Abstract:
An atomic layer deposition (ALD) apparatus is, suitable for thermal ALD and plasma-enhanced ALD of conductive and non-conductive films. The ALD apparatus can maintain electrical insulation of a gas dispersion structure, such as a showerhead assembly, which acts as an RF electrode to generate plasma inside a reaction chamber while depositing electrically conductive films in the reaction chamber. Fine tubules of micro-feeding tube assembly prevents plasma generation in them and reactive gases each have separate flow paths through the micro-feeding tube assembly. Process gases out of the micro-feeding tube assembly enter narrow grooves of a helical flow inducing plate and form helical flows which mix well each other. Symmetrically mounted pads on showerhead assembly and flow guiding plate maintain a symmetrical gap through which an inert gas flows continuously to keep reactive gases outside the gap and unwanted film deposition in the gap. Longer operating time before maintenance (cleaning) and thus higher productivity can be achieved.

Description:
REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the priority benefit under 35 U.S.C. §119(a) of Korean Application No. 10-2004-0113898, filed Dec. 28, 2004. This application is also related to U.S. utility application Ser. No. 10/486,311, filed Feb. 6, 2004, attorney docket no. ASMGEN.001APC. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to an atomic layer deposition apparatus capable of depositing a uniform thin film. In particular, the present invention relates to the reaction chamber structure of a plasma enhanced atomic layer deposition apparatus which is designed to prevent electrical short between plasma generating electrode and electrically grounded other parts, despite use of conductive elements during deposition.  
       BACKGROUND AND SUMMARY OF THE INVENTION  
       [0003]     As semiconductor integration technologies advance, methods for depositing ultra thin films in a uniform and conformal manner, such as in a via or trench pattern, become increasingly important. Currently, the most advanced process for ensuring such a nano-scale thickness ultra thin film in semiconductor device fabrication is known as atomic layer deposition (ALD), a variant of conventional CVD processes.  
         [0004]     Unlike a conventional CVD method, where all process gases are simultaneously supplied (in flow) or removed (outflow), in an ALD method each atomic layer of thin film is formed by alternate and sequential supply of process gases, which are separated in space and in time. Thus mutually reactive reactant gases do not meet each other in the gas phase. Rather, typically one reactant pulse adsorbs in a self-limiting manner, excess reactant is removed from the reaction space, and a subsequent reactant pulse reacts with the adsorbed reactant. Frequently inert gases are supplied to purge the inside of the ALD apparatus between reactant pulses in order to prevent their mixing in the gas phase. These inert gases are called purge gases. Some ALD recipes involve three or more different reactant pulses in one cycle, with purge or other removal steps between pulses.  
         [0005]     When the aforementioned ALD method is used, material is adsorbed on the surface of the substrate. A thin film is formed uniformly over the entire surface of a substrate regardless of the quantity of the process gas in each cycle because the amount of adsorbed molecules on the surface of a substrate is limited up to a maximum of a monolayer. Therefore, a uniform thickness of thin film can be formed even in the areas of high aspect ratio or large step difference, even when the thin film is formed with a thickness of several nanometers. Furthermore, the thickness of the thin film can be easily controlled by the number of process gas supply cycles, or ALD cycles, due to the self-saturating nature of the adsorption and the reactions.  
         [0006]     Adding to the aforementioned advantages of ALD, we can get more useful benefits when a plasma is generated during the ALD cycles. For example, the use of plasma can help to broaden the choice of source chemicals. Plasma as an additional energy source to thermal energy can activate the reaction between the chemicals which are not otherwise reactive. For example, tantalum halides (e.g. TaCl 5 , TaF 5 ) do not react readily with H 2  at low temperatures (400° C. or lower). This implies that one cannot use tantalum halides and H 2  for ALD of Ta by the conventional thermal ALD at a temperature less than 400° C., making the process unsuitable for many back end of line (BEOL) metallization processes. However, atomic hydrogen or hydrogen ions from hydrogen plasma can react very effectively with tantalum halides to form Ta metal film at low temperatures. H 2  gas plasma contains the neutral hydrogen radicals and/or the hydrogen ions and can be generated, e.g., by applying a radio frequency (RF) power to H 2  gas or a mixture of H 2  gas and an inert gas. Pure Ta metal film can thus be deposited by using plasma enhanced atomic layer deposition (PEALD).  
         [0007]     Therefore, in the PEALD technique, a thin film can be deposited by using chemical species which do not react readily with each other using only thermal energy. PEALD of TaN is described in detail as an example. Ta source TaF 5  is vaporized and supplied to the reaction chamber to be adsorbed on a substrate. After the adsorption is completed, an inert gas flows into the reaction chamber and on the substrate to purge excess gaseous or weakly adsorbed TaF 5  and vent them from the reaction chamber. Subsequently, H 2  gas is supplied to the substrate where TaF 5  is adsorbed, and H 2  gas plasma is generated. Hydrogen atoms or hydrogen ions generated in the plasma react with TaF 5  (or adsorbed fragments of TaF 5 ) on the substrate to form the tantalum metal and a reaction by-product HCl, which is volatile and removed from the substrate in a gaseous state. When the reaction of the substrate is completed, the plasma is turned off, and the remaining HCl is removed. At this time, an inert purge gas may be optionally supplied in order to facilitate the removal of the HCl. By repeating this ALD cycle, a Ta metal thin film can be deposited up to a desired thickness. On the other hand, in the above example, instead of using a separate inert gas, H 2  gas flow can continue after the plasma power is shut off and may be used as a purge gas. In this case, Ta source gas pulses and plasma power pulses alternate while H 2  gas flows continuously.  
