Abstract:
A method of forming a trench in a semiconductor substrate by gas plasma etching having the steps of detecting and analyzing emission spectrums of gas plasma etching products and controlling the gas plasma etching based on the ratio of the emission spectrums. The etching uses a HBr/O 2  gas. The ratio of emission spectrums is preferable from 1 to 2.

Description:
FIELD OF THE INVENTION 
     This invention relates to an etching method for forming a trench on a semiconductor substrate by gas plasma etching, and more particularly to, a device-separating trench etching method. 
     BACKGROUND OF THE INVENTION 
     In a conventional etching technique using gas plasma, observing the state of a sample to be etched by some way is known. As the conventional etching technique, for example, Japanese patent application laid-open No.1-183124 (1989) discloses an etching method that, in a process of forming a trench in a treated material, prior to conducting the plasma etching, an oxide film for detecting the end point of etching on the treated material is formed and then a polysilicon film equal to the depth of trench is formed thereon, thereafter removing the oxide film and polysilicon film to open the trench. A product by the reaction of the oxide film and ion gas plasma is detected by emission spectrum analysis in the process of conducting the plasma etching. 
     Also, Japanese patent application laid-open No.1-278728 (1989) discloses an etching method that, in forming a trench in a treated material, prior to conducting the plasma etching, predetermined ions are ion-implanted up to a position equal to the depth of the trench to be measured from the upper surface at a position to form the trench in the treated material, and a product formed by the ions is detected by emission spectrum analysis or mass spectrum analysis in the process of conducting the plasma etching. 
     When plasma etching, further to the detection of the end point of etching being solved by Japanese patent application laid-open Nos.1-183124 (1989) and 1-278728 (1989), “a method for conducting the etching” that does not generate any etch residue or etch stop is also important. 
     Also, it is desired that the taper angle of a trench to be formed by etching be constant regardless of the etch area. 
     Further, a problem in the plasma etching is that it can easily performed according to a variation in the ratio (i.e., a specification required to etching) of etch area to an etched sample. 
     Also, the above problems need to be solved, with succeeding the conventional plasma etching technique, i.e., by means to be within the range of conventional facilities. 
     However, means to solve the above problems need to have such a property that can be performed even by introducing additionally a larger-scale automatic control unit as the case may be, i.e., expandability to be prepared to the prospects for the future. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the invention to provide an etching method that does not generate any etch residue or etch stop. 
     According to the invention, a method of forming a trench in a semiconductor substrate by gas plasma etching, comprises the steps of: 
     conducting a predetermined pre-treatment for etching to the semiconductor substrate; 
     conducting the gas plasma etching under an etching condition with a predetermined initial setting; 
     detecting and analyzing an emission spectrum occurred with the gas plasma etching; 
     monitoring the analyzing result; and 
     controlling the gas plasma etching on the basis of the monitored result of the analyzing result of the emission spectrum in the process of conducting the etching, wherein the controlling step of the gas plasma etching includes a renewal of etching condition. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained in more detail in conjunction with the appended drawings, wherein: 
     FIG. 1 is an illustration showing the composition of an etching system used for an etching method in a first preferred embodiment according to the invention, 
     FIGS. 2A and 2B are cross sectional views showing an etching sample to be sequentially processed by the etching method in the first embodiment according to the present invention, 
     FIG. 3 is a graph obtained summarizing the experimental results data in searching the correlation among SiBr/O peak intensity ratio, etch residue and taper angle, 
     FIG. 4 is a graph obtained summarizing the experimental results data in searching the etch area dependency of SiBr/O peak intensity ratio, 
     FIG. 5 is a graph obtained summarizing the experimental results data in searching the oxygen mixing ratio dependency of SiBr/O peak intensity ratio, 
     FIGS. 6A and 6B are graphs obtained summarizing the experimental results data in searching the oxygen mixing ratio dependency of taper angle, 
     FIG. 7 is a cross sectional view showing an etched sample used for the experiment to measure the effect of etch product SiBr x  to SiO 2 -etched amount, 
     FIG. 8 is a graph obtained summarizing the experimental results data in searching the etching time dependency of SiO 2 -etched amount and SiBr/O peak intensity ratio, 
     FIG. 9 is a graph obtained summarizing the experimental results data in searching the SiF 4 -gas addition amount dependency of SiBr/O peak intensity ratio and taper angle, 
     FIG. 10 is a graph obtained summarizing the experimental results data in searching the SiF 4 -gas addition amount dependency of SiO 2  etch rate, 
     FIG. 11 is an illustration showing the composition of an etching system used for the etching method in a third preferred embodiment according to the present invention, and 
     FIGS. 12A and 12B are cross sectional views showing an etched sample to be sequentially processed by an etching method in a fourth preferred embodiment according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the present invention will be explained below referring to the drawings. 
