Patent Publication Number: US-10312055-B2

Title: Method of depositing film by PEALD using negative bias

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention generally relates to a method of forming a film on a substrate by plasma-enhanced atomic layer deposition (PEALD) using a capacitively coupled plasma (CCP) modified by a negative bias voltage. 
     Description of the Related Art 
     A dielectric film deposited by plasma-enhanced atomic layer deposition (PEALD) is widely used in the semiconductor manufacturing industry, since the conformality of such a film is very high, e.g., 100%, when being deposited in a trench having an aspect ratio of about 10. However, conventional PEALD using a capacitively coupled plasma (CCP) has the following problems. A plasma is an ionized gas containing positive ions and free electrons in proportions resulting in more or less no overall electric charge, and in PEALD using CCP, film formation is accomplished using ions and radicals. Ion bombardment is important to film growth and film quality. However, strong ion bombardment exerted on a substrate surface causes problems. That is, in many PEALD processes, ion bombardment worsens film properties.  FIG. 5  illustrate problems in deposition of a film caused by strong ion bombardment  45  wherein (a) shows breakage of bonding between a deposited film  42  and a substrate  41 , (b) shows formation of blisters  43   a  along an interface between a deposited film  42  and a substrate  41  (also blisters  43   b  are formed on a surface of the deposited film), and (c) shows damage or etching of an underlying layer  44  deposited between a deposited film  42  and a substrate  41 . 
     It may be possible to alleviate adverse effects of strong ion bombardment by decreasing plasma power and/or increasing process pressure. However, when plasma power is decreased, plasma density is reduced, resulting in an uneven or non-uniform plasma distribution, and when process pressure is increased, ignition of plasma becomes difficult and unstable. 
     In light of the above-discussed problems, the present inventors have completed the present invention, at least some embodiments of which can effectively resolve some or all of the above-discussed problems. 
     Any discussion of problems and solutions including those discussed above in relation to the related art has been included in this disclosure solely for the purposes of providing a context for the present invention, and should not be taken as an admission that any or all of the discussion was known at the time the invention was made. 
     SUMMARY OF THE INVENTION 
     Some embodiments provide a method of forming a film on a substrate by plasma-enhanced atomic layer deposition (PEALD) in a reaction space defined by capacitively coupled first and second electrodes facing each other wherein the substrate is placed on the first electrode facing the second electrode, which PEALD comprises deposition cycles, each cycle comprising: (i) feeding a precursor in a pulse to the reaction space to adsorb the precursor on a surface of the substrate; (ii) after step (i), applying RF power to the second electrode to generate in the reaction space a plasma to which the precursor-adsorbed surface is exposed, thereby forming a sublayer on the surface; and (iii) applying a bias voltage to the second electrode while applying RF power in step (ii), said bias voltage being negative with reference to a potential on a surface of the first electrode, wherein the cycle is repeated to deposit multiple sublayers until a film constituted by the sublayers has a desired thickness. The above method can be conducted using any suitable CCP type PEALD apparatuses including any conventional apparatuses, with minimum modifications, so as to generate a high-density plasma with low ion energy. Some embodiments can be characterized by the following features: (1) a lower ion energy process with a lower plasma potential; (2) higher throughput with a higher plasma density; and (3) less particle generation on the upper electrode. Especially, the ion energy control capability at a lower region in which the substrate is placed is notable. As discussed above in relation to  FIG. 5 , ion bombardment may induce defects such as interlayer separation, blistering, or etching, and worsen PEALD film properties; however, some embodiments can effectively eliminate these problems and are particularly beneficial for ion-bombardment-sensitive PEALD processes, achieving film formation with good film properties. 
     For purposes of summarizing aspects of the invention and the advantages achieved over the related art, certain objects and advantages of the invention are described in this disclosure. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein. 
     Further aspects, features and advantages of this invention will become apparent from the detailed description which follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features of this invention will now be described with reference to the drawings of preferred embodiments which are intended to illustrate and not to limit the invention. The drawings are greatly simplified for illustrative purposes and are not necessarily to scale. 
         FIG. 1  is a schematic representation of a PEALD (plasma-enhanced atomic layer deposition) apparatus for depositing a dielectric film using a single-frequency RF power with a negative bias voltage according to an embodiment of the present invention. 
         FIG. 2  is a schematic representation of a PEALD (plasma-enhanced atomic layer deposition) apparatus for depositing a dielectric film using a dual-frequency RF power with a negative bias voltage according to an embodiment of the present invention. 
