Abstract:
A method for etching of sub-quarter micron openings in insulative layers for contacts and vias is described. The method uses hardmask formed of carbon enriched titanium nitride. The hardmask has a high selectivity for etching contact and via openings in relatively thick insulative layers. The high selectivity requires a relatively thin hardmask which can be readily patterned by thin photoresist masks, making the process highly desirable for DUV photolithography. The hardmask is formed by MOCVD using a metallorganic titanium precursor. By proper selection of the MOCVD deposition conditions, a controlled amount of carbon is incorporated into the TiN film. The carbon is released as the hardmask erodes during plasma etching and participates in the formation of a protective polymer coating along the sidewalls of the opening being etched in the insulative layer. The protective sidewall polymer inhibits lateral chemical etching and results in openings with smooth, straight, and near-vertical sidewalls without loss of dimensional integrity.

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 processes for forming contacts and Vias. 
     (2) Description of Prior Art 
     The integrated circuit (IC) manufacturing industry continues relentlessly towards smaller device geometries and greater circuit densities. This trend is made possible by the development of new manufacturing techniques as well as innovative improvements of existing procedures thereby extending their utility further towards miniaturization and higher density. The benefits and rewards of these efforts in very large scale integrated circuit technology development are extraordinary. Not only are the integrated circuits of today cheaper to produce, they continue to reward both end user and manufacturer with improved reliability and performance. 
     One such discipline wherein the limits of technology are constantly tested is the formation of openings in insulative layers wherein contacts to subjacent semiconductive elements are made. These openings generally represent the smallest photolithographically defined features of the integrated circuit. The openings are typically formed by reactive ion etching (RIE) through the insulative layer using a patterned photoresist mask. RIE is a well known anisotropic etching technique which can provide deep vertical openings having high aspect ratios. The aspect ratio in this regard is defined as the depth of the opening divided by its width. 
     The evolution in the capabilities of advanced optical lithography systems using laser light sources with wavelengths in the deep ultraviolet (DUV) spectrum has enabled the resolution of features below 0.25 microns. In spite of the improvements in lens quality, however, these resolutions come at the expense of very shallow depth of focus. 
     The formation of contact openings for sub-quarter micron IC technology requires such a fine photolithographic resolution. The narrow depth of focus dictates the use of photoresist layers having a thickness no greater than, and preferably less than the depth of focus. In order to achieve a resolution of less that 0.25 microns it is therefore necessary that the photoresist thickness be less than about 5,000 Angstroms. The etch rate selectivities of typically used insulative materials, such as silicon oxide and borophosphosilicate glass (BPSG), with respect to the photoresist materials used in deep-ultraviolet (DUV) photolithography are not sufficiently high to permit such thin photoresist layers to be used alone to accomplish the RIE patterning of contact or via openings. 
     Instead, a more durable material must be deposited over the insulative layer. This material is then patterned by a thin photoresist mask to form a hardmask. The hardmask material, having a substantially lower etch rate in the fluorocarbon gases used to etch the insulative layer, may be deposited relatively thinly and can therefore be easily patterned with a thin photoresist mask. One of the current inventors, has disclosed titanium nitride(TiN) as a material which forms an effective hardmask for etching of insulative layers and can be conveniently patterned with thin photoresist layers. Whereas the selectivity of silicon oxide to photoresist is only about 2-3:1, the selectivity of silicon oxide to TiN is about 10:1. 
     A problem with the use of sputtered TiN as a hardmask for anisotropic etching of ILD and IMD layers is that the sputter deposited TiN films lack the carbon content necessary for the formation of a protective polymer on the sidewalls of the contact/via openings during the plasma etch. As a result, the sidewalls are not vertical, suffer from unacceptable surface roughness and image distortion. 
