Abstract:
A semiconductor device manufacture method includes the steps of forming a resist layer above a work target layer; exposing and developing the resist layer to form resist patterns including isolated pattern and dense patterns; monitoring widths of isolated and dense pattern of the resist patterns to determine trimming amounts of linewidths to be reduced; determining etching conditions for realizing the trimming amounts of both the isolated and dense patterns, the etching conditions using mixed gas of a gas having a function of mainly enhancing etching and a gas having a function of mainly suppressing etching; trimming the resist pattern under said determined etching conditions; and etching the work target layer by using said trimmed resist patterns. A desired pattern width an be realized stably by trimming using plasma etching.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based on and claims priority of Japanese Patent Applications No. 2003-194380 filed on Jul. 9, 2003 and No. 2004-158165 filed on May 27, 2004, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     A) Field of the Invention 
     The present invention relates to a semiconductor device manufacture method and an etching system, and more particularly to a semiconductor device manufacture method including a process of trimming fine resist pattern such as a pattern used for patterning gate electrodes, and an etching system suitable for embodying this method. 
     B) Description of the Related Art 
     The operation speed of a MOS transistor is greatly influenced by a gate length. The shorter the gate length, the high speed operation of the transistor becomes easier. As the gate length is shortened by scaling, the gate length of 100 nm or shorter has been adopted. Such a short gate length is in excess of a photography resolution in some cases. If a gate electrode is etched by using a resist pattern itself, a gate electrode having a longer gate length than a desired size is formed. 
     A method of trimming a gate electrode once formed has been proposed by which after a gate electrode of polysilicon is formed, an oxide film is grown on side walls of the gate electrode through oxidation and then the oxide films are selectively etched to obtain a desired gate length (e.g., refer to Japanese Patent Laid-open Publication No. 2003-31557). With this method, a controlled oxidation process and a selective etching process are required to be added. 
     If the underlying layer of a resist layer including a layer to be etched has a high reflection factor such as polysilicon, a standing wave or halation may occur in the resist layer due to reflected light. In order to eliminate a standing wave and halation, a bottom anti-reflection coating (BARC) is formed under the resist layer. As BARC, an organic compound layer is widely used which has a similar composition to that of the resist layer although it has no photosensitivity. 
     After a BARC layer and a resist layer are formed, the resist layer is exposed and developed to form resist pattern, and then by using the resist pattern as a mask, the BARC layer is etched. If the BARC layer has a similar composition to that of the resist layer, the resist pattern is etched while the BARC layer is etched so that the size of the resist pattern is reduced. 
     It is said that thinning of a resist pattern can be suppressed by providing adhesion to side walls of the resist pattern by using mixed gas of oxygen gas and gas having a deposition nature such as freon. However, adhesion given to the side walls of a resist pattern is dependent upon the density of resist patterns. It is also said that there occurs a size difference between resist patterns after etching, depending upon the density of resist patterns. 
     A method of reducing a size difference depending on the density of resist patterns has been proposed (e.g., refer to Japanese Patent Laid-open Publication No. 2000-77386). According to this method, first main etching is executed by using mixed gas of chlorine and oxygen to minimize a conversion difference due to the pattern density, and next over etching is executed by using only halogen hydrocarbon gas, to sputter resist of the mask in an area where the patterns are dense to deposit it on side walls so that the pattern size is made thick so that a conversion difference between the resist pattern width and the anti-reflection film width is removed. 
     The phenomenon that a resist pattern width is reduced during patterning a BARC layer can be used for trimming a resist pattern. If a resist pattern for gate electrode patterning is to be trimmed, after a resist pattern is formed on the gate electrode layer, the width of the resist pattern is trimmed and reduced by oxygen plasma or the like, and the gate electrode layer is etched by using a narrowed resist pattern as a mask. Also in this case, the problem is the density difference between final resist pattern widths. 
     On the assumption that a trimming amount is dependent upon the density difference, it is possible to correct a reticle pattern for exposing resist, in order to make uniform the fine resist pattern widths. However, in the etching process, there exists a long term variation that etching products deposited on the wall of an etching system are released in a later etching process, and this long term variation changes the pattern widths due to a density difference. If this variation is to be dealt with reticle correction, the number of reticles becomes enormous, which is not practical. The pattern width of a resist layer obtained by an exposure/development process may be shifted from an expected pattern width. In this case, reticle correction provides no significant effects. 
     SUMMARY OF THE INVENTION 
     An object of this invention is to provide a semiconductor device manufacture method capable of stably realizing a target pattern width through trimming using plasma etching. 
     Another object of this invention is to provide a semiconductor device manufacture method capable of stably realizing a target pattern width through trimming using plasma etching, independently from the pattern density. 
     Another object of this invention is to provide an etching system suitable for the semiconductor device manufacture method. 
     According to one aspect of the present invention, there is provided a semiconductor device manufacture method includes the steps of forming a resist layer above a work target layer; exposing and developing the resist layer to form a resist pattern including isolated patterns and dense patterns; monitoring widths of each isolated pattern and each dense pattern of the resist pattern to determine trimming amounts of widths to be reduced; determining etching conditions realizing the trimming amounts of both the isolated and dense patterns, the etching conditions using a mixed gas of a gas having a function of mainly enhancing etching and a gas having a function of mainly suppressing etching; trimming the resist pattern under said determined etching conditions; and etching the work target layer by using said trimmed resist pattern. 
     According to another aspect of the present invention, there is provided an etching system comprising: a measuring apparatus capable of measuring a pattern width; an etching apparatus capable of trimming a resist pattern by using a mixed gas of a gas having a function of mainly enhancing etching and a gas having a function of mainly suppressing etching, and thereafter etching a work target layer under the resist pattern; and a controller for storing data representative of a relation of an isolated pattern width and a dense pattern width of the resist pattern relative to an isolated pattern width and a dense pattern width of the work target layer after etching, the relation depending upon etching conditions of the etching using the mixed gas, and etching conditions capable of realizing desired trimming amounts of the isolated pattern width and the dense pattern width of a given resist pattern, wherein the measuring apparatus has a feed forward series of measuring the pattern width of the resist pattern and transfer the measurement results to the controller, and the controller determines the etching conditions in accordance with the transferred resist pattern width and controls the etching apparatus. 
