Patent Publication Number: US-8524606-B2

Title: Chemical mechanical planarization with overburden mask

Description:
RELATED APPLICATION INFORMATION 
     This application claims priority to U.S. Provisional Ser. No. 61/389,546 filed on Oct. 4, 2010, incorporated herein by reference in its entirety. 
     This application is related to commonly assigned applications: “SHALLOW TRENCH ISOLATION CHEMICAL MECHANICAL PLANARIZATION”, Ser. No. 13/012,142, filed concurrently herewith; “CHEMICAL MECHANICAL PLANARIZATION PROCESSES FOR FABRICATION OF FINFET DEVICES”, Ser. No. 13/012,836, filed concurrently herewith; and “FABRICATION OF REPLACEMENT METAL GATE DEVICES”, Ser. No. 13/012,879, filed concurrently herewith, all incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to semiconductor fabrication and more particularly to systems and methods for chemical mechanical planarization (CMP) using an overburden mask to achieve higher planarity. 
     RELATED ART 
     Shallow trench isolation (STI) chemical mechanical planarization (CMP) is a process technology that enables the fabrication of advanced microprocessor chips. Current STI planarization processes involve the use of ceria/surfactant slurry polish followed by a fixed abrasive polish. There are several problems with this method. The ceria/surfactant system exhibits an unstable polish rate problem that contributes to significant variability in the final topography. The fixed abrasive process step is expensive and has high defect counts due to micro scratching. Furthermore, for 22 nm technology nodes and beyond, the planarity requirements are very stringent (&lt;10 nm) for devices with high k metal gate transistors and these planarity requirements are difficult to achieve by conventional CMP processes. 
     Irrespective of the polish process and the slurry systems used, the observed non-planarity is around 200 to 300 Å for most designs, with large STI features recessed with respect to the active area. For 32 nm technology node and beyond, less than 100 Å final topography may be required to achieve better SRAM yields. The importance of controlling the with-in-die (WID) thickness variations and with-in-wafer (WIW) uniformity in STI polish has been emphasized for future devices. It may be difficult to meet these requirements by improved slurry chemistry alone. Other approaches may also become necessary to achieve the high levels of planarity needed for the performance of future devices. 
     SUMMARY 
     Planarization methods include depositing a mask material on top of an overburden layer on a semiconductor wafer. The mask material is planarized to remove the mask material from up areas of the overburden layer to expose the overburden layer without removing the mask material from down areas. The exposed overburden layer is wet etched and leaves a thickness remaining over an underlying layer. Remaining portions of the mask layer and the exposed portions of the overburden layer are planarized to expose the underlying layer. 
     A planarization method includes depositing a mask material on top of an overburden layer on a semiconductor wafer; planarizing the mask material to remove the mask material from up areas of the overburden layer to expose the overburden layer without removing the mask material from down areas; wet etching the exposed overburden layer leaving a thickness over an underlying layer; wet etching the mask material to remove the mask material from the down areas; and performing a touch up planarization to further improve a final planarity, obtain a defect free, smooth surface and expose the underlying layer. 
     These and other features and advantages will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The disclosure will provide details in the following description of preferred embodiments with reference to the following figures wherein: 
         FIGS. 1A-1F  are schematic diagrams showing cross-sectional views of a semiconductor device to illustrate a shallow trench isolation (STI) chemical mechanical planarization (CMP) process in accordance with the present principles; 
         FIG. 2  is a plot showing an effect of pH on polish rates of oxide and nitride by adjusting the pH with phosphoric acid and KOH; and 
         FIG. 3  is a flow chart showing an illustrative method in accordance with the present principles. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In accordance with the present principles, methods for planarization of semiconductor structures, such as, e.g., shallow trench isolation (STI) structures are described. An overburden mask is deposited over a dielectric overburden layer. Top positions of the mask layer are removed by polishing. Next, a wet etch removes an underlying layer below the overburden mask. Then, another polishing process is performed which results in a highly planar surface. 
