SUBSURFACE INSPECTION METHOD AND SYSTEM

Abstract

A subsurface inspection method and system (1) for detecting internal defects (2) and/or overlay/misalignment in a semiconductor wafer (4). A measurement laser beam (6) is split into a laser probe beam (8) and a reference laser beam (10). The laser probe beam is transmitted to a wafer surface (12). A laser excitation pulse (14) is transmitted impinging the wafer surface for causing an ultrasound wave propagating through the wafer and causing wafer surface movement when reflected back from an encountered subsurface feature. The laser probe beam and the reference laser beam are recombined in an optical interference detector (18) and the subsurface feature inside the wafer is detected by a deviation of a detected phase difference. The laser probe beam and the reference laser beam are guided through an optic (20) prior to arriving at the optical interference detector. The optic has a dispersive characteristic dimensioned to enlarge the phase difference between the reference beam and the wave length shifted probe beam.

Claims

1. A subsurface inspection method comprising: splitting a measurement laser beam into a laser probe beam and a reference laser beam, transmitting the laser probe beam to a surface of a wafer, transmitting a laser excitation pulse impinging upon a target location on the surface of the wafer to generate an ultrasound wave propagating through the wafer, wherein a reflection of the ultrasound wave from an encountered subsurface feature inside the wafer causes a wafer surface movement at or near the target location, recombining the laser probe beam and the reference laser beam in an optical interference detector, and detecting the encountered subsurface feature inside the wafer by a deviation of a detected phase difference, wherein at least the laser probe beam is guided through an optic prior to arriving at the optical interference detector, wherein the optic has a dispersive characteristic dimensioned to enlarge the deviation of the detected phase difference between the reference laser beam and the laser probe beam to detect a phase difference between the reflected laser probe beam and the reference laser beam as a result of a wavelength shift of the probe beam during or after the laser excitation pulse.

2. The subsurface inspection method according to claim 1, wherein the laser probe beam and the laser reference beam are both transmitted to the surface of the wafer, wherein the reference laser beam formed by a first pulse configured to arrive at the surface of the wafer is synchronized with the laser excitation pulse, and wherein the laser probe beam formed by a second pulse is configured to arrive at the surface of the wafer after a predetermined delay, during which the surface of the wafer is moved as a result of the ultrasound wave reflected back from an encountered subsurface defect inside the wafer.

3. The subsurface inspection method according to claim 2, wherein the first pulse and the second pulse are guided through a common path, wherein the optic is provided in the common path, wherein the second pulse is delayed relative to the first pulse at a fixed temporal delay; and wherein the optic is selected in dimension and/or refractive index to suitably measure a phase difference between the first pulse and the second pulse.

4. The subsurface inspection method according to claim 2, wherein the measurement laser beam is formed by a single pulse that is split into two optical branches of different optical path lengths, and the two optical branches are recombined in a same optical path, to result in the first pulse and the second pulse of a fixed temporal delay.

5. The subsurface inspection method according to claim 1, wherein a resulting phase difference between the reflected wavelength shifted second pulse and the first pulse obtained as a result of at least the second pulse being guided through the optic is greater than a resulting phase difference obtained as a result of a static deflection of the wafer surface induced by the laser excitation pulse.

6. The subsurface inspection method according to claim 1, wherein the resulting phase difference obtained as a result of at least the second pulse being guided through the optic is at least 2 times greater than the resulting phase difference obtained as a result of a static deflection of the wafer surface induced by the laser excitation pulse.

7. The subsurface inspection method according to claim 1, wherein the optic has a thickness of at least 20 mm.

8. The subsurface inspection method according to claim 1, wherein the optic has a thickness in the range of 20-250 mm.

9. The subsurface inspection method according to claim 1, wherein the optic provides a chromatic dispersion of at least 0.005 m.sup.1.

10. The subsurface inspection method according to claim 1, wherein the optic is made out of a transparent solid or wherein the optic is made out of a transparent glass unit at least partially filled with a transparent solid, liquid, gas and/or vapor.

11. The subsurface inspection method according to claim 1, wherein the first pulse and the second pulse are a multiple number of times guided through the same optic prior to arriving at the optical interference detector.