         [0008]     In addition, the area of reaction between sequentially supplied chemical can be confined only to the area where plasma-activated species reach (usually on the substrate) so that extraneous film deposition at other parts of reaction chamber can be suppressed where plasma is not generated.  
         [0009]     In addition to the advantages aforementioned, PEALD usually can produce the film which has higher density and less impurity than conventional thermal ALD. Because of these reasons, PEALD has received attention from the semiconductor industry (see for example, Sherman, U.S. Pat. No. 6,342,277).  
         [0010]     Lee et al.&#39;s Korean Patent No. 273473 and U.S. Pat. No. 6,645,574, disclose an ALD method in which plasma is generated periodically during the ALD cycles to activate one of the reactants.  
         [0011]     Lee et al. further describes an example of a PEALD chamber in Korean Patent Application 2001-46802 filed on Aug. 2, 2001 and U.S. patent application Ser. No. 10/486,311 filed on Feb. 6, 2004, expressly incorporated herein by reference.  
         [0012]     This type of apparatus disclosed by Lee et al. and reproduced here as in  FIG. 1  is designed for and well suited for PEALD. However, there is a limitation that it cannot be used for multi-layer film deposition or graded film deposition, which includes both PEALD and thermal ALD of a metallic film. Once thermal ALD of metallic film is performed in the reaction chamber, PEALD is not possible in the reaction chamber before removing the metallic film due to shorting of the plasma-generating electrodes. Any metallic film deposition due to thermal activation without plasma, whether during thermal ALD or PEALD, can coat insulator parts and electrically short the electrodes. Thus apparatus service time between reaction chamber cleanings may be shortened.  
         [0013]      FIG. 2  illustrates the structure of the gas inlet parts of U.S. application Ser. No. 10/486,311. The disclosed micro-feeding tube assembly  14 , made of non-conducting material, is disposed on the insulation wall  24  in order to prevent the plasma generation above the showerhead. Without the micro-feeding tube construction, such plasma generation might occur due to electrical potential difference between the showerhead assembly ( 26 ,  28 ), which is connected to the RF terminal (not shown), and the gas inlet tube  10 , which is electrically grounded. Although such a structural design can suppress the plasma generation above the showerhead, some risk remains for film deposition  16  on the surface of micro-feeding tube assembly  14  due to the thermal reaction. It is because all the process gases share the same inlet tube and the parts are heated by conduction, which may allow thermal reaction on the micro-feeding tube assembly  14 . If the deposited films  16  are electrically conductive, the electrical insulation between the showerhead assembly ( 26 ,  28 ) and the gas inlet tube  10  will be no longer effective and thus the plasma density will be reduced or the plasma will not be generated at all where it is desired over the substrate, thus causing detrimental effects on film deposition such as lower deposition rate, poorer uniformity, or no deposition at all. So it is desirable to prevent film deposition on the micro-feeding tube assembly  14 .  
         [0014]     Another potential problem with the structure of U.S. application Ser. No. 10/486,311 is the build-up of film deposition at the insulation wall as shown in  FIG. 3 . Since the insulation wall  24  abuts the showerhead assembly ( 26 ,  28 ) tightly and the wall fringe  25  is very close to the reaction region  27 , a metal film  23  may be deposited after numerous process runs on the bottom of insulation wall  24  and also maybe on the bottom of the plasma generation barrier wall  22 , which is electrically grounded. If the metal film  23  continues to grow on the showerhead insulation wall  24  and/or the plasma generation barrier wall  22 , it may cause an electrical short between the showerhead assembly ( 26 ,  28 ) and the plasma generation barrier wall  22  to hinder the plasma generation in the reaction region  27 . Even a slight deposition of metallic film on the wall fringe  25  may disturb the local electrical field. This may cause non-uniform and/or asymmetric plasma, particularly at the substrate edge, and thus non-uniform deposition of the film on the substrates  32 .  
         [0015]     In addition, another problem with the structure of U.S. application Ser. No. 10/486,311 is that a circular gap  544  (see  FIG. 1 ) through which purge gas flows is difficult to control due to assembly variation. For example, where the reaction chamber is designed so as to maintain the circular gap  544  in a width of 2 mm, variation during assembly may be as large as 0.5 mm, and then the narrowest and the widest sides of the circular gap  544  become 1.5 mm and 2.5 mm, respectively. Such an asymmetric circular gap makes the purge gas flow through it asymmetrically. Accordingly, the flow of gas at the edge of the substrate becomes asymmetric, and causes non-uniformity of film deposition on the substrate, in particular at the substrate edge.  
         [0016]     In order to continue to deposit conductive film on the substrate using either PEALD or conventional thermal ALD, it is desirable to maintain electrical insulation despite deposition of metallic film due to thermal reaction between reactants.  
         [0017]     For an example of Ru ALD, an ALD film growth rate during a thermal ALD cycle is higher than of PEALD. However, the ALD process may have a long incubation time before film growth, whereas the PEALD process has a short incubation time. In this case, the ALD apparatus according to the present invention can be employed. Namely, the PEALD process is firstly performed to form a thin Ru film with a short incubation time, and then, the ALD process having a higher film growth rate is performed, so that it is possible to deposit maximum thickness of Ru film in a short time.  