         FIG. 3  is a schematic representation of a circuit for measuring a direct current component included in components constituting potential to determine a bias voltage according to an embodiment of the present invention. 
         FIG. 4  illustrates a schematic representation of a precursor supply system using a flow-pass system (FPS) usable in an embodiment of the present invention, wherein (a) illustrates a gas flow with a precursor, and (b) illustrates a gas flow without a precursor. 
         FIG. 5  illustrate problems in deposition of a film caused by strong ion bombardment wherein (a) shows breakage of bonding between a deposited film and a substrate, (b) shows formation of blisters along an interface between a deposited film and a substrate, and (c) shows damage or etching of an underlying layer deposited between a deposited film and a substrate. 
         FIG. 6  illustrates an embodiment wherein a DC bias voltage is changed with progress of process cycles in (a), i.e., ion energy is changed with progress of process cycles in (b). 
         FIG. 7  illustrates an embodiment wherein (a) shows potential profiles between electrodes when a negative DC bias voltage is applied to an upper electrode (“w/DC bias”) and when no negative DC bias voltage is applied (“w/o DC bias”), and (b) is a schematic representation of a plasma distribution between the electrodes. Note that for ease of illustration, the orientation has been rotated 90 degrees counterclockwise. 
         FIG. 8  shows a schematic process sequence according to an embodiment of the present invention wherein a step-up line represents an ON state or an increased-quantity state whereas a step-down line represents an OFF state or a decreased-quantity state, and the height and duration of each section are not necessarily to scale. 
         FIG. 9  is a graph showing greatly simplified relationships between the amount of negative bias voltage and damage to an underlying organic film and between the amount of negative bias voltage and wet etch rate of SiCNO film deposited on the organic film according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In this disclosure, “gas” may include vaporized solid and/or liquid and may be constituted by a single gas or a mixture of gases, depending on the context. Likewise, an article “a” or “an” refers to a species or a genus including multiple species, depending on the context. In this disclosure, a process gas for deposition introduced to a reaction chamber through a showerhead may be comprised of, consist essentially of, or consist of a silicon-containing precursor and an additive gas. The additive gas may include a reactant gas for oxidizing, nitriding and/or carbonizing the precursor, and an inert gas (e.g., noble gas) for exciting the precursor, when RF power is applied to the additive gas. The inert gas may be fed to a reaction chamber as a carrier gas and/or a dilution gas. Further, in some embodiments, no reactant gas is used, and only noble gas (as a carrier gas and/or a dilution gas) is used. The precursor and the additive gas can be introduced as a mixed gas or separately to a reaction space. The precursor can be introduced with a carrier gas such as a rare gas. A gas other than the process gas, i.e., a gas introduced without passing through the showerhead, may be used for, e.g., sealing the reaction space, which includes a seal gas such as a rare gas. In some embodiments, the term “precursor” refers generally to a compound that participates in the chemical reaction that produces another compound, and particularly to a compound that constitutes a film matrix or a main skeleton of a film, whereas the term “reactant” refers to a compound, other than precursors, that is used in association with a precursor and activates the precursor, modifies the precursor, or catalyzes a reaction of the precursor, wherein the reactant may provide an element (such as O, N, and/or C) to a film matrix and become a part of the film matrix, when RF power is applied. The term “inert gas” refers to a gas that is inactive when RF power (or other electromagnetic energy) is not applied but can become a plasma state to excite a precursor or reform a film when RF power (or other electromagnetic energy) is applied, but unlike a reactant, it may not become a part of or incorporated into a film matrix. 
     In some embodiments, “film” refers to a layer continuously extending in a direction perpendicular to a thickness direction substantially without pinholes to cover an entire target or concerned surface, or simply a layer covering a target or concerned surface. In some embodiments, “layer” refers to a structure having a certain thickness formed on a surface or a synonym of film or a non-film structure. A film or layer may be constituted by a discrete single film or layer having certain characteristics or multiple films or layers, and a boundary between adjacent films or layers may or may not be clear and may be established based on physical, chemical, and/or any other characteristics, formation processes or sequence, and/or functions or purposes of the adjacent films or layers. Further, in this disclosure, any two numbers of a variable can constitute a workable range of the variable as the workable range can be determined based on routine work, and any ranges indicated may include or exclude the endpoints. Additionally, any values of variables indicated (regardless of whether they are indicated with “about” or not) may refer to precise values or approximate values and include equivalents, and may refer to average, median, representative, majority, etc. in some embodiments. Further, in this disclosure, the terms “constituted by” and “having” refer independently to “typically or broadly comprising”, “comprising”, “consisting essentially of”, or “consisting of” in some embodiments. In this disclosure, any defined meanings do not necessarily exclude ordinary and customary meanings in some embodiments. 