     In one embodiment, a nitride mask is created not by lithography but by a blanket deposition of a nitride layer, and then the nitride layer is selectively removed from “up” areas by chemical mechanical planarization/polishing (CMP). Slurries that have high selectivity towards nitride and/or a nitride to oxide selectivity ˜1:1 are preferred. A bulk of oxide overburden is removed by wet etching in the presence of the nitride mask. A next step is to remove the top nitride layers by CMP or by wet etching. A final touch up polish with about a 1:1 nitride to oxide selectivity slurry completes the planarization process. Excellent planarity is observed in STI structures planarized with this method. The slurry compositions employed for this process are also disclosed. 
     The flowchart and block diagrams in the Figures may, in some alternative implementations, occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. 
     It is to be understood that the present invention will be described in terms of given illustrative architectures; however, other architectures, structures, substrate materials and process features and steps may be varied within the scope of the present invention. Throughout this disclosure oxide, nitride and polysilicon materials are described. However, these materials are illustrative and other materials are also contemplated and within the scope of the invention. In addition, thickness dimensions are described throughout this disclosure. These thickness dimensions are illustrative and other dimensions may be employed in accordance with the present principles. 
     Devices as described herein may be part of a design for an integrated circuit chip. The chip design may be created in a graphical computer programming language, and stored in a computer storage medium (such as a disk, tape, physical hard drive, or virtual hard drive such as in a storage access network). If the designer does not fabricate chips or the photolithographic masks used to fabricate chips, the designer may transmit the resulting design by physical means (e.g., by providing a copy of the storage medium storing the design) or electronically (e.g., through the Internet) to such entities, directly or indirectly. The stored design is then converted into the appropriate format (e.g., GDSII) for the fabrication of photolithographic masks, which typically include multiple copies of the chip design in question that are to be formed on a wafer. The photolithographic masks are utilized to define areas of the wafer (and/or the layers thereon) to be etched or otherwise processed. 
     The methods as described herein may be used in the fabrication of integrated circuit chips. The resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (that is, as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. In the latter case the chip is mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a motherboard or other higher level carrier) or in a multichip package (such as a ceramic carrier that has either or both surface interconnections or buried interconnections). In any case the chip is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor. 
     Referring now to the drawings in which like numerals represent the same or similar elements and initially to  FIGS. 1A-1F , cross-sectional views of a CMP process are shown in accordance with one illustrative embodiment. This process uses slurries that are capable of providing different polish rates for a top layer and an underlying layer which can be varied to achieve highly planar post polish surfaces. 
     For purposes of explanation, a top layer of oxide and an underlying layer of nitride will be described. These materials represent a commonly employed pair of materials and are particularly useful in shallow trench isolation fabrication processes. Other materials and pairs of materials may also be employed. Slurry compositions for an STI CMP process will also be illustratively described. 
     Initially, a semiconductor substrate  10  has trenches  12  formed therein to be employed in forming shallow trench isolation. The trenches  12  are etched into substrate  10  by forming and patterning a photoresist mask and additional layers which may include a pad oxide  14  and a pad nitride  16 . In  FIG. 1A , an overburdened top layer (e.g., oxide)  18  may include, e.g., tetraethyl orthosilicate (TEOS), high density plasma (HDP) oxide, high-aspect-ratio process (HARP) oxide, etc. is deposited in the trenches  12  and over the pad nitride  16 . It should be understood that other structures may be employed. 
     One goal is to create a highly planar STI structure after CMP. This is accomplished by reducing the dishing and erosion associated with the CMP process by forming a nitride mask  20  for an STI CMP process in  FIG. 1B . The nitride mask  20  may have a thickness of between about 400 to 700 Å of silicon nitride, which is deposited on top of the layer  18  (e.g., oxide) in an STI structure. An amount of nitride  20  needed depends on the initial topography and the thickness of the oxide overburden of layer  18 . This nitride  20  is then polished off to expose the underlying oxide structures in “up” areas  22  of a pattern in  FIG. 1C . No significant amount of nitride in the “down” areas  24  should be removed during this CMP process. A nitride selective slurry with a high polish rate for nitride and near zero polish rate for oxide can be used during this step. Alternatively, a slurry with an approximate 1:1 nitride to oxide selectivity can also be employed. It is preferable to ensure that all the nitride  20  from the “up” areas  22  is removed during this CMP step. 
