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 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 <BR> <BR> lines. Kohl, et al. , IEEE Electron Device Letters, Vol. 21, No. 12, December 2000, p557-559 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 (for example, 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 September 1995, pl37-140.

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 styrene (or a styrene derivative such as alpha methyl styrene) and vinylbenzocyclobutene (or a derivative of vinylbenzocyclobutene). In addition, a semiconductor structure, comprising a sacrificial polymer positioned between conductor lines, wherein the sacrificial polymer is a copolymer comprising styrene (or a styrene derivative) and vinylbenzocyclobutene (or a vinylbenzocyclobutene derivative).

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 styrene (or a derivative of styrene) and vinylbenzocyclobutene (or a vinylbenzocyclobutene derivative). The term"derivative of styrene"used herein means Ar-CR=CH2 wherein R is an alkyl group containing from 1-6 carbon atoms which may be mono or multisubstituted with functional groups such as nitro, amino, cyano, carbonyl and carboxyl and wherein Ar is phenyl, alkylphenyl, naphthyl, pyridinyl or anthracenyl. Formula (a) below shows the schematic chemical formula for 4-vinylbenzocyclobutene wherein hydrogen atoms are assumed. Formula (b) below shows the schematic chemical formula for alpha methyl styrene wherein hydrogen atoms are assumed.

The term"derivative of vinylbenzocyclobutene"means that one or more of the hydrogens of vinylbenzocyclobutene are replaced with an alkyl, aryl, alkylaryl or hetero atom group (s) which may be mono or multisubstituted with functional groups such as nitro, amino, cyano, carbonyl and carboxyl. The term"vinybenzocyclobutene"means bicyclo [4.2. 0] octa-1, 3,5-trene, 2-ethenyl and bicyclo [4.2. 0] octa-1, 3,5-triene, 3-ethenyl, that is, the

ethylene group is attached to the benzene ring and not the cyclobutene ring.

Preferably, the sacrificial polymer is a copolymer comprising from 99 to 40 mole styrene or styrene derivative and from 1 to 60 mole percent vinylbenzocyclobutene or a vinylbenzocyclobutene derivative 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 styrene or styrene derivative and from 1 to 60 mole percent vinylbenzocyclobutene or a vinylbenzocyclobutene derivative based on total moles of incorporated monomers of the polymer. Yet more preferably, the sacrificial polymer is a copolymer comprising from 85 to 55 mole percent styrene or styrene derivative and from 15 to 45 mole percent vinylbenzocyclobutene or a vinylbenzocyclobutene derivative based on total moles of incorporated monomers of the polymer.

Even more preferably, the sacrificial polymer is a copolymer consisting essentially of from 85 to 55 mole percent styrene or styrene derivative and from 15 to 45 mole percent vinylbenzocyclobutene or a vinylbenzocyclobutene derivative based on total moles of incorporated monomers of the polymer. In a highly preferred embodiment, the sacrificial polymer is a copolymer comprising about 70 mole percent styrene or styrene derivative and about 30 mole percent vinylbenzocyclobutene or a vinylbenzocyclobutene derivative 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 70 mole percent styrene or styrene derivative and about 30 mole percent vinylbenzocyclobutene or a vinylbenzocyclobutene derivative based on total moles of incorporated monomers of the polymer.

The relative amount of styrene or styrene derivative and vinylbenzocyclobutene or a vinylbenzocyclobutene derivative 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 vinylbenzocyclobutene or a vinylbenzocyclobutene derivative 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 70 mole styrene or styrene derivative and about 30 mole percent vinylbenzocyclobutene or a vinylbenzocyclobutene derivative 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 a styrene type monomer) selected to give the polymer the desired decomposition temperature and a second monomer (such as a vinylbenzocyclobutene 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 dispersible 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.

Copolymers of alpha methyl styrene and vinylbenzocyclobutene are known, see for example U. S. Pat.

No. 4,698, 394. The copolymers of alpha methyl styrene and vinylbenzocyclobutene of the instant invention are preferably prepared by anionic polymerization as illustrated in the Example below.

