Abstract:
Method and structure for minimizing the downsides associated with microelectronic device processing adjacent porous dielectric materials are disclosed. In particular, chemical protocols are disclosed wherein porous dielectric materials may be sealed by attaching coupling agents to the surfaces of pores. The coupling agents may form all or part of caps on reactive groups in the dielectric surface or may crosslink to seal pores in the dielectric.

Description:
BACKGROUND  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates to dielectric layers with low dielectric constants, and more particularly to forming barriers across exposed pores in porous low dielectric constant dielectric layers.  
         [0003]     2. Background of the Invention  
         [0004]     Low dielectric constant (“k”) materials are used as interlayer dielectrics in microelectronic devices, such as semiconductor devices, to reduce the resistance-capacitance (“RC”) delay and improve device performance. As device sizes continue to shrink, the dielectric constant of the material between metal lines must also decrease to maintain the improvement. Certain low-k materials have been proposed, including various carbon-containing materials such as organic polymers and carbon-doped oxides. The eventual limit for a dielectric constant is k=1, which is the value for a vacuum.  
         [0005]     One of the challenges encountered in microelectronic device processing relates to the diffusion of wet chemical and processes gases through dielectric films leading to increased k values (water has a k value of about 80). While pores left in dielectric thin films after certain process steps may be advantageous from a k-value perspective when dry (dry air or nitrogen having relatively low k values), they may also facilitate fast diffusion of unwanted moisture, and may increase surface reactivity with such moisture due to the increased accessible surface area provided by pores, along with possible hydrophilic SiOH formation to propagate such a problem. SiOH available to react near the surface of dielectric pores may result in an increased k value. Pores may also cause additional challenges to subsequent process steps due to geometric considerations. For example, forming a thin sidewall upon a trench cut into a highly porous dielectric material presents obvious challenge—particularly if one of the sides of the trench upon which a sidewall is to be formed lies in the middle of a large pore.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     The present invention is illustrated by way of example and is not limited in the figures of the accompanying drawings, in which like references indicate similar elements. Features shown in the drawings are not intended to be drawn to scale, nor are they intended to be shown in precise positional relationship.  
         [0007]      FIGS. 1   a  and  1   b  are cross sectional side views of a porous dielectric layer.  
         [0008]      FIG. 2  is a flow chart that describes how porous dielectric may be sealed.  
         [0009]      FIG. 3   a  is an illustration of a coupling agent used according to one embodiment.  
         [0010]      FIG. 3   b  illustrates the linkage of the coupling agent to the pore surface.  
         [0011]      FIG. 3   c  illustrates the formation of the barrier across the opening of the pore.  
         [0012]      FIG. 4   a  illustrates the dielectric exposed to a coupling agent and the results of various reactions that may occur.  
         [0013]      FIG. 4   b  illustrates exposure of coupling structures to an additional sealing agent.  
         [0014]      FIG. 5  illustrates another example of how a porous dielectric may be sealed.  
         [0015]      FIG. 6  illustrates yet another example of how a porous dielectric may be sealed.  
     
    
     DETAILED DESCRIPTION  
       [0016]     In the following detailed description of embodiments of the invention, reference is made to the accompanying drawings in which like references indicate similar elements. The illustrative embodiments described herein are disclosed in sufficient detail to enable those skilled in the art to practice the invention. The following detailed description is therefore not to be taken in a limiting sense, and the scope of the invention is defined only by the appended claims.  
         [0017]      FIGS. 1   a  and  1   b  are cross sectional side views of a porous dielectric layer. Referring to  FIG. 1   a , there is a porous dielectric layer  104  above a substrate layer  102 . The porous dielectric layer  104  has pores  108 , some of which are within the dielectric layer  104 , and some of which are exposed at the surface. In an embodiment, the pores  108  typically have a size ranging from about 20 angstroms to about 100 angstroms, although the pores may also be other sizes. There is also a trench  106  formed in the dielectric layer  104 . As shown in  FIG. 1   a , the pores  108  at the surface of the dielectric layer  104  or at the sidewalls of the trench  108  may have openings that expose the interior of the pores  108  to the surrounding environment. The pores  108  may make it easier for materials in the environment to diffuse into the dielectric material  104 , may increase the difficulty of forming thin films on the dielectric layer  104 . The porous dielectric material  104  may have reactive groups, such as SiOH groups, near the surface that may cause an increase the dielectric constant (“k”) value of the dielectric layer  104 .  
