IES83326Y1 - An optical spectrometer - Google Patents

Abstract

ABSTRACT An optical spectrometer (1,100,200) has a collector (15) for collecting a reflected probe, and a separate collector (23) for collecting luminescence. In one spectrometer (200) the collimated luminescence and reflectance beams are routed through an optical subsystem (6), a monochromator (7), and further optical components (8). Thus reflectance (16), calibration (11), and luminescence (24) detectors receive inputs from the same optical subsystem (8). This arrangement allows calibration (spectral accuracy and even correction or repair), as well as modulation reflectance spectroscopy and luminescence spectroscopy to be simultaneously performed without the need for a filter such as a notch filter.

Description

An optical spectrometer INTRODUCTION Field of the Invention The invention relates to an optical spectrometer.

Prior Art Discussion Modulation spectroscopy is a class of spectroscopy in which the reflectance (or transmission) of a material such as a semiconductor, or an organic material, or a polymer is altered at some parts of the electromagnetic spectrum by means of an external perturbation. Generally, this perturbation is applied in a periodic Thanner, such that the reflectance of the semiconductor at the wavelengths where it changes in response to the external perturbation, periodically alternates between the value in the absence of external perturbation, and that which it has in the presence of the external perturbation. In many methods of modulation spectroscopy, the perturbation is optically applied by means of a light beam. In such methods, the light beam used to perform the spectroscopy measurement is often referred to as the "probe" beam and the light beam which perturbs the reflectance of the material is generally referred to as the "pump" beam. The pump beam is generally coincident with the probe beam on the sample and is modulated between being present and absent at the area of coincidence with the probe beam.

Modulated reflectance spectroscopy in which the application of the external perturbation, in a periodic fashion, by means of a periodically modulated light beam directed on the material at the same point as the light beam used to perform reflectance spectroscopy, is commonly referred to as photoreflectance spectroscopy.

United States Patent Number 5,172,191 describes a method of photoreflectance spectroscopy in which the modulation of the reflectance of the sample material at the point of incidence of the probe beam is performed by sweeping the (modulating) pump light beam used as the external perturbation laterally in and out of coincidence with the reflected probe beam at the sample.

The invention is directed towards providing a spectrometer for more comprehensive analysis, so that more information about a sample material can be determined.

SUMMARY OF THE INVENTION According to the invention, there is provided an optical spectrometer comprising: an optical probe supply for delivering a probe beam to a sample; an optical pump supply for delivering a modulated pump beam to the sample coincident with probe beam; an optical probe receiver for receiving a reflected probe beam; an optical receiver for receiving luminescence from the area of incidence of the pump beam on the sample; and a detector and a processor for processing the receiver outputs to perform both modulation reflectance spectroscopy and luminescence spectroscopy.

In one embodiment, the probe supply comprises a monochromator, and the luminescence receiver routes its output through the same monochromator.

In another embodiment, the spectrometer further comprises a calibration light supply for directing a calibration beam through the same monochromator, and a detector for detecting the calibration beam after passing through the monochromator.

In a further embodiment, the probe receiver directs a modulated reflected beam through an optical channel of the same monochromator which is separate from luminescence, incident probe supply, and calibration beam optical channels.

In one embodiment, the spectrometer further comprises means for performing repair of spectral shifts of dispersed modulated reflectance and luminescence spectra optical channels.

In another embodiment, the spectrometer further comprises a cylindrical lens located before the monochromator for diverging the light onto a grating such that it illuminates a plurality of lines of the grating.

In a further embodiment, the monochromator further comprises a slit aperture for defining a line source which diverges onto the grating.

In one embodiment, the spectrometer further comprises means for directing a plurality of independent beams along the length of the cylindrical lens.

In another embodiment, the spectrometer further comprises means for recording the pump beam reflected from a sample, and a processor for fully normalising modulated reflectance.

In a further embodiment, the pump beam modulator is a spatial modulator, and the spectrometer further comprises a detector for feeding back position of the pump beam in a feedback loop, and the modulator varies pump beam spot position according to the feedback to suppress luminescence.

In one embodiment, the spectrometer further comprises a detector for feeding back pump beam intensity in a feedback loop, and the modulator varies pump beam spot intensity to suppress luminescence.

In another embodiment, said detector comprises a quadrant photodiode operating in summation mode for intensity detection and operating in differential mode for position detection.

