WO1998035255A1

WO1998035255A1 - Selective interferometry

Info

Publication number: WO1998035255A1
Application number: PCT/GB1998/000221
Authority: WO
Inventors: Andrew Michael Christian Smout; Michael John Hammond; Nigel Peter Smith; Andrew Peter Keens
Original assignee: Bio-Rad Micromeasurements Ltd.
Priority date: 1997-02-06
Filing date: 1998-01-26
Publication date: 1998-08-13
Also published as: GB9702388D0; AU5771998A

Abstract

The invention relates to a novel interference microscope and also novel parts for use therein. The microscope is adapted (to enable it), in a first selected condition, to function as an interference microscope and in a second, alternative condition, to function as a bright-field microscope.

Description

SELECTIVE INTERFEROMETRY

The invention relates to a novel interference microscope and also novel parts for use therein.

Interference imaging of a sample is particularly, but not exclusively, undertaken where contrast is low. A number of automated imaging machines use interference imaging in order to take advantage of the improved data provided by interference techniques. Many of these machines are used as inspection tools in order to determine whether, for example, small components in a system have been correctly positioned or orientated with respect to neighbouring components. In addition, these machines may also be used to take measurements and may thus be used, for example, as metrology tools which in turn may be used for measuring overlay error.

In a semiconductor multilayer wafer fabrication process, there is a repeated step involving the printing of a new layer on top of a previous reference layer through a process of photolithography. Technology requirements make ever increasing demands on the degree of accuracy to which these layers should be aligned. The error in alignment is known as overlay error or misregistration error. This overlay error is carefully monitored. Some manual monitoring techniques exist, but in recent years this has become mostly an automated process performed by machines known as Overlay Metrology Tools.

An overlay metrology tool operates on an overlay target which often comprises two rectilinear marks. Typically, the bigger mark which is called the Outer, is printed as part of the selected reference layer. The smaller mark, called the Inner, is then printed inside the outer as part of the new layer that is being superimposed. A typical overlay mark is illustrated in Figure 1.

The tool determines the overlay error by first determining the centre point of each mark, and then computing the distance between these two centre points.

Optical inspection of small overlay marks or patterns has been performed using imaging apparatus of a conventional nature such as a microscope in combination with electronic image processing. In this way images have been viewed in order to determine the nature of their alignment. However, particularly in the semiconductor industry, chemical and mechanical processes are now being used to polish and so planarize wafer surfaces and these techniques undesirably degrade the optical appearance of the mark on the wafer surface. Thus using conventional apparatus it is difficult to view the marks and thus make the appropriate measurements. Moreover the polishing process itself may introduce asymmetries into the appearance of the mark that undesirably distort any measurements of overlay error that are made.

A problem in the measurement process may occur as a result of poor contrast in the image of either mark, for example where wafer fabrication processes, such as the chemical and/or mechanical polishing

(CMP) process or other planarization processes, have degraded the structure and hence the contrast of the optical image of the overlay marks.

Under these circumstances, the precision of overlay measurement of such marks, by means of metrology tools equipped with conventional optics, may degrade considerably. To overcome this, optical systems that are sensitive to variations in the phase as well as the amplitude of the target illumination may be used. Interferometers are such systems. In our co-pending UK patent application number 9608178.1 we describe the use of a Mirau interferometer in a overlay metrology tool.

The Mirau interferometer consists of a special objective fitted to an otherwise standard bright-field metallurgical microscope. This objective has a plate beamsplitter after its final element that allows a camera to simultaneously focus on the surface of the wafer and a small plane mirror spot deposited on a transparent surface placed above the beamsplitter. The Mirau interferometer is illustrated in Figure 2. Constructive and destructive interference between these two images occurs, depending on the exact phase of the signal and reference rays. By scanning the wafer surface slowly through focus and observing the variations in brightness of each pixel in the image, it is possible to create a synthetic image demonstrating much greater contrast variations, corresponding to topography variations on the wafer, than would be possible with a bright- field microscope. Such an image can consequently be used for overlay metrology, defect inspection, and critical dimension measurement, producing superior results.

