Patent Publication Number: US-6986851-B2

Title: Dry developing method

Description:
FIELD OF THE INVENTION 
   The present invention relates to a dry developing method; and, more particularly, to a dry developing method capable of etching a lower layer resist of a double-layer resist without destroying an upper layer resist thereof, while maintaining a vertical sidewall of a pattern being etched. 
   BACKGROUND OF THE INVENTION 
   In a recent trend of high integration of semiconductor devices, design rule is becoming stricter. Accordingly, a lithography process performed during a manufacturing process requires a high degree of accuracy. 
   For this reason, a resist used in the lithography process includes a resist layer subject to a wet developing process (hereinafter, referred to as “lower layer resist”) provided beneath a wet-developed resist layer (hereinafter, referred to as “upper layer resist”), for example. Such resist structure enables a high density patterning, despite, if any, a stepped portion in an etching layer therebelow. 
   However, in order to maintain the accuracy when the lower layer resist of the double-layer structure resist is dry-developed, the lower layer resist needs to be sufficiently etched without destroying the upper layer resist by etching. Additionally, it is crucial that sidewalls of a pattern formed in the lower layer resist are vertically formed in a designed dimensions in order to improve an accuracy of machining. 
     FIG. 4  is a schematic crosssectional view of a processing object  10  developed by using a conventional method.  FIG. 4A  shows a case where a reactive ion etching is performed by using, e.g., an oxygen gas, whereas  FIG. 4B  illustrates a case where an etching is carried out by using a gaseous mixture of, e.g., a nitrogen gas and a hydrogen gas. Herein, the processing object  10  includes an upper layer resist  16  forming a top surface thereof, a lower layer resist  14  formed under the upper layer resist  16  and an etching layer  12  formed beneath the lower layer resist  14 . 
   Since, however, the etching by using the oxygen gas is isotropically performed with minimal anisotropy, and thus, an erosion phenomenon, e.g., an undercut or a bowing, occurs at the sidewall of the pattern in the lower layer resist  14 , as illustrated in  FIG. 4A . 
   To that end, there is provided a method of low power etching to avoid the erosion phenomenon described above. However, such low power etching method suffers from drawbacks of reduced etching rateand deteriorated efficiency. 
   On the other hand, by using the gaseous mixture of nitrogen gas and hydrogen gas in etching, the undercutting of the sidewall or the like can be avoided. Since, however, an etching selectivity between the lower layer resist  14  and the upper layer resist  16  is small, when using such gaseous mixture in performing etching, a pattern formed in the upper layer resist  16  is damaged, thereby deteriorating the machining accuracy of the lower layer resist  14  as well as the etching accuracy of the etching layer  12  formed beneath the lower layer resist  14 . 
   SUMMARY OF THE INVENTION 
   The present invention has been developed to overcome the above-mentioned drawbacks of conventional dry developing methods. It is, therefore, an object of the present invention to provided a new and improved dry developing method having satisfactory machining accuracy and efficiency. 
   To achieve the aforementioned object in accordance with the present invention, there is provided a dry developing method for patterning a resist formed on a processing object by supplying a processing gas in a vacuum processing container and forming plasma of the processing gas by applying a high frequency power, wherein the resist has a first resist layer containing silicon and having a pattern formed therein and a second resist layer provided beneath the first resist layer, and the second resist layer is plasma processed by a first step using a gaseous mixture of carbon monoxide gas and an oxygen gas. In accordance with this configuration, silicon contained in a surface of the silicon-containing first resist layer is oxidized and the first resist layer can be prevented from being destroyed during a developing process. 
   The dry developing method may further include, after the first step, a second step in which a further plasma processing is carried out by using a gaseous mixture of a nitrogen gas and a hydrogen gas. In accordance with this configuration, it is possible to avoid an undercut or a bowing of the second resist layer and form a vertical pattern with high accuracy. 