         [0018]     The preferred embodiments of the present invention provide an apparatus capable of PEALD, thermal ALD, and a combined process of PEALD and thermal ALD for metallic or other conductive film deposition. The embodiments also provide multi-layer or graded film deposition by any combination of PEALD and/or thermal ALD of dielectric films and/or metallic films.  
         [0019]     The preferred embodiments also provide a deposition apparatus capable of maintaining an electrical insulation in a reaction chamber and continuously generating plasma by preventing unnecessary film deposition within the reaction chamber when a conductive thin film is deposited by using PEALD and/or ALD processes.  
         [0020]     The preferred embodiments of the present invention also provide a deposition apparatus capable of continuously forming a thin film by employing a combination of PEALD and ALD processes or a series thereof.  
         [0021]     The preferred embodiments of the present invention also provide a deposition apparatus capable of depositing a thin film by using a PEALD or ALD process by separately supplying a plurality of process gases to a reaction chamber and mixing the process gases in the reaction chamber.  
         [0022]     According to an aspect of the present invention, there is provided an ALD apparatus for depositing a thin film on a substrate, comprising a substrate support for supporting the substrate, a reaction chamber wall defining a reaction chamber, a gas inflow tube connected to a source of process gas and communicating with the reaction chamber, a showerhead assembly which defines a reaction space together with the substrate support and includes a plurality of holes connected to the gas inflow tube to supply gas to the reaction space, a showerhead insulating plate made of an insulating material and disposed on the showerhead assembly, a gas flow guiding plate disposed on the showerhead insulating plate, a gas outlet for venting gas from the reaction chamber, and a RF connection port connected to the showerhead assembly to supply RF power, wherein purge gas passages are defined between the showerhead assembly and the showerhead insulating plate, between the showerhead insulating plate and the gas flow guiding plate, and between the gas flow guiding plate and the reaction chamber wall.  
         [0023]     In the above aspect of the present invention, the ALD apparatus may further comprise a plurality of pads symmetrically mounted between the showerhead assembly and the showerhead insulating plate, the height of which determines a gap between the showerhead assembly and showerhead assembly insulating plate.  
         [0024]     In addition, the pads may be machined directly on the showerhead insulating plate or the showerhead assembly.  
         [0025]     In addition, the ALD apparatus may further comprise a plurality of pads symmetrically mounted between the gas flow guiding plate and the reaction chamber wall, the height of which determines a gap between the gas flow guiding plate and the reaction chamber wall.  
         [0026]     In addition, the pads may be machined directly on the gas flow guiding plate or the reaction chamber wall.  
         [0027]     In addition, the ALD apparatus may further comprise a flanged cylinder type gas manifold having gas inlets and outlets.  
         [0028]     In addition, the RF connection port may pass through the reaction chamber wall to be connected to the showerhead assembly and electrically insulated from the reaction chamber wall.  
         [0029]     In addition, the ALD apparatus may further comprise a heating plate disposed under the substrate support to heat the substrate.  
         [0030]     In addition, the ALD apparatus may further comprise a heater provided on the reaction chamber wall.  
         [0031]     In addition, the substrate support may be a pedestal that can be lifted up to contact the reaction chamber wall to define the reaction chamber, and the pedestal may be dropped to be separated from the reaction chamber wall, so that the substrate can be mounted or detached.  
         [0032]     According to another aspect of the present invention, there is provided an ALD apparatus for depositing a thin film on a substrate, comprising a substrate support for supporting the substrate, a reaction chamber wall mounted above the substrate support and defining a reaction chamber, a gas inflow tube having a plurality of gas inlets configured to allow a plurality of reactive gases to separately communicate with into the reaction chamber, a gas dispersion structure which defines a reaction space together with the substrate support and supplies the process gases to the reaction space, a micro-feeding tube assembly disposed between the gas inflow tube and the gas dispersion structure and having a plurality of fine tubules, and a helical flow inducing plate disposed between the micro-feeding tube assembly and the gas dispersion structure.  
         [0033]     In the above aspect of the present invention, the micro-feeding tube assembly may comprise an electrically conductive micro-feeding tube sub-assembly connected to the gas inflow tube, and an insulating micro-feeding tube sub-assembly connected to the helical flow inducing plate.  
         [0034]     In addition, inner diameters of the fine tubules of the electrically conductive micro-feeding tube sub-assembly and the insulating micro-feeding tube sub-assembly may be in a range from 0.1 mm to 1.2 mm.  
         [0035]     In addition, fine tubules of the electrically conductive micro-feeding tube sub-assembly and fine tubules of the insulating micro-feeding tube sub-assembly may be of the same size and at the same position and aligned with each other to form a plurality of single conduits.  
         [0036]     In addition, the ALD apparatus may further comprise an insulating plate made of an insulating material and disposed on the gas dispersion structure, a gas flow guiding plate disposed on the insulating plate, a gas outlet for venting the gas out of the reaction chamber, and a RF connection port connected to the gas dispersion structure to supply a RF power, wherein purge gas passages are defined between the gas dispersion structure and the insulating plate, between the insulating plate and the gas flow guiding plate, and between the gas flow guiding plate and the reaction chamber wall.  
         [0037]     In addition, the ALD apparatus may further comprise a plurality of pads symmetrically formed between the gas dispersion structure and the insulating plate, wherein a width of the gas passage between the gas dispersion structure and the insulating plate is determined by heights of the pads.  
         [0038]     In addition, the pads may be machined directly on the insulating plate or the gas dispersion structure.  
