Abstract:
A method is described for cleaning freshly etched dual damascene via openings and preparing them for copper fill without damage or contamination of exposed organic or other porous low-k insulative layers. The method is entirely dry and does not expose the porous materials to contamination from moisture or solvents. The method is effective for removing all traces of residual polymer deposits from an in-process substrate wafers after via or damascene trench etching. The method employs an in-situ three-step treatment comprising a first step of exposing the electrically biased substrate wafer to a O 2 /N 2  ashing plasma to remove photoresist and polymers, a second step immediately following the first step of remove silicon nitride etch stop layers, and a final step of treating the wafer with H 2 /N 2  to remove copper polymer deposits formed during nitride removal. The H 2 /N 2  plasma is capable of removing the difficult polymer residues which are otherwise only removable by wet stripping procedures. The H 2 /N 2  plasma is not harmful to exposed porous low-k dielectric layers as well as copper metallurgy.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The invention relates to processes for the manufacture of semiconductor devices and more particularly to the formation of copper interconnective metallization by a damascene process. 
     (2) Background of the Invention and Description of Related Art 
     Integrated circuits are manufactured by forming discrete semiconductor devices in the surface of silicon wafers. A multi-level metallurgical interconnection network is then formed over the devices, contacting their active elements, and wiring them together to create the desired circuits. The wiring layers are formed by depositing an insulating layer over the discrete devices, patterning and etching contact openings into this layer, and then depositing conductive material into the openings. A conductive layer is applied over the insulating layer and patterned to form wiring interconnections between the device contacts, thereby creating a first level of basic circuitry. The circuits are then further interconnected by utilizing additional wiring levels laid out over additional insulating layers with conductive via pass throughs. Depending upon the complexity of the overall integrated circuit, several levels of wiring interconnections are used. 
     A method for forming the interconnection layer is the damascene process, whereby openings and trenches, comprising an image of the interconnection pattern are formed in an insulative layer. A metal layer is then deposited into the openings and over the insulative layer. Finally, the metal is polished back to the insulative layer leaving the metal pattern inlaid within the insulative layer. Polishing back of the metal layer is accomplished by CMP (chemical mechanical polishing), a relatively old process which has found new application in planarization of insulative layers and more recently in the damascene process. In a single damascene process a metal line pattern is generated which connects to subjacent vias or contacts. In a dual damascene process, both vias/contacts and an interconnective wiring pattern are formed by a single metal deposition and CMP. A description of both single and dual damascene processes may be found in Chang, C. Y. and Sze, S. M., “ULSI Technology” McGraw-Hill, N.Y., (1996), p444-445 and in El-Kareh, B., “Fundamentals of Semiconductor Processing Technologies”, Kluwer, Boston(1995), p563-4. The dual damascene process has been particularly favored for the manufacture of integrated circuits using copper metallurgy. Copper is rapidly replacing aluminum as the metallurgy of choice in integrated circuit manufacturing because it has a higher conductivity than aluminum. The use of copper results in greatly improved circuit performance. 
     In dual damascene processing, the deep via openings are first partially patterned with a first photoresist mask. Then, using a second photoresist mask, the shallower metal interconnective wiring channels are etched as the vias openings are etched to completion. Often, etch stop layers are used to limit the etching at both the deep and shallow levels. Dai, et.al., U.S. Pat. Nos. 5,877,075 and 5,882,996 show methods for etching both levels using a single photoresist layer, which is patterned twice. Silicon nitride is typically used as an etch stop. 
     In order to further improve circuit performance, low dielectric constant (low-k) insulative materials have been incorporated into the dielectric layers of modern integrated circuits. These materials provide a lower capacitance than conventional silicon oxide and consequently, an increase in circuit speed. Lou, et.al., U.S. Pat. No. 5,916,823 shows a method for forming a dual damascene structure using a low-k SOG (spin-on-glass). SOGs are alcohol soluble siloxanes or silicates which are spin deposited and baked to drive off solvents resulting in a relatively porous silicon oxide structure. Other porous silica structures such as xerogels have been developed, notably by Texas Instruments Inc. and incorporated into dual damascene processes to obtain insulative layers with dielectric constants as low as 1.3. This is to be compared with a dielectric constant of about 4 for conventional silicon oxide. Other organic and quasi-organic materials such as polysilsesquioxanes and polyarylene ethers have been added to the growing family of low-k and ultra low-k insulative materials. The materials are deposited either by spin-on deposition or by CVD (Chemical vapor deposition). 
