Sealing porous dielectric materials

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.

BACKGROUND

1. Field of the Invention

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.

2. Background of the Invention

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.

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.

DETAILED DESCRIPTION

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.

FIGS. 1aand1bare cross sectional side views of a porous dielectric layer. Referring toFIG. 1a, there is a porous dielectric layer104above a substrate layer102. The porous dielectric layer104has pores108, some of which are within the dielectric layer104, and some of which are exposed at the surface. In an embodiment, the pores108typically have a size ranging from about 20 angstroms to about 100 angstroms, although the pores may also be other sizes. There is also a trench106formed in the dielectric layer104. As shown inFIG. 1a, the pores108at the surface of the dielectric layer104or at the sidewalls of the trench108may have openings that expose the interior of the pores108to the surrounding environment. The pores108may make it easier for materials in the environment to diffuse into the dielectric material104, may increase the difficulty of forming thin films on the dielectric layer104. The porous dielectric material104may have reactive groups, such as SiOH groups, near the surface that may cause an increase the dielectric constant (“k”) value of the dielectric layer104.

Referring toFIG. 1b, barriers110have been formed over some of the openings of exposed pores108at the surface of the dielectric layer104. These barriers110may be considered to “seal” the pore108or “seal” the dielectric layer104and may prevent problems such as those described above. A pore112illustrated inFIG. 1bdoes not have a barrier formed covering it, but a reaction has caused reactive SiOH groups formerly found near the surface of the dielectric104to be capped, which may prevent an increased k value of the dielectric layer104. Such a capping of SiOH or other reactive chemical structures on the surface of the dielectric104may also be considered as a “sealing” of the pore108or a “sealing” of the dielectric104.

FIG. 2is a flow chart200that describes various ways that a porous dielectric, such as dielectric104, 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 barriers110over exposed pores108. To seal the dielectric104, the dielectric104may be exposed202to a coupling agent. The coupling agent may then link204to the dielectric104surface. This linking may result from a reaction between the coupling agent and chemical groups at the dielectric104surface to form a coupling structure attached to the dielectric104surface.

In a first sealing embodiment, the coupling agent and the dielectric104may be exposed206to a crosslinking agent. This crosslinking agent exposure206may cause the coupling agent or coupling structure attached to the dielectric104to crosslink in a reaction. Such crosslinks may seal208the dielectric104. This seal208may result from crosslinking that form barriers to prevent external material from penetrating the pores108, or crosslinked groups capping reactive groups on the surface of the dielectric104.

In a second sealing embodiment, the coupling agent and the dielectric104may be exposed210to a capping agent. This capping agent exposure210may cause the coupling agent or coupling structure attached to the dielectric104to react to form212a cap structure to seal the dielectric104. Note that both a crosslinking agent and a capping agent may be referred to as a “sealing agent.”

In a third sealing embodiment, the coupling agent may seal214the dielectric104without additional capping or crosslinking agents. The coupling agent may react with the surface of the dielectric104to form a cap to seal the dielectric104. Alternatively, coupling structures linked204to the dielectric104surface may crosslink with other coupling structures linked204to the dielectric104to form a barrier and seal214the dielectric104. This seal214may result from crosslinks that form barriers to prevent external material from penetrating the pores108, or from crosslinked groups or capped groups that cap reactive groups on the surface of the dielectric104.

In other embodiments, the dielectric104may 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 dielectric104may be exposed202to a coupling agent that may form coupling structures linked204to a pore108surface. Some coupling structures may partially seal214the pore108by crosslinking or other processes, without being exposed to a crosslinking or other agent. Other coupling structures may crosslink in response to being exposed206to a crosslinking agent, and form208a barrier to seal the pore108, or cap reactive groups on the pore108surface. Other coupling structures may alternatively form212a cap structure in response to being exposed210to a capping agent. Thus, in the fourth sealing embodiment, the dielectric104may be sealed partially via the first or second sealing embodiments and partially via the third sealing embodiments. Other embodiments of sealing a dielectric material104by capping or crosslinking may also be used.

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 dielectric104. 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.