   In the first step, it is preferable that an etching rate ratio of the second resist layer to the first resist layer during a plasma process is greater than or equal to 10. Such a condition can be achieved by setting a flow rate ratio of the carbon monoxide gas to the oxygen gas to range from 0.2 to 5; an applied high frequency power density to range from 0.32 W/cm 2  to 3.18 W/cm 2 ; and a temperature of a susceptor, for mounting thereon the processing object, of a lower electrode to which the high frequency power is applied to range from −30° C. to 20° C. 
   In the first step, an undercut is formed around a top portion of the second resist layer and, in the second step, a pattern having an approximately equal size to that of the first resist layer is formed at a lower portion of the second resist layer. The undercut can be controlled in the first step by a processing time, a flow rate ratio between the carbon monoxide gas and the oxygen gas and the high frequency power applied to the lower electrode. Further, a trimming of the pattern can be performed by the first and the second step. 
   The first resist layer is thicker than a total thickness of the first resist layer etched by the first and the second step, thereby preventing the first resist layer from being locally removed completely during the second step. In the dry developing method described above the plasma can be formed between parallel flat electrodes installed in a vacuum processing container. In accordance with the above configuration, a destruction of the first resist layer can be prevented and, further, it is possible to perform a dry developing method having an improved etching rate of the second resist layer with high accuracy and efficiency. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a schematic cross sectional view of an etching apparatus in accordance with the present invention; 
       FIG. 2  illustrates a schematic cross sectional view of a processing object before being drydeveloped; 
       FIG. 3  depicts a schematic cross sectional view of the processing object after being drydeveloped; and 
       FIG. 4  describes a schematic cross sectional view of a processing object after being dry-developed by using a conventional method. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, preferred embodiments of the dry developing method in accordance with the present invention will be described in detail with reference to the accompanying drawings. Further, in the specification and the accompanying drawings, like reference numerals will be given to like parts having substantially the same functions, and redundant description thereof will be omitted. 
   First, an overall arrangement of an etching apparatus  100  to which the present invention can be applied will be described in detail with reference to  FIG. 1 . A processing container  102  of the etching apparatus  100  illustrated in  FIG. 1  is grounded and made of aluminum, having a surface coated with an aluminum oxide film layer by performing, e.g., an anodizing process. 
   Disposed inside the processing container  102  is a lower electrode  104  also serving as a susceptor for mounting thereon a processing object, e.g., a semiconductor wafer W (hereinafter, referred to as “wafer”). Further, in the example shown in  FIG. 1 , portions other than a mounting surface of the lower electrode  104  are covered with an insulating member  105  made of, e.g., ceramic, and a conductive member  107  made of, e.g., aluminum. 
   The lower electrode  104  is vertically movable by an operation of an elevating column  106 . Furthermore, bellows  109  made of, e.g., stainless steel, are installed between the conductive member  107  and the processing container  102 . 
   An aluminum oxide film layer is removed from parts of surfaces of the conductive member  107  and the processing container  102 , which are electrically in contact with bellows  109 , and, thus, enabling the conductive member  107  to be grounded via the bellows  109  and the processing container  102 . Moreover, a bellows cover  111  is installed to surround a side surface of the conductive member  107  and bellows  109 . 
   As shown in  FIG. 1 , an electrostatic chuck  110  connected to a high voltage DC power supply  108  is installed at the mounting surface of the lower electrode  104  and, moreover, an insulating focus ring  112  is provided to surround the electrostatic chuck  110 . 
   The lower electrode  104  is connected to a high frequency power supply  118  outputting a high frequency power via a matching unit  116 . An exhaust ring  120  is disposed at a side portion of the lower electrode  104 . In the illustrated example, the exhaust ring  120  is inserted between the focus ring  112  and the conductive member  107 , while being fixed at a top portion of the conductive member  107  by conductive screws (not shown). 