     In the present disclosure where conditions and/or structures are not specified, the skilled artisan in the art can readily provide such conditions and/or structures, in view of the present disclosure, as a matter of routine experimentation. 
     In all of the disclosed embodiments, any element used in an embodiment can be replaced with any elements equivalent thereto, including those explicitly, necessarily, or inherently disclosed herein, for the intended purposes. Further, the present invention can equally be applied to apparatuses and methods. 
     The embodiments will be explained with respect to preferred embodiments. However, the present invention is not limited to the preferred embodiments. 
     As discussed above, in some embodiments, in a method of forming a film on a substrate by plasma-enhanced atomic layer deposition (PEALD) in a reaction space defined by capacitively coupled first and second electrodes facing each other wherein the substrate is placed on the first electrode facing the second electrode, the PEALD comprises deposition cycles, each cycle comprising: (i) feeding a precursor in a pulse to the reaction space to adsorb the precursor on a surface of the substrate; (ii) after step (i), applying RF power to the second electrode to generate in the reaction space a plasma to which the precursor-adsorbed surface is exposed, thereby forming a sublayer on the surface; and (iii) applying a bias voltage to the second electrode while applying RF power in step (ii), said bias voltage being negative with reference to a potential on a surface of the first electrode, wherein the cycle is repeated to deposit multiple sublayers until a film constituted by the sublayers has a desired thickness. 
     In some embodiments, the bias voltage is a DC voltage. In some embodiments, the bias voltage is an AC voltage having a frequency of 1 MHz or less, whose average voltage is not zero. Typically, the bias voltage is a DC voltage, but as long as ions can follow changes of current, the bias voltage can be an AC voltage or RF power such as those having a frequency of 1 MHz or less. 
     In some embodiments, the first electrode is grounded wherein RF power as well as the bias voltage are applied to the second electrode only. In some embodiments, RF power having different frequencies is applied to the first and second electrodes, respectively, and the bias voltage is applied to the second electrode only. 
     In some embodiments, in step (ii), the plasma is a hydrogen plasma. In some embodiments, in step (ii), the plasma is a plasma of one or more gases selected from the group consisting of H 2 , N 2 , O 2 , NH 3 , N x H y  (x and y are integers), N 2 O, NO 2 , CO 2  and noble gas, and a mixture of any of the foregoing. In some embodiments, in step (ii), the plasma is a plasma of any suitable gas, since effect by a reduction of plasma potential using a negative bias voltage, i.e., a reduction of ion energy, can be realized regardless of the type of gas. 
     In some embodiments, an average value of the bias voltage is more than 0 V (e.g., 10 V or more) but no more than 1,000 V, wherein the bias voltage is applied in a manner where the bias voltage is negative with reference to a potential on a surface of the first electrode (in this disclosure, the bias voltage is expressed as an absolute value, and the term “negative” refers to the direction of application to reduce the potential between the first and second electrodes, unless stated otherwise). Since the process conditions for PEALD (e.g., RF power (13.56 MHz) of 50 to 1,000 W, a pressure of 30 to 3,000 Pa) vary, the average value of the bias voltage may be adjusted accordingly, based on the quality of a film deposited under the selected process conditions (e.g., wet etch rate using HF solution) and/or the degree of damage to an underlying layer (e.g., organic film) evaluated by a TEM image of a cross section of the layer. In order to reduce the degree of damage to the underlying layer (e.g., organic film), a high bias voltage may be used, whereas in order to improve the quality of a deposited film, a low bias voltage may be used (which includes zero voltage, e.g., under high pressure conditions, e.g., a pressure of 3,000 Pa).  FIG. 9  is a graph schematically representing the above relationship, which shows greatly simplified relationships between the amount of negative bias voltage and damage to an underlying organic film and between the amount of negative bias voltage and wet etch rate of SiCNO film deposited on the organic film according to an embodiment of the present invention. In light of the above-discussed relationships, in some embodiments, the average bias voltage is in a range of 10 to 500 V, typically 10 to 300 V. In this disclosure, the property of the film includes, but is not limited to, resistance to wet etching which can be evaluated by immersing a film in a solution of DHF (diluted hydrogen fluoride) having a dilution ratio of typically 1/100 to 1/200 (in some embodiments, using an etching solution of HF 0.05-5%, at an etching solution temperature of 10 to 50° C. (preferably 15 to 30° C.), for a duration of etching of 1 sec to 5 min (preferably 1 to 3 min), at an etching rate of 0.1 to 5 nm/min (preferably 0.5 to 2 nm/min)). 
     In some embodiments, the use of the bias voltage is effective especially when the target film is ion-bombardment sensitive, such as an oxide film, nitride film, or carbon film, and/or when an underlying film on which the target film is directly deposited is a polymer film or an amorphous silicon film. 