12. A subsurface wafer inspection system comprising a laser interferometer having a controller configured to cause the system to carry out a wafer inspection operation comprising: splitting a measurement laser beam into a laser probe beam and a reference laser beam, transmitting the laser probe beam to a surface of a wafer, transmitting a laser excitation pulse impinging upon a target location on the surface of the wafer to generate an ultrasound wave propagating through the wafer, wherein a reflection of the ultrasound wave from an encountered subsurface feature inside the wafer causes a wafer surface movement at or near the target location, recombining the laser probe beam and the reference laser beam in an optical interference detector, and detecting the encountered subsurface feature inside the wafer by a deviation of a detected phase difference, wherein at least the laser probe beam is guided through an optic prior to arriving at the optical interference detector, wherein the optic has a dispersive characteristic dimensioned to enlarge the deviation of the detected phase difference between the reference laser beam and the laser probe beam to detect a phase difference between the reflected laser probe beam and the reference laser beam as a result of a wavelength shift of the probe beam during or after the laser excitation pulse.

13. The subsurface inspection method according to claim 6, wherein the phase difference obtained as a result of at least the second pulse being guided through the optic is at least 5 times greater than the resulting phase difference obtained as a result of a static deflection of the wafer surface induced by the laser excitation pulse.

14. The subsurface inspection method according to claim 6, wherein the phase difference obtained as a result of at least the second pulse being guided through the optic is at least 10 times greater than the resulting phase difference obtained as a result of a static deflection of the wafer surface induced by the laser excitation pulse.

15. The subsurface inspection method according to claim 6, wherein the phase difference obtained as a result of at least the second pulse being guided through the optic is at least 100 times greater than the resulting phase difference obtained as a result of a static deflection of the wafer surface induced by the laser excitation pulse.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0052] The invention will further be elucidated on the basis of exemplary embodiments which are represented in a drawing. The exemplary embodiments are given by way of non-limitative illustration. It is noted that the figures are only schematic representations of embodiments of the invention that are given by way of non-limiting example.

[0053] In the drawing:

[0054] FIG. 1 shows a schematic diagram of an embodiment of a system;

[0055] FIG. 2(a and b) shows a schematic diagram of an embodiment of a wafer being probed;

[0056] FIG. 3 shows a schematic diagram of an embodiment of a system; and

[0057] FIG. 4 shows a schematic diagram of a method.

DETAILED DESCRIPTION

[0058] FIG. 1 shows a schematic diagram of an embodiment of a subsurface wafer inspection system 1. The method can be employed for identifying internal subsurface features, such as defects or anomalies 2 in a semiconductor wafer 4, and/or for identifying overlay or misalignment errors in the semiconductor wafer 4.

[0059] The system 1 comprises a laser interferometer having a controller configured to carry out the steps of: splitting a measurement laser beam 6 into a laser probe beam 8 and a reference laser beam 10; transmitting the laser probe beam 8 to a surface 12 of the wafer 4; transmitting a laser excitation pulse 14 impinging upon a target location 16 on the surface 12 of the wafer 4 to generate an ultrasound wave propagating through the wafer 4, wherein the ultrasound wave causes a wafer surface movement at or near the target location 16 when reflected back from an encountered subsurface feature, such as defect 2 inside the wafer 4; recombining the laser probe beam 8 and the reference laser beam 10 in an optical interference detector 18 arranged for detecting a phase difference between the reflected laser probe beam 8 and the reference beam 10 as a result of a wavelength shift of the probe pulse during or after the laser excitation pulse 14; and detecting the subsurface feature 2 inside the wafer by a deviation of a detected phase difference; wherein at least the laser probe beam 8 is guided through an optic 20 prior to arriving at the optical interference detector 18, the optic 20 having a dispersive characteristic dimensioned to enlarge the phase difference between the reference beam 10 and the wave length shifted probe beam 8. In this example, both the reference laser beam 10 and the laser probe beam 8 are guided through the optic 20. However, it is also possible that only the (reflected) laser probe beam 8 is guided through the optic 20 (reference laser beam 10 not guided through the optic 20).

[0060] The system 1 enables a non-destructive wafer 4 subsurface characterization technique with a nanoscale resolution. Non-destructive testing is more feasible in terms of price, convenience and reliability. The subsurface characterization is carried out by determining the interaction of the wafer with acoustic waves, involving monitoring the effect of the acoustic wave passing through the material and reflecting as it is influenced by flaws or inhomogeneities in the wafer. Monitoring the effects can be done remotely without structural contact (using light). The laser excitation pulse 14 locally pumps sufficient energy in a short period of time causing an acoustic wave (e.g. ultrasound). The displacement of the surface 4 at or near the target location 16 of the wafer 4, which is caused by the induced acoustic wave reflected back from the internal subsurface feature 2, is determined on the basis of interference between the reflected laser probe beam 8 and reference beam 10.

