Abstract:
A sublitohgraphic trench is formed in a semiconductor substrate by first forming a microtrench which is subsequently used as a mask. Filled sublithographic trenches are formed with a width corresponding to the width of the microtrenches.

Description:
BACKGROUND OF INVENTION 
     1. Technical Field 
     The present invention relates generally to semiconductor manufacture, and more particularly, to the formation of a sublithographic trench in a semiconductor substrate. 
     2. Related Art 
     A number of processes have been proposed for patterning, featuring dimensions that are smaller than those obtainable by conventional photolithography. For example, sidewall image transfer is available for patterning layers by subtractive etching. However, this process is not suitable for producing sublithographic trenches. Because trench patterns are widely used in a number of applications, such as making damascene gates, damascene interconnects, isolation trenches, etc., it would be desirable to have a process that could be employed to achieve trenches with sublithographic dimensions. 
     Typically, when trenches having sublithographic dimensions have been formed, it has occurred as an undesirable artifact of an etching process. For example, during polysilicon gate etch with a high density plasma reactor, ions reflecting off sidewalls can cause increased etching at the periphery of the feature and possibly trenching into the underlying substrate. Such microtrenching into the substrate can adversely affect device performance and reliability. See, for example, U.S. Pat. No. 5,895,273, for a discussion of microtrenching problems. In this context, microtrenching has been studied, with the objective being to avoid or overcome it as undesirable. See also, J. -H. Lee, et al. Effects Of Buffer Layer Structure On Polysilicon Buffer LOCOS For The Isolation Of Submicron Silicon Devices, IEEE Transactions On Electron Devices, Vol. 45, No. 10, October 1998, pp. 2153-2160; I. J. Gupta, et al., A Comprehensive Assessment of Microtrenching During High Density Polysilicon Etch, 3rd International Symposium on Plasma Process-Induced Damage, June 1998, pp. 84-87, and Morioka, et al., Effect of Wafer Bias Frequency On Microtrenching During High Selective Gate Etching, 4th International Symposium On Plasma-Process Induced Damage, May 1999, pp. 159-162. 
     On the contrary, approaches attempting to use microtrenching, rather than avoiding it, are described in U.S. Pat. No. 5,736,418 and Eur. Pat. Appln., Pub. No. EP 0 632 495 A2. In the first reference, Microtrenches are formed in an effort to control hot electron effects and in the second, microtrenches are formed, then filled, to form isolation structures. Neither, however, uses the microtrenches for further etching definition and control. 
     Therefore, there exists a need in the industry for an improved method of making a sublithographic trench, without adding an unreasonable number of steps, while at the same time addressing other manufacturing problems such as microtrenching. 
     SUMMARY OF INVENTION 
     It is against this background that the present invention introduces a method of forming a sublithographic trench by first forming a microtrench, which is subsequently used as a mask. In particular, in accordance with the invention, there is provided a method of forming a sublithographic trench in a semiconductor substrate, comprising the steps of: forming a selectively etchable layer on the substrate; forming a masking layer on the selectively etchable layer; forming a pattern in the masking layer; using the pattern to partially etch the selectively etchable layer to form a microtrench therein; and using the microtrench as a mask to etch a sublithographic trench in the substrate. 
     Further, in accordance with the invention, there is provided a trench formed in a semiconductor substrate, the trench having a width which is sublithographic and corresponds to the width of a micotrench etched into a selectively etchable layer over the substrate. 
