Abrading – Abrading process – With tool treating or forming
Reexamination Certificate
2002-07-01
2004-11-30
Wilson, Lee D. (Department: 3723)
Abrading
Abrading process
With tool treating or forming
C451S051000, C451S155000, C451S312000, C408S158000, C409S143000
Reexamination Certificate
active
06824451
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a process for the abrasive machining of surfaces of semiconductors.
Chemical mechanical polishing (CMP) is frequently used in the production of large scale integrated circuits for the purpose of planarizing dielectrics or indirect structuring of wiring planes, i.e. for removing elevated areas from a structured surface. A liquid that has been mixed with polishing grains, preferably of a high hardness, and in some cases contains basic chemicals, known as a slurry, is often introduced between the surface of a semiconductor wafer that is to be machined and a polishing pad. The pad and the surface that is to be machined are in surface-to-surface contact with one another and are moved relative to one another, so that the polishing grains which move between the two surfaces abrade the surface which is to be machined.
High topography selectivity is desirable for efficient planarization of evenly structured surfaces. Therefore, elevated areas should be abraded to a greater extent than areas that lie at a lower level. This cannot be ensured under all circumstances by the slurry method, particularly when both large and very small structures occur together. The polishing grains that are entrained with the slurry are able to exert an abrading action even in the lower-lying regions, so that overall a greater amount of material than just the layer thickness of the elevated structures has to be removed for complete planarization.
Better results have recently been achieved by what is known as fixed abrasive CMP. In this process, the polishing pad is covered with a polishing device, e.g. a polishing cloth, in which the polishing grains are fixed in a polishing-grain carrier and only project above the surface of the latter in certain regions. In fixed abrasive CMP, the polishing device and the surface that is to be machined are brought into contact with one another and set in motion relative to one another. Depending on the specific device, this can be achieved by moving only one surface or by moving both surfaces. In addition, depending on the particular requirements it is possible to add suitable liquid chemicals, in order to produce chemical abrasion at the same time as the mechanical abrasion. Since the polishing grains only interact with the surface that is to be machined at the actual points of contact between the polishing devices and the surface which is to be machined, fixed abrasive CMP is able to achieve a particularly high topography selectivity.
In the strictest mechanical sense, fixed abrasive CMP is a grinding process rather than a polishing process, since the grinding or polishing grains cannot move freely, but rather are fixed randomly in a carrier, in particular at the surface of the latter. Nevertheless, in the present context the term “polishing” has become the accepted word, and consequently it will also be used in the present description.
It is inevitable that an in some cases considerable number of polishing grains will become detached from the carrier during the machining operation, depending on the type of wafer and/or polishing device, so that, first, a “true” polishing process also always takes place and, second, over the course of time the polishing device becomes blunt or aggressive, i.e. the amount of material removed per unit time drops or increases.
The latter phenomenon is extremely undesirable in series production, in which a large number of wafers are successively subjected to the same CMP working step, since the same, presettable parameters for a working step, such as for example machining time, chemicals selected, etc. would lead to different results depending on the degree of wear to the polishing device. Particularly as structures become ever smaller, fluctuations of this nature cannot be tolerated.
A phenomenon that has a similar result also occurs with the slurry method explained above. However, the processes that lead to the abrasive becoming blunt are different. Specifically, in the slurry method the surface of the pad, which is actually elastic, “vitrifies”, i.e. its pores become blocked with the relatively small polishing grains and in particular with material that has been abraded from the surface that is to be machined. This leads to a hard and flat pad surface, leading to a considerable change in abrasion rates. This is generally counteracted by a cleaning roughening of the pad surface with the aid of a diamond needle. However, this method is too coarse for the fixed abrasive method, since it would lead to the substantially pore-free polishing-grain carrier being destroyed, and therefore it cannot be used for this method.
Therefore, the problem is currently combated by replacing the polishing device in steps in each case before machining of a new wafer. For example, certain CMP devices offer an automatic feed of polishing devices (roll to roll polisher). However, such a configuration is expensive in two respects. First, a device of this type requires considerable mechanical outlay. Second, the consumption of the polishing device is excessive. This is a significant cost factor. On account of the extremely small size of the structures that are to be machined, polishing cloth that is customarily used has to satisfy extremely high demands on accuracy both with regard to its mechanical properties and with regard to the number, size and uniformity of the polishing grains. This makes production complex and correspondingly expensive.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a process for the abrasive machining of surfaces, in particular of semiconductor wafers that overcomes the above-mentioned disadvantages of the prior art methods of this general type, in which fluctuations in abrasion caused by the polishing device becoming blunt or aggressive are avoided as far as possible while the cost of replacing the polishing device is considerably reduced.
With the foregoing and other objects in view there is provided, in accordance with the invention, a process for abrasive machining. The process includes subjecting successively a plurality of wafers having surfaces of a first type to a polishing step. During the polishing step the surfaces of the first type are in each case brought into contact with a sheet-like polishing device. The polishing device has a polishing-grain carrier with polishing grains fixed therein. The surfaces of the first type are moved relative to the polishing device, so that, as a result of an interaction between the polishing grains fixed in the polishing-grain carrier and a surface being machined, material is removed from the surface, and during the polishing step the polishing grains can become at least partially detached from the polishing-grain carrier. A conditioning step is performed for regenerating the polishing device before performing the polishing step on at least one of the wafers having the surfaces of the first type. The conditioning step includes bringing the polishing device and a conditioning surface having a strong structure into contact with one another and moved relative to one another, with a result that a regenerated state of a polishing device surface is achieved. The regenerated state is a starting state of the polishing device surface before the polishing step is performed and starting states of different polishing steps are comparable with each other at a beginning of each of a series of the polishing steps. Wafers having a surface of a second type are used to provide the conditioning surface.
Specifically, these features have the following significance. Before each individual polishing step, i.e. in each case between the machining operations carried out on the surfaces which are to be machined in succession or in each case before a sequence of individual polishing steps, an intermediate step is carried out, in which the polishing device is brought into contact with and moved relative to a special conditioning surface having a strong topography, referred to below as the “dummy wafer”. The result i
Hollatz Mark
Römer Andreas
Greenberg Laurence A.
Infineon - Technologies AG
Locher Ralph E.
Ojini Anthony
Stemer Werner H.
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