[0061] Advantageously, the wavelength shift or Doppler shift is converted to an additional path-length difference between the reference beam 10 and the laser probe beam 8 by means of the dispersive optic 20 having tuned dispersive properties. In this way, the wavelength shift can be used for enhancing or amplifying the detected measurement signal at the optical interference detector 18.

[0062] In an advantageous way, subsurface nano-imaging may be enabled for detection and/or characterization of nanostructures buried below the surface of the wafer 4.

[0063] In an example, the optic 20 is a block or rod of a transparent material, such as glass. Many shapes and forms can be employed. It is also envisaged that a plurality of optics are employed, for instance sequentially. A holder may be provided for holding the plurality of optics together.

[0064] FIG. 2 shows a schematic diagram of an embodiment of a wafer 4 being probed by system 1. In this embodiment, the laser probe beam 8 and the laser reference beam 10 are both transmitted to the surface of the wafer 4 in the direction of arrow A in FIG. 2(a). The laser reference beam 10 is formed by a first pulse 10 configured to arrive at the surface 12 of the wafer 4 synchronized with the laser excitation pulse 14 (not shown). The laser probe beam 8 is formed by a second pulse 8 configured to arrive at the surface 12 of the wafer 4 after a predetermined delay, during which the surface of the wafer 4 is moved as a result of the ultrasound wave reflected back from an encountered subsurface feature 2 inside the wafer 4.

[0065] The laser excitation pulse 14 irradiates the surface 12 of the wafer 4. As a result, the surface 12 is locally heated up during a relatively short period of time (e.g. femto- or picoseconds) at or near the target location 16. After this, an acoustic pressure pulse is generated resulting in an acoustic shock wave 22 traveling or propagating from near the surface 12 towards the inside (i.e. subsurface) of the wafer 4, see arrow B in FIG. 2(a). When subsequently a subsurface feature 2 is encountered in an inner volume of the wafer 4, the acoustic wave 22 can at least partially be reflected back and cause movement of the surface 12 of the wafer 4 (deflection) at or near the target location 12, see arrow B in FIG. 2(b). A time resolved deflection of the surface 12 of the wafer 4 at or near the target location 12 may provide information about the internal structure of the subsurface feature 2 inside the wafer 4. Typically the movement of the surface 12 of the wafer 4 (i.e. deflection) is in the range of a few picometers occurs in the range of a few picoseconds. As a result, the instantaneous velocity of the surface 12 of the wafer 4 moving is in a meters/second range.

[0066] At the time the first pulse 10 arrives at the surface 12 of the wafer 4, the surface 12 is not moving. At the time of the second pulse 8 arrives at the surface of the wafer, the surface 12 is moving with a velocity in the order of meters per second. As a result, the second pulse 8 can have a wavelength shift. When light reflects on a (transversely) moving object, its wavelength or frequency will experience a Doppler frequency shift which can be calculated by means of the equation F=F.Math.((cv)/(c+v)), wherein v is the velocity of the surface 12 of the wafer 4 reflecting the light (acting as a mirror), c is the velocity of light. F is the frequency of the source, and F is the shifted Doppler frequency. For example, when light of 1550 nm (1.94e8 Hz) is used for probing the surface 12 of the wafer 4 moving at 1 meter/second, the resulting Doppler frequency-shift corresponds 1.3 Hz. The first pulse 10 and second pulse 8 are reflected as a reflected first pulse 10 and a reflected second pulse 8, see arrow A in FIG. 2(b). The reflected second pulse 8 will have a different frequency, and accordingly a different wavelength, than the second pulse 8 prior to arriving at the surface 12 of the sample 4. The reflected first pulse 10 will not have a wavelength shift. Advantageously, the detectability of the induced wavelength shift can be enhanced by means of the optic 20. For example, when the optic 20 is formed by a fused silica rod having a length of 10 centimeter, the optical path length difference between the nominal and the Doppler shifted beam amounts to approximately 975 picometer, which is much larger than the corresponding displacement (e.g. in picometer range) of the surface 12 of the wafer 4 as a result of the acoustic wave 22 reflected back from the encountered subsurface feature 2 within the wafer 4. In this example, an optional focusing lens 24 is arranged.