         [0039]     In addition, the ALD apparatus may further comprise a plurality of pads symmetrically formed between the gas flow guiding plate and the reaction chamber wall, wherein a width of the purge gas passage between the gas flow guiding plate and the reaction chamber wall is determined by height of the pads.  
         [0040]     In addition, the pads may be machined directly on the gas flow guiding plate or the reaction chamber wall.  
         [0041]     In addition, the helical flow inducing plate may be electrically and mechanically connected to the gas dispersion structure to have an electrical potential equal to that of the gas dispersion structure.  
         [0042]     In addition, a plurality of fine holes facing a plurality of the fine tubules of the insulating micro-feeding tube sub-assembly may be formed on the upper side of the helical flow inducing plate, and wherein a plurality of grooves are formed on the lower side of the helical flow inducing plate, which deflects the direction of gas flows out of the fine holes into a mixing region formed at the center of the helical flow inducing plate.  
         [0043]     In addition, the grooves may be skewed clockwisely or counter-clockwisely, wherein the mixing region has a shape of disc, and wherein the inducing grooves are connected to the mixing region so as to contact a circumference of the mixing region.  
         [0044]     In addition, the gas dispersion structure may comprise a volume adjusting horn. The volume adjusting horn has a shape of funnel, the diameter of which increases from an upper portion to a lower portion thereof. The shape of the volume adjusting horn allows the process gas to distribute uniformly, evenly and smoothly over the substrate and, at the same time, minimizes the volume of the inner part of the gas dispersion structure.  
         [0045]     In addition, the gas dispersion structure may be a showerhead assembly, further including a gas dispersion perforated grid downstream of the volume adjusting horn.  
         [0046]     In addition, the helical flow inducing plate may be fixed at an upper opening of the volume adjusting horn, wherein the helical flow inducing plate is electrically and mechanically connected to the showerhead assembly to have an electrical potential equal to that of the showerhead assembly.  
         [0047]     In addition, the ALD apparatus may further comprise a flanged cylinder type gas manifold having gas inlets and outlets.  
         [0048]     In addition, the RF connection port may pass through the reaction chamber wall to be connected to the showerhead assembly and electrically insulated from the reaction chamber wall.  
         [0049]     In addition, the gas inflow tube and the micro-feeding tube assembly may be configured to introduce gases substantially perpendicular to the helical flow inducing plate.  
         [0050]     In addition, the helical flow inducing plate may include a plurality of grooves extending in a plane substantially parallel to the substrate support, where the grooves are configured to direct gases in a spiral prior to entering the gas dispersion structure in a direction substantially perpendicular to the substrate support 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0051]     The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings, which are meant to illustrate and not to limit the invention, and in which:  
         [0052]      FIG. 1  is a cross sectional view showing a prior-art ALD apparatus;  
         [0053]      FIG. 2  is a cross sectional view showing a gas inflow portion of the prior-ALD apparatus of  FIG. 1 ;  
         [0054]      FIG. 3  is a detailed cross sectional view showing a portion of the prior-art ALD apparatus of  FIG. 1 ;  
         [0055]      FIG. 4  is a schematic cross sectional view showing an ALD apparatus according to an embodiment of the present invention;  
         [0056]      FIG. 5  is an enlarged cross sectional view showing a process gas inflow unit of the ALD apparatus of  FIG. 4 ;  
         [0057]      FIG. 6  is a schematic perspective view showing upper and lower portions of a helical flow inducing plate of the process gas inflow unit of  FIG. 5 ;  
         [0058]      FIG. 7  is a schematic isometric view showing a gas flow in the process gas inflow unit, the micro-feeding tube assembly and the helical flow inducing plate of the ALD apparatus according to an embodiment of the present invention;  
         [0059]      FIG. 8  is a schematic partially cut-away, isometric view showing an inert gas flow in the ALD apparatus according to the embodiment of  FIG. 4 ; and  
         [0060]      FIG. 9  is a schematic, enlarged, cut-away, perspective view showing an inert gas flow for preventing unwanted deposition and particle generation in the ALD apparatus according to the embodiment of  FIG. 4 .  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0061]     Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the attached drawings such that the present invention can be easily put into practice by those skilled in the art. The present invention can be embodied in various forms, but is not limited to the embodiments described herein.  
         [0062]     In the drawings, thicknesses are enlarged for the purpose of clearly illustrating layers and areas. In addition, like elements are denoted by like reference numerals in the whole specification.  
         [0063]     An ALD apparatus according to an embodiment of the present invention will be described in detail with reference to  FIG. 4 .  FIG. 4  is a schematic cross sectional view showing an ALD apparatus according to an embodiment of the present invention.  
         [0064]     Referring to  FIG. 4 , the ALD apparatus according to the embodiment of the present invention includes an outer apparatus wall  300 , a gas manifold  315 , a gas inflow tube  310 , an electrically conductive micro-feeding tube sub-assembly  321 , an insulating micro-feeding tube sub-assembly  320 , a helical flow inducing plate  332 , a reaction chamber wall  361 , heaters  366  and  367 , a powered gas dispersion structure in the form of a showerhead assembly  330 ,  335 , a substrate support in the form of a pedestal  360 , a pedestal driver  380 , a gas flow guiding plate  345 , a showerhead insulating plate  340 , a showerhead assembly insulating pipe  349 , pads  350  and  336 , and a RF connection port  325 .  
         [0065]     Now, these components will be described in detail.  
         [0066]     A substrate  370  which is subject to deposition is mounted on the pedestal  360 , and a heating plate  365  is disposed under the substrate  370  to increase a temperature of the substrate up to a desired process temperature.  