     The semiconductor manufacture method including an etching process capable of realizing a desired etching, amount even if there are dense and coarse pattern distributions. 
     Even if a resist pattern formed by photolithography has a pattern width shifted from an expected value, a desired pattern width can be realized eventually. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A to 1E  are plan views and cross sectional views illustrating a process of manufacturing an experimental sample. 
         FIGS. 2A and 2B  are graphs showing experiment results. 
         FIGS. 3A and 3B  are distribution diagrams showing the experiment results. 
         FIG. 4  is a graph showing a control equation of a mode II. 
         FIG. 5  is a block diagram showing the structure of an etching system. 
         FIG. 6  is a block diagram showing an example of an etching apparatus. 
         FIG. 7  is a graph illustrating another control method. 
         FIGS. 8A and 8B  are a table and a graph showing the experiment results of a change in a gate length reduction amount when an inert gas dilution ratio is changed. 
         FIG. 9  is a table showing the contents of samples. 
         FIG. 10  is a graph showing the experiment results of a change in a gate length reduction amount when an SO 2  gas-flow-rate fraction is changed for isolated patterns. 
         FIG. 11  is a graph showing the experiment results of a change in a gate length reduction amount when the SO 2  gas-flow-rate fraction is changed for dense patterns. 
         FIG. 12  is a graph showing the experiment results of a change in a gate length reduction amount when an He dilution ratio is changed for isolated patterns. 
         FIG. 13  is a graph showing the experiment results of a change in a gate length reduction amount when an He dilution ratio is changed for dense patterns.  FIGS. 14A to 14D  are tables, a graph and equations illustrating the procedure of transforming the experiment results shown in  FIGS. 10 to 13  into the equations. 
         FIGS. 15A to 15C  are solid graphs and a table showing the relation of a gate length reduction amount of isolated patterns relative to an SO 2  gas-flow-rate fraction X and an He dilution ratio Z. 
         FIGS. 16A to 16C  are solid graphs and a table showing the relation of a gate length reduction amount of dense patterns relative to the SO 2  gas-flow-rate fraction X and He dilution ratio Z. 
         FIGS. 17A and 17B  are a solid graph and a table showing the relation of a gate length reduction amount difference between isolated and dense patterns and the SO 2  gas-flow-rate fraction X and He dilution ratio Z. 
         FIG. 18  is a table showing an example of a recipe for obtaining the SO 2  gas-flow-rate fraction X and He dilution ratio Z from the gate length reduction amount difference between isolated and dense patterns and the gate length reduction amount of dense patterns. 
         FIG. 19  is a block diagram showing the structure of another etching system. 
         FIGS. 20A to 20C  are graphs showing the experiment results of a change in a gate length reduction amount when an over etch time is changed. 
         FIGS. 21A to 21C  are graphs showing the experiment results of a change in a gate length reduction amount when a total gas flow rate is changed. 
         FIG. 22  is a flow chart illustrating an etching process using a pilot wafer. 
         FIGS. 23A and 23B  are graphs showing the experiment results of the etching processes with and without a pilot wafer. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present inventor has studied a method of forming a gate electrode having a gate length narrower than a scale limit of photolithography. If a resist pattern is trimmed by plasma etching, an etching rate changes with whether the density of patterns is dense or coarse, and gate lengths in a wafer become irregular. 
     If a resist pattern is formed on a BARC layer, the BARC layer is etched by using the resist pattern as a mask. Etching the resist pattern while the BARC layer is etched can be utilized for trimming. However, if the resist pattern is etched by plasma etching, the etching rate changes with the pattern density and resultant pattern widths are often irregular due to the influence of the pattern density. 
     As a method of correcting a difference among etching amounts to be caused by the pattern density, it is known that a mixed gas of a plurality of gasses having different etching characteristics is used. The present inventor has checked how a pattern width changes with the etching conditions of etching the BARC layer by using as an etching gas a mixed gas of a gas for mainly enhancing etching and a gas for mainly suppressing etching. 
       FIGS. 1A to 1E  are plan views and cross sectional views illustrating a sample used for experiments. 
       FIG. 1A  is a plan view showing an isolated pattern. The isolated pattern ISO is a single resist pattern isolated in a broad area. 
       FIG. 1B  shows an example of the layout of dense patterns. The dense patterns DNS has the layout of a plurality of patterns disposed densely. 
       FIGS. 1C ,  1 D and  1 E are cross sectional views illustrating the manufacture processes for a sample. As shown in  FIG. 1C , a gate oxide film  12  of about 1 to 2 nm is formed on a silicon substrate  11 . A gate electrode layer  13  of polysilicon having a thickness of 80 to 150 nm is formed on the gate oxide film. The polysilicon film  13  is, for example, 100 nm thick. A hard mask layer  14  of silicon oxide having a thickness of 10 to 70 nm is formed on the polysilicon film  13 , the hard mask layer functioning as a hard mask when the polysilicon film  13  is etched. The hard mask layer  14  is, for example, 30 nm thick. The polysilicon layer  13  and hard mask layer  14  are target layers to be worked. 
     On the hard mask layer  14 , a BARC layer  18  is coated to a thickness of 30 to 80 nm. Although the BARC layer  18  has no photosensitivity, it has the composition almost equivalent to resist. A resist layer of 200 to 300 nm in thickness is formed on the BARC layer  18  and exposed and developed to form a resist pattern  19  which is disposed in the layouts shown in  FIGS. 1A and 1B . A width of the resist pattern  19  is represented by d 1 . 