     Although these materials offer welcome improvement in circuit performance, they also pose problems in processing. Because they are very porous, they are not only prone to absorb and retain contaminants, in particular moisture and solvents, but they also react to a large extent with wet processing chemistries such as those used for cleaning and removing residues left after plasma etching and photoresist stripping. In Dai, solvents are removed by baking and a cap oxide liner is deposited on the walls of the etched damascene openings to seal the SOG surfaces against contamination. Whereas Dai uses an oxygen ashing process followed by wet stripping with H 2 SO 4 , H 2 O 2 , and NH 4 OH to remove photoresist and other residues after the via etch such treatment would be damaging to OSOGs (organic spin-on-glasses) and some other low-k insulative materials including those deposited by CVD. 
     Jeng, U.S. Pat. No. 5,453,157 teaches an anisotropic, low temperature (−40° C. to 20° C.), oxygen plasma ashing process which does not harm the exposed edges of low-k OSOG underlayers. The ashing plasma is made directional with respect to the wafer by biasing the wafer to attract the positively charged oxygen species. In addition, the wafer must be cooled during the ashing, for example by liquid nitrogen. 
     Fujimura, et.al., U.S. Pat. No. 5,773,201 teaches another photoresist ashing process wherein ashing takes place in a chamber located downstream of an oxygen plasma source. It is shown that hydrogen or water vapor, added to the oxygen plasma, lowers the activation energy of the ashing process. However, hydrogen addition, even in small amounts, creates an explosion hazard. The addition of nitrogen or water vapor to the oxygen plasma increases the ashing rate by increasing the concentration of oxygen atoms in the plasma. 
     Jain, et.al., U.S. Pat. No. 5,741,626 teaches the use of Ta 3 O 5  as an insulative ARC (anti-reflective coating) and etch stop material for forming dual damascene structures. The insulative property is advantageous when the ARC is used other than directly beneath the photoresist layer, for example between the two main dielectric layers of the dual damascene structure. Ta 2 O 5  reduces the reflectivity of a subjacent aluminum layer by about a factor of 5. 
     Porous doped silicates such as fluorinated and carbon doped silicate glasses and totally organic, non silicaceous, materials such as fluorinated polyarlyene ethers, are seeing an increased usage in semiconductor processing technology because of their favorable dielectric characteristics and ease of application. These materials are, however, particularly susceptible to etch erosion and contamination from wet etchant and stripping chemistries which have heretofore not been particularly problematic. Complex polymeric residues often result from the reactions of dry etchant components and a variety of organic and inorganic (primarily silicaceous), materials exposed during plasma etching. CVD deposited fluorinated silicate glasses and carbon doped silicates are also susceptible to erosion and contamination by wet etchants. 
     In addition, wet etchant chemistries also cause damage to exposed copper surfaces which are exposed in certain copper damascene via processes wherein the wet etch clean is used after copper is exposed at the base of the via openings. These residues may no longer be removed by conventional wet etching because of the interaction of the wet etchants with exposed low-k dielectric layers. Oftentimes, for reasons of design or process integration, the edges of low-k dielectric layers, cannot be sealed in the manner of Lou, &#39;823, to prevent exposure of the edges to a wet cleaning step. 
     It is therefore desirable to have a totally dry process which provides a highly but not totally anisotropic cleaning action without damaging or contaminating exposed lateral edges of low-k organic and doped silicaceous structural layers. In addition, the process should provide means to protect metal exposed at the base of the via opening. The method taught by this invention is such a process. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of this invention to provide a totally dry method for removing photoresist and sidewall polymer from a freshly etched damascene via opening, removing an etch stop at the base of the via opening, and cleaning out copper polymers left by the etch stop removal process, without damaging or contaminating exposed low-k dielectric layers. 
     It is another object of this invention to provide a dry method for cleaning a via opening to copper metallization without damage or contamination of exposed copper. 
     It is yet another object of this invention to provide an all-dry method for removing residual polymer deposits from in-process substrate wafers. 
