Abstract:
A hard mask is produced from spacer structures. The spacer structures are formed from a conformal deposition on elevated structures produced lithographically in a projection process. The conformal deposition is etched back laterally on the elevated structures resulting in the spacer structures. The elevated structures between the spacer structures are subsequently etched away, so that the spacer structures remain in an isolated fashion as sublithographic structures of a hard mask with a doubled structure density compared with that originally produced in lithographic projection. In a regularly disposed two-dimensional array of structures in the hard mask for forming trenches—for instance for trench capacitors—the method achieves a doubling of the structure density in the array. A further iteration step is formed by forming further spacer structures on the first and second spacer structures, thereby achieving an even higher increase in structure density in the hard mask. REL/rit

Description:
BACKGROUND OF THE INVENTION  
         [0001]    Field of the Invention  
           [0002]    The present invention relates to a method for forming a hard mask in a layer on a planar device, in particular semiconductor wafers, masks or flat panels.  
           [0003]    In the fabrication of integrated circuits it is generally the aim to achieve the highest possible number of components on a predetermined substrate area. In particular in the fabrication of memory modules, a higher clock frequency and hence a faster module can be made available by shortening lengths of interconnects. The production of smaller structures is nowadays achieved essentially using optical lithography methods. The minimum achievable or resolvable structure width is  
           
         b 
         min 
         =x×λ/NA,  
       
           [0004]    where b min  is the minimum resolvable structure width, x is a parameter which describes the lithography technique, λ is the wavelength of the light for the optical projection, and NA is the numerical aperture of the projection system. Integrated circuits produced at the present time can be fabricated with minimum resolvable structure widths of 110 nm. However, in order to attain for example the 70 nm technology generation, a transition must be realized in light wavelengths used in the exposure apparatuses, to those apparatuses where λ=157 nm. However, such exposure apparatuses are currently still in development and are not available for production at this time.  
           [0005]    Such leaps in technology are usually associated with high costs. The transition to the 157 nm exposure apparatuses requires, by way of example, the use of exotic lens materials such as CaF 2 , which equally have first to be developed. A further problem is posed by the processes connected with the exposure step. A decrease in the exposure wavelength also leads to a necessary limitation of the permissible focus range for an exposure. This requirement in turn entails the use of even thinner resists for the exposure.  
           [0006]    In addition to the costs that are necessary for a leap to the next technology generation, however, the time taken to implement the leap in technology also increases on account of the complexity of the mutual dependencies of the respectively different process techniques. Although it is possible, with the aid of the process steps that usually follow the lithographic projection in the fabrication sequence, to produce feature sizes below the resolution limit—for instance by spacer deposition onto the side areas surrounding a gap in order to limit the gap size—a structure density that can be produced, and which is composed of a gap width and a line width in the example, cannot thereby be increased.  
         SUMMARY OF THE INVENTION  
         [0007]    It is accordingly an object of the invention to provide a method for forming a hard mask in a layer on a planar device which overcomes the above-mentioned disadvantages of the prior art methods of this general type, which requires a small structure spacing in conjunction with low costs and a short development time.  
           [0008]    With the foregoing and other objects in view there is provided, in accordance with the invention, a first method for forming a hard mask in a first layer on a planar device. The first method includes the steps of depositing a second layer on the first layer, using a lithographic projection process for forming at least one elevated first structure from the second layer disposed above the first layer, conformally depositing a third layer on the elevated first structure and on the first layer, etching back the third layer for forming a first spacer structure and a second spacer structure each disposed on a respective side area of the elevated first structure, and etching the second layer selectively with respect to the first and third layers to remove the elevated first structure disposed between the first and second spacer structures, resulting in the hard mask being formed from the first and second spacer structures.  
           [0009]    With the foregoing and other objects in view there is provided, in accordance with the invention, a second method for forming a hard mask in a first layer on a planar device. The second method includes the steps of depositing a fourth layer on the first layer, using a lithographic projection process for forming a hole structure in the fourth layer, conformally depositing a third layer, etching back the third layer for forming a first spacer structure and a second spacer structure on a respective side area of the hole structure, depositing and filling the hole structure with a second layer, planarizing the second layer, etching the fourth layer selectively with respect to the first, second and third layers for forming an elevated first structure enclosed by the first and second spacer structures on the side areas, and etching the second layer selectively with respect to the first and third layers for removing the elevated first structure between the first and second spacer structures, resulting in the hard mask being formed from the spacer structures.  