     In some embodiments, in step (iii), the bias voltage is gradually reduced when the cycle is repeated. In this disclosure, the term “gradually increasing (or decreasing)” refers to continuously increasing (or decreasing), increasing (or decreasing) at a certain (determinable) rate, continuously increasing (or decreasing) when a regression equation is drawn using data points, or increasing (or decreasing) as if data points are plotted along a curved line or approximately straight line, or continuously increasing (or decreasing) when smoothing the data, depending on the deposited film, the underlying layer, the deposition process, etc. according to embodiments.  FIG. 6  illustrates an embodiment wherein a DC bias voltage is changed with progress of process cycles in (a), i.e., ion energy is changed with progress of process cycles in (b). As shown in  FIG. 6 , when depositing the film on a certain film such as a polymer film, the ion energy may be low at the beginning as shown in (b) (i.e., the bias voltage may be high at the beginning as shown in (a)) so as to inhibit polymer etching at the beginning where the underlying layer is particularly vulnerable to ion bombardment since the depositing film is still thin. Thereafter, since the influence of ion bombardment is lessens as the process cycle is repeated, the bias voltage can be reduced so as to improve the quality of the depositing film. By supplying the bias voltage at the beginning and then gradually decreasing it as shown in (a), it is possible to minimize damage to the underlying layer while improving the quality of the depositing film. 
     In some embodiments, as shown in  FIG. 6 , the deposition cycles are conducted as first deposition cycles (step “a” in (a)), and the PEALD further comprises second deposition cycles (step “c” in (a)) each comprising steps (i) and (ii) without step (iii) (or substantially without step (iii), i.e., the bias voltage is substantially or nearly zero) conducted after the first deposition cycles via an intermediate or transitional step “b” in (a). In some embodiments, the intermediate deposition cycles comprise steps (i) to (iii) in which the bias voltage is gradually reduced (e.g., about −1 to −20 V per cycle, typically about −10 V per cycle) when the cycle is repeated, wherein the intermediate deposition cycles are conducted continuously after the first deposition cycles, and the second deposition cycles are conducted continuously after the intermediate deposition cycles. In this disclosure, “continuously” refers to without breaking a vacuum, without interruption as a timeline, without any material intervening step, without changing treatment conditions, immediately thereafter, as a next step, or without an intervening discrete physical or chemical structure between two structures other than the two structures, depending on the embodiment. In some embodiments, the first deposition cycles are terminated when a thickness of the film reaches 10 nm or less (e.g., 5 nm or less, 3 nm or less, at least 1 nm, 2 nm or more). When the sequence illustrated in  FIG. 6  is applied to double-patterning processes using a SiO film on a carbon film, step “a” is constituted by about 30 cycles (to form a protective film having a thickness of about 2 nm), step “b” is constituted by about 30 cycles (to continuously form a SiO film having a thickness of about 2 nm by increasing ion energy to the extent that the underlying layer, the carbon film, is not damaged), and step “c” is constituted by about 240 cycles (to continuously form a SiO film having a thickness of about 16 nm under conditions optimized for film deposition). 
     In some embodiments, a reactant gas and a carrier/dilution gas are continuously fed to the reaction space throughout steps (i) to (iii). In some embodiments, the cycle further comprises purging after every step (i) and every step (ii).  FIG. 8  shows a schematic process sequence according to an embodiment of the present invention wherein a step-up line represents an ON state or an increased-quantity state whereas a step-down line represents an OFF state or a decreased-quantity state, and the height and duration of each section are not necessarily to scale. In this process sequence, the deposition process is conducted in a CCP-type reaction chamber accommodating a Si wafer. Steps “Feed”, “Purge 1”, “RF”, and “Purge 2” constitute one cycle of PEALD. In “Feed”, a precursor (such as alkylaminosilane) is fed in a pulse to the reaction chamber to chemisorb a precursor on a surface of the wafer while continuously feeding a reactant gas (e.g., O 2 ) and dilution/carrier gas (e.g., Ar) through “Feed”, “Purge 1”, “RF”, and “Purge 2”. In “RF”, RF power is applied to the reaction chamber to expose the precursor-adsorbed wafer to a plasma of the reaction gas so as to form a monolayer on the surface of the wafer. “Purge 1” and “Purge 2” are purging steps to remove non-reacted components and by-products from the substrate, wherein the continuous flows of the reactant gas and the dilution/carrier gas function as purging gases. In this embodiment, a DC bias voltage is applied to an upper electrode entirely while RF power is applied in “RF”, i.e., the timing of “ON” of RF power and the timing of “ON” of the DC bias voltage are entirely or substantially synchronized, so that ion energy in the plasma can be controlled throughout the entire period of “RF”. In this disclosure, “substantially the same”, “substantially uniform”, or the like may refer to an immaterial difference or a difference recognized by a skilled artisan such as those of less than 10%, less than 5%, less than 1%, or any ranges thereof depending on the embodiment. Further, the term “substantially synchronized” includes an unavoidable response or time delay in a control circuit. Alternatively, the DC bias voltage can be applied in pulses or in a shorter pulse during a period of the pulse of RF power when ion energy is not high. The one cycle is repeated until a desired thickness of film is obtained on the wafer. 
     In some embodiments, a PEALD cycle is conducted under conditions shown in Table 1 below. 
     