   Since the exhaust ring  120  is grounded via the conductive member  107 , the bellows  109 , and the processing container  102 , the exhaust ring  120  and an inner wall of the processing container  102  can have an approximately equal electric potential (ground potential). Consequently, the exhaust ring  120  and an upper portion of the inner wall of the processing container  102  that is above the exhaust ring  120  function as counterpart electrodes, so that a plasma can be trapped in an upper portion of the exhaust ring  120 , i.e., a processing space  122  to be described below. 
   An upper electrode  126  is installed on an inner wall surface of the processing container  102 , which faces the mounting surface of the lower electrode  104 . Further, a plurality of gas inlet holes  126   a  that are connected to a gas supply source (not shown) for supplying a processing gas, are provided at the upper electrode  126 . Thus, the processing gas is supplied into the processing space  122  through the gas inlet holes  126   a.    
   An exhaust line  128  connected to a vacuum device (not shown) is provided at a lower portion of the processing container  102 . Therefore, an inner space of the processing container  122  is evacuated via slits  120   a  of the exhaust ring  120 , an exhaust channel  124  and the exhaust line  128 . Further, there is provided a magnet  130  disposed outside the processing container  102  to surround a plasma area formed between the lower electrode  104  and the upper electrode  126 . 
   In the following, a resist used in carrying out a dry developing method in accordance with the embodiment and etching conditions therefor will be described in detail.  FIG. 2  depicts a schematic cross sectional view of a processing object  150  before the dry development thereof. As for the object to be processed, a wafer W made of silicon having a diameter of, e.g., 200 mm, is used. 
   As illustrated in  FIG. 2 , a silicon-containing resist layer (hereinafter, referred to as “upper layer resist”)  156  having a pattern formed by, e.g., a wet process, is provided on a surface. A method of forming the silicon-containing upper layer resist  156  includes, e.g., a method of patterning a previously formed silicon-containing resist film; and a method of patterning a resist film formed of widely used resist materials and then silylating a surface thereof. 
   In the former method, i.e., the method of patterning an already formed silicon-containing resist film, materials of resist films can be, e.g., a positive resist obtained by chemically adhering a photoactive dimazo compound to aqueous base soluble polysilsesquioxane; a positive resist obtained by chemically adhering acid-susceptible t-butyloxylcarbonyl to aqueous base soluble polysilsesquiozane; a negative resist obtained by chemically adhering an azido functional group to aqueous base soluble polysilsesquioxane; a negative resist including aqueous base soluble silicon-containing polymer having phenolic group, a crosslinking agent and an acid generator; and the like. 
   In the latter method, i.e., the method of patterning a typical resist film and then silylating a surface thereof, a silylation method includes, e.g., a method in which a resist film is exposed to a gas atmosphere including hexamethyldisilazane or tetramethyldisilazane as a silylating agent, and a method in which a resist film is exposed to a plasma of a silicon-containing gas such as silane, disilane, dichlorosilane, and the like. 
   A thickness of the upper layer resist  156  is chosen to be, e.g., 250 nm, to ensure that none of the parts are completely removed locally after the dry development, whereas a width R 1  of a pattern is chosen to be, e.g., 150 nm. 
   Formed under the upper layer resist  156  is a lower layer resist  154  to be developed by using the dry developing method in accordance with the present invention. The lower layer resist  154  can be made of a typical resist material having as a main component thereof, e.g., phenol novolak resin, cresol novolak resin, 1-methoxy-2-propanol and the like. A thickness of the lower layer resist  154  is, e.g., 820 nm. 
   Furthermore, an etching layer  152 , e.g., a silicon oxide film (SiO 2  film), is formed beneath the lower layer resist  154 . The etching layer  152  can also be, e.g., an Al film or the like for metal wiring. 
   The dry developing method in accordance with the present invention has two steps. A first step is performed by using a gaseous mixture of a carbon monoxide gas and an oxygen gas to ensure that a surface of the upper layer resist  156  is oxidized to form a silicon oxide film, thereby preventing the upper layer resist  156  from being damaged during a second step. 
