Abstract:
Disclosed herein are techniques for using diblock copolymer (DBCP) films as etch masks to form small dots or holes in integrated circuit layers. In an embodiment, the DBCP film is deposited on the circuit layer to be etched. Then the DCBP film is confined to define an area of interest in the DCBP film in which hexagonal domains will eventually be formed. Such confinement can constitute masking and exposing the DCBP film using photolithographic techniques. Such masking preferably incorporates knowledge of the domain spacing and/or grain size of the to-be-formed domains in the area of interest to ensure that a predictable number and/or orientation of the domains will result in the area of interest, although this is not strictly necessary in all useful embodiments. Domains are then formed in the area of interest in the DBCP film which comprises a hexagonal array of cylindrical domains in a matrix. The film is then treated (e.g., with osmium or ozone) to render either the domains or the matrix susceptible to removal, while the other component is then used as a mask to etch either dots or holes in the underlying circuit layer.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to the use of a diblock copolymer, and particularly to the use of a diblock copolymer selectively provided in a defined area to form a predictable etch mask.  
       BACKGROUND  
       [0002]     As is well known in the field of polymer chemistry, diblock copolymers (DBCPs) comprise molecules having two polymer blocks joined together by a covalent bond. DBCPs can come in many forms, such as those disclosed in M. Park et al., “Block Copolymer Lithography: Periodic Arrays of ˜10 11  Holes in 1 Square Centimeter,” Science, Vol. 276, pg. 1401, May 30, 1997) (the “Park Reference”), which is incorporated herein by reference. As disclosed in the Park Reference, DBCPs can include molecules with blocks of polystyrene and polybutadiene (PS-PB), or polystyrene and polyisoprene (PS-PI). Other references disclose DBCP molecules with blocks of polystyrene and polymethylmethacrylate (PS-PMMA), yielding a conjoined molecule having a molecular weight of 70 kg/mol and a 7-to-3 mass ratio of PS to PMMA.  
         [0003]     Because the block components are not miscible, the DBCP molecules will tend to align to bring like blocks (or ends of the molecule) together when energy is added to the film. Thus, when the DBCP solution is placed on a support structure (such as a silicon substrate or other film placed thereon), like ends of the molecules will draw together to form small cylindrical domains in the DBCP film. Such domains may be cylindrical or spherical in nature depending on the relative polymer chain lengths and on the surface binding energies. More specifically, the molecules will aligned when the glass transition temperature is exceed for the DBCP film in question (e.g., approximately 125° C. for a PS-PB DBCP).  
         [0004]     Such cylindrical domains in a thin film will naturally tend to become as closely packed as possible, and hence generally take on a hexagonal or “honeycomb” appearance in the DBCP film. This is shown in  FIG. 1 , which shows from a top view cylindrical domains of PB  16   a  in a matrix of PS  16   b . The domains  16   a  typically have a diameter of approximately 10-20 nanometers (“d”). Moreover, typical domains are spaced from one another at their centers by 40 nanometers (“x”), what is referred to as the domain spacing. The sizes and spacing of the diameters of the domains (“d”) as well as the domain spacing (“x”) depend on the relative sizes (e.g., chain lengths) the polymers blocks used in the DBCP, and will vary between different DBCP formulations. The exact dimensions “d” and “x” for a given DBCP film are usually well known and can be well tailored for a given application.  
         [0005]     Because each of the block components are sensitive to chemicals that the other is not, one of the block components can be selectively removed in the DBCP film, leaving either the cylindrical domains (e.g., of PB) or holes where the cylindrical domains used to appear (e.g., of PS), which provides creative masking solutions for underlying structures.  
