Patent Application
20060017936
The invention relates to measurement systems and methods. More specially, the present invention relates to transparent height object measurement based on a Fast Moiré Interferometry method.
In the field of semi-conductor fabrication, the inspection of the quality of the semi-conductor surface, as the different components and circuit layers are added, is very important.
One method that is used to measure the height or thickness of a pellicle deposited on a semi-conductor surface is based on an interferometric method where the variation of a wavelength spectrum is analyzed. In this method, a white light source is projected on the pellicle and a reflected spectrum from the pellicle is measured. The reflected spectrum from the pellicle contains a component which corresponds to the interference between the reflected light at the surface of the pellicle and the reflected light at the interface between the pellicle and the semi-conductor. By comparing this spectrum to a reference spectrum obtained without the pellicle, it is possible to infer the pellicle thickness.
One drawback with this method is that it is based on the spectrum analysis of the light, which involves the use of a spectrum analyzer.
The invention provides a method of determining a height of a substantially transparent object having a refractive index. The method comprises obtaining an image of the object corresponding to an intensity pattern projected on the object, establishing a phase associated to the object using the image and determining the height using the object phase, the refractive index and a reference phase. The method further comprises determining an angle between a projection axis, along which is projected said intensity pattern, and a normal axis being substantially normal to a surface of the object. The method comprises determining the height of at least one of a pellicle, a coating, a liquid and a semi-opaque object.
The invention further provides a method of determining a height of a substantially transparent object having a refractive index. The method comprises obtaining at least two images of the object, each image being associated with a corresponding intensity pattern projected on the object, establishing a phase associated to the object using the images and determining the height using the object phase, the refractive index and a reference phase. The method further comprises intensity patterns that are phase-shifted with respect to each others. Also, the method comprises obtaining simultaneously a first and a second image of the object, the first image corresponding to a first bandwidth of a first intensity pattern and the second image corresponding to a second bandwidth of a second intensity pattern.
The invention further provides a method of determining a variation of a height of a substantially transparent object having a refractive index. The method comprises obtaining at least one image of a first layer of the object corresponding to an intensity pattern projected on the first layer and establishing a phase associated to the first object layer using the at least one image of the first layer. The method also comprises obtaining at least one image of a second layer of the object corresponding to an intensity pattern projected on the second layer and establishing a phase associated to the second object layer using the at least one image of the second layer. The method also comprises determining the variation of the height of the object using the phases of the first and second object layers and the refractive index.
The invention further provides a method of determining a presence of a substantially transparent object on at least a part of a reference object. The method comprises obtaining an image of the reference object corresponding to an intensity pattern projected on the reference object and determining the presence of the substantially transparent object by comparing the image to a reference image.
The invention further provides a method of determining a presence of a substantially transparent object on at least a part of a reference object. The method comprises obtaining at least one image of the reference object corresponding to an intensity pattern projected on the reference object, establishing an object phase using the at least one image, and determining the presence of the object by comparing the object phase to a reference phase.
The invention further provides an interferometric system for determining a height of a substantially transparent object having a refractive index. The system comprises a pattern projection assembly for projecting along a projection axis an intensity pattern on the object and a detection assembly for obtaining at least one image of the object. The system also comprises a processor for establishing a phase of the object using the at least one image and for determining the height of the object using the object phase, the refractive index and a reference phase.
In order that the invention may be readily understood, embodiments of the invention are illustrated by way of example in the accompanying drawings.
Further details of the invention and its advantages will be apparent from the detailed description included below.
In the following description of the embodiments, reference to the accompanying drawings are by way of illustration of an example by which the invention may be practiced. It will be understood that other embodiments may be made without departing from the scope of the invention disclosed.
In one embodiment of the present invention, the height of a substantially transparent object such as, for example a pellicle, is measured using a Fast Moiré Interferometry phase stepping method.
The Fast Moiré Interferometry phase-stepping method (FMI) is based on the combination of structured light projection and phase-shift method for the extraction of 3D information at each point of an image, I(x,y).
An image is taken of an object and the 3D information is extracted from this image by evaluating an intensity variation at each point of the image due to the height profile of the object. The height profile information of the object, zobject(x,y), can be found in the phase map φobject(x,y) associated with the variation of the image intensity, I(x,y). A phase-shifting technique with different images taken for different grating projections is used to determine, from the images, the phase map φobject(x,y) for both the object and for a reference surface φref(x,y). As it is well known in the art, depending on the situation, the phase map may be determined with only two images (meaning that there are only two intensity pattern projections, each pattern projections being phase-shifted from the other) or with more than two images (in this case, more phase-shifted projections of the intensity pattern are needed).
