1. Field of the Invention
The present invention relates to a method for measuring film thickness of a thin film formed in, for example, a semiconductor manufacturing process.
2. Background Information
Conventionally, the following methods are known as methods for measuring the thickness of a thin film which is formed in a semiconductor manufacturing process:
(1) In the method shown in FIG. 5(A), the film thickness of a thin film is measured using an ellipsometer. In this measuring method, incident light IO irradiates a transparent thin film 512 placed on a substrate 511, and the intensity of the reflected light l1 is measured. At this time, the reflected light l1 is interference light between the light l11 reflected on the upper surface of the transparent thin film and the light l12 reflected on the lower surface thereof. Therefore, the intensity of the reflected light is maximized when a difference in an optical path length between the light l11 and the light l12 is a value an integer multiple of times larger than the irradiation light wavelength xcex. Here, a difference in an optical path length L is obtained from L=2d/cos xcex8, wherein xcex8 is an incident angle of the light I/O and D is a film thickness. Therefore, by obtaining an incident angle xcex8 when xcex=L is held, the film thickness D of a transparent thin film can be obtained.
(2) According to the method shown in FIG. 5(B), a needle is placed on the surface of a thin film for measuring film thickness. In this measuring method, steps are formed between respective thin films 521, 521, which constitute a laminated film, using etching techniques. Then, a needle 522 is placed on the surfaces of the respective thin films 521, 521, and the positions of the needle 522 tip on the respective surfaces are detected for measurement of the film thicknesses of the respective thin films 521, 521.
(3) According to the method shown in FIG. 5(C), a film thickness is measured through observation of an etching cross section 531 formed on the thin film. In this method, an etching cross section 531 is initially formed on a laminated film by using, for example, a focusing ion beam device etc. Then, the cross section is observed by using a scanning electron microscope or a transmission electron microscope etc, so that a film thickness is measured.
(4) According to the method shown in FIG. 5(D), component analysis is applied to secondary ions while etching a laminated film. In the example. shown in FIG. 5(D), Si ions are discharged while etching a silicon thin film 541, and Al ions are discharged while etching an aluminum thin film 542. Therefore, etching times for the thin films 541, 542 can be known through component analysis applied to the secondary ions in parallel to the etching. Also, etching rates of the thin films 541, 542 are measured by using another method. Then, using the etching times and rates, the film thicknesses of the thin films 541, 542 are calculated.
The above described thin film measuring methods, however, have following drawbacks.
The method (1) suffers from the drawbacks that the film thickness of a thin film which is not photo transmissive cannot be measure, and that the film thickness of a thin film smaller than a light wavelength cannot be measured. For example, this method cannot measure the film thickness of an oxide film of about 10 nm, which as been recently used in practice for semiconductor devices.
The method (2) has a drawback that a larger burden is imposed on sample production due to very complicated etching processing. Therefore, it is substantially impossible to apply this method to a thin film having a complicated laminated structure.
The method (3) has a defect that the film thickness of a thin film smaller than the resolution of a scanning or transmission electron microscope cannot be measured. For example, as the limit of the resolution of a scanning electron microscope is 1 to 3 nm, the thickness of a thin film thinner than 1 to 3 nm cannot be measured using this method. This method has another drawback in that it requires an expensive measurement device when using a transmission electron microscope.
The method (4) has a drawback in that it requires an expensive measurement device as it requires component analysis.
The present invention has been conceived to overcome the above problems of the related art, and aims to provide a film thickness measuring method capable of measuring the film thickness of a very thin film and realized using an inexpensive measurement device.
According to the present invention, there is provided a film thickness measuring method using a focused ion beam device, comprising the steps of etching a thin film by irradiating charged particles to a surface of the thin film; measuring a change as time passes of strength of secondary charged particles discharged from the thin film during the step of etching; calculating an etching time of the thin film, using a point at which the strength changes quickly; and determining a film thickness of the thin film using the etching time.
A measurement method relative to the present invention can be realized using a very inexpensive device, compared to a case where an etching time is detected through component analysis, as an etching time is detected through observation of secondary charged particles. In addition, as a film thickness is determined using an etching time, resolution can be improved.