Patent Publication Number: US-7585785-B2

Title: Sacrificial benzocyclobutene copolymers for making air gap semiconductor devices

Description:
The invention herein described relates generally to the fabrication of semiconductor devices and more specifically to such devices that use air gaps to reduce capacitive coupling between conductors in such devices. 
     As a consequence of the progress made in integrated circuit technology, the spacing between the metal lines on any given plane of an integrated circuit has become less and less, now extending into the submicrometer range. By reducing the spacing between conductive members in the integrated circuit, an increase in capacitive coupling occurs. This increase in capacitive coupling causes greater crosstalk, higher capacitive losses and increased RC time constant. 
     In order to reduce capacitive coupling, much effort has been directed toward developing low dielectric constant (low-K) materials to replace conventional dielectric materials that are interposed between the metal lines on a given layer and between layers. Many conventional electronic insulators have dielectric constants in the 3.5 to 4.2 range. For example, silicon dioxide has a dielectric constant of 4.2 and polyimides typically have dielectric constants from 2.9 to 3.5. Some advanced polymers have dielectric constants in the 2.5 to 3.0 range. Materials in the 1.8 to 2.5 range are also known. 
     The lowest possible, or ideal, dielectric constant is 1.0, which is the dielectric constant of a vacuum. Air is almost as good with a dielectric constant of 1.001. With this recognition of the low dielectric constant of air, attempts have been made to fabricate semiconductor devices with air gaps between metal leads to reduce the capacitive coupling between the electrically conducting members. The air gap forming techniques that have been developed have varying degrees of complexity and include subtractive and damascene techniques. 
     U.S. Pat. No. 4,987,101 describes a method and structure for providing an insulating electrical space between two lines on a layer of material or between lines on adjacent superposed layers of material. A base member is formed having a plurality of support members extending upwardly from the base member. A removable material is deposited on the base member and around the support members. A cap member of insulating material is then deposited over said support members and the removable material. Access openings are formed in at least one of the base member or the cap member communicating with the removable material. The removable material is removed through the access openings to thereby define a space between the cap member and the base member and between the support members. During this step a partial vacuum (in which some inert gas may be dispersed) may be created in the space vacated by the removable material. The access openings are then filled in so as to provide a sealed space between the cap member which has a very low dielectric constant. 
     U.S. Pat. No. 5,324,683 describes several techniques for forming air gaps or regions in a semiconductor device. The air regions are formed by either selectively removing a sacrificial spacer or by selectively removing a sacrificial layer. The air regions are sealed, enclosed or isolated by either a selective growth process or by a non-conformal deposition technique. The air regions may be formed under any pressure, gas concentration or processing condition. 
     The techniques disclosed in the aforesaid patents rely on holes or other passageways for effecting removal of the sacrificial material. In U.S. Pat. No. 5,461,003, a sacrificial material is removed through a porous dielectric layer. According to this patent, metal leads are formed on a substrate, after which a disposable solid layer is deposited on the metal leads and substrate. The disposable solid layer is then etched back to expose the tops of the metal leads. Then a porous dielectric layer is deposited over the metal leads and disposable layer. This is followed by removal of the disposable layer which is said to be preferably accomplished by exposing the device to oxygen or oxygen-plasma at a high temperature (greater than 100 degrees Celsius) to vaporize, or burn off, the disposable layer. The oxygen moves through the porous dielectric layer to reach and react with the disposable layer and thereby convert it to a gas that moves back out of the porous dielectric layer. Upon removal of the disposable layer, air gaps are left. Finally, a non-porous dielectric layer is deposited on top of the porous dielectric layer to seal the porous dielectric layer from moisture, provide improved structural support and thermal conductivity, and passivate the porous dielectric layer. This procedure results in an air gap that does not extend the full height of the adjacent metal leads or lines. The &#39;003 patent discloses a modified method to remedy this problem and to increase the process margin. This modified method involves a further process step wherein an oxide layer is formed on top of the metal leads so that the disposable dielectric layer can extend higher than the metal leads. 
     It is also noted that the exposure of the device to oxygen plasma which must diffuse through a porous layer is not only inefficient, it also exposes other elements of the device to potentially damaging oxygen plasma for an extended period of time. In particular, exposure of oxygen plasma to copper lines can prove deleterious. Copper is becoming an increasingly important metal in semiconductor manufacturing due to its lower resistivity when compared to aluminum. 
     In U.S. Pat. No. 6,165,890 a sacrificial norbornene polymer is used between the metal lines of a semiconductor device and then the device is heated to decompose and vaporize said polymer leaving an air gap between said metal lines. Kohl, et al., IEEE Electron Device Letters, Vol. 21, No. 12, December 2000, p. 557-559 teach that critical material properties of such a sacrificial polymer include: (a) a glass transition temperature sufficiently high to provide dimensional stability during processing (for example, greater than 350 degrees Celsius); (b) a sufficiently slow decomposition rate to mitigate problems of pressure build-up during air gap formation; (c) no objectionable residue after decomposition; and (d) a temperature of decomposition sufficiently low (e.g., 450 degrees Celsius) to mitigate device damage that may occur at higher temperatures. Many polymers are not suitable for such an application, IBM Technical Disclosure Bulletin, Vol. 38, No. 9 Sep. 1995, p 137-140. 
     A method of forming an air gap within a semiconductor structure comprising the steps of: (a) using a sacrificial polymer to occupy a space in a semiconductor structure; and (b) heating the semiconductor structure to decompose the sacrificial polymer leaving an air gap within the semiconductor structure, wherein the sacrificial polymer is a copolymer comprising bis[3-(4-benzocyclobutenyl)]1,n (n=2-12) alkyldiol diacrylate and 1,3 bis 2[4-benzocyclobutenyl(ethenyl)]benzene monomers. In addition, a semiconductor structure, comprising a sacrificial polymer positioned between conductor lines, wherein the sacrificial polymer is a copolymer comprising bis[3-(4-benzocyclobutenyl)]1,n (n=2-12) alkyldiol diacrylate and 1,3 bis 2[4-benzocyclobutenyl(ethenyl)]benzene monomers. 
    
