DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] The conventional multilevel interconnect structure illustrated in FIG. 1 provides air gaps as the dielectric material separating the wiring lines and other conductors within the multilevel interconnect structure. For example, air gaps 32 are provided as the insulation between first level wiring lines 20 , 22 . The low dielectric constant (k≈1) of air provides an improvement of a factor of 2˜4 over interconnect structures that use solid dielectric materials. The FIG. 1 structure accordingly has significant advantages with respect to reducing the capacitive and inductive coupling between adjacent wiring lines in the wiring required by a high density integrated circuit. Because the capacitive and inductive coupling of the FIG. 1 structure are reduced as compared to conventional interconnect structures, the operating speed and power consumption of high density integrated circuits which incorporate aspects of the FIG. 1 interconnect structure are improved. There is, however, an important aspect of the conventional FIG. 1 interconnect structure that hinders its application to such high density integrated circuits. In particular, the FIG. 1 structure is incompatible with the formation of unlanded vias, which are often a feature of high density integrated circuit designs.

[0017] The conventional FIG. 1 structure facilitates the formation of vertical interconnects between different levels of wiring lines by enlarging the wiring lines in the region where the vertical interconnects are to be formed. This enlarged “landing pad” area can be seen in FIG. 1 by comparing the larger width of the sections 20 of the first level wiring lines where interconnects 36 are formed with the sections 22 of the first level wiring lines not associated with an interconnect. Such a landing pad area for the first level wiring lines provides a margin for possible errors in alignment and in the lithography process used to define the interconnects 36 . If, for the FIG. 1 structure, enlarged sections of the first level wiring lines were not provided, then the vertical interconnects 36 and the first level wiring lines 22 beneath the vertical interconnects would be the same size. Any misalignments or other lithography errors would then cause the etching used to define the vias to remove the first oxide layer 30 from above the air gaps 32 , possibly allowing the oxide layer 30 to collapse over the affected air gap 32 . Subsequent cleaning processes could leave contaminants within the affected air gap in a manner that could foul any subsequently formed interconnects. Subsequent metal deposition processes could expand the first level wiring lines in an undesired manner or could even introduce a short across the first level wiring lines. As such, the FIG. 1 structure and the methods used in forming the FIG. 1 structure are not compatible with the formation of unlanded vias.

[0018] As the design rules for integrated circuits shrink, it becomes increasingly undesirable to provide oversized landing pad areas within lower level wiring lines to accommodate the misalignments or lithography errors that lead to the formation of unlanded vias. Such oversized sections preclude the associated wiring lines from being spaced as closely as is possible for the design rule applicable to the given process. To fully utilize the advantages of the smaller design rules, it is desirable to use both vias and wiring lines comparable in size to the design rule. Such a design choice necessarily means that unlanded vias will be formed in the course of forming the interconnect structure. Preferred embodiments of the present invention provide a method of forming a multilevel interconnect structure that incorporates air gaps as the dielectric material between adjacent wiring lines and which is compatible with designs that produce unlanded vias. In particular, some preferred embodiments of the present invention deposit a layer of carbon, form trenches and fill the trenches with metal wiring lines, deposit a layer of silicon oxide over the metal wiring lines and the carbon layer and then perform ashing to remove the carbon layer from between the wiring lines to form air gaps. An etch stop layer is preferably provided over the oxide layer and the metal wiring lines before forming further layers such as an intermetal insulating layer over the wiring line structures. By providing an etch stop layer over the first oxide layer, vias can be reliably formed through a thick internetal insulating layer to the first oxide layer, whether that intermetal insulating layer be carbon, oxide or another dielectric material, with a far reduced likelihood of the via etch process opening onto an air gap in an unlanded portion of the via. As such, methods in accordance with the present invention have improved compatibility with the processes that are most desirably used for formation of high density integrated circuit devices.