In a further embodiment, the modulator varies the pump beam intensities and positions either independently or together.

DETAILED DESCRIPTION OF THE INVENTION Detailed Description of the Drawings The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only with reference to the accompanying drawings in which:- Fig. l is a diagrammatic overview of a system of the invention; Fig. 2 is a diagrammatic overview of an alternative system of the invention having a calibration means for a monochromator through which three light beams pass; Fig. 3 is a diagrammatic overview of an alternative system having passage of a double monochromated reflected beam through a monochromator through which four light beams pass; Fig. 4 is a diagram showing the passage of multiple light beams in spatially separate optical paths through one monochromator using cylindrical optics to couple the light beams through the monochromator; Fig. 5 shows delivery of multiple parallel light beams to the slit entrance of a single grating or wavelength-dispersive element monochromator by means of optical coupling of light beams to a monochromator using cylindrical optics; and Fig. 6 is a diagrammatic overview of another system of the invention.

Description of the Embodiments A system 1 of the invention comprises subsystems referred to below and illustrated in Fig. l. The system 1 comprises a reflectance (probe) light source 2 for producing a light beam having a broad spectrum of wavelengths. Optical components 3 shape the light beam and couple it to other components of the system. Optical components 6 including cylindrical optics combine several light beams together such that the beams are aligned parallel to each other along an axis perpendicular to that of a slit aperture of a monochromator 7, such that the beams converge towards the slit and expand beyond the slit only in the plane perpendicular to the axis of the slit.

The monochromator 7 disperses the wavelengths of light from one or more light beam produced by the probe light source 2 and/ or from other light sources or subsystems within an assembly such that only a narrow range of wavelengths of the light are selected and transmitted. Optical components 8 including cylindrical optics shape several light beams exiting the monochromator 7 and diverging in the plane perpendicular to the long axis of the monochromator exit slit aperture, such that the beams are shaped to near circular cross-sectional profiles and directed separately to different components of the system. The components 8 also include an additional cylindrical lens perpendicular to the first one to provide a collimated beam. An input probe beamsteering subsystem 12 comprises optical components for shaping a light beam 9, steering it, and coupling it to a sample material 13 as a probe beam.

A pump light source 17 produces a pump beam 19 having a single wavelength or a narrow spectrum of wavelengths. Optical components 18 shape the pump light beam and couple it to other components of the system. In one method of operation, the wavelength of the pump beam is chosen such that the corresponding photon energy is greater than that of the bandgap of a semiconductor to be inspected, or is greater than the separation of two molecular electronic energy levels of a chemical substance to be inspected, or is otherwise sufficient in photon energy to cause the creation of photo- induced charge carrier in the material to be inspected.

A pump beam optical modulator 20 modulates the amplitude or direction and point of incidence of the pump light beam (21, as modulated) on the sample material, and couple it to the sample material 13.

An output probe beam collection subsystem 15 comprises optical components for coupling both a reflected probe beam 14 and also some luminescence 22 from the sample 13, shaping the light, and coupling it to other components of the system. The subsystem operates such that it minimises the collection of luminescence 22. A luminescence collection subsystem 23 comprises optical components for coupling part of the luminescence light 22 scattered from the area of incidence of the pump light beam 21 on the sample 13.

The system also comprises a mechanical assembly to which the components are mounted, such that the beamsteering subsystems 12 and 15 are mounted with their optical axes at equal angles relative to the sample 13 such that the light beam 9 is reflected from the sample into the optical path of the output probe beam subsystem 15.

A reflectance beam detector 16, which may in some embodiments of the invention form part of the output probe beam subsystem, detects light from the output probe beam subsystem 15.

A luminescence detector 24 detects light 22 from the luminescence collection subsystem 23 via the components 6, 7, and 8. A pump beam detector 60 is mounted to collect and detect pump beam light reflected from the sample 13. .

An electronic controller records and processes electrical signals from the detectors l6 and 24 and controls the systems components. An electrical power subsystem provides mains and low voltage electrical power to the system.

A sample mounting subsystem supports the sample, and moves the sample 13 relative to the light beams.

Referring to Figs. 2 and 3 variations of the system 1 are shown, indicated by the numerals 100 and 200 respectively. Like parts are assigned the same reference numerals.