     These objects are accomplished by an in-situ three-step plasma treatment comprising a first step of exposing the substrate wafer, after via etching, to a O 2 /N 2  ashing plasma, a second step immediately following the first step in the same chamber, and without breaking vacuum, of removing the etch stop layer and a third step of treating the wafer with a H 2 /N 2  plasma to remove copper polymer residues which remain after the etch stop removal. The H 2 /N 2  plasma is capable of removing difficult polymer residues which are otherwise only removable by wet stripping procedures. The H 2 /N 2  plasma is not harmful to the exposed porous low-k dielectric layers. Because the process is completely dry, the absorption of moisture or solvents by porous low-k layers is avoided. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG.  1 A through FIG. 1F are cross sections of a silicon wafer illustrating a sequence of processing steps for forming a via and a layer of copper metallization in the presence of low-k insulative layers by a dual damascene process according to an embodiment of this invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In a first embodiment of this invention a p-type &lt;100&gt; oriented monocrystalline silicon wafer substrate is provided. Referring to FIG. 1A, silicon substrate  10  is provided with a substructure of devices formed in the substrate and/or metal layers thereof, and as they are not significant to the invention, they are not described in detail in order not to unnecessarily obscure the present invention. An insulative layer  12  is formed on the substrate  10  and a copper wiring layer is formed in the upper portion of the layer  12 . The copper wiring layer  14  is shown embedded in the figure as it is formed by a damascene process. Alternately, the wiring layer  14  may comprise aluminum, an aluminum alloy, tungsten, a tungsten alloy, or a copper alloy. The surface of the insulative layer  12  and the embedded metal wiring  14  is planarized by CMP. 
     Alternately, the copper wiring layer may be formed by another process. In the present embodiment the insulative layer  12  is preferably silicon oxide, although it may alternatively be formed of another insulative material, for example BPSG (borophosphosilicate glass), PSG (phosphosilicate glass), an organic polymer, an organosilicate glass, a fluorinated silicate glass, a xerogel, an aerogel, or a polysilsesquioxane. The insulative layer  12  is formed by PECVD (plasma enhanced chemical vapor deposition) although other deposition means may alternately be used to deposit layer  12 . However, PECVD is preferred because of the low deposition temperature. The wiring layer  14  is interconnected to elements of the devices located in the wafer  10  substructure. 
     A layer of silicon nitride  16  is deposited on the insulative layer  12  by PECVD to a thickness of between about 500 and 1,000 Angstroms. Alternately, another of a number of well known deposition methods may be employed to deposit the layer  16 . The silicon nitride layer will serve as an etch stop during subsequent via etching and also as a chemical barrier. 
     A low-k dielectric layer  18  is next formed over the wafer  10  by a CVD method, preferably by HDP (high density plasma) CVD or by PECVD to a thickness of between about 0.5 and 0.7 microns. CVD deposited low-k materials which may be used include fluorinated silicate glasses (FSG), sometimes called fluorinated oxide, Organosilicate glasses (OSG), for example Black Diamond™, from Applied Materials Corporation of Santa Clara Calif., films formed from a methylated silane, and the more recently investigated FLAC (fluorinated amorphous carbon) films. 
     A second layer of silicon nitride  20  is next deposited on the low-k layer  18  by PECVD. Again, another deposition means may alternately be used to deposit this layer, however, PECVD is preferred because of the beneficial low deposition temperature. The silicon nitride layer  20  is deposited to a thickness of between about 300 and 500 Angstroms. A second low-k dielectric layer  22  is deposited on the silicon nitride layer  20  using the same procedures as are employed in the formation of the first low-k dielectric layer  18 . The second low-k layer  22  is between about 0.5 and 1.0 microns thick and may be formed of the same low-k material as the first low-k layer  18  or of one of the alternate low-k material cited above for the layer  18 . 
     A layer of silicon oxynitride  24  is deposited on the second low-k dielectric layer  22 . The silicon oxynitride layer  24  is deposited by PECVD using SiH 4  and N 2 O in a He carrier gas and is between about 200 and 600 Angstroms thick. Process parameters for PECVD deposition of silicon oxynitride are well known to those skilled in the art. 
     Photoresist  26  is applied and patterned to form a mask which defines an opening  8  overlying the copper wiring  14 . The silicon oxynitride layer  24  serves as a BARC (bottom anti-reflective coating) during the photoresist exposure. The wafer  10  is then inserted in the etching chamber of a plasma etching tool, for example, a MERIE (magnetically enhanced reactive ion etching) tool. 