   The following etching conditions were applied to the first step: a vacuum level in the processing container  102  of 15 mT; a processing gas of a gaseous mixture of a CO gas and an O 2  gas respectively having a flow rate of 60 sccm; a temperature of the inner wall surface of the processing container  102  including the upper electrode  126  controlled at 60° C.; a temperature of the mounting surface of the lower electrode  104  controlled at 0° C.; a distance of 27 mm between the upper electrode  126  and the lower electrode  104 ; a pressure of a cooling gas on a backside of a center of a wafer at 7 Torr; a pressure of a cooling gas on a backside of a wafer edge at 40 Torr; a high frequency power of 240 W (0.76 W/cm 2 ) applied to the lower electrode  104 ; and a processing time of 30 seconds. 
   Since a bowing or an undercut is likely to occur during the first step, a processing time is shortened in a controlled manner such that during the processing time a surface of the upper layer resist  156  is oxidized to form a silicon oxide film and further an appropriate undercut is formed at the lower layer resist  154 . Since the formation of the undercut proportionally varies with an amount of processing time, the amount of undercutting can be controlled by adjusting the processing time and the like. In addition, the upper layer resist  156  is prevented from being damaged during the first and the second step by adjusting a high frequency power, a temperature condition of the mounting surface, and a CO/O 2  mixing ratio which increases a ratio of etching rates (hereinafter, referred to as etching selectivity) of the upper layer resist  156  and the lower layer resist  154 . The undercut mentioned above refers to a horizontal etching of the lower layer resist  154  near an interface of the lower layer resist  154  and the upper layer resist  154 . The bowing refers to an etching of the lower layer resist  154  in a shape of a beer keg. 
   The second step is carried out by using a gaseous mixture of a nitrogen gas and a hydrogen gas. By using such gaseous mixture, a highly anisotropic etching can be performed, but an etched width tends to be slightly narrower than that of the pattern. Moreover, as described in the description of the prior art, in using such gaseous mixture, the etching selectivity between the upper layer resist  156  and the lower layer resist  154  tends to be poor, and the upper layer resist  156  gets damaged. 
   However, in accordance with the dry developing method of the preferred embodiment, silicon contained in the surface of the upper layer resist  156  is oxidized in the first step, thereby improving the etching selectivity. Further, a formation of a small undercut in the first step, combined with a narrower etched width in the second step, enables an accurate etching according to a pattern of the upper layer resist  156 . 
   The following etching conditions were applied to the second step: vacuum level in the processing container  102  of 70 mT; a processing gas of a gaseous mixture of a N 2  gas and a H 2  gas each having a flow rate of 200 sccm; a temperature of the inner wall surface of the processing container  102  including the upper electrode  126  controlled at 60° C.; a temperature of the mounting surface of the lower electrode  104  controlled at 0° C.; a distance of 47 nm between the upper electrode  126  and the lower electrode  104 ; a pressure of a cooling gas on the backside at the center of the wafer centerat 7 Torr; a pressure of a cooling gas on the backside at the wafer edge at 40 Torr; a high frequency power of 1000 W (3.18 W/cm 2 ) applied to the lower electrode  104 ; and a processing time of 90 seconds. 
   In the second step, a highly anisotropic etching, yielding a width slightly narrower than that of the pattern, is performed. However, due to the formation of small undercut in the first step, the etching can be performed according to the pattern width of the upper layer resist  156 . Moreover, the etching selectivity is improved by oxidizing silicon contained in the surface of the upper layer resist  156 , which in turn enables the lower layer resist  154  to be effectively etched. 
   Table 1 shows a comparison of etching results in accordance with different dry developing methods including: the method 1 of performing a single process by using a gaseous mixture of carbon monoxide gas and oxygen gas; the method 2 of performing a single process by using a gaseous mixture of nitrogen gas and hydrogen gas; a method 3 of executing two-step process including the first and the second step. 
   