       
         
           
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 (numbers are approximate) 
               
               
                 Conditions for PEALD cycle 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 Substrate temperature 
                 100 to 500° C. (preferably 200 to 400° C.) 
               
               
                 Electrode gap (a thickness 
                 3 to 15 mm (preferably 7 to 12 mm) 
               
               
                 of a substrate is about 
               
               
                 0.7 mm) 
               
               
                 Pressure 
                 50 to 3000 Pa (preferably 200 to 1000 Pa) 
               
               
                 Flow rate of reactant 
                 0.1 to 4 sccm (preferably 0.5 to 1 sccm) 
               
               
                 gas (continuous) 
               
               
                 Flow rate of carrier 
                 1 to 4 sccm (preferably 1.5 to 3 sccm) 
               
               
                 gas (continuous) 
               
               
                 Flow rate of dilution 
                 0.1 to 3 slm (preferably 0.5 to 1.5 slm) 
               
               
                 gas (continuous) 
               
               
                 Flow rate of 
                 Corresponding to the flow rate of carrier gas 
               
               
                 precursor 
               
               
                 RF power for a 
                 13.56 to 100 MHz (preferably 13.56 MHz); 
               
               
                 300-mm wafer 
                 50 to 1000 W (preferably 100 to 400 W) 
               
               
                 Bias voltage 
                 DC 10 to 1000 V (preferably 10 to 200 V); 
               
               
                   
                 AC (frequency of 1 MHz or less) average 
               
               
                   
                 10 to 1000 V (preferably 10 to 200 V) 
               
               
                 Duration of “Feed” 
                 0.1 to 1 sec. (preferably 0.1 to 0.5 sec.) 
               
               
                 Duration of “Purge 1” 
                 0.5 to 2 sec. (preferably 0.5 to 1 sec.) 
               
               
                 Duration of “RF” 
                 0.5 to 10 sec. (preferably 1 to 5 sec.) 
               
               
                 Duration of “Purge 2” 
                 0.1 to 2 sec. (preferably 0.1 to 1 sec.) 
               