     FIG. 1 is a cross section of a wafer  10  of with a contact opening  16  etched in an insulative layer  12  with a conventional fluorocarbon etchant containing CF 4  and CHF 3  using a sputtered TiN hardmask  14  which has been patterned by DUV photolithography. The sputtered TiN hardmask does not contain carbon and therefore a sidewall polymer is not formed during the insulator etching. The absence of sidewall protection permits lateral chemical etching resulting in rough and irregular wall surfaces as well as loss of image dimensional integrity. 
     The use of hardmasks for patterning layers by plasma etching is not new. Dennison, U.S. Pat. No. 5,362,666 shows a method of producing a self-aligned contact penetrating a cell plate using a silicon nitride or polysilicon hardmask to etch a contact hole. Chang, U.S. Pat. No. 5,612,240 shows a contact method using a silicon nitride spacer as a hardmask. 
     MOCVD(metal organic chemical vapor deposition) methods for forming conductive layers have been reported. For example Chao, et.al, U.S. Pat. No. 5,385,868 cites the formation of aluminum/silicon and aluminum/copper alloy layers by MOCVD. Sandhu, U.S. Pat No. 5,254,499 cites the formation of conformal high density TiN barrier layer films by MOCVD. Shapiro, et.al. U.S. Pat. No. 5,603,988 likewise cites the formation of TaN and TiN films by MOCVD. Generally, the carbon content of these films is problematic because it causes the films to be more resistive than sputtered TaN and TiN films. The predominant use of these films is for metal diffusion barriers in contacts. In this application a high conductivity is sought. Sandhu, U.S. Pat. No. 5,480,684 cites a method for reducing the carbon content of MOCVD TiN films from 21 atomic percent to 12 atomic percent by ion implantation of ions of a “late transition metal”, for example platinum, into the MOCVD TiN followed by annealing in hydrogen. 
     C. Y. Chang and S. M. Sze, in ULSI Technology, McGraw-Hill, New York, (1996) p.388-389 cite the formation of MOCVD TiN films from TDMAT and TDEAT precursors, indicating that these films have low densities and high resistivities because of carbon and oxygen inclusion, compared with sputtered TiN films making them unsuitable as barrier layers. 
     In the current invention, wherein MOCVD TiN films are used as hardmasks, the incorporated carbon is used advantageously to form a protective sidewall polymer coating during RIE of insulative layers. Conductivity and high conformality are of little or no consequence to a RIE hardmask. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of this invention to provide an improved TiN hardmask for forming contact or via openings in sub-micron integrated circuits using thin photoresist thereby gaining full benefit of the 250 nm. or better resolution available with DUV photolithography. 
     It is another object of this invention to provide a method for forming a high etch rate selectivity RIE hardmask for etching patterns in insulative layers. 
     It is another object of this invention to provide a method for forming a carbon enriched TiN hardmask for etching oxide films with etchants containing fluorocarbons without mask undercutting and sidewall roughness caused by lack of polymer formation. 
     It is another object of this invention to provide a method for introducing a controlled amount of carbon into an inorganic hardmask material so that sidewall protective polymer is formed when the hardmask is used to anisotropically etch silicaceous layers. 
     These objects are accomplished by forming a carbon enriched refractory metal nitride hardmask by MOCVD. The metal organic precursors provide sufficient carbon in the deposited nitride hardmask which is released as the hardmask erodes during plasma etching. The released carbon participates in the formation of a polymer layer along the sidewalls of the silicaceous material being etched, thereby preventing lateral erosion by chemical reaction. The hardmask material disclosed by the embodiments of the current invention is TiN x C y  and is formed by MOCVD using tetrakis-(dimethylamido)-titanium (TDMAT) or tetrakis(diethylamido)-titanium (TDEAT) precursors. The TiN x C y  films can be deposited at temperatures between about 250 and 480° C. The carbon content of the MOCVD TiN films can be adjusted from about 5% to 20% by variation of the deposition conditions. 