The Mirau interferometer requires that a plane beamsplitter be placed after the final element of the objective, and thus that the objective needs to be positioned relatively farther from the sample to allow room for this beamsplitter.

There is a problem associated with the use of an interferometer in an automated imaging machine which is brought about because contrast in the image reverses for small changes in the focus of the object. This makes the image of the object unpredictable and consequently systems equipped with machine vision have difficulty locating the object. Accordingly, many such machines are supplemented with a bright-field imaging system where a bright-field objective is first used to locate the object and after this step has been undertaken the interference objective is used in order to provide interference image data. In automated machinery employing these two systems the throughput of samples and the speed of operation is of critical importance. The time to move from one objective to another objective is detrimental to the throughput of the system. Moreover, the use of two objectives presents an additional problem. Typically two different objectives will not have the same image centre, any difference in their respective centres is known as parcentering error. Values in the order of 20 microns are not untypical. Consequently having located the object with the bright-field objective one must not only change to the interferometric objective one must also move the sample so as to compensate for the parcentering error between the objectives. Given that the location of the object may need to be known within 0.5 microns with respect to the interference objective this places stringent requirements on the absolute accuracy of the positioning equipment which might not otherwise be required.

It is therefore an object of the invention to provide a microscope that is used typically in an automated imaging system which is simple in construction and/or able to accommodate a large throughput of samples with the minimum wastage of time.

It is a further object of the invention to provide a novel objective and/or associated means for selectively controlling the character of electromagnetic radiation for use with said objective.

According to a first aspect of the invention there is therefore provided a microscope adapted for interference imaging using a Mirau interferometer comprising an objective and down stream thereof a beamsplitter such that electromagnetic radiation from a sample and electromagnetic radiation from a reference mirror can undergo interference so as to provide interference image data of said sample; characterised in that said beamsplitter is adapted such that the reflectivity of same varies according to the character of the electromagnetic radiation, and means is provided for selectively controlling the character of said electromagnetic radiation whereby use of a first selected electromagnetic radiation, or range thereof, enables said microscope to undertake interference imaging and use of a second, alternative, selected electromagnetic radiation, or range thereof, enables said microscope to undertake alternative imaging.

In a preferred embodiment of the invention said reflectivity varies such that said first selected electromagnetic radiation provides for a first selected condition comprising at least 20% reflectivity and no more than ideally 50% reflectivity. Optimum reflectivity is governed by the need to achieve completely destructive and correspondingly completely constructive interference between the object and reference means in the interferometer.

In this preferred embodiment of the invention, or in an alternative preferred embodiment of the invention said reflectivity varies as a function of wavelength so that the use of said second selected electromagnetic radiation provides for a second selected condition comprising less than 20% reflectivity and more preferably less than 10% reflectivity and more preferably further still less than 5% reflectivity. Optimally the reflectivity should be as close to zero as can be arranged.

In a yet further alternative embodiment of the invention said reflectivity varies such that a change in wavelength of the electromagnetic radiation of 200 nm results in a change in reflectivity between said first and said second aforementioned conditions and more preferably a change in wavelength of 100-200 nm results in said changed conditions. More preferably still a change in wave energy of 20-100 nm results in said changed conditions.

It is preferred that there is a relatively sharp, or steep, change in reflectivity over a relatively narrow change in wavelength or wave energy, thus representing a steep division between the ability of said microscope to undertake interference imaging and alternative imaging. The more discrete this cut-off the better because it provides for flexibility insofar as the spectral selection available is greater for both types of imaging.

In a preferred embodiment of the invention, said means provided for selectively controlling the character of said electromagnetic radiation comprises at least one filter whose property/properties is/are selected having regard to said reflectivity. For example, in one embodiment of the invention ie when said first selected condition is provided (as shown in Figure 3) said reflectivity is approximately 50% between 400-575 nm and when said second selected condition is provided approximately 1% between 600-1000 nm. In this embodiment we use a green filter in order to use the microscope for interference imaging at maximum reflectivity and a red filter in order to use the microscope as a bright-field microscope at minimum reflectivity.