[0067] It is appreciated that a difference in path length between the first pulse 10 to the detector 18 and the second pulse 8 to the detector 18 can include a static and a dynamic contribution. The static contribution is a result of the static deformation/deflection of the surface of the sample 4, which causes the second pulse 8 to travel a smaller distance until reaching the detector 18, than the first pulse 10. The dynamic contribution is a result of the surface of the sample 4 moving with a certain velocity. Such motion of the surface of the sample 4 causes a wavelength shift (Doppler) in the reflected second pulse 8. The resulting wavelength shift in the reflected second pulse 8 results in an additional temporal distance between the first pulse 10 and the second pulse 8 as a result of the first pulse 10 and the second pulse 8 being guided through the dispersive optic 20. This additional temporal distance between the first pulse 10 and the second pulse 8 can correspond to an additional path length difference between the two pulses 10, 8, cf. the additional path length (distance) that the second pulse 8 should have traveled to result in the obtained additional temporal distance between the first pulse 10 and the second pulse 8 resulting due to the wavelength shift in the dispersive optic 20.

[0068] FIG. 3 shows a schematic diagram of an embodiment of a system 1. As shown in the previous embodiments, the system 1 may be configured to generate a photo-thermally generated acoustic wave (e.g. ultrasound wave) which can reflect back from an encountered subsurface feature 2, subsequently causing motion of the surface 12 of the wafer 4. The first pulse 10 and the second pulse 8 are optically guided such that after reflection they arrive at a same moment at the optical interference detector 18 in case the surface of the sample is not deformed or deflected (cf. common path length interferometer). When a deflection occurs at or near the target location 16, resulting from the photo-thermally generated acoustic wave reflected back from an encountered subsurface feature 2, the reflected first pulse 10 and the second pulse 8 will arrive at a different moment at the optical interference detector 18. Moreover, since the second pulse 8 reaches the surface of the sample in motion (deflection), the reflected second pulse 8 can have a wavelength shift with respect to the reflected first pulse 10, so that by means of the (dispersive) optic 20 the phase difference between the first pulse 10 and the wavelength shifted second pulse 8 (i.e. reflected back) can be enlarged.

[0069] At least the reflected second pulse 8 is guided through the optic 20 prior to arriving at the interference detector 18. It is possible that both the reflected first pulse 10 and the reflected second pulse 8 are guided through the optic 20 (as shown). However, it is also envisaged that only the reflected second pulse 8 is guided through the optic 20. The skilled person can make the necessary adjustments to obtain a common-path interferometric system.

[0070] In this embodiment, the first and second pulses 10, 8 are guided through a common path. The optic 20 is provided in the common path. The second pulse 8 is delayed relative to the first pulse 10 at a fixed temporal delay. The optic 20 is selected in dimension and/or refractive index to suitably measure a phase difference between the two pulses 10, 8. The measurement laser beam 6 is formed by a single pulse that is split into two optical branches of different optical path lengths, and recombined in a same optical path, to result in first and second pulses 10, 8 of a fixed temporal delay.

[0071] A first beam splitter 30 is arranged for splitting the first pulse 10 and the second pulse 8. The first pulse 10 travels to a first mirror 32. After reflection from the first mirror 32, the first pulse 10 traverses the first beam splitter 30 and is directed onto the surface 12 of the sample 4. Optionally, a focusing lens (not shown) is provided for focusing the light on the sample. The first pulse 10 is then reflected from the surface 12 of the sample 4. The reflected first pulse 10 is then directed via the first beam splitter 30 to the second mirror 36 and then passes the first beam splitter 30 again to return to the second beam splitter 34. The reflected first pulse 10 then traverses the dispersive optic 20 prior to reaching the optical interference detector 18. The second beam splitter 34 may for example be a non-polarizing beam splitter. The first beam splitter 30 may be a polarizing beam splitter.

