Abstract:
An atmospheric pressure plasma etching reactor has a table holding a wafer to be processed and which moves the wafer to be processed under at least one electrode that is mounted in close proximity to the table and defines an entry of a gas mixture. With a radio-frequency voltage connected between the table and the at least one electrode, a plasma is created between the at least one electrode and the wafer to be processed processing the wafer to be processed as it is moved under the at least one electrode by the table.

Description:
[0001] This invention was made with Government support under Contract No. W-7405-ENG-36 awarded by the U.S. Department of Energy. The Government has certain rights in the invention. 
     
    
     
         [0002]    The present invention generally relates to plasma generation for use in material etching processes, and, more specifically to a reactor for generating a plasma at atmospheric pressure.  
         BACKGROUND OF THE INVENTION  
         [0003]    Integrated circuits have become pervasive components of myriad products the world uses everyday. They are found in household products, cell phones, computers, radios and virtually thousands of additional application. Because of the demand for these products, it is imperative that the manufacture of integrated circuits produces efficacious and reliable devices in the most efficient and cost effective manner possible.  
           [0004]    One of the critical steps in the manufacture of integrated circuits is the step of plasma ashing of photoresist. Photoresist is a thin film compound that is applied to a wafer in order to photographically transfer a circuit pattern to the surface of a wafer. The photoresist is “developed” with the circuit pattern and then the developed photoresist is used as a mask to selectively define regions of the wafer that will be etched using a chemically-reactive plasma. After the silicon etching process is complete, the residual photoresist mask must be removed, or “ashed” off the surface of the wafer, in preparation for the next process step. It is important that removal of all the photoresist material from the wafer be done in this ashing step, to avoid contamination in subsequent process steps.  
           [0005]    Present systems for providing the wafer ashing process include wet processes, done using solvents, and dry processes accomplished by oxidation of the photoresist layer using ozone or oxygen-containing plasmas. Wet photoresist removal steps generate chemical waste, which must be disposed of properly. And dry processes, such as plasma ashing, involve the use of a vacuum chamber in which the plasma is generated, which increases the cost of the equipment. A drawback in the use of ozone for photoresist removal is the danger and toxicity of this relatively unstable, noxious gas.  
           [0006]    Plasma ashing is the generally preferred means of photoresist removal. However, because the wafers are individually processed in vacuum, each step requires a separate vacuum chamber so that a single process chemistry can be effected in a single chamber, in order to avoid chemical contamination between the steps. This means that, should multiple process steps be necessary, multiple vacuum chambers are required. Naturally, with multiple vacuum chambers, a wafer must be moved from one chamber to the next. This increases the cost and complexity of the process. Multiple process steps are often desirable to use in photoresist ashing as described herein. While the use of multiple processing steps is possible using the prior art, the need for separate vacuum process chambers to accommodate the different chemistries adds to the cost and complexity of the present method.  
           [0007]    The present invention simplifies this process, and provides cleaning ability far superior to the present processes. The invention does this at less cost than the conventional technology because of the much higher efficiency attained. It accomplishes these improvements through an atmospheric pressure system that permits it to complete several process steps without the need for vacuum transfers and without the risk of cross contamination. It therefore is an object of the present invention to provide a substrate processing system capable of providing multiple processing steps to a given substrate within a single process enclosure. For purposes of discussion herein, a vacuum chamber is defined as a vacuum-tight, sealed unit capable of being pumped down to a low base pressure and refilled with the process gas for the purpose of generating a plasma. It also would be fitted with necessary vacuum pumps and vacuum gauges and would be constructed of material compatible with vacuum operation. An enclosure is defined as leak-tight box that can contain a mix of process gas without contamination from outside air. An enclosure does not need the structural stability required for vacuum operation and does not require vacuum pumps, vacuum gauges or load-locks capable of transferring substrates from room air to a vacuum chamber.  
           [0008]    The present invention is loosely related to a recently filed U.S. patent application Ser. No. 09/776,086, filed Feb. 2, 2001, for Processing Materials Inside an Atmospheric-Pressure Radio Frequency Nonthermal Plasma Discharge.  
           [0009]    It is an object of the present invention to provide substrate processing that is capable of processing multiple substrates in sequence.  
           [0010]    It is another object of the present invention to provide substrate processing that is capable of using different plasma chemistries within the same enclosure.  
           [0011]    Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.  