         [0018]     Referring to  FIG. 1   b , barriers  110  have been formed over some of the openings of exposed pores  108  at the surface of the dielectric layer  104 . These barriers  110  may be considered to “seal” the pore  108  or “seal” the dielectric layer  104  and may prevent problems such as those described above. A pore  112  illustrated in  FIG. 1   b  does not have a barrier formed covering it, but a reaction has caused reactive SiOH groups formerly found near the surface of the dielectric  104  to be capped, which may prevent an increased k value of the dielectric layer  104 . Such a capping of SiOH or other reactive chemical structures on the surface of the dielectric  104  may also be considered as a “sealing” of the pore  108  or a “sealing” of the dielectric  104 .  
         [0019]      FIG. 2  is a flow chart  200  that describes various ways that a porous dielectric, such as dielectric  104 , may be sealed according to some embodiments of the present invention. This sealing may include capping reactive chemical structures, such as SiOH groups, of the dielectric material. The sealing may include forming barriers  110  over exposed pores  108 . To seal the dielectric  104 , the dielectric  104  may be exposed  202  to a coupling agent. The coupling agent may then link  204  to the dielectric  104  surface. This linking may result from a reaction between the coupling agent and chemical groups at the dielectric  104  surface to form a coupling structure attached to the dielectric  104  surface.  
         [0020]     In a first sealing embodiment, the coupling agent and the dielectric  104  may be exposed  206  to a crosslinking agent. This crosslinking agent exposure  206  may cause the coupling agent or coupling structure attached to the dielectric  104  to crosslink in a reaction. Such crosslinks may seal  208  the dielectric  104 . This seal  208  may result from crosslinking that form barriers to prevent external material from penetrating the pores  108 , or crosslinked groups capping reactive groups on the surface of the dielectric  104 .  
         [0021]     In a second sealing embodiment, the coupling agent and the dielectric  104  may be exposed  210  to a capping agent. This capping agent exposure  210  may cause the coupling agent or coupling structure attached to the dielectric  104  to react to form  212  a cap structure to seal the dielectric  104 . Note that both a crosslinking agent and a capping agent may be referred to as a “sealing agent.” 
         [0022]     In a third sealing embodiment, the coupling agent may seal  214  the dielectric  104  without additional capping or crosslinking agents. The coupling agent may react with the surface of the dielectric  104  to form a cap to seal the dielectric  104 . Alternatively, coupling structures linked  204  to the dielectric  104  surface may crosslink with other coupling structures linked  204  to the dielectric  104  to form a barrier and seal  214  the dielectric  104 . This seal  214  may result from crosslinks that form barriers to prevent external material from penetrating the pores  108 , or from crosslinked groups or capped groups that cap reactive groups on the surface of the dielectric  104 .  
         [0023]     In other embodiments, the dielectric  104  may be sealed using multiple ones of the first through third embodiments, or may be sealed through other methods. For example, in a fourth sealing embodiment, the dielectric  104  may be exposed  202  to a coupling agent that may form coupling structures linked  204  to a pore  108  surface. Some coupling structures may partially seal  214  the pore  108  by crosslinking or other processes, without being exposed to a crosslinking or other agent. Other coupling structures may crosslink in response to being exposed  206  to a crosslinking agent, and form  208  a barrier to seal the pore  108 , or cap reactive groups on the pore  108  surface. Other coupling structures may alternatively form  212  a cap structure in response to being exposed  210  to a capping agent. Thus, in the fourth sealing embodiment, the dielectric  104  may be sealed partially via the first or second sealing embodiments and partially via the third sealing embodiments. Other embodiments of sealing a dielectric material  104  by capping or crosslinking may also be used.  
         [0024]     In some embodiments, a cleaning step (not shown) may also be performed. In such a cleaning step, products of chemical reactions, excess coupling or crosslinking agents, or other substances may be removed from the vicinity of the dielectric  104 . This may be accomplished by a flow of gas or fluid removing the substances, causing a reaction of the substances with another material to result in an acceptable result material, or by other methods. Other additional steps may also be performed in some embodiments.  