     The foregoing and other features and advantages of the invention will be apparent from the following more particular description of embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The embodiments of this invention will be described in detail, with reference to the following figures, wherein like designations denote like elements, and wherein: 
     FIGS. 1A-1F are schematic section views illustrating the method steps of one embodiment according to the present invention; 
     FIGS. 2A-2F are schematic section views illustrating the method steps of another embodiment according to the present invention; 
     FIGS. 3A-3F are schematic section views illustrating the method steps of yet another embodiment according to the present invention; 
     FIGS. 4A-4F are schematic section views illustrating the method steps of a further embodiment according to the present invention; and 
     FIGS. 5A-5F are schematic section views illustrating the method steps of a still further embodiment according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to the drawings, FIGS. 1A-1F show an example of a method sequence, which provides a sublithographic trench in inorganic and organic insulator layers, which is subsequently filled with a conductive material. As shown in FIG. 1A, there is shown a processed semiconductor structure  1 , which may typically include a semiconductor substrate comprising silicon, GaAs or the like, on which devices such as capacitors and transistors are formed. Deposited on the structure  1  is an insulator  2 , which in this example is a low-k organic material, generally having a dielectric constant less than 4. An example of a suitable low-k organic material is disclosed in U.S. Pat. No. 5,965,679, the entire contents of which are incorporated herein by reference, as well as details concerning its preparation and use. Other low-k organic materials that can be employed will be known to those skilled in the art. An additional insulator  3 , such as an inorganic material, e.g. silicon carbide, is deposited on the insulator  2 . Typically, layers  2  and  3  are applied by spin coating, although dip coating, spray coating, extrusion coating, etc. may be suitably employed. Typically, the organic insulator  2  is about 100-1,000, or preferably 400-600 (e.g. 500) nanometers (nm) thick, while the inorganic insulator  3  is about 20-200, or preferably 40-60 (e.g. 50), nm thick. 
     A two-layer hard mask is then provided on the inorganic insulator  3 , which includes a silicon nitride layer  4  and a silicon oxide layer  5 . These layers  4  and  5  can be deposited, for example, by chemical vapor deposition (CVD) so that the layers  4  and  5  are about 10-100, preferably 40-60 (e.g. 50), nm thick and 50-200, preferably 75-125 (e.g. 100), nm thick, respectively. Conventional photolithography was employed to form a photoresist pattern that was transferred into silicon oxide layer  5  by reactive ion etching (RIE). 
     As shown in FIG. 1B, an RIE process, such as one that is fluorine-based (e.g. CF 4 , CHF 3 , or C 4 F 8 ) with high bias voltage, is used to partially etch the silicon nitride layer  4 . During etching, an enhanced reaction occurs as a result of ion scattering from the sidewalls to form microtrenches  6 . If desired, a blocking mask (e.g. a photoresist)  7  is applied as to cover some of the microtrenches but not all to define a pattern where sublithographic trenches are to be formed, as shown in FIG.  1 C. Sublithographic trenches then will be formed only at the edges of the silicon oxide hardmask  5  which are not covered by the blocking mask  7 . Additional etching of the silicon nitride layer  4  defines the widths of the sublithographic trenches  8  t be formed, as shown in FIG.  1 D. Next, sublithographic trenches  8  are etched into the underlying layers  3  and  2  or substrate, using the microtrenches as a mask pattern, using conventional RIE processing as will be apparent to those skilled in the art. For example, an oxygen/nitrogen-based process can be used, such as N 2 /H 2 . It should be noted, that during the etching, the blocking mask  7  is removed, as shown in FIG.  1 E. 
     Following formation of the sublithographic trenchs  8 , further processing would include depositing a conductor  9  over the insulator  3  and into the sublithographic trenches  8 . The conductor  9  may comprise copper in a particularly preferred embodiment, which would usually be deposited by electroplating, although other techniques, such as electroless plating, could be employed. Once the sublithographic trenches  8  have been filled with conductor  9 , the surface of conductor  9  with insulator  3  is coplanarized using chemical mechanical polishing or other suitable technique, as shown in FIG.  1 F. During coplanarization, the silicon oxide and silicon nitride hardmasks,  5  and  4 , respectively, would be removed to provide a filled sublithographic trench  8  in the inorganic insulator  3  and organic insulator  2 . 
     FIGS. 2A-2F show another example of a method sequence, which is similar to that of FIGS. 1A-1F, but does not involve use of an insulator  3 . This sequence also provides a sublithographic trench in inorganic and organic insulator layers which is subsequently filled with a conductive material. As shown in FIG. 2A, a two-layer hard mask is provided on an insulator  2 , which comprises a low-k organic material, where the hardmask includes a silicon nitride layer  4 , deposited on the insulator  2 , and a silicon oxide layer  3 . Conventional photolithography was employed to form a photoresist pattern  10  that was transferred into silicon oxide layer  5  by RIE, e.g. using fluorine-based (such as CF 4 , CHF 3  or C 4 F 8 ) with high bias voltage to partially etch the silicon oxide layer  5 , as shown in FIG.  2 B. The etch conditions are selected so that silicon oxide is removed to form microtrenches  6 , while leaving behind silicon oxide at the centers of the etch patterns. As shown in FIG. 2C, if desired, a blocking mask (e.g. photoresist)  7  is applied as to cover some of the microtrenches but not all, to define a pattern where sublithographic trenches are to be formed. 