           [0010]    According to the present invention, structures are formed in lithographic projection steps, which structures naturally have a structure width above the resolution limit of the projection apparatus respectively used. Spacers are subsequently formed laterally on these structures, which spacers may afterward, through removal of the structure formed lithographically, themselves in turn serve as a hard mask in the form of isolated structures. The structure width of the spacer structures is given by the layer thickness during the deposition of the third layer, in which case, through the conformal deposition, for instance in a CVD, LPCVD or ALD method, in particular on the side areas of an existing elevated structure, it is necessary to measure the thickness of the deposition parallel to the surface of the substrate used. Since the thicknesses of deposited layers may be a few nanometers, it is also possible, in particular, to produce structure widths in the sublithographic range, i.e. to produce structure widths below the minimum resolution limit of the projection system.  
           [0011]    Moreover, since spacers are formed on a respective side of a line formed lithographically, that is to say the elevated structures or the hole structures, what is advantageously achieved is the production of two identical structures (in each case both elevated or both as a hole structure) in an area region which, by lithographic projection, could accommodate just a single structure produced. Consequently, in comparison with the prior art, the present invention achieves a miniaturization of structures down into the sublithographic range in conjunction with an increase in the structure density on the substrate of a planar device. Moreover, since this is done using simple process engineering, a considerable cost and time advantage results from the application of the present invention compared with the development of higher-resolution exposure techniques for planar devices such as, for instance, wafers or masks.  
           [0012]    The two methods according to the invention differ in that, in the first method, using lithographic techniques, the spacer structures are formed on the side areas of an elevated structure, while in the second method hole structures are formed, the spacer structures being formed on the inner side areas of the hole structures. However, the elevated structures are attained in the second method, too, if the holes provided with the spacer structures are filled and the layer originally surrounding the holes is removed. Once this state has been reached, the sequence in the two methods is essentially identical.  
           [0013]    In a further refinement, a fifth layer deposited conformally on the first layer and the spacer structures is etched back after the removal of the structures produced lithographically between the now isolated spacer structures. The first and second spacer structures then again act as a master structure for forming a third and fourth and, respectively, fifth and sixth spacer structure on a respective side of the first and second spacer structures. As a result, the structure width of the spacer structure that is composed of the first, third and fourth spacer structures, for example, can be varied in this method step. It thus becomes possible, in an advantageous manner, to adapt the line to gap ratio in a dense periodic grid of lines.  
           [0014]    In a further refinement, the spacer structure formed first, i.e. the first or second spacer structure, is removed from the composite spacer structure. This step corresponds to a further iteration for doubling the number of structures per predetermined substrate area. Even further iteration steps are possible. In a further refinement, the first layer is essentially composed of silicon nitride, the second layer is essentially composed of SiO 2  and the third layer is essentially composed of polysilicon. The choice of these assignments to the three layers ensures a high selectivity for the respective etching steps and hence a high quality of the respective structure formation. According to the invention, it is also possible to exchange the assignments of the constituents to the layers.  
           [0015]    However, the choice of silicon nitride for the first layer affords the particular advantage of serving as an etching stop for an end point identification during isotropic or anisotropic etching. The spacer structures are advantageously etched back in an anisotropic etching process.  
           [0016]    In a further refinement, the third and fifth layers essentially contain the same material, with the advantage that the composite spacer structure is more resistant or more stable with respect to subsequent etching steps.  
           [0017]    In a further refinement, the first and second or third to sixth spacer structures or the spacer structures composed thereof are produced with a structure width that lies below the minimum resolution limit of the lithographic projection apparatus.  
           [0018]    In accordance with an added mode of the invention, during the lithographic projection process, an elevated second structure is formed from the second layer. The elevated first and second structures have a given width and a mutual spacing there-between, the mutual spacing not being less than the given width and not being more than three times the given width.  
           [0019]    In accordance with an additional mode of the invention, there are the steps of carrying out the depositing of the third layer with a first thickness, and carrying out the depositing of the fifth layer with a second thickness. The first and second thicknesses are implemented in a manner dependent on the given width and the mutual spacing. The first thickness is formed to be essentially half a difference between the mutual spacing and the given width of the first and second structure. The second thickness is formed to be essentially an eighth of a difference between three times the given width and the mutual spacing.  