[0019] These and other aspects of the present invention will now be described in further detail with reference to FIGS. 6 - 9 . Referring to FIG. 6 , formation of the multilevel interconnect structure according to the present invention begins in a fashion similar to that discussed above for the formation of the FIG. 1 interconnect structure. Thus, a semiconductor substrate 60 having a variety of devices formed in and on its surface is provided and the substrate and the various devices are covered by an interlayer dielectric 62 . The interlayer dielectric 62 is typically an oxide of silicon, but may take several forms including, for example, a single layer of CVD silicon oxide or a combination of oxide layers and spin on glass (SOG) layers. Vias are provided through the layer 62 as appropriate and metal or polysilicon interconnects are provided either in a separate process or as part of the process for forming the first level wiring lines. The interlayer dielectric 62 is then covered with a layer of carbon deposited, for example, in a high density plasma CVD (HDPCVD) process using a CH ₄ or C ₂ H ₂ source gas. An appropriate system for performing this deposition is available commercially from Applied Materials Corporation of Santa Clara, Calif., and so is not described further herein. The carbon layer 64 is deposited to a thickness appropriate to first level metal wiring lines, for example, between approximately 5,000-12,000 A. A photoresist mask or, more preferably, a hard mask consisting of either silicon oxide or silicon nitride is provided over the carbon layer 64 , exposing the carbon layer 64 over those regions where first level wiring lines are to be formed. Anisotropic etching is performed to form trenches into the carbon layer 64 and the etch mask is removed. Metal is then deposited over the carbon layer and within the trenches in the carbon layer 64 . The metal used for the first level wring lines might be aluminum or a refractory metal or a multilayer combination of such metals or different conductive materials. It is preferred that the first level wiring lines and other conductors that are separated by air gaps formed by the removal of carbon be capable of withstanding an ashing process at a temperature of between 400-450° C. Either an etch back process or, more preferably, a chemical mechanical polishing (CMP) process is used to remove excess metal from the surface of the carbon layer 64 , defining the first level wiring lines 66 shown in FIG. 6 . As illustrated, the upper surfaces of the wiring lines 66 are preferably coplanar with the surfaces of the carbon layer 64 , which is most easily achieved using CMP.

[0020] Next, recesses are preferably formed in the carbon layer 64 by providing the FIG. 6 structure to an ashing or etching system and exposing the carbon layer 64 to an oxygen plasma ashing or etching process. This will produce recessed carbon regions 68 with surfaces spaced from the upper surfaces of first level wiring lines 66 by a few hundred angstroms. The resulting structure is shown in FIG. 7 . Next, an oxide layer 70 is deposited over the recessed carbon structures 68 and the first level wiring lines 66 so that the oxide layer extends partially down the sides of each of the first level wiring lines 66 . The oxide layer 70 serves different purposes in the structure and processes of the present invention. Forming the oxide layer to extend partially along the walls of the metal lines provides additional process latitude for future etching processes used to expose and to contact the surface of the first level wiring lines. As such, the oxide layer facilitates the formation of unlanded vias. A consumption reaction, which consists primarily of oxidation in the illustrated embodiment, is preferably performed through the oxide layer 70 to consume the sacrificial carbon layer 68 to create the air gaps 74 between the illustrated first level wiring lines 66 . After the air gaps are defined, the oxide layer 70 defines the upper extent of the air gaps 74 and supports any subsequently deposited layers. The oxide layer 70 is preferably formed to a thickness appropriate to the preferred ashing process while still being sufficiently thick to provide a reasonable level of structural support for later deposited layers. As discussed in the Anand article, an appropriate oxide layer might be about 500 Å in thickness. A particularly preferred method for forming the oxide capping layer 70 is by the deposition of a liquid polymer material that is cured to form a planarized oxide layer. For example, the material known as “HSQ” (hydrogen silsesquioxane) may be provided in its liquid form over the FIG. 7 structure and then the HSQ can be annealed at a temperature of approximately 400° C. for approximately 90 minutes in a nitrogen environment to form the oxide layer 70 (nominally SiO _3/2 ). The preferred method of forming the oxide layer 70 by spinning on liquid HSQ and curing the HSQ has the particular advantage of automatically providing a planarized surface for the oxide layer 70 . This is so because a spin on process is used to provide the liquid polymer HSQ to the surface before curing. Other polymeric or other liquids that can be deposited and cured or otherwise solidified to form a layer of dielectric material might be used in place of the illustrative HSQ material. The liquid used in forming the capping layer might be selected so that a chemical reaction or a process such as the deposition of material from a saturated solvent occurs in or from the liquid to provide material to form the oxide layer or an equivalent capping layer. If layer 70 is formed by a process which does not naturally produce a planarized surface for the oxide layer, such as by a CVD or HDPCVD process, then it is preferred that a planarization process be performed so that layer 70 is provided with a substantially planar surface. For example, if a CVD oxide is used, it is preferred that the CVD oxide be planarized using CMP before further processing because planarized surface provides much wider process margins for later via etch steps.