     The incorporated carbon in the MOCVD TiN x C y  films is used advantageously by this invention in forming a protective polymer coating on the sidewalls of contact or via openings in ILD and IMD layers during the anisotropic etching. The carbon content of these MOCVD films may be controlled by plasma annealing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross section of a contact opening formed in an ILD layer using a TiN hardmask formed by sputtering. 
     FIGS.  2  through FIG. 4 are a cross sections illustrating processing steps used in a first embodiment of this invention. 
     FIG. 5 is a line drawing made from a scanning electron micrograph showing a cross section of a contact opening made according to the first embodiment of this invention. 
     FIGS. 6 and 7 are a cross sections illustrating additional processing steps used in a first embodiment of this invention. 
     FIG.  8  through FIG. 10 are a cross sections illustrating processing steps used in a second embodiment of this invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In a first embodiment of this invention a contact opening is formed to a semiconductive element by etching an interlevel dielectric layer(ILD) using a hardmask formed of TiN x C y . Contact openings having aspect ratios between about 1.5 and 5 may be formed by the process of this embodiment. The TiN x C y  hardmask is formed by MOCVD and is patterned by DUV photolithography using thin photoresist. 
     Referring to FIG. 2 a p-type, monocrystalline silicon wafer  20  is provided. Semiconductor devices, for example, MOSFETs and bipolar transistors, are formed within the surface of wafer  20 . A region  21  is an element of such a device, for example a source or drain of a MOSFET, to which an electrical contact is to be formed. The devices, which may also have elements(not shown) formed over the silicon wafer surface, for example insulated gate structures, are typically isolated by regions of field oxide(not shown). The devices are formed according to procedures well known and widely practiced by those in the art. 
     An ILD layer  22  comprising silicon oxide is formed over the wafer  20  to a thickness of between about 2,000 to 5,000 Angstroms. The deposition is performed preferably by low pressure chemical vapor deposition(LPCVD) using tetraethylorthosilicate(TEOS) as a precursor. Alternatively the ILD layer  22  may be formed of other insulative materials, for example, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG) or combination of silicon oxide and BPSG or PSG. The layer  22  may alternatively be formed by other deposition methods, for example, plasma enhanced chemical vapor deposition(PECVD). The formation of such layers or films is common in the semiconductor industry and the deposition procedures are familiar to those in the art. 
     A layer  24  comprising TiN x C y  is next deposited over the ILD layer  22  by a MOCVD process using TDMAT as a precursor. Alternatively, other titanium organometallic precursors, for example TDEAT, may be employed. The substrate is maintained at a temperature of between about 400 and 450° C. in a conventional MOCVD tool. A carrier gas, preferably He or N 2  is bubbled at a flow rate of between about 275 and 300 SCCM (Standard cubic centimeters per minute) through a reservoir of TDMAT contained in a vessel and heated to a temperature of about 60° C. The layer  24  is deposited to a thickness between about 1,000 and 3,000 Angstroms. The selected thickness depends upon the etch depth in layer to which the hardmask is applied. 
     The carbon content of the resultant film can be adjusted to between about 5 to 20 atomic % by in-situ plasma annealing in the MOCVD chamber in which the film is deposited. The annealing is accomplished in a gas mixture containing N 2  at a flow rate of 200 SCCM or thereabout and H 2  at a flow rate of 100 SCCM or thereabout at a chamber pressure of 1.3 Torr or thereabout. The substrate temperature is maintained at about 400° C. and the rf power at 750 Watts or thereabout. The optimal carbon enrichment is determined by experiment to accommodate the subsequent etchant chemistry and the polymer formation rate. 
     A bottom anti reflective coating (BARC) layer  26  is next deposited over the TiN x C y  layer  24 . The BARC layer  26 , preferably an organic BARC layer, is deposited by well known spin coating procedures. A thin photoresist layer  28  between about 1,000 and 8,000 Angstroms thick is deposited over the TiN x C y  layer  24 . The photoresist  28  is patterned to define a contact opening  30  using well known photolithographic procedures, preferably high resolution (less than 0.25 micron) DUV photolithography. 
