Abstract:
A plasma reactor for PECVD treatment of large-size substrates according to the invention comprises a vacuum process chamber as an outer chamber and at least one inner reactor with an electrode showerhead acting as RF antenna, said inner reactor again comprising a reactor bottom and a reactor top, being sealingly connected at least during treatment of substrates in the plasma reactor and separated at least during loading/unloading of the substrates. Further embodiments comprise a sealing for said reactor to/bottom and a suspender for the RF antenna/electrode showerhead.

Description:
[0001]     This invention relates to vacuum processing equipment for very large area substrates, especially a PECVD process chamber (respectively an inner reactor) with compensation means for the deviation from flatness.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to large area PECVD process chambers in general and to such chambers which themselves are enclosed again in a second surrounding vacuum chamber in particular. Such “boxes within a box”, (Plasma Box™) are known in the art and described in U.S. Pat. No. 4,798,739. The major advantage of such “boxes within a box” is that a lower pressure may be maintained in the outer airtight chamber than within the inner reactor chamber such that a controlled gas flow may be maintained from the inner- to the outer chamber (“differential pumping”). A further advantage of such a “boxes within a box” system is that the inner chamber may be maintained at a constant process temperature of typically around 250-350° C. (isothermal reactor). By thus being constantly held at process temperature, such an inner reactor allows for uniform temperature distribution and thus for uniform overall deposition rates. With the appearance of larger and larger substrates (over 2 m×2 m) however, it becomes more and more difficult to keep the inner reactor substantially flat and consequently to be able to comply with the required production specifications and to load and unload the substrates.  
         [0003]     Due to the aggressive nature of the chemical agents involved in PECVD, aluminum alloys are the economic material of choice: Aluminum is one of the few materials known to be able to resist the attack of the chemical agents used in PECVD processes, such as fluorine containing gasses and species. Unfortunately however, aluminum alloys tend to exhibit creep deformation at elevated temperatures and even creep resistant alloys cannot fully eliminate such deformation over time.  
         [0004]     Any deformation and deviation from flatness of the reactor also causes non uniform deposition on the substrate, since the deposition rate is (among other factors) a function of the plasma gap—i.e. the distance between the top and bottom electrodes of the reactor. Furthermore, in order to load and unload substrates, it is necessary to be able to open both the outside chamber and the inner reactor and access them through a load lock. Any such opening must again be quickly and reliably sealable in a gas tight manner for the actual deposition process in order to avoid leakage.  
         [0005]     1. Prior Art  
         [0006]     In the PECVD reactors of the “boxes within a box” type known in the art (U.S. Pat. No. 4,798,739), stainless steel bars known as “stiffeners” are used to suspend the inner reactors from the outer chamber. The inner reactors themselves (for example the reactors of the Unaxis KAI 1200 system) are machined from two gas tight near symmetrical halves which are opened merely for maintenance and not for loading/unloading purposes. For loading/unloading purposes, a slit is machined in a side wall of the inner reactor which may be opened and closed by a slit valve in a gas tight manner. A fork holding a substrate is introduced into the inner chamber through such a slit. Then the substrate is accommodated by a set of vertical pins. Upon retraction of the fork, these (lifting-) pins may be retracted vertically until the substrate rests in its designated position. The slit is then sealed by a slit valve known in the art.  
         [0007]     2. Disadvantages in Prior Art  
         [0008]     The biggest disadvantage of the current reactor design is the side-slit/fork/pin-type of loading and unloading the substrates as described above. This requires a uniform inner height of the reactor to accommodate the fork and the pins. With very large substrate sizes however, the fork tends to bend under the combination of its own weight and the substrate weight. The used loading/unloading mechanism dictates an increasingly large inner height of the reactor and dictates a large slit height.  
         [0009]     Simple stainless steel stiffeners, such as T- or H shaped bars as known in the art, cannot fully compensate the deformation and distortion of very large reactors, especially when these reactors reach side lengths of over two meters. Simple stiffeners would not only fail to provide a flat reactor at room temperature, but especially so at operating temperature, since even stainless steel tends to loose strength at elevated temperature. Simple stiffener solutions tend to sag under the weight of the reactor as well as under their own weight, at room temperature as well as at operating temperatures of about 300° C.  
         [0010]     The aforementioned issues, resulting mainly from different form accuracy issues that must be faced when using large reactor sizes of more than 2 meter side length, ask for a new reactor design.  
         [0011]     So far the inner reactor is conceived as a one-piece vacuum chamber in the prior art. The loading and unloading of the reactor is done through a side slit machined in a side wall. The new reactor design has to meet requirements of optimal height while processing the substrate and the aforementioned loading issue resulting from the loading fork being bent. These requirements are no longer fulfilled by the traditional reactor design. Additionally, the reactors achieve larger and larger dimensions and have to comply with increasing deformation and expansion issues.  