         [0006]     Exemplary processes for removing one of the block components and for etching an underlying support structure are disclosed in the Park Reference, and are briefly illustrated in  FIG. 2 .  FIG. 2A  shows initially that a DBCP layer  16  has been placed on a layer  11  to be etched upon substrate  10 . Should it be desired to leave domains  16   a  (i.e., to remove the PS) as an etch mask, the DBCP layer  16  can be treated with osmium, and more specifically an OsO 4  vapor (referred to herein as “Process I”). This treatment “stains” the PB domains  16   a  by adding osmium across the carbon-carbon double bonds in the PB backbone, making the domains more resilient to the plasma etchant used to etch the underlying layer  11 , as shown in  FIG. 2B . After such plasma etching, the remaining domains  16   a  are removed ( FIG. 2C ), leaving “dots” of layer  11 . Should it be desired to remove domains  16   a  (i.e., to remove the PB) as an etch mask, the DBCP layer  16  can be treated with ozone (referred to herein as “Process II”). Ozone predominantly attacks the carbon-carbon double bonds in the PB domains  16   a , cutting the bonds and producing PB fragments that can be removed with water. This results in voids in the PS matrix  16   b , thus largely exposing the underlying layer  11  in the locations where the PB domains  16   a  used to be present ( FIG. 2D ). The layer  11  can then be plasma etched using the PS matrix  16   b  as a mask ( FIG. 2E ). After such plasma etching, the remaining portions of the PS matrix  16   b  are removed ( FIG. 2F ), leaving “holes” in layer  11 .  
         [0007]     While DBCPs can be used in a variety of masking applications to fabricate different types of structures for differing purposes, they find particular utility in the manufacture of memory cells. For example,  FIG. 3  demonstrates using a DBCP layer to form flash EPROM memory cells using Process I discussed above. In this example, as shown in  FIG. 3A , a silicon crystalline substrate  10  is layered with a tunnel dielectric  12  (e.g., silicon dioxide or nitride) and a polysilicon layer  14 . A DBCP layer  16  is deposited on the polysilicon layer  14  and processed to form domains  16   a  therein. The DBCP layer  16  is treated (with osmium) and the structure is etched ( FIG. 3B ), thus etching the polysilicon to leave polysilicon dots  14   a . Once the domains  16   a  are removed ( FIG. 3C ), a control gate dielectric  17  (e.g., silicon dioxide or nitride) is formed over the resulting structure ( FIG. 3D ). Then a second layer of polysilicon  18  is deposited and etched using traditional patterning and etching techniques ( FIG. 3E ), and which might have a width (w) as small as 100 nanometers or so. Thus, a flash memory cell is formed, in which charge is selectively storable on the polysilicon dots  14   a , which in tandem act like a single floating gate in a traditional flash memory cell and which are controllable by control gate layer  18 .  
         [0008]     A modification to the process flow for forming a flash memory cell using a DBCP layer and Process II is illustrated in  FIG. 4 . As shown in  FIG. 4A , a thick dielectric layer  12  (which eventually will become in part the tunnel dielectric  12 ) is formed on the silicon crystalline substrate  10 . The DBCP layer  16  is formed on the dielectric layer  12 , and again separated into its constituent components  16   a  and  16   b . In this modified process, the PB domains  16   a  are treated (with ozone) and removed ( FIG. 4B ). Remaining portions  16   b  act as the mask for the underlying dielectric layer  12 , which is plasma etched to form cylindrical holes  12   a  whose bottoms constitutes tunnel oxide  12  ( FIG. 4C ). Thereafter, polysilicon is deposited on the resulting structure and etched back to at least partially fill the cylindrical holes  12   a  to form polysilicon dots  14   a  ( FIG. 4D ). Then the control gate dielectric  17  is deposited ( FIG. 4E ) and the control gate layer  18  of polysilicon is patterned and etched ( FIG. 4F ).  