     Referring now to FIG. 3, the substrate wafer  20  is loaded into an RIE tool. 
     The BARC layer  26  and the TiN x C y  layer  24  are etched by RIE using well known TiN etchant gas mixtures, for example mixtures containing HBr, BCI 3 , fluorocarbon gases, and/or Cl 2  Endpoint is detected, preferably by optical emission spectroscopy, detecting the appearance O 2  which signals the exposure of the ILD layer  22 . Alternatively a timed etch period may be employed. The TiN x C y  hardmask is now patterned. The residual photoresist  28  and the BARC layer  26  are stripped preferably by plasma ashing or alternatively by liquid strippers. These procedures are well known to those familiar with the art and are comparable to the etching procedures used to pattern TiN. 
     Referring now to FIG. 4, the contact opening  30  is formed in the ILD layer  22  by RIE using the TiN x C y  hardmask  24 . The etching is accomplished using a mixture of fluorocarbons containing, for example, CF 4 , C 2 F 6 , C 4 F 8 , CHF 3  or combinations thereof. The respective flow rates and etching parameters are experimentally optimized by etchant and parameter selection to obtain high etch rate selectivities for the ILD layer material with respect to the TiN x C y  hardmask. These optimization procedures are well known to those skilled in the art. In the current embodiment a gas mixture consisting of CHF 3  at 60 SCCM(standard cubic centimeters per minute), CF 4  at 30 SCCM or thereabout, Argon at 100 SCCM or thereabout, and N 2  at 20 SCCM or thereabout is used to etch the opening shown in FIG.  5 . The total chamber pressure is maintained at 150 mTorr or thereabout. 
     As the TiN x C y  hardmask  24  erodes during RIE, carbon from the hardmask reacts with the etchant gases to form a carbonaceous polymer  32  which deposits along the sidewalls of the opening  30  being etched in the ILD layer  22 . The polymer  32  protects the sidewalls from lateral chemical etching in a similar manner as does the polymer formed when an ILD layer is etched using a photoresist mask. The formation of a protective sidewall polymer during RIE of oxide layers with photoresist masking is widely accepted. The ability to achieve smooth, near vertical sidewalls is generally attributed to this protective polymer. 
     The cross section of the ILD opening  30  as depicted in FIG. 4 is typical of the shape of contact openings formed in the ILD layer using a TiN x C y  hardmask formed according to the method of this invention. FIG. 5 is a drawing made from a scanning electron micrograph showing a contact opening  40  etched in a 1,05 micron thick silicon oxide layer  42  using a 1,400 Å thick TiN x C y  hardmask  44 . Much of the hardmask  44  still remains. However, the high carbon content of the hardmask of between about 5 and 12 atomic percent was sufficient to form adequate sidewall polymer protection so that a smooth, near vertical contact opening  40  was formed. The dimension at the base of the opening  40  is 0.6 microns. An etchant gas consisting of CHF 3  at 60 SCCM(standard cubic centimeters per minute), CF 4  at 30 SCCM, Argon at 100 SCCM, and N 2  at 20 SCCM was used to etch the opening  40 . The total chamber pressure was 150 mTorr. 
     Referring back to FIG. 4, the polymer  32  formed during RIE of the contact opening  30 , is removed by a liquid stripper or, alternatively, by plasma ashing. These procedures are well know by those in the art. After the polymer removal a Ti/TiN glue/barrier layer is formed, preferably by sputtering. The formation of glue/barrier layers to line the walls of contact openings is well known and widely practiced in the art. Referring now to FIG. 6 a layer of titanium  50 , between about 100 and 800 Angstroms thick, is deposited over the wafer  20  and into the opening  30 . This is immediately followed by a barrier layer of TiN  52  deposited in the same tool without breaking vacuum. The TiN layer  52  is between about 300 and 500 Angstroms thick. 
