Abstract:
Etching method applicable to a semiconductor device fabrication and an MEMS(Micro-Electro-Mechanical System) process, including the steps of forming an etching mask on a substrate, forming a plurality of patterns in the etching mask corresponding to depths of the plurality of trenches; and etching the substrate using the etching mask having the plurality of patterns formed therein, whereby eliminating an alignment error in respective photolithography, that permits to form a precise structure, simplify a fabrication process, and reduce a production cost.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an etching method, and more particularly, to an etching method applicable to a semiconductor device fabrication and an MEMS(Micro-Electro-Mechanical System) process. 
     2. Background of the Related Art 
     In general, the etching is an essential process in fabrication of a semiconductor device as well as in fabrication of the MEMS currently under research widely, which occupy great portions of the fabrication processes. In the etching, an etching mask is always required for separate a portion to be etched from a portion to be protected from the etching. As the etching mask, a photoresist thin film, a nitride thin film, such as SiN, an oxide thin film, such as SiO2, and a metal thin film, such as Cr, can be used, selectively. 
     As one example, a related art wet etching will be explained. Referring to FIG. 1, in fabrication of a device having different etching depths ‘A’, ‘B’, and ‘C’, the following two methods are used in the related art. 
     Referring to FIG. 2A, in the first method, a first SiN etching mask layer  2  and the first photoresist layer  3  are formed in succession on the silicon substrate  1 , and a portion of the first SiN etching mask layer  2  is removed by photolithography and etching, to expose a region of the silicon substrate  1 . Then, as shown in FIG. 2B, the first SiN etching mask layer  2  is used as a mask in etching the silicon substrate  1  to an ‘A’ depth in FIG. 1, and the first SiN etching mask layer  2  and the first photoresist layer  3  are removed. Then, as shown in FIG. 2C, a second SiN etching mask layer  4  and a second photoresist layer  5  are formed in succession on an entire surface of the substrate  1 , and a portion of the second SiN etching mask layer is removed by photolithography and etching, to expose a region of the silicon substrate  1 . Then, as shown in FIG. 2D, the second SiN etching mask layer  4  is used as a mask in etching the silicon substrate  1  to a depth of ‘B’ in FIG. 1, and the second SiN etching mask layer  4  and the second photoresist layer  5  are removed. And, as shown in FIGS. 2E and 2F, a third SiN etching mask layer  6  and a third photoresist layer  7  are used in etching to ‘C’ depth in FIG. 1 in the same method as the above methods, to complete fabrication of a device having different etching depths of ‘A’, ‘B’, and ‘C’ as shown in FIG.  2 G. 
     Referring to FIG. 3, in the second method, a first SiN etching mask layer  12  and a first photoresist layer  13  are formed on a silicon substrate  11 , and portions of the first SiN etching mask layer  12  are removed by photolithography and etching, to expose regions of the silicon substrate  1  to be etched as ‘A’, ‘B’ and ‘C’ in FIG.  1 . Then, as shown in FIG. 3B, the first SiN etching mask layer  12  is used as a mask in etching the exposed silicon substrate  11  in overall for the first time to a depth ‘C’ in FIG. 1 which has the shallowest etching depth, and the first SiN etching mask layer  12  and the first photoresist layer  13  are removed. And, as shown in FIG. 3C, a second SiN etching mask layer  14  and a second photoresist layer  15  are formed in succession on an entire surface of the substrate  11 , and a portion of the second SiN mask layer  14  is removed by photolithography, to expose the silicon substrate  11  only in the ‘B’ region of FIG.  1 . Then, as shown in FIG. 3D, the second SiN etching mask layer  14  is used as a mask in etching the silicon substrate  11  etched for the first time for the second time, and the second SiN etching mask layer  14  and the second photoresist layer  15  are removed. And, as shown in FIGS. 3E and 3F, a third SiN mask layer  16  and a third photoresist layer  17  are used in etching ‘A’ region in FIG. 1 etched for the first time for the second time by a method the same as the foregoing method, to complete fabrication of a device having different etching depths of ‘A’, ‘B’ and ‘C’ as shown in FIG.  3 G. 