         [0009]     However, while useful as masking layers, DBCP films as used in the prior art are not ideal, especially when applied to the formation of memory cells such as those illustrated above. First, the number of cylindrical domains formed in the patterned layer (e.g., polysilicon domains  14   a ) will not always appear in a predictable relationship with respect to other structures traditionally patterned elsewhere on the device. For example, and referring to  FIG. 5A , the relationship between the polysilicon dots  14   a  and the overlying control gate  18  (in dotted lines) are shown. As can be seen, the control gate  18  does not entirely cover a discrete number of polysilicon dots  14   a : some dots  14   a  (e.g.,  19 ) are only partially covered by the control gate  18  at its edge. Thus, as alignment varies from control gate to control gate (or from device to device), the numbers of polysilicon dots  14   a  covered (and thus affected) by the control gate  18 , and/or the extent of that coverage, will change. Even if the control gate could be consistently aligned and patterned with extreme precision (+/−1 nanometer or so), the position of the polysilicon dots  14   a  will change from control gate to control gate because they are defined by the DBCP film, which in turns grows its domains at random starting locations on the device. The effect is that each control gate  18  (or memory cell) will have slightly different electrical properties due to the different numbers and orientations of the dots  14   a . Such variability in the finished device is of course not optimal.  
         [0010]     Another problem (which exacerbates the first) is that the domains in the DBCP layer will not form on the device with perfect uniformity. In this regard, the DBCP layer is akin to polycrystals. Thus, the DBCP layer will establish local regions of perfect order, or “grains”  20 , but on the whole will have inconsistency in its ordering, as is shown in  FIG. 5B . (Typically, the grain size of such grains  20  may be on the order of ten domains or so, but this is variable and depends on the temperatures and times used during domain formation). Thus, it is seen that the DBCP layer in  FIG. 5B  (as reflected in the eventual locations of the polysilicon dots  14   a ) had at least three fairly distinct grains  20   a - 20   c  defining grain boundaries  21  in between. More generally however (and possibly as a result of grain formation), certain domains or areas of domains  22  are disordered when compared to the predominant local ordering that is present. The salient point is that the domains in the DBCP layer, and hence resulting structures etched thereby such as the polysilicon dots  14   a , will vary in their order. Again, this injects variance into devices formed using such structures (e.g., memory cells), as each control gate  18  may have slightly different numbers or arrangements of polysilicon dots  14   a  that it can influence (or that it is influenced by).  
         [0011]     Richard D. Peters et al., “Combining Advanced Lithographic Techniques and Self-Assembly of Thin Films of Diblock Copolymers to Produce Templates for Nanofabrication,” J. Vac. Sci. Tech, B 18(6), pp. 3530-34 (November/December 2000), which is hereby incorporated by reference, suggests a method to more accurately order the domains as they form in the matrix. In Peters, a substrate was treated with an “imaging layer,” such as an alkylsiloxane layer. The imaging layer is patterned using extreme ultra-violet (EUV) or X-ray radiation to form chemically-altered stripes whose period roughly match that of the domain spacing, x. Because these chemically-altered stripes selectively wet to the domains, the domains will tend to form above them, leaving the matrix portions of the copolymer in the unexposed portions of the imaging layer between the chemically-altered stripes (anti-stripes). The diblock copolymer is then formed over the imaging layer and heated to promote domain formation, such that the domains form in straight lines over the chemically-altered stripes, and the matrix portion forms in straight lines over the anti-stripes. Then, either the domains or the matrix are removed, and used as a template.  
         [0012]     However, Peters&#39; approach is not suitable for some applications. First, it requires the use of an imaging layer, which introduces potential contamination and complexity to the process. Second, while promoting domain order, such domain ordering is formed along straight lines. This is useful for patterning lines in the underlying circuit layer, but not dots or holes, and thus is not useful in all applications.  
       SUMMARY  
       [0013]     Disclosed herein are techniques for using diblock copolymer (DBCP) films as etch masks to form small dots or holes in integrated circuit layers. In an embodiment, the DBCP film is deposited on the circuit layer to be etched. Then the DCBP film is confined to define an area of interest in the DCBP film in which hexagonal domains will eventually be formed. Such confinement can constitute masking and exposing the DCBP film using photolithographic techniques. Such masking preferably incorporates knowledge of the domain spacing and/or grain size of the to-be-formed domains in the area of interest to ensure that a predictable number and/or orientation of the domains will result in the area of interest, although this is not strictly necessary in all useful embodiments. Domains are then formed in the area of interest in the DBCP film which comprises a hexagonal array of cylindrical domains in a matrix. The film is then treated (e.g., with osmium or ozone) to render either the domains or the matrix susceptible to removal, while the other component is then used as a mask to etch either dots or holes in the underlying circuit layer. The disclosed process is particularly useful in the formation of flash EPROM cells, but has other uses in integrated processes benefiting from arrays of well-aligned dots or holes.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     Embodiments of the inventive aspects of this disclosure will be best understood with reference to the following detailed description, when read in conjunction with the accompanying drawings, in which:  
         [0015]      FIG. 1  illustrates a prior art diblock copolymer (DBCP) film and the formation of cylindrical domains.  