[0021] An ashing process is performed to remove the carbon layer 68 from beneath the oxide layer. For example, the ashing process might be performed in an oxygen ambient at a temperature of about 400-450° C. for a sufficiently long time to completely consume the thickness of the carbon layer through the oxide layer, approximately one to two hours. At the end of this process, the first level wiring lines 66 are separated by air gaps 74 that are bounded on the upper surface by capping oxide layer 70 and on the lower surface by interlayer dielectric 62 . The air gaps form the dielectric isolation between the first level wiring lines and so ensure a relatively low level of capacitive and inductive coupling between adjacent ones of the first level wiring lines 66 . In this particularly preferred embodiment, carbon is the sacrificial material that is consumed by the preferred oxidation reaction conducted through the preferred oxide capping layer. The oxide layer has the desired properties of a capping layer in accordance with the present invention in that the preferred oxidation consumption reaction readily proceeds through the oxide capping layer without overly attacking the oxide capping layer. Other systems of sacrificial materials, capping layers and consumption reaction are possible, and aspects of the present invention would also be applicable to such other systems.

[0022] Within the present context, the terms capping layer, sacrificial layer and consumption reaction are defined with reference to each other. More explicitly, in terms of the present invention, a consumption reaction or process is one that can remove a sacrificial layer by performing some form of reaction or process through the capping layer with the end result that the sacrificial layer is removed from beneath the capping layer. Alternate systems could likely be identified which would achieve certain of the advantages of aspects of the present invention. For example, photoresist could be substituted for the preferred carbon sacrificial layer in the illustrative process flow, using a similar oxide capping layer and a similar ashing consumption reaction, to achieve many of the same advantages as are provided by at least some aspects of the present invention. At this time, photoresist is less preferred than carbon primarily because carbon has a lower level of trace metal contamination and because the preferred oxidation or ashing processes more completely convert the carbon to CO ₂ without other, less volatile reaction by-products. Photoresist might nevertheless be desirable in some instances because there is less carbon to be consumed and the consumption reaction is likely to proceed more quickly.

[0023] A layer of an etch stop material 72 ( FIG. 8 ) is next provided over the oxide capping layer 70 . Generally, the etch stop layer will be different in composition than the oxide capping layer. Importantly, the etch stop layer is selected to be sufficiently different from the layer, which might be an intermetal dielectric layer, that is deposited over the etch stop layer. In this way, the etch stop layer can act as a stop for a via etching process performed through the intermetal dielectric or other layer. Often, the intermetal dielectric layer is an oxide of silicon and so silicon nitride is an appropriate etch stop material. For such embodiments, the etch stop layer is silicon nitride and may be deposited by CVD to a thickness of approximately 200-500 Å. The thickness of the nitride layer is chosen to be no thicker than necessary to provide an adequate etch stop. Excessive nitride thickness is undesirable because nitride deposits slowly and excess nitride requires unnecessary etching time in those subsequent processes like via etching which require that openings be formed through the etch stop layer 72 . It might, in some instances, be possible for the oxide capping layer to serve as an etch stop layer, if the intermetal dielectric layer has a composition significantly different from the oxide capping layer. The intermetal dielectric layer is most often an oxide, however, so it is most often desirable to use a silicon nitride etch stop layer in conjunction with an oxide capping layer.

[0024] After deposition of the etch stop layer 72 , additional layers of wiring lines could be formed in a manner similar to that used in forming the first level wiring lines and the separating air gaps 74 . In such embodiments, the etch stop layer would be covered by a layer of carbon which would then be patterned for the formation of wiring lines and further processing would proceed in the manner described above. For the purposes of the present discussion, however, the process for forming an intermetal dielectric, a via and vertical interconnect and a second level wiring line described next is sufficient to illustrate the practice and advantages of the present invention. It is nevertheless important to keep in mind that various air gap, carbon or other materials might be included in the intermetal dielectric layer through which vertical interconnects might be formed to connect wiring lines formed on either side of the first and second level wiring lines.