         SUMMARY OF THE INVENTION  
         [0012]    To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, an atmospheric pressure plasma etching reactor comprises a table for holding and moving a wafer to be processed, with at least one electrode being situated in close proximity to the table and defining an entry for introduction of a gas mixture. Wherein, with a radio-frequency voltage connected between the translatable table and the at least one electrode and the gas mixture introduced into the at least one electrode, a plasma is created between the wafer to be processed and the at least one electrode for processing the wafer to be processed as it is moved under the at least one electrode by the table. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    The accompanying drawings, which are incorporated in and form a part of the specification, illustrate the embodiments of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings:  
         [0014]    [0014]FIG. 1 is a schematical side view of the one embodiment of the present invention showing two processing stations.  
         [0015]    [0015]FIG. 2 is an end view of an embodiment of the present invention.  
         [0016]    [0016]FIG. 3 is a top view of an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0017]    The present invention provides plasma processing of substrates and allows each substrate to undergo sequential processing by multiple plasma processors using a single enclosure and a robotic stage. The invention can be understood most easily through reference to the drawings.  
         [0018]    In FIG. 1, a schematical plan view of one embodiment of the invention is shown where plasma-etching reactor  10  has wafer table  11  for transporting wafer  12  to be processed by an atmospheric pressure plasma jet. This atmospheric pressure plasma  13   a  is created in atmospheric pressure plasma jet processors  13 , in this figure showing two atmospheric plasma jet processors  13 . Atmospheric pressure plasma processors  13 , each contain an electrode  14 , shown in side-view in FIG. 1. Each electrode  14  has optional temperature control channels  16  and gas baffles  17 . An appropriate processing gas is introduced between the two electrodes  14  through gas inlets  18 . With the application of a voltage between either electrode  14  and wafer table  11 , and introduction of an appropriate gas through gas inlets  18 , a plasma  13   a  will be created for processing wafer  12  as it is carried through the plasma by wafer table  11 . Appropriate temperature control fluids such as air, water or oil, at some desired temperature, are circulated through temperature control channels  16  when necessary to regulate the temperature of electrode  14 . In some cases, it also might be desirable to heat the electrodes  14 , by passing a heated fluid through the fluid channels  16 . In either case, fluid channels  16  are used together with a circulating fluid to control the temperature of gas striking the wafer  12 .  
         [0019]    Wafer table  11  incorporates electric heating rods  19 . Heating rods  19  serve to heat wafer  12  to an appropriate temperature for processing when such action is required. Wafer table  11  is supported by ceramic thermal insulators  20 , which, in turn, are attached to slide carriage  21 . Slide carriage  21  slides along translating slide rails  22  when slide carriage  21  is moved as described below.  
         [0020]    Referring now to FIG. 2, there can be seen an illustration of an end view of this embodiment of the present invention, where many elements are shown that were hidden in FIG. 1. Here, it can be seen that wafer table  11  with wafer is moved under electrode  14  by conventional slide drive screw  23 . Slide drive screw  23  can be turned in any convenient manner such as by hand or by a variable-speed motor. Also shown, here in cross section, are electric heating rods  19 , which can be controlled by a thermostat (not shown) to regulate the temperature of wafer  12  for a particular processing regimen.  
         [0021]    Turning now to FIG. 3, there can be seen a top view of this embodiment of the present invention in which two atmospheric pressure plasma processors are shown. This FIG. 3 shows clearly how wafer table  11  transports wafer  12  under electrodes  14 . This transport of wafer table  11  is provided by slide drive screw  23 , while sliding along slide rails  22 . Also shown are the protective electrically conductive shields  15  inside which the processing of wafer  12  is accomplished.  
         [0022]    Although the FIGS.  1 - 3  illustrate an embodiment of the present invention utilizing two electrodes  14 , the invention is not limited to two electrodes  14 . Any appropriate number could be utilized, from one to many, depending on the processes to be employed for a particular wafer  12 . These electrodes  14  could be employed along with subsequent process steps, including wet rinses, all within the traverse of slide carriage  21 .  
         [0023]    In the present invention, electrode  14  is one electrode and wafer table  11  is the other electrode for connection of the RF energy for creation of a plasma. Either one may be rf-powered, and typically, one is grounded. In most cases, it is convenient to have electrode  14  be rf-powered and wafer table  11  be grounded for safety reasons. The specific frequency of the RF energy and its voltage level are to be determined for the particular process step to be employed for a particular wafer  12 .  