     Referring to FIG. 2D, RIE is used to etch silicon nitride layer  4  with high selectivity to silicon oxide layer  5 , e.g. using a fluorine-based RIE process. Sublithographic trenches  8  are formed in the silicon nitride layer  4 , using the microtrenches as a mask pattern, in regions not covered by the blocking mask  7 . The depths of the sublithographic trenches  8  are extended by further etching into the insulator  2  using, for example, an oxygen/nitrogen-based process, such as N 2 /H 2 . It should again be noted that during the etching, the blocking mask  7  is removed, as shown in FIG.  2 E. 
     Following formation of the sublithographic trenches  8 , further processing can be carried out as, for example, in accordance with the first example. In that case, the result is a metal  9  filled, sublithographic trench  8  in the hard mask silicon nitride layer  4  an organic insulator  2 , with the silicon nitride layer  4  having served as a polish stop. 
     FIGS. 3A-3F show another example of a method sequence, which is similar to that of the second example of FIGS. 2A-2F, but using a different material, an anti-reflective coating (ARC) instead of silicon oxide, and an inorganic insulator in place of the organic insulator. This sequence provides a sublithographic trench in inorganic insulator layers which is subsequently filled with a conductive material. As shown in FIG. 3A, deposited on the structure  1  is an insulator  11 , which in this example is an inorganic material, such as silicon oxide. A silicon nitride layer  4  is deposited on the insulator  11 , and an ARC  12  (e.g. “Duv30” available from Japan Synthetic Rubber Company) is deposited on the silicon nitride layer using, for example, standard spin apply and baking conditions. Conventional photolithography was employed to form a photoresist pattern  10  that was transferred into ARC layer  12  by RIE, e.g. using oxygen-based RIE with high bias voltage to partially etch the ARC layer  12 , as shown in FIG.  3 B. The etch conditions are selected so that ARC is removed to form microtrenches  6 , while leaving behind ARC at the centers of the etch patterns. As shown in FIG. 3C, if desired, a blocking mask (e.g. photoresist)  7  is applied as to cover some of the microtrenches but not all, to define a pattern where sublithographic trenches are to be formed. 
     Referring to FIG. 3D, RIE is used to etch silicon nitride layer  4  with high selectivity to silicon oxide layer  11 , e.g. using a fluorine-based RIE process. Sublithographic trenches  8  are formed in the silicon nitride layer  4 , using the microtrenches as a mask pattern, in regions not covered by the blocking mask  7 . The depths of the sublithographic trenches  8  are extended by further etching into the insulator  11  using, for example, a fluorine-based RIE process that is selective to silicon nitride  4 , such as C4F8/CH3/Ar. It should again be noted that during the etching, the blocking mask  7  is removed, as shown in FIG.  3 E. 
     Following formation of the sublithographic trenches  8 , further processing can be carried out as, for example, in accordance with the second example. In that case, the result is a metal  9  filled, sublithographic trench  8  in the hard mask silicon nitride layer  4  and inorganic insulator  11 , with the silicon nitride layer  4  having served as a polish stop. 
     FIGS. 4A-4F show another example of a method sequence, which is similar to that of the third example of FIGS. 3A-3F, but in particular the inorganic insulator  11  is applied over a substrate  13  comprising silicon. This sequence also provides a sublithographic trench in inorganic insulator layers which is subsequently filled with a conductive material. As shown in FIG. 4A, an inorganic material such as silicon oxide is deposited, as to provide an insulator  11  on substrate  13 . As in the third example, a silicon nitride layer  4  is deposited on the insulator  11 , and an ARC  12  (e.g. “DUV30”) is deposited on the silicon nitride layer using, for example, standard spin apply and baking conditions. Convention photolithography was employed to form a photoresist pattern  10  that was transferred into ARC layer  12  by RIE, e.g. using oxygen-based RIE with high bias voltage to partially etch the ARC layer  12 , as shown in FIG.  4 B. The etch conditions are selected so that ARC is removed to form microtrenches  6 , while leaving behind ARC at the centers of the etch patterns. As shown in FIG. 4C, if desired, a blocking mask (e.g. photoresist)  7  is applied as to cover some of the microtrenches but not all, to define a pattern where sublithographic trenches are to be formed. 