           [0020]    In accordance with a further mode of the invention, during the lithographic projection process, elevated second, third and fourth structures are formed from the second layer. The elevated first to fourth structures are not disposed contiguously on the first layer and the elevated first structure has, in a first direction, a given spacing with respect to the elevated second structure and, in a second direction substantially perpendicular to the first direction, having the given spacing with respect to the elevated third structure. The etching of the second layer step is performed to the first to fourth structures in a same way to form a hole structure enclosed by the third to sixth spacer structures of the fifth layer in the hard mask.  
           [0021]    In accordance with another mode of the invention, a silicon substrate is provided and the first layer bears on the silicon substrate and is formed of silicon nitride. The second layer is formed from polysilicon, and the third layer and the fifth layer are formed from silicon oxide. The hard mask transferred to the first layer is used in an etching step for forming trench capacitors in a memory cell array.  
           [0022]    In accordance with the invention, the planar device is a semiconductor wafer, a mask, or a flat panel.  
           [0023]    In accordance with a concomitant feature of the invention, the step of etching the first layer selectively with respect to the third layer, is performed so that the first and second spacer structures are transferred to the first layer for forming a further hard mask.  
           [0024]    Other features which are considered as characteristic for the invention are set forth in the appended claims.  
           [0025]    Although the invention is illustrated and described herein as embodied in a method for forming a hard mask in a layer on a planar device, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.  
           [0026]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    FIGS.  1 A- 1 D are diagrammatic, sectional views of a first exemplary embodiment of a method according to the invention for forming a hard mask with doubled structure density;  
         [0028]    FIGS.  2 A- 2 D are sectional views of a second exemplary embodiment of the method according to the invention for forming the hard mask with doubled structure density with an identical width of lines and gaps;  
         [0029]    [0029]FIGS. 3A, 3C, and  3 E are plan views of a third exemplary embodiment of the method according to the invention for forming the hard mask with regular configurations of holes for patterning trenches in a memory cell array;  
         [0030]    [0030]FIGS. 3B, 3D, and  3 F are sectional views of the third exemplary embodiment of the method according to the invention for forming the hard mask with the regular configurations of holes for patterning trenches in a memory cell array;  
         [0031]    [0031]FIGS. 4A, 4C, and  4 E are plan views of the third exemplary embodiment of the method according to the invention for forming the hard mask with the regular configurations of holes for patterning trenches in a memory cell array but on the basis of hole structures;  
         [0032]    [0032]FIGS. 4B, 4D, and  4 F are sectional views of the third exemplary embodiment of the method according to the invention for forming the hard mask with the regular configurations of holes for patterning trenches in the memory cell array, but on the basis of hole structures; and  
         [0033]    [0033]FIGS. 5A and 5B are illustrations showing a comparison of the hole structures produced lithographically with the hole structures of the hard mask that are produced therefrom in the method according to the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]    Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown a first exemplary embodiment of a method according to the invention with reference to a cross section through a semiconductor wafer. Situated on a substrate  5 , which is essentially composed of monocrystalline silicon, is a first layer  10 , which essentially contains silicon nitride. A second layer  20  made of silicon oxide (SiO 2 ) is deposited thereon. The semiconductor wafer coated in this way is coated with a resist  30  in order to carry out a lithographic projection step, the resist being exposed in a projection step in order to produce structures  21 . The exposed parts are developed and removed (FIG. 1A), so that the exposed structures are transferred to the second layer  20  in a subsequent etching step (FIG. 1B).  
         [0035]    In the present exemplary embodiment, the structure  21  of the second layer  20  has a spacing  27  from a nearest further elevated structure  23  of the second layer  20 , which spacing is equal to three times the minimum resoluble structure width of the projection apparatus, i.e. 3 F, where F=b min . A structure width  28  of the structures  21 ,  23  is embodied at the resolution limit with 1 F in the example.  
         [0036]    A third layer  40  made of polysilicon is subsequently deposited onto the silicon nitride layer  10  and the structures  21 ,  23 . By etching back as far as an etching stop with end point identification on the silicon nitride layer  10 , the third layer  40  is removed from the structures  21 ,  23  and the silicon nitride layer  10 , except at those locations at which it has a vertical orientation bearing conformally on side areas of the structures  21 ,  23 . This region is left behind as spacer structures  41 ,  42  in the anisotropic etching-back step (FIG. 1C).  
         [0037]    In a further etching step, the silicon oxide of the second layer  20  is removed selectively with respect to the silicon nitride of the first layer  10  and the polysilicbn of the third layer  40  between the spacer structures  41 ,  42 . The spacer structures  41 ,  42  then lie in an isolated manner on the first layer of the silicon nitride layer  10 . The spacer structures  41 ,  42  have a structure width  48  corresponding to the deposition thickness of the polysilicon for forming the third layer  40  (FIG. 1D).  