     By using the resist pattern  19  as a mask, the underlying BARC layer  18  is etched. After the wafer to be etched is transported into an etching chamber, preparatory He gas plasma is ignited. The preparatory plasma trims by about 5 nm. Thereafter, main plasma etching of He/SO 2 /O 2  is executed. 
       FIG. 1D  shows the state that the BARC layer was etched by main etching and over etching. The resist pattern  19  reduces its height and width by etching while the BARC layer  18  is etched. The “main etching” is generally the etching until the surface of an underlying layer is exposed, and the “over etching” is the etching to be executed thereafter. Even if some underlying surfaces are exposed by the main etching, this does not mean that the predetermined whole area was completely etched. The over etching is therefore necessary. 
     The resist pattern  19  and underlying BARC layer pattern  18  are an etching mask for the work target layers. In this state, the hard mask layer  14  is etched to transfer the mask pattern. The polysilicon layer  13  under the hard mask layer  14  is etched. Even if the resist pattern  19  and BARC pattern  18  are extinguished, the hard mask layer  14  can function as the etching mask.  FIG. 1E  shows the state after the polysilicon layer  13  is etched. A width of a patterned polysilicon layer is represented by d 2 . The width d 1  of the resist pattern  19  was trimmed to the width d 2  of the polysilicon layer. 
     In the etching process for the BARC layer  18  shown in  FIG. 1D , a mixed gas which contained O 2  gas and SO 2  gas was used as the etching gas. It was checked how the pattern width d 2  of the finally obtained polysilicon layer pattern  13  was changed by adjusting the gas-flow-rate fraction (mixture ratio) of the mixed etching gas and an over etching time. 
       FIG. 2A  is a graph obtained when a mixed gas of He/SO 2 /O 2 =60/06/24 was used as the etching gas. The abscissa represents a ratio of an over etching time to a main etching time in the unit of %. The ordinate represents a gate length (pattern width) reduction amount in the unit of nm. 
     A total plasma-on time after the ignition of plasma was 337.5 hours. Radiation of a CO bond emitted during etching the BARC layer was monitored and the time when the monitored radiation was reduced to 90% was considered as the completion of the main etching. The main etching was completed in 16.63 seconds. 
     It was checked how the pattern width changed with the over etching. As shown in  FIG. 2A , at the start of the over etching, the width of the isolated pattern ISO is much narrower than the width of the dense pattern DNS. As the over etching progresses, a width difference is gradually reduced, the widths become equal and then an inverse width difference appears. 
     It is therefore possible to make the pattern width (gate length) reduction amounts of the isolated pattern and dense pattern have the same value, by selecting the over etching time or the ratio of the over etching time to the main etching time. The same pattern width reduction amount (trimming amount) was about 46 nm. 
       FIG. 2B  is a graph obtained when a mixed gas of He/SO 2 /O 2 =60/08/22 was used as the etching gas. The main etching ME was terminated in 17.76 seconds. The total plasma-on time is 353.2 hours, which is not so different from the plasma-on time of 337.5 hours so that age deterioration in the plasma chamber can be almost neglected. 
     At the start of the over etching, a pattern length reduction amount difference between the isolated pattern ISO and dense pattern DNS is larger than that shown in  FIG. 2A . Similar to  FIG. 2A , as the over etching progresses, the difference reduces gradually, the reduction amounts become equal, and then an inverse length difference appears. The pattern length reduction amount when both the isolated and dense patterns become coincident was about 43 nm. 
     As above, while the main etching and over etching are performed, the peripheral status changes with the main etching and over etching. Therefore, a pattern length reduction amount difference between the isolated and dense patterns takes a positive value at the start of the over etching, and the positive value becomes small as the over etching progresses and inverted to a negative value. During this etching, there is the condition that both the pattern length reduction amounts are made equal. 
     If the developed resist pattern widths have the same value for the isolated pattern and dense pattern, the isolated pattern and dense pattern having the same length can be finally formed by selecting the over etching time. This method is particularly suitable for working gate patterns of a mixed logic circuit including an SRAM area. It is expected that the pattern length reduction amount can be determined by selecting a mixture ratio of a mixed gas. 
     If the developed resist pattern widths have different values for the isolated and dense patterns, it is desired that the isolated pattern and dense pattern have different pattern length reduction amounts in order to finally form the patterns of the same length. 
     Each pattern width (gate length) reduction amount and a pattern width reduction amount difference between the isolated pattern and dense pattern were measured by using as parameters a time period of the over etching and the gas-flow-rate fraction of O 2  to O 2 +SO 2  of the mixed gas. 
       FIG. 3A  is a graph showing the experiment results of checking how the gate length reduction amount of a dense pattern changes with a change in the over etching time and the O 2  gas-flow-rate fraction. In  FIG. 3A , an upper left area L 1  has the gate length reduction amount in the range of 20 nm to about 25 nm. A lower right area L 8  has the gate length reduction amount in the range of about 58 nm to 60 nm. Areas L 2 , L 3 , L 4 , L 5 , L 6  and L 7  are stripe areas continuously distributed between the areas L 1  and L 8  and have the gate length reduction amounts gradually increasing their values. 
     As apparent from  FIG. 3A , since each area has a spread of the over etching time and gas-flow-rate fraction, the same gate length reduction amount can be obtained by selecting the same area. 
     Similar to  FIG. 3A ,  FIG. 3B  is a graph showing the experiment results of checking the gate length reduction amount difference between the isolated pattern and dense pattern by using as parameters the over etching time and the O 2  gas-flow-rate fraction. In a right side area D 1 , the gate length reduction amount of the dense pattern is larger than that of the isolated pattern by 3 nm to about 1.5 nm. 
     In an area D 2  surrounding the area D 1 , the gate length reduction amount of the dense pattern is larger than that of the isolated pattern by about 1.5 nm to about 0.3 nm. 