         [0067]     The pedestal driver  380  moves the pedestal  360  up and down. The pedestal driver  380  includes a central supporting pin  372  for supporting the pedestal  360 , and a moving plate  378  linked to pneumatic cylinders  384 , the other ends of which are fixed at a lower portion of the outer apparatus wall  300  of the ALD apparatus.  
         [0068]     Before or after the deposition process, the pedestal  360 , which is connected to the pneumatic cylinders  384 , is moved down and the reaction chamber wall  361  and the pedestal  360  are detached, so that the reaction chamber opens. While the reaction chamber opens, the central supporting pin  372  may be lifted up or moved down, so that the substrate  370  can be detached from the pedestal  360  or mounted on the pedestal  360 . The substrate  370  can be loaded or unloaded while the central supporting pin  372  is lifted up relative to the pedestal  370 .  
         [0069]     After placing a new substrate for deposition, the central supporting pin  372  is dropped down and the substrate  370  is mounted on the pedestal  360 . Then the pedestal  360  is lifted up by the pneumatic cylinders  384  close to the reaction chamber wall  361 , so that the reaction chamber is closed and reaction space is defined between the pedestal  360  and the showerhead assembly  330 ,  335 .  
         [0070]     In order to maintain a suitable inner temperature of the reaction chamber, the separate heaters  366  and  367  are provided to outer surfaces of the reaction chamber wall  361 . In order to prevent the loss of heat, generated by the heaters  366  and  367 , to the outer apparatus wall  300 , the reaction chamber wall  361  has a minimum head conduction path to the outer wall  300 , i.e., it is fixed to the outer apparatus wall  300  through the flanged cylinder-type gas manifold  315 . Due to such a structure, even though the inner temperature of the reaction chamber is, for example, about 300° C., the temperature of the outer apparatus wall  300  can be maintained at about 65° C. or below. Additional heaters (not shown) may be attached to the gas manifold  315  or inserted into the gas manifold  315 .  
         [0071]     The gas inflow tube  310  having a plurality of gas inlets  311 ,  312 , and  313  for supplying a plurality of process gases are positioned in the central portion of the gas manifold  315 . The electrically conductive micro-feeding tube sub-assembly  321  having a plurality of fine tubules is disposed under and downstream of the gas inflow tube  310 .  
         [0072]     The insulating micro-feeding tube sub-assembly  320  has a plurality of fine tubules which have the same geometries as those of the electrically conductive micro-feeding tube sub-assembly  321 . It is disposed under and downstream of the electrically conductive micro-feeding tube sub-assembly  321 . The fine tubules of the electrically conductive micro-feeding tube sub-assembly  321  and the insulating micro-feeding tube sub-assembly  320  are aligned and may be of a size in a range from 0.1 mm to 1.2 mm.  
         [0073]     The helical flow inducing plate  332  made of a conductive material is electrically and mechanically to a downstream gas dispersion structure for distributing process gas across the major face of the substrate  370 . In the illustrated embodiment, the plate  332  is connected to a volume adjusting horn  330  which constitutes an upper portion of the showerhead assembly  330 ,  335 . The showerhead assembly  330 ,  335  is constructed with the volume adjusting horn  330  and a gas dispersion perforated grid or faceplate  335 . The gas dispersion perforated grid or faceplate  335  is disposed above the substrate  370  in parallel thereto and has perforations  334 . The volume adjusting horn  330  has two internal funnels of which the upper end matches with the diameter of the helical flow inducing plate  332 , downstream of which the internal passage first narrows and then widens to the lower end, which matches with the diameter of the faceplate  335 . The shape of the volume adjusting horn  330  allows the process gas to distribute uniformly, evenly and smoothly over the wafer substrate  370  and, at the same time, minimizes the volume of the inner part of the showerhead assembly  330 ,  335 .  
         [0074]     The showerhead assembly  330 ,  335  is electrically connected to the RF connection port  325 , which is constructed with a bar-shaped metal. The RF connection port  325  functions to apply RF power generated by an external RF power generator (not shown) to the showerhead assembly  330 ,  335 . The RF connection port  325  is surrounded with a covering insulating member  326  to avoid short-circuiting with other connection portions.  
         [0075]     In order to maintain electrical insulation of the showerhead assembly  330 ,  335 , the showerhead insulating plate  340  is disposed on upper face of the volume adjusting horn  330 , and the showerhead assembly insulating pipe  349  is disposed at the center of showerhead insulating plate  340  (see also  FIG. 8 ).  
         [0076]     The gas flow guiding plate  345  is disposed on the showerhead insulating plate  340  to provide purge and process gas passages  347  and  341 . The pads  350  are disposed on the flow guiding plate  345  to define the guiding plate upper gap  347 . Similarly a plurality of the pads  336  are symmetrically disposed on the volume adjusting horn  330  to define the insulating plate lower gap  342  between the volume adjusting horn  330  and the showerhead insulating plate  340 .  
         [0077]     The gas flow guiding plate  345 , the showerhead insulating plate  340 , and the pads  336 ,  350  will be described later in detail with reference to  FIG. 8 .  
         [0078]     The reaction chamber wall  361  is constructed with double walls of inner and outer walls. The inner wall is separated from the outer wall to form an inner-chamber-wall gas passage  362  between the inner wall and the outer wall and between the inner wall and the pedestal  360 . In addition, a groove is formed along the lower edge of the inner wall to form a gas flow buffering channel  363 .  