It will therefore be apparent from the above that by selecting the character of the electromagnetic radiation, and thus by selectively controlling the nature of filter down stream of the source of said electromagnetic radiation one can effectively switch between a microscope that is able to undertake interference imaging and a microscope which can undertake alternative imaging such as, in the example afore described, bright- field imaging. In use, with the red filter in place - bright-field imaging - one can firstly focus on the sample and then without need to change the objective one can simply replace the red filter with a green filter and use the same objective to undertake interference imaging.

The speed of this operation is extremely quick and thus the throughput of samples is optimised. Moreover, the need to compensate for different centre points between different objectives is overcome, so that the complexity of the system is minimised.

In yet a further preferred embodiment of the invention said beamsplitter is made of, or coated with, a reflective material having the above referred to characteristics.

More preferably still said Mirau objective is provided with a small spot mirror ideally positioned in the centre of same, thus representing said Mirau reference mirror.

According to a yet further embodiment of the invention there is provided an automated imaging system comprising a microscope in accordance with the invention.

According to a yet further embodiment of the invention there is provided a Mirau interferometer comprising an objective and downstream thereof a beamsplitter such that electromagnetic radiation from a sample and electromagnetic radiation from a reference mirror can undergo interference so as to provide interference image data of said sample; characterised in that said beamsplitter is adapted such that the reflectivity of same varies according to the character of the electromagnetic radiation whereby use of a first selected electromagnetic radiation, or range thereof, enables a microscope including said interferometer to undertake interference imaging and use of a second, alternative, selected electromagnetic radiation, or range thereof, enables said microscope to undertake alternative imaging.

It will be apparent to those skilled in the art that the exact nature of the reflective material may vary with respect to composition and thus it is not intended that this invention should be limited by any particular reflective material. Rather, the invention lies in the realisation that the use of a variable reflective material in a Mirau interferometer, and more particularly on or in the beamsplitter of same, provides an elegant arrangement where one can selectively determine whether one undertakes interference imaging or alternative imaging. However, with a view to providing a full disclosure of the invention examples of suitable materials for providing the reflectivity above described will be disclosed herein.

Thus, an embodiment of the invention will now be described by way of example only with reference to the following figures wherein;

Figure 1 is an illustration of typical Bar-in-Bar type overlay marks and respective horizontal intensity profiles. Right: good contrast inner and outer marks. Left: case of low contrast outer mark;

Figure 2 shows a diagrammatic illustration of a microscope including a Mirau interferometer; Figure 3 is a graph showing change in reflectivity as a function of wavelength for a Mirau interferometer in accordance with the invention;

Figure 4 is a graph showing change in reflectivity as a function of wavelength using the coating specified therebelow where i s 420nm (i.e. for example magnesium fluoride layer will have a thickness of 0.25 of the wavelength (in magnesium fluoride) of that light whose wavelength in vacuo is 420nm);

Figure 5 is a graph showing change in reflectivity as a function of wavelength using the second, alternative, coating specified therebelow where is 830nm; and

Figure 6 is a graph showing optimised conditions for a system in accordance with the invention.

Referring to the figures and firstly to Figure 2 there is shown a microscope incorporating a Mirau interferometer in accordance with the invention.

Briefly, the microscope comprises an incoherent illumination source (1) upstream of a condenser (2) and an illumination beamsplitter (3). A Mirau interferometer (4) is positioned between aforementioned features (1), (2) and (3) and sample (5).

The Mirau interferometer comprises an objective (6) of a conventional nature having a small spot mirror (7) in the centre of same. Associated with said objective is a beamsplitter (8). Notably, said beamsplitter (8) is positioned between said objective (6) and said sample (5). Associated imaging means, such as a tube lens (9) and camera (10) are provided down stream of said Mirau interferometer (4).