The system 100 shown in Fig. 2 comprises a calibration light source 4 for producing a calibration light beam 10 having in the spectrum of its emission at least one and preferably several welldefined characteristic unique wavelengths which are stable against significant spectral drift on a spectral scale resolvable by the monochromator 7, and which may be used for verifying at least one and preferably a number of spectral positions within the range of operation of the monochromator 7. Optical components 5 shape the calibration light beam 10 and couple it to other subsystems via optical components 6 and the monochromator 7. A calibration detector ll detects the calibration light beam 10 after it has been passed through the monochromator 7 on a spatially separate (independent) optical path to other light beams.

Referring to Fig. 3 the system 200 has similar functionality to the system l0O, except that in this case the output of the beamsteering subsystem 15 is routed through the optical components 6, which comprises optics for routing this beam to the reflectance detector 16 via the monochromator and the optical components 8.

Any of the systems 1, l00 or 200 may also include a wafer manipulation subsystem for selecting as sample 13 a semiconductor wafer which may have one of a range of diameters, from one or more cassettes or trays of such wafers, and placing the semiconductor wafer on the sample mounting subsystem such that a selected point on the wafer is at the point of incidence of the light beam from the input probe beam subsystem.

In operation, the reflectance or probe light beam 9 emerging from the monochromator 7 is optically coupled to the sample 13 by the set of optical components 8 which separates out the reflectance light beam 9 from the other light beams emerging in a parallel but spatially separate manner from the monochromator 7. The optical components 12 shape and steer the beam onto the sample 13.

The pump light source 17 is optically coupled to the sample 13 via the modulator 20 and the optical components 18 such that the modulated pump light beam 21 is periodically present and absent on an area of incidence which is co-incident with the area of incidence of the probe beam 9 on the sample 13, and for any angle of incidence of the probe beam on the sample which can be achieved using the mechanical assembly, at least fully covers the probe beam spot area on the sample 13. The optical components 18 may be mounted such that the angle of incidence of the pump beam 21 is normal to the sample surface 13 or is at some other angle of incidence.

The optical components 23 collect a portion of the luminescence from the area of co- incidence of the pump light beam 21 with the area of incidence of the reflectance beam 9 on the sample 13. The luminescence 22 beam emerging from the optical components 23 is optically coupled to the same monochromator 7 and the optical components 6, using a different optical path to that for the incident reflectance beam 9. The optics 23 select luminescence from one of two discrete modulation spots, whereas the optics 15 should collect equally for both spots. In this case the modulator 20 performs spatial modulation, whereby the pump beam is alternately incident at the probe spot on the sample and incident at a different location. This is achieved by use of an acousto—optic modulator which is driven with alternately different drive frequencies, the output pump beam 21 being a diffracted first order at one angle for one drive frequency and another angle for In general, the modulator operating parameters are as the other drive frequency. follows: rf drive frequency of the acousto-optic modulator crystal, of the order of l0O’s MHZ.

The frequency at which the beam is switched between the two discrete paths, namely the modulation or toggle frequency. This is typically in the range of hundreds of Hz to low MHz for modulation spectroscopy applications.

The angle between the two discrete paths (and thus the spatial separation on the sample) may be varied by varying one or both of the drive frequencies. Also, changing of both drive frequencies can be performed to achieve an equal shift of both beams with no mutual angle difference. Also, intensity of either or both beams may be varied by changing the amplitude of one or both of the drive frequencies. Furthermore, the duty cycle may be varied from 50:50 to any desired ratio by changing the modulation (toggling) duty cycle.

The use of a programmable controller for the acousto-optic modulator allows the intensity of the pump beam to be controlled at one or both locations with particular ease and versatility. A photosensitive detector positioned 60 to detect all or part of the pump beam reflected from the sample can form part of a feedback device of an intensity control mechanism. Such an intensity control mechanism can be used in modulation spectroscopy applications to vary the intensity of the modulated pump laser beam. The use of such a laser intensity feedback loop ensures the stability of the intensity of the laser spot in each of its two spatial positions of incidence on the sample. Beam spot position feedback may be performed to vary the beam positions on the sample (by drive frequency control as set out above). In particular, the spot position may be detected by a position sensitive detector (PSD) of the type having a quadrant photodiode. A PSD may be used also for intensity detection for intensity feedback. For position detection the PSD is operated in a differential mode, and for intensity feedback it is operated in a summation mode.

However, the modulation may alternatively be spatial modulation with a scanning mirror or alternatively an on-off modulation on the one path/ sample spot. Where spatial modulation is used, the optics 15 have positional symmetry with a centroid between the two spots. Also, irrespective of the form of modulation, the f-number (f/ D) of the optics 23 should be less than the f-number (f/ D) of the optics 15.