     An area D 3  distributed broadly in a central area covers the area starting from that the gate length reduction amount difference is almost 0 to that the gate length reduction amount of the isolated pattern is larger than that of the dense pattern. In areas D 4 , D 5 , D 6 , D 7  and D 8 , the gate length reduction amount of the isolated pattern gradually becomes larger than that of the dense pattern. The area D 8  partially shown on the left side has the gate length reduction amount difference of near 9 nm. 
     An adjustment method of a mode I will be described in which the isolated and dense patterns have different resist pattern widths. Consider for example that polysilicon patterns having the same width of 47 nm are formed under the conditions that the isolated pattern has a resist pattern width of 84 nm and the dense pattern has a resist pattern width of 80 nm. A width to be trimmed is 37 nm for the isolated pattern and 33 nm for the dense pattern. A difference therebetween is 4 nm. 
     In  FIG. 3(B) , the area where the gate length reduction amount of the isolated pattern is larger than that of the dense pattern by 4 nm is approximately the border area between the areas D 5  and D 6 . By using the over etching time and O 2  gas-flow-rate fraction in this border area, it becomes possible to obtain the gate length reduction amount of the isolated pattern larger than that of the dense pattern by about 4 nm. 
     In  FIG. 3A , a central area of the area L 3  slightly shifted to the area L 4  has the gate length reduction amount of 33 nm for the dense pattern. By using the over etching time and O 2  gas-flow-rate fraction in this area, it becomes possible to obtain the gate length reduction amount of the desired 33 nm for the dense pattern. 
     An area is then determined, superposed on which area are the area in  FIG. 3A  satisfying the desired gate length reduction amount of the dense pattern and the area in  FIG. 3B  where the desired value of the gate length reduction amount difference between the isolated and dense patterns can be obtained. In this superposed area, both the isolated and dense patterns can obtain the desired gate length reduction amounts. If the superposed area includes a lot of suitable conditions, for example the shorter etching time is selected from the viewpoint of a production efficiency. 
     The desired etching conditions can be determined by changing the two parameters, the over etching time and the gas-flow-rate fraction. 
     Next, an adjustment method of a mode II will be described in which the isolated and dense patterns have the same resist pattern width. In  FIG. 3B , a central area of the area D slightly shifted to the area D 2  has the gate length reduction amount difference of almost 0 between the isolated and dense patterns. The etching under this condition can obtain the same gate reduction amount for both the isolated and dense patterns. 
     Namely, the resist pattern having the same width is formed for the isolated and dense patterns, and the condition that the isolated and dense patterns have the same gate length reduction amount is used so that the desired gate length can be formed. 
     In  FIG. 3B , the condition indicated by a broken line is selected which sets the over etching time to 8.8 seconds. In this case, approximately the same gate length reduction amount can be obtained for the isolated and dense patterns. 
       FIG. 4  is a graph showing the experiment results in which experiments were conducted by changing the O 2  gas-flow-rate fraction under the above-described conditions, a straight line fitting plots was drawn, and an optimum recipe was obtained which indicates the relation between a desired gate length reduction amount y and an O 2  gas-flow-rate fraction X. In  FIG. 4 , plots indicated by a square show the experiment results and the straight line most properly approximating these plots is obtained. This straight line is given by:
   y= 0.5219  X+ 4.153 
where X is the O 2  gas-flow-rate fraction and y is the gate length reduction amount.
 
     Several gate length reduction amounts were selected, and the etching was performed at the corresponding O 2  gas-flow-rate fraction and the lengths of the obtained gates were measured. Plots indicated by a circle are these measurement results. The measurement results of samples 1 to 5 are very near to the straight line y, which indicates that this approximation is proper. Under the above-described conditions, polysilicon patterns can be obtained at an error of 1 nm or smaller at least in the gate length reduction amount range of 21 to 46 nm. 
     The adjustment method of the mode II has a fixed over etching time and selects only the O 2  gas-flow-rate fraction so that the control of this method is easy. If the isolated and dense patterns have different developed resist patterns, the mode II cannot be used and the mode I is used for the adjustment. 
       FIG. 5  is a diagram showing an etching system based upon the knowing acquired from the above-described experiments. A measuring apparatus  1  is, for example, a scanning electron microscope (SEM) for measuring a micro size. The measuring apparatus  1  measures a resist pattern width and feeds forward the measurement results to a controller  5 . An etching apparatus  2  etches a BARC layer under a resist pattern, a hard mask layer under the BARC layer, a polysilicon layer under the hard mask layer. The etching apparatus  2  can change the gas-flow-rate fraction of a mixed gas and adjust the etching time. 
       FIG. 6  shows an example of the etching apparatus. Gas flow paths  21 ,  22  and  23  flow He, SO 2  and O 2 , respectively and include mass flow controllers MFC 1 , MFC 2  and MFC 3 , respectively. An optional number of other flow paths (not shown) having similar structures are also provided for flowing other gasses (e.g., HBr, Cl 2 , CF 4 , etc for etching a silicon oxide film and a polysilicon film). An etching chamber  26  is made of aluminum subjected to anodic oxidation, and has a diameter of 500 mm and a height of 230 mm for example. The top of the aluminum chamber is sealed with a quartz cover  27 , and a helical coil  28  is provided on the quartz cover  27 . The helical coil  25  has a rectangular cross section having a side of 12.5 mm, and 3.5 turns with a diameter of 330 mm. 
     The coil  28  is supplied with a high frequency power at a frequency of, for example, 27 MHz from a high frequency power unit  29 . The high frequency power unit  29  has a high frequency power source HF, variable capacitors VC 1  and VC 3  and capacitors C 2  and C 4 , and can automatically perform impedance matching. 
     The paths  21 ,  22  and  23  become confluent to a path  24  which passes through a through hole  26  in the quartz cover  27  to introduce an etching gas into the etching chamber. The etching chamber is connected to a turbo molecule pump  35  and a dry pump  36  via an exhaust pipe  33  and a valve  34 . 