     First Embodiment 
     First, an etching method in the first embodiment according to the present invention will be explained. 
     FIG. 1 is an illustration showing the composition of an etching system used for the etching method in the first embodiment according to the present invention. The etching system shown in FIG. 1 takes the composition that an emission spectrum analyzing unit is connected with an inductively-coupled plasma etching system to produce low-pressure and high-density plasma 
     The composition of the etching system will be explained below, referring to FIG.  1 . 
     A dielectric plate  2  is disposed at the top of a chamber  1 , a lower electrode  5  is disposed at the bottom and inside of the chamber  1 , and a wafer  6  to be etched is put on the lower electrode  5 . A coil  3  is disposed, surrounding the chamber  1 , at a corresponded position over the dielectric plate  2 , and an alternating voltage from an RF generator  8  is applied to the coil  3 . An alternating voltage by a RF generator  7  is applied to the lower electrode  6 . 
     Also, a view port  4  of quartz is disposed on the sidewall of the chamber  1 . Rays of light in the chamber  1  are detected through the view port  4  by a head  9  and sent out through an optical fiber  10  to an emission spectrum analyzer  11 . The analysis result of the emission spectrum analyzer  11  is monitored by an operating part  12  to control the etching, and it is used to control the whole etching work including a variation in etching conditions. 
     Meanwhile, the work to be operated by the operating part  12  is conducted by its operator. However, in some cases, it may be conducted by an automatic control system. 
     FIGS. 2A and 2B are cross sectional views showing an etched sample to be sequentially processed by the etching method in the first embodiment according to the present invention. 
     Referring to FIG.  1  and FIGS. 2A and 2B, the steps of the etching method in the first embodiment according to the present invention will be explained below. 
     As shown in FIG. 2A, an oxide film (SiO 2 )  14  of about 20 nm thick is first formed on a single-crystal silicon substrate  13 , and a silicon nitride film (Si 3 N 4 )  15  of about 200 nm thick is formed thereon. Then, patterning is conducted by the photolithography process and the silicon nitride film  15  and the oxide film  14  are dry etched where a silicon trench will be formed. 
     Thereafter, where the silicon trench will be formed, the photoresist film is removed and the silicon nitride film  14  is removed, thereby exposing a Si substrate exposed part  16 . 
     Then, by using the above-mentioned “inductively-coupled plasma etching system to produce low-pressure and high-density plasma” in FIG. 1, the silicon-trench etching is conducted to form a silicon trench  17 . 
     For this silicon-trench etching, HBr/O 2  system gas is used. In conducting this etching, as shown in FIG. 1, an emission spectrum, which is detected through the view port  4  disposed at the side wall of the chamber  1  by the head  9 , is monitored. In particular, for example, the peak intensity ratio (hereinafter referred to as “SiBr/O”) of an etch product SiBr with an emission wavelength of 504 nm and oxygen with an emission wavelength of 617 nm is monitored. The SiBr/O peak intensity ratio is controlled to be 1 to 2 by varying the etching conditions etc. By controlling the SiBr/O peak intensity ratio to be 1 to 2, a sample shape with a taper angle of 80° at a separation width of 0.24 μm can be obtained, without the etch residue. 
     FIG. 3 is a graph obtained summarizing the experimental results data in searching the correlation among SiBr/O peak intensity ratio, etch residue and taper angle. 
     The correlation among SiBr/O peak intensity ratio, etch residue and taper angle will be explained below, referring to the experimental data in FIG.  3 . 
     In the above experiment, the etch residue increases with a decrease in etch area. On the other hand, the taper angle is almost constant, not depending on the etch area. However, all the etchings are conducted under same conditions below. 
     (Etching Conditions) 
     gas; HBr/O 2 =90/10 sccm 
     pressure; 5 mTorr 
     source power; 600 W 
     bias power; 150 W. 
     From this analysis result of emission spectrum, with a decrease in etch area, the peak intensity of SiBr (emission wavelength 503 nm) as the etch product decreases and the peak intensity of oxygen O (emission wavelength 617 nm) increases. As a result, as shown in FIG. 4, the SiBr/O peak intensity decreases (meanwhile, FIG. 4 is a graph obtained summarizing the experimental results data in searching the etch area dependency of SiBr/O peak intensity ratio). 
     Further, when the SiBr/O peak intensity ratio is 1 or less, the etch residue or etch stop occurs. 
     Next, with samples with various etch areas, by changing the SiBr/O peak intensity ratio by varying the oxygen mixing ratio in etch gas, it is measured whether the residue exists or not (refer to a graph obtained summarizing the experimental results data in searching the oxygen mixing ratio dependency of SiBr/O peak intensity ratio in FIG.  5 ). 
     In this case, in the samples with any etch areas, the residue occurs when the SiBr/O peak intensity ratio is one or less. Namely, it is proved that whether the residue exists or not is determined by the SiBr/O peak intensity ratio. 