    
     
         FIGS. 1-6  are diagrammatic cross-sections of a portion of a semiconductor structure, illustrating several steps of a method according to an aspect of the instant invention 
     
    
    
     In one embodiment the instant invention is a method of forming an air gap within a semiconductor structure comprising the steps of: (a) using a sacrificial polymer to occupy a space in a semiconductor structure; (b) heating the semiconductor structure to decompose the sacrificial polymer leaving an air gap within the semiconductor structure, wherein the sacrificial polymer of step (a) is a copolymer comprising bis[3-(4-benzocyclobutenyl)]1,n (n=2-12) alkyldiol diacrylate and 1,3 bis 2[4-benzocyclobutenyl(ethenyl)]benzene monomers. 
     Preferably, the sacrificial polymer is a copolymer is comprising from 99 to 40 mole bis[3-(4-benzocyclobutenyl)]1,n (n=2-12) alkyldiol diacrylate and from 1 to 60 mole percent 1,3 bis 2[4-benzocyclobutenyl(ethenyl)]benzene based on total moles of incorporated monomers of the polymer. More preferably, the sacrificial polymer is a copolymer consisting essentially of from 99 to 40 mole percent bis[3-(4-benzocyclobutenyl)]1,n (n=2-12) alkyldiol diacrylate and from 1 to 60 mole percent 1,3 bis 2[4-benzocyclobutenyl(ethenyl)]benzene based on total moles of incorporated monomers of the polymer. Yet more preferably, the sacrificial polymer is a copolymer comprising from 95 to 75 mole percent bis[3-(4-benzocyclobutenyl)]1,n (n=2-12) alkyldiol diacrylate and from 5 to 25 mole percent 1,3 bis 2[4-benzocyclobutenyl(ethenyl)]benzene based on total moles of incorporated monomers of the polymer. 
     Formula (a) below shows the schematic chemical formula for bis[3-(4-benzocyclobutenyl)]1,n (n=2-12) alkyldiol diacrylate wherein hydrogen atoms are assumed. 
                         