     Referring to FIG. 7, the wafer  20  is then subjected to a rapid thermal annealing procedure in nitrogen wherein the Ti layer  50  at the base of the opening  30  reacts with the substrate silicon to form a titanium silicide bonding layer  54 . A tungsten plug contact  56  is next formed in the opening  30  by conventional methods well known to those skilled in the art. A layer of LPCVD tungsten is deposited over the wafer to fill the contact opening  30 . The tungsten layer is then blanket etched by RIE until the ILD layer  22  is exposed leaving a tungsten plug contact  56  in the opening  30 .. The etchant gases used for the tungsten RIE contain chlorine and also etch away the residual TiN x C y  hardmask  24 . 
     In a second embodiment of this invention a via opening is formed in a inter metal dielectric(IMD) layer exposing a subjacent conductive element of an integrated circuit interconnection level. The interconnection level used in the present embodiment is an aluminum wiring pattern although vias openings to other patterned conductive materials, for example polysilicon or copper, may also formed by the method of this embodiment. Via openings having aspect ratios between about 1 and 4 may be formed by the process of this embodiment. A TiN x C y  hardmask is used to form the via. An organic BARC layer is next deposited over the TiN x C y  layer. After the opening is etched, the residual hardmask remaining after the via has been etched is subsequently removed during the steps which form the via fill metallization. 
     Referring to FIG. 8, an p-type, monocrystalline silicon wafer  20  is provided. Semiconductor devices, for example, MOSFETs and bipolar transistors(not shown), are formed within the surface of wafer  20 . FIG. 8 shows an active region  21  of a semiconductor device which is isolated by as region of field oxide  58 . The procedures for the formation of semiconductor devices and the regions of field oxide are well known by those skilled in the art. 
     An ILD layer  22  comprising silicon oxide is formed over the wafer  20  to a thickness of between about 2,000 to 5,000 Angstroms. The deposition is performed preferably by low pressure chemical vapor deposition(LPCVD) using tetraethylorthosilicate(TEOS) as a precursor. Alternatively the ILD layer  22  may be formed of other insulative materials, for example, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG) or combination of silicon oxide and BPSG or PSG. The layer  22  may alternatively be formed by other deposition methods, for example, plasma enhanced chemical vapor deposition(PECVD). The formation of such layers or films is common in the semiconductor industry and the deposition procedures are familiar to those in the art. A tungsten plug contact  56  is formed to the active element  21  preferably by the method described in the first embodiment of this invention. 
     An aluminum metallization layer  60  is deposited on the insulative layer  22  using methods well known to those skilled in the art, for example, by sputtering or by vacuum evaporation. The layer  60  is then patterned using well known photolithographic procedures commensurate with the technology. The interconnective metallization layer  60 , as used in this embodiment is a first interconnective layer and is electrically connected to semiconductive device element  21  through the contact  56 . 
     An inter metal dielectric (IMD) layer  62 , preferably comprising BPTEOS (a BPSG formed from using a TEOS precursor) deposited over the patterned metallization layer  60 . The IMD layer may alternatively be formed of another insulative material, for example silicon oxide. The IMD layer  62  is formed by LPCVD although PECVD may alternatively be used. The formation of IMD layers is a well known procedure in the manufacture of integrated circuits and the insulative material chosen for this application is typically some form of silicate glass. The IMD layer  62  is between about 5,000 and 15,000 Angstroms thick. 
     A layer  64  comprising TiN x C y  is next deposited over the IMD layer  62  by a MOCVD process using TDMAT as a precursor. Alternatively other titanium organometallic precursors, for example TDEAT, may be employed. The layer  24  is deposited to a thickness between about 1,000 and 3,000 Angstroms. 