     The via opening  8  is etched in a single operation, by reactive ion etching sequentially, through the silicon oxynitride layer  24 , the low-k dielectric layer  22 , the silicon nitride layer  20  and the low-k dielectric layer  18 , stopping on the silicon nitride layer  16 . Reactant gas mixtures and etching parameters are adjusted for each layer to achieve a high etch rate for each layer as it is reached. An endpoint sensor, such as an optical emission spectrometer, provides continuous monitoring of the etching process and indicates when etchant gases are to be changed to accommodate either a nitride layer or an insulative layer. The nitride layers are etched with a gas mixture containing a fluorocarbon such as CF 4  and oxygen, while the insulative layers are etched with fluorocarbons alone, for example CHF 3  or C 4 F 8 . Etchant gas mixtures and plasma parameters for etching the various layers are well known to those in the art and can be experimentally optimized for each application. Stopping on the silicon nitride layer  16  is easily achieved by utilizing etching parameters which provide a high insulative layer-to-silicon nitride selectivity. 
     The wafer  10  is next transferred from the etching tool into a plasma ashing chamber. The plasma ashing chamber may be in a separate tool or it may also be a second chamber of the RIE tool. Residual photoresist  26  and polymer deposits developed during RIE are removed by ashing in with a O 2 /N 2  gas mixture. O 2  is flowed at a rate of between about 20 and 80 SCCM (standard cubic centimeters per minute) and N 2  at a flow rate of between about 20 and 80 SCCM. The pumping speed is throttled to maintain the ashing chamber pressure at between about 50 and 100 milliTorr. The temperature of the wafer is maintained at between about 40 and 60° C. by controlling the temperature of a pedestal in the ashing tool upon which the wafer rests. In addition, the wafer is electrically biased to provide directionality to the plasma. 
     After the residual photoresist has been removed, the via opening may be optionally subjected to a hydrogen plasma in order to remove any residual polymer which may be left after the ashing. After the ashing and without breaking vacuum, the gas mixture is changed to one of H 2  and N 2  while maintaining a plasma. H 2  is flowed at a rate of between about 100 and 500 SCCM and N 2  at a rate of between about 100 and 500 SCCM. The pumping speed is throttled to maintain a chamber pressure of between about 50 and 100 milliTorr and the wafer temperature maintained between about 40 and 60° C. 
     The H 2 /N 2  plasma cleans away any polymer, formed during via etching, which may be difficult to remove by the oxygen ashing alone. The H 2 /N 2  plasma treatment is applied for a period of between about 20 and 40 seconds. The completed via opening  8  is shown in FIG.  1 B. The silicon nitride etch stop layer  16  is retained at the bottom of the opening  8  to protect the subjacent metal from corrosion or other chemical attack. 
     Referring next to FIG. 1C, an organic BARC (bottom anti reflective coating)  28  is applied over the wafer  10  by a spin-on technique. Any conventional organic BARC material may be used. The BARC  28  is formed by first depositing a liquid monomer onto the wafer from a nozzle as the wafer  10  is spun on a wafer spinner. This method of film deposition is well known and widely practice in the art. The thickness of the BARC  28  is determined by the viscosity of the liquid monomer and the spinning speed of the wafer. The organic BARC  28  is thermally cured resulting in a final thickness of between about 1,000 and 2,000 Angstroms. 
     Although the silicon oxynitride BARC  24  still remains over the low-k layer  22 , the spun-on BARC  28  has the added purpose of protecting the nitride etch stop layer  16  at the bottom of the trench  8 . When the BARC  28  is spun onto the wafer, a portion  28 A is captured and retained in the opening  8  during the spin-on process. The portion  28 A is much thicker than the BARC  28  over the planar regions and subsequently, enough remains to protect the silicon nitride layer  16  at the base of the opening  8  during subsequent etching of the wider or trench portion of the dual damascene structure. 
     A photoresist layer  30  is next patterned on the wafer  10  to define the wider or trench portion  9  of the dual damascene structure. Referring to FIG. 1D, the BARC  28 , the silicon oxynitride layer  24 , and the low-k organic layer  22  are etched to the silicon nitride etch stop layer  20 , forming the trench  9 . The etching process is accomplished in the same etching chamber that was previously used to form the via opening  8 . The wafer  10  is next transferred to the ashing chamber and the residual photoresist layer  30 , the thin BARC  28  under the photoresist, as well as the BARC portion  28 A in the via opening are removed by ashing in a O 2 /N 2  plasma. The O 2  is flowed at a rate of between about 20 and 80 SCCM and N 2  at a flow rate of between about 20 and 80 SCCM. The chamber pumping speed is throttled to maintain a chamber pressure of between about 50 and 100 milliTorr. During ashing a bias power of between about 500 and 700 Watts is applied to the substrate to provide plasma directionality and the temperature of the wafer is maintained at between about 40 and 60° C. by controlling the temperature of a pedestal in the ashing tool upon which the wafer rests. 