Formula (b) below shows the schematic chemical formula for, 3 bis 2[4-benzocyclobutenyl(ethenyl)]benzene wherein hydrogen atoms are assumed.
 
     
       
         
         
             
             
         
       
     
     Even more preferably, the sacrificial polymer is a copolymer consists essentially of from 95 to 75 mole percent bis[3-(4-benzocyclobutenyl)]1,n (n=2-12) alkyldiol diacrylate and from 5 to 25 mole percent 1,3 bis 2[4-benzocyclobutenyl(ethenyl)]benzene based on total moles of incorporated monomers of the polymer. In a highly preferred embodiment, the sacrificial polymer is a copolymer comprising about 90 mole percent bis[3-(4-benzocyclobutenyl)]1,6 hexanediol diacrylate and about 10 mole percent 1,3 bis 2[4-benzocyclobutenyl(ethenyl)]benzene based on total moles of incorporated monomers of the polymer. In another highly preferred embodiment, the sacrificial polymer is a copolymer consisting essentially of about 90 mole percent bis[3-(4-benzocyclobutenyl)]1,6 hexanediol diacrylate and about 10 mole percent 1,3 bis 2[4-benzocyclobutenyl(ethenyl)]benzene based on total moles of incorporated monomers of the polymer. 
     The relative amount of bis[3-(4-benzocyclobutenyl)]1,n (n=2-12) alkyldiol diacrylate and 1,3 bis 2[4-benzocyclobutenyl (ethenyl)]benzene used in the copolymer of the instant invention depends on the glass transition temperature and decomposition temperature that is desired. When the copolymer contains more 1,3 bis 2[4-benzocyclobutenyl(ethenyl)]benzene monomer then the copolymer will have a higher glass transition temperature and a higher decomposition temperature. The highly preferred copolymer of the instant invention consisting essentially of about 90 mole bis[3-(4-benzocyclobutenyl)]1,n (n=2-12) alkyldiol diacrylate and about 10 mole percent 1,3 bis 2[4-benzocyclobutenyl(ethenyl)]benzene based on total moles of incorporated monomers of the polymer shows a glass transition temperature prior to benzocyclobutene cure of about 200 degrees Celsius, a glass transition temperature after benzocyclobutene cure of about 350 degrees Celsius and a decomposition temperature of about 450 degrees Celsius. 
     The full scope of the instant invention is realized when the sacrificial polymer comprises a first monomer (such as an alkyl diacrylate type monomer with or without terminal benzocyclobutene groups) selected to give the polymer the desired decomposition temperature and a second monomer (such as a benzocyclobutene type monomer) selected to give the polymer the desired glass transition temperature. Using this approach, it is possible to obtain a polymer having a decomposition and glass transition temperature tailored to the specific temperature requirements for processing air gap semiconductor structures 
     The copolymers of the instant invention are dispersable in common solvents, such as toluene, xylenes or mesitylene. Dispersions of the copolymers of the instant invention in such solvents can be used to apply the copolymers of the instant invention to a semiconductor structure by any suitable coating technique, for example by spin coating. 
     Referring now to  FIGS. 1-6 , wherein is shown diagrammatic cross-sections of a portion of a semiconductor structure, illustrating several steps of a method according to one aspect of the instant invention. In  FIGS. 1 and 2  a patterned layer of sacrificial polymer  30  comprising bis[3-(4-benzocyclobutenyl)]1,n (n=2-12) alkyldiol diacrylate and 1,3 bis 2[4-benzocyclobutenyl(ethenyl)]benzene monomers is formed on a substrate  32 . The substrate  32  may have patterns already on it, or it may be an unpatterned material. The substrate may be a base layer or a layer of material overlaying a base layer such as an insulating layer of silicon dioxide that may overlie the devices on an integrated circuit chip (not shown). By way is of specific example, the substrate may be a semiconductor wafer that may, for example, contain transistors, diodes, and other semiconductor elements (as are well known in the art). 