         [0038]    The grid width of the structures  21 ,  23  produced lithographically is composed of the structure width  28  and the structure spacing  27  of the structures  21 ,  23 . It is thus 4 F in the present example. Through a suitable choice of the deposition thickness—corresponding approximately to the minimum resolvable structure width 1 F—the structure width  48  of the spacer structures  41 ,  42  is also about 1 F. However, the mutual spacing of the spacer structures  41 ,  42  is thus also about 1 F. Consequently, given an approximately identical structure width  28 ,  48  of lithographically produced elevated structures  21 ,  23  and spacer structures  41 ,  42 , the grid width of 4 F can be reduced to a value of 2 F for the hard mask formed by the spacer structures  41 ,  42 .  
         [0039]    A particular advantage arises, then, by virtue of the fact that lines with a grid width of less than 2 F can be fabricated according to the present invention. In this case, a grid having a grid width of 2 F is fabricated by conventional lithographic methods, but the line structures may also have a width of less than 1 F and the gap structures may have a width of greater than 1 F. The lines are somewhat overexposed in this case. It is thus possible to dispense with the double spacer technique in a restricted process window.  
         [0040]    In this case, the spacer structures  41 ,  42  either themselves constitute the hard mask or the latter is formed by an anisotropic etching step for transferring the spacer structures  41 ,  42  to the first layer  10  made of silicon nitride (FIG. 1D).  
         [0041]    A further exemplary embodiment is shown in FIGS.  2 A- 2 D. In this example, a particularly advantageous effect is achieved through a suitable choice of deposition thicknesses in different steps of the spacer formation. In order to provide a better understanding, the method sequence will be described.  
         [0042]    In a manner similar to that illustrated in FIGS. 1A and 1B, according to FIGS. 2A and 2B, the monocrystalline silicon substrate  5  is coated with the first layer  10  made of silicon nitride and the second layer  20  made of silicon oxide (SiO 2 ) and subsequently coated with the resist layer  30 , which, for the formation of the structures  21 ,  23 , is exposed, developed and etched in order to transfer structures to the second layer  20 . It is an object of the present example to produce a dense structure of lines and gaps in which the gap and line widths are in an identical ratio to one another. The structure width  28  and the structure spacing  27  of the elevated structures  21 ,  23  produced lithographically are initially arbitrary, but must meet the following conditions: three times the structure width  28  must be more than the structure spacing  27 , but the structure width  28  must not be greater than the structure spacing  27 . If f is the structure spacing  27 , then the following holds true:  
         [0043]    F&lt;f&lt;3 F.  
         [0044]    As in the previous example, in a deposition step, the spacer structures  41 ,  42  are then formed from a third layer  40  made of polysilicon, by anisotropic etching back. The structure width  48  of the spacer structures  41 ,  42  corresponds to the deposition thickness  49 . The second layer  20  is subsequently removed in a selective etching step (FIG. 2C). Through the deposition of a fifth layer  50 , further spacer structures, for instance third, fourth, fifth and sixth spacer structures  51  to  54 , are formed on the existing spacer structures  41 ,  42  by polysilicon once again being deposited conformally with a deposition thickness  59  and etched back. The structure width  48  is thus enlarged into a new structure width  48 ′, since at least one of the resulting spacer structures is composed of the originally second spacer structure  42 , the fifth spacer structure  53  formed on its left-hand side, and the sixth spacer structure  54  formed on its right-hand side. The structure width  48 ′ then amounts to the sum of the deposition thickness  49 , of the third layer  40 , and twice the deposition thickness  59  of the fifth layer  50 , which are both formed from polysilicon.  
         [0045]    An identical width ratio of lines and gaps can be achieved by choosing the deposition thicknesses  49 ,  59  as follows: if d1 is the deposition thickness  49  and d2 the deposition thickness  59 , then the following holds true:  
         [0046]    d1=½ (f-F); and  
         [0047]    d2=⅛ (3F-f).  
         [0048]    A condition is that in order to form spacer structure spacings  127 ,  127 ′ of essentially the same magnitude, as early as during the lithographic projection or in the design of the mask structures for projection onto a wafer, the structure widths  28  are less than the structure spacing  27  by precisely twice the deposition thickness  49  of the third layer  40 .  