     FIG. 1E shows the structure after the O 2 /N 2  ashing treatment. In order to prepare the dual damascene opening  8 , 9  for copper deposition, the silicon nitride layer  16  must be removed at the base of the via opening  8 . This is accomplished directly after the O 2 /N 2  ashing treatment, and in the same chamber without breaking vacuum, by plasma etching in a plasma containing fluorocarbons and oxygen. Fluorocarbon/oxygen etchant gas mixtures and plasma parameters for etching silicon nitride are well known to those in the art and can be experimentally optimized to etch the nitride with minimum over-etch. A bias power of between about 500 and 700 Watts is applied to the substrate to provide plasma directionality. The silicon nitride layer  16  in the via opening  8  as well as the exposed portions of the silicon nitride layer  20  and the silicon oxynitride BARC  24  are removed with negligible attack of the low-k dielectric layers. The silicon oxynitride  24  and the exposed silicon nitride  20  are concurrently removed along with the exposed etch stop nitride  16 . 
     After the exposed nitride and oxynitride have been removed by the fluorocarbon etch, polymeric copper residues remain in the opening  8 . These residues comprise Cu—F and Cu—O polymers which are formed at the end of the nitride etching by reaction of the fluorocarbon etchant gases with the freshly exposed copper wiring  14  at the base of the via opening  8 . In an alternate instance when the wiring layer  14  is of another metal, corresponding residues may be formed by attack of the metal by the silicon nitride etchant gas. These residues must be removed to from within the opening  8  prior to the copper damascene metallization in order to achieve a good ohmic contact. 
     Directly after the fluorocarbon nitride etch the copper polymer residues are removed with a H 2 /N 2  plasma etch within the same chamber and without breaking vacuum. The a gas mixture is changed to one consisting of H 2  at a flow rate of between about 100 and 500 SCCM and N 2  at a flow rate of between about 100 and 500 SCCM. The chamber pumping speed is throttled to maintain a chamber pressure of between about 50 and 100 milliTorr while the wafer temperature is maintained between about 40 and 60° C. The H 2 /N 2  plasma cleans away the copper polymer residues leaving a clean copper surface at the base of the via opening  8 . The H 2 /N 2  treatment is applied for a period of between about 20 and 40 seconds. 
     Referring to FIG. 1F, the damascene trench  9  and via  8  are lined with a barrier layer  32  comprising TaN. Alternately another barrier layer configuration such as TaN/Ta or TiN may be used. A layer of copper  34  is deposited to fill the opening  8 , 9 . Deposition of the copper layer can be by PVD (physical vapor deposition) methods such as sputtering or vacuum evaporation, or by CVD (chemical vapor deposition), or by ECD (electrochemical deposition). The ECD method involves placing the wafer into an electrolyte bath and electro plating a metal layer onto the wafer surface by applying of an electric field between the wafer and the electrolyte. The ECD method has been found to be particularly desirable for the deposition of copper. Methods for barrier formation and copper deposition on dual damascene structures are well known in the art and need not be described in detail here. Following deposition the layer is planarized by CMP to expose the dielectric layer  22  completing the formation of the damascene metallization. 
     While the embodiment of the invention utilizes a CVD deposited low-k dielectric layers for each of the two insulative sections of the dual damascene structure, the procedures taught by this invention for forming and cleaning a dual damascene structure can be applied equally well using other low-k insulative materials such as the various porous low-k silicaceous dielectric materials deposited either by CVD or by spin-on techniques, such as aerogels, silica gels, or polysilsesquioxanes. In addition the all dry via forming process taught by this invention may be applied when conventional dielectrics such as silicon oxide, BPSG, PSG, and are used to form the layers which abut the via. 
     While the embodiment describes the formation of a damascene structure, it is to be understood that the all-dry, in-situ processing sequence of O 2 /N 2  ashing, plasma nitride removal, and final H 2 /N 2  copper polymer removal, taught by this invention, may be equally well applied to the formation of a simple via in a single insulative layer having a silicon nitride etch stop over a subjacent copper pattern. After the via is patterned to the etch stop, the all dry sequence is applied to remove the etch stop and clean the via in preparation for the metallization. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention. The terms “dielectric” and “insulative” are used interchangeably, the word “dielectric” being used when the context refers to the dielectric property of an insulative material.