Chemical-mechanical planarization

Chemical- mechanical polishing, and chemical- mechanical planarization (CMP, English: chemical mechanical polishing, chemical and mechanical planarization ) is a polishing process in the wafer processing to remove thin layers evenly.

History and Motivation

For the manufacture of microelectronic circuits, inter alia, thin layers are applied in several steps to a very flat and smooth the wafer and patterned. As illustrated in Figure 2, there arise after a few layers thick unevenness, which can lead to tearing of a higher layer and so can be difficult to achieve multi-layer systems. This problem increases with increasing complexity of the circuits, because more metallization levels are required for the wiring device. Another point is the negative influence of the photolithographic patterning by an uneven surface. An exact figure is possible only on planar surfaces and the requirements for the photolithographic patterning rise additionally with each technology node, in which the structures are to be fabricated steadily reduced.

Procedures were very early in the semiconductor technology used for leveling, such as the " BPSG reflow process " in which a layer of borophosphosilicate glass (BPSG ) was applied and melted. However, the planarity achieved this method ranged early 1990s no longer made ​​, so that was previously searched for alternative methods.

The polishing of glass is a practiced art for centuries. Chemical mechanical polishing was proposed early / mid 1980s for use in semiconductor technology for the first time. Then as now, the silicon dioxide layers and their leveling were an important aspect in the semiconductor technology. In the last 20 years, CMP has developed in the semiconductor industry to a standard procedure and key technology. The advantage of the CMP is essentially the " simplicity " of the basic principle, the principle works with all materials: After applying a layer it is polished back and bumps are so balanced. Then, it has a smooth flat surface on which to apply photoresists and can accurately exposed.

Principle of operation

The wafer to be polished is received by the wafer carrier (carrier ) and pressed with a defined pressure to the polishing plate with the polishing cloth. Meanwhile, wafer carrier and polishing cloth (even a non -directional rotation direction, it is possible ) start in the same direction to turn. There are many ways to vary the speed or the wafer carrier to put in an oscillating motion so as to optimize the removal and its uniformity.

The polishing cloth is usually made of polyurethane foams or polyurethane- treated nonwoven fabric. We have here the opportunity between different hardnesses and perforations to choose.

During the whole polishing operation is via a pump system to be a colloidal polish suspension ( slurry), in the grave isolation, for example 30 to 200 nm silica, cerium (IV ) oxide or aluminum oxide particles directed to the inner area of the polishing cloth, the is distributed by the rotary motion on the polishing cloth. Is produced between the wafer and a polishing cloth, a thin film of the polishing agent to the polishing layer chemically attack, makes the abrasive particles contained in it for a mechanical treatment of the surface and thus removes the material.

The polishing result depends essentially on a uniform pressure distribution. Thus, for example, have the density of the structures on the wafer and the planarity of the "Chucks " and the polishing plate impact on the polish, because every bump and deformation results in a change of the pressure conditions on the wafer result and, therefore, the result will deteriorate ( inhomogeneity ). Therefore, the bottom of the chuck is covered with a backing film, the soft fiber compensates for irregularities and transfers the rotation of the chuck by means of adhesion to the wafer.

After a defined time, the wafer is removed from the polishing cloth, and (within the CMP machine ) subjected to a first pre-cleaning with ultrapure water. The complete removal of the polishing agent prevents possible crystal and scratching and prevents the continuous etching of the wafer surface.

Meanwhile, the conditioner begins the polishing cloth for the next wafer preparation ( ex-situ conditioning). This is done by a rotating diamond-studded disc is driven by the addition of deionized water over the also rotating polishing cloth, it anrauht in this way and frees the pores in the polishing cloth polishing agent residues and material of the wafer layers. This step can be driven during polishing one or more times after polishing take place as well. In the latter case one speaks of in situ conditioning. For which type of conditioning you decide depends strongly on the process.