     
       
         
             
             
             
             
           
             
                 
               TABLE 1 
             
           
          
             
                 
                 
             
             
                 
               Method 1 
               Method 2 
               Method 3 
             
          
         
         
             
             
             
             
             
          
             
                 
               CO + O 2   
                N 2  + H 2   
               First step 
               Second step 
             
             
                 
                 
             
          
         
         
             
             
             
             
             
          
             
               Lower layer 
               267 
               532 
               460 
               515 
             
             
               E/R (nm/min.) 
             
             
               Upper layer 
               5 
               137 
               25 
               75 
             
             
               E/R (nm/min.) 
             
             
               Selectivity 
               53.5 
               3.9 
               18.4 
               6.9 
             
          
         
         
             
             
             
             
          
             
               CD bias (nm) 
               50 
               −75 
               −32 
             
             
                 
             
          
         
       
     
   
   Herein, E/R indicates an etching rate in nanometer/minute. Since corner portions of the upper layer resist  156  are etched more relative to other portions, the upper layer E/R represents the amount of thickness T 1  (illustrated in  FIG. 3 ) etched per unit time. The selectivity is a ratio of the upper layer E/R and the lower layer E/R. All of these are obtained in a direction of a short axis of an etched pattern (an oval shaped hole) of a wafer center portion. The CD bias, which is a value obtained by subtracting a pattern width (R 1  of  FIG. 2 ) of the upper layer resist before the etching from that (R 2  of  FIG. 3 ) of a lower portion of the lower layer resist  154  after the etching, represents a change in pattern width. In the present invention, the CD bias is −32 nm, representing a slight reduction in line-width. 
   The etching conditions of the method 1 are as follows: first, in order to ignite a plasma, the following process were performed. That is, a vacuum level in the processing container  102  was established at 30 mT. A processing gas used was a gaseous mixture of CO gas and O 2  gas having a respective flow rate of 60 sccm. A temperature of the inner wall surface of the processing container  102  including the upper electrode  126  was controlled at 60° C. and a temperature of the mounting surface of the lower electrode  104  was controlled at −20° C. A high frequency power of 300 W (0.96 W/cm 2 ) was applied to the lower electrode  104  and a processing time was 3 seconds. 
   Thereafter, the etching was performed under the following conditions: a vacuum level in the processing container  102  of 15 mT; a processing gas having a gaseous mixture of CO gas and O 2  gas having a respective flow rate of 60 sccm; a temperature of the inner wall surface of the processing container  102  including the upper electrode  126  controlled at 60° C.; a temperature of the mounting surface of the lower electrode  104  controlled at −20° C.; a distance of 27 mm between the upper electrode  126  and the lower electrode  104 ; a high frequency power of 120 W (0.38 W/cm 2 ) applied to the lower electrode  104 ; and a processing time of 273 seconds. A rather large selectivity of 53.5 and CD bias of 50 nm can be observed as well as a large undercut. 
   The following conditions were applied to the etching in accordance with the method 2: a vacuum level in the processing container  102  of 70 mT; a processing gas a gaseous mixture of N 2  gas and H 2  gas having a respective flow rate of 200 sccm ; a temperature of the inner wall surface of the processing container  102  including the upper electrode  126  controlled at 60° C.; a temperature of the mounting surface of the lower electrode  104  controlled at −20° C.; a distance of 47 nm between the upper electrode  126  and the lower electrode  104 ; a high frequency power of 1000 W applied to the lower electrode  104 ; and a processing time of 210 seconds. A rather small selectivity of 3.9 and the CD bias of −75 nm can be observed as well as a smaller etched pattern width. 
   The first step of the method 3 has main purposes of oxidizing silicon contained in a silicon-containing upper layer resist and generating a proper undercut, and can be performed in a short period of time. Since the silicon contained in a surface of the upper layer resist  156  is oxidized in the first step of the method 3, a SiO 2  film formed at the surface of the upper layer resist  156  serves as a protective layer and, therefore, the etching selectivity of the second step, i.e., 6.9, is improved in comparison with that of the method 2, i.e., 3.9. Furthermore, due to a formation of an appropriate amount of undercut in the first step of the method 3, combined with a narrower etched pattern width in the second step, etching can be performed according to the pattern width of the resist. 
   Hereinafter, processing conditions of the method 1 and those of the first step of the method 3 will be examined in detail. Table 2 illustrates an etching result obtained by changing a flow rate ratio between the carbon monoxide gas and the oxygen gas. After performing a plasma ignition process under the same conditions as in the method 1, the etching was carried out under the following conditions: a temperature of the inner wall surface of the processing container  102  including the upper electrode  126  controlled at 60° C.; a temperature of the mounting surface of the lower electrode  104  controlled at −20° C.; a distance of 27 mm between the upper electrode  126  to the lower electrode  104 ; a high frequency power of 120 W (0.38 W/cm 2 ) applied to the lower electrode  104 ; and a processing time of 273 seconds, while a gaseous mixture of CO gas and O 2  gas having varying flow rates of 60/60, 80/40 and 100/20 sccm/sccm was used as a processing gas under a vacuum level of 15 mT in the processing container  102 . 
   
