Process for imaging laserinterferometry and a laserinterferomete

Details Process for imaging laserinterferometry and a laserinterferomete Process for imaging laserinterferometry and a laserinterferomete

1987-09-09

1989-06-13

Willis, Davis L.

Optics: measuring and testing

By dispersed light spectroscopy

Utilizing a spectrometer

156626, G01B 902

Patent

active

048386944

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to a laserinterferometric process intended for use in etching layers and to a laserinterferometer for the examination of a layer, in particular for carrying out said laserinterferometric process.
Interferometric examinations are often employed in the examination of thin layers. In this application, laserinterferometers have become quite predominant. They are used, e.g. in the etching of thin layers to determine the etch rate and the termination of etching, which will be made more apparent with reference to FIGS. 1 and 2. The layer to be etched 2, which is placed on a substrate 1, is radiated by a laser. With transparent layers, the incident laser beam 10 is partially reflected at the surface of the layer 2 and partially at the interface of layer 2 to substrate 1. The reflected laser beam, thus, is composed of two beams, designated 20 and 22. The phase difference of the two reflected beams depends on the thickness of the layer 2 to be etched. Depending on the phase relation or phase difference, the beams 20 and 22 become stronger or weaker. If the thickness of the layer 2 changes, as is the case in etching, the resulting reflected laser is of changing intensity. A periodicity of the intensity of the reflected laser beam ensues, which is illustrated in FIG. 2. The etch rate can be ascertained from the temporal course of the intensity of the reflected laser beam, i.e. from the period T. When the layer 2 has been etched away, so that the incident beam 10 only strikes the substrate 1, the result is usually a lack of change in the reflectivity, so that the intensity of the reflected laser beam produces a bend in the intensity curve. This bend or stand value indicates the termination point of the etching process, designated 3 in FIG. 2.
Furthermore, deflecting a laser beam by a modulator is known. Such devices, called laser scanners, are employed, by way of illustration, in health care technology or in certain holographic processes.
A disadvantage in the known laserinterferometer devices is that the fixing must be precise in order to analyze specific locations of the layer to be examined.
The object of the present invention is, proceeding from the aforegoing state of the art, to provide a laserinterferometric process and a laserinterferometer, which permits an "on-line" analysis of the surface to be examined, particularly at selected points.
With the present invention, it is possible to obtain a visual representation of the surface to be examined by a laserinterferometer. The laser beam passes over a defined surface of the layer to be examined, whereby it is deflected two-dimensionally. The intensity of the reflected laser beam can be ascertained for the entire examined surface point by point according to the laser passing over point by point or line by line and evaluated via the linked-up data processing system. The invented process simplifies the use of laserinterferometers as no complicated fixing of the location to be analyzed is required prior to the examination, but rather an electronic localization. Thus even very small zones can now be analyzed, e.g. contact holes. Laserinterferometers can now also be employed in cases of a very small ratio of etching surface to masking surface.
In the invented laserinterferometer, it is expedient to employ a HeNe laser or a UV laser. Naturally other types of lasers can also be used. For example in a number of cases, a laser with a wavelength of variable frequency has proven to be suitable.
In the invented process, the laser beam is deflected electro-mechanically, permitting low power input and rapid scanning of the surfaces to be examined.
In an expedient variant of the process, the laser beam is deflected by an electrically modulated crystal.
Optical means are preferably used for the deflection or imaging of the laser beam. In this connection semi-permeable mirrors, by way of illustration, can be employed. It is particularly expedient to direct the laser beam to the layer to be examined via glass fib

REFERENCES:
patent: 3461261 (1969-08-01), Lewis et al.
patent: 3465114 (1969-09-01), Bleackley et al.
patent: 3861804 (1975-01-01), Lehmbeck et al.
patent: 4207452 (1980-06-01), Arai
patent: 4417116 (1983-11-01), Black
patent: 4618262 (1986-10-01), Maydan et al.
IEEE Transactions on Industrial Electronics and Control Instrumentation, vol. IECI-27, No. 1, Feb. 1980.
Patent Abstracts of Japan, vol. 8, #72 (P-265), (1509), Apr. 4, 1984 (Abstract).
Article "Thin Film Monitor for In Situ Measuring of Silicon Etch Rates", from IBM Technical Disl. Bulletin, vol. 24, #9.
Article from Solid State Technology, "Methods of End Point Detection for Plasma Etching", vol. 24 (1981).

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