         [0079]     Now, flows of process gases of the ALD apparatus according to the embodiment of the present invention will be described.  
         [0080]     In  FIG. 4 , the arrows denote the flows of the process gases. The process gases pass, in sequence, though the gas inflow tube  310 , the electrically conductive micro-feeding tube sub-assembly  321 , the insulating micro-feeding tube sub-assembly  320 , and the helical flow inducing plate  332 . Process gases are dispersed inside the volume adjusting horn  330 , and then pass through the spray holes or perforations  334  of the faceplate  335  to meet the surface of the substrate  370 . The process and any byproduct gases pass the edge of the substrate  370 . Next, the process gases pass through the gap  314  between the side edge of the volume adjusting horn  330  and the reaction chamber wall  361 . The process gases pass through the guiding plate upper gap  347  to the gas manifold  315 . Finally, the process gases are vented out through the gas outlet  316  to an external vacuum pump (not shown).  
         [0081]     Now, the flow of process gases passing through the gas inlets  311 ,  312 ,  313  to be supplied to the substrate  370  will be described in detail with reference to FIGS.  5  to  7 .  
         [0082]      FIG. 5  is an enlarged cross sectional view showing a process gas inflow unit of the ALD apparatus according to the embodiment of the present invention;  FIG. 6  is an schematic view showing upper and lower portions of helical flow inducing plate  332  of the process gas inflow unit of the ALD apparatus according to the embodiment of the present invention; and  FIG. 7  is a schematic view showing gas flow in the process gas inflow unit, including the inlets  311 ,  312 ,  313 , the micro-feeding tube assembly  321 ,  322  and the helical flow inducing plate  332  of the ALD apparatus according to the embodiment of the present invention.  
         [0083]     In  FIG. 5 , the arrows denote the flowing direction of the process gases. The process gases are supplied from process gas sources (not shown) through the gas inlets  311 ,  312 ,  313  separated from each other in the gas inflow tube  310  and passes through the electrically conductive micro-feeding tube sub-assembly  321  made of a conductive material and having a plurality of fine tubules. After that, the process gases pass through the insulating micro-feeding tube sub-assembly  320  made of a non-conductive material and having a plurality of fine tubules which have the same number, positions (alignment), and diameters as those of the fine tubules of the electrically conductive micro-feeding tube sub-assembly  321 . The process gases passing through the electrically conductive micro-feeding tube sub-assembly  321  and the insulating micro-feeding tube sub-assembly  320  pass through the helical flow inducing plate  332  which is preferably made of a conductive material and connected to the volume adjusting horn  330  electrically and mechanically.  
         [0084]     The gas inlets  311 ,  312 ,  313  are separated from each other so as to separately supply each of a plurality of process gases. The electrically conductive micro-feeding tube sub-assembly  321  and the insulating micro-feeding tube sub-assembly  320  have a plurality of the fine tubules which are disposed in parallel to each other. Each of the fine tubules of the electrically conductive micro-feeding tube sub-assembly  321  connect and align with one of fine tubules of the insulating micro-feeding tube sub-assembly  320  to form a plurality of single continuous, fine conduits. A plurality of fine holes which have the same number, positions, and diameters as the fine tubules of the electrically conductive micro-feeding tube sub-assembly  321  and insulating micro-feeding tube sub-assembly  320  are formed in an upper portion of the helical flow inducing plate  332  to be aligned to the fine tubules of the micro-feeding tube assembly  321  and  320 .  
         [0085]     The plurality of the fine tubules in the micro-feeding tube sub-assemblies  321 ,  320  suppresses generation of plasma within the fine conduits because electrons in such a narrow space cannot be accelerated enough to ionize other molecules or atoms, and thus do not generate plasma.  
         [0086]     The insulating micro-feeding tube sub-assembly  320  maintains electrical insulation between the electrically conductive micro-feeding tube sub-assembly  321  and the helical flow inducing plate  332  while allowing the process gases to pass through the fine tubules.  
         [0087]     The helical flow inducing plate  332  is electrically connected to the showerhead assembly  330 ,  335  so as to have an electrical potential equal to that of the volume adjusting horn  330 . Accordingly, when a RF power is supplied to the showerhead assembly  330 ,  335 , there is no potential difference between the volume adjusting horn  330  and the helical flow inducing plate  332 . Therefore, plasma is not generated in a space between the volume adjusting horn  330  and the helical flow inducing plate  332 . It is therefore possible to prevent unnecessary film deposition on inner surfaces of the showerhead assembly  330 ,  335  and the helical flow inducing plate  332  while performing PEALD. The gap between lower ends of the fine tubules of the insulating micro-feeding tube sub-assembly  320  and the helical flow inducing plate  332  is designed to be narrow (for example, 2 mm or less) enough to prevent or suppress plasma generation.  
         [0088]     On the other hand, if the process gases are mixed outside the volume adjusting horn  330  of the ALD apparatus, conductive materials or contaminants may be generated due to chemical reactions of the process gases.  
         [0089]     Therefore, it is important to keep the mixing of the process gases only inside the showerhead assembly  330 ,  335 .  
