The system includes at least a showerhead style structure for use as a precursor inlet to provide a uniform precursor flux across the entire surface of the substrate. The porous substrate is also preferably positioned relative to the showerhead structure in a fixture that forces a flow-through geometry wherein the precursor flux and a carrier gas are required to flow through the pores of the porous substrate. In a most preferred embodiment, the flow-through reactor system includes a second showerhead style structure located downstream of the porous substrate to maintain a uniform flow that is everywhere parallel to the axis of the substrate pores. These features serve to improve the uniformity of the precursor flux, resulting in improved thickness and compositional uniformity of the deposited layers on the porous substrate. In yet another embodiment plural porous substrates can be disposed between the dual showerheads to enable deposition of material on multiple substrates. 
         [0006]    These features and other advantages of the invention, together with arrangement and manner of operation thereof, will become apparent from the following detailed description when taken in conjunction with the accompanying drawings, wherein like elements have like numerals throughout the several drawings described below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1   a  shows a top view schematic of a conventional ALD cross-flow reactor; and  FIG. 1   b  shows an elevation view of the reactor of  FIG. 1   a;    
           [0008]      FIG. 2   a  shows a commercial ALD reactor;  FIG. 2   b  shows a view of a reaction chamber of the reactor of  FIG. 2   a ; and  FIG. 2   c  shows placement of a cross-flow reaction chamber suitable for 300 mm substrates in the reactor; 
           [0009]      FIG. 3   a  shows an as received 8″×8″ capillary glass array mounted in a stainless steel frame;  FIG. 3   b  shows the capillary glass array after ALD functionalization with a nanocomposite of Mo:Al 2 O 3  to produce a microchannel plate (MCP) and  FIG. 3   c  shows a 33 mm diameter MCP after the same ALD functionalization in  FIG. 3   b;    
           [0010]      FIG. 4   a  shows the cross-flow reaction chamber of  FIGS. 2   b  and  2   c  with a 300 mm Si wafer before Al 2 O 3  ALD deposition; and  FIG. 4   b  shows the 300 mm Si wafer after ALD deposition of an Al 2 O 3  coating with Trimethyl Aluminum (“TMA”) and water (H 2 O) vapour precursors; 
           [0011]      FIG. 5  shows a thickness contour plot of the ALD Al 2 O 3  coating across the 300 mm Si wafer after the ALD deposition of  FIG. 4   b  for 1000 ALD cycles of TMA-N 2  purge-H 2 O—N 2  purge; 
           [0012]      FIGS. 6   a - 6   c  show a cross-flow reaction chamber for the scale up of the ALD deposition process of  FIG. 4   b  with  FIG. 6   a  being an overview,  FIG. 6   b  a close-up of the inside view of the cross-flow reaction chamber and  FIG. 6   c  shows a 1″×1″ to 12″×12″ plane glass substrate with ITO (indium tin oxide) coating deposited by ALD in the cross-flow reaction chamber of  FIGS. 6   a  and  6   b;    
           [0013]      FIG. 7  shows a plot of resistivity measured at different locations across a 300 mm Si wafer for a Chem-2 (Mo—Al 2 O 3 ) coating done with the system of  FIGS. 6   a - 6   c;    
           [0014]      FIG. 8  shows a photograph of a 300 mm Si wafer after Al 2 O 3  ALD performed in the presence of an 8″×8″ capillary glass array using the system of  FIGS. 2   c  and  4   a;    
           [0015]      FIG. 9  shows the thickness profiles, expressed as the growth rate versus the position across 300 mm wafers, following the ALD deposition of Al 2 O 3  in the cross-flow reaction chamber system of  FIGS. 2   c  and  4   a  both with and without the presence of an 8″×8″ capillary glass array using a variety of deposition conditions; 
           [0016]      FIG. 10  shows the thickness profiles, expressed as the growth rate versus the position across 300 mm wafers, following the ALD deposition of MgO using bis-cyclopentadienyl magnesium (MgCp2) and H 2 O precursors in the cross-flow reaction chamber of system of  FIGS. 2   c  and  4   a  in the presence of an 8″×8″ capillary glass array using a variety of deposition conditions. The solid data points and line indicate the MgO growth rate measurements performed under identical conditions but with the capillary glass array removed; 
           [0017]      FIG. 11  shows thickness trend for MgO ALD deposited without the capillary glass array (black dots) and with the capillary glass array (colored dots) using different Mg(Cp) 2  dose times performed using the cross-flow reaction chamber of  FIGS. 6   a  and  6   b.    
