GB2241350A - Raman spectrometer - Google Patents

Abstract

A Raman spectrometer (10) comprises a measuring unit with a light microscope (11), and an evaluation unit (12) separate from the measuring unit (11), in particular an infrared spectrometer. There are further provided a light source (13), first optical ray guiding means (21, 23, 25, 31 to 37) for guiding a ray (30) between the light source (13) and a sample (39), and second optical ray guiding means (22, 24, 26, 34 to 38, 41, 42) for guiding the light ray (40) emanating from the sample (39) to the evaluation unit (12). At least one of the ray guiding means comprises a light guide in the form of a fiber-optic cable (23, 24). <IMAGE>

Description

2 --> -cl iL --3 ----1 c) 11.

1 Raman Spectrometer The present invention relates to a Raman spectrometer comprising a measuring unit and an evaluation unit separate from the measuring unit, in particular an infrared spectrometer, further a light source, first optical ray guiding means for guiding a ray between the light source and a sample, and second optical ray guiding means for guiding the light emanating from the sample to the evaluation unit.

A Raman spectrometer of the type described above has been known from the German Publication entitled "Bruker Report of 1/1989 ' pages 2 and 3.

1 1 The known Raman spectrometer consists of an additional unit and a commercial Fourier transform (FT) infrared spectrometer. The known Raman spectrometer comprises an integrated Nd:YAG laser operating at a wave length of 1 064 nm, and further the necessary ray guiding means, a sample holder and a detector having high sensitivity and a low noise factor and operating at close infrared. The known Raman spectrometer enables measurements to be carried out at a scattering angle of 900 or 1800, and comprises filters intended to suppress any Rayleigh scattered light.

For signal processing and signal evaluation, the known Raman spectrometer attachment makes use of the infrared spectrometer whose evaluation system is supplied with the detector signal of the Raman spectrometer.

However, it has been found to be a disadvantage in certain applications that the Raman spectrometer attachment must be arranged in the immediate neighborhood of the evaluation unit, i.e. of the FT infrared spectrometer. This is also true for the Nd:YAG laser employed which requires particular care with respect to its installation and operation as the high intensity of the light of this laser, which is required for Raman experiments, may be harmful to the human eye.

EP-OS 116 321 describes an infrared spectrometer where on the one hand the sample under examination is exposed to the measuring light, while on the other hand the measuring spot on the sample can be observed visually at the same time. One obtains in this manner a so-called "infrared microscope", the lens system of the known spectrometer being designed in such a way that a microscopically small area only can be defined by visual observation which can then be examined 1 1 1 - 3 - with the aid of infrared rays. In order to be absolutely sure that the visually defined area is identical to the area exposed to the measuring light, a mirror is, or can be, positioned in the path of the measuring light rays so as to make the ray path of the observation light and that of the measuring light coincide, from the observation mirror to the sample. The observation mirror either may be designed to enable it to be moved out of the ray path of the measuring light, or else - as in the case of the known spectrometer a semi-permeable mirror may be employed which may then remain in the ray path during both the observation and the measuring phases.

From EP-PS 183 706 there has been known an electrically switchable mask for opto-spectroscopic measurements. This masks allows a sample to be exposed to a matrix of measuring-light dots. The dot matrix is generated by a switchable mask, and the position of the dots is connected through according to a predetermined code. It is thus possible, by suitable coding of the dot matrix, to attain an improvement of the signal-to-noise ratio due to the fact that a plurality of measuring spots are always illuminated simultaneously.

Now, it is the object of the present invention to improve a Raman spectrometer of the kind described at the outset in such a way as to permit visual observation of the measuring spot and to enable the Raman spectrometer attachment to be set up in a modular and flexible manner relative to the evaluation unit.

The invention solves this object by the fact that the measuring unit consists of a largely unmodified, commercial light microscope equipped with a coupling and decoupling 1 device for the light ray arriving from the light source and the Raman light emanating from the sample, and that at least one of the ray guiding means comprises a light guide in the form of a fiber-optic cable.

This solves the object underlying the invention fully and perfectly because the use of a light microscope and the connection via a fiberoptic cable allow the Raman spectrometer to be set up in a modular and highly flexible way. The fiber-optic cable may be provided for connecting the light source to the measuring unit and/or for connecting the measuring unit to the evaluation unit.

In both cases, the necessary units can be exchanged for different measuring purposes with only a few manipulations, it being only necessary to change the connections of the fiber-optic cables using suitable optical fiber connectors. So, it is for example possible to use different light sources, or to connect the Raman spectrometer to evaluation units of different kinds.