     A susceptor  31  is mounted on the bottom of the etching chamber  26  and places thereon a work target (wafer). The susceptor  31  is connected to a high frequency bias power unit  41 . The high frequency bias power unit  41  includes a high frequency power source LF having a relatively low frequency of 2 MHz, variable capacitors VC 5  and VC 6  and inductors L 1  and L 2 , and can automatically perform impedance matching. 
     A vacuum degree in the etching chamber  26  is monitored with a manometer  38 , and the measurement result is sent to a control circuit  40 . The control circuit  40  supplies control signals to the high frequency power units  29  and  41 , mass flow controllers MFC 1 , MFC 2  and MFC 4 , valve  34  and the like. 
     Reverting to  FIG. 5 , a measuring apparatus  3  is an SEM capable of measuring a micro size similar to the measuring apparatus  1 . The measuring apparatus  3  may be replaced by the measuring apparatus  1 . The measuring apparatus  3  measures the polysilicon pattern length after etching and feeds back the measurement results to the controller  5 . 
     A resist pattern width of each wafer in a test element group to be etched is first measured with the measuring apparatus  1 . The measuring apparatus  1  feeds forward the measurement results to the controller  5 . Upon reception of the resist pattern width from the measuring apparatus  1 , the controller  5  determines the etching conditions to realize a preset pattern length, and supplies the etching conditions to the control circuit  40  of the etching apparatus  2 . 
     In the mode I, both the etching time and gas-flow-rate fraction are used as variables and determined so as to realize a desired etching process. In the mode II, the over etching time is fixed so that the gas-flow-rate fraction is determined so as to realize a desired etching process. 
     The etching apparatus  2  performs etching under the supplied etching conditions. The measuring apparatus  3  measures the polysilicon pattern length after etching and feeds the measurement results back to the controller  5 . If the conditions of the etching apparatus change after a long term use and there is any displacement from a desired gate length, the controller  5  corrects data in accordance with the measurement results of the pattern length after etching. For the succeeding etching, the etching conditions are determined from the corrected data so that a desired result can be obtained. 
     The above-described control does not operate properly if a length difference between the isolated and dense patterns is too large. In this case, the succeeding control becomes easy if the length of one of the isolated and dense patterns can be adjusted. 
       FIG. 7  is a graph showing a change in a resist pattern length when the resist pattern is processed by He gas at 40 mtorr. Although the length reduction amount of the isolated pattern ISO changes hardly, the length reduction amount of the dense pattern DNS increases as the process time prolongs. By using a combination of the above-described adjustment and the resist pattern width adjustment for one of the isolated and dense patterns shown in  FIG. 7 , the length reduction amount difference between the isolated and dense patterns can be introduced into the easy-to-control range so that a desired adjustment becomes possible. 
     In the above-description, the O 2  gas-flow-rate fraction (=[O 2 ]/([SO 2 ]=[O 2 ] where [ ] indicates a flow rate) and the over etching time are mainly used as the control variables. The etching gas essentially consists of the O 2  gas for mainly enhancing etching, the SO 2  gas for mainly suppressing etching, and the inert He gas which is unable to chemically react with the work target. In the above-description, the He dilution ratio (=[He]/[total flow rate]) is made constant. The He dilution ratio may be used as the control variable. How the gate length reduction amount changes with the He dilution ratio was checked. 
       FIGS. 8A and 8B  are a table and a graph showing the experiment conditions and results.  FIG. 8A  shows the experiment results. Mixture gas of He, SO 2 , and O 2  was used as the etching gas, and the He dilution ratio was changed to four values, 34%, 46%, 57% and 80%. The total flow rate was made constant at 87 sccm, and the SO 2  gas-flow-rate fraction was made constant at 26%. The following description is made by using the SO 2  gas-flow-rate fraction instead of the O 2  gas-flow-rate fraction. The pressure in the plasma processing chamber is 5 mtorr, the plasma generation high frequency power is 300 W and the bias voltage is 100 Vpp. The gate length reduction amounts of the isolated and dense patterns and the electron temperature were measured. The electron temperature was estimated from an intensity ratio between an optical emission intensity at the wavelength of 706 nm of an He atom and that at the wavelength of 728 nm. 
       FIG. 8B  is the graph showing the experiment results. The abscissa represents an He dilution ratio in the unit of % and the ordinate represents a gate length reduction amount difference between the isolated and dense patterns in the unit of nm. The electron temperature Te is represented by a relative value of the He optical emission intensity I (706 nm) at the wavelength of 706 nm relative to the He optical emission intensity I (728 nm) at the wavelength of 728 nm. It can be seen that as the He dilution ratio increases, the gate length reduction amount difference between the isolated and dense patterns increases almost linearly. This indicates the possibility that the He dilution ratio can be used as a suitable variable for etching control. It is expected that the electron temperature can be used instead of the He dilution ratio. For example, a similar change can be expected by a pressure change. 
       FIG. 3A  indicates that the trimming amount can be controlled by the function of the O 2  gas-flow-rate fraction and over etching time. A change in the trimming amount shows a rather complicated mode relative to the over etching time. It has been found that the trimming amount by etching can be controlled by changing the dilution ratio of inert gas such as He and Ne. This has been studied more in detail. 
       FIG. 9  is a table schematically showing the structure of samples under experiments. The He dilution ratio was changed to five values, 50, 60, 70, 80 and 90 (%) and the SO 2  gas-flow-rate fraction was changed to five values, 15.47, 20.62, 25.32, 29.72 and 34.91 (%). As shown in the table, these twenty five samples are represented by S 11 , . . . , S 15 , . . . , S 51 , . . . , S 55 , and sample groups with a constant dilution ratio and different SO 2  gas-flow-rate fraction are represented by VF 1 , VF 2 , VF 3 , VF 4  and VF 5 . Sample groups with a constant SO 2  gas-flow-rate fraction and different He dilution ratios are represented by VD 1 , VD 2 , VD 3 , VD 4  and VD 5 . The total etching gas flow rate was fixed to 130.5 sccm for all samples and the over etching time was fixed to 50%. Each sample has isolated and dense patterns. 