In the system 100 (Fig. 2) a calibration light beam 10 passes from the calibration source 4 through optical components 5 and 6 through the monochromator 7 and optical components 8 to the calibration detector ll.

In the system 200 (Fig. 3) the reflectance beam 14 is passed back through the monochromator 7. This assists with excluding residual scattered light from the pump beam 21 from the beam 14 and excluding luminescence at wavelengths other than the current wavelength of the reflectance beam (9 and I4) from the reflectance beam. This improves the signal-to-noise, avoids the need for a filter such as a notch or long pass filter and minimises also the spectrometer detection ‘blind-spot’.

The optical components 6 function to provide several discrete and spatially separate optical channels for light beams to pass through the monochromator 7. With reference to Fig. 4, parallel light beams (9, 10, 22) enter a cylindrical lens 25 of the optical components 6. The beams 9, l0, and 22 are focused in one dimension to pass through a slit 26 and then (in some cases reflecting off monochromator optics 27) the beams 9, l0, and 22 expand to fill the grating or other dispersive element 28 across its width, covering all grating rulings to obtain the best spectral resolution and throughput possible. The beams then (after in some cases reflecting off monochromator optics 29) exit through a slit 30 and are then reshaped as parallel light beams by the cylindrical lens 31. These three parallel beams are the reflectance beam 9, the calibration beam 10, and the luminescence beam 22 from the sample. In the system 200 the reflectance beam after reflection from the sample 14 is also passed through the monochromator. -1]- Optical components in an arrangement as shown in Fig. 5 form part or all of the optical components 6. A similar arrangement of optical components acting in the reverse manner forms part or all of the optical components 8. The several beams (9, 10, 14, and 22) to be passed through the monochromator 7 are aligned at different heights relative to the centre of the monochromator entrance slit 26 by means of optics and opto- mechanical components. Among such optics and opto-mechanical components may be included a series of mounts such as those shown at 32, 33, 34 in Fig. 5. Each of these mounts (for example 33, which comprises a number of clear apertures through which collimated beams (for example 9 and 22 in the case of mount 33) which are already aligned correctly for passage through the monochromator can pass through the mount 33, and a mirror can be placed in one or more of the clear apertures of the mount 33 at a height or heights where it is intended to introduce one or more extra parallel beams (for example 10) into the array of beams entering the monochromator. By this means, a full array of spatially separate parallel beams is assembled and enters a cylindrical lens 25 which focuses the beams through the entrance slit 26 such that they expand as required to fill all of the grating or dispersive element 28. The focusing action only occurs in the plane normal to the direction of the grating rulings or dispersive element axis.

An advantageous aspect of the invention is the combination of modulation reflectance spectroscopy and luminescence spectroscopy in a single apparatus using a single pump source and a single monochromator simultaneously, with the optional additional facility to have a continuously available calibration beam passing through the same monochromator. This affords the facility to calibrate every spectrum measured against a standard spectral light source, and with the additional facility to pass the modulated reflected beam back through another discrete optical channel of the same monochromator to reduce scattered modulated pump beam and modulated luminescence interferences in the beam.

This represents an improvement over prior art apparatus for both modulated reflectance spectroscopy and photo-induced luminescence or photoluminescence spectroscopy. The invention can be used to collect a periodically modulated luminescence signal from the same area of measurement as used from modulation spectroscopy, and is therefore amenable to the use of frequency—specific signal recovery techniques to discriminate against stray and scattered light interferences in the luminescence signal. The apparatus can also perform both methods of spectroscopy in parallel, saving time over the sequential measurement of each spectrum. Also, since the system uses the same dispersive elements (monochromator and grating), this ensures the exact same spectral response for both luminescence and modulation reflectance signals, in particular with the latter (modulated reflectance spectroscopy) highly localized spectrally.

Multiple passes of different light beams may be made through the same monochromator, assuming uniform grating action along the grating. Fig. 4 shows three beams being passed through the same monochromator, using cylindrical optics to expand each beam only in the direction perpendicular to the direction of the grating lines or rulings in the monochromator, in order to produce each beam distinctly onto a different region of the same grating. However, it is envisaged that there may be an arrangement with some overlap without crosstalk between all separate optical channels.