[0025] In the illustrated embodiments, an intermetal dielectric layer 76 is deposited over the surface of the etch stop layer 72 , for example, by CVD of silicon oxide from a TEOS source gas. A via mask is provided on the intermetal dielectric layer 76 , for example, by providing a layer of photoresist which is formed into an etching mask by conventional photolithography. A via is then formed through the intermetal dielectric layer by etching. The via etch might be performed in any of a number of dielectric etchers including the LAM Rainbow system, which is particularly preferred because it allows the user to adjust the level of selectivity of the etching process between oxide and nitride materials. This is advantageous for performing the various stages of the via etch in a single processing system using a continuous series of processing steps while still achieving good process controls. Vias are most preferably etched through the intermetal dielectric layer using an etchant which, at least near the end of the via etch through the intermetal dielectric, is highly selective to etching oxide without quickly etching the nitride etch stop layer.

[0026] In some embodiments, a comparatively high speed but less selective etching process might be used in an initial stage of the via etching process, followed by a second etching stage using a more selective and typically slower etching process. For example, the first, higher speed, lower selectivity portion of the via etching process might be performed using an etchant derived in a plasma process from source gases including CF ₄ . The second, lower speed, higher selectivity etching process might use an etchant derived from source gases including C ₂ F ₆ or C ₃ F ₈ . The first portion of the via etching process is used to etch partially through the intermetal dielectric and is discontinued well before the etching process nears the etch stop layer. The via etching process is then continued using the high selectivity via etch, stopping on the etch stop layer. Other appropriate etching systems are known to those of ordinary skill and are commercially available. Providing an etch stop layer and using an appropriate etching system is highly desirable for performing the via etch because the intermetal dielectric layer 76 can have thickness variations and varying etch characteristics that make the results of fixed-time etching processes unpredictable and unreliable. The subsequent etching steps used to complete definition of the via proceed through thinner layers in a manner that allows those steps to be more carefully controlled than is possible or practical in the etch process through the thick intermetal dielectric layer. In addition, the etch stop layer and the capping layer are planarized, so that the etching process to be performed is much more predictable and uniform over the different portions of the wafer than it is for the intermetal dielectric layer.

[0027] After the via is defined through the intermetal dielectric layer 76 extending vertically toward the first level metal wiring line 66 , the via etch is continued through the etch stop layer 72 and capping layer 70 to reach the surface of the first level wiring line 66 . For the preferred nitride etch stop layer 72 , the etch stop layer can be removed where exposed within the via using a nitride etchant derived, for example, in a plasma process from an SF ₆ source gas. A portion of the oxide capping layer 70 is then removed where it is exposed within the interconnect via in a fixed time etching process using a conventional oxide etchant such as those discussed above. This etch through the thin, planarized oxide capping layer 70 can be easily controlled to proceed in a manner that satisfactorily clears the surface of the wiring line 66 yet avoids etching completely through the capping layer 70 . This aspect of the present invention relates to the formation in preferred embodiments of a capping layer that extends partially downward along the walls of first level wiring lines, providing a sufficient margin for completion of the via etch process. As discussed above, this feature is formed because the carbon sacrificial layer is etched back or recessed to expose the walls of the first level wiring lines prior to depositing the capping layer.

[0028] Processing continues after the via is formed and the surface of the wiring line 66 has been cleared by forming a metal plug 78 to fill the via. The metal plug might be formed from aluminum but is more typically formed from tungsten in a CVD process using WF ₆ as a source gas. In many instances, plug formation begins by providing a glue or adhesion layer, which might be titanium or titanium nitride, within the via and over the surface of the intermetal dielectric layer 76 . Sputtering or CVD processes may be used to form the glue or adhesion layer. Tungsten is then provided in a CVD process to fill the via, and a chemical mechanical polishing (CMP) or etch back process is performed to define the vertical extent of the tungsten plug 78 and to remove excess tungsten from the surface of the intermetal dielectric layer 76 . The polishing or etch back process also removes unnecessary portions of the glue layer. Further processing continues to form second level wiring lines such as the wiring line 80 shown in FIG. 9 . The second level wiring line might be formed by blanket metal deposition and conventional photolithography or by a damascene process.

[0029] The present invention has been described in terms of certain preferred embodiments, but those of ordinary skill in the art will appreciate that variations of and modifications to this process and structure might be made without altering the basic teachings of the present invention. For example, while the present invention has been described in terms of forming a first level wiring line connected to other conductors, the present invention might be implemented in multiple ones or all of the layers of a multilevel interconnect structure. Alternately, the present invention might be implemented for various levels of multilevel wiring structures other than the first level. As such, the present invention is not to be limited to any particular embodiment described, but instead the scope of the invention should be determined from the claims, which follow.