       FIG. 10  is a graph showing the measurement results of checking how the gate length reduction amount y (ISO) of the isolated pattern changes with the SO 2  gas-flow-rate fraction X. The abscissa represents an SO 2  gas-flow-rate fraction in the unit of % and the ordinate represents a gate length reduction amount in the unit of nm. The measurement results of the sample group VF 1  are indicated by rhomboid plots and a straight line approximating these points is represented by vf 1 . Similarly, the measurement results Δ of the sample group VF 2  are approximated by a straight line vf 2 , the measurement results×of the sample group VF 3  are approximated by a straight line vf 3 , the measurement results+of the sample group VF 4  are approximated by a straight line vf 4 , and the measurement results−of the sample group VF 5  are approximated by a straight line vf 5 . In all the sample groups, as the SO 2  gas-flow-rate fraction X is increased, the gate length reduction amount reduces approximately linearly and its slope is generally equal. Although the characteristics vf 1  to vf 4  change almost regularly, the characteristic vf 5  saturates. In this graph, the formulas of the approximated straight lines are shown. 
       FIG. 11  is a graph showing the measurement results of the gate length reduction amount y (DNS) of the dense pattern. In all the sample groups, as the SO 2  gas-flow-rate fraction X is increased, the gate length reduction amount reduces approximately linearly. Although the characteristics vf 1  to vf 4  of the sample groups VF 1  to VF 4  change almost regularly, the characteristic vf 5  of the sample group VF 5  saturates. The slope of the reduction in the gate length reduction amount y (DNS) relative to an increase in the SO 2  gas-flow-rate fraction X is gentler than the isolated pattern and the change amount relative to a change in the dilution ratio is small. Similar to  FIG. 10 , the formulas of the approximated straight lines are shown in this graph. 
     In  FIG. 4 , the trimming amount increases generally linearly as the O 2  gas-flow-rate fraction increases. In  FIGS. 10 and 11 , a slope of the change is in an opposite direction because the SO 2  gas-flow-rate fraction is used instead of the O 2  gas-flow-rate fraction. 
       FIG. 12  is a graph showing the gate length reduction amount y (ISO) of the isolated pattern as a function of an He dilution ratio Z. The abscissa represents an He dilution ratio Z in the unit of %, and the ordinate represents a gate length reduction amount y (ISO) of the isolated pattern. The characteristics vd 1  to vd 5  correspond to sample groups VD 1  to VD 5 . It appears that the characteristics change particularly at the He dilution ratio Z=90%. Regular characteristics appear in the He dilution ratio range Z=50 to 80%, particularly in the range of 60 to 80%. 
       FIG. 13  is a graph showing a change in the gate length reduction amount y (DNS) of the dense pattern relative to a change in the He dilution ratio Z. Similar to  FIG. 12 , it appears that the characteristics change at the He dilution rate Z=90%. Regular characteristics can be observed in the He dilution ratio range Z=50 to 80%, particularly in the range of 60 to 80%. 
     As shown in  FIGS. 10 to 13 , by controlling the SO 2  gas-flow-rate fraction X and He dilution ratio Z, it can be expected that a desired gate length reduction amount can be obtained for both the isolated and dense patterns. The procedure of obtaining a desired gate length reduction amount will be described in detail hereinafter. 
       FIG. 14A  is a table showing a set of characteristics of the gate length reduction amount obtained by controlling the SO 2  gas-flow-rate fraction X. The ISO column shows a slope of the gate length reduction amount of the isolated pattern relative to the SO 2  gas-flow-rate fraction X, and the DNS column shows a slope of the gate length reduction amount of the dense pattern relative to the SO 2  gas-flow-rate fraction X. The slope value is obtained by approximating the values of the five sample groups. An average value is 0.741 for the isolated pattern and 0.623 for the dense pattern. By using this average value, all samples can be processed by the same formula. 
       FIG. 14B  is a table showing a set of slopes of the gate length reduction amount obtained by controlling the He dilution ratio. The ISO column shows a slope of the gate length reduction amount of the isolated pattern relative to the He dilution ratio, and the DNS column shows a slope of the gate length reduction amount of the dense pattern relative to the He dilution ratio. These values are also obtained by approximating the values of the five sample groups, and the average values thereof are 0.201 and 0.135. 
       FIG. 14C  shows the values of a constant term of the function of the SO 2  gas-flow-rate fraction X and He dilution ratio Z indicating the gate length reduction amount y. As an error of real data is calculated from a formula without the constant term, the minimum error value of the isolated pattern is 45.63 and that of the dense pattern is 38.10. These values can therefore be adopted as the constant terms. 
       FIG. 14D  shows the formulas indicating the gate length reduction amount y (ISO) of the isolated pattern and the gate length reduction amount y (DNS) of the dense pattern as the function of the SO 2  gas-flow-rate fraction X and He dilution ratio Z.
   y ( ISO )=−0.741  X+ 0.201  Z+ 45.63  (1)   y ( DNS )=−0.623  X+ 0.135  Z+ 38.10  (2) 
     Two quadratic equations are obtained by giving a desired isolated pattern gate length reduction amount y (ISO) and a desired dense pattern gate length reduction amount y (DNS). By solving these simultaneous equations, solutions for the variables X and Z can be obtained. 
       FIG. 15A  is a solid graph expressing stepwise the equation (1) of the gate length reduction amount y (ISO) of the isolated pattern. The gate length reduction amount reduces linearly as the SO 2  gas-flow-rate fraction increases, and increases linearly as the He dilution ratio increases. 