         [0016]      FIGS. 2A-2F  illustrate prior art processes for using the DBCP film as a mask to pattern either small dots or small holes in an underlying film.  
         [0017]      FIGS. 3A-3E  illustrate a prior art process for forming a Flash EPROM memory cell using a DBCP film.  
         [0018]      FIGS. 4A-4F  illustrate a modified prior art process for forming a Flash EPROM memory cell using a DBCP film.  
         [0019]      FIGS. 5A and 5B  illustrate problems with the prior art processes of  FIGS. 3A-3E  and  4 A- 4 F, specifically problems with partial domain overlap, the formation of grains, and unpredictability in the number of domains formed when compared to an overlying control gate.  
         [0020]      FIGS. 6-11B  illustrate an exemplary process for forming a flash EPROM cell using a confined area of a DBCP film. 
     
    
     DETAILED DESCRIPTION  
       [0021]      FIGS. 6-11B  illustrates a process for using a diblock copolymer (DBCP) film as a mask to form domains in a controlled manner and in a confined region. The confined region is preferably sized and shaped so that a predictable number of domains are formed. Because the confined region is relatively small in size, the possibility of domains forming in a non-uniform manner (e.g., with grain boundaries or other non-uniformities) is reduced. Moreover, the disclosed process is also beneficial in that structures, such as control gate, will not affect (or by affected by) the presence of partial domains at the edges of such structures. In the context of fabricating a Flash EPROM cell, which illustrates an exemplary device whose fabrication is assisted by the disclosed processing techniques, the result is a controlled, well-ordered, non-partial domain structure that will improve cell-to-cell uniformity and that exhibits improved predictability in the electrical behavior in the finished cell.  
         [0022]     The exemplary process for  FIGS. 6-11B  employs processing the DBCP layer according to Process I as discussed earlier. However, one skilled in the art will realize that Process II could also be used, although it is not illustrated.  
         [0023]      FIG. 6  shows a silicon substrate  10 , a gate oxide  12 , and a presently unpatterned polysilicon layer  14 . A DBCP layer  16  has been placed on to the wafer, e.g., by spinning it in a toluene solution to a thickness of 50 nm or so. The DBCP layer  40  is then patterned using radiation  30  and an etch mask  32 , as shown in  FIG. 7A . (The radiation  30  and etch mask  32  can be those typically used in semiconductor processing). In so doing, portions  40   b  of the DBCP layer become degraded by the radiation  30 ; other portions (area of interest)  40   a  protected by the mask  32  remain unaffected. As will be seen, polysilicon dots  14   a  will eventually be formed underneath the area of interest  40   a.    
         [0024]      FIG. 7B  shows further details of the mask  32  and the area of interest  40   a . One preferable aspect of the disclosed technique is to ensure that the polysilicon dots  14   a  eventually formed in the area of interest are uniform and well ordered. As noted earlier, this would give rise to devices (e.g., flash memory cells) with more predictable performance. To achieve this result, knowledge of the domain spacing (see  FIG. 1 ; “x”) that will eventually be formed in the DBCP layer  40  is beneficial. As noted earlier, the domain spacing “x” is generally well known for a given DBCP formulation, and/or the domain spacing can be experimentally determined for a given formulation and thickness.  