       SUMMARY OF THE INVENTION  
       [0012]     A plasma reactor for PECVD treatment of large-size substrates according to the invention will comprise a vacuum process chamber  19  as an outer chamber and at least one inner reactor with process gas feed  22  and a RF feed  24  electrically connected to an electrode showerhead  25  acting as RF antenna, said inner reactor again comprising a reactor bottom  6  and a reactor top  2 , being sealingly connected at least during treatment of substrates in the plasma reactor and separated at least during loading/unloading of the substrates. Further useful embodiments and features are described below and in respective dependant claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  shows a reactor according to one embodiment of the invention in opened ( 1   a ) and closed condition ( 1   b ).  
         [0014]      FIG. 2  shows stiffeners in sideview ( 2   a ) and lengthwise ( 2   b  and  2   c ) at two different sections.  
         [0015]      FIG. 3  shows a sealing plate used for sealingly closing a reactor according to the invention.  
         [0016]      FIG. 4  shows an implementaion of an inventive sealing plate/sealing spacer combination.  
         [0017]      FIG. 5  is a detail of an end of a sealing plate  
         [0018]      FIG. 6  shows a suspender for an RF antenna according to a further embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]     Therefore, the present invention is based on a new reactor concept. The reactor is divided in two parts; a reactor bottom  6  and a reactor top  2  (see  FIG. 1 ). The reactor top  2  is attached to the outer vacuum process chamber  19  preferably by stiffeners  1  (connection not shown in  FIG. 1 ). The reactor bottom  6  (or bottoms in the case of multiple reactor systems in a single outer chamber) is movable vertically such that a slit opens between the reactors side wall  11  and the sealing plate  9 . When the reactor is fully opened the slit broadens and the lifting pins  8  start sticking out. The loading fork (not shown in  FIG. 1 ) is then able to deposit the substrate on the lifting pins for loading, or to retract the substrate from the lifting pins  8  by lifting the substrate from underneath through the chamber gate valve  20 . This “inverted shoe box” type of opening has the major advantage that the height of the reactor walls  11  and thus accordingly the plasma gap can be relatively small. If a loading/unloading solution would be chosen by accommodating a slit valve (as opposed to the present invention and as is known in the art) in the reactor wall, the height of the wall  11  would have had to be massively increased to accommodate the entrance of a loading/unloading fork which may bend and vibrate with large substrates. Hence, an economic deposition process would be highly limited.  
         [0020]     Beside the new reactor concept additional measures may be taken to assure the proper working of the plasma device. A further embodiment of the invention includes measures to compensate the deformation and expansion of the reactor that also result in sealing issues of the two-piece reactor. A first step to compensate for the deviation from flatness according to the present invention is the use of compensation spacers ( FIG. 2 , reference  4 ).  
         [0021]      FIG. 2   a - c  illustrate how the sagging of the stiffeners  1  and thus of the reactor top  4  by gravity can be compensated for by compensation spacers  4 . Screws  5  join reactor top  2  (e.g. made from an aluminum alloy) with stiffeners  1 . Stiffener clips  3  engage e.g. with a groove machined into stiffener  1 . Compensation spacers  4  of different thickness are arranged between the stiffener ( 1 ) and the reactor top ( 2 ) and allow compensating the sagging of the stiffeners  1  during operation of the plasma reactor at elevated temperatures. The stiffeners  1  again are attached to cross plates at their ends (not shown). By using a reactor top stiffener  1  which is supported and attached to the outer chamber at the ends, and by carefully choosing the thickness of the compensation spacers  4 , which are thicker at the ends of the stiffeners ( FIG. 2   b ), thinner towards the middle ( FIG. 2   c ) and absent in the center, sagging at operation temperature can be compensated for. The sagging of the reactor top needs to be more compensated in the middle than on the ends as illustrated by the curvature of the stiffener  1  in  FIG. 2   a ). Accordingly the stiffeners at the bottom (bottom side) of the reactor also show a slightly downwards curvature towards the center, but have the thickest compensation spacers, arranged between the stiffener ( 1 ) and the reactor bottom ( 6 ) in the center of the stiffener. The machined grooves in the stiffener  1  and in the stiffener clip  3  can further accommodate the thermal expansion between the reactor top and the stiffener.  
         [0022]      FIG. 3  illustrates a further means of compensation from deviation of flatness: by employing a sealing plate  9   a  with plate springs  10 . Further deviation from flatness of the reactor side wall  11  against the reactor bottom  6 , which could not be compensated for by the stiffener compensation spacers, will negatively influence the gas tightness of the reactor. The sealing plate  9   a  is conceived to compensate for this deviation, since the plate  9   a  is elastic to some extent and is pressed to the reactor bottom  6  on the inner side of the reactor. Furthermore, a sealing spacer  9   b  under the center of the sealing plate ( FIG. 4 ) serves to avoid that the sealing plate would be pinched over all the length between the reactor wall  11  and the reactor bottom  6 . Thus the actual sealing is achieved in two places: between the sealing plate  9   a  and the reactor bottom on the inner side, and between the sealing plate  9   a  and the reactor wall  11  on the outer (top) side of the reactor.  
         [0023]     The sealing spacers  9   b  offer a well defined close position, they enable the sealing plates  9   a —which are fully pressed to the wall  11  by the plate spring  10 —to freely contract or expand away form the center. With a reactor side length of about 2.5 meters, a maximum distortion of about 2 mm can thus be compensated.  