       FIG. 15B  is a solid graph showing an error δy (ISO) between a real gate length reduction amount and a gate length reduction amount obtained by the single equation (1): y(ISO)=−0.741 X+0.201 Z+45.63, relative to the SO 2  gas-flow-rate fraction X and He dilution ratio Z. 
       FIG. 15C  is a table showing a set of errors. 
     Similar to  FIG. 15A ,  FIG. 16A  is a solid graph showing the gate length reduction amount y (DNS) of the dense pattern as the function of the SO 2  gas-flow-rate fraction and He dilution ratio. The gate length reduction amount reduces linearly as the SO 2  gas-flow-rate fraction increases, and increases linearly as the He dilution ratio increases. A change rate is gentler than that of the isolated pattern. 
     Similar to  FIG. 15B ,  FIG. 16B  is a solid graph showing an error δy (DNS) between a real gate length reduction amount and a gate length reduction amount obtained by the single equation (2): y(DNS)=−0.623 X+0.135 Z+38.10, as the function of the SO 2  gas-flow-rate fraction X and He dilution ratio Z. 
       FIG. 16C  is a table showing a summary of errors shown in  FIG. 16B . 
     If the gate length reduction amounts of the isolated and dense patterns are each approximated by a single linear expression, how the SO 2  gas-flow-rate fraction X and He dilution ratio Z are selected can be easily determined analytically to obtain desired gate length reduction amounts. The SO 2  gas-flow-rate fraction X and He dilution ratio Z for realizing desired gate length reduction amounts can be obtained by solving the simultaneous equations shown in  FIG. 14D  using a desired isolated pattern gate length reduction amount y (ISO) and a desired dense pattern gate length reduction amount y (DNS). However, if the amounts are each approximated by a single linear expression, errors occur as shown in  FIGS. 15B and 15C  and  FIGS. 16B and 16C . After X and Z are obtained once analytically, the obtained solutions can be further corrected by considering the errors from real data. 
     There is the case that it is more convenient to use the gate length reduction amount difference between the isolated and dense patterns and one of the gate length reduction amounts, that using the gate length reduction amounts of the isolated and dense patterns. 
       FIG. 17A  is a solid graph showing stepwise a change in a gate length reduction amount difference Δy (ISO-to-DNS) between the isolated and dense patterns, obtained by subtracting a value shown in  FIG. 16A  from a value shown in  FIG. 15A , relative to the SO 2  gas-flow-rate fraction and He dilution ratio Z. Broken lines A, B and C shown in  FIG. 17A  are straight lines for the gate length reduction amount differences between the isolated and dense patterns of 10 nm, 9 nm and 8 nm, respectively. For example, the characteristic of the broken line B can be expressed by:
   Z= 1.54321  X+ 26.1265  (3) 
The equations of the relation between X and Z of the other gate length reduction amount differences between the isolated and dense patterns can be obtained similarly. As this equation is substituted into the equation (1) or (2), X or Z can be obtained. As the result is substituted in the equation (3), the remaining Z or X can be obtained.
 
       FIG. 18  is a recipe table showing the SO 2  gas-flow-rate fraction and He dilution ratios to be used when a desired gate length reduction amount of the dense pattern is obtained at the gate length reduction amount difference between the isolated and dense patterns of 9 nm. 
     In this manner, the SO 2  gas-flow-rate fraction X and He dilution ratio Z realizing a desired gate length reduction amount can be obtained by obtaining the gate length reduction amount difference between the isolated and dense patterns and using the equation of the He dilution ratio Z and SO 2  gas-flow-rate fraction at this gate length reduction amount difference and the equation of the He dilution ratio Z and SO 2  gas-flow-rate fraction for one of the isolated and dense patterns. After X and Z are obtained once, correction can be made by considering errors, similar to the above-described case. As the gas not chemically reacting with a work target layer, another inert gas such as Ne may be used in addition to He. 
       FIG. 19  is a block diagram showing the structure of an etching system using the above-described control method. Measuring apparatuses  1  and  3  and an etching apparatus  2  have similar structures to those shown in  FIG. 5 . A controller  5  obtains the SO 2  gas-flow-rate fraction X and He dilution ratio Z realizing desired gate length reduction amounts by using any of the methods described above and two of a desired isolated pattern gate length reduction amount y (ISO), a desired dense pattern gate length reduction amount y (DNS) and a desired gate length amount difference Δy between the isolated and dense patterns. In accordance with the obtained results, the controller controls the etching apparatus  2 . Other points are similar to the etching system shown in  FIG. 5 . 
     The layouts of semiconductor integrated circuit devices vary one after another depending on the product type. If the layouts are very different, a slope of a change in the gate length reduction amount relative to a change in the SO 2  gas-flow-rate fraction and He dilution ratio changes. In order to retain a desired adjustment range, it is desired in some cases to control the adjustment range by using other parameters. In addition to the SO 2  gas-flow-rate fraction (O 2  partial ratio) and inert gas dilution ratio, the over etching time and a total gas flow rate (gas-flow-rate fraction) may be used as the etching parameters. 
       FIG. 20A  is a graph showing how the gate length reduction amount y (ISO) of the isolated pattern relative to the He dilution ratio changes when the over etching time is changed. As the over etching time is prolonged, there is the tendency that the gate length reduction amount increases as a whole. The total gas flow rate TF is fixed to 216 sccm and the SO 2  gas-flow-rate fraction is fixed to 23.5%. 
       FIG. 20B  is a graph showing a change in the gate length reduction amount y (DNS) of the dense pattern relative to a change in the He dilution ratio when the over etching time is changed. As the over etching time is prolonged, there is the tendency that the gate length reduction amount increases. However, it cannot be said that this change is very regular. 