     
       
         
             
             
           
             
                 
               TABLE 2 
             
           
          
             
                 
                 
             
             
                 
               CO/O 2  flow ratio 
             
             
                 
               (sccm/sccm) 
             
          
         
         
             
             
             
             
          
             
                 
               60/60 
               80/40 
               100/20 
             
             
                 
                 
             
          
         
         
             
             
             
             
             
          
             
                 
               Lower layer E/R 
               267 
               222 
               175 
             
             
                 
               (nm/min.) 
             
             
                 
               Upper layer E/R 
               5 
               7 
               12 
             
             
                 
               (nm/min.) 
             
             
                 
               Selectivity 
               53.5 
               30.0 
               14.0 
             
             
                 
                 
             
          
         
       
     
   
   As shown in Table 2, all the etching selectivities are greater than or equal to 10 under the above conditions. The conditions of the above flow rate ratios of the gaseous mixture can be applied to the method 1 and the first step of the method 3. However, as the CO/O 2  flow rate ratio increases, the selectivity and the etching rate are decreased. Thus, it is preferable to set the CO/O 2  flow rate ratio to be smaller than or equal to 100/20, i.e., 5. Meanwhile, as the CO/O 2  flow rate ratio decreases, the selectivity and the etching rate increase, while an undercut tends to be larger. Accordingly, the CO/O 2  flow rate ratio is preferably greater than or equal to 20/100, i.e., 0.2 and, more preferably, 60/60, i.e., 1.0. 
   In the following, the conditions for a high frequency power will be examined. After performing the plasma ignition process under the same conditions as in the method 1, the etching was carried out under the following conditions: a processing gas of a gaseous mixture of CO gas and O 2  gas of 60/60 (sccm/sccm) under a vacuum level of 15 mT in the processing container  102 ; a temperature of the inner wall surface of the processing container  102  including the upper electrode  126  controlled at 60° C.; a temperature of the mounting surface of the lower electrode  104  controlled at −20° C.; and a distance of 27 mm between the upper electrode  126  and the lower electrode  104 . Under such conditions, a high frequency powers of 120 W, 240 W and 360 W were applied to the lower electrode  104 , and by taking into account the etching rate a processing time was chosen to yield an equal amount of etching (over-etching rate). Results obtained by varying the applied high frequency power is given in Table 3. 
   
     
       
         
             
             
           
             
                 
               TABLE 3 
             
           
          
             
                 
                 
             
             
                 
               High frequency 
             
             
                 
               power (W) 
             
          
         
         
             
             
             
             
          
             
                 
               120 
               240 
               360 
             
             
                 
                 
             
          
         
         
             
             
             
             
             
          
             
                 
               Lower layer E/R 
               267 
               480 
               625 
             
             
                 
               (nm/min.) 
             
             
                 
               Upper layer E/R 
               5 
               25 
               32 
             
             
                 
               (nm/min.) 
             
             
                 
               Selectivity 
               53.5 
               19.2 
               19.2 
             
             
                 
                 
             
          
         
       
     