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 alpha methyl styrene and vinylbenzocyclobutene 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 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 build-up 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. Those skilled in the art will also appreciate that many techniques may be employed to remove or decompose the sacrificial polymer. 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 890 patent. The critical aspect of the instant invention is the sacrificial polymer. The sacrificial polymer of the instant invention is a copolymer comprising styrene or styrene derivative and vinylbenzocyclobutene or a vinylbenzocyclobutene derivative.

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 styrene or styrene derivative and vinylbenzocyclobutene or a vinylbenzocyclobutene derivative. FIG. 5 shows such a structure.

EXAMPLE 1 A one liter round bottom flask is equipped with a magnetic stirring bar, a thermometer, a gas inlet adapter and a septum. The flask is heated to 110 degrees Celsius and dried with a stream of dry nitrogen. The flask is then cooled in an ice/water bath. 400 milliliters of dry cyclohexane and 38 milliliters of dry tetrahydrofuran are added to the flask. 82.6 grams of alpha methyl styrene (passed over alumina to remove inhibitor and then distilled from calcium hydride) is added to the flask by cannula. 39 grams of vinylbenzocyclobutene (purified by distillation from calcium hydride, treated with butyl lithium to a persistent color and then distilled again) is added to the flask by syringe.

The temperature of the flask is brought to 8 degrees Celsius. 4.6 milliliters of 0.725 molar sec butyl lithium solution in cyclohexane is added to the flask with stirring to initiate polymerization. The temperature of the flask rises to 12 degrees Celsius. The flask is stirred for two hours. 2 milliliters of isopropanol is then added to the flask. The polymer produced in the flask is precipitated by adding more isopropanol, dried, dissolved in methylene chloride, precipitated in methanol and then dried.

The dried polymer weighs 82 grams. Proton NMR analysis indicates that the dried polymer is a copolymer of alpha methyl styrene and vinylbenzocyclobutene. Proton NMR analysis also indicates that the dried polymer is 29.7

weight percent vinylbenzocyclobutene. Gel Permeation Chromatography analysis indicates that the number average molecular weight of the dried polymer is 16,300 grams per mole compared to polystyrene standards with a polydispersity of 1.11. 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 with 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.

EXAMPLE 2 This polymerization is performed in a 1 liter 2 neck round bottom flask. This flask is equipped with magnetic stirring, a reflux condenser topped with a gas inlet adapter and supplied with a positive pressure of dry nitrogen, and a thermometer. This apparatus is heated in a forced air oven, assembled hot, and allowed to dry under a stream of dry nitrogen. To this flask is added 0.303 g benzoyl peroxide (99%, FW 242., 0.0012 moles) and 200 mL toluene (HPCL grade, d=0.865,). 82.4 g styrene is purified by passing over activated alumina and Q5 catalyst, isolating in a graduated cylinder, and adding to the reaction flask by

cannula. 30.6 g vinylbenzocyclobutene is purified by distillation away from CaH2 and added by syringe. The contents are stirred using the magnetic stirrer, and the flask is heated to 84°C overnight (15 hours) using a heating mantle. After this time the flask is removed from the heat and the reactor contents are cooled to room temperature.

The polymer is isolated from solution by precipitation from methanol, dried, redissolved in methylene chloride, and precipitated again from methanol. The polymer is then dried in a vacuum oven at 120°C for three hours. The isolated polymer weighs 78.2 g. Gel permeation chromatographic analysis (THF eluent, 1.0 mL/minute flow rate, molecular weights vs. polystyrene standards) shows a number average molecular weight of 44,850 g/mol and a weight average molecular weight of 100,500 g/mol. 1H NMR in CDC13 shows broad singlets at 1.4 and 1.85 ppm due to backbone methylene and methine protons, a broad singlet at 3.0 ppm due to the benzocyclobutene cyclobutene ring methylenes, and two broad multiple peaks between 6.2 and 7.4 ppm due to the aromatic ring protons from both the phenyl ring and the benzocyclobutene ring. Integration of the four-membered ring methylene protons and comparison with the combined aromatic ring region gave a composition of 22.4 mole percent vinylbenzocyclobutene, corresponding to 27.1 weight percent vinylbenzocyclobutene (there was 27. 1% vinylbenzocyclobutene monomer in the feed).

Differential Scanning Calorimetry shows a glass transition at 94. 6°C on the first scan from room temperature to 280°C (heating rate 10°C/minute), and no glass transition was observed on a second heating scan from room temperature to 300°C.