Semiconductor device manufacturing: process – Including control responsive to sensed condition – Optical characteristic sensed
Reexamination Certificate
1999-06-17
2001-03-13
Powell, William (Department: 1765)
Semiconductor device manufacturing: process
Including control responsive to sensed condition
Optical characteristic sensed
C216S059000, C216S079000, C438S714000, C438S719000
Reexamination Certificate
active
06200822
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for detecting the transition between different materials in semiconductor structures during alternating etching steps and covering or deposition steps for anisotropic depthwise etching of defined patterns performed using a plasma.
BACKGROUND INFORMATION
German Patent No. 42 41 045 describes a plasma etching process for silicon in which a Teflon-like protective film is applied onto the side walls of etched structures, and in a subsequent inherently isotropic etching step the silicon is etched down with fluorine radicals. By removal of the overlying side wall protective film, subsequent transport downward, and redeposition during the individual etching steps, the newly produced side wall sections are always protected from the next etching attack, resulting in a smooth side wall. If layers of different materials are present in a semiconductor structure, etching must often be performed through one layer down to the layer lying below it; i.e. the etching operation is performed until a new layer begins, and then stopped. For example, if the upper layer is made of silicon and the layer lying below it of silicon dioxide, it is usual to use an optical spectroscopy method to stop the etching process after the silicon layer has been etched through and directly after the silicon dioxide layer has been reached. The optical method involves examining the strength of the plasma emission in terms of a specific substance, by way of its characteristic emission wavelength(s). During the etching of silicon as defined in German Patent No. 42 41 045, a relatively large quantity of fluorine radicals is consumed in the etching reaction, i.e. the concentration of fluorine radicals in the plasma is relatively low. At the same time gaseous reaction products such as SiF
2
, SiF
3
, SiFP
4
, etc., which in turn exhibit a characteristic emission, are produced. When etching reaches the dielectric intermediate layer, i.e. the silicon dioxide layer, the result is more or less an etching stoppage, since the etching operation proceeds much more slowly in silicon dioxide than in silicon. Because of this etching stoppage, the fluorine consumption decreases, and the quantity of free fluorine in the plasma rises correspondingly. This is detected in the optical measurement by a corresponding light emission at characteristic wavelengths. The slowing of the etching process when the silicon dioxide layer is reached moreover causes the SiF
x
concentration to decline, so that its light emission at characteristic wavelengths decreases, and emission of oxygen from the silicon dioxide takes place at its characteristic wavelengths. These effects can also be sensed with the optical measurement method. Using the aforementioned optical method, it is thus possible to terminate the etching process or continue it with modified parameters. This is necessary because uncontrolled overetching results in an undesired etching profile, for example in incipient etching of the vertical side walls, so that undercutting of the mask occurs with pattern loss. The originally achieved accuracy thus no longer exists, and pockets can form at the dielectric interface (between silicon and silicon dioxide).
SUMMARY OF THE INVENTION
The method according to the present invention provides, for detection of the transition between different materials in semiconductor structures with alternating etching and covering steps as defined in German Patent No. 42 41 045, a method with which precise, reproducible results can be achieved. (The terms “concentration” and “concentration maximum” will be used hereinafter; these may be replaced by the terms “intensity” and “intensity maximum,” respectively, since the concentration of a substance contained in the plasma and the intensity by way of an intensity measurement of the characteristic wavelengths are proportionally related to one another.) For this purpose, it is provided that by way of an intensity measurement of at least one specific substance contained in the plasma, via its emission strength and thus its concentration, the beginning of each etching step be ascertained by the fact that a characteristic threshold is reached; that then, when the threshold value is reached, a delay time be started which is longer than the course of a first uncharacteristic concentration maximum; that a second concentration maximum then be ascertained after this delay time has elapsed; and that the second concentration maxima of the etching steps be monitored as to whether they exceed or fall below a predefined value, in order to detect the material transition. Instead of the fact of exceeding or falling below a threshold value, it is also optionally possible to ascertain the end or the beginning of the etching step by way of a synchronization signal, the generation of which will not be discussed in further detail. The present invention utilizes proposes to utilizes as an essential variable a second concentration maximum, occurring at each etching step, of a specific substance contained in the plasma. Detection of this second maximum is accomplished by the fact that by waiting for the delay time, an uncharacteristic signal peak located in each case before the maximum to be analyzed, namely a first concentration maximum, is excluded from consideration. The delay time is selected so that it is longer than the course of the first concentration maximum, so that this first concentration maximum is, e.g., blanked out. Analysis of the second characteristic concentration maximum makes available a characteristic and thus informative value for detecting the transition between different materials at each etching step. This value is preferably ascertained continuously, i.e. at each etching step. Alternatively, however, it is also possible for this value to be ascertained not at each etching step but, for example, at every other etching step. The individual second concentration maxima change in magnitude over the course of the plurality of etching steps; a dependence on the etched material thus exists. During the etching operations, as one material is left and etching of a second material begins, the informative second concentration values change, i.e. the fact of exceeding or falling below a predefined value allows the conclusion that the underlying second material has been reached. The method according to the present invention moreover permits a conclusion in terms of trend, since the change in the second concentration maximum in the course of the individual etching steps proceeds essentially continuously, thus resulting in a rise or fall in the maxima; it is evident from this that the material transition is about to be reached. It is thus possible, by way of a suitable continuous analysis, to draw conclusions as to trends.
According to another embodiment of the present invention, provision is made for a peak value of the second concentration maximum to be ascertained. The greatest value in each case is thus acquired, and utilized as an analysis criterion. Alternatively, it is also possible for several values of the second concentration maximum lying in the region of the peak to be utilized for the analysis.
Such is the case, for example, if the second concentration maximum is ascertained using a sample-and-hold method. A corresponding sample-and-hold apparatus can be used for this purpose.
It is furthermore advantageous if the semiconductor structure to be etched using fluorine radicals has at least one region made of silicon and at least one other region made of silicon dioxide, the silicon being the material first processed from the etching side and the silicon dioxide the material to be reached by the etching process as it proceeds, the rise in the second fluorine concentration to more than the predefined value resulting in detection of the material transition from silicon to silicon dioxide. The second fluorine concentration maxima thus constitute the aforementioned second concentration maxima. Alternatively, in the case of the etching of silicon down to an underlying si
Becker Volker
Laermer Franz
Schilp Andrea
Kenyon & Kenyon
Powell William
Robert & Bosch GmbH
LandOfFree
Method for detecting the transition between different... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for detecting the transition between different..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for detecting the transition between different... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2434688