     In  FIG. 1D , a next step is to etch the oxide  18  from exposed areas  22  to form open areas  26  in the oxide below the nitride  20 . A wet etch with a dilute solution of buffered HF can be used for this purpose. Only about 70 to 80% of the oxide is removed by wet etching leaving about 100 to 300 Å of oxide  18  above pad nitride  16 . This is followed by a CMP step with a nitride selective slurry or a slurry with approximately 1:1 selectivity to complete a final planarization and form surface  28  in  FIG. 1E . 
     Alternatively, the nitride  24  can also be removed by a hot phosphoric acid wet etch. An oxide  18  touch up polish is used to smooth the surface and remove any surface blemishes. This is followed by the usual buffered HF etch (deglaze) and nitride strip steps to remove the pad nitride  16  in  FIG. 1F . 
     Composition of the Nitride Selective Slurry: 
     The slurry according to one embodiment includes: i) Abrasive 5 to 10 W %, ii) Acid 0.1 to 10 g/L, iii) pH in the range of 2 to 5 adjusted with KOH and/or NH 4 OH. In another embodiment the slurry includes i) 5 to 10 W % of colloidal silica abrasive, ii) 0.1 to 10 g/L of phosphoric acid, iii) 0.1 to 15 g/L of citric acid, iv) pH in the range of 2 to 5 adjusted with KOH and/or NH 4 OH. 
     Slurry Components: a) Abrasives: The abrasive may be at least one type of abrasive selected from inorganic and organic particles. Examples of the inorganic particles may include silica, alumina, titania, zirconia, ceria, and the like. Examples of the silica abrasives may include fumed silica, silica synthesized by sol-gel methods, colloidal silica, and the like. The fumed silica may be obtained by reacting silicon tetrachloride or other compounds of silicon, with oxygen and water in a gaseous phase. The silica synthesized by the sol-gel methods may be obtained by hydrolysis and/or condensation of an alkoxysilicon compounds as a raw materials. The precipitated colloidal silica may be obtained by an inorganic colloid method using raw materials purified in advance. Commercially available monodispersed, spherical colloidal silica slurries are suitable for this purpose. 
     Examples of the organic particles may include polyvinyl chloride, styrene (co)polymers, polyacetal, polyester, polyamide, polycarbonate, olefin (co)polymers, phenoxy resins, acrylic (co)polymers, and the like. Examples of the olefin (co)polymers include polyethylene, polypropylene, poly-1-butene, poly-4-methyl-1-pentene, and the like. Examples of the acrylic (co)polymers include polymethyl methacrylate polymers, copolymers, and the like. An average particle diameter of the abrasive may be 5 to 500 nm, preferably in the range 10 to 200 nm. Appropriate polishing rates can be achieved by using the abrasive particles mentioned above having an average particle diameter within this range. Combinations of one or more of the inorganic and/or organic abrasives may also be employed to achieve desired results. 
     b) Acids: Organic and inorganic acids may be employed to increase nitride polish rates (accelerators) and decrease oxide polish rates (inhibitors). Examples of the inorganic acids may include nitric acid, sulfuric acid, phosphoric acid and the like. Use of phosphoric acid is preferred. Various organic acids such as monobasic acids (e.g., monocarboxylic acid), dibasic acids (e.g., dicarboxylic acid), polybasic acids (e.g., polycarboxylic acid), substituted acids (hydroxyl, amino groups) may be employed. Examples of such organic acids may include saturated acids, unsaturated acids, aromatic acids and aliphatic acids, and the like. Examples of the saturated acid may include formic acid, acetic acid, butyric acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, and the like. Examples of the carboxylic acids with hydroxyl groups may include lactic acid, malic acid, tartaric acid, citric acid, and the like. Examples of the unsaturated acid may include maleic acid, fumaric acid, and the like. Examples of the aromatic acid may include benzoic acid, phthalic acid, and the like. It is preferable to use an organic acid having two or more carboxylic acid groups to obtain high polish rates of nitride. Potassium salt or ammonium salt of these acids can also be used in the pH range of 2 to 5. 