[0072] The second pulse 8 is first reflected off the second mirror 36 before incidence on the surface 12 of the wafer 4. The optical length of the first arm 38 between the first beam splitter 30 and the second mirror 36 is configured to be longer than the optical length of the second arm 40 between the first beam splitter 30 and the first mirror 32 by a, preferably fixed, predetermined amount. In this way, it can be ensured that the second pulse 8 arrives at the surface 12 of the wafer 4 after (i.e. time delay) the first pulse 10. The second pulse 8 travels from the first beam splitter 30 to the second mirror 36. After reflection from the second mirror 36, the second pulse 8 traverses the first beam splitter 30 and is directed onto the surface 12 of the sample 4. The second pulse 8 is then reflected from the surface 12 of the sample 4. The reflected second pulse 8 is then directed via the first beam splitter 30 to the first mirror 32 and then passes the first beam splitter 30 again so as to return to the second beam splitter 34. The reflected second pulse 8 then also traverses the dispersive optic 20 prior to reaching the optical interference detector 18. Hence, a common path interferometric set-up can be employed. The second pulse 8 is guided to the optical interference detector 18 in which optical interference with the first pulse 10 can occur because of the common-path arrangement of the system 1. It is appreciated that the optical interference detector may comprise photodetectors. For example, variations in the optical phase difference between the first pulse 10 and the second pulse 8 reflected back from the wafer 4 can be monitored by taking the difference in output voltages of two photodetectors using a differential amplifier. Advantageously, because of the common-path arrangement, the exact temporal coincidence and interference of the reflected first pulse 10 and the reflected second pulse 8 can be guaranteed. In this example, optional waveplates 42 are arranged which are arranged for changing the polarization of light going therethrough. Such waveplates 42 may also be omitted.

[0073] Advantageously, by means of the optic 20 a nanoscale resolution can be obtained. The method can thus be used to detect subsurface features in depths of for example nanometers, or micrometers, or even deeper.

[0074] Furthermore, the common-path system 1 has no moving parts, providing an advantageous mechanical stability. The system 1 may have one or more moving optical components, for example to scan the time delay between the reference and the probe pulse onto the sample. The system 1 may be arranged to measure or image small changes in optical phase over a wide frequency range.

[0075] The system 1 may be configured to employ a train of first pulses and delayed second pulses on a target location 16 at the surface 12 of the wafer 4.

[0076] The reference beam 10 and the laser probe beam 8 originate from a same laser source 26. The laser excitation pulse 14 originates from a different laser source 28 and is guided to the surface 12 of the wafer by means of a mirror 15. However, in an example, the reference beam 10, laser probe beam 8 and the laser excitation pulse 14 may originate from a same light source.

[0077] In this example, the first pulse 10 and the second pulse 8 are incident normally on the surface of the wafer. However, it is also possible that the first pulse 10 and the second pulse 8 are at a different angle with respect to the surface of the wafer.

[0078] FIG. 4 shows a schematic diagram of a method 1000 for subsurface inspection method for identifying internal or subsurface features, such as for example defects or anomalies, in a semiconductor wafer and/or for determining overlay/misalignment errors in said semiconductor wafer 4. In a first step 1001, a measurement laser beam is split into a laser probe beam and a reference laser beam. In a second step 1002, a laser excitation pulse is transmitted impinging upon a target location on the surface of the wafer such as to generate an ultrasound wave which propagates through the wafer. In a third step 1003, the laser probe beam is transmitted to a surface of the wafer. It is appreciated that the third step 1003 may be performed simultaneously with or prior to the second step 1002, or vice versa. The generated ultrasound wave causes a wafer surface movement at or near the target location when reflected back from an encountered subsurface feature inside the wafer. In a fourth step 1004, at least the laser probe beam is guided through an optic having a dispersive characteristic dimensioned to enlarge the phase difference between the reference beam and the wave length shifted probe beam. Optionally, also the reference laser beam is guided through said optic. In a fifth step 1005, the laser probe beam and the reference laser beam are recombined in an optical interference detector which is arranged for detecting a phase difference between the reflected laser probe beam and the reference beam as a result of a wavelength shift of the probe beam during or after the laser excitation pulse. In a sixth step 1006, a feature inside the wafer is detected by a deviation of a detected phase difference.

[0079] When the generated ultrasound acoustic wave is reflected on a subsurface feature or structure inside the sample, the reflected acoustic wave will propagate back to the surface, and cause the surface to deform, wherein information about the three-dimensional structure of the sample can be inferred by measuring a time and position resolved wafer surface shape. By means of the optic, the precision can be improved.

[0080] The dispersive optic can be configured to transfer a difference in wavelength between the reference beam and the laser probe beam in a path-length difference therebetween. The path-length difference is added to the path-length difference resulting from a difference in the surface height of the wafer as a result of the deflection. It can be measured on the basis of interference between the reference beam and the laser probe beam.