FIGS. 3athrough3cillustrate how porous dielectric material104may be sealed according to an example of the first sealing embodiment described above with respect toFIG. 2.FIG. 3ais an illustration of a coupling agent300to which a pore108in the dielectric104may be exposed202in one embodiment. The coupling agent300depicted inFIG. 3ais a silane coupling reagent300with a thiol (“SH”) termini or “end cap”302that may facilitate disulfide bonding to seal an exposed pore108. The silane coupling reagent300is depicted with the thiol end cap302comprising a sulfur atom that may form one half of a disulfide bond at the end of the chemical progression. The thiol end cap302may be coupled to a silicon atom308by a chain306of CH2moieties. The length of the chain306may be selected to facilitate micromotion of the thiol end cap302for sulfur-sulfur (“disulfide”) bonding. In other words, the chain of CH2moieties may provide a relatively “floppy” construct to allow the thiol end cap302freedom to move into electrical contact with another nearby thiol end cap302, enabling formation of a disulfide bond. Such a floppy construct may be known as a flexible chain. In some embodiments, the chain306may be relatively long. In an embodiment, the chain306may have 4 CH2moieties, and in some other embodiments, the chain306may have from about 3 to about 5 CH2moieties.

The depicted embodiment of the silane coupling reagent300also comprises three surface coupling groups304selected to react with SiOH groups or other groups that might be found on the surface of a silicate glass pore108. These surface coupling groups304need not be the exact OCH3(or CH3O) 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 pore108in 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 reagent300as surface coupling groups304. In an embodiment, the alkoxy (oxygen) groups support pore surface reaction.

Referring now toFIG. 3b, the linking204of the coupling agent300to the pore108surface is illustrated according to one embodiment. In an embodiment, the material104may be exposed to nitrogen or helium gas containing the coupling agent300at a concentration below the coupling agent's300lower flammability limit and at a temperature just below the flash point of the coupling agent300. As the silane coupling agent300is introduced, nearby water310may react with the surface coupling groups304, OCH3in this embodiment, to form methanol (“CH3OH”)312. The methanol312may 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 pore108to form “Si—O—Si” linkages, as depicted inFIG. 3b. The silicon atom308is therefore coupled to the surface of the pore108. Thus, the reaction of the coupling agent300has resulted in a coupling structure linked204to the surface of the pore108.

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 pore108may be substantially occupied by variations of the silane coupling reagent300bonded to the surface of the pore108. As the pore108may be substantially occupied by the coupling reagent300bonded to the surface of the pore108, this may have resulted in the capping of the SiOH groups and a sealed pore.

Pore108sizes may generally be on the order of 20-100 angstroms in some porous dielectric materials used to form the dielectric layer104. Silane coupling reagents300such as that depicted may have molecular radii of about 3 angstroms. For a pore108size of about 20-100 angstroms, therefore, about 6-30 molecules of the silane coupling agent300may be adequate to link204with the pore108surface and form a barrier for the pore108that may prevent external material from penetrating into the pore108.

To form a barrier, the linked coupling agent300and pore108may be exposed206to a crosslinking agent314. In an embodiment, the crosslinking agent314may be a mild oxidizing agent314, such as formaldehyde (“H2CO”). In an embodiment, this exposure206may occur when the pore108is substantially covered with linked204coupling agent300molecules. The oxidizing agent314may be selected to be strong enough to oxidize the “SH” bonds of the thiol end caps302without substantially oxidizing other adjacent sensitive materials, such as copper, in an embodiment. In an embodiment, the pore108may be exposed to nitrogen or helium gas containing formaldehyde gas as the crosslinking agent314, at a concentration below formaldehyde's lower flammability limit, and at a temperature just below formaldehyde's flash point. In an embodiment, the concentration may be below about 7% formaldehyde, and the temperature may be below about 59 degrees Celsius.

FIG. 3cillustrates sealing208the dielectric104pore108by formation of a barrier across the opening of the exposed pore108according to one embodiment. The crosslinking agent314may oxidize the “SH” bonds of the thiol end caps302of the linked204coupling agent300molecules. This enables the sulfur atoms in the end caps302to fulfill their bonding capabilities and form a disulfide bond316by 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 atom308, a flexible chain306, and a sulfur atom. The resultant “bridge” structure across the pore108may substantially fill the pore opening, creating a barrier110to the pore108. 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 layer104. Thus, the exposed pore108has been sealed. In some embodiments, the crosslinking may not result in a bridge that substantially fills the opening of the pore108. The dielectric104may still be sealed, since the crosslinked structures may effectively cap reactive groups, such as SiOH groups, at the surface of the dielectric.

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.