When used for interference imaging electromagnetic radiation from said source (1) passes through condenser (2) and beamsplitter (3) so as to provide Kohler illumination. The Mirau objective refracts said electromagnetic radiation so directing it towards beamsplitter (8) which splits each ray of the electromagnetic radiation into two parts, one of which illuminates the object being imaged and the other, known as the reference, illuminates the small spot mirror (7) in the centre of objective (6) and goes on to interfere with the electromagnetic radiation from the object. This can be clearly seen by tracing the path of the arrows shown in Figure 2. The interference may be constructive or destructive dependant upon the relative phases of the rays involved.

If sample (5) is moved through the focus of the objective a cyclic change in interference is observed, i.e. to and from constructive/destructive interference. Because of this last fact it is not possible to use the image from a Mirau objective for pattern recognition without placing unrealistically stringent requirements on the accuracy of the positioning equipment that controls the focus.

Thus, as mentioned, using conventional prior art equipment the microscope shown in Figure 2 would additionally be provided with a conventional imaging system such as a bright-field imaging system so that the sample could firstly be viewed using same in order to determine the location of the object and then the hardware would switch to use the Mirau interferometer with a view to improving the image of the sample using interference imaging. However, in our invention we have adapted a Mirau interferometer by replacing the beamsplitter with a spectrally selective beamsplitter. In other words, we have provided a beamsplitter in a Mirau interferometer of variable reflectivity. Moreover, this reflectivity varies having regard to the wavelength of the electromagnetic radiation used to view the sample.

This is best illustrated having regard to Figure 3. In the particular example illustrated the maximum reflectivity of the new beamsplitter is approximately 50%. However, the maximum reflectivity may vary according to the nature of the reflective material. It is thus not intended to limit the invention by the data provided in Figure 3. However, it is preferred that a reflectivity of at least 20% is provided and more preferably still 40% is provided. It can be seen that a reflectivity of approximately 50% is represented by a wave energy of between 400 and approximately 575 nm. In contrast, at a wave energy greater than 600 nm the reflectivity substantially falls to approximately 1%. There is a relatively steep change in reflectivity from 50%) to 1% between approximately 575 nm and 625 nm, this steep change is preferred because it enables a discrete transition between maximum and minimum reflectivity. This discrete transition can be used to advantage in the invention in that one can reliably use a microscope incorporating the invention to provide, for example, bright-field imaging data at wavelengths in excess of 600 nm and interference imaging data at wavelengths of 550 nm or less.

In use, when taking imaging data we firstly use a red filter in association with illumination source (1) so as to provide a spectrum with wavelengths in the region of 700 nm thus considerably reducing the reflectivity of beamsplitter (8) and ensuring that the microscope operates, in this particular embodiment, as a bright-field microscope. Thus using a red filter we are able to accurately determine the location of the sample (5). Once this has been determined we simply replace the red filter with a green filter thus ensuring that a spectrum with wavelengths in the region of 550 nm is used. This spectrum ensures that the beamsplitter operates at approximately 50% reflectivity and so allows for interference to take place and thus for interference image data to be provided.

In Figure 4 there is illustrated one example only of a coating in accordance with the invention. It can be seen having regard to the graph illustrated in Figure 4 that at a wavelength greater than 650nm maximum reflectivity occurs and thus the Mirau interferometer can be used for interference imaging. In contrast at a wavelength between 420nm and 570nm the coating is anti-reflective and the beamsplitter has a reflectivity similar to that of plain glass, thus bright field imaging can be undertaken.

In Figure 5 there is described an alternative coating in accordance with the invention. It can be seen having regard to the graph illustrated in Figure 5 that a wavelength between 420 and 600nm maximum reflectivity occurs and thus the Mirau interferometer can be used for interference imaging. In contrast at a wavelength greater than 650nm minimum reflectivity occurs thus, the coating is anti-reflective and the beamsplitter has a reflectivity similar to that of plain glass, thus bright field imaging can be undertaken.

It follows from the information provided above that the choice of coating and/or the filter for use in connection with same will be selected in order to optimise the system. An example of an ideal system is shown in Figure 6 where it can be seen that 50% reflectivity is achieved over a wavelength of between 400nm and 600nm and zero reflectivity is achieved at a wavelength above 600nm. Thus there is shown a square shaped curve which represents a very sharply defined cut off between reflectivity and lack of reflectivity.