     As depicted in  FIG. 1 , a relatively uniform layer of the sacrificial polymer  30  is deposited on the substrate  32 . This may be done in any suitable manner, for example, by spin coating, spraying, meniscus, extrusion or other coating methods, by pressing or laying a dry film laminate onto the substrate, etc. 
     In  FIG. 2 , the layer of sacrificial polymer is patterned to produce the patterned layer of the sacrificial polymer  30 , the pattern of which corresponds to the desired pattern of one or more air gaps to be formed in the semiconductor device. Any suitable technique can be used to pattern the layer of sacrificial polymer, including, for example, laser ablating, etching, etc. The sacrificial polymer may be made photosensitive to facilitate patterning. 
     In  FIG. 3 , a layer of conductive material  34 , usually a metal such as copper or aluminum is deposited over the patterned layer of sacrificial polymer  30 . This may be done by any suitable technique including, for example, metal sputtering, chemical vapor deposition (CVD), physical vapor deposition (PVD), electroplating, electroless plating, etc. 
     In  FIG. 4 , the layer  34  is planarized if needed by any suitable technique including, for example, chemical mechanical polishing (CMP). If CMP is used, an etch stop (such as a layer of silicon dioxide or other suitable material) is preferably applied to the surface of the sacrificial polymer. 
     In  FIG. 5 , a permanent dielectric  36  is deposited over the patterned layer of sacrificial polymer  30  with the metal inlay  34 . The permanent dielectric  36  is deposited as a solid layer and covers the sacrificial layer  30  and at least the tops of the metal leads  34 . The permanent dielectric layer may be planarized before or after removal of the sacrificial material. The permanent dielectric layer, for example, may be silicon dioxide, polyimide or other material. The permanent dielectric layer may be deposited by spin coating, spray coating or meniscus coating, chemical vapor deposition, plasma enhanced chemical vapor deposition, sol-gel process, or other method. 
     As seen in  FIG. 5 , the metal layer can be conveniently formed with a height less than the height of the adjacent sacrificial material. As will be appreciated, this will result in air gaps that extend above the tops of the metal leads, as is desirable to reduce capacitive coupling. Also, the substrate could have trenches formed therein in a pattern corresponding to the pattern of the sacrificial material, so that the resultant air gaps will extend below the metal leads located on lands on the substrate between the trenches. 
     The sacrificial polymer  30  is removed through the permanent dielectric layer  36  to form the air gaps  38  shown in  FIG. 6 . The removal of the sacrificial polymer preferably is accomplished by thermal decomposition and passage of one or more of the decomposition products through the permanent dielectric layer  36  by diffusion. An important benefit of a polymer of the instant invention is that the molecular weight of the degraded polymer is relatively low, thereby facilitating removal of the degradation products from the semiconductor structure. As above indicated, polymers of the instant invention can undergo thermal decomposition at temperatures on the order of about 450 degrees Centigrade, and lower, with essentially no residue being left in the air gaps of the resultant semiconductor structure  40 . Also, the decomposition products are diffusible through many dielectric materials useful in forming the permanent dielectric layer, including in particular polyimides. 
     The rate of decomposition should be slow enough so that diffusion through the permanent dielectric will occur. Diffusion typically arises from a pressure buildup within the air gap. This pressure build up should not be so great as to exceed the mechanical strength of the permanent dielectric. Increased temperature will generally aid diffusion as diffusivity of gas through the permanent dielectric will normally increase with temperature. 
