Chemical-mechanical polishing

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Chemical-mechanical polishing , and chemical mechanical planarization (CMP, engl : chemical mechanical polishing , and chemical mechanical planarization ) is a polishing method in the wafer processing to thin layers uniformly ablate.

Story and motivation

Fig. 1: On the motivation of the CMP. Left: without CMP, right: with CMP according to the purple and red color-coded sputtering / vapor deposition processes

For the production of microelectronic circuits , among other things, thin layers are applied and structured in several steps on a very flat and smooth wafer. As illustrated in Fig. 1, after a few layers, strong unevenness occurs which can lead to the tearing off of a higher layer and thus make multilayer systems difficult to implement. This problem increases with the increasing complexity of the circuits, since more metallization levels are required for the component wiring. Another point is the negative influence of an uneven surface on the photolithographic structuring. An exact image is only possible on planar surfaces and the requirements for the photolithographic structuring increase with each technology node in which the structures to be manufactured are continuously reduced.

Processes for leveling were used very early in semiconductor technology, such as the "BPSG reflow process" in which a layer of borophosphosilicate glass ( BPSG ) was applied and melted. However, the planarity achieved by these methods was no longer sufficient at the beginning of the 1990s, so that alternative methods had already been sought beforehand.

Polishing glass has been a technique that has been practiced for centuries. Chemical-mechanical polishing was first proposed for use in semiconductor technology in the early / mid-1980s. Then as now, silicon dioxide layers and their leveling were an important aspect in semiconductor technology. In the last 20 years, CMP has developed into a standard process and key technology in semiconductor technology. The main advantage of the CMP is the “simplicity” of the basic principle, which works with all materials: After a layer has been applied, it is polished back and unevenness is evened out. Then you have a smooth even surface on which you can apply photoresist and expose it precisely.

Working principle

Fig. 2: Functional principle of the CMP

The wafer to be polished is picked up by the wafer carrier and pressed onto the polishing plate with the polishing cloth with a defined pressure. In the meantime, the wafer carrier and the polishing cloth begin to rotate in the same direction (an opposite direction of rotation is also possible). There are various possibilities here to vary the speeds or to set the wafer carrier in an oscillating movement in order to optimize the removal and its evenness.

The polishing cloth mostly consists of polyurethane foams or non-woven fabric treated with polyurethane . You have the option to choose between different hardnesses and perforations .

During the entire polishing process, a colloidal polishing agent suspension (slurry), in the case of trench insulation for example 30 to 200 nm silicon dioxide , cerium (IV) oxide or aluminum oxide particles, is fed to the inner area of ​​the polishing cloth via a pump system is distributed over the polishing cloth by the rotational movement. This creates a thin film of polishing agent between the wafer and the polishing cloth, which chemically attacks the layers to be polished, uses the abrasive particles it contains to mechanically process the surface and thus removes the material.

The polishing result depends largely on an even distribution of pressure. For example, the density of the structures on the wafer as well as the planarity of the " Chuck " and the polishing plate influence the polishing, because every unevenness and deformation results in a change in the pressure conditions on the wafer and can therefore worsen the result ( inhomogeneity ). The underside of the chuck is therefore covered with a backing film , the soft fiber of which compensates for unevenness and transfers the rotation of the chuck to the wafer by means of adhesion .

After a defined time, the wafer is removed from the polishing cloth and (within the CMP machine) subjected to an initial pre-cleaning with ultrapure water . The complete removal of the polishing agent prevents the formation of crystals and scratches and prevents the continued etching of the wafer surface.

Fig. 3: Diamond-studded underside of a polishing cloth conditioner

In the meantime, the conditioner begins to prepare the polishing cloth for the next wafer (ex-situ conditioning). This is done by moving a rotating disc with diamonds while adding deionized water over the also rotating polishing cloth, roughening it in this way and removing polishing agent residues and material from the wafer layers from the pores in the polishing cloth. This work step can take place once or several times after polishing, but it can also be carried out during polishing. In the latter case, one speaks of in-situ conditioning. The type of conditioning you choose depends heavily on the process.

literature

  • Y. Li (Ed.): Microelectronic Applications of Chemical Mechanical Planarization. Wiley-Interscience, Hoboken, NJ 2008.
  • H. Liang, D. Craven: Tribology in Chemical Mechanical Planarization. CRC Press, Boca Raton, FL, 2005.
  • JM Steigerwald (Ed.): Chemical Mechanical Planarization of Microelectronic Materials. John Wiley & Sons, Inc., New York 1997, p. 181 ff.

Individual evidence

  1. ↑ Ulrich Hilleringmann: silicon semiconductor technology . Vieweg + Teubner Verlag, 2008, ISBN 978-3-8351-0245-3 , p. 153 ff .
  2. ^ KD Beyer: A “Dirty” Risk. In: Innovative Leader. Volume 8, No. 6, 1999, p. 407 ( HTML , accessed April 12, 2010).
  3. ^ MA Fury: The early days of CMP . In: Solid State Technology . tape 40 , no. 4 , 1997, p. 81-88, ( HTML version [accessed April 13, 2010]).