               
                 Duration of one cycle 
                 1.1 to 15 sec. (preferably 1.7 to 7.5 sec.) 
               
               
                 Glow rate per 
                 0.01 to 0.1 on top surface 
               
               
                 cycle (nm/min) 
               
               
                   
               
            
           
         
       
     
     The bias voltage is applied to the second electrode while applying RF power, as a negative voltage with reference to a potential on a surface of the first electrode. 
     The above indicated RF power for a 300-mm wafer can be converted to W/cm 2  (wattage per unit area of a wafer) which can apply to a wafer having a different diameter such as 200 mm or 450 mm. 
     Typically, the thickness of the dielectric film is in a range of about 50 nm to about 500 nm (a desired film thickness can be selected as deemed appropriate according to the application and purpose of film, etc.). The dielectric film may be used for double patterning. 
     The above process can be used in various applications, including spacer-defined double patterning (SDDP), wherein a silicon oxide film according to any of the disclosed embodiments or equivalents thereto can be used as a vertical spacer. 
     In some embodiments, the substrate has a recess pattern, each recess being defined between adjacent vertical spacers and constituted by a bottom and sidewalls, referred to as a “trench”. That is, a trench is any recess pattern including a pattern formed by vertical spacers and which has, in some embodiments, a width of about 10 nm to about 100 nm (typically about 14 nm to about 30 nm) (wherein when the trench has a length substantially the same as the width, it is referred to as a hole/via, and a diameter thereof is about 10 nm to about 100 nm), a depth of about 30 nm to about 100 nm (typically about 40 nm to about 60 nm), and an aspect ratio of about 2 to about 20 (typically about 2 to about 5). The proper dimensions of the trench may vary depending on the process conditions, film compositions, intended applications, etc. 
     In some embodiments, the film deposited in the recess pattern has a conformality (a ratio of thickness of film deposited on sidewalls to thickness of film deposited on a top surface or on a bottom surface) of 80% to 100%, typically approximately 90% or higher. 
     The process cycle can be performed using any suitable apparatus including an apparatus illustrated in  FIG. 1 , for example.  FIG. 1  is a schematic representation of a PEALD (plasma-enhanced atomic layer deposition) apparatus for depositing a dielectric film using a single-frequency RF power with a negative bias voltage according to an embodiment of the present invention, desirably in conjunction with controls programmed to conduct the sequences described below. 
     In this figure, by providing a pair of electrically conductive flat-plate electrodes  4 ,  2  in parallel and facing each other in the interior  11  (reaction zone) of a reaction chamber  3 , applying HRF power (e.g., 2 to 100 MHz including 13.56 MHz, 27 MHz, 60 MHz, 100 MHz, and any values between any two number of the foregoing)  20  to one side, and electrically grounding the other side  12 , a plasma is excited between the electrodes. A temperature regulator is provided in a lower stage or susceptor  2  (the lower electrode), and a temperature of a substrate  1  placed thereon is kept constant at a given temperature. The upper electrode  4  serves as a shower plate as well, and reactant gas and/or dilution gas, if any, and precursor gas are introduced into the reaction chamber  3  through a gas line  21  and a gas line  22 , respectively, and through the shower plate  4 . Additionally, in the reaction chamber  3 , a circular duct  13  with an exhaust line  7  is provided, through which gas in the interior  11  of the reaction chamber  3  is exhausted. Additionally, a transfer chamber  5  disposed below the reaction chamber  3  is provided with a seal gas line  24  to introduce seal gas into the interior  11  of the reaction chamber  3  via the interior  16  (transfer zone) of the transfer chamber  5  wherein a separation plate  14  for separating the reaction zone and the transfer zone is provided (a gate valve through which a wafer is transferred into or from the transfer chamber  5  is omitted from this figure). The transfer chamber is also provided with an exhaust line  6 . In some embodiments, the deposition of multi-element film and post-deposition treatment are performed in the same reaction space, so that all the steps can continuously be conducted without exposing the substrate to air or other oxygen-containing atmosphere. 