     The present principles including the functions of the components of the slurry are further described below by way of examples. Note that the invention is not limited to the following examples. Examples 1-2 may illustratively be employed in accordance with the present principles. It should be understood that other slurries and etching processes in accordance with the present principles may be employed other then those presented in the examples. 
     Example 1, a slurry for polishing nitride selective to oxide includes:
         i. 5 to 10 W % of colloidal silica abrasive,   ii. 0.1 to 10 g/L of phosphoric acid, and   iii. pH in the range of 2 to 5 adjusted with KOH and/or NH 4 OH.       

     Example 2, another slurry for polishing nitride selective to oxide includes:
         i. 5 to 10 (W %) of colloidal silica abrasive,   ii. 0.1 to 10 g/L of phosphoric acid,   iii. 0.1 to 15 g/L of citric acid, and   iv. pH in the range of 2 to 5 adjusted with KOH and/or NH 4 OH.       

     Referring to  FIG. 2 , a plot shows removal rate (Å/min) versus pH for a 5% abrasive silicon slurry. This slurry has a higher nitride removal rate than oxide removal rate in a pH range 2 to 5. One advantage of the method of  FIG. 1  is that the method transfers non-uniformities associated with the nitride CMP processes to a thickness variation in a top sacrificial nitride layer ( 20 ). Since this nitride layer  20  acts only as a mask during the wet etching of oxide layers  18 , the variation in its thickness does not affect the final planarity. The final touch up polish ( FIG. 1E ) can introduce some small amount of pattern dependent topography. To minimize this, polishing should be kept to a minimum. To reduce this even further, the nitride  24  ( FIG. 1D ) can be wet etched with hot phosphoric acid and eliminate the final touch up polish ( FIG. 1E ), if desired. 
     The silicon nitride layer  20  may be replaced by one of many materials that can be used as a mask. For example, TiN, TaN, silicon carbide, diamond like carbon, carbon doped silicon, polymer layers such as PMMA, polyimide, polystyrene and photo resists, carbon doped oxide such as SiCOH, OMCTS, and a variety of other materials may be used as the mask layer  20 . 
     A test structure was employed for the evaluation of the nitride mask STI CMP process. A first structure, called a house structure, included 100 μm wide features that are covered with nitride. Another structure called a dishing macro had 130 μm wide structures that were completely filled with oxide. These two extremes were chosen to illustrate the effectiveness of the present principles. 
     A post CMP topography was achieved in the house structures by 1) a STI CMP process and 2) the nitride mask process in accordance with the present principles. The nitride in the house structure was recessed by ˜140 Å for the conventional CMP process with a ceria/surfactant system. For the nitride mask STI CMP process in accordance with the present principles, no significant loss of nitride was observed. 
     A post CMP topography was achieved in the dishing macro structures by 1) the conventional STI CMP process and 2) the nitride mask process in accordance with the present principles. The oxide in the dishing macro structure was recessed by ˜600 Å for the conventional CMP process with ceria/surfactant system. For the nitride mask STI CMP process in accordance with the present principles, no significant oxide recess was observed. The oxide in the macro was nearly planar with the field surrounding the structure. 