           [0018]      FIG. 12  shows Mo growth rate versus distance across a 300 mm Si wafer for Mo ALD deposited with and without the capillary glass array under different dose and purge times with deposition performed at 200° C. using MoF 6  and Si 2 H 6  precursors in the cross-flow reaction chamber of  FIGS. 2   c  and  4   a;    
           [0019]      FIG. 13   a  shows a schematic side view of one embodiment of a through-flow (or flow-through) system for an ALD/CVD reactor with a single porous substrate;  FIG. 13   b  shows a schematic side view of another embodiment of a through-flow system with multiple porous substrates; 
           [0020]      FIG. 14   a  shows a side view cross section of the reactor of  FIG. 13   a ; and  FIG. 14   b  shows a top view of the top showerhead portion of  FIG. 14   a;    
           [0021]      FIG. 15   a  shows a photograph of a flow-through reactor with dual showerheads; and  FIG. 15   b  shows another view of the embodiment of  FIG. 15   a;    
           [0022]      FIG. 16   a  shows Al 2 O 3  deposition profiles using various conditions (with and without capillary glass arrays) for the system of  FIG. 13   a ;  FIG. 16   b  shows MgO deposition growth rate with the Si coupons placed below the MCP; and 
           [0023]      FIG. 17   a  shows thickness and refractive index of W:Al 2 O 3 , (chem-1) on Si (100) coupons using the system of  FIG. 13   a  with deposition for an 8″×8″ capillary glass array disposed on top at 150° C. using WF 6  and Si 2 H 6  for W and with TMA and H 2 O for Al 2 O 3 ;  FIG. 17   b  shows further results for deposition of W:Al 2 O 3 , (chem-1) on an 8″×8″ MCP (no Si) using the system of  FIG. 13   a ; and  FIG. 17   c  shows X-ray fluorescence analysis of the relative W concentration from front and back sides of the MCP versus location on the MCP, deposited uniformly and done at 150° C. using WF 6  and Si 2 H 6  for W and TMA and H 2 O for Al 2 O 3 . 
           [0024]      FIGS. 18A-B  illustrate one embodiment of a 8″×8″ porous capillary glass array substrate (MCP) placement inside the dual showerhead ALD reaction chamber:  FIG. 18A  MCP placed on bottom showerhead elevated with ceramic beads (side view);  FIG. 18B  MCP placement with monitor Si witness coupons on the bottom showerhead (view from top) 
           [0025]      FIGS. 19A-B  illustrate one embodiment of a 8″×8″ porous capillary glass array substrate (MCP) placement with side pieces of similar porous substrate inside the dual showerhead ALD reaction chamber (monitor silicon pieces are positioned below the MCP);  FIG. 19A  illustrates a MCP substrate holder with side cut pieces of similar type of MCP;  FIG. 19B  illustrates a MCP placed with side pieces (view from top). 
           [0026]      FIG. 20A  Growth rate of Al 2 O 3  with and without side pieces of MCP in the dual showerhead ALD reaction chamber during coating of 8″×8″ MCP for different reaction chamber configurations;  FIG. 20B  Growth rate of Al 2 O 3  with side pieces of similar MCP in the dual showerhead ALD reaction chamber;  FIG. 20C  illustrates measurement of the growth rate of Al 2 O 3  along the diagonal of the 8″×8″ MCP with side pieces of similar type of MCP in the dual showerhead ALD reaction chamber. 
           [0027]      FIG. 21  illustrates a 8″×8″ porous capillary glass array substrate (MCP) with side pieces of similar type of MCP cut to surround the 8″×8″ MCP. One border of the 8″×8″ MCP is sealed with kapton tape to the adjacent side piece. 