         [0025]      FIGS. 3   a  through  3   c  illustrate how porous dielectric material  104  may be sealed according to an example of the first sealing embodiment described above with respect to  FIG. 2 .  FIG. 3   a  is an illustration of a coupling agent  300  to which a pore  108  in the dielectric  104  may be exposed  202  in one embodiment. The coupling agent  300  depicted in  FIG. 3   a  is a silane coupling reagent  300  with a thiol (“SH”) termini or “end cap”  302  that may facilitate disulfide bonding to seal an exposed pore  108 . The silane coupling reagent  300  is depicted with the thiol end cap  302  comprising a sulfur atom that may form one half of a disulfide bond at the end of the chemical progression. The thiol end cap  302  may be coupled to a silicon atom  308  by a chain  306  of CH 2  moieties. The length of the chain  306  may be selected to facilitate micromotion of the thiol end cap  302  for sulfur-sulfur (“disulfide”) bonding. In other words, the chain of CH 2  moieties may provide a relatively “floppy” construct to allow the thiol end cap  302  freedom to move into electrical contact with another nearby thiol end cap  302 , enabling formation of a disulfide bond. Such a floppy construct may be known as a flexible chain. In some embodiments, the chain  306  may be relatively long. In an embodiment, the chain  306  may have 4 CH 2  moieties, and in some other embodiments, the chain  306  may have from about 3 to about 5 CH 2  moieties.  
         [0026]     The depicted embodiment of the silane coupling reagent  300  also comprises three surface coupling groups  304  selected to react with SiOH groups or other groups that might be found on the surface of a silicate glass pore  108 . These surface coupling groups  304  need not be the exact OCH 3  (or CH 3 O) groups depicted—indeed, they may be referred to as “alkoxy,” “ether,” or “alkoxides” to one skilled in the art. For example, anywhere from one to three ethoxy groups would work to reactively link with the surface chemistry at the pore  108  in an embodiment. Other reagent surface coupling groups, including but not limited to those known as tert-butoxy and isopropoxy, may be substituted into the silane coupling reagent  300  as surface coupling groups  304 . In an embodiment, the alkoxy (oxygen) groups support pore surface reaction.  
         [0027]     Referring now to  FIG. 3   b , the linking  204  of the coupling agent  300  to the pore  108  surface is illustrated according to one embodiment. In an embodiment, the material  104  may be exposed to nitrogen or helium gas containing the coupling agent  300  at a concentration below the coupling agent&#39;s  300  lower flammability limit and at a temperature just below the flash point of the coupling agent  300 . As the silane coupling agent  300  is introduced, nearby water  310  may react with the surface coupling groups  304 , OCH 3  in this embodiment, to form methanol (“CH 3 OH”)  312 . The methanol  312  may readily evaporate or may be vented away. This results in SiOH groups (not shown) that react with other SiOH groups already hanging off the surface of the pore  108  to form “Si—O—Si” linkages, as depicted in  FIG. 3   b . The silicon atom  308  is therefore coupled to the surface of the pore  108 . Thus, the reaction of the coupling agent  300  has resulted in a coupling structure linked  204  to the surface of the pore  108 .  
         [0028]     Additional water produced by such reactions (not shown) may help to propagate more of these reactions by facilitating production of more SiOH groups, and eventually the pore  108  may be substantially occupied by variations of the silane coupling reagent  300  bonded to the surface of the pore  108 . As the pore  108  may be substantially occupied by the coupling reagent  300  bonded to the surface of the pore  108 , this may have resulted in the capping of the SiOH groups and a sealed pore.  
         [0029]     Pore  108  sizes may generally be on the order of 20-100 angstroms in some porous dielectric materials used to form the dielectric layer  104 . Silane coupling reagents  300  such as that depicted may have molecular radii of about 3 angstroms. For a pore  108  size of about 20-100 angstroms, therefore, about 6-30 molecules of the silane coupling agent  300  may be adequate to link  204  with the pore  108  surface and form a barrier for the pore  108  that may prevent external material from penetrating into the pore  108 .  
         [0030]     To form a barrier, the linked coupling agent  300  and pore  108  may be exposed  206  to a crosslinking agent  314 . In an embodiment, the crosslinking agent  314  may be a mild oxidizing agent  314 , such as formaldehyde (“H 2 CO”). In an embodiment, this exposure  206  may occur when the pore  108  is substantially covered with linked  204  coupling agent  300  molecules. The oxidizing agent  314  may be selected to be strong enough to oxidize the “SH” bonds of the thiol end caps  302  without substantially oxidizing other adjacent sensitive materials, such as copper, in an embodiment. In an embodiment, the pore  108  may be exposed to nitrogen or helium gas containing formaldehyde gas as the crosslinking agent  314 , at a concentration below formaldehyde&#39;s lower flammability limit, and at a temperature just below formaldehyde&#39;s flash point. In an embodiment, the concentration may be below about 7% formaldehyde, and the temperature may be below about 59 degrees Celsius.  