[0081] Subsurface imaging can be carried out with increased accuracy in comparison to current subsurface measurement techniques. It may be possible to characterize subsurface features with nanometer resolution or even better, using interference measurement methods. The method can for example be utilized in semiconductor industry for inspection and/or characterization of produced wafers.

[0082] The method can improve the resolution, precision, signal-to-noise ratio, measurement time, throughput and/or detection limits of the photo-thermal acoustic imaging method.

[0083] It will be appreciated that an interferometer can be regarded as an optical system configured to split light from a single source into at least two beams that travel different optical paths, then combined again to produce interference. The resulting interference fringes in an interference detector can give information about the difference in optical path length.

[0084] It is appreciated that instead of a laser excitation beam/pulse also a different kind of optical beam/pulse can be employed, for example a LED beam pulse, a flashlamp beam/pulse, an infrared beam/pulse, etc. It is appreciated that instead of optical excitation, other forms of excitation can be employed, for example an acoustic transmitter or a mechanical device such as a piezo-type actuator.

[0085] The terms first, second, third and the like, as used herein may not denote any order, quantity, or importance, but rather are used to distinguish one element from another. They may be used to solely distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions.

[0086] Herein, the invention is described with reference to specific examples of embodiments of the invention. It will, however, be evident that various modifications and changes may be made therein, without departing from the essence of the invention. For the purpose of clarity and a concise description features are described herein as part of the same or separate examples or embodiments, however, alternative embodiments having combinations of all or some of the features described in these separate embodiments are also envisaged.

[0087] The subsurface wafer inspection may be implemented in an automated wafer inspection system arranged to inspect wafers in batch or continuously.

[0088] It will be appreciated that the method may include computer implemented steps. All above mentioned steps can be computer implemented steps. Embodiments may comprise computer apparatus, wherein processes performed in computer apparatus. The invention also extends to computer programs, particularly computer programs on or in a carrier, adapted for putting the invention into practice. The program may be in the form of source or object code or in any other form suitable for use in the implementation of the processes according to the invention. The carrier may be any entity or device capable of carrying the program. For example, the carrier may comprise a storage medium, such as a ROM, for example a semiconductor ROM or hard disk. Further, the carrier may be a transmissible carrier such as an electrical or optical signal which may be conveyed via electrical or optical cable or by radio or other means, e.g. via the internet or cloud.

[0089] Some embodiments may be implemented, for example, using a machine or tangible computer-readable medium or article which may store an instruction or a set of instructions that, if executed by a machine, may cause the machine to perform a method and/or operations in accordance with the embodiments.

[0090] Various embodiments may be implemented using hardware elements, software elements, or a combination of both. Examples of hardware elements may include processors, microprocessors, circuits, application specific integrated circuits (ASIC), programmable logic devices (PLD), digital signal processors (DSP), field programmable gate array (FPGA), logic gates, registers, semiconductor device, microchips, chip sets, et cetera. Examples of software may include software components, programs, applications, computer programs, application programs, system programs, machine programs, operating system software, mobile apps, middleware, firmware, software modules, routines, subroutines, functions, computer implemented methods, procedures, software interfaces, application program interfaces (API), methods, instruction sets, computing code, computer code, et cetera.

[0091] The sequence of operations (or steps) is not limited to the order presented in the figures and/or claims unless specifically indicated otherwise. Furthermore, the order in which various described method steps are performed may be changed in alternative embodiments, and in other alternative embodiments one or more method steps may be skipped altogether.

[0092] Herein, the invention is described with reference to specific examples of embodiments of the invention. It will, however, be evident that various modifications, variations, alternatives and changes may be made therein, without departing from the essence of the invention. For the purpose of clarity and a concise description features are described herein as part of the same or separate embodiments, however, alternative embodiments having combinations of all or some of the features described in these separate embodiments are also envisaged and understood to fall within the framework of the invention as outlined by the claims. The specifications, figures and examples are, accordingly, to be regarded in an illustrative sense rather than in a restrictive sense. The invention is intended to embrace all alternatives, modifications and variations which fall within the spirit and scope of the appended claims. Further, many of the elements that are described are functional entities that may be implemented as discrete or distributed components or in conjunction with other components, in any suitable combination and location.

[0093] In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word comprising does not exclude the presence of other features or steps than those listed in a claim. Furthermore, the words a and an shall not be construed as limited to only one, but instead are used to mean at least one, and do not exclude a plurality. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to an advantage.