Leveling Extreme Topography
Bulk micromachining and surface micromachining are used to create 3-D structures. As semiconductor devices have advanced, the aspect ratio of these structures has increased from a 1:1 height-to-width ratio to a 10:1 ratio. The processes used to make these structures, however, can severely damage microelectronic circuitry that already exists on the wafer.
In many cases, it is easier to create the high-aspect-ratio features before starting the fabrication of the microelectronic circuits. The disadvantage of this alternative is that spin-coating additional materials is difficult once the 3-D structures have been created. Having the ability to temporarily fill in the structures and return the substrate’s surface to a flat profile allows the use of existing photolithography tools, processes, and materials. This capability, in turn, broadens the design latitude that an engineer has in crafting a device.
Many methods exist for planarizing the substrate. Three methods that can be used with lithography tracks are:
- Dry etch back
- Wet develop back
- Expose and develop
In all of these processes, success depends upon the combination of materials, processes, and tool sets used.
Dry Etch Back
The dry-etch method of planarizing is straightforward. Typically, an organic film is coated on the wafer. This material must have self-leveling properties to minimize the difference in overburden between areas with high feature density and areas with low feature density. Overburden is the thickness of material extending beyond the top of the trenches, and “self-leveling” refers to the tendency of the material to flow or reflow during processing to create a uniform flat surface across the wafer. Once the material is coated and baked, the wafer is placed in a dry-etch chamber and is oxygen-ashed to remove the overburden.
The advantage of this method is that a wide range of materials can be used. Depending on the material selected, methods for removing the fill include dry etching, wet etching (with solvent or developer), or thermal decomposition.
The disavantage of dry etching is that it does require a dry-etch tool. Also, during the dry-etch-back process, other organic films on the substrate could be damaged.
Wet Develop Back
The wet-develop method is similar to the dry-etch-back process except that either TMAH developer or a solvent is used to remove the overburden. For many of our materials, we find that the etch rate of the overburden is much faster than the etch rate of the material in the vias or trenches. This characteristic reduces the iso-dense variance.
An advantage of the wet-develop method is that all the processing can be performed on a standard lithography track. Also, the film can be removed later by the same wet processing. The disadvantage is that the the film’s ability to be removed in developer or solvent may limit downstream processing. In some cases, the material can be cured after developing so that it is no longer soluble. However, curing limits the removal method to dry etch only.
Expose and Develop
The last method for leveling topography is expose and develop. In this method, a photosensitive material that has self-leveling properties is used. The material is coated on the wafer and baked. The wafer is then exposed by photolithography (the material is typically negative acting) to set the material in the vias or trenches. The overburden in the open areas is removed by either a TMAH developer or a solvent developer, depending on the planarizing material being used. After exposure, the material in the vias is normally removed by plasma etching.