       FIG. 20C  is a graph showing a change in the gate length reduction amount difference Δy relative to a change in the He dilution ratio when the over etching time is changed. In the over etching range of 30 to 50%, as the over etching time is prolonged, there is the tendency that the gate length reduction amount difference between the isolated and dense patterns increases. However, at the over etching time of 60%, a slope of the gate length reduction amount difference relative to the He dilution ratio disappears. 
     As shown in  FIGS. 20A ,  20 B and  20 C, by changing the over etching time, the etching mode can be changed. It is however preferable that the adjustment is performed by limiting the adjustment range. If the over etching time is changed in excess of the proper adjustment range, an unexpected phenomenon may occur. 
       FIG. 21A  is a graph showing how the gate length reduction amount y (ISO) of the isolated pattern relative to the He dilution ratio changes when the total gas flow rate is changed. As the total gas flow rate is increased, there is the tendency that the gate length reduction amount y (ISO) of the isolated pattern reduces. 
       FIG. 21B  is a graph showing how the gate length reduction amount y (DNS) of the dense pattern relative to the He dilution ratio changes when the total gas flow rate increased. As the total gas flow rate is increased, there is the tendency that the gate length reduction amount y (DNS) of the dense pattern reduces. A change in the gate length reduction amount y (DNS) of the dense pattern is smaller than a change in the gate length reduction amount of the isolated pattern, and it cannot be said that the change is very regular. Particularly at the total gas flow rate of 216 sccm, the change becomes larger than at the total flow rate of 174 sccm. 
       FIG. 21C  is a graph showing how the gate length reduction amount difference Δy between the isolated and dense patterns relative to the He dilution ratio changes with a change in the total gas flow rate. As the total gas flow rate is increased, the gate length reduction amount difference between the isolated and dense patterns reduces. 
     As shown in  FIGS. 20A ,  20 B and  20 C and  FIGS. 21A ,  21 B and  21 C, at least one of the over etching time and total gas flow rate can be adopted as an auxiliary parameter to be used for changing the adjustment range or the like. 
     The etching performance of the etching apparatus has the tendency that it changes with a lapse of the etching time. As the product type of a target semiconductor integrated circuit changes, the etching characteristic also changes in some cases. Description will be made on an etching process adopting a pilot wafer for dealing with such changes. 
       FIG. 22  is a flow chart illustrating an etching method using a pilot wafer. At Step S 11 , the pattern widths of a wafer group (lot) formed with resist patterns are first measured by a measuring SEM. An optimum recipe  1  is selected which realizes desired pattern widths relative to those measured resist pattern widths. At a next Step S 12 , the lot is divided into a pilot wafer and other wafers. At a next Step S 13 , the pilot wafer is etched by the selected optimum recipe  1 . 
     At a next Step S 14 , the resist of the etched pilot wafer is ashed and washed to expose actually obtained patterns. 
     At Step S 15 , the obtained pattern width is measured by a measuring SEM. If there is a difference from a desired pattern width, it is checked how the optimum recipe is corrected in order to remove the difference, and a corrected optimum recipe  2  is obtained. At Step S 16 , the remaining wafers are etched by the corrected optimum recipe  2 . 
     At Step S 17 , the resist after the etching is ashed and the whole surfaces of the wafers are washed. At Step S 18 , the obtained pattern width is measured by a measuring SEM and the measurement results are fed back to the controller. 
     The pilot wafer is used for checking the results in advance, and if the first selected optimum recipe  1  is not proper, the optimum recipe is corrected to obtain a new optimum recipe and process the remaining wafers. 
       FIG. 23A  is a graph showing an example of the characteristics of the etching not using the pilot wafer. The abscissa represents the number of lapsed days, and A′, A″ and D′ represent the types of the layouts of semiconductor integrated circuit devices. The layouts A′ and A″ are similar and the layout D′ is very different from the layouts A′ and A″. The ordinate represents a shift of the gate length reduction amount from the model equation. The position of a broken line indicates a standard value for the layout A′. 
     It can be recognized that if the etching for the product type D′ having a very different layout is mixed, a shift of the gate length reduction amount of the layout A′ changes greatly. In the last half of the graph without the etching for D′, the disturbance is gradually relaxed and takes a constant value. In this manner, if the layout of a wafer to be processed changes, a large disturbance of the etching results may occur. The etching characteristics after the long term use may gradually drift. The pilot wafer scheme can deal with these changes. 
       FIG. 23B  shows the results of etching a lot after the pilot wafer is etched. The lower area of the graph shows a difference between a pilot wafer value and a lot average value. A difference between the lot average value and pilot wafer value is converged in a narrow range. Therefore, the results are obtained from the pilot wafer, and the correction is made in accordance with the results so that the lot average value can be converged in a narrow range. 
     A difference between the lot average value and the pilot wafer value is about 1 nm. This value is called first wafer effects. As a number of wafers are processed, there is the tendency that the results gradually drift. It is preferable to adopt the first wafer effects when the etching conditions of a lot are to be determined by feeding back the results of the pilot wafer. 
     Although pilot wafers may be adopted for all lots, if the lots of the same or similar product type are to be continuously processed, it is expected that adopting pilot wafers for all lots is rare. If the obtained results shift from the desired value by 1 nm or larger after the etching process, it is desired to adopt a pilot wafer. When the product type having a very different layout is to be processed, it is desired to adopt a pilot wafer. It is not limited to only a single pilot wafer per lot, but a plurality of pilot wafers per lot may be used. A pilot wafer may be a wafer at the middle of a lot. Measuring a resist pattern may not be executed for all wafers. Measuring may be executed for the pilot wafer and only a portion of remaining wafers to perform data feed forward and data feedback. 
     The present invention has been described in connection with the preferred embodiments. The invention is not limited only to the above embodiments. For example, the etching gas is not limited to the mixed gas of He/SO 2 /O 2 , but other mixed gases having similar characteristics may also be used. It will be apparent to those skilled in the art that other various modifications, improvements, combinations, and the like can be made.