     As will be appreciated, the air gaps may contain residual gas from the decomposition although generally such residual gas will eventually exchange with air. However, steps may be taken to prevent such exchange, or dispose a different gas (a noble gas for example) or a vacuum in the air gaps. For example, the semiconductor structure may be subjected to vacuum conditions to extract any residual gas from the air gaps by diffusion or otherwise after which the semiconductor structure may be coated by a suitable sealing material. Before the semiconductor structure is sealed, it may be subjected to a controlled gas atmosphere, such as one containing a noble gas, to fill the air gaps with such gas. Further processing steps may be performed on the semiconductor structure  40 , for example to form additional layer(s) of interconnection in the semiconductor device having air gaps above and below conductor lines as well as air gaps on the sides of conductor lines. Thus, the polymer of the instant invention may be decomposed as a single layer before each next interconnect level is built or the polymer of the instant invention may be decomposed in multiple layers simultaneously after multiple interconnect levels have been built. Preferably, the entire multiple layer interconnect structure is built and the polymer of the instant invention is decomposed simultaneously. However, thermal decomposition is the preferred technique. 
     The method of the instant invention is not limited to the specific steps outlined above with reference to FIGS. 1-6. U.S. Pat. No. 6,165,890, for example, shows a number of specific steps for forming air gaps in semiconductor structures using sacrificial polymers and the polymers of the instant invention are also applicable to the steps and structures outlined in the &#39;890 patent. The critical aspect of the instant invention is the sacrificial polymer. The sacrificial polymer of the instant invention is a copolymer comprising bis[3-(4-benzocyclobutenyl)]1,n (n=2-12) alkyldiol diacrylate and 1,3 bis 2[4-benzocyclobutenyl(ethenyl)]benzene monomers. 
     In another embodiment, the instant invention is a semiconductor structure comprising a sacrificial polymer positioned between conductor lines, wherein the sacrificial polymer is a copolymer comprising bis[3-(4-benzocyclobutenyl)]1,n (n=2-12) alkyldiol diacrylate and 1,3 bis 2[4-benzocyclobutenyl(ethenyl)]benzene monomers.  FIG. 5  shows such a structure. 
     EXAMPLE 1 
     Ninety grams of bis[3-(4-benzocyclobutenyl)]1,6 hexanediol diacrylate monomer, ten grams of 1,3 bis 2[4-benzocyclobutenyl (ethenyl)]benzene monomer are mixed with five hundred milliliters of toluene and heated in a sealed container at 160 degrees Celsius for five days to thermally polymerize the monomers. The resulting polymer is precipitated by adding isopropanol, dried, dissolved in methylene chloride, precipitated in methanol and then dried. 
     The dried polymer weighs 87 grams. Proton NMR analysis indicates that the dried polymer is a copolymer of bis[3-(4-benzocyclobutenyl)]1,6 hexanediol diacrylate and 1,3 bis 2[4-benzocyclobutenyl(ethenyl)]benzene. Differential Scanning Calorimitery shows a glass transition temperature of 200 degrees Celsius prior to benzocyclobutene cure, then a glass transition temperature of 350 degrees Celsius after curing at 280 degrees Celsius (the cyclobutene group on the polymer opens at 280 degrees Celsius and cross-links the polymer by way of reaction with a neighboring cyclobutene group, see Kirchhoff et al., Prog. Polym. Sci. Vol 18, 85-185, 1993, the cyclobutene rings on the benzocyclobutene moieties begin to undergo ring opening at a significant rate at 200 degrees Celsius with a polymerization exotherm maximum temperature at 250-280 degrees Celsius). Six grams of the dried polymer (uncured) is dissolved in fourteen grams of mesitylene and filtered through a one micron pore size filter. Two milliliters of the filtered polymer solution is spin coated (3,500 rpm) on a semiconductor substrate to produce a system like that shown in  FIG. 1  which is then processed as shown in  FIGS. 2-6  to produce a semiconductor device having air gaps.