According to a preferred embodiment of the spectrometer according to the invention, the first optical ray guiding means and the second optical ray guiding means comprise a common dichroic filter which is permeable to the light beam arriving from the light source, and which reflects the Raman light emanating from the sample, the latter having a different wave length.

This feature provides the advantage to permit accurate spectral separation. In particular, this filter also has a suppressing effect with respect to Raman lines produced in the glass as the light rays pass through the fiber-optic cable.

i i i The light emanating from the sample, preferably, can be fed to both the second optical ray guiding means and the visual observation means.

This feature provides the advantage that the measuring site of the sample can be exactly adjusted with the aid of the visual observation means. 4 This is true in particular because the visual observation means comprises a microscope and this permits geometrically selective measurements to be carried out even on extremely small structures, such as semiconductor components or other miniature structures, by positioning the light beam arriving from the light source on a geometrically defined site of the sample, under visual observation through the microscope, and by carrying out the measurement thereafter.

The same applies by analogy if the visual observation means comprises a camera, preferably a video camera with CCD image converter. The use of a CCD image converter is particularly advantageous because the wave length of the light required for Raman measurements is such that CCD elements also respond to it.

Further, an arrangement is particularly preferred where the light emanating from the sample is guided via a beam splitter and where the latter transmits to the visual observation means less than 10 %, preferably 4 % of the light emanating from the sample.

This feature provides the advantage that on the one hand visual observation is rendered possible using the measuring light, the intensity of which can be reduced by the beam splitter to such a degree that it can be regarded by the CCD - 6 camera or else, without any risk, by the human eye, while on the other hand the intensity of the measuring light is not notably reduced.

Other advantages of the invention will appear from the specification and the attached drawing.

It is understood that the features that have been described before and will be explained hereafter may be used not only in the described combinations, but also in any other combination, or individually, without leaving the scope and intent of the present invention.

One embodiment of the invention will now be described in more detail with reference to the drawing in which:

Fig. 1 shows a top view - cut in part - through one embodiment of a Raman spectrometer according to the invention, taken along line I-I in fig. 2; and fig. 2 shows a side view of part of the Raman spectrometer illustrated in fig. 1, along line II-II in fig. 1.

Regarding now the figures, a Raman spectrometer is indicated generally by reference numeral 10. The spectrometer 10.comprises a measuring unit 11 which will be discussed in more detail below. The measuring unit 11 has its output connected to an evaluation unit 12. The evaluation unit 12 may be either a Fourier infrared spectrometer, in which case it must comprise a detector suited for Raman wave lengths, or a usual Raman spectrometer.

A light source 13 supplies the measuring unit 11 with k - 7 measuring light. The light source 13, preferably, consists of a laser, in particular a Nd:YAG laser.

The measuring unit 11 comprises a housing 20, which is attached to a light microscope 15 and provided with a first connection 21 and a second connection 22 for a first fiberoptic cable 23 and a second fiber-optic cable 24, respectively. The first fiber-optic cable 23 connects the first connection 21 to a corresponding third connection 25 at the light source 13. The second fiber optic cable 24 connects, analogously, the second connection 22 to a corresponding fourth connection 26 on the evaluation unit 12.

It will be readily appreciated that it is easily possible in this manner to connect the measuring unit 11 to the most different evaluation units 12 or light sources 13, by simply changing the connections in the usual manner. It is also in this sense that it has been said above that the evaluation unit 12 may also be a Raman spectrometer of the usual kind because in this case measuring light can be directed from the measuring unit 11 via the second fiber-optic cable 24 to the Raman spectrometer serving as evaluation unit 12, in order to be finally supplied to a Raman detector.

An arriving light ray 30 is supplied from the light source 13 to the first connection 21 via the first fiber-optic cable 23, and then into the interior of the housing 20.

The arriving light ray 30 passes initially a first magnifying lens 31 which preferably can be exchanged to allow for magnification ratios of between 1:1 and 1:5.

The light ray 30 then passes a, preferably fixed, first diaphragm 32 and a second, preferably also fixed, magnifying lens 33 having a magnification ratio of 1:10, for example.

Thereafter, the light ray 30 passes a dichroic filter 34 which is permeable to the wave length of the arriving light ray 30. The light ray 30 then passes a second, preferably variable, diaphragm 35 which, being an iris diaphragm, is capable of adjusting stop values of between 1 and 8, for example.