     A bottom anti reflective coating (BARC) layer  66  is next deposited over the TiN x C y  layer  64 . The BARC layer  66 , preferably an organic BARC layer, is deposited by well known spin coating procedures. A thin photoresist layer  68  between about 1,000 and 8,000 Angstroms thick is deposited over the TiN x C y  layer  64 . The photoresist  68  is patterned to define a via opening  70  using well known photolithographic procedures, preferably high resolution (less than 0.25 micron) DUV photolithography. 
     Referring now to FIG. 9, the wafer  20  is mounted the chamber of an RIE tool and the BARC layer  66  and the TiN x C y  layer  64  are etched using well known TiN etchant gas mixtures, for example mixtures containing HBr, BCI 3 , and/or Cl 2 . Endpoint is detected, preferably by optical emission spectroscopy, detecting the appearance O 2  which signals the exposure of the IMD layer  62 . Alternatively a timed etch period may be employed. The residual photoresist  68  and the BARC layer  66  are stripped preferably by plasma ashing or alternatively by commercially available liquid strippers leaving behind the patterned TiN x C y  hardmask  24 . These procedures are well known to those familiar with the art. 
     Referring now to FIG. 10, the via opening  70  is formed in the IMD layer  62  by RIE using the TiN x C y  hardmask  64 . The etching is accomplished using a mixture of fluorocarbons containing, for example, CF 4 , C 2 F 6 , C 4 F 8 , CHF 3  or combinations thereof. The respective flow rates and etching parameters are experimentally optimized by etchant and parameter selection to obtain high etch rate selectivities for the IMD layer material with respect to  the TiNXCY  hardmask. These optimization procedures are well known to those skilled in the art. In the current embodiment a gas mixture consisting of CHF 3  at 60 SCCM(standard cubic centimeters per minute), CF 4  at 30 SCCM or thereabout, Argon at 100 SCCM or thereabout, and N 2  at 20 SCCM or thereabout is used. The total chamber pressure is maintained at 150 mTorr or thereabout. 
     As the TiN x C y  hardmask  64  erodes during RIE, carbon from the hardmask reacts with the etchant gases causing a carbonaceous polymer  32  to deposit along the sidewalls of the opening  70  in the IMD layer  62 . The polymer  32  protects the sidewalls from lateral chemical etching in a similar manner as does the polymer formed when an IMD layer is etched using a photoresist mask. The formation of a protective sidewall polymer during RIE of oxide layers with photoresist masking is widely accepted. The ability to achieve smooth, near vertical sidewalls is generally attributed to this protective polymer. After the etching is completed the polymer  32  is removed by a commercially available liquid stripper or by plasma ashing. These procedures are well know by those in the art. 
     The next metallization level wiring (not shown) is then patterned and connected to the first metallization level wiring  60  either by means of a tungsten plug formed in a similar manner as the plug  56  or by deposition of the next metallization layer material directly into the via opening  70 . 
     The via formed in the second embodiment interconnects a first metallization level with a second. Integrated circuits can have additional metallization levels. It should be understood that The method of the second embodiment can be equally applied to vias between any two metallization levels. 
     The preferred embodiments of this invention teach the formation of sub-quarter micron contacts using high resolution DUV photolithography. By using a thin layer of photoresist, full advantage of the high resolution photolithography can be realized to pattern a hardmask. At the same time the hardmask provides a high selectivity for etching the insulative layer. In addition the hardmask also provides sufficient carbon to plasma etching ambient to cause the formation of a protective polymer along the sidewalls of the opening thereby greatly improving the surfaces of the opening as well as maintaining the dimensional integrity of the mask. 
     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. 
     Whereas the embodiments of this invention utilize a p-type silicon substrate, an n-type silicon substrate could also be used without departing from the concepts therein provided. It should be further understood that the substrate conductivity type as referred to herein does not necessarily refer to the conductivity of the starting wafer but could also be the conductivity of a diffused region within a wafer wherein the semiconductor devices are incorporated.