         [0049]    The formation of a hard mask for patterning a dense, regular array of trenches is shown in FIGS.  3 A- 4 F. FIG. 3A shows as a starting point a configuration of four elevated structures  21 - 24  on the first layer  10  made of silicon nitrite, the so-called pad nitrite. The elevated structures  21  to  24  are composed e.g. of polysilicon. FIG. 3A is a plan view and FIG. 3B is a cross-sectional illustration taken along the line IIIB-IIIB shown in FIG. 3A. In a manner similar to that in the previous examples, the third layer  40  made of silicon oxide is deposited conformally onto the second layer  20 —forming the elevated structures  21 - 24 —and etched back, so that the structures  21 - 24  are enclosed by the spacer structures  41 ,  42 , which—as can be seen in the plan view of FIG. 3C—are connected to one another.  
         [0050]    Since the structures  21 - 24  are disposed according to the invention in such a way that each of the structures has the same spacing  27 ′ from two further structures, an initially unstructured interspace is produced in a quadrangle formed by the four structures. The formation of the spacer structures  41 ,  42 , which are formed with the deposition thickness  49  such that the spacer structures  41 ,  42  are connected to one another, results in that a region  125  enclosed by the spacer structures  41 ,  42  forms within the quadrangle described. The region  125  thus forms a hole structure between the structures  21 - 24  enclosed by the spacer structures  41 ,  42 . By selectively removing the second layer  20  between the spacer structures  41 ,  42 , further hole structures  121 ,  123  are also formed at the locations of the previous structures  21 - 24 . Widths of the hole structures  121 ,  123 ,  125  can be dimensioned further by depositing a fifth layer  50  (FIGS.  3 E- 3 F). It follows as a particularly advantageous result, by contrast, that the hard mask formed by the spacer structures  41 ,  42 ,  51 - 54 , which hard mask can also be transferred to the silicon nitride layer  10 , has been formed from the originally four structures  21 - 24  now five hole structures  121 - 125  by the method according to the invention. If the structures  21 - 24  are now considered as an excerpt from a regularly disposed array of a multiplicity of structures, the result is a doubling of the density of hole structures on a wafer, for example, compared with the structures  21 - 24  originally formed lithographically on the first layer  10 , as can be seen from the diagrammatic comparison of FIGS. 5A and 5B. It should be taken into consideration, however, that the grid orientation is rotated through 45 degrees in the case of the processing by the method according to the invention. Furthermore, it should be taken into account owing to the two-dimensional mode of consideration in this exemplary embodiment that the structure spacing  27  is no longer halved here, but rather is only divided by the factor {square root}{square root over (2)}. This example can advantageously be used to form dense memory cell arrays, as are planned particularly in the technology generations below 110 nm, namely in trench structures or memory cells disposed in a matrix-like fashion.  
         [0051]    The same success can also be achieved on the basis of hole structures  29  formed in a fourth layer  60  formed, for example, from borophosphosilicate glass, as can be seen in FIG. 4A. The requirements made of the configuration of the hole structure  29  are the same as those made of the elevated structures  21 - 24 . It should be noted, however, that in a memory cell array having a multiplicity of hole structures  29 —or elevated structures  21 - 24 —there is the condition that each of the structures has the same spacing from four further structures with the exception of those structures which are located at the edge of the memory cell array.  
         [0052]    The third layer  40  is deposited conformally into the hole structures  29  and etched back, thereby forming spacer structures  41 ,  42  on the inner side areas of the hole structures  29  (FIG. 4B). The hole structures  29  are filled with a second layer  20  made of polysilicon, for example, and planarized in a chemical mechanical polishing process. The planarization process is ended on the borophosphosilicate glass. The silicate glass  60  is subsequently removed selectively with respect to the polysilicon and silicon oxide (FIGS. 4C and 4D). Afterward, silicon oxide is again deposited and etched back with an etching stop on the silicon nitride. Afterward, the polysilicon  20  is removed, so that the spacer structures are isolated on the silicon nitrite of the first layer  10 . In this method, too, proceeding from hole structures produced lithographically, an additional hole structure  125  is formed between the spacer structures  41 ,  42 ,  51 ,  54 . In contrast to the method proceeding from the elevated structures, in the case of this aspect, the removal of the filling of the second layer  20  only after the second spacer formation enables the production of the spacer structures  51 ,  54  on only one side of the first and second spacer structures  41 ,  42 . This simultaneously leads to a displacement of the center point of composite spacer structures.