         [0031]      FIG. 3   c  illustrates sealing  208  the dielectric  104  pore  108  by formation of a barrier across the opening of the exposed pore  108  according to one embodiment. The crosslinking agent  314  may oxidize the “SH” bonds of the thiol end caps  302  of the linked  204  coupling agent  300  molecules. This enables the sulfur atoms in the end caps  302  to fulfill their bonding capabilities and form a disulfide bond  316  by pairing with another sulfur atom missing an electron. This bond may result in a “bridge” structure of multiple coupled barrier molecules, which may each include a silicon atom  308 , a flexible chain  306 , and a sulfur atom. The resultant “bridge” structure across the pore  108  may substantially fill the pore opening, creating a barrier  110  to the pore  108 . The bridge structure may be crosslinked to have enough density to act as a physical barrier to other chemicals getting through as a result of subsequent process treatments of the dielectric layer  104 . Thus, the exposed pore  108  has been sealed. In some embodiments, the crosslinking may not result in a bridge that substantially fills the opening of the pore  108 . The dielectric  104  may still be sealed, since the crosslinked structures may effectively cap reactive groups, such as SiOH groups, at the surface of the dielectric.  
         [0032]     Optionally in some embodiments, the structure may be made more hydrophobic by introducing, for example, hexamethyldisilazane (“HMDS”) or other chemicals. Such a process would further prevent pore reactivity or formation of further SiOH. HMDS forms strong surface bonds, which creates a locally hydrophobic environment.  
         [0033]      FIGS. 4   a  and  4   b  illustrate how the dielectric  104  may be sealed according to another example of the first sealing embodiment described above with respect to  FIG. 2 , according to an example of the second sealing embodiment described above with respect to  FIG. 2 , and according to an example of the third sealing embodiment described above with respect to  FIG. 2 .  
         [0034]      FIG. 4   a  illustrates the dielectric  104  exposed  202  to a coupling agent and the results of various reactions that may occur. In the embodiment illustrated in  FIG. 4   a , the coupling agent  400  is phosgene, which may be present in a vapor form. Note that care should be taken when using phosgene. Even brief exposures to phosgene at a concentration above 50 ppm may be fatal to humans. The dielectric  104  may have reactive SiOH groups  404  at the surface. In an embodiment, phosgene may be introduced in a vapor phase at a temperature in a range from about zero degrees Celsius to about 50 degrees Celsius at a concentration of about 5 ppm to about 1%. The phosgene coupling agent  400  may react with the SiOH groups  404 . This reaction may result in HCl  402 , and the phosgene  400  linking  204  to the surface of the dielectric  104 , which may be in a pore  108 . The linked  204  phosgene  400  may result in a pendant phosgene functional group  408 , which may be considered a coupling structure, linked  204  to the pore  108  or dielectric  104  surface. Nearby coupling structures may react to form disilyl carbonate  406 . The disilyl carbonate  406  may seal  214  the pore  108 , and may comprise an example of the third sealing embodiment described above with respect to  FIG. 2   a . The disilyl carbonate  406  may seal the dielectric  104  by forming a pore  108  barrier  110  to prevent external material from penetrating the pores  108 , and/or by capping reactive groups on the surface of the dielectric  104 .  
         [0035]      FIG. 4   b  illustrates exposure of coupling structures, such as the pendant phosgene functional groups  408 , to an additional sealing agent  410 . The agent  410  may be either or both of a crosslinking or capping agent, so that the coupling agent structures may be exposed  206  to a crosslinking agent according to the first sealing embodiment described above with respect to  FIG. 2  and/or exposed  210  to a capping agent according to an example of the second sealing embodiment described above with respect to  FIG. 2 . Both a crosslinking agent and a capping agent may be considered “sealing agents”  410 .  
         [0036]     In an embodiment, the agent  410  may be a crosslinking agent such as a multifunctional alcohol. Examples of such a multifunctional alcohol may include ethylene glycol, propylene glycol, glycerol, erythritol, and pentaerythritol. The alcohol may be in a vapor form. These crosslinking agents may react to attach to the coupling structures  408 , then may form crosslinks  414  to connect two or more coupling structures  408 . In an embodiment where one desires to seal exposed surfaces of the material  104 , the sealing agent  410  may be introduced as a liquid or in a solvent at a temperature in a range from about zero degrees Celsius to about 100 degrees Celsius at a concentration of about 0.1% to about 100% (liquid state). In an embodiment where one desires the sealant to penetrate further into the material  104 , the sealing agent  410  may be introduced as a solution in a supercritical fluid at a concentration of about 0.1% to about 100% or in a vapor phase at a temperature in a range from about 100 degrees Celsius to about 300 degrees Celsius at a concentration of about 5 ppm to about 5%. Such crosslinking may effectively seal  208  the dielectric  104  by removing reactive SiOH groups at the surface of the dielectric  104  and/or forming a barrier to prevent external material from penetrating the pore  108 .  