A first deflector 36 then deflects the arriving light ray 30 to the right in fig. 1, and toward a second deflector 37 of, preferably, partially permeable design.

As can be clearly seen in fig. 2, the second deflector 37 is inclined in downward direction by 450 so that the arriving light ray 30 is now deflected in vertically downward direction where it will come to impinge upon the lens system 38 of the light microscope 15 - which is indicated extremely diagrammatically in the drawing - from where it will finally be directed upon a sample 39.

The light emanating from the sample 39 is initially directed, via a light path 40, upon the same optical ray guiding means, namely the second deflector 37 and the first deflector 36, from where the reflected light is guided to the filter 34 - still along the same optical path as the arriving light ray 30, though in opposite direction - the filter 34 being designed in such a way as to be reflecting to the wave length of the transmitted Raman light, but permeable to any scattered light simultaneously transmitted from the sample 39.

In this connection, the following should be noted:

The Nd:YAG laser serving as light source 13 emits a light ray 30 having a wave length of 1.06 pm, corresponding approximately to 9 400 wave numbers. In the case of Raman spectroscopy, lines are now produced in the spectrum which are somewhat offset against the line of the incoming light. These lines are the result of oscillations of the molecules of the sample 39. The wave lengths of the transmitted Raman light, therefore, are offset relative to the wave length of the arriving light ray 30, this offset being equal to approximately 50 to 4000 wave numbers for usual Raman spectra.

Now, the dichroic filter 34 is designed in such a way as to be permeable to the arriving light ray 30, but reflecting to the wave lengths of the transmitted Raman light.

Consequently, the transmitted light is deflected, on its light path 40, by the dichroic filter 34 and directed upon another, preferably fixed deflector 41 from where it is guided to the second connection 22, via a convex lens 42, and from there to the evaluation unit 12 via the second fiber-optic cable 24.

It has already been mentioned that the second deflector 37 is partially permeable. As will be readily appreciated from fig. 2, this has the consequence that part of the light emanating from the sample 39 passes through the second deflector 37 along the light path 40' and moves on in vertically upward direction where there are provided, in the case of the Raman spectrometer 10 illustrated in the drawings, on the one hand the ocular 51 of the microscope 15 and, on the other hand, a video camera 52.

In order to enable the microscope 15 to be further used for normal lightmicroscopic tasks, a common slide 55 is provided by means of which the usual bright-field 53 and dark-field illumination devices 54 of the microscope 15 can be displaced in such a way that one of the units 37, 53 or 54, in certain cases also a plurality of them, are at any time positioned in the ray path 40' of the microscope 15.

The Raman spectrometer 10 allows the following operating mode:

When a sample 39, in particular a sample having a microstructure, is brought into the position shown in fig. 2, then the sample 39 can be observed through the ocular 51 or the video camera 52. The light required for this purpose may be derived from an light source that can be switched on separately, in which case lamps not shown in the drawing would have to be switched on; on the other hand, however, it is also possible to observe the sample 39 with the aid of the scattered light, i.e. via the light path 40', as indicated in fig. 2, or to make use of the illumination means 53, 54 of the microscope 15.

Given the fact that for physical reasons, Raman experiments have to be carried out with light rays of high intensity, the light intensity necessarily has to be drastically reduced in the area of the microscope 50 in order to avoid damage to the observer's eye. The second deflector 37, therefore, is designed in such a way as to permit only an extremely small part of the measuring light to pass, i.e. less than 10 %, preferably approximately 4 %. This also has the advantage that less light is lost for the measuring process.

1 k_ If the video camera 52 is equipped with usual CCD components for image conversion, the laser light of the Nd:YAG laser can be utilized directly, usual CCD components being responsive to this wave length.

It is understood that the illustrated embodiment is meant only to illustrate, and not to restrict the invention. In fact, numerous modifications or changes of the embodiments illustrated in the figures are possible without leaving the scope of the present invention.

According to one further improvement of the invention, a single-core or monomode cable may be used as fiber-optic cable 23 or 24. With the aid of these cables, the laser light can be focussed optimally upon a small measuring spot on the sample.

If laser light is guided through a fiber-optic cable, scattering phenomena in the glass also give rise to Raman lines which, according to other preferred embodiments of the invention, are filtered out. These filters may coincide with the dichroic filter 34, as regards their function, but may also be arranged at a different point in the ray path of the light rays 30, depending on the circumstances of the particular case.