         [0024]     Since both the inner reactor and the outer vessel are under vacuum during operation, the sealing only needs to be gas tight to the pressure difference between both, which is typically in the range of 10 −2  to 10 −3  mbar.  
         [0025]     In  FIG. 4 , the thin arrows illustrate how the thermal expansion of the sealing is accounted for. Generally, the sealing plate  9   a  is fixedly attached in the center and can contract and expand towards the corners.  
         [0026]      FIG. 5  illustrates a detail of the end of a seal plate  9   a  where it joins another seal plate  9   a  at a corner: a lip is provided to compensate thermal expansion.  
         [0027]     In another, however less preferred embodiment, the sealing may alternatively be achieved by an elastic O-ring accommodated in a trapezoidal groove on the lower side (bottom) of the reactor wall  11 . Since the reactor is intended to be opened and closed many thousand times, since the temperature in the reactor is high, and since the chemical species in the plasma are very aggressive, the material of such an O-ring is highly stressed. Today&#39;s materials for such an O-ring barely fulfill such requirements.  
         [0028]      FIG. 6  shows another part of the reactor where thermal expansion needs to be compensated for: the suspension of the radio—frequency (RF) antenna  12 . Arrows in  FIG. 6  indicate freedom to contract/expand. The suspenders hold the antenna in place; they do not feed the actual RF power. The RF power is fed through the antenna into the plasma which thereby considerably heats up and thermally expands accordingly. If the suspender were not used in a plasma reactor, the expansion/contraction problem could be readily solved by adding a dilatation groove as shown with the arrows between the reactor top  2  (grounded) and the suspender  14 , and then electrically isolating the reactor top from the antenna by employing isolating ceramics on an appropriate part of the suspender. Since the reactor is operated under vacuum however, gaps and large potential drops must be avoided in order to avoid the ignition of parasitic plasma. Since in this case a gap between the reactor top  2  and the suspender  14  cannot be avoided because of the thermal expansion/contraction, a potential drop is avoided by bringing the top part  14  of the suspender to the same potential as the reactor top  2 , by isolating the lower part  17  of the suspender (which has the same potential as the RF-antenna  12 ) by means of a ceramics cylinder (middle part,  16 ), which has a thread on the inside to attach the top  14  and the bottom  17  part of the suspender to each other. The top  14  and the bottom  17  part of the suspender are additionally separated by a small gap, which is too small to be susceptible to parasitic plasma. Additionally, RF spacers  18 , with a floating potential are employed over the antenna to avoid parasitic plasma in the space between the reactor top  2  and the antenna  12 .  
         [0029]     In another, less preferred embodiment, the equivalent of the ceramic part in the middle of the suspender is a ceramic cylinder with two screw threads protruding at its ends. Screws in ceramic however are prone to break easily.  
         [0000]     Advantages of the Invention  
         [0030]     The reactor according to the present invention is intended for very large substrate sizes (such as substrates for liquid crystal displays) and for use in a outer vacuum chamber (like a Plasma Box™). Due to its large size—thermal expansion (which can be in the range of centimeters with reactor lengths in the range of meters) and general deformation (such as creep deformation)—pose severe problems to gas tightness and to suspensions of the elements which have to be attached to the outer chamber. The major advantage of the present invention is that the reactor is gas tight from ambient temperature up to operating temperature (about 300° C.). Another major advantage is that by using the “inverted shoebox” opening principle of the reactor, large slits in the reactor wall (as known in the art) can be avoided and thus the plasma gap can be kept small, which is essential to the productivity of the reactor.  
         [0031]     The reactor according to the present invention is thus efficient, cheap, easy to manufacture and to maintain.  
         [0032]     References used in the figures:  
         [0033]      1  Stiffener (e.g. from stainless steel)  
         [0034]      2  reactor top (e.g. from aluminum alloy)  
         [0035]      3  stiffener clip  
         [0036]      4  compensation spacer  
         [0037]      5  screw  
         [0038]      6  reactor bottom  
         [0039]      7  substrate  
         [0040]      8  (lifting) pin for substrate support  
         [0041]      9   9   a  sealing plate 
         9   b  sealing spacer          
         [0043]      10  Plate spring  
         [0044]      11  (reactor) side wall  
         [0045]      12  RF antenna (e.g. from aluminum)  
         [0046]      13  Suspension lid  
         [0047]      14  Suspender top (e.g. from aluminum)  
         [0048]      15  Friction and particle reducing rings (e.g. from ceramics)  
         [0049]      16  Suspender middle (e.g. from ceramics)  
         [0050]      17  Suspender bottom (e.g. from aluminum)  
         [0051]      18  RF spacers  
         [0052]      19  Vacuum process chamber  
         [0053]      20  Chamber valve gate  
         [0054]      21  Suspender  
         [0055]      22  Process gas feed  
         [0056]      23  Pumping grid  
         [0057]      24  RF feed  
         [0058]      25  Electrode showerhead  
         [0059]      26  Exhaust