     On the other hand, as shown in FIGS. 6A and 6B, the taper angle depends on only the oxygen mixing ratio and little depends on the etch area (meanwhile, FIGS. 6A and 6B are graphs obtained summarizing the experimental results data in searching the oxygen mixing ratio dependency of taper angle). 
     It is proved that the deposition amount of the sidewall of trench, which is a factor to form the taper, is determined by the amount of oxygen and the oxygen mixing ratio needs to be 8% or more to obtain a shape with a taper angle of about 80° at a separation width of 0.22 μm. 
     As the cause of residue occurred when the etch area decreases, it is supposed that a micro mask composed of silicon oxide (SiOx) is likely to form, due to the increase in the emission peak intensity of oxygen. Other than this, it is supposed that the etch product SiBr x  affects the removal of the micro mask (oxide), due to the decrease in the SiBr peak intensity. From these reasons, the effect of SiBr x  to SiO 2  etch rate is searched. 
     Fig. 7 shows an etched sample used for the experiment to measure the effect of etch product SiBr x  to SiO 2 -etched amount. 
     Using the sample shown in FIG. 7, the etching time dependency of SiO 2 -etched amount is measured. By using this sample, the etch product SiBr x  is supplied while polysilicon exists. However, after completing the polysilicon etching, the supply of SiBr x  stops. Thus, the effect of SiBr x  to SiO 2 -etched amount can be found. 
     As shown in FIG. 8, the slopes of graphs are different between during the polysilicon etching and after the polysilicon etching. When SiBr x  exists in the chamber, the etch rate of SiO 2  becomes higher. From this, it can be judged that SiBr x  has the ability to remove the oxide (meanwhile, FIG. 8 is a graph obtained summarizing the experimental results data in searching the etching time dependency of SiO 2 -etched amount and SiBr/O peak intensity ratio). 
     From the above matters, under the condition that the amount of oxygen added is constant, the amount of SiBr to be discharged in the plasma decreases with the decrease in etch area. In this case, the gaseous atmosphere in plasma is classified into oxidizing atmosphere or reducing atmosphere, according to the ratio of SiBr and O. In a sample that a small amount of SiBr is discharged, i.e., a sample with a small etch area, the micro mask is formed due to the oxidizing atmosphere, thereby causing the residue. On the other hand, in a sample with a large etch area, it is supposed that the reducing atmosphere occurs due to the existence of much SiBr and the micro mask is therefore not formed, thereby generating no residue. Namely, by monitoring the SiBr/O peak intensity ratio, the oxidizing atmosphere and reducing atmosphere can be identified. In particular, when the SiBr/O peak intensity ratio is made to be 1 or more, the reducing atmosphere can be obtained, thereby enabling the etching not to generate any residue. 
     Second Embodiment 
     Next, an etching method in the second embodiment according to the present invention will be explained. 
     An etching system used in the etching method of the second embodiment has the same composition as the etching system used in the etching method of the first embodiment. 
     The steps of the etching method in the second embodiment according to the present invention will be explained below, on the basis of the above-mentioned matters. 
     As described earlier, the taper angle depends on only the oxygen mixing ratio in etch gas, and the oxygen mixing ratio needs to be 8% or more to obtain a shape with a taper angle of about 80° at a separation width of 0.22 μm. Under this condition, in case of a small etch area, the SiBr/O peak intensity ratio becomes 1 or less, i.e., oxidizing atmosphere, therefore generating the residue. In case of a small etch area, the SiBr/O peak intensity ratio needs to be increased to more than 1. 
     So, the second embodiment of the present invention is applied to the case of a small etch area, adding silicon-halide-system gas, such as SiF 4 , SiCl 4  and SiBr 4 , to etch gas HBr/O 2  (more than 8% mixed) to make the SiBr/O peak intensity ratio 1 or more. 
     FIG. 9 is a graph obtained summarizing the experimental results data in searching the SiF 4 -gas addition amount dependency of SiBr/O peak intensity ratio and taper angle. 
     The SiBr/O peak intensity ratio increases with an increase of SiF 4 -gas addition amount, and, when it becomes 1 or more, the residue disappears. Also, the taper angle gradually increases. Here, as shown in FIG. 10, when SiF 4  gas is added, the etch rate of SiO 2  increases. Therefore, it is thought that the taper angle decreases because deposition composed of SiO x  deposited on the sidewall of trench decreases (meanwhile, FIG. 10 is a graph obtained summarizing the experimental results data in searching the SiF 4 -gas addition amount dependency of SiO 2  etch rate). 