         [0037]     In another embodiment, the agent  410  may be a capping agent such as a monofunctional alcohol. Such a monofunctional alcohol may be methanol. The alcohol may be in a vapor form. In an embodiment, a methanol capping agent may be introduced as a liquid or in a solvent at a temperature in a range from about zero degrees Celsius to about 50 degrees Celsius at a concentration of about 0.1% to about 100% (liquid state). In an embodiment where one desires the sealant to penetrate further into the material  104 , the methanol capping agent may be introduced as a solution in a supercritical fluid at a concentration of about 0.1% to about 100% or in a vapor phase at a temperature in a range from about 50 degrees Celsius to about 100 degrees Celsius at a concentration of about 5 ppm to about 5%. The capping agent  410  may react to attach to the coupling structures  408  and form  212  a cap  412  on them. For example, when methanol is used as a capping agent, the methanol may react to a coupling structure  408  to form mehtyl silyl carbonate, which may seal the pore  108  or dielectric  104 .  
         [0038]      FIG. 5  illustrates how dielectric  104  or pores  108  may be sealed according to an example of the third sealing embodiment described above with respect to  FIG. 2 . The dielectric  104  may be exposed  202  to a coupling agent  500 . The coupling agent  500  may be an acyl dichloride, such as succinyl chloride, and may include one or more R groups  502  that may react to form crosslinks. The R group  502  may be different for different coupling agents  500 . For example, in an embodiment where the coupling agent  500  is succinyl chloride, the R group  502  may be CH 2 CH 2 . In an embodiment where succinyl chloride acts as a coupling agent  500 , the succinyl chloride may be in a solution of a supercritical fluid at a temperature in a range from about zero degrees Celsius to about 100 degrees Celsius. The coupling agent  500  may both link  204  to the dielectric surface  104  and crosslink to seal  214  the pore. The dielectric  104  may have reactive SiOH groups  504  at the surface. The coupling agent  500  may react with the SiOH groups  504  to form coupling structures  506  linked to the dielectric  104  surface, and crosslinking groups  502  of the structures  506  may react to form a crosslink  508 . Such crosslinked structures  506  may seal  214  the dielectric  104 . Such crosslinking may effectively seal  214  the dielectric  104  by removing reactive groups at the surface of the dielectric  104  and/or forming a barrier to prevent external material from penetrating the pore  108 .  
         [0039]      FIG. 6  illustrates how dielectric  104  or pores  108  may be sealed according to another example of the third sealing embodiment described above with respect to  FIG. 2 . The dielectric  104  may be exposed  202  to a coupling agent  600 . The coupling agent  600  may be an acyl chloride, such as acetyl chloride, and may include a capping group  602 . The coupling agent  600  may be introduced under conditions similar to those described above with respect to the sealing agent  410 . The coupling agent  600  may link  204  the dielectric surface  104 . The dielectric  104  may have reactive SiOH groups  604  at the surface. The coupling agent  600  may react with the SiOH groups  604  to form structures  606  linked to the dielectric  104  surface. The capping groups  602  may comprise caps  608  to seal  214  the dielectric  104 .  
         [0040]     In the embodiments described above, larger coupling agents with more crosslinking groups may be used to increase the crosslinking density and form a pore  108  barrier  110  to prevent external material from penetrating the pores  108 . Smaller coupling agents with fewer functional crosslinking groups may be used to cap reactive groups on the surface of the dielectric  104 . Further, the depth of the sealing into the dielectric  104  may be controlled by the time which the dielectric  104  may be exposed to coupling agents. A smaller exposure time may result in sealing the surface of dielectric  104  and pores  108  near the surface, while a larger exposure time may result in sealing the surface of pores  108  deeper within the dielectric material.  
         [0041]     Thus, a novel pore sealing solution is disclosed. Although the invention is described herein with reference to specific embodiments, many modifications therein will readily occur to those of ordinary skill in the art. Accordingly, all such variations and modifications are included within the intended scope of the invention as defined by the following claims.