         [0090]     In the ALD apparatus according to the illustrated embodiment of the present invention, a plurality of the fine tubules are provided to the electrically conductive micro-feeding tube sub-assembly  321  and the insulating micro-feeding tube sub-assembly  320 , and a plurality of the fine holes are provided in the upper portion of the helical flow inducing plate  332 . Therefore, the flow rate of the process gases in the fine tubules  321 ,  320 , and the plate  332  having a smaller diameter is higher than the flow rate of the process gases in the gas inlets  311 ,  312 ,  313  having a larger diameter. This higher flow rate prevents back-diffusion of the process gases into the gas inlets  311 ,  312 ,  313 , and thus prevents mixing of those gases outside the showerhead assembly  330 ,  335 . Also there is no mixing of reactive gases inside the fine conduits because the fine tubules are separated for each process gas flow.  
         [0091]     In the ALD apparatus according to the embodiment of the present invention, the helical flow inducing plate  332  has a function of effectively mixing the process gases passing through the separate fine conduits by inducing helical flows. Note that, in operation, only one reactant is typically flowed at a time, but the others of the inlets  311 ,  312 ,  313  typically include a flowing inert gas while a reactant flows through one of the inlets  311 ,  312 ,  313 . Thus, the inert and reactant flows mix, rather than mutually reactive reactants. The inert gas may also serve as a reactant, but only upon activation by plasma below the gas inflow unit. Now, the helical flow inducing plate will be described in detail with reference to  FIG. 6 .  
         [0092]     In  FIG. 6 , (a) is a schematic view of the top view of the helical flow inducing plate  332 , and (b) is the bottom view of the helical flow inducing plate  332 . Grooves are formed in the lower face of the helical flow inducing plate  332 , which grooves are skewed clockwisely or counter-clockwisely. The grooves direct gas flows to a central disc-shaped mixing region. Process gases passing through the grooves form helical flow and mix each other well at the mixing region. The grooves shown in (b) of  FIG. 6  are turned about 90° within a horizontal plane; however, they may have a shape of a straight line, an arc, or other shapes.  
         [0093]     The process gases passing through the electrically conductive micro-feeding tube sub-assembly  321 , the insulating micro-feeding tube sub-assembly  320 , and the fine holes in the upper portion of the helical flow inducing plate  332  are accelerated at a high flow rate when passing through the narrow helical flow inducing grooves.  
         [0094]     In  FIG. 7 , the arrows denote the flow direction of the process gases. As shown in  FIG. 7 , the process gases flowing into the gas inlets  311 ,  312 ,  313 , substantially perpendicular to the substrate surface, pass through the electrically conductive micro-feeding tube sub-assembly  321 , the insulating micro-feeding tube sub-assembly  320 , and the fine holes in the upper portion of the helical flow inducing plate  332 . The flows of process gases are turned roughly parallel to the substrate, rotate clockwise or counterclockwise when passing through the narrow inducing grooves in the lower portion of the helical flow inducing plate  332 , and are again provided with a flow component vector substantially perpendicular to the substrate when passing from the central disc-shaped mixing region into the volume adjusting horn. These helical flows mix the gases flowing from the various inlets  311 ,  312 ,  313  well inside the volume adjusting horn  330 .  
         [0095]     The inner portion of the volume adjusting horn  330  has a shape of funnel so as to induce a laminar flow and smooth dispersion of the mixed process gases. The horn shape also minimizes the inner surface area of the volume adjusting horn  330 . Laminar flow and minimum surface area facilitate rapid switching of process gases inside the showerhead assembly  330 ,  335 . Rapid gas switching allows more ALD cycles per unit time, and thus higher film growth rate. Together with the helical flow inducing plate  332 , the volume adjusting horn  330  produces a more uniformly distributed (across the substrate surface) and well mixed process gas during each of the relatively short ALD pulses.  
         [0096]     In addition, according to the illustrated embodiment, the gas dispersion perforated grid or faceplate  335  allows more uniform process gas supply onto the substrate  370  by passing the gases through the spray holes or perforations  334 .  
         [0097]     Advantageously, the helical flow inducing plate  332  provides swirling action that distributes the process gas or gas mixture symmetrically with respect to the downstream gas dispersion structure and facing substrate, even though the reactive gas may be asymmetrically introduced through one of the gas inlets  311 ,  312 ,  313 . Additionally, if during one pulse a reactant is introduced through one of the gas inlets  311 ,  312 ,  313  and inert gas is introduced through another of the gas inlets  311 ,  312 ,  313 , the swirling action mixes these process gases to improve uniformity of the exposure of the substrate to the reactant within the mixture. Accordingly, the skilled artisan will readily appreciate that the helical flow inducing plate  332 , downstream of the separate gas inlets  311 ,  312 ,  313 , provides advantages to distribution uniformity regardless of the particular gas dispersion structure between the plate  332  and the face of the substrate  370 . For example, the perforated faceplate  335  can be omitted, and the helical flow inducing plate  332  together with the volume adjusting horn  330  ensure good distribution of process gases introduced perpendicularly to the substrate surface.  
         [0098]     With reference again to  FIG. 4 , in the ALD apparatus according to the illustrated embodiment of the present invention, the RF power is supplied to the showerhead assembly  330 ,  335  through the RF connection port  325 , and the plasma is generated between the electrically grounded pedestal  360  and the faceplate  335 , so that the thin film is deposited on the substrate  370 .  
         [0099]     A film may be deposited if the process gases flow between the showerhead insulating plate  340  and the showerhead assembly  330 ,  335  to which a RF voltage is applied. Also a film may deposited on the lower portion of the inner wall  361  of the reaction chamber adjacent to on the substrate  370  and the faceplate  335  to which the process gases are supplied. In the ALD apparatus according to the embodiment of the present invention, an inert gas purge is used to prevent such undesirable film deposition.  