     However, the related art methods are complicated since photolithography and etching mask removal are required every time ‘A’, ‘B’ and ‘C’ regions are etched. And, an etching mask used in a prior etching should be removed before a following etching process is started, and if the etching mask is not removed perfectly, formation of an etching mask for the following etching can not be done properly. That is, in the removal of the etching mask used in the prior etching, if the etching mask is not removed properly after the prior etching process is finished, there are over-hangs of the etching mask left at edges of the etched regions as shown in FIG. 4, which causes a shadow effect in formation of an etching mask for the following etching, to impede formation of an etching mask thin film at the edges of the etched regions. And, as shown in FIG. 5B, in the photolithography, misalignment of a mask pattern is occurred, which impedes fabrication of a precise device, particularly, as shown in FIG. 5A, in an MEMS fabrication, in order to enhance a device precision, an improvement of an etching process that can reduce an alignment error of the mask pattern is required. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to an etching method that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. 
     An object of the present invention is to provide an etching method which can reduce a misalignment error of a mask pattern in etching. 
     Other object of the present invention is to provide a simple etching method which permits to etch to a variety of depths by using only one etching mask. 
     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the etching method for forming a plurality of trenches or recesses having depths different from one another in a substrate, includes the steps of forming an etching mask on the substrate, forming a plurality of patterns in the etching mask corresponding to depths of the plurality of trenches, and etching the substrate using the etching mask having the plurality of patterns formed therein. 
     In other aspect of the present invention, there is provided an etching method, including the steps of forming an etching mask on a substrate, etching a first, a second, and a third regions in the etching mask to a first depth on the same time, forming a first mask pattern in the first region, and using the first mask pattern as a mask in etching the second and third regions of the etching mask on the same time to a second depth, removing the first mask pattern, and forming second mask patterns in the first and second regions, etching the third region of the etching mask using the second mask patterns as a mask, to expose the substrate, removing the second mask patterns, and using the etching mask as a mask in etching the exposed substrate to a third depth, etching the second region of the etching mask to expose the substrate, and etching the exposed substrate to a fourth depth using the etching mask, and etching the first region of the etching mask to expose the substrate, and using the etching mask as a mask in etching the exposed substrate to a fifth depth. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention: 
     In the drawings: 
     FIG. 1 illustrates a silicon structure formed by a related art etching method; 
     FIGS.  2 A˜ 2 G illustrate a first related art example of an etching process for fabricating the silicon structure in FIG. 1; 
     FIGS.  3 A˜ 3 G illustrate a second related art example of an etching process for fabricating the silicon structure in FIG. 1; 
     FIG. 4 illustrates overhangs of an etching mask occurred by the related art etching method; 
     FIGS. 5A and 5B illustrate an aligned mask pattern and non-aligned mask pattern, respectively; 
     FIG. 6 illustrates a silicon structure formed by a wet etching method in accordance with a preferred embodiment of the present invention; 
     FIGS.  7 A˜ 7 K illustrate sections showing the steps of an etching method for fabricating the structure in FIG. 6 in accordance with a preferred embodiment of the present invention; 
     FIG. 8 illustrates a perspective view of a silicon structure formed by a dry etching method in accordance with a preferred embodiment of the present invention; 
     FIGS.  9 A˜ 9 K illustrate sections showing the steps of an etching method for fabricating the structure in FIG. 8 in accordance with a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. The present invention provides an etching method, in which an etching mask formed for the first time is used in the next etching as it is. That is, as shown in FIG. 6, in fabrication of a device having different etch depths ‘A’, ‘B’ and ‘C’, the present invention provides an etching method which permits a simple and precise etching by using, not many etching masks as the case of the related art, but only one etching mask, applicable both to an wet etching and dry etching, and all kinds of wafers, etching solutions, and etching gases. For example, in a case when a silicon structure is wet etched, an isotropic or an anisotropic etching of a silicon single crystal is possible depending on selected etching solution, which can be selected according to an etching purpose. HNA(HF+HNO 3 +Acetic Acid) is used as an isotropic etching solution, and EDP(Ethylenediamine Pyrocatechol Water), TMAH(Tetramethyl Ammonium Hydroxide), or KOH(Potassium Hydroxide) is used as an anisotropic etching solution. 
     In general, as silicon single crystal used in fabrication of MEMS, a wafer with an ( 100 ) plane and an ( 110 ) plane is used. In the present invention, an etching method of wet etching a ( 100 ) silicon wafer by using KOH solution will be explained as one embodiment. 