FIGS. 4aand4billustrate how the dielectric104may be sealed according to another example of the first sealing embodiment described above with respect toFIG. 2, according to an example of the second sealing embodiment described above with respect toFIG. 2, and according to an example of the third sealing embodiment described above with respect toFIG. 2.

FIG. 4aillustrates the dielectric104exposed202to a coupling agent and the results of various reactions that may occur. In the embodiment illustrated inFIG. 4a, the coupling agent400is 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 dielectric104may have reactive SiOH groups404at 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 agent400may react with the SiOH groups404. This reaction may result in HCl402, and the phosgene400linking204to the surface of the dielectric104, which may be in a pore108. The linked204phosgene400may result in a pendant phosgene functional group408, which may be considered a coupling structure, linked204to the pore108or dielectric104surface. Nearby coupling structures may react to form disilyl carbonate406. The disilyl carbonate406may seal214the pore108, and may comprise an example of the third sealing embodiment described above with respect toFIG. 2a. The disilyl carbonate406may seal the dielectric104by forming a pore108barrier110to prevent external material from penetrating the pores108, and/or by capping reactive groups on the surface of the dielectric104.

FIG. 4billustrates exposure of coupling structures, such as the pendant phosgene functional groups408, to an additional sealing agent410. The agent410may be either or both of a crosslinking or capping agent, so that the coupling agent structures may be exposed206to a crosslinking agent according to the first sealing embodiment described above with respect toFIG. 2and/or exposed210to a capping agent according to an example of the second sealing embodiment described above with respect toFIG. 2. Both a crosslinking agent and a capping agent may be considered “sealing agents”410.

In an embodiment, the agent410may 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 structures408, then may form crosslinks414to connect two or more coupling structures408. In an embodiment where one desires to seal exposed surfaces of the material104, the sealing agent410may 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 material104, the sealing agent410may 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 seal208the dielectric104by removing reactive SiOH groups at the surface of the dielectric104and/or forming a barrier to prevent external material from penetrating the pore108.

In another embodiment, the agent410may 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 material104, 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 agent410may react to attach to the coupling structures408and form212a cap412on them. For example, when methanol is used as a capping agent, the methanol may react to a coupling structure408to form methyl silyl carbonate, which may seal the pore108or dielectric104.

FIG. 5illustrates how dielectric104or pores108may be sealed according to an example of the third sealing embodiment described above with respect toFIG. 2. The dielectric104may be exposed202to a coupling agent500. The coupling agent500may be an acyl dichloride, such as succinyl chloride, and may include one or more R groups502that may react to form crosslinks. The R group502may be different for different coupling agents500. For example, in an embodiment where the coupling agent500is succinyl chloride, the R group502may be CH2CH2. In an embodiment where succinyl chloride acts as a coupling agent500, 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 agent500may both link204to the dielectric surface104and crosslink to seal214the pore. The dielectric104may have reactive SiOH groups504at the surface. The coupling agent500may react with the SiOH groups504to form coupling structures506linked to the dielectric104surface, and crosslinking groups502of the structures506may react to form a crosslink508. Such crosslinked structures506may seal214the dielectric104. Such crosslinking may effectively seal214the dielectric104by removing reactive groups at the surface of the dielectric104and/or forming a barrier to prevent external material from penetrating the pore108.

FIG. 6illustrates how dielectric104or pores108may be sealed according to another example of the third sealing embodiment described above with respect toFIG. 2. The dielectric104may be exposed202to a coupling agent600. The coupling agent600may be an acyl chloride, such as acetyl chloride, and may include a capping group602. The coupling agent600may be introduced under conditions similar to those described above with respect to the sealing agent410. The coupling agent600may link204the dielectric surface104. The dielectric104may have reactive SiOH groups604at the surface. The coupling agent600may react with the SiOH groups604to form structures606linked to the dielectric104surface. The capping groups602may comprise caps608to seal214the dielectric104.

In the embodiments described above, larger coupling agents with more crosslinking groups may be used to increase the crosslinking density and form a pore108barrier110to prevent external material from penetrating the pores108. Smaller coupling agents with fewer functional crosslinking groups may be used to cap reactive groups on the surface of the dielectric104. Further, the depth of the sealing into the dielectric104may be controlled by the time which the dielectric104may be exposed to coupling agents. A smaller exposure time may result in sealing the surface of dielectric104and pores108near the surface, while a larger exposure time may result in sealing the surface of pores108deeper within the dielectric material.

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.