           [0028]      FIG. 22  illustrates growth rate of Al 2 O 3  with side pieces of similar MCP in the dual showerhead ALD reaction chamber configured as in  FIG. 21 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0029]    Prior art systems, such as shown in  FIGS. 1   a - 1   b , consist of an ALD cross-flow reactor  10  wherein a precursor and carrier are input through precursor inlets  20  disposed within a flow distributor plate  30 . The reactor  10  further includes a precursor distributor  40 , a precursor outlet  50  disposed within a second flow distributor plate  60  and a planar substrate  70 . As noted by arrows  100 , precursor/carrier gas  110  flows from the precursor inlets  20 , flow across the planar substrate  70 , and out the precursor outlet  50 . As will be shown hereinafter, example data from performance of this conventional prior art reactor  10  show how such systems suffer from a number of deficiencies, including irregular layer thickness across the substrate. 
         [0030]      FIGS. 2   a - 2   c  illustrate various views of a commercial Beneq ALD deposition system  11 . As shown in  FIG. 2   b , a chamber  185  has subchamber  190  for carrying out conventional cross-flow ALD deposition. In such a system certain ALD functionalities can be performed, such as the scale-up of substrate size for 33 mm MCP disks  170  (see  FIG. 3   c ) up to 300 mm MCP.  FIG. 3   a  shows an 8″×8:capillary glass array (MCP)  200  before deposition and  FIG. 3   b  after ALD functionalization with a nanocomposite of Mo:Al 2 O 3  layer  210  and  FIG. 3   c  for a 33 mm diameter MCP also functionalized with the ALD nanocomposite layer.  FIGS. 4   a  and  4   b  show photographs of 300 mm silicon wafer  201  and  211  before and after ALD functionalization in the subchamber  190  (see  FIG. 4   a ).  FIG. 5  shows a schematic of precursor input and output with a topograph of Al 2 O 3  layer thickness across the 300 mm Si wafer  211  after ALD deposition for 1000 cycles. The referenced Si wafer is for monitoring the reaction and providing a compariable between embodiments with the MCP installed and without the MCP installed. In particular, the layer thickness is uniform on the 300 mm Si wafer coated in the cross-flow reactor without the MCP installed as evidenced by the coating thickness variation of &lt;1% across the wafer. 
         [0031]      FIGS. 6   a - 6   c  show a scale-up of a conventional reactor with  FIG. 6   a  being a photograph of a closed cross-flow reaction chamber  230  for depositing an ALD coating on substrates with up to 12″×18″ dimensions.  FIG. 6   b  shows an inside view of the cross-flow reaction chamber  230 .  FIG. 6   c  shows photographs of a 1″×1″ scale-up to a 12″×12″ glass substrate with an ALD indium tin oxide coating that is uniform in thickness within 5%. The ITO coatings on these planar substrates  240  and  250  are very uniform in thickness in the absence of a porous substrate 
         [0032]    FIG. 7 shows a plot of electrical resistivity of a Chem-2 coating (Chem-2=ALD Mo/Al 2 O 3  nanocomposite) at different locations on a 300 mm Si wafer. The system of  FIGS. 2   c  and  4   a  was used to carry out the ALD cross-flow deposition. The resistivity of the chem-2 coatings are extremely sensitive to the composition of the film, with the resistivity decreasing exponentially with increasing Mo content. The very uniform resistivity of the chem-2 film across the 300 mm Si wafer is evidence that the composition of the chem-2 coatings is very uniform in the conventional cross-flow reactor without the MCP installed. 
         [0033]      FIG. 8  shows a photograph of a 300 mm Si (100) wafer after Al 2 O 3  deposition with a capillary glass array substrate (MCP) placed in close proximity to the wafer (note the precursor inlet and outlet) throughout the coating process. The thickness of the coating was measured at various points by spectroscopic ellipsometry, and the thickness values are indicated on  FIG. 8 . The data shows substantial non-uniformity of the ALD coating. In particular, the coating thickness varies by 300% (from 50 nm to 150 nm) with the MCP installed, in comparison to the &lt;1% thickness variation obtained in the absence of the MCP as shown in  FIG. 5 . These depositions were performed at 200C using (Trimethyl Aluminum) “TMA” and H 2 O precursor. 