Once the object and reference phase maps have been determined, the height profile of the object relative to the reference surface, h(x,y)=zobject(x,y)−zref(x,y) is calculated on the basis of the difference of the phase values, δ(x,y), for each point of the image:
Thus, the FMI method offers the possibility to measure a solid object height versus any reference surface. For example, it could be a plane reference, or a model object without any defects.
As described in the following, the FMI method may be also applied to determine the height or thickness of a substantially transparent object such as, for example a pellicle, a coating, a liquid, a semi-opaque object, or any other substantially transparent matter.
Then, the projection of the intensity pattern on the transparent object is phase-shifted and another image is taken. This sequence of measurements is repeated until enough images are acquired. From these images, a phase map of the object φobject(x,y) is calculated and, as mentioned above, when the phase map is compared to a reference phase map, φref(x,y), a height profile h(x,y) can be determined. As illustrated on
By knowing the angle θ (the angle between the projection axis and the normal of the surface of the object) and the transparent object refraction index, “n”, it is possible to measure the object thickness “d”. Indeed, because of the presence of the transparent object, the measured phase difference δ(x,y) is equivalent to the presence of a virtual solid object having a height “h”.
The person with knowledge in the art can find the relationship between “d” and “h” on the basis of the projection angle, θ, and the object refraction index, n.
As it will be obvious for someone skilled in the art, the transparent object only need to be substantially transparent. The present method can thus be applied to many type of objects such as semi-opaque objects, pellicle, films, liquids, and so on.
Although we described in the above embodiment a Fast Moiré Interferometry method based on phase-shifting (or phase-stepping) of an intensity pattern, it will be obvious for someone skilled in the art, that other ways, without departing from the scope of the invention disclosed, can be used to extract, from an image, the phase map information, such as for example to use of Fast Fourier Transform to determine the phase map of the transparent object. The present invention comprises all techniques by which the height information of a transparent object can be extracted from one or more images, the images being characterizing the object on which is projected a structured intensity (intensity patterns).
According to an embodiment of the present invention, a method 10 of determining a height of a substantially transparent object, as illustrated in FIG. 3, will be described. At least one intensity pattern is projected on the object (step 11) and at least one image is acquired (step 12). Then an object phase map, φobject(x,y), is determined at step 13 using the acquired images at step 12. By comparing the object phase map φobject(x,y) to a reference phase map φref(x,y) corresponding to a reference surface, and knowing the refractive index value of the object, the object height is determined at step 14.
The height can be a measurement of the height at one or several points of the object surface, it can be a measurement along a cross-section line of the object, or it can correspond to a map of the entire object thickness.
Turning now to
Turning now to
As illustrated in
The software 63 comprises the necessary algorithms to extract from the acquired images the object phase. If this information is extracted by using a FFT processing of the images, then software 63 will include a processing module comprising a FFT algorithm to perform a FFT on an image an provide a spectrum, a selection algorithm to select automatically a portion of the spectrum, an inverse FFT algorithm to perform an inverse FFT on the selected portion of the spectrum, and an algorithm to extract, from the imaginary and real components resulting from the inverse FFT, the phase map.
The above-described method 10 and system 20 can be used to map the thickness of a transparent object deposited on a reference object or that is above the reference object. They may also be provided for detecting defects on an pellicle in comparison with a similar coated object used as a model or to detect changes of a coated object surface with time. In all cases, the above-described method 10 and system 20 can further include the selection of an appropriate intensity pattern and of an appropriate acquisition resolution that will be in accordance with the thickness of the pellicle or with the height of the transparent object to be measured.
The above-described method 10 can naturally be applied in discrete steps in order to perform the pellicle thickness measurement or the object height measurement layer by layer. This technique—also called image unwrapping—enables one to measure the net transparent object height while keeping a good image resolution.
As mentioned earlier, the present method 10 and system 20 can be used to determine the height map of substantially transparent objects of different natures. The substantially transparent object may be a solid coating as well as a liquid object.
The above-described method 10 and system 20 can also be used to determine the shape and the volume of an object or of a portion of an object, since this method provides information, not only about the height of the object, but also about its length and width. This method can be advantageously applied, for example, in the semiconductor industry.
All the above presented applications of the invention can be used to further assess the quality of a coated object under inspection.
Although the present invention has been described hereinabove by way of specific embodiments thereof, it can be modified, without departing from the spirit and nature of the subject invention as defined herein.
Although the above description relates to a specific preferred embodiment as presently contemplated by the inventor, it will be understood that the invention in its broad aspect includes mechanical and functional equivalents of the elements described herein.