The probe beam is monochromated prior to its incidence on the sample, and the output probe beam is not monochromated after its reflection from the sample. Conventionally, a notch filter is used to eliminate a narrow band of wavelengths about the pump beam wavelength, to eliminate scattered pump light, which is modulated at the pump modulation frequency and therefore would contribute an a.c. interference additional to the a.c. modulated reflectance signal. In some cases, a long—wavelength pass filter is used instead of the notch filter, in cases where there is no interest in collecting the modulated reflectance signal at wavelengths shorter than that of the pump wavelength.

The effect of these schemes is that the entire spectral range of the portion of the a.c. luminescence signal collected by the output optics contributes another interference additional to the a.c. modulated reflectance signal.

With reference to Fig. 3, a second pass of the reflectance light beam l4 is made through the monochromator instead of a notch optical filter or long—wavelength pass optical filter to eliminate most of the wavelengths of the a.c. luminescence signal, apart from those at the specific wavelength of measurement to which the monochromator is adjusted. This aspect of the invention has an advantage over the use of an auxiliary monochromator which would have to be driven synchronously in wavelength with the monochromator 7, such that the auxiliary monochromator central wavelength would be within the engineering tolerances of both pieces of apparatus, that of the monochromator 7. Thus, the arrangement of the invention suppresses laser pump scatter and there is no need for a filter such as a notch filter. Also, there is the exact same spectral (dispersion) response as for modulated reflectance spectrometry, assuming a uniform grating.

The f-number of the spherical lens for the luminescence collection is less than the f- number of the cylindrical lens for collecting the modulated reflectance signal. The advantage is for modulation reflectance signal isolation as well as efficient a.c. luminescence signal recovery.

The invention may employ use of a linear scanning monochromator to achieve faster data acquisition. The monochromator may be a synchronously scannable monochromator whose central wavelength of transmission may be scanned continuously such that the rate of change of central transmission wavelength remains constant with time. This arrangement permits an improvement in the speed of spectral data acquisition compared to the prior art, by allowing the continual acquisition of modulated reflectance data by the detection and electronic subsystems, as the monochromator scans. This data may be accessed complete or downloaded after the acquisition is completed.

In the systems 1, 100, and 200 the probe beam is steered to the sample and the reflected probe beam is detected by the detector 16. The probe beam is modulated by the periodic illumination of the pump beam at a modulation frequency FL The detection of the time- invariant reflected probe beam intensity (denoted R) and any amplitude modulated time- variant component of the reflected probe beam intensity (denoted AR) at the spatial modulation frequency of the pump beam such that their ratio (denoted AR/ R) is known, at a number of different wavelengths of the probe beam. The above is performed such |\) (J: that the minimum of scattered light (including luminescence 22) from the sample is collected in the reflectance detector 16. Signal AR is detected by a lock-in amplifier 55 and R is detected by a meter 54 as a function of time. AR is typically several orders of magnitude smaller than the R signal.

The reflectance beam 9 is delivered to the sample as a monochromated beam, in order to expose the sample to the minimum possible intensity of light in the condition in which the pump beam 21 is extinguished.

The reflectance beam after reflection from the sample (14) is once again passed through a discrete optical path of the monochromator (7) to eliminate stray light and reduce any residual luminescence in the reflected beam to that at the specific wavelength of the monochromator. Pump beam induced scatter suppression is achieved without need for a filter such as a notch or long pass filter.

In a simple mode of operation luminescence spectroscopy is performed with: collection of the luminescence 22 signal arising from the area of incidence of the pump beam 21 on the sample 13; and measurement of the intensity of the luminescence 22 as a function of wavelength using the monochromator 7.

A calibration method of the invention comprises: detection of the calibration beam 10 passed through the monochromator 7 using the calibration detector 11; measurement of the relative intensity of the calibration beam 10 as a function of wavelength using the monochromator 7, and verification that one or more local intensity maxima which are expected to occur at wavelengths within the range over which the monochromator scans, are present at the correct wavelengths within the capability of the monochromator 7 to so resolve them.

Referring to Fig. 6 another system 300 comprises optical components in an arrangement of the type shown in Fig. 5. Again, like parts are assigned the same reference numerals.