The same applies analogously to scattered light (Rayleigh light) from the sample space which may likewise be filtered out by suitable filters in the ray path 40.

Further, it is also possible to vary the size of the "focal spot" of the laser on the sample 39 by exchanging and/or varying the elements in the ray path of the arriving light ray 30, as indicated before.

Moreover, it is also possible to displace the sample 39 in a defined manner, by means of a suitable mechanical stage, along at least two coordinate directions for the purpose of either selecting a desired measuring spot, or for carrying out, if desired automatically, a plurality of measurements on different spots.

Other details which may be employed with advantage in connection with the Raman spectrometer 10 according to the present invention have been described, for example, by EP-OS 116 321 which discloses an infrared spectrometer permitting visual observations to be performed on microscopic areas of the sample and infrared measurements to be carried out either at the same time or in succession.

Further, if several almost point-shaped areas of the sample surface are to be measured, a position coding system may be used with advantage for determining and processing the measuring points. In this case, approximately half of the sample is illuminated at any time by means of a corresponding electrically switchable mask, and the pattern of the illuminated spots is varied (permutated) according to a predetermined code (so-called Hadamard code). A mask of this type has been known from EP-OS 183 706. With respect to the use of such masks in spectrometers, it has been a problem heretofore that they only have a very limited useful spectral range. However, this limitation is of no importance for the present purpose, as in this case only light of a single wave length has to be coded. Consequently, the use of Hadamard masks is of particular advantage in a Raman spectrometer with light microscope. The mask may be installed, for example, in the housing 11, in place of the diaphragm 32.

1 1 i i i i - 13 c 1 a i m s:

Raman spectrometer comprising a measuring unit (11) and An evaluation unit (12) separate from the measuring unit (11), in particular a Fourier infrared spectrometer, further a light source (13), first optical ray guiding means (21, 23, 25, 31 to 38) for guiding a ray (30) between the light source (13) and a sample (39), and second optical ray guiding means (22, 24, 26, 34 to 38, 41, 42) for guiding the light ray (40) emanating from the sample (39) to the evaluation unit (12), w h e r e i n the said measuring unit (11) consists of a largely unmodified, commercial light microscope (15)-equipped with a coupling and decoupling device for the light ray (30) arriving from the said light source (13) and the light emanating from the said sample (39), and at least one of the ray guiding means comprises a light guide in the form of a fiber-optic cable (23, 24).

2) Raman spectrometer according to claim 1, - w h e r e i n a first fiber-optic cable (23) is provided for connecting the said light source (13) to the said measuring unit (12).

3) Raman spectrometer according to claim 1 or 2, w h e r e i n a second fiber-optic cable (24) is provided for connecting the said measuring unit (11) to the said evaluation unit (12).

1 t.

j 4) 5) Raman spectrometer according to one or more of claims 1 to 3, w h e r e i n said first optical ray guiding means and said second optical ray guiding means comprise a common dichroic filter (34) which is permeable to the light beam (30) arriving from the said light source (13), and which reflects the Raman light emanating from the said sample (39) along a path (40) leading away from the said sample (39).

Raman spectrometer according to one or more of claims 1 to 4, w h e r e i n the light emanating from the said sample (39), along a path (40) leading away from the said sample (39), can be fed to the said second optical ray guiding means (22, 24, 26, 34 to 38, 41, 42) and to visual observation means.

6) Raman spectrometer according to claim 5, w h e r e i n the light emanating from the said sample (39) along the said path (40) is-guided through a beam splitter (37).

7) Raman spectrometer according to claim 6, w h e r e i n the said beam splitter (37) transmits to the said visual observation means less than 10 %, preferably 4 % of the light emanating from the said sample (39).

8) Raman spectrometer according to one or more of claims 5 to 7, w h e r e i n the said visual observation means comprise a camera (52), preferably a video camera having a CCD image converter.

I 1 1 IC - is - 9) Raman spectrometer according to one or more of the preceding claims, w h e r e i n an electrically switchable, socalled Hadamard mask is arranged in the said housing (11) in the ray path (30), for local coding purposes.

10) A Raman spectrometer substantially as herein described with reference to the accompanying drawings h Published 1991 at The Patent office. Concept House. CardifT Road, Newport. Gwent NP9 1 RH. Further copies maY beSotbMtained from 7HZ. Printed by Multiplex techniques ltd. ary Cray. Rent.

Sales Branch. Unit 6. Nine Mile Point. Cvmfelinfach. Cross Revs, Newport. NPI