Forming semiconductor structures

A semiconductor structure may be covered with a thermally decomposing film. That film may then be covered by a sealing cover. Subsequently, the thermally decomposing material may be decomposed, forming a cavity.

BACKGROUND

This invention relates generally to semiconductor structures.

A complementary metal oxide semiconductor (CMOS) device is generally a delicate electronic structure formed by a combination of lithographic and etching techniques that allow the device to be formed and exist in a microscopically clean, contamination free environment with precisely controlled physical properties to ensure reliable and efficient high speed operation. As a result, there is a need to control the dielectric constant of materials used to separate the electronic components and interconnections within the device.

To this end, an interlayer dielectric (ILD) material is deposited around the structures (transistors, passives, etc.) and between the layers of interconnections that make up the CMOS device for the purpose of establishing a dielectric constant. The dielectric constant affects the speed with which signals may propagate through the interconnection of the device.

While many dielectric materials have been studied, for the lowest dielectric constants, increasing amounts of void space and hence air have been incorporated within ILD materials. Indeed, true air gaps have been engineered into the devices directly to optimize the lowest effective dielectric constants. Air gap structures may be formed and encapsulated to protect such structures from the deleterious effects of environmental contamination.

Similarly, in a variety of other circumstances, it may be desirable to form air gap structures in various microelectronic, micromechanical, microbiological, and microoptical systems, as well as, microelectromechanical system (MEMS) device.

Thus, there is a need to make airgaps in semiconductor structures.

DETAILED DESCRIPTION

Referring toFIG. 1, a semiconductor wafer15may have a thermally decomposing sacrificial layer14formed thereon. Defined within the layer14may be an interconnect16in one embodiment of the present invention. For example, the interconnect may be a copper interconnect in accordance with the dual damascene process. A porous etch stop capping layer12may be formed over the entire structure.

In some embodiments, the structure10may be a portion of a complementary metal oxide semiconductor (CMOS) integrated circuit. In other embodiments, it may be a portion of a microelectromechanical system.

The layer14may be driven off by heating the structure10after the capping layer12has been deposited on the surface of the polymer. The capping layer12can also serve as an etch stop for the creation of the next layer. If the layer12is porous, the thermally decomposed sacrificial layer14may be driven off without removing the layer12. However, in other embodiments, apertures of any type may be formed in the layer12as desired.

As a result of the thermally driven decomposition and removal material of the forming the layer14, an air gap region, pocket, or cavity18of low dielectric constant may be formed as shown inFIG. 2. Any technique for heating the layer14can be used, including baking or exposure to infrared or other energy sources.

Advantageously, the sacrificial layer14may be made of a material that may be decomposed by temperatures greater than those normally encountered during conventional semiconductor fabrication processes. A film that decomposes at relatively high temperatures (e.g., greater than about 260° C.) into smaller molecular weight effluents is advantageous. Advantageously, the decomposing film exhibits a high decomposition temperature and generally lower molecular weight byproducts on decomposition so that those byproducts can diffuse away through the layer12.

The following chart provides a list of components and their thermal decomposition temperatures (Td):

In accordance with another embodiment of the present invention, the layer12may be sufficiently porous to facilitate the exhaustion of the decomposed sacrificial layer14upon heating. A thin layer of hydrogen silsesquioxane (HSQ) or methylsilsesquioxane (MSQ) spin-on glass (SOG) may be utilized as the capping layer12. After being cured, the HSQ or MSQ layer14may be exposed to electron beam or plasma conditions to densify the HSQ or MSQ film to be like a silicon dioxide film.

In some cases, a silicon dioxide chemical vapor deposition layer may be applied as an option to seal all remaining pores in the HSQ or MSQ film for subsequent metal interconnect processes. The deposited silicon dioxide layer may enhance the mechanical properties of the HSQ or MSQ layer and/or seal the remaining pores in the HSQ or MSQ films.

In some embodiments, the stress that is caused on the layer12during decomposition may be reduced. HSQ or MSQ may be sufficiently porous to enhance the ability of the thermally decomposing polymers to diffuse out of the air gap pocket or cavity18, through the layer12, without building up pressure to excessively deform the layer12. The layer12performance may be comparable to silicon dioxide films after electron beam or plasma treatment of the layer12. In some cases the mechanical performance of the layer12may be enhanced by forming a sealing material, such as deposited silicon dioxide, on top of the HSQ/MSQ layer12.

In some cases the receiving surface may be hydrophilic while the polymer decomposing film such as those described herein may be relatively hydrophobic. Because of the energy mismatch, when the decomposing film is applied over surface irregularities, such as trenches, there may be incomplete filling of those trenches or bridging.

As a result, it is desirable to energy match the decomposing film to the underlying surface. In other words, if the underlying surface or the polymer are not both hydrophilic or both hydrophobic, it may be desirable to convert one of the surfaces to energy match the other.

In the case of hydrophobic decomposing polymers such as those described previously herein, it may be most feasible to simply modify a hydrophilic surface to which they are to be applied to make that surface hydrophobic. Thus, referring toFIG. 3, a substrate52may be covered by an oxide40having a trench44formed therein. A surface coating42may be applied to the oxide40to present an energy match with the decomposing film that may be applied. The film42may convert the hydrophilic material40to present a more hydrophobic surface which energy matches with the applied decomposing film.

In one embodiment, the surface40may be treated in the atmosphere of hexamethyldisilazane (HMDS) saturated nitrogen for 100 seconds at a temperature of 40° C. Following a cooling step, the surface40may be spin coated with the sacrificial polymer46, as shown inFIG. 4, and baked. As shown inFIG. 5, as a result of the surface treatment, the surface energy of the sacrificial material “A” better matches the surface energy of the underlying substrate “B” compared to the substrate surface energy before treatment “C”.

Other materials that may be utilized for surface energy modification include alkyl and fluoroalkyl functionalized silylhalides, alkoxysilanes, and nitrogen containing silation agents. With different types of substrates such as SiON, SiOF, carbon doped oxide (CDO), and metal, appropriate surface tension modifying agents may be utilized. These may include self-assembled monolayers (SAMs) formed from precursors including, but not limited to, thiols, sulfides, phosphates, phosphites, alkenes, chelation agents (benzotriazole (BTA), crown ethers, kryptands, cyclodextrins, poly and oligothiophenes, poly and oligoanalines) to mention a few examples.