     Referring to FIG. 7A, an SiN etching mask layer  22  is formed on a silicon substrate  21 , and a plurality of photoresist patterns  23  are formed on the SiN etching mask layer  22 . Then, as shown in FIG. 7B, the first photoresist patterns  23  are used as masks in removing the SiN etching mask layer  22  to a required depth by dry etching. The depth is determined taking a depth in the silicon substrate  21  to be etched in a later step. As shown in FIG. 7C, the first photoresist pattern  23  is removed, a second photoresist pattern is formed in a region to be etched the shallowest the same as the ‘C’ region in FIG. 6, and the second photoresist pattern  24  is used as a mask in etching the SiN etching mask layer  22  to a required depth. Then, as shown in FIG. 7D, the second photoresist pattern  24  is removed, a third photoresist pattern  25  is respectively formed in the ‘B’ region and the ‘C’ region in FIG. 6 as shown in FIG. 7E, and the third photoresist patterns  25  are used as masks in dry etching as shown in FIG. 7F, to remove the SiN etching mask layer  22  only from ‘A’ region in FIG. 6, to expose the substrate  21 . And, as shown in FIG. 7G, the third photoresist pattern  25  is removed, and the SiN etching mask layer  22  is used as a mask in wet etching the exposed substrate  21  to a depth. Then, as shown in FIG. 7H, upon etching the substrate  21  on the whole without photolithography process, the SiN etching mask layer  22  is removed from the ‘B’ region in FIG. 6, to expose the substrate  21 . As shown in FIG. 7I, identical to the above step, the SiN etching mask layer  22  is used as a mask in wet etching the exposed substrate  21  to a depth, and the substrate  21  is dry etched on the whole without a photolithography process as shown in FIG. 7J, to remove the SiN etching mask layer  22  from the ‘C’ region in FIG. 6, to expose the substrate  21 . Then, as shown in FIG. 7K, the SiN etching mask layer  22  is used as a mask again in wet etching the exposed substrate  21  to a depth, to obtain a silicon structure as shown in FIG.  6 . In the wet etching, 30 wt % KOH water solution is used at a temperature of approx. 80° C. 
     Though the alignment error in the related art etching method caused by a requirement for formation of the etching mask layer and conduction of photolithography without fail every time before the ‘A’, ‘B’, and ‘C’ portions are respectively wet etched has impeded fabrication of a precise device, since a precision of the photolithography is of no great significance for the present invention, fabrication of a precise device is possible in the present invention. Because an outline of a photoresist pattern is defined in the first dry etching, what is required for the following dry etching is only reduction of a thickness of the etching mask layer. And, the use of only one etching mask layer required in the present invention can simplify the fabrication process compared to the related art. 
     In the anisotropic wet etching of silicon, a surface of an ( 100 ) wafer is an ( 100 ) plane, and a side surface of the ( 100 ) wafer is an ( 111 ) plane, wherein the ( 111 ) plane is tilted by approx. 54.7° from the ( 100 ) plane. However, in MEMS fabrication process, there are many cases when a side surface at a right angle from the ( 100 ) plane wafer surface is required. In these cases, though a dry etching can make such a vertical side surface available, it is very difficult to conduct the dry etching by only using a photoresist pattern the same as the related art for obtaining the structure in FIG.  8 . Accordingly, a dry etching by a method as shown in FIGS. 7A-7B can make the structure as shown in FIG. 8 available with easy. 
     FIGS.  9 A˜ 9 K illustrate sections showing the steps of a process for dry etching a silicon substrate, which are identical to FIGS. 7A 7 K, detailed explanations of which will be omitted. The etching mask layer  22  is formed of ITO(Indium Tin Oxide), and removed by using Cl 2 +BCl 3  gases. The silicon substrate  21  is etched using SF 6  gas and utilizing a high etching selectivity of the silicon substrate  21  over the etching mask layer  22 . 
     Thus, in a wet etching, the present invention permits to form an SiN thin film as an etching mask only once for conducting etching over three times in formation of a silicon structure having three different depths, which is applicable to a dry etching, the present invention has a wide application. 
     The elimination of an alignment error from respective photolithography by the present invention permits fabrication of a precise structure, to simplify a fabrication process, and to reduce a production cost. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the etching method of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.