     In this apparatus, a negative bias voltage is applied from a DC power supply  31  to the shower plate  4 , through a blocking capacitor (RF cut filter or low-path filter)  32 . The timing of applying RF power and the timing of applying the bias voltage are substantially synchronized using a control unit  34  which controls a matching box  30  disposed on the output side of the RF power source  20 , the power source  20 , the blocking capacitor  32 , and the DC power supply  31 . The control unit  34  receives a signal from an electrode potential measuring unit  33  which measures a potential between the electrodes, so that the control unit can control the above components according to the electrode potential. The value of bias voltage to be applied to the upper electrode can be measured using a DC voltage meter.  FIG. 3  is a schematic representation of a circuit for measuring a direct current component of potential to determine a bias voltage according to an embodiment of the present invention. A DC voltage meter  27  is placed after a choke coil  28  (low-path filter  32 ) so that only a DC component of the potential can be extracted and pass through the DC voltage meter  27 . 
     The above configuration can be applied to a dual-frequency RF power system.  FIG. 2  is a schematic representation of a PEALD (plasma-enhanced atomic layer deposition) apparatus for depositing a dielectric film using a dual-frequency RF power with a negative bias voltage according to an embodiment of the present invention. In this apparatus, a second RF power source  36  is connected to the lower electrode  2  through a matching box  37 , and a control unit  35  controls the matching box  37  and the second RF power source  36  in addition to the above-described components. 
       FIG. 7  (orientation rotated 90° counterclockwise) illustrates an embodiment wherein (a) shows potential profiles between electrodes when a negative DC bias voltage is applied to an upper electrode (“w/ DC bias”) and when no negative DC bias voltage is applied (“w/o DC bias”), and (b) is a schematic representation of a plasma distribution between the electrodes. In general, because an electron temperature of a plasma is higher than that of ions, the wall potential is normally negative with reference to the plasma potential due to light and fast-moving electrons. In other words, the plasma potential is normally positive with reference to the wall potential. In this disclosure, the value of the voltage is determined with reference to the wall potential of the lower electrode  2 , i.e., the wall potential of the lower electrode  2  is considered to be zero. On the wall surface, an ion sheath is generated, which is a space-charge layer which slows down the speed of electrons or reflects electrons and increases the speed of ions, wherein excessive ions are present, so that the plasma is kept electrically neutral as a whole. As shown in (a) of  FIG. 7 , when no DC bias voltage is applied to the upper electrode  4  (“w/o DC bias”) while RF power is applied to the upper electrode  4 , the wall potential of the upper electrode  4  is V0dc which is negative with reference to the wall potential (zero) of the lower electrode  2 , and the plasma potential is positive excepting regions adjacent to the walls of the upper and lower electrodes (the plasma potential is expressed using the maximum and minimum lines). When a DC bias voltage is applied to the upper electrode  4  (“w/ DC bias”) while RF power is applied to the upper electrode  4 , the wall potential of the upper electrode  4  is Vdc which is negative with reference to the wall potential (zero) of the lower electrode  2 , and the plasma potential is positive excepting regions adjacent to the walls of the upper and lower electrodes (the plasma potential is expressed using the maximum and minimum lines in the upper ion sheath, but is represented by one line in the remaining region since the difference between the maximum and minimum is small and can be simplified and approximate by one line). Vdc is the sum of V0dc and ΔVdc (=V1dc) which is the value of the bias voltage. By applying the bias voltage as illustrated in (a) of  FIG. 7 , the thickness of the upper ion sheath becomes thicker, whereas the thickness of the lower ion sheath becomes thinner. As a result, as illustrated in (b) of  FIG. 7 , in the upper ion sheath  53  formed in the interior  11  of the reaction chamber  3 , the wall surface is exposed to strong ion bombardment by ions  45  (high ion energy) which induces an increase of secondary electrons  51 , resulting in higher plasma density. On the other hand, in the lower ion sheath  54  formed on the other side of the plasma  52  near the lower electrode  2 , the thickness of the lower ion sheath  54  becomes thin, and a substrate  1  on the wall surface is exposed to less ion bombardment by ions. 