         [0024]    It is to be understood that in utilizing individual electrodes  14 , each electrode  14  can be controlled independently, both with respect to RF energy and process chemistry, while wafer  12  is moved below each electrode  14 . A true plasma, including ions and electrons, as well as reactive chemical neutral species, exists in the space between electrodes  14  and wafer  12  (FIGS. 1 and 2).  
         [0025]    It is a clear advantage of the present invention that individual electrodes  14  can be powered differently than others, and can employ different process gas mixtures for particular etching situations. For example, one electrode  14  could have a He/CF 4  gas mixture introduced through its gas inlet  18  (FIGS. 1 and 2), while a second electrode  14  could have a He/O 2  gas mixture introduced through its gas inlet  18 . As wafer  12  is moved under each electrode  14  it is processed for two process steps instead of the one step in the conventional reactor. In this embodiment, a third electrode  14  could be used for passivation of wafer  12 , with use of a gas mixture of He/H 2  for the plasma. Such an arrangement may be useful for removal of photoresist that has been “hardened” or carbonized, by exposure to an ion implantation step. The intense energy of the ion beam causes a hard “skin” to form on the surface of the photoresist. This surface film must be removed using a chemically-aggressive plasma, such as fluorine-containing feedgas (i.e., CF 4 ) and He feedgas plasma, but it is desirable to avoid the use of such plasmas after the surface skin has been removed, and instead use an O 2  and He-based plasma to finish ashing the photoresist. The oxygen plasma has better selectivity to silicon (i.e., it will preferentially etch the photoresist without etching the silicon under the photoresist, whereas the fluorine-based plasma will etch both). In conventional plasma systems operating in vacuum, this requires two processes chambers (one for the fluorine plasma and one for the oxygen plasma to avoid cross contamination. This invention improves operation of the ashing process by eliminating the need for separate process chambers.  
         [0026]    Also, because the present invention processes a single wafer  12 , it is not subject to the accumulation of particles and etch products, as might occur in a solvent cleaning process, such as wet chemical etching systems. Thus the present invention is inherently both dry and clean. Operational savings result because there is no need to dry wafer  12  or to dispose of solvents. In addition, the present invention can perform multiple process steps nearly simultaneously, a feat that is not possible with wet processes, and can do so with lower capital equipment cost and with a smaller footprint, or equipment size.  
         [0027]    The present invention offers other advantages over the prior art. First, it eliminates the need for any vacuum equipment, simplifying maintenance of the equipment. Second, it etches or cleans wafers or substrates faster because of high reactive species gas density and in-situ exposure to the plasma, so its throughput is greater. Third, it has the ability to run multiple process steps almost simultaneously, even those requiring different process chemistries, so it results in reduced equipment and process complexity.  
         [0028]    As previously mentioned, it was desirable in the use of prior art vacuum-based plasmas, to operate a single process in a single vacuum chamber for each wafer or substrate. This was done because the use of different process chemistries in the same vacuum chamber causes particle contamination to occur, which is a leading cause of defects during wafer processing. As previously mentioned, the use of different process chemistries was helpful in removing hardened, or carbonized, photoresist. Thus, to use different process chemistries and to avoid contamination problems requires that multiple vacuum chambers be used. When multiple vacuum chambers are used, it means that the wafer must be moved from one chamber to the next, requiring extra handling in addition to the extra process steps and the associated time and expense.  
         [0029]    The present invention does not require different vacuum chambers or any vacuum chamber at all. It utilizes a single manipulator to move the wafer through multiple process units, each having the same or different plasma chemistry, and without the associated need for vacuum loadlocks in between. A single process enclosure is used. However, the effect of multiple vacuum chambers is achieved through the use of multiple independently controlled electrodes  14 . The close proximity of electrodes  14  to wafer  12  allows wafer  12  to receive multiple process steps as it progresses under each electrode  14 . Because the gas pressure in the plasma region of each process unit is slightly in excess of atmospheric pressure (to achieve gas flow) the likelihood of cross contamination resulting from gas flow in one process unit entering the adjacent process unit is minimal. Diffusion is slow in this situation, owing to the high pressure operation of each process unit, so cross contamination problems are avoided.  
         [0030]    Applications of the present invention are many and varied. For example, it can be used to etch photoresists, silicon and metal from semiconductor wafers. It can also be used to deposit thin films, including especially large area deposition for thin-film transistor passivation, coatings used for architectural window glass, and deposition of hermetic coatings on magnetic media. Additional applications exist and still others are likely to be discovered through use of the present invention.  
         [0031]    The foregoing description of the embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.