         [0100]     Now, the inert gas flow in the ALD apparatus according to a preferred embodiment of the present invention will be described in detail with reference to  FIGS. 8 and 9 .  FIG. 8  is a schematic perspective view showing the inert gas flow in the ALD apparatus according to the preferred embodiment of the present invention; and  FIG. 9  is a schematic view showing the inert gas flow for preventing unnecessary deposition and particle generation in the ALD apparatus according to the embodiment of the present invention. In  FIGS. 8 and 9 , arrows denote the inert gas flow.  
         [0101]     Firstly, returning to  FIG. 4 , the inert gas is supplied through a gap between the RF connection port  325  and the gas flow guiding plate  345 , shown in  FIG. 4  to the left of the RF connection port  325 . The inert gas may be such as argon (Ar), helium (He) or nitrogen (N 2 ).  
         [0102]     Referring to  FIG. 8 , the supplied inert gas flows into a circular channel  343  through a gas passage  344  of the RF connection port. Next, the inert gas approaching to the circular channel  343  is uniformed dispersed in the radial direction from the circular channel  343  to flow through the insulating plate lower gap  342  between the volume adjusting horn  330  and the showerhead insulating plate  340 . In addition, the inert gas is divided into passages  346  which are formed through the showerhead insulating plate  340  to flow through the insulating plate upper gap  341  between the showerhead insulating plate  340  and the gas flow guiding plate  345 . The inert gas passing over the upper and lower surfaces of the showerhead insulating plate  340  is combined with the process and byproduct gases exhausted from the substrate surface. The combined purge gas and process gases passes through the gap between volume adjusting horn  330  and the reaction chamber wall  361 , pass through the guiding plate upper gap  347 , and then are vented to the gas outlet  316 .  
         [0103]     An inert gas continuously flows through the gas passages  341  and  342  disposed on the upper and lower surfaces of the showerhead insulating plate  340  to prevent the process gases from forming a thin film on the showerhead insulating plate  340 .  
         [0104]     As described above, the insulating plate lower gap  342  is defined by the heights of the pads  336 , which are symmetrically distributed. The symmetrically disposed pads  336  may be attached to or machined from the upper surface of the volume adjusting horn  330 , as illustrated. All the upper surfaces of pads are at the same height so as to closely contact the lower surface of the showerhead insulating plate  340 . Alternatively, the pads  336  may be attached to or machined from the underside of the showerhead insulating plate  340 . Therefore, assembly variation of the ALD apparatus does not occur, and the insulating plate lower gap  342  is uniformly maintained.  
         [0105]     Similarly, a plurality of pads  350  are symmetrically distributed over the gas flow guiding plate  345  so as to define the guiding plate upper gap  347 . The pads  350  are precisely attached to or directly machined from the upper portion of the guiding plate  345 . Alternatively, the pads may be attached to or machined from the underside of the reaction chamber wall  361  (see  FIG. 4 ). Therefore, without influence of the assembly variation, the guiding plate upper gap  347  (see  FIG. 4 ) is uniformly maintained.  
         [0106]     In addition the pads  336  and  350 , defining the purge gas passage gaps, conduct heat effectively from the heaters  366  and  367  to the showerhead assembly  330 ,  335 .  
         [0107]     Referring to  FIGS. 4 and 9 , in the double layers of the reaction chamber wall  361 , the inner wall is slightly separated from the outer wall. The inert gas can flow through the inner-chamber-wall gas passage  362  formed between the inner wall and the outer wall and between the inner wall and the pedestal  360 . In addition, the groove is formed along the lower edge of the inner wall to define the gas flow buffering channel  363  at the contact area  364  between the pedestal  360  and the outer wall of the chamber wall  361 . The buffering channel  363  is allowed to have a gas pressure higher than the process pressure of the reaction chamber, so that the inert gas can uniformly flow into the reaction chamber.  
         [0108]     Inert gas continuously flows into the gas passage  362  and the buffering channel  363  during the deposition process in order to prevent a thin film from being formed at the contact area  364 , where substantial mechanical contact is formed. Films deposited at the contact area may peel off during repetitive contact and detachment from opening and closing the chamber, which may generate contaminant particles in the inner portion of the reaction chamber.  
         [0109]     In an ALD apparatus according to the preferred embodiment, when a conductive thin film is deposited by using any of a PEALD process, an ALD process, a combination thereof, and a series thereof, plasma can be stably generated in an inner portion of a reaction chamber without occurrence of electrical short-circuit, so that it is possible to deposit a thin film having an excellent step coverage with precise thickness control.  
         [0110]     In an ALD apparatus according to the preferred embodiment, a plurality of process gases for depositing a thin film by using the PEALD or thermal ALD process are separately supplied to the reaction chamber, so that it is possible to prevent a thin film from being deposited outside the reaction space and supply suitably mixed process gases in the reaction chamber.  
         [0111]     According to the disclosure herein, it is possible to provide an ALD apparatus using PEALD and thermal ALD processes capable of reducing occurrence of contaminant particles caused by an unnecessary deposition in an inner portion of a reaction chamber and preventing a thin film from being deposited on a rear surface of a substrate.  
         [0112]     Although the exemplary embodiments and the modified examples of the present invention have been described, the present invention is not limited to the embodiments and examples, but may be modified in various forms without departing from the scope of the appended claims, the detailed description, and the accompanying drawings of the present invention. Therefore, it is natural that such modifications belong to the scope of the present invention.