         [0025]     In any event, knowing the domain spacing “x” before hand, the extent of the area of interest  40   a , and hence the extent of the mask  32  to be used, can be confined so that a set number of domains will form in the area. As noted earlier, the domains in the DBCP layer  40  will preferably form in a hexagonal pattern, and (for a given formulation) with a domain spacing of “x” between the domains. Knowing this, the area of interest  40   a  can be tailored so that a discrete number of domains will form within the area of interest. For example, and as shown in  FIG. 7B , assume that the area of interest  40   a  is hexagonal in shape, which is a particularly beneficial shape because the to-be-formed domains will naturally match this shape. By defining the area of interest  40   a  to include a discrete and quantized number of these domains, it likely that the domains will form in the DBCP layer  40  in an ordered and predictable fashion, especially if the area of interest is smaller than the normally-exhibited grain size (see  FIG. 5B ;  20 ) for the film.  
         [0026]     Thus the area of interest  40   a  has been defined so as to include 19 potential domains (more specifically two rows of three domains, two rows of four domains, and a row of five domains. Using geometry, it can be seen that the area of interest  40  thus must be 5*x along its longest diameter  50 , and 5*{square root}3/2*x along its shortest diameter  51 .  FIG. 7B  shows these dimensions more generically to include N numbers of domains along these diameters.  
         [0027]     Of course, constraining the area of interest  40   a  in this fashion is ultimately accomplished by appropriate sizing and shaping of the mask  32 . However, it is not necessary that the mask  32  need exactly match the size and shape of the patterned area of interest  40   a , although this is shown for ease of understanding. As one skilled in the art will recognize, masks  32  are often made larger than the desired area to be patterned (e.g., 5 or 10 times larger), and then scaled down by optics to the appropriate size for patterning. Moreover, other structures may be added to masks  32  (not shown) to reduce diffractive effects and to allow for crisp patterning, and which will change the shape of the mask relative to the to-be-patterned area  40   a . In any event, what is critical is to ensure that the patterned area of interest  40   a  is well defined and well constrained, and those skilled in the art will know how to make an appropriate mask  32  to achieve that result.  
         [0028]     Note that the area of interest  40   a , measuring approximately 200 nm at its largest diameter  50 , is relatively small in the sense that it can encompass a relatively small number of domains (e.g., a maximum length of five as shown). This is beneficial because the domains are likely to form uniformly because the number of domains is less than would normally appear in a grain ( FIG. 5B ;  20 ) of the DBCP layer were that layer unbounded. However, the area of interest  40   a  is also suitably large to be patterned by traditional photolithography techniques.  
         [0029]     This being said however, a relatively large and traditionally-patternable area of interest  40   a  and/or the area of interest encompassing a relatively small number of domains are not limitations of the disclosed technique. Benefits are had through use of the disclosed technique even should grain boundaries eventually form in the area of interest  40   a . For example, even if large areas of interest  40   a  are formed, and even if grain boundaries are formed, the area of interest  40   a  will still likely contain a predictable, quantized number of domains, and in any event will not exhibit any partial domains at its edges. Likewise, benefits exist even if the area of interest is quite small and not patternable using traditional techniques. For example, using more advanced techniques, areas of interest as small as a few domains, or even a single domain, are possible.  
         [0030]     The area of interest  40   a  is shown as hexagonal in shape, which as noted provides a natural match for the way the domains would preferably align themselves. However, it should be noted that this is not strictly necessary. Other shapes could be formed as well, including more traditional squares or rectangles. In this regard, although the domains may not form in such areas with perfect ordering, the ordering exhibited is expected to be predictable even if not perfect by virtue of confining the area of interest  40   a . Moreover, even in a non-hexagonal area, predictable numbers of complete non-partial domains should form, promoting device uniformity. Routine experimentation may be required for a given area of interest shape to empirically determine the nature of domain formation.  
         [0031]     After patterning the areas of interest  40   a , the exposed portions  40   b  of the DBCP layer  40  can either be removed using traditional photolithography stripping techniques ( FIG. 9 ), or can remain and be removed later after domain formation, or after the domains are treated (with osmium or ozone). ( FIG. 8A  shown portions  40   b  remaining).  