     Referring to  FIG. 3 , a method for the chemical mechanical planarization of semiconductor wafer structures, e.g., shallow trench isolation structures, employs the following steps. In block  102 , a mask material is deposited on top of an overburden layer. The overburden layer may include an oxide layer employed in forming STI structures. The mask material may include a nitride or other material such that the overburden layer can be selectively etched relative to the mask layer. The mask materials may include but are not limited to, e.g., SiN, TiN, TaN, silicon carbide, diamond like carbon, polymers such as poly methyl methacrylic acid, polyamides, polystyrene, and carbon doped oxides such as SiCOH and OMCTS (Octamethylcyclotetrasiloxane). A preferred mask material includes silicon nitride. The thickness of the mask layer depends on the topography that needs to be planarized. This may vary from about 400 to 800 Å, although other thicknesses may be employed. The mask materials may be deposited by a plasma deposition process such as rapid thermal chemical vapor deposition (RTCVD), low pressure CVD (LPCVD), atomic layer deposition (ALD), etc. The mask materials may be deposited by various spin coating methods. 
     In block  104 , a chemical mechanical planarization (CMP) process is employed to remove the mask layer from “up” areas (e.g., peaks) without removing significant amounts of mask material from “down” areas by the use of hard polishing pads such as an industry standard IC-1000 pad or equivalent. The slurry used in the CMP may include a very high selectivity (e.g., 100:1) towards nitride in comparison to oxide, or the slurry used may have a selectivity of 1:1 to 2:1 towards nitride in comparison to oxide. The CMP step is to completely remove all the nitride from the “up” areas using a hard polish pad in conjunction with the high selectivity slurry and/or 1:1 to 2:1 nitride to oxide selectivity. 
     In block  106 , a wet etch is employed to remove exposed portions of the overburden layer (e.g., oxide) leaving about 200 to 300 Å of material remaining over an underlying layer or structure. For the wet etch process, the etching solution may include a dilute buffered HF (BHF) in the ratio of 1:10 to 1:1000 of (BHF) with water. 
     In block  108 , a chemical mechanical planarization process is employed to remove the remaining portions of the mask layer and to planarize the overburden layer to expose the underlying layer (e.g., nitride) covered surfaces. The slurries for blocks  104  and  108  provide, e.g., a very high polish rate for nitride and a low polish rate for oxide and may include, e.g., 5 to 10 W % of colloidal silica abrasive, 0.1 to 10 g/L of phosphoric acid, pH in the range of 2 to 5 adjusted with KOH and/or NH 4 OH. The slurry with a selectivity of ˜1:1 or 2:1 may include, e.g., 5 to 10 W % of colloidal silica abrasive, 0.1 to 10 g/L of phosphoric acid, 0.1 to 15 g/L of citric acid, pH in the range of 2 to 5 adjusted with KOH and/or NH 4 OH. These slurries may be used as a single mix or a two part system depending on the desired outcome. 
     The mask and overburden layers or structures may be removed by etching or planarization. The etching embodiments may include the following. From block  106 , the wet etch is performed to remove the exposed overburden layer (e.g., oxide) leaving a thickness of, e.g., about 200 to 300 Å remaining over the underlying layer. In block  112 , wet etching is performed to remove the mask layer (e.g., nitride) in down areas by, e.g., a hot phosphoric acid etch 
     In block  114 , a touch up CMP may be performed to further improve final planarity and obtain a defect free, smooth oxide and nitride surface. The slurry for the additional touch up CMP to improve planarity and obtain a defect free, smooth oxide and nitride surface can be a 1:1 to 2:1 nitride to oxide selectivity slurry or another oxide CMP slurry. 
     In block  116 , the underlying layers or structures may be removed by etching or planarization. The underlying structures may include a mask layer or layers, such as a pad nitride layer and/or a pad oxide layer. The portions of the overburden materials may be formed in trenches in a substrate, e.g., to form STI structures. Processing can continue to complete chip fabrication. 
     Having described preferred embodiments for chemical mechanical planarization with an overburden mask (which are intended to be illustrative and not limiting), it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments disclosed which are within the scope of the invention as outlined by the appended claims. Having thus described aspects of the invention, with the details and particularity required by the patent laws, what is claimed and desired protected by Letters Patent is set forth in the appended claims.