         [0034]      FIG. 9  shows detailed plots of ALD Al 2 O 3  layer growth rate versus distance across 300 mm Si wafer substrates coated using the cross-flow system of  FIGS. 2   c  and  4   a.  Note the plot labeled 1421x3-5-1 FR has no capillary glass array substrate (MCP) installed, whereas all others do. Deposition conditions were as for  FIG. 8 . The presence of the MCP resulted in highly non-uniform layer thickness across the Si wafer, regardless of the many changes in process conditions made in an effort to obtain a uniform layer thickness. Although the MCP is referenced as an example, similar porous substrates will produce the same non-uniform coatings, for instance substrates manufactured by sintering glass beads. Other porous substrates may be comprised of aerogels, xerogels, microfiber plates, nanotube-based mesh or cloth, and 3D-printed materials. Further, the holes need not be a regular array of holes, in certain embodiments the array will be irregular and the pores will be tortuous. These porous substrates may be intended for use in catalysis, separations, or electron multipliers. 
         [0035]      FIG. 10  shows growth rate versus location on a Si wafer for MgO coatings prepared by ALD with and without an MCP installed, and deposited using MgCp 2  and H 2 O precursors with precursor inlet located on the left side at location “0”. These depositions used the cross-flow system of  FIGS. 2   c  and  4   a.  The conditions were varied in an attempt to obtain a uniform MgO coating with the MCP installed. For instance, deposition was performed with flow restrictors installed, precursor was supplied under both flow-boost conditions and constant N 2  flow conditions. For reference, the baseline trace shows the MgO thickness without the MCP installed. Again, the MgO layer thickness is highly non-uniform with the MCP installed, as for the Al 2 O 3  in  FIG. 9 , regardless of the process conditions used.  FIG. 11  likewise shows non-uniform thickness for MgO deposition using the reactor system in  FIGS. 6   a  and  6   b  with the MCP substrate installed, regardless of process conditions, whereas the thickness is uniform in the absence of the MCP. 
         [0036]      FIG. 12  is shows the variation in Mo growth rate across a 300 mm Si wafer using the cross-flow system of  FIGS. 2   c  and  4   a . As for the Al 2 O 3  and MgO deposition, the thickness is highly non-uniform for ALD performed with the MCP substrate installed regardless of process conditions. But again, the Mo thickness is uniform in the absence of the MCP. Depositions were carried out at 200C using MoF 6  and Si 2 H 6  precursors. 
         [0037]    In preferred embodiment of the invention shown in  FIG. 13   a , a dual showerhead flow-through reactor  120  includes a top showerhead (or first plate)  130 , a porous substrate  145 , a bottom showerhead (or second plate)  140  with a central precursor injection port  150  for a precursor and the carrier gas  110 . The top showerhead  130  and bottom showerhead  140  can be made out of any metal, alloy, or ceramic compatible with the process temperatures and chemical precursors for the intended ALD or CVD growth. Each of the top shower head  130  and bottom shower head  140  have a plurality of holes. The hole size and the density of the holes is selected for the desired delivery of the precursor flux. In one embodiment, the top showerhead  130  includes smaller diameter holes than the bottom shower head  140 . The ratio of the hole diameters between the top showerhead  130  and the bottom showerhead  140  can be adjusted to adjust the relative conductance. Likewise the overall thickness of the holes (the showerhead thickness) for the showerheads  130 ,  140  dictates the conductance of the holes. In one embodiment, the top showerhead  130  and the bottom showerhead  140  are sealed to or integral with a side of the reactor  120  such that precursor cannot flow “around” the shower head but must flow-through the showerhead  130 ,  140 . 
         [0038]    The precursor injection portion  150  is preferably spaced sufficiently from the top showerhead  130  to allow for uniform distribution of the precursor with respect to the top showerhead  130  prior to passing through the holes in the showerhead. The precursor supply arrangement comprised of the precursor injection  150 , the precursors  120 , and the inert purge gas  110  can be connected only above the top showerhead  130  as shown in  FIG. 13   a , or can be connected both above the top showerhead  130  and below the bottom showerheads  140 . Likewise, the exhaust portion  155  can be connected only below the bottom showerhead  140  as shown in  FIG. 13   a , or can be connected both below the bottom showerhead  140  and above the top showerhead  130 . By connecting the precursor supply and the exhaust portion to both the top and bottom, the direction of flow through the porous substrate  180  can be reversed. By reversing the flow periodically throughout the deposition of the film, the film thickness can be made more uniform through the pores of the substrate. 