     Namely, SiF 4  gas has a reducing property to pull out oxygen. Thus, in case of the small etch area, adding SiF 4  gas to etch gas HBr/O 2  (more than 8% mixed) to increase the SiBr/O peak intensity ratio and control the taper angle is an effective means. Meanwhile, though only data when adding SiF 4  gas are shown herein, the addition of SiCl 4 , SiBr 4  etc. can also bring the same effect. 
     Third Embodiment 
     Next, an etching method in the third embodiment according to the present invention will be explained. 
     FIG. 11 shows the composition of an etching system used in the etching method of the third embodiment according to the present invention. 
     The etching system used in the etching method of the third embodiment has the same composition as the etching system used in the etching method of the first embodiment according to the present invention, except a wafer  66  put on the lower electrode  5 . 
     The steps of the etching method in the third embodiment will be explained below, on the basis of the above-mentioned matters. 
     In the second embodiment mentioned above, in case of the small etch area, silicon-halide-system gas, such as SiF 4 , as etch gas is added to etch gas HBr/O 2  (more than 8% mixed). However, in this embodiment, as shown in FIG. 11, a member  42  of silicon with a larger area than the area of the wafer  66  is disposed in the chamber  1 , planning the SiBr/O peak intensity ratio not to depend on etch area. In particular, by disposing the doughnut-like Si member  42  on an edge ring disposed surrounding the wafer  66  and on the lower electrode  5 , a process not depending on etch area is enabled. Namely, by disposing the doughnut-like Si member  42  on the lower electrode  5 , the Si member  42  becomes easy to etch, and therefore SiBr becomes easy to feed. 
     In the third embodiment, silicon-halid-system gas, such as SiF 4 , as etch gas may be added to etch gas HBr/O 2  (more than 8% mixed), like the second embodiment mentioned above. 
     Fourth Embodiment 
     Next, an etching method in the fourth embodiment according to the present invention will be explained. 
     The etching system used in the etching method of this embodiment has the same composition as the etching system (refer to FIG. 1) used in the etching method of the first embodiment according to the present invention. 
     The steps of the etching method in the fourth embodiment will be explained below, on the basis of the above-mentioned matters. Meanwhile, though the Si member is disposed outside the wafer in the third embodiment, in the fourth embodiment, by using polysilicon as etch mask, an etching process that the SiBr/O peak intensity ratio does not depend on etch area is enabled. 
     FIGS. 12A and 12B are cross sectional views showing an etched sample to be sequentially processed by the etching method in the fourth embodiment according to the present invention. 
     Referring to FIG.  1  and FIGS. 12A and 12B, the steps of the etching method in the fourth embodiment according to the present invention will be explained below. 
     In the etching method of the fourth embodiment, as shown in FIG. 12A, an oxide film (SiO 2 )  45  of about 20 nm thick is first formed on a single-crystal silicon substrate  63 , and then a silicon nitride film (Si 3 N 4 )  47  of about 200 nm thick is formed thereon, and further a polysilicon film  43  with a film thickness corresponding to a desired trench depth to be etched is formed thereon. Then, patterning is conducted by the photolithography process and the dry etching process of polysilicon film  43 , silicon nitride film  47  and oxide film  45 , and part where a silicon trench will be formed is exposed. 
     Thereafter, at part (refer to FIG. 12B) where a silicon trench  44  will be formed, the photoresist film is removed, the silicon nitride film  47  as the mask when conducting the trench etching is removed, and the oxide film (SiO 2 )  45  is also removed, thereby exposing a Si substrate exposed part  48 . 
     In the step in FIG. 12A, by using the etching system shown in FIG. 1, the silicon trench etching is conducted using the same control method as that disclosed in the first embodiment. Then, as shown in FIG. 12B, the silicon trench  44  is formed. 
     In the fourth embodiment, the etching process not depending on the etch area of sample is enabled. 
     Also, as a pre-treatment for etching, the polysilicon film  43  with a film thickness corresponding to a desired trench depth is formed in advance. Therefore, there is the merit that the end point of trench etching can be easily detected. 
     Advantages of the Invention: 
     As described above, by the etching method according to the present invention, in gas plasma etching, emission inside the chamber is spectrum-analyzed and the results are continuously fed back to the etching conditions etc. Thereby, the etching does not generate any etch residue or etch stop. 
     Also, there occurs the effect that the taper angle of a trench to be formed by etching can be constant regardless of the etch area. 
     Also, the above effects can be obtained by using means to cope with a variation in a specification required to etching, despite the variation in a specification, such as the ratio of etch area to an etched sample. 
     Further, they can be realized succeeding the conventional plasma etching technique, i.e., by means within the range of conventional facilities. Also, they can be realized by introducing additionally a larger-scale automatic control unit as the case may be, i.e., to have an expandability to be prepared to the prospects for the future. 
     Although the invention has been described with respect to specific embodiment for complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modification and alternative constructions that may be occurred to one skilled in the art which fairly fall within the basic teaching here is set forth.