Thus it can be seen that by altering the reflective properties of the Mirau interferometer, and more particularly the beamsplitter associated with the Mirau objective, and also by providing means, such as filters, for selectively controlling the spectrum of the electromagnetic radiation used for illumination we are able to determine the nature of the image data that is provided and thus we can effectively switch between providing interference image data and conventional image data such as bright-field image data. In this way a number of sequential operations can be undertaken with minimum adjustment to the components of the system.

Claims

1. A microscope adapted for interference imaging using a Mirau interferometer comprising an objective and downstream thereof a beamsplitter such that electromagnetic radiation from a sample and electromagnetic radiation from a reference mirror can undergo interference to provide interference image data of said sample; characterised in that said beamsplitter is adapted such that the reflectivity of same varies according to the character of the electromagnetic radiation, and means is provided for selectively controlling the character of said electromagnetic radiation whereby use of a first selected electromagnetic radiation, or range thereof, enables said microscope to undertake interference imaging and use of a second, alternative, selected electromagnetic radiation, or range thereof, enables said microscope to undertake alternative imaging.

2. A microscope adapted for interference imaging according to Claim 1 wherein, reflectivity varies such that said first selected electromagnetic radiation provides for a first selected condition comprising at least 20% reflectivity and no more than ideally 50% reflectivity.

3. A microscope adapted for interference imaging according to Claim 1 wherein reflectivity varies as a function of wavelength so that the use of said second selected electromagnetic radiation provides for a second selected condition comprising less than 20% reflectivity.

4. A microscope adapted for interference imaging according to Claim 3 wherein said reflectivity is less than 10% reflectivity.

5. A microscope adapted for interference imaging according to Claim 3 or 4 wherein said reflectivity is less than 5% reflectivity.

6. A microscope adapted for interference imaging according to any preceding Claim wherein said reflectivity varies such that a change in wavelength of electromagnetic radiation of 200 nm results in a change in reflectivity between said first and said second conditions.

7. A microscope adapted for interference imaging according to

Claim 1-5 wherein said reflectivity varies such that a change in wavelength of electromagnetic radiation of 100-200 nm results in a change in reflectivity between said first and said second conditions.

8. A microscope adapted for interference imaging according to Claim 1 wherein said reflectivity varies such that a change in wavelength of electromagnetic radiation of 20-100 nm, or less, results in a change in reflectivity between said first and said second conditions.

9. A microscope adapted for interference imaging according to any preceding Claim wherein said means provided for selectively controlling the character of said electromagnetic radiation comprises at least one filter whose property/properties is/are selected having regard to said reflectivity.

10. A microscope adapted for interference imaging according to any preceding Claim comprising a beamsplitter composed or coated with a reflective material.

11. A microscope adapted for interference imaging according to

Claim 10 comprising a Mirau objective which is provided with a spot mirror.

12. A microscope adapted for interference imaging according to Claim 11 wherein said spot mirror is suitably sized, shaped and positioned to provide a Mirau reference mirror.

13. An automated imaging system comprising a microscope adapted for interference imaging according to Claims 1-12.

14. A Mirau interferometer comprising an objective and downstream thereof a beamsplitter such that electromagnetic radiation from a sample and electromagnetic radiation from a reference mirror can undergo interference so as to provide interference image data of said sample characterised in that said beamsplitter is adapted such that the reflectivity of same vary according to the character of the electromagnetic radiation, whereby use of a first selected electromagnetic radiation, or range thereof, enables a microscope, including said interferometer, to undertake interference imaging and use of a second, alternative, selected electromagnetic radiation, or range thereof, enables said microscope to undertake alternative imaging.

15. A Mirau interferometer according to Claim 14 comprising an objective which is characterised by a small spot mirror ideally positioned to provide a Mirau reference mirror.

16. A microscope according to Claims 1-12, or an imaging system according to Claim 13, or an interferometer according to Claim 14 wherein said alternative imaging comprises bright-field imaging.