   
   As shown in Table 3, all of the etching selectivities are greater than or equal to 10 under the above conditions. The above conditions of the high frequency power can be applied to the method 1 and the first step of the method 3. As the high frequency power increases, a drawing energy of ion increases, and as a result an increase in etching rate can be seen, as well as well as an increase in anisotropy of an etching shape, however with that the selectivity tends to decline. Accordingly, the high frequency power is preferably chosen to be smaller than or equal to about 1000 W (3.18 W/cm 2 ) and, more preferably about 400 W (1.27 W/cm 2 ) As the high frequency power decreases, the etching rate declines, while the selectivity increases, however, with that the anisotropy is slightly reduced and an undercut tends to be larger, and thus the high frequency power is preferably chosen to be greater than or equal to about 100 W (0.32 W/cm 2 ). 
   Hereinafter, a various temperature conditions of the mounting surface of the lower electrode  104  will be examined. After performing the plasma ignition process under the same conditions as the method 1, the etching was carried out under the following conditions: a processing gas having a gaseous mixture of CO gas and O 2  gas having a flow rate ratio of 80/40 (sccm/sccm) under a vacuum level of 15 mT in the processing container  102 ; a distance of 27 mm between the upper electrode  126  and the lower electrode  104 ; a high frequency power of 120 W applied to the lower electrode  104 ; and by taking into account an etching rate a processing time was chosen to yield an equal amount of etching (over etching rate). Under such conditions, a temperature of the inner wall surface of the processing container  102  including the upper electrode  126  was controlled at 60° C.; and a temperature of the mounting surface of the lower electrode  104  was controlled at −20 and 0° C., respectively. Table 4 illustrates a result obtained by treating the temperature of the mounting surface of the lower electrode  104  as a variable. 
   
     
       
         
             
             
             
           
             
                 
               TABLE 4 
             
           
          
             
                 
                 
             
             
                 
               Temperature 
                 
             
             
                 
               of lower 
             
             
                 
               electrode 
             
             
                 
               (° C.) 
             
          
         
         
             
             
             
          
             
                 
               0 
               −20 
             
             
                 
                 
             
          
         
         
             
             
             
             
          
             
                 
               Lower layer E/R (nm/min.) 
               205 
               230 
             
             
                 
               Upper layer E/R (nm/min.) 
               12 
               10 
             
             
                 
               Selectivity 
               16.4 
               23.0 
             
             
                 
                 
             
          
         
       
     
   
   As shown in Table 4, all of the etching selectivities are greater than or equal to 10 under the above conditions. The conditions of the temperature of the lower electrode can be applied to the method 1 and the first step of the method 3. However, an increase in the temperature of the mounting surface of the lower electrode reduces a deposition, which causes a reduction in selectivity. And also, the anisotropy is slightly deteriorated, so that the undercut tends to be larger. Accordingly, it is preferable that the temperature of the mounting surface of the lower electrode ranges from about −30° C. to 20° C. and, more preferably smaller than or equal to about 0° C. 
   Based on the above results, an appropriate undercut is formed near a top portion of the lower layer resist  154  in the method 1 and the first step of the method 3, and the etching can be performed according to the pattern of the upper layer resist  156  in the second step of the method 3 under the following preferable conditions: a flow rate ratio of carbon monoxide gas and oxygen gas ranging from about 0.2 to 5; a high frequency power density applied to the lower electrode ranging from about 120 W (0.32 W/cm 2 ) to 1000 W (3.18 W/cm2); and a temperature of the susceptor, for mounting thereon the processing object, of the lower electrode to which the high frequency power is applied ranging from about −30° C. to 20° C. 
   In accordance with the dry developing method of the present invention, such method provides a satisfactory etching efficiency while maintaining a vertical sidewall of a pattern formed on a resist. Further, in the present invention, a trimming for controlling a lower pattern width (R 2  in  FIG. 3 ) of the lower layer resist  154 , which is obtained after the second step, can be performed by changing the CO/O 2  flow rate ratio, the high frequency power applied to the lower electrode, the temperature of the lower electrode and the processing time of the first step of the method 3. 
   While the preferred embodiments of the dry developing method in accordance with the invention have been shown and described with reference to the accompanying drawings, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention. 
   Etching conditions and a thickness of a resist film, for example, vary depending on devices and, therefore, they are not limited to the aforementioned examples. Such conditions should be determined depending on devices so as to obtain the same effects. 
   INDUSTRIAL APPLICABILITY 
   The present invention is applied to a dry developing method for etching a lower layer resist of a double-layer resist with a high degree of accuracy without destroying an upper layer resist while maintaining a vertical sidewall of a pattern being etched.