A probe beam light source 2 coupled by means of a fibre optic 35 and a pair of lenses 36 and 25. The lens 36 has a suitable f-number compared to the f-number of the fibre optic , and the lens 25 is a cylindrical lens of focal length to cause parallel beams of a similar diameter entering it to converge to a line focus along the entrance slit 26 of the monochromator 7 and hence to diverge to fill the width of the grating 28 within the monochromator 7. This achieves a near-optimum spectral resolution and throughput from the monochromator 7. This produces monochromated light beams which are reshaped to parallel beams by a cylindrical lens 31. The probe beam 9 is optically coupled by a lens 37 into a fibre optic 38, and hence to the input optical probe beam subsystem 12 comprising a pair of lenses 39 and 40, the second of which 40 is the objective lens of a Galilean telescope, the eyepiece lens 41 of which is placed such that a parallel probe beam 9 of demagnified diameter is produced at the output of lens 41, and is focused using a high f-number lens 42 onto a sample material 13.

The pump optical source 17 is coupled by a mirror 18 to a synchronously alternating spatial modulator 20, which modulates the pump beam 19 into a pair of spatially distinct, alternately modulated pump beams 21 and 43. The beam 21 is directed to an area of coincidence with the area of incidence of the probe beam on the sample 13, and the beam 43 is directed to a different area of incidence on the sample 13.

The output optical probe beam subsystem 15 comprises a cylindrical lens 45 oriented such that it has a line focus or oblong near-focus area of confusion 44 overlapping both pump beam areas of incidence at the sample 13, and a spherical lens 46. In a modification of this embodiment a cylindrical lens 46 is used instead. The lens couple the beam through a filter 47 into a fibre optic 48, the output of which is coupled through a pair of lenses of suitable f-number 49 and 50 onto a photo-sensitive detector 51, which is a silicon photodiode detector or an indium gallium arsenide photodiode detector. The filter 47 is generally a notch filter or a long—pass filter having negligible transmission at the wavelength of the pump light source 17 but high transmission at least over a wide spectrum of wavelengths longer than the wavelength of the pump light source l7 and extending over the wavelengths at which the modulated reflectance of the sample l3 is to be measured.

In some embodiments of the invention, lenses other than cylindrical lenses are insofar as possible achromatic pair lenses, in order to reduce aberrations in the optical system of the invention.

The electrical signal produced by the detector 51 is coupled through a transimpedance stage 52 and a pre—amplifier device 53 to a lock-in amplifier 55 which uses a reference frequency signal derived from the same source as that driving the modulator 20. The signals read by the lock-in amplifier are read by the controlling computer 5 6, which may control several of the other modules of the system.

The lock-in amplifier 55 is used to measure and record the magnitude and phase of the modulated reflectance signal AR in the form of an a.c. voltage or current signals from the detector 51 at the frequency of modulation. The magnitude of the constant d.c. voltage or current signal from the detector 51 is also measured and recorded by the meter 54, whose output is read by the controlling computer 56. This constant d.c. voltage or current signal from the detector 51 is the unmodulated reflectance R of the sample at the transmission wavelength 7» of the monochromator (7), with a very small additional constant luminescence signal, which is negligible by comparison to the size of the reflectance signal.

Fig. 6 also shows that the pump beam reflected signal (RIM) 60 from the detector 62 is collected by an optic 61 and thus the ratio AR/ R is fully normalised by AR/ R x l/RIM. .17.

The detector 62 includes a quadrant photodiode detector as described above for intensity feedback in summation mode and position feedback in differential mode.

The result of the measurement is expressed as the dimensionless quantity AR/R x l/RIM. The measurement of AR/ R is repeated at a number of wavelengths by programmably adjusting the transmission wavelength of the monochromator 7, to acquire a spectrum of the modulated reflectance AR/ R R x l/Rm, of the sample l3.

In some modes of the invention, one or more phase shifts may be introduced into the modulated reflected probe beam intensity component electrical signal from the photodetector, such that the signal may be measured under several different phase conditions and a phase analysis may be performed. The lock-in amplifier may contain the necessary electronic devices to perform this phase shifting.

It will be appreciated that the invention provides an apparatus and method which improve upon the prior art by providing a means of combining, within a single spectrometer apparatus, using a single monochromator, the methods of modulated reflectance spectroscopy (called photoreflectance spectroscopy when the modulation of the reflection is performed by optical means) and luminescence or photoluminescence spectroscopy. In addition, the apparatus provides for the implementation of automatic calibration of the monochromator simultaneously with its operation (including possible spectral repair), and also provides for an improved method of performing photoreflectance spectroscopy with monochromation of both the incident and reflected light beam using the same monochromator.