     Referring to FIG. 4D, RIE is used to etch silicon nitride layer  4  with high selectivity to silicon oxide layer  11 , e.g. using a fluorine-based RIE process. Sublithographic trenches  8  are formed in the silicon nitride layer  4 , using the microtrenches as a mask pattern, in regions not covered by the blocking mask  7 . The depths of the sublithographic trenches  8  are extended by further etching into the insulator  11  using, for example, a fluorine-based RIE process that is selective to silicon nitride  4 , such as C 4 F 8 /CH 3 /Ar. It should again be noted, that during the etching, the blocking mask  7  is removed, as shown in FIG. 4E, and that the etching extends down to the substrate  13 , or to an etch stop layer, such as silicon nitride, applied to the substrate  13 . 
     Following formation of the sublithographic trenches  8 , further processing can be carried out to grow a gate dielectric (e.g. silicon oxide, silicon nitride, silicon oxynitride, etc.) on the substrate  13 , then depositing a conductor  15  over the insulator  4  and into the sublithographic trenches  8 . The conductor  15  may include, for example, doped polysilicon, refractory metal such as tungsten, titanium nitride, etc. As in the third example, the surface is coplanarized to provide a filled sublithographic trench  8  in the silicon nitride layer  4  and inorganic insulator  11 , with the silicon nitride layer  4  having served as a polish stop. 
     FIGS. 5A-5F show another example of a method sequence, which is similar to that of the third example of FIGS. 3A-3F, but provides a trench in the substrate which is subsequently filled with an insulative material. As shown in FIG. 5A, a pad oxide  16  is grown over substrate  13 , to a thickness usually about 10-100, preferably 40-60 (e.g. 50) nm, then a silicon nitride layer  4 , with a thickness usually about 50-500, preferably 150-250 (e.g. 200) nm, and a silicon oxide layer  5 , with a thickness usually about 10-100, preferably 40-60 (e.g. 50) nm, are deposited, such as by CVD. Next, an ARC  12  (e.g. “DUV30”) is deposited on the silicon oxide layer  5  using, for example, standard spin apply and baking conditions. Conventional photolithography was employed to form a photoresist pattern  10  that was transferred into ARC layer  12  by RIE, e.g. using oxygen-based RIE with high bias voltage to partially etch the ARC layer  12 , as shown in FIG.  5 B. The etch conditions are selected so that ARC is removed to form microtrenches  6 , while leaving behind ARC at the centers of the etch patterns. As shown in FIG. 5C, if desired, a blocking mask (e.g. photoresist)  7  is applied as to cover some of the microtrenches but not all, to define a pattern where sublithographic trenches are to be formed. 
     Referring to FIG. 5D, RIE is used to etch silicon oxide layer  5  with high selectivity to silicon nitride layer  4 , e.g. using a fluorine-based RIE process. Sublithographic trenches  8  are formed in the silicon oxide layer  5 , using the microtrenches as a mask pattern, in regions not covered by the blocking mask  7 . The depths of the sublithographic trenches  8  are extended by further etching into the silicon nitride layer  4  using, for example, a fluorine-based RIE process that is selective to silicon oxide  16 , and into the pad oxide  16  by further etching with, for example, CF 4 , CHF 3  or C 4 F 8 . Etching is then continued into the substrate  13  using, for instance, a chlorine-based RIE process that is selective to silicon oxide. 
     Following formation of the sublithographic trenches  8 , further processing can be carried out as, for example, in accordance with the third example, except that an insulative material  17 , such as silicon oxide is deposited over the silicon nitride layer  4  into the sublithographic trenches  8 . During coplanarization, the silicon nitride layer  4  would serve as a polish stop. 
     In a variation of the previous example, no blocking mask is used, so that oxide filled, sublithographic trenches are formed only around the perimeter of the isolation regions. As will be apparent, this would reduce the oxide pattern factor in large isolation areas, i.e. self-aligned shallow trench isolation (STI) fill. 
     While this invention has been described in conjunction with the specific embodiments outlined above, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. For example, a wide variety of materials may be used for the various layers over a wide range of thicknesses, and etching conditions may be extensively modified, depending upon the structures being formed, and so forth. Accordingly, the embodiments of the invention as set forth above are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention as defined in the following claims.