     As a result of application of the bias voltage, since the lower ion sheath  54  has low ion energy (less ion bombardment), damage or etching of an underlying layer can effectively be suppressed or reduced. On the other hand, since the upper ion sheath  53  has a high plasma density induced by released secondary electrons due to strong ion bombardment, and the state of high plasma density can be maintained through the plasma  52  and it can increase the plasma density in the lower ion sheath  54 , resulting in higher throughput. In the lower ion sheath  54 , although the substrate surface is exposed to less ion bombardment, the ion density is high due to high plasma density (high ion dose), and thus, the duration of RF power can be shortened, resulting in higher throughput. Further, since the surface of the upper electrode  4  is exposed to strong ion bombardment, film growth on the surface is inhibited, resulting in less particle generation. Further, since the released secondary electrons in the upper ion sheath  53  are increased and plasma species generated therefrom are dispersed throughout the upper ion sheath  53 , plasma uniformity can significantly be improved. 
     In some embodiments, since the surface of the upper electrode is exposed to strong ion bombardment, metal contaminants may be generated from the surface by sputtering due to the strong ion bombardment, resulting in an increase of metal contamination of a film formed on a substrate. This tendency is more manifested as the bias voltage is increased. Thus, in some embodiments, the surface of the upper electrode is covered in advance with a film as a pre-coat which is the same film as the one to be formed on the substrate. 
     The present invention is further explained with reference to working examples below. However, the examples are not intended to limit the present invention. In the examples where conditions and/or structures are not specified, the skilled artisan in the art can readily provide such conditions and/or structures, in view of the present disclosure, as a matter of routine experimentation. Also, the numbers applied in the specific examples can be modified by a range of at least ±50% in some embodiments, and the numbers are approximate. 
     EXAMPLES 
     Example 1 (Prophetic) 
     On a Si substrate (having a diameter of 300 mm and a thickness of 0.7 mm) having an organic film (polyimide film), a SiOCN film having a thickness of about 30 nm is deposited by PEALD using the apparatus illustrated in  FIGS. 1, 3, and 4  using the sequence illustrated in  FIG. 8 , under conditions using an aminosilane ((3-aminopropyl)trimethoxysilane, APTMS) as a precursor, H 2  as a reactant (H 2  plasma), and Ar as a carrier gas, wherein the temperature of a bottle for the precursor is set at room temperature, the flow rate of H 2  gas is about 100 sccm, and the flow rate of Ar gas is about 600 sccm, and the pressure is about 4 Torr, and the substrate temperature is about 200° C. RF power (a frequency of 13.56 MHz) applied to the upper electrode is about 200 W. 
     The bias voltage is applied to the upper electrode, using a voltage of 0 V, −50V, −100 V, −200 V, and −300 V, to overly the RF power in examples, respectively. After completion of the film deposition, the quality of the resultant SiOCN film and the degree of damage to the underlying polyimide film are evaluated in order to determine overall improvement by the bias voltage. The quality of the SiOCN film is evaluated by wet etch rate using DHF, and the degree of damage to the polyimide film is evaluated based on a TEM (Transmission Electron Microscopy) image of a cross section of the film. By plotting each evaluation point, the relationship illustrated in  FIG. 9  or the like can be obtained, and accordingly, optimum conditions can be determined. 
     Example 2 (Prophetic) 
     A SiOCN film having a thickness of about 30 nm is deposited on a polyimide film formed on a 300-mm Si wafer) using APTMS as a precursor, H 2  as a reactant (H 2  plasma), and Ar as a carrier gas, in a manner similar to that in Example 1, except that the bias voltage is changed as illustrated in  FIG. 6 . That is, in step “a”, the deposition cycle is repeated until the thickness of the SiOCN film reaches about 3 nm using the bias voltage determined in Example 1 where the degree of damage to the organic film is minimal, and then, in step “b”, the deposition cycle is continuously repeated until the thickness of the SiOCN film reaches about 5 nm (adding about 2 nm) using the bias voltage which is gradually increased in a negative direction (e.g., about −10 V/cycle) to the bias voltage determined in Example 1 where the quality of the SiOCN film is maximum (optimum), and then, in step “c”, the deposition cycle is continuously repeated until the thickness of the SiOCN film reaches a desired value using the bias voltage (including zero) determined in Example 1 where the quality of the SiOCN film is maximum (optimum). As a result, a high-quality SiOCN film is successfully deposited while minimizing the damage to the underlying organic film. 
     It will be understood by those of skill in the art that numerous and various modifications can be made without departing from the spirit of the present invention. Therefore, it should be clearly understood that the forms of the present invention are illustrative only and are not intended to limit the scope of the present invention.