The apparatus may be used for the performance of any method of modulated reflectance spectroscopy, using any method of modulation of the reflectance of the sample material, and is well suited to the implementation of those methods of modulated reflectance spectroscopy in which the luminescence signal is to be suppressed in the optics coupling the reflected beam from the sample to the photodetector for that beam. The apparatus provides further suppression of the luminescence signal in the reflected beam and pump to (Jr .13. scatter by passing the said beam through the same monochromator a second time, thus avoiding use of a filter such as a long pass or notch filter. Also the system ensures recovery and repair of spectral recording from the simultaneous calibration recording.

The apparatus may be used for the performance of any method of luminescence spectroscopy, and is well suited to the implementation of those methods of luminescence spectroscopy in which the luminescence signal is modulated by means of modulation of the photoexcitation or pump beam by any method, as well as those methods in which the photoluminescence signal appears as a time-invariant signal.

The invention finds application in the following technical fields, among others:- Measurement of the bandgap of semiconductor layers at room temperature.

Measurement of the alloy mole fraction in compound semiconductor layers and wafers.

Measurement of surface and interfacial electric fields in semiconductor layers and wafers.

Characterisation of electronic transitions and band structure in semiconductor layers and wafers.

Characterisation of semiconductor surfaces and interfaces.

Characterisation of chemical, ion, electron, or plasma induced damage or modification effects in semiconductor layers and wafers or at their surfaces and interfaces.

Characterisation of lattice mismatch in epitaxially grown semiconductor structures.

Characterisation of stress-induced strain effects in semiconductor layers and wafers.

Characterisation of quantum well semiconductor structures.

Characterisation of quantum dot semiconductor structures.

Characterisation of semiconductor heterostructures and related devices.

Characterisation of semiconductor laser and light-emitting structures and related devices.

Characterisation of organic and polymer light emitting devices and materials.

Characterisation of the active laser layer and etalon or reflecting cavity layers in a vertical cavity surface emitting laser or light emitting compound semiconductor structure Whether on an epitaxial wafer or in a material sample or device, such that their modulation spectroscopy responses are distinguished by means of performing the modulated reflectance measurement at more than one angle of incidence.

Measurement of the base-collector and base~emitter internal electric fields in a heterojunction bipolar transistor epitaxial wafer, or other internal, interfacial or surface electric fields in a multilayer epitaxial structure. It may also be applied to corresponding base-emitter and base-collector light intensity analysis.

Any of the measurements or characterisation applications listed above may be performed as a function of the application of an external stress to the sample, such as the heat applied during solder reflow or bonding or brasing of a semiconductor or other material of device to a second material or device.

The invention is not limited to the embodiments described but may be varied in construction and detail.

Claims (1)

1.Claims An optical spectrometer comprising: an optical probe supply for delivering a probe beam to a sample; an optical pump supply for delivering a modulated pump beam to the sample coincident with probe beam; an optical probe receiver for receiving a reflected probe beam; an optical receiver for receiving luminescence from the area of incidence of the pump beam on the sample; and a detector and a processor for processing the receiver outputs to perform both modulation reflectance spectroscopy and luminescence spectroscopy. An optical spectrometer as claimed in claim 1, wherein the probe supply comprises a monochromator, and the luminescence receiver routes its output through the same monochromator; and further comprising a calibration light supply for directing a calibration beam through the same monochromator, and a detector for detecting the calibration beam after passing through the monochromator; and wherein the probe receiver directs a modulated reflected beam through an optical channel of the same monochromator which is separate from luminescence, incident probe supply, and calibration beam optical channels. An optical spectrometer as claimed in any preceding claim further comprising means for recording the pump beam reflected from a sample, and a processor for fully normalising modulated reflectance; and wherein the pump beam modulator is a spatial modulator, and the spectrometer further comprises a detector for feeding back position of the pump beam in a feedback loop, and the modulator varies pump beam spot position according to the feedback to suppress luminescence; and wherein the spectrometer further comprises a detector for feeding back pump beam intensity in a feedback loop, and the modulator varies pump beam spot intensity to suppress luminescence. An optical spectrometer as claimed in claim 3, wherein said detector comprises a quadrant photodiode operating in summation mode for intensity detection and operating in differential mode for position detection. An optical spectrometer substantially as described with reference to the drawings. 52 51 50 49