         [0039]    The reactor  120  further includes showerhead spacers  160  separating the top showerhead  130  and the bottom showerhead  140  and an exhaust port  155 . In one embodiment, the spacers provide a spacing of up to 3-5 mm. The showerhead spacers are preferably positioned about the periphery to allow an internal space defined by the showerhead spacers and the top showerhead  130  and bottom showerhead  140  in which the MCP  145  can be placed. In one embodiment, the MCP  145  is positioned on ceramic supports  180  (see  FIG. 15   b ) and also disposed between the top showerhead  130  and the bottom showerhead  140 . In one embodiment, the exhaust port  155  is in communication with a pump to assist in evacuating the reactor and in moving the precursor through the showheads  130 ,  140 . Note also a conical output section  185  of the reactor  120  provides improved pumping. In another embodiment shown in  FIG. 13   b  a plurality of flow-through substrates  145  can be used in the flow-through reactor  120 . 
         [0040]    Design considerations for one embodiment of the top showerhead  130  in  FIG. 14   a  are given below. The diameter of the holes  185  ( d ) is selected to be much larger than the mean free path of the carrier gas to ensure viscous flow through the pores (e.g. d=1 mm). The spacing between the holes (L) is selected to be several times smaller than the average lateral diffusion length for the ALD precursor to ensure that the precursor exiting the pore spreads out evenly before encountering the porous substrate  145 . For instance, L=9 mm. The gap between the top showerhead  130  and the porous substrate  145  (G) should be minimized to ensure that the carrier gas sweep is effective, and that the overall residence time of the precursors in the showerhead reactor is short. At the same time, G should be sufficiently large to permit lateral diffusion of the precursor given the choice of L. For instance, with L=9 mm, G=10 mm. The gap between the precursor inlet tube  155  and the top showerhead  130  (H) should be minimized to reduce the overall residence time of the precursors in the showerhead reactor. However, H must be sufficiently large that the gas from the precursor inlet tube  155  spreads out uniformly before entering the top showerhead  130 . That is to say that the conductance of the volume above the top showerhead must be much larger than the conductance of the top showerhead itself  130 . For instance, H=10 mm. The bottom showerhead  140  should have a similar design to the top showerhead  130  (d=1 mm, L=9 mm) to maintain a parallel flow direction before and after the porous substrate  145 . Similarly, the gap heights below the porous substrate  145  should be similar (G=10 mm, H=10 mm) for the same reasons given above. 
         [0041]    Further details of the reactor  120  are shown in  FIGS. 14   a  and  14   b . As noted in  FIG. 14   b , the top view of the top showerhead  130 , there is a matrix of through holes  185  which enable controlled flow-through of the precursor/carrier gas  155  to impact on the intervening MCP  145  for deposition of material. In  FIGS. 16   a  and  16   b  are shown example data with  FIG. 16   a  showing ALD deposition of Al 2 O 3 , with and without the MCP  145 . Likewise,  FIG. 16   b  shows MgO ALD deposition under various conditions with and without MCP  145 .  FIGS. 17   a - 17   c  further show ALD deposition of W:Al 2 O 3  (Chem-1) with various system features with and without MCP  145 .  FIG. 17   a  shows thickness and refractive index of W:Al 2 O 3 , on Si (100) using the system of  FIGS. 15   a ,  15   b  with deposition for an 8″×8″ MCP disposed on top at 150° C. using WF 6  and Si 2 H 6  for W and with TMA and H 2 O for Al 2 O 3 ;  FIG. 17   b  shows further results for deposition of W:Al 2 O 3 , (Chem — 1) on an 8″×8″ MCP (no Si) using the system of  FIGS. 15   a ,  15   b  and using an 8″×8″ MCP performed at 150° C.; and  FIG. 17   c  shows X-ray fluorescence analysis from the front and back sides of the MCP versus location on the MCP, deposited uniformly and done at 150° C. using WF 6  and Si 2 H 6  for W and TMA and H 2 O for Al 2 O 3 . Taken together, we see that cross-flow reaction chambers, whether commercial or scale-up, are suitable for depositing uniform films onto planar substrates of a variety of materials (MgO, Al 2 O 3 , Mo) in the absence of a high surface area porous substrate. However, in the presence of a high surface area porous substrate, the cross-flow reaction chambers consistently produce non-uniform coatings regardless of the process conditions. In contrast, the dual-showerhead flow-through reaction chamber gives uniform coatings of these materials even in the presence of the porous, high surface area substrates such as the MCP. 