         [0032]     Next, and still referring to  FIG. 8A , the DBCP layer  40 , and specifically the area of interest portion  40   a  of that layer, are heated to promote domain  41   a  formation. As described earlier, this requires the DBCP layer  40  to be heated beyond the glass transition temperature for the formulation in question, normally above 125° C. If portions  40   b  still remain at this time, they will not phase segregate upon heating. For the exemplary hexagonal area of interest  40   a , the result of domain formation is shown in  FIG. 8B , which shows from a top view that the domains have formed with good ordering and in a predictable number. Again, this results from tailoring the size of the area of interest  40  with knowledge of the domain spacing “x” and/or forming the area of interest  40   a  smaller than a typical DBCP grain size, as discussed earlier.  
         [0033]     After treatment (with osmium) to etch-harden the domains  41   a  pursuant to Process I, the resulting structure is plasma etched to selectively remove the polysilicon layer  14  to form polysilicon dots  14   a , as shown in  FIG. 9 . As noted above, osmium treatment makes domains  41   a  (e.g., PB) more resilient to plasma etching than non-domain portions  41   b  (e.g., PS). Then domains  41   a  are removed, and a control gate dielectric  17  is formed ( FIG. 10 ). Thereafter, the control gate  18  can be patterned over the polysilicon dots  14   a  and etched, as shown in  FIG. 11A . Although shown as hexagonal in shape, the control gate  18  can take on other shapes (e.g., rectangular), even if the underlying polysilicon dots  14   a  were patterned within a different shape. At this point, processing continues to finish fabrication of the device. For example, the control gate  18  can be used as an ion implantation mask to form N+ source/drain regions for the formed transistors. (One skilled in the art will also appreciate that isolation regions formed in or on the silicon substrate  10  would also normally be present and useful to define the source/drain regions, but these have not been shown because such structures are well known and not useful to illustrate the disclosed inventive techniques).  
         [0034]     A comparison of  FIG. 11B  with the prior art illustrations of  FIGS. 5A and 5B  show the utility of the disclosed technique. First, it is seen that a predictable number of polysilicon dots  14   a  appear within the confines of the control gate  18  using the disclosed technique. Moreover, the dots  14   a  do not appear only partially within the confines of the control gate  18  (i.e., at its edges). Lastly, the dots  14   a  are well ordered within the control gate  18 . In sum, a device with improved performance predictability results.  
         [0035]     The disclosed technique allowing for selective provision of a DBCP layer has been illustrated in the context of the fabrication of a flash memory cell. However, other structures and devices requiring small ordered dots of material (“Process I”), or ordered holes (“Process II”), can benefit from the disclosed technique, such as but not limited to those disclosed in the Park Reference cited earlier.  
         [0036]     Although in a preferred embodiment diblock coplymers are used, other types of block copolymers could be used as well (e.g., triblock copolymers with a middle block for joining the two primary blocks of interest).  
         [0037]     Moreover, while disclosed herein as being usable as a plasma etch mask, it should be understood that the DBCP layer can be used with wet etchants as well.  
         [0038]     Other references disclosing DBCPs and having relevance to the present disclosure, and which are incorporated herein by reference, include: M. Reed et al., “Molecular Random Access Memory Cell,” App. Phys. Lett., Vol. 78, No. 22, pg. 3735 (Jun. 4, 2001); G. Hadziioannou, “Semiconductive Block Copolymers for Self-Assembled Photovoltaic Devices,” MRS Bull., pg. 456 (June 2002); C. Zhou et al., “Nanoscale Metal/Self-Assembled Monolayer/Metal Heterostructures,” App. Phys. Lett., Vol. 71, No.  5 , pg. 611 (Aug. 4, 2001); and IBM Research, “IBM Nanotechnology Announcement at IEDM,” published at http://domino.research.ibm.com /Comm/bios.nsf/pages/selfassembly-iedm.html (including animation) (date unknown).  
         [0039]     It should be understood that the inventive concepts disclosed herein are capable of many modifications. To the extent such modifications fall within the scope of the appended claims and their equivalents, they are intended to be covered by this patent.