# Patent

## Title
Planarization for integrated circuits

## Abstract

A method of planarizing a layer of an integrated circuit. In one embodiment, a liquid film is applied over the layer, using extrusion coating techniques. In another embodiment, the layer itself may be applied as a liquid film, using extrusion techniques.

## Background
RELATED APPLICATION
This application is a continuation-in-part application of application Ser. No. 10/113,008 filed Mar. 29, 2002 and entitled, “Via Formation in Integrated Circuits By Use of Sacrificial Structures”.


TECHNICAL FIELD OF THE INVENTION
This invention relates to fabrication of integrated circuits, and more particularly to a method for planarizing surfaces of integrated circuit layers.
BACKGROUND OF THE INVENTION
A typical digital integrated circuit comprises a number of transistors and other electrical elements. The integrated circuit chip is a sandwiched structure made up of the silicon substrate, dielectric layers, metal interconnects, devices and so on. The layers are formed by various deposition, photolithographic, and etching techniques.
Advances in integrated circuit capacity and complexity depend on increases in the density of semiconductor devices and layering of metal circuitry. Each layer must be planarized prior to the next lithography step to achieve desired device performance. Deposition means such as chemical vapor deposition or spin coating are conformal, which requires them to be subsequently planarized.
To ensure flatness, manufacturers typically use chemical mechanical planarization (CMP), which is essentially a chemical polishing process. Another approach to planarization is contact planarization, which involves the application of a malleable coating on the surface of a chip; the coating then is pressed against an optically clear, flat surface and cured with ultraviolet light. The coating may be left in place or removed, leaving the flat substrate.
SUMMARY OF THE INVENTION
One aspect of the invention is a method of planarizing a layer of an integrated circuit. A liquid film is applied over the layer, using extrusion coating techniques. In another embodiment, the layer itself may be applied as a liquid film, using extrusion coating techniques.
An advantage of the invention is that the extrusion coating is “self-planarizing”. No subsequent planarization steps are needed. As a result, clean-up steps are eliminated and defects are reduced.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates the formation of pillar-like structures over a metal lead of an integrated circuit.
FIG. 2 illustrates the application of an insulating layer over the pillar-like structure.
FIG. 3 illustrates the removal of a portion of the insulating layer to expose the top of the pillar-like structure.
FIG. 4 illustrates removal of the pillar-like structure.
FIG. 5 illustrates formation of a conductive line.



DETAILED DESCRIPTION OF THE INVENTION
The following description is directed to a method of planarizing one or more layers of an integrated circuit. For purposes of example, the planarization is performed for an insulating layer, through which there is a via. In the example below, the layer in question is applied using extrusion techniques. In other embodiments, a layer of another material could be planarized by applying an extrusion coating on the surface of that layer.
“Extrusion” coating is meant here in its conventional sense. The extruder is mounted above the substrate, and liquid is forced through a die onto the substrate. In the extrusion coating of a hot melt liquid, the liquid is fed into an extruder, and pumped to the substrate. Alternatively, granules of solid form of the liquid are fed into an extruder, where they are melted and homogenised before being pumped through the die. Typically, then, the coating is a hot melt liquid applied at an elevated temperature. The extruder is motion relative to the substrate, thinner coatings can be produced where the line speed is faster than the speed of the extrusion.
FIG. 1 illustrates the formation of a pillar 21 over a metal line 22, which has already been patterned on a semiconductor substrate 23. Pillar 21 may be formed from photoresist, using conventional lithographic techniques, including patterning and etching. The material used to form pillar 21 is typically photoresist, but may be any “sacrificial” material, that may subsequently be removed as explained below. Pillar 21 need not be column-shaped, but rather may be any structure having a shape such that when sacrificially removed, will form a via.
FIG. 2 illustrates the application of an insulating layer 31. As illustrated, insulating layer 31 encapsulates pillar 21. Insulating layer 31 may be any material suitable for an interlevel dielectric layer of an integrated circuit.
Extrusion coating is especially desirable for applying layer 31 because of its ability to provide a planarized surface. In contrast, other deposition methods, such as chemical vapor deposition or spin coating, are conformal and require an etch back or chemical mechanical planarization to achieve a desired planar surface.
For extrusion coating, the desired material may be applied in the form of a solution gel or liquid film. For liquid films applied by extrusion coating, application of the film may be followed by other processing, such as thermal or photochemical steps, in which the uniformity achieved through deposition is maintained. During these subsequent process steps, the chemical or physical structures of the film or underlying layers or interfaces may change, that is, these steps have a curing effect.
Experimental testing with extrusion coating has indicated that surface features may be coated and planarized to less than 250 angstroms. The same features covered by a spin coating typically result in nonplanarities of approximately 1800 angstroms.
FIG. 3 illustrates an etch back of insulating layer 31 to expose pillar 21. The etch may be either a wet or dry etch, such as a wet chemical or dry plasma etch. The etching is performed for a duration sufficient to expose at least the top surface of pillar 21.
FIG. 4 illustrates the removal of pillar 21. This may be achieved with a relatively gentle etch, such as a gentle plasma etch. This type of plasma etch is commonly called an ash process, but other selective isotropic etches may be used, if suitable for removing the material from which pillar 21 is made may be used. The avoidance of anisotropic etching eliminates etch residue issues. After the sacrificial structure, that is, pillar 21, has been removed, a via 51 is formed in layer 31 and extends to metal line 22. As shown in FIG. 5, a conductive line 61 can be formed over via 51 and layer 31.
OTHER EMBODIMENTS
Although the present invention has been described in detail, it should be understood that various changes, substitutions, and alterations can be made hereto without departing from the spirit and scope of the invention as defined by the appended claims.

## Claims

1. A method of planarizing a layer of an integrated circuit, the method comprising:
providing a layer of material that is to be planarized; 
forming a substantially planar layer over the layer of material by applying a liquid film over the layer, wherein the applying is performed by extrusion coating; and 
etching at least a portion of the substantially planar layer.

2. The method of claim 1 and further comprising curing the liquid film by performing a thermal step.

3. The method of claim 1 and further comprising curing the liquid film by performing a photochemical step.

4. The method of claim 1 wherein the substantially planar layer is planarized to less than 250 angstroms.

5. The method of claim 1 wherein etching at least a portion of the substantially planar layer comprises etching back the substantially planar layer.

6. The method of claim 1 and further comprising forming a via within the substantially planar layer.