         [0042]    In one embodiment, illustrated in  FIGS. 18   a - b , the porous substrate  145  is placed on the bottom shower head  140 . In the illustrated embodiment, the porous substrate  145  is elevated from the bottom shower head  140  by spacer elements  143 . The spacer elements may comprise, for example ceramic beads or cylinders. The spacer elements  143  physically space the porous substrate from the shower head  140 . In one embodiment, the spacer elements may be intergral with the bottom shower head  140 . In alternative embodiment, the spacer elements  143  may be separate components from the bottom shower head  140 . For applications where the porous substrate  145  has a different size and/or shape from one or both of the bottom shower head  140  and the upper shower head  130 . 
         [0043]    Any void or leak around the porous substrate  145  can result in non-uniformity in the film deposited on or near the porous substrate. In an alternative embodiment shown in  FIGS. 19A-B , the porous substrate  145  may be placed inside the reactor  120  with side pieces  147  of the same porous substrate material selected so as to essentially fill the cross-sectional area of the reactor with the porous substrate material. In other words, the substrate, combined with the side pieces  147 , should have the same size and shape as the showerheads  130  and  140 . The side pieces  147  can be comprised of the same material as the substrate, or of a different porous material having the same surface area and conductance. That is to say that the side pieces  147  should consume the precursors at the same rate as the substrate, but also maintain the same gas flow as the substrate. The silicon monitor is placed below the plane of the porous substrate  145 . 
         [0044]      FIG. 20A  illustrates the growth rate (in Angstrom per cycle) under a number of conditions (see legend).  FIG. 20A  illustrates GR data with and without side pieces  147 . For the embodiment using side pieces  147 , effectively a 300 mm round MCP is presented which comprises a  8 ″× 8 ″ square MCP surrounded with arc-shaped pieces of MCP which make in total an MCP size of 300 mm diameter. In the graph the this data point is marked in circle and 300 mm MCP. 
         [0045]    The ALD Al 2 O 3  growth rate is higher at the edge of the porous substrate  145 . In order to reduce the growth rate disparity, side pieces  147  of porous substrate were added. Preferably the gap between the side pieces  147  and the MCP is minimized, for example less than about 0.1 mm. In one embodiment, this gap is minimized by making exact size cut pieces with the edges polished so as to minimize the gap.  FIGS. 20   b - c  illustrate the growth rate with side pieces  147 . The measurements across the diagonal of  FIG. 19   b  is show in  FIG. 20   c . As can be seen in  FIG. 20   c , the extra growth rate is not observed at the 5 and 25 cm locations but rather is associated with the edge between the porous substrate plate and the side pieces  147 . 
         [0046]      FIG. 21  illustrates a further embodiment where the border between the porous substrate plate and the side pieces  147  is sealed. In the illustrated embodiment, one side is sealed using tape  149 . The seal indicates the advantage of minimizing the gap. 
         [0047]    For example, in one embodiment a polyimide film, such as Kapton™ (poly(4,4′-oxydiphenylene-pyromellitimide) can be utilized. Where a seal is provided between the porous substrate  145  and the side pieces  147 , appreciable additional growth rate was not observed.  FIG. 22  is a growth rate graph indicating the relative growth rates of baseline samples and a sample with poly(4,4′-oxydiphenylene-pyromellitimide tape applied at the 25 cm border of the porous substrate and adjacent side piece. When the MCP gap is sealed with Kapton™ tape at one edge of the MCP, there was no thickness bump. All three other sides shows thickness bump, i.e. increased growth rate. 
         [0048]    In one embodiment, there may be overlap between the MCP plate and the side pieces  147 . For example, the MCP plate may be placed on top of the side pieces  147  such that there is about 1-2 mm overlap at the edges of the MCP plate. 
         [0049]    The foregoing description of embodiments of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the present invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the present invention. The embodiments were chosen and described in order to explain the principles of the present invention and its practical application to enable one skilled in the art to utilize the present invention in various